TBX3 is essential to establish the posterior boundary of anterior genes and up-regulate posterior genes with HAND2 during onset of limb bud development

Abstract

During limb bud formation, axes polarities are established as evidenced by the spatially restricted expression of key regulator genes. In particular, the mutually antagonistic interaction between the GLI3 repressor and HAND2 results in the distinct and non-overlapping anterior-distalGli3and posteriorHand2expression domains. This hallmarks establishment of antero-posterior (AP) limb axis polarity together with spatially restricted expression of homeodomain and other transcriptional regulators. Here, we identify TBX3 as the transcription factor to initiate AP axis polarity in mouse limb buds. ChIP-seq and differential gene expression analysis of wildtype and mutant limb buds identifies the TBX3-specific and shared TBX3-HAND2 target genes. High sensitivity fluorescent whole mountin situhybridisation shows that the posterior expression boundaries of anterior genes are positioned by TBX3-mediated repression, which excludes anterior genes such asGli3,Alx4,Hand1andIrx3/5from the posterior limb bud mesenchyme. This exclusion delineates the posterior mesenchymal territory competent to establish theShh-expressing limb bud organiser. In turn, HAND2 is required forShhactivation and cooperates with TBX3 to up-regulate shared posterior identity target genes in early limb buds.