Topographic comparison of subneural apparatuses at neuromuscular junctions in normal and dystrophic (mdx) mice: a scanning electron microscope study

Early Developmental Changes of Muscle Acetylcholine Receptors Are Little Influenced by Dystrophin Absence in mdx Mouse

Dystrophin is a cytoskeletal protein contributing to the organization of the neuromuscular junction. In Duchenne muscular dystrophy, due to dystrophin absence, the distribution of endplate acetylcholine receptors (AChRs) becomes disorganized. It is still debated whether this is due to the absence of dystrophin or to the repeated damage/regeneration cycles typical of dystrophic muscle. We addressed this controversy studying the endplate in the first 3 postnatal weeks, when muscle damage in dystrophic (mdx) mice is minimal. By synaptic and extra-synaptic patch-clamp recordings in acutely dissociated mdx and wt muscle fibers, we recorded unitary events due to openings of AChR-channels containing the γ and ε subunit. We also examined AChR distribution at the endplate by immunofluorescence assays. No differences between wt and mdx fibers were found in the γ/ε switch, nor in the AChR organization at the endplates up to 21 postnatal days. Conversely, we detected a delayed appearance and disappearance of patches with high channel opening frequency in mdx fibers. Our data emphasize that the innervation-dependent γ/ε switch and AChR organization in the endplate are not affected by the absence of dystrophin, while extra-synaptic AChR cluster formation and disassembly could be differentially regulated in mdx mice.

The role of free radicals in the pathophysiology of muscular dystrophy

Null mutation of any one of several members of the dystrophin protein complex can cause progressive, and possibly fatal, muscle wasting. Although these muscular dystrophies arise from mutation of a single gene that is expressed primarily in muscle, the resulting pathology is complex and multisystemic, which shows a broader disruption of homeostasis than would be predicted by deletion of a single-gene product. Before the identification of the deficient proteins that underlie muscular dystrophies, such as Duchenne muscular dystrophy (DMD), oxidative stress was proposed as a major cause of the disease. Now, current knowledge supports the likelihood that interactions between the primary genetic defect and disruptions in the normal production of free radicals contribute to the pathophysiology of muscular dystrophies. In this review, we focus on the pathophysiology that results from dystrophin deficiency in humans with DMD and the mdx mouse model of DMD. Current evidence indicates three general routes through which free radical production can be disrupted in dystrophin deficiency to contribute to the ensuing pathology. First, constitutive differences in free radical production can disrupt signaling processes in muscle and other tissues and thereby exacerbate pathology. Second, tissue responses to the presence of pathology can cause a shift in free radical production that can promote cellular injury and dysfunction. Finally, behavioral differences in the affected individual can cause further changes in the production and stoichiometry of free radicals and thereby contribute to disease. Unfortunately, the complexity of the free radical-mediated processes that are perturbed in complex pathologies such as DMD will make it difficult to develop therapeutic approaches founded on systemic administration of antioxidants. More mechanistic knowledge of the specific disruptions of free radicals that underlie major features of muscular dystrophy is needed to develop more targeted and successful therapeutic approaches.

Variability and failure of neurotransmission in the diaphragm of mdx mice

Loss of specific muscle force and evidence of myopathy are present in the diaphragm of mdx mice by 4 weeks of age. The neuromuscular junction of dystrophic muscle also shows structural abnormalities at this age. Whether these structural alterations result in neural transmission abnormalities is currently unclear, particularly at physiological firing frequencies. Thus, we investigated the extent of neurotransmission variability and failure during 35 and 100 Hz stimulation in the diaphragm of 6 to 8-month-old mdx mice in comparison to age-matched controls. Neurotransmission failure was similar across groups at both stimulation frequencies, despite the presence of disrupted post-synaptic acetylcholine receptors (AChRs). Neural transmission variability, however, measured by comparing variation in force production during direct muscle stimulation compared to variation in force production during phrenic nerve stimulation was significantly greater in dystrophic muscle. Together, these results suggest that neurotransmission is maintained at physiologic firing frequencies in dystrophic muscle, but the precision of neurotransmission is attenuated. A reduced density of functional AChRs likely underlies the increase in neurotransmission variability.

Scanning electron microscopic study of the neuromuscular junctions of the cricothyroid and thyroarytenoid muscles in rats

Neuromuscular junctions were observed in the cricothyroid (CT) and thyroarytenoid (TA) muscles of adult rats by scanning electron microscopy after removing the intramuscular connective tissue components using the HCI hydrolysis method. Morphologically, the junctions were classified into three types in the CT muscle and two types in the TA muscle, based on the structural characteristics of the subneural apparatuses, including junctional folds. In the CT muscle, type 1 junctions (32%) consisted of more than 15 cup-like depressions with slit-like junctional folds. Type 2 junctions (20%) were characterized by approximately 10 cup-like depressions with a small number of pit- or slit-like junctional folds. Type 3 junctions (48%) had irregular labyrinthine gutters with slit-like junctional folds. In the TA muscle, type 1 (82%) and 2 (18%) junctions had similar structures to type 1 and 2 junctions in the CT muscle, respectively. Histochemical studies using myosin adenosine triphosphatase staining showed that both CT and TA muscles predominantly consisted of type II muscle fibers (78% and 82%, respectively), and that the diameter of type II fibers was larger than that of type I fibers. These findings suggest that the type 2 junction belongs to type I muscle fibers, while both type 1 and type 3 junctions belong to type II fibers, and that the type 3 junction is a structural variation of the type 1 junction. The significance of the structural differences of the subneural apparatuses in the intrinsic laryngeal muscles is discussed briefly.

Postnatal morphodifferentiation of the subneural apparatuses of the posterior cricoarytenoid muscle in rats: a scanning electron microscopy study

The subneural apparatus, i. e., the post-synaptic component of the neuromuscular junction, in the posterior cricoarytenoid muscle of the rat was studied by scanning electron microscopy, with special attention given to its postnatal differentiation along with the functional development of the muscle. Primitive synaptic troughs observed in the first postnatal week consisted of single cup-like depressions 5-6 microm in diameter. On the 7th day, low sarcoplasmic ridges appeared in the trough. In the second postnatal week, muscle fibers could be classified into two groups: large (10-15 microm in diameter) and small (less than 10 microm in diameter). In the large muscle fibers, many low ridges became circular and protruded to transform the single trough into numerous cup-like depressions (2-5 microm in diameter). In contrast, the subneural apparatus in the small muscle fibers consisted of a small number of cup-like depressions. The two types of subneural apparatus differentiated into adult forms by the 28th postnatal day, although they remained smaller in size than those of adults. In the large muscle fibers, the number of pit-like or elongated invaginations increased and gradually transformed into slit-like junctional folds by the 28th postnatal day, while the small muscle fibers still possessed a few pit-like or elongated junctional folds at this point in time. The two types of morphodifferentiation of the subneural apparatus are thought to reflect the two types of muscle fibers in the rat posterior cricoarytenoid muscle.

Dystrophin is required for organizing large acetylcholine receptor aggregates

Dystrophin is a cytoplasmic protein underlying the plasma membrane in normal skeletal muscle. Its absence leads to muscle degeneration as seen in Duchenne muscular dystrophy (DMD) and in mdx mice. One puzzling question in the study of dystrophinopathies is that in mdx muscles the neuromuscular junctions (NMJs) show little, if any, developmental defect, but morphological and functional abnormalities of NMJs are obvious after muscle damage and regeneration begin. This phenomenon leads us to hypothesize that dystrophin may be required for endplate maintenance and/or endplate remodeling in regenerating fibers. Here we show that the absence of dystrophin causes NMJ fragmentation in adult muscle fibers, and greatly reduces both spontaneous and agrin-induced acetylcholine receptor (AChR) clustering activities on cultured myotubes derived from satellite cells. The lower AChR clustering in mdx myotubes originates in the smaller size of each cluster and from a 72% reduction in the occurrence of large (> 10 micron 2) AChR clusters. Our results suggest dystrophin is involved in organizing small AChR clusters into large AChR aggregates during muscle regeneration, although it is not required for initiating the original AChR clustering activity.

Three-dimensional structure of the synaptic contact of the neuromuscular junction in the rat lumbrical muscle

This study examined the three-dimensional structures of the synaptic contact in rat lumbrical muscles by scanning electron microscopy using three different methods: the aldehyde prefix-osmium-dimethyl sulfoxide-osmium method (A-O-D-O method), the cell-extraction method, and the NaOH-digestion method. These three methods visualized the motor nerve endings, subneural basal lamina and postsynaptic sarcolemma, respectively. The motor nerve endings were composed of a cluster of spherical and cylindrical terminals. Pores on the presynaptic membrane were considered openings of exocytotic vesicles. The postsynaptic side of the subneural basal lamina showed numerous ridges, corresponding to junctional folds. Most of the ridges rose vertically from their base. The ridges showed widening, narrowing, and branching. The subneural basal lamina appeared to be composed of small granular substances. The basal lamina of the primary synaptic clefts had pores 25-30 nm in diameter, which may facilitate the transport of acetylcholine (ACh) without being hydrolyzed by ACh esterase in the lamina. On the outer surface of the postsynaptic sarcolemma in a sole plate, the primary synaptic clefts were composed of a mixture of depressions and gutters; so far as we know, this represents the only example of such a phenomenon. These depressions and gutters seem to fit respectively into the spherical and cylindrical terminals of the motor nerve endings. The openings of the junctional folds consisted of a mixture of many slits and a few pits in the primary synaptic clefts.

Characteristics of skeletal muscle in mdx mutant mice

We review the extensive research conducted on the mdx mouse since 1987, when demonstration of the absence of dystrophin in mdx muscle led to X-chromosome-linked muscular dystrophy (mdx) being considered as a homolog of Duchenne muscular dystrophy. Certain results are contradictory. We consider most aspects of mdx skeletal muscle: (i) the distribution and roles of dystrophin, utrophin, and associated proteins; (ii) morphological characteristics of the skeletal muscle and hypotheses put forward to explain the regeneration characteristic of the mdx mouse; (iii) special features of the diaphragm; (iv) changes in basic fibroblast growth factor, ion flux, innervation, cytoskeleton, adhesive proteins, mastocytes, and metabolism; and (v) different lines of therapeutic research.