Séminaire Institut

Friday 21 May 2010 à 14h00.

Intracellular Transport Phenomena and Cellular Migration Processes

''Living cells exhibit exceptional dynamic properties, caused by the presence of ATP-driven motion. In particular, intracellular transport of cargos proceeds by successive phases of diffusion and active movement along microtubules via dynein and kinesin motors. While passive Brownian motion allows for intracellular transport of molecules on the nanoscale, it becomes inefficient for transport of large proteins, vesicles and organelles on the scale of a whole cell. Thus, motor-driven movement is crucial for many physiological processes, such as axonal transport in neurons. We developed an automated and reliable time-resolved identification method for motility state signatures of cytoplasmic tracers in living cells. Such an approach is both experimentally challenging and of fundamental importance for our understanding of intracellular transport processes. Our rolling-average algorithm [1] is based on the analysis of the local mean-square displacement (MSD) and directional persistence of the tracer path. The algorithm is able to reliably separate the active and passive motion of particles in cells. We analyze the particle motion in terms of a two-state motility model: this yields the distribution of active and passive state durations as well as the distribution of the state parameters, i.e. the velocity during active phases and the diffusion coefficient of the passive motion. We are able to extend this analysis to sub-diffusive intracellular transport states and further to motion states of the entire cell, migrating on structured surfaces. We investigated the motion of micron- and nanosized particles (NPs) in the amoeba Dictyostelium discoideum (Dd). Dd is a very suitable cell model because of its cytoskeleton simplicity and because of its ability to phagocytose micro-and nanoparticles. The distribution of active transport durations is found to decay exponentially with a characteristic time tA = 0.65 s. The velocity distribution of active events exhibits several peaks, revealing the signature of a finite number of molecular motors working collectively. In contrast, after depolymerization of the microtubule network, the analyzed paths exhibit no significant active event, proving that active states are due to tracer transport along the microtubules exclusively. We manipulate intracellular transport states via controlled external stimuli, by exerting magnetic force pulses to phagocytosed magnetic NPs in living cells [2]. By further applying spatially and temporally defined external boundary conditions to these cells, like precisely monitored chemotaxis gradients or by cell motility essays on pre-ordered 3D topologies [3], we induce changes in cellular function and control cell migration properties. [1] D. Arcizet, B. Meier, E. Sackmann, J. Rädler and D. Heinrich, Phys. Rev. Lett. 101, 248103, 2008 [2] J. Mahowald, D. Arcizet, and D. Heinrich, ChemPhysChem 10, 1559 (2009) [3] C. Pelzl, D. Arcizet, G. Piontek, J. Schlegel, and D. Heinrich, ChemPhysChem 10, 2884 (2009)''