Fatty acid-binding proteins and fatty acid synthase influence glial reactivity and promote the formation of Müller glia-derived progenitor cells in the avian retina
The capacity for retinal regeneration varies greatly across vertebrates species. A recent comparative epigenetic and transcriptomic investigation of Muller glial (MG) in the retinas of fish, birds and mice revealed that Fatty Acid Binding Proteins (FABPs) are among the most highly up-regulated genes in activated chick MG (Hoang et al., 2020). Herein we provide an in-depth follow-up investigation to describe patterns of expression and how FABPs and fatty acid synthase (FASN) influence glial cells in the chick retina. During development, FABP7 is highly expressed by embryonic retinal progenitor cells (eRPCs) and maturing MG, whereas FABP5 is gradually up-regulated in maturing MG and remains elevated in mature glial cells. PMP2 (FABP8) is expressed by oligodendrocytes and FABP5 is expressed by non-astrocytic inner retinal glial cells, and both of these FABPs are significantly up-regulated in activated MG in damaged or growth factor-treated retinas. In addition to suppressing the formation of MGPCs, we find that FABP-inhibition suppressed the accumulation of proliferating microglia, although the microglia appeared highly reactive. scRNA-seq analyses of cells treated with FABP-inhibitor revealed distinct changes in patterns of expression suggesting that FABPs are involved in the transitions of MG from a resting state to a reactive state and conversion from reactive MG to MGPCs. Inhibition of FABPs in undamaged retinas had a significant impact upon the transcriptomic profiles of MG, with up-regulation of genes associated with gliogenesis, decreases in genes associated with neurogenesis, and suppression of the ability of MG to become MGPCs. scRNA-seq analyses of microglia indicated that FABP inhibition enhances gene modules related to reactivity, proliferation and cytokine signaling. We find that the proliferation of retinal progenitors in the circumferential marginal zone (CMZ) is unaffected by FABP-inhibitor. Upstream of FABP activity, we inhibited FASN in damaged retinas, which reduced numbers of dying cells, increased the proliferation of microglia, and potently suppressed the formation MGPCs in damaged retinas. We conclude that the activity of FASN and FABPs are required early during the formation of proliferating MGPCs. Fatty acid metabolism and cell signaling involving fatty acids are important in regulating glial homeostasis in the retina, and the dedifferentiation and proliferation of microglia and MGPCs.