THE MURINE HOMEOBOX GENE MSX-3 SHOWS HIGHLY RESTRICTED EXPRESSION IN THE DEVELOPING NEURAL-TUBE

The mouse homeobox-genes Msx-1 and Msx-2 are expressed in several area s of the developing embryo, including the neural tube, neural crest, f acial processes and limb buds. Here we report the characterisation of a third mouse Msx gene, which we designate Msx-3. The embryonic expres sion of Msx-3 was found to differ from that of Msx-1 and -2 in that it was confined to the dorsal neural tube. In embryos with 5-8 somites a segmental pattern of expression was observed in the hindbrain, with r hombomeres 3 and 5 lacking Msx-3 while other rhombomeres expressed Msx -3. This pattern was transient, however, such that in embryos with 18 or more somites expression was continuous throughout the dorsal hindbr ain and anterior dorsal spinal cord. Differentiation of dorsal cell ty pes in the neural tube can be induced by addition of members of the Tg f-beta family. Additionally, Msx-1 and -2 have been shown to be activa ted by addition of the Tgf-beta family member Bmp-4. To determine if B mp-4 could activate Msx-3, we incubated embryonic hindbrain explants w ith exogenous Bmp-4. The dorsal expression of Msx-3 was seen to expand into more ventral regions of the neurectoderm in Bmp-4-treated cultur es, implying that Bmp-4 may be able to mimic an in vivo signal that in duces Msx-3.