T-box transcription factor interactions in the zebrafish mesoderm: regulation of gene expression and cell fate.

T-box genes encode related transcription factors that regulate tissue specification and morphogenesis in vertebrates. Whereas individual T-box genes are commonly expressed across broad domains of precursor cells, phenotypic analyses of mutant embryos indicate that T-box genes often have regionally proscribed functions within their expression domains. As multiple T-box genes are often coexpressed within developmental fields, I propose that interactions among T-box genes may alter the activity of an individual T-box gene within a portion of its expression domain. The T-box genes no tail, spadetail and tbx6 are expressed in overlapping domains within the mesoderm of the zebrafish embryo, and regions marked by the coexpression of these genes correlate well with domains of known cell fate. Additionally, the phenotypes of no tail and spadetail mutants indicate that these broadly expressed genes have tissue-specific functions within their continuous expression domains. I hypothesized that in domains of coexpression, T-box genes may interact to direct the region-specific activities of an individual family member. I demonstrate that both No Tail and Spadetail regulate distinct target genes in sub-regions of their expression domains. Additionally, I show that interactions among the three T-box genes account for some of the region-specific functions of No Tail and Spadetail. These findings indicate that the mesoderm T-box genes function as an interacting network to direct region-specific gene expression and development. The identified T-box gene interactions do not account for all the region-specific activities of spadetail and no tail. The expression of tbx6 within the ventral mesoderm, for example, requires spadetail function; however, as spadetail is initially expressed throughout the entire mesoderm, there must be additional factors that limit the expression of tbx6 to the ventral subdomain of the spadetail expression domain. Here I demonstrate that the correct expression of tbx6 requires the combined functions of Spadetail and signaling through the BMP pathway. Thus, an interaction with the BMP pathway is an additional mechanism that directs the region-specific activities of a T-box transcription factor. Finally, I show that regulation of tbx6 by Spadetail is direct and mediated through the interaction of Spadetail with a novel T-site motif present in the tbx6 promoter.