RTG 2756: Cytoskeletal elements of active matter – from molecular interactions to cellular biophysics

During the last decade, physical approaches entirely changed our view on biological systems by regarding them as active matter and unraveling some of the underlying universal principles, i.e., how structure and diversity emerge even with a limited set of different molecules. All active matter has in common that scalar chemical energy is converted in to directed mechanical work, enabling, e.g., motility, adhesion, division, and contraction of cells, and – on the tissue scale – tumorigenesis, embryogenesis and wound healing.

An intricate system of active filament networks inside eukaryotic cells, referred to as the cytoskeleton, carries out the following biophysical functions: it forms a scaffold to organize the content of the cell as well as to position and steer cellular components; it connects the cell with its external environment such as the extracellular matrix or neighboring cells; and it generates coordinated forces that enable the cell to become motile and to respond to external and internal cues. From a physical view point, the cytoskeleton can be regarded as a composite, where three filamentous components contribute distinctly different properties. Structurally, it is - to begin with - a passive composite network of actin filaments, microtubules and intermediate filaments.

Owing to their diverse chemical composition and molecular architecture, the physical properties of these biopolymers differ significantly. In conjunction with various cross-linkers, they fulfill specific mechanical and biochemical functions in the cell. However, the hallmark of the cytoskeleton is that molecular motors convert the passive biopolymer network into a contractile and highly active entity that is capable of responding to external stimuli, thus enabling locomotion of cells and transport of cargo within the cell.

While ample expertise has been acquired for the individual classes of cytoskeletal biopolymers, less emphasis has been put on the interface between the different filament types and the transformation from a passive to an active network, in particular in the context of an enclosing membrane, which essentially confines the space in which the cytoskeleton exists and thereby provides topological boundary conditions. In this RTG we are now in the position to focus our attention on the structure and dynamics of the cytoskeleton as a whole, as a paradigm for active matter systems.