The regeneration-responsive element careg monitors activation of Müller glia after MNU-induced damage of photoreceptors in the zebrafish retina

by Selleckchem | Oct 31 2023

In contrast to mammals, zebrafish can regenerate their damaged photoreceptors. This capacity depends on the intrinsic plasticity of Müller glia (MG). Here, we identified that the transgenic reporter careg, a marker of regenerating fin and heart, also participates in retina restoration in zebrafish. After methylnitrosourea (MNU) treatment, the retina became deteriorated and contained damaged cell types including rods, UV-sensitive cones and the outer plexiform layer. This phenotype was associated with the induction of careg expression in a subset of MG until the reconstruction of the photoreceptor synaptic layer. Single-cell RNA sequencing (scRNAseq) analysis of regenerating retinas revealed a population of immature rods, defined by high expression of rhodopsin and the ciliogenesis gene meig1, but low expression of phototransduction genes. Furthermore, cones displayed deregulation of metabolic and visual perception genes in response to retina injury. Comparison between careg:EGFP expressing and non-expressing MG demonstrated that these two subpopulations are characterized by distinct molecular signatures, suggesting their heterogenous responsiveness to the regenerative program. Dynamics of ribosomal protein S6 phosphorylation showed that TOR signaling became progressively switched from MG to progenitors. Inhibition of TOR with rapamycin reduced the cell cycle activity, but neither affected careg:EGFP expression in MG, nor prevented restoration of the retina structure. This indicates that MG reprogramming, and progenitor cell proliferation might be regulated by distinct mechanisms. In conclusion, the careg reporter detects activated MG, and provides a common marker of regeneration-competent cells in diverse zebrafish organs, including the retina.

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The passage you provided describes a study conducted on zebrafish to investigate the regenerative abilities of their photoreceptors, which are light-sensing cells in the retina. Unlike mammals, zebrafish have the remarkable ability to regenerate their damaged photoreceptors. The regeneration process relies on the intrinsic plasticity of a specific type of glial cells called Müller glia (MG).

The researchers used a transgenic zebrafish line that expressed a reporter gene called careg, which is known to be activated during regeneration of the fin and heart tissues. They discovered that this careg reporter gene was also expressed in the regenerating retina after treatment with methylnitrosourea (MNU), a chemical that induced retinal damage.

Following MNU treatment, the zebrafish retinas exhibited deterioration and damage to various cell types, including rods (responsible for vision in dim light), UV-sensitive cones (responsible for detecting ultraviolet light), and the outer plexiform layer (a layer involved in synaptic connections). The researchers observed that the expression of the careg gene was induced in a subset of MG cells during the reconstruction of the photoreceptor synaptic layer.

To gain further insights into the regenerative process, the researchers employed single-cell RNA sequencing (scRNAseq) analysis on the regenerating retinas. This technique allows the identification and characterization of individual cells based on their gene expression patterns. They identified a population of immature rods characterized by high expression of the rhodopsin gene (a light-sensitive protein in rods) and the meig1 gene (involved in ciliogenesis, the formation of cellular projections called cilia). However, these immature rods showed low expression of phototransduction genes, which are involved in the conversion of light into electrical signals.

Additionally, the researchers observed that the cones, another type of photoreceptor cells, displayed changes in the expression of metabolic and visual perception genes in response to retinal injury. This suggests that cone cells also undergo regulatory changes during the regenerative process.

By comparing MG cells that expressed the careg:EGFP reporter with those that did not, the researchers identified distinct molecular signatures in these two subpopulations. This finding suggests that MG cells have heterogeneous responsiveness to the regenerative program, meaning that not all MG cells respond to injury in the same way.

The researchers also investigated the dynamics of TOR signaling, a cellular pathway involved in various biological processes, including cell growth and proliferation. They found that during the regenerative process, the TOR signaling activity shifted from MG cells to progenitor cells, which are cells capable of differentiating into specialized cell types. Inhibition of TOR signaling using a drug called rapamycin reduced the cell cycle activity (the process of cell division) but did not affect the expression of the careg:EGFP reporter in MG cells, nor did it prevent the restoration of the retina structure. This suggests that the reprogramming of MG cells and the proliferation of progenitor cells might be regulated by different mechanisms.

In conclusion, the researchers identified the careg reporter gene as a marker of activated MG cells involved in the regeneration of various organs in zebrafish, including the retina. They observed distinct molecular signatures between careg-expressing and non-expressing MG cells, suggesting differences in their responsiveness to regeneration. The study also highlighted the involvement of TOR signaling in the regenerative process, specifically in the proliferation of progenitor cells.