Distribution of Xrel in the early Xenopus embryo: a cytoplasmic and nuclear gradient

In Drosophila embryos, the dorsal gene, a member of the rel family of transcription factors, is a maternal gene required for dorsal/ventral axis specification and mesoderm and neural migration and differentiation. In this report, the presence and distribution of Xrel-1 protein, a Xenopus homologue of the rel family of transcription factors, is described during early embryogenesis. Antiserum to v-rel, an avian homologue of dorsal, was used in immunoblot to detect rel proteins in Xenopus embryos. Anti-v-rel serum recognized a single band of approximately 57 kDa in Western blots of extracts of Xenopus embryos. Antiserum was also generated against bacterially expressed Xrel-1 fusion protein, a Xenopus rel homologue expressed during oogenesis. Anti-Xrel-1 antiserum recognized the same approximately 57 kDa band in Western blots as anti-v-rel. In developmental Westerns, this 57 kDa protein was present throughout early embryogenesis. Whole mounts and histologic sections of staged embryos were stained with anti-rel antiserum. Asymmetric staining of the cytoplasm of the animal cap became apparent by the eight-cell stage, with little or no staining of the vegetal hemisphere. Staining of nuclei in cells of the animal cap down to the equatorial zone became apparent between Stage 7 and 8, while vegetal nuclei never stained. Unlike Drosophila dorsal, no dorsal-ventral gradient of Xrel-1 could be detected. Anti-actin antibodies stained embryos uniformly, while preimmune serum or anti-v-rel antiserum immunoabsorbed against bacterially expressed Xrel-1 protein failed to stain. Nuclear staining diminished during gastrulation, but reappeared during neurulation. In conclusion, Xrel-1 protein is enriched in the cytoplasm of the animal pole of early Xenopus embryos, and enters the nuclei of the cells of the animal cap and presumptive mesoderm at Stage 7-1/2. The presence of this putative transcription factor in the nuclei of these cells prior to mid-blastula transition suggests that Xrel-1 may be involved in programming animal cells to respond to vegetal-inducing factors.