Myosin heavy chain composition of muscle fibers in spinal muscular atrophy

Characterization of SMA type II skeletal muscle from treated patients shows OXPHOS deficiency and denervation

Spinal muscular atrophy (SMA) is a recessive developmental disorder caused by the genetic loss or mutation of the gene SMN1 (survival of motor neuron 1). SMA is characterized by neuromuscular symptoms and muscle weakness. Several years ago, SMA treatment underwent a radical transformation, with the approval of 3 different SMN-dependent disease-modifying therapies. This includes 2 SMN2 splicing therapies - risdiplam and nusinersen. One main challenge for type II SMA patients treated with these drugs is ongoing muscle fatigue, limited mobility, and other skeletal problems. To date, few molecular studies have been conducted on SMA patient-derived tissues after treatment, limiting our understanding of what targets remain unchanged after the spinal cord-targeted therapies are applied. Therefore, we collected paravertebral muscle from 8 type II patients undergoing spinal surgery for scoliosis and 7 controls. We used RNA-seq to characterize their transcriptional profiles and correlate these molecular changes with muscle histology. Despite the limited cohort size and heterogeneity, we observed a consistent loss of oxidative phosphorylation (OXPHOS) machinery of the mitochondria, a decrease in mitochondrial DNA copy number, and a correlation between signals of cellular stress, denervation, and increased fibrosis. This work provides new putative targets for combination therapies for type II SMA.

Surface Electromyography Thresholds as a Measure for Performance Fatigability During Incremental Cycling in Patients With Neuromuscular Disorders

In healthy persons, there is an excellent relation between the timing of the (two) surface electromyography (sEMG) thresholds and the (two) ventilatory thresholds during exercise. The primary aim of this study was to determine the relative timing of both sEMG and ventilatory thresholds in patients with neuromuscular disorders compared with healthy subjects during a maximal ergospirometry cycling test. We hypothesized that in patients with neuromuscular disorders, the sEMG thresholds would occur relatively earlier in time than the ventilatory thresholds, compared to healthy subjects, because performance fatigability occurs more rapidly. In total, 24 healthy controls and 32 patients with a neuromuscular disorder performed a cardiopulmonary exercise test on a bicycle using a 10-min ramp protocol, during which we collected ergospirometry data: power at both ventilatory and sEMG thresholds, and sEMG data of lower leg muscles. In line with our hypothesis, normalized values for all thresholds were lower for patients than healthy subjects. These differences were significant for the first ventilatory (p = 0.008) and sEMG threshold (p < 0.001) but not for the second sEMG (p = 0.053) and ventilatory threshold (p = 0.238). Most parameters for test–retest reliability of all thresholds did not show any fixed bias, except for the second ventilatory threshold. The feasibility of the sEMG thresholds was lower than the ventilatory thresholds, particularly of the first sEMG threshold. As expected, the sEMG thresholds, particularly the first threshold, occurred relatively earlier in time than the ventilatory thresholds in patients compared with healthy subjects. A possible explanation could be (a combination of) a difference in fiber type composition, disuse, and limited muscle-specific force in patients with neuromuscular disorders. sEMG measurements during submaximal dynamic exercises are needed to generalize the measurements to daily life activities for future use in prescribing and evaluating rehabilitation interventions.

Fast and slow-twitching muscles are differentially affected by reduced cholinergic transmission in mice deficient for VAChT: A mouse model for congenital myasthenia

Congenital myasthenic syndromes (CMS) result from reduced cholinergic transmission at neuromuscular junctions (NMJs). While the etiology of CMS varies, the disease is characterized by muscle weakness. To date, it remains unknown if CMS causes long-term and irreversible changes to skeletal muscles. In this study, we examined skeletal muscles in a mouse line with reduced expression of Vesicular Acetylcholine Transporter (VAChT, mouse line herein called VAChT-KDHOM). We examined this mouse line for several reasons. First, VAChT plays a central function in loading acetylcholine (ACh) into synaptic vesicles and releasing it at NMJs, in addition to other cholinergic nerve endings. Second, loss of function mutations in VAChT causes myasthenia in humans. Importantly, VAChT-KDHOM present with reduced ACh and muscle weakness, resembling CMS. We evaluated the morphology, fiber type (myosin heavy chain isoforms), and expression of muscle-related genes in the extensor digitorum longus (EDL) and soleus muscles. This analysis revealed that while muscle fibers atrophy in the EDL, they hypertrophy in the soleus muscle of VAChT-KDHOM mice. Along with these cellular changes, skeletal muscles exhibit altered levels of markers for myogenesis (Pax-7, Myogenin, and MyoD), oxidative metabolism (PGC1-α and MTND1), and protein degradation (Atrogin1 and MuRF1) in VAChT-KDHOM mice. Importantly, we demonstrate that deleterious changes in skeletal muscles and motor deficits can be partially reversed following the administration of the cholinesterase inhibitor, pyridostigmine in VAChT-KDHOM mice. These findings reveal that fast and slow type muscles differentially respond to cholinergic deficits. Additionally, this study shows that the adverse effects of cholinergic transmission, as in the case of CMS, on fast and slow type skeletal muscles are reversible.

Myosin heavy chain composition of the human sternocleidomastoid muscle

The sternocleidomastoid (SCM) muscle is one of the neck muscles responsible for head posture and control of head movement. It functions in rotation, inclination, protraction, extension and flexion of the head, whilst chewing and in exerting increased respiratory efforts. This study is the first one describing the myosin heavy chain (MyHC) isoform composition of the SCM muscle of presumably healthy young males for the purpose of better understanding the contractile properties of the muscle as well as to help in evaluation of pathologically altered structure of the muscle. Autopsy samples were processed immunohistochemically to reveal the MyHC isoform composition. The muscle fibres expressed MyHC-1 (31.5%), -2a (29.7%) and -2x (4.3%) or co-expressed MyHC-2a with MyHC-2x (26.8%), MyHC-1 with MyHC-2a (4.1%) and/or MyHC-1, -2a with -2x (1.1%). In addition to the MyHC isoforms, characteristic of adult limb muscles, a very low percentage of muscle fibres (0.2-2.7%) expressed MyHC-neo, which is normally not found in adult limb muscles. Only two samples exhibited MyHC-neo at a rather higher percentage (6.3% and 7.5%) of muscle fibres. The high share of hybrid fibres and the presence of MyHC-neo in the SCM muscle differ from that of adult limb muscles where hybrid fibres are rare and the expression of immature MyHC isoforms occurs only in pathological or experimental conditions. Since the SCM muscle shares the same embryogenic potential as limb muscles, its distinct MyHC expression appears to be associated with twin innervation and with the intrinsic specialisation to perform multiple functions.

Level of muscle regeneration in limb-girdle muscular dystrophy type 2I relates to genotype and clinical severity

Background

The balance between muscle regeneration and ongoing degeneration is a relationship that greatly influences the progression of muscular dystrophy. Numerous factors may influence the muscle regeneration, but more information about the relationship between genotype, clinical severity and the ability to regenerate is needed.

Methods

Muscle biopsies were obtained from the tibialis anterior muscle, and frozen sections were stained for general histopathological and immunohistological evaluation. Differences between groups were considered statistical significant at P < 0.05 using Student's unpaired t-test.

Results

We found that all patients with limb-girdle muscular dystrophy type 2I (LGMD2I) had a large number of internally nucleated fibers, a sign of previous regeneration. The level of expression of muscle-specific developmental proteins, such as neonatal myosin heavy chain (nMHC) and myogenin, was related to the clinical severity. Additionally, we found that the majority of nMHC-positive fibers did not stain positively for utrophin in patients who were compound heterozygous for the L276I mutation, suggesting that the predominant form of regeneration in these patients is fiber repair rather than formation of new fibers. Double staining showed that many smaller nMHC-positive fibers were positive for antibodies against the glycosylation on α-dystroglycan, suggesting that such glycosylation may be a result of muscle regeneration.

Conclusion

Severely affected patients with LGMD2I have a high level of muscle degeneration, which leads to a high rate of regeneration, but this is insufficient to change the imbalance between degeneration and regeneration, ultimately leading to progressive muscle wasting. Detailed information regarding the level and rate of muscle regeneration and potential obstructions of the regenerative pathway should be of use for future therapies involving satellite-cell activation.

Myosin heavy-chain composition of the human hyoglossus muscle

The human tongue muscle hyoglossus (HG) is active in oromotor behaviors encompassing a wide range of tongue movement speeds. Here we test the hypothesis that the human HG is composed of "uncommon" myosin heavy-chain (MHC) isoforms MHCembryonic, MHCneonatal, and MHCslow tonic as has been reported for other head and neck muscles active during kinematically diverse behaviors. Following reaction of human HG with antibodies specific for MHCI, MHCIIA, MHCII, MHCembryonic, MHCextraocular, MHCneonatal, and MHCslow tonic, only antibodies to MHCI, MHCIIA, and MHCII label more than occasional muscle fibers. These antibodies describe five phenotypes with prevalence MHCIIA > MHCI > MHCI-IIX > MHCI-IIA > MHCIIX. In MHC composition, the human HG is thus similar to human appendicular muscles and many human head and neck muscles but different from human masseter and extraocular muscles which contain five or more MHC isoforms.

Abnormal palatopharyngeal muscle morphology in sleep-disordered breathing

The aim of the present study was to investigate whether histopathological changes can be detected in two soft palate muscles, the palatopharyngeus and the uvula, in 11 patients with long duration of sleep-disordered breathing (SDB). Muscle samples were collected from patients undergoing uvulo-palatopharyngoplasty (UPPP). Reference samples from the corresponding areas were obtained at autopsy from five previously healthy subjects. Muscle morphology, fibre type and myosin heavy chain (MyHC) compositions were analysed with enzyme-histochemical, immunohistochemical and biochemical techniques. The muscle samples from the patients, and especially those from the palatopharyngeus, showed several morphological abnormalities. The most striking findings were (i) increased amount of connective tissue, (ii) abnormal variability in fibre size, (iii) increased proportion of small-sized fibres, (iv) alterations in fibre type and MyHC compositions, (v) increased frequency of fibres containing developmental MyHC isoforms. Our findings point towards a pathological process of denervation and degeneration in the patient samples. Conclusively, the morphological abnormalities suggest a neuromuscular disorder of the soft palate in SDB patients.

Spinal muscular atrophy: present state

Spinal muscular atrophy (SMA) is a hereditary neurodegenerative disease caused by homozygous deletions or mutations in the SMN1 gene on Chr.5q13. SMA spans from severe Werdnig-Hoffmann disease (SMA 1) to relatively benign Kugelberg-Welander disease (SMA 3). Onset before birth possibly aggravates the clinical course, because immature motoneurons do not show compensatory sprouting and collateral reinnervation, and motor units in SMA 1, in contrast to those in SMA 3, are not enlarged. Genetic evidence indicates that SMN2, a gene 99% identical to SMN1, can attenuate SMA severity: in patients, more SMN2 copies and higher SMN protein levels are correlated with milder SMA. There is evidence that SMN plays a role in motoneuron RNA metabolism, but it has also been linked to apoptosis. Several mouse models with motoneuron disease have been successfully treated with neurotrophic factors. None of these models is, however, homologous to SMA. Recently, genetic mouse models of SMA have been created by introducing human SMN2 transgenes into Smn knockout mice or by targeting the Smn gene knockout to neurons. These mice not only provide important insights into the pathogenesis of SMA but are also crucial for testing new therapeutic strategies. These include SMN gene transfer, molecules capable to up-regulate SMN expression and trophic or antiapoptotic factors.

DNA fragmentation and BCL-2 expression in infantile spinal muscular atrophy

Chromatin cleavage, a hallmark of apoptosis, was identified by in situ labeling in 55 +/- 7% of the muscle fibers in infantile spinal muscular atrophy (ISMA) and, to a lesser extent, in peripheral neuropathy indicating that DNA fragmentation is not specific to ISMA but a common feature in defect innervation. However, as DNA breaks are also known as a temporary process in differentiating myotubes DNA fragmentation may not always proceed to cell death. Therefore, it is currently not certain whether high rates of DNA fragmentation in ISMA are part of delayed muscle maturation due to neuronal defect or part of fibre breakdown. While atrophic muscle fibres in peripheral neuropathy displayed strong expression of bcl-2, a protein delaying onset of apoptosis, only 30% of the ISMA cases revealed weak bcl-2 expression assuming that immature muscle fibers are not able to produce a sufficient level of bcl-2.

Expression of developmentally regulated cytoskeleton and cell surface proteins in childhood spinal muscular atrophies

Expression of some developmentally regulated cytoskeleton components (desmin, vimentin and myosin heavy chain isoforms) and cell surface proteins (including neural cell adhesion molecule (NCAM), its polysialylated (PSA) isoform and CD24) have been studied by immunohistochemical detection in a series of 23 infantile spinal muscular atrophies (SMA). According to the clinical classification established by Byers and Banker in 1961, 8 cases were type I SMA (Werdnig-Hoffmann's disease), 10 cases were type II (intermediate form), and 5 cases were type III (Kugelberg-Welander's disease). In 15 cases, the percentage of immunoreactive fibers with the various antibodies used has been quantified and the results correlated with clinical data. The aim of the study was to search for variations in the pattern of expression of the proteins to improve the accuracy of diagnosis and prognosis, and to gain an understanding of the pathological processes involved in SMA. The results showed that the pattern of expression of these cytoskeleton and cell surface proteins is abnormal in all types of SMA. However, it was strikingly different in type I and II SMA as opposed to type III. In type I and II SMA, strong NCAM and developmental myosin heavy chain (MHC) expression was observed in atrophic fibers. Numerous atrophic fibers co-expressed desmin and vimentin as well as slow and fast adult MHC. Very few of them expressed PSA NCAM, fetal MHC and CD24. In type III SMA, the number of fibers expressing NCAM, developmental MHC and co-expressing slow and fast adult MHC was low and virtually none of them expressed vimentin or desmin. These findings are in favor of a denervation process occurring very early in life, probably even in utero, in type I and II SMA and leading to a severe impairment of muscle fibers maturation. In contrast, in type III SMA, the process is initiated well after birth and affects mature muscle fibers. In all types of SMA, the ability of muscle fibers to regenerate is low, although some fibers may be reinnervated. Immunohistochemical data was not related to the patients follow-up and thus has no prognostic value.

Biochemical and immunocytochemical analysis in chronic proximal spinal muscular atrophy

Immunocytochemical and biochemical analyses were carried out on patients affected by chronic SMA. Three groups of patients were identified. In group I, the muscle presented a fascicular atrophy; a high percentage of atrophic type II fibers; and fibers expressing fast, slow, embryonic, and fetal myosin isoforms. In group II, the muscle was characterized by atrophic fibers and normal/hypertrophic fibers expressing only slow myosin isoforms. In group III, the muscle was characterized by fiber type grouping and fibers coexpressing fast and slow myosin isoforms but never embryonic or fetal MHC isoforms. The muscles of groups I and III contained both fast and slow myosins whereas group II muscles were predominantly slow by immunocytochemical analysis or only slow by biochemical analysis. In view of these results, immunocytochemical and histochemical analyses could help to classify chronic SMA and help to understand the different pathogenic processes which seem to be related to the maturational stage of the muscle at the age of onset of the disease.

Evolution of muscle specific proteins in Werdnig-Hoffman's disease

The pattern of expression of desmin, vimentin, titin and different myosin isoforms expressed in atrophic and hypertrophic type I and type II muscle fibers was investigated in 7 biopsies from patients of various ages all diagnosed as suffering from Werdnig-Hoffman's disease. The results revealed that there was a progressive atrophy affecting both type I and type II muscle fibers. The proportion of atrophic type II fibers increased with age. These atrophic fibers expressed predominantly fast MHC together with variable amounts of embryonic and fetal abnormal concentrations of desmin, vimentin and titin were also observed in some of these fibers. Hypertrophic type I fibers expressed exclusively slow MHC. These results are in good agreement with the hypothesis that Werdnig-Hoffman's disease is associated with a persistence of slow twitch type I motor units and a loss of phasic type II motor units. They also confirm that the atrophic fibers were frequently immature although embryonic MLC was never detected in these muscles. In addition we have demonstrated that the hypertrophic fibers were not completely normal since they frequently contained abnormal concentrations of desmin and titin at their periphery.

Disuse of anterior tibial muscle during locomotion and increased proportion of type II fibres in hemiplegia

The tibialis anterior (TA) is a muscle activated mainly during walking. Its use during the step cycle was studied in 10 patients (55.8 +/- 8.8 years) with chronic hemiplegia (duration 3-18 years) and related to the muscle fibre composition, size and expression of isoforms of myosin heavy chains (MHCs). In the average step cycle the integrated surface EMG of the paralysed TA did in the majority of the hemiplegic patients not exceed 10% of that recorded during maximal contraction of the normal leg. The type I fibre percentage in the paralysed TA subject was 57.4% as compared with 79.4% in normal muscles (P less than 0.05). The range of axonal conduction velocities in the peroneal nerve did not differ in paralysed and non-paralysed leg, suggesting that there was no selective loss of one class of motoneurons. The type II fibres consisted of IIA (66%) and IIB (31%), in contrast to the normal TA muscle where less than 1% of the muscle fibres are of type IIB. The incidence of fibres in the biopsies with both slow and fast MHCs had a mean value of 3.5% (range 0.7-9%). The type I and type II muscle fibres had normal sizes with cross-sectional area 4511 +/- 962 microns 2 and 6181 +/- 1062 microns 2. No selective type II atrophy was seen. Occasional hypertrophic type I and II fibres were seen in 4 patients.(ABSTRACT TRUNCATED AT 250 WORDS)