Defective Hand1 phosphoregulation uncovers essential roles for Hand1 in limb morphogenesis [RESEARCH ARTICLE]

Beth A. Firulli, Hannah Milliar, Kevin P. Toolan, Jade Harkin, Robyn K. Fuchs, Alex G. Robling, and Anthony B. Firulli

The morphogenesis of the vertebrate limbs is a complex process where cell signaling and transcriptional regulation coordinate diverse structural adaptations across species. In this study, we examine the consequences of altering Hand1 dimer choice regulation within the developing vertebrate limbs. Although Hand1 deletion via the limb-specific Prx1-Cre reveals a non-essential role for Hand1 in limb morphogenesis, altering Hand1 phosphoregulation, and consequently Hand1 dimerization affinities, results in a severe truncation of anterior-proximal limb elements. Molecular analysis reveals a non-cell autonomous mechanism that causes widespread cell death within embryonic limb bud. In addition, we observe changes in proximal anterior gene regulation including a reduction in the expression of Irx3&5, Gli3, and Alx4, all of which are upregulated in Hand2 limb conditional knockouts. A reduction of Hand2 and Shh gene dosage improves the integrity of anterior limb structures validating this proposed mechanism.

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