Regulating fibrinolysis to engineer skeletal muscle from the C2C12 cell line

Current approaches for the recreation of cardiac ischaemic environment in vitro

Myocardial ischaemia is one of the leading dead causes worldwide. Although animal experiments have historically provided a wealth of information, animal models are time and money consuming, and they usually miss typical human patient's characteristics associated with ischemia prevalence, including aging and comorbidities. Generating reliable in vitro models that recapitulate the human cardiac microenvironment during an ischaemic event can boost the development of new drugs and therapeutic strategies, as well as our understanding of the underlying cellular and molecular events, helping the optimization of therapeutic approaches prior to animal and clinical testing. Although several culture systems have emerged for the recreation of cardiac physiology, mimicking the features of an ischaemic heart tissue in vitro is challenging and certain aspects of the disease process remain poorly addressed. Here, current in vitro cardiac culture systems used for modelling cardiac ischaemia, from self-aggregated organoids to scaffold-based constructs and heart-on-chip platforms are described. The advantages of these models to recreate ischaemic hallmarks such as oxygen gradients, pathological alterations of mechanical strength or fibrotic responses are highlighted. The new models represent a step forward to be considered, but unfortunately, we are far away from recapitulating all complexity of the clinical situations.

Recent trends in bioartificial muscle engineering and their applications in cultured meat, biorobotic systems and biohybrid implants

Recent advances in tissue engineering and biofabrication technology have yielded a plethora of biological tissues. Among these, engineering of bioartificial muscle stands out for its exceptional versatility and its wide range of applications. From the food industry to the technology sector and medicine, the development of this tissue has the potential to affect many different industries at once. However, to date, the biofabrication of cultured meat, biorobotic systems, and bioartificial muscle implants are still considered in isolation by individual peer groups. To establish common ground and share advances, this review outlines application-specific requirements for muscle tissue generation and provides a comprehensive overview of commonly used biofabrication strategies and current application trends. By solving the individual challenges and merging various expertise, synergetic leaps of innovation that inspire each other can be expected in all three industries in the future.

nNOS-derived NO modulates force production and iNO-derived NO the excitability in C2C12-derived 3D tissue engineering skeletal muscle via different NO signaling pathways

Nitric oxide (NO) is a bioactive gas produced by one of the three NO synthases: neuronal NOS (nNOS), inducible (iNOS), and endothelial NOS (eNOS). NO has a relevant modulatory role in muscle contraction; this takes place through two major signaling pathways: (i) activation of soluble guanylate cyclase and, thus, protein kinase G or (ii) nitrosylation of sulfur groups of cysteine. Although it has been suggested that nNOS-derived NO is the responsible isoform in muscle contraction, the roles of eNOS and iNOS and their signaling pathways have not yet been clarified. To elucidate the action of each pathway, we optimized the generation of myooids, an engineered skeletal muscle tissue based on the C2C12 cell line. In comparison with diaphragm strips from wild-type mice, 180 myooids were analyzed, which expressed all relevant excitation–contraction coupling proteins and both nNOS and iNOS isoforms. Along with the biochemical results, myooids treated with NO donor (SNAP) and unspecific NOS blocker (L-NAME) revealed a comparable NO modulatory effect on force production as was observed in the diaphragm strips. Under the effects of pharmacological tools, we analyzed the myooids in response to electrical stimulation of two possible signaling pathways and NO sources. The nNOS-derived NO exerted its negative effect on force production via the sGG-PKG pathway, while iNOS-derived NO increased the excitability in response to sub-threshold electrical stimulation. These results strengthen the hypotheses of previous reports on the mechanism of action of NO during force production, showed a novel function of iNOS-derived NO, and establish the myooid as a novel and robust alternative model for pathophysiological skeletal muscle research.

Changes in Elastic Moduli of Fibrin Hydrogels Within the Myogenic Range Alter Behavior of Murine C2C12 and Human C25 Myoblasts Differently

Fibrin hydrogels have proven highly suitable scaffold materials for skeletal muscle tissue engineering in the past. Certain parameters of those types of scaffolds, however, greatly affect cellular mechanobiology and therefore the myogenic outcome. The aim of this study was to identify the influence of apparent elastic properties of fibrin scaffolds in 2D and 3D on myoblasts and evaluate if those effects differ between murine and human cells. Therefore, myoblasts were cultured on fibrin-coated multiwell plates (“2D”) or embedded in fibrin hydrogels (“3D”) with different elastic moduli. Firstly, we established an almost linear correlation between hydrogels’ fibrinogen concentrations and apparent elastic moduli in the range of 7.5 mg/ml to 30 mg/ml fibrinogen (corresponds to a range of 7.7–30.9 kPa). The effects of fibrin hydrogel elastic modulus on myoblast proliferation changed depending on culture type (2D vs 3D) with an inhibitory effect at higher fibrinogen concentrations in 3D gels and vice versa in 2D. The opposite effect was evident in differentiating myoblasts as shown by gene expression analysis of myogenesis marker genes and altered myotube morphology. Furthermore, culture in a 3D environment slowed down proliferation compared to 2D, with a significantly more pronounced effect on human myoblasts. Differentiation potential was also substantially impaired upon incorporation into 3D gels in human, but not in murine, myoblasts. With this study, we gained further insight in the influence of apparent elastic modulus and culture type on cellular behavior and myogenic outcome of skeletal muscle tissue engineering approaches. Furthermore, the results highlight the need to adapt parameters of 3D culture setups established for murine cells when applied to human cells.

Skeletal muscle differentiation of human iPSCs meets bioengineering strategies: perspectives and challenges

Although skeletal muscle repairs itself following small injuries, genetic diseases or severe damages may hamper its ability to do so. Induced pluripotent stem cells (iPSCs) can generate myogenic progenitors, but their use in combination with bioengineering strategies to modulate their phenotype has not been sufficiently investigated. This review highlights the potential of this combination aimed at pushing the boundaries of skeletal muscle tissue engineering. First, the overall organization and the key steps in the myogenic process occurring in vivo are described. Second, transgenic and non-transgenic approaches for the myogenic induction of human iPSCs are compared. Third, technologies to provide cells with biophysical stimuli, biomaterial cues, and biofabrication strategies are discussed in terms of recreating a biomimetic environment and thus helping to engineer a myogenic phenotype. The embryonic development process and the pro-myogenic role of the muscle-resident cell populations in co-cultures are also described, highlighting the possible clinical applications of iPSCs in the skeletal muscle tissue engineering field.

Mechanical unloading in 3D-engineered muscle leads to muscle atrophy by suppressing protein synthesis

Three dimensional (3D)-engineered muscle is an useful approach to a more comprehensive understanding of molecular mechanisms underlying unloading-induced muscle atrophy. We investigated the effects of mechanical unloading on molecular muscle protein synthesis (MPS)- and muscle protein breakdown (MPB)-related signaling pathways involved in muscle atrophy in 3D-engineered muscle, and to better understand in vitro model of muscle disuse. The 3D-engineered muscle consisting of C2C12 myoblasts and type-1 collagen gel was allowed to differentiate for 2 weeks and divided into three groups: 0 days of stretched-on control (CON), 2 and/or 7 days of stretched-on (ON), in which both ends of the muscle were fixed with artificial tendons, and the stretched-off group (OFF), in which one side of the artificial tendon was detached. Muscle weight (-38.1 to -48.4%), length (-67.0 to -73.5%), twitch contractile force (-70.5 to -75.0%) and myosin heavy chain expression (-32.5 to -50.5%) in the OFF group were significantly decreased on days 2 and 7 compared with the ON group (P < 0.05, respectively), despite that ON group was stable over time. Although determinative molecular signaling could not be identified, the MPS rate reflected by puromysin labeled protein was significantly decreased following mechanical unloading (P < 0.05, -38.5 to -51.1%). Meanwhile, MPB, particularly the ubiquitin-proteasome pathway, was not impacted. Hence, mechanical unloading of 3D-engineered muscle in vitro leads to muscle atrophy by suppressing MPS, cell differentiation, and cell growth rather than the promotion of MPB.

Unraveling Muscle Impairment Associated With COVID-19 and the Role of 3D Culture in Its Investigation

COVID-19, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has been considered a public health emergency, extensively investigated by researchers. Accordingly, the respiratory tract has been the main research focus, with some other studies outlining the effects on the neurological, cardiovascular, and renal systems. However, concerning SARS-CoV-2 outcomes on skeletal muscle, scientific evidence is still not sufficiently strong to trace, treat and prevent possible muscle impairment due to the COVID-19. Simultaneously, there has been a considerable amount of studies reporting skeletal muscle damage in the context of COVID-19. Among the detrimental musculoskeletal conditions associated with the viral infection, the most commonly described are sarcopenia, cachexia, myalgia, myositis, rhabdomyolysis, atrophy, peripheral neuropathy, and Guillain-Barré Syndrome. Of note, the risk of developing sarcopenia during or after COVID-19 is relatively high, which poses special importance to the condition amid the SARS-CoV-2 infection. The yet uncovered mechanisms by which musculoskeletal injury takes place in COVID-19 and the lack of published methods tailored to study the correlation between COVID-19 and skeletal muscle hinder the ability of healthcare professionals to provide SARS-CoV-2 infected patients with an adequate treatment plan. The present review aims to minimize this burden by both thoroughly exploring the interaction between COVID-19 and the musculoskeletal system and examining the cutting-edge 3D cell culture techniques capable of revolutionizing the study of muscle dynamics.

Differential microRNA profiles of intramuscular and secreted extracellular vesicles in human tissue-engineered muscle

Exercise affects the expression of microRNAs (miR/s) and muscle-derived extracellular vesicles (EVs). To evaluate sarcoplasmic and secreted miR expression in human skeletal muscle in response to exercise-mimetic contractile activity, we utilized a three-dimensional tissue-engineered model of human skeletal muscle ("myobundles"). Myobundles were subjected to three culture conditions: no electrical stimulation (CTL), chronic low frequency stimulation (CLFS), or intermittent high frequency stimulation (IHFS) for 7 days. RNA was isolated from myobundles and from extracellular vesicles (EVs) secreted by myobundles into culture media; miR abundance was analyzed by miRNA-sequencing. We used edgeR and a within-sample design to evaluate differential miR expression and Pearson correlation to evaluate correlations between myobundle and EV populations within treatments with statistical significance set at p < 0.05. Numerous miRs were differentially expressed between myobundles and EVs; 116 miRs were differentially expressed within CTL, 3 within CLFS, and 2 within IHFS. Additionally, 25 miRs were significantly correlated (18 in CTL, 5 in CLFS, 2 in IHFS) between myobundles and EVs. Electrical stimulation resulted in differential expression of 8 miRs in myobundles and only 1 miR in EVs. Several KEGG pathways, known to play a role in regulation of skeletal muscle, were enriched, with differentially overrepresented miRs between myobundle and EV populations identified using miEAA. Together, these results demonstrate that in vitro exercise-mimetic contractile activity of human engineered muscle affects both their expression of miRs and number of secreted EVs. These results also identify novel miRs of interest for future studies of the role of exercise in organ-organ interactions in vivo.

A Cylindrical Molding Method for the Biofabrication of Plane-Shaped Skeletal Muscle Tissue

Muscle tissues can be fabricated in vitro by culturing myoblast-populated hydrogels. To counter the shrinkage of the myoblast-populated hydrogels during culture, a pair of anchors are generally utilized to fix the two ends of the hydrogel. Here, we propose an alternative method to counter the shrinkage of the hydrogel and fabricate plane-shaped skeletal muscle tissues. The method forms myoblast-populated hydrogel in a cylindrical cavity with a central pillar, which can prevent tissue shrinkage along the circumferential direction. By eliminating the usages of the anchor pairs, our proposed method can produce plane-shaped skeletal muscle tissues with uniform width and thickness. In experiments, we demonstrate the fabrication of plane-shaped (length: ca. 10 mm, width: 5~15 mm) skeletal muscle tissue with submillimeter thickness. The tissues have uniform shapes and are populated with differentiated muscle cells stained positive for myogenic differentiation markers (i.e., myosin heavy chains). In addition, we show the assembly of subcentimeter-order tissue blocks by stacking the plane-shaped skeletal muscle tissues. The proposed method can be further optimized and scaled up to produce cultured animal products such as cultured meat.

Mechanical loading of bioengineered skeletal muscle in vitro recapitulates gene expression signatures of resistance exercise in vivo

Understanding the role of mechanical loading and exercise in skeletal muscle (SkM) is paramount for delineating the molecular mechanisms that govern changes in muscle mass. However, it is unknown whether loading of bioengineered SkM in vitro adequately recapitulates the molecular responses observed after resistance exercise (RE) in vivo. To address this, the transcriptional and epigenetic (DNA methylation) responses were compared after mechanical loading in bioengineered SkM in vitro and after RE in vivo. Specifically, genes known to be upregulated/hypomethylated after RE in humans were analyzed. Ninety-three percent of these genes demonstrated similar changes in gene expression post-loading in the bioengineered muscle when compared to acute RE in humans. Furthermore, similar differences in gene expression were observed between loaded bioengineered SkM and after programmed RT in rat SkM tissue. Hypomethylation occurred for only one of the genes analysed (GRIK2) post-loading in bioengineered SkM. To further validate these findings, DNA methylation and mRNA expression of known hypomethylated and upregulated genes post-acute RE in humans were also analyzed at 0.5, 3, and 24 h post-loading in bioengineered muscle. The largest changes in gene expression occurred at 3 h, whereby 82% and 91% of genes responded similarly when compared to human and rodent SkM respectively. DNA methylation of only a small proportion of genes analyzed (TRAF1, MSN, and CTTN) significantly increased post-loading in bioengineered SkM alone. Overall, mechanical loading of bioengineered SkM in vitro recapitulates the gene expression profile of human and rodent SkM after RE in vivo. Although some genes demonstrated differential DNA methylation post-loading in bioengineered SkM, such changes across the majority of genes analyzed did not closely mimic the epigenetic response to acute-RE in humans.

Focus on the road to modelling cardiomyopathy in muscular dystrophy

Alterations in the DMD gene, which codes for the protein dystrophin, cause forms of dystrophinopathies such as Duchenne muscular dystrophy, an X-linked disease. Cardiomyopathy linked to DMD mutations is becoming the leading cause of death in patients with dystrophinopathy. Since phenotypic pathophysiological mechanisms are not fully understood, the improvement and development of new disease models, considering their relative advantages and disadvantages, is essential. The application of genetic engineering approaches on induced pluripotent stem cells, such as gene editing technology, enables the development of physiologically relevant human cell models for in vitro dystrophinopathy studies. The combination of induced pluripotent stem cells-derived cardiovascular cell types and 3 D bioprinting technologies hold great promise for the study of dystrophin-linked cardiomyopathy. This combined approach enables the assessment of responses to physical or chemical stimuli, and the influence of pharmaceutical approaches. The critical objective of in vitro microphysiological systems is to more accurately reproduce the microenvironment observed in vivo. Ground-breaking methodology involving the connection of multiple microphysiological systems comprised of different tissues would represent a move toward precision body-on-chip disease modelling could lead to a critical expansion in what is known about inter-organ responses to disease and novel therapies that have the potential to replace animal models. In this review, we will focus on the generation, development, and application of current cellular, animal and potential for bio-printed models, in the study of the pathophysiological mechanisms underlying dystrophin-linked cardiomyopathy in the direction of personalized medicine.

3D in vitro models of skeletal muscle: myopshere, myobundle and bioprinted muscle construct

Typical two-dimensional (2D) culture models of skeletal muscle-derived cells cannot fully recapitulate the organization and function of living muscle tissues, restricting their usefulness in in-depth physiological studies. The development of functional 3D culture models offers a major opportunity to mimic the living tissues and to model muscle diseases. In this respect, this new type of in vitro model significantly increases our understanding of the involvement of the different cell types present in the formation of skeletal muscle and their interactions, as well as the modalities of response of a pathological muscle to new therapies. This second point could lead to the identification of effective treatments. Here, we report the significant progresses that have been made the last years to engineer muscle tissue-like structures, providing useful tools to investigate the behavior of resident cells. Specifically, we interest in the development of myopshere- and myobundle-based systems as well as the bioprinting constructs. The electrical/mechanical stimulation protocols and the co-culture systems developed to improve tissue maturation process and functionalities are presented. The formation of these biomimetic engineered muscle tissues represents a new platform to study skeletal muscle function and spatial organization in large number of physiological and pathological contexts.

Physiological and pathophysiological concentrations of fatty acids induce lipid droplet accumulation and impair functional performance of tissue engineered skeletal muscle

Fatty acids (FA) exert physiological and pathophysiological effects leading to changes in skeletal muscle metabolism and function, however, in vitro models to investigate these changes are limited. These experiments sought to establish the effects of physiological and pathophysiological concentrations of exogenous FA upon the function of tissue engineered skeletal muscle (TESkM). Cultured initially for 14 days, C2C12 TESkM was exposed to FA-free bovine serum albumin alone or conjugated to a FA mixture (oleic, palmitic, linoleic, and α-linoleic acids [OPLA] [ratio 45:30:24:1%]) at different concentrations (200 or 800 µM) for an additional 4 days. Subsequently, TESkM morphology, functional capacity, gene expression and insulin signaling were analyzed. There was a dose response increase in the number and size of lipid droplets within the TESkM (p < .05). Exposure to exogenous FA increased the messenger RNA expression of genes involved in lipid storage (perilipin 2 [p < .05]) and metabolism (pyruvate dehydrogenase lipoamide kinase isozyme 4 [p < .01]) in a dose dependent manner. TESkM force production was reduced (tetanic and single twitch) (p < .05) and increases in transcription of type I slow twitch fiber isoform, myosin heavy chain 7, were observed when cultured with 200 µM OPLA compared to control (p < .01). Four days of OPLA exposure results in lipid accumulation in TESkM which in turn results in changes in muscle function and metabolism; thus, providing insight ito the functional and mechanistic changes of TESkM in response to exogenous FA.

Tissue-Engineered Skeletal Muscle Models to Study Muscle Function, Plasticity, and Disease

Skeletal muscle possesses remarkable plasticity that permits functional adaptations to a wide range of signals such as motor input, exercise, and disease. Small animal models have been pivotal in elucidating the molecular mechanisms regulating skeletal muscle adaptation and plasticity. However, these small animal models fail to accurately model human muscle disease resulting in poor clinical success of therapies. Here, we review the potential of in vitro three-dimensional tissue-engineered skeletal muscle models to study muscle function, plasticity, and disease. First, we discuss the generation and function of in vitro skeletal muscle models. We then discuss the genetic, neural, and hormonal factors regulating skeletal muscle fiber-type in vivo and the ability of current in vitro models to study muscle fiber-type regulation. We also evaluate the potential of these systems to be utilized in a patient-specific manner to accurately model and gain novel insights into diseases such as Duchenne muscular dystrophy (DMD) and volumetric muscle loss. We conclude with a discussion on future developments required for tissue-engineered skeletal muscle models to become more mature, biomimetic, and widely utilized for studying muscle physiology, disease, and clinical use.

Msx1 cooperates with Runx1 for inhibiting myoblast differentiation

Myogenesis is an important and complicated biological process, especially during the process of embryonic development. The homeoprotein Msx1 is a crucial transcriptional repressor of myogenesis and maintains myogenic precursor cells in an undifferentiated, proliferative state. However, the molecular mechanism through which Msx1 coordinates myogenesis remains to be elucidated. Here, we determine the interacting partner proteins of Msx1 in myoblast cells by a proteomic screening method. Msx1 is found to interact with 55 proteins, among which our data demonstrate that the cooperation of Runt-related transcription factor 1 (Runx1) with Msx1 is required for myoblast cell differentiation. Our findings provide important insights into the mechanistic roles of Msx1 in myoblast cell differentiation, and lays foundation for the myogenic differentiation process.

Molecular analysis of muscle progenitor cells on extracellular matrix coatings and hydrogels

Development of an ex vivo culture system to expand satellite cells, the resident muscle stem cell population, will be necessary for the development of their use as therapeutics. The loss of the niche environment is often cited as the reason that culture results in both the loss of myogenic potential and low re-engraftment rates of these cells. Studies have shown that culture of satellite cells on more elastic substrates maintained their quiescence and potential and increased re-engraftment, but there was limited proliferation. We examined whether substrates composed of extracellular matrix proteins, as either coatings or hydrogels, could support expansion of this population whilst maintaining the potency of these cells. The collagen based hydrogels were very pliant and our studies demonstrate that stiffer substrates are necessary for in vitro proliferation and differentiation of satellite cells, and the ECM composition was not significantly important. Our data further indicates that culture on highly elastic substrates allowed satellite cells to down-regulate myogenic specific transcription factors, resulting in an expression profile similar to a G_alert state. These satellite cells could be subsequently cultured on Matrigel and induced to differentiate. Proliferation and gene expression data further indicated that C2C12 cells are not a good proxy for studies of satellite cell proliferation and differentiation on alternative substrates. STATEMENT OF SIGNIFICANCE: Although biomaterials and muscle stem cell-based therapeutics for muscle loss due to trauma and disease have been studied intensely for the last decade, significant challenges remain; satellite cells lose their myogenic potential after in vitro culture, and do not re-engraft efficiently when delivered to skeletal muscle. We cultured adult mouse derived satellite cells and C2C12 cells on biomimetic, extracellular matrix based, hydrogels and coated plates and carried out a detailed analysis of proliferation, differentiation, and gene expression. These studies were designed to allow for the examination of the roles of both matrix composition and elasticity. The data demonstrated that satellite cells were most affected by elasticity. Examining the expression of lineage and cell cycle specific genes provided important insight into the behavior of both cell types, and point to fundamental differences that affect the interpretation of studies aimed at understanding the in vitro requirements of muscle progenitor cells. Understanding how satellite cells respond to various biochemical and biophysical cues at the molecular level will inform future efforts to design therapeutics for muscle regeneration.

Engineered skeletal muscles for disease modeling and drug discovery

Skeletal muscle is the largest organ of human body with several important roles in everyday movement and metabolic homeostasis. The limited ability of small animal models of muscle disease to accurately predict drug efficacy and toxicity in humans has prompted the development in vitro models of human skeletal muscle that fatefully recapitulate cell and tissue level functions and drug responses. We first review methods for development of three-dimensional engineered muscle tissues and organ-on-a-chip microphysiological systems and discuss their potential utility in drug discovery research and development of new regenerative therapies. Furthermore, we describe strategies to increase the functional maturation of engineered muscle, and motivate the importance of incorporating multiple tissue types on the same chip to model organ cross-talk and generate more predictive drug development platforms. Finally, we review the ability of available in vitro systems to model diseases such as type II diabetes, Duchenne muscular dystrophy, Pompe disease, and dysferlinopathy.

Human induced pluripotent stem cell models for the study and treatment of Duchenne and Becker muscular dystrophies

Duchenne and Becker muscular dystrophies are the most common muscle diseases and are both currently incurable. They are caused by mutations in the dystrophin gene, which lead to the absence or reduction/truncation of the encoded protein, with progressive muscle degeneration that clinically manifests in muscle weakness, cardiac and respiratory involvement and early death. The limits of animal models to exactly reproduce human muscle disease and to predict clinically relevant treatment effects has prompted the development of more accurate in vitro skeletal muscle models. However, the challenge of effectively obtaining mature skeletal muscle cells or satellite stem cells as primary cultures has hampered the development of in vitro models. Here, we discuss the recently developed technologies that enable the differentiation of skeletal muscle from human induced pluripotent stem cells (iPSCs) of Duchenne and Becker patients. These systems recapitulate key disease features including inflammation and scarce regenerative myogenic capacity that are partially rescued by genetic and pharmacological therapies and can provide a useful platform to study and realize future therapeutic treatments. Implementation of this model also takes advantage of the developing genome editing field, which is a promising approach not only for correcting dystrophin, but also for modulating the underlying mechanisms of skeletal muscle development, regeneration and disease. These data prove the possibility of creating an accurate Duchenne and Becker in vitro model starting from iPSCs, to be used for pathogenetic studies and for drug screening to identify strategies capable of stopping or reversing muscular dystrophinopathies and other muscle diseases.

Electrical stimulation increases hypertrophy and metabolic flux in tissue-engineered human skeletal muscle

In vitro models of contractile human skeletal muscle hold promise for use in disease modeling and drug development, but exhibit immature properties compared to native adult muscle. To address this limitation, 3D tissue-engineered human muscles (myobundles) were electrically stimulated using intermittent stimulation regimes at 1 Hz and 10 Hz. Dystrophin in myotubes exhibited mature membrane localization suggesting a relatively advanced starting developmental maturation. One-week stimulation significantly increased myobundle size, sarcomeric protein abundance, calcium transient amplitude (∼2-fold), and tetanic force (∼3-fold) resulting in the highest specific force generation (19.3mN/mm2) reported for engineered human muscles to date. Compared to 1 Hz electrical stimulation, the 10 Hz stimulation protocol resulted in greater myotube hypertrophy and upregulated mTORC1 and ERK1/2 activity. Electrically stimulated myobundles also showed a decrease in fatigue resistance compared to control myobundles without changes in glycolytic or mitochondrial protein levels. Greater glucose consumption and decreased abundance of acetylcarnitine in stimulated myobundles indicated increased glycolytic and fatty acid metabolic flux. Moreover, electrical stimulation of myobundles resulted in a metabolic shift towards longer-chain fatty acid oxidation as evident from increased abundances of medium- and long-chain acylcarnitines. Taken together, our study provides an advanced in vitro model of human skeletal muscle with improved structure, function, maturation, and metabolic flux.

Biomaterializing the promise of cardiac tissue engineering

During an average individual's lifespan, the human heart pumps nearly 200 million liters of blood delivered by approximately 3 billion heartbeats. Therefore, it is not surprising that native myocardium under this incredible demand is extraordinarily complex, both structurally and functionally. As a result, successful engineering of adult-mimetic functional cardiac tissues is likely to require utilization of highly specialized biomaterials representative of the native extracellular microenvironment. There is currently no single biomaterial that fully recapitulates the architecture or the biochemical and biomechanical properties of adult myocardium. However, significant effort has gone toward designing highly functional materials and tissue constructs that may one day provide a ready source of cardiac tissue grafts to address the overwhelming burden of cardiomyopathic disease. In the near term, biomaterial-based scaffolds are helping to generate in vitro systems for querying the mechanisms underlying human heart homeostasis and disease and discovering new, patient-specific therapeutics. When combined with advances in minimally-invasive cardiac delivery, ongoing efforts will likely lead to scalable cell and biomaterial technologies for use in clinical practice. In this review, we describe recent progress in the field of cardiac tissue engineering with particular emphasis on use of biomaterials for therapeutic tissue design and delivery.

Exercising Bioengineered Skeletal Muscle In Vitro: Biopsy to Bioreactor

The bioengineering of skeletal muscle tissue in-vitro has enabled researchers to more closely mimic the in-vivo skeletal muscle niche. The three-dimensional (3-D) structure of the tissue engineered systems employed to date enable the generation of highly aligned and differentiated myofibers within a representative biological matrix. The use of electrical stimulation to model concentric contraction, via innervation of the myofibers, and the use of mechanical loading to model passive lengthening or stretch has begun to provide a manipulable environment to investigate the cellular and molecular responses following exercise mimicking stimuli in-vitro. Currently available bioreactor systems allow either electrical stimulation or mechanical loading to be utilized at any given time. In the present manuscript, we describe in detail the methodological procedures to create 3-D bioengineered skeletal muscle using both cell lines and/or primary human muscle derived cells from a tissue biopsy, through to modeling exercising stimuli using a bioreactor that can provide both electrical stimulation and mechanical loading simultaneously within the same in-vitro system.

In Vitro Tissue-Engineered Skeletal Muscle Models for Studying Muscle Physiology and Disease

Healthy skeletal muscle possesses the extraordinary ability to regenerate in response to small-scale injuries; however, this self-repair capacity becomes overwhelmed with aging, genetic myopathies, and large muscle loss. The failure of small animal models to accurately replicate human muscle disease, injury and to predict clinically-relevant drug responses has driven the development of high fidelity in vitro skeletal muscle models. Herein, the progress made and challenges ahead in engineering biomimetic human skeletal muscle tissues that can recapitulate muscle development, genetic diseases, regeneration, and drug response is discussed. Bioengineering approaches used to improve engineered muscle structure and function as well as the functionality of satellite cells to allow modeling muscle regeneration in vitro are also highlighted. Next, a historical overview on the generation of skeletal muscle cells and tissues from human pluripotent stem cells, and a discussion on the potential of these approaches to model and treat genetic diseases such as Duchenne muscular dystrophy, is provided. Finally, the need to integrate multiorgan microphysiological systems to generate improved drug discovery technologies with the potential to complement or supersede current preclinical animal models of muscle disease is described.

An Assessment of Myotube Morphology, Matrix Deformation, and Myogenic mRNA Expression in Custom-Built and Commercially Available Engineered Muscle Chamber Configurations

There are several three-dimensional (3D) skeletal muscle (SkM) tissue engineered models reported in the literature. 3D SkM tissue engineering (TE) aims to recapitulate the structure and function of native (in vivo) tissue, within an in vitro environment. This requires the differentiation of myoblasts into aligned multinucleated myotubes surrounded by a biologically representative extracellular matrix (ECM). In the present work, a new commercially available 3D SkM TE culture chamber manufactured from polyether ether ketone (PEEK) that facilitates suitable development of these myotubes is presented. To assess the outcomes of the myotubes within these constructs, morphological, gene expression, and ECM remodeling parameters were compared against a previously published custom-built model. No significant differences were observed in the morphological and gene expression measures between the newly introduced and the established construct configuration, suggesting biological reproducibility irrespective of manufacturing process. However, TE SkM fabricated using the commercially available PEEK chambers displayed reduced variability in both construct attachment and matrix deformation, likely due to increased reproducibility within the manufacturing process. The mechanical differences between systems may also have contributed to such differences, however, investigation of these variables was beyond the scope of the investigation. Though more expensive than the custom-built models, these PEEK chambers are also suitable for multiple use after autoclaving. As such this would support its use over the previously published handmade culture chamber system, particularly when seeking to develop higher-throughput systems or when experimental cost is not a factor.

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.

Investigating the Life Expectancy and Proteolytic Degradation of Engineered Skeletal Muscle Biological Machines

A combination of techniques from 3D printing, tissue engineering and biomaterials has yielded a new class of engineered biological robots that could be reliably controlled via applied signals. These machines are powered by a muscle strip composed of differentiated skeletal myofibers in a matrix of natural proteins, including fibrin, that provide physical support and cues to the cells as an engineered basement membrane. However, maintaining consistent results becomes challenging when sustaining a living system in vitro. Skeletal muscle must be preserved in a differentiated state and the system is subject to degradation by proteolytic enzymes that can break down its mechanical integrity. Here we examine the life expectancy, breakdown, and device failure of engineered skeletal muscle bio-bots as a result of degradation by three classes of proteases: plasmin, cathepsin L, and matrix metalloproteinases (MMP-2 and MMP-9). We also demonstrate the use of gelatin zymography to determine the effects of differentiation and inhibitor concentration on protease expression. With this knowledge, we are poised to design the next generation of complex biological machines with controllable function, specific life expectancy and greater consistency. These results could also prove useful for the study of disease-specific models, treatments of myopathies, and other tissue engineering applications.

Hypoxia Impairs Muscle Function and Reduces Myotube Size in Tissue Engineered Skeletal Muscle

Contemporary tissue engineered skeletal muscle models display a high degree of physiological accuracy compared with native tissue, and therefore may be excellent platforms to understand how various pathologies affect skeletal muscle. Chronic obstructive pulmonary disease (COPD) is a lung disease which causes tissue hypoxia and is characterized by muscle fiber atrophy and impaired muscle function. In the present study we exposed engineered skeletal muscle to varying levels of oxygen (O₂; 21–1%) for 24 h in order to see if a COPD like muscle phenotype could be recreated in vitro, and if so, at what degree of hypoxia this occurred. Maximal contractile force was attenuated in hypoxia compared to 21% O₂; with culture at 5% and 1% O₂ causing the most pronounced effects with 62% and 56% decrements in force, respectively. Furthermore at these levels of O₂, myotubes within the engineered muscles displayed significant atrophy which was not seen at higher O₂ levels. At the molecular level we observed increases in mRNA expression of MuRF‐1 only at 1% O₂ whereas MAFbx expression was elevated at 10%, 5%, and 1% O₂. In addition, p70S6 kinase phosphorylation (a downstream effector of mTORC1) was reduced when engineered muscle was cultured at 1% O₂, with no significant changes seen above this O₂ level. Overall, these data suggest that engineered muscle exposed to O₂ levels of ≤5% adapts in a manner similar to that seen in COPD patients, and thus may provide a novel model for further understanding muscle wasting associated with tissue hypoxia. J. Cell. Biochem. 118: 2599–2605, 2017. © 2017 The Authors. Journal of Cellular Biochemistry Published by Wiley Periodicals, Inc.

Leucine elicits myotube hypertrophy and enhances maximal contractile force in tissue engineered skeletal muscle in vitro

The amino acid leucine is thought to be important for skeletal muscle growth by virtue of its ability to acutely activate mTORC1 and enhance muscle protein synthesis, yet little data exist regarding its impact on skeletal muscle size and its ability to produce force. We utilized a tissue engineering approach in order to test whether supplementing culture medium with leucine could enhance mTORC1 signaling, myotube growth, and muscle function. Phosphorylation of the mTORC1 target proteins 4EBP-1 and rpS6 and myotube hypertrophy appeared to occur in a dose dependent manner, with 5 and 20 mM of leucine inducing similar effects, which were greater than those seen with 1 mM. Maximal contractile force was also elevated with leucine supplementation; however, although this did not appear to be enhanced with increasing leucine doses, this effect was completely ablated by co-incubation with the mTOR inhibitor rapamycin, showing that the augmented force production in the presence of leucine was mTOR sensitive. Finally, by using electrical stimulation to induce chronic (24 hr) contraction of engineered skeletal muscle constructs, we were able to show that the effects of leucine and muscle contraction are additive, since the two stimuli had cumulative effects on maximal contractile force production. These results extend our current knowledge of the efficacy of leucine as an anabolic nutritional aid showing for the first time that leucine supplementation may augment skeletal muscle functional capacity, and furthermore validates the use of engineered skeletal muscle for highly-controlled investigations into nutritional regulation of muscle physiology.

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.

Factors That Affect Tissue-Engineered Skeletal Muscle Function and Physiology

Tissue-engineered skeletal muscle has the promise to be a tool for studying physiology, screening muscle-active drugs, and clinical replacement of damaged muscle. To maximize the potential benefits of engineered muscle, it is important to understand the factors required for tissue formation and how these affect muscle function. In this review, we evaluate how biomaterials, cell source, media components, and bioreactor interventions impact muscle function and phenotype.

Striated muscle function, regeneration, and repair

As the only striated muscle tissues in the body, skeletal and cardiac muscle share numerous structural and functional characteristics, while exhibiting vastly different size and regenerative potential. Healthy skeletal muscle harbors a robust regenerative response that becomes inadequate after large muscle loss or in degenerative pathologies and aging. In contrast, the mammalian heart loses its regenerative capacity shortly after birth, leaving it susceptible to permanent damage by acute injury or chronic disease. In this review, we compare and contrast the physiology and regenerative potential of native skeletal and cardiac muscles, mechanisms underlying striated muscle dysfunction, and bioengineering strategies to treat muscle disorders. We focus on different sources for cellular therapy, biomaterials to augment the endogenous regenerative response, and progress in engineering and application of mature striated muscle tissues in vitro and in vivo. Finally, we discuss the challenges and perspectives in translating muscle bioengineering strategies to clinical practice.

Role of contraction duration in inducing fast-to-slow contractile and metabolic protein and functional changes in engineered muscle

The role of factors such as frequency, contraction duration and active time in the adaptation to chronic low-frequency electrical stimulation (CLFS) is widely disputed. In this study we explore the ability of contraction duration (0.6, 6, 60, and 600 sec) to induce a fast-to-slow shift in engineered muscle while using a stimulation frequency of 10 Hz and keeping active time constant at 60%. We found that all contraction durations induced similar slowing of time-to-peak tension. Despite similar increases in total myosin heavy (MHC) levels with stimulation, increasing contraction duration resulted in progressive decreases in total fast myosin. With contraction durations of 60 and 600 sec, MHC IIx levels decreased and MHC IIa levels increased. All contraction durations resulted in fast-to-slow shifts in TnT and TnC but increased both fast and slow TnI levels. Half-relaxation slowed to a greater extent with contraction durations of 60 and 600 sec despite similar changes in the calcium sequestering proteins calsequestrin and parvalbumin and the calcium uptake protein SERCA. All CLFS groups resulted in greater fatigue resistance than control. Similar increases in GLUT4, mitochondrial enzymes (SDH and ATPsynthase), the fatty acid transporter CPT-1, and the metabolic regulators PGC-1α and MEF2 were found with all contraction durations. However, the mitochondrial enzymes cytochrome C and citrate synthase were increased to greater levels with contraction durations of 60 and 600 sec. These results demonstrate that contraction duration plays a pivotal role in dictating the level of CLFS-induced contractile and metabolic adaptations in tissue-engineered skeletal muscle.

Neuromuscular Junction Formation in Tissue-Engineered Skeletal Muscle Augments Contractile Function and Improves Cytoskeletal Organization

Neuromuscular and neurodegenerative diseases are conditions that affect both motor neurons and the underlying skeletal muscle tissue. At present, the majority of neuromuscular research utilizes animal models and there is a growing need to develop novel methodologies that can be used to help understand and develop treatments for these diseases. Skeletal muscle tissue-engineered constructs exhibit many of the characteristics of the native tissue such as accurate fascicular structure and generation of active contractions. However, to date, there has been little consideration toward the integration of engineered skeletal muscle with motor neurons with the aim of neuromuscular junction (NMJ) formation, which would provide a model to investigate neuromuscular diseases and basic biology. In the present work we isolated primary embryonic motor neurons and neonatal myoblasts from Sprague-Dawley rats, and cocultured the two cell types in three-dimensional tissue-engineered fibrin hydrogels with the aim of NMJ formation. Immunohistochemistry revealed myotube formation in a fascicular arrangement and neurite outgrowth from motor neuron cell bodies toward the aligned myotubes. Furthermore, colocalization of pre- and postsynaptic proteins and chemical inhibition of spontaneous myotube twitch indicated the presence of NMJs in the innervated constructs. When electrical field stimulation was employed to evoke isometric contractions, maximal twitch and tetanic force were higher in the constructs cocultured with motor neurons, which may, in part, be explained by improved myotube cytoskeletal organization in these constructs. The fabrication of such constructs may be useful tools for investigating neuromuscular pharmaceuticals and improving the understanding of neuromuscular pathologies.

A novel bioreactor for the generation of highly aligned 3D skeletal muscle-like constructs through orientation of fibrin via application of static strain

The generation of functional biomimetic skeletal muscle constructs is still one of the fundamental challenges in skeletal muscle tissue engineering. With the notion that structure strongly dictates functional capabilities, a myriad of cell types, scaffold materials and stimulation strategies have been combined. To further optimize muscle engineered constructs, we have developed a novel bioreactor system (MagneTissue) for rapid engineering of skeletal muscle-like constructs with the aim to resemble native muscle in terms of structure, gene expression profile and maturity. Myoblasts embedded in fibrin, a natural hydrogel that serves as extracellular matrix, are subjected to mechanical stimulation via magnetic force transmission. We identify static mechanical strain as a trigger for cellular alignment concomitant with the orientation of the scaffold into highly organized fibrin fibrils. This ultimately yields myotubes with a more mature phenotype in terms of sarcomeric patterning, diameter and length. On the molecular level, a faster progression of the myogenic gene expression program is evident as myogenic determination markers MyoD and Myogenin as well as the Ca(2+) dependent contractile structural marker TnnT1 are significantly upregulated when strain is applied. The major advantage of the MagneTissue bioreactor system is that the generated tension is not exclusively relying on the strain generated by the cells themselves in response to scaffold anchoring but its ability to subject the constructs to individually adjustable strain protocols. In future work, this will allow applying mechanical stimulation with different strain regimes in the maturation process of tissue engineered constructs and elucidating the role of mechanotransduction in myogenesis.

STATEMENT OF SIGNIFICANCE

Mechanical stimulation of tissue engineered skeletal muscle constructs is a promising approach to increase tissue functionality. We have developed a novel bioreactor-based 3D culture system, giving the user the possibility to apply different strain regimes like static, cyclic or ramp strain to myogenic precursor cells embedded in a fibrin scaffold. Application of static mechanical strain leads to alignment of fibrin fibrils along the axis of strain and concomitantly to highly aligned myotube formation. Additionally, the pattern of myogenic gene expression follows the temporal progression observed in vivo with a more thorough induction of the myogenic program when static strain is applied. Ultimately, the strain protocol used in this study results in a higher degree of muscle maturity demonstrated by enhanced sarcomeric patterning and increased myotube diameter and length. The introduced bioreactor system enables new possibilities in muscle tissue engineering as longer cultivation periods and different strain applications will yield tissue engineered muscle-like constructs with improved characteristics in regard to functionality and biomimicry.

Contractile and metabolic properties of engineered skeletal muscle derived from slow and fast phenotype mouse muscle

Satellite cells derived from fast and slow muscles have been shown to adopt contractile and metabolic properties of their parent muscle. Mouse muscle shows less distinctive fiber-type profiles than rat or rabbit muscle. Therefore, in this study we sought to determine whether three-dimensional muscle constructs engineered from slow soleus (SOL) and fast tibialis anterior (TA) from mice would adopt the contractile and metabolic properties of their parent muscle. Time-to-peak tension (TPT) and half-relaxation time (1/2RT) was significantly slower in SOL constructs. In agreement with TPT, TA constructs contained significantly higher levels of fast myosin heavy chain (MHC) and fast troponin C, I, and T isoforms. Fast SERCA protein, both slow and fast calsequestrin isoforms and parvalbumin were found at higher levels in TA constructs. SOL constructs were more fatigue resistant and contained higher levels of the mitochondrial proteins SDH and ATP synthase and the fatty acid transporter CPT-1. SOL constructs contained lower levels of the glycolytic enzyme phosphofructokinase but higher levels of the β-oxidation enzymes LCAD and VLCAD suggesting greater fat oxidation. Despite no changes in PGC-1α protein, SOL constructs contained higher levels of SIRT1 and PRC. TA constructs contained higher levels of the slow-fiber program repressor SOX6 and the six transcriptional complex (STC) proteins Eya1 and Six4 which may underlie the higher in fast-fiber and lower slow-fiber program proteins. Overall, we have found that muscles engineered from predominantly slow and fast mouse muscle retain contractile and metabolic properties of their native muscle.

The effect of serum origin on tissue engineered skeletal muscle function

Skeletal muscle phenotype is regulated by a complex interaction between genetic, hormonal, and electrical inputs. However, because of the interrelatedness of these factors in vivo it is difficult to determine the importance of one over the other. Over the last 5 years, we have engineered skeletal muscles in the European Union (EU) and the United States (US) using the same clone of C2C12 cells. Strikingly, the dynamics of contraction of the muscles was dramatically different. Therefore, in this study we sought to determine whether the hormonal milieu (source of fetal bovine serum (FBS)) could alter engineered muscle phenotype. In muscles engineered in serum of US origin time-to-peak tension (2.2-fold), half relaxation (2.6-fold), and fatigue resistance (improved 25%) all showed indications of a shift towards a slower phenotype. Even though there was a dramatic shift in the rate of contraction, myosin heavy chain expression was the same. The contraction speed was instead related to a shift in calcium release/sensitivity proteins (DHPR = 3.1-fold lower, slow CSQ = 3.4-fold higher, and slow TnT = 2.4-fold higher) and calcium uptake proteins (slow SERCA = 1.7-fold higher and parvalbumin = 41-fold lower). These shifts in calcium dynamics were accompanied by a partial shift in metabolic enzymes, but could not be explained by purported regulators of muscle phenotype. These data suggest that hormonal differences in serum of USDA and EU origin cause a shift in calcium handling resulting in a dramatic change in engineered muscle function.

Biomimetic 3D Tissue Models for Advanced High-Throughput Drug Screening

Most current drug screening assays used to identify new drug candidates are 2D cell-based systems, even though such in vitro assays do not adequately re-create the in vivo complexity of 3D tissues. Inadequate representation of the human tissue environment during a preclinical test can result in inaccurate predictions of compound effects on overall tissue functionality. Screening for compound efficacy by focusing on a single pathway or protein target, coupled with difficulties in maintaining long-term 2D monolayers, can serve to exacerbate these issues when using such simplistic model systems for physiological drug screening applications. Numerous studies have shown that cell responses to drugs in 3D culture are improved from those in 2D, with respect to modeling in vivo tissue functionality, which highlights the advantages of using 3D-based models for preclinical drug screens. In this review, we discuss the development of microengineered 3D tissue models that accurately mimic the physiological properties of native tissue samples and highlight the advantages of using such 3D microtissue models over conventional cell-based assays for future drug screening applications. We also discuss biomimetic 3D environments, based on engineered tissues as potential preclinical models for the development of more predictive drug screening assays for specific disease models.

Factors affecting the structure and maturation of human tissue engineered skeletal muscle

Tissue engineered skeletal muscle has great utility in experimental studies of physiology, clinical testing and its potential for transplantation to replace damaged tissue. Despite recent work in rodent tissue or cell lines, there is a paucity of literature concerned with the culture of human muscle derived cells (MDCs) in engineered constructs. Here we aimed to tissue engineer for the first time in the literature human skeletal muscle in self-assembling fibrin hydrogels and determine the effect of MDC seeding density and myogenic proportion on the structure and maturation of the constructs. Constructs seeded with 4 × 10(5) MDCs assembled to a greater extent than those at 1 × 10(5) or 2 × 10(5), and immunostaining revealed a higher fusion index and a higher density of myotubes within the constructs, showing greater structural semblance to in vivo tissue. These constructs primarily expressed perinatal and slow type I myosin heavy chain mRNA after 21 days in culture. In subsequent experiments MACS(®) technology was used to separate myogenic and non-myogenic cells from their heterogeneous parent population and these cells were seeded at varying myogenic (desmin +) proportions in fibrin based constructs. Only in the constructs seeded with 75% desmin + cells was there evidence of striations when immunostained for slow myosin heavy chain compared with constructs seeded with 10 or 50% desmin + cells. Overall, this work reveals the importance of cell number and myogenic proportions in tissue engineering human skeletal muscle with structural resemblance to in vivo tissue.

The effect of in vitro formation of acetylcholine receptor (AChR) clusters in engineered muscle fibers on subsequent innervation of constructs in vivo

Timely innervation of muscle tissue is critical in the recovery of function, and this time-sensitive process relies heavily on the host tissue microenvironment after implantation. However, restoration of muscle tissue mass and function has been a challenge. We investigated whether pre-forming acetylcholine receptor (AChR) clusters on engineered muscle fibers using an AChR cluster-inducing factor (agrin) prior to implantation would facilitate established contacts between implanted muscle tissues and nerves and result in rapid innervation of engineered muscle in vivo. We showed that agrin treatment significantly increased the formation of AChR clusters on culture differentiated myotubes (C2C12), enhanced contacts with nerves in vitro and in vivo, and increased angiogenesis. Pre-fabrication of AChR clusters on engineered skeletal muscle using a released neurotrophic factor can accelerate innervations following implantation in vivo. This technique has considerable potential for enhancing muscle tissue function.

Effects of B-cell lymphoma 2 gene transfer to myoblast cells on skeletal muscle tissue formation using magnetic force-based tissue engineering

Tissue-engineered skeletal muscle should possess a high cell-dense structure with unidirectional cell alignment. However, limited nutrient and/or oxygen supply within the artificial tissue constructs might restrict cell viability and muscular functions. In this study, we genetically modified myoblast cells with the anti-apoptotic B-cell lymphoma 2 (Bcl-2) gene and evaluated their function in artificial skeletal muscle tissue constructs. Magnetite cationic liposomes were used to magnetically label C2C12 myoblast cells for the construction of skeletal muscle bundles by applying a magnetic force. Bcl-2-overexpressing muscle bundles formed highly cell-dense and viable tissue constructs, while muscle bundles without Bcl-2 overexpression exhibited substantial necrosis/apoptosis at the central region of the bundle. Bcl-2-overexpressing muscle bundles contracted in response to electrical pulses and generated a significantly higher physical force. These findings indicate that the incorporation of anti-apoptotic gene-transduced myoblast cells into tissue constructs significantly enhances skeletal muscle formation and function.

The role of fibroblasts in self-assembled skeletal muscle

Small facial skeletal muscles often have no autologous donor source to effect surgical reconstruction. Autologously derived muscles could be engineered for replacement tissue, but must be vascularized and innervated to be functional. As a critical step, engineered muscle must mimic the morphology, protein and gene expression, and function of native muscle. This study utilized a self-assembly process to engineer three-dimensional (3D) muscle from a statically strained muscle cell monolayer. Primary mouse myoblasts (PMMs) and mouse embryonic fibroblasts (MEFs) were separately proliferated and coseeded on a fibrin sheet with anchored sutures. Within 10 days of initiating PMM differentiation, the cell-gel layer contracted, lifted, and rolled into a cylindrical 3D structure around the tendon-like suture anchors; the myotubes longitudinally aligned along the lines of tensile force. The objectives of this study were to characterize these engineered muscles and to elucidate the role of the fibroblasts in the self-assembly process. Fibroblasts maintained myotube viability, mediated fibrin degradation, and assisted in muscle self-assembly. The optimal 1:1 PMM:MEF ratio resulted in tissue morphology remarkably similar to native muscle. Through gene and protein expression assays, the development and maturation of the engineered muscle tissue was demonstrated to recapitulate normal skeletal muscle development.

The role of extracellular matrix composition in structure and function of bioengineered skeletal muscle

One of the obstacles to the potential clinical utility of bioengineered skeletal muscle is its limited force generation capacity. Since engineered muscle, unlike most native muscle tissue, is composed of relatively short myofibers, we hypothesized that, its force production and transmission would be profoundly influenced by cell-matrix interactions. To test this hypothesis, we systematically varied the matrix protein type (collagen I/fibrin/Matrigel) and concentration in engineered, hydrogel-based neonatal rat skeletal muscle bundles and assessed the resulting tissue structure, generation of contractile force, and intracellular Ca(2+) handling. After two weeks of culture, the muscle bundles consisted of highly aligned and cross-striated myofibers and exhibited standard force-length and force-frequency relationships achieving tetanus at 40 Hz. The use of 2 mg/ml fibrin (control) yielded isometric tetanus amplitude of 1.4 ± 0.3 mN as compared to 0.9 ± 0.4 mN measured in collagen I-based bundles. Higher fibrin and Matrigel concentrations synergistically yielded further increase in active force generation to 2.8 ± 0.5 mN without significantly affecting passive mechanical properties, tetanus-to-twitch ratio, and twitch kinetics. Optimized matrix composition yielded significant cellular hypertrophy (protein/DNA ratio = 11.4 ± 4.1 vs. 6.5 ± 1.9 μg/μg in control) and a prolonged Ca(2+) transient half-width (Ca(50) = 232.8 ± 33.3 vs. 101.7 ± 19.8 ms). The use of growth-factor-reduced Matrigel, instead of standard Matrigel did not alter the obtained results suggesting enhanced cell-matrix interactions rather than growth factor supplementation as an underlying cause for the measured increase in contractile force. In summary, biomaterial-based manipulation of cell-matrix interactions represents an important target for improving contractile force generation in engineered skeletal muscle.

The effect of cell density on the maturation and contractile ability of muscle derived cells in a 3D tissue-engineered skeletal muscle model and determination of the cellular and mechanical stimuli required for the synthesis of a postural phenotype

The successful engineering of a truly biomimetic model of skeletal muscle could have a significant impact on a number of biomedical disciplines. Although a variety of techniques are currently being developed, there is, as of yet, no widely available and easily reproducible culture system for the synthesis of 3D artificial muscle tissues. In attempting to generate such a model it is essential to optimise any protocol in order to generate a tissue that best represents the in vivo environment. Since the maturation of muscle derived cells in culture is critically dependent on density, a major factor to be addressed in the development of these models is the ideal concentration at which to seed cells in order to generate an optimal response. In studying the effect of cell density on the performance of cells in an established 3D collagen based model of skeletal muscle, we demonstrate that an optimum density does exist in terms of peak force generation and myogenic gene expression data. Greater densities however, lead to the formation of a more physiologically relevant tissue with a phenotype characteristic of slow, postural muscle.