Genetic inactivation of acetylcholinesterase causes functional and structural impairment of mouse soleus muscles

Does the perigestational exposure to chlorpyrifos and/or high-fat diet affect respiratory parameters and diaphragmatic muscle contractility in young rats?

The perinatal period is characterized by developmental stages with high sensitivity to environmental factors. Among the risk factors, maternal High-Fat Diet (HFD) consumption and early-life pesticide exposure can induce metabolic disorders at adulthood. We established the effects of perigestational exposure to Chlorpyrifos (CPF) and/or HFD on respiratory parameters, sleep apnea and diaphragm contractility in adult rats. Four groups of female rats were exposed starting from 4 months before gestation till the end of lactation period to CPF (1 mg/kg/day vs. vehicle) with or without HFD. Sleep apnea and respiratory parameters were measured by whole-body plethysmography in male offspring at postnatal day 60. Then diaphragm strips were dissected for the measurement of contractility, acetylcholinesterase (AChE) activity, and gene expression. The perigestational exposure to CPF and/or HFD increased the sleep apnea index but decreased the respiratory frequency. The twitch tension and the fatigability index were also increased, associated with reduced AChE activity and elevated mRNA expression of AChE, ryanodine receptor, and myosin heavy chain isoforms. Therefore, the perigestational exposure to either CPF and/or HFD could program the risks for altered ventilatory parameters and diaphragm contractility in young adult offspring despite the lack of direct contact to CPF and/or HFD.

Advances in the understanding of skeletal muscle weakness in murine models of diseases affecting nerve-evoked muscle activity, motor neurons, synapses and myofibers

Disease processes and trauma affecting nerve-evoked muscle activity, motor neurons, synapses and myofibers cause different levels of muscle weakness, i.e., reduced maximal force production in response to voluntary activation or nerve stimulation. However, the mechanisms of muscle weakness are not well known. Using murine models of amyotrophic lateral sclerosis (SOD1(G93A) transgenic mice), congenital myasthenic syndrome (AChE knockout mice and Musk(V789M/-) mutant mice), Schwartz-Jampel syndrome (Hspg2(C1532YNEO/C1532YNEO) mutant mice) and traumatic nerve injury (Neurotomized wild-type mice), we show that the reduced maximal activation capacity (the ability of the nerve to maximally activate the muscle) explains 52%, 58% and 100% of severe weakness in respectively SOD1(G93A), Neurotomized and Musk mice, whereas muscle atrophy only explains 37%, 27% and 0%. We also demonstrate that the impaired maximal activation capacity observed in SOD1, Neurotomized, and Musk mice is not highly related to Hdac4 gene upregulation. Moreover, in SOD1 and Neurotomized mice our results suggest LC3, Fn14, Bcl3 and Gadd45a as candidate genes involved in the maintenance of the severe atrophic state. In conclusion, our study indicates that muscle weakness can result from the triggering of different signaling pathways. This knowledge may be helpful in designing therapeutic strategies and finding new drug targets for amyotrophic lateral sclerosis, congenital myasthenic syndrome, Schwartz-Jampel syndrome and nerve injury.

Molecular and phenotypic characterization of a mouse model of oculopharyngeal muscular dystrophy reveals severe muscular atrophy restricted to fast glycolytic fibres

Oculopharyngeal muscular dystrophy (OPMD) is an adult-onset disorder characterized by ptosis, dysphagia and proximal limb weakness. Autosomal-dominant OPMD is caused by a short (GCG)(8-13) expansions within the first exon of the poly(A)-binding protein nuclear 1 gene (PABPN1), leading to an expanded polyalanine tract in the mutated protein. Expanded PABPN1 forms insoluble aggregates in the nuclei of skeletal muscle fibres. In order to gain insight into the different physiological processes affected in OPMD muscles, we have used a transgenic mouse model of OPMD (A17.1) and performed transcriptomic studies combined with a detailed phenotypic characterization of this model at three time points. The transcriptomic analysis revealed a massive gene deregulation in the A17.1 mice, among which we identified a significant deregulation of pathways associated with muscle atrophy. Using a mathematical model for progression, we have identified that one-third of the progressive genes were also associated with muscle atrophy. Functional and histological analysis of the skeletal muscle of this mouse model confirmed a severe and progressive muscular atrophy associated with a reduction in muscle strength. Moreover, muscle atrophy in the A17.1 mice was restricted to fast glycolytic fibres, containing a large number of intranuclear inclusions (INIs). The soleus muscle and, in particular, oxidative fibres were spared, even though they contained INIs albeit to a lesser degree. These results demonstrate a fibre-type specificity of muscle atrophy in this OPMD model. This study improves our understanding of the biological pathways modified in OPMD to identify potential biomarkers and new therapeutic targets.

Restoration of muscle functionality by genetic suppression of glycogen synthesis in a murine model of Pompe disease

Glycogen storage disease type II (GSDII) or Pompe disease is an autosomal recessive disorder caused by acid alpha-glucosidase (GAA) deficiency, leading to lysosomal glycogen accumulation. Affected individuals store glycogen mainly in cardiac and skeletal muscle tissues resulting in fatal hypertrophic cardiomyopathy and respiratory failure in the most severe infantile form. Enzyme replacement therapy has already proved some efficacy, but results remain variable especially in skeletal muscle. Substrate reduction therapy was successfully used to improve the phenotype in several lysosomal storage disorders. We have recently demonstrated that shRNA-mediated reduction of glycogen synthesis led to a significant reduction of glycogen accumulation in skeletal muscle of GSDII mice. In this paper, we analyzed the effect of a complete genetic elimination of glycogen synthesis in the same GSDII model. GAA and glycogen synthase 1 (GYS1) KO mice were inter-crossed to generate a new double-KO model. GAA/GYS1-KO mice exhibited a profound reduction of the amount of glycogen in the heart and skeletal muscles, a significant decrease in lysosomal swelling and autophagic build-up as well as a complete correction of cardiomegaly. In addition, the abnormalities in glucose metabolism and insulin tolerance observed in the GSDII model were corrected in double-KO mice. Muscle atrophy observed in 11-month-old GSDII mice was less pronounced in GAA/GYS1-KO mice, resulting in improved exercise capacity. These data demonstrate that long-term elimination of muscle glycogen synthesis leads to a significant improvement of structural, metabolic and functional defects in GSDII mice and offers a new perspective for the treatment of Pompe disease.

The effects of exercise-induced fatigue on acetylcholinesterase expression and activity at rat neuromuscular junctions

Acetylcholinesterase is the enzyme that terminates neurotransmission by hydrolyzing the acetylcholine released by the motoneurons at the neuromuscular junctions. Although acetylcholinesterase has been studied for almost a century, the underlying relationship between exercise-induced fatigue and acetylcholinesterase activity at the synaptic cleft is not clear. The purpose of this study was to assess the effects of exercise-induced fatigue on the expression and activity of acetylcholinesterase at the neuromuscular junctions. The expression and activity of acetylcholinesterase at the gastrocnemius neuromuscular junctions was decreased transiently by exercise-induced fatigue and then gradually increased over 24 hr. The expression of acetylcholinesterase in the 24 hr recovery group returned to the level of the control (non-exercised) group, but the activity of acetylcholinesterase remained significantly lower. These data suggest that the decrease of acetylcholinesterase expression and activity may be involved in the production and/or maintenance of exercise-induced fatigue.

Slow myosin heavy chain expression in the absence of muscle activity

Innervation has been generally accepted to be a major factor involved in both triggering and maintaining the expression of slow myosin heavy chain (MHC-1) in skeletal muscle. However, previous findings from our laboratory have suggested that, in the mouse, this is not always the case (30). Based on these results, we hypothesized that neurotomy would not markedly reduced the expression of MHC-1 protein in the mouse soleus muscles. In addition, other cellular, biochemical, and functional parameters were also studied in these denervated soleus muscles to complete our study. Our results show that denervation reduced neither the relative amount of MHC-1 protein, nor the percentage of muscle fibers expressing MHC-1 protein (P > 0.05). The fact that MHC-1 protein did not respond to muscle inactivity was confirmed in three different mouse strains (129/SV, C57BL/6, and CD1). In contrast, all of the other histological, biochemical, and functional muscle parameters were markedly altered by denervation. Cross-sectional area (CSA) of muscle fibers, maximal tetanic isometric force, maximal velocity of shortening, maximal power, and citrate synthase activity were all reduced in denervated muscles compared with innervated muscles (P < 0.05). Contraction and one-half relaxation times of the twitch were also increased by denervation (P < 0.05). Addition of tenotomy to denervation had no further effect on the relative expression of MHC-1 protein (P > 0.05), despite a greater reduction in CSA and citrate synthase activity (P < 0.05). In conclusion, a deficit in neural input leads to marked atrophy and reduction in performance in mouse soleus muscles. However, the maintenance of the relative expression of slow MHC protein is independent of neuromuscular activity in mice.