<p>Recent research has shown that enhanced focal adhesion between cells and extracellular matrix (ECM) and intracellular actin polymerization can accelerate cellular functions like proliferation and differentiation. It is a desirable and necessary technique to modulate cellular functions in the desired lineage in tissue engineering. Previously, we have shown that topographical effects of micropatterns on a cell culture substrate promoted osteogenic differentiation of mesenchymal stem cells (MSCs) without administration of osteogenic growth factors. In this study, bone marrow mesenchymal stem cells from rats were cultured with combined biophysical stimuli, such as surface topography of biomaterials and uniaxial tensile stress and induced osteogenic differentiation. We evaluated MSCs by assessment of alkaline phosphatase (ALP). We demonstrated that a polydimethylsiloxane substrate with microgroove patterns (2 μm of ridge thickness, 1 μm of depth and 1 μm of groove distance) could suppress osteogenic differentiation compared to a flat substrate. Changes of cell adhesion and shape were observed at microgroove substrates by immunofluorescence staining of focal adhesion and actin filaments. We showed that to apply the stretching force promoted differentiation at microgroove substrates but inhibited differentiation at the flat surface. Through this study, a more efficient method to control cellular fate is expected to be established for tissue regeneration <i>in vitro</i>.</p> <div style="text-align:center;"> </div>

Keywords

Mesenchymal stem cells, Osteoblast differentiation, Uniaxial tensile stress, Surface topography, Micropattern, Alkaline phosphatase, Focal adhesion

Paper information

[Advance Publication] (Proper information for citation will be announced after formal publication)

Adherent cells generate traction forces, which are generated by and transmitted along the actin cytoskeleton to the underlying external substrate via focal adhesions. These cell generated forces are fundamental for maintaining cell shape and driving cell migration as well as triggering signaling pathways to promote processes such as differentiation and proliferation. Here we investigate how mechanical stretch affects traction forces. Following moderate mechanical stretching and release, traction forces are increased and then return to basal levels. However, when cells are stretched with relatively large strains, &gt;10%, traction forces fail to return to basal levels and are attenuated. In this study, we investigate whether cells experiencing attenuated traction forces, following a large strain, can still respond to subsequent mechanical stretching. Traction forces were measured in cells following a consecutive 12% increase in mechanical stretch. We show that, even after traction force attenuation occurs, cells are able to respond to consecutive mechanical stretching by increasing traction force. Visualization of stress fibers, with GFP-actin, indicates that the attenuation of traction force is accompanied by a decrease in stress fiber integrity during mechanical stretching. The ability of cells to increase traction force during a second round of stretching, following attenuation, maybe generated by intact stress fibers that escape damage.

Keywords

Traction force, Mechanical stretch, Actin stress fibers, HUVEC

Paper information

Akira TSUKAMOTO, Katie R. RYAN, Yusuke MITSUOKA, Katsuko S. FURUKAWA, Takashi USHIDA, “Cellular traction forces increase during consecutive mechanical stretching following traction force attenuation”, Journal of Biomechanical Science and Engineering, Vol.12, No.3 (2017), p.17-00118. doi:10.1299/jbse.17-00118. Final Version Released on July 21, 2017, Advance Publication Released on March 06, 2017.

Cells under cyclic stretch sense their environments and induce responses such as actin stress fiber (SF) reorientation and morphological changes. These physiological responses are thought to occur when cells sense incompatibility between SF orientation and stretching direction. This hypothesis requires existence of SFs. However, such existence of SFs in cells under cyclic stretch remains unclear since few studies attempted to track the existence of SFs throughout cyclic stretch. In order to track the existence of SFs throughout cyclic stretch, high time resolution time-lapse imaging was improved in two points. First, SFs were clearly imaged with coexpression of DsRed-zyxin and GFP-actin. Second, time resolution was improved so that fluorescence images were obtained every 28 sec. With the improved high time resolution time-lapse imaging, it was revealed, for the first time, that SFs could exist continuously throughout cyclic stretch. Moreover, physiological responses including morphological change as well as SF reorientation occurred during the time when SFs formed incompatibility between SF orientation and stretching direction. These results demonstrated that SFs continuously existed in cells under cyclic stretch and in turn suggested that continuous presence of incompatibility between orientation of long-lasting SFs and the stretching direction might be important for mechanosensing which induces physiological responses.

Keywords

Cyclic Stretch, Stress Fiber, Orientation Dynamics, Morphological Change, Time-Lapse Imaging, HUVEC

Paper information

Yusuke MITSUOKA, Akira TSUKAMOTO, Shunsuke IWAYOSHI, Katsuko S. FURUKAWA and Takashi USHIDA, “High Time Resolution Time-Lapse Imaging Reveals Continuous Existence and Rotation of Stress Fibers under Cyclic Stretch in HUVEC”, Journal of Biomechanical Science and Engineering, Vol. 7, No. 2 (2012), pp.188-198 . doi:10.1299/jbse.7.188