Neuromuscular junction disorders mimicking myopathy

Effects of a ketogenic diet on motor function and motor unit number estimation in aged C57BL/6 mice

OBJECTIVE

Pathological, age-related loss of muscle function, commonly referred to as sarcopenia, contributes to loss of mobility, impaired independence, as well as increased risk of adverse health events. Sarcopenia has been attributed to changes in both neural and muscular integrity during aging. Current treatment options are primarily limited to exercise and dietary protein fortification, but the therapeutic impact of these approaches are often inadequate. Prior work has suggested that a ketogenic diet (KD) might improve healthspan and lifespan in aging mice. Thus, we sought to investigate the effects of a KD on neuromuscular indices of sarcopenia in aged C57BL/6 mice.

DESIGN

A randomized, controlled pre-clinical experiment consisting of longitudinal assessments performed starting at 22-months of age (baseline) as well as 2, 6 and 10 weeks after the start of a KD vs. regular chow intervention.

SETTING

Preclinical laboratory study.

SAMPLE SIZE

Thirty-six 22-month-old mice were randomized into 2 dietary groups: KD [n = 22 (13 female and 9 male)], and regular chow [n = 15 (7 female and 8 male)].

MEASUREMENTS

Measures included body mass, hindlimb and all limb grip strength, rotarod for motor performance, plantarflexion muscle contractility, motor unit number estimations (MUNE), and repetitive nerve stimulation (RNS) as an index of neuromuscular junction transmission efficacy recorded from the gastrocnemius muscle. At end point, muscle wet weight and blood samples were collected to assess blood beta-hydroxybutyrate levels.

STATISTICAL ANALYSIS

Primary analyses were two-way mixed effects ANOVA (diet and time × diet) to determine the effect of a KD on indices of motor function (grip, rotarod) and indices of motor unit (MUNE) and muscle (contractility) function.

RESULTS

Beta-hydroxybutyrate (BHB) was significantly higher at 10 weeks in mice on a KD vs control group (0.83 ± 0.44 mmol/l versus 0.42 ± 0.21 mmol/l, η2 = 0.265, unpaired t-test, p = 0.0060). Mice on the KD intervention demonstrated significantly increased hindlimb grip strength (diet, p = 0.0001; time × diet, p = 0.0030), all limb grip strength (diet, p = 0.0005; time × diet, p = 0.0523), and rotarod latency to fall (diet, p = 0.0126; time × diet, p = 0.0021). Mice treated with the KD intervention also demonstrated increased MUNE (diet, p = 0.0465; time × diet, p = 0.0064), but no difference in muscle contractility (diet, p = 0.5248; time × diet, p = 0.5836) or RNS (diet, p = 0.3562; time × diet, p = 0.9871).

CONCLUSION

KD intervention improved neuromuscular and motor function in aged mice. This pre-clinical work suggests that further research is needed to assess the efficacy and physiological effects of a KD on indices of sarcopenia.

Electrophysiological evaluation of the neuromuscular junction: a brief review

The nerve terminal and muscle membrane compose the neuromuscular junction. After opening the voltage-gated calcium channels, action potentials from the motor axons provoke a cascade for the acetylcholine release from synaptic vesicles to the synaptic cleft, where it binds to its receptor at the muscle membrane for depolarization. Low amplitude compound muscle action potential typically presents in presynaptic disorders, increasing by more than 100% after a 10-second effort in the Lambert-Eaton myasthenic syndrome and less in botulism. Needle electromyography may show myopathic motor unit action potentials and morphological instability ("jiggle") due to impulse blocking. Low-frequency repetitive nerve stimulation (RNS) is helpful in postsynaptic disorders, such as myasthenia gravis and most congenital myasthenic syndromes, where the number of functioning acetylcholine receptors is reduced. Low-frequency RNS with a decrement >10% is abnormal when comparing the 4th to the first compound muscle action potential amplitude. High-frequency RNS is helpful in presynaptic disorders like Lambert-Eaton myasthenic syndrome, botulism, and some rare congenital myasthenic syndromes. The high-frequency RNS releases more calcium, increasing the acetylcholine with a compound muscle action potential increment. Concentric needle records apparent single-fiber action potentials (spikes). A voluntary activation measures the jitter between spikes from two endplates. An electrical activation measures the jitter of one spike (one endplate). The jitter is the most sensitive test for detecting a neuromuscular junction dysfunction. Most neuromuscular junction disorders are responsive to treatment.

Ischemic stroke-induced polyaxonal innervation at the neuromuscular junction is attenuated by robot-assisted mechanical therapy

Ischemic stroke is a leading cause of disability world-wide. Mounting evidence supports neuromuscular pathology following stroke, yet mechanisms of dysfunction and therapeutic action remain undefined. The objectives of our study were to investigate neuromuscular pathophysiology following ischemic stroke and to evaluate the therapeutic effect of Robot-Assisted Mechanical massage Therapy (RAMT) on neuromuscular junction (NMJ) morphology. Using an ischemic stroke model in male rats, we demonstrated longitudinal losses of muscle contractility and electrophysiological estimates of motor unit number in paretic hindlimb muscles within 21 days of stroke. Histological characterization demonstrated striking pre- and postsynaptic alterations at the NMJ. Stroke prompted enlargement of motor axon terminals, acetylcholine receptor (AChR) area, and motor endplate size. Paretic muscle AChRs were also more homogenously distributed across motor endplates, exhibiting fewer clusters and less fragmentation. Most interestingly, NMJs in paretic muscle exhibited increased frequency of polyaxonal innervation. This finding of increased polyaxonal innervation in stroke-affected skeletal muscle suggests that reduction of motor unit number following stroke may be a spurious artifact due to overlapping of motor units rather than losses. Furthermore, we tested the effects of RAMT - which we recently showed to improve motor function and protect against subacute myokine disturbance - and found significant attenuation of stroke-induced NMJ alterations. RAMT not only normalized the post-stroke presentation of polyaxonal innervation but also mitigated postsynaptic expansion. These findings confirm complex neuromuscular pathophysiology after stroke, provide mechanistic direction for ongoing research, and inform development of future therapeutic strategies. SIGNIFICANCE: Ischemic stroke is a leading contributor to chronic disability, and there is growing evidence that neuromuscular pathology may contribute to the impact of stroke on physical function. Following ischemic stroke in a rat model, there are progressive declines of motor unit number estimates and muscle contractility. These changes are paralleled by striking pre- and postsynaptic maladaptive changes at the neuromuscular junction, including polyaxonal innervation. When administered to paretic hindlimb muscle, Robot-Assisted Mechanical massage Therapy - previously shown to improve motor function and protect against subacute myokine disturbance - prevents stroke-induced neuromuscular junction alterations. These novel observations provide insight into the neuromuscular response to cerebral ischemia, identify peripheral mechanisms of functional disability, and present a therapeutic rehabilitation strategy with clinical relevance.

Slow-Channel Congenital Myasthenic Syndrome due to a Novel Mutation in the Acetylcholine Receptor Alpha Subunit in a South Asian: A Case Report

Congenital myasthenic syndromes (CMS) result from genetic mutations that cause aberrations in structure and/or function of proteins involved in neuromuscular transmission. The slow-channel CMS (SCCMS) is an autosomal dominant postsynaptic defect caused by mutations in genes encoding alpha, beta, delta, or epsilon subunits of the acetylcholine receptor resulting in a functional defect which is an increase of the opening time of the receptor. We report a case of SCCMS due to a heterozygous mutation in the M2 domain of the AChR alpha subunit - CHRNA1:ENST00000348749.6:exon7:c.806T>G:p.Val269Gly and corresponding kinetic defect. A substitution of valine with phenylalanine in the same position has been previously described. This is the first reported case of a new CHRNA1 variant in a patient with SCCMS from South Asia. We also highlight the phenotype that would favour a genetic basis over an autoimmune one, in an adult presenting with fatigable weakness.

Utility and practice of electrodiagnostic testing in the pediatric population: An AANEM consensus statement

Nerve conduction studies and needle electromyography, collectively known as electrodiagnostic (EDX) studies, have been available for pediatric patients for decades, but the accessibility of this diagnostic modality and the approach to testing vary significantly depending on the physician and institution. The maturation of molecular diagnostic approaches and other diagnostic technologies such as neuromuscular ultrasound indicate that an analysis of current needs and practices for EDX studies in the pediatric population is warranted. The American Association of Neuromuscular & Electrodiagnostic Medicine convened a consensus panel to perform literature searches, share collective experiences, and develop a consensus statement. The panel found that electrodiagnostic studies continue to have high utility for the diagnosis of numerous childhood neuromuscular disorders, and that standardized approaches along with the use of high-quality reference values are important to maximize the diagnostic yield of these tests in infants, children, and adolescents.

Trouble at the junction: When myopathy and myasthenia overlap

Although myopathies and neuromuscular junction disorders are typically distinct, their coexistence has been reported in several inherited and acquired conditions. Affected individuals have variable clinical phenotypes but typically display both a decrement on repetitive nerve stimulation and myopathic findings on muscle biopsy. Inherited causes include myopathies related to mutations in BIN1, DES, DNM2, GMPPB, MTM1, or PLEC and congenital myasthenic syndromes due to mutations in ALG2, ALG14, COL13A1, DOK7, DPAGT1, or GFPT1. Additionally, a decrement due to muscle fiber inexcitability is observed in certain myotonic disorders. The identification of a defect of neuromuscular transmission in an inherited myopathy may assist in establishing a molecular diagnosis and in selecting patients who would benefit from pharmacological correction of this defect. Acquired cases meanwhile stem from the co-occurrence of myasthenia gravis or Lambert-Eaton myasthenic syndrome with an immune-mediated myopathy, which may be due to paraneoplastic disorders or exposure to immune checkpoint inhibitors.

Myasthenia gravis induced by avelumab

Neurological immune-related adverse events are potentially life-threatening complications of immune checkpoint inhibitors. Myasthenia gravis (MG) is a rare complication of treatment with inhibitors of programmed cell death protein 1 (PD)-1 and PD ligand 1 (PD-L1). We present a patient who developed seronegative MG resulting in respiratory failure while being treated with avelumab for metastatic, treatment-refractory ovarian cancer. Her MG went into remission following steroids and maintenance intravenous immunoglobulin treatment. We conclude that MG is a rare, but potentially life-threatening immune-related adverse event of avelumab therapy. This case provides support to the hypothesis that PD-1/PD-L1 signaling may have a protective role in MG.

Guidelines for single fiber EMG

This document is the consensus of international experts on the current status of Single Fiber EMG (SFEMG) and the measurement of neuromuscular jitter with concentric needle electrodes (CNE - CN-jitter). The panel of authors was chosen based on their particular interests and previous publications within a specific area of SFEMG or CN-jitter. Each member of the panel was asked to submit a section on their particular area of interest and these submissions were circulated among the panel members for edits and comments. This process continued until a consensus was reached. Donald Sanders and Erik Stålberg then edited the final document.

Collagen VI is required for the structural and functional integrity of the neuromuscular junction

The synaptic cleft of the neuromuscular junction (NMJ) consists of a highly specialized extracellular matrix (ECM) involved in synapse maturation, in the juxtaposition of pre- to post-synaptic areas, and in ensuring proper synaptic transmission. Key components of synaptic ECM, such as collagen IV, perlecan and biglycan, are binding partners of one of the most abundant ECM protein of skeletal muscle, collagen VI (ColVI), previously never linked to NMJ. Here, we demonstrate that ColVI is itself a component of this specialized ECM and that it is required for the structural and functional integrity of NMJs. In vivo, ColVI deficiency causes fragmentation of acetylcholine receptor (AChR) clusters, with abnormal expression of NMJ-enriched proteins and re-expression of fetal AChRγ subunit, both in Col6a1 null mice and in patients affected by Ullrich congenital muscular dystrophy (UCMD), the most severe form of ColVI-related myopathies. Ex vivo muscle preparations from ColVI null mice revealed altered neuromuscular transmission, with electrophysiological defects and decreased safety factor (i.e., the excess current generated in response to a nerve impulse over that required to reach the action potential threshold). Moreover, in vitro studies in differentiated C2C12 myotubes showed the ability of ColVI to induce AChR clustering and synaptic gene expression. These findings reveal a novel role for ColVI at the NMJ and point to the involvement of NMJ defects in the etiopathology of ColVI-related myopathies.

The Curse of Apneic Spells

A 6-year-old girl had reduced fetal movements, numerous apneic spells, muscle hypotonia, and developmental motor delay. Her muscle biopsy tissue showed variation in myofiber diameters, small minicores by electron microscopy, and near-uniformity of type I fibers. Although no mutations were detected in RYR1, SEPN1, and DMPK genes, the RAPSN gene revealed one known mutation, p.Asn88Lys, from the mother, and one novel mutation, p.Cys366Gly, from the father. Life-saving pyridostigmine treatment suppressed her apneic spells and improved her motor development.

Anticholinesterase Therapy Worsening Head Drop and Limb Weakness Due to a Novel DOK7 Mutation

Dok-7 myasthenia is an autosomal recessive congenital myasthenic syndrome due to DOK7 mutations. Anticholinesterase therapy is ineffective and may worsen the weakness in patients with Dok-7 myasthenia or few other forms of congenital myasthenic syndromes. We describe a 31-year-old man previously diagnosed with seronegative myasthenia gravis. Repetitive stimulation of the right spinal accessory nerve showed 51% decrement. Needle electromyography revealed myopathic changes in clinically affected muscles. Muscle biopsy was normal. The patient was referred to us for worsening weakness after taking pyridostigmine. We searched for DOK7 mutations and identified compound heterozygous mutations of a common c.1124_1127dupTGCC mutation and a novel splice site mutation, c.772+2_+4delinsCCGGGCAGGCGGGCA. Discontinuation of pyridostigmine improved weakness. He further regained strength with oral albuterol therapy and decrement was reduced to 25%. Worsening of symptoms with anticholinesterase therapy in patients with "seronegative myasthenia gravis" should prompt clinicians to consider a possibility of congenital myasthenic syndromes to avoid unnecessary use of immunosuppressive therapy. Patients with Dok-7 myasthenia respond well to oral albuterol treatment.