PROJECT SUMMARY Stroke is one of the most common causes of death and disability in the United States and worldwide. The vascular system is meticulously regulated throughout life to adapt to changes in metabolic demand and blood flow under widely variable conditions. Many ischemic stroke patients however fail to fully recover following an acute attack. This impaired recovery is related in part to the limited return of perfusion within the brain microcirculation, even after restoring the patency of occluded vessels – a scenario referred to as the “no-reflow” phenomenon. Blood circulating within the vascular system exerts different types of forces on the surrounding vessels. These forces are sensed and interpreted by the vascular cells to guide their development during embryogenesis and regulate remodeling during postnatal and adult life. It has been also suggested in recent years that there are signals downstream of mechanical changes that are exchanged between vascular cells. Specifically, pericytes and endothelial cells integrate these cues to dynamically regulate blood vessel physiology, capillary permeability, and changes in microvascular tone in health and in disease. Despite recent advances in our knowledge of flow-mediated biomechanical inputs, the underlying molecular processes and their link to hemodynamic forces in vivo are still emerging, in part due to limitations in the tools and models to measure these forces. To help fill this gap in knowledge, the proposed study aims to investigate the impact of abrupt changes in blood flow on two components of the blood-brain barrier -- pericytes and endothelial cells -- and their interaction in mature brain vessels under static conditions following the loss of flow. We will utilize both ex vivo and in vivo models to establish the mechanistic interactions underlying how pericytes and endothelial cells process, interpret, and organize various mechanical signals. Additionally, we will look at corresponding changes in the surrounding extracellular matrix that might accompany this cellular interplay, specifically interactions between endothelial cell integrin α5 and pericyte-derived vitronectin within the capillary wall. Our preliminary data suggests a two-phase response over time following an acute shift towards static conditions. We propose that an early stage marked by a rapid inflammatory response, involving elevated interleukin-1beta expression, is overlaid by a hypoxia-driven response in a subsequent phase, both contributing to cerebrovascular instability and an increased risk for hemorrhagic conversion of ischemic stroke patients after re-establishing cerebral blood flow. Identifying the key mechanistic determinants responsible for blood vessel destabilization in the brain during the hyper-acute phase of stroke will provide targetable signals that could be clinically significant in advancing stroke therapies.