Effects of eccentric treadmill running on mouse soleus: degeneration/regeneration studied with Myf-5 and MyoD probes

Impact of elevated serum advanced glycation end products and exercise on intact and injured murine tendons

OVERVIEW

Delayed tendon healing is a significant clinical challenge for those with diabetes. We explored the role of advanced glycation end-products (AGEs), a protein modification present at elevated levels in serum of individuals with diabetes, on injured and intact tendons using a mouse model. Cell proliferation following tissue injury is a vital component of healing. Based on our previous work demonstrating that AGEs limit cell proliferation, we proposed that AGEs are responsible for the delayed healing process commonly observed in diabetic patients. Further, in pursuit of interventional strategies, we suggested that moderate treadmill exercise may support a healing environment in the presence of AGEs as exercise has been shown to stimulate cell proliferation in tendon tissue.

MATERIALS AND METHODS

Mice began receiving daily intraperitoneal injections of bovine serum albumin (BSA)-Control or AGE-BSA injections (200μg/ml) at 16-weeks of age. A tendon injury was created in the central third of both patellar tendons. Animals assigned to an exercise group began a moderate treadmill protocol one week following injury. The intact Achilles tendon and soleus muscle were also evaluated to assess the effect of BSA and AGE-BSA on un-injured muscle and tendon.

RESULTS

We demonstrate that our injection dosing and schedule lead to an increase in serum AGEs. Our findings imply that AGEs indeed modulate gene expression following a patellar tendon injury and have modest effects on gene expression in intact muscle and tendon.

CONCLUSIONS

While additional biomechanical analysis is warranted, these data suggest that elevated serum AGEs in persons with diabetes may impact tendon health.

Skeletal Muscle Regeneration in Cardiotoxin-Induced Muscle Injury Models

Skeletal muscle injuries occur frequently in daily life and exercise. Understanding the mechanisms of regeneration is critical for accelerating the repair and regeneration of muscle. Therefore, this article reviews knowledge on the mechanisms of skeletal muscle regeneration after cardiotoxin-induced injury. The process of regeneration is similar in different mouse strains and is inhibited by aging, obesity, and diabetes. Exercise, microcurrent electrical neuromuscular stimulation, and mechanical loading improve regeneration. The mechanisms of regeneration are complex and strain-dependent, and changes in functional proteins involved in the processes of necrotic fiber debris clearance, M1 to M2 macrophage conversion, SC activation, myoblast proliferation, differentiation and fusion, and fibrosis and calcification influence the final outcome of the regenerative activity.

The biphasic and age-dependent impact of klotho on hallmarks of aging and skeletal muscle function

Aging is accompanied by disrupted information flow, resulting from accumulation of molecular mistakes. These mistakes ultimately give rise to debilitating disorders including skeletal muscle wasting, or sarcopenia. To derive a global metric of growing 'disorderliness' of aging muscle, we employed a statistical physics approach to estimate the state parameter, entropy, as a function of genes associated with hallmarks of aging. Escalating network entropy reached an inflection point at old age, while structural and functional alterations progressed into oldest-old age. To probe the potential for restoration of molecular 'order' and reversal of the sarcopenic phenotype, we systemically overexpressed the longevity protein, Klotho, via AAV. Klotho overexpression modulated genes representing all hallmarks of aging in old and oldest-old mice, but pathway enrichment revealed directions of changes were, for many genes, age-dependent. Functional improvements were also age-dependent. Klotho improved strength in old mice, but failed to induce benefits beyond the entropic tipping point.

Longitudinal hypertrophic and transcriptional responses to high-load eccentric-concentric vs concentric training in males

High-load eccentric training reputedly produces greater muscle hypertrophy than concentric training, possibly due to greater loading and/or inflammation. We quantified the temporal impact of combined maximal concentric-eccentric training vs maximal concentric training on muscle cross-sectional area (CSA), volume, and targeted mRNA expression (93 transcripts). Eight recreationally active males (24 ± 5 years, BMI 23.5 ± 2.5 kg/m² ) performed 3 x 30 maximal eccentric isokinetic knee extensions and 2 x 30 maximal concentric knee extensions in dominant limb (ECC + CON) and 5 x 30 maximal concentric contractions (CON) in the non-dominant limb for 12 weeks (all 90°/s, 3x/wk). Quadriceps muscle CSA and volume were measured at baseline, 28 days (d), and 84 d in both limbs (3T MRI). Resting vastus lateralis biopsies were obtained from both limbs at baseline, 24 hours (h), 7, 28, and 84 d for mRNA abundance measurements (RT-PCR microfluidic cards). Work output was greater throughout training in ECC + CON vs CON (20.8 ± 9.7%, P < .001). Muscle CSA increased from baseline in both limbs at 28 d (CON 4.3 ± 2.6%, ECC + CON 4.0 ± 1.9%, both P < .001) and 84d (CON 3.9 ± 2.3%, ECC + CON 4.0 ± 3.1%, both P < .001), and muscle volume and isometric strength at 84 d (CON 44.8 ± 40.0%, P < .001; ECC + CON 36.9 ± 40.0%, P < .01), but no between-limb differences existed in any parameter. Ingenuity Pathway Analysis identified several cellular functions associated with regulation of muscle mass and metabolism as altered by both modalities at 24 h and 7 d, but particularly with ECC + CON. However, mRNA responses waned thereafter, regardless of modality. Initial muscle mRNA responses to training did not reflect chronic training-induced hypertrophy. Moreover, ECC + CON did not produce greater hypertrophy than CON, despite greater loading throughout and a differential mRNA response during the initial training week.

Effect of single bout downhill running on the serum irisin concentrations in rats

This study aimed to characterize the effect of different running modes on serum irisin concentrations in rats. A total of 18, 10-week-old rats were divided into three groups; control group, 16° uphill running group (concentric exercise; CON) and, -16° downhill running group (eccentric exercise; ECC). The running group's rats ran on the inclined treadmill at 16 m/min, for a total of 90 min. Blood was drawn from the rats, 48 h after running, after which the rats were anesthetized. The serum concentrations of irisin were measured using enzyme-linked immunosorbent assays. Vastus intermedius was collected for immunohistochemical analysis. After multiple comparisons, the ECC showed a significantly high serum irisin concentration (ECC: 28.42 ± 6.31 ng/ml, CON: 21.27 ± 3.03 ng/ml) and a larger irisin antibody reactive cross-sectional area in vastus intermedius compared to the CON (p < 0.05). This is the first study to reveal that single bout downhill running increases serum irisin concentrations in rats.

Eccentric exercise results in a prolonged increase in interleukin-6 and tumor necrosis factor-α levels in rat skeletal muscle

Interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α) are well-known cytokines with pro-inflammatory capabilities, and have been shown to be involved in adaptation to exercise as multifaceted myokines. However, the precise role of IL-6 and TNF-α during exercise-induced skeletal muscle injury and subsequent repair processes is not fully understood. In this study, IL-6 and TNF-α were examined in soleus muscles at the gene and protein levels using in situ hybridization and immunohistochemical staining, respectively, and serum levels of IL-6 and TNF-α were determined before and after a 90-min downhill running session in rats. There were no changes in serum levels of IL-6 and TNF-α after exercise, but IL-6 and TNF-α mRNA increased and maintained high expression in muscles for 1-2 weeks after exercise. IL-6 and TNF-a mRNAs were identified in both the cytoplasm and the nuclei of myocytes, as well as in invading inflammatory cells. IL-6 and TNF-α protein mainly distributed in cytoplasm unevenly and had a prolonged expression until 2 weeks after eccentric exercise. Our results demonstrate that there is increased IL-6 and TNF-α expression in skeletal muscle that is induced by eccentric exercise and that the high expression of IL-6 and TNF-α in the long-term phase after eccentric exercise may be more involved in the subsequent recovery of damaged muscle.

Decreased Satellite Cell Number and Function in Humans and Mice With Type 1 Diabetes Is the Result of Altered Notch Signaling

Type 1 diabetes (T1D) negatively influences skeletal muscle health; however, its effect on muscle satellite cells (SCs) remains largely unknown. SCs from samples from rodents (Akita) and human subjects with T1D were examined to discern differences in SC density and functionality compared with samples from their respective control subjects. Examination of the Notch pathway was undertaken to investigate its role in changes to SC functionality. Compared with controls, Akita mice demonstrated increased muscle damage after eccentric exercise along with a decline in SC density and myogenic capacity. Quantification of components of the Notch signaling pathway revealed a persistent activation of Notch signaling in Akita SCs, which could be reversed with the Notch inhibitor DAPT. Similar to Akita samples, skeletal muscle from human subjects with T1D displayed a significant reduction in SC content, and the Notch ligand, DLL1, was significantly increased compared with control subjects, supporting the dysregulated Notch pathway observed in Akita muscles. These data indicate that persistent activation in Notch signaling impairs SC functionality in the T1D muscle, resulting in a decline in SC content. Given the vital role played by the SC in muscle growth and maintenance, these findings suggest that impairments in SC capacities play a primary role in the skeletal muscle myopathy that characterizes T1D.

Effects of eccentric exercise in rehabilitation of phasic and tonic muscles after leg immobilization in rats

Eccentric exercise is an essential resource for skeletal muscle rehabilitation following muscle disuse however, abnormalities linked to the tissue recuperation require further research. Our aim was analyze the adaptation ability of rehabilitated muscular tissue in rats during different periods of eccentric training after 10 days of limb immobilization. Twenty-seven Wistar rats were divided into six groups: immobilized 10 days, immobilized and eccentric trained for 10 days, immobilized and eccentric trained for 21 days, and three age-matched control groups. After sacrifice, soleus and plantaris muscles were frozen, cut and stained for general histology using hematoxylin and eosin and Gomori trichrome methods and immunohistochemical methods for fiber typing (mATPase, NADH2-TR), for capillaries (CD31) and intermediate filaments (desmin, vimentin) and high resolution microscopy of resin embedded material. Immobilization resulted in more intense morphological alterations in soleus muscles such as formation of target fibers, nuclear centralization, a reduction in the number of type I fibers, diameter of type I, IIA, IIAD fibers, and capillaries. After 10 days of eccentric training, increases in the nuclear centralization and the number of lobulated fibers were observed. This period was insufficient to reestablish the capillary/fiber (C/F) ratio and distribution of fiber types as that observed in the control group. However, 21 days of rehabilitation allowed the reversal of all morphological and quantitative abnormalities. For the plantaris muscles, 10-days of training restored their basic characteristics. Despite the fact that immobilization affected soleus and plantaris muscles, 10 days of eccentric training was insufficient to restore the morphological characteristics of soleus muscles, which was not the case observed in plantaris muscle.

L-arginine supplementation protects exercise performance and structural integrity of muscle fibers after a single bout of eccentric exercise in rats

Eccentric exercise is known to disrupt sarcolemmal integrity and induce damage of skeletal muscle fibers. We hypothesized that L-arginine (L-Arg; nitric oxide synthase (NOS) substrate) supplementation prior to a single bout of eccentric exercise would diminish exercise-induced damage. In addition, we used N-nitro-L-arginine methyl ester hydrochloride (L-NAME; NOS inhibitor) to clarify the role of native NOS activity in the development of exercise-induced muscle damage. Rats were divided into four groups: non-treated control (C), downhill running with (RA) or without (R) L-Arg supplementation and downhill running with L-NAME supplementation (RN). Twenty four hours following eccentric exercise seven rats in each group were sacrificed and soleus muscles were dissected and frozen for further analysis. The remaining seven rats in each group were subjected to the exercise performance test. Our experiments showed that L-Arg supplementation prior to a single bout of eccentric exercise improved subsequent exercise performance capacity tests in RA rats when compared with R, RN and C rats by 37%, 27% and 13%, respectively. This outcome is mediated by L-Arg protection against post-exercise damage of sarcolemma (2.26- and 0.87-fold less than R and RN groups, respectively), reduced numbers of damaged muscle fibers indicated by the reduced loss of desmin content in the muscle (15% and 25% less than R and RN groups, respectively), and diminished µ-calpain mRNA up-regulation (42% and 30% less than R and RN groups, respectively). In conclusion, our study indicates that L-Arg supplementation prior to a single bout of eccentric exercise alleviates muscle fiber damage and preserves exercise performance capacity.

GSK3β inhibition and LEF1 upregulation in skeletal muscle following a bout of downhill running

Canonical Wnt signaling is important in skeletal muscle repair but has not been well characterized in response to physiological stimuli. The objective of this study was to assess the effect of downhill running (DHR) on components of Wnt signaling. Young, male C57BL/J6 mice were exposed to DHR. Muscle injury and repair (MCadherin) were measured in soleus. Gene and protein expression of Wnt3a, active β-catenin, GSK3β, and LEF1 were measured in gastrocnemius. Muscle injury increased 6 days post-DHR and MCadherin protein increased 5 days post-DHR. Total and active GSK3β protein decreased 3 days (9-fold and 3.6-fold, respectively) post-DHR. LEF1 protein increased 6 days (5-fold) post-DHR. DHR decreased GSK3β and increased LEF1 protein expression, but did not affect other components of Wnt signaling. Due to their applicability, using models of physiological stimuli such as DHR will provide significant insight into cellular mechanisms within muscle.

Defining a role for non-satellite stem cells in the regulation of muscle repair following exercise

Skeletal muscle repair is essential for effective remodeling, tissue maintenance, and initiation of beneficial adaptations post-eccentric exercise. A series of well characterized events, such as recruitment of immune cells and activation of satellite cells, constitute the basis for muscle regeneration. However, details regarding the fine-tuned regulation of this process in response to different types of injury are open for investigation. Muscle-resident non-myogenic, non-satellite stem cells expressing conventional mesenchymal stem cell (MSC) markers, have the potential to significantly contribute to regeneration given the role for bone marrow-derived MSCs in whole body tissue repair in response to injury and disease. The purpose of this mini-review is to highlight a regulatory role for Pnon-satellite stem cells in the process of skeletal muscle healing post-eccentric exercise. The non-myogenic, non-satellite stem cell fraction will be defined, its role in tissue repair will be briefly reviewed, and recent studies demonstrating a contribution to eccentric exercise-induced regeneration will be presented.

Eccentric exercise transiently affects mice skeletal muscle mitochondrial function

Eccentric exercise (EE) is known to induce damage and dysfunction in skeletal muscle. However, the possible role of mitochondrial (dys)function, including the vulnerability to mitochondrial permeability transition pore (MPTP) opening, is unclear. Therefore, this study aimed to analyze the impact of a single acute bout of downhill running on skeletal muscle mitochondrial function. Thirty 12-week-old Charles River CD1 male mice were randomly assigned into control (C) or exercised groups. EE consisted of 120 min of downhill treadmill running at a –16° gradient. Exercised animals were sacrificed immediately (Ecc0h) and 48 h (Ecc48h) after the end of the running bout. Plasma and skeletal muscles were then obtained. Muscle mitochondrial function, including oxygen consumption prior to and after anoxia and reoxygenation, membrane potential, and MPTP opening, were evaluated. Respiratory chain complexI, II, and V activities were determined. EE significantly increased plasma creatine kinase activity (119.4 ± 5.6 vs. 1061.3 ± 46.3 vs. 256.8 ± 15.3 U·L–1, C, Ecc0h and Ecc48h, respectively) and myoglobin and interleukin-6 content. Impaired state 3 and respiratory control ratio (8.4 ± 0.4 vs. 5.6 ± 0.9 vs. 8.4 ± 0.5, C, Ecc0h and Ecc48h, respectively), as well as increased susceptibility to MPTP opening, seen by cyclosporin A-sensitive high swelling amplitude, lower time to maximal swelling velocity (313.8 ± 17.7 vs. 244.5 ± 19.4 vs. 298.5 ± 8.7 s, C, Ecc0h and Ecc48h, respectively), and calcium release immediately after the end of exercise (C vs. Ecc0h) were observed. EE induced a transient impairment in the activity of complex V (C vs. Ecc0h). No significant changes from the C group were observed 48 h after the end of EE (C vs. Ecc48h) in any analyzed parameters. In conclusion, prolonged EE transiently impaired mice skeletal muscle mitochondrial function and increased susceptibility to calcium-induced MPTP opening.

Acute heat stress prior to downhill running may enhance skeletal muscle remodeling

Heat shock proteins (HSPs) are chaperones that are known to have important roles in facilitating protein synthesis, protein assembly and cellular protection. While HSPs are known to be induced by damaging exercise, little is known about how HSPs actually mediate skeletal muscle adaption to exercise. The purpose of this study was to determine the effects of a heat shock pretreatment and the ensuing increase in HSP expression on early remodeling and signaling (2 and 48 h) events of the soleus (Sol) muscle following a bout of downhill running. Male Wistar rats (10 weeks old) were randomly assigned to control, eccentric exercise (EE; downhill running) or heat shock + eccentric exercise (HS; 41°C for 20 min, 48 h prior to exercise) groups. Markers of muscle damage, muscle regeneration and intracellular signaling were assessed. The phosphorylation (p) of HSP25, Akt, p70s6k, ERK1/2 and JNK proteins was also performed. As expected, following exercise the EE group had increased creatine kinase (CK; 2 h) and mononuclear cell infiltration (48 h) compared to controls. The EE group had an increase in p-HSP25, but there was no change in HSP72 expression, total protein concentration, or neonatal MHC content. Additionally, the EE group had increased p-p70s6k, p-ERK1/2, and p-JNK (2 h) compared to controls; however no changes in p-Akt were seen. In contrast, the HS group had reduced CK (2 h) and mononuclear cell infiltration (48 h) compared to EE. Moreover, the HS group had increased HSP72 content (2 and 48 h), total protein concentration (48 h), neonatal MHC content (2 and 48 h), p-HSP25 and p-p70s6k (2 h). Lastly, the HS group had reduced p-Akt (48 h) and p-ERK1/2 (2 h). These data suggest that heat shock pretreatment and/or the ensuing HSP72 response may protect against muscle damage, and enhance increases in total protein and neonatal MHC content following exercise. These changes appear to be independent of Akt and MAPK signaling pathways.

The α₇β₁-integrin increases muscle hypertrophy following multiple bouts of eccentric exercise

Mechanical stimuli increase skeletal muscle growth in a mammalian target of rapamycin (mTOR)- and p70(S6K)-dependent manner. It has been proposed that costameric proteins at Z bands may sense and transfer tension to these initiators of protein translation, but few candidates have been identified. The purpose of this study was to determine whether a role exists for the α(7)-integrin in the activation of hypertrophic signaling and growth following eccentric exercise training. Five-week-old, wild-type (WT) and α(7)BX2-integrin transgenic (α(7)Tg) mice were randomly assigned to one of two groups: 1) sedentary (SED), or 2) exercise training (EX). Exercise training consisted of downhill running 3 sessions/wk for 4 wk (-20°, 17 m/min, 30 min). Downhill running was used to induce physiological mechanical strain. Twenty-four hours following the final training session, maximal isometric hindlimb plantar flexor force was measured. Gastrocnemius-soleus complexes were collected for further analysis of signaling changes, which included AKT, mTOR and p70(S6K), and muscle growth. Despite increased p70(S6K) activity in WT/EX, no significant changes in cross-sectional area or force were observed in WT/EX compared with WT/SED. AKT, mTOR, and p70(S6K) activation was higher, and whole muscle hypertrophy, relative muscle weight, myofibrillar protein, and force were significantly elevated in α(7)Tg/EX compared with α(7)Tg/SED. A marked increase in average myofiber cross-sectional area was observed in α(7)Tg/EX compared with all groups. Our findings demonstrate that the α(7)β(1)-integrin sensitizes skeletal muscle to mechanical strain and subsequent growth. Thus the α(7)β(1)-integrin may represent a novel molecular therapy for the treatment of disuse muscle atrophy.

The α7β1-integrin accelerates fiber hypertrophy and myogenesis following a single bout of eccentric exercise

The α(7)β(1)-integrin is a heterodimeric transmembrane protein that adheres to laminin in the extracellular matrix, representing a critical link that maintains structure in skeletal muscle. In addition to preventing exercise-induced skeletal muscle injury, the α(7)-integrin has been proposed to act as an intrinsic mechanosensor, initiating cellular growth in response to mechanical strain. The purpose of this study was to determine the extent to which the α(7)-integrin regulates muscle hypertrophy following eccentric exercise. Wild-type (WT) and α(7)-integrin transgenic (α(7)Tg) mice completed a single bout of downhill running exercise (-20°, 17 m/min, 60 min), and gastrocnemius-soleus complexes were collected 1, 2, 4, and 7 days (D) postexercise (PE). Maximal isometric force was maintained and macrophage accumulation was suppressed in α(7)Tg muscle 1D PE. Mean fiber cross-sectional area was unaltered in WT mice but increased 40% in α(7)Tg mice 7D PE. In addition, a rapid and striking fivefold increase in embryonic myosin heavy chain-positive fibers appeared in α(7)Tg mice 2D PE. Although Pax7-positive satellite cells were increased in α(7)Tg muscle 1D PE, the number of nuclei per myofiber was not altered 7D PE. Phosphorylation of mammalian target of rapamycin (mTOR) was significantly elevated in α(7)Tg 1D PE. This study provides the first demonstration that the presence of the α(7)β(1)-integrin in skeletal muscle increases fiber hypertrophy and new fiber synthesis in the early time course following a single bout of eccentric exercise. Further studies are necessary to elucidate the precise mechanism by which the α(7)-integrin can enhance muscle hypertrophy following exercise.

Eccentric stimulation reveals an involvement of FGF6 in muscle resistance to mechanical stress

The objective of this report was to analyse a potential role for FGF6 in muscle resistance to mechanical stress. Normal or regenerating muscles of FGF6 (-/-) mice versus wild-type mice were submitted to different protocols of damaging eccentric contractions (eccentric electrostimulation and intermittent downhill exercise). Then muscular structural properties were analysed by histological and immunochemistry techniques to evaluate the post-injury muscle recovery; their muscle contractile parameters (maximal tetanic force, kinetics properties and fatigue resistance) were assessed. The absence of FGF6 causes (1) a fast-to-slow myofibre type switch in adult control and regenerating Tibialis anterior (TA) muscle; (2) muscle weakness in regenerating muscles in animals submitted to eccentric exercise protocols due to aberrant extensive necrotic zones. These observations point out a crucial and unexpected role for FGF6 in muscle integrity and muscle protection against mechanical stress.

Are human and mouse satellite cells really the same?

Satellite cells are quiescent cells located under the basal lamina of skeletal muscle fibers that contribute to muscle growth, maintenance, repair, and regeneration. Mouse satellite cells have been shown to be muscle stem cells that are able to regenerate muscle fibers and self-renew. As human skeletal muscle is also able to regenerate following injury, we assume that the human satellite cell is, like its murine equivalent, a muscle stem cell. In this review, we compare human and mouse satellite cells and highlight their similarities and differences. We discuss gaps in our knowledge of human satellite cells, compared with that of mouse satellite cells, and suggest ways in which we may advance studies on human satellite cells, particularly by finding new markers and attempting to re-create the human satellite cell niche in vitro.

Notch and Wnt signaling, physiological stimuli and postnatal myogenesis

Adult skeletal muscle stem cells, termed satellite cells are imperative to muscle regeneration. Much work has been performed on satellite cell identification and the subsequent activation of the myogenic response but the regulation of satellite cells including its activation is not well elucidated. The purpose of this review article is to synthesize what the literature reveals in regards to the current understanding of satellite cells including their contribution to muscle repair and growth following physiological stimuli. In addition, this review article will describe the recent findings on the roles of the classic developmental signaling pathways, Notch and Wnt, to the myogenic response in various muscle injury models. This purpose of this summary is to bring awareness of the impact that muscle contraction models have on the local and systemic environment of adult muscle stem cells which will be beneficial for comprehending and treatment development for muscle -associated ailments and other organ diseases.

Gross ultrastructural changes and necrotic fiber segments in elbow flexor muscles after maximal voluntary eccentric action in humans

Eccentric muscle actions are associated with ultrastructural changes. The severity and types of change depend on the nature of the stimulation protocol, and on the method for assessing such changes, and can be regarded as a continuum from mild changes to pathological-like changes. Most studies describing more severe changes have been performed on animals and only a few in humans, some using electrical stimuli. Hence, a debate has emerged on whether voluntary actions are associated with the pathological-like end of the continuum. The aim of this study was to determine whether severe muscle damage, i.e., extensive ultrastructural changes, is confined to animal studies and studies on humans using electrical stimuli. Second, because there is no generally approved method to quantify the degree of muscle damage, we compared two published methods, analyzing the Z disks or sarcomeres, as well as novel analyses of pathological-like changes. A group of untrained subjects performed 70 voluntary maximal eccentric muscle actions using the elbow flexors. On the basis of large reductions in maximal force-generating capacity (on average, −62 ± 3% immediately after exercise, and −35 ± 6% 9 days later), five subjects were selected for further analysis. Biopsies were taken from m. biceps brachii in both the exercised and nonexercised arm. In exercised muscle, more disrupted (13 ± 4 vs. 3 ± 3%) and destroyed (15 ± 6 vs. 0%) Z disks were found compared with nonexercised muscle. A significant proportion of exercised myofibers had focal (85 ± 5 vs. 11 ± 7%), moderate (65 ± 7 vs. 11 ± 6%), and extreme (38 ± 9 vs. 0%) myofibrillar disruptions. Hypercontracted myofibrils, autophagic vacuoles, granular areas, central nuclei, and necrotic fiber segments were found to various degrees. The present study demonstrates that the more severe end of the continuum of ultrastructural changes occurs in humans after voluntary exercise when maximal eccentric muscle actions are involved.

Skeletal muscle as a paradigm for regenerative biology and medicine

Tissue development and regeneration share common features, since modules of regulatory pathways and transcription factors that are crucial for prenatal development are redeployed for tissue reconstruction after trauma. Regenerative medicine has therefore gained important insights through the study of developmental and regenerative biology. Moreover, diverse experimental models have been used to investigate the regeneration process in different tissues and organs. Paradoxically, little is known regarding the relative contribution of stem cells with respect to the supporting tissue during tissue regeneration. Particular attention will be given to mouse models using distinct injury paradigms to investigate the regenerative biology of skeletal muscle. An understanding of the response of stem and parenchymal cells is crucial for the development of clinical strategies to combat the normal decline in tissue performance during aging or its reconstitution after trauma and during disease. This review addresses these issues, focusing on muscle regeneration and how different factors, including genes, cells and the environment, impinge on this process.

Effects of fatiguing jumping exercise on mRNA expression of titin-complex proteins and calpains

Eccentric exercise induced by electrostimulation increases mRNA expression of titin-complex proteins in rodent skeletal muscle. In this study, mRNA expression of titin, muscle LIM protein (MLP), cardiac ankyrin repeat protein (CARP), ankyrin repeat domain protein 2 (Ankrd2), diabetes-related ankyrin repeat protein (DARP), and calcium-activated proteinases, calpains, were investigated in human skeletal muscle after fatiguing jumping exercise. Fatiguing jumping exercise did not change mRNA expression of titin, DARP, calpain 1, or calpain 3. MLP, Ankrd2 and calpain 2 mRNA levels were increased 2 days postexercise. CARP mRNA level was already elevated 30 min and remained elevated 2 days postexercise. Increased mRNA expression of MLP, CARP, and Ankrd2, observed for the first time in human skeletal muscle, may be part of the signaling activated by physical exercise. The rapid increase in the level of CARP mRNA nominates CARP as one of the first genes to respond to exercise. The increase in the mRNA level of calpain 2 suggests its involvement in myofiber remodeling after strenuous jumping exercise.

Analysis of skeletal muscle function in the C57BL6/SV129 syncoilin knockout mouse

Syncoilin is a 64-kDa intermediate filament protein expressed in skeletal muscle and enriched at the perinucleus, sarcolemma, and myotendinous and neuromuscular junctions. Due to its pattern of cellular localization and binding partners, syncoilin is an ideal candidate to be both an important structural component of myocytes and a potential mediator of inherited myopathies. Here we present a report of a knockout mouse model for syncoilin and the results of an investigation into the effect of a syncoilin null state on striated muscle function in 6–8-week-old mice. An analysis of proteins known to associate with syncoilin showed that ablation of syncoilin had no effect on absolute expression or spatial localization of desmin or alpha dystrobrevin. Our syncoilin-null animal exhibited no differences in cardiotoxin-induced muscle regeneration, voluntary wheel running, or enforced treadmill exercise capacity, relative to wild-type controls. Finally, a mechanical investigation of isolated soleus and extensor digitorum longus indicated a potential differential reduction in muscle strength and resilience. We are the first to present data identifying an increased susceptibility to muscle damage in response to an extended forced exercise regime in syncoilin-deficient muscle. This study establishes a second viable syncoilin knockout model and highlights the importance of further investigations to determine the role of syncoilin in skeletal muscle.

Exercise, oxidative stress and hormesis

Physical inactivity leads to increased incidence of a variety of diseases and it can be regarded as one of the end points of the exercise-associated hormesis curve. On the other hand, regular exercise, with moderate intensity and duration, has a wide range of beneficial effects on the body including the fact that it improves cardio-vascular function, partly by a nitric oxide-mediated adaptation, and may reduce the incidence of Alzheimer's disease by enhanced concentration of neurotrophins and by the modulation of redox homeostasis. Mechanical damage-mediated adaptation results in increased muscle mass and increased resistance to stressors. Physical inactivity or strenuous exercise bouts increase the risk of infection, while moderate exercise up-regulates the immune system. Single bouts of exercise increases, and regular exercise decreases the oxidative challenge to the body, whereas excessive exercise and overtraining lead to damaging oxidative stress and thus are an indication of the other end point of the hormetic response. Based upon the genetic setup, regular moderate physical exercise/activity provides systemic beneficial effects, including improved physiological function, decreased incidence of disease and a higher quality of life.

Impact of repeated bouts of eccentric exercise on myogenic gene expression

Evidence indicates that repeated-bouts of eccentric exercise (EE) do not exacerbate the extent of muscle damage indices, as compared to a single-bout. We hypothesized that molecular adaptations, under repeated-bouts of EE, would include suppression of muscle repair inhibitory factors such as myostatin and up-regulation of muscle repair positive regulatory factors such as myogenic regulatory factors (MRFs). Fifteen males were recruited for this study. The exercise group (n=9) successfully completed six sets of 15 reps of maximum voluntary eccentric contractions, for six consecutive days, using a dynamometer (Multicont-II). Blood and muscle biopsy samples were obtained from each subject 1 week prior to exercise, 2 days post the first training session, and 24 h after the last training session. Gene expression levels were determined using real-time RT-PCR. Blood samples were analyzed for creatine kinase (CK) and lactate-dehydrogenase (LDH) activity. Repeated-bouts of EE induced a large down-regulation of myostatin mRNA (-73%) which persisted throughout the study. The responses of MRFs were mild. At day 3 only myogenin increased significantly (1.9 fold) while MyoD decreased by 45%. Surprisingly, at day 7, despite the presence of muscle damage indices, all MRFs returned to the pre-exercise levels. The results of the present study showed that repeated-bouts of EE, for six consecutive days, dramatically decreased Myostatin mRNA expression but impaired the expression patterns of MRFs such that, with the exception of myogenin that showed a moderate non-sustained increase, MyoD and MYf5 response was minimal.

Interaction of contractile activity and training history on mRNA abundance in skeletal muscle from trained athletes

Skeletal muscle displays enormous plasticity to respond to contractile activity with muscle from strength- (ST) and endurance-trained (ET) athletes representing diverse states of the adaptation continuum. Training adaptation can be viewed as the accumulation of specific proteins. Hence, the altered gene expression that allows for changes in protein concentration is of major importance for any training adaptation. Accordingly, the aim of the present study was to quantify acute subcellular responses in muscle to habitual and unfamiliar exercise. After 24-h diet/exercise control, 13 male subjects (7 ST and 6 ET) performed a random order of either resistance (8 x 5 maximal leg extensions) or endurance exercise (1 h of cycling at 70% peak O2 uptake). Muscle biopsies were taken from vastus lateralis at rest and 3 h after exercise. Gene expression was analyzed using real-time PCR with changes normalized relative to preexercise values. After cycling exercise, peroxisome proliferator-activated receptor-gamma coactivator-1alpha (ET approximately 8.5-fold, ST approximately 10-fold, P < 0.001), pyruvate dehydrogenase kinase-4 (PDK-4; ET approximately 26-fold, ST approximately 39-fold), vascular endothelial growth factor (VEGF; ET approximately 4.5-fold, ST approximately 4-fold), and muscle atrophy F-box protein (MAFbx) (ET approximately 2-fold, ST approximately 0.4-fold) mRNA increased in both groups, whereas MyoD (approximately 3-fold), myogenin (approximately 0.9-fold), and myostatin (approximately 2-fold) mRNA increased in ET but not in ST (P < 0.05). After resistance exercise PDK-4 (approximately 7-fold, P < 0.01) and MyoD (approximately 0.7-fold) increased, whereas MAFbx (approximately 0.7-fold) and myostatin (approximately 0.6-fold) decreased in ET but not in ST. We conclude that prior training history can modify the acute gene responses in skeletal muscle to subsequent exercise.

The molecular regulation of exercised-induced muscle fibre hypertrophy in the common carp: Expression of MyoD, PCNA and components of the calcineurin-signalling pathway

Growth was investigated over 16 d in juvenile common carp (Cyprinus carpio L.) held in either static water (tank rested, TR16) or exercised in a flume at 2.5-3.2 body lengths s-1 for 18 h a day (exercised, E16). Relative to the start of the experiment (TR0), the TR16 group showed a 31% increase in body mass (specific growth rate, 1.57% d-1), whereas there was no net change in the E16 group. There was, however, a significant exercise-induced hypertrophy of slow muscle fibres with average fibre cross-sectional area (FCSA) increasing by 35% in the E16 group, compared with 11% in the TR16 group. In contrast, FCSA of fast muscle fibres increased by 34% in the TR16 group compared to just 18% in the E16 group. The relative concentrations and subcellular localisation of proteins hypothesised to play a role in the regulation of muscle growth were measured. MyoD concentration was similar in the TR0, TR16 and E16 groups in both slow and fast muscle. However, there was a small (5%-10%) but statistically significant increase in nuclear localisation of MyoD in those groups showing a significant increase in FCSA over the time course of the experiment. PCNA concentration was 31% and 12% higher in the TR16 than in either the TR0 or E16 groups for slow and fast muscle, respectively. Exercise resulted in a approximately 10% increase in nuclear factor of T-cells (NFAT2) concentration in slow muscle but no change in NFAT2 localisation. Calcineurin B concentration was similar in tank rested and exercised groups. The results do not support a major role for the calcineurin-signalling pathway in the regulation of muscle hypertrophy in the common carp.

Resting and load-induced levels of myogenic gene transcripts differ between older adults with demonstrable sarcopenia and young men and women

Regenerative capacity appears to be impaired in sarcopenic muscle. As local growth factors and myogenic regulatory factors (MRFs) modulate repair/regeneration responses after overload, we hypothesized that resistance loading (RL)-induced expression of MRFs and muscle IGF-I-related genes would be blunted in older (O) males (M) and females (F) with demonstrable sarcopenia vs. young (Y) adults. Y (20-35 yr, 10 YF, 10 YM) and O (60-75 yr, 9 OF, 9 OM) underwent vastus lateralis biopsy before and 24 h after knee extensor RL. Sarcopenia was assessed by cross-sectional area of type I, IIa, and IIx myofibers. Transcript levels were assessed by relative RT-PCR and analyzed by age x gender x load repeated-measures ANOVA. O were sarcopenic based on type II atrophy with smaller type IIa (P < 0.05) and IIx (P < 0.001) myofibers. Within-gender cross-sectional area differences were more marked in F (OF < YF: IIa 21%, IIx 42%). Load effects (P < 0.05) were seen for four of seven mRNAs as IGF-IEa (34%), myogenin (53%), and MyoD (20%) increased, and myf-6 declined 10%. Increased IGF-IEa was driven by O (48%) and/or M (43%). An age x gender x load interaction was found for MyoD (P < 0.05). An age x load interaction for type 1 IGF receptor (P < 0.05) was driven by a small increase in O (16%, P < 0.05). A gender x load interaction (P < 0.05) was noted for IGF binding protein-4. Age effects (P < 0.05) resulted from higher MyoD (54%), myf-5 (21%), and IGF binding protein-4 (17%) in O and were primarily localized to F at baseline (OF > YF; MyoD 94%, myf-5 47%, P < 0.05). We conclude that RL acutely increases mRNA expression of IGF-IEa and myogenin, which may promote growth/regeneration in both Y and O. Higher resting levels of MRFs in OF vs. YF suggest elevated basal regenerative activity in sarcopenic muscle of OF.

Response of the ubiquitin-proteasome pathway to changes in muscle activity

The ubiquitin-proteasome pathway plays a critical role in the adaptation of skeletal muscle to persistent decreases or increases in muscle activity. This article outlines the basics of pathway function and reviews what we know about pathway responses to altered muscle use. The ubiquitin-proteasome pathway regulates proteolysis in mammalian cells by attaching ubiquitin polymers to damaged proteins; this targets the protein for degradation via the 26S proteasome. The pathway is constitutively active in muscle and continually regulates protein turnover. Conditions of decreased muscle use, e.g., unloading, denervation, or immobilization, stimulate general pathway activity. This activity increase is caused by upregulation of regulatory components in the pathway and leads to accelerated proteolysis, resulting in net loss of muscle protein. Pathway activity is also increased in response to exercise, a two-phase response. An immediate increase in selective ubiquitin conjugation by constitutive pathway components contributes to exercise-stimulated signal transduction. Over hours-to-days, exercise also stimulates a delayed increase in general ubiquitin conjugating activity by inducing expression of key components in the pathway. This increase mediates a late-phase rise in protein degradation that is required for muscle adaptation to exercise. Thus the ubiquitin-proteasome pathway functions as an essential mediator of muscle remodeling, both in atrophic states and exercise training.