Transient Expression of Fez Family Zinc Finger 2 Protein Regulates the Brn3b Gene in Developing Retinal Ganglion Cells

Retinal ganglion cells (RGCs) are projection neurons in the neural retina that relay visual information from the environment to the central nervous system. The early expression of MATH5 endows the post-mitotic precursors with RGC competence and leads to the activation of Brn3b that marks committed RGCs. Nevertheless, this fate commitment process and, specifically, regulation of Brn3b remain elusive. To explore the molecular mechanisms underlying RGC generation in the mouse retina, we analyzed the expression and function of Fez family zinc finger 2 (FEZF2), a transcription factor critical for the development of projection neurons in the cerebral cortex. Fezf2 mRNA and protein were transiently expressed at embryonic day 16.5 in the inner neuroblast layer and the prospective ganglion cell layer of the retina, respectively. Knockout of Fezf2 in the developing retina reduced BRN3B+ cells and increased apoptotic cell markers. Fezf2 knockdown by retinal in utero electroporation diminished BRN3B but not the coexpressed ISLET1 and BRN3A, indicating that the BRN3B decrease was the cause, not the result, of the overall reduction of BRN3B+ RGCs in the Fezf2 knockout retina. Moreover, the mRNA and promoter activity of Brn3b were increased in vitro by FEZF2, which bound to a 5' regulatory fragment in the Brn3b genomic locus. These results indicate that transient expression of Fezf2 in the retina modulates the transcription of Brn3b and the survival of RGCs. This study improves our understanding of the transcriptional cascade required for the specification of RGCs and provides novel insights into the molecular basis of retinal development.