Vasopressin receptor signaling — V1a/V1b Gq/11-phospholipase C coupling and the renal V2 Gs/ADCY6/cAMP/AKAP-anchored PKA pathway driving aquaporin-2 exocytosis and water reabsorption

Arginine vasopressin (AVP), a neurohypophyseal nonapeptide hormone, acts through three G protein-coupled receptors that fall into two signalling classes. The V1a receptor (AVPR1A; vascular smooth muscle, liver, platelets, brain) and the V1b receptor (AVPR1B; anterior pituitary corticotrophs) couple to the Gq/11 alpha subunits GNAQ and GNA11, stimulating phospholipase C beta, inositol trisphosphate-driven calcium release and diacylglycerol/PKC signalling, which underlie vasoconstriction and pituitary ACTH release. The V2 receptor (AVPR2), on the basolateral membrane of renal collecting-duct principal cells, couples to the stimulatory G alpha subunit GNAS, which activates adenylyl cyclase type 6 (ADCY6), the dominant inner-medullary cyclase. cAMP activates protein kinase A, which is tethered to intracellular aquaporin-2 (AQP2)-bearing vesicles by the A-kinase anchoring protein AKAP18 (AKAP7) together with the cAMP phosphodiesterase PDE4D. PKA phosphorylates AQP2 at serine 256, which is required for the regulated exocytosis of AQP2 vesicles. The vesicles move through a RAB11A-positive recycling compartment with the motor myosin Vb (MYO5B) and its Rab11 adaptor RAB11FIP2, the Rap1 GTPase-activating protein SPA-1 (SIPA1) binds AQP2 and is required for its apical delivery, and the R-SNARE VAMP2 on the vesicle fuses with the apical Q-SNAREs SNAP23 and syntaxin-3 (or syntaxin-4). AQP2 water channels in the apical membrane then let water follow the osmotic gradient out of the tubular lumen, concentrating the urine. PKA also phosphorylates and activates PDE4D on the same vesicles, providing local negative feedback, and PKA-phosphorylated CREB drives AQP2 transcription. Loss-of-function mutations in AVPR2 or AQP2 cause nephrogenic diabetes insipidus.

Derived from the RGD Pathway Ontology diagrams PW:0000491 (vasopressin signaling pathway) and PW:0000713 (its receptor type 2 branch) through the RGD_PATHWAY_ONTOLOGY project. Both PW ids are listed as evidence because the module covers both. The other PW subpathways linked from the PW:0000491 diagram are not claimed: PW:0000712 (receptor type 1 branch) has no separate RGD record in the project and its content is the V1 variant here; the generic PW:0000227 (Galphas), PW:0000229 (Galphaq) and PW:0000543 (PKA) pathways are represented by reference to modules/gpcr_camp_pka_signaling.yaml and modules/gpcr_plc_calcium_pkc_signaling.yaml. All 17 RGD-curated human genes (ADCY6 AKAP7 AQP2 AVP AVPR1A AVPR1B AVPR2 GNA11 GNAQ GNAS MYO5B PDE4D RAB11A RAB11FIP2 SIPA1 SNAP23 VAMP2) are modelled. Three proteins not on the RGD gene list are added because the RGD text names their step and the step has no enzyme otherwise: PRKACA (the PKA catalytic subunit that phosphorylates AQP2 Ser256; PKA is drawn only as a pathway node) and the syntaxins STX3 and STX4 ("syntaxin (3 or 4)" in the PW:0000713 text). Molecular functions come from each protein's experimental (or, where none exists, IBA/TAS) GO annotations in QuickGO and from the primary literature, verified against go-basic releases/2026-07-26. Receptor variant set. The root module has one shared part (the AVP ligand) and a variant_set on receptor coupling class, mirroring modules/dopamine_receptor_signaling.yaml and modules/acetylcholine_signaling.yaml. V1 receptors: AVPR1A and AVPR1B both carry IDA vasopressin receptor activity (GO:0005000) and couple via pertussis-toxin-insensitive Gq/11 to phospholipase C (PMID:8106369, PMID:7929452, PMID:9322919); GNAQ and GNA11 carry G protein activity (GO:0003925) as their reviewed core function. No cached paper assays the human V1 receptors with GNAQ or GNA11 by name, so the Gq/11 assignment rests on the PTX-insensitive PLC coupling plus the RGD diagram's Gq alpha node; downstream PLC beta, IP3/calcium and DAG/PKC are included by reference to gpcr_plc_calcium_pkc_signaling#plc_beta_activation, not re-modelled. Vasoconstriction, hepatic glycogenolysis, platelet and behavioural effects of V1a and ACTH release by V1b are physiological outputs of other cell types and are left as prose. AVPR1B can couple to cAMP at very high expression (PMID:9322919); this is treated as an overexpression effect, not a separate branch. V2 receptor: AVPR2 (GO:0005000 IDA PMID:1534149; GO:0007189 IDA) couples to GNAS (P63092, Gs alpha short form, as in the GO-CAM) and ADCY6. ADCY6 rather than the generic module's ADCY5 is the cyclase because the RGD diagram names Adcy6 and Adcy6-null mice lose dDAVP-stimulated inner-medullary cAMP (PMID:20864687). AVP as ligand, not as a pathway participant. Per the CLAUDE.md substrate/participant rule, AVP is given its molecular function only: neuropeptide hormone activity (GO:0005184, IDA; a child of hormone activity and receptor ligand activity). It is not given cellular response to vasopressin, which is the response of the target cell to AVP, nor the receptor-signalling process terms; no human AVP row in GOA carries them either. The curated GO-CAM gomodel:67e5e74400004286 models AVP (the Arg-vasopressin chain P01185-PRO_0000020515) the same way: an extracellular hormone activity that directly positively regulates AVPR2 vasopressin receptor activity and has AVPR2 as input, part of renal water absorption (GO:0070295, a physiological process rather than the signalling pathway). The module annoton uses the parent accession P01185 with that chain noted. AQP2 is a participant, not merely a PKA substrate or a vesicle cargo. AQP2 has two roles in this module and the two are treated differently. As the phosphoacceptor of PKA and the cargo of the regulated vesicles it is an input: it is listed as a PKA target and is not annotated to exocytosis or to the signalling pathway, since cargo does none of the work of vesicle transport or fusion. As the apical water channel, it does the work of the module's output step itself: water channel activity (GO:0015250, EXP/IMP; nonfunctional NDI mutants in PMID:8140421) moves water across the apical membrane, which is the transport step of renal water absorption. That passes the "does the gene do any of the work" test (it is the channel, not a substrate of one), so the AQP2 annoton is a leaf effector with GO:0015250 and process GO:0070295 renal water absorption. Comparator check: GO:0070295 in human GOA is carried by AVPR2 (IDA) and HAS2/HYAL2, while AQP2 carries renal water homeostasis (GO:0003091, IMP) and water transport (GO:0006833, IDA/IMP) but not GO:0070295. Since AQP2 is itself the channel that carries out the transport, the module asserts GO:0070295 on the AQP2 annoton and records the absence from GOA as a possible curation gap rather than evidence against the role. PKA and the AKAP18/PDE4D compartment. The generic cAMP/PKA cascade is not re-modelled: the cyclase and PKA tiers point at modules/gpcr_camp_pka_signaling.yaml by module_ref (adenylyl_cyclase_activation and pka_creb_output), and the CREB-driven transcription of AQP2 that both RGD records describe is covered by that module's PKA/CREB node rather than duplicated here. What is specific to this pathway, and therefore modelled, is the vesicle compartment: AKAP18 anchors PKA on AQP2-bearing vesicles with PDE4D (PMID:15037626, PMID:17135396), PKA phosphorylates AQP2 Ser256 to permit exocytosis (PMID:9169447) and phosphorylates PDE4D to activate it (PMID:8663227, PMID:17135396), giving local negative feedback on cAMP. Gene-product caveats: the AQP2-vesicle AKAP is the rat AKAP18delta splice variant, and human AKAP7 has two reviewed entries (O43687 alpha/beta and Q9P0M2 gamma) neither of which is the delta isoform, so the annoton uses the main human AKAP7 entry O43687 and the isoform mismatch is recorded as a knowledge gap. Its function is taken as GO:0035591 signaling adaptor activity, the core function in the AKAP7 gene review, rather than the GOA row GO:0051018 protein kinase A binding, which is a protein-binding descendant and says less about the role; the review's own evidence is for cardiac CaV1.2 anchoring, not the AQP2 vesicle, which is why the renal claim is carried by the vesicle-specific papers here. PRKACA is the PKA exemplar with cAMP-dependent protein kinase activity (GO:0004691); the RGD diagrams draw PKA only as a pathway node, so no specific catalytic subunit is named by RGD. Trafficking machinery. The recycling step is modelled as one part with four annotons: RAB11A (G protein activity GO:0003925, IDA, as in the curated FIP3-Rab11 GO-CAM), its effector RAB11FIP2 (small GTPase binding GO:0031267), the actin-based motor MYO5B (microfilament motor activity GO:0000146) and SIPA1/SPA-1 (GTPase activator activity GO:0005096, EXP), with PMID:17156409 for the myosin Vb/Rab11/Rab11-FIP2 requirement and PMID:15196935 for the SPA-1 requirement. SIPA1 was flagged for checking in the task: it is retained, because its role is directly supported (it binds AQP2, its Rap1GAP-dead mutant and constitutively active Rap1 block apical trafficking, and trafficking is impaired in Spa-1-null mice) and the activity asserted is its own experimentally established Rap1 GAP activity, not a vasopressin-specific invention. RAB11FIP2's GO:0031267 rests on IEA plus IPI protein-kinase and Rab11 interaction rows rather than a direct-assay GTPase-binding annotation, which is noted here rather than hidden. The fusion step keeps the vesicle R-SNARE VAMP2 (SNAP receptor activity GO:0005484) and the apical Q-SNAREs SNAP23 and syntaxin-3/syntaxin-4 as a variant set, with the generic zippering/fusion mechanism left to modules/snare_fusion_cycle.yaml. VAMP3 and Munc18b (PMID:18505797) are named in the literature but are not on either RGD gene list and are left out; syntaxin-3 and syntaxin-4 are included because the PW:0000713 text names "syntaxin (3 or 4)" even though neither has an RGD gene link on the diagram. Deliberately outside the module. Vasopressin biosynthesis, processing and axonal transport (modelled by the curated GO-CAM gomodel:69729a3800003304) are upstream ligand supply. Vasoconstriction, pituitary ACTH release, platelet and behavioural effects are downstream physiology of the V1 arm in other cell types. AQP3 and AQP4, the constitutive basolateral exit route for the water that AQP2 admits, are not on either RGD gene list and are mentioned only here. The calcium-dependent, cAMP-dependent but PKA-independent route to AQP2 translocation that both RGD records mention is recorded as a knowledge gap, not asserted, because neither record names the mediating gene products.