Molecular and Biomechanical Adaptations to Mechanical Stretch in Cultured Myotubes

Tensile Forces and Nanofiber Alignment Influence Both Innervated and Non-Innervated Skeletal Myofiber Formation in Custom Mechanobioreactors

While it is understood that muscle tissue generates contractile forces, it is less appreciated that muscle dynamically responds to applied forces during development. We previously fabricated tissue engineered muscle comprised of skeletal myocytes in co-culture with spinal motor neurons on aligned nanofiber poly-caprolactone scaffolding, demonstrating that innervation elicited more robust myofibers and formation of neuromuscular junctions. The current study utilized custom mechanobioreactors to apply tensile elongation to this engineered muscle platform to explore the effects of exogenous forces and scaffold topology on innervated versus non-innervated myocytes. Nanofiber scaffold alignment played a significant role in myocyte thickness, width, and fusion under both innervated and non-innervated conditions. A combination of tensile loading and nanofiber alignment increased myocyte fusion, suggesting these parameters work together to expedite and enhance myofiber formation and maturation. Overall, this multi-faceted paradigm, featuring biomechanical loading, substrate topology, and innervation, mimics key features of the developmental microenvironment experienced by myocytes in vivo. These findings may facilitate more sophisticated studies on muscle development, function, and responses to trauma while also elucidating principles to support the fabrication of engineered muscle to repair major muscle defects.

Promoting and accelerating muscle regeneration through cell therapy in a mouse model

Objectives

Skeletal muscle injuries and disorders are universal clinical challenges with direct and indirect mechanisms and notable residual effects, such as prolonged, intense pain and physical disability. Stem cells, an innovative tool for cell therapy for musculoskeletal disorders, specifically promote skeletal muscle regeneration. This study was aimed at investigating the use of mesenchymal stem cells (MSCs) and their differentiated myocytes as a cell-based therapy to promote regeneration in damaged or diseased skeletal muscle.

Methods

Bone marrow mesenchymal stem cells (BM-MSCs) were isolated from the bone marrow of adult mice and grown in tissue culture flasks. The BM-MSCs were positive for CD90 and CD105, and negative for CD45 and CD34. These cells were induced with specific differentiation medium in vitro to differentiate into a skeletal muscle cell lineage over 7 days. Skeletal muscle differentiation was characterized according to morphology through hematoxylin and eosin staining, and scanning electron microscopy. Immunostaining for Myf-6, myosin heavy chain (MHC), and desmin-specific factors for skeletal muscle development-was performed to confirm skeletal muscle differentiation. An in vivo study in a muscle injury model was used to evaluate cell therapy based on naïve stem cells and differentiated myocytes.

Results

Cultured mouse BM-MSCS were positive for CD90 and CD105, and negative for CD45 and CD34. These cells developed into skeletal muscle with strong skeletal muscle differentiation potential, as confirmed by immunohistochemistry for the markers Myf6, MHC, and desmin. The differentiated myocytes showed better repair enhancement than undifferentiated stem cells after transplantations into a mouse model of skeletal muscle atrophy.

Conclusions

Myocytes derived from BM-MSCs may be incorporated into muscular atrophy treatment as a biological strategy for managing skeletal muscle diseases and injuries, thus advancing cell-based clinical treatments.

Generating fast-twitch myotubes in vitro with an optogenetic-based, quantitative contractility assay

The composition of fiber types within skeletal muscle impacts the tissue's physiological characteristics and susceptibility to disease and ageing. In vitro systems should therefore account for fiber-type composition when modelling muscle conditions. To induce fiber specification in vitro, we designed a quantitative contractility assay based on optogenetics and particle image velocimetry. We submitted cultured myotubes to long-term intermittent light-stimulation patterns and characterized their structural and functional adaptations. After several days of in vitro exercise, myotubes contract faster and are more resistant to fatigue. The enhanced contractile functionality was accompanied by advanced maturation such as increased width and up-regulation of neuron receptor genes. We observed an up-regulation in the expression of fast myosin heavy-chain isoforms, which induced a shift towards a fast-twitch phenotype. This long-term in vitro exercise strategy can be used to study fiber specification and refine muscle disease modelling.

Transcriptomic Response of Differentiating Porcine Myotubes to Thermal Stress and Donor Piglet Age

Climate change is a current concern that directly and indirectly affects agriculture, especially the livestock sector. Neonatal piglets have a limited thermoregulatory capacity and are particularly stressed by ambient temperatures outside their optimal physiological range, which has a major impact on their survival rate. In this study, we focused on the effects of thermal stress (35 °C, 39 °C, and 41 °C compared to 37 °C) on differentiating myotubes derived from the satellite cells of Musculus rhomboideus, isolated from two different developmental stages of thermolabile 5-day-old (p5) and thermostable 20-day-old piglets (p20). Analysis revealed statistically significant differential expression genes (DEGs) between the different cultivation temperatures, with a higher number of genes responding to cold treatment. These DEGs were involved in the macromolecule degradation and actin kinase cytoskeleton categories and were observed at lower temperatures (35 °C), whereas at higher temperatures (39 °C and 41 °C), the protein transport system, endoplasmic reticulum system, and ATP activity were more pronounced. Gene expression profiling of HSP and RBM gene families, which are commonly associated with cold and heat responses, exhibited a pattern dependent on temperature variability. Moreover, thermal stress exhibited an inhibitory effect on cell cycle, with a more pronounced downregulation during cold stress driven by ADGR genes. Additionally, our analysis revealed DEGs from donors with an undeveloped thermoregulation capacity (p5) and those with a fully developed thermoregulation capacity (p20) under various cultivation temperature. The highest number of DEGs and significant GO terms was observed under temperatures of 35 °C and 37 °C. In particular, under 35 °C, the DEGs were enriched in insulin, thyroid hormone, and calcium signaling pathways. This result suggests that the different thermoregulatory capacities of the donor piglets determined the ability of the primary muscle cell culture to differentiate into myotubes at different temperatures. This work sheds new light on the underlying molecular mechanisms that govern piglet differentiating myotube response to thermal stress and can be leveraged to develop effective thermal management strategies to enhance skeletal muscle growth.

A novel bistable device to study mechanosensitive cell responses to instantaneous stretch

The behavior of cells and tissues in vivo is determined by the integration of multiple biochemical and mechanical signals. Of the mechanical signals, stretch has been studied for decades and shown to contribute to pathophysiological processes. Several different stretch devices have been developed for in vitro investigations of cell stretch. In this work, we describe a new 3D-printed uniaxial stretching device for studying cell response to rapid deformation. The device is a bistable compliant mechanism holding two equilibrium states-an unstretched and stretched configuration-without the need of an external actuator. Furthermore, it allows multiple simultaneous measurements of different levels of stretch on a single substrate and is compatible with standard immunofluorescence imaging of fixed cells as well as live-cell imaging. To demonstrate the effectiveness of the device to stretch cells, a test case using aligned myotubes is presented. Leveraging material area changes associated with deformation of the substrate, changes in nuclei density provided evidence of affine deformation between cells and substrate. Furthermore, intranuclear deformations were also assessed and shown to deform non-affinely. As a proof-of-principle of the use of the device for mechanobiological studies, we uniaxially stretched aligned healthy and dystrophic myotubes that displayed different passive mechanical responses, consistent with previous literature in the field. We also identified a new feature in the mechanoresponse of dystrophic myotubes, which is of potential interest for identifying the diseased cells based on a quick mechanical readout. While some applications of the device for elucidating passive mechanical responses are demonstrated, the simplicity of the device allows it to be potentially used for other modes of deformation with little modifications.

Stretching muscle cells induces transcriptional and splicing transitions and changes in SR proteins

Alternative splicing is an RNA processing mechanism involved in skeletal muscle development and pathology. Muscular diseases exhibit splicing alterations and changes in mechanobiology leading us to investigate the interconnection between mechanical forces and RNA processing. We performed deep RNA-sequencing after stretching muscle cells. First, we uncovered transcriptional changes in genes encoding proteins involved in muscle function and transcription. Second, we observed that numerous mechanosensitive genes were part of the MAPK pathway which was activated in response to stretching. Third, we revealed that stretching skeletal muscle cells increased the proportion of alternatively spliced cassette exons and their inclusion. Fourth, we demonstrated that the serine and arginine-rich proteins exhibited stronger transcriptional changes than other RNA-binding proteins and that SRSF4 phosphorylation is mechanosensitive. Identifying SRSF4 as a mechanosensitive RNA-binding protein that might contribute to crosstalk between mechanotransduction, transcription, and splicing could potentially reveal novel insights into muscular diseases, particularly those with unknown etiologies.

FRET Visualization of Cyclic Stretch-Activated ERK via Calcium Channels Mechanosensation While Not Integrin β1 in Airway Smooth Muscle Cells

Mechanical stretch is one type of common physiological activities such as during heart beating, lung breathing, blood flow through the vessels, and physical exercise. The mechanical stimulations regulate cellular functions and maintain body homeostasis. It still remains to further characterize the mechanical-biomechanical coupling mechanism. Here we applied fluorescence resonance energy transfer (FRET) technology to visualize ERK activity in airway smooth muscle (ASM) cells under cyclic stretch stimulation in airway smooth muscle (ASM) cells, and studied the mechanosensing pathway. FRET measurements showed apparent ERK activation by mechanical stretch, which was abolished by ERK inhibitor PD98059 pretreatment. Inhibition of extracellular Ca²⁺ influx reduced ERK activation, and selective inhibition of inositol 1,4,5-trisphosphate receptor (IP₃R) Ca²⁺ channel or SERCA Ca²⁺ pump on endoplasmic reticulum (ER) blocked the activation. Chemical inhibition of the L-type or store-operated Ca²⁺ channels on plasma membrane, or inhibition of integrin β1 with siRNA had little effect on ERK activation. Disruption of actin cytoskeleton but not microtubule one inhibited the stretch-induced ERK activation. Furthermore, the ER IP₃R-dependent ERK activation was not dependent on phospholipase C-IP₃ signal, indicating possibly more mechanical mechanism for IP₃R activation. It is concluded from our study that the mechanical stretch activated intracellular ERK signal in ASM cells through membrane Ca²⁺ channels mechanosensation but not integrin β1, which was mediated by actin cytoskeleton.

Customized bioreactor enables the production of 3D diaphragmatic constructs influencing matrix remodeling and fibroblast overgrowth

The production of skeletal muscle constructs useful for replacing large defects in vivo, such as in congenital diaphragmatic hernia (CDH), is still considered a challenge. The standard application of prosthetic material presents major limitations, such as hernia recurrences in a remarkable number of CDH patients. With this work, we developed a tissue engineering approach based on decellularized diaphragmatic muscle and human cells for the in vitro generation of diaphragmatic-like tissues as a proof-of-concept of a new option for the surgical treatment of large diaphragm defects. A customized bioreactor for diaphragmatic muscle was designed to control mechanical stimulation and promote radial stretching during the construct engineering. In vitro tests demonstrated that both ECM remodeling and fibroblast overgrowth were positively influenced by the bioreactor culture. Mechanically stimulated constructs also increased tissue maturation, with the formation of new oriented and aligned muscle fibers. Moreover, after in vivo orthotopic implantation in a surgical CDH mouse model, mechanically stimulated muscles maintained the presence of human cells within myofibers and hernia recurrence did not occur, suggesting the value of this approach for treating diaphragm defects.

The Evolution of Complex Muscle Cell In Vitro Models to Study Pathomechanisms and Drug Development of Neuromuscular Disease

Many neuromuscular disease entities possess a significant disease burden and therapeutic options remain limited. Innovative human preclinical models may help to uncover relevant disease mechanisms and enhance the translation of therapeutic findings to strengthen neuromuscular disease precision medicine. By concentrating on idiopathic inflammatory muscle disorders, we summarize the recent evolution of the novel in vitro models to study disease mechanisms and therapeutic strategies. A particular focus is laid on the integration and simulation of multicellular interactions of muscle tissue in disease phenotypes in vitro. Finally, the requirements of a neuromuscular disease drug development workflow are discussed with a particular emphasis on cell sources, co-culture systems (including organoids), functionality, and throughput.