The cell membrane and cytoskeleton are dynamic structures that are strongly influenced by the thermo-mechanical background in addition to biologically driven mechanical processes. We used atomic force microscopy (AFM) to measure the local membrane motion of human foreskin fibroblasts (HFFs) which were found to be governed by random and non-random correlated mechanical processes. Interphase cells displayed distinct membrane pulsations in which the membrane was observed to slowly contract upwards followed by a recovery to its initial position. These pulsations occurred one to three times per minute with variable amplitudes (20-100 pN) separated by periods of random baseline fluctuations with amplitudes of <20 pN. Cells were exposed to actin and microtubule (MT) destabilizing drugs and induced into early apoptosis. Mechanical pulsations (20-80 pN) were not prevented by actin or MT depolymerization but were prevented in early apoptotic cells which only displayed small amplitude baseline fluctuations (<20 pN). Correlation analysis revealed that the cell membrane motion is largely random; however several non-random processes, with time constants varying between approximately 2 and 35 s are present. Results were compared to measured cardiomyocyte motion which was well defined and highly correlated. Employing automated positioning of the AFM tip, interphase HFF correlation time constants were also mapped over a 10 microm2 area above the nucleus providing some insights into the spatial variability of membrane correlations. Here, we are able to show that membrane pulsations and fluctuations can be linked to physiological state and cytoskeletal dynamics through distinct sets of correlation time constants in human cells.
Abstract:The aim of this study was to probe the morphological response of single mouse embryonic stem cells (mESC) to precisely delivered nanomechanical forces. Plating mESC as single cells gave rise to either round compact or flattened fibroblastic morphologies. The expression of OCT4 and Nanog was reduced in flattened cells, indicating that this population had begun to differentiate. Upon application of .5 nN of force, using atomic force microscopy and simultaneous laser scanning confocal microscopy, round cells, but not flattened cells, were capable of forming mechanically induced blebs (miBlebs). Flattened cells appeared to have a more highly developed cytoskeleton than undifferentiated stem cells as characterized by the distribution of phospho-ezrin-radixin-moesin (pERM). Higher levels of pERM and an inability to form miBlebs in flattened cells imply that the earliest stages of embryonic stem cell differentiation are associated with the development of stronger mechanical links between the plasma membrane and the cytoskeleton.
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