Role of Toll-like receptors in the pathogenesis of dystrophin-deficient skeletal and heart muscle.

Although the cause of Duchenne muscular dystrophy (DMD) is known, the specific factors that initiate and perpetuate disease progression are not well understood. We hypothesized that leaky dystrophin-deficient skeletal muscle releases endogenous danger signals (TLR ligands), which bind to Toll-like receptors (TLRs) on muscle and immune cells and activate downstream processes that facilitate degeneration and regeneration in dystrophic skeletal muscle. Here, we demonstrate that dystrophin-deficient mouse muscle cells show increased expression of several cell-surface and endosomal TLRs. In vitro screening identified ssRNA as a relevant endogenous TLR7 ligand. TLR7 activation led to myd88-dependent production of pro-inflammatory cytokines in dystrophin-deficient muscle cells, and cause significant degeneration/regeneration in vivo in mdx mouse muscle. Also, knockout of the central TLR adaptor protein, myd88 in mdx mice significantly improved skeletal and cardiac muscle function. Likewise, proof-of-concept experiments showed that treating young mdx mice with a TLR7/9 antagonist significantly reduced skeletal muscle inflammation and increased muscle force, suggesting that blocking this pathway may have therapeutic potential for DMD.

The effects of MyD88 deficiency on disease phenotype in dysferlin-deficient A/J mice: role of endogenous TLR ligands.

An absence of dysferlin leads to activation of innate immune receptors such as Toll-like receptors (TLRs) and skeletal muscle inflammation. Myeloid differentiation primary response gene 88 (MyD88) is a key mediator of TLR-dependent innate immune signalling. We hypothesized that endogenous TLR ligands released from the leaking dysferlin-deficient muscle fibres engage TLRs on muscle and immune cells and contribute to disease progression. To test this hypothesis, we generated and characterized dysferlin and MyD88 double-deficient mice. Double-deficient mice exhibited improved body weight, grip strength, and maximum muscle contractile force at 6-8 months of age when compared to MyD88-sufficient, dysferlin-deficient A/J mice. Double-deficient mice also showed a decrease in total fibre number, which contributed to the observed increase in the number of central nuclei/fibres. These results indicate that there was less regeneration in the double-deficient mice. We next tested the hypothesis that endogenous ligands, such as single-stranded ribonucleic acids (ssRNAs), released from damaged muscle cells bind to TLR-7/8 and perpetuate the disease progression. We found that injection of ssRNA into the skeletal muscle of pre-symptomatic mice (2 months old) resulted in a significant increase in degenerative fibres, inflammation, and regenerating fibres in A/J mice. In contrast, characteristic histological features were significantly decreased in double-deficient mice. These data point to a clear role for the TLR pathway in the pathogenesis of dysferlin deficiency and suggest that TLR-7/8 antagonists may have therapeutic value in this disease.

The molecular basis of skeletal muscle weakness in a mouse model of inflammatory myopathy.

OBJECTIVE: It is generally believed that muscle weakness in patients with polymyositis and dermatomyositis is due to autoimmune and inflammatory processes. However, it has been observed that there is a poor correlation between the suppression of inflammation and a recovery of muscle function in these patients. This study was undertaken to examine whether nonimmune mechanisms also contribute to muscle weakness. In particular, it has been suggested that an acquired deficiency of AMP deaminase 1 (AMPD1) may be responsible for muscle weakness in myositis. METHODS: We performed comprehensive functional, behavioral, histologic, molecular, enzymatic, and metabolic assessments before and after the onset of inflammation in a class I major histocompatibility complex (MHC)-transgenic mouse model of autoimmune inflammatory myositis. RESULTS: Muscle weakness and metabolic disturbances were detectable in the mice prior to the appearance of infiltrating mononuclear cells. Force contraction analysis of muscle function revealed that weakness was correlated with AMPD1 expression and was myositis specific. Decreasing AMPD1 expression resulted in decreased muscle strength in healthy mice. Fiber typing suggested that fast-twitch muscles were converted to slow-twitch muscles as myositis progressed, and microarray results indicated that AMPD1 and other purine nucleotide pathway genes were suppressed, along with genes essential to glycolysis. CONCLUSION: These data suggest that an AMPD1 deficiency is acquired prior to overt muscle inflammation and is responsible, at least in part, for the muscle weakness that occurs in the mouse model of myositis. AMPD1 is therefore a potential therapeutic target in myositis.

Inflammasome up-regulation and activation in dysferlin-deficient skeletal muscle.

A deficiency of the dysferlin protein results in limb girdle muscular dystrophy type 2B and Miyoshi myopathy, with resulting plasma membrane abnormalities in myofibers. Many patients show muscle inflammation, but the molecular mechanisms that initiate and perpetuate this inflammation are not well understood. We previously showed abnormal activation of macrophages and hypothesized that activation of the inflammasome pathway may play a role in disease progression. To test this, we studied the inflammasome molecular platform in dysferlin-deficient human and mouse muscle. Consistent with our model, components of the NACHT, LRR and PYD-containing proteins (NALP)-3 inflammasome pathway were specifically up-regulated and activated in dysferlin-deficient but not in dystrophin-deficient and normal muscle. We demonstrate for the first time that normal primary skeletal muscle cells are capable of secreting IL-1beta in response to combined treatment with lipopolysaccharide and the P2X7 receptor agonist, benzylated ATP, suggesting that not only immune cells but also muscle cells can actively participate in inflammasome formation. In addition, we show that dysferlin-deficient primary muscle cells express toll-like receptors (TLRs; TLR-2 and TLR-4) and can efficiently produce IL-1beta in response to lipopolysaccharide and benzylated ATP. These data indicate that skeletal muscle is an active contributor of IL-1beta and strategies that interfere with this pathway may be therapeutically useful for patients with limb girdle muscular dystrophy type 2B.

Intracellular localization of dysferlin and its association with the dihydropyridine receptor.

Mutations in the dysferlin gene underlie two phenotypically distinct muscular dystrophies: Miyoshi myopathy and limb-girdle muscular dystrophy 2B. Dysferlin was proposed to have a putative functional role in mediating the fusion of intracellular vesicles to the sarcolemma during injury-induced membrane repair, but dysferlin has been found not only at the sarcolemma but also within the cytoplasm of skeletal muscle fibers by immunohistochemistry. In this study, we examined the subcellular localization of dysferlin in skeletal muscle by immunohistochemical and biochemical analyses to elucidate other functional roles of dysferlin. Immunohistochemistry confirmed granular cytoplasmic expression pattern of dysferlin in muscle fibers. Subcellular membrane fractionation revealed that a portion of dysferlin associated with a T-tubule-enriched intracellular membrane fraction as well as a sarcolemmal fraction. This indication was consistent with subsequent results that dysferlin coprecipitates by immunoprecipitation with the dihydropyridine receptor (DHPR), a protein complex localized in T-tubules. Moreover, both proteins were observed to partially colocalize by double immunofluorescent labeling in skeletal muscle fibers. We also found that caveolin-3, previously shown to interact with dysferlin, coprecipitates with DHPR. These results demonstrated that dysferlin may be involved in the formation of an oligomeric complex with DHPR and caveolin-3. Caveolin-3 has been also reported to participate in an insulin-regulated transport mechanism in muscle, and caveolin-3-containing vesicles might traffic between intracellular sites and target sites on the sarcolemma and T-tubules. Therefore, it is very intriguing to assume that dysferlin might be involved in the fusion of caveolin-3-containing vesicles with T-tubules.

Inhibition of inflammation with celastrol fails to improve muscle function in dysferlin-deficient A/J mice.

The dysferlin-deficient A/J mouse strain represents a homologous model for limb-girdle muscular dystrophy 2B. We evaluated the disease phenotype in 10 month old A/J mice compared to two dysferlin-sufficient, C57BL/6 and A/JOlaHsd, mouse lines to determine which functional end-points are sufficiently sensitive to define the disease phenotype for use in preclinical studies in the A/J strain. A/J mice had significantly lower open field behavioral activity (horizontal activity, total distance, movement time and vertical activity) when compared to C57BL/6 and A/JoIaHsd mice. Both A/J and A/JOIaHsd mice showed decreases in latency to fall with rotarod compared to C57BL/6. No changes were detected in grip strength, force measurements or motor coordination between these three groups. Furthermore, we have found that A/J muscle shows significantly increased levels of the pro-inflammatory cytokine TNF-α when compared to C57BL/6 mice, indicating an activation of NF-κB signaling as part of the inflammatory response in dysferlin-deficient muscle. Therefore, we assessed the effect of celastrol (a potent NF-κB inhibitor) on the disease phenotype in female A/J mice. Celastrol treatment for four months significantly reduced the inflammation in A/J muscle; however, it had no beneficial effect in improving muscle function, as assessed by grip strength, open field activity, and in vitro force contraction. In fact, celastrol treated mice showed a decrease in body mass, hindlimb grip strength and maximal EDL force. These findings suggest that inhibition of inflammation alone may not be sufficient to improve the muscle disease phenotype in dysferlin-deficient mice and may require combination therapies that target membrane stability to achieve a functional improvement in skeletal muscle.

Effect of hypothyroidism on beta-adrenoceptor-mediated relaxation in the rat thoracic aortae. A time-dependent study.

We investigated the time dependency of hypothyroid-induced changes in beta-adrenoceptor-mediated relaxation of vascular smooth muscle. Methimazole (0.03%) was administered to male Wistar-Imamichi rats for 3 days, 1, 2 or 6 weeks. This treatment led to significant increases in thyroid weight while inhibiting growth rate. Tension in isolated rings of thoracic aortae from control and hypothyroid rats was measured isometrically. Responses of aortic rings to cumulative doses of acetylcholine (ACh) and sodium nitroprusside (SNP) were not significantly different between control and hypothyroid groups. After 3 days and 1 week of treatment, isoprenaline (ISO)-induced relaxation was unchanged, but after 2 and 6 weeks, a marked increase was observed as compared to controls. Removal of the endothelium and pretreatment with NG-nitro-L-arginine (L-NOARG) inhibited the ISO-induced relaxation in both groups; but whereas this degree of inhibition was the same for both groups after 3 days and 1 week, it was significantly less pronounced in hypothyroid rats after 2 and 6 weeks as compared to their controls. These results suggest that hypothyroidism has a time-dependent influence on beta-adrenoceptor-mediated relaxation in the rat thoracic aortae and that the enhancement after 2 and 6 weeks of methimazole treatment may be due to a hypothyroid-induced alteration in arterial smooth muscle function.

Recombinant adeno-associated virus type 8-mediated extensive therapeutic gene delivery into skeletal muscle of alpha-sarcoglycan-deficient mice.

Autosomal recessive limb-girdle muscular dystrophy type 2D (LGMD 2D) is caused by mutations in the alpha-sarcoglycan gene (alpha-SG). The absence of alpha-SG results in the loss of the SG complex at the sarcolemma and compromises the integrity of the sarcolemma. To establish a method for recombinant adeno-associated virus (rAAV)-mediated alpha-SG gene therapy into alpha-SG-deficient muscle, we constructed rAAV serotypes 2 and 8 expressing the human alpha-SG gene under the control of the ubiquitous cytomegalovirus promoter (rAAV2-alpha-SG and rAAV8-alpha-SG). We compared the transduction profiles and evaluated the therapeutic effects of a single intramuscular injection of rAAVs into alpha-SG-deficient (Sgca(-/-)) mice. Four weeks after rAAV2 injection into the tibialis anterior (TA) muscle of 10-day-old Sgca(-/-) mice, transduction of the alpha-SG gene was localized to a limited area of the TA muscle. On the other hand, rAAV8-mediated alpha-SG expression was widely distributed in the hind limb muscle, and persisted for 7 months without inducing cytotoxic and immunological reactions, with a reversal of the muscle pathology and improvement in the contractile force of the Sgca(-/-) muscle. This extensive rAAV8-mediated alpha-SG transduction in LGMD 2D model animals paves the way for future clinical application.