tbx6,aBrachyury-Related Gene Expressed by Ventral Mesendodermal Precursors in the Zebrafish Embryo

Abstract

Classical embryology experiments have indicated the existence of dorsal-type and ventral-type mesoderms that arise as a consequence of mesoderm induction during vertebrate development. Here we report that the zebrafish tbx6 gene, a member of the Brachyury-related T-box family of genes, is exclusively expressed by ventral mesendoderm. Three observations link the expression of tbx6 to ventral mesoderm specification. First, the gene is initially expressed at the onset of gastrulation within a ventrolateral subpopulation of cells that express the pan-mesodermal gene, no tail (Brachyury). Second, the mesoderm-inducing factors activin and bFGF activate tbx6 expression in animal caps. Third, dorsalization of the mesendodermal precursor population following exposure of embryos to lithium ions causes down-regulation of tbx6 transcription. tbx6 is expressed transiently in the involuting derivatives of the ventral mesendoderm, which give rise to nonaxial mesodermal tissues; its expression is extinguished as tissue differentiation progresses. Transcription of tbx6 commences about an hour after initiation of expression of the pan-mesendodermal gene no tail and the organizer gene goosecoid. The dependence of tbx6 expression on no tail activity was examined in no tail mutant embryos. The activation of tbx6 transcription in ventral mesoderm does not depend on no tail gene activity. However, no tail appears to contribute to the maintenance of normal levels of tbx6 transcription and may be required for tbx6 transcription in the developing tail.

T‐box genes in early embryogenesis

“…The expression pattern of tbx6 is similar to that of spadetail, i.e., in the trunk and tail paraxial mesoderm progenitors of the hypoblast layer ( Fig. 3a; Hug et al, 1997). In spadetail mutant embryos, tbx6 expression is greatly reduced in the paraxial mesoderm progenitors of the trunk but is still expressed in the paraxial mesoderm of the tail at later stages, when tail formation is under way.…”

An interacting network of T-box genes directs gene expression and fate in the zebrafish mesoderm

“…ntl, spt, and tbx6 are expressed exclusively in the developing zebrafish mesoderm (20,21,23). Overlapping expression of the three genes demarcates distinct regions of mesoderm identities (Fig.…”

“…Myc-epitope-tagged protein was detected with the 9E10 antibody (Santa Cruz Biotechnology). To measure gene expression in animal caps, caps were removed at 3.5 h from injected embryos and incubated for 5 h at 28°C (21), and RNA expression was detected by RT-PCR and quantified (see Supporting Materials and Methods). Relative MyoD expression was determined as the ratio of MyoD PCR product to cytokeratin8 PCR product for each experimental condition.…”

“…We examined individual and combined functions of three T-box genes, no tail (ntl), spadetail (spt), and tbx6, which are expressed exclusively in the developing zebrafish mesoderm (20)(21)(22)(23)(24). The three genes are expressed in broad domains that overlap and together mark all of the mesoderm (Fig.…”

pMesogenin1 and 2 function directly downstream of Xtbx6 in Xenopus somitogenesis and myogenesis

“…A member of this family, tbx6, is expressed in primitive streaks, paraxial mesoderms, muscle-lineage blastomeres, and tailbuds in various species (Chapman et al, 1996;Yasuo et al, 1996;Hug et al, 1997;Griffin et al, 1998;Uchiyama et al, 2001). It is also involved in mesoderm specification and differentiation.…”