Differential muscle hypertrophy is associated with satellite cell numbers and Akt pathway activation following activin type IIB receptor inhibition in Mtm1 p.R69C mice

Different Mouse Models of Nemaline Myopathy Harboring Acta1 Mutations Display Differing Abnormalities Related to Mitochondrial Biology

ACTA1 encodes skeletal muscle-specific α-actin, which polymerizes to form the thin filament of the sarcomere. Mutations in ACTA1 are responsible for approximately 30% of nemaline myopathy (NM) cases. Previous studies of weakness in NM have focused on muscle structure and contractility, but genetic issues alone do not explain the phenotypic heterogeneity observed in patients with NM or NM mouse models. To identify additional biological processes related to NM phenotypic severity, proteomic analysis was performed using muscle protein isolates from wild-type mice in comparison to moderately affected knock-in (KI) Acta1H40Y and the minimally affected transgenic (Tg) ACTA1D286G NM mice. This analysis revealed abnormalities in mitochondrial function and stress-related pathways in both mouse models, supporting an in-depth assessment of mitochondrial biology. Interestingly, evaluating each model in comparison to its wild-type counterpart identified different degrees of mitochondrial abnormality that correlated well with the phenotypic severity of the mouse model. Muscle histology, mitochondrial respiration, electron transport chain function, and mitochondrial transmembrane potential were all normal or minimally affected in the TgACTA1D286G mouse model. In contrast, the more severely affected KI.Acta1H40Y mice displayed significant abnormalities in relation to muscle histology, mitochondrial respirometry, ATP, ADP, and phosphate content, and mitochondrial transmembrane potential. These findings suggest that abnormal energy metabolism is related to symptomatic severity in NM and may constitute a contributor to phenotypic variability and a novel treatment target.

Aberrations in Energetic Metabolism and Stress-Related Pathways Contribute to Pathophysiology in the Neb Conditional Knockout Mouse Model of Nemaline Myopathy

Nemaline myopathy (NM) is a genetically and clinically heterogeneous disease that is diagnosed on the basis of the presence of nemaline rods on skeletal muscle biopsy. Although NM has typically been classified by causative genes, disease severity or prognosis cannot be predicted. The common pathologic end point of nemaline rods (despite diverse genetic causes) and an unexplained range of muscle weakness suggest that shared secondary processes contribute to the pathogenesis of NM. We speculated that these processes could be identified through a proteome-wide interrogation using a mouse model of severe NM in combination with pathway validation and structural/functional analyses. A proteomic analysis was performed using skeletal muscle tissue from the Neb conditional knockout mouse model compared with its wild-type counterpart to identify pathophysiologically relevant biological processes that might impact disease severity or provide new treatment targets. A differential expression analysis and Ingenuity Pathway Core Analysis predicted perturbations in several cellular processes, including mitochondrial dysfunction and changes in energetic metabolism and stress-related pathways. Subsequent structural and functional studies demonstrated abnormal mitochondrial distribution, decreased mitochondrial respiratory function, an increase in mitochondrial transmembrane potential, and extremely low ATP content in Neb conditional knockout muscles relative to wild type. Overall, the findings of these studies support a role for severe mitochondrial dysfunction as a novel contributor to muscle weakness in NM.

A New Signature of Sarcoma Based on the Tumor Microenvironment Benefits Prognostic Prediction

Sarcomas are a group of malignant tumors derived from mesenchymal tissues that display complex and variable pathological types. The impact of the immune properties of the tumor microenvironment (TME) on the prognosis, treatment, and management of sarcomas has attracted attention, requiring the exploration of sensitive and accurate signatures. In this study, The Cancer Genome Atlas (TCGA) database was searched to screen for an RNA sequencing dataset, retrieving 263 sarcoma and 2 normal samples with survival data. Genes associated with immune regulation in sarcomas were retrieved from the Tumor Immune Estimation Resource database to estimate tumor purity and immune cell infiltration levels. The samples were then divided into the immune-high and immune-low groups. Then, we screened for differentially expressed genes (DEGs) between the two groups. The intersection between immune-related genes and DEGs was then determined. Univariate Cox and least absolute shrinkage and selection operator analyses were used to select ideal genes for prognostic prediction and subsequent construction of a risk signature. A survival analysis was performed to reveal the dissimilarity in survival between the high- and low-score groups. Finally, a nomogram was generated to verify the accuracy and reliability of the signature. Through Estimation of STromal and Immune cells in MAlignant Tumour tissues using Expression (ESTIMATE) analysis, high ESTIMATE, and low tumor purity were significantly associated with a favorable prognosis. Moreover, a total of 5259 DEGs were retrieved, the majority of which were downregulated. In total, 590 immune-associated genes overlapped with the DEGs, among which nine hub genes were identified. Finally, two candidate genes, ACVR2B and NFYA, were identified, based on which a risk signature was constructed. The risk signature constructed in this study is accurate and reliable for the prognostic prediction and phenotyping of sarcomas.

Therapeutic approaches in different congenital myopathies

Congenital myopathies are rare and severe genetic diseases affecting the skeletal muscle function in children and adults. They present a variable spectrum of phenotypes and a genetic heterogeneity. Subgroups are defined according to the clinical and histopathological features and encompass core myopathy, centronuclear myopathy, nemaline myopathy and other rare congenital myopathies. No approved treatment exists to date for any congenital myopathies. To tackle this important unmet need, an increased number of proof-of-concept studies recently assessed the therapeutic potential of various strategies, either pharmacological or genetic-based, aiming at counteracting muscle weakness or/and cure the pathology. Here, we list the implicated genes and cellular pathways, and review the therapeutic approaches preclinically tested and the ongoing/completed clinical trials for the different types of congenital myopathies.

Common Pathogenic Mechanisms in Centronuclear and Myotubular Myopathies and Latest Treatment Advances

Centronuclear myopathies (CNM) are rare congenital disorders characterized by muscle weakness and structural defects including fiber hypotrophy and organelle mispositioning. The main CNM forms are caused by mutations in: the MTM1 gene encoding the phosphoinositide phosphatase myotubularin (myotubular myopathy), the DNM2 gene encoding the mechanoenzyme dynamin 2, the BIN1 gene encoding the membrane curvature sensing amphiphysin 2, and the RYR1 gene encoding the skeletal muscle calcium release channel/ryanodine receptor. MTM1, BIN1, and DNM2 proteins are involved in membrane remodeling and trafficking, while RyR1 directly regulates excitation-contraction coupling (ECC). Several CNM animal models have been generated or identified, which confirm shared pathological anomalies in T-tubule remodeling, ECC, organelle mispositioning, protein homeostasis, neuromuscular junction, and muscle regeneration. Dynamin 2 plays a crucial role in CNM physiopathology and has been validated as a common therapeutic target for three CNM forms. Indeed, the promising results in preclinical models set up the basis for ongoing clinical trials. Another two clinical trials to treat myotubular myopathy by MTM1 gene therapy or tamoxifen repurposing are also ongoing. Here, we review the contribution of the different CNM models to understanding physiopathology and therapy development with a focus on the commonly dysregulated pathways and current therapeutic targets.

Association between yoga, physiologic and psychologic health: A cross sectional study

PURPOSE

To compare markers of health associated with chronic diseases between yoga and non-yoga participants.

METHODS

30 participants were categorized as either: 1) "Yoga" engaging in yoga ≥2 times/week for ≥6 months, or 2) "Non-yoga" not engaging in yoga.

RESULTS

Perceived Stress Scale (PSS) and Beck Depression Inventory-II (BDI-II) scores were significantly different between the yoga and non-yoga groups (PSS: 8.0 vs. 17.5, respectively, p < 0.05; BDI-II: 1.0 vs. 5.5, respectively, p < 0.05). No significant differences were evident between groups for inflammatory markers nor Complex V of the mitochondrial electron transport chain. The erythrocyte sedimentation rate values differed between groups based on clinical cutoffs, with yoga participants categorized as normal (11.0 mm) and non-yoga above normal (21.5 mm).

CONCLUSION

This research supports that yoga participation is associated with lower PSS and BDI-II scores but does not support a relationship with markers of inflammation. Further research is warranted.

Antimyostatin Treatment in Health and Disease: The Story of Great Expectations and Limited Success

In the past 20 years, myostatin, a negative regulator of muscle mass, has attracted attention as a potential therapeutic target in muscular dystrophies and other conditions. Preclinical studies have shown potential for increasing muscular mass and ameliorating the pathological features of dystrophic muscle by the inhibition of myostatin in various ways. However, hardly any clinical trials have proven to translate the promising results from the animal models into patient populations. We present the background for myostatin regulation, clinical and preclinical results and discuss why translation from animal models to patients is difficult. Based on this, we put the clinical relevance of future antimyostatin treatment into perspective.

Striated Preferentially Expressed Protein Kinase (SPEG)-Deficient Skeletal Muscles Display Fewer Satellite Cells with Reduced Proliferation and Delayed Differentiation

Centronuclear myopathies (CNMs) are a subtype of congenital myopathies characterized by skeletal muscle weakness and an increase in the number of central myonuclei. SPEG (striated preferentially expressed protein kinase) has been identified as the sixth gene associated with CNM, and it has been shown that striated muscle-specific Speg-knockout (KO) mice have defective triad formation, abnormal excitation-contraction coupling, and calcium mishandling. The impact of SPEG deficiency on the survival and function of myogenic cells remains to be deciphered. In this study, the authors examined the overall population, proliferation, and differentiation of myogenic cells obtained from striated muscle-specific Speg-KO mice and compared them with wild-type (WT) controls. SPEG-deficient skeletal muscles contained fewer myogenic cells, which on further study demonstrated reduced proliferation and delayed differentiation compared with those from WT muscles. Regenerative response to skeletal muscle injury in Speg-KO mice was compared with that of WT mice, leading to the identification of similar abnormalities including fewer satellite cells, fewer dividing cells, and an increase in apoptotic cells in KO mice. Overall, these results reveal specific abnormalities in myogenic cell number and behavior associated with SPEG deficiency. Similar satellite cell defects have been reported in mouse models of MTM1- and DNM2-associated CNM, suggestive of shared underlying pathways.

Myostatin: a Circulating Biomarker Correlating with Disease in Myotubular Myopathy Mice and Patients

Myotubular myopathy, also called X-linked centronuclear myopathy (XL-CNM), is a severe congenital disease targeted for therapeutic trials. To date, biomarkers to monitor disease progression and therapy efficacy are lacking. The Mtm1 ^-/y mouse is a faithful model for XL-CNM, due to myotubularin 1 (MTM1) loss-of-function mutations. Using both an unbiased approach (RNA sequencing [RNA-seq]) and a directed approach (qRT-PCR and protein level), we identified decreased Mstn levels in Mtm1 ^-/y muscle, leading to low levels of myostatin in muscle and plasma. Myostatin (Mstn or growth differentiation factor 8 [Gdf8]) is a protein released by myocytes and inhibiting muscle growth and differentiation. Decreasing Dnm2 by genetic cross with Dnm2 ^+/- mice or by antisense oligonucleotides blocked or postponed disease progression and resulted in an increase in circulating myostatin. In addition, plasma myostatin levels inversely correlated with disease severity and with Dnm2 mRNA levels in muscles. Altered Mstn levels were associated with a generalized disruption of the myostatin pathway. Importantly, in two different forms of CNMs we identified reduced circulating myostatin levels in plasma from patients. This provides evidence of a blood-based biomarker that may be used to monitor disease state in XL-CNM mice and patients and supports monitoring circulating myostatin during clinical trials for myotubular myopathy.

Myostatin Inhibition Using ActRIIB-mFc Does Not Produce Weight Gain or Strength in the Nebulin Conditional KO Mouse

Mutations in at least 12 genes are responsible for a group of congenital skeletal muscle diseases known as nemaline myopathies (NMs). NMs are associated with a range of clinical symptoms and pathological changes often including the presence of cytoplasmic rod-like structures (nemaline bodies) and myofiber hypotrophy. Our recent work has identified a variable degree of behavioral benefit when treating 2 NM mouse models due to mutations in Acta1 with myostatin inhibition. This study is focused on the effects of delivering ActRIIB-mFc (Acceleron; a myostatin inhibitor) to the nebulin conditional knockout KO (Neb cKO) mouse model of NM. Treatment of Neb cKO mice with ActRIIB-mFc did not produce increases in weight gain, strength, myofiber size, or hypertrophic pathway signaling. Overall, our studies demonstrate a lack of response in Neb cKO mice to myostatin inhibition, which differs from the response observed when treating other NM models.

AAV-Mediated Gene Transfer Restores a Normal Muscle Transcriptome in a Canine Model of X-Linked Myotubular Myopathy

Multiple clinical trials employing recombinant adeno-associated viral (rAAV) vectors have been initiated for neuromuscular disorders, including Duchenne and limb-girdle muscular dystrophies, spinal muscular atrophy, and recently X-linked myotubular myopathy (XLMTM). Our previous work on a canine model of XLMTM showed that a single rAAV8-cMTM1 systemic infusion corrected structural abnormalities within the muscle and restored contractile function, with affected dogs surviving more than 4 years post injection. This remarkable therapeutic efficacy presents a unique opportunity to identify the downstream molecular drivers of XLMTM pathology and to what extent the whole muscle transcriptome is restored to normal after gene transfer. Herein, RNA-sequencing was used to examine the transcriptomes of the Biceps femoris and Vastus lateralis in a previously described canine cohort that showed dose-dependent clinical improvements after rAAV8-cMTM1 gene transfer. Our analysis confirmed several dysregulated genes previously observed in XLMTM mice but also identified transcripts linked to XLMTM pathology. We demonstrated XLMTM transcriptome remodeling and dose-dependent normalization of gene expression after gene transfer and created metrics to pinpoint potential biomarkers of disease progression and correction.

Sdha+/- Rats Display Minimal Muscle Pathology Without Significant Behavioral or Biochemical Abnormalities

Mitochondrial diseases (MDs) result from alteration of the mitochondrial respiratory chain (MRC) function. Despite the prevalence of MDs in the population, the paucity of animal models available limits the understanding of these disorders. Mutations in SDHA, a gene that codes for the alpha subunit of succinate dehydrogenase (SDH), can cause some forms of MD. SDHA is a crucial contributor to MRC function. In order to expand the range of MD animal models available, we attempted to generate a Sdha knockout rat. Since homozygous Sdha-/- rats could neither be identified in newborn litters, nor as early as embryonic day 14, we evaluated wild-type (WT) and heterozygous Sdha+/- genotypes. No differences in behavioral, biochemical, or molecular evaluations were observed between WT and Sdha+/- rats at 6 weeks or 6 months of age. However, 30% of Sdha+/- rats displayed mild muscle fiber atrophy with rare fibers negative for cytochrome oxidase and SDH on histochemical staining. Collectively, our data provide additional evidence that modeling SDH mutations in rodents may be challenging due to animal viability, and heterozygous rats are insufficiently symptomatic at a phenotypic and molecular level to be of significant use in the study of SDH deficiency.

Towards Fine Whole-Slide Skeletal Muscle Image Segmentation through Deep Hierarchically Connected Networks

Automatic skeletal muscle image segmentation (MIS) is crucial in the diagnosis of muscle-related diseases. However, accurate methods often suffer from expensive computations, which are not scalable to large-scale, whole-slide muscle images. In this paper, we present a fast and accurate method to enable the more clinically meaningful whole-slide MIS. Leveraging on recently popular convolutional neural network (CNN), we train our network in an end-to-end manner so as to directly perform pixelwise classification. Our deep network is comprised of the encoder and decoder modules. The encoder module captures rich and hierarchical representations through a series of convolutional and max-pooling layers. Then, the multiple decoders utilize multilevel representations to perform multiscale predictions. The multiscale predictions are then combined together to generate a more robust dense segmentation as the network output. The decoder modules have independent loss function, which are jointly trained with a weighted loss function to address fine-grained pixelwise prediction. We also propose a two-stage transfer learning strategy to effectively train such deep network. Sufficient experiments on a challenging muscle image dataset demonstrate the significantly improved efficiency and accuracy of our method compared with recent state of the arts.

High throughput automatic muscle image segmentation using parallel framework

BACKGROUND

Fast and accurate automatic segmentation of skeletal muscle cell image is crucial for the diagnosis of muscle related diseases, which extremely reduces the labor-intensive manual annotation. Recently, several methods have been presented for automatic muscle cell segmentation. However, most methods exhibit high model complexity and time cost, and they are not adaptive to large-scale images such as whole-slide scanned specimens.

METHODS

In this paper, we propose a novel distributed computing approach, which adopts both data and model parallel, for fast muscle cell segmentation. With a master-worker parallelism manner, the image data in the master is distributed onto multiple workers based on the Spark cloud computing platform. On each worker node, we first detect cell contours using a structured random forest (SRF) contour detector with fast parallel prediction and generate region candidates using a superpixel technique. Next, we propose a novel hierarchical tree based region selection algorithm for cell segmentation based on the conditional random field (CRF) algorithm. We divide the region selection algorithm into multiple sub-problems, which can be further parallelized using multi-core programming.

RESULTS

We test the performance of the proposed method on a large-scale haematoxylin and eosin (H &E) stained skeletal muscle image dataset. Compared with the standalone implementation, the proposed method achieves more than 10 times speed improvement on very large-scale muscle images containing hundreds to thousands of cells. Meanwhile, our proposed method produces high-quality segmentation results compared with several state-of-the-art methods.

CONCLUSIONS

This paper presents a parallel muscle image segmentation method with both data and model parallelism on multiple machines. The parallel strategy exhibits high compatibility to our muscle segmentation framework. The proposed method achieves high-throughput effective cell segmentation on large-scale muscle images.

Centronuclear myopathies under attack: A plethora of therapeutic targets

Centronuclear myopathies are a group of congenital myopathies characterized by severe muscle weakness, genetic heterogeneity, and defects in the structural organization of muscle fibers. Their names are derived from the central position of nuclei on biopsies, while they are at the fiber periphery under normal conditions. No specific therapy exists yet for these debilitating diseases. Mutations in the myotubularin phosphoinositides phosphatase, the GTPase dynamin 2, or amphiphysin 2 have been identified to cause respectively X-linked centronuclear myopathies (also called myotubular myopathy) or autosomal dominant and recessive forms. Mutations in additional genes, as RYR1, TTN, SPEG or CACNA1S, were linked to phenotypes that can overlap with centronuclear myopathies. Numerous animal models of centronuclear myopathies have been studied over the last 15 years, ranging from invertebrate to large mammalian models. Their characterization led to a partial understanding of the pathomechanisms of these diseases and allowed the recent validation of therapeutic proof-of-concepts. Here, we review the different therapeutic strategies that have been tested so far for centronuclear myopathies, some of which may be translated to patients.

Myostatin inhibition using mRK35 produces skeletal muscle growth and tubular aggregate formation in wild type and TgACTA1D286G nemaline myopathy mice

Nemaline myopathy (NM) is a heterogeneous congenital skeletal muscle disease with cytoplasmic rod-like structures (nemaline bodies) in muscle tissue. While weakness in NM is related to contractile abnormalities, myofiber smallness is an additional abnormality in NM that may be treatable. We evaluated the effects of mRK35 (a myostatin inhibitor developed by Pfizer) treatment in the TgACTA1D286G mouse model of NM. mRK35 induced skeletal muscle growth that led to significant increases in animal bodyweight, forelimb grip strength and muscle fiber force, although it should be noted that animal weight and forelimb grip strength in untreated TgACTA1D286G mice was not different from controls. Treatment was also associated with an increase in the number of tubular aggregates found in skeletal muscle. These findings suggest that myostatin inhibition may be useful in promoting muscle growth and strength in Acta1-mutant muscle, while also further establishing the relationship between low levels of myostatin and tubular aggregate formation.

Electrical impedance myography as a biomarker of myostatin inhibition with ActRIIB-mFc: a study in wild-type mice

Aim:

We sought to determine the sensitivity of electrical impedance myography (EIM) to myofiber hypertrophy induced by treatment with various doses of ActRIIB-mFc, an inhibitor of myostatin signaling.

Methods:

Wild-type C57BL/6 J mice (n = 40, male) were treated with three different doses of ActRIIB-mFc (i.e., RAP-031) or vehicle twice weekly for 5 weeks. End point assessments included gastrocnemius EIM, force measurements, muscle mass and myofiber size quantification.

Results:

ActRIIB-mFc increased body mass, muscle mass and myofiber size across all doses. Alterations in EIM 50 kHz phase and center frequency (fc) were also present, with trends in a dose-dependent fashion. Significant correlations between EIM parameters and myofiber/functional data were identified.

Conclusion:

EIM outcomes can serve as effective biomarkers of myostatin signaling inhibition, demonstrating a dose sensitivity and correlation to standard assessments.

We were interested in studying the sensitivity of a technique, called electrical impedance myography (EIM), to noninvasively assess the size of muscle fibers. In this technique a minute electrical current is used to probe the tissue. To do so, we gave a drug (ActRIIB-mFc) to mice that enlarges muscle fibers at three different doses. We were able to show that the EIM technique was able to detect this differential effect and functional changes induced by the drug correlated to the EIM data. This work suggests that EIM will be useful as a noninvasive marker muscle health.

Single Intramuscular Injection of AAV-shRNA Reduces DNM2 and Prevents Myotubular Myopathy in Mice

Myotubular myopathy, or X-linked centronuclear myopathy, is a severe muscle disorder representing a significant burden for patients and their families. It is clinically characterized by neonatal and severe muscle weakness and atrophy. Mutations in the myotubularin (MTM1) gene cause myotubular myopathy, and no specific curative treatment is available. We previously found that dynamin 2 (DNM2) is upregulated in both Mtm1 knockout and patient muscle samples, whereas its reduction through antisense oligonucleotides rescues the clinical and histopathological features of this myopathy in mice. Here, we propose a novel approach targeting Dnm2 mRNA. We screened and validated in vitro and in vivo several short hairpin RNA (shRNA) sequences that efficiently target Dnm2 mRNA. A single intramuscular injection of AAV-shDnm2 resulted in long-term reduction of DNM2 protein level and restored muscle force, mass, histology, and myofiber ultrastructure and prevented molecular defects linked to the disease. Our results demonstrate a robust DNM2 knockdown and provide an alternative strategy based on reduction of DNM2 to treat myotubular myopathy.

Myotubularin-related protein 7 inhibits insulin signaling in colorectal cancer

Phosphoinositide (PIP) phosphatases such as myotubularins (MTMs) inhibit growth factor receptor signaling. However, the function of myotubularin-related protein 7 (MTMR7) in cancer is unknown. We show that MTMR7 protein was down-regulated with increasing tumor grade (G), size (T) and stage (UICC) in patients with colorectal cancer (CRC) (n=1786). The presence of MTMR7 in the stroma correlated with poor prognosis, whereas MTMR7 expression in the tumor was not predictive for patients' survival. Insulin reduced MTMR7 protein levels in human CRC cell lines, and CRC patients with type 2 diabetes mellitus (T2DM) or loss of imprinting (LOI) of insulin-like growth factor 2 (IGF2) had an increased risk for MTMR7 loss. Mechanistically, MTMR7 lowered PIPs and inhibited insulin-mediated AKT-ERK1/2 signaling and proliferation in human CRC cell lines. MTMR7 provides a novel link between growth factor signaling and cancer, and may thus constitute a potential marker or drug target for human CRC.

Mechanically patterned neuromuscular junctions-in-a-dish have improved functional maturation

Motor neuron (MN) diseases are progressive disorders resulting from degeneration of neuromuscular junctions (NMJs), which form the connection between MNs and muscle fibers. NMJ-in-a-dish models have been developed to examine human MN-associated dysfunction with disease; however such coculture models have randomly oriented myotubes with immature synapses that contract asynchronously. Mechanically patterned (MP) extracellular matrix with alternating soft and stiff stripes improves current NMJ-in-a-dish models by inducing both mouse and human myoblast durotaxis to stripes where they aligned, differentiated, and fused into patterned myotubes. Compared to conventional culture on rigid substrates or unpatterned hydrogels, MP substrates supported increased differentiation and fusion, significantly larger acetylcholine (ACh) receptor clusters, and increased expression of MuSK and Lrp4, two cell surface receptors required for NMJ formation. Robust contractions were observed when mouse myotubes were stimulated by ACh, with twitch duration and frequency most closely resembling those for mature muscle on MP substrates. Fused myotubes, when cocultured with MNs, were able to form even larger NMJs. Thus MP matrices produce more functionally active NMJs-in-a-dish, which could be used to elucidate disease pathology and facilitate drug discovery.

Systemic AAV8-Mediated Gene Therapy Drives Whole-Body Correction of Myotubular Myopathy in Dogs

X-linked myotubular myopathy (XLMTM) results from MTM1 gene mutations and myotubularin deficiency. Most XLMTM patients develop severe muscle weakness leading to respiratory failure and death, typically within 2 years of age. Our objective was to evaluate the efficacy and safety of systemic gene therapy in the p.N155K canine model of XLMTM by performing a dose escalation study. A recombinant adeno-associated virus serotype 8 (rAAV8) vector expressing canine myotubularin (cMTM1) under the muscle-specific desmin promoter (rAAV8-cMTM1) was administered by simple peripheral venous infusion in XLMTM dogs at 10 weeks of age, when signs of the disease are already present. A comprehensive analysis of survival, limb strength, gait, respiratory function, neurological assessment, histology, vector biodistribution, transgene expression, and immune response was performed over a 9-month study period. Results indicate that systemic gene therapy was well tolerated, prolonged lifespan, and corrected the skeletal musculature throughout the body in a dose-dependent manner, defining an efficacious dose in this large-animal model of the disease. These results support the development of gene therapy clinical trials for XLMTM.

Skeletal Muscle Pathology in X-Linked Myotubular Myopathy: Review With Cross-Species Comparisons

X-linked myotubular myopathy (XLMTM) is a devastating, rare, congenital myopathy caused by mutations in the MTM1 gene, resulting in a lack of or dysfunction of the enzyme myotubularin. This leads to severe perinatal weakness and distinctive muscle pathology. It was originally thought that XLMTM was related to developmental arrest in myotube maturation; however, the generation and characterization of several animal models have significantly improved our understanding of clinical and pathological aspects of this disorder. Myotubularin is now known to participate in numerous cellular processes including endosomal trafficking, excitation-contraction coupling, cytoskeletal organization, neuromuscular junction structure, autophagy, and satellite cell proliferation and survival. The available vertebrate models of XLMTM, which vary in severity from complete absence to reduced functional levels of myotubularin, recapitulate features of the human disease to a variable extent. Understanding how pathological endpoints in animals with XLMTM translate to human patients will be essential to interpret preclinical treatment trials and translate therapies into human clinical studies. This review summarizes the published animal models of XLMTM, including those of zebrafish, mice, and dogs, with a focus on their pathological features as compared to those seen in human XLMTM patients.

A Study of a Cohort of X-Linked Myotubular Myopathy at the Clinical, Histologic, and Genetic Levels

BACKGROUND

Myotubular myopathy is a rare X-linked congenital myopathy characterized by marked neonatal hypotonia and respiratory insufficiency, facial and ocular involvement, and muscle biopsy with prominent central nuclei in the majority of muscle fibers. It is caused by mutations in MTM1, which codes for the phosphoinositides phosphatase myotubularin. In this work, we established and detailed a new cohort of six patients at the clinical, histologic, and genetic levels.

PATIENTS AND METHODS

Patients were recruited after screening 3065 muscle biopsy reports from two large biopsy banks in Sao Paulo, Brazil from the years 2008 to 2013, and from referrals to a neuromuscular outpatient clinic between 2011 and 2013. We reviewed biopsy slides, evaluated patients, and Sanger sequenced MTM1 in the families.

RESULTS

All patients but one had classic phenotypes with a stable course after a severe onset. Two patients died suddenly from hypovolemic shock. Muscle biopsies had been performed in five patients, all of whom showed a classic pattern with a predominance of centrally located nuclei and increased oxidative activity in the center of the fibers. Two patients showed necklace fibers, and two families had novel truncating mutations in MTM1.

CONCLUSIONS

X-linked myotubular myopathy is rare in the Brazilian population. Necklace fibers might be more prevalent in this condition than previously reported. Direct Sanger sequencing of MTM1 on clinical suspicion avoids the need of a muscle biopsy.

Treatment with ActRIIB-mFc Produces Myofiber Growth and Improves Lifespan in the Acta1 H40Y Murine Model of Nemaline Myopathy

Nemaline myopathies (NMs) are a group of congenital muscle diseases caused by mutations in at least 10 genes and associated with a range of clinical symptoms. NM is defined on muscle biopsy by the presence of cytoplasmic rod-like structures (nemaline rods) composed of cytoskeletal material. Myofiber smallness is also found in many cases of NM and may represent a cause of weakness that can be counteracted by treatment. We have used i.p. injection of activin type IIB receptor (ActRIIB)-mFc (an inhibitor of myostatin signaling) to promote hypertrophy and increase strength in our prior murine work; we therefore tested whether ActRIIB-mFc could improve weakness in NM mice through myofiber hypertrophy. We report a study of ActRIIB-mFc treatment in the Acta1 H40Y mouse model of NM. Treatment of Acta1 H40Y mice produced significant increases in body mass, muscle mass, quadriceps myofiber size, and survival, but other measurements of strength (forelimb grip strength, ex vivo measurements of contractile function) did not improve. Our studies also identified that the complications of urethral obstruction are associated with mortality in male hemizygote Acta1 H40Y mice. The incidence of urethral obstruction and histologic evidence of chronic obstruction (inflammation) were significantly lower in Acta1 H40Y mice that had been treated with ActRIIB-mFc. ActRIIB-mFc treatment produces a mild benefit to the disease phenotype in Acta1 H40Y mice.

In vivo imaging of skeletal muscle in mice highlights muscle defects in a model of myotubular myopathy

Skeletal muscle structure and function are altered in different myopathies. However, the understanding of the molecular and cellular mechanisms mainly rely on in vitro and ex vivo investigations in mammalian models. In order to monitor in vivo the intracellular structure of the neuromuscular system in its environment under normal and pathological conditions, we set-up and validated non-invasive imaging of ear and leg muscles in mice. This original approach allows simultaneous imaging of different cellular and intracellular structures such as neuromuscular junctions and sarcomeres, reconstruction of the 3D architecture of the neuromuscular system, and video recording of dynamic events such as spontaneous muscle fiber contraction. Second harmonic generation was combined with vital dyes and fluorescent-coupled molecules. Skin pigmentation, although limiting, did not prevent intravital imaging. Using this versatile toolbox on the Mtm1 knockout mouse, a model for myotubular myopathy which is a severe congenital myopathy in human, we identified several hallmarks of the disease such as defects in fiber size and neuromuscular junction shape. Intravital imaging of the neuromuscular system paves the way for the follow-up of disease progression or/and disease amelioration upon therapeutic tests. It has also the potential to reduce the number of animals needed to reach scientific conclusions.

Nebulin deficiency in adult muscle causes sarcomere defects and muscle-type-dependent changes in trophicity: novel insights in nemaline myopathy

Nebulin is a giant filamentous protein that is coextensive with the actin filaments of the skeletal muscle sarcomere. Nebulin mutations are the main cause of nemaline myopathy (NEM), with typical adult patients having low expression of nebulin, yet the roles of nebulin in adult muscle remain poorly understood. To establish nebulin's functional roles in adult muscle, we studied a novel conditional nebulin KO (Neb cKO) mouse model in which nebulin deletion was driven by the muscle creatine kinase (MCK) promotor. Neb cKO mice are born with high nebulin levels in their skeletal muscles, but within weeks after birth nebulin expression rapidly falls to barely detectable levels Surprisingly, a large fraction of the mice survive to adulthood with low nebulin levels (<5% of control), contain nemaline rods and undergo fiber-type switching toward oxidative types. Nebulin deficiency causes a large deficit in specific force, and mechanistic studies provide evidence that a reduced fraction of force-generating cross-bridges and shortened thin filaments contribute to the force deficit. Muscles rich in glycolytic fibers upregulate proteolysis pathways (MuRF-1, Fbxo30/MUSA1, Gadd45a) and undergo hypotrophy with smaller cross-sectional areas (CSAs), worsening their force deficit. Muscles rich in oxidative fibers do not have smaller weights and can even have hypertrophy, offsetting their specific-force deficit. These studies reveal nebulin as critically important for force development and trophicity in adult muscle. The Neb cKO phenocopies important aspects of NEM (muscle weakness, oxidative fiber-type predominance, variable trophicity effects, nemaline rods) and will be highly useful to test therapeutic approaches to ameliorate muscle weakness.