A cell autonomous function of Brachyury in T/T embryonic stem cell chimaeras

Abstract

Developmental genetics has shown that the Brachyury (T) gene has a key role in mesoderm formation during gastrulation in the mouse. Homozygous embryos have a defective allantois, degenerate or absent notochord and disrupted primitive streak and node. The neural tube is kinked and somite formation interrupted. The T gene has been cloned and is expressed during the early stages of gastrulation, being restricted to the primitive streak region, nascent mesoderm and notochord. Neither the sequence of the gene nor its expression pattern define its developmental function. To study the cell autonomy of the T mutation we have isolated and genetically characterized embryonic stem cell lines and studied their behaviour in chimaeras. T/+ embryonic stem cells form normal chimaeras, whereas T/T in equilibrium with +/+ chimaeras mimic the T/T mutant phenotype. The results indicate that the T gene acts cell autonomously in the primitive streak and notochord but may activate a signalling pathway involved in the specification of other mesodermal tissues.

T‐box genes in early embryogenesis

“…In support of such a hypothesis, measurements of mesodermal migration on an extracellular matrix show a reduction in the migration of mutant-derived mesodermal cells relative to wild-type (Hashimoto et al, 1987). A role for Brachyury in cell adhesion is further supported by in vivo analysis showing T -/-mutant embryonic stem (ES) cells are compromised in their ability to migrate away from the primitive streak and, therefore, unable to carry out the morphogenetic movements performed by their wild-type counterparts during gastrulation, leading to their accumulation in the primitive streak (Rashbass et al, 1991;Wilson et al, 1993Wilson et al, , 1995Wilson and Beddington, 1997). As shown in homozygous T mutant embryos, this accumulation eventually leads to loss by programmed cell death ( Fig.…”

A primary requirement for nodal in the formation and maintenance of the primitive streak in the mouse

“…Loss of T gene function seems to affect the migratory properties of posterior mesoderm cells, as shown by the non-uniform distribution of T mutant cells in chimeric embryos (Beddington et al, 1992;Rashbass et al, 1991;Wilson et al, 1993). Similarly in X e n o p u s, over-expression of g s c has been shown to be associated with enhanced migratory properties of cells (Neihrs et al, 1993).…”

The murine allantois: emerging paradigms in development of the mammalian umbilical cord and its relation to the fetus

“…Although it was thought that the foreshortened allantois in conceptuses bearing a deletion of the Brachyury (T) gene was the result of faulty mesoderm translocation from the streak (Rashbass et al, 1991;Wilson et al, 1993Wilson et al, , 1995, later studies showed that T/T mutant cells could contribute to the conceptus at all levels, including the allantois, and thus, short T/T allantoises were not the result of a defect in apparent migration. Limited in situ fate mapping has recently confirmed that the primitive streak of T C /T C mutants produces mesoderm that translocates into the allantois with kinetics similar to wildtype conceptuses (Inman and Downs, 2006b).…”

“…Moreover, it seems to be present only during the period of allantoic elongation. Allantoises homozygous for T/T (Rashbass et al, 1991), or T C /T C (Inman and Downs, 2006b) tend to spread out toward the amnion rather than elongate toward the chorion.…”