Matrix metalloproteinases and skeletal muscle: A brief review

Cellular and molecular signatures of muscle regeneration: current concepts and controversies in adult myogenesis

Adult skeletal muscle generates force in a controlled and directed manner through the contraction of highly specialized, postmitotic, multinucleated myofibers. Life-long muscle function relies on maintenance and regeneration of myofibers through a highly regulated process beginning with activation of normally quiescent muscle precursor cells and proceeding with formation of proliferating progenitors that fuse to generate differentiated myofibers. In this review, we describe the historical basis and current evidence for the identification of satellite cells as adult muscle stem cells, critically evaluate contributions of other cells to adult myogenesis, and summarize existing data regarding the origins, genetic markers, and molecular regulation of satellite cells in normal, diseased, and aged muscle.

Regulation of TIMP-2, MT1-MMP, and MMP-2 expression during C2C12 differentiation

Matrix metalloproteinases (MMPs) are zinc-dependent proteases capable of degrading extracellular matrix components. The activity of these proteases is tightly regulated through the actions of the tissue inhibitors of metalloproteinases (TIMPs). Although the regulation of MMPs and TIMPs during physiological and pathological remodeling has been investigated in a number of systems, almost nothing is known about their role in skeletal muscle differentiation. To investigate the role of MMP-mediated proteolysis during myogenesis, the regulation of TIMP-2, MT1-MMP, and MMP-2 expression was investigated during differentiation of the mouse myoblastic C2C12 cell line. We show that this trio is upregulated coincident with myogenesis. The more diffuse spatial distribution of TIMP-2 relative to MT1-MMP and MMP-2 suggests that TIMP-2 may exert MMP-independent functions during myogenesis. Elucidating the regulation of these molecules during muscle differentiation in vitro may lead to a better understanding of their role in pathological processes in muscle tissue in vivo.

Arteriolization of capillaries and FGF-2 upregulation in skeletal muscles of patients with chronic peripheral arterial disease

OBJECTIVE

Microvascular changes in ischemic skeletal muscle are described derived from patients with long-lasting peripheral arterial disease (PAD).

METHODS

Skeletal muscles from the lower limb of 17 patients (obtained after amputation) with chronic PAD and 4 asymptomatic controls (obtained from biopsies after bypass surgery) were evaluated by electron microscopy and immunohistochemistry.

RESULTS

The capillaries in skeletal muscles of PAD patients were surrounded by a more than 1 microm-thick coat, which was positively stained for basement membrane pericapillary coat collagen type IV. Thickness of the coat correlated with presence of PAD (p < .0001), and less strongly with diabetes mellitus (p = .023) and age of patients (p = .019). The majority of the capillaries in skeletal muscles of PAD patients (71.1 +/- 15.3%) were covered with cells positive for smooth muscle cell actin (sma) as compared to samples from asymptomatic controls (22.8% +/- 9.6%; p < .0001) suggesting advanced arteriolization. Semiquantitative analysis revealed that patients with PAD demonstrate a higher expression of FGF-2 in capillary endothelial cells (67.8 +/- 17.5%) as compared to controls (10.2 +/- 8.4%; p < .0001), whereas VEGF immunoreactivity was only occasionally present in extravascular cells.

CONCLUSION

Thickened collagen type IV-positive basement membranes in combination with a significant increase in sma-coverage indicate arteriolization of capillaries characteristic for chronic PAD, what may be related to high FGF-2 expression in capillary endothelial cells.

Immunobiology of muscle: advances in understanding an immunological microenvironment

Skeletal muscle, which is the largest cellular compartment of the body, lacks detectable MHC expression under physiological conditions. Therefore, immune reactions triggered by, or directed against, muscle cells proceed along specific pathways. Recently, the expression and functioning of classical MHC, non-classical MHC, adhesion and co-stimulatory molecules have been shown to support the concept that muscle cells can act as facultative antigen-presenting cells and should be considered as active participants, rather than passive targets, of immune reactions. Here, we summarize current knowledge on the immunological capabilities of skeletal muscle cells and discuss how these characteristics might contribute to inflammatory muscle disorders, as well as therapeutic strategies, such as gene or myoblast transfer.

Specific matrix metalloproteinase expression in focal myositis: an immunopathological study

OBJECTIVES

The aim of our study was to investigate immunoexpression of matrix metalloproteinases MMP2, MMP7 and MMP9 in muscle specimens from patients with focal myositis (FM) vs polymyositis (PM) and dermatomyositis.

MATERIALS AND METHODS

We studied muscle biopsy samples from seven patients affected by FM; samples from five patients each with PM and dermatomyositis were studied as disease controls.

RESULTS

MMP2 immunoreactivity was present in PM and dermatomyositis, MMP7 only in PM, and MMP9 in PM, dermatomyositis and FM.

CONCLUSIONS

Our results confirm that increased MMP9 immunoreactivity in muscle fibres is a common feature of all inflammatory myopathies and suggest that MMP2 and MMP7 cannot be implicated in the inflammatory events of FM.

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.

Enhanced matrix metalloproteinase activity in skeletal muscles of rats with congestive heart failure

Patients with congestive heart failure (CHF) are prone to increased skeletal muscle fatigue. Elevated circulatory concentrations of tumor necrosis factor (TNF)-alpha and monocyte chemoattractant protein-1, which may stimulate matrix metalloproteinase (MMP) activity and, thereby, contribute to skeletal muscle dysfunction, are frequently found in CHF. However, whether skeletal muscle MMP activity is altered in CHF is unknown. Hence, we have used a gelatinase assay to assess the activity of MMP and tissue inhibitors of MMP in single skeletal muscles of rats with CHF 6 wk after induction of myocardial infarction. Sham-operated (Sham) rats were used as controls. We also measured the gene expression and protein contents of MMP-2 and MMP-9 in skeletal muscles of these rats. Plasma MMP activity was nearly seven times higher (P < 0.05) in CHF than in Sham rats. Concomitantly, the MMP activity within single slow- and fast-twitch skeletal muscles of CHF rats increased two- to fourfold compared with Sham animals, whereas tissue inhibitor of MMP activity did not differ (P > 0.05). Preformed MMP-2 and MMP-9 were probably activated in CHF, because neither their gene expression nor protein levels were altered (P > 0.05). Serum concentrations of TNF-alpha and monocyte chemoattractant protein-1 remained unchanged (P > 0.05) between CHF and Sham rats during the 6-wk observation period. We conclude that development of CHF in rats enhances MMP activity, which in turn may distort the normal contractile function of skeletal muscle, thereby contributing to increased skeletal muscle fatigue.

The effect of hindlimb immobilization on acid phosphatase, metalloproteinases and nuclear factor-κB in muscles of young and old rats

Age-associated muscle wasting (sarcopenia of old age) is a major problem in elderly people, however, the mechanisms of muscle proteolysis in aging remain obscure and enigmatic. Possible reasons for loss of skeletal muscle mass with aging may be attributed to multiple and complex proteolytic systems. The purpose of the present study was to explore the kinetics of activation of extracellular hydrolytic and proteolytic systems in muscles of hindlimbs immobilized by external fixation of 24-month-old female Wistar rats, in comparison with those of 6-month-old rats. Results show that elevated acid phosphatase activities (lysosomal hydrolytic enzyme activated mainly in macrophages) in immobilized limb muscles of young animals, differ from old animals. In young rats external fixation resulted in significantly elevated acid phosphatase activities (50-55%; p<0.05) after 4 weeks of immobilization, whereas in old animals similar increases were observed already during the first and second weeks of immobilization. The extracellular proteolytic enzymes, matrix metalloproteinases (MMP-2 and -9), were also differentially activated in old animals compared to young animals. In young animals, as shown in previous studies, both MMP-2 and -9 activities were elevated significantly in immobilized muscles. In this study of old animals, only MMP-2 activity was detected, with no significant elevation in the immobilized muscles of old animals. In addition, the levels of the transcription factor Nuclear Factor-kappaB (NF-kappaB) in nuclear extracts of old rat muscles, as detected by ELISA, showed a biphasic pattern after immobilization, suggesting that NF-kappaB could be activated by different processes in the atrophy process, at least in the old age. In conclusion, it seems that the kinetics of activation of extracellular hydrolytic and proteolytic systems differ in muscles of old animals compared to young animals.