Adverse effects of myasthenia gravis on rat phrenic diaphragm contractile performance

Brain derived neurotrophic factor/tropomyosin related kinase B signaling impacts diaphragm neuromuscular transmission in a novel rat chemogenetic model

The neuromuscular junction (NMJ) mediates neural control of skeletal muscle fibers. Neurotrophic signaling, specifically brain derived neurotrophic factor (BDNF) acting through its high-affinity tropomyosin related kinase B (TrkB) receptor is known to improve neuromuscular transmission. BDNF/TrkB signaling also maintains the integrity of antero- and retrograde communication between the motor neuron soma, its distal axons and pre-synaptic terminals and influences neuromuscular transmission. In this study, we employed a novel rat chemogenetic mutation (TrkB F616), in which a 1-naphthylmethyl phosphoprotein phosphatase 1 (1NMPP1) sensitive knock-in allele allowed specific, rapid and sustained inhibition of TrkB kinase activity. In adult female and male TrkB F616 rats, treatment with either 1NMPP1 (TrkB kinase inhibition) or DMSO (vehicle) was administered in drinking water for 14 days. To assess the extent of neuromuscular transmission failure (NMTF), diaphragm muscle isometric force evoked by nerve stimulation at 40 Hz (330 ms duration trains repeated each s) was compared to isometric forces evoked by superimposed direct muscle stimulation (every 15 s). Chronic TrkB kinase inhibition (1NMPP1 group) markedly worsened NMTF compared to vehicle controls. Acute BDNF treatment did not rescue NMTF in the 1NMPP1 group. Chronic TrkB kinase inhibition did not affect the apposition of pre-synaptic terminals (labeled with synaptophysin) and post-synaptic endplates (labeled with α-Bungarotoxin) at diaphragm NMJs. We conclude that inhibition of BDNF/TrkB signaling in TrkB F616 rats disrupts diaphragm neuromuscular transmission in a similar manner to TrkB F616A mice, likely via a pre-synaptic mechanism independent of axonal branch point failure.

A review of the histopathological findings in myasthenia gravis: Clues to the pathogenesis of treatment-resistance in extraocular muscles

In myasthenia gravis autoantibodies target components of the neuromuscular junction causing variable degrees of weakness. In most cases, autoantibodies trigger complement-mediated endplate damage and extraocular muscles may be most susceptible. A proportion of MG cases develop treatment-resistant ophthalmoplegia. We reviewed publications spanning 65 years reporting the histopathological findings in the muscles and extraocular muscles of myasthenic patients to determine whether pathological changes in extraocular muscles differ from non-ocular muscles. As extraocular muscles represent a unique muscle allotype we also compared their histopathology in myasthenia to those in strabismus. We found that in myasthenia gravis, the non-ocular muscles frequently demonstrate neurogenic changes regardless of myasthenic serotype. Mitochondrial stress/damage was also frequent in myasthenic muscles and possibly more evident in muscle-specific kinase antibody-positive MG. Although myasthenia-associated paralysed extraocular muscles demonstrated prominent fibro-fatty replacement and mitochondrial alterations, these features appeared commonly in paralysed extraocular muscles of any cause. We postulate that extraocular muscles may be more susceptible than limb muscles to poor contractility as a consequence of myasthenia, resulting in a cascade of atrophy signaling pathways and altered mitochondrial homeostasis which contribute to the tipping point in developing treatment-resistant myasthenic ophthalmoplegia. Early strategies to improve force generation in extraocular muscles are critical.

Measuring Neuromuscular Junction Functionality

Neuromuscular junction (NMJ) functionality plays a pivotal role when studying diseases in which the communication between motor neuron and muscle is impaired, such as aging and amyotrophic lateral sclerosis (ALS). Here we describe an experimental protocol that can be used to measure NMJ functionality by combining two types of electrical stimulation: direct muscle membrane stimulation and the stimulation through the nerve. The comparison of the muscle response to these two different stimulations can help to define, at the functional level, potential alterations in the NMJ that lead to functional decline in muscle. Ex vivo preparations are suited to well-controlled studies. Here we describe an intensive protocol to measure several parameters of muscle and NMJ functionality for the soleus-sciatic nerve preparation and for the diaphragm-phrenic nerve preparation. The protocol lasts approximately 60 min and is conducted uninterruptedly by means of a custom-made software that measures the twitch kinetics properties, the force-frequency relationship for both muscle and nerve stimulations, and two parameters specific to NMJ functionality, i.e. neurotransmission failure and intratetanic fatigue. This methodology was used to detect damages in soleus and diaphragm muscle-nerve preparations by using SOD1^G93A transgenic mouse, an experimental model of ALS that ubiquitously overexpresses the mutant antioxidant enzyme superoxide dismutase 1 (SOD1).

Measuring Neuromuscular Junction Functionality in the SOD1(G93A) Animal Model of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease that leads to motor neuron degeneration, alteration in neuromuscular junctions (NMJs), muscle atrophy, and paralysis. To investigate the NMJ functionality in ALS we tested, in vitro, two innervated muscle types excised from SOD1^G93A transgenic mice at the end-stage of the disease: the Soleus, a postural muscle almost completely paralyzed at that stage, and the diaphragm, which, on the contrary, is functional until death. To this aim we employed an experimental protocol that combined two types of electrical stimulation: the direct stimulation and the stimulation through the nerve. The technique we applied allowed us to determine the relevance of NMJ functionality separately from muscle contractile properties in SOD1^G93A animal model. Functional measurements revealed that the muscle contractility of transgenic diaphragms is almost unaltered in comparison to control muscles, while transgenic Soleus muscles were severely compromised. In contrast, when stimulated via the nerve, both transgenic muscle types showed a strong decrease of the contraction force, a slowing down of the kinetic parameters, as well as alterations in the neurotransmission failure parameter. All together, these results confirm a severely impaired functionality in the SOD1^G93A neuromuscular junctions.

Deficits in endogenous adenosine formation by ecto-5'-nucleotidase/CD73 impair neuromuscular transmission and immune competence in experimental autoimmune myasthenia gravis

AMP dephosphorylation via ecto-5'-nucleotidase/CD73 is the rate limiting step to generate extracellular adenosine (ADO) from released adenine nucleotides. ADO, via A2A receptors (A2ARs), is a potent modulator of neuromuscular and immunological responses. The pivotal role of ecto-5'-nucleotidase/CD73, in controlling extracellular ADO formation, prompted us to investigate its role in a rat model of experimental autoimmune myasthenia gravis (EAMG). Results show that CD4(+)CD25(+)FoxP3(+) regulatory T cells express lower amounts of ecto-5'-nucleotidase/CD73 as compared to controls. Reduction of endogenous ADO formation might explain why proliferation of CD4(+) T cells failed upon blocking A2A receptors activation with ZM241385 or adenosine deaminase in EAMG animals. Deficits in ADO also contribute to neuromuscular transmission failure in EAMG rats. Rehabilitation of A2AR-mediated immune suppression and facilitation of transmitter release were observed by incubating the cells with the nucleoside precursor, AMP. These findings, together with the characteristic increase in serum adenosine deaminase activity of MG patients, strengthen our hypothesis that the adenosinergic pathway may be dysfunctional in EAMG. Given that endogenous ADO formation is balanced by ecto-5'-nucleotidase/CD73 activity and that A2ARs exert a dual role to restore use-dependent neurocompetence and immune suppression in myasthenics, we hypothesize that stimulation of the two mechanisms may have therapeutic potential in MG.

RNA expression analysis of passive transfer myasthenia supports extraocular muscle as a unique immunological environment

PURPOSE

Myasthenia gravis demonstrates a distinct predilection for involvement of the extraocular muscles (EOM), and we have hypothesized that this may be due to a unique immunological environment. To assess this hypothesis, we took an unbiased approach to analyze RNA expression profiles in EOM, diaphragm, and extensor digitorum longus (EDL) in rats with experimentally acquired myasthenia gravis (EAMG).

METHODS

Experimentally acquired myasthenia gravis was induced in rats by intraperitoneal injection of antibody directed against the acetylcholine receptor (AChR), whereas control rats received antibody known to bind the AChR but not induce disease. After 48 hours, animals were killed and muscles analyzed by RNA expression profiling. Profiling results were validated using qPCR and immunohistochemical analysis.

RESULTS

Sixty-two genes common among all muscle groups were increased in expression. These fell into four major categories: 12.8% stress response, 10.5% immune response, 10.5% metabolism, and 9.0% transcription factors. EOM expressed 212 genes at higher levels, not shared by the other two muscles, and a preponderance of EOM gene changes fell into the immune response category. EOM had the most uniquely reduced genes (126) compared with diaphragm (26) and EDL (50). Only 18 downregulated genes were shared by the three muscles. Histological evaluation and disease load index (sum of fold changes for all genes) demonstrated that EOM had the greatest degree of pathology.

CONCLUSIONS

Our studies demonstrated that consistent with human myasthenia gravis, EOM demonstrates a distinct RNA expression signature from EDL and diaphragm, which is based on differences in the degree of muscle injury and inflammatory response.

A Diagnostic and Management Dilemma

BACKGROUND

Neuromuscular junction disorders are usually categorized as either presynaptic or postsynaptic. The most frequently encountered disorder of the postsynaptic neuromuscular junction is acquired myasthenia gravis. Lambert-Eaton myasthenic syndrome is a well-known prototype of the presynaptic autoimmune disorders of neuromuscular transmission. These major disorders of neuromuscular transmission are relatively common and distinctly recognized, but co-occurrence of these disorders (overlap myasthenic syndrome) is rare and has so far attracted little attention.

REVIEW SUMMARY

This report describes a patient with acquired myasthenia gravis and immunologic coexistence of Lambert-Eaton myasthenic syndrome (overlap myasthenic syndrome) in association with abdominal/uterine leiomyosarcoma. The patient presented with acute respiratory failure, making identification and management of her illness challenging. A general overview of the complexities associated with overlap between myasthenia gravis and Lambert-Eaton myasthenic syndrome is provided and this patient's complicated clinical course and response to therapy are discussed.

CONCLUSION

To our knowledge, this is the first report of overlap myasthenic syndrome in conjunction with abdominal leiomyosarcoma. The immunologic coexistence of acquired myasthenia gravis and Lambert-Eaton myasthenic syndrome in a patient with a malignant smooth-muscle tumor is intriguing and suggests that a common paraneoplastic process targeting 2 different onconeural antigens was the underlying pathogenic mechanism in this patient.

[The relationship between lung function and health-related quality of life in patients with generalized myasthenia gravis]

OBJECTIVE

To study lung function abnormalities and health-related quality of life (HRQL) in a group of patients diagnosed with generalized myasthenia gravis, and to analyze the relationship between these 2 sets of variables.

PATIENTS AND METHODS

In a prospective study of 20 patients diagnosed with generalized myasthenia gravis, the following parameters were measured: spirometry, static lung volumes, breathing pattern, maximal respiratory pressures, and maximum voluntary ventilation. HRQL was assessed using the 36-item short form general health questionnaire (SF-36).

RESULTS

The mean (SD) age of the patients was 64 (11) years. Patients were classified into 2 groups depending on whether they had IIa (12 patients) or IIb (8 patients) type disease. A small decrease in total lung capacity (86%) and slight reductions in maximal inspiratory pressure (88%) and maximum voluntary ventilation (63% in group IIb) were observed. The HRQL domains most affected were those related to physical activity and self-perceived health status in all groups, although women were more affected. The scores relating to vitality and physical activity were found to be significantly associated with forced vital capacity and lung volumes. Tidal volume was associated with maximal inspiratory and expiratory pressures, inspiratory capacity, and maximum voluntary ventilation. The respiratory rate to tidal volume ratio was inversely associated with the first three of these variables.

CONCLUSIONS

A very slight restrictive pattern and a reduction in inspiratory muscle strength were observed. The HRQL domains most affected were those related to physical activity and the patients' self-perceived health status. The weakness of the respiratory muscles contributes to the abnormalities observed in lung function and to the deterioration of health-related quality of life.

Outcome of acetylcholinesterase deficiency for neuromuscular functioning

Acetylcholinesterase (AChE) plays an essential role in neuromuscular transmission, therefore it is surprising that AChE knockout (KO) mice could live to the adulthood. Neuromuscular functioning in KO and normal (wild type, WT) mice were studied, at different age (1.5-, 4- and 9-month-old). Hindlimb muscle force productions in response to nerve or muscle electric stimulation were recorded in situ and in vitro. Our results show that contrary to WT mice, 1.5-, 4- and 9-month-old KO mice exhibited a decreased in tetanic force during short periods (500 ms) of repetitive nerve stimulations (tetanic fade). Nevertheless submaximal muscle forces in response to single or repetitive nerve stimulation were increased (potentiation) in 1.5-, 4- and 9-month-old KO mice as compared to WT mice (p<0.05). Tetanic fade and potentiation were absent when muscles were directly stimulated, indicating neuromuscular transmission alterations in KO mice. Contrary to younger mice, muscle weight and maximal tetanic force in response to repetitive nerve stimulation were not reduced in 4- and 9-month-old KO mice as compared to WT mice (p>0.05). In conclusion AChE deficit leads to marked neuromuscular alterations in hind limb muscle functioning and a prominent symptom is the lack of resistance to fatigue.