ADGRA2

ADGRA2 (GPR124/TEM5) is an endothelial adhesion GPCR that functions as a cell-autonomous regulator of CNS-specific angiogenesis and blood-brain barrier formation, as established by global and endothelial-specific knockout mice that die embryonically with defective forebrain and spinal cord vascularization, failed vessel invasion of the neuroepithelium, and loss of barrier markers including Glut-1 (PMID:21421844). Its CNS angiogenic role is mediated through canonical Wnt signaling: ADGRA2 acts as a co-receptor that, together with FZD5, FZD8, LRP6, and RECK, drives WNT7B-mediated synergistic β-catenin signaling downstream of β-catenin stabilization, correlating with increased β-catenin acetylation (PMID:28289266). ADGRA2 and RECK traffic to the plasma membrane independently and meet at the cell surface, with the leucine-rich repeat domain required for correct receptor trafficking (PMID:27979830, PMID:27979884). At the membrane, ADGRA2 promotes adhesion and cytoskeletal remodeling by coupling Gβγ to a Rho-GEF module: it forms direct complexes with Elmo/Dock and intersectin-1, activates Rac and Cdc42, and concentrates with phospho-Elmo and ITSN1 at lamellipodia to direct polarity during migration (PMID:28600358). ADGRA2 also mediates Rac-dependent contact inhibition of endothelial proliferation (PMID:19853600) and is required for VEGF-induced tumor angiogenesis (PMID:24730523). The receptor is proteolytically shed by MMP-9 and by thrombin—the latter requiring cell-surface protein disulfide-isomerase—to expose a cryptic RGD motif that engages integrin αvβ3 and supports survival of growth-factor-deprived endothelial cells (PMID:16982628, PMID:22013897). Its C-terminal PDZ-binding motif recruits the scaffold hDlg to the membrane (PMID:15021905).

1. 2004 Medium

   Established that ADGRA2 has an intracellular scaffolding output, linking the receptor's C-terminus to a membrane-associated guanylate kinase scaffold.

   Evidence Direct pulldown of hDlg PDZ domains by the TEM5 C-terminal PDZ-binding motif plus co-localization in embryonic liver endothelium

   PMID:15021905

   Open questions at the time

   • No reciprocal Co-IP or mutagenesis of the PDZ motif
   • Functional consequence of hDlg scaffolding not defined
2. 2006 High

   Defined an integrin-coupled extracellular function, showing that proteolytic shedding converts ADGRA2 into a pro-survival ligand via a cryptic RGD motif.

   Evidence In vitro capillary assays, MMP-9 shedding, recombinant binding, function-blocking αvβ3 antibody, survival assays

   PMID:16982628

   Open questions at the time

   • Relationship between shed-fragment signaling and full-length receptor signaling unresolved
   • In vivo relevance of αvβ3 engagement not demonstrated
3. 2009 Medium

   Connected ADGRA2 to GTPase control of endothelial growth, identifying Rac as both an inducer of its expression and an effector of its contact-inhibition function.

   Evidence Capillary morphogenesis assays with GTPase inhibitors and antibody/soluble-domain blockade, proliferation assays

   PMID:19853600

   Open questions at the time

   • Molecular link between receptor and Rac not yet defined at this stage
   • Pharmacological inhibitor specificity limits conclusions
4. 2011 High

   Established the cell-autonomous, organ-specific physiological role, showing ADGRA2 is essential for CNS angiogenesis and BBB formation.

   Evidence Global and endothelial-specific knockout mice with histology, immunostaining, and barrier-marker readouts

   PMID:21421844

   Open questions at the time

   • Did not identify the ligand or downstream signaling pathway
   • Mechanism of CNS specificity not defined
5. 2012 High

   Refined the shedding mechanism, identifying thrombin as a protease and cell-surface PDI as a regulator that controls RGD exposure.

   Evidence In vitro thrombin cleavage of recombinant TEM5, cell-based shedding with PDI inhibitor/activator, disulfide bond analysis

   PMID:22013897

   Open questions at the time

   • In vivo contribution of thrombin/PDI-dependent shedding unknown
   • How PDI accesses the heterodimer disulfide not structurally resolved
6. 2014 Medium

   Extended ADGRA2 function to pathological angiogenesis, showing it is required for VEGF-driven tumor vessel formation.

   Evidence siRNA knockdown in human endothelial cells, xenograft tumor model, in vitro angiogenesis assays

   PMID:24730523

   Open questions at the time

   • Mechanistic link between ADGRA2 and VEGF signaling not defined
   • Single lab, knockdown-based
7. 2016 Medium

   Assigned the ouchless phenotype to adgra2 and showed the LRR domain governs receptor trafficking independently of RECK.

   Evidence ENU splice allele characterization, genetic complementation, CRISPR cell lines, subcellular localization assays in zebrafish and cells

   PMID:27979830 PMID:27979884

   Open questions at the time

   • How the LRR mediates trafficking mechanistically unknown
   • Where/how ADGRA2 and RECK assemble at the surface not resolved
8. 2017 High

   Defined the receptor's proximal signaling output, showing Gβγ-dependent coupling to Elmo/Dock and ITSN1 Rho-GEFs activates Rac and Cdc42 for adhesion and polarity.

   Evidence Reciprocal endogenous Co-IP, GTPase activation assays, dominant-negative fragment competition, Elmo phosphorylation, lamellipodial co-localization, wound healing

   PMID:28600358

   Open questions at the time

   • Activating ligand for the Gβγ output not identified
   • Integration with the Wnt co-receptor function unclear
9. 2017 Medium

   Placed ADGRA2 within canonical Wnt signaling, showing it acts with FZD/LRP6/RECK as a co-receptor for WNT7B-driven β-catenin activation.

   Evidence β-catenin luciferase reporters, siRNA and dominant-negative receptor requirement tests, multi-cell-type co-expression, β-catenin acetylation biochemistry

   PMID:28289266

   Open questions at the time

   • Functional significance of β-catenin acetylation not established
   • How co-receptor assembly is spatially organized unknown

• How the Wnt co-receptor activity and the Gβγ/Rho-GEF adhesion output are mechanistically integrated, and what physiological agonist engages the receptor, remain open.
• No structural model of the receptor or its co-receptor complex
• Endogenous activating ligand for the GPCR signaling arm undefined
• Cross-talk between shedding, Wnt, and Rho-GEF arms unresolved