[Show abstract][Hide abstract] ABSTRACT: Cholesterol is a major component of eukaryotic cell membranes. It is well-accepted that cholesterol depletion triggers a complicated cascade of biochemical reactions which may affect many cell processes. However, the effect of cholesterol depletion on the deformability of cell membranes is still controversial. In this study, depth-sensing nano-indentation is performed on the lamellipodium of adherent NIH-3T3fibroblastcells with normal, depleted, and restored membranecholesterol contents. By extracting data from contact stiffness measurement, nano-mechanical characterizations are focused at a depth within the superficial 20 nm of the tested cells. Our results show that cholesterol depletion indeed decreases membrane stiffness, while the membrane stiffness decreases exponentially with the increase of cholesterol-depletion time. In addition, the effect of cholesterol restoration following depletion is further examined, showing that cholesterol restoration reverses the effect of cholesterol depletion on both cellular morphology and membrane stiffness. This is the first study, focused on nano-mechanical characterization of cellular outermost layers, demonstrating the effect of altered cholesterol content on the stiffness of cell membranes.
[Show abstract][Hide abstract] ABSTRACT: Continuous depth-sensing nano-indentation on living, fixed and dehydrated fibroblast cells was performed using a dynamic contact module and vertically measured from a pre-contact state to the glass substrate. The nano-indentation tip-on-cell approaches took advantage of finding a contact surface, followed by obtaining a continuous nano-mechanical profile along the nano-indentation depths. In the experiment, serial indentations from the leading edge, i.e., the lamellipodium to nucleus regions of living, fixed and dehydrated fibroblast cells were examined. Nano-indentations on a living cell anchored upon glass substrate were competent in finding the tip-on-cell contact surfaces and cell heights. For the result on the fixed and the dehydrated cells, cellular nano-mechanical properties were clearly characterized by continuous harmonic contact stiffness (HCS) measurements. The relations of HCS versus measured displacement, varied from the initial tip-on-cell contact to the glass substrate, were presumably divided into three stages, respectively induced by cellular intrinsic behavior, the substrate-dominant property, and the substrate property. This manifestation is beneficial to elucidate how the underlying substrate influences the interpretation of the nano-mechanical property of thin soft matter on a hard substrate. These findings, based upon continuous depth-sensing nano-indentations, are presumably valuable as a reference to related work, e.g., accomplished by atomic force microscopy.
[Show abstract][Hide abstract] ABSTRACT: Dipalmitoylphosphatic acid was chosen as a model to interpret how molecules physically adsorbed upon glass responded to an infinitesimal oscillation force at the surface contact level. Oscillation of a nano-indentation tip toward the phospholipid layers was driven by a dynamic contact module at a constant harmonic frequency; the phase angle of the oscillation frequency was exponentially relaxed along the nano-scale displacement. The tip-on-molecule contact was thereafter identified and influenced by the characteristic of the physically adsorbed phospholipids. By applying the harmonic displacement of the nano-indentation tip and making a distinction between full contact displacements, the thickness of the phospholipid layers was thereafter estimated. Moreover, the additional force required to penetrate through the physically adsorbed molecules was minor compared to the analogous process for the chemically adsorbed ones. The importance of recognizing the physically adsorbed molecules is relevant to applications of contact mechanics for the distinction of various phospholipids. Furthermore it is very promising to interpret the mechanism by which cells convert mechanical stimuli into biochemical responses on the channels of phospholipids.