Restoration of skeletal muscle defects with adult human cells delivered on fibrin microthreads

Extrusion-Based Printing of Myoblast-Loaded Fibrin Microthreads to Induce Myogenesis

Large skeletal muscle injuries such as volumetric muscle loss (VML) disrupt native tissue structures, including biophysical and biochemical signaling cues that promote the regeneration of functional skeletal muscle. Various biofabrication strategies have been developed to create engineered skeletal muscle constructs that mimic native matrix and cellular microenvironments to enhance muscle regeneration; however, there remains a need to create scalable engineered tissues that provide mechanical stability as well as structural and spatiotemporal signaling cues to promote cell-mediated regeneration of contractile skeletal muscle. We describe a novel strategy for bioprinting multifunctional myoblast-loaded fibrin microthreads (myothreads) that recapitulate the cellular microniches to drive myogenesis and aligned myotube formation. We characterized myoblast alignment, myotube formation, and tensile properties of myothreads as a function of cell-loading density and culture time. We showed that increasing myoblast loading densities enhances myotube formation. Additionally, alignment analyses indicate that the bioprinting process confers myoblast alignment in the constructs. Finally, tensile characterizations suggest that myothreads possess the structural stability to serve as a potential platform for developing scalable muscle scaffolds. We anticipate that our myothread biofabrication approach will enable us to strategically investigate biophysical and biochemical signaling cues and cellular mechanisms that enhance functional skeletal muscle regeneration for the treatment of VML.

Neuromuscular Regeneration of Volumetric Muscle Loss Injury in Response to Agrin-Functionalized Tissue Engineered Muscle Grafts and Rehabilitative Exercise

Neuromuscular deficits compound the loss of contractile tissue in volumetric muscle loss (VML). Two avenues for promoting recovery are neuromuscular junction (NMJ)-promoting substrates (e.g., agrin) and endurance exercise. Although mechanical stimulation enhances agrin-induced NMJ formation, the two modalities have yet to be evaluated combinatorially. It is hypothesized that the implantation of human myogenic progenitor-seeded tissue-engineered muscle grafts (hTEMGs) in combination with agrin treatment and/or exercise will enhance neuromuscular recovery after VML. The hTEMGs alone transplant into VML defects promote significant regeneration with minimal scarring. A sex-appropriate, low-intensity continuous running exercise paradigm increases acetylcholine receptor (AChR) cluster density in male mice twofold relative to hTEMG alone after 7 weeks of treadmill training (p < 0.05). To further promote neuromuscular recovery, agrin is incorporated into the scaffolds via covalent tethering. In vitro, agrin increases the proliferation of hMPs, and trends toward greater myogenic maturity and AChR clustering. Upon transplantation, both hTEMGs + agrin and hTEMGs + exercise induce near 100% recovery of muscle mass and increase twitch and tetanic force output (p > 0.05). However, agrin treatment in combination with exercise produces no additional benefit. These data highlight the unprecedented regenerative potential of using hTEMGs together with either agrin or exercise supplementation to treat VML injuries.

Tissue Engineered 3D Constructs for Volumetric Muscle Loss

Severe injuries to skeletal muscles, including cases of volumetric muscle loss (VML), are linked to substantial tissue damage, resulting in functional impairment and lasting disability. While skeletal muscle can regenerate following minor damage, extensive tissue loss in VML disrupts the natural regenerative capacity of the affected muscle tissue. Existing clinical approaches for VML, such as soft-tissue reconstruction and advanced bracing methods, need to be revised to restore tissue function and are associated with limitations in tissue availability and donor-site complications. Advancements in tissue engineering (TE), particularly in scaffold design and the delivery of cells and growth factors, show promising potential for regenerating damaged skeletal muscle tissue and restoring function. This article provides a brief overview of the pathophysiology of VML and critiques the shortcomings of current treatments. The subsequent section focuses on the criteria for designing TE scaffolds, offering insights into various natural and synthetic biomaterials and cell types for effectively regenerating skeletal muscle. We also review multiple TE strategies involving both acellular and cellular scaffolds to encourage the development and maturation of muscle tissue and facilitate integration, vascularization, and innervation. Finally, the article explores technical challenges hindering successful translation into clinical applications.

Biomaterials-Based Technologies in Skeletal Muscle Tissue Engineering

For many clinically prevalent severe injuries, the inherent regenerative capacity of skeletal muscle remains inadequate. Skeletal muscle tissue engineering (SMTE) seeks to meet this clinical demand. With continuous progress in biomedicine and related technologies including micro/nanotechnology and 3D printing, numerous studies have uncovered various intrinsic mechanisms regulating skeletal muscle regeneration and developed tailored biomaterial systems based on these understandings. Here, the skeletal muscle structure and regeneration process are discussed and the diverse biomaterial systems derived from various technologies are explored in detail. Biomaterials serve not merely as local niches for cell growth, but also as scaffolds endowed with structural or physicochemical properties that provide tissue regenerative cues such as topographical, electrical, and mechanical signals. They can also act as delivery systems for stem cells and bioactive molecules that have been shown as key participants in endogenous repair cascades. To achieve bench-to-bedside translation, the typical effect enabled by biomaterial systems and the potential underlying molecular mechanisms are also summarized. Insights into the roles of biomaterials in SMTE from cellular and molecular perspectives are provided. Finally, perspectives on the advancement of SMTE are provided, for which gene therapy, exosomes, and hybrid biomaterials may hold promise to make important contributions.

Therapeutic applications of biological macromolecules and scaffolds for skeletal muscle regeneration: A review

Skeletal muscle (SM) mass and strength maintenance are important requirements for human well-being. SM regeneration to repair minor injuries depends upon the myogenic activities of muscle satellite (stem) cells. However, losses of regenerative properties following volumetric muscle loss or severe trauma or due to congenital muscular abnormalities are not self-restorable, and thus, these conditions have major healthcare implications and pose clinical challenges. In this context, tissue engineering based on different types of biomaterials and scaffolds provides an encouraging means of structural and functional SM reconstruction. In particular, biomimetic (able to transmit biological signals) and several porous scaffolds are rapidly evolving. Several biological macromolecules/biomaterials (collagen, gelatin, alginate, chitosan, and fibrin etc.) are being widely used for SM regeneration. However, available alternatives for SM regeneration must be redesigned to make them more user-friendly and economically feasible with longer shelf lives. This review aimed to explore the biological aspects of SM regeneration and the roles played by several biological macromolecules and scaffolds in SM regeneration in cases of volumetric muscle loss.

A biomimetic approach to modulating the sustained release of fibroblast growth factor 2 from fibrin microthread scaffolds

Aim

The pleiotropic effect of fibroblast growth factor 2 (FGF2) on promoting myogenesis, angiogenesis, and innervation makes it an ideal growth factor for treating volumetric muscle loss (VML) injuries. While an initial delivery of FGF2 has demonstrated enhanced regenerative potential, the sustained delivery of FGF2 from scaffolds with robust structural properties as well as biophysical and biochemical signaling cues has yet to be explored for treating VML. The goal of this study is to develop an instructive fibrin microthread scaffold with intrinsic topographic alignment cues as well as regenerative signaling cues and a physiologically relevant, sustained release of FGF2 to direct myogenesis and ultimately enhance functional muscle regeneration.

Methods

Heparin was passively adsorbed or carbodiimide-conjugated to microthreads, creating a biomimetic binding strategy, mimicking FGF2 sequestration in the extracellular matrix (ECM). It was also evaluated whether FGF2 incorporated into fibrin microthreads would yield sustained release. It was hypothesized that heparin-conjugated and co-incorporated (co-inc) fibrin microthreads would facilitate sustained release of FGF2 from the scaffold and enhance in vitro myoblast proliferation and outgrowth.

Results

Toluidine blue staining and Fourier transform infrared spectroscopy confirmed that carbodiimide-conjugated heparin bound to fibrin microthreads in a dose-dependent manner. Release kinetics revealed that heparin-conjugated fibrin microthreads exhibited sustained release of FGF2 over a period of one week. An in vitro assay demonstrated that FGF2 released from microthreads remained bioactive, stimulating myoblast proliferation over four days. Finally, a cellular outgrowth assay suggests that FGF2 promotes increased outgrowth onto microthreads.

Conclusions

It was anticipated that the combined effects of fibrin microthread structural properties, topographic alignment cues, and FGF2 release profiles will facilitate the fabrication of a biomimetic scaffold that enhances the regeneration of functional muscle tissue for the treatment of VML injuries.

Porous biomaterial scaffolds for skeletal muscle tissue engineering

Volumetric muscle loss is a traumatic injury which overwhelms the innate repair mechanisms of skeletal muscle and results in significant loss of muscle functionality. Tissue engineering seeks to regenerate these injuries through implantation of biomaterial scaffolds to encourage endogenous tissue formation and to restore mechanical function. Many types of scaffolds are currently being researched for this purpose. Scaffolds are typically made from either natural, synthetic, or conductive polymers, or any combination therein. A major criterion for the use of scaffolds for skeletal muscle is their porosity, which is essential for myoblast infiltration and myofiber ingrowth. In this review, we summarize the various methods of fabricating porous biomaterial scaffolds for skeletal muscle regeneration, as well as the various types of materials used to make these scaffolds. We provide guidelines for the fabrication of scaffolds based on functional requirements of skeletal muscle tissue, and discuss the general state of the field for skeletal muscle tissue engineering.

Tissue Engineering Challenges for Cultivated Meat to Meet the Real Demand of a Global Market

Cultivated meat (CM) technology has the potential to disrupt the food industry-indeed, it is already an inevitable reality. This new technology is an alternative to solve the environmental, health and ethical issues associated with the demand for meat products. The global market longs for biotechnological improvements for the CM production chain. CM, also known as cultured, cell-based, lab-grown, in vitro or clean meat, is obtained through cellular agriculture, which is based on applying tissue engineering principles. In practice, it is first necessary to choose the best cell source and type, and then to furnish the necessary nutrients, growth factors and signalling molecules via cultivation media. This procedure occurs in a controlled environment that provides the surfaces necessary for anchor-dependent cells and offers microcarriers and scaffolds that favour the three-dimensional (3D) organisation of multiple cell types. In this review, we discuss relevant information to CM production, including the cultivation process, cell sources, medium requirements, the main obstacles to CM production (consumer acceptance, scalability, safety and reproducibility), the technological aspects of 3D models (biomaterials, microcarriers and scaffolds) and assembly methods (cell layering, spinning and 3D bioprinting). We also provide an outlook on the global CM market. Our review brings a broad overview of the CM field, providing an update for everyone interested in the topic, which is especially important because CM is a multidisciplinary technology.

W. Lo E, Lim H, Dias S, Wagner KR, Mao HQ, Cahan P, Lee G, Grayson WL. Engineering Skeletal Muscle Grafts with PAX7::GFP-Sorted Human Pluripotent Stem Cell-Derived Myogenic Progenitors on Fibrin Microfiber Bundles for Tissue Regeneration

Tissue engineering strategies that combine human pluripotent stem cell-derived myogenic progenitors (hPDMs) with advanced biomaterials provide promising tools for engineering 3D skeletal muscle grafts to model tissue development in vitro and promote muscle regeneration in vivo. We recently demonstrated (i) the potential for obtaining large numbers of hPDMs using a combination of two small molecules without the overexpression of transgenes and (ii) the application of electrospun fibrin microfiber bundles for functional skeletal muscle restoration following volumetric muscle loss. In this study, we aimed to demonstrate that the biophysical cues provided by the fibrin microfiber bundles induce hPDMs to form engineered human skeletal muscle grafts containing multinucleated myotubes that express desmin and myosin heavy chains and that these grafts could promote regeneration following skeletal muscle injuries. We tested a genetic PAX7 reporter line (PAX7::GFP) to sort for more homogenous populations of hPDMs. RNA sequencing and gene set enrichment analyses confirmed that PAX7::GFP-sorted hPDMs exhibited high expression of myogenic genes. We tested engineered human skeletal muscle grafts derived from PAX7::GFP-sorted hPDMs within in vivo skeletal muscle defects by assessing myogenesis, engraftment and immunogenicity using immunohistochemical staining. The PAX7::GFP-sorted groups had moderately high vascular infiltration and more implanted cell association with embryonic myosin heavy chain (eMHC) regions, suggesting they induced pro-regenerative microenvironments. These findings demonstrated the promise for the use of PAX7::GFP-sorted hPDMs on fibrin microfiber bundles and provided some insights for improving the cell-biomaterial system to stimulate more robust in vivo skeletal muscle regeneration.

New developments in the biology of fibroblast growth factors

The fibroblast growth factor (FGF) family is composed of 18 secreted signaling proteins consisting of canonical FGFs and endocrine FGFs that activate four receptor tyrosine kinases (FGFRs 1-4) and four intracellular proteins (intracellular FGFs or iFGFs) that primarily function to regulate the activity of voltage-gated sodium channels and other molecules. The canonical FGFs, endocrine FGFs, and iFGFs have been reviewed extensively by us and others. In this review, we briefly summarize past reviews and then focus on new developments in the FGF field since our last review in 2015. Some of the highlights in the past 6 years include the use of optogenetic tools, viral vectors, and inducible transgenes to experimentally modulate FGF signaling, the clinical use of small molecule FGFR inhibitors, an expanded understanding of endocrine FGF signaling, functions for FGF signaling in stem cell pluripotency and differentiation, roles for FGF signaling in tissue homeostasis and regeneration, a continuing elaboration of mechanisms of FGF signaling in development, and an expanding appreciation of roles for FGF signaling in neuropsychiatric diseases. This article is categorized under: Cardiovascular Diseases > Molecular and Cellular Physiology Neurological Diseases > Molecular and Cellular Physiology Congenital Diseases > Stem Cells and Development Cancer > Stem Cells and Development.

Hydrogel-Based Fiber Biofabrication Techniques for Skeletal Muscle Tissue Engineering

The functional capabilities of skeletal muscle are strongly correlated with its well-arranged microstructure, consisting of parallelly aligned myotubes. In case of extensive muscle loss, the endogenous regenerative capacity is hindered by scar tissue formation, which compromises the native muscle structure, ultimately leading to severe functional impairment. To address such an issue, skeletal muscle tissue engineering (SMTE) attempts to fabricate in vitro bioartificial muscle tissue constructs to assist and accelerate the regeneration process. Due to its dynamic nature, SMTE strategies must employ suitable biomaterials (combined with muscle progenitors) and proper 3D architectures. In light of this, 3D fiber-based strategies are gaining increasing interest for the generation of hydrogel microfibers as advanced skeletal muscle constructs. Indeed, hydrogels possess exceptional biomimetic properties, while the fiber-shaped morphology allows for the creation of geometrical cues to guarantee proper myoblast alignment. In this review, we summarize commonly used hydrogels in SMTE and their main properties, and we discuss the first efforts to engineer hydrogels to guide myoblast anisotropic orientation. Then, we focus on presenting the main hydrogel fiber-based techniques for SMTE, including molding, electrospinning, 3D bioprinting, extrusion, and microfluidic spinning. Furthermore, we describe the effect of external stimulation (i.e., mechanical and electrical) on such constructs and the application of hydrogel fiber-based methods on recapitulating complex skeletal muscle tissue interfaces. Finally, we discuss the future developments in the application of hydrogel microfibers for SMTE.

Laminin-111-Enriched Fibrin Hydrogels Enhance Functional Muscle Regeneration Following Trauma

Volumetric muscle loss (VML) is the surgical or traumatic loss of skeletal muscle, which can cause loss of limb function or permanent disability. VML injuries overwhelms the endogenous regenerative capacity of skeletal muscle and results in poor functional healing outcomes. Currently, there are no approved tissue engineering treatments for VML injuries. In this study, fibrin hydrogels enriched with laminin-111 (LM-111; 50-450 μg/mL) were used for the treatment of VML of the tibialis anterior in a rat model. Treatment with fibrin hydrogel containing 450 μg/mL of LM-111 (FBN450) improved muscle regeneration following VML injury. FBN450 hydrogel treatment increased the relative proportion of contractile to fibrotic tissue as indicated by the myosin: collagen ratio on day 28 post-VML injury. FBN450 hydrogels also enhanced myogenic protein expression and increased the quantity of small to medium size myofibers (500-2000 μm²) as well as innervated myofibers. Improved contractile tissue deposition due to FBN450 hydrogel treatment resulted in a significant improvement (∼60%) in torque production at day 28 postinjury. Taken together, these results suggest that the acellular FBN450 hydrogels provide a promising therapeutic strategy for VML that is worthy of further investigation.

Controllable assembly of skeletal muscle-like bundles through 3D bioprinting

3D printing is an effective technology for recreating skeletal muscle tissuein vitro. To achieve clinical skeletal muscle injury repair, relatively large volumes of highly aligned skeletal muscle cells are required; obtaining these is still a challenge. It is currently unclear how individual skeletal muscle cells and their neighbouring components co-ordinate to establish anisotropic architectures in highly homogeneous orientations. Here, we demonstrated a 3D printing strategy followed by sequential culture processes to engineer skeletal muscle tissue. The effects of confined printing on the skeletal muscle during maturation, which impacted the myotube alignment, myogenic gene expression, and mechanical forces, were observed. Our findings demonstrate the dynamic changes of skeletal muscle tissue duringin vitro3D construction and reveal the role of physical factors in the orientation and maturity of muscle fibres.

Scaffold biomaterials and nano-based therapeutic strategies for skeletal muscle regeneration

Skeletal muscle is integral to the functioning of the human body. Several pathological conditions, such as trauma (primary lesion) or genetic diseases such as Duchenne muscular dystrophy (DMD), can affect and impair its functions or exceed its regeneration capacity. Tissue engineering (TE) based on natural, synthetic and hybrid biomaterials provides a robust platform for developing scaffolds that promote skeletal muscle regeneration, strength recovery, vascularization and innervation. Recent 3D-cell printing technology and the use of nanocarriers for the release of drugs, peptides and antisense oligonucleotides support unique therapeutic alternatives. Here, the authors present recent advances in scaffold biomaterials and nano-based therapeutic strategies for skeletal muscle regeneration and perspectives for future endeavors.

Fast photocurable thiol-ene elastomers with tunable biodegradability, mechanical and surface properties enhance myoblast differentiation and contractile function

Biodegradable elastomers are important emerging biomaterials for biomedical applications, particularly in the area of soft-tissue engineering in which scaffolds need to match the physicochemical properties of native tissues. Here, we report novel fast photocurable elastomers with readily tunable mechanical properties, surface wettability, and degradability. These elastomers are prepared by a 5-min UV-irradiation of thiol-ene reaction systems of glycerol tripentenoate (GTP; a triene) or the combination of GTP and 4-pentenyl 4-pentenoate (PP; a diene) with a carefully chosen series of di- or tri-thiols. In the subsequent application study, these elastomers were found to be capable of overcoming delamination of myotubes, a technical bottleneck limiting the in vitro growth of mature functional myofibers. The glycerol-based elastomers supported the proliferation of mouse and human myoblasts, as well as myogenic differentiation into contractile myotubes. More notably, while beating mouse myotubes detached from conventional tissue culture plates, they remain adherent on the elastomer surface. The results suggest that these elastomers as novel biomaterials may provide a promising platform for engineering functional soft tissues with potential applications in regenerative medicine or pharmacological testing.

Engineering 3D skeletal muscle primed for neuromuscular regeneration following volumetric muscle loss

Volumetric muscle loss (VML) overwhelms the native regenerative capabilities of skeletal muscle and has few effective treatments to regain lost muscle mass and function. Tissue engineered muscle constructs designed to promote neuromuscular regeneration are a promising therapeutic avenue. To date, there has been no engineered muscle construct for VML treatment that has incorporated a pharmacologic agent to promote neuromuscular regeneration. Here, we have modified electrospun fibrin microfiber bundles, which have demonstrated muscle regenerative potential, with the heparan sulfate proteoglycan, agrin, to stimulate innervation post-VML. Myoblasts cultured on microfiber bundles with either soluble or chemically tethered agrin demonstrated statistically significant increased clustering of acetylcholine receptors (AChRs) with soluble agrin displaying AChR clusters throughout the myofiber bundles, and tethered agrin displaying AChR clusters only at 10 μm from the substrate surface. Following implantation into murine VML defects for 4 weeks, constructs pre-treated with soluble or tethered agrin resulted in statistically significant increased neuromuscular junctions, regenerating myofibers, vascular infiltration, neural infiltration, and nuclear yes-associated protein (YAP) expression within the defect site compared to the control without agrin. The agrin-tethered microfiber bundles provided sustained agrin signaling within the regenerating site during the 4-week post-implantation periods and further augmented the density of regenerating myofibers in regenerated tissue with statistical significance compared to constructs with soluble agrin. These data demonstrate the neuromuscular regenerative potential of engineered muscle constructs pre-treated to induce AChR clustering with locally delivered agrin at the site of VML regeneration.

Skeletal Muscle Tissue Engineering: Biomaterials-Based Strategies for the Treatment of Volumetric Muscle Loss

Millions of Americans suffer from skeletal muscle injuries annually that can result in volumetric muscle loss (VML), where extensive musculoskeletal damage and tissue loss result in permanent functional deficits. In the case of small-scale injury skeletal muscle is capable of endogenous regeneration through activation of resident satellite cells (SCs). However, this is greatly reduced in VML injuries, which remove native biophysical and biochemical signaling cues and hinder the damaged tissue's ability to direct regeneration. The current clinical treatment for VML is autologous tissue transfer, but graft failure and scar tissue formation leave patients with limited functional recovery. Tissue engineering of instructive biomaterial scaffolds offers a promising approach for treating VML injuries. Herein, we review the strategic engineering of biophysical and biochemical cues in current scaffold designs that aid in restoring function to these preclinical VML injuries. We also discuss the successes and limitations of the three main biomaterial-based strategies to treat VML injuries: acellular scaffolds, cell-delivery scaffolds, and in vitro tissue engineered constructs. Finally, we examine several innovative approaches to enhancing the design of the next generation of engineered scaffolds to improve the functional regeneration of skeletal muscle following VML injuries.

Recent Trends in Injury Models to Study Skeletal Muscle Regeneration and Repair

Skeletal muscle injuries that occur from traumatic incidents, such as those caused by car accidents or surgical resections, or from injuries sustained on the battlefield, result in the loss of functionality of the injured muscle. To understand skeletal muscle regeneration and to better treat these large scale injuries, termed volumetric muscle loss (VML), in vivo injury models exploring the innate mechanisms of muscle injury and repair are essential for the creation of clinically applicable treatments. While the end result of a muscle injury is often the destruction of muscle tissue, the manner in which these injuries are induced as well as the response from the innate repair mechanisms found in muscle in each animal models can vary. This targeted review describes injury models that assess both skeletal muscle regeneration (i.e., the response of muscle to myotoxin or ischemic injury) and skeletal muscle repair (i.e., VML injury). We aimed to summarize the injury models used in the field of skeletal muscle tissue engineering, paying particular attention to strategies to induce muscle damage and how to standardize injury conditions for future experiments.

Etching anisotropic surface topography onto fibrin microthread scaffolds for guiding myoblast alignment

To regenerate functional muscle tissue, engineered scaffolds should impart topographical features to induce myoblast alignment by a phenomenon known as contact guidance. Myoblast alignment is an essential step towards myotube formation, which is guided in vivo by extracellular matrix structure and micron-scale grooves between adjacent muscle fibers. Fibrin microthread scaffolds mimic the morphological architecture of native muscle tissue and have demonstrated promise as an implantable scaffold for treating skeletal muscle injuries. While these scaffolds promote modest myoblast alignment, it is not sufficient to generate highly functional muscle tissue. The goal of this study is to develop and characterize a new method of etching the surface of fibrin microthreads to incorporate aligned, sub-micron grooves that promote myoblast alignment. To generate these topographic features, we placed fibrin microthreads into 2-(N-morpholino)ethane-sulfonic acid (MES) acidic buffer and evaluated the effect of buffer pH on the generation of these features. Surface characterization with atomic force microscopy and scanning electron microscopy indicated the generation of aligned, sub-micron sized grooves on microthreads in MES buffer with pH 5.0. Microthreads etched with surface features had tensile mechanical properties comparable to controls, indicating that the surface treatment does not inhibit scaffold bulk properties. Our data demonstrate that etching threads in MES buffer with pH 5.0 enhanced alignment and filamentous actin stress fiber organization of myoblasts on the surface of scaffolds. The ability to tune topographic features on the surfaces of scaffolds independent of mechanical properties provides a valuable tool for designing microthread-based scaffolds to enhance regeneration of functional muscle tissue.

Horseradish Peroxidase-Catalyzed Crosslinking of Fibrin Microthread Scaffolds

Horseradish peroxidase (HRP) has been investigated as a catalyst to crosslink tissue-engineered hydrogels because of its mild reaction conditions and ability to modulate the mechanical properties of the matrix. Here, we report the results of the first study investigating the use of HRP to crosslink fibrin scaffolds. We examined the effect of varying HRP and hydrogen peroxide (H2O2) incorporation strategies on the resulting crosslink density and structural properties of fibrin in a microthread scaffold format. Primary (1°) and secondary (2°) scaffold modification techniques were evaluated to crosslink fibrin microthread scaffolds. A primary scaffold modification technique was defined as incorporating crosslinking agents into the microthread precursor solutions during extrusion. A secondary scaffold modification technique was defined as incubating the microthreads in a postprocessing crosslinker bath. Fibrin microthreads were enzymatically crosslinked through primary, secondary, or a combination of both approaches. All fibrin microthread scaffolds crosslinked with HRP and H2O2 via primary and/or secondary methods exhibited an increase in dityrosine crosslink density compared with uncrosslinked control microthreads, demonstrated by scaffold fluorescence. Fourier transform infrared spectroscopy indicated the formation of isodityrosine bonds in 1° HRP crosslinked microthreads. Characterization of tensile mechanical properties revealed that all HRP crosslinked microthreads were significantly stronger than control microthreads. Primary (1°) HRP crosslinked microthreads also demonstrated significantly slower degradation than control microthreads, suggesting that incorporating HRP and H2O2 during extrusion yields scaffolds with increased resistance to proteolytic degradation. Finally, cells seeded on HRP crosslinked microthreads retained a high degree of viability, demonstrating that HRP crosslinking yields biocompatible scaffolds that are suitable for tissue engineering. The goal of this work was to facilitate the logical design of enzymatically crosslinked fibrin microthreads with tunable structural properties, enabling their application for engineered tissue constructs with varied mechanical and structural properties.

mRNA-activated matrices encoding transcription factors as primers of cell differentiation in tissue engineering

Gene-activated matrices (GAMs) encoding pivotal transcription factors (TFs) represent a powerful tool to direct stem cell specification for tissue engineering applications. However, current TF-based GAMs activated with pDNA, are challenged by their low transfection efficiency and delayed transgene expression. Here, we report a GAM technology activated with mRNAs encoding TFs SOX9 (cartilage) and MYOD (muscle). We find that these mRNA-GAMs induce a higher and faster TF expression compared to pDNA-GAMs, especially in the case of RNase resistant mRNA sequences. This potent TF expression was translated into a high synthesis of cartilage- and muscle-specific markers, and ultimately, into successful tissue specification in vitro. Additionally, we show that the expression of tissue-specific markers can be further modulated by altering the properties of the mRNA-GAM environment. These results highlight the value of this GAM technology for priming cell lineage specification, a key centerpiece for future tissue engineering devices.

Microcarriers for Upscaling Cultured Meat Production

Due to the considerable environmental impact and the controversial animal welfare associated with industrial meat production, combined with the ever-increasing global population and demand for meat products, sustainable production alternatives are indispensable. In 2013, the world's first laboratory grown hamburger made from cultured muscle cells was developed. However, coming at a price of $300.000, and being produced manually, substantial effort is still required to reach sustainable large-scale production. One of the main challenges is scalability. Microcarriers (MCs), offering a large surface/volume ratio, are the most promising candidates for upscaling muscle cell culture. However, although many MCs have been developed for cell lines and stem cells typically used in the medical field, none have been specifically developed for muscle stem cells and meat production. This paper aims to discuss the MCs' design criteria for skeletal muscle cell proliferation and subsequently for meat production based on three scenarios: (1) MCs are serving only as a temporary substrate for cell attachment and proliferation and therefore they need to be separated from the cells at some stage of the bioprocess, (2) MCs serve as a temporary substrate for cell proliferation but are degraded or dissolved during the bioprocess, and (3) MCs are embedded in the final product and therefore need to be edible. The particularities of each of these three bioprocesses will be discussed from the perspective of MCs as well as the feasibility of a one-step bioprocess. Each scenario presents advantages and drawbacks, which are discussed in detail, nevertheless the third scenario appears to be the most promising one for a production process. Indeed, using an edible material can limit or completely eliminate dissociation/degradation/separation steps and even promote organoleptic qualities when embedded in the final product. Edible microcarriers could also be used as a temporary substrate similarly to scenarios 1 and 2, which would limit the risk of non-edible residues.

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.

Therapeutic Approaches for Volumetric Muscle Loss Injury: A Systematic Review and Meta-Analysis

Our goal was to understand the impact of regenerative therapies on the functional capacity of skeletal muscle following volumetric muscle loss (VML) injury. An extensive database search (e.g., PubMed, Cochrane Library, and ClinicalTrials.gov) was conducted up through January 2019 to evaluate the following: "In humans or animals with VML injury, is treatment better than no treatment at recovering functional capacity?" Study eligibility criteria required studies to have both an untreated and at least one treated VML injury group. From 2312 study reports, 44 studies met the inclusion criteria. Quantitative functional capacity data (absolute and/or normalized strength) or proportional measures (histological analysis quantifying viable muscle tissue, mitochondrial function, and/or exhaustive treadmill running) were extracted for use. While both human and animal studies were included in the searches, only animal studies met the eligibility criteria. Using a random-effects model, Hedges' g was used as the effect size (ES) and calculated such that a positive ES indicated treatment efficacy. The overall ES was 0.75 (95% confidence interval: 0.53-0.96; p < 0.0000001), indicating that the treatments, on average, resulted in a significant improvement in functional capacity. From network meta-analyses, it was determined that an acellular biomaterial combined with stem and/or progenitor cells had the greatest treatment effectiveness. The findings indicate that various treatments in animal models of VML improve the functional capacity of muscle compared to leaving the injury untreated; however, the ∼16% beneficial effect is small. Our results suggest that current regenerative therapy paradigms require further maturation to achieve clinically meaningful improvements in the functional capacity of the muscle. Impact Statement Our most salient findings are that (1) various treatment approaches used in animal models of volumetric muscle loss (VML) injury improve functional capacity compared to leaving the injury untreated and (2) an acellular biomaterial in combination with cellular components was the most effective treatment to improve functional capacity following VML injury to date. The nature of our findings has substantial implications for regenerative medicine, biomedical engineering, and rehabilitative techniques currently being evaluated and developed for VML injury repair, and are pivotal to the progression of the regenerative medicine effort aimed at restoring maximal function to traumatized and disabled limbs.

Immune responses towards bioengineered tissues and strategies to control them

PURPOSE OF REVIEW

Research into development of artificial tissues and bioengineered organs to replace physiological functions of injured counterparts has highlighted a previously underestimated challenge for its clinical translatability: the immune response against biomaterials. Herein, we will provide an update and review current knowledge regarding this important barrier to regenerative medicine.

RECENT FINDINGS

Although a clear understanding of the immune reactivity against biomaterials remains elusive, accumulating evidence indicates that innate immune cells, primarily neutrophils and macrophages, play a key role in the initial phases of the immune response. More recently, data have shown that in later phases, T and B cells are also involved. The use of physicochemical modifications of biomaterials and cell-based strategies to modulate the host inflammatory response is being actively investigated for effective biomaterial integration.

SUMMARY

The immune response towards biomaterials and bioengineered organs plays a crucial role in determining their utility as transplantable grafts. Expanding our understanding of these responses is necessary for developing protolerogenic strategies and delivering on the ultimate promise of regenerative medicine.

The Effect of Laminin-111 Hydrogels on Muscle Regeneration in a Murine Model of Injury

IMPACT STATEMENT

Extremity injuries make up the most common survivable injuries in vehicular accidents and modern military conflicts. A majority of these injuries involve volumetric muscle loss (VML). The potential for donor site morbidity may limit the clinical use of autologous muscle grafts for VML injuries. Treatments that can improve the regeneration of functional muscle tissue are critically needed to improve limb salvage and reduce the rate of delayed amputations. The development of a laminin-111-enriched fibrin hydrogel will offer a potentially transformative and "off-the-shelf" clinically relevant therapy for functional skeletal muscle regeneration.

Muscle fibrosis in the soft palate: Delivery of cells, growth factors and anti-fibrotics

The healing of skeletal muscle injuries after major trauma or surgical reconstruction is often complicated by the development of fibrosis leading to impaired function. Research in the field of muscle regeneration is mainly focused on the restoration of muscle mass while far less attention is paid to the prevention of fibrosis. In this review, we take as an example the reconstruction of the muscles in the soft palate of cleft palate patients. After surgical closure of the soft palate, muscle function during speech is often impaired by a shortage of muscle tissue as well as the development of fibrosis. We will give a short overview of the most common approaches to generate muscle mass and then focus on strategies to prevent fibrosis. These include anti-fibrotic strategies that have been developed for muscle and other organs by the delivery of small molecules, decorin and miRNAs. Anti-fibrotic compounds should be delivered in aligned constructs in order to obtain the organized architecture of muscle tissue. The available techniques for the preparation of aligned muscle constructs will be discussed. The combination of approaches to generate muscle mass with anti-fibrotic components in an aligned muscle construct may greatly improve the functional outcome of regenerative therapies for muscle injuries.

Biomaterial and stem cell-based strategies for skeletal muscle regeneration

Adult skeletal muscle can regenerate effectively after mild physical or chemical insult. Muscle trauma or disease can overwhelm this innate capacity for regeneration and result in heightened inflammation and fibrotic tissue deposition resulting in loss of structure and function. Recent studies have focused on biomaterial and stem cell-based therapies to promote skeletal muscle regeneration following injury and disease. Many stem cell populations besides satellite cells are implicated in muscle regeneration. These stem cells include but are not limited to mesenchymal stem cells, adipose-derived stem cells, hematopoietic stem cells, pericytes, fibroadipogenic progenitors, side population cells, and CD133⁺ stem cells. However, several challenges associated with their isolation, availability, delivery, survival, engraftment, and differentiation have been reported in recent studies. While acellular scaffolds offer a relatively safe and potentially off-the-shelf solution to cell-based therapies, they are often unable to stimulate host cell migration and activity to a level that would result in clinically meaningful regeneration of traumatized muscle. Combining stem cells and biomaterials may offer a viable therapeutic strategy that may overcome the limitations associated with these therapies when they are used in isolation. In this article, we review the stem cell populations that can stimulate muscle regeneration in vitro and in vivo. We also discuss the regenerative potential of combination therapies that utilize both stem cell and biomaterials for the treatment of skeletal muscle injury and disease. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1246-1262, 2019.

Engineering Biomimetic Materials for Skeletal Muscle Repair and Regeneration

Although skeletal muscle is highly regenerative following injury or disease, endogenous self-regeneration is severely impaired in conditions of volume traumatic muscle loss. Consequently, tissue engineering approaches are a promising means to regenerate skeletal muscle. Biological scaffolds serve as not only structural support for the promotion of cellular ingrowth but also impart potent modulatory signaling cues that may be beneficial for tissue regeneration. In this work, the progress of tissue engineering approaches for skeletal muscle engineering and regeneration is overviewed, with a focus on the techniques to create biomimetic engineered tissue using extracellular cues. These factors include mechanical and electrical stimulation, geometric patterning, and delivery of growth factors or other bioactive molecules. The progress of evaluating the therapeutic efficacy of these approaches in preclinical models of muscle injury is further discussed.

Encapsulation of bone marrow-MSCs in PRP-derived fibrin microbeads and preliminary evaluation in a volumetric muscle loss injury rat model: modular muscle tissue engineering

Repair of volumetric muscle loss (VML) injuries is a complicated endeavour which necessitates the collaborative use of different regenerative approaches and technologies. Herein is proposed the development of fibrin-based microbeads (FMs) alone or as a bone marrow mesenchymal stem cell (MSC) encapsulation matrix for modular muscle engineering. FMs were generated through the ionotropic gelation of alginate and fibrinogen obtained from the platelet-rich plasma of whole blood, and then removing the alginate by citrate treatment. FMs were first characterized by FT-IR, SEM and water uptake tests. Then, the stability of FMs and the mitochondrial dehydrogenase activity of the MSCs encapsulated in FMs were evaluated under in vitro culture conditions. Eventually, the regenerative capacity of the cell-devoid and MSCs-encapsulated FMs was evaluated in a rat VML injury model involving 8 × 4×4 mm³-size bilateral defects in the biceps femoris muscles. The histochemical, immunohistochemical and semi-quantitative histomorphological scoring results retrieved at 30, 60 and 180 days demonstrated that the cell-devoid FMs supported muscle regeneration to a great extent. Moreover, MSCs-encapsulated FMs were more effective in shortening the regeneration period of the injured tissue of the rat VML, resulting in good myofibre orientation, while the Sham group resulted in incomplete repair with fibrotic scar tissue formations.

Investigation of the batch-to-batch inconsistencies of Collagen in PCL-Collagen nanofibers

The application of tissue engineered constructs is an approach in regenerative therapies to support regeneration of damaged muscular tissue. Therefore the development of highly aligned electrospun scaffolds based on polycaprolactone (PCL) and collagen enables a versatile development towards tailor made applications. However, the application of natural polymers like collagen brings the risk of batch-to-batch inconsistencies, which influence the reproducibility of the electrospinning process. Aligned PCL-Collagen nanofibers were fabricated via electrospinning using benign solvents. The spinnability of different collagen batches and polymer concentrations in diluted acetic acid as solvent was investigated. Furthermore spinning parameters and fiber morphology were investigated in order to determine the most stable spinning conditions and analyze the batch-to-batch variations. Finally the effect of the solution temperature and the time of pure collagen in solution were investigated, to complete the analysis of the influences on the spinning behavior.

Adipose-derived Stem/Stromal Cells on Electrospun Fibrin Microfiber Bundles Enable Moderate Muscle Reconstruction in a Volumetric Muscle Loss Model

Current treatment options for volumetric muscle loss (VML) are limited due to donor site morbidity, lack of donor tissue, and insufficient functional recovery. Tissue-engineered skeletal muscle grafts offer the potential to significantly improve functional outcomes. In this study, we assessed the potential pro-myogenic effects of human adipose-derived stem cells (ASCs) seeded onto electrospun uniaxially aligned fibrin hydrogel microfiber bundles. Although both uninduced and 5-azacytidine-induced ASCs exhibited alignment, elongation, and diffuse muscle marker expression when grown on microfiber bundles for 2 months in vitro, both groups failed to fully recapitulate myotube characteristics. To assess the muscle regeneration potential of ASCs in vivo, ASC-seeded fibrin microfiber bundles were implanted in a robust murine VML defect model. Minimal fibrosis was observed surrounding implanted acellular hydrogel fibers at 2 and 4 weeks, and fibers seeded with ASCs exhibited up to 4 times higher volume retention than acellular fibers. We observed increased numbers of cells positive for the regenerating muscle marker embryonic myosin and the mature muscle marker myosin heavy chain in ASC-seeded fibers compared with acellular fibers at 1 and 3 months post-transplantation. Regenerating muscle cells were closely associated with ASC-derived cells and in some cases had potentially fused with them. These findings demonstrate that despite failing to undergo myogenesis in vitro, ASCs combined with electrospun fibrin microfibers moderately increased muscle reconstruction in vivo compared with acellular fibers following a severe VML defect.

Rehabilitative exercise and spatially patterned nanofibrillar scaffolds enhance vascularization and innervation following volumetric muscle loss

Muscle regeneration can be permanently impaired by traumatic injuries, despite the high regenerative capacity of skeletal muscle. Implantation of engineered biomimetic scaffolds to the site of muscle ablation may serve as an attractive off-the-shelf therapeutic approach. The objective of the study was to histologically assess the therapeutic benefit of a three-dimensional spatially patterned collagen scaffold, in conjunction with rehabilitative exercise, for treatment of volumetric muscle loss. To mimic the physiologic organization of skeletal muscle, which is generally composed of myofibers aligned in parallel, three-dimensional parallel-aligned nanofibrillar collagen scaffolds were fabricated. When implanted into the ablated murine tibialis anterior muscle, the aligned nanofibrillar scaffolds, in conjunction with voluntary caged wheel exercise, significantly improved the density of perfused microvessels, in comparison to treatments of the randomly oriented nanofibrillar scaffold, decellularized scaffold, or in the untreated control group. The abundance of neuromuscular junctions was 19-fold higher when treated with aligned nanofibrillar scaffolds in conjunction with exercise, in comparison to treatment of aligned scaffold without exercise. Although, the density of de novo myofibers was not significantly improved by aligned scaffolds, regardless of exercise activity, the cross-sectional area of regenerating myofibers was increased by > 60% when treated with either aligned and randomly oriented scaffolds, in comparison to treatment of decellularized scaffold or untreated controls. These findings demonstrate that voluntary exercise improved the regenerative effect of aligned scaffolds by augmenting neurovascularization, and have important implications in the design of engineered biomimetic scaffolds for treatment of traumatic muscle injury.

A collagen scaffold designed to mimic skeletal muscle, together with rehabilitative exercise, can help regenerate nerves and blood vessels following traumatic muscle injury. Ngan Huang from Stanford University, California, USA, and colleagues created scaffolding composed of collagen nanofibers aligned in parallel, as natural muscle fibers are. They implanted these specially patterned collagen constructs into the shins of mice that had no tibialis anterior muscles. Mice given the opportunity to exercise formed far more nerve connections in their injured muscles compared to mice without exercise wheels in their cages. Active mice also developed significantly more blood vessels in their injured muscles with the parallel-aligned scaffolds compared to other animals with randomly oriented scaffolds, decellularized scaffolds or no implant at all. The findings highlight the potential of combining exercise and biomimetic scaffolds to treat muscle trauma.

Utility of extracellular matrix powders in tissue engineering

Extracellular matrix (ECM) materials have had remarkable success as scaffolds in tissue engineering (TE) and as therapies for tissue injury whereby the ECM microenvironment promotes constructive remodeling and tissue regeneration. ECM powder and solubilized derivatives thereof have novel applications in TE and RM afforded by the capacity of these constructs to be dynamically modulated. The powder form allows for effective incorporation and penetration of reagents; hence, ECM powder is an efficacious platform for 3D cell culture and vehicle for small molecule delivery. ECM powder offers minimally invasive therapy for tissue injury and successfully treatment for wounds refractory to first-line therapies. Comminution of ECM and fabrication of powder-derived constructs, however, may compromise the biological integrity of the ECM. The current lack of optimized fabrication protocols prevents a more extensive and effective clinical application of ECM powders. Further study on methods of ECM powder fabrication and modification is needed.

The Importance of Biophysical and Biochemical Stimuli in Dynamic Skeletal Muscle Models

Classical approaches to engineer skeletal muscle tissue based on current regenerative and surgical procedures still do not meet the desired outcome for patient applications. Besides the evident need to create functional skeletal muscle tissue for the repair of volumetric muscle defects, there is also growing demand for platforms to study muscle-related diseases, such as muscular dystrophies or sarcopenia. Currently, numerous studies exist that have employed a variety of biomaterials, cell types and strategies for maturation of skeletal muscle tissue in 2D and 3D environments. However, researchers are just at the beginning of understanding the impact of different culture settings and their biochemical (growth factors and chemical changes) and biophysical cues (mechanical properties) on myogenesis. With this review we intend to emphasize the need for new in vitro skeletal muscle (disease) models to better recapitulate important structural and functional aspects of muscle development. We highlight the importance of choosing appropriate system components, e.g., cell and biomaterial type, structural and mechanical matrix properties or culture format, and how understanding their interplay will enable researchers to create optimized platforms to investigate myogenesis in healthy and diseased tissue. Thus, we aim to deliver guidelines for experimental designs to allow estimation of the potential influence of the selected skeletal muscle tissue engineering setup on the myogenic outcome prior to their implementation. Moreover, we offer a workflow to facilitate identifying and selecting different analytical tools to demonstrate the successful creation of functional skeletal muscle tissue. Ultimately, a refinement of existing strategies will lead to further progression in understanding important aspects of muscle diseases, muscle aging and muscle regeneration to improve quality of life of patients and enable the establishment of new treatment options.

Decellularized Tissue for Muscle Regeneration

Several acquired or congenital pathological conditions can affect skeletal muscle leading to volumetric muscle loss (VML), i.e., an irreversible loss of muscle mass and function. Decellularized tissues are natural scaffolds derived from tissues or organs, in which the cellular and nuclear contents are eliminated, but the tridimensional (3D) structure and composition of the extracellular matrix (ECM) are preserved. Such scaffolds retain biological activity, are biocompatible and do not show immune rejection upon allogeneic or xenogeneic transplantation. An increase number of reports suggest that decellularized tissues/organs are promising candidates for clinical application in patients affected by VML. Here we explore the different strategies used to generate decellularized matrix and their therapeutic outcome when applied to treat VML conditions, both in patients and in animal models. The wide variety of VML models, source of tissue and methods of decellularization have led to discrepant results. Our review study evaluates the biological and clinical significance of reported studies, with the final aim to clarify the main aspects that should be taken into consideration for the future application of decellularized tissues in the treatment of VML conditions.

Synthetic matrix enhances transplanted satellite cell engraftment in dystrophic and aged skeletal muscle with comorbid trauma

Muscle satellite cells (MuSCs) play a central role in muscle regeneration, but their quantity and function decline with comorbidity of trauma, aging, and muscle diseases. Although transplantation of MuSCs in traumatically injured muscle in the comorbid context of aging or pathology is a strategy to boost muscle regeneration, an effective cell delivery strategy in these contexts has not been developed. We engineered a synthetic hydrogel-based matrix with optimal mechanical, cell-adhesive, and protease-degradable properties that promotes MuSC survival, proliferation, and differentiation. Furthermore, we establish a biomaterial-mediated cell delivery strategy for treating muscle trauma, where intramuscular injections may not be applicable. Delivery of MuSCs in the engineered matrix significantly improved in vivo cell survival, proliferation, and engraftment in nonirradiated and immunocompetent muscles of aged and dystrophic mice compared to collagen gels and cell-only controls. This platform may be suitable for treating craniofacial and limb muscle trauma, as well as postoperative wounds of elderly and dystrophic patients.

Mesenchymal stem cells and extracellular matrix scaffold promote muscle regeneration by synergistically regulating macrophage polarization toward the M2 phenotype

Background

Skeletal muscle plays an important role in the body’s physiology but there are still no effective treatments for volumetric muscle loss (VML) resulting from severe traumatic injury or tumor excision. Recent studies show that a tissue engineering strategy using a compound containing mesenchymal stem cells (MSCs) and decellularized extracellular matrix (ECM) scaffold generates significant regenerative effects on VML injury, but the underlying mechanisms are not fully understood.

Methods

The characteristics of human umbilical cord MSCs, including multiplication capacity and multidifferentiation ability, were determined. We constructed a compound containing MSCs and decellularized ECM scaffold which was used for tissue regeneration in a VML model.

Results

We found that MSCs and decellularized ECM scaffold generated synergistic effects on promoting skeletal muscle tissue regeneration. Interestingly, both MSCs and decellularized ECM scaffold could promote macrophage polarization toward the M2 phenotype and suppress macrophage polarization toward the M1 phenotype, which is widely regarded as an important promoting factor in tissue regeneration. More importantly, MSCs and decellularized ECM scaffold generate synergistic promoting effects on macrophage polarization toward the M2 phenotype, not just an additive effect.

Conclusions

Our findings uncover a previously unknown mechanism that MSCs and decellularized ECM scaffold promote tissue regeneration via collaboratively regulating macrophage polarization.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-0821-5) contains supplementary material, which is available to authorized users.

Engineering functional and histological regeneration of vascularized skeletal muscle

Tissue engineering strategies to treat patients with volumetric muscle loss (VML) aim to recover the structure and contractile function of lost muscle tissue. Here, we assessed the capacity of novel electrospun fibrin hydrogel scaffolds seeded with murine myoblasts to regenerate the structure and function of damaged muscle within VML defects to the mouse tibialis anterior muscle. The electrospun fibrin scaffolds provide pro-myogenic alignment and stiffness cues, myomimetic hierarchical structure, suturability, and scale-up capabilities. Myoblast-seeded scaffolds enabled remarkable muscle regeneration with high myofiber and vascular densities after 2 and 4 weeks, mimicking that of native skeletal muscle, while acellular scaffolds lacked muscle regeneration. Both myoblast-seeded and acellular scaffolds fully recovered muscle contractile function to uninjured values after 2 and 4 weeks. Electrospun scaffolds pre-vascularized with co-cultured human endothelial cells and human adipose-derived stem cells implanted into VML defects for 2 weeks anastomosed with host vasculature and were perfused with host red blood cells. These data demonstrate the significant potential of electrospun fibrin scaffolds seeded with myoblasts to fully regenerate the structure and function of volumetric muscle defects and these scaffolds offer a promising treatment option for patients with VML.

Volumetric muscle loss injury repair using in situ fibrin gel cast seeded with muscle-derived stem cells (MDSCs)

Volumetric muscle defect, caused by trauma or combat injuries, is a major health concern leading to severe morbidity. It is characterized by partial or full thickness loss of muscle and its bio-scaffold, resulting in extensive fibrosis and scar formation. Therefore, the ideal therapeutic option is to use stem cells combined with bio-scaffolds to restore muscle. For this purpose, muscle-derived stem cells (MDSCs) are a great candidate due to their unique multi-lineage differentiation potential.

In this study, we evaluated the regeneration potential of MDSCs for muscle loss repair using a novel in situ fibrin gel casting. Muscle defect was created by a partial thickness wedge resection in the tibialis anterior (TA)muscles of NSG mice which created an average of 25% mass loss. If untreated, this defect leads to severe muscle fibrosis. Next, MDSCs were delivered using a novel in situ fibrin gel casting method.

Our results demonstrated MDSCs are able to engraft and form new myofibers in the defect when casted along with fibrin gel. LacZ labeled MDSCs were able to differentiate efficiently into new myofibers and significantly increase muscle mass. This was also accompanied by significant reduction of fibrotic tissue in the engrafted muscles. Furthermore, transplanted cells also contributed to new vessel formation and satellite cell seeding.

These results confirmed the therapeutic potential of MDSCs and feasibility of direct in situ casting of fibrin/MDSC mixture to repair muscle mass defects.

Hydrogel biomaterials and their therapeutic potential for muscle injuries and muscular dystrophies

Muscular diseases such as muscular dystrophies and muscle injuries constitute a large group of ailments that manifest as muscle weakness, atrophy or fibrosis. Although cell therapy is a promising treatment option, the delivery and retention of cells in the muscle is difficult and prevents sustained regeneration needed for adequate functional improvements. Various types of biomaterials with different physical and chemical properties have been developed to improve the delivery of cells and/or growth factors for treating muscle injuries. Hydrogels are a family of materials with distinct advantages for use as cell delivery systems in muscle injuries and ailments, including their mild processing conditions, their similarities to natural tissue extracellular matrix, and their ability to be delivered with less invasive approaches. Moreover, hydrogels can be made to completely degrade in the body, leaving behind their biological payload in a process that can enhance the therapeutic process. For these reasons, hydrogels have shown great potential as cell delivery matrices. This paper reviews a few of the hydrogel systems currently being applied together with cell therapy and/or growth factor delivery to promote the therapeutic repair of muscle injuries and muscle wasting diseases such as muscular dystrophies.

Pro-myogenic and low-oxygen culture increases expression of contractile smooth muscle markers in human fibroblasts

Smooth muscle cells (SMCs) are essential for tissue engineering strategies to fabricate organs such as blood vessels, the oesophagus and bladder, and to create disease models of these systems. In order for such therapies and models to be feasible, SMCs must be sourced effectively to enable production of large numbers of functional cells. In vitro, SMCs divide slowly and demonstrate short proliferative lifespans compared with other types of cells, including stem cells and fibroblasts, limiting the number of cells that can be derived from expansion in culture of a primary isolation. As such, it would be beneficial to better understand the factors underlying induction and maintenance of SMC phenotypes, in order to produce new sources of SMCs for tissue engineering and disease modelling. Here we report the ability of human dermal fibroblasts to display patterns of gene expression resembling contractile SMCs when cultured under conditions that are known to promote a contractile phenotype in SMCs, including culture on collagen IV, low-serum culture, TGF-β1 treatment and hypoxia. These factors drive expression of the myogenic transcription factor myocardin, as well as expression of several of its gene targets that are known contributors to contractile phenotype in SMCs, including smooth muscle alpha actin, calponin, and myosin heavy chain. Our results suggest that culture conditions associated with culture of SMCs may be sufficient to induce myogenic gene expression patterns and potential myogenic function in non-muscle cells.

Laminin-111 enriched fibrin hydrogels for skeletal muscle regeneration

Laminin (LM)-111 supplementation has improved muscle regeneration in several models of disease and injury. This study investigated a novel hydrogel composed of fibrinogen and LM-111. Increasing LM-111 concentration (50-450 μg/mL) in fibrin hydrogels resulted in highly fibrous scaffolds with progressively thinner interlaced fibers. Rheological testing showed that all hydrogels had viscoelastic behavior and the Young's modulus ranged from 2-6KPa. C₂C₁₂ myobalsts showed a significant increase in VEGF production and decrease in IL-6 production on LM-111 enriched fibrin hydrogels as compared to pure fibrin hydrogels on day 4. Western blotting results showed a significant increase in MyoD and desmin protein quantity but a significant decrease in myogenin protein quantity in myoblasts cultured on the LM-111 (450 μg/mL) enriched fibrin hydrogel. Combined application of electromechanical stimulation significantly enhanced the production of VEGF and IGF-1 from myoblast seeded fibrin-LM-111 hydrogels. Taken together, these observations offer an important first step toward optimizing a tissue engineered constructs for skeletal muscle regeneration.

Emerging Biofabrication Strategies for Engineering Complex Tissue Constructs

The demand for organ transplantation and repair, coupled with a shortage of available donors, poses an urgent clinical need for the development of innovative treatment strategies for long-term repair and regeneration of injured or diseased tissues and organs. Bioengineering organs, by growing patient-derived cells in biomaterial scaffolds in the presence of pertinent physicochemical signals, provides a promising solution to meet this demand. However, recapitulating the structural and cytoarchitectural complexities of native tissues in vitro remains a significant challenge to be addressed. Through tremendous efforts over the past decade, several innovative biofabrication strategies have been developed to overcome these challenges. This review highlights recent work on emerging three-dimensional bioprinting and textile techniques, compares the advantages and shortcomings of these approaches, outlines the use of common biomaterials and advanced hybrid scaffolds, and describes several design considerations including the structural, physical, biological, and economical parameters that are crucial for the fabrication of functional, complex, engineered tissues. Finally, the applications of these biofabrication strategies in neural, skin, connective, and muscle tissue engineering are explored.

Exploratory study on the effect of osteoactivin on muscle regeneration in a rat volumetric muscle loss model

Wounds causing extensive injury loss of muscle, also known as volumetric muscle loss (VML), are frequently associated with high-energy civilian trauma and combat-related extremity injuries. Currently, no effective clinical therapy is available for promoting de novo muscle tissue regeneration to restore muscle function following VML. Recent studies have shown evidence that osteoactivin (OA), a transmembrane glycoprotein, has the ability to prevent skeletal muscle atrophy in response to denervation. Therefore the objective of this study is to investigate the potential regenerative effect of OA embedded and delivered via a cross-linked gelatin hydrogel within a volumetric tibialis anterior muscle defect in a rat model. After 4 weeks, however, no evidence for muscle formation was found in defects treated with either low (5 μg/ml) or high (50 μg/ml) OA. It is possible that a different delivery scaffold, delivery kinetics, or OA concentration may have yielded an alternate outcome, or it is also possible that the spaciostructural environment of VML, or the local (versus systemic) delivery of OA, simply does not support any potential regenerative activity of OA in VML. Together with prior work, this study demonstrates that an efficacious and scalable therapy for regenerating muscle volume and function in VML remains a veritable clinical challenge worthy of continued future research efforts.

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.

Fibrin nanostructures for biomedical applications

Fibrin is a versatile biopolymer that has been extensively used in tissue engineering. In this paper fibrin nanostructures prepared using a technique based on the catalytic effect of fibrin-bound thrombin are presented. This technique enables surface-attached thin fibrin networks to form with precisely regulated morphology without the development of fibrin gel in bulk solution. Moreover, the influence of changing the polymerization time, along with the antithrombin III and heparin concentrations on the morphology of fibrin nanostructures was explored. The binding of bioactive molecules (fibronectin, laminin, collagen, VEGF, bFGF, and heparin) to fibrin nanostructures was confirmed. These nanostructures can be used for the surface modification of artificial biomaterials designed for different biomedical applications (e.g. artificial vessels, stents, heart valves, bone and cartilage constructs, skin grafts, etc.) in order to promote the therapeutic outcome.