Tbx5 and Tbx4 trigger limb initiation through activation of the Wnt/Fgf signaling cascade

Abstract

A tight loop between members of the fibroblast growth factor and the Wnt families plays a key role in the initiation of vertebrate limb development. We show for the first time that Tbx5 and Tbx4 are directly involved in this process. When dominant-negative forms of these Tbx genes were misexpressed in the chick prospective limb fields, a limbless phenotype arose with repression of both Wnt and Fgf genes By contrast, when Tbx5 and Tbx4 were misexpressed in the flank, an additional wing-like and an additional leg-like limbs were induced, respectively. This additional limb formation was accompanied by the induction of both Wnt and Fgf genes These results highlight the pivotal roles of Tbx5 and Tbx4 during limb initiation, specification of forelimb/hindlimb and evolution of tetrapod limbs, placing Tbx genes at the center of a highly conserved genetic program.

Finger or toe: the molecular basis of limb identity

“…In support of these conclusions, in the chick, dominant-negative forms of Tbx5 misexpressed in the limb led to the absence of Wnt2b and Fgf10 expression in the prospective wing region (Takeuchi et al, 2003). In addition, misexpression of a dominant-negative form of Lef1 initially had no effect on Tbx5 expression (Takeuchi et al, 2003). These observations suggest that Tbx5 acts upstream of Wnt signalling (Agarwal et al, 2003;Takeuchi et al, 2003), although contradictory data in chick (Ng et al, 2002) indicates that this may have to be confirmed.…”

“…These results indicate that, in the mouse, Tbx5 acts upstream of Wnt signalling in the forelimb field, and that Wnt signalling, via β-catenin, Lef1 and Tcf1, is required to maintain normal levels of Fgf10 expression. In support of these conclusions, in the chick, dominant-negative forms of Tbx5 misexpressed in the limb led to the absence of Wnt2b and Fgf10 expression in the prospective wing region (Takeuchi et al, 2003). In addition, misexpression of a dominant-negative form of Lef1 initially had no effect on Tbx5 expression (Takeuchi et al, 2003).…”

“…To understand the role of Tbx5 in early limb formation, it is necessary to review our current understanding of the molecules involved in limb initiation. A number of experimental approaches in a range of model organisms indicate that Tbx5 is essential for forelimb bud formation, and that it interacts with the Fgf and Wnt signalling pathways to initiate limb bud outgrowth (Agarwal et al, 2003;Ahn et al, 2002;Ng et al, 2002;Rallis et al, 2003;Takeuchi et al, 2003). In the following sections, the current understanding of the interactions between Tbx5 and Fgf and Wnt family proteins, which initiate and maintain limb bud formation, is outlined.…”

“…Similarly, following knockdown of tbx5 function in zebrafish (Ahn et al, 2002;Ng et al, 2002), or misexpression of dominant-negative forms of Tbx5 in the chick (Ng et al, 2002;Takeuchi et al, 2003), Fgf10 expression is either absent or downregulated in the developing limb. As a consequence of failure of normal Fgf10 signalling in Tbx5 mutants, Fgf8 expression is also disrupted in the limb (Agarwal et al, 2003;Ng et al, 2002;Rallis et al, 2003;Takeuchi et al, 2003).…”

Role of paraxial mesoderm in limb/flank regionalization of the trunk lateral plate

“…An asterisk indicates the ablated side. for forelimb bud initiation (Ng et al, 2002;Takeuchi et al, 2003;Rallis et al, 2003;Hasson et al, 2007). Tbx5 is not expressed in the LPM before stage 13 in the chick embryo (Saito et al, 2002), and the LPM fate for forelimb-flank-hindlimb regionalization can be altered up to stage 13 (Saito et al, 2002(Saito et al, , 2006.…”