Elucidating the roles of the cardiogenic factors Tbx20, Tbx5 and Hsp27 in vertebrate heart development

The development of a functional multi-chambered contractile heart from a simple patterned tube involves a number of interacting transcriptional networks acting within well-defined temporal and spatial parameters. While many of the regulatory transcription factors have been identified and shown to be required for proper cardiogenesis, much remains to be deciphered regarding how these pathways interact at different times and within different cell populations during cardiogenesis. In addition, very little is currently known as to the specific downstream pathways affected by the various transcriptional regulators known to be involved in cardiac morphogenesis.; In this dissertation, I describe the identification of several novel Xenopus laevis (X. laevis) genes expressed during heart development, including T-box 20 (Tbx20), titin novex 3 (XTn3), and heat shock protein 27 (XHsp27), thus providing evidence that these genes may be involved in cardiogenesis. By depleting X. laevis embryos of TBX20 and the related T-box protein, TBX5, I demonstrate that these proteins are both critical for proper cardiac morphogenesis. In addition, I provide evidence for a physical and functional interaction between TBX20 and TBX5 in cardiogenesis, thus adding one potential point at which multiple pathways may converge during heart development. Results from a microarray-based screen I conducted to identify genes downstream of ii Tbx5 function led to the identification of several cell cycle genes putatively misregulated in response to loss of TBX5. This work led directly to a study conducted by Sarah Goetz in which she has demonstrated a requirement for Tbx5 for proper cell cycle progression, thus further elucidating downstream pathways in heart development. A similar screen I performed to identify genes misregulated in response to loss of Tbx20 yielded a host of novel cardiac-specific genes, setting up the basis for future studies to come. This microarray experiment further resulted in the identification of Hsp27, which I subsequently demonstrated is required for proper fusion of cardiac precursors and for actin cytoskeleton integrity. Thus, this dissertation significantly advances our understanding of cardiogenesis at multiple levels, including the identification of factors involved in cardiogenesis, interactions between these factors, and downstream cellular behaviors affected by these factors.