Confocal Endomicroscopy of Neuromuscular Junctions Stained with Physiologically Inert Protein Fragments of Tetanus Toxin

Live imaging of neuromuscular junctions (NMJs) in situ has been constrained by the suitability of ligands for inert vital staining of motor nerve terminals. Here, we constructed several truncated derivatives of the tetanus toxin C-fragment (TetC) fused with Emerald Fluorescent Protein (emGFP). Four constructs, namely full length emGFP-TetC (emGFP-865:TetC) or truncations comprising amino acids 1066-1315 (emGFP-1066:TetC), 1093-1315 (emGFP-1093:TetC) and 1109-1315 (emGFP-1109:TetC), produced selective, high-contrast staining of motor nerve terminals in rodent or human muscle explants. Isometric tension and intracellular recordings of endplate potentials from mouse muscles indicated that neither full-length nor truncated emGFP-TetC constructs significantly impaired NMJ function or transmission. Motor nerve terminals stained with emGFP-TetC constructs were readily visualised in situ or in isolated preparations using fibre-optic confocal endomicroscopy (CEM). emGFP-TetC derivatives and CEM also visualised regenerated NMJs. Dual-waveband CEM imaging of preparations co-stained with fluorescent emGFP-TetC constructs and Alexa647-α-bungarotoxin resolved innervated from denervated NMJs in axotomized WldS mouse muscle and degenerating NMJs in transgenic SOD1G93A mouse muscle. Our findings highlight the region of the TetC fragment required for selective binding and visualisation of motor nerve terminals and show that fluorescent derivatives of TetC are suitable for in situ morphological and physiological characterisation of healthy, injured and diseased NMJs.

Salbutamol modifies the neuromuscular junction in a mouse model of ColQ myasthenic syndrome

The β-adrenergic agonists salbutamol and ephedrine have proven to be effective as therapies for human disorders of the neuromuscular junction, in particular many subsets of congenital myasthenic syndromes. However, the mechanisms underlying this clinical benefit are unknown and improved understanding of the effect of adrenergic signalling on the neuromuscular junction is essential to facilitate the development of more targeted therapies. Here, we investigated the effect of salbutamol treatment on the neuromuscular junction in the ColQ deficient mouse, a model of end-plate acetylcholinesterase deficiency. ColQ-/- mice received 7 weeks of daily salbutamol injection, and the effect on muscle strength and neuromuscular junction morphology was analysed. We show that salbutamol leads to a gradual improvement in muscle strength in ColQ-/- mice. In addition, the neuromuscular junctions of salbutamol treated mice showed significant improvements in several postsynaptic morphological defects, including increased synaptic area, acetylcholine receptor area and density, and extent of postjunctional folds. These changes occurred without alterations in skeletal muscle fibre size or type. These findings suggest that β-adrenergic agonists lead to functional benefit in the ColQ-/- mouse and to long-term structural changes at the neuromuscular junction. These effects are primarily at the postsynaptic membrane and may lead to enhanced neuromuscular transmission.

'Fragmentation' of NMJs: a sign of degeneration or regeneration? A long journey with many junctions

Mammalian neuromuscular junctions (NMJs) often consist of curved bands of synaptic contact, about 3-6 μm wide, which resemble pretzels. This contrasts with the NMJs of most animal species which consist of a cluster of separate synaptic spots, each of which is also about 3-6 μm across. In a number of situations, including a variety of disease states as well as normal ageing, mammalian NMJs acquire a more 'fragmented' appearance that resembles somewhat that of other species. This 'fragmentation' of the NMJ has sometimes been interpreted as a 'disintegration' or 'degeneration', with the suggestion that it might be associated with impaired neuromuscular transmission. An alternative view is that NMJ fragmentation is the outcome of a normal process by which the NMJ is maintained in an effective state. In this highly personal commentary, I cite a number of examples of this and point out that although the 'pretzel' form arises during normal development as a result of the sculpting of an immature synaptic 'plaque', in virtually all situations where new synaptic contact is established in adult mammals this occurs by the addition of new synaptic 'spots' rather than by the extension, or neoformation, of 'pretzels'. Further, where appropriate studies have been performed, no evidence of a correlation between the degree of fragmentation and the efficacy of transmission has emerged. It may therefore be more appropriate to consider NMJ 'fragmentation' as a form of regeneration, rather than of degeneration. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

GFPT1 deficiency in muscle leads to myasthenia and myopathy in mice

Glutamine-fructose-6-phosphate transaminase 1 (GFPT1) is the rate-limiting enzyme in the hexosamine biosynthetic pathway which yields precursors required for protein and lipid glycosylation. Mutations in GFPT1 and other genes downstream of this pathway cause congenital myasthenic syndrome (CMS) characterized by fatigable muscle weakness owing to impaired neurotransmission. The precise pathomechanisms at the neuromuscular junction (NMJ) owing to a deficiency in GFPT1 is yet to be discovered. One of the challenges we face is the viability of Gfpt1^−/− knockout mice. In this study, we use Cre/LoxP technology to generate a muscle-specific GFPT1 knockout mouse model, Gfpt1^tm1d/tm1d, characteristic of the human CMS phenotype. Our data suggest a critical role for muscle derived GFPT1 in the development of the NMJ, neurotransmission, skeletal muscle integrity and highlight that a deficiency in skeletal muscle alone is sufficient to cause morphological postsynaptic NMJ changes that are accompanied by presynaptic alterations despite the conservation of neuronal GFPT1 expression. In addition to the conventional morphological NMJ changes and fatigable muscle weakness, Gfpt1^tm1d/tm1d mice display a progressive myopathic phenotype with the presence of tubular aggregates in muscle, characteristic of the GFPT1-CMS phenotype. We further identify an upregulation of skeletal muscle proteins glypican-1, farnesyltransferase/geranylgeranyltransferase type-1 subunit α and muscle-specific kinase, which are known to be involved in the differentiation and maintenance of the NMJ. The Gfpt1^tm1d/tm1d model allows for further investigation of pathophysiological consequences on genes and pathways downstream of GFPT1 likely to involve misglycosylation or hypoglycosylation of NMJs and muscle targets.

Age-related fragmentation of the motor endplate is not associated with impaired neuromuscular transmission in the mouse diaphragm

As mammals age, their neuromuscular junctions (NMJs) gradually change their form, acquiring an increasingly fragmented appearance consisting of numerous isolated regions of synaptic differentiation. It has been suggested that this remodelling is associated with impairment of neuromuscular transmission, and that this contributes to age-related muscle weakness in mammals, including humans. The underlying hypothesis, that increasing NMJ fragmentation is associated with impaired transmission, has never been directly tested. Here, by comparing the structure and function of individual NMJs, we show that neuromuscular transmission at the most highly fragmented NMJs in the diaphragms of old (26-28 months) mice is, if anything, stronger than in middle-aged (12-14 months) mice. We suggest that NMJ fragmentation per se is not a reliable indicator of impaired neuromuscular transmission.

Glutamatergic modulation of synaptic-like vesicle recycling in mechanosensory lanceolate nerve terminals of mammalian hair follicles

Our aim in the present study was to determine whether a glutamatergic modulatory system involving synaptic-like vesicles (SLVs) is present in the lanceolate ending of the mouse and rat hair follicle and, if so, to assess its similarity to that of the rat muscle spindle annulospiral ending we have described previously. Both types of endings are formed by the peripheral sensory terminals of primary mechanosensory dorsal root ganglion cells, so the presence of such a system in the lanceolate ending would provide support for our hypothesis that it is a general property of fundamental importance to the regulation of the responsiveness of the broad class of primary mechanosensory endings. We show not only that an SLV-based system is present in lanceolate endings, but also that there are clear parallels between its operation in the two types of mechanosensory endings. In particular, we demonstrate that, as in the muscle spindle: (i) FM1-43 labels the sensory terminals of the lanceolate ending, rather than the closely associated accessory (glial) cells; (ii) the dye enters and leaves the terminals primarily by SLV recycling; (iii) the dye does not block the electrical response to mechanical stimulation, in contrast to its effect on the hair cell and dorsal root ganglion cells in culture; (iv) SLV recycling is Ca²⁺ sensitive; and (v) the sensory terminals are enriched in glutamate. Thus, in the lanceolate sensory ending SLV recycling is itself regulated, at least in part, by glutamate acting through a phospholipase D-coupled metabotropic glutamate receptor.

Mutations in DPAGT1 cause a limb-girdle congenital myasthenic syndrome with tubular aggregates

Congenital myasthenic syndromes are a heterogeneous group of inherited disorders that arise from impaired signal transmission at the neuromuscular synapse. They are characterized by fatigable muscle weakness. We performed whole-exome sequencing to determine the underlying defect in a group of individuals with an inherited limb-girdle pattern of myasthenic weakness. We identify DPAGT1 as a gene in which mutations cause a congenital myasthenic syndrome. We describe seven different mutations found in five individuals with DPAGT1 mutations. The affected individuals share a number of common clinical features, including involvement of proximal limb muscles, response to treatment with cholinesterase inhibitors and 3,4-diaminopyridine, and the presence of tubular aggregates in muscle biopsies. Analyses of motor endplates from two of the individuals demonstrate a severe reduction of endplate acetylcholine receptors. DPAGT1 is an essential enzyme catalyzing the first committed step of N-linked protein glycosylation. Our findings underscore the importance of N-linked protein glycosylation for proper functioning of the neuromuscular junction. Using the DPAGT1-specific inhibitor tunicamycin, we show that DPAGT1 is required for efficient glycosylation of acetylcholine-receptor subunits and for efficient export of acetylcholine receptors to the cell surface. We suggest that the primary pathogenic mechanism of DPAGT1 mutations is reduced levels of acetylcholine receptors at the endplate region. These individuals share clinical features similar to those of congenital myasthenic syndrome due to GFPT1 mutations, and their disorder might be part of a larger subgroup comprising the congenital myasthenic syndromes that result from defects in the N-linked glycosylation pathway and that manifest through impaired neuromuscular transmission.

Mutations in DPAGT1 cause a limb-girdle congenital myasthenic syndrome with tubular aggregates

Congenital myasthenic syndromes are a heterogeneous group of inherited disorders that arise from impaired signal transmission at the neuromuscular synapse. They are characterized by fatigable muscle weakness. We performed whole-exome sequencing to determine the underlying defect in a group of individuals with an inherited limb-girdle pattern of myasthenic weakness. We identify DPAGT1 as a gene in which mutations cause a congenital myasthenic syndrome. We describe seven different mutations found in five individuals with DPAGT1 mutations. The affected individuals share a number of common clinical features, including involvement of proximal limb muscles, response to treatment with cholinesterase inhibitors and 3,4-diaminopyridine, and the presence of tubular aggregates in muscle biopsies. Analyses of motor endplates from two of the individuals demonstrate a severe reduction of endplate acetylcholine receptors. DPAGT1 is an essential enzyme catalyzing the first committed step of N-linked protein glycosylation. Our findings underscore the importance of N-linked protein glycosylation for proper functioning of the neuromuscular junction. Using the DPAGT1-specific inhibitor tunicamycin, we show that DPAGT1 is required for efficient glycosylation of acetylcholine-receptor subunits and for efficient export of acetylcholine receptors to the cell surface. We suggest that the primary pathogenic mechanism of DPAGT1 mutations is reduced levels of acetylcholine receptors at the endplate region. These individuals share clinical features similar to those of congenital myasthenic syndrome due to GFPT1 mutations, and their disorder might be part of a larger subgroup comprising the congenital myasthenic syndromes that result from defects in the N-linked glycosylation pathway and that manifest through impaired neuromuscular transmission.

Congenital myasthenic syndromes and the formation of the neuromuscular junction

The congenital myasthenic syndromes (CMS) are a heterogeneous group of disorders affecting neuromuscular transmission. Underlying mutations have been identified in at least 11 different genes. The majority of CMS patients have disorders due to mutations in postsynaptic proteins. Initial studies focused on dysfunction of the acetylcholine receptor (AChR) itself as the major cause of CMS. However, it is becoming apparent that mutations of proteins involved in clustering the AChR and maintaining neuromuscular junction structure form important subgroups. Analysis of the mutations in the AChR-clustering protein, rapsyn, show diverse causes for defective AChR localization and suggest that the common mutation rapsyn-N88K results in AChR clusters that are less stable than those generated by wild-type rapsyn. More recently, mutations in the newly identified endplate protein Dok-7 have been shown to affect AChR clustering and the generation and maintenance of specialized structures at the endplate. Dok-7 binds MuSK and many of the mutations of DOK7 impair the MuSK signaling pathway. Components of this pathway will provide attractive gene candidates for additional forms of CMS. The phenotypic characteristics of the different CMS in which muscle groups may be differentially affected not only provide clues for targeted genetic screening, but also pose further intriguing questions about underlying molecular mechanisms.

Recovery of mouse neuromuscular junctions from single and repeated injections of botulinum neurotoxin A

Botulinum neurotoxin type A (BoNT/A) paralyses muscles by blocking acetylcholine (ACh) release from motor nerve terminals. Although highly toxic, it is used clinically to weaken muscles whose contraction is undesirable, as in dystonias. The effects of an injection of BoNT/A wear off after 3–4 months so repeated injections are often used. Recovery of neuromuscular transmission is accompanied by the formation of motor axon sprouts, some of which form new synaptic contacts. However, the functional importance of these new contacts is unknown. Using intracellular and focal extracellular recording we show that in the mouse epitrochleoanconeus (ETA), quantal release from the region of the original neuromuscular junction (NMJ) can be detected as soon as from new synaptic contacts, and generally accounts for > 80% of total release. During recovery the synaptic delay and the rise and decay times of endplate potentials (EPPs) become prolonged approximately 3-fold, but return to normal after 2–3 months. When studied after 3–4 months, the response to repetitive stimulation at frequencies up to 100 Hz is normal. When two or three injections of BoNT/A are given at intervals of 3–4 months, quantal release returns to normal values more slowly than after a single injection (11 and 15 weeks to reach 50% of control values versus 6 weeks after a single injection). In addition, branching of the intramuscular muscular motor axons, the distribution of the NMJs and the structure of many individual NMJs remain abnormal. These findings highlight the plasticity of the mammalian NMJ but also suggest important limits to it.

Pre- and post-synaptic abnormalities associated with impaired neuromuscular transmission in a group of patients with 'limb-girdle myasthenia'

The properties of neuromuscular junctions (NMJs) were studied in motor-point biopsy samples from eight patients with congenital myasthenic syndromes affecting primarily proximal limb muscles ['limb-girdle myasthenia' (LGM)]. All had moderate to severe weakness of the proximal muscles, without short-term clinical fatigability but with marked variation in strength over periods of weeks or months, with little or no facial weakness or ptosis and no ophthalmoplegia. Most had a characteristic gait and stance. All patients showed decrement of the compound muscle action potential (CMAP) on repetitive stimulation at 3 Hz, and increased jitter and blocking was detected by SFEMG, confirming the presence of impaired neuromuscular transmission. None of the patients had serum antibodies against acetylcholine receptors (AChRs). Two of the patients had similarly affected siblings. Intracellular recording from isolated nerve-muscle preparations revealed that the quantal content (the number of ACh quanta released per nerve impulse) was only approximately 50% of that in controls. However, the quantal size (amplitude of miniature end-plate currents) and the kinetic properties of synaptic potentials and currents were similar to control values. The area of synaptic contact and extent of post-synaptic folding were approximately 50% of control values. Thus, the quantal content per unit area of synaptic contact was normal. The number of AChRs per NMJ was also reduced to approximately 50% of normal, so the local AChR density was normal. Immunolabelling studies revealed qualitatively normal distributions and abundance of each of 14 proteins normally concentrated at the NMJ, including components of the basal lamina, post-synaptic membrane and post-synaptic cytoskeleton. DNA analysis failed to detect mutations in the genes encoding any of the following proteins: AChR subunits, rapsyn, ColQ, ChAT or muscle-specific kinase. Response of these patients to treatment was varied: few showed long-term improvement with pyridostigmine and some even deteriorated with treatments, while others had intolerable side-effects. Several patients showed improvement with 3,4-diaminopyridine, but this was generally only transient. Ephedrine was helpful in half of the patients. We conclude that impaired neuromuscular transmission in these LGM patients results from structural abnormalities of the NMJ, including reduced size and post-synaptic folding, rather from any abnormality in the immediate events of neuromuscular transmission.

Accumulation of Nav1 mRNAs at differentiating postsynaptic sites in rat soleus muscles

Acetylcholine receptors (AChRs) and voltage-gated sodium channels (Na(V)1s) accumulate at different times in the development of the murine neuromuscular junction (NMJ). We used in situ hybridization to study the relationship of Na(V)1 mRNA accumulation to this difference. mRNAs encoding both muscle Na(V)1 isoforms, Na(v)1.4 and Na(v)1.5, were first concentrated at NMJs at birth, when the proteins start to accumulate. Within 4 weeks, Na(v)1.4 mRNA increased 5-fold at the NMJ while Na(v)1.5 mRNA became undetectable. Na(V)1 mRNA accumulation occurred even if the nerve was cut at birth. Like AChR mRNA, Na(V)1 mRNA accumulated at denervated synaptic sites on regenerating muscles and in response to ectopically expressed neural agrin. Clustering of Na(V)1 at the NMJ follows that of its mRNA while AChR clustering precedes its mRNA clustering by several days. This suggests that factors other than local mRNA upregulation determine the timing of clustering of these two important postsynaptic ion channels.

Structural determinants of the reliability of synaptic transmission at the vertebrate neuromuscular junction

The reliability of neuromuscular transmission depends on the size and molecular organization of the neuromuscular junction. Comparative studies show that the quantal release per unit area is similar at neuromuscular junctions in a number of species in spite of wide variation in synaptic area. They also show an inverse relationship between the size of the nerve terminal and the extent of postsynaptic folding. Evidence is presented supporting the view that the folds, and the voltage-gated sodium channels present in them, effectively amplify synaptic currents. How are the size and molecular organization of the neuromuscular junction determined? Studies with botulinum toxin, including our new work on humans, reveal striking "adaptive plasticity" of the nerve terminal. However, the links between synaptic size and effective transmission remain unclear. On the postsynaptic side, we have shown that mRNA encoding sodium channels is concentrated at the adult junction. During development, mRNA accumulates just before the protein it encodes. Throughout development the sodium channels are associated with ankyrinG and both proteins are initially excluded from the junctional acetylcholine receptor cluster, possibly accounting for the formation of the boundary between the domains occupied by the two key postsynaptic ion channels. These findings have important clinical implications. Reduced transmitter release may result from small nerve terminals as much as from defective release. Abnormal folding is likely to reduce the reliability of transmission. A better understanding of how the structural features that influence the reliability of the neuromuscular transmission are controlled should be of general interest to neuroscientists and of use to clinicians.

Voltage-gated sodium channels and ankyrinG occupy a different postsynaptic domain from acetylcholine receptors from an early stage of neuromuscular junction maturation in rats

Spatial segregation of membrane proteins is a feature of many excitable cells. In skeletal muscle, clusters of acetylcholine receptors (AChRs) and voltage-gated sodium channels (Na(V)1s) occupy distinct domains at the neuromuscular junction (NMJ). We used quantitative immunolabeling of developing rat soleus muscles to study the mechanism of ion channel segregation and Na(V)1 clustering at NMJs. When Na(V)1s can first be detected, at birth, they already occupy a postsynaptic domain that is distinct from that occupied by AChRs. At this time, Na(V)1s are expressed only in a diffuse area that extends 50-100 microm from the immature NMJ. However, in the region of the high-density AChR cluster at NMJ itself, Na(V)1s are actually present in lower density than in the immediately surrounding membrane. These distinctive features of the Na(V)1 distribution at birth are closely correlated with the distribution of ankyrinG immunolabeling. This suggests that an interaction with ankyrinG plays a role in the initial segregation of Na(V)1s from AChRs. Both Na(V)1 and ankyrinG become clustered at the NMJ itself 1-2 weeks after birth, coincident with the formation of postsynaptic folds. Syntrophin immunolabeling codistributes with AChRs and never resembles that for Na(V)1 or ankyrinG. Therefore, syntrophin is unlikely to play an important part in the initial accumulation of Na(V)1 at the NMJ. These findings suggest that the segregation of Na(V)1 from AChRs begins early in NMJ formation and occurs as a result of the physical exclusion of Na(V)1 and ankyrinG from the region of nerve-muscle contact rather than by a process of active clustering.

Sodium channel mRNAs at the neuromuscular junction: distinct patterns of accumulation and effects of muscle activity

Voltage-gated sodium channels (VGSCs) are highly concentrated at the neuromuscular junction (NMJ) in mammalian skeletal muscle. Here we test the hypothesis that local upregulation of mRNA contributes to this accumulation. We designed radiolabeled antisense RNA probes, specific for the "adult" Na(V)1.4 and "fetal" Na(V)1.5 isoforms of VGSC in mammalian skeletal muscle, and used them in in situ hybridization studies of rat soleus muscles. Na(V)1.4 mRNA is present throughout normal adult muscles but is highly concentrated at the NMJ, in which the amount per myonucleus is more than eightfold greater than away from the NMJ. Na(V)1.5 mRNA is undetectable in innervated muscles but is dramatically upregulated by denervation. In muscles denervated for 1 week, both Na(V)1.4 and Na(V)1.5 mRNAs are present throughout the muscle, and both are concentrated at the NMJ. No Na(V)1.5 mRNA was detectable in denervated muscles stimulated electrically for 1 week in vivo. Neither denervation nor stimulation had any significant effect on the level or distribution of Na(V)1.4 mRNA. We conclude that factors, probably derived from the nerve, lead to the increased concentration of VGSC mRNAs at the NMJ. In addition, the expression of Na(V)1.5 mRNA is downregulated by muscle activity, both at the NMJ and away from it.

Safety factor at the neuromuscular junction

Reliable transmission of activity from nerve to muscle is necessary for the normal function of the body. The term 'safety factor' refers to the ability of neuromuscular transmission to remain effective under various physiological conditions and stresses. This is a result of the amount of transmitter released per nerve impulse being greater than that required to trigger an action potential in the muscle fibre. The safety factor is a measure of this excess of released transmitter. In this review we discuss the practical difficulties involved in estimating the safety factor in vitro. We then consider the factors that influence the safety factor in vivo. While presynaptic transmitter release may be modulated on a moment to moment basis, the postsynaptic features that determine the effect of released transmitter are not so readily altered to meet changing demands. Different strategies are used by different species to ensure reliable neuromuscular transmission. Some, like frogs, rely on releasing a large amount of transmitter while others, like man, rely on elaborate postsynaptic specialisations to enhance the response to transmitter. In normal adult mammals, the safety factor is generally 3-5. Both pre- and postsynaptic components change during development and may show plasticity in response to injury or disease. Thus, both acquired autoimmune and inherited congenital diseases of the neuromuscular junction (NMJ) can significantly reduce, or even transiently increase, safety factor.

Ectopic expression of NCAM in skeletal muscle of transgenic mice results in terminal sprouting at the neuromuscular junction and altered structure but not function

The neuromuscular system provides an excellent model for the analysis of molecular interactions involved in the development and plasticity of synaptic contacts. The neural cell adhesion molecule (NCAM) is believed to be involved in the development and plasticity of the neuromuscular junction, in particular the axonal sprouting response observed in paralyzed and denervated muscle. In order to explore the role of myofiber NCAM in modulating the differentiation of motor neurons, we generated transgenic mice expressing a GPI-anchored NCAM isoform that is normally found in developing and denervated muscle, under the control of a skeletal muscle-specific promoter. This results in the constitutive expression of NCAM at postnatal ages, a time when the endogenous mouse NCAM is absent from the myofiber. We found that a significant number of neuromuscular junctions in adult transgenic animals displayed terminal sprouting (>20%) reminiscent of that elicited in response to cessation of neuromuscular activity. Additionally, a significant increase in the size and complexity of neuromuscular synapses as a result of extensive intraterminal sprouting was detected. Electrophysiological studies, however, revealed no significant alterations of neuromuscular transmission at this highly efficient synapse. Sprouting in response to paralysis or following nerve crush was also significantly enhanced in transgenic animals. These results suggest that in this ectopic expression model NCAM can directly modulate synaptic structure and motor neuron-muscle interactions. The results contrast with knockout experiments of the NCAM gene, where very limited changes in the neuromuscular system were observed.

Synaptic vesicle dynamics in rat fast and slow motor nerve terminals

We have investigated whether rat motor nerve terminals with different in vivo activity patterns also have different vesicle trafficking characteristics. To do this, we monitored, using combined optical and electrical techniques, the rate of exocytosis (during different frequencies and patterns of activity), the releasable pool size, and the recycle time of synaptic vesicles in terminals on soleus (slow-twitch) and extensor digitorum longus [(EDL); fast-twitch] muscle fibers. EDL terminals had a higher initial quantal content (QC) than soleus, but during tonic or phasic stimulation at 20-80 Hz, EDL QC ran down to a greater extent than soleus QC. By recording loss of fluorescence from exocytosing vesicles labeled with the dye FM1-43, EDL terminals were found to destain faster than those in soleus. Simultaneous intracellular recording of end plate potentials, to count the number of vesicles released, permitted estimation of the total vesicle pool (VP) size and the recycle time by combining the optical and electrophysiological data. Soleus vesicle pool was larger than EDL, but recycle time was not significantly different. These terminals, therefore, are adapted to their in vivo activity patterns by alterations in QC and VP size but not recycle time.

An early stage in sodium channel clustering at developing rat neuromuscular junctions

Voltage-gated sodium channels (VGSCs) are concentrated in the postsynaptic membrane at the adult rat neuromuscular junction (NMJ). We have used immunolabelling to determine the pattern of initial VGSC accumulation during development. At birth, but not 3 days before, VGSC labelling is detectable at the NMJ and in the perijunctional (periJ) membrane but not elsewhere. A much higher density cluster of VGSCs forms at the NMJ itself 1-2 weeks later. If the nerve is cut 2 days after birth, VGSC labelling persists in the periJ region for at least 4 weeks but the clustering of VGSCs at the NMJ fails to develop. Thus an early, stable accumulation of VGSCs develops near the NMJ at least a week before high density postsynaptic VGSC clusters form.

Utrophin mRNA expression in muscle is not restricted to the neuromuscular junction

Utrophin is normally present exclusively in synaptic regions of skeletal muscle fibers, although it is expressed extrasynaptically in certain pathological situations, where it has been proposed to compensate for the absence of dystrophin in Duchenne muscular dystrophy patients and mdx mice. Recently there have been conflicting reports regarding the preferential expression of utrophin mRNA at the neuromuscular junction. Using in situ hybridization with RNA probes, we show a clear accumulation of autoradiographic labeling at more than 90% of neuromuscular junctions (identified by histochemical demonstration of cholinesterase activity). The intensity of this labeling is proportional to the number of junctional myonuclei in the section. Some clusters of labeling were found associated with nonmuscle nuclei (e.g., blood vessels, nerves), where utrophin is present. In addition, labeling for utrophin mRNA was associated with about 25% of extrajunctional myonuclei, where the protein is not present. The mean labeling per nucleus at junctional myonuclei was at least 10 times greater than at extrajunctional myonuclei. We discuss the possible regulatory mechanisms involved in the heterogeneous expression of utrophin mRNA in skeletal muscle.

beta-Spectrin is colocalized with both voltage-gated sodium channels and ankyrinG at the adult rat neuromuscular junction

Voltage-gated sodium channels (VGSCs) are concentrated in the depths of the postsynaptic folds at mammalian neuromuscular junctions (NMJs) where they facilitate action potential generation during neuromuscular transmission. At the nodes of Ranvier and the axon hillocks of central neurons, VGSCs are associated with the cytoskeletal proteins, beta-spectrin and ankyrin, which may help to maintain the high local density of VGSCs. Here we show in skeletal muscle, using immunofluorescence, that beta-spectrin is precisely colocalized with both VGSCs and ankyrinG, the nodal isoform of ankyrin. In en face views of rat NMJs, acetylcholine receptors (AChRs), and utrophin immunolabeling are organized in distinctive linear arrays corresponding to the crests of the postsynaptic folds. In contrast, beta-spectrin, VGSCs, and ankyrinG have a punctate distribution that extends laterally beyond the AChRs, consistent with a localization in the depths of the folds. Double antibody labeling shows that beta-spectrin is precisely colocalized with both VGSCs and ankyrinG at the NMJ. Furthermore, quantification of immunofluorescence in labeled transverse sections reveals that beta-spectrin is also concentrated in perijunctional regions, in parallel with an increase in labeling of VGSCs and ankyrinG, but not of dystrophin. These observations suggest that interactions with beta-spectrin and ankyrinG help to maintain the concentration of VGSCs at the NMJ and that a common mechanism exists throughout the nervous system for clustering VGSCs at a high density.

An improved method for the simultaneous demonstration of mRNA and esterase activity at the human neuromuscular junction

The aim of this study was to develop a simple means of studying the distribution of mRNA coding for post-synaptic proteins at the human neuromuscular junction. A reliable method by which to identify the junctions in tissue sections after in situ hybridization was essential. A method is described for combining the histochemical demonstration of esterase activity at the neuromuscular junction with autoradiographic localization of mRNA by in situ hybridization in the same cryostat section of skeletal muscle. The indigogenic esterase method of Strum and Hall-Craggs (1982) was modified in such a way that it is able to survive the multiple steps involved in in situ hybridization and autoradiography. The protocol is simple and reproducible and has been used successfully on sections of both rat and human skeletal muscle. To demonstrate the method, sections were reacted to reveal esterase activity and were then processed for in situ hybridization using a 35S-labelled probe specific for the epsilon-subunit of the acetylcholine receptor. The reaction product was retained after the lengthy in situ hybridization and autoradiographic procedures. To our knowledge, this is the first demonstration of acetylcholine receptor mRNA by in situ hybridization at human neuromuscular junctions.

Utrophin abundance is reduced at neuromuscular junctions of patients with both inherited and acquired acetylcholine receptor deficiencies

Congenital myasthenic syndromes are a heterogeneous group of conditions in which muscle weakness resulting from impaired neuromuscular transmission is often present from infancy. One form of congenital myasthenic syndrome is due to a reduction of the number of acetylcholine receptors (AChRs) at the neuromuscular junction. We describe four new cases of AChR deficiency, characterized by a reduction in both miniature endplate potential amplitude and AChR abundance accompanied by elongation of the neuromuscular junction and some decrease in postsynaptic folding. A number of cytoplasmic proteins are normally associated with the postsynaptic membrane and may contribute to the clustering of AChRs at the neuromuscular junction. We therefore investigated the expression of several of these proteins in these AChR-deficiency patients. In each patient, immunolabelling of the neuromuscular junction for rapsyn, dystrophin, beta-dystroglycan and a form of beta-spectrin was strong but that for utrophin was markedly reduced or absent. This suggested that a defect in utrophin expression might underlie the congenital AChR deficiency. However, a reduction in utrophin labelling was also seen in three patients with adult acquired autoimmune myasthenia gravis in whom AChR loss results directly from the extracellular binding of autoantibodies. We conclude that the loss of AChRs in AChR deficiency does not result from the absence of rapsyn or beta-dystroglycan and that reduction of utrophin is probably secondary to the loss of AChRs. The possible role of AChRs and/or utrophin in determining the extent of postsynaptic folding is discussed.

Microdosimetry spectra of the Loma Linda proton beam and relative biological effectiveness comparisons

Protons have long been recognized as low LET radiation in radiotherapy. However, a detailed account of LET (linear energy transfer) and RBE (relative biological effectiveness) changes with incident beam energy and depth in tissue is still unresolved. This issue is particularly important for treatment planning, where the physical dose prescription is calculated from a RBE using cobalt as the reference radiation. Any significant RBE changes with energy or depth will be important to incorporate in treatment planning. In this paper we present microdosimetry spectra for the proton beam at various energies and depths and compare the results to cell survival studies performed at Loma Linda. An empirically determined biological weighting function that depends on lineal energy is used to correlate the microdosimetry spectra with cell survival data. We conclude that the variations in measured RBE with beam energy and depth are small until the distal edge of the beam is reached. On the distal edge, protons achieve stopping powers as high as 100 keV/micron, which is reflected in the lineal energy spectra taken there. Lineal energy spectra 5 cm beyond the distal edge of the Bragg peak also show a high LET component but at a dose rate 600 times smaller than observed inside the proton field.

Differential localization of voltage-dependent calcium channel alpha1 subunits at the human and rat neuromuscular junction

Neurotransmitter release is regulated by voltage-dependent calcium channels (VDCCs) at synapses throughout the nervous system. At the neuromuscular junction (NMJ) electrophysiological and pharmacological studies have identified a major role for P- and/or Q-type VDCCs in controlling acetylcholine release from the nerve terminal. Additional studies have suggested that N-type channels may be involved in neuromuscular transmission. VDCCs consist of pore-forming alpha1 and regulatory beta subunits. In this report, using fluorescence immunocytochemistry, we provide evidence that immunoreactivity to alpha1A, alpha1B, and alpha1E subunits is present at both rat and human adult NMJs. Using control and denervated rat preparations, we have been able to establish that the subunit thought to correspond to P/Q-type channels, alpha1A, is localized presynaptically in discrete puncta that may represent motor nerve terminals. We also demonstrate for the first time that alpha1A and alpha1B (which corresponds to N-type channels) may be localized in axon-associated Schwann cells and, further, that the alpha1B subunit may be present in perisynaptic Schwann cells. In addition, the alpha1E subunit (which may correspond to R/T-type channels) seems to be localized postsynaptically in the muscle fiber membrane and concentrated at the NMJ. The possibility that all three VDCCs at the NMJ are potential targets for circulating autoantibodies in amyotrophic lateral sclerosis is discussed.

The contribution of postsynaptic folds to the safety factor for neuromuscular transmission in rat fast- and slow-twitch muscles

1. At the rat neuromuscular junction, the postsynaptic folds and the voltage-gated sodium channels (VGSCs) within them are thought to amplify the effects of postsynaptic currents. In this study, the contribution of this effect to the safety factor for neuromuscular transmission, the ratio of the normal quantal content to the number of quanta required to reach threshold, has been estimated. 2. Normal quantal content was determined in isolated nerve-muscle preparations of rat soleus and extensor digitorum longus (EDL) muscles in which muscle action potentials were blocked by mu-conotoxin. The quantal content estimated from voltage recordings was 61.8 and 79.4 in soleus and EDL, respectively, and from charge measurements derived from current recordings was 46.3 (soleus) and 65.1 (EDL). 3. The threshold for action potential generation in response to nerve stimulation was determined from endplate potentials (EPPs) and endplate currents (EPCs) in preparations partially blocked with d-tubocurarine. The number of quanta required to reach threshold was estimated from voltage recordings to be 19.7 (soleus) and 23.2 (EDL) and from charge measurements derived from current recordings to be 13.3 (soleus) and 13.0 (EDL). 4. When intracellular electrodes were used to inject current into the muscle fibre, the total charge required to reach threshold was approximately twice that of the nerve-evoked threshold EPC. 5. The safety factor for nerve-evoked responses at the junction was 3.5 (soleus) and 5.0 (EDL). In the extrajunctional region the safety factor estimated from injected currents was 1.7 (soleus) and 2.5 (EDL). 6. It is concluded that the effect of the postsynaptic folds and the VGSCs within them is to double the safety factor. At normal frequencies of nerve impulse activity in vivo, this effect is likely to be crucial for ensuring effective neuromuscular transmission.

Postsynaptic abnormalities at the neuromuscular junctions of utrophin-deficient mice

Utrophin is a dystrophin-related cytoskeletal protein expressed in many tissues. It is thought to link F-actin in the internal cytoskeleton to a transmembrane protein complex similar to the dystrophin protein complex (DPC). At the adult neuromuscular junction (NMJ), utrophin is precisely colocalized with acetylcholine receptors (AChRs) and recent studies have suggested a role for utrophin in AChR cluster formation or maintenance during NMJ differentiation. We have disrupted utrophin expression by gene targeting in the mouse. Such mice have no utrophin detectable by Western blotting or immunocytochemistry. Utrophin-deficient mice are healthy and show no signs of weakness. However, their NMJs have reduced numbers of AChRs (alpha-bungarotoxin [alpha-BgTx] binding reduced to approximately 60% normal) and decreased postsynaptic folding, though only minimal electrophysiological changes. Utrophin is thus not essential for AChR clustering at the NMJ but may act as a component of the postsynaptic cytoskeleton, contributing to the development or maintenance of the postsynaptic folds. Defects of utrophin could underlie some forms of congenital myasthenic syndrome in which a reduction of postsynaptic folds is observed.

Spatial relationships of utrophin, dystrophin, beta-dystroglycan and beta-spectrin to acetylcholine receptor clusters during postnatal maturation of the rat neuromuscular junction

At the adult mammalian neuromuscular junction, acetylcholine receptors are concentrated at the tops of the postsynaptic folds and voltage-gated sodium channels are concentrated in their depths. It is likely that this arrangement involves linkage of the ion channels to components of the underlying membrane cytoskeleton. In rats, the mature distribution of acetylcholine receptors arises as part of the developmental remodelling of the junctional region during the first few weeks after birth. We have followed the changes during this period in the distribution of four proteins associated with the postsynaptic cytoskeleton at mature neuromuscular junctions (utrophin, dystrophin, beta-dystroglycan and beta-spectrin) to see if any of them co-localizes with acetylcholine receptors during the remodelling process, as would be required if it serves to link acetylcholine receptors to the cytoskeleton. Each protein was visualized with specific monoclonal antibodies and its distribution at various stages was compared with that of the acetylcholine receptors, labelled with alpha-bungarotoxin. We also related the changes in distribution of these postsynaptic proteins to the main stages in fold formation and, in the Discussion, to reported observations of the accumulation of voltage gated sodium channels during development. Our results show that utrophin labelling is closely co-localized with that of acetylcholine receptors throughout postnatal maturation. beta-dystroglycan labelling is present at most sites of high acetylcholine receptors density throughout maturation although it often extends beyond the region of highest acetylcholine receptors labelling density. By contrast, dystrophin and beta-spectrin labelling is not consistently concentrated at most neuromuscular junctions until after P7 and P14 respectively.

Action potential generation in rat slow- and fast-twitch muscles

1. In skeletal muscle fibres, voltage-gated sodium channels are concentrated at the neuromuscular junction. The effect of this accumulation of sodium channels on action potential generation was investigated in rat slow- and fast-twitch muscle fibres. 2. Intracellular microelectrodes were used to generate and record action potentials, from an imposed membrane potential of -75 and -90 mV, in junctional and extrajunctional regions of the muscle fibre. To identify junctional regions, preparations were incubated with 5 x 10(-7) M d-tubocurarine (dTC) to block muscle contraction in response to nerve stimulation whilst allowing endplate potentials (EPPs) to be recorded. Injection of rectangular depolarizing current pulses initiated action potentials at the endplate with threshold values several millivolts lower than those generated elsewhere in the fibre. In addition, the maximum rate of rise of the action potential was greater at the endplate than in extrajunctional regions. 3. In other muscles, neuromuscular transmission was partially blocked with dTC (2 x 10(-7) M), such that repetitive nerve stimulation evoked action potentials and EPPs in the same fibre. The threshold of these nerve-evoked action potentials was approximately 50% lower than values derived from action potentials generated by current injection. 4. It is concluded that the threshold for action potential generation is significantly lower at the neuromuscular junction than in extrajunctional regions of skeletal muscle fibres. Furthermore, nerve-evoked current is more effective at generating an action potential than is injected current.

Release and synthesis of acetylcholine at ectopic neuromuscular junctions in the rat

1. The ability of axons in the superficial fibular nerve to synthesize and release acetylcholine (ACh) has been studied before and after the formation of ectopic neuromuscular junctions (NMJs) with denervated soleus muscles of adult rats. 2. The central end of the severed fibular nerve was transplanted to the surface of the soleus muscle. After 3.5-5 weeks the soleus muscle was denervated in one group of rats by cutting the tibial nerve, allowing the formation of functional ectopic NMJs within a few days. In other rats the tibial nerve remained intact, preventing the formation of functional ectopic NMJs. 3. A month later the content of ACh, the levels of activity of choline acetyltransferase (ChAT) and acetylcholinesterase (AChE), and the amount of ACh released by depolarization by exposure to 50 mM KCl were measured in segments of isolated muscles that (i) contained normal or ectopic NMJs, (ii) were free of nerve or (iii) contained nerve that had not made NMJs. 4. Regions of muscles with ectopic nerve growth in which new NMJs had not formed contained substantial amounts of ACh and ChAT but no AChE. No detectable release of ACh could be evoked from these regions. 5. In muscles in which ectopic NMJs had formed after cutting the tibial nerve, the amounts of ACh and ChAT were about one-fifth of those in the regions of innervation of control muscles. ACh release could be evoked from the region of ectopic nerve growth in amounts nearly as great as those released from NMJs in normal and contralateral control muscles. 6. We conclude that the ability of the terminal parts of mature motor axons to synthesize and store ACh is largely independent of functional contact with muscle fibres. By contrast, the ability to release ACh in substantial amounts only develops when NMJs are formed. The possible significance of this situation for the development of synapses is discussed.

Relationship of a dystrophin-associated glycoprotein to junctional acetylcholine receptor clusters in rat skeletal muscle

The relationship of a member of the transmembrane dystrophin-associated glycoprotein (DAG) complex to acetylcholine receptors (AChRs) was investigated using immunofluorescence techniques at rat neuromuscular junctions (NMJs) viewed en face. These results were compared with those from a similar previous study of dystrophin and an autosomal homologue (utrophin or dystrophin-related protein, DRP) (Bewick et al. Neuro Report 1992; 3: 857-860). The region of highest 43 K DAG (43DAG) labelling projected beyond the AChRs by approximately 0.3 microns, as does that for dystrophin. By contrast DRP labelling precisely co-localizes with the AChRs. These results suggest that at the NMJ, the region of high 43DAG concentration encompasses the area of highest intensity labelling for both DRP and dystrophin.

Different distributions of dystrophin and related proteins at nerve-muscle junctions

The distributions of dystrophin, 'dystrophin-related protein' (DRP) and beta-spectrin were compared with that of acetylcholine receptors (AChRs) at rat nerve-muscle junctions (NMJs) using immunofluorescence techniques. In sections, monoclonal antibodies (MAbs) to dystrophin and beta-spectrin labelled the entire sarcolemma but were concentrated at the NMJs while those to DRP labelled only NMJs. In permeabilized muscle fibres, DRP was precisely co-localized with the AChRs, whereas the zone of high density labelling of dystrophin and beta-spectrin extended 0.3-0.4 microns beyond the AChRs. Within the NMJ, the labelling of DRP appeared as a series of interconnecting lines similar to that of AChRs. However, labelling of dystrophin and beta-spectrin was consistently more punctate. These data suggest DRP is more closely associated with AChRs than are dystrophin or beta-spectrin.

Synapse-specific expression of acetylcholine receptor genes and their products at original synaptic sites in rat soleus muscle fibres regenerating in the absence of innervation

To test the hypothesis that synaptic basal lamina can induce synapse-specific expression of acetylcholine receptor (AChR) genes, we examined the levels mRNA for the alpha- and epsilon-subunits of the AChR in regenerating rat soleus muscles up to 17 days of regeneration. Following destruction of all muscle fibres and their nuclei by exposure to venom of the Australian tiger snake, new fibres regenerated within the original basal lamina sheaths. Northern blots showed that original mRNA was lost during degeneration. Early in regeneration, both alpha- and epsilon-subunit mRNAs were present throughout the muscle fibres but in situ hybridization showed them to be concentrated primarily at original synaptic sites, even when the nerve was absent during regeneration. A similar concentration was seen in denervated regenerating muscles kept active by electrical stimulation and in muscles frozen 41–44 hours after venom injection to destroy all cells in the synaptic region of the muscle. Acetylcholine-gated ion channels with properties similar to those at normal neuromuscular junctions were concentrated at original synaptic sites on denervated stimulated muscles. Taken together, these findings provide strong evidence that factors that induce the synapse-specific expression of AChR genes are stably bound to synaptic basal lamina.

Structure and function of neuromuscular junctions in the vastus lateralis of man. A motor point biopsy study of two groups of patients

The properties of neuromuscular junctions (NMJs) in the vastus lateralis of man have been studied in motor point biopsy samples and compared with those reported for lower vertebrates (frogs and mice). The patients studied had no convincing evidence of a primary disturbance of neuromuscular transmission or other neurogenic component. Morphological studies were made using a variety of methods at the light- and electron-microscope levels. The size of the presynaptic nerve terminal and the area of postsynaptic specialization were smaller, relative to the size of the muscle fibres, than in the lower vertebrates. In contrast, the extent of postsynaptic folding was greater. Intracellular recordings from single muscle fibres showed that the duration of synaptic currents was longer than in most other vertebrates so far studied and that the number of transmitter 'quanta' released by a single nerve impulse, about 20, was lower, probably reflecting the small size of the presynaptic terminals. The hypothesis is discussed that in man, a relatively weak effect of transmitter on the muscle fibre surface is amplified by voltage-dependent sodium channels which have been shown in the rat to be concentrated in the depths of the synaptic folds. The implications of this hypothesis for the interpretation of pathological findings in myasthenic syndromes are also discussed.

Structure and function of the neuromuscular junction in young adult mdx mice

Dystrophin, the protein product of the gene responsible for X-linked muscular dystrophies, shares structural features with the cytoskeletal proteins spectrin and alpha-actinin. Like spectrin, it is localized at the cytoplasmic surface of the sarcolemma and is particularly concentrated in the subsynaptic region of the neuromuscular junction. Mdx mice have a profound deficiency of dystrophin and develop a necrotizing myopathy in the first weeks of life. Abnormalities of the neuromuscular junction, including a redistribution of postsynaptic molecules and reduction in synaptic folding, are also observed. We have studied these mice to see whether the lack of dystrophin has a specific effect on the structure and function of their neuromuscular junctions. Using a fore-limb muscle from 8 week old mdx mice we confirm the previously described postsynaptic structural changes and in addition show that many nerve terminals are abnormally complex. We demonstrate that these structural abnormalities are found exclusively at neuromuscular junctions on regenerated muscle fibres. Despite these structural abnormalities, miniature endplate potential frequency, the quantal content of endplate potentials, the amplitude and time course of miniature endplate currents and the number of acetylcholine receptors at the postsynaptic membrane are normal in mdx mice of this age. We conclude that in the mdx mouse the absence of dystrophin from the postsynaptic membrane has little direct effect on the function of the neuromuscular junction but that degeneration and regeneration of muscle fibres leads to remodelling of both its pre- and postsynaptic components.

The effect of age on motor neurone death following axotomy in the mouse

The ability of mouse motor neurones to survive axotomy during the first month of life was studied. The motor neurones that lie in the dorsolateral columns of spinal segments C7 and C8 and supply the flexor muscles of the forepaw were axotomized by cutting and removing part of the median and ulnar nerves above the elbow. The number and position of cell bodies with axons in these nerves were confirmed by retrograde labelling of the cut axons with horseradish peroxidase. The ability of these neurones to survive axotomy varies with the age of the animal at the time of axotomy. When the axons are sectioned within the first four postnatal days, 80–90% of the cell bodies will die, more than half of this death occurring in less than one week after axotomy. If the animals are one week old at the time the nerves are cut, a significantly smaller number (50%) die (P = 0.013), and the time-course of death is different, with eight to ten days elapsing before half the death has occurred. 40% of the neurones will die if sectioned at two weeks of age, and it is not until four weeks of age that more than 90% of the cells can survive axotomy. We conclude, therefore, that the kinetics of motor neurone death, as well as the final extent of neuronal loss, are affected by the age at which the animal is axotomized.

Is dystrophin labelling always discontinuous in Becker muscular dystrophy?

It has been reported that immunofluorescent labelling of dystrophin in muscle from patients with Becker muscular dystrophy (BMD) is invariably patchy or discontinuous. This observation has led to the suggestion that BMD dystrophin molecules, which are usually smaller than normal due to the presence of "in frame" gene deletions, cannot be assembled into a complete lattice network under the plasma membrane and instead form isolated patches. Our experience with immunoperoxidase labelling of BMD muscle indicates that complete gaps in the reaction around fibres are uncommon. We have therefore compared immunofluorescence and immunoperoxidase labelling patterns on sets of serial sections from 6 BMD patients using a monoclonal antibody to dystrophin. No difference was detected between the two types of label used: the incidence of discontinuous labelling was rare in both cases. We suggest that significantly different patterns of dystrophin labelling may be obtained using different primary antibodies, and that caution needs to be exercised in extrapolating models of structure/function relationships from observations of antibody binding patterns.

Loss of motor neurons from the median nerve motor nucleus of the mutant mouse 'wobbler'

This paper describes the location and number of motor neurons in the median nerve pool of wobbler mice and normal littermates as determined by retrograde labelling of the cut median nerve with horseradish peroxidase (HRP) in animals from 3 weeks to 1-year-old. The median nerve motor nucleus is located in spinal segments C5-T1, and in normal animals contains 199 (6) (mean (SEM] motor neurons. Three-week-old wobbler mice have the same number of labelled neurons as control animals, and this number falls to 75% of normal values by 4 weeks of age, and to approximately 60% by 6 weeks of age and older. Numerous swollen, pale and frequently vacuolated perikarya are present in the same 3-6-week-old mice. In the 3-week-old mutants these comprise on average 17% of the total large (greater than 20 microns) neuronal cell bodies counted in segments C5-T1. By 6 weeks this figure has fallen to 10%, and to less than 4% in adult wobblers. We conclude that the most active period in the expression of the wobbler phenotype is from 3 to 6 weeks of age.

Dystrophin in skeletal muscle. I. Western blot analysis using a monoclonal antibody

The value of analysing dystrophin on Western blots of skeletal muscle for the differential diagnosis of Xp21 muscular dystrophies is now fairly well established. Here we describe a sensitive system based on monoclonal antibodies to dystrophin. The specificity of the antibodies was established and experiments were undertaken to identify the source of dystrophin-related protein bands which were detected on blots of normal skeletal muscle. These investigations formed a necessary preliminary study to the application of the assay to samples of muscle obtained at biopsy from patients with Duchenne and Becker muscular dystrophy.

Dystrophin in skeletal muscle. II. Immunoreactivity in patients with Xp21 muscular dystrophy

In the preceding paper a sensitive Western blotting analysis system based on the use of a monoclonal antibody to dystrophin was described. Here we report the immunoreactivity on blots and on unfixed frozen sections of muscle from patients with Duchenne (DMD) and Becker (BMD) muscular dystrophy. Muscle from 3 BMD patients showed variation both in the band pattern observed on blots and in the immunocytochemical labelling of dystrophin on frozen sections. In contrast to previous reports, we were able to detect some minor dystrophin bands on blots from 6 of 9 DMD biopsy samples. Tissue sections from 8 of the 9 contained isolated fibres with dystrophin-positive labelling. We conclude that the majority of DMD patients have muscle fibres which can synthesize dystrophin in a limited manner.

The influence of basal lamina on the accumulation of acetylcholine receptors at synaptic sites in regenerating muscle

If skeletal muscles are damaged in ways that spare the basal lamina sheaths of the muscle fibers, new myofibers develop within the sheaths and neuromuscular junctions form at the original synaptic sites on them. At the regenerated neuromuscular junctions, as at the original ones, the muscle fiber plasma membrane is characterized by infoldings and a high concentration of acetylcholine receptors (AChRs). The aim of this study was to determine whether or not the synaptic portion of the myofiber basal lamina sheath plays a direct role in the formation of the subsynaptic apparatus on regenerating myofibers, a question raised by the results of earlier experiments. The junctional region of the frog cutaneous pectoris muscle was crushed or frozen, which resulted in disintegration and phagocytosis of all cells at the synapse but left intact much of the myofiber basal lamina. Reinnervation was prevented. When new myofibers developed within the basal lamina sheaths, patches of AChRs and infoldings formed preferentially at sites where the myofiber membrane was apposed to the synaptic region of the sheaths. Processes from unidentified cells gradually came to lie on the presynaptic side of the basal lamina at a small fraction of the synaptic sites, but there was no discernible correlation between their presence and the effectiveness of synaptic sites in accumulating AChRs. We therefore conclude that molecules stably attached to the myofiber basal lamina at synaptic sites direct the formation of subsynaptic apparatus in regenerating myofibers. An analysis of the distribution of AChR clusters at synaptic sites indicated that they formed as a result of myofiber-basal lamina interactions that occurred at numerous places along the synaptic basal lamina, that their presence was not dependent on the formation of plasma membrane infoldings, and that the concentration of receptors within clusters could be as great as the AChR concentration at normal neuromuscular junctions.

Spatial distribution of acetylcholine receptors at developing chick neuromuscular junctions

The development of high-density clusters of acetylcholine receptors (AChRs) and the relationship of these clusters to nerve contacts on embryonic chick wing muscle fibres has been studied. Fluorescent labelling of AChRs with rhodamine-conjugated alpha-bungarotoxin (R-Bgt) revealed the presence of irregularly shaped AChR clusters in wing buds at 4 1/2-5 days of incubation. This is within a day of when myotubes first appear in the wing bud, and close to the time when functional innervation becomes established. At 10 days of incubation AChR clusters present on muscle cells in anterior and posterior latissimus dorsi appear as round or oval, uniformly labelled plaques. At about the time of hatching, however, these plaques break into numerous smaller clusters. Similar changes in the morphology of AChR clusters have been observed previously in mammalian skeletal muscle during development. Using horseradish peroxidase labelled alpha-bungarotoxin (HRP-Bgt), the relationship between AChR clusters and motor nerve terminals was studied at the ultrastructural level. At all stages of development nerve-muscle contacts were labelled with HRP-Bgt. In wing buds, however, the majority (90%) of labelled clusters observed were not in contact with a motor nerve terminal. The incidence of AChR clusters with axon contacts increased sharply during development such that by 10 days more than 50% and by hatching more than 90% of all sections through labelled AChR clusters contained nerve terminal profiles. At all times studied nerve-contacted receptor clusters were longer (about 5 micron) than non-contacted clusters (about 2 micron).