Mechanical signals and IGF-I gene splicing in vitro in relation to development of skeletal muscle

Engineering interfacial tissues: The myotendinous junction

The myotendinous junction (MTJ) is the interface connecting skeletal muscle and tendon tissues. This specialized region represents the bridge that facilitates the transmission of contractile forces from muscle to tendon, and ultimately the skeletal system for the creation of movement. MTJs are, therefore, subject to high stress concentrations, rendering them susceptible to severe, life-altering injuries. Despite the scarcity of knowledge obtained from MTJ formation during embryogenesis, several attempts have been made to engineer this complex interfacial tissue. These attempts, however, fail to achieve the level of maturity and mechanical complexity required for in vivo transplantation. This review summarizes the strategies taken to engineer the MTJ, with an emphasis on how transitioning from static to mechanically inducive dynamic cultures may assist in achieving myotendinous maturity.

Cyclic Tensile Stress Induces Skeletal Muscle Hypertrophy and Myonuclear Accretion in a 3D Model

Skeletal muscle is highly adaptive to mechanical stress due to its resident stem cells and the pronounced level of myotube plasticity. Herein, we study the adaptation to mechanical stress and its underlying molecular mechanisms in a tissue-engineered skeletal muscle model. We subjected differentiated 3D skeletal muscle-like constructs to cyclic tensile stress using a custom-made bioreactor system, which resulted in immediate activation of stress-related signal transducers (Erk1/2, p38). Cell cycle re-entry, increased proliferation, and onset of myogenesis indicated subsequent myoblast activation. Furthermore, elevated focal adhesion kinase and β-catenin activity in mechanically stressed constructs suggested increased cell adhesion and migration. After 3 days of mechanical stress, gene expression of the fusogenic markers MyoMaker and MyoMixer, myotube diameter, myonuclear accretion, as well as S6 activation, were significantly increased. Our results highlight that we established a promising tool to study sustained adaptation to mechanical stress in healthy, hypertrophic, or regenerating skeletal muscle.

Mechanical loading of bioengineered skeletal muscle in vitro recapitulates gene expression signatures of resistance exercise in vivo

Understanding the role of mechanical loading and exercise in skeletal muscle (SkM) is paramount for delineating the molecular mechanisms that govern changes in muscle mass. However, it is unknown whether loading of bioengineered SkM in vitro adequately recapitulates the molecular responses observed after resistance exercise (RE) in vivo. To address this, the transcriptional and epigenetic (DNA methylation) responses were compared after mechanical loading in bioengineered SkM in vitro and after RE in vivo. Specifically, genes known to be upregulated/hypomethylated after RE in humans were analyzed. Ninety-three percent of these genes demonstrated similar changes in gene expression post-loading in the bioengineered muscle when compared to acute RE in humans. Furthermore, similar differences in gene expression were observed between loaded bioengineered SkM and after programmed RT in rat SkM tissue. Hypomethylation occurred for only one of the genes analysed (GRIK2) post-loading in bioengineered SkM. To further validate these findings, DNA methylation and mRNA expression of known hypomethylated and upregulated genes post-acute RE in humans were also analyzed at 0.5, 3, and 24 h post-loading in bioengineered muscle. The largest changes in gene expression occurred at 3 h, whereby 82% and 91% of genes responded similarly when compared to human and rodent SkM respectively. DNA methylation of only a small proportion of genes analyzed (TRAF1, MSN, and CTTN) significantly increased post-loading in bioengineered SkM alone. Overall, mechanical loading of bioengineered SkM in vitro recapitulates the gene expression profile of human and rodent SkM after RE in vivo. Although some genes demonstrated differential DNA methylation post-loading in bioengineered SkM, such changes across the majority of genes analyzed did not closely mimic the epigenetic response to acute-RE in humans.

Tissue-Engineered Skeletal Muscle Models to Study Muscle Function, Plasticity, and Disease

Skeletal muscle possesses remarkable plasticity that permits functional adaptations to a wide range of signals such as motor input, exercise, and disease. Small animal models have been pivotal in elucidating the molecular mechanisms regulating skeletal muscle adaptation and plasticity. However, these small animal models fail to accurately model human muscle disease resulting in poor clinical success of therapies. Here, we review the potential of in vitro three-dimensional tissue-engineered skeletal muscle models to study muscle function, plasticity, and disease. First, we discuss the generation and function of in vitro skeletal muscle models. We then discuss the genetic, neural, and hormonal factors regulating skeletal muscle fiber-type in vivo and the ability of current in vitro models to study muscle fiber-type regulation. We also evaluate the potential of these systems to be utilized in a patient-specific manner to accurately model and gain novel insights into diseases such as Duchenne muscular dystrophy (DMD) and volumetric muscle loss. We conclude with a discussion on future developments required for tissue-engineered skeletal muscle models to become more mature, biomimetic, and widely utilized for studying muscle physiology, disease, and clinical use.

Role of Alternatively Spliced Messenger RNA (mRNA) Isoforms of the Insulin-Like Growth Factor 1 (IGF1) in Selected Human Tumors

Insulin-like growth factor 1 (IGF1) is a key regulator of tissue growth and development that is also implicated in the initiation and progression of various cancers. The human IGF1 gene contains six exons and five long introns, the transcription of which is controlled by two promoters (P1 and P2). Alternate promoter usage, as well as alternative splicing (AS) of IGF1, results in the expression of six various variants (isoforms) of mRNA, i.e., IA, IB, IC, IIA, IIB, and IIC. A mature 70-kDa IGF1 protein is coded only by exons 3 and 4, while exons 5 and 6 are alternatively spliced code for the three C-terminal E peptides: Ea (exon 6), Eb (exon 5), and Ec (fragments of exons 5 and 6). The most abundant of those transcripts is IGF1Ea, followed by IGF1Eb and IGF1Ec (also known as mechano-growth factor, MGF). The presence of different IGF1 transcripts suggests tissue-specific auto- and/or paracrine action, as well as separate regulation of both of these gene promoters. In physiology, the role of different IGF1 mRNA isoforms and pro-peptides is best recognized in skeletal muscle tissue. Their functions include the development and regeneration of muscles, as well as maintenance of proper muscle mass. In turn, in nervous tissue, a neuroprotective function of short peptides, produced as a result of IGF1 expression and characterized by significant blood-brain barrier penetrance, has been described and could be a potential therapeutic target. When it comes to the regulation of carcinogenesis, the potential biological role of different var iants of IGF1 mRNAs and pro-peptides is also intensively studied. This review highlights the role of IGF1 isoform expression (mRNAs, proteins) in physiology and different types of human tumors (e.g., breast cancer, cervical cancer, colorectal cancer, osteosarcoma, prostate and thyroid cancers), as well as mechanisms of IGF1 spliced variants involvement in tumor biology.

Transplantation of insulin-like growth factor-1 laden scaffolds combined with exercise promotes neuroregeneration and angiogenesis in a preclinical muscle injury model

The regeneration of skeletal muscle can be permanently impaired by traumatic injuries, despite the high regenerative capacity of native muscle. An attractive therapeutic approach for treating severe muscle inuries is the implantation of off-the-shelf engineered biomimetic scaffolds into the site of tissue damage to enhance muscle regeneration. Anisotropic nanofibrillar scaffolds provide spatial patterning cues to create organized myofibers, and growth factors such as insulin-like growth factor-1 (IGF-1) are potent inducers of both muscle regeneration as well as angiogenesis. The aim of this study was to test the therapeutic efficacy of anisotropic IGF-1-releasing collagen scaffolds combined with voluntary exercise for the treatment of acute volumetric muscle loss, with a focus on histomorphological effects. To enhance the angiogenic and regenerative potential of injured murine skeletal muscle, IGF-1-laden nanofibrillar scaffolds with aligned topography were fabricated using a shear-mediated extrusion approach, followed by growth factor adsorption. Individual scaffolds released a cumulative total of 1244 ng ± 153 ng of IGF-1 over the course of 21 days in vitro. To test the bioactivity of IGF-1-releasing scaffolds, the myotube formation capacity of murine myoblasts was quantified. On IGF-1-releasing scaffolds seeded with myoblasts, the resulting myotubes formed were 1.5-fold longer in length and contained 2-fold greater nuclei per myotube, when compared to scaffolds without IGF-1. When implanted into the ablated murine tibialis anterior muscle, the IGF-1-laden scaffolds, in conjunction with voluntary wheel running, significantly increased the density of perfused microvessels by greater than 3-fold, in comparison to treatment with scaffolds without IGF-1. Enhanced myogenesis was also observed in animals treated with the IGF-1-laden scaffolds combined with exercise, compared to control scaffolds transplanted into mice that did not receive exercise. Furthermore, the abundance of mature neuromuscular junctions was greater by approximately 2-fold in muscles treated with IGF-1-laden scaffolds, when paired with exercise, in comparison to the same treatment without exercise. These findings demonstrate that voluntary exercise improves the regenerative effect of growth factor-laden scaffolds by augmenting neurovascular regeneration, and have important translational implications in the design of off-the-shelf therapeutics for the treatment of traumatic muscle injury.

Effect of amino acids on IGF1 gene expression in human myotubes and skeletal muscle

OBJECTIVE

Insulin-like growth factor I (IGF1) is an important regulator of collagen and extracellular matrix protein expression. We aimed to evaluate the effect of amino acids (AAs) on expression of IGF1 and IGF1-dependent genes in human myotubes and skeletal muscle and supposed that AAs administration increases IGF1 levels in blood and expression of IGF1 and IGF1-dependent genes in trained skeletal muscle, thereby reducing training-induced muscle damage.

DESIGN

Human myotubes were incubated with Arg and Leu for 24 h. Then, the effects of long-term branched chain AAs administration (10 weeks, 0.1 g/kg body mass/day) to volunteers (six subjects per AAs and placebo groups) performing large training volumes regularly (cross country skiers, training twice a day) were examined.

RESULTS

Incubating the myotubes with AAs increases expression of IGF1 mRNA isoforms and IGF1 secretion by 2-3 times. In athletes, long-term AAs administration increased basal blood levels of IGF1 (~50%) and expression of IGF1Ea mRNA slightly in skeletal muscle. There is no marked increase in expression of COL1A1, COL3A1, COL5A1, and LOX genes in skeletal muscle after AAs administration. However, expression of these genes in the combined group (placebo + AAs; n = 12) significantly correlated with the expression of IGF1Ea mRNA in muscle and did not correlate with IGF1 levels in the blood.

CONCLUSIONS

AAs administration increases IGF1 expression in vitro and in vivo. To obtain more pronounced changes in expression of IGF1 and IGF1-dependent genes in skeletal muscle, it may be necessary to increase the dose and/or duration of AAs administration.

Implications of Insulin-Like Growth Factor-1 in Skeletal Muscle and Various Diseases

Skeletal muscle is an essential tissue that attaches to bones and facilitates body movements. Insulin-like growth factor-1 (IGF-1) is a hormone found in blood that plays an important role in skeletal myogenesis and is importantly associated with muscle mass entity, strength development, and degeneration and increases the proliferative capacity of muscle satellite cells (MSCs). IGF-1R is an IGF-1 receptor with a transmembrane location that activates PI3K/Akt signaling and possesses tyrosine kinase activity, and its expression is significant in terms of myoblast proliferation and normal muscle mass maintenance. IGF-1 synthesis is elevated in MSCs of injured muscles and stimulates MSCs proliferation and myogenic differentiation. Mechanical loading also affects skeletal muscle production by IGF-1, and low IGF-1 levels are associated with low handgrip strength and poor physical performance. IGF-1 is potentially useful in the management of Duchenne muscular dystrophy, muscle atrophy, and promotes neurite development. This review highlights the role of IGF-1 in skeletal muscle, its importance during myogenesis, and its involvement in different disease conditions.

Insulin-like growth factor 1 signaling in motor neuron and polyglutamine diseases: From molecular pathogenesis to therapeutic perspectives

The pleiotropic peptide insulin-like growth factor 1 (IGF-I) regulates human body homeostasis and cell growth. IGF-I activates two major signaling pathways, namely phosphoinositide-3-kinase (PI3K)/protein kinase B (PKB/Akt) and Ras/extracellular signal-regulated kinase (ERK), which contribute to brain development, metabolism and function as well as to neuronal maintenance and survival. In this review, we discuss the general and tissue-specific effects of the IGF-I pathways. In addition, we present a comprehensive overview examining the role of IGF-I in neurodegenerative diseases, such as spinal and muscular atrophy, amyotrophic lateral sclerosis, and polyglutamine diseases. In each disease, we analyze the disturbances of the IGF-I pathway, the modification of the disease protein by IGF-I signaling, and the therapeutic strategies based on the use of IGF-I developed to date. Lastly, we highlight present and future considerations in the use of IGF-I for the treatment of these disorders.

Engineered skeletal muscles for disease modeling and drug discovery

Skeletal muscle is the largest organ of human body with several important roles in everyday movement and metabolic homeostasis. The limited ability of small animal models of muscle disease to accurately predict drug efficacy and toxicity in humans has prompted the development in vitro models of human skeletal muscle that fatefully recapitulate cell and tissue level functions and drug responses. We first review methods for development of three-dimensional engineered muscle tissues and organ-on-a-chip microphysiological systems and discuss their potential utility in drug discovery research and development of new regenerative therapies. Furthermore, we describe strategies to increase the functional maturation of engineered muscle, and motivate the importance of incorporating multiple tissue types on the same chip to model organ cross-talk and generate more predictive drug development platforms. Finally, we review the ability of available in vitro systems to model diseases such as type II diabetes, Duchenne muscular dystrophy, Pompe disease, and dysferlinopathy.

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

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

The Importance of Biophysical and Biochemical Stimuli in Dynamic Skeletal Muscle Models

Classical approaches to engineer skeletal muscle tissue based on current regenerative and surgical procedures still do not meet the desired outcome for patient applications. Besides the evident need to create functional skeletal muscle tissue for the repair of volumetric muscle defects, there is also growing demand for platforms to study muscle-related diseases, such as muscular dystrophies or sarcopenia. Currently, numerous studies exist that have employed a variety of biomaterials, cell types and strategies for maturation of skeletal muscle tissue in 2D and 3D environments. However, researchers are just at the beginning of understanding the impact of different culture settings and their biochemical (growth factors and chemical changes) and biophysical cues (mechanical properties) on myogenesis. With this review we intend to emphasize the need for new in vitro skeletal muscle (disease) models to better recapitulate important structural and functional aspects of muscle development. We highlight the importance of choosing appropriate system components, e.g., cell and biomaterial type, structural and mechanical matrix properties or culture format, and how understanding their interplay will enable researchers to create optimized platforms to investigate myogenesis in healthy and diseased tissue. Thus, we aim to deliver guidelines for experimental designs to allow estimation of the potential influence of the selected skeletal muscle tissue engineering setup on the myogenic outcome prior to their implementation. Moreover, we offer a workflow to facilitate identifying and selecting different analytical tools to demonstrate the successful creation of functional skeletal muscle tissue. Ultimately, a refinement of existing strategies will lead to further progression in understanding important aspects of muscle diseases, muscle aging and muscle regeneration to improve quality of life of patients and enable the establishment of new treatment options.

Modelling multi-scale cell-tissue interaction of tissue-engineered muscle constructs

Expectation on engineered tissue substitute continues to grow, and for an effective development of a functional tissue and to control its quality, cellular mechanoresponse plays a key role. Although the mechanoresponse – in terms of cell–tissue interaction across scales – has been understood better in recent years, there are still technical limitations to quantitatively monitor the processes involved in the development of both native and engineered tissues. Computational (in silico) studies have been utilised to complement the experimental limitations and successfully applied to the prediction of tissue growth. We here review recent activities in the area of combined experimental and computational analyses of tissue growth, especially in the tissue engineering context, and highlight the advantages of such an approach for the future of the tissue engineering, using our own case study of predicting musculoskeletal tissue engineering construct development.

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

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

An Assessment of Myotube Morphology, Matrix Deformation, and Myogenic mRNA Expression in Custom-Built and Commercially Available Engineered Muscle Chamber Configurations

There are several three-dimensional (3D) skeletal muscle (SkM) tissue engineered models reported in the literature. 3D SkM tissue engineering (TE) aims to recapitulate the structure and function of native (in vivo) tissue, within an in vitro environment. This requires the differentiation of myoblasts into aligned multinucleated myotubes surrounded by a biologically representative extracellular matrix (ECM). In the present work, a new commercially available 3D SkM TE culture chamber manufactured from polyether ether ketone (PEEK) that facilitates suitable development of these myotubes is presented. To assess the outcomes of the myotubes within these constructs, morphological, gene expression, and ECM remodeling parameters were compared against a previously published custom-built model. No significant differences were observed in the morphological and gene expression measures between the newly introduced and the established construct configuration, suggesting biological reproducibility irrespective of manufacturing process. However, TE SkM fabricated using the commercially available PEEK chambers displayed reduced variability in both construct attachment and matrix deformation, likely due to increased reproducibility within the manufacturing process. The mechanical differences between systems may also have contributed to such differences, however, investigation of these variables was beyond the scope of the investigation. Though more expensive than the custom-built models, these PEEK chambers are also suitable for multiple use after autoclaving. As such this would support its use over the previously published handmade culture chamber system, particularly when seeking to develop higher-throughput systems or when experimental cost is not a factor.

Factors That Affect Tissue-Engineered Skeletal Muscle Function and Physiology

Tissue-engineered skeletal muscle has the promise to be a tool for studying physiology, screening muscle-active drugs, and clinical replacement of damaged muscle. To maximize the potential benefits of engineered muscle, it is important to understand the factors required for tissue formation and how these affect muscle function. In this review, we evaluate how biomaterials, cell source, media components, and bioreactor interventions impact muscle function and phenotype.

Creating Interactions between Tissue-Engineered Skeletal Muscle and the Peripheral Nervous System

Effective models of mammalian tissues must allow and encourage physiologically (mimetic) correct interactions between co-cultured cell types in order to produce culture microenvironments as similar as possible to those that would normally occur in vivo. In the case of skeletal muscle, the development of such a culture model, integrating multiple relevant cell types within a biomimetic scaffold, would be of significant benefit for investigations into the development, functional performance, and pathophysiology of skeletal muscle tissue. Although some work has been published regarding the behaviour of in vitro muscle models co-cultured with organotypic slices of CNS tissue or with stem cell-derived neurospheres, little investigation has so far been made regarding the potential to maintain isolated motor neurons within a 3D biomimetic skeletal muscle culture platform. Here, we review the current state of the art for engineering neuromuscular contacts in vitro and provide original data detailing the development of a 3D collagen-based model for the co-culture of primary muscle cells and motor neurons. The devised culture system promotes increased myoblast differentiation, forming arrays of parallel, aligned myotubes on which areas of nerve-muscle contact can be detected by immunostaining for pre- and post-synaptic proteins. Quantitative RT-PCR results indicate that motor neuron presence has a positive effect on myotube maturation, suggesting neural incorporation influences muscle development and maturation in vitro. The importance of this work is discussed in relation to other published neuromuscular co-culture platforms along with possible future directions for the field.

Engineered matrices for skeletal muscle satellite cell engraftment and function

Regeneration of traumatically injured skeletal muscles is severely limited. Moreover, the regenerative capacity of skeletal muscle declines with aging, further exacerbating the problem. Recent evidence supports that delivery of muscle satellite cells to the injured muscles enhances muscle regeneration and reverses features of aging, including reduction in muscle mass and regenerative capacity. However, direct delivery of satellite cells presents a challenge at a translational level due to inflammation and donor cell death, motivating the need to develop engineered matrices for muscle satellite cell delivery. This review will highlight important aspects of satellite cell and their niche biology in the context of muscle regeneration, and examine recent progresses in the development of engineered cell delivery matrices designed for skeletal muscle regeneration. Understanding the interactions of muscle satellite cells and their niche in both native and engineered systems is crucial to developing muscle pathology-specific cell- and biomaterial-based therapies.

Microgravity Stress: Bone and Connective Tissue

The major alterations in bone and the dense connective tissues in humans and animals exposed to microgravity illustrate the dependency of these tissues' function on normal gravitational loading. Whether these alterations depend solely on the reduced mechanical loading of zero g or are compounded by fluid shifts, altered tissue blood flow, radiation exposure, and altered nutritional status is not yet well defined. Changes in the dense connective tissues and intervertebral disks are generally smaller in magnitude but occur more rapidly than those in mineralized bone with transitions to 0 g and during recovery once back to the loading provided by 1 g conditions. However, joint injuries are projected to occur much more often than the more catastrophic bone fracture during exploration class missions, so protecting the integrity of both tissues is important. This review focuses on the research performed over the last 20 years in humans and animals exposed to actual spaceflight, as well as on knowledge gained from pertinent ground-based models such as bed rest in humans and hindlimb unloading in rodents. Significant progress has been made in our understanding of the mechanisms for alterations in bone and connective tissues with exposure to microgravity, but intriguing questions remain to be solved, particularly with reference to biomedical risks associated with prolonged exploration missions.

Increased IGF-IEc expression and mechano-growth factor production in intestinal muscle of fibrostenotic Crohn's disease and smooth muscle hypertrophy

The igf1 gene is alternatively spliced as IGF-IEa and IGF-IEc variants in humans. In fibrostenotic Crohn's disease, the fibrogenic cytokine TGF-β1 induces IGF-IEa expression and IGF-I production in intestinal smooth muscle and results in muscle hyperplasia and collagen I production that contribute to stricture formation. Mechano-growth factor (MGF) derived from IGF-IEc induces skeletal and cardiac muscle hypertrophy following stress. We hypothesized that increased IGF-IEc expression and MGF production mediated smooth muscle hypertrophy also characteristic of fibrostenotic Crohn's disease. IGF-IEc transcripts and MGF protein were increased in muscle cells isolated from fibrostenotic intestine under regulation by endogenous TGF-β1. Erk5 and MEF2C were phosphorylated in vivo in fibrostenotic muscle; both were phosphorylated and colocalized to nucleus in response to synthetic MGF in vitro. Smooth muscle-specific protein expression of α-smooth muscle actin, γ-smooth muscle actin, and smoothelin was increased in affected intestine. Erk5 inhibition or MEF2C siRNA blocked smooth muscle-specific gene expression and hypertrophy induced by synthetic MGF. Conditioned media of cultured fibrostenotic muscle induced muscle hypertrophy that was inhibited by immunoneutralization of endogenous MGF or pro-IGF-IEc. The results indicate that TGF-β1-dependent IGF-IEc expression and MGF production in patients with fibrostenotic Crohn's disease regulates smooth muscle cell hypertrophy a critical factor that contributes to intestinal stricture formation.

Mechanical cues in orofacial tissue engineering and regenerative medicine

Cleft lip and palate patients suffer from functional, aesthetical, and psychosocial problems due to suboptimal regeneration of skin, mucosa, and skeletal muscle after restorative cleft surgery. The field of tissue engineering and regenerative medicine (TE/RM) aims to restore the normal physiology of tissues and organs in conditions such as birth defects or after injury. A crucial factor in cell differentiation, tissue formation, and tissue function is mechanical strain. Regardless of this, mechanical cues are not yet widely used in TE/RM. The effects of mechanical stimulation on cells are not straight-forward in vitro as cellular responses may differ with cell type and loading regime, complicating the translation to a therapeutic protocol. We here give an overview of the different types of mechanical strain that act on cells and tissues and discuss the effects on muscle, and skin and mucosa. We conclude that presently, sufficient knowledge is lacking to reproducibly implement external mechanical loading in TE/RM approaches. Mechanical cues can be applied in TE/RM by fine-tuning the stiffness and architecture of the constructs to guide the differentiation of the seeded cells or the invading surrounding cells. This may already improve the treatment of orofacial clefts and other disorders affecting soft tissues.

Pretreatment with mechano-growth factor E peptide protects bone marrow mesenchymal cells against damage by fluid shear stress

Improper fluid shear stress (FSS) can cause serious damages to bone marrow mesenchymal stem cells (MSCs). Mechano-growth factor (MGF) E peptide pretreatment was proposed to protect MSCs against FSS damage in this study. MSCs were exposed to FSS for 30 min after they were pretreated with MGF E peptide for 24 h. Then, the effects of MGF E peptide on the viability, proliferation and cell apoptosis of MSCs were investigated. MGF E peptide pretreatment could recover the cellular metabolic activity of MSCs reduced by 72 dyne cm(-2) FSS and had a synergistic effect with FSS on the cellular metabolic viability of MSCs under 24 and 72 dyne cm(-2) FSS. These results suggested that MGF E peptide pretreatment could be an effective method for the protection of FSS damage in bone tissue engineering.

Acute mechanical overload increases IGF-I and MMP-9 mRNA in 3D tissue-engineered skeletal muscle

Skeletal muscle (SkM) is a tissue that responds to mechanical load following both physiological (exercise) or pathophysiological (bed rest) conditions. The heterogeneity of human samples and the experimental and ethical limitations of animal studies provide a rationale for the study of SkM plasticity in vitro. Many current in vitro approaches of mechanical loading of SkM disregard the three-dimensional (3D) structure in vivo. Tissue engineered 3D SkM, that displays highly aligned and differentiated myotubes, was used to investigate mechano-regulated gene transcription of genes implicated in hypertrophy/atrophy. Static loading (STL) and ramp loading (RPL) at 10 % strain for 60 min were used as mechano-stimulation with constructs sampled immediately for RNA extraction. STL increased IGF-I mRNA compared to both RPL and CON (control, p = 0.003 and 0.011 respectively) whilst MMP-9 mRNA increased in STL and RPL compared to CON (both p < 0.05). IGFBP-2 mRNA was differentially regulated in RPL and STL compared to CON (p = 0.057), whilst a reduction in IGFBP-5 mRNA was found for STL and RPL compared to CON (both p < 0.05). There was no effect in the expression of putative atrophic genes, myostatin, MuRF-1 and MAFBx (all p > 0.05). These data demonstrate a transcriptional signature associated with SkM hypertrophy within a tissue-engineered model that more greatly recapitulates the in vivo SkM structure compared previously published studies.

Use of flow, electrical, and mechanical stimulation to promote engineering of striated muscles

The field of tissue engineering involves design of high-fidelity tissue substitutes for predictive experimental assays in vitro and cell-based regenerative therapies in vivo. Design of striated muscle tissues, such as cardiac and skeletal muscle, has been particularly challenging due to a high metabolic demand and complex cellular organization and electromechanical function of the native tissues. Successful engineering of highly functional striated muscles may thus require creation of biomimetic culture conditions involving medium perfusion, electrical and mechanical stimulation. When optimized, these external cues are expected to synergistically and dynamically activate important intracellular signaling pathways leading to accelerated muscle growth and development. This review will discuss the use of different types of tissue culture bioreactors aimed at providing conditions for enhanced structural and functional maturation of engineered striated muscles.

The therapeutic potential of IGF-I in skeletal muscle repair

Skeletal muscle loss due to aging, motor-neuron degeneration, cancer, heart failure, and ischemia is a serious condition for which currently there is no effective treatment. Insulin-like growth factor 1 (IGF-I) plays an important role in muscle maintenance and repair. Preclinical studies have shown that IGF-I is involved in increasing muscle mass and strength, reducing degeneration, inhibiting the prolonged and excessive inflammatory process due to toxin injury, and increasing the proliferation potential of satellite cells. However, clinical trials have not been successful due to ineffective delivery methods. Choosing the appropriate isoforms or peptides and developing targeted delivery techniques can resolve this issue. Here we discuss the latest development in the field with special emphasis on novel therapeutic approaches.

Skeletal muscle adaptation in response to mechanical stress in p130cas−/− mice

Mammalian skeletal muscles undergo adaptation in response to changes in the functional demands upon them, involving mechanical-stress-induced cellular signaling called “mechanotransduction.” We hypothesized that p130Cas, which is reported to act as a mechanosensor that transduces mechanical extension into cellular signaling, plays an important role in maintaining and promoting skeletal muscle adaptation in response to mechanical stress via the p38 MAPK signaling pathway. We demonstrate that muscle-specific p130Cas−/− mice express the contractile proteins normally in skeletal muscle. Furthermore, muscle-specific p130Cas−/− mice show normal mechanical-stress-induced muscle adaptation, including exercise-induced IIb-to-IIa muscle fiber type transformation and hypertrophy. Finally, we provide evidence that exercise-induced p38 MAPK signaling is not impaired by the muscle-specific deletion of p130Cas. We conclude that p130Cas plays a limited role in mechanical-stress-induced skeletal muscle adaptation.

Calcium-dependent deceleration of the cell cycle in muscle cells by simulated microgravity

Of all our mechanosensitive tissues, skeletal muscle is the most developmentally responsive to physical activity. Conversely, restricted mobility due to injury or disease results in muscle atrophy. Gravitational force is another form of mechanical input with profound developmental consequences. The mechanical unloading resulting from the reduced gravitational force experienced during spaceflight results in oxidative muscle loss. We examined the early stages of myogenesis under conditions of simulated microgravity (SM). C2C12 mouse myoblasts in SM proliferated more slowly (2.23× less) as a result of their being retained longer within the G2/M phase of the cell cycle (2.10× more) relative to control myoblasts at terrestrial gravity. Blocking calcium entry via TRP channels with SKF-96365 (10-20 μM) accumulated myoblasts within the G2/M phase of the cell cycle and retarded their proliferation. On the genetic level, SM resulted in the reduced expression of TRPC1 and IGF-1 isoforms, transcriptional events regulated by calcium downstream of mechanical input. A decrease in TRPC1-mediated calcium entry thus appears to be a pivotal event in the muscle atrophy brought on by gravitational mechanical unloading. Hence, relieving the constant force of gravity on cells might prove one valid experimental approach to expose the underlying mechanisms modulating mechanically regulated developmental programs.

Modelling in vivo skeletal muscle ageing in vitro using three-dimensional bioengineered constructs

Degeneration of skeletal muscle (SkM) with age (sarcopenia) is a major contributor to functional decline, morbidity and mortality. Methodological implications often make it difficult to embark on interventions in already frail and diseased elderly individuals. Using in vitro three-dimensional (3D) bioengineered skeletal muscle constructs that model aged phenotypes and incorporate a representative extracellular matrix (collagen), are under tension, and display morphological and transcript expression of mature skeletal muscle may more accurately characterize the SkM niche. Furthermore, an in vitro model would provide greater experimental manipulation with regard to gene, pharmacological and exercise (mechanical stretch/electrical stimulation) therapies and thus strategies for combating muscle wasting with age. The present study utilized multiple population-doubled (MPD) murine myoblasts compared with parental controls (CON), previously shown to have an aged phenotype in monolayer cultures (Sharples et al., 2011), seeded into 3D type I collagen matrices under uniaxial tension. 3D bioengineered constructs incorporating MPD cells had reduced myotube size and diameter vs. CON constructs. MPD constructs were characterized by reduced peak force development over 24 h after cell seeding, reduced transcript expression of remodelling matrix metalloproteinases, MMP2 and MMP9, with reduced differentiation/hypertrophic potential shown by reduced IGF-I, IGF-IR, IGF-IEa, MGF mRNA. Increased IGFBP2 and myostatin in MPD vs. CON constructs also suggested impaired differentiation/reduced regenerative potential. Overall, 3D bioengineered skeletal muscle constructs represent an in vitro model of the in vivo cell niche with MPD constructs displaying similar characteristics to ageing/atrophied muscle in vivo, thus potentially providing a future test bed for therapeutic interventions to contest muscle degeneration with age.

Differential effects of cyclic uniaxial stretch on human mesenchymal stem cell into skeletal muscle cell

Both fetal and adult skeletal muscle cells are continually being subjected to biomechanical forces. Biomechanical stimulation during cell growth affects proliferation, differentiation and maturation of skeletal muscle cells. Bone marrow-derived hMSCs [human MSCs (mesenchymal stem cells)] can differentiate into a variety of cell types, including skeletal muscle cells that are potentially a source for muscle regeneration. Our investigations involved a 10% cyclic uniaxial strain at 1 Hz being applied to hMSCs grown on collagen-coated silicon membranes with or without IGF-I (insulin-like growth factor-I) for 24 h. Results obtained from morphological studies confirmed the rearrangement of cells after loading. Comparison of MyoD and MyoG mRNA levels between test groups showed that mechanical loading alone can initiate myogenic differentiation. Furthermore, comparison of Myf5, MyoD, MyoG and Myf6 mRNA levels between test groups showed that a combination of mechanical loading and growth factor results in the highest expression of myogenic genes. These results indicate that cyclic strain may be useful in myogenic differentiation of stem cells, and can accelerate the differentiation of hMSCs into MSCs in the presence of growth factor.

Stimulation of mechano-growth factor expression by myofibrillar proteins in murine myoblasts and myotubes

Mechano-growth factor (MGF) is a product of alternative splicing of the insulin-like growth factor 1 (IGF-1) mRNA. MGF is known to stimulate myoblast proliferation and to protect neurons and cardiomyocytes from apoptosis. MGF expression is dramatically increased in response to mechanical stimuli and tissue damage. The mechanisms of induction of MGF expression are as yet imperfectly understood. There is certain evidence that some protein factors able to stimulate MGF synthesis in normal myoblasts are released from damaged muscle. This study was undertaken to explore the nature of these protein inductors of MGF expression and to investigate the mechanism of their action. We report here that myofibrillar fraction of skeletal muscle homogenate activated MGF expression in murine myoblasts and myotubes in culture. The expression of another splice form of IGF-1 gene, IGF-1Ea, was also stimulated by myofibrils. Three myofibrillar proteins able to stimulate MGF synthesis were isolated. These proteins were identified by MALDI and immunoblotting as myomesin, myosin-binding protein C, and titin. The activation of MGF expression was associated with the increase of cAMP level in the cells. Inhibitor of adenylyl cyclase dideoxyadenosine arrested stimulation of MGF synthesis by all three myofibrillar proteins.

Effect of passive stretch on intracellular nitric oxide and superoxide activities in single skeletal muscle fibres: influence of ageing

Skeletal muscle is repeatedly exposed to passive stretches due to the activation of antagonist muscles and to external forces. Stretch has multiple effects on muscle mass and function, but the initiating mechanisms and intracellular signals that modulate those processes are not well understood. Mechanical stretch applied to some cell types induces production of reactive oxygen species (ROS) and nitric oxide that modulate various cellular signalling pathways. The aim of this study was to assess whether intracellular activities of ROS and nitric oxide were modulated by passive stretches applied to single mature muscle fibres isolated from young and old mice. We developed a novel approach to apply passive stretch to single mature fibres from the flexor digitorum brevis muscle in culture and to monitor the activities of ROS and nitric oxide in situ by fluorescence microscopy. Passive stretch applied to single skeletal muscle fibres from young mice induced an increase in dihydroethidium oxidation (reflecting intracellular superoxide) with no increase in intracellular DAF-FM oxidation (reflecting nitric oxide activity) or CM-DCFH oxidation. In contrast, in fibres isolated from muscles of old mice passive stretch was found to induce an increase in intracellular nitric oxide activities with no change in DHE oxidation.

Evolution of oxygen utilization in multicellular organisms and implications for cell signalling in tissue engineering

Oxygen is one of the critically defining elements resulting in the existence of eukaryotic life on this planet. The rise and fall of this element can be tracked through time and corresponds with the evolution of diverse life forms, development of efficient energy production (oxidative phosphorylation) in single cell organisms, the evolution of multicellular organisms and the regulation of complex cell phenotypes. By understanding these events, we can plot the effect of oxygen on evolution and its direct influence on different forms of life today, from the whole organism to specific cells within multicellular organisms. In the emerging field of tissue engineering, understanding the role of different levels of oxygen for normal cell function as well as control of complex signalling cascades is paramount to effectively build 3D tissues in vitro and their subsequent survival when implanted.

The evaluation of cyclic uniaxial strain on myogenic differentiation of adipose-derived stem cells

It has been revealed that skeletal muscle cells have the potential to generate, sense and respond to biomechanical signals and that, mechanical force is one of the important factors influencing proliferation, differentiation, regeneration and homeostasis of skeletal muscle cells and myoblasts. The aim of this study was to illustrate the effect of cyclic uniaxial strain on myogenic differentiation of adipose-derived stem cells (ASCs). This study was designed to investigate this effect within 3 days in 4 groups: control (untreated), chemical, chemical-mechanical and mechanical based on exposure of ASCs to chemical growth factors for 3 days or to mechanical strain just on the 2nd day. Finally, cell orientation, muscle-related gene expression, myosin protein synthesis and the number of myosin-positive cells were examined to estimate the rate of differentiation. By studying the cells before and after exposure to uniaxial strain, it could be observed that by exerting the load, the cells were organized almost perpendicularly to strain direction. Real-time RT-PCR demonstrated that uniaxial strain had a significant effect on up-regulation of muscle-related genes in chemical-mechanical group (P < 0.001) as compared to mechanical or chemical groups. Immunocytochemistry confirmed the myosin-positive cells in treated groups and the numbers of these cells were enumerated by flow cytometry. These data suggest that uniaxial cyclic strain could affect ASCs and cause their myogenic differentiation and that the combination of chemical myogenic differentiation factors with mechanical signals promotes differentiation much more than differentiation by chemical myogenic differentiation factors or mechanical signals alone.

Expression of insulin-like growth factor 1 isoforms in the rabbit oculomotor system

OBJECTIVE

The insulin-like growth factor-1 (IGF-1) gene encodes two isoforms, IGF-1Ea and IGF-1Eb. Both isoforms can regulate skeletal muscle growth and strength. It has been suggested that IGF-Eb may be more potent in promoting skeletal muscle hypertrophy. Precise contractile force regulation is particularly important in the oculomotor system. However, expression of these isoforms in mammalian extraocular muscles (EOMs) is unknown. Here, we examined their expression in rabbit EOMs and the innervating nerve, two potential sources for myogenic growth factors, and compared isoform expression between EOMs and limb skeletal muscles.

DESIGN

Expression of IGF-1 isoforms was quantified by real-time RT-PCR in adult rabbit EOMs, trochlear and ophthalmic nerves, and compared with expression in rabbit limb skeletal muscles. The presence of mature IGF-1 peptide in the muscles was further examined by Western blot.

RESULTS

Both IGF-1Ea and IGF-1Eb were expressed in the EOM and the trochlear nerve. Both isoforms were expressed at significantly higher levels (9-fold) in EOM than in limb skeletal muscle. Transcripts of IGF-1 isoforms, of IGF-1 receptor and of IGF binding proteins showed a gradient distribution along the EOM from proximal to distal. The mature IGF-1 protein showed the same gradient distribution in the EOM.

CONCLUSIONS

Expression of relatively abundant amounts of both IGF-1 splicing isoforms in EOMs, and at a significantly higher level than in limb skeletal muscle, underscores the potential relevance of these myogenic growth factors in EOM plasticity and force regulation.

How to make rapid eye movements "rapid": the role of growth factors for muscle contractile properties

Different muscle functions require different muscle contraction properties. Saccade-generating extraocular muscles (EOMs) are the fastest muscles in the human body, significantly faster than limb skeletal muscles. Muscle contraction speed is subjected to plasticity, i.e., contraction speed can be adjusted to serve different demands, but little is known about the molecular mechanisms that control contraction speed. Therefore, we examined whether myogenic growth factors modulate contractile properties, including twitch contraction time (onset of force to peak force) and half relaxation time (peak force to half relaxation). We examined effects of three muscle-derived growth factors: insulin-like growth factor 1 (IGF1), cardiotrophin-1 (CT1), and glial cell line-derived neurotrophic factor (GDNF). In gain-of-function experiments, CT1 or GDNF injected into the orbit shortened contraction time, and IGF1 or CT1 shortened half relaxation time. In loss-of-function experiments with binding proteins or neutralizing antibodies, elimination of endogenous IGFs prolonged both contraction time and half relaxation time, while eliminating endogenous GDNF prolonged contraction time, with no effect on half relaxation time. Elimination of endogenous IGFs or CT1, but not GDNF, significantly reduced contractile force. Thus, IGF1, CT1, and GDNF have partially overlapping but not identical effects on muscle contractile properties. Expression of these three growth factors was measured in chicken and/or rat EOMs by real-time PCR. The "fast" EOMs express significantly more message encoding these growth factors and their receptors than skeletal muscles with slower contractile properties. Taken together, these findings indicate that EOM contractile kinetics is regulated by the amount of myogenic growth factors available to the muscle.

Effect of aging on cellular mechanotransduction

Aging is becoming a critical heath care issue and a burgeoning economic burden on society. Mechanotransduction is the ability of the cell to sense, process, and respond to mechanical stimuli and is an important regulator of physiologic function that has been found to play a role in regulating gene expression, protein synthesis, cell differentiation, tissue growth, and most recently, the pathophysiology of disease. Here we will review some of the recent findings of this field and attempt, where possible, to present changes in mechanotransduction that are associated with the aging process in several selected physiological systems, including musculoskeletal, cardiovascular, neuronal, respiratory systems and skin.

The role of GH and IGF-I in mediating anabolic effects of testosterone on androgen-responsive muscle

Testosterone (T) supplementation increases skeletal muscle mass, circulating GH, IGF-I, and im IGF-I expression, but the role of GH and IGF-I in mediating T's effects on the skeletal muscle remains poorly understood. Here, we show that T administration increased body weight and the mass of the androgen-dependent levator ani muscle in hypophysectomized as well as castrated plus hypophysectomized adult male rats. T stimulated the proliferation of primary human skeletal muscle cells (hSKMCs) in vitro, an effect blocked by transfecting hSKMCs with small interference RNA targeting human IGF-I receptor (IGF-IR). In differentiation conditions, T promoted the fusion of hSKMCs into larger myotubes, an effect attenuated by small interference RNA targeting human IGF-IR. Notably, MKR mice, which express a dominant negative form of the IGF-IR in skeletal muscle fibers, treated with a GnRH antagonist (acyline) to suppress endogenous T, responded to T administration by an attenuated increase in the levator ani muscle mass. In conclusion, circulating GH and IGF-I are not essential for mediating T's effects on an androgen-responsive skeletal muscle. IGF-I signaling plays an important role in mediating T's effects on skeletal muscle progenitor cell growth and differentiation in vitro. However, IGF-IR signaling in skeletal muscle fibers does not appear to be obligatory for mediating the anabolic effects of T on the mass of androgen-responsive skeletal muscles in mice.

Stimulation of mechano-growth factor expression by second messengers

The effect of second messengers on the expression of mechano-growth factor (MGF) synthesis by myoblasts and differentiated myotubes in culture was investigated. cAMP stimulates MGF expression both in murine and human cells. CNG- and HCN-channel blockers slightly activated MGF synthesis, while an activator of Epac protein had no effect. It is assumed that cAMP activates MGF synthesis via protein kinase A. Phorbol ester (PMA) activates MGF synthesis in human myoblasts and myotubes only. The expression of another splice form of IGF-1 gene, IGF-1Ea, was also stimulated in human cells by db-cAMP and PMA and in murine cells by db-cAMP only. Stimulation of MGF expression in human cells by db-cAMP and PMA demonstrated different time dependences but showed additivity when the compounds were applied in a combination. Inhibitors specific to protein kinase A did not affect PMA-mediated activation, while inhibitors specific to protein kinase C did not affect db-cAMP-mediated process. Ca²+ ionophore and ROS inductor strongly inhibited synthesis of the growth factor. PGE2 known as physiological stimulator of cAMP synthesis was shown to stimulate MGF expression both in murine and human cells. Implication of protein kinase A and protein kinase C in MGF synthesis stimulation and a cross-talk between two signaling systems is discussed.

The effect of cell density on the maturation and contractile ability of muscle derived cells in a 3D tissue-engineered skeletal muscle model and determination of the cellular and mechanical stimuli required for the synthesis of a postural phenotype

The successful engineering of a truly biomimetic model of skeletal muscle could have a significant impact on a number of biomedical disciplines. Although a variety of techniques are currently being developed, there is, as of yet, no widely available and easily reproducible culture system for the synthesis of 3D artificial muscle tissues. In attempting to generate such a model it is essential to optimise any protocol in order to generate a tissue that best represents the in vivo environment. Since the maturation of muscle derived cells in culture is critically dependent on density, a major factor to be addressed in the development of these models is the ideal concentration at which to seed cells in order to generate an optimal response. In studying the effect of cell density on the performance of cells in an established 3D collagen based model of skeletal muscle, we demonstrate that an optimum density does exist in terms of peak force generation and myogenic gene expression data. Greater densities however, lead to the formation of a more physiologically relevant tissue with a phenotype characteristic of slow, postural muscle.

Insulin-like growth factor-I E peptides: implications for aging skeletal muscle

In skeletal muscle there is good evidence to suggest that locally produced insulin-like growth factor-1 (IGF-I), rather than circulating IGF-I, is important in regard to muscle mass maintenance, repair and hypertrophy. This "mature" IGF-I comprises exons 3 and 4 of the IGF-I gene, but during processing the full length gene (which contains six exons) is subject to a process of alternative splicing. As a result smaller peptides (E peptides) are believed to be cleaved from the mature IGF-I peptide during processing of the prohormone and the likelihood is that they have different biological roles. In human skeletal muscle three transcripts encoding for these splice variants (IGF-IEa, IGF-IEb and IGF-IEc, also known as MGF) can be identified. When studied at the mRNA level these three transcripts are known to be upregulated in the muscles of elderly people following high resistance exercise, albeit with different time courses. However, compared with mature IGF-I relatively little is known about the mechanism of action of the different E peptides.

Investigation of MGF mRNA expression in patients with amyotrophic lateral sclerosis using parallel in vivo and in vitro approaches

In an animal model of ALS, intramuscular administration of MGF, the IGF-I Ec gene splice variant, improved muscle strength and increased both motor unit and motor neuron survival. Here we investigated whether there is a deficit in MGF production in the muscles of patients with ALS. We used complementary in vivo and in vitro techniques to study the IGF-I splice variant response of human muscle to exercise or mechanical stretch. We assessed the levels of MGF and IGF-IEa mRNA in muscle biopsy samples from healthy subjects and patients with ALS, before and after exercise. We used primary muscle cells to build three-dimensional collagen constructs and subjected them to a ramp stretch. Patients with ALS had similar baseline levels of MGF and IGF-IEa mRNA to healthy controls. No up-regulation was seen in either group within a short time of a single bout of low intensity exercise. Three-dimensional human muscle constructs also detected no response to a mechanical stretch from either control subjects or ALS. We conclude that the pathology of ALS does not include a deficit in baseline levels of MGF and IGF-IEa mRNA splice variants in muscle.