Creating Interactions between Tissue-Engineered Skeletal Muscle and the Peripheral Nervous System

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.

Upscaled Skeletal Muscle Engineered Tissue with In Vivo Vascularization and Innervation Potential

Engineering functional tissues of clinically relevant size (in mm-scale) in vitro is still a challenge in tissue engineering due to low oxygen diffusion and lack of vascularization. To address these limitations, a perfusion bioreactor was used to generate contractile engineered muscles of a 3 mm-thickness and a 8 mm-diameter. This study aimed to upscale the process to 50 mm in diameter by combining murine skeletal myoblasts (SkMbs) with human adipose-derived stromal vascular fraction (SVF) cells, providing high neuro-vascular potential in vivo. SkMbs were cultured on a type-I-collagen scaffold with (co-culture) or without (monoculture) SVF. Large-scale muscle-like tissue showed an increase in the maturation index over time (49.18 ± 1.63% and 76.63 ± 1.22%, at 9 and 11 days, respectively) and a similar force of contraction in mono- (43.4 ± 2.28 µN) or co-cultured (47.6 ± 4.7 µN) tissues. Four weeks after implantation in subcutaneous pockets of nude rats, the vessel length density within the constructs was significantly higher in SVF co-cultured tissues (5.03 ± 0.29 mm/mm²) compared to monocultured tissues (3.68 ± 0.32 mm/mm²) (p < 0.005). Although no mature neuromuscular junctions were present, nerve-like structures were predominantly observed in the engineered tissues co-cultured with SVF cells. This study demonstrates that SVF cells can support both in vivo vascularization and innervation of contractile muscle-like tissues, making significant progress towards clinical translation.

Biomedical applications of three-dimensional bioprinted craniofacial tissue engineering

Anatomical complications of the craniofacial regions often present considerable challenges to the surgical repair or replacement of the damaged tissues. Surgical repair has its own set of limitations, including scarcity of the donor tissues, immune rejection, use of immune suppressors followed by the surgery, and restriction in restoring the natural aesthetic appeal. Rapid advancement in the field of biomaterials, cell biology, and engineering has helped scientists to create cellularized skeletal muscle-like structures. However, the existing method still has limitations in building large, highly vascular tissue with clinical application. With the advance in the three-dimensional (3D) bioprinting technique, scientists and clinicians now can produce the functional implants of skeletal muscles and bones that are more patient-specific with the perfect match to the architecture of their craniofacial defects. Craniofacial tissue regeneration using 3D bioprinting can manage and eliminate the restrictions of the surgical transplant from the donor site. The concept of creating the new functional tissue, exactly mimicking the anatomical and physiological function of the damaged tissue, looks highly attractive. This is crucial to reduce the donor site morbidity and retain the esthetics. 3D bioprinting can integrate all three essential components of tissue engineering, that is, rehabilitation, reconstruction, and regeneration of the lost craniofacial tissues. Such integration essentially helps to develop the patient-specific treatment plans and damage site-driven creation of the functional implants for the craniofacial defects. This article is the bird's eye view on the latest development and application of 3D bioprinting in the regeneration of the skeletal muscle tissues and their application in restoring the functional abilities of the damaged craniofacial tissue. We also discussed current challenges in craniofacial bone vascularization and gave our view on the future direction, including establishing the interactions between tissue-engineered skeletal muscle and the peripheral nervous system.

Design of Recombinant Spider Silk Proteins for Cell Type Specific Binding

Cytophilic (cell-adhesive) materials are very important for tissue engineering and regenerative medicine. However, for engineering hierarchically organized tissue structures comprising different cell types, cell-specific attachment and guidance are decisive. In this context, materials made of recombinant spider silk proteins are promising scaffolds, since they exhibit high biocompatibility, biodegradability, and the underlying proteins can be genetically functionalized. Here, previously established spider silk variants based on the engineered Araneus diadematus fibroin 4 (eADF4(C16)) are genetically modified with cell adhesive peptide sequences from extracellular matrix proteins, including IKVAV, YIGSR, QHREDGS, and KGD. Interestingly, eADF4(C16)-KGD as one of 18 tested variants is cell-selective for C2C12 mouse myoblasts, one out of 11 tested cell lines. Co-culturing with B50 rat neuronal cells confirms the cell-specificity of eADF4(C16)-KGD material surfaces for C2C12 mouse myoblast adhesion.

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.

Three-dimensional in vitro models of neuromuscular tissue

Skeletal muscle is a dynamic tissue in which homeostasis and function are guaranteed by a very defined three-dimensional organization of myofibers in respect to other non-muscular components, including the extracellular matrix and the nervous network. In particular, communication between myofibers and the nervous system is essential for the overall correct development and function of the skeletal muscle. A wide range of chronic, acute and genetic-based human pathologies that lead to the alteration of muscle function are associated with modified preservation of the fine interaction between motor neurons and myofibers at the neuromuscular junction. Recent advancements in the development of in vitro models for human skeletal muscle have shown that three-dimensionality and integration of multiple cell types are both key parameters required to unveil pathophysiological relevant phenotypes. Here, we describe recent achievement reached in skeletal muscle modeling which used biomaterials for the generation of three-dimensional constructs of myotubes integrated with motor neurons.

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.

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.

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.

Bioengineered model of the human motor unit with physiologically functional neuromuscular junctions

Investigations of the human neuromuscular junction (NMJ) have predominately utilised experimental animals, model organisms, or monolayer cell cultures that fail to represent the physiological complexity of the synapse. Consequently, there remains a paucity of data regarding the development of the human NMJ and a lack of systems that enable investigation of the motor unit. This work addresses this need, providing the methodologies to bioengineer 3D models of the human motor unit. Spheroid culture of iPSC derived motor neuron progenitors augmented the transcription of OLIG2, ISLET1 and SMI32 motor neuron mRNAs ~ 400, ~ 150 and ~ 200-fold respectively compared to monolayer equivalents. Axon projections of adhered spheroids exceeded 1000 μm in monolayer, with transcription of SMI32 and VACHT mRNAs further enhanced by addition to 3D extracellular matrices in a type I collagen concentration dependent manner. Bioengineered skeletal muscles produced functional tetanic and twitch profiles, demonstrated increased acetylcholine receptor (AChR) clustering and transcription of MUSK and LRP4 mRNAs, indicating enhanced organisation of the post-synaptic membrane. The number of motor neuron spheroids, or motor pool, required to functionally innervate 3D muscle tissues was then determined, generating functional human NMJs that evidence pre- and post-synaptic membrane and motor nerve axon co-localisation. Spontaneous firing was significantly elevated in 3D motor units, confirmed to be driven by the motor nerve via antagonistic inhibition of the AChR. Functional analysis outlined decreased time to peak twitch and half relaxation times, indicating enhanced physiology of excitation contraction coupling in innervated motor units. Our findings provide the methods to maximise the maturity of both iPSC motor neurons and primary human skeletal muscle, utilising cell type specific extracellular matrices and developmental timelines to bioengineer the human motor unit for the study of neuromuscular junction physiology.

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.

Cross-talk between motor neurons and myotubes via endogenously secreted neural and muscular growth factors

Neuromuscular junction (NMJ) research is vital to advance the understanding of neuromuscular patho‐physiology and development of novel therapies for diseases associated with NM dysfunction. In vivo, the micro‐environment surrounding the NMJ has a significant impact on NMJ formation and maintenance via neurotrophic and differentiation factors that are secreted as a result of cross‐talk between muscle fibers and motor neurons. Recently we showed the formation of functional NMJs in vitro in a co‐culture of immortalized human myoblasts and motor neurons from rat‐embryo spinal‐cord explants, using a culture medium free from serum and neurotrophic or growth factors. The aim of this study was to assess how functional NMJs were established in this co‐culture devoid of exogenous neural growth factors. To investigate this, an ELISA‐based microarray was used to compare the composition of soluble endogenously secreted growth factors in this co‐culture with an a‐neural muscle culture. The levels of seven neurotrophic factors brain‐derived neurotrophic factor (BDNF), glial‐cell‐line‐derived neurotrophic factor (GDNF), insulin‐like growth factor‐binding protein‐3 (IGFBP‐3), insulin‐like growth factor‐1 (IGF‐1), neurotrophin‐3 (NT‐3), neurotrophin‐4 (NT‐4), and vascular endothelial growth factor (VEGF) were higher (p < 0.05) in the supernatant of NMJ culture compared to those in the supernatant of the a‐neural muscle culture. This indicates that the cross‐talk between muscle and motor neurons promotes the secretion of soluble growth factors contributing to the local microenvironment thereby providing a favourable regenerative niche for NMJs formation and maturation.

Temporal mechanisms of myogenic specification in human induced pluripotent stem cells

Understanding the mechanisms of myogenesis in human induced pluripotent stem cells (hiPSCs) is a prerequisite to achieving patient-specific therapy for diseases of skeletal muscle. hiPSCs of different origin show distinctive kinetics and ability to differentiate into myocytes. To address the unique cellular and temporal context of hiPSC differentiation, we perform a longitudinal comparison of the transcriptomic profiles of three hiPSC lines that display differential myogenic specification, one robust and two blunted. We detail temporal differences in mechanisms that lead to robust myogenic specification. We show gene expression signatures of putative cell subpopulations and extracellular matrix components that may support myogenesis. Furthermore, we show that targeted knockdown of ZIC3 at the outset of differentiation leads to improved myogenic specification in blunted hiPSC lines. Our study suggests that β-catenin transcriptional cofactors mediate cross-talk between multiple cellular processes and exogenous cues to facilitate specification of hiPSCs to mesoderm lineage, leading to robust myogenesis.

Neuromuscular Junction Model Optimized for Electrical Platforms

Neuromuscular junctions (NMJs), specialized synapses between motor neurons and muscle fibers, are essential for muscle activity. A simple and reproducible cell-based in vitro NMJ platform is needed to test the impact of chemicals on the neuron-muscle communication. Our platform utilizes genetically modified neurons and muscle cells, optimized culture conditions, and commercially available multielectrode array system for recording action potentials. Neuronal cells (NSC34) were optogenetically modified with channelrhodopsin chimera to allow for simultaneous, light-mediated, millisecond-precise activation of neuronal population. This signal is propagated through functional synapses to the muscle fibers. Muscle cells (C2C12) were modified by incorporating gap junction protein (Connexin-43) to improve intracellular communication without affecting muscle differentiation. This communication between muscle fibers resulted in better signal propagation and signal strength. Optimized culture medium facilitated the growth and differentiation of both cell types together. Our system was validated using vecuronium, a muscle relaxant, which abolished the muscle response. This in vitro model provides a unique tool for establishing a NMJ platform that is easy to record and analyze. Potential applications include nondestructive long-term screening of drugs affecting the NMJ.

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.

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.

Emergence of functional neuromuscular junctions in an engineered, multicellular spinal cord-muscle bioactuator

Three-dimensional (3D) biomimetic systems hold great promise for the study of biological systems in vitro as well as for the development and testing of pharmaceuticals. Here, we test the hypothesis that an intact segment of lumbar rat spinal cord will form functional neuromuscular junctions (NMJs) with engineered, 3D muscle tissue, mimicking the partial development of the peripheral nervous system (PNS). Muscle tissues are grown on a 3D-printed polyethylene glycol (PEG) skeleton where deflection of the backbone due to muscle contraction causes the displacement of the pillar-like "feet." We show that spinal cord explants extend a robust and complex arbor of motor neurons and glia in vitro. We then engineered a "spinobot" by innervating the muscle tissue with an intact segment of lumbar spinal cord that houses the hindlimb locomotor central pattern generator (CPG). Within 7 days of the spinal cord being introduced to the muscle tissue, functional neuromuscular junctions (NMJs) are formed, resulting in the development of an early PNS in vitro. The newly innervated muscles exhibit spontaneous contractions as measured by the displacement of pillars on the PEG skeleton. Upon chemical excitation, the spinal cord-muscle system initiated muscular twitches with a consistent frequency pattern. These sequences of contraction/relaxation suggest the action of a spinal CPG. Chemical inhibition with a blocker of neuronal glutamate receptors effectively blocked contractions. Overall, these data demonstrate that a rat spinal cord is capable of forming functional neuromuscular junctions ex vivo with an engineered muscle tissue at an ontogenetically similar timescale.

Skeletal muscle cells derived from mouse skin cultures

We have established a novel, simple, and highly reproducible method to generate skeletal muscle cells from mouse skin. Small pieces of skin from the back of mice were cultured in extracellular material-coated dishes in typical culture medium for about 3 weeks. Myotubes formed after about a week, grew into twitching myotubes, and became twitching myotube clumps after 3 weeks. Skeletal muscle cells are formed spontaneously with no induction. Myotubes were immunologically positive for myosin heavy chains, MyoD, and myogenin. Ultrastructural analysis revealed the presence of the sarcomere structure. Furthermore, PAX7⁺/MyoD^- muscle stem cells proliferated around these myotubes, and MyoD⁺/myogenin⁺/MHC^-- cells were also observed. Moreover, we investigated the formation of skeletal muscle cells from the sialidosis mouse skin, and showed that it is decreased compared to that of the wild type. Our method to generate skeletal muscle cells from skin is thought to be useful for the investigation of muscle cell development and muscle-related disorders.

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.

In vitro models of neuromuscular junctions and their potential for novel drug discovery and development

Introduction: Neuromuscular Junctions (NMJs) are the synapses between motor neurons and skeletal muscle fibers, and they are responsible for voluntary motor function. NMJs are affected at early stages of numerous neurodegenerative and neuroimmunological diseases. Due to the difficulty of systematically studying and manipulating NMJs in live subjects, in vitro systems with human tissue models would provide a powerful complement to simple cell cultures and animal models for mechanistic and drug development studies.Areas covered: The authors review the latest advances in in vitro models of NMJs, from traditional cell co-culture systems to novel tissue culture approaches, with focus on disease modeling and drug testing.Expert opinion: In recent years, more sophisticated in vitro models of human NMJs have been established. The combination of human stem cell technology with advanced tissue culture systems has resulted in systems that better recapitulate the human NMJ structure and function, and thereby allow for high-throughput quantitative functional measurements under both healthy and diseased conditions. Although they still have limitations, these advanced systems are increasingly demonstrating their utility for evaluating new therapies for motoneuron and autoimmune neuromuscular diseases, and we expect them to become an integral part of the drug discovery process in the near future.

Differentiation of Bioengineered Skeletal Muscle within a 3D Printed Perfusion Bioreactor Reduces Atrophic and Inflammatory Gene Expression

Bioengineered skeletal muscle tissues benefit from dynamic culture environments which facilitate the appropriate provision of nutrients and removal of cellular waste products. Biologically compatible perfusion systems hold the potential to enhance the physiological biomimicry of in vitro tissues via dynamic culture, in addition to providing technological advances in analytical testing and live cellular imaging for analysis of cellular development. To meet such diverse requirements, perfusion systems require the capacity and adaptability to incorporate multiple cell laden constructs of both monolayer and bioengineered tissues. This work reports perfusion systems produced using additive manufacturing technology for the in situ phenotypic development of myogenic precursor cells in monolayer and bioengineered tissue. Biocompatibility of systems 3D printed using stereolithography (SL), laser sintering (LS), and PolyJet outlined preferential morphological development within both SL and LS devices. When exposed to intermittent perfusion in the monolayer, delayed yet physiologically representative cellular proliferation, MyoD and myogenin transcription of C₂C₁₂ cells was evident. Long-term (8 days) intermittent perfusion of monolayer cultures outlined viable morphological and genetic in situ differentiation for the live cellular imaging of myogenic development. Continuous perfusion cultures (13 days) of bioengineered skeletal muscle tissues outlined in situ myogenic differentiation, forming mature multinucleated myotubes. Here, reductions in IL-1β and TNF-α inflammatory cytokines, myostatin, and MuRF-1 atrophic mRNA expression were observed. Comparable myosin heavy chain (MyHC) isoform transcription profiles were evident between conditions; however, total mRNA expression was reduced in perfusion conditions. Decreased transcription of MuRF1 and subsequent reduced ubiquitination of the MyHC protein allude to a decreased requirement for transcription of MyHC isoform transcripts. Together, these data appear to indicate that 3D printed perfusion systems elicit enhanced stability of the culture environment, resulting in a reduced basal requirement for MyHC gene expression within bioengineered skeletal muscle tissue.

Toward Building the Neuromuscular Junction: In Vitro Models To Study Synaptogenesis and Neurodegeneration

The neuromuscular junction (NMJ) is a unique, specialized chemical synapse that plays a crucial role in transmitting and amplifying information from spinal motor neurons to skeletal muscles. NMJ complexity ensures closely intertwined interactions between numerous synaptic vesicles, signaling molecules, ion channels, motor neurons, glia, and muscle fibers, making it difficult to dissect the underlying mechanisms and factors affecting neurodegeneration and muscle loss. Muscle fiber or motor neuron cell death followed by rapid axonal degeneration due to injury or disease has a debilitating effect on movement and behavior, which adversely affects the quality of life. It thus becomes imperative to study the synapse and intercellular signaling processes that regulate plasticity at the NMJ and elucidate mechanisms and pathways at the cellular level. Studies using in vitro 2D cell cultures have allowed us to gain a fundamental understanding of how the NMJ functions. However, they do not provide information on the intricate signaling networks that exist between NMJs and the biological environment. The advent of 3D cell cultures and microfluidic lab-on-a-chip technologies has opened whole new avenues to explore the NMJ. In this perspective, we look at the challenges involved in building a functional NMJ and the progress made in generating models for studying the NMJ, highlighting the current and future applications of these models.

A 3D culture model of innervated human skeletal muscle enables studies of the adult neuromuscular junction

Two-dimensional (2D) human skeletal muscle fiber cultures are ill-equipped to support the contractile properties of maturing muscle fibers. This limits their application to the study of adult human neuromuscular junction (NMJ) development, a process requiring maturation of muscle fibers in the presence of motor neuron endplates. Here we describe a three-dimensional (3D) co-culture method whereby human muscle progenitors mixed with human pluripotent stem cell-derived motor neurons self-organize to form functional NMJ connections. Functional connectivity between motor neuron endplates and muscle fibers is confirmed with calcium imaging and electrophysiological recordings. Notably, we only observed epsilon acetylcholine receptor subunit protein upregulation and activity in 3D co-cultures. Further, 3D co-culture treatments with myasthenia gravis patient sera shows the ease of studying human disease with the system. Hence, this work offers a simple method to model and evaluate adult human NMJ de novo development or disease in culture.

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

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

Challenges and Future Prospects on 3D in-vitro Modeling of the Neuromuscular Circuit

Movement of skeletal-muscle fibers is generated by the coordinated action of several cells taking part within the locomotion circuit (motoneurons, sensory-neurons, Schwann cells, astrocytes, microglia, and muscle-cells). Failures in any part of this circuit could impede or hinder coordinated muscle movement and cause a neuromuscular disease (NMD) or determine its severity. Studying fragments of the circuit cannot provide a comprehensive and complete view of the pathological process. We trace the historic developments of studies focused on in-vitro modeling of the spinal-locomotion circuit and how bioengineered innovative technologies show advantages for an accurate mimicking of physiological conditions of spinal-locomotion circuit. New developments on compartmentalized microfluidic culture systems (cμFCS), the use of human induced pluripotent stem cells (hiPSCs) and 3D cell-cultures are analyzed. We finally address limitations of current study models and three main challenges on neuromuscular studies: (i) mimic the whole spinal-locomotion circuit including all cell-types involved and the evaluation of independent and interdependent roles of each one; (ii) mimic the neurodegenerative response of mature neurons in-vitro as it occurs in-vivo; and (iii) develop, tune, implement, and combine cμFCS, hiPSC, and 3D-culture technologies to ultimately create patient-specific complete, translational, and reliable NMD in-vitro model. Overcoming these challenges would significantly facilitate understanding the events taking place in NMDs and accelerate the process of finding new therapies.

Establishment of primary myoblast cell cultures from cryopreserved skeletal muscle biopsies to serve as a tool in related research & development studies

BACKGROUND

Primary myoblast cell cultures display the phenotypic characteristics and genetic defects of the donor tissue and represent an in vitro model system reflecting the disease pathology. They have been generated only from freshly harvested tissue biopsies. Here, we describe a novel technique to establish myoblast cell cultures from cryopreserved skeletal muscle biopsy tissues that are useful for diagnostic and research purposes.

METHODS AND RESULTS

This protocol was performed on seven gradually frozen muscle biopsy specimens from various neuromuscular disorders that were stored in dimethylsulfoxide (DMSO)-supplemented freezing media at -80 °C for up to one year. After storage for varying periods of time, primary myoblast cultures were successfully established from all cryopreserved biopsy tissues without any chromosomal abnormality. Desmin immunoreactivity confirmed that the cell cultures contained >90% pure myoblasts. The myoblasts differentiated into multinucleated myotubes successfully. Furthermore, there were no statistically significant differences in cell viability, metabolic activity, population doubling time, and myocyte enhancer factor 2 (MEF2C) expression between cell cultures established from freshly harvested and one year-stored frozen tissue specimens.

CONCLUSIONS

This protocol opens up new horizons for basic research and the pre-clinical studies of novel therapies by using cryopreserved skeletal muscle biopsies stored under suitable conditions in tissue banks.

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

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