Oscillatory Microrheology, Creep Compliance and Stress Relaxation of Biological Cells Reveal Strong Correlations as Probed by Atomic Force Microscopy

The mechanical properties of cells are important for many biological processes, including wound healing, cancers, and embryogenesis. Currently, our understanding of cell mechanical properties remains incomplete. Different techniques have been used to probe different aspects of the mechanical properties of cells, among them microplate rheology, optical tweezers, micropipette aspiration, and magnetic twisting cytometry. These techniques have given rise to different theoretical descriptions, reaching from simple Kelvin-Voigt or Maxwell models to fractional such as power law models, and their combinations. Atomic force microscopy (AFM) is a flexible technique that enables global and local probing of adherent cells. Here, using an AFM, we indented single retinal pigmented epithelium cells adhering to the bottom of a culture dish. The indentation was performed at two locations: above the nucleus, and towards the periphery of the cell. We applied creep compliance, stress relaxation, and oscillatory rheological tests to wild type and drug modified cells. Considering known fractional and semi-fractional descriptions, we found the extracted parameters to correlate. Moreover, the Young’s modulus as obtained from the initial indentation strongly correlated with all of the parameters from the applied power-law descriptions. Our study shows that the results from different rheological tests are directly comparable. This can be used in the future, for example, to reduce the number of measurements in planned experiments. Apparently, under these experimental conditions, the cells possess a limited number of degrees of freedom as their rheological properties change

Fate of intravenously administered high-density lipoprotein labeled with radioiodinated cholesteryl oleate in normal and hypolipidemic rats

Radioiodinated cholesteryl oleate (125I-CO) was found to associate rapidly with plasma lipoproteins following intravenous administration to rats. The high-density lipoprotein (HDL) fraction was observed to contain the highest amount of radioiodinated ester. Isolation and purification of this HDL fraction (125I-CO-HDL) and subsequent administration to rats demonstrated a plasma clearance similar to that previously observed for HDL labeled by direct iodination. Moreover, the concentration of radioactivity appearing in the adrenal cortex and ovary 0.5 h after intravenous administration of 125I-CO-HDL was greater than that observed after administration of 125I-CO, and the uptake of radioactivity by these tissues was considerably greater in hypolipidemic rats. These findings are consistent with existing knowledge relating to the metabolic fate of HDL and radioiodinated cholesterol derivatives in the rat, and suggest that radioiodinated cholesteryl esters may become useful probes for labeling lipoproteins.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25293/1/0000736.pd