Occurrence of a species-specific nuclear antigen in the germ line of Xenopus and its expression from paternal genes in hybrid frogs

A cell cycle arrest is necessary for bottle cell formation in the early Xenopus gastrula: Integrating cell shape change, local mitotic control and mesodermal patterning

During development cell proliferation and morphogenetic movements are tightly intermingled. Both processes depend on the same cytoskeletal elements. Therefore, precise regulation of local mitotic activity seems to be basic for proper embryogenesis. Here, I report on bottle cells as an early non-mitotic cell population in the Xenopus gastrula. Endogenous and activin/BVg1-induced ectopic bottle cells do not proliferate. Overexpression of the mitosis-promoting phosphatase cdc25C increases the proliferation rate and interferes with bottle cell formation whereas the phosphatase-dead mutant cdc25C(C457A) does not. Cdc25C also affects other gastrulation processes such as epiboly, vegetal rotation or tissue separation as inferred from histological inspection of early gastrulae. Double stainings of gsc/Xbra transcripts and mitotic nuclei in ectopic and endogenous lips demonstrated that non-mitotic cells occur in the bottle cell region and, to a lesser extent, in the gsc domain which both are indicative of high TGF-beta signalling. In contrast, the Xbra-region and the remainder of the animal cap appear to be permissive for higher rates of cell proliferation. These data suggest inhibition of cell proliferation by high levels of activin-type signals and a close link of mesodermal and mitotic patterning. Finally, coexpression of eFGF together with activin/BVg1 interferes with TGF-beta-induced bottle cell formation. This inhibitory effect correlates with increased cell proliferation as compared to embryos injected with activin/BVg1 alone. Taken together, these data suggest that TGF-beta and FGF signals play antagonistic roles in bottle cell formation and the spatial control of the cell cycle in early Xenopus gastrulae.

Cell specificity of nuclear protein antigens in the development of Xenopus species

Protein antigens accumulated in the oocyte nucleus of Xenopus spp. are found later in all nuclei of the embryo. During and after organogenesis, the fate of different nuclear antigens diverges: Some are permanently expressed in all nuclei, some are no longer detectable in any nuclei, and some show a high degree of specificity relative to cell type. Of the class of antigens permanently expressed in all cells, a nucleolar antigen has been studied that is relatively enriched in the gonia of both sexes. A possible correlations of its occurrence with ribosomal DNA amplification and transcription is examined. The cell-specific expression of 4 antigens has been analyzed in detail, using indirect immunofluorescent staining methods as well as conventional histology. The tissue-specificity of 2 of these antigens does not change during metamorphosis, whereas the pattern of others is dynamic until after metamorphosis. We conclude that the observed tissue-specificity does not correlate with descendence from one of the 3 primary germ layers: Some antigens appear to accumulate in the nucleus when a cell takes up its specific function.

The function of the nuclear envelope in nuclear protein accumulation

The mechanism by which proteins accumulate in the cell nucleus is not yet known. Two alternative mechanisms are discussed here: (a) selective unidirectional entry of karyophilic proteins through the nuclear pores, and (b) free diffusion of all proteins through the nuclear pores and specific binding of nuclear proteins to nondiffusible components of the nucleoplasm. We present experiments designed to distinguish between these alternatives. After mechanical injury of the Xenopus oocyte nuclear envelope, nuclear proteins were detected in the cytoplasm by immunohistochemical methods. In a second approach, nuclei from X. borealis oocytes were isolated under oil, the nuclear envelopes were removed, and the pure nucleoplasm was injected into the vegetal pole of X. laevis oocytes. With immunohistochemical methods, it was found that each of five nuclear proteins rapidly diffuses out of the injected nucleoplasm into the surrounding cytoplasm. The subsequent transport and accumulation in the intact host nucleus could be shown for the nuclear protein N1 with the aid of a species-specific mAb that reacts only with X. borealis N1. Purified and iodinated nucleoplasmin was injected into the cytoplasm of Xenopus oocytes and its uptake into the nucleus was studied by biochemical methods.

Immunological relationship between oocyte nuclear proteins of Xenopus laevis and X. borealis

Monoclonal antibodies (mABs) have been raised against oocyte nuclear proteins of Xenopus laevis and X. borealis and have been screened for species specificity. Although about 40% of all germinal vesicle polypeptides differ between the two species as judged by two-dimensional gel analysis, most mABs react with polypeptides of both species. Biochemical analysis of the antigens by immunoblotting revealed that a homologue of each antigen of one species could be detected in the other species, despite frequent differences in molecular structure. Nevertheless, five strictly species-specific mABs have been identified. All five are directed against the same acidic polypeptide B3 of X. borealis, which is a structurally altered homologue of the protein N1, previously described in X. laevis germinal vesicles. In oocyte nuclei of hybrids between X. laevis females and X. borealis males, polypeptide of both species appear to be accumulated equivalently. Exceptions to this rule are most easily explained by differences between individuals and by the loss of certain alleles resulting from the cross.