Embryonic stem (ES) cells and induced pluripotent stem (iPS) cells are expected to play important roles in regenerative therapy. Cell surface markers, the epitopes of which are carbohydrates, are useful for distinguishing these pluripotent cells from other cells and following the process of their differentiation. The most established marker of mouse ES cells is SSEA-1. The epitope of SSEA-1 is Lewis X, namely Galβ1-4(Fucα1-3)GlcNAc. The markers of human ES cells are SSEA-3, SSEA-4, TRA-1-60 and TRA-1-81. SSEA-3 and SSEA-4 are antigens of globo-series glycolipids, while TRA-1-60 and TRA-1-81 are antigens related to keratan sulfate. iPS cells express the same set of carbohydrate markers found on ES cells of the same species. EG cells, pluripotent stem cells derived from primordial germ cells, express SSEA-1 both in humans and in mice. Embryoglycan, a high molecular weight, branched poly-N-acetyllactosamine of early embryonic cells, is expressed in mouse ES cells and in human and mouse embryonal carcinoma (EC) cells. Embryoglycan is a carrier of many carbohydrate markers, including SSEA-1. ES cells derived from mice deficient in Iβ1,6-N-acetylglucosaminyltransferase (IGnT) exhibited decreased synthesis of embryoglycan, loss of 4C9 antigen, and decreased adhesion to laminin-coated plates. The results raised the possibility that embryoglycan enhances integrin's actions.
The identities and amounts of glycan structures present on cell surfaces can change dramatically during development. Undifferentiated embryonic stem (ES) cells also express glycan structures on their surfaces and these determine some of the characteristics of the cells. Some of the glycan structures on ES cells are specific to these cells and can be used as markers for ES cells. Others, such as heparan sulfate, are not specific to ES cells but nevertheless have an important function in the cells. In this review, I describe the use of carbohydrate antigens as markers of ES cells and also consider the function of heparan sulfate, a sulfated carbohydrate structure on ES cells, in the maintenance of self-renewal and pluripotency.
N-glycosylation is now recognized as one of the most important modification reactions in eukaryotic cells, and it has been demonstrated that N-glycans on glycoproteins have crucial roles on their physicochemical properties such as solubility or stability of proteins, as well as their physiological properties such as bioactivity or intra- and intercellular trafficking. Explosive progress of glycobiology during the past decades unveiled most, if not all, of the biosynthetic pathway for N-glycans. On the other hand, how the N-glycans are catabolized in cells is relatively unexplored even in this “post-genome” era. Here I would like to outline what is known and what should be clarified regarding the catabolic pathway of N-glycans.
DC-SIGN (DC-specific intercellular adhesion molecule-3-grabbing nonintegrin), which is a type II transmembrane C-type lectin expressed on dendritic cells (DCs), recognizes cell-surface carbohydrates on invading pathogens such as bacteria and viruses and plays an important role in the uptake and presentation of these antigens. To date, little is known about the correlation of DC-SIGN with cancer. Recently, we reported that DC-SIGN bound to colorectal carcinoma cells by recognizing colorectal tumor-associated Lewis (Le) glycan ligands. The interaction of DC-SIGN with colorectal cancer cells increased LPS-induced IL-6 and IL-10 secretion from monocyte-derived DCs (MoDCs), resulting in inhibition of the maturation of MoDCs and differentiation of naïve T cells into Th1 cells. Furthermore, DC-SIGN-expressing cells were shown to colocalize with colorectal cancer-associated Le glycan ligands in primary colon cancer tissues. These results indicate that DC-SIGN-mediated dysfunction of DCs in tumor immunity is one of the critical mechanisms for escaping immune surveillance.
Sialic acid (Sia) occupies the outermost terminus of glycans. Owing to their position, negative charge, and structural diversity, sialoglycans are key determinants in various molecular recognition events. Recently, we revealed that a monoclonal antibody, GL7, recognizes glycans with a terminal α2,6-linked Sia and that this binding is specific to Sia species. Although its epitope is unclear, GL7 has been used as a marker for activated B cells and germinal centers. In a germinal center, immunoglobulins undergo affinity maturation and class switch recombination, making it an important microenvironment for acquired immune response mediated by B cells. Our results indicated that GL7 probes the activation-dependent conversion of major Sia species, from N-glycolylneuraminic acid (Neu5Gc) to N-acetylneuraminic acid (Neu5Ac), in mouse germinal center B cells. This change in Sia species occurred with the concomitant loss of the CD22 ligand on B cells. Furthermore, the phenotypes of Neu5Gc-deficient mice suggested that Neu5Gc negatively modulates B cell proliferation.