Chie Koga

Cloning and expression studies of cDNA for a novelXenopus cadherin (XmN-cadherin), expressed maternally and later neural-specifically in embryogenesis

From a Xenopus tailbud cDNA library, we obtained the cDNA for a novel cadherin which was named as XmN-cadherin (Xenopus maternally expressed neural cadherin). The cDNA consisted of 3690 bp and encoded 922 amino acid residues. XmN-cadherin preserved five extracellular cadherin motifs, a single transmembrane domain, and a cytoplasmic domain, and was closely related by its sequence to R- and N-cadherin. In the adult frog, XmN-cadherin mRNA was detected strongly in ovary, testis, brain, eye, and kidney, and weakly in stomach, and intestine. In the egg, the mRNA occurred as a maternal mRNA at a relatively high level, and its level became very low by the neurula stage, then increased steadily thereafter. Dissection experiments with 8-cell stage and neurula stage embryos revealed that the maternally inherited mRNA was relatively uniformly distributed within the embryo. By a sharp contrast, whole mount in situ hybridization revealed that the zygotically expressed mRNA occurred almost exclusively in neural tissues such as brain, the anterior part of spinal cord, and the optic and otic vesicles. Thus, XmN-cadherin appears to have at least triple functions; it probably contributes in early embryos to cell-type non-specific cell adhesion, but in post-neurula embryos may be responsible for the development and/or maintenance of anterior neural tissues, and may be used in adult frog for the development and/or maintenance of neural, endodermal and reproductive organs.

Different spatial distribution of mRNAs for activin receptors (type IIA and IIB) and follistatin in developing embryos of Xenopus laevis

Spatial distribution of mRNAs for activin receptors and follistatin was studied by Northern blot hybridization using RNAs from different parts of dissected Xenopus embryos. mRNAs of two activin receptors (type IIA and IIB) occurred uniformly in pre-gastrular embryos, but occurred in larger amounts in ectoderm (in gastrulae), neural plate (in neurulae) and anterior (head) regions (in tailbud embryos) than in other embryonic regions. By contrast, follistatin mRNA appeared almost exclusively in the dorsal mesoderm including invaginating organizer region at the gastrula stage, in notochord and in dorsal ectoderm at the neurula stage, then in anterior part at the tailbud stage. The localized patterns of the distribution of these mRNAs may be due to the regionally different zygotic expression of genes in embryos at later stages. From the relatively widespread pattern of distribution of their mRNAs, we assume that both type IIA and type IIB activin receptors have broad functions in ectodermal and neural differentiation. On the other hand, follistatin mRNA showed quite a restricted pattern of expression, and therefore, we assume that follistatin may have functions more specifically related to the sites of expression of its mRNA. Thus, follistatin may be involved in the differentiation of notochord itself and/or directly be responsible for organizer functions such as neural induction and subsequent differentiation of induced neural tissues at the gastrula and later stages.

ASSIGNMENT OF THE HUMAN GENE FOR KBF2/RBP-Jk TO CHROMOSOME 9p12-13 AND 9q13 BY FLUORESCENCE IN SITU HYBRIDIZATION

SummaryThe transcription factor KBF2 has been characterized as a factor that binds to the NFkB site of mouse major histocompatibility complex (MHC) class I genes and its amino acid sequence has been shown to be identical to those of members of the recombination signalsequence binding protein (RBP-Jk) family. Previous studies by Amakawa et al. (Genomics17, 306–315, 1993) demonstrated that the functional gene is localized at human chromosome 3q25. However, in the present study we showed by in situ hybridization with the functional KBF2/RBPJk cosmid clone that the gene is localized at 9p12-13 and 9q13, namely, at the same loci as pseudogenes that were reported previously (Zhang et al., Jpn J Human Genet 39, 391–401, 1994).

The developing Xenopus embryo as a complex system: Maternal and zygotic contribution of gene products in nucleo-cytoplasmic and cell-to-cell interactions

The developing animal embryo constitutes a complex system in which various nucleo-cytoplasmic (N-C) and cell-to-cell (C-C) interactions take place. In that sense, it is possible to define early embryogenesis as a function of these interactions, as for instance is expressed by a formula “Development = f (N-C, C-C)”. We present here our recent studies on temporal and spatial control of the expression of genes in zygotic nucleus and of genes exogenously introduced in Xenopus embryos. For zygotic gene expression, our studies revealed that the syntheses of mRNA, tRNA and rRNA are initiated at the cleavage stage, the stage of midblastula transition (MBT) and late blastula stage, respectively. For exogenously-injected genes, we summarize their expression pattern which is controlled by the promoter they carry in addition to the cytological effects of the injection. We also briefly present our recent results obtained with embryos which had been injected with in vitro-transcribed mRNAs.