Fine structure of prenatal and early postnatal dystrophic mouse muscle

Muscular transverse relaxation time measurement by magnetic resonance imaging at 4 Tesla in normal and dystrophic dy/dy and dy(2j)/dy(2j) mice

Muscular transverse relaxation times values were measured in vivo in normal mice (strain C57BL6/J, n=14) and in murine models of human congenital muscular dystrophy (dy/dy, n=9; dy(2j)/dy(2j), n=8). A single-slice multi-echo sequence was used. Gastrocnemius/soleus complex, thigh and buttock muscles were studied. Muscular transverse relaxation times values were compared between different muscle groups in each type of animal and between animal groups. Differences were observed between normal and dy(2j)/dy(2j) mice from 3 to 12 weeks of age, and between normal and dy/dy mice at 6 weeks. In specific age ranges, the values of muscular transverse relaxation times in two dystrophic models are different from those in normal mice, and could thus be used as an index of modifications in dystrophic muscle to evaluate therapies.

Age-related changes and tissue distribution of parvalbumin in normal and dystrophic mice of strain 129 ReJ

In murine muscular dystrophy, hindlimb muscle contains a functionally defective thiol protease inhibitor (TPI) which has been implicated in the onset and progression of the disease in mice. More recently, this protease inhibitor has been identified as parvalbumin, a calcium binding protein. In this study, a polyclonal antibody against mouse muscle parvalbumin was used to study the concentration and distribution of this protein in normal and dystrophic male mice at various ages. Immunodetection assays were used to screen extracts of hindlimb, forelimb, brain, heart, lung, liver, and kidney in 60-day-old normal and dystrophic male mice for parvalbumin content. Parvalbumin was detected in relatively high amounts in both hindlimb and forelimb muscle extracts, while much lower concentrations were detected in brains of normal and dystrophic animals. No parvalbumin was detected in the lung, liver, heart, or kidney extracts using the immunoassay. With aging, the parvalbumin concentration in hindlimb muscle of normal mice remained fairly constant for 90 days, whereupon the level increased at 120 days. In contrast, the parvalbumin concentration in hindlimb muscle of dystrophic mice decreased steadily with age to about 22%% of normal animals at 120 days. The parvalbumin content was also reduced in dystrophic brain.

Muscle fiber growth and necrosis in dystrophic muscles: a comparative study between dy and mdx mice

Histopathological and morphometric studies of murine dystrophic muscles in the early postnatal period were performed. In dy mice, obtained by in vitro fertilization, scattered necrotic fibers were noted at 10 days of age followed by poorly compensated regeneration leading to muscle fiber loss, marked variation in fiber size, and fibrous and adipose tissue proliferation. In mdx mice, clusters of necrotic muscle fibers seen at 10-15 days were followed by well compensated regeneration with little fibrosis. There appeared to be no delay in muscle fiber growth and fiber type differentiation in both dy and mdx mice up to 10 days of age since there was no distinct difference in muscle fiber size, fiber type differentiation and the incidence of myosatellite cells between dystrophic muscles and those from age-matched control mice. Muscle fibers appeared to undergo necrosis only when they had reached a certain stage of maturation since at early stages of development or regeneration they rarely became necrotic. The difference in clinical symptoms between dy and mdx mice may result from differences in their regenerative response to necrosis.

Alterations in muscle spindle morphology in advanced stages of murine muscular dystrophy

Muscle spindles in the soleus of 1-year-old dystrophic mice of the C57BL/6J dy2J/dy2J strain were studied by microscopic and morphometric methods, and comparisons were made with those in age-matched normal tissue. Transverse epon sections were cut through various regions of an individual receptor, and subsequent 90 degrees reorientation enabled longitudinal examination of the same spindle. In dystrophy, alterations were detected in the outer capsule and consisted of a significant increase in its overall thickness in equatorial regions. Perineurial proliferation accompanied histiocyte and collagen infiltration. Within the equator, intrafusal fibers and sensory terminals appeared unaffected by dystrophy. Alterations in the intrafusal fibers were restricted to polar zones where the mean diameters of chain and bag fibers were significantly reduced. Polar chain fibers exhibited a greater degree of atrophy in dystrophy with a 40% diminution in size. Ultrastructural changes in intrafusal fiber polar regions were less pronounced compared to the surrounding dystrophic muscle. Mitochondrial alterations in affected intrafusal fibers included intramatrix inclusions and glycogen deposition. Vacuolization of the sarcoplasmic reticulum and subsarcolemmal tubular aggregates were also observed in polar regions of dystrophic chain fibers. Regional variation in spindle involvement in advanced murine dystrophy provides evidence that the equatorial contents of this receptor are sequestered from the deleterious effects of the disease. Capsular thickening in the equator may be an adaptive response, preventing the intrafusal fibers from undergoing the moderate change and atrophy observed at their polar ends.

Elevations of cathepsin B and cathepsin L activities in forelimb and hind limb muscles of genetically dystrophic mice

The combined activities of cathepsin B and cathepsin L were studied in the forelimb and hind limb muscles of dystrophic mice. The activities of these proteases in the forelimb and hind limb muscles of young and adult dystrophic mice were significantly higher than those in normal mice. However, clinical involvement of dystrophy appeared in the hind limbs but not in the forelimbs. We therefore suggest that the increase in protease activity begins at a very early age and that the clinical involvement is not linked with the increase in cathepsin B and L.

Natural killer cell activity in murine muscular dystrophy. III. NK-sensitive myoblast cells and lack of NK activity in beige/dystrophic hybrid mice

The NK-susceptibility of dystrophic mouse myoblast cells was investigated. Spleen cells from 8- to 10-week-old normal (+/+) and dystrophic (dy2J/dy2J) male C57BL/6J mice were fractionated on Percoll density gradients and the cells at each density interface were incubated with either 51Cr-labeled YAC-1 or myoblast cells in a 6 hr 51Cr-release assay. Myoblast target cells were obtained from either heterozygous (+/dy2J) or homozygous (dy2J/dy2J) muscle cultures or a transformed tetraploid myoblast line (M14D2). The data indicate that the interface between the 50 and 60% (1.060-1.075 g/ml) Percoll density fractions of spleen cells from either normal or dystrophic mice contains the largest proportion of asialo GM-1 positive and NK-1 positive cells displaying NK activity. Myoblast cells from either heterozygous (phenotypically normal) or homozygous dystrophic mice were not significantly different in susceptibility to NK-mediated lysis by Percoll enriched normal or dystrophic mouse NK cells. However, dystrophic mouse spleen cells had the highest NK activity against both myoblast targets as compared with normal mouse spleen cells. The transformed myoblast cell line, M14D2, was significantly less susceptible to NK-mediated lysis by dystrophic mouse spleen cells when compared with freshly cultured myoblast target cells. Target cell binding studies revealed that conjugate forming cells from the 50% Percoll density interface of dystrophic mouse spleen cells were approximately twofold greater than that of normal mouse spleen cells against either heterozygous or homozygous dystrophic mouse myoblast targets. Cold target inhibition studies revealed that the natural killing of dystrophic mouse myoblast cells was due to a YAC-1 reactive NK cell. Breeding experiments between C57BL/6J homozygous "beige" (bgJ/bgJ) mutant mice and dystrophic (dy2J/dy2J) mice produced beige/dystrophic hybrid mice which displayed clinical symptoms of the dystrophy process by 3 to 4 weeks of age. Spleen cells from these hybrid mice showed no significant differences in NK activity against YAC-1 target cells when compared with homozygous beige mice. Taken together, these results demonstrate the first reported evidence that murine myoblasts are susceptible to NK-mediated lysis. In addition, the data indicate that although dystrophic mouse NK cells recognize myoblast cells as targets, the NK cell studies with the beige/dystrophic hybrid mice do not indicate a direct in vivo role for NK cells in the dystrophy process.

Successful treatment of murine muscular dystrophy with the protease inhibitor bestatin

Continued administration of the dipeptide protease inhibitor Bestatin to 34 mice with genetic muscular dystrophy from the onset of clinical deficit, cured about half of the animals within 3 months. Cessation of treatment in the recovered mice at age 4 months was not followed by relapse. Examinations of these mice revealed recovery of (1) weight gain and life span, (2) muscle strength, and (3) marker enzyme activities in skeletal muscle and serum, as well as (4) disappearance of myopathological features characteristic of the disease such as necrosis of muscle fibers, centralization or a chain like arrangement of nuclei, or a marked infiltration of collagenous fibers. Finally, (5) the genetic confirmation of the animals which attained remission was confirmed to be dy/dy.

Electron microscopic and histochemical studies on dystrophic masseter muscle

Masseter muscles from normal and dystrophic mice were investigated with the electron microscope and by the use of histochemical techniques for myosin ATPase and succinic dehydrogenase activity. The observations show that the muscle fibers are severely affected in the masseter of the dystrophic mouse.

Ca2+-dependent slow action potentials in normal and dystrophic mouse skeletal muscle

Slowly rising action potentials (APs), previously described in amphibian skeletal muscle, were examined in skeletal muscle of normal and dystrophic mice (129/ReJ strain). A standard two-microelectrode recording technique was used. Muscles were bathed in a solution that was Cl- free (methanesulfonate substituted), high in K+ (20 mM), and contained 15 mM tetraethylammonium. The slow APs were elicited under conditions in which the fast Na+ channels were voltage inactivated (by partial depolarization) and in which the external Na+ concentration was only 10 mM. Increases in external Ca2+ concentration produced increases in slow AP amplitude and duration. Mn2+ (4 mM), La3+ (4 mM), and detubulation with osmotic shock blocked the slow APs. When slow APs were generated at 30-s intervals, their amplitude stayed constant. When they were generated at 15-s intervals, their amplitude decreased progressively and then fell to zero by the 11th stimulus. The Ca antagonists verapamil (10(-5) M) and bepridil (10(-5) M) caused this decrease in amplitude to occur more quickly. Voltage inactivation of the slow APs occurred between -45 and -10 mV. Slow APs recorded from dystrophic muscle fibers were decreased in amplitude and duration compared with those in normal fibers, and there was a reduced incidence of occurrence; 96% of the fibers in normal muscle exhibited slow APs compared with only 46% of dystrophic muscle fibers. In summary, slow Ca2+ APs in mammalian muscle are similar to those in cardiac and amphibian skeletal muscle, and these slow APs are depressed in dystrophic skeletal muscle.

Mitochondrial calcium content and oxidative phosphorylation in heart and skeletal muscle of dystrophic mice

Mitochondrial calcium overloading was investigated in the genetically dystrophic mouse (strains 129/ReJ dy/dy) as a possible contributing factor to the development of muscle fiber necrosis. Mitochondrial calcium concentrations were significantly elevated in both skeletal muscle and heart organelles. Because mitochondria were isolated in the presence of ruthenium red this finding was not the result of an artefact of isolation. State 3 respiration rates and concomitantly the respiratory control ratios were slightly decreased in skeletal muscle, but not in heart mitochondria. This abnormality could result from calcium overloading in a small fraction of the mitochondria. Fractionation of skeletal muscle mitochondria on sucrose gradients gave two distinct populations of dystrophic organelles, one with high calcium, whereas normal skeletal muscle mitochondria and heart organelles showed only one broad band on the gradient. The results support the idea that both skeletal muscle and heart are affected in dystrophic mice, strain 129/ReJ dy/dy and also that in the dystrophic mouse the process of cell necrosis is associated with cellular calcium overloading.

Membrane alterations in skeletal muscle fibers of dystrophic mice

Skeletal muscle fibers from dystrophic mice and littermate controls (ReJ-129) were characterized electrically and then injected with an intracellular marker. In this way they could be identified for examination with an electron microscope to correlate the relative time course of electrical and ultrastructural alterations resulting from the dystrophic process. On the average, dystrophic muscle fibers displayed decreased membrane potentials (-59 +/- 1.2 vs -79 +/- 0.7 mV for normals), decreased specific membrane resistivity (517 +/- 27 vs 642 +/- 34 omega-cm2 for normals), and depressed action potential (AP) maximum rates of rise (+Vmax) (352 +/- 9 vs 417 +/- 9 V/s for normals) and amplitudes (92 +/- 1.2 vs 102 +/- 1.0 mV for normals) at an experimentally polarized membrane potential of -90 mV. Membrane resistivity and AP +Vmax were decreased even in those fibers from dystrophic muscles that displayed normal ultrastructure (classified visually and by ratio of sarcoplasmic reticulum to total cell volume). These findings support the membrane hypothesis of muscular dystrophy that membrane lesions are the primary lesion in the disease process.

Beneficial effects of transplanting normal limb-bud mesenchyme into dystrophic mouse muscles

A new technique is being developed to remedy muscle weakness of hereditary myopathies. Mesenchymal cells dissected from limb-buds of day-12 normal mouse embryos were transplanted into the right solei of 20-day-old normal or dystrophic C56BL/6J-dy2J mice. Host and donors were immunocompatible. Unoperated left solei served as controls. Sham control solei receiving similar surgical treatment but no mesenchyme transplant did not differ from contralateral, unoperated solei. Six to seven months postoperatively the test solei (8 normal and 15 dystrophic) exhibited greater cross-sectional area, total fiber number, and twitch and tetanus tensions than their contralateral controls. Test dystrophic solei contained more normal-appearing and less abnormal-appearing fibers than their controls. Their mean fiber resting potential was intermediate between those of normal and dystrophic controls. There is no difference in twitch time course between test and control solei. The results indicate that such transplantation improves the structure and function of the dystrophic muscles.

A quantitative assessment of dystrophic mouse (129 ReJ dy/dy) myogenesis in vitro

A quantitative assessment was made of the myogenic capability in vitro of muscle cells from dystrophic (129 Rej dy/dy) and normal mice from birth to 5 months old. Seeding efficiency was increased in dystrophic cells from neonatal and 1-week-old mice compared to age-matched controls. The extent of myogenesis in cultures from neonatal and 1-week-old dystrophic mice did not differ from controls. Muscle colony formation in cultures established from 5-month-old dystrophic mice was reduced by 80% compared with normal cultures. Normal and dystrophic cultures established from 5-month-old mice contained equal numbers of fibroblast colonies. The results suggest that decreased myogenesis in cultures from 5-month-old dystrophic mice is due to a relative absence of myogenic cells rather than a numerical dilution of the cultures by fibroblasts. This may be due to population of nonviable satellite cells or to necrosis of dystrophic myotubes in vitro.

An electron microscopic study of satellite-cells and regeneration in dystrophic mouse muscle

Triceps and gastrocnemius muscles from dystrophic (129 Rej dy/dy) and normal mice were examined by electron microscopy at different stages in development for evidence of regeneration. Mitotic satellite cells were present only in dystrophic muscle. Myoblasts containing myofilaments, and multinucleate myotubes were observed within foci of regeneration. Approximately 50% of the myotubes showed features indicative of degeneration or abnormal development. These features included the presence of membrane whorls, and myofibrillar and sarcolemma breakdown. Quantitative studies suggest that the number of satellite cells is increased in dystrophic muscle. It is concluded that there are sufficient satellite cells in dystrophic mouse muscle to allow regeneration, and they are able to proliferate and form well differentiated myotubes. However, subsequent development of the myotubes can be ineffective, or 'abortive', reducing the regeneration capacity to the muscle.

A histochemical and electron microscopic study of a fast- and a slow-twitch muscle in genetically spastic mice

Fast and slow muscle fibers were studied in the flexor digitorum longus (FDL) and soleus (SOL) muscles, respectively, in control and spastic mice. HIstochemical and electron microscopic studies indicated an increased number of mitochondria, a decreased deposition of glycogen and a vesiculation and distension of the sarcoplasmic reticulum in many fast-twitch fibers of the spastic FDL. Similar findings were not evident in the slow-twitch fibers of the spastic SOL. Since the spastic condition causes increased muscular activity as a result of more rapid and prolonged nerve impulse firing, these findings reinforce the idea that a muscle fiber's oxidative capabilities are a function of its activity.

Necrotic extrafusal muscle fibers of the dystrophic mutant mouse: the ultrastructure of the myoneural junction

At 28 days postpartum, the extensor digitorum longus muscle of the dy2J mutant mouse contains a population of myofibers which exhibit coagulation necrosis for approximately 90% of their length. Using the electron microscope, motor endplates were found on more than half of the necrotic fibers studied, occurring in mildly, moderately, and severely necrotic regions of these fibers. The ultrastructural features of the axonal terminals did not vary with the condition of the fiber segment at which the endplate occurred. No morphological criteria could be established for distinguishing between the axonal terminals of necrotic fibers and those of "healthy" fibers in the dystrophic animal. The principle morphological changes at motor endplates of necrotic fibers involved not the axonal terminal, but the muscle fiber itself. This study demonstrates that the necrotic myofibers, which are present at the onset of the first clinical symptoms of murine dystrophy, are innervated. Therefore, necrosis is not precipitated by structural denervation. Furthermore, observations of motor endplates on mildly, moderately, and severely necrotic regions of the myofibers indicate that regional changes along the necrotic fiber's length are not a function of distance from the motor endplate.

Aspects of normal and dystrophic chicken muscle grown in vitro

Pectoral muscle from normal and dystrophic New Hampshire chicken embryos was dissociated and grown in vitro. Marked differences between the two types of cell cultures were observed with the light and electron microscopes during early myogenic stages. The diseased myoblasts assumed a polarized affect and fused into smaller and fewer myotubes. Pseudostraps rather than true muscle straps were often seen in diseased cultures. There was also a delay in the appearance of myosin containing thick myofilaments in differentiating dystrophic muscle cells.

Ultrastructural evidence of "abortive" regeneration in murine muscular dystrophy

Triceps and gastrocnemius muscles from dystrophic (129 ReJ dy/dy) and normal mice were examined by electron microscopy at different stages in development (birth to 6 months). A qualitative assessment was made of the incidence and success of regeneration. Mitotic figures were seen in satellite cells in two-week-old dystrophic muscle. Well-differentiated myotubes were seen in dystrophic muscles at all ages but the incidence of regeneration was greatly reduced in the older dystrophic muscles. Myotubes in areas of regeneration up to two months of age frequently showed atypical or degenerative features. It is concluded that regeneration of dystrophic mouse muscle proceeds normally to the myotube stage after which differentiation becomes aberrant and degeneration may occur. This "abortive" regeneration probably contributes to the progression of the myopathy.