Altered expression of the transcription factor Mef2c during retinal degeneration in Rpe65-/- mice

To investigate the role of the myocyte enhancer factor 2 (Mef2) transcription factor family in retinal diseases, Mef2c expression was assessed during retinal degeneration in the Rpe65(-/-) mouse model of Leber's congenital amaurosis (LCA). Mef2c-dependent expression of photoreceptor-specific genes was further addressed.

Expression of Mef2 members was analyzed by oligonucleotide microarray, quantitative PCR (qPCR), and in situ hybridization. Mef2c-dependent transcriptional activity was assayed by luciferase assay in HEK293T cells.

Mef2c was the only Mef2 member markedly downregulated during retinal degeneration in Rpe65(-/-) mice. Mef2c mRNA level was decreased by more than 2-fold at 2 and 4 months and by 3.5-fold at 6 months in retinas of Rpe65(-/-) mice. Downregulation of Mef2c at the protein level was confirmed in Rpe65(-/-) retinas. The decrease in Mef2c mRNA levels in the developing Rpe65(-/-) retinas from postnatal day (P) 13 onward was concomitant with the decreased expression of the rod-specific transcription factors Nrl and Nr2e3. Nrl was further shown to drive Mef2c transcriptional activity, supporting a physiological role for Mef2c in the retina. In addition, Mef2c appeared to act as a transcriptional repressor of its own expression and the expression of the retina-specific retinal G-protein coupled receptor (Rgr), rhodopsin, and M-opsin genes.

These findings highlight the early altered regulation of the rod-specific transcriptional network in Rpe65-related disease. They also indicate that Mef2c may act as a novel transcription factor involved in the development and the maintenance of photoreceptor cells.