Three-dimensional neuron-muscle constructs with neuromuscular junctions

Dynamic and Static Workout of In Vitro Skeletal Muscle Tissue through a Weight Training Device

Enhancing muscle strength through workouts is a key factor in improving physical activity and maintaining metabolic profiles. The controversial results concerning the impacts of weight training often arise from clinical experiments that require controlled experimental conditions. In this study, a weight training system for a muscle development model is presented, which is capable of performing weight training motions with adjustable weight loads. Through the implementation of cultured skeletal muscle tissue with floating structures and a flexible ribbon, both isotonic (dynamic change in muscle length) and isometric (static in muscle length) exercises can be performed without the deflection of the tissue. Quantitative analysis of contraction force, changes in metabolic processes, and muscle morphology under different weight training conditions demonstrates the effectiveness of the proposed system. Our proposed system holds potential for establishing effective muscle development and for further applications in rehabilitation training methods.

Motor neurons and endothelial cells additively promote development and fusion of human iPSC-derived skeletal myocytes

BACKGROUND

Neurovascular cells have wide-ranging implications on skeletal muscle biology regulating myogenesis, maturation, and regeneration. Although several in vitro studies have investigated how motor neurons and endothelial cells interact with skeletal myocytes independently, there is limited knowledge about the combined effect of neural and vascular cells on muscle maturation and development.

METHODS

Here, we report a triculture system comprising human-induced pluripotent stem cell (iPSC)-derived skeletal myocytes, human iPSC-derived motor neurons, and primary human endothelial cells maintained under controlled media conditions. Briefly, iPSCs were differentiated to generate skeletal muscle progenitor cells (SMPCs). These SMPCs were seeded at a density of 5 × 104 cells/well in 12-well plates and allowed to differentiate for 7 days before adding iPSC-derived motor neurons at a concentration of 0.5 × 104 cells/well. The neuromuscular coculture was maintained for another 7 days in coculture media before addition of primary human umbilical vein endothelial cells (HUVEC) also at 0.5 × 104 cells/well. The triculture was maintained for another 7 days in triculture media comprising equal portions of muscle differentiation media, coculture media, and vascular media. Extensive morphological, genetic, and molecular characterization was performed to understand the combined and individual effects of neural and vascular cells on skeletal muscle maturation.

RESULTS

We observed that motor neurons independently promoted myofiber fusion, upregulated neuromuscular junction genes, and maintained a molecular niche supportive of muscle maturation. Endothelial cells independently did not support myofiber fusion and downregulated expression of LRP4 but did promote expression of type II specific myosin isoforms. However, neurovascular cells in combination exhibited additive increases in myofiber fusion and length, enhanced production of Agrin, along with upregulation of several key genes like MUSK, RAPSYN, DOK-7, and SLC2A4. Interestingly, more divergent effects were observed in expression of genes like MYH8, MYH1, MYH2, MYH4, and LRP4 and secretion of key molecular factors like amphiregulin and IGFBP-4.

CONCLUSIONS

Neurovascular cells when cultured in combination with skeletal myocytes promoted myocyte fusion with concomitant increase in expression of various neuromuscular genes. This triculture system may be used to gain a deeper understanding of the effects of the neurovascular niche on skeletal muscle biology and pathophysiology.

Scalable, optically-responsive human neuromuscular junction model reveals convergent mechanisms of synaptic dysfunction in familial ALS

Neuromuscular junctions (NMJs) are specialized synapses that mediate communication between motor neurons and skeletal muscles and are essential for movement. The degeneration of this system can lead to symptoms observed in neuromuscular and motor neuron diseases. Studying these synapses and their degeneration has proven challenging. Prior NMJ studies heavily relied upon the use of mouse, chick, or isolated primary human cells, which have demonstrated limited fidelity for disease modeling. To enable the study of NMJ dysfunction and model genetic diseases, we, and others, have developed methods to generate human NMJs from pluripotent stem cells (PSCs), embryonic stem cells, and induced pluripotent stem cells. However, published studies have highlighted technical limitations associated with these complex in vitro NMJ models. In this study, we developed a robust PSC-derived motor neuron and skeletal muscle co-culture method, and demonstrated its sensitivity in modeling motor neuron disease. Our method spontaneously and reproducibly forms human NMJs. We developed multiwell-multielectrode array (MEA) parameters to quantify the activity of PSC-derived skeletal muscles, as well as measured the electrophysiological activity of functional human PSC-derived NMJs. We further leveraged our method to morphologically and functionally assess NMJs from the familial amyotrophic lateral sclerosis (fALS) PSCs, C9orf72 hexanucleotide (G4C2)n repeat expansion (HRE), SOD1 A5V , and TDP43 G298S to define the reproducibility and sensitivity of our system. We observed a significant decrease in the numbers and activity of PSC-derived NMJs developed from the different ALS lines compared to their respective controls. Furthermore, we evaluated a therapeutic candidate undergoing clinical trials and observed a variant-dependent rescue of functionality of NMJs. Our newly developed method provides a platform for the systematic investigation of genetic causes of NMJ neurodegeneration and highlights the need for therapeutic avenues to consider patient genotype.

Human Motor System-Based Biohybrid Robot-On-a-Chip for Drug Evaluation of Neurodegenerative Disease

Biohybrid robots have been developed for biomedical applications and industrial robotics. However, the biohybrid robots have limitations to be applied in neurodegenerative disease research due to the absence of a central nervous system. In addition, the organoids-on-a-chip has not yet been able to replicate the physiological function of muscle movement in the human motor system, which is essential for evaluating the accuracy of the drugs used for treating neurodegenerative diseases. Here, a human motor system-based biohybrid robot-on-a-chip composed of a brain organoid, multi-motor neuron spheroids, and muscle bundle on solid substrateis proposed to evaluate the drug effect on neurodegenerative diseases for the first time. The electrophysiological signals from the cerebral organoid induced the muscle bundle movement through motor neuron spheroids. To evaluate the drug effect on Parkinson's disease (PD), a patient-derived midbrain organoid is generated and incorporated into a biohybrid robot-on-a-chip. The drug effect on PD is successfully evaluated by measuring muscle bundle movement. The muscle bundle movement of PD patient-derived midbrain organoid-based biohybrid robot-on-a-chip is increased from 4.5 ± 0.99 µm to 18.67 ± 2.25 µm in response to levodopa. The proposed human motor system-based biohybrid robot-on-a-chip can serve as a standard biohybrid robot model for drug evaluation.

Advances in In Vitro Models of Neuromuscular Junction: Focusing on Organ-on-a-Chip, Organoids, and Biohybrid Robotics

The neuromuscular junction (NMJ) is a peripheral synaptic connection between presynaptic motor neurons and postsynaptic skeletal muscle fibers that enables muscle contraction and voluntary motor movement. Many traumatic, neurodegenerative, and neuroimmunological diseases are classically believed to mainly affect either the neuronal or the muscle side of the NMJ, and treatment options are lacking. Recent advances in novel techniques have helped develop in vitro physiological and pathophysiological models of the NMJ as well as enable precise control and evaluation of its functions. This paper reviews the recent developments in in vitro NMJ models with 2D or 3D cultures, from organ-on-a-chip and organoids to biohybrid robotics. Related derivative techniques are introduced for functional analysis of the NMJ, such as the patch-clamp technique, microelectrode arrays, calcium imaging, and stimulus methods, particularly optogenetic-mediated light stimulation, microelectrode-mediated electrical stimulation, and biochemical stimulation. Finally, the applications of the in vitro NMJ models as disease models or for drug screening related to suitable neuromuscular diseases are summarized and their future development trends and challenges are discussed.

Organ-on-Chip: Advancing Nutraceutical Testing for Improved Health Outcomes

The recent global wave of organic food consumption and the vitality of nutraceuticals for human health benefits has driven the need for applying scientific methods for phytochemical testing. Advanced in vitro models with greater physiological relevance than conventional in vitro models are required to evaluate the potential benefits and toxicity of nutraceuticals. Organ-on-chip (OOC) models have emerged as a promising alternative to traditional in vitro models and animal testing due to their ability to mimic organ pathophysiology. Numerous studies have demonstrated the effectiveness of OOC models in identifying pharmaceutically relevant compounds and accurately assessing compound-induced toxicity. This review examines the utility of traditional in vitro nutraceutical testing models and discusses the potential of OOC technology as a preclinical testing tool to examine the biomedical potential of nutraceuticals by reducing the need for animal testing. Exploring the capabilities of OOC models in carrying out plant-based bioactive compounds can significantly contribute to the authentication of nutraceuticals and drug discovery and validate phytochemicals medicinal characteristics. Overall, OOC models can facilitate a more systematic and efficient assessment of nutraceutical compounds while overcoming the limitations of current traditional in vitro models.

Biohybrid Soft Robots Powered by Myocyte: Current Progress and Future Perspectives

Myocyte-driven robots, a type of biological actuator that combines myocytes with abiotic systems, have gained significant attention due to their high energy efficiency, sensitivity, biocompatibility, and self-healing capabilities. These robots have a unique advantage in simulating the structure and function of human tissues and organs. This review covers the research progress in this field, detailing the benefits of myocyte-driven robots over traditional methods, the materials used in their fabrication (including myocytes and extracellular materials), and their properties and manufacturing techniques. Additionally, the review explores various control methods, robot structures, and motion types. Lastly, the potential applications and key challenges faced by myocyte-driven robots are discussed and summarized.

Building blocks of microphysiological system to model physiology and pathophysiology of human heart

Microphysiological systems (MPS) are drawing increasing interest from academia and from biomedical industry due to their improved capability to capture human physiology. MPS offer an advanced in vitro platform that can be used to study human organ and tissue level functions in health and in diseased states more accurately than traditional single cell cultures or even animal models. Key features in MPS include microenvironmental control and monitoring as well as high biological complexity of the target tissue. To reach these qualities, cross-disciplinary collaboration from multiple fields of science is required to build MPS. Here, we review different areas of expertise and describe essential building blocks of heart MPS including relevant cardiac cell types, supporting matrix, mechanical stimulation, functional measurements, and computational modelling. The review presents current methods in cardiac MPS and provides insights for future MPS development with improved recapitulation of human physiology.

3D human induced pluripotent stem cell-derived bioengineered skeletal muscles for tissue, disease and therapy modeling

Skeletal muscle is a complex tissue composed of multinucleated myofibers responsible for force generation that are supported by multiple cell types. Many severe and lethal disorders affect skeletal muscle; therefore, engineering models to reproduce such cellular complexity and function are instrumental for investigating muscle pathophysiology and developing therapies. Here, we detail the modular 3D bioengineering of multilineage skeletal muscles from human induced pluripotent stem cells, which are first differentiated into myogenic, neural and vascular progenitor cells and then combined within 3D hydrogels under tension to generate an aligned myofiber scaffold containing vascular networks and motor neurons. 3D bioengineered muscles recapitulate morphological and functional features of human skeletal muscle, including establishment of a pool of cells expressing muscle stem cell markers. Importantly, bioengineered muscles provide a high-fidelity platform to study muscle pathology, such as emergence of dysmorphic nuclei in muscular dystrophies caused by mutant lamins. The protocol is easy to follow for operators with cell culture experience and takes between 9 and 30 d, depending on the number of cell lineages in the construct. We also provide examples of applications of this advanced platform for testing gene and cell therapies in vitro, as well as for in vivo studies, providing proof of principle of its potential as a tool to develop next-generation neuromuscular or musculoskeletal therapies.

In Vitro 3D Modeling of Neurodegenerative Diseases

The study of neurodegenerative diseases (such as Alzheimer's disease, Parkinson's disease, Huntington's disease, or amyotrophic lateral sclerosis) is very complex due to the difficulty in investigating the cellular dynamics within nervous tissue. Despite numerous advances in the in vivo study of these diseases, the use of in vitro analyses is proving to be a valuable tool to better understand the mechanisms implicated in these diseases. Although neural cells remain difficult to obtain from patient tissues, access to induced multipotent stem cell production now makes it possible to generate virtually all neural cells involved in these diseases (from neurons to glial cells). Many original 3D culture model approaches are currently being developed (using these different cell types together) to closely mimic degenerative nervous tissue environments. The aim of these approaches is to allow an interaction between glial cells and neurons, which reproduces pathophysiological reality by co-culturing them in structures that recapitulate embryonic development or facilitate axonal migration, local molecule exchange, and myelination (to name a few). This review details the advantages and disadvantages of techniques using scaffolds, spheroids, organoids, 3D bioprinting, microfluidic systems, and organ-on-a-chip strategies to model neurodegenerative diseases.

Optimization of Application-Driven Development of In Vitro Neuromuscular Junction Models

Neuromuscular junctions (NMJs) are specialized synapses responsible for signal transduction between motor neurons (MNs) and skeletal muscle tissue. Malfunction at this site can result from developmental disorders, toxic environmental exposures, and neurodegenerative diseases leading to severe neurological dysfunction. Exploring these conditions in human or animal subjects is restricted by ethical concerns and confounding environmental factors. Therefore, in vitro NMJ models provide exciting opportunities for advancements in tissue engineering. In the last two decades, multiple NMJ prototypes and platforms have been reported, and each model system design is strongly tied to a specific application: exploring developmental physiology, disease modeling, or high-throughput screening. Directing the differentiation of stem cells into mature MNs and/or skeletal muscle for NMJ modeling has provided critical cues to recapitulate early-stage development. Patient-derived inducible pluripotent stem cells provide a personalized approach to investigating NMJ disease, especially when disease etiology cannot be resolved down to a specific gene mutation. Having reproducible NMJ culture replicates is useful for high-throughput screening to evaluate drug toxicity and determine the impact of environmental threat exposures. Cutting-edge bioengineering techniques have propelled this field forward with innovative microfabrication and design approaches allowing both two-dimensional and three-dimensional NMJ culture models. Many of these NMJ systems require further validation for broader application by regulatory agencies, pharmaceutical companies, and the general research community. In this summary, we present a comprehensive review on the current state-of-art research in NMJ models and discuss their ability to provide valuable insight into cell and tissue interactions. Impact statement In vitro neuromuscular junction (NMJ) models reveal the specialized mechanisms of communication between neurons and muscle tissue. This site can be disrupted by developmental disorders, toxic environmental exposures, or neurodegenerative diseases, which often lead to fatal outcomes and is therefore of critical importance to the medical community. Many bioengineering approaches for in vitro NMJ modeling have been designed to mimic development and disease; other approaches include in vitro NMJ models for high-throughput toxicology screening, providing a platform to limit or replace animal testing. This review describes various NMJ applications and the bioengineering advancements allowing for human NMJ characteristics to be more accurately recapitulated. While the extensive range of NMJ device structures has hindered standardization attempts, there is still a need to harmonize these devices for broader application and to continue advancing the field of NMJ modeling.

Development of High-Cell-Density Tissue Method for Compressed Modular Bioactuator

Bioactuators have been developed in many studies in the recent decade for actuators of micro-biorobots. However, bioactuators have not shown the same power as animal muscles. Centrifugal force was used in this study to increase the cell density of cultured muscle cells that make up the bioactuator. The effect of the centrifugal force on cells in the matrix gel before curing was investigated, and the optimal centrifugal force was identified to be around 450× g. The compressed modular bioactuator (C-MBA) fabricated in this study exhibited 1.71 times higher cell density than the conventional method. In addition, the contractile force per unit cross-sectional area was 1.88 times higher. The proposed method will contribute to new bioactuators with the same power as living muscles in animals.

Wirelessly Powered 3D Printed Hierarchical Biohybrid Robots with Multiscale Mechanical Properties

The integration of flexible and stretchable electronics into biohybrid soft robotics can spur the development of new approaches to fabricate biohybrid soft machines, thus enabling a wide variety of innovative applications. Inspired by flexible and stretchable wireless-based bioelectronic devices, we have developed untethered biohybrid soft robots that can execute swimming motions, which are remotely controllable by the wireless transmission of electrical power into a cell simulator. To this end, wirelessly-powered, stretchable, and lightweight cell stimulators were designed to be integrated into muscle bodies without impeding the robots' underwater swimming abilities. The cell stimulators function by generating controlled monophasic pulses of up to ∼9 V in biological environments. By differentiating induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) directly on the cell stimulators using an accordion-inspired, three-dimensional (3D) printing construct, we have replicated the native myofiber architecture with comparable robustness and enhanced contractibility. Wirelessly modulated electrical frequencies enabled us to control the speed and direction of the biohybrid soft robots. A maximum locomotion speed of ∼580 μm/s was achieved in robots possessing a large body size by adjusting the pacing frequency. This innovative approach will provide a platform for building untethered and biohybrid systems for various biomedical applications.

Bio-actuated microvalve in microfluidics using sensing and actuating function of Mimosa pudica

Bio-actuators and sensors are increasingly employed in microscale devices for numerous applications. Unlike other artificial devices actuated by living cells or tissues, here we introduce a microvalve system actuated by the stimuli-responsive action plant, Mimosa pudica (sleepy plant). This system realizes the control of the valve to open and close by dropping and recovering responses of Mimosa pudica branch upon external physical stimulations. The results showed that one matured single uncut Mimosa pudica branch produced average force of 15.82 ± 0.7 mN. This force was sufficient for actuating and keeping the valve open for 8.46 ± 1.33 min in a stimulation-recovering cycle of 30 min. Additionally, two separately cut Mimosa pudica branches were able to keep the valve open for 2.28 ± 0.63 min in a stimulating-recovering cycle of 20min. The pressure resistance and the response time of the valve were 4.2 kPa and 1.4 s, respectively. This demonstration of plant-microfluidics integration encourages exploiting more applications of microfluidic platforms that involve plant science and plant energy harvesting.

Validation of Functional Connectivity of Engineered Neuromuscular Junction With Recombinant Monosynaptic Pseudotyped ΔG-Rabies Virus Tracing

Current preclinical models of neurodegenerative disease, such as amyotrophic lateral sclerosis (ALS), can significantly benefit from in vitro neuroengineering approaches that enable the selective study and manipulation of neurons, networks, and functional units of interest. Custom-designed compartmentalized microfluidic culture systems enable the co-culture of different relevant cell types in interconnected but fluidically isolated microenvironments. Such systems can thus be applied for ALS disease modeling, as they enable the recapitulation and study of neuromuscular junctions (NMJ) through co-culturing of motor neurons and muscle cells in separate, but interconnected compartments. These in vitro systems are particularly relevant for investigations of mechanistic aspects of the ALS pathological cascade in engineered NMJ, as progressive loss of NMJ functionality may constitute one of the hallmarks of disease related pathology at early onset, in line with the dying back hypothesis. In such models, ability to test whether motor neuron degeneration in ALS starts at the nerve terminal or at the NMJ and retrogradely progresses to the motor neuron cell body largely relies on robust methods for verification of engineered NMJ functionality. In this study, we demonstrate the functionality of engineered NMJs within a microfluidic chip with a differentially perturbable microenvironment using a designer pseudotyped ΔG-rabies virus for retrograde monosynaptic tracing.

Biobased Elastomer Nanofibers Guide Light-Controlled Human-iPSC-Derived Skeletal Myofibers

Generating skeletal muscle tissue that mimics the cellular alignment, maturation, and function of native skeletal muscle is an ongoing challenge in disease modeling and regenerative therapies. Skeletal muscle cultures require extracellular guidance and mechanical support to stabilize contractile myofibers. Existing microfabrication-based solutions are limited by complex fabrication steps, low throughput, and challenges in measuring dynamic contractile function. Here, the synthesis and characterization of a new biobased nanohybrid elastomer, which is electrospun into aligned nanofiber sheets to mimic the skeletal muscle extracellular matrix, is presented. The polymer exhibits remarkable hyperelasticity well-matched to that of native skeletal muscle (≈11-50 kPa), with ultimate strain ≈1000%, and elastic modulus ≈25 kPa. Uniaxially aligned nanofibers guide myoblast alignment, enhance sarcomere formation, and promote a ≈32% increase in myotube fusion and ≈50% increase in myofiber maturation. The elastomer nanofibers stabilize optogenetically controlled human induced pluripotent stem cell derived skeletal myofibers. When activated by blue light, the myofiber-nanofiber hybrid constructs maintain a significantly higher (>200%) contraction velocity and specific force (>280%) compared to conventional culture methods. The engineered myofibers exhibit a power density of ≈35 W m-3 . This system is a promising new skeletal muscle tissue model for applications in muscular disease modeling, drug discovery, and muscle regeneration.

Biohybrid Soft Robots Driven by Contractions of Skeletal Muscle Tissue

In this letter, we introduce biohybrid robots powered by skeletal muscle tissue. Culturing myoblast-laden extracellular matrix structures enables the construction of skeletal muscle tissue in vitro. Biohybrid robots constructed by the integration of such fabricated muscle tissue with robot skeletons have achieved various movements, according to the configuration of the skeleton. We believe that biohybrid robots will increasingly become available in the field of robotics.

Lab-on-Chip Microsystems for Ex Vivo Network of Neurons Studies: A Review

Increasing population is suffering from neurological disorders nowadays, with no effective therapy available to treat them. Explicit knowledge of network of neurons (NoN) in the human brain is key to understanding the pathology of neurological diseases. Research in NoN developed slower than expected due to the complexity of the human brain and the ethical considerations for in vivo studies. However, advances in nanomaterials and micro-/nano-microfabrication have opened up the chances for a deeper understanding of NoN ex vivo, one step closer to in vivo studies. This review therefore summarizes the latest advances in lab-on-chip microsystems for ex vivo NoN studies by focusing on the advanced materials, techniques, and models for ex vivo NoN studies. The essential methods for constructing lab-on-chip models are microfluidics and microelectrode arrays. Through combination with functional biomaterials and biocompatible materials, the microfluidics and microelectrode arrays enable the development of various models for ex vivo NoN studies. This review also includes the state-of-the-art brain slide and organoid-on-chip models. The end of this review discusses the previous issues and future perspectives for NoN studies.

Hydrogel-Based Fiber Biofabrication Techniques for Skeletal Muscle Tissue Engineering

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

Current Progress in the Creation, Characterization, and Application of Human Stem Cell-derived in Vitro Neuromuscular Junction Models

Human pluripotent stem cells (PSCs) such as embryonic stem cells (ESCs) or induced pluripotent stem cells (iPSCs) are of great value for studying developmental processes, disease modeling, and drug testing. One area in which the use of human PSCs has become of great interest in recent years is for in vitro models of the neuromuscular junction (NMJ). The NMJ is a synapse at which a motor neuron releases acetylcholine to bind to skeletal muscle and stimulate contraction. Degeneration of the NMJ and subsequent loss of muscle function is a common feature of many neuromuscular diseases such as myasthenia gravis, spinal muscular atrophy, and amyotrophic lateral sclerosis. In order to develop new therapies for patients with neuromuscular diseases, it is essential to understand mechanisms taking place at the NMJ. However, we have limited ability to study the NMJ in living human patients, and animal models are limited by physiological relevance. Therefore, an in vitro model of the NMJ consisting of human cells is of great value. The use of stem cells for in vitro NMJ models is still in progress and requires further optimization in order to yield reliable, reproducible results. The objective of this review is (1) to outline the current progress towards fully PSC-derived in vitro co-culture models of the human NMJ and (2) to discuss future directions and challenges that must be overcome in order to create reproducible fully PSC-derived models that can be used for developmental studies, disease modeling, and drug testing.

Contraction Control of Aligned Myofiber Sheet Tissue by Parallel Oriented iPS Cell-derived Neurons

Fabrication and application of engineered complex tissues composed of different types of cells is a crucial milestone in the next phase of tissue engineering. The delicate organization structure of each tissue component and their physiological connections enable all the functions in the human body. In this study, cell sheet-based engineering allowed us to fabricate a complex myofiber sheet tissue using motor neurons derived from human iPS cells. In contrast with previous studies of other groups, a myofiber sheet with a biomimetic aligned structure was produced from human myoblasts using a striped-patterned thermoresponsive dish, which enabled manipulation of the sheet tissue by simply lowering the culture temperature. The myofiber sheet was transferred onto a gel that promotes functional maturation of human myofibers, resulting in production of contractile human muscle tissue. Just by seeding motor neurons onto the sheet tissue, all the neurons physically contacted to the aligned myofibers, and autonomously elongated in parallel to the myofiber orientation. In addition, the neurite outgrowth was enlarged by co-culturing on the myofiber sheet. The presence of the neurons enhanced clustering of myofiber acetylcholine receptors (AChR), typically found at the neuromuscular junctions (NMJs). Consequently, contraction behaviors of the myofiber sheet were regulated by neuronal signal transduction through NMJs. Muscle contraction was induced when the motor neurons were stimulated by glutamic acid, and effectively blocked by administration of d-tubocurarine as an antagonistic inhibitor for the AChR. The fibrin-based gel was useful as a culture environment for tissue maturation and as a favorable substrate for unobstructed contractions. Our neuron-muscle sheet tissue will be scalable by simply enlarging the micropatterned substrate and manipulable three-dimensionally; fabrication of a thick tissue and a bundle-like structured tissue will be possible just by layering multiple sheets or rolling up the sheet. Simplified control over self-orientation of neurite elongation will be advantageous for fabrication of such a large and complex tissue. Therefore, our methodology, established in this study, will be instrumental in future applications of regenerative medicine for locomotion apparatus.

Nano-sized graphene oxide coated nanopillars on microgroove polymer arrays that enhance skeletal muscle cell differentiation

The degeneration or loss of skeletal muscles, which can be caused by traumatic injury or disease, impacts most aspects of human activity. Among various techniques reported to regenerate skeletal muscle tissue, controlling the external cellular environment has been proven effective in guiding muscle differentiation. In this study, we report a nano-sized graphene oxide (sGO)-modified nanopillars on microgroove hybrid polymer array (NMPA) that effectively controls skeletal muscle cell differentiation. sGO-coated NMPA (sG-NMPA) were first fabricated by sequential laser interference lithography and microcontact printing methods. To compensate for the low adhesion property of polydimethylsiloxane (PDMS) used in this study, graphene oxide (GO), a proven cytophilic nanomaterial, was further modified. Among various sizes of GO, sGO (< 10 nm) was found to be the most effective not only for coating the surface of the NM structure but also for enhancing the cell adhesion and spreading on the fabricated substrates. Remarkably, owing to the micro-sized line patterns that guide cellular morphology to an elongated shape and because of the presence of sGO-modified nanostructures, mouse myoblast cells (C2C12) were efficiently differentiated into skeletal muscle cells on the hybrid patterns, based on the myosin heavy chain expression levels. Therefore, the developed sGO coated polymeric hybrid pattern arrays can serve as a potential platform for rapid and highly efficient in vitro muscle cell generation.

Neuromuscular Development and Disease: Learning From in vitro and in vivo Models

The neuromuscular junction (NMJ) is a specialized cholinergic synaptic interface between a motor neuron and a skeletal muscle fiber that translates presynaptic electrical impulses into motor function. NMJ formation and maintenance require tightly regulated signaling and cellular communication among motor neurons, myogenic cells, and Schwann cells. Neuromuscular diseases (NMDs) can result in loss of NMJ function and motor input leading to paralysis or even death. Although small animal models have been instrumental in advancing our understanding of the NMJ structure and function, the complexities of studying this multi-tissue system in vivo and poor clinical outcomes of candidate therapies developed in small animal models has driven the need for in vitro models of functional human NMJ to complement animal studies. In this review, we discuss prevailing models of NMDs and highlight the current progress and ongoing challenges in developing human iPSC-derived (hiPSC) 3D cell culture models of functional NMJs. We first review in vivo development of motor neurons, skeletal muscle, Schwann cells, and the NMJ alongside current methods for directing the differentiation of relevant cell types from hiPSCs. We further compare the efficacy of modeling NMDs in animals and human cell culture systems in the context of five NMDs: amyotrophic lateral sclerosis, myasthenia gravis, Duchenne muscular dystrophy, myotonic dystrophy, and Pompe disease. Finally, we discuss further work necessary for hiPSC-derived NMJ models to function as effective personalized NMD platforms.

Skeletal muscle-adipose cocultured tissue fabricated using cell-laden microfibers and a hydrogel sheet

The emerging interest in skeletal muscle tissue originates from its unique properties that control body movements. In particular, recent research advances in engineered skeletal muscle tissue have broadened the possibilities of applications in nonclinical models. However, due to the lack of adipose tissue, current engineered skeletal muscle tissue has the limitation of satisfying in vivo-like position and proportion of intermuscular fat. Adipose tissue within the skeletal muscle affects their functional properties. Here, a fabrication method for cocultured tissue composed of skeletal muscle and adipose tissues is proposed to reproduce the functional and morphological characteristics of muscle. By implementing prematured adipose microfibers in a myoblast-laden hydrogel sheet, both the accumulation of large lipid droplets and control of the position of adipose tissue within the skeletal muscle tissue becomes feasible. The findings of this study provide helpful information regarding engineered skeletal muscle, which has strong potential in drug screening models.

Recent Trends in Biofabrication Technologies for Studying Skeletal Muscle Tissue-Related Diseases

In native skeletal muscle, densely packed myofibers exist in close contact with surrounding motor neurons and blood vessels, which are embedded in the fibrous connective tissue. In comparison to conventional two-dimensional (2D) cultures, the three-dimensional (3D) engineered skeletal muscle models allow structural and mechanical resemblance with native skeletal muscle tissue by providing geometric confinement and physiological matrix stiffness to the cells. In addition, various external stimuli applied to these models enhance muscle maturation along with cell-cell and cell-extracellular matrix interaction. Therefore, 3D in vitro muscle models can adequately recapitulate the pathophysiologic events occurring in tissue-tissue interfaces inside the native skeletal muscle such as neuromuscular junction. Moreover, 3D muscle models can induce pathological phenotype of human muscle dystrophies such as Duchenne muscular dystrophy by incorporating patient-derived induced pluripotent stem cells and human primary cells. In this review, we discuss the current biofabrication technologies for modeling various skeletal muscle tissue-related diseases (i.e., muscle diseases) including muscular dystrophies and inflammatory muscle diseases. In particular, these approaches would enable the discovery of novel phenotypic markers and the mechanism study of human muscle diseases with genetic mutations.

Engineering skeletal muscle tissues with advanced maturity improves synapse formation with human induced pluripotent stem cell-derived motor neurons

To develop effective cures for neuromuscular diseases, human-relevant in vitro models of neuromuscular tissues are critically needed to probe disease mechanisms on a cellular and molecular level. However, previous attempts to co-culture motor neurons and skeletal muscle have resulted in relatively immature neuromuscular junctions (NMJs). In this study, NMJs formed by human induced pluripotent stem cell (hiPSC)-derived motor neurons were improved by optimizing the maturity of the co-cultured muscle tissue. First, muscle tissues engineered from the C2C12 mouse myoblast cell line, cryopreserved primary human myoblasts, and freshly isolated primary chick myoblasts on micromolded gelatin hydrogels were compared. After three weeks, only chick muscle tissues remained stably adhered to hydrogels and exhibited progressive increases in myogenic index and stress generation, approaching values generated by native muscle tissue. After three weeks of co-culture with hiPSC-derived motor neurons, engineered chick muscle tissues formed NMJs with increasing co-localization of pre- and postsynaptic markers as well as increased frequency and magnitude of synaptic activity, surpassing structural and functional maturity of previous in vitro models. Engineered chick muscle tissues also demonstrated increased expression of genes related to sarcomere maturation and innervation over time, revealing new insights into the molecular pathways that likely contribute to enhanced NMJ formation. These approaches for engineering advanced neuromuscular tissues with relatively mature NMJs and interrogating their structure and function have many applications in neuromuscular disease modeling and drug development.

Engineered whole cut meat-like tissue by the assembly of cell fibers using tendon-gel integrated bioprinting

With the current interest in cultured meat, mammalian cell-based meat has mostly been unstructured. There is thus still a high demand for artificial steak-like meat. We demonstrate in vitro construction of engineered steak-like tissue assembled of three types of bovine cell fibers (muscle, fat, and vessel). Because actual meat is an aligned assembly of the fibers connected to the tendon for the actions of contraction and relaxation, tendon-gel integrated bioprinting was developed to construct tendon-like gels. In this study, a total of 72 fibers comprising 42 muscles, 28 adipose tissues, and 2 blood capillaries were constructed by tendon-gel integrated bioprinting and manually assembled to fabricate steak-like meat with a diameter of 5 mm and a length of 10 mm inspired by a meat cut. The developed tendon-gel integrated bioprinting here could be a promising technology for the fabrication of the desired types of steak-like cultured meats.

Creating stem cell-derived neuromuscular junctions in vitro

Recent development of novel therapies has improved mobility and quality of life for people suffering from inheritable neuromuscular disorders. Despite this progress, the majority of neuromuscular disorders are still incurable, in part due to a lack of predictive models of neuromuscular junction (NMJ) breakdown. Improvement of predictive models of a human NMJ would be transformative in terms of expanding our understanding of the mechanisms that underpin development, maintenance, and disease, and as a testbed with which to evaluate novel therapeutics. Induced pluripotent stem cells (iPSCs) are emerging as a clinically relevant and non‐invasive cell source to create human NMJs to study synaptic development and maturation, as well as disease modeling and drug discovery. This review will highlight the recent advances and remaining challenges to generating an NMJ capable of eliciting contraction of stem cell‐derived skeletal muscle in vitro. We explore the advantages and shortcomings of traditional NMJ culturing platforms, as well as the pioneering technologies and novel, biomimetic culturing systems currently in use to guide development and maturation of the neuromuscular synapse and extracellular microenvironment. Then, we will explore how this NMJ‐in‐a‐dish can be used to study normal assembly and function of the efferent portion of the neuromuscular arc, and how neuromuscular disease‐causing mutations disrupt structure, signaling, and function.

Zonisamide upregulates neuregulin-1 expression and enhances acetylcholine receptor clustering at the in vitro neuromuscular junction

Decreased acetylcholine receptor (AChR) clustering compromises signal transmission at the neuromuscular junction (NMJ) in myasthenia gravis, congenital myasthenic syndromes, and motor neuron diseases. Although the enhancement of AChR clustering at the NMJ is a promising therapeutic strategy for these maladies, no drug is currently available for this enhancement. We previously reported that zonisamide (ZNS), an anti-epileptic and anti-Parkinson's disease drug, enhances neurite elongation of the primary spinal motor neurons (SMNs). As nerve sprouting occurs to compensate for the loss of AChR clusters in human diseases, we examined the effects of ZNS on AChR clustering at the NMJ. To this end, we established a simple and quick co-culture system to reproducibly make in vitro NMJs using C2C12 myotubes and NSC34 motor neurons. ZNS at 1-20 μM enhanced the formation of AChR clusters dose-dependently in co-cultured C2C12 myotubes but not in agrin-treated single cultured C2C12 myotubes. We observed that molecules that conferred responsiveness to ZNS were not secreted into the co-culture medium. We found that 10 μM ZNS upregulated the expression of neuregulin-1 (Nrg1) in co-cultured cells but not in single cultured C2C12 myotubes or single cultured NSC34 motor neurons. In accordance with this observation, inhibition of the Nrg1/ErbB signaling pathways nullified the effect of 10 μM ZNS on the enhancement of AChR clustering in in vitro NMJs. Although agrin was not induced by 10 μM ZNS in co-cultured cells, anti-agrin antibody attenuated ZNS-mediated enhancement of AChR clustering. We conclude that ZNS enhances agrin-dependent AChR-clustering by upregulating the Nrg1/ErbB signaling pathways in the presence of NMJs.

Development of a human neuromuscular tissue-on-a-chip model on a 24-well-plate-format compartmentalized microfluidic device

Engineered three-dimensional models of neuromuscular tissues are promising for use in mimicking their disorder states in vitro. Although several models have been developed, it is still challenging to mimic the physically separated structures of motor neurons (MNs) and skeletal muscle (SkM) fibers in the motor units in vivo. In this study, we aimed to develop microdevices for precisely compartmentalized coculturing of MNs and engineered SkM tissues. The developed microdevices, which fit a well of 24 well plates, had a chamber for MNs and chamber for SkM tissues. The two chambers were connected by microtunnels for axons, permissive to axons but not to cell bodies. Human iPSC (hiPSC)-derived MN spheroids in one chamber elongated their axons into microtunnels, which reached the tissue-engineered human SkM in the SkM chamber, and formed functional neuromuscular junctions with the muscle fibers. The cocultured SkM tissues with MNs on the device contracted spontaneously in response to spontaneous firing of MNs. The addition of a neurotransmitter, glutamate, into the MN chamber induced contraction of the cocultured SkM tissues. Selective addition of tetrodotoxin or vecuronium bromide into either chamber induced SkM tissue relaxation, which could be explained by the inhibitory mechanisms. We also demonstrated the application of chemical or mechanical stimuli to the middle of the axons of cocultured tissues on the device. Thus, compartmentalized neuromuscular tissue models fabricated on the device could be used for phenotypic screening to evaluate the cellular type specific efficacy of drug candidates and would be a useful tool in fundamental research and drug development for neuromuscular disorders.

Development of Cultured Muscles with Tendon Structures for Modular Bio-Actuators

In this article, we propose a new actuator named the modular bio-actuator (MBA). The MBA has two tendon structures made of polydimethylsiloxane (PDMS) at both ends of the bio-actuator. The MBA can be easily handled and fixed on an artificial micro-robot body to increase its design flexibility and output power. The tendon structures were connected to a bio-actuator in the form of a chain structure, and the connection between the tendon structures and the bio-actuator was maintained for more than three weeks. The contraction length of the MBA was linearly increased when the DC voltage applied to the MBA was increased. The MBA contracted over 200 µm when a DC voltage of 10 V and 1 Hz was applied to the bio-actuator. The output power of the MBA was measured using a PDMS cantilever, and the total output power of the MBA increased linearly when multiple MBAs were stacked on a PDMS cantilever. This study was aimed at improving the design flexibility and controllability of micro-robots and bionic systems.

Formation of contractile 3D bovine muscle tissue for construction of millimetre-thick cultured steak

Owing to the increase in the global demand of meat, cultured meat technology is being developed to circumvent a shortage of meat in the future. However, methods for construction of millimetre-thick bovine muscle tissues with highly aligned myotubes have not yet been established. Here, we propose a culture method for constructing 3D-cultured bovine muscle tissue containing myotubes aligned along its long-axial direction, which contracted in response to electrical stimulation. First, we optimised the composition of biomaterials used in the construction and the electrical stimulation applied to the tissue during culture. Subsequently, we fabricated millimetre-thick bovine muscle tissues containing highly aligned myotubes by accumulating bovine myoblast-laden hydrogel modules. The microbial content of the bovine muscle tissue cultured for 14 days was below the detection limit, indicating that the muscle tissues were sterile, unlike commercial meat. Therefore, the proposed construction method for bovine muscle tissues will be useful for the production of clean cultured steak meat simulating real meat.

Neuromuscular junction-on-a-chip: ALS disease modeling and read-out development in microfluidic devices

Amyotrophic lateral sclerosis (ALS) is a fatal and progressive neurodegenerative disease affecting upper and lower motor neurons with no cure available. Clinical and animal studies reveal that the neuromuscular junction (NMJ), a synaptic connection between motor neurons and skeletal muscle fibers, is highly vulnerable in ALS and suggest that NMJ defects may occur at the early stages of the disease. However, mechanistic insight into how NMJ dysfunction relates to the onset and progression of ALS is incomplete, which hampers therapy development. This is, in part, caused by a lack of robust in vitro models. The ability to combine microfluidic and induced pluripotent stem cell (iPSC) technologies has opened up new avenues for studying molecular and cellular ALS phenotypes in vitro. Microfluidic devices offer several advantages over traditional culture approaches when modeling the NMJ, such as the spatial separation of different cell types and increased control over the cellular microenvironment. Moreover, they are compatible with 3D cell culture, which enhances NMJ functionality and maturity. Here, we review how microfluidic technology is currently being employed to develop more reliable in vitro NMJ models. To validate and phenotype such models, various morphological and functional read-outs have been developed. We describe and discuss the relevance of these read-outs and specifically illustrate how these read-outs have enhanced our understanding of NMJ pathology in ALS. Finally, we share our view on potential future directions and challenges.

3D Biofabrication Using Living Cells for Applications in Biohybrid Sensors and Actuators

In this paper, we highlight the concept of biohybrid sensors and actuators built by incorporating living cells into artificial systems. Instead of using the materials extracted from cells, these approaches utilize cells to dynamically generate functional materials and to provide the native intracellular environment for the proper functioning of the materials. By incorporating the functional cells into artificial devices/chips, the cell-based biohybrid approaches can be applied to create portable odorant sensors with high sensitivity and to create biohybrid muscle actuators for applications in both drug screening and soft robotics.

Engineered Heterochronic Parabiosis in 3D Microphysiological System for Identification of Muscle Rejuvenating Factors

Exposure of aged mice to a young systemic milieu revealed remarkable rejuvenation effects on aged tissues, including skeletal muscle. Although some candidate factors have been identified, the exact identity and the underlying mechanisms of putative rejuvenating factors remain elusive, mainly due to the complexity of in vivo parabiosis. Here, we present an in vitro muscle parabiosis system that integrates young- and old-muscle stem cell vascular niche on a three-dimensional microfluidic platform designed to recapitulate key features of native muscle stem cell microenvironment. This innovative system enables mechanistic studies of cellular dynamics and molecular interactions within the muscle stem cell niche, especially in response to conditional extrinsic stimuli of local and systemic factors. We demonstrate that vascular endothelial growth factor (VEGF) signaling from endothelial cells and myotubes synergistically contribute to the rejuvenation of the aged muscle stem cell function. Moreover, with the adjustable on-chip system, we can mimic both blood transfusion and parabiosis and detect the time-varying effects of anti-geronic and pro-geronic factors in a single organ or multi-organ systems. Our unique approach presents a complementary in vitro model to supplement in vivo parabiosis for identifying potential anti-geronic factors responsible for revitalizing aging organs.

Modeling the neuromuscular junction in vitro: an approach to study neuromuscular junction disorders

The neuromuscular junction (NMJ) is a specialized structure that works as an interface to translate the action potential of the presynaptic motor neuron (MN) in the contraction of the postsynaptic myofiber. The design of appropriate experimental models is essential to have efficient and reliable approaches to study NMJ development and function, but also to generate conditions that recapitulate distinct features of diseases. Initial studies relied on the use of tissue slices maintained under the same environment and in which single motor axons were difficult to trace. Later, MNs and muscle cells were obtained from primary cultures or differentiation of progenitors and cocultured as monolayers; however, the tissue architecture was lost. Current approaches include self-assembling 3D structures or the incorporation of biomaterials with cells to generate engineered tissues, although the incorporation of Schwann cells remains a challenge. Thus, numerous investigations have established different NMJ models, some of which are quite complex and challenging. Our review summarizes the in vitro models that have emerged in recent years to coculture MNs and skeletal muscle, trying to mimic the healthy and diseased NMJ. We expect our review may serve as a reference for choosing the appropriate experimental model for the required purposes of investigation.

A Novel Bioengineered Functional Motor Unit Platform to Study Neuromuscular Interaction

Background: In many neurodegenerative and muscular disorders, and loss of innervation in sarcopenia, improper reinnervation of muscle and dysfunction of the motor unit (MU) are key pathogenic features. In vivo studies of MUs are constrained due to difficulties isolating and extracting functional MUs, so there is a need for a simplified and reproducible system of engineered in vitro MUs. Objective: to develop and characterise a functional MU model in vitro, permitting the analysis of MU development and function. Methods: an immortalised human myoblast cell line was co-cultured with rat embryo spinal cord explants in a serum-free/growth fact media. MUs developed and the morphology of their components (neuromuscular junction (NMJ), myotubes and motor neurons) were characterised using immunocytochemistry, phase contrast and confocal microscopy. The function of the MU was evaluated through live observations and videography of spontaneous myotube contractions after challenge with cholinergic antagonists and glutamatergic agonists. Results: blocking acetylcholine receptors with α-bungarotoxin resulted in complete, cessation of myotube contractions, which was reversible with tubocurarine. Furthermore, myotube activity was significantly higher with the application of L-glutamic acid. All these observations indicate the formed MU are functional. Conclusion: a functional nerve-muscle co-culture model was established that has potential for drug screening and pathophysiological studies of neuromuscular interactions.

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

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

New volumetric CNT-doped gelatin-cellulose scaffolds for skeletal muscle tissue engineering

Currently, the fabrication of scaffolds for engineered skeletal muscle tissues is unable to reach the millimeter size. The main drawbacks are the poor nutrient diffusion, lack of an internal structure to align the precursor cells, and poor mechanical and electric properties. Herein, we present a combination of gelatin-carboxymethyl cellulose materials polymerised by a cryogelation process that allowed us to reach scaffold fabrication up to millimeter size and solve the main problems related to the large size muscle tissue constructs. (1) By incorporating carbon nanotubes (CNT), we can improve the electrical properties of the scaffold, thereby enhancing tissue maturation when applying an electric pulse stimulus (EPS). (2) We have fabricated an anisotropic internal three-dimensional microarchitecture with good pore distribution and highly aligned morphology to enhance the cell alignment, cell fusion and myotube formation. With this set up, we were able to generate a fully functional skeletal muscle tissue using a combination of EPS and our doped-biocomposite scaffold and obtain a mature tissue on the millimeter scale. We also characterized the pore distribution, swelling, stiffness and conductivity of the scaffold. Moreover, we proved that the cells were viable and could fuse in three-dimensional (3D) functional myotubes throughout the scaffold. In conclusion, we fabricated a biocompatible and customizable scaffold for 3D cell culture suitable for a wide range of applications such as organ-on-a-chip, drug screening, transplantation and disease modelling.

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.

Pre-innervated tissue-engineered muscle promotes a pro-regenerative microenvironment following volumetric muscle loss

Volumetric muscle loss (VML) is the traumatic or surgical loss of skeletal muscle beyond the inherent regenerative capacity of the body, generally leading to severe functional deficit. Formation of appropriate somato-motor innervations remains one of the biggest challenges for both autologous grafts as well as tissue-engineered muscle constructs. We aim to address this challenge by developing pre-innervated tissue-engineered muscle comprised of long aligned networks of spinal motor neurons and skeletal myocytes on aligned nanofibrous scaffolds. Motor neurons led to enhanced differentiation and maturation of skeletal myocytes in vitro. These pre-innervated tissue-engineered muscle constructs when implanted in a rat VML model significantly increased satellite cell density, neuromuscular junction maintenance, graft revascularization, and muscle volume over three weeks as compared to myocyte-only constructs and nanofiber scaffolds alone. These pro-regenerative effects may enhance functional neuromuscular regeneration following VML, thereby improving the levels of functional recovery following these devastating injuries.

Biohybrid robot with skeletal muscle tissue covered with a collagen structure for moving in air

Biohybrid robots composed of biological and synthetic components have been introduced to reconstruct biological functions in mechanical systems and obtain better understanding of biological designs. For example, biohybrid robots powered by skeletal muscle tissue have already succeeded in performing various movements. However, it has been difficult for the conventional biohybrid robots to actuate in air, as the skeletal muscle tissue often dries out in air and is damaged. To overcome this limitation, we propose a biohybrid robot in which the skeletal muscle tissue is encapsulated in a collagen structure to maintain the required humidity conditions when operated in air. As the skeletal muscle tissue maintains high cell viability and contractility, even after encapsulation within the collagen structure, the biohybrid robot can move in air through contractions of the skeletal muscle tissue. To demonstrate the applicability of the developed biohybrid robot, we demonstrate its use in object manipulation. In addition, to prove its capability of functionality enhancement, we show that the biohybrid robot can actuate for a long term when perfusable tubes are set inside the collagen structure; it can actuate even while culturing cells on its surface. The developed biohybrid robot composed of skeletal muscle tissue and collagen structure can be employed within platforms used to replicate various motions of land animals.

Skeletal muscle explants: ex-vivo models to study cellular behavior in a complex tissue environment

Purpose/Aim: Skeletal muscle tissue explants have been cultured and studied for nearly 100 years. These cultures, which retain complex tissue structure in an environment suited to precision manipulation and measurement, have led to seminal discoveries of the extrinsic and intrinsic mechanisms regulating contractility, metabolism and regeneration. This review discusses the two primary models of muscle explant: isolated myofiber and intact muscle.Materials and Methods: Relevant literature was reviewed and synthesized with a focus on the unique challenges and capabilities of each explant model.Results: Impactful past, current and future novel applications are discussed.Conclusions: Experiments using skeletal muscle explants have been integral to our understanding of the fundamentals of muscle physiology. As they are refined and adapted, they are poised to continue to inform the field for years to come.

Graft alignment impacts the regenerative response of skeletal muscle after volumetric muscle loss in a rat model

A key event in the etiology of volumetric muscle loss (VML) injury is the bulk loss of structural cues provided by the underlying extracellular matrix (ECM). To re-establish the lost cues, there is broad consensus within the literature supporting the utilization of implantable scaffolding. However, while scaffold based regenerative medicine strategies have shown potential, there remains a significant amount of outcome variability observed across the field. We suggest that an overlooked source of outcome variability is differences in scaffolding architecture. The goal of this study was to test the hypothesis that implant alignment has a significant impact on genotypic and phenotypic outcomes following the repair of VML injuries. Using a rat VML model, outcomes across three autograft implant treatment groups (aligned implants, 45° misaligned, and 90° misaligned) and two recovery time points (2 weeks and 12 weeks) were examined (n = 6-8/group). At 2 weeks post-repair there were no significant differences in muscle mass and torque recovery between the treatment groups, however we did observe a significant upregulation of MyoD (2.5 fold increase) and Pax7 (2 fold increase) gene expression as well as the presence of immature myofibers at the implant site for those animals repaired with aligned autografts. By 12 weeks post-repair, functional and structural differences between the treatment groups could be detected. Aligned autografts had significantly greater mass and torque recovery (77 ± 10% of normal) when compared to 45° and 90° misaligned autografts (64 ± 10% and 61 ± 11%, respectively). Examination of tissue structure revealed extensive fibrosis and a significant increase in non-contractile tissue area fraction for only those animals treated using misaligned autografts. When taken together, the results suggest that implant graft orientation has a significant impact on in-vivo outcomes and indicate that the effect of graft alignment on muscle phenotype may be mediated through genotypic changes to myogenesis and fibrosis at the site of injury and repair. STATEMENT OF SIGNIFICANCE: A key event in the etiology of volumetric muscle loss injury is the bulk loss of architectural cues provided by the underlying extracellular matrix. To re-establish the lost cues, there is broad consensus within the literature supporting the utilization of implantable scaffolding. Yet, although native muscle is a highly organized tissue with network and cellular alignment in the direction of contraction, there is little evidence within the field concerning the importance of re-establishing native architectural alignment. The results of this study suggest that critical interactions exist between implant and native muscle alignment cues during healing, which influence the balance between myogenesis and fibrosis. Specifically, it appears that alignment of implant architectural cues with native muscle cues is necessary to create a pro-myogenic environment and contractile force recovery. The results also suggest that misaligned cues may be pathological, leading to fibrosis and poor contractile force recovery.

Polymer- and Hybrid-Based Biomaterials for Interstitial, Connective, Vascular, Nerve, Visceral and Musculoskeletal Tissue Engineering

In this review, materials based on polymers and hybrids possessing both organic and inorganic contents for repairing or facilitating cell growth in tissue engineering are discussed. Pure polymer based biomaterials are predominantly used to target soft tissues. Stipulated by possibilities of tuning the composition and concentration of their inorganic content, hybrid materials allow to mimic properties of various types of harder tissues. That leads to the concept of "one-matches-all" referring to materials possessing the same polymeric base, but different inorganic content to enable tissue growth and repair, proliferation of cells, and the formation of the ECM (extra cellular matrix). Furthermore, adding drug delivery carriers to coatings and scaffolds designed with such materials brings additional functionality by encapsulating active molecules, antibacterial agents, and growth factors. We discuss here materials and methods of their assembly from a general perspective together with their applications in various tissue engineering sub-areas: interstitial, connective, vascular, nervous, visceral and musculoskeletal tissues. The overall aims of this review are two-fold: (a) to describe the needs and opportunities in the field of bio-medicine, which should be useful for material scientists, and (b) to present capabilities and resources available in the area of materials, which should be of interest for biologists and medical doctors.

Neural cell integration into 3D bioprinted skeletal muscle constructs accelerates restoration of muscle function

A bioengineered skeletal muscle construct that mimics structural and functional characteristics of native skeletal muscle is a promising therapeutic option to treat extensive muscle defect injuries. We previously showed that bioprinted human skeletal muscle constructs were able to form multi-layered bundles with aligned myofibers. In this study, we investigate the effects of neural cell integration into the bioprinted skeletal muscle construct to accelerate functional muscle regeneration in vivo. Neural input into this bioprinted skeletal muscle construct shows the improvement of myofiber formation, long-term survival, and neuromuscular junction formation in vitro. More importantly, the bioprinted constructs with neural cell integration facilitate rapid innervation and mature into organized muscle tissue that restores normal muscle weight and function in a rodent model of muscle defect injury. These results suggest that the 3D bioprinted human neural-skeletal muscle constructs can be rapidly integrated with the host neural network, resulting in accelerated muscle function restoration.

3D bioprinting of skeletal muscle using primary human muscle progenitor cells results in correct muscle architecture, but functional restoration in rodent models is limited. Here the authors include human neural stem cells into bioprinted skeletal muscle and observe improved architecture and function in vivo.

Enzymatically crosslinked gelatin-laminin hydrogels for applications in neuromuscular tissue engineering

We report a water-soluble and non-toxic method to incorporate additional extracellular matrix proteins into gelatin hydrogels, while obviating the use of chemical crosslinkers such as glutaraldehyde. Gelatin hydrogels were fabricated using a range of gelatin concentrations (4%-10%) that corresponded to elastic moduli of approximately 1 kPa-25 kPa, respectively, a substrate stiffness relevant for multiple cell types. Microbial transglutaminase was then used to enzymatically crosslink a layer of laminin on top of gelatin hydrogels, resulting in 2-component gelatin-laminin hydrogels. Human induced pluripotent stem cell derived spinal spheroids readily adhered and rapidly extended axons on GEL-LN hydrogels. Axons displayed a more mature morphology and superior electrophysiological properties on GEL-LN hydrogels compared to the controls. Schwann cells on GEL-LN hydrogels adhered and proliferated normally, displayed a healthy morphology, and maintained the expression of Schwann cell specific markers. Lastly, skeletal muscle cells on GEL-LN hydrogels achieved long-term culture for up to 28 days without delamination, while expressing higher levels of terminal genes including myosin heavy chain, MyoD, MuSK, and M-cadherin suggesting enhanced maturation potential and myotube formation compared to the controls. Future studies will employ the superior culture outcomes of this hybrid substrate for engineering functional neuromuscular junctions and related organ on a chip applications.

Vascularized and Innervated Skeletal Muscle Tissue Engineering

Volumetric muscle loss (VML) is a devastating loss of muscle tissue that overwhelms the native regenerative properties of skeletal muscle and results in lifelong functional deficits. There are currently no treatments for VML that fully recover the lost muscle tissue and function. Tissue engineering presents a promising solution for VML treatment and significant research has been performed using tissue engineered muscle constructs in preclinical models of VML with a broad range of defect locations and sizes, tissue engineered construct characteristics, and outcome measures. Due to the complex vascular and neural anatomy within skeletal muscle, regeneration of functional vasculature and nerves is vital for muscle recovery following VML injuries. This review aims to summarize the current state of the field of skeletal muscle tissue engineering using 3D constructs for VML treatment with a focus on studies that have promoted vascular and neural regeneration within the muscle tissue post-VML.

Tissue Engineering for Musculoskeletal Regeneration and Disease Modeling

Musculoskeletal injuries and associated conditions are the leading cause of physical disability worldwide. The concept of tissue engineering has opened up novel approaches to repair musculoskeletal defects in a fast and/or efficient manner. Biomaterials, cells, and signaling molecules constitute the tissue engineering triad. In the past 40 years, significant progress has been made in developing and optimizing all three components, but only a very limited number of technologies have been successfully translated into clinical applications. A major limiting factor of this barrier to translation is the insufficiency of two-dimensional cell cultures and traditional animal models in informing the safety and efficacy of in-human applications. In recent years, microphysiological systems, often referred to as organ or tissue chips, generated according to tissue engineering principles, have been proposed as the next-generation drug testing models. This chapter aims to first review the current tissue engineering-based approaches that are being applied to fabricate and develop the individual critical elements involved in musculoskeletal organ/tissue chips. We next highlight the general strategy of generating musculoskeletal tissue chips and their potential in future regenerative medicine research. Exemplary microphysiological systems mimicking musculoskeletal tissues are described. With sufficient physiological accuracy and relevance, the human cell-derived, three-dimensional, multi-tissue systems have been used to model a number of orthopedic disorders and to test new treatments. We anticipate that the novel emerging tissue chip technology will continually reshape and improve our understanding of human musculoskeletal pathophysiology, ultimately accelerating the development of advanced pharmaceutics and regenerative therapies.