Evidence for a mitochondrial regulatory pathway defined by peroxisome proliferator-activated receptor-gamma coactivator-1 alpha, estrogen-related receptor-alpha, and mitofusin 2

Possible role of mitochondrial remodelling on cellular triacylglycerol accumulation

Mitochondrial fusion and fission processes play a role in a variety of cell functions, including energy metabolism, cell differentiation and programmed cell death. Still, it is not clear how these processes contribute to the cell functions. Here, we investigated the role of mitochondrial remodelling on lipid metabolism in adipocytes. In 3T3-L1 pre-adipocytes, the morphology of mitochondria is organized as a continuous reticulum. Upon differentiation of adipocytes manifested by cellular triacylglycerol (TG) accumulation, mitochondrial morphology altered from filamentous to fragmented and/or punctate structures. When the mitochondrial fusion was induced in adipocytes by silencing of mitochondrial fission proteins including Fis1 and Drp1, the cellular TG content was decreased. In contrast, the silencing of mitochondrial fusion proteins including mitofusin 2 and Opa1 increased the cellular TG content followed by fragmentation of mitochondria. It also appears that polyphenolic phytochemicals, negative regulators of lipid accumulation, have mitochondrial fusion activity and that there is a good correlation between mitochondrial fusion activity and the cellular TG accumulation-reducing activity of the phytochemicals. These results suggest that cellular TG accumulation is regulated, at least in part, via mitochondrial fusion and fission processes.

Genes involved in mitochondrial biogenesis/function are induced in response to bilio-pancreatic diversion in morbidly obese individuals with normal glucose tolerance but not in type 2 diabetic patients

AIMS/HYPOTHESIS

The mechanisms allowing normalisation of insulin sensitivity and reversal of type 2 diabetes after bilio-pancreatic diversion (BPD) have not been elucidated. We studied whether the expression of genes relevant to mitochondrial biogenesis/function is induced in response to BPD and whether the response differs between morbidly obese patients with normal glucose tolerance (NGT) and patients with type 2 diabetes.

METHODS

The effect of stable weight reduction after BPD on metabolic variables and expression of nuclear genes encoding for mitochondrial proteins or regulators of mitochondrial function was investigated in skeletal muscle. Insulin sensitivity was assessed by euglycaemic-hyperinsulinaemic clamp and substrate oxidation by indirect calorimetry.

RESULTS

Both NGT and type 2 diabetic patients showed a net improvement of insulin sensitivity, with the latter also showing blood glucose normalisation. NGT patients had a large increase in glucose oxidation and substantial reduction in lipid oxidation. In contrast, type 2 diabetic patients had a blunted response to BPD in terms of glucose oxidation. NGT patients showed increased expression of genes encoding mitofusin 2, porin or citrate synthase; no significant changes were detected in diabetic patients. The expression of genes regulating mitochondrial activity (PGC-1beta [also known as PPARGC1B], PGC-1alpha [also known as PPARGC1A], PPARdelta [also known as PPARD], SIRT1) was induced only in NGT patients.

CONCLUSIONS/INTERPRETATION

These findings indicate that weight loss after BPD exerts a beneficial effect on insulin sensitivity via mechanisms that are independent of the expression of genes involved in mitochondrial biogenesis/activity. Furthermore, the observation that gene expression is not altered with weight loss in type 2 diabetic patients while it is induced in NGT patients suggests a heritable component.

PGC-1{alpha} and PGC-1{beta} regulate mitochondrial density in neurons

Recent studies indicate that regulation of cellular oxidative capacity through enhancing mitochondrial biogenesis may be beneficial for neuronal recovery and survival in human neurodegenerative disorders. The peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) has been shown to be a master regulator of mitochondrial biogenesis and cellular energy metabolism in muscle and liver. The aim of our study was to establish whether PGC-1alpha and PGC-1beta control mitochondrial density also in neurons and if these coactivators could be up-regulated by deacetylation. The results demonstrate that PGC-1alpha and PGC-1beta control mitochondrial capacity in an additive and independent manner. This effect was observed in all studied subtypes of neurons, in cortical, midbrain, and cerebellar granule neurons. We also observed that endogenous neuronal PGC-1alpha but not PGC-1beta could be activated through its repressor domain by suppressing it. Results demonstrate also that overexpression of SIRT1 deacetylase or suppression of GCN5 acetyltransferase activates transcriptional activity of PGC-1alpha in neurons and increases mitochondrial density. These effects were mediated exclusively via PGC-1alpha, since overexpression of SIRT1 or suppression of GCN5 was ineffective where PGC-1alpha was suppressed by short hairpin RNA. Moreover, the results demonstrate that overexpression of PGC-1beta or PGC-1alpha or activation of the latter by SIRT1 protected neurons from mutant alpha-synuclein- or mutant huntingtin-induced mitochondrial loss. These evidences demonstrate that activation or overexpression of the PGC-1 family of coactivators could be used to compensate for neuronal mitochondrial loss and suggest that therapeutic agents activating PGC-1 would be valuable for treating neurodegenerative diseases in which mitochondrial dysfunction and oxidative damage play an important pathogenic role.

Regulation of mitofusin-2 expression in skeletal muscleThis paper is one of a selection of papers published in this Special Issue, entitled 14th International Biochemistry of Exercise Conference – Muscles as Molecular and Metabolic Machines, and has undergone the Journal’s usual peer review process

Fusion and fission of mitochondria regulate their morphology and distribution. Mitofusin-2 (Mfn2) is a mitochondrial protein involved in such fusion. Recent observations indicate that Mfn2 is a multifunctional protein that participates in cell proliferation and metabolism and that it is required for normal endoplasmic reticulum morphology. In relation to the metabolic role of Mfn2, alterations in activity have been reported to modify cell respiration, substrate oxidation, and oxidative phosphorylation subunit expression in cultured nonmuscle and muscle cells. Mfn2 expression in skeletal muscle is subject to regulation and conditions characterized by reduced mitochondrial activity, such as obesity or type 2 diabetes, and are associated with repressed Mfn2 . In contrast, cold-exposure treatment with β3-adrenergic agonists or exercise induce the expression of this gene in muscle. Estrogen-related receptor-α transcription factor is a key regulator of Mfn2 transcription and recruits peroxisome proliferator-activated receptor γ coactivator (PGC)-1β and PGC-1α. These 2 nuclear coactivators are potent, positive regulators of Mfn2 expression in muscle cells, and ablation of PGC-1β causes Mfn2 downregulation in skeletal muscle and in the heart. We propose that PGC-1β is a regulator of normal expression of Mfn2 in muscle, whereas PGC-1α participates in the stimulation of Mfn2 expression under a variety of conditions characterized by enhanced energy expenditure.

The role of PGC-1alpha on mitochondrial function and apoptotic susceptibility in muscle

Mitochondria are critical for cellular bioenergetics, and they mediate apoptosis within cells. We used whole body peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) knockout (KO) animals to investigate its role on organelle function, apoptotic signaling, and cytochrome-c oxidase activity, an indicator of mitochondrial content, in muscle and other tissues (brain, liver, and pancreas). Lack of PGC-1alpha reduced mitochondrial content in all muscles (17-44%; P < 0.05) but had no effect in brain, liver, and pancreas. However, the tissue expression of proteins involved in mitochondrial DNA maintenance [transcription factor A (Tfam)], import (Tim23), and remodeling [mitofusin 2 (Mfn2) and dynamin-related protein 1 (Drp1)] did not parallel the decrease in mitochondrial content in PGC-1alpha KO animals. These proteins remained unchanged or were upregulated (P < 0.05) in the highly oxidative heart, indicating a change in mitochondrial composition. A change in muscle organelle composition was also evident from the alterations in subsarcolemmal and intermyofibrillar mitochondrial respiration, which was impaired in the absence of PGC-1alpha. However, endurance-trained KO animals did not exhibit reduced mitochondrial respiration. Mitochondrial reactive oxygen species (ROS) production was not affected by the lack of PGC-1alpha, but subsarcolemmal mitochondria from PGC-1alpha KO animals released a greater amount of cytochrome c than in WT animals following exogenous ROS treatment. Our results indicate that the lack of PGC-1alpha results in 1) a muscle type-specific suppression of mitochondrial content that depends on basal oxidative capacity, 2) an alteration in mitochondrial composition, 3) impaired mitochondrial respiratory function that can be improved by training, and 4) a greater basal protein release from subsarcolemmal mitochondria, indicating an enhanced mitochondrial apoptotic susceptibility.

Diabetes mellitus exacerbates motor and sensory impairment in CMT1A

Charcot-Marie-Tooth disease type 1A (CMT1A) is caused by a duplication of PMP22 on chromosome 17 and is the most commonly inherited demyelinating neuropathy. Diabetes frequently causes predominantly sensory neuropathy. Whether diabetes exacerbates CMT1A is unknown. We identified 10 patients with CMT1A and diabetes and compared their impairment with 48 age-matched control patients with CMT1A alone. Comparisons were made with the Charcot-Marie-Tooth disease (CMT) neuropathy score (CMTNS) and by electrophysiology. The CMTNS was significantly higher in patients with diabetes (20.25 +/- 2.35) compared with controls (15.19 +/- 0.69; p = 0.01). Values were particularly higher for motor signs and symptoms. Seven of the 10 diabetic patients had CMTNS >20 (severe CMT), while only 7 of the 48 age-matched controls had scores >20. There was a trend for CMT1A patients with diabetes to have low compound muscle action potentials and sensory nerve action potentials, although nerve conduction velocities were not slower in diabetic patients compared with controls. Diabetes was associated with more severe motor and sensory impairment in patients with CMT1A.

Peroxisome proliferator-activated receptor delta regulates mitofusin 2 expression in the heart

Mitofusin 2 (Mfn2) has been proposed as an important mitochondrial protein in maintaining mitochondrial network and bioenergetics. Mfn2 is highly expressed in the heart, but is downregulated in response to hypertrophic stimuli. However, little is known about how Mfn2's expression is regulated in cardiomyocytes. Here, we have investigated how Mfn2 expression in the heart responds to fasting condition and determined if Mfn2 is one of those PPARdelta-selective target genes that are involved in myocardial energy metabolism. Fasting for 48 h in mice led to a robust increase of Mfn2 expression in the heart. On the other hand, cardiomyocyte-restricted PPARdelta deficiency in mice led to substantially diminished cardiac expression of Mfn2 transcript and protein compared to that of controls. Fasting induced cardiac expression of Mfn2 was blunted in cardiomyocyte-restricted PPARdelta deficient hearts. Moreover, PPARdelta-selective ligand treatment in cultured cardiomyocytes induced elevated Mfn2 expression. A functional PPRE consensus sequence located at -837 to -817 bp upstream of the mouse Mfn2 promoter was identified and confirmed by Electrophoretic Mobility Shift Assays and Luciferase Promoter Reporter Assays. We conclude that Mfn2 is a PPARdelta-selective target, which may play an important role in regulating myocardial energy homeostasis.

Beneficial effects of exercise on muscle mitochondrial function in diabetes mellitus

The physiopathology of diabetes mellitus has been closely associated with a variety of alterations in mitochondrial histology, biochemistry and function. Generally, the alterations comprise increased mitochondrial reactive oxygen and nitrogen species (RONS) generation, resulting in oxidative stress and damage; decreased capacity to metabolize lipids, leading to intramyocyte lipid accumulation; and diminished mitochondrial density and reduced levels of uncoupling proteins (UCPs), with consequent impairment in mitochondrial function. Chronic physical exercise is a physiological stimulus able to induce mitochondrial adaptations that can counteract the adverse effects of diabetes on muscle mitochondria. However, the mechanisms responsible for mitochondrial adaptations in the muscles of diabetic patients are still unclear. The main mechanisms by which exercise may be considered an important non-pharmacological strategy for preventing and/or attenuating diabetes-induced mitochondrial impairments may involve (i) increased mitochondrial biogenesis, which is dependent on the increased expression of some important proteins, such as the 'master switch' peroxisome proliferator-activated receptor (PPAR)-gamma-coactivator-1alpha (PGC-1alpha) and heat shock proteins (HSPs), both of which are severely downregulated in the muscles of diabetic patients; and (ii) the restoration or attenuation of the low UCP3 expression in skeletal muscle mitochondria of diabetic patients, which is suggested to play a pivotal role in mitochondrial dysfunction.There is evidence that chronic exercise and lifestyle interventions reverse impairments in mitochondrial density and size, in the activity of respiratory chain complexes and in cardiolipin content; however, the mechanisms by which chronic exercise alters mitochondrial respiratory parameters, mitochondrial antioxidant systems and other specific proteins involved in mitochondrial metabolism in the muscles of diabetic patients remain to be elucidated.

Mitochondrial fusion is increased by the nuclear coactivator PGC-1beta

BACKGROUND

There is no evidence to date on whether transcriptional regulators are able to shift the balance between mitochondrial fusion and fission events through selective control of gene expression.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we demonstrate that reduced mitochondrial size observed in knock-out mice for the transcriptional regulator PGC-1beta is associated with a selective reduction in Mitofusin 2 (Mfn2) expression, a mitochondrial fusion protein. This decrease in Mfn2 is specific since expression of the remaining components of mitochondrial fusion and fission machinery were not affected. Furthermore, PGC-1beta increases mitochondrial fusion and elongates mitochondrial tubules. This PGC-1beta-induced elongation specifically requires Mfn2 as this process is absent in Mfn2-ablated cells. Finally, we show that PGC-1beta increases Mfn2 promoter activity and transcription by coactivating the nuclear receptor Estrogen Related Receptor alpha (ERRalpha).

CONCLUSIONS/SIGNIFICANCE

Taken together, our data reveal a novel mechanism by which mammalian cells control mitochondrial fusion. In addition, we describe a novel role of PGC-1beta in mitochondrial physiology, namely the control of mitochondrial fusion mainly through Mfn2.

Transcriptional control of energy homeostasis by the estrogen-related receptors

Transcriptional control of cellular energy metabolic pathways is achieved by the coordinated action of numerous transcription factors and associated coregulators. Several members of the nuclear receptor superfamily have been shown to play important roles in this process because they can translate hormonal, nutrient, and metabolite signals into specific gene expression networks to satisfy energy demands in response to distinct physiological cues. Estrogen-related receptor (ERR) alpha, ERRbeta, and ERRgamma are nuclear receptors that have yet to be associated with a natural ligand and are thus considered as orphan receptors. However, the transcriptional activity of the ERRs is exquisitely sensitive to the presence of coregulatory proteins known to be essential for the control of energy homeostasis, and for all intents and purposes, these coregulators function as protein ligands for the ERRs. In particular, functional genomics and biochemical studies have shown that ERRalpha and ERRgamma operate as the primary conduits for the activity of members of the family of PGC-1 coactivators. As transcription factors, the ERRs control vast gene networks involved in all aspects of energy homeostasis, including fat and glucose metabolism as well as mitochondrial biogenesis and function. Phenotypic analyses of knockout mouse models have shown that all three ERRs are indispensable for proper development and/or survival of the organism when subjected to a variety of physiological challenges. The focus of this review is on the recent and rapid advances in understanding the functions of the ERRs in regulating bioenergetic pathways, with an emphasis on their roles in the specification of energetic properties required for cell- and tissue-specific functions.

ERRalpha: a metabolic function for the oldest orphan

Estrogen receptor related receptor (ERR)alpha was one of the first identified (1988) orphan nuclear receptors. Many of the orphan receptors identified after ERRalpha were deorphanized in a timely manner and appreciated as key transcriptional regulators of metabolic pathways. ERRalpha, however, remains an orphan. Nevertheless, recent studies have defined regulatory mechanisms and transcriptional targets of ERRalpha, allowing this receptor to join ranks with other nuclear receptors that control metabolism. Notably, mice lacking ERRalpha show defects when challenged with stressors that require a 'shift of gears' in energy metabolism, such as exposure to cold, cardiac overload or infection. These findings establish the importance of ERRalpha for adaptive energy metabolism, and suggest that strategies targeting ERRalpha may be useful in fighting metabolic diseases.

Regulation of PGC-1alpha and PGC-1alpha-responsive genes with forskolin-induced Schwann cell differentiation

Recent evidence indicates that mitochondrial homeostasis is critical for myelination and maintenance of peripheral nerve function. Mice lacking the metabolic transcriptional coactivator peroxisome proliferator activated receptor gamma coactivator 1alpha (PGC-1alpha) show reductions in expression of myelin-related proteins and exhibit myelin-associated lesions, so we identified PGC-1alpha target genes in Schwann cells (SCs) in vitro to determine potential roles for PGC-1alpha in glia and tested whether PGC-1alpha was sufficient for SC differentiation and myelination. Forskolin-induced differentiation was associated with an upregulation of PGC-1alpha mRNA and protein, and while overexpression of PGC-1alpha upregulated genes such as manganese superoxide dismutase and estrogen-related receptor alpha, it was not sufficient for induction of differentiation. Both PGC-1alpha overexpression and forskolin exposure caused an increase in the mitochondrial fusion-related protein mitofusin 1. These studies suggest that PGC-1alpha might be a potential target to promote mitochondrial stability during differentiation and myelination.

The orphan nuclear receptor, NOR-1, a target of beta-adrenergic signaling, regulates gene expression that controls oxidative metabolism in skeletal muscle

beta 1-3-Adrenoreceptor (AR)-deficient mice are unable to regulate energy expenditure and develop diet-induced obesity on a high-fat diet. We determined previously that beta2-AR agonist treatment activated expression of the mRNA encoding the orphan nuclear receptor, NOR-1, in muscle cells and plantaris muscle. Here we show that beta2-AR agonist treatment significantly and transiently activated the expression of NOR-1 (and the other members of the NR4A subgroup) in slow-twitch oxidative soleus muscle and fast-twitch glycolytic tibialis anterior muscle. The activation induced by beta-adrenergic signaling is consistent with the involvement of protein kinase A, MAPK, and phosphorylation of cAMP response element-binding protein. Stable cell lines transfected with a silent interfering RNA targeting NOR-1 displayed decreased palmitate oxidation and lactate accumulation. In concordance with these observations, ATP production in the NOR-1 silent interfering RNA (but not control)-transfected cells was resistant to (azide-mediated) inhibition of oxidative metabolism and expressed significantly higher levels of hypoxia inducible factor-1alpha. In addition, we observed the repression of genes that promote fatty acid oxidation (peroxisomal proliferator-activated receptor-gamma coactivator-1alpha/beta and lipin-1alpha) and trichloroacetic acid cycle-mediated carbohydrate (pyruvate) oxidation [pyruvate dehydrogenase phosphatase 1 regulatory and catalytic subunits (pyruvate dehydrogenase phosphatases-1r and -c)]. Furthermore, we observed that beta2-AR agonist administration in mouse skeletal muscle induced the expression of genes that activate fatty acid oxidation and modulate pyruvate use, including PGC-1alpha, lipin-1alpha, FOXO1, and PDK4. Finally, we demonstrate that NOR-1 is recruited to the lipin-1alpha and PDK-4 promoters, and this is consistent with NOR-1-mediated regulation of these genes. In conclusion, NOR-1 is necessary for oxidative metabolism in skeletal muscle.

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

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.

Adopting new orphans into the family of metabolic regulators

The importance of the adopted metabolite receptors, such as peroxisome proliferator-activated receptor, liver X receptor, and farnesoid X receptor, in transcriptional control of metabolic pathways has been appreciated for many years. However, it is becoming increasingly clear that the number of nuclear receptors with roles in metabolism is much larger than initially suspected. Recent years have brought an intense effort to define the biological functions of the most enigmatic group of the nuclear receptor superfamily, the true orphan receptors, including nuclear receptor 4As, estrogen-related receptors, retinoid-related orphan receptors, and Rev-erbs. Unexpectedly, several of these receptors also turn out to have important functions in various aspects of metabolic control.

The molecular bases of training adaptation

Skeletal muscle is a malleable tissue capable of altering the type and amount of protein in response to disruptions to cellular homeostasis. The process of exercise-induced adaptation in skeletal muscle involves a multitude of signalling mechanisms initiating replication of specific DNA genetic sequences, enabling subsequent translation of the genetic message and ultimately generating a series of amino acids that form new proteins. The functional consequences of these adaptations are determined by training volume, intensity and frequency, and the half-life of the protein. Moreover, many features of the training adaptation are specific to the type of stimulus, such as the mode of exercise. Prolonged endurance training elicits a variety of metabolic and morphological changes, including mitochondrial biogenesis, fast-to-slow fibre-type transformation and substrate metabolism. In contrast, heavy resistance exercise stimulates synthesis of contractile proteins responsible for muscle hypertrophy and increases in maximal contractile force output. Concomitant with the vastly different functional outcomes induced by these diverse exercise modes, the genetic and molecular mechanisms of adaptation are distinct. With recent advances in technology, it is now possible to study the effects of various training interventions on a variety of signalling proteins and early-response genes in skeletal muscle. Although it cannot presently be claimed that such scientific endeavours have influenced the training practices of elite athletes, these new and exciting technologies have provided insight into how current training techniques result in specific muscular adaptations, and may ultimately provide clues for future and novel training methodologies. Greater knowledge of the mechanisms and interaction of exercise-induced adaptive pathways in skeletal muscle is important for our understanding of the aetiology of disease, maintenance of metabolic and functional capacity with aging, and training for athletic performance. This article highlights the effects of exercise on molecular and genetic mechanisms of training adaptation in skeletal muscle.

Orphan nuclear receptor estrogen-related receptor-beta suppresses in vitro and in vivo growth of prostate cancer cells via p21(WAF1/CIP1) induction and as a potential therapeutic target in prostate cancer

Recent studies indicate that estrogen-related receptors (ERRs) are involved in similar estrogen receptor (ER) regulatory pathways and play roles in energy and lipid metabolism. Here, we analysed the functional role of ERRbeta in prostate cancer cell growth regulation in an androgen-sensitive and androgen-insensitive prostate cancer cell lines. ERRbeta was expressed in normal human prostates, but exhibited a reduced expression in prostate cancer lesions. Stable ERRbeta expression suppressed significantly cell proliferation and tumorigenicity of LNCaP and DU145 cells, accompanied by an S-phase suppression and increased p21 expression. Reporter and chromatin immunoprecipitation assays showed that ERRbeta could directly transactivate p21 gene promoter, which could be further enhanced by peroxisome proliferator-activated receptor-gamma coactivator-1alpha. Truncation analysis showed that ERRbeta-mediated p21 transactivation and prostate cancer cell growth inhibition required intact DNA-binding domain and AF2 domains in ERRbeta. Interestingly, ERRbeta displayed a cell cycle associated downregulated expression pattern in ERRbeta-transduced and non-transduced cells. Finally, we showed that ERRbeta-mediated growth inhibition could be potentiated by an ERRbeta/gamma agonist DY131. Knockdown of ERRbeta by RNA interference could reduce the DY131-induced growth inhibition in prostate cancer cells. Taken together, our findings indicate that ERRbeta performs a tumor suppressing function in prostate cancer cells, and targeting ERRbeta could be a potential therapeutic strategy for prostate cancer.

The NR3B subgroup: an ovERRview

Members of the NR3B group of the nuclear receptor superfamily, known as the estrogen-related receptors (ERRs), were the first orphan receptors to be identified two decades ago. Despite the fact that a natural ligand has yet to be associated with the ERRs, considerable knowledge about their mode of action and biological functions has emerged through extensive biochemical, genetic and functional genomics studies. This review describes our current understanding of how the ERRs work as transcription factors and as such, how they control diverse developmental and physiological programs.

Mitochondrial-nuclear communications

Mitochondria cannot be made de novo but replicate by a mechanism of recruitment of new proteins, which are added to preexisting subcompartments. Although mitochondria have their own DNA, more than 98% of the total protein complement of the organelle is encoded by the nuclear genome. Mitochondrial biogenesis requires a coordination of expression of two genomes and therefore cross talk between the nucleus and mitochondria. In mammals, regulation of mitochondrial biogenesis and proliferation is influenced by external factors, such as nutrients, hormones, temperature, exercise, hypoxia, and aging. This complexity points to the existence of a coordinated and tightly regulated network connecting different pathways. Communications are also required for eliciting mitochondrial responses to specific stress pathways. This review covers the mechanisms of mitochondrial biogenesis and the way cells respond to external signals to maintain mitochondrial function and cellular homeostasis.

Overexpression of Mitofusin 2 inhibited oxidized low-density lipoprotein induced vascular smooth muscle cell proliferation and reduced atherosclerotic lesion formation in rabbit

Our previous studies have implies that Mitofusin 2 (Mfn2), which was progressively reduced in arteries from ApoE(-/-) mice during the development of atherosclerosis, may take part in pathogenesis of atherosclerosis. In this study, we found that overexpression of Mfn2 inhibited oxidized low-density lipoprotein or serum induced vascular smooth muscle cell proliferation by down-regulation of Akt and ERK phosphorylation. Then we investigated the in vivo role of Mfn2 on the development of atherosclerosis in rabbits using adenovirus expressing Mitofusin 2 gene (AdMfn2). By morphometric analysis we found overexpression of Mfn2 inhibited atherosclerotic lesion formation and intima/media ratio by 66.7% and 74.6%, respectively, compared with control group. These results suggest that local Mfn2 treatment suppresses the development of atherosclerosis in vivo in part by attenuating the smooth muscle cell proliferation induced by lipid deposition and vascular injury.

The molecular bases of training adaptation

Skeletal muscle is a malleable tissue capable of altering the type and amount of protein in response to disruptions to cellular homeostasis. The process of exercise-induced adaptation in skeletal muscle involves a multitude of signalling mechanisms initiating replication of specific DNA genetic sequences, enabling subsequent translation of the genetic message and ultimately generating a series of amino acids that form new proteins. The functional consequences of these adaptations are determined by training volume, intensity and frequency, and the half-life of the protein. Moreover, many features of the training adaptation are specific to the type of stimulus, such as the mode of exercise. Prolonged endurance training elicits a variety of metabolic and morphological changes, including mitochondrial biogenesis, fast-to-slow fibre-type transformation and substrate metabolism. In contrast, heavy resistance exercise stimulates synthesis of contractile proteins responsible for muscle hypertrophy and increases in maximal contractile force output. Concomitant with the vastly different functional outcomes induced by these diverse exercise modes, the genetic and molecular mechanisms of adaptation are distinct. With recent advances in technology, it is now possible to study the effects of various training interventions on a variety of signalling proteins and early-response genes in skeletal muscle. Although it cannot presently be claimed that such scientific endeavours have influenced the training practices of elite athletes, these new and exciting technologies have provided insight into how current training techniques result in specific muscular adaptations, and may ultimately provide clues for future and novel training methodologies. Greater knowledge of the mechanisms and interaction of exercise-induced adaptive pathways in skeletal muscle is important for our understanding of the aetiology of disease, maintenance of metabolic and functional capacity with aging, and training for athletic performance. This article highlights the effects of exercise on molecular and genetic mechanisms of training adaptation in skeletal muscle.

Mitochondrial coupling defect in Charcot-Marie-Tooth type 2A disease

OBJECTIVE

Mutations of the mitofusin 2 gene (MFN2) may account for at least a third of the cases of Charcot-Marie-Tooth disease type 2 (CMT2). This study investigates mitochondrial cellular bioenergetics in MFN2-related CMT2A.

METHODS

Mitochondrial network morphology and metabolism were studied in cultures of skin fibroblasts obtained from four CMT2A patients harboring novel missense mutations of the MFN2 gene.

RESULTS

Although the mitochondrial network appeared morphologically unaltered, there was a significant defect of mitochondrial coupling associated with a reduction of the mitochondrial membrane potential.

INTERPRETATION

Our results suggest that the sharply reduced efficacy of oxidative phosphorylation in MFN2-related CMT2A may contribute to the pathophysiology of the axonal neuropathy.

Down-regulation of mitofusin-2 expression in cardiac hypertrophy in vitro and in vivo

Mitofusin-2 (Mfn2) suppresses smooth muscle cell proliferation through inhibition of the Ras-extracellular signal-regulated kinases (ERK1/2) pathway. Since the ERK1/2 pathway is implicated in mediating hypertrophic signaling, we studied the changes in Mfn2 in cardiac hypertrophy using in vitro and in vivo models. Phenylephrine was used to induce hypertrophy in neonatal rat ventricular myocytes (NRVMs). In vivo hypertrophy models included spontaneously hypertensive rats (SHR), pressure-overload hypertrophy by transverse aortic constriction (TAC), hypertrophy of non-infarcted myocardium following myocardial infarction (MI), and cardiomyopathy due to cardiac-restricted overexpression of beta(2)-adrenergic receptors (beta(2)-TG). We determined hypertrophic parameters and analysed expression of atrial natriuretic peptide (ANP) and Mfn2 by real-time PCR. Phosphorylated-ERK1/2 (phospho-ERK) was measured by Western blot. Mfn2 was downregulated in phenylephrine treated NRCMs (by approximately 40%), hypertrophied hearts from SHR (by approximately 80%), mice with TAC (at 1 and 3 weeks, by approximately 50%), and beta(2)-TG mice (by approximately 20%). However, Mfn2 was not downregulated in hypertrophied hearts with 15 weeks of TAC, nor in hypertrophied non-infarcted myocardium following MI. phospho-ERK1/2 was increased in hypertrophied myocardium at 1 week post-TAC, but not in non-infarcted myocardium after MI, indicating that downregulated Mfn2 may be accompanied by an increase of phospho-ERK1/2. This study shows, for the first time, downregulated Mfn2 expression in hypertrophied hearts, which depends on the etiology and time course of hypertrophy. Further study is required to examine the causal relationship between Mfn2 and cardiac hypertrophy.

[Transcriptional regulation of metabolic switching PDK4 gene under various physiological conditions]

Pyruvate dehydrogenase kinase 4 (PDK4) phosphorylates and inactivates the pyruvate dehydrogenase complex to respond to physiologic conditions. This response switches the energy source from glucose to fatty acids to maintain blood glucose levels. Transcription of the PDK4 gene is activated by fasting or by the administration of a peroxisome proliferator-activated receptor alpha (PPARalpha) ligand in a tissue-specific manner. However, the two mechanisms to induce PDK4 mRNA as well as the relationship between the two have not been studied in detail. In this study, we show that the two mechanisms are independent, at least in the mouse skeletal muscle, and that estrogen-related receptor alpha (ERRalpha) is directly involved in the PPARalpha-independent transcriptional activation of the PDK4 gene with peroxisome proliferator-activated receptor gamma co-activator 1alpha (PGC-1alpha) as a specific partner. The latter conclusion is based on the following evidence: 1) Deletion and point mutation analyses of the cloned mouse PDK4 gene promoter sequence identified an exact possible ERRalpha-binding motif as the PGC-1alpha responsive element. 2) The overexpression of ERRalpha by cotransfection enhanced, and the knocking down of it by specific shRNAs diminished, the PGC-1alpha-dependent activation. 3) Specific binding of ERRalpha to the identified PGC-1alpha-responsive sequence of the mouse PDK4 promoter was confirmed in the electrophoresis mobility shift assay using anti-ERRalpha antibodies. These results suggest that PGC-1alpha plays an essential role not only in regulating the amounts of energy creating enzymes, but also at the step of metabolic switching with unevenly distributed tissue transcription factors such as ERRalpha in the skeletal muscle, thus harmonizing tissue-specific functions and energy metabolism.