GPR124 regulates murine brain embryonic angiogenesis and BBB formation by an intracellular domain-independent mechanism.

The GPR124/RECK/WNT7 pathway is an essential regulator of CNS angiogenesis and blood-brain barrier (BBB) function. GPR124, a brain endothelial adhesion 7-pass transmembrane protein, associates with RECK, which binds and stabilizes newly synthesized WNT7, which is transferred to Frizzled (FZD) to initiate canonical b-catenin signaling. GPR124 remains enigmatic; while its extracellular domain (ECD) is essential, the poorly conserved intracellular domain (ICD) appears variably required in mammals versus zebrafish, potentially via adaptor protein bridging of GPR124/FZD ICDs. GPR124 ICD deletion impairs zebrafish angiogenesis, but paradoxically retains WNT7 signaling upon mammalian transfection. We thus investigated GPR124 ICD function by mouse deletion (Gpr124ΔC). Despite inefficiently expressed GPR124ΔC protein, Gpr124ΔC/ΔC mice could be born with normal cerebral cortex angiogenesis, versus Gpr124-/- embryonic lethality, forebrain avascularity and hemorrhage. Gpr124ΔC/ΔC vascular phenotypes were restricted to sporadic ganglionic eminence angiogenic defects, attributable to impaired GPR124ΔC protein expression. Further, Gpr124ΔC and recombinant GPR124 ECD rescued WNT7 signaling in culture upon brain endothelial Gpr124 knockdown. Thus, in mice, GPR124-regulated CNS forebrain angiogenesis and BBB function is exerted by ICD-independent functionality, extending the signaling mechanisms used by adhesion 7-pass transmembrane receptors.