Metabolic alterations in reactive astrocytes and their roles in ischemic stroke

[EMBARGOED UNTIL 12/1/2024] Focal ischemic stroke (FIS) is a severe neurological disease that lacks effective remedy. Astrocytes are the predominant glial cell types in the central nervous system (CNS) and exert critical influences over neuronal function and neuronal viability under different pathological conditions. Accumulating evidence highlights the capacity of astrocytes to couple neuronal activity and energy metabolism and adjust their metabolic patterns in neurodegenerative disorders, including FIS. However, the precise genetic factors or pathways underlying metabolic alterations in astrocytes remain inadequately understood. Moreover, the effect of astrocytic metabolic reprogramming on neuronal survival following FIS is still obscure. Astrocytes emerge as principal consumers of glucose in the brain and utilize it to produce energy and vital metabolites through glycolysis and the pentose phosphate pathway (PPP). Nicotinamide adenine dinucleotide (NAD+) and reduced nicotinamide adenine dinucleotide phosphate (NADPH) serve as crucial cofactors in glycolysis and PPP, respectively. Nicotinamide phosphoribosyl transferase (NAMPT) and glucose-6-phosphate dehydrogenase (G6PD) play pivotal roles in mediating NAD+ and NADPH levels. However, the functions of these two enzymes in modulating astrocyte metabolism after ischemia are not clear. It also remains uncertain whether astrocytic NAMPT or G6PD could promote neuronal survival and stroke recovery. In Chapter one, I conducted an extensive review of FIS pathology, reactive astrogliosis, metabolic alterations in reactive astrocytes after brain injury and the biological functions of NAMPT and G6PD in the CNS. In Chapter two, I delved into the impacts of NAMPT-mediated NAD+ synthesis in different subcompartments on neuronal bioenergetics. Then, in Chapter three, I unveiled the roles of astrocytic NAMPT in regulating astrocyte metabolism and neuronal protection following FIS. Employing diverse methods encompassing immunostaining, western blot analysis, extracellular flux assays and inducible and conditional NAMPT knockout mice, i.e., ALDH1L1-CreERT2: NAMPTf/f mice, I found that astrocytic NAMPT effectively facilitated neuronal survival following ischemic stroke by upregulating glycolysis and GSH production within astrocytes. In Chapter four, I studied the effect of astrocytic G6PD on stroke outcomes through utilizing both in vivo and in vitro ischemia models. I discovered that overexpression of G6PD in astrocytes significantly reduced neuronal death following ischemic injury through removal of oxidative stress. These findings provide novel insights into the development of therapeutic strategies for FIS, based on modulating metabolic routes in astrocytes. Finally, in Chapter five, I discussed the significance of my studies and future directions.