Myofiber Damage Research Articles

Hydrodynamic Limb Vein Delivery of a Xenogeneic DNA Cancer Vaccine Effectively Induces Antitumor Immunity

Tumor-associated antigens (TAA) are typically poorly immunogenic "self" antigens. An effective strategy to break tolerance and induce antitumor immunity is by genetic vaccination, employing the orthologous TAA-sequence from a different species. We recently developed a clinically relevant approach for intravascular hydrodynamic limb vein (HLV) delivery of nucleic acids to skeletal muscle. Using the human gp100 xenogeneic TAA in the murine B16 melanoma model, we show that genetic vaccination of mice by HLV plasmid DNA delivery was highly effective at breaking tolerance against the homologous murine gp100 (mgp100) TAA and induced prophylactic antitumor protection. HLV vaccination resulted in an anti-hgp100 humoral and cellular response, with 4-5% of CD8(+) T cells being gp100(25-33)-epitope-specific. Vaccinated animals demonstrated in vivo cytolytic activity against human and mgp100(25-33) peptide-pulsed targets. Antitumor immunity could be adoptively transferred by splenocytes from human gp100-vaccinated animals. Furthermore, a durable antitumor memory response was established as approximately 3% of CD8(+) T cells were gp100(25-33) antigen-specific in mice 6 months after vaccination. Following a single HLV human gp100 DNA boost, this level increased to approximately 17% and protected animals from subsequent B16 tumor rechallenge. Our results warrant further consideration of HLV as a clinically relevant method for cancer gene therapy.

Intrinsic laryngeal muscles are spared from myonecrosis in the mdx mouse model of Duchenne muscular dystrophy

Intrinsic laryngeal muscles share many anatomical and physiological properties with extraocular muscles, which are unaffected in both Duchenne muscular dystrophy and mdx mice. We hypothesized that intrinsic laryngeal muscles are spared from myonecrosis in mdx mice and may serve as an additional tool to understand the mechanisms of muscle sparing in dystrophinopathy. Intrinsic laryngeal muscles and tibialis anterior (TA) muscle of adult and aged mdx and control C57Bl/10 mice were investigated. The percentage of central nucleated fibers, as a sign of muscle fibers that had undergone injury and regeneration, and myofiber labeling with Evans blue dye, as a marker of myofiber damage, were studied. Except for the cricothyroid muscle, none of the intrinsic laryngeal muscles from adult and old mdx mice showed signs of myofiber damage or Evans blue dye labeling, and all appeared to be normal. Central nucleation was readily visible in the TA of the same mdx mice. A significant increase in the percentage of central nucleated fibers was observed in adult cricothyroid muscle compared to the other intrinsic laryngeal muscles, which worsened with age. Thus, we have shown that the intrinsic laryngeal muscles are spared from the lack of dystrophin and may serve as a useful model to study the mechanisms of muscle sparing in dystrophinopathy.

Role of Nitric Oxide in the Apoptosis of Structural Damage in Rat Skeletal Muscle

PURPOSE: Apoptosis is a common mechanism of programmed cell death that has been implicated in the development of skeletal muscle damage. We examined the role of nitric oxide (NO) in the apoptosis of skeletal muscle damage. METHODS: A standardized eccentric stretch injury was created to the tibialis anterior (TA) muscle of 40 male Wistar rats (200 to 240 g), using a customized isokinetic test device and electrical stimulation. We evaluated the effects of the nitric oxide synthase inhibitor N(G)-nitro-L-arginine-methyl ester (L-NAME; 85 mg/kg/day) on apoptosis in rat skeletal muscle cell. The TA muscle was isolated immediately, 1, 3 and 7 days after muscle stretch injury. The animals were divided into two groups: (1) structural muscle damage (SMD), without L-NAME infusion (SMD/−L-NAME) (n=5 at each day); (2) structural muscle damage, with L-NAME infusion (SMD/+L-NAME) (n=5 at each day). Apoptosis was assessed by Caspase-3 and Cytochrome-C assays. RESUITS: NO content was greater in the SDM/−L-NAME group at all time points (p<0.05). Three days following the bout of SMD there was greater myofiber damage and leukocyte invasion in the SMD/−L-NAME group as compared to the SMD/+L-NAME group. Both groups showed evidence for myofiber repair, however the degree of regeneration was less in the SMD/+L-NAME group. SMD/+L-NAME group compared with SMD/−L-NAME group showed a significantly higher apoptotic index in Cytochrome-C protein (p < 0.01) and Caspase-3 activity (p < 0.05), respectively. CONCLUSIONS: We conclude that NO produces an anti-apoptotic effect by suppression of caspase activity in rat skeletal muscle following a single stretch injury.

Rapid Intravascular Injection into Limb Skeletal Muscle: A Damage Assessment Study

We have recently developed a simple and highly efficient methodology for delivering plasmid DNA (pDNA) to skeletal muscle cells of mammalian limbs. The procedure involves the rapid intravascular injection of a large volume of saline (containing pDNA) into the vasculature of the distal limb. As a result of the robust delivery methodology involved, it is important to understand the effects of the injection procedure on the skeletal muscle tissue in the targeted limb. In previous studies, only modest and transient muscle damage was noted. In this study we quantitatively assessed the degree of muscle damage in rat limbs following intravascular injections using muscle histology (H&E staining), membrane integrity (Evans blue staining), and leukocyte infiltration (immunohistochemistry) assays. The rapid extravasation of fluid during the injection process resulted in edema of the muscle tissue of the targeted limb; however, the edema was transient and resolved within 24 h. Consistent with observations from previous studies, minimal levels of myofiber damage were detected. Immunohistochemical labeling indicated that increased numbers of neutrophils (CD43+) and macrophages (ED1+ and ED2+) were present in the muscle tissue interstitium shortly after injection but that elevations were relatively modest and resolved by 2 weeks postinjection.

Strength at the extracellular matrix–muscle interface

Mechanical force is generated within skeletal muscle cells by contraction of specialized myofibrillar proteins. This paper explores how the contractile force generated at the sarcomeres within an individual muscle fiber is transferred through the connective tissue to move the bones. The initial key point for transfer of the contractile force is the muscle cell membrane (sarcolemma) where force is transferred laterally to the basement membrane (specialized extracellular matrix rich in laminins) to be integrated within the connective tissue (rich in collagens) before transmission to the tendons. Connections between (1) key molecules outside the myofiber in the basement membrane to (2) molecules within the sarcolemma of the myofiber and (3) the internal cytoplasmic structures of the cytoskeleton and sarcomeres are evaluated. Disturbances to many components of this complex interactive system adversely affect skeletal muscle strength and integrity, and can result in severe muscle diseases. The mechanical aspects of these crucial linkages are discussed, with particular reference to defects in laminin-alpha2 and integrin-alpha7. Novel interventions to potentially increase muscle strength and reduce myofiber damage are mentioned, and these are also highly relevant to muscle diseases and aging muscle.

Reconstruction of Ablated Rat Rectus Abdominis by Muscle Regeneration

Skeletal muscle regeneration is a powerful, naturally occurring process of tissue reconstruction that follows myofiber damage secondary to myotoxic injury that does not normally affect the tissue circulation and scaffold. The ablated tissue, in traumatology and free muscle grafts, is frequently replaced by scars. The final outcome is poor even after in situ myoblast seeding of the harvested muscle. The goal of this study was to identify protocols to reconstruct muscle tissue, even in such adverse environments. The authors applied a step-by-step approach to identify factors favoring the survival of autologous satellite cells and, thus, muscle regeneration. In a rat model of full-thickness rectus abdominis muscle ablation, autologous myoblasts were isolated from the explanted rectus abdominis and seeded in a homologous acellular matrix immediately after wall reconstruction (group 1, five animals). In group 2 (five animals), the ablated rectus abdominis was autografted in situ. In a third group of five rats, Marcaine was injected into both the autograft and the surrounding abdominal wall muscle. Three weeks after surgery, serial cross-sections of the reconstructed abdominal wall were stained with hematoxylin and eosin or embryonic myosin antibody, a well-characterized molecular marker of early myogenesis in development and regeneration. Percentages of the patch area covered by regenerated myofibers were determined by morphometry. When autologous myoblasts were seeded in a homologous acellular matrix, the only myofibers observed to regenerate were those along the border of the patch. Autografting of the middle third of the rectus abdominis muscle similarly resulted in scar formation. The few muscle cells in the graft core were scanty myoblasts that could be detected only by monoclonal embryonic myosin antibody. Although negative for myofiber regeneration, the results in both cases confirmed the mechanical patency of the patches with regard to abdominal organ support. Myofibers were successfully regenerated in the graft by injecting Marcaine into both the autograft and the surrounding muscles. Three weeks after surgery, the patch was paved with young, centrally nucleated myofibers intermixed with young myofibers and myotubes expressing embryonic myosin. The difference in percentage of patch area covered by regenerated myofibers in group 3 (Marcaine injection around the patch, 81.6 +/- 3.0 percent) (mean +/- SD) versus either group 1 (Myoblast-seeded acellular patch, 18.0 +/- 3.0 percent) or group 2 (Autograft, 25.8 +/- 7.0 percent) was statistically significant on independent t test analysis (p < 0.0001). Even an acellular matrix showed some myofiber regeneration after surrounding muscles had been injected with Marcaine. This is the first successful evidence of muscle reconstruction after full-thickness ablation of the middle third of the rectus abdominis. Muscle regeneration seems to be the result of successive waves of migration of angioblasts and then satellite cell-derived myoblasts from the muscles surrounding the patch. The results strongly suggest that vascularization of the scaffold and successive coordinate proliferation of the seeded cells are required for myoblasts to be able to migrate into the patch and differentiate up to myofiber stage.

Targeted expression of insulin-like growth factor-i reduces early myofiber necrosis in dystrophic mdx mice

Necrosis of dystrophic myofibers in Duchenne muscular dystrophy and mdx mice results from defects in the subsarcolemmal protein dystrophin that cause membrane fragility and tears in the sarcolemma, and these lead to the destruction of the myofibers. The present study specifically tests whether overexpression of mIGF-1 in mdx/mIGF-1 transgenic mice reduces myofiber breakdown during the acute onset phase of dystrophy (at 21 days). The extent of muscle damage and Evans blue dye (EBD) staining of myofibers was quantitated histologically for mdx/mIGF-1 and their mdx littermates from 15 to 30 days of age. Overexpression of mIGF-1 strikingly reduced the extent of myofiber damage (histology and EBD staining) by up to 97% in tibialis anterior and quadriceps muscles at 21–22 days after birth. In the mdx diaphragm, the onset of muscle breakdown was earlier (by 15 days after birth) but no significant protective effect of IGF-1 was apparent within the first month of age in mdx/IGF-1 mice. These novel observations show that increased mIGF-1 within mdx myofibers specifically reduces the breakdown of dystrophic muscle during the acute onset of muscle degeneration. This mechanism of action can account for the long-term reduced severity of the dystropathology in mdx mice that overexpress mIGF-1 and provides promising opportunities for therapeutic strategies.

Effects of Range of Motion on Skeletal Muscle Morphology due to Stretch-Shortening Cycle-Induced Injury

0055 Quantification of skeletal muscle damage in response to injurious stretch-shortening cycles (SSC's) would be beneficial in elucidating the effect of different biomechanical exposures on the amount of muscle damage. Purpose: To investigate the effect of stretch-shortening cycle range of motion (ROM) on skeletal muscle damage in rats. Methods: Testing was performed on the dorsiflexor muscles of Sprague-Dawley rats in vivo. Animals (n = 36) were randomly assigned to a long ROM injury group (L-Inj), short ROM injury group (S-Inj), or isometric group (Iso). The injury protocol consisted of 7sets of 10 SSC's at 500°/s. The S-Inj group received SSC's between 70°-120° ankle angle, whereas the L-Inj group received SSC's between 90°-140° ankle angle. The Iso group received equivalent muscle stimulation at 90° ankle angle. Dorsiflexor muscles were stimulated for 2.8 s each set and administered every minute. Rats were sacrificed at 6 and 48 hours post exposure. Following sacrifice, tissue was excised, weighed, sectioned, quick-frozen, and stored at −80°C. Transverse sections (12 μm) were cut, mounted on pre-coated microscope slides, and stained using a routine procedure with Harris Hematoxylin & Eosin. Tissue sections were evaluated on a Leica DMLB microscope. Stereology was used to quantify the degree of myofiber damage in muscle from each group; and, also was used to measure the volume fraction, surface densities, and average thickness of normal myofibers, degenerative myofibers, and the interstitial space. Results: No degenerative fibers were present in the contra-lateral control tissue, Iso group at 6h or 48h post-injury, nor the S-inj and L-inj at 6h post injury. In contrast, there was an increased volume of degenerative myofibers in the S-inj and L-inj at 48h post injury. The volume of cellular interstitium (cell infiltrates) and non-cellular interstitium (edema) increased (p = 0.0082 and p = 0.0053; respectively) from muscle exposed to the S-inj and L-inj at 48h post injury; and, thus the average thickness of the extracellular matrix increased with these groups. Conclusions: Increased degenerative myofibers, cellular infiltrates, and edema occur temporally in response to both S-inj and L-inj. However, a more dramatic increase occurs with the L-inj at 48h as assessed by the increase in volume of cellular and non-cellular interstitium.

Effects of Repetitive Stretch-Shortening Cycles on Muscle Morphology in Rats

0056 Quantification of skeletal muscle damage after exposure to injurious stretch-shortening cycles (SSC's) would be beneficial to elucidate the temporal muscle response after contraction-induced injury. Purpose: To investigate the effect of injurious SSC exposure on the temporal myofiber response Methods: Testing was performed on the dorsiflexor muscles of Sprague-Dawley rats in vivo. Briefly, rats were anesthetized with isoflurane and exposed to 15 sets of 10 SSC's. Control rats were exposed to 15 isometric contractions of equal intensity. Changes in muscle morphometry were assessed at 0.5, 24, 48, 72, and 240 hours post exposure. The tibialis anterior muscle tissue was excised and weighed, and the midbelly region was cut from the muscle and mounted on cork, immersed in OCT, frozen in isopentane cooled with liquid nitrogen, and stored at −80°C. Next, transverse sections (12 μm) were cut, mounted on precoated microscope slides, air dried, and stained using a routine procedure with Harris Hematoxlin & Eosin. Tissue sections were evaluated on a Leica DMLB microscope. Stereology was used to quantify the degree of myofiber damage in muscle from each group; and, also was used to measure the volume fraction, surface densities and average thickness of normal myofibrils, degenerative myofibers and the interstitial space. Results: Analyses of degeneration indicated that almost no degenerative fibers were present in muscle collected from animals in the isometric-control groups. In contrast, there was an increase in the volume and thickness of degenerating myofibers over time in the muscle collected from rats exposed to the SSC's (p < 0.0001). Further analyses revealed that the thickness and volume of degenerating fibers from muscle exposed to SSC's was significantly increased above muscle exposed to isometric contractions at 24, 48 and 72 h post exposure (p = 0.003). There were no degenerative myofibers seen in muscle at 240 h post exposure. Conclusion: Stereological analysis indicated that muscles exposed to damaging SSC's showed significant myofiber disruption that was not resolved until 240 hours post exposure, while muscles exposed to isometric contractions did not exhibit any degenerative myofibers.

Effects of Range of Motion on Skeletal Muscle Morphology due to Stretch-Shortening Cycle-Induced Injury

0055 Quantification of skeletal muscle damage in response to injurious stretch-shortening cycles (SSC's) would be beneficial in elucidating the effect of different biomechanical exposures on the amount of muscle damage. Purpose: To investigate the effect of stretch-shortening cycle range of motion (ROM) on skeletal muscle damage in rats. Methods: Testing was performed on the dorsiflexor muscles of Sprague-Dawley rats in vivo. Animals (n = 36) were randomly assigned to a long ROM injury group (L-Inj), short ROM injury group (S-Inj), or isometric group (Iso). The injury protocol consisted of 7sets of 10 SSC's at 500°/s. The S-Inj group received SSC's between 70°-120° ankle angle, whereas the L-Inj group received SSC's between 90°-140° ankle angle. The Iso group received equivalent muscle stimulation at 90° ankle angle. Dorsiflexor muscles were stimulated for 2.8 s each set and administered every minute. Rats were sacrificed at 6 and 48 hours post exposure. Following sacrifice, tissue was excised, weighed, sectioned, quick-frozen, and stored at −80°C. Transverse sections (12 μm) were cut, mounted on pre-coated microscope slides, and stained using a routine procedure with Harris Hematoxylin & Eosin. Tissue sections were evaluated on a Leica DMLB microscope. Stereology was used to quantify the degree of myofiber damage in muscle from each group; and, also was used to measure the volume fraction, surface densities, and average thickness of normal myofibers, degenerative myofibers, and the interstitial space. Results: No degenerative fibers were present in the contra-lateral control tissue, Iso group at 6h or 48h post-injury, nor the S-inj and L-inj at 6h post injury. In contrast, there was an increased volume of degenerative myofibers in the S-inj and L-inj at 48h post injury. The volume of cellular interstitium (cell infiltrates) and non-cellular interstitium (edema) increased (p = 0.0082 and p = 0.0053; respectively) from muscle exposed to the S-inj and L-inj at 48h post injury; and, thus the average thickness of the extracellular matrix increased with these groups. Conclusions: Increased degenerative myofibers, cellular infiltrates, and edema occur temporally in response to both S-inj and L-inj. However, a more dramatic increase occurs with the L-inj at 48h as assessed by the increase in volume of cellular and non-cellular interstitium.

M1/70 attenuates blood-borne neutrophil oxidants, activation, and myofiber damage following stretch injury

The purpose of this study was to determine the role of the CD11b-dependent respiratory burst in neutrophil oxidant generation and activation, interleukin-8 (IL-8) production, and myofiber damage after muscle stretch injury by using the monoclonal antibody M1/70 to block this pathway. Twelve male New Zealand White rabbits were randomly assigned to a treatment group: M1/70 (n = 6), IgG isotype control (n = 3), or saline control (n = 3). After intravenous injection of the assigned agent under gas anesthesia, a standardized single-stretch injury was created in the right tibialis anterior, whereas the left tibialis anterior underwent a sham surgery. Blood-borne neutrophil oxidant generation and CD11b receptor density and plasma IL-8 levels were measured pre- and 24 h postinjury. Damage was assessed histologically at the hematoma site by counting torn myofibers. M1/70 group demonstrated decreased blood-borne neutrophil oxidant generation (P < 0.05) and CD11b receptor density (P < 0.05), an increase in plasma IL-8 concentration (P < 0.01), and less torn myofibers (P < 0.01) compared with IgG isotype or saline control groups. These data indicate that 1). CD11b-dependent respiratory burst is a major source of oxidants produced by the neutrophil, and that treatment with M1/70 2). attenuates neutrophil activation status, 3). increases plasma IL-8 concentration, and 4). minimizes myofiber damage 24 h postmuscle stretch injury.

NEUTROPHILS AND MYOFIBER DAMAGE FOLLOWING STRETCH INJURY

Muscle stretch injury is characterized by immediate fiber disruption and hemorrhage at the muscle-tendon junction followed by a cascade of events leading to further myofiber damage and contractile deficits 24h post-injury. PURPOSE To investigate the role of neutrophils in myofiber damage by using the monoclonal antibody M1/70 to block the CD11b β2 integrin receptor. METHODS 12 male New Zealand white rabbits were randomly assigned to one of three treatment groups: M1/70 (M), n=6; IgG isotype control (I), n=3; saline control (S), n=3. Following injection of the appropriate agent, a standardized single stretch injury was created on the tibialis anterior (TA) muscle while the contralateral limb underwent a sham surgery. Animals were sacrificed 24h post-injury and the hematoma in the injured TA and corresponding site in the uninjured TA were evaluated for damage using the following methods: presence of torn fibers and loss of desmin and/or dystrophin. Infiltrating inflammatory cells (CD11b+ and RPN3/57+) were also quantified. ANOVA was used to compare the differences among means of torn fibers and injured minus uninjured cellular concentrations for the treatment groups (P < 0.05). RESULTS Group M demonstrated less torn fibers 24h post-injury (P < 0.01). Desmin and dystrophin ghost fibers were present in all injured TA muscles from groups I and S, while ghost fibers existed in only half the injured TA muscles from group M. Ghost fibers were always surrounded or infiltrated by inflammatory cells. Group M has less CD11b inflammatory cells (P < 0.05), which appeared to be mostly RPN3/57+ neutrophils (P < 0.01). CONCLUSION Neutrophils appear to mediate myofiber damage following stretch injury through the CD11b-dependent pathway and may target the cytoskeletal proteins desmin and dystrophin.

Inspiratory loading does not accelerate dystrophy inmdxmouse diaphragm: implications for regenerative therapy

Since the finding that the mdx mouse diaphragm, in contrast to limb muscles, undergoes progressive degeneration analogous to that seen in Duchenne muscular dystrophy, the relationship between the workload on a muscle and the pathogenesis of dystrophy has remained controversial. We increased the work performed by the mdx mouse diaphragm in vivo by tracheal banding and evaluated the progression of dystrophic changes in that muscle. Despite the establishment of dramatically increased respiratory workload and accelerated myofiber damage documented by Evans blue dye, no change in the pace of progression of dystrophy was seen in banded animals vs. unbanded, sham-operated controls. At the completion of the study, more centrally nucleated fibers were evident in the diaphragms of banded mdx mice than in sham-operated mdx controls, indicating that myofiber regeneration increases to meet the demands of the work-induced damage. These data suggest that there is untapped regenerative capacity in dystrophin-deficient muscle and validates experimental efforts aimed at augmenting regeneration within skeletal muscle as a therapeutic strategy in the treatment of dystrophinopathies.

THE EARLY PHASE OF CHANGES IN DYSTROPHIN AFTER LENGTHENING CONTRACTIONS IN SKELETAL MUSCLE

Exercise-induced myofiber damage is rather a consequence of the mechanical stress than the number of contractions. The mechanical stress during lengthening contractions (LC) is mainly concentrated on the intermediate filament system, dystrophin, and specialized plasma membrane complexes. Disturbances in these structures might in turn render muscle fibers more susceptible to necrotic changes. PURPOSE: The present study was designed to evaluate the early phase of changes in proteins of the contractile system and sarcolemma after LC in rat skeletal muscle. METHODS: Tibialis anterior (TA) muscles of anesthetized rats were subjected to 240 LC. The contra lateral leg was used as a non-exercised control. Muscle samples were taken 0 h and 4 d post-exercise. Muscle β-glucuronidase activity (β-GU, a reliable indicator of muscle damage) was determined and antibodies against actin, dystrophin (C-terminus and rod domain), and β-dystroglycan were used for double immunohistochemical stainigs and were studied with confocal microscopy. RESULTS: The prominent increase in β-GU as well as a great number of necrotic fibers were observed at 4 d. Immediately after LC swollen fibers and dissappearence of actin staining with discontinuous staining of dystrophin c-terminus and fragmental β-dystroglycan staining was observed while in serial sections no changes in dystrophin rod domain were detected. CONCLUSION: These observations indicate the segmental sensitiveness of dystrophin molecule against physical stress and disturbed structural intergity in all studied proteins at the early phase after the LC. Supported by LIKES-Foundation and The Finnish Ministry of Education.

Consequences of skeletal muscle injury induced by unaccustomed exercise.

Unaccustomed exercise is exercise that has never been performed before or has not been performed previously at a certain intensity. Unaccustomed exercise commonly leads to an acute skeletal muscle injury. Several negative consequences develop as a result of this injury and can interfere with daily and recreational physical activities. This article describes what the consequences are, when they occur and recover, and what might cause them to occur. Also, the article discusses the nursing implications of these consequences for healthy young and older individuals and for those individuals who have disuse muscle atrophy.

Suicidal ideation among outpatients at general neurology clinics: prospective study

<h3>Abstract</h3> Specialized pro-resolving mediators (SPMs) actively limit inflammation and expedite its resolution. Here we profiled intramuscular lipid mediators following injury and investigated the role of SPMs in skeletal muscle inflammation...

Controlled lengthening or shortening contraction-induced damage is followed by fiber hypertrophy in rat skeletal muscle.

To study the hypothesis that more severe damage, caused by controlled lengthening (L) contractions, results in greater myofiber hypertrophy compared to increase in fiber size followed shortening (S) contractions, tibialis anterior muscles of anesthesized male Wistar rats were subjected to 240 either L or S contractions. The highest increase in muscle beta-glucuronidase activity, an indicator of muscle damage, was observed in L (7.1-fold) 4 days and in S (2.6-fold) 8 days postexercise. Dystrophin- and desmin-negative as well as fibronectin-positive fibers (signs of the early phase of damage) were observed immediately after exercise in the L group. At 4 days, massive myofiber injury was visible, and internally localized nuclei were present at 15-80 days after exercise in the L group. The shift towards more glycolytic fiber types (p<0.05 in L and S) and an increased mean cross-sectional area of type IIX/B fibers (p < 0.001 in L and S) at 80 days were observed in both groups. The observed minor damage with unchanged myofiber structures following S induced, however, an increase in myofiber cross-sectional area of nearly the same magnitude as that following L, which was more damaging. The results do not support the hypothesis that fiber hypertrophy depends on the extent of the myofiber damage upon the exercised muscles.

Disruption of thelama2Gene in Embryonic Stem Cells: Laminin α2 Is Necessary for Sustenance of Mature Muscle Cells

Mutations in the gene coding for the α2 chain of laminin-2 and -4 (merosin) cause a severe form of congenital muscular dystrophy in humans and mice. To establish a defined model forin vitroandin vivostudies of the role of laminin α2/merosin in development and cell and tissue function, we generated several lines of mutant embryonic stem (ES) cell with disruption of the laminin α2 chain gene. We find that homozygous mutant ES cells differentiate normallyin vitro,giving rise to cardiomyocytes, myotubes, and smooth muscle cells in addition to many other cell types. However, the myotubes that are formed are unstable. They detach, collapse, and degenerate, a process which is initiated at the appearance of the mature, contractile phenotype of the cells. We propose that the detachment and death of contracting myotubesin vitrohas its counterpartin vivoand that contraction-induced myofiber damage, along with the lack of survival cues provided by laminin α2/merosin, is a significant contribution to muscle degeneration in merosin-deficient muscular dystrophy.

Corticosteroid therapy does not alter the threshold for contraction-induced injury in dystrophic (MDX) mouse diaphragm

The effects of methylprednisolone therapy on the susceptibility of dystrophin-deficient myofibers to contraction-induced injury were evaluated in the mdx mouse diaphragm model of Duchenne dystrophy. Mdx myofibers were abnormally vulnerable to injury induced by high-stress eccentric contractions. However, methylprednisolone therapy did not significantly alter the degree of contraction-induced injury. These data suggest that beneficial effects of corticosteroid therapy in Duchenne dystrophy are unlikely to be related to a change in the threshold for contraction-induced myofiber damage.

Corticosteroid therapy does not alter the threshold for contraction-induced injury in dystrophic (mdx) mouse diaphragm.

The effects of methylprednisolone therapy on the susceptibility of dystrophin-deficient myofibers to contraction-induced injury were evaluated in the mdx mouse diaphragm model of Duchenne dystrophy. Mdx myofibers were abnormally vulnerable to injury induced by high-stress eccentric contractions. However, methylprednisolone therapy did not significantly alter the degree of contraction-induced injury. These data suggest that beneficial effects of corticosteroid therapy in Duchenne dystrophy are unlikely to be related to a change in the threshold for contraction-induced myofiber damage.