The membrane morphology of the neuromuscular junction, sarcolemma, sarcoplasmic reticulum and transverse tubule system in murine muscular dystrophy studied by freeze-fracture electron microscopy

Presynaptic mechanisms controlling calcium-triggered transmitter release at the neuromuscular junction

Calcium-triggered neurotransmission underlies most communication in the nervous system. Yet, despite the conserved and essential nature of this process, the molecular underpinnings of calcium-triggered neurotransmission have been difficult to study directly and our understanding to this date remains incomplete. Here we frame more recent efforts to understand this process with a historical perspective of the study of neurotransmitter release at the neuromuscular junction. We focus on the role of calcium channel distribution and organization relative to synaptic vesicles, as well as the nature of the calcium sensors that trigger release. Importantly, we provide a framework for understanding how the function of neurotransmitter release sites, or active zones, contributes to the function of the synapse as a whole.

Structure and functions of aquaporin-4-based orthogonal arrays of particles

Orthogonal arrays or assemblies of intramembranous particles (OAPs) are structures in the membrane of diverse cells which were initially discovered by means of the freeze-fracturing technique. This technique, developed in the 1960s, was important for the acceptance of the fluid mosaic model of the biological membrane. OAPs were first described in liver cells, and then in parietal cells of the stomach, and most importantly, in the astrocytes of the brain. Since the discovery of the structure of OAPs and the identification of OAPs as the morphological equivalent of the water channel protein aquaporin-4 (AQP4) in the 1990s, a plethora of morphological work on OAPs in different cells was published. Now, we feel a need to balance new and old data on OAPs and AQP4 to elucidate the interrelationship of both structures and molecules. In this review, the identity of OAPs as AQP4-based structures in a diversity of cells will be described. At the same time, arguments are offered that under pathological or experimental circumstances, AQP4 can also be expressed in a non-OAP form. Thus, we attempt to project classical work on OAPs onto the molecular biology of AQP4. In particular, astrocytes and glioma cells will play the major part in this review, not only due to our own work but also due to the fact that most studies on structure and function of AQP4 were done in the nervous system.

Finite element simulations of acetylcholine diffusion in neuromuscular junctions

A robust infrastructure for solving time-dependent diffusion using the finite element package FEtk has been developed to simulate synaptic transmission in a neuromuscular junction with realistic postsynaptic folds. Simplified rectilinear synapse models serve as benchmarks in initial numerical studies of how variations in geometry and kinetics relate to endplate currents associated with fast-twitch, slow-twitch, and dystrophic muscles. The flexibility and scalability of FEtk affords increasingly realistic and complex models that can be formed in concert with expanding experimental understanding from electron microscopy. Ultimately, such models may provide useful insight on the functional implications of controlled changes in processes, suggesting therapies for neuromuscular diseases.

Overexpression of the CT GalNAc transferase in skeletal muscle alters myofiber growth, neuromuscular structure, and laminin expression

Carbohydrates have been shown to mediate or modulate a number of important events in the development of the nervous system; however, there is little evidence that they participate directly in the development of synapses. One carbohydrate structure that is likely to be important in synaptic development of the neuromuscular junction is the CT carbohydrate antigen [GalNAcbeta1,4[NeuAcalpha2,3]Galbeta1(-3GalNAc or -4GlcNAc)]. The synaptic localization of the CT antigen is due to the presence of the terminal beta1,4 GalNAc linkage, and such linkages are localized to the neuromuscular junction in many species. Here we show that an enzyme that can create the synaptic CT structure, the CT GalNAc transferase, is also confined to the neuromuscular junction in mice. Using transgenic mice, we show that overexpression of the CT GalNAc transferase in extrasynaptic regions in skeletal myofibers caused as much as a 60% reduction in the diameter of adult myofibers and an order of magnitude increase in satellite cells. Neuromuscular junctions of transgenic mice had severely reduced numbers of secondary folds, Schwann cell processes were present in the synaptic cleft, and secondary folds were often misaligned with active zones. In addition, multiple presynaptic specializations occurred on individual myofibers. In addition, some normally synaptic proteins, including laminin alpha4, laminin alpha5, utrophin, and NCAM, were expressed along extrasynaptic regions of myofibers. One of the muscle proteins that displayed increased glycosylation with the CT antigen in the transgenic mice was alpha-dystroglycan. These experiments provide the first in vivo evidence that a synaptic carbohydrate antigen has important roles in the development of the neuromuscular synapse and suggest that the CT antigen is involved in controlling the expression of synaptic molecules.

[Neuromuscular junction changes in experimental myopathies in mice]

Morphological abnormalities of the neuromuscular junction in two murine models with primary myopathy were studied by combined cholinesterase-silver impregnation techniques and electron microscopy. In both situations the results were similar showing that the neuromuscular junction remained unaffected even when innervating necrotic muscle fibres. In regenerated muscle fibres, however, there was marked simplification of the post-synaptic membrane with reduction in number and depth of folds up to 50% of normal values confirmed by morphometric analysis. Since after regeneration succeeded no detectable clinical or physiological alterations were observed in these experiments it seems reasonable to assume that the prominent branching of post-synaptic folds in normal skeletal muscles might represent an increased anatomical safety mechanism in chemical transmission.

Scanning electron microscopic study of the neuromuscular junction of dystrophic mice

The structure of the end-plate regions of normal and dystrophic 3-month-old mice were studied by scanning electron microscopy after the presynaptic terminals were removed by hydrochloric acid treatment. Quantitative analysis revealed that the end-plate area correlated positively with the muscle fiber diameter in both the normal and dystrophic animals. However, the motor end-plate area was significantly smaller in the dystrophic mice. The total length of the primary cleft of an end-plate correlated positively with the end-plate area and with the muscle fiber diameter in both normal and dystrophic mice. However, the total length of the primary cleft of an end-plate was significantly shorter in dystrophic mice, especially in large-diameter muscle fibers. Finally, the end-plate of dystrophic mice was characterized by shorter primary clefts with less branching points. These changes of several morphometric characteristics of the postsynaptic membrane suggest that the functional denervation of the mouse dystrophic neuromuscular junction has a postsynaptic origin.

Reduction of postjunctional fold density and depth in dystrophic mice

A total of 769 neuromuscular junction profiles (on 221 muscle fibers) from 21 normal (C57BL/6J +/+), 6 heterozygous (C57BL/6J dy2J/+) and 22 homozygous dystrophic (C57BL/6J dy2J/dy2J) gastrocnemius were photographed in electron microscopy. The fast and slow twitch muscle fibers were differentiated by the width of the Z lines. The number of secondary postjunctional folds per unit length of synaptic cleft was lower in dystrophic than in normal animals. Intermediate values were obtained in heterozygous animals. The length of the postsynaptic membrane was measured and normalized by dividing it by the length of the cross-section through the synaptic cleft. This normalized length of postjunctional membrane was significantly reduced by 45% in dystrophic animals relative to normal animals and by 41% relative to heterozygous animals. The depth of the secondary folds was also significantly reduced in dystrophic animals relative to normal animals in both fast and slow twitch fibers. These morphometric changes of the postjunctional membrane were observed on NMJs whose nerve terminals were not significantly modified by the dystrophic process and were sometimes observed on apparently normal muscle fibers.

Identification of polypeptides associated with sarcolemmal vesicles enriched in orthogonal arrays

Electron microscopy of freeze-fracture replicas from the sarcolemmas of fast-twitch muscle fibers reveals orthogonal arrays of particles. The biochemical nature of macromolecules associated with the sarcolemmal orthogonal array was investigated using muscle fragments and isolated sarcolemmal vesicles. Muscle fragments incubated in vitro with the lectin concanavalin A exhibited a clustering of orthogonal arrays into local patches. Treatment with other lectins did not result in the clustering of arrays. Clustering was inhibited by the addition of alpha-methyl-D-mannoside, a ligand which also binds concanavalin A. These results suggest that the orthogonal arrays (or associated components) specifically bind concanavalin A. Sarcolemmal vesicles from rabbit sacrospinalis (SAC) and rat extensor digitorum longus (EDL) (both primarily fast-twitch) and rat soleus (SOL) (primarily slow-twitch) were obtained by a combination of low-salt fractionation and sucrose density gradient centrifugation. SDS-polyacrylamide gel electrophoresis (SDS-PAGE) of proteins and glycoproteins solubilized from these vesicles revealed several bands. Four of these bands were present in gels from both the rabbit and rat fast-twitch muscle sarcolemmal preparations (that contained arrays), yet were absent in gels from rat slow-twitch muscle sarcolemmal preparations (not bearing arrays). An enrichment in vesicles containing arrays was achieved by binding SAC sarcolemmal vesicles to Con A-Sepharose 4B beads. SDS-PAGE analysis of array-enriched vesicles from the concanavalin A beads revealed enrichment of three major bands at Mr 93,000, 54,000 and 49,000. These enriched bands correlate with three of the four bands common to fast-twitch EDL and SAC, yet absent in slow-twitch SOL sarcolemmal preparations. We conclude that at least one macromolecular component associated with the sarcolemmal orthogonal array is a concanavalin A binding glycoprotein. We further conclude that three candidates for this component co-purify with the morphological array, and have approximate molecular weights of 93,000, 54,000 and 49,000.

A freeze-fracture analysis of intramembrane particle densities on dystrophic hamster heart sarcolemma

The intramembrane particle (IMP) profile of control and dystrophic (Bio 14.6) hamster cardiac muscle plasma membrane was assessed in freeze-fracture replicas to determine whether this animal model of muscular dystrophy exhibits the same membrane characteristics found in skeletal muscle from other more thoroughly studied dystrophic animals, and to test the hypothesis that the plasma membrane of the cardiac muscle cell is the site of a defect associated with the disease. Samples of cardiac muscle tissue from hamsters ranging in age from 1 to 13 months were freeze-fractured. Intramembrane particle numbers were determined for all tissue samples by counting randomly selected areas of P- and E-face surfaces. Up to the age of 1 month, the particle density was the same in both strains of hamster, after which time, the population of IMPs was about 30% lower in dystrophic than in normal heart sarcolemma. This 30% difference in particle frequency in dystrophic hamster heart membrane is consistent with values published for cell membrane from other muscular dystrophies and supports the theory that there is a defect in the plasma membrane of dystrophic cells. In addition, this study has shown for the first time that a presumed membrane defect related to muscular dystrophy (reduced number of IMPs) may be present throughout the life of the animal (1-13 months), and expressed in every cell sampled.

Intense ultraterminal sprouting from motor nerves and ultrastructural aspects of the neuromuscular junction and non-junctional sarcolemma of the soleus (slow-twitch) muscle in motor endplate disease in the mouse

Motor end-plate disease (med) in the mouse is an hereditary defect of the neuromuscular system, with partial functional denervation and muscle inactivity in late stages of the disease. Motor end-plate disease is characterized by an intense ultraterminal sprouting of the motor nerves from swollen nerve terminal branches in the soleus muscle. At the ultrastructural level, the neuromuscular junctions extend to very wide territories, often outside the original motor end-plate, in regions where the nerve sprouts are in simple apposition to the muscle fiber, with no secondary synaptic folds. The nerve terminals are rich in neurofilaments and poor in synaptic vesicles. Freeze fracture analysis of the pre-synaptic and post-synaptic membrane specializations fails to reveal any important structural alteration which could suggest a defect in acetylcholine release or in muscle membrane excitability. However, the non-junctional sarcolemmal specializations (the so-called 'square arrays') are found with a frequency slightly higher than in normal muscle. The nerve abnormalities at the neuromuscular junction may be either a consequence of muscle inactivity or the morphological expression of some primary nerve abnormality. Further studies of the soleus muscle at early stages of the disease may provide evidence in favor of either possibility.

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.