delta-Aminolaevulinate synthase expression in muscle after contractions and recovery

Delta-aminolevulinate synthase 2 polymorphism is associated with maximal oxygen uptake after Living-high exercise-high training-low in a male Chinese population

OBJECTIVE

Each individual shows great variations to hypoxic training, which may be due to individual differences in genotype or gene polymorphism. δ-aminolevulinic acid synthase 2 (ALAS2 ) polymorphism is used as a marker for X-linked sideroblastic anemia. This study assessed the ALAS2 polymorphism for an association with response to Living-high exercise-high training-low (HiHiLo) training in Han Chinese males.

METHODS

A total of 244 healthy young male subjects of Han nationality were recruited from Northern China for detection of the ALAS2 polymorphism, 72 of whom were then selected for undergoing a four-week HiHiLo training program (i.e., subject to 10 h of hypoxia training daily at 14.5-14.8% O2 exposure, three occurrences of hypoxic training every week at 15.4% O2 level, and normal training at sea level). GeneScan and DNA sequencing were used to analyze ALAS2 polymorphism. Before and after training, the maximal oxygen uptake (VO2 max) in each individual was recorded.

RESULTS

A successive cut-point analysis showed that the initial hemoglobin value in individuals with dinucleotide repeats ≤ 166 bp was significantly higher than in individuals with dinucleotide repeats > 166-bp (P = 0.05). The ∆VO2 max and ∆rVO2 max after HiHiLo training were significantly higher in individuals with dinucleotide repeats ≤ 166 bp than in individuals with dinucleotide repeats > 166 bp (P < 0.01).

CONCLUSIONS

The compound dinucleotide repeat polymorphism in ALAS2 intron 7 correlated with response to HiHiLo training. Further study will evaluate this ALAS2 SNP as a genetic marker to predict responses to HiHiLo training.

Plasticity of TOM complex assembly in skeletal muscle mitochondria in response to chronic contractile activity

We investigated the assembly of the TOM complex within skeletal muscle under conditions of chronic contractile activity-induced mitochondrial biogenesis. Tom40 import into mitochondria was increased by chronic contractile activity, as was its time-dependent assembly into the TOM complex. These changes coincided with contractile activity-induced augmentations in the expression of key protein import machinery components Tim17, Tim23, and Tom22, as well as the cytosolic chaperone Hsp90. These data indicate the adaptability of the TOM protein import complex and suggest a regulatory role for the assembly of this complex in exercise-induced mitochondrial biogenesis.

Effect of chronic contractile activity on mRNA stability in skeletal muscle

Repeated bouts of exercise promote the biogenesis of mitochondria by multiple steps in the gene expression patterning. The role of mRNA stability in controlling the expression of mitochondrial proteins is relatively unexplored. To induce mitochondrial biogenesis, we chronically stimulated (10 Hz; 3 or 6 h/day) rat muscle for 7 days. Chronic contractile activity (CCA) increased the protein expression of PGC-1alpha, c-myc, and mitochondrial transcription factor A (Tfam) by 1.6-, 1.7- and 2.0-fold, respectively. To determine mRNA stability, we incubated total RNA with cytosolic extracts using an in vitro cell-free system. We found that the intrinsic mRNA half-lives (t(1/2)) were variable within control muscle. Peroxisome proliferator-activated receptor-gamma, coactivator-1alpha (PGC-1alpha) and Tfam mRNAs decayed more rapidly (t(1/2) = 22.7 and 31.4 min) than c-myc mRNA (t(1/2) = 99.7 min). Furthermore, CCA resulted in a differential response in degradation kinetics. After CCA, PGC-1alpha and Tfam mRNA half-lives decreased by 48% and 44%, respectively, whereas c-myc mRNA half-life was unchanged. CCA induced an elevation of both the cytosolic RNA-stabilizing human antigen R (HuR) and destabilizing AUF1 (total) by 2.4- and 1.8-fold, respectively. Increases in the p37(AUF1), p40(AUF1), and p45(AUF1) isoforms were most evident. Thus these data indicate that CCA results in accelerated turnover rates of mRNAs encoding important mitochondrial biogenesis regulators in skeletal muscle. This adaptation is likely beneficial in permitting more rapid phenotypic plasticity in response to subsequent contractile activity.

Biogenesis of the mitochondrial Tom40 channel in skeletal muscle from aged animals and its adaptability to chronic contractile activity

Evidence exists that mitochondrial content and/or function is reduced in muscle of aging individuals. The purposes of this study were to investigate the contribution of outer membrane protein import and assembly processes to this decline and to determine whether the assembly process could adapt to chronic contractile activity (CCA). Tom40 assembly into the translocases of the outer membrane (TOM complex) was measured in subsarcolemmal mitochondria obtained from young (6 mo old) and aged (36 mo old) Fischer 344 x Brown Norway animals. While the initial import of Tom40 did not differ between young and aged animals, its subsequent assembly into the final approximately 380 kDa complex was 2.2-fold higher (P < 0.05) in mitochondria from aged compared with young animals. This was associated with a higher abundance of Tom22, a protein vital for the assembly process. CCA induced a greater initial import and subsequent assembly of Tom40 in mitochondria from young animals, resulting in a CCA-induced 75% increase (P < 0.05) in Tom40 within mitochondria. This effect of CCA was attenuated in mitochondria from old animals. These data suggest that the import and assembly of proteins into the outer membrane do not contribute to reduced mitochondrial content or function in aged animals. Indeed, the greater assembly rate in mitochondria from aged animals may be a compensatory mechanism attempting to offset any decrements in mitochondrial content or function within aged muscle. Our data also indicate the potential of CCA to contribute to increased mitochondrial biogenesis in muscle through changes in the outer membrane import and assembly pathway.

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-γ (PPARγ) coactivator (PGC)-1α 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.

Nuclear hormone receptors for heme: REV-ERBalpha and REV-ERBbeta are ligand-regulated components of the mammalian clock

The nuclear hormone receptors (NHRs), REV-ERBalpha and REV-ERBbeta, regulate a number of physiological functions including the circadian rhythm, lipid metabolism, and cellular differentiation. These two receptors lack the activation function-2 region that is associated with the ability of NHRs to recruit coactivators and activate target gene transcription. These NHRs have been characterized as constitutive repressors of transcription due to their lack of an identified ligand and their strong ability to recruit the corepressor, nuclear receptor corepressor. Recently, the porphyrin heme was demonstrated to function as a ligand for both REV-ERBs. Heme binds directly to the ligand-binding domain and regulates the ability of these NHRs to recruit nuclear receptor corepressor to target gene promoters. This review focuses on the physiological roles that these two receptors play and the implications of heme functioning as their ligand. The prospect that these NHRs, now known to be regulated by small molecule ligands, may be targets for development of drugs for treatment of diseases associated with aberrant circadian rhythms including metabolic and psychiatric disorders as well as cancer is also addressed.

Coordination of metabolic plasticity in skeletal muscle

Skeletal muscle is a highly malleable tissue, capable of pronounced metabolic and morphological adaptations in response to contractile activity (i.e. exercise). Each bout of contractile activity results in a coordinated alteration in the expression of a variety of nuclear DNA and mitochondrial DNA (mtDNA) gene products, leading to phenotypic adaptations. This results in an increase in muscle mitochondrial volume and changes in organelle composition, referred to as mitochondrial biogenesis. The functional consequence of this biogenesis is an improved resistance to fatigue. Signals initiated by the exercise bout involve changes in intracellular Ca2+ as well as alterations in energy status (i.e. ATP/ADP ratio) and the consequent activation of downstream kinases such as AMP kinase and Ca2+-calmodulin-activated kinases. These kinases activate transcription factors that bind DNA to affect the transcription of genes, the most evident manifestation of which occurs during the post-exercise recovery period when energy metabolism is directed toward anabolism, rather than contractile activity. An important protein that is affected by exercise is the transcriptional coactivator PGC-1alpha, which cooperates with multiple transcription factors to induce the expression of nuclear genes encoding mitochondrial proteins. Once translated in the cytosol, these mitochondrially destined proteins are imported into the mitochondrial outer membrane, inner membrane or matrix space via specific import machinery transport components. Contractile activity affects the expression of the import machinery, as well as the kinetics of import, thus facilitating the entry of newly synthesized proteins into the expanding organelle. An important set of proteins that are imported are the mtDNA transcription factors, which influence the expression and replication of mtDNA. While mtDNA contributes only 13 proteins to the synthesis of the organelle, these proteins are vital for the proper assembly of multi-subunit complexes of the respiratory chain, when combined with nuclear-encoded protein subunits. The expansion of skeletal muscle mitochondria during organelle biogenesis involves the assembly of an interconnected network system (i.e. a mitochondrial reticulum). This expansion of membrane size is influenced by the balance between mitochondrial fusion and fission. Thus, mitochondrial biogenesis is an adaptive process that requires the coordination of multiple cellular events, including the transcription of two genomes, the synthesis of lipids and proteins and the stoichiometric assembly of multisubunit protein complexes into a functional respiratory chain. Impairments at any step can lead to defective electron transport, a subsequent failure of ATP production and an inability to maintain energy homeostasis.

Application of Animal Models: Chronic Electrical Stimulation-Induced Contractile Activity

Unilateral, chronic low-frequency electrical stimulation (CLFS) is an experimental model that evokes numerous biochemical and physiological adaptations in skeletal muscle. These occur within a short time frame and are restricted to the stimulated muscle. The humoral effects of whole body exercise are eliminated and the nonstimulated contralaterai limb can often be used as a control muscle, if possible effects on the contralateral side are considered. CLFS induces a fast-to-slow transformation of muscle because of alterations in calcium dynamics and myofibrillar proteins, and a white-to-red transformation because of changes in mitochondrial enzymes, myoglobin, and the induction of angiogenesis. These adaptations occur in a coordinated time-dependent manner and result from altered gene expression, including transcriptional and posttranscriptional processes. CLFS techniques have also been applied to myocytes in cell culture, which provide a greater opportunity for the delivery of pharmacological agents or for the application of gene transfer methodologies. Clinical applications of the CLFS technique have been limited, but they have shown potential therapeutic value in patients in whom voluntary muscle contraction is not possible due to debilitating disease and/or injury. Thus the CLFS technique has great value for studying various aspects of muscle adaptation, and its wider scientific application to a variety of neuromuscular-based disorders in humans appears to be warranted. Key words: skeletal muscle, muscle plasticity, endurance training, mitochondrial biogenesis, fiber types

Regulation of Egr-1, SRF, and Sp1 mRNA expression in contracting skeletal muscle cells

The early cellular signals associated with contractile activity initiate the activation and induction of transcription factors that regulate changes in skeletal muscle phenotype. The transcription factors Egr-1, Sp1, and serum response factor (SRF) are potentially important mediators of mitochondrial biogenesis based on the prevalence of binding sites for them in the promoter regions of genes encoding mitochondrial proteins, including PGC-1α, the important regulator of mitochondrial biogenesis. Thus, to further define a role for transcription factors at the onset of contractile activity, we examined the time-dependent alterations in Egr-1, Sp1, and SRF mRNA and the levels in electrically stimulated mouse C2C12skeletal muscle cells. Early transient increases in Egr-1 mRNA levels within 30 min ( P < 0.05) of contractile activity led to threefold increases ( P < 0.05) in Egr-1 protein by 60 min. The increase in Egr-1 mRNA was not because of increased stability, as Egr-1 mRNA half-life after 30 min of stimulation showed only a 58% decline. Stimulation of muscle cells had no effect on Sp1 mRNA but led to progressive increases ( P < 0.05) in SRF mRNA by 30 and 60 min. This was not matched by increases in SRF protein but occurred coincident with increases ( P < 0.05) in SRF-serum response element DNA binding at 30 and 60 min as a result of SRF phosphorylation on serine-103. To assess the importance of the recovery period, 12 h of continuous contractile activity was compared with four successive 3-h bouts, with an intervening 21-h recovery period after each bout. Continuous contractile activity led to a twofold increase ( P < 0.05) in Egr-1 mRNA, no change in SRF mRNA, and a 43% decrease in Sp1 mRNA expression. The recovery period prevented the decline in Sp1 mRNA, produced a decrease in Egr-1 mRNA, and had no effect on SRF mRNA. Thus continuous and intermittent contractile activity evoked different specific transcription factor expression patterns, which may ultimately contribute to divergent qualitative, or temporal patterns of, phenotypic adaptation in muscle.

AMP kinase is required for mitochondrial biogenesis in skeletal muscle in response to chronic energy deprivation

Mitochondrial biogenesis is a critical adaptation to chronic energy deprivation, yet the signaling mechanisms responsible for this response are poorly understood. To examine the role of AMP-activated protein kinase (AMPK), an evolutionarily conserved fuel sensor, in mitochondrial biogenesis we studied transgenic mice expressing a dominant-negative mutant of AMPK in muscle (DN-AMPK). Both DN-AMPK and WT mice were treated with beta-guanidinopropionic acid (GPA), a creatine analog, which led to similar reductions in the intramuscular ATPAMP ratio and phosphocreatine concentrations. In WT mice, GPA treatment resulted in activation of muscle AMPK and mitochondrial biogenesis. However, the same GPA treatment in DN-AMPK mice had no effect on AMPK activity or mitochondrial content. Furthermore, AMPK inactivation abrogated GPA-induced increases in the expression of peroxisome proliferator-activated receptor gamma coactivator 1alpha and calciumcalmodulin-dependent protein kinase IV (both master regulators of mitochondrial biogenesis). These data demonstrate that by sensing the energy status of the muscle cell, AMPK is a critical regulator involved in initiating mitochondrial biogenesis.

Chronic activation of AMP kinase results in NRF-1 activation and mitochondrial biogenesis

The underlying mechanism by which skeletal muscle adapts to exercise training or chronic energy deprivation is largely unknown. To examine this question, rats were fed for 9 wk either with or without beta-guanadinopropionic acid (beta-GPA; 1% enriched diet), a creatine analog that is known to induce muscle adaptations similar to those induced by exercise training. Muscle phosphocreatine, ATP, and ATP/AMP ratios were all markedly decreased and led to the activation of AMP-activated protein kinase (AMPK) in the beta-GPA-fed rats compared with control rats. Under these conditions, nuclear respiratory factor-1 (NRF-1) binding activity, measured using a cDNA probe containing a sequence encoding for the promoter of delta-aminolevulinate (ALA) synthase, was increased by about eightfold in the muscle of beta-GPA-fed rats compared with the control group. Concomitantly, muscle ALA synthase mRNA and cytochrome c content were also increased. Mitochondrial density in both extensor digitorum longus and epitrochlearis from beta-GPA-fed rats was also increased by more than twofold compared with the control group. In conclusion, chronic phosphocreatine depletion during beta-GPA supplementation led to the activation of muscle AMPK that was associated with increased NRF-1 binding activity, increased cytochrome c content, and increased muscle mitochondrial density. Our data suggest that AMPK may play an important role in muscle adaptations to chronic energy stress and that it promotes mitochondrial biogenesis and expression of respiratory proteins through activation of NRF-1.

Invited Review: contractile activity-induced mitochondrial biogenesis in skeletal muscle

Chronic contractile activity produces mitochondrial biogenesis in muscle. This adaptation results in a significant shift in adenine nucleotide metabolism, with attendant improvements in fatigue resistance. The vast majority of mitochondrial proteins are derived from the nuclear genome, necessitating the transcription of genes, the translation of mRNA into protein, the targeting of the protein to a mitochondrial compartment via the import machinery, and the assembly of multisubunit enzyme complexes in the respiratory chain or matrix. Putative signals involved in initiating this pathway of gene expression in response to contractile activity likely arise from combinations of accelerations in ATP turnover or imbalances between mitochondrial ATP synthesis and cellular ATP demand, and Ca(2+) fluxes. These rapid events are followed by the activation of exercise-responsive kinases, which phosphorylate proteins such as transcription factors, which subsequently bind to upstream regulatory regions in DNA, to alter transcription rates. Contractile activity increases the mRNA levels of nuclear-encoded proteins such as cytochrome c and mitochondrial transcription factor A (Tfam) and mRNA levels of upstream transcription factors like c-jun and nuclear respiratory factor-1 (NRF-1). mRNA level changes are often most evident during the postexercise recovery period, and they can occur as a result of contractile activity-induced increases in transcription or mRNA stability. Tfam is imported into mitochondria and controls the expression of mitochondrial DNA (mtDNA). mtDNA contributes only 13 protein products to the respiratory chain, but they are vital for electron transport and ATP synthesis. Contractile activity increases Tfam expression and accelerates its import into mitochondria, resulting in increased mtDNA transcription and replication. The result of this coordinated expression of the nuclear and the mitochondrial genomes, along with poorly understood changes in phospholipid synthesis, is an expansion of the muscle mitochondrial reticulum. Further understanding of 1) regulation of mtDNA expression, 2) upstream activators of NRF-1 and other transcription factors, 3) the identity of mRNA stabilizing proteins, and 4) potential of contractile activity-induced changes in apoptotic signals are warranted.

Effect of contractile activity on protein turnover in skeletal muscle mitochondrial subfractions

To determine the role of intramitochondrial protein synthesis (PS) and degradation (PD) in contractile activity-induced mitochondrial biogenesis, we evaluated rates of [(35)S]methionine incorporation into protein in isolated rat muscle subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria. Rates of PS ranged from 47 to 125% greater (P < 0.05) in IMF compared with SS mitochondria. Intense, acute in situ contractile activity (10 Hz, 5 min) of fast-twitch gastrocnemius muscle resulted in a 50% decrease in PS (P < 0.05) in SS but not IMF mitochondria. Recovery, or continued contractile activity (55 min), reestablished PS in SS mitochondria. In contrast, PS was not affected in either SS or IMF mitochondria after prolonged (60-min) contractile activity in the presence or absence of a recovery period. PD was not influenced by 5 min of contractile activity in the presence or absence of recovery but was reduced after 60 min of contractions followed by recovery. Chronic stimulation (10 Hz, 3 h/day, 14 days) increased muscle cytochrome-c oxidase activity by 2.2-fold but reduced PS in IMF mitochondria by 29% (P < 0.05; n = 4). PS in SS mitochondria and PD in both subfractions were not changed by chronic stimulation. Thus acute contractile activity exerts differential effects on protein turnover in IMF and SS mitochondria, and it appears that intramitochondrial PS does not limit the extent of chronic contractile activity-induced mitochondrial biogenesis.

Cytochrome c transcriptional activation and mRNA stability during contractile activity in skeletal muscle

We evaluated contractile activity-induced alterations in cytochrome c transcriptional activation and mRNA stability with unilateral chronic stimulation (10 Hz, 3 h/day) of the rat tibialis anterior (TA) muscle for 1, 2, 3, 4, 5, and 7 days (n = 3-11/group). Transcriptional activation was assessed by direct plasmid DNA injection into the TA with a chloramphenicol acetyltransferase (CAT) reporter gene linked to 326 bp of the cytochrome c promoter. Cytochrome c mRNA in stimulated muscles increased by 1.3- to 1. 7-fold above control between 1 and 7 days. Cytochrome c protein was increased after 5 days of stimulation to reach levels that were 1. 9-fold higher than control by 7 days. Cytochrome c mRNA stability, determined with an in vitro decay assay, was greater in stimulated TA than in control between 2 and 4 days, likely mediated by the induction of a cytosolic factor. In contrast, cytochrome c transcriptional activation was elevated only after 5 days of stimulation when mRNA stability had returned to control levels. Thus the contractile activity-induced increase in cytochrome c mRNA was due to an early increase in mRNA stability, followed by an elevation in transcriptional activation, leading to an eventual increase in cytochrome c protein levels.

Contractile activity-induced adaptations in the mitochondrial protein import system

We previously demonstrated that subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondrial subfractions import proteins at different rates. This study was undertaken to investigate 1) whether protein import is altered by chronic contractile activity, which induces mitochondrial biogenesis, and 2) whether these two subfractions adapt similarly. Using electrical stimulation (10 Hz, 3 h/day for 7 and 14 days) to induce contractile activity, we observed that malate dehydrogenase import into the matrix of the SS and IMF mitochondia isolated from stimulated muscle was significantly increased by 1.4-to 1.7-fold, although the pattern of increase differed for each subfraction. This acceleration of import may be mitochondrial compartment specific, since the import of Bcl-2 into the outer membrane was not affected. Contractile activity also modified the mitochondrial content of proteins comprising the import machinery, as evident from increases in the levels of the intramitochondrial chaperone mtHSP70 as well as the outer membrane import receptor Tom20 in SS and IMF mitochondria. Addition of cytosol isolated from stimulated or control muscles to the import reaction resulted in similar twofold increases in the ability of mitochondria to import malate dehydrogenase, despite elevations in the concentration of mitochondrial import-stimulating factor within the cytosol of chronically stimulated muscle. These results suggest that chronic contractile activity modifies the extra- and intramitochondrial environments in a fashion that favors the acceleration of precursor protein import into the matrix of the organelle. This increase in protein import is likely an important adaptation in the overall process of mitochondrial biogenesis.

Induction of heme oxygenase-1 (HSP32) mRNA in skeletal muscle following contractions

The capacity of preexisting antioxidant pathways to handle oxidative stress during exercise may be complemented by the synthesis of inducible heat stress proteins (HSP). Our purpose was to determine if the amount of mRNA for HSP32, a major oxidative stress protein, was increased in muscle after repetitive contractions. Reverse transcriptase-polymerase chain reaction analysis showed that HSP32 mRNA (normalized to alpha-actin mRNA) was increased about seven- and about fourfold (P < 0.35) immediately after 1 h of exhaustive running and after 3 h of muscle contractions (10 Hz nerve stimulation), respectively. Northern blot analysis revealed that HSP70 mRNAs were 3.5- to 15.5-fold above control value (P < 0.05), whereas the content of another oxidative stress protein mRNA (macrophage stress protein 23) was unchanged 0 h after contractions. The relative increase in HSP32 mRNA was found to be dependent on active tension generation; passive tension did not increase the HSP32-to-actin mRNA ratio. Increases in HSP32 mRNA may underlie an inducible antioxidant pathway in muscle responsive to metabolic stresses associated with repeated muscle contractions.

Increased contractile activity decreases RNA-protein interaction in the 3'-UTR of cytochrome c mRNA

This study was designed to gain an insight into mechanisms by which cytochrome c gene expression is enhanced by increased contractile activity in skeletal muscle. When rat tibialis anterior muscles were stimulated (10 Hz, 0.25 ms) for 0, 2, 6, 12, or 24 h or 2, 5, 9, or 13 days (n = 4 for each time point), cytochrome c protein (enzyme-linked immunosorbent assay) and mRNA (Northern blot analysis) concentrations started to increase by 9 days, and this was associated with concurrent decreases in cytochrome c mRNA-protein interaction (RNA gel mobility shift assay). We found that the decreased RNA-protein interaction in the stimulated muscle extract was restored by ultracentrifugation (150,000 g, 1 h) in the supernatant fraction. The 150,000 g pellet fraction of stimulated muscle was capable of inhibiting the RNA-protein interaction in control tibialis anterior muscles. These results provide evidence of an inhibitory factor that is responsible for decreasing RNA-protein interaction in the 3'-untranslated region of cytochrome c mRNA in continuously stimulated muscle.

Mitochondrial biogenesis in striated muscle

Mitochondrial biogenesis (synthesis) has been observed to occur in skeletal muscle in response to chronic use. It also occurs in cardiac muscle during growth and hypertrophy, and it may be impaired during the aging process. This review summarizes the literature on the processes of mitochondrial biogenesis at the biochemical and molecular levels, with particular reference to striated muscles. Mitochondrial biogenesis involves the expression of nuclear and mitochondrial genes and the coordination of these two genomes, the synthesis of proteins and phospholipids and their import into the organelle, and the incorporation of these lipids and proteins into their appropriate locations within the matrix, inner or outer membranes. The emphasis is on the regulation of these events, with information derived in part from other cellular systems. Although descriptions of mitochondrial content changes in heart and skeletal muscle during altered physiological states are plentiful, much work is needed at the molecular level to investigate the regulatory processes involved. A knowledge of biochemical and molecular biology techniques is essential for continued progress in the field. This is a promising area, and potential new avenues for future research are suggested.