Type IV collagen and its degradation in paralyzed human muscle: effect of functional electrical stimulation

Validation of bioimpedance phase angle in lower extremity of male patients with chronic spinal cord injury

[Purpose] This study aimed to evaluate the relationship between lower extremity phase angle and muscle thickness/echo intensity in males with chronic spinal cord injury. It also compared bioelectrical impedance analysis measurements to investigate skeletal muscle degeneration between individuals with spinal cord injury and healthy controls. [Participants and Methods] This cross-sectional study included 12 male patients with chronic spinal cord injury and 14 healthy male controls. We used bioelectrical impedance analysis and ultrasonography to measure the lower extremity phase angle and muscle thickness/echo intensity of the rectus femoris muscle, respectively. We also compared the bioelectrical impedance analysis measurements between individuals with spinal cord injury and healthy controls. [Results] Lower extremity phase angle was strongly correlated with muscle thickness and echo intensity of the rectus femoris muscle in individuals with spinal cord injury. All measures differed significantly between individuals with spinal cord injury and healthy controls. [Conclusion] These findings suggest that lower extremity phase angle is a valuable skeletal muscle indicator in spinal cord injury. Furthermore, bioelectrical impedance analysis revealed degeneration of the lower extremity skeletal muscles in individuals with chronic spinal cord injury.

Protein Turnover in Skeletal Muscle: Looking at Molecular Regulation towards an Active Lifestyle

Skeletal muscle is a highly plastic tissue, able to change its mass and functional properties in response to several stimuli. Skeletal muscle mass is influenced by the balance between protein synthesis and breakdown, which is regulated by several signaling pathways. The relative contribution of Akt/mTOR signaling, ubiquitin-proteasome pathway, autophagy among other signaling pathways to protein turnover and, therefore, to skeletal muscle mass, differs depending on the wasting or loading condition and muscle type. By modulating mitochondria biogenesis, PGC-1α has a major role in the cell's bioenergetic status and, thus, on protein turnover. In fact, rates of protein turnover regulate differently the levels of distinct protein classes in response to atrophic or hypertrophic stimuli. Mitochondrial protein turnover rates may be enhanced in wasting conditions, whereas the increased turnover of myofibrillar proteins triggers muscle mass gain. The present review aims to update the knowledge on the molecular pathways implicated in the regulation of protein turnover in skeletal muscle, focusing on how distinct muscle proteins may be modulated by lifestyle interventions with emphasis on exercise training. The comprehensive analysis of the anabolic effects of exercise programs will pave the way to the tailored management of muscle wasting conditions.

Exercise builds the scaffold of life: muscle extracellular matrix biomarker responses to physical activity, inactivity, and aging

Skeletal muscle extracellular matrix (ECM) is critical for muscle force production and the regulation of important physiological processes during growth, regeneration, and remodelling. ECM remodelling is a tightly orchestrated process, sensitive to multi-directional tensile and compressive stresses and damaging stimuli, and its assessment can convey important information on rehabilitation effectiveness, injury, and disease. Despite its profound importance, ECM biomarkers are underused in studies examining the effects of exercise, disuse, or aging on muscle function, growth, and structure. This review examines patterns of short- and long-term changes in the synthesis and concentrations of ECM markers in biofluids and tissues, which may be useful for describing the time course of ECM remodelling following physical activity and disuse. Forces imposed on the ECM during physical activity critically affect cell signalling while disuse causes non-optimal adaptations, including connective tissue proliferation. The goal of this review is to inform researchers, and rehabilitation, medical, and exercise practitioners better about the role of ECM biomarkers in research and clinical environments to accelerate the development of targeted physical activity treatments, improve ECM status assessment, and enhance function in aging, injury, and disease.

3D synchrotron imaging of muscle tissues at different atrophic stages in stroke and spinal cord injury: a proof-of-concept study

Synchrotron X-ray computed tomography (SXCT) allows 3D imaging of tissue with a very large field of view and an excellent micron resolution and enables the investigation of muscle fiber atrophy in 3D. The study aimed to explore the 3D micro-architecture of healthy skeletal muscle fibers and muscle fibers at different stages of atrophy (stroke sample = muscle atrophy; spinal cord injury (SCI) sample = severe muscle atrophy). Three muscle samples: a healthy control sample; a stroke sample (atrophic sample), and an SCI sample (severe atrophic sample) were imaged using SXCT, and muscle fiber populations were segmented and quantified for microarchitecture and morphology differences. The volume fraction of muscle fibers was 74.7%, 70.2%, and 35.3% in the healthy, stroke (atrophic), and SCI (severe atrophic) muscle fiber population samples respectively. In the SCI (severe atrophic sample), 3D image analysis revealed fiber splitting and fiber swelling. In the stroke sample (atrophic sample) muscle fiber buckling was observed but was only visible in the 3D analysis. 3D muscle fiber population analysis revealed new insights into the different stages of muscle fiber atrophy not to be observed nor quantified with a 2D histological analysis including fiber buckling, loss of fibers and fiber splitting.

Insights on the Potential Mechanisms of Action of Functional Electrical Stimulation Therapy in Combination With Task-Specific Training: A Scoping Review

OBJECTIVES

This scoping review was undertaken to synthetize and appraise the literature on the potential mechanisms of action of functional electrical stimulation therapy in combination with task-specific training (FEST + TST) in the rehabilitation following stroke, spinal cord injury, traumatic brain injury, or multiple sclerosis.

MATERIALS AND METHODS

The literature search was performed using multiple databases (including APA, PsycInfo, Medline, PubMed, EMBASE, CCRCT, and Cochrane Database of Systematic Reviews) from 1946 to June 2020. The literature search used the following terms: (spinal cord injury, paraplegia, tetraplegia, quadriplegia, stroke, multiple sclerosis, traumatic brain injury, or acquired brain injury) AND (functional electrical stimulation or FES). The search included clinical and preclinical studies without limits to language.

RESULTS

Of the 8209 titles retrieved from the primary search, 57 publications fulfilled the inclusion and exclusion criteria for this scoping review. While most publications were clinical studies (n = 50), there were only seven preclinical studies using animal models. The results of this review suggest that FEST + TST can result in multiple effects on different elements from the muscle to the cerebral cortex. However, most studies were focused on the muscle changes after FEST + TST.

CONCLUSIONS

The results of this scoping review suggest that FEST + TST can result in multiple effects on different elements of the neuromuscular system, while most research studies were focused on the muscle changes after FEST + TST. Despite the efficacy of the FEST + TST in the neurorehabilitation after CNS injury or disease, the results of this review underline an important knowledge gap with regards to the actual mechanism of action of FEST + TST.

Extracellular matrix remodelling induced by alternating electrical and mechanical stimulations increases the contraction of engineered skeletal muscle tissues

Engineered skeletal muscles are inferior to natural muscles in terms of contractile force, hampering their potential use in practical applications. One major limitation is that the extracellular matrix (ECM) not only impedes the contraction but also ineffectively transmits the forces generated by myotubes to the load. In the present study, ECM remodelling improves contractile force in a short time, and a coordinated, combined electrical and mechanical stimulation induces the desired ECM remodelling. Notably, the application of single and combined stimulations to the engineered muscles remodels the structure of their ECM networks, which determines the mechanical properties of the ECM. Myotubes in the tissues are connected in parallel and in series to the ECM. The stiffness of the parallel ECM must be low not to impede contraction, while the stiffness of the serial ECM must be high to transmit the forces to the load. Both the experimental results and the mechanistic model suggest that the combined stimulation through coordination reorients the ECM fibres in such a way that the parallel ECM stiffness is reduced, while the serial ECM stiffness is increased. In particular, 3 and 20 minutes of alternating electrical and mechanical stimulations increase the force by 18% and 31%, respectively.

Effect of different resistance-training protocols on the extracellular matrix of the calcaneal tendon of rats

The calcaneal tendon extracellular matrix (ECM) is composed of collagen, non-collagenous glycoproteins and proteoglycans, and able to adapt to various biomechanical stimuli. The objective of this study was to analyze the response of different resistance-training protocols, such as hypertrophy, strength and resistance, on the organization of the calcaneal tendon after training. Wistar rats were divided into four groups: untrained (UT), resistance training (RT), hypertrophy training (HT), and strength training (ST). The protocol in a vertical climbing platform was performed thrice per week over twelve weeks. For biochemical study, the tendons of each group were minced and analyzed for gelatinases, quantification of non-collagenous proteins, sulfated glycosaminoglycans, and hydroxyproline. For morphological analysis, sections were stained with HE and toluidine blue. Non-stained sections were used for birefringence analysis under polarization microscopy. The highest hydroxyproline concentrations were found in HT (154.8±14.2) and RT (173.6±25.2) compared with UT (122.4±27.0). A higher concentration of non-collagenous proteins was detected in the RT group (14.98mg/g) compared with the other groups. In polarization microscopy, major birefringence was observed in HT and the lowest in ST compared with UT, indicating higher organization of collagen bundles in HT. In analysis for zymography, the presence of latent MMP-9 was more prominent in the ST group and the active MMP-9 more prominent in the HT group. For MMP-2, significant differences in the latent isoform between the HT (184,867±6765) and UT (173,018±9696) groups were found. In sections stained with toluidine blue (TB), higher metachromasia was observed in the tendon's distal region in HT and RT groups, indicating a greater amount of proteoglycans. We conclude that the different training protocols produced different responses in the ECM. The remarkable presence of MMP-2 and -9 in the hypertrophy training group may be related to the highest organization of collagen bundles and possibly a more efficient remodeling process, observed in that group, as demonstrated by images and measurements of birefringence.

Therapeutic potential of matrix metalloproteinases in Duchenne muscular dystrophy

Matrix metalloproteinases (MMPs) are secreted proteinases that have physiologic roles in degradation and remodeling of extracellular matrix (ECM) in almost all tissues. However, their excessive production in disease conditions leads to many pathological features including tissue breakdown, inflammation, cell death, and fibrosis. Duchenne Muscular dystrophy (DMD) is a devastating genetic muscle disorder caused by partial or complete loss of cytoskeletal protein dystrophin. Progressive muscle wasting in DMD is accompanied by myofiber necrosis followed by cycles of regeneration and degeneration and inflammation that eventually result in replacement of myofiber by connective and adipose tissues. Emerging evidence suggests that gene expression and the activity of various MMPs are aberrantly regulated in muscle biopsies from DMD patients and in skeletal muscle of animal models of DMD. Moreover, a few studies employing genetic mouse models have revealed that different MMPs play distinct roles in disease progression in DMD. Modulation of the activity of MMPs improves myofiber regeneration and enhances the efficacy of transplantation and engraftment of muscle progenitor cells in dystrophic muscle in mouse models of DMD. Furthermore, recent reports also suggest that some MMPs especially MMP-9 can serve as a biomarker for diagnosis and prognosis of DMD. In this article, we provide a succinct overview of the regulation of various MMPs and their therapeutic importance in DMD.

Wasting mechanisms in muscular dystrophy

Muscular dystrophy is a group of more than 30 different clinical genetic disorders that are characterized by progressive skeletal muscle wasting and degeneration. Primary deficiency of specific extracellular matrix, sarcoplasmic, cytoskeletal, or nuclear membrane protein results in several secondary changes such as sarcolemmal instability, calcium influx, fiber necrosis, oxidative stress, inflammatory response, breakdown of extracellular matrix, and eventually fibrosis which leads to loss of ambulance and cardiac and respiratory failure. A number of molecular processes have now been identified which hasten disease progression in human patients and animal models of muscular dystrophy. Accumulating evidence further suggests that aberrant activation of several signaling pathways aggravate pathological cascades in dystrophic muscle. Although replacement of defective gene with wild-type is paramount to cure, management of secondary pathological changes has enormous potential to improving the quality of life and extending lifespan of muscular dystrophy patients. In this article, we have reviewed major cellular and molecular mechanisms leading to muscle wasting in muscular dystrophy. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

Elevated levels of active matrix metalloproteinase-9 cause hypertrophy in skeletal muscle of normal and dystrophin-deficient mdx mice

Matrix metalloproteinases (MMPs) are a group of extracellular proteases involved in tissue remodeling in several physiological and pathophysiological conditions. While increased expression of MMPs (especially MMP-9) has been observed in skeletal muscle in numerous conditions, their physiological significance remains less-well understood. By generating novel skeletal muscle-specific transgenic (Tg) mice expressing constitutively active mutant of MMP-9 (i.e. MMP-9G100L), in this study, we have investigated the effects of elevated levels of MMP-9 on skeletal muscle structure and function in vivo. Tg expression of enzymatically active MMP-9 protein significantly increased skeletal muscle fiber cross-section area, levels of contractile proteins and force production in isometric contractions. MMP-9 stimulated the activation of the Akt signaling pathway in Tg mice. Moreover, expression of active MMP-9 increased the proportion of fast-type fiber in soleus muscle of mice. Overexpression of MMP-9 also considerably reduced the deposition of collagens I and IV in skeletal muscle in vivo. In one-year-old mdx mice (a model for Duchenne muscular dystrophy, DMD), deletion of the Mmp9 gene reduced fiber hypertrophy and phosphorylation of Akt and p38 mitogen-activated protein kinase. Collectively, our study suggests that elevated levels of active MMP-9 protein cause hypertrophy in skeletal muscle and that the modulation of MMP-9 levels may have therapeutic value in various muscular disorders including DMD.

Effects of 660 and 780 nm low-level laser therapy on neuromuscular recovery after crush injury in rat sciatic nerve

BACKGROUND AND OBJECTIVE

Post-traumatic nerve repair is still a challenge for rehabilitation. It is particularly important to develop clinical protocols to enhance nerve regeneration. The present study investigated the effects of 660 and 780 nm low-level laser therapy (LLLT) using different energy densities (10, 60, and 120 J/cm²) on neuromuscular and functional recovery as well as on matrix metalloproteinase (MMP) activity after crush injury in rat sciatic nerve.

MATERIALS AND METHODS

Rats received transcutaneous LLLT irradiation at the lesion site for 10 consecutive days post-injury and were sacrificed 28 days after injury. Both the sciatic nerve and tibialis anterior muscles were analyzed. Nerve analyses consisted of histology (light microscopy) and measurements of myelin, axon, and nerve fiber cross-sectional area (CSA). S-100 labeling was used to identify myelin sheath and Schwann cells. Muscle fiber CSA and zymography were carried out to assess the degree of muscle atrophy and MMP activity, respectively. Statistical significance was set at 5% (P≤0.05).

RESULTS

Six hundred sixty nanometer LLLT either using 10 or 60 J/cm² restored muscle fiber, myelin and nerve fiber CSA compared to the normal group (N). Furthermore, it increased MMP-2 activity in nerve and decreased MMP-2 activity in muscle and MMP-9 activity in nerve. In contrast, 780 nm LLLT using 10 J/cm² decreased MMP-9 activity in nerve compared to the crush group (CR) and N; it also restored normal levels of myelin and nerve fiber CSA. Both 60 and 120 J/cm² decreased MMP-2 activity in muscle compared to CR and N. 780 nm did not prevent muscle fiber atrophy. Functional recovery in the irradiated groups did not differ from the non-irradiated CR.

CONCLUSION

Data suggest that 660 nm LLLT with low (10 J/cm²) or moderate (60 J/cm²) energy densities is able to accelerate neuromuscular recovery after nerve crush injury in rats.

Influence of laser photobiomodulation on collagen IV during skeletal muscle tissue remodeling after injury in rats

OBJECTIVE

The aim of the present study was to determine the effect of GaAlAs low-level laser therapy (LLLT) on collagen IV remodeling of the tibialis anterior (TA) muscle in rats after cryolesion.

BACKGROUND

Considerable interest exists in skeletal muscle regeneration in situations such as repair after exercise-induced muscle injury, after muscle transplantation, in muscular dystrophy, exercise-induced muscle injury, and the recovery of strength after atrophy due to disuse. A number of studies have demonstrated the potential of LLLT in facilitating the muscle-healing process; however, no consensus is found in the literature regarding the best laser-irradiation parameters.

METHODS

Adult male Wistar rats (n = 45) were used and randomly divided into three groups: control (n = 5); nontreated cryolesioned group (n = 20), and LLLT-cryolesioned group (n = 20). The cryolesioned groups were analyzed at 1, 7, 14, and 21 days after the injury procedure. Laser irradiation was performed 3 times per week on the injured region by using the GaAlAs laser (660 nm; beam spot of 0.04 cm(2), output power of 20 mW, power density of 500 mW/cm(2), and energy density of 5 J/cm(2), for 10 sec). The muscles were removed, frozen, cryosectioned, and then stained with hematoxylin-eosin for the visualization of general morphology or used for immunohistochemical analysis of collagen IV.

RESULTS

It was demonstrated that LLLT promotes an increase in collagen IV immunolabeling in skeletal muscle in the first 7 days after acute trauma caused by cryoinjury, but does not modify the duration of the tissue-repair process. Even with LLLT, the injured muscle tissue needs ∼21 days to achieve the same state of organization as that in the noninjured muscle.

CONCLUSION

The collagen IV content is modulated in regenerating skeletal muscle under LLLT, which might be associated with better tissue outcome, although the histologic analysis did not detect tissue improvement in the LLLT group.

Muscle after spinal cord injury

The morphological and contractile changes of muscles below the level of the lesion after spinal cord injury (SCI) are dramatic. In humans with SCI, a fiber-type transformation away from type I begins 4-7 months post-SCI and reaches a new steady state with predominantly fast glycolytic IIX fibers years after the injury. There is a progressive drop in the proportion of slow myosin heavy chain (MHC) isoform fibers and a rise in the proportion of fibers that coexpress both the fast and slow MHC isoforms. The oxidative enzymatic activity starts to decline after the first few months post-SCI. Muscles from individuals with chronic SCI show less resistance to fatigue, and the speed-related contractile properties change, becoming faster. These findings are also present in animals. Future studies should longitudinally examine changes in muscles from early SCI until steady state is reached in order to determine optimal training protocols for maintaining skeletal muscle after paralysis.

Muscle expression of genes associated with inflammation, growth, and remodeling is strongly correlated in older adults with resistance training outcomes

A group (n = 8) of healthy older (68 +/- 6 yr) adults participated in a 36-session progressive resistance exercise training program targeting the thigh muscles to determine the relationship between muscle gene expression and gains in muscle size and strength. Biopsies were obtained from the vastus lateralis at baseline 72 h after an acute bout of exercise and 72 h after completion of the training program. Training increased thigh muscle size (7%) and strength for the three exercises performed: knee extension (30%) and curl (28%) and leg press (20%). We quantified 18 transcripts encoding factors that function in inflammation, growth, and muscle remodeling that were demonstrated previously to be regulated by aging and acute exercise. The gain in extension strength and muscle size showed a high number of significant correlations with gene expression. These gains were most strongly correlated (P < or = 0.003, R > or = 0.89) with the baseline mRNA levels for insulin-like growth factor-1, matrix metalloproteinase-2 and its inhibitor TIMP1, and ciliary neurotrophic factor. Moreover, strength gains were inversely correlated with the change in these mRNA levels after training (P < or = 0.002 and R < or = -0.90). Changes in gene expression after acute exercise were not associated with training outcomes. These results suggest that higher baseline expression for key genes in muscle conveys an adaptive advantage for certain older adults. Individuals with lower baseline expression of these genes show less adaptation to exercise despite increased gene expression in response to training. These genes hold promise as useful predictors of training outcomes that could be used to design more effective exercise regimens for maintaining muscle function in older adults.

Matrix metalloproteinase-9 inhibition ameliorates pathogenesis and improves skeletal muscle regeneration in muscular dystrophy

Duchenne muscular dystrophy (DMD) is a fatal X-linked genetic disorder of skeletal muscle caused by mutation in dystrophin gene. Although the degradation of skeletal muscle extracellular matrix, inflammation and fibrosis are the common pathological features in DMD, the underlying mechanisms remain poorly understood. In this study, we have investigated the role and the mechanisms by which increased levels of matrix metalloproteinase-9 (MMP-9) protein causes myopathy in dystrophin-deficient mdx mice. The levels of MMP-9 but not tissue inhibitor of MMPs were drastically increased in skeletal muscle of mdx mice. Besides skeletal muscle, infiltrating macrophages were found to contribute significantly to the elevated levels of MMP-9 in dystrophic muscle. In vivo administration of a nuclear factor-kappa B inhibitory peptide, NBD, blocked the expression of MMP-9 in dystrophic muscle of mdx mice. Deletion of Mmp9 gene in mdx mice improved skeletal muscle structure and functions and reduced muscle injury, inflammation and fiber necrosis. Inhibition of MMP-9 increased the levels of cytoskeletal protein beta-dystroglycan and neural nitric oxide synthase and reduced the amounts of caveolin-3 and transforming growth factor-beta in myofibers of mdx mice. Genetic ablation of MMP-9 significantly augmented the skeletal muscle regeneration in mdx mice. Finally, pharmacological inhibition of MMP-9 activity also ameliorated skeletal muscle pathogenesis and enhanced myofiber regeneration in mdx mice. Collectively, our study suggests that the increased production of MMP-9 exacerbates dystrophinopathy and MMP-9 represents as one of the most promising therapeutic targets for the prevention of disease progression in DMD.

Structural and biochemical analysis of the effect of immobilization followed by stretching on the calcaneal tendon of rats

Little is known about the stretching effects on the biochemical and morphological features of tendons submitted to a long period of immobilization. Our purpose was to evaluate the response of rat tendons to stretching procedures after immobilization. The animals were separated into five experimental groups: GI--control of immobilized and euthanized animals; GII--immobilized and euthanized animals; GIII--control of immobilized animals and afterward stretched or allowed free cage activity; GIV--immobilized and stretched animals; and GV--immobilized and allowed free cage activity. Analysis in SDS-PAGE showed no remarkable differences among the groups, but a prominent collagen band was observed in GV, as compared to GIV and the control group, both in the compression and tension regions. Hydroxyproline content was highest in the compression region of GII. No differences among the groups were observed in the tension region. In regard to the concentration of noncollagenous proteins, differences were detected only in the tension region, where larger concentrations were found in the GII. When GII and GIV were compared, highest values were found in the GII. A more abundant presence of sulfated glycosaminoglycans, especially chondroitin sulfate, was detected in GIV, at the compression region of tendons. The presence of dermatan sulfate was outstanding in the compression and tension regions of the GII and GV groups. In the Ponceau SS stained sections, analyzed under polarization microscopy, GII exhibited the highest disorganization of the collagen bundles, partially recovered after stretching or with only remobilization. Our results indicate that a revision in the stretching procedures, in terms of duration and periodicity of the sessions, could benefit the efficiency of the stretching in cases of previous immobilization of tendons.

Human muscle gene expression responses to endurance training provide a novel perspective on Duchenne muscular dystrophy

Global gene expression profiling is used to generate novel insight into a variety of disease states. Such studies yield a bewildering number of data points, making it a challenge to validate which genes specifically contribute to a disease phenotype. Aerobic exercise training represents a plausible model for identification of molecular mechanisms that cause metabolic-related changes in human skeletal muscle. We carried out the first transcriptome-wide characterization of human skeletal muscle responses to 6 wk of supervised aerobic exercise training in 8 sedentary volunteers. Biopsy samples before and after training allowed us to identify approximately 470 differentially regulated genes using the Affymetrix U95 platform (80 individual hybridization steps). Gene ontology analysis indicated that extracellular matrix and calcium binding gene families were most up-regulated after training. An electronic reanalysis of a Duchenne muscular dystrophy (DMD) transcript expression dataset allowed us to identify approximately 90 genes modulated in a nearly identical fashion to that observed in the endurance exercise dataset. Trophoblast noncoding RNA, an interfering RNA species, was the singular exception-being up-regulated by exercise and down-regulated in DMD. The common overlap between gene expression datasets may be explained by enhanced alpha7beta1 integrin signaling, and specific genes in this signaling pathway were up-regulated in both datasets. In contrast to these common features, OXPHOS gene expression is subdued in DMD yet elevated by exercise, indicating that more than one major mechanism must exist in human skeletal muscle to sense activity and therefore regulate gene expression. Exercise training modulated diabetes-related genes, suggesting our dataset may contain additional and novel gene expression changes relevant for the anti-diabetic properties of exercise. In conclusion, gene expression profiling after endurance exercise training identified a range of processes responsible for the physiological remodeling of human skeletal muscle tissue, many of which were similarly regulated in DMD. Furthermore, our analysis demonstrates that numerous genes previously suggested as being important for the DMD disease phenotype may principally reflect compensatory integrin signaling.

High intensity exercise increases expression of matrix metalloproteinases in fast skeletal muscle fibres

Metalloproteinases (MMPs) are proteolytic enzymes that function in the extracellular matrix to degrade connective tissues. While it is clear that exercise-induced injury in skeletal muscle promotes increased expression of MMPs, the relationship between exercise intensity and expression of MMPs in muscles is unknown. These experiments tested the hypothesis that exercise-induced expression of matrix metalloproteinases (MMP-2 and MMP-9) is dose-dependent such that high-intensity endurance exercise increases MMP expression whereas low-intensity endurance exercise will not promote MMP expression in skeletal muscles. Female rats (4 months old) completed 2 weeks of treadmill running at either low (18 m min(-1); approximately 50% maximum oxygen consumption rate ) or high intensity (32 m min(-1); approximately 70% ; up to 50 min day(-1)). Non-running, sedentary animals served as controls. Muscle mRNA and protein levels of MMP-2 and MMP-9 were assessed in gastrocnemius, quadriceps and soleus muscles by reverse transcriptase-polymerase chain reaction and Western blotting, respectively. Results indicate that exercise did not alter MMP-9 in any of these skeletal muscles. Further, our data reveal that low-intensity exercise did not alter the expression of MMP-2 in any of the muscles investigated. In contrast, high-intensity exercise increased both mRNA and protein levels of MMP-2 in skeletal muscles containing a high percentage of fast type II fibres (i.e. gastronemius and superficial quadriceps). These results support the hypothesis that high-intensity exercise is required to promote the expression of MMP-2 in skeletal muscles and that the influence of exercise on MMP-2 expression is dominant in muscles containing a high percentage of fast fibres.

Role of extracellular matrix in adaptation of tendon and skeletal muscle to mechanical loading

The extracellular matrix (ECM), and especially the connective tissue with its collagen, links tissues of the body together and plays an important role in the force transmission and tissue structure maintenance especially in tendons, ligaments, bone, and muscle. The ECM turnover is influenced by physical activity, and both collagen synthesis and degrading metalloprotease enzymes increase with mechanical loading. Both transcription and posttranslational modifications, as well as local and systemic release of growth factors, are enhanced following exercise. For tendons, metabolic activity, circulatory responses, and collagen turnover are demonstrated to be more pronounced in humans than hitherto thought. Conversely, inactivity markedly decreases collagen turnover in both tendon and muscle. Chronic loading in the form of physical training leads both to increased collagen turnover as well as, dependent on the type of collagen in question, some degree of net collagen synthesis. These changes will modify the mechanical properties and the viscoelastic characteristics of the tissue, decrease its stress, and likely make it more load resistant. Cross-linking in connective tissue involves an intimate, enzymatical interplay between collagen synthesis and ECM proteoglycan components during growth and maturation and influences the collagen-derived functional properties of the tissue. With aging, glycation contributes to additional cross-linking which modifies tissue stiffness. Physiological signaling pathways from mechanical loading to changes in ECM most likely involve feedback signaling that results in rapid alterations in the mechanical properties of the ECM. In developing skeletal muscle, an important interplay between muscle cells and the ECM is present, and some evidence from adult human muscle suggests common signaling pathways to stimulate contractile and ECM components. Unaccostumed overloading responses suggest an important role of ECM in the adaptation of myofibrillar structures in adult muscle. Development of overuse injury in tendons involve morphological and biochemical changes including altered collagen typing and fibril size, hypervascularization zones, accumulation of nociceptive substances, and impaired collagen degradation activity. Counteracting these phenomena requires adjusted loading rather than absence of loading in the form of immobilization. Full understanding of these physiological processes will provide the physiological basis for understanding of tissue overloading and injury seen in both tendons and muscle with repetitive work and leisure time physical activity.

Matrix metalloproteinases and skeletal muscle: A brief review

Matrix metalloproteinases (MMPs) are a family of zinc- dependent proteolytic enzymes that function mainly in the extracellular matrix, where they contribute to the development, functioning, and pathology of a wide range of tissues. This mini-review describes the MMPs and tissue inhibitors of MMPs (TIMPs) in skeletal muscle, and considers their involvement in muscle development, ischemia, myonecrosis, angiogenesis, denervation, exercise-induced injuries, disuse atrophy, muscle repair and regeneration, and inflammatory myopathies and dystrophies. Despite the very limited information currently available on MMPs and their inhibitors in skeletal muscle, it is becoming increasingly clear that they have important physiological functions in maintenance of the integrity and homeostasis of muscle fibers and of the extracellular matrix. Understanding the roles of MMPs and TIMPs may lead to the development of new drug-related treatments for various muscle disorders based on suppression or upregulation of their expression.

Expression of matrix metalloproteinases, inhibitor, and acid phosphatase in muscles of immobilized hindlimbs of rats

External fixation procedures of limb immobilization provide excellent experimental models to study mechanisms involved in muscle disuse atrophy and recovery. Female Wistar rats (7-8 months old) had their right hindlimbs immobilized by an external fixation procedure for 5, 10, 21, and 30 days. Muscle mass of the gastrocnemius and quadriceps muscles was reduced by 41-46% in comparison with contralateral nonimmobilized legs. Acid phosphatase activities were significantly increased after 21 and 30 days of hindlimb immobilization. Histochemical staining for acid phosphatase activities increased in myofibers after the external fixation and also in macrophages in the adjacent extracellular matrix. Matrix metalloproteinase (MMP-2 and MMP-9) activities assessed by gel zymography and also a tissue inhibitor of metalloproteinases (TIMP-1) assessed by Western blot were elevated in the immobilized hindlimb muscles. Our study demonstrated that metalloproteinases are expressed relatively late after limb immobilization and appear to be responsible to a large degree for degradation of the extracellular matrix in experimental disuse atrophy.

Training-induced changes in peritendinous type I collagen turnover determined by microdialysis in humans

1. Acute exercise is found to increase collagen type I formation locally in peritendinous connective tissue of the Achilles' tendon in humans, as determined from changes in interstitial concentrations of collagen propeptide (PICP) and a collagen degradation product (ICTP) by the use of microdialysis catheters. However, the local collagen type I turnover response to training is unknown. 2. Nineteen young males were studied before and after 4 and 11 weeks of physical training. Microdialysis catheters with a high molecular mass cut-off value (3000 kDa), allowing the determination of PICP and ICTP, were placed in the peritendinous space ventral to the Achilles' tendon, under ultrasound guidance, in both legs. The catheters were perfused with a Ringer-acetate solution containing (3)H-labelled human type IV collagen for in vivo recovery determination (relative recovery: 79 +/- 2 %, mean +/- S.E.M.). 3. The PICP concentration in the peritendinous tissue increased in response to training (from 5 +/- 1 to 35 +/- 5 microg l(-1) (4 weeks), P < 0.05) and remained elevated throughout the training period (28 +/- 6 microg l(-1), 11 weeks). Tissue ICTP only rose transiently with training (from 2.2 +/- 0.1 to 2.8 +/- 0.2 microg l(-1) (4 weeks), P < 0.05, and 2.5 +/- 0.2 microg l(-1) (11 weeks), P > 0.05 vs. basal). Plasma PICP was unchanged whereas plasma ICTP declined by 17 % in response to training. 4. The findings indicate that physical training results in an increased turnover of collagen type I in local connective tissue of the peritendinous Achilles' region. Early in the process both synthesis and degradation are elevated, whereas later, the anabolic processes are dominating causing a net synthesis of type I collagen in tendon-related tissue in humans.

Acute exercise induced changes in rat skeletal muscle mRNAs and proteins regulating type IV collagen content

This experiment tested the hypothesis that running-induced damage to rat skeletal muscle causes changes in synthesis and degradation of basement membrane type IV collagen and to proteins regulating its degradation. Samples from soleus muscle and red and white parts of quadriceps femoris muscle (MQF) were collected 6 h or 1, 2, 4, or 7 days after downhill running. Increased muscle beta-glucuronidase activity indicated greater muscle damage in the red part of MQF than in the white part of MQF or soleus. In the red part of MQF, type IV collagen expression was upregulated at the pretranslational level and the protein concentration decreased, whereas matrix metalloproteinase-2 (MMP-2), a protein that degrades type IV collagen, and tissue inhibitor of metalloproteinase-2 (TIMP-2), a protein that inhibits degradation, were increased in parallel both at mRNA and protein levels. Type IV collagen mRNA level increased in the white part of MQF and soleus muscle. The protein concentration increased in the white part of MQF and was unchanged in soleus muscle. MMP-2 and TIMP-2 changed only slightly in the white part of MQF and soleus muscle. The changes seem to depend on the severity of myofiber injury and thus probably reflect reorganization of basement membrane compounds.

Lack of type XV collagen causes a skeletal myopathy and cardiovascular defects in mice

Type XV collagen occurs widely in the basement membrane zones of tissues, but its function is unknown. To understand the biological role of this protein, a null mutation in the Col15a1 gene was introduced into the germ line of mice. Despite the complete lack of type XV collagen, the mutant mice developed and reproduced normally, and they were indistinguishable from their wild-type littermates. However, Col15a1-deficient mice showed progressive histological changes characteristic for muscular diseases after 3 months of age, and they were more vulnerable than controls to exercise-induced muscle injury. Despite the antiangiogenic role of type XV collagen-derived endostatin, the development of the vasculature appeared normal in the null mice. Nevertheless, ultrastructural analyses revealed collapsed capillaries and endothelial cell degeneration in the heart and skeletal muscle. Furthermore, perfused hearts showed a diminished inotropic response, and exercise resulted in cardiac injury, changes that mimic early or mild heart disease. Thus, type XV collagen appears to function as a structural component needed to stabilize skeletal muscle cells and microvessels.

Lack of type XV collagen causes a skeletal myopathy and cardiovascular defects in mice

Type XV collagen occurs widely in the basement membrane zones of tissues, but its function is unknown. To understand the biological role of this protein, a null mutation in the Col15a1 gene was introduced into the germ line of mice. Despite the complete lack of type XV collagen, the mutant mice developed and reproduced normally, and they were indistinguishable from their wild-type littermates. However, Col15a1-deficient mice showed progressive histological changes characteristic for muscular diseases after 3 months of age, and they were more vulnerable than controls to exercise-induced muscle injury. Despite the antiangiogenic role of type XV collagen-derived endostatin, the development of the vasculature appeared normal in the null mice. Nevertheless, ultrastructural analyses revealed collapsed capillaries and endothelial cell degeneration in the heart and skeletal muscle. Furthermore, perfused hearts showed a diminished inotropic response, and exercise resulted in cardiac injury, changes that mimic early or mild heart disease. Thus, type XV collagen appears to function as a structural component needed to stabilize skeletal muscle cells and microvessels.