We are interested in secondary induction, and are focusing on the function of Msx homeobox genes in this process. We have generated a number of mutant alleles for both Msx1 and Msx2 in the mouse. Mutant analysis confirms a role for Msx genes in inductive signal transduction. In addition, we are investigating the function of Msx genes in the adult mouse vascular smooth muscle.

Msx genes and anterior identity in the limb

In Msx1:Msx2 double null mutants, anterior elements of the limb skeleton (radius, tibia, thumb) fail to form. This is due to a defect in limb dorso-ventral polarization that precludes apical ectodermal ridge formation anteriorly, and likely derives from impairment in BMP signaling. In Msx1^-/-;Msx2^+/- compound mutants, all limb structures form, but

they fail to adopt an anterior identity (fig. 1). In the limb, antero-posterior polarization depends on Shh / Gli3 signaling. Indeed, analysis of Shh, Gli3 and Msx gene interactions demonstrates that Msx genes are downstream effectors of Gli3 in anterior identity specification. Our work sheds light on the interplay between the different signaling pathways involved in limb outgrowth and patterning.

Role of Msx genes in the formation of the dorsal midline (roof plate) of the neural tube

Msx1 and Msx2 are expressed in the dorsal midline of the neural tube. In the Msx1 mutant, the dorsal midline of the diencephalon adopts a more lateral identity, in correlation with epistasis of Msx1 over Wnt1. In the Msx1:Msx2 double mutant, these defects also encompass the mesencephalon and frequently lead to exencephaly. Conversely, in the spinal cord, the roof plate is maintained, indicating that different mechanisms are involved in dorsal midline formation in the cephalic and spinal cord regions.

Role of Msx genes in mural cells of blood vessels

Both Msx1 and Msx2 are expressed in the smooth muscle layer of peripheral arteries, up to the latest stages of life. In the retina, Msx1 expression is restri­cted to a tiny fraction of pericytes that are associated with the branching points in the arterioles. These cells constitute the pre-capillary sphincters and are critical for the regulation of blood flow into micro­circulation. They also play a major role in stabilizing newly formed endothelial tubes that branch out. This suggests possible roles for Msx genes in vessel physiology, homeostasis and regeneration.