Anomalous Subdiffusive Measurements by Fluorescence Correlations Spectroscopy and Simulations of Translational Diffusive Behavior in Live Cells

Abstract

Using a combination of optical experiments and computer simulations, we found that live cells act as traps to produce anomalous subdiffusive translational motion of molecules exemplified by the glucocorticoid receptor α. The glucocorticoid receptor α was expressed as a fusion protein with the green fluorescent protein in living mammalian U2OS cells. The measurements were carried out by fluorescence correlation spectroscopy. The glucocorticoid receptor α was either a homodimer or a monomer in the nucleoplasm depending on the presence or absence of the stimulus dexamethasone. Our simulations showed that the experimentally measured rates of translational diffusion could be attributed to spatial and temporal traps. Thus, traps acted by spatial and temporal heterogeneity and randomness, respectively. As we prove here for the first time, anomalous translational diffusion caused by spatial randomness was different from anomalous translational diffusion caused by heterogeneous temporal randomness. For this purpose, we explored two classes of transformations in the time domain for which the assumption that they are stable and have probability density functions was satisfied. The first one was the Inverse Gamma distribution and the second class was a stable Levy distribution. The spatial exponent α that was extracted from time series expressed scale invariance in the space domain. The time exponent γ measured limited time scaling of the embedding complex dynamics. Hence, both exponents must not be mixed up by a single exponent. Fluorescence fluctuation spectroscopy (FCS) is the method of choice for measuring the exponents of anomalous subdiffusive translational motion of molecules in live cells.