Major alteration of the pathological phenotype in gamma irradiated mdx soleus muscles

Dystrophic changes in extraocular muscles after gamma irradiation in mdx:utrophin(+/-) mice

Extraocular muscles (EOM) have a strikingly different disease profile than limb skeletal muscles. It has long been known that they are spared in Duchenne (DMD) and other forms of muscular dystrophy. Despite many studies, the cause for this sparing is not understood. We have proposed that differences in myogenic precursor cell properties in EOM maintain normal morphology over the lifetime of individuals with DMD due to either greater proliferative potential or greater resistance to injury. This hypothesis was tested by exposing wild type and mdx:utrophin(+/-) (het) mouse EOM and limb skeletal muscles to 18 Gy gamma irradiation, a dose known to inhibit satellite cell proliferation in limb muscles. As expected, over time het limb skeletal muscles displayed reduced central nucleation mirrored by a reduction in Pax7-positive cells, demonstrating a significant loss in regenerative potential. In contrast, in the first month post-irradiation in the het EOM, myofiber cross-sectional areas first decreased, then increased, but ultimately returned to normal compared to non-irradiated het EOM. Central nucleation significantly increased in the first post-irradiation month, resembling the dystrophic limb phenotype. This correlated with decreased EECD34 stem cells and a concomitant increase and subsequent return to normalcy of both Pax7 and Pitx2-positive cell density. By two months, normal het EOM morphology returned. It appears that irradiation disrupts the normal method of EOM remodeling, which react paradoxically to produce increased numbers of myogenic precursor cells. This suggests that the EOM contain myogenic precursor cell types resistant to 18 Gy gamma irradiation, allowing return to normal morphology 2 months post-irradiation. This supports our hypothesis that ongoing proliferation of specialized regenerative populations in the het EOM actively maintains normal EOM morphology in DMD. Ongoing studies are working to define the differences in the myogenic precursor cells in EOM as well as the cellular milieu in which they reside.

Donor satellite cell engraftment is significantly augmented when the host niche is preserved and endogenous satellite cells are incapacitated

Stem cell transplantation is already in clinical practice for certain genetic diseases and is a promising therapy for dystrophic muscle. We used the mdx mouse model of Duchenne muscular dystrophy to investigate the effect of the host satellite cell niche on the contribution of donor muscle stem cells (satellite cells) to muscle regeneration. We found that incapacitation of the host satellite cells and preservation of the muscle niche promote donor satellite cell contribution to muscle regeneration and functional reconstitution of the satellite cell compartment. But, if the host niche is not promptly refilled, or is filled by competent host satellite cells, it becomes nonfunctional and donor engraftment is negligible. Application of this regimen to aged host muscles also promotes efficient regeneration from aged donor satellite cells. In contrast, if the niche is destroyed, yet host satellite cells remain proliferation-competent, donor-derived engraftment is trivial. Thus preservation of the satellite cell niche, concomitant with functional impairment of the majority of satellite cells within dystrophic human muscles, may improve the efficiency of stem cell therapy.

Cellular and molecular regulation of muscle regeneration

Under normal circumstances, mammalian adult skeletal muscle is a stable tissue with very little turnover of nuclei. However, upon injury, skeletal muscle has the remarkable ability to initiate a rapid and extensive repair process preventing the loss of muscle mass. Skeletal muscle repair is a highly synchronized process involving the activation of various cellular responses. The initial phase of muscle repair is characterized by necrosis of the damaged tissue and activation of an inflammatory response. This phase is rapidly followed by activation of myogenic cells to proliferate, differentiate, and fuse leading to new myofiber formation and reconstitution of a functional contractile apparatus. Activation of adult muscle satellite cells is a key element in this process. Muscle satellite cell activation resembles embryonic myogenesis in several ways including the de novo induction of the myogenic regulatory factors. Signaling factors released during the regenerating process have been identified, but their functions remain to be fully defined. In addition, recent evidence supports the possible contribution of adult stem cells in the muscle regeneration process. In particular, bone marrow-derived and muscle-derived stem cells contribute to new myofiber formation and to the satellite cell pool after injury.

A-utrophin up-regulation in mdx skeletal muscle is independent of regeneration

Duchenne muscular dystrophy is a fatal childhood disease caused by mutations that abolish the expression of dystrophin in muscle. Utrophin is a paralogue of dystrophin and can functionally replace it in skeletal muscle. A method to induce utrophin up-regulation in muscle should therefore be therapeutically useful in Duchenne muscular dystrophy. The search for such a method needs to be informed by an understanding of the mechanisms controlling utrophin expression in muscle. Two full length utrophin isoforms are expressed: A and B. A-utrophin is up-regulated in dystrophin deficient skeletal muscle and we sought to test the hypothesis that this up-regulation occurs as a consequence of ongoing regeneration. We measured utrophin expression by immunohistochemistry and immunoblotting in the oesophageal outer muscular layer and in gamma-irradiated limb muscle from mdx mice. Skeletal muscle in these tissues is dystrophin deficient but not regenerating; we found that A-utrophin up-regulation still occurred. We conclude that utrophin up-regulation in skeletal muscle does not depend on regeneration. An alternative hypothesis involving competition for binding sites between utrophin and dystrophin is discussed. These results have important implications for future studies aiming to effect therapeutic utrophin up-regulation in Duchenne muscular dystrophy patients.

Irradiation of dystrophic host tissue prior to myoblast transfer therapy enhances initial (but not long-term) survival of donor myoblasts

There is a massive and rapid death of donor myoblasts (<20% surviving) within hours after intramuscular injection in myoblast transfer therapy (MTT), due to host immune cells, especially natural killer (NK) cells. To investigate the role of host immune cells in the dramatic death of donor myoblasts, MTT experiments were performed in irradiated host mice. Cultured normal C57BL/10ScSn male donor myoblasts were injected into muscles of female C57BL/10ScSn-Dmdmdx host mice after one of three treatments: whole body irradiation (WBI) to eliminate all circulating leukocytes, WBI and bone marrow reconstitution (BMR), or local irradiation (or protection) of one limb. Similar experiments were performed in host mice after antibody depletion of NK cells. Numbers of male donor myoblasts were quantified using a Y-chromosome-specific (male) probe following total DNA extraction of injected muscles. WBI prior to MTT resulted in dramatically enhanced survival (approximately 80%) of donor myoblasts at 1 hour after MTT, supporting a central role for host inflammatory cells in the initial death of donor myoblasts seen in untreated host mice. BMR restored the massive and rapid loss (approximately 25% surviving) of donor myoblasts at 1 hour after MTT. Local pre-irradiation also resulted in increased donor myoblast numbers (approximately 35-40%) compared with untreated controls (approximately 10%) at 3 weeks after MTT. Preirradiation of host muscle with 10 Gy did not significantly stimulate proliferation of the injected donor myoblasts. Serum protein levels of TNFalpha, IL-1beta, IL-6 and IL-12 fluctuated following irradiation treatments. These combined results strongly reinforce a major role for host immune cells in the rapid death of injected cultured donor myoblasts.

Stem cell route to neuromuscular therapies

As applied to skeletal muscle, stem cell therapy is a reincarnation of myoblast transfer therapy that has resulted from recent advances in the cell biology of skeletal muscle. Both strategies envisage the reconstruction of damaged muscle from its precursors, but stem cell therapy employs precursors that are earlier in the developmental hierarchy. It is founded on demonstrations of apparently multipotential cells in a wide variety of tissues that can assume, among others, a myogenic phenotype. The main demonstrated advantage of such cells is that they are capable of colonizing many tissues, including skeletal and cardiac muscle via the blood vascular system, thereby providing the potential for a body-wide distribution of myogenic progenitors. From a practical viewpoint, the chief disadvantage is that such colonization has been many orders of magnitude too inefficient to be useful. Proposals for overcoming this drawback are the subject of much speculation but, so far, relatively little experimentation. This review attempts to give some perspective to the status of the stem cell as a therapeutic instrument for neuromuscular disease and to identify issues that need to be addressed for application of this technology.

Myoblast transplantation

Myoblast transplantation was the first quasi-gene therapy to be suggested for Duchenne muscular dystrophy. Animal experiments established the principles that the missing gene could be targeted to muscle by grafting of genetically normal myoblasts that were able to repair the disease-damaged muscle fibres. In the recipient muscle the gene was expressed and the resultant protein provided some functional benefit in protecting the fibres against necrosis. However, these effects were limited to a small region around the injection site and there was some evidence of immunological problems. Human trials provided little evidence of effectiveness probably, in part due to immune rejection, and in part to the inadequacy of the cells implanted. Most work since this time has been directed at preventing immune rejection, improving dispersion of the injected cells, and selecting more 'stem cell-like' myogenic cells which might be more effective at reconstituting large regions of muscle. Most recently, a number of sources of 'stem cell' with myogenic potential have been described, some of which have been found to be dispersed via the blood vascular system but none of which have been very efficient at generating new muscle.

Transplanted primary neonatal myoblasts can give rise to functional satellite cells as identified using the Myf5nlacZl+ mouse

Myoblast transplantation is a potential therapeutic approach for the genetic modification of host skeletal muscle tissue. To be considered an effective, long-lived method of delivery, however, it is essential that at least a proportion of the transplanted cells also retain their proliferative potential. We sought to investigate whether transplanted neonatal myoblasts can contribute to the satellite cell compartment of adult skeletal muscle by using the Myf5nlacZ/+ mouse. The Myf5nlacZ/+ mouse has nlacZ targeted to the Myf5 locus resulting in beta-galactosidase activity in quiescent satellite cells. Following transplantation, beta-galactosidase-labelled nuclei were detected in host muscles, showing that donor cells had been incorporated. Significantly, beta-galactosidase-positive, and therefore donor-derived, satellite cells were detected. When placed in culture, beta-galactosidase marked myogenic cells emanated from the parent fibre. These observations demonstrate that cell transplantation not only results in the incorporation of donor nuclei into the host muscle syncytia, but also that the donor cells can become functional satellite cells. The Myf5nlacZ/+ mouse therefore provides a novel and specific marker for determining the contribution of transplanted cells to the satellite cell pool.

Dermal fibroblasts participate in the formation of new muscle fibres when implanted into regenerating normal mouse muscle

Both in vitro and in vivo studies have described the conversion of fibroblasts to myogenesis when in the presence of dysfunctional myogenic cells. Myogenic conversion of fibroblasts subjected to a normal, as opposed to a diseased muscle environment has only been reported in vitro. The primary aim of this work was to determine if fibroblasts can convert to a myogenic lineage and contribute to new fibre formation when implanted into the regenerating muscle of a normal mouse. Dermal fibroblasts were prepared from neonatal mouse skin and labelled prior to implantation with the fluorescent nuclear marker 4',6-diamidino-2-phenylindole (DAPI). Cells were implanted into muscles of host mice that had been subjected to either cold/crush or minced muscle injury. Some host muscles were x-irradiated to deplete the muscle of endogenous muscle precursor cells. Muscles were removed at 3 wk postimplantation and analysed both histologically and for the presence of DAPI labelled nuclei. Fibres containing DAPI labelled central nuclei indicated that the implanted cells had participated in the regenerative process. Mouse dermal fibroblasts therefore do contribute to muscle fibre formation in regenerating normal mouse muscle but the extent of their contribution is dependent on the nature of the trauma induced in the host muscle. The study also showed that regeneration was more successful in muscles which had not been irradiated, which is contrary to the previous studies where dermal fibroblasts were introduced into myopathic mouse muscle.

Thymic myoid cells as a source of cells for myoblast transfer

Transplantation of disaggregated myoblasts from normal donor to the muscles of a diseased host, or reimplantation of genetically modified host myoblasts, has been suggested as a possible route to therapy for inherited myopathies such as Duchenne muscular dystrophy, or to supply missing proteins that are required systemically in diseases such as hemophilia. With two exceptions, studies of myoblast transfer in the mouse have involved transplantation of donor myoblasts isolated from adult or neonatal skeletal muscle satellite cells. In this study we present evidence that thymic myoid cells are capable of participating in the regeneration of postnatal skeletal muscle, resulting in the expression of donor-derived proteins such as dystrophin and retrovirally encoded proteins such as beta-galactosidase within host muscles. This leads us to conclude that thymic myoid cells may provide an alternative to myoblasts derived from skeletal muscle as a source of myogenic cells for myoblast transfer.

Evidence for a myogenic stem cell that is exhausted in dystrophic muscle

Injection of the myotoxin notexin, was found to induce regeneration in muscles that had been subjected to 18 Gy of radiation. This finding was unexpected as irradiation doses of this magnitude are known to block regeneration in dystrophic (mdx) mouse muscle. To investigate this phenomenon further we subjected mdx and normal (C57Bl/10) muscle to irradiation and notexin treatment and analysed them in two ways. First by counting the number of newly regenerated myofibres expressing developmental myosin in cryosections of damaged muscles. Second, by isolating single myofibres from treated muscles and counting the number of muscle precursor cells issuing from these over 2 day and 5 day periods. After irradiation neither normal nor dystrophic muscles regenerate to any significant extent. Moreover, single myofibres cultured from such muscles produce very few muscle precursor cells and these undergo little or no proliferation. However, when irradiated normal and mdx muscles were subsequently treated with notexin, regeneration was observed. In addition, some of the single myofibres produced rapidly proliferative muscle precursor cells when cultured. This occurred more frequently, and the myogenic cells proliferated more extensively, with fibres cultured from normal compared with dystrophic muscles. Even after 25 Gy, notexin induced some regeneration but no proliferative myogenic cells remained associated with the muscle fibres. Thus, skeletal muscles contain a number of functionally distinct populations of myogenic cells. Most are radiation sensitive. However, some survive 18 Gy as proliferative myogenic cells that can be evoked by extreme conditions of muscle damage; this population is markedly diminished in muscles of the mdx mouse. A small third population survives 25 Gy and forms muscle but not proliferative myogenic cells.

The use of adeno-associated virus to circumvent the maturation-dependent viral transduction of muscle fibers

Muscle-based gene therapy using adenovirus, retrovirus, and herpes simplex virus has been hindered by viral cytotoxicity, host immune response, and the maturation-dependent viral transduction of muscle fibers. The development of new mutant vectors has greatly reduced the toxicity and the immune rejection problems, but the inability of viral vectors to penetrate and transduce mature myofibers remains an important issue. Research has been focused on the characterization of barriers to viral transduction in mature myofibers to develop strategies to circumvent the maturation-dependent viral transduction of myofibers. Here, we report that adeno-associated virus (AAV) can be used to overcome the maturation-dependent viral transduction of myofibers. We have investigated by which mechanism AAV can penetrate and efficiently transduce mature muscle fibers, and have shown that this viral vector is not blocked by the basal lamina and that AAV transduction of myofibers is independent of myoblast mediation. Although AAV can efficiently transduce mature myofibers, a differential transduction is still observed among the different types of myofibers that correlates with the expression of the heparan sulfate proteoglycan receptors, the muscle maturity, the number of viral particles used, and the time postinjection. The identification of the mechanisms by which AAV transduces mature myofibers will help in the development of strategies to achieve an efficient muscle-based gene therapy for inherited and acquired diseases.

Extended tropism of an adenoviral vector does not circumvent the maturation-dependent transducibility of mouse skeletal muscle

BACKGROUND

Efficient adenoviral gene delivery to mature skeletal muscle has been hindered by different factors. The low levels of adenoviral attachment receptor (CAR) that have been reported in this tissue may be a limiting factor. Therefore, adenoviral transduction of mature muscle may be improved by extending the tropism of the adenoviral vectors to attachment receptors that are highly expressed in mature myofibers. In this study, we have investigated whether an extended tropism adenoviral vector which additionally attaches to the broadly expressed heparan-containing receptors (AdPK) can bypass the maturation-dependent adenoviral transducibility of mouse skeletal muscle.

METHODS

The adenoviral vector AdPK carrying the LacZ gene was evaluated as a gene delivery vehicle in mouse skeletal muscle at different maturities in vitro and in vivo. The viral transduction efficiencies were determined by histochemical and ONPG analysis of the beta-galactosidase activity level.

RESULTS

Higher transduction efficiencies were detected in immature muscle from normal mice, and in mature muscle from merosin-deficient dy/dy mice (carrying myofibers with an impaired extracellular matrix) and dystrophin-deficient mdx mice (showing a high level of myoblast activity) when compared to mature muscle from normal mice.

CONCLUSION

Despite the enhanced attachment characteristics, the extended tropism adenoviral vector is, similarly to the wild-type adenoviral vector in previous studies, still hindered by both a protective extracellular matrix and the diminished myoblast-mediation in mature muscle.

Patterns of repair of dystrophic mouse muscle: studies on isolated fibers

Repair of damaged skeletal muscle fibers by muscle precursor cells (MPC) is central to the regeneration that occurs after injury or disease of muscle and is vital to the success of myoblast transplantation to treat inherited myopathies. However, we lack a detailed knowledge of the mechanisms of this muscle repair. Here, we have used a novel combination of techniques to study this process, marking MPC with nuclear-localizing LacZ and tracing their contribution to regeneration of muscle fibers after grafting into preirradiated muscle of the mdx nu/nu mouse. In this model system, there is muscle degeneration, but little or no regeneration from endogenous MPC. Incorporation of donor MPC into injected muscles was analyzed by preparing single viable muscle fibers at various times after cell implantation. Fibers were either stained immediately for beta-gal, or cultured to allow their associated satellite cells to migrate from the fiber and then stained for beta-gal. Marked myonuclei were located in discrete segments of host muscle fibers and were not incorporated preferentially at the ends of the fibers. All branches on host fibers were also found to be composed of myonuclei carrying the beta-gal marker. There was no significant movement of donor myonuclei within myofibers for up to 7 weeks after MPC implantation. Although donor-derived dystrophin was usually located coincidentally with donor myonuclei, in some fibers, the dystrophin protein had spread further along the mosaic myofibers than had the myonuclei of donor origin. In addition to repairing segments of the host fiber, the implanted MPC also gave rise to satellite cells, which may contribute to future muscle repair.

Covert persistence of mdx mouse myopathy is revealed by acute and chronic effects of irradiation

To compare muscle fiber loss in young and old mdx mice, we have blocked regeneration in one leg with a high dose (18 Gy) of X-rays administered at two ages; 16 days, just prior to the onset of the myopathy, and 15 weeks, when the myopathy is considered to be quiescent. Mice were examined 4 days after irradiation to look for acute effects, or after 6 weeks to look for cumulative effects. Tibial length, muscle weight, muscle fiber size, fiber number and histological changes were recorded. Signs of acute damage to muscle fibers, leakage of Procion Orange dye into fibers and loss of creatine kinase from the fibers were also examined. Irradiation caused no acute or chronic damage to muscle fibers; on the contrary, in the youngest mdx mice, irradiation delayed the onset of the disease. However, in mdx but not in normal mice, there was a loss of muscle mass and fiber number in irradiated by comparison with the non-irradiated contra-lateral muscles. This loss, attributed to fiber necrosis in the absence of regeneration, was as great in animals irradiated at 15 weeks as in those irradiated at 16 days. Such persistence of muscle fiber necrosis contradicts the standard view of the mdx mouse and establishes it as a closer model of Duchenne muscular dystrophy than is generally appreciated.

Muscle precursor cells injected into irradiated mdx mouse muscle persist after serial injury

Muscle of donor origin was formed after implantation of H-2Kb-tsA58 muscle precursor cells (mpc) into irradiated mdx nu/nu mouse muscles. A series of injections of the myotoxin, notexin, which destroys mature muscle fibers but spares muscle precursor cells and other tissues, was made into the mpc-injected muscles, leaving time for regeneration to occur between each injection. New muscle fibers of donor origin were formed after up to four notexin treatments, providing evidence that some of the implanted mpc reentered an undifferentiated, quiescent, stem cell-like state and were capable of myogenesis after further injuries to the muscle. A similar model could be used to assay whether preparations of human mpc contain long-lasting precursor cells, prior to their implantation into patients. In control mdx muscles, which had been irradiated, injected with tissue culture medium, and given three notexin injections, regeneration also occurred, indicating that radiation-resistant mpc were present, presumably within the treated muscle.

Regeneration-blocked mdx muscle: in vivo model for testing treatments

We have refined the mdx mouse as a clinical model for Duchenne dystrophy. Our power estimates, primary measures, regular sacrifice intervals, and quality checks constitute a high-speed, low-cost system for preclinically testing therapies designed to slow muscle destruction in Duchenne dystrophy. Irradiated (18 Gy) and contralateral shielded anterior tibial muscles were studied in 21-day-old mdx and normal mice at the time of irradiation and 4, 8, 12, 16, and 20 weeks thereafter. Regeneration-blocked mdx (irradiated) muscle expressed muscular dystrophy as progressive wasting after a brief (4 week) period of growth. Regeneration-blocked normal muscle showed stunted growth but neither progressive wasting nor microscopic pathological changes.

Radiation inhibition of mdx mouse muscle regeneration: dose and age factors

A single hind limb was irradiated with 12, 18, 24, or 30 Gy in mdx and C57 mice aged 12, 21, or 42 days to determine regeneration inhibition dose-response curves in different aged dystrophic mice and to characterize radiation side-effects in normal mice. The anterior tibial muscle mass (8 weeks postirradiation) and percent central nucleated (i.e., regenerated) muscle fibers were measures of regeneration inhibition. Twenty-one-day-old mdx mice irradiated with 18 Gy had complete inhibition of muscle regeneration, but 30 Gy only partially blocked regeneration in mdx mice irradiated at 12 and 42 days in age. In working to produce a clinically relevant model for Duchenne dystrophy, it is crucial to regard mouse age as a major factor in determining radiation effects on mdx muscle regeneration.

Clenbuterol, a beta 2-agonist, retards wasting and loss of contractility in irradiated dystrophic mdx muscle

Treatment with the adrenergic beta 2-receptor agonist clenbuterol prevented, in dystrophic muscle from mdx mice, a pronounced loss of contractile strength that is observed after blockade of muscle regeneration with gamma irradiation. In addition, muscle mass and myosin content were greater (62-109%) in irradiated hindlimbs from clenbuterol-treated mdx mice, whereas the effects of the beta 2-agonist were relatively smaller (12-21%) in the nonirradiated hindlimbs. Together, these results suggest that beta 2-agonists can antagonize degenerative processes occurring in muscle fibers lacking dystrophin.

The potential for gene therapy in Duchenne muscular dystrophy and other genetic muscle diseases

Dystrophin cDNAs have been introduced into skeletal muscle fibers of dystrophin-deficient mice (mdx) through direct DNA injection in plasmid expression vectors and by replication-defective recombinant adenovirus vectors. The introduced genes appear to protect those muscle fibers from necrosis in which they become expressed. By direct injection of dystrophin cDNA in plasmid expression vector, only 1-2% of adult mdx muscle fibers of the injected muscle expressed dystrophin. On the other hand, by recombinant adenovirus injection into very young mdx muscle, a better efficiency has been reported. We have discussed several putative and proven factors that may contribute to the thus far demonstrated relatively low efficiency of dystrophin gene transfer. These include poor uptake of gene constructs by muscle fibers, degradation of the injected DNA, and poor access of gene constructs to the nuclear compartment. Neutralization or elimination of these factors could improve the efficiency of gene transfer so that it might, in the future, qualify as an effective therapy for DMD and some other genetic diseases of muscle.

Biological effects of high energy shock waves in mouse skeletal muscle: correlation between 31P magnetic resonance spectroscopic and microscopic alterations

To investigate the in vivo effects of electromagnetically generated high energy shock waves (HESW) on skeletal muscle, we used in vivo 31P nuclear magnetic resonance (NMR) spectroscopy (MRS) measurements and correlated the results with microscopical studies. Mouse skeletal muscle (calf muscle) was exposed to 200 or 800 HESW (Pmax: 37.5 MPa, Pmin: 5.2 MPa, tr: 30-120 ns, tw: 340 ns, frequency: 1.25 Hz). In the 31P MRS spectra, transient alterations were observed. A prominent increase of inorganic phosphate (Pi) peaks was found, as well as the appearance of Pi with different chemical shifts, reflecting the presence of different pH values (5.4-7.1) in cellular or tissue compartments. Within 20-96 h after exposure, pH values and Pi levels returned to normal. The changes were more pronounced in the animals treated with 800 HESW as compared to 200 HESW. Light and electron microscopy demonstrated focal degenerations of muscle fibers. This process consisted of disorganization of myofilaments and structural changes in sarcoplasmic organelles and was progressive in time. The (ultra)structural changes were not present in all myofibers (i.e., between affected degenerating fibers unaffected intact fibers were seen). Several ultrastructural abnormalities were also found in capillaries even up to severe dilatation and disruption, as well as in the peripheral nerves. The degeneration of the preexisting myofibers was predominantly confined to type 1 fibers and was followed by a regeneration of the muscle tissue by proliferation of myoblasts. A notable amount of myotubes still showed vacuolization. We conclude that in vivo HESW exposure of skeletal muscle tissue results in a degeneration of myofibers. The cellular effects are present in foci and associated with changes in the 31P NMR spectra. The NMR spectroscopy technique provides us with a noninvasive method to evaluate in a longitudinal way the biological effects of HESW.