Müller glial and microglial responses coupled to recovery of cone photoreceptors following limited cone ablation in zebrafish retina

Zebrafish can spontaneously regenerate retinal neurons after acute damage. Ablation and regeneration of retinal neurons can be achieved in transgenic zebrafish with cell-selective expression of bacterial nitroreductase (NTR/nfsb) upon exposure to pro-drugs such as metronidazole (Mtz). We found that Mtz exposure of gnat2:nfsb-mCherry adult zebrafish at certain dose/duration could limit death to only a small number of cones, while causing transient stress to the surviving cone photoreceptors, providing insight into glial and immune cell responses to transient neuronal perturbations. Using this approach, we identified molecular changes in multiple retinal cell types following Mtz washout. Cones exhibited evidence of stress including downregulation of phototransduction genes and upregulation of oxidative stress response genes. Even with only limited cone death detected, the Müller glia (MG) response involved features known to occur during retinal regeneration, including proliferating progenitor production. Microglia/myeloid cells showed evidence of two response waves. The first response occurred within a day of drug washout while the second response occurred around 4 days after drug washout and was associated with cone photoreceptor stress recovery. Single cell RNA-seq suggested two subpopulations of microglia/myeloid cells participated in this response. Our results provide insight into triggers of proliferative responses from the MG, microglial/myeloid responses coupled to neuronal stress recovery, and potential molecular changes involved with cone recovery to stress.