MEDIOLATERAL PATTERNING OF SOMITES - MULTIPLE AXIAL SIGNALS, INCLUDING SONIC HEDGEHOG, REGULATE NKX-3.1 EXPRESSION

The axial structures, the notochord and the neural tube, play an essen tial role in the dorsoventral patterning of somites and in the differe ntiation of their many cell lineages. Here, we investigated the role o f the axial structures in the mediolateral patterning of the somite by using a newly identified murine homeobox gene, Nkr-3.1, as a medial s emitic marker in explant in vitro assays. Nkx-3.1 is dynamically expre ssed during somitogenesis only in the youngest, most newly-formed somi tes at the caudal end of the embryo. We found that the expression of N kx-3.1 in pre-semitic tissue explants is induced by the notochord and maintained in newly-differentiated somites by the notochord and both v entral and dorsal parts of the neural tube. We showed that Sonic hedge hog (Shh) is one of the signaling molecules that can reproduce the eff ect of the axial structures by exposing explants to either COS cells t ransfected with a Shh expression construct or to recombinant SHH. Shh could induce and maintain Nkx-3.1 expression in pre-semitic mesoderm a nd young somites but not in more mature, differentiated ones. The effe cts of Shh on Nkx-3.1 expression were antagonized by a forskolin-induc ed increase in the activity of cyclic AMP-dependent protein kinase A. Additionally, we confirmed that the expression of the earliest express ed murine myogenic marker, myf5, is also regulated by the axial strucu tres but that Shh by itself is not capable of inducing or maintaining it. We suggest that the establishment of semitic medial and lateral co mpartments and the early events in myogenesis are governed by a combin ation of positive and inhibitory signals derived from the neighboring structures, as has previously been proposed for the dorsoventral patte rning of somites. (C) 1998 Elsevier Science Ireland Ltd.