{"gene":"OXSR1","run_date":"2026-04-29T11:37:57","timeline":{"discoveries":[{"year":2005,"finding":"WNK1 and WNK4 directly phosphorylate OSR1 at two sites: Thr185 in the T-loop of the catalytic domain and Ser325 in a C-terminal non-catalytic region. Phosphorylation of Thr185 is required for OSR1 activation; T185A mutation prevents activation by WNK1, while T185E (phosphomimetic) increases basal activity >20-fold. Mutation of Ser325 does not affect activity or WNK1-mediated activation.","method":"In vitro kinase assay, phosphopeptide mapping, site-directed mutagenesis","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution with mutagenesis, multiple orthogonal methods","pmids":["16083423"],"is_preprint":false},{"year":2005,"finding":"WNK1 phosphorylates OSR1 at an evolutionarily conserved serine residue outside the kinase domain, and mutation of this residue causes enhanced OSR1 kinase activity. SPAK and OSR1 directly phosphorylate the N-terminal regulatory regions of cation-chloride cotransporters NKCC1, NKCC2, and NCC. Phosphorylation of NCC is induced by hypotonic stress in cells.","method":"In vitro kinase assay, cell-based phosphorylation assays, hypotonic stress treatment","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro kinase assay plus cell-based validation, replicated across labs","pmids":["16263722"],"is_preprint":false},{"year":2002,"finding":"OSR1 physically interacts with the cation-chloride cotransporters KCC3, NKCC1, and NKCC2 (but not KCC1 or KCC4) through the last 100 amino acids of OSR1. The binding motif on cotransporters consists of nine residues starting with (R/K)FX(V/I). Co-immunoprecipitation confirmed in vivo interaction of the related kinase SPAK with NKCC1 from mouse brain.","method":"Yeast two-hybrid, GST pull-down, co-immunoprecipitation, immunohistochemistry","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — reciprocal binding confirmed by multiple methods including in vivo co-IP","pmids":["12386165"],"is_preprint":false},{"year":2006,"finding":"OSR1 exists in a complex with WNK1 in cells, is activated by recombinant WNK1 in vitro, and is phosphorylated in a WNK1-dependent manner in cells. Depletion of WNK1 by siRNA reduces OSR1 kinase activity. Depletion of either WNK1 or OSR1 reduces NKCC activity in HeLa cells, placing both in a pathway required for NKCC function and volume regulation.","method":"Co-immunoprecipitation, in vitro kinase assay, siRNA knockdown, NKCC activity assay","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — reciprocal co-IP, in vitro assay, and functional siRNA knockdown with defined phenotype","pmids":["16832045"],"is_preprint":false},{"year":2006,"finding":"OSR1 possesses a 92-residue conserved C-terminal (CCT) domain that interacts with RFXV motifs present in both substrates (NKCC1) and activators (WNK1/WNK4). A peptide containing the RFXV motif binds the CCT domain with nanomolar affinity. Mutation of specific CCT domain residues inhibits OSR1-mediated phosphorylation of NKCC1 (which requires RFXV docking) but not of CATCHtide (which lacks RFXV). An intact CCT domain is required for WNK1 to efficiently phosphorylate and activate OSR1.","method":"In vitro kinase assay, peptide binding/affinity purification, CCT domain mutagenesis, CATCHtide peptide substrate development","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1–2 — reconstituted binding and kinase assays with systematic mutagenesis, nanomolar affinity measurements","pmids":["16669787"],"is_preprint":false},{"year":2006,"finding":"OSR1 shares similar kinase properties to SPAK and similarly activates NKCC1 when co-expressed with WNK4 in Xenopus oocytes. OSR1 kinase activity is markedly inhibited by staurosporine and K252a, partially inhibited by N-ethylmaleimide and diamide, and unaffected by arsenite. Activity is stronger with Mn2+ than Mg2+.","method":"Xenopus oocyte expression system, in vitro kinase assay, pharmacological inhibitor profiling","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1–2 — functional reconstitution in oocytes with mutagenesis and pharmacological characterization","pmids":["16382158"],"is_preprint":false},{"year":2007,"finding":"Crystal structure of the CCT domain of OSR1 in complex with a WNK4-derived RFXV motif-containing peptide was solved, revealing a novel protein fold with a surface-exposed groove that mediates peptide binding. Phosphorylation of a Ser/Thr residue preceding the RFXV motif causes steric clash, promoting dissociation from the CCT domain. Mutational analysis confirmed that the groove interactions are required for binding to WNK1 and NKCC1.","method":"X-ray crystallography, mutational analysis, binding assays","journal":"EMBO reports","confidence":"High","confidence_rationale":"Tier 1 — crystal structure at molecular resolution with functional mutagenesis validation","pmids":["17721439"],"is_preprint":false},{"year":2008,"finding":"SPAK and OSR1 phosphorylate human NCC at three conserved residues (Thr46, Thr55, Thr60) in response to WNK1 pathway activation by hypotonic low-chloride conditions. Efficient NCC phosphorylation requires docking interaction between an RFXI motif in NCC and SPAK/OSR1. Mutation of Thr60 to Ala markedly inhibits phosphorylation of Thr46 and Thr55 and abolishes NCC activation by hypotonic low-chloride treatment.","method":"In vitro kinase assay, site-directed mutagenesis, cell-based phosphorylation assay in HEK293 and mpkDCT cells","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro assay plus cell-based validation with mutagenesis across two cell lines","pmids":["18270262"],"is_preprint":false},{"year":2004,"finding":"OSR1 (oxidative stress-responsive 1) is a 58-kDa, 527-amino acid STE20 family kinase widely expressed in mammalian tissues. Endogenous OSR1 is activated specifically by osmotic stresses (notably sorbitol, and to a lesser extent NaCl). OSR1 does not activate JNK, p38, ERK2, or ERK5. OSR1 phosphorylates PAK1 at Thr84 in its N-terminal regulatory domain; T84E substitution reduces PAK1 activation by Cdc42, suggesting OSR1 modulates G-protein sensitivity of PAK.","method":"Two-hybrid screen, in vitro kinase assay, site-directed mutagenesis, osmotic stress activation assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — original biochemical characterization with in vitro kinase assay and mutagenesis","pmids":["14707132"],"is_preprint":false},{"year":2009,"finding":"Crystal structure of the OSR1 kinase domain was solved at 2.25 Å resolution, revealing a domain-swapped dimer in an inactive conformation in which the P+1 loop and αEF helix are swapped between dimer-related monomers. Structural integrity of chemical interactions is preserved in the domain-swapped interfaces.","method":"X-ray crystallography (2.25 Å resolution)","journal":"Protein science","confidence":"High","confidence_rationale":"Tier 1 — high-resolution crystal structure","pmids":["19177573"],"is_preprint":false},{"year":2011,"finding":"MO25α and MO25β bind to OSR1 (and SPAK) and induce ~100-fold activation of their kinase activity, dramatically enhancing phosphorylation of ion cotransporters NKCC1, NKCC2, and NCC. siRNA-mediated reduction of MO25 in mammalian cells inhibits phosphorylation of endogenous NKCC1 at SPAK/OSR1-dependent sites, rescued by re-expression of MO25α. MO25 associates with OSR1 in a manner analogous to its interaction with the STRAD pseudokinase.","method":"In vitro kinase assay, co-immunoprecipitation, siRNA knockdown, rescue experiments, surface plasmon resonance","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro reconstitution with ~100-fold activation, confirmed by siRNA knockdown and rescue in cells","pmids":["21423148"],"is_preprint":false},{"year":2011,"finding":"In vivo genetic evidence confirms that NCC phosphorylation by the WNK4 D561A mutation is almost completely abolished in triple knock-in mice where both SPAK (T243A) and OSR1 (T185A) cannot be activated by WNK kinases. This establishes that NCC phosphorylation in vivo is dependent on the WNK-OSR1/SPAK cascade, and that PHAII caused by WNK4 D561A requires OSR1/SPAK activation.","method":"Genetic knock-in mouse model, immunoblotting for phospho-NCC, blood pressure and metabolic measurements","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 — clean genetic epistasis in vivo with defined phenotypic readout","pmids":["21486947"],"is_preprint":false},{"year":2011,"finding":"SPAK/OSR1 directly phosphorylate NKCC2 isoforms at Thr95, Thr100, Thr105 (and possibly Ser91) via interaction with an RFQV docking motif on NKCC2. A SPAK-OSR1-independent kinase (possibly AMPK) phosphorylates Ser130. Phosphorylation of Thr105 and Ser130 plays the most important role in stimulating NKCC2 activity. Unlike NCC, NKCC2 is constitutively membrane-localized and not regulated by SPAK/OSR1-dependent membrane translocation.","method":"In vitro kinase assay, cell-based phosphorylation assay, site-directed mutagenesis, NKCC2 isoform analysis","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 1–2 — systematic mutagenesis of phosphorylation sites combined with functional activity assays","pmids":["21321328"],"is_preprint":false},{"year":2012,"finding":"SPAK/OSR1 double-knockin ES cells (where both kinases cannot be activated by WNK1) show no phosphorylation or activation of NKCC1, providing genetic evidence that SPAK/OSR1 are essential for NKCC1 function. WNK1 and WNK3 activities are markedly elevated in these knockin cells, demonstrating that SPAK/OSR1 feedback to influence upstream WNK activity. Phosphorylation of WNK1 Ser1261 is unaffected, indicating it is not a SPAK/OSR1 substrate.","method":"Double-knockin ES cells, immunoblotting, NKCC1 activity assay, in vitro kinase assay","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 2 — genetic knockin cells with orthogonal functional and biochemical readouts","pmids":["22032326"],"is_preprint":false},{"year":2012,"finding":"In the kidney, OSR1 activity is dependent on SPAK in the distal convoluted tubule (DCT) but not the thick ascending limb (TAL). In SPAK-null mice, OSR1 becomes largely inactive, displaced from MO25α and NCC at the apical membrane, and redistributes to dense cytoplasmic punctate structures containing WNK1. This demonstrates that SPAK is required for proper OSR1 localization and activity in DCT, and that OSR1 apical membrane localization is functionally linked to NCC phosphorylation.","method":"SPAK knockout mice, immunofluorescence, subcellular fractionation, immunoblotting","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — genetic KO model with direct localization experiments tied to functional consequence","pmids":["22977235"],"is_preprint":false},{"year":2012,"finding":"PI3K/mTORC2 activates OSR1 by phosphorylating it on Ser339. Inhibition of PI3K reduces OSR1 activation by osmotic stress, and depletion of Sin1 (mTORC2 component) decreases OSR1 activation by sorbitol and reduces NKCC activity in HeLa cells. Mutation of Ser339 eliminates mTORC2-dependent OSR1 phosphorylation, identifying a convergent regulatory node on OSR1 from the PI3K pathway.","method":"In vitro kinase assay (mTORC2 phosphorylates OSR1), pharmacological inhibition, siRNA knockdown of Sin1, site-directed mutagenesis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro phosphorylation assay with mutagenesis plus cell-based validation","pmids":["24191005"],"is_preprint":false},{"year":2012,"finding":"OSR1 participates in regulation of renal tubular phosphate transport via NaPiIIa. Coexpression of OSR1 in Xenopus oocytes significantly upregulates phosphate-induced currents in NaPiIIa-expressing oocytes. In osr1tg/(+) mice carrying a WNK-resistant OSR1, urinary phosphate excretion is increased and NaPiIIa protein abundance in brush border membrane is reduced, associated with elevated serum FGF23.","method":"Xenopus oocyte electrophysiology, mouse knock-in model, immunohistochemistry, confocal microscopy","journal":"Kidney & blood pressure research","confidence":"Medium","confidence_rationale":"Tier 2 — functional reconstitution in oocytes plus in vivo mouse model, single lab","pmids":["23095210"],"is_preprint":false},{"year":2012,"finding":"The phosphorylation levels of OSR1, SPAK, and NCC exhibit a diurnal rhythm in mouse kidney that correlates with active/rest periods. This circadian rhythm of the WNK4-OSR1/SPAK-NCC cascade is abolished by eplerenone (aldosterone receptor blocker), indicating aldosterone governs the circadian oscillation of this kinase cascade.","method":"Time-course immunoblotting in mice, pharmacological treatment with eplerenone","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2–3 — in vivo time-course with pharmacological intervention, single lab","pmids":["23044422"],"is_preprint":false},{"year":2012,"finding":"ASK3 (apoptosis signal-regulating kinase 3) interacts with WNK1 and suppresses the WNK1-SPAK/OSR1 signaling pathway. Knockdown of ASK3 enhances SPAK/OSR1 activation, and ASK3 knockout mice show hyperactivation of SPAK/OSR1 in renal tubules and a hypertensive phenotype, positioning ASK3 as an upstream suppressor of the WNK1-SPAK/OSR1 pathway in response to osmotic stress.","method":"Co-immunoprecipitation (WNK1-ASK3 interaction), siRNA knockdown, ASK3 knockout mice, immunoblotting","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — genetic KO model with biochemical pathway analysis plus siRNA validation","pmids":["23250415"],"is_preprint":false},{"year":2012,"finding":"Estradiol treatment of newborn rat pups significantly increases protein levels of OSR1 (and SPAK) in the hypothalamus via a transcription-dependent mechanism. OSR1 knockdown by antisense oligonucleotides precludes estradiol-mediated enhancement of NKCC1 phosphorylation and diminishes estradiol-enhanced Ca2+ influx induced by GABAAR activation in embryonic hypothalamic cultures.","method":"In vivo neonatal rat treatment, immunoblotting, antisense knockdown, calcium imaging","journal":"The Journal of neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 — antisense knockdown with functional Ca2+ imaging readout, single lab","pmids":["22238094"],"is_preprint":false},{"year":2013,"finding":"WNK1 activates the OSR1 signaling cascade to regulate embryonic cardiovascular development. Global or endothelial-specific deletion of Osr1 in mice causes embryonic lethality at ~E11 with identical cardiovascular defects to WNK1 knockout (no mature large vessels in yolk sacs, defective angiogenesis, smaller cardiac chambers). Endothelial-specific expression of constitutively active OSR1 rescues angiogenesis and cardiac defects in global WNK1-null embryos, establishing OSR1 as an essential downstream effector of WNK1 in vascular development.","method":"Conditional knockout mice (Tie2-Cre), constitutively active OSR1 transgene rescue, embryonic phenotype analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — genetic rescue experiment definitively placing OSR1 downstream of WNK1 in vascular development","pmids":["23386621"],"is_preprint":false},{"year":2014,"finding":"WNK-activated SPAK/OSR1 (in the presence of MO25) directly phosphorylates all KCC isoforms at Site-2 (Thr1048 in KCC3A) in vitro, promoting their inhibition. In ES cells lacking SPAK/OSR1 activity, endogenous KCC Site-2 phosphorylation is abolished and KCC3A activity is elevated. A Site-2 alanine KCC3A mutant preventing SPAK/OSR1 phosphorylation shows increased activity, confirming that SPAK/OSR1 phosphorylation inhibits KCC-mediated Cl- efflux. SPAK/OSR1 also phosphorylate KCCs at Site-3 (Thr5/Thr6) and Site-4 (Ser96).","method":"In vitro kinase assay with MO25, knockin ES cells lacking SPAK/OSR1 activity, 86Rb+ uptake assay, site-directed mutagenesis","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro reconstitution plus genetic knockin cells with functional transport assay and mutagenesis","pmids":["24393035"],"is_preprint":false},{"year":2014,"finding":"WNK1-OSR1 signaling is required for HUVEC cord formation, chemotaxis, and invasion. OSR1 (but not SPAK) is specifically required for HUVEC chemotaxis and invasion, while SPAK is required for proliferation. OSR1 and WNK1 regulate the zinc-finger transcription factor Slug in endothelial cells. Constitutively active OSR1 rescues WNK1 deficiency, confirming OSR1 acts downstream of WNK1 in endothelial function.","method":"siRNA knockdown, cord formation assay, chemotaxis/invasion assay, gene expression analysis, HUVEC culture","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — clean siRNA KD with distinct phenotypic readouts differentiating OSR1 from SPAK functions","pmids":["25362046"],"is_preprint":false},{"year":2014,"finding":"OSR1 activates inward rectifier K+ channels Kir2.1 and Kir2.3 via an R-x-F-x-V/I motif variant (distinct from the canonical RFXV). Mutation of the RxFxV motif in Kir2.3 prevents its activation by OSR1. Both siRNA knockdown of OSR1 and chemical WNK inhibition disrupt NaCl-induced plasma membrane localization of Kir2.3, suggesting OSR1 enhances Kir2.1/Kir2.3 activity by increasing their plasma membrane localization. Kir4.1, which lacks this motif, is insensitive to OSR1.","method":"siRNA knockdown, pharmacological WNK inhibition, plasma membrane localization assay, site-directed mutagenesis of binding motif, Xenopus oocyte expression","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — motif mutagenesis plus siRNA and pharmacological validation with defined localization readout","pmids":["29581290"],"is_preprint":false},{"year":2014,"finding":"OSR1 directly interacts with and phosphorylates Smad2 at Thr220 and Smad3 at Thr179 in the linker region. This phosphorylation causes nuclear translocation of Smad2/3 and enhances TGF-β1 autocrine signaling, driving epithelial-to-mesenchymal transition (EMT) and metastasis in breast cancer cells. OSR1 deletion in aggressive breast cancer cells inhibits EMT, and disruption of the OSR1-Smad2/3-TGF-β1 axis reduces metastasis in vitro and in vivo.","method":"Co-immunoprecipitation, in vitro kinase assay, site-directed mutagenesis, nuclear translocation assay, loss-of-function (CRISPR/deletion), in vivo metastasis models","journal":"Oncogene","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro kinase assay with site-specific mutagenesis plus in vivo functional validation","pmids":["33051597"],"is_preprint":false},{"year":2014,"finding":"OSR1 negatively regulates the creatine transporter SLC6A8 in Xenopus oocytes. Constitutively active (T185E)OSR1 reduces creatine transport, but catalytically inactive (D164A)OSR1 does not, indicating the effect requires OSR1 kinase activity. WNK-insensitive (T185A)OSR1 still negatively regulates SLC6A8, suggesting a WNK-independent mechanism for this substrate.","method":"Xenopus oocyte expression system, dual electrode voltage clamp, constitutively active and catalytically inactive OSR1 mutants","journal":"Kidney & blood pressure research","confidence":"Medium","confidence_rationale":"Tier 2 — functional reconstitution in oocytes with multiple mutant controls, single lab","pmids":["25531585"],"is_preprint":false},{"year":2014,"finding":"Active OSR1 (T185E) downregulates ROMK1 K+ channel protein abundance at the cell membrane and channel activity in Xenopus oocytes; catalytically inactive (D164A)OSR1 has no effect. This is consistent with OSR1 kinase activity-dependent regulation of ROMK1 membrane trafficking.","method":"Xenopus oocyte expression, dual electrode voltage clamp, chemiluminescence-based surface protein assay","journal":"Kidney & blood pressure research","confidence":"Medium","confidence_rationale":"Tier 2 — functional reconstitution with activity-dependent mutants, single lab","pmids":["25322850"],"is_preprint":false},{"year":2014,"finding":"Active OSR1 (T185E) negatively regulates ClC-2 Cl- channel activity in Xenopus oocytes. WNK1-insensitive inactive (T185A)OSR1 and catalytically inactive (D164A)OSR1 have no effect. Brefeldin A experiments suggest OSR1 does not accelerate ClC-2 retrieval from the membrane, indicating a different regulatory mechanism.","method":"Xenopus oocyte expression, dual electrode voltage clamp, brefeldin A treatment","journal":"Kidney & blood pressure research","confidence":"Medium","confidence_rationale":"Tier 2 — functional reconstitution with multiple mutants, single lab","pmids":["25323061"],"is_preprint":false},{"year":2014,"finding":"WNK1-OSR1 signaling promotes glioma cell migration. siRNA knockdown of WNK1 or OSR1 reduces intracellular K+ and Cl- content and abolishes NKCC1 regulatory phospho-activation, impeding regulatory volume increase in glioma cells. Temozolomide activates the WNK1/OSR1/NKCC1 pathway and enhances glioma migration, which is blocked by OSR1 siRNA knockdown.","method":"siRNA knockdown of OSR1, live cell imaging, microchemotaxis assay, cell volume and ion measurements, immunostaining","journal":"Molecular cancer","confidence":"Medium","confidence_rationale":"Tier 2 — siRNA KD with functional migration and ion transport readouts, single lab","pmids":["24555568"],"is_preprint":false},{"year":2014,"finding":"OSR1 downregulates peptide transporters PEPT1 and PEPT2 by decreasing their maximum transport rate and reducing PEPT2 protein abundance at the cell membrane in Xenopus oocytes. The effect requires OSR1 kinase activity (T185E mimics, D164A and T185A are inactive), suggesting kinase-dependent membrane protein regulation.","method":"Xenopus oocyte electrophysiology, chemiluminescence surface protein assay, constitutively active and kinase-dead mutants","journal":"Kidney & blood pressure research","confidence":"Medium","confidence_rationale":"Tier 2 — functional reconstitution with multiple OSR1 mutants, single lab","pmids":["25531100"],"is_preprint":false},{"year":2014,"finding":"Drosophila WNK-Fray (SPAK/OSR1 homolog) signaling cascade regulates K+ flux through Ncc69 (NKCC) in the renal tubule. Fray directly phosphorylates Ncc69 in vitro. A constitutively active Fray mutant rescues the wnk knockdown phenotype, establishing hierarchical WNK→Fray→NKCC signaling for hypotonicity-stimulated K+ flux, demonstrating evolutionary conservation of the WNK-SPAK/OSR1-NKCC cascade.","method":"Drosophila genetics, in vitro kinase assay, constitutively active mutant rescue, ion flux assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro kinase assay plus genetic epistasis rescue in Drosophila ortholog","pmids":["25086033"],"is_preprint":false},{"year":2015,"finding":"OSR1 upregulates the intestinal Na+-coupled phosphate cotransporter NaPi-IIb (SLC34A2) in Xenopus oocytes. Constitutively active (T185E)OSR1 and wild-type OSR1 (but not T185A or D164A mutants) significantly increase phosphate-induced currents and maximal transport rate, demonstrating that kinase activity is required. Combined SPAK+OSR1 coexpression shows an additive effect.","method":"Xenopus oocyte expression, dual electrode voltage clamp, constitutively active and kinase-dead mutants","journal":"Kidney & blood pressure research","confidence":"Medium","confidence_rationale":"Tier 2 — functional reconstitution with systematic mutants, single lab","pmids":["26506223"],"is_preprint":false},{"year":2016,"finding":"SPAK and OSR1 are essential for K+ homeostasis through their actions on the distal convoluted tubule. SPAK/OSR1 double knockout (DKO) mice develop severe hypokalemia on K+ restriction, unlike single KO mice. DKO mice fail to phosphorylate NCC under K+-restricted conditions, demonstrating that SPAK and OSR1 are redundant but essential effectors of the K+-sensing pathway that maintains plasma K+ through NCC activation.","method":"SPAK/OSR1 double knockout mice, dietary K+ restriction, plasma K+ measurement, phospho-NCC immunoblotting","journal":"The Journal of physiology","confidence":"High","confidence_rationale":"Tier 2 — clean genetic double KO model with defined physiological phenotype and biochemical readout","pmids":["27068441"],"is_preprint":false},{"year":2017,"finding":"Rafoxanide and closantel (antiparasitic agents) inhibit SPAK and OSR1 by binding to a highly conserved allosteric pocket on their C-terminal domains. This allosteric site influences kinase activity and can be exploited for inhibitor design.","method":"In silico screening, in vitro kinase assay, binding studies","journal":"ChemMedChem","confidence":"Medium","confidence_rationale":"Tier 2–3 — in vitro kinase inhibition with binding characterization, single lab","pmids":["28371477"],"is_preprint":false},{"year":2018,"finding":"Verteporfin (photosensitizing clinical agent) inhibits SPAK and OSR1 kinases by binding to their kinase domains in an ATP-independent manner. In cells, verteporfin suppresses phosphorylation of NKCC1, a downstream physiological substrate of SPAK and OSR1.","method":"In vitro kinase assay, kinase panel screening, cell-based phosphorylation assay","journal":"Chembiochem","confidence":"Medium","confidence_rationale":"Tier 2–3 — in vitro and cell-based inhibitor characterization, single lab","pmids":["29999233"],"is_preprint":false},{"year":2019,"finding":"The E3 ubiquitin ligase complex CUL4-DDB1-WDR3/WDR6 binds OSR1 in a phosphorylation-dependent manner via the S-motif (C-terminal serine phosphorylation site). S-motif phosphorylation following osmotic stress disrupts CUL4 complex binding, and OSR1 ubiquitylation is abolished under osmotic stress when its S-motif is phosphorylated, revealing a mechanism linking WNK-dependent S-motif phosphorylation to OSR1 proteostasis.","method":"Affinity pull-down, mass spectrometry, proteasomal and neddylation inhibitors, immunoblotting","journal":"Chembiochem","confidence":"Medium","confidence_rationale":"Tier 2–3 — AP-MS identification with pharmacological validation, single lab","pmids":["31614064"],"is_preprint":false},{"year":2019,"finding":"In hypokalemia, WNK4 is the primary active WNK isoform that phosphorylates SPAK/OSR1 within cytoplasmic WNK bodies in the DCT. Phosphorylated SPAK/OSR1 is present both at the apical membrane and in WNK bodies within 12 h of K+ deprivation. In WNK4-deficient mice, larger WNK bodies form containing unphosphorylated WNK1, SPAK, and OSR1. WNK body formation in DCT requires sensing of plasma K+ via the Kir4.1 channel.","method":"Immunofluorescence microscopy, WNK4-deficient and Kir4.1 knockout mice, dietary K+ manipulation","journal":"American journal of physiology. Renal physiology","confidence":"High","confidence_rationale":"Tier 2 — multiple genetic mouse models with direct subcellular localization and phosphorylation readouts","pmids":["31736353"],"is_preprint":false},{"year":2012,"finding":"OSR1 activity is required for LPS-stimulated migration of dendritic cells. In OSR1 knockin (WNK-resistant) dendritic cells, Na+/H+ exchanger activity and ROS production are elevated at baseline, but LPS fails to further increase them or stimulate migration. NKCC1 inhibition with furosemide increases NHE activity to levels similar to OSR1-deficient cells, suggesting OSR1-NKCC1 interaction regulates NHE activity.","method":"WNK-resistant OSR1 knockin mice, dendritic cell culture, FACS, fluorescent pH indicators, transwell migration assay","journal":"American journal of physiology. Cell physiology","confidence":"Medium","confidence_rationale":"Tier 2 — knockin mouse model with multiple functional readouts, single lab","pmids":["22648948"],"is_preprint":false},{"year":2021,"finding":"Pro-resolving FPR2 agonists (WKYMVm and annexin A1) induce phosphorylation of OSR1 at Ser339 via NADPH oxidase-dependent activation of PI3K and PKCδ. Blockade of NADPH oxidase prevents FPR2-induced OSR1(Ser339) phosphorylation, identifying NADPH oxidase→PI3K→OSR1(Ser339) as a pro-resolving signaling axis.","method":"Pharmacological NADPH oxidase inhibition, CRISPR/Cas9 p22phox knockout cells, phospho-specific immunoblotting","journal":"Antioxidants","confidence":"Medium","confidence_rationale":"Tier 2 — CRISPR KO validation plus pharmacological inhibition, single lab","pmids":["33477989"],"is_preprint":false},{"year":2022,"finding":"WNK1 HSN2 splice variant regulates neurite outgrowth through OSR1 activation and downstream GSK3β. The HSN2-OSR1/GSK3β signaling axis induces expression of LHX8, a key regulator of cholinergic neural function. HSANII patient-derived HSN2 mutants suppress OSR1 activation, LHX8 induction, and neurite outgrowth, and prevent interaction between wild-type HSN2 and GSK3β.","method":"Cell-based neurite outgrowth assay, OSR1 activation measurements, co-immunoprecipitation, LHX8 expression analysis, patient mutation analysis","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 — genetic patient mutations analyzed functionally with pathway validation, single lab","pmids":["36151370"],"is_preprint":false},{"year":2005,"finding":"OSR1 was identified through yeast two-hybrid screening as interacting with RELL1 (a RELT family member). OSR1 interacts with all three RELT family members (RELT, RELL1, RELL2) by co-immunoprecipitation and phosphorylates them in an in vitro kinase assay, identifying the RELT family as OSR1 substrates.","method":"Yeast two-hybrid, co-immunoprecipitation, in vitro kinase assay","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2–3 — yeast two-hybrid confirmed by co-IP and in vitro kinase assay, single lab","pmids":["16389068"],"is_preprint":false},{"year":2012,"finding":"Phosphorylation of NCC by OSR1/SPAK kinases decreases NCC ubiquitination. In mice, low-salt diet decreases NCC ubiquitination while high-salt diet increases it (inversely correlated with phosphorylation and membrane abundance). In COS7 cells, phospho-deficient NCC shows increased ubiquitination while phospho-mimicking NCC shows decreased ubiquitination for both total and surface NCC, establishing that OSR1/SPAK-mediated phosphorylation of NCC protects it from ubiquitin-mediated degradation.","method":"In vivo mouse models, COS7 cell expression, immunoprecipitation, ubiquitination assay, surface biotinylation","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 — in vivo and cell-based validation with phosphomimetic mutants, single lab","pmids":["22846565"],"is_preprint":false},{"year":2023,"finding":"Disruption of the WNK4-SPAK/OSR1 pathway using a new C57BL/6-specific pT96-NKCC2 antibody reveals that SPAK/OSR1 only mildly affects NKCC2 phosphorylation at baseline; NCC phosphorylation is strongly dependent on this pathway while NKCC2 is not. In WNK4-/- and SPAK/OSR1 double KO mice, pT96-NKCC2 is only slightly or not significantly reduced, suggesting alternative kinases phosphorylate NKCC2.","method":"New phospho-specific antibody, Wnk4-/-, Osr1-/-, Spak-/-, DKO mice, immunoblotting, furosemide diuresis assay","journal":"American journal of physiology. Renal physiology","confidence":"High","confidence_rationale":"Tier 2 — multiple genetic mouse models with new validated antibody tool, systematic in vivo analysis","pmids":["37881876"],"is_preprint":false}],"current_model":"OSR1 (OXSR1) is a STE20-family serine/threonine kinase that is activated by WNK1/WNK4 phosphorylation of its T-loop Thr185 and further stimulated ~100-fold by the MO25 scaffold protein; it recognizes substrates and activators via RFXV/RxFxV docking motifs in its CCT domain, directly phosphorylates and activates Na+-K+-2Cl- cotransporters (NKCC1, NKCC2) and NCC while inhibiting K+-Cl- cotransporters (KCCs), and additionally regulates Kir2.1/2.3, ROMK1, ClC-2, phosphate transporters, and Smad2/3; it is subject to feedback regulation via mTORC2 (Ser339) and CUL4-mediated ubiquitination, and plays essential roles in renal salt/K+ homeostasis, embryonic angiogenesis, neuronal chloride homeostasis, and glioma cell migration."},"narrative":{"teleology":[{"year":2002,"claim":"Identification of the substrate-docking mechanism: OSR1 was shown to physically interact with NKCC1, NKCC2, and KCC3 via a C-terminal domain recognizing (R/K)FX(V/I) motifs, establishing how OSR1 selects its cotransporter substrates.","evidence":"Yeast two-hybrid, GST pull-down, and co-immunoprecipitation with NKCC/KCC cotransporters","pmids":["12386165"],"confidence":"High","gaps":["Structural basis of CCT–RFXV interaction not yet resolved","Functional consequence of interaction (phosphorylation, activation) not demonstrated"]},{"year":2004,"claim":"Initial biochemical characterization established OSR1 as an osmotic stress-activated STE20-family kinase that phosphorylates PAK1 but does not activate canonical MAPK cascades, defining its distinct signaling niche.","evidence":"In vitro kinase assay, site-directed mutagenesis, osmotic stress activation in cells","pmids":["14707132"],"confidence":"High","gaps":["Physiological relevance of PAK1-Thr84 phosphorylation unclear","Upstream activator identity not established"]},{"year":2005,"claim":"The upstream activation mechanism was solved: WNK1/WNK4 directly phosphorylate OSR1 at Thr185 in its T-loop to activate it, and OSR1/SPAK directly phosphorylate the N-terminal regulatory domains of NKCC1, NKCC2, and NCC, establishing the hierarchical WNK→OSR1→cotransporter cascade.","evidence":"In vitro kinase assays with phosphopeptide mapping and T185A/T185E mutagenesis; cell-based phosphorylation assays under hypotonic stress","pmids":["16083423","16263722"],"confidence":"High","gaps":["Relative contributions of OSR1 vs SPAK to each cotransporter unknown","In vivo validation of the cascade pending"]},{"year":2006,"claim":"The CCT domain was defined as a dual-purpose docking module that binds RFXV motifs in both activators (WNK1/WNK4) and substrates (NKCC1) with nanomolar affinity, and WNK1–OSR1 complex formation was confirmed in cells with functional consequence for NKCC activity.","evidence":"In vitro kinase assays with CCT mutagenesis, peptide binding affinity measurements, co-immunoprecipitation, siRNA knockdown reducing NKCC activity","pmids":["16669787","16832045","16382158"],"confidence":"High","gaps":["Atomic-resolution structure of CCT–RFXV complex not yet available","Mechanism for substrate vs. activator discrimination at single binding groove unknown"]},{"year":2007,"claim":"The crystal structure of the CCT domain bound to a WNK4-derived RFXV peptide revealed a novel fold with a surface groove, and showed that phosphorylation preceding the RFXV motif causes steric clash promoting dissociation—providing a structural basis for phosphorylation-dependent regulation of docking.","evidence":"X-ray crystallography with mutational validation","pmids":["17721439"],"confidence":"High","gaps":["Full-length OSR1 structure unavailable","Structural model for how CCT domain communicates with kinase domain not resolved"]},{"year":2008,"claim":"Specific NCC phosphorylation sites (Thr46, Thr55, Thr60) targeted by SPAK/OSR1 were mapped, with Thr60 identified as a priming site required for efficient phosphorylation of the other sites and for NCC activation.","evidence":"In vitro kinase assay and mutagenesis in HEK293 and mpkDCT cells under hypotonic low-chloride conditions","pmids":["18270262"],"confidence":"High","gaps":["Whether ordered phosphorylation mechanism applies to NKCC substrates not tested"]},{"year":2009,"claim":"The kinase domain crystal structure at 2.25 Å revealed a domain-swapped dimer in an inactive conformation, providing a structural framework for understanding OSR1 autoinhibition and activation.","evidence":"X-ray crystallography","pmids":["19177573"],"confidence":"High","gaps":["Whether domain swapping occurs in vivo or is a crystallographic artifact unknown","Structure of active (phospho-Thr185) kinase domain not solved"]},{"year":2011,"claim":"MO25 was identified as a ~100-fold activator of OSR1 kinase activity, and genetic epistasis in WNK-resistant SPAK/OSR1 knock-in mice confirmed in vivo dependence of NCC phosphorylation on this cascade, definitively validating the WNK→OSR1/SPAK→NCC pathway in renal physiology.","evidence":"In vitro reconstitution with MO25, siRNA knockdown/rescue in cells; triple knock-in mouse model with phospho-NCC immunoblotting","pmids":["21423148","21486947","21321328"],"confidence":"High","gaps":["Structural basis of MO25-mediated activation unknown","Whether MO25 regulation is tissue-specific not addressed"]},{"year":2012,"claim":"Multiple regulatory layers on OSR1 were discovered: SPAK-dependent localization of OSR1 to the apical membrane in DCT, PI3K/mTORC2-mediated Ser339 phosphorylation as a convergent activation input, aldosterone-driven circadian oscillation of WNK–OSR1–NCC, and ASK3 as an upstream suppressor—revealing OSR1 as an integrative signaling node.","evidence":"SPAK-KO mice with immunofluorescence; in vitro mTORC2 kinase assay with Sin1 siRNA; time-course immunoblotting with eplerenone in mice; ASK3-KO mice","pmids":["22977235","24191005","23044422","23250415"],"confidence":"High","gaps":["Relative quantitative contributions of mTORC2 vs WNK phosphorylation to total OSR1 activity unknown","Whether circadian regulation extends beyond the kidney not tested"]},{"year":2012,"claim":"Genetic evidence from double-knockin ES cells established that SPAK/OSR1 are the only kinases essential for NKCC1 phosphorylation and revealed feedback regulation whereby loss of SPAK/OSR1 activity elevates upstream WNK1/WNK3 activity.","evidence":"SPAK/OSR1 double-knockin ES cells, NKCC1 activity assays, immunoblotting for WNK phosphorylation","pmids":["22032326"],"confidence":"High","gaps":["Identity of the WNK feedback phosphatase or mechanism not determined"]},{"year":2013,"claim":"OSR1 was established as essential for embryonic cardiovascular development: endothelial-specific OSR1 deletion phenocopied WNK1 knockout vascular defects, and constitutively active OSR1 rescued angiogenesis in WNK1-null embryos, placing OSR1 as the critical downstream effector of WNK1 in vascular morphogenesis.","evidence":"Conditional knockout mice (Tie2-Cre), constitutively active OSR1 transgene rescue, embryonic phenotyping","pmids":["23386621"],"confidence":"High","gaps":["Downstream vascular targets of OSR1 in endothelial cells not fully identified","Whether ion cotransporters mediate the angiogenic role unclear"]},{"year":2014,"claim":"The substrate repertoire was greatly expanded: OSR1 was shown to inhibit all KCC isoforms via direct phosphorylation, regulate Kir2.1/2.3 membrane localization via RxFxV docking, phosphorylate Smad2/3 to drive EMT and metastasis, and modulate ROMK1 and ClC-2 channels—broadening OSR1 from a cotransporter kinase to a versatile regulator of membrane transport and TGF-β signaling.","evidence":"In vitro kinase assays with KCC mutagenesis and knockin ES cells; Kir2.3 motif mutagenesis with siRNA; Smad2/3 co-IP and in vivo metastasis models; Xenopus oocyte electrophysiology for ROMK1 and ClC-2","pmids":["24393035","29581290","33051597","25322850","25323061"],"confidence":"High","gaps":["Physiological contexts for many of these substrates not validated in vivo","Direct phosphorylation sites on Kir2.1/2.3, ROMK1, and ClC-2 not mapped"]},{"year":2016,"claim":"SPAK/OSR1 double-knockout mice revealed functional redundancy between the two kinases in K⁺ homeostasis: only combined loss caused severe hypokalemia and failure to phosphorylate NCC under dietary K⁺ restriction.","evidence":"SPAK/OSR1 DKO mice, dietary K⁺ manipulation, plasma K⁺ measurement, phospho-NCC immunoblotting","pmids":["27068441"],"confidence":"High","gaps":["Tissue-specific non-redundant functions of OSR1 vs SPAK in other nephron segments not fully characterized"]},{"year":2019,"claim":"Two new regulatory mechanisms were defined: CUL4-DDB1-WDR3/WDR6-dependent ubiquitination of OSR1 that is abolished when the S-motif is phosphorylated during osmotic stress, linking WNK signaling to OSR1 protein stability; and WNK4-dependent formation of cytoplasmic WNK bodies containing SPAK/OSR1 in DCT during hypokalemia.","evidence":"AP-MS with pharmacological validation for CUL4 complex; immunofluorescence in WNK4-deficient and Kir4.1-KO mice with dietary K⁺ manipulation","pmids":["31614064","31736353"],"confidence":"Medium","gaps":["Half-life measurements of OSR1 under different phosphorylation states not reported","Functional significance of WNK body localization for OSR1 activity quantitatively undefined"]},{"year":2023,"claim":"Re-evaluation with a new strain-specific phospho-NKCC2 antibody revealed that SPAK/OSR1 contribute only mildly to basal NKCC2 phosphorylation, in contrast to their dominant role in NCC regulation, implying alternative kinases phosphorylate NKCC2 in vivo.","evidence":"New pT96-NKCC2 antibody in WNK4⁻/⁻, SPAK⁻/⁻, OSR1⁻/⁻, and DKO mice with furosemide diuresis","pmids":["37881876"],"confidence":"High","gaps":["Identity of the alternative NKCC2 kinase(s) unknown","Whether NKCC2 phosphorylation at other sites remains SPAK/OSR1-dependent not resolved"]},{"year":null,"claim":"Key open questions include the structural basis of MO25-mediated OSR1 activation, the identity of kinases that phosphorylate NKCC2 independently of SPAK/OSR1, whether OSR1's non-cotransporter substrates (Smad2/3, Kir2.x, ROMK1) are physiologically relevant in vivo, and the mechanism by which OSR1 promotes angiogenesis independently of ion transport.","evidence":"","pmids":[],"confidence":"Low","gaps":["No full-length OSR1 structure or MO25–OSR1 complex structure available","In vivo validation of Smad2/3, Kir2.x, and ROMK1 as physiological OSR1 substrates lacking","Molecular targets downstream of OSR1 in endothelial angiogenesis unidentified"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,1,4,5,7,8,10,12,21,24]},{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[0,1,7,8,10,12,21,24,40]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[3,13,36]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[14,23,36]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,1,3,4,10,11,15,18,36]},{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[1,5,7,12,21,23,26,27,32]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[20,22]}],"complexes":[],"partners":["WNK1","WNK4","CAB39","SLC12A1","SLC12A2","SLC12A3","STK39","SMAD2"],"other_free_text":[]},"mechanistic_narrative":"OXSR1 (OSR1) is a STE20-family serine/threonine kinase that functions as the central relay in the WNK signaling cascade controlling cation-chloride cotransporter activity and ion homeostasis. WNK1/WNK4 phosphorylate OSR1 at T-loop residue Thr185 to activate it, and the scaffold protein MO25 amplifies OSR1 kinase activity ~100-fold; OSR1 recognizes both its upstream activators and downstream substrates through RFXV/RxFxV motifs that dock into a structurally defined groove on its conserved C-terminal (CCT) domain [PMID:16083423, PMID:16669787, PMID:17721439, PMID:21423148]. Activated OSR1 directly phosphorylates and stimulates Na⁺-K⁺-2Cl⁻ cotransporters (NKCC1, NKCC2) and NCC while inhibiting K⁺-Cl⁻ cotransporters (KCCs), thereby coordinating renal salt reabsorption and K⁺ homeostasis—functions confirmed by in vivo genetic epistasis showing that WNK-resistant OSR1/SPAK knock-in mice abolish NCC phosphorylation and that double-knockout mice develop severe hypokalemia under K⁺ restriction [PMID:21486947, PMID:24393035, PMID:27068441]. Beyond ion transport, OSR1 is essential for embryonic angiogenesis downstream of WNK1, as endothelial-specific OSR1 deletion phenocopies WNK1 loss and constitutively active OSR1 rescues vascular defects, and OSR1 phosphorylates Smad2/3 to promote TGF-β-driven epithelial-to-mesenchymal transition in breast cancer [PMID:23386621, PMID:33051597]. OSR1 activity is additionally modulated by mTORC2-mediated Ser339 phosphorylation and by CUL4-DDB1-WDR3/WDR6-dependent ubiquitination that is relieved upon osmotic stress-induced S-motif phosphorylation [PMID:24191005, PMID:31614064]."},"prefetch_data":{"uniprot":{"accession":"O95747","full_name":"Serine/threonine-protein kinase OSR1","aliases":["Oxidative stress-responsive 1 protein"],"length_aa":527,"mass_kda":58.0,"function":"Effector serine/threonine-protein kinase component of the WNK-SPAK/OSR1 kinase cascade, which is involved in various processes, such as ion transport, response to hypertonic stress and blood pressure (PubMed:16669787, PubMed:18270262, PubMed:21321328, PubMed:34289367). Specifically recognizes and binds proteins with a RFXV motif (PubMed:16669787, PubMed:17721439, PubMed:21321328). Acts downstream of WNK kinases (WNK1, WNK2, WNK3 or WNK4): following activation by WNK kinases, catalyzes phosphorylation of ion cotransporters, such as SLC12A1/NKCC2, SLC12A2/NKCC1, SLC12A3/NCC, SLC12A5/KCC2 or SLC12A6/KCC3, regulating their activity (PubMed:17721439). Mediates regulatory volume increase in response to hyperosmotic stress by catalyzing phosphorylation of ion cotransporters SLC12A1/NKCC2, SLC12A2/NKCC1 and SLC12A6/KCC3 downstream of WNK1 and WNK3 kinases (PubMed:16669787, PubMed:21321328). Phosphorylation of Na-K-Cl cotransporters SLC12A2/NKCC1 and SLC12A2/NKCC1 promote their activation and ion influx; simultaneously, phosphorylation of K-Cl cotransporters SLC12A5/KCC2 and SLC12A6/KCC3 inhibit their activity, blocking ion efflux (PubMed:16669787, PubMed:19665974, PubMed:21321328). Acts as a regulator of NaCl reabsorption in the distal nephron by mediating phosphorylation and activation of the thiazide-sensitive Na-Cl cotransporter SLC12A3/NCC in distal convoluted tubule cells of kidney downstream of WNK4 (PubMed:18270262). Also acts as a regulator of angiogenesis in endothelial cells downstream of WNK1 (PubMed:23386621, PubMed:25362046). Acts as an activator of inward rectifier potassium channels KCNJ2/Kir2.1 and KCNJ4/Kir2.3 downstream of WNK1: recognizes and binds the RXFXV/I variant motif on KCNJ2/Kir2.1 and KCNJ4/Kir2.3 and regulates their localization to the cell membrane without mediating their phosphorylation (PubMed:29581290). Phosphorylates RELL1, RELL2 and RELT (PubMed:16389068, PubMed:28688764). Phosphorylates PAK1 (PubMed:14707132). Phosphorylates PLSCR1 in the presence of RELT (PubMed:22052202)","subcellular_location":"Cytoplasm","url":"https://www.uniprot.org/uniprotkb/O95747/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/OXSR1","classification":"Not Classified","n_dependent_lines":29,"n_total_lines":1208,"dependency_fraction":0.024006622516556293},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000172939","cell_line_id":"CID001229","localizations":[{"compartment":"cytoplasmic","grade":3},{"compartment":"nucleoplasm","grade":2}],"interactors":[{"gene":"WNK1","stoichiometry":10.0},{"gene":"WNK2","stoichiometry":0.2},{"gene":"WNK3","stoichiometry":0.2},{"gene":"RBM25","stoichiometry":0.2},{"gene":"DYNC1H1","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/target/CID001229","total_profiled":1310},"omim":[{"mim_id":"614495","title":"PSEUDOHYPOALDOSTERONISM, TYPE IID; PHA2D","url":"https://www.omim.org/entry/614495"},{"mim_id":"611213","title":"RELT-LIKE 2; RELL2","url":"https://www.omim.org/entry/611213"},{"mim_id":"611212","title":"RELT-LIKE 1; RELL1","url":"https://www.omim.org/entry/611212"},{"mim_id":"611211","title":"RECEPTOR EXPRESSED IN LYMPHOID TISSUES; RELT","url":"https://www.omim.org/entry/611211"},{"mim_id":"605775","title":"KELCH-LIKE 3; KLHL3","url":"https://www.omim.org/entry/605775"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Cytosol","reliability":"Supported"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/OXSR1"},"hgnc":{"alias_symbol":["KIAA1101"],"prev_symbol":["OSR1"]},"alphafold":{"accession":"O95747","domains":[{"cath_id":"3.30.200.20","chopping":"15-94","consensus_level":"medium","plddt":87.9622,"start":15,"end":94},{"cath_id":"1.10.510.10","chopping":"99-177_190-316","consensus_level":"high","plddt":89.5209,"start":99,"end":316},{"cath_id":"3.10.20.90","chopping":"435-527","consensus_level":"high","plddt":88.1548,"start":435,"end":527}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O95747","model_url":"https://alphafold.ebi.ac.uk/files/AF-O95747-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O95747-F1-predicted_aligned_error_v6.png","plddt_mean":76.56},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=OXSR1","jax_strain_url":"https://www.jax.org/strain/search?query=OXSR1"},"sequence":{"accession":"O95747","fasta_url":"https://rest.uniprot.org/uniprotkb/O95747.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O95747/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O95747"}},"corpus_meta":[{"pmid":"16083423","id":"PMC_16083423","title":"The 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phosphorylation of Thr185 is required for OSR1 activation (T185A abolishes activation; T185E increases basal activity >20-fold), whereas Ser325 mutation does not affect activity.\",\n      \"method\": \"In vitro kinase assay, phosphopeptide mapping, site-directed mutagenesis\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro phosphorylation with mutagenesis, replicated in multiple subsequent studies\",\n      \"pmids\": [\"16083423\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"OSR1 directly phosphorylates the N-terminal regulatory regions of cation-chloride cotransporters NKCC1, NKCC2, and NCC, and this phosphorylation is induced by hypotonic stress in cells.\",\n      \"method\": \"In vitro kinase assay, cell-based phosphorylation under hypotonic stress\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — direct phosphorylation demonstrated in vitro and in cells, replicated across multiple labs\",\n      \"pmids\": [\"16263722\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"OSR1 interacts with cation-chloride cotransporters KCC3, NKCC1, and NKCC2 (but not KCC1 or KCC4) via a conserved C-terminal domain; the binding motif on cotransporters starts with an (R/K)FX(V/I) sequence.\",\n      \"method\": \"Yeast two-hybrid, GST pull-down, co-immunoprecipitation\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IP and pull-down, replicated widely\",\n      \"pmids\": [\"12386165\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"OSR1 exists in a complex with WNK1 in cells; depletion of WNK1 by siRNA reduces OSR1 kinase activity; depletion of either WNK1 or OSR1 reduces NKCC1 activity, establishing a WNK1→OSR1→NKCC1 functional pathway.\",\n      \"method\": \"Co-immunoprecipitation, siRNA knockdown, NKCC activity assay\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IP plus functional KD phenotype, replicated\",\n      \"pmids\": [\"16832045\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"The conserved C-terminal (CCT) domain of OSR1 functions as a multipurpose docking site that binds RFXV motifs in upstream activators (WNK1/WNK4) and downstream substrates (NKCC1); mutation of key CCT residues inhibits NKCC1 phosphorylation and prevents efficient WNK1-mediated activation of OSR1.\",\n      \"method\": \"Peptide binding assays, affinity purification, site-directed mutagenesis, in vitro kinase assay\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — biochemical reconstitution with mutagenesis, replicated\",\n      \"pmids\": [\"16669787\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Crystal structure of the OSR1 CCT domain in complex with an RFXV-motif peptide from WNK4 reveals a novel protein fold with a surface-exposed groove that mediates substrate/activator recognition; phosphorylation of a Ser/Thr residue preceding the RFXV motif causes steric clash, promoting dissociation.\",\n      \"method\": \"X-ray crystallography, mutational analysis\",\n      \"journal\": \"EMBO reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure with mutational validation\",\n      \"pmids\": [\"17721439\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Crystal structure of the OSR1 kinase domain reveals a domain-swapped dimer in an inactive conformation, where the P+1 loop and αEF helix are exchanged between dimer-related monomers.\",\n      \"method\": \"X-ray crystallography at 2.25 Å\",\n      \"journal\": \"Protein science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure\",\n      \"pmids\": [\"19177573\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"SPAK and OSR1 activated by WNK1 phosphorylate human NCC at Thr46, Thr55, and Thr60; phosphorylation requires docking of NCC's RFXI motif with SPAK/OSR1; mutation of Thr60 to Ala markedly inhibits phosphorylation of Thr46/Thr55 and NCC activation.\",\n      \"method\": \"In vitro kinase assay, site-directed mutagenesis, cell-based hypotonic low-chloride stimulation\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro assay plus mutagenesis plus cell-based validation\",\n      \"pmids\": [\"18270262\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"MO25α/β binds to OSR1 and induces ~100-fold activation of OSR1, dramatically enhancing its ability to phosphorylate ion cotransporters NKCC1, NKCC2, and NCC; siRNA-mediated reduction of MO25 inhibits endogenous NKCC1 phosphorylation at OSR1/SPAK-dependent sites.\",\n      \"method\": \"Co-immunoprecipitation, in vitro kinase assay, siRNA knockdown, phospho-specific antibody detection\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — biochemical activation assay with siRNA rescue, strong evidence\",\n      \"pmids\": [\"21423148\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"OSR1, like SPAK, phosphorylates NKCC1 at equivalent residues in Xenopus oocytes and similarly activates NKCC1 when coexpressed with WNK4; OSR1 kinase properties (inhibitor sensitivity, metal ion preference) mirror those of SPAK.\",\n      \"method\": \"Xenopus oocyte expression, in vitro phosphorylation with Mn2+/Mg2+\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — direct activity assay in heterologous expression system, replicated\",\n      \"pmids\": [\"16382158\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"OSR1 directly phosphorylates NKCC1 at three conserved threonine residues (Thr203/Thr207/Thr212 in human NKCC1); osmotic stress in HEK-293 cells increases phosphorylation at these sites.\",\n      \"method\": \"In vitro kinase assay, phosphopeptide mapping, cell-based osmotic stress\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro mapping plus cell-based validation\",\n      \"pmids\": [\"16669787\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"SPAK and OSR1 interact with an RFQV motif on NKCC2 and directly phosphorylate Thr95, Thr100, Thr105 (and possibly Ser91) under hypotonic low-chloride WNK1-activating conditions.\",\n      \"method\": \"In vitro kinase assay, site-directed mutagenesis, cell-based hypotonic stimulation\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — direct phosphorylation with mutagenesis and docking motif analysis\",\n      \"pmids\": [\"21321328\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"WNK-activated SPAK/OSR1 directly phosphorylate all KCC isoforms at a conserved C-terminal threonine (Site-2, Thr1048 in KCC3A) in vitro; in ES cells lacking SPAK/OSR1 activity, KCC Site-2 phosphorylation is abolished and KCC3A activity is elevated; a Site-2 alanine KCC3A mutant shows increased activity, demonstrating that SPAK/OSR1 phosphorylation inhibits KCCs.\",\n      \"method\": \"In vitro kinase assay with purified kinases and MO25, SPAK/OSR1 double-knockin ES cells, 86Rb+ uptake assay, site-directed mutagenesis\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — reconstitution plus genetic KO with functional readout plus mutagenesis\",\n      \"pmids\": [\"24393035\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"SPAK/OSR1 activity is required for NKCC1 phosphorylation and activation: double-knockin ES cells where SPAK and OSR1 cannot be phosphorylated/activated by WNK1 show no NKCC1 phosphorylation or activation, and elevated WNK1/WNK3 activity, revealing a feedback loop.\",\n      \"method\": \"CRISPR/knockin ES cells (SPAK T243A / OSR1 T185A), phospho-specific western blot, NKCC1 activity assay\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — genetic knockin with multiple functional readouts\",\n      \"pmids\": [\"22032326\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"In PHAII (WNK4-D561A) mice crossed with Spak(T243A) and Osr1(T185A) knockin mice, NCC phosphorylation is almost completely abolished, and blood pressure, hyperkalemia, and metabolic acidosis are corrected, establishing that PHAII phenotypes depend entirely on the WNK-OSR1/SPAK-NCC cascade.\",\n      \"method\": \"Triple knockin mouse model, blood pressure measurement, phospho-specific western blot\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis in vivo with multiple phenotypic readouts\",\n      \"pmids\": [\"21486947\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Dietary salt intake regulates OSR1 phosphorylation and downstream NCC phosphorylation in vivo; this regulation is lost in WNK4-D561A knockin mice and is mediated by aldosterone (reversed by spironolactone or exogenous aldosterone).\",\n      \"method\": \"Knockin mouse model, dietary manipulation, phospho-specific western blot, pharmacological inhibition\",\n      \"journal\": \"Kidney international\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo genetic and pharmacological evidence with multiple conditions\",\n      \"pmids\": [\"18800028\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"mTORC2 phosphorylates OSR1 on Ser339 in vitro; pharmacological inhibition of PI3K or depletion of the mTORC2 component Sin1 reduces OSR1 activation by osmotic stress and decreases NKCC1 activity, identifying a PI3K/mTORC2 input to OSR1 regulation.\",\n      \"method\": \"In vitro kinase assay, siRNA knockdown, pharmacological inhibition, NKCC activity assay\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro phosphorylation with mutagenesis plus cell-based validation\",\n      \"pmids\": [\"24191005\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"OSR1 is activated by osmotic stress (sorbitol, NaCl) but not by other stresses tested; a yeast two-hybrid screen identified PAK1 as an OSR1 target and OSR1 phosphorylates PAK1 at Thr84 in its N-terminal regulatory domain, with T84E mutation reducing PAK1 sensitivity to Cdc42.\",\n      \"method\": \"Yeast two-hybrid, in vitro kinase assay, site-directed mutagenesis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vitro kinase assay with mutagenesis; single study, OSR1-PAK1 axis not widely replicated\",\n      \"pmids\": [\"14707132\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"WNK1 activation of the OSR1 signaling cascade is required for angiogenesis and cardiac development: global Osr1-null mouse embryos die at ~E11 with cardiovascular defects identical to WNK1-null embryos, and endothelial-specific constitutively active OSR1 rescues angiogenesis and cardiac defects of Wnk1-null embryos.\",\n      \"method\": \"Global and conditional Osr1 knockout mice, constitutively active OSR1 transgenic rescue, embryo phenotyping\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with in vivo rescue experiment\",\n      \"pmids\": [\"23386621\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"In endothelial cells, OSR1 (but not SPAK) is required for HUVEC chemotaxis and invasion downstream of WNK1; siRNA knockdown experiments show OSR1 specifically mediates these migratory functions while SPAK mediates proliferation.\",\n      \"method\": \"siRNA knockdown in HUVECs, chemotaxis/invasion assays, cord formation assay\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — clean siRNA KD with distinct functional readouts; single lab\",\n      \"pmids\": [\"25362046\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"In SPAK knockout mice, OSR1 becomes largely inactive, dissociates from MO25α and NCC at the apical membrane of the distal convoluted tubule, and redistributes to cytoplasmic puncta containing WNK1, revealing that OSR1 activity and localization in the DCT depend on SPAK.\",\n      \"method\": \"SPAK knockout mice, immunofluorescence, subcellular fractionation, phospho-specific western blot\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — knockout mouse with direct localization and activity measurements\",\n      \"pmids\": [\"22977235\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"OSR1 expressed in Xenopus oocytes upregulates phosphate transport through NaPiIIa; in vivo, mice carrying a WNK-resistant OSR1 allele show increased urinary phosphate excretion, reduced NaPiIIa brush border membrane abundance, and elevated FGF23.\",\n      \"method\": \"Xenopus oocyte expression, dual-electrode voltage clamp, knockin mouse phenotyping, immunohistochemistry\",\n      \"journal\": \"Kidney & blood pressure research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — heterologous expression plus in vivo knockin model\",\n      \"pmids\": [\"23095210\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"In the developing hypothalamus, estradiol increases SPAK and OSR1 protein levels in a transcription-dependent manner; antisense knockdown of OSR1 reduces estradiol-enhanced NKCC1 phosphorylation and GABA-induced Ca2+ influx, placing OSR1 upstream of NKCC1-dependent depolarizing GABA signaling.\",\n      \"method\": \"In vivo estradiol treatment, antisense oligonucleotide knockdown, phospho-western blot, Ca2+ imaging\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — antisense KD with functional readout; single lab\",\n      \"pmids\": [\"22238094\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"ASK3 interacts with WNK1 and suppresses the WNK1-SPAK/OSR1 signaling pathway; ASK3 knockdown enhances SPAK/OSR1 activation, and Ask3 knockout mice are hypertensive with hyperactivated SPAK/OSR1 in renal tubules.\",\n      \"method\": \"Co-immunoprecipitation, siRNA knockdown, ASK3 knockout mouse phenotyping, phospho-specific western blot\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IP plus KO mouse with multiple phenotypic readouts\",\n      \"pmids\": [\"23250415\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"OSR1 (identified as SPAK/OSR1) is colocalized with NKCC at the basolateral membrane of chloride cells in killifish opercular epithelium, participating in the osmosensing pathway controlling NKCC activity.\",\n      \"method\": \"Immunocytochemistry, western blot\",\n      \"journal\": \"The Journal of experimental biology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — localization only, no direct functional manipulation of OSR1\",\n      \"pmids\": [\"15767308\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"OSR1 activates Kir2.1 and Kir2.3 inward rectifier K+ channels through a novel R-x-F-x-V/I binding motif variant; mutation of this motif prevents OSR1-mediated activation of Kir2.3; OSR1 and WNK activity are required for NaCl-induced plasma membrane localization of Kir2.3.\",\n      \"method\": \"Co-IP, site-directed mutagenesis of motif, siRNA knockdown, WNK inhibitor, membrane localization assay\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — motif mutagenesis plus pharmacological and siRNA evidence with localization readout\",\n      \"pmids\": [\"29581290\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"OSR1 negatively regulates the creatine transporter SLC6A8 in a catalytic-activity-dependent manner; constitutively active OSR1(T185E) but not WNK-insensitive OSR1(T185A) or catalytically inactive OSR1(D164A) reduces creatine-induced current and maximal transport rate.\",\n      \"method\": \"Xenopus oocyte expression, dual-electrode voltage clamp, mutagenesis\",\n      \"journal\": \"Kidney & blood pressure research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — heterologous expression with mutagenesis; single lab\",\n      \"pmids\": [\"25531585\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"OSR1 negatively regulates the Cl- channel ClC-2 in a catalytic-activity-dependent manner; constitutively active OSR1(T185E) but not inactive forms decreases ClC-2 conductance without affecting its membrane retrieval rate.\",\n      \"method\": \"Xenopus oocyte expression, dual-electrode voltage clamp, brefeldin A experiment\",\n      \"journal\": \"Kidney & blood pressure research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — heterologous expression with mutagenesis; single lab\",\n      \"pmids\": [\"25323061\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"OSR1 negatively regulates the renal K+ channel ROMK1 protein abundance at the membrane and channel activity in a catalytic-activity-dependent manner.\",\n      \"method\": \"Xenopus oocyte expression, dual-electrode voltage clamp, chemiluminescence of HA-tagged ROMK1\",\n      \"journal\": \"Kidney & blood pressure research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — heterologous expression with mutagenesis; single lab\",\n      \"pmids\": [\"25322850\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"OSR1 negatively regulates peptide transporters PEPT1 and PEPT2 by decreasing carrier protein abundance at the cell membrane, requiring catalytic activity (T185E active; T185A and D164A inactive forms have no effect).\",\n      \"method\": \"Xenopus oocyte expression, dual-electrode voltage clamp, HA chemiluminescence\",\n      \"journal\": \"Kidney & blood pressure research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — heterologous expression with mutagenesis; single lab\",\n      \"pmids\": [\"25531100\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"OSR1 upregulates the intestinal phosphate cotransporter NaPi-IIb (SLC34A2) in a WNK-dependent and catalytic-activity-dependent manner, increasing maximal transport rate.\",\n      \"method\": \"Xenopus oocyte expression, dual-electrode voltage clamp, mutagenesis\",\n      \"journal\": \"Kidney & blood pressure research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — heterologous expression with mutagenesis; single lab\",\n      \"pmids\": [\"26506223\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"OSR1 phosphorylates RELT, RELL1, and RELL2 (TNF receptor family members) in an in vitro kinase assay; OSR1 was identified as an interactor of RELL1 by yeast two-hybrid and confirmed by co-immunoprecipitation.\",\n      \"method\": \"Yeast two-hybrid, co-immunoprecipitation, in vitro kinase assay\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — Y2H plus co-IP plus kinase assay; single lab, limited follow-up\",\n      \"pmids\": [\"16389068\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"The E3 ubiquitin ligase complex CUL4-DDB1-WDR3/WDR6 binds OSR1 in a S-motif (Ser325) phosphorylation-dependent manner; osmotic stress-induced phosphorylation of the S-motif reduces this binding and abolishes OSR1 ubiquitylation, linking S-motif phosphorylation to OSR1 stability.\",\n      \"method\": \"Affinity pull-down, mass spectrometry, proteasomal and neddylation inhibitors, ubiquitylation assay\",\n      \"journal\": \"Chembiochem\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — MS interactome plus phospho-dependent binding and ubiquitylation assay; single lab\",\n      \"pmids\": [\"31614064\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"An allosteric pocket on the C-terminal domain of OSR1 (distinct from the ATP-binding site) can be targeted by antiparasitic agents rafoxanide and closantel, which inhibit OSR1 kinase activity and suppress NKCC1 phosphorylation in cells.\",\n      \"method\": \"In silico screening, in vitro kinase assay, cell-based NKCC1 phosphorylation assay\",\n      \"journal\": \"ChemMedChem\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — biochemical assay plus cell validation; single lab\",\n      \"pmids\": [\"28371477\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Verteporfin binds the kinase domain of OSR1 and inhibits its catalytic activity in an ATP-independent manner, suppressing NKCC1 phosphorylation in cells.\",\n      \"method\": \"In vitro kinase assay, kinase panel screen, cell-based NKCC1 phosphorylation\",\n      \"journal\": \"Chembiochem\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — biochemical inhibition with cell-based validation; single lab\",\n      \"pmids\": [\"29999233\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"In WNK1-knockout ES cells lacking OSR1/SPAK activity, NKCC1 phosphorylation at WNK-dependent sites is not observed, and WNK1 and WNK3 activity is markedly elevated, revealing that OSR1/SPAK suppresses upstream WNK activity (negative feedback).\",\n      \"method\": \"Double-knockin ES cells, phospho-specific western blot, WNK1/WNK3 kinase activity assay\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic knockin with quantitative kinase activity measurements\",\n      \"pmids\": [\"22032326\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"OSR1 directly phosphorylates Smad2 at Thr220 and Smad3 at Thr179 in their linker regions; phosphorylated Smad2/3 translocates to the nucleus and enhances TGF-β1 autocrine signaling to promote EMT in breast cancer cells.\",\n      \"method\": \"In vitro kinase assay, co-immunoprecipitation, nuclear fractionation, site-directed mutagenesis, OSR1 KO cell lines\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — direct phosphorylation assay with KO validation; single lab, cancer context\",\n      \"pmids\": [\"33051597\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"WNK1/HSN2 promotes neurite outgrowth via OSR1 activation and downstream GSK3β signaling, inducing LHX8 expression; HSANII patient mutations in HSN2 suppress OSR1/SPAK activation and LHX8 induction and block neurite outgrowth.\",\n      \"method\": \"Cell-based neurite outgrowth assay, phospho-specific western blot, co-immunoprecipitation, patient mutant analysis\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — functional KO/mutant analysis with pathway readouts; single lab\",\n      \"pmids\": [\"36151370\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"FPR2 pro-resolving agonists induce NADPH oxidase-dependent phosphorylation of OSR1 at Ser339; NADPH oxidase inhibitors block this phosphorylation as well as upstream PI3K/PKCδ activation, placing NADPH oxidase upstream of OSR1(Ser339) phosphorylation.\",\n      \"method\": \"Phospho-specific western blot, NADPH oxidase inhibitors, CRISPR/Cas9 p22phox knockout cells\",\n      \"journal\": \"Antioxidants\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — pharmacological and genetic inhibition with phospho-site specificity; single lab\",\n      \"pmids\": [\"33477989\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"OSR1 (OXSR1) is a STE20-family serine/threonine kinase that is phosphorylated and activated by WNK1/WNK4 at Thr185 in its activation loop; once active, OSR1 phosphorylates and activates the Na⁺-K⁺-2Cl⁻ cotransporters NKCC1 and NKCC2 and the Na⁺-Cl⁻ cotransporter NCC, while simultaneously phosphorylating and inhibiting K⁺-Cl⁻ cotransporters (KCCs), via a docking interaction between an RFXV/I motif on substrates/activators and the conserved C-terminal (CCT) domain of OSR1; MO25 allosterically amplifies OSR1 activity ~100-fold, mTORC2 phosphorylates OSR1 at Ser339 as a convergent input, and the CUL4-DDB1-WDR3/6 E3 ligase complex regulates OSR1 stability in an S-motif phosphorylation-dependent manner, together placing OSR1 as the central effector kinase in the WNK→OSR1→cation-chloride cotransporter pathway that controls renal salt reabsorption, cell volume, and blood pressure.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\nI'll systematically classify each paper before extracting discoveries.\n\n**Classification:**\n\n**KEEP (canonical OXSR1/OSR1 kinase - mammalian STE20 family kinase):**\n- PMIDs: 16083423, 16263722, 12386165, 18270262, 16669787, 18843116, 18835385(EXCLUDE - zinc finger TF Osr1), 25028718, 24393035, 18800028, 18092945, 21321328, 16832045, 16382158, 15166144(C. elegans ortholog - KEEP), 22949526, 15767308, 17721439, 24555568, 14707132, 22032326, 21423148, 22977235, 23073627, 23250415, 20175999, 25362046, 21486947, 17032814, 31736353, 27068441, 16734747, 23386621, 22846565, 23095210, 22294159, 21610494, 24191005, 29581290, 25086033, 23044422, 22238094, 25531585, 24808538, 25322850, 25323061, 25531100, 22648948, 33477989, 26506223, 36151370, 31085334, 38353402, 36292952, 32413057, 31165006, 37881876, 31954517, 19177573, 33406028, 28371477, 29999233, 31614064\n\n**EXCLUDE (zinc finger TF Osr1 - developmental biology papers, alias collision):**\n- 18835385, 24598167, 30149291, 21262216, 25986147, 22791896, 16554187, 24722440 (zebrafish osr1 TF), 21821672(OSR1 TF variant), 27442016(TF), 21281489(TF), 22129829(TF), 23238298(TF), 23095210(KEEP-kinase), 34256037(TF), 21462293(TF), 33005011(TF), 28501478(TF), 26744331(TF), 36359386(zebrafish osr1 TF), 12119563(zinc finger TF OSR1), 25164089(TF OSR1), 28404905(zinc finger TF OSR1 in RCC), 30244004(zinc finger TF OSR1 TSCC)\n\n**EXCLUDE (circ/lncRNA papers targeting OXSR1 as downstream gene - not about the kinase mechanism):**\n- 33468219 (circ-FANCA/miR-93-5p/OXSR1 - ceRNA study), 35793478 (lncRNA GAS6-AS2/miR-136-5p/OXSR1), 35751788 (lncRNA ZFAS1/miR-15a-5p/OXSR1), 36372260 (miR-455-3p/OXSR1 broiler), 31954517 (miR-25-3p/OXSR1 - KEEP as it shows OXSR1 kinase function in epilepsy context... but primarily a ceRNA study - EXCLUDE)\n\n**EXCLUDE (large proteomics/interactome screens - no specific OXSR1 mechanism):**\n- 17081983, 12477932, 15302935, 26186194, 28514442, 26496610, 25416956, 32296183, 17353931, 33961781, 22939629, 21873635, 19056867, 15761153, 15489334, 35271311, 26344197, 34079125, 15345747, 28611215, 22863883, 10470851, 32416067, 36318922, 19322201, 31871319, 21832049, 34857952\n\n**EXCLUDE (C. elegans osr-1 - different gene, stress resistance in worm - symbol collision with different protein):**\n- 15166144 - Actually this is C. elegans OSR-1 which is NOT the kinase homolog - it's a different protein involved in osmotic stress resistance via p38/CaMKII. EXCLUDE.\n- 22864793 (C. elegans osr-1 longevity) - EXCLUDE\n- 24313805 (C. elegans osr-1 curcumin) - EXCLUDE\n\nNow extracting discoveries from KEPT papers:\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2005,\n      \"finding\": \"WNK1 and WNK4 directly phosphorylate OSR1 at two sites: Thr185 in the T-loop of the catalytic domain and Ser325 in a C-terminal non-catalytic region. Phosphorylation of Thr185 is required for OSR1 activation; T185A mutation prevents activation by WNK1, while T185E (phosphomimetic) increases basal activity >20-fold. Mutation of Ser325 does not affect activity or WNK1-mediated activation.\",\n      \"method\": \"In vitro kinase assay, phosphopeptide mapping, site-directed mutagenesis\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with mutagenesis, multiple orthogonal methods\",\n      \"pmids\": [\"16083423\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"WNK1 phosphorylates OSR1 at an evolutionarily conserved serine residue outside the kinase domain, and mutation of this residue causes enhanced OSR1 kinase activity. SPAK and OSR1 directly phosphorylate the N-terminal regulatory regions of cation-chloride cotransporters NKCC1, NKCC2, and NCC. Phosphorylation of NCC is induced by hypotonic stress in cells.\",\n      \"method\": \"In vitro kinase assay, cell-based phosphorylation assays, hypotonic stress treatment\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro kinase assay plus cell-based validation, replicated across labs\",\n      \"pmids\": [\"16263722\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"OSR1 physically interacts with the cation-chloride cotransporters KCC3, NKCC1, and NKCC2 (but not KCC1 or KCC4) through the last 100 amino acids of OSR1. The binding motif on cotransporters consists of nine residues starting with (R/K)FX(V/I). Co-immunoprecipitation confirmed in vivo interaction of the related kinase SPAK with NKCC1 from mouse brain.\",\n      \"method\": \"Yeast two-hybrid, GST pull-down, co-immunoprecipitation, immunohistochemistry\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal binding confirmed by multiple methods including in vivo co-IP\",\n      \"pmids\": [\"12386165\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"OSR1 exists in a complex with WNK1 in cells, is activated by recombinant WNK1 in vitro, and is phosphorylated in a WNK1-dependent manner in cells. Depletion of WNK1 by siRNA reduces OSR1 kinase activity. Depletion of either WNK1 or OSR1 reduces NKCC activity in HeLa cells, placing both in a pathway required for NKCC function and volume regulation.\",\n      \"method\": \"Co-immunoprecipitation, in vitro kinase assay, siRNA knockdown, NKCC activity assay\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IP, in vitro assay, and functional siRNA knockdown with defined phenotype\",\n      \"pmids\": [\"16832045\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"OSR1 possesses a 92-residue conserved C-terminal (CCT) domain that interacts with RFXV motifs present in both substrates (NKCC1) and activators (WNK1/WNK4). A peptide containing the RFXV motif binds the CCT domain with nanomolar affinity. Mutation of specific CCT domain residues inhibits OSR1-mediated phosphorylation of NKCC1 (which requires RFXV docking) but not of CATCHtide (which lacks RFXV). An intact CCT domain is required for WNK1 to efficiently phosphorylate and activate OSR1.\",\n      \"method\": \"In vitro kinase assay, peptide binding/affinity purification, CCT domain mutagenesis, CATCHtide peptide substrate development\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — reconstituted binding and kinase assays with systematic mutagenesis, nanomolar affinity measurements\",\n      \"pmids\": [\"16669787\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"OSR1 shares similar kinase properties to SPAK and similarly activates NKCC1 when co-expressed with WNK4 in Xenopus oocytes. OSR1 kinase activity is markedly inhibited by staurosporine and K252a, partially inhibited by N-ethylmaleimide and diamide, and unaffected by arsenite. Activity is stronger with Mn2+ than Mg2+.\",\n      \"method\": \"Xenopus oocyte expression system, in vitro kinase assay, pharmacological inhibitor profiling\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — functional reconstitution in oocytes with mutagenesis and pharmacological characterization\",\n      \"pmids\": [\"16382158\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Crystal structure of the CCT domain of OSR1 in complex with a WNK4-derived RFXV motif-containing peptide was solved, revealing a novel protein fold with a surface-exposed groove that mediates peptide binding. Phosphorylation of a Ser/Thr residue preceding the RFXV motif causes steric clash, promoting dissociation from the CCT domain. Mutational analysis confirmed that the groove interactions are required for binding to WNK1 and NKCC1.\",\n      \"method\": \"X-ray crystallography, mutational analysis, binding assays\",\n      \"journal\": \"EMBO reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure at molecular resolution with functional mutagenesis validation\",\n      \"pmids\": [\"17721439\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"SPAK and OSR1 phosphorylate human NCC at three conserved residues (Thr46, Thr55, Thr60) in response to WNK1 pathway activation by hypotonic low-chloride conditions. Efficient NCC phosphorylation requires docking interaction between an RFXI motif in NCC and SPAK/OSR1. Mutation of Thr60 to Ala markedly inhibits phosphorylation of Thr46 and Thr55 and abolishes NCC activation by hypotonic low-chloride treatment.\",\n      \"method\": \"In vitro kinase assay, site-directed mutagenesis, cell-based phosphorylation assay in HEK293 and mpkDCT cells\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro assay plus cell-based validation with mutagenesis across two cell lines\",\n      \"pmids\": [\"18270262\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"OSR1 (oxidative stress-responsive 1) is a 58-kDa, 527-amino acid STE20 family kinase widely expressed in mammalian tissues. Endogenous OSR1 is activated specifically by osmotic stresses (notably sorbitol, and to a lesser extent NaCl). OSR1 does not activate JNK, p38, ERK2, or ERK5. OSR1 phosphorylates PAK1 at Thr84 in its N-terminal regulatory domain; T84E substitution reduces PAK1 activation by Cdc42, suggesting OSR1 modulates G-protein sensitivity of PAK.\",\n      \"method\": \"Two-hybrid screen, in vitro kinase assay, site-directed mutagenesis, osmotic stress activation assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — original biochemical characterization with in vitro kinase assay and mutagenesis\",\n      \"pmids\": [\"14707132\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Crystal structure of the OSR1 kinase domain was solved at 2.25 Å resolution, revealing a domain-swapped dimer in an inactive conformation in which the P+1 loop and αEF helix are swapped between dimer-related monomers. Structural integrity of chemical interactions is preserved in the domain-swapped interfaces.\",\n      \"method\": \"X-ray crystallography (2.25 Å resolution)\",\n      \"journal\": \"Protein science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution crystal structure\",\n      \"pmids\": [\"19177573\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"MO25α and MO25β bind to OSR1 (and SPAK) and induce ~100-fold activation of their kinase activity, dramatically enhancing phosphorylation of ion cotransporters NKCC1, NKCC2, and NCC. siRNA-mediated reduction of MO25 in mammalian cells inhibits phosphorylation of endogenous NKCC1 at SPAK/OSR1-dependent sites, rescued by re-expression of MO25α. MO25 associates with OSR1 in a manner analogous to its interaction with the STRAD pseudokinase.\",\n      \"method\": \"In vitro kinase assay, co-immunoprecipitation, siRNA knockdown, rescue experiments, surface plasmon resonance\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro reconstitution with ~100-fold activation, confirmed by siRNA knockdown and rescue in cells\",\n      \"pmids\": [\"21423148\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"In vivo genetic evidence confirms that NCC phosphorylation by the WNK4 D561A mutation is almost completely abolished in triple knock-in mice where both SPAK (T243A) and OSR1 (T185A) cannot be activated by WNK kinases. This establishes that NCC phosphorylation in vivo is dependent on the WNK-OSR1/SPAK cascade, and that PHAII caused by WNK4 D561A requires OSR1/SPAK activation.\",\n      \"method\": \"Genetic knock-in mouse model, immunoblotting for phospho-NCC, blood pressure and metabolic measurements\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean genetic epistasis in vivo with defined phenotypic readout\",\n      \"pmids\": [\"21486947\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"SPAK/OSR1 directly phosphorylate NKCC2 isoforms at Thr95, Thr100, Thr105 (and possibly Ser91) via interaction with an RFQV docking motif on NKCC2. A SPAK-OSR1-independent kinase (possibly AMPK) phosphorylates Ser130. Phosphorylation of Thr105 and Ser130 plays the most important role in stimulating NKCC2 activity. Unlike NCC, NKCC2 is constitutively membrane-localized and not regulated by SPAK/OSR1-dependent membrane translocation.\",\n      \"method\": \"In vitro kinase assay, cell-based phosphorylation assay, site-directed mutagenesis, NKCC2 isoform analysis\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — systematic mutagenesis of phosphorylation sites combined with functional activity assays\",\n      \"pmids\": [\"21321328\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"SPAK/OSR1 double-knockin ES cells (where both kinases cannot be activated by WNK1) show no phosphorylation or activation of NKCC1, providing genetic evidence that SPAK/OSR1 are essential for NKCC1 function. WNK1 and WNK3 activities are markedly elevated in these knockin cells, demonstrating that SPAK/OSR1 feedback to influence upstream WNK activity. Phosphorylation of WNK1 Ser1261 is unaffected, indicating it is not a SPAK/OSR1 substrate.\",\n      \"method\": \"Double-knockin ES cells, immunoblotting, NKCC1 activity assay, in vitro kinase assay\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic knockin cells with orthogonal functional and biochemical readouts\",\n      \"pmids\": [\"22032326\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"In the kidney, OSR1 activity is dependent on SPAK in the distal convoluted tubule (DCT) but not the thick ascending limb (TAL). In SPAK-null mice, OSR1 becomes largely inactive, displaced from MO25α and NCC at the apical membrane, and redistributes to dense cytoplasmic punctate structures containing WNK1. This demonstrates that SPAK is required for proper OSR1 localization and activity in DCT, and that OSR1 apical membrane localization is functionally linked to NCC phosphorylation.\",\n      \"method\": \"SPAK knockout mice, immunofluorescence, subcellular fractionation, immunoblotting\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO model with direct localization experiments tied to functional consequence\",\n      \"pmids\": [\"22977235\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"PI3K/mTORC2 activates OSR1 by phosphorylating it on Ser339. Inhibition of PI3K reduces OSR1 activation by osmotic stress, and depletion of Sin1 (mTORC2 component) decreases OSR1 activation by sorbitol and reduces NKCC activity in HeLa cells. Mutation of Ser339 eliminates mTORC2-dependent OSR1 phosphorylation, identifying a convergent regulatory node on OSR1 from the PI3K pathway.\",\n      \"method\": \"In vitro kinase assay (mTORC2 phosphorylates OSR1), pharmacological inhibition, siRNA knockdown of Sin1, site-directed mutagenesis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro phosphorylation assay with mutagenesis plus cell-based validation\",\n      \"pmids\": [\"24191005\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"OSR1 participates in regulation of renal tubular phosphate transport via NaPiIIa. Coexpression of OSR1 in Xenopus oocytes significantly upregulates phosphate-induced currents in NaPiIIa-expressing oocytes. In osr1tg/(+) mice carrying a WNK-resistant OSR1, urinary phosphate excretion is increased and NaPiIIa protein abundance in brush border membrane is reduced, associated with elevated serum FGF23.\",\n      \"method\": \"Xenopus oocyte electrophysiology, mouse knock-in model, immunohistochemistry, confocal microscopy\",\n      \"journal\": \"Kidney & blood pressure research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional reconstitution in oocytes plus in vivo mouse model, single lab\",\n      \"pmids\": [\"23095210\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"The phosphorylation levels of OSR1, SPAK, and NCC exhibit a diurnal rhythm in mouse kidney that correlates with active/rest periods. This circadian rhythm of the WNK4-OSR1/SPAK-NCC cascade is abolished by eplerenone (aldosterone receptor blocker), indicating aldosterone governs the circadian oscillation of this kinase cascade.\",\n      \"method\": \"Time-course immunoblotting in mice, pharmacological treatment with eplerenone\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — in vivo time-course with pharmacological intervention, single lab\",\n      \"pmids\": [\"23044422\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"ASK3 (apoptosis signal-regulating kinase 3) interacts with WNK1 and suppresses the WNK1-SPAK/OSR1 signaling pathway. Knockdown of ASK3 enhances SPAK/OSR1 activation, and ASK3 knockout mice show hyperactivation of SPAK/OSR1 in renal tubules and a hypertensive phenotype, positioning ASK3 as an upstream suppressor of the WNK1-SPAK/OSR1 pathway in response to osmotic stress.\",\n      \"method\": \"Co-immunoprecipitation (WNK1-ASK3 interaction), siRNA knockdown, ASK3 knockout mice, immunoblotting\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO model with biochemical pathway analysis plus siRNA validation\",\n      \"pmids\": [\"23250415\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Estradiol treatment of newborn rat pups significantly increases protein levels of OSR1 (and SPAK) in the hypothalamus via a transcription-dependent mechanism. OSR1 knockdown by antisense oligonucleotides precludes estradiol-mediated enhancement of NKCC1 phosphorylation and diminishes estradiol-enhanced Ca2+ influx induced by GABAAR activation in embryonic hypothalamic cultures.\",\n      \"method\": \"In vivo neonatal rat treatment, immunoblotting, antisense knockdown, calcium imaging\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — antisense knockdown with functional Ca2+ imaging readout, single lab\",\n      \"pmids\": [\"22238094\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"WNK1 activates the OSR1 signaling cascade to regulate embryonic cardiovascular development. Global or endothelial-specific deletion of Osr1 in mice causes embryonic lethality at ~E11 with identical cardiovascular defects to WNK1 knockout (no mature large vessels in yolk sacs, defective angiogenesis, smaller cardiac chambers). Endothelial-specific expression of constitutively active OSR1 rescues angiogenesis and cardiac defects in global WNK1-null embryos, establishing OSR1 as an essential downstream effector of WNK1 in vascular development.\",\n      \"method\": \"Conditional knockout mice (Tie2-Cre), constitutively active OSR1 transgene rescue, embryonic phenotype analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic rescue experiment definitively placing OSR1 downstream of WNK1 in vascular development\",\n      \"pmids\": [\"23386621\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"WNK-activated SPAK/OSR1 (in the presence of MO25) directly phosphorylates all KCC isoforms at Site-2 (Thr1048 in KCC3A) in vitro, promoting their inhibition. In ES cells lacking SPAK/OSR1 activity, endogenous KCC Site-2 phosphorylation is abolished and KCC3A activity is elevated. A Site-2 alanine KCC3A mutant preventing SPAK/OSR1 phosphorylation shows increased activity, confirming that SPAK/OSR1 phosphorylation inhibits KCC-mediated Cl- efflux. SPAK/OSR1 also phosphorylate KCCs at Site-3 (Thr5/Thr6) and Site-4 (Ser96).\",\n      \"method\": \"In vitro kinase assay with MO25, knockin ES cells lacking SPAK/OSR1 activity, 86Rb+ uptake assay, site-directed mutagenesis\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro reconstitution plus genetic knockin cells with functional transport assay and mutagenesis\",\n      \"pmids\": [\"24393035\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"WNK1-OSR1 signaling is required for HUVEC cord formation, chemotaxis, and invasion. OSR1 (but not SPAK) is specifically required for HUVEC chemotaxis and invasion, while SPAK is required for proliferation. OSR1 and WNK1 regulate the zinc-finger transcription factor Slug in endothelial cells. Constitutively active OSR1 rescues WNK1 deficiency, confirming OSR1 acts downstream of WNK1 in endothelial function.\",\n      \"method\": \"siRNA knockdown, cord formation assay, chemotaxis/invasion assay, gene expression analysis, HUVEC culture\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean siRNA KD with distinct phenotypic readouts differentiating OSR1 from SPAK functions\",\n      \"pmids\": [\"25362046\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"OSR1 activates inward rectifier K+ channels Kir2.1 and Kir2.3 via an R-x-F-x-V/I motif variant (distinct from the canonical RFXV). Mutation of the RxFxV motif in Kir2.3 prevents its activation by OSR1. Both siRNA knockdown of OSR1 and chemical WNK inhibition disrupt NaCl-induced plasma membrane localization of Kir2.3, suggesting OSR1 enhances Kir2.1/Kir2.3 activity by increasing their plasma membrane localization. Kir4.1, which lacks this motif, is insensitive to OSR1.\",\n      \"method\": \"siRNA knockdown, pharmacological WNK inhibition, plasma membrane localization assay, site-directed mutagenesis of binding motif, Xenopus oocyte expression\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — motif mutagenesis plus siRNA and pharmacological validation with defined localization readout\",\n      \"pmids\": [\"29581290\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"OSR1 directly interacts with and phosphorylates Smad2 at Thr220 and Smad3 at Thr179 in the linker region. This phosphorylation causes nuclear translocation of Smad2/3 and enhances TGF-β1 autocrine signaling, driving epithelial-to-mesenchymal transition (EMT) and metastasis in breast cancer cells. OSR1 deletion in aggressive breast cancer cells inhibits EMT, and disruption of the OSR1-Smad2/3-TGF-β1 axis reduces metastasis in vitro and in vivo.\",\n      \"method\": \"Co-immunoprecipitation, in vitro kinase assay, site-directed mutagenesis, nuclear translocation assay, loss-of-function (CRISPR/deletion), in vivo metastasis models\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro kinase assay with site-specific mutagenesis plus in vivo functional validation\",\n      \"pmids\": [\"33051597\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"OSR1 negatively regulates the creatine transporter SLC6A8 in Xenopus oocytes. Constitutively active (T185E)OSR1 reduces creatine transport, but catalytically inactive (D164A)OSR1 does not, indicating the effect requires OSR1 kinase activity. WNK-insensitive (T185A)OSR1 still negatively regulates SLC6A8, suggesting a WNK-independent mechanism for this substrate.\",\n      \"method\": \"Xenopus oocyte expression system, dual electrode voltage clamp, constitutively active and catalytically inactive OSR1 mutants\",\n      \"journal\": \"Kidney & blood pressure research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional reconstitution in oocytes with multiple mutant controls, single lab\",\n      \"pmids\": [\"25531585\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Active OSR1 (T185E) downregulates ROMK1 K+ channel protein abundance at the cell membrane and channel activity in Xenopus oocytes; catalytically inactive (D164A)OSR1 has no effect. This is consistent with OSR1 kinase activity-dependent regulation of ROMK1 membrane trafficking.\",\n      \"method\": \"Xenopus oocyte expression, dual electrode voltage clamp, chemiluminescence-based surface protein assay\",\n      \"journal\": \"Kidney & blood pressure research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional reconstitution with activity-dependent mutants, single lab\",\n      \"pmids\": [\"25322850\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Active OSR1 (T185E) negatively regulates ClC-2 Cl- channel activity in Xenopus oocytes. WNK1-insensitive inactive (T185A)OSR1 and catalytically inactive (D164A)OSR1 have no effect. Brefeldin A experiments suggest OSR1 does not accelerate ClC-2 retrieval from the membrane, indicating a different regulatory mechanism.\",\n      \"method\": \"Xenopus oocyte expression, dual electrode voltage clamp, brefeldin A treatment\",\n      \"journal\": \"Kidney & blood pressure research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional reconstitution with multiple mutants, single lab\",\n      \"pmids\": [\"25323061\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"WNK1-OSR1 signaling promotes glioma cell migration. siRNA knockdown of WNK1 or OSR1 reduces intracellular K+ and Cl- content and abolishes NKCC1 regulatory phospho-activation, impeding regulatory volume increase in glioma cells. Temozolomide activates the WNK1/OSR1/NKCC1 pathway and enhances glioma migration, which is blocked by OSR1 siRNA knockdown.\",\n      \"method\": \"siRNA knockdown of OSR1, live cell imaging, microchemotaxis assay, cell volume and ion measurements, immunostaining\",\n      \"journal\": \"Molecular cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — siRNA KD with functional migration and ion transport readouts, single lab\",\n      \"pmids\": [\"24555568\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"OSR1 downregulates peptide transporters PEPT1 and PEPT2 by decreasing their maximum transport rate and reducing PEPT2 protein abundance at the cell membrane in Xenopus oocytes. The effect requires OSR1 kinase activity (T185E mimics, D164A and T185A are inactive), suggesting kinase-dependent membrane protein regulation.\",\n      \"method\": \"Xenopus oocyte electrophysiology, chemiluminescence surface protein assay, constitutively active and kinase-dead mutants\",\n      \"journal\": \"Kidney & blood pressure research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional reconstitution with multiple OSR1 mutants, single lab\",\n      \"pmids\": [\"25531100\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Drosophila WNK-Fray (SPAK/OSR1 homolog) signaling cascade regulates K+ flux through Ncc69 (NKCC) in the renal tubule. Fray directly phosphorylates Ncc69 in vitro. A constitutively active Fray mutant rescues the wnk knockdown phenotype, establishing hierarchical WNK→Fray→NKCC signaling for hypotonicity-stimulated K+ flux, demonstrating evolutionary conservation of the WNK-SPAK/OSR1-NKCC cascade.\",\n      \"method\": \"Drosophila genetics, in vitro kinase assay, constitutively active mutant rescue, ion flux assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro kinase assay plus genetic epistasis rescue in Drosophila ortholog\",\n      \"pmids\": [\"25086033\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"OSR1 upregulates the intestinal Na+-coupled phosphate cotransporter NaPi-IIb (SLC34A2) in Xenopus oocytes. Constitutively active (T185E)OSR1 and wild-type OSR1 (but not T185A or D164A mutants) significantly increase phosphate-induced currents and maximal transport rate, demonstrating that kinase activity is required. Combined SPAK+OSR1 coexpression shows an additive effect.\",\n      \"method\": \"Xenopus oocyte expression, dual electrode voltage clamp, constitutively active and kinase-dead mutants\",\n      \"journal\": \"Kidney & blood pressure research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional reconstitution with systematic mutants, single lab\",\n      \"pmids\": [\"26506223\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"SPAK and OSR1 are essential for K+ homeostasis through their actions on the distal convoluted tubule. SPAK/OSR1 double knockout (DKO) mice develop severe hypokalemia on K+ restriction, unlike single KO mice. DKO mice fail to phosphorylate NCC under K+-restricted conditions, demonstrating that SPAK and OSR1 are redundant but essential effectors of the K+-sensing pathway that maintains plasma K+ through NCC activation.\",\n      \"method\": \"SPAK/OSR1 double knockout mice, dietary K+ restriction, plasma K+ measurement, phospho-NCC immunoblotting\",\n      \"journal\": \"The Journal of physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean genetic double KO model with defined physiological phenotype and biochemical readout\",\n      \"pmids\": [\"27068441\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Rafoxanide and closantel (antiparasitic agents) inhibit SPAK and OSR1 by binding to a highly conserved allosteric pocket on their C-terminal domains. This allosteric site influences kinase activity and can be exploited for inhibitor design.\",\n      \"method\": \"In silico screening, in vitro kinase assay, binding studies\",\n      \"journal\": \"ChemMedChem\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — in vitro kinase inhibition with binding characterization, single lab\",\n      \"pmids\": [\"28371477\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Verteporfin (photosensitizing clinical agent) inhibits SPAK and OSR1 kinases by binding to their kinase domains in an ATP-independent manner. In cells, verteporfin suppresses phosphorylation of NKCC1, a downstream physiological substrate of SPAK and OSR1.\",\n      \"method\": \"In vitro kinase assay, kinase panel screening, cell-based phosphorylation assay\",\n      \"journal\": \"Chembiochem\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — in vitro and cell-based inhibitor characterization, single lab\",\n      \"pmids\": [\"29999233\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"The E3 ubiquitin ligase complex CUL4-DDB1-WDR3/WDR6 binds OSR1 in a phosphorylation-dependent manner via the S-motif (C-terminal serine phosphorylation site). S-motif phosphorylation following osmotic stress disrupts CUL4 complex binding, and OSR1 ubiquitylation is abolished under osmotic stress when its S-motif is phosphorylated, revealing a mechanism linking WNK-dependent S-motif phosphorylation to OSR1 proteostasis.\",\n      \"method\": \"Affinity pull-down, mass spectrometry, proteasomal and neddylation inhibitors, immunoblotting\",\n      \"journal\": \"Chembiochem\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — AP-MS identification with pharmacological validation, single lab\",\n      \"pmids\": [\"31614064\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"In hypokalemia, WNK4 is the primary active WNK isoform that phosphorylates SPAK/OSR1 within cytoplasmic WNK bodies in the DCT. Phosphorylated SPAK/OSR1 is present both at the apical membrane and in WNK bodies within 12 h of K+ deprivation. In WNK4-deficient mice, larger WNK bodies form containing unphosphorylated WNK1, SPAK, and OSR1. WNK body formation in DCT requires sensing of plasma K+ via the Kir4.1 channel.\",\n      \"method\": \"Immunofluorescence microscopy, WNK4-deficient and Kir4.1 knockout mice, dietary K+ manipulation\",\n      \"journal\": \"American journal of physiology. Renal physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple genetic mouse models with direct subcellular localization and phosphorylation readouts\",\n      \"pmids\": [\"31736353\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"OSR1 activity is required for LPS-stimulated migration of dendritic cells. In OSR1 knockin (WNK-resistant) dendritic cells, Na+/H+ exchanger activity and ROS production are elevated at baseline, but LPS fails to further increase them or stimulate migration. NKCC1 inhibition with furosemide increases NHE activity to levels similar to OSR1-deficient cells, suggesting OSR1-NKCC1 interaction regulates NHE activity.\",\n      \"method\": \"WNK-resistant OSR1 knockin mice, dendritic cell culture, FACS, fluorescent pH indicators, transwell migration assay\",\n      \"journal\": \"American journal of physiology. Cell physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — knockin mouse model with multiple functional readouts, single lab\",\n      \"pmids\": [\"22648948\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Pro-resolving FPR2 agonists (WKYMVm and annexin A1) induce phosphorylation of OSR1 at Ser339 via NADPH oxidase-dependent activation of PI3K and PKCδ. Blockade of NADPH oxidase prevents FPR2-induced OSR1(Ser339) phosphorylation, identifying NADPH oxidase→PI3K→OSR1(Ser339) as a pro-resolving signaling axis.\",\n      \"method\": \"Pharmacological NADPH oxidase inhibition, CRISPR/Cas9 p22phox knockout cells, phospho-specific immunoblotting\",\n      \"journal\": \"Antioxidants\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — CRISPR KO validation plus pharmacological inhibition, single lab\",\n      \"pmids\": [\"33477989\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"WNK1 HSN2 splice variant regulates neurite outgrowth through OSR1 activation and downstream GSK3β. The HSN2-OSR1/GSK3β signaling axis induces expression of LHX8, a key regulator of cholinergic neural function. HSANII patient-derived HSN2 mutants suppress OSR1 activation, LHX8 induction, and neurite outgrowth, and prevent interaction between wild-type HSN2 and GSK3β.\",\n      \"method\": \"Cell-based neurite outgrowth assay, OSR1 activation measurements, co-immunoprecipitation, LHX8 expression analysis, patient mutation analysis\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic patient mutations analyzed functionally with pathway validation, single lab\",\n      \"pmids\": [\"36151370\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"OSR1 was identified through yeast two-hybrid screening as interacting with RELL1 (a RELT family member). OSR1 interacts with all three RELT family members (RELT, RELL1, RELL2) by co-immunoprecipitation and phosphorylates them in an in vitro kinase assay, identifying the RELT family as OSR1 substrates.\",\n      \"method\": \"Yeast two-hybrid, co-immunoprecipitation, in vitro kinase assay\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — yeast two-hybrid confirmed by co-IP and in vitro kinase assay, single lab\",\n      \"pmids\": [\"16389068\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Phosphorylation of NCC by OSR1/SPAK kinases decreases NCC ubiquitination. In mice, low-salt diet decreases NCC ubiquitination while high-salt diet increases it (inversely correlated with phosphorylation and membrane abundance). In COS7 cells, phospho-deficient NCC shows increased ubiquitination while phospho-mimicking NCC shows decreased ubiquitination for both total and surface NCC, establishing that OSR1/SPAK-mediated phosphorylation of NCC protects it from ubiquitin-mediated degradation.\",\n      \"method\": \"In vivo mouse models, COS7 cell expression, immunoprecipitation, ubiquitination assay, surface biotinylation\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo and cell-based validation with phosphomimetic mutants, single lab\",\n      \"pmids\": [\"22846565\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Disruption of the WNK4-SPAK/OSR1 pathway using a new C57BL/6-specific pT96-NKCC2 antibody reveals that SPAK/OSR1 only mildly affects NKCC2 phosphorylation at baseline; NCC phosphorylation is strongly dependent on this pathway while NKCC2 is not. In WNK4-/- and SPAK/OSR1 double KO mice, pT96-NKCC2 is only slightly or not significantly reduced, suggesting alternative kinases phosphorylate NKCC2.\",\n      \"method\": \"New phospho-specific antibody, Wnk4-/-, Osr1-/-, Spak-/-, DKO mice, immunoblotting, furosemide diuresis assay\",\n      \"journal\": \"American journal of physiology. Renal physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple genetic mouse models with new validated antibody tool, systematic in vivo analysis\",\n      \"pmids\": [\"37881876\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"OSR1 (OXSR1) is a STE20-family serine/threonine kinase that is activated by WNK1/WNK4 phosphorylation of its T-loop Thr185 and further stimulated ~100-fold by the MO25 scaffold protein; it recognizes substrates and activators via RFXV/RxFxV docking motifs in its CCT domain, directly phosphorylates and activates Na+-K+-2Cl- cotransporters (NKCC1, NKCC2) and NCC while inhibiting K+-Cl- cotransporters (KCCs), and additionally regulates Kir2.1/2.3, ROMK1, ClC-2, phosphate transporters, and Smad2/3; it is subject to feedback regulation via mTORC2 (Ser339) and CUL4-mediated ubiquitination, and plays essential roles in renal salt/K+ homeostasis, embryonic angiogenesis, neuronal chloride homeostasis, and glioma cell migration.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"OXSR1 (OSR1) is a STE20-family serine/threonine kinase that serves as a central effector in the WNK signaling cascade, transducing osmotic and ionic signals into coordinated regulation of cation-chloride cotransporters and ion channels to control cell volume, renal salt reabsorption, and blood pressure. WNK1 and WNK4 activate OSR1 by phosphorylating its activation-loop residue Thr185, and the scaffold protein MO25 allosterically amplifies OSR1 catalytic activity ~100-fold; activated OSR1 then phosphorylates and stimulates Na⁺-K⁺-2Cl⁻ cotransporters (NKCC1, NKCC2) and Na⁺-Cl⁻ cotransporter NCC while simultaneously phosphorylating and inhibiting K⁺-Cl⁻ cotransporters (KCCs), with substrate and activator recognition mediated by RFXV/I motifs docking into its conserved C-terminal (CCT) domain [PMID:16083423, PMID:16263722, PMID:24393035, PMID:16669787, PMID:21423148]. Genetic epistasis in mice demonstrates that inactivation of OSR1 together with SPAK abolishes NCC phosphorylation and corrects the hypertension, hyperkalemia, and metabolic acidosis of pseudohypoaldosteronism type II, and Osr1-null embryos phenocopy Wnk1-null cardiovascular lethality, establishing OSR1 as essential for both renal electrolyte homeostasis and embryonic angiogenesis [PMID:21486947, PMID:23386621]. Additional regulatory inputs converge on OSR1: mTORC2 phosphorylates Ser339 to modulate osmotic stress responses, and the CUL4-DDB1-WDR3/6 E3 ligase complex ubiquitylates OSR1 in an S-motif phosphorylation-dependent manner to regulate its stability [PMID:24191005, PMID:31614064].\",\n  \"teleology\": [\n    {\n      \"year\": 2002,\n      \"claim\": \"Identifying how OSR1 recognizes its cotransporter substrates: the discovery of a conserved C-terminal domain that binds (R/K)FX(V/I) motifs on NKCC1, NKCC2, and KCC3 established a docking mechanism for kinase–substrate interaction.\",\n      \"evidence\": \"Yeast two-hybrid, GST pull-down, and co-immunoprecipitation with cation-chloride cotransporters\",\n      \"pmids\": [\"12386165\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequence of the interaction (phosphorylation, activation) not yet demonstrated\", \"Structural basis of the docking interaction unknown at this stage\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Establishing OSR1 as an osmotic stress-activated kinase: OSR1 was shown to be activated specifically by osmotic stress (sorbitol, NaCl) and not by other cellular stresses, and PAK1 was identified as an in vitro substrate.\",\n      \"evidence\": \"Yeast two-hybrid, in vitro kinase assay, and mutagenesis showing OSR1 phosphorylates PAK1 at Thr84\",\n      \"pmids\": [\"14707132\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"OSR1–PAK1 axis not independently replicated\", \"Upstream activator of OSR1 under osmotic stress not yet identified\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Defining the activation mechanism: WNK1/WNK4 were identified as the direct upstream kinases that phosphorylate OSR1 at Thr185 in the activation loop, with T185 phosphorylation being necessary and sufficient for catalytic activation, and OSR1 was shown to directly phosphorylate NKCC1, NKCC2, and NCC.\",\n      \"evidence\": \"In vitro kinase assays, phosphopeptide mapping, site-directed mutagenesis (T185A abolishes; T185E activates), cell-based hypotonic stress phosphorylation\",\n      \"pmids\": [\"16083423\", \"16263722\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structural explanation for how T185 phosphorylation activates the kinase\", \"In vivo significance of WNK→OSR1→cotransporter axis not yet demonstrated\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Establishing the linear WNK1→OSR1→NKCC1 signaling pathway in cells and demonstrating that the CCT domain serves as a dual-purpose docking site for both upstream activators and downstream substrates.\",\n      \"evidence\": \"Co-immunoprecipitation of endogenous WNK1–OSR1 complex, siRNA knockdown reducing NKCC1 activity, peptide binding assays and mutagenesis of CCT domain residues, identification of NKCC1 phosphorylation sites (Thr203/207/212)\",\n      \"pmids\": [\"16832045\", \"16669787\", \"16382158\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative contributions of OSR1 vs. SPAK to NKCC1 regulation unclear\", \"Whether feedback from cotransporter phosphorylation regulates OSR1 unknown\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Solving the structural basis of substrate recognition: the crystal structure of the CCT domain bound to a WNK4 RFXV peptide revealed a novel fold with a surface groove, and showed that phosphorylation preceding the RFXV motif causes steric clash and dissociation—providing a mechanism for regulated release of substrates/activators.\",\n      \"evidence\": \"X-ray crystallography of CCT–peptide complex with mutational validation\",\n      \"pmids\": [\"17721439\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full-length OSR1 structure not determined\", \"Whether phosphorylation-dependent dissociation occurs in vivo not demonstrated\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Extending OSR1 substrate specificity to NCC and demonstrating in vivo physiological regulation: OSR1/SPAK phosphorylate NCC at Thr46/55/60 via RFXI docking, and dietary salt intake regulates OSR1 phosphorylation and NCC activity in mouse kidney via aldosterone signaling.\",\n      \"evidence\": \"In vitro kinase assay with NCC mutagenesis, knockin mouse models with dietary salt manipulation and spironolactone treatment\",\n      \"pmids\": [\"18270262\", \"18800028\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Individual contributions of OSR1 vs. SPAK to NCC regulation in vivo not separated\", \"Mechanism linking aldosterone to WNK–OSR1 activation not defined\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Revealing the kinase domain architecture: the crystal structure of the OSR1 kinase domain showed a domain-swapped dimer in an inactive conformation, suggesting a structural basis for autoinhibition.\",\n      \"evidence\": \"X-ray crystallography at 2.25 Å resolution\",\n      \"pmids\": [\"19177573\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether domain swapping occurs in vivo or is a crystallographic artifact unknown\", \"Active-state kinase domain structure not determined\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Proving that the entire pseudohypoaldosteronism type II disease phenotype depends on the WNK–OSR1/SPAK–NCC cascade: triple knockin mice showed that inactivating OSR1/SPAK abolishes NCC phosphorylation and corrects hypertension, hyperkalemia, and acidosis caused by a WNK4 gain-of-function mutation.\",\n      \"evidence\": \"Triple knockin mouse model (WNK4-D561A × SPAK-T243A × OSR1-T185A), blood pressure measurement, electrolyte analysis, phospho-specific western blot\",\n      \"pmids\": [\"21486947\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether pharmacological inhibition of OSR1/SPAK alone suffices to treat PHAII not tested\", \"Individual contribution of OSR1 vs. SPAK not genetically separated in this model\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Identifying MO25 as a potent allosteric activator: MO25α/β binding to OSR1 produces ~100-fold enhancement of catalytic activity, and MO25 knockdown reduces endogenous NKCC1 phosphorylation, establishing MO25 as an essential cofactor.\",\n      \"evidence\": \"Co-immunoprecipitation, in vitro kinase assay with purified proteins, siRNA knockdown with phospho-specific antibody readout\",\n      \"pmids\": [\"21423148\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of MO25-mediated activation of OSR1 not determined\", \"Whether MO25 levels are regulated by osmotic stress unknown\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Demonstrating negative feedback and SPAK dependency: double-knockin ES cells revealed that loss of OSR1/SPAK activation elevates upstream WNK1/WNK3 activity (negative feedback), and SPAK knockout mice showed that OSR1 activity and apical membrane localization in the distal convoluted tubule depend on SPAK.\",\n      \"evidence\": \"SPAK-T243A/OSR1-T185A knockin ES cells with kinase activity measurements; SPAK knockout mice with immunofluorescence and subcellular fractionation\",\n      \"pmids\": [\"22032326\", \"22977235\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism of WNK feedback suppression by OSR1/SPAK not identified\", \"Whether OSR1 can function independently of SPAK in any tissue not established\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Establishing OSR1 as essential for embryonic cardiovascular development: Osr1-null embryos die at ~E11 with angiogenesis and cardiac defects phenocopying Wnk1 nulls, and constitutively active OSR1 rescues the Wnk1-null vascular phenotype, proving OSR1 is the critical WNK1 effector in endothelial cells.\",\n      \"evidence\": \"Global and conditional Osr1 knockout mice; constitutively active OSR1 transgenic rescue of Wnk1-null embryos\",\n      \"pmids\": [\"23386621\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream substrates mediating OSR1's role in angiogenesis not identified (NKCC1 vs. other targets)\", \"Whether adult vascular maintenance requires OSR1 unknown\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Expanding OSR1 substrate range to include inhibitory phosphorylation of KCC cotransporters and regulation of diverse ion channels: OSR1 phosphorylates KCC isoforms at a conserved Site-2 threonine to inhibit K⁺-Cl⁻ cotransport, and separately regulates Kir2.1/2.3 channels, ROMK1, ClC-2, and creatine/peptide/phosphate transporters.\",\n      \"evidence\": \"In vitro kinase assays with SPAK/OSR1 double-knockin ES cells showing abolished KCC phosphorylation and elevated KCC activity; Xenopus oocyte expression with mutagenesis for diverse transporters\",\n      \"pmids\": [\"24393035\", \"25322850\", \"25323061\", \"25531585\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Many non-NKCC/NCC substrates characterized only in heterologous oocyte systems by a single lab\", \"Physiological relevance of OSR1 regulation of ClC-2, ROMK1, PEPT1/2, SLC6A8 in vivo not established\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identifying a convergent regulatory input from mTORC2: mTORC2 phosphorylates OSR1 at Ser339, and PI3K or Sin1 inhibition reduces osmotic stress-induced OSR1 activation and NKCC1 activity, revealing PI3K/mTORC2 as a parallel activating pathway.\",\n      \"evidence\": \"In vitro kinase assay with mTOR complex components, siRNA knockdown of Sin1, PI3K inhibitor treatment with NKCC activity readout\",\n      \"pmids\": [\"24191005\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether mTORC2 and WNK phosphorylation of OSR1 are additive or synergistic not resolved\", \"Ser339 phosphorylation's effect on OSR1 structure/conformation unknown\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Demonstrating OSR1 regulation of inward rectifier K⁺ channels via a variant RFXV motif: OSR1 activates Kir2.1/2.3 through direct interaction with an R-x-F-x-V/I motif, and WNK-OSR1 activity is required for NaCl-induced Kir2.3 plasma membrane localization.\",\n      \"evidence\": \"Co-IP, RFXV motif mutagenesis, siRNA and WNK inhibitor treatment with membrane localization assay\",\n      \"pmids\": [\"29581290\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct phosphorylation site on Kir2.1/2.3 by OSR1 not mapped\", \"In vivo cardiac or neuronal significance not demonstrated\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Revealing ubiquitin-dependent stability control: the CUL4-DDB1-WDR3/6 E3 ligase complex binds OSR1 in an S-motif (Ser325) phosphorylation-dependent manner, and osmotic stress-induced Ser325 phosphorylation blocks ubiquitylation, stabilizing OSR1.\",\n      \"evidence\": \"Affinity pull-down with mass spectrometry, neddylation and proteasomal inhibitors, ubiquitylation assay\",\n      \"pmids\": [\"31614064\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab; independent replication pending\", \"Whether CUL4 complex regulation of OSR1 turnover affects renal physiology in vivo unknown\", \"Quantitative contribution of ubiquitin-mediated degradation vs. dephosphorylation to OSR1 inactivation not measured\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Linking OSR1 to TGF-β/Smad signaling: OSR1 directly phosphorylates Smad2 (Thr220) and Smad3 (Thr179) linker regions, promoting nuclear translocation and TGF-β1 autocrine signaling to drive epithelial-mesenchymal transition in breast cancer cells.\",\n      \"evidence\": \"In vitro kinase assay, co-IP, nuclear fractionation, mutagenesis, OSR1 KO cell lines\",\n      \"pmids\": [\"33051597\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab; independent replication pending\", \"In vivo relevance of OSR1-Smad axis to metastasis not demonstrated\", \"Whether this is a general or cancer-specific function unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key open questions include: the full-length active-state structure of OSR1 (with MO25 and/or WNK), the tissue-specific partitioning of function between OSR1 and SPAK, the in vivo relevance of OSR1's broader substrate repertoire beyond cation-chloride cotransporters, and whether therapeutic OSR1 inhibition can safely lower blood pressure.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No full-length OSR1 structure in active conformation\", \"OSR1 vs. SPAK tissue-specific roles not genetically resolved in conditional knockouts for most organs\", \"Pharmacological OSR1 inhibitors lack selectivity and in vivo validation\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 1, 7, 10, 12, 36]},\n      {\"term_id\": \"GO:0140657\", \"supporting_discovery_ids\": [0, 6]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [20, 24, 25]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [20]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [1, 7, 10, 11, 12, 25]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [3, 16, 36, 37]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"WNK1\",\n      \"WNK4\",\n      \"NKCC1\",\n      \"NCC\",\n      \"NKCC2\",\n      \"MO25\",\n      \"SPAK\",\n      \"KCC3\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"OXSR1 (OSR1) is a STE20-family serine/threonine kinase that functions as the central relay in the WNK signaling cascade controlling cation-chloride cotransporter activity and ion homeostasis. WNK1/WNK4 phosphorylate OSR1 at T-loop residue Thr185 to activate it, and the scaffold protein MO25 amplifies OSR1 kinase activity ~100-fold; OSR1 recognizes both its upstream activators and downstream substrates through RFXV/RxFxV motifs that dock into a structurally defined groove on its conserved C-terminal (CCT) domain [PMID:16083423, PMID:16669787, PMID:17721439, PMID:21423148]. Activated OSR1 directly phosphorylates and stimulates Na⁺-K⁺-2Cl⁻ cotransporters (NKCC1, NKCC2) and NCC while inhibiting K⁺-Cl⁻ cotransporters (KCCs), thereby coordinating renal salt reabsorption and K⁺ homeostasis—functions confirmed by in vivo genetic epistasis showing that WNK-resistant OSR1/SPAK knock-in mice abolish NCC phosphorylation and that double-knockout mice develop severe hypokalemia under K⁺ restriction [PMID:21486947, PMID:24393035, PMID:27068441]. Beyond ion transport, OSR1 is essential for embryonic angiogenesis downstream of WNK1, as endothelial-specific OSR1 deletion phenocopies WNK1 loss and constitutively active OSR1 rescues vascular defects, and OSR1 phosphorylates Smad2/3 to promote TGF-β-driven epithelial-to-mesenchymal transition in breast cancer [PMID:23386621, PMID:33051597]. OSR1 activity is additionally modulated by mTORC2-mediated Ser339 phosphorylation and by CUL4-DDB1-WDR3/WDR6-dependent ubiquitination that is relieved upon osmotic stress-induced S-motif phosphorylation [PMID:24191005, PMID:31614064].\",\n  \"teleology\": [\n    {\n      \"year\": 2002,\n      \"claim\": \"Identification of the substrate-docking mechanism: OSR1 was shown to physically interact with NKCC1, NKCC2, and KCC3 via a C-terminal domain recognizing (R/K)FX(V/I) motifs, establishing how OSR1 selects its cotransporter substrates.\",\n      \"evidence\": \"Yeast two-hybrid, GST pull-down, and co-immunoprecipitation with NKCC/KCC cotransporters\",\n      \"pmids\": [\"12386165\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of CCT–RFXV interaction not yet resolved\", \"Functional consequence of interaction (phosphorylation, activation) not demonstrated\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Initial biochemical characterization established OSR1 as an osmotic stress-activated STE20-family kinase that phosphorylates PAK1 but does not activate canonical MAPK cascades, defining its distinct signaling niche.\",\n      \"evidence\": \"In vitro kinase assay, site-directed mutagenesis, osmotic stress activation in cells\",\n      \"pmids\": [\"14707132\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological relevance of PAK1-Thr84 phosphorylation unclear\", \"Upstream activator identity not established\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"The upstream activation mechanism was solved: WNK1/WNK4 directly phosphorylate OSR1 at Thr185 in its T-loop to activate it, and OSR1/SPAK directly phosphorylate the N-terminal regulatory domains of NKCC1, NKCC2, and NCC, establishing the hierarchical WNK→OSR1→cotransporter cascade.\",\n      \"evidence\": \"In vitro kinase assays with phosphopeptide mapping and T185A/T185E mutagenesis; cell-based phosphorylation assays under hypotonic stress\",\n      \"pmids\": [\"16083423\", \"16263722\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative contributions of OSR1 vs SPAK to each cotransporter unknown\", \"In vivo validation of the cascade pending\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"The CCT domain was defined as a dual-purpose docking module that binds RFXV motifs in both activators (WNK1/WNK4) and substrates (NKCC1) with nanomolar affinity, and WNK1–OSR1 complex formation was confirmed in cells with functional consequence for NKCC activity.\",\n      \"evidence\": \"In vitro kinase assays with CCT mutagenesis, peptide binding affinity measurements, co-immunoprecipitation, siRNA knockdown reducing NKCC activity\",\n      \"pmids\": [\"16669787\", \"16832045\", \"16382158\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic-resolution structure of CCT–RFXV complex not yet available\", \"Mechanism for substrate vs. activator discrimination at single binding groove unknown\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"The crystal structure of the CCT domain bound to a WNK4-derived RFXV peptide revealed a novel fold with a surface groove, and showed that phosphorylation preceding the RFXV motif causes steric clash promoting dissociation—providing a structural basis for phosphorylation-dependent regulation of docking.\",\n      \"evidence\": \"X-ray crystallography with mutational validation\",\n      \"pmids\": [\"17721439\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full-length OSR1 structure unavailable\", \"Structural model for how CCT domain communicates with kinase domain not resolved\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Specific NCC phosphorylation sites (Thr46, Thr55, Thr60) targeted by SPAK/OSR1 were mapped, with Thr60 identified as a priming site required for efficient phosphorylation of the other sites and for NCC activation.\",\n      \"evidence\": \"In vitro kinase assay and mutagenesis in HEK293 and mpkDCT cells under hypotonic low-chloride conditions\",\n      \"pmids\": [\"18270262\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether ordered phosphorylation mechanism applies to NKCC substrates not tested\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"The kinase domain crystal structure at 2.25 Å revealed a domain-swapped dimer in an inactive conformation, providing a structural framework for understanding OSR1 autoinhibition and activation.\",\n      \"evidence\": \"X-ray crystallography\",\n      \"pmids\": [\"19177573\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether domain swapping occurs in vivo or is a crystallographic artifact unknown\", \"Structure of active (phospho-Thr185) kinase domain not solved\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"MO25 was identified as a ~100-fold activator of OSR1 kinase activity, and genetic epistasis in WNK-resistant SPAK/OSR1 knock-in mice confirmed in vivo dependence of NCC phosphorylation on this cascade, definitively validating the WNK→OSR1/SPAK→NCC pathway in renal physiology.\",\n      \"evidence\": \"In vitro reconstitution with MO25, siRNA knockdown/rescue in cells; triple knock-in mouse model with phospho-NCC immunoblotting\",\n      \"pmids\": [\"21423148\", \"21486947\", \"21321328\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of MO25-mediated activation unknown\", \"Whether MO25 regulation is tissue-specific not addressed\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Multiple regulatory layers on OSR1 were discovered: SPAK-dependent localization of OSR1 to the apical membrane in DCT, PI3K/mTORC2-mediated Ser339 phosphorylation as a convergent activation input, aldosterone-driven circadian oscillation of WNK–OSR1–NCC, and ASK3 as an upstream suppressor—revealing OSR1 as an integrative signaling node.\",\n      \"evidence\": \"SPAK-KO mice with immunofluorescence; in vitro mTORC2 kinase assay with Sin1 siRNA; time-course immunoblotting with eplerenone in mice; ASK3-KO mice\",\n      \"pmids\": [\"22977235\", \"24191005\", \"23044422\", \"23250415\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative quantitative contributions of mTORC2 vs WNK phosphorylation to total OSR1 activity unknown\", \"Whether circadian regulation extends beyond the kidney not tested\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Genetic evidence from double-knockin ES cells established that SPAK/OSR1 are the only kinases essential for NKCC1 phosphorylation and revealed feedback regulation whereby loss of SPAK/OSR1 activity elevates upstream WNK1/WNK3 activity.\",\n      \"evidence\": \"SPAK/OSR1 double-knockin ES cells, NKCC1 activity assays, immunoblotting for WNK phosphorylation\",\n      \"pmids\": [\"22032326\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the WNK feedback phosphatase or mechanism not determined\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"OSR1 was established as essential for embryonic cardiovascular development: endothelial-specific OSR1 deletion phenocopied WNK1 knockout vascular defects, and constitutively active OSR1 rescued angiogenesis in WNK1-null embryos, placing OSR1 as the critical downstream effector of WNK1 in vascular morphogenesis.\",\n      \"evidence\": \"Conditional knockout mice (Tie2-Cre), constitutively active OSR1 transgene rescue, embryonic phenotyping\",\n      \"pmids\": [\"23386621\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream vascular targets of OSR1 in endothelial cells not fully identified\", \"Whether ion cotransporters mediate the angiogenic role unclear\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"The substrate repertoire was greatly expanded: OSR1 was shown to inhibit all KCC isoforms via direct phosphorylation, regulate Kir2.1/2.3 membrane localization via RxFxV docking, phosphorylate Smad2/3 to drive EMT and metastasis, and modulate ROMK1 and ClC-2 channels—broadening OSR1 from a cotransporter kinase to a versatile regulator of membrane transport and TGF-β signaling.\",\n      \"evidence\": \"In vitro kinase assays with KCC mutagenesis and knockin ES cells; Kir2.3 motif mutagenesis with siRNA; Smad2/3 co-IP and in vivo metastasis models; Xenopus oocyte electrophysiology for ROMK1 and ClC-2\",\n      \"pmids\": [\"24393035\", \"29581290\", \"33051597\", \"25322850\", \"25323061\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological contexts for many of these substrates not validated in vivo\", \"Direct phosphorylation sites on Kir2.1/2.3, ROMK1, and ClC-2 not mapped\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"SPAK/OSR1 double-knockout mice revealed functional redundancy between the two kinases in K⁺ homeostasis: only combined loss caused severe hypokalemia and failure to phosphorylate NCC under dietary K⁺ restriction.\",\n      \"evidence\": \"SPAK/OSR1 DKO mice, dietary K⁺ manipulation, plasma K⁺ measurement, phospho-NCC immunoblotting\",\n      \"pmids\": [\"27068441\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Tissue-specific non-redundant functions of OSR1 vs SPAK in other nephron segments not fully characterized\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Two new regulatory mechanisms were defined: CUL4-DDB1-WDR3/WDR6-dependent ubiquitination of OSR1 that is abolished when the S-motif is phosphorylated during osmotic stress, linking WNK signaling to OSR1 protein stability; and WNK4-dependent formation of cytoplasmic WNK bodies containing SPAK/OSR1 in DCT during hypokalemia.\",\n      \"evidence\": \"AP-MS with pharmacological validation for CUL4 complex; immunofluorescence in WNK4-deficient and Kir4.1-KO mice with dietary K⁺ manipulation\",\n      \"pmids\": [\"31614064\", \"31736353\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Half-life measurements of OSR1 under different phosphorylation states not reported\", \"Functional significance of WNK body localization for OSR1 activity quantitatively undefined\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Re-evaluation with a new strain-specific phospho-NKCC2 antibody revealed that SPAK/OSR1 contribute only mildly to basal NKCC2 phosphorylation, in contrast to their dominant role in NCC regulation, implying alternative kinases phosphorylate NKCC2 in vivo.\",\n      \"evidence\": \"New pT96-NKCC2 antibody in WNK4⁻/⁻, SPAK⁻/⁻, OSR1⁻/⁻, and DKO mice with furosemide diuresis\",\n      \"pmids\": [\"37881876\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the alternative NKCC2 kinase(s) unknown\", \"Whether NKCC2 phosphorylation at other sites remains SPAK/OSR1-dependent not resolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key open questions include the structural basis of MO25-mediated OSR1 activation, the identity of kinases that phosphorylate NKCC2 independently of SPAK/OSR1, whether OSR1's non-cotransporter substrates (Smad2/3, Kir2.x, ROMK1) are physiologically relevant in vivo, and the mechanism by which OSR1 promotes angiogenesis independently of ion transport.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No full-length OSR1 structure or MO25–OSR1 complex structure available\", \"In vivo validation of Smad2/3, Kir2.x, and ROMK1 as physiological OSR1 substrates lacking\", \"Molecular targets downstream of OSR1 in endothelial angiogenesis unidentified\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 1, 4, 5, 7, 8, 10, 12, 21, 24]},\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [0, 1, 7, 8, 10, 12, 21, 24, 40]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [3, 13, 36]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [14, 23, 36]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 1, 3, 4, 10, 11, 15, 18, 36]},\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [1, 5, 7, 12, 21, 23, 26, 27, 32]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [20, 22]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"WNK1\",\n      \"WNK4\",\n      \"CAB39\",\n      \"SLC12A1\",\n      \"SLC12A2\",\n      \"SLC12A3\",\n      \"STK39\",\n      \"SMAD2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}