CADASIL (Cerebral Autosomal Dominant Arteriopathy with Subcortical Infarcts and Leukoencephalopathy) is the most common monogenic form of small vessel diseases (SVDs) which underlie a large portion of vascular cognitive impairment cases and a quarter of all stroke cases. Despite the identification of mutated NOTCH3 as the site of pathology in two vascular cell types, endothelial cells (ECs) and smooth muscle cells, there is currently no cure for CADASIL. Surprisingly, a third vascular cell type—pericytes—are virtually unstudied in the context of CADASIL, despite ample expression of NOTCH3 protein and known roles in the control of cerebral blood flow (CBF), which CADASIL patients exhibit early deficits in. Pericytes are found in the brain surrounding capillaries and at least two forms of pericytes exist along distinct sections of the capillary tree. Contractile pericytes, one such subtype, are located on the capillaries initially branching from the penetrating arterioles (PAs) that dive into the brain parenchyma. Contractile pericytes can contract and relax rapidly to directly regulate capillary diameter and thus modulate blood flow delivery to the downstream capillary tree. Pericyte contraction is in turn regulated by membrane potential, which ECs can modulate via transcellular communication through gap junctions that can be found in pericyte invaginations into EC membranes known as peg-and-socket junctions. Contractile pericyte hyperpolarization collectively lowers Ca2+ levels which leads to contractile pericyte relaxation and dilation of the underlying capillary. Our preliminary data indicates that contractile pericyte-mediated control of blood flow is impaired earlier than other known CBF deficits in a classically used mouse model of CADASIL, thus pericytes could be a target for early clinical intervention. Intriguingly, we observed that other mechanisms of pericyte blood flow control are intact at this early timepoint despite diminished blood flow and capillary diameter responses to EC hyperpolarization in capillaries covered in contractile pericytes. Our data strongly suggests that contractile pericytes are unable to respond to capillary electrical signaling to induce membrane hyperpolarization. As gap junctions between ECs and contractile pericytes control communication of signals between these cells, I propose to elucidate whether EC-contractile pericyte gap junctions are altered in CADASIL and the downstream ramifications of this disruption with pharmacological and optogenetic interventions in a transgenic mouse model of CADASIL. To achieve this, I will use two-photon laser scanning microscopy in awake mice, a capillary- parenchymal arteriole (CaPA) ex vivo preparation, electron microscopy, and electrophysiology, all to measure perturbations of normal contractile pericyte physiology in this disease context. This work will provide novel insight into a potential early-stage pathogenic mechanism of an early marker of CADASIL, positioning con...