{"gene":"MC2R","run_date":"2026-04-28T18:30:28","timeline":{"discoveries":[{"year":2007,"finding":"MRAP isoforms alpha and beta are essential for ACTH binding to MC2R and ACTH-induced cAMP production, but are not required for MC2R localization at the plasma membrane. MRAPα and MRAPβ differentially regulate cell membrane density and functional properties of MC2R, with MRAPβ-expressing cells showing higher cell surface MC2R density and higher maximal cAMP responses.","method":"Stable expression of Myc-MC2R with MRAP isoforms in isogenic HEK293/Flp cells; ACTH binding assays; cAMP production assays; immunofluorescence","journal":"Molecular endocrinology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (binding, cAMP, localization) in clean isogenic system","pmids":["17456795"],"is_preprint":false},{"year":2010,"finding":"The N-terminal segment and the third and fourth transmembrane regions of MC2R contain signals responsible for its intracellular retention in non-adrenal cells; the fourth and fifth transmembrane domains are involved in ACTH binding selectivity. MRAP interaction bypasses these retention signals.","method":"Systematic chimeric MC2R/MC4R receptor mutagenesis; confocal fluorescence microscopy quantification of cell-membrane localization; ACTH binding assays; cAMP response assays","journal":"Molecular and cellular endocrinology","confidence":"High","confidence_rationale":"Tier 1 — systematic mutagenesis with 15 chimeric receptors, multiple orthogonal functional readouts","pmids":["20206229"],"is_preprint":false},{"year":2011,"finding":"ACTH induces concentration-dependent, arrestin-, clathrin-, and dynamin-dependent MC2R/MRAP1 internalization, followed by recycling via Rab4-, Rab5-, and Rab11-positive endosomes. Specific intracellular Ser/Thr residues (T131, S140, T143, T147, T204, S208, S280) regulate plasma membrane targeting, internalization, and cAMP signaling; the second intracellular loop is critical for MC2R expression and functional regulation.","method":"Mutagenesis of Ser/Thr residues; internalization assays with monensin/brefeldin A; colocalization with Rab GTPases; cAMP accumulation assays in HEK293/FRT cells","journal":"Molecular endocrinology","confidence":"High","confidence_rationale":"Tier 1 — site-directed mutagenesis combined with multiple functional and localization readouts","pmids":["21920850"],"is_preprint":false},{"year":2010,"finding":"ACTH binding to MC2R activates p44/p42 MAPK and p38 MAPK phosphorylation through a cAMP/PKA-dependent pathway, but not via cAMP/Epac1/2 or arrestin-coupled internalization. Recycling inhibitors brefeldin A and monensin attenuated both cAMP accumulation and MAPK phosphorylation proportionally.","method":"PKA inhibitors (H89, KI-6-22, Rp-cAMPS); selective Epac1/2 activator; dominant-negative arrestin constructs; recycling inhibitors; phospho-MAPK immunoblot in HEK293 cells and primary human fasciculata cells","journal":"Molecular and cellular endocrinology","confidence":"High","confidence_rationale":"Tier 2 — multiple pharmacological tools and genetic constructs with consistent results across two cell systems","pmids":["21195128"],"is_preprint":false},{"year":2003,"finding":"MC2R desensitization is protein kinase A-dependent, whereas internalization is PKA-independent and dependent on a G protein receptor kinase (GRK). MC2R requires a cofactor (later identified as MRAP) for cell surface expression in heterologous cells.","method":"Y1 and Y6 cell expression systems; PKA inhibitor studies; GRK-dependent internalization assays","journal":"Annals of the New York Academy of Sciences","confidence":"Medium","confidence_rationale":"Tier 2 — defined mechanistic findings but single lab, partial characterization of internalization pathway","pmids":["12851305"],"is_preprint":false},{"year":2004,"finding":"Two naturally occurring MC2R missense mutations (C21R and S247G), each individually causing loss of ligand binding and signaling, together produce constitutive cAMP accumulation when present in the same receptor molecule, demonstrating a synergistic interaction leading to constitutive activation.","method":"Stable expression in Y6 cells; basal cAMP accumulation measurement; ACTH dose-response assays","journal":"Molecular and cellular endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 — defined functional epistasis between two mutations with quantitative cAMP readout","pmids":["15062562"],"is_preprint":false},{"year":2010,"finding":"Loss of the MC2R C-terminus (K289fs and M290X mutations) impairs cell surface expression by disrupting transport from the ER to the cell membrane, without altering interaction of MC2R with MRAP as shown by co-immunoprecipitation.","method":"Cell surface assays; confocal localization studies; co-immunoprecipitation; OS3 cell-based reporter assay for cAMP","journal":"The Journal of clinical endocrinology and metabolism","confidence":"Medium","confidence_rationale":"Tier 2 — multiple orthogonal methods (CoIP, cell surface assay, imaging, functional assay) in single study","pmids":["20962024"],"is_preprint":false},{"year":2012,"finding":"The C-terminal domains of MRAP1 isoforms dictate their intracellular localization: MRAPα localizes near the nuclear envelope and intracellular endosomes while MRAPβ localizes strongly at the plasma membrane. MRAPβ induces the highest ACTH-stimulated cAMP accumulation. MRAP1 is required for ACTH to activate MC2R; its presence enhances MC2R cell-surface expression. MRAPβ exhibits dual topology (N-cyto/C-exo and N-exo/C-cyto) at the plasma membrane.","method":"Stable expression in HEK293/FRT and B16-G4F cells; confocal localization; cAMP accumulation assays; topology assays","journal":"General and comparative endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 — multiple cell systems and orthogonal methods, single lab","pmids":["22366472"],"is_preprint":false},{"year":2014,"finding":"Both the ligand-recognition specificity determinant and the intracellular retention signal of MC2R are formed by a specific extracellular structure supported by correct alignment of transmembrane domains. The aromatic-residue-rich segment of the second extracellular loop is involved in effects mediated by the second ACTH pharmacophore (-K-K-R-R-).","method":"Directed mutagenesis replacing 2-5 amino acid segments of MC2R with MC4R equivalents; 20 recombinant EGFP-fusion receptors; membrane trafficking efficiency assays; cAMP response to α-MSH and ACTH(1-24) with/without MRAP coexpression","journal":"Journal of molecular endocrinology","confidence":"Medium","confidence_rationale":"Tier 1 — systematic mutagenesis with functional readouts, single lab","pmids":["25074265"],"is_preprint":false},{"year":2000,"finding":"Two SF-1 binding sites at -209 and -35 in the human ACTHR/MC2R promoter are required for adrenal-specific expression; AP-1 binding at -764 to -503 is required for cAMP/forskolin-dependent induction of MC2R transcription. Both AP-1 and SF-1 are necessary for full cAMP-dependent MC2R gene expression.","method":"Promoter deletion analysis; luciferase reporter assays in Y1, JEG3, and Cos-1 cells; electrophoretic mobility shift assay (EMSA); site-directed mutagenesis of AP-1 and SF-1 sites; SF-1 cotransfection","journal":"Endocrine journal","confidence":"High","confidence_rationale":"Tier 1 — reconstitution via cotransfection, EMSA, mutagenesis, and multiple cell lines","pmids":["10811295"],"is_preprint":false},{"year":2004,"finding":"In murine adipocytes, a PPRE-like sequence in the MC2R promoter binds PPARγ and RXRα; PPARγ2/RXRα cotransfection activates MC2R transcription in preadipocytes, and mutation of the PPRE abolishes MC2R promoter activity in differentiated 3T3-L1 adipocytes. This defines a novel adipocyte-specific SF-1-independent mechanism of MC2R transcription.","method":"5' deletion analysis of mc2-r promoter; luciferase reporter in undifferentiated and differentiated 3T3-L1 cells; EMSA with adipocyte nuclear extracts; PPARγ2/RXRα cotransfection; PPRE mutagenesis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — multiple methods including EMSA, mutagenesis, and functional reporter assays","pmids":["15028712"],"is_preprint":false},{"year":2010,"finding":"FOXL2 and NR5A1 (SF-1) synergistically activate the Mc2r promoter. FOXL2 alone activates Mc2r promoter activity dose-dependently, and the synergistic effect with NR5A1 requires distal NR5A1 response elements at -1410 and -975 bp.","method":"Promoter mapping; luciferase reporter assays in Y1 adrenocortical cells; cotransfection of FOXL2 and NR5A1 expression plasmids; deletion constructs of Mc2r promoter","journal":"Biology of reproduction","confidence":"Medium","confidence_rationale":"Tier 2 — promoter mapping with multiple deletion constructs and cotransfection, single lab","pmids":["20650879"],"is_preprint":false},{"year":2004,"finding":"An E-box element at -1020 bp of the human MC2R promoter represses MC2R transcription in granulosa cells through interactions with transcription factors including AP-4; this contributes to adrenal-specific expression independently of SF-1.","method":"Promoter luciferase reporter assays in bovine adrenocortical and granulosa cells; EMSA; serial promoter deletions","journal":"Journal of molecular endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 — EMSA and functional reporter assays with mutagenesis, single lab","pmids":["15171714"],"is_preprint":false},{"year":2017,"finding":"JDP2 activates Mc2r promoter transcription dose-dependently via cAMP response elements (particularly at -830 bp), as demonstrated by ChIP. Phosphorylation of JDP2 is required for its transcriptional activation of Mc2r, and SUMOylation of JDP2 modulates this activity.","method":"Luciferase reporter assays; real-time ChIP; promoter mapping; mutations of CRE sites; phosphorylation-deficient and SUMOylation mutants of JDP2 in Y1 adrenocortical cells","journal":"International journal of molecular sciences","confidence":"Medium","confidence_rationale":"Tier 2 — ChIP, promoter mapping, mutagenesis, and PTM analysis in single lab","pmids":["28146118"],"is_preprint":false},{"year":2010,"finding":"MC2R promoter haplotype TCCT confers ~4-fold higher basal transcriptional activity and ~5-fold greater ACTH-induced MC2R expression compared to TCCC haplotype, as shown by luciferase reporter and RT-PCR assays, providing a molecular basis for variable ACTH responsiveness.","method":"Luciferase reporter assay; real-time quantitative RT-PCR of MC2R cDNA expression driven by TCCT vs TCCC promoters in presence and absence of ACTH","journal":"Pharmacogenetics and genomics","confidence":"Medium","confidence_rationale":"Tier 2 — functional reporter and expression assays with two orthogonal methods, single lab","pmids":["20042918"],"is_preprint":false},{"year":1995,"finding":"A homozygous Y254C mutation in the third extracellular loop of MC2R causes isolated glucocorticoid deficiency, consistent with disruption of ligand binding at this position.","method":"Direct sequencing of MC2R gene; family analysis confirming autosomal recessive inheritance; functional inference from receptor structure","journal":"The Journal of clinical endocrinology and metabolism","confidence":"Low","confidence_rationale":"Tier 3 — genetic identification with structural inference, no direct functional assay in this paper","pmids":["7608277"],"is_preprint":false},{"year":2020,"finding":"Ginsenoside Rd inhibits ACTH-induced corticosterone biosynthesis by blocking the MC2R-cAMP/PKA/CREB signaling pathway and downregulating MC2R and MRAP expression in adrenocortical Y1 cells.","method":"cAMP content measurement; PKA activity assay; Western blot and qPCR for MC2R and MRAP in Y1 cells treated with ACTH ± ginsenoside Rd","journal":"Life sciences","confidence":"Medium","confidence_rationale":"Tier 2 — multiple biochemical readouts of the MC2R-cAMP/PKA/CREB pathway in single cell system","pmids":["31972205"],"is_preprint":false},{"year":2013,"finding":"In rat bone marrow stromal cells expressing MC2R and MRAP, ACTH via MC2-R signaling promotes chondrogenic nodule formation and induces transient elevations in intracellular calcium; neither α-MSH (MC5-R agonist) nor γ2-MSH (MC3-R agonist) replicate these effects, defining MC2-R as the specific mediator.","method":"MC-R-specific peptide agonists; calcium flux assays; immunoblot of membrane fractions; chondrogenic nodule formation assay; dexamethasone modulation of MC2-R and MRAP expression","journal":"Cell and tissue research","confidence":"Medium","confidence_rationale":"Tier 2 — receptor-specific pharmacological dissection with multiple readouts, single lab","pmids":["23358747"],"is_preprint":false},{"year":2012,"finding":"In human prostate cancer cell lines, ACTH acting via MC2R induces cAMP production, increases androgen receptor nuclear localization, and promotes concentration-dependent cell proliferation, with the effect partially blocked by a non-selective MCR antagonist (SHU9119).","method":"MTT cell proliferation assay; cAMP production assay; immunocytochemistry for AR nuclear localization; MCR antagonist (SHU9119) inhibition","journal":"International journal of oncology","confidence":"Medium","confidence_rationale":"Tier 2 — multiple functional readouts with pharmacological antagonist validation, single lab","pmids":["22842514"],"is_preprint":false},{"year":2017,"finding":"Activation of tetrapod MC2R orthologs (human, Anolis carolinensis, Xenopus tropicalis) requires both the H6F7R8W9 (HFRW) pharmacophore and the K15K16R17R18P19 motifs of ACTH(1-24); alanine substitution at these positions has more severe effects on non-mammalian MC2Rs than on human MC2R, pointing to conserved but differentially tuned dual binding sites on the receptor.","method":"Alanine-substituted ACTH(1-24) analog stimulation; cAMP assays in cells coexpressing tetrapod MC2R orthologs with MRAP1","journal":"General and comparative endocrinology","confidence":"Medium","confidence_rationale":"Tier 1 — systematic alanine scan of ligand motifs with functional cAMP readout across multiple orthologs","pmids":["23639234"],"is_preprint":false},{"year":2017,"finding":"The second extracellular loop (EC2)/TM5 region of MC2R is a contact site for the K/KKRRP motif of ACTH; specifically V159 in TM4, F171 in TM5, and F175 in TM5 are critical for receptor activation—triple alanine substitution at these positions completely abolishes ACTH-stimulated activation without impairing receptor trafficking to the plasma membrane.","method":"Single and triple alanine mutagenesis of rainbow trout MC2R; cAMP luciferase reporter assay; cell surface ELISA for trafficking","journal":"General and comparative endocrinology","confidence":"Medium","confidence_rationale":"Tier 1 — mutagenesis with functional and trafficking readouts, single lab","pmids":["28495271"],"is_preprint":false},{"year":2025,"finding":"ACTH directly stimulates MC2R in brown adipocytes to promote thermogenesis; glucocorticoids act permissively to enhance ACTH-mediated energy expenditure via MC2R, while also separately driving cold-induced hyperphagia.","method":"In vitro brown adipocyte assays; in vivo cold exposure model; MC2R-specific signaling readouts","journal":"bioRxiv","confidence":"Low","confidence_rationale":"Tier 3 — preprint with in vitro and in vivo evidence but limited mechanistic detail in abstract","pmids":[],"is_preprint":true}],"current_model":"MC2R (ACTH receptor) is a G protein-coupled receptor that is selectively activated by ACTH to generate cAMP via adenylyl cyclase, driving steroidogenesis; its functional expression at the cell surface requires MRAP1 (which is essential for ACTH binding and signaling but not for basal plasma membrane targeting), while intracellular retention signals in TM3/TM4 restrict its surface expression in non-adrenal cells; ACTH binding involves dual pharmacophore contacts (HFRW and K/KKRRP motifs of ACTH engaging TM2/TM3/TM6 and EC2/TM4/TM5 of MC2R respectively); upon agonist stimulation, MC2R undergoes PKA-dependent desensitization and GRK/clathrin/dynamin-dependent internalization followed by Rab4/5/11-mediated recycling; downstream signaling proceeds through cAMP/PKA to activate both steroidogenic gene transcription and MAPK (p44/42, p38) phosphorylation; adrenal-specific transcription is coordinated by SF-1/NR5A1, AP-1, FOXL2, PPARγ/RXRα (in adipocytes), and JDP2 acting on distinct promoter elements."},"narrative":{"teleology":[{"year":1995,"claim":"Establishing that MC2R mutations are causal in familial glucocorticoid deficiency linked the receptor directly to adrenal ACTH responsiveness in humans, though without functional proof of the mutation's biochemical effect.","evidence":"Homozygous Y254C mutation identified by direct sequencing in a family with isolated glucocorticoid deficiency","pmids":["7608277"],"confidence":"Low","gaps":["No functional assay performed for Y254C; causality inferred from segregation and structural position only","No binding or signaling data for this mutation"]},{"year":2000,"claim":"Defining the transcriptional architecture of the MC2R promoter resolved how adrenal-specific and cAMP-inducible expression are achieved, identifying SF-1 and AP-1 as essential elements.","evidence":"Promoter deletion, EMSA, site-directed mutagenesis, and luciferase reporter assays in Y1, JEG3, and Cos-1 cells","pmids":["10811295"],"confidence":"High","gaps":["Chromatin-level regulation and histone marks at the MC2R locus not addressed","In vivo validation of promoter element requirements not performed"]},{"year":2003,"claim":"Dissecting desensitization and internalization pathways revealed that MC2R desensitization is PKA-dependent whereas internalization is GRK-dependent, and established that a then-unknown cofactor was needed for heterologous surface expression.","evidence":"PKA inhibitor studies and GRK-dependent internalization assays in Y1 and Y6 adrenocortical cells","pmids":["12851305"],"confidence":"Medium","gaps":["Identity of the cofactor (later MRAP) was unknown at this stage","GRK isoform specificity not determined"]},{"year":2004,"claim":"Parallel studies expanded understanding of MC2R regulation: (1) intramolecular epistasis between two loss-of-function mutations producing constitutive activity revealed conformational constraints on receptor activation; (2) PPARγ/RXRα-dependent transcription defined an SF-1-independent adipocyte-specific mechanism; (3) an E-box repressor element further explained tissue-restricted expression.","evidence":"Constitutive activity measured by cAMP accumulation with double-mutant receptor in Y6 cells; PPRE mutagenesis and PPARγ2/RXRα cotransfection in 3T3-L1 adipocytes; E-box/AP-4 identified by EMSA and promoter reporter in granulosa vs adrenocortical cells","pmids":["15062562","15028712","15171714"],"confidence":"Medium","gaps":["Structural basis for C21R/S247G intramolecular synergy unknown","In vivo relevance of PPARγ-driven MC2R in adipose tissue not tested","AP-4 binding to E-box not confirmed by ChIP"]},{"year":2007,"claim":"Identification of MRAP as the essential cofactor for ACTH binding resolved the long-standing puzzle of why MC2R was non-functional in non-adrenal cells, and showed that MRAP isoforms differentially tune receptor surface density and signaling capacity.","evidence":"ACTH binding assays, cAMP production, and immunofluorescence in isogenic HEK293/Flp cells stably expressing MC2R with MRAPα or MRAPβ","pmids":["17456795"],"confidence":"High","gaps":["Structural basis of MRAP–MC2R interaction not resolved","Whether MRAP isoform ratio varies across adrenal zones not determined"]},{"year":2010,"claim":"Systematic chimeric receptor analysis mapped intracellular retention signals to TM3/TM4 and the N-terminus, and ACTH selectivity determinants to TM4/TM5 and EC2, defining how MC2R structure encodes both trafficking restriction and ligand discrimination.","evidence":"15 MC2R/MC4R chimeras assessed by confocal microscopy, ACTH binding, and cAMP response ± MRAP coexpression","pmids":["20206229"],"confidence":"High","gaps":["Atomic-level contacts between MRAP and the retention signal not resolved","Contribution of individual residues within TM3/TM4 to retention not mapped"]},{"year":2010,"claim":"Downstream of cAMP, MC2R was shown to activate p44/42 and p38 MAPK via a cAMP/PKA-dependent rather than Epac- or arrestin-dependent route, establishing the signaling logic downstream of the receptor.","evidence":"PKA inhibitors, selective Epac activator, dominant-negative arrestin, recycling inhibitors, and phospho-MAPK immunoblots in HEK293 and primary human fasciculata cells","pmids":["21195128"],"confidence":"High","gaps":["Direct PKA substrates linking to MAPK cascade not identified","Whether MAPK activation drives specific steroidogenic gene programs through MC2R remains unclear"]},{"year":2010,"claim":"Loss-of-function C-terminal truncation mutations (K289fs, M290X) showed that the MC2R C-terminus is required for ER-to-plasma membrane transport but not for MRAP binding, separating trafficking from accessory protein interaction.","evidence":"Co-immunoprecipitation, cell-surface ELISA, confocal microscopy, and cAMP reporter in OS3 cells","pmids":["20962024"],"confidence":"Medium","gaps":["Specific trafficking machinery interacting with MC2R C-terminus not identified","Only two mutations tested; generalizability of C-terminal role uncertain"]},{"year":2011,"claim":"Mapping intracellular Ser/Thr residues governing internalization and recycling defined the receptor's post-activation trafficking itinerary through Rab4/5/11-positive compartments, revealing how MC2R/MRAP complexes are dynamically regulated.","evidence":"Site-directed mutagenesis of seven Ser/Thr residues combined with Rab GTPase colocalization and cAMP assays in HEK293/FRT cells","pmids":["21920850"],"confidence":"High","gaps":["Kinase(s) phosphorylating specific Ser/Thr residues not identified beyond GRK","Fate of MRAP during recycling not tracked"]},{"year":2012,"claim":"MRAP isoform-specific localization was resolved: MRAPα concentrates at endosomes and the nuclear envelope while MRAPβ strongly localizes to the plasma membrane, explaining their differential effects on MC2R signaling output.","evidence":"Confocal localization, topology assays, and cAMP accumulation in HEK293/FRT and B16-G4F cells","pmids":["22366472"],"confidence":"Medium","gaps":["Determinants of differential MRAP isoform sorting not identified","Dual topology of MRAPβ not mechanistically explained"]},{"year":2017,"claim":"Alanine scanning of ACTH and mutagenesis of MC2R orthologs defined dual pharmacophore contacts: HFRW engages one receptor region while KKRRP contacts EC2/TM4/TM5 residues (V159, F171, F175), with triple alanine substitution abolishing activation without affecting trafficking.","evidence":"Systematic alanine substitution of ACTH(1-24) tested on human, reptile, and amphibian MC2Rs; single and triple alanine mutagenesis of rainbow trout MC2R with cAMP reporter and cell-surface ELISA","pmids":["23639234","28495271"],"confidence":"Medium","gaps":["No co-crystal or cryo-EM structure of MC2R–ACTH–MRAP complex available","Precise contacts of the HFRW motif on the receptor not mapped at residue resolution for mammalian MC2R"]},{"year":2017,"claim":"JDP2 was identified as a transcriptional activator of MC2R through CRE elements, with its activity regulated by phosphorylation and SUMOylation, adding a stress-responsive transcriptional layer to MC2R regulation.","evidence":"ChIP, luciferase reporter, CRE mutagenesis, and phosphorylation/SUMOylation mutants of JDP2 in Y1 cells","pmids":["28146118"],"confidence":"Medium","gaps":["In vivo relevance of JDP2 to MC2R expression in adrenal not demonstrated","Whether JDP2 cooperates with SF-1/AP-1 at the endogenous promoter not tested"]},{"year":null,"claim":"A high-resolution structure of the MC2R–MRAP–ACTH signaling complex is lacking, and the precise mechanism by which MRAP overcomes ER retention signals, the in vivo roles of MC2R in extra-adrenal tissues (adipose, bone, prostate), and the kinase specificity for trafficking-regulatory phosphosites remain undefined.","evidence":"","pmids":[],"confidence":"Low","gaps":["No cryo-EM or crystal structure of MC2R–MRAP complex","In vivo validation of extra-adrenal MC2R functions (thermogenesis, chondrogenesis, prostate proliferation) lacking","Complete phosphosite-to-kinase mapping for internalization/recycling residues not performed"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,1,3,19,20]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[3,16]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,1,2,6,7]},{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[1,6]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[2,7]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,3,4,16,19]},{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[9,10,11,13]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[3,16]}],"complexes":["MC2R–MRAP1 receptor complex"],"partners":["MRAP","NR5A1","FOXL2","PPARG","RXRA","JDP2"],"other_free_text":[]},"mechanistic_narrative":"MC2R is a Gs-coupled receptor uniquely selective for ACTH that drives cAMP/PKA-dependent steroidogenesis and MAPK signaling in adrenocortical cells. Functional cell-surface expression of MC2R requires the accessory protein MRAP1, which overcomes intrinsic ER-retention signals encoded in TM3/TM4 and the N-terminus; MRAP1 isoforms differentially regulate receptor density and maximal signaling capacity [PMID:17456795, PMID:20206229]. ACTH engages MC2R through dual pharmacophore contacts — the HFRW motif interacting with TM2/TM3/TM6 and the KKRRP motif contacting the second extracellular loop and TM4/TM5 — and stimulation triggers PKA-dependent desensitization, GRK/clathrin/dynamin-dependent internalization, and Rab4/5/11-mediated recycling [PMID:21920850, PMID:28495271, PMID:23639234]. Adrenal-specific MC2R transcription is governed by SF-1 and AP-1 elements required for basal and cAMP-induced expression, with additional regulation by FOXL2, PPARγ/RXRα in adipocytes, and JDP2 via CRE sites [PMID:10811295, PMID:15028712, PMID:20650879, PMID:28146118]."},"prefetch_data":{"uniprot":{"accession":"Q01718","full_name":"Adrenocorticotropic hormone receptor","aliases":["Adrenocorticotropin receptor","Melanocortin receptor 2","MC2-R"],"length_aa":297,"mass_kda":33.9,"function":"G protein-coupled receptor for corticotropin/ACTH, primarily expressed in adrenal cortex where it plays a key role in the regulation of adrenocortical function (PubMed:1325670, PubMed:17596328, PubMed:36588120). Upon activation, couples to G(s) protein, stimulating adenylate cyclase and activating the cAMP-dependent signaling pathway, the MAPK cascade as well as the PKA pathway, leading to steroidogenic factor 1/NR5A1-mediated transcriptional activation (PubMed:1325670). Activation by ACTH facilitates the release of adrenal glucocorticoids, including cortisol and corticosterone (PubMed:12213892, PubMed:8636348). In addition, MC2R is required for fetal and neonatal adrenal gland development (By similarity)","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/Q01718/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/MC2R","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/MC2R","total_profiled":1310},"omim":[{"mim_id":"615549","title":"ARMADILLO REPEAT-CONTAINING PROTEIN 5; ARMC5","url":"https://www.omim.org/entry/615549"},{"mim_id":"615410","title":"MELANOCORTIN 2 RECEPTOR ACCESSORY PROTEIN 2; MRAP2","url":"https://www.omim.org/entry/615410"},{"mim_id":"614736","title":"GLUCOCORTICOID DEFICIENCY 4 WITH OR WITHOUT MINERALOCORTICOID DEFICIENCY; GCCD4","url":"https://www.omim.org/entry/614736"},{"mim_id":"609197","title":"GLUCOCORTICOID DEFICIENCY 3; GCCD3","url":"https://www.omim.org/entry/609197"},{"mim_id":"609196","title":"MELANOCORTIN 2 RECEPTOR ACCESSORY PROTEIN; MRAP","url":"https://www.omim.org/entry/609196"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"adrenal gland","ntpm":50.6}],"url":"https://www.proteinatlas.org/search/MC2R"},"hgnc":{"alias_symbol":["ACTHR"],"prev_symbol":[]},"alphafold":{"accession":"Q01718","domains":[{"cath_id":"1.20.1070.10","chopping":"1-15_26-282","consensus_level":"medium","plddt":85.6953,"start":1,"end":282}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q01718","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q01718-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q01718-F1-predicted_aligned_error_v6.png","plddt_mean":85.38},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=MC2R","jax_strain_url":"https://www.jax.org/strain/search?query=MC2R"},"sequence":{"accession":"Q01718","fasta_url":"https://rest.uniprot.org/uniprotkb/Q01718.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q01718/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q01718"}},"corpus_meta":[{"pmid":"17456795","id":"PMC_17456795","title":"Differential regulation of the human adrenocorticotropin receptor [melanocortin-2 receptor (MC2R)] by human MC2R accessory protein isoforms alpha and beta in isogenic human embryonic kidney 293 cells.","date":"2007","source":"Molecular endocrinology (Baltimore, Md.)","url":"https://pubmed.ncbi.nlm.nih.gov/17456795","citation_count":85,"is_preprint":false},{"pmid":"17223989","id":"PMC_17223989","title":"Severe loss-of-function mutations in the adrenocorticotropin receptor (ACTHR, MC2R) can be found in patients diagnosed with salt-losing adrenal hypoplasia.","date":"2007","source":"Clinical endocrinology","url":"https://pubmed.ncbi.nlm.nih.gov/17223989","citation_count":61,"is_preprint":false},{"pmid":"7608277","id":"PMC_7608277","title":"A novel mutation of the adrenocorticotropin receptor (ACTH-R) gene in a family with the syndrome of isolated glucocorticoid deficiency, but no ACTH-R abnormalities in two families with the triple A syndrome.","date":"1995","source":"The Journal of clinical endocrinology and metabolism","url":"https://pubmed.ncbi.nlm.nih.gov/7608277","citation_count":60,"is_preprint":false},{"pmid":"28220105","id":"PMC_28220105","title":"ACTH Receptor (MC2R) Specificity: What Do We Know About Underlying Molecular Mechanisms?","date":"2017","source":"Frontiers in endocrinology","url":"https://pubmed.ncbi.nlm.nih.gov/28220105","citation_count":58,"is_preprint":false},{"pmid":"15062562","id":"PMC_15062562","title":"Constitutive activation of the human ACTH receptor resulting from a synergistic interaction between two naturally occurring missense mutations in the MC2R gene.","date":"2004","source":"Molecular and cellular endocrinology","url":"https://pubmed.ncbi.nlm.nih.gov/15062562","citation_count":43,"is_preprint":false},{"pmid":"23724142","id":"PMC_23724142","title":"Molecular characterization and functional regulation of melanocortin 2 receptor (MC2R) in the sea bass. 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MRAPα and MRAPβ differentially regulate cell membrane density and functional properties of MC2R, with MRAPβ-expressing cells showing higher cell surface MC2R density and higher maximal cAMP responses.\",\n      \"method\": \"Stable expression of Myc-MC2R with MRAP isoforms in isogenic HEK293/Flp cells; ACTH binding assays; cAMP production assays; immunofluorescence\",\n      \"journal\": \"Molecular endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (binding, cAMP, localization) in clean isogenic system\",\n      \"pmids\": [\"17456795\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"The N-terminal segment and the third and fourth transmembrane regions of MC2R contain signals responsible for its intracellular retention in non-adrenal cells; the fourth and fifth transmembrane domains are involved in ACTH binding selectivity. MRAP interaction bypasses these retention signals.\",\n      \"method\": \"Systematic chimeric MC2R/MC4R receptor mutagenesis; confocal fluorescence microscopy quantification of cell-membrane localization; ACTH binding assays; cAMP response assays\",\n      \"journal\": \"Molecular and cellular endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis with 15 chimeric receptors, multiple orthogonal functional readouts\",\n      \"pmids\": [\"20206229\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"ACTH induces concentration-dependent, arrestin-, clathrin-, and dynamin-dependent MC2R/MRAP1 internalization, followed by recycling via Rab4-, Rab5-, and Rab11-positive endosomes. Specific intracellular Ser/Thr residues (T131, S140, T143, T147, T204, S208, S280) regulate plasma membrane targeting, internalization, and cAMP signaling; the second intracellular loop is critical for MC2R expression and functional regulation.\",\n      \"method\": \"Mutagenesis of Ser/Thr residues; internalization assays with monensin/brefeldin A; colocalization with Rab GTPases; cAMP accumulation assays in HEK293/FRT cells\",\n      \"journal\": \"Molecular endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — site-directed mutagenesis combined with multiple functional and localization readouts\",\n      \"pmids\": [\"21920850\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"ACTH binding to MC2R activates p44/p42 MAPK and p38 MAPK phosphorylation through a cAMP/PKA-dependent pathway, but not via cAMP/Epac1/2 or arrestin-coupled internalization. Recycling inhibitors brefeldin A and monensin attenuated both cAMP accumulation and MAPK phosphorylation proportionally.\",\n      \"method\": \"PKA inhibitors (H89, KI-6-22, Rp-cAMPS); selective Epac1/2 activator; dominant-negative arrestin constructs; recycling inhibitors; phospho-MAPK immunoblot in HEK293 cells and primary human fasciculata cells\",\n      \"journal\": \"Molecular and cellular endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple pharmacological tools and genetic constructs with consistent results across two cell systems\",\n      \"pmids\": [\"21195128\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"MC2R desensitization is protein kinase A-dependent, whereas internalization is PKA-independent and dependent on a G protein receptor kinase (GRK). MC2R requires a cofactor (later identified as MRAP) for cell surface expression in heterologous cells.\",\n      \"method\": \"Y1 and Y6 cell expression systems; PKA inhibitor studies; GRK-dependent internalization assays\",\n      \"journal\": \"Annals of the New York Academy of Sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — defined mechanistic findings but single lab, partial characterization of internalization pathway\",\n      \"pmids\": [\"12851305\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Two naturally occurring MC2R missense mutations (C21R and S247G), each individually causing loss of ligand binding and signaling, together produce constitutive cAMP accumulation when present in the same receptor molecule, demonstrating a synergistic interaction leading to constitutive activation.\",\n      \"method\": \"Stable expression in Y6 cells; basal cAMP accumulation measurement; ACTH dose-response assays\",\n      \"journal\": \"Molecular and cellular endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — defined functional epistasis between two mutations with quantitative cAMP readout\",\n      \"pmids\": [\"15062562\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Loss of the MC2R C-terminus (K289fs and M290X mutations) impairs cell surface expression by disrupting transport from the ER to the cell membrane, without altering interaction of MC2R with MRAP as shown by co-immunoprecipitation.\",\n      \"method\": \"Cell surface assays; confocal localization studies; co-immunoprecipitation; OS3 cell-based reporter assay for cAMP\",\n      \"journal\": \"The Journal of clinical endocrinology and metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (CoIP, cell surface assay, imaging, functional assay) in single study\",\n      \"pmids\": [\"20962024\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"The C-terminal domains of MRAP1 isoforms dictate their intracellular localization: MRAPα localizes near the nuclear envelope and intracellular endosomes while MRAPβ localizes strongly at the plasma membrane. MRAPβ induces the highest ACTH-stimulated cAMP accumulation. MRAP1 is required for ACTH to activate MC2R; its presence enhances MC2R cell-surface expression. MRAPβ exhibits dual topology (N-cyto/C-exo and N-exo/C-cyto) at the plasma membrane.\",\n      \"method\": \"Stable expression in HEK293/FRT and B16-G4F cells; confocal localization; cAMP accumulation assays; topology assays\",\n      \"journal\": \"General and comparative endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple cell systems and orthogonal methods, single lab\",\n      \"pmids\": [\"22366472\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Both the ligand-recognition specificity determinant and the intracellular retention signal of MC2R are formed by a specific extracellular structure supported by correct alignment of transmembrane domains. The aromatic-residue-rich segment of the second extracellular loop is involved in effects mediated by the second ACTH pharmacophore (-K-K-R-R-).\",\n      \"method\": \"Directed mutagenesis replacing 2-5 amino acid segments of MC2R with MC4R equivalents; 20 recombinant EGFP-fusion receptors; membrane trafficking efficiency assays; cAMP response to α-MSH and ACTH(1-24) with/without MRAP coexpression\",\n      \"journal\": \"Journal of molecular endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis with functional readouts, single lab\",\n      \"pmids\": [\"25074265\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Two SF-1 binding sites at -209 and -35 in the human ACTHR/MC2R promoter are required for adrenal-specific expression; AP-1 binding at -764 to -503 is required for cAMP/forskolin-dependent induction of MC2R transcription. Both AP-1 and SF-1 are necessary for full cAMP-dependent MC2R gene expression.\",\n      \"method\": \"Promoter deletion analysis; luciferase reporter assays in Y1, JEG3, and Cos-1 cells; electrophoretic mobility shift assay (EMSA); site-directed mutagenesis of AP-1 and SF-1 sites; SF-1 cotransfection\",\n      \"journal\": \"Endocrine journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution via cotransfection, EMSA, mutagenesis, and multiple cell lines\",\n      \"pmids\": [\"10811295\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"In murine adipocytes, a PPRE-like sequence in the MC2R promoter binds PPARγ and RXRα; PPARγ2/RXRα cotransfection activates MC2R transcription in preadipocytes, and mutation of the PPRE abolishes MC2R promoter activity in differentiated 3T3-L1 adipocytes. This defines a novel adipocyte-specific SF-1-independent mechanism of MC2R transcription.\",\n      \"method\": \"5' deletion analysis of mc2-r promoter; luciferase reporter in undifferentiated and differentiated 3T3-L1 cells; EMSA with adipocyte nuclear extracts; PPARγ2/RXRα cotransfection; PPRE mutagenesis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple methods including EMSA, mutagenesis, and functional reporter assays\",\n      \"pmids\": [\"15028712\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"FOXL2 and NR5A1 (SF-1) synergistically activate the Mc2r promoter. FOXL2 alone activates Mc2r promoter activity dose-dependently, and the synergistic effect with NR5A1 requires distal NR5A1 response elements at -1410 and -975 bp.\",\n      \"method\": \"Promoter mapping; luciferase reporter assays in Y1 adrenocortical cells; cotransfection of FOXL2 and NR5A1 expression plasmids; deletion constructs of Mc2r promoter\",\n      \"journal\": \"Biology of reproduction\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — promoter mapping with multiple deletion constructs and cotransfection, single lab\",\n      \"pmids\": [\"20650879\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"An E-box element at -1020 bp of the human MC2R promoter represses MC2R transcription in granulosa cells through interactions with transcription factors including AP-4; this contributes to adrenal-specific expression independently of SF-1.\",\n      \"method\": \"Promoter luciferase reporter assays in bovine adrenocortical and granulosa cells; EMSA; serial promoter deletions\",\n      \"journal\": \"Journal of molecular endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — EMSA and functional reporter assays with mutagenesis, single lab\",\n      \"pmids\": [\"15171714\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"JDP2 activates Mc2r promoter transcription dose-dependently via cAMP response elements (particularly at -830 bp), as demonstrated by ChIP. Phosphorylation of JDP2 is required for its transcriptional activation of Mc2r, and SUMOylation of JDP2 modulates this activity.\",\n      \"method\": \"Luciferase reporter assays; real-time ChIP; promoter mapping; mutations of CRE sites; phosphorylation-deficient and SUMOylation mutants of JDP2 in Y1 adrenocortical cells\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ChIP, promoter mapping, mutagenesis, and PTM analysis in single lab\",\n      \"pmids\": [\"28146118\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"MC2R promoter haplotype TCCT confers ~4-fold higher basal transcriptional activity and ~5-fold greater ACTH-induced MC2R expression compared to TCCC haplotype, as shown by luciferase reporter and RT-PCR assays, providing a molecular basis for variable ACTH responsiveness.\",\n      \"method\": \"Luciferase reporter assay; real-time quantitative RT-PCR of MC2R cDNA expression driven by TCCT vs TCCC promoters in presence and absence of ACTH\",\n      \"journal\": \"Pharmacogenetics and genomics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional reporter and expression assays with two orthogonal methods, single lab\",\n      \"pmids\": [\"20042918\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"A homozygous Y254C mutation in the third extracellular loop of MC2R causes isolated glucocorticoid deficiency, consistent with disruption of ligand binding at this position.\",\n      \"method\": \"Direct sequencing of MC2R gene; family analysis confirming autosomal recessive inheritance; functional inference from receptor structure\",\n      \"journal\": \"The Journal of clinical endocrinology and metabolism\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — genetic identification with structural inference, no direct functional assay in this paper\",\n      \"pmids\": [\"7608277\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Ginsenoside Rd inhibits ACTH-induced corticosterone biosynthesis by blocking the MC2R-cAMP/PKA/CREB signaling pathway and downregulating MC2R and MRAP expression in adrenocortical Y1 cells.\",\n      \"method\": \"cAMP content measurement; PKA activity assay; Western blot and qPCR for MC2R and MRAP in Y1 cells treated with ACTH ± ginsenoside Rd\",\n      \"journal\": \"Life sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple biochemical readouts of the MC2R-cAMP/PKA/CREB pathway in single cell system\",\n      \"pmids\": [\"31972205\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"In rat bone marrow stromal cells expressing MC2R and MRAP, ACTH via MC2-R signaling promotes chondrogenic nodule formation and induces transient elevations in intracellular calcium; neither α-MSH (MC5-R agonist) nor γ2-MSH (MC3-R agonist) replicate these effects, defining MC2-R as the specific mediator.\",\n      \"method\": \"MC-R-specific peptide agonists; calcium flux assays; immunoblot of membrane fractions; chondrogenic nodule formation assay; dexamethasone modulation of MC2-R and MRAP expression\",\n      \"journal\": \"Cell and tissue research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — receptor-specific pharmacological dissection with multiple readouts, single lab\",\n      \"pmids\": [\"23358747\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"In human prostate cancer cell lines, ACTH acting via MC2R induces cAMP production, increases androgen receptor nuclear localization, and promotes concentration-dependent cell proliferation, with the effect partially blocked by a non-selective MCR antagonist (SHU9119).\",\n      \"method\": \"MTT cell proliferation assay; cAMP production assay; immunocytochemistry for AR nuclear localization; MCR antagonist (SHU9119) inhibition\",\n      \"journal\": \"International journal of oncology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple functional readouts with pharmacological antagonist validation, single lab\",\n      \"pmids\": [\"22842514\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Activation of tetrapod MC2R orthologs (human, Anolis carolinensis, Xenopus tropicalis) requires both the H6F7R8W9 (HFRW) pharmacophore and the K15K16R17R18P19 motifs of ACTH(1-24); alanine substitution at these positions has more severe effects on non-mammalian MC2Rs than on human MC2R, pointing to conserved but differentially tuned dual binding sites on the receptor.\",\n      \"method\": \"Alanine-substituted ACTH(1-24) analog stimulation; cAMP assays in cells coexpressing tetrapod MC2R orthologs with MRAP1\",\n      \"journal\": \"General and comparative endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — systematic alanine scan of ligand motifs with functional cAMP readout across multiple orthologs\",\n      \"pmids\": [\"23639234\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"The second extracellular loop (EC2)/TM5 region of MC2R is a contact site for the K/KKRRP motif of ACTH; specifically V159 in TM4, F171 in TM5, and F175 in TM5 are critical for receptor activation—triple alanine substitution at these positions completely abolishes ACTH-stimulated activation without impairing receptor trafficking to the plasma membrane.\",\n      \"method\": \"Single and triple alanine mutagenesis of rainbow trout MC2R; cAMP luciferase reporter assay; cell surface ELISA for trafficking\",\n      \"journal\": \"General and comparative endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis with functional and trafficking readouts, single lab\",\n      \"pmids\": [\"28495271\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"ACTH directly stimulates MC2R in brown adipocytes to promote thermogenesis; glucocorticoids act permissively to enhance ACTH-mediated energy expenditure via MC2R, while also separately driving cold-induced hyperphagia.\",\n      \"method\": \"In vitro brown adipocyte assays; in vivo cold exposure model; MC2R-specific signaling readouts\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — preprint with in vitro and in vivo evidence but limited mechanistic detail in abstract\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"MC2R (ACTH receptor) is a G protein-coupled receptor that is selectively activated by ACTH to generate cAMP via adenylyl cyclase, driving steroidogenesis; its functional expression at the cell surface requires MRAP1 (which is essential for ACTH binding and signaling but not for basal plasma membrane targeting), while intracellular retention signals in TM3/TM4 restrict its surface expression in non-adrenal cells; ACTH binding involves dual pharmacophore contacts (HFRW and K/KKRRP motifs of ACTH engaging TM2/TM3/TM6 and EC2/TM4/TM5 of MC2R respectively); upon agonist stimulation, MC2R undergoes PKA-dependent desensitization and GRK/clathrin/dynamin-dependent internalization followed by Rab4/5/11-mediated recycling; downstream signaling proceeds through cAMP/PKA to activate both steroidogenic gene transcription and MAPK (p44/42, p38) phosphorylation; adrenal-specific transcription is coordinated by SF-1/NR5A1, AP-1, FOXL2, PPARγ/RXRα (in adipocytes), and JDP2 acting on distinct promoter elements.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"MC2R is a Gs-coupled receptor uniquely selective for ACTH that drives cAMP/PKA-dependent steroidogenesis and MAPK signaling in adrenocortical cells. Functional cell-surface expression of MC2R requires the accessory protein MRAP1, which overcomes intrinsic ER-retention signals encoded in TM3/TM4 and the N-terminus; MRAP1 isoforms differentially regulate receptor density and maximal signaling capacity [PMID:17456795, PMID:20206229]. ACTH engages MC2R through dual pharmacophore contacts — the HFRW motif interacting with TM2/TM3/TM6 and the KKRRP motif contacting the second extracellular loop and TM4/TM5 — and stimulation triggers PKA-dependent desensitization, GRK/clathrin/dynamin-dependent internalization, and Rab4/5/11-mediated recycling [PMID:21920850, PMID:28495271, PMID:23639234]. Adrenal-specific MC2R transcription is governed by SF-1 and AP-1 elements required for basal and cAMP-induced expression, with additional regulation by FOXL2, PPARγ/RXRα in adipocytes, and JDP2 via CRE sites [PMID:10811295, PMID:15028712, PMID:20650879, PMID:28146118].\",\n  \"teleology\": [\n    {\n      \"year\": 1995,\n      \"claim\": \"Establishing that MC2R mutations are causal in familial glucocorticoid deficiency linked the receptor directly to adrenal ACTH responsiveness in humans, though without functional proof of the mutation's biochemical effect.\",\n      \"evidence\": \"Homozygous Y254C mutation identified by direct sequencing in a family with isolated glucocorticoid deficiency\",\n      \"pmids\": [\"7608277\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No functional assay performed for Y254C; causality inferred from segregation and structural position only\", \"No binding or signaling data for this mutation\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Defining the transcriptional architecture of the MC2R promoter resolved how adrenal-specific and cAMP-inducible expression are achieved, identifying SF-1 and AP-1 as essential elements.\",\n      \"evidence\": \"Promoter deletion, EMSA, site-directed mutagenesis, and luciferase reporter assays in Y1, JEG3, and Cos-1 cells\",\n      \"pmids\": [\"10811295\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Chromatin-level regulation and histone marks at the MC2R locus not addressed\", \"In vivo validation of promoter element requirements not performed\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Dissecting desensitization and internalization pathways revealed that MC2R desensitization is PKA-dependent whereas internalization is GRK-dependent, and established that a then-unknown cofactor was needed for heterologous surface expression.\",\n      \"evidence\": \"PKA inhibitor studies and GRK-dependent internalization assays in Y1 and Y6 adrenocortical cells\",\n      \"pmids\": [\"12851305\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Identity of the cofactor (later MRAP) was unknown at this stage\", \"GRK isoform specificity not determined\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Parallel studies expanded understanding of MC2R regulation: (1) intramolecular epistasis between two loss-of-function mutations producing constitutive activity revealed conformational constraints on receptor activation; (2) PPARγ/RXRα-dependent transcription defined an SF-1-independent adipocyte-specific mechanism; (3) an E-box repressor element further explained tissue-restricted expression.\",\n      \"evidence\": \"Constitutive activity measured by cAMP accumulation with double-mutant receptor in Y6 cells; PPRE mutagenesis and PPARγ2/RXRα cotransfection in 3T3-L1 adipocytes; E-box/AP-4 identified by EMSA and promoter reporter in granulosa vs adrenocortical cells\",\n      \"pmids\": [\"15062562\", \"15028712\", \"15171714\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Structural basis for C21R/S247G intramolecular synergy unknown\", \"In vivo relevance of PPARγ-driven MC2R in adipose tissue not tested\", \"AP-4 binding to E-box not confirmed by ChIP\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Identification of MRAP as the essential cofactor for ACTH binding resolved the long-standing puzzle of why MC2R was non-functional in non-adrenal cells, and showed that MRAP isoforms differentially tune receptor surface density and signaling capacity.\",\n      \"evidence\": \"ACTH binding assays, cAMP production, and immunofluorescence in isogenic HEK293/Flp cells stably expressing MC2R with MRAPα or MRAPβ\",\n      \"pmids\": [\"17456795\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of MRAP–MC2R interaction not resolved\", \"Whether MRAP isoform ratio varies across adrenal zones not determined\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Systematic chimeric receptor analysis mapped intracellular retention signals to TM3/TM4 and the N-terminus, and ACTH selectivity determinants to TM4/TM5 and EC2, defining how MC2R structure encodes both trafficking restriction and ligand discrimination.\",\n      \"evidence\": \"15 MC2R/MC4R chimeras assessed by confocal microscopy, ACTH binding, and cAMP response ± MRAP coexpression\",\n      \"pmids\": [\"20206229\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic-level contacts between MRAP and the retention signal not resolved\", \"Contribution of individual residues within TM3/TM4 to retention not mapped\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Downstream of cAMP, MC2R was shown to activate p44/42 and p38 MAPK via a cAMP/PKA-dependent rather than Epac- or arrestin-dependent route, establishing the signaling logic downstream of the receptor.\",\n      \"evidence\": \"PKA inhibitors, selective Epac activator, dominant-negative arrestin, recycling inhibitors, and phospho-MAPK immunoblots in HEK293 and primary human fasciculata cells\",\n      \"pmids\": [\"21195128\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct PKA substrates linking to MAPK cascade not identified\", \"Whether MAPK activation drives specific steroidogenic gene programs through MC2R remains unclear\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Loss-of-function C-terminal truncation mutations (K289fs, M290X) showed that the MC2R C-terminus is required for ER-to-plasma membrane transport but not for MRAP binding, separating trafficking from accessory protein interaction.\",\n      \"evidence\": \"Co-immunoprecipitation, cell-surface ELISA, confocal microscopy, and cAMP reporter in OS3 cells\",\n      \"pmids\": [\"20962024\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Specific trafficking machinery interacting with MC2R C-terminus not identified\", \"Only two mutations tested; generalizability of C-terminal role uncertain\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Mapping intracellular Ser/Thr residues governing internalization and recycling defined the receptor's post-activation trafficking itinerary through Rab4/5/11-positive compartments, revealing how MC2R/MRAP complexes are dynamically regulated.\",\n      \"evidence\": \"Site-directed mutagenesis of seven Ser/Thr residues combined with Rab GTPase colocalization and cAMP assays in HEK293/FRT cells\",\n      \"pmids\": [\"21920850\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Kinase(s) phosphorylating specific Ser/Thr residues not identified beyond GRK\", \"Fate of MRAP during recycling not tracked\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"MRAP isoform-specific localization was resolved: MRAPα concentrates at endosomes and the nuclear envelope while MRAPβ strongly localizes to the plasma membrane, explaining their differential effects on MC2R signaling output.\",\n      \"evidence\": \"Confocal localization, topology assays, and cAMP accumulation in HEK293/FRT and B16-G4F cells\",\n      \"pmids\": [\"22366472\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Determinants of differential MRAP isoform sorting not identified\", \"Dual topology of MRAPβ not mechanistically explained\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Alanine scanning of ACTH and mutagenesis of MC2R orthologs defined dual pharmacophore contacts: HFRW engages one receptor region while KKRRP contacts EC2/TM4/TM5 residues (V159, F171, F175), with triple alanine substitution abolishing activation without affecting trafficking.\",\n      \"evidence\": \"Systematic alanine substitution of ACTH(1-24) tested on human, reptile, and amphibian MC2Rs; single and triple alanine mutagenesis of rainbow trout MC2R with cAMP reporter and cell-surface ELISA\",\n      \"pmids\": [\"23639234\", \"28495271\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No co-crystal or cryo-EM structure of MC2R–ACTH–MRAP complex available\", \"Precise contacts of the HFRW motif on the receptor not mapped at residue resolution for mammalian MC2R\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"JDP2 was identified as a transcriptional activator of MC2R through CRE elements, with its activity regulated by phosphorylation and SUMOylation, adding a stress-responsive transcriptional layer to MC2R regulation.\",\n      \"evidence\": \"ChIP, luciferase reporter, CRE mutagenesis, and phosphorylation/SUMOylation mutants of JDP2 in Y1 cells\",\n      \"pmids\": [\"28146118\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"In vivo relevance of JDP2 to MC2R expression in adrenal not demonstrated\", \"Whether JDP2 cooperates with SF-1/AP-1 at the endogenous promoter not tested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"A high-resolution structure of the MC2R–MRAP–ACTH signaling complex is lacking, and the precise mechanism by which MRAP overcomes ER retention signals, the in vivo roles of MC2R in extra-adrenal tissues (adipose, bone, prostate), and the kinase specificity for trafficking-regulatory phosphosites remain undefined.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No cryo-EM or crystal structure of MC2R–MRAP complex\", \"In vivo validation of extra-adrenal MC2R functions (thermogenesis, chondrogenesis, prostate proliferation) lacking\", \"Complete phosphosite-to-kinase mapping for internalization/recycling residues not performed\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 1, 3, 19, 20]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [3, 16]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 1, 2, 6, 7]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [1, 6]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [2, 7]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 3, 4, 16, 19]},\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [9, 10, 11, 13]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [3, 16]}\n    ],\n    \"complexes\": [\n      \"MC2R–MRAP1 receptor complex\"\n    ],\n    \"partners\": [\n      \"MRAP\",\n      \"NR5A1\",\n      \"FOXL2\",\n      \"PPARG\",\n      \"RXRA\",\n      \"JDP2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}