The Early Expression Control of Xepsin by Nonaxial and Planar Posteriorizing Signals in Xenopus Epidermis

The control mechanism of the anteroposterior axis specification in Xenopus epidermis was studied by comparing the expression of a novel anterior marker, Xepsin, with that of a panepidermal marker, type I keratin. Xepsin mRNA, which encodes a novel Xenopus serine protease, is transcribed zygotically with the expression peak in neurula stages. In normal development, its expression is limited to the anterior and anterior–dorsal portions within epidermis during neurula and tailbud stages, respectively. In UV-irradiated ventralized embryos (dorsoanterior index, DAI 0 and 1), an expression boundary for Xepsin is apparently formed within the epidermis. In contrast, Xepsin expression was observed throughout the epidermis in LiCl-treated dorsalized embryos (DAI 10), as seen from an expression pattern indistinguishable from that of type I keratin. These data suggest that posteriorizing signals which suppress the transcription of Xepsin are present in nonaxial regions and absent in the anterior dorsal mesoderm. That posteriorizing signals were present in nonaxial regions was also supported by a conjugation experiment in which Xepsin expression was suppressed in ectodermal explants conjugated with lateral or ventral marginal zone. Moreover, the partly suppressed expression of Xepsin in the epidermal region of exogastrulae indicates that the signals may travel horizontally within the plane of the epidermis. We also present data showing that both treatment with retinoic acid and the overexpression of a constitutively active form of a retinoic acid receptor caused the suppression of Xepsin mRNA transcription, suggesting that anterior–posterior patterning in the central nervous system and in the epidermis may share common endogenous factors, i.e., retinoids, in the Xenopus embryo.