Reduced neuromuscular quantal content with normal synaptic release time course and depression in canine motor neuron disease

Towards early diagnosis of amyotrophic lateral sclerosis using near fibre EMG

OBJECTIVE

Amyotrophic lateral sclerosis (ALS) is characterized by progressive loss of motor neurons and diagnosis is usually delayed several months. The continuous denervation and reinnervation associated with ALS are manifest in EMG signals as changes in motor unit potential (MUP) size, temporal dispersion and instability. Near Fibre EMG is a novel method to assess early changes in MUP temporal dispersion and instability using routinely recorded EMG signals in a semi-automated manner.

METHODS

Near Fibre EMG values from 2318 MUs, retrospectively sampled at the time of ALS diagnosis, from 96 muscles of 15 patients were compared with values from 3954 MUs sampled from 109 muscles of 84 reference subjects.

RESULTS

30.1% and 46.1% of ALS MUs had MUPs with increased complexity or instability, respectively, and 17.4% had both. The potential importance and heightened sensitivity of NFEMG was highlighted when analyzing normal-sized motor units; as many as 24% of the normal-sized MUPs actually had significant instability, while 14% had increased complexity, and 7.4% had both.

CONCLUSIONS

Near Fibre EMG can characterize motor unit electrophysiological status and hence help quantify the degree, and course of denervation and reinnervation.

SIGNIFICANCE

Near-Fiber EMG offers the potential to facilitate earlier ALS diagnosis, which, as promising therapies become available, can be consequential.

Age-Related Homeostatic Plasticity at Rodent Neuromuscular Junctions

Motor ability decline remains a major threat to the quality of life of the elderly. Although the later stages of aging co-exist with degenerative pathologies, the long process of aging is more complicated than a simple and gradual degeneration. To combat senescence and the associated late-stage degeneration of the neuromuscular system, it is imperative to examine changes that occur during the long process of aging. Prior to late-stage degeneration, age-induced changes in the neuromuscular system trigger homeostatic plasticity. This unique phenomenon may be important for the maintenance of the neuromuscular system during the early stages of aging. In this review, we will focus on age-induced changes in neurotransmission at the neuromuscular junction, providing the potential mechanisms responsible for these changes. The goal is to highlight these key elements and their role in regulating neurotransmission, facilitating future research efforts to combat late-stage degeneration in the neuromuscular system by preserving the functional and structural integrity of these elements prior to the late stage of aging.

Creating stem cell-derived neuromuscular junctions in vitro

Recent development of novel therapies has improved mobility and quality of life for people suffering from inheritable neuromuscular disorders. Despite this progress, the majority of neuromuscular disorders are still incurable, in part due to a lack of predictive models of neuromuscular junction (NMJ) breakdown. Improvement of predictive models of a human NMJ would be transformative in terms of expanding our understanding of the mechanisms that underpin development, maintenance, and disease, and as a testbed with which to evaluate novel therapeutics. Induced pluripotent stem cells (iPSCs) are emerging as a clinically relevant and non‐invasive cell source to create human NMJs to study synaptic development and maturation, as well as disease modeling and drug discovery. This review will highlight the recent advances and remaining challenges to generating an NMJ capable of eliciting contraction of stem cell‐derived skeletal muscle in vitro. We explore the advantages and shortcomings of traditional NMJ culturing platforms, as well as the pioneering technologies and novel, biomimetic culturing systems currently in use to guide development and maturation of the neuromuscular synapse and extracellular microenvironment. Then, we will explore how this NMJ‐in‐a‐dish can be used to study normal assembly and function of the efferent portion of the neuromuscular arc, and how neuromuscular disease‐causing mutations disrupt structure, signaling, and function.

Synaptic Failure: Focus in an Integrative View of ALS

From early description by Charcot, the classification of the Amyotrophic Lateral Sclerosis (ALS) is evolving from a subtype of Motor Neuron (MN) Disease to be considered rather a multi-systemic, non-cell autonomous and complex neurodegenerative disease. In the last decade, the huge amount of knowledge acquired has shed new insights on the pathological mechanisms underlying ALS from different perspectives. However, a whole vision on the multiple dysfunctional pathways is needed with the inclusion of information often excluded in other published revisions. We propose an integrative view of ALS pathology, although centered on the synaptic failure as a converging and crucial player to the etiology of the disease. Homeostasis of input and output synaptic activity of MNs has been proved to be severely and early disrupted and to definitively contribute to microcircuitry alterations at the spinal cord. Several cells play roles in synaptic communication across the MNs network system such as interneurons, astrocytes, microglia, Schwann and skeletal muscle cells. Microglia are described as highly dynamic surveying cells of the nervous system but also as determinant contributors to the synaptic plasticity linked to neuronal activity. Several signaling axis such as TNFα/TNFR1 and CX3CR1/CX3CL1 that characterize MN-microglia cross talk contribute to synaptic scaling and maintenance, have been found altered in ALS. The presence of dystrophic and atypical microglia in late stages of ALS, with a decline in their dynamic motility and phagocytic ability, together with less synaptic and neuronal contacts disrupts the MN-microglia dialogue, decreases homeostatic regulation of neuronal activity, perturbs “on/off” signals and accelerates disease progression associated to impaired synaptic function and regeneration. Other hotspot in the ALS affected network system is the unstable neuromuscular junction (NMJ) leading to distal axonal degeneration. Reduced neuromuscular spontaneous synaptic activity in ALS mice models was also suggested to account for the selective vulnerability of MNs and decreased regenerative capability. Synaptic destabilization may as well derive from increased release of molecules by muscle cells (e.g. NogoA) and by terminal Schwann cells (e.g. semaphorin 3A) conceivably causing nerve terminal retraction and denervation, as well as inhibition of re-connection to muscle fibers. Indeed, we have overviewed the alterations on the metabolic pathways and self-regenerative capacity presented in skeletal muscle cells that contribute to muscle wasting in ALS. Finally, a detailed footpath of pathologic changes on MNs and associated dysfunctional and synaptic alterations is provided. The oriented motivation in future ALS studies as outlined in the present article will help in fruitful novel achievements on the mechanisms involved and in developing more target-driven therapies that will bring new hope in halting or delaying disease progression in ALS patients.

Reversible Recruitment of a Homeostatic Reserve Pool of Synaptic Vesicles Underlies Rapid Homeostatic Plasticity of Quantal Content

Homeostatic regulation is essential for the maintenance of synaptic strength within the physiological range. The current study is the first to demonstrate that both induction and reversal of homeostatic upregulation of synaptic vesicle release can occur within seconds of blocking or unblocking acetylcholine receptors at the mouse neuromuscular junction. Our data suggest that the homeostatic upregulation of release is due to Ca(2+)-dependent increase in the size of the readily releasable pool (RRP). Blocking vesicle refilling prevented upregulation of quantal content (QC), while leaving baseline release relatively unaffected. This suggested that the upregulation of QC was due to mobilization of a distinct pool of vesicles that were rapidly recycled and thus were dependent on continued vesicle refilling. We term this pool the "homeostatic reserve pool." A detailed analysis of the time course of vesicle release triggered by a presynaptic action potential suggests that the homeostatic reserve pool of vesicles is normally released more slowly than other vesicles, but the rate of their release becomes similar to that of the major pool during homeostatic upregulation of QC. Remarkably, instead of finding a generalized increase in the recruitment of vesicles into RRP, we identified a distinct homeostatic reserve pool of vesicles that appear to only participate in synchronized release following homeostatic upregulation of QC. Once this small pool of vesicles is depleted by the block of vesicle refilling, homeostatic upregulation of QC is no longer observed. This is the first identification of the population of vesicles responsible for the blockade-induced upregulation of release previously described. Significance statement: The current study is the first to demonstrate that both the induction and reversal of homeostatic upregulation of synaptic vesicle release can occur within seconds. Our data suggest that homeostatic upregulation of release is due to Ca(2+)-dependent priming/docking of a small homeostatic reserve pool of vesicles that normally have slow-release kinetics. Following priming, the reserve pool of vesicles is released synchronously with the normal readily releasable pool of synaptic vesicles. This is the first description of this unique pool of synaptic vesicles.

Selective disruption of acetylcholine synthesis in subsets of motor neurons: a new model of late-onset motor neuron disease

Motor neuron diseases are characterized by the selective chronic dysfunction of a subset of motor neurons and the subsequent impairment of neuromuscular function. To reproduce in the mouse these hallmarks of diseases affecting motor neurons, we generated a mouse line in which ~40% of motor neurons in the spinal cord and the brainstem become unable to sustain neuromuscular transmission. These mice were obtained by conditional knockout of the gene encoding choline acetyltransferase (ChAT), the biosynthetic enzyme for acetylcholine. The mutant mice are viable and spontaneously display abnormal phenotypes that worsen with age including hunched back, reduced lifespan, weight loss, as well as striking deficits in muscle strength and motor function. This slowly progressive neuromuscular dysfunction is accompanied by muscle fiber histopathological features characteristic of neurogenic diseases. Unexpectedly, most changes appeared with a 6-month delay relative to the onset of reduction in ChAT levels, suggesting that compensatory mechanisms preserve muscular function for several months and then are overwhelmed. Deterioration of mouse phenotype after ChAT gene disruption is a specific aging process reminiscent of human pathological situations, particularly among survivors of paralytic poliomyelitis. These mutant mice may represent an invaluable tool to determine the sequence of events that follow the loss of function of a motor neuron subset as the disease progresses, and to evaluate therapeutic strategies. They also offer the opportunity to explore fundamental issues of motor neuron biology.

Spinal muscular atrophy, John Griffin, and mentorship

Hereditary canine spinal muscular atrophy is an inherited motor neuron disease that occurs in Brittany Spaniels and has remarkable similarities with human spinal muscular atrophy. Both disorders are characterized by proximal limb and truncal muscle weakness of variable severity. Detailed pathological studies indicate that there is early dysfunction of motor neuron synapses, particularly the neuromuscular junction synapse, prior to motor neuron death. This period of synaptic dysfunction may define a critical window of opportunity for disease reversibility in motor neuron disease.

Morphological and functional changes in innervation of a fast forelimb muscle in SOD1-G85R mice

Muscle endplates become denervated in mice that express mutations of human superoxide dismutase 1 (hSOD1), models of familial amyotrophic lateral sclerosis. This denervation is especially marked in fast limb muscles, and precedes death of motor neuron somata. This study used mice that expressed yellow fluorescent protein (YFP) in neurons to investigate changes in the morphology and function of axons and motor terminals innervating a fast forelimb muscle (epitrochleoanconeus, ETA) in presymptomatic and symptomatic hSOD1-G85R mice, compared to those in mice that express wild-type (wt) hSOD1. The percentage of endplates (identified using fluorescently-labeled α-bungarotoxin) innervated by motor terminals remained high in presymptomatic SOD1-G85R mice, but fell to ~50% in symptomatic mice. The number of large diameter (≥4 μm) axons in the ETA nerve also decreased as mice became symptomatic, and endplate innervation correlated best with the number of large diameter axons. Motor terminal function was assessed using changes in terminal YFP fluorescence evoked by trains of action potentials; different components of the pH-dependent YFP signals reflect stimulation-induced Ca2+ entry and vesicular exo/endocytosis. Most visible motor terminals (>90%) remained capable of responding to nerve stimulation in both pre- and symptomatic hSOD1-G85R mice, but with functional alterations. Responses in presymptomatic terminals suggested reduced acidification and increased vesicular release, whereas symptomatic terminals exhibited increased acidification and reduced vesicular release. The fact that most remaining terminals were able to respond to nerve stimulation suggests that motor terminal-protective therapies might contribute to preserving neuromuscular function in fALS mice.

Motor terminal degeneration unaffected by activity changes in SOD1(G93A) mice; a possible role for glycolysis

This study examined whether activity is a contributing factor to motor terminal degeneration in mice that overexpress the G93A mutation of the SOD1 enzyme found in humans with inherited motor neuron disease. Previously, we showed that overload of muscles accomplished by synergist denervation accelerated motor terminal degeneration in dogs with hereditary canine spinal muscular atrophy (HCSMA). In the present study, we found that SOD1 plantaris muscles overloaded for 2months showed no differences of neuromuscular junction innervation status when compared with normally loaded, contralateral plantaris muscles. Complete elimination of motor terminal activity using blockade of sciatic nerve conduction with tetrodotoxin cuffs for 1month also produced no change of plantaris innervation status. To assess possible effects of activity on motor terminal function, we examined the synaptic properties of SOD1 soleus neuromuscular junctions at a time when significant denervation of close synergists had occurred as a result of natural disease progression. When examined in glucose media, SOD1 soleus synaptic properties were similar to wildtype. When glycolysis was inhibited and ATP production limited to mitochondria, however, blocking of evoked synaptic transmission occurred and a large increase in the frequency of spontaneous mEPCs was observed. Similar effects were observed at neuromuscular junctions in muscle from dogs with inherited motor neuron disease (HCSMA), although significant defects of synaptic transmission exist at these neuromuscular junctions when examined in glucose media, as reported previously. These results suggest that glycolysis compensates for mitochondrial dysfunction at motor terminals of SOD1 mice and HCSMA dogs. This compensatory mechanism may help to support resting and activity-related metabolism in the presence of dysfunctional mitochondria and prolong the survival of SOD1 motor terminals.

Using induced pluripotent stem cells (iPSC) to model human neuromuscular connectivity: promise or reality?

Motor neuron diseases (MND) such as amyotrophic lateral sclerosis and spinal muscular atrophy are devastating, progressive and ultimately fatal diseases for which there are no effective treatments. Recent evidence from systematic studies of animal models and human patients suggests that the neuromuscular junction (NMJ) is an important early target in MND, demonstrating functional and structural abnormalities in advance of pathological changes occurring in the motor neuron cell body. The ability to study pathological changes occurring at the NMJ in humans is therefore likely to be important for furthering our understanding of disease pathogenesis, and also for designing and testing new therapeutics. However, there are many practical and technical reasons why it is not possible to visualise or record from NMJs in pre- and early-symptomatic MND patients in vivo. Other approaches are therefore required. The development of stem cell technologies has opened up the possibility of creating human NMJs in vitro, using pluripotent cells generated from healthy individuals and patients with MND. This review covers historical attempts to develop mature and functional NMJs in vitro, using co-cultures of muscle and nerve from animals, and discusses how recent developments in the generation and specification of human induced pluripotent stem cells provides an opportunity to build on these previous successes to recapitulate human neuromuscular connectivity in vitro.

Neural cell adhesion molecule is required for stability of reinnervated neuromuscular junctions

Studies examining the etiology of motoneuron diseases usually focus on motoneuron death as the defining pathophysiology of the disease. However, impaired neuromuscular transmission and synapse withdrawal often precede cell death, raising the possibility that abnormalities in synaptic function contribute to disease onset. Although little is known about the mechanisms maintaining the synaptic integrity of neuromuscular junctions (NMJs), Drosophila studies suggest that Fasciclin II plays an important role. Inspired by these studies we used a reinnervation model of synaptogenesis to analyze neuromuscular function in mice lacking neural cell adhesion molecule (NCAM), the Fasciclin II vertebrate homolog. Our results showed that the recovery of contractile force was the same in wild-type and NCAM-/- mice at 1 month after nerve injury, indicating that endplates were appropriately reformed. This normality was only transient because the contractile force and myofiber number decreased at 3 months after injury in NCAM-/- mice. Both declined further 3 months later. Myofibers degenerated, not because motoneurons died but because synapses were withdrawn. Although neurotransmission was initially normal at reinnervated NCAM-/- NMJs, it was significantly compromised 3 months later. Interestingly, the selective ablation of NCAM from motoneurons, or muscle fibers, did not mimic the deficits observed in reinnervated NCAM-/- mice. Taken together, these results indicate that NCAM is required to maintain normal synaptic function at reinnervated NMJs, although its loss pre-synaptically or post-synaptically is not sufficient to induce synaptic destabilization. Consideration is given to the role of NCAM in terminal Schwann cells for maintaining synaptic integrity and how NCAM dysfunction may contribute to motoneuron disorders.

Ca2+ dependence of the binomial parameters p and n at the mouse neuromuscular junction

The Ca(2+) dependence of synaptic quantal release is generally thought to be restricted to probability of vesicular release. However, some studies have suggested that the number of release sites (n) at the neuromuscular junction (NMJ) is also Ca(2+) dependent. In this study, we recorded endplate currents over a wide range of extracellular Ca(2+) concentrations and found the expected Ca(2+) dependency of release. A graphical technique was used to estimate p (probability of release) and n using standard binomial assumptions. The results suggested n was Ca(2+) dependent. The data were simulated using compound binomial statistics with variable n (Ca(2+) dependent) or fixed n (Ca(2+) independent). With fixed n, successful simulation of increasing Ca(2+) required that p increase abruptly at some sites from very low to high values. Successful simulation with variable n required the introduction of previously silent release sites (p = 0) with high values of p. Thus the success of both simulations required abrupt, large increases of p at a subset of release sites with initially low or zero p. Estimates of the time course of release obtained by deconvolving evoked endplate currents with average miniature endplate currents decreased slightly as Ca(2+) increased, thus arguing against sequential release of multiple quanta at higher Ca(2+) levels. Our results suggest that the apparent Ca(2+) dependence of n at the NMJ can be explained by an underlying Ca(2+) dependence of a spatially variable p such that p increases abruptly at a subset of sites as Ca(2+) is increased.

Motor unit number estimate as a predictor of motor dysfunction in an animal model of type 1 diabetes

Peripheral neuropathy is a common complication of diabetes that leads to severe morbidity. In this study, we investigated the sensitivity of motor unit number estimate (MUNE) to detect early motor axon dysfunction in streptozotocin (STZ)-treated mice. We compared the findings with in vitro changes in the morphology and electrophysiology of the neuromuscular junction. Adult Thy1-YFP and Swiss Webster mice were made diabetic following three interdaily intraperitoneal STZ injections. Splay testing and rotarod performance assessed motor activity for 6 wk. Electromyography was carried out in the same time course, and compound muscle action potential (CMAP) amplitude, latency, and MUNE were estimated. Two-electrode voltage clamp was used to calculate quantal content (QC) of evoked transmitter release. We found that an early reduction in MUNE was evident before a detectable decline of motor activity. CMAP amplitude was not altered. MUNE decrease accompanied a drop of end-plate current amplitude and QC. We also observed small axonal loss, sprouting of nerve endings, and fragmentation of acetylcholine receptor clusters at the motor end plate. Our results suggest an early remodeling of motor units through the course of diabetic neuropathy, which can be readily detected by the MUNE technique. The early detection of MUNE anomalies is significant because it suggests that molecular changes associated with pathology and leading to neurodegeneration might already be occurring at this stage. Therefore, trials of interventions to prevent motor axon dysfunction in diabetic neuropathy should be administered at early stages of the disorder.

Impaired synaptic vesicle release and immaturity of neuromuscular junctions in spinal muscular atrophy mice

The motor neuron disease spinal muscular atrophy (SMA) causes profound muscle weakness that most often leads to early death. At autopsy, SMA is characterized by loss of motor neurons and muscle atrophy, but the initial cellular events that precipitate motor unit dysfunction and loss remain poorly characterized. Here, we examined the function and corresponding structure of neuromuscular junction (NMJ) synapses in a mouse model of severe SMA (hSMN2/delta7SMN/mSmn-/-). Surprisingly, most SMA NMJs remained innervated even late in the disease course; however they showed abnormal synaptic transmission. There was a two-fold reduction in the amplitudes of the evoked endplate currents (EPCs), but normal spontaneous miniature EPC (MEPC) amplitudes. These features in combination indicate reduced quantal content. SMA NMJs also demonstrated increased facilitation suggesting a reduced probability of vesicle release. By electron microscopy, we found a decreased density of synaptic vesicles that is likely to contribute to the reduced release probability. In addition to presynaptic defects, there were postsynaptic abnormalities. EPC and MEPC decay time constants were prolonged because of a slowed switch from the fetal acetylcholine receptor (AChR) gamma-subunit to the adult epsilon-subunit. There was also reduced size of AChR clusters and small myofibers, which expressed an immature pattern of myosin heavy chains. Together these results indicate that impaired synaptic vesicle release at NMJs in severe SMA is likely to contribute to failed postnatal maturation of motor units and muscle weakness.

Mutant SOD1 impairs axonal transport of choline acetyltransferase and acetylcholine release by sequestering KAP3

Mutations in the superoxide dismutase 1 (sod1) gene cause familial amyotrophic lateral sclerosis (FALS), likely due to the toxic properties of misfolded mutant SOD1 protein. Here we demonstrated that, starting from the pre-onset stage of FALS, misfolded SOD1 species associates specifically with kinesin-associated protein 3 (KAP3) in the ventral white matter of SOD1^G93A-transgenic mouse spinal cord. KAP3 is a kinesin-2 subunit responsible for binding to cargos including choline acetyltransferase (ChAT). Motor axons in SOD1^G93A-Tg mice also showed a reduction in ChAT transport from the pre-onset stage. By employing a novel FALS modeling system using NG108-15 cells, we showed that microtubule-dependent release of acetylcholine was significantly impaired by misfolded SOD1 species. Furthermore, such impairment was able to be normalized by KAP3 overexpression. KAP3 was incorporated into SOD1 aggregates in human FALS cases as well. These results suggest that KAP3 sequestration by misfolded SOD1 species and the resultant inhibition of ChAT transport play a role in the dysfunction of ALS.

Axonal degeneration in motor neuron disease

Growing evidence from animal models and patients with amyotrophic lateral sclerosis (ALS) suggests that distal axonal degeneration begins very early in this disease, long before symptom onset and motor neuron death. The cause of axonal degeneration is unknown, and may involve local axonal damage, withdrawal of trophic support from a diseased cell body, or both. It is increasingly clear that axons are not passive extensions of their parent cell bodies, and may die by mechanisms independent of cell death. This is supported by studies in which protection of motor neurons in models of ALS did not significantly improve symptoms or prolong lifespan, likely due to a failure to protect axons. Here, we will review the evidence for early axonal degeneration in ALS, and discuss possible mechanisms by which it might occur, with a focus on oxidative stress. We contend that axonal degeneration may be a primary feature in the pathogenesis of motor neuron disease, and that preventing axonal degeneration represents an important therapeutic target that deserves increased attention.

The control of neuromuscular transmission in health and disease

Multiple techniques are available to study failure of neuromuscular transmission. Electrophysiological techniques used in patients are well suited to detect failure of neuromuscular transmission; however, these methods offer little insight into the mechanisms underlying failure of transmission. More detailed techniques that are better suited for studying the underlying mechanisms can be performed in animal models of neuromuscular disease. However, it is often difficult to compare studies using different techniques to measure neuromuscular transmission. In this review, I discuss different techniques that are available to study failure of neuromuscular transmission. The strengths and weaknesses of various techniques are compared using several diseases as examples. The review concludes with a discussion of mechanisms that may contribute to failure of neuromuscular transmission during repetitive stimulation.

Activity-dependent presynaptic regulation of quantal size at the mammalian neuromuscular junction in vivo

Changes in synaptic activity alter quantal size, but the relative roles of presynaptic and postsynaptic cells in these changes are only beginning to be understood. We examined the mechanism underlying increased quantal size after block of synaptic activity at the mammalian neuromuscular junction in vivo. We found that changes in neither acetylcholinesterase activity nor acetylcholine receptor density could account for the increase. By elimination, it appears likely that the site of increased quantal size after chronic block of activity is presynaptic and involves increased release of acetylcholine. We used mice with muscle hyperexcitability caused by mutation of the ClC-1 muscle chloride channel to examine the role of postsynaptic activity in controlling quantal size. Surprisingly, quantal size was increased in ClC mice before block of synaptic activity. We examined the mechanism underlying increased quantal size in ClC mice and found that it also appeared to be located presynaptically. When presynaptic activity was completely blocked in both control and ClC mice, quantal size was large in both groups despite the higher level of postsynaptic activity in ClC mice. This suggests that postsynaptic activity does not regulate quantal size at the neuromuscular junction. We propose that presynaptic activity modulates quantal size at the neuromuscular junction by modulating the amount of acetylcholine released from vesicles.

Increased persistent Na(+) current and its effect on excitability in motoneurones cultured from mutant SOD1 mice

Mutations in the enzyme superoxide dismutase 1 (SOD1) initiate a progressive motoneurone degeneration in amyotrophic lateral sclerosis (ALS). Transgenic mice overexpressing this mutation develop a similar progressive motoneurone degeneration. In spinal motoneurones cultured from presymptomatic mice expressing the glycine to alanine mutation at base pair 93 (G93A) SOD1 mutation, a marked increase in the persistent component of the Na(+) current was observed, without changes in passive properties. This increase only enhanced neuronal excitability in high input conductance cells, as low input conductance cells exhibited a compensatory outward shift in the current remaining after Na(+) blockade. High input conductance motoneurones tend to be large, so these results may explain the tendency of large motoneurones to degenerate first in ALS. Riluzole, at the therapeutic concentration used to treat ALS, decreased neuronal excitability and persistent Na(+) current in G93A motoneurones to levels observed in the control motoneurones. Aberrations in the intrinsic electrical properties may be among the first symptoms to emerge in SOD1-linked ALS.

Activity-Driven Synaptic and Axonal Degeneration in Canine Motor Neuron Disease

The role of neuronal activity in the pathogenesis of neurodegenerative disease is largely unknown. In this study, we examined the effects of increasing motor neuron activity on the pathogenesis of a canine version of inherited motor neuron disease (hereditary canine spinal muscular atrophy). Activity of motor neurons innervating the ankle extensor muscle medial gastrocnemius (MG) was increased by denervating close synergist muscles. In affected animals, 4 wk of synergist denervation accelerated loss of motor-unit function relative to control muscles and decreased motor axon conduction velocities. Slowing of axon conduction was greatest in the most distal portions of motor axons. Morphological analysis of neuromuscular junctions (NMJs) showed that these functional changes were associated with increased loss of intact innervation and with the appearance of significant motor axon and motor terminal sprouting. These effects were not observed in the MG muscles of age-matched, normal animals with synergist denervation for 5 wk. The results indicate that motor neuron action potential activity is a major contributing factor to the loss of motor-unit function and degeneration in inherited canine motor neuron disease.

Dysmyelinated lower motor neurons retract and regenerate dysfunctional synaptic terminals

Axonal degeneration is the major cause of permanent neurological disability in individuals with inherited diseases of myelin. Axonal and neuronal changes that precede axonal degeneration, however, are not well characterized. We show here that dysmyelinated lower motor neurons retract and regenerate dysfunctional presynaptic terminals, leading to severe neurological disability before axonal degeneration. In addition, dysmyelination led to a decreased synaptic quantal content, an indicator of synaptic dysfunction. The amplitude and rise time of miniature endplate potentials were also increased, but these changes were primarily consistent with an increase in the passive membrane properties of the transgenic muscle fibers. Maintenance of synaptic connections should be considered as a therapeutic target for slowing progression of neurological disability in primary diseases of myelin.

Amyotrophic lateral sclerosis is a distal axonopathy: evidence in mice and man

The SOD1 mutant mouse is the most widely used model of human amyotrophic lateral sclerosis (ALS). To determine where and when the pathological changes of motor neuron disease begins, we performed a comprehensive spatiotemporal analysis of disease progression in SOD1(G93A) mice. Quantitative pathological analysis was performed in the same mice at multiple ages at neuromuscular junctions (NMJ), ventral roots, and spinal cord. In addition, a patient with sporadic ALS who died unexpectedly was examined at autopsy. Mice became clinically weak at 80 days and died at 131 +/- 5 days. At 47 days, 40% of end-plates were denervated whereas there was no evidence of ventral root or cell body loss. At 80 days, 60% of ventral root axons were lost but there was no loss of motor neurons. Motor neuron loss was well underway by 100 days. Microglial and astrocytic activation around motor neurons was not identified until after the onset of distal axon degeneration. Autopsy of the ALS patient demonstrated denervation and reinnervation changes in muscle but normal appearing motor neurons. We conclude that in this widely studied animal model of human ALS, and in this single human case, motor neuron pathology begins at the distal axon and proceeds in a "dying back" pattern.

Dual action of hydrogen peroxide on synaptic transmission at the frog neuromuscular junction

There is evidence that reactive oxygen species (ROS) are produced and released during neuromuscular activity, but their role in synaptic transmission is not known. Using a two-electrode voltage-clamp technique, at frog neuromuscular junctions, the action H2O2 on end-plate currents (EPC) was studied to determine the targets for this membrane-permeable ROS. In curarized or cut muscles, micromolar concentrations of H2O2 increased the amplitude of EPCs. Higher (> 30 microM) doses inhibited EPCs and prolonged current decay. These effects were presynaptic since H2O2 did not change the amplitude or duration of miniature EPCs (although it reduced the rate of spontaneous release at high concentrations). Quantal analysis and deconvolution methods showed that facilitation of EPCs was due to increased quantal release, while depression was accompanied by temporal dispersion of evoked release. Extracellular recordings revealed prolonged presynaptic Ca2+ entry in the presence of high H2O2. Both low and high H2O2 increased presynaptic potentiation during high-frequency stimulation. Pro-oxidant Fe2+ did not affect facilitation by low doses of H2O2 but augmented the inhibition of EPCs by high H2O2, indicating involvement of hydroxyl radicals. High Mg2+ and the ROS scavenger N-acetylcysteine eliminated both the facilitatory and depressant effects of H2O2. The facilitatory effect of H2O2 was prevented by protein kinase C (PKC) inhibitors and 4beta-phorbol 12-myristate, 13-acetate (PMA), an activator of PKC. PKC inhibitors but not PMA also abolished the depressant effect of H2O2. Our data suggest complex presynaptic actions of H2O2, which could serve as a fast feedback modulator of intense neuromuscular transmission.

Reduced endplate currents underlie motor unit dysfunction in canine motor neuron disease

Hereditary canine spinal muscular atrophy (HCSMA) is an autosomal dominant degenerative disorder of motor neurons. In homozygous animals, motor units produce decreased force output and fail during repetitive activity. Previous studies suggest that decreased efficacy of neuromuscular transmission underlies these abnormalities. To examine this, we recorded muscle fiber endplate currents (EPCs) and found reduced amplitudes and increased failures during nerve stimulation in homozygotes compared with wild-type controls. Comparison of EPC amplitudes with muscle fiber current thresholds indicate that many EPCs from homozygotes fall below threshold for activating muscle fibers but can be raised above threshold following potentiation. To determine whether axonal abnormalities might play a role in causing motor unit dysfunction, we examined the postnatal maturation of axonal conduction velocity in relation to the appearance of tetanic failure. We also examined intracellularly labeled motor neurons for evidence of axonal neurofilament accumulations, which are found in many instances of motor neuron disease including HCSMA. Despite the appearance of tetanic failure between 90 and 120 days, average motor axon conduction velocity increased with age in homozygotes and achieved adult levels. Normal correlations between motor neuron properties (including conduction velocity) and motor unit properties were also observed. Labeled proximal motor axons of several motor neurons that supplied failing motor units exhibited little or no evidence of axonal swellings. We conclude that decreased release of transmitter from motor terminals underlies motor unit dysfunction in HCSMA and that the mechanisms determining the maturation of axonal conduction velocity and the pattern of correlation between motor neuron and motor unit properties do not contribute to the appearance or evolution of motor unit dysfunction.