The synergistic effect of treadmill running on stem-cell transplantation to heal injured skeletal muscle

Age-associated functional healing of musculoskeletal trauma through regenerative engineering and rehabilitation

Traumatic musculoskeletal injuries that lead to volumetric muscle loss (VML) are challenged by irreparable soft tissue damage, impaired regenerative ability, and reduced muscle function. Regenerative rehabilitation strategies involving the pairing of engineered therapeutics with exercise have guided considerable advances in the functional repair of skeletal muscle following VML. However, few studies evaluate the efficacy of regenerative rehabilitation across the lifespan. In the current study, young and aged mice are treated with an engineered muscle, consisting of nanofibrillar-aligned collagen laden with myogenic cells, in combination with voluntary running activity following a VML injury. Overall, young mice perform at higher running volumes and intensities compared to aged mice but exhibit similar volumes relative to age-matched baselines. Additionally, young mice are highly responsive to the dual treatment showing enhanced force production (p < 0.001), muscle mass (p < 0.05), and vascular density (p < 0.01) compared to age-matched controls. Aged mice display upregulation of circulating inflammatory cytokines and show no significant regenerative response to treatment, suggesting a diminished efficacy of regenerative rehabilitation in aged populations. These findings highlight the restorative potential of regenerative engineering and rehabilitation for the treatment of traumatic musculoskeletal injuries in young populations and the complimentary need for age-specific interventions and studies to serve broader patient demographics.

Regenerative rehabilitation: a novel multidisciplinary field to maximize patient outcomes

Regenerative rehabilitation is a novel and rapidly developing multidisciplinary field that converges regenerative medicine and rehabilitation science, aiming to maximize the functions of disabled patients and their independence. While regenerative medicine provides state-of-the-art technologies that shed light on difficult-to-treated diseases, regenerative rehabilitation offers rehabilitation interventions to improve the positive effects of regenerative medicine. However, regenerative scientists and rehabilitation professionals focus on their aspects without enough exposure to advances in each other's field. This disconnect has impeded the development of this field. Therefore, this review first introduces cutting-edge technologies such as stem cell technology, tissue engineering, biomaterial science, gene editing, and computer sciences that promote the progress pace of regenerative medicine, followed by a summary of preclinical studies and examples of clinical investigations that integrate rehabilitative methodologies into regenerative medicine. Then, challenges in this field are discussed, and possible solutions are provided for future directions. We aim to provide a platform for regenerative and rehabilitative professionals and clinicians in other areas to better understand the progress of regenerative rehabilitation, thus contributing to the clinical translation and management of innovative and reliable therapies.

Biomaterial-Based Regenerative Strategies for Volumetric Muscle Loss: Challenges and Solutions

Significance: Volumetric muscle loss (VML) is caused by the loss of significant amounts of skeletal muscle tissue. VML cannot be repaired by intrinsic regenerative processes, resulting in permanent loss of muscle function and disability. Current rehabilitative-focused treatment strategies lack efficacy and do not restore muscle function, indicating the need for the development of effective regenerative strategies. Recent Advances: Recent developments implicate biomaterial-based approaches for promoting muscle repair and functional restoration post-VML. Specifically, bioscaffolds transplanted in the injury site have been utilized to mimic endogenous cues of the ablated tissue to promote myogenic pathways, increase neo-myofiber synthesis, and ultimately restore contractile function to the injured unit. Critical Issues: Despite the development and preclinical testing of various biomaterial-based regenerative strategies, effective therapies for patients are not available. The unique challenges posed for biomaterial-based treatments of VML injuries, including its scalability and clinical applicability beyond small-animal models, impede progress. Furthermore, production of tissue-engineered constructs is technically demanding, with reproducibility issues at scale and complexities in achieving vascularization and innervation of large constructs. Future Directions: Biomaterial-based regenerative strategies designed to comprehensively address the pathophysiology of VML are needed. Considerations for clinical translation, including scalability and regulatory compliance, should also be considered when developing such strategies. In addition, an integrated approach that combines regenerative and rehabilitative strategies is essential for ensuring functional improvement.

The Complementary Roles of Neurological and Musculoskeletal Physical Therapy and Regenerative Medicine: A Comprehensive Review

Regenerative medicine, encompassing various therapeutic approaches aimed at tissue repair and regeneration, has emerged as a promising field in the realm of physical therapy. Aim: This comprehensive review seeks to explore the evolving role of regenerative medicine within the domain of physical therapy, highlighting its potential applications, challenges, and current trends. Researchers selected publications of pertinent studies from 2015 to 2024 and performed an exhaustive review of electronic databases such as PubMed, Embase, and Google Scholar using the targeted keywords "regenerative medicine", "rehabilitation", "tissue repair", and "physical therapy" to screen applicable studies according to preset parameters for eligibility, then compiled key insights from the extracted data. Several regenerative medicine methods that are applied in physical therapy, in particular, stem cell therapy, platelet-rich plasma (PRP), tissue engineering, and growth factor treatments, were analyzed in this research study. The corresponding efficacy of these methods in the recovery process were also elaborated, including a discussion on facilitating tissue repair, alleviating pain, and improving functional restoration. Additionally, this review reports the challenges concerning regenerative therapies, among them the standardization of protocols, safety concerns, and ethical issues. Regenerative medicine bears considerable potential as an adjunctive therapy in physiotherapy, providing new pathways for improving tissue repair and functional results. Although significant strides have been made in interpreting the potential of regenerative techniques, further research is warranted to enhance protocols, establish safety profiles, and increase access and availability. Merging regenerative medicine into the structure of physical therapy indicates a transformative alteration in clinical practice, with the benefit of increasing patient care and improving long-term results.

Mice from lines selectively bred for voluntary exercise are not more resistant to muscle injury caused by either contusion or wheel running

Muscle injury can be caused by strenuous exercise, repetitive tasks or external forces. Populations that have experienced selection for high locomotor activity may have evolutionary adaptations that resist exercise-induced injury and/or enhance the ability to cope with injury. We tested this hypothesis with an experiment in which mice are bred for high voluntary wheel running. Mice from four high runner lines run ~three times more daily distance than those from four non-selected control lines. To test recovery from injury by external forces, mice experienced contusion via weight drop on the calf. After injury, running distance and speed were reduced in high runner but not control lines, suggesting that the ability of control mice to run exceeds their motivation. To test effects of injury from exercise, mice were housed with/without wheels for six days, then trunk blood was collected and muscles evaluated for injury and regeneration. Both high runner and control mice with wheels had increased histological indicators of injury in the soleus, and increased indicators of regeneration in the plantaris. High runner mice had relatively more central nuclei (regeneration indicator) than control in the soleus, regardless of wheel access. The subset of high runner mice with the mini-muscle phenotype (characterized by greatly reduced muscle mass and type IIb fibers) had lower plasma creatine kinase (indicator of muscle injury), more markers of injury in the deep gastrocnemius, and more markers of regeneration in the deep and superficial gastrocnemius than normal-muscled individuals. Contrary to our expectations, high runner mice were not more resistant to either type of injury.

Mechanical strain drives exosome production, function, and miRNA cargo in C2C12 muscle progenitor cells

Mesenchymal stem cells (MSCs) have been proven to promote tissue repair. However, concerns related to their clinical application and regulatory hurdles remain. Recent data has demonstrated the proregenerative secretome of MSCs can result in similar effects in the absence of the cells themselves. Within the secretome, exosomes have emerged as a promising regenerative component. Exosomes, which are nanosized lipid vesicles secreted by cells, encapsulate micro-RNA (miRNA), RNA, and proteins that drive MSCs regenerative potential with cell specific content. As such, there is an opportunity to optimize the regenerative potential of MSCs, and thus their secreted exosome fraction, to improve clinical efficacy. Exercise is one factor that has been shown to improve muscle progenitor cell function and regenerative potential. However, the effect of exercise on MSC exosome content and function is still unclear. To address this, we used an in vitro culture system to evaluate the effects of mechanical strain, an exercise mimetic, on C2C12 (muscle progenitor cell) exosome production and proregenerative function. Our results indicate that the total exosome production is increased by mechanical strain and can be regulated with different tensile loading regimens. Furthermore, we found that exosomes from mechanically stimulated cells increase proliferation and myogenic differentiation of naïve C2C12 cells. Lastly, we show that exosomal miRNA cargo is differentially expressed following strain. Gene ontology mapping suggests positive regulation of bone morphogenetic protein signaling, regulation of actin-filament-based processes, and muscle cell apoptosis may be at least partially responsible for the proregenerative effects of exosomes from mechanically stimulated C2C12 muscle progenitor cells.

The effect of a rehabilitation program after mesenchymal stromal cell transplantation for advanced osteonecrosis of the femoral head: a 10-year follow-up study

Objective

To assess the status of 10 patients with advanced osteonecrosis of the femoral head who underwent mesenchymal stromal cell transplants and a 12-week rehabilitation program 10 years earlier.

Design

Retrospective study.

Setting

University clinical research laboratory.

Participants

Patients (N=10) who had undergone mesenchymal stromal cell transplantation and rehabilitation for a single hip osteonecrosis of the femoral head 10 years prior to the current study were recruited by telephone. The average age was 31.7 years and all participants were men; radiographic stages were 3A in 6 patients and 3B in 4 patients before treatment.

Intervention

A 12-week rehabilitation program with follow-up once every 1 to 2 years was performed after mesenchymal stromal cell transplantation.

Main Outcome Measures

Radiographic analysis, clinical score, timed Up and Go test, hip function (range of motion, muscle strength), and Short Form-36 scores were assessed before treatment and 1 and 10 years after treatment.

Results

Upon imaging, 5 hips were found to be stable (stable group) and 5 had progressed (progressed group); 2 of the latter group required a total hip arthroplasty. The pretreatment radiographic stage of the progressed group was more advanced than that of the stable group. Body mass index was higher in the progressed group than in the stable group. Hip function and clinical score at 1 and 10 years after treatment improved in the hips of 8 patients without total hip arthroplasty. There were no severe adverse events during the rehabilitation.

Conclusions

The 12-week rehabilitation program and annual follow-up after mesenchymal stromal cell transplantation for osteonecrosis of the femoral head was associated with pain reduction, maintaining hip muscle strength, widening range of motion, and improving quality of life. The level and timing of weight-bearing and social activity should be planned according to the individual's lifestyle and body composition.

Muscle Precursor Cells Enhance Functional Muscle Recovery and Show Synergistic Effects With Postinjury Treadmill Exercise in a Muscle Injury Model in Rats

BACKGROUND

Skeletal muscle injuries represent a major concern in sports medicine. Cell therapy has emerged as a promising therapeutic strategy for muscle injuries, although the preclinical data are still inconclusive and the potential clinical use of cell therapy has not yet been established.

PURPOSE

To evaluate the effects of muscle precursor cells (MPCs) on muscle healing in a small animal model.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 27 rats were used in the study. MPCs were isolated from rat (n = 3) medial gastrocnemius muscles and expanded in primary culture. Skeletal muscle injury was induced in 24 rats, and the animals were assigned to 3 groups. At 36 hours after injury, animals received treatment based on a single ultrasound-guided MPC (10⁵ cells) injection (Cells group) or MPC injection in combination with 2 weeks of daily exercise training (Cells+Exercise group). Animals receiving intramuscular vehicle injection were used as controls (Vehicle group). Muscle force was determined 2 weeks after muscle injury, and muscles were collected for histological and immunofluorescence evaluation.

RESULTS

Red fluorescence-labeled MPCs were successfully transplanted in the site of the injury by ultrasound-guided injection and were localized in the injured area after 2 weeks. Transplanted MPCs participated in the formation of regenerating muscle fibers as corroborated by the co-localization of red fluorescence with developmental myosin heavy chain (dMHC)-positive myofibers by immunofluorescence analysis. A strong beneficial effect on muscle force recovery was detected in the Cells and Cells+Exercise groups (102.6% ± 4.0% and 101.5% ± 8.5% of maximum tetanus force of the injured vs healthy contralateral muscle, respectively) compared with the Vehicle group (78.2% ± 5.1%). Both Cells and Cells+Exercise treatments stimulated the growth of newly formed regenerating muscles fibers, as determined by the increase in myofiber cross-sectional area (612.3 ± 21.4 µm² and 686.0 ± 11.6 µm², respectively) compared with the Vehicle group (247.5 ± 10.7 µm²), which was accompanied by a significant reduction of intramuscular fibrosis in Cells and Cells+Exercise treated animals (24.2% ± 1.3% and 26.0% ± 1.9% of collagen type I deposition, respectively) with respect to control animals (40.9% ± 4.1% in the Vehicle group). MPC treatment induced a robust acceleration of the muscle healing process as demonstrated by the decreased number of dMHC-positive regenerating myofibers (enhanced replacement of developmental myosin isoform by mature myosin isoforms) (4.3% ± 2.6% and 4.1% ± 1.5% in the Cells and Cells+Exercise groups, respectively) compared with the Vehicle group (14.8% ± 13.9%).

CONCLUSION

Single intramuscular administration of MPCs improved histological outcome and force recovery of the injured skeletal muscle in a rat injury model that imitates sports-related muscle injuries. Cell therapy showed a synergistic effect when combined with an early active rehabilitation protocol in rats, which suggests that a combination of treatments can generate novel therapeutic strategies for the treatment of human skeletal muscle injuries.

CLINICAL RELEVANCE

Our study demonstrates the strong beneficial effect of MPC transplant and the synergistic effect when the cell therapy is combined with an early active rehabilitation protocol for muscle recovery in rats; this finding opens new avenues for the development of effective therapeutic strategies for muscle healing and clinical trials in athletes undergoing MPC transplant and rehabilitation protocols.

Vascularized and Innervated Skeletal Muscle Tissue Engineering

Volumetric muscle loss (VML) is a devastating loss of muscle tissue that overwhelms the native regenerative properties of skeletal muscle and results in lifelong functional deficits. There are currently no treatments for VML that fully recover the lost muscle tissue and function. Tissue engineering presents a promising solution for VML treatment and significant research has been performed using tissue engineered muscle constructs in preclinical models of VML with a broad range of defect locations and sizes, tissue engineered construct characteristics, and outcome measures. Due to the complex vascular and neural anatomy within skeletal muscle, regeneration of functional vasculature and nerves is vital for muscle recovery following VML injuries. This review aims to summarize the current state of the field of skeletal muscle tissue engineering using 3D constructs for VML treatment with a focus on studies that have promoted vascular and neural regeneration within the muscle tissue post-VML.

Skeletal muscle cell transplantation: models and methods

Xenografts of skeletal muscle are used to study muscle repair and regeneration, mechanisms of muscular dystrophies, and potential cell therapies for musculoskeletal disorders. Typically, xenografting involves using an immunodeficient host that is pre-injured to create a niche for human cell engraftment. Cell type and method of delivery to muscle depend on the specific application, but can include myoblasts, satellite cells, induced pluripotent stem cells, mesangioblasts, immortalized muscle precursor cells, and other multipotent cell lines delivered locally or systemically. Some studies follow cell engraftment with interventions to enhance cell proliferation, migration, and differentiation into mature muscle fibers. Recently, several advances in xenografting human-derived muscle cells have been applied to study and treat Duchenne muscular dystrophy and Facioscapulohumeral muscular dystrophy. Here, we review the vast array of techniques available to aid researchers in designing future experiments aimed at creating robust muscle xenografts in rodent hosts.

Cell therapy to improve regeneration of skeletal muscle injuries

Diseases that jeopardize the musculoskeletal system and cause chronic impairment are prevalent throughout the Western world. In Germany alone, ~1.8 million patients suffer from these diseases annually, and medical expenses have been reported to reach 34.2bn Euros. Although musculoskeletal disorders are seldom fatal, they compromise quality of life and diminish functional capacity. For example, musculoskeletal disorders incur an annual loss of over 0.8 million workforce years to the German economy. Among these diseases, traumatic skeletal muscle injuries are especially problematic because they can occur owing to a variety of causes and are very challenging to treat. In contrast to chronic muscle diseases such as dystrophy, sarcopenia, or cachexia, traumatic muscle injuries inflict damage to localized muscle groups. Although minor muscle trauma heals without severe consequences, no reliable clinical strategy exists to prevent excessive fibrosis or fatty degeneration, both of which occur after severe traumatic injury and contribute to muscle degeneration and dysfunction. Of the many proposed strategies, cell-based approaches have shown the most promising results in numerous pre-clinical studies and have demonstrated success in the handful of clinical trials performed so far. A number of myogenic and non-myogenic cell types benefit muscle healing, either by directly participating in new tissue formation or by stimulating the endogenous processes of muscle repair. These cell types operate via distinct modes of action, and they demonstrate varying levels of feasibility for muscle regeneration depending, to an extent, on the muscle injury model used. While in some models the injury naturally resolves over time, other models have been developed to recapitulate the peculiarities of real-life injuries and therefore mimic the structural and functional impairment observed in humans. Existing limitations of cell therapy approaches include issues related to autologous harvesting, expansion and sorting protocols, optimal dosage, and viability after transplantation. Several clinical trials have been performed to treat skeletal muscle injuries using myogenic progenitor cells or multipotent stromal cells, with promising outcomes. Recent improvements in our understanding of cell behaviour and the mechanistic basis for their modes of action have led to a new paradigm in cell therapies where physical, chemical, and signalling cues presented through biomaterials can instruct cells and enhance their regenerative capacity. Altogether, these studies and experiences provide a positive outlook on future opportunities towards innovative cell-based solutions for treating traumatic muscle injuries-a so far unmet clinical need.

Stem cells, blood vessels, and angiogenesis as major determinants for musculoskeletal tissue repair

This manuscript summarizes 20 years of research from my laboratories at the University of Pittsburgh and more recently, at the University of Texas Health Science Center at Houston and the Steadman Philippon Research Institute in Vail, Colorado. The discovery of muscle-derived stem cells (MDSCs) did not arise from a deliberate search to find a novel population of muscle cells with high regenerative potential, but instead was conceived in response to setbacks encountered while working in muscle cell transplantation for Duchenne muscular dystrophy (DMD). DMD is a devastating inherited X-linked muscle disease characterized by progressive muscle weakness due to lack of dystrophin expression in muscle fiber sarcolemma.¹ Although the transplantation of normal myoblasts into dystrophin-deficient muscle can restore dystrophin, this approach has been hindered by limited survival (less than 1%) of the injected cells.¹ The fact that 99% of the cells were not surviving implantation was seen as a major weakness with this technology by most. My research team decided to investigate which cells represent the 1% of the cells surviving post-implantation. We have subsequently confirmed that the few cells which exhibit high survival post-implantation also display stem cell characteristics, and were termed "muscle-derived stem cells" or MDSCs. Herein, I will describe the origin of these MDSCs, the mechanisms of MDSC action during tissue repair, and finally the development of therapeutic strategies to improve regeneration and repair of musculoskeletal tissues. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1212-1220, 2019.

Non-invasive Assessment of Changes in Muscle Injury by Ultrasound Shear Wave Elastography: An Experimental Study in Contusion Model

The present study investigated the potential of ultrasound shear wave elastography (SWE) in assessment of muscle stiffness in muscle injury. SWE was performed on the injured muscle in 30 New Zealand rabbits that were randomly assigned to three groups: the contusion group, which was not treated with an efficient therapeutic strategy after muscle injury; the treatment group, which was treated with a therapeutic scheme after muscle injury; and the healthy group, which was not injured and served as a control. Both the mean Young's modulus (E_mean) and the maximum Young's modulus (E_max) were obtained pre-injury and 0.5, 1, 3, 5, 7, 14 and 28 d post-injury. At these time points, a rabbit in each group was randomly selected for biopsy for histopathological observation as well as comparison with Young's modulus. Eventually, all muscle tissues were collected for histologic analysis of collagen fiber formation. The contusion group had the highest Young's modulus, followed by the treatment group and then the healthy group (p < 0.05). In both the contusion and treatment groups, E_mean and E_max gradually increased within 1-3 d after injury, followed by a gradual decrease. Compared with the healthy group, histopathologic analysis of the contusion and treatment groups revealed the myofibril destruction process, inflammatory reaction and myofibril regeneration. The amount of collagen fibers in the contusion group was maximal compared with the treated and healthy groups (p = 0.001 and p < 0.001, respectively). There were more collagen fibers in the treatment group than in the healthy group (p = 0.003). The abundance of collagen fibers was positively correlated with the value of Young's modulus (E_mean: r = 0.706, p < 0.001; E_max: r = 0.761, p < 0.001). Thus, SWE can be used to detect pathologic changes in injured muscle and to monitor therapeutic effects.

Effects of aging on basement membrane of the soleus muscle during recovery following disuse atrophy in rats

Aging is known to lead to the impaired recovery of muscle after disuse as well as the increased susceptibility of the muscle to damage. Here, we show that, in the older rats, reloading after disuse atrophy, causes the damage of the muscle fibers and the basement membrane (BM) that structurally support the muscle fibers. Male Wistar rats of 3-(young) and 20-(older) months of age were subjected to hindlimb-unloading for 2weeks followed by reloading for a week. In the older rats, the soleus muscles showed necrosis and central nuclei fiber indicating the regeneration of muscle fibers. Furthermore, ectopic immunoreactivity of collagen IV, a major component of the BM, remained mostly associated with the necrotic appearance, suggesting that the older rats were impaired with the ability of repairing the damaged BM. Further, after unloading and reloading, the older rats did not show a significant alteration, although the young rats showed clear response of Col4a1 and Col4a2 genes, both coding for collagen IV. In addition, during the recovery phase, the young rats showed increase in the amount of Hsp47 and Sparc mRNA, which are protein folding-related factor genes, while the older rats did not show any significant variation. Taken together, our findings suggest that the atrophic muscle fibers of the older rats induced by unloading were vulnerable to the weight loading, and that attenuated reactivity of the BM-synthesizing fibroblast to gravity contributes to the fragility of muscle fibers in the older animals.

Regenerative Rehabilitation: Combining Stem Cell Therapies and Activity-Dependent Stimulation

The number of clinical trials in regenerative medicine is burgeoning, and stem cell/tissue engineering technologies hold the possibility of becoming the standard of care for a multitude of diseases and injuries. Advances in regenerative biology reveal novel molecular and cellular targets, with potential to optimize tissue healing and functional recovery, thereby refining rehabilitation clinical practice. The purpose of this review is to (1) highlight the potential for synergy between the fields of regenerative medicine and rehabilitation, a convergence of disciplines known as regenerative rehabilitation; (2) provide translational examples of regenerative rehabilitation within the context of neuromuscular injuries and diseases; and (3) offer recommendations for ways to leverage activity dependence via combined therapy and technology, with the goal of enhancing long-term recovery. The potential clinical benefits of regenerative rehabilitation will likely become a critical aspect in the standard of care for many neurological and musculoskeletal disorders.

Muscle damage, metabolism, and oxidative stress in mdx mice: Impact of aerobic running

INTRODUCTION

We tested how a treadmill exercise program influences oxygen consumption, oxidative stress, and exercise capacity in the mdx mouse, a model of Duchenne muscular dystrophy.

METHODS

At age 4 weeks mdx mice were subjected to 4 weeks of twice-weekly treadmill exercise. Sedentary mdx and wild-type mice served as controls. Oxygen consumption, time to exhaustion, oxidative stress, and myofiber damage were assessed.

RESULTS

At age 4 weeks, there was a significant difference in exercise capacity between mdx and wild-type mice. After exercise, mdx mice had lower basal oxygen consumption and exercise capacity, but similar maximal oxygen consumption. Skeletal muscle from these mice displayed increased oxidative stress. Collagen deposition was higher in exercised versus sedentary mice.

CONCLUSIONS

Exercised mdx mice exhibit increased oxidative stress, as well as deficits in exercise capacity, baseline oxygen consumption, and increased myofiber fibrosis. Muscle Nerve 54: 110-117, 2016.

Creation of a contusion injury method for skeletal muscle in rats with differing impacts

Purpose:

To realize a morphological examination of the musculoskeletal tissue, assessing the effect of a contusion method for the production in rat gastrocnemius, comparing the inflammatory responses generated by different impacts.

Methods:

For the analysis of a contusion method, twelve female Wistar rats were distributed into four groups. The lesion was generated by 324 g of mass that was dropped from different predetermined heights for each group (30, 45, 60 and 70 cm).

Results:

In the analysis of musculoskeletal tissue, the response to injury varied according to the mass of the height drop onto the muscle. Only the group that was injured from 70 cm responded with uniform and severe inflammation, whereas the groups 30, 45 and 60 cm showed inflammation in some regions of the tissue with mild and moderate infiltrates.

Conclusion:

The method with the 324-gram mass dropped from a 70-cm height onto the gastrocnemius muscle of rats seems to be the most suitable for the production of muscle injury in these animals after 72 hours, showing an important inflammatory infiltrate.

Aging of the skeletal muscle extracellular matrix drives a stem cell fibrogenic conversion

Age‐related declines in skeletal muscle regeneration have been attributed to muscle stem cell (MuSC) dysfunction. Aged MuSCs display a fibrogenic conversion, leading to fibrosis and impaired recovery after injury. Although studies have demonstrated the influence of in vitro substrate characteristics on stem cell fate, whether and how aging of the extracellular matrix (ECM) affects stem cell behavior has not been investigated. Here, we investigated the direct effect of the aged muscle ECM on MuSC lineage specification. Quantification of ECM topology and muscle mechanical properties reveals decreased collagen tortuosity and muscle stiffening with increasing age. Age‐related ECM alterations directly disrupt MuSC responses, and MuSCs seeded ex vivo onto decellularized ECM constructs derived from aged muscle display increased expression of fibrogenic markers and decreased myogenicity, compared to MuSCs seeded onto young ECM. This fibrogenic conversion is recapitulated in vitro when MuSCs are seeded directly onto matrices elaborated by aged fibroblasts. When compared to young fibroblasts, fibroblasts isolated from aged muscle display increased nuclear levels of the mechanosensors, Yes‐associated protein (YAP)/transcriptional coactivator with PDZ‐binding motif (TAZ), consistent with exposure to a stiff microenvironment in vivo. Accordingly, preconditioning of young fibroblasts by seeding them onto a substrate engineered to mimic the stiffness of aged muscle increases YAP/TAZ nuclear translocation and promotes secretion of a matrix that favors MuSC fibrogenesis. The findings here suggest that an age‐related increase in muscle stiffness drives YAP/TAZ‐mediated pathogenic expression of matricellular proteins by fibroblasts, ultimately disrupting MuSC fate.

The Effect of Mechanical Loading Upon Extracellular Matrix Bioscaffold-Mediated Skeletal Muscle Remodeling

Mounting evidence suggests that site-appropriate loading of implanted extracellular matrix (ECM) bioscaffolds and the surrounding microenvironment is an important tissue remodeling determinant, although the role at the cellular level in ECM-mediated skeletal muscle remodeling remains unknown. This study evaluates crosstalk between progenitor cells and macrophages during mechanical loading in ECM-mediated skeletal muscle repair. Myoblasts were exposed to solubilized ECM bioscaffolds and were mechanically loaded at 10% strain, 1 Hz for 5 h. Conditioned media was collected and applied to bone marrow-derived macrophages followed by immunolabeling for proinflammatory M1-like markers and proremodeling M2-like markers. Macrophages were subjected to the same loading protocol and their secreted products were collected for myoblast migration, proliferation, and differentiation analysis. A mouse hind limb unloading volumetric muscle loss model was used to evaluate the effect of loading upon the skeletal muscle microenvironment after ECM implantation. Animals were sacrificed at 14 or 180 days. Isometric torque production was tested and tissue sections were immunolabeled for macrophage phenotype and muscle fiber content. Results show that loading augments the ability of myoblasts to promote an M2-like macrophage phenotype following exposure to ECM bioscaffolds. Mechanically loaded macrophages promote myoblast chemotaxis and differentiation. Lack of weight bearing impaired muscle remodeling as indicated by Masson's Trichrome stain. Isometric torque was significantly increased following ECM implantation when compared to controls, a response not present in the hind limb-unloaded group. This work provides an important mechanistic insight of the effects of rehabilitation upon ECM-mediated remodeling and could have broader implications in clinical practice, advocating multidisciplinary approaches to regenerative medicine, emphasizing rehabilitation.

The Rotator Cuff Organ: Integrating Developmental Biology, Tissue Engineering, and Surgical Considerations to Treat Chronic Massive Rotator Cuff Tears

The torn rotator cuff remains a persistent orthopedic challenge, with poor outcomes disproportionately associated with chronic, massive tears. Degenerative changes in the tissues that comprise the rotator cuff organ, including muscle, tendon, and bone, contribute to the poor healing capacity of chronic tears, resulting in poor function and an increased risk for repair failure. Tissue engineering strategies to augment rotator cuff repair have been developed in an effort to improve rotator cuff healing and have focused on three principal aims: (1) immediate mechanical augmentation of the surgical repair, (2) restoration of muscle quality and contractility, and (3) regeneration of native enthesis structure. Work in these areas will be reviewed in sequence, highlighting the relevant pathophysiology, developmental biology, and biomechanics, which must be considered when designing therapeutic applications. While the independent use of these strategies has shown promise, synergistic benefits may emerge from their combined application given the interdependence of the tissues that constitute the rotator cuff organ. Furthermore, controlled mobilization of augmented rotator cuff repairs during postoperative rehabilitation may provide mechanotransductive cues capable of guiding tissue regeneration and restoration of rotator cuff function. Present challenges and future possibilities will be identified, which if realized, may provide solutions to the vexing condition of chronic massive rotator cuff tears.

Expression levels of TGF-β1 and CTGF are associated with the severity of Duchenne muscular dystrophy

The present study aimed to analyze the association of transforming growth factor-β1 (TGF-β1) and connective tissue growth factor (CTGF) expression levels in skeletal muscle with the clinical manifestation of Duchenne muscular dystrophy (DMD). A total of 18 cases of DMD, which were confirmed by routine pathological diagnosis were recruited into the present study, along with 8 subjects who suffered from acute trauma but did not present any neuromuscular diseases and were enrolled as the healthy controls. Immunohistochemical staining was used to detect the expression levels of CTGF and TGF-β1 in muscle biopsy specimens. Furthermore, Spearman rank correlation analysis was conducted among the expression levels of CTGF and TGF-β1, age, clinical severity and pathological severity in DMD patients. The immunohistochemical staining results revealed that the expression levels of CTGF and TGF-β1 were significantly increased in the DMD group compared with those in the control group (P<0.05). These levels were not found to be significantly correlated with the onset age (P>0.05), but there was a significant correlation with the degree of pathology and clinical severity (P<0.05). In conclusion, an upregulated expression of CTGF and TGF-β1 was revealed in the skeletal muscle of DMD patients, which were in positive correlation with the degree of pathology and clinical severity. These two factors may be involved in the pathophysiology of fibrosis in DMD.

A Murine Model of Robotic Training to Evaluate Skeletal Muscle Recovery after Injury

PURPOSE

In vivo studies have suggested that motor exercise can improve muscle regeneration after injury. Nevertheless, preclinical investigations still lack reliable tools to monitor motor performance over time and to deliver optimal training protocols to maximize force recovery. Here, we evaluated the utility of a murine robotic platform (i) to detect early impairment and longitudinal recovery after acute skeletal muscle injury and (ii) to administer varying intensity training protocols to enhance forelimb motor performance.

METHODS

A custom-designed robotic platform was used to train mice to perform a forelimb retraction task. After an acute injury to bilateral biceps brachii muscles, animals performed a daily training protocol in the platform at high (HL) or low (LL) loading levels over the course of 3 wk. Control animals were not trained (NT). Motor performance was assessed by quantifying force, time, submovement count, and number of movement attempts to accomplish the task. Myofiber number and cross-sectional area at the injury site were quantified histologically.

RESULTS

Two days after injury, significant differences in the time, submovement count, number of movement attempts, and exerted force were observed in all mice, as compared with baseline values. Interestingly, the recovery time of muscle force production differed significantly between intervention groups, with HL group showing a significantly accelerated recovery. Three weeks after injury, all groups showed motor performance comparable with baseline values. Accordingly, there were no differences in the number of myofibers or average cross-sectional area among groups after 3 wk.

CONCLUSION

Our findings demonstrate the utility of our custom-designed robotic device for the quantitative assessment of skeletal muscle function in preclinical murine studies. Moreover, we demonstrate that this device may be used to apply varying levels of resistance longitudinally as a means manipulate physiological muscle responses.

Rehabilitation Considerations in Regenerative Medicine

Rehabilitation and regenerative medicine therapies has shown improved outcomes for tissue regeneration. Regenerative rehabilitation guides protocols regarding when to start therapy, types of stimuli administered, and graded exercise programs, taking into account biological factors and technologies designed to optimize healing potential. Although there are currently no evidence-based guidelines for rehabilitation, fundamental physical therapy principles likely apply. Immobilization tends to have deleterious effects on musculoskeletal tissues; mechanical loading promotes tissue healing and regeneration. Common physical therapy interventions may provide beneficial effects after the application of regenerative therapies. Research is needed to determine optimal rehabilitation protocols to enhance tissue healing and regeneration.

Platelet-Rich Plasma in a Murine Model: Leukocytes, Growth Factors, Flt-1, and Muscle Healing

BACKGROUND

It is well known that platelet-rich plasma (PRP) preparations are not the same and that not all preparations include white blood cells, but the part that leukocytes play on the healing role of PRP is still unknown.

PURPOSE

The primary aim of this study was to evaluate the influence of leukocytes in different PRP preparations with a special emphasis on growth factor concentrations. The secondary aim was to evaluate the influence of PRP on muscle healing.

STUDY DESIGN

Controlled laboratory study.

METHODS

Two PRP preparation procedures were evaluated. Blood fractions were stained with Rapid Panoptic, and growth factors (transforming growth factor beta 1 [TGF-β1], vascular endothelial growth factor [VEGF], insulin-like growth factor [IGF], epidermal growth factor [EGF], hepatocyte growth factor [HGF], and platelet-derived growth factor [PDGF]) were quantified by enzyme-linked immunosorbent assay. Western blotting analysis was performed for Fms-related tyrosine kinase 1 (Flt-1). A muscle contusion injury was created and treated with PRP at different time points.

RESULTS

Leukocytes were the main source of VEGF, and all other growth factors measured had a higher concentration in the preparations that included the buffy coat and consequently had a higher concentration of white blood cells. Flt-1 was also found in platelet-poor plasma (PPP). There were higher concentrations of PDGF and HGF in the preparations that encompassed the buffy coat. A PRP injection 7 days after the injury provided significantly increased exercise performance and decreased the fibrotic area when compared with other PRP-treated groups.

CONCLUSION

VEGF is only present in PRP's buffy coat, while Flt-1 is present in PPP. A PRP injection 7 days after an injury resulted in improved exercise performance.

CLINICAL RELEVANCE

The presence of Flt-1 in PRP provides yet another explanation for results described in the literature after a PRP injection. This information is relevant for selecting the best PRP for each type of injury.

Mesenchymal stem cells in regenerative rehabilitation

[Purpose] Regenerative medicine and rehabilitation contribute in many ways to a specific plan of care based on a patient’s medical status. The intrinsic self-renewing, multipotent, regenerative, and immunosuppressive properties of mesenchymal stem cells offer great promise in the treatment of numerous autoimmune, degenerative, and graft-versus-host diseases, as well as tissue injuries. As such, mesenchymal stem cells represent a therapeutic fortune in regenerative medicine. The aim of this review is to discuss possibilities, limitations, and future clinical applications of mesenchymal stem cells. [Subjects and Methods] The authors have identified and discussed clinically and scientifically relevant articles from PubMed that have met the inclusion criteria. [Results] Direct treatment of muscle injuries, stroke, damaged peripheral nerves, and cartilage with mesenchymal stem cells has been demonstrated to be effective, with synergies seen between cellular and physical therapies. Over the past few years, several researchers, including us, have shown that there are certain limitations in the use of mesenchymal stem cells. Aging and spontaneous malignant transformation of mesenchymal stem cells significantly affect the functionality of these cells. [Conclusion] Definitive conclusions cannot be made by these studies because limited numbers of patients were included. Studies clarifying these results are expected in the near future.

Developmental Biology and Regenerative Medicine: Addressing the Vexing Problem of Persistent Muscle Atrophy in the Chronically Torn Human Rotator Cuff

Persistent muscle atrophy in the chronically torn rotator cuff is a significant obstacle for treatment and recovery. Large atrophic changes are predictive of poor surgical and nonsurgical outcomes and frequently fail to resolve even following functional restoration of loading and rehabilitation. New insights into the processes of muscle atrophy and recovery gained through studies in developmental biology combined with the novel tools and strategies emerging in regenerative medicine provide new avenues to combat the vexing problem of muscle atrophy in the rotator cuff. Moving these treatment strategies forward likely will involve the combination of surgery, biologic/cellular agents, and physical interventions, as increasing experimental evidence points to the beneficial interaction between biologic therapies and physiologic stresses. Thus, the physical therapy profession is poised to play a significant role in defining the success of these combinatorial therapies. This perspective article will provide an overview of the developmental biology and regenerative medicine strategies currently under investigation to combat muscle atrophy and how they may integrate into the current and future practice of physical therapy.

Neural Stem Cell Therapy and Rehabilitation in the Central Nervous System: Emerging Partnerships

The goal of regenerative medicine is to restore function through therapy at levels such as the gene, cell, tissue, or organ. For many disorders, however, regenerative medicine approaches in isolation may not be optimally effective. Rehabilitation is a promising adjunct therapy given the beneficial impact that physical activity and other training modalities can offer. Accordingly, "regenerative rehabilitation" is an emerging concentration of study, with the specific goal of improving positive functional outcomes by enhancing tissue restoration following injury. This article focuses on one emerging example of regenerative rehabilitation-namely, the integration of clinically based protocols with stem cell technologies following central nervous system injury. For the purposes of this review, the state of stem cell technologies for the central nervous system is summarized, and a rationale for a synergistic benefit of carefully orchestrated rehabilitation protocols in conjunction with cellular therapies is provided. An overview of practical steps to increase the involvement of physical therapy in regenerative rehabilitation research also is provided.

Cell death, clearance and immunity in the skeletal muscle

The skeletal muscle is an immunologically unique tissue. Leukocytes, virtually absent in physiological conditions, are quickly recruited into the tissue upon injury and persist during regeneration. Apoptosis, necrosis and autophagy coexist in the injured/regenerating muscles, including those of patients with neuromuscular disorders, such as inflammatory myopathies, dystrophies, metabolic and mitochondrial myopathies and drug-induced myopathies. Macrophages are able to alter their function in response to microenvironment conditions and as a consequence coordinate changes within the tissue from the early injury throughout regeneration and eventual healing, and regulate the activation and the function of stem cells. Early after injury, classically activated macrophages ('M1') dominate the picture. Alternatively activated M2 macrophages predominate during resolution phases and regulate the termination of the inflammatory responses. The dynamic M1/M2 transition is increasingly felt to be the key to the homeostasis of the muscle. Recognition and clearance of debris originating from damaged myofibers and from dying stem/progenitor cells, stromal cells and leukocytes are fundamental actions of macrophages. Clearance of apoptotic cells and M1/M2 transition are causally connected and represent limiting steps for muscle healing. The accumulation of apoptotic cells, which reflects their defective clearance, has been demonstrated in various tissues to prompt autoimmunity against intracellular autoantigens. In the muscle, in the presence of type I interferon, apoptotic myoblasts indeed cause the production of autoantibodies, lymphocyte infiltration and continuous cycles of muscle injury and regeneration, mimicking human inflammatory myopathies. The clearance of apoptotic cells thus modulates the homeostatic response of the skeletal muscle to injury. Conversely, defects in the process may have deleterious local effects, guiding maladaptive tissue remodeling with collagen and fat accumulation and promoting autoimmunity itself. There is strong promise for novel treatments based on new knowledge of cell death, clearance and immunity in the muscle.

Strategies for functional bioscaffold-based skeletal muscle reconstruction

Tissue engineering and regenerative medicine-based strategies for the reconstruction of functional skeletal muscle tissue have included cellular and acellular approaches. The use of acellular biologic scaffold material as a treatment for volumetric muscle loss (VML) in five patients has recently been reported with a generally favorable outcome. Further studies are necessary for a better understanding of the mechanism(s) behind acellular bioscaffold-mediated skeletal muscle repair, and for combination cell-based/bioscaffold based approaches. The present overview highlights the current thinking on bioscaffold-based remodeling including the associated mechanisms and the future of scaffold-based skeletal muscle reconstruction.

An acellular biologic scaffold does not regenerate appreciable de novo muscle tissue in rat models of volumetric muscle loss injury

Extracellular matrix (ECM) derived scaffolds continue to be investigated for the treatment of volumetric muscle loss (VML) injuries. Clinically, ECM scaffolds have been used for lower extremity VML repair; in particular, MatriStem™, a porcine urinary bladder matrix (UBM), has shown improved functional outcomes and vascularization, but limited myogenesis. However, efficacy of the scaffold for the repair of traumatic muscle injuries has not been examined systematically. In this study, we demonstrate that the porcine UBM scaffold when used to repair a rodent gastrocnemius musculotendinous junction (MTJ) and tibialis anterior (TA) VML injury does not support muscle tissue regeneration. In the MTJ model, the scaffold was completely resorbed without tissue remodeling, suggesting that the scaffold may not be suitable for the clinical repair of muscle-tendon injuries. In the TA VML injury, the scaffold remodeled into a fibrotic tissue and showed functional improvement, but not due to muscle fiber regeneration. The inclusion of physical rehabilitation also did not improve functional response or tissue remodeling. We conclude that the porcine UBM scaffold when used to treat VML injuries may hasten the functional recovery through the mechanism of scaffold mediated functional fibrosis. Thus for appreciable muscle regeneration, repair strategies that incorporate myogenic cells, vasculogenic accelerant and a myoconductive scaffold need to be developed.

Physical rehabilitation improves muscle function following volumetric muscle loss injury

Background

Given the clinical practice of prescribing physical rehabilitation for the treatment of VML injuries, the present study examined the functional and histomorphological adaptations in the volumetric muscle loss (VML) injured muscle to physical rehabilitation.

Methods

Tibialis anterior muscle VML injury was created in Lewis rats (n = 32), and were randomly assigned to either sedentary (SED) or physical rehabilitation (RUN) group. After 1 week, RUN rats were given unlimited access to voluntary running wheels either 1 or 7 weeks (2 or 8 weeks post-injury). At 2 weeks post-injury, TA muscles were harvested for molecular analyses. At 8 weeks post-injury, the rats underwent in vivo function testing. The explanted tissue was analyzed using histological and immunofluorescence procedures.

Results

The primary findings of the study are that physical rehabilitation in the form of voluntary wheel running promotes ~ 17% improvement in maximal isometric torque, and a ~ 13% increase in weight of the injured muscle, but it did so without significant morphological adaptations (e.g., no hypertrophy and hyperplasia). Wheel running up-regulated metabolic genes (SIRT-1, PGC-1α) only in the uninjured muscles, and a greater deposition of fibrous tissue in the defect area of the injured muscle preceded by an up-regulation of pro-fibrotic genes (Collagen I, TGF-β1). Therefore, it is plausible that the wheel running related functional improvements were due to improved force transmission and not muscle regeneration.

Conclusions

This is the first study to demonstrate improvement in functional performance of non-repaired VML injured muscle with physical rehabilitation in the form of voluntary wheel running. This study provides information for the first time on the basic changes in the VML injured muscle with physical rehabilitation, which may aid in the development of appropriate physical rehabilitation regimen(s).

Treadmill exercise induces murine cardiac allograft survival and generates regulatory T cell

Exercise therapy has been associated with improvement in functional capacity and quality of life. The role of exercise therapy in heart transplant recipients is of great interest for the transplant society, although concerning the effect of exercise therapy, there is little knowledge at present. We analyzed the effects of exercise on alloimmune responses in murine cardiac allograft transplantation. CBA mice (H2^k) underwent transplantation of C57Bl/6 (H2^b) hearts and exercised on a treadmill. Untreated CBA recipients rejected C57Bl/6 cardiac grafts acutely (median survival time [MST], 7 days). CBA recipients treated with treadmill for 1 week after transplantation, and for 1 week both before and after transplantation prolonged allograft survivals (MSTs, 35 and 18 days, respectively). However, treadmill exercise recipients for 1 week before transplantation were not effective to allograft survival (MST, 8 days). Adoptive transfer of whole splenocytes and CD4⁺ cells from treadmill exercise recipients significantly prolonged allograft survival in naive secondary recipients (MSTs, 30 and 52 days, respectively), suggesting that regulatory cells was generated after treadmill exercise. Moreover, flow cytometry studies showed that CD4⁺CD25⁺Foxp3⁺ cell population increased in treadmill exercise recipients. Therefore, postoperative but not pre-operative exercise could induce prolongation of survival of fully allogeneic cardiac allografts and generate CD4⁺CD25⁺Foxp3⁺ regulatory T cells.

Rehabilitation program after mesenchymal stromal cell transplantation augmented by vascularized bone grafts for idiopathic osteonecrosis of the femoral head: a preliminary study

OBJECTIVE

To determine the feasibility and safety of implementing a 12-week rehabilitation program after mesenchymal stromal cell (MSC) transplantation augmented by vascularized bone grafting for idiopathic osteonecrosis (ION) of the femoral head.

DESIGN

A prospective case series.

SETTING

University clinical research laboratory.

PARTICIPANTS

Participants (N=10) with ION who received MSC transplantation augmented by vascularized bone grafting.

INTERVENTION

A 12-week exercise program, which included range-of-motion (ROM) exercises, muscle-strengthening exercises, and aerobic training.

MAIN OUTCOME MEASURES

Measures of ROM, muscle strength, Timed Up and Go test, and Medical Outcomes Study 36-Item Short-Form Health Survey (SF-36) were collected before surgery and again at 6 and 12 months after surgery.

RESULTS

All participants completed the 12-week program. External rotation ROM as well as extensor and abductor muscle strength significantly improved 6 months after treatment compared with that before treatment (P<.05). Significant improvements were also seen in physical function, role physical, and bodily pain subgroup scores of the SF-36 (P<.05). No serious adverse events occurred.

CONCLUSIONS

This study demonstrates the feasibility and safety of a multiplex rehabilitation program after MSC transplantation and provides support for further study on the benefits of rehabilitation programs in regenerative medicine.

Arsenic induces sustained impairment of skeletal muscle and muscle progenitor cell ultrastructure and bioenergetics

Over 4 million individuals in the United States, and over 140 million individuals worldwide, are exposed daily to arsenic-contaminated drinking water. Human exposures can range from below the current limit of 10 μg/L to over 1mg/L, with 100 μg/L promoting disease in a large portion of those exposed. Although increased attention has recently been paid to myopathy following arsenic exposure, the pathogenic mechanisms underlying clinical symptoms remain poorly understood. This study tested the hypothesis that arsenic induces lasting muscle mitochondrial dysfunction and impairs metabolism. Compared to nonexposed controls, mice exposed to drinking water containing 100 μg/L arsenite for 5 weeks demonstrated impaired muscle function, mitochondrial myopathy, and altered oxygen consumption that were concomitant with increased mitochondrial fusion gene transcription. There were no differences in the levels of inorganic arsenic or its monomethyl and dimethyl metabolites between controls and exposed muscles, confirming that arsenic does not accumulate in muscle. Nevertheless, muscle progenitor cells isolated from exposed mice recapitulated the aberrant myofiber phenotype and were more resistant to oxidative stress, generated more reactive oxygen species, and displayed autophagic mitochondrial morphology, compared to cells isolated from nonexposed mice. These pathological changes from a possible maladaptive oxidative stress response provide insight into declines in muscle functioning caused by exposure to this common environmental contaminant.

Laminin-111 improves skeletal muscle stem cell quantity and function following eccentric exercise

Laminin-111 (α1, β1, γ1; LM-111) is an important component of the extracellular matrix that is required for formation of skeletal muscle during embryonic development. Recent studies suggest that LM-111 supplementation can enhance satellite cell proliferation and muscle function in mouse models of muscular dystrophy. The purpose of this study was to determine the extent to which LM-111 can alter satellite and nonsatellite stem cell quantity following eccentric exercise-induced damage in young adult, healthy mice. One week following injection of LM-111 or saline, mice either remained sedentary or were subjected to a single bout of downhill running (EX). While one muscle was preserved for evaluation of satellite cell number, the other muscle was processed for isolation of mesenchymal stem cells (MSCs; Sca-1+CD45-) via FACS at 24 hours postexercise. Satellite cell number was approximately twofold higher in LM-111/EX compared with all other groups (p<.05), and the number of satellite cells expressing the proliferation marker Ki67 was 50% to threefold higher in LM-111/EX compared with all other groups (p<.05). LM-111 also increased the quantity of embryonic myosin heavy chain-positive (eMHC+) fibers in young mice after eccentric exercise (p<.05). Although MSC percentage and number were not altered, MSC proinflammatory gene expression was decreased, and hepatocyte growth factor gene expression was increased in the presence of LM-111 (p<.05). Together, these data suggest that LM-111 supplementation provides a viable solution for increasing skeletal muscle stem cell number and/or function, ultimately allowing for improvements in the regenerative response to eccentric exercise.

Tendon proper- and peritenon-derived progenitor cells have unique tenogenic properties

INTRODUCTION

Multipotent progenitor populations exist within the tendon proper and peritenon of the Achilles tendon. Progenitor populations derived from the tendon proper and peritenon are enriched with distinct cell types that are distinguished by expression of markers of tendon and vascular or pericyte origins, respectively. The objective of this study was to discern the unique tenogenic properties of tendon proper- and peritenon-derived progenitors within an in vitro model. We hypothesized that progenitors from each region contribute differently to tendon formation; thus, when incorporated into a regenerative model, progenitors from each region will respond uniquely. Moreover, we hypothesized that cell populations like progenitors were capable of stimulating tenogenic differentiation, so we generated conditioned media from these cell types to analyze their stimulatory potentials.

METHODS

Isolated progenitors were seeded within fibrinogen/thrombin gel-based constructs with or without supplementation with recombinant growth/differentiation factor-5 (GDF5). Early and late in culture, gene expression of differentiation markers and matrix assembly genes was analyzed. Tendon construct ultrastructure was also compared after 45 days. Moreover, conditioned media from tendon proper-derived progenitors, peritenon-derived progenitors, or tenocytes was applied to each of the three cell types to determine paracrine stimulatory effects of the factors secreted from each of the respective cell types.

RESULTS

The cell orientation, extracellular domain and fibril organization of constructs were comparable to embryonic tendon. The tendon proper-derived progenitors produced a more tendon-like construct than the peritenon-derived progenitors. Seeded tendon proper-derived progenitors expressed greater levels of tenogenic markers and matrix assembly genes, relative to peritenon-derived progenitors. However, GDF5 supplementation improved expression of matrix assembly genes in peritenon progenitors and structurally led to increased mean fibril diameters. It also was found that peritenon-derived progenitors secrete factor(s) stimulatory to tenocytes and tendon proper progenitors.

CONCLUSIONS

Data demonstrate that, relative to peritenon-derived progenitors, tendon proper progenitors have greater potential for forming functional tendon-like tissue. Furthermore, factors secreted by peritenon-derived progenitors suggest a trophic role for this cell type as well. Thus, these findings highlight the synergistic potential of including these progenitor populations in restorative tendon engineering strategies.

Skeletal muscle as a regulator of the longevity protein, Klotho

Klotho is a powerful longevity protein that has been linked to the prevention of muscle atrophy, osteopenia, and cardiovascular disease. Similar anti-aging effects have also been ascribed to exercise and physical activity. While an association between muscle function and Klotho expression has been previously suggested from longitudinal cohort studies, a direct relationship between circulating Klotho and skeletal muscle has not been investigated. In this paper, we present a review of the literature and preliminary evidence that, together, suggests Klotho expression may be modulated by skeletal muscle activity. Our pilot clinical findings performed in young and aged individuals suggest that circulating Klotho levels are upregulated in response to an acute exercise bout, but that the response may be dependent on fitness level. A similar upregulation of circulating Klotho is also observed in response to an acute exercise in young and old mice, suggesting that this may be a good model for mechanistically probing the role of physical activity on Klotho expression. Finally, we highlight overlapping signaling pathways that are modulated by both Klotho and skeletal muscle and propose potential mechanisms for cross-talk between the two. It is hoped that this review will stimulate further consideration of the relationship between skeletal muscle activity and Klotho expression, potentially leading to important insights into the well-documented systemic anti-aging effects of exercise.

Tendon and ligament regeneration and repair: clinical relevance and developmental paradigm

As dense connective tissues connecting bone to muscle and bone to bone, respectively, tendon and ligament (T/L) arise from the somitic mesoderm, originating in a recently discovered somitic compartment, the syndetome. Inductive signals from the adjacent sclerotome and myotome upregulate expression of Scleraxis, a key transcription factor for tenogenic and ligamentogenic differentiation. Understanding T/L development is critical to establishing a knowledge base for improving the healing and repair of T/L injuries, a high-burden disease due to the intrinsically poor natural healing response. Current treatment of the three most common tendon injuries-tearing of the rotator cuff of the shoulder, flexor tendon of the hand, and Achilles tendon-include mostly surgical repair and/or conservative approaches, including biophysical modalities such as rehabilitation and cryotherapy. Unfortunately, the fibrovascular scar formed during healing possesses inferior mechanical and biochemical properties, resulting in compromised tissue functionality. Regenerative approaches have sought to augment the injured tissue with cells, scaffolds, bioactive agents, and mechanical stimulation to improve the natural healing response. The key challenges in restoring full T/L function following injury include optimal combination of these biological agents as well as their delivery to the injury site. A greater understanding of the molecular mechanisms involved in T/L development and natural healing, coupled with the capability of producing complex biomaterials to deliver multiple biofactors with high spatiotemporal resolution and specificity, should lead to regenerative procedures that more closely recapitulate T/L morphogenesis, thereby offering future patients the prospect of T/L regeneration, as opposed to simple tissue repair.

Rejuvenation: an integrated approach to regenerative medicine

The word "rejuvenate" found in the Merriam-Webster dictionary is (1) to make young or youthful again: give new vigor to, and (2) to restore to an original or new state. Regenerative medicine is the process of creating living, functional tissues to repair or replace tissue or organ function lost due to age, disease, damage, or congenital defects. To accomplish this, approaches including transplantation, tissue engineering, cell therapy, and gene therapy are brought into action. These all use exogenously prepared materials to forcefully mend the failed organ. The adaptation of the materials in the host and their integration into the organ are all uncertain. It is a common sense that tissue injury in the younger is easily repaired and the acute injury is healed better and faster. Why does the elder have a diminished capacity of self-repairing, or why does chronic injury cause the loss of the self-repairing capacity? There must be some critical elements that are involved in the repair process, but are suppressed in the elder or under the chronic injury condition. Rejuvenation of the self-repair mechanism would be an ideal solution for functional recovery of the failed organ. To achieve this, it would involve renewal of the injury signaling, reestablishment of the communication and transportation system, recruitment of the materials for repairing, regeneration of the failed organ, and rehabilitation of the renewed organ. It thus would require a comprehensive understanding of developmental biology and a development of new approaches to activate the critical players to rejuvenate the self-repair mechanism in the elder or under chronic injury condition. Efforts focusing on rejuvenation would expect an alternative, if not a better, accomplishment in the regenerative medicine.

Muscle injury: review of experimental models

Skeletal muscle is the most abundant tissue in the human body. Its main characteristic is the capacity to regenerate after injury independent of the cause of injury through a process called inflammatory response. Mechanical injuries are the most common type of the skeletal muscle injuries and are classified into one of three areas strain, contusion, and laceration. First, this review aims to describe and compare the main experimental methods that replicate the mechanical muscle injuries. There are several ways to replicate each kind of mechanical injury; there are, however, specific characteristics that must be taken into account when choosing the most appropriate model for the experiment. Finally, this review discusses the context of mechanical injury considering types, variability of methods, and the ability to reproduce injury models.

Neuromuscular electrical stimulation as a method to maximize the beneficial effects of muscle stem cells transplanted into dystrophic skeletal muscle

Cellular therapy is a potential approach to improve the regenerative capacity of damaged or diseased skeletal muscle. However, its clinical use has often been limited by impaired donor cell survival, proliferation and differentiation following transplantation. Additionally, functional improvements after transplantation are all-too-often negligible. Because the host microenvironment plays an important role in the fate of transplanted cells, methods to modulate the microenvironment and guide donor cell behavior are warranted. The purpose of this study was to investigate whether the use of neuromuscular electrical stimulation (NMES) for 1 or 4 weeks following muscle-derived stem cell (MDSC) transplantation into dystrophic skeletal muscle can modulate the fate of donor cells and enhance their contribution to muscle regeneration and functional improvements. Animals submitted to 4 weeks of NMES after transplantation demonstrated a 2-fold increase in the number of dystrophin+ myofibers as compared to control transplanted muscles. These findings were concomitant with an increased vascularity in the MDSC+NMES group when compared to non-stimulated counterparts. Additionally, animals subjected to NMES (with or without MDSC transplantation) presented an increased maximal specific tetanic force when compared to controls. Although cell transplantation and/or the use of NMES resulted in no changes in fatigue resistance, the combination of both MDSC transplantation and NMES resulted in a faster recovery from fatigue, when compared to non-injected and non-stimulated counterparts. We conclude that NMES is a viable method to improve MDSC engraftment, enhance dystrophic muscle strength, and, in combination with MDSC transplantation, improve recovery from fatigue. These findings suggest that NMES may be a clinically-relevant adjunct approach for cell transplantation into skeletal muscle.

The role of fibrosis in Duchenne muscular dystrophy

Muscular dystrophies such as Duchenne muscular dystrophy (DMD) are usually approached as dysfunctions of the affected skeletal myofibres and their force transmission. Comparatively little attention has been given to the increase in connective tissue (fibrosis) which accompanies these muscular changes. Interestingly, an increase in endomysial tissue is apparent long before any muscular degeneration can be observed. Fibrosis is the result of a reactive or reparative process involving mechanical, humoral and cellular factors. Originating from vulnerable myofibres, muscle cell necrosis and inflammatory processes are present in DMD. Muscular recovery is limited due to the limited number and capacity of satellite cells. Hence, a proactive and multimodal approach is necessary in order to activate protective mechanisms and to hinder catabolic and tissue degrading pathways. Several avenues are discussed in terms of potential antifibrotic therapy approaches. These include pharmaceutical, nutritional, exercise-based and other mechanostimulatory modalities (such as massage or yoga-like stretching) with the intention of exerting an anti-inflammatory and antifibrotic effect on the affected muscular tissues. A preventive intervention at an early age is crucial, based on the early and seemingly non-reversible nature of the fibrotic tissue changes. Since consistent assessment is essential, different measurement technologies are discussed.

Stem cells, angiogenesis and muscle healing: a potential role in massage therapies?

Skeletal muscle injuries are among the most common and frequently disabling injuries sustained by athletes. Repair of injured skeletal muscle is an area that continues to present a challenge for sports medicine clinicians and researchers due, in part, to complete muscle recovery being compromised by development of fibrosis leading to loss of function and susceptibility to re-injury. Injured skeletal muscle goes through a series of coordinated and interrelated phases of healing including degeneration, inflammation, regeneration and fibrosis. Muscle regeneration initiated shortly after injury can be limited by fibrosis which affects the degree of recovery and predisposes the muscle to reinjury. It has been demonstrated in animal studies that antifibrotic agents that inactivate transforming growth factor (TGF)-β1 have been effective at decreasing scar tissue formation. Several studies have also shown that vascular endothelial growth factor (VEGF) can increase the efficiency of skeletal muscle repair by increasing angiogenesis and, at the same time, reducing the accumulation of fibrosis. We have isolated and thoroughly characterised a population of skeletal muscle-derived stem cells (MDSCs) that enhance repair of damaged skeletal muscle fibres by directly differentiating into myofibres and secreting paracrine factors that promote tissue repair. Indeed, we have found that MDSCs transplanted into skeletal and cardiac muscles have been successful at repair probably because of their ability to secrete VEGF that works in a paracrine fashion. The application of these techniques to the study of sport-related muscle injuries awaits investigation. Other useful strategies to enhance skeletal muscle repair through increased vascularisation may include gene therapy, exercise, neuromuscular electrical stimulation and, potentially, massage therapy. Based on recent studies showing an accelerated recovery of muscle function from intense eccentric exercise through massage-based therapies, we believe that this treatment modality offers a practical and non-invasive form of therapy for skeletal muscle injuries. However, the biological mechanism(s) behind the beneficial effect of massage are still unclear and require further investigation using animal models and potentially randomised, human clinical studies.

Physical exercise affects cell proliferation in lumbar intervertebral disc regions in rats

STUDY DESIGN

Descriptive experimental study.

OBJECTIVE

The aim of this study was to investigate the effect of exercise on cell proliferation in different areas of the intervertebral disc (IVD) and recruitment of cells possibly active in regeneration of normal rat lumbar IVDs.

SUMMARY OF BACKGROUND DATA

Little is known about the effects of physical exercise on lumbar IVD tissue. Recently, stem cell niches in the perichondrium area of the IVD were identified and cells in these niches have been suggested to be involved in the normal regeneration of the IVD.

METHODS

Thirty Sprague-Dawley rats were exposed to 5-bromo-2-deoxyuridine (BrdU) diluted in the drinking water during 14 days. Fifteen rats ran on a treadmill daily for 50 min/d, 5 d/wk (exercise group), and 15 nonexercised rats served as controls. Immunohistochemical analyses (anti-BrdU antibody) were performed at 9, 14, 28, 56, and 105 days after the start of the exercise protocol. BrdU positive cells were counted in the stem cell niche area, the peripheral region of epiphyseal cartilage area, and the annulus fibrous outer and inner areas. Data were analyzed by 2-way analysis of variance (significance level; P < 0.05).

RESULTS

The BrdU positive cell numbers in the stem cell niche and annulus fibrous outer regions were increased in discs from the exercising group on days 14 (P < 0.01) and 105 (P < 0.05) and at day 14 (P < 0.01) in the peripheral epiphyseal cartilage region compared with controls.

CONCLUSION

Physical exercise was shown to have positive effects on cell proliferation in IVDs, with involvement of various disc regions, indicating a differential response by disc tissue to exercise depending on anatomical location and tissue characteristics.