Capillary control of cerebral blood flow, and its disruption in small vessel disease

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Summary Neurons lack energy reserves and thus their survival depends on an uninterrupted, dynamically regulated supply of blood-borne nutrients, which are delivered through a dense capillary network. Precise control of the blood flow through the brain microcirculation is therefore essential for neuronal health. However, the mechanisms through which blood is distributed within the capillary network remains poorly understood. Furthering our understanding of this process is critical, as it is increasingly appreciated that disruption of brain hemodynamics is one of the earliest pathological events in cerebral small vessel diseases. Pericytes are mural cells that wrap around the endothelial cells forming the capillaries. Our extensive preliminary data show for the first time that pericytes located on the first to fourth order capillary branches constrict and relax in response to luminal pressure changes. This observation implies that the resistance created by the capillary network is not constant and homogeneous, but rather variable and dynamic, casting a new light on blood flow regulation. Specifically, we have found that pressure-induced constriction in pericytes engages the autocrine activation of the epidermal growth factor receptor (EGFR), subsequent inositol trisphosphate (IP3) signaling, and transient receptor potential canonical 3 (TRPC3) activation. Using a well-established genetic mouse model of CADASIL, a hereditary form of small vessel disease, we further propose that pathogenic mechanisms depress the EGFR activation in pericytes, resulting in impaired capillary blood flow autoregulation. To test these ideas, we engage a wide variety of novel, state-of-the-art experimental approaches using intact animals, native tissue, and freshly isolated cells, complemented by sophisticated computational modeling. Taking advantage of our newly developed pressurized arteriole- capillary ex vivo preparation, Aim 1 will explore how EGFR activation by intraluminal pressure and agonist-induced vasoconstriction contributes to γ1 phospholipase C (PLCγ1) activation and IP3- dependent Ca2+ signals. Aim 2 will determine the mechanism linking EGFR and PLCγ1 activation to TRPC3 channel opening to cause membrane depolarization and constriction. Finally, using extracellular matrix disruptions characteristic of CADASIL as a framework, Aim 3 will provide the first insights into the mechanisms by which pericyte contractility is regulated by EGFR and its upstream regulators TIMP3, a matrix metalloproteinase inhibitor, and ADAM17, a metalloproteinase that mediates shedding of the EGFR ligand, HB-EGF. The proposed work has the potential to provide a paradigm-shifting view on how pericytes control capillary blood flow distribution, and as such, should provide the foundation for understanding small vessel diseases of the brain.