Development and evaluation of a removable tissue-engineered muscle with artificial tendons

Quercetin induces a slow myofiber phenotype in engineered human skeletal muscle tissues

Skeletal muscle comprises slow and fast myofibers, with slow myofibers excelling in aerobic metabolism and endurance. Quercetin, a polyphenol, is reported to induce slow myofibers in rodent skeletal muscle both in vitro and in vivo. However, its effect on human myofiber types remains unexplored. In this study, we evaluated quercetin's impact on slow myofiber induction using human skeletal muscle satellite cells. In a two-dimensional culture, quercetin enhanced gene expression, contributing to muscle differentiation, and significantly expanded the area of slow-type myosin heavy chain positive cells. It also elevated the gene expression of Pgc1α, an inducer of slow myofibers. Conversely, quercetin did not affect mitochondrial abundance, fission, or fusion, but it did increase the gene expression of Cox7A2L, which aids in promoting mitochondrial supercomplexity and endurance, and Mb, which contributes to oxidative phosphorylation. In a three-dimensional culture, quercetin significantly extended the time to peak tension and half relaxation time of the engineered human skeletal muscle tissues constructed on microdevices. Moreover, quercetin enhanced the muscle endurance of the tissues and curbed the rise in lactate secretion from the exercised tissues. These findings suggest that quercetin may induce slow myofibers in human skeletal muscle.

Investigating the Combined Effects of Mechanical Stress and Nutrition on Muscle Hypertrophic Signals Using Contractile 3D-Engineered Muscle (3D-EM)

Since 3D-EM closely resembles in vivo muscles, the aim of this study was to investigate the effects of exercise (electrical pulse stimulation (EPS)) and nutrition (maca), which contains triterpenes, on muscle hypertrophy by using 3D-EM for the first time. The 3D-EM was composed of C2C12 cells and type 1 collagen gel, was differentiated for 14 days, and was divided into four groups: control, maca, EPS, and maca + EPS. The medium was replaced every two days before each EPS intervention, and the concentration of maca in the culture solution was 1 mg/mL. The intervention conditions of the EPS were 30 V, 1 Hz, and 2 ms (24 h on, 24 h off, for one week). The expression levels of proteins were examined by Western blotting. The intervention of maca and EPS upregulated the expression of MHC-fast/slow (both p < 0.05) compared with the control group, and the addition of maca had no effect on the phosphorylation of mTOR (p = 0.287) but increased the AMPK phosphorylation (p = 0.001). These findings suggest that intervention with maca and EPS has a positive effect on muscle hypertrophy, which has a positive impact on sarcopenia. However, the underlying mechanisms remain to be further explored.

Removal of collagen three-dimensional scaffold bubbles utilizing a vacuum suction technique

The process of generating type I/II collagen scaffolds is fraught with bubble formation, which can interfere with the three-dimensional structure of the scaffold. Herein, we applied low-temperature vacuum freeze-drying to remove mixed air bubbles under negative pressure. Type I and II rubber sponges were acid-solubilized via acid lysis and enzymolysis. Thereafter, vacuum negative pressure was applied to remove bubbles, and the cover glass press method was applied to shape the type I/II original scaffold. Vacuum negative pressure was applied for a second time to remove any residual bubbles. Subsequent application of carbamide/N-hydroxysuccinimide cross-linked the scaffold. The traditional method was used as the control group. The structure and number of residual bubbles and pore sizes of the two scaffolds were compared. Based on the relationship between the pressure and the number of residual bubbles, a curve was created, and the time of ice formation was calculated. The bubble content of the experimental group was significantly lower than that of the control group (P < 0.05). The pore diameter of the type I/II collagen scaffold was higher in the experimental group than in the control group. The time of icing effect of type I and II collagen solution was 136.54 ± 5.26 and 144.40 ± 6.45 s, respectively. The experimental scaffold had a more regular structure with actively proliferating chondrocytes that possessed adherent pseudopodia. The findings indicated that the vacuum negative pressure method did not affect the physical or chemical properties of collagen, and these scaffolds exhibited good biocompatibility with chondrocytes.

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

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

Contractile force assessment methods for in vitro skeletal muscle tissues

Over the last few years, there has been growing interest in measuring the contractile force (CF) of engineered muscle tissues to evaluate their functionality. However, there are still no standards available for selecting the most suitable experimental platform, measuring system, culture protocol, or stimulation patterns. Consequently, the high variability of published data hinders any comparison between different studies. We have identified that cantilever deflection, post deflection, and force transducers are the most commonly used configurations for CF assessment in 2D and 3D models. Additionally, we have discussed the most relevant emerging technologies that would greatly complement CF evaluation with intracellular and localized analysis. This review provides a comprehensive analysis of the most significant advances in CF evaluation and its critical parameters. In order to compare contractile performance across experimental platforms, we have used the specific force (sF, kN/m2), CF normalized to the calculated cross-sectional area (CSA). However, this parameter presents a high variability throughout the different studies, which indicates the need to identify additional parameters and complementary analysis suitable for proper comparison. We propose that future contractility studies in skeletal muscle constructs report detailed information about construct size, contractile area, maturity level, sarcomere length, and, ideally, the tetanus-to-twitch ratio. These studies will hopefully shed light on the relative impact of these variables on muscle force performance of engineered muscle constructs. Prospective advances in muscle tissue engineering, particularly in muscle disease models, will require a joint effort to develop standardized methodologies for assessing CF of engineered muscle tissues.

Investigation of Brain Function-Related Myokine Secretion by Using Contractile 3D-Engineered Muscle

Brain function-related myokines, such as lactate, irisin, and cathepsin B (CTSB), are upstream factors that control brain-derived neurotrophic factor (BDNF) expression and are secreted from skeletal muscle by exercise. However, whether irisin and CTSB are secreted by muscle contraction remains controversial. Three-dimensional (3D)-engineered muscle (3D-EM) may help determine whether skeletal muscle contraction leads to the secretion of irisin and CTSB, which has never been identified with the addition of drugs in conventional 2D muscle cell cultures. We aimed to investigate the effects of electrical pulse stimulation (EPS)-evoked muscle contraction on irisin and CTSB secretion in 3D-EM. The 3D-EM, which consisted of C2C12 myoblasts and type-1 collagen gel, was allowed to differentiate for 2 weeks and divided into the control and EPS groups. EPS was applied at 13 V, 66 Hz, and 2 msec for 3 h (on: 5 s/off: 5 s). Irisin and CTSB secretion into the culture medium was measured by Western blotting. Irisin secretion was significantly increased following EPS (p < 0.05). However, there was no significant difference in CTSB secretion between the two groups. The present study suggests that irisin may be a contractile muscle-derived myokine, but CTSB is not secreted by EPS-evoked muscle contractile stimulation in 3D-EM.

Mechanical unloading in 3D-engineered muscle leads to muscle atrophy by suppressing protein synthesis

Three dimensional (3D)-engineered muscle is an useful approach to a more comprehensive understanding of molecular mechanisms underlying unloading-induced muscle atrophy. We investigated the effects of mechanical unloading on molecular muscle protein synthesis (MPS)- and muscle protein breakdown (MPB)-related signaling pathways involved in muscle atrophy in 3D-engineered muscle, and to better understand in vitro model of muscle disuse. The 3D-engineered muscle consisting of C2C12 myoblasts and type-1 collagen gel was allowed to differentiate for 2 weeks and divided into three groups: 0 days of stretched-on control (CON), 2 and/or 7 days of stretched-on (ON), in which both ends of the muscle were fixed with artificial tendons, and the stretched-off group (OFF), in which one side of the artificial tendon was detached. Muscle weight (-38.1 to -48.4%), length (-67.0 to -73.5%), twitch contractile force (-70.5 to -75.0%) and myosin heavy chain expression (-32.5 to -50.5%) in the OFF group were significantly decreased on days 2 and 7 compared with the ON group (P < 0.05, respectively), despite that ON group was stable over time. Although determinative molecular signaling could not be identified, the MPS rate reflected by puromysin labeled protein was significantly decreased following mechanical unloading (P < 0.05, -38.5 to -51.1%). Meanwhile, MPB, particularly the ubiquitin-proteasome pathway, was not impacted. Hence, mechanical unloading of 3D-engineered muscle in vitro leads to muscle atrophy by suppressing MPS, cell differentiation, and cell growth rather than the promotion of MPB.

Hypoxia transactivates cholecystokinin gene expression in 3D-engineered muscle

At high altitudes, the hypoxic atmosphere decreases the oxygen partial pressure in the body, inducing several metabolic changes in tissues and cells. Furthermore, it exerts potent anorectic effects, thus causing an energy deficit. Two decades ago, a marked increase in the resting level of plasma cholecystokinin (CCK) was observed in humans at the Mt. Kanchenjunga basecamp, located at 5100 m above the sea level, compared to sea-level control values. Interestingly, acute exercise also raises plasma CCK and exerts potent anorectic effects under normoxic conditions. However, the transcriptional regulations of Cck gene underlying these effects have not yet been established. Here, we employed acute electrical pulse stimulation (EPS) followed by microarray analysis to discover novel myokines in 3D-engineered muscle. Acute EPS affects the contractile function, inducing a decline in the contractile force. Surprisingly, microarray analysis revealed an EPS-induced activation of cholecystokinin receptor (CCKR)-mediated signaling. Furthermore, Cck was constitutively upregulated in 3D-engineered muscle, and its expression increased under hypoxic conditions. Notably, a hypoxia-responsive element was detected in the Cck promoters of mice and humans. Our results suggested that hypoxia transactivated Cck expression in 3D-engineered muscle. Furthermore, the elevation in plasma CCK levels following acute exercise or at high altitude might be partly attributed to myogenic cells.

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

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

Regenerative medicine for skeletal muscle loss: a review of current tissue engineering approaches

Skeletal muscle is capable of regeneration following minor damage, more significant volumetric muscle loss (VML) however results in permanent functional impairment. Current multimodal treatment methodologies yield variable functional recovery, with reconstructive surgical approaches restricted by limited donor tissue and significant donor morbidity. Tissue-engineered skeletal muscle constructs promise the potential to revolutionise the treatment of VML through the regeneration of functional skeletal muscle. Herein, we review the current status of tissue engineering approaches to VML; firstly the design of biocompatible tissue scaffolds, including recent developments with electroconductive materials. Secondly, we review the progenitor cell populations used to seed scaffolds and their relative merits. Thirdly we review in vitro methods of scaffold functional maturation including the use of three-dimensional bioprinting and bioreactors. Finally, we discuss the technical, regulatory and ethical barriers to clinical translation of this technology. Despite significant advances in areas, such as electroactive scaffolds and three-dimensional bioprinting, along with several promising in vivo studies, there remain multiple technical hurdles before translation into clinically impactful therapies can be achieved. Novel strategies for graft vascularisation, and in vitro functional maturation will be of particular importance in order to develop tissue-engineered constructs capable of significant clinical impact.

Large three-dimensional cell constructs for tissue engineering

Much research has been conducted on fabricating biomimetic biomaterials in vitro. Tissue engineering approaches are often conducted by combining cells, scaffolds, and growth factors. However, the degradation rate of scaffolds is difficult to control and the degradation byproducts occasionally limit tissue regeneration. To overcome these issues, we have developed a novel system using a thermo-responsive hydrogel that forms scaffold-free, three-dimensional (3D) cell constructs with arbitrary size and morphology. 3D cell constructs prepared using bone marrow-derived stromal stem cells (BMSCs) exhibited self-organizing ability and formed bone-like tissue with endochondral ossification. Endothelial cells were then introduced into the BMSC construct and a vessel-like structure was formed within the constructs. Additionally, the bone formation ability was promoted by endothelial cells and cell constructs could be freeze-dried to improve their clinical application. A pre-treatment with specific protein protectant allowed for the fabrication of novel bone substitutes composed only of cells. This 3D cell construct technology using thermo-responsive hydrogels was then applied to other cell species. Cell constructs composed of dental pulp stem cells were fabricated, and the resulting construct regenerated pulp-like tissue within a human pulpless tooth. In this review, we demonstrate the approaches for the in vitro fabrication of bone and dental pulp-like tissue using thermo-responsive hydrogels and their potential applications.

A multi-chamber tissue culture device for load-dependent parallel evaluation of tendon explants

Background

Injuries in the musculoskeletal system, such as tendon and ligament ruptures, are challenging to manage and often require surgical reconstructions with limited long-term success. Thus, characterizations of these tissues are urgently needed to better understand cellular mechanisms that regulate tissue homeostasis and healing. Explant culturing systems allow for ex vivo analysis of tissues in an environment that mimics the native microenvironment in vivo.

Methods

Collaborative efforts within our institution facilitated the establishment of a novel explant culturing system. Tissue specimens cultured in single wells, with individual applied loading and/or biological environment, allowed characterization of tissue cultured under a variety of biological loading conditions. Quantitative PCR analysis for selected gene markers was our primary outcome.

Results

Data were stratified for analysis by either culture environment or loading condition. Our gene expression results show that specimens clustered by culture condition may differ in molecular markers related to ECM production (e.g., Col1a1, Adamts4) and/or organization (e.g., Tnc, Dnc). In contrast, loading condition did significantly alter the median gene expression levels of tissues in comparison to unloaded control samples, although gene expression values related to ECM degradation (e.g., Mmp1, Mmp10) were altered in tendons cultured under tension in the device.

Conclusion

Our study demonstrates promising utility of a novel explant culturing system for further characterization of musculoskeletal tissues such as native tendons and ligaments, as well as pathologic fibrotic tissues resulting from arthrofibrosis or Dupuytren’s disease.