Diffusion and directed motion in cellular transport

Abstract

We study the motion of a probe driven by microtubule-associated motors within a living eukaryotic cell. The measured mean square displacement, <x(t)2> of engulfed 2 and 3 microm diameter microspheres shows enhanced diffusion scaling as t(3/2) at short times, with a clear crossover to ordinary or subdiffusive scaling, i.e., t(gamma) with gamma less than or equal to 1, at long times. Using optical tweezers we tried to move the engulfed bead within the cell in order to relate the anomalous diffusion scaling to the density of the network in which the bead is embedded. Results show that the larger beads, 2 and 3 microm diameter, must actively push the cytoskeleton filaments out of the way in order to move, whereas smaller beads of 1 microm diameter can be "rattled" within a cage. The 1 microm beads also perform an enhanced diffusion but with a smaller and less consistent exponent 1.2<gamma<1.45. We interpret the half-integer power observed with large beads based on two diverse phenomena widely studied in purified cytoskeleton filaments: (1) the motion of the intracellular probe results from random forces generated by motor proteins rather than thermal collisions for classical Brownian particles, and (2) thermal bending modes of these semiflexible polymers lead to anomalous subdiffusion of particles embedded in purified gel networks or attached to single filaments, with <x(t)2> approximately t(3/4). In the case of small beads, there may also be a Brownian contribution to the motion that results in a smaller exponent.

Intracellular transport driven by cytoskeletal motors: General mechanisms and defects

“…This is not necessarily a drawback, as these symmetric bimodal distributions have never been clearly evidenced experimentally. Still, the stochastic model reproduces well that the tug-of-war leads to anomalous diffusion in agreement with some experimental observations [213,68,308,318]. Indeed, at very short time scales and if thermal fluctuations are taken into account, subdiffusion is observed as a result of the trapping of the cargo by oppositely pulling motors [193].…”

Quantitating intracellular transport of polyplexes by spatio-temporal image correlation spectroscopy

“…The idea of two temporal regimes of intracellular motion is not unique to this system; indeed, similar findings have been reported for m beads moving in the cytoplasm and chromosome motion in the nucleus (34,36,37) describing random walk processes. The benefit of our analysis is that it provides information about the system while requiring minimal fit parameters.…”

Active mechanical stabilization of the viscoplastic intracellular space of Dictyostelia cells by microtubule–actin crosstalk

“…Since the beads perform actively driven local irregular walks and directed motions over distances on the order of micrometers we think it is more appropriate to characterize their motion in terms of local velocity distributions instead of mean square displacement-versus-time plots [24]. Velocity distributions of nine wild-type cells with a total observation time of about 1.5 h, six myosin II null mutants (observation time about 1 h), four latrunkulin treated cells (observation time about 45 min) and four benomyl treated cells (observation time about 45 min) were investigated in this work.…”

“…Moreover, studies of the local viscoelastic moduli of the cell envelope or the cytoplasm can yield valuable insights into the structure of the membrane associated actin cortex and structural changes induced by cell stimulating or damaging agents [7,10], deceases [21] or by application of external forces [22]. A variety of colloidal probe microrheometry techniques have been developed over the last few years based on the analysis of Brownian motion [10,23,24] or of the viscoelastic responses evoked by local forces applied through optical traps [10,23] and magnetic tweezers. With magnetic tweezers, viscoelastic responses can be evoked by linear [25] or by torsional [26,27] excitations.…”