MCP-1 deficiency causes altered inflammation with impaired skeletal muscle regeneration

Single-cell mass cytometry in immunological skin diseases

Immune-related skin diseases represent a collective of dermatological disorders intricately linked to dysfunctional immune system processes. These conditions are primarily characterized by an immoderate activation of the immune system or deviant immune responses, involving diverse immune components including immune cells, antibodies, and inflammatory mediators. However, the precise molecular dysregulation underlying numerous individual cases of these diseases and unique subsets respond under disease conditions remains elusive. Comprehending the mechanisms and determinants governing the homeostasis and functionality of diseases could offer potential therapeutic opportunities for intervention. Mass cytometry enables precise and high-throughput quantitative measurement of proteins within individual cells by utilizing antibodies labeled with rare heavy metal isotopes. Imaging mass cytometry employs mass spectrometry to obtain spatial information on cell-to-cell interactions within tissue sections, simultaneously utilizing more than 40 markers. The application of single-cell mass cytometry presents a unique opportunity to conduct highly multiplexed analysis at the single-cell level, thereby revolutionizing our understanding of cell population heterogeneity and hierarchy, cellular states, multiplexed signaling pathways, proteolysis products, and mRNA transcripts specifically in the context of many autoimmune diseases. This information holds the potential to offer novel approaches for the diagnosis, prognostic assessment, and monitoring responses to treatment, thereby enriching our strategies in managing the respective conditions. This review summarizes the present-day utilization of single-cell mass cytometry in studying immune-related skin diseases, highlighting its advantages and limitations. This technique will become increasingly prevalent in conducting extensive investigations into these disorders, ultimately yielding significant contributions to their accurate diagnosis and efficacious therapeutic interventions.

Unilateral hindlimb ischaemia-induced systemic inflammation is associated with non-ischaemic skeletal muscle inflammation

Skeletal muscle atrophy and dysfunction commonly accompany cardiovascular diseases such as peripheral arterial disease and may be partially attributable to systemic inflammation. We sought to determine whether acute systemic inflammation in a model of hindlimb ischaemia (HLI) could affect skeletal muscle macrophage infiltration, fibre size, or capillarization, independent of the ischaemia. Eight-week-old C57BL/6 male mice underwent either Sham or HLI surgery, and were killed 1, 3, or 7 days post-surgery. Circulating inflammatory cytokine concentrations were measured, as well as immune cell infiltration and morphology of skeletal muscle from both limbs of HLI and Sham mice. In HLI compared with Sham mice at day 1, plasma interleukin-1β levels were 216% higher (0.48 ± 0.10 vs. 0.15 ± 0.01 pg/μL, P = 0.005) and decreased by day 3. This was followed by increased macrophage presence in muscle from both ischaemic and non-ischaemic limbs of HLI mice by day 7 (7.3- and 2.3-fold greater than Sham, respectively, P < 0.0001). In HLI mice, muscle from the ischaemic limb had 21% lower fibre cross-sectional area than the non-ischaemic limb (724 ± 28 vs. 916 ± 46 μm2, P = 0.01), but the non-ischaemic limb of HLI mice was no different from Sham. This shows that HLI induces acute systemic inflammation accompanied by immune infiltration in both ischaemic and remote skeletal muscle; however, this did not induce skeletal muscle atrophy in remote muscle within the 7-day time course of this study. This effect of local skeletal muscle ischaemia on the inflammatory status of remote skeletal muscle may signal a priming of muscle for subsequent atrophy over a longer time course. HIGHLIGHTS: What is the central question of this study? Does hindlimb ischaemia-induced inflammation cause acute immune, inflammatory and morphological alterations in remote non-ischaemic skeletal muscle? What is the main finding and its importance? Hindlimb ischaemia induced systemic inflammation with subsequent neutrophil and macrophage infiltration in both ischaemic and non-ischaemic skeletal muscle; however, morphological changes did not occur in non-ischaemic muscle within 7 days. These immune alterations may have functional implications that take longer than 7 days to manifest, and subsequent or prolonged systemic inflammation and immune infiltration of muscle could lead to morphological changes and functional decline.

Inflammatory pathway communication with skeletal muscle-Does aging play a role? A topical review of the current evidence

Skeletal muscle plays an integral role in locomotion, but also as part of the integrative physiological system. Recent progress has identified crosstalk between skeletal muscle and various physiological systems, including the immune system. Both the musculoskeletal and immune systems are impacted by aging. Increased age is associated with decreased muscle mass and function, while the immune system undergoes "inflammaging" and immunosenescence. Exercise is identified as a preventative medicine that can mitigate loss of function for both systems. This review summarizes: (1) the inflammatory pathways active in skeletal muscle; and (2) the inflammatory and skeletal muscle response to unaccustomed exercise in younger and older adults. Compared to younger adults, it appears older individuals have a muted pro-inflammatory response and elevated anti-inflammatory response to exercise. This important difference could contribute to decreased regeneration and recovery following unaccustomed exercise in older adults, as well as in chronic disease. The current research provides specific information on the role inflammation plays in altering skeletal muscle form and function, and adaptation to exercise; however, the pursuit of more knowledge in this area will delineate specific interventions that may enhance skeletal muscle recovery and promote resiliency in this tissue particularly with aging.

IL-6 and TNF are Potential Inflammatory Biomarkers in Facioscapulohumeral Muscular Dystrophy

Background

FSHD is a highly prevalent inherited myopathy with a still poorly understood pathology.

Objective

To investigate whether proinflammatory cytokines are associated with FSHD and which specific innate immune cells are involved in its pathology.

Methods

First, we measured circulating cytokines in serum samples: IL-6 (FSHD, n = 150; HC, n = 98); TNF (FSHD, n = 150; HC, n = 59); IL-1α (FSHD, n = 150; HC, n = 66); IL-1β (FSHD, n = 150; HC, n = 98); MCP-1 (FSHD, n = 14; HC, n = 14); VEGF-A (FSHD, n = 14; HC, n = 14). Second, we tested trained immunity in monocytes (FSHD, n = 15; HC, n = 15) and NK cells (FSHD, n = 11; HC, n = 11). Next, we explored the cytokine production capacity of NK cells in response to different stimuli (FSHD, n = 39; HC, n = 22). Lastly, we evaluated the cytokine production of ex vivo stimulated MRI guided inflamed (TIRM+) and paired MRI guided non inflamed (TIRM-) muscle biopsies of 21 patients and of 8 HC muscle biopsies.

Results

We included a total of 190 FSHD patients (N = 190, 48±14 years, 49% men) and of 135 HC (N = 135, 44±15 years, 47% men). We found that FSHD patients had higher concentrations of IL-6 and TNF measured (a) in the circulation, (b) after ex-vivo stimulation of NK cells, and (c) in muscle specimens. Besides, IL-6 circulating concentrations, as well as its production by NK cells and IL-6 content of FSHD muscle specimens, showed a mild correlation with disease duration, disease severity, and muscle weakness.

Conclusion

These results show that IL-6 and TNF may contribute to FSHD pathology and suggest novel therapeutic targets. Additionally, the activation of NK cells in FSHD may be a novel pathway contributing to FSHD pathology.

A discrete 'early-responder' stromal-cell subtype orchestrates immunocyte recruitment to injured tissue

Following acute injury, stromal cells promote tissue regeneration by a diversity of mechanisms. Time-resolved single-cell RNA sequencing of muscle mesenchymal stromal cells (MmSCs) responding to acute injury identified an 'early-responder' subtype that spiked on day 1 and expressed a notable array of transcripts encoding immunomodulators. IL-1β, TNF-α and oncostatin M each strongly and rapidly induced MmSCs transcribing this immunomodulatory program. Macrophages amplified the program but were not strictly required for its induction. Transfer of the inflammatory MmSC subtype, tagged with a unique surface marker, into healthy hindlimb muscle induced inflammation primarily driven by neutrophils and macrophages. Among the abundant inflammatory transcripts produced by this subtype, Cxcl5 was stroma-specific and highly upregulated with injury. Depletion of this chemokine early after injury revealed a substantial impact on recruitment of neutrophils, a prolongation of inflammation to later times and an effect on tissue regeneration. Mesenchymal stromal cell subtypes expressing a comparable inflammatory program were found in a mouse model of muscular dystrophy and in several other tissues and pathologies in both mice and humans. These 'early-responder' mesenchymal stromal cells, already in place, permit rapid and coordinated mobilization and amplification of critical cell collaborators in response to injury.

Genetic diversity modulates the physical and transcriptomic response of skeletal muscle to simulated microgravity in male mice

Developments in long-term space exploration necessitate advancements in countermeasures against microgravity-induced skeletal muscle loss. Astronaut data shows considerable variation in muscle loss in response to microgravity. Previous experiments suggest that genetic background influences the skeletal muscle response to unloading, but no in-depth analysis of genetic expression has been performed. Here, we placed eight, male, inbred founder strains of the diversity outbred mice (129S1/SvImJ, A/J, C57BL/6J, CAST/EiJ, NOD/ShiLtJ, NZO/HILtJ, PWK/PhJ, and WSB/EiJ) in simulated microgravity (SM) via hindlimb unloading for three weeks. Body weight, muscle morphology, muscle strength, protein synthesis marker expression, and RNA expression were collected. A/J and CAST/EiJ mice were most susceptible to SM-induced muscle loss, whereas NOD/ShiLtJ mice were the most protected. In response to SM, A/J and CAST/EiJ mice experienced reductions in body weight, muscle mass, muscle volume, and muscle cross-sectional area. A/J mice had the highest number of differentially expressed genes (68) and associated gene ontologies (328). Downregulation of immunological gene ontologies and genes encoding anabolic immune factors suggest that immune dysregulation contributes to the response of A/J mice to SM. Several muscle properties showed significant interactions between SM and mouse strain and a high degree of heritability. These data imply that genetic background plays a role in the degree of muscle loss in SM and that more individualized programs should be developed for astronauts to protect their skeletal muscles against microgravity on long-term missions.

Pathogenesis of Musculotendinous and Fascial Injuries After Physical Exercise - Short Review

Purpose

The identification of sports and physical exercises with injury risk is necessary to preserve the capacity of athletes and people who perform physical education and also to prevent the installation of functional deficiencies.

Methods

We have selected the articles related to the pathogenic mechanisms involved in musculotendinous and fascial injuries produced as a result of physical exercise.

Results and Discussions

The lesional pathogenesis is complex and incompletely clarified. Recent theories put in a new light the mechanisms of muscle pain and tendinopathy production. The accumulation of lactate anion, known to be a residue that induces fatigue and muscle pain, has been reconsidered by some authors. It appears that lactate anion is an excellent fuel for the myocardial fiber. Moreover, the accumulation of lactic acid after intense physical exercise could prevent the inexcitability of the sarcolemma induced by the increased concentration of interstitial K+. Most of the time, overuse injuries are not limited to muscles. They can cause myofascial, myotendinous or purely muscular injuries. The muscular fascia is more susceptible to injuries produced under the action of large external forces. Also, fascia is more sensitive to pain compared to muscle when external forces act eccentrically. Overloading the tendon and putting it under tension repeatedly is followed by ruptures of the tendon fibers. The regeneration of the degenerated tendon is defective in the context of the inflammation produced by the injury. Tendon fibers undergo a process of fibrosis, scarring, adhesion and heterogeneous calcification. Oxidative stress is responsible for inflammation, degeneration and apoptosis of tenocytes.

Conclusion

The benefits brought by physical education and sports are indisputable, but their practice requires a coordinated program to prevent possible traumatic and overuse injuries.

Platelet-derived chemokines promote skeletal muscle regeneration by guiding neutrophil recruitment to injured muscles

Skeletal muscle regeneration involves coordinated interactions between different cell types. Injection of platelet-rich plasma is circumstantially considered an aid to muscle repair but whether platelets promote regeneration beyond their role in hemostasis remains unexplored. Here, we find that signaling via platelet-released chemokines is an early event necessary for muscle repair in mice. Platelet depletion reduces the levels of the platelet-secreted neutrophil chemoattractants CXCL5 and CXCL7/PPBP. Consequently, early-phase neutrophil infiltration to injured muscles is impaired whereas later inflammation is exacerbated. Consistent with this model, neutrophil infiltration to injured muscles is compromised in male mice with Cxcl7-knockout platelets. Moreover, neo-angiogenesis and the re-establishment of myofiber size and muscle strength occurs optimally in control mice post-injury but not in Cxcl7ko mice and in neutrophil-depleted mice. Altogether, these findings indicate that platelet-secreted CXCL7 promotes regeneration by recruiting neutrophils to injured muscles, and that this signaling axis could be utilized therapeutically to boost muscle regeneration.

Response to Mechanical Properties and Physiological Challenges of Fascia: Diagnosis and Rehabilitative Therapeutic Intervention for Myofascial System Disorders

Damage to the fascia can cause significant performance deficits in high-performance sports and recreational exercise and may contribute to the development of musculoskeletal disorders and persistent potential pain. The fascia is widely distributed from head to toe, encompassing muscles, bones, blood vessels, nerves, and internal organs and comprising various layers of different depths, indicating the complexity of its pathogenesis. It is a connective tissue composed of irregularly arranged collagen fibers, distinctly different from the regularly arranged collagen fibers found in tendons, ligaments, or periosteum, and mechanical changes in the fascia (stiffness or tension) can produce changes in its connective tissue that can cause pain. While these mechanical changes induce inflammation associated with mechanical loading, they are also affected by biochemical influences such as aging, sex hormones, and obesity. Therefore, this paper will review the current state of knowledge on the molecular level response to the mechanical properties of the fascia and its response to other physiological challenges, including mechanical changes, innervation, injury, and aging; imaging techniques available to study the fascial system; and therapeutic interventions targeting fascial tissue in sports medicine. This article aims to summarize contemporary views.

Allogenic Synovia-Derived Mesenchymal Stem Cells for Treatment of Equine Tendinopathies and Desmopathies-Proof of Concept

Tendon and ligament injuries are frequent in sport horses and humans, and such injuries represent a significant therapeutic challenge. Tissue regeneration and function recovery are the paramount goals of tendon and ligament lesion management. Nowadays, several regenerative treatments are being developed, based on the use of stem cell and stem cell-based therapies. In the present study, the preparation of equine synovial membrane mesenchymal stem cells (eSM-MSCs) is described for clinical use, collection, transport, isolation, differentiation, characterization, and application. These cells are fibroblast-like and grow in clusters. They retain osteogenic, chondrogenic, and adipogenic differentiation potential. We present 16 clinical cases of tendonitis and desmitis, treated with allogenic eSM-MSCs and autologous serum, and we also include their evaluation, treatment, and follow-up. The concerns associated with the use of autologous serum as a vehicle are related to a reduced immunogenic response after the administration of this therapeutic combination, as well as the pro-regenerative effects from the growth factors and immunoglobulins that are part of its constitution. Most of the cases (14/16) healed in 30 days and presented good outcomes. Treatment of tendon and ligament lesions with a mixture of eSM-MSCs and autologous serum appears to be a promising clinical option for this category of lesions in equine patients.

Global deletion of CCL2 has adverse impacts on recovery of skeletal muscle fiber size and function and is muscle specific

Timely and complete recovery of muscle mass and function following a bout of physical disuse are critical components of returning to normal activities of daily living and lifestyle. Proper cross talk between the muscle tissue and myeloid cells (e.g., macrophages) throughout the recovery period from disuse atrophy plays a significant role in the complete resolution of muscle size and function. Chemokine C-C motif ligand 2 (CCL2) has a critical function of recruiting macrophages during the early phase of muscle damage. However, the importance of CCL2 has not been defined in the context of disuse and recovery. Here, we utilized a mouse model of whole body CCL2 deletion (CCL2KO) and subjected them to a period of hindlimb unloading followed by reloading to investigate the importance of CCL2 on the regrowth of muscle following disuse atrophy using ex vivo muscle tests, immunohistochemistry, and fluorescence-activated cell sorting approaches. We show mice that lack CCL2 display an incomplete recovery of gastrocnemius muscle mass, myofiber cross-sectional area, and EDL muscle contractile characteristics during the recovery from disuse atrophy. The soleus and plantaris had limited impact as a result of CCL2 deficiency suggesting a muscle-specific effect. Mice that lack CCL2 have decreased skeletal muscle collagen turnover, which may be related to defects in muscle function and stiffness. In addition, we show that the recruitment of macrophages to gastrocnemius muscle was dramatically reduced in CCL2KO mice during the recovery from disuse atrophy, which likely precipitated poor recovery of muscle size and function and aberrant collagen remodeling.NEW & NOTEWORTHY We provide evidence that the whole body loss of CCL2 in mice has adverse impacts on whole body function and skeletal muscle-specific contractile characteristics and collagen content. These defects in muscle function worsened during the recovery from disuse atrophy and corresponded with decreased recovery of muscle mass. We conclude that the absence of CCL2 decreased recruitment of proinflammatory macrophages to the muscle during the regrowth phase following disuse atrophy resulting in impaired collagen remodeling events and full resolution of muscle morphology and function.

HuMSC-EV induce monocyte/macrophage mobilization to orchestrate neovascularization in wound healing process following radiation injury

This study aims to investigate the mechanisms of human mesenchymal stem cell-derived extracellular vesicles (HuMSC-EV)-induced proangiogenic paracrine effects after radiation injury. HuMSC-EV were locally administered in mice hindlimb following 80-Gy X-ray irradiation and animals were monitored at different time points. HuMSC-EV improved neovascularization of the irradiated tissue, by stimulating angiogenesis, normalizing cutaneous blood perfusion, and increasing capillary density and production of proangiogenic factors. HuMSC-EV also stimulated vasculogenesis by promoting the recruitment and differentiation of bone marrow progenitors. Moreover, HuMSC-EV improved arteriogenesis by increasing the mobilization of monocytes from the spleen and the bone marrow and their recruitment into the muscle, with a pro-inflammatory potential. Importantly, monocyte depletion by clodronate treatment abolished the proangiogenic effect of HuMSC-EV. The critical role of Ly6C(hi) monocyte subset in HuMSC-EV-induced neovascularization process was further confirmed using Ccr2^-/- mice. This study demonstrates that HuMSC-derived EV enhances the neovascularization process in the irradiated tissue by increasing the production of proangiogenic factors, promoting the recruitment of vascular progenitor cells, and the mobilization of innate cells to the injured site. These results support the concept that HuMSC-EV might represent a suitable alternative to stem cells for therapeutic neovascularization in tissue repair.

Protective action of ultrasound-guided electrolysis technique on the muscle damage induced by notexin in rats

It is known that exercise can be one of the causes of muscular damage. In recent times, physiotherapists and medical professionals have been employing USGET techniques to stimulate muscle recovery to improve its performance after the injury. We pretend to analyse if the Ultrasound-guided electrolysis (USGET) technique could reduce muscle damage, inflammation, and pain in the present study. Female Wistar rats were assigned to one of three different groups: control (C), notexin (NOT) and notexin with USGET (electrolysis at 6mA) (NOT+USGET). We used the USGT technique, based on electrical stimulation with a continuous current of 4 pulses at an intensity of 6 mA for 5 seconds, conveyed to the muscle. The response was tested with motor function tests. In these tests, we could observe an increase in time and foot faults when crossing a beam in the NOT group compared to C group rats. On the other hand, a significant decrease in both variables was detected in the NOT+USGET compared to the NOT group. Muscle power was measured with a grip strength test, obtaining far better performances in NOT+USGET rats when compared to NOT rats. Moreover, the USGET technique prevented the increase of pro-inflammatory proteins IL-6 and chemokines CCL3 (Chemokine (C-C motif) ligand 3), CCL4 (Chemokine (C-C motif) ligand 4), and CCL5 (Chemokine (C-C motif) ligand 5) with their receptor CCR5 (C-C chemokine receptor type 5), induced by notexin in the quadriceps. At the same time, the study evidenced a decrease in both CCR8 (C-C chemokine receptor type 5,) and NF-ᴋB (nuclear factor- ᴋB) expressions after USGET treatment. On the other hand, we obtained evidence that demonstrated anti-inflammatory properties of the USGET technique, thus being the increase in IL-10 (Interleukin 10) and IL-13 (Interleukin 13) in the NOT+USGET group compared to the NOT group. Furthermore, when applying NSGET after damage, an increase in anti-inflammatory mediators and reduction of pro-inflammatory mediators, which, overall, promoted muscle regeneration, was observed. These results support the idea that the NSGET technique improves muscle recovery after toxic damages, which would justify its employment.

Aging is associated with an altered macrophage response during human skeletal muscle regeneration

Skeletal muscle injury in aged rodents is characterized by an asynchronous infiltration of pro- and anti-inflammatory macrophage waves, leading to improper and incomplete regeneration. It is unclear whether this aberration also occurs in aged human muscle. In this study, we quantified the macrophage responses in a human model of muscle damage and regeneration induced by electrical stimulation in 7 young and 21 older adults. At baseline, total resident macrophage (CD68⁺/DAPI⁺) content was not different between young and old subjects, but pro-inflammatory (CD206^-/CD68⁺/DAPI⁺) macrophage content was lower in the old. Following damage, muscle Infiltration of CD206^-/CD68⁺/DAPI⁺ macrophages was lower in old relative to young subjects. Further, only the increase in CD206^-/CD68⁺ macrophages correlated with the change in muscle satellite cell content. Our data show that older individuals have a compromised macrophage response during muscle regeneration, pointing to an altered inflammatory response as a potential mechanism for reduced muscle regenerative efficacy in aged humans.

Role of macrophages during skeletal muscle regeneration and hypertrophy-Implications for immunomodulatory strategies

Skeletal muscle is a plastic tissue that regenerates ad integrum after injury and adapts to raise mechanical loading/contractile activity by increasing its mass and/or myofiber size, a phenomenon commonly refers to as skeletal muscle hypertrophy. Both muscle regeneration and hypertrophy rely on the interactions between muscle stem cells and their neighborhood, which include inflammatory cells, and particularly macrophages. This review first summarizes the role of macrophages in muscle regeneration in various animal models of injury and in response to exercise-induced muscle damage in humans. Then, the potential contribution of macrophages to skeletal muscle hypertrophy is discussed on the basis of both animal and human experiments. We also present a brief comparative analysis of the role of macrophages during muscle regeneration versus hypertrophy. Finally, we summarize the current knowledge on the impact of different immunomodulatory strategies, such as heat therapy, cooling, massage, nonsteroidal anti-inflammatory drugs and resolvins, on skeletal muscle regeneration and their potential impact on muscle hypertrophy.

CCL2 Gene Expression and Protein Level Changes Observed in Response to Wingate Anaerobic Test in High-Trained Athletes and Non-Trained Controls

Intensive, acute exercise may bring a large systemic inflammatory response marked by substantial increases in inflammatory cytokines and chemokines. One such chemokines-CCL2-is a key factor involved in inflammatory reaction to exercise. The direct aim of the study was to describe the changes in the CCL2 expression levels after anaerobic exercise in well-trained athletes adapted to long-term training and in non-trained participants. The expression of CCL2 mRNA was evaluated in peripheral blood MNCs and CCL2 protein level was observed in blood plasma. The changes were assessed as the response to an acute, intensive bout of exercise (Wingate Anaerobic Test) in two groups of participants: well-trained soccer players and non-trained individuals. An increase of CCL2 expression inn both mRNA and protein levels was observed. The response was greater in non-trained individuals and elevated levels of CCL2 transcripts persisted for more than 24 h after exercise. Well-trained individuals responded more modestly and the effect was attenuated relatively quickly. This shows muscular adaptation to a continuous training regime in well-trained individuals and better control of immune reactions to muscular injury. In non-training individuals, the induction of the inflammatory response was greater, suggesting presence of more serious myotrauma.

SNAI1 is upregulated during muscle regeneration and represses FGF21 and ATF3 expression by directly binding their promoters

During skeletal myogenesis, the zinc-finger transcription factors SNAI1 and SNAI2, are expressed in proliferating myoblasts and regulate the transition to terminally differentiated myotubes while repressing pro-differentiation genes. Here, we demonstrate that SNAI1 is upregulated in vivo during the early phase of muscle regeneration induced by bupivacaine injury. Using shRNA-mediated gene silencing in C2C12 myoblasts and whole-transcriptome microarray analysis, we identified a collection of genes belonging to the endoplasmic reticulum (ER) stress pathway whose expression, induced by myogenic differentiation, was upregulated in absence of SNAI1. Among these, key ER stress genes, such as Atf3, Ddit3/Chop, Hspa5/Bip, and Fgf21, a myokine involved in muscle differentiation, were strongly upregulated. Furthermore, by promoter mutant analysis and Chromatin immune precipitation assay, we demonstrated that SNAI1 represses Fgf21 and Atf3 in proliferating myoblasts by directly binding to multiple E boxes in their respective promoter regions. Together, these data describe a new regulatory mechanism of myogenic differentiation involving the direct repressive action of SNAI1 on ER stress and Fgf21 expression, ultimately contributing to maintaining the proliferative and undifferentiated state of myoblasts.

Sex-Based Differences in the Myogenic Response and Inflammatory Gene Expression Following Eccentric Contractions in Humans

After muscle injury, the interaction between muscle satellite cells (SC) and the immune response is instrumental for the repair and regeneration of skeletal muscle tissue. Studies have reported sex-based differences in the skeletal muscle inflammatory and regenerative response following injury. However, many of these studies investigated such differences by manipulating the concentration of estradiol, in rodents and humans, without directly comparing males to females. We sought to explore differences in the myogenic and inflammatory response following unaccustomed eccentric exercise in males and females. We hypothesized that females would have a blunted myogenic and inflammatory response as compared to males.

Methods: 26 (13 male, 13 female) healthy young adults (22 ± 0.4 years [mean ± SEM]) performed 300 maximal eccentric contractions (180°/s) of the knee extensors. Muscle biopsies were taken before (pre) and 48 h (post) following eccentric damage. SC content and activation were determined by immunohistochemical and real time-polymerase chain reaction (rt-PCR) analysis. Inflammatory markers were analyzed using rt-PCR.

Results: Following eccentric damage, males had a greater expansion of type I-associated SC (p < 0.05), and there was a trend for a greater expansion in total SC (type I + II fibers) (p = 0.06) compared to females. There was a trend for a greater increase in Pax7 and CCL2 gene expression in males compared to females (p = 0.09).

Conclusion: We conclude that there are sex-based differences in the myogenic and inflammatory response, where females have a blunted SC and inflammatory response.

Boosting the peripheral immune response in the skeletal muscles improved motor function in ALS transgenic mice

Monocyte chemoattractant protein-1 (MCP1) is one of the most powerful pro-inflammatory chemokines. However, its signalling is pivotal in driving injured axon and muscle regeneration.

Dynamic Multiscale Regulation of Perfusion Recovery in Experimental Peripheral Arterial Disease: A Mechanistic Computational Model

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A first-of-a-kind systems biology computational model is presented that describes multiscale regulation of perfusion recovery in experimental peripheral arterial disease.

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Multilevel model calibration and validation enable high-resolution model simulations for experimental peripheral arterial disease (mouse HLI).

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An integrative model-based mechanistic characterization of the intracellular, cellular, and tissue-level features critical for the dynamic reconstitution of perfusion following different patterns of occlusion-induced ischemia in HLI is described.

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Using a model-based virtual HLI mouse population, pharmacologic inhibition of cell necrosis is predicted as a strategy with high therapeutic potential to improve perfusion recovery; in real HLI mice, the positive impact of this new strategy is then experimentally studied and confirmed.

In peripheral arterial disease (PAD), the degree of endogenous capacity to modulate revascularization of limb muscle is central to the management of leg ischemia. To characterize the multiscale and multicellular nature of revascularization in PAD, we have developed the first computational systems biology model that mechanistically incorporates intracellular, cellular, and tissue-level features critical for the dynamic reconstitution of perfusion after occlusion-induced ischemia. The computational model was specifically formulated for a preclinical animal model of PAD (mouse hindlimb ischemia [HLI]), and it has gone through multilevel model calibration and validation against a comprehensive set of experimental data so that it accurately captures the complex cellular signaling, cell–cell communication, and function during post-HLI perfusion recovery. As an example, our model simulations generated a highly detailed description of the time-dependent spectrum-like macrophage phenotypes in HLI, and through model sensitivity analysis we identified key cellular processes with potential therapeutic significance in the pathophysiology of PAD. Furthermore, we computationally evaluated the in vivo effects of different targeted interventions on post-HLI tissue perfusion recovery in a model-based, data-driven, virtual mouse population and experimentally confirmed the therapeutic effect of a novel model-predicted intervention in real HLI mice. This novel multiscale model opens up a new avenue to use integrative systems biology modeling to facilitate translational research in PAD.

Reversal of deficits in aged skeletal muscle during disuse and recovery in response to treatment with a secrotome product derived from partially differentiated human pluripotent stem cells

Aged individuals are at risk to experience slow and incomplete muscle recovery following periods of disuse atrophy. While several therapies have been employed to mitigate muscle mass loss during disuse and improve recovery, few have proven effective at both. Therefore, the purpose of this study was to examine the effectiveness of a uniquely developed secretome product (STEM) on aged skeletal muscle mass and function during disuse and recovery. Aged (22 months) male C57BL/6 were divided into PBS or STEM treatment (n = 30). Mice within each treatment were assigned to either ambulatory control (CON; 14 days of normal cage ambulation), 14 days of hindlimb unloading (HU), or 14 days of hindlimb unloading followed by 7 days of recovery (recovery). Mice were given an intramuscular delivery into the hindlimb muscle of either PBS or STEM every other day for the duration of their respective treatment group. We found that STEM-treated mice compared to PBS had greater soleus muscle mass, fiber cross-sectional area (CSA), and grip strength during CON and recovery experimental conditions and less muscle atrophy and weakness during HU. Muscle CD68 +, CD11b + and CD163 + macrophages were more abundant in STEM-treated CON mice compared to PBS, while only CD68 + and CD11b + macrophages were more abundant during HU and recovery conditions with STEM treatment. Moreover, STEM-treated mice had lower collagen IV and higher Pax7 + cell content compared to PBS across all experimental conditions. As a follow-up to examine the cell autonomous role of STEM on muscle, C2C12 myotubes were given STEM or horse serum media to examine myotube fusion/size and effects on muscle transcriptional networks. STEM-treated C2C12 myotubes were larger and had a higher fusion index and were related to elevated expression of transcripts associated with extracellular matrix remodeling. Our results demonstrate that STEM is a unique cocktail that possesses potent immunomodulatory and cytoskeletal remodeling properties that may have translational potential to improve skeletal muscle across a variety of conditions that adversely effect aging muscle.

Redressing the interactions between stem cells and immune system in tissue regeneration

Skeletal muscle has an extraordinary regenerative capacity reflecting the rapid activation and effective differentiation of muscle stem cells (MuSCs). In the course of muscle regeneration, MuSCs are reprogrammed by immune cells. In turn, MuSCs confer immune cells anti-inflammatory properties to resolve inflammation and facilitate tissue repair. Indeed, MuSCs can exert therapeutic effects on various degenerative and inflammatory disorders based on their immunoregulatory ability, including effects primed by interferon-γ (IFN-γ) and tumor necrosis factor-α (TNF-α). At the molecular level, the tryptophan metabolites, kynurenine or kynurenic acid, produced by indoleamine 2,3-dioxygenase (IDO), augment the expression of TNF-stimulated gene 6 (TSG6) through the activation of the aryl hydrocarbon receptor (AHR). In addition, insulin growth factor 2 (IGF2) produced by MuSCs can endow maturing macrophages oxidative phosphorylation (OXPHOS)-dependent anti-inflammatory functions. Herein, we summarize the current understanding of the immunomodulatory characteristics of MuSCs and the issues related to their potential applications in pathological conditions, including COVID-19.

Stem Cell and Macrophage Roles in Skeletal Muscle Regenerative Medicine

In severe muscle injury, skeletal muscle tissue structure and functionality can be repaired through the involvement of several cell types, such as muscle stem cells, and innate immune responses. However, the exact mechanisms behind muscle tissue regeneration, homeostasis, and plasticity are still under investigation, and the discovery of pathways and cell types involved in muscle repair can open the way for novel therapeutic approaches, such as cell-based therapies involving stem cells and peripheral blood mononucleate cells. Indeed, peripheral cell infusions are a new therapy for muscle healing, likely because autologous peripheral blood infusion at the site of injury might enhance innate immune responses, especially those driven by macrophages. In this review, we summarize current knowledge on functions of stem cells and macrophages in skeletal muscle repairs and their roles as components of a promising cell-based therapies for muscle repair and regeneration.

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.

Dental Pulp Mesenchymal Stem Cells Attenuate Limb Ischemia via Promoting Capillary Proliferation and Collateral Development in a Preclinical Model

Critical limb ischemia (CLI), an end-stage manifestation of peripheral artery disease (PAD), still lacks effective therapeutic strategies. Recently, dental pulp-derived mesenchymal stem cells (DP-MSCs) have been attracting more and more attentions in therapeutic applications due to their high proliferation ability, powerful osteogenic differentiation potential, and effective anti-inflammatory effects. In this study, we compared the therapeutic effects of MSCs derived from different sources in a femoral artery-ligated preclinical ischemic model. We found that treatments with MSCs, including bone marrow- (BM-), adipose- (AD-), dental pulp- (DP-), and umbilical cord- (UC-) derived MSCs, improved limb functions, reduced inflammatory responses, increased angiogenesis, and promoted regeneration of muscle fiber. Among them, DP-MSCs and BM-MSCs produced much more impressive effects in restoring limb functions and promoting angiogenesis. The flow velocity restored to nearly 20% of the normal level at 3 weeks after treatments with DP-MSCs and BM-MSCs, and obvious capillary proliferation and collateral development could be observed. Although neovascularization was induced in the ischemic limb after ligation, MSCs, especially DP-MSCs, significantly enhanced the angiogenesis. In vitro experiments showed that serum deprivation improved the expression of angiogenic factors, growth factors, and chemokines in DP-MSCs and UC-MSCs, but not in BM-MSCs and AD-MSCs. However, DP-MSCs produced stronger therapeutic responses than UC-MSCs, which might be due to the higher expression of hepatocyte growth factor (HGF) and hypoxia-inducible factor-1 α (HIF-1α). We speculated that DP-MSCs might stimulate angiogenesis and promote tissue repair via expressing and secreting angiogenic factors, growth factors, and chemokines, especially HGF and HIF-1α. In conclusion, DP-MSCs might be a promising approach for treating CLI.

Myeloid cell-mediated targeting of LIF to dystrophic muscle causes transient increases in muscle fiber lesions by disrupting the recruitment and dispersion of macrophages in muscle

Leukemia inhibitory factor (LIF) can influence development by increasing cell proliferation and inhibiting differentiation. Because of its potency for expanding stem cell populations, delivery of exogenous LIF to diseased tissue could have therapeutic value. However, systemic elevations of LIF can have negative, off-target effects. We tested whether inflammatory cells expressing a LIF transgene under control of a leukocyte-specific, CD11b promoter provide a strategy to target LIF to sites of damage in the mdx mouse model of Duchenne muscular dystrophy, leading to increased numbers of muscle stem cells and improved muscle regeneration. However, transgene expression in inflammatory cells did not increase muscle growth or increase numbers of stem cells required for regeneration. Instead, transgene expression disrupted the normal dispersion of macrophages in dystrophic muscles, leading to transient increases in muscle damage in foci where macrophages were highly concentrated during early stages of pathology. The defect in inflammatory cell dispersion reflected impaired chemotaxis of macrophages to C-C motif chemokine ligand-2 and local increases of LIF production that produced large aggregations of cytolytic macrophages. Transgene expression also induced a shift in macrophage phenotype away from a CD206+, M2-biased phenotype that supports regeneration. However, at later stages of the disease when macrophage numbers declined, they dispersed in the muscle, leading to reductions in muscle fiber damage, compared to non-transgenic mdx mice. Together, the findings show that macrophage-mediated delivery of transgenic LIF exerts differential effects on macrophage dispersion and muscle damage depending on the stage of dystrophic pathology.

Inflammation during post-injury skeletal muscle regeneration

The adult skeletal muscle fully regenerates after injury thanks to the properties of muscle stem cells that follow the adult myogenic program to replace damaged myofibers. Muscle regeneration also relies upon the coordinated actions of several other cell types, among which immune cells. Leukocytes infiltrate the damaged muscle soon after injury and support the regeneration process in a variety of ways, from the activation of muscle stem cells to the maturation of newly formed myofibers. Leukocytes also interact with other cell types such as fibroadipogenic precursors and endothelial cells. This review presents the interactions that leukocytes develop with the cells present in their vicinity and the impact they have on skeletal muscle regeneration.

The Role of Innate and Adaptive Immune Cells in Skeletal Muscle Regeneration

Skeletal muscle regeneration is highly dependent on the inflammatory response. A wide variety of innate and adaptive immune cells orchestrate the complex process of muscle repair. This review provides information about the various types of immune cells and biomolecules that have been shown to mediate muscle regeneration following injury and degenerative diseases. Recently developed cell and drug-based immunomodulatory strategies are highlighted. An improved understanding of the immune response to injured and diseased skeletal muscle will be essential for the development of therapeutic strategies.

Dermal macrophage and its potential in inducing hair follicle regeneration

Hair follicle (HF) is an excellent mini-model to study adult tissue regeneration, since it can regenerate itself under appropriate stress settings via interaction with niche components. Dermal macrophages, a group of heterogeneous cell populations, serve as key regulators in this microenvironment. Recent advances in phenotype identification and lineage tracing have unveiled various dermal macrophage subsets involved in stress-induced hair regeneration through different mechanisms, where HF structural integrity is impaired to varying degrees. This review summarized current knowledge regarding the distribution, sources, phenotypes of dermal macrophages in association with HF, as well as the mechanisms underlying macrophage-mediated hair regeneration in response to different internal-stress settings. Further investigation on macrophage dynamics will provide novel cell-targeting therapies for HF engineering and hair loss.

Does C-C Motif Chemokine Ligand 2 (CCL2) Link Obesity to a Pro-Inflammatory State?

The mechanisms of how obesity contributes to the development of cardio-metabolic diseases are not entirely understood. Obesity is frequently associated with adipose tissue dysfunction, characterized by, e.g., adipocyte hypertrophy, ectopic fat accumulation, immune cell infiltration, and the altered secretion of adipokines. Factors secreted from adipose tissue may induce and/or maintain a local and systemic low-grade activation of the innate immune system. Attraction of macrophages into adipose tissue and altered crosstalk between macrophages, adipocytes, and other cells of adipose tissue are symptoms of metabolic inflammation. Among several secreted factors attracting immune cells to adipose tissue, chemotactic C-C motif chemokine ligand 2 (CCL2) (also described as monocyte chemoattractant protein-1 (MCP-1)) has been shown to play a crucial role in adipose tissue macrophage infiltration. In this review, we aimed to summarize and discuss the current knowledge on CCL2 with a focus on its role in linking obesity to cardio-metabolic diseases.

Aging of the immune system and impaired muscle regeneration: A failure of immunomodulation of adult myogenesis

Skeletal muscle regeneration that follows acute injury is strongly influenced by interactions with immune cells that invade and proliferate in the damaged tissue. Discoveries over the past 20 years have identified many of the key mechanisms through which myeloid cells, especially macrophages, regulate muscle regeneration. In addition, lymphoid cells that include CD8+ T-cells and regulatory T-cells also significantly affect the course of muscle regeneration. During aging, the regenerative capacity of skeletal muscle declines, which can contribute to progressive loss of muscle mass and function. Those age-related reductions in muscle regeneration are accompanied by systemic, age-related changes in the immune system, that affect many of the myeloid and lymphoid cell populations that can influence muscle regeneration. In this review, we present recent discoveries that indicate that aging of the immune system contributes to the diminished regenerative capacity of aging muscle. Intrinsic, age-related changes in immune cells modify their expression of factors that affect the function of a population of muscle stem cells, called satellite cells, that are necessary for normal muscle regeneration. For example, age-related reductions in the expression of growth differentiation factor-3 (GDF3) or CXCL10 by macrophages negatively affect adult myogenesis, by disrupting regulatory interactions between macrophages and satellite cells. Those changes contribute to a reduction in the numbers and myogenic capacity of satellite cells in old muscle, which reduces their ability to restore damaged muscle. In addition, aging produces changes in the expression of molecules that regulate the inflammatory response to injured muscle, which also contributes to age-related defects in muscle regeneration. For example, age-related increases in the production of osteopontin by macrophages disrupts the normal inflammatory response to muscle injury, resulting in regenerative defects. These nascent findings represent the beginning of a newly-developing field of investigation into mechanisms through which aging of the immune system affects muscle regeneration.

Macrophage-Derived Vascular Endothelial Growth Factor-A Is Integral to Neuromuscular Junction Reinnervation after Nerve Injury

Functional recovery in the end target muscle is a determinant of outcome after peripheral nerve injury. The neuromuscular junction (NMJ) provides the interface between nerve and muscle and includes non-myelinating terminal Schwann cells (tSCs). After nerve injury, tSCs extend cytoplasmic processes between NMJs to guide axon growth and NMJ reinnervation. The mechanisms related to NMJ reinnervation are not known. We used multiple mouse models to investigate the mechanisms of NMJ reinnervation in both sexes, specifically whether macrophage-derived vascular endothelial growth factor-A (Vegf-A) is crucial to establishing NMJ reinnervation at the end target muscle. Both macrophage number and Vegf-A expression increased in end target muscles after nerve injury and repair. In mice with impaired recruitment of macrophages and monocytes (Ccr2-/- mice), the absence of CD68+ cells (macrophages) in the muscle resulted in diminished muscle function. Using a Vegf-receptor 2 (VegfR2) inhibitor (cabozantinib; CBZ) via oral gavage in wild-type (WT) mice resulted in reduced tSC cytoplasmic process extension and decreased NMJ reinnervation compared with saline controls. Mice with Vegf-A conditionally knocked out in macrophages (Vegf-Afl/fl; LysMCre mice) demonstrated a more prolonged detrimental effect on NMJ reinnervation and worse functional muscle recovery. Together, these results show that contributions of the immune system are integral for NMJ reinnervation and functional muscle recovery after nerve injury.SIGNIFICANCE STATEMENT This work demonstrates beneficial contributions of a macrophage-mediated response for neuromuscular junction (NMJ) reinnervation following nerve injury and repair. Macrophage recruitment occurred at the NMJ, distant from the nerve injury site, to support functional recovery at the muscle. We have shown hindered terminal Schwann cell (tSC) injury response and NMJ recovery with inhibition of: (1) macrophage recruitment after injury; (2) vascular endothelial growth factor receptor 2 (VegfR2) signaling; and (3) Vegf secretion from macrophages. We conclude that macrophage-derived Vegf is a key component of NMJ recovery after injury. Determining the mechanisms active at the end target muscle after motor nerve injury reveals new therapeutic targets that may translate to improve motor recovery following nerve injury.

Dehydrated Human Amnion Chorion Membrane as Treatment for Pediatric Burns

Objective: Pediatric burns are a major source of injury and in the absence of adequate care can lead to lifelong functional loss and disfigurement. While split thickness skin autografts are the current standard of care for deep partial and full-thickness burns, this approach is associated with considerable morbidity. For this reason, alternative skin substitutes such as allografts have gained interest. Approach: In the present study, we present a case series of 30 children with various types of burns treated with dehydrated human amnion chorion membrane (dHACM). Results: We show that treatment with dHACM is associated with an excellent rate of healing comparable to split thickness skin grafts with less rate of hypertrophic scar and contracture. Innovation: Treatment with dHACM is particularly attractive as it consists of many tissue regenerative factors, such as growth factors and immune modulators, thus it will reduce the risk of scaring. Conclusion: While dHACM is associated with an increased upfront cost, treating patients with small to moderate-sized burns with dHACM in their regional centers works to decrease downstream costs such as management of prolonged pain from donor-site morbidity, revisional surgeries from scar and contractures of split thickness grafts, and avoiding the cost of transfer to higher level centers of care. Our findings challenge the current standard of care, suggesting that dHACM provides an alternative to the current use of split thickness skin grafting and is a safe, feasible, and potentially superior substitute for the management of small to moderate total body surface area partial and full-thickness pediatric burns.

Differential Effects of Myeloid Cell PPARδ and IL-10 in Regulating Macrophage Recruitment, Phenotype, and Regeneration following Acute Muscle Injury

Changes in macrophage phenotype in injured muscle profoundly influence regeneration. In particular, the shift of macrophages from a proinflammatory (M1 biased) phenotype to a proregenerative (M2 biased) phenotype characterized by expression of CD206 and CD163 is essential for normal repair. According to the current canonical mechanism regulating for M1/M2 phenotype transition, signaling through PPARδ is necessary for obtaining the M2-biased phenotype. Our findings confirm that the murine myeloid cell-targeted deletion of Ppard reduces expression in vitro of genes that are activated in M2-biased macrophages; however, the mutation in mice in vivo increased numbers of CD206+ M2-biased macrophages and did not reduce the expression of phenotypic markers of M2-biased macrophages in regenerating muscle. Nevertheless, the mutation impaired CCL2-mediated chemotaxis of macrophages and slowed revascularization of injured muscle. In contrast, null mutation of IL-10 diminished M2-biased macrophages but produced no defects in muscle revascularization. Our results provide two significant findings. First, they illustrate that mechanisms that regulate macrophage phenotype transitions in vitro are not always predictive of mechanisms that are most important in vivo. Second, they show that mechanisms that regulate macrophage phenotype transitions differ in different in vivo environments.

Water immersion methods do not alter muscle damage and inflammation biomarkers after high-intensity sprinting and jumping exercise

Purpose

The aim of this study was to compare the efficacy of three water immersion interventions performed after active recovery compared to active recovery only on the resolution of inflammation and markers of muscle damage post-exercise.

Methods

Nine physically active men (n = 9; age 20‒35 years) performed an intensive loading protocol, including maximal jumps and sprinting on four occasions. After each trial, one of three recovery interventions (10 min duration) was used in a random order: cold-water immersion (CWI, 10 °C), thermoneutral water immersion (TWI, 24 °C), contrast water therapy (CWT, alternately 10 °C and 38 °C). All of these methods were performed after an active recovery (10 min bicycle ergometer), and were compared to active recovery only (ACT). 5 min, 1, 24, 48, and 96 h after exercise bouts, immune response and recovery were assessed through leukocyte subsets, monocyte chemoattractant protein-1, myoglobin and high-sensitivity C-reactive protein concentrations.

Results

Significant changes in all blood markers occurred at post-loading (p < 0.05), but there were no significant differences observed in the recovery between methods. However, retrospective analysis revealed significant trial-order effects for myoglobin and neutrophils (p < 0.01). Only lymphocytes displayed satisfactory reliability in the exercise response, with intraclass correlation coefficient > 0.5.

Conclusions

The recovery methods did not affect the resolution of inflammatory and immune responses after high-intensity sprinting and jumping exercise. It is notable that the biomarker responses were variable within individuals. Thus, the lack of differences between recovery methods may have been influenced by the reliability of exercise-induced biomarker responses.

Electronic supplementary material

The online version of this article (10.1007/s00421-020-04481-8) contains supplementary material, which is available to authorized users.

Surface Tethering of Inflammation-Modulatory Nanostimulators to Stem Cells for Ischemic Muscle Repair

Stem cell transplantation has been a promising treatment for peripheral arterial diseases in the past decade. Stem cells act as living bioreactors of paracrine factors that orchestrate tissue regeneration. Prestimulated adipose-derived stem cells (ADSCs) have been proposed as potential candidates but have been met with challenges in activating their secretory activities for clinical use. Here, we propose that tethering the ADSC surface with nanoparticles releasing tumor necrosis factor α (TNFα), named nanostimulator, would stimulate cellular secretory activity in situ. We examined this hypothesis by complexing octadecylamine-grafted hyaluronic acid onto a liposomal carrier of TNFα. Hyaluronic acid increased the liposomal stability and association to CD44 on ADSC surface. ADSCs tethered with these TNFα carriers exhibited up-regulated secretion of proangiogenic vascular endothelial growth factor and immunomodulatory prosteoglandin E2 (PGE₂) while decreasing secretion of antiangiogenic pigment epithelium-derived factors. Accordingly, ADSCs tethered with nanostimulators promoted vascularization in a 3D microvascular chip and enhanced recovery of perfusion, walking, and muscle mass in a murine ischemic hindlimb compared to untreated ADSCs. We propose that this surface tethering strategy for in situ stimulation of stem cells would replace the costly and cumbersome preconditioning process and expedite clinical use of stem cells for improved treatments of various injuries and diseases.

Inhibition of the migration of MCP-1 positive cells by icing applied soon after crush injury to rat skeletal muscle

Migration of the macrophages to the injured site soon after the skeletal muscle injury is crucial for subsequent regeneration of the muscle fibers. The Monocyte chemoattractant protein-1 (MCP-1) is important chemokine for regulating migration of the monocytes/macrophages. Earlier reports have discussed that icing applied soon after muscle crush injury retards muscle regeneration through retardation of macrophage migration. The MCP-1+ cells and neutrophils might promote the migration of the macrophages. To test the hypothesis that icing soon after the skeletal muscle injury affects MCP-1+ cells and neutrophils, we examined the effect of icing on MCP-1+ cells and neutrophils after crush injury to skeletal muscle in rats. Owing to the icing application for 20 min soon after the injury, accumulation of the macrophages was inhibited until 12 h after injury. Numbers of the neutrophils at 3 h after the injury and the MCP-1+ cells at 6 h and later after the injury in the icing group were significantly lower than those in the non-icing group, suggesting that these phenomena contribute to the retardation of macrophage migration.

RG, Adler KA, Cook G, Molina PE, Simon L. Chronic Alcohol Dysregulates Skeletal Muscle Myogenic Gene Expression after Hind Limb Immobilization in Female Rats

Alcohol use and aging are risk factors for falls requiring immobilization and leading to skeletal muscle atrophy. Skeletal muscle regeneration is integral to post-immobilization recovery. This study aimed to elucidate the effects of alcohol and ovarian hormone loss on the expression of genes implicated in muscle regeneration. Three-month-old female rats received an ovariectomy or a sham surgery, consumed an alcohol-containing or control diet for 10 weeks, were subjected to unilateral hind limb immobilization for seven days, and finally were allowed a three (3d)- or 14 (14d)-day recovery. Immobilization decreased the quadriceps weight at 3d and 14d, and alcohol decreased the quadriceps weight at 14d in the nonimmobilized hind limb (NI). At 3d, alcohol decreased gene expression of myoblast determination protein (MyoD) in the immobilized hind limb (IMM) and myocyte enhancer factor (Mef)2C and tumor necrosis factor (TNF)α in NI, and ovariectomy increased MyoD and decreased TNFα expression in NI. At 14d, alcohol increased the gene expression of Mef2C, MyoD, TNFα, and transforming growth factor (TFG)β in IMM and decreased monocyte chemoattractant protein (MCP)1 expression in NI; ovariectomy increased TNFα expression in NI, and alcohol and ovariectomy together increased Mef2C expression in NI. Despite increased TGFβ expression, there was no concomitant alcohol-mediated increase in collagen in IMM at 14d. Overall, these data indicate that alcohol dysregulated the post-immobilization alteration in the expression of genes implicated in regeneration. Whether alcohol-mediated molecular changes correspond with post-immobilization functional alterations remains to be determined.

Changes in gene expression in human skeletal stem cells transduced with constitutively active Gsα correlates with hallmark histopathological changes seen in fibrous dysplastic bone

Fibrous dysplasia (FD) of bone is a complex disease of the skeleton caused by dominant activating mutations of the GNAS locus encoding for the α subunit of the G protein-coupled receptor complex (Gsα). The mutation involves a substitution of arginine at position 201 by histidine or cysteine (Gsα^{R201H or R201C}), which leads to overproduction of cAMP. Several signaling pathways are implicated downstream of excess cAMP in the manifestation of disease. However, the pathogenesis of FD remains largely unknown. The overall FD phenotype can be attributed to alterations of skeletal stem/progenitor cells which normally develop into osteogenic or adipogenic cells (in cis), and are also known to provide support to angiogenesis, hematopoiesis, and osteoclastogenesis (in trans). In order to dissect the molecular pathways rooted in skeletal stem/progenitor cells by FD mutations, we engineered human skeletal stem/progenitor cells with the Gsα^R201C mutation and performed transcriptomic analysis. Our data suggest that this FD mutation profoundly alters the properties of skeletal stem/progenitor cells by pushing them towards formation of disorganized bone with a concomitant alteration of adipogenic differentiation. In addition, the mutation creates an altered in trans environment that induces neovascularization, cytokine/chemokine changes and osteoclastogenesis. In silico comparison of our data with the signature of FD craniofacial samples highlighted common traits, such as the upregulation of ADAM (A Disintegrin and Metalloprotease) proteins and other matrix-related factors, and of PDE7B (Phosphodiesterase 7B), which can be considered as a buffering process, activated to compensate for excess cAMP. We also observed high levels of CEBPs (CCAAT-Enhancer Binding Proteins) in both data sets, factors related to browning of white fat. This is the first analysis of the reaction of human skeletal stem/progenitor cells to the introduction of the FD mutation and we believe it provides a useful background for further studies on the molecular basis of the disease and for the identification of novel potential therapeutic targets.

Accelerated Muscle Recovery After In Vivo Curcumin Supplementation

The currently available treatment options for muscle injuries are suboptimal and often delay muscle recovery. In this study, the effects of curcumin on inflammation and skeletal muscle regeneration after contusion-induced injury in mice were investigated. The mice were randomly assigned to 4 groups, namely normal control (NC), with induced injury (mass-drop injury, MDI) and without treatment (MDI [M]), with induced injury and diclofenac (DCF) treatment (MDI + DCF [M + D]), and with induced injury and curcumin treatment (MDI + curcumin [M + C]). Contusion-induced injury was inflicted on the left gastrocnemius muscle, and DCF or curcumin was orally administered after injury once per day for 7 days. The M group exhibited significantly higher lipid peroxidation, myeloperoxidase (MPO), and desmin than the NC group. The M + D and M + C groups have lower lipid peroxidation and neutrophils (decrease in MPO protein) and higher muscle satellite cell regeneration (increase in desmin protein) than the M group. Additionally, for the contusion-induced muscle injury, curcumin could affect the specific proteins of inflammation, neutrophils, and differentiation of satellite cells, including Ikk-α/ß, MPO, and myogenin. In conclusion, curcumin potentially accelerates muscle recovery; therefore, it may be a potential candidate for further research as an effective treatment to enhance muscle repair.

Modulation of Klotho expression in injured muscle perturbs Wnt signalling and influences the rate of muscle growth

NEW FINDINGS

What is the central question of this study? Does modulating the expression of Klotho affect myogenesis following acute injury of healthy, non-senescent muscle? What is the main finding and its importance? Klotho can accelerate muscle growth following acute injury of healthy, adult mice, which supports the possibility that increased delivery of Klotho could have therapeutic value for improving repair of damaged muscle.

ABSTRACT

Skeletal muscle injuries activate a complex programme of myogenesis that can restore normal muscle structure. We tested whether modulating the expression of klotho influenced the response of mouse muscles to acute injury. Our findings show that klotho expression in muscle declines at 3 days post-injury. That reduction in klotho expression coincided with elevated expression of targets of Wnt signalling (Ccnd1; Myc) and increased MyoD+ muscle cell numbers, reflecting the onset of myogenic cell differentiation. klotho expression subsequently increased at 7 days post-injury with elevated expression occurring primarily in inflammatory lesions, which was accompanied by reduced expression of Wnt target genes (Ccnd1: 91%; Myc: 96%). Introduction of a klotho transgene maintained high levels of klotho expression over the course of muscle repair and attenuated the increases in Ccnd1 and Myc expression that occurred at 3 days post-injury. Correspondingly, transgene expression reduced Wnt signalling in Pax7+ cells, reflected by reductions in Pax7+ cells expressing active β-catenin, and reduced the numbers of MyoD+ cells at 3 days post-injury. At 21 days post-injury, muscles in klotho transgenic mice showed increased Pax7+ and decreased myogenin+ cell densities and large increases in myofibre size. Likewise, treating myogenic cells in vitro with Klotho reduced Myod expression but did not affect Pax7 expression. Muscle inflammation was only slightly modulated by increased klotho expression, initially reducing the expression of M2-biased macrophage markers Cd163 and Cd206 at 3 days post-injury and later increasing the expression of pan-macrophage marker F480 and Cd68 at 21 days post-injury. Collectively, our study shows that Klotho modulates myogenesis and that increased expression accelerates muscle growth after injury.

Prostaglandin E2 Receptor 2 Modulates Macrophage Activity for Cardiac Repair

Background

Prostaglandin E₂ has long been known to be an immune modulator. It is released after tissue injury and plays a role in modulating macrophage activities, which are essential for tissue regeneration. However, the involvement of prostaglandin E₂ receptor 2 (EP2)–dependent regulation of macrophages in postischemic heart is unclear. This study aims to evaluate the role of EP2 in damaged heart.

Methods and Results

The effect of EP2 in postischemic heart was evaluated using EP2‐deficient transgenic mice. We demonstrated that cardiac function was worse after myocardial injury on loss of EP2. Furthermore, EP2 deficiency also altered proinflammatory response and resulted in a defect in macrophage recruitment to the injured myocardium. Transcriptome analysis revealed that the expression of erythroid differentiation regulator 1 (Erdr1) was significantly induced in EP2‐deficient macrophages. Knocking down Erdr1 expression restored migration ability of EP2‐deficient cells both in vitro and in vivo. By using a genetic fate‐mapping approach, we showed that abolishment of EP2 expression effectively attenuated cell replenishment.

Conclusions

The EP2‐dependent signaling pathway plays a critical role in regulating macrophage recruitment to the injured myocardium, thereby exerting a function in modulating the inflammatory microenvironment for cardiac repair.

Macrophages fine tune satellite cell fate in dystrophic skeletal muscle of mdx mice

Satellite cells (SCs) are muscle stem cells that remain quiescent during homeostasis and are activated in response to acute muscle damage or in chronic degenerative conditions such as Duchenne Muscular Dystrophy. The activity of SCs is supported by specialized cells which either reside in the muscle or are recruited in regenerating skeletal muscles, such as for instance macrophages (MΦs). By using a dystrophic mouse model of transient MΦ depletion, we describe a shift in identity of muscle stem cells dependent on the crosstalk between MΦs and SCs. Indeed MΦ depletion determines adipogenic conversion of SCs and exhaustion of the SC pool leading to an exacerbated dystrophic phenotype. The reported data could also provide new insights into therapeutic approaches targeting inflammation in dystrophic muscles.

Muscular dystrophies are a heterogenous group of genetic disorders characterized by muscle wasting, leading to loss of mobility and eventually to death due to respiratory or cardiac failure. Duchenne Muscular Dystrophy (DMD) is one of the most severe dystrophies and is caused by the loss of functional dystrophin protein owing to genetic mutations, consequently, the sarcolemma becomes fragile and susceptible to muscle damage induced by contraction.

Satellite cells (SCs) are skeletal muscle stem cells that mediate the repair process leading to muscle regeneration. Dystrophic muscles undergo continuous cycles of degeneration and regeneration eventually culminating in myofiber loss and deposition of fibrous and fatty connective tissue. Inflammation is always associated with the muscle regeneration process. Among different types of inflammatory cells, mainly macrophages (MΦs) are present in regenerating skeletal muscles and are involved in the regenerative process both after an acute injury and during pathological conditions such as DMD. We focused on the cross-talk between MΦs and SCs in a mouse model of DMD and highlighted a role of MΦs in preserving the SC identity.

Featured characteristics and pivotal roles of satellite cells in skeletal muscle regeneration

Skeletal muscle, the essential organ for locomotion, as well as energy reservoir and expenditure, has robust regenerative capacity in response to mechanical stress and injury. As muscle-specific stem cells, satellite cells are responsible for providing new myoblasts during the process of muscle growth and regeneration. Self-renewal capacity and the fate of satellite cells are highly regulated and influenced by their surrounding factors, such as extracellular matrix and soluble proteins. The strong myogenic potential of satellite cells makes them a potential resource for stem cell therapy to cure genetic muscle disease and repair injured muscle. Here, we both review key features of satellite cells during skeletal muscle development and regeneration and summarize recent outcomes of satellite cell transplantation studies.

Macrophages in inflammation, repair and regeneration

Tissue injury triggers a complex series of cellular responses, starting from inflammation activated by tissue and cell damage and proceeding to healing. By clearing cell debris, activating and resolving inflammation and promoting fibrosis, macrophages play key roles in most, if not all, phases of the response to injury. Recent studies of the mechanisms underlying the initial inflammation and later tissue regeneration and repair revealed that macrophages bridge these processes in part by supporting and activating stem/progenitor cells, clearing damaged tissue, remodeling extracellular matrix to prepare scaffolding for regeneration and promoting angiogenesis. However, macrophages also have a central role in the development of pathology induced by failed resolution (e.g. chronic inflammation) and excessive scarring. In this review, we summarize the activities of macrophages in inflammation and healing in response to acute injury in tissues with differing regenerative capacities. While macrophages lead similar processes in response to tissue injury in these tissues, their priorities and the consequences of their activities differ among tissues. Moreover, the magnitude, nature and duration of injury also greatly affect cellular responses and healing processes. In particular, continuous injury and/or failed resolution of inflammation leads to chronic ailments in which macrophage activities may become detrimental.

Functional regeneration of tissue engineered skeletal muscle in vitro is dependent on the inclusion of basement membrane proteins

Skeletal muscle has a high regenerative capacity, injuries trigger a regenerative program which restores tissue function to a level indistinguishable to the pre-injury state. However, in some cases where significant trauma occurs, such as injuries seen in military populations, the regenerative process is overwhelmed and cannot restore full function. Limited clinical interventions exist which can be used to promote regeneration and prevent the formation of non-regenerative defects following severe skeletal muscle trauma. Robust and reproducible techniques for modelling complex tissue responses are essential to promote the discovery of effective clinical interventions. Tissue engineering has been highlighted as an alternative method, allowing the generation of three-dimensional in vivo like tissues without laboratory animals. Reducing the requirement for animal models promotes rapid screening of potential clinical interventions, as these models are more easily manipulated, genetically and pharmacologically, and reduce the associated cost and complexity, whilst increasing access to models for laboratories without animal facilities. In this study, an in vitro chemical injury using barium chloride is validated using the C2C12 myoblast cell line, and is shown to selectively remove multinucleated myotubes, whilst retaining a regenerative mononuclear cell population. Monolayer cultures showed limited regenerative capacity, with basement membrane supplementation or extended regenerative time incapable of improving the regenerative response. Conversely tissue engineered skeletal muscles, supplemented with basement membrane proteins, showed full functional regeneration, and a broader in vivo like inflammatory response. This work outlines a freely available and open access methodology to produce a cell line-based tissue engineered model of skeletal muscle regeneration.

Glucocorticoids Shape Macrophage Phenotype for Tissue Repair

Inflammation is a complex process which is highly conserved among species. Inflammation occurs in response to injury, infection, and cancer, as an allostatic mechanism to return the tissue and to return the organism back to health and homeostasis. Excessive, or chronic inflammation is associated with numerous diseases, and thus strategies to combat run-away inflammation is required. Anti-inflammatory drugs were therefore developed to switch inflammation off. However, the inflammatory response may be beneficial for the organism, in particular in the case of sterile tissue injury. The inflammatory response can be divided into several parts. The first step is the mounting of the inflammatory reaction itself, characterized by the presence of pro-inflammatory cytokines, and the infiltration of immune cells into the injured area. The second step is the resolution phase, where immune cells move toward an anti-inflammatory phenotype and decrease the secretion of pro-inflammatory cytokines. The last stage of inflammation is the regeneration process, where the tissue is rebuilt. Innate immune cells are major actors in the inflammatory response, of which, macrophages play an important role. Macrophages are highly sensitive to a large number of environmental stimuli, and can adapt their phenotype and function on demand. This change in phenotype in response to the environment allow macrophages to be involved in all steps of inflammation, from the first mounting of the pro-inflammatory response to the post-damage tissue repair.

Glucose Ingestion Inhibits Endurance Exercise-Induced IL-6 Producing Macrophage Infiltration in Mice Muscle

Background: Carbohydrate (CHO) supplementation during exercise attenuates exercise-induced increases in plasma Interleukin (IL)-6 concentration. However, the effects of CHO supplementation on muscle IL-6 production during endurance exercise is controversial. The purpose of this study was to investigate the effects of CHO supplementation on muscle IL-6 production during endurance exercise with a special focus on the IL-6 producing cells. Methods: C57BL/6J mice were divided into three groups—sedentary with water ingestion group as the control (Con; n = 10), exercise with water ingestion group (Ex; n = 10), and exercise with 6% glucose ingestion group (Ex + glucose; n = 10). The Ex and Ex + glucose groups completed 3 h of treadmill running (24 m/min, 7% incline) and were sacrificed immediately after exercise. Results: The exercise-induced increases of plasma IL-6 concentration and gastrocnemius IL-6 gene expression were attenuated by glucose ingestion. However, the increases of soleus IL-6 gene expression and gastrocnemius and soleus IL-6 protein expression were not attenuated by glucose ingestion. Furthermore, we observed that macrophages that infiltrated muscle produce IL-6 and glucose ingestion attenuated the infiltration of IL-6-producing macrophages. Conclusion: This study revealed that infiltrating macrophages may be one type of IL-6-producing cells during endurance exercise, and the infiltration of these cells in muscle was attenuated by glucose ingestion. However, the effects of glucose ingestion on muscle IL-6 production were limited.

Targeting a therapeutic LIF transgene to muscle via the immune system ameliorates muscular dystrophy

Many potentially therapeutic molecules have been identified for treating Duchenne muscular dystrophy. However, targeting those molecules only to sites of active pathology is an obstacle to their clinical use. Because dystrophic muscles become extensively inflamed, we tested whether expressing a therapeutic transgene in leukocyte progenitors that invade muscle would provide selective, timely delivery to diseased muscle. We designed a transgene in which leukemia inhibitory factor (LIF) is under control of a leukocyte-specific promoter and transplanted transgenic cells into dystrophic mice. Transplantation diminishes pathology, reduces Th2 cytokines in muscle and biases macrophages away from a CD163+/CD206+ phenotype that promotes fibrosis. Transgenic cells also abrogate TGFβ signaling, reduce fibro/adipogenic progenitor cells and reduce fibrogenesis of muscle cells. These findings indicate that leukocytes expressing a LIF transgene reduce fibrosis by suppressing type 2 immunity and highlight a novel application by which immune cells can be genetically modified as potential therapeutics to treat muscle disease.

An altered response in macrophage phenotype following damage in aged human skeletal muscle: implications for skeletal muscle repair

The purpose of this study was to test the hypothesis that macrophage polarization is altered in old compared to young skeletal muscle, possibly contributing to the poor satellite cell response observed in older muscle tissue. Muscle biopsies were collected prior to and at 3, 24, and 72 h following a muscle-damaging exercise in young and old individuals. Immunohistochemistry was used to measure i.m. macrophage content and phenotype, and cell culture experiments tested macrophage behavior and influence on primary myoblasts from older individuals. We found that macrophage infiltration was similar between groups at 24 (young: 3712 ± 2407 vs. old: 5035 ± 2978 cells/mm³) and 72 (young: 4326 ± 2622 vs. old: 5287 ± 2248 cells/mm³) hours postdamage, yet the proportion of macrophages that expressed the proinflammatory marker CD11b were markedly lower in the older subjects (young: 74.5 ± 15 vs. old: 52.6 ± 17%). This finding was coupled with a greater overall proportion of CD206⁺, anti-inflammatory macrophages in the old (group: P = 0.0005). We further demonstrate in vitro that proliferation, and in some cases differentiation, of old primary human myoblasts increase as much as 30% when exposed to a young macrophage-conditioned environment. Collectively, the data suggest that old macrophages appear less capable of adapting and maintaining inflammatory function, which may contribute to poor satellite cell activation and delayed recovery from muscle damage.-Sorensen, J. R., Kaluhiokalani, J. P., Hafen, P. S., Deyhle, M. R., Parcell, A. C., Hyldahl, R. D. An altered response in macrophage phenotype following damage in aged human skeletal muscle: implications for skeletal muscle repair.