Glycosaminoglycans (GAGs) such as chondroitin sulfates and heparan sulfates are linear polysaccharide chains that are covalently attached to multiple core proteins to form proteoglycans (PGs). GAG chain assembly begins with the synthesis of common protein linkage region tetrasaccharide (glucuronic acid-galactose-galactose-xylose; GlcA-Gal-Gal-Xyl); this linkage region tetrasaccharide is attached to specific serine residues of core proteins. The GAG-protein linkage region tetrasaccharide of PGs transiently exhibited 2-O-phosphorylation of xylose. We identified a Xyl kinase that phosphorylates C-2 of the xylose residue and phosphoxylose phosphatase. This review focuses on the biological significance of phosphorylation and dephosphorylation of Xyl residues in the linkage region tetrasaccharide of PGs.

Glycosphingolipids that play crucial roles in various physiological contexts are ubiquitous components of the eukaryotic plasma membrane. Complex structures of glycan chains found in glycosphingolipids are accurately synthesized by glycosyltransferases including the GM3 and GM2 synthases in the Golgi apparatus. We have identified three isoforms of mouse GM3 synthase (M1-, M2-, and M3-GM3S), which have distinct lengths of an NH₂-terminal cytoplasmic domain. These isoforms are co-expressed within a single cell, although they exhibit significant differences in their subcellular localization and protein stability. We have also found that GM2 synthase has two protein isoforms (M1- and M2-GM2S) with varying length of the NH₂-terminal. Notably, M1-GM2S enhances the stability of M2-GM2S through disulfide-linked heterodimerization. These isoforms could participate in spatiotemporally fine regulation of the functions of GM3 and GM2 synthases and contribute to the maintenance of the balance of glycosphingolipid synthesis. This review summarizes our current understanding of regulatory mechanisms underlying the glycosphingolipid biosynthesis, particularly focusing on the intracellular dynamics and isoform production of GM3 and GM2 synthases.

O-linked N-acetylglucosaminylation (O-GlcNAcylation) is a glycosylation characterized by the attachment of a single N-acetylglucosamine (GlcNAc) to the serine/threonine residues of nuclear, mitochondrial, and cytoplasmic proteins. Proteins modified by O-GlcNAc include signaling components, transcription factors, epigenetic regulators, and histones. O-GlcNAc has various functions such as inhibition of phosphorylation, regulation of transcriptional activity, stabilization of proteins, and regulation of intracellular localization. In recent years, O-GlcNAc has been drawing attention as a key factor for regulating the undifferentiated/differentiated state in mouse/human pluripotent stem cells, and the functions of O-GlcNAc have been gradually clarified. In this review, we introduce the versatile functions of O-GlcNAc in mouse/human pluripotent stem cells.

In demyelinating diseases such as multiple sclerosis, the regeneration of lost myelin sheaths (remyelination) is clinically important. N-acetyl glucosaminyl transferase IX (GnT-IX, GnT-Vb) is a branching enzyme on O-mannosyl glycan that is expressed exclusively in the brain in vivo. GnT-IX-deficient mice showed enhanced remyelination compared to wild-type mice in a cuprizone-induced demyelination model. In GnT-IX-deficient mice, astrocyte activation was attenuated while oligodendrocyte differentiation was simultaneously promoted, suggesting they are the causes of enhanced remyelination. HNK-1-capped branched O-mannosyl glycan is attached to should-be receptor-type protein tyrosine phosphatase ζ (PTPRZ) and expressed in reactive astrocytes. Moreover, GnT-IX was shown to be involved in PTPRZ lipid raft targeting. GnT-IX-deficient mice showed no obvious abnormalities; therefore, GnT-IX is a potential therapeutic target that promotes remyelination in demyelinating diseases.

Glycosaminoglycans (GAGs) such as chondroitin sulfates and heparan sulfates are linear polysaccharide chains that are covalently attached to multiple core proteins to form proteoglycans (PGs). GAG chain assembly begins with the synthesis of common protein linkage region tetrasaccharide (glucuronic acid-galactose-galactose-xylose; GlcA-Gal-Gal-Xyl); this linkage region tetrasaccharide is attached to specific serine residues of core proteins. The GAG-protein linkage region tetrasaccharide of PGs transiently exhibited 2-O-phosphorylation of xylose. We identified a Xyl kinase that phosphorylates C-2 of the xylose residue and phosphoxylose phosphatase. This review focuses on the biological significance of phosphorylation and dephosphorylation of Xyl residues in the linkage region tetrasaccharide of PGs.

Glycosphingolipids that play crucial roles in various physiological contexts are ubiquitous components of the eukaryotic plasma membrane. Complex structures of glycan chains found in glycosphingolipids are accurately synthesized by glycosyltransferases including the GM3 and GM2 synthases in the Golgi apparatus. We have identified three isoforms of mouse GM3 synthase (M1-, M2-, and M3-GM3S), which have distinct lengths of an NH₂-terminal cytoplasmic domain. These isoforms are co-expressed within a single cell, although they exhibit significant differences in their subcellular localization and protein stability. We have also found that GM2 synthase has two protein isoforms (M1- and M2-GM2S) with varying length of the NH₂-terminal. Notably, M1-GM2S enhances the stability of M2-GM2S through disulfide-linked heterodimerization. These isoforms could participate in spatiotemporally fine regulation of the functions of GM3 and GM2 synthases and contribute to the maintenance of the balance of glycosphingolipid synthesis. This review summarizes our current understanding of regulatory mechanisms underlying the glycosphingolipid biosynthesis, particularly focusing on the intracellular dynamics and isoform production of GM3 and GM2 synthases.

O-linked N-acetylglucosaminylation (O-GlcNAcylation) is a glycosylation characterized by the attachment of a single N-acetylglucosamine (GlcNAc) to the serine/threonine residues of nuclear, mitochondrial, and cytoplasmic proteins. Proteins modified by O-GlcNAc include signaling components, transcription factors, epigenetic regulators, and histones. O-GlcNAc has various functions such as inhibition of phosphorylation, regulation of transcriptional activity, stabilization of proteins, and regulation of intracellular localization. In recent years, O-GlcNAc has been drawing attention as a key factor for regulating the undifferentiated/differentiated state in mouse/human pluripotent stem cells, and the functions of O-GlcNAc have been gradually clarified. In this review, we introduce the versatile functions of O-GlcNAc in mouse/human pluripotent stem cells.

In demyelinating diseases such as multiple sclerosis, the regeneration of lost myelin sheaths (remyelination) is clinically important. N-acetyl glucosaminyl transferase IX (GnT-IX, GnT-Vb) is a branching enzyme on O-mannosyl glycan that is expressed exclusively in the brain in vivo. GnT-IX-deficient mice showed enhanced remyelination compared to wild-type mice in a cuprizone-induced demyelination model. In GnT-IX-deficient mice, astrocyte activation was attenuated while oligodendrocyte differentiation was simultaneously promoted, suggesting they are the causes of enhanced remyelination. HNK-1-capped branched O-mannosyl glycan is attached to should-be receptor-type protein tyrosine phosphatase ζ (PTPRZ) and expressed in reactive astrocytes. Moreover, GnT-IX was shown to be involved in PTPRZ lipid raft targeting. GnT-IX-deficient mice showed no obvious abnormalities; therefore, GnT-IX is a potential therapeutic target that promotes remyelination in demyelinating diseases.