Total Absence of Dystrophin Expression Exacerbates Ectopic Myofiber Calcification and Fibrosis and Alters Macrophage Infiltration Patterns

The BALB/c.mdx62 mouse exhibits a dystrophic muscle pathology and is a model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a devastating monogenic skeletal muscle-wasting disorder. Although many pharmacological and genetic interventions have been reported in preclinical studies, few have progressed to clinical trials with meaningful benefit. Identifying therapeutic potential can be limited by availability of suitable preclinical mouse models. More rigorous testing across models with varied background strains and mutations can identify treatments for clinical success. Here, we report the generation of a DMD mouse model with a CRISPR-induced deletion within exon 62 of the dystrophin gene (Dmd) and the first generated in BALB/c mice. Analysis of mice at 3, 6 and 12 months of age confirmed loss of expression of the dystrophin protein isoform Dp427 and resultant dystrophic pathology in limb muscles and the diaphragm, with evidence of centrally nucleated fibers, increased inflammatory markers and fibrosis, progressive decline in muscle function, and compromised trabecular bone development. The BALB/c.mdx62 mouse is a novel model of DMD with associated variations in the immune response and muscle phenotype, compared with those of existing models. It represents an important addition to the preclinical model toolbox for developing therapeutic strategies.

Role of platelet factor 4 in arteriovenous fistula maturation failure: What do we know so far?

The rate of arteriovenous fistula (AVF) maturation failure remains unacceptably high despite continuous efforts on technique improvement and careful pre-surgery planning. In fact, half of all newly created AVFs are unable to be used for hemodialysis (HD) without a salvage procedure. While vascular stenosis in the venous limb of the access is the culprit, the underlying factors leading to vascular narrowing and AVF maturation failure are yet to be determined. We have recently demonstrated that AVF non-maturation is associated with post-operative medial fibrosis and fibrotic stenosis, and post-operative intimal hyperplasia (IH) exacerbates the situation. Multiple pathological processes and signaling pathways are underlying the stenotic remodeling of the AVF. Our group has recently indicated that a pro-inflammatory cytokine platelet factor 4 (PF4/CXCL4) is upregulated in veins that fail to mature after AVF creation. Platelet factor 4 is a fibrosis marker and can be detected in vascular stenosis tissue, suggesting that it may contribute to AVF maturation failure through stimulation of fibrosis and development of fibrotic stenosis. Here, we present an overview of the how PF4-mediated fibrosis determines AVF maturation failure.

MEK-inhibitors decrease Nfix in muscular dystrophy but induce unexpected calcifications, partially rescued with Cyanidin diet

Muscular dystrophies (MDs) are incurable genetic myopathies characterized by progressive degeneration of skeletal muscles. Dystrophic mice lacking the transcription factor Nfix display morphological and functional improvements of the disease. Recently, we demonstrated that MAPK signaling pathway positively regulates Nfix in muscle development and that Cyanidin, a natural antioxidant molecule, strongly ameliorates the pathology. To explore a synergistic approach aimed at treating MDs, we administered Trametinib, a clinically approved MEK inhibitor, alone or combined with Cyanidin to adult Sgca null mice. We observed that chronic treatment with Trametinib and Cyanidin reduced Nfix in myogenic cells but, unexpectedly, caused ectopic calcifications exclusively in dystrophic muscles. The combined treatment with Cyanidin resulted in histological improvements by preventing Trametinib-induced calcifications in Diaphragm and Soleus. Collectively, this first pilot study revealed that Nfix is modulated by the MAPK pathway in MDs, and that Cyanidin partly rescued the unexpected ectopic calcifications caused by MEK inhibition.

The Role of P2X7 Purinoceptors in the Pathogenesis and Treatment of Muscular Dystrophies

Muscular dystrophies are inherited neuromuscular diseases, resulting in progressive disability and often affecting life expectancy. The most severe, common types are Duchenne muscular dystrophy (DMD) and Limb-girdle sarcoglycanopathy, which cause advancing muscle weakness and wasting. These diseases share a common pathomechanism where, due to the loss of the anchoring dystrophin (DMD, dystrophinopathy) or due to mutations in sarcoglycan-encoding genes (LGMDR3 to LGMDR6), the α-sarcoglycan ecto-ATPase activity is lost. This disturbs important purinergic signaling: An acute muscle injury causes the release of large quantities of ATP, which acts as a damage-associated molecular pattern (DAMP). DAMPs trigger inflammation that clears dead tissues and initiates regeneration that eventually restores normal muscle function. However, in DMD and LGMD, the loss of ecto-ATPase activity, that normally curtails this extracellular ATP (eATP)-evoked stimulation, causes exceedingly high eATP levels. Thus, in dystrophic muscles, the acute inflammation becomes chronic and damaging. The very high eATP over-activates P2X7 purinoceptors, not only maintaining the inflammation but also tuning the potentially compensatory P2X7 up-regulation in dystrophic muscle cells into a cell-damaging mechanism exacerbating the pathology. Thus, the P2X7 receptor in dystrophic muscles is a specific therapeutic target. Accordingly, the P2X7 blockade alleviated dystrophic damage in mouse models of dystrophinopathy and sarcoglycanopathy. Therefore, the existing P2X7 blockers should be considered for the treatment of these highly debilitating diseases. This review aims to present the current understanding of the eATP-P2X7 purinoceptor axis in the pathogenesis and treatment of muscular dystrophies.

The P2X7 purinoceptor in pathogenesis and treatment of dystrophino- and sarcoglycanopathies

Dystrophinopathy and sarcoglycanopathies are incurable diseases caused by mutations in the genes encoding dystrophin or members of the dystrophin associated protein complex (DAPC). Restoration of the missing dystrophin or sarcoglycans via genetic approaches is complicated by the downsides of personalised medicines and immune responses against re-expressed proteins. Thus, the targeting of disease mechanisms downstream from the mutant protein has a strong translational potential. Acute muscle damage causes release of large quantities of ATP, which activates P2X7 purinoceptors, resulting in inflammation that clears dead tissues and triggers regeneration. However, in dystrophic muscles, loss of α-sarcoglycan ecto-ATPase activity further elevates extracellular ATP (eATP) levels, exacerbating the pathology. Moreover, seemingly compensatory P2X7 upregulation in dystrophic muscle cells, combined with high eATP leads to further damage. Accordingly, P2X7 blockade alleviated dystrophic damage in mouse models of both dystrophinopathy and sarcoglycanopathy. Existing P2X7 blockers could be re-purposed for the treatment of these highly debilitating diseases.

Downregulation of Dystrophin Expression Occurs across Diverse Tumors, Correlates with the Age of Onset, Staging and Reduced Survival of Patients

Altered dystrophin expression was found in some tumors and recent studies identified a developmental onset of Duchenne muscular dystrophy (DMD). Given that embryogenesis and carcinogenesis share many mechanisms, we analyzed a broad spectrum of tumors to establish whether dystrophin alteration evokes related outcomes. Transcriptomic, proteomic, and mutation datasets from fifty tumor tissues and matching controls (10,894 samples) and 140 corresponding tumor cell lines were analyzed. Interestingly, dystrophin transcripts and protein expression were found widespread across healthy tissues and at housekeeping gene levels. In 80% of tumors, DMD expression was reduced due to transcriptional downregulation and not somatic mutations. The full-length transcript encoding Dp427 was decreased in 68% of tumors, while Dp71 variants showed variability of expression. Notably, low expression of dystrophins was associated with a more advanced stage, older age of onset, and reduced survival across different tumors. Hierarchical clustering analysis of DMD transcripts distinguished malignant from control tissues. Transcriptomes of primary tumors and tumor cell lines with low DMD expression showed enrichment of specific pathways in the differentially expressed genes. Pathways consistently identified: ECM-receptor interaction, calcium signaling, and PI3K-Akt are also altered in DMD muscle. Therefore, the importance of this largest known gene extends beyond its roles identified in DMD, and certainly into oncology.

Ion Channels of the Sarcolemma and Intracellular Organelles in Duchenne Muscular Dystrophy: A Role in the Dysregulation of Ion Homeostasis and a Possible Target for Therapy

Duchenne muscular dystrophy (DMD) is caused by the absence of the dystrophin protein and a properly functioning dystrophin-associated protein complex (DAPC) in muscle cells. DAPC components act as molecular scaffolds coordinating the assembly of various signaling molecules including ion channels. DMD shows a significant change in the functioning of the ion channels of the sarcolemma and intracellular organelles and, above all, the sarcoplasmic reticulum and mitochondria regulating ion homeostasis, which is necessary for the correct excitation and relaxation of muscles. This review is devoted to the analysis of current data on changes in the structure, functioning, and regulation of the activity of ion channels in striated muscles in DMD and their contribution to the disruption of muscle function and the development of pathology. We note the prospects of therapy based on targeting the channels of the sarcolemma and organelles for the correction and alleviation of pathology, and the problems that arise in the interpretation of data obtained on model dystrophin-deficient objects.

Histological Methods to Assess Skeletal Muscle Degeneration and Regeneration in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a progressive disease caused by the loss of function of the protein dystrophin. This protein contributes to the stabilisation of striated cells during contraction, as it anchors the cytoskeleton with components of the extracellular matrix through the dystrophin-associated protein complex (DAPC). Moreover, absence of the functional protein affects the expression and function of proteins within the DAPC, leading to molecular events responsible for myofibre damage, muscle weakening, disability and, eventually, premature death. Presently, there is no cure for DMD, but different treatments help manage some of the symptoms. Advances in genetic and exon-skipping therapies are the most promising intervention, the safety and efficiency of which are tested in animal models. In addition to in vivo functional tests, ex vivo molecular evaluation aids assess to what extent the therapy has contributed to the regenerative process. In this regard, the later advances in microscopy and image acquisition systems and the current expansion of antibodies for immunohistological evaluation together with the development of different spectrum fluorescent dyes have made histology a crucial tool. Nevertheless, the complexity of the molecular events that take place in dystrophic muscles, together with the rise of a multitude of markers for each of the phases of the process, makes the histological assessment a challenging task. Therefore, here, we summarise and explain the rationale behind different histological techniques used in the literature to assess degeneration and regeneration in the field of dystrophinopathies, focusing especially on those related to DMD.

Specific Dystrophins Selectively Associate with Inhibitory and Excitatory Synapses of the Mouse Cerebellum and their Loss Alters Expression of P2X7 Purinoceptors and Pro-Inflammatory Mediators

Duchenne muscular dystrophy (DMD) patients, having mutations of the DMD gene, present with a range of neuropsychiatric disorders, in addition to the quintessential muscle pathology. The neurobiological basis remains poorly understood because the contributions of different DMD gene products (dystrophins) to the different neural networks underlying such symptoms are yet to be fully characterised. While full-length dystrophin clusters in inhibitory synapses, with inhibitory neurotransmitter receptors, the precise subcellular expression of truncated DMD gene products with excitatory synapses remains unresolved. Furthermore, inflammation, involving P2X purinoceptor 7 (P2RX7) accompanies DMD muscle pathology, yet any association with brain dystrophins is yet to be established. The aim of this study was to investigate the comparative expression of different dystrophins, alongside ionotropic glutamate receptors and P2RX7s, within the cerebellar circuitry known to express different dystrophin isoforms. Immunoreactivity for truncated DMD gene products was targeted to Purkinje cell (PC) distal dendrites adjacent to, or overlapping with, signal for GluA1, GluA4, GluN2A, and GluD2 receptor subunits. P2X7R immunoreactivity was located in Bergmann glia profiles adjacent to PC-dystrophin immunoreactivity. Ablation of all DMD gene products coincided with decreased mRNA expression for Gria2, Gria3, and Grin2a and increased GluD2 immunoreactivity. Finally, dystrophin-null mice showed decreased brain mRNA expression of P2rx7 and several inflammatory mediators. The data suggest that PCs target different dystrophin isoforms to molecularly and functionally distinct populations of synapses. In contrast to muscle, dystrophinopathy in brain leads to the dampening of the local immune system.

Loss of full-length dystrophin expression results in major cell-autonomous abnormalities in proliferating myoblasts

Duchenne muscular dystrophy (DMD) affects myofibers and muscle stem cells, causing progressive muscle degeneration and repair defects. It was unknown whether dystrophic myoblasts—the effector cells of muscle growth and regeneration—are affected. Using transcriptomic, genome-scale metabolic modelling and functional analyses, we demonstrate, for the first time, convergent abnormalities in primary mouse and human dystrophic myoblasts. In Dmd^mdx myoblasts lacking full-length dystrophin, the expression of 170 genes was significantly altered. Myod1 and key genes controlled by MyoD (Myog, Mymk, Mymx, epigenetic regulators, ECM interactors, calcium signalling and fibrosis genes) were significantly downregulated. Gene ontology analysis indicated enrichment in genes involved in muscle development and function. Functionally, we found increased myoblast proliferation, reduced chemotaxis and accelerated differentiation, which are all essential for myoregeneration. The defects were caused by the loss of expression of full-length dystrophin, as similar and not exacerbated alterations were observed in dystrophin-null Dmd^mdx-βgeo myoblasts. Corresponding abnormalities were identified in human DMD primary myoblasts and a dystrophic mouse muscle cell line, confirming the cross-species and cell-autonomous nature of these defects. The genome-scale metabolic analysis in human DMD myoblasts showed alterations in the rate of glycolysis/gluconeogenesis, leukotriene metabolism, and mitochondrial beta-oxidation of various fatty acids. These results reveal the disease continuum: DMD defects in satellite cells, the myoblast dysfunction affecting muscle regeneration, which is insufficient to counteract muscle loss due to myofiber instability. Contrary to the established belief, our data demonstrate that DMD abnormalities occur in myoblasts, making these cells a novel therapeutic target for the treatment of this lethal disease.

P2X7 Purinoceptor Affects Ectopic Calcification of Dystrophic Muscles

Ectopic calcification (EC) of myofibers is a pathological feature of muscle damage in Duchenne muscular dystrophy (DMD). Mineralisation of muscle tissue occurs concomitantly with macrophage infiltration, suggesting a link between ectopic mineral deposition and inflammation. One potential link is the P2X7 purinoceptor, a key trigger of inflammation, which is expressed on macrophages but also up-regulated in dystrophic muscle cells. To investigate the role of P2X7 in dystrophic calcification, we utilised the Dmdmdx-βgeo dystrophin-null mouse model of DMD crossed with a global P2X7 knockout (P2rx7−/−) or with our novel P2X7 knockin-knockout mouse (P2x7KiKo), which expresses P2X7 in macrophages but not muscle cells. Total loss of P2X7 increased EC, indicating that P2X7 overexpression is a protective mechanism against dystrophic mineralisation. Given that muscle-specific P2X7 ablation did not affect dystrophic EC, this underlined the role of P2X7 receptor expression on the inflammatory cells. Serum phosphate reflected dystrophic calcification, with the highest serum phosphate levels found in genotypes with the most ectopic mineral. To further investigate the underlying mechanisms, we measured phosphate release from cells in vitro, and found that dystrophic myoblasts released less phosphate than non-dystrophic cells. Treatment with P2X7 antagonists increased phosphate release from both dystrophic and control myoblasts indicating that muscle cells are a potential source of secreted phosphate while macrophages protect against ectopic mineralisation. Treatment of cells with high phosphate media engendered mineral deposition, which was decreased in the presence of the P2X7 agonist BzATP, particularly in cultures of dystrophic cells, further supporting a protective role for P2X7 against ectopic mineralisation in dystrophic muscle.

The Mechanism and Regulation of the NLRP3 Inflammasome during Fibrosis

Fibrosis is often the end result of chronic inflammation. It is characterized by the excessive deposition of extracellular matrix. This leads to structural alterations in the tissue, causing permanent damage and organ dysfunction. Depending on the organ it effects, fibrosis can be a serious threat to human life. The molecular mechanism of fibrosis is still not fully understood, but the NLRP3 (NOD-, LRR- and pyrin–domain–containing protein 3) inflammasome appears to play a significant role in the pathogenesis of fibrotic disease. The NLRP3 inflammasome has been the most extensively studied inflammatory pathway to date. It is a crucial component of the innate immune system, and its activation mediates the secretion of interleukin (IL)-1β and IL-18. NLRP3 activation has been strongly linked with fibrosis and drives the differentiation of fibroblasts into myofibroblasts by the chronic upregulation of IL-1β and IL-18 and subsequent autocrine signaling that maintains an activated inflammasome. Both IL-1β and IL-18 are profibrotic, however IL-1β can have antifibrotic capabilities. NLRP3 responds to a plethora of different signals that have a common but unidentified unifying trigger. Even after 20 years of extensive investigation, regulation of the NLRP3 inflammasome is still not completely understood. However, what is known about NLRP3 is that its regulation and activation is complex and not only driven by various activators but controlled by numerous post-translational modifications. More recently, there has been an intensive attempt to discover NLRP3 inhibitors to treat chronic diseases. This review addresses the role of the NLRP3 inflammasome in fibrotic disorders across many different tissues. It discusses the relationships of various NLRP3 activators to fibrosis and covers different therapeutics that have been developed, or are currently in development, that directly target NLRP3 or its downstream products as treatments for fibrotic disorders.

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.

Macrophages in Skeletal Muscle Dystrophies, An Entangled Partner

While skeletal muscle remodeling happens throughout life, diseases that result in its dysfunction are accountable for many deaths. Indeed, skeletal muscle is exceptionally capable to respond to stimuli modifying its homeostasis, such as in atrophy, hypertrophy, regeneration and repair. In particular conditions such as genetic diseases (muscular dystrophies), skeletal muscle’s capacity to remodel is strongly affected and undergoes continuous cycles of chronic damage. This induces scarring, fatty infiltration, as well as loss of contractibility and of the ability to generate force. In this context, inflammation, primarily mediated by macrophages, plays a central pathogenic role. Macrophages contribute as the primary regulators of inflammation during skeletal muscle regeneration, affecting tissue-resident cells such as myogenic cells and endothelial cells, but also fibro-adipogenic progenitors, which are the main source of the fibro fatty scar. During skeletal muscle regeneration their function is tightly orchestrated, while in dystrophies their fate is strongly disturbed, resulting in chronic inflammation. In this review, we will discuss the latest findings on the role of macrophages in skeletal muscle diseases, and how they are regulated.

Arecanut-induced fibrosis display dual phases of reorganising glycans and amides in skin extracellular matrix

The habit of chewing arecanut leads to fibrosis in the oral tissues, which can lead to cancer. Despite high mortality, fibrosis has limited clinical success owing to organ-specific variations, genetic predispositions, and slow progression. Fibrosis is a progressive condition that is unresponsive to medications in the severe phase. To understand underlying macromolecular changes we studied the extracellular matrix's (ECM) key molecular modifications in the early and late phase of arecanut-induced fibrosis in skin. To study the fibrosis, we topically applied arecanut extract on the mice skin. We observed that the matrix changes observe early and late phases based on ECM characteristics including the matrix proteins and the glycans. A spike in the levels of proteoglycans and β-sheet structures are noted in the early phase. A significant drop in the proteoglycans and strengthening of amide covalent interactions is observed in the late phase. Although, almost no physical changes are noticeable only in the early phase; the late phase observes thick collagen bundling and a 4-fold stiffening of the skin tissue. The study indicates that the temporal interplay of proteins and glycans determine the matrix's severity state while opening avenues to research directed towards the phase-specific clinical discovery.

Fibrosis: from mechanisms to medicines

Fibrosis can affect any organ and is responsible for up to 45% of all deaths in the industrialized world. It has long been thought to be relentlessly progressive and irreversible, but both preclinical models and clinical trials in various organ systems have shown that fibrosis is a highly dynamic process. This has clear implications for therapeutic interventions that are designed to capitalize on this inherent plasticity. However, despite substantial progress in our understanding of the pathobiology of fibrosis, a translational gap remains between the identification of putative antifibrotic targets and conversion of this knowledge into effective treatments in humans. Here we discuss the transformative experimental strategies that are being leveraged to dissect the key cellular and molecular mechanisms that regulate fibrosis, and the translational approaches that are enabling the emergence of precision medicine-based therapies for patients with fibrosis.