THE EXPRESSION PATTERN OF XENOPUS MOX-2 IMPLIES A ROLE IN INITIAL MESODERMAL DIFFERENTIATION

We have isolated a Xenopus homolog of the murine Mox-2 gene. As is the case for the mouse homolog, mesoderm specific expression of Xenopus M ox-2 (X. Mox-2) expression begins during gastrulation. Using whole mou nt in situ hybridization, we show that X. Mox-2 is expressed in undiff erentiated dorsal, lateral and ventral mesoderm in the posterior of ne urula/tailbud embryos, with expression more anteriorly detected in the dermatomes. In the tailbud tadpole, X. Mox-2 is expressed in tissues of the tailbud itself that represent a site of continued gastrulation- like processes resulting in mesoderm formation. X. Mox-2 is not expres sed in the marginal zone of blastula, nor in the dorsal lip of gastrul a, nor midline tissues (i.e, prospective notochord). Treatments that a ffect mesodermal patterning during embryonic development, including Li Cl and ultraviolet light, and injection of mRNAs encoding BMP-4, or do minant negative activin and FGF receptors, produce changes in X. Mox-2 expression consistent with the types of tissues affected by these man ipulations. X. Mox-2 expression is induced more in animal caps treated with FGF than those treated with activin. Together with the fact that X. Mox-2 activation in animal caps requires protein synthesis, our da ta suggest that X. Mox-2 is involved in initial mesodermal differentia tion, downstream of molecules affecting mesoderm induction and determi nation such as Brachyury and goosecoid, and upstream of factors contro lling terminal differentiation such as MyoD and myf5. X. Mox-2, theref ore, is another useful marker for understanding the formation of mesod erm in amphibian development.