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Pattern formation processes in active systems give rise to a plethora of collective structures. Predicting how the emergent structures depend on the microscopic interactions between the moving agents remains a challenge. By introducing a high-density actin gliding assay on a fluid membrane, we demonstrate the emergence of polar structures in a regime of nematic binary interactions dominated by steric repulsion. The transition from a microscopic nematic symmetry to a macroscopic polar structure is linked to microscopic polarity sorting mechanisms, including accumulation in wedge-like topological defects. Our results should be instrumental for a better understanding of pattern formation and polarity sorting processes in active matter.

A. Sciortino, A. R. Bausch (2014)

Pattern formation and polarity sorting of driven actin filaments on lipid membranes

PNAS, 118 (6).

Arp2/3 complex-mediated actin assembly at cell membranes drives the formation of protrusions or endocytic vesicles. To identify the mechanism by which different membrane deformations can be achieved, we reconstitute the basic membrane deformation modes of inward and outward bending in a confined geometry by encapsulating a minimal set of cytoskeletal proteins into giant unilamellar vesicles. Formation of membrane protrusions is favoured at low capping protein (CP) concentrations, whereas the formation of negatively bent domains is promoted at high CP concentrations. Addition of non-muscle myosin II results in full fission events in the vesicle system. The different deformation modes are rationalized by simulations of the underlying transient nature of the reaction kinetics. The relevance of the regulatory mechanism is supported by CP overexpression in mouse melanoma B16-F1 cells and therefore demonstrates the importance of the quantitative understanding of microscopic kinetic balances to address the diverse functionality of the cytoskeleton.

K. Dürre , F. C. Keber, P. Bleicher, F. Brauns, C. J. Cyron, J. Faix and A. R. Bausch (2018)

Capping protein-controlled actin polymerization shapes lipid membranes

Nature Communications, 9:1630

Engineering synthetic materials that mimic the remarkable complexity of living organisms is a fundamental challenge in science and technology. We studied the spatiotemporal patterns that emerge when an active nematic film of microtubules and molecular motors is encapsulated within a shape-changing lipid vesicle. Unlike in equilibrium systems, where defects are largely static structures, in active nematics defects move spontaneously and can be described as self-propelled particles. The combination of activity, topological constraints, and vesicle deformability produces a myriad of dynamical states. We highlight two dynamical modes: a tunable periodic state that oscillates between two defect configurations, and shape-changing vesicles with streaming filopodia-like protrusions. These results demonstrate how biomimetic materials can be obtained when topological constraints are used to control the non-equilibrium dynamics of active matter.

F. C. Keber, E. Loiseau, T. Sanchez, S. J. DeCamp, L. Giomi, M. J. Bowick, M. C. Marchetti, Z. Dogic and A. R. Bausch (2014)

Topology and Dynamics of Active Nematic Vesicles

Science, 345, 6201.

Morphological transformations of lipid membranes such as shape adaptation to external stimuli, blebbing, invagination or tethering re- sult from an intricate interplay of an active tension generating shear elastic cytoskeleton with a fluid lipid membrane. Here we present a minimal in vitro model system which demonstrates that tension generation of an encapsulated active acto-myosin network suffices for global shape transformation of cell-sized lipid vesicles, which are reminiscent to morphological adaptations in living cells. Our in vitro model system uses purified cytoskeletal elements inside cell-sized vesicles, in which coupling to the membrane, elasticity of the cytoskeletal network, and contractile activity can all be precisely controlled and tuned.  The identification of the physical mechanisms for shape transformations of active cytoskeletal vesicles sets a conceptual and quantitative benchmark for the further exploration of the adaptation mechanisms of cells. 

E. Loiseau, J. A. M. Schneider, F. C. Keber, C. Pelzl, G. Massiera, G. Salbreux, A. R. Bausch (2016)

Shape remodeling and blebbing of active cytoskeletal vesicles

Science Advances; 2 : e1500465

Cells set up contractile actin arrays to drive various shape changes and to exert forces to their environment. To understand their assembly process, we present here a reconstituted contractile system, comprising F-actin and myosin II filaments, where we can control the local activation of myosin by light. By stimulating different symmetries, we show that the force balancing at the boundaries determine the shape changes as well as the dynamics of the global contraction. Spatially anisotropic attachment of initially isotropic networks leads to a self-organization of highly aligned contractile fibres, being reminiscent of the order formation in muscles or stress fibres. The observed shape changes and dynamics are fully recovered by a minimal physical model.

M. Schuppler, F.C. Keber, M. Kröger, A.R. Bausch (2016)

Boundaries steer the contraction of active gels

Nature Communications 7:13120

Living matter has the extraordinary ability to behave in a concerted manner,

which is exemplified throughout nature ranging from the self-organisation of

the cytoskeleton to flocks of animals. The microscopic dynamics of constituents

have been linked to the system’s meso- or macroscopic behaviour in

silico via the Boltzmann equation for propelled particles. Thereby, simplified

binary collision rules between the constituents had to be assumed due

to the lack of experimental data. We report here experimentally determined

binary collision statistics by studying the recently introduced molecular system,

the high density actomyosin motility assay . We demonstrate that the

alignment effect of the binary collision statistics is too weak to account for the

observed ordering transition. The transition density for polar pattern formation

decreases quadratically with filament length, which indicates that multi-filament

collisions drive the observed ordering phenomenon and that a gas-like picture

cannot explain the transition of the system to polar order. The presented findings

demonstrate that the unique properties of biological active matter systems

require a description that goes well beyond a gas-like picture developed in the

framework of kinetic theories.

R. Suzuki, C. A. Weber, E. Frey and A. R. Bausch (2015)

Polar pattern formation in driven filament systems requires non-binary particle collisions

Nature Physics 11, 839–843

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