Every organ in the body requires blood vessels for efficient delivery of oxygen and nutrients, but independent vascular beds are highly specialized to meet the individual needs of specific organs. The vasculature of the brain is tightly sealed, with blood–brain barrier (BBB) properties developing coincident with neural vascularization. G protein-coupled receptor 124 (GPR124) (tumor endothelial marker 5, TEM5), an orphan member of the adhesion family of G protein-coupled receptors, was previously identified on the basis of its overexpression in tumor vasculature. Here, we show that global deletion or endothelial-specific deletion of GPR124 in mice results in embryonic lethality associated with abnormal angiogenesis of the forebrain and spinal cord. Expression of GPR124 was found to be required for invasion and migration of blood vessels into neuroepithelium, establishment of BBB properties, and expansion of the cerebral cortex. Thus, GPR124 is an important regulator of neurovasculature development and a potential drug target for cerebrovascular diseases.

Tumor endothelial marker (TEM)8 was uncovered as a gene expressed predominantly in tumor endothelium, and its protein product was recently identified as the receptor for anthrax toxin. Here, we demonstrate that TEM8 protein is preferentially expressed in endothelial cells of neoplastic tissue. We used the extracellular domain of TEM8 to search for ligands and identified the α3 subunit of collagen VI as an interacting partner. The TEM8-interacting region on collagen α3(VI) was mapped to its COOH-terminal C5 domain. Remarkably, collagen α3(VI) is also preferentially expressed in tumor endothelium in a pattern concordant with that of TEM8. These results suggest that the TEM8/C5 interaction may play an important biological role in tumor angiogenesis.

G protein-coupled receptor 124 (GPR124) is an orphan receptor in the adhesion family of GPCRs, and previous global or endothelial-specific disruption of Gpr124 in mice led to defective CNS angiogenesis and blood-brain barriergenesis. Similar developmental defects were observed following dual deletion of Wnt7a/Wnt7b or deletion of β-catenin in endothelial cells, suggesting a possible relationship between GPR124 and canonical WNT signaling. Here, we show using in vitro reporter assays, mutation analysis, and genetic interaction studies in vivo that GPR124 functions as a WNT7A/WNT7B-specific costimulator of β-catenin signaling in brain endothelium. WNT7-stimulated β-catenin signaling was dependent upon GPR124's intracellular PDZ binding motif and a set of leucine-rich repeats in its extracellular domain. This study reveals a vital role for GPR124 in potentiation of WNT7-induced canonical β-catenin signaling with important implications for understanding and manipulating CNS-specific angiogenesis and blood-brain barrier-genesis.

Antiangiogenic agents that block vascular endothelial growth factor (VEGF) signaling are important components of current cancer treatment modalities but are limited by alternative ill-defined angiogenesis mechanisms that allow persistent tumor vascularization in the face of continued VEGF pathway blockade. We identified prostaglandin E₂ (PGE₂) as a soluble tumor-derived angiogenic factor associated with VEGF-independent angiogenesis. PGE₂ production in preclinical breast and colon cancer models was tightly controlled by cyclooxygenase-2 (COX-2) expression, and COX-2 inhibition augmented VEGF pathway blockade to suppress angiogenesis and tumor growth, prevent metastasis, and increase overall survival. These results demonstrate the importance of the COX-2/PGE₂ pathway in mediating resistance to VEGF pathway blockade and could aid in the rapid development of more efficacious anticancer therapies.