Deletion of Notch Signaling in Pericytes Results in Retinal Arteriovenous Malformations in Mice

Pericytes, mural cells of microvascular capillaries, are regulators of vascular morphogenesis. They are closely associated with the endothelium and function as mediators of angiogenesis and blood barrier integrity. Their absence or dysfunction is implicated in several vascular human pathologies such as diabetic retinopathy, Alzheimer’s disease, and arteriovenous malformations (AVMs). Notch signaling regulates vascular morphogenesis and remodeling. In contrast to endothelial cells, in which Notch signaling has been studied extensively, current understanding of the physiological contributions of the Notch pathway in pericytes is limited. To examine the function of Notch in pericyte biology, we generated a novel mouse line expressing Cre in a tamoxifen inducible fashion under the platelet-derived growth factor beta promoter, PDGFR-P2A-CreERT2. We used this mouse line to block Notch signaling (via removal of Rbpj, a transcription factor critical for Notch signaling) in pericytes. We inhibited Notch perinatally and analyzed the retinal vasculature of mice at multiple time points: postnatal day (P)5, P14, and 6 weeks. We observed that during sprouting angiogenesis at P5, mild but not significant reductions were observed in pericyte coverage and count in mutant mice (Rbpj i△PC). Vascular smooth muscle cell (vSMC) coverage, however, was significantly reduced on arteries. By P14, pericyte coverage was significantly reduced in Rbpj i△PC and pericyte absence led to enlarged blood vessels. Additionally at P14, we began to observe an interesting shift in αSMA+ cell distribution, notably the loss of αSMA+ vSMCs on arteries and an accumulation αSMA+ cells on veins. By 6-weeks, absence of Notch signaling in pericytes resulted in marked disruptions in vasculature architecture and function. Rbpj i△PC mice with deficient pericyte Notch signaling presented significantly increased vessel diameter and developed AVMs. Interestingly, retinas from Rbpj i△PC mice also exhibited abnormal αSMA+ cell distribution, and a significant reduction in pericyte coverage when compared to control littermates. Altogether, our data indicates that an absence of Notch-dependent input in pericytes results in pericyte dysfunction leading to AVMs. Further understanding of how Notch signaling in pericytes contributes to vascular stability will provide fundamental insight into pericyte biology and will help us identify new approaches to combat AVMs.