A pivotal Wnt antagonist role promoting digit joint specification by constraining Wnt activity

Bmps and Wnts often act antagonistically. Here we report that in mouse digit progenitors, unlike long bone joints, they cooperate to promote chondrogenic over joint (interzone) commitment. Elevated Bmp signaling prevents 5'Hoxd^Δ/Δ digit progenitors from forming interzones, causing joint loss. We show that constitutive βCatenin activation (βCatCA) in 5'Hoxd^Δ/Δ interdigits restores digit joints indirectly and cell non-autonomously. RNA profiling reveals βCatCA induces secreted Wnt antagonists that restore 5'Hoxd^Δ/Δ digit joints by reducing digit-tip Bmp activity. Deleting the βCatCA-induced Wnt antagonist Dkk2 in 5'Hoxd^Δ/Δ interdigits abolishes joint rescue by βCatCA. Wnts inhibit Gsk3β kinase, which phosphorylates and destabilizes both βCatenin and Bmp-activated receptors pSmad1/5. In cultured limb buds, Gsk3β antagonists stabilize pSmad1/5, enhancing digit-tip Bmp activity. We propose that Wnt antagonists prevent precocious pSmad1/5 accumulation by stabilizing Gsk3β, favoring joint fate. Excess Bmp-pSmad1/5 activity in 5'Hoxd^Δ/Δ digit-tips accelerates chondrogenic commitment, impeding a switch to joint fate. Wnt antagonists maintain mesenchymal plasticity for normal phalanx-joint specification by slowing the pace of chondrogenic commitment.