A simplified method for tissue engineering skeletal muscle organoids in vitro

Viscoelastic Biomaterials for Tissue Regeneration

The extracellular matrix (ECM) mechanical properties regulate key cellular processes in tissue development and regeneration. The majority of scientific investigation has focused on ECM elasticity as the primary mechanical regulator of cell and tissue behavior. However, all living tissues are viscoelastic, exhibiting both solid- and liquid-like mechanical behavior. Despite increasing evidence regarding the role of ECM viscoelasticity in directing cellular behavior, this aspect is still largely overlooked in the design of biomaterials for tissue regeneration. Recently, with the emergence of various bottom-up material design strategies, new approaches can deliver unprecedented control over biomaterial properties at multiple length scales, thus enabling the design of viscoelastic biomaterials that mimic various aspect of the native tissue ECM microenvironment. This review describes key considerations for the design of viscoelastic biomaterials for tissue regeneration. We provide an overview of the role of matrix viscoelasticity in directing cell behavior towards regenerative outcomes, highlight recent strategies utilizing viscoelastic hydrogels for regenerative therapies, and outline remaining challenges, potential solutions, and emerging applications for viscoelastic biomaterials in tissue engineering and regenerative medicine.

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.

Available In Vitro Models for Human Satellite Cells from Skeletal Muscle

Skeletal muscle accounts for almost 40% of the total adult human body mass. This tissue is essential for structural and mechanical functions such as posture, locomotion, and breathing, and it is endowed with an extraordinary ability to adapt to physiological changes associated with growth and physical exercise, as well as tissue damage. Moreover, skeletal muscle is the most age-sensitive tissue in mammals. Due to aging, but also to several diseases, muscle wasting occurs with a loss of muscle mass and functionality, resulting from disuse atrophy and defective muscle regeneration, associated with dysfunction of satellite cells, which are the cells responsible for maintaining and repairing adult muscle. The most established cell lines commonly used to study muscle homeostasis come from rodents, but there is a need to study skeletal muscle using human models, which, due to ethical implications, consist primarily of in vitro culture, which is the only alternative way to vertebrate model organisms. This review will survey in vitro 2D/3D models of human satellite cells to assess skeletal muscle biology for pre-clinical investigations and future directions.

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.

Next Stage Approach to Tissue Engineering Skeletal Muscle

Large-scale muscle injury in humans initiates a complex regeneration process, as not only the muscular, but also the vascular and neuro-muscular compartments have to be repaired. Conventional therapeutic strategies often fall short of reaching the desired functional outcome, due to the inherent complexity of natural skeletal muscle. Tissue engineering offers a promising alternative treatment strategy, aiming to achieve an engineered tissue close to natural tissue composition and function, able to induce long-term, functional regeneration after in vivo implantation. This review aims to summarize the latest approaches of tissue engineering skeletal muscle, with specific attention toward fabrication, neuro-angiogenesis, multicellularity and the biochemical cues that adjuvate the regeneration process.

Effects of extracellular matrix viscoelasticity on cellular behaviour

Substantial research over the past two decades has established that extracellular matrix (ECM) elasticity, or stiffness, affects fundamental cellular processes, including spreading, growth, proliferation, migration, differentiation and organoid formation. Linearly elastic polyacrylamide hydrogels and polydimethylsiloxane (PDMS) elastomers coated with ECM proteins are widely used to assess the role of stiffness, and results from such experiments are often assumed to reproduce the effect of the mechanical environment experienced by cells in vivo. However, tissues and ECMs are not linearly elastic materials-they exhibit far more complex mechanical behaviours, including viscoelasticity (a time-dependent response to loading or deformation), as well as mechanical plasticity and nonlinear elasticity. Here we review the complex mechanical behaviours of tissues and ECMs, discuss the effect of ECM viscoelasticity on cells, and describe the potential use of viscoelastic biomaterials in regenerative medicine. Recent work has revealed that matrix viscoelasticity regulates these same fundamental cell processes, and can promote behaviours that are not observed with elastic hydrogels in both two- and three-dimensional culture microenvironments. These findings have provided insights into cell-matrix interactions and how these interactions differentially modulate mechano-sensitive molecular pathways in cells. Moreover, these results suggest design guidelines for the next generation of biomaterials, with the goal of matching tissue and ECM mechanics for in vitro tissue models and applications in regenerative medicine.

Neuromuscular disease modeling on a chip

Organs-on-chips are broadly defined as microfabricated surfaces or devices designed to engineer cells into microscale tissues with native-like features and then extract physiologically relevant readouts at scale. Because they are generally compatible with patient-derived cells, these technologies can address many of the human relevance limitations of animal models. As a result, organs-on-chips have emerged as a promising new paradigm for patient-specific disease modeling and drug development. Because neuromuscular diseases span a broad range of rare conditions with diverse etiology and complex pathophysiology, they have been especially challenging to model in animals and thus are well suited for organ-on-chip approaches. In this Review, we first briefly summarize the challenges in neuromuscular disease modeling with animal models. Next, we describe a variety of existing organ-on-chip approaches for neuromuscular tissues, including a survey of cell sources for both muscle and nerve, and two- and three-dimensional neuromuscular tissue-engineering techniques. Although researchers have made tremendous advances in modeling neuromuscular diseases on a chip, the remaining challenges in cell sourcing, cell maturity, tissue assembly and readout capabilities limit their integration into the drug development pipeline today. However, as the field advances, models of healthy and diseased neuromuscular tissues on a chip, coupled with animal models, have vast potential as complementary tools for modeling multiple aspects of neuromuscular diseases and identifying new therapeutic strategies.

Summary: Modeling neuromuscular diseases is challenging due to their complex etiology and pathophysiology. Here, we review the cell sources and tissue-engineering procedures that are being integrated as emerging neuromuscular disease models.

Vascularization of tissue-engineered skeletal muscle constructs

Skeletal muscle tissue can be created in vitro by tissue engineering approaches, based on differentiation of muscle stem cells. Several approaches exist and generally result in three dimensional constructs composed of multinucleated myofibers to which we refer as myooids. Engineering methods date back to 3 decades ago and meanwhile a wide range of cell types and scaffold types have been evaluated. Nevertheless, in most approaches, myooids remain very small to allow for diffusion-mediated nutrient supply and waste product removal, typically less than 1 mm thick. One of the shortcomings of current in vitro skeletal muscle organoid development is the lack of a functional vascular structure, thus limiting the size of myooids. This is a challenge which is nowadays applicable to almost all organoid systems. Several approaches to obtain a vascular structure within myooids have been proposed. The purpose of this review is to give a concise overview of these approaches.

Mechanical loading stimulates hypertrophy in tissue-engineered skeletal muscle: Molecular and phenotypic responses

Mechanical loading of skeletal muscle results in molecular and phenotypic adaptations typified by enhanced muscle size. Studies on humans are limited by the need for repeated sampling, and studies on animals have methodological and ethical limitations. In this investigation, three‐dimensional skeletal muscle was tissue‐engineered utilizing the murine cell line C2C12, which bears resemblance to native tissue and benefits from the advantages of conventional in vitro experiments. The work aimed to determine if mechanical loading induced an anabolic hypertrophic response, akin to that described in vivo after mechanical loading in the form of resistance exercise. Specifically, we temporally investigated candidate gene expression and Akt‐mechanistic target of rapamycin 1 signalling along with myotube growth and tissue function. Mechanical loading (construct length increase of 15%) significantly increased insulin‐like growth factor‐1 and MMP‐2 messenger RNA expression 21 hr after overload, and the levels of the atrophic gene MAFbx were significantly downregulated 45 hr after mechanical overload. In addition, p70S6 kinase and 4EBP‐1 phosphorylation were upregulated immediately after mechanical overload. Maximal contractile force was augmented 45 hr after load with a 265% increase in force, alongside significant hypertrophy of the myotubes within the engineered muscle. Overall, mechanical loading of tissue‐engineered skeletal muscle induced hypertrophy and improved force production.

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

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

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.

Heterocellular molecular contacts in the mammalian stem cell niche

Adult tissue homeostasis and repair relies on prompt and appropriate intervention by tissue-specific adult stem cells (SCs). SCs have the ability to self-renew; upon appropriate stimulation, they proliferate and give rise to specialized cells. An array of environmental signals is important for maintenance of the SC pool and SC survival, behavior, and fate. Within this special microenvironment, commonly known as the stem cell niche (SCN), SC behavior and fate are regulated by soluble molecules and direct molecular contacts via adhesion molecules providing connections to local supporting cells and the extracellular matrix. Besides the extensively discussed array of soluble molecules, the expression of adhesion molecules and molecular contacts is another fundamental mechanism regulating niche occupancy and SC mobilization upon activation. Some adhesion molecules are differentially expressed and have tissue-specific consequences, likely reflecting the structural differences in niche composition and design, especially the presence or absence of a stromal counterpart. However, the distribution and identity of intercellular molecular contacts for adhesion and adhesion-mediated signaling within stromal and non-stromal SCN have not been thoroughly studied. This review highlights common details or significant differences in cell-to-cell contacts within representative stromal and non-stromal niches that could unveil new standpoints for stem cell biology and therapy.

Skeletal muscle-on-a-chip: an in vitro model to evaluate tissue formation and injury

Engineered skeletal muscle tissues can be used for in vitro studies that require physiologically relevant models of native tissues. Herein, we describe the development of a three-dimensional (3D) skeletal muscle tissue that recapitulates the architectural and structural complexities of muscle within a microfluidic device. Using a 3D photo-patterning approach, we spatially confined a cell-laden gelatin network around two bio-inert hydrogel pillars, which induce uniaxial alignment of the cells and serve as anchoring sites for the encapsulated cells and muscle tissues as they form and mature. We have characterized the tissue morphology and strain profile during differentiation of the cells and skeletal muscle tissue formation by using a combination of fluorescence microscopy and computational tools. The time-dependent strain profile suggests the existence of individual cells within the gelatin matrix, which differentiated to form a multinucleated skeletal muscle tissue bundle as a function of culture time. We have also developed a method to calculate the passive tension generated by the engineered muscle tissue bundles suspended between two pillars. Finally, as a proof-of-concept we have examined the applicability of the skeletal muscle-on-chip system as a screening platform and in vitro muscle injury model. We studied the dose-dependent effect of cardiotoxin on the engineered muscle tissue architecture and its subsequent effect on the passive tension. This simple yet effective tool can be appealing for studies that necessitate the analysis of skeletal muscle structure and function, including preclinical drug discovery and development.

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.

Synergizing Engineering and Biology to Treat and Model Skeletal Muscle Injury and Disease

Although skeletal muscle is one of the most regenerative organs in our body, various genetic defects, alterations in extrinsic signaling, or substantial tissue damage can impair muscle function and the capacity for self-repair. The diversity and complexity of muscle disorders have attracted much interest from both cell biologists and, more recently, bioengineers, leading to concentrated efforts to better understand muscle pathology and develop more efficient therapies. This review describes the biological underpinnings of muscle development, repair, and disease, and discusses recent bioengineering efforts to design and control myomimetic environments, both to study muscle biology and function and to aid in the development of new drug, cell, and gene therapies for muscle disorders. The synergy between engineering-aided biological discovery and biology-inspired engineering solutions will be the path forward for translating laboratory results into 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.

Endothelial Network Formation Within Human Tissue-Engineered Skeletal Muscle

The size of in vitro engineered skeletal muscle tissue is limited due to the lack of a vascular network in vitro. In this article, we report tissue-engineered skeletal muscle consisting of human aligned myofibers with interspersed endothelial networks. We extend our bioartificial muscle (BAM) model by coculturing human muscle progenitor cells with human umbilical vein endothelial cells (HUVECs) in a fibrin extracellular matrix (ECM). First, the optimal medium conditions for coculturing myoblasts with HUVECs were determined in a fusion assay. Endothelial growth medium proved to be the best compromise for the coculture, without affecting the myoblast fusion index. Second, both cell types were cocultured in a BAM maintained under tension to stimulate myofiber alignment. We then tested different total cell numbers containing 50% HUVECs and found that BAMs with a total cell number of 2 × 10(6) resulted in well-aligned and densely packed myofibers while allowing for improved interspersed endothelial network formation. Third, we compared different myoblast-HUVEC ratios. Including higher numbers of myoblasts improved endothelial network formation at lower total cell density; however, improvement of network characteristics reached a plateau when 1 × 10(6) or more myoblasts were present. Finally, addition of Matrigel to the fibrin ECM did not enhance overall myofiber and endothelial network formation. Therefore, in our BAM model, we suggest the use of a fibrin extracellular matrix containing 2 × 10(6) cells of which 50-70% are muscle cells. Optimizing these coculture conditions allows for a physiologically more relevant muscle model and paves the way toward engineering of larger in vitro muscle constructs.

Biomaterial-based delivery for skeletal muscle repair

Skeletal muscle possesses a remarkable capacity for regeneration in response to minor damage, but severe injury resulting in a volumetric muscle loss can lead to extensive and irreversible fibrosis, scarring, and loss of muscle function. In early clinical trials, the intramuscular injection of cultured myoblasts was proven to be a safe but ineffective cell therapy, likely due to rapid death, poor migration, and immune rejection of the injected cells. In recent years, appropriate therapeutic cell types and culturing techniques have improved progenitor cell engraftment upon transplantation. Importantly, the identification of several key biophysical and biochemical cues that synergistically regulate satellite cell fate has paved the way for the development of cell-instructive biomaterials that serve as delivery vehicles for cells to promote in vivo regeneration. Material carriers designed to spatially and temporally mimic the satellite cell niche may be of particular importance for the complete regeneration of severely damaged skeletal muscle.

Bio-hybrid cell-based actuators for microsystems

As we move towards the miniaturization of devices to perform tasks at the nano and microscale, it has become increasingly important to develop new methods for actuation, sensing, and control. Over the past decade, bio-hybrid methods have been investigated as a promising new approach to overcome the challenges of scaling down robotic and other functional devices. These methods integrate biological cells with artificial components and therefore, can take advantage of the intrinsic actuation and sensing functionalities of biological cells. Here, the recent advancements in bio-hybrid actuation are reviewed, and the challenges associated with the design, fabrication, and control of bio-hybrid microsystems are discussed. As a case study, focus is put on the development of bacteria-driven microswimmers, which has been investigated as a targeted drug delivery carrier. Finally, a future outlook for the development of these systems is provided. The continued integration of biological and artificial components is envisioned to enable the performance of tasks at a smaller and smaller scale in the future, leading to the parallel and distributed operation of functional systems at the microscale.

Roles of adherent myogenic cells and dynamic culture in engineered muscle function and maintenance of satellite cells

Highly functional engineered skeletal muscle constructs could serve as physiological models of muscle function and regeneration and have utility in therapeutic replacement of damaged or diseased muscle tissue. In this study, we examined the roles of different myogenic cell fractions and culturing conditions in the generation of highly functional engineered muscle. Fibrin-based muscle bundles were fabricated using either freshly-isolated myogenic cells or their adherent fraction pre-cultured for 36 h. Muscle bundles made of these cells were cultured in both static and dynamic conditions and systematically characterized with respect to early myogenic events and contractile function. Following 2 weeks of culture, we observed both individual and synergistic benefits of using the adherent cell fraction and dynamic culture on muscle formation and function. In particular, optimal culture conditions resulted in significant increase in the total cross-sectional muscle area (- 3-fold), myofiber size (- 1.6-fold), myonuclei density (- 1.2-fold), and force generation (- 9-fold) compared to traditional use of freshly-isolated cells and static culture. Curiously, we observed that only a simultaneous use of the adherent cell fraction and dynamic culture resulted in accelerated formation of differentiated myofibers which were critical for providing a niche-like environment for maintenance of a satellite cell pool early during culture. Our study identifies key parameters for engineering large-size, highly functional skeletal muscle tissues with improved ability for retention of functional satellite cells.

Biomimetic engineered muscle with capacity for vascular integration and functional maturation in vivo

Tissue-engineered skeletal muscle can serve as a physiological model of natural muscle and a potential therapeutic vehicle for rapid repair of severe muscle loss and injury. Here, we describe a platform for engineering and testing highly functional biomimetic muscle tissues with a resident satellite cell niche and capacity for robust myogenesis and self-regeneration in vitro. Using a mouse dorsal window implantation model and transduction with fluorescent intracellular calcium indicator, GCaMP3, we nondestructively monitored, in real time, vascular integration and the functional state of engineered muscle in vivo. During a 2-wk period, implanted engineered muscle exhibited a steady ingrowth of blood-perfused microvasculature along with an increase in amplitude of calcium transients and force of contraction. We also demonstrated superior structural organization, vascularization, and contractile function of fully differentiated vs. undifferentiated engineered muscle implants. The described in vitro and in vivo models of biomimetic engineered muscle represent enabling technology for novel studies of skeletal muscle function and regeneration.

Utilization and control of bioactuators across multiple length scales

In this review, we summarize the recent developments in the emerging field of bioactuators across a multitude of length scales. First, we discuss the use and control of biomolecules as nanoscale actuators. Molecular motors, such as DNA, kinesin, myosin, and F1-ATPase, have been shown to exert forces in the range between 1 pN to 45 pN. Second, we discuss the use and control of single and small clusters of cells to power microscale devices. Microorganisms, such as flagellated bacteria, protozoa, and algae, can naturally swim at speeds between 20 μm s(-1) to 2 mm s(-1) and produce thrust forces between 0.3 pN to 200 pN. Individual and clustered mammalian cells, such as cardiac and skeletal cells, can produce even higher contractile forces between 80 nN to 3.5 μN. Finally, we discuss the use and control of 2D- and 3D-assembled muscle tissues and muscle tissue explants as bioactuators to power devices. Depending on the size, composition, and organization of these hierarchical tissue constructs, contractile forces have been demonstrated to produce between 25 μN to 1.18 mN.

Integrating in vitro organ-specific function with the microcirculation

There is significant interest within the tissue engineering and pharmaceutical industries to create 3D microphysiological systems of human organ function. The interest stems from a growing concern that animal models and simple 2D culture systems cannot replicate essential features of human physiology that are critical to predict drug response, or simply to develop new therapeutic strategies to repair or replace damaged organs. Central to human organ function is a microcirculation that not only enhances the rate of nutrient and waste transport by convection, but also provides essential additional physiological functions that can be specific to each organ. This review highlights progress in the creation of in vitro functional microvessel networks, and emphasizes organ-specific functional and structural characteristics that should be considered in the future mimicry of four organ systems that are of primary interest: lung, brain, liver, and muscle (skeletal and cardiac).

Generation of eX vivo-vascularized Muscle Engineered Tissue (X-MET)

The object of this study was to develop an in vitro bioengineered three-dimensional vascularized skeletal muscle tissue, named eX-vivo Muscle Engineered Tissue (X-MET). This new tissue contains cells that exhibit the characteristics of differentiated myotubes, with organized contractile machinery, undifferentiated cells, and vascular cells capable of forming "vessel-like" networks. X-MET showed biomechanical properties comparable with that of adult skeletal muscles; thus it more closely mimics the cellular complexity typical of in vivo muscle tissue than myogenic cells cultured in standard monolayer conditions. Transplanted X-MET was able to mimic the activity of the excided EDL muscle, restoring the functionality of the damaged muscle. Our results suggest that X-MET is an ideal in vitro 3D muscle model that can be employed to repair muscle defects in vivo and to perform in vitro studies, limiting the use of live animals.

Murine muscle engineered from dermal precursors: an in vitro model for skeletal muscle generation, degeneration, and fatty infiltration

Skeletal muscle can be engineered by converting dermal precursors into muscle progenitors and differentiated myocytes. However, the efficiency of muscle development remains relatively low and it is currently unclear if this is due to poor characterization of the myogenic precursors, the protocols used for cell differentiation, or a combination of both. In this study, we characterized myogenic precursors present in murine dermospheres, and evaluated mature myotubes grown in a novel three-dimensional culture system. After 5-7 days of differentiation, we observed isolated, twitching myotubes followed by spontaneous contractions of the entire tissue-engineered muscle construct on an extracellular matrix (ECM). In vitro engineered myofibers expressed canonical muscle markers and exhibited a skeletal (not cardiac) muscle ultrastructure, with numerous striations and the presence of aligned, enlarged mitochondria, intertwined with sarcoplasmic reticula (SR). Engineered myofibers exhibited Na(+)- and Ca(2+)-dependent inward currents upon acetylcholine (ACh) stimulation and tetrodotoxin-sensitive spontaneous action potentials. Moreover, ACh, nicotine, and caffeine elicited cytosolic Ca(2+) transients; fiber contractions coupled to these Ca(2+) transients suggest that Ca(2+) entry is activating calcium-induced calcium release from the SR. Blockade by d-tubocurarine of ACh-elicited inward currents and Ca(2+) transients suggests nicotinic receptor involvement. Interestingly, after 1 month, engineered muscle constructs showed progressive degradation of the myofibers concomitant with fatty infiltration, paralleling the natural course of muscular degeneration. We conclude that mature myofibers may be differentiated on the ECM from myogenic precursor cells present in murine dermospheres, in an in vitro system that mimics some characteristics found in aging and muscular degeneration.

Engineering skeletal muscle tissues from murine myoblast progenitor cells and application of electrical stimulation

Engineered muscle tissues can be used for several different purposes, which include the production of tissues for use as a disease model in vitro, e.g. to study pressure ulcers, for regenerative medicine and as a meat alternative (1). The first reported 3D muscle constructs have been made many years ago and pioneers in the field are Vandenburgh and colleagues (2,3). Advances made in muscle tissue engineering are not only the result from the vast gain in knowledge of biochemical factors, stem cells and progenitor cells, but are in particular based on insights gained by researchers that physical factors play essential roles in the control of cell behavior and tissue development. State-of-the-art engineered muscle constructs currently consist of cell-populated hydrogel constructs. In our lab these generally consist of murine myoblast progenitor cells, isolated from murine hind limb muscles or a murine myoblast cell line C2C12, mixed with a mixture of collagen/Matrigel and plated between two anchoring points, mimicking the muscle ligaments. Other cells may be considered as well, e.g. alternative cell lines such as L6 rat myoblasts (4), neonatal muscle derived progenitor cells (5), cells derived from adult muscle tissues from other species such as human (6) or even induced pluripotent stem cells (iPS cells) (7). Cell contractility causes alignment of the cells along the long axis of the construct (8,9) and differentiation of the muscle progenitor cells after approximately one week of culture. Moreover, the application of electrical stimulation can enhance the process of differentiation to some extent (8). Because of its limited size (8 x 2 x 0.5 mm) the complete tissue can be analyzed using confocal microscopy to monitor e.g. viability, differentiation and cell alignment. Depending on the specific application the requirements for the engineered muscle tissue will vary; e.g. use for regenerative medicine requires the up scaling of tissue size and vascularization, while to serve as a meat alternative translation to other species is necessary.

A tissue-engineered muscle repair construct for functional restoration of an irrecoverable muscle injury in a murine model

There are no effective clinical treatments for volumetric muscle loss (VML) resulting from traumatic injury, tumor excision, or other degenerative diseases of skeletal muscle. The goal of this study was to develop and characterize a more clinically relevant tissue-engineered muscle repair (TE-MR) construct for functional restoration of a VML injury in the mouse lattissimus dorsi (LD) muscle. To this end, TE-MR constructs developed by seeding rat myoblasts on porcine bladder acellular matrix were preconditioned in a bioreactor for 1 week and implanted in nude mice at the site of a VML injury created by excising 50% of the native LD. Two months postinjury and implantation of TE-MR, maximal tetanic force was ∼72% of that observed in native LD muscle. In contrast, injured LD muscles that were not repaired, or were repaired with scaffold alone, produced only ∼50% of native LD muscle force after 2 months. Histological analyses of LD tissue retrieved 2 months after implantation demonstrated remodeling of the TE-MR construct as well as the presence of desmin-positive myofibers, blood vessels, and neurovascular bundles within the TE-MR construct. Overall, these encouraging initial observations document significant functional recovery within 2 months of implantation of TE-MR constructs and provide clear proof of concept for the applicability of this technology in a murine VML injury model.

High-content drug screening with engineered musculoskeletal tissues

Tissue engineering for in vitro drug-screening applications based on tissue function is an active area of translational research. Compared to targeted high-throughput drug-screening methods that rapidly analyze hundreds of thousands of compounds affecting a single biochemical reaction or gene expression, high-content screening (HCS) with engineered tissues is more complex and based on the cumulative positive and negative effects of a compound on the multiple pathways altering tissue function. It may therefore serve as better predictor of in vivo activity and serve as a bridge between high-throughput drug screening and in vivo animal studies. In the case of the musculoskeletal system, tissue function includes determining improvements in the mechanical properties of bone, tendon, cartilage, and, for skeletal muscle, contractile properties such as rate of contraction/relaxation, force generation, fatigability, and recovery from fatigue. HCS of compound banks with engineered tissues requires miniature musculoskeletal organs as well as automated functional testing. The resulting technologies should be rapid, cost effective, and reduce the number of small animals required for follow-on in vivo studies. Identification of compounds that improve the repair/regeneration of damaged tissues in vivo would have extensive clinical applications for treating musculoskeletal disorders.

Preparation of artificial skeletal muscle tissues by a magnetic force-based tissue engineering technique

Artificial muscle tissues composed of mouse myoblast C2C12 cells were prepared using a magnetic force-based tissue engineering technique. C2C12 cells labeled with magnetite nanoparticles were seeded into the wells of 24-well ultralow-attachment culture plates. When a magnet was positioned underneath each plate, the cells accumulated evenly on the culture surface and formed multilayered cell sheets. Since the shapes of artificial tissue constructs can be controlled by magnetic force, cellular string-like assemblies were formed by using a linear magnetic field concentrator with a magnet. However, the resulting cellular sheets and strings shrank considerably and did not retain their shapes during additional culture periods for myogenic differentiation. On the other hand, when a silicone plug was positioned at the center of the well during the fabrication of a cell sheet, the cell sheet shrank drastically and formed a ring-like assembly around the plug. A histological examination revealed that the cells in the cellular ring were highly oriented in the direction of the circumference by the tension generated within the structure. Individual cellular rings were hooked around two pins separated by 10 mm, and successfully cultured for 6 d without breakage. After a 6-d culture in differentiation medium, the C2C12 cells differentiated to form myogenin-positive multinucleated myotubes. Highly dense and oriented skeletal muscle tissues were obtained using this technique, suggesting that this procedure may represent a novel strategy for muscle tissue engineering.

Essential environmental cues from the satellite cell niche: optimizing proliferation and differentiation

The use of muscle progenitor cells (MPCs) for regenerative medicine has been severely compromised by their decreased proliferative and differentiative capacity after being cultured in vitro. We hypothesized the loss of pivotal niche factors to be the cause. Therefore, we investigated the proliferative and differentiative response of passage 0 murine MPCs to varying substrate elasticities and protein coatings and found that proliferation was influenced only by elasticity, whereas differentiation was influenced by both elasticity and protein coating. A stiffness of 21 kPa optimally increased the proliferation of MPCs. Regarding differentiation, we demonstrated that fusion of MPCs into myotubes takes place regardless of elasticity. However, ongoing maturation with cross-striations and contractions occurred only on elasticities higher than 3 kPa. Furthermore, maturation was fastest on poly-d-lysine and laminin coatings.

Microfeature guided skeletal muscle tissue engineering for highly organized 3-dimensional free-standing constructs

Engineering tissue similar in structure to their natural equivalents is a major challenge and crucial to function. Despite attempts to engineer skeletal muscle, it is still difficult to effectively mimic tissue architecture. Rigid scaffolds can guide cell alignment but have the critical drawback of hindering mechanical function of the resultant tissue. We present a method for creating highly ordered tissue-only constructs by using rigid microtopographically patterned surfaces to first guide myoblast alignment, followed by transfer of aligned myotubes into a degradable hydrogel and self-organization of the ordered cells into a functional, 3-dimensional, free-standing construct independent of the initial template substrate. Histology revealed an intracellular organization resembling that of native muscle. Aligned cell constructs exhibited a 2-fold increase in peak force production compared to controls. Effective specific force, or force normalized over cross-sectional area, was increased by 23%. This template, transfer, and self-organization strategy is envisioned to be broadly useful in improving construct function and clinical applicability for highly ordered tissues like muscle.

Bioreactors for guiding muscle tissue growth and development

Muscle tissue bioreactors are devices which are employed to guide and monitor the development of engineered muscle tissue. These devices have a modern history that can be traced back more than a century, because the key elements of muscle tissue bioreactors have been studied for a very long time. These include barrier isolation and culture of cells, tissues and organs after isolation from a host organism; the provision of various stimuli intended to promote growth and maintain the muscle, such as electrical and mechanical stimulation; and the provision of a perfusate such as culture media or blood derived substances. An accurate appraisal of our current progress in the development of muscle bioreactors can only be made in the context of the history of this endeavor. Modern efforts tend to focus more upon the use of computer control and the application of mechanical strain as a stimulus, as well as substrate surface modifications to induce cellular organization at the early stages of culture of isolated muscle cells.

Drug-screening platform based on the contractility of tissue-engineered muscle

A tissue-based approach to in vitro drug screening allows for determination of the cumulative positive and negative effects of a drug at the tissue rather than the cellular or subcellular level. Skeletal muscle myoblasts were tissue-engineered into three-dimensional muscle with parallel myofibers generating directed forces. When grown attached to two flexible micro-posts (mu posts) acting as artificial tendons in a 96-well plate format, the miniature bioartificial muscles (mBAMs) generated tetanic (active) forces upon electrical stimulation measured with a novel image-based motion detection system. mBAM myofiber hypertrophy and active force increased in response to insulin-like growth factor 1. In contrast, mBAM deterioration and weakness was observed with a cholesterol-lowering statin. The results described in this study demonstrate the integration of tissue engineering and biomechanical testing into a single platform for the screening of compounds affecting muscle strength.

The influence of serum-free culture conditions on skeletal muscle differentiation in a tissue-engineered model

The influence of differentiation medium (DM) components on C2C12 murine myoblast differentiation has only been studied in monolayer cultures. Serum-free formulations have been applied that omit the use of sera with unknown composition. The goal of the present study was to compare the influence of serum-free media on C2C12 differentiation in 3-dimensional tissue-engineered muscle constructs. Myoblast proliferation and differentiation in media containing Ultroser G (DMU), insulin-like growth factor (IGF)-I (DMI), or both (DMUI) were compared with those induced by more-traditional media containing horse serum (HS) or horse serum and IGF-I (HSI). Effects of the applied media were assessed from gross construct morphology, total protein content, creatine kinase activity, and tissue viability. Addition of IGF-I (HSI) to the standard DM (HS) improved myoblast differentiation in muscle constructs. Even better results were obtained using DMU and DMUI culture conditions. DMI could not induce differentiation or maintain cell viability. Serum-free culture medium supplemented with DMU or DMUI accelerates and improves myoblast differentiation in engineered muscle tissue better than the gold standard HS.

Rapid formation of functional muscle in vitro using fibrin gels

The transition of a muscle cell from a differentiated myotube into an adult myofiber is largely unstudied. This is primarily due to the difficulty of isolating specific developmental stimuli in vivo and the inability to maintain viable myotubes in culture for sufficient lengths of time. To address these limitations, a novel method for rapidly generating three-dimensional engineered muscles using fibrin gel casting has been developed. Myoblasts were seeded and differentiated on top of a fibrin gel. Cell-mediated contraction of the gel around artificial anchors placed 12 mm apart culminates 10 days after plating in a tubular structure of small myotubes (10-microm diameter) surrounded by a fibrin gel matrix. These tissues can be connected to a force transducer and electrically stimulated between parallel platinum electrodes to monitor physiological function. Three weeks after plating, the three-dimensional engineered muscle generated a maximum twitch force of 329 +/- 26.3 microN and a maximal tetanic force of 805.8 +/- 55 microN. The engineered muscles demonstrated normal physiological function including length-tension and force-frequency relationships. Treatment with IGF-I resulted in a 50% increase in force production, demonstrating that these muscles responded to hormonal interventions. Although the force production was maximal at 3 wk, constructs can be maintained in culture for up to 6 wk with no intervention. We conclude that fibrin-based gels provide a novel method to engineer three-dimensional functional muscle tissue and that these tissues may be used to model the development of skeletal muscle in vitro.

Spaceflight bioreactor studies of cells and tissues

Studies of the fundamental role of gravity in the development and function of biological organisms are a central component of the human exploration of space. Microgravity affects numerous physical phenomena relevant to biological research, including the hydrostatic pressure in fluid filled vesicles, sedimentation of organelles, and buoyancy-driven convection of flow and heat. These physical phenomena can in turn directly and indirectly affect cellular morphology, metabolism, locomotion, secretion of extracellular matrix and soluble signals, and assembly into functional tissues. Studies aimed at distinguishing specific effects of gravity on biological systems require the ability to: (i) control and systematically vary gravity, e.g. by utilizing the microgravity environment of space in conjunction with an in-flight centrifuge; and (ii) maintain constant all other factors in the immediate environment, including in particular concentrations and exchange rates of biochemical species and hydrodynamic shear. The latter criteria imply the need for gravity-independent mechanisms to provide for mass transport between the cells and their environment. Available flight hardware has largely determined the experimental design and scientific objectives of spaceflight cell and tissue culture studies carried out to date. Simple culture vessels have yielded important quantitative data, and helped establish in vitro models of cell locomotion, growth and differentiation in various mammalian cell types including embryonic lung cells [6], lymphocytes [2,8], and renal cells [7,31]. Studies done using bacterial cells established the first correlations between gravity-dependent factors such as cell settling velocity and diffusional distance and the respective cell responses [12]. The development of advanced bioreactors for microgravity cell and tissue culture and for tissue engineering has benefited both research areas and provided relevant in vitro model systems for studies of astronaut well-being (loss of muscle and skeletal tissues [15-17]) and gene- and cell-level responses to the mechanical environment [13,14,18]. All five of the spaceflight bioreactor studies described above utilized three-dimensional cell culture systems in which the cells were associated with biodegradable polymer scaffolds [17], collagen gel [16], or microcarrier beads [13-15,18] in order to promote the expression of differentiated cell function. In four of the five spaceflight bioreactor studies [15-18], cells were cultured in perfused vessels (cartridges or rotating bioreactors) within recirculating loops designed to maintain medium composition within target ranges by a combination of gas exchange and fresh medium supply. Future spaceflight studies of cells and tissues are likely to involve a three-dimensional culture system, to promote cellular differentiation, and perfusion with or without rotation, to provide a gravity-independent mechanism for fluid mixing and mass transport. Previous spaceflight studies have guided the ongoing development of NASA flight hardware for the ISS (e.g. the EDU-2 and the CCU). This next generation of hardware will have extended operational capabilities including on-line microscopy, in-line sensors for the monitoring and control of metabolic parameters, modular design for replicate cultures, and, perhaps most importantly of all, compatibility with the ISS centrifuge. The latter will permit in-flight, 1 g control cultures, and thereby allow the experimental variable to be gravity itself rather than the more general "spaceflight environment". Technical limitations of spaceflight studies (e.g. allowable size, mass, and power) continue to motivate a creative approach to system design and to result in "spin-off" technologies (e.g. the STLV) for ground-based cell and tissue culture research. The increasing scientific and medical relevance of this work is evidenced by the growing number of publications in which advanced bioreactors are used for in vitro studies in physiologically relevant cell and tissue models.

Monitoring local cell viability in engineered tissues: a fast, quantitative, and nondestructive approach

Assessment of cell viability is a key issue in monitoring in vitro engineered tissue constructs. In this study we describe a fully automated, quantitative, and nondestructive approach, which is particularly suitable for tissue engineering. The approach offers several advantages above existing methods. Living and dead cell numbers can be separately determined for both isolated cells and cells that form networks during tissue formation. Moreover, viability can be locally monitored in time throughout the three-dimensional tissue. The viability assay is based on a dual fluorescent staining technique using CellTracker Green (CTG) for detection of living cells and propidium iodide (PI) for dead cells. CTG and PI images are created with a confocal laser scanning microscope. To determine the number of living cells, CTG fluorescence intensity is determined from the CTG image. Thereby, novel image-processing techniques have been developed, normalizing for various undesired influences that alter measurements of absolute CTG fluorescence intensities. Dead cell numbers are determined from the PI image, using an improved computerized counting method. The approach was first evaluated on C2C12 monolayers, of which images were taken directly after probe addition and 24 h later. Results show that at both times, computed living and dead cell numbers highly correlate with manually counted cell numbers (r > 0.996). Next, the approach was applied for monitoring viability in three-dimensional engineered skeletal muscle tissue constructs, which were subjected to unfavorable environmental conditions. This example illustrated that local viability can be quantitatively, nondestructively, and locally monitored in three-dimensional tissue constructs, making it a promising tool in the field of tissue engineering.

In vitro edible muscle protein production system (MPPS): stage 1, fish

The working efficiency and state-of-mind of a Space vehicle crew on long-term missions is dependent on the suitability of living conditions including food. Our purpose was to establish the feasibility of an in vitro muscle protein production system (MPPS) for the fabrication of surrogate muscle protein constructs as food products for Space travelers. In the experimental treatments, we cultivated the adult dorsal abdominal skeletal muscle mass of Carassius (Gold fish). An ATCC fish fibroblast cell line was used for tissue engineering investigations. No antibiotics were used during any phase of the research. Our four treatments produced these results: a low contamination rate, self-healing, cell proliferation, a tissue engineered construct of non-homologous co-cultured cells with explants, an increase in tissue size in homologous co-cultures of explants with crude cell mixtures, maintenance of explants in media containing fetal bovine serum substitutes, and harvested explants which resembled fresh fish filets. We feel that not only have we pointed the way to an innovative, viable means of supplying safe, healthy, nutritious food to Space voyagers on long journeys, but our research also points the way to means of alleviating food supply and safety problems in both the public and private sectors worldwide.

A Finite Element Model of Left Ventricular Cellular Transplantation in Dilated Cardiomyopathy

Surgical therapies for heart failure that reduce left ventricular (LV) size have failed to improve LV function. Recent reports describe the direct injection of myocytes into the LV wall and suggest that myocyte transplantation improves regional contractile ability and improves LV function. Using a previously described finite element model, we simulated myocyte transplantation in the failing LV and tested the hypothesis that myocyte transplantation improves LV function (Starling relationship). An elastance model for active fiber stress was incorporated in an axisymmetric geometric model of the dilated, poorly contractile LV (dilated cardiomyopathy [DCM]). The nonlinear stress-strain relationship for the diastolic myocardium was anisotropic with respect to the local muscle fiber direction. Systolic material properties were depressed by assigning a peak intracellular calcium concentration (Ca2+) of 1.8 micromoL (normal value: 4.2 micromoL). Six different simulations of myocyte transplantation were performed in which transplanted areas were assigned a peak intracellular calcium concentration (Ca2+) of 4.2 micromoL. The pattern of myocyte engraftment was varied (transmural versus subepicardial; confluent versus heterogeneous), as was the amount of the LV free wall that was transplanted (17% vs 33%). Models were created and loaded with a range of physiologic LV pressures. Simulated myocyte transplantation increased the slope of the end-systolic elastance curve, improved the Starling relationship, and improved stroke volume and ejection fraction compared with DCM. This study demonstrated an improvement in LV function after myocyte transplantation.

Salt fusion: an approach to improve pore interconnectivity within tissue engineering scaffolds

Macroporous scaffolds composed of biodegradable polymers have found extensive use as three-dimensional substrates either for in vitro cell seeding followed by transplantation, or as conductive substrates for direct implantation in vivo. Methods abound for creation of macroporous scaffolds for tissue engineering, and common methods typically employ a solid porogen within a three-dimensional polymer matrix to create a well-defined pore size, pore structure, and total scaffold porosity. This study describes an approach to impart improved pore interconnectivity to polymer scaffolds for tissue engineering by partially fusing the solid porogen together prior to creation of a continuous polymer matrix. Three dimensional, porous scaffolds of the copolymer 85:15 poly(lactide-co-glycolide) were fabricated via either a solvent casting/particulate leaching process, or a gas foaming/particulate leaching process. Prior to creation of a continuous polymer matrix the NaCl crystals, which serve as the solid porogen, are partially fused via treatment in 95% humidity. Scanning electron micrographs clearly display fused salt crystals and an enhancement in pore interconnectivity in the salt fused scaffolds prepared via both solvent casting and gas foaming, and the extent of pore interconnectivity is enhanced with longer treatment times. Fusion of salt crystal for 24 h increased the radius of curvature of salt crystals, and led to a twofold increase in the compressive modulus of solvent cast scaffolds (total porosity of 97 +/- 1%). Fusion of NaCl crystals prior to gas foaming resulted in a decrease in scaffold compressive modulus from 277 +/- 60k Pa to 187 +/- 30k Pa (total porosity of 94 +/- 1%). The resulting highly interconnected scaffolds have implications for facilitated cell migration, abundant cell-cell interaction, and potentially improved neural and vascular growth within tissue engineering scaffolds.

Recombinant vascular endothelial growth factor secreted from tissue-engineered bioartificial muscles promotes localized angiogenesis

BACKGROUND

Therapeutic angiogenesis by the administration of recombinant vascular endothelial growth factor (rVEGF) is a novel strategy for the treatment of ischemic disorders. rVEGF has been delivered as a protein, by plasmid DNA, and by genetically engineered cells with different pharmacokinetic and physiological properties. In the present study, we examined a new method for delivery of rVEGF using implantable bioartificial muscle (BAM) tissues made from genetically modified primary skeletal myoblasts. Our goal was to determine whether the rVEGF delivered by this technique promoted controlled angiogenesis in nonischemic and/or ischemic adult mouse tissue.

METHODS AND RESULTS

Primary adult mouse myoblasts were retrovirally transduced to secrete human or mouse rVEGF and tissue-engineered into implantable 1x10 to 15-mm BAMs containing parallel arrays of postmitotic myofibers. In vitro, they secreted 290 to 511 ng of bioactive mouse or human VEGF/BAM per day. rVEGF BAMs implanted subcutaneously into syngeneic mice caused a 30-fold increase in the number of CD31-positive capillary cells within the BAM by 1 week compared with control BAMs. Implantation of rVEGF-secreting BAMs into ischemic hindlimbs resulted in a 2- to 3-fold increase in capillary density of neighboring host muscle by 1 week and out to 4 weeks with no evidence of hemangioma formation.

CONCLUSIONS

Local delivery of rVEGF from BAMs rapidly increases capillary density both within the BAM itself and in adjacent ischemic muscle tissue. Genetically engineered muscle tissue provides a method for therapeutic protein delivery in a dose-regulated fashion.

Identification of a novel stretch-responsive skeletal muscle gene (Smpx)

Skeletal muscle is able to respond to a range of stimuli, including stretch and increased load, by increasing in diameter and length in the absence of myofiber division. This type of cellular growth (hypertrophy) is a highly complex process involving division of muscle precursor cells (myoblasts) and their fusion to existing muscle fibers as well as increased protein synthesis and decreased protein degradation. Underlying the alterations in protein levels are increases in a range of specific mRNAs including those coding for structural proteins and proteins that regulate the hypertrophic process. Seven days of passive stretch in vivo of tibialis anterior (TA) muscle has been shown to elicit muscle hypertrophy. We have identified a cDNA corresponding to an mRNA that exhibits increased expression in response to 7 days of passive stretch imposed on TA muscles in vivo. This 944-bp novel murine transcript is expressed primarily in cardiac and skeletal muscle and to a lesser extent in brain. Translation of the transcript revealed an open reading frame of 85 amino acids encoding a nuclear localization signal and two overlapping casein kinase II phosphorylation sites. This gene has been called "small muscle protein (X chromosome)" (Smpx; HGMW-approved human gene symbol SMPX) and we hypothesize that it plays a role in skeletal muscle hypertrophy.

Excitability and contractility of skeletal muscle engineered from primary cultures and cell lines

The purpose of this study was to compare the excitability and contractility of three-dimensional skeletal muscle constructs, termed myooids, engineered from C2C12 myoblast and 10T1/2 fibroblast cell lines, primary muscle cultures from adult C3H mice, and neonatal and adult Sprague-Dawley rats. Myooids were 12 mm long, with diameters of 0.1-1 mm, were excitable by transverse electrical stimulation, and contracted to produce force. After approximately 30 days in culture, myooid cross-sectional area, rheobase, chronaxie, resting baseline force, twitch force, time to peak tension, one-half relaxation time, and peak isometric force were measured. Specific force was calculated by dividing peak isometric force by cross-sectional area. The specific force generated by the myooids was 2-8% of that generated by skeletal muscles of control adult rodents. Myooids engineered from C2C12-10T1/2 cells exhibited greater rheobase, time to peak tension, and one-half relaxation time than myooids engineered from adult rodent cultures, and myooids from C2C12-10T1/2 and neonatal rat cells had greater resting baseline forces than myooids from adult rodent cultures.

Identification of Ankrd2, a novel skeletal muscle gene coding for a stretch-responsive ankyrin-repeat protein

Mechanically induced hypertrophy of skeletal muscles involves shifts in gene expression leading to increases in the synthesis of specific proteins. Full characterization of the regulation of muscle hypertrophy is a prerequisite for the development of novel therapies aimed at treating muscle wasting (atrophy) in human aging and disease. Using suppression subtractive hybridization, cDNAs corresponding to mRNAs that increase in relative abundance in response to mechanical stretch of mouse skeletal muscles in vivo were identified. A novel 1100-bp transcript was detected exclusively in skeletal muscle. This exhibited a fourfold increase in expression after 7 days of stretch. The transcript had an open reading frame of 328 amino acids encoding an ATP/GTP binding domain, a nuclear localization signal, two PEST protein-destabilization motifs, and a 132-amino-acid ankyrin-repeat region. We have named this gene ankyrin-repeat domain 2 (stretch-responsive muscle) (Ankrd2). We hypothesize that Ankrd2 plays an important role in skeletal muscle hypertrophy.

Excitability and isometric contractile properties of mammalian skeletal muscle constructs engineered in vitro

Our purpose was to engineer three-dimensional skeletal muscle tissue constructs from primary cultures of adult rat myogenic precursor cells, and to measure their excitability and isometric contractile properties. The constructs, termed myooids, were muscle-like in appearance, excitability, and contractile function. The myooids were 12 mm long and ranged in diameter from 0.1 to 1 mm. The myooids were engineered with synthetic tendons at each end to permit the measurement of isometric contractile properties. Within each myooid the myotubes and fibroblasts were supported by an extracellular matrix generated by the cells themselves, and did not require a preexisting scaffold to define the size, shape, and general mechanical properties of the resulting structure. Once formed, the myooids contracted spontaneously at approximately 1 Hz, with peak-to-peak force amplitudes ranging from 3 to 30 microN. When stimulated electrically the myooids contracted to produce force. The myooids (n = 14) had the following mean values: diameter of 0.49 mm, rheobase of 1.0 V/mm, chronaxie of 0.45 ms, twitch force of 215 microN, maximum isometric force of 440 microN, resting baseline force of 181 microN, and specific force of 2.9 kN/m2. The mean specific force was approximately 1% of the specific force generated by control adult rat muscle. Based on the functional data, the myotubes in the myooids appear to remain arrested in an early developmental state due to the absence of signals to promote expression of adult myosin isoforms.