Tumor angiogenesis occurs through regulation of genes that orchestrate endothelial sprouting and vessel maturation, including deposition of a vessel-associated extracellular matrix. CD93 is a transmembrane receptor that is upregulated in tumor vessels in many cancers, including high-grade glioma. Here, we demonstrate that CD93 regulates β1 integrin signaling and organization of fibronectin fibrillogenesis during tumor vascularization. In endothelial cells and mouse retina, CD93 was found to be expressed in endothelial filopodia and to promote filopodia formation. The CD93 localization to endothelial filopodia was stabilized by interaction with multimerin-2 (MMRN2), which inhibited its proteolytic cleavage. The CD93-MMRN2 complex was required for activation of β1 integrin, phosphorylation of focal adhesion kinase (FAK), and fibronectin fibrillogenesis in endothelial cells. Consequently, tumor vessels in gliomas implanted orthotopically in CD93-deficient mice showed diminished activation of β1 integrin and lacked organization of fibronectin into fibrillar structures. These findings demonstrate a key role of CD93 in vascular maturation and organization of the extracellular matrix in tumors, identifying it as a potential target for therapy.

Regulation of vascular permeability to plasma is essential for tissue and organ homeostasis and is mediated by endothelial cell-to-cell junctions that tightly regulate the trafficking of molecules between blood and tissue. The single-pass transmembrane glycoprotein CD93 is upregulated in endothelial cells during angiogenesis and controls cytoskeletal dynamics. However, its role in maintaining homeostasis by regulating endothelial barrier function has not been elucidated yet. Here, we demonstrate that CD93 interacts with vascular endothelial (VE)cadherin and limits its phosphorylation and turnover. CD93 deficiency in vitro and in vivo induces phosphorylation of VE-cadherin under basal conditions, displacing it from endothelial cell-cell contacts. Consistent with this, endothelial junctions are defective in CD93 −/− mice, and the blood-brain barrier permeability is enhanced. Mechanistically, CD93 regulates VE-cadherin phosphorylation and turnover at endothelial junctions through the Rho/Rho kinase-dependent pathway. In conclusion, our results identify CD93 as a key regulator of VE-cadherin stability at endothelial junctions, opening up possibilities for therapeutic strategies directed to control vascular permeability.

Determining the levels of protein–protein interactions is essential for the analysis of signaling within the cell, characterization of mutation effects, protein function and activation in health and disease, among others. Herein, we describe MolBoolean – a method to detect interactions between endogenous proteins in various subcellular compartments, utilizing antibody-DNA conjugates for identification and signal amplification. In contrast to proximity ligation assays, MolBoolean simultaneously indicates the relative abundances of protein A and B not interacting with each other, as well as the pool of A and B proteins that are proximal enough to be considered an AB complex. MolBoolean is applicable both in fixed cells and tissue sections. The specific and quantifiable data that the method generates provide opportunities for both diagnostic use and medical research.