Inference of the force pattern acting on a semiflexible filament from shape analysis.

The study of the active forces acting on semiflexible filaments networks such as the cytoskeleton requires noninvasive tools able to explore the deformation of single filaments in their natural environment. We propose here a practical method based on the solution of the hydrodynamic beam equation in the presence of transverse forces. We found that the derivative of the local curvature presents discontinuities that match the location of the applied forces, in contrast to the smooth curvature function obtained for the case of compressing longitudinal forces. These patterns can be easily appreciated in a kymograph of the curvature, which also reflects the temporal behavior of the forces. We assessed the method performance with numerical simulations describing the deformation of single microtubules provoked by the action of intracellular active forces.

Morphological fluctuations of individual mitochondria in living cells.

Uncovering the link between mitochondrial morphology, dynamics, positioning and function is challenging. Mitochondria are very flexible organelles that are subject to tension and compression within cells. Recent findings highlighted the importance of these mechanical aspects in the regulation of mitochondria dynamics, arising the question on which are the processes and mechanisms involved in their shape remodeling. In this work we explored in detail the morphological changes and spatio-temporal fluctuations of these organelles in livingXenopus laevismelanophores, a well-characterized cellular model. We developed an automatic method for the classification of mitochondria shapes based on the analysis of the curvature of the contour shape from confocal microscopy images. A persistence length of 2.1µm was measured, quantifying, for the first time, the bending plasticity of mitochondria in their cellular environment. The shape evolution at the single organelle level was followed during a few minutes revealing that mitochondria can bend and unbend in the seconds timescale. Furthermore, the inspection of confocal movies simultaneously registering fluorescent mitochondria and microtubules suggests that the cytoskeleton network architecture and dynamics play a significant role in mitochondria shape remodeling and fluctuations. For instance changes from sinuous to elongated organelles related to transitions from confined behavior to fast directed motion along microtubule tracks were observed.