The nSMase2/Smpd3 gene modulates the severity of muscular dystrophy and the emotional stress response in mdx mice.

BACKGROUND: Duchenne muscular dystrophy (DMD) is a progressive, degenerative muscular disorder and cognitive dysfunction caused by mutations in the dystrophin gene. It is characterized by excess inflammatory responses in the muscle and repeated degeneration and regeneration cycles. Neutral sphingomyelinase 2/sphingomyelin phosphodiesterase 3 (nSMase2/Smpd3) hydrolyzes sphingomyelin in lipid rafts. This protein thus modulates inflammatory responses, cell survival or apoptosis pathways, and the secretion of extracellular vesicles in a Ca2+-dependent manner. However, its roles in dystrophic pathology have not yet been clarified. METHODS: To investigate the effects of the loss of nSMase2/Smpd3 on dystrophic muscles and its role in the abnormal behavior observed in DMD patients, we generated mdx mice lacking the nSMase2/Smpd3 gene (mdx:Smpd3 double knockout [DKO] mice). RESULTS: Young mdx:Smpd3 DKO mice exhibited reduced muscular degeneration and decreased inflammation responses, but later on they showed exacerbated muscular necrosis. In addition, the abnormal stress response displayed by mdx mice was improved in the mdx:Smpd3 DKO mice, with the recovery of brain-derived neurotrophic factor (Bdnf) expression in the hippocampus. CONCLUSIONS: nSMase2/Smpd3-modulated lipid raft integrity is a potential therapeutic target for DMD.

Contracting C2C12 myotubes release CCL2 in an NF-κB-dependent manner to induce monocyte chemoattraction.

Muscle inflammation following exercise is characterized by expression of inflammatory cytokines and chemokines. Exercise also increases muscle macrophages derived from circulating monocytes. However, it is unknown whether muscle cells themselves attract circulating monocytes, or what is the underlying mechanism. We used an in vitro system of electrical stimulation (ES) causing C2C12 myotube contraction to explore whether monocyte chemoattraction ensues and investigated the mediating chemoattractants. Conditioned medium from ES-contracted myotubes caused robust chemoattraction of THP-1 monocytes across Boyden chambers. Following ES, expression of several known monocyte chemokines [C-C motif ligand 2 (CCL2) and C-X-C motif ligand (CXCL)1, -2, and -5] was elevated, but of these, only recombinant CCL2 effectively reproduced monocyte migration. Electrically stimulated myotubes secreted CCL2, and neutralization of CCL2 in conditioned medium or antagonizing the CCL2 receptor (CCR2) in THP-1 monocytes inhibited ES-induced monocyte migration. N-benzyl-p-toluene sulfonamide (BTS), a myosin II-ATPase inhibitor, prevented ES-induced myotube contraction but not CCL2 gene expression and secretion. The membrane-permeant calcium chelator BAPTA-AM reduced ES-induced CCL2 secretion. Hence, electrical depolarization, rather than mechanical contraction, drives the rise in CCL2, with partial calcium input. ES activated the NF-κB pathway; NF-κB inhibitors reduced ES-induced CCL2 gene expression and secretion and repressed ES-induced THP-1 chemoattraction. Thus, electrically stimulated myotubes chemoattract monocytes through NF-κB-regulated CCL2 secretion.

An improved glucose transport assay system for isolated mouse skeletal muscle tissues.

There is a growing demand for a system in the field of sarcopenia and diabetes research that could be used to evaluate the effects of functional food ingredients that enhance muscle mass/contractile force or muscle glucose uptake. In this study, we developed a new type of in vitro muscle incubation system that systemizes an apparatus for muscle incubation, using an electrode, a transducer, an incubator, and a pulse generator in a compact design. The new system enables us to analyze the muscle force stimulated by the electric pulses and glucose uptake during contraction and it may thus be a useful tool for analyzing the metabolic changes that occur during muscle contraction. The system may also contribute to the assessments of new food ingredients that act directly on skeletal muscle in the treatment of sarcopenia and diabetes.

Macrophage migration inhibitory factor diminishes muscle glucose transport induced by insulin and AICAR in a muscle type-dependent manner.

Skeletal muscle is a primary organ that uses blood glucose. Insulin- and 5'AMP-activated protein kinase (AMPK)-regulated intracellular signaling pathways are known as major mechanisms that regulate muscle glucose transport. It has been reported that macrophage migration inhibitory factor (MIF) is secreted from adipose tissue and heart, and affects these two pathways. In this study, we examined whether MIF is a myokine that is secreted from skeletal muscles and affects muscle glucose transport induced by these two pathways. We found that MIF is expressed in several different types of skeletal muscle. Its secretion was also confirmed in C2C12 myotubes, a skeletal muscle cell line. Next, the extensor digitorum longus (EDL) and soleus muscles were isolated from mice and treated with recombinant MIF in an in vitro muscle incubation system. MIF itself did not have any effect on glucose transport in both types of muscles. However, glucose transport induced by a submaximal dose of insulin was diminished by co-incubation with MIF in the soleus muscle. MIF also diminished glucose transport induced by a maximal dose of 5-aminoimidazole-4-carboxyamide ribonucleoside (AICAR), an AMPK activator, in the EDL muscle. These results suggest that MIF is a negative regulator of insulin- and AICAR-induced glucose transport in skeletal muscle. Since MIF secretion from C2C12 myotubes to the culture medium decreased during contraction evoked by electrical stimulations, MIF may be involved in the mechanisms underlying exercise-induced sensitization of glucose transport in skeletal muscle.

Visualization of dynamic change in contraction-induced lipid composition in mouse skeletal muscle by matrix-assisted laser desorption/ionization imaging mass spectrometry.

Lipids in skeletal muscle play a fundamental role both in normal muscle metabolism and in disease states. Skeletal muscle lipid accumulation is associated with several chronic metabolic disorders, including obesity, insulin resistance, and type 2 diabetes. However, it is poorly understood whether the lipid composition of skeletal muscle changes by contraction, due to the complexity of lipid molecular species. In this study, we used matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS) to investigate changes in skeletal muscle lipid composition induced by contraction. We successfully observed the reduction of diacylglycerol and triacylglycerol, which are generally associated with muscle contraction. Interestingly, we found the accumulation of some saturated and mono-unsaturated fatty acids and poly-unsaturated fatty acids containing phosphatidylcholine in contracted muscles. Moreover, the distributions of several types of lipid were changed by contraction. Our results show that changes in the lipid amount, lipid composition, and energy metabolic activity can be evaluated in each local spot of cells and tissues at the same time using MALDI-IMS. In conclusion, MALDI-IMS is a powerful tool for studying lipid changes associated with contractions.

Fine Tuning of Phosphorothioate Inclusion in 2'-O-Methyl Oligonucleotides Contributes to Specific Cell Targeting for Splice-Switching Modulation.

Splice-switching antisense oligonucleotide- (SSO-) mediated correction of framedisrupting mutation-containing premessenger RNA (mRNA) transcripts using exon skipping is a highly promising treatment method for muscular diseases such as Duchenne muscular dystrophy (DMD). Phosphorothioate (PS) chemistry, a commonly used oligonucleotide modification, has been shown to increase the stability of and improve the pharmacokinetics of SSOs. However, the effect of PS inclusion in 2'-O-methyl SSOs (2OMe) on cellular uptake and splice switching is less well-understood. At present, we demonstrate that the modification of PS facilitates the uptake of 2OMe in H2k-mdx myoblasts. Furthermore, we found a dependency of SSO nuclear accumulation and high splice-switching activity on PS inclusion in 2OMe (2OMePS), as tested in various reporter cell lines carrying pLuc/705. Increased exon-inclusion activity was observed in muscle, neuronal, liver, and bone cell lineages via both the gymnotic uptake and lipofection of 2OMePS. Using the photoactivatable ribonucleoside-enhanced crosslinking and a subsequent proteomic approach, we identified several 2OMePS-binding proteins, which are likely to play a role in the trafficking of 2OMePS to the nucleus. Ablation of one of them, Ncl by small-interfering RNA (siRNA) enhanced 2OMePS uptake in C2C12 myoblasts and upregulated luciferase RNA splicing in the HeLa Luc/705 reporter cell line. Overall, we demonstrate that PS inclusion increases nuclear delivery and splice switching in muscle, neuronal, liver, and bone cell lineages and that the modulation of 2OMePS-binding partners may improve SSO delivery.

Scavenger Receptor Class A1 Mediates Uptake of Morpholino Antisense Oligonucleotide into Dystrophic Skeletal Muscle.

Exon skipping using phosphorodiamidate morpholino oligomers (PMOs) is a promising treatment strategy for Duchenne muscular dystrophy (DMD). The most significant limitation of these clinically used compounds is their lack of delivery systems that target muscles; thus, cell-penetrating peptides are being developed to enhance uptake into muscles. Recently, we reported that uptake of peptide-conjugated PMOs into myofibers was mediated by scavenger receptor class A (SR-A), which binds negatively charged ligands. However, the mechanism by which the naked PMOs are taken up into fibers is poorly understood. In this study, we found that PMO uptake and exon-skipping efficiency were promoted in dystrophin-deficient myotubes via endocytosis through a caveolin-dependent pathway. Interestingly, SR-A1 was upregulated and localized in juxtaposition with caveolin-3 in these myotubes and promoted PMO-induced exon skipping. SR-A1 was also upregulated in the skeletal muscle of mdx52 mice and mediated PMO uptake. In addition, PMOs with neutral backbones had negative zeta potentials owing to their nucleobase compositions and interacted with SR-A1. In conclusion, PMOs with negative zeta potential were taken up into dystrophin-deficient skeletal muscle by upregulated SR-A1. Therefore, the development of a drug delivery system targeting SR-A1 could lead to highly efficient exon-skipping therapies for DMD.

In Vivo Evaluation of Single-Exon and Multiexon Skipping in mdx52 Mice.

Exon-skipping therapy is an emerging approach that uses synthetic DNA-like molecules called antisense oligonucleotides (ASOs) to splice out frame-disrupting parts of mRNA, restore the reading frame, and produce truncated yet functional proteins. Phosphorodiamidate morpholino oligomer (PMO) is one of the safest among therapeutic ASOs for patients and has recently been approved under the accelerated approval program by the US Food and Drug Administration (FDA) as the first ASO-based drug for Duchenne muscular dystrophy (DMD). Multi-exon skipping utilizing ASOs can theoretically treat 80-90% of patients with DMD. Here, we describe the systemic delivery of a cocktail of ASOs to skip exon 51 and exons 45-55 in the mdx52 mouse, an exon 52 deletion model of DMD produced by gene targeting, and the evaluation of their efficacies in vivo.

Exon Skipping Using Antisense Oligonucleotides for Laminin-Alpha2-Deficient Muscular Dystrophy.

Phosphorodiamidate morpholino oligomer (PMO)-mediated exon skipping is among the more promising approaches available for the treatment of several neuromuscular disorders, including Duchenne muscular dystrophy. The main weakness of this treatment arises from the low efficiency and sporadic nature of delivery of the neutrally charged PMO into muscle fibers, the mechanism of which is unknown.Recently, using wild-type and dystrophic mdx52 mice, we showed that muscle fibers took up PMO more efficiently during myotube formation. Interestingly, through in situ hybridization, we detected PMO mainly in embryonic myosin heavy chain-positive regenerating fibers. Next, we tested the therapeutic potential of PMO in laminin-alpha2 (laminin-α2) chain-null dy 3K/dy 3K mice, a model of merosin-deficient congenital muscular dystrophy 1A (MDC1A) with active muscle regeneration. We confirmed the recovery of the laminin-α2 chain following skipping of the mutated exon 4 in dy 3K/dy 3K mice, which prolonged the life span of the animals slightly. These findings support the theory that PMO entry into fibers is dependent on the developmental stage in myogenesis rather than on dystrophinless muscle membranes, and provide a platform for the future development of PMO-mediated therapies for a variety of muscular disorders, such as MDC1A, that involve active muscle regeneration. Herein, we describe the methods for PMO transfection/injection and evaluation of the efficacy of exon skipping in the laminin-α2-deficient dy 3K/dy 3K mouse model both in vitro and in vivo.

Exon Skipping Therapy Using Phosphorodiamidate Morpholino Oligomers in the mdx52 Mouse Model of Duchenne Muscular Dystrophy.

Exon skipping therapy using synthetic DNA-like molecules called antisense oligonucleotides (ASOs) is a promising therapeutic candidate for overcoming the dystrophin mutation that causes Duchenne muscular dystrophy (DMD). This treatment involves splicing out the frame-disrupting segment of the dystrophin mRNA, which restores the reading frame and produces a truncated yet functional dystrophin protein. Phosphorodiamidate morpholino oligomer (PMO) is the safest ASO for patients among ASOs and has recently been approved under the accelerated approval pathway by the U.S. Food and Drug Administration (FDA) as the first drug for DMD. Here, we describe the methodology and protocol of PMO transfection and evaluation of the exon skipping efficacy in the mdx52 mouse, an exon 52 deletion model of DMD produced by gene targeting. The mdx52 mouse model offers advantages over the mdx mouse, a spontaneous DMD model with a nonsense mutation in exon 23, in terms of the deletion in a hotspot of deletion mutations in DMD patients, the analysis of caveolae and also Dp140 and Dp260, shorter dystrophin isoforms.

Recent advances in innovative therapeutic approaches for Duchenne muscular dystrophy: from discovery to clinical trials.

Duchenne muscular dystrophy (DMD) is an X-linked progressive degenerative muscle disorder caused by the absence of dystrophin. There is no curative therapy, although innovative therapeutic approaches have been aggressively investigated over recent years. Currently, the international clinical trial registry platform for this disease has been constructed and clinical trials for innovative therapeutic approaches are underway. Among these, exon skipping and read-through of nonsense mutations are in the most advanced stages, with exon skipping theoretically applicable to a larger number of patients. To date, exon skipping that targets exons 51, 44, 45, and 53 is being globally investigated including in USA, EU, and Japan. The latest announcement from Japan was made, demonstrating successful dystrophin production in muscles of patients with DMD after treating with exon 53 skipping antisense oligonucleotides (ASOs). However, the innovative therapeutic approaches have demonstrated limited efficacy. To address this issue in exon skipping, studies to unveil the mechanism underlying gymnotic delivery of ASO uptake in living cells have been conducted in an effort to improve in vivo delivery. Further, establishing the infrastructures to integrate multi-institutional clinical trials are needed to facilitate the development of successful therapies for DMD, which ultimately is applicable to other myopathies and neurodegenerative diseases, including spinal muscular atrophy and motor neuron diseases.

Anti-inflammatory drugs for Duchenne muscular dystrophy: focus on skeletal muscle-releasing factors.

Duchenne muscular dystrophy (DMD), an incurable and a progressive muscle wasting disease, is caused by the absence of dystrophin protein, leading to recurrent muscle fiber damage during contraction. The inflammatory response to fiber damage is a compelling candidate mechanism for disease exacerbation. The only established pharmacological treatment for DMD is corticosteroids to suppress muscle inflammation, however this treatment is limited by its insufficient therapeutic efficacy and considerable side effects. Recent reports show the therapeutic potential of inhibiting or enhancing pro- or anti-inflammatory factors released from DMD skeletal muscles, resulting in significant recovery from muscle atrophy and dysfunction. We discuss and review the recent findings of DMD inflammation and opportunities for drug development targeting specific releasing factors from skeletal muscles. It has been speculated that nonsteroidal anti-inflammatory drugs targeting specific inflammatory factors are more effective and have less side effects for DMD compared with steroidal drugs. For example, calcium channels, reactive oxygen species, and nuclear factor-κB signaling factors are the most promising targets as master regulators of inflammatory response in DMD skeletal muscles. If they are combined with an oligonucleotide-based exon skipping therapy to restore dystrophin expression, the anti-inflammatory drug therapies may address the present therapeutic limitation of low efficiency for DMD.

Characterization of an acute muscle contraction model using cultured C2C12 myotubes.

A cultured C2C12 myotube contraction system was examined for application as a model for acute contraction-induced phenotypes of skeletal muscle. C2C12 myotubes seeded into 4-well rectangular plates were placed in a contraction system equipped with a carbon electrode at each end. The myotubes were stimulated with electric pulses of 50 V at 1 Hz for 3 ms at 997-ms intervals. Approximately 80% of the myotubes were observed to contract microscopically, and the contractions lasted for at least 3 h with electrical stimulation. Calcium ion (Ca²âº) transient evoked by the electric pulses was detected fluorescently with Fluo-8. Phosphorylation of protein kinase B/Akt (Akt), 5' AMP-activated protein kinase (AMPK), p38 mitogen-activated protein kinase (p38), and c-Jun NH2-terminal kinase (JNK)1/2, which are intracellular signaling proteins typically activated in exercised/contracted skeletal muscle, was observed in the electrically stimulated C2C12 myotubes. The contractions induced by the electric pulses increased glucose uptake and depleted glycogen in the C2C12 myotubes. C2C12 myotubes that differentiated after exogenous gene transfection by a lipofection or an electroporation method retained their normal contractile ability by electrical stimulation. These findings show that our C2C12 cell contraction system reproduces the muscle phenotypes that arise invivo (exercise), in situ (hindlimb muscles in an anesthetized animal), and invitro (dissected muscle tissues in incubation buffer) by acute muscle contraction, demonstrating that the system is applicable for the analysis of intracellular events evoked by acute muscle contraction.