Functional Analysis of β1,3-N-Acetylglucosaminyltransferases and Regulation of Immunological Function by Polylactosamine

We have cloned many genes of beta 1,3-glycosyltransferases, which transfer sugars via a beta 1,3-linkage, and have characterized their biological functions. Among beta 1,3-glycosyltransferases, beta 1,3-N-acetylglucosaminyltransferases (beta 3GnTs) synthesize a unique carbohydrate structure known as "polylactosamine (poly-N-acetyllactosamine)". Polylactosamine is carried on N- and O-glycans, and on glycolipids. Polylactosamine structures are considered to be integral components serving as a fundamental structure and backbone for carbohydrate structures. However, most of their biological functions are still unknown. To investigate the in vivo function of polylactosamine on glycoconjugates, we generated and analyzed two mouse lines of beta 1,3-N-acetylglucosaminyltransferase (B3gnt)-deficient (B3gnt2(-/-) or B3gnt5(-/-)) mice lacking the polylactosamine structure. First, to investigate the in vivo function of polylactosamine on glycoproteins, we analyzed gene knockout mice lacking B3gnt2, which synthesizes polylactosamine on glycoproteins. In B3gnt2(-/-) mice, glycan analysis demonstrated that the amount of long polylactosamine chains on N-glycan was greatly reduced in the tissues of B3gnt2(-/-) mice. We also examined immunological responses in B3gnt2(-/-) mice. B3gnt2(-/-) lymphocytes showed hyperactivation via TCR/CD28 or BCR stimulation. Next, to investigate the in vivo function of polylactosamine on glycosphingolipids (glycolipid), we analyzed B3gnt5(-/-) mice lacking lacto/neolacto-series glycolipids. B3gnt5(-/-) B cells showed an abnormality of glycolipid-enriched microdomains (GEMs; also known as glycolipid rafts) and showed hyperactivation via BCR-related molecules in GEMs, as compared with wild-type (WT) B cells. Polylactosamine deficiency seems to be involved in the immunological disorders observed in these mice. Taken together, these studies suggest that the polylactosamine chain is a putative immune regulatory factor that presumably suppresses excessive responses during immune reactions and has an important biological role in the immune system.