Msx genes and BMP signaling in mammalian digit regeneration

Manjong Han and Ken Muneoka

In mammals, including humans, digit tips readily regenerate following amputation injury. Using a mouse model we have characterized the regenerating fetal digit tip and show that regeneration correlates with the expression of many distally restricted genes, including Msx1, Msx2 and Bmp4. We show that the regeneration response can occur in cultured digits and we have used an in vitro screen to characterize regeneration in mutants carrying a targeted deletion of the Msx genes, Msx1 and Msx2. When regeneration does occur in Msx1 mutant digits, we provide evidence that Msx2 compensates for the loss of Msx1 function. Our studies also identify Bmp4 as a gene downstream of Msx function in the developing and regenerating digit tip. Further, we find that the Msx1 mutant phenotype is rescued in a dose-dependent manner by exogenous application of BMP4, thus providing evidence that along with Msx function, BMP signaling is critical for the regeneration response. Consistent with this conclusion, we find that treatment of amputated wildtype digits with the BMP binding protein, NOGGIN, inhibits the regeneration response without inhibiting endogenous Msx or Bmp4 expression. Our studies provide both loss of function and gain of function evidence implicating Msx function and BMP signaling in the regulation of mammalian digit regeneration.