Cilia are hair like appendages of many cells that can generate motion and that can propel the swimming of microorganisms and sperm. We study the interplay between force generating processes and the three dimensional geometry of curved cilia. We show that torques generated by molecular motors in the cilium provide conditions in which local curvature can influence motor activity. Such curvature control provides a key mechanism for the generation of wave-like beats in short cilia.
The segmented body plan of vertebrate animals forms during the development of the embryo in a sequential process by which segment after segment is added. We presents a dynamic model for the interplay of dynamic oscillators and spatially graded signaling profiles that can account for the segmentation process as a self-organized pattern forming system in an active medium.
The cell cytoplams is a complex mixture of many molecules including proteins and RNA. Phase separation provides a physical mechanism to spatially organize the cytoplasm via the fromation of liquid-like droplets. Here we show that phase separation of certain RNA-protein droplets is regulated by RNA binding competition.
S. Saha, C. A. Weber, M. Nousch, O. Adame-Arana, C. Hoege, M. Y. Hein, E. Osborne-Nishimura,
J. Mahamid, M. Jahnel, L. Jawerth, A. Pozniakovski, C. R. Eckmann, F. Jülicher, and A. A. Hyman
Cell 166, 1572 (2016)
[PDF (6 MB)]
Compartment boundaries separate different compartments in tissues. A local increase of mechanical tension at cell bonds governs the shape and roughness of such compartment boundaries. We show that specific signals are involved in the upregulation of tension. A physical model of tissue mechanics reveals the role of cell bond tension in shaping clones of mutant cells.
The cell cortex is a thin film of an active gel below the cell membrane that generates an active tension. We study the material properties of this active film during cell division. We find that stress relaxation is governed by a range of relaxation processes that provide the cortex with dynamic response functions in response to tension changes and liquid like rheology at long times.
We provide a computational tool to study the collective dynamics of cells in tissues. Starting from microscopy data, large scale tissue deformations can be decomposen in contributions from cellular processes such as cell shape changes and cell rearrangments.
The periodic bending motion of cilia and flagella arises from mechanical feedback: dynein motors generate sliding forces that bend the flagellum, and bending leads to deformations and stresses, which feed back and regulate the motors. Different possible feedback mechanisms have been proposed: regulation by the sliding forces, regulation by the curvature of the flagellum, and regulation by the normal forces that deform the cross-section of the flagellum. Here, we combined theoretical and experimental approaches to show that the curvature control mechanism accords best with the bending waveforms of Chlamydomonas flagella.
Using a continuum description of active visco-elastic gels, we study the response of a contractile layer of an actomyosin gel to mechanical perturbations. This work is motivated by laser ablation experiments in cell biophysics. Our work shows that from the observation of the flow response to a linear later cut, key biophysical parameters of the contractile layer can be inferred.
Using a numerical approach, we investigate the spatiotemporal dynamics of an active polar gel in two dimensions. We find that when increasing the magnitude of active stresses, the system first undergoes a flow instability to stationary flow patterns. Subsequently, traveling wave solutions appear. Finally, for further increased active stress the system undergoes a transition to spatiotemporal chaos.
We study the dynamics of gene expression of cells from the zebrafish segmentation clock in vitro. We show that single cells can behave as autonomous noisy cellular oscillations. The observed variability of cell behaviors can be captured by a generic oscillator model with correlated noise. We find that single cells have longer periods and lower precision than the tissue, highlighting the role of collective processes in the segmentation clock.
We use a novel generalized vertex model to analyze epithelial morphogenesis in three dimensions. We show that increased actomyosin contractility at the interface between normal and aberrantly specified cells drives the formation of cysts while single cells are eliminated from the tissue.
C. Bielmeier, S. Alt, V. Weichselberger, M. La Fortezza, H. Harz, F. Jülicher,
G. Salbreux, A. Classen
Current Biology 26, 1 (2016)
[PDF (31,5 MB)]