Unsticking the Broken Diabetic Heart: O-GlcNAcylation and Calcium Sensitivity

Cardiovascular disease is the leading cause of death in patients with diabetes. However, identifying mechanisms of disease progression has proven difficult as diabetes involves dysfunction of numerous pathways including cellular signaling and mitochondrial capacity. This results in the alteration of circulating hormones and nutrients that can lead to inflammation, oxidative stress, endoplasmic reticulum stress, lipotoxicity, and glucotoxicity (1). One hallmark of diabetes is the development of hyperglycemia and one goal of patient care is to control glycemia through the modification of lifestyle and drug therapy. However, the effectiveness of antihyperglycemia drugs on the development of heart failure in individuals with diabetes is not fully understood (2). Part of this challenge results from an incomplete understanding of how glucose regulates molecular function. Over 30 years ago, Torres and Hart (3) identified a unique posttranslational modification (PTM) by glycosylation of intracellular proteins via addition of an O-linked b-N-acetyl-D-glucosamine (O-GlcNAc), termed O-GlcNAcylation. Owing to the input of multiple metabolic pathways to generate uridine diphosphate (UDP)GlcNAc, the substrate of O-GlcNAcylation, this PTM gained attention as the link between metabolism and chronic disease (4). Increases in glucose associated with diabetes lead to excess O-GlcNAcylation and represent a crucial link between glucose and molecular regulation. Similar to protein phosphorylation, O-GlcNAcylation occurs at serine and threonine residues in proteins. It has been identified on;4,000 targets across most cellular pathways, as reviewed by Bond and Hanover (5). Unlike other PTMs that may have dozens if not hundreds of proteins modulating levels, two enzymes control O-GlcNAc cycling: O-GlcNAc transferase (OGT) for its addition and OGlcNAcase (OGA) for its removal. One point of regulation comes from the few splice variants of the genes encoding these proteins that show differing subcellular localization, including the nucleus to alter gene expression (6) and the mitochondria to regulate metabolism (7). The apparent promiscuous nature of this modification has made defining specificity of regulation difficult. In this issue of Diabetes, Ramirez-Correa et al. (8) identify a new mechanism of subcellular redistribution of the O-GlcNAcylation machinery, providing a means by which this pathway may be regulated in the diabetic heart. Examining myofilaments from the hearts of streptozotocin-induced diabetic rats, the authors found increased O-GlcNAcylation associated with decreased Ca sensitivity. Using b-elimination/Michael addition with Tandem Mass Tags (TMT), they specifically map the O-GlcNAcylation to 63 sites on six different myofilament proteins. Removal of excessO-GlcNAc levels with the bacterial glycosidase Clostridium perfringens N-acetyl-glycosidase J (CpOGA) restores myofilament Ca sensitivity in skinned cardiac muscles isolated from diabetic rats. However, the most novel discovery of the current study is the finding that basal levels of O-GlcNAc in both rodent and human myocardium localize to the Z-line and that this localization spreads toward the A-band and intensifies in streptozotocin-induced diabetes. This suggests that the increased O-GlcNAcylation found in pan-O-GlcNAc blots might be missing part of the picture. To begin to define the mechanism of this redistributed pattern, the authors used a series of immunoprecipitations and double immunoelectron microscopy to show that OGT and OGA sarcomeric distribution is inverted, altering O-GlcNAc levels and calcium sensitivity in the diabetic myocardium (Fig. 1). This suggests that the location of the O-GlcNAcylation machinery within the cell partially dictates its function and might explain some of the physiological and pathological roles of this PTM. O-GlcNAcylation modifies a number of pathways to regulate cellular function. It therefore becomes difficult to define cause and effect. The changes in calcium signaling may also result from O-GlcNAc regulation of Ca cycling either through modification of regulating transcription factors decreasing expression of sarcoendoplasmic reticulum Ca ATPase 2a (SERCA2a) (9) or