HOW TO DRIVE EMBRYONIC STEM CELLS TOWARD A RETINAL FATE: THE ROLE OF THE NEURAL INDUCER NOGGIN

Millions of people worldwide suffer of irreversible vision loss because of retinal degenerative diseases such as Retinitis pigmentosa (RP). Embryonic stem cells (ESCs) have been considered as an important source for the replacement of the retinal neurons following degeneration since they have the ability to differentiate into any cell type of the organism. The animal cap tissue of the blastula stage Xenopus embryo consists of a few layers of ectodermal cells, which are pluripotent and can, as ESCs, be induced to form any cell type. Noggin, a small secreted protein, can induce ectoderm to form neural tissue. In this study, we explored the potential of noggin to drive pluripotent Xenopus animal cap embryonic stem (ACES) cells toward retinal cell fates. Using RT-PCR, in situ hybridization and immunohistochemistry, we found that different doses of noggin have different effects on the expression, in cultured ACES cells, of terminal differentiation markers of specific retinal cell types. ACES cells expressing high doses of noggin efficiently induce the expression of retinal differentiation markers, and, following in vivo transplantation, form a retina both in the presumptive eye field and in ectopic posterior regions. The eyes originating from the transplants in the eye field region are functionally equivalent to normal eyes, as revealed by electrophysiology and c-fos expression in response to light. By contrast, ACES cells expressing low doses of noggin induce retina formation only in a low percentage of cases. Our studies show that in Xenopus embryos, appropriate doses of noggin, can effectively drive ACES cells toward retinal cell differentiation. Moreover, we performed microarray analysis to identify the factors necessary for Noggin-mediated retinal induction. The preliminary results we have show the involvement of other signaling pathways that act synergistically with noggin to promote retinal differentiation of ACES cells. This provides a link between BMP and other signaling pathways in eye specification. The further elucidation of the pathways through which Noggin elicits retinal fates will help us to establish improved protocols for the differentiation of mammalian ESCs toward retinal neurons.