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

Self-association of helical peptides in a lipid environment

The self-association of two model transmembrane helical peptides, differing in their surface topography, was compared in mixed micelles containing 3-([3-cholamidopropyl]dimethylammonio)-1-propanesulfonate (CHAPS) and dimyristoylphosphatidylcholine (DMPC). One peptide, Ac-KKL24KK-amide (L24), has large, rotationally mobile leucine side chains and a relatively rough surface. The other peptide, Ac-KKLLLLLLAALLALLAALLALLLLLLKK-amide (L18A6), has a patch of small alanines on one side of the helix that forms a smooth surface. The aggregation state of the peptides was sampled by chemical cross-linking with bis-sulfosuccinimidyl suberate (B53). A monomer-aggregate association constant was obtained from the cross-linking results in the range of 2 x 10(5) M(-1) to 3 x I0(5) M(-1) for both peptides. Kinetics of formation of cross-linked dimers indicated that the ratio of dimerization constants for L18A6 to L24 was between 10 and 20. This suggests that the alanine patch contributes about 1.5 Kcal/mol more stabilization free energy to dimer formation of L18A6 compared to L24.

Cloning and expression of an alpha-2,8-polysialyltransferase (STX) from Xenopus laevis

Polysialic acid (polySia) is a carbohydrate structure found on neural cell adhesion molecules (N-CAM). Two polysialyltransferases (polySiaTs) that catalyze synthesis of polySia have been described, and designated PST-1/PST/ST8SiaIV and STX/ST8SiaII. We cloned a polySiaT (xSTX) from a nonmammalian vertebrate, Xenopus laevis . xSTX had 80% amino acid similarity to the rat STX. This clone induced polySia expression when transfected into polySia-negative COS-1 cells. Northern blot analysis of whole embryos at different stages of development revealed that xSTX mRNA was most abundantly expressed in premetamorphic stages. The relative level of xSTX and N-CAM mRNAs was also examined and found to change in parallel to the extent of polysialylation on N-CAM. In adult tissues, the expression of xSTX mRNA was restricted to brain, eye and heart, which also expressed polySia. These results suggest that xSTX is the major enzyme responsible for the synthesis of polysialylated N-CAM in embryos at certain stages of development and also in adult tissues.

Exogastrula formation in Xenopus laevis embryos depleted with maternal XmN-cadherin mRNA by antisense S-oligo DNA

Xenopus XmN-cadherin gene appears to have dual functions, since its mRNA is maternally provided in unfertilized eggs, once disappears almost completely during gastrula stage, then accumulates again specifically in neural tissues in later stage embryos. In the present experiment, we first followed the change in XmN-cadherin mRNA level during oogenesis by RT-PCR and showed that this mRNA exists from the earliest stage of oogenesis and at least one third of it is inherited as a maternal mRNA. We then carried out an experiment to deplete the maternal XmN-cadherin mRNA by injecting its antisense S-oligo DNA into full grown oocytes. When mRNA-depleted oocytes were matured in vitro and fertilized eggs obtained therefrom by host transfer technique were allowed to develop, embryos cleaved normally and developed until blastula stage. Such XmN-cadherin mRNA-depleted blastulae initiated invagination, but further involution did not take place, and exogastrulae were formed. These results suggest that the main function of maternally provided XmN-cadherin mRNA is to support cell movement or rearrangement required later during gastrulation, rather than to maintain adhesion of blastomeres during cleavage and blastula formation.

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.