The zebrafish galectin Drgal1-L2 is expressed by proliferating Muller glia and photoreceptor progenitors and regulates the regeneration of rod photoreceptors

Purpose: The purpose of this study was to identify secreted proteins in the retina of the adult zebrafish that are induced by the selective death of photoreceptors and experimentally test the function of these proteins during the regeneration of photoreceptors. Methods: Induced selective death of photoreceptors in the retina of the adult zebrafish was combined with in situ hybridization and immunocytochemistry to identify the induced cellular expression of the secreted beta-galactoside binding protein, Galectin 1-like 2 (Drgal1-L2). Electroporation of morpholino oligonucleotides was used to knock down protein synthesis, and regenerated photoreceptors were counted in control and experimental retinas following labeling with cell type-specific RNA probes. Results: Expression analysis and immunocytochemistry showed that Drgal1-L2 is induced de novo by photoreceptor death and is synthesized by microglia and proliferating Müller glia and their mitotic progeny. Knockdown of Drgal1-L2 expression in Müller glia results in reduced regeneration of rod photoreceptors, without affecting injury-induced proliferation or the regeneration of cone photoreceptors. Conclusions: From these data we conclude that Drgal1-L2 is induced by photoreceptor cell death, secreted by stem cells and neuronal progenitors and regulates the regeneration of rod photoreceptors. Drgal1-L2 is the first secreted factor shown to regulate aspects of regenerative neurogenesis in the teleost retina.