Galectins from Amphibian Species: Carbohydrate Specificity, Molecular Structure, and Evolution.
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概要
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Galectins appear to be ubiquitous in homeotherm vertebrates and presumably mediate several biological processes related to cell-cell and cell-extracellular matrix interactions. However, the detailed mechanisms of their function(s), as well as the stability of their binding activity in the extracellular environment, still remain unclear. Further, although their presence has been reported in poikilotherm vertebrates and invertebrates, very little is known about their molecular properties, nature of their endogenous carbohydrate ligands, biological role(s), and the overall evolutionary history of the galectin family in these taxa. Among amphibian species, galectins have been isolated from the toad Bufo arenarum, the frogs Rana tigerina and R. catesbeiana, the African clawed frog Xenopus laevis and the axolotl Ambystoma mexicanum, but only those from X. laevis and B. arenarum have been characterized in detail. Our recent work on a galectin from toad (B. arenarum) ovary suggest that it is closer in primary and three-dimensional structure and carbohydrate specificity, to the bovine galectin-1 than to the galectin from X. laevis, a species considered as exhibiting archaic features. This observation raises several questions: How prevalent are galectin-1-like lectins in oocytes from extant species within the "modern" amphibian taxa such as Bufo sp. (Neobatrachia), as compared to the "primitive" groups that include Xenopus sp. (Archaeobatrachia)? Is the presence or absence of these lectins the result of the species' life history or environmental factors? Or does it truly reveal the fact that galectin-1-like lectins arising in the Neobatrachia have merited such striking evolutionary conservation throughout the vertebrate lineages in structure and specificity, due to their involvement in a critical biological function? In contrast with X. laevis, where galectins are mainly confined to adult skin, in the toad B. arenarum galectins are detected in oocytes and further post-fertilization stages, raising the possibility that although present in phylogenetically related taxa, they might mediate mechanisms associated with substantially different biological functions, such as host defense in X. laevis vs. developmental processes in B. arenarum. In homeotherm vertebrates galectin-1 is expressed at the earliest in the blastula stage and has been postulated to participate in trophoblast implantation on the endometrium, whereas galectin activity in B. arenarum can be detected in oocytes, fertilized eggs and stages prior to blastula. It is possible that it mediates attachment of the protective jelly coat that is typical of egg masses of Bufo sp., and further participates in cell-cell or cell-intercellular matrix interactions in the developing embryo. Our preliminary work suggests that amphibian species may constitute useful models for gaining insight in the phylogenetic and functional aspects of galectins.
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