Search

Forelimbs (FLs) and hindlimbs (HLs) develop complex musculoskeletal structures that rely on the deployment of a conserved developmental program. Pitx1, a transcription factor gene with expression restricted to HL and absent from FL, plays an important role in generating HL features. The genomic mechanisms by which Pitx1 effects HL identity remain poorly understood. Here, we use expression profiling and analysis of direct Pitx1 targets to characterize the HL- and FL-restricted genetic programs in mouse and situate the Pitx1-dependent gene network within the context of limb-specific gene regulation. We show that Pitx1 is a crucial component of a narrow network of HL-restricted regulators, acting on a developmental program that is shared between FL and HL. Pitx1 targets sites that are in a similar chromatin state in FL and HL and controls expression of patterning genes as well as the chondrogenic program, consistent with impaired chondrogenesis in Pitx1-/- HL. These findings support a model in which multifactorial actions of a limited number of HL regulators redirect the generic limb development program in order to generate the unique structural features of the limb. [ABSTRACT FROM AUTHOR]

Limb development relies on an exquisite coordination between growth and patterning, but the underlying mechanisms remain elusive. Anterior-posterior and proximal-distal specification initiates in early limb bud concomitantly with the proliferative expansion of limb cells. Previous studies have shown that limb bud growth initially relies on fibroblast growth factors (FGFs) produced in the apical ectodermal ridge (AER-FGFs), the maintenance of which relies on a positive-feedback loop involving sonic hedgehog (Shh) and the BMP antagonist gremlin 1 (Grem1). The positive cross-regulation between Shh and the HoxA and HoxD clustered genes identified an indirect effect of Hox genes on the maintenance of AER-FGFs but the respective function of Shh and Hox genes in this process remains unknown. Here, by uncoupling Hox and Shh function, we show that HoxA and HoxD genes are required for proper AER-FGFs expression, independently of their function in controlling Shh expression. In addition, we provide evidence that the Hoxdependent control of AER-FGF expression is achieved through the regulation of key mesenchymal signals, namely Grem1 and Fgf10, ensuring proper epithelial-mesenchymal interactions. Notably, HoxA and HoxD genes contribute to both the initial activation of Grem1 and the subsequent anterior expansion of its expression domain. We propose that the intricate interactions between Hox genes and the FGF and Shh signaling pathways act as a molecular network that ensures proper limb bud growth and patterning, probably contributing to the coordination of these two processes. [ABSTRACT FROM AUTHOR]