Dystrophin is a large protein of 427 kDa that is expressed at its highest level in normal skeletal muscle (18). This protein is altered or absent in skeletal muscle of patients with Becker and Duchenne muscular dystrophies, leading to progressive skeletal muscle deterioration. Dystrophin is localized at the cytoplasmic surface of the subsarcolemmal membrane of skeletal muscle fibers, and in conjugation with surface glycoproteins, forms a scaffold thought to play an important role in maintaining skeletal muscle architecture and the integrity of the sarcolemma between the contractile apparatus and the extracellular matrix(6). Skeletal muscle possesses extensive and highly specialized subsarcolemmal adherens junctions, such as neuromuscular and myotendinous junctions, as well as costameres which are myofibril-to-sarcolemma attachment sites. These junctions represent membrane-microfilament contact zones that appear to be involved in the uniform transmission of contraction forces, as well as the lateral association of myofibrils with the sarcolemma (31). Several proteins of adherens junctions, namely vinculin, meta-vinculin, talin, aciculin,α-actinin, and integrin(s), are found in skeletal muscle and appear to mediate a linkage between actin filaments and the plasma membrane(1,2,7,9). Immunofluorescent studies have also localized dystrophin to costameres, of which vinculin is an ubiquitous marker (28). Aciculin, a newly identified cytoskeletal component of adherens junctions, is particularly enriched in the myotendinous junction (1,2), where dystrophin is also found (3). Thus, the cytoskeletal proteins dystrophin, vinculin and aciculin appear to be intimately related to the maintenance of adherens junctions in skeletal muscle.
The effects of chronic muscle use and disuse on the muscle phenotype has been well established for myofibrillar proteins and Z-line proteins(26,30), as well as those involved in the function of the sarcoplasmic reticulum and in energy metabolism(13,14,27). Less well established are the effects of chronic muscle use and disuse on structural proteins such as those of the cytoskeleton. In the mdx mouse, an animal model in which dystrophin is lacking in skeletal muscle, a chronic functional overload results in muscle deterioration (4,22) and an increased susceptibility to contraction-induced sarcolemmal rupture(25). Thus, it seems reasonable to hypothesize that chronic exercise of normal muscle possessing dystrophin might result in the induction of this protein as a compensatory mechanism designed to subvert muscle damage. The present study was designed to evaluate changes in the levels of dystrophin and other related cytoskeletal proteins (vinculin and aciculin) in different models of skeletal muscle use and disuse.
Male Sprague-Dawley rats (Charles River, Canada) were used. They were housed individually with a 12:12-h light-dark cycle. All procedures used were in accordance with the guiding principles in the care and use of animals of the Canadian Council on Animal Care, and the American College of Sports Medicine.
Individual rats were placed in cages adjoining a running wheel. As found in other studies (29), animals had to be moderately food restricted to attain running distances in which a training effect could be expected. Thus, each runner (R) was matched with a food restricted sedentary(S) animal, as well as a free-eating sedentary (FS) animal (N = 7/group). Animal weight was monitored regularly, and running distances were measured using a revolution counter and converted to km distances attained per day. The animals used in the present study ran an average of 9.6 ± 1.0 km·d-1. At the end of 8 wk, animals were anesthetized and the red gastrocnemius muscle was removed and frozen for further processing. This tissue was selected because it was expected to be recruited during this moderately intense exercise condition.
The surgical procedure was carried out as described previously(32,33). The left tibialis anterior (TA) muscle(N = 5 rats) was stimulated at 10 Hz, 24 h·d-1. The contralateral muscle served as a nonstimulated control. After 10 d, muscles were excised and frozen as above. Chronic stimulation for this period of time results in an approximate 15% decline in muscle mass(32).
Frozen forelimb muscles (triceps and biceps brachii) of six rats which were subject to 6 d of microgravity during the spaceflight of the NASA shuttle“Endeavour” in January 1993 were obtained. Identical tissues from control animals (N = 6) subject to normal gravity conditions were also excised simultaneously.
The surgical procedure for the induction of denervation was described previously (34). Tibialis anterior muscles were denervated for 7 or 21 d (N = 4-6 rats), after which tissues were removed and frozen. Sham-operated animals served as controls. This treatment period resulted in approximately 36% and 64% declines in muscle mass, respectively (5).
Tissue Sampling and Extraction
Muscle tissues were stored at -80°C. Muscles were subsequently powdered using a stainless steel mortar which was precooled to the temperature of liquid N2. Samples of muscle powder (10-15 mg) were suspended in a 20-fold (w/v) dilution of 0.1 M KH2PO4/Na2PO4 buffer (pH 7.2) containing 2 mM EDTA for the measurement of cytochrome c oxidase activity (12). Muscle powders were also used to generate tissue extracts in the same buffer using a 40-fold dilution(15) for total protein measurements(19) and for application to gradient gels for electrophoresis.
Gradient Gel Electrophoresis
A 3-12% gradient polyacrylamide gel was used for the separation of dystrophin. Extracts of samples containing 150 μg of total protein were combined with an equal volume of sample buffer (10% glycerol (v/v), 2.3% SDS(v/v), 62.5 mM Tris-HCl (pH 6.8), 5% mercaptoethanol (v/v)) containing bromphenol blue, denatured 5 min at 95°C, and applied to the gel. A 4-10% gradient polyacrylamide gel and a 3% stacking gel were used for the separation of aciculin from samples containing 100 μg of total protein. Vinculin was separated on 4-15% gradient polyacrylamide gels with a 3% stacking gel. In this case, 50 μg of total protein/lane were applied to the gel. All separations were achieved using low voltage (40-50 V) overnight.
Proteins were transferred from the gel onto Hybond-C Super nitrocellulose membrane (Amersham, Canada) for 1.5-2 h using an ISS semi-dry electroblotter. The transfer buffer used for dystrophin electroblotting was prepared according to Otter et al. (24), while that used for vinculin and aciculin analyses was made according to Harlow and Lane(10). Membranes were incubated with a primary monoclonal antibody directed against dystrophin (donated by Dr. R. G. Worton, or Novocastra NCL-Dys2, Newcastle upon Tyne, U.K.) diluted to 1:300 for 3 h at room temperature. The affinity-purified monoclonal antibody specific for aciculin 60/63 kDa (XIVF8, Ref. 1) was diluted to 1:200, and incubated with the membrane for 2 h at room temperature. Vinculin and meta-vinculin (M-vinculin) were detected by using a hybridoma supernate(VIIF9) at room temperature for 2 h as well. Primary antibodies were revealed with sheep anti-mouse IgG alkaline phosphatase-coupled secondary antibody(Sigma, St. Louis) diluted to 1:1000. The reaction was detected using a 5-15 min incubation with nitro-blue-tetrazolium and 5-bromo-4-chloro-3-indolyl phosphate. Incubation times were chosen such that signals were within the linear range of the laser densitometry detection system (Zeineh SL-504-XL, Fullerton, CA). Scanning was performed using a vertical line through the midpoint of the sample signal. Relative values were expressed as the ratio of treated samples over control.
Absolute values of integration peaks obtained via laser densitometry were compared using Student's t-test for paired or unpaired samples, as appropriate. All values are reported as means ± SEM, and values in brackets in the tables refer to the number of animals analyzed.
The effects of the two chronic muscle use and disuse models on cytochrome c oxidase activity are illustrated in Table 1. As expected, 10 d of chronic stimulation was much more effective than voluntary locomotion for 8 wk in producing adaptive changes within the muscle. Of the muscle disuse models, denervation for 21 d led to a slightly greater reduction in mitochondrial content compared to 6 d of microgravity conditions. This analysis was performed to indicate the extent of the adaptive changes evident in the muscles under the experimental situations employed, as done previously(5,16,34).
The protein levels of dystrophin, vinculin and aciculin are shown as representative blots in Figs. 1-4, and quantified as a larger sample size in Table 2. Dystrophin levels were not altered by voluntary running (Fig. 1). The ratio of dystrophin levels in muscle of running animals relative to sedentary weight matched controls was 1.59 ± 0.63 (N = 7). The greater stimulus of chronic stimulation also resulted in no significant change in dystrophin levels (Fig. 2, Table 2). In contrast, muscle disuse imposed by microgravity conditions resulted in values which were 1.82 ± 0.21-fold (N = 6) above those in control animals (P < 0.05; Fig. 3). Denervation (21 d) led to a greater than three-fold increase in dystrophin levels within the muscle (Fig. 4, Table 2). Because the two muscle disuse models were divergent in duration (6 vs 21 d) and with respect to the presence (microgravity) or absence (denervation) of the nerve, we subsequently investigated dystrophin levels in 7-d denervated muscle, which approximated the duration of the microgravity condition. Dystrophin levels were increased 1.9 ±0.2 fold (N = 4; P < 0.05, immunoblot not shown) under these conditions, similar to the results of the microgravity treatment.
Two distinct cytoskeletal proteins are detectable on the blots probed with the vinculin antibody. These are the closely related proteins M-vinculin (150 kDa) and vinculin (130 kDa). Vinculin levels were markedly increased by both chronic stimulation and denervation to values which were greater than three-fold above nontreated control levels. Changes in vinculin levels appear to occur partly at the expense of reductions in M-vinculin levels, since these decreased by approximately 25-50% as a result of these treatments(Figs. 2 and 4, Table 2)
Aciculin exists in two different isoforms with relative molecular weights of 63 kDa and 60 kDa. Aciculin 60 kDa was markedly increased to six-fold higher levels in stimulated as compared with control tibialis anterior muscles. In contrast, aciculin 63 kDa levels were unchanged in stimulated as compared to control muscles (Fig. 2). This latter protein was not detectable in 21 d denervated muscles, but a 3.3-fold increase in aciculin 60 kDa was observed in this condition of muscle disuse(Fig. 4, Table 2).
The purpose of this study was to investigate the adaptability of cytoskeletal proteins in skeletal muscle subject to conditions of chronic muscle use or disuse. While much attention has been focused on changes in proteins of the myofibrils, sarcoplasmic reticulum, and mitochondria (for reviews, see (13,14,26,27), little is known regarding alterations in the gene expression of muscle cell structural proteins under conditions of altered muscle contractile activity. The cytoskeletal proteins chosen for the present work appear to be intimately associated, and are involved in the maintenance of subsarcolemmal adherens junctions in muscle cells (7). For example, dystrophin colocalizes with β-spectrin (28) and its N-terminal end binds F-actin (20) in subsarcolemmal domains within muscle cells. Vinculin is also present in the same structural complexes as dystrophin and β-spectrin (28). The C-terminus of dystrophin appears to be attached to a large sarcolemmal oligomeric complex consisting mainly of glycoproteins, called the dystrophin-glycoprotein complex(21). This complex acts as a link between intracellular proteins such as actin, and extracellular matrix proteins such as laminin(6). This lattice of proteins may serve to stabilize the plasma membrane during muscle contraction (20). Aciculin is a newly discovered protein of adherens junctions, and its role within muscle is unknown. However, it appears to be concentrated in similar areas as dystrophin, and it also interacts with actin filaments(1,2). Because these proteins colocalize at adherens junctions, it was hypothesized that levels of these may change similarly under conditions of muscle adaptation. Our results reveal that this was not the case. For example, while dystrophin was unaltered under conditions of chronic muscle use, both aciculin and vinculin were markedly increased by contractile activity. However, they did not increase in parallel, indicating independent regulation of these two proteins under conditions of chronic muscle use. In contrast, these three proteins exhibited a coordinated three-fold increase following 21 d of denervation. Muscle disuse imposed for a shorter time period in the continuing presence (microgravity conditions for 6 d) or absence (denervation for 7 d) of the nerve resulted in similar 1.8- and 1.9-fold increases in dystrophin levels. These data suggest that the duration of muscle disuse is more important than the presence or absence of the nerve in modifying dystrophin levels in muscle.
In contrast, muscle use imposed by running or 10 d of chronic stimulation had no impact on the dystrophin levels in muscle. A similar lack of effect of chronic stimulation has recently been reported in the dog(23). However, it is known that dog muscle reacts very differently to chronic stimulation (17) than do other species such as the rat or the rabbit (15,16), and therefore we felt that it was important to clarify the role of chronic contractile activity, either through voluntary running or chronic stimulation, on dystrophin levels in the rat. Thus, despite the apparent role of dystrophin in protecting the sarcolemma during the stresses of muscle contraction(25), sufficient levels of this protein may be present within normal muscle without the need for invoking cellular mechanisms of protein induction in response to contractile activity. It is unlikely that the increase in dystrophin levels which we observed under conditions of chronic muscle disuse would be of any benefit to muscular dystrophy patients possessing a defective dystrophin gene. However, it may be of benefit for muscle cell structure and function in those normal individuals forced into conditions of muscle disuse (i.e., immobilization, denervation).
Dystrophin levels have been shown to exist at a higher concentration in slow-twitch, compared with fast-twitch muscle (11). Based upon this observation, we expected that denervation might increase the level of dystrophin in muscle, since this treatment results in a muscle with slower contractile properties (34). Our results indicate that changes in dystrophin levels are associated with the transformation to a slower phenotype. In support of this, chronic stimulation does not result in a fast-to-slow transformation in rat skeletal muscle (27), and we did not find an increase in dystrophin levels as a result of this treatment.
The increases in protein levels observed are likely due either to alterations in the expression of genes that encode these proteins, or to decreased rates of protein turnover. Although it is possible that increases in the degradation of other proteins in atrophying muscle could result in an increase in the concentration of cytoskeletal proteins, the large increases of greater than 3- to 6-fold cannot be entirely accounted for by the decreases in muscle mass observed. Further, even though aciculin, vinculin, and M-vinculin are highly expressed in vascular smooth muscle(1,9), it seems unlikely that the increases in concentration noted could be due solely to changes in skeletal muscle vascularity. This is because any increase in vascularity brought about by the treatment would be expected to enhance the measured levels of all three proteins. However, M-vinculin tended to decrease in response to both denervation and stimulation, rather than increase. Since vinculin and M-vinculin appear to arise from alternative splicing of the same primary transcript (8), the results potentially illustrate an interesting example of splicing regulation. Thus, it seems appropriate to suggest that these experimental conditions are useful for the study of the regulation of the expression and turnover of cytoskeletal proteins in muscle. The changes observed are large and rapid, and reveal a new area of muscle phenotypic plasticity, which could have important implications for muscle cell structure and function.
Figure 2-Immunoblot of dystrophin (427 kDa), vinculin (130 kDa), M-vinculin (150 kDa), and aciculin (63 and 60 kDa) in chronically stimulated and control muscles. St, stimulated; C, control; Dys, dystrophin; M-vin, meta-vinculin; Vin, vinculin; Aci-63, aciculin 63 kDa; Aci-60, aciculin 60 kDa.
Figure 4-Immunoblot of dystrophin, vinculin, M-vinculin, and aciculin in denervated and control muscles. D, denervated; C, control; Dys, dystrophin; M-vin, meta-vinculin; vin, vinculin; Aci-63, aciculin 63 kDa; Aci-60, aciculin 60 kDa.
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