Book Description

T-box genes encode a family of transcription factors that contain a conserved DNA-binding domain, the T-box, and have been shown to play a crucial role in various developmental processes. Two zebrafish T-box genes, no tail (ntl) and spadetail (spt), as well as their orthologs in other vertebrates, have been shown to play important roles in the specification and patterning of posterior mesoderm. While wild-type zebrafish embryos develop ~30 somites that will later differentiate into muscle and vertebrae, spt mutants lack the anterior 15-17 trunk somites, and ntl mutants lack a notochord and the posterior 15 tail somites. Interestingly, embryos mutant for both ntl and spt lack all trunk and tail mesoderm, including tissues that form in both single mutants, indicating Ntl and Spt have overlapping functions in specifying these structures. Despite the obvious importance of T-box factors in development, relatively few of their transcriptional targets have been identified and tested to examine their role in mediating posterior mesoderm development. Recently, microarray results published by our lab and others have generated an extensive list of genes up- or down-regulated by Ntl and Spt. For my thesis I have chosen to focus on characterizing the role of several of these potential targets: mesogenin (msgn1), which belongs to the bHLH family of transcription factors; T-box gene 6-like (tbx6l), itself a T-box protein; RNA binding motif protein 38 (rbm38), an RNA-binding protein; and integrin beta 5 (itgb5), an adhesion and signaling molecule. Through characterization of a null mutant, I have shown that msgn1 functions with spt to promote cell migration out of the tailbud, but is not essential for zebrafish development. Additionally, I have shown that depleting tbx6l by morpholino oligonucleotide results in perturbation of dorsal-ventral patterning during gastrulation as well as dose-dependent loss of tail mesoderm. I have also characterized the spatial and temporal expression patterns of rbm38 and itgb5 and shown that both genes are expressed in the presumptive mesoderm and tailbud, where they overlap with ntl and spt expression, consistent with a role in mesodermal development. Characterizing the functions of downstream targets will add to the gene regulatory network for specification of posterior mesoderm and help to detail the molecular mechanism of vertebrate posterior development in general.