Repair of traumatic skeletal muscle injury with bone-marrow-derived mesenchymal stem cells seeded on extracellular matrix

Cell and Growth Factor-Loaded Keratin Hydrogels for Treatment of Volumetric Muscle Loss in a Mouse Model

Wounds to the head, neck, and extremities have been estimated to account for ∼84% of reported combat injuries to military personnel. Volumetric muscle loss (VML), defined as skeletal muscle injuries in which tissue loss results in permanent functional impairment, is common among these injuries. The present standard of care entails the use of muscle flap transfers, which suffer from the need for additional surgery when using autografts or the risk of rejection when cadaveric grafts are used. Tissue engineering (TE) strategies for skeletal muscle repair have been investigated as a means to overcome current therapeutic limitations. In that regard, human hair-derived keratin (KN) biomaterials have been found to possess several favorable properties for use in TE applications and, as such, are a viable candidate for use in skeletal muscle repair. Herein, KN hydrogels with and without the addition of skeletal muscle progenitor cells (MPCs) and/or insulin-like growth factor 1 (IGF-1) and/or basic fibroblast growth factor (bFGF) were implanted in an established murine model of surgically induced VML injury to the latissimus dorsi (LD) muscle. Control treatments included surgery with no repair (NR) as well as implantation of bladder acellular matrix (BAM). In vitro muscle contraction force was evaluated at two months postsurgery through electrical stimulation of the explanted LD in an organ bath. Functional data indicated that implantation of KN+bFGF+IGF-1 (n = 8) enabled a greater recovery of contractile force than KN+bFGF (n = 8)***, KN+MPC (n = 8)**, KN+MPC+bFGF+IGF-1 (n = 8)**, BAM (n = 8)*, KN+IGF-1 (n = 8)*, KN+MPCs+bFGF (n = 9)*, or NR (n = 9)**, (*p < 0.05, **p < 0.01, ***p < 0.001). Consistent with the physiological findings, histological evaluation of retrieved tissue revealed much more extensive new muscle tissue formation in groups with greater functional recovery (e.g., KN+IGF-1+bFGF) when compared with observations in tissue from groups with lower functional recovery (i.e., BAM and NR). Taken together, these findings further indicate the general utility of KN biomaterials in TE and, moreover, specifically highlight their potential application in the treatment of VML injuries.

Achieving Acetylcholine Receptor Clustering in Tissue-Engineered Skeletal Muscle Constructs In vitro through a Materials-Directed Agrin Delivery Approach

Volumetric muscle loss (VML) can result from trauma, infection, congenital anomalies, or surgery, and produce permanent functional and cosmetic deficits. There are no effective treatment options for VML injuries, and recent advances toward development of muscle constructs lack the ability to achieve innervation necessary for long-term function. We sought to develop a proof-of-concept biomaterial construct that could achieve acetylcholine receptor (AChR) clustering on muscle-derived cells (MDCs) in vitro. The approach consisted of the presentation of neural (Z+) agrin from the surface of microspheres embedded with a fibrin hydrogel to muscle cells (C2C12 cell line or primary rat MDCs). AChR clustering was spatially restricted to areas of cell (C2C12)-microsphere contact when the microspheres were delivered in suspension or when they were incorporated into a thin (2D) fibrin hydrogel. AChR clusters were observed from 16 to 72 h after treatment when Z+ agrin was adsorbed to the microspheres, and for greater than 120 h when agrin was covalently coupled to the microspheres. Little to no AChR clustering was observed when agrin-coated microspheres were delivered from specially designed 3D fibrin constructs. However, cyclic stretch in combination with agrin-presenting microspheres led to dramatic enhancement of AChR clustering in cells cultured on these 3D fibrin constructs, suggesting a synergistic effect between mechanical strain and agrin stimulation of AChR clustering in vitro. These studies highlight a strategy for maintaining a physiological phenotype characterized by motor endplates of muscle cells used in tissue engineering strategies for muscle regeneration. As such, these observations may provide an important first step toward improving function of tissue-engineered constructs for treatment of VML injuries.

Autologous Minced Muscle Grafts Improve Muscle Strength in a Porcine Model of Volumetric Muscle Loss Injury

OBJECTIVES

The traumatic loss of muscle tissue, defined as volumetric muscle loss (VML) injury, has no definitive therapy. The purposes of this study were: (1) to develop a porcine model of VML and (2) to investigate autologous minced muscle grafts (1-mm pieces of muscle) as a potential therapeutic. Minced grafts were evaluated because they have promoted fiber regeneration and functional recovery in rat VML models and do not require US Food and Drug Administration approval for clinical use.

METHODS

In 5 female Yorkshire-cross pigs, ≈5 g (≈20%) of tissue was excised from the peroneous tertius muscle (≈3 × 3 × 1.5-cm defect) of each leg. The defect in one leg was treated with autologous minced grafts derived from the contralateral leg. Maximal isometric tetanic strength assessments of the dorsiflexor muscles (ie, the peroneous tertius muscle) were performed before and biweekly up to 12 weeks postinjury.

RESULTS

VML injury resulted in a -43.5% ± 7.2% strength deficit 12 weeks postinjury in nonrepaired legs. Autologous minced muscle graft repair significantly improved strength over 12 weeks (32% strength increase 12 weeks postinjury vs. nonrepaired muscles with a remaining -27.8% ± 7.0% strength deficit; P < 0.001). Nonrepaired muscles developed extensive fibrosis and presented no evidence of muscle fiber regeneration within the defect area. Minced graft-treated muscles presented areas of putative de novo muscle fiber regeneration within the defect area, although extensive fibrotic tissue deposition was also present.

CONCLUSION

Autologous minced muscle grafts partially restored neuromuscular strength in a novel porcine model of VML.

The Potential of Combination Therapeutics for More Complete Repair of Volumetric Muscle Loss Injuries: The Role of Exogenous Growth Factors and/or Progenitor Cells in Implantable Skeletal Muscle Tissue Engineering Technologies

Despite the robust regenerative capacity of skeletal muscle, there are a variety of congenital and acquired conditions in which the volume of skeletal muscle loss results in major permanent functional and cosmetic deficits. These latter injuries are referred to as volumetric muscle loss (VML) injuries or VML-like conditions, and they are characterized by the simultaneous absence of multiple tissue components (i.e., nerves, vessels, muscles, satellite cells, and matrix). There are currently no effective treatment options. Regenerative medicine/tissue engineering technologies hold great potential for repair of these otherwise irrecoverable VML injuries. In this regard, three-dimensional scaffolds have been used to deliver sustained amounts of growth factors into a variety of injury models, to modulate host cell recruitment and extracellular matrix remodeling. However, this is a nascent field of research, and more complete functional improvements require more precise control of the spatiotemporal distribution of critical growth factors over a physiologically relevant range. This is especially true for VML injuries where incorporation of a cellular component into the scaffolds might provide not only a source of new tissue formation but also additional signals for host cell migration, recruitment, and survival. To this end, we review the major features of muscle repair and regeneration for largely recoverable injuries, and then discuss recent cell- and/or growth factor-based approaches to repair the more profound and irreversible VML and VML-like injuries. The underlying supposition is that more rationale incorporation of exogenous growth factors and/or cellular components will be required to optimize the regenerative capacity of implantable therapeutics for VML repair.

Guidelines for Models of Skeletal Muscle Injury and Therapeutic Assessment

Volumetric muscle loss (VML) injuries present a large clinical challenge with a significant need for new interventions. While there have been numerous reviews on muscle injury models, few have critically evaluated VML models. The objective of this review is to discuss current preclinical models of VML in terms of models, analytical outcomes, and therapeutic interventions, and to provide guidelines for the future use of preclinical VML models. This is a work of the US Government and is not subject to copyright protection in the USA. Foreign copyrights may apply. Published by S. Karger AG, Basel.

Codelivery of Infusion Decellularized Skeletal Muscle with Minced Muscle Autografts Improved Recovery from Volumetric Muscle Loss Injury in a Rat Model

Skeletal muscle is capable of robust self-repair following mild trauma, yet in cases of traumatic volumetric muscle loss (VML), where more than 20% of a muscle's mass is lost, this capacity is overwhelmed. Current autogenic whole muscle transfer techniques are imperfect, which has motivated the exploration of implantable scaffolding strategies. In this study, the use of an allogeneic decellularized skeletal muscle (DSM) scaffold with and without the addition of minced muscle (MM) autograft tissue was explored as a repair strategy using a lower-limb VML injury model (n = 8/sample group). We found that the repair of VML injuries using DSM + MM scaffolds significantly increased recovery of peak contractile force (81 ± 3% of normal contralateral muscle) compared to unrepaired VML controls (62 ± 4%). Similar significant improvements were measured for restoration of muscle mass (88 ± 3%) in response to DSM + MM repair compared to unrepaired VML controls (79 ± 3%). Histological findings revealed a marked decrease in collagen dense repair tissue formation both at and away from the implant site for DSM + MM repaired muscles. The addition of MM to DSM significantly increased MyoD expression, compared to isolated DSM treatment (21-fold increase) and unrepaired VML (37-fold) controls. These findings support the further exploration of both DSM and MM as promising strategies for the repair of VML injury.

Immunomodulatory capacity of the local mesenchymal stem cells transplantation after severe skeletal muscle injury in female rats

CONTEXT

Cell therapy technique with stem cells is a very attractive strategy for the treatment of muscle disorders.

OBJECTIVE

The objective of this study was to investigate the mechanism of local transplantation of mesenchymal stem cells (MSCs) which could contribute to skeletal muscle healing.

MATERIALS AND METHODS

Female rats were divided into three equal groups as the following: group 1, the negative control group (untreated group), group 2, sham-treated group, rats with muscle injuries involving volumetric muscle loss (VML) of adductor brevis muscle and injected locally with phosphate-buffered saline (PBS) 0.5 ml without stem cells after 7 d of muscle injury, group 3, treated group, rats with VML and injected locally (intramuscular) with 1.5 × 10⁶ bone marrow MSCs suspended in PBS 0.5 ml (1) after 7 d of muscle tissue injury. All animals were sacrificed after 4 weeks of stem cell transplantation.

RESULTS

In vitro culture the morphology of MSCs reached confluence and appeared as long spindle in shape on 9-14 d. Most of the cells did not express the hematopoietic cell marker, CD34 and CD45 but expressed MSCs marker CD44, CD90 and CD105. The remarkable increase of proliferating cell nuclear antigen positive nucleus was recorded in MSCs group as compared to PBS group. After 28 d of injection, administration of only PBS into the site of muscle injury caused up-regulation in the levels of interleukins IL-1β, IL-6, tumor necrosis factor alpha (TNF-α), transforming growth factor beta (TGF-β1), interferon alpha (IFN-α) and down-regulate the level of IL-10 in muscular tissue comparing to the untreated control. Bone marrow MSCs + PBS injected at the site of muscle injury significantly down-regulate the inflammatory cytokines levels IL-1β and IL-6 and TNF-α, TGF-β1 and IFN-α and up-regulate the level of IL-10. Collagen concentrations in the injured skeletal muscle estimated by enzyme-linked immuno sorbent assay and stained with Masson trichrome stain were increased with PBS group and decreased after transplantation of bone marrow MSCs in the site of injury. Muscle sections stained with H&E showed a higher number of centronucleated regenerating myofibers in the stem-cell-treated group than in the (PBS) and untreated control group. Microvasculature of skeletal muscle was decreased as demonstrated by immunostaining technique for CD34 in PBS group from untreated control. The MSCs group showed angiogenesis and marked increase of skeletal muscle microvasculature than PBS group.

CONCLUSION

MSCs can modify the local immunological responses and improve muscle regeneration by suppressing of inflammatory cytokines, activating of the anti-inflammatory cytokine, restoration of muscle fibers and angiogenesis. By means of increase in TGF-β production in response to muscle injury prevent the repair of injured fibers and increase connective tissue production (collagen fibers), thus propagating skeletal muscle weakness and fibrosis whereas MSCs + PBS injected at the site of muscle injury significantly down-regulate (TGF-β1) and hence the level of collagen (fibrosis or scar areas). MSCs are able to block the fibrotic signaling cascade by declining TGF-β1 and scar areas in the injured muscle.

The characterization of decellularized human skeletal muscle as a blueprint for mimetic scaffolds

The use of decellularized skeletal muscle (DSM) as a cell substrate and scaffold for the repair of volumetric muscle loss injuries has shown therapeutic promise. The performance of DSM materials motivated our interest in exploring the chemical and physical properties of this promising material. We suggest that these properties could serve as a blueprint for the development of next generation engineered materials with DSM mimetic properties. In this study, whole human lower limb rectus femoris (n = 10) and upper limb supraspinatus muscle samples (n = 10) were collected from both male and female tissue donors. Skeletal muscle samples were decellularized and nine property values, capturing key compositional, architectural, and mechanical properties, were measured and statistically analyzed. Mean values for each property were determined across muscle types and sexes. Additionally, the influence of muscle type (upper vs lower limb) and donor sex (male vs female) on each of the DSM material properties was examined. The data suggests that DSM materials prepared from lower limb rectus femoris samples have an increased modulus and contain a higher collagen content then upper limb supraspinatus muscles. Specifically, lower limb rectus femoris DSM material modulus and collagen content was approximately twice that of lower limb supraspinatus DSM samples. While muscle type did show some influence on material properties, we did not find significant trends related to sex. The material properties reported herein may be used as a blueprint for the data-driven design of next generation engineered scaffolds with muscle mimetic properties, as well as inputs for computational and physical models of skeletal muscle.

Recovery from volumetric muscle loss injury: A comparison between young and aged rats

Termed volumetric muscle loss (VML), the bulk loss of skeletal muscle tissue either through trauma or surgery overwhelms the capacity for repair, leading to the formation of non-contractile scar tissue. The myogenic potential, along with other factors that influence wound repair are known to decline with age. In order to develop effective treatment strategies for VML injuries that are effective across a broad range of patient populations, it is necessary to understand how the response to VML injury is affected by aging. Towards this end, this study was conducted to compare the response of young and aged animal groups to a lower extremity VML injury. Young (3months, n=12) and aged (18months, n=8) male Fischer 344 rats underwent surgical VML injury of the tibialis anterior muscle. Three months after VML injury it was found that young TA muscle was on average 16% heavier than aged muscle when no VML injury was performed and 25% heavier when comparing VML treated young and aged animals (p<0.0001, p<0.0001). Peak contractile force for both the young and aged groups was found to decrease significantly following VML injury, producing 65% and 59% of the contralateral limbs' peak force, respectively (p<0.0001). However, there were no differences found for peak contractile force based on age, suggesting that VML affects muscle's ability to repair, regardless of age. In this study, we used the ratio of collagen I to MyoD expression as a metric for fibrosis vs. myogenesis. Decreasing fiber cross-sectional area with advancing age (p<0.005) coupled with the ratio of collagen I to MyoD expression, which increased with age, supports the thought that regeneration is impaired in the aged population in favor of fibrosis (p=0.0241). This impairment is also exacerbated by the contribution of VML injury, where a 77-fold increase in the ratio of collagen I to MyoD was observed in the aged group (p<0.0002). The aged animal model described in this study provides a tool for investigators exploring not only the development of VML injury strategies but also the effect of aging on muscle regeneration.

Development of an infusion bioreactor for the accelerated preparation of decellularized skeletal muscle scaffolds

The implantation of decellularized tissue has shown effectiveness as a strategy for the treatment of volumetric muscle loss (VML) injuries. The preparation of decellularized tissue typically relies on the diffusion driven removal of cellular debris. For bulky tissues like muscle, the process can be lengthy, which introduces opportunities for both tissue contamination and degradation of key ECM molecules. In this study we report on the accelerated preparation of decellularized skeletal muscle (DSM) scaffolds using a infusion system and examine scaffold performance for the repair of VML injuries. The preparation of DSM scaffolds using infusion was dramatically accelerated. As the infusion rate (1% SDS) was increased from 0.1 to 1 and 10ml/hr, the time needed to remove intracellular myoglobin and actin decreased from a maximum of 140 ± 3hrs to 45 ± 3hrs and 10 ± 2hrs respectively. Although infusion appeared to remove cellular debris more aggressively, it did not significantly decrease the collagen or glycosaminoglycan composition of DSM samples when compared to un-infused controls. Infusion prepared DSM samples retained the aligned network structure and mechanical integrity of control samples. Infusion prepared DSM samples supported the attachment and in-vitro proliferation of myoblast cells and was well tolerated by the host when examined in-vivo. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:745-755, 2016.

Evaluation of adipose-derived stem cells for tissue-engineered muscle repair construct-mediated repair of a murine model of volumetric muscle loss injury

Volumetric muscle loss (VML) can occur from congenital defects, muscle wasting diseases, civilian or military injuries, and as a result of surgical removal of muscle tissue (eg, cancer), all of which can lead to irrevocable functional and cosmetic defects. Current tissue engineering strategies to repair VML often employ muscle-derived progenitor cells (MDCs) as one component. However, there are some inherent limitations in their in vitro culture expansion. Thus, this study explores the potential of adipose-derived stem cells (ADSCs) as an alternative cell source to MDCs for tissue engineering of skeletal muscle. A reproducible VML injury model in murine latissimus dorsi muscle was used to evaluate tissue-engineered muscle repair (TEMR) constructs incorporating MDCs or ADSCs. Importantly, histological analysis revealed that ADSC-seeded constructs displayed regeneration potential that was comparable to those seeded with MDCs 2 months postrepair. Furthermore, morphological analysis of retrieved constructs demonstrated signs of neotissue formation, including cell fusion, fiber formation, and scaffold remodeling. Immunohistochemistry demonstrated positive staining for both structural and functional proteins. Positive staining for vascular structures indicated the potential for long-term neotissue survival and integration with existing musculature. Qualitative observation of lentivirus-Cherry-labeled donor cells by immunohistochemistry indicates that participation of ADSCs in new hybrid myofiber formation incorporating donor cells was relatively low, compared to donor MDCs. However, ADSCs appear to participate in vascularization. In summary, I have demonstrated that TEMR constructs generated with ADSCs displayed skeletal muscle regeneration potential comparable to TEMR–MDC constructs as previously reported.

Mesenchymal stem cells augment the adaptive response to eccentric exercise

PURPOSE

The α7β1 integrin is a transmembrane protein expressed in the skeletal muscle that can link the actin cytoskeleton to the surrounding basal lamina. We have previously demonstrated that transgenic mice overexpressing the α7B integrin in the skeletal muscle (MCK:α7B; α7Tg) mount an enhanced satellite cell and growth response to single or multiple bouts of eccentric exercise. In addition, interstitial stem cells characterized as mesenchymal stem cells (MSCs) accumulate in α7Tg muscle (mMSCs) in the sedentary state and after exercise. The results from these studies prompted us to determine the extent to which mMSC underlie the beneficial adaptive responses observed in α7Tg skeletal muscle after exercise.

METHODS

mMSCs (Sca-1CD45) were isolated from α7Tg mice, dye-labeled, and intramuscularly injected into adult wild type recipient mice. After injection of mMSCs or saline, mice remained sedentary (SED) or were subjected to eccentric exercise training (TR) (downhill running) on a treadmill (three times per week) for 2 or 4 wk. Gastrocnemius-soleus complexes were collected 24 h after the last bout of exercise.

RESULTS

mMSCs did not directly fuse with existing fibers; however, mMSCs injection enhanced Pax7 satellite cell number and myonuclear content compared with all other groups at 2 wk after exercise. Mean CSA, percentage of larger caliber fibers (>3000 μm), and grip strength were increased in mMSCs/TR compared with saline/SED and mMSCs/SED at 4 wk. mMSC transplantation did not enhance repair or growth in the absence of exercise.

CONCLUSIONS

The results from this study demonstrate that mMSCs contribute to beneficial changes in satellite cell expansion and growth in α7Tg muscle after eccentric exercise. Thus, MSCs that naturally accumulate in the muscle after eccentric contractions may enhance the adaptive response to exercise.

Impaired primary mouse myotube formation on crosslinked type I collagen films is enhanced by laminin and entactin

In skeletal muscle, the stem cell niche is important for controlling the quiescent, proliferation and differentiation states of satellite cells, which are key for skeletal muscle regeneration after wounding. It has been shown that type I collagen, often used as 3D-scaffolds for regenerative medicine purposes, impairs myoblast differentiation. This is most likely due to the absence of specific extracellular matrix proteins providing attachment sites for myoblasts and/or myotubes. In this study we investigated the differentiation capacity of primary murine myoblasts on type I collagen films either untreated or modified with elastin, laminin, type IV collagen, laminin/entactin complex, combinations thereof, and Matrigel as a positive control. Additionally, increased reactive oxygen species (ROS) and ROCK signaling might also be involved. To measure ROS levels with live-cell microscopy, fibronectin-coated glass coverslips were additionally coated with type I collagen and Matrigel onto which myoblasts were differentiated. On type I collagen-coated coverslips, myotube formation was impaired while ROS levels were increased. However, anti-oxidant treatment did not enhance myotube formation. ROCK inhibition, which generally improve cellular attachment to uncoated surfaces or type I collagen, enhanced myoblast attachment to type I collagen-coated coverslips and -films, but slightly enhanced myotube formation. Only modification of type I collagen films by Matrigel and a combination of laminin/entactin significantly improved myotube formation. Our results indicate that type I collagen scaffolds can be modified by satellite cell niche factors of which specifically laminin and entactin enhanced myotube formation. This offers a promising approach for regenerative medicine purposes to heal skeletal muscle wounds.

STATEMENT OF SIGNIFICANCE

In this manuscript we show for the first time that impaired myotube formation on type I collagen scaffolds can be completely restored by modification with laminin and entactin, two extracellular proteins from the satellite cell niche. This offers a promising approach for regenerative medicine approaches to heal skeletal muscle wounds.

Biomimetic scaffolds for regeneration of volumetric muscle loss in skeletal muscle injuries

Skeletal muscle injuries typically result from traumatic incidents such as combat injuries where soft-tissue extremity injuries are present in one of four cases. Further, about 4.5 million reconstructive surgical procedures are performed annually as a result of car accidents, cancer ablation, or cosmetic procedures. These combat- and trauma-induced skeletal muscle injuries are characterized by volumetric muscle loss (VML), which significantly reduces the functionality of the injured muscle. While skeletal muscle has an innate repair mechanism, it is unable to compensate for VML injuries because large amounts of tissue including connective tissue and basement membrane are removed or destroyed. This results in a significant need to develop off-the-shelf biomimetic scaffolds to direct skeletal muscle regeneration. Here, the structure and organization of native skeletal muscle tissue is described in order to reveal clear design parameters that are necessary for scaffolds to mimic in order to successfully regenerate muscular tissue. We review the literature with respect to the materials and methodologies used to develop scaffolds for skeletal muscle tissue regeneration as well as the limitations of these materials. We further discuss the variety of cell sources and different injury models to provide some context for the multiple approaches used to evaluate these scaffold materials. Recent findings are highlighted to address the state of the field and directions are outlined for future strategies, both in scaffold design and in the use of different injury models to evaluate these materials, for regenerating functional skeletal muscle.

STATEMENT OF SIGNIFICANCE

Volumetric muscle loss (VML) injuries result from traumatic incidents such as those presented from combat missions, where soft-tissue extremity injuries are represented in one of four cases. These injuries remove or destroy large amounts of skeletal muscle including the basement membrane and connective tissue, removing the structural, mechanical, and biochemical cues that usually direct its repair. This results in a significant need to develop off-the-shelf biomimetic scaffolds to direct skeletal muscle regeneration. In this review, we examine current strategies for the development of scaffold materials designed for skeletal muscle regeneration, highlighting advances and limitations associated with these methodologies. Finally, we identify future approaches to enhance skeletal muscle regeneration.

Strategies for skeletal muscle tissue engineering: seed vs. soil

The most commonly used tissue engineering approach includes the ex vivo combination of site-appropriate cell(s) and scaffold material(s) to create three-dimensional constructs for tissue replacement or reconstruction. These three-dimensional combinations are typically subjected to a period of culture and conditioning (i.e., self-assembly and maturation) to promote the development of ex vivo constructs which closely mimic native target tissue. This cell-based approach is challenged by the host response to the engineered tissue construct following surgical implantation. As an alternative to the cell-based approach, acellular biologic scaffolds attract endogenous cells and remodel into partially functional mimics of native tissue upon implantation. The present review examines cell-types (i.e., seed), scaffold materials (i.e., soil), and challenges associated with functional tissue engineering. Skeletal muscle is used as the target tissue prototype but the discussed principles will largely apply to most body systems.

Rapid release of growth factors regenerates force output in volumetric muscle loss injuries

A significant challenge in the design and development of biomaterial scaffolds is to incorporate mechanical and biochemical cues to direct organized tissue growth. In this study, we investigated the effect of hepatocyte growth factor (HGF) loaded, crosslinked fibrin (EDCn-HGF) microthread scaffolds on skeletal muscle regeneration in a mouse model of volumetric muscle loss (VML). The rapid, sustained release of HGF significantly enhanced the force production of muscle tissue 60 days after injury, recovering more than 200% of the force output relative to measurements recorded immediately after injury. HGF delivery increased the number of differentiating myoblasts 14 days after injury, and supported an enhanced angiogenic response. The architectural morphology of microthread scaffolds supported the ingrowth of nascent myofibers into the wound site, in contrast to fibrin gel implants which did not support functional regeneration. Together, these data suggest that EDCn-HGF microthreads recapitulate several of the regenerative cues lost in VML injuries, promote remodeling of functional muscle tissue, and enhance the functional regeneration of skeletal muscle. Further, by strategically incorporating specific biochemical factors and precisely tuning the structural and mechanical properties of fibrin microthreads, we have developed a powerful platform technology that may enhance regeneration in other axially aligned tissues.

In Vivo Bioluminescence Imaging - A Suitable Method to Track Mesenchymal Stromal Cells in a Skeletal Muscle Trauma

Cell-based therapies have emerged during the last decade in various clinical fields. Especially mesenchymal stromal cells (MSCs) have been used in pre-clinical and clinical applications in cardiovascular, neurodegenerative and musculoskeletal disorders. In order to validate survival and viability as well as possible engraftment of MSCs into the host tissue a live cell imaging technique is needed that allows non-invasive, temporal imaging of cellular kinetics as well as evaluation of cell viability after transplantation. In this study we used luciferase-based bioluminescence imaging (BLI) to investigate the survival of autologous MSCs transplanted into a severely crushed soleus muscle of the rats. Furthermore we compared local as well as intra-arterial (i.a.) administration of cells and analyzed if luciferase transduced MSCs depict the same characteristics in vitro as non-transduced MSCs. We could show that transduction of MSCs does not alter their in vitro characteristics, thus, transduced MSCs display the same differentiation, proliferation and migration capacity as non-transduced cells. Using BLI we could track MSCs transplanted into a crushed soleus muscle until day 7 irrespective of local or i.a.

APPLICATION

Hence, our study proves that luciferase-based BLI is a suitable method for in vivo tracking of MSCs in skeletal muscle trauma in rats.

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.

Decellularized Human Skeletal Muscle as Biologic Scaffold for Reconstructive Surgery

Engineered skeletal muscle tissues have been proposed as potential solutions for volumetric muscle losses, and biologic scaffolds have been obtained by decellularization of animal skeletal muscles. The aim of the present work was to analyse the characteristics of a biologic scaffold obtained by decellularization of human skeletal muscles (also through comparison with rats and rabbits) and to evaluate its integration capability in a rabbit model with an abdominal wall defect. Rat, rabbit and human muscle samples were alternatively decellularized with two protocols: n.1, involving sodium deoxycholate and DNase I; n.2, trypsin-EDTA and Triton X-NH₄OH. Protocol 2 proved more effective, removing all cellular material and maintaining the three-dimensional networks of collagen and elastic fibers. Ultrastructural analyses with transmission and scanning electron microscopy confirmed the preservation of collagen, elastic fibres, glycosaminoglycans and proteoglycans. Implantation of human scaffolds in rabbits gave good results in terms of integration, although recellularization by muscle cells was not completely achieved. In conclusion, human skeletal muscles may be effectively decellularized to obtain scaffolds preserving the architecture of the extracellular matrix and showing mechanical properties suitable for implantation/integration. Further analyses will be necessary to verify the suitability of these scaffolds for in vitro recolonization by autologous cells before in vivo implantation.

Interactions between Skeletal Muscle Myoblasts and their Extracellular Matrix Revealed by a Serum Free Culture System

Decellularisation of skeletal muscle provides a system to study the interactions of myoblasts with muscle extracellular matrix (ECM). This study describes the efficient decellularisation of quadriceps muscle with the retention of matrix components and the use of this matrix for myoblast proliferation and differentiation under serum free culture conditions. Three decellularisation approaches were examined; the most effective was phospholipase A2 treatment, which removed cellular material while maximizing the retention of ECM components. Decellularised muscle matrices were then solubilized and used as substrates for C2C12 mouse myoblast serum free cultures. The muscle matrix supported myoblast proliferation and differentiation equally as well as collagen and fibronectin. Immunofluorescence analyses revealed that myoblasts seeded on muscle matrix and fibronectin differentiated to form long, well-aligned myotubes, while myoblasts seeded on collagen were less organized. qPCR analyses showed a time dependent increase in genes involved in skeletal muscle differentiation and suggested that muscle-derived matrix may stimulate an increased rate of differentiation compared to collagen and fibronectin. Decellularized whole muscle three-dimensional scaffolds also supported cell adhesion and spreading, with myoblasts aligning along specific tracts of matrix proteins within the scaffolds. Thus, under serum free conditions, intact acellular muscle matrices provided cues to direct myoblast adhesion and migration. In addition, myoblasts were shown to rapidly secrete and organise their own matrix glycoproteins to create a localized ECM microenvironment. This serum free culture system has revealed that the correct muscle ECM facilitates more rapid cell organisation and differentiation than single matrix glycoprotein substrates.

Surgical treatment for muscle injuries

Muscle injury causes functional impairment. The healing process takes time and fibrotic tissue can result. Recurrence and delayed recovery remain as unsolved problems. Surgical intervention can be a feasible alternative to avoid early and late complications associated with complete muscle tear in attempt to improve functional results. This article hopes to provide an update about surgical treatments for muscle tears in different scenarios.

Engineering muscle tissue for the fetus: getting ready for a strong life

Congenital malformations frequently involve either skeletal, smooth or cardiac tissues. When large parts of those tissues are damaged, the repair of the malformations is challenged by the fact that so much autologous tissue is missing. Current treatments require the use of prostheses or other therapies and are associated with a significant morbidity and mortality. Nonetheless, affected children have generally good survival rates and mostly normal schooling. As such, new therapeutic modalities need to represent significant improvements with clear safety profiles. Regenerative medicine and tissue engineering technologies have the potential to dramatically improve the treatment of any disease or disorder involving a lack of viable tissue. With respect to congenital soft tissue anomalies, the development of, for example, implantable muscle constructs would provide not only the usual desired elasticity and contractile proprieties, but should also be able to grow with the fetus and/or in the postnatal life. Such an approach would eliminate the need for multiple surgeries. However, the more widespread clinical applications of regenerative medicine and tissue engineering technologies require identification of the optimal indications, as well as further elucidation of the precise mechanisms and best methods (cells, scaffolds/biomaterials) for achieving large functional tissue regeneration in those clinical indications. In short, despite some amazing scientific progress, significant safety and efficacy hurdles remain. However, the rapid preclinical advances in the field bode well for future applications. As such, translational researchers and clinicians alike need be informed and prepared to utilize these new techniques for the benefit of their patients, as soon as they are available. To this end, we review herein, the clinical need(s), potential applications, and the relevant preclinical studies that are currently guiding the field toward novel therapeutics.

Biomaterials based strategies for skeletal muscle tissue engineering: existing technologies and future trends

Skeletal muscles have a robust capacity to regenerate, but under compromised conditions, such as severe trauma, the loss of muscle functionality is inevitable. Research carried out in the field of skeletal muscle tissue engineering has elucidated multiple intrinsic mechanisms of skeletal muscle repair, and has thus sought to identify various types of cells and bioactive factors which play an important role during regeneration. In order to maximize the potential therapeutic effects of cells and growth factors, several biomaterial based strategies have been developed and successfully implemented in animal muscle injury models. A suitable biomaterial can be utilized as a template to guide tissue reorganization, as a matrix that provides optimum micro-environmental conditions to cells, as a delivery vehicle to carry bioactive factors which can be released in a controlled manner, and as local niches to orchestrate in situ tissue regeneration. A myriad of biomaterials, varying in geometrical structure, physical form, chemical properties, and biofunctionality have been investigated for skeletal muscle tissue engineering applications. In the current review, we present a detailed summary of studies where the use of biomaterials favorably influenced muscle repair. Biomaterials in the form of porous three-dimensional scaffolds, hydrogels, fibrous meshes, and patterned substrates with defined topographies, have each displayed unique benefits, and are discussed herein. Additionally, several biomaterial based approaches aimed specifically at stimulating vascularization, innervation, and inducing contractility in regenerating muscle tissues are also discussed. Finally, we outline promising future trends in the field of muscle regeneration involving a deeper understanding of the endogenous healing cascades and utilization of this knowledge for the development of multifunctional, hybrid, biomaterials which support and enable muscle regeneration under compromised conditions.

An Autologous Muscle Tissue Expansion Approach for the Treatment of Volumetric Muscle Loss

Volumetric muscle loss (VML) is a hallmark of orthopedic trauma with no current standard of care. As a potential therapy for some VML indications, autologous minced muscle grafts (1 mm(3) pieces of muscle) are effective in promoting remarkable de novo fiber regeneration. But they require ample donor muscle tissue and therefore may be limited in their application for large clinical VML. Here, we tested the hypothesis that autologous minced grafts may be volume expanded in a collagen hydrogel, allowing for the use of lesser autologous muscle while maintaining regenerative and functional efficacy. The results of the study indicate that 50% (but not 75%) less minced graft tissue suspended in a collagen hydrogel promoted a functional improvement similar to that of a 100% minced graft repair. However, approximately half of the number of fibers regenerated de novo with 50% graft repair. Moreover, the fibers that regenerated had a smaller cross-sectional area. These findings support the concept of using autologous minced grafts for the regeneration of muscle tissue after VML, but indicate the need to identify optimal carrier materials for expansion.

Development of a biological scaffold engineered using the extracellular matrix secreted by skeletal muscle cells

The performance of implantable biomaterials derived from decellularized tissue, including encouraging results with skeletal muscle, suggests that the extracellular matrix (ECM) derived from native tissue has promising regenerative potential. Yet, the supply of biomaterials derived from donated tissue will always be limited, which is why the in-vitro fabrication of ECM biomaterials that mimic the properties of tissue is an attractive alternative. Towards this end, our group has utilized a novel method to collect the ECM that skeletal muscle myoblasts secrete and form it into implantable scaffolds. The cell derived ECM contained several matrix constituents, including collagen and fibronectin that were also identified within skeletal muscle samples. The ECM was organized into a porous network that could be formed with the elongated and aligned architecture observed within muscle samples. The ECM material supported the attachment and in-vitro proliferation of cells, suggesting effectiveness for cell transplantation, and was well tolerated by the host when examined in-vivo. The results suggest that the ECM collection approach can be used to produce biomaterials with compositions and structures that are similar to muscle samples, and while the physical properties may not yet match muscle values, the in-vitro and in-vivo results indicate it may be a suitable first generation alternative to tissue derived biomaterials.

Transplantation of devitalized muscle scaffolds is insufficient for appreciable de novo muscle fiber regeneration after volumetric muscle loss injury

Volumetric muscle loss (VML) is a traumatic and functionally debilitating muscle injury with limited treatment options. Developmental regenerative therapies for the repair of VML typically comprise an ECM scaffold. In this study, we tested if the complete reliance on host cell migration to a devitalized muscle scaffold without myogenic cells is sufficient for de novo muscle fiber regeneration. Devitalized (muscle ECM with no living cells) and, as a positive control, vital minced muscle grafts were transplanted to a VML defect in the tibialis anterior muscle of Lewis rats. Eight weeks post-injury, devitalized grafts did not appreciably promote de novo muscle fiber regeneration within the defect area, and instead remodeled into a fibrotic tissue mass. In contrast, transplantation of vital minced muscle grafts promoted de novo muscle fiber regeneration. Notably, pax7+ cells were absent in remote regions of the defect site repaired with devitalized scaffolds. At 2 weeks post-injury, the devitalized grafts were unable to promote an anti-inflammatory phenotype, while vital grafts appeared to progress to a pro-regenerative inflammatory response. The putative macrophage phenotypes observed in vivo were supported in vitro, in which soluble factors released from vital grafts promoted an M2-like macrophage polarization, whereas devitalized grafts failed to do so. These observations indicate that although the remaining muscle mass serves as a source of myogenic cells in close proximity to the defect site, a devitalized scaffold without myogenic cells is inadequate to appreciably promote de novo muscle fiber regeneration throughout the VML defect.

Tissue engineered scaffolds for an effective healing and regeneration: reviewing orthotopic studies

It is commonly stated that tissue engineering is the most promising approach to treat or replace failing tissues/organs. For this aim, a specific strategy should be planned including proper selection of biomaterials, fabrication techniques, cell lines, and signaling cues. A great effort has been pursued to develop suitable scaffolds for the restoration of a variety of tissues and a huge number of protocols ranging from in vitro to in vivo studies, the latter further differentiating into several procedures depending on the type of implantation (i.e., subcutaneous or orthotopic) and the model adopted (i.e., animal or human), have been developed. All together, the published reports demonstrate that the proposed tissue engineering approaches spread toward multiple directions. The critical review of this scenario might suggest, at the same time, that a limited number of studies gave a real improvement to the field, especially referring to in vivo investigations. In this regard, the present paper aims to review the results of in vivo tissue engineering experimentations, focusing on the role of the scaffold and its specificity with respect to the tissue to be regenerated, in order to verify whether an extracellular matrix-like device, as usually stated, could promote an expected positive outcome.

Repairing damaged tendon and muscle: are mesenchymal stem cells and scaffolds the answer?

Mesenchymal stem cells (MSCs) have become an area of intense interest in the treatment of musculoskeletal conditions, such as muscle and tendon injury, as various animal and human trials have demonstrated that implantation with MSCs leads to improved healing and function. However, these trials have usually been relatively small scale and lacking in adequate controls. Additionally, the optimum source of these cells has yet to be determined, partly due to a lack of understanding as to how MSCs produce their beneficial effects when implanted. Scaffolds have been shown to improve tissue-engineering repairs but require further work to optimize their interactions with both native tissue and implanted MSCs. Robust, well-controlled trials are therefore required to determine the usefulness of MSCs in musculoskeletal tissue repair.

Therapeutic approaches to skeletal muscle repair and healing

Context:

Skeletal muscle is comprised of a highly organized network of cells, neurovascular structures, and connective tissue. Muscle injury is typically followed by a well-orchestrated healing response that consists of the following phases: inflammation, regeneration, and fibrosis. This review presents the mechanisms of action and evidence supporting the effectiveness of various traditional and novel therapies at each phase of the skeletal muscle healing process.

Evidence Acquisition:

Relevant published articles were identified using MEDLINE (1978-2013).

Study Design:

Clinical review.

Level of Evidence:

Level 3.

Results:

To facilitate muscle healing, surgical techniques involving direct suture repair, as well as the implantation of innovative biologic scaffolds, have been developed. Nonsteroidal anti-inflammatory drugs may be potentially supplanted by nitric oxide and curcumin in modulating the inflammatory pathway. Studies in muscle regeneration have identified stem cells, myogenic factors, and β-agonists capable of enhancing the regenerative capabilities of injured tissue. Furthermore, transforming growth factor-β1 (TGF-β1) and, more recently, myostatin and the rennin-angiotensin system have been implicated in fibrous tissue formation; several antifibrotic agents have demonstrated the ability to disrupt these systems.

Conclusion:

Effective repair of skeletal muscle after severe injury is unlikely to be achieved with a single intervention. For full functional recovery of muscle there is a need to control inflammation, stimulate regeneration, and limit fibrosis.

Strength-of-Recommendation Taxonomy (SORT):

B

Native extracellular matrix: a new scaffolding platform for repair of damaged muscle

Effective clinical treatments for volumetric muscle loss resulting from traumatic injury or resection of a large amount of muscle mass are not available to date. Tissue engineering may represent an alternative treatment approach. Decellularization of tissues and whole organs is a recently introduced platform technology for creating scaffolding materials for tissue engineering and regenerative medicine. The muscle stem cell niche is composed of a three-dimensional architecture of fibrous proteins, proteoglycans, and glycosaminoglycans, synthesized by the resident cells that form an intricate extracellular matrix (ECM) network in equilibrium with the surrounding cells and growth factors. A consistent body of evidence indicates that ECM proteins regulate stem cell differentiation and renewal and are highly relevant to tissue engineering applications. The ECM also provides a supportive medium for blood or lymphatic vessels and for nerves. Thus, the ECM is the nature's ideal biological scaffold material. ECM-based bioscaffolds can be recellularized to create potentially functional constructs as a regenerative medicine strategy for organ replacement or tissue repopulation. This article reviews current strategies for the repair of damaged muscle using bioscaffolds obtained from animal ECM by decellularization of small intestinal submucosa (SIS), urinary bladder mucosa (UB), and skeletal muscle, and proposes some innovative approaches for the application of such strategies in the clinical setting.

Enhancement of muscle repair using human mesenchymal stem cells with a magnetic targeting system in a subchronic muscle injury model

BACKGROUND

A magnetic cell targeting system was previously developed to promote the accumulation of transplanted cells in sites of injury in order to effectively treat injured tissues. However, the optimum time of exposure to the magnetic field and the strength of the magnetic force have not yet been clarified. In this study, we investigated the optimum conditions of the magnetic force required to retain iron-labeled human mesenchymal stem cells (hMSCs) at the site of transplantation for muscle repair in a subchronic skeletal muscle injury nude rat model.

METHODS

First, the optimum strength and time of exposure to the magnetic force for cell retention at the transplantation site were investigated 2 days after cell transplantation (1 × 10(5) cells). Second, the degree of enhancement of muscle repair was investigated at 3 weeks after cell transplantation in the group treated without a magnetic force and two typical magnetic condition groups that exhibited different levels of cell integration in first part of the study.

RESULTS

On the basis of the results of the first investigation, it was concluded that a magnetic strength of 1.5 T and 10 min of exposure to the magnetic force were efficient conditions to induce the retention of transplanted cells at the site of transplantation. In the second study, the groups exposed to a 1.5-T magnetic field for 10 min demonstrated significant enhancement of muscle repair, both histologically and electromechanically.

CONCLUSIONS

This study identified the optimal conditions required to retain transplanted hMSCs at the site of transplantation using a magnetic targeting system. This study also showed that the restoration of subchronic muscle injuries can be enhanced by magnetically labeled hMSCs following the application of a magnetic force.

Decellularized tissue and cell-derived extracellular matrices as scaffolds for orthopaedic tissue engineering

The reconstruction of musculoskeletal defects is a constant challenge for orthopaedic surgeons. Musculoskeletal injuries such as fractures, chondral lesions, infections and tumor debulking can often lead to large tissue voids requiring reconstruction with tissue grafts. Autografts are currently the gold standard in orthopaedic tissue reconstruction; however, there is a limit to the amount of tissue that can be harvested before compromising the donor site. Tissue engineering strategies using allogeneic or xenogeneic decellularized bone, cartilage, skeletal muscle, tendon and ligament have emerged as promising potential alternative treatment. The extracellular matrix provides a natural scaffold for cell attachment, proliferation and differentiation. Decellularization of in vitro cell-derived matrices can also enable the generation of autologous constructs from tissue specific cells or progenitor cells. Although decellularized bone tissue is widely used clinically in orthopaedic applications, the exciting potential of decellularized cartilage, skeletal muscle, tendon and ligament cell-derived matrices has only recently begun to be explored for ultimate translation to the orthopaedic clinic.

Implantation of in vitro tissue engineered muscle repair constructs and bladder acellular matrices partially restore in vivo skeletal muscle function in a rat model of volumetric muscle loss injury

The frank loss of a large volume of skeletal muscle (i.e., volumetric muscle loss [VML]) can lead to functional debilitation and presents a significant problem to civilian and military medicine. Current clinical treatment for VML involves the use of free muscle flaps and physical rehabilitation; however, neither are effective in promoting regeneration of skeletal muscle to replace the tissue that was lost. Toward this end, skeletal muscle tissue engineering therapies have recently shown great promise in offering an unprecedented treatment option for VML. In the current study, we further extend our recent progress (Machingal et al., 2011, Tissue Eng; Corona et al., 2012, Tissue Eng) in the development of tissue engineered muscle repair (TEMR) constructs (i.e., muscle-derived cells [MDCs] seeded on a bladder acellular matrix (BAM) preconditioned with uniaxial mechanical strain) for the treatment of VML. TEMR constructs were implanted into a VML defect in a tibialis anterior (TA) muscle of Lewis rats and observed up to 12 weeks postinjury. The salient findings of the study were (1) TEMR constructs exhibited a highly variable capacity to restore in vivo function of injured TA muscles, wherein TEMR-positive responders (n=6) promoted an ≈61% improvement, but negative responders (n=7) resulted in no improvement compared to nonrepaired controls, (2) TEMR-positive and -negative responders exhibited differential immune responses that may underlie these variant responses, (3) BAM scaffolds (n=7) without cells promoted an ≈26% functional improvement compared to uninjured muscles, (4) TEMR-positive responders promoted muscle fiber regeneration within the initial defect area, while BAM scaffolds did so only sparingly. These findings indicate that TEMR constructs can improve the in vivo functional capacity of the injured musculature at least, in part, by promoting generation of functional skeletal muscle fibers. In short, the degree of functional recovery observed following TEMR implantation (BAM+MDCs) was 2.3×-fold greater than that observed following implantation of BAM alone. As such, this finding further underscores the potential benefits of including a cellular component in the tissue engineering strategy for VML injury.

Functional analysis of limb recovery following autograft treatment of volumetric muscle loss in the quadriceps femoris

Severe injuries to the extremities often result in muscle trauma and, in some cases, significant volumetric muscle loss (VML). These injuries continue to be challenging to treat, with few available clinical options, a high rate of complications, and often persistent loss of limb function. To facilitate the testing of regenerative strategies for skeletal muscle, we developed a novel quadriceps VML model in the rat, specifically addressing functional recovery of the limb. Our outcome measures included muscle contractility measurements to assess muscle function and gait analysis for evaluation of overall limb function. We also investigated treatment with muscle autografts, whole or minced, to promote regeneration of the defect area. Our defect model resulted in a loss of muscle function, with injured legs generating less than 55% of muscle strength from the contralateral uninjured control legs, even at 4 weeks post-injury. The autograft treatments did not result in significant recovery of muscle function. Measures of static and dynamic gait were significantly decreased in the untreated, empty defect group, indicating a decrease in limb function. Histological sections of the affected muscles showed extensive fibrosis, suggesting that this scarring of the muscle may be in part the cause of the loss of muscle function in this VML model. Taken together, these data are consistent with clinical findings of reduced muscle function in large VML injuries. This new model with quantitative functional outcome measures offers a platform on which to evaluate treatment strategies designed to regenerate muscle tissue volume and restore limb function.

Autologous minced muscle grafts: a tissue engineering therapy for the volumetric loss of skeletal muscle

Volumetric muscle loss (VML) results in a large void deficient in the requisite materials for regeneration for which there is no definitive clinical standard of care. Autologous minced muscle grafts (MG), which contain the essential components for muscle regeneration, may embody an ideal tissue engineering therapy for VML. The purpose of this study was to determine if orthotopic transplantation of MG acutely after VML in the tibialis anterior muscle of male Lewis rats promotes functional tissue regeneration. Herein we report that over the first 16 wk postinjury, MG transplantation 1) promotes remarkable regeneration of innervated muscle fibers within the defect area (i.e., de novo muscle fiber regeneration); 2) reduced evidence of chronic injury in the remaining muscle mass compared with nonrepaired muscles following VML (i.e., transplantation attenuated chronically upregulated transforming growth factor-β1 gene expression and the presence of centrally located nuclei in 30% of fibers observed in nonrepaired muscles); and 3) significantly improves net torque production (i.e., ∼55% of the functional deficit in nonrepaired muscles was restored). Additionally, voluntary wheel running was shown to reduce the heightened accumulation of extracellular matrix deposition observed within the regenerated tissue of MG-repaired sedentary rats 8 wk postinjury (collagen 1% area: sedentary vs. runner, ∼41 vs. 30%), which may have been the result of an augmented inflammatory response [i.e., M1 (CCR7) and M2 (CD163) macrophage expression was significantly greater in runner than sedentary MG-repaired muscles 2 wk postinjury]. These findings support further exploration of autologous minced MGs for the treatment of VML.

Muscle-derived decellularised extracellular matrix improves functional recovery in a rat latissimus dorsi muscle defect model

PURPOSE

Craniofacial maxillary injuries represent nearly 30% of all battlefield wounds, often involving volumetric muscle loss (VML). The physical loss of muscle results in functional deficits and cosmetic disfigurement. Although surgical solutions are limited, advances in biomaterials offer great promise for the restoration of form and function following VML. The primary purpose of this study was to determine whether muscle function could be restored in a novel VML rat model using muscle-derived extracellular matrix (M-ECM).

METHODS

Ten percent of the mass of the latissimus dorsi (LD) was excised. Three groups were examined: 1) no repair of defect (DEF), 2) repair with M-ECM and 3) sham (all procedures except muscle excision). Four and 8 weeks post-surgery, the isometric contractile properties of the LD were assessed in situ and selected histological properties were evaluated.

RESULTS

The defect resulted in an initial reduction in peak isometric force (Po) of 30%. At 8 weeks the difference between DEF and sham was 20.5%. At the same time, M-ECM was only 8.4% below sham. Although the histological analysis revealed a narrow, but well-formed band of muscle running along the middle of the M-ECM, it was judged to be too small to account for the observed improvement in muscle force.

CONCLUSIONS

Repair of VML with M-ECM can dramatically improve muscle function independent of muscle regeneration by providing a physical bridge that accommodates force transmission across the injury site. This method of repair may provide an easily translatable surgical method for selected forms of VML.

Transplantated mesenchymal stem cells derived from embryonic stem cells promote muscle regeneration and accelerate functional recovery of injured skeletal muscle

We previously established that mesenchymal stem cells originating from mouse embryonic stem (ES) cells (E-MSCs) showed markedly higher potential for differentiation into skeletal muscles in vitro than common mesenchymal stem cells (MSCs). Further, the E-MSCs exhibited a low risk for teratoma formation. Here we evaluate the potential of E-MSCs for differentiation into skeletal muscles in vivo and reveal the regeneration and functional recovery of injured muscle by transplantation. E-MSCs were transplanted into the tibialis anterior (TA) muscle 24 h following direct clamping. After transplantation, the myogenic differentiation of E-MSCs, TA muscle regeneration, and re-innervation were morphologically analyzed. In addition, footprints and gaits of each leg under spontaneous walking were measured by CatWalk XT, and motor functions of injured TA muscles were precisely analyzed. Results indicate that >60% of transplanted E-MSCs differentiated into skeletal muscles. The cross-sectional area of the injured TA muscles of E-MSC–transplanted animals increased earlier than that of control animals. E-MSCs also promotes re-innervation of the peripheral nerves of injured muscles. Concerning function of the TA muscles, we reveal that transplantation of E-MSCs promotes the recovery of muscles. This is the first report to demonstrate by analysis of spontaneous walking that transplanted cells can accelerate the functional recovery of injured muscles. Taken together, the results show that E-MSCs have a high potential for differentiation into skeletal muscles in vivo as well as in vitro. The transplantation of E-MSCs facilitated the functional recovery of injured muscles. Therefore, E-MSCs are an efficient cell source in transplantation.

A standardized rat model of volumetric muscle loss injury for the development of tissue engineering therapies

Soft tissue injuries involving volumetric muscle loss (VML) are defined as the traumatic or surgical loss of skeletal muscle with resultant functional impairment and represent a challenging clinical problem for both military and civilian medicine. In response, a variety of tissue engineering and regenerative medicine treatments are under preclinical development. A wide variety of animal models are being used, all with critical limitations. The objective of this study was to develop a model of VML that was reproducible and technically uncomplicated to provide a standardized platform for the development of tissue engineering and regenerative medicine solutions to VML repair. A rat model of VML involving excision of ∼20% of the muscle's mass from the superficial portion of the middle third of the tibialis anterior (TA) muscle was developed and was functionally characterized. The contralateral TA muscle served as the uninjured control. Additionally, uninjured age-matched control rats were also tested to determine the effect of VML on the contralateral limb. TA muscles were assessed at 2 and 4 months postinjury. VML muscles weighed 22.7% and 19.5% less than contralateral muscles at 2 and 4 months postinjury, respectively. These differences were accompanied by a reduction in peak isometric tetanic force (Po) of 28.4% and 32.5% at 2 and 4 months. Importantly, Po corrected for differences in body weight and muscle wet weights were similar between contralateral and age-matched control muscles, indicating that VML did not have a significant impact on the contralateral limb. Lastly, repair of the injury with a biological scaffold resulted in rapid vascularization and integration with the wound. The technical simplicity, reliability, and clinical relevance of the VML model developed in this study make it ideal as a standard model for the development of tissue engineering solutions for VML.

Promotion of skeletal muscle repair in a rat skeletal muscle injury model by local injection of human adipose tissue-derived regenerative cells

Human adipose tissue-derived regenerative cells (ADRCs) can be isolated easily and aseptically from unwanted subcutaneous fat without culturing. ADRCs have been used in clinical cosmetic therapy. In addition, they are expected to be an attractive and feasible source of cell-based therapies in regenerative medicine. Therefore, this paper investigates whether transplantation of human adult ADRCs into skeletal muscle injury models promotes the repair of muscle tissues. This was done by locally injecting human ADRCs into an injured site after laceration of the nude-rat tibialis anterior muscle. Phosphate-buffered saline (PBS) and bone marrow mononuclear cells (MNCs) were injected as negative and positive controls, respectively. After injury, recovery of muscle strength was accelerated by transplantation of ADRCs compared to administration of PBS and MNCs. Moreover, transplantation of ADRCs also enhanced angiogenesis and myogenesis, but the number of vascular and muscular cells labeled with the human cell-specific maker was limited at the injury site. Results showed that transplantation of ADRCs into a skeletal muscle injury model promoted repair of muscle tissues in a paracrine manner rather than differentiation of itself into blood vessels and myofibres. Thus, it is believed that ADRCs are a useful and feasible cell source not only for cosmetic therapy but also for regenerative therapy.

Induction of cardiac myogenic lineage development differs between mesenchymal and satellite cells and is accelerated by bone morphogenetic protein-4

Our aim was to further elucidate the cardiac lineage development of bone marrow-derived mesenchymal stem cells (MSC) and to identify cells which had the potential for cardiac myogenic differentiation when compared to skeletal muscle satellite (Sk-sat) myogenesis. Unlike Sk-sat, MSC expressed the early cardiac markers Nkx2.5 and GATA4. Their expression was significantly increased by culturing MSC with Bone Morphogenetic Protein 4 (BMP4). Enhanced cardiac myogenic lineage differentiation and loss of stem cell characteristics induced by BMP4 were further confirmed by flow cytometry of cells stained for Nkx2.5 and Sca-1 expression. MSC also expressed skeletal genes (MyoG, ssTnI, Sk-Act) early in culture but their expression was suppressed when BMP4 was added from day 0 to day 6 (p<0.05). BMP4 treated MSC also exhibited a 6-fold increase in cTnI expression by day 12 in culture. The average MSC action potential time duration at 90% (APD90) was 32.3±4ms, with some cells exhibiting action potentials closer to Sk-sat APD90 of 13.7±0.9ms. After treatment with BMP4, MSC significantly increased their APD90 to 54.4±7.6ms, shifting from the shorter skeletal-like signature, towards a longer action potential duration more characteristic of a cardiomyocyte signature. Our results show that MSC and Sk-sat exhibit similarities in myogenic lineage development early in culture but that BMP4 clearly enhances cardiac myogenic development, suppresses skeletal myogenesis, and leads to loss of "stemness" in MSC. These findings provide novel information regarding the use of BMP4 to accelerate cardiac myogenic development in harvested MSC and further support the use of MSC in cardiac regenerative therapy.

Further development of a tissue engineered muscle repair construct in vitro for enhanced functional recovery following implantation in vivo in a murine model of volumetric muscle loss injury

Volumetric muscle loss (VML) can result from trauma and surgery in civilian and military populations, resulting in irrecoverable functional and cosmetic deficits that cannot be effectively treated with current therapies. Previous work evaluated a bioreactor-based tissue engineering approach in which muscle derived cells (MDCs) were seeded onto bladder acellular matrices (BAM) and mechanically preconditioned. This first generation tissue engineered muscle repair (TEMR) construct exhibited a largely differentiated cellular morphology consisting primarily of myotubes, and moreover, significantly improved functional recovery within 2 months of implantation in a murine latissimus dorsi (LD) muscle with a surgically created VML injury. The present report extends these initial observations to further document the importance of the cellular phenotype and composition of the TEMR construct in vitro to the functional recovery observed following implantation in vivo. To this end, three distinct TEMR constructs were created by seeding MDCs onto BAM as follows: (1) a short-term cellular proliferation of MDCs to generate primarily myoblasts without bioreactor preconditioning (TEMR-1SP), (2) a prolonged cellular differentiation and maturation period that included bioreactor preconditioning (TEMR-1SPD; identical to the first generation TEMR construct), and (3) similar treatment as TEMR-1SPD but with a second application of MDCs during bioreactor preconditioning (TEMR-2SPD); simulating aspects of "exercise" in vitro. Assessment of maximal tetanic force generation on retrieved LD muscles in vitro revealed that TEMR-1SP and TEMR-1SPD constructs promoted either an accelerated (i.e., 1 month) or a prolonged (i.e., 2 month postinjury) functional recovery, respectively, of similar magnitude. Meanwhile, TEMR-2SPD constructs promoted both an accelerated and prolonged functional recovery, resulting in twice the magnitude of functional recovery of either TEMR-1SP or TEMR-1SPD constructs. Histological and molecular analyses indicated that TEMR constructs mediated functional recovery via regeneration of functional muscle fibers either at the interface of the construct and the native tissue or within the BAM scaffolding independent of the native tissue. Taken together these findings are encouraging for the further development and clinical application of TEMR constructs as a VML injury treatment.