Renée Ventura-Clapier

Transcriptional control of mitochondrial biogenesis: the central role of PGC-1α

Although the concept of energy starvation in the failing heart was proposed decades ago, still very little is known about the origin of energetic failure. Recent advances in molecular biology have started to elucidate the transcriptional events governing mitochondrial biogenesis. In particular, a great step was taken with the discovery that peroxisome proliferator-activated receptor gamma co-activator (PGC-1alpha) is the master regulator of mitochondrial biogenesis. The molecular mechanisms underlying the downregulation of PGC-1alpha and the consequent decrease in mitochondrial function in heart failure are, however, still poorly understood. Indeed, the main pathways involved in mitochondrial biogenesis are thought to be up- rather than down-regulated in pathological hypertrophy and heart failure. The current review summarizes recent advances in this field and is restricted to the heart when cardiac data are available.

Depressed mitochondrial transcription factors and oxidative capacity in rat failing cardiac and skeletal muscles

Congestive heart failure (CHF) induces alterations in energy metabolism and mitochondrial function that span cardiac as well as skeletal muscles. Whether these defects originate from altered mitochondrial DNA copy number and/or mitochondrial gene transcription is not known at present, nor are the factors that control mitochondrial capacity in different muscle types completely understood. We used an experimental model of CHF induced by aortic banding in the rat and investigated mitochondrial respiration and enzyme activity of biochemical mitochondrial markers in cardiac, slow and fast skeletal muscles. We quantified mitochondrial DNA (mtDNA), expression of nuclear (COX IV) and mitochondrial (COX I) encoded cytochrome c oxidase subunits as well as nuclear factors involved in mitochondrial biogenesis and in the necessary coordinated interplay between nuclear and mitochondrial genomes in health and CHF. CHF induced a decrease in oxidative capacity and mitochondrial enzyme activities with a parallel decrease in the mRNA level of COX I and IV, but no change in mtDNA content. The expression of the peroxisome proliferator activated receptor gamma co‐activator 1α (PGC‐1α) gene was downregulated in CHF, as well as nuclear respiratory factor 2 and mitochondrial transcription factor A, which act downstream from PGC‐1α. Most interestingly, only the level of PGC‐1α expression was strongly correlated with muscle oxidative capacity in cardiac and skeletal muscles, both in healthy and CHF rats. Mitochondrial gene transcription is reduced in CHF, and PGC‐1α appears as a potential modulator of muscle oxidative capacity under these experimental conditions.

Energy metabolism in heart failure.

Heart failure (HF) is a syndrome resulting from the inability of the cardiac pump to meet the energy requirements of the body. Despite intensive work, the pathogenesis of the cardiac intracellular abnormalities that result from HF remains incompletely understood. Factors that lead to abnormal contraction and relaxation in the failing heart include metabolic pathway abnormalities that result in decreased energy production, energy transfer and energy utilization. Heart failure also affects the periphery. Patients suffering from heart failure always complain of early muscular fatigue and exercise intolerance. This is linked in part to intrinsic alterations of skeletal muscle, among which decreases in the mitochondrial ATP production and in the transfer of energy through the phosphotransfer kinases play an important role. Alterations in energy metabolism that affect both cardiac and skeletal muscles argue for a generalized metabolic myopathy in heart failure. Recent evidence shows that decreased expression of mitochondrial transcription factors and mitochondrial proteins are involved in mechanisms causing the energy starvation in heart failure. This review will focus on energy metabolism alterations in long‐term chronic heart failure with only a few references to compensated hypertrophy when necessary. It will briefly describe the energy metabolism of normal heart and skeletal muscles and their alterations in chronic heart failure. It is beyond the scope of this review to address the metabolic switches occurring in compensated hypertrophy; readers could refer to well‐documented reviews on this subject.

AMPK : Lessons from transgenic and knockout animals.

AMP-activated protein kinase (AMPK), a phylogenetically conserved serine/threonine protein kinase, has been proposed to function as a fuel gauge to monitor cellular energy status in response to nutritional environmental variations. AMPK system is a regulator of energy balance that, once activated by low energy status, switches on ATP-producing catabolic pathways (such as fatty acid oxidation and glycolysis), and switches off ATP-consuming anabolic pathways (such as lipogenesis), both by short-term effect on phosphorylation of regulatory proteins and by long-term effect on gene expression. Numerous observations obtained with pharmacological activators and agents that deplete intracellular ATP have been supportive of AMPK playing a role in the control of energy metabolism but none of these studies have provided conclusive evidence. Relatively recent developments in our understanding of precisely how AMPK complexes might operate to control energy metabolism is due in part to the development of transgenic and knockout mouse models. Although there are inevitable caveats with genetic models, some important findings have emerged. In the present review, we discuss recent findings obtained from animal models with inhibition or activation of AMPK signaling pathway.

Subcellular Creatine Kinase Alterations Implications in Heart Failure

We have tested the hypothesis that decreased functioning of creatine kinase (CK) at sites of energy production and utilization may contribute to alterations in energy fluxes and calcium homeostasis in congestive heart failure (CHF). Heart failure was induced by aortic banding in 3-week-old rats. Myofilaments, sarcoplasmic reticulum (SR), mitochondrial functions, and CK compartmentation were studied in situ using selective membrane permeabilization of left ventricular fibers with detergents (saponin for mitochondria and SR and Triton X-100 for myofibrils). Seven months after surgery, animals were in CHF. A decrease in total CK activity could be accounted for by a 4-fold decrease in activity and content (Western blots) of mitochondrial CK and a 30% decrease in M isoform of CK (MM-CK) activity. In myofibrils, maximal force, crossbridge kinetics, and alpha-myosin heavy-chain expression decreased, whereas calcium sensitivity of tension development remained unaltered. Myofibrillar CK efficacy was unchanged. Calcium uptake capacities of SR were estimated from the surface of caffeine-induced tension transient (SCa) after loading with different substrates. In CHF, SCa decreased by 23%, and phosphocreatine was 2 times less efficient in enhancing calcium uptake. Oxidative capacities of the failing myocardium measured as oxygen consumption per gram of fiber dry weight decreased by 28%. Moreover, the control of respiration by creatine, ADP, and AMP was severely impaired. Our observations provide evidence that alterations in CK compartmentation may contribute to alterations of energy fluxes and calcium homeostasis in CHF.

Physical activity changes the regulation of mitochondrial respiration in human skeletal muscle

This study explores the importance of creatine kinase (CK) in the regulation of muscle mitochondrial respiration in human subjects depending on their level of physical activity. Volunteers were classified as sedentary, active or athletic according to the total activity index as determined by the Baecke questionnaire in combination with maximal oxygen uptake values (peak V̇O2, expressed in ml min−1 kg−1). All volunteers underwent a cyclo‐ergometric incremental exercise test to estimate their peak V̇O2 and V̇O2 at the ventilatory threshold (VT). Muscle biopsy samples were taken from the vastus lateralis and mitochondrial respiration was evaluated in an oxygraph cell on saponin permeabilised muscle fibres in the absence (V̇0) or in the presence (V̇max) of saturating [ADP]. While V̇0 was similar, V̇max differed among groups (sedentary, 3.7 ± 0.3, active, 5.9 ± 0.9 and athletic, 7.9 ± 0.5 μmol O2 min−1 (g dry weight)−1). V̇max was correlated with peak V̇O2 (P < 0.01, r= 0.63) and with V̇T (P < 0.01, r= 0.57). There was a significantly greater degree of coupling between oxidation and phosphorylation (V̇max/V̇0) in the athletic individuals. The mitochondrial Km for ADP was significantly higher in athletic subjects (P < 0.01). Mitochondrial CK (mi‐CK) activation by addition of creatine induced a marked decrease in Km in athletic individuals only, indicative of an efficient coupling of mi‐CK to ADP rephosphorylation in the athletic subjects only. It is suggested that increasing aerobic performance requires an enhancement of both muscle oxidative capacity and mechanisms of respiratory control, attesting to the importance of temporal co‐ordination of energy fluxes by CK for higher efficacy.

Bioenergetics of the failing heart

The heart is responsible for pumping blood throughout the blood vessels to the periphery by repeated, rhythmic contractions at variable intensity. As such the heart should permanently adjust energy production to energy utilization and is a high-energy consumer. For this the heart mainly depends on oxidative metabolism for adequate energy production and on efficient energy transfer systems. In heart failure, there is disequilibrium between the work the heart has to perform and the energy it is able to produce to fulfill its needs. This has led to the concept of energy starvation of the failing heart. This includes decreased oxygen and substrate supply, altered substrate utilization, decreased energy production by mitochondria and glycolysis, altered energy transfer and inefficient energy utilization. Mitochondrial biogenesis and its transcription cascade are down-regulated. Disorganization of the cytoarchitecture of the failing cardiomyocyte also participates in energy wastage. Finally, the failing of the cardiac pump, by decreasing oxygen and substrate supply, leads to a systemic energy starvation. Metabolic therapy has thus emerged as an original and promising approach in the treatment heart failure. This article is part of a Special Issue entitled: Mitochondria and Cardioprotection.

Functional coupling of creatine kinases in muscles: Species and tissue specificity

Creatine kinase (CK) isoenzymes are present in all vertebrates. An important property of the creatine kinase system is that its total activity, its isoform distribution, and the concentration of guanidino substrates are highly variable among species and tissues. In the highly organized structure of adult muscles, it has been shown that specific CK isoenzymes are bound to intracellular compartments, and are functionally coupled to enzymes and transport systems involved in energy production and utilization. It is however, not established whether functional coupling and intracellular compartmentation are present in all vertebrates. Furthermore, these characteristics seem to be different among different muscle types within a given species. This study will review some of these aspects.It has been observed that: (1) In heart ventricle, CK compartmentation and coupling characterize adult mammalian cells. It is almost absent in frogs, and is weakly present in birds. (2) Efficient coupling of MM-CK to myosin ATPase is seen in adult mammalian striated muscles but not in frog and bird heart where B-CK is expressed instead of M-CK. Thus, the functional efficacy of bound MM-CK to regulate adenine nucleotide turnover within the myofibrillar compartment seems to be specific for muscles expressing M-CK as an integral part of the sarcomere. (3) Mi-CK expression and/or functional coupling are highly tissue and species specific; moreover, they are subject to short term and long term adaptations, and are present late in development. The mitochondrial form of CK (mi-CK) can function in two modes depending on the tissue: (i) in an ≪ADP regeneration mode≫ and (ii) in an ≪ADP amplification mode≫. The mode of action of mi-CK seems to be related to its precise localization within the mitochondrial intermembrane space, whereas its amount might control the quantitative aspects of the coupling. Mi-CK is highly plastic, making it a strong candidate for fine regulation of excitation-contraction coupling in muscles and for energy transfer in cells with large and fluctuating energy demands in general. (4) Although CK isoforms show a binding specificity, the presence of a given isoform within a tissue or a species only, does not predict its functional role. For example, M-CK is expressed before it is functionally compartmentalized within myofibrils during development. Similarly, the presence of ubiquitous or sarcomeric mi-CK isoforms, is not an index of functional coupling of mi-CK to oxidative phosphorylation. (5) Amongst species or muscles, it appears that a large buffering action of the CK system is associated with rapid contraction and high glycolytic activity. On the other hand, an oxidative metabolism is associated with isoform diversity, increased compartmentation, a subsequent low buffering action and efficient phosphotransfer between mitochondria and energy utilization sites.It can be concluded that, in addition to a high variation of total activity and isoform expression, the role of the CK system also critically depends on its intracellular organization and interaction with energy producing and utilizing pathways. This compartmentation will determine the high cellular efficiency and fine specialization of highly organized and differentiated muscle cells.

Metabolic control and metabolic capacity: two aspects of creatine kinase functioning in the cells.

In this short review, the merits and limits of three theoretical concepts explaining the functional role of the creatine kinase system in muscle and brain cells are analysed. In addition to the usual concept of an energy buffer system and the recently proposed metabolic capacity theory (Sweeney, H.L. (1994) Med. Sci. Sports Exerc. 26, 30-36), it is proposed that coupled creatine kinase systems are involved in effective metabolic regulation of energy fluxes and oxidative phosphorylation, beside their energy transfer function. This aspect of the system is considered on the basis of metabolic control analysis. It is shown by using the results of mathematical modelling that, due to amplification of ADP fluxes from the cytoplasm by the mechanism of metabolic channelling, coupled mitochondrial creatine kinase may exert a flux control coefficient significantly exceeding 1.

Heart Failure Affects Mitochondrial but Not Myofibrillar Intrinsic Properties of Skeletal Muscle

BackgroundCongestive heart failure (CHF) induces abnormalities in skeletal muscle that are thought to in part explain exercise intolerance. The aim of the present study was to determine whether these changes actually result in contractile or metabolic functional alterations and whether they are muscle type specific. Methods and ResultsWith a rat model of CHF (induced by aortic banding), we studied mitochondrial function, mechanical properties, and creatine kinase (CK) compartmentation in situ in permeabilized fibers from soleus (SOL), an oxidative slow-twitch muscle, and white gastrocnemius (GAS), a glycolytic fast-twitch muscle. Animals were studied 7 months after surgery, and CHF was documented on the basis of anatomic data. Alterations in skeletal muscle phenotype were documented with an increased proportion of fast-type fiber and fast myosin heavy chain, decreased capillary-to-fiber ratio, and decreased citrate synthase activity. Despite a slow-to-fast phenotype transition in SOL, no change was observed in contractile capacity or calcium sensitivity. However, muscles from CHF rats exhibited a dramatic decrease in oxidative capacities (oxygen consumption per gram of fiber dry weight) of 35% for SOL and 45% for GAS (P <0.001). Moreover, the regulation of respiration with ADP and mitochondrial CK and adenylate kinase was impaired in CHF SOL. Mitochondrial CK activity and content (Western blots) were dramatically decreased in both muscles. ConclusionsCHF results in alterations in both mitochondrial function and phosphotransfer systems but unchanged myofibrillar function in skeletal muscles, which suggests a myopathy of metabolic origin in CHF.