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Background: Adult zebrafish spontaneously regenerate their retinas after damage. Although a number of genes and signaling pathways involved in regeneration have been identified, the exact mechanisms regulating various aspects of regeneration are unclear. microRNAs (miRNAs) were examined for their potential roles in regulating zebrafish retinal regeneration. Results: To investigate the requirement of miRNAs during zebrafish retinal regeneration, we knocked down the expression of Dicer in retinas prior to light-induced damage. Reduced Dicer expression significantly decreased the number of proliferating Muller glia-derived neuronal progenitor cells during regeneration. To identify individual miRNAs with roles in neuronal progenitor cell proliferation, we collected retinas at different stages of light damage and performed small RNA high-throughput sequencing. We identified subsets of miRNAs that were differentially expressed during active regeneration but returned to basal levels once regeneration was completed. We then knocked down five different miRNAs that increased in expression and assessed the effects on retinal regeneration. Reduction of miR-142b and miR-146a expression significantly reduced INL proliferation at 51h of light treatment, while knockdown of miR-7a, miR-27c, and miR-31 expression significantly reduced INL proliferation at 72h of constant light. Conclusions: miRNAs exhibit dynamic expression profiles during retinal regeneration and are necessary for neuronal progenitor cell proliferation. Developmental Dynamics 243:1591–1605, 2014. © 2014 The Authors. Developmental Dynamics published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists

Zebrafish are capable of robust and spontaneous regeneration of injured retina. Constant intense light exposure to adult albino zebrafish specifically causes apoptosis of rod and cone photoreceptor cells and is an excellent model to study the molecular mechanisms underlying photoreceptor regeneration. However, this paradigm has only been applied to lesion zebrafish of the nonpigmented albino genetic background, which precludes the use of numerous transgenic reporter lines that are widely used to study regeneration. Here, we explored the effectiveness of constant intense light exposure in causing photoreceptor apoptosis and stimulating regeneration in normally pigmented zebrafish retinas. We show that constant intense light exposure causes widespread photoreceptor damage in the dorsal-central retinas of pigmented zebrafish. Photoreceptor loss triggers dedifferentiation and proliferation of Müller glia as well as progenitor cell proliferation. We also demonstrate that the timeline of regeneration response is comparable between the albino and the pigmented retinas.

Damage of the zebrafish retina triggers a spontaneous regeneration response that is initiated by Müller Glia (MG) dedifferentiation and asymmetric cell division to produce multipotent progenitor cells. Subsequent expansion of the progenitor pool by proliferation is critical for retina regeneration. Pax6b expression in the progenitor cells is necessary for their proliferation, but exact regulation of its expression is unclear. Here, we show that miR-203 is downregulated during regeneration in proliferating progenitor cells. Elevated miR-203 levels inhibit progenitor cell expansion without affecting MG dedifferentiation or progenitor cell generation. Using GFP-reporter assays and gain and loss of function experiments in the retina, we show that miR-203 expression must be suppressed to allow pax6b expression and subsequent progenitor cell proliferation.