{"gene":"MPL","run_date":"2026-04-28T18:30:28","timeline":{"discoveries":[{"year":1994,"finding":"MPL (c-mpl) is the receptor for thrombopoietin (TPO) and specifically regulates megakaryocytopoiesis and thrombopoiesis; c-mpl-deficient mice have 85% reduction in platelets and megakaryocytes with elevated circulating TPO, demonstrating a feedback loop where platelet mass regulates TPO levels through receptor-mediated clearance.","method":"Gene targeting / knockout mice with hematological phenotyping","journal":"Science","confidence":"High","confidence_rationale":"Tier 1 — foundational KO study, highly cited, replicated in multiple subsequent studies","pmids":["8073287"],"is_preprint":false},{"year":1995,"finding":"TPO binding to c-Mpl induces rapid tyrosine phosphorylation of JAK2, Shc, and c-Mpl itself; JAK2 physically associates with c-Mpl relatively late (20–60 min) after ligand binding, suggesting JAK2 may not be the initiating kinase in the signaling cascade.","method":"Immunoprecipitation, Western blotting, co-immunoprecipitation in BaF3/mMpl cells","journal":"The Journal of Biological Chemistry","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP and Western blot with time-course in engineered cell line, highly cited","pmids":["7534285"],"is_preprint":false},{"year":1995,"finding":"Activation of MPL by TPO rapidly induces tyrosine phosphorylation of JAK2 and TYK2 (but not JAK1 or JAK3), followed by phosphorylation of STAT1, STAT3, and STAT5, and formation of specific DNA-binding complexes.","method":"In vitro kinase assays, immunoprecipitation, gel-shift assays in factor-dependent hematopoietic cell lines","journal":"Experimental Hematology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods, replicated across cell lines","pmids":["7543416"],"is_preprint":false},{"year":1996,"finding":"A point mutation (Ser498→Asn498) in the transmembrane domain of MPL constitutively activates the receptor, conferring factor-independent growth and activating SHC-Raf-MAPK and JAK2-STAT3/STAT5 signaling pathways.","method":"Retrovirus-mediated gene transfer with PCR-driven random mutagenesis, signaling pathway analysis in Ba/F3 cells, tumorigenicity assay","journal":"Blood","confidence":"High","confidence_rationale":"Tier 1 — active-site mutagenesis with functional reconstitution and in vivo tumorigenicity","pmids":["8695859"],"is_preprint":false},{"year":1996,"finding":"The MPL promoter contains binding sites for GATA-1 and Ets family proteins (Ets-1, Fli-1) that cooperatively regulate megakaryocyte-specific MPL expression; an Ets site at –15 and another downstream of the GATA motif are critical for promoter activity.","method":"Promoter deletion analysis, EMSA/in vitro binding, transactivation assays in HEL cells","journal":"Blood","confidence":"High","confidence_rationale":"Tier 1 — multiple methods including mutagenesis and transactivation, replicated with two Ets factors","pmids":["8639837"],"is_preprint":false},{"year":1998,"finding":"c-Mpl (TPO receptor) is required for hematopoietic stem cell maintenance; mpl-/- mice have 4–12-fold fewer preprogenitor cells and severely impaired long-term hematopoietic repopulating capacity, with a stem cell-intrinsic self-renewal defect.","method":"Bone marrow transplantation, CFU-S assays, competitive repopulation assays in mpl-/- mice","journal":"Proceedings of the National Academy of Sciences","confidence":"High","confidence_rationale":"Tier 2 — multiple functional stem cell assays, highly cited, replicated","pmids":["9448308"],"is_preprint":false},{"year":2004,"finding":"The adaptor protein Lnk negatively regulates TPO/c-mpl signaling through its SH2 domain; Lnk deficiency causes enhanced and prolonged TPO-induced STAT3, STAT5, Akt, and MAPK signaling in megakaryocytes, leading to increased megakaryocyte numbers and ploidy in vivo.","method":"Overexpression and loss-of-function (Lnk-/- mice), signaling assays, domain mutagenesis","journal":"The Journal of Experimental Medicine","confidence":"High","confidence_rationale":"Tier 2 — genetic KO mice plus domain mutagenesis and signaling pathway analysis","pmids":["15337790"],"is_preprint":false},{"year":2004,"finding":"AMG531, a recombinant Fc-linked Mpl-binding protein, competes with TPO for binding to Mpl on BaF3-Mpl cells and platelets, and induces rapid tyrosine phosphorylation of Mpl, JAK2, and STAT5.","method":"Competitive radioligand binding assay (125I-TPO), phosphorylation assays, in vitro megakaryocyte culture","journal":"Cytokine","confidence":"Medium","confidence_rationale":"Tier 2 — direct binding and signaling assays in defined cell system","pmids":["14693160"],"is_preprint":false},{"year":2007,"finding":"THPO/MPL signaling in the osteoblastic niche regulates HSC quiescence; MPL+ LT-HSCs are associated with THPO-producing osteoblasts, and MPL signaling upregulates β1-integrin and CDK inhibitors; anti-MPL antibody reduces quiescent LT-HSCs and enables HSC engraftment without irradiation.","method":"Anti-MPL neutralizing antibody treatment, exogenous THPO administration, flow cytometry, cell cycle analysis in vivo","journal":"Cell Stem Cell","confidence":"High","confidence_rationale":"Tier 2 — reciprocal gain/loss-of-function with multiple cellular readouts, highly cited","pmids":["18371409"],"is_preprint":false},{"year":2009,"finding":"After TPO stimulation, c-Mpl is ubiquitinated on two intracellular lysine residues (K553 and K573) by the E3 ubiquitin ligase c-Cbl, and degraded by both lysosomal and proteasomal pathways; mutation of these lysines reduces ubiquitination, degradation, and causes hyperproliferation.","method":"Site-directed mutagenesis, siRNA knockdown, dominant-negative overexpression, ubiquitination assays","journal":"Blood","confidence":"High","confidence_rationale":"Tier 1 — active-site mutagenesis plus RNAi and dominant-negative with functional readout","pmids":["19880496"],"is_preprint":false},{"year":2002,"finding":"A novel MPL mutation (W508S) in the intracellular domain constitutively activates three signaling pathways: SHC-Ras-Raf-MAPK/JNK, JAK-STAT, and PI3K-Akt-Bad, conferring factor-independent growth.","method":"Ba/F3 cell factor-independence assay, signaling pathway analysis","journal":"Leukemia","confidence":"Medium","confidence_rationale":"Tier 2 — functional reconstitution with pathway dissection","pmids":["12145691"],"is_preprint":false},{"year":2002,"finding":"MPL receptor signaling through Mpl ligand activates IKK transiently then suppresses its activity; proliferating megakaryocytes display constitutive NF-κB (p50 homodimer and p50-p65 heterodimer) DNA-binding activity that is reduced upon TPO-induced differentiation.","method":"IKK activity assays, EMSA, NF-κB reporter assay in megakaryocytic cell lines","journal":"Journal of Cellular Biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 — multiple signaling assays in defined cell system","pmids":["11967992"],"is_preprint":false},{"year":2006,"finding":"JAK2 acts as a chaperone for Mpl, responsible for its cell-surface expression; there is a reciprocal relationship between JAK2 V617F allele burden and platelet Mpl expression in myeloproliferative disorder patients.","method":"Quantitative allele analysis and flow cytometry of patient samples; mechanistic chaperone function inferred from clinical correlates","journal":"Blood","confidence":"Medium","confidence_rationale":"Tier 3 — clinical correlative data supporting chaperone role, single lab","pmids":["16912229"],"is_preprint":false},{"year":2012,"finding":"Wild-type MPL expression promotes RUNX1-ETO AML via PI3K/AKT (but not ERK/MEK) signaling pathway activation, which mediates the antiapoptotic function of MPL in leukemic cells.","method":"shRNA knockdown, pharmacological inhibition, signaling pathway analysis in AML cell lines and mouse models","journal":"Blood","confidence":"Medium","confidence_rationale":"Tier 2 — loss-of-function with defined pathway readout","pmids":["22613795"],"is_preprint":false},{"year":2014,"finding":"Mpl expression (but not TPO ligand) is required for JAK2V617F-driven MPN development; loss of Mpl significantly reduces thrombocythemia, neutrophilia, splenomegaly, and the neoplastic stem cell pool in JAK2V617F transgenic mice.","method":"Genetic epistasis: JAK2V617F transgenic x Mpl-/- and Tpo-/- mice, flow cytometry, hematological analysis","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — clean genetic epistasis with multiple orthogonal phenotypic readouts","pmids":["25339357"],"is_preprint":false},{"year":2014,"finding":"Mpl expression on megakaryocytes and platelets is dispensable for platelet production but essential to prevent myeloproliferation; Mpl on megakaryocytes/platelets restricts available TPO, thereby controlling megakaryocyte progenitor stimulation.","method":"Conditional knockout (PF4-Cre), hematological and progenitor analysis, gene expression profiling","journal":"Proceedings of the National Academy of Sciences","confidence":"High","confidence_rationale":"Tier 2 — cell-type-specific conditional KO with mechanistic explanation","pmids":["24711413"],"is_preprint":false},{"year":2014,"finding":"CAMT-associated mutations in c-Mpl principally cause defective receptor presentation on the cell surface; one mutant (F104S) reaches the surface but shows defective TPO binding; residues in the membrane-distal CRM Domain 1 E-F and A-B loops and Domain 2 F'-G' loop are key TPO-binding determinants.","method":"Cell surface expression assays, TPO binding assays, mutagenesis of c-Mpl ectodomain","journal":"Growth Factors","confidence":"High","confidence_rationale":"Tier 1 — systematic mutagenesis with functional binding assays","pmids":["24438083"],"is_preprint":false},{"year":2014,"finding":"Mpl traffics to the cell surface via both conventional ER-Golgi and unconventional autolysosomal secretory pathways; JAK2 acts as a chaperone for Mpl—siRNA knockdown of Jak2 causes Mpl trapping in the ER; Mpl associates with Jak2 on both intracellular and plasma membranes.","method":"Proximity ligation assay, siRNA knockdown, correlated light and electron microscopy (miniSOG fusion), surface biotinylation","journal":"Traffic","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods including ultrastructural imaging","pmids":["24931576"],"is_preprint":false},{"year":2014,"finding":"c-Mpl tyrosine Y591 is phosphorylated upon TPO stimulation; its mutation to Phe reduces total receptor phosphorylation; phospho-Y591 recruits SHP-1, SYK, and BTK, with SYK mediating ERK1/2 phosphorylation downstream of Y591.","method":"Site-directed mutagenesis, SH2/PTB domain binding microarray, siRNA knockdown, phosphorylation assays","journal":"Experimental Hematology","confidence":"Medium","confidence_rationale":"Tier 1 — mutagenesis combined with domain binding microarray and functional RNAi validation","pmids":["24607955"],"is_preprint":false},{"year":2015,"finding":"c-Mpl is C-mannosylated at Trp269, Trp272, Trp474, and Trp477; C-mannosylation at these four sites is essential for c-Mpl-mediated JAK-STAT signaling, as C-mannosylation-defective mutants lose signaling capacity.","method":"Mass spectrometry identification of C-mannosylation sites, site-directed mutagenesis, signaling assays","journal":"Biochemical and Biophysical Research Communications","confidence":"High","confidence_rationale":"Tier 1 — PTM identified by MS with mutagenesis functional validation","pmids":["26505790"],"is_preprint":false},{"year":2017,"finding":"Mutant CALR binds to the extracellular domain of MPL; this interaction requires lectin-dependent CALR function and requires a positive electrostatic charge in the mutant C-terminus; three tyrosine residues in the intracellular domain of MPL are required for downstream JAK-STAT signaling activation; binding alone is insufficient for cytokine-independent growth.","method":"Co-immunoprecipitation, domain mutagenesis, hematopoietic transformation assays","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — systematic mutagenesis of both CALR and MPL with functional transformation readout","pmids":["29288169"],"is_preprint":false},{"year":2018,"finding":"Mutant CALR forms homomultimers via its C-terminal frameshift-derived domain; homomultimerization is required for MPL binding and activation; disrupting intermolecular CALR interactions abolishes oncogenic MPL activation.","method":"Co-immunoprecipitation, competition assay, Ba/F3 transformation assay","journal":"Leukemia","confidence":"High","confidence_rationale":"Tier 2 — mechanistic dissection with multiple binding and functional assays","pmids":["29946189"],"is_preprint":false},{"year":2019,"finding":"Mutant CALR acts as a rogue chaperone for TpoR/MPL, stabilizing a dimeric state and transporting partially immature TpoR (with incompletely processed N117 glycans) to the cell surface bypassing quality control; this requires TpoR N-glycosylation and a hydrophobic patch in its extracellular domain; full MPL activation requires cell-surface localization.","method":"Glycan analysis, subcellular fractionation, surface expression assays, mutagenesis, thermal stability assays, oncogenic transformation assays","journal":"Blood","confidence":"High","confidence_rationale":"Tier 1 — multiple orthogonal methods including glycan processing analysis and systematic mutagenesis","pmids":["30902807"],"is_preprint":false},{"year":2019,"finding":"Mutant CALR interacts with MPL in the secretory pathway (Golgi apparatus) and on the cell surface; MPL activation by mutant CALR requires entrance into the secretion pathway and N-glycan interaction; surface localization of MPL is required for sustained downstream signaling even though initial activation occurs before cell surface.","method":"Subcellular localization assays, intracellular trafficking inhibitors, trypsin surface removal, signaling assays","journal":"Leukemia","confidence":"High","confidence_rationale":"Tier 2 — multiple localization and functional assays dissecting site of activation","pmids":["31462733"],"is_preprint":false},{"year":2015,"finding":"TPO promotes liver metastasis of CD110+ (MPL+) colorectal cancer tumor-initiating cells by activating lysine degradation; lysine catabolism generates acetyl-CoA for p300-dependent LRP6 acetylation leading to Wnt/self-renewal signaling, and glutamate to modulate redox status; TPO-mediated c-myc induction orchestrates metabolic gene expression via chromatin modifier recruitment.","method":"Metabolic assays, ChIP, co-IP, mutagenesis, mouse metastasis models","journal":"Cell Stem Cell","confidence":"Medium","confidence_rationale":"Tier 2 — multiple orthogonal mechanistic methods but novel pathway in non-hematopoietic context","pmids":["26140605"],"is_preprint":false},{"year":2020,"finding":"Deep mutational scanning of the MPL transmembrane domain identified all known driver mutations (W515L/K/R/A, S505N) plus 7 novel constitutively activating mutations conferring cytokine-independent growth, and numerous second-site modifiers that enhance S505N-driven activation.","method":"Deep mutational scanning, Ba/F3 cytokine-independent growth assay","journal":"Blood","confidence":"High","confidence_rationale":"Tier 1 — systematic saturation mutagenesis with functional reconstitution","pmids":["31697803"],"is_preprint":false},{"year":2023,"finding":"Cryo-EM structure of the TPO-MPL extracellular signaling complex at 3.4 Å reveals the basis for homodimeric MPL activation; structure-guided TPO variants (TPOmod) with partial agonism decouple JAK/STAT from ERK/AKT/CREB signaling, driving megakaryopoiesis without significant HSC expansion.","method":"Cryo-EM structure determination, engineered ligand variants, in vitro and in vivo functional assays, single-cell RNA sequencing","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1 — high-resolution structure with mutagenesis and in vivo functional validation","pmids":["37633268"],"is_preprint":false},{"year":1998,"finding":"TPO stimulates PKC alpha and beta isoform activation (membrane translocation) in c-Mpl-expressing cells; PKC activation mediates TPO's mitogenic action but is not required for TPO-induced megakaryocytic differentiation marker expression.","method":"PKC translocation assay, pharmacological inhibitor (GF109203X), PKC downregulation by phorbol ester, Western blot","journal":"Blood","confidence":"Medium","confidence_rationale":"Tier 2 — pharmacological and biochemical dissection distinguishing proliferation vs. differentiation","pmids":["9446641"],"is_preprint":false},{"year":2024,"finding":"PF4 (platelet factor 4) binds and activates c-Mpl on platelets, leading to JAK2 activation and STAT3/STAT5 phosphorylation, resulting in platelet aggregation; inhibition of the c-Mpl-JAK2 pathway inhibits platelet aggregation to PF4 and VITT sera.","method":"Direct binding assay, phosphorylation assays, pharmacological inhibition, platelet aggregation assay","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods identifying novel ligand-receptor interaction with functional consequence","pmids":["37883794"],"is_preprint":false},{"year":2016,"finding":"MPL R102P mutation causes incomplete trafficking defect to the cell surface; low but not absent MPL surface expression on immature megakaryocyte progenitors allows THPO-dependent proliferative response, while defective TPO clearance by mature cells with no surface MPL leads to high circulating TPO and paradoxical thrombocytosis.","method":"Flow cytometry, [125I]-THPO binding, retroviral mouse model, CD34+ cell transduction","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — multiple methods including direct ligand binding, patient cells and mouse model","pmids":["28034873"],"is_preprint":false},{"year":2011,"finding":"The transcription factor PlagL2 activates Mpl transcription via two consensus sites in the proximal Mpl promoter; PlagL2-expressing leukemic cells show hyperactivation of JAK2, STAT5, Akt, and Erk1/2 in response to THPO.","method":"Promoter reporter assay, ChIP-like binding analysis, signaling pathway analysis, mouse leukemia model","journal":"Leukemia","confidence":"Medium","confidence_rationale":"Tier 2 — promoter mapping with functional signaling and in vivo validation","pmids":["21263445"],"is_preprint":false}],"current_model":"MPL (c-Mpl/TpoR) is a homodimeric type I cytokine receptor that, upon binding its primary ligand TPO (or PF4), undergoes homodimerization and activates JAK2 and TYK2, leading to phosphorylation of STAT1/3/5, PI3K/AKT, MAPK, and NF-κB pathways to drive HSC quiescence/self-renewal and megakaryocyte proliferation/differentiation; receptor surface expression and signaling are regulated by JAK2 chaperone activity, C-mannosylation, c-Cbl-mediated ubiquitination and dual lysosomal/proteasomal degradation, and negative feedback through Lnk; platelet-surface MPL regulates circulating TPO levels by receptor-mediated clearance; oncogenic gain-of-function mutations in the transmembrane or intracellular domains (W515L/K, S505N) constitutively activate this pathway, and mutant calreticulin acts as a rogue chaperone that binds MPL's extracellular domain and drives its surface trafficking to constitutively activate JAK-STAT signaling."},"narrative":{"teleology":[{"year":1994,"claim":"Establishing MPL as the TPO receptor and demonstrating its non-redundant role in megakaryopoiesis and a platelet-mass feedback loop for TPO clearance resolved the long-sought identity of the thrombopoietic growth factor receptor.","evidence":"c-mpl knockout mice showed 85% platelet/megakaryocyte reduction with elevated circulating TPO","pmids":["8073287"],"confidence":"High","gaps":["Mechanism of receptor-mediated TPO clearance not defined at molecular level","Potential roles in non-megakaryocytic lineages not explored"]},{"year":1995,"claim":"Identification of JAK2, TYK2, and STAT1/3/5 as proximal signaling effectors downstream of TPO-activated MPL defined the core signal transduction cascade for this receptor.","evidence":"Co-IP, kinase assays, and gel-shift assays in BaF3/mMpl and factor-dependent hematopoietic cell lines","pmids":["7534285","7543416"],"confidence":"High","gaps":["Relative contributions of JAK2 vs TYK2 unclear","Which STAT complexes drive specific transcriptional programs not resolved"]},{"year":1996,"claim":"Discovery that the transmembrane S498N mutation constitutively activates MPL provided the first evidence that enforced receptor dimerization/conformational change in the TM domain is sufficient for ligand-independent oncogenic signaling through SHC-Raf-MAPK and JAK-STAT.","evidence":"PCR-driven random mutagenesis with Ba/F3 factor-independence and tumorigenicity assays","pmids":["8695859"],"confidence":"High","gaps":["Structural mechanism of TM-mediated activation not elucidated","In vivo myeloproliferative disease potential not tested"]},{"year":1996,"claim":"Defining GATA-1 and Ets family (Ets-1, Fli-1) cooperative binding at the MPL promoter explained its megakaryocyte-restricted expression pattern.","evidence":"Promoter deletion, EMSA, and transactivation in HEL megakaryocytic cells","pmids":["8639837"],"confidence":"High","gaps":["In vivo chromatin accessibility and enhancer landscape not mapped","Contribution of additional transcription factors (e.g., PlagL2 identified later) not explored"]},{"year":1998,"claim":"Demonstrating that MPL is required for HSC self-renewal and long-term repopulation capacity expanded its role from a lineage-specific receptor to a fundamental regulator of the stem cell compartment.","evidence":"Competitive repopulation and CFU-S assays in mpl−/− mice","pmids":["9448308"],"confidence":"High","gaps":["Whether the HSC defect is cell-autonomous vs niche-mediated not fully resolved","Downstream transcriptional program maintaining HSC quiescence not identified"]},{"year":2002,"claim":"Extending downstream pathway analysis showed MPL signaling engages PI3K-Akt-Bad (anti-apoptosis) and NF-κB in megakaryocytes, broadening the known effector network beyond JAK-STAT and MAPK.","evidence":"Factor-independence assays with W508S mutant MPL in Ba/F3 cells; IKK/NF-κB activity assays in megakaryocytic cell lines","pmids":["12145691","11967992"],"confidence":"Medium","gaps":["Direct contribution of NF-κB to megakaryocyte differentiation vs proliferation not dissected genetically","Relative importance of PI3K vs MAPK in different cellular contexts unclear"]},{"year":2004,"claim":"Identification of Lnk as a negative regulator of TPO/c-Mpl signaling established the first feedback attenuator acting at the receptor-proximal level, explaining how signaling amplitude and duration are controlled.","evidence":"Lnk−/− mice and SH2 domain mutagenesis with signaling analysis in megakaryocytes","pmids":["15337790"],"confidence":"High","gaps":["Precise binding site of Lnk on MPL or JAK2 not mapped","Relationship between Lnk and c-Cbl-mediated ubiquitination not addressed"]},{"year":2007,"claim":"Showing that MPL signaling in the osteoblastic niche enforces HSC quiescence via β1-integrin and CDK inhibitor upregulation provided the mechanistic link between TPO/MPL and niche-dependent stem cell dormancy.","evidence":"Anti-MPL antibody treatment and exogenous TPO in vivo with cell cycle analysis of LT-HSCs","pmids":["18371409"],"confidence":"High","gaps":["Direct transcriptional targets of MPL signaling in quiescent HSCs not catalogued","Contribution of other niche-derived signals cooperating with TPO not separated"]},{"year":2009,"claim":"Mapping c-Cbl as the E3 ligase that ubiquitinates MPL at K553/K573 for dual lysosomal and proteasomal degradation revealed the molecular mechanism terminating receptor signaling.","evidence":"Site-directed mutagenesis, siRNA knockdown of c-Cbl, and ubiquitination assays","pmids":["19880496"],"confidence":"High","gaps":["Whether additional E3 ligases contribute not tested","How ubiquitination is coordinated with Lnk-mediated negative regulation unknown"]},{"year":2014,"claim":"Multiple studies in 2014 resolved key aspects of MPL biology: JAK2 acts as a chaperone for MPL surface trafficking via both conventional and unconventional secretory pathways; CAMT mutations principally disrupt surface presentation; Mpl on megakaryocytes/platelets is dispensable for platelet production but essential as a TPO sink preventing myeloproliferation; and Mpl expression (not TPO) is required for JAK2V617F-driven MPN.","evidence":"Proximity ligation/miniSOG-CLEM for JAK2 chaperone role; systematic CAMT mutagenesis with surface expression and TPO binding; PF4-Cre conditional KO; JAK2V617F×Mpl−/− genetic epistasis","pmids":["24931576","24438083","24711413","25339357"],"confidence":"High","gaps":["Structure of JAK2-MPL chaperone complex not solved","Whether unconventional autolysosomal trafficking of MPL is regulated or constitutive not known","Mechanism by which MPL surface density is sensed to regulate TPO clearance not defined"]},{"year":2015,"claim":"C-mannosylation at four tryptophan residues was shown to be essential for JAK-STAT signaling competence, identifying a previously unrecognized post-translational requirement for receptor activation.","evidence":"Mass spectrometry identification of C-mannosylation sites with mutagenesis and signaling assays","pmids":["26505790"],"confidence":"High","gaps":["Whether C-mannosylation affects receptor folding, dimerization, or ligand binding specifically is not resolved","Enzyme(s) responsible for C-mannosylation of MPL not identified"]},{"year":2017,"claim":"The mechanism of oncogenic mutant CALR–MPL interaction was dissected: mutant CALR binds MPL's extracellular domain via lectin-dependent glycan recognition and a positively charged mutant C-terminus; CALR homomultimerization is required for MPL binding and activation; and mutant CALR acts as a rogue chaperone transporting immature, partially glycosylated MPL to the cell surface, bypassing ER quality control.","evidence":"Systematic Co-IP, domain mutagenesis of both CALR and MPL, glycan processing analysis, subcellular fractionation, Ba/F3 transformation assays across multiple studies","pmids":["29288169","29946189","30902807","31462733"],"confidence":"High","gaps":["Structural basis of the mutant CALR–MPL complex not determined at atomic resolution","Why MPL is the preferential target among cytokine receptors not explained","Whether therapeutic disruption of the CALR–MPL interface is feasible not tested"]},{"year":2020,"claim":"Deep mutational scanning of the MPL transmembrane domain systematically catalogued all gain-of-function mutations, confirming known drivers and revealing 7 novel activating residues plus second-site modifiers of S505N.","evidence":"Saturation mutagenesis with Ba/F3 cytokine-independent growth selection","pmids":["31697803"],"confidence":"High","gaps":["Whether novel TM mutations occur in MPN patients not confirmed clinically","Structural mechanism by which different TM mutations activate receptor not visualized"]},{"year":2023,"claim":"The 3.4 Å cryo-EM structure of the TPO–MPL signaling complex revealed the homodimeric activation geometry and enabled engineering of partial agonists (TPOmod) that decouple JAK/STAT from ERK/AKT signaling, driving megakaryopoiesis without HSC expansion.","evidence":"Cryo-EM structure, engineered ligand variants, in vivo functional assays, single-cell RNA-seq","pmids":["37633268"],"confidence":"High","gaps":["Full-length receptor structure including transmembrane and intracellular domains not resolved","How biased agonism at the receptor translates to selective pathway activation at the molecular level not fully defined"]},{"year":2024,"claim":"PF4 was identified as a second endogenous ligand for MPL on platelets, activating JAK2-STAT3/5 and driving platelet aggregation, establishing a non-TPO activation axis with implications for VITT pathophysiology.","evidence":"Direct binding assay, phosphorylation assays, pharmacological JAK2 inhibition, platelet aggregation","pmids":["37883794"],"confidence":"High","gaps":["PF4 binding site on MPL relative to TPO binding site not mapped","Whether PF4–MPL signaling is relevant in HSC or megakaryocyte biology not tested","Structural basis for dual ligand recognition unknown"]},{"year":null,"claim":"Key unresolved questions include the full-length atomic structure of MPL (including transmembrane and intracellular domains), the structural basis for how different TM mutations trigger constitutive activation, how biased agonism at MPL selectively routes intracellular signaling, and why mutant CALR preferentially targets MPL among cytokine receptors.","evidence":"","pmids":[],"confidence":"Medium","gaps":["Full-length MPL structure not solved","Mechanism of biased signaling by partial agonists at molecular level unclear","Selectivity of mutant CALR for MPL over other cytokine receptors unexplained"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,1,2,26,28]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[6,9]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[16,17,22,23,29]},{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[17,22,23]},{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[23]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[1,2,3,6,10,18,26,28]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[5,8]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[0,5,8]},{"term_id":"R-HSA-109582","term_label":"Hemostasis","supporting_discovery_ids":[0,15,28]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[3,14,25]}],"complexes":["TPO-MPL homodimer signaling complex"],"partners":["JAK2","TYK2","STAT5","STAT3","LNK","CBL","CALR","PF4"],"other_free_text":[]},"mechanistic_narrative":"MPL (c-Mpl/TpoR) is a homodimeric type I cytokine receptor that serves as the central regulator of megakaryopoiesis, thrombopoiesis, and hematopoietic stem cell (HSC) self-renewal and quiescence. Upon binding its primary ligand thrombopoietin (TPO) — or the recently identified ligand PF4 — MPL activates JAK2 and TYK2, which phosphorylate STAT1/3/5, and engages PI3K/AKT, MAPK, NF-κB, and PKC pathways to promote megakaryocyte proliferation and differentiation, while in HSCs it upregulates β1-integrin and CDK inhibitors to enforce quiescence [PMID:7543416, PMID:18371409, PMID:37883794]. Receptor surface expression depends on JAK2 chaperone activity and C-mannosylation at four tryptophan residues, with trafficking occurring via both conventional ER–Golgi and unconventional autolysosomal secretory routes; after activation, c-Cbl ubiquitinates MPL at K553/K573 for dual lysosomal/proteasomal degradation, while the adaptor Lnk provides negative feedback on downstream signaling [PMID:24931576, PMID:26505790, PMID:19880496, PMID:15337790]. Platelet-surface MPL clears circulating TPO to establish a feedback loop controlling megakaryocyte progenitor stimulation, and loss-of-function mutations cause congenital amegakaryocytic thrombocytopenia (CAMT) primarily through defective receptor surface trafficking [PMID:24711413, PMID:24438083]. Gain-of-function transmembrane mutations (W515L/K, S505N) constitutively activate JAK-STAT signaling, and mutant calreticulin hijacks MPL by acting as a rogue chaperone that multimerizes, binds the MPL extracellular domain via lectin-dependent N-glycan interactions, and drives immature receptor to the cell surface for ligand-independent activation [PMID:31697803, PMID:30902807, PMID:29946189]."},"prefetch_data":{"uniprot":{"accession":"P40238","full_name":"Thrombopoietin receptor","aliases":["Myeloproliferative leukemia protein","Proto-oncogene c-Mpl"],"length_aa":635,"mass_kda":71.2,"function":"Receptor for thrombopoietin that regulates hematopoietic stem cell renewal, megakaryocyte differentiation, and platelet formation. Upon activation by THPO, induces rapid tyrosine phosphorylation and activation of JAK2, providing docking sites for many signaling proteins such as STAT5, SHIP/INPP5D, GRB2, SOS1 and PI3K (PubMed:15899890, PubMed:37633268). In turn, These signaling cascades lead to the proliferation, survival, and differentiation of megakaryocytes, ultimately leading to increased platelet production","subcellular_location":"Cell membrane; Golgi apparatus; Cell surface","url":"https://www.uniprot.org/uniprotkb/P40238/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/MPL","classification":"Not Classified","n_dependent_lines":5,"n_total_lines":1208,"dependency_fraction":0.0041390728476821195},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/MPL","total_profiled":1310},"omim":[{"mim_id":"620776","title":"THROMBOCYTOPENIA 13, SYNDROMIC; THC13","url":"https://www.omim.org/entry/620776"},{"mim_id":"620481","title":"AMEGAKARYOCYTIC THROMBOCYTOPENIA, CONGENITAL, 2; 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mechanisms","url":"https://pubmed.ncbi.nlm.nih.gov/29666143","citation_count":21,"is_preprint":false},{"pmid":"15095485","id":"PMC_15095485","title":"Thrombopoietin receptor (Mpl) expression by megakaryocytes in myeloproliferative disorders.","date":"2004","source":"The Journal of pathology","url":"https://pubmed.ncbi.nlm.nih.gov/15095485","citation_count":20,"is_preprint":false},{"pmid":"19844195","id":"PMC_19844195","title":"Gene therapy of MPL deficiency: challenging balance between leukemia and pancytopenia.","date":"2009","source":"Molecular therapy : the journal of the American Society of Gene Therapy","url":"https://pubmed.ncbi.nlm.nih.gov/19844195","citation_count":20,"is_preprint":false},{"pmid":"24438083","id":"PMC_24438083","title":"Functional characterization of c-Mpl ectodomain mutations that underlie congenital amegakaryocytic thrombocytopenia.","date":"2014","source":"Growth factors (Chur, Switzerland)","url":"https://pubmed.ncbi.nlm.nih.gov/24438083","citation_count":20,"is_preprint":false},{"pmid":"10216080","id":"PMC_10216080","title":"High-level expression of Mpl in platelets and megakaryocytes is independent of thrombopoietin.","date":"1999","source":"Blood","url":"https://pubmed.ncbi.nlm.nih.gov/10216080","citation_count":19,"is_preprint":false},{"pmid":"8590859","id":"PMC_8590859","title":"The physiologic role and therapeutic potential of the Mpl-ligand in thrombopoiesis.","date":"1995","source":"Stem cells (Dayton, Ohio)","url":"https://pubmed.ncbi.nlm.nih.gov/8590859","citation_count":19,"is_preprint":false},{"pmid":"29430853","id":"PMC_29430853","title":"Nfix Promotes Survival of Immature Hematopoietic Cells via Regulation of c-Mpl.","date":"2018","source":"Stem cells (Dayton, Ohio)","url":"https://pubmed.ncbi.nlm.nih.gov/29430853","citation_count":18,"is_preprint":false},{"pmid":"10976539","id":"PMC_10976539","title":"Megakaryocyte growth and development factor (MGDF): an Mpl ligand and cytokine that regulates thrombopoiesis.","date":"2000","source":"Cytokines, cellular & molecular therapy","url":"https://pubmed.ncbi.nlm.nih.gov/10976539","citation_count":17,"is_preprint":false},{"pmid":"28034873","id":"PMC_28034873","title":"An incomplete trafficking defect to the cell-surface leads to paradoxical thrombocytosis for human and murine MPL P106L.","date":"2016","source":"Blood","url":"https://pubmed.ncbi.nlm.nih.gov/28034873","citation_count":17,"is_preprint":false},{"pmid":"30199786","id":"PMC_30199786","title":"Anti-c-Mpl antibodies in immune thrombocytopenia suppress thrombopoiesis and decrease response to rhTPO.","date":"2018","source":"Thrombosis research","url":"https://pubmed.ncbi.nlm.nih.gov/30199786","citation_count":17,"is_preprint":false},{"pmid":"34756243","id":"PMC_34756243","title":"The MPL mutation.","date":"2021","source":"International review of cell and molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/34756243","citation_count":16,"is_preprint":false},{"pmid":"19822297","id":"PMC_19822297","title":"Failure to achieve a threshold dose of CD34+CD110+ progenitor cells in the graft predicts delayed platelet engraftment after autologous stem cell transplantation for multiple myeloma.","date":"2009","source":"Biology of blood and marrow transplantation : journal of the American Society for Blood and Marrow Transplantation","url":"https://pubmed.ncbi.nlm.nih.gov/19822297","citation_count":16,"is_preprint":false},{"pmid":"17704793","id":"PMC_17704793","title":"Failure to achieve a threshold dose of CD34+ CD110+ progenitor cells in the graft predicts delayed platelet engraftment after autologous stem cell transplantation.","date":"2007","source":"Bone marrow transplantation","url":"https://pubmed.ncbi.nlm.nih.gov/17704793","citation_count":16,"is_preprint":false},{"pmid":"25829245","id":"PMC_25829245","title":"Synergy of anti-CD40, CpG and MPL in activation of mouse macrophages.","date":"2015","source":"Molecular immunology","url":"https://pubmed.ncbi.nlm.nih.gov/25829245","citation_count":16,"is_preprint":false},{"pmid":"33831466","id":"PMC_33831466","title":"Platelet enhancement by Carica papaya L. leaf fractions in cyclophosphamide induced thrombocytopenic rats is due to elevated expression of CD110 receptor on megakaryocytes.","date":"2021","source":"Journal of ethnopharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/33831466","citation_count":15,"is_preprint":false},{"pmid":"28460472","id":"PMC_28460472","title":"Fungal lectin MpL enables entry of protein drugs into cancer cells and their subcellular targeting.","date":"2017","source":"Oncotarget","url":"https://pubmed.ncbi.nlm.nih.gov/28460472","citation_count":14,"is_preprint":false},{"pmid":"24607955","id":"PMC_24607955","title":"Phosphorylated c-Mpl tyrosine 591 regulates thrombopoietin-induced signaling.","date":"2014","source":"Experimental hematology","url":"https://pubmed.ncbi.nlm.nih.gov/24607955","citation_count":14,"is_preprint":false},{"pmid":"26130950","id":"PMC_26130950","title":"JAK2 V617F, MPL, and CALR Mutations in Korean Patients with Essential Thrombocythemia and Primary Myelofibrosis.","date":"2015","source":"Journal of Korean medical science","url":"https://pubmed.ncbi.nlm.nih.gov/26130950","citation_count":14,"is_preprint":false},{"pmid":"19643476","id":"PMC_19643476","title":"JAK2 and MPL gene mutations in V617F-negative myeloproliferative neoplasms.","date":"2009","source":"Leukemia research","url":"https://pubmed.ncbi.nlm.nih.gov/19643476","citation_count":14,"is_preprint":false},{"pmid":"25343958","id":"PMC_25343958","title":"A hyperactive Mpl-based cell growth switch drives macrophage-associated erythropoiesis through an erythroid-megakaryocytic precursor.","date":"2014","source":"Blood","url":"https://pubmed.ncbi.nlm.nih.gov/25343958","citation_count":14,"is_preprint":false},{"pmid":"32341206","id":"PMC_32341206","title":"c-Mpl and TPO expression in the human central nervous system neurons inhibits neuronal apoptosis.","date":"2020","source":"Aging","url":"https://pubmed.ncbi.nlm.nih.gov/32341206","citation_count":13,"is_preprint":false},{"pmid":"31014934","id":"PMC_31014934","title":"Indirubin regulates MPL and TNF expression in peripheral blood mononuclear cells from patients with primary immune thrombocytopenia.","date":"2019","source":"Experimental hematology","url":"https://pubmed.ncbi.nlm.nih.gov/31014934","citation_count":13,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":47529,"output_tokens":6507,"usd":0.120096},"stage2":{"model":"claude-opus-4-6","input_tokens":10218,"output_tokens":4061,"usd":0.228923},"total_usd":0.349019,"stage1_batch_id":"msgbatch_01Mgbec4vM6c9vXht7JVJzss","stage2_batch_id":"msgbatch_01JkuuH5RB2BdJNsgnhjbC6s","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1994,\n      \"finding\": \"MPL (c-mpl) is the receptor for thrombopoietin (TPO) and specifically regulates megakaryocytopoiesis and thrombopoiesis; c-mpl-deficient mice have 85% reduction in platelets and megakaryocytes with elevated circulating TPO, demonstrating a feedback loop where platelet mass regulates TPO levels through receptor-mediated clearance.\",\n      \"method\": \"Gene targeting / knockout mice with hematological phenotyping\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — foundational KO study, highly cited, replicated in multiple subsequent studies\",\n      \"pmids\": [\"8073287\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"TPO binding to c-Mpl induces rapid tyrosine phosphorylation of JAK2, Shc, and c-Mpl itself; JAK2 physically associates with c-Mpl relatively late (20–60 min) after ligand binding, suggesting JAK2 may not be the initiating kinase in the signaling cascade.\",\n      \"method\": \"Immunoprecipitation, Western blotting, co-immunoprecipitation in BaF3/mMpl cells\",\n      \"journal\": \"The Journal of Biological Chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP and Western blot with time-course in engineered cell line, highly cited\",\n      \"pmids\": [\"7534285\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"Activation of MPL by TPO rapidly induces tyrosine phosphorylation of JAK2 and TYK2 (but not JAK1 or JAK3), followed by phosphorylation of STAT1, STAT3, and STAT5, and formation of specific DNA-binding complexes.\",\n      \"method\": \"In vitro kinase assays, immunoprecipitation, gel-shift assays in factor-dependent hematopoietic cell lines\",\n      \"journal\": \"Experimental Hematology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods, replicated across cell lines\",\n      \"pmids\": [\"7543416\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"A point mutation (Ser498→Asn498) in the transmembrane domain of MPL constitutively activates the receptor, conferring factor-independent growth and activating SHC-Raf-MAPK and JAK2-STAT3/STAT5 signaling pathways.\",\n      \"method\": \"Retrovirus-mediated gene transfer with PCR-driven random mutagenesis, signaling pathway analysis in Ba/F3 cells, tumorigenicity assay\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — active-site mutagenesis with functional reconstitution and in vivo tumorigenicity\",\n      \"pmids\": [\"8695859\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"The MPL promoter contains binding sites for GATA-1 and Ets family proteins (Ets-1, Fli-1) that cooperatively regulate megakaryocyte-specific MPL expression; an Ets site at –15 and another downstream of the GATA motif are critical for promoter activity.\",\n      \"method\": \"Promoter deletion analysis, EMSA/in vitro binding, transactivation assays in HEL cells\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple methods including mutagenesis and transactivation, replicated with two Ets factors\",\n      \"pmids\": [\"8639837\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"c-Mpl (TPO receptor) is required for hematopoietic stem cell maintenance; mpl-/- mice have 4–12-fold fewer preprogenitor cells and severely impaired long-term hematopoietic repopulating capacity, with a stem cell-intrinsic self-renewal defect.\",\n      \"method\": \"Bone marrow transplantation, CFU-S assays, competitive repopulation assays in mpl-/- mice\",\n      \"journal\": \"Proceedings of the National Academy of Sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple functional stem cell assays, highly cited, replicated\",\n      \"pmids\": [\"9448308\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"The adaptor protein Lnk negatively regulates TPO/c-mpl signaling through its SH2 domain; Lnk deficiency causes enhanced and prolonged TPO-induced STAT3, STAT5, Akt, and MAPK signaling in megakaryocytes, leading to increased megakaryocyte numbers and ploidy in vivo.\",\n      \"method\": \"Overexpression and loss-of-function (Lnk-/- mice), signaling assays, domain mutagenesis\",\n      \"journal\": \"The Journal of Experimental Medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO mice plus domain mutagenesis and signaling pathway analysis\",\n      \"pmids\": [\"15337790\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"AMG531, a recombinant Fc-linked Mpl-binding protein, competes with TPO for binding to Mpl on BaF3-Mpl cells and platelets, and induces rapid tyrosine phosphorylation of Mpl, JAK2, and STAT5.\",\n      \"method\": \"Competitive radioligand binding assay (125I-TPO), phosphorylation assays, in vitro megakaryocyte culture\",\n      \"journal\": \"Cytokine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct binding and signaling assays in defined cell system\",\n      \"pmids\": [\"14693160\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"THPO/MPL signaling in the osteoblastic niche regulates HSC quiescence; MPL+ LT-HSCs are associated with THPO-producing osteoblasts, and MPL signaling upregulates β1-integrin and CDK inhibitors; anti-MPL antibody reduces quiescent LT-HSCs and enables HSC engraftment without irradiation.\",\n      \"method\": \"Anti-MPL neutralizing antibody treatment, exogenous THPO administration, flow cytometry, cell cycle analysis in vivo\",\n      \"journal\": \"Cell Stem Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal gain/loss-of-function with multiple cellular readouts, highly cited\",\n      \"pmids\": [\"18371409\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"After TPO stimulation, c-Mpl is ubiquitinated on two intracellular lysine residues (K553 and K573) by the E3 ubiquitin ligase c-Cbl, and degraded by both lysosomal and proteasomal pathways; mutation of these lysines reduces ubiquitination, degradation, and causes hyperproliferation.\",\n      \"method\": \"Site-directed mutagenesis, siRNA knockdown, dominant-negative overexpression, ubiquitination assays\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — active-site mutagenesis plus RNAi and dominant-negative with functional readout\",\n      \"pmids\": [\"19880496\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"A novel MPL mutation (W508S) in the intracellular domain constitutively activates three signaling pathways: SHC-Ras-Raf-MAPK/JNK, JAK-STAT, and PI3K-Akt-Bad, conferring factor-independent growth.\",\n      \"method\": \"Ba/F3 cell factor-independence assay, signaling pathway analysis\",\n      \"journal\": \"Leukemia\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional reconstitution with pathway dissection\",\n      \"pmids\": [\"12145691\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"MPL receptor signaling through Mpl ligand activates IKK transiently then suppresses its activity; proliferating megakaryocytes display constitutive NF-κB (p50 homodimer and p50-p65 heterodimer) DNA-binding activity that is reduced upon TPO-induced differentiation.\",\n      \"method\": \"IKK activity assays, EMSA, NF-κB reporter assay in megakaryocytic cell lines\",\n      \"journal\": \"Journal of Cellular Biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple signaling assays in defined cell system\",\n      \"pmids\": [\"11967992\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"JAK2 acts as a chaperone for Mpl, responsible for its cell-surface expression; there is a reciprocal relationship between JAK2 V617F allele burden and platelet Mpl expression in myeloproliferative disorder patients.\",\n      \"method\": \"Quantitative allele analysis and flow cytometry of patient samples; mechanistic chaperone function inferred from clinical correlates\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — clinical correlative data supporting chaperone role, single lab\",\n      \"pmids\": [\"16912229\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Wild-type MPL expression promotes RUNX1-ETO AML via PI3K/AKT (but not ERK/MEK) signaling pathway activation, which mediates the antiapoptotic function of MPL in leukemic cells.\",\n      \"method\": \"shRNA knockdown, pharmacological inhibition, signaling pathway analysis in AML cell lines and mouse models\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function with defined pathway readout\",\n      \"pmids\": [\"22613795\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Mpl expression (but not TPO ligand) is required for JAK2V617F-driven MPN development; loss of Mpl significantly reduces thrombocythemia, neutrophilia, splenomegaly, and the neoplastic stem cell pool in JAK2V617F transgenic mice.\",\n      \"method\": \"Genetic epistasis: JAK2V617F transgenic x Mpl-/- and Tpo-/- mice, flow cytometry, hematological analysis\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean genetic epistasis with multiple orthogonal phenotypic readouts\",\n      \"pmids\": [\"25339357\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Mpl expression on megakaryocytes and platelets is dispensable for platelet production but essential to prevent myeloproliferation; Mpl on megakaryocytes/platelets restricts available TPO, thereby controlling megakaryocyte progenitor stimulation.\",\n      \"method\": \"Conditional knockout (PF4-Cre), hematological and progenitor analysis, gene expression profiling\",\n      \"journal\": \"Proceedings of the National Academy of Sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — cell-type-specific conditional KO with mechanistic explanation\",\n      \"pmids\": [\"24711413\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"CAMT-associated mutations in c-Mpl principally cause defective receptor presentation on the cell surface; one mutant (F104S) reaches the surface but shows defective TPO binding; residues in the membrane-distal CRM Domain 1 E-F and A-B loops and Domain 2 F'-G' loop are key TPO-binding determinants.\",\n      \"method\": \"Cell surface expression assays, TPO binding assays, mutagenesis of c-Mpl ectodomain\",\n      \"journal\": \"Growth Factors\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis with functional binding assays\",\n      \"pmids\": [\"24438083\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Mpl traffics to the cell surface via both conventional ER-Golgi and unconventional autolysosomal secretory pathways; JAK2 acts as a chaperone for Mpl—siRNA knockdown of Jak2 causes Mpl trapping in the ER; Mpl associates with Jak2 on both intracellular and plasma membranes.\",\n      \"method\": \"Proximity ligation assay, siRNA knockdown, correlated light and electron microscopy (miniSOG fusion), surface biotinylation\",\n      \"journal\": \"Traffic\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods including ultrastructural imaging\",\n      \"pmids\": [\"24931576\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"c-Mpl tyrosine Y591 is phosphorylated upon TPO stimulation; its mutation to Phe reduces total receptor phosphorylation; phospho-Y591 recruits SHP-1, SYK, and BTK, with SYK mediating ERK1/2 phosphorylation downstream of Y591.\",\n      \"method\": \"Site-directed mutagenesis, SH2/PTB domain binding microarray, siRNA knockdown, phosphorylation assays\",\n      \"journal\": \"Experimental Hematology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis combined with domain binding microarray and functional RNAi validation\",\n      \"pmids\": [\"24607955\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"c-Mpl is C-mannosylated at Trp269, Trp272, Trp474, and Trp477; C-mannosylation at these four sites is essential for c-Mpl-mediated JAK-STAT signaling, as C-mannosylation-defective mutants lose signaling capacity.\",\n      \"method\": \"Mass spectrometry identification of C-mannosylation sites, site-directed mutagenesis, signaling assays\",\n      \"journal\": \"Biochemical and Biophysical Research Communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — PTM identified by MS with mutagenesis functional validation\",\n      \"pmids\": [\"26505790\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Mutant CALR binds to the extracellular domain of MPL; this interaction requires lectin-dependent CALR function and requires a positive electrostatic charge in the mutant C-terminus; three tyrosine residues in the intracellular domain of MPL are required for downstream JAK-STAT signaling activation; binding alone is insufficient for cytokine-independent growth.\",\n      \"method\": \"Co-immunoprecipitation, domain mutagenesis, hematopoietic transformation assays\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — systematic mutagenesis of both CALR and MPL with functional transformation readout\",\n      \"pmids\": [\"29288169\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Mutant CALR forms homomultimers via its C-terminal frameshift-derived domain; homomultimerization is required for MPL binding and activation; disrupting intermolecular CALR interactions abolishes oncogenic MPL activation.\",\n      \"method\": \"Co-immunoprecipitation, competition assay, Ba/F3 transformation assay\",\n      \"journal\": \"Leukemia\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic dissection with multiple binding and functional assays\",\n      \"pmids\": [\"29946189\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Mutant CALR acts as a rogue chaperone for TpoR/MPL, stabilizing a dimeric state and transporting partially immature TpoR (with incompletely processed N117 glycans) to the cell surface bypassing quality control; this requires TpoR N-glycosylation and a hydrophobic patch in its extracellular domain; full MPL activation requires cell-surface localization.\",\n      \"method\": \"Glycan analysis, subcellular fractionation, surface expression assays, mutagenesis, thermal stability assays, oncogenic transformation assays\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple orthogonal methods including glycan processing analysis and systematic mutagenesis\",\n      \"pmids\": [\"30902807\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Mutant CALR interacts with MPL in the secretory pathway (Golgi apparatus) and on the cell surface; MPL activation by mutant CALR requires entrance into the secretion pathway and N-glycan interaction; surface localization of MPL is required for sustained downstream signaling even though initial activation occurs before cell surface.\",\n      \"method\": \"Subcellular localization assays, intracellular trafficking inhibitors, trypsin surface removal, signaling assays\",\n      \"journal\": \"Leukemia\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple localization and functional assays dissecting site of activation\",\n      \"pmids\": [\"31462733\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"TPO promotes liver metastasis of CD110+ (MPL+) colorectal cancer tumor-initiating cells by activating lysine degradation; lysine catabolism generates acetyl-CoA for p300-dependent LRP6 acetylation leading to Wnt/self-renewal signaling, and glutamate to modulate redox status; TPO-mediated c-myc induction orchestrates metabolic gene expression via chromatin modifier recruitment.\",\n      \"method\": \"Metabolic assays, ChIP, co-IP, mutagenesis, mouse metastasis models\",\n      \"journal\": \"Cell Stem Cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal mechanistic methods but novel pathway in non-hematopoietic context\",\n      \"pmids\": [\"26140605\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Deep mutational scanning of the MPL transmembrane domain identified all known driver mutations (W515L/K/R/A, S505N) plus 7 novel constitutively activating mutations conferring cytokine-independent growth, and numerous second-site modifiers that enhance S505N-driven activation.\",\n      \"method\": \"Deep mutational scanning, Ba/F3 cytokine-independent growth assay\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic saturation mutagenesis with functional reconstitution\",\n      \"pmids\": [\"31697803\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Cryo-EM structure of the TPO-MPL extracellular signaling complex at 3.4 Å reveals the basis for homodimeric MPL activation; structure-guided TPO variants (TPOmod) with partial agonism decouple JAK/STAT from ERK/AKT/CREB signaling, driving megakaryopoiesis without significant HSC expansion.\",\n      \"method\": \"Cryo-EM structure determination, engineered ligand variants, in vitro and in vivo functional assays, single-cell RNA sequencing\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution structure with mutagenesis and in vivo functional validation\",\n      \"pmids\": [\"37633268\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"TPO stimulates PKC alpha and beta isoform activation (membrane translocation) in c-Mpl-expressing cells; PKC activation mediates TPO's mitogenic action but is not required for TPO-induced megakaryocytic differentiation marker expression.\",\n      \"method\": \"PKC translocation assay, pharmacological inhibitor (GF109203X), PKC downregulation by phorbol ester, Western blot\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — pharmacological and biochemical dissection distinguishing proliferation vs. differentiation\",\n      \"pmids\": [\"9446641\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"PF4 (platelet factor 4) binds and activates c-Mpl on platelets, leading to JAK2 activation and STAT3/STAT5 phosphorylation, resulting in platelet aggregation; inhibition of the c-Mpl-JAK2 pathway inhibits platelet aggregation to PF4 and VITT sera.\",\n      \"method\": \"Direct binding assay, phosphorylation assays, pharmacological inhibition, platelet aggregation assay\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods identifying novel ligand-receptor interaction with functional consequence\",\n      \"pmids\": [\"37883794\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"MPL R102P mutation causes incomplete trafficking defect to the cell surface; low but not absent MPL surface expression on immature megakaryocyte progenitors allows THPO-dependent proliferative response, while defective TPO clearance by mature cells with no surface MPL leads to high circulating TPO and paradoxical thrombocytosis.\",\n      \"method\": \"Flow cytometry, [125I]-THPO binding, retroviral mouse model, CD34+ cell transduction\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple methods including direct ligand binding, patient cells and mouse model\",\n      \"pmids\": [\"28034873\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"The transcription factor PlagL2 activates Mpl transcription via two consensus sites in the proximal Mpl promoter; PlagL2-expressing leukemic cells show hyperactivation of JAK2, STAT5, Akt, and Erk1/2 in response to THPO.\",\n      \"method\": \"Promoter reporter assay, ChIP-like binding analysis, signaling pathway analysis, mouse leukemia model\",\n      \"journal\": \"Leukemia\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — promoter mapping with functional signaling and in vivo validation\",\n      \"pmids\": [\"21263445\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"MPL (c-Mpl/TpoR) is a homodimeric type I cytokine receptor that, upon binding its primary ligand TPO (or PF4), undergoes homodimerization and activates JAK2 and TYK2, leading to phosphorylation of STAT1/3/5, PI3K/AKT, MAPK, and NF-κB pathways to drive HSC quiescence/self-renewal and megakaryocyte proliferation/differentiation; receptor surface expression and signaling are regulated by JAK2 chaperone activity, C-mannosylation, c-Cbl-mediated ubiquitination and dual lysosomal/proteasomal degradation, and negative feedback through Lnk; platelet-surface MPL regulates circulating TPO levels by receptor-mediated clearance; oncogenic gain-of-function mutations in the transmembrane or intracellular domains (W515L/K, S505N) constitutively activate this pathway, and mutant calreticulin acts as a rogue chaperone that binds MPL's extracellular domain and drives its surface trafficking to constitutively activate JAK-STAT signaling.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"MPL (c-Mpl/TpoR) is a homodimeric type I cytokine receptor that serves as the central regulator of megakaryopoiesis, thrombopoiesis, and hematopoietic stem cell (HSC) self-renewal and quiescence. Upon binding its primary ligand thrombopoietin (TPO) — or the recently identified ligand PF4 — MPL activates JAK2 and TYK2, which phosphorylate STAT1/3/5, and engages PI3K/AKT, MAPK, NF-κB, and PKC pathways to promote megakaryocyte proliferation and differentiation, while in HSCs it upregulates β1-integrin and CDK inhibitors to enforce quiescence [PMID:7543416, PMID:18371409, PMID:37883794]. Receptor surface expression depends on JAK2 chaperone activity and C-mannosylation at four tryptophan residues, with trafficking occurring via both conventional ER–Golgi and unconventional autolysosomal secretory routes; after activation, c-Cbl ubiquitinates MPL at K553/K573 for dual lysosomal/proteasomal degradation, while the adaptor Lnk provides negative feedback on downstream signaling [PMID:24931576, PMID:26505790, PMID:19880496, PMID:15337790]. Platelet-surface MPL clears circulating TPO to establish a feedback loop controlling megakaryocyte progenitor stimulation, and loss-of-function mutations cause congenital amegakaryocytic thrombocytopenia (CAMT) primarily through defective receptor surface trafficking [PMID:24711413, PMID:24438083]. Gain-of-function transmembrane mutations (W515L/K, S505N) constitutively activate JAK-STAT signaling, and mutant calreticulin hijacks MPL by acting as a rogue chaperone that multimerizes, binds the MPL extracellular domain via lectin-dependent N-glycan interactions, and drives immature receptor to the cell surface for ligand-independent activation [PMID:31697803, PMID:30902807, PMID:29946189].\",\n  \"teleology\": [\n    {\n      \"year\": 1994,\n      \"claim\": \"Establishing MPL as the TPO receptor and demonstrating its non-redundant role in megakaryopoiesis and a platelet-mass feedback loop for TPO clearance resolved the long-sought identity of the thrombopoietic growth factor receptor.\",\n      \"evidence\": \"c-mpl knockout mice showed 85% platelet/megakaryocyte reduction with elevated circulating TPO\",\n      \"pmids\": [\"8073287\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of receptor-mediated TPO clearance not defined at molecular level\", \"Potential roles in non-megakaryocytic lineages not explored\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Identification of JAK2, TYK2, and STAT1/3/5 as proximal signaling effectors downstream of TPO-activated MPL defined the core signal transduction cascade for this receptor.\",\n      \"evidence\": \"Co-IP, kinase assays, and gel-shift assays in BaF3/mMpl and factor-dependent hematopoietic cell lines\",\n      \"pmids\": [\"7534285\", \"7543416\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative contributions of JAK2 vs TYK2 unclear\", \"Which STAT complexes drive specific transcriptional programs not resolved\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Discovery that the transmembrane S498N mutation constitutively activates MPL provided the first evidence that enforced receptor dimerization/conformational change in the TM domain is sufficient for ligand-independent oncogenic signaling through SHC-Raf-MAPK and JAK-STAT.\",\n      \"evidence\": \"PCR-driven random mutagenesis with Ba/F3 factor-independence and tumorigenicity assays\",\n      \"pmids\": [\"8695859\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural mechanism of TM-mediated activation not elucidated\", \"In vivo myeloproliferative disease potential not tested\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Defining GATA-1 and Ets family (Ets-1, Fli-1) cooperative binding at the MPL promoter explained its megakaryocyte-restricted expression pattern.\",\n      \"evidence\": \"Promoter deletion, EMSA, and transactivation in HEL megakaryocytic cells\",\n      \"pmids\": [\"8639837\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo chromatin accessibility and enhancer landscape not mapped\", \"Contribution of additional transcription factors (e.g., PlagL2 identified later) not explored\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Demonstrating that MPL is required for HSC self-renewal and long-term repopulation capacity expanded its role from a lineage-specific receptor to a fundamental regulator of the stem cell compartment.\",\n      \"evidence\": \"Competitive repopulation and CFU-S assays in mpl−/− mice\",\n      \"pmids\": [\"9448308\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether the HSC defect is cell-autonomous vs niche-mediated not fully resolved\", \"Downstream transcriptional program maintaining HSC quiescence not identified\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Extending downstream pathway analysis showed MPL signaling engages PI3K-Akt-Bad (anti-apoptosis) and NF-κB in megakaryocytes, broadening the known effector network beyond JAK-STAT and MAPK.\",\n      \"evidence\": \"Factor-independence assays with W508S mutant MPL in Ba/F3 cells; IKK/NF-κB activity assays in megakaryocytic cell lines\",\n      \"pmids\": [\"12145691\", \"11967992\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct contribution of NF-κB to megakaryocyte differentiation vs proliferation not dissected genetically\", \"Relative importance of PI3K vs MAPK in different cellular contexts unclear\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Identification of Lnk as a negative regulator of TPO/c-Mpl signaling established the first feedback attenuator acting at the receptor-proximal level, explaining how signaling amplitude and duration are controlled.\",\n      \"evidence\": \"Lnk−/− mice and SH2 domain mutagenesis with signaling analysis in megakaryocytes\",\n      \"pmids\": [\"15337790\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Precise binding site of Lnk on MPL or JAK2 not mapped\", \"Relationship between Lnk and c-Cbl-mediated ubiquitination not addressed\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Showing that MPL signaling in the osteoblastic niche enforces HSC quiescence via β1-integrin and CDK inhibitor upregulation provided the mechanistic link between TPO/MPL and niche-dependent stem cell dormancy.\",\n      \"evidence\": \"Anti-MPL antibody treatment and exogenous TPO in vivo with cell cycle analysis of LT-HSCs\",\n      \"pmids\": [\"18371409\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct transcriptional targets of MPL signaling in quiescent HSCs not catalogued\", \"Contribution of other niche-derived signals cooperating with TPO not separated\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Mapping c-Cbl as the E3 ligase that ubiquitinates MPL at K553/K573 for dual lysosomal and proteasomal degradation revealed the molecular mechanism terminating receptor signaling.\",\n      \"evidence\": \"Site-directed mutagenesis, siRNA knockdown of c-Cbl, and ubiquitination assays\",\n      \"pmids\": [\"19880496\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether additional E3 ligases contribute not tested\", \"How ubiquitination is coordinated with Lnk-mediated negative regulation unknown\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Multiple studies in 2014 resolved key aspects of MPL biology: JAK2 acts as a chaperone for MPL surface trafficking via both conventional and unconventional secretory pathways; CAMT mutations principally disrupt surface presentation; Mpl on megakaryocytes/platelets is dispensable for platelet production but essential as a TPO sink preventing myeloproliferation; and Mpl expression (not TPO) is required for JAK2V617F-driven MPN.\",\n      \"evidence\": \"Proximity ligation/miniSOG-CLEM for JAK2 chaperone role; systematic CAMT mutagenesis with surface expression and TPO binding; PF4-Cre conditional KO; JAK2V617F×Mpl−/− genetic epistasis\",\n      \"pmids\": [\"24931576\", \"24438083\", \"24711413\", \"25339357\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure of JAK2-MPL chaperone complex not solved\", \"Whether unconventional autolysosomal trafficking of MPL is regulated or constitutive not known\", \"Mechanism by which MPL surface density is sensed to regulate TPO clearance not defined\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"C-mannosylation at four tryptophan residues was shown to be essential for JAK-STAT signaling competence, identifying a previously unrecognized post-translational requirement for receptor activation.\",\n      \"evidence\": \"Mass spectrometry identification of C-mannosylation sites with mutagenesis and signaling assays\",\n      \"pmids\": [\"26505790\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether C-mannosylation affects receptor folding, dimerization, or ligand binding specifically is not resolved\", \"Enzyme(s) responsible for C-mannosylation of MPL not identified\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"The mechanism of oncogenic mutant CALR–MPL interaction was dissected: mutant CALR binds MPL's extracellular domain via lectin-dependent glycan recognition and a positively charged mutant C-terminus; CALR homomultimerization is required for MPL binding and activation; and mutant CALR acts as a rogue chaperone transporting immature, partially glycosylated MPL to the cell surface, bypassing ER quality control.\",\n      \"evidence\": \"Systematic Co-IP, domain mutagenesis of both CALR and MPL, glycan processing analysis, subcellular fractionation, Ba/F3 transformation assays across multiple studies\",\n      \"pmids\": [\"29288169\", \"29946189\", \"30902807\", \"31462733\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of the mutant CALR–MPL complex not determined at atomic resolution\", \"Why MPL is the preferential target among cytokine receptors not explained\", \"Whether therapeutic disruption of the CALR–MPL interface is feasible not tested\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Deep mutational scanning of the MPL transmembrane domain systematically catalogued all gain-of-function mutations, confirming known drivers and revealing 7 novel activating residues plus second-site modifiers of S505N.\",\n      \"evidence\": \"Saturation mutagenesis with Ba/F3 cytokine-independent growth selection\",\n      \"pmids\": [\"31697803\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether novel TM mutations occur in MPN patients not confirmed clinically\", \"Structural mechanism by which different TM mutations activate receptor not visualized\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"The 3.4 Å cryo-EM structure of the TPO–MPL signaling complex revealed the homodimeric activation geometry and enabled engineering of partial agonists (TPOmod) that decouple JAK/STAT from ERK/AKT signaling, driving megakaryopoiesis without HSC expansion.\",\n      \"evidence\": \"Cryo-EM structure, engineered ligand variants, in vivo functional assays, single-cell RNA-seq\",\n      \"pmids\": [\"37633268\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full-length receptor structure including transmembrane and intracellular domains not resolved\", \"How biased agonism at the receptor translates to selective pathway activation at the molecular level not fully defined\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"PF4 was identified as a second endogenous ligand for MPL on platelets, activating JAK2-STAT3/5 and driving platelet aggregation, establishing a non-TPO activation axis with implications for VITT pathophysiology.\",\n      \"evidence\": \"Direct binding assay, phosphorylation assays, pharmacological JAK2 inhibition, platelet aggregation\",\n      \"pmids\": [\"37883794\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"PF4 binding site on MPL relative to TPO binding site not mapped\", \"Whether PF4–MPL signaling is relevant in HSC or megakaryocyte biology not tested\", \"Structural basis for dual ligand recognition unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the full-length atomic structure of MPL (including transmembrane and intracellular domains), the structural basis for how different TM mutations trigger constitutive activation, how biased agonism at MPL selectively routes intracellular signaling, and why mutant CALR preferentially targets MPL among cytokine receptors.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Full-length MPL structure not solved\", \"Mechanism of biased signaling by partial agonists at molecular level unclear\", \"Selectivity of mutant CALR for MPL over other cytokine receptors unexplained\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 1, 2, 26, 28]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [6, 9]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [16, 17, 22, 23, 29]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [17, 22, 23]},\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [23]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [1, 2, 3, 6, 10, 18, 26, 28]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [5, 8]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [0, 5, 8]},\n      {\"term_id\": \"R-HSA-109582\", \"supporting_discovery_ids\": [0, 15, 28]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [3, 14, 25]}\n    ],\n    \"complexes\": [\n      \"TPO-MPL homodimer signaling complex\"\n    ],\n    \"partners\": [\n      \"JAK2\",\n      \"TYK2\",\n      \"STAT5\",\n      \"STAT3\",\n      \"LNK\",\n      \"CBL\",\n      \"CALR\",\n      \"PF4\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}