Muscular dystrophy in the mouse caused by an allele at the dy-locus

Impaired biosynthesis of highly unsaturated phosphatidylcholines: a hypothesis on the molecular etiology of some muscular dystrophies

A brief review of the literature concerning the synthesis of phosphatidylcholine and phosphatidylethanolamine in muscle suggests that the cytidine pathways are replaced by the recently proposed acyl-specific de novo and salvage glycerolphosphodiester pathways (Infante, 1984) in fully differentiated muscle. An analysis of published data suggests an impaired synthesis of 4,7,10,13,16,19-docosahexaenoic phosphatidylcholine, at the level of de novo sn-3-glycerolphosphorylcholine synthesis, as the primary defect in Duchenne and (dy) murine muscular dystrophies. This phosphatidylcholine species is postulated to be required for optimum sarcoplasmic Ca2+ transport activity. It is proposed that this impairment initiates the secondary series of events which lead to the observed pathology of these diseases. Based on some predictions of the hypothesis, potential diagnosis and treatments are suggested.

Natural killer (NK) cell activity in murine muscular dystrophy. II. Age-related tissue distribution and enhanced NK activity in the thymus of dystrophic mice

The age-related tissue distribution of natural killer (NK) cell activity in murine muscular dystrophy was investigated. Lymphoid tissues including the spleen, thymus, mediastinal (or bronchial) lymph nodes (BLN), mesenteric lymph nodes (MLN), inguinal/popliteal lymph nodes (PLN1), and axillary/brachial lymph nodes (PLN2) were obtained from various aged normal (+/+) and dystrophic (dy2J/dy2J) C57BL/6J mice. Cell suspensions were incubated with 51Cr-labeled YAC-1 lymphoma target cells in a 4-hr 51Cr-release assay. The data indicated that dystrophic mice, at all ages studied, had elevated levels of NK activity in the spleen, BLN, MLN, PLN1, and PLN2 as compared with the normal age- and sex-matched control group. The NK activity in the thymocyte population from dystrophic mice at 2 weeks of age was found to be negligible, while at 8 weeks of age it was two-fold higher than that for the normal control group. In addition, dystrophic mice had an age-related decline in NK activity in all tissues after 10 weeks of age but the activity was still elevated at 40 weeks of age as compared with the normal control group. Target cell binding studies revealed that the number of conjugate-forming cells in thymocytes from 8-week-old dystrophic mice were found to be significantly higher than that found in normal mouse thymocytes using NK-sensitive YAC-1 tumor target cells. The number of cells bound per YAC-1 target cell ranged from 1 to 3 for dystrophic mouse thymocytes as compared with 1 to 2 for the normal control group. Thus, the data indicate an elevated NK activity in all lymphoid tissues studied from dystrophic mice of different ages. In addition, the thymus from dystrophic mice at 8 weeks of age contains an enhanced number of conjugate-forming NK cells and NK activity.

Myosatellite cells, growth, and regeneration in murine dystrophic muscle: a quantitative study

Patterns of growth and regeneration in 2-, 4-, 8-, and 17-week-old murine dystrophic (129 ReJ dy/dy) extensor digitorum longus muscles have been determined. Necrosis and myofiber loss, hypertrophy, and regeneration result in a reduced population of myofibers whose diameter distribution is more extensive than that found in the extensor digitorum longus muscles of age-matched normal mice. At the onset of dystrophic symptoms (2 weeks postnatal), the ratio of myosatellite cell nuclei to the total sublaminal nuclear population (myonuclei + myosatellite cells) is similar to that found in 2-week-old control muscles. The frequency of finding myosatellite cells decreases with age in both control and dystrophic muscles. Myosatellite cells account for 11%, 6%, 5%, and 3% of the total sublaminal nuclear population in control muscle and 12%, 8%, 6%, and 5% of the total sublaminal nuclear population in dystrophic muscle at 2, 4, 8, and 17 weeks, respectively. No preferential association of myosatellite cells with myofibers of a particular diameter is found in control muscle or in the two youngest dystrophic groups. At 8 and 17 weeks, myosatellite cells are less frequently encountered on small-diameter, regenerating myofibers of dystrophic muscle, and they are preferentially associated with large diameter, hypertrophied myofibers. The labeling index of myosatellite cells decreases with age in both normal and dystrophic muscle. At all ages the myosatellite cell labeling index is higher in dystrophic muscle (23%, 7%, 5%, and 2% at 2, 4, 8, and 17 weeks, respectively) than in normal muscle (5%, less than 1% at 2 and 4 weeks, respectively), with no labeled myosatellite cells being found in 8- and 17-week-old normal muscles. It is suggested that the magnitude of the regenerative response of dystrophic murine muscle decreases with age and that this factor may be responsible for the inability of the regenerative response of dystrophic muscle to keep pace with the rapid muscle deterioration.

Enhanced natural killer (NK) cell activity and NK-sensitive thymic cells in murine muscular dystrophy

Studies have shown that there is an abnormality in the thymus of dystrophic mice with respect to age-dependent thymus weight changes and altered morphology (T. DeKretser and B. Livett, Nature (London) 263, 682, 1976). Recently, others have shown that natural killer (NK) cells can lyse cells of a large, immature, rapidly dividing cell subpopulation within the thymus of normal young (3 weeks of age) mice (M. Hansson, K. Karre, R. Kiessling, J. Roder, B. Anderson, and P. Hayry, J. Immunol. 123, 765, 1979). The NK susceptibility of dystrophic mouse thymocytes as targets was therefore studied. Spleen cells from normal (+/+) and dystrophic (dy2J/dy2J) male C57BL/6J mice 8-10 weeks old were passed over nylon wool and the nonadherent cells were incubated with 51Cr-labeled YAC-1 lymphoma target cells or thymocytes in a 51Cr-release assay. Spleen cells from dystrophic mice killed twofold more YAC-1 target cells than did spleen cells from normal mice. Thymocytes from 3- to 4-week-old dystrophic mice were three to four times more susceptible to NK lysis by dystrophic mouse spleen cells as compared with normal mouse spleen cells. Spleen cells from dystrophic mice had the same NK activity against dystrophic and normal mouse thymocytes as targets. Normal mouse spleen cells killed three- to fourfold more dystrophic mouse thymocytes than that of normal mouse thymocytes as targets. Target cell-binding studies revealed that conjugate-forming cells from nylon nonadherent dystrophic mouse spleen cells were found to be two- to fourfold greater than for normal mouse spleen cells using YAC-1 tumor cells as targets. The number of lymphocytes bound per YAC-1 target cell ranged from 2 to 5 for dystrophic mouse spleen cells as compared with 1 to 2 for the normal control group. Using both normal and dystrophic mouse thymocytes as targets, the conjugate-forming cells from dystrophic mouse spleen cells were also found to be twofold greater than in the normal control group. Cold target inhibition studies revealed that the natural killing of dystrophic mouse thymocytes was due to a YAC-1-reactive NK cell. Effector cell depletion studies using monoclonal anti-Thy-1.2 plus complement treatment and plastic petri dish adherence also revealed that the natural killing of dystrophic mouse thymocytes was not due to either T lymphocytes or macrophages. Taken together, these results show an increase in NK-sensitive thymocyte targets in dystrophic mice, in combination with an increase in splenic NK activity.

Abnormal distribution of fiber types in the slow-twitch soleus muscle of the C57BL/6J dy2J/dy2J dystrophic mouse during postnatal development

The postnatal development of extrafusal fibers in the slow-twitch soleus muscle of genetically dystrophic C57BL/6J dy2J/dy2J mice and their normal age-matched controls was investigated by histochemical and quantitative methods at selected ages of 4, 8, 12, and 32 weeks. The majority of fibers in the soleus consisted of two kinds, fast-twitch oxidative-glycolytic (FOG) and slow-twitch oxidative (SO), according to reactions for alkaline-stable and acid-stable myosin ATPase and the oxidative enzyme, NADH-tetrazolium reductase. A minor population of fibers, stable for both alkaline- and acid-preincubated ATPase, but variable in staining intensity for NADH-TR, were designated "atypical" fibers. With age, the normal soleus exhibited a gradual increase in the number and proportion of SO fibers and a reciprocal, steady decline in the percentage of FOG fibers. Atypical fibers were numerous at 4 weeks, but were substantially diminished at later ages. Since total extrafusal fiber number remained relatively constant between the periods examined, this change in relative proportions reflects an adaptive transformation of fiber types characteristic of normal postnatal growth. A striking alteration in the number and distribution of fiber types was associated with the dystrophic soleus. At 4 weeks an 18% reduction in total fiber number was already noted. Subsequently, by 32 weeks a further 22% diminution in overall fiber number had occurred. With age, the absolute number and proportion of dystrophic SO fibers were drastically reduced. In contrast, the percentage of dystrophic FOG fibers increased significantly while their absolute numbers between 4 and 32 weeks remained relatively constant. Atypical fibers in the dystrophic solei were found in elevated numbers at all age groups, particularly at 12 weeks. They may, in part, represent attempts at regeneration or an intermediate stage in fiber-type transformation. Microscopically, both of the major fiber types appeared affected, albeit differently, by the dystrophic process. We suggest that a failure or retardation in the normal postnatal conversion of fiber types within the soleus muscle occurs in this murine model for muscular dystrophy.

Elimination of polyneuronal innervation in a fast muscle of normal and dystrophic mice

The changes in the pattern of innervation of extensor digitorum longus (e.d.l.) during post-natal development was studied in normal and dystrophic mice. As in other mammals, individual muscle fibres of new-born mice are supplied by more than one axon. Up to 10 days after birth there was no difference in the extent of this polyneuronal innervation between normal and dystrophic muscle fibres. During post-natal development the polyneuronal innervation gradually disappeared. In normal e.d.l. muscles the rate of the elimination of polyneuronal innervation was faster during the first 10 post-natal days and then slowed down. By 16 days the final value of less than 10% of muscle fibres receiving more than one input was reached. In the dystrophic muscles the rate of elimination was similar to normal up to 10 days of age, but continued to decrease rapidly so that already by 11 days of age polyneuronal innervation was reduced to its final level of less than 10%. Thus the elimination of polyneuronal innervation was completed at least 3 days earlier in the dystrophic animals. It is suggested that the increased nerve activity said to be present in dystrophic mice could account for this finding.

The multiple molecular forms of acetylcholinesterase in "motor end-plate disease" in the mouse (medJ and med allelic forms): sensitivity of the 10 S form to partial or total loss of muscle activity

Motor end-plate disease in the mouse is a mutation, lethal at the time of weaning. Two alleles exist, med and medJ, with medJ/medJ surviving slightly longer. The multiple molecular forms of acetylcholinesterase show an abnormal developmental pattern during the course of the disease. A decrease in the 10 S AChE proportion to total AChE activity is the major change in gastrocnemius muscle. Similar AChE changes occur after total short-term denervation, tenotomy, and in other genetic diseases. Thus it appears that AChE is modified in med/med muscle as the result of a partial or total loss of muscle activity.

Abnormal distribution of proteins in the soleus and extensor digitorum longus of dystrophic mice

Proteins of the whole muscle homogenates of the slow-twitch soleus (SOL) and fast-twitch extensor digitorum longus (EDL) of normal and dystrophic C57BL/6J mice at 4, 8, 12, and 32 weeks of age were resolved on polyacrylamide isoelectric focusing gels. Gels of the normal SOL proteins at all ages contained two bands specific to SOL and not represented in EDL. Gels of normal EDL contained three bands highly amplified in EDL but barely detectable in SOL. The distribution of proteins in dystrophic SOL was abnormal at all age groups studied due, in part, to a decrease in the proportion of SOL-specific proteins relative to other proteins in the muscle. The distribution of proteins in dystrophic EDL appeared abnormal first at 12 weeks due to a decrease in the relative proportion of EDL-amplified proteins. Due to these and other changes, at 32 weeks the dystrophic SOL and EDL were almost indistinguishable on the basis of their proteins' distributions.

Purine nucleotide cycle enzymes in dystrophic and normal mouse muscle

A comparative study of the operation of the purine nucleotide cycle and of the activity of adenylosuccinase in extracts of muscle from the two strains of dystrophic mouse shows that the cycle is defective in both cases in the conversion of adenylosuccinate to AMP. However, adenylosuccinase activity is only slightly reduced in the standard conditions for its direct assay.

Successful treatment of murine muscular dystrophy with the proteinase inhibitor leupeptin

Mice with genetic muscular dystrophy were treated with intraperitoneal injections of the proteinase inhibitor leupeptin, beginning before the onset of weakness. A significant number of the treated animals failed to develop histological evidence of dystrophy, compared with controls. Leupeptin treatment prevented (or delayed) the onset of muscular dystrophy in this experiment.

Myelination-dependent axonal membrane specializations demonstrated in insufficiently myelinated nerves of the dystrophic mouse

"Dystrophic' mice of the 129/ReJ-Dy strain have a genetic defect affecting Schwann cell proliferation. Spinal nerve roots of these animals contain myelinated and unmyelinated axons in addition to groups of large "amyelinated' axons. In affected regions of the spinal roots, myelinated axons are missing their myelin sheaths. Where the myelination terminates or begins, half-nodes are created. Freeze-fracture analysis of these half-nodes shows that only the myelinated side contains rows of dimeric particles in the axonal P-face of the paranode. The P-face on the amyelinated side of a half-node, and the remainder of the amyelinated axon. contains a dense even distribution of particles, many of which are the size of dimeric-particle subunits, but only a few of which are arranged into short rows. As the long circumferential rows are not found on the unmyelinated side of the myelinated side of the half-node we conclude that the paranodal rows of dimeric particles are dependent upon myelination for their organization.

The three-dimensional cytoarchitecture and pattern of motor innervation of branched striated myotubes

Three-dimensional reconstructions of "regenerating" myotubes in the "degeneration-regeneration" regions and in the "regenerative" foci of the extensor digitorum longus muscle of the C57BL6J/dy2J myopathic mutant mouse were made from spaced serial ultrathin section. Complex branching and recombination occurred, involving myotubes which for extensive regions along their length appeared to be independent. No accumulation of specialized organelles occurred at the branching site. Continuous branches displayed multiple discrete motor endplates.

Muscle fiber necrosis in murine dystrophy

A sampling technique based on spaced serial ultrathin sections was used to examine the ultrastructure and cytoarchitecture of myofibers, which were necrotic throughout their length, in the dystrophic mutant mouse (C57Bl6J/dy2J). Regional variations along the length of the myofibers were described. The necrotic fibers extended the full length of the muscle fascicle and did not branch. In necrotic fibers which were free of invasive cells, the only nuclei found under the basal lamina were peripherally placed, euchromatic myonuclei, or nuclear membrane remnants. In these fibers, the nuclear population was approximately 14% of the nuclear population of "healthy" fibers from the same animals. No myosatellite cells were observed to be associated with these necrotic fibers. In necrotic fibers, which had undergone foreign cell invasion, the existence of a pleomorphic population of mononucleated invasive cells, many of which had not begun to express phagocytic characteristics, caused considerable confusion, because their appearance tended to mimic that of myonuclei or satellite cells. No heterochromatic nuclei or true myosatellite cells were found associated with necrotic fibers displaying macrophage invasion.

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.

Some factors affecting spontaneous transmitter release in dystrophic mice

Neuromuscular junctions of slow- and fast-twitch skeletal muscles from dystrophic (dy2J/dy2J) and control mice of the C57BL/6J strain were used to investigate the effect of muscular dystrophy on nerve-terminal regulation of their intracellular concentration of free calcium ions. The frequency of spontaneous miniature endplate potentials (MEPPs) was taken as an indicator of the intraterminal free calcium ion concentration. Dicoumarol, 2,4-dinitrophenol, ruthenium red, and the calcium ionophore A-23187 all potentiated the MEPP frequency in dystrophic muscles at concentrations which had negligible effects on normal muscles. Dystrophic muscle preparations were also more sensitive to an increased extracellular calcium concentration. Usually, these manipulations had more effect on the nerve terminals of dystrophic slow muscle than on those of dystrophic fast muscle. We conclude that muscular dystrophy alters the nerve terminal's ability to regulate the concentration of intracellular free calcium ions.

New muscle transplant method produces normal twitch tension in dystrophic muscle

Grafting newborn muscle is an innovative method of muscle transplant. This method overcomes hypoxia in the deeper fibers and facilitates reinnervation and revascularization of the grafted muscle fibers, thus promoting the survival and development of the characteristics of the donor muscle. The result achieved is superior to that obtained from mature muscle grafts or from minced muscle transplants. When an intact soleus from a 1-day-old normal mouse was grafted into a recipient soleus of a 20-day-old dystrophic C57BL/6J-dy2J mouse, the actively developing normal graft helped to improve the structure and function of the dystrophic muscle. When compared to the intact dystrophic solei, the test dystrophic muscles five to six months after operation showed increases in cross-sectional area, in wet weight, in twitch and tetanic tension, and in the number of muscle fibers with high resting membrane potentials. This is the first procedure to have raised the muscle twitch tension in an adult dystrophic mouse to the normal level.

In vitro radioisotope precursor incorporation into ribonucleic acid and protein in dystrophic mouse gastrocnemius muscle (C57 BL/6J dy2J/dy2J)

An in vitro tissue slice technique has been developed and used to compare radioisotope precurosr incorporation into RNA and protein in normal and dystrophic mouse gastrophic muscle. Significant differences are observed with both RNA and protein when incorporation is measured on a fresh weight of muscle basis. Specific activity comparisons show significantly increased incorporation with protein but not with RNA. A comparison of the results with in vivo studies has been made. The in vitro system developed is applicable to the study of macromolecular metabolism in normal and diseased human muscle tissue.

Ultrastructure of muscle spindles in C57BL/6J dy2J/dy2J dystrophic mice

The sensory organs of skeletal muscles, the muscle spindles, were examined using electron microscopy in dy2J/dy2J dystrophic mice. Despite widespread damage to the extrafusal (skeletomotor) fibres the intrafusal (spindle) fibres appeared normal and seemed resistant to the aetiological factors for murine dystrophy.

Dystrophic mice show age related muscle fibre and myelinated axon losses

Although many differences between age matched normal and dystrophic animals have been found, there have been few demonstrations of a time related quantitative change from normal to a characteristically dystrophic situation within the dystrophic strain itself. Here we report such a change-normal numbers of muscle fibres present in young dystrophic mice were rapidly lost, such that older animals showed the reduced number of muscle fibres characteristic of murine dystrophy. Although these losses began after a demonstrable loss of myelinated axons had occurred, it is not possible to say if the loss of muscle fibres was a result of the loss of nerve fibres.

Electron-microscopic, cytochemical and biochemical studies of acetylcholinesterase and butyrylcholinesterase activity in muscle of dystrophic mice

We have studied extrajunctional muscle of control and dystrophic mice by electron microscopic-cytochemistry and radiometric assay. We have found both a soluble and particulate AChE activity, which is similar proportionally in control and dystrophic muscle. The particulate AChE activity is probably due to the enzyme localized in the sarcotubular system. These sites are more numerous in muscle adjacent to the motor end-plant than in distally located extrajunctional muscle, and are increased markedly in the dystrophic mouse. Myoblasts and small muscle fibers in the dystrophic mouse also have AChE activity in the reticulum similar to fetal muscle. The soluble AChE activity identified radiometrically may represent those sites exhibiting random cytochemical end-product, such as some muscle nuclei, satellite cells, myogenic mononuclear cells in the connective tissue, and degenerating axonal boutons no longer associated with junctional folds of muscle. Enzyme activity is present in degenerating fibers, but it is randomly dispersed in the sarcoplasm rather than membrane-bound. AChE activity has not been found in debris of completely necrotic muscle. BuChE activity is higher and the number of BuChE-active sites in the sarcotubular system adjacent to the motor end-plates is greater in dystrophic muscle than in control muscle.