Permeability of C2C12 myotube membranes is influenced by stretch velocity

Molecular and Biomechanical Adaptations to Mechanical Stretch in Cultured Myotubes

Myotubes are mature muscle cells that form the basic structural element of skeletal muscle. When stretching skeletal muscles, myotubes are subjected to passive tension as well. This lead to alterations in myotube cytophysiology, which could be related with muscular biomechanics. During the past decades, much progresses have been made in exploring biomechanical properties of myotubes in vitro. In this review, we integrated the studies focusing on cultured myotubes being mechanically stretched, and classified these studies into several categories: amino acid and glucose uptake, protein turnover, myotube hypertrophy and atrophy, maturation, alignment, secretion of cytokines, cytoskeleton adaption, myotube damage, ion channel activation, and oxidative stress in myotubes. These biomechanical adaptions do not occur independently, but interconnect with each other as part of the systematic mechanoresponse of myotubes. The purpose of this review is to broaden our comprehensions of stretch-induced muscular alterations in cellular and molecular scales, and to point out future challenges and directions in investigating myotube biomechanical manifestations.

Life is determined by its environment

A well-developed theory of evolutionary biology requires understanding of the origins of life on Earth. However, the initial conditions (ontology) and causal (epistemology) bases on which physiology proceeded have more recently been called into question, given the teleologic nature of Darwinian evolutionary thinking. When evolutionary development is focused on cellular communication, a distinctly different perspective unfolds. The cellular communicative-molecular approach affords a logical progression for the evolutionary narrative based on the basic physiologic properties of the cell. Critical to this appraisal is recognition of the cell as a fundamental reiterative unit of reciprocating communication that receives information from and reacts to epiphenomena to solve problems. Following the course of vertebrate physiology from its unicellular origins instead of its overt phenotypic appearances and functional associations provides a robust, predictive picture for the means by which complex physiology evolved from unicellular organisms. With this foreknowledge of physiologic principles, we can determine the fundamentals of Physiology based on cellular first principles using a logical, predictable method. Thus, evolutionary creativity on our planet can be viewed as a paradoxical product of boundary conditions that permit homeostatic moments of varying length and amplitude that can productively absorb a variety of epigenetic impacts to meet environmental challenges.

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.

Stretch-induced ERK2 phosphorylation requires PLA2 activity in skeletal myotubes

Mechanical stretch rapidly activates multiple signaling cascades, including phospholipases and kinases, to stimulate protein synthesis and growth. The purpose of this study was to determine whether PLA2 activation contributes to stretch-induced phosphorylation of ERK2 in skeletal muscle myotubes. Myotubes derived from neonatal C57 mice were cultured on silicone membranes and subjected to brief cyclic stretch. Inhibition of PLA2 prevented ERK2 phosphorylation, while inhibition of prostaglandin or leukotriene synthesis did not. ERK2 phosphorylation was also blocked by genistein and PD98059, implicating the canonical raf-MEK-ERK cassette. It appears that PLA2, but not further metabolism of arachidonic acid, is required for stretch-induced activation of ERK2. Exposure to exogenous arachidonic acid had no effect on ERK2 phosphorylation, but exposure to lysophosphatidylcholine, the other metabolite of PLA2, caused a dose-dependent increase in ERK2 phosphorylation. These results suggest that stretch-induced activation of ERK2 may result from an interaction between PLA2 derived lysophosphatidylcholine and membrane receptors.

Modulation of luciferase activity using high intensity focused ultrasound in combination with bioluminescence imaging, magnetic resonance imaging and histological analysis in muscle tissue

This study investigates the effect of high intensity focused ultrasound (HIFU) to muscle tissue transfected with a luciferase reporter gene under the control of a CMV-promoter. HIFU was applied to the transfected muscle tissue using a dual HIFU system. In a first group four different intensities (802 W/cm2, 1401 W/cm2, 2117 W/cm2, 3067 W/cm2) of continuous HIFU were applied 20 s every other week for four times. In a second group two different intensities (802 W/cm2, 1401 W/cm2) were applied 20 s every fourth day for 20 times. The luciferase activity was determined by bioluminescence imaging. The effect of HIFU to the muscle tissue was assessed by T1-weighted +/- Gd-DTPA, T2-weighted and a diffusion-weighted STEAM sequence obtained on a 1.5-T GE-MRI scanner. Histology of the treated tissue was done at the end. In the first group the photon emission was at 3067.6 W/cm2 1.28 x 10(7) +/- 3.1 x 10(6) photon/s (5.5 +/- 1.2-fold), of 2157.9 W/cm2 8.1 +/- 2.7 x 10(6) photon/s (3.2 +/- 1.1-fold), of 1401.9 W/cm2 9.3 +/- 1.3 x 10(6) photon/s (4.9 +/- 0.4-fold) and of 802.0 W/cm2 8.6x +/- 1.2 x 10(6) photon/s (4.5 +/- 0.6-fold) compared to baseline. In the second group the photon emission was at 1401.9 W/cm2 and 802.0 W/cm2 14.1 +/- 3.6 x 10(6) photon/s (6.1 +/- 1.5-fold), respectively, 5.1 +/- 4.7 x 10(6) photon/s (6.5 +/- 2.0-fold). HIFU can enhance the luciferase activity controlled by a CMV-promoter.

Investigation of gene expression in C(2)C(12) myotubes following simvastatin application and mechanical strain

AIM

The 3-hydroxy-3methylgutaryl coenzyme A (HMG-CoA) reductase inhibitors (statins) are the most effective prescribed drugs for lowering serum cholesterol; however, although statins are extremely safe medications and have brought significant benefits to patients with hypercholesterolemia, they have been shown to produce myalgia, cramps, exercise intolerance and fatigue. The aim of the study was to investigate the molecular mechanisms that may mediate statin myopathy.

METHODS

We used DNA microarray analysis to examine the changes in gene expression profiles induced by 1 hour and 6 hours of statin treatment on differentiated C(2)C(12) myotubes. Four genes were selected for analysis at the protein level using Western blot analysis on myotubes treated with statin with or without additional mechanical stretching.

RESULTS

Eighty-five genes exhibited more than a 2-fold up- or down-regulation in expression, of which 46 have known biological functions related primarily to transmembrane transport, signal transduction, cell growth/maintenance, protein metabolism, or apoptosis. At protein level, three of the four proteins were induced (Adrb1, Socs4 and Cflar) and one was repressed (Birc4). Changes in protein expression largely mirrored the changes in their corresponding transcripts, although the fold-change was less dramatic. The addition of imposed muscle fiber stretching did not exacerbate the expression of these genes at the protein level with the exception of Cflar, a pro-apoptotic protein.

CONCLUSION

These data suggested that alterations in the expressions of some statin-regulated genes could be causative factors for statin toxicity in muscle. Repression of the anti-apoptosis gene (Birc4) and activation of the pro-apoptosis gene (Cflar) indicated that cell death may play an important role in statin-induced myopathy.

Mechanical strain increases gene transfer to skeletal muscle cells

Gene transfer techniques possess tremendous potential for treating diseases and for facilitating the study of basic physiological processes. However, further development of efficient and safe methods for gene transfer is needed. The purpose of this study was to test the hypothesis that mechanical strain increases the transfer of DNA to differentiated skeletal muscle cells. We tested this hypothesis by applying cyclic strain to cultured skeletal myotubes either prior to or immediately after the introduction of exogenous DNA complexed with lipids, with strains of varying magnitude (10%, 20% and 30%), number (1800, 3600 and 7200 strain cycles) and frequency (0.5, 1.0 and 1.5 Hz). Results demonstrated that DNA transfection was increased by exposing muscle cells to cyclic strain, and that strain magnitude, number and frequency each influenced DNA transfection. Optimal strain conditions (20% strain magnitude, 3600 cycles applied at 1 Hz) were utilized to examine the role of membrane transport systems in strain-induced increases in DNA transfection. Filipin III was used to inhibit caveolar transport and was found to inhibit strain-mediated increases in DNA transfection, whereas chlorpromazine, used to inhibit clathrin-coated vesicle transport, had no effect. These results indicate that mechanical strain may be an effective method for increasing DNA transfection in skeletal muscle through enhanced caveolar transport.

Ecto-ADP-ribose transferases: cell-surface response to local tissue injury

Ecto-ADP-ribose transferases (ecto-ARTs) catalyze the transfer of ADP-ribose from NAD(+) to arginine residues in cell-surface proteins. Since the concentration of extracellular NAD(+) is very low under normal physiological conditions but rises significantly upon tissue injury or membrane stress, it is postulated that the main role of ecto-ARTs is to ADP-ribosylate and regulate the function of certain membrane receptors in response to elevated levels of NAD(+).

Stretch-induced myoblast proliferation is dependent on the COX2 pathway

Skeletal muscle increases in size due to weight bearing loads or passive stretch. This growth response is dependent in part upon myoblast proliferation. Although skeletal muscles are responsive to mechanical forces, the effect on myoblast proliferation remains unknown. To investigate the effects of mechanical stretch on myoblast proliferation, primary myoblasts isolated from Balb/c mice were subjected to 25% cyclical uniaxial stretch for 5 h at 0.5 Hz. Stretch stimulated myoblast proliferation by 32% and increased cell number by 41% 24 and 48 h after stretch, respectively. COX2 mRNA increased 3.5-fold immediately poststretch. Prostaglandin E2 and F2alpha increased 2.4- and 1.6-fold 6 h after stretch, respectively. Because COX2 has been implicated in regulating muscle growth and regeneration, we hypothesized that stretched myoblasts may proliferate via a COX2-dependent mechanism. We employed two different models to disrupt COX2 activity: (1) treatment with a COX2-selective drug, and (2) transgenic mice null for COX2. Treating myoblasts with a COX2-specific inhibitor blocked stretch-induced proliferation. Likewise, stretched COX2-/- myoblasts failed to proliferate compared to controls. However, supplementing stretched, COX2-/- myoblasts with prostaglandin E2 or fluprostenol increased proliferation. These data suggest that the COX2 pathway is critical for myoblast proliferation in response to stretch.

Pulsed High-Intensity Focused Ultrasound Enhances Systemic Administration of Naked DNA in Squamous Cell Carcinoma Model: Initial Experience

PURPOSE

To determine whether exposures to pulsed high-intensity focused ultrasound can enhance local delivery and expression of a reporter gene, administered with systemic injection of naked DNA, in tumors in mice.

MATERIALS AND METHODS

The study was performed according to an approved animal protocol and in compliance with guidelines of the institutional animal care and use committee. Squamous cell carcinoma (SCC7) tumors were induced subcutaneously in both flanks of female C3H mice (n = 3) and allowed to grow to average size of 0.4 cm(3). In each mouse, one tumor was exposed to pulsed high-intensity focused ultrasound while a second tumor served as a control. Immediately after ultrasound exposure, a solution containing a cytomegalovirus-green fluorescent protein (GFP) reporter gene construct was injected intravenously via the tail vein. The mouse was sacrificed 24 hours later. Tissue specimens were viewed with fluorescence microscopy to determine the presence of GFP expression, and Western blot analysis was performed, at which signal intensities of expressed GFP were quantitated. A paired Student t test was used to compare mean values in controls with those in treated tumors. Histologic analyses were performed with specific techniques (hematoxylin-eosin staining, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling) to determine whether tumor cells had been damaged by ultrasound exposure.

RESULTS

GFP expression was present in all sections of tumors that received ultrasound exposure but not in control tumors. Results of signal intensity measurement at Western blot analysis showed expressed GFP to be nine times greater in ultrasound-exposed tumors (160.2 +/- 24.5 [standard deviation]) than in controls (17.4 +/- 11.8) (P = .004, paired Student t test). Comparison of histologic sections from treated tumors with those from controls revealed no destructive effects from ultrasound exposure.

CONCLUSION

Local exposure to pulsed high-intensity focused ultrasound in tumors can enhance the delivery and expression of systemically injected naked DNA.

Reduction of caveolin-3 expression does not inhibit stretch-induced phosphorylation of ERK2 in skeletal muscle myotubes

Mechanotransduction is critical to the maintenance and growth of skeletal muscle, but the mechanism by which cellular deformations are converted to biochemical signals remains unclear. Among the earliest and most ubiquitous responses to mechanical stimulation is the phosphorylation and activation of mitogen-activated protein kinases, in particular ERK2. Caveolin-3 (CAV-3) binds ERK2 and its upstream activators in inactive states on the caveolae of resting muscle. Caveolae are deformed by stretch, and it was hypothesized that this deformation might disrupt the CAV-3-dependent inhibition of ERK2 to affect stretch-induced activation. Stretch-induced phosphorylation of ERK2 in myotubes was both amplitude and velocity dependent, consistent with a viscoelastic mechanism, such as deformation of caveolae. Chemical disruption of caveolae by cholesterol depletion increased ERK2 activation by up to 176%. Small interfering RNA oligomers were then used to knock down expression of CAV-3 in cultured myotubes before mechanical stimulation, with the expectation that reducing CAV-3 expression would eliminate the stretch-induced activation of ERK2. Knockdown reduced CAV-3 protein content by 55% but did not significantly alter the stretch-induced increase in ERK2 phosphorylation, suggesting that CAV-3 is not an essential element of the mechanotransduction pathway, although the limited extent of knockdown limits the strength of this conclusion.