Regulation of neural plate border formation by distal-less proteins

This thesis investigates the process of neural plate border formation in Xenopus laevis. In the first part of this thesis (Chapter Two), I examine the role of Distal-less (D1x) proteins in positioning the mediolateral boundary between the neural plate and the epidermis and in inducing border cell fates. Functional analysis using activating and repressing versions of Dlx reveals Dlx proteins are both sufficient and necessary to inhibit neural induction along the neuralmonneural border. While activation of Dlx activity inhibits neural tissue, repression of Dlx activity results in expansion of the neural plate. Interestingly, the stereotypic pattern of border cell fates is reconstituted along the new neural plate border. Through transplantation of the neural plate into ventral ectoderm, I show that, while interactions between the neural plate and ectoderm can normally induce border cell fates such as neural crest and placodes, inhibition of Dlx activity in the non-neural ectoderm prevents the induction of these cell fates. These findings demonstrate that Dlx activity is required in the non-neural ectoderm for the induction of border cell fates. Additional experiments using conditional versions of the activating and repressing Dlx constructs show that Dlx activity is required prior to midgastrulation to properly position the neural plate border.; The second part of this thesis (Chapter Three) examines the mechanism through which Dlx positions the neural plate border. Previous studies had suggested that modulation of BMP signaling patterns the early ectoderm. To determine if Dlx activity influences BMP signaling levels within the ectoderm, I analyzed the effects of modulating Dlx activity on endogenous BMP mRNA expression and signaling. The results of these analyses demonstrate that inhibiting Dlx activity results in a loss of BMP signaling and mRNA expression within the ventral ectoderm. Activation of Dlx activity, however, does not upregulate BMP signaling or mRNA expression within the dorsal ectoderm. These results suggest that Dlx activity is required to maintain BMP signaling within the non-neural ectoderm but is insufficient to independently induce ectopic BMP signaling.; Taken together, the work presented in this thesis demonstrates that Dlx activity plays two roles in positioning and forming the neural plate border: first, Dlx inhibits neural induction downstream of BMP signaling; and second, Dlx is required within the non-neural ectoderm for the maintenance of BMP signaling and the induction of adjacent neural plate border cell fates.