Various biological functions of carbohydrate chains learned from glycosyltransferase-deficient mice

Abstract

Carbohydrate chains are attached to various proteins and lipids and modify their functions. The complex structures of carbohydrate chains, which have various biological functions, are involved not only in regulating protein conformation, transport, and stability but also in cell-cell and cell-matrix interactions. These functional carbohydrate structures are designated as "glyco-codes." Carbohydrate chains are constructed through complex reactions of glycosyltransferases, glycosidases, nucleotide sugars, and protein and lipid substrates in a cell. To elucidate the functions of carbohydrate chains, I and my colleagues generated and characterized knockout (KO) mice of galactosyltransferase family genes. In this review, I introduce our studies about galactosyltransferase family genes together with related studies performed by other researchers, which I presented in my award lecture for the Ando-Tajima Prize of the Japanese Association for Laboratory Animal Science (JALAS) in 2019.

Keywords: carbohydrate chains; galactosyltransferase; glyco-code; glycobiology; knockout mice.

Fig. 1.

Phylogenetic tree of the β-1,4-galactosyltransferase gene family. Phylogenetic tree of B4galt-1 to -7 genes. The mouse chromosome number in which each β4GalT gene is located is indicated in parentheses.

Fig. 2.

A model of stepwise adhesion of leukocytes and endothelial cells during inflammation and lymphocyte homing to peripheral lymph nodes (PLNs). In the first step, leukocytes in the bloodstream start to roll by the interaction of sialyl Lewis x (sLe^x) on leukocytes and E/P-selectins on endothelial cells near inflammatory sites. During lymphocyte homing to PLNs, the interaction of L-selectin on lymphocytes and sialyl 6-sulfo Le^x on the high endothelial venule (HEV) in PLNs is important. Then, chemokines expressed on proteoglycans of endothelial cells bind to chemokine receptors induced on leukocytes. The chemokine signals induce integrins and integrin ligands on leukocytes and endothelial cells, respectively. Finally, activated leukocytes infiltrate into inflammatory sites.

Fig. 3.

Sialyl Lewis x (sLe^x) on core 2 O-glycans and sialyl 6-sulfo Lewis x (Le^x) on core 1 O-glycans. Composed of sialic acid, galactose, GlcNAc, and fucose, sLe^x is mostly expressed at the terminus of N-acetyl lactosamine repeats on core 2 O-glycans. Sialyl 6-sulfo Le^x, in which fucose of sLe^x is sulfated, is on core 1 O-glycans.

Fig. 4.

Hematopoietic stem/progenitor cell (HSPC) differentiation. Illustration of HSPC differentiation to terminal blood cells. LT-HSC, long-term hematopoietic stem cell (HSC); ST-HSC, short-term HSC; MPP, multipotent progenitor; CLP, common lymphoid progenitor; CMP, common myeloid progenitor; GMP, granulocyte-macrophage progenitor; MEP, megakaryocyte-erythroid progenitor.

Fig. 5.

Illustration of human IgA1 and mouse IgA with O- and N-glycosylation sites. Human IgA1 (right) has several O-glycosylation sites in the hinge region and two N-glycosylation sites in the CH2 region and C-terminus. Mouse IgA (left) has two N-glycosylation sites in the CH1 and CH3 regions but no O-glycosylation sites in the hinge region.

Fig. 6.

Biosynthetic pathway of glycosphingolipids (GSLs). Ceramide (Cer) is the biosynthetic origin of GSLs. Glucosylceramide (GlcCer) is synthesized from Cer by GlcCer synthetase, and subsequently, lactosylceramide (LacCer) is synthesized from GlcCer by LacCer synthetase. Our results indicate that LacCer synthetase is encoded by B4galt-5 and B4galt-6 genes in mice. LacCer is the starting point of various gangliosides, including o-, a-, b-, and c-series gangliosides.

Fig. 7.

Tetraploid chimeric analysis. A wild-type (wt, +/+) two-cell stage egg marked by green fluorescent protein (GFP) is electrofused to make a one-cell tetraploid egg. The 4-cell stage wt tetraploid embryo is aggregated with a diploid mutant (mt, −/−) 8-cell stage embryo to make a chimeric embryo. When the chimeric embryo differentiates into a blastocyst and further, the trophectoderm layer and placental tissues are mostly derived from the wt tetraploid egg, respectively, although the embryo is derived from the mt egg. This is because the tetraploid egg cannot differentiate toward epiblast lineages. If the embryonic lethal phenotype is caused by a defect in extraembryonic tissues, these chimeric embryos could be rescued and develop further.