Hybrid Muscular Tissues: Preparation of Skeletal Muscle Cell-Incorporated Collagen Gels

Unraveling Muscle Impairment Associated With COVID-19 and the Role of 3D Culture in Its Investigation

COVID-19, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has been considered a public health emergency, extensively investigated by researchers. Accordingly, the respiratory tract has been the main research focus, with some other studies outlining the effects on the neurological, cardiovascular, and renal systems. However, concerning SARS-CoV-2 outcomes on skeletal muscle, scientific evidence is still not sufficiently strong to trace, treat and prevent possible muscle impairment due to the COVID-19. Simultaneously, there has been a considerable amount of studies reporting skeletal muscle damage in the context of COVID-19. Among the detrimental musculoskeletal conditions associated with the viral infection, the most commonly described are sarcopenia, cachexia, myalgia, myositis, rhabdomyolysis, atrophy, peripheral neuropathy, and Guillain-Barré Syndrome. Of note, the risk of developing sarcopenia during or after COVID-19 is relatively high, which poses special importance to the condition amid the SARS-CoV-2 infection. The yet uncovered mechanisms by which musculoskeletal injury takes place in COVID-19 and the lack of published methods tailored to study the correlation between COVID-19 and skeletal muscle hinder the ability of healthcare professionals to provide SARS-CoV-2 infected patients with an adequate treatment plan. The present review aims to minimize this burden by both thoroughly exploring the interaction between COVID-19 and the musculoskeletal system and examining the cutting-edge 3D cell culture techniques capable of revolutionizing the study of muscle dynamics.

Available In Vitro Models for Human Satellite Cells from Skeletal Muscle

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

Biomimetic sponges improve muscle structure and function following volumetric muscle loss

Skeletal muscle is inept in regenerating after traumatic injuries such as volumetric muscle loss (VML) due to significant loss of various cellular and acellular components. Currently, there are no approved therapies for the treatment of muscle tissue following trauma. In this study, biomimetic sponges composed of gelatin, collagen, laminin-111, and FK-506 were used for the treatment of VML in a rodent model. We observed that biomimetic sponge treatment improved muscle structure and function while modulating inflammation and limiting the extent of fibrotic tissue deposition. Specifically, sponge treatment increased the total number of myofibers, type 2B fiber cross-sectional area, myosin: collagen ratio, myofibers with central nuclei, and peak isometric torque compared to untreated VML injured muscles. As an acellular scaffold, biomimetic sponges may provide a promising clinical therapy for VML.

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

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

Biomimetic sponges for regeneration of skeletal muscle following trauma

Skeletal muscle is inept in regenerating after traumatic injuries due to significant loss of basal lamina and the resident satellite cells. To improve regeneration of skeletal muscle, we have developed biomimetic sponges composed of collagen, gelatin, and laminin (LM)-111 that were crosslinked with 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide (EDC). Collagen and LM-111 are crucial components of the muscle extracellular matrix and were chosen to impart bioactivity whereas gelatin and EDC were used to provide mechanical strength to the scaffold. Morphological and mechanical evaluation of the sponges showed porous structure, water-retention capacity and a compressive modulus of 590-808 kPa. The biomimetic sponges supported the infiltration and viability of C₂ C₁₂ myoblasts over 5 days of culture. The myoblasts produced higher levels of myokines such as VEGF, IL-6, and IGF-1 and showed higher expression of myogenic markers such as MyoD and myogenin on the biomimetic sponges. Biomimetic sponges implanted in a mouse model of volumetric muscle loss (VML) supported satellite, endothelial, and inflammatory cell infiltration but resulted in limited myofiber regeneration at 2 weeks post-injury. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 92-103, 2019.

In Vitro Tissue-Engineered Skeletal Muscle Models for Studying Muscle Physiology and Disease

Healthy skeletal muscle possesses the extraordinary ability to regenerate in response to small-scale injuries; however, this self-repair capacity becomes overwhelmed with aging, genetic myopathies, and large muscle loss. The failure of small animal models to accurately replicate human muscle disease, injury and to predict clinically-relevant drug responses has driven the development of high fidelity in vitro skeletal muscle models. Herein, the progress made and challenges ahead in engineering biomimetic human skeletal muscle tissues that can recapitulate muscle development, genetic diseases, regeneration, and drug response is discussed. Bioengineering approaches used to improve engineered muscle structure and function as well as the functionality of satellite cells to allow modeling muscle regeneration in vitro are also highlighted. Next, a historical overview on the generation of skeletal muscle cells and tissues from human pluripotent stem cells, and a discussion on the potential of these approaches to model and treat genetic diseases such as Duchenne muscular dystrophy, is provided. Finally, the need to integrate multiorgan microphysiological systems to generate improved drug discovery technologies with the potential to complement or supersede current preclinical animal models of muscle disease is described.

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.

Muscular Tissue Engineering: Capillary-Incorporated Hybrid Muscular Tissues in Vivo Tissue Culture

Requirements for a functional hybrid muscular tissue are 1) a high density of multinucleated cells, 2) a high degree of cellular orientation, and 3) the presence of a capillary network in the hybrid tissue. Rod-shaped hybrid muscular tissues composed of C2C12 cells (skeletal muscle myoblast cell line) and type I collagen, which were prepared using the centrifugal cell-packing method reported in our previous article, were implanted into nude mice. The grafts, comprised three hybrid tissues (each dimension, diameter, approximately 0.3 mm, length, approximately 1 mm, respectively), were inserted into the subcutaneous spaces on the backs of nude mice. All nude mice that survived the implantation were sacrificed at 1, 2, and 4 wk after the implantation. The grafts were easily distinguishable from the subcutaneous tissues of host mice with implantation time. The grafts increased in size with time after implantation, and capillary networks were formed in the vicinities and on the surfaces of the grafts. One week after implantation, many capillaries formed in the vicinities of the grafts. In the central portion of the graft, few capillaries and necrotic cells were observed. Mononucleated myoblasts were densely distributed and a low number of multinucleated myotubes were scattered. Two weeks after implantation, the formation of a capillary network was induced, resulting in the surfaces of the grafts being covered by capillaries. Numerous elongated multinucleated myotubes and mononucleated myoblasts were densely distributed and numerous capillaries were observed throughout the grafts. Four weeks after implantation a dense capillary network was formed in the vicinities and on the surfaces of the grafts. In the peripheral portion of the graft, multinucleated myotubes in the vicinities of the rich capillaries were observed. Thus, hybrid muscular tissues in vitro preconstructed was remodeled in vivo, which resulted in facilitating the incorporation of capillary networks into the tissues. © 1998 Elsevier Science Inc.

Tissue Engineered Skeletal Muscle: Preparation of Highly Dense, Highly Oriented Hybrid Muscular Tissues

We prepared highly dense, highly oriented hybrid muscular tissues that are composed of C2C12 cells (skeletal muscle myoblast cell line) and type I collagen. A cold mixture of C2C12 cells suspended in DMEM and type I collagen solution was poured into capillary tube molds of two different sizes (inner diameters; 0.90 and 0.53 mm, respectively). One end of each mold was sealed. Upon centrifugation (1000 rpm, 5 min) and subsequent thermal gelation, a rod-shaped gel was obtained. It was cultured in an agarose gel-coated dish for 7 days (first for 3 days in a growth medium and then for 4 days in a differentiation medium), during which time it shrank to become a highly dense tissue. Small-diameter rod-shaped, highly dense cellular assemblages with multinucleated myotubes were formed and only few necrotic cells at the core of the tissue were observed. On the other hand, a ring-shaped tissue prepared using a specially devised agarose gel mold was subjected to cyclic stretching at 60 rpm, resulting in the formation of a highly dense, highly oriented hybrid muscular tissue in which both densely accumulated cells and collagen fiber bundles tended to be aligned in the direction of stretching. The hybrid muscular tissues that were prepared using via sequential procedures of a centrifugal cell packing method and a mechanical stress-loading method became closer to native muscular tissues in terms of cell density and orientation.

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

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

Morphology and ultrastructure of differentiating three-dimensional mammalian skeletal muscle in a collagen gel

Because previous studies of three-dimensional skeletal muscle cultures have shown limited differentiation, the goal of this study was to establish conditions that would produce mature sarcomeres in a mammalian-derived skeletal muscle construct. We evaluated the differentiation of bioartificial muscles generated from C(2)C(12) myoblasts in a collagen gel cultured under steady, passive tension for up to 36 days. Staining for alpha-actinin, myosin, and F-actin indicated the presence of striated fibers as early as 6 days post-differentiation. Electron microscopy at 16 days post-differentiation revealed multinucleated myotubes with ordered, striated myofibers. At 33 days, the cultures contained collagen fibers and showed localization of paxillin at the fiber termini, suggesting that myotendinous junctions were forming. The present study demonstrates mature muscle synthesis in a three-dimensional system using a pure mammalian myoblast cell line. Our results suggest that this culture model can be used to evaluate the effects of various mechanical and biochemical cues on muscle development under normal and pathological conditions.

A composite gene delivery system consisting of polyethylenimine and an amphipathic peptide KALA

BACKGROUND

Animal viruses such as enveloped virus carry multi-functional proteins in the virion that can mediate more than two distinct steps of a gene delivery process during the transfer of viral genome into host cells. We tested if the aspects of the viral gene delivery mechanism could be mimicked by forming composite formulae from multi-functional synthetic gene carriers having complementary action modes.

METHODS

Polyethylenimine (PEI) was chosen as the component responsible for endosome escape and DNA condensation and KALA for cellular entry and DNA condensation. Compact DNA-carrier particles consisting of the core part where DNA chains were tightly condensed by PEI and the outer layer lined with KALA were formulated, characterized and compared with monolithic cationic formulae in terms of gene delivery efficiency and mechanism.

RESULTS

High-level gene expression was observed when C2C12 cells were transfected with DNA that was first partially condensed with PEI and, then, fully with KALA. In these formulae KALA mediated enhanced cellular entry of DNA by facilitating endocytic vesicle formation, while PEI provided an effective endosomolytic capacity. An optimal PEI/KALA formula showed transfection efficiencies better than or comparable to the commercial cationic liposome in various cell types in culture and in vivo.

CONCLUSIONS

Gene delivery by combining the membrane-active property of KALA with the endosomolytic activity of PEI can be more efficient than that by either of the properties alone. It appears that, in these formulae, the predominant role of KALA is to facilitate cellular entry of DNA by providing a fusogenic capability, rather than an endosomolytic activity.

Rapid formation of functional muscle in vitro using fibrin gels

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

Tissue engineered muscle implantation for tongue reconstruction: a preliminary report

OBJECTIVES/HYPOTHESIS

Because current tongue reconstructive methods introduce adynamic, variably sensate tissue into the mouth, the critical functions of the tongue in articulation and deglutition may be compromised. The objective of this work was to introduce a combination of myoblasts and scaffolding material into rat hemiglossectomy defects and to examine the extent of neomuscle formation in the reconstructed area, under the hypothesis that the presence of myoblasts leads to formation of new muscle.

STUDY DESIGN

Randomized, prospective animal study.

METHODS

Myoblasts were harvested from neonatal Lewis rats, and a growth factor enriched collagen gel was prepared. Syngeneic adult animals received either hemiglossectomy alone or reconstruction with one of four experimental reconstructive preparations: collagen gel alone, collagen gel with suspended myoblasts, the gel-cell combination in undifferentiated muscle construct form by way of tissue culture for 7 days in a preformed mold, or differentiated constructs, cultured in myoblast fusion medium. After 6 or 16 weeks, animal weight gain was recorded, animals were killed, and the tongues harvested. The tissue was examined histologically, and quality of the muscular regenerate was rated on a scale according to predefined criteria.

RESULTS

Animals in all groups gained weight appropriately. In groups receiving hemiglossectomy alone or acellular (gel only) reconstruction, there was significant scarring and lack of neomuscle formation. In groups receiving myoblast transplantation, either by way of gel suspension or in the form of undifferentiated or differentiated constructs, muscle quality was superior to controls.

CONCLUSIONS

Myoblast transplantation into hemiglossectomy defects appears to lead to new muscle formation and does not inhibit normal weight gain in animals after tongue implantation.

Novel therapeutic strategy for prevention of malignant tumor recurrence after surgery: Local delivery and prolonged release of adenovirus immobilized in photocured, tissue-adhesive gelatinous matrix

We have been developing a new gene delivery method using a styrenated gelatin-based tissue-adhesive matrix that allows in situ adenovirus-immobilized gel formation on living tissue and sustained virus release to permeate carcinoma tissue. Styrenated gelatin was synthesized by the condensation reaction of gelatin with 4-vinylbenzoic acid. Aqueous styrenated gelatin solution premixed with AdLacZ, adenovirus encoding beta-galactosidase cDNA, and carboxylated camphorquinone (CQ) as a photoinitiator was irradiated with visible light to form a styrenated gelatin gel. The in vitro adenovirus release from the styrenated gelatin gel to a medium strongly depended not on styrenated gelatin concentration but on CQ concentration. Maximal beta-galactosidase expression was observed on day 1, followed by a rapid decrease that continued up to 1 month for a styrenated gelatin gel prepared with a low styrenated gelatin concentration and a low CQ concentration. Dose-dependent reduced expression of beta-galactosidase activity with increasing CQ under photoirradiation was observed. AdLacZ-immobilized styrenated gelatin gel was formed on a hybrid tissue, which is a cell traction-induced collagenous gel entrapped with fibroblasts, and lacZ gene expression of fibroblasts in the hybrid tissue was observed for more than one month. The result of this in vitro model experiment implies that the tissue-adhesive styrenated gelatin may be applicable for the delivery of adenovirus encoding cDNA for tumor dormancy therapy into malignant tissue to prevent tumor recurrence after surgery when cDNA is properly selected.

Mechanical stimulation improves tissue-engineered human skeletal muscle

Human bioartificial muscles (HBAMs) are tissue engineered by suspending muscle cells in collagen/MATRIGEL, casting in a silicone mold containing end attachment sites, and allowing the cells to differentiate for 8 to 16 days. The resulting HBAMs are representative of skeletal muscle in that they contain parallel arrays of postmitotic myofibers; however, they differ in many other morphological characteristics. To engineer improved HBAMs, i.e., more in vivo-like, we developed Mechanical Cell Stimulator (MCS) hardware to apply in vivo-like forces directly to the engineered tissue. A sensitive force transducer attached to the HBAM measured real-time, internally generated, as well as externally applied, forces. The muscle cells generated increasing internal forces during formation which were inhibitable with a cytoskeleton depolymerizer. Repetitive stretch/relaxation for 8 days increased the HBAM elasticity two- to threefold, mean myofiber diameter 12%, and myofiber area percent 40%. This system allows engineering of improved skeletal muscle analogs as well as a nondestructive method to determine passive force and viscoelastic properties of the resulting tissue.

In situ hydrogelation of photocurable gelatin and drug release

We devised an in situ tissue-adhesive, drug-release technology based on a photoreactive gelatin, which allows in situ drug-incorporated gel formation on living tissues and sustained drug release directly on diseased tissues. Styrene-derivatized gelatins, synthesized by condensation reaction of gelatin with 4-vinylbenzoic acid, were photopolymerized in the presence of a water-soluble camphorquinone derivative as a photoinitiator upon visible-light irradiation to form swollen gels. Using albumin as a drug model, gelation characteristics and drug-release characteristics easily were manipulated by material variables, formulation variables, and operation variables. Tissue adhesivity of the gel was superior to that of fibrin glue. The biologic response, which was evaluated by intraperitoneal implantation in rats, showed that the gel was biodegraded and biosorbed, without cytotoxicity, within a few months after implantation. An in situ processable tissue-adhesive local drug release system effectively may be used to help inhibit tumor recurrence.

Fabrication of compliant hybrid grafts supported with elastomeric meshes

We devised tubular hybrid medial tissues with mechanical properties similar to those of native arteries, which were composed of bovine smooth muscle cells (SMCs) and type I collagen with minimal reinforcement with knitted fabric meshes made of synthetic elastomers. Three hybrid medial tissue models that incorporated segmented polyester (mesh A) or polyurethane-nylon (mesh B) meshes were designed: the inner, sandwich, and wrapping models. Hybrid medial tissues were prepared by pouring a cold mixed solution of SMCs and collagen into a tubular glass mold consisting of an inner mandrel and an outer sheath and subsequent thermal gelation, followed by further culture for 7 days. For the inner model, the mandrel was wrapped with a mesh. For the sandwich model, a cylindrically shaped mesh was incorporated into a space between the mandrel and the sheath. The wrapping model was prepared by wrapping a 7-day-incubated nonmesh gel with a mesh. The inner diameter was 3 mm, irrespective of the model, and the length was 2.5-4.0 cm, depending on the model. The intraluminal pressure-external diameter relationship showed that nonmesh and inner models had a very low burst strength below 50 mmHg, while the sandwich model ruptured at around 110-120 mmHg; no rupturing below 240 mmHg was observed for the wrapping model, regardless of the type of mesh used. Compliance values of wrapping and sandwich models were close to those of native arteries. Pressure-dependent distensibility characteristics similar to native arteries were observed for a mesh A wrapping model, whereas a mesh B wrapping model expanded almost linearly as intraluminal pressure increased, which appeared to be due to elasticity of the incorporated mesh. Thus, design criteria for hybrid vascular grafts with appropriate biomechanical matching with host arteries were established. Such hybrid grafts may be mechanically adapted in an arterial system.