A Dynamic Change of Microglial States Occurs During the Transition From Photoreceptor Degeneration to Regeneration in Zebrafish <i>pde6c</i> Mutants

Microglia, brain-resident immune cells, maintain brain homeostasis. However, homeostatic microglia dynamically change into disease-associated microglia in human neurodegenerative diseases. Furthermore, microglia-mediated inflammation is required to initiate neuronal regeneration in zebrafish brain. To understand how functional states of microglia are regulated in response to neuronal degeneration and regeneration, we focused on pde6c mutants, a chronic photoreceptor degeneration zebrafish model. We conducted scRNA-seq analysis on microglia in wild-type sibling and pde6c mutant retinas at the onset of photoreceptor degeneration (5 dpf) and Müller glia-mediated neuronal regeneration (4 wpf). At 5 dpf, retinal microglia consist of three clusters, which correspond to homeostatic, degeneration-response, and stress-response microglia, respectively. The degeneration-response cluster expands in pde6c mutants and expresses genes for neuroprotection and tissue repair. At 4 wpf, retinal microglia comprise four clusters, two of which are specifically produced in pde6c mutants and approach the photoreceptor layer. Furthermore, another cluster is prominently localized in the retinal stem cell niche and shows a transcriptomic profile similar to that of neurogenic-associated microglia (NAM). Comparison of transcriptomic similarity between 4 wpf and 5 dpf microglial clusters revealed that each 4 wpf microglial cluster inherits characteristics of homeostatic, degeneration-response, and stress-response state of 5 dpf microglia in different combinations. Thus, there is a unique heterogeneity of microglia in the initial stage of Müller glia-mediated regeneration. Taken together, our findings reveal a dynamic change of retinal microglia during the transition from photoreceptor degeneration to Müller glia-mediated neuronal regeneration in zebrafish.