The number of N-glycans (n) is a distinct feature of each glycoprotein sequence and cooperates with the physical properties of the Golgi N-glycan-branching pathway to regulate surface glycoprotein levels. The Golgi pathway is ultrasensitive to hexosamine flux for the production of tri- and tetra-antennary N-glycans, which bind to galectins and form a molecular lattice that opposes glycoprotein endocytosis. Glycoproteins with few N-glycans (e.g., TbetaR, CTLA-4, and GLUT4) exhibit enhanced cell-surface expression with switch-like responses to increasing hexosamine concentration, whereas glycoproteins with high numbers of N-glycans (e.g., EGFR, IGFR, FGFR, and PDGFR) exhibit hyperbolic responses. Computational and experimental data reveal that these features allow nutrient flux stimulated by growth-promoting high-n receptors to drive arrest/differentiation programs by increasing surface levels of low-n glycoproteins. We have identified a mechanism for metabolic regulation of cellular transition between growth and arrest in mammals arising from apparent coevolution of N-glycan number and branching.

Genetic information flows from DNA to macromolecular structures—the dominant force in the molecular organization of life. However, recent work suggests that metabolite availability to the hexosamine and Golgi N-glycosylation pathways exerts control over the assembly of macromolecular complexes on the cell surface, and in this capacity, acts upstream of signaling and gene expression. The structure and number of N-glycans per protein molecule cooperate to regulate lectin binding and thereby the distribution of glycoproteins at the cell surface. Congenital disorders of glycosylation provide insight as extreme hypomorphisms, whereas milder deficiencies may encompass many common chronic conditions including autoimmunity, metabolic syndrome, and aging.

Autoimmunity is a complex trait disease where the environment influences susceptibility to disease by unclear mechanisms. T cell receptor clustering and signaling at the immune synapse, T cell proliferation, CTLA-4 endocytosis, T H 1 differentiation, and autoimmunity are negatively regulated by ␤1,6GlcNAc-branched N-glycans attached to cell surface glycoproteins. ␤1,6GlcNAc-branched N-glycan expression in T cells is dependent on metabolite supply to UDP-GlcNAc biosynthesis (hexosamine pathway) and in turn to Golgi N-acetylglucosaminyltransferases Mgat1, -2, -4, and -5. In Jurkat T cells, ␤1,6Glc-NAc-branching in N-glycans is stimulated by metabolites supplying the hexosamine pathway including glucose, GlcNAc, acetoacetate, glutamine, ammonia, or uridine but not by control metabolites mannosamine, galactose, mannose, succinate, or pyruvate. Hexosamine supplementation in vitro and in vivo also increases ␤1,6GlcNAc-branched N-glycans in naïve mouse T cells and suppresses T cell receptor signaling, T cell proliferation, CTLA-4 endocytosis, T H 1 differentiation, experimental autoimmune encephalomyelitis, and autoimmune diabetes in non-obese diabetic mice. Our results indicate that metabolite flux through the hexosamine and N-glycan pathways conditionally regulates autoimmunity by modulating multiple T cell functionalities downstream of ␤1,6GlcNAc-branched N-glycans. This suggests metabolic therapy as a potential treatment for autoimmune disease.Complex trait diseases such as autoimmunity are determined by poorly understood genetic and environmental interactions. The T cell-mediated autoimmune diseases multiple sclerosis (MS) 3 and type 1 diabetes exemplify this problem, where identical twins of Northern European descent are discordant ϳ60 -70% of the time despite displaying an ϳ150 -300 times higher risk than the general population prevalence of ϳ0.1 and ϳ0.4%, respectively (1, 2). Genetic-environmental interactions have been established between disease-associated major histocompatibility complex haplotypes and specific pathogen peptides that mimic disease self antigens (3, 4). The prevalence of MS and type 1 diabetes changes along north-south gradients, implicating ultraviolet light exposure and production of vitamin D3 in the skin (5-7), a hormone known to negatively regulate T cell function, the MS animal model experimental autoimmune encephalomyelitis (EAE), and spontaneous autoimmune diabetes in the non-obese diabetic (NOD) mouse (5, 8 -11). However, molecular mechanisms for genetic-environmental interactions are poorly understood.Salvage of glucosamine by the hexosamine pathway to UDPGlcNAc is reported to suppress T cell function and EAE in mice by an unknown mechanism (12, 13). De novo biosynthesis of UDP-GlcNAc by the hexosamine pathway utilizes glucose, acetyl-CoA, glutamine, and UTP (see Fig. 1), key allosteric regulators of basic metabolism, suggesting regulation of UDP-GlcNAc supply is integrated with down-stream pathways requiring this sugar-nucleotide. In this regard, the Golgi pathway to ␤1,6Glc-NAc-b...

How environmental factors combine with genetic risk at the molecular level to promote complex trait diseases such as multiple sclerosis (MS) is largely unknown. In mice, N-glycan branching by the Golgi enzymes Mgat1 and/or Mgat5 prevents T cell hyperactivity, cytotoxic T-lymphocyte antigen 4 (CTLA-4) endocytosis, spontaneous inflammatory demyelination and neurodegeneration, the latter pathologies characteristic of MS. Here we show that MS risk modulators converge to alter N-glycosylation and/or CTLA-4 surface retention conditional on metabolism and vitamin D3, including genetic variants in interleukin-7 receptor-α (IL7RA*C), interleukin-2 receptor-α (IL2RA*T), MGAT1 (IVAVT−T) and CTLA-4 (Thr17Ala). Downregulation of Mgat1 by IL7RA*C and IL2RA*T is opposed by MGAT1 (IVAVT−T) and vitamin D3, optimizing branching and mitigating MS risk when combined with enhanced CTLA-4 N-glycosylation by CTLA-4 Thr17. Our data suggest a molecular mechanism in MS whereby multiple environmental and genetic inputs lead to dysregulation of a final common pathway, namely N-glycosylation.

Rapidly proliferating cells switch from oxidative phosphorylation to aerobic glycolysis plus glutaminolysis, markedly increasing glucose and glutamine catabolism. Although Otto Warburg first described aerobic glycolysis in cancer cells >90 years ago, the primary purpose of this metabolic switch remains controversial. The hexosamine biosynthetic pathway requires glucose and glutamine for de novo synthesis of UDP-GlcNAc, a sugar-nucleotide that inhibits receptor endocytosis and signaling by promoting N-acetylglucosamine branching of Asn (N)-linked glycans. Here, we report that aerobic glycolysis and glutaminolysis co-operatively reduce UDP-GlcNAc biosynthesis and N-glycan branching in mouse T cell blasts by starving the hexosamine pathway of glucose and glutamine. This drives growth and pro-inflammatory TH17 over anti-inflammatory-induced T regulatory (iTreg) differentiation, the latter by promoting endocytic loss of IL-2 receptor-α (CD25). Thus, a primary function of aerobic glycolysis and glutaminolysis is to co-operatively limit metabolite supply to N-glycan biosynthesis, an activity with widespread implications for autoimmunity and cancer.DOI: http://dx.doi.org/10.7554/eLife.21330.001