Myotube formation on micro-patterned glass: intracellular organization and protein distribution in C2C12 skeletal muscle cells

Impact of Uniaxial Static Strain on Myoblast Differentiation in Collagen-Coated PCL Microfilament Scaffolds: Role of Onset Time of Mechanical Stimulation

Tissue engineering endeavors to create in vitro constructs that replicate the properties of native tissue, such as skeletal muscle. This study investigated the use of mechanical stimulation to promote myogenic differentiation and enhance the functionality of bioengineered tissues. Specifically, it aimed to facilitate the differentiation of myoblasts within a three-dimensional scaffold using a defined pattern of mechanical stimulation. C2C12 cells were cultured on a collagen-coated PCL microfilament scaffold and subjected to 24 h of uniaxial static strain using a biomechanical stimulation system. Two onset times of stimulation, 72 h and 120 h post-seeding, were evaluated. Cell proliferation, myogenic marker expression, and alterations in cell morphology and orientation were assessed. Results indicate that static strain on the scaffold promoted myoblast differentiation, evidenced by morphological and molecular changes. Notably, strain initiated at 72 h induced an early differentiation stage marked by MyoD expression, whereas stimulation beginning at 120 h led to a mid-stage differentiation characterized by the co-expression of MyoD and Myogenin, culminating in myotube formation. These results highlight the critical influence of myoblast maturity at the time of strain application on the differentiation outcome. This study provides insights that could guide the optimization of mechanical stimulation protocols in tissue engineering applications.

Effect of mTORC Agonism via MHY1485 with and without Rapamycin on C2C12 Myotube Metabolism

The mechanistic target of rapamycin complex (mTORC) regulates protein synthesis and can be activated by branched-chain amino acids (BCAAs). mTORC has also been implicated in the regulation of mitochondrial metabolism and BCAA catabolism. Some speculate that mTORC overactivation by BCAAs may contribute to insulin resistance. The present experiments assessed the effect of mTORC activation on myotube metabolism and insulin sensitivity using the mTORC agonist MHY1485, which does not share structural similarities with BCAAs.

METHODS

C2C12 myotubes were treated with MHY1485 or DMSO control both with and without rapamycin. Gene expression was assessed using qRT-PCR and insulin sensitivity and protein expression by western blot. Glycolytic and mitochondrial metabolism were measured by extracellular acidification rate and oxygen consumption. Mitochondrial and lipid content were analyzed by fluorescent staining. Liquid chromatography-mass spectrometry was used to assess extracellular BCAAs.

RESULTS

Rapamycin reduced p-mTORC expression, mitochondrial content, and mitochondrial function. Surprisingly, MHY1485 did not alter p-mTORC expression or cell metabolism. Neither treatment altered indicators of BCAA metabolism or extracellular BCAA content.

CONCLUSION

Collectively, inhibition of mTORC via rapamycin reduces myotube metabolism and mitochondrial content but not BCAA metabolism. The lack of p-mTORC activation by MHY1485 is a limitation of these experiments and warrants additional investigation.

Decoding the forces that shape muscle stem cell function

Skeletal muscle is a force-producing organ composed of muscle tissues, connective tissues, blood vessels, and nerves, all working in synergy to enable movement and provide support to the body. While robust biomechanical descriptions of skeletal muscle force production at the body or tissue level exist, little is known about force application on microstructures within the muscles, such as cells. Among various cell types, skeletal muscle stem cells reside in the muscle tissue environment and play a crucial role in driving the self-repair process when muscle damage occurs. Early evidence indicates that the fate and function of skeletal muscle stem cells are controlled by both biophysical and biochemical factors in their microenvironments, but much remains to accomplish in quantitatively describing the biophysical muscle stem cell microenvironment. This book chapter aims to review current knowledge on the influence of biophysical stresses and landscape properties on muscle stem cells in heath, aging, and diseases.

Advances and Challenges in Cell Biology for Cultured Meat

Cultured meat is an emerging biotechnology that aims to produce meat from animal cell culture, rather than from the raising and slaughtering of livestock, on environmental and animal welfare grounds. The detailed understanding and accurate manipulation of cell biology are critical to the design of cultured meat bioprocesses. Recent years have seen significant interest in this field, with numerous scientific and commercial breakthroughs. Nevertheless, these technologies remain at a nascent stage, and myriad challenges remain, spanning the entire bioprocess. From a cell biological perspective, these include the identification of suitable starting cell types, tuning of proliferation and differentiation conditions, and optimization of cell-biomaterial interactions to create nutritious, enticing foods. Here, we discuss the key advances and outstanding challenges in cultured meat, with a particular focus on cell biology, and argue that solving the remaining bottlenecks in a cost-effective, scalable fashion will require coordinated, concerted scientific efforts. Success will also require solutions to nonscientific challenges, including regulatory approval, consumer acceptance, and market feasibility. However, if these can be overcome, cultured meat technologies can revolutionize our approach to food.

3D Co-Printing and Substrate Geometry Influence the Differentiation of C2C12 Skeletal Myoblasts

Cells are influenced by several biomechanical aspects of their microenvironment, such as substrate geometry. According to the literature, substrate geometry influences the behavior of muscle cells; in particular, the curvature feature improves cell proliferation. However, the effect of substrate geometry on the myogenic differentiation process is not clear and needs to be further investigated. Here, we show that the 3D co-printing technique allows the realization of substrates. To test the influence of the co-printing technique on cellular behavior, we realized linear polycaprolactone substrates with channels in which a fibrinogen-based hydrogel loaded with C2C12 cells was deposited. Cell viability and differentiation were investigated up to 21 days in culture. The results suggest that this technology significantly improves the differentiation at 14 days. Therefore, we investigate the substrate geometry influence by comparing three different co-printed geometries-linear, circular, and hybrid structures (linear and circular features combined). Based on our results, all structures exhibit optimal cell viability (>94%), but the linear pattern allows to increase the in vitro cell differentiation, in particular after 14 days of culture. This study proposes an endorsed approach for creating artificial muscles for future skeletal muscle tissue engineering applications.

Global Quantitative Proteomics Analysis Reveals the Downstream Signaling Networks of Msx1 and Msx2 in Myoblast Differentiation

The msh homeobox 1 (Msx1) and msh homeobox 2 (Msx2) coordinate in myoblast differentiation and also contribute to muscle defects if altered during development. Deciphering the downstream signaling networks of Msx1 and Msx2 in myoblast differentiation will help us to understand the molecular events that contribute to muscle defects. Here, the proteomics characteristics in Msx1- and Msx2-mediated myoblast differentiation was evaluated  using isobaric tags for the relative and absolute quantification labeling technique (iTRAQ). The downstream regulatory proteins of Msx1- and Msx2-mediated differentiation were identified. Bioinformatics analysis revealed that these proteins were primarily associated with xenobiotic metabolism by cytochrome P450, fatty acid degradation, glycolysis/gluconeogenesis, arginine and proline metabolism, and apoptosis. In addition, our data show Acta1 was probably a core of the downstream regulatory networks of Msx1 and Msx2 in myoblast differentiation.

Supplementary Information

The online version contains supplementary material available at 10.1007/s43657-022-00049-y.

Myogenic Potential of Extracellular Matrix Derived from Decellularized Bovine Pericardium

Skeletal muscles represent 40% of body mass and its native regenerative capacity can be permanently lost after a traumatic injury, congenital diseases, or tumor ablation. The absence of physiological regeneration can hinder muscle repair preventing normal muscle tissue functions. To date, tissue engineering (TE) represents one promising option for treating muscle injuries and wasting. In particular, hydrogels derived from the decellularized extracellular matrix (dECM) are widely investigated in tissue engineering applications thanks to their essential role in guiding muscle regeneration. In this work, the myogenic potential of dECM substrate, obtained from decellularized bovine pericardium (Tissuegraft Srl), was evaluated in vitro using C2C12 murine muscle cells. To assess myotubes formation, the width, length, and fusion indexes were measured during the differentiation time course. Additionally, the ability of dECM to support myogenesis was assessed by measuring the expression of specific myogenic markers: α-smooth muscle actin (α-sma), myogenin, and myosin heavy chain (MHC). The results obtained suggest that the dECM niche was able to support and enhance the myogenic potential of C2C12 cells in comparison of those grown on a plastic standard surface. Thus, the use of extracellular matrix proteins, as biomaterial supports, could represent a promising therapeutic strategy for skeletal muscle tissue engineering.

Bio-essential Inorganic Molecular Nanowires as a Bioactive Muscle Extracellular-Matrix-Mimicking Material

Many physiochemical properties of the extracellular matrix (ECM) of muscle tissues, such as nanometer scale dimension, nanotopography, negative charge, and elasticity, must be carefully reproduced to fabricate scaffold materials mimicking muscle tissues. Hence, we developed a muscle tissue ECM-mimicking scaffold using Mo₆S₃I₆ inorganic molecular wires (IMWs). Composed of bio-essential elements and having a nanofibrous structure with a diameter of ∼1 nm and a negative surface charge with high stability, Mo₆S₃I₆ IMWs are ideal for mimicking natural ECM molecules. Once Mo₆S₃I₆ IMWs were patterned on a polydimethylsiloxane surface with an elasticity of 1877.1 ± 22.2 kPa, that is, comparable to that of muscle tissues, the proliferation and α-tubulin expression of myoblasts enhanced significantly. Additionally, the repetitive one-dimensional patterns of Mo₆S₃I₆ IMWs induced the alignment and stretching of myoblasts with enhanced α-tubulin expression and differentiation into myocytes. This study demonstrates that Mo₆S₃I₆ IMWs are promising for mimicking the ECM of muscle tissues.

Promoting endogenous repair of skeletal muscle using regenerative biomaterials

Skeletal muscles normally have a remarkable ability to repair themselves; however, large muscle injuries and several myopathies diminish this ability leading to permanent loss of function. No clinical therapy yet exists that reliably restores muscle integrity and function following severe injury. Consequently, numerous tissue engineering techniques, both acellular and with cells, are being investigated to enhance muscle regeneration. Biomaterials are an essential part of these techniques as they can present physical and biochemical signals that augment the repair process. Successful tissue engineering strategies require regenerative biomaterials that either actively promote endogenous muscle repair or create an environment supportive of regeneration. This review will discuss several acellular biomaterial strategies for skeletal muscle regeneration with a focus on those under investigation in vivo. This includes materials that release bioactive molecules, biomimetic materials and immunomodulatory materials.

Micro/nano-hierarchical scaffold fabricated using a cell electrospinning/3D printing process for co-culturing myoblasts and HUVECs to induce myoblast alignment and differentiation

Human skeletal muscle is composed of intricate anatomical structures, including uniaxially arranged myotubes and widely distributed blood capillaries. In this regard, vascularization is an essential part of the successful development of an engineered skeletal muscle tissue to restore its function and physiological activities. In this paper, we propose a method to obtain a platform for co-culturing human umbilical vein endothelial cells (HUVECs) and C2C12 cells using cell electrospinning and 3D bioprinting. To elaborate, on the surface of mechanical supporters (polycaprolactone and collagen struts) with a topographical cue, HUVECs-laden alginate bioink was uniaxially electrospun. The electrospun HUVECs showed high cell viability (90%), homogeneous cell distribution, and efficient HUVEC growth. Furthermore, the myoblasts (C2C12 cells), which were seeded on the vascularized structure (HUVECs-laden fibers), were co-cultured to facilitate myoblast regeneration. As a result, the scaffold that included myoblasts and HUVECs represented a high degree of the myosin heavy chain (MHC) with striated patterns and enhanced myogenic-specific gene expressions (MyoD, troponin T, MHC and myogenin) as compared to the scaffold that included only myoblasts. STATEMENT OF SIGNIFICANCE: Cell electrospinning is an advanced electrospinning method that improves cell-matrix interactions by embedding cells directly into micro/nanofibers. Here, cell electrospinning was employed to achieve not only the homogeneous human umbilical vein endothelial cells (HUVECs) distribution with a high cell-viability (~90%), but also highly aligned topographical cue. Moreover, the uniaxially micropatterned PCL/collagen struts as a physical support were generated using three-dimensional (3D) printing, and was covered with HUVEC-laden micro/nanofibers. This hierarchical structure provided meaningful mechanical stability, homogeneous cell distribution, and HUVEC transformation into a narrow, elongated structure. Furthermore, the myoblasts (C2C12 cells) were seeded on the HUVECs-laden fibers and cocultured to facilitate myogenesis. In brief, a myosin heavy chain with striated patterns and enhanced myogenic specific gene expressions were represented.

A Myoblast-Laden Collagen Bioink with Fully Aligned Au Nanowires for Muscle-Tissue Regeneration

Contact guidance can promote cell alignment and is thus widely employed in tissue regeneration. In particular, skeletal muscle consists of long fibrous bundles of multinucleated myotubes formed by the fusion and differentiation of the satellite cells of myoblasts. Herein, a functional bioink and cell-printing process supplemented with an electric field are proposed for obtaining highly aligned myoblasts in a collagen-based bioink. To achieve the goal, we mixed Au nanowires (GNWs) with the collagen-based bioink to provide aligned topographical cues to the laden cells. Because the aligned GNWs could clearly provide topographical cues to the cells, we adjusted various processing parameters (flow rate, nozzle speed, and processing temperature) and applied an external electric field to optimally align the GNWs. By selecting an appropriate condition, the GNWs in the printed C2C12-laden structure were well aligned in the printing direction, and they eventually induced a high degree of myoblast alignment and efficient myotube formation. Through the several in vitro cellular activities and in vivo works revealing the myogenesis of the cell-laden structure, we conclude that the collagen/GNW-based cell-laden structure fabricated using the proposed method is a new prospective platform for the effective formation of muscle tissues.

Directional topography gradients drive optimum alignment and differentiation of human myoblasts

Tissue engineering of skeletal muscle aims to replicate the parallel alignment of myotubes on the native tissue. Directional topography gradients allow the study of the influence of topography on cellular orientation, proliferation, and differentiation, resulting in yield cues and clues to develop a proper in vitro environment for muscle tissue engineering. In this study, we used a polydimethylsiloxane-based substrate containing an aligned topography gradient with sinusoidal features ranging from wavelength (λ) = 1,520 nm and amplitude (A) =176 nm to λ = 9,934 nm and A = 2,168 nm. With this topography gradient, we evaluated the effect of topography on human myoblasts distribution, dominant orientation, cell area, nuclei coverage, cell area per number of nuclei, and nuclei area of myotubes. We showed that human myoblasts aligned and differentiated irrespective of the topography section. In addition, aligned human myotubes showed functionality and maturity by contracting spontaneously and nuclei peripheral organization resembling natural myotubes.

(-)-Epicatechin stimulates mitochondrial biogenesis and cell growth in C2C12 myotubes via the G-protein coupled estrogen receptor

We have reported on the capacity of (-)-epicatechin ((-)-EPI) to stimulate mitochondrial biogenesis (MiB) in mouse skeletal muscle (SkM). However, the mechanisms mediating the effects of (-)-EPI are not fully understood. We previously identified a role of the G-protein coupled estrogen receptor (GPER) in modulating the vascular effects of (-)-EPI. We therefore tested the hypothesis that GPER mediates (at least in part) the stimulatory effects of (-)-EPI on MiB in SkM cells. As an in vitro model, we employed mouse SkM-derived C2C12 myoblasts differentiated into myotubes. Using confocal microscopy, we detected GPER at the cell surface and cytoplasm in C2C12 myotubes. Treatment with (-)-EPI (3 and 10μM) resulted in the stimulation of MiB as per increases in mitochondrial inner (MitoTracker Red FM fluorescence staining) and outer membrane (porin protein levels) markers, transcription factors involved in MiB stimulation (i.e., nuclear respiratory factor-2 [NRF-2] and mitochondrial transcription factor A [TFAM] protein levels) and citrate synthase (CS) activity levels. (-)-EPI-treated myotubes were longer and wider compared to vehicle-treated myotubes. The effects of (-)-EPI on myotube mitochondria and cell size were larger in magnitude to those observed with the GPER agonist G-1. The chemical blockade and down-regulation (siRNA) of GPER evidenced a partial and complete blockade of measured endpoints following (-)-EPI- or G-1-treatment, respectively. Altogether, results indicate that GPER is expressed in muscle cells and appears to mediate to a significant extent, the stimulatory effects of (-)-EPI on MiB. Thus, GPER activation may account for the stimulatory effects of (-)-EPI on SkM structure/function.

MyoScreen, a High-Throughput Phenotypic Screening Platform Enabling Muscle Drug Discovery

Despite the need for more effective drug treatments to address muscle atrophy and disease, physiologically accurate in vitro screening models and higher information content preclinical assays that aid in the discovery and development of novel therapies are lacking. To this end, MyoScreen was developed: a robust and versatile high-throughput high-content screening (HT/HCS) platform that integrates a physiologically and pharmacologically relevant micropatterned human primary skeletal muscle model with a panel of pertinent phenotypic and functional assays. MyoScreen myotubes form aligned, striated myofibers, and they show nerve-independent accumulation of acetylcholine receptors (AChRs), excitation-contraction coupling (ECC) properties characteristic of adult skeletal muscle and contraction in response to chemical stimulation. Reproducibility and sensitivity of the fully automated MyoScreen platform are highlighted in assays that quantitatively measure myogenesis, hypertrophy and atrophy, AChR clusterization, and intracellular calcium release dynamics, as well as integrating contractility data. A primary screen of 2560 compounds to identify stimulators of myofiber regeneration and repair, followed by further biological characterization of two hits, validates MyoScreen for the discovery and testing of novel therapeutics. MyoScreen is an improvement of current in vitro muscle models, enabling a more predictive screening strategy for preclinical selection of the most efficacious new chemical entities earlier in the discovery pipeline process.

Ternary Aligned Nanofibers of RGD Peptide-Displaying M13 Bacteriophage/PLGA/Graphene Oxide for Facilitated Myogenesis

Recently, there have been tremendous efforts to develop the biofunctional scaffolds by incorporating various biochemical factors. In the present study, we fabricated poly(lactic-co-glycolic acid) (PLGA) nanofiber sheets decorated with graphene oxide (GO) and RGD peptide. The decoration of GO and RGD peptide was readily achieved by using RGD peptide-displaying M13 bacteriophage (RGD-M13 phage) and electrospinning. Furthermore, the aligned GO-decorated PLGA/RGD peptide (GO-PLGA/RGD) ternary nanofiber sheets were prepared by magnetic field-assisted electrospinning, and their potentials as bifunctional scaffolds for facilitating myogenesis were explored. We characterized the physicochemical and mechanical properties of the sheets by scanning electron microscopy, Raman spectroscopy, contact angle measurement, and tensile test. In addition, the C2C12 skeletal myoblasts were cultured on the aligned GO-PLGA/RGD nanofiber sheets, and their cellular behaviors, including initial attachment, proliferation and myogenic differentiation, were evaluated. Our results revealed that the GO-PLGA/RGD nanofiber sheets had suitable physicochemical and mechanical properties for supporting cell growth, and could significantly promote the spontaneous myogenic differentiation of C2C12 skeletal myoblasts. Moreover, it was revealed that the myogenic differentiation was further accelerated on the aligned GO-PLGA/RGD nanofiber sheets due to the synergistic effects of RGD peptide, GO and aligned nanofiber structure. Therefore, , it is suggested that the aligned GO-PLGA/RGD ternary nanofiber sheets are one of the most promising approaches for facilitating myogenesis and promoting skeletal tissue regeneration.

Gelatin-genipin-based biomaterials for skeletal muscle tissue engineering

Skeletal muscle engineering aims at tissue reconstruction to replace muscle loss following traumatic injury or in congenital muscle defects. Skeletal muscle can be engineered by using biodegradable and biocompatible scaffolds that favor myogenic cell adhesion and subsequent tissue organization. In this study, we characterized scaffolds made of gelatin cross-linked with genipin, a natural derived cross-linking agent with low cytotoxicity and high biocompatibility, for tissue engineering of skeletal muscle. We generated gelatin-genipin hydrogels with a stiffness of 13 kPa to reproduce the mechanical properties characteristic of skeletal muscle and we show that their surface can be topographically patterned through soft lithography to drive myogenic cells differentiation and unidirectional orientation. Furthermore, we demonstrate that these biomaterials can be successfully implanted in vivo under dorsal mouse skin, showing good biocompatibility and slow biodegradation rate. Moreover, the grafting of this biomaterial in partially ablated tibialis anterior muscle does not impair muscle regeneration, supporting future applications of gelatin-genipin biomaterials in the field of skeletal muscle tissue repair. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 2763-2777, 2018.

Non-equivalence of nuclear import among nuclei in multinucleated skeletal muscle cells

Skeletal muscle is primarily composed of large myofibers containing thousands of post-mitotic nuclei distributed throughout a common cytoplasm. Protein production and localization in specialized myofiber regions is crucial for muscle function. Myonuclei differ in transcriptional activity and protein accumulation, but how these differences among nuclei sharing a cytoplasm are achieved is unknown. Regulated nuclear import of proteins is one potential mechanism for regulating transcription spatially and temporally in individual myonuclei. The best-characterized nuclear localization signal (NLS) in proteins is the classical NLS (cNLS), but many other NLS motifs exist. We examined cNLS and non-cNLS reporter protein import using multinucleated muscle cells generated in vitro, revealing that cNLS and non-cNLS nuclear import differs among nuclei in the same cell. Investigation of cNLS nuclear import rates in isolated myofibers ex vivo confirmed differences in nuclear import rates among myonuclei. Analyzing nuclear import throughout myogenesis revealed that cNLS and non-cNLS import varies during differentiation. Taken together, our results suggest that both spatial and temporal regulation of nuclear import pathways are important in muscle cell differentiation and protein regionalization in myofibers.

Study of myoblast differentiation using multi-dimensional scaffolds consisting of nano and micropatterns

BACKGROUND

The topographical cue is major influence on skeletal muscle cell culture because the structure is highly organized and consists of long parallel bundles of multinucleated myotubes that are formed by differentiation and fusion of myoblast satellite cells. In this technical report, we fabricated a multiscale scaffold using electrospinning and poly (ethylene glycol) (PEG) hydrogel micropatterns to monitor the cell behaviors on nano- and micro-alignment combined scaffolds with different combinations of angles.

RESULTS

We fabricated multiscale scaffolds that provide biocompatible and extracellular matrix (ECM)-mimetic environments via electrospun nanofiber and PEG hydrogel micro patterning. MTT assays demonstrated an almost four-fold increase in the proliferation rate during the 7 days of cell culture for all of the experimental groups. Cell orientation and elongation were measured to confirm the myogenic potential. On the aligned fibrous scaffolds, more than 90% of the cells were dispersed ± 20° of the fiber orientation. To determine cell elongation, we monitored nuclei aspect ratios. On a random nanofiber, the cells demonstrated an aspect ratio of 1.33, but on perpendicular and parallel nanofibers, the aspect ratio was greater than 2. Myosin heavy chain (MHC) expression was significantly higher i) on parallel compared to random fibers, ii) the 100 μm compared to the 200 μm line pattern. We confirmed the disparate trends of myotube formation that can be provoked through multi-dimensional scaffolds.

CONCLUSION

We studied more favorable environments that induce cell alignment and elongation for myogenesis by combining nano- and micro-scale patterns. The fabricated system can serve as a novel multi-dimensional platform to study in vitro cell behaviors.

Anisotropic Materials for Skeletal-Muscle-Tissue Engineering

Repair of damaged skeletal-muscle tissue is limited by the regenerative capacity of the native tissue. Current clinical approaches are not optimal for the treatment of large volumetric skeletal-muscle loss. As an alternative, tissue engineering represents a promising approach for the functional restoration of damaged muscle tissue. A typical tissue-engineering process involves the design and fabrication of a scaffold that closely mimics the native skeletal-muscle extracellular matrix (ECM), allowing organization of cells into a physiologically relevant 3D architecture. In particular, anisotropic materials that mimic the morphology of the native skeletal-muscle ECM, can be fabricated using various biocompatible materials to guide cell alignment, elongation, proliferation, and differentiation into myotubes. Here, an overview of fundamental concepts associated with muscle-tissue engineering and the current status of muscle-tissue-engineering approaches is provided. Recent advances in the development of anisotropic scaffolds with micro- or nanoscale features are reviewed, and how scaffold topographical, mechanical, and biochemical cues correlate to observed cellular function and phenotype development is examined. Finally, some recent developments in both the design and utility of anisotropic materials in skeletal-muscle-tissue engineering are highlighted, along with their potential impact on future research and clinical applications.

Homologous Transcription Factors DUX4 and DUX4c Associate with Cytoplasmic Proteins during Muscle Differentiation

Hundreds of double homeobox (DUX) genes map within 3.3-kb repeated elements dispersed in the human genome and encode DNA-binding proteins. Among these, we identified DUX4, a potent transcription factor that causes facioscapulohumeral muscular dystrophy (FSHD). In the present study, we performed yeast two-hybrid screens and protein co-purifications with HaloTag-DUX fusions or GST-DUX4 pull-down to identify protein partners of DUX4, DUX4c (which is identical to DUX4 except for the end of the carboxyl terminal domain) and DUX1 (which is limited to the double homeodomain). Unexpectedly, we identified and validated (by co-immunoprecipitation, GST pull-down, co-immunofluorescence and in situ Proximal Ligation Assay) the interaction of DUX4, DUX4c and DUX1 with type III intermediate filament protein desmin in the cytoplasm and at the nuclear periphery. Desmin filaments link adjacent sarcomere at the Z-discs, connect them to sarcolemma proteins and interact with mitochondria. These intermediate filament also contact the nuclear lamina and contribute to positioning of the nuclei. Another Z-disc protein, LMCD1 that contains a LIM domain was also validated as a DUX4 partner. The functionality of DUX4 or DUX4c interactions with cytoplasmic proteins is underscored by the cytoplasmic detection of DUX4/DUX4c upon myoblast fusion. In addition, we identified and validated (by co-immunoprecipitation, co-immunofluorescence and in situ Proximal Ligation Assay) as DUX4/4c partners several RNA-binding proteins such as C1QBP, SRSF9, RBM3, FUS/TLS and SFPQ that are involved in mRNA splicing and translation. FUS and SFPQ are nuclear proteins, however their cytoplasmic translocation was reported in neuronal cells where they associated with ribonucleoparticles (RNPs). Several other validated or identified DUX4/DUX4c partners are also contained in mRNP granules, and the co-localizations with cytoplasmic DAPI-positive spots is in keeping with such an association. Large muscle RNPs were recently shown to exit the nucleus via a novel mechanism of nuclear envelope budding. Following DUX4 or DUX4c overexpression in muscle cell cultures, we observed their association with similar nuclear buds. In conclusion, our study demonstrated unexpected interactions of DUX4/4c with cytoplasmic proteins playing major roles during muscle differentiation. Further investigations are on-going to evaluate whether these interactions play roles during muscle regeneration as previously suggested for DUX4c.

Microfabrication and microfluidics for muscle tissue models

The relatively recent development of microfluidic systems with wide-ranging capabilities for generating realistic 2D or 3D systems with single or multiple cell types has given rise to an extensive collection of platform technologies useful in muscle tissue engineering. These new systems are aimed at (i) gaining fundamental understanding of muscle function, (ii) creating functional muscle constructs in vitro, and (iii) utilizing these constructs a variety of applications. Use of microfluidics to control the various stimuli that promote differentiation of multipotent cells into cardiac or skeletal muscle is first discussed. Next, systems that incorporate muscle cells to produce either 2D sheets or 3D tissues of contractile muscle are described with an emphasis on the more recent 3D platforms. These systems are useful for fundamental studies of muscle biology and can also be incorporated into drug screening assays. Applications are discussed for muscle actuators in the context of microrobotics and in miniaturized biological pumps. Finally, an important area of recent study involves coculture with cell types that either activate muscle or facilitate its function. Limitations of current designs and the potential for improving functionality for a wider range of applications is also discussed, with a look toward using current understanding and capabilities to design systems of greater realism, complexity and functionality.

Insulin-Based Regulation of Glucose-functionalized Nanoparticle Uptake in Muscle Cells

Effective regulation of nanoparticle (NP) uptake facilitates the NP-based therapeutics and diagnostics. Here, we report the use of insulin and 2-deoxyglucose (2-DG) to modulate the cellular uptake of glucose-functionalized quantum dots (Glc-QDs) in C2C12 muscle cells. The cellular uptake of Glc-QDs can be modulated up to almost two-fold under insulin stimulation while be down-regulated in the presence of 2-DG. These results demonstrate the use of secondary regulators to control the cellular uptake of NPs through membrane protein recognition in a specific and fine-tunable fashion.

Are cultured human myotubes far from home?

Satellite cells can be isolated from skeletal muscle biopsies, activated to proliferating myoblasts and differentiated into multinuclear myotubes in culture. These cell cultures represent a model system for intact human skeletal muscle and can be modulated ex vivo. The advantages of this system are that the most relevant genetic background is available for the investigation of human disease (as opposed to rodent cell cultures), the extracellular environment can be precisely controlled and the cells are not immortalized, thereby offering the possibility of studying innate characteristics of the donor. Limitations in differentiation status (fiber type) of the cells and energy metabolism can be improved by proper treatment, such as electrical pulse stimulation to mimic exercise. This review focuses on the way that human myotubes can be employed as a tool for studying metabolism in skeletal muscles, with special attention to changes in muscle energy metabolism in obesity and type 2 diabetes.

Injectable skeletal muscle matrix hydrogel promotes neovascularization and muscle cell infiltration in a hindlimb ischemia model

Peripheral artery disease (PAD) currently affects approximately 27 million patients in Europe and North America, and if untreated, may progress to the stage of critical limb ischemia (CLI), which has implications for amputation and potential mortality. Unfortunately, few therapies exist for treating the ischemic skeletal muscle in these conditions. Biomaterials have been used to increase cell transplant survival as well as deliver growth factors to treat limb ischemia; however, existing materials do not mimic the native skeletal muscle microenvironment they are intended to treat. Furthermore, no therapies involving biomaterials alone have been examined. The goal of this study was to develop a clinically relevant injectable hydrogel derived from decellularized skeletal muscle extracellular matrix and examine its potential for treating PAD as a stand-alone therapy by studying the material in a rat hindlimb ischemia model. We tested the mitogenic activity of the scaffold's degradation products using an in vitro assay and measured increased proliferation rates of smooth muscle cells and skeletal myoblasts compared to collagen. In a rat hindlimb ischemia model, the femoral artery was ligated and resected, followed by injection of 150 µL of skeletal muscle matrix or collagen 1 week post-injury. We demonstrate that the skeletal muscle matrix increased arteriole and capillary density, as well as recruited more desmin-positive and MyoD-positive cells compared to collagen. Our results indicate that this tissue-specific injectable hydrogel may be a potential therapy for treating ischemia related to PAD, as well as have potential beneficial effects on restoring muscle mass that is typically lost in CLI.

An image analysis method for the precise selection and quantitation of fluorescently labeled cellular constituents: application to the measurement of human muscle cells in culture

The accurate measurement of the morphological characteristics of cells with nonuniform conformations presents difficulties. We report here a straightforward method using immunofluorescent staining and the commercially available imaging program Adobe Photoshop, which allows objective and precise information to be gathered on irregularly shaped cells. We have applied this measurement technique to the analysis of human muscle cells and their immunologically marked intracellular constituents, as these cells are prone to adopting a highly branched phenotype in culture. Use of this method can be used to overcome many of the long-standing limitations of conventional approaches for quantifying muscle cell size in vitro. In addition, wider applications of Photoshop as a quantitative and semiquantitative tool in immunocytochemistry are explored.

In vitro expression profiling of myostatin, follistatin, decorin and muscle-specific transcription factors in adult caprine contractile myotubes

Skeletal muscle is one of the several adult postmitotic tissues that retain the capacity to regenerate, which relies on a population of quiescent precursors, termed satellite cells. Proliferation and differentiation of myoblasts to form mature myotubes in vitro has been a valuable tool in the characterization of the cellular events during myogenesis, which is a multistep process starting with progenitor cell proliferation, followed by their exit from the cell cycle, differentiation, alignment, and fusion to form multinucleated myotubes. A typical feature during muscle differentiation is the variation in expression of various genes along with myogenic factors. In this experiment, mRNA level of myostatin, follistatin, decorin and three muscle-specific transcription factors in adult caprine contractile myotubes have been studied through quantitative real time PCR. We observed that the expression level of myostatin, decorin, Myf5 and myogenin transcripts were significantly higher in contractile myotubes compared to myoblast monolayer (P < 0.05), and follistatin level was similar in both types of cells, whereas MyoD transcript level was significantly high in monolayer culture which might be due heterogeneity of myoblast population. It is concluded that the information generated would provide the base line information as well as monitoring markers to undertake experiments aimed at modulating muscle growth.

Micropatterning contractile C2C12 myotubes embedded in a fibrin gel

Contractile C(2)C(12) myotube line patterns embedded in a fibrin gel have been developed to afford a physiologically relevant and stable bioassay system. The C(2)C(12) myotube/fibrin gel system was prepared by transferring a myotube monolayer from a glass substrate to a fibrin gel while retaining the original line patterns of myotubes. To endow the myotubes with contractile activity, a series of electrical pulses was applied through a pair of carbon electrodes placed at either side of a fibrin gel separately. The frequency and magnitude of myotube contraction were functions of the pulse frequency and duration, respectively. We found that the myotubes supported by an elastic fibrin gel maintained their line patterns and contractile activities for a longer period of time (1 week) than myotubes adhered on a conventional culture dish.

Controlling the orientation and synaptic differentiation of myotubes with micropatterned substrates

Micropatterned poly(dimethylsiloxane) substrates fabricated by soft lithography led to large-scale orientation of myoblasts in culture, thereby controlling the orientation of the myotubes they formed. Fusion occurred on many chemically identical surfaces in which varying structures were arranged in square or hexagonal lattices, but only a subset of patterned surfaces yielded aligned myotubes. Remarkably, on some substrates, large populations of myotubes oriented at a reproducible acute angle to the lattice of patterned features. A simple geometrical model predicts the angle and extent of orientation based on maximizing the contact area between the myoblasts and patterned topographic surfaces. Micropatterned substrates also provided short-range cues that influenced higher-order functions such as the localization of focal adhesions and accumulation of postsynaptic acetylcholine receptors. Our results represent what we believe is a new approach for musculoskeletal tissue engineering, and our model sheds light on mechanisms of myotube alignment in vivo.

Direct cell surface modification with DNA for the capture of primary cells and the investigation of myotube formation on defined patterns

Previously, we reported a method for the attachment of living cells to surfaces through the hybridization of synthetic DNA strands attached to their plasma membrane. The oligonucleotides were introduced using metabolic carbohydrate engineering, which allowed reactive tailoring of the cell surface glycans for chemoselective bioconjugation. While this method is highly effective for cultured mammalian cells, we report here a significant improvement of this technique that allows the direct modification of cell surfaces with NHS-DNA conjugates. This method is rapid and efficient, allowing virtually any mammalian cell to be patterned on surfaces bearing complementary DNA in under 1 h. We demonstrate this technique using several types of cells that are generally incompatible with integrin-targeting approaches, including red blood cells and primary T-cells. Cardiac myoblasts were also captured. The immobilization procedure itself was found not to activate primary T-cells, in contrast to previously reported antibody- and lectin-based methods. Myoblast cells were patterned with high efficiency and remained undifferentiated after surface attachment. Upon changing to differentiation media, myotubes formed in the center of the patterned areas with an excellent degree of edge alignment. The availability of this new protocol greatly expands the applicability of the DNA-based attachment strategy for the generation of artificial tissues and the incorporation of living cells into device settings.

Effect of Electromechanical Stimulation on the Maturation of Myotubes on Aligned Electrospun Fibers

Tissue engineering may provide an alternative to cell injection as a therapeutic solution for myocardial infarction. A tissue-engineered muscle patch may offer better host integration and higher functional performance. This study examined the differentiation of skeletal myoblasts on aligned electrospun polyurethane (PU) fibers and in the presence of electromechanical stimulation. Skeletal myoblasts cultured on aligned PU fibers showed more pronounced elongation, better alignment, higher level of transient receptor potential cation channel-1 (TRPC-1) expression, upregulation of contractile proteins and higher percentage of striated myotubes compared to those cultured on random PU fibers and film. The resulting tissue constructs generated tetanus forces of 1.1 mN with a 10-ms time to tetanus. Additional mechanical, electrical, or synchronized electromechanical stimuli applied to myoblasts cultured on PU fibers increased the percentage of striated myotubes from 70 to 85% under optimal stimulation conditions, which was accompanied by an upregulation of contractile proteins such as α-actinin and myosin heavy chain. In describing how electromechanical cues can be combined with topographical cue, this study helped move towards the goal of generating a biomimetic microenvironment for engineering of functional skeletal muscle.