Pattern of protein synthesis in dissociated embryonic cells of Xenopus laevis whose reaggregation was inhibited by Ca2+-deprivation and by mechanical interference

Cloning and expression studies of cDNA for a novel Xenopus 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.

Molecular cloning of cDNA for XTCAD-1, a novel Xenopus cadherin, and its expression in adult tissues and embryos of Xenopus laevis

We have isolated from a Xenopus tailbud cDNA library a novel cadherin cDNA, denoted as XTCAD-1, which contained an open reading frame including the entire coding region. XTCAD-1 codes for 714 amino acids (molecular mass: 96 kDa), which include five characteristic extracellular cadherin motifs, a single putative transmembrane domain, and a cytoplasmic domain. In each domain, XTCAD-1 shared extensive homologies with other cadherins, and was related to EP-, E-, and P-cadherins more closely than to N- and M-cadherins. In adult Xenopus, XTCAD-1 mRNA was strongly expressed in intestine/stomach, kidney and skin, which are respectively derived from endoderm, mesoderm, and ectoderm. In Xenopus embryogenesis, expression of XTCAD-1 mRNA was first detected at blastula stage, and the level of the expression increased gradually during gastrula stage, reached a peak at tailbud stage and then decreased slightly at tadpole stage. These results suggest that in Xenopus laevis XTCAD-1 plays an important role in the maintenance of adult tissues that contain epithelial cells abundantly and also in morphogenesis in early embryonic development.

Studies on cellular adhesion of Xenopus laevis melanophores: modulation of cell-cell and cell-substratum adhesion in vitro by endogenous Xenopus galactoside-binding lectin

We have investigated cell-cell and cell-substratum adhesion of Xenopus laevis neural crest cells at various stages of melanophore differentiation. Single-cell suspensions were obtained by trypsinization and aggregated in a cell-cell adhesion assay. Unpigmented cells did not adhere while the rate of adhesion of melanophores correlated with the degree of melanization. Melanophore cell-cell adhesion decreased significantly in the presence of beta-galactosidase, which suggests that cell-surface galactose is involved. Beta-galactoside-binding lectin has been isolated and purified from embryos at the stage of neural crest migration. When added to aggregating cells smaller, looser clusters formed compared to controls. When lectin was added to cells in stationary culture to test cell-substratum adhesion, melanophores spread more smoothly and formed more regular spacing patterns. These results suggest that this lectin can modulate receptors used in cell-cell and cell-substratum adhesion of melanophores.

Ammonium ion as a possible regulator of the commencement of rRNA synthesis in Xenopus laevis embryogenesis

Recently, we have shown that ammonium salts and amines at an external concentration of 3 mM selectively inhibit rRNA synthesis in Xenopus disaggregated neurula cells. We studied here the change in the amount of ammonia within the embryo and its inhibitory action on the commencement of rRNA synthesis which normally occurs at the blastula stage of development. Ammonia exists at ca. 50 ng/egg (or ca. 3.0 mM at an intra-egg concentration) in the unfertilized egg. This level was maintained during cleavage and then sharply decreased during the blastula stage to the level of ca. 20 ng/embryo (or 1.2 mM) in postblastular stages. Ammonia was extracted from cleavage embryos in a form of ammonium chloride and confirmed to selectively inhibit rRNA synthesis in neurula cells. With authentic ammonium chloride, ammonia was found to be promptly incorporated into cells and to inhibit rRNA synthesis within 1 hr after treatment. In blastula cells, ammonium salts reversibly inhibited the commencement of the synthesis of rRNA, but not hnRNA, 5 S RNA and U1, U2, and U5 snRNAs. The inhibition was at the step of transcription of 40 S pre-rRNA but not the processing or degradation of the processed rRNA. Ammonium salts did not inhibit DNA synthesis, protein synthesis, cell division, and cellular reaggregation. These observations suggest that ammonium ion may be involved in the regulation of the commencement of rRNA synthesis in Xenopus embryogenesis, although it is not yet clear if the ammonium ion exerts its effect directly upon the rDNA transcription system.