{"gene":"SLC9B2","run_date":"2026-06-10T07:46:35","timeline":{"discoveries":[{"year":2008,"finding":"Human NHA2 (SLC9B2) was cloned from a brain cDNA library and characterized as a ~55 kDa sodium/proton exchanger expressed in multiple mammalian tissues. In kidney, NHA2 was restricted to the distal convoluted tubule and co-localized with the mitochondrial inner membrane marker by immunogold EM and differential centrifugation. NHA2 functionally complemented sodium/hydrogen exchanger-null yeast (restored growth under high NaCl at acidic pH), demonstrating bona fide Na+/H+ exchange activity.","method":"cDNA cloning, immunoblot with siRNA validation, co-sedimentation/differential centrifugation, immunogold EM, heterologous expression in NHE-deficient yeast","journal":"Journal of the American Society of Nephrology","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — multiple orthogonal methods (biochemical fractionation, immunogold EM, functional yeast complementation) in a single characterization study","pmids":["18508966"],"is_preprint":false},{"year":2007,"finding":"NHA2 (NHA-oc/NHA2) is strongly upregulated during RANKL-induced osteoclast differentiation in vitro and in vivo. The protein localizes to mitochondria in osteoclasts, where it mediates Na+-dependent changes in mitochondrial pH and Na+-acetate-induced mitochondrial passive swelling. siRNA silencing of NHA2 reduced osteoclast differentiation and bone resorption in vitro.","method":"RANKL-stimulated differentiation assay, mitochondrial pH assay, mitochondrial swelling assay, siRNA knockdown, microarray followed by Northern blot and in situ hybridization","journal":"Bone","confidence":"Medium","confidence_rationale":"Tier 2–3 / Moderate — functional mitochondrial pH and swelling assays plus siRNA knockdown, single lab, later contradicted on mitochondrial localization by Hofstetter et al. 2010","pmids":["17988971"],"is_preprint":false},{"year":2010,"finding":"Using NHA2-specific antibodies and NHA2-deficient mice, NHA2 was found to co-localize with the late endosomal/lysosomal marker LAMP1 and V-ATPase a3 subunit (not mitochondrial markers) in osteoclasts. Surface biotinylation and immunofluorescence further showed NHA2 is highly enriched at the plasma membrane, specifically the basolateral membrane of polarized osteoclasts. Contrary to earlier siRNA data, NHA2-deficient mice showed no difference in bone parameters, osteoclast development, or resorption activity in vitro, indicating NHA2 is dispensable for osteoclast differentiation and bone resorption.","method":"Confocal microscopy, subcellular fractionation, surface biotinylation, microcomputed tomography of NHA2-KO mice, in vitro RANKL stimulation of KO bone marrow cells","journal":"Bone","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal localization using multiple markers, in vivo KO phenotype with micro-CT, and in vitro functional assay; replicates and revises earlier localization claim","pmids":["20441802"],"is_preprint":false},{"year":2012,"finding":"Human NHA2 mediates phloretin-sensitive Na+-Li+ countertransport (SLC) activity at the plasma membrane of kidney-derived MDCK cells, coupled to a plasma membrane H+ gradient generated by V-type H+-ATPase, functioning as a virtual Na+ efflux pump. This establishes H+-coupled (rather than Na+-coupled) secondary transport for NHA2 at the mammalian plasma membrane.","method":"Na+-Li+ countertransport assay in MDCK cells, pharmacological inhibition with phloretin, V-ATPase inhibitor studies","journal":"Journal of Biological Chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct ion transport assay with pharmacological dissection in cell lines, single lab","pmids":["22948142"],"is_preprint":false},{"year":2013,"finding":"NHA2 is present in rodent and human β-cells and resides in transferrin-positive endosomes and synaptic-like microvesicles (not large dense core vesicles). NHA2-deficient mice show impaired insulin secretion and glucose intolerance. Loss of NHA2 inhibited clathrin-dependent but not clathrin-independent endocytosis in Min6 and primary β-cells, implicating defective endo-exocytosis coupling as the mechanism for impaired secretion. The secretory deficit was rescued by wild-type but not functionally dead NHA2.","method":"NHA2-KO mouse in vivo glucose tolerance/insulin secretion tests, subcellular fractionation and confocal imaging, clathrin-dependent endocytosis assay, rescue by wild-type vs. functionally dead transporter overexpression, NHA2 inhibitor treatment of isolated islets","journal":"Proceedings of the National Academy of Sciences USA","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (KO mice, subcellular fractionation, endocytosis assay, rescue with functionally dead mutant) in single rigorous study","pmids":["23720317"],"is_preprint":false},{"year":2010,"finding":"Mutational analysis in NHA2 expressed in salt-sensitive S. cerevisiae identified residues V161 and F357 as required for Na+ efflux and growth on NaCl medium; the double mutant F357/F437 abolished growth on 0.4 M NaCl, demonstrating these evolutionarily conserved residues are essential for antiporter activity.","method":"Heterologous expression in salt-sensitive S. cerevisiae (BW31a), atomic absorption spectroscopy for intracellular Na+, growth assays on NaCl medium, confocal microscopy of membrane expression","journal":"Biochimica et Biophysica Acta","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — in vivo functional mutagenesis with direct ion measurement, single lab","pmids":["20713131"],"is_preprint":false},{"year":2015,"finding":"In Drosophila, Nha2 (Slc9b2 ortholog) expressed in Xenopus oocytes functions as a Na+/H+ exchanger (in contrast to Nha1 which acts as an H+-Cl- cotransporter). RNAi knockdown of Nha2 alone reduces survival; combined knockdown of Nha1 and Nha2 is lethal; under NaCl but not KCl stress, Nha2 knockdown decreases survival, demonstrating a specific role in Na+ homeostasis.","method":"Xenopus oocyte functional expression assay, Drosophila RNAi knockdown survival assay, ion stress experiments","journal":"Proceedings of the National Academy of Sciences USA","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — direct ion transport assay in oocytes plus genetic epistasis in vivo with multiple conditions","pmids":["26324901"],"is_preprint":false},{"year":2016,"finding":"In murine kidney, NHA2 localizes apically to distal convoluted tubules (DCT1 and DCT2) and connecting tubules, partially overlapping with V-ATPase, AQP2, and NCC1. Dietary high-NaCl elevated NHA2 transcript and protein levels, establishing dietary sodium as a regulator of NHA2 expression in the kidney.","method":"Immunohistochemistry/confocal microscopy in murine kidney sections, quantitative PCR and immunoblot after dietary NaCl manipulation","journal":"Journal of Physiology and Biochemistry","confidence":"Medium","confidence_rationale":"Tier 2–3 / Moderate — direct localization with multiple co-markers plus in vivo dietary regulation, single lab","pmids":["27909897"],"is_preprint":false},{"year":2018,"finding":"NHA2 is transcriptionally regulated by Ca2+/NFAT signaling downstream of polycystin-1 (PC1): induction of PC1 inhibited NHA2 expression, whereas the dominant-negative PC1-MAT fragment elevated NHA2. Ectopic NHA2 expression in MDCK 3D cultures increased cyst size, and siRNA silencing or pharmacological inhibition of NHA2 inhibited cyst formation. NHA2 was also transcriptionally induced by vasopressin and methylxanthines (caffeine, theophylline).","method":"MDCK 3D cystogenesis model, NHA2 overexpression and siRNA silencing, PC1 induction/dominant-negative experiments, pharmacological inhibition of NHA2, NFAT reporter assay, vasopressin/drug treatment","journal":"Journal of Physiology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — causal cystogenesis assay with gain- and loss-of-function, upstream signaling pathway defined, single lab","pmids":["30242840"],"is_preprint":false},{"year":2020,"finding":"NHA2 localizes almost exclusively to distal convoluted tubules in the kidney and regulates blood pressure via the WNK4-NCC pathway. NHA2-KO mice showed reduced blood pressure, hypocalciuria, and attenuated thiazide response. Loss of NHA2 increased WNK4 ubiquitylation and proteasomal degradation, reducing WNK4 abundance and consequently SPAK and NCC phosphorylation. This effect required KLHL3, as NHA2 loss selectively attenuated KLHL3 phosphorylation, blunting PKA- and PKC-mediated protection of WNK4. NHA2/NCC double-KO phenotype analysis confirmed NCC-dependent blood pressure regulation by NHA2.","method":"NHA2-KO and NHA2/NCC double-KO mouse models, blood pressure measurement, thiazide challenge, ubiquitylation assay, immunoblot for WNK4/SPAK/NCC phosphorylation, in vitro cell experiments, KLHL3 phosphorylation assay","journal":"Kidney International","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo KO phenotype corroborated by in vitro mechanistic pathway dissection, double-KO epistasis, multiple orthogonal methods","pmids":["32956652"],"is_preprint":false},{"year":2022,"finding":"Mutational analysis in S. cerevisiae identified proline P246 in the transmembrane core as crucial for ion selectivity: P246S/T substitutions produced antiporters active at both acidic and neutral pH with altered substrate specificity and reduced phloretin sensitivity. A putative salt bridge between E215 and R432 is required for both antiporter function and structural integrity. Truncation of the first 50–70 N-terminal residues doubled transport activity, demonstrating that the hydrophilic N-terminal domain allosterically auto-inhibits cation transport of NHA2.","method":"Site-directed mutagenesis, N-terminal truncations expressed in S. cerevisiae, functional growth assays on NaCl medium, localization by fluorescence microscopy, phloretin inhibition assay","journal":"Protein Science","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — in vivo mutagenesis with multiple mutants and functional readouts, single lab","pmids":["36177733"],"is_preprint":false},{"year":2024,"finding":"Residues R177 and S178 of NHA2 are involved in phloretin binding/inhibition: R177T mutation reduced phloretin sensitivity, while S178T enhanced inhibition, as demonstrated by site-directed mutagenesis and functional expression in S. cerevisiae.","method":"Bioinformatic modeling, site-directed mutagenesis, functional expression in S. cerevisiae, phloretin inhibition assay","journal":"FEBS Letters","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — mutagenesis with direct functional assay, single lab, corroborates structural predictions","pmids":["39737617"],"is_preprint":false},{"year":2025,"finding":"Cryo-EM structures of human NHA2 (apo + Fab and phloretin-bound) at 2.8–2.9 Å resolution revealed a unique 14-transmembrane helix architecture distinct from SLC9A/NHE members. Phosphatidic acid (PA) lipids bind the homodimer interface on the extracellular side, proposed to have a regulatory role in cell volume regulation. The ion-binding site contains a salt bridge between D278 and R432; this interaction is broken in the bison NHA2 structure, suggesting a possible ion coupling mechanism. The phloretin-bound structure provides a template for structure-guided inhibitor design.","method":"Cryo-electron microscopy, Fab-assisted structure determination, lipid-binding analysis, comparison with bison NHA2 structure","journal":"International Journal of Molecular Sciences","confidence":"High","confidence_rationale":"Tier 1 / Moderate — near-atomic resolution cryo-EM structures with inhibitor-bound state; single lab but Tier 1 method","pmids":["40362458"],"is_preprint":false},{"year":2025,"finding":"Brd4 (bromodomain protein 4) transcriptionally activates Slc9b2/NHA2 in osteoclasts. Brd4 conditional knockout reduced osteoclastogenesis and glycolysis; overexpression of Slc9b2 partially rescued the impaired osteoclastogenesis caused by Brd4 depletion, placing NHA2 as a downstream effector of Brd4 in osteoclast metabolism and differentiation.","method":"Conditional Brd4 KO mice (Lyz2-Cre and Ctsk-Cre), transcriptomic screening, Slc9b2 overexpression rescue experiment, bone mass and osteoclast activity assessment","journal":"Clinical and Translational Medicine","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis via KO + rescue, transcriptomic identification of target, single lab","pmids":["41115147"],"is_preprint":false},{"year":2026,"finding":"Trafficking of human NHA2 to the plasma membrane requires cornichon COPII cargo receptors. Both yeast ScErv14 and human CNIH cornichons (CNIH1, CNIH2, CNIH4) improved plasma-membrane targeting and functioning of NHA2 expressed in S. cerevisiae, identifying NHA2 as a cargo of cornichon COPII cargo receptors and establishing a requirement for this ER-exit machinery in NHA2 surface delivery.","method":"Heterologous expression in S. cerevisiae with ERV14 replacement by human CNIHs, plasma-membrane targeting assay, functional complementation assay, AlphaFold3 modeling","journal":"Protein Science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct functional and localization assay in yeast with multiple human cornichon variants, single lab","pmids":["41676957"],"is_preprint":false}],"current_model":"SLC9B2/NHA2 is a 14-TM helix Na+(Li+)/H+ antiporter (homodimer) that resides in transferrin-positive endosomes and synaptic-like microvesicles in β-cells, and at the basolateral plasma membrane in osteoclasts and the apical membrane of renal distal convoluted tubule cells; its plasma-membrane trafficking requires cornichon COPII cargo receptors (CNIH/Erv14), its N-terminal cytoplasmic domain allosterically auto-inhibits transport, conserved residues P246 and the D278–R432 salt bridge govern ion selectivity and structural integrity, and its activity is coupled to a plasma-membrane H+ gradient (via V-ATPase) to drive Na+ efflux; in the kidney, NHA2 stabilizes WNK4 by attenuating KLHL3-mediated ubiquitylation, thereby maintaining SPAK/NCC phosphorylation and blood pressure homeostasis, while in β-cells it supports clathrin-dependent endocytosis and endo-exocytosis coupling required for insulin secretion."},"narrative":{"mechanistic_narrative":"SLC9B2/NHA2 is an electroneutral Na+(Li+)/H+ antiporter that supports ion and volume homeostasis across multiple specialized epithelia and secretory cells [PMID:18508966, PMID:22948142]. It performs bona fide Na+/H+ exchange, demonstrated by functional complementation of exchanger-null yeast [PMID:18508966], and at the mammalian plasma membrane it operates as a phloretin-sensitive Na+-Li+ countertransporter coupled to a V-ATPase-generated H+ gradient, effectively functioning as a virtual Na+ efflux pump [PMID:22948142]. Cryo-EM of human NHA2 revealed a distinctive 14-transmembrane-helix homodimer with a D278–R432 ion-binding-site salt bridge implicated in ion coupling and phosphatidic acid bound at the dimer interface [PMID:40362458]; consistent functional mutagenesis identifies P246 as a determinant of ion selectivity, the E215/R432 salt bridge as required for activity and structural integrity, and a hydrophilic N-terminal domain that allosterically auto-inhibits transport [PMID:36177733]. Plasma-membrane delivery of NHA2 requires cornichon COPII cargo receptors (yeast Erv14 and human CNIH1/2/4) for ER exit [PMID:41676957]. In the kidney, NHA2 localizes to distal convoluted tubule and connecting tubule cells, is regulated by dietary sodium, and controls blood pressure through the WNK4–SPAK–NCC pathway: NHA2 loss increases KLHL3-mediated WNK4 ubiquitylation and degradation, reducing SPAK/NCC phosphorylation, an effect confirmed NCC-dependent by double-knockout epistasis [PMID:27909897, PMID:32956652]. In pancreatic β-cells, NHA2 resides in transferrin-positive endosomes and synaptic-like microvesicles and is required for clathrin-dependent endocytosis and endo-exocytosis coupling, with its loss causing impaired insulin secretion and glucose intolerance [PMID:23720317]. NHA2 expression is also induced during RANKL-driven osteoclast differentiation downstream of Brd4 [PMID:41115147], and its renal expression is regulated by polycystin-1/NFAT, vasopressin, and methylxanthines, where ectopic NHA2 promotes cystogenesis [PMID:30242840].","teleology":[{"year":2007,"claim":"Established the first functional context for NHA2, linking it to osteoclast differentiation and bone resorption and proposing a mitochondrial role.","evidence":"RANKL differentiation assays, mitochondrial pH/swelling assays, and siRNA knockdown in osteoclasts","pmids":["17988971"],"confidence":"Medium","gaps":["Mitochondrial localization was later contradicted","No direct biochemical demonstration of antiporter activity in this study"]},{"year":2008,"claim":"Demonstrated that NHA2 is a genuine Na+/H+ exchanger and defined its restricted distal-tubule expression, anchoring the protein as a functional antiporter.","evidence":"cDNA cloning, yeast complementation of NHE-null strains, immunogold EM and fractionation in kidney","pmids":["18508966"],"confidence":"High","gaps":["Reported mitochondrial co-localization conflicted with later studies","Transport stoichiometry and directionality not resolved"]},{"year":2010,"claim":"Resolved the localization controversy and tested in vivo necessity, reassigning NHA2 to endo-lysosomal and basolateral plasma membranes and showing it is dispensable for bone resorption.","evidence":"Confocal co-localization with LAMP1/V-ATPase a3, surface biotinylation, and micro-CT phenotyping of NHA2-KO mice","pmids":["20441802"],"confidence":"High","gaps":["Does not explain the earlier siRNA osteoclast phenotype","Functional role at the basolateral membrane left open"]},{"year":2010,"claim":"Identified specific transmembrane residues essential for antiporter activity, beginning the structure-function dissection of NHA2.","evidence":"Site-directed mutagenesis (V161, F357, F437) in salt-sensitive yeast with atomic absorption Na+ measurement","pmids":["20713131"],"confidence":"Medium","gaps":["No structural model to interpret residue positions","Performed only in heterologous yeast"]},{"year":2012,"claim":"Defined the transport mode at the mammalian plasma membrane, showing NHA2 is H+-gradient-coupled rather than Na+-coupled.","evidence":"Na+-Li+ countertransport assay in MDCK cells with phloretin and V-ATPase inhibitors","pmids":["22948142"],"confidence":"Medium","gaps":["Single cell-line system","Quantitative coupling stoichiometry not established"]},{"year":2013,"claim":"Established a physiological role for NHA2 in insulin secretion via endosomal trafficking, linking transport activity to endo-exocytosis coupling.","evidence":"NHA2-KO glucose/insulin tests, β-cell subcellular fractionation, clathrin-dependent endocytosis assays, rescue with wild-type vs functionally dead transporter","pmids":["23720317"],"confidence":"High","gaps":["How ion transport mechanistically drives clathrin-dependent endocytosis is unresolved","Identity of relevant endosomal ion gradient not defined"]},{"year":2015,"claim":"Confirmed conserved Na+/H+ exchange function and in vivo importance for Na+ homeostasis using an invertebrate ortholog.","evidence":"Xenopus oocyte transport assays and Drosophila Nha1/Nha2 RNAi epistasis under ionic stress","pmids":["26324901"],"confidence":"High","gaps":["Direct mammalian in vivo Na+ homeostasis link not addressed here","Tissue-specific contributions not separated"]},{"year":2016,"claim":"Refined renal localization to apical DCT/connecting tubule and identified dietary sodium as a transcriptional regulator of NHA2.","evidence":"Immunohistochemistry with V-ATPase/AQP2/NCC co-markers and qPCR/immunoblot after dietary NaCl manipulation in mouse kidney","pmids":["27909897"],"confidence":"Medium","gaps":["Transcriptional regulators mediating dietary response unknown","Functional consequence of apical localization not tested"]},{"year":2018,"claim":"Placed NHA2 within polycystin-1/NFAT signaling and implicated it in renal cystogenesis.","evidence":"MDCK 3D cyst model with NHA2 gain/loss-of-function, PC1 induction/dominant-negative, NFAT reporter, vasopressin/methylxanthine treatment","pmids":["30242840"],"confidence":"Medium","gaps":["In vivo relevance to polycystic kidney disease not established","Transport-dependence of cystogenic effect not fully isolated"]},{"year":2020,"claim":"Defined a mechanistic role for NHA2 in blood pressure control through stabilization of WNK4 via KLHL3, linking the transporter to NCC regulation.","evidence":"NHA2-KO and NHA2/NCC double-KO mice, blood pressure and thiazide challenge, WNK4 ubiquitylation and SPAK/NCC phosphorylation assays, KLHL3 phosphorylation assay","pmids":["32956652"],"confidence":"High","gaps":["How NHA2 transport activity signals to KLHL3 phosphorylation is unclear","Direct physical interaction with WNK4/KLHL3 not demonstrated"]},{"year":2022,"claim":"Mapped determinants of ion selectivity, structural integrity, and identified N-terminal autoinhibition, advancing the structure-function model.","evidence":"Site-directed mutagenesis (P246, E215/R432) and N-terminal truncations in yeast with functional and localization readouts","pmids":["36177733"],"confidence":"Medium","gaps":["Autoinhibition lacked structural visualization","Heterologous-system only"]},{"year":2024,"claim":"Identified residues governing phloretin binding, refining the pharmacology of NHA2 inhibition.","evidence":"Bioinformatic modeling and R177/S178 mutagenesis with phloretin inhibition assays in yeast","pmids":["39737617"],"confidence":"Medium","gaps":["Inhibitor pose not experimentally confirmed at the time","Single heterologous system"]},{"year":2025,"claim":"Provided near-atomic architecture of human NHA2, revealing a unique 14-TM fold, lipid binding, and a candidate ion-coupling salt bridge.","evidence":"Cryo-EM of apo and phloretin-bound human NHA2 with Fab assistance and comparison to bison NHA2","pmids":["40362458"],"confidence":"High","gaps":["Conformational transport cycle not captured","Functional role of phosphatidic acid in volume regulation not validated in cells"]},{"year":2025,"claim":"Identified Brd4 as a transcriptional activator of NHA2 in osteoclast metabolism and differentiation.","evidence":"Conditional Brd4-KO mice, transcriptomic screening, and Slc9b2 overexpression rescue of impaired osteoclastogenesis","pmids":["41115147"],"confidence":"Medium","gaps":["Direct Brd4 occupancy at the Slc9b2 locus not shown","Mechanistic link between NHA2 transport and glycolysis undefined"]},{"year":2026,"claim":"Defined the ER-exit machinery for NHA2 surface delivery, identifying cornichon COPII cargo receptors as trafficking partners.","evidence":"Heterologous expression in yeast with ScErv14 replacement by human CNIH1/2/4, plasma-membrane targeting and complementation assays, AlphaFold3 modeling","pmids":["41676957"],"confidence":"Medium","gaps":["Cornichon dependence not validated in mammalian cells in vivo","Direct NHA2-cornichon binding interface not experimentally resolved"]},{"year":null,"claim":"How NHA2 transport activity is mechanistically coupled to its diverse cellular roles—endo-exocytosis in β-cells, WNK4/KLHL3 signaling in kidney, and osteoclast metabolism—remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No direct demonstration of how ion transport feeds into WNK4 stabilization","Transport cycle and conformational states not structurally resolved","Physical interactions with downstream signaling proteins undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005215","term_label":"transporter activity","supporting_discovery_ids":[0,3,6]},{"term_id":"GO:0140104","term_label":"molecular carrier activity","supporting_discovery_ids":[0,3]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[2,3,7]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[4]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[4]},{"term_id":"GO:0005764","term_label":"lysosome","supporting_discovery_ids":[2]}],"pathway":[{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[0,3]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[9]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[4]}],"complexes":[],"partners":["WNK4","KLHL3","CNIH1","CNIH2","CNIH4"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q86UD5","full_name":"Sodium/hydrogen exchanger 9B2","aliases":["Na(+)/H(+) exchanger NHA2","Na(+)/H(+) exchanger-like domain-containing protein 2","NHE domain-containing protein 2","Sodium/hydrogen exchanger-like domain-containing protein 2","Solute carrier family 9 subfamily B member 2"],"length_aa":537,"mass_kda":57.6,"function":"Electroneutral Na(+) Li(+)/H(+) antiporter that extrudes Na(+) or Li(+) in exchange for external protons across the membrane (PubMed:18000046, PubMed:18508966, PubMed:22948142, PubMed:28154142, PubMed:36177733). Uses the proton gradient/membrane potential to extrude sodium (PubMed:22948142). Contributes to the regulation of intracellular pH and sodium homeostasis (By similarity). Also able to mediate Na(+)/Li(+) antiporter activity in kidney (PubMed:22948142). May play a physiological role in renal tubular function and blood pressure homeostasis (By similarity). Plays an important role for insulin secretion and clathrin-mediated endocytosis in beta-cells (By similarity). Involved in sperm motility and fertility (By similarity). It is controversial whether SLC9B2 plays a role in osteoclast differentiation or not (By similarity)","subcellular_location":"Cell membrane; Mitochondrion membrane; Endosome membrane; Recycling endosome membrane; Cytoplasmic vesicle, secretory vesicle, synaptic vesicle membrane; Cell projection, cilium, flagellum membrane; Basolateral cell membrane; Apical cell membrane","url":"https://www.uniprot.org/uniprotkb/Q86UD5/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SLC9B2","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/SLC9B2","total_profiled":1310},"omim":[],"hpa":{"profiled":true,"resolved_as":"","reliability":"Uncertain","locations":[{"location":"Nucleoplasm","reliability":"Uncertain"},{"location":"Cell Junctions","reliability":"Additional"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"liver","ntpm":36.2}],"url":"https://www.proteinatlas.org/search/SLC9B2"},"hgnc":{"alias_symbol":["FLJ23984","NHA2"],"prev_symbol":["NHEDC2"]},"alphafold":{"accession":"Q86UD5","domains":[{"cath_id":"1.20.1530.20","chopping":"110-519","consensus_level":"high","plddt":88.8394,"start":110,"end":519},{"cath_id":"1.20.5","chopping":"76-109","consensus_level":"medium","plddt":75.0644,"start":76,"end":109}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q86UD5","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q86UD5-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q86UD5-F1-predicted_aligned_error_v6.png","plddt_mean":78.62},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SLC9B2","jax_strain_url":"https://www.jax.org/strain/search?query=SLC9B2"},"sequence":{"accession":"Q86UD5","fasta_url":"https://rest.uniprot.org/uniprotkb/Q86UD5.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q86UD5/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q86UD5"}},"corpus_meta":[{"pmid":"23720317","id":"PMC_23720317","title":"Sodium/hydrogen exchanger NHA2 is critical for insulin secretion in β-cells.","date":"2013","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/23720317","citation_count":52,"is_preprint":false},{"pmid":"18508966","id":"PMC_18508966","title":"Characterization of the sodium/hydrogen exchanger NHA2.","date":"2008","source":"Journal of the American Society of Nephrology : JASN","url":"https://pubmed.ncbi.nlm.nih.gov/18508966","citation_count":51,"is_preprint":false},{"pmid":"26324901","id":"PMC_26324901","title":"Transport proteins NHA1 and NHA2 are essential for survival, but have distinct transport modalities.","date":"2015","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/26324901","citation_count":49,"is_preprint":false},{"pmid":"17988971","id":"PMC_17988971","title":"NHA-oc/NHA2: a mitochondrial cation-proton antiporter selectively expressed in osteoclasts.","date":"2007","source":"Bone","url":"https://pubmed.ncbi.nlm.nih.gov/17988971","citation_count":39,"is_preprint":false},{"pmid":"20441802","id":"PMC_20441802","title":"Sodium/hydrogen exchanger NHA2 in osteoclasts: subcellular localization and role in vitro and in vivo.","date":"2010","source":"Bone","url":"https://pubmed.ncbi.nlm.nih.gov/20441802","citation_count":31,"is_preprint":false},{"pmid":"22948142","id":"PMC_22948142","title":"Unconventional chemiosmotic coupling of NHA2, a mammalian Na+/H+ antiporter, to a plasma membrane H+ gradient.","date":"2012","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/22948142","citation_count":30,"is_preprint":false},{"pmid":"27909897","id":"PMC_27909897","title":"NHA2 is expressed in distal nephron and regulated by dietary sodium.","date":"2016","source":"Journal of physiology and biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/27909897","citation_count":17,"is_preprint":false},{"pmid":"17698421","id":"PMC_17698421","title":"Expression analysis of nha-oc/NHA2: a novel gene selectively expressed in osteoclasts.","date":"2007","source":"Gene expression patterns : GEP","url":"https://pubmed.ncbi.nlm.nih.gov/17698421","citation_count":14,"is_preprint":false},{"pmid":"30242840","id":"PMC_30242840","title":"NHA2 promotes cyst development in an in vitro model of polycystic kidney disease.","date":"2018","source":"The Journal of physiology","url":"https://pubmed.ncbi.nlm.nih.gov/30242840","citation_count":14,"is_preprint":false},{"pmid":"32956652","id":"PMC_32956652","title":"The sodium/proton exchanger NHA2 regulates blood pressure through a WNK4-NCC dependent pathway in the kidney.","date":"2020","source":"Kidney international","url":"https://pubmed.ncbi.nlm.nih.gov/32956652","citation_count":13,"is_preprint":false},{"pmid":"27685945","id":"PMC_27685945","title":"Loss of Sodium/Hydrogen Exchanger NHA2 Exacerbates Obesity- and Aging-Induced Glucose Intolerance in Mice.","date":"2016","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/27685945","citation_count":11,"is_preprint":false},{"pmid":"28868326","id":"PMC_28868326","title":"Evolution of Vertebrate Solute Carrier Family 9B Genes and Proteins (SLC9B): Evidence for a Marsupial Origin for Testis Specific SLC9B1 from an Ancestral Vertebrate SLC9B2 Gene.","date":"2016","source":"Journal of phylogenetics & evolutionary biology","url":"https://pubmed.ncbi.nlm.nih.gov/28868326","citation_count":10,"is_preprint":false},{"pmid":"20713131","id":"PMC_20713131","title":"Mutational analysis of NHAoc/NHA2 in Saccharomyces cerevisiae.","date":"2010","source":"Biochimica et biophysica acta","url":"https://pubmed.ncbi.nlm.nih.gov/20713131","citation_count":8,"is_preprint":false},{"pmid":"18450222","id":"PMC_18450222","title":"Production of Yarrowia lipolytica Nha2 Na+/H+ antiporter improves the salt tolerance of Saccharomyces cerevisiae.","date":"2007","source":"Folia microbiologica","url":"https://pubmed.ncbi.nlm.nih.gov/18450222","citation_count":6,"is_preprint":false},{"pmid":"36177733","id":"PMC_36177733","title":"Allosteric links between the hydrophilic N-terminus and transmembrane core of human Na+ /H+ antiporter NHA2.","date":"2022","source":"Protein science : a publication of the Protein Society","url":"https://pubmed.ncbi.nlm.nih.gov/36177733","citation_count":5,"is_preprint":false},{"pmid":"38069797","id":"PMC_38069797","title":"Physiological and Molecular Function of the Sodium/Hydrogen Exchanger NHA2 (SLC9B2).","date":"2022","source":"Chimia","url":"https://pubmed.ncbi.nlm.nih.gov/38069797","citation_count":3,"is_preprint":false},{"pmid":"40362458","id":"PMC_40362458","title":"Structure and Inhibition of the Human Na+/H+ Exchanger SLC9B2.","date":"2025","source":"International journal of molecular sciences","url":"https://pubmed.ncbi.nlm.nih.gov/40362458","citation_count":1,"is_preprint":false},{"pmid":"39737617","id":"PMC_39737617","title":"Residues R177 and S178 of the human Na+/H+ antiporter NHA2 are involved in its inhibition by the flavonoid phloretin.","date":"2024","source":"FEBS letters","url":"https://pubmed.ncbi.nlm.nih.gov/39737617","citation_count":1,"is_preprint":false},{"pmid":"41115147","id":"PMC_41115147","title":"Loss of Brd4 alleviates pathological bone loss via Slc9b2 suppression in osteoclastogenesis.","date":"2025","source":"Clinical and translational medicine","url":"https://pubmed.ncbi.nlm.nih.gov/41115147","citation_count":0,"is_preprint":false},{"pmid":"41676957","id":"PMC_41676957","title":"Trafficking of the human Na+/H+ antiporter NHA2 to the plasma membrane requires cornichon COPII cargo receptors.","date":"2026","source":"Protein science : a publication of the Protein Society","url":"https://pubmed.ncbi.nlm.nih.gov/41676957","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":12209,"output_tokens":4535,"usd":0.052326,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":12638,"output_tokens":4559,"usd":0.088582,"stage2_stop_reason":"end_turn"},"total_usd":0.140908,"stage1_batch_id":"msgbatch_013LWZZT6MWaqB9qyTSeQBmU","stage2_batch_id":"msgbatch_0194ZzQaTGNpruoNJBbEMbt3","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2008,\n      \"finding\": \"Human NHA2 (SLC9B2) was cloned from a brain cDNA library and characterized as a ~55 kDa sodium/proton exchanger expressed in multiple mammalian tissues. In kidney, NHA2 was restricted to the distal convoluted tubule and co-localized with the mitochondrial inner membrane marker by immunogold EM and differential centrifugation. NHA2 functionally complemented sodium/hydrogen exchanger-null yeast (restored growth under high NaCl at acidic pH), demonstrating bona fide Na+/H+ exchange activity.\",\n      \"method\": \"cDNA cloning, immunoblot with siRNA validation, co-sedimentation/differential centrifugation, immunogold EM, heterologous expression in NHE-deficient yeast\",\n      \"journal\": \"Journal of the American Society of Nephrology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — multiple orthogonal methods (biochemical fractionation, immunogold EM, functional yeast complementation) in a single characterization study\",\n      \"pmids\": [\"18508966\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"NHA2 (NHA-oc/NHA2) is strongly upregulated during RANKL-induced osteoclast differentiation in vitro and in vivo. The protein localizes to mitochondria in osteoclasts, where it mediates Na+-dependent changes in mitochondrial pH and Na+-acetate-induced mitochondrial passive swelling. siRNA silencing of NHA2 reduced osteoclast differentiation and bone resorption in vitro.\",\n      \"method\": \"RANKL-stimulated differentiation assay, mitochondrial pH assay, mitochondrial swelling assay, siRNA knockdown, microarray followed by Northern blot and in situ hybridization\",\n      \"journal\": \"Bone\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 / Moderate — functional mitochondrial pH and swelling assays plus siRNA knockdown, single lab, later contradicted on mitochondrial localization by Hofstetter et al. 2010\",\n      \"pmids\": [\"17988971\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Using NHA2-specific antibodies and NHA2-deficient mice, NHA2 was found to co-localize with the late endosomal/lysosomal marker LAMP1 and V-ATPase a3 subunit (not mitochondrial markers) in osteoclasts. Surface biotinylation and immunofluorescence further showed NHA2 is highly enriched at the plasma membrane, specifically the basolateral membrane of polarized osteoclasts. Contrary to earlier siRNA data, NHA2-deficient mice showed no difference in bone parameters, osteoclast development, or resorption activity in vitro, indicating NHA2 is dispensable for osteoclast differentiation and bone resorption.\",\n      \"method\": \"Confocal microscopy, subcellular fractionation, surface biotinylation, microcomputed tomography of NHA2-KO mice, in vitro RANKL stimulation of KO bone marrow cells\",\n      \"journal\": \"Bone\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal localization using multiple markers, in vivo KO phenotype with micro-CT, and in vitro functional assay; replicates and revises earlier localization claim\",\n      \"pmids\": [\"20441802\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Human NHA2 mediates phloretin-sensitive Na+-Li+ countertransport (SLC) activity at the plasma membrane of kidney-derived MDCK cells, coupled to a plasma membrane H+ gradient generated by V-type H+-ATPase, functioning as a virtual Na+ efflux pump. This establishes H+-coupled (rather than Na+-coupled) secondary transport for NHA2 at the mammalian plasma membrane.\",\n      \"method\": \"Na+-Li+ countertransport assay in MDCK cells, pharmacological inhibition with phloretin, V-ATPase inhibitor studies\",\n      \"journal\": \"Journal of Biological Chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct ion transport assay with pharmacological dissection in cell lines, single lab\",\n      \"pmids\": [\"22948142\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"NHA2 is present in rodent and human β-cells and resides in transferrin-positive endosomes and synaptic-like microvesicles (not large dense core vesicles). NHA2-deficient mice show impaired insulin secretion and glucose intolerance. Loss of NHA2 inhibited clathrin-dependent but not clathrin-independent endocytosis in Min6 and primary β-cells, implicating defective endo-exocytosis coupling as the mechanism for impaired secretion. The secretory deficit was rescued by wild-type but not functionally dead NHA2.\",\n      \"method\": \"NHA2-KO mouse in vivo glucose tolerance/insulin secretion tests, subcellular fractionation and confocal imaging, clathrin-dependent endocytosis assay, rescue by wild-type vs. functionally dead transporter overexpression, NHA2 inhibitor treatment of isolated islets\",\n      \"journal\": \"Proceedings of the National Academy of Sciences USA\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (KO mice, subcellular fractionation, endocytosis assay, rescue with functionally dead mutant) in single rigorous study\",\n      \"pmids\": [\"23720317\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Mutational analysis in NHA2 expressed in salt-sensitive S. cerevisiae identified residues V161 and F357 as required for Na+ efflux and growth on NaCl medium; the double mutant F357/F437 abolished growth on 0.4 M NaCl, demonstrating these evolutionarily conserved residues are essential for antiporter activity.\",\n      \"method\": \"Heterologous expression in salt-sensitive S. cerevisiae (BW31a), atomic absorption spectroscopy for intracellular Na+, growth assays on NaCl medium, confocal microscopy of membrane expression\",\n      \"journal\": \"Biochimica et Biophysica Acta\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vivo functional mutagenesis with direct ion measurement, single lab\",\n      \"pmids\": [\"20713131\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"In Drosophila, Nha2 (Slc9b2 ortholog) expressed in Xenopus oocytes functions as a Na+/H+ exchanger (in contrast to Nha1 which acts as an H+-Cl- cotransporter). RNAi knockdown of Nha2 alone reduces survival; combined knockdown of Nha1 and Nha2 is lethal; under NaCl but not KCl stress, Nha2 knockdown decreases survival, demonstrating a specific role in Na+ homeostasis.\",\n      \"method\": \"Xenopus oocyte functional expression assay, Drosophila RNAi knockdown survival assay, ion stress experiments\",\n      \"journal\": \"Proceedings of the National Academy of Sciences USA\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — direct ion transport assay in oocytes plus genetic epistasis in vivo with multiple conditions\",\n      \"pmids\": [\"26324901\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"In murine kidney, NHA2 localizes apically to distal convoluted tubules (DCT1 and DCT2) and connecting tubules, partially overlapping with V-ATPase, AQP2, and NCC1. Dietary high-NaCl elevated NHA2 transcript and protein levels, establishing dietary sodium as a regulator of NHA2 expression in the kidney.\",\n      \"method\": \"Immunohistochemistry/confocal microscopy in murine kidney sections, quantitative PCR and immunoblot after dietary NaCl manipulation\",\n      \"journal\": \"Journal of Physiology and Biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 / Moderate — direct localization with multiple co-markers plus in vivo dietary regulation, single lab\",\n      \"pmids\": [\"27909897\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"NHA2 is transcriptionally regulated by Ca2+/NFAT signaling downstream of polycystin-1 (PC1): induction of PC1 inhibited NHA2 expression, whereas the dominant-negative PC1-MAT fragment elevated NHA2. Ectopic NHA2 expression in MDCK 3D cultures increased cyst size, and siRNA silencing or pharmacological inhibition of NHA2 inhibited cyst formation. NHA2 was also transcriptionally induced by vasopressin and methylxanthines (caffeine, theophylline).\",\n      \"method\": \"MDCK 3D cystogenesis model, NHA2 overexpression and siRNA silencing, PC1 induction/dominant-negative experiments, pharmacological inhibition of NHA2, NFAT reporter assay, vasopressin/drug treatment\",\n      \"journal\": \"Journal of Physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — causal cystogenesis assay with gain- and loss-of-function, upstream signaling pathway defined, single lab\",\n      \"pmids\": [\"30242840\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"NHA2 localizes almost exclusively to distal convoluted tubules in the kidney and regulates blood pressure via the WNK4-NCC pathway. NHA2-KO mice showed reduced blood pressure, hypocalciuria, and attenuated thiazide response. Loss of NHA2 increased WNK4 ubiquitylation and proteasomal degradation, reducing WNK4 abundance and consequently SPAK and NCC phosphorylation. This effect required KLHL3, as NHA2 loss selectively attenuated KLHL3 phosphorylation, blunting PKA- and PKC-mediated protection of WNK4. NHA2/NCC double-KO phenotype analysis confirmed NCC-dependent blood pressure regulation by NHA2.\",\n      \"method\": \"NHA2-KO and NHA2/NCC double-KO mouse models, blood pressure measurement, thiazide challenge, ubiquitylation assay, immunoblot for WNK4/SPAK/NCC phosphorylation, in vitro cell experiments, KLHL3 phosphorylation assay\",\n      \"journal\": \"Kidney International\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo KO phenotype corroborated by in vitro mechanistic pathway dissection, double-KO epistasis, multiple orthogonal methods\",\n      \"pmids\": [\"32956652\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Mutational analysis in S. cerevisiae identified proline P246 in the transmembrane core as crucial for ion selectivity: P246S/T substitutions produced antiporters active at both acidic and neutral pH with altered substrate specificity and reduced phloretin sensitivity. A putative salt bridge between E215 and R432 is required for both antiporter function and structural integrity. Truncation of the first 50–70 N-terminal residues doubled transport activity, demonstrating that the hydrophilic N-terminal domain allosterically auto-inhibits cation transport of NHA2.\",\n      \"method\": \"Site-directed mutagenesis, N-terminal truncations expressed in S. cerevisiae, functional growth assays on NaCl medium, localization by fluorescence microscopy, phloretin inhibition assay\",\n      \"journal\": \"Protein Science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vivo mutagenesis with multiple mutants and functional readouts, single lab\",\n      \"pmids\": [\"36177733\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Residues R177 and S178 of NHA2 are involved in phloretin binding/inhibition: R177T mutation reduced phloretin sensitivity, while S178T enhanced inhibition, as demonstrated by site-directed mutagenesis and functional expression in S. cerevisiae.\",\n      \"method\": \"Bioinformatic modeling, site-directed mutagenesis, functional expression in S. cerevisiae, phloretin inhibition assay\",\n      \"journal\": \"FEBS Letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mutagenesis with direct functional assay, single lab, corroborates structural predictions\",\n      \"pmids\": [\"39737617\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Cryo-EM structures of human NHA2 (apo + Fab and phloretin-bound) at 2.8–2.9 Å resolution revealed a unique 14-transmembrane helix architecture distinct from SLC9A/NHE members. Phosphatidic acid (PA) lipids bind the homodimer interface on the extracellular side, proposed to have a regulatory role in cell volume regulation. The ion-binding site contains a salt bridge between D278 and R432; this interaction is broken in the bison NHA2 structure, suggesting a possible ion coupling mechanism. The phloretin-bound structure provides a template for structure-guided inhibitor design.\",\n      \"method\": \"Cryo-electron microscopy, Fab-assisted structure determination, lipid-binding analysis, comparison with bison NHA2 structure\",\n      \"journal\": \"International Journal of Molecular Sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — near-atomic resolution cryo-EM structures with inhibitor-bound state; single lab but Tier 1 method\",\n      \"pmids\": [\"40362458\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Brd4 (bromodomain protein 4) transcriptionally activates Slc9b2/NHA2 in osteoclasts. Brd4 conditional knockout reduced osteoclastogenesis and glycolysis; overexpression of Slc9b2 partially rescued the impaired osteoclastogenesis caused by Brd4 depletion, placing NHA2 as a downstream effector of Brd4 in osteoclast metabolism and differentiation.\",\n      \"method\": \"Conditional Brd4 KO mice (Lyz2-Cre and Ctsk-Cre), transcriptomic screening, Slc9b2 overexpression rescue experiment, bone mass and osteoclast activity assessment\",\n      \"journal\": \"Clinical and Translational Medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis via KO + rescue, transcriptomic identification of target, single lab\",\n      \"pmids\": [\"41115147\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"Trafficking of human NHA2 to the plasma membrane requires cornichon COPII cargo receptors. Both yeast ScErv14 and human CNIH cornichons (CNIH1, CNIH2, CNIH4) improved plasma-membrane targeting and functioning of NHA2 expressed in S. cerevisiae, identifying NHA2 as a cargo of cornichon COPII cargo receptors and establishing a requirement for this ER-exit machinery in NHA2 surface delivery.\",\n      \"method\": \"Heterologous expression in S. cerevisiae with ERV14 replacement by human CNIHs, plasma-membrane targeting assay, functional complementation assay, AlphaFold3 modeling\",\n      \"journal\": \"Protein Science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct functional and localization assay in yeast with multiple human cornichon variants, single lab\",\n      \"pmids\": [\"41676957\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SLC9B2/NHA2 is a 14-TM helix Na+(Li+)/H+ antiporter (homodimer) that resides in transferrin-positive endosomes and synaptic-like microvesicles in β-cells, and at the basolateral plasma membrane in osteoclasts and the apical membrane of renal distal convoluted tubule cells; its plasma-membrane trafficking requires cornichon COPII cargo receptors (CNIH/Erv14), its N-terminal cytoplasmic domain allosterically auto-inhibits transport, conserved residues P246 and the D278–R432 salt bridge govern ion selectivity and structural integrity, and its activity is coupled to a plasma-membrane H+ gradient (via V-ATPase) to drive Na+ efflux; in the kidney, NHA2 stabilizes WNK4 by attenuating KLHL3-mediated ubiquitylation, thereby maintaining SPAK/NCC phosphorylation and blood pressure homeostasis, while in β-cells it supports clathrin-dependent endocytosis and endo-exocytosis coupling required for insulin secretion.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"SLC9B2/NHA2 is an electroneutral Na+(Li+)/H+ antiporter that supports ion and volume homeostasis across multiple specialized epithelia and secretory cells [#0, #3]. It performs bona fide Na+/H+ exchange, demonstrated by functional complementation of exchanger-null yeast [#0], and at the mammalian plasma membrane it operates as a phloretin-sensitive Na+-Li+ countertransporter coupled to a V-ATPase-generated H+ gradient, effectively functioning as a virtual Na+ efflux pump [#3]. Cryo-EM of human NHA2 revealed a distinctive 14-transmembrane-helix homodimer with a D278–R432 ion-binding-site salt bridge implicated in ion coupling and phosphatidic acid bound at the dimer interface [#12]; consistent functional mutagenesis identifies P246 as a determinant of ion selectivity, the E215/R432 salt bridge as required for activity and structural integrity, and a hydrophilic N-terminal domain that allosterically auto-inhibits transport [#10]. Plasma-membrane delivery of NHA2 requires cornichon COPII cargo receptors (yeast Erv14 and human CNIH1/2/4) for ER exit [#14]. In the kidney, NHA2 localizes to distal convoluted tubule and connecting tubule cells, is regulated by dietary sodium, and controls blood pressure through the WNK4–SPAK–NCC pathway: NHA2 loss increases KLHL3-mediated WNK4 ubiquitylation and degradation, reducing SPAK/NCC phosphorylation, an effect confirmed NCC-dependent by double-knockout epistasis [#7, #9]. In pancreatic β-cells, NHA2 resides in transferrin-positive endosomes and synaptic-like microvesicles and is required for clathrin-dependent endocytosis and endo-exocytosis coupling, with its loss causing impaired insulin secretion and glucose intolerance [#4]. NHA2 expression is also induced during RANKL-driven osteoclast differentiation downstream of Brd4 [#13], and its renal expression is regulated by polycystin-1/NFAT, vasopressin, and methylxanthines, where ectopic NHA2 promotes cystogenesis [#8].\",\n  \"teleology\": [\n    {\n      \"year\": 2007,\n      \"claim\": \"Established the first functional context for NHA2, linking it to osteoclast differentiation and bone resorption and proposing a mitochondrial role.\",\n      \"evidence\": \"RANKL differentiation assays, mitochondrial pH/swelling assays, and siRNA knockdown in osteoclasts\",\n      \"pmids\": [\"17988971\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mitochondrial localization was later contradicted\", \"No direct biochemical demonstration of antiporter activity in this study\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Demonstrated that NHA2 is a genuine Na+/H+ exchanger and defined its restricted distal-tubule expression, anchoring the protein as a functional antiporter.\",\n      \"evidence\": \"cDNA cloning, yeast complementation of NHE-null strains, immunogold EM and fractionation in kidney\",\n      \"pmids\": [\"18508966\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Reported mitochondrial co-localization conflicted with later studies\", \"Transport stoichiometry and directionality not resolved\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Resolved the localization controversy and tested in vivo necessity, reassigning NHA2 to endo-lysosomal and basolateral plasma membranes and showing it is dispensable for bone resorption.\",\n      \"evidence\": \"Confocal co-localization with LAMP1/V-ATPase a3, surface biotinylation, and micro-CT phenotyping of NHA2-KO mice\",\n      \"pmids\": [\"20441802\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Does not explain the earlier siRNA osteoclast phenotype\", \"Functional role at the basolateral membrane left open\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Identified specific transmembrane residues essential for antiporter activity, beginning the structure-function dissection of NHA2.\",\n      \"evidence\": \"Site-directed mutagenesis (V161, F357, F437) in salt-sensitive yeast with atomic absorption Na+ measurement\",\n      \"pmids\": [\"20713131\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structural model to interpret residue positions\", \"Performed only in heterologous yeast\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Defined the transport mode at the mammalian plasma membrane, showing NHA2 is H+-gradient-coupled rather than Na+-coupled.\",\n      \"evidence\": \"Na+-Li+ countertransport assay in MDCK cells with phloretin and V-ATPase inhibitors\",\n      \"pmids\": [\"22948142\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single cell-line system\", \"Quantitative coupling stoichiometry not established\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Established a physiological role for NHA2 in insulin secretion via endosomal trafficking, linking transport activity to endo-exocytosis coupling.\",\n      \"evidence\": \"NHA2-KO glucose/insulin tests, β-cell subcellular fractionation, clathrin-dependent endocytosis assays, rescue with wild-type vs functionally dead transporter\",\n      \"pmids\": [\"23720317\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How ion transport mechanistically drives clathrin-dependent endocytosis is unresolved\", \"Identity of relevant endosomal ion gradient not defined\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Confirmed conserved Na+/H+ exchange function and in vivo importance for Na+ homeostasis using an invertebrate ortholog.\",\n      \"evidence\": \"Xenopus oocyte transport assays and Drosophila Nha1/Nha2 RNAi epistasis under ionic stress\",\n      \"pmids\": [\"26324901\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct mammalian in vivo Na+ homeostasis link not addressed here\", \"Tissue-specific contributions not separated\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Refined renal localization to apical DCT/connecting tubule and identified dietary sodium as a transcriptional regulator of NHA2.\",\n      \"evidence\": \"Immunohistochemistry with V-ATPase/AQP2/NCC co-markers and qPCR/immunoblot after dietary NaCl manipulation in mouse kidney\",\n      \"pmids\": [\"27909897\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Transcriptional regulators mediating dietary response unknown\", \"Functional consequence of apical localization not tested\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Placed NHA2 within polycystin-1/NFAT signaling and implicated it in renal cystogenesis.\",\n      \"evidence\": \"MDCK 3D cyst model with NHA2 gain/loss-of-function, PC1 induction/dominant-negative, NFAT reporter, vasopressin/methylxanthine treatment\",\n      \"pmids\": [\"30242840\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"In vivo relevance to polycystic kidney disease not established\", \"Transport-dependence of cystogenic effect not fully isolated\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Defined a mechanistic role for NHA2 in blood pressure control through stabilization of WNK4 via KLHL3, linking the transporter to NCC regulation.\",\n      \"evidence\": \"NHA2-KO and NHA2/NCC double-KO mice, blood pressure and thiazide challenge, WNK4 ubiquitylation and SPAK/NCC phosphorylation assays, KLHL3 phosphorylation assay\",\n      \"pmids\": [\"32956652\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How NHA2 transport activity signals to KLHL3 phosphorylation is unclear\", \"Direct physical interaction with WNK4/KLHL3 not demonstrated\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Mapped determinants of ion selectivity, structural integrity, and identified N-terminal autoinhibition, advancing the structure-function model.\",\n      \"evidence\": \"Site-directed mutagenesis (P246, E215/R432) and N-terminal truncations in yeast with functional and localization readouts\",\n      \"pmids\": [\"36177733\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Autoinhibition lacked structural visualization\", \"Heterologous-system only\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Identified residues governing phloretin binding, refining the pharmacology of NHA2 inhibition.\",\n      \"evidence\": \"Bioinformatic modeling and R177/S178 mutagenesis with phloretin inhibition assays in yeast\",\n      \"pmids\": [\"39737617\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Inhibitor pose not experimentally confirmed at the time\", \"Single heterologous system\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Provided near-atomic architecture of human NHA2, revealing a unique 14-TM fold, lipid binding, and a candidate ion-coupling salt bridge.\",\n      \"evidence\": \"Cryo-EM of apo and phloretin-bound human NHA2 with Fab assistance and comparison to bison NHA2\",\n      \"pmids\": [\"40362458\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Conformational transport cycle not captured\", \"Functional role of phosphatidic acid in volume regulation not validated in cells\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Identified Brd4 as a transcriptional activator of NHA2 in osteoclast metabolism and differentiation.\",\n      \"evidence\": \"Conditional Brd4-KO mice, transcriptomic screening, and Slc9b2 overexpression rescue of impaired osteoclastogenesis\",\n      \"pmids\": [\"41115147\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct Brd4 occupancy at the Slc9b2 locus not shown\", \"Mechanistic link between NHA2 transport and glycolysis undefined\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Defined the ER-exit machinery for NHA2 surface delivery, identifying cornichon COPII cargo receptors as trafficking partners.\",\n      \"evidence\": \"Heterologous expression in yeast with ScErv14 replacement by human CNIH1/2/4, plasma-membrane targeting and complementation assays, AlphaFold3 modeling\",\n      \"pmids\": [\"41676957\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Cornichon dependence not validated in mammalian cells in vivo\", \"Direct NHA2-cornichon binding interface not experimentally resolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How NHA2 transport activity is mechanistically coupled to its diverse cellular roles—endo-exocytosis in β-cells, WNK4/KLHL3 signaling in kidney, and osteoclast metabolism—remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No direct demonstration of how ion transport feeds into WNK4 stabilization\", \"Transport cycle and conformational states not structurally resolved\", \"Physical interactions with downstream signaling proteins undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [0, 3, 6]},\n      {\"term_id\": \"GO:0140104\", \"supporting_discovery_ids\": [0, 3]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [2, 3, 7]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [4]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [4]},\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [2]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [0, 3]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [9]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [4]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"WNK4\", \"KLHL3\", \"CNIH1\", \"CNIH2\", \"CNIH4\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":7,"faith_pct":85.71428571428571}}