SUMMARY AND ABSTRACT Astrocytes are the most diverse glial cell type and maintain essential interactions with endothelial cells to form the blood-brain barrier (BBB). The BBB plays a major role in CNS homeostatic function, while dysregulation of the astrocytic BBB contributes to a spectrum of neurological disorders, ranging from neurodevelopmental and degenerative diseases to CNS injury and malignancy. Particularly, pathologic astrocyte-BBB interactions contribute to ischemic stroke, which is the 5th leading cause of death in the U.S. However, the mechanisms underlying maintenance of BBB integrity, both in health and in diseases such as stroke, remain poorly defined. The overarching goal of this proposal is to elucidate novel targets and associated signaling pathways influencing normal astrocyte-BBB function and to leverage this knowledge to define key mechanisms for BBB disruption after ischemic cerebral stroke. This proposal focuses on an astrocyte-enriched sodium-bicarbonate cotransporter 1, Slc4a4, which was previously identified as a glial-specific regulator of both intracellular and extracellular pH. While pH homeostasis is essential for brain function and patients carrying Slc4a4 variants can suffer ischemic stroke, a regulatory role of Slc4a4 in astrocyte-BBB integrity remains unknown. To begin addressing this knowledge gap, we generated new transgenic mouse lines that temporally ablate Slc4a4 in astrocytes. Using this genetic mouse model, we showed that loss of Slc4a4 significantly reduces astrocytic morphological complexity and generates enlarged blood vessels with disrupted endothelial junctions. Our profiling analyses of astrocytic Slc4a4-deficient mice and conditioned media of Slc4a4 ablated astrocytes revealed increased CCL2 production and secretion coupled with dysregulated arginine-nitro oxide (NO) metabolism, further supporting a crucial role for Slc4a4 in astrocyte-BBB integrity. Using an ischemic stroke mouse model, we found loss of Slc4a4 exacerbates stroke-induced motor dysfunction and increases infarct area coupled with impaired reactive gliosis which results in BBB disruption, which is rescued by inhibition of CCL2 in vivo. Based on these preliminary data, our central hypothesis is that Slc4a4 functions to maintain astrocyte-BBB interactions and prevent progressive BBB leakage in ischemic stroke, in part by inhibiting the chemokine CCL2. To address our hypothesis, we will first determine how Slc4a4 regulates astrocyte morphology and physiology in the adult brain (Aim 1). Second, we will determine the role of CCL2 in Slc4a4-dependent BBB maintenance in the adult brain (Aim 2). Upon completion, these studies will establish whether and how Slc4a4-deficient astrocytes influence anatomical and physiological properties of astrocytes, and will elucidate how Slc4a4 regulates astrocytic modulation of endothelial cell and BBB integrity via NO metabolism that drive astrocytic CCL2 signaling in the adult brain. Lastly, we will define the Slc4a4 pathway in the maintenance of BBB integrity following ischemic stroke (Aim 3). Together, elucidating the Slc4a4 pathway could reveal fundamental mechanisms controlling astrocyte morphology, physiology, and secreted molecules, and it will advance our understanding of astrocyte-BBB interactions after stroke.

PROJECT NARRATIVE This project will reveal the critical roles of astrocyte-endothelial cell communication in the normal brain and in ischemic stroke by determining how the dynamics of astrocyte anatomy, physiology, and paracrine signaling regulate blood-brain barrier function through molecular, cellular, and physiological mechanisms.