Conditional and biased regeneration of cone photoreceptor types in the zebrafish retina

Abstract

A major challenge in regenerative medicine is replacing cells lost through injury or disease. While significant progress has been made, much remains unknown about the accuracy of native regenerative programs in cell replacement. Here, we capitalized on the regenerative capacity and stereotypic retinal organization of zebrafish to determine the specificity with which retinal Müller glial cells replace lost neuronal cell types. By utilizing a targeted genetic ablation technique, we restricted death to all or to distinct cone photoreceptor types (red, blue, or UV-sensitive cones), enabling us to compare the composition of cones that are regenerated. We found that Müller glia produce cones of all types upon nondiscriminate ablation of these photoreceptors, or upon selective ablation of red or UV cones. Pan-ablation of cones led to regeneration of the various cone types in relative abundances that resembled those of nonablated controls, that is, red > green > UV blue cones. Moreover, selective loss of red or UV cones biased production toward the cone type that was ablated. In contrast, ablation of blue cones alone largely failed to induce cone production at all, although it did induce cell division in Müller glia. The failure to produce cones upon selective elimination of blue cones may be due to their low abundance compared to other cone types. Alternatively, it may be that blue cone death alone does not trigger a change in progenitor competency to support cone genesis. Our findings add to the growing notion that cell replacement during regeneration does not perfectly mimic programs of cell generation during development.

The Regenerating Adult Zebrafish Retina Recapitulates Developmental Fate Specification Programs

“…The reduced fraction of proliferating thrb:Tomato-positive cells in NMDA relative to light-exposed retinas further suggests regulation of cell type production in a damage-dependent manner, similar to previous reports (Fraser et al, 2013;Powell et al, 2016;D'Orazi et al, 2020). Surprisingly, the fraction of ptf1a:EGFP-positive NPCs that yields amacrine cells did not change between light-and NMDA-damaged retinas, although NMDA induced death of a subset of amacrine cells (Powell et al, 2016).…”

Disruption ofmiR-18aalters proliferation, photoreceptor replacement kinetics, inflammatory signaling and microglia/macrophage numbers during retinal regeneration in zebrafish

“…At least some of these excess progenitors remain in the INL or migrate to the GCL, suggesting that they may differentiate into other types of neurons besides photoreceptors. This is consistent with data showing that following damage to retinal neurons, Müller glia derived progenitors are multipotent and generate multiple cell types even though they preferentially generate the neurons that were lost [54][55][56]. The presence of additional MG-derived cells in the INL and GCL could also be due to aberrant migration of progenitor cells.…”

Transcriptomic comparison of two selective retinal cell ablation paradigms in zebrafish reveals shared and cell-specific regenerative responses

“…It is likely that novel pathways were easier to associate with bipolar cell regeneration due to the fact that this paradigm has been comparatively understudied compared to rod photoreceptor regeneration. These findings expand upon previous studies demonstrating that selective retinal cell loss can trigger a fate biased regenerative responses (8)(9)(10)(11), and provide potential insights useful for developing selective cell regeneration therapies. Collectively, these findings emphasize that regeneration need not always "recapitulate development", particularly for paradigms that mimic the selective cell loss attending most neurodegenerative diseases.…”

“…In contrast, the responsiveness and regenerative properties of stem cells following the discrete loss of retinal cell subtypes remain less well undefined. Intriguingly, recent studies suggest that selective retinal cell loss in zebrafish elicits a fate-biased regenerative response; i.e., selective cell replacement (8)(9)(10)(11). To increase understanding of fate-biased regenerative responses at the transcriptomic level, we used time-resolved gene expression profiling to compare two cell-specific ablation paradigms in the zebrafish retina involving the targeted loss of 1) bipolar cells, versus, 2) rod photoreceptors.…”

“…Coexpression of a fluorescent reporter in NTR-targeted cells allows the regenerative process to be characterized in detail using in vivo time-lapse imaging (32,35) or quantified using highthroughput methods (36,37). Using this approach, recent studies have shown that selective retinal cell loss does not trigger a developmental response but rather a fate-biased regenerative process (8)(9)(10)(11). These data suggest differential regulation of selective and non-selective cellular regeneration.…”