The intra-mitochondrial O-GlcNAcylation system rapidly modulates OXPHOS function and ROS release in the heart

Protein O-GlcNAcylation is increasingly recognized as an important cellular regulatory mechanism, in multiple organs including the heart. However, the mechanisms leading to O-GlcNAcylation in mitochondria and the consequences on their function remain poorly understood. In this study, we use an in vitro reconstitution assay to characterize the intra-mitochondrial O-GlcNAc system without potential cytoplasmic confounding effects. We compare the O-GlcNAcylome of isolated cardiac mitochondria with that of mitochondria acutely exposed to NButGT, a specific inhibitor of glycoside hydrolase. Amongst the 409 O-GlcNAcylated mitochondrial proteins identified, 191 display increased O-GlcNAcylation in response to NButGT. This is associated with enhanced Complex I (CI) activity, increased maximal respiration in presence of pyruvate-malate, and a striking reduction of mitochondrial ROS release, which could be related to O-GlcNAcylation of specific subunits of ETC complexes (CI, CIII) and TCA cycle enzymes. In conclusion, our work underlines the existence of a dynamic mitochondrial O-GlcNAcylation system capable of rapidly modifying mitochondrial function.

An in vitro assay in isolated heart mitochondria reveals that O-GlcNAcase inhibitor NButGT rapidly increases protein O-GlcNAcylation leading to increased respiratory capacity and complex I activity and decreased ROS release.

Evaluation of muscle oxygenation by near infrared spectroscopy in patients with facioscapulohumeral muscular dystrophy

The purpose of the study was to determine muscle metabolism adaptation to exercise in facioscapulohumeral muscular dystrophy patients (FSHD) and to study the correlation with clinical functional status (6-min walk test). 8 FSHD patients and 15 age-matched healthy controls (Controls) performed two isokinetic constant-load knee extension exercises: (1) at 20% of their maximal extensors' peak torque (i.e., the same relative workload) and (2) at (20N⋅m) (the same absolute workload) for up to 4 min. All exercises consisted of rhythmic, voluntary, isokinetic, concentric contractions of the quadriceps femoris at 90°/s, whereas the flexion was performed passively at the same speed. Muscle oxygenation in the vastus lateralis was evaluated using near-infrared spectroscopy (NIRS). The FSHD patients displayed a lower maximal peak torque than controls (-41%, p < 0.05). During the two-exercise modalities, deoxygenated haemoglobin (HHb) and total haemoglobin volume (tHb) were lower in the FSHD patients (p < 0.05). The initial muscle deoxygenation time delay was shorter in the control group (FSHD: 15.1 ± 4.1 s vs.

CONTROLS

10.4 ± 2.1 s, p < 0.05). Mean response time and maximal peak torque were both correlated with functional impairment (walking endurance). The results suggest that FSHD patients present an impairment in their capacity to deliver or to use oxygen.

Microbiopsies versus Bergström needle for skeletal muscle sampling: impact on maximal mitochondrial respiration rate

INTRODUCTION

Microbiopsies are increasingly used as an alternative to the standard Bergström technique for skeletal muscle sampling. The potential impact of these two different procedures on mitochondrial respiration rate is unknown. The objective of this work was to compare microbiopsies versus Bergström procedure on mitochondrial respiration in skeletal muscle.

METHODS

52 vastus lateralis muscle samples were obtained from 13 anesthetized pigs, either with a Bergström [6 gauges (G)] needle or with microbiopsy needles (12, 14, 18G). Maximal mitochondrial respiration (V GM-ADP) was assessed using an oxygraphic method on permeabilized fibers.

RESULTS

The weight of the muscle samples and V GM-ADP decreased with the increasing gauge of the needles. A positive nonlinear relationship was observed between the weight of the muscle sample and the level of maximal mitochondrial respiration (r = 0.99, p < 0.05) and between needle size and maximal mitochondrial respiration (r = 0.99, p < 0.05).

CONCLUSION

Microbiopsies give lower muscle sample weight and maximal rate of mitochondrial respiration compared to the standard Bergström needle.Therefore, the higher the gauge (i.e. the smaller the size) of the microbiopsy needle, the lower is the maximal rate of respiration. Microbiopsies of skeletal muscle underestimate the maximal mitochondrial respiration rate, and this finding needs to be highlighted for adequate interpretation and comparison with literature data.

Peroxisome proliferator-activated receptor γ coactivator1- gene α transfer restores mitochondrial biomass and improves mitochondrial calcium handling in post-necrotic mdx mouse skeletal muscle

Alterations of mitochondrial function have been implicated in the pathogenesis of Duchenne muscular dystrophy. In the present study, mitochondrial respiratory function, reactive oxygen species (ROS) dynamics and susceptibility to Ca(2+)-induced permeability transition pore (PTP) opening were investigated in permeabilized skeletal muscle fibres of 6-week-old mdx mice, in order to characterize the magnitude and nature of mitochondrial dysfunction at an early post-necrotic stage of the disease. Short-term overexpression of the transcriptional co-activator PGC1α, achieved by in vivo plasmid transfection, was then performed to determine whether this intervention could prevent mitochondrial impairment and mitigate associated biochemical abnormalities. Compared with normal mice, mdx mice exhibited a lower mitochondrial biomass and oxidative capacity, greater ROS buffering capabilities, and an increased vulnerability to Ca(2+)-induced opening of the mitochondrial permeability transition pore complex. PGC1α gene transfer restored mitochondrial biomass, normalized the susceptibility to PTP opening and increased the capacity of mitochondria to buffer Ca(2+)(.) This was associated with reductions in the activity levels of the Ca(2+)-dependent protease calpain as well as caspases 3 and 9. Overall, these results suggest that overexpression of PGC1α in dystrophin-deficient muscles, after the onset of necrosis, has direct beneficial effects upon multiple aspects of mitochondrial function, which may in turn mitigate the activation of proteolytic and apoptotic signalling pathways associated with disease progression.

AMPK activation stimulates autophagy and ameliorates muscular dystrophy in the mdx mouse diaphragm

Duchenne muscular dystrophy (DMD) is characterized by myofiber death from apoptosis or necrosis, leading in many patients to fatal respiratory muscle weakness. Among other pathological features, DMD muscles show severely deranged metabolic gene regulation and mitochondrial dysfunction. Defective mitochondria not only cause energetic deficiency, but also play roles in promoting myofiber atrophy and injury via opening of the mitochondrial permeability transition pore. Autophagy is a bulk degradative mechanism that serves to augment energy production and eliminate defective mitochondria (mitophagy). We hypothesized that pharmacological activation of AMP-activated protein kinase (AMPK), a master metabolic sensor in cells and on-switch for the autophagy-mitophagy pathway, would be beneficial in the mdx mouse model of DMD. Treatment of mdx mice for 4 weeks with an established AMPK agonist, AICAR (5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside), potently triggered autophagy in the mdx diaphragm without inducing muscle fiber atrophy. In AICAR-treated mdx mice, the exaggerated sensitivity of mdx diaphragm mitochondria to calcium-induced permeability transition pore opening was restored to normal levels. There were associated improvements in mdx diaphragm histopathology and in maximal force-generating capacity, which were not linked to increased mitochondrial biogenesis or up-regulated utrophin expression. These findings suggest that agonists of AMPK and other inducers of the autophagy-mitophagy pathway can help to promote the elimination of defective mitochondria and may thus serve as useful therapeutic agents in DMD.

Effect of angiotensin-converting enzyme inhibition on skeletal muscle oxidative function and exercise capacity in streptozotocin-induced diabetic rats

Since exercise capacity is related to the mitochondrial respiration rate in skeletal muscle and both parameters are potentially modulated by the onset of diabetes and by inhibition of the angiotensin-converting enzyme (ACE), we investigated whether skeletal muscle oxidative functions and exercise capacities are impaired in chronic streptozotocin-induced diabetic (STZ) rats and whether ACE inhibition could reverse such abnormalities. The ACE inhibitor perindopril (2 mg kg(-1) day(-1)) was given for a period of 5 weeks to 7-month-old STZ rats (DIA-PE, n = 8) whose haemodynamic function, skeletal muscle mitochondrial function and exercise capacity were compared with those of untreated diabetic (DIA, n = 8) and control rats (CONT, n = 8). Increased arterial blood pressure (157 +/- 12 versus 130 +/- 6 mmHg, P < 0.05) and reduced exercise capacity (29 +/- 2 versus 91 +/- 2 min, respectively, P < 0.01) were observed in DIA compared with CONT. The oxidative capacity of the gastrocnemius muscle was significantly reduced in DIA compared with CONT rats (5.4 +/- 0.5 versus 10.6 +/- 0.7 micromol O(2) min(-1)(g dry weight)(-1), respectively, P < 0.001). Moreover, the coupling between oxidation and phosphorylation was significantly impaired in DIA (-52%, P < 0.001). Angiotensin-converting enzyme inhibition (ACEi) normalized blood pressure without improving mitochondrial function (4.3 +/- 0.8 micromol O(2) min(-1) (g dry weight)(-1) in DIA-PE rats) but reduced exercise capacity to even lower levels (10 +/- 1 min, P < 0.01). Exercise capacity correlated positively with blood pressure in DIA-PE (r = 0.79, P < 0.05). In experimental type 1 diabetic rats, both skeletal muscle mitochondrial respiration and exercise capacity are impaired. The ACEi failed to restore the muscular function and worsened exercise capacity. Further studies will be useful to determine whether an inadequate muscular blood flow secondary to the reduction in mean systemic blood pressure can explain these results.

Resveratrol improves mitochondrial function and protects against metabolic disease by activating SIRT1 and PGC-1alpha

Diminished mitochondrial oxidative phosphorylation and aerobic capacity are associated with reduced longevity. We tested whether resveratrol (RSV), which is known to extend lifespan, impacts mitochondrial function and metabolic homeostasis. Treatment of mice with RSV significantly increased their aerobic capacity, as evidenced by their increased running time and consumption of oxygen in muscle fibers. RSV's effects were associated with an induction of genes for oxidative phosphorylation and mitochondrial biogenesis and were largely explained by an RSV-mediated decrease in PGC-1alpha acetylation and an increase in PGC-1alpha activity. This mechanism is consistent with RSV being a known activator of the protein deacetylase, SIRT1, and by the lack of effect of RSV in SIRT1(-/-) MEFs. Importantly, RSV treatment protected mice against diet-induced-obesity and insulin resistance. These pharmacological effects of RSV combined with the association of three Sirt1 SNPs and energy homeostasis in Finnish subjects implicates SIRT1 as a key regulator of energy and metabolic homeostasis.

Purification, properties, and immunological characterization of GalT-3 (UDP-galactose: GM2 ganglioside, beta 1-3 galactosyltransferase) from embryonic chicken brain

A beta 1-3 galactosyltransferase (GalT-3; UDP-Gal; GM2 beta 1-3 galactosyltransferase) was purified over 5100-fold from 19-day-old embryonic chicken brain homogenate employing detergent solubilization, alpha-lactalbumin Sepharose, Q-Sepharose, UDP-hexanolamine Sepharose, and GalNAc beta 1-4Gal beta-Synsorb column chromatography. The purified enzyme was resolved into two bands on reducing gels with apparent molecular weights of 62 kDa and 65 kDa, respectively. GalT-3 activity was also localized in the same regions by activity gel analysis and sucrose-density gradient centrifugation of a detergent-solubilized extract of 19-day-old embryonic chicken brain. Purified GalT-3 exhibited apparent Kms of 33 microM, and 14.4 mM with respect to the substrates GM2, UDP-galactose, and MnCl2, respectively. Substrate specificity studies with the purified enzyme and a variety of glycosphingolipids, glycoproteins, and synthetic substrates revealed that the enzyme was highly specific only for the glycosphingolipid acceptors, GM2 and GgOse3Cer (asialo-GM2). Ovine-asialo-agalacto submaxillary mucin inhibited the transfer of galactose to GM2 but did not act as an acceptor in the range of concentrations tested. Polyclonal antibodies raised against purified GalT-3 inhibited GalT-3 activity in vitro and Western-immunoblot analysis of purified GalT-3 showed immunopositive bands at 62 and 65 kDa.

Effect of a fatty acid moiety of phospholipid and ceramide on purified GalT-3 (UDP-Gal:GM2 beta 1-3 galactosyltransferase) activity from embryonic chicken brain

Galactosyltransferase, GalT-3 (UDP-Gal:GM2 beta 1-3 galactosyltransferase) has been characterized and solubilized from 19-day-old embryonic chicken brain, and purified to over 2000-fold using mixed-modal chromatography on a omega-aminohexyl Sepharose column and affinity chromatography on a UDP-hexanolamine Sepharose column. The activity of purified GalT-3 was modulated by phospholipids in vitro with stimulation observed specifically with dipalmitoyl phosphatidylethanolamine (PE). All natural phospholipids tested (PE, PC and PI) inhibited GalT-3 activity. Enzyme activity was affected by the structure of the phospholipid vesicle. It was stabilized by the hexagonal (dipalmitoyl PE) structure and inhibited by the bilayer (dielaidoyl PE) structure. The long-chain fatty acid moiety of the glycosphingolipid substrate, GM2, was found to be necessary for optimum enzyme activity. In the absence of fatty acid, the modified substrates, lyso-GM2 and acetyl-GM2, had a 10-fold increased Km and a 4-8 fold decreased Vmax compared to the normal substrate. We postulate that GalT-3 belongs to a group of glycosyltransferases having recognition for both the carbohydrate as well as the hydrophobic domains (HY-CARS) of their substrates and that the fatty acid moiety of either the substrate (GM2) or a heterotropic effector (phospholipid) plays an important role in regulating the activity of this enzyme.

Solubilized glycosyltransferases and biosynthesis in vitro of glycolipids

The assembly of most of the ceramide-linked glycolipids (GSLs) in eukaryotic cells occurs in Golgi bodies. At least 18 different glycolipid:glycosyltransferases (GSL:GLTs) have been characterized, 10 of which have been solubilized. These GLTs can be classified into 2 distinct groups: 1) GLTs dedicated to either Dol-P-P-sugar(s) or ceramide-linked sugar(s); and 2) GLTs with dual loyalties (i.e., they compete with glycolipid- and glycoprotein-bound oligosaccharides). Studies with solubilized and purified GalNAcT-1 and GalNAcT-2 from embryonic chicken brains prove that GalNAcT-1 (UDP-GalNAc:GM3 beta 1-4GalNAcT) is specific for GSL, whereas GalNAcT-2 (UDP-GalNAc:Gb3 beta 1-3GalNAcT) can transfer to an oligosaccharide containing the alpha-linked terminal galactose. Similarly, GalT-3 (UDP-Gal:GM2 beta 1-3GalT) is more specific for ganglio-oligosaccharide and GalT-4 (UDP-Gal:Lc3 beta 1-4GalT) can transfer galactose to N-acetylglucosamine linked to p-nitrophenol, glycolipid or glycoprotein. Both GalT-3 and GalT-4 have been separated and purified from embryonic chicken brains. Studies with solubilized SAT-4 and SAT-3, from bovine spleen and embryonic chicken brains, respectively, suggest the existence of 2 different gene-expressed alpha 2-3SATs. The newly discovered FucT-3 (GDP-Fuc:NeuGc-iLc6-alpha 1-3FucT) from human colon carcinoma (Colo-205) has also been solubilized and separated from other GSL:GLTs. Using a new activity gel-Western blot combined technique, the molecular mass of this FucT-3 was determined to be 105 kDa.

p-Benzylthiocarbamoyl-aspartyl-daunorubicin-substituted polytrisacryl. A new drug acid-labile arm-carrier conjugate

A new type of drug acid-labile arm-carrier is described. A hydrophilic polymer [polytrisacryl, or poly(acryloyl 2-amido-2-hydroxymethyl-1,3-propanediol)], used as a model, is substituted with p-nitrobenzyl groups. The nitrobenzyl groups are reduced to aminobenzyl and transformed into benzylisothiocyanate groups which are allowed to react with aminoacyl daunorubicin. The excess of benzylisothiocyanate is transformed into benzylthiocarbamoyl N-methyl glucamine. A conjugate containing benzylthiocarbamoyl-aspartyl daunorubicin is stable at neutral pH, and releases free daunorubicin when it is exposed to pH 5 or below. This conjugate is about 200-fold less toxic than free daunorubicin, on a molecular basis, when it is added to the medium of Lewis lung carcinoma (3LL) cells in culture.