The role of rx1 and pax6 in Xenopus retinal cell fate specification

The specification process that guides a cell from a retinal stem cell state to a differentiated retinal cell is the result of a combination of intrinsic gene expression and extrinsic cues. Transcription factors involved in retinal specification are mainly thought to function based on the timing of their expression---either early in the determination of retinal stem cell pools or later in the differentiation of retinal cell types. The purpose of the research described herein is to determine how the eye field transcription factors rx1 and pax6 function at eye field stages to influence cell type specification.; Chapter II reports that rx1 and pax6 function differentially in the specification of retinal cell types. Increased Rx1 expression reduced the production of all cell types whereas decreased Rx1 expression increased nearly all cell types. In contrast, increased Pax6 expression promoted ganglion cells and repressed photoreceptor and horizontal cells in a time-dependent manner. These observations were most consistently explained by the effects on proliferation and on markers of stem cells and differentiation markers reported in Chapter III. Rx1 expression increased proliferation and repressed retinal (pax6 and six3 ) and neural (notch1 and sox2) stem cell markers. Its expression also repressed the expression of differentiation markers, yet loss-of-function experiments demonstrate that it is necessary for their initial expression. These findings are consistent with a role for rx1 in the maintenance of a stem cell state and the repression of differentiation. pax6 also promotes proliferation but does not act to maintain retinal stem cells. Rather, it specifies retinal progenitor cells that give rise to specific cell types by regulating sox2 in a time-dependent manner and by differentially regulating the expression of differentiation markers.; This dissertation contributes to the understanding of the genetic interactions that direct specification of retinal stem cells into specific retinal progenitor cells and subsequent mature retinal cell types. These findings will be important for future endeavors to produce retinal cells for transplantation and treatment of retinal disease.