Molecular mechanisms underlying retinal astrocyte death during development
Abstract
Developmental cell death is essential for nervous system development, sculpting the developing tissue by controlling cell numbers. While developmental neuron death has been studied extensively, the most abundant cell type of the nervous system – the astrocyte – has often been overlooked. Our lab recently showed that astrocytes in the developing retina undergo an unusual non-apoptotic form of death that eliminates a vast proportion of the original population. Further, we found that microglia are the major effectors of astrocyte death. However, the mechanisms that induce microglia to kill astrocytes remain mysterious. It is important to understand these astrocyte death mechanisms because astrocytes play a crucial role in patterning the retinal blood vessel network. Developmental perturbations to astrocyte number have large effects on their patterning, and in turn cause severe vascular patterning defects – some of which resemble vasculopathies typical of human blinding disorders. Because death has such a major impact on astrocyte number, it presumably has an outsized impact on this critical patterning process. We therefore sought to identify the non-apoptotic mechanisms that drive astrocyte death. Previously, we showed that astrocyte numbers modulate microglial phagocytic activity – increasing this activity as astrocyte numbers rise and decreasing it as astrocyte numbers decline. This observation suggested that astrocytes themselves are the source of cues that drive their own death via recruitment of phagocytic microglia. Here we identify the membrane lipid phosphatidylserine (PtdSer) as one such astrocyte-derived “eat-me” cue. PtdSer is best known as an “eat-me” signal expressed on the surface of apoptotic cells. We show that PtdSer is also externalized on the cell surface of apparently normal astrocytes during the developmental death period. Moreover, using a genetic approach to increase cell-surface PtdSer, we show that it is sufficient to drive astrocyte death. For these studies, we used an astrocyte-specific mouse knockout of Tmem30a, an obligate subunit of the flippase enzymes that normally remove PtdSer from the cell surface. In these knockout animals, microglia are recruited to Tmem30a mutant astrocytes, engulf them, and cause a significant acceleration of cell number decline. This excess astrocyte loss has functional consequences for the development of the vasculature: The astrocytic template for angiogenesis is overly sparse, which leads to vascular patterning defects and delayed angiogenesis. Interestingly, these defects can be rescued by blocking the function of a phagocytic signaling pathway that can recognize PtdSer exposure, suggesting that the excess PtdSer exposure in the Tmem30a knockout animals is responsible for the increase in astrocyte death. Altogether our findings highlight the broad impact of dysregulated astrocyte death. Understanding how astrocyte population size is controlled will provide new insights into death mechanisms that are crucial for development not only in the retina but may also sculpt glial populations elsewhere in the central nervous system.
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Paisley, Caitlin Elizabeth Gorse (2023). Molecular mechanisms underlying retinal astrocyte death during development. Dissertation, Duke University. Retrieved from https://hdl.handle.net/10161/29142.
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