RhoA mediates defective stem cell function and heterotopic ossification in dystrophic muscle of mice

Misregulation of the Ubiquitin-Proteasome System and Autophagy in Muscular Dystrophies Associated with the Dystrophin-Glycoprotein Complex

The stability of the sarcolemma is severely impaired in a series of genetic neuromuscular diseases defined as muscular dystrophies. These are characterized by the centralization of skeletal muscle syncytial nuclei, the replacement of muscle fibers with fibrotic tissue, the release of inflammatory cytokines, and the disruption of muscle protein homeostasis, ultimately leading to necrosis and loss of muscle functionality. A specific subgroup of muscular dystrophies is associated with genetic defects in components of the dystrophin-glycoprotein complex (DGC), which plays a crucial role in linking the cytosol to the skeletal muscle basement membrane. In these cases, dystrophin-associated proteins fail to correctly localize to the sarcolemma, resulting in dystrophy characterized by an uncontrolled increase in protein degradation, which can ultimately lead to cell death. In this review, we explore the role of intracellular degradative pathways-primarily the ubiquitin-proteasome and autophagy-lysosome systems-in the progression of DGC-linked muscular dystrophies. The DGC acts as a hub for numerous signaling pathways that regulate various cellular functions, including protein homeostasis. We examine whether the loss of structural stability within the DGC affects key signaling pathways that modulate protein recycling, with a particular emphasis on autophagy.

Exercise Attenuate Diaphragm Atrophy in COPD Mice via Inhibiting the RhoA/ROCK Signaling

Background

Exercise is an indispensable component of pulmonary rehabilitation with strong anti-inflammatory effects. However, the mechanisms by which exercise prevents diaphragmatic atrophy in COPD (chronic obstructive pulmonary disease) remain unclear.

Methods

Forty male C57BL/6 mice were assigned to the control (n=16) and smoke (n=24) groups. Mice in the smoke group were exposed to the cigarette smoke (CS) for six months. They were then divided into model and exercise training groups for 2 months. Histological changes were observed in lung and diaphragms. Subsequently, agonist U46639 and antagonist Y27632 of RhoA/ROCK were subjected to mechanical stretching in LPS-treated C2C12 myoblasts. The expression levels of Atrogin-1, MuRF-1, MyoD, Myf5, IL-1β, TNF-α, and RhoA/ROCK were determined by Western blotting.

Results

Diaphragmatic atrophy and increased RhoA/ROCK expression were observed in COPD mice. Exercise training attenuated diaphragmatic atrophy, decreased the expression of MuRF-1, and increased MyoD expression in COPD diaphragms. Exercise also affects the upregulation of RhoA/ROCK and inflammation-related proteins. In in vitro experiments with C2C12 myoblasts, LPS remarkably increased the level of inflammation and protein degradation, whereas Y27632 or combined with mechanical stretching prevented this phenomenon considerably.

Conclusion

RhoA/ROCK plays an important role in the prevention of diaphragmatic atrophy in COPD.

The Role of MicroRNA in the Pathogenesis of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked progressive disorder associated with muscle wasting and degeneration. The disease is caused by mutations in the gene that encodes dystrophin, a protein that links the cytoskeleton with cell membrane proteins. The current treatment methods aim to relieve the symptoms of the disease or partially rescue muscle functionality. However, they are insufficient to suppress disease progression. In recent years, studies have uncovered an important role for non-coding RNAs (ncRNAs) in regulating the progression of numerous diseases. ncRNAs, such as micro-RNAs (miRNAs), bind to their target messenger RNAs (mRNAs) to suppress translation. Understanding the mechanisms involving dysregulated miRNAs can improve diagnosis and suggest novel treatment methods for patients with DMD. This review presents the available evidence on the role of altered expression of miRNAs in the pathogenesis of DMD. We discuss the involvement of these molecules in the processes associated with muscle physiology and DMD-associated cardiomyopathy.

Low-dose radiation induces unstable gene expression in developing human iPSC-derived retinal ganglion organoids

The effects of low-dose radiation on undifferentiated cells carry important implications. However, the effects on developing retinal cells remain unclear. Here, we analyzed the gene expression characteristics of neuronal organoids containing immature human retinal cells under low-dose radiation and predicted their changes. Developing retinal cells generated from human induced pluripotent stem cells (iPSCs) were irradiated with either 30 or 180 mGy on days 4-5 of development for 24 h. Genome-wide gene expression was observed until day 35. A knowledge-based pathway analysis algorithm revealed fluctuations in Rho signaling and many other pathways. After a month, the levels of an essential transcription factor of eye development, the proportion of paired box 6 (PAX6)-positive cells, and the proportion of retinal ganglion cell (RGC)-specific transcription factor POU class 4 homeobox 2 (POU4F2)-positive cells increased with 30 mGy of irradiation. In contrast, they decreased after 180 mGy of irradiation. Activation of the "development of neurons" pathway after 180 mGy indicated the dedifferentiation and development of other neural cells. Fluctuating effects after low-dose radiation exposure suggest that developing retinal cells employ hormesis and dedifferentiation mechanisms in response to stress.

RhoA/ROCK signalling activated by ARHGEF3 promotes muscle weakness via autophagy in dystrophic mdx mice

BACKGROUND

Duchenne muscular dystrophy (DMD), caused by dystrophin deficiency, leads to progressive and fatal muscle weakness through yet-to-be-fully deciphered molecular perturbations. Emerging evidence implicates RhoA/Rho-associated protein kinase (ROCK) signalling in DMD pathology, yet its direct role in DMD muscle function, and related mechanisms, are unknown.

METHODS

Three-dimensionally engineered dystrophin-deficient mdx skeletal muscles and mdx mice were used to test the role of ROCK in DMD muscle function in vitro and in situ, respectively. The role of ARHGEF3, one of the RhoA guanine nucleotide exchange factors (GEFs), in RhoA/ROCK signalling and DMD pathology was examined by generating Arhgef3 knockout mdx mice. The role of RhoA/ROCK signalling in mediating the function of ARHGEF3 was determined by evaluating the effects of wild-type or GEF-inactive ARHGEF3 overexpression with ROCK inhibitor treatment. To gain more mechanistic insights, autophagy flux and the role of autophagy were assessed in various conditions with chloroquine.

RESULTS

Inhibition of ROCK with Y-27632 improved muscle force production in 3D-engineered mdx muscles (+25% from three independent experiments, P < 0.05) and in mice (+25%, P < 0.001). Unlike suggested by previous studies, this improvement was independent of muscle differentiation or quantity and instead related to increased muscle quality. We found that ARHGEF3 was elevated and responsible for RhoA/ROCK activation in mdx muscles, and that depleting ARHGEF3 in mdx mice restored muscle quality (up to +36%, P < 0.01) and morphology without affecting regeneration. Conversely, overexpressing ARHGEF3 further compromised mdx muscle quality (-13% vs. empty vector control, P < 0.01) in GEF activity- and ROCK-dependent manner. Notably, ARHGEF3/ROCK inhibition exerted the effects by rescuing autophagy which is commonly impaired in dystrophic muscles.

CONCLUSIONS

Our findings uncover a new pathological mechanism of muscle weakness in DMD involving the ARHGEF3-ROCK-autophagy pathway and the therapeutic potential of targeting ARHGEF3 in DMD.

Selective ablation of Nfix in macrophages attenuates muscular dystrophy by inhibiting fibro-adipogenic progenitor-dependent fibrosis

Muscular dystrophies are genetic diseases characterized by chronic inflammation and fibrosis. Macrophages are immune cells that sustain muscle regeneration upon acute injury but seem deleterious in the context of chronic muscle injury such as in muscular dystrophies. Here, we observed that the number of macrophages expressing the transcription factor Nfix increases in two distinct mouse models of muscular dystrophies. We showed that the deletion of Nfix in macrophages in dystrophic mice delays the establishment of fibrosis and muscle wasting, and increases grasp force. Macrophages lacking Nfix expressed more TNFα and less TGFβ1, thus promoting apoptosis of fibro‐adipogenic progenitors. Moreover, pharmacological treatment of dystrophic mice with a ROCK inhibitor accelerated fibrosis through the increase of Nfix expression by macrophages. Thus, we have identified Nfix as a macrophage profibrotic factor in muscular dystrophies, whose inhibition could be a therapeutic route to reduce severity of the dystrophic disease. © 2022 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Immunologic Contributions Following Rotator Cuff Injury and Development of Cuff Tear Arthropathy

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Rotator cuff tear arthropathy (RCTA) describes a pattern of glenohumeral degenerative changes following chronic rotator cuff tears that is characterized by superior humeral head migration, erosion of the greater tuberosity of the humeral head, contouring of the coracoacromial arch to create a socket for the humeral head, and eventual glenohumeral arthritis.

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Acute and chronic inflammatory changes following rotator cuff tears are thought to contribute to cartilage damage, muscle fibrosis, and fatty infiltration in the glenohumeral joint.

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In vitro animal studies targeting various inflammatory modulators, including macrophages, insulin-like growth factor-I, and transforming growth factor-beta pathways, provide promising therapeutic targets to improve healing after rotator cuff tears.

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The role of platelet-rich plasma in the treatment and prevention of RCTA has been investigated, with conflicting results.

Rho GTPases in Skeletal Muscle Development and Homeostasis

Rho guanosine triphosphate hydrolases (GTPases) are molecular switches that cycle between an inactive guanosine diphosphate (GDP)-bound and an active guanosine triphosphate (GTP)-bound state during signal transduction. As such, they regulate a wide range of both cellular and physiological processes. In this review, we will summarize recent work on the role of Rho GTPase-regulated pathways in skeletal muscle development, regeneration, tissue mass homeostatic balance, and metabolism. In addition, we will present current evidence that links the dysregulation of these GTPases with diseases caused by skeletal muscle dysfunction. Overall, this information underscores the critical role of a number of members of the Rho GTPase subfamily in muscle development and the overall metabolic balance of mammalian species.

Partial Ablation of Non-Myogenic Progenitor Cells as a Therapeutic Approach to Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD), caused by the loss of dystrophin, remains incurable. Reduction in muscle regeneration with DMD is associated with the accumulation of fibroadipogenic progenitors (FAPs) differentiating into myofibroblasts and leading to a buildup of the collagenous tissue aggravating DMD pathogenesis. Mesenchymal stromal cells (MSCs) expressing platelet-derived growth factor receptors (PDGFRs) are activated in muscle during DMD progression and give rise to FAPs promoting DMD progression. Here, we hypothesized that muscle dysfunction in DMD could be delayed via genetic or pharmacologic depletion of MSC-derived FAPs. In this paper, we test this hypothesis in dystrophin-deficient mdx mice. To reduce fibro/adipose infiltration and potentiate muscle progenitor cells (MPCs), we used a model for inducible genetic ablation of proliferating MSCs via a suicide transgene, viral thymidine kinase (TK), expressed under the Pdgfrb promoter. We also tested if MSCs from fat tissue, the adipose stromal cells (ASCs), contribute to FAPs and could be targeted in DMD. Pharmacological ablation was performed with a hunter-killer peptide D-CAN targeting ASCs. MSC depletion with these approaches resulted in increased endurance, measured based on treadmill running, as well as grip strength, without significantly affecting fibrosis. Although more research is needed, our results suggest that depletion of pathogenic MSCs mitigates muscle damage and delays the loss of muscle function in mouse models of DMD.

Generation and Characterization of a Skeletal Muscle Cell-Based Model Carrying One Single Gne Allele: Implications in Actin Dynamics

UDP-N-Acetyl glucosamine-2 epimerase/N-acetyl mannosamine kinase (GNE) catalyzes key enzymatic reactions in the biosynthesis of sialic acid. Mutation in GNE gene causes GNE myopathy (GNEM) characterized by adult-onset muscle weakness and degeneration. However, recent studies propose alternate roles of GNE in other cellular processes beside sialic acid biosynthesis, particularly interaction of GNE with α-actinin 1 and 2. Lack of appropriate model system limits drug and treatment options for GNEM as GNE knockout was found to be embryonically lethal. In the present study, we have generated L6 rat skeletal muscle myoblast cell-based model system carrying one single Gne allele where GNE gene is knocked out at exon-3 using AAV mediated SEPT homology recombination (SKM-GNEHz). The cell line was heterozygous for GNE gene with one wild type and one truncated allele as confirmed by sequencing. The phenotype showed reduced GNE epimerase activity with little reduction in sialic acid content. In addition, the heterozygous GNE knockout cells revealed altered cytoskeletal organization with disrupted actin filament. Further, we observed increased levels of RhoA leading to reduced cofilin activity and causing reduced F-actin polymerization. The disturbed signaling cascade resulted in reduced migration of SKM-GNEHz cells. Our study indicates possible role of GNE in regulating actin dynamics and cell migration of skeletal muscle cell. The skeletal muscle cell-based system offers great potential in understanding pathomechanism and target identification for GNEM.

Improved Bone Quality and Bone Healing of Dystrophic Mice by Parabiosis

Duchenne muscular dystrophy (DMD) is a degenerative muscle disorder characterized by a lack of dystrophin expression in the sarcolemma of muscle fibers. DMD patients acquire bone abnormalities including osteopenia, fragility fractures, and scoliosis indicating a deficiency in skeletal homeostasis. The dKO (dystrophin/Utrophin double knockout) is a more severe mouse model of DMD than the mdx mouse (dystrophin deficient), and display numerous clinically-relevant manifestations, including a spectrum of degenerative changes outside skeletal muscle including bone, articular cartilage, and intervertebral discs. To examine the influence of systemic factors on the bone abnormalities and healing in DMD, parabiotic pairing between dKO mice and mdx mice was established. Notably, heterochronic parabiosis with young mdx mice significantly increased bone mass and improved bone micro-structure in old dKO-hetero mice, which showed progressive bone deterioration. Furthermore, heterochronic parabiosis with WT C56/10J mice significantly improved tibia bone defect healing in dKO-homo mice. These results suggest that systemic blood-borne factor(s) and/or progenitors from WT and young mdx mice can influence the bone deficiencies in dKO mice. Understanding these circulating factors or progenitor cells that are responsible to alleviate the bone abnormalities in dKO mice after heterochronic parabiosis might be useful for the management of poor bone health in DMD.

Therapeutic aspects of cell signaling and communication in Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a devastating chromosome X-linked disease that manifests predominantly in progressive skeletal muscle wasting and dysfunctions in the heart and diaphragm. Approximately 1/5000 boys and 1/50,000,000 girls suffer from DMD, and to date, the disease is incurable and leads to premature death. This phenotypic severity is due to mutations in the DMD gene, which result in the absence of functional dystrophin protein. Initially, dystrophin was thought to be a force transducer; however, it is now considered an essential component of the dystrophin-associated protein complex (DAPC), viewed as a multicomponent mechanical scaffold and a signal transduction hub. Modulating signal pathway activation or gene expression through epigenetic modifications has emerged at the forefront of therapeutic approaches as either an adjunct or stand-alone strategy. In this review, we propose a broader perspective by considering DMD to be a disease that affects myofibers and muscle stem (satellite) cells, as well as a disorder in which abrogated communication between different cell types occurs. We believe that by taking this systemic view, we can achieve safe and holistic treatments that can restore correct signal transmission and gene expression in diseased DMD tissues.

The microRNA miR-133b functions to slow Duchenne muscular dystrophy pathogenesis

KEY POINTS

Impairment of muscle biogenesis contributes to the progression of Duchenne muscular dystrophy (DMD). As a muscle enriched microRNA that has been implicated in muscle biogenesis, the role of miR-133b in DMD remains unknown. To assess miR-133b function in DMD-affected skeletal muscles, we genetically ablated miR-133b in the mdx mouse model of DMD. We show that deletion of miR-133b exacerbates the dystrophic phenotype of DMD-afflicted skeletal muscle by dysregulating muscle stem cells involved in muscle biogenesis, in addition to affecting signalling pathways related to inflammation and fibrosis. Our results provide evidence that miR-133b may underlie DMD pathology by affecting the proliferation and differentiation of muscle stem cells.

ABSTRACT

Duchenne muscular dystrophy (DMD) is characterized by progressive skeletal muscle degeneration. No treatments are currently available to prevent the disease. While the muscle enriched microRNA miR-133b has been implicated in muscle biogenesis, its role in DMD remains unknown. To assess miR-133b function in DMD-affected skeletal muscles, we genetically ablated miR-133b in the mdx mouse model of DMD. In the absence of miR-133b, the tibialis anterior muscle of P30 mdx mice is smaller in size and exhibits a thickened interstitial space containing more mononucleated cells. Additional analysis revealed that miR-133b deletion influences muscle fibre regeneration, satellite cell proliferation and differentiation, and induces widespread transcriptomic changes in mdx muscle. These include known miR-133b targets as well as genes involved in cell proliferation and fibrosis. Altogether, our data demonstrate that skeletal muscles utilize miR-133b to mitigate the deleterious effects of DMD.

ARHGEF3 Regulates Skeletal Muscle Regeneration and Strength through Autophagy

Skeletal muscle regeneration after injury is essential for maintaining muscle function throughout aging. ARHGEF3, a RhoA/B-specific GEF, negatively regulates myoblast differentiation through Akt signaling independently of its GEF activity in vitro. Here, we report ARHGEF3's role in skeletal muscle regeneration revealed by ARHGEF3-KO mice. These mice exhibit indiscernible phenotype under basal conditions. Upon acute injury, however, ARHGEF3 deficiency enhances the mass/fiber size and function of regenerating muscles in both young and regeneration-defective middle-aged mice. Surprisingly, these effects occur independently of Akt but via the GEF activity of ARHGEF3. Consistently, overexpression of ARHGEF3 inhibits muscle regeneration in a Rho-associated kinase-dependent manner. We further show that ARHGEF3 KO promotes muscle regeneration through activation of autophagy, a process that is also critical for maintaining muscle strength. Accordingly, ARHGEF3 depletion in old mice prevents muscle weakness by restoring autophagy. Taken together, our findings identify a link between ARHGEF3 and autophagy-related muscle pathophysiology.

Aberrant RhoA activation in macrophages increases senescence-associated secretory phenotypes and ectopic calcification in muscular dystrophic mice

Duchenne Muscular Dystrophy (DMD) patients often suffer from both muscle wasting and osteoporosis. Our previous studies have revealed reduced regeneration potential in skeletal muscle and bone, concomitant with ectopic calcification of soft tissues in double knockout (dKO, dystrophin-/-; utrophin-/-) mice, a severe murine model for DMD. We found significant involvement of RhoA/ROCK (Rho-Associated Protein Kinase) signaling in mediating ectopic calcification of muscles in dKO mice. However, the cellular identity of these RhoA+ cells, and the role that RhoA plays in the chronic inflammation-associated pathologies has not been elucidated. Here, we report that CD68+ macrophages are highly prevalent at the sites of ectopic calcification of dKO mice, and that these macrophages highly express RhoA. Macrophages from dKO mice feature a shift towards a more pro-inflammatory M1 polarization and an increased expression of various senescence-associated secretory phenotype (SASP) factors that was reduced with the RhoA/ROCK inhibitor Y-27632. Further, systemic inhibition of RhoA activity in dKO mice led to reduced number of RhoA+/CD68+ cells, as well as a reduction in fibrosis and ectopic calcification. Together, these data revealed that RhoA signaling may be a key regulator of imbalanced mineralization in the dystrophic musculoskeletal system and consequently a therapeutic target for the treatment of DMD or other related muscle dystrophies.

Higher BMP Expression in Tendon Stem/Progenitor Cells Contributes to the Increased Heterotopic Ossification in Achilles Tendon With Aging

Although the mineralization in tendon tissue has been reported in a series of aging and disease models, the underlying mechanisms remain unknown. This study aimed to describe the appearance of heterotopic ossification in rat Achilles tendon and further verify whether this tissue metaplasia is related to the enhanced osteogenic differentiation of tendon stem/progenitor cells (TSPCs) owing to the higher expression of bone morphogenetic proteins (BMP-2/4/7) with aging. The male SD rats, aged 4, 8, and 20 months (M), were used. The analyses of ossification and BMP expression in tendon were tested by radiological view (X-ray and CT), histological staining [hematoxylin and eosin (HE), Alcian blue, and Alizarin red], immunohistochemistry, and Western blot. The osteogenic differentiation potential and BMP expression of TSPCs were examined by Alizarin red S staining and real-time PCR. TSPCs were treated with BMP-2 or noggin, and the osteogenic differentiation potential was also examined. X-ray and CT showed the appearance of heterotopic ossification in tendon, and the volume and density of ossification was increased with aging. Histological staining showed the appearance of calcified region surrounded by chondrocyte-like cells and the increased osteogenesis-related gene and BMP expression in ossified tendon with aging. Moreover, the osteogenic differentiation potential and BMP expression in TSPCs isolated from ossified tendon were increased with aging. Additionally, BMP-2 increased the calcium nodule formation and osteogenesis-related gene expression in TSPCs. The addition of noggin inhibited BMP-induced enhancement of osteogenic differentiation. Thus, these findings suggested that the enhanced osteogenic differentiation of TSPCs contributes to the increased heterotopic ossification in aged tendon, which might be induced by the higher expression of BMPs with aging.

Impaired bone defect and fracture healing in dystrophin/utrophin double-knockout mice and the mechanism

This study investigated the role of muscle damage in bone defect healing using skull and tibial double-defect and tibial fracture models in dystrophin^-/-/Utrophin^-/- double-knockout (dKO-Hom) mice. The skull and tibia bone defect and fracture healing was monitored using micro-CT, histology, immuohistochemistry and quantitative PCR. We found the skull defect healing is not impaired while the tibial defect healing was delayed at day 7 in the dKO-Hom group compared to wild-type (WT) group as revealed by micro-CT. Mechanistically, the number of osteoclasts and osteoblasts significantly decreased in the defect area in dKO-Hom group compared to WT group on day 21. DKO-Hom mice showed higher mortality after fracture (6/12) and significantly impaired fracture healing compared to the other groups as revealed by the micro-CT parameters of the calluses. Histology showed higher osteoclast number in the calluses of dKO-Hom mice than other groups. Furthermore, dKO-Hom mice showed down-regulation of 15-Pgdh, Il-4, Bmp7, and Bmp9 at 10 days after tibia fracture and BMP6 and 7 in the muscle. In conclusion, the long bone defect and fracture healing are impaired in dKO-Hom mice which demonstrated significantly muscle sarcopenia and related with disturbance of osteoclastogenesis and osteoblastogenesis. The impaired tibial fracture healing was associated with down-regulation of several genes in the muscle.

Cytoskeleton stiffness regulates cellular senescence and innate immune response in Hutchinson-Gilford Progeria Syndrome

Hutchinson-Gilford progeria syndrome (HGPS) is caused by the accumulation of mutant prelamin A (progerin) in the nuclear lamina, resulting in increased nuclear stiffness and abnormal nuclear architecture. Nuclear mechanics are tightly coupled to cytoskeletal mechanics via lamin A/C. However, the role of cytoskeletal/nuclear mechanical properties in mediating cellular senescence and the relationship between cytoskeletal stiffness, nuclear abnormalities, and senescent phenotypes remain largely unknown. Here, using muscle-derived mesenchymal stromal/stem cells (MSCs) from the Zmpste24-/- (Z24-/- ) mouse (a model for HGPS) and human HGPS fibroblasts, we investigated the mechanical mechanism of progerin-induced cellular senescence, involving the role and interaction of mechanical sensors RhoA and Sun1/2 in regulating F-actin cytoskeleton stiffness, nuclear blebbing, micronuclei formation, and the innate immune response. We observed that increased cytoskeletal stiffness and RhoA activation in progeria cells were directly coupled with increased nuclear blebbing, Sun2 expression, and micronuclei-induced cGAS-Sting activation, part of the innate immune response. Expression of constitutively active RhoA promoted, while the inhibition of RhoA/ROCK reduced cytoskeletal stiffness, Sun2 expression, the innate immune response, and cellular senescence. Silencing of Sun2 expression by siRNA also repressed RhoA activation, cytoskeletal stiffness and cellular senescence. Treatment of Zmpste24-/- mice with a RhoA inhibitor repressed cellular senescence and improved muscle regeneration. These results reveal novel mechanical roles and correlation of cytoskeletal/nuclear stiffness, RhoA, Sun2, and the innate immune response in promoting aging and cellular senescence in HGPS progeria.

Systemic investigation of bone and muscle abnormalities in dystrophin/utrophin double knockout mice during postnatal development and the mechanisms

The dystrophin-/-/utrophin-/-/ double knockout (dKO-Hom) mouse is a murine model of human Duchenne muscular dystrophy. This study investigated the bone and muscle abnormalities of dKO-Hom mouse and mechanisms. We collected bone and skeletal muscle samples from control mice and three muscular dystrophic mouse models at different ages and performed micro-computer tomography and histological analyses of both bone and skeletal muscle tissues. Serum receptor activator of nuclear factor kappa-Β ligand (RANKL) and sclerostin (SOST) levels, osteoclastogenesis and serum proteomics were also analyzed. Our results indicated that dKO-Hom mice developed skeletal muscle histopathologies by 5 days of age, whereas bone abnormalities developed at 4 weeks of age. Furthermore, our results indicated that the numbers of osteoblasts and osteoclasts were decreased in the proximal tibia and spine trabecular bone of dKO-Hom mice compared to wild-type (WT) mice, which correlated with a significant reduction in serum RANKL levels. The number of tibia cortical osteocytes also decreased, whereas serum SOST levels increased significantly in dKO-Hom mice than WT mice. Osteoblastic number was significantly lower, but osteoclast number increased, in the spine L6 of dKO-Hom mice than WT mice at 6 weeks of age, resulting in a decrease in bone formation and an increase in bone resorption. Serum proteomics results revealed abnormal proteome profiles in dKO-Hom mice compared to control mice. In conclusion, our study elucidated the timing of development of bone and muscle abnormalities. The bone abnormalities in dKO-Hom mice are correlated with lower serum RANKL and higher SOST levels that resulted in dysregulation of osteogenesis and osteoclastogenesis and bone loss.

CRISPR/Cas9-Based Dystrophin Restoration Reveals a Novel Role for Dystrophin in Bioenergetics and Stress Resistance of Muscle Progenitors

Although the lack of dystrophin expression in muscle myofibers is the central cause of Duchenne muscular dystrophy (DMD), accumulating evidence suggests that DMD may also be a stem cell disease. Recent studies have revealed dystrophin expression in satellite cells and demonstrated that dystrophin deficiency is directly related to abnormalities in satellite cell polarity, asymmetric division, and epigenetic regulation, thus contributing to the manifestation of the DMD phenotype. Although metabolic and mitochondrial dysfunctions have also been associated with the DMD pathophysiology profile, interestingly, the role of dystrophin with respect to stem cells dysfunction has not been elucidated. In the past few years, editing of the gene that encodes dystrophin has emerged as a promising therapeutic approach for DMD, although the effects of dystrophin restoration in stem cells have not been addressed. Herein, we describe our use of a clustered regularly interspaced short palindromic repeats/Cas9‐based system to correct the dystrophin mutation in dystrophic (mdx) muscle progenitor cells (MPCs) and show that the expression of dystrophin significantly improved cellular properties of the mdx MPCs in vitro. Our findings reveal that dystrophin‐restored mdx MPCs demonstrated improvements in cell proliferation, differentiation, bioenergetics, and resistance to oxidative and endoplasmic reticulum stress. Furthermore, our in vivo studies demonstrated improved transplantation efficiency of the corrected MPCs in the muscles of mdx mice. Our results indicate that changes in cellular energetics and stress resistance via dystrophin restoration enhance muscle progenitor cell function, further validating that dystrophin plays a role in stem cell function and demonstrating the potential for new therapeutic approaches for DMD. stem cells2019;37:1615–1628

SPECT Imaging of Muscle Injury with [99mTc]MDP in a Mouse Model of Muscular Dystrophy

PURPOSE

Tc-99m methylene diphosphonate ([^99mTc]MDP) is an in vivo bone imaging agent that also accumulates in injured skeletal muscle cells. The objective of this study was to investigate if [^99mTc]MDP could be used to detect muscle injury in the mdx mouse model of Duchenne muscular dystrophy (DMD).

PROCEDURES

Static whole-body single-photon emission computed tomography/computed tomography (CT) scans were acquired at 2 h post-injection of [^99mTc]MDP in two cohorts of animals at different sites: one cohort of mice at 6, 15, and 19 weeks of age, and a separate cohort at 16 weeks. The second cohort was also imaged with high-resolution CT at 8 weeks.

RESULTS

mdx mice had higher [^99mTc]MDP uptake and significantly higher [^99mTc]MDP concentrations in muscle than controls.

CONCLUSIONS

Higher uptake of [^99mTc]MDP in muscle of mdx mice agrees with histological reports of muscle calcification in mdx mice, and suggests the potential translational use of [^99mTc]MDP imaging for tracking DMD progression and therapeutic response.

Fibro-adipogenic progenitors of dystrophic mice are insensitive to NOTCH regulation of adipogenesis

Fibro-adipogenic progenitors (FAPs) promote satellite cell differentiation in adult skeletal muscle regeneration. However, in pathological conditions, FAPs are responsible for fibrosis and fatty infiltrations. Here we show that the NOTCH pathway negatively modulates FAP differentiation both in vitro and in vivo. However, FAPs isolated from young dystrophin-deficient mdx mice are insensitive to this control mechanism. An unbiased mass spectrometry-based proteomic analysis of FAPs from muscles of wild-type and mdx mice suggested that the synergistic cooperation between NOTCH and inflammatory signals controls FAP differentiation. Remarkably, we demonstrated that factors released by hematopoietic cells restore the sensitivity to NOTCH adipogenic inhibition in mdx FAPs. These results offer a basis for rationalizing pathological ectopic fat infiltrations in skeletal muscle and may suggest new therapeutic strategies to mitigate the detrimental effects of fat depositions in muscles of dystrophic patients.

RhoA/ROCK inhibition improves the beneficial effects of glucocorticoid treatment in dystrophic muscle: implications for stem cell depletion

Glucocorticoid treatment represents a standard palliative treatment for Duchenne muscular dystrophy (DMD) patients, but various adverse effects have limited this treatment. In an effort to understand the mechanism(s) by which glucocorticoids impart their effects on the dystrophic muscle, and potentially reduce the adverse effects, we have studied the effect of prednisolone treatment in dystrophin/utrophin double knockout (dKO) mice, which exhibit a severe dystrophic phenotype due to rapid muscle stem cell depletion. Our results indicate that muscle stem cell depletion in dKO muscle is related to upregulation of mTOR, and that prednisolone treatment reduces the expression of mTOR and other pro-inflammatory mediators, consequently slowing down muscle stem cell depletion. However, prednisolone treatment was unable to improve the myogenesis of stem cells and reduce fibrosis in dKO muscle. We then studied whether glucocorticoid treatment can be improved by co-administration of an inhibitor of RhoA/ROCK signaling, which can be activated by glucocorticoids and was found in our previous work to be over-activated in dystrophic muscle. Our results indicate that the combination of RhoA/ROCK inhibition and glucocorticoid treatment in dystrophic muscle have a synergistic effect in alleviating the dystrophic phenotype. Taken together, our study not only shed light on the mechanism by which glucocorticoid imparts its beneficial effect on dystrophic muscle, but also revealed the synergistic effect of RhoA/ROCK inhibition and glucocorticoid treatment, which could lead to the development of more efficient therapeutic approaches for treating DMD patients.

Lysophosphatidic acid-induced RhoA signaling and prolonged macrophage infiltration worsens fibrosis and fatty infiltration following rotator cuff tears

Previous studies have suggested that macrophage-mediated chronic inflammation is involved in the development of rotator cuff muscle atrophy and degeneration following massive tendon tears. Increased RhoA signaling has been reported in chronic muscle degeneration, such as muscular dystrophy. However, the role of RhoA signaling in macrophage infiltration and rotator muscle degeneration remains unknown. Using a previously established rat model of massive rotator cuff tears, we found RhoA signaling is upregulated in rotator cuff muscle following a massive tendon-nerve injury. This increase in RhoA expression is greatly potentiated by the administration of a potent RhoA activator, lysophosphatidic acid (LPA), and is accompanied by increased TNFα and TGF-β1 expression in rotator cuff muscle. Boosting RhoA signaling with LPA significantly worsened rotator cuff muscle atrophy, fibrosis, and fatty infiltration, accompanied with massive monocytic infiltration of rotator cuff muscles. Co-staining of RhoA and the tissue macrophage marker CD68 showed that CD68+ tissue macrophages are the dominant cell source of increased RhoA signaling in rotator cuff muscles after tendon tears. Taken together, our findings suggest that LPA-mediated RhoA signaling in injured muscle worsens the outcomes of atrophy, fibrosis, and fatty infiltration by increasing macrophage infiltraion in rotator cuff muscle. Clinically, inhibiting RhoA signaling may represent a future direction for developing new treatments to improve muscle quality following massive rotator cuff tears. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1539-1547, 2017.

The sarcomeric M-region: a molecular command center for diverse cellular processes

The sarcomeric M-region anchors thick filaments and withstands the mechanical stress of contractions by deformation, thus enabling distribution of physiological forces along the length of thick filaments. While the role of the M-region in supporting myofibrillar structure and contractility is well established, its role in mediating additional cellular processes has only recently started to emerge. As such, M-region is the hub of key protein players contributing to cytoskeletal remodeling, signal transduction, mechanosensing, metabolism, and proteasomal degradation. Mutations in genes encoding M-region related proteins lead to development of severe and lethal cardiac and skeletal myopathies affecting mankind. Herein, we describe the main cellular processes taking place at the M-region, other than thick filament assembly, and discuss human myopathies associated with mutant or truncated M-region proteins.

Activation of non-myogenic mesenchymal stem cells during the disease progression in dystrophic dystrophin/utrophin knockout mice

Ectopic calcification as well as fatty and fibrotic tissue accumulation occurs in skeletal muscle during the disease progression of Duchenne muscular dystrophy (DMD), a degenerative muscle disorder caused by mutations in the dystrophin gene. The cellular origin and the environmental cues responsible for this ectopic calcification, fatty and fibrotic infiltration during the disease progression, however, remain unknown. Based on a previously published preplate technique, we isolated two distinct populations of muscle-derived cells from skeletal muscle: (i) a rapidly adhering cell population, which is non-myogenic, Pax7(-) and express the mesenchymal stem cell (MSC) marker platelet-derived growth factor receptor alpha; hence, we termed this population of cells non-myogenic MSCs (nmMSCs); and (ii) a slowly adhering cell population which is Pax7(+) and highly myogenic, termed muscle progenitor cells (MPCs). Previously, we demonstrated that the rapid progression of skeletal muscle histopathologies in dystrophin/utrophin knockout (dys(-/-) utro(-/-) dKO) mice is closely associated with a rapid depletion of the MPC population pool. In the current study, we showed that in contrast to the MPCs, the nmMSCs become activated during the disease progression in dKO mice, displaying increased proliferation and differentiation potentials (adipogenesis, osteogenesis and fibrogenesis). We also found that after co-culturing the dKO-nmMSCs with dKO-MPCs, the myogenic differentiation potential of the dKO-MPCs was reduced. This effect was found to be potentially mediated by the secretion of secreted frizzled-related protein 1 by the dKO-nmMSCs. We therefore posit that the rapid occurrence of fibrosis, ectopic calcification and fat accumulation, in dKO mice, is not only attributable to the rapid depletion of the MPC pool, but is also the consequence of nmMSC activation. Results from this study suggest that approaches to alleviate muscle weakness and wasting in DMD patients should not only target the myogenic MPCs but should also attempt to prevent the activation of the nmMSCs.

Pathogenesis and prevention strategies of heterotopic ossification in total hip arthroplasty: a narrative literature review and results of a survey in Germany

INTRODUCTION

Heterotopic ossification (HO) after THA can lead to pain, impaired range of motion and possibly revision surgery. This article summarizes current literature on the pathogenesis of HO in THA and trauma. Second, it presents the results of a survey on prophylactic concepts for HO in Germany.

MATERIALS AND METHODS

A narrative literature review was conducted by searching three databases (Pubmed, ScienceDirect, the Cochrane library) on the aetiology of HO. Between 2013 and 2014, a questionnaire was sent to 119 orthopaedic and trauma surgery departments in Germany.

RESULTS

The acquired form of HO seems to develop after tissue trauma, which induces a local inflammation. A change in tissue conditions, multiple signalling pathways and involvement of several different cell types seem to promote enchondral ossification and finally HO formation. The feed back rate of the survey was 67%. Eighty-seven percent of all departments currently administer NSAIDs with a mean time span of 3 weeks after surgery for oral prophylaxis. Prophylactic perioperative irradiation is performed in 64% of trauma/orthopaedic departments if the patient is at risk for HO with a mean dosage of 7 Gy.

CONCLUSIONS

Basic research detected new pathways and cell signalling mechanisms of HO pathogenesis, which could offer new treatment and prophylaxis options in the near future. So far, there is no uniform strategy for the clinical prophylaxis of HO in THA. Guidelines and new clinical trials need to be developed to further reduce HO rates in THA.

The role of Notch signaling in muscle progenitor cell depletion and the rapid onset of histopathology in muscular dystrophy

Although it has been speculated that stem cell depletion plays a role in the rapid progression of the muscle histopathology associated with Duchenne Muscular Dystrophy (DMD), the molecular and cellular mechanisms responsible for stem cell depletion remain poorly understood. The rapid depletion of muscle stem cells has not been observed in the dystrophin-deficient model of DMD (mdx mouse), which may explain the relatively mild dystrophic phenotype observed in this animal model. In contrast, we have observed a rapid occurrence of stem cell depletion in the dystrophin/utrophin double knockout (dKO) mouse model, which exhibits histopathological features that more closely recapitulate the phenotype observed in DMD patients compared with the mdx mouse. Notch signaling has been found to be a key regulator of stem cell self-renewal and myogenesis in normal skeletal muscle; however, little is known about the role that Notch plays in the development of the dystrophic histopathology associated with DMD. Our results revealed an over-activation of Notch in the skeletal muscles of dKO mice, which correlated with sustained inflammation, impaired muscle regeneration and the rapid depletion and senescence of the muscle progenitor cells (MPCs, i.e. Pax7+ cells). Consequently, the repression of Notch in the skeletal muscle of dKO mice delayed/reduced the depletion and senescence of MPCs, and restored the myogenesis capacity while reducing inflammation and fibrosis. We suggest that the down-regulation of Notch could represent a viable approach to reduce the dystrophic histopathologies associated with DMD.

Pericytes: multitasking cells in the regeneration of injured, diseased, and aged skeletal muscle

Pericytes are perivascular cells that envelop and make intimate connections with adjacent capillary endothelial cells. Recent studies show that they may have a profound impact in skeletal muscle regeneration, innervation, vessel formation, fibrosis, fat accumulation, and ectopic bone formation throughout life. In this review, we summarize and evaluate recent advances in our understanding of pericytes' influence on adult skeletal muscle pathophysiology. We also discuss how further elucidating their biology may offer new approaches to the treatment of conditions characterized by muscle wasting.

Molecular and cellular mechanisms of heterotopic ossification

Heterotopic ossification (HO) is a debilitating condition in which cartilage and bone forms in soft tissues such as muscle, tendon, and ligament causing immobility. This process is induced by inflammation associated with traumatic injury. In an extremely rare genetic disorder called fibrodysplasia ossificans progessiva (FOP), a combination of inflammation associated with minor soft tissue injuries and a hereditary genetic mutation causes massive HO that progressively worsens throughout the patients' lifetime leading to the formation of an ectopic skeleton. An activating mutation in the BMP type I receptor ALK2 has been shown to contribute to the heterotopic lesions in FOP patients, yet recent studies have shown that other events are required to stimulate HO including activation of sensory neurons, mast cell degranulation, lymphocyte infiltration, skeletal myocyte cell death, and endothelial-mesenchymal transition (EndMT). In this review, we discuss the recent evidence and mechanistic data that describe the cellular and molecular mechanisms that give rise to heterotopic bone.

Dystrophic changes in extraocular muscles after gamma irradiation in mdx:utrophin(+/-) mice

Extraocular muscles (EOM) have a strikingly different disease profile than limb skeletal muscles. It has long been known that they are spared in Duchenne (DMD) and other forms of muscular dystrophy. Despite many studies, the cause for this sparing is not understood. We have proposed that differences in myogenic precursor cell properties in EOM maintain normal morphology over the lifetime of individuals with DMD due to either greater proliferative potential or greater resistance to injury. This hypothesis was tested by exposing wild type and mdx:utrophin(+/-) (het) mouse EOM and limb skeletal muscles to 18 Gy gamma irradiation, a dose known to inhibit satellite cell proliferation in limb muscles. As expected, over time het limb skeletal muscles displayed reduced central nucleation mirrored by a reduction in Pax7-positive cells, demonstrating a significant loss in regenerative potential. In contrast, in the first month post-irradiation in the het EOM, myofiber cross-sectional areas first decreased, then increased, but ultimately returned to normal compared to non-irradiated het EOM. Central nucleation significantly increased in the first post-irradiation month, resembling the dystrophic limb phenotype. This correlated with decreased EECD34 stem cells and a concomitant increase and subsequent return to normalcy of both Pax7 and Pitx2-positive cell density. By two months, normal het EOM morphology returned. It appears that irradiation disrupts the normal method of EOM remodeling, which react paradoxically to produce increased numbers of myogenic precursor cells. This suggests that the EOM contain myogenic precursor cell types resistant to 18 Gy gamma irradiation, allowing return to normal morphology 2 months post-irradiation. This supports our hypothesis that ongoing proliferation of specialized regenerative populations in the het EOM actively maintains normal EOM morphology in DMD. Ongoing studies are working to define the differences in the myogenic precursor cells in EOM as well as the cellular milieu in which they reside.

Drug repositioning for orphan genetic diseases through Conserved Anticoexpressed Gene Clusters (CAGCs)

Background

The development of new therapies for orphan genetic diseases represents an extremely important medical and social challenge. Drug repositioning, i.e. finding new indications for approved drugs, could be one of the most cost- and time-effective strategies to cope with this problem, at least in a subset of cases. Therefore, many computational approaches based on the analysis of high throughput gene expression data have so far been proposed to reposition available drugs. However, most of these methods require gene expression profiles directly relevant to the pathologic conditions under study, such as those obtained from patient cells and/or from suitable experimental models. In this work we have developed a new approach for drug repositioning, based on identifying known drug targets showing conserved anti-correlated expression profiles with human disease genes, which is completely independent from the availability of ‘ad hoc’ gene expression data-sets.

Results

By analyzing available data, we provide evidence that the genes displaying conserved anti-correlation with drug targets are antagonistically modulated in their expression by treatment with the relevant drugs. We then identified clusters of genes associated to similar phenotypes and showing conserved anticorrelation with drug targets. On this basis, we generated a list of potential candidate drug-disease associations. Importantly, we show that some of the proposed associations are already supported by independent experimental evidence.

Conclusions

Our results support the hypothesis that the identification of gene clusters showing conserved anticorrelation with drug targets can be an effective method for drug repositioning and provide a wide list of new potential drug-disease associations for experimental validation.