Differential effects of exercise training on skeletal muscle SERCA gene expression

Small peptides: could they have a big role in metabolism and the response to exercise?

Exercise is a powerful non-pharmacological intervention for the treatment and prevention of numerous chronic diseases. Contracting skeletal muscles provoke widespread perturbations in numerous cells, tissues and organs, which stimulate multiple integrated adaptations that ultimately contribute to the many health benefits associated with regular exercise. Despite much research, the molecular mechanisms driving such changes are not completely resolved. Technological advancements beginning in the early 1960s have opened new avenues to explore the mechanisms responsible for the many beneficial adaptations to exercise. This has led to increased research into the role of small peptides (<100 amino acids) and mitochondrially derived peptides in metabolism and disease, including those coded within small open reading frames (sORFs; coding sequences that encode small peptides). Recently, it has been hypothesized that sORF-encoded mitochondrially derived peptides and other small peptides play significant roles as exercise-sensitive peptides in exercise-induced physiological adaptation. In this review, we highlight the discovery of mitochondrially derived peptides and newly discovered small peptides involved in metabolism, with a specific emphasis on their functions in exercise-induced adaptations and the prevention of metabolic diseases. In light of the few studies available, we also present data on how both single exercise sessions and exercise training affect expression of sORF-encoded mitochondrially derived peptides. Finally, we outline numerous research questions that await investigation regarding the roles of mitochondrially derived peptides in metabolism and prevention of various diseases, in addition to their roles in exercise-induced physiological adaptations, for future studies.

Functional sympatholysis in mouse skeletal muscle involves sarcoplasmic reticulum swelling in arterial smooth muscle cells

The vasoconstrictive effect of sympathetic activity is attenuated in contracting skeletal muscle (functional sympatholysis), allowing increased blood supply to the working muscle but the underlying mechanisms are incompletely understood. The purpose of this study was to examine α-adrenergic receptor responsiveness in isolated artery segments from non-exercised and exercised mice, using wire myography. Isometric tension recordings performed on femoral artery segments from exercised mice showed decreased α-adrenergic receptor responsiveness compared to non-exercised mice (logEC50 -5.2 ± 0.04 M vs. -5.7 ± 0.08 M, respectively). In contrast, mesenteric artery segments from exercised mice displayed similar α-adrenergic receptor responses compared to non-exercised mice. Responses to the vasoconstrictor serotonin (5-HT) and vasodilator isoprenaline, were similar in femoral artery segments from non-exercised and exercised mice. To study sarcoplasmic reticulum (SR) function, we examined arterial contractions induced by caffeine, which depletes SR Ca2+ and thapsigargin, which inhibits SR Ca2+ -ATPase (SERCA) and SR Ca2+ uptake. Arterial contractions to both caffeine and thapsigargin were increased in femoral artery segment from exercised compared to non-exercised mice. Furthermore, 3D electron microscopy imaging of the arterial wall showed SR volume/length ratio increased 157% in smooth muscle cells of the femoral artery from the exercised mice, whereas there was no difference in SR volume/length ratio in mesenteric artery segments. These results show that in arteries surrounding exercising muscle, the α-adrenergic receptor constrictions are blunted, which can be attributed to swollen smooth muscle cell SR's, likely due to increased Ca2+ content that is possibly reducing free intracellular Ca2+ available for contraction. Overall, this study uncovers a previously unknown mechanism underlying functional sympatholysis.

The regulation of sarco(endo)plasmic reticulum calcium-ATPases (SERCA)

The sarco(endo)plasmic reticulum calcium ATPase (SERCA) is responsible for transporting calcium (Ca(2+)) from the cytosol into the lumen of the sarcoplasmic reticulum (SR) following muscular contraction. The Ca(2+) sequestering activity of SERCA facilitates muscular relaxation in both cardiac and skeletal muscle. There are more than 10 distinct isoforms of SERCA expressed in different tissues. SERCA2a is the primary isoform expressed in cardiac tissue, whereas SERCA1a is the predominant isoform expressed in fast-twitch skeletal muscle. The Ca(2+) sequestering activity of SERCA is regulated at the level of protein content and is further modified by the endogenous proteins phospholamban (PLN) and sarcolipin (SLN). Additionally, several novel mechanisms, including post-translational modifications and microRNAs (miRNAs) are emerging as integral regulators of Ca(2+) transport activity. These regulatory mechanisms are clinically relevant, as dysregulated SERCA function has been implicated in the pathology of several disease states, including heart failure. Currently, several clinical trials are underway that utilize novel therapeutic approaches to restore SERCA2a activity in humans. The purpose of this review is to examine the regulatory mechanisms of the SERCA pump, with a particular emphasis on the influence of exercise in preventing the pathological conditions associated with impaired SERCA function.

Differential regulation of the fiber type-specific gene expression of the sarcoplasmic reticulum calcium-ATPase isoforms induced by exercise training

The regulatory role of adenosine monophosphate-activated protein kinase (AMPK)-α2 on sarcoplasmic reticulum calcium-ATPase (SERCA) 1a and SERCA2a in different skeletal muscle fiber types has yet to be elucidated. Sedentary (Sed) or exercise-trained (Ex) wild-type (WT) and AMPKα2-kinase dead (KD) transgenic mice, which overexpress a mutated and inactivated AMPKα2 subunit, were utilized to characterize how genotype or exercise training influenced the regulation of SERCA isoforms in gastrocnemius. As expected, both Sed and Ex KD mice had >40% lower AMPK phosphorylation and 30% lower SERCA1a protein than WT mice (P < 0.05). In contrast, SERCA2a protein was not different among KD and WT mice. Exercise increased SERCA1a and SERCA2a protein content among WT and KD mice, compared with their Sed counterparts. Maximal SERCA activity was lower in KD mice, compared with WT. Total phospholamban protein was higher in KD mice than in WT and lower in Ex compared with Sed mice. Exercise training increased phospholamban Ser(16) phosphorylation in WT mice. Laser capture microdissection and quantitative PCR indicated that SERCA1a mRNA expression among type I fibers was not altered by genotype or exercise, but SERCA2a mRNA was increased 30-fold in WT+Ex, compared with WT+Sed. In contrast, the exercise-stimulated increase for SERCA2a mRNA was blunted in KD mice. Exercise upregulated SERCA1a and SERCA2a mRNA among type II fibers, but was not altered by genotype. Collectively, these data suggest that exercise differentially influences SERCA isoform expression in type I and type II fibers. Additionally, AMPKα2 influences the regulation of SERCA2a mRNA in type I skeletal muscle fibers following exercise training.

Aerobic exercise training improves Ca2+ handling and redox status of skeletal muscle in mice

Exercise training is known to promote relevant changes in the properties of skeletal muscle contractility toward powerful fibers. However, there are few studies showing the effect of a well-established exercise training protocol on Ca2+ handling and redox status in skeletal muscles with different fiber-type compositions. We have previously standardized a valid and reliable protocol to improve endurance exercise capacity in mice based on maximal lactate steady-state workload (MLSSw). The aim of this study was to investigate the effect of exercise training, performed at MLSSw, on the skeletal muscle Ca2+ handling-related protein levels and cellular redox status in soleus and plantaris. Male C57BL/6J mice performed treadmill training at MLSSw over a period of eight weeks. Muscle fiber-typing was determined by myosin ATPase histochemistry, citrate synthase activity by spectrophotometric assay, Ca2+ handling-related protein levels by Western blot and reduced to oxidized glutathione ratio (GSH:GSSG) by high-performance liquid chromatography. Trained mice displayed higher running performance and citrate synthase activity compared with untrained mice. Improved running performance in trained mice was paralleled by fast-to-slow fiber-type shift and increased capillary density in both plantaris and soleus. Exercise training increased dihydropyridine receptor (DHPR) α2 subunit, ryanodine receptor and Na+/Ca2+ exchanger levels in plantaris and soleus. Moreover, exercise training elevated DHPR β1 subunit and sarcoplasmic reticulum Ca2+-ATPase (SERCA) 1 levels in plantaris and SERCA2 levels in soleus of trained mice. Skeletal muscle GSH content and GSH:GSSG ratio was increased in plantaris and soleus of trained mice. Taken together, our findings indicate that MLSSw exercise-induced better running performance is, in part, due to increased levels of proteins involved in skeletal muscle Ca2+ handling, whereas this response is partially dependent on specificity of skeletal muscle fiber-type composition. Finally, we demonstrated an augmented cellular redox status and GSH antioxidant capacity in trained mice.

Initiating treadmill training in late middle age offers modest adaptations in Ca2+ handling but enhances oxidative damage in senescent rat skeletal muscle

Aging skeletal muscle shows an increased time to peak force and relaxation and a decreased specific force, all of which could relate to changes in muscle Ca(2+) handling. The purpose of this study was to determine if Ca(2+)-handling protein content and function are decreased in senescent gastrocnemius muscle and if initiating a training program in late middle age (LMA, 29 mo old) could improve function in senescent (34- to 36-mo-old) muscle. LMA male Fischer 344 x Brown-Norway rats underwent 5-7 mo of treadmill training. Aging resulted in a decrease in maximal sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) activity and a decrease in Ca(2+) release rate but no change in Ca(2+) uptake rate. Efficiency of the Ca(2+) pump was increased with age, as was the content of SERCA2a. Training caused a further increase in SERCA2a content. Aging also caused an increase in protein carbonyl and reactive nitrogen species damage accumulation, and both further increased with training. Consistent with the increase in oxidative damage, heat shock protein 70 content was increased with age and further increased with training. Together, these results suggest that while initiating exercise training in LMA augments the age-related increase in expression of heat shock protein 70 and the more efficient SERCA2a isoform, it did not prevent the decrease in SERCA activity and exacerbated oxidative damage in senescent gastrocnemius muscle.

Moderate intensity of regular exercise improves cardiac SR Ca2+ uptake activity in ovariectomized rats

The impact of regular exercise in protecting cardiac deteriorating results of female sex hormone deprivation was evaluated by measuring changes in intracellular Ca2+ removal activity of sarcoplasmic reticulum (SR) in ovariectomized rats following 9-wk treadmill running exercise at moderate intensity. Despite induction of cardiac hypertrophy in exercised groups of both sham-operated and ovariectomized rats, exercise training had no effect on SR Ca2+ uptake and SR Ca(2+)-ATPase (SERCA) in hormone intact rat heart. However, exercise training normalized the suppressed maximum SR Ca2+ uptake and SERCA activity in ovariectomized rat heart. While exercise training normalized the leftward shift in pCa (-log[Ca2+])-SR Ca2+ uptake relation in ovariectomized rats, no effect was detected in exercised sham-operated rats. Similar phenomena were also observed on SERCA and on phospholamban (PLB) phosphorylation levels; exercise training in ovariectomized rats enhanced SERCA expression to reach the level as that in sham-operated rats, in which there were no differences in SERCA and phospho-PLB levels between sedentary and exercised groups. In addition, the reduction in phospho-Thr(17) PLB in myocardium of ovariectomized rats was abolished by exercise training. These results showed that regular exercise maintains the molecular activation of cardiac SR Ca2+ uptake under normal physiological conditions and is able to induce a protective impact on cardiac SR Ca2+ uptake in ovarian sex hormone-deprived status.

Muscle sarcoplasmic reticulum calcium regulation in humans during consecutive days of exercise and recovery

The study investigated the hypothesis that three consecutive days of prolonged cycle exercise would result in a sustained reduction in the Ca(2+)-cycling properties of the vastus lateralis in the absence of changes in the sarcoplasmic (endoplasmic) reticulum Ca(2+)-ATPase (SERCA) protein. Tissue samples were obtained at preexercise (Pre) and postexercise (Post) on day 1 (E1) and day 3 (E3) and during recovery day 1 (R1), day 2 (R2), and day 3 (R3) in 12 active but untrained volunteers (age 19.2 +/- 0.27 yr; mean +/- SE) and analyzed for changes (nmol.mg protein(-1).min(-1)) in maximal Ca(2+)-ATPase activity (V(max)), Ca(2+) uptake and Ca(2+) release (phase 1 and phase 2), and SERCA isoform expression (SERCA1a and SERCA2a). At E1, reductions (P < 0.05) from Pre to Post in V(max) (150 +/- 7 vs. 121 +/- 7), Ca(2+) uptake (7.79 +/- 0.28 vs. 5.71 +/- 0.33), and both phases of Ca(2+) release (phase 1, 20.3 +/- 1.3 vs. 15.2 +/- 1.1; phase 2, 7.70 +/- 0.60 vs. 4.99 +/- 0.48) were found. In contrast to V(max), which recovered at Pre E3 and then remained stable at Post E3 and throughout recovery, Ca(2+) uptake remained depressed (P < 0.05) at E3 Pre and Post and at R1 as did phase 2 of Ca(2+) release. Exercise resulted in an increase (P < 0.05) in SERCA1a (14% at R2) but not SERCA2a. It is concluded that rapidly adapting mechanisms protect V(max) following the onset of regular exercise but not Ca(2+) uptake and Ca(2+) release.

Interaction between signalling pathways involved in skeletal muscle responses to endurance exercise

The purpose of this review is to summarise the latest literature on the signalling pathways involved in transcriptional modulations of genes that encode contractile and metabolic proteins in response to endurance exercise. A special attention has been paid to the cooperation between signalling pathways and coordinated expression of protein families that establish myofibre phenotype. Calcium acts as a second messenger in skeletal muscle during exercise, conveying neuromuscular activity into changes in the transcription of specific genes. Three main calcium-triggered regulatory pathways acting through calcineurin, Ca(2+)-calmodulin-dependent protein kinases (CaMK) and Ca(2+)-dependent protein kinase C, transduce alterations in cytosolic calcium concentration to target genes. Calcineurin signalling, the most important of these Ca(2+)-dependent pathways, stimulates the activation of many slow-fibre gene expression, including genes encoding proteins involved in contractile process, Ca(2+) uptake and energy metabolism. It involves the interaction between multiple transcription factors and the collaboration of other Ca(2+)-dependent CaMKs. Although members of mitogen-activated protein kinase (MAPK) pathways are activated during exercise, their integration into other signalling pathways remains largely unknown. The peroxisome proliferator-activated receptor gamma (PPARgamma) coactivator-1alpha (PGC-1alpha) constitutes a pivotal factor of the circuitry which coordinates mitochondrial biogenesis and which couples to the expression of contractile and metabolic genes with prolonged exercise.

UCP-3: regulação da expressão gênica no músculo esquelético e possível relação com o controle do peso corporal

As UCPs constituem um subgrupo das proteínas carreadoras mitocondriais que estão localizadas na membrana mitocondrial interna. Por meio da dissipação do gradiente de próton, elas desacoplam a fosforilação oxidativa e convertem combustível em calor. Foram identificadas quatro isoformas da UCP. A UCP-1 foi a primeira a ser descoberta, sendo encontrada exclusivamente no tecido adiposo marrom, a UCP-2 é encontrada em vários tecidos, a UCP-3 encontra-se no músculo esquelético em humanos e tecido adiposo marrom e músculo esquelético em roedores, enquanto a UCP-4 é expressa no cérebro. A expressão da UCP-3 no músculo esquelético e no tecido adiposo marrom pode fazer destes tecidos importantes mediadores da termogênese adaptativa. No entanto, o papel da UCP-3 quanto ao gasto de energia e como causa da obesidade ainda não passa de uma hipótese. Há evidências de que a UCP-3 seja regulada pelos substratos energéticos, tais como ácidos graxos e glicose que, ao entrarem no músculo, provocam aumento da UCP-3 e aumento no gasto de energia. Nosso objetivo nesta revisão foi descrever e discutir as informações disponíveis sobre a regulação da UCP-3, e sua possível relação com o controle do peso corporal.