Quantitative Studies of Subdiffusion in Living Cells and Actin Networks

Abstract

Optical tweezers are a versatile tool in biophysics and have matured from a tool of manipulation to a tool of precise measurements. We argue here that the data analysis with advantage can be developed to a level of sophistication that matches that of the instrument. We review methods of analysis of optical tweezers data, primarily based on the power spectra of time series of positions for trapped spherical objects. The majority of precise studies in the literature are performed on in vitro systems, whereas in the present work, an example of an in vivo system is presented for which precise power spectral analysis is both useful and necessary. The biological system is the cytoplasm of fission yeast, Schizosaccharomyces pombe in which we observe subdiffusion of lipid granules. In a search for the cause of subdiffusion, we chemically disrupt the actin network in the cytoplasm and further consider in vitro networks of filamenteous actin undergoing similar chemical disruption.

Variety in intracellular diffusion during the cell cycle

“…Earlier work on the diffusion of lipid granules in interphase S. pombe cells using optical tweezers and multiple-particle tracking showed that different types of diffusion can be detected in the intracellular space, with a rheological exponent, α, that depends on the timescale in question: on timescales shorter than 10 −3 s, only subdiffusion was observed. For timescales longer than 10 −1 s, also other types of motion have been detected, e.g., confined motion, normal Brownian motion and superdiffusive motion [13,18]. All these types of motion were interpreted as indicative of specific biological processes in the cell.…”

“…In recent years, in vivo studies of the viscoelastic properties of the intracellular space have attracted more and more interest. In many experiments, the diffusion of endogenous lipid granules is studied [8,13,18]. One big advantage of the lipid granules is that they occur naturally and are therefore not regarded as foreign objects by the cell, whereas non-endogenous particles can induce active intracellular transport processes [19].…”