Measurement of strain in cultured bone and fetal muscle and lung cells

Emerging Development of Microfluidics-Based Approaches to Improve Studies of Muscle Cell Migration

IMPACT STATEMENT

The essential interactions between and among cells in the three types of muscle tissue in development, wound healing, and regeneration of tissues, are underpinned by the ability of cardiac, smooth, and skeletal muscle cells to migrate in maintaining functional capacity after pathologies such as myocardial infarction, tissue grafting, and traumatic and postsurgical injury. Microfluidics-based devices now offer significant enhancement over conventional approaches to studying cell chemotaxis and haptotaxis that are inherent in migration. Advances in experimental approaches to muscle cell movement and tissue formation will contribute to innovations in tissue engineering for patching wound repair and muscle tissue replacement.

A new look at cytoskeletal NOS-1 and β-dystroglycan changes in developing muscle and brain in control and mdx dystrophic mice

BACKGROUND

Loss of dystrophin profoundly affects muscle function and cognition. Changes in the dystrophin-glycoprotein complex (DGC) including disruption of nitric oxide synthase (NOS-1) may result from loss of dystrophin or secondarily after muscle damage. Disruptions in NOS-1 and beta-dystroglycan (bDG) were examined in developing diaphragm, quadriceps, and two brain regions between control and mdx mice at embryonic day E18 and postnatal days P1, P10, and P28. Age-dependent differential muscle loading allowed us to test the hypothesis that DGC changes are dependent on muscle use.

RESULTS

Muscle development, including loss of central nucleation and the localization of NOS-1 and bDG, was earlier in diaphragm than quadriceps; these features were differentially disrupted in dystrophic muscles. The NOS-1/bDG ratio, an index of DGC stability, was higher in dystrophic diaphragm (P10-P28) and quadriceps (P28) than controls. There were also distinct regional differences in NOS-1 and bDG in brain tissues with age and strain. NOS-1 increased with age in control forebrain and cerebellum, and in mdx cerebellum; NOS-1 and bDG were higher in control than mdx mouse forebrain.

CONCLUSIONS

Important developmental changes in structure and muscle DGC preceded the hallmarks of dystrophy, and are consistent with the impact of muscle-specific differential loading during maturation.

Single muscle-fiber isolation and culture for cellular, molecular, pharmacological, and evolutionary studies

The technique of single muscle-fiber cultures has already proven valuable in extending knowledge of myogenesis, stem cell heterogeneity, the stem cell niche in skeletal muscle, and satellite cell activation. This report reviews the background of the model and applications, and details the procedures of muscle dissection, fiber digestion and isolation, cleaning the fiber preparation, plating fibers, and extensions of the technique for studying activation from stable quiescence of satellite cells, mRNA expression by in situ hybridization and regulation of satellite cell activation in zebrafish muscle by nitric oxide, hepatocyte growth factor.

Effects of respiratory mechanical forces on the pharmacological response of lung cancer cells to chemotherapeutic agents

In vitro screening of chemotherapeutic agents is routinely carried out in static monolayer cell cultures. However, drugs administered to patients act in the presence of various microenvironments in vivo. For example, in lung tumors, mechanical forces are constantly present and do affect the physiological response of the lung tissue to a variety of therapeutic agents. We hypothesized that mechanical forces may affect the response of lung tumors to chemotherapeutic agents and studied the effects under simulated conditions. First, we examined the effects of simulated forces that approximate normal respiration on the proliferation and morphology of NCI-H358 and A549 cell lines. Then, we studied the effects of the simulated forces on the ability of Paclitaxel, Doxorubicin, Cisplatin, Zactima and an experimental drug to induce cytotoxicity in both cell lines. Cells were treated with the drugs in the presence or absence of simulated forces (20% maximum strain and 15 cycles/minute) that approximate human lung expansion and contraction. Cell proliferation and the effectiveness of the drugs were assessed. Using a standard exponential cell growth model, it was determined that mechanical forces significantly reduced the proliferation of both cell lines. Interestingly, forces also significantly lowered the effectiveness of all drugs except Zactima in A549 cells, while in NCI-H358 cells, Zactima was the only drug that demonstrated an increase in effectiveness owing to applied forces. Our results demonstrate that mechanical forces have significant impact on cell survival and chemotherapeutic efficacy and may be of significance in engineering improved screening assays for antitumor drug discovery.

Nitric oxide-dependence of satellite stem cell activation and quiescence on normal skeletal muscle fibers

Satellite cells (quiescent precursors in normal adult skeletal muscle) are activated for growth and regeneration. Signaling by nitric oxide (NO) and hepatocyte growth factor (HGF) during activation has not been examined in a model that can distinguish quiescent from activated satellite cells. We tested the hypothesis that NO and HGF are required to regulate activation using the single-fiber culture model. In normal fibers, HGF and inhibition of NO synthase (NOS) each increased activation without stretching, and NOS inhibition reduced stretch-activation. Activation in unstretched mdx and NOS-I(-/-) fibers was three- to fourfold higher than normal, and was reduced by stretching. Distinctions were not due to different pax7-expressing populations on normal and mdx fibers. The population of c-met-expressing satellite cells on normal fibers was increased by stretch, demonstrating functional heterogeneity among normal satellite cells. Cycloheximide did not prevent the stretch-related increase in c-met expression, suggesting c-met may be an immediate-early gene in satellite cell activation. Results have important implications for designing new therapies that target the role of exercise in health, aging, and disease.

Satellite cell activation on fibers: modeling events in vivo — an invited review

Knowledge of the events underlying satellite cell activation and the counterpart maintenance of quiescence is essential for planning therapies that will promote the growth and regeneration of skeletal muscle in healthy, disease and aging. By modeling those events of satellite cell activation in studies of single muscle fibers or muscles in culture, the roles of mechanical stretching and nitric oxide are becoming understood. Recent studies demonstrated that stretch-induced activation is very rapid and exhibits some features of satellite cell heterogeneity. As well, gene expression studies showed that expression of the c-met receptor gene rises rapidly after stretching muscles in culture compared to those without stretch. This change in gene expression during activation, and the maintenance of quiescence in both normal and dystrophic muscles are dependent on NO, as they are blocked by inhibition of nitric oxide synthase (NOS). Mechanical, contractile activity is the defining feature of muscle function. Therefore, ongoing studies of stretch effects in satellite cell activation and quiescence in quiescent fiber and muscle cultures provides appropriate models by which to explore the regulatory steps in muscle in vivo under many conditions related to disease, repair, rehabilitation, growth and the prevention or treatment of atrophy.Key words: regeneration, stretch, myofiber culture, muscular dystrophy, quiescence.

C-Met expression and mechanical activation of satellite cells on cultured muscle fibers

Single-fiber cultures can be used to model satellite cell activation in vivo. Although technical deficiencies previously prevented study of stretch-induced events, here we describe a method developed to study satellite cell gene expression by in situ hybridization (ISH) using protocol modifications for fiber adhesion and fixation. The hypothesis that mechanical stretching activates satellite cells was tested. Fiber cultures were established from normal flexor digitorum brevis muscles and plated on FlexCell dishes with a layer of Vitrogen. After 2 hr of stretch in the presence of BrdU, satellite cells on fibers attached to Vitrogen were activated above control levels. In the absence of activating treatments or mechanical stretch, ISH studies showed 0-6 c-Met+ satellite cells per fiber. Time course experiments demonstrated stable quiescence in the absence of stretch and significant peaks in activation after 30 min and 2 hr of stretch. Frequency distributions for unstretched fiber cultures showed a significantly greater number of quiescent c-Met+ satellite cells than were activated by stretching, suggesting that typical activation stimuli did not trigger cycling in the entire c-Met+ population of satellite cells. These methods have a strong potential to further dissect the nature of stretch-induced activation and gene expression among characterized populations of individual quiescent and activated satellite cells.

Mechanical stimuli enhances the expression of RGD-CAP/betaig-h3 in the periodontal ligament

RGD-CAP, a member of the fasciclin family, is expressed in the periodontal ligament (PDL). Since the PDL is continually subjected to mechanical forces from such orofacial functions as mastication, biting, speech and swallowing, the mechanical stimuli is thought to be associated with the expression of RGD-CAP. Furthermore, the adhesive functions of RGD-CAP may contribute to the maintenance or regeneration of PDL architecture. The objective of this study was to examine whether mechanical stimuli modulate the expression of RGD-CAP in the human PDL, and to examine the effects of recombinant RGD-CAP on the adhesion of PDL cells. During experimental tooth movement, the expression of RGD-CAP was significantly enhanced in the PDL. In vitro experiments with cultured PDL cells showed that the expression of RGD-CAP mRNA was significantly enhanced by mechanical tensile force of 15.4kPa for 48h. The induction of RGD-CAP mRNA, meanwhile, was completely inhibited by cycloheximide which is an inhibitor of protein synthesis. Furthermore, neutralising antibody against TGF-beta also suppressed the mechanical induction of RGD-CAP. The adhesion of cultured PDL cells onto plates coated with recombinant RGD-CAP increased significantly compared with the controls. These findings suggest that RGD-CAP, induced by TGF-beta expressed in response to mechanical stimuli, plays an important role in modulating the homeostasis of PDL.

The effects of high magnitude cyclic tensile load on cartilage matrix metabolism in cultured chondrocytes

Excessive mechanical load is thought to be responsible for the onset of osteoarthrosis (OA), but the mechanisms of cartilage destruction caused by mechanical loads remain unknown. In this study we applied a high magnitude cyclic tensile load to cultured chondrocytes using a Flexercell strain unit, which produces a change in cell morphology from a polygonal to spindle-like shape, and examined the protein level of cartilage matrixes and the gene expression of matrix metalloproteinases (MMPs), tissue inhibitors of matrix metalloproteinases (TIMPs) and proinflammatory cytokines such as IL-1beta and TNF-alpha. Toluidine blue staining, type II collagen immunostaining, and an assay of the incorporation of [35S]sulfate into proteoglycans revealed a decrease in the level of cartilage-specific matrixes in chondrocyte cultures subjected to high magnitude cyclic tensile load. PCR-Southern blot analysis showed that the high magnitude cyclic tensile load increased the mRNA level of MMP-1, MMP-3, MMP-9, IL-1beta, TNF-alpha and TIMP-1 in the cultured chondrocytes, while the mRNA level of MMP-2 and TIMP-2 was unchanged. Moreover, the induction of MMP-1, MMP-3 and MMP-9 mRNA expression was observed in the presence of cycloheximide, an inhibitor of protein synthesis. These findings suggest that excessive mechanical load directly changes the metabolism of cartilage by reducing the matrix components and causing a quantitative imbalance between MMPs and TIMPs.

The effects of mechanical forces on lung functions

The lung is a dynamic organ that is subjected to mechanical forces throughout development and adult life. This review article addresses the types of mechanical forces in the lung and their effects on development and normal lung functions. The effects of mechanical forces on the various different cell types of the lung are discussed, as are the mechanisms underlying mechanotransduction.

Mechanical force-induced signal transduction in lung cells

The lung is a unique organ in that it is exposed to physical forces derived from breathing, blood flow, and surface tension throughout life. Over the past decade, significant progress has been made at the cellular and molecular levels regarding the mechanisms by which physical forces affect lung morphogenesis, function, and metabolism. With the use of newly developed devices, mechanical forces have been applied to a variety of lung cells including fetal lung cells, adult alveolar epithelial cells, fibroblasts, airway epithelial and smooth muscle cells, pulmonary endothelial and smooth muscle cells, and mesothelial cells. These studies have led to new insights into how cells sense mechanical stimulation, transmit signals intra- and intercellularly, and regulate gene expression at the transcriptional and posttranscriptional levels. These advances have significantly increased our understanding of the process of mechanotransduction in lung cells. Further investigation in this exciting research field will facilitate our understanding of pulmonary physiology and pathophysiology at the cellular and molecular levels.

Skeletal myogenesis on elastomeric substrates: implications for tissue engineering

Studies geared towards understanding the interaction between skeletal muscle and biomaterials may provide useful information for the development of various emerging technologies, ranging from novel delivery vehicles for genetically modified cells to fully functional skeletal muscle tissue. To determine the utility of elastomeric materials as substrates for such applications, we asked whether skeletal myogenesis would be supported on a commercially available polyurethane, Tecoflex SG-80A. G8 skeletal myoblasts were cultured on Tecoflex two-dimensional solid thin films fabricated by a spin-casting method. Myoblasts attached, proliferated, displayed migratory activity and differentiated into multinucleated myotubes which expressed myosin heavy chain on solid thin films indicating that Tecoflex SG-80A was permissive for skeletal myogenesis. Porous three-dimensional (3-D) cell scaffolds were fabricated in a variety of shapes, thicknesses, and porosities by an immersion precipitation method, and where subsequently characterized with microscopic and mechanical methods. Mechanical analysis revealed that the constructs were elastomeric, recovering their original length following 100% elongation. The 3-D substrates were seeded with muscle precursors to determine if muscle differentiation could be obtained within the porous network of the fabricated constructs. Following several weeks in culture, histological studies revealed the presence of multinucleated myotubes within the elastomeric material. In addition, immunohistochemical analysis indicated that the myotubes expressed the myosin heavy chain protein suggesting that the myotubes had reached a state of terminal differentiation. Together the results of the study suggest that it is indeed feasible to engineer bioartificial systems consisting of skeletal muscle cultivated on a 3-D elastomeric substrate.

Glycosaminoglycan synthesis in the rat articular disk in response to mechanical stress

The mechanism by which compressive mechanical stress affects glycosaminoglycan synthesis in the articular disk was investigated with a modified organ culture technique. Forty-eight male Sprague-Dawley rats were divided into three experimental groups and one control group of 12 animals each, aged 7 and 9 weeks. The experimental groups followed different regimens of stress applied for 25%, 75%, or 100% of the time during the total test period of 24 hours. Articular disks were stressed with flexible bottomed dishes (Flex I dishes, Flexcell Corp., McKeesport, Pa.) using the Flexercell Strain Unit (Flexcell Corp., McKeesport, Pa.) and incubated with [3H]-glucosamine for 24 hours. Samples were then collected, digested with Pronase-E, and after precipitation with cetylpyridinium chloride (CPC) and ethanol, the different glycosaminoglycans (GAGs) were separated by using cellulose acetate electrophoresis. The significant GAG types with stress were chondroitin6sulfate (C6S), hyaluronic acid (HA), and dermatan sulfate (DS). There was no significant relationship in the experimental groups between age and regimen of stress applied in either age. Higher stress regimens showed significantly higher proportions of C6S when compared with the controls, whereas HA appeared to decrease slightly and DS was not affected. Since C6S is the major component of hyaline cartilage, the results of this study suggest that compressive forces in the articular disk may stimulate the development of more cartilagenous-like properties with respect to GAG content.

The effects of mechanical stimulation on the distribution of beta 1 integrin and expression of beta 1-integrin mRNA in TE-85 human osteosarcoma cells

Mechanical stimulation of the skeleton alters the metabolism of bone cells, but the effects of mechanical strain on the cytoskeleton of osteoblasts are poorly understood. While changes in the distribution of the cytoskeleton in mechanically strained cells have been reported, little is known about the pathways by which these changes are transduced into cell functions. Human osteosarcoma (HOS) TE-85 cells were cultured in Dubelcco's modified Eagle's medium/F-12 and grown to confluency in Flexercell type I dishes in a humidified incubator with 5% CO2 and 95% air. Intermittent strain (3 cycles/min) was applied to the cells for periods of 15 and 30 min, 2, 4 and 24 h, and 3, 5, 7, 10, 14, 20 and 28 days. Unstrained cells were used as controls. The distribution of beta 1 integrin was studied immunocytochemically. Total RNA was isolated at every period of time and Northern blots were used to study the effects of strain on the levels of beta 1-integrin expression. The results indicated that mechanical strain increased the synthesis of beta 1 integrin. Northern blots showed that beta 1 mRNA expression was increased significantly (p < 0.005) at 30 min and 3 days of strain application. Strain also affected beta 1 distribution markedly in 24-h cultures. The response of HOS cells to mechanical strain demonstrates that the cytoskeleton of the osteoblast adapts to strain through the stimulation of specific cytoskeletal and receptor proteins. These results suggest a pathway through which mechanical strain is transmitted to the osteoblastic-like cells.