Bone morphogenetic protein-4 induction of mouse retinal progenitor cell differentiation may involve the activation of smad-1/5 and inhibitors of differentiation

Abstract

Bone morphogenetic proteins (BMPs), a member of the transforming growth factor (TGF)-β superfamily, has been implicated in the development of many different organs. Previous work showed that BMP-4 could induce cell proliferation, differentiation and apoptosis. Inhibitors of differentiation (Id) proteins have emerged recently as critical targets of BMPs. It was also known that the up-regulation of Id protein by BMPs is mainly through a Smad-dependent pathway. Expression of BMP-2, -4 and -7 mRNAs have been observed in the mouse retina by in situ hybridization. We previously demonstrated that both mRNA and protein of BMP-2, -4 and -7 were expressed during the development of the mouse retina. In addition, the expression of Id1-4 was also observed in the developing mouse retina showing unique and yet overlapping expression patterns. Our results support a possible role for Id in inner retina development and in the terminal differentiation and/or maintenance of ganglion cells and INL interneurons in the adult. In this study, we assessed (1) the co-expression of Ids and BMPs in the mouse retina; (2) the regulation of Id expression by BMPs in the mouse retina as reported in the other cell types; (3) and the effects of BMPs on retinal cell development. By using real-time PCR and western blotting, we found that BMP-4, BMPRIa, Ib and II and Id1-4 were highly expressed in the mouse retina at the embryonic (E13.5-E18.5) and early postnatal (P1) stage and then decreased at adult stage. In the adult, Co-expression of BMPs and BMPRs with Ids was mainly localized in the ganglion cells and amacrine cells in the INL suggesting that BMPs may cooperate with Ids in regulating retinal cell development and retinal cell fate determination. By using a retinal progenitor cell culture, we found that exogenous BMP-4 increased Id promoter activity and the mRNA expression of Id1-3. The effect of BMP-4 on Id-1 promoter was blocked by Noggin, a BMP antagonist. By immunocytochemistry, BMP-4 stimulation led to the nuclear localization of phospho-Smad1/5/8 in the retinal progenitor cells. Furthermore, BMP-4 induced the differentiation of retinal progenitor cells expressing neuronal markers. These results point to a potentially new pathway for regulating neurogenesis in the developing retina through precise interaction between BMP-4, Ids and their downstream signal molecules.