Golgi self-correction generates bioequivalent glycans to preserve cellular homeostasis

Abstract

Essential biological systems employ self-correcting mechanisms to maintain cellular homeostasis. Mammalian cell function is dynamically regulated by the interaction of cell surface galectins with branched N-glycans. Here we report that N-glycan branching deficiency triggers the Golgi to generate bioequivalent N-glycans that preserve galectin-glycoprotein interactions and cellular homeostasis. Galectins bind N-acetyllactosamine (LacNAc) units within N-glycans initiated from UDP-GlcNAc by the medial-Golgi branching enzymes as well as the trans-Golgi poly-LacNAc extension enzyme β1,3-N-acetylglucosaminyltransferase (B3GNT). Marginally reducing LacNAc content by limiting N-glycans to three branches results in T-cell hyperactivity and autoimmunity; yet further restricting branching does not produce a more hyperactive state. Rather, new poly-LacNAc extension by B3GNT maintains galectin binding and immune homeostasis. Poly-LacNAc extension is triggered by redistribution of unused UDP-GlcNAc from the medial to trans-Golgi via inter-cisternal tubules. These data demonstrate the functional equivalency of structurally dissimilar N-glycans and suggest a self-correcting feature of the Golgi that sustains cellular homeostasis.DOI: http://dx.doi.org/10.7554/eLife.14814.001

Glycolysis and glutaminolysis cooperatively control T cell function by limiting metabolite supply to N-glycosylation

“…However, GlcNAc or kifunensine did not acutely alter the metabolic state of the T cell, as measured by the oxygen consumption rate and extracellular acidification rate. Consistent with this, we have previously shown that lowering branching does not alter UDP-GlcNAc levels in T cells (Mkhikian et al, 2016). GlcNAc does raise UDP-GlcNAc levels, which may impact other glycan pathways such as O-glycans, O-GlcNAc or CMP-sialic scid, in addition to N-glycan branching.…”

“…Branching in N-glycans depends on UDP-GlcNAc biosynthesis via the hexosamine pathway (Dennis et al, 2009; Grigorian et al, 2011, 2007; Lau et al, 2007; Mkhikian et al, 2016). De novo synthesis of UDP-GlcNAc, the sugar-nucleotide donor substrate required by the N-glycan branching Golgi enzymes Mgat1, 2, 4 and 5, begins with the conversion of fructose-6-phosphate to glucosamine-6-phosphate by the rate-limiting enzyme glutamine-fructose-6-phosphate transaminase (GFPT, Figure 1A).…”

CDG and immune response: From bedside to bench and back

“…All in all, the ultimate question persists: “If glycans are so important for immunity, why is immunological involvement not more common within CDG?”. A few light‐shedding assumptions can be advanced: Since total glycosylation abrogation is incompatible with life, patients still retain residual levels of glycosylating capacity, which may prevent the development of more severe phenotypes, preserving reasonable immune responses; The possible development of bioequivalence glycosylation mechanisms which create alternative pathways, allowing to achieve similar functionality; The differential enzymatic expression and existence of cell/tissue/organ specific defects which leads to site‐specific anomalies, thus restricting systemic and more severe phenotypes …”

NMR-Based metabolomic analysis of the anticancer effect of metformin on cholangiocarcinoma cells

“…To our knowledge, this observation represents the rst report that the anticancer effect of metformin is related to UDP-GlcNAc. As is known, UDP-GlcNAc is a common donor substrate for the N-glycosylation of most cell-surface receptors and transporters in eukaryotes [45]. Previous works indicated that glycoproteins with few N-glycans were signi cantly increased in a switchlike response to the enhanced UDP-GlcNAc level, such as TβR, CTLA-4 and GLUT4 which mediated organogenesis, differentiation and cell cycle arrest [46,47].…”