Tina Wenz

Increased muscle PGC-1α expression protects from sarcopenia and metabolic disease during aging

Aging is a major risk factor for metabolic disease and loss of skeletal muscle mass and strength, a condition known as sarcopenia. Both conditions present a major health burden to the elderly population. Here, we analyzed the effect of mildly increased PGC-1α expression in skeletal muscle during aging. We found that transgenic MCK-PGC-1α animals had preserved mitochondrial function, neuromuscular junctions, and muscle integrity during aging. Increased PGC-1α levels in skeletal muscle prevented muscle wasting by reducing apoptosis, autophagy, and proteasome degradation. The preservation of muscle integrity and function in MCK-PGC-1α animals resulted in significantly improved whole-body health; both the loss of bone mineral density and the increase of systemic chronic inflammation, observed during normal aging, were prevented. Importantly, MCK-PGC-1α animals also showed improved metabolic responses as evident by increased insulin sensitivity and insulin signaling in aged mice. Our results highlight the importance of intact muscle function and metabolism for whole-body homeostasis and indicate that modulation of PGC-1α levels in skeletal muscle presents an avenue for the prevention and treatment of a group of age-related disorders.

RETRACTED: Activation of the PPAR/PGC-1α Pathway Prevents a Bioenergetic Deficit and Effectively Improves a Mitochondrial Myopathy Phenotype

Neuromuscular disorders with defects in the mitochondrial ATP-generating system affect a large number of children and adults worldwide, but remain without treatment. We used a mouse model of mitochondrial myopathy, caused by a cytochrome c oxidase deficiency, to evaluate the effect of induced mitochondrial biogenesis on the course of the disease. Mitochondrial biogenesis was induced either by transgenic expression of peroxisome proliferator-activated receptor gamma (PPARgamma) coactivator alpha (PGC-1alpha) in skeletal muscle or by administration of bezafibrate, a PPAR panagonist. Both strategies successfully stimulated residual respiratory capacity in muscle tissue. Mitochondrial proliferation resulted in an enhanced OXPHOS capacity per muscle mass. As a consequence, ATP levels were conserved resulting in a delayed onset of the myopathy and a markedly prolonged life span. Thus, induction of mitochondrial biogenesis through pharmacological or metabolic modulation of the PPAR/PGC-1alpha pathway promises to be an effective therapeutic approach for mitochondrial disorders.

Mitochondrial myopathy induces a starvation-like response

Mitochondrial respiratory chain (RC) deficiency is among the most common causes of inherited metabolic disease, but its physiological consequences are poorly characterized. We studied the skeletal muscle gene expression profiles of mice with late-onset mitochondrial myopathy. These animals express a dominant patient mutation in the mitochondrial replicative helicase Twinkle, leading to accumulation of multiple mtDNA deletions and progressive subtle RC deficiency in the skeletal muscle. The global gene expression pattern of the mouse skeletal muscle showed induction of pathways involved in amino acid starvation response and activation of Akt signaling. Furthermore, the muscle showed induction of a fasting-related hormone, fibroblast growth factor 21 (Fgf21). This secreted regulator of lipid metabolism was also elevated in the mouse serum, and the animals showed widespread changes in their lipid metabolism: small adipocyte size, low fat content in the liver and resistance to high-fat diet. We propose that RC deficiency induces a mitochondrial stress response, with local and global changes mimicking starvation, in a normal nutritional state. These results may have important implications for understanding the metabolic consequences of mitochondrial myopathies.

Role of phospholipids in respiratory cytochrome bc1 complex catalysis and supercomplex formation

Specific protein-lipid interactions have been identified in X-ray structures of membrane proteins. The role of specifically bound lipid molecules in protein function remains elusive. In the current study, we investigated how phospholipids influence catalytic, spectral and electrochemical properties of the yeast respiratory cytochrome bc(1) complex and how disruption of a specific cardiolipin binding site in cytochrome c(1) alters respiratory supercomplex formation in mitochondrial membranes. Purified yeast cytochrome bc(1) complex was treated with phospholipase A(2). The lipid-depleted enzyme was stable but nearly catalytically inactive. The absorption maxima of the reduced b-hemes were blue-shifted. The midpoint potentials of the b-hemes of the delipidated complex were shifted from -52 to -82 mV (heme b(L)) and from +113 to -2 mV (heme b(H)). These alterations could be reversed by reconstitution of the delipidated enzyme with a mixture of asolectin and cardiolipin, whereas addition of the single components could not reverse the alterations. We further analyzed the role of a specific cardiolipin binding site (CL(i)) in supercomplex formation by site-directed mutagenesis and BN-PAGE. The results suggested that cardiolipin stabilizes respiratory supercomplex formation by neutralizing the charges of lysine residues in the vicinity of the presumed interaction domain between cytochrome bc(1) complex and cytochrome c oxidase. Overall, the study supports the idea, that enzyme-bound phospholipids can play an important role in the regulation of protein function and protein-protein interaction.

Regulation of mitochondrial biogenesis and PGC-1α under cellular stress

Cell function relies on the constant supply of ATP and it is crucial that mitochondrial ATP production adapts to environmental and cellular challenges to maintain cellular function. Key molecules in sensing cellular stress situations seem to be the PGC-family of transcriptional co-activators, which are key regulators of mitochondrial biogenesis. Recent work has identified several stress-regulated pathways that affect mitochondrial biogenesis through modulation of the activity of PGC-1α. This review focuses on caloric restriction, hypoxia as well as the role of reactive oxygen species in regulating mitochondrial biogenesis and how this process is linked to other cellular stress responses.

mTERF2 regulates oxidative phosphorylation by modulating mtDNA transcription

Regulation of mitochondrial protein expression is crucial for the function of the oxidative phosphorylation (OXPHOS) system. Although the basal machinery for mitochondrial transcription is known, the regulatory mechanisms are not completely understood. Here, we characterized mTERF2, a mitochondria-localized homolog of the mitochondrial transcription termination factor mTERF1. We show that inactivation of mTERF2 in the mouse results in a myopathy and memory deficits associated with decreased levels of mitochondrial transcripts and imbalanced tRNA pool. These aberrations were associated with decreased steady-state levels of OXPHOS proteins causing a decrease in respiratory function. mTERF2 binds to the mtDNA promoter region, suggesting that it affects transcription initiation. In vitro interaction studies suggest that mtDNA mediates interactions between mTERF2 and mTERF3. Our results indicate that mTERF1, mTERF2, and mTERF3 regulate transcription by acting in the same site in the mtDNA promoter region and thereby mediate fine-tuning of mitochondrial transcription and hence OXPHOS function.

PGC‐1α activation as a therapeutic approach in mitochondrial disease

Mitochondria play a central role in cellular homeostasis. Hence, mitochondrial dysfunction resulting in impaired ATP supply is detrimental to cell viability and causes severe and often fatal disorders. Current treatment of these mitochondrial disorders is limited and mostly addresses the symptoms, but not the deficiency itself. Recent work in cell and animal models of mitochondrial disease shows that increased mitochondrial biogenesis can boost residual OXPHOS capacity and thereby prevent the bioenergetic crisis. In animal models, this strategy results in increased health and lifespan. Here, I review which mitochondrial processes are affected in known mitochondrial disorders and discuss why PGC‐1α mediated mitochondrial biogenesis offers a promising venue for treatment of mitochondrial disorders.

Endurance exercise is protective for mice with mitochondrial myopathy.

Defects in the mitochondrial ATP-generating system are one of the most commonly inherited neurological disorders, but they remain without treatment. We have recently shown that modulation of the peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) level in skeletal muscle of a mitochondrial myopathy mouse model offers a therapeutic approach. Here we analyzed if endurance exercise, which is known to be associated with an increased PGC-1alpha level in muscle, offers the same beneficial effect. We subjected male and female mice that develop a severe mitochondrial myopathy due to a cytochrome-c oxidase deficiency at 3 mo of age to endurance exercise training and monitored phenotypical and metabolic changes. Sedentary myopathy and wild-type mice were used as controls. Exercise increased PGC-1alpha in muscle, resulting in increased mitochondrial biogenesis, and successfully stimulated residual respiratory capacity in muscle tissue. As a consequence, ATP levels were increased in exercised mice compared with sedentary myopathy animals, which resulted in a delayed onset of the myopathy and a prolonged lifespan of the exercised mice. As an added benefit, endurance exercise induced antioxidant enzymes. The overall protective effect of endurance exercise delayed the onset of the mitochondrial myopathy and increased life expectancy in the mouse model. Thus stimulating residual oxidative phosphorylation function in the affected muscle by inducing mitochondrial biogenesis through endurance exercise might offer a valuable therapeutic intervention for mitochondrial myopathy patients.

Mutational Analysis of Cytochrome b at the Ubiquinol Oxidation Site of Yeast Complex III

The cytochrome bc1 complex is a dimeric enzyme of the inner mitochondrial membrane that links electron transfer from ubiquinol to cytochrome c by a protonmotive Q cycle mechanism in which ubiquinol is oxidized at one center in the enzyme, referred to as center P, and ubiquinone is rereduced at a second center, referred to as center N. To better understand the mechanism of ubiquinol oxidation, we have examined catalytic activities and pre-steady-state reduction kinetics of yeast cytochrome bc1 complexes with mutations in cytochrome b that we expected would affect oxidation of ubiquinol. We mutated two residues thought to be involved in proton conduction linked to ubiquinol oxidation, Tyr132 and Glu272, and two residues proposed to be involved in docking ubiquinol into the center P pocket, Phe129 and Tyr279. Substitution of Phe129 by lysine or arginine yielded a respiration-deficient phenotype and lipid-dependent catalytic activity. Increased bypass reactions were detectable for both variants, with F129K showing the more severe effects. Substitution with lysine leads to a disturbed coordination of a b heme as deduced from changes in the midpoint potential and the EPR signature. Removal of the aromatic side chain in position Tyr279 lowers the catalytic activity accompanied by a low level of bypass reactions. Pre-steady-state kinetics of the enzymes modified at Glu272 and Tyr132 confirmed the importance of their functional groups for electron transfer. Altered center N kinetics and activation of ubiquinol oxidation by binding of cytochrome c in the Y132F and E272D enzymes indicate long range effects of these mutations.

Emerging therapeutic approaches to mitochondrial diseases.

Mitochondrial diseases are very heterogeneous and can affect different tissues and organs. Moreover, they can be caused by genetic defects in either nuclear or mitochondrial DNA as well as by environmental factors. All of these factors have made the development of therapies difficult. In this review article, we will discuss emerging approaches to the therapy of mitochondrial disorders, some of which are targeted to specific conditions whereas others may be applicable to a more diverse group of patients.