Molecular and functional aspects of astrocytes at the brain-blood interface

Abstract

This thesis is comprised of four papers that make up my doctoral degree thesis at the University of Oslo (UiO). These papers focus on the molecular and functional aspects of the brain-blood interface, with special attention on the astrocytes, the predominant glial cell in the brain. Water is essential for life, and water dyshomeostasis is a hallmark of many diseases that affect the central nervous system (CNS). Astrocytic processes at the brainblood and the brain-liquor interfaces - coined endfeet - exhibit a dense expression of aquaporin-4 (AQP4) water channels. This thesis aims to provide a deeper insight into the roles of astrocytic endfeet in brain water transport and signaling. In the first study (Paper I), we provided evidence that pericytes regulate AQP4 anchoring to perivascular astrocytic endfoot membranes. In the second study (Paper II), we generated and characterized a glial-specific Aqp4 knockout mouse line, in which the Aqp4 gene is deleted specifically from astrocytes and ependymal cells. By characterizing this mouse line and comparing it with controls, we provided evidence that endothelial cells are devoid of AQP4. We found that deleting Aqp4 from glial membranes reduced blood-to-brain water uptake as well as clearance of brain interstitial water. We concluded that the astrocytic endfoot sheath can serve as a barrier for water transport. In the third study (Paper III), we investigated whether global and targeted removal of AQP4 from perivascular and ependymal membranes affected basal brain water content. We found that only global Aqp4 deletion increased brain water content. Measurements of intracranial pressure during intracisternal infusion of tracer in wildtype and Aqp4 mutant mice suggested that Aqp4 gene deletion does not compromise extracerebral drainage pathways. Finally, our last study (Paper IV) showed that astrocytes in acute hippocampal slices respond to Schaffer collateral stimulation with Ca2+ elevations. The stimulation-evoked Ca2+ signals were modulated by the intensity of the stimulation. Experiments in IP3R2 mutant mice revealed that the astrocytic Ca2+ response was dependent on Ca2+ release from internal stores. Through these four projects, we have learned that astrocytes are important for the regulation of brain water, that there are different levels of involvement for the different pools of astrocytic water channels, and that astrocytic Ca2+ signaling occurs in response to neuronal signaling through release from internal stores – possibly triggered by cell swelling. Still, more studies are needed to unravel molecular targets in glia for novel treatment of neurological disorders with water dyshomeostasis.