Biological condensates are membrane-less organelles that organize material inside biological cells. It is now understood that the often form by the physical process of liquid-liquid phase separation, but how cells regulate this process is still elusive. We use concepts from theoretical physics to predict the behavior of biological condensates to understand their behavior. For instance, we investigate how cells can control droplets by chemical reactions.
Spontaneously dividing active droplets as a model for protocells
The first lifeforms on earth must have been simple enough to emerge spontaneously, but they also needed to divide and propagate, such that natural selection and evolution could lead to more complex lifeforms. So far, the physical properties of such early cells are unclear, but it has been proposed almost a century ago that they could have been liquid-like droplets. We showed that such simple liquid droplets can indeed divide spontaneously if the chemical reaction that builds the droplet material from precursors is driven by an external energy input, like sun light or heat from thermal vents. Here, the chemical reaction of these active droplets plays the role of a prebiotic metabolism.
Centrosomes described as active droplets
Centrosomes are small organelles present in all animal cells. They are important for organizing the mitotic spindle, which segregates the DNA during cell division. Cancer cells often contain more than two centrosomes, which impairs normal cell division. It is thus important to understand the assembly of centrosomes in order to tackle failures in their formation and function.
The centrosome is a dynamic aggregate, which forms and dissolves in synchrony with the cell cycle. Beside reactions between and diffusion of centrosome components, aspects of non-equilibrium thermodynamics have to be considered to describe the observed behavior. In fact, we developed a physical description of centrosomes as autocatalytic droplets, whose formation is controlled by chemical reactions. This project was carried out in close collaboration with the group of Tony Hyman at the MPI of Molecular Cell Biology and Genetics.
Recently, we have also shown that the same model also explains why centrioles are always found at the center of the centrosome: The chemical reactions in the centrosome and at the centriole surface induce compositional fluxes, which lead to the effective centering of the centrioles.