MitoNews Volume 8, Issue 06



MitoNews Volume 8, Issue 06

Dear Colleague,

Metabolic Syndrome, Diabetes and Mitochondria June, 2012

Edited by Elisa Oquendo, MD PhD, and James Murray, PhD.

Thousands of researchers around the world are studying the connections between mitochondria, metabolism and disease. MitoNews summarizes a selection of the latest published findings and highlights how Abcam's MitoSciences range of research tools has contributed to this effort. The full list of 40 original research papers published this month using the MitoSciences range of products can be found here.

Past issues are available for review in the archives.

Table of Contents

I. The role of mitochondria in metabolic syndrome

II. Glycosylation of mitochondrial proteins

III. ATP synthase – IF1 regulation of activity and MCL1 regulation of oligomerization

I. The role of mitochondria in metabolic syndrome

The metabolic syndrome is a constellation of metabolic disorders encompassing obesity (with and without insulin resistance), hypertension, dyslipidaemia and hyperglycaemia, all of which are risk factors for diabetes and cardiovascular disease. Diabetes is considered a global growing epidemic, with WHO currently estimating that more than 300 million people worldwide will suffer from it by the year 2025. Since cellular metabolic homeostasis is largely dependent on mitochondria, researchers have recently focused their attention on the pathogenic role of mitochondrial dysfunction, ROS production and inflammation in the etiologies of obesity, insulin resistance and complications of diabetes. Understanding the underlying molecular mechanisms of these conditions and developing new therapeutic strategies that target muscle lipid balance, mitochondrial energetics and insulin action is currently a major challenge to the scientific community. Several papers were published this month using products from the MitoSciences range that underscore the importance of mitochondria in the development of obesity and diabetes.

Metabolic inflexibility, a term that describes the failure of adjusting mitochondrial fuel selection based on nutritional cues, has gained recently increased attention in the field of metabolic syndrome. This month Muoio et al, have proposed Carnitine acetyltransferase (CAT, ‘CrAT') as a new player in the regulation of substrate switching and glucose tolerance. The authors found in a muscle-specific CrAT KO mouse model (CrAT M-/-): (1) complete alteration of whole body energy metabolism with glucose intolerance in the presence of a normal Akt pathway, (2) disruption of mitochondrial pyruvate metabolism without inhibition of the respiratory function and (3) abnormal fuel selection with increased beta oxidation even in the presence of high levels of pyruvate. The metabolic disruption was associated with aberrant control of pyruvate dehydrogenase (PDH) activity which was inhibited by carnitine in the KO model rather than stimulated. Despite the large metabolic aberrations in these animals, the protein levels of citrate synthase, complex I, complex II and complex V were all normal. The authors proposed a new model of diet induced metabolic dysfunction where CrAT combats nutrient stress by permitting mitochondrial efflux of excess acetyl moieties that otherwise would inhibit key enzymes such as PDH. The authors hypothesized that this gating process enhances insulin action and promotes metabolic flexibility.

Muscle-Specific Deletion of Carnitine Acetyltransferase Compromises Glucose Tolerance and Metabolic Flexibility. Cell Metab 2012. Muoio DM, Noland RC, Kovali JP, Seiler SE, Davies MN, DeBalsi KL, Ilkayeva OR, Stevens RD, Kheterpal I, Zhang J, Covington JD, Bajpeyi S, Ravussin E, Kraus W, Koves TR, Mynatt RL

Epicardial adipose tissue (EAT) has been described in humans as a functioning brown adipose tissue (BAT) and has been shown in animal models to have a lower glucose oxidation rate and higher fatty acid (FA) metabolism. However in the presence of obesity, EAT may be a source of cardiotoxic fatty acids and proinflamatory cytokines. This month, Distel et al have shown in a rat model of obesity and insulin resistance that short treatment with rosiglitazone promoted a BAT phenotype in the EAT depot characterized by: (1)increase in the levels of UCP-1, PGC-1 α, mitochondrial biogenesis (shown by the levels of expression of Tfam and (COXIV), (2)increase in fatty acid oxidation (shown by the levels of VLCAD and ATGL) and (3) a decrease in the release of esterified FA from the EAT depot. The authors conclude that the activation of a thermogenesis program in EAT by rosiglitazone decreases the release of FA and may benefit cardiac metabolism.

Early induction of a brown-like phenotype by rosiglitazne in the epicardial adipose tissue of fatty Zucker rats. Biochimie. 2012 Distel E, Penot G, Cadoudal T, Balguy I, Durant S, Benelli C.

The role of the AMPK/PGC1α axis in the muscle from insulin resistant and type 2 diabetes mellitus patients is well known, but the understanding of this axis in the peripheral nervous system has been partial. In a recent publication, Chowdhury et al have shown in sensory neurons of diabetic mice with marked signs of thermal hypoalgesia, a significant reduction in phospho-AMPK, phopho-ACC, total PGC-1α, NDUFS3 and COXIV in sensory neurons of the dorsal root ganglia of 14 week old diabetic mice with marked signs of thermal hypoalgesia. These results were associated with an impaired neuronal bioenergetic profile and a decrease in the activity of complex I, complex IV and citrate synthase. Furthermore, the authors demonstrated that treatment with resveratrol induced neurite outgrowth and normalized all proteomic and bioenergetic changes analyzed. The authors hypothesized that suboptimal functioning of mitochondria due to down-regulation of AMPK/PGC-1 α has the potential to contribute to axonal structural pathology.

Impaired adenosine monophosphate-activated protein kinase signaling in dorsal root ganglia neurons is linked to mitochondrial dysfunction and peripheral neuropathy in diabetes. Brain. 2012 Chowdhury SKR, Smith D. R., Saleh, A, Schapansky J, Marquez A, Gomes S, Akude E, Morrow D, Calcutt NA, Fernyhough P.

ROS generated in the ETC has been considered until recently a major contributor in the development of diabetic complications such as nephropathy, retinopathy, cardiomyophathy and vascular disease. In a recent paper Rosca et al, identified increased fatty acid oxidation as the source of ROS in mitochondria from renal proximal tubules of streptozotocin induced 8 – 9 week diabetic rats. The authors found changes in ETC substrate processing, with high formation of NADH from glutamate and no alteration in electron transport or in the formation of ROS from pyruvate oxidation. Instead, the authors showed that mitochondria from the diabetic tubule oxidizes fatty acids at a higher rate than controls. This finding was associated the finding with an increase in the activity of MCAD and LCAD but not in the levels of these proteins. All western blot data was normalized with SDHB, which is known to be unchanged in kidney mitochondria from type 1 diabetes. ROS production was increased in the presence of FA and not pyruvate and the site of leakage was found at the ETF protein. The authors hypothesized that this leakage may damage ETC protein subunits by peroxynitration in long term diabetes.

Oxidation of fatty acids is the source of increased mitochondrial reactive oxygen species production in kidney cortical tubules in early diabetes. Diabetes 2012 Rosca M, Vazquez E, Chen Q, Kerner J, Timothy K, Hoppel C.

See also:

Nonsynonymouse variants in mt-Nd2, mt-Nd4, and mt-Nd5 are linked to effects on oxidative phosphorylation and insulin sensitivity in rat conplastic strains Physiol Genomics 2012 Houštěk J, Hejzlarová K, Vrbacký M, Drahota Z, Landa V, Zídek V, Mlejnek P, Šimáková M, Šilhavý J, Mikšík I, Kazdová L, Oliyarnyk O, Kurtz T, and Pravenec M.

The p47phox- and NADPH oxidase organiser 1 (NOXO1)-dependent activation of NADPH oxidase 1 (NOX1) mediates endothelial nitric oxide synthase (eNOS) uncoupling and endothelial dysfunction in a streptozotocin-induced murine model of diabetes. Diabetologia 2012. Youn JY, Gao L and Cai H.

Overfeeding Reduces Insulin Sensitivity and Increases Oxidative Stress, without Altering Markers of Mitochondrial Content and Function in Humans. PLOS one 2012. Samocha-Bonet D, Campbell LV, Mori TA, Croft KD, Greenfield JR, Turner N, Heilbronn LK

High-fat diet feeding induces a depot-dependent response on the pro-inflammatory state and mitochondrial function of gonadal white adipose tissue. British Journal of Nutrition 2012. Amengual-Cladera E., Llado I, Proenza AM and Gianotti M.

Early cardiovascular changes occurring in diet-induced, obese insulin-resistant rats. Mol Cell Biochem 2012. Huisamen B, Dietrich D, Bezuidenhout N, Lopes J, Flepisi B, Blackhurst D and Lochner A

II. Glycosylation of mitochondrial proteins

For many years, glycosylation of proteins has been considered a process which occurs exclusively in the ER and golgi. While, N-glycosylation is commonly associated with ER and golgi organelles, O-glycosylation has been found to occur beyond these two compartments and is becoming a hot topic of debate in the field of diabetes, cancer and epigenetics. This month, Burnham-Marusich et al, found at least 3 proteins with established mitochondrial function to be glycosylated: PDH E1α, ATP synthase OSCP subunit and the NADH dehydrogenase [ubiquinone] iron-sulfur protein 3 (NDUFS3). Using a combined approach of wheat germ agglutinin agarose beads, click chemistry, Western blot detection and in-silico analysis, the authors concluded that O-GlcNAc modification of these proteins was likely and hypothesized that such post-translational modification may be of importance either in the modification of their metabolic function or in a potential extra mitochondrial unconventional localization.

Multiple proteins with essential mitochondrial functions have glycosylated isoforms Mitochondrion 2012. Burnham-Marusich AR and Berninsone PM

See also:

A role for ubiquitinylation and the cytosolic proteasome in turnover of mitochondrial uncoupling protein 1 (UCP1). Biochim Biophys Acta. 2012 Clarke KJ, Adams AE, Manzke LH, Pearson TW, Borchers CH, Porter RK

Nicotinamide-induced mitophagy: An event mediated by high NAD+/NADH ratio and SIRT1 activation. J Biol Chem. 2012 Jang SY, Kang HT, Hwang ES.

III. ATP synthase – IF1 regulation of activity and MCL1 regulation of oligomerization

The IF1 subunit has been proposed as an inhibitor protein of the mammalian ATP synthase. Presumably this regulatory mechanism is in place to prevent futile ATP cycling if the mitochondrial proton motive force (pmf) becomes insufficient to generate ATP, as may occur during ischemia. This month Fujikawa et al. described a HeLa cell line with stable, permanent knockdown of IF1 demonstrated by Western blotting with alpha, beta and IF1 monoclonal antibodies. These researchers showed this knockdown does not affect cell growth, mitochondrial morphology or ATP synthase oligomerization. However using a novel real-time ATP monitoring method, adding an uncoupler collapse the pmf in IF1 deficient cells, results in a rapid drop in ATP concentration, presumably due to ATP hydrolysis. This was not seen in control cells and demonstrates the regulatory role of IF1 in cellular energy crisis. The IF1 deficient cells in the presence of the uncoupler do establish normal ATP levels after 10 minutes by increasing glycolysis via 3 fold increase in glucose consumption, the authors propose that the loss of pmf itself may directly trigger an increase in glycolysis though an as yet unknown mechanism.

Assessing Actual Contribution of IF1, Inhibitor of Mitochondrial FoF1, to ATP Homeostasis, Cell Growth, Mitochondrial Morphology, and Cell Viability. J Biol Chem. 2012 Fujikawa M, Imamura H, Nakamura J, Yoshida M.

Also this month Perciavalle et al. show that MCL-1, a Bcl2 family member, may have a dual role in mitochondria: an established role in preventing apoptosis when localized to the outer mitochondrial membrane and a second, novel role when localized to the mitochondrial matrix. By deletion experimentation these researchers showed that MCL1 was necessary to maintain inner mitochondrial membrane structure, mitochondrial morphology and fusion. Loss of MCL-1 leads to decreased OXPHOS super-complex assembly and ATP synthase oligmerization, as demonstrated by BNPAGE and Western blotting with anti-OXPHOS antibodies. These complexes are known to be essential for proper organization of the inner mitochondrial membrane, cristae structure and disruption of mitochondrial fusion.

Anti-apoptotic MCL-1 localizes to the mitochondrial matrix and couples mitochondrial fusion to respiration. Nat Cell Biol. 2012. Perciavalle RM, Stewart DP, Koss B, Lynch J, Milasta S, Bathina M, Temirov J, Cleland MM, Pelletier S, Schuetz JD, Youle RJ, Green DR, Opferman JT.

Additive effects of mitochondrion-targeted cytochrome CYP2E1 and alcohol toxicity on cytochrome c oxidase function and stability of respirosome complexes. J Biol Chem. 2012. Bansal S, Srinivasan S, Anandasadagopan S, Chowdhury AR, Selvaraj V, Kalyanaraman B, Joseph J, Avadhani NG

Metabolic adaptation to chronic hypoxia in cardiac mitochondria. Res Cardiol. 2012 Heather LC, Cole MA, Tan JJ, Ambrose LJ, Pope S, Abd-Jamil AH, Carter EE, Dodd MS, Yeoh KK, Schofield CJ, Clarke K.

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