Dystrophic mice show age related muscle fibre and myelinated axon losses

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.

Identification of homozygous and heterozygous dy2J mice by PCR

The dystrophia muscularis dy2J/dy2J mouse is an animal model for one form of human congenital muscular dystrophy. A point mutation in the gene coding for the laminin-2 alpha 2 chain leads to the expression of a truncated, partially functional protein. We developed a simple assay for the detection of the dy2J allele, which contains a new NdeI restriction site. Genomic DNA was prepared from animals of known status and amplified by PCR. The digestion of the PCR product with the restriction enzyme resulted in characteristic profiles. Then, this technique was used to identify heterozygous mice among unaffected animals of unknown status. Subsequently, the heterozygous genotype of these mice was confirmed by the birth of dystrophic offspring after mating. This technique allows the detection of the dy2J allele in heterozygous and homozygous animals at any age.

Partial laminin alpha2 chain restoration in alpha2 chain-deficient dy/dy mouse by primary muscle cell culture transplantation

Laminin-2 is a component of skeletal and cardiac basal lamina expressed in normal mouse and human. Laminin alpha2 chain (LAMA2), however, is absent from muscles of some congenital muscular dystrophy patients and the dystrophia muscularis (dy/dy) mouse model. LAMA2 restoration was investigated following cell transplantation in vivo in dy/dy mouse. Allogeneic primary muscle cell cultures expressing the beta-galactosidase transgene under control of a muscular promoter, or histocompatible primary muscle cell cultures, were transplanted into dy/dy mouse muscles. FK506 immunosuppression was used in noncompatible models. All transplanted animals expressed LAMA2 in these immunologically-controlled models, and the degrees of LAMA2 restoration were shown to depend on the age of the animal at transplantation, on muscle pretreatment, and on duration time after transplantation in some cases. LAMA2 did not always colocalize with new or hybrid muscle fibers formed by the fusion of donor myoblasts. LAMA2 deposition around muscle fibers was often segmental and seemed to radiate from the center to the periphery of the injection site. Allogeneic conditionally immortalized pure myogenic cells expressing the beta-galactosidase transgene were characterized in vitro and in vivo. When injected into FK506-immunosuppressed dy/dy mice, these cells formed new or hybrid muscle fibers but essentially did not express LAMA2 in vivo. These data show that partial LAMA2 restoration is achieved in LAMA2-deficient dy/dy mouse by primary muscle cell culture transplantation. However, not all myoblasts, or myoblasts alone, or the muscle fibers they form are capable of LAMA2 secretion and deposition in vivo.

Effect of low Ca2+ solution on muscle contraction of developing, preclinical dystrophic (dy2j) mice

EDL muscles from normal and dystrophic (dy2j) mice aged 7 to 21 days of postnatal life were examined. Muscles were divided into 2 groups according to age, 7 to 14 days and 16 to 21 days postnatal, so as to assess age- and/or phenotype-related differences in the muscle response to low Ca2+ solution. Tension production was already much impaired in "predystrophic" muscles. At this stage, however, there was essentially no difference in twitch kinetics between normal and dystrophic muscles. Upon exposure to low Ca2+ solution, twitch responses of both normal and dystrophic muscles declined in a similar manner. In the youngest animals studied (7 to 14 days), the tetanic responses of both normal and dystrophic muscles to low Ca2+ solution were also similar. In animals 15 to 21 days old, however, the tetanic tension developed in low Ca2+ solution by dystrophic muscles, was significantly less than that of normal. Moreover, under these conditions (i.e., in low Ca2+ solution), and following tetanic stimulation, the membrane potential of dystrophic muscles in this age group was significantly more depolarized than that of normal muscles. Our results suggest that the ability of the cell to deal with extracellular Ca2+ is normal in predystrophic muscles up to 21 days of postnatal life. The results also clearly point to the fragility of the membrane in these muscles.

Co-cultures of inferior olive and cerebellum: electrophysiological evidence for multiple innervation of Purkinje cells by olivary axons

Slices of inferior olive (IO) and cerebellum were co-cultured for several weeks by means of the roller tube technique. Recordings were carried out intracellularly from Purkinje cells (PCs) which were identified morphologically by intracellular injection of the fluorescent dye Lucifer yellow, or by immunohistochemical stainings with antibodies raised against the 28 kD Ca(2+)-binding protein calbindin. Following stimulation of olivary tissue, an all-or-none full complex spike response was recorded in some PCs consisting of a fast rising spike followed by a depolarizing potential. In other PCs, graded stimulation of the olivary explant induced synaptic potentials which were characterized by step-wise variation in their amplitude and resembled the ones occurring spontaneously. In contrast, only smoothly graded synaptic potentials were observed in cerebellar mono-cultures. These results indicate that some of the PCs in olivo-cerebellar co-cultures are innervated by several olivary neurons.

Normal myoblast injections provide genetic treatment for murine dystrophy

A treatment has been developed to alleviate muscle weakness in murine dystrophy. Cultured myoblasts from genetically normal mouse embryos were injected into the right soleus of 20-day-old normal or dystrophic mice. Hosts and donors were immunocompatible but exhibited different genotype markers. Donor cells produced GPl-1CC. Host cells produced GPl-1BB. When compared with contralateral controls 6 months postoperatively, test dystrophic solei exhibited greater cross-sectional area, total fiber number, wet weight, and twitch and tetanus tensions. They contained more normal-appearing and less abnormal-appearing fibers. Their mean fiber resting potential was similar to that of normal controls. Presence of GPl-1CC with or without the hybrid isozyme GPl-lBC in these muscles implied the survival and development of donor myoblasts into normal myofibers, and fusion of normal myoblasts with dystrophic satellite cells to form genetically mosaic myofibers. Injection of fibroblasts instead of myoblasts caused detrimental effects.

Acetylcholine receptor density and binding in murine dystrophy

The concentration and binding characteristics of acetylcholine receptors (AChR) in sarcolemmal fractions from normal and dystrophic mice have been measured. Unlike the results following denervation, AChR concentration (per g wet weight) is unchanged and AChR affinity for two different alpha-neurotoxins is also unchanged. In contrast sarcolemmal ATPase is significantly reduced in dystrophic tissues. These data reaffirm that denervation is a poor model of dystrophy.

Developmental changes in succinate dehydrogenase activity in muscle fibers from normal and dystrophic mice

The question of whether or not the development of dystrophic muscles is similar to that of normal muscles, prior to the manifestations of the symptoms of the disease, is investigated here. The developmental change in the activity of succinate dehydrogenase was therefore measured in individual fibers of prospectively dystrophic muscles from 10- to 28-day-old mice (strain C57Bl/6J dy2j) and compared with that of muscles from normal mice of the same age. It was found that up to 10 days of age, muscle fibers from normal and prospective dystrophic animals had low succinate dehydrogenase activities, and were all more or less uniform. Thereafter in the normal muscle the overall activity of the enzyme increased and the fibers became more heterogeneous with age. By 21 days the extensor digitorum longus muscle resembled that of the adult. At that time, fibers from prospectively dystrophic muscles had lower succinate dehydrogenase activities and were more homogeneous. Thus fibers from prospectively dystrophic muscles fail to achieve their adult characteristics by 21 days. On the basis of these results, it is suggested that muscle maturation is retarded in dystrophic animals.

Long-term peripheral nerve and muscle recordings from normal and dystrophic mice

A method for long-term recording of electrical activity from small mammalian nerves and muscles is described. Electrodes for stimulating and recording activity were implanted on nerves and muscles subserving ankle flexion and extension in normal and dystrophic mice. Activity was monitored on a regular basis for up to 200 days following implantation. Neural compound action potentials, compound EMG potentials and twitch tension were recorded. Shortly after implantation, evoked EMG and twitch tension declined, but recovered progressively to values measured at the time of implantation and subsequently remained steady in normal mice. However, while dystrophic mice did recover, with EMG levels reaching 50-60% of the values recorded at implantation, tension eventually dropped to 10% in flexor muscles and 25% in extensors.

Number and size of motoneurons in a forelimb motor nucleus of normal and dystrophic (C57BL/6J dy2j/dy2j) mice

The method of retrograde axonal transport of horseradish peroxidase (HRP) was used to identify the motoneurons that innervate the distal forelimb muscles via the ulnar nerve in normal and dystrophic (C56BL/6J dy2j/dy2j) mice. In both normal and dystrophic mice this motor nucleus was located in spinal segments C6 through T1. No clear division, on the basis of size, into alpha and gamma motoneuron populations was apparent. The motoneurons of dystrophic mice were fewer in number (26.5 vs. 35) but larger in cross-sectional area (780 vs. 674 microns2) than those of age-matched control mice. These results are quantitatively similar to those reported for the hind limb soleus muscle of dystrophic mice and suggest that the motoneuronal changes are a reflection of the dystrophic process rather than the associated spontaneous action potential generation seen in the dystrophic hind limb muscles.

Acetylcholine-sensitivity in fast and slow twitch muscle of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice

Small bundles of muscle fibres were isolated from diaphragm, extensor digitorum longus (EDL) and soleus (SOL) muscles of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice, and their isometric tension developed in response to acetylcholine (ACh) was recorded. For each type of muscle the relationship between the maximum amplitude of the ACh-contracture and log [ACh] was similar in normal and dystrophic animals. However, this relationship was steeper for normal and dystrophic SOL than for EDL and diaphragm muscles. Dystrophy did not induce changes in the time course of the ACh-contractures, except a significant 'speeding' of dystrophic SOL that appeared in the time to peak of the contractile response. The amplitude of ACh-contractures of both normal and dystrophic diaphragm preparations increased by about 50% after perfusion for 80-90 min in physiological solution containing phospholipase C 5 mU/ml. ACh-sensitivity was measured in normal and dystrophic diaphragm preparations by iontophoretic application of ACh from high resistance pipettes. ACh-potentials were similar in time course in the two types of muscle fibres, and there was also no significant difference in the length of sensitive fibre segments and maximum sensitivity values. Extrajunctional ACh-sensitivity was absent in normal as well as in dystrophic fibres. It is concluded that the absence in dystrophic muscles of stronger ACh-contractures and of extrajunctional sensitivity can be considered as evidence against a primary neuronal involvement in murine dystrophy of the dy2J strain.

Regeneration and reinnervation of the dystrophic mouse soleus muscle. A light- and electron-microscopic study

The regeneration and reinnervation of the dystrophic mouse soleus muscle was investigated in response to a double crush-lesion, which causes degeneration of muscle fibres leaving the innervation intact. In normal and dystrophic muscles, injury produced degeneration of muscle fibres, proliferation and fusion of muscle satellite cells, and growth and reinnervation of regenerating fibres. Four, 6 and 21 days after injury, regenerating dystrophic fibres were 50% smaller in cross-sectional area than regenerating normal fibres and showed several pathological changes. Nerve terminal morphology was initially unaffected by the crush, and nerve terminals were associated with degenerating muscle fibres 2 days after injury and with regenerating muscle fibres 6-28 days after crushing. In intact muscles dystrophic endplates were longer and showed increased ultraterminal sprouting compared to normal endplates. At 28 days after crushing normal nerve terminal sprouting was significantly increased compared to the contralateral control. The extent of nerve terminal sprouting and endplate length in dystrophic muscles was not affected by the degeneration and subsequent regeneration of the muscle fibres. We conclude that a proportion of dystrophic mouse soleus muscle fibres can regenerate after a crush when the innervation is left intact.

Axonal transport of nerve growth factor and serum albumin in the dystrophic mouse

Specific (nerve growth factor, NGF) and non-specific (serum albumin, BSA) uptake followed by retrograde axonal transport was studied in the sciatic nerves of normal (C57BL/6J) and dystrophic (dy2J/dy2J) mice. In the dystrophic mouse there was a significant decrease in the amount of NGF transported to dorsal root ganglia or BSA accumulating at a ligature placed on the sciatic nerve.

X-ray diffraction from striated muscles and nerves in normal and dystrophic mice

The structure of striated muscle (thick and thin filaments, filament lattice, and collagen), peripheral nerve myelin, and tendon collagen were studied in tissues from dystrophic and normal mice using small-angle x-ray diffraction. There were increases in the amount of disorganized tissue in the dystrophic mice, and the time course of the changes was monitored over the first 42 weeks of life. As the dystrophic mice became older, the contractile apparatus of the muscles appeared to atrophy, while the amount of collagen increased. In general, the molecular structure and packing appeared to remain unchanged as the disease progressed, although changes in the relative amounts and the organization of proteins were noted. In both normal and dystrophic mice, the collagen periodicity (65.7 nm) was 2% smaller when detected in muscle tissue compared with that detected in tendon tissue.

In vivo morphometric analysis of muscle microcirculation in dystrophic mice

In order to test the vascular hypothesis of muscular dystrophy, the gracilis muscle in 6- to 7-week-old C57BL/6J-dy2J normal and dystrophic mice was studied using in vivo quantitative morphometric techniques to determine the total length and surface area of capillaries in which blood was flowing per unit volume of muscle. Individual capillary lengths, diameters, and red blood cell velocities were also quantified. During resting conditions, the capillary density(length per unit volume of muscle) and surface area are increased significantly in dystrophic muscle compared to normal muscle. Under fully vasodilated conditions, the capillary density and surface area are similar in normal and dystrophic muscle. Individual capillary lengths, diameters, and red blood cell velocities are also similar in normal and dystrophic muscle under resting conditions. These results indicate that, contrary to the vascular hypothesis, dystrophic muscle at rest has increased capillary density, surface area, and blood flow. It is postulated that the increased capillary density in dystrophic muscle at rest is secondary to muscle fiber breakdown.

Effects of slow frequency electrical stimulation on muscles of dystrophic mice

The hind leg muscles of dystrophic mice (C57 BL dy2J/dy2J) wer chronically stimulated at 10 Hz for 30 minutes six times a day. After 14 days of such activity a clinical improvement in the use of the stimulated leg was noticed. The twitch and tetanic tensions developed by the stimulated tibialis anterior and extensor digitorum longus muscles were higher than those developed by the control, unstimulated muscles on the contralateral side. Histochemically visualised activity of the oxidative enzyme succinic dehydrogenase was greater in fibres of the stimulated muscles. The stimulated muscles contained more muscle fibres than unstimulated controls. It is concluded that slow frequency activity has a beneficial effect on muscles of dystrophic mice.

Motor innervation of the gastrocnemius muscle of wobbler and dystrophic mice

Innervation of the gastrocnemius muscle of mice from the wobbler, dystrophic and C57/BL colonies has been studied. It was found that phenotypically normal mice from each of the colonies did not differ in their innervation properties, hence suggesting no heterozygote penetrance. However, the end-plate complexity increased with age of normal mice. Functional terminal innervation ratio for both the wobbler and dystrophic gastrocnemius muscle was raised above that of the normal and many dystrophic end-plates also appeared abnormal. A study of mice from the wobbler colony manifesting a late onset hind-limb muscle degeneration (Wr/HLD) has been included and the results suggest a relatively benign form of spinal muscular atrophy.

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.

Muscular dystrophy in the mouse: neuromuscular transmission and the concept of functional denervation

The results of recent investigations by ourselves and others indicate that no form of denervation exists to any remarkable degree in dystrophic mouse skeletal muscles. This conclusion is based on the following information: Dystrophic nerve terminals liberate normal amounts of transmitter both spontaneously and during impulse-mediated activity. The characteristics of the release process, the size of the available store of transmitter, and the probability of release of transmitter in response to the invasion of an action potential appear to be normal. The sensitivity of the postsynaptic membrane to the transmitter is normal. Action potential generation in response to both direct and indirect excitation is normal. There is no unequivocal pharmacologic evidence of denervation in dystrophic skeletal muscle, even though dystrophic muscle fibers respond to surgical denervation in a normal fashion. Nerve terminal sprouting is extensive, but there is no evidence of collateral reinnervation.

Further observations on myelinated axon numbers in normal and dystrophic mice

Counts of the number of myelinated axons in the nerves to slow-twitch soleus and fast-twitch plantaris muscles of 129 ReJ dy/dy dystrophic mice at 4, 15 and 24 weeks of age have shown statistically significant reductions when compared to normal values. In addition muscle fibre losses had occurred in both muscles at the earliest age point studied. There was no suggestion of a progressive loss of myelinated axons. There was a greater percentage reduction in the soleus than in the plantaris nerves. The number of myelinated axons in the nerves to the plantaris and soleus muscles of the 129 ReJ +/+ normal animals was approximately the same, in contrast to the C57BL/6J strain where the soleus nerve always contained a greater number. Statistically significant reductions in the number of myelinated axons in the nerves to the medial gastrocnemius muscles of C57BL/6J dy2J/dy2J dystrophic mice have been observed at both 15 and 72 weeks of age. Muscle fibre losses have also been recorded in 72 weeks dystrophic medial gastrocnemius muscles.