| The Nk-like homeobox genes, Msx1 and Msx2 are immediate downstream effectors of Bmp signaling pathway. They are expressed in largely overlapping domains and functioning cooperatively during embryonic development.;It is well known that the formation of primordial germ cells (PGC) requires active Bmp signaling and mutations in Bmp ligands, receptors or transducers results in germ cell loss. Due to the embryonic lethality of Msx1-/- and Msx1-/- ;Msx2-/- mutants, the function of Msx genes in germ cell development has not received much attention. In this study, we described their expression patterns during gastrulation, showed that Msx1-/-;Msx2-/- mutant contains significantly less PGCs than the controls, suggesting that Msx1 and Msx2 may play a minor role in PGC specification. More importantly, PGC migration is affected in Msx1-/- ;Msx2-/- mutant since E8.5, in association with an upregulation of Fibronectin. We further suggest that Msx genes act non-autonomously through ECM in regulating PGC migration.;To gain more knowledge on the function of Msx genes in skull vault development, we reexamined the expression patterns of Msx1 and Msx2 in the calvarial mesenchyme and showed an intriguing phenomena in which the open space between the frontal rudiments was filled by ectopic bony islands in Wnt1-Cre;Msx1,2cko/cko mutant, suggesting expression of Msx genes protects the apical mesenchymal cells from osteogenic differentiation. We further indicated that a dysregulation of Bmp signaling pathway likely caused the ectopic osteogenesis in this mutant. Then, we showed that the expression domain of Msx genes, in particular Msx2 is tightly controlled and is critical for the bone primordium to form properly. Thus ectopic osteoblast differentiation marked by an expansion of ALP activity was observed in the presumptive frontal bone primordium in Foxc1 ch/ch mutant, accompanied by expansions of Msx2 as well as Bmp2 and Bmp4 in the same manner. Finally, we presented evidence that Foxc1 directly regulates Msx2 by showing Foxc1 is recruited to a 52bp Bmp responsive element in the 5' flanking region of Msx2, and inactivation of Msx2 in a Foxc1 mutant background rescues its phenotype. |
