EGF mediated suppression of ROD photoreceptor differentiation is accompanied by a decrease in MASH-1 expression

PURPOSE: Mash-1 is acandidale transcription factor involved in the differentiation of the rod photoreceptor. Since Mash-1 and other members of bHLH class of transcription factors have been observed to be regulated in response to growth factors we tested the hypothesis that Mash-1 plays a role in the mediation of intracellular response during the specification of progenitors into rod photoreceptors. METHODS: The effect of EGF on retinal cell proliferation and differentiation was studied in culture of relinal explants obtained from E-18 embryos. The expiants were treated with EGF ( 10ng/ml) for three days. Cell-proliferation was assayed by tritiated thymidine incorporation. Immunocytochemical analyses were carried out using MashI and opsin antibodies. Mash-1 and opsin transcript levels were analyzed by RTPCR. RESULTS: Treatment of retinal expiants with EGF led to cell proliferation These proliferating cells were predominantly localized in the outer neuroblastic layer which harbors rod precursors. Mash-I positive cells, which are also preferentially localized in the outer neuroblastic layer in the E-18 retina, decreased in response to EGF treatment. The decrease in Mash-1 expression was accompanied by a decrease in opsin immunoreactivity and opsin transcript levels. Withdrawal of EGF from the culture led to an increase in Mash-l immunoreactivity and transcript levels. Similar increases in opsin immunoreactivity and transcript levels were observed following EGF withdrawal. CONCLUSIONS: In retinal explant culture, EOF promotes proliferation of progenitors at the expense of their differentiation into rod photoreceptors. The concomitant decrease in Mash-l and opsin expression in the presence of EGF and their increase following EGF withdrawal suggest that Mash-1 may be involved in the EGF mediated suppression of rod photoreceptur differentiation. Supported by NIH grant EY 10313. NONE.