Ayesha Saleem

Mitochondrial function and apoptotic susceptibility in aging skeletal muscle.

During aging, skeletal muscle undergoes sarcopenia, a condition characterized by a loss of muscle cell mass and alterations in contractile function. The origin of these decrements is unknown, but evidence suggests that they can be partly attributed to mitochondrial dysfunction. To characterize the nature of this dysfunction, we investigated skeletal muscle contractile properties, subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondrial biogenesis and function, as well as apoptotic susceptibility in young (6 months old) and senescent (36 months old) Fischer 344 Brown Norway rats. Muscle mass and maximal force production were significantly lower in the 36‐month group, which is indicative of a sarcopenic phenotype. Furthermore, contractile activity in situ revealed greater fatigability in the 36‐month compared to the 6‐month animals. This decrement could be partially accounted for by a 30% lower mitochondrial content in fast‐twitch muscle from 36‐month animals, as well as lower protein levels of the transcriptional coactivator peroxisome proliferator‐activated receptor γ coactivator‐1α. Enzyme activities and glutamate‐induced oxygen consumption rates in isolated SS and IMF mitochondria were similar between age groups. However, mitochondrial reactive oxygen species (ROS) production during state 3 respiration was ~1.7‐fold greater in mitochondria isolated from 36‐month compared to 6‐month animals, and was accompanied by a 1.8‐fold increase in the DNA repair enzyme 8‐oxoguanine glycosylase 1 in fast‐twitch muscle. Basal rates of release of cytochrome c and endonuclease G in SS mitochondria were 3.5‐ to 7‐fold higher from senescent animals. These data suggest that the age‐related sarcopenia and muscle fatigability are associated with enhanced ROS production, increased mitochondrial apoptotic susceptibility and reduced transcriptional drive for mitochondrial biogenesis.

Role of p53 in mitochondrial biogenesis and apoptosis in skeletal muscle

p53 is a tumor suppressor protein that also plays a role in regulating aerobic metabolism. Since skeletal muscle is a major source of whole body aerobic respiration, it is important to delineate the effects of p53 on muscle metabolism. In p53 knockout (KO) mice, we observed diminished mitochondrial content in mixed muscle and lowered peroxisome proliferator-activated receptor-gamma (PPARgamma) coactivator (PGC)-1alpha protein levels in gastrocnemius muscle. In intermyofibrillar (IMF) mitochondria, lack of p53 was associated with reduced respiration and elevated reactive oxygen species production. Permeability transition pore kinetics remained unchanged; however, IMF mitochondrial cytochrome c release was reduced and DNA fragmentation was lowered, illustrating a resistance to mitochondrially driven apoptosis in muscle of KO mice. p53-null animals displayed similar muscle strength but greater fatigability and less locomotory endurance than wild-type (WT) animals. Surprisingly, the adaptive responses in mitochondrial content to running were similar in WT and KO mice. Thus p53 may be important, but not necessary, for exercise-induced mitochondrial biogenesis. In WT animals, acute muscle contractions induced the phosphorylation of p53 in concert with increased activation of upstream kinases AMP-activated protein kinase and p38, indicating a pathway through which p53 may initiate mitochondrial biogenesis in response to contractile activity. These data illustrate a novel role for p53 in maintaining mitochondrial biogenesis, apoptosis, and performance in skeletal muscle.

Transcriptional and post-transcriptional regulation of mitochondrial biogenesis in skeletal muscle: Effects of exercise and aging

Acute contractile activity of skeletal muscle initiates the activation of signaling kinases. This promotes the phosphorylation of transcription factors, leading to enhanced DNA binding and transcriptional activation and/or repression. The mRNA products of nuclear genes encoding mitochondrial proteins are translated in the cytosol and imported into pre-existing mitochondria. When contractile activity is repeated, the recapitulation of these cellular events progressively leads to an expansion of the mitochondrial reticulum within muscle. This has physiologically relevant health benefit, including enhanced lipid metabolism and reduced muscle fatigability. In aging skeletal muscle, the response to contractile activity appears to be attenuated, suggesting that a greater contractile stimulus is required to attain a similar phenotype adaptation. This review summarizes our current understanding of the effects of exercise on the gene expression pathway leading to organelle biogenesis in muscle.

Sirtuin 1-mediated Effects of Exercise and Resveratrol on Mitochondrial Biogenesis

Background: SirT1 regulates mitochondrial biogenesis in various tissues. Results: Exercise combined with resveratrol has a SirT1-dependent synergistic effect on mitochondrial biogenesis, despite individual treatments being SirT1-independent. Conclusion: SirT1 is important for maintaining muscle mitochondrial content and function. Significance: The dependence of muscle mitochondrial biogenesis on SirT1 depends on the metabolic state of the muscle. The purpose of this study was to evaluate the role of sirtuin 1 (SirT1) in exercise- and resveratrol (RSV)-induced skeletal muscle mitochondrial biogenesis. Using muscle-specific SirT1-deficient (KO) mice and a cell culture model of differentiated myotubes, we compared the treatment of resveratrol, an activator of SirT1, with that of exercise in inducing mitochondrial biogenesis. These experiments demonstrated that SirT1 plays a modest role in maintaining basal mitochondrial content and a larger role in preserving mitochondrial function. Furthermore, voluntary exercise and RSV treatment induced mitochondrial biogenesis in a SirT1-independent manner. However, when RSV and exercise were combined, a SirT1-dependent synergistic effect was evident, leading to enhanced translocation of PGC-1α and SirT1 to the nucleus and stimulation of mitochondrial biogenesis. Thus, the magnitude of the effect of RSV on muscle mitochondrial biogenesis is reliant on SirT1, as well as the cellular environment, such as that produced by repeated bouts of exercise.

Acute exercise induces tumour suppressor protein p53 translocation to the mitochondria and promotes a p53-Tfam-mitochondrial DNA complex in skeletal muscle.

•  p53 is one of several important proteins that regulate the synthesis and function of mitochondria in muscle, and mice lacking p53 have impaired aerobic capacity. •  The role of p53 in response to physiological stress such as exercise has not been investigated. •  We demonstrate that acute exercise induces the translocation of p53 to mitochondria, and promotes a subsequent interaction with mitochondrial transcription factor A (Tfam) and mitochondrial DNA (mtDNA) to positively affect mtDNA transcription. •  This rapid effect on p53 translocation probably represents an early exercise response which can promote long‐term beneficial mitochondrial and metabolic adaptations in muscle. •  The results could have clinical implications in light of the growing recognition of exercise as an alternative therapy in cancer treatment, and the central role of p53 in halting tumorigenesis.

p53 is necessary for the adaptive changes in cellular milieu subsequent to an acute bout of endurance exercise

An acute bout of exercise activates downstream signaling cascades that ultimately result in mitochondrial biogenesis. In addition to inducing mitochondrial synthesis, exercise triggers the removal of damaged cellular material via autophagy and of dysfunctional mitochondria through mitophagy. Here, we investigated the necessity of p53 to the changes that transpire within the muscle upon an imposed metabolic and physiological challenge, such as a bout of endurance exercise. We randomly assigned wild-type (WT) and p53 knockout (KO) mice to control, acute exercise (AE; 90 min at 15 m/min), and AE + 3 h recovery (AER) groups and measured downstream alterations in markers of mitochondrial biogenesis, autophagy, and mitophagy. In the absence of p53, activation of p38 MAPK upon exercise was abolished, whereas CaMKII and AMP-activated protein kinase only displayed an attenuated enhancement in the AER group compared with WT mice. The translocation of peroxisome proliferator-activated receptor-γ coactivator-1 α to the nucleus was diminished and only observed in the AER group, and the subsequent increase in messenger RNA transcripts related to mitochondrial biogenesis with exercise and recovery was absent in the p53 KO animals. Whole-muscle autophagic and lysosomal markers did not respond to exercise, irrespective of the genotype of the exercised mice, with the exception of increased ubiquitination observed in KO mice with exercise. Markers of mitophagy were elevated in response to AE and AER conditions in both WT and p53 KO runners. The data suggest that p53 is important for the exercise-induced activation of mitochondrial synthesis and is integral in regulating autophagy during control conditions but not in response to exercise.

Exercise-induced mitochondrial p53 repairs mtDNA mutations in mutator mice

BackgroundHuman genetic disorders and transgenic mouse models have shown that mitochondrial DNA (mtDNA) mutations and telomere dysfunction instigate the aging process. Epidemiologically, exercise is associated with greater life expectancy and reduced risk of chronic diseases. While the beneficial effects of exercise are well established, the molecular mechanisms instigating these observations remain unclear.ResultsEndurance exercise reduces mtDNA mutation burden, alleviates multisystem pathology, and increases lifespan of the mutator mice, with proofreading deficient mitochondrial polymerase gamma (POLG1). We report evidence for a POLG1-independent mtDNA repair pathway mediated by exercise, a surprising notion as POLG1 is canonically considered to be the sole mtDNA repair enzyme. Here, we show that the tumor suppressor protein p53 translocates to mitochondria and facilitates mtDNA mutation repair and mitochondrial biogenesis in response to endurance exercise. Indeed, in mutator mice with muscle-specific deletion of p53, exercise failed to prevent mtDNA mutations, induce mitochondrial biogenesis, preserve mitochondrial morphology, reverse sarcopenia, or mitigate premature mortality.ConclusionsOur data establish a new role for p53 in exercise-mediated maintenance of the mtDNA genome and present mitochondrially targeted p53 as a novel therapeutic modality for diseases of mitochondrial etiology.

Role of p53 within the regulatory network controlling muscle mitochondrial biogenesis.

The tumor suppressor protein p53 is recognized to contribute significantly to the regulation of mitochondrial content. Mice without p53 have reduced endurance capacity and muscle performance. However, the function of p53 in muscle remains to be fully established. Understanding how p53 coordinates mitochondrial homeostasis will facilitate a better comprehension of how exercise could constitute as a therapy for cancer treatment.

Effect of p53 on mitochondrial morphology, import, and assembly in skeletal muscle

The purpose of this study was to investigate whether p53 regulates mitochondrial function via changes in mitochondrial protein import, complex IV (COX) assembly, or the expression of key proteins involved in mitochondrial dynamics and degradation. Mitochondria from p53 KO mice displayed ultra-structural alterations and were more punctate in appearance. This was accompanied by protein-specific alterations in fission, fusion, and mitophagy-related proteins. However, matrix-destined protein import into subsarcolemmal or intermyofibrillar mitochondria was unaffected in the absence of p53, despite mitochondrial subfraction-specific reductions in Tom20, Tim23, mtHsp70, and mtHsp60 in the knockout (KO) mitochondria. Complex IV activity in isolated mitochondria was also unchanged in KO mice, but two-dimensional blue native-PAGE revealed a reduction in the assembly of complex IV within the IMF fractions from KO mice in tandem with lower levels of the assembly protein Surf1. This observed defect in complex IV assembly may facilitate the previously documented impairment in mitochondrial function in p53 KO mice. We suspect that these morphological and functional impairments in mitochondria drive a decreased reliance on mitochondrial respiration as a means of energy production in skeletal muscle in the absence of p53.

The regulation of mitochondrial transcription factor A (Tfam) expression during skeletal muscle cell differentiation

The present study highlights the transcriptional and post-transcriptional regulation of mitochondrial transcription factor A (Tfam) during myogenesis and the relationship this has with the increase in mitochondrial content during differentiation.