Thesis (Ph.D.)--University of Rochester. School of Medicine and Dentistry. Dept. of Biomedical Genetics, 2009.
The evolutionary conserved BarH family of homeodomain transcription factors plays essential roles in cell fate specification, cell differentiation, migration and survival by either activation or repression mechanisms. Barhl2, a member of the Barh gene family, is expressed restrictively in the central nervous system. In the retina, Barhl2 is expressed in developing and mature retinal ganglion cells (RGCs), amacrine cells (ACs) and horizontal cells. Here, to investigate the role of Barhl2 in retinal development, Barhl2 deficient mice were generated. Analysis of AC subtypes in Barhl2 deficient retinas suggests that Barhl2 plays a critical role in AC subtype determination. A significant reduction of glycinergic and GABAergic ACs with a substantial increase in the number of cholinergic ACs was observed in Barhl2-null retinas. Barhl2 is also critical for the development of a normal complement of RGCs. Barhl2 deficiency resulted in the approximate 35% apoptotic RGCs during development. Genetic analysis revealed that Barhl2 functions downstream of the Atoh7-Pou4f2 regulatory pathway and regulates the maturation and/or survival of RGCs. Thus, BARHL2 appears to have numerous roles in retinal development, including regulating neuronal subtype specification, differentiation, and survival.
In the spinal cord part, we present evidence showing that BARHL2 is required for the development of dI1 interneurons. Targeted deletion of Barhl2 resulted in a significant reduction in a subset of dI1 interneurons, dI1i, that project the axons ipsilaterally. Utilizing genetic axonal tracing and retrograde tracing method, we found that Barhl2-null dI1i interneurons adopted a dI1c-like contralateral axonal projection and ectopically expressed Lhx2, indicating the dI1i interneurons were transfated to a dI1c identity. Moreover, BARHL2 binds to conserved motifs in Lhx2, suggesting that BARHL2 directly regulates the expression of Lhx2. These results demonstrate the requirement for BARHL2 in the acquisition of dI1 subtype identities and also reveal a genetic hierarchy between progenitors and post-mitotic neurons that drives dI1 interneuron differentiation.
This study provides insights into the problem of how the identities of distinct classes of neurons at different positions are controlled within the nervous systems. Our results contribute to a better understanding of transcriptional programs that define neuronal subtype identities and molecular mechanisms that guide distinctive aspects of their subtype-specific properties.
