Pericyte and Endothelial Cell Responses within Murine Cerebral Capillaries After Blood Flow Cessation

ABSTRACT/SUMMARYBlood flow provides critical inputs for mechanisms governing vascular homeostasis. Altered hemodynamics can therefore trigger a wide range of cellular responses in blood vessels. Endothelial cells (ECs) downstream of atherosclerotic plaques for instance are exposed to turbulent flow, activating inflammatory pathways that promote immune cell infiltration. In conditions like stroke and myocardial infarction, the abrupt loss of blood flow prompts responses in vascular cells such as ECs and pericytes (PCs) to adapt to ischemic or no-flow conditions. To better understand how cerebral capillary ECs and PCs react to the sudden loss of blood flow, we used a murine brain slice model cultured for 12- and 24-hours in artificial cerebrospinal fluid (aCSF) with 95% oxygen supplementation. As expected, inflammation mediators were upregulated in cultured slices compared to non-cultured samples, particularly those associated with leukocyte recruitment. Additionally, transcriptional markers of extracellular matrix (ECM) remodeling and cell-ECM interactions were elevated, consistent with reduced PC coverage along capillaries. We initially presumed these changes reflected blood-brain barrier (BBB) degradation, but instead we found an increase in mRNA transcripts for EC junctions and stable protein levels for junction molecules, with an apparent rearrangement of Claudin5-based tight junctions. Some capillaries also exhibited reduced diameters, suggesting constriction by PCs or a subset thereof. Consistent with these observations, we found an upregulation of the vasoconstrictor Endothelin-1 (ET-1) with its receptors and contractile proteins found in a subpopulation of PCs. Suppressing ET-1 activity prevented Claudin5 upregulation, indicating that ET-1 might regulate microvascular constriction and associated changes in endothelial tight junctions. Overall, these results suggest that in the absence of blood flow, PCs contribute to capillary wall remodeling by (i) potentially mediating a mechanism driven by ET-1 that affects EC Claudin5 dynamics, and (ii) reducing capillary ECM and detaching from microvessel walls.