Immune-mediated pathology in Duchenne muscular dystrophy

High mobility group box 1 (HMGB1) is a potential disease biomarker in cell and mouse models of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a progressive muscle wasting disorder affecting 1:3500 male births and is associated with myofiber degeneration, regeneration, and inflammation. Glucocorticoid treatments have been the standard of care due to immunomodulatory/immunosuppressive properties but novel genetic approaches, including exon skipping and gene replacement therapy, are currently being developed. The identification of additional biomarkers to assess DMD-related inflammatory responses and the potential efficacy of these therapeutic approaches are thus of critical importance. The current study uses RNA sequencing of skeletal muscle from two mdx mouse models to identify high mobility group box 1 (HMGB1) as a candidate biomarker potentially contributing to DMD-related inflammation. HMGB1 protein content was increased in a human iPSC-derived skeletal myocyte model of DMD and microdystrophin treatment decreased HMGB1 back to control levels. In vivo, HMGB1 protein levels were increased in vehicle treated B10-mdx skeletal muscle compared to B10-WT and significantly decreased in B10-mdx animals treated with adeno-associated virus (AAV)-microdystrophin. However, HMGB1 protein levels were not increased in D2-mdx skeletal muscle compared to D2-WT, demonstrating a strain-specific difference in DMD-related immunopathology.

Ultrasound attenuation imaging as a strategy for evaluation of early and late ambulatory functions in Duchenne muscular dystrophy

BACKGROUND

Duchenne muscular dystrophy (DMD) is a genetic neuromuscular disorder that leads to mobility loss and life-threatening cardiac or respiratory complications. Quantitative ultrasound (QUS) envelope statistics imaging, which characterizes fat infiltration and fibrosis in muscles, has been extensively used for DMD evaluations.

PURPOSE

Notably, changes in muscle microstructures also result in acoustic attenuation, potentially serving as another crucial imaging biomarker for DMD. Expanding upon the reference frequency method (RFM), this study contributes to the field by introducing the robust RFM (RRFM) as a novel approach for ultrasound attenuation imaging in DMD.

METHODS

The RRFM algorithm was developed using an iterative reweighted least squares technique. We conducted standard phantom measurements with a clinical ultrasound system equipped with a linear array transducer to assess the improvement in attenuation estimation bias by RRFM. Additionally, 161 DMD patients, included in both a validation dataset (n = 130) and a testing dataset (n = 31), underwent ultrasound scanning of the gastrocnemius for RRFM-based attenuation imaging. The diagnostic performances for ambulatory functions and discrimination between early and late ambulatory stages were evaluated and compared with those of QUS envelope statistics imaging (involving Nakagami distribution, homodyned K distribution, and entropy values) using the area under the receiver operating characteristic curve (AUROC).

RESULTS

The results indicated that the RRFM method more closely matched the actual attenuation properties of the phantom, reducing measurement bias by 50% compared to conventional RFM. The AUROCs for RRFM-based attenuation imaging, used to discriminate between early and late ambulatory stages, were 0.88 and 0.92 for the validation and testing datasets, respectively. These performances significantly surpassed those of QUS envelope statistics imaging (p < 0.05).

CONCLUSIONS

Ultrasound attenuation imaging employing RRFM may serve as a sensitive tool for evaluating the progression of ambulatory function deterioration, offering substantial potential for the health management and follow-up care of DMD patients.

Leucyl-tRNA Synthetase Contributes to Muscle Weakness through Mammalian Target of Rapamycin Complex 1 Activation and Autophagy Suppression in a Mouse Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD), caused by loss-of-function mutations in the dystrophin gene, results in progressive muscle weakness and early fatality. Impaired autophagy is one of the cellular hallmarks of DMD, contributing to the disease progression. Molecular mechanisms underlying the inhibition of autophagy in DMD are not well understood. In the current study, the DMD mouse model mdx was used for the investigation of signaling pathways leading to suppression of autophagy. Mammalian target of rapamycin complex 1 (mTORC1) was hyperactive in the DMD muscles, accompanying muscle weakness and autophagy impairment. Surprisingly, Akt, a well-known upstream regulator of mTORC1, was not responsible for mTORC1 activation or the dystrophic muscle phenotypes. Instead, leucyl-tRNA synthetase (LeuRS) was overexpressed in mdx muscles compared with the wild type. LeuRS activates mTORC1 in a noncanonical mechanism that involves interaction with RagD, an activator of mTORC1. Disrupting LeuRS interaction with RagD by the small-molecule inhibitor BC-LI-0186 reduced mTORC1 activity, restored autophagy, and ameliorated myofiber damage in the mdx muscles. Furthermore, inhibition of LeuRS by BC-LI-0186 improved dystrophic muscle strength in an autophagy-dependent manner. Taken together, our findings uncovered a noncanonical function of the housekeeping protein LeuRS as a potential therapeutic target in the treatment of DMD.

Investigating the Involvement of C-X-C Motif Chemokine 5 and P2X7 Purinoceptor in Ectopic Calcification in Mouse Models of Duchenne Muscular Dystrophy

Ectopic calcification of myofibers is an early pathogenic feature in patients and animal models of Duchenne muscular dystrophy (DMD). In previous studies using the Dmdmdx-βgeo mouse model, we found that the dystrophin-null phenotype exacerbates this abnormality and that mineralised myofibers are surrounded by macrophages. Furthermore, the P2X7 purinoceptor, functioning in immune cells offers protection against dystrophic calcification. In the present study, by exploring transcriptomic data from Dmdmdx mice, we hypothesised these effects to be mediated by C-X-C motif chemokine 5 (CXCL5) downstream of P2X7 activation. We found that CXCL5 is upregulated in the quadriceps muscles of Dmdmdx-βgeo mice compared to wild-type controls. In contrast, at the cell level, dystrophic (SC5) skeletal muscle cells secreted less CXCL5 chemokine than wild-type (IMO) controls. Although release from IMO cells was increased by P2X7 activation, this could not explain the elevated CXCL5 levels observed in dystrophic muscle tissue. Instead, we found that CXCL5 is released by dystrophin-null macrophages in response to P2X7 activation, suggesting that macrophages are the source of CXCL5 in dystrophic muscles. The effects of CXCL5 upon mineralisation were investigated using the Alizarin Red assay to quantify calcium deposition in vitro. In basal (low phosphate) media, CXCL5 increased calcification in IMO but not SC5 myoblasts. However, in cultures treated in high phosphate media, to mimic dysregulated phosphate metabolism occurring in DMD, CXCL5 decreased calcification in both IMO and SC5 cells. These data indicate that CXCL5 is part of a homoeostatic mechanism regulating intracellular calcium, that CXCL5 can be released by macrophages in response to the extracellular ATP damage-associated signal, and that CXCL5 can be part of a damage response to protect against ectopic calcification. This mechanism is affected by DMD gene mutations.

Benfotiamine improves dystrophic pathology and exercise capacity in mdx mice by reducing inflammation and fibrosis

Duchenne Muscular Dystrophy (DMD) is a progressive and fatal neuromuscular disease. Cycles of myofibre degeneration and regeneration are hallmarks of the disease where immune cells infiltrate to repair damaged skeletal muscle. Benfotiamine is a lipid soluble precursor to thiamine, shown clinically to reduce inflammation in diabetic related complications. We assessed whether benfotiamine administration could reduce inflammation related dystrophic pathology. Benfotiamine (10 mg/kg/day) was fed to male mdx mice (n = 7) for 15 weeks from 4 weeks of age. Treated mice had an increased growth weight (5-7 weeks) and myofibre size at treatment completion. Markers of dystrophic pathology (area of damaged necrotic tissue, central nuclei) were reduced in benfotiamine mdx quadriceps. Grip strength was increased and improved exercise capacity was found in mdx treated with benfotiamine for 12 weeks, before being placed into individual cages and allowed access to an exercise wheel for 3 weeks. Global gene expression profiling (RNAseq) in the gastrocnemius revealed benfotiamine regulated signalling pathways relevant to dystrophic pathology (Inflammatory Response, Myogenesis) and fibrotic gene markers (Col1a1, Col1a2, Col4a5, Col5a2, Col6a2, Col6a2, Col6a3, Lum) towards wildtype levels. In addition, we observed a reduction in gene expression of inflammatory gene markers in the quadriceps (Emr1, Cd163, Cd4, Cd8, Ifng). Overall, these data suggest that benfotiamine reduces dystrophic pathology by acting on inflammatory and fibrotic gene markers and signalling pathways. Given benfotiamine's excellent safety profile and current clinical use, it could be used in combination with glucocorticoids to treat DMD patients.

The role of interleukin-6 family cytokines in cancer cachexia

Cachexia is a wasting syndrome that manifests in more than half of all cancer patients. Cancer-associated cachexia negatively influences the survival of patients and their quality of life. It is characterized by a rapid loss of adipose and skeletal muscle tissues, which is partly mediated by inflammatory cytokines. Here, we explored the crucial roles of interleukin-6 (IL-6) family cytokines, including IL-6, leukemia inhibitory factor, and oncostatin M, in the development of cancer cachexia. These cytokines have been shown to exacerbate cachexia by promoting the wasting of adipose and muscle tissues, activating mechanisms that enhance lipolysis and proteolysis. Overlapping effects of the IL-6 family cytokines depend on janus kinase/signal transducer and activator of transcription 3 signaling. We argue that the blockade of these cytokine pathways individually may fail due to redundancy and future therapeutic approaches should target common downstream elements to yield effective clinical outcomes.

The role of TGF-β signaling in muscle atrophy, sarcopenia and cancer cachexia

Skeletal muscle, comprising a significant proportion (40 to 50 percent) of total body weight in humans, plays a critical role in maintaining normal physiological conditions. Muscle atrophy occurs when the rate of protein degradation exceeds protein synthesis. Sarcopenia refers to age-related muscle atrophy, while cachexia represents a more complex form of muscle wasting associated with various diseases such as cancer, heart failure, and AIDS. Recent research has highlighted the involvement of signaling pathways, including IGF1-Akt-mTOR, MuRF1-MAFbx, and FOXO, in regulating the delicate balance between muscle protein synthesis and breakdown. Myostatin, a member of the TGF-β superfamily, negatively regulates muscle growth and promotes muscle atrophy by activating Smad2 and Smad3. It also interacts with other signaling pathways in cachexia and sarcopenia. Inhibition of myostatin has emerged as a promising therapeutic approach for sarcopenia and cachexia. Additionally, other TGF-β family members, such as TGF-β1, activin A, and GDF11, have been implicated in the regulation of skeletal muscle mass. Furthermore, myostatin cooperates with these family members to impair muscle differentiation and contribute to muscle loss. This review provides an overview of the significance of myostatin and other TGF-β signaling pathway members in muscular dystrophy, sarcopenia, and cachexia. It also discusses potential novel therapeutic strategies targeting myostatin and TGF-β signaling for the treatment of muscle atrophy.

Recent Advances in Pre-Clinical Development of Adiponectin Receptor Agonist Therapies for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is caused by genetic mutations in the cytoskeletal-sarcolemmal anchor protein dystrophin. Repeated cycles of sarcolemmal tearing and repair lead to a variety of secondary cellular and physiological stressors that are thought to contribute to weakness, atrophy, and fibrosis. Collectively, these stressors can contribute to a pro-inflammatory milieu in locomotor, cardiac, and respiratory muscles. Given the many unwanted side effects that accompany current anti-inflammatory steroid-based approaches for treating DMD (e.g., glucocorticoids), there is a need to develop new therapies that address inflammation and other cellular dysfunctions. Adiponectin receptor (AdipoR) agonists, which stimulate AdipoR1 and R2 isoforms on various cell types, have emerged as therapeutic candidates for DMD due to their anti-inflammatory, anti-fibrotic, and pro-myogenic properties in pre-clinical human and rodent DMD models. Although these molecules represent a new direction for therapeutic intervention, the mechanisms through which they elicit their beneficial effects are not yet fully understood, and DMD-specific data is limited. The overarching goal of this review is to investigate how adiponectin signaling may ameliorate pathology associated with dystrophin deficiency through inflammatory-dependent and -independent mechanisms and to determine if current data supports their future progression to clinical trials.

Inhibition of miR-25 ameliorates cardiac and skeletal muscle dysfunction in aged mdx/utrn haploinsufficient (+/-) mice

Dystrophic cardiomyopathy is a significant feature of Duchenne muscular dystrophy (DMD). Increased cardiomyocyte cytosolic calcium (Ca2+) and interstitial fibrosis are major pathophysiological hallmarks that ultimately result in cardiac dysfunction. MicroRNA-25 (miR-25) has been identified as a suppressor of both sarcoplasmic reticulum calcium ATPase 2a (SERCA2a) and mothers against decapentaplegic homolog-7 (Smad7) proteins. In this study, we created a gene transfer using an miR-25 tough decoy (TuD) RNA inhibitor delivered via recombinant adeno-associated virus serotype 9 (AAV9) to evaluate the effect of miR-25 inhibition on cardiac and skeletal muscle function in aged dystrophin/utrophin haploinsufficient mice mdx/utrn (+/-), a validated transgenic murine model of DMD. We found that the intravenous delivery of AAV9 miR-25 TuD resulted in strong and stable inhibition of cardiac miR-25 levels, together with the restoration of SERCA2a and Smad7 expression. This was associated with the amelioration of cardiomyocyte interstitial fibrosis as well as recovered cardiac function. Furthermore, the direct quadricep intramuscular injection of AAV9 miR-25 TuD significantly restored skeletal muscle Smad7 expression, reduced tissue fibrosis, and enhanced skeletal muscle performance in mdx/utrn (+/-) mice. These results imply that miR-25 TuD gene transfer may be a novel therapeutic approach to restore cardiomyocyte Ca2+ homeostasis and abrogate tissue fibrosis in DMD.

Three-dimensional imaging studies in mice identify cellular dynamics of skeletal muscle regeneration

The function of many organs, including skeletal muscle, depends on their three-dimensional structure. Muscle regeneration therefore requires not only reestablishment of myofibers but also restoration of tissue architecture. Resident muscle stem cells (SCs) are essential for regeneration, but how SCs regenerate muscle architecture is largely unknown. We address this problem using genetic labeling of mouse SCs and whole-mount imaging to reconstruct, in three dimensions, muscle regeneration. Unexpectedly, we found that myofibers form via two distinct phases of fusion and the residual basement membrane of necrotic myofibers is critical for promoting fusion and orienting regenerated myofibers. Furthermore, the centralized myonuclei characteristic of regenerated myofibers are associated with myofibrillogenesis and endure months post injury. Finally, we elucidate two cellular mechanisms for the formation of branched myofibers, a pathology characteristic of diseased muscle. We provide a synthesis of the cellular events of regeneration and show that these differ from those used during development.

A Novel MAO-B/SSAO Inhibitor Improves Multiple Aspects of Dystrophic Phenotype in mdx Mice

Duchenne muscular dystrophy (DMD) is one of the most frequent and severe childhood muscle diseases. Its pathophysiology is multifaceted and still incompletely understood, but we and others have previously shown that oxidative stress plays an important role. In particular, we have demonstrated that inhibition of mitochondrial monoamine oxidases could improve some functional and biohumoral markers of the pathology. In the present study we report the use of dystrophic mdx mice to evaluate the efficacy of a dual monoamine oxidase B (MAO-B)/semicarbazide-sensitive amine oxidase (SSAO) inhibitor, PXS-5131, in reducing inflammation and fibrosis and improving muscle function. We found that a one-month treatment starting at three months of age was able to decrease reactive oxygen species (ROS) production, fibrosis, and inflammatory infiltrate in the tibialis anterior (TA) and diaphragm muscles. Importantly, we also observed a marked improvement in the capacity of the gastrocnemius muscle to maintain its force when challenged with eccentric contractions. Upon performing a bulk RNA-seq analysis, PXS-5131 treatment affected the expression of genes involved in inflammatory processes and tissue remodeling. We also studied the effect of prolonged treatment in older dystrophic mice, and found that a three-month administration of PXS-5131 was able to greatly reduce the progression of fibrosis not only in the diaphragm but also in the heart. Taken together, these results suggest that PXS-5131 is an effective inhibitor of fibrosis and inflammation in dystrophic muscles, a finding that could open a new therapeutic avenue for DMD patients.

Circadian Clock in Muscle Disease Etiology and Therapeutic Potential for Duchenne Muscular Dystrophy

Circadian clock and clock-controlled output pathways exert temporal control in diverse aspects of skeletal muscle physiology, including the maintenance of muscle mass, structure, function, and metabolism. They have emerged as significant players in understanding muscle disease etiology and potential therapeutic avenues, particularly in Duchenne muscular dystrophy (DMD). This review examines the intricate interplay between circadian rhythms and muscle physiology, highlighting how disruptions of circadian regulation may contribute to muscle pathophysiology and the specific mechanisms linking circadian clock dysregulation with DMD. Moreover, we discuss recent advancements in chronobiological research that have shed light on the circadian control of muscle function and its relevance to DMD. Understanding clock output pathways involved in muscle mass and function offers novel insights into the pathogenesis of DMD and unveils promising avenues for therapeutic interventions. We further explore potential chronotherapeutic strategies targeting the circadian clock to ameliorate muscle degeneration which may inform drug development efforts for muscular dystrophy.

Late-Stage Skeletal Muscle Transcriptome in Duchenne muscular dystrophy shows a BMP4-Induced Molecular Signature

Duchenne muscular dystrophy (DMD) is a fatal X-linked recessive disease due to loss-of-function mutations in the DYSTROPHIN gene. DMD-related skeletal muscle wasting is typified by an aberrant immune response involving upregulation of TGFβ family of cytokines. We previously demonstrated that bone morphogenetic protein 4 (BMP4) is increased in DMD and BMP4 stimulation induces a 20-fold upregulation of Smad8 transcription. However, the role of BMP4 in severely affected DMD skeletal muscle is unknown. We hypothesized that transcriptomic signatures in severely affected human DMD skeletal muscle are driven by BMP4 signaling. Transcriptomes from skeletal muscle biopsies of late-stage DMD vs. non-DMD controls and C2C12 muscle cells with or without BMP4 stimulation were generated by RNA-Seq and analyzed for single transcript differential expression as well as by Ingenuity Pathway Analysis and weighted gene co-expression network analyses. A total of 2,328 and 5,291 transcripts in the human muscle and C2C12 muscle cells, respectively, were differentially expressed. We identified an overlapping molecular signature of 1,027 genes dysregulated in DMD muscle that were induced in BMP4-stimulated C2C12 muscle cells. Highly upregulated DMD transcripts that overlapped with BMP4-stimulated C2C12 muscle cells included ADAMTS3, HCAR2, SERPING1, SMAD8 , and UNC13C. The DMD transcriptome was characterized by dysregulation of pathways involving immune function, extracellular matrix remodeling, and metabolic/mitochondrial function. In summary, we define a late-stage DMD skeletal muscle transcriptome that substantially overlaps with the BMP4-induced molecular signature in C2C12 muscle cells. This supports BMP4 as a disease-driving regulator of transcriptomic changes in late-stage DMD skeletal muscle and expands our understanding of the evolution of dystrophic signaling pathways and their associated gene networks that could be explored for therapeutic development.

Oncostatin M signaling drives cancer-associated skeletal muscle wasting

Progressive weakness and muscle loss are associated with multiple chronic conditions, including muscular dystrophy and cancer. Cancer-associated cachexia, characterized by dramatic weight loss and fatigue, leads to reduced quality of life and poor survival. Inflammatory cytokines have been implicated in muscle atrophy; however, available anticytokine therapies failed to prevent muscle wasting in cancer patients. Here, we show that oncostatin M (OSM) is a potent inducer of muscle atrophy. OSM triggers cellular atrophy in primary myotubes using the JAK/STAT3 pathway. Identification of OSM targets by RNA sequencing reveals the induction of various muscle atrophy-related genes, including Atrogin1. OSM overexpression in mice causes muscle wasting, whereas muscle-specific deletion of the OSM receptor (OSMR) and the neutralization of circulating OSM preserves muscle mass and function in tumor-bearing mice. Our results indicate that activated OSM/OSMR signaling drives muscle atrophy, and the therapeutic targeting of this pathway may be useful in preventing muscle wasting.

Regenerative inflammation: When immune cells help to re-build tissues

Inflammation is an essential immune response critical for responding to infection, injury and maintenance of tissue homeostasis. Upon injury, regenerative inflammation promotes tissue repair by a timed and coordinated infiltration of diverse cell types and the secretion of growth factors, cytokines and lipids mediators. Remarkably, throughout evolution as well as mammalian development, this type of physiological inflammation is highly associated with immunosuppression. For instance, regenerative inflammation is the consequence of an in situ macrophage polarization resulting in a transition from pro-inflammatory to anti-inflammatory/pro-regenerative response. Immune cells are the first responders upon injury, infiltrating the damaged tissue and initiating a pro-inflammatory response depleting cell debris and necrotic cells. After phagocytosis, macrophages undergo multiple coordinated metabolic and transcriptional changes allowing the transition and dictating the initiation of the regenerative phase. Differences between a highly efficient, complete ad integrum tissue repair, such as, acute skeletal muscle injury, and insufficient regenerative inflammation, as the one developing in Duchenne Muscular Dystrophy (DMD), highlight the importance of a coordinated response orchestrated by immune cells. During regenerative inflammation, these cells interact with others and alter the niche, affecting the character of inflammation itself and, therefore, the progression of tissue repair. Comparing acute muscle injury and chronic inflammation in DMD, we review how the same cells and molecules in different numbers, concentration and timing contribute to very different outcomes. Thus, it is important to understand and identify the distinct functions and secreted molecules of macrophages, and potentially other immune cells, during tissue repair, and the contributors to the macrophage switch leveraging this knowledge in treating diseases.

A review on mechanistic insights into structure and function of dystrophin protein in pathophysiology and therapeutic targeting of Duchenne muscular dystrophy

Duchenne Muscular Dystrophy (DMD) is an X-linked recessive genetic disorder characterized by progressive and severe muscle weakening and degeneration. Among the various forms of muscular dystrophy, it stands out as one of the most common and impactful, predominantly affecting boys. The condition arises due to mutations in the dystrophin gene, a key player in maintaining the structure and function of muscle fibers. The manuscript explores the structural features of dystrophin protein and their pivotal roles in DMD. We present an in-depth analysis of promising therapeutic approaches targeting dystrophin and their implications for the therapeutic management of DMD. Several therapies aiming to restore dystrophin protein or address secondary pathology have obtained regulatory approval, and many others are ongoing clinical development. Notably, recent advancements in genetic approaches have demonstrated the potential to restore partially functional dystrophin forms. The review also provides a comprehensive overview of the status of clinical trials for major therapeutic genetic approaches for DMD. In addition, we have summarized the ongoing therapeutic approaches and advanced mechanisms of action for dystrophin restoration and the challenges associated with DMD therapeutics.

Retention of stress susceptibility in the mdx mouse model of Duchenne muscular dystrophy after PGC-1α overexpression or ablation of IDO1 or CD38

Duchenne muscular dystrophy (DMD) is a lethal degenerative muscle wasting disease caused by the loss of the structural protein dystrophin with secondary pathological manifestations including metabolic dysfunction, mood and behavioral disorders. In the mildly affected mdx mouse model of DMD, brief scruff stress causes inactivity, while more severe subordination stress results in lethality. Here, we investigated the kynurenine pathway of tryptophan degradation and the nicotinamide adenine dinucleotide (NAD+) metabolic pathway in mdx mice and their involvement as possible mediators of mdx stress-related pathology. We identified downregulation of the kynurenic acid shunt, a neuroprotective branch of the kynurenine pathway, in mdx skeletal muscle associated with attenuated peroxisome proliferator-activated receptor-gamma coactivator 1 alpha (PGC-1α) transcriptional regulatory activity. Restoring the kynurenic acid shunt by skeletal muscle-specific PGC-1α overexpression in mdx mice did not prevent scruff -induced inactivity, nor did abrogating extrahepatic kynurenine pathway activity by genetic deletion of the pathway rate-limiting enzyme, indoleamine oxygenase 1. We further show that reduced NAD+ production in mdx skeletal muscle after subordination stress exposure corresponded with elevated levels of NAD+ catabolites produced by ectoenzyme cluster of differentiation 38 (CD38) that have been implicated in lethal mdx response to pharmacological β-adrenergic receptor agonism. However, genetic CD38 ablation did not prevent mdx scruff-induced inactivity. Our data do not support a direct contribution by the kynurenine pathway or CD38 metabolic dysfunction to the exaggerated stress response of mdx mice.

Muscle stem cell dysfunction in rhabdomyosarcoma and muscular dystrophy

Muscle stem cells (MuSCs) are crucial to the repair and homeostasis of mature skeletal muscle. MuSC dysfunction and dysregulation of the myogenic program can contribute to the development of pathology ranging from cancers like rhabdomyosarcoma (RMS) or muscle degenerative diseases such as Duchenne muscular dystrophy (DMD). Both diseases exhibit dysregulation at nearly all steps of myogenesis. For instance, MuSC self-renewal processes are altered. In RMS, this leads to the creation of tumor propagating cells. In DMD, impaired asymmetric stem cell division creates a bias towards producing self-renewing stem cells instead of committing to differentiation. Hyperproliferation of these cells contribute to tumorigenesis in RMS and symmetric expansion of the self-renewing MuSC population in DMD. Both diseases also exhibit a repression of factors involved in terminal differentiation, halting RMS cells in the proliferative stage and thus driving tumor growth. Conversely, the MuSCs in DMD exhibit impaired differentiation and fuse prematurely, affecting myonuclei maturation and the integrity of the dystrophic muscle fiber. Finally, both disease states cause alterations to the MuSC niche. Various elements of the niche such as inflammatory and migratory signaling that impact MuSC behavior are dysregulated. Here we show how these seemingly distantly related diseases indeed have similarities in MuSC dysfunction, underlying the importance of considering MuSCs when studying the pathophysiology of muscle diseases.

Insight into the Role of Gut Microbiota in Duchenne Muscular Dystrophy: An Age-Related Study in Mdx Mice

Dystrophin deficiency alters the sarcolemma structure, leading to muscle dystrophy, muscle disuse, and ultimately death. Beyond limb muscle deficits, patients with Duchenne muscular dystrophy have numerous transit disorders. Many studies have highlighted the strong relationship between gut microbiota and skeletal muscle. The aims of this study were: i) to characterize the gut microbiota composition over time up to 1 year in dystrophin-deficient mdx mice, and ii) to analyze the intestine structure and function and expression of genes linked to bacterial-derived metabolites in ileum, blood, and skeletal muscles to study interorgan interactions. Mdx mice displayed a significant reduction in the overall number of different operational taxonomic units and their abundance (α-diversity). Mdx genotype predicted 20% of β-diversity divergence, with a large taxonomic modification of Actinobacteria, Proteobacteria, Tenericutes, and Deferribacteres phyla and the included genera. Interestingly, mdx intestinal motility and gene expressions of tight junction and Ffar2 receptor were down-regulated in the ileum. Concomitantly, circulating inflammatory markers related to gut microbiota (tumor necrosis factor, IL-6, monocyte chemoattractant protein-1) and muscle inflammation Tlr4/Myd88 pathway (Toll-like receptor 4, which recognizes pathogen-associated molecular patterns) were up-regulated. Finally, in mdx mice, adiponectin was reduced in blood and its receptor modulated in muscles. This study highlights a specific gut microbiota composition and highlights interorgan interactions in mdx physiopathology with gut microbiota as the potential central metabolic organ.

A phase 2 open-label study of the safety and efficacy of weekly dosing of ATL1102 in patients with non-ambulatory Duchenne muscular dystrophy and pharmacology in mdx mice

BACKGROUND

ATL1102 is a 2'MOE gapmer antisense oligonucleotide to the CD49d alpha subunit of VLA-4, inhibiting expression of CD49d on lymphocytes, reducing survival, activation and migration to sites of inflammation. Children with DMD have dystrophin deficient muscles susceptible to contraction induced injury, which triggers the immune system, exacerbating muscle damage. CD49d is a biomarker of disease severity in DMD, with increased numbers of high CD49d expressing T cells correlating with more severe and progressive weakess, despite corticosteroid treatment.

METHODS

This Phase 2 open label study assessed the safety, efficacy and pharmacokinetic profile of ATL1102 administered as 25 mg weekly by subcutaneous injection for 24 weeks in 9 non-ambulatory boys with DMD aged 10-18 years. The main objective was to assess safety and tolerability of ATL1102. Secondary objectives included the effect of ATL1102 on lymphocyte numbers in the blood, functional changes in upper limb function as assessed by Performance of Upper Limb test (PUL 2.0) and upper limb strength using MyoGrip and MyoPinch compared to baseline.

RESULTS

Eight out of nine participants were on a stable dose of corticosteroids. ATL1102 was generally safe and well tolerated. No serious adverse events were reported. There were no participant withdrawals from the study. The most commonly reported adverse events were injection site erythema and skin discoloration. There was no statistically significant change in lymphocyte count from baseline to week 8, 12 or 24 of dosing however, the CD3+CD49d+ T lymphocytes were statistically significantly higher at week 28 compared to week 24, four weeks past the last dose (mean change 0.40x109/L 95%CI 0.05, 0.74; p = 0.030). Functional muscle strength, as measured by the PUL2.0, EK2 and Myoset grip and pinch measures, and MRI fat fraction of the forearm muscles were stable throughout the trial period.

CONCLUSION

ATL1102, a novel antisense drug being developed for the treatment of inflammation that exacerbates muscle fibre damage in DMD, appears to be safe and well tolerated in non-ambulant boys with DMD. The apparent stabilisation observed on multiple muscle disease progression parameters assessed over the study duration support the continued development of ATL1102 for the treatment of DMD.

TRIAL REGISTRATION

Clinical Trial Registration. Australian New Zealand Clinical Trials Registry Number: ACTRN12618000970246.

How Can Proteomics Help to Elucidate the Pathophysiological Crosstalk in Muscular Dystrophy and Associated Multi-System Dysfunction?

This perspective article is concerned with the question of how proteomics, which is a core technique of systems biology that is deeply embedded in the multi-omics field of modern bioresearch, can help us better understand the molecular pathogenesis of complex diseases. As an illustrative example of a monogenetic disorder that primarily affects the neuromuscular system but is characterized by a plethora of multi-system pathophysiological alterations, the muscle-wasting disease Duchenne muscular dystrophy was examined. Recent achievements in the field of dystrophinopathy research are described with special reference to the proteome-wide complexity of neuromuscular changes and body-wide alterations/adaptations. Based on a description of the current applications of top-down versus bottom-up proteomic approaches and their technical challenges, future systems biological approaches are outlined. The envisaged holistic and integromic bioanalysis would encompass the integration of diverse omics-type studies including inter- and intra-proteomics as the core disciplines for systematic protein evaluations, with sophisticated biomolecular analyses, including physiology, molecular biology, biochemistry and histochemistry. Integrated proteomic findings promise to be instrumental in improving our detailed knowledge of pathogenic mechanisms and multi-system dysfunction, widening the available biomarker signature of dystrophinopathy for improved diagnostic/prognostic procedures, and advancing the identification of novel therapeutic targets to treat Duchenne muscular dystrophy.

Identifying hub genes and dysregulated pathways in Duchenne muscular dystrophy

PURPOSE

The aim of this study was to identify the hub genes and dysregulated pathways in the progression of duchenne muscular dystrophy (DMD) and to unveil detailedly the cellular and molecular mechanisms associated with DMD for developing efficacious treatments in the future.

MATERIAL AND METHODS

Three mRNA microarray datasets (GSE13608, GSE38417 and GSE109178) were downloaded from Gene Expression Omnibus (GEO). The differentially expressed genes (DEGs) between DMD and normal tissues were obtained via R package. Function enrichment analyses were implemented respectively using DAVID online database. The network analysis of protein-protein interaction network (PPI) was conducted using String. Cytoscape and String were used to analyse modules and screen hub genes. The expression of the identified hub genes was confirmed in mdx mice through using qRT-PCR.

RESULTS

In total, 519 DEGs were identified, consisting of 393 upregulated genes and 126 downregulated genes. The enriched functions and pathways of the DEGs mainly involve extracellular matrix organization, collagen fibril organization, interferon-gamma-mediated signaling pathway, muscle contraction, endoplasmic reticulum lumen, MHC class II receptor activity, phagosome, graft-versus-host disease, cardiomyocytes, calcium signaling pathway. Twelve hub genes were discovered and biological process analysis proved that these genes were mainly enriched cell cycle, cell division. The result of qRT-PCR suggested that increase in expression of CD44, ECT2, TYMS, MAGEL2, HLA-DMA, SERPINH1, TNNT2 was confirmed in mdx mice and the downregulation of ASB2 and LEPREL1 was also observed.

CONCLUSION

In conclusion, DEGs and hub genes identified in the current research help us probe the molecular mechanisms underlying the pathogenesis and progression of DMD, and provide candidate targets for diagnosis and treatment of DMD.

Evaluation of pro-regenerative and anti-inflammatory effects of isolecanoric acid in the muscle: Potential treatment of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a devastating degenerative disease of skeletal muscles caused by loss of dystrophin, a key protein that maintains muscle integrity, which leads to progressive muscle degeneration aggravated by chronic inflammation, muscle stem cells' (MuSCs) reduced regenerative capacity and replacement of muscle with fibroadipose tissue. Previous research has shown that pharmacological GSK-3β inhibition favors myogenic differentiation and plays an important role in modulating inflammatory processes. Isolecanoric acid (ILA) is a natural product isolated from a fungal culture displaying GSK-3β inhibitory properties. The present study aimed to investigate the proregenerative and anti-inflammatory properties of this natural compound in the DMD context. Our results showed that ILA markedly promotes myogenic differentiation of myoblasts by increasing β-Catenin signaling and boosting the myogenic potential of mouse and human stem cells. One important finding was that the GSK-3β/β-Catenin pathway is altered in dystrophic mice muscle and ILA enhances the myofiber formation of dystrophic MuSCs. Treatment with this natural compound improves muscle regeneration of dystrophic mice by, in turn, improving functional performance. Moreover, ILA ameliorates the inflammatory response in both muscle explants and the macrophages isolated from dystrophic mice to, thus, mitigate fibrosis after muscle damage. Overall, we show that ILA modulates both inflammation and muscle regeneration to, thus, contribute to improve the dystrophic phenotype.

A fat- and sucrose-enriched diet causes metabolic alterations in mdx mice

Duchenne muscular dystrophy (DMD), a progressive muscle disease caused by the absence of functional dystrophin protein, is associated with multiple cellular, physiological, and metabolic dysfunctions. As an added complication to the primary insult, obesity/insulin resistance (O/IR) is frequently reported in patients with DMD; however, how IR impacts disease severity is unknown. We hypothesized a high-fat, high-sucrose diet (HFHSD) would induce O/IR, exacerbate disease severity, and cause metabolic alterations in dystrophic mice. To test this hypothesis, we treated 7-wk-old mdx (disease model) and C57 mice with a control diet (CD) or an HFHSD for 15 wk. The HFHSD induced insulin resistance, glucose intolerance, and hyperglycemia in C57 and mdx mice. Of note, mdx mice on CD were also insulin resistant. In addition, visceral adipose tissue weights were increased with HFHSD in C57 and mdx mice though differed by genotype. Serum creatine kinase activity and histopathological analyses using Masson's trichrome staining in the diaphragm indicated muscle damage was driven by dystrophin deficiency but was not augmented by diet. In addition, markers of inflammatory signaling, mitochondrial abundance, and autophagy were impacted by disease but not diet. Despite this, in addition to disease signatures in CD-fed mice, metabolomic and lipidomic analyses demonstrated a HFHSD caused some common changes in C57 and mdx mice and some unique signatures of O/IR within the context of dystrophin deficiency. In total, these data revealed that in mdx mice, 15 wk of HFHSD did not overtly exacerbate muscle injury but further impaired the metabolic status of dystrophic muscle.

Beneficial immune-modulatory effects of the N-163 strain of Aureobasidium pullulans-produced 1,3-1,6 Beta glucans in Duchenne muscular dystrophy: Results of an open-label, prospective, exploratory case-control clinical study

Background

This exploratory case-control study is to evaluate the effects of supplementation of Aureobasidium pullulans-N-163 strain produced 1,3-1,- 6 beta glucan in young patients with Duchenne muscular dystrophy (DMD).

Methods

Twenty-seven male subjects aged 5-19 years with DMD were included, nine in the control arm and 18 in the treatment arm to receive N-163 beta glucan along with conventional therapies for 45 days. While performing the analysis, steroid usage was also taken into consideration, those not administered steroids (Steroid -ve) (Control, n = 5; treatment, n = 9), those administered steroids (Steroid +ve) (Control, n = 4; treatment, n = 9).

Results

IL-6 showed a significant decrease in the treatment groups, especially the N-163 Steroid -ve group. IL-13 decreased in both treatment groups and TGF-β levels showed a significant decrease in the treatment groups, especially the N-163 Steroid -ve group, (p < 0.05). Dystrophin levels increased by up to 32% in the treatment groups compared to the control. Medical research council (MRC) grading showed slight improvement in muscle strength improvement in 12 out of 18 patients (67%) in the treatment group and four out of nine (44%) subjects in the control group.

Conclusion

Supplementation with the N-163 beta glucan food supplement produced beneficial effects: a significant decrease in inflammation and fibrosis markers, increase in serum dystrophin and slight improvement in muscle strength in DMD subjects over 45 days, thus making this a potential adjunct treatment for DMD after validation.

Trial registration

The study was registered in Clinical trials registry of India, CTRI/2021/05/033346. Registered on 5th May, 2021.

Lethal immunotoxicity in high-dose systemic AAV therapy

High-dose systemic gene therapy with adeno-associated virus (AAV) is in clinical trials to treat various inherited diseases. Despite remarkable success in spinal muscular atrophy and promising results in other diseases, fatality has been observed due to liver, kidney, heart, or lung failure. Innate and adaptive immune responses to the vector play a critical role in the toxicity. Host factors also contribute to patient death. This mini-review summarizes clinical findings and calls for concerted efforts from all stakeholders to better understand the mechanisms underlying lethality in AAV gene therapy and to develop effective strategies to prevent/treat high-dose systemic AAV-gene-therapy-induced immunotoxicity.

T cell biology in neuromuscular disorders: a focus on Duchenne Muscular Dystrophy and Amyotrophic Lateral Sclerosis

Growing evidence demonstrates a continuous interaction between the immune system, the nerve and the muscle in neuromuscular disorders of different pathogenetic origins, such as Duchenne Muscular Dystrophy (DMD) and Amyotrophic Lateral Sclerosis (ALS), the focus of this review. Herein we highlight the complexity of the cellular and molecular interactions involving the immune system in neuromuscular disorders, as exemplified by DMD and ALS. We describe the distinct types of cell-mediated interactions, such as cytokine/chemokine production as well as cell-matrix and cell-cell interactions between T lymphocytes and other immune cells, which target cells of the muscular or nervous tissues. Most of these interactions occur independently of exogenous pathogens, through ligand-receptor binding and subsequent signal transduction cascades, at distinct levels of specificity. Although this issue reveals the complexity of the system, it can also be envisioned as a window of opportunity to design therapeutic strategies (including synthetic moieties, cell and gene therapy, as well as immunotherapy) by acting upon one or more targets. In this respect, we discuss ongoing clinical trials using VLA-4 inhibition in DMD, and in ALS, with a focus on regulatory T cells, both revealing promising results.

Cellular pathogenesis of Duchenne muscular dystrophy: progressive myofibre degeneration, chronic inflammation, reactive myofibrosis and satellite cell dysfunction

Duchenne muscular dystrophy is a highly progressive muscle wasting disease of early childhood and characterized by complex pathophysiological and histopathological changes in the voluntary contractile system, including myonecrosis, chronic inflammation, fat substitution and reactive myofibrosis. The continued loss of functional myofibres and replacement with non-contractile cells, as well as extensive tissue scarring and decline in tissue elasticity, leads to severe skeletal muscle weakness. In addition, dystrophic muscles exhibit a greatly diminished regenerative capacity to counteract the ongoing process of fibre degeneration. In normal muscle tissues, an abundant stem cell pool consisting of satellite cells that are localized between the sarcolemma and basal lamina, provides a rich source for the production of activated myogenic progenitor cells that are involved in efficient myofibre repair and tissue regeneration. Interestingly, the self-renewal of satellite cells for maintaining an essential pool of stem cells in matured skeletal muscles is increased in dystrophin-deficient fibres. However, satellite cell hyperplasia does not result in efficient recovery of dystrophic muscles due to impaired asymmetric cell divisions. The lack of expression of the full-length dystrophin isoform Dp427-M, which is due to primary defects in the DMD gene,  appears to affect key regulators of satellite cell polarity causing a reduced differentiation of myogenic progenitors, which are essential for myofibre regeneration. This review outlines the complexity of dystrophinopathy and describes the importance of the pathophysiological role of satellite cell dysfunction. A brief discussion of the bioanalytical usefulness of single cell proteomics for future studies of satellite cell biology is provided.

Anti-RANKL Therapy Prevents Glucocorticoid-Induced Bone Loss and Promotes Muscle Function in a Mouse Model of Duchenne Muscular Dystrophy

Bisphosphonates prevent bone loss in glucocorticoid (GC)-treated boys with Duchenne muscular dystrophy (DMD) and are recommended as standard of care. Targeting receptor activator of nuclear factor kappa-B ligand (RANKL) may have advantages in DMD by ameliorating dystrophic skeletal muscle function in addition to their bone anti-resorptive properties. However, the potential effects of anti-RANKL treatment upon discontinuation in GC-induced animal models of DMD are unknown and need further investigation prior to exploration in the clinical research setting. In the first study, the effects of anti-RANKL and deflazacort (DFZ) on dystrophic skeletal muscle function and bone microstructure were assessed in mdx mice treated with DFZ or anti-RANKL, or both for 8 weeks. Anti-RANKL and DFZ improved grip force performance of mdx mice but an additive effect was not noted. However, anti-RANKL but not DFZ improved ex vivo contractile properties of dystrophic muscles. This functional improvement was associated with a reduction in muscle damage and fibrosis, and inflammatory cell number. Anti-RANKL treatment, with or without DFZ, also improved trabecular bone structure of mdx mice. In a second study, intravenous zoledronate (Zol) administration (1 or 2 doses) following 2 months of discontinuation of anti-RANKL treatment was mostly required to record an improvement in bone microarchitecture and biomechanical properties in DFZ-treated mdx mice. In conclusion, the ability of anti-RANKL therapy to restore muscle function has profound implications for DMD patients as it offers the possibility of improving skeletal muscle function without the steroid-related skeletal side effects.

Imaging in inflammatory arthritis: progress towards precision medicine

Imaging techniques such as ultrasonography and MRI have gained ground in the diagnosis and management of inflammatory arthritis, as these imaging modalities allow a sensitive assessment of musculoskeletal inflammation and damage. However, these techniques cannot discriminate between disease subsets and are currently unable to deliver an accurate prediction of disease progression and therapeutic response in individual patients. This major shortcoming of today's technology hinders a targeted and personalized patient management approach. Technological advances in the areas of high-resolution imaging (for example, high-resolution peripheral quantitative computed tomography and ultra-high field MRI), functional and molecular-based imaging (such as chemical exchange saturation transfer MRI, positron emission tomography, fluorescence optical imaging, optoacoustic imaging and contrast-enhanced ultrasonography) and artificial intelligence-based data analysis could help to tackle these challenges. These new imaging approaches offer detailed anatomical delineation and an in vivo and non-invasive evaluation of the immunometabolic status of inflammatory reactions, thereby facilitating an in-depth characterization of inflammation. By means of these developments, the aim of earlier diagnosis, enhanced monitoring and, ultimately, a personalized treatment strategy looms closer.

Mimicking sarcolemmal damagein vitro: a contractile 3D model of skeletal muscle for drug testing in Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is the most prevalent neuromuscular disease diagnosed in childhood. It is a progressive and wasting disease, characterized by a degeneration of skeletal and cardiac muscles caused by the lack of dystrophin protein. The absence of this crucial structural protein leads to sarcolemmal fragility, resulting in muscle fiber damage during contraction. Despite ongoing efforts, there is no cure available for DMD patients. One of the primary challenges is the limited efficacy of current preclinical tools, which fail in modeling the biological complexity of the disease. Human-based three-dimensional (3D) cell culture methods appear as a novel approach to accelerate preclinical research by enhancing the reproduction of pathophysiological processes in skeletal muscle. In this work, we developed a patient-derived functional 3D skeletal muscle model of DMD that reproduces the sarcolemmal damage found in the native DMD muscle. These bioengineered skeletal muscle tissues exhibit contractile functionality, as they responded to electrical pulse stimulation. Sustained contractile regimes induced the loss of myotube integrity, mirroring the pathological myotube breakdown inherent in DMD due to sarcolemmal instability. Moreover, damaged DMD tissues showed disease functional phenotypes, such as tetanic fatigue. We also evaluated the therapeutic effect of utrophin upregulator drug candidates on the functionality of the skeletal muscle tissues, thus providing deeper insight into the real impact of these treatments. Overall, our findings underscore the potential of bioengineered 3D skeletal muscle technology to advance DMD research and facilitate the development of novel therapies for DMD and related neuromuscular disorders.

In vivo shear wave elasticity imaging for assessment of diaphragm function in muscular dystrophy

Duchenne muscular dystrophy (DMD) causes patients to suffer from ambulatory disability and cardiorespiratory failure, the latter of which leads to premature death. Due to its role in respiration, the diaphragm is an important muscle for study. A common method for evaluating diaphragm function is ex vivo force testing, which only allows for an end point measurement. In contrast, ultrasound shear wave elastography imaging (US-SWEI) can assess diaphragm function over time; however, US-SWEI studies in dystrophic patients to date have focused on the limbs without preclinical studies. In this work, we used US-SWEI to estimate the shear wave speed (SWS) in diaphragm muscles of healthy (WT) mice, mdx mice, and mdx mice haploinsufficient for utrophin (mdx-utr) at 6 and 12 months of age. Diaphragms were then subjected to ex vivo force testing and histological analysis at 12 months of age. Between 6 and 12 months, a 23.8% increase in SWS was observed in WT mice and a 27.8% increase in mdx mice, although no significant difference was found in mdx-utr mice. Specific force generated by mdx-utr diaphragms was lower than that of WT diaphragms following twitch stimulus. A strong correlation between SWS and collagen deposition was observed, as well as between SWS and muscle fiber size. Together, these data demonstrate the ability of US-SWEI to evaluate dystrophic diaphragm functionality over time and predict the biochemical and morphological make-up of the diaphragm. Additionally, our results highlight the advantage of US-SWEI over ex vivo testing by obtaining longitudinal measurements in the same subject. STATEMENT OF SIGNIFICANCE: In DMD patients, muscles experience cycles of regeneration and degeneration that contribute to chronic inflammation and muscle weakness. This pathology only worsens with time and leads to muscle wasting, including in respiratory and cardiac muscles. Because respiratory failure is a major contributor to premature death in DMD patients, the diaphragm muscle is an important muscle to evaluate and treat over time. Currently, diaphragm function is assessed using ex vivo force testing, a technique that only allows measurement at sacrifice. In contrast, ultrasonography, particularly shear wave elasticity imaging (USSWEI), is a promising tool for longitudinal assessment; however, most US-SWEI in DMD patients aimed for limb muscles only with the absence of preclinical studies. This work broadens the applications of US-SWE imaging by demonstrating its ability to track properties and function of dystrophic diaphragm muscles longitudinally in multiple dystrophic mouse models.

Sodium hydrosulfide moderately alleviates the hallmark symptoms of Duchenne muscular dystrophy in mdx mice

Duchenne muscular dystrophy (DMD) is an incurable disease caused by mutations in the X-linked DMD gene that encodes a structural muscle protein, dystrophin. This, in turn, leads to progressive degeneration of the skeletal muscles and the heart. Hydrogen sulfide (H2S), the pleiotropic agent with antioxidant, anti-inflammatory, and pro-angiogenic activities, could be considered a promising therapeutic factor for DMD. In this work, we studied the effect of daily intraperitoneal administration of the H2S donor, sodium hydrosulfide (NaHS, 100 μmol/kg/day for 5 weeks) on skeletal muscle (gastrocnemius, diaphragm and tibialis anterior) pathology in dystrophin-deficient mdx mice, characterized by decreased expression of H2S-generating enzymes. NaHS reduced the level of muscle damage markers in plasma (creatine kinase, lactate dehydrogenase and osteopontin). It lowered oxidative stress by affecting the GSH/GSSG ratio, up-regulating the level of cytoprotective heme oxygenase-1 (HO-1) and down-regulating the NF-κB pathway. In the gastrocnemius muscle, it also increased angiogenic vascular endothelial growth factor (Vegf) and its receptor (Kdr) expression, accompanied by the elevated number of α-SMA/CD31/lectin-positive blood vessels. The expression of fibrotic regulators, like Tgfβ, Col1a1 and Fn1 was decreased by NaHS in the tibialis anterior, while the level of autophagy markers (AMPKα signalling and Atg genes), was mostly affected in the gastrocnemius. Histological and molecular analysis showed no effect of H2S donor on regeneration and the muscle fiber type composition. Overall, the H2S donor modified the gene expression and protein level of molecules associated with the pathophysiology of DMD, contributing to the regulation of oxidative stress, inflammation, autophagy, and angiogenesis.

Microbes, metabolites and muscle: Is the gut-muscle axis a plausible therapeutic target in Duchenne muscular dystrophy?

NEW FINDINGS

What is the topic of this review? The contribution of gut microbial signalling to skeletal muscle maintenance and development and identification of potential therapeutic targets in progressive muscle degenerative diseases such as Duchenne muscular dystrophy. What advances does it highlight? Gut microbe-derived metabolites are multifaceted signalling molecules key to muscle function, modifying pathways contributing to skeletal muscle wasting, making them a plausible target for adjunctive therapy in muscular dystrophy.

ABSTRACT

Skeletal muscle is the largest metabolic organ making up ∼50% of body mass. Because skeletal muscle has both metabolic and endocrine properties, it can manipulate the microbial populations within the gut. In return, microbes exert considerable influence on skeletal muscle via numerous signalling pathways. Gut bacteria produce metabolites (i.e., short chain fatty acids, secondary bile acids and neurotransmitter substrates) that act as fuel sources and modulators of inflammation, influencing host muscle development, growth and maintenance. The reciprocal interactions between microbes, metabolites and muscle establish a bidirectional gut-muscle axis. The muscular dystrophies constitute a broad range of disorders with varying disabilities. In the profoundly debilitating monogenic disorder Duchenne muscular dystrophy (DMD), skeletal muscle undergoes a reduction in muscle regenerative capacity leading to progressive muscle wasting, resulting in fibrotic remodelling and adipose infiltration. The loss of respiratory muscle in DMD culminates in respiratory insufficiency and eventually premature death. The pathways contributing to aberrant muscle remodelling are potentially modulated by gut microbial metabolites, thus making them plausible targets for pre- and probiotic supplementation. Prednisone, the gold standard therapy for DMD, drives gut dysbiosis, inducing a pro-inflammatory phenotype and leaky gut barrier contributing to several of the well-known side effects associated with chronic glucocorticoid treatment. Several studies have observed that gut microbial supplementation or transplantation exerts positive effects on muscle, including mitigating the side effects of prednisone. There is growing evidence in support of the potential for an adjunctive microbiota-directed regimen designed to optimise gut-muscle axis signalling, which could alleviate muscle wasting in DMD.

Nanoparticles systemically biodistribute to regenerating skeletal muscle in DMD

Skeletal muscle disease severity can often progress asymmetrically across muscle groups and heterogeneously within tissues. An example is Duchenne Muscular Dystrophy (DMD) in which lack of dystrophin results in devastating skeletal muscle wasting in some muscles whereas others are spared or undergo hypertrophy. An efficient, non-invasive approach to identify sites of asymmetry and degenerative lesions could enable better patient monitoring and therapeutic targeting of disease. In this study, we utilized a versatile intravenously injectable mesoporous silica nanoparticle (MSNP) based nanocarrier system to explore mechanisms of biodistribution in skeletal muscle of mdx mouse models of DMD including wildtype, dystrophic, and severely dystrophic mice. Moreover, MSNPs could be imaged in live mice and whole muscle tissues enabling investigation of how biodistribution is altered by different types of muscle pathology such as inflammation or fibrosis. We found MSNPs were tenfold more likely to aggregate within select mdx muscles relative to wild type, such as gastrocnemius and quadriceps. This was accompanied by decreased biodistribution in off-target organs. We found the greatest factor affecting preferential delivery was the regenerative state of the dystrophic skeletal muscle with the highest MSNP abundance coinciding with the regions showing the highest level of embryonic myosin staining and intramuscular macrophage uptake. To demonstrate, muscle regeneration regulated MSNP distribution, we experimentally induced regeneration using barium chloride which resulted in a threefold increase of intravenously injected MSNPs to sites of regeneration 7 days after injury. These discoveries provide the first evidence that nanoparticles have selective biodistribution to skeletal muscle in DMD to areas of active regeneration and that nanoparticles could enable diagnostic and selective drug delivery in DMD skeletal muscle.

A cellular and molecular spatial atlas of dystrophic muscle

Asynchronous skeletal muscle degeneration/regeneration is a hallmark feature of Duchenne muscular dystrophy (DMD); however, traditional -omics technologies that lack spatial context make it difficult to study the biological mechanisms of how asynchronous regeneration contributes to disease progression. Here, using the severely dystrophic D2-mdx mouse model, we generated a high-resolution cellular and molecular spatial atlas of dystrophic muscle by integrating spatial transcriptomics and single-cell RNAseq datasets. Unbiased clustering revealed nonuniform distribution of unique cell populations throughout D2-mdx muscle that were associated with multiple regenerative timepoints, demonstrating that this model faithfully recapitulates the asynchronous regeneration observed in human DMD muscle. By probing spatiotemporal gene expression signatures, we found that propagation of inflammatory and fibrotic signals from locally damaged areas contributes to widespread pathology and that querying expression signatures within discrete microenvironments can identify targetable pathways for DMD therapy. Overall, this spatial atlas of dystrophic muscle provides a valuable resource for studying DMD disease biology and therapeutic target discovery.

Extracellular Matrix Proteomics: The mdx-4cv Mouse Diaphragm as a Surrogate for Studying Myofibrosis in Dystrophinopathy

The progressive degeneration of the skeletal musculature in Duchenne muscular dystrophy is accompanied by reactive myofibrosis, fat substitution, and chronic inflammation. Fibrotic changes and reduced tissue elasticity correlate with the loss in motor function in this X-chromosomal disorder. Thus, although dystrophinopathies are due to primary abnormalities in the DMD gene causing the almost-complete absence of the cytoskeletal Dp427-M isoform of dystrophin in voluntary muscles, the excessive accumulation of extracellular matrix proteins presents a key histopathological hallmark of muscular dystrophy. Animal model research has been instrumental in the characterization of dystrophic muscles and has contributed to a better understanding of the complex pathogenesis of dystrophinopathies, the discovery of new disease biomarkers, and the testing of novel therapeutic strategies. In this article, we review how mass-spectrometry-based proteomics can be used to study changes in key components of the endomysium, perimysium, and epimysium, such as collagens, proteoglycans, matricellular proteins, and adhesion receptors. The mdx-4cv mouse diaphragm displays severe myofibrosis, making it an ideal model system for large-scale surveys of systematic alterations in the matrisome of dystrophic fibers. Novel biomarkers of myofibrosis can now be tested for their appropriateness in the preclinical and clinical setting as diagnostic, pharmacodynamic, prognostic, and/or therapeutic monitoring indicators.

Comprehensive analysis of m6A regulators characterized by the immune microenvironment in Duchenne muscular dystrophy

BACKGROUND

Duchenne muscular dystrophy (DMD) is an X-linked, incurable, degenerative neuromuscular disease that is exacerbated by secondary inflammation. N⁶-methyladenosine (m⁶A), the most common base modification of RNA, has pleiotropic immunomodulatory effects in many diseases. However, the role of m⁶A modification in the immune microenvironment of DMD remains elusive.

METHODS

Our study retrospectively analyzed the expression data of 56 muscle tissues from DMD patients and 26 from non-muscular dystrophy individuals. Based on single sample gene set enrichment analysis, immune cells infiltration was identified and the result was validated by flow cytometry analysis and immunohistochemical staining. Then, we described the features of genetic variation in 26 m⁶A regulators and explored their relationship with the immune mircoenvironment of DMD patients through a series of bioinformatical analysis. At last, we determined subtypes of DMD patients by unsupervised clustering analysis and characterized the molecular and immune characteristics in different subgroups.

RESULTS

DMD patients have a sophisticated immune microenvironment that is significantly different from non-DMD controls. Numerous m⁶A regulators were aberrantly expressed in the muscle tissues of DMD and inversely related to most muscle-infiltrating immune cell types and immune response-related signaling pathways. A diagnostic model involving seven m⁶A regulators was established using LASSO. Furthermore, we determined three m⁶A modification patterns (cluster A/B/C) with distinct immune microenvironmental characteristics.

CONCLUSION

In summary, our study demonstrated that m⁶A regulators are intimately linked to the immune microenvironment of muscle tissues in DMD. These findings may facilitate a better understanding of the immunomodulatory mechanisms in DMD and provide novel strategies for the treatment.

Altered muscle niche contributes to myogenic deficit in the D2-mdx model of severe DMD

Lack of dystrophin expression is the underlying genetic basis for Duchenne muscular dystrophy (DMD). However, disease severity varies between patients, based on specific genetic modifiers. D2-mdx is a model for severe DMD that exhibits exacerbated muscle degeneration and failure to regenerate even in the juvenile stage of the disease. We show that poor regeneration of juvenile D2-mdx muscles is associated with an enhanced inflammatory response to muscle damage that fails to resolve efficiently and supports the excessive accumulation of fibroadipogenic progenitors (FAPs), leading to increased fibrosis. Unexpectedly, the extent of damage and degeneration in juvenile D2-mdx muscle is significantly reduced in adults, and is associated with the restoration of the inflammatory and FAP responses to muscle injury. These improvements enhance regenerative myogenesis in the adult D2-mdx muscle, reaching levels comparable to the milder B10-mdx model of DMD. Ex vivo co-culture of healthy satellite cells (SCs) with juvenile D2-mdx FAPs reduces their fusion efficacy. Wild-type juvenile D2 mice also manifest regenerative myogenic deficit and glucocorticoid treatment improves their muscle regeneration. Our findings indicate that aberrant stromal cell responses contribute to poor regenerative myogenesis and greater muscle degeneration in juvenile D2-mdx muscles and reversal of this reduces pathology in adult D2-mdx muscle, identifying these responses as a potential therapeutic target for the treatment of DMD.

Is it time for genetic modifiers to predict prognosis in Duchenne muscular dystrophy?

Patients with Duchenne muscular dystrophy (DMD) show clinically relevant phenotypic variability, despite sharing the same primary biochemical defect (dystrophin deficiency). Factors contributing to this clinical variability include allelic heterogeneity (specific DMD mutations), genetic modifiers (trans-acting genetic polymorphisms) and variations in clinical care. Recently, a series of genetic modifiers have been identified, mostly involving genes and/or proteins that regulate inflammation and fibrosis - processes increasingly recognized as being causally linked with physical disability. This article reviews genetic modifier studies in DMD to date and discusses the effect of genetic modifiers on predicting disease trajectories (prognosis), clinical trial design and interpretation (inclusion of genotype-stratified subgroup analyses) and therapeutic approaches. The genetic modifiers identified to date underscore the importance of progressive fibrosis, downstream of dystrophin deficiency, in driving the disease process. As such, genetic modifiers have shown the importance of therapies aimed at slowing this fibrotic process and might point to key drug targets.

Duchenne muscular dystrophy: disease mechanism and therapeutic strategies

Duchenne muscular dystrophy (DMD) is a severe, progressive, and ultimately fatal disease of skeletal muscle wasting, respiratory insufficiency, and cardiomyopathy. The identification of the dystrophin gene as central to DMD pathogenesis has led to the understanding of the muscle membrane and the proteins involved in membrane stability as the focal point of the disease. The lessons learned from decades of research in human genetics, biochemistry, and physiology have culminated in establishing the myriad functionalities of dystrophin in striated muscle biology. Here, we review the pathophysiological basis of DMD and discuss recent progress toward the development of therapeutic strategies for DMD that are currently close to or are in human clinical trials. The first section of the review focuses on DMD and the mechanisms contributing to membrane instability, inflammation, and fibrosis. The second section discusses therapeutic strategies currently used to treat DMD. This includes a focus on outlining the strengths and limitations of approaches directed at correcting the genetic defect through dystrophin gene replacement, modification, repair, and/or a range of dystrophin-independent approaches. The final section highlights the different therapeutic strategies for DMD currently in clinical trials.

Trained immunity as a potential target for therapeutic immunomodulation in Duchenne muscular dystrophy

Dysregulated inflammation involving innate immune cells, particularly of the monocyte/macrophage lineage, is a key contributor to the pathogenesis of Duchenne muscular dystrophy (DMD). Trained immunity is an evolutionarily ancient protective mechanism against infection, in which epigenetic and metabolic alterations confer non-specific hyperresponsiveness of innate immune cells to various stimuli. Recent work in an animal model of DMD (mdx mice) has shown that macrophages exhibit cardinal features of trained immunity, including the presence of innate immune system "memory". The latter is reflected by epigenetic changes and durable transmissibility of the trained phenotype to healthy non-dystrophic mice by bone marrow transplantation. Mechanistically, it is suggested that a Toll-like receptor (TLR) 4-regulated, memory-like capacity of innate immunity is induced at the level of the bone marrow by factors released from the damaged muscles, leading to exaggerated upregulation of both pro- and anti-inflammatory genes. Here we propose a conceptual framework for the involvement of trained immunity in DMD pathogenesis and its potential to serve as a new therapeutic target.

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.

A Protocol for Simultaneous In Vivo Imaging of Cardiac and Neuroinflammation in Dystrophin-Deficient MDX Mice Using [18F]FEPPA PET

Duchenne muscular dystrophy (DMD) is a neuromuscular disorder caused by dystrophin loss-notably within muscles and the central neurons system. DMD presents as cognitive weakness, progressive skeletal and cardiac muscle degeneration until pre-mature death from cardiac or respiratory failure. Innovative therapies have improved life expectancy; however, this is accompanied by increased late-onset heart failure and emergent cognitive degeneration. Thus, better assessment of dystrophic heart and brain pathophysiology is needed. Chronic inflammation is strongly associated with skeletal and cardiac muscle degeneration; however, neuroinflammation's role is largely unknown in DMD despite being prevalent in other neurodegenerative diseases. Here, we present an inflammatory marker translocator protein (TSPO) positron emission tomography (PET) protocol for in vivo concomitant assessment of immune cell response in hearts and brains of a dystrophin-deficient mouse model [mdx:utrn(+/-)]. Preliminary analysis of whole-body PET imaging using the TSPO radiotracer, [¹⁸F]FEPPA in four mdx:utrn(+/-) and six wildtype mice are presented with ex vivo TSPO-immunofluorescence tissue staining. The mdx:utrn(+/-) mice showed significant elevations in heart and brain [¹⁸F]FEPPA activity, which correlated with increased ex vivo fluorescence intensity, highlighting the potential of TSPO-PET to simultaneously assess presence of cardiac and neuroinflammation in dystrophic heart and brain, as well as in several organs within a DMD model.

Altered muscle niche contributes to myogenic deficit in the D2- mdx model of severe DMD

Lack of dystrophin is the genetic basis for the Duchenne muscular dystrophy (DMD). However, disease severity varies between patients, based on specific genetic modifiers. D2- mdx is a model for severe DMD that exhibits exacerbated muscle degeneration and failure to regenerate even in the juvenile stage of the disease. We show that poor regeneration of juvenile D2- mdx muscles is associated with enhanced inflammatory response to muscle damage that fails to resolve efficiently and supports excessive accumulation of fibroadipogenic progenitors (FAPs). Unexpectedly, the extent of damage and degeneration of juvenile D2- mdx muscle is reduced in adults and is associated with the restoration of the inflammatory and FAP responses to muscle injury. These improvements enhance myogenesis in the adult D2- mdx muscle, reaching levels comparable to the milder (B10- mdx ) mouse model of DMD. Ex vivo co-culture of healthy satellite cells (SCs) with the juvenile D2- mdx FAPs reduced their fusion efficacy and in vivo glucocorticoid treatment of juvenile D2 mouse improved muscle regeneration. Our findings indicate that aberrant stromal cell response contributes to poor myogenesis and greater muscle degeneration in dystrophic juvenile D2- mdx muscles and reversal of this reduces pathology in adult D2- mdx mouse muscle, identifying these as therapeutic targets to treat dystrophic DMD muscles.

The Bi-directional Relationship Between Sleep and Inflammation in Muscular Dystrophies: A Narrative Review

Muscular dystrophies vary in presentation and severity, but are associated with profound disability in many people. Although characterised by muscle weakness and wasting, there is also a very high prevalence of sleep problems and disorders which have significant impacts on quality of life in these individuals. There are no curative therapies for muscular dystrophies, with the only options for patients being supportive therapies to aid with symptoms. Therefore, there is an urgent need for new therapeutic targets and a greater understanding of pathogenesis. Inflammation and altered immunity are factors which have prominent roles in some muscular dystrophies and emerging roles in others such as type 1 myotonic dystrophy, signifying a link to pathogenesis. Interestingly, there is also a strong link between inflammation/immunity and sleep. In this review, we will explore this link in the context of muscular dystrophies and how it may influence potential therapeutic targets and interventions.

Aberrant Adenosine Triphosphate Release and Impairment of P2Y2-Mediated Signaling in Sarcoglycanopathies

Sarcoglycanopathies, limb-girdle muscular dystrophies (LGMD) caused by genetic loss-of-function of the membrane proteins sarcoglycans (SGs), are characterized by progressive degeneration of skeletal muscle. In these disorders, muscle necrosis is associated with immune-mediated damage, whose triggering and perpetuating molecular mechanisms are not fully elucidated yet. Extracellular adenosine triphosphate (eATP) seems to represent a crucial factor, with eATP activating purinergic receptors. Indeed, in vivo blockade of the eATP/P2X7 purinergic pathway ameliorated muscle disease progression. P2X7 inhibition improved the dystrophic process by restraining the activity of P2X7 receptors on immune cells. Whether P2X7 blockade can display a direct action on muscle cells is not known yet. In this study, we investigated eATP effects in primary cultures of myoblasts isolated from patients with LGMDR3 (α-sarcoglycanopathy) and in immortalized cells isolated from a patient with LGMDR5 (γ-sarcoglycanopathy). Our results demonstrated that, owing to a reduced ecto-ATPase activity and/or an enhanced release of ATP, patient cells are exposed to increased juxtamembrane concentrations of eATP and display a higher susceptivity to eATP signals. The purinoceptor P2Y2, which proved to be overexpressed in patient cells, was identified as a pivotal receptor responsible for the enhanced ATP-induced or UTP-induced Ca²⁺ increase in affected myoblasts. Moreover, P2Y2 stimulation in LDMDR3 muscle cells induced chemotaxis of immune cells and release of interleukin-8. In conclusion, a higher eATP concentration and sensitivity in primary human muscle cells carrying different α-SG or γ-SG loss-of-function mutations indicate that eATP/P2Y2 is an enhanced signaling axis in cells from patients with α-/γ-sarcoglycanopathy. Understanding the basis of the innate immune-mediated damage associated with the dystrophic process may be critical in overcoming the immunologic hurdles associated with emerging gene therapies for these disorders.

Transcriptomic characterization of clinical skeletal muscle biopsy from late-onset Pompe patients

Pompe disease is a rare lysosomal storage disorder arising from recessive mutations in the acid α-glucosidase gene and resulting in the accumulation of glycogen, particularly in the cardiac and skeletal muscle. The current standard of care is administration of enzyme replacement therapy in the form of alglucosidase alfa or the recently approved avalglucosidase alfa. In order to better understand the underlying cellular processes that are disrupted in Pompe disease, we conducted gene expression analysis on skeletal muscle biopsies obtained from late-onset Pompe disease patients (LOPD) prior to treatment and following six months of enzyme replacement with avalglucosidase alfa. The LOPD patients had a distinct transcriptomic signature as compared to control patient samples, largely characterized by perturbations in pathways involved in lysosomal function and energy metabolism. Although patients were highly heterogeneous, they collectively exhibited a strong trend towards attenuation of the dysregulated genes following just six months of treatment. Notably, the enzyme replacement therapy had a strong stabilizing effect on gene expression, with minimal worsening in genes that were initially dysregulated. Many of the cellular process that were altered in LOPD patients were also affected in the more clinically severe infantile-onset (IOPD) patients. Additionally, both LOPD and IOPD patients demonstrated enrichment across several inflammatory pathways, despite a lack of overt immune cell infiltration. This study provides further insight into Pompe disease biology and demonstrates the positive effects of avalglucosidase alfa treatment.

Eccentric contraction-induced strength loss in dystrophin-deficient muscle: Preparations, protocols, and mechanisms

The absence of dystrophin hypersensitizes skeletal muscle of lower and higher vertebrates to eccentric contraction (ECC)-induced strength loss. Loss of strength can be accompanied by transient and reversible alterations to sarcolemmal excitability and disruption, triad dysfunction, and aberrations in calcium kinetics and reactive oxygen species production. The degree of ECC-induced strength loss, however, appears dependent on several extrinsic and intrinsic factors such as vertebrate model, skeletal muscle preparation (in vivo, in situ, or ex vivo), skeletal muscle hierarchy (single fiber versus whole muscle and permeabilized versus intact), strength production, fiber branching, age, and genetic background, among others. Consistent findings across research groups show that dystrophin-deficient fast(er)-twitch muscle is hypersensitive to ECCs relative to wildtype muscle, but because preparations are highly variable and sensitivity to ECCs are used repeatedly to determine efficacy of many preclinical treatments, it is critical to evaluate the impact of skeletal muscle preparations on sensitivity to ECC-induced strength loss in dystrophin-deficient skeletal muscle. Here, we review and discuss variations in skeletal muscle preparations to evaluate the factors responsible for variations and discrepancies between research groups. We further highlight that dystrophin-deficiency, or loss of the dystrophin-glycoprotein complex in skeletal muscle, is not a prerequisite for accelerated strength loss-induced by ECCs.

Nano-Immunomodulation: A New Strategy for Skeletal Muscle Diseases and Aging?

The skeletal muscle has a very remarkable ability to regenerate upon injury under physiological conditions; however, this regenerative capacity is strongly diminished in physio-pathological conditions, such as those present in diseased or aged muscles. Many muscular dystrophies (MDs) are characterized by aberrant inflammation due to the deregulation of both the lymphoid and myeloid cell populations and the production of pro-inflammatory cytokines. Pathological inflammation is also observed in old muscles due to a systemic change in the immune system, known as "inflammaging". Immunomodulation represents, therefore, a promising therapeutic opportunity for different skeletal muscle conditions. However, the use of immunomodulatory drugs in the clinics presents several caveats, including their low stability in vivo, the need for high doses to obtain therapeutically relevant effects, and the presence of strong side effects. Within this context, the emerging field of nanomedicine provides the powerful tools needed to control the immune response. Nano-scale materials are currently being explored as biocarriers to release immunomodulatory agents in the damaged tissues, allowing therapeutic doses with limited off-target effects. In addition, the intrinsic immunomodulatory properties of some nanomaterials offer further opportunities for intervention that still need to be systematically explored. Here we exhaustively review the state-of-the-art regarding the use of nano-sized materials to modulate the aberrant immune response that characterizes some physio-pathological muscle conditions, such as MDs or sarcopenia (the age-dependent loss of muscle mass). Based on our learnings from cancer and immune tolerance induction, we also discuss further opportunities, challenges, and limitations of the emerging field of nano-immunomodulation.