The multifaceted role of intracellular glycosylation in cytoprotection and heart disease

Abstract

The modification of nuclear, cytoplasmic, and mitochondrial proteins by O-linked β-N-actylglucosamine (O-GlcNAc) is an essential posttranslational modification that is common in metozoans. O-GlcNAc is cycled on and off proteins in response to environmental and physiological stimuli impacting protein function, which, in turn, tunes pathways that include transcription, translation, proteostasis, signal transduction, and metabolism. One class of stimulus that induces rapid and dynamic changes to O-GlcNAc is cellular injury, resulting from environmental stress (for instance, heat shock), hypoxia/reoxygenation injury, ischemia reperfusion injury (heart attack, stroke, trauma hemorrhage), and sepsis. Acute elevation of O-GlcNAc before or after injury reduces apoptosis and necrosis, suggesting that injury-induced changes in O-GlcNAcylation regulate cell fate decisions. However, prolonged elevation or reduction in O-GlcNAc leads to a maladaptive response and is associated with pathologies such as hypertrophy and heart failure. In this review, we discuss the impact of O-GlcNAc in both acute and prolonged models of injury with a focus on the heart and biological mechanisms that underpin cell survival.

Keywords: ER stress; autophagy; cardioprotection; cellular stress response; chaperone; glycoprotein; heart failure; hypertrophy; integrated stress response.

Figure 1

Cellular stress/injury models impacted by O-GlcNAcylation. Stress-induced changes in O-GlcNAc-cycling and O-GlcNAc–mediated cytoprotection have been observed in a broad range of environmental and physiological models. While global changes in O-GlcNAc levels are detected (both increased and decreased) in response to injury, protein specific studies identify a more nuanced model in which O-GlcNAc is cycled on and off proteins differentially. That is, while global levels of O-GlcNAc are increased, O-GlcNAcylation can be reduced on a subset of proteins. Similarly, in models where global O-GlcNAc levels are decreased, an elevation of O-GlcNAcylation is observed on a subset of proteins. Nonetheless, in all these models manipulating O-GlcNAc impacts survival, specifically acutely depressing O-GlcNAc promotes cell death, whereas acutely increasing O-GlcNAc augments survival. One interpretation of these data is that loss of O-GlcNAc on a subset of proteins promotes cell death, whereas elevation of O-GlcNAc on others promotes cell survival pathways. OGA, O-GlcNAcase; OGT, O-GlcNAc-transferase.

Figure 2

The biosynthesis of O-GlcNAc. O-GlcNAc transferase (OGT; O15294, EC:2.4.1.255) and O-GlcNAcase (OGA; O60502; EC:3.2.1.169), respectively, catalyze the addition and removal of O-GlcNAc. OGT uses the nucleotide sugar UDP-GlcNAc, which is synthesized by the hexosamine biosynthetic pathway (HBP, boxed). Select OGT and OGA inhibitors are indicated, as well as inhibitors of glutamine-fructose-6-phosphate aminotransferase (GFPT1/2; Q06210/O94808, EC:2.6.1.16), the rate limiting enzyme of the HBP. Other enzymes include: hexokinase (HK; EC:2.7.1.1), glucose-6-phosphate isomerase (GPI; EC:5.3.1.9), glucosamine-6-phosphate isomerase (GNPDA1/2; P46926/Q8TDQ7, EC:3.5.99.6), N-acetyl-D-glucosamine kinase (NAGK; Q9UJ70, EC:2.7.1.59), glucosamine 6-phosphate N-acetyltransferase (GNPAT; Q96EK6, EC:2.3.1.4), N-acetylglucosamine-6-phosphate deacetylase (AMDHD2; Q9Y303, EC:3.5.1.25), phosphoacetylglucosamine mutase (PGM3; O95394; EC:5.4.2.3), UDP-N-acetylhexosamine pyrophosphorylase (UAP; Q16222; EC:2.7.7.83). UniProt identification numbers are listed for human proteins/genes. 5AcSGlcNAc, (2S,3R,4R,5S,6R)-3-acetamido-6-(acetoxymethyl)tetrahydro-2H-thiopyran-2,4,5-triyl triacetate; AZA, azaserine; DON, 6-diazo-5-oxo-L-norleucine; NButGT, (3aR,5R,6S,7R,7aR)-3a,6,7,7a-tetrahydro-5-(hydroxymethyl)-2-propyl-5H-pyrano[3,2-d]thiazole-6,7-diol; OMSI: (αR)-α-[[(1,2-dihydro-2-oxo-6-quinolinyl)sulfonyl]amino]-N-(2-furanylmethyl)-2-methoxy-N-(2-thienylmethyl)-benzeneacetamide; PUGNAc, O-(2-acetamido-2-deoxy-D-glucopyranosylidenamino) N-phenylcarbamate; TMG, thiamet-G.

Figure 3

An overview of the autophagy pathway. Autophagy can be broken down into five steps: induction, nucleation, maturation, fusion, and degradation. The first committed step of autophagy is activation of Ulk1, which results in the formation of a complex with Atg13, Fip200, and Atg101. In turn, the ULK1 complex can activate the PI3 kinase complex (VPS34, VPS15, Beclin1, ATG14). The VPS34 complex generates phosphatidylinositol (3,4,5)-trisphosphate (PIP₃) on the cytosolic face of the ER membrane to form the omegasome, which recruits effector proteins WIPI2/DFCP1. In turn, WIPI1/DFCP1 recruits a ubiquitin-like conjugation system, ATG12-AT5-ATG16L1, that cleaves LC3-1 and subsequently conjugates it with phosphatidylethanoloamine to form LC3-II. The latter is critical for the elongation and closure of the phagophore. Several cargo proteins, including p62/sequestosome, recruit cargo to the growing phagophore. Ultimately, the autophagosome fuses with the lysosome, resulting in the degradation of contents. Many proteins within this pathway are O-GlcNAc–modified (156), including central players indicated with a blue square (): AMPK (249), ULK1, Beclin1, and p62/sequestosome (56). Steps promoted and inhibited by O-GlcNAcylation are identified. AMPK, 5′-AMP-activated protein kinase; ATG, autophagy; CaMK II, calcium/calmodulin-dependent protein kinase kinase 2; DFCP1, double FYVE-containing protein 1; GRASP55, Golgi reassembly-stacking protein of 55 kDa; LC3, microtubule-associated proteins 1A/1B light chain 3B; LKB1, liver kinase B1; mTOR, mammalian target of rapamycin; PIP₃, phosphatidylinositol-3,4,5-triphosphate; SNAP29, synaptosomal-associated protein 29; ULK1, Unc-51–like kinase 1; WIPI1, WD repeat domain phosphoinositide-interacting protein 1.

Figure 4

The impact of sustained changes in cardiac O-GlcNAcylation. Pressure overload, diabetes, and other heart derangements lead to elevated O-GlcNAcylation that is associated with maladaptation. Pathologic phenotypes can be recapitulated by genetic and pharmacological upregulation of O-GlcNAc (indicated in blue) and counteracted by blocking the HBP or reducing O-GlcNAcylation (indicated in gray). Changes in O-GlcNAc are associated with pathology via changes in transcription, reactive oxygen and nitrogen species, calcium handling, ER stress, signal transduction, and mitochondrial energetics and function. ER, endoplasmic reticulum; HBP, hexosamine biosynthetic pathway.