Adhesion-contractile balance in myocyte differentiation

Mechano-metabolism of metastatic breast cancer cells in 2D and 3D microenvironments

Cells regulate their shape and metabolic activity in response to the mechano-chemical properties of their microenvironment. To elucidate the impact of matrix stiffness and ligand density on the bioenergetics of mesenchymal cells, we developed a nonequilibrium, active chemo-mechanical model that accounts for the mechanical energy of the cell and matrix, chemical energy from ATP hydrolysis, interfacial energy, and mechano-sensitive regulation of stress fiber assembly through signaling. By integrating the kinetics and energetics of these processes, we define the cell "metabolic potential" that, when minimized, provides testable predictions of cell contractility, shape, and ATP consumption. Specifically, we show that the morphology of MDA-MB-231 breast cancer cells in 3D collagen changes from spherical to elongated to spherical with increasing matrix stiffness, which is consistent with experimental observations. On 2D hydrogels, our model predicts a hemispherical-to-spindle-to-disc shape transition with increasing gel stiffness. In both cases, we show that these shape transitions emerge from competition between the energy of ATP hydrolysis associated with increased contractility that drives cell elongation and the interfacial energy that favors a rounded shape. Furthermore, our model can predict how increased energy demand in stiffer microenvironments is met by AMPK activation, which is confirmed experimentally in both 2D and 3D microenvironments and found to correlate with the upregulation of mitochondrial potential, glucose uptake, and ATP levels, as well as provide estimates of changes in intracellular adenosine nucleotide concentrations with changing environmental stiffness. Overall, we present a framework for relating adherent cell energy levels and contractility through biochemical regulation of underlying physical processes.

Statement of Significance

Increasing evidence indicates that cellular metabolism is regulated by mechanical cues from the extracellular environment. Forces transmitted from the microenvironment activate mechanotransduction pathways in the cell, which trigger a cascade of biochemical events that impact cytoskeletal tension, cellular morphology and energy budget available to the cell. Using a nonequilibrium free energy-based theory, we can predict the ATP consumption, contractility, and shape of mesenchymal cancer cells, as well as how cells regulate energy levels dependent on the mechanosensitive metabolic regulator AMPK. The insights from our model can be used to understand the mechanosensitive regulation of metabolism during metastasis and tumor progression, during which cells experience dynamic changes in their microenvironment and metabolic state.

Translation of nanotechnology-based implants for orthopedic applications: current barriers and future perspective

The objective of bioimplant engineering is to develop biologically compatible materials for restoring, preserving, or altering damaged tissues and/or organ functions. The variety of substances used for orthopedic implant applications has been substantially influenced by modern material technology. Therefore, nanomaterials can mimic the surface properties of normal tissues, including surface chemistry, topography, energy, and wettability. Moreover, the new characteristics of nanomaterials promote their application in sustaining the progression of many tissues. The current review establishes a basis for nanotechnology-driven biomaterials by demonstrating the fundamental design problems that influence the success or failure of an orthopedic graft, cell adhesion, proliferation, antimicrobial/antibacterial activity, and differentiation. In this context, extensive research has been conducted on the nano-functionalization of biomaterial surfaces to enhance cell adhesion, differentiation, propagation, and implant population with potent antimicrobial activity. The possible nanomaterials applications (in terms of a functional nanocoating or a nanostructured surface) may resolve a variety of issues (such as bacterial adhesion and corrosion) associated with conventional metallic or non-metallic grafts, primarily for optimizing implant procedures. Future developments in orthopedic biomaterials, such as smart biomaterials, porous structures, and 3D implants, show promise for achieving the necessary characteristics and shape of a stimuli-responsive implant. Ultimately, the major barriers to the commercialization of nanotechnology-derived biomaterials are addressed to help overcome the limitations of current orthopedic biomaterials in terms of critical fundamental factors including cost of therapy, quality, pain relief, and implant life. Despite the recent success of nanotechnology, there are significant hurdles that must be overcome before nanomedicine may be applied to orthopedics. The objective of this review was to provide a thorough examination of recent advancements, their commercialization prospects, as well as the challenges and potential perspectives associated with them. This review aims to assist healthcare providers and researchers in extracting relevant data to develop translational research within the field. In addition, it will assist the readers in comprehending the scope and gaps of nanomedicine's applicability in the orthopedics field.

Recent developments in nanomaterials for upgrading treatment of orthopedics diseases

Nanotechnology has changed science in the last three decades. Recent applications of nanotechnology in the disciplines of medicine and biology have enhanced medical diagnostics, manufacturing, and drug delivery. The latest studies have demonstrated this modern technology's potential for developing novel methods of disease detection and treatment, particularly in orthopedics. According to recent developments in bone tissue engineering, implantable substances, diagnostics and treatment, and surface adhesives, nanomedicine has revolutionized orthopedics. Numerous nanomaterials with distinctive chemical, physical, and biological properties have been engineered to generate innovative medication delivery methods for the local, sustained, and targeted delivery of drugs with enhanced therapeutic efficacy and minimal or no toxicity, indicating a very promising strategy for effectively controlling illnesses. Extensive study has been carried out on the applications of nanotechnology, particularly in orthopedics. Nanotechnology can revolutionize orthopedics cure, diagnosis, and research. Drug delivery precision employing nanotechnology using gold and liposome nanoparticles has shown especially encouraging results. Moreover, the delivery of drugs and biologics for osteosarcoma is actively investigated. Different kind of biosensors and nanoparticles has been used in the diagnosis of bone disorders, for example, renal osteodystrophy, Paget's disease, and osteoporosis. The major hurdles to the commercialization of nanotechnology-based composite are eventually examined, thus helping in eliminating the limits in connection to some pre-existing biomaterials for orthopedics, important variables like implant life, quality, cure cost, and pain and relief from pain. The potential for nanotechnology in orthopedics is tremendous, and most of it looks to remain unexplored, but not without challenges. This review aims to highlight the up tp date developments in nanotechnology for boosting the treatment modalities for orthopedic ailments. Moreover, we also highlighted unmet requirements and present barriers to the practical adoption of biomimetic nanotechnology-based orthopedic treatments.

Matured Myofibers in Bioprinted Constructs with In Vivo Vascularization and Innervation

For decades, the study of tissue-engineered skeletal muscle has been driven by a clinical need to treat neuromuscular diseases and volumetric muscle loss. The in vitro fabrication of muscle offers the opportunity to test drug-and cell-based therapies, to study disease processes, and to perhaps, one day, serve as a muscle graft for reconstructive surgery. This study developed a biofabrication technique to engineer muscle for research and clinical applications. A bioprinting protocol was established to deliver primary mouse myoblasts in a gelatin methacryloyl (GelMA) bioink, which was implanted in an in vivo chamber in a nude rat model. For the first time, this work demonstrated the phenomenon of myoblast migration through the bioprinted GelMA scaffold with cells spontaneously forming fibers on the surface of the material. This enabled advanced maturation and facilitated the connection between incoming vessels and nerve axons in vivo without the hindrance of a scaffold material. Immunohistochemistry revealed the hallmarks of tissue maturity with sarcomeric striations and peripherally placed nuclei in the organized bundles of muscle fibers. Such engineered muscle autografts could, with further structural development, eventually be used for surgical reconstructive purposes while the methodology presented here specifically has wide applications for in vitro and in vivo neuromuscular function and disease modelling.

Bioactive micropatterned platform to engineer myotube-like cells from stem cells

Skeletal muscle has the capacity to repair and heal itself after injury. However, this self-healing ability is diminished in the event of severe injuries and myopathies. In such conditions, stem cell-based regenerative treatments can play an important part in post injury restoration. We herein report the development of a bioactive (integrin-β1 antibody immobilized) gold micropatterned platform to promote human mesenchymal stem cells (hMSCs) differentiation into the myotube-like cells. hMSCs grown on bioactive micropattern differentiated into the myotube-like cells within two weeks. Further, up-regulation of myogenic markers, multi-nucleated state with continuous actin cytoskeleton and absence of proliferation marker confirmed the formation of myotube-like cells on bioactive micropattern. Prominent expression of elongated integrin-β1 focal adhesions (ITG-β1 FAs) and development of anisotropic stress fibres in those differentiated cells elucidated their importance in stem cell myogenesis. Together these findings delineate the synergistic role of engineered cell anisotropy and ITG-β1 mediated signaling in the development of myotube-like cells from hMSCs.

π-SACS: pH Induced Self-Assembled Cell Sheets Without the Need for Modified Surfaces

The ability to form tissue-like constructs that have high cell density with proper cell-cell and cell-ECM interactions is critical for many applications including tissue models for drug discovery and tissue regeneration. Newly emerging bioprinting methods sometimes lack the high cellular density needed to provide biophysical cues to orchestrate cellular behavior to recreate tissue architecture and function. Alternate methods using self-assembly can be used to create tissue-like constructs with high cellular density and well-defined microstructure in the form of spheroids, organoids, or cell sheets. Cell sheets have a particularly interesting architecture in the context of tissue regeneration and repair as they can be applied as patches to integrate with surrounding tissues. Until now, the preparation of these sheets has involved culturing on specialized substrates that can be triggered by temperature or phase change (hydrophobic to hydrophilic) to release cells growing on them and form sheets. Here a new technique is proposed that allows delamination of cells and secreted ECM and rapid self-assembly into a cell sheet using a simple pH trigger and without the need to use responsive surfaces or applying external stimuli such as electrical and magnetic fields, only with routine tissue culture plates. This technique can be used with cells that are capable of syncytialization and fusion such as skeletal muscle cells and placenta cells. Using C2C12 myoblast cells we show that the pH trigger induces a rapid delamination of the cells as a continuous layer that self-assembles into a thick dense sheet. The delamination process has little effect on cell viability and maturation and preserves the ECM components that allow sheets to adhere to each other within a short incubation time enabling formation of thicker constructs when multiple sheets are stacked (double- and quadruple-layer constructs are formed here). These thick grafts can be used for regeneration purposes or as in vitro models.

Biomaterials in Tendon and Skeletal Muscle Tissue Engineering: Current Trends and Challenges

Tissue engineering is a promising approach to repair tendon and muscle when natural healing fails. Biohybrid constructs obtained after cells’ seeding and culture in dedicated scaffolds have indeed been considered as relevant tools for mimicking native tissue, leading to a better integration in vivo. They can also be employed to perform advanced in vitro studies to model the cell differentiation or regeneration processes. In this review, we report and analyze the different solutions proposed in literature, for the reconstruction of tendon, muscle, and the myotendinous junction. They classically rely on the three pillars of tissue engineering, i.e., cells, biomaterials and environment (both chemical and physical stimuli). We have chosen to present biomimetic or bioinspired strategies based on understanding of the native tissue structure/functions/properties of the tissue of interest. For each tissue, we sorted the relevant publications according to an increasing degree of complexity in the materials’ shape or manufacture. We present their biological and mechanical performances, observed in vitro and in vivo when available. Although there is no consensus for a gold standard technique to reconstruct these musculo-skeletal tissues, the reader can find different ways to progress in the field and to understand the recent history in the choice of materials, from collagen to polymer-based matrices.

The Preservation of Bone Cell Viability in a Human Femoral Head through a Perfusion Bioreactor

Current methods for drug development and discovery involve pre-clinical analyses that are extremely expensive and time consuming. Animal models are not the best precedent to use, when comparing to human models as they are not synonymous with the human response, thus, alternative methods for drug development are needed. One of which could be the use of an ex vivo human organ where drugs could be tested and the effects of those drugs could be observed. Finding a viable human organ to use in these preliminary ex vivo studies is difficult due to the availability, cost, and viability. Bone tissue and marrow contain a plethora of both bone and stem cells, however, these cells need constant perfusion to be viable over a longer time range. Here we maintain bone cell sustainability in an ex vivo model, through the use of human femoral heads in a novel bioreactor. This bioreactor was designed to directly perfuse cell culture media (DMEM) through the vasculature of a femoral head, providing ideal nutrients and conditions required for maintaining organ viability. We show, for the first time, that cells within a femoral head can stay alive up to 12 h. Further development could be used to determine the effects of drugs on a human organ system and could aid in the understanding of the progression of bone diseases and pathologies.

Prestressed cells are prone to cytoskeleton failures under localized shear strain: an experimental demonstration on muscle precursor cells

We report on a wavelet based space-scale decomposition method for analyzing the response of living muscle precursor cells (C2C12 myoblasts and myotubes) upon sharp indentation with an AFM cantilever and quantifying their aptitude to sustain such a local shear strain. Beyond global mechanical parameters which are currently used as markers of cell contractility, we emphasize the necessity of characterizing more closely the local fluctuations of the shear relaxation modulus as they carry important clues about the mechanisms of cytoskeleton strain release. Rupture events encountered during fixed velocity shear strain are interpreted as local disruptions of the actin cytoskeleton structures, the strongest (brittle) ones being produced by the tighter and stiffer stress fibers or actin agglomerates. These local strain induced failures are important characteristics of the resilience of these cells, and their aptitude to maintain their shape via a quick recovery from local strains. This study focuses on the perinuclear region because it can be considered as a master mechanical organizing center of these muscle precursor cells. Using this wavelet-based method, we combine the global and local approaches for a comparative analysis of the mechanical parameters of normal myoblasts, myotubes and myoblasts treated with actomyosin cytoskeleton disruptive agents (ATP depletion, blebbistatin).

A cell-printing approach for obtaining hASC-laden scaffolds by using a collagen/polyphenol bioink

In the cell-printing process, bioink has been considered as an extremely important component for successful fabrication of macroscale cell-laden structures. Bioink should be non-toxic, biocompatible, and printable. To date, alginate has been widely used as a whole or partial component of bioink because it is non-toxic to embedded cells and even it can provide good printability with rapid gelation under calcium ions. However, alginate bioinks do not possess cell-activating ability. To overcome the shortcomings of alginate-based bioinks, a new collagen bioink, which was mixed with human adipose stem cells (hASCs) and crosslinked with a polyphenol (tannic acid), was proposed. The feasibility of the bioink was demonstrated using several in vitro assessments for comparison of the macroscale porous cell-laden collagen/polyphenol structure containing the hASCs with the conventional alginate-based cell-laden structure. The levels of the metabolic activity, including the cell viability and cell proliferation, of the cell-laden collagen structure were significantly higher than those of the control (alginate-based cell-laden structure). The results show that the newly designed bioink and cell-laden structure are potentially new outstanding components for regeneration of various tissues.

Validation of Ultrasound Elastography Imaging for Nondestructive Characterization of Stiffer Biomaterials

Ultrasound elastography (UE) has been widely used as a "digital palpation" tool to characterize tissue mechanical properties in the clinic. UE benefits from the capability of noninvasively generating 2-D elasticity encoded maps. This spatial distribution of elasticity can be especially useful in the in vivo assessment of tissue engineering scaffolds and implantable drug delivery platforms. However, the detection limitations have not been fully characterized and thus its true potential has not been completely discovered. Characterization studies have focused primarily on the range of moduli corresponding to soft tissues, 20-600 kPa. However, polymeric biomaterials used in biomedical applications such as tissue scaffolds, stents, and implantable drug delivery devices can be much stiffer. In order to explore UE's potential to assess mechanical properties of biomaterials in a broader range of applications, this work investigated the detection limit of UE strain imaging beyond soft tissue range. To determine the detection limit, measurements using standard mechanical testing and UE on the same polydimethylsiloxane samples were compared and statistically evaluated. The broadest detection range found based on the current optimized setup is between 47 kPa and 4 MPa which exceeds the modulus of normal soft tissue suggesting the possibility of using this technique for stiffer materials' mechanical characterization. The detectable difference was found to be as low as 157 kPa depending on sample stiffness and experimental setup.

Biodegradable Polymers Influence the Effect of Atorvastatin on Human Coronary Artery Cells

Drug-eluting stents (DES) have reduced in-stent-restenosis drastically. Yet, the stent surface material directly interacts with cascades of biological processes leading to an activation of cellular defense mechanisms. To prevent adverse clinical implications, to date almost every patient with a coronary artery disease is treated with statins. Besides their clinical benefit, statins exert a number of pleiotropic effects on endothelial cells (ECs). Since maintenance of EC function and reduction of uncontrolled smooth muscle cell (SMC) proliferation represents a challenge for new generation DES, we investigated the effect of atorvastatin (ATOR) on human coronary artery cells grown on biodegradable polymers. Our results show a cell type-dependent effect of ATOR on ECs and SMCs. We observed polymer-dependent changes in IC50 values and an altered ATOR-uptake leading to an attenuation of statin-mediated effects on SMC growth. We conclude that the selected biodegradable polymers negatively influence the anti-proliferative effect of ATOR on SMCs. Hence, the process of developing new polymers for DES coating should involve the characterization of material-related changes in mechanisms of drug actions.

Nanofiber Yarn/Hydrogel Core-Shell Scaffolds Mimicking Native Skeletal Muscle Tissue for Guiding 3D Myoblast Alignment, Elongation, and Differentiation

Designing scaffolds that can mimic native skeletal muscle tissue and induce 3D cellular alignment and elongated myotube formation remains an ongoing challenge for skeletal muscle tissue engineering. Herein, we present a simple technique to generate core-shell composite scaffolds for mimicking native skeletal muscle structure, which comprise the aligned nanofiber yarn (NFY) core and the photocurable hydrogel shell. The aligned NFYs are prepared by the hybrid composition including poly(caprolactone), silk fibroin, and polyaniline via a developed dry-wet electrospinning method. A series of core-shell column and sheet composite scaffolds are ultimately obtained by encapsulating a piece and layers of aligned NFY cores within the hydrogel shell after photo-cross-linking. C2C12 myoblasts are seeded within the core-shell scaffolds, and the good biocompatibility of these scaffolds and their ability to induce 3D cellular alignment and elongation are successfully demonstrated. Furthermore, the 3D elongated myotube formation within core-shell scaffolds is also performed after long-term cultivation. These data suggest that these core-shell scaffolds combine the aligned NFY core that guides the myoblast alignment and differentiation and the hydrogel shell that provides a suitable 3D environment for nutrition exchange and mechanical protection to perform a great practical application for skeletal muscle regeneration.

Vascular smooth muscle cell functional contractility depends on extracellular mechanical properties

Vascular smooth muscle cells' primary function is to maintain vascular homeostasis through active contraction and relaxation. In diseases such as hypertension and atherosclerosis, this function is inhibited concurrent to changes in the mechanical environment surrounding vascular smooth muscle cells. It is well established that cell function and extracellular mechanics are interconnected; variations in substrate modulus affect cell migration, proliferation, and differentiation. To date, it is unknown how the evolving extracellular mechanical environment of vascular smooth muscle cells affects their contractile function. Here, we have built upon previous vascular muscular thin film technology to develop a variable-modulus vascular muscular thin film that measures vascular tissue functional contractility on substrates with a range of pathological and physiological moduli. Using this modified vascular muscular thin film, we found that vascular smooth muscle cells generated greater stress on substrates with higher moduli compared to substrates with lower moduli. We then measured protein markers typically thought to indicate a contractile phenotype in vascular smooth muscle cells and found that phenotype is unaffected by substrate modulus. These data suggest that mechanical properties of vascular smooth muscle cells' extracellular environment directly influence their functional behavior and do so without inducing phenotype switching.

Role of Cytoskeletal Tension in the Induction of Cardiomyogenic Differentiation in Micropatterned Human Mesenchymal Stem Cell

The role of biophysical induction methods such as cell micropatterning in stem cell differentiation has been well documented previously. However, the underlying mechanistic linkage of the engineered cell shape to directed lineage commitment remains poorly understood. Here, it is reported that micropatterning plays an important role in regulating the optimal cytoskeletal tension development in human mesenchymal stem cell (hMSC) via cell mechanotransduction pathways to induce cardiomyogenic differentiation. Cells are grown on fibronectin strip patterns to control cell polarization and morphology. These patterned cells eventually show directed commitment toward the myocardial lineage. The cell's mechanical properties (cell stiffness and cell traction forces) are observed to be very different for cells that have committed to the myocardial lineage when compared with that of control. These committed cells have mechanical properties that are significantly lower indicating a correlation between the micropatterning-induced differentiation and actomyosin-generated cytoskeletal tension within patterned cells. To study this correlation, patterned cells are treated with RhoA pathway inhibitor. Severely down-regulated cardiomyogenic marker expression is observed in those treated patterned cells, thus emphasizing the direct dependence of hMSCs differentiation fate on the cytoskeletal tension.

Elasticity of nanoparticles influences their blood circulation, phagocytosis, endocytosis, and targeting

The impact of physical and chemical modifications of nanoparticles on their biological function has been systemically investigated and exploited to improve their circulation and targeting. However, the impact of nanoparticles' flexibility (i.e., elastic modulus) on their function has been explored to a far lesser extent, and the potential benefits of tuning nanoparticle elasticity are not clear. Here, we describe a method to synthesize polyethylene glycol (PEG)-based hydrogel nanoparticles of uniform size (200 nm) with elastic moduli ranging from 0.255 to 3000 kPa. These particles are used to investigate the role of particle elasticity on key functions including blood circulation time, biodistribution, antibody-mediated targeting, endocytosis, and phagocytosis. Our results demonstrate that softer nanoparticles (10 kPa) offer enhanced circulation and subsequently enhanced targeting compared to harder nanoparticles (3000 kPa) in vivo. Furthermore, in vitro experiments show that softer nanoparticles exhibit significantly reduced cellular uptake in immune cells (J774 macrophages), endothelial cells (bEnd.3), and cancer cells (4T1). Tuning nanoparticle elasticity potentially offers a method to improve the biological fate of nanoparticles by offering enhanced circulation, reduced immune system uptake, and improved targeting.

CDH2 and CDH11 act as regulators of stem cell fate decisions

Accumulating evidence suggests that the mechanical and biochemical signals originating from cell-cell adhesion are critical for stem cell lineage specification. In this review, we focus on the role of cadherin mediated signaling in development and stem cell differentiation, with emphasis on two well-known cadherins, cadherin-2 (CDH2) (N-cadherin) and cadherin-11 (CDH11) (OB-cadherin). We summarize the existing knowledge regarding the role of CDH2 and CDH11 during development and differentiation in vivo and in vitro. We also discuss engineering strategies to control stem cell fate decisions by fine-tuning the extent of cell-cell adhesion through surface chemistry and microtopology. These studies may be greatly facilitated by novel strategies that enable monitoring of stem cell specification in real time. We expect that better understanding of how intercellular adhesion signaling affects lineage specification may impact biomaterial and scaffold design to control stem cell fate decisions in three-dimensional context with potential implications for tissue engineering and regenerative medicine.

Engineered skeletal muscle tissue for soft robotics: fabrication strategies, current applications, and future challenges

Skeletal muscle is a scalable actuator system used throughout nature from the millimeter to meter length scales and over a wide range of frequencies and force regimes. This adaptability has spurred interest in using engineered skeletal muscle to power soft robotics devices and in biotechnology and medical applications. However, the challenges to doing this are similar to those facing the tissue engineering and regenerative medicine fields; specifically, how do we translate our understanding of myogenesis in vivo to the engineering of muscle constructs in vitro to achieve functional integration with devices. To do this researchers are developing a number of ways to engineer the cellular microenvironment to guide skeletal muscle tissue formation. This includes understanding the role of substrate stiffness and the mechanical environment, engineering the spatial organization of biochemical and physical cues to guide muscle alignment, and developing bioreactors for mechanical and electrical conditioning. Examples of engineered skeletal muscle that can potentially be used in soft robotics include 2D cantilever-based skeletal muscle actuators and 3D skeletal muscle tissues engineered using scaffolds or directed self-organization. Integration into devices has led to basic muscle-powered devices such as grippers and pumps as well as more sophisticated muscle-powered soft robots that walk and swim. Looking forward, current, and future challenges include identifying the best source of muscle precursor cells to expand and differentiate into myotubes, replacing cardiomyocytes with skeletal muscle tissue as the bio-actuator of choice for soft robots, and vascularization and innervation to enable control and nourishment of larger muscle tissue constructs.

Cross talk between matrix elasticity and mechanical force regulates myoblast traction dynamics

Growing evidence suggests that critical cellular processes are profoundly influenced by the cross talk between extracellular nanomechanical forces and the material properties of the cellular microenvironment. Although many studies have examined either the effect of nanomechanical forces or the material properties of the microenvironment on biological processes, few have investigated the influence of both. Here, we performed simultaneous atomic force microscopy and traction force microscopy to demonstrate that muscle precursor cells (myoblasts) rapidly generate a significant increase in traction when stimulated with a local 10 nN force. Cells were cultured and nanomechanically stimulated on hydrogel substrates with controllable local elastic moduli varying from ~16-89 kPa, as confirmed with atomic force microscopy. Importantly, cellular traction dynamics in response to nanomechanical stimulation only occurred on substrates that were similar to the elasticity of working muscle tissue (~64-89 kPa) as opposed to substrates mimicking resting tissue (~16-51 kPa). The traction response was also transient, occurring within 30 s, and dissipating by 60 s, during constant nanomechanical stimulation. The observed biophysical dynamics are very much dependent on rho-kinase and myosin-II activity and likely contribute to the physiology of these cells. Our results demonstrate the fundamental ability of cells to integrate nanoscale information in the cellular microenvironment, such as nanomechanical forces and substrate mechanics, during the process of mechanotransduction.

Calponin 3 regulates stress fiber formation in dermal fibroblasts during wound healing

Skin wound healing is an intricate process involving various cell types and molecules. In granulation tissue, fibroblasts proliferate and differentiate into myofibroblasts and generate mechanical tension for wound closure and contraction. Actin stress fibers formed in these cells, especially those containing α-smooth muscle actin (α-SMA), are the central machinery for contractile force generation. In the present study, calponin 3 (CNN3), which has a phosphorylation-dependent actin-binding property, was identified in the molecular mechanism underlying stress fiber formation. CNN3 was expressed by fibroblasts/myofibroblasts in the proliferation phase of wound healing, and was associated with α-SMA in stress fibers formed by cultured dermal fibroblasts. CNN3 expression was post-transcriptionally regulated by tension, as demonstrated by disruption of actin filament organization under floating culture or blebbistatin treatment. CNN3 knockdown in primary fibroblasts impaired stress fiber formation, resulting in a phenotype of decreased cellular dynamics such as cell motility and contractile ability. These findings indicate that CNN3 participates in actin stress fiber remodeling, which is required for cell motility and contraction of dermal fibroblasts in the wound healing process.

Inhibition of CK2 binding to BMPRIa induces C2C12 differentiation into osteoblasts and adipocytes

BMP2 is a growth factor that regulates the cell fate of mesenchymal stem cells into osteoblast and adipocytes. However, the detailed signaling pathways and mechanism are unknown. We previously reported a new interaction of Casein kinase II (CK2) with the BMP receptor type-Ia (BMPRIa) and demonstrated using mimetic peptides CK2.1, CK2.2 and CK2.3 that the release of CK2 from BMPRIa activates Smad signaling and osteogenesis. Previously, we showed that mutation of these CK2 sites on BMPRIa (MCK2.1 (476S-A), MCK2.2 (324S-A) and MCK2.3 (214S-A)) induced osteogenesis. However, one mutant MCK2.1 induced osteogenesis similar to overexpression of wild type BMPRIa, suggesting that the effect of this mutant on mineralization was due to overexpression. In this paper we investigated the signaling pathways involved in the CK2-BMPRIa mediated osteogenesis and identified a new signaling pathway activating adipogenesis dependent on the BMPRIa and CK2 association. Further the mechanism for adipogenesis and osteogenesis is specific to the CK2 interaction site on BMPRIa. In detail our data show that overexpression of MCK2.2 induced osteogenesis was dependent on Caveolin-1 (Cav1) and the activation of the Smad and mTor pathways, while overexpression of MCK2.3 induced osteogenesis was independent of Caveolin-1 without activation of Smad pathway. However, MCK2.3 induced osteogenesis via the MEK pathway. The adipogenesis induced by the overexpression of MCK2.2 in C2C12 cells was dependent on the p38 and ERK pathways as well as Caveolin-1. These data suggest that signaling through BMPRIa used two different signaling pathways to induce osteogenesis dependent on CK2. Additionally the data supports a signaling pathway initiated in caveolae and one outside of caveolae to induce mineralization. Moreover, they reveal the signaling pathway of BMPRIa mediated adipogenesis.

A generic micropatterning platform to direct human mesenchymal stem cells from different origins towards myogenic differentiation

Human mesenchymal stem cells (MSCs) derived from various origins show varied differentiation capability. Recent work shows that cell shape manipulation via micropatterning can modulate the differentiation of bone-marrow-derived MSCs. Herein, the effect of micropatterning on the myogenesis of MSCs isolated from three different sources (bone marrow, fetal tissue, and adipose) is reported. All the well-aligned cells, regardless of source, predominantly commit to myogenic lineage, as shown by the significant upregulation of myogenic gene markers and positive myosin heavy chain staining. It is demonstrated that our novel micropattern can be used as a generic platform for inducing myogenesis of MSCs from different sources and may also have the potential to be extended to induce other lineage commitment.

Skeletal muscle-derived cell cultures as potent models in regenerative medicine research

Cell cultures have been used extensively by many scientists in recent decades to study various cell and tissue mechanisms. The use of cell cultures has many advantages over use of in vivo experimental models, but there are also limitations. As skeletal muscle-derived cell cultures become more commonly utilized in studies of muscle regeneration processes the question of their relevance in experimentation is highlighted with regard to in vivo experimental models. This article reviews studies that have been performed simultaneously in in vivo and in vitro experiments on skeletal muscle and assesses the correlation of results. Although they seem to correlate, no such studies on humans have been performed so far.

A bio-inspired platform to modulate myogenic differentiation of human mesenchymal stem cells through focal adhesion regulation

The use of human mesenchymal stem cells (hMSCs) in cardiac-tissue engineering has gained widespread attention and many reports have shown that matrix compliance, micro/nano-patterns could be some of the important biophysical cues that drive hMSCs differentiation. Regardless of the type of biophysical induction cues, cells mainly explore their environment via focal adhesion (FA) and FA plays an important role in many cellular behaviours. Therefore, it is hypothesized that FA modulation through materials manipulation could be an important cue for modulation that would result in the stem cell lineage commitment. In this work, the FA of hMSCs is modulated by a novel microcontact printing method using polyvinyl alcohol as a trans-print media which can successfully print proteins on soft polydimethylsiloxane (PDMS). The FA is successfully modified into dense FA and elongated FA by micropatterning square and rectangular patterns on 12.6 kPa PDMS respectively. Additionally, the combined effects of stiffness of PDMS substrates (hard (308 kPa), intermediate (12.6 kPa)) and FA patterning on hMSCs differentiation are studied. The results indicate that dense FA does not induce myogenesis while elongated FA can promote cytoskeleton alignment and further myogenesis on PDMS with intermediate stiffness of 12.6 kPa. However, on stiff substrate (308 kPa), with or without patterns, the cytoskeleton alignment and myogenesis are not obvious. This demonstrates for the first time that it is possible to induce the differentiation of hMSCs by regulating the FA using a materials platform even in the absence of any biochemical factors. It also shows that there is a synergistic effect between FA regulation and matrix stiffness that results in a more specific and higher up-regulated myogenesis. This platform presents a new chemical/biological-free method to engineer the myogenic differentiation of hMSCs.

Compliance-induced adherens junction formation in epithelial cells and tissues is regulated by JNK

We demonstrate that JNK responds to substrate stiffness and regulates AJ formation in epithelial cells in 2D cultures and in 3D tissues in vitro and in vivo. Rigid substrates led to JNK activation and AJ disassembly, while soft matrices suppressed JNK activity leading to AJ formation. Expression of constitutively active JNK (MKK7-JNK1) induced AJ dissolution even on soft substrates, while JNK knockdown (shJNK) induced AJ formation even on hard substrates. In human epidermis, basal cells expressed phosphorylated (p)-JNK but lacked AJ, while suprabasal keratinocytes contained strong AJ but lacked p-JNK. AJ formation was significantly impaired even in the upper suprabasal layers of bioengineered epidermis when prepared with stiffer scaffold or MKK7-JNK1 expressing keratinocytes. In contrast, shJNK1 or shJNK2 epidermis exhibited strong AJ even in the basal layer. The results with bioengineered epidermis were in full agreement with the epidermis of jnk1−/− or jnk2−/− mice. In conclusion, we propose that JNK mediates the effects of substrate stiffness on AJ formation in 2D and 3D context in vitro as well as in vivo.

Chondrogenesis on sulfonate-coated hydrogels is regulated by their mechanical properties

Many studies have demonstrated that sulfur-containing acidic groups induce chondrogenesis in vitro and in vivo. Recently, it is increasingly clear that mechanical properties of cell substrates largely influence cell differentiation. Thus, the present study investigated how mechanical properties of sulfonate-coated hydrogels influences chondrogenesis of mesenchymal stem cells (MSCs). Sulfonate-coated polyacrylamide gels (S-PAAm gels) which have the elastic modulus, E, of about 1, 15 and 150 kPa, were used in this study. MSCs cultured on the high stiffness S-PAAm gels (E=∼150 kPa) spread out with strong expression of stress fibers, while MSCs cultured on the low stiffness S-PAAm gels (E=∼1 kPa) had round shapes with less stress fibers but more cortical actins. Importantly, even in the absence of differentiation supplements, the lower stiffness S-PAAm gels led to the higher mRNA levels of chondrogenic markers such as Col2a1, Agc and Sox9 and the lower mRNA levels of an undifferentiation marker Sca1, indicating that the mechanical properties of S-PAAm gels strongly influence chondrogenesis. Blebbistatin which blocks myosin II-mediated mechanical sensing suppressed chondrogenesis induced by the low stiffness S-PAAm gels. The present study demonstrates that the soft S-PAAm gels effectively drive MSC chondrogenesis even in the absence of soluble differentiation factors and thus suggests that sulfonate-containing hydrogels with low stiffness could be a powerful tool for cartilage regeneration.

The physical interaction of myoblasts with the microenvironment during remodeling of the cytoarchitecture

Integrins, focal adhesions, the cytoskeleton and the extracellular matrix, form a structural continuum between the external and internal environment of the cell and mediate the pathways associated with cellular mechanosensitivity and mechanotransduction. This continuum is important for the onset of muscle tissue generation, as muscle precursor cells (myoblasts) require a mechanical stimulus to initiate myogenesis. The ability to sense a mechanical cue requires an intact cytoskeleton and strong physical contact and adhesion to the microenvironment. Importantly, myoblasts also undergo reorientation, alignment and large scale remodeling of the cytoskeleton when they experience mechanical stretch and compression in muscle tissue. It remains unclear if such dramatic changes in cell architecture also inhibit physical contact and adhesion with the tissue microenvironment that are clearly important to myoblast physiology. In this study, we employed interference reflection microscopy to examine changes in the close physical contact of myoblasts with a substrate during induced remodeling of the cytoarchitecture (de-stabilization of the actin and microtubule cytoskeleton and inhibition of acto-myosin contractility). Our results demonstrate that while each remodeling pathway caused distinct effects on myoblast morphology and sub-cellular structure, we only observed a ∼13% decrease in close physical contact with the substrate, regardless of the pathway inhibited. However, this decrease did not correlate well with changes in cell adhesion strength. On the other hand, there was a close correlation between cell adhesion and β1-integrin expression and the presence of cell-secreted fibronectin, but not with the presence of intact focal adhesions. In this study, we have shown that myoblasts are able to maintain a large degree of physical contact and adhesion to the microenvironment, even during shot periods (<60 min) of large scale remodeling and physiological stress, which is essential to their in-vivo functionality.

Nanopatterning reveals an ECM area threshold for focal adhesion assembly and force transmission that is regulated by integrin activation and cytoskeleton tension

Integrin-based focal adhesions (FA) transmit anchorage and traction forces between the cell and the extracellular matrix (ECM). To gain further insight into the physical parameters of the ECM that control FA assembly and force transduction in non-migrating cells, we used fibronectin (FN) nanopatterning within a cell adhesion-resistant background to establish the threshold area of ECM ligand required for stable FA assembly and force transduction. Integrin-FN clustering and adhesive force were strongly modulated by the geometry of the nanoscale adhesive area. Individual nanoisland area, not the number of nanoislands or total adhesive area, controlled integrin-FN clustering and adhesion strength. Importantly, below an area threshold (0.11 µm(2)), very few integrin-FN clusters and negligible adhesive forces were generated. We then asked whether this adhesive area threshold could be modulated by intracellular pathways known to influence either adhesive force, cytoskeletal tension, or the structural link between the two. Expression of talin- or vinculin-head domains that increase integrin activation or clustering overcame this nanolimit for stable integrin-FN clustering and increased adhesive force. Inhibition of myosin contractility in cells expressing a vinculin mutant that enhances cytoskeleton-integrin coupling also restored integrin-FN clustering below the nanolimit. We conclude that the minimum area of integrin-FN clusters required for stable assembly of nanoscale FA and adhesive force transduction is not a constant; rather it has a dynamic threshold that results from an equilibrium between pathways controlling adhesive force, cytoskeletal tension, and the structural linkage that transmits these forces, allowing the balance to be tipped by factors that regulate these mechanical parameters.

Engineering muscle tissues on microstructured polyelectrolyte multilayer films

The use of surface coating on biomaterials can render the original substratum with new functionalities that can improve the chemical, physical, and mechanical properties as well as enhance cellular cues such as attachment, proliferation, and differentiation. In this work, we combined biocompatible polydimethylsiloxane (PDMS) with a biomimetic polyelectrolyte multilayer (PEM) film made of poly(L-lysine) and hyaluronic acid (PLL/HA) for skeletal muscle tissue engineering. By microstructuring PDMS in grooves of a different width (5, 10, 30, and 100 μm) and by modulating the stiffness of the (PLL/HA) films, we guided skeletal muscle cell differentiation into myotubes. We found optimal conditions for both the formation of parallel-oriented myotubes and their maturation. Significantly, the myoblasts were collectively prealigned to the grooves before their differentiation. Before fusion, the highest aspect ratio and orientation of nuclei were observed for the 5 and 10 μm wide micropatterns. The formation of myotubes was observed regardless of the size of the micropatterns, and we found that their typical width was 10-12 μm. Their maturation was characterized by the immunolabeling of type II isomyosin. The amount of myosin striation was not affected by the topography, except for the 5 μm wide micropatterns. We highlighted the spatial constraints that led to an important nuclei deformation and further impairment of maturation within the 5 μm grooves. Altogether, our results show that the PEM film combined with PDMS is a powerful tool that is used for skeletal muscle engineering. This work opens perspectives for the development of skeletal muscle tissue in contact with films containing bioactive peptides or growth factors as well as for the study of pathogenic myotubes.

Cellular self-organization by autocatalytic alignment feedback

Myoblasts aggregate, differentiate and fuse to form skeletal muscle during both embryogenesis and tissue regeneration. For proper muscle function, long-range self-organization of myoblasts is required to create organized muscle architecture globally aligned to neighboring tissue. However, how the cells process geometric information over distances considerably longer than individual cells to self-organize into well-ordered, aligned and multinucleated myofibers remains a central question in developmental biology and regenerative medicine. Using plasma lithography micropatterning to create spatial cues for cell guidance, we show a physical mechanism by which orientation information can propagate for a long distance from a geometric boundary to guide development of muscle tissue. This long-range alignment occurs only in differentiating myoblasts, but not in non-fusing myoblasts perturbed by microfluidic disturbances or other non-fusing cell types. Computational cellular automata analysis of the spatiotemporal evolution of the self-organization process reveals that myogenic fusion in conjunction with rotational inertia functions in a self-reinforcing manner to enhance long-range propagation of alignment information. With this autocatalytic alignment feedback, well-ordered alignment of muscle could reinforce existing orientations and help promote proper arrangement with neighboring tissue and overall organization. Such physical self-enhancement might represent a fundamental mechanism for long-range pattern formation during tissue morphogenesis.

Cell-biomaterial mechanical interaction in the framework of tissue engineering: insights, computational modeling and perspectives

Tissue engineering is an emerging field of research which combines the use of cell-seeded biomaterials both in vitro and/or in vivo with the aim of promoting new tissue formation or regeneration. In this context, how cells colonize and interact with the biomaterial is critical in order to get a functional tissue engineering product. Cell-biomaterial interaction is referred to here as the phenomenon involved in adherent cells attachment to the biomaterial surface, and their related cell functions such as growth, differentiation, migration or apoptosis. This process is inherently complex in nature involving many physico-chemical events which take place at different scales ranging from molecular to cell body (organelle) levels. Moreover, it has been demonstrated that the mechanical environment at the cell-biomaterial location may play an important role in the subsequent cell function, which remains to be elucidated. In this paper, the state-of-the-art research in the physics and mechanics of cell-biomaterial interaction is reviewed with an emphasis on focal adhesions. The paper is focused on the different models developed at different scales available to simulate certain features of cell-biomaterial interaction. A proper understanding of cell-biomaterial interaction, as well as the development of predictive models in this sense, may add some light in tissue engineering and regenerative medicine fields.

β-Adrenergic inhibition of contractility in L6 skeletal muscle cells

The β-adrenoceptors (β-ARs) control many cellular processes. Here, we show that β-ARs inhibit calcium depletion-induced cell contractility and subsequent cell detachment of L6 skeletal muscle cells. The mechanism underlying the cell detachment inhibition was studied by using a quantitative cell detachment assay. We demonstrate that cell detachment induced by depletion of extracellular calcium is due to myosin- and ROCK-dependent contractility. The β-AR inhibition of L6 skeletal muscle cell detachment was shown to be mediated by the β₂-AR and increased cAMP but was surprisingly not dependent on the classical downstream effectors PKA or Epac, nor was it dependent on PKG, PI3K or PKC. However, inhibition of potassium channels blocks the β₂-AR mediated effects. Furthermore, activation of potassium channels fully mimicked the results of β₂-AR activation. In conclusion, we present a novel finding that β₂-AR signaling inhibits contractility and thus cell detachment in L6 skeletal muscle cells by a cAMP and potassium channel dependent mechanism.

Upregulation of paxillin and focal adhesion signaling follows Dystroglycan Complex deletions and promotes a hypertensive state of differentiation

Anchorage to matrix is mediated for many cells not only by integrin-based focal adhesions but also by a parallel assembly of integral and peripheral membrane proteins known as the Dystroglycan Complex. Deficiencies in either dystrophin (mdx mice) or γ-sarcoglycan (γSG(-/-) mice) components of the Dystroglycan Complex lead to upregulation of numerous focal adhesion proteins, and the phosphoprotein paxillin proves to be among the most prominent. In mdx muscle, paxillin-Y31 and Y118 are both hyper-phosphorylated as are key sites in focal adhesion kinase (FAK) and the stretch-stimulatable pro-survival MAPK pathway, whereas γSG(-/-) muscle exhibits more erratic hyper-phosphorylation. In cultured myotubes, cell tension generated by myosin-II appears required for localization of paxillin to adhesions while vinculin appears more stably integrated. Overexpression of wild-type (WT) paxillin has no obvious effect on focal adhesion density or the physical strength of adhesion, but WT and a Y118F mutant promote contractile sarcomere formation whereas a Y31F mutant shows no effect, implicating Y31 in striation. Self-peeling of cells as well as Atomic Force Microscopy (AFM) probing of cells with or without myosin-II inhibition indicate an increase in cell tension within paxillin-overexpressing cells. However, prednisolone, a first-line glucocorticoid for muscular dystrophies, decreases cell tension without affecting paxillin at adhesions, suggesting a non-linear relationship between paxillin and cell tension. Hypertension that results from upregulation of integrin adhesions is thus a natural and treatable outcome of Dystroglycan Complex down-regulation.

Intercellular and extracellular mechanotransduction in cardiac myocytes

Adult cardiomyocytes are terminally differentiated with minimal replicative capacity. Therefore, long-term preservation or enhancement of cardiac function depends on structural adaptation. Myocytes interact with the extracellular matrix, fibroblasts, and vascular cells and with each other (end to end; side to side). We review the current understanding of the mechanical determinants and environmental sensing systems that modulate and regulate myocyte molecular machinery and its structural organization. We feature the design and application of engineered cellular microenvironments to demonstrate the ability of cardiac cells to remodel their cytoskeletal organization and shape, including sarcomere/myofibrillar architectural topography. Cell shape-dependent functions result from complex mechanical interactions between the cytoskeleton architecture and external conditions, be they cell-cell or cell-extracellular matrix (ECM) adhesion contact-mediated. This mechanobiological perspective forms the basis for viewing the cardiomyocyte as a mechanostructural anisotropic continuum, exhibiting constant mechanosensory-driven self-regulated adjustment of the cytoskeleton through tight interplay between its force generation activity and concurrent cytoarchitectural remodeling. The unifying framework guiding this perspective is the observation that these emerging events and properties are initiated by and respond to cytoskeletal reorganization, regulated by cell-cell and cell-ECM adhesion and its corresponding (mutually interactive) signaling machinery. It is important for future studies to elucidate how cross talk between these mechanical signals is coordinated to control myocyte structure and function. Ultimately, understanding how the highly interactive mechanical signaling can give rise to phenotypic changes is critical for targeting the underlying pathways that contribute to cardiac remodeling associated with various forms of dilated and hypertrophic myopathies, myocardial infarction, heart failure, and reverse remodeling.

Microcontact printing of fibronectin on a biodegradable polymeric surface for skeletal muscle cell orientation

BACKGROUND AND OBJECTIVES

Micropatterning and microfabrication techniques have been widely used to control cell adhesion and proliferation along a preferential direction according to contact guidance theory. One of these techniques is microcontact printing, a soft lithographic technique based on the transfer of a "molecular ink" from an elastomeric stamp to a surface. This method allows the useful attachment of biomolecules in a few seconds on a variety of surfaces with sub-micrometer resolution and control, without modifying the biomolecule properties. The aim of this study is to develop an easy and versatile technique for in vitro production of arrays of skeletal muscle myofibers using microcontact printing technique on biodegradable substrata.

METHODS

Microcontact printing of fibronectin stripes (10, 25, 50 μm in width) was performed onto biodegradable L-lactide/trimethylene carbonate copolymer (PLLA-TMC) films. C2C12, a murine myoblast cell line, was used for the production of parallel myofibers.

RESULTS

This approach proved to be simple, reliable and effective in obtaining a stable pattern of fibronectin on the PLLA-TMC surface as observed by fluorescence microscopy. C2C12 cells were well aligned along the pattern 24 hours after seeding, especially on fibronectin stripes 10 and 25 μm in width. Seven days after confluence cells fused and formed aligned multinucleated cells expressing a-actinin.

CONCLUSIONS

Fibronectin patterning seems to be a useful method to induce cell alignment and to improve myotube formation. Further studies will be focused on the possibility of applying external stimuli to these structures to obtain healthy myotubes and to induce myofiber development.

Cardiac myocyte remodeling mediated by N-cadherin-dependent mechanosensing

Cell-to-cell adhesions are crucial in maintaining the structural and functional integrity of cardiac cells. Little is known about the mechanosensitivity and mechanotransduction of cell-to-cell interactions. Most studies of cardiac mechanotransduction and myofibrillogenesis have focused on cell-extracellular matrix (ECM)-specific interactions. This study assesses the direct role of intercellular adhesion, specifically that of N-cadherin-mediated mechanotransduction, on the morphology and internal organization of neonatal ventricular cardiac myocytes. The results show that cadherin-mediated cell attachments are capable of eliciting a cytoskeletal network response similar to that of integrin-mediated force response and transmission, affecting myofibrillar organization, myocyte shape, and cortical stiffness. Traction forces mediated by N-cadherin were shown to be comparable to those sustained by ECM. The directional changes in predicted traction forces as a function of imposed loads (gel stiffness) provide the added evidence that N-cadherin is a mechanoresponsive adhesion receptor. Strikingly, the mechanical sensitivity response (gain) in terms of the measured cell-spread area as a function of imposed load (adhesive substrate rigidity) was consistently higher for N-cadherin-coated surfaces compared with ECM protein-coated surfaces. In addition, the cytoskeletal architecture of myocytes on an N-cadherin adhesive microenvironment was characteristically different from that on an ECM environment, suggesting that the two mechanotransductive cell adhesion systems may play both independent and complementary roles in myocyte cytoskeletal spatial organization. These results indicate that cell-to-cell-mediated force perception and transmission are involved in the organization and development of cardiac structure and function.

Role of material-driven fibronectin fibrillogenesis in cell differentiation

Fibronectin (FN) is a ubiquitous extracellular matrix protein (ECM) protein that is organized into fibrillar networks by cells through an integrin-mediated process that involves contractile forces. This assembly allows for the unfolding of the FN molecule, exposing cryptic domains that are not available in the native globular FN structure and activating intracellular signalling complexes. However, organization of FN into a physiological fibrillar network upon adsorption on a material surface has not been observed. Here we demonstrate cell-free, material-induced FN fibrillogenesis into a biological matrix with enhanced cellular activities. We found that simple FN adsorption onto poly(ethyl acrylate) surfaces, but not control polymers, triggered FN organization into a fibrillar network via interactions in the amino-terminal 70 kDa fragment, which is involved in the formation of cell-mediated FN fibrils. Moreover, the material-driven FN fibrils exhibited enhanced biological activities in terms of myogenic differentiation compared to individual FN molecules and even type I collagen. Our results demonstrate that molecular assembly of FN can take place at the material interface, giving rise to a physiological protein network similar to fibrillar matrices assembled by cells. This research identifies material surfaces that trigger the organization of extracellular matrix proteins into biological active fibrils and establishes a new paradigm to engineer ECM-mimetic biomaterials.

Influence of Electromechanical Activity on Cardiac Differentiation of Mouse Embryonic Stem Cells

During differentiation, mouse embryonic stem cell-derived cardiomyocytes (mESC-CMs) receive electromechanical cues from spontaneous beating. Therefore, promoting electromechanical activity via electrical pacing or suppressing it by drug treatment might affect the cellular functional development. Electrical pacing was applied to confluent monolayers of mESC-CMs during late-stage differentiation (days 16-18). Alternatively, spontaneous contraction was suppressed by (a) blocking ion currents with CsCl (HCN channel), trazodone (T-type Ca2+ channel), or both CsCl and trazodone on days 11-18; or (b) applying blebbistatin (excitation-contraction uncoupler) on days 11-14. Electrophysiological properties and gene expression were examined on day 19 and 18, respectively. Optical mapping revealed no significant difference in conduction velocity (CV)in paced vs. non-pacedmonolayers, nor were there significant changes in gene expression of connexin-43, Na-Ca exchanger (NCX), or myosin heavy chain (MHC). However, CV variability among differentiation batches and CV heterogeneity within individual monolayers were significantly lower in paced mESC-CMs. Alternatively, while the four drug treatments suppressed contraction with varying degrees (up to complete inhibition), there was no significant difference in CV for any of the treatments compared with controls. Trazodone treatment significantly reduced CV variability as compared to controls, whereas CsCl treatment significantly reduced CV heterogeneity. Distinct changes in gene expression of connexin-43, MHC, HCNl, Cav3.1/3.2 were not observed. Electrical pacing, but not suppression of spontaneous contraction, during late-stage differentiation reduces the intrinsic variability of CV among differentiation batches and across individual monolayers, which can be beneficial in the application of ESCs for myocardial tissue repair.

Electrically induced contraction of C2C12 myotubes cultured on a porous membrane-based substrate with muscle tissue-like stiffness

A porous membrane-based cell culture device was developed to electrically stimulate a confluent monolayer of C2C12 myotubes. The device's cell culture substrate is a microporous alumina membrane-modified by attaching an atelocollagen membrane on the upperside and a hole-spotted poly(dimethylsiloxane) (PDMS) film on the underside. When electric current is generated between the device's Pt ring electrodes--one of which is placed above the cells and the other below the PDMS layer--the focused current at the PDMS hole can electrically stimulate the cells. C2C12 myoblasts were cultured on the substrate and differentiated into myotubes. When the electrical pulses were applied, myotubes started to contract slightly in and near the hole, and that the continuous stimulation increased both the number of stimuli-responding myotubes and the magnitude of the contraction considerably owing to the underlying atelocollagen membrane with muscle tissue-like stiffness. Also, the generation of contractile myotubes on a wider region of the membrane substrate was possible by applying the electrical pulses through the array of holes in the PDMS film. Using the present system, the glucose uptake by contractile myotubes was examined with fluorescence-labeled glucose, 2-NBDG, which displayed a positive correlation between the contractile activity of myotubes and the uptake of 2-NBDG.

Triggering cell detachment from patterned electrode arrays by programmed subcellular release

Programmed subcellular release is an in vitro technique for the quantitative study of cell detachment. The dynamics of cell contraction are measured by releasing cells from surfaces to which they are attached with spatial and temporal control. Release of subcellular regions of cells is achieved by plating cells on an electrode array created by standard microfabrication methods. The electrodes are then biochemically functionalized with an arginine-glycine-aspartic acid (RGD)-terminated thiol. Application of a voltage pulse results in electrochemical desorption of the RGD-terminated thiols, triggering cell detachment. This method allows for the study of the full cascade of events from detachment to subsequent subcellular reorganization. Fabrication of the electrode arrays may take 1-2 d. Preparation for experiments, including surface functionalization and cell plating, can be completed in 10 h. A series of cell release experiments on one device may last several hours.