Synthetic Cells: Controlling Shapes and Movements
Jülich/Zurich, 1 October 2020. Living cells are anything but rigid structures. They can take on many different forms, in order to move around, worm their way through narrow spaces, or to absorb nutrients. Pathogens, such as the food-borne germ Listeria monocytogenes or the plague-related bacterium Yersinia pseudotuberculosis use these abilities for active locomotion to penetrate healthy tissue, for instance. Scientists at Forschungszentrum Jülich and ETH Zurich have now studied the physical principles of these complex processes using a new synthetic model system. The results help to better understand biological processes and could be important for the development of synthetic cells, which might one day serve as miniature factories or work as micro-robots.
The outer sheath of cells – the membrane – has no muscles or comparable structures. Instead, cell shapes and dynamic deformations are created in response to forces inside the cell. How these forces are generated, how the membrane reacts to these forces and just which cell formations are created and when – these and similar questions were explored by scientists at Forschungszentrum Jülich and ETH Zurich, by combining two well-researched model systems and studying them together, both experimentally and by using computer simulations.
Because the cell membrane does not play an active role in shaping cells, the researchers were able to replace it in their studies with simpler vesicles – tiny blisters whose flexible membrane is very similar to cell membranes in structure and mechanical behaviour. When in equilibrium, such vesicles are often almost spherical. The researchers replaced the complex cytoskeleton, which in living cells exerts forces on the membranes from the inside and thus causes the cells to move and change shape, with active particles. These spherical Janus particles develop their own propulsion when chemical energy is supplied. The theoretical investigation of active particles has been a focus of the Jülich researchers led by Prof. Dr. Gerhard Gompper, Director at the Institute for Advanced Simulation and the Institute of Biological Information Processing, for several years now.
In Zurich, during experimental studies of the system using different particle concentrations, the researchers found a surprising variety of unexpected vesicle shapes – as they were never observed in thermal equilibrium. In order to explain how they are created, the Jülich researchers have developed a new program for extensive calculations on a supercomputer at Forschungszentrum Jülich, which enables a high spatial resolution of the membrane shapes and deformations to be achieved.
“The computer simulation of the entire system of vesicles and active particles and their interactions with each other enables us to carry out studies that are experimentally difficult or completely impossible,” explains Prof. Dr. Gerhard Gompper. For example, in experiments with a high concentration of the fuel hydrogen peroxide, bubbles are formed which themselves generate strong forces through surface tension. Moreover, gravity causes the vesicles to sink to the bottom of the sample containers, where they are then deformed. Such unwanted “side effects” can be avoided in simulations. Furthermore, the findings from the simulations can be used to better analyse the experiments.
The researchers thus succeeded in identifying the three essential factors that determine the shape and dynamics of vesicles: firstly, the membrane tension, secondly, the propulsive force of the active particles and thirdly, their concentration in the vesicle. The researchers were in particular surprised by the observation that the greatest variety of shapes occurred at quite low particle concentrations. Further studies on vesicles with more complex membranes will be carried out in the next step to bring the model systems closer to their biological counterparts.
Original publication: Hanumantha Rao Vutukuri et al.;
Active particles induce large shape deformations in giant lipid vesicles;
Nature, 30. September 2020, DOI: 10.1038/s41586-020-2730-x
A video simulation made by the researchers showing the transformation of an initially almost spherical cell into a star-shaped cell. Certain nerve cells in the brain, the astrocytes for example, are similarly star-shaped.
Source: Forschungszentrum Jülich
Prof. Dr. Gerhard Gompper
Theoretical Physics of Living Matter/Theoretical Soft Matter and Biophysics (IBI-5/IAS-2)
Tel: 02461 61-4012
Angela Wenzik, Science Journalist
Tel: 02461 61-6048