Transcription factors and docosahexaenoic acid interactions during photoreceptor development

POLITI, L, GARELLI A., ROTSTEIN N.

Buenos Aires

Congreso; XVII International Congress of Eye Research (ICER); 2006

International Congress of Eye Research

Pax6 and Crx play critical roles during eye and photoreceptor development, respectively; however, their precise functions still remain ill-defined. Pax6, an eye determinant essential for optic vesicle development, also affects proliferation rate in the developing brain. Crx, a transcription factor that binds to, and trans-activates photoreceptor specific genes, including rhodopsin, is essential for progenitor cells to become photoreceptors; however, it has been proposed that its expression might not be sufficient to complete differentiation in these cells. We have previously shown that docosahexaenoic acid (DHA), a major polyunsaturated fatty acid in photoreceptors, delays apoptosis, and promotes cell cycle exit and differentiation in photoreceptor progenitors. We now investigated the pattern of Pax6 expression during rat retina development and the interrelationship between Crx and DHA during photoreceptor differentiation. Two waves of Pax6 expression were observed. The first one occurred in all retina progenitors while they progressed in the cell cycle, at early stages of eye development, and disappeared after they exited the cell cycle. Expression of Pax6 corresponded with that of nestin, an intermediate filament found in retina multipotent progenitors, and with other markers of cell cycle progression, suggesting that Pax6 might participate in controlling cell cycle progression in the developing retina. Pax6 second wave occurred in postmitotic amacrine, ganglion and horizontal cells; it co-expressed with N-CAM, which concentrated in growth cones of postmitotic amacrine neurons, and in ganglion cells at the time at which these cells projected their axons toward the brain, suggesting it might regulate neuronal differentiation in these cells. On the other hand, as photoreceptor progenitors became postmitotic, they turned off Pax6 expression, no longer took up BrdU, and started expressing Crx. While in vivo Crx expression closely matched opsin expression, in culture only a few Crx-positive photoreceptors expressed opsin, and most of them did not develop their characteristic outer segments. Addition of DHA did not alter Crx expression in photoreceptors; however, it significantly enhanced opsin expression and axonal length and promoted further apical differentiation. These results show that Pax6 and Crx expression are mutually exclusive during photoreceptor differentiation. They also suggest that Crx expression is necessary but not sufficient to achieve fully differentiated photoreceptors. It seems to provide a permissive stage, essential to initiate photoreceptor differentiation, but additional support of DHA, among other environmental signals, is required to accomplish further differentiation. This suggests that photoreceptors depend on the sequential involvement of intrinsic and extrinsic factors, among which the contribution of DHA is essential to develop differentiated and functional photoreceptors.