Congenital myopathies are genetically and clinically heterogeneous conditions causing severe muscle weakness, and mutations in the ryanodine receptor gene (RYR1) represent the most frequent cause of these conditions. A common feature of diseases caused by recessive RYR1 mutations is a decrease of ryanodine receptor 1 protein content in muscle. The aim of the present investigation was to gain mechanistic insight into the causes of this reduced ryanodine receptor 1. We found that muscle biopsies of patients with recessive RYR1 mutations exhibit decreased expression of muscle-specific microRNAs, increased DNA methylation and increased expression of class II histone deacetylases. Transgenic mouse muscle fibres over-expressing HDAC-4/HDAC-5 exhibited decreased expression of RYR1 and of muscle-specific miRNAs, whereas acute knock-down of RYR1 in mouse muscle fibres by siRNA caused up-regulation of HDAC-4/HDAC-5. Intriguingly, increased class II HDAC expression and decreased ryanodine receptor protein and miRNAs expression were also observed in muscles of patients with nemaline myopathy, another congenital neuromuscular disorder. Our results indicate that a common pathophysiological pathway caused by epigenetic changes is activated in some forms of congenital neuromuscular disorders.

Various insults cause ototoxicity in mammals by increasing oxidative stress leading to apoptosis of auditory hair cells (HCs). The thiazolidinediones (TZDs; e.g., pioglitazone) and fibrate (e.g., fenofibrate) drugs are used for the treatment of diabetes and dyslipidemia. These agents target the peroxisome proliferator-activated receptors, PPARγ and PPARα, which are transcription factors that influence glucose and lipid metabolism, inflammation, and organ protection. In this study, we explored the effects of pioglitazone and other PPAR agonists to prevent gentamicin-induced oxidative stress and apoptosis in mouse organ of Corti (OC) explants. Western blots showed high levels of PPARγ and PPARα proteins in mouse OC lysates. Immunofluorescence assays indicated that PPARγ and PPARα proteins are present in auditory HCs and other cell types in the mouse cochlea. Gentamicin treatment induced production of reactive oxygen species (ROS), lipid peroxidation, caspase activation, PARP-1 cleavage, and HC apoptosis in cultured OCs. Pioglitazone mediated its anti-apoptotic effects by opposing the increase in ROS induced by gentamicin, which inhibited the subsequent formation of 4-hydroxy-2-nonenal (4-HNE) and activation of pro-apoptotic mediators. Pioglitazone mediated its effects by upregulating genes that control ROS production and detoxification pathways leading to restoration of the reduced:oxidized glutathione ratio. Structurally diverse PPAR agonists were protective of HCs. Pioglitazone (PPARγ-specific), tesaglitazar (PPARγ/α-specific), and fenofibric acid (PPARα-specific) all provided >90% protection from gentamicin toxicity by regulation of overlapping subsets of genes controlling ROS detoxification. This study revealed that PPARs play important roles in the cochlea, and that PPAR-targeting drugs possess therapeutic potential as treatment for hearing loss.

Malignant hyperthermia is a potentially fatal hypermetabolic disorder triggered by halogenated anesthetics and the myorelaxant succinylcholine in genetically predisposed individuals.Approximately 50% of Malignant Hyperthermia susceptible individuals carry dominant, gain of function mutations in RYR1 (which encodes ryanodine receptor type 1), though they have normal muscle function and no overt clinical symptoms. RyR1 is predominantly expressed in skeletal muscle but also to a lower extent in some immune and smooth muscle cells, suggesting that RYR1 mutations may have a wider range of effects than previously suspected. Consistently, reports describing mild bleeding abnormalities in patients with malignant hyperthermia carrying gain of function RYR1 mutations have appeared. In the present report we sought to determine the 3 frequency and molecular basis for this symptom. We found that some patients with specific RYR1 mutations had abnormally high bleeding scores, whereas their healthy relatives did not. Knock-in mice with the Malignant Hyperthermia Susceptibility RYR1 mutation Y522S (MHS RYR1 Y522S ) had bleeding times that were 3 times longer than their wild-type littermates. Primary vascular smooth muscle cells from RYR1 Y522S knock-in mice exhibited a higher frequency of subplasmalemmal Ca 2+ sparks leading to a more negative resting membrane potential. The bleeding defect of RYR1 Y522S mice and of one patient was reversed by treatment with the RYR1 antagonist dantrolene, and Ca 2+ sparks in primary vascular smooth muscle cells from the MHS RYR1 Y522S mice were blocked by ryanodine or dantrolene. Thus, RYR1 mutations may lead to prolonged bleeding by altering vascular smooth muscle cell function. The reversibility of the bleeding phenotype emphasizes the potential therapeutic value of dantrolene in the treatment of such bleeding disorders.4

Excitation-contraction coupling (ECC) is the physiological mechanism whereby an electrical signal detected by the dihydropyridine receptor, is converted into an increase in [Ca2+], via activation of ryanodine receptors (RyRs). Mutations in RYR1, the gene encoding RyR1, are the underlying cause of various congenital myopathies including central core disease, multiminicore disease (MmD), some forms of centronuclear myopathy (CNM) and congenital fibre-type disproportion. Interestingly, patients with recessive, but not dominant, RYR1 mutations show a significant reduction in RyR protein in muscle biopsies as well as ophthalmoplegia. This specific involvement of the extraocular muscles (EOMs) indicates that this group of muscles may express different amounts of proteins involved in ECC compared with limb muscles. In the present paper, we report that this is indeed the case; in particular the transcripts encoding RyR3, cardiac calsequestrin (CSQ2) and the α1 subunit of the cardiac dihydropyridine receptor are up-regulated by at least 100-fold, whereas excitation-coupled Ca2+ entry is 3-fold higher. These findings support the hypothesis that EOMs have a unique mode of calcium handling.