Superstatistical analysis and modelling of heterogeneous random walks

Stochastic time series are ubiquitous in nature. In particular, random walks with time-varying statistical properties are found in many scientific disciplines. Here we present a superstatistical approach to analyse and model such heterogeneous random walks. The time-dependent statistical parameters can be extracted from measured random walk trajectories with a Bayesian method of sequential inference. The distributions and correlations of these parameters reveal subtle features of the random process that are not captured by conventional measures, such as the mean-squared displacement or the step width distribution. We apply our new approach to migration trajectories of tumour cells in two and three dimensions, and demonstrate the superior ability of the superstatistical method to discriminate cell migration strategies in different environments. Finally, we show how the resulting insights can be used to design simple and meaningful models of the underlying random processes.

Conventional methods to quantify the migratory behaviour of cells assume that underlying parameters are constant. Mark et al. apply a superstatistical approach to extract time-dependent parameters of motile cells, and demonstrate an enhanced ability to distinguish between different migration strategies.

Differential interference contrast microscopy using light-emitting diode illumination in conjunction with dual optical traps

Differential interference contrast (DIC) microscopy is a common mode of biological light microscopy used to achieve maximal resolution and contrast with label-free, weakly absorbing specimens such as cells. Maintaining the polarization state of the illuminating light is essential for the technique, and this requirement can conflict with optical trapping. We describe how to optimize DIC imaging using a light-emitting diode illumination source in a microscope while integrating a dual optical trap into the set up. Every time a polarized light beam reflects off or transmits through a dichroic mirror in the beam path, its polarization state will change if it is not polarized exactly parallel (p) or perpendicular (s) to the plane of incidence. We observe wavelength-dependent optical rotation and depolarization effects in our illumination light upon reflection from/transmission through dichroic mirrors in the beam path, resulting in significant degradation of image quality. We describe a method to compensate for these effects by introducing quarter-waveplates and a laser clean-up filter into the imaging pathway. We show that this approach achieves a full recovery of image quality.

Inhibition of Rho kinases increases directional motility of microvascular endothelial cells

Rho kinases are major regulators of actin cytoskeletal organization and cell motility. Depending on the model system, inhibitors of Rho kinases (ROCK) have been reported to increase or decrease endothelial cell migration. In the present study we investigated the effect of Rho kinase inhibitors on microvascular endothelial cell migration with a special focus on the isoform ROCK2. Migration of microvascular endothelial cells was analyzed in a wound-healing, a spheroid-on-collagen migration assay and in cells embedded in collagen-1 gels. The non-selective Rho kinase inhibitor H1152 was compared to the selective ROCK2 inhibitor SLX2119 and to siRNA knock down. Non-selective inhibition of Rho kinases decreased cell-spanning F-actin fibers, loosened cell-cell contacts visualized by VE cadherin staining, and reduced cell-matrix interactions as shown by reduced Hic-5 expression in focal contacts. Rho kinase inhibitors facilitated directed migration of endothelial cells away from spheroids on fibronectin-coated plates and in collagen-1 gels. By contrast, migration of firmly attached endothelial cells, resembling intact vessels, was not promoted by Rho kinase inhibition. Selective inhibition of ROCK2 mimicked the cytoskeletal effects of H1152 and also increased cell motility, although to a lesser extent. In summary, Rho kinase inhibition enhanced the migration and cytoskeletal restructuring preferentially in freshly attached endothelial cells. ROCK2 may be a potential target to manipulate endothelial cell migration after vessel injury.