Developmental wave of Brn3b expression leading to RGC fate specification is synergistically maintained by miR‐23a and miR‐374

Differential regulation of Brn3b is essential for the Retinal Ganglion Cell (RGC) development in the two phases of retinal histogenesis. This biphasic Brn3b regulation is required first, during early retinal histogenesis for RGC fate specification and secondly, during late histogenesis, where Brn3b is needed for RGC axon guidance and survival. Here, we have looked into how the regulation of Brn3b at these two stages happens. We identified two miRNAs, miR‐23a and miR‐374, as regulators of Brn3b expression, during the early stage of RGC development. Temporal expression pattern of miR‐23a during E10–19, PN1–7, and adult retina revealed an inverse relation with Brn3b expression. Though miR‐374 did not show such a pattern, its co‐expression with miR‐23a evidently inhibited Brn3b. We further substantiated these findings by ex vivo overexpression of these miRNAs in E14 mice retina and found that miR‐23a and miR‐374 together brings about a change in Brn3b expression pattern in ganglion cell layer (GCL) of the developing retina. From our results, it appears that the combined expression of these miRNAs could be regulating the timing of the wave of Brn3b expression required for early ganglion cell fate specification and later for its survival and maturation into RGCs. Taken together, here we provide convincing evidences for the existence of a co‐ordinated mechanism by miRNAs to down regulate Brn3b that will ultimately regulate the development of RGCs from their precursors. © 2014 Wiley Periodicals, Inc. Develop Neurobiol 74: 1155–1171, 2014     

Conserved and divergent functions of Drosophila atonal,amphibian, and mammalian Ath5 genes

Insect and vertebrate eyes differ in their formation, cellular composition, neural connectivity, and visual function. Despite this diversity, Drosophila atona and its vertebrate Ortholog in the eye, Ath5, each regulate determination of the first retinal neuron class-R8 photo-receptors and retinal ganglion cells (RGCs)-in their respective organisms. We have performed a cross-species functional comparison of these genes. In ato mutant Drosophila, ectopic Xenopus Ath5 (Xath5) rescues photoreceptor cell development comparably with atonaI. In contrast, mouse Ath5 (Math5) induces formation of very few ommatidia, and most of these lack R8 cells. In the developing frog eye, ectopic atonal, like Xath5, promotes the differentiation RGCs. Despite strong conservation of atonaI, Xath5, and Math5 structure and shared function, other factors must contribute to the species specificity of retinal neuron determination. These observations suggest that the atonaI family may occupy a position in a gene hierarchy where differences in gene regulation or function can be correlated with evolutionary diversity of eye development.