Functions of Rx in early vertebrate ocular development

Abstract

Functions of Rx in Early Vertebrate Ocular Development Brian G. Zamora Patterning of the optic vesicle is a crucial step in early vertebrate eye development that organizes uncommitted optic vesicle cells into distinct distal, dorsal, and proximal regions that will give rise to the neural retina, retinal pigment epithelium (RPE), and optic stalk, respectively. Originating from the same sheet of anterior neuroectoderm, uncommitted optic vesicle cells are patterned into neural and non-neural (RPE) retinal domains through the coordinated activities of extrinsic signaling molecules and intrinsic transcription factors. Neural retinal specification is driven by FGF signals emanating from the surface ectoderm and developing lens, while RPE is specified by signals from the extraocular mesenchyme overlying the dorsal optic vesicle, likely a TGF-β superfamily protein. Similarly, signaling from the optic vesicle has been implicated in directing lens formation from cells in the surface ectoderm. Various experiments have shown that patterning of neural retina and RPE can be altered through the ectopic introduction of signaling molecules, or the manipulation of various developmentally regulated transcription factors. Indeed, in frogs, chicks, and rodents, determination of neural retina and RPE can be interchanged for some time after their initial specification: presumptive RPE cells can transdifferentiate into neural retinal cells, and vice versa, demonstrating the bipotentiality of optic vesicle cells. Specifically, numerous studies demonstrate FGFs, and downstream effectors of FGF signaling, are important mediators of RPE-to-neural retinal transdifferentiation. Rx is a paired-like homeobox gene that encodes a transcription factor that is expressed in retinal progenitor cells (RPCs) of the developing optic vesicle/cup. As the optic cup forms, Rx expression is restricted to the inner layer of developing optic cup (presumptive neural retina) and is terminated as retinal progenitors exit the cell cycle and differentiate into a neural retinal cell type. Rx expression is maintained through adulthood in Müller glial cells, which have been shown to function like neural retinal stem cells. During embryogenesis, mice homozygous for a targeted Rx-null allele fail to form optic vesicles, demonstrating Rx is intrinsically required for the earliest stage of eye formation-evagination of the optic vesicles. Given the expression of Rx in proliferating retinal progenitors, we hypothesize that following optic vesicle evagination, Rx plays a role in promoting the proliferation of retinal progenitors, which could impact retinal and lens morphogenesis, and is required for the specification and/or maintenance of neural retinal identity. In this dissertation, the functions of Rx during the early stages of vertebrate ocular development following optic vesicle evagination are ascertained through Rx loss-of-function studies using Cre/loxP conditional inactivation strategies. We demonstrate that inactivation of Rx during the optic vesicle stage, via the...