mTORC1 signaling in brain endothelial progenitors contributes to CCM pathogenesis in a Pdcd10-deficient mouse model

Cerebral vascular malformations (CCM) are primarily found within brain, where they result in increased risk for stroke, seizures and focal neurological deficits. The unique feature of the brain vasculature is the blood-brain barrier (BBB) formed by the brain neurovascular unit, and recent studies suggest that loss of CCM genes causes disruptions of BBB integrity as the inciting event for the development of CCM. CCM lesion is proposed to be initially derived from a single clonal expansion of a subset of angiogenic venous capillary endothelial cells (ECs) and respective resident endothelial progenitor cells (EPCs). However, the critical signaling in the subclass of brain EC/EPC for the CCM lesion initiation and progression are unclear. Here we employed single-cell RNA-sequencing (scRNA-seq) analyses in early stages of the brain EC-specific Ccm3 deletion (Pdcd10BECKO) microvessels and identified a unique EPC cluster with high stem cell markers but low BBB-associated genes. mTORC1, but not mTORC2, is activated in mouse and human CCM lesions and EPCs. Mechanistic studies suggest that CCM3 suppresses Cav1/caveolae-mediated cellular trafficking and complex formation of the mTORC1 signaling proteins. Genetic deficiency of Raptor (mTORC1), but not of Rictor (mTORC2), prevents CCM lesion formation in the Pdcd10BECKO model. Importantly, mTORC1 pharmacological inhibitor Rapamycin ameliorate the CCM pathogenesis in the Pdcd10BECKO mice, by suppressing lesion-forming EPC expansion while enhancing BBB maturation. Our data suggest that CCM3 is critical for maintaining BBB integrity, and CCM3 loss-induced mTORC1 signaling in brain EPCs initiate the CCM pathogenesis.