Michael Sinnreich

Sustained Activation of mTORC1 in Skeletal Muscle Inhibits Constitutive and Starvation-Induced Autophagy and Causes a Severe, Late-Onset Myopathy

Autophagy is a catabolic process that ensures homeostatic cell clearance and is deregulated in a growing number of myopathological conditions. Although FoxO3 was shown to promote the expression of autophagy-related genes in skeletal muscle, the mechanisms triggering autophagy are unclear. We show that TSC1-deficient mice (TSCmKO), characterized by sustained activation of mTORC1, develop a late-onset myopathy related to impaired autophagy. In young TSCmKO mice, constitutive and starvation-induced autophagy is blocked at the induction steps via mTORC1-mediated inhibition of Ulk1, despite FoxO3 activation. Rapamycin is sufficient to restore autophagy in TSCmKO mice and improves the muscle phenotype of old mutant mice. Inversely, abrogation of mTORC1 signaling by depletion of raptor induces autophagy regardless of FoxO inhibition. Thus, mTORC1 is the dominant regulator of autophagy induction in skeletal muscle and ensures a tight coordination of metabolic pathways. These findings may open interesting avenues for therapeutic strategies directed toward autophagy-related muscle diseases.

Rapamycin attenuates the progression of tau pathology in P301S tau transgenic mice.

Altered autophagy contributes to the pathogenesis of Alzheimer’s disease and other tauopathies, for which curative treatment options are still lacking. We have recently shown that trehalose reduces tau pathology in a tauopathy mouse model by stimulation of autophagy. Here, we studied the effect of the autophagy inducing drug rapamycin on the progression of tau pathology in P301S mutant tau transgenic mice. Rapamycin treatment resulted in a significant reduction in cortical tau tangles, less tau hyperphosphorylation, and lowered levels of insoluble tau in the forebrain. The favourable effect of rapamycin on tau pathology was paralleled by a qualitative reduction in astrogliosis. These effects were visible with early preventive or late treatment. We further noted an accumulation of the autophagy associated proteins p62 and LC3 in aged tangle bearing P301S mice that was lowered upon rapamycin treatment. Thus, rapamycin treatment defers the progression of tau pathology in a tauopathy animal model and autophagy stimulation may constitute a therapeutic approach for patients suffering from tauopathies.

Characterization of lipid binding specificities of dysferlin C2 domains reveals novel interactions with phosphoinositides.

Dysferlin is a type II transmembrane protein implicated in Ca(2+)-dependent sarcolemmal membrane repair. Dysferlin has seven C2 domains, which are lipid and protein binding modules. In this study, we sought to characterize the lipid binding specificity of dysferlins seven C2 domains. Dysferlins C2A domain was able to bind to phosphatidylserine (PS), phosphatidylinositol 4-phosphate [PtdIns(4)P], and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] in a Ca(2+)-dependent fashion. The remainder of the C2 domains exhibited weaker and Ca(2+)-independent binding to PS and no significant binding to phosphoinositides.

Quantitative muscle MRI: A powerful surrogate outcome measure in Duchenne muscular dystrophy

In muscular dystrophies quantitative muscle MRI (qMRI) detects disease progression more sensitively than clinical scores. This prospective one year observational study compared qMRI with clinical scores in Duchenne muscular dystrophy (DMD) to investigate if qMRI can serve as a surrogate outcome measure in clinical trials. In 20 DMD patients the motor function measure (MFM) total and subscores (D1-D3) were done for physical examination, and the fat fraction (MFF) of thigh muscle qMRI was obtained using the two-point Dixon method. Effect sizes (ES) were calculated for all measures. Sample size estimation (SS) was done modelling assumed treatment effects. Ambulant patients <7 years at inclusion improved in the MFM total and D1 score (ES 1.1 and 1.0). Ambulant patients >7 years (highest ES in the MFM D1 subscore (1.2)), and non-ambulant patients (highest ES in the total MFM score (0.7)) worsened. In comparison the ES of QMRI was much larger, e.g. SS estimations for qMRI data were up to 17 fold smaller compared to the MFM total score and up to 7 fold to the D1 subscore, respectively. QMRI shows pathophysiological changes in DMD and might serve as a surrogate outcome measure in clinical trials.

Proteasomal inhibition restores biological function of mis-sense mutated dysferlin in patient-derived muscle cells

Background: Dysferlin encoded by mis-sense alleles is rapidly degraded in skeletal muscle. Results: Proteasomal inhibitors increase dysferlin levels, restore membrane repair and myotube formation in patient-derived myoblasts harboring mis-sense mutated dysferlin. Conclusion: Proteasomal inhibition restores function of mis-sense mutated dysferlin. Significance: Inhibiting the degradation of mis-sense mutated dysferlin may be a therapeutic strategy for dysferlinopathies with certain mis-sense mutations. Dysferlin is a transmembrane protein implicated in surface membrane repair of muscle cells. Mutations in dysferlin cause the progressive muscular dystrophies Miyoshi myopathy, limb girdle muscular dystrophy 2B, and distal anterior compartment myopathy. Dysferlinopathies are inherited in an autosomal recessive manner, and many patients with this disease harbor mis-sense mutations in at least one of their two pathogenic DYSF alleles. These patients have significantly reduced or absent dysferlin levels in skeletal muscle, suggesting that dysferlin encoded by mis-sense alleles is rapidly degraded by the cellular quality control system. We reasoned that mis-sense mutated dysferlin, if salvaged from degradation, might be biologically functional. We used a dysferlin-deficient human myoblast culture harboring the common R555W mis-sense allele and a DYSF-null allele, as well as control human myoblast cultures harboring either two wild-type or two null alleles. We measured dysferlin protein and mRNA levels, resealing kinetics of laser-induced plasmalemmal wounds, myotube formation, and cellular viability after treatment of the human myoblast cultures with the proteasome inhibitors lactacystin or bortezomib (Velcade). We show that endogenous R555W mis-sense mutated dysferlin is degraded by the proteasomal system. Inhibition of the proteasome by lactacystin or Velcade increases the levels of R555W mis-sense mutated dysferlin. This salvaged protein is functional as it restores plasma membrane resealing in patient-derived myoblasts and reverses their deficit in myotube formation. Bortezomib and lactacystin did not cause cellular toxicity at the regimen used. Our results raise the possibility that inhibition of the degradation pathway of mis-sense mutated dysferlin could be used as a therapeutic strategy for patients harboring certain dysferlin mis-sense mutations.

Improved Muscle Function in Duchenne Muscular Dystrophy through L-Arginine and Metformin: An Investigator-Initiated, Open-Label, Single-Center, Proof-Of-Concept-Study

Altered neuronal nitric oxide synthase function in Duchenne muscular dystrophy leads to impaired mitochondrial function which is thought to be one cause of muscle damage in this disease. The study tested if increased intramuscular nitric oxide concentration can improve mitochondrial energy metabolism in Duchenne muscular dystrophy using a novel therapeutic approach through the combination of L-arginine with metformin. Five ambulatory, genetically confirmed Duchenne muscular dystrophy patients aged between 7–10 years were treated with L-arginine (3 x 2.5 g/d) and metformin (2 x 250 mg/d) for 16 weeks. Treatment effects were assessed using mitochondrial protein expression analysis in muscular biopsies, indirect calorimetry, Dual-Energy X-Ray Absorptiometry, quantitative thigh muscle MRI, and clinical scores of muscle performance. There were no serious side effects and no patient dropped out. Muscle biopsy results showed pre-treatment a significantly reduced mitochondrial protein expression and increased oxidative stress in Duchenne muscular dystrophy patients compared to controls. Post-treatment a significant elevation of proteins of the mitochondrial electron transport chain was observed as well as a reduction in oxidative stress. Treatment also decreased resting energy expenditure rates and energy substrate use shifted from carbohydrates to fatty acids. These changes were associated with improved clinical scores. In conclusion pharmacological stimulation of the nitric oxide pathway leads to improved mitochondria function and clinically a slowing of disease progression in Duchenne muscular dystrophy. This study shall lead to further development of this novel therapeutic approach into a real alternative for Duchenne muscular dystrophy patients. Trial Registration ClinicalTrials.gov NCT02516085

Targeting deregulated AMPK/mTORC1 pathways improves muscle function in myotonic dystrophy type I

Myotonic dystrophy type I (DM1) is a disabling multisystemic disease that predominantly affects skeletal muscle. It is caused by expanded CTG repeats in the 3′-UTR of the dystrophia myotonica protein kinase (DMPK) gene. RNA hairpins formed by elongated DMPK transcripts sequester RNA-binding proteins, leading to mis-splicing of numerous pre-mRNAs. Here, we have investigated whether DM1-associated muscle pathology is related to deregulation of central metabolic pathways, which may identify potential therapeutic targets for the disease. In a well-characterized mouse model for DM1 (HSALR mice), activation of AMPK signaling in muscle was impaired under starved conditions, while mTORC1 signaling remained active. In parallel, autophagic flux was perturbed in HSALR muscle and in cultured human DM1 myotubes. Pharmacological approaches targeting AMPK/mTORC1 signaling greatly ameliorated muscle function in HSALR mice. AICAR, an AMPK activator, led to a strong reduction of myotonia, which was accompanied by partial correction of misregulated alternative splicing. Rapamycin, an mTORC1 inhibitor, improved muscle relaxation and increased muscle force in HSALR mice without affecting splicing. These findings highlight the involvement of AMPK/mTORC1 deregulation in DM1 muscle pathophysiology and may open potential avenues for the treatment of this disease.

Modular dispensability of dysferlin C2 domains reveals rational design for mini-dysferlin molecules.

Background: Full-length dysferlin exceeds adeno-associated virus encapsulation capacity, requiring the generation of mini-dysferlin molecules. Results: By studying the modular dispensability of the dysferlin C2 domains regarding plasmalemmal repair and localization, functional mini-dysferlin constructs were designed. Conclusion: Mini-dysferlin constructs retained a similar capacity for plasmalemmal localization and repair as full-length dysferlin. Significance: These mini-dysferlins can now be used to study their therapeutic effect in animal models of dysferlinopathy. Dysferlin is a large transmembrane protein composed of a C-terminal transmembrane domain, two DysF domains, and seven C2 domains that mediate lipid- and protein-binding interactions. Recessive loss-of-function mutations in dysferlin lead to muscular dystrophies, for which no treatment is currently available. The large size of dysferlin precludes its encapsulation into an adeno-associated virus (AAV), the vector of choice for gene delivery to muscle. To design mini-dysferlin molecules suitable for AAV-mediated gene transfer, we tested internally truncated dysferlin constructs, each lacking one of the seven C2 domains, for their ability to localize to the plasma membrane and to repair laser-induced plasmalemmal wounds in dysferlin-deficient human myoblasts. We demonstrate that the dysferlin C2B, C2C, C2D, and C2E domains are dispensable for correct plasmalemmal localization. Furthermore, we show that the C2B, C2C, and C2E domains and, to a lesser extent, the C2D domain are dispensable for dysferlin membrane repair function. On the basis of these results, we designed small dysferlin molecules that can localize to the plasma membrane and reseal laser-induced plasmalemmal injuries and that are small enough to be incorporated into AAV. These results lay the groundwork for AAV-mediated gene therapy experiments in dysferlin-deficient mouse models.

Gene expression profiling in nerve biopsy of vasculitic neuropathy

To investigate molecular mechanisms of peripheral nerve vasculitis, gene expression patterns in archived frozen sural nerve biopsies from patients with vasculitic neuropathy were compared to control nerves by DNA microarray technology. There was a striking upregulation of mRNA of genes involved in immune system processes. Of special interest was the activation of immunoglobulin genes, such as IGLJ3, IGHG3, IGKC, and IGL, and of several chemokines, such as CXCL9 or CCR2. Genes involved in vascular proliferation or remodelling such as CXC31 and AIF were also upregulated. Among the downregulated genes were the Krüppel-Like Transcription Factors KLF2, KLF4 and the nuclear orphan receptor NR4A1 genes known to be involved in endothelial cell activation. Thus, this gene expression profile analysis revealed that in peripheral nerve vasculitis a prominent activation of immune response related genes as well as genes involved in vascular proliferation is taken place, while genes inhibiting endothelial cell activation are down regulated. These data point to interesting mechanistic clues to the molecular pathogenesis of vasculitic neuropathies.

The role of muscle biopsy in the age of genetic testing.

Purpose of reviewThe purpose of this review is to discuss the role of muscle biopsy in the current age of genetic testing. Recent findingsThe diagnostic approach to patients with suspected genetically determined myopathies has been altered by recent advances in molecular diagnostic technologies and by the increased number of conditions for which the genetic alterations have been identified. Myopathological aspects can narrow down the differential diagnosis when the clinical phenotype is not informative enough and can help guide the molecular investigation. SummaryHere, we review genetic and myopathological aspects of selected genetically determined myopathies.