Investigating neurogenesis and cell type specification in the mammalian thalamus.

Abstract

The thalamus mediates a variety of important brain functions that are critical for behavior and survival. A key feature that enables the thalamus to perform such diverse functions is its parcellation into anatomically and functionally distinct groups of neurons called nuclei. The purpose of this project was to identify the origin of neuronal diversity within the thalamus by investigating the process of neurogenesis. During neurogenesis, proliferating progenitor cells begin to divide asymmetrically to generate neurons. The central hypothesis of the research presented herein is that thalamic organization requires the appropriate number and types of neurons to be generated and that these critical processes are regulated during neurogenesis. This work has characterized the different types of progenitor cells present during thalamic neurogenesis. We confirmed the existence of a special population of thalamic progenitor cells, intermediate (or basal) progenitor cells, and identified transcription factors that regulate their formation and/or maintenance. We also addressed the origin of distinct subtypes of neurons. The spatial organization of thalamic progenitor cells into two distinct progenitor domains during neurogenesis is thought to drive the formation of different subtypes of thalamic neurons. Signaling molecules have been proposed to induce the formation of distinct progenitor domains in numerous brain areas, including the thalamus. We provided a detailed characterization of components of the Wnt/β-catenin-mediated transcriptional pathway during thalamic neurogenesis. Based on the pattern of signaling activity, we hypothesized that Wnt/β-catenin-mediated transcription has a function in forming the two progenitor domains during thalamic neurogenesis. Using conditional genetic manipulations of β-catenin, we found that β-catenin-mediated transcription is required for the specification of thalamic progenitor domains. Furthermore, we found that the Wnt/β-catenin signaling pathway functions in parallel with the sonic hedgehog (Shh) signaling pathway, which had been previously shown to specify thalamic progenitor identity in an opposing manner, by independently regulating transcriptional networks in thalamic progenitor cells. Collectively, the process of neurogenesis involves the generation of the correct number of neurons by regulating asymmetric progenitor divisions and generation of appropriate neuronal subtypes through the functions of signaling pathways and transcriptional networks. These mechanisms provide a broad map for the generation and positioning of appropriate types of neurons in the correct locations within the thalamus.