The microRNA miR-18a links proliferation and inflammation during photoreceptor regeneration in the injured zebrafish retina

In mammals, photoreceptor loss causes permanent blindness, but in zebrafish (Danio rerio), photoreceptor loss reprograms Muller glia to function as stem cells, producing progenitors that fully regenerate photoreceptors. MicroRNAs (miRNAs) regulate neurogenesis in the CNS, but the roles of miRNAs in injury-induced neuronal regeneration are largely unknown. In the embryonic zebrafish retina, miRNA miR-18a regulates photoreceptor differentiation. The purpose of the current study was to determine in zebrafish the function of miR-18a during injury-induced photoreceptor regeneration. RT-qPCR, in-situ hybridization and immunohistochemistry showed that miR-18a expression increases throughout the retina by 1-day post-injury (dpi) and increases through 5 dpi. To test miR-18a function during photoreceptor regeneration, we used homozygous miR-18a mutants (miR-18a[mi5012]), and knocked down miR-18a with morpholino oligonucleotides. During photoreceptor regeneration, miR-18a[mi5012] retinas have fewer mature photoreceptors than WT at 7 and 10 dpi, but there is no difference at 14 dpi, indicating that photoreceptor regeneration is delayed. Labeling dividing cells with bromodeoxyuridine (BrdU) showed that at 7 and 10 dpi, there are excess Muller glia-derived progenitors in both mutants and morphants, indicating that miR-18a negatively regulates injury-induced proliferation. Tracing BrdU-labeled cells showed that in miR-18a[mi5012] retinas excess progenitors migrate to other retinal layers in addition to the photoreceptor layer. Inflammation is critical for photoreceptor regeneration, and RT-qPCR showed that in miR-18a[mi5012] retinas, inflammatory gene expression and microglia activation are prolonged. Suppressing inflammation with dexamethasone rescues the miR-18a[mi5012] phenotype. Together, these data show that during photoreceptor regeneration in zebrafish, miR-18a regulates proliferation and photoreceptor regeneration by regulating the inflammatory response.