Tbx6 and the Specification of Somitic Mesoderm

During gastrulation, the dorsal mesoderm is formed, consisting of the dorsalmost notochord (chordamesoderm) and the two parasegmental cords that flank the neural tube. The cells of these paraxial mesodermal cords eventually coalesce into the somites.

It had not been known how the paraxial mesoderm was specified, but a striking result from a gene knockout experiment in mice shows that the Tbx6 protein is of critical importance.

In situ hybridization experiments (Chapman et al., 1996) had shown that Tbx6 mRNA is expressed during the formation of the paraxial mesoderm, and this suggested that the protein encoded by this message might be important in specifying the paraxial mesodermal lineages. Tbx6 is thought to encode a DNA-binding protein that might be a transcription factor.

Chapman and Papaioannou (1998) constructed gene knockouts wherein the region of the gene encoding the Tbx6 DNA-binding region was interrupted by a neomycin resistance gene. The homozygous Tbx6 mutant mice generated by the gene knockout technique all died in utero. They lacked trunk somites, but had enormous neural regions. Histological sections showed the presence of two tubular structures flanking the neural tube. These tubes looked similar to the actual neural tube, itself. When in situ hybridization was performed using probes for genes expressed in the neural tube, Chapman and Papaioannou demonstrated that these paraxial tubes were indeed neural epithelia. Moreover, they were patterned appropriately with respect to the notochord and dorsal ectoderm. Thus, the HNF-3b message, which is induced in floorplate cells by the notochord, was seen in the region of all three tubes which was closest to the notochord. The expression of the Pax3 gene, which is normally seen far from the notochord, was also seen far from the notochord in the ectopic paraxial tubes (Figure 1).

Figure 1 Mice were constructed which were homozygous for the mutated form of Tbx6. Probes that recognized specific mRNAs were incubated with microscopic cross-sections of control (left) and mutant (right) mice. The Tbx6 mutant mice had three neural tube-like structures and in situ hybridization showed that HNF-3b, Pax3, and other neural tube markers recognized specific regions of these paraxial tubes. (After Chapman and Papaioannou, 1998; with permission of the authors).

Further differentiation of the ectopic neural tubes was demonstrated by the binding of probes for motor neuron proteins neurofilament-L and islet-1. Moreover, in situ hybridization experiments using markers of normal paraxial mesoderm failed to find messages in these paraxial tubes. It appears that in the absence of Tbx6, the paraxial epithelium is converted into neural tubes rather than into presomitic mesoderm.

While it is not known whether Tbx6 normally inhibits the neural pathway or if it normally stimulates the mesodermal pathway, two other mutations are interesting in this regard. First, mutations of Wnt3a result in the failure of mesodermal precursor cells to migrate laterally away from the primitive streak. Once they have migrated through the streak, they differentiate as a ventral midline neural tube. Second, mutations of fibroblast growth factor receptor-1 also create ectopic neural tube-like structures from cells that otherwise would be paraxial mesoderm. It is possible that all three of these genes are involved in the pathway that ensures that the paraxial cells become mesodermal and not neural epithelia.

Literature Cited

Chapman, D. L., Agulnik, I., Hancock, S., Silver, L. M., and Papaionnou, V. E. 1996. Tbx6, a mouse T-box gene implicated in paraxial mesoderm formation at gastrulation. Dev. Biol. 180: 534-542.

Chapman, D. L. and Papaionnou, V. E. 1998. Three neural tubes in mouse embryos with mutations in the T-box gene Tbx6. Nature 391: 695-697.

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