Skeletal muscle atrophy in bioengineered skeletal muscle: a new model system

Simulated microgravity attenuates myogenesis and contractile function of 3D engineered skeletal muscle tissues

While the effects of microgravity on inducing skeletal muscle atrophy have been extensively studied, the impacts of microgravity on myogenesis and its mechanisms remain unclear. In this study, we developed a microphysiological system of engineered muscle tissue (EMT) fabricated using a collagen / Matrigel composite hydrogel and murine skeletal myoblasts. This 3D EMT model allows non-invasive quantitative assessment of contractile function. After applying a 7-day differentiation protocol to induce myotube formation, the EMTs clearly exhibited sarcomerogenesis, myofilament formation, and synchronous twitch and tetanic contractions with electrical stimuli. Using this 3D EMT system, we investigated the effects of simulated microgravity at 10-3 G on myogenesis and contractile function utilizing a random positioning machine. EMTs cultured for 5 days in simulated microgravity exhibited significantly reduced contractile forces, myofiber size, and differential expression of muscle contractile, myogenesis regulatory, and mitochondrial biogenesis-related proteins. These results indicate simulated microgravity attenuates myogenesis, resulting in impaired muscle function.

Biomanufacturing of 3D Tissue Constructs in Microgravity and their Applications in Human Pathophysiological Studies

The growing interest in bioengineering in-vivo-like 3D functional tissues has led to novel approaches to the biomanufacturing process as well as expanded applications for these unique tissue constructs. Microgravity, as seen in spaceflight, is a unique environment that may be beneficial to the tissue-engineering process but cannot be completely replicated on Earth. Additionally, the expense and practical challenges of conducting human and animal research in space make bioengineered microphysiological systems an attractive research model. In this review, published research that exploits real and simulated microgravity to improve the biomanufacturing of a wide range of tissue types as well as those studies that use microphysiological systems, such as organ/tissue chips and multicellular organoids, for modeling human diseases in space are summarized. This review discusses real and simulated microgravity platforms and applications in tissue-engineered microphysiological systems across three topics: 1) application of microgravity to improve the biomanufacturing of tissue constructs, 2) use of tissue constructs fabricated in microgravity as models for human diseases on Earth, and 3) investigating the effects of microgravity on human tissues using biofabricated in vitro models. These current achievements represent important progress in understanding the physiological effects of microgravity and exploiting their advantages for tissue biomanufacturing.

Ionically Modified Gelatin Hydrogels Maintain Murine Myogenic Cell Viability and Fusion Capacity

For tissue engineering of skeletal muscles, there is a need for biomaterials which do not only allow cell attachment, proliferation, and differentiation, but also support the physiological conditions of the tissue. Next to the chemical nature and structure of the biomaterial, its response to the application of biophysical stimuli, such as mechanical deformation or application of electrical pulses, can impact in vitro tissue culture. In this study, gelatin methacryloyl (GelMA) is modified with hydrophilic 2-acryloxyethyltrimethylammonium chloride (AETA) and 3-sulfopropyl acrylate potassium (SPA) ionic comonomers to obtain a piezoionic hydrogel. Rheology, mass swelling, gel fraction, and mechanical characteristics are determined. The piezoionic properties of the SPA and AETA-modified GelMA are confirmed by a significant increase in ionic conductivity and an electrical response as a function of mechanical stress. Murine myoblasts display a viability of >95% after 1 week on the piezoionic hydrogels, confirming their biocompatibility. The GelMA modifications do not influence the fusion capacity of the seeded myoblasts or myotube width after myotube formation. These results describe a novel functionalization providing new possibilities to exploit piezo-effects in the tissue engineering field.

Tissue Engineering Applied to Skeletal Muscle: Strategies and Perspectives

Muscle tissue is formed by elongated and contractile cells with specific morphofunctional characteristics. Thus, it is divided into three basic types: smooth muscle tissue, cardiac striated muscle tissue and skeletal striated muscle tissue. The striated skeletal muscle tissue presents high plasticity, regeneration and growth capacity due to the presence of satellite cells, quiescent myoblasts that are activated in case of injury to the tissue and originate new muscle fibers when they differentiate. In more severe deficiencies or injuries there is a loss of their regenerative capacity, thus compromising the body's functionality at different levels. Tissue engineering studies the development of biomaterials capable of stimulating the recovery of cellular activity in injured body tissues, as well as the activity of cells with muscle differentiation potential in injury repair. However, the need for three-dimensional re-assembly in a complex organization makes it difficult to mimic this tissue and fully regenerate it for the sake of precise and effective movements. Thus, this article aims to provide a narrative review of tissue engineering strategies applied to the regeneration of skeletal muscle, in a critical evaluation of research, whether aimed at injury or atrophies such as spinal muscular atrophy.

Organotypic cultures as aging associated disease models

Aging remains a primary risk factor for a host of diseases, including leading causes of death. Aging and associated diseases are inherently multifactorial, with numerous contributing factors and phenotypes at the molecular, cellular, tissue, and organismal scales. Despite the complexity of aging phenomena, models currently used in aging research possess limitations. Frequently used in vivo models often have important physiological differences, age at different rates, or are genetically engineered to match late disease phenotypes rather than early causes. Conversely, routinely used in vitro models lack the complex tissue-scale and systemic cues that are disrupted in aging. To fill in gaps between in vivo and traditional in vitro models, researchers have increasingly been turning to organotypic models, which provide increased physiological relevance with the accessibility and control of in vitro context. While powerful tools, the development of these models is a field of its own, and many aging researchers may be unaware of recent progress in organotypic models, or hesitant to include these models in their own work. In this review, we describe recent progress in tissue engineering applied to organotypic models, highlighting examples explicitly linked to aging and associated disease, as well as examples of models that are relevant to aging. We specifically highlight progress made in skin, gut, and skeletal muscle, and describe how recently demonstrated models have been used for aging studies or similar phenotypes. Throughout, this review emphasizes the accessibility of these models and aims to provide a resource for researchers seeking to leverage these powerful tools.

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.

Factors mediating spaceflight-induced skeletal muscle atrophy

Skeletal muscle atrophy is a well-known consequence of spaceflight. Because of the potential significant impact of muscle atrophy and muscle dysfunction on astronauts and to their mission, a thorough understanding of the mechanisms of this atrophy and the development of effective countermeasures is critical. Spaceflight-induced muscle atrophy is similar to atrophy seen in many terrestrial conditions, and therefore our understanding of this form of atrophy may also contribute to the treatment of atrophy in humans on Earth. The unique environmental features humans encounter in space include the weightlessness of microgravity, space radiation, and the distinctive aspects of living in a spacecraft. The disuse and unloading of muscles in microgravity are likely the most significant factors that mediate spaceflight-induced muscle atrophy, and have been extensively studied and reviewed. However, there are numerous other direct and indirect effects on skeletal muscle that may be contributing factors to the muscle atrophy and dysfunction seen as a result of spaceflight. This review offers a novel perspective on the issue of muscle atrophy in space by providing a comprehensive overview of the unique aspects of the spaceflight environment and the various ways in which they can lead to muscle atrophy. We systematically review the potential contributions of these different mechanisms of spaceflight-induced atrophy and include findings from both actual spaceflight and ground-based models of spaceflight in humans, animals, and in vitro studies.

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.

Global and local tension measurements in biomimetic skeletal muscle tissues reveals early mechanical homeostasis

Tension and mechanical properties of muscle tissue are tightly related to proper skeletal muscle function, which makes experimental access to the biomechanics of muscle tissue formation a key requirement to advance our understanding of muscle function and development. Recently developed elastic in vitro culture chambers allow for raising 3D muscle tissue under controlled conditions and to measure global tissue force generation. However, these chambers are inherently incompatible with high-resolution microscopy limiting their usability to global force measurements, and preventing the exploitation of modern fluorescence based investigation methods for live and dynamic measurements. Here, we present a new chamber design pairing global force measurements, quantified from post-deflection, with local tension measurements obtained from elastic hydrogel beads embedded in muscle tissue. High-resolution 3D video microscopy of engineered muscle formation, enabled by the new chamber, shows an early mechanical tissue homeostasis that remains stable in spite of continued myotube maturation.

A Pound of Flesh: What Cachexia Is and What It Is Not

Body weight loss, mostly due to the wasting of skeletal muscle and adipose tissue, is the hallmark of the so-called cachexia syndrome. Cachexia is associated with several acute and chronic disease states such as cancer, chronic obstructive pulmonary disease (COPD), heart and kidney failure, and acquired and autoimmune diseases and also pharmacological treatments such as chemotherapy. The clinical relevance of cachexia and its impact on patients’ quality of life has been neglected for decades. Only recently did the international community agree upon a definition of the term cachexia, and we are still awaiting the standardization of markers and tests for the diagnosis and staging of cancer-related cachexia. In this review, we discuss cachexia, considering the evolving use of the term for diagnostic purposes and the implications it has for clinical biomarkers, to provide a comprehensive overview of its biology and clinical management. Advances and tools developed so far for the in vitro testing of cachexia and drug screening will be described. We will also evaluate the nomenclature of different forms of muscle wasting and degeneration and discuss features that distinguish cachexia from other forms of muscle wasting in the context of different conditions.

Extracellular Matrix-Derived Hydrogels as Biomaterial for Different Skeletal Muscle Tissue Replacements

Recently, skeletal muscle represents a complex and challenging tissue to be generated in vitro for tissue engineering purposes. Several attempts have been pursued to develop hydrogels with different formulations resembling in vitro the characteristics of skeletal muscle tissue in vivo. This review article describes how different types of cell-laden hydrogels recapitulate the multiple interactions occurring between extracellular matrix (ECM) and muscle cells. A special attention is focused on the biochemical cues that affect myocytes morphology, adhesion, proliferation, and phenotype maintenance, underlining the importance of topographical cues exerted on the hydrogels to guide cellular orientation and facilitate myogenic differentiation and maturation. Moreover, we highlight the crucial role of 3D printing and bioreactors as useful platforms to finely control spatial deposition of cells into ECM based hydrogels and provide the skeletal muscle native-like tissue microenvironment, respectively.

A 96-well culture platform enables longitudinal analyses of engineered human skeletal muscle microtissue strength

Three-dimensional (3D) in vitro models of human skeletal muscle mimic aspects of native tissue structure and function, thereby providing a promising system for disease modeling, drug discovery or pre-clinical validation, and toxicity testing. Widespread adoption of this research approach is hindered by the lack of easy-to-use platforms that are simple to fabricate and that yield arrays of human skeletal muscle micro-tissues (hMMTs) in culture with reproducible physiological responses that can be assayed non-invasively. Here, we describe a design and methods to generate a reusable mold to fabricate a 96-well platform, referred to as MyoTACTIC, that enables bulk production of 3D hMMTs. All 96-wells and all well features are cast in a single step from the reusable mold. Non-invasive calcium transient and contractile force measurements are performed on hMMTs directly in MyoTACTIC, and unbiased force analysis occurs by a custom automated algorithm, allowing for longitudinal studies of function. Characterizations of MyoTACTIC and resulting hMMTs confirms the capability of the device to support formation of hMMTs that recapitulate biological responses. We show that hMMT contractile force mirrors expected responses to compounds shown by others to decrease (dexamethasone, cerivastatin) or increase (IGF-1) skeletal muscle strength. Since MyoTACTIC supports hMMT long-term culture, we evaluated direct influences of pancreatic cancer chemotherapeutics agents on contraction competent human skeletal muscle myotubes. A single application of a clinically relevant dose of Irinotecan decreased hMMT contractile force generation, while clear effects on myotube atrophy were observed histologically only at a higher dose. This suggests an off-target effect that may contribute to cancer associated muscle wasting, and highlights the value of the MyoTACTIC platform to non-invasively predict modulators of human skeletal muscle function.

Bioinspired Three-Dimensional Human Neuromuscular Junction Development in Suspended Hydrogel Arrays

The physical connection between motoneurons and skeletal muscle targets is responsible for the creation of neuromuscular junctions (NMJs), which allow electrical signals to be translated to mechanical work. NMJ pathology contributes to the spectrum of neuromuscular, motoneuron, and dystrophic disease. Improving in vitro tools that allow for recapitulation of the physiology of the neuromuscular connection will enable researchers to better understand the development and maturation of NMJs, and will help to decipher mechanisms leading to NMJ degeneration. In this work, we first describe robust differentiation of bungarotoxin-positive human myotubes, as well as a reproducible method for encapsulating and aligning human myoblasts in three-dimensional (3D) suspended culture using bioprinted silk fibroin cantilevers as cell culture supports. Further analysis with coculture of motoneuron-like cells demonstrates feasibility of fully human coculture using two-dimensional and 2.5-dimensional culture methods, with appropriate differentiation of both cell types. Using these coculture differentiation conditions with motoneuron-like cells added to monocultures of 3D suspended human myotubes, we then demonstrate synaptic colocalization in coculture as well as acetylcholine and glutamic acid stimulation of human myocytes. This method represents a unique platform to coculture suspended human myoblast-seeded 3D hydrogels with integrated motoneuron-like cells derived from human induced neural stem cells. The platform described is fully customizable using 3D freeform printing into standard laboratory tissue culture materials, and allows for human myoblast alignment in 3D with precise motoneuron integration into preformed myotubes. The coculture method will ideally be useful in observation and analysis of neurite outgrowth and myogenic differentiation in 3D with quantification of several parameters of muscle innervation and function.

Development and application of human skeletal muscle microphysiological systems

A number of major disease states involve skeletal muscle, including type 2 diabetes, muscular dystrophy, sarcopenia and cachexia arising from cancer or heart disease. Animals do not accurately represent many of these disease states. Human skeletal muscle microphysiological systems derived from primary or induced pluripotent stem cells (hPSCs) can provide an in vitro model of genetic and chronic diseases and assess individual variations. Three-dimensional culture systems more accurately represent skeletal muscle function than do two-dimensional cultures. While muscle biopsies enable culture of primary muscle cells, hPSCs provide the opportunity to sample a wider population of donors. Recent advances to promote maturation of PSC-derived skeletal muscle provide an alternative to primary cells. While contractile function is often measured in three-dimensional cultures and several systems exist to characterize contraction of small numbers of muscle fibers, there is a need for functional measures of metabolism suited for microphysiological systems. Future research should address generation of well-differentiated hPSC-derived muscle cells, enabling muscle repair in vitro, and improved disease models.

Engineering of Human Skeletal Muscle With an Autologous Deposited Extracellular Matrix

Adult skeletal muscle progenitor cells can be embedded in an extracellular matrix (ECM) and tissue-engineered to form bio-artificial muscles (BAMs), composed of aligned post-mitotic myofibers. The ECM proteins which have been used most commonly are collagen type I and fibrin. Fibrin allows for in vitro vasculogenesis, however, high concentrations of fibrinolysis inhibitors are needed to inhibit degradation of the ECM and subsequent loss of BAM tissue structure. For in vivo implantation, fibrinolysis inhibition may prove difficult or even harmful to the host. Therefore, we adapted in vitro culture conditions to enhance the deposition of de novo synthesized collagen type I gradually replacing the degrading fibrin ECM. The in vitro viscoelastic properties of the fibrin BAMs and deposition of collagen were characterized. BAMs engineered with the addition of proline, hydroxyproline, and ascorbic acid in the tissue culture medium had a twofold increase in Young’s Modulus, a 2.5-fold decrease in maximum strain, and a 1.6-fold increase in collagen deposition. Lowering the fibrin content of the BAMs also increased Young’s Modulus, decreased maximum strain, and increased collagen deposition. Tissue engineering of BAMs with autologous ECM may allow for prolonged in vivo survival.

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.

Vascularized microfluidic organ-chips for drug screening, disease models and tissue engineering

Vascularization of micro-tissues in vitro has enabled formation of tissues larger than those limited by diffusion with appropriate nutrient/gas exchange as well as waste elimination. Furthermore, angiocrine signaling from the vasculature may be essential in mimicking organ-level functions in these micro-tissues. In drug screening applications, the presence of an appropriate blood-organ barrier in the form of a vasculature and its supporting cells (pericytes, appropriate stromal cells) may be essential to reproducing organ-scale drug delivery pharmacokinetics. Cutting-edge techniques including 3D bioprinting and in vitro angiogenesis and vasculogenesis could be applied to vascularize a range of tissues and organoids. Herein, we describe the latest developments in vascularization and prevascularization of micro-tissues and provide an outlook on potential future strategies.

Controlling Differentiation of Stem Cells for Developing Personalized Organ-on-Chip Platforms

Organ-on-chip (OOC) platforms have attracted attentions of pharmaceutical companies as powerful tools for screening of existing drugs and development of new drug candidates. OOCs have primarily used human cell lines or primary cells to develop biomimetic tissue models. However, the ability of human stem cells in unlimited self-renewal and differentiation into multiple lineages has made them attractive for OOCs. The microfluidic technology has enabled precise control of stem cell differentiation using soluble factors, biophysical cues, and electromagnetic signals. This study discusses different tissue- and organ-on-chip platforms (i.e., skin, brain, blood-brain barrier, bone marrow, heart, liver, lung, tumor, and vascular), with an emphasis on the critical role of stem cells in the synthesis of complex tissues. This study further recaps the design, fabrication, high-throughput performance, and improved functionality of stem-cell-based OOCs, technical challenges, obstacles against implementing their potential applications, and future perspectives related to different experimental platforms.

Label-Free, High-Throughput Purification of Satellite Cells Using Microfluidic Inertial Separation

Skeletal muscle satellite cells have tremendous therapeutic potential in cell therapy or skeletal muscle tissue engineering. Obtaining a sufficiently pure satellite cell population, however, presents a significant challenge. We hypothesized that size differences between satellite cells and fibroblasts, two primary cell types obtained from skeletal muscle dissociation, would allow for label-free, inertial separation in a microfluidic device, termed a "Labyrinth," and that these purified satellite cells could be used to engineer skeletal muscle. Throughout tissue fabrication, Labyrinth-purified cells were compared with unsorted controls to assess the efficiency of this novel sorting process and to examine potential improvements in myogenic proliferation, differentiation, and tissue function. Immediately after dissociation and Labyrinth sorting, cells were immunostained to identify myogenic cells and fibroblast progenitors. Remaining cells were cultured for 14 days to form a confluent monolayer that was induced to delaminate and was captured as a 3D skeletal muscle construct. During monolayer development, myogenic proliferation (BrdU assay on Day 4), differentiation and myotube fusion index (α-actinin on Day 11), and myotube structural development (light microscopy on Day 14) were assessed. Isometric tetanic force production was measured in 3D constructs on Day 16. Immediately following sorting, unsorted cells exhibited a myogenic purity of 39.9% ± 3.99%, and this purity was enriched approximately two-fold to 75.5% ± 1.59% by microfluidic separation. The BrdU assay on Day 4 similarly showed significantly enhanced myogenic proliferation: in unsorted controls 47.0% ± 2.77% of proliferating cells were myogenic, in comparison to 61.7% ± 2.55% following purification. Myogenic differentiation and fusion, assessed by fusion index quantification, showed improvement from 82.7% ± 3.74% in control to 92.3% ± 2.04% in the purified cell population. Myotube density in unsorted controls, 18.6 ± 3.26 myotubes/mm², was significantly enriched in the purified cell population to 33.9 ± 3.74 myotubes/mm². Constructs fabricated from Labyrinth-purified cells also produced significantly greater tetanic forces (143.6 ± 16.9 μN) than unsorted controls (70.7 ± 8.03 μN). These results demonstrate the promise of microfluidic sorting in purifying isolated satellite cells. This unique technology could assist researchers in translating the regenerative potential of satellite cells to cell therapies and engineered tissues.

In vitro drug testing based on contractile activity of C2C12 cells in an epigenetic drug model

Skeletal muscle tissue engineering holds great promise for pharmacological studies. Herein, we demonstrated an in vitro drug testing system using tissue-engineered skeletal muscle constructs. In response to epigenetic drugs, myotube differentiation of C2C12 myoblast cells was promoted in two-dimensional cell cultures, but the levels of contractile force generation of tissue-engineered skeletal muscle constructs prepared by three-dimensional cell cultures were not correlated with the levels of myotube differentiation in two-dimensional cell cultures. In contrast, sarcomere formation and contractile activity in two-dimensional cell cultures were highly correlated with contractile force generation of tissue-engineered skeletal muscle constructs. Among the epigenetic drugs tested, trichostatin A significantly improved contractile force generation of tissue-engineered skeletal muscle constructs. Follistatin expression was also enhanced by trichostatin A treatment, suggesting the importance of follistatin in sarcomere formation of muscular tissues. These observations indicate that contractility data are indispensable for in vitro drug screening.

Elderly Patient-Derived Endothelial Cells for Vascularization of Engineered Muscle

In vitro prevascularization of engineered tissue constructs promises to enhance their clinical applicability. We hypothesize that adult endothelial cells (ECs), isolated from limb veins of elderly patients, bear the vasculogenic properties required to form vascular networks in vitro that can later integrate with the host vasculature upon implantation. Here, we show that adult ECs formed vessel networks that were more developed and complex than those formed by human umbilical vein endothelial cells (HUVECs) seeded with various supporting cells on three-dimensional (3D) biodegradable polymer scaffolds. In parallel, secreted levels of key proangiogenic cytokines were significantly higher in adult EC-bearing scaffolds as compared to HUVEC scaffolds. As a proof of concept for applicability of this model, adult ECs were co-seeded with human myoblasts as well as supporting cells and successfully formed a branched network, which was surrounded by aligned human myotubes. The vascularized engineered muscle tissue implanted into a full-thickness defect in immunodeficient mice remained viable and anastomosed with the host vasculature within 9 days of implantation. Functional "chimeric" blood vessels and various types of anastomosis were observed. These findings provide strong evidence of the applicability of adult ECs in construction of clinically relevant autologous vascularized tissue.

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.

Growth Factors for Skeletal Muscle Tissue Engineering

Tissue-engineered skeletal muscle holds promise as a source of graft tissue for repair of volumetric muscle loss and as a model system for pharmaceutical testing. To reach this potential, engineered tissues must advance past the neonatal phenotype that characterizes the current state of the art. In this review, we describe native skeletal muscle development and identify important growth factors controlling this process. By comparing in vivo myogenesis to in vitro satellite cell cultures and tissue engineering approaches, several key similarities and differences that may potentially advance tissue-engineered skeletal muscle were identified. In particular, hepatocyte and fibroblast growth factors used to accelerate satellite cell activation and proliferation, followed by addition of insulin-like growth factor as a potent inducer of differentiation, are proven methods for increased myogenesis in engineered muscle. Additionally, we review our recent novel application of dexamethasone (DEX), a glucocorticoid that stimulates myoblast differentiation, in skeletal muscle tissue engineering. Using our established skeletal muscle unit (SMU) fabrication protocol, timing- and dose-dependent effects of DEX were measured. The supplemented SMUs demonstrated advanced sarcomeric structure and significantly increased myotube diameter and myotube fusion compared to untreated controls. Most significantly, these SMUs exhibited a fivefold rise in force production. Thus, we concluded that DEX may serve to improve myogenesis, advance muscle structure, and increase force production in engineered skeletal muscle.

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.

Design, evaluation, and application of engineered skeletal muscle

For over two decades, research groups have been developing methods to engineer three-dimensional skeletal muscle tissues. These tissues hold great promise for use in disease modeling and pre-clinical drug development, and have potential to serve as therapeutic grafts for functional muscle repair. Recent advances in the field have resulted in the engineering of regenerative muscle constructs capable of survival, vascularization, and functional maturation in vivo as well as the first-time creation of functional human engineered muscles for screening of therapeutics in vitro. In this review, we will discuss the methodologies that have progressed work in the muscle tissue engineering field to its current state. The emphasis will be placed on the existing procedures to generate myogenic cell sources and form highly functional muscle tissues in vitro, techniques to monitor and evaluate muscle maturation and performance in vitro and in vivo, and surgical strategies to both create diseased environments and ensure implant survival and rapid integration into the host. Finally, we will suggest the most promising methodologies that will enable continued progress in the field.

Effects of Dexamethasone on Satellite Cells and Tissue Engineered Skeletal Muscle Units

Tissue engineered skeletal muscle has potential for application as a graft source for repairing soft tissue injuries, a model for testing pharmaceuticals, and a biomechanical actuator system for soft robots. However, engineered muscle to date has not produced forces comparable to native muscle, limiting its potential for repair and for use as an in vitro model for pharmaceutical testing. In this study, we examined the trophic effects of dexamethasone (DEX), a glucocorticoid that stimulates myoblast differentiation and fusion into myotubes, on our tissue engineered three-dimensional skeletal muscle units (SMUs). Using our established SMU fabrication protocol, muscle isolates were cultured with three experimental DEX concentrations (5, 10, and 25 nM) and compared to untreated controls. Following seeding onto a laminin-coated Sylgard substrate, the administration of DEX was initiated on day 0 or day 6 in growth medium or on day 9 after the switch to differentiation medium and was sustained until the completion of SMU fabrication. During this process, total cell proliferation was measured with a BrdU assay, and myogenesis and structural advancement of muscle cells were observed through immunostaining for MyoD, myogenin, desmin, and α-actinin. After SMU formation, isometric tetanic force production was measured to quantify function. The histological and functional assessment of the SMU showed that the administration of 10 nM DEX beginning on either day 0 or day 6 yielded optimal SMUs. These optimized SMUs exhibited formation of advanced sarcomeric structure and significant increases in myotube diameter and myotube fusion index, compared with untreated controls. Additionally, the optimized SMUs matured functionally, as indicated by a fivefold rise in force production. In conclusion, we have demonstrated that the addition of DEX to our process of engineering skeletal muscle tissue improves myogenesis, advances muscle structure, and increases force production in the resulting SMUs.

Microfluidic organs-on-chips

An organ-on-a-chip is a microfluidic cell culture device created with microchip manufacturing methods that contains continuously perfused chambers inhabited by living cells arranged to simulate tissue- and organ-level physiology. By recapitulating the multicellular architectures, tissue-tissue interfaces, physicochemical microenvironments and vascular perfusion of the body, these devices produce levels of tissue and organ functionality not possible with conventional 2D or 3D culture systems. They also enable high-resolution, real-time imaging and in vitro analysis of biochemical, genetic and metabolic activities of living cells in a functional tissue and organ context. This technology has great potential to advance the study of tissue development, organ physiology and disease etiology. In the context of drug discovery and development, it should be especially valuable for the study of molecular mechanisms of action, prioritization of lead candidates, toxicity testing and biomarker identification.

Bioengineered human myobundles mimic clinical responses of skeletal muscle to drugs

Existing in vitro models of human skeletal muscle cannot recapitulate the organization and function of native muscle, limiting their use in physiological and pharmacological studies. Here, we demonstrate engineering of electrically and chemically responsive, contractile human muscle tissues (‘myobundles’) using primary myogenic cells. These biomimetic constructs exhibit aligned architecture, multinucleated and striated myofibers, and a Pax7⁺ cell pool. They contract spontaneously and respond to electrical stimuli with twitch and tetanic contractions. Positive correlation between contractile force and GCaMP6-reported calcium responses enables non-invasive tracking of myobundle function and drug response. During culture, myobundles maintain functional acetylcholine receptors and structurally and functionally mature, evidenced by increased myofiber diameter and improved calcium handling and contractile strength. In response to diversely acting drugs, myobundles undergo dose-dependent hypertrophy or toxic myopathy similar to clinical outcomes. Human myobundles provide an enabling platform for predictive drug and toxicology screening and development of novel therapeutics for muscle-related disorders.

DOI:http://dx.doi.org/10.7554/eLife.04885.001

Scientists have developed realistic models of the human liver, lung, and heart that allow them to observe living tissue in the laboratory. These models have helped us to better understand how these organs work and what goes wrong in diseases that affect these organs. The models can also be used to test how new drugs may affect a particular organ without the risk of exposing patients to the drug.

Efforts to develop a realistic laboratory model of human muscle tissues that can contract like real muscles have not been as successful to date. This shortcoming has potentially hindered the development of drugs to treat numerous disorders that affect muscles and movement in humans—such as muscular dystrophies, which are diseases in which people progressively lose muscle strength.

Some important drugs, like cholesterol-lowering statins, have detrimental effects on muscle tissue; one statin was so harmful to muscles that it had to be withdrawn from the market. As such, it would be useful to have experimental models that would allow scientists to test whether potential drugs damage or treat muscle tissue.

Madden et al. have now bioengineered a three-dimensional laboratory model of living muscle tissue made of cells taken from biopsies of several different human patients. These tissues were grown into bundles of muscle fibers on special polymer frames in the laboratory. The bioengineered muscle bundles respond to electrical and chemical signals and contract just like normal muscle. They also exhibit the same structure and signaling as healthy muscle tissue in humans.

Madden et al. exposed the muscle tissue bundles to three drugs known to affect muscles to determine if the model could be used to test whether drugs have harmful effects. This revealed that the bundles had weaker contractions in response to statins and the malaria drug chloroquine, just like normal muscles do—and that this effect worsened if more of each drug was used. Madden et al. also found that a drug that strengthens muscle contractions at low doses and damages muscle at high doses in humans has similar effects in the model.

As well as this model being used to screen for harmful effects of drugs before clinical trials, the technique used to create the model could be used to grow muscle tissue from patients with muscle diseases. This would help researchers and doctors to better understand the patient's condition and potentially develop more efficient therapies. Also, the technique could be eventually developed to grow healthy muscle tissue to implant in patients who have been injured.

DOI:http://dx.doi.org/10.7554/eLife.04885.002

Isolation and Purification of Satellite Cells for Skeletal Muscle Tissue Engineering

Engineered skeletal muscle holds promise as a source of graft tissue for the repair of traumatic injuries such as volumetric muscle loss. The resident skeletal muscle stem cell, the satellite cell, has been identified as an ideal progenitor for tissue engineering due to its role as an essential player in the potent skeletal muscle regeneration mechanism. A significant challenge facing tissue engineers, however, is the isolation of sufficiently large satellite cell populations with high purity. The two common isolation techniques, single fiber explant culture and enzymatic dissociation, can yield either a highly pure satellite cell population or a suitably large number or cells but fail to do both simultaneously. As a result, it is often necessary to use a purification technique such as pre-plating or cell sorting to enrich the satellite cell population post-isolation. Furthermore, the absence of complex chemical and biophysical cues influencing the in vivo satellite cell "niche" complicates the culture of isolated satellite cells. Techniques under investigation to maximize myogenic proliferation and differentiation in vitro are described in this article, along with current methods for isolating and purifying satellite cells.

Physiology and metabolism of tissue-engineered skeletal muscle

Skeletal muscle is a major target for tissue engineering, given its relative size in the body, fraction of cardiac output that passes through muscle beds, as well as its key role in energy metabolism and diabetes, and the need for therapies for muscle diseases such as muscular dystrophy and sarcopenia. To date, most studies with tissue-engineered skeletal muscle have utilized murine and rat cell sources. On the other hand, successful engineering of functional human muscle would enable different applications including improved methods for preclinical testing of drugs and therapies. Some of the requirements for engineering functional skeletal muscle include expression of adult forms of muscle proteins, comparable contractile forces to those produced by native muscle, and physiological force-length and force-frequency relations. This review discusses the various strategies and challenges associated with these requirements, specific applications with cultured human myoblasts, and future directions.