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

Brain Dysfunction in LAMA2-Related Congenital Muscular Dystrophy: Lessons From Human Case Reports and Mouse Models

Laminin α2 gene (LAMA2)-related Congenital Muscular Dystrophy (CMD) was distinguished by a defining central nervous system (CNS) abnormality—aberrant white matter signals by MRI—when first described in the 1990s. In the past 25 years, researchers and clinicians have expanded our knowledge of brain involvement in LAMA2-related CMD, also known as Congenital Muscular Dystrophy Type 1A (MDC1A). Neurological changes in MDC1A can be structural, including lissencephaly and agyria, as well as functional, including epilepsy and intellectual disability. Mouse models of MDC1A include both spontaneous and targeted LAMA2 mutations and range from a partial loss of LAMA2 function (e.g., dy^2J/dy^2J), to a complete loss of LAMA2 expression (dy^3K/dy^3K). Diverse cellular and molecular changes have been reported in the brains of MDC1A mouse models, including blood-brain barrier dysfunction, altered neuro- and gliogenesis, changes in synaptic plasticity, and decreased myelination, providing mechanistic insight into potential neurological dysfunction in MDC1A. In this review article, we discuss selected studies that illustrate the potential scope and complexity of disturbances in brain development in MDC1A, and as well as highlight mechanistic insights that are emerging from mouse models.

A Light and Electron Microscopic Study in Serial Sections of Skeletal and Extraocular Muscles in Mouse Myotonic Dystrophy

A suitable animal model (Mouse Strain Re-129 dy2j/dy2j) has been reported for myotonic dystrophy, a hereditary disease in which skeletal muscles degenerate. In the present study, another strain of mouse (Bar Harbor Strain C57BL/6J dy2j/dy2j), carrying this same myotonic gene (dy2j) was studied by light and electron microscopy (EM) in serial sections of epon embedded tissue.

Mouse models for muscular dystrophies: an overview

Muscular dystrophies (MDs) encompass a wide variety of inherited disorders that are characterized by loss of muscle tissue associated with a progressive reduction in muscle function. With a cure lacking for MDs, preclinical developments of therapeutic approaches depend on well-characterized animal models that recapitulate the specific pathology in patients. The mouse is the most widely and extensively used model for MDs, and it has played a key role in our understanding of the molecular mechanisms underlying MD pathogenesis. This has enabled the development of therapeutic strategies. Owing to advancements in genetic engineering, a wide variety of mouse models are available for the majority of MDs. Here, we summarize the characteristics of the most commonly used mouse models for a subset of highly studied MDs, collated into a table. Together with references to key publications describing these models, this brief but detailed overview would be useful for those interested in, or working with, mouse models of MD.

Cellular rescue in a zebrafish model of congenital muscular dystrophy type 1A

Laminins comprise structural components of basement membranes, critical in the regulation of differentiation, survival and migration of a diverse range of cell types, including skeletal muscle. Mutations in one muscle enriched Laminin isoform, Laminin alpha2 (Lama2), results in the most common form of congenital muscular dystrophy, congenital muscular dystrophy type 1A (MDC1A). However, the exact cellular mechanism by which Laminin loss results in the pathological spectrum associated with MDC1A remains elusive. Here we show, via live tracking of individual muscle fibres, that dystrophic myofibres in the zebrafish model of MDC1A maintain sarcolemmal integrity and undergo dynamic remodelling behaviours post detachment, including focal sarcolemmal reattachment, cell extension and hyper-fusion with surrounding myoblasts. These observations imply the existence of a window of therapeutic opportunity, where detached cells may be “re-functionalised” prior to their delayed entry into the cell death program, a process we show can be achieved by muscle specific or systemic Laminin delivery. We further reveal that Laminin also acts as a pro-regenerative factor that stimulates muscle stem cell-mediated repair in lama2-deficient animals in vivo. The potential multi-mode of action of Laminin replacement therapy suggests it may provide a potent therapeutic axis for the treatment for MDC1A.

Laminin-deficient muscular dystrophy: Molecular pathogenesis and structural repair strategies

Laminins are large heterotrimers composed of the α, β and γ subunits with distinct tissue-specific and developmentally regulated expression patterns. The laminin-α2 subunit, encoded by the LAMA2 gene, is expressed in skeletal muscle, Schwann cells of the peripheral nerve and astrocytes and pericytes of the capillaries in the brain. Mutations in LAMA2 cause the most common type of congenital muscular dystrophies, called LAMA2 MD or MDC1A. The disorder manifests mostly as a muscular dystrophy but slowing of nerve conduction contributes to the disease. There are severe, non-ambulatory or milder, ambulatory variants, the latter resulting from reduced laminin-α2 expression and/or deficient laminin-α2 function. Lm-211 (α2β1γ1) is responsible for initiating basement membrane assembly. This is primarily accomplished by anchorage of Lm-211 to dystroglycan and α7β1 integrin receptors, polymerization, and binding to nidogen and other structural components. In LAMA2 MD, Lm-411 replaces Lm-211; however, Lm-411 lacks the ability to polymerize and bind to receptors. This results in a weakened basement membrane leading to the disease. The possibility of introducing structural repair proteins that correct the underlying abnormality is an attractive therapeutic goal. Recent studies in mouse models for LAMA2 MD reveal that introduction of laminin-binding linker proteins that restore lost functional activities can substantially ameliorate the disease. This review discusses the underlying mechanism of this repair and compares this approach to other developing therapies employing pharmacological treatments.

The role of laminins in the organization and function of neuromuscular junctions

The synapse between motor neurons and skeletal muscle is known as the neuromuscular junction (NMJ). Proper alignment of presynaptic and post-synaptic structures of motor neurons and muscle fibers, respectively, is essential for efficient motor control of skeletal muscles. The synaptic cleft between these two cells is filled with basal lamina. Laminins are heterotrimer extracellular matrix molecules that are key members of the basal lamina. Laminin α4, α5, and β2 chains specifically localize to NMJs, and these laminin isoforms play a critical role in maintenance of NMJs and organization of synaptic vesicle release sites known as active zones. These individual laminin chains exert their role in organizing NMJs by binding to their receptors including integrins, dystroglycan, and voltage-gated calcium channels (VGCCs). Disruption of these laminins or the laminin-receptor interaction occurs in neuromuscular diseases including Pierson syndrome and Lambert-Eaton myasthenic syndrome (LEMS). Interventions to maintain proper level of laminins and their receptor interactions may be insightful in treating neuromuscular diseases and aging related degeneration of NMJs.

Laminin regulates PDGFRβ(+) cell stemness and muscle development

Muscle-resident PDGFRβ⁺ cells, which include pericytes and PW1⁺ interstitial cells (PICs), play a dual role in muscular dystrophy. They can either undergo myogenesis to promote muscle regeneration or differentiate into adipocytes and other cells to compromise regeneration. How the differentiation and fate determination of PDGFRβ⁺ cells are regulated, however, remains unclear. Here, by utilizing a conditional knockout mouse line, we report that PDGFRβ⁺ cell-derived laminin inhibits their proliferation and adipogenesis, but is indispensable for their myogenesis. In addition, we show that laminin alone is able to partially reverse the muscle dystrophic phenotype in these mice at the molecular, structural and functional levels. Further RNAseq analysis reveals that laminin regulates PDGFRβ⁺ cell differentiation/fate determination via gpihbp1. These data support a critical role of laminin in the regulation of PDGFRβ⁺ cell stemness, identify an innovative target for future drug development and may provide an effective treatment for muscular dystrophy.

Muscle PDGFRβ⁺ cells are interstitial stem/progenitor cells with myogenic potential. Here, Yao et al. show that PDGFRβ⁺ cell-derived laminin actively regulates their proliferation, differentiation and fate determination.

Mesoangioblast delivery of miniagrin ameliorates murine model of merosin-deficient congenital muscular dystrophy type 1A

Background

Merosin-deficient congenital muscular dystrophy type-1A (MDC1A) is characterized by progressive muscular dystrophy and dysmyelinating neuropathy caused by mutations of the α2 chain of laminin-211, the predominant laminin isoform of muscles and nerves. MDC1A has no available treatment so far, although preclinical studies showed amelioration of the disease by the overexpression of miniagrin (MAG). MAG reconnects orphan laminin-211 receptors to other laminin isoforms available in the extracellular matrix of MDC1A mice.

Methods

Mesoangioblasts (MABs) are vessel-associated progenitors that can form the skeletal muscle and have been shown to restore defective protein levels and motor skills in animal models of muscular dystrophies. As gene therapy in humans still presents challenging technical issues and limitations, we engineered MABs to overexpress MAG to treat MDC1A mouse models, thus combining cell to gene therapy.

Results

MABs synthesize and secrete only negligible amount of laminin-211 either in vitro or in vivo. MABs engineered to deliver MAG and injected in muscles of MDC1A mice showed amelioration of muscle histology, increased expression of laminin receptors in muscle, and attenuated deterioration of motor performances. MABs did not enter the peripheral nerves, thus did not affect the associated peripheral neuropathy.

Conclusions

Our study demonstrates the potential efficacy of combining cell with gene therapy to treat MDC1A.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-015-0055-5) contains supplementary material, which is available to authorized users.

Differential expression of genes involved in the degeneration and regeneration pathways in mouse models for muscular dystrophies

The genetically determined muscular dystrophies are caused by mutations in genes coding for muscle proteins. Differences in the phenotypes are mainly the age of onset and velocity of progression. Muscle weakness is the consequence of myofiber degeneration due to an imbalance between successive cycles of degeneration/regeneration. While muscle fibers are lost, a replacement of the degraded muscle fibers by adipose and connective tissues occurs. Major investigation points are to elicit the involved pathophysiological mechanisms to elucidate how each mutation can lead to a specific degenerative process and how the regeneration is stimulated in each case. To answer these questions, we used four mouse models with different mutations causing muscular dystrophies, Dmd (mdx), SJL/J, Large (myd) and Lama2 (dy2J) /J, and compared the histological changes of regeneration and fibrosis to the expression of genes involved in those processes. For regeneration, the MyoD, Myf5 and myogenin genes related to the proliferation and differentiation of satellite cells were studied, while for degeneration, the TGF-β1 and Pro-collagen 1α2 genes, involved in the fibrotic cascade, were analyzed. The result suggests that TGF-β1 gene is activated in the dystrophic process in all the stages of degeneration, while the activation of the expression of the pro-collagen gene possibly occurs in mildest stages of this process. We also observed that each pathophysiological mechanism acted differently in the activation of regeneration, with distinctions in the induction of proliferation of satellite cells, but with no alterations in stimulation to differentiation. Dysfunction of satellite cells can, therefore, be an important additional mechanism of pathogenesis in the dystrophic muscle.

A laminin-2, dystroglycan, utrophin axis is required for compartmentalization and elongation of myelin segments

Animal and plant cells compartmentalize to perform morphogenetic functions. Compartmentalization of myelin-forming Schwann cells may favor elongation of myelin segments to the size required for efficient conduction of nerve impulses. Compartments in myelinated fibers were described by Ramón y Cajal and depend on periaxin, mutated in the hereditary neuropathy Charcot-Marie-Tooth disease type 4F (Charcot-Marie-Tooth 4F). Lack of periaxin in mice causes loss of compartments, formation of short myelin segments (internodes) and reduced nerve conduction velocity. How compartments are formed and maintained, and their relevance to human neuropathies is largely unknown. Here we show that formation of compartments around myelin is driven by the actin cytoskeleton, and maintained by actin and tubulin fences through linkage to the dystroglycan complex. Compartmentalization and establishment of correct internodal length requires the presence of glycosylated dystroglycan, utrophin and extracellular laminin-2/211. A neuropathic patient with reduced internodal length and nerve conduction velocity because of absence of laminin-2/211 (congenital muscular dystrophy 1A) also shows abnormal compartmentalization. These data link formation of compartments through a laminin2, dystroglycan, utrophin, actin axis to internodal length, and provide a common pathogenetic mechanism for two inherited human neuropathies. Other cell types may exploit dystroglycan complexes in similar fashions to create barriers and compartments.

Laminins in peripheral nerve development and muscular dystrophy

Laminins are extracellular matrix (ECM) proteins that play an important role in cellular function and tissue morphogenesis. In the peripheral nervous system (PNS), laminins are expressed in Schwann cells and participate in their development. Mutations in laminin subunits expressed in the PNS and in skeleton muscle may cause peripheral neuropathies and muscular dystrophy in both humans and mice. Recent studies using gene knockout technology, such as cell-type specific gene targeting techniques, revealed that laminins and their receptors mediate Schwann cell and axon interactions. Schwann cells with disrupted laminin expression exhibit impaired proliferation and differentiation and also undergo apoptosis. In this review, we focus on the potential molecular mechanisms by which laminins participate in the development of Schwann cells.

Overexpression of mini‐agrin in skeletal muscle increases muscle integrity and regenerative capacity in laminin‐α2‐deficient mice

Mutations in the gene encoding the alpha2 subunit of laminins cause the severe "merosin-deficient congenital muscular dystrophy" (MDC1A). We have recently shown that overexpression of a miniaturized form of the molecule agrin (mini-agrin) counteracts the disease in dy(W)/dy(W) mice, a model for MDC1A. However, these mice express some residual truncated laminin-alpha2, suggesting that the observed amelioration might be due to mini-agrin's presenting the residual laminin-alpha2 to its receptors. Here we show that the mini-agrin counteracts the disease in dy(3K)/dy(3K) mice, which are null for laminin-alpha2. As in dy(W)/dy(W) mice, mini-agrin improves both the function and structure of muscle. We show that muscle regeneration after injury is severely impaired in dy(3K)/dy(3K) mice but is restored in the mini-agrin-expressing littermates. In summary, our results 1) show that the direct linkage of muscle basal lamina with the sarcolemma is the basis of mini-agrin-mediated amelioration and 2) provide unprecedented evidence that this linkage is important for proper regeneration of muscle fibers after injury. Our findings thus suggest that treatment with mini-agrin might be beneficial over the entire spectrum of the MDC1A disease, whose severity inversely correlates with expression levels and the size of the truncation in laminin-alpha2.

Laminins and their receptors in Schwann cells and hereditary neuropathies

This review focuses on the influence of laminins, mediated through laminin receptors present on Schwann cells, on peripheral nerve development and pathology. Laminins influence multiple aspects of cell differentiation and tissue morphogenesis, including cell survival, proliferation, cytoskeletal rearrangements, and polarity. Peripheral nerves are no exception, as shown by the discovery that defective laminin signals contribute to the pathogenesis of diverse neuropathies such as merosin-deficient congenital muscular dystrophy and Charcot-Marie-Tooth 4F, neurofibromatosis, and leprosy. In the last 5 years, advanced molecular and cell biological techniques and conditional mutagenesis in mice began revealing the role of different laminins and receptors in developing nerves. In this way, we are starting to explain morphological and pathological observations beginning at the start of the last century. Here, we review these recent advances and show how the roles of laminins and their receptors are surprisingly varied in both time and place.

Elimination of myostatin does not combat muscular dystrophy in dy mice but increases postnatal lethality

Myostatin is a TGF-beta family member and a negative regulator of skeletal muscle growth. It has been proposed that reduction or elimination of myostatin could be a treatment for degenerative muscle diseases such as muscular dystrophy. Laminin-deficient congenital muscular dystrophy is one of the most severe forms of muscular dystrophy. To test the possibility of ameliorating the dystrophic phenotype in laminin deficiency by eliminating myostatin, we crossed dy(W) laminin alpha2-deficient and myostatin null mice. The resulting double-deficient dy(W)/dy(W);Mstn(-/-) mice had a severe clinical phenotype similar to that of dy(W)/dy(W) mice, even though muscle regeneration was increased. Degeneration and inflammation of muscle were not alleviated. The pre-weaning mortality of dy(W)/dy(W);Mstn(-/-) mice was increased compared to dy(W)/dy(W), most likely due to significantly less brown and white fat in the absence of myostatin, and postweaning mortality was not significantly improved. These results show that eliminating myostatin in laminin-deficiency promotes muscle formation, but at the expense of fat formation, and does not reduce muscle pathology. Any future therapy based on myostatin may have undesirable side effects.

Canine and feline models of human inherited muscle diseases

Animal models are of immense importance for studying mechanisms of disease and testing new therapies, and rodents have been used extensively in the field of neuromuscular disorders. Mice and rats can be genetically manipulated to over-express or not express genes that are important to muscle function, and these animals can be available in large numbers for analysis. Other species, such as cats and dogs, cannot be manipulated in the same ways or be used in large numbers, but they have spontaneously occurring muscle diseases with clinical presentations more closely resembling those of the human disorders. Therefore, cats and dogs may become valuable as intermediate disease models. This review focuses on canine and feline models of human inherited muscle diseases with comparisons to rodent models and an emphasis on the muscular dystrophies.

Histopathological Characterization of the Skeletal Myopathy in rasH2 Mice Carrying Human Prototype c-Ha-ras Gene

A skeletal myopathy is found in approximately 100% of rasH2 mice. To confirm detailed features of the rasH2 skeletal myopathy, the biceps femoris, diaphragm, triceps brachii, gastrocnemial (types I and II fiber-mixed muscles) and soleus muscle (type I fiber-dominant muscle) obtained from male rasH2 and non-transgenic littermates aged 10-13 and 34 weeks were examined. Variations in the muscle fiber size, early-scattered degeneration/necrosis and regeneration of muscle fibers were detected in 10-13-week-old rasH2 mice. The severity of the above muscular lesions was more prominent in older rasH2 mice. These lesions were noted in the type II myofiber dominant muscles (biceps femoris, triceps brachii and gastrocnemial). NADH-TR stain clearly demonstrated a disorganized intermyofibrillar network and necrotic change in muscle fibers. No specific morphological changes, like rod structure or tubular aggregation seen in some types of myopathy, were noted in Gomori trichrome and NADH-TR stains in the rasH2 mouse like in many types of muscular dystrophy. Electronmicroscopically, occasional muscle fiber degeneration/regeneration, invaded phagocytic cells, indistinct Z-band suggesting excessive contraction and dilatation of the sarcoplasmic reticulum were observed. In summary, the skeletal myopathy occurring in rasH2 mice is consistent with muscular dystrophy characterized morphologically by progressive degeneration and regeneration of myofibers. The myopathy is confined to the type II myofiber predominant muscles and is not associated with any pathognomonic lesions. These characteristics will provide us with a useful model for research in muscular dystrophy of diverse myofibers.

Spontaneous muscular dystrophy caused by a retrotransposal insertion in the mouse laminin alpha2 chain gene

We identified a novel spontaneous mouse model of human congenital muscular dystrophy with laminin alpha2 chain deficiency, named dy(Pas)/dy(Pas). Homozygous animals rapidly developed a progressive muscular dystrophy leading to premature death. Immunohistological and biochemical analyses demonstrated the absence of laminin alpha2 chain expression in skeletal muscle. Analysis of the laminin alpha2 chain cDNA showed the insertion of the long terminal repeat of an intracisternal A-particle gene. In addition, a 6.1 kb insertion composed of retrotransposon elements was identified in the Lama2 sequence. The dy(Pas)/dy(Pas) mouse is thus the first spontaneous mutant with a complete laminin alpha2 chain deficiency in which the mutation has been identified.

Laminin alpha2 deficiency and muscular dystrophy; genotype-phenotype correlation in mutant mice

Deficiency of laminin alpha2 is the cause of one of the most severe muscular dystrophies in humans and other species. It is not yet clear how particular mutations in the laminin alpha2 chain gene affect protein expression, and how abnormal levels or structure of the protein affect disease. Animal models may be valuable for such genotype-phenotype analysis and for determining mechanism of disease as well as function of laminin. Here, we have analyzed protein expression in three lines of mice with mutations in the laminin alpha2 chain gene and in two lines of transgenic mice overexpressing the human laminin alpha2 chain gene in skeletal muscle. The dy(3K)/dy(3K) experimental mutant mice are completely deficient in laminin alpha2; the dy/dy spontaneous mutant mice have small amounts of apparently normal laminin; and the dy(W)/dy(W) mice express even smaller amounts of a truncated laminin alpha2, lacking domain VI. Interestingly, all mutants lack laminin alpha2 in peripheral nerve. We have demonstrated previously, that overexpression of the human laminin alpha2 in skeletal muscle in dy(2J)/dy(2J) and dy(W)/dy(W) mice under the control of a striated muscle-specific creatine kinase promoter substantially prevented the muscular dystrophy in these mice. However, dy(W)/dy(W) mice, expressing the human laminin alpha2 under the control of the striated muscle-specific portion of the desmin promoter, still developed muscular dystrophy. This failure to rescue is apparently because of insufficient production of laminin alpha2. This study provides additional evidence that the amount of laminin alpha2 is most critical for the prevention of muscular dystrophy. These data may thus be of significance for attempts to treat congenital muscular dystrophy in human patients.

Skeletal myopathy in transgenic mice carrying human prototype c-Ha-ras gene

Skeletal myopathy was found in almost all-transgenic mice carrying the human prototype c-Ha-ras gene (rasH2 mouse). Microscopically, variation of the muscle fiber size, centrally placed nuclei, regenerating fibers, and interstitial fibrosis were evident; hyalinization and necrosis were sometimes observed in the skeletal muscle (femoralis and pectoralis) of the rasH2 mice. Inflammatory changes in the skeletal muscle or abnormality of adjacent peripheral nerve were not observed. The features were essentially similar to those of muscular dystrophy. Although the severity was relatively mild compared to 34-week-old rasH2 mice, the skeletal myopathy was also observed in younger male (10 weeks of age) rasH2 mice. In nontransgenic littermates, skeletal myopathy was not observed. The mRNA of human c-Ha-ras product was detected in femoral muscle from the rasH2 mice by RT-PCR. In conclusion, these data suggest that skeletal myopathy is occurring in almost all rasH2 mice. Integration of c-Ha-ras gene is thought to be crucial to pathogenesis of skeletal myopathy in the rasH2 mice. Further characterization of the muscular lesion and its pathogenesis are needed to explore the possibility of rasH2 mouse becoming a new model for muscular dystrophy.

Dystrophin and muscular dystrophy: past, present, and future

Duchenne muscular dystrophy was described in the medical literature in the early 1850s but the molecular basis of the disease was not determined until the late 1980s. The cloning of dystrophin led to the identification of a large complex of proteins that plays an important, although not yet well understood, role in muscle biology. Concomitant with the elucidation of the function of dystrophin and its associated proteins has been the pursuit of therapeutic options for muscular dystrophy. Although there is still no cure for this disorder, great advances are being made in the areas of gene introduction and cell transplant therapy.

Laminin alpha 2 (merosin)-deficient muscular dystrophy and demyelinating neuropathy in two cats

We report laminin alpha 2 (merosin) deficiency associated with muscular dystrophy and demyelinating neuropathy in two cats. The cats developed progressive muscle weakness, and atrophy. Either hypotonia or contractures resulted in recumbency, necessitating euthanasia. Muscle biopsies showed dystrophic changes including marked endomysial fibrosis, myofiber necrosis, variability of fiber size, and perimysial lipid accumulation. Immunohistochemistry showed that laminin alpha 2 chain was absent or reduced, while dystrophin and all the components of the dystrophin-associated glycoprotein complex were present and normal. One cat was examined in detail. Motor nerve conduction velocity (MNCV) was decreased, and ultrastructurally the peripheral nerves showed Schwann cell degeneration and demyelination. Brain imaging was not performed, but white matter changes were not apparent in the brain at necropsy. The disease in these cats is similar to primary or secondary merosin (laminin alpha 2)-deficient congenital muscular dystrophy (CMD) in humans and to dystrophia muscularis in mice.

Electrotransfer of naked DNA in the skeletal muscles of animal models of muscular dystrophies

The electrotransfer of naked DNA has recently been adapted to the transduction of skeletal muscle fibers. We investigated the short- and long-term efficacy of this methodology in wild-type animals and in mouse models of congenital muscular dystrophy (dy/dy, dy(2J)/dy(2J)), or Duchenne muscular dystrophy (mdx/mdx). Using a reporter construct, the short-term efficacy of fiber transduction reached 40% and was similar in wild-type, dy/dy and dy(2J)/dy(2J) animals, indicating that ongoing muscle fibrosis was not a major obstacle to the electrotransfer-mediated gene transfer. Although the complete rejection of transduced fibers was observed within 3 weeks in the absence of immunosuppression, the persistency was prolonged over 10 weeks when transient or continuous immunosuppressive regimens were used. Using therapeutic plasmids, we demonstrated that electrotransfer also allowed the transduction of large constructs encoding the laminin alpha2 chain in dy/dy mouse, or a chimeric dystrophin-EGFP protein in mdx/mdx mouse. The correct sarcolemmal localization of these structural proteins demonstrated the functional relevance of their expression in vivo, with a diffusion domain estimated to be 300 to 500 microm. However, degeneration-regeneration events hampered the long-term stability of transduced fibers. Given its efficacy for naked DNA transfer in these models of muscular dystrophies, and despite some limitations, gene electrotransfer methodology should be further explored as a potential avenue for treatment of muscular dystrophies.

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.

Merosin and congenital muscular dystrophy

Merosin (also called as Laminin-2) is an isoform of laminin comprised of the alpha2, beta1 and gamma1 chains. In European populations, half of the patients with classical congenital muscular dystrophy have mutations of the LAMA2 gene (6q22-23) and present reduced or absence of laminin alpha2 chain. This form is generally referred to as merosin-deficient CMD. Merosin-deficient CMD is characterized by involvement of not only skeletal muscle but also central and peripheral nervous systems: Extensive brain white matter abnormalities are found by magnetic resonance imaging (MRI). However, most patients show no mental retardation. Recent case studies reported that some patients have several structural abnormalities such as abnormal cerebral cortical gyration, hypoplasia of cerebellum and pons, and dilation of ventricles. At present, functions of merosin related to muscle degeneration have not been fully elucidated. In addition, the mechanisms responsible for pathogenesis of diffuse brain white matter abnormalities remain to be determined. As mouse models for merosin-deficient CMD, three spontaneous mutants(dy, dy(2J), dy(PAS1)) and two mutants named dy(W) and dy(3K) by targeted gene disruption have been reported. These mice will help to elucidate the pathogenesis of merosin-deficient CMD and serve to develop therapy.

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.

Myoblast transplantations lead to the expression of the laminin alpha 2 chain in normal and dystrophic (dy/dy) mouse muscles

Laminin-2 is part of the basement membrane of the skeletal muscle fibers. The laminin alpha 2 chain is absent or drastically reduced in a subgroup of congenital muscular dystrophy patients, and in the severely affected dystrophic dy/dy mouse. We previously reported that heterogeneous primary mouse muscle cell cultures conferred laminin alpha 2 chain expression in dy/dy mice muscles upon cell transplantation. In the present study we investigated whether pure myoblast cell lines were able to confer laminin alpha 2 chain expression in vivo. We observed that: (1) xeno-transplantation of non-immortalized human myoblast in SCID mouse muscles allows human laminin alpha 2 chain expression; (2) allotransplantation of the permanent G8 mouse myoblast cell line in dy/dy muscles allows the expression of the murine laminin alpha 2 chain; and (3) allo-transplantation of the D7 dystrophic dy/dy cell line allows the formation of new and hybrid muscle fibers in dy/dy muscle in the absence of laminin alpha 2 chain expression. We conclude that normal myoblasts are able to restore the expression of an extracellular skeletal muscle protein and that the absence of laminin-2 does not prevent transplanted muscle cells from participating in the formation of myofibers. Myoblasts are, therefore, attractive tools for further exploration of gene complementation strategies in the animal models of congenital muscular dystrophy.

Merosin-deficient congenital muscular dystrophy. Partial genetic correction in two mouse models

Humans and mice with deficiency of the alpha2 subunit of the basement membrane protein laminin-2/merosin suffer from merosin-deficient congenital muscular dystrophy (MCMD). We have expressed a human laminin alpha2 chain transgene under the regulation of a muscle-specific creatine kinase promoter in mice with complete or partial deficiency of merosin. The transgene restores the synthesis and localization of merosin in skeletal muscle, and greatly improves muscle morphology and integrity and the health and longevity of the mice. However, the transgenic mice share with the nontransgenic dystrophic mice a progressive lameness of hind legs, suggestive of a nerve defect. These results indicate that the absence of merosin in tissues other than the muscle, such as nervous tissue, is a critical component of MCMD. Future gene therapies of human MCMD, and perhaps of other forms of muscular dystrophy, may require restoration of the defective gene product in multiple tissues.

Electrophysiology of the neuromuscular junction of the laminin-2 (merosin) deficient C57 BL/6J dy2J/dy2J dystrophic mouse

The C57 BL/6J dy2J/dy2J dystrophic mouse expresses an abnormal truncated form of the alpha2 subunit of the protein laminin-2 (or merosin), which is unable to form a stable link between the extracellular matrix and the dystrophin-associated proteins, resulting in muscular dystrophy. Morphological abnormalities of the peripheral nervous system and neuromuscular junction have also been reported. The electrophysiological properties of the neuromuscular junctions of diaphragm, extensor digitorum longus (EDL), and soleus from C57 BL/6J dy2J/dy2J mice and controls are described. No evidence for the presence of denervated fibres were found. Mean MEPP amplitudes were significantly increased in EDL and soleus but reduced in the diaphragm from affected mice. Mean MEPP frequencies were raised in all the dy2J/dy2J muscles studied. dy2J/dy2J muscles were paralysed by low concentrations of mu-conotoxin suggesting that embryonic (tetrodotoxin and mu-conotoxin resistant) sodium channels are not widespread on dy2J/dy2J muscle as has previously been reported. EPP latencies were significantly prolonged in the diaphragm and EDL but not soleus from dy2J/dy2J mice. Quantal contents were higher in all dy2J/dy2J muscles. In the dy2J/dy2J diaphragm failures in neurotransmission occurred and a faster rate of rundown of EPPs were apparent. Some changes appear from a direct effect of dystrophy, whilst increased MEPP frequency and quantal content, and failures in neurotransmission indicate neuronal abnormalities.

The effects of muscular dystrophy on craniofacial growth in mice: a study of heterochrony and ontogenetic allometry

Mechanical loading of muscles on bones at their sites of attachment can regulate skeletal morphology. The present study examined the effects of muscle degeneration on craniofacial growth, using two strains of muscular dystrophic mice, Mus musculus, differing in pathological severity. We collected radiographic and weight data longitudinally and digitized radiographs to obtain distances between anatomical landmarks in different functional regions of the skull. We then quantified heterochronic and allometric differences among genotypes and between sexes. Because growth is nonlinear with respect to time, we first used the Gompertz model to obtain heterochronic growth parameters, which were then tested with ANOVA. Ontogenetic allometric analyses examined the scaling relationships between various measurements with linear regressions. For most measurements the severely dystrophic mice are significantly smaller in final size than both the control and the mildly dystrophic mice, which are statistically indistinguishable. Measures of total growth and the neurocranium exhibit more differences among groups in heterochronic parameters of early ontogeny because growth in these regions is controlled primarily by brain expansion that ceases early in development. In contrast, the face and mandible exhibit more differences in later growth parameters possibly because of the increased influence of muscles on these regions as growth progresses. The severely dystrophic mice have flatter, more elongate skulls and mandibles than those of the other two genotypes, concurrent with an absence of muscular forces to stimulate growth in a superior-inferior direction.

Linkage of familial dilated cardiomyopathy with conduction defect and muscular dystrophy to chromosome 6q23

Inherited cardiomyopathies may arise from mutations in genes that are normally expressed in both heart and skeletal muscle and therefore may be accompanied by skeletal muscle weakness. Phenotypically, patients with familial dilated cardiomyopathy (FDC) show enlargement of all four chambers of the heart and develop symptoms of congestive heart failure. Inherited cardiomyopathies may also be accompanied by cardiac conduction-system defects that affect the atrioventricular node, resulting in bradycardia. Several different chromosomal regions have been linked with the development of autosomal dominant FDC, but the gene defects in these disorders remain unknown. We now characterize an autosomal dominant disorder involving dilated cardiomyopathy, cardiac conduction-system disease, and adult-onset limb-girdle muscular dystrophy (FDC, conduction disease, and myopathy [FDC-CDM]). Genetic linkage was used to exclude regions of the genome known to be linked to dilated cardiomyopathy and muscular dystrophy phenotypes and to confirm genetic heterogeneity of these disorders. A genomewide scan identified a region on the long arm of chromosome 6 that is significantly associated with the presence of myopathy (D6S262; maximum LOD score [Z(max)] 4.99 at maximum recombination fraction [theta(max)] .00), identifying FDC-CDM as a genetically distinct disease. Haplotype analysis refined the interval containing the genetic defect, to a 3-cM interval between D6S1705 and D6S1656. This haplotype analysis excludes a number of striated muscle-expressed genes present in this region, including laminin alpha2, laminin alpha4, triadin, and phospholamban.

Genetic regulation of slowly progressing mild muscle atrophy in fast-twitch muscles of BUF/Mna rats

BUF/Mna strain rats spontaneously develop slowly progressing mild-moderate muscle atrophy of extensor digitorum longus, tibialis, and extraocular muscles, which consist mainly of fast-twitch type fibers, at nearly 100% incidence. They have lighter extensor digitorum longus muscles than soleus muscles, when alive for more than 6 weeks. Genetic segregation of the development of the muscle atrophy was studied by crossing the BUF/ Mna strain with three other strains, ACI/NMs, WKY/NCrj, and BDIX, which were free of muscle atrophy. Two autosomal dominant susceptible genes, Mas-1 and Mas-2, determine the development of the muscle atrophy in these combinations of crosses.

Inherited muscular disorder in mutant Japanese quail (Coturnix coturnix japonica): an ultrastructural study

Ultrastructural study of muscles taken from a mutant (LWC) strain of Japanese quail with myotonia showed type 2 fibre atrophy, ring fibre formation, sarcoplasmic masses, and "moth-eaten" fibres. In these abnormal fibres, the most characteristic feature was the loss of interconnection among the myofibrils, mitochondria, and T tubules. Apparently normal muscle fibres often showed mild changes, such as proliferation of T tubules and enlarged sarcoplasmic areas with increased glycogen granules and ribosomes at the periphery of the fibres. The study suggested that one possible cause of these ultrastructural changes was a defect in cytoskeleton of muscle cells, especially in intermediate filaments.

Spontaneous opening of the acetylcholine receptor channel in developing muscle cells from normal and dystrophic mice

Single-channel activity was recorded from cell-attached patches on skeletal muscle cells isolated from wild-type mice and from mice carrying the dy or mdx mutations. Spontaneous openings of the nicotinic acetylcholine receptor channel (nAChR) were detected in virtually all recordings from either dy/dy or dy/+ myotubes, but only infrequently from wild-type or mdx myotubes. Spontaneous openings were also present in most recordings from undifferentiated myoblasts from all of the mouse strains studied. The biophysical properties of the spontaneous activity were similar to those of the embryonic form of the nAChR in the presence of acetylcholine (ACh). Examination of the single-channel currents evoked by low concentrations of ACh showed a reduced sensitivity to the agonist in the dystrophic dy and mdx myotubes, but not in wild-type myotubes. The results suggest that alterations in nAChR function are associated with the pathogenesis of muscular dystrophy in the dy mouse.

Mechanosensitive ion channels in skeletal muscle from normal and dystrophic mice

1. We examined the activity of single mechanosensitive ion channels in recordings from cell-attached patches on myoblasts, differentiated myotubes and acutely isolated skeletal muscle fibres from wild-type and mdx and dy mutant mice. The experiments were concerned with the role of these channels in the pathophysiology of muscular dystrophy. 2. The predominant form of channel activity recorded with physiological saline in the patch electrode arose from an approximately 25 pS mechanosensitive ion channel. Channel activity was similar in undifferentiated myoblasts isolated from all three strains of mice. By contrast, channel activity in mdx myotubes was approximately 3-4 times greater than in either wild-type or dy myotubes and arose from a novel mode of mechanosensitive gating. 3. Single mechanosensitive channels in acutely isolated flexor digitorum brevis fibres had properties indistinguishable from those of muscle cells grown in tissue culture. The channel open probability in mdx fibres was approximately 2 times greater than the activity recorded from wild-type fibres. The overall level of activity in fibres, however, was roughly an order of magnitude smaller than in myoblasts or myotubes. 4. Histological examination of the flexor digitorum brevis fibres from mdx mice showed no evidence of myonecrosis or regenerating fibres, suggesting that the elevated channel activity in dystrophin-deficient muscle precedes the onset of fibre degeneration. 5. An early step in the dystrophic process of the mdx mouse, which leads to pathophysiological Ca2+ entry, may be an alteration in the mechanisms that regulate mechanosensitive ion channel activity.

Murine muscular dystrophy caused by a mutation in the laminin alpha 2 (Lama2) gene

The classic murine muscular dystrophy strain, dy, was first described almost 40 years ago. We have identified the molecular basis of an allele of dy, called dy2J, by detecting a mutation in the laminin alpha 2 chain gene--the first identified mutation in laminin-2. The G to A mutation in a splice site consensus sequence causes abnormal splicing and expression of multiple mRNAs. One mRNA is translated into an alpha 2 polypeptide with a deletion in domain VI. The truncated protein apparently lacks important qualities of the wild type protein and is unable to provide sufficient muscle stability.

Modification of the dystrophic phenotype after transient neonatal denervation: role of MHC isoforms

While it recently has been demonstrated that it is possible to modify the phenotypic expression of murine dystrophy (dy/dy) (i.e., prevent myofiber loss) by subjecting the extensor digitorum longus (EDL) muscle of 14-day-old dy/dy mice to transient neonatal denervation (Moschella and Ontell, 1987), the mechanism responsible for this phenomenon has not been determined. Since it has been suggested that the effects of dystrophy vary according to fiber type, the fiber type frequency in 100-day-old normal (+/+) and dy/dy EDL muscles subjected to transient neonatal denervation has been determined by immunohistochemical analysis of their myosin heavy chain (MHC) composition. This frequency has been compared with that found in the EDL muscles of 14- and 100-day-old unoperated +/+ and dy/dy mice, in order to determine whether the reinnervation of transiently denervated neonatal muscle results in a preponderance of fibers of the type that might be spared dystrophic deterioration. In unoperated dy/dy muscle there is a progressive decrease in the frequency and in the absolute number of fibers that express MHC2B, with 100-day-old dy/dy muscles having approximately 32% of the number of myofibers fibers containing MHC2B as is found in age-matched +/+ muscles. The number of fibers containing the other fast isoforms (MHC2A and MHC2X) is similar in +/+ and dy/dy muscles at this age, indicating that fibers with MHC2B are most affected by the dystrophic process. Reinnervation following transient neonatal denervation of both the +/+ and the dy/dy EDL muscles results in a similar decrease (approximately 62%) in the number of myofibers containing MHC2B and an increase in myofibers containing the other fast MHC isoforms (MHC2A and MHC2X). The selective effect of dy/dy on fibers containing MHC2B and the sparing of myofibers in transiently denervated dy/dy muscle (which contains a reduced frequency of fibers containing MHC2B) are consistent with, although not direct proof of, the hypothesis that alterations in the fiber type may play a role in the failure of myofibers in transiently denervated dy/dy muscles to undergo dystrophic deterioration. Evidence is presented suggesting that neurons that supply myofibers containing MHC2B may be at a selective disadvantage in their ability to reinnervate neonatally denervated muscles.

Animal models of muscular dystrophy--what can they teach us?

The discovery and characterization of the X-linked gene which is defective in Duchenne muscular dystrophy (DMD) and of its protein product, dystrophin, has led to the identification of biochemical homologues of this disease in the mouse, the dog and the cat. All three animal models resemble DMD in that they lack dystrophin and that their skeletal muscle fibres undergo spontaneous necrosis and regeneration. In the dog and man, the degenerative and fibrotic aspects predominate, leading to a progressive loss of muscle structure and function, and to severe clinical disability. By contrast, in the mouse and the cat there is little fibrosis and the regenerative process seems to overcompensate, producing a true muscle hypertrophy and little or no clinical deficit. This interspecies variation in pathological response limits the usefulness of these animals as models for therapeutic testing, calling into question the strength of linkage between a given biochemical lesion and a particular pattern of pathology. However, these differences do give a valuable perspective to the pathology of the dystrophin-deficiency diseases, permitting identification of the immediate and secondary consequences of the lack of dystrophin. Moreover, the dystrophic mouse and dog are readily bred as colonies, thus providing consistent material for investigating the function of dystrophin and for testing methods of replacing its function or compensating for the absence of this function in the muscles of DMD patients. The fact that a lack of dystrophin is compatible, in some species, with only minor muscle dysfunction, raises hopes for an effective therapy in man.

Invited review: myoblast transfer: a possible therapy for inherited myopathies?

A potential therapeutic strategy for genetic diseases is to alter the genetic constitution of the affected tissues by means of grafts of normal precursor or stem cells. Over several years, evidence has accumulated to suggest that primary diseases of skeletal muscle, such as Duchenne muscular dystrophy, may be susceptible to this approach. This review makes a critical examination of such background evidence, and also of more recent data directly addressing the concept of therapy by means of grafts of normal myogenic cells. It is concluded that the data establish the principle that such grafts effect an alteration of the genetic constitution and phenotype of skeletal muscle and, therefore, might be used to alleviate recessively inherited myopathies. Several obstacles to the therapeutic application of this method to human disease are also identified; these seem to be problems of a technical nature rather than of basic principle, and none appears insuperable.

Age-related changes in oxidative capacity of the gastrocnemius muscle in normal and dystrophic (dy2J/dy2J) mice

The hind limb muscles of dy2J/dy2J mice appear more oxidative than normal hind limb muscles when assayed histochemically. In some cases, biochemical assay of oxidative capacity does not match histochemical data, and in some dy2J mice, particularly old animals, there appears to be a decline in biochemically assayed oxidative enzymes. The current study cyto-biochemically assayed succinic dehydrogenase activity of the gastrocnemius muscle in dy2J and normal mice. The assays were conducted over a maturation time course of 1 to 6 months of age. Additionally, the gastrocnemius muscle was divided into two distinct regions, a superficial region, containing largely glycolytic fast-twitch fibers (SGM), and a deeper region of mixed, largely oxidative fibers (DGM) in normal animals. Assay of the whole muscle and the two regions of each individual muscle showed that the dy2J muscle increased in SDH activity with maturation and was significantly greater than the change observed in normal muscle during the same period. The increase in the whole muscle SDH activity was accounted for increases noted in the SGM. The dy2J DGM, which showed marked morphologic degeneration, had neither an increase nor a decrease in measured SDH activity. Michaelis-Menton analysis of the enzymatic assay indicated that the Vmax, and not the Km, of the dystrophic muscle enzyme system was higher than normal, suggesting a change in quantity of enzyme present, and not altered function of the system. The observed increases occurred for 4 months of maturation and then began a decline, which was noted at 6 months. Coupled with the time course of gastrocnemius muscle degeneration (the glycolytic region was slow--very little by 4 months; the mixed was fast--highly degenerated by 1 to 2 months), the results of this study suggest that oxidative capacity in the dy2J gastrocnemius muscle increased with maturation until it became obscured by inherant muscle wasting in a particular dy2J muscle region. By comparison, analysis of the large glycolytic triceps muscle, a forelimb muscle which does not receive pseudomyotonia, indicated that this muscle did not increase in SDH activity in dystrophic animals, which showed the abnormal increase in the activity of the hind limb gastrocnemius. Thus the relationship of pseudomyotonia to increasing oxidative activity in the hind limb muscles of dystrophic animals was evident.

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.

Natural killer cells in murine muscular dystrophy. IV. Characterization of Percoll fractionated splenic and thymic natural killer cells and natural killer-sensitive thymocyte targets

The Natural Killer (NK) activity in the thymus and NK-sensitive thymocyte targets of dystrophic mice was investigated. Dystrophic and normal mouse thymocytes or spleen cells were layered on discontinuous Percoll gradients (5 or 10% increments, respectively) between 40 and 70% and centrifuged at 1700 g for 30 min. All fractions were tested for either NK activity or used a 51Cr-labeled NK-sensitive targets in a 6-hr 51Cr release assay. The density interface between the 50% (1.060 g/ml) and 60% (1.075 g/ml) Percoll fractions of either dystrophic or normal mouse spleen cells and the 40% (1.050 g/ml) and 50% (1.060 g/ml) Percoll fractions of either dystrophic or normal mouse thymocytes were found to contain the largest proportion of NK activity using YAC-1 lymphoma tumor cells as targets. In addition, the NK activity in dystrophic mouse spleen cells and thymocytes was significantly greater when compared with normal mouse controls. Target binding cell studies revealed that these Percoll fractions of dystrophic mouse spleen cells and thymocytes had greater numbers of conjugate-forming cells when compared with normal control groups. Cell depletion experiments using either anti-Thy 1.2, anti-asialo-GM 1 or anti-NK-1 plus complement treatment revealed that the cell responsible for NK activity in the 50% Percoll fraction interface of dystrophic mouse spleen cells was asialo-GM 1 positive. NK-1 positive, and partially Thy 1.2 positive. However, the cells displaying NK-activity in the thymus of normal or dystrophic mice were found to be highly Thy-1.2 positive and peanut agglutinin (PNA) negative. The density interface between the 60% (1.075 g/ml) and 65% (1.081 g/ml) Percoll fractions of either normal or dystrophic mouse thymocytes contained the largest proportion of NK-sensitive target cells. Interestingly, the 60% Percoll fraction of dystrophic mouse thymocyte targets was significantly more susceptible to NK-mediated lysis than that of the normal mouse thymocyte population. Cell depletion experiments revealed that the NK-sensitive thymocyte population was similar in both mice, that is, Thy-1.2 positive, cortisone sensitive, PNA positive, Dolichos biflorus (DBA) negative and asialo GM-1 negative. The results indicate that there are density differences between splenic and thymic NK cells. In addition, there are density and phenotypic differences between thymic NK cells and thymic NK-sensitive target cells. The findings support the hypothesis that there are different populations of NK cells.

Muscular dystrophy and muscle regeneration

An animal model of muscular dystrophy, the dystrophic (129ReJ dy/dy) mutant mouse, was used to evaluate the regenerative phenomenon in dystrophic muscle. The effect of age on "spontaneous" regeneration (i.e., regeneration in the absence of secondary trauma) was assessed by quantitative morphometric analysis and evaluation of myosatellite cell dynamics (i.e., myosatellite cell frequency, proliferative activity, and fusion capability). Spontaneous regeneration ceased by the time the mice were 8 weeks old. The findings suggested that the small "regenerating" myofibers found in older dystrophic muscle had been formed earlier in the time course of the disease and were growth-inhibited. To determine the cause of the cessation of regeneration, dystrophic muscle was subjected to the severe trauma of whole-muscle transplantation, a trauma that results in total myofiber necrosis followed by de novo myotube formation. When young dystrophic muscle (from 4- to 6-week-old dystrophic mice) was orthotopically transplanted, the time course of degeneration-regeneration was similar to that seen in age-matched normal muscle. Moreover, the regenerated dystrophic myofibers were capable of long-term survival (200 days or longer after transplantation), and they failed to show evidence of histologic changes consistent with murine dystrophy. When older dystrophic muscle (from 17-week-old dystrophic mice), muscle that failed to display spontaneous regeneration, was transplanted, it displayed remarkable regenerative capacity. It was suggested that the cessation of spontaneous regeneration in older dystrophic murine muscle is due not to exhaustion of myosatellite cell proliferative capacity, but rather to age-related loss of the mitogenic effect of dystrophy on the myosatellite cells of dystrophic muscle.

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.

Calcium and strontium activation of single skinned muscle fibres of normal and dystrophic mice

Differences in contractile activation by Ca2+ and Sr2+ between various types of normal and dystrophic murine muscle fibres were investigated using mechanically skinned fibres derived from soleus and extensor digitorum longus (e.d.l.) muscles of normal and dystrophic mice of strain 129ReJ. In terms of contractile activation, the normal e.d.l. muscle was found to consist of one relatively homogeneous population of muscle fibres characterized by steep force-pCa and force-pSr curves, low sensitivity to Ca2+ and very low sensitivity to Sr2+. Normal soleus muscles contained two fibre populations of similar size which could be distinguished on the basis of their contractile activation properties. The first fibre population was characterized mainly by its shallow force-pCa and force-pSr curves, high Ca2+ sensitivity, high Sr2+ sensitivity and the occurrence of large, slow force oscillations of myofibrillar origin. The second fibre population was characterized by force-pCa and force-pSr curves of steepness intermediate between those of normal e.d.l. and those of the first fibre population of normal soleus, by faster myofibrillar force oscillations and by low sensitivity to Ca2+ and Sr2+. The dystrophic e.d.l. fibre population had contractile characteristics which were distinct from those of the three types of normal fibre populations. However, some characteristics of the dystrophic e.d.l. fibres were very similar to those of the normal e.d.l. fibre population. Of all the fibre types investigated, dystrophic e.d.l. fibres were the least sensitive to Ca2+. Dystrophic soleus muscle contained a single homogeneous population of fibres which shared some common contractile activation characteristics with both of the fibre populations present in normal soleus muscle. However, of all fibre types investigated, the dystrophic soleus fibres were the most sensitive to Ca2+. Because of this characteristic, these fibres formed a distinct population. The maximum tensions induced by Ca2+ and Sr2+ were usually smaller in dystrophic fibres than in normal fibres obtained equivalent muscles. In conclusion, various normal murine muscle fibre types can be identified on the basis of differences in the mechanism of force activation by Ca2+ and Sr2+. Furthermore, it is possible to detect significant physiological differences in the mechanism of force activation brought about by murine muscular dystrophy.