Rules in acto-myosin contraction

Eukaryotic cells control their shape via tight coordination of the dynamic self-assembly of cytoskeletal polymers (e.g. actin filaments) and modulation of intracellular tension exerted among these filaments by molecular motor proteins (e.g. myosins). Using geometrically controlled and polarized in vitro actin networks we establish a set of general rules that govern the spatial organization of actin filaments and biases introduced by myosin-dependent contraction and disassembly.

HFSP Program Grant holders Laurent Blanchoin and Enrique de la Cruz and colleagues
authored on Thu, 12 July 2012

The cytoplasm of eukaryotic cells is the site of constant reorganization of a system of fiber-like polymers – the cytoskeleton. In this system, the assembly of actin monomers into filaments generates dynamic subcellular structures that coordinate diverse biological events ranging from cell migration to cell division. Molecular motors of the myosin family are key effectors during these processes, but the precise characterization of this complex orchestration in space and time has remained experimentally elusive. Indeed, the identification of the biochemical mechanisms and physical laws governing the macroscopic organization of the cytoskeleton is rendered difficult by the complexity of the systems.

Figure: Image reconstruction of time series of myosin-induced contraction of actin networks nucleated from a dotted ring (red actin filament), (green myosin). Scale bar: 5 µm

To gain insight into the macroscopic organization of actin filaments in response to myosin-induced contraction, we designed new, minimal component experimental systems that reproduce myosin-induced contractility among defined actin architectures. Actin organization was modulated biochemically by reconstituted systems comprised of a subset of specific actin interacting proteins, and geometrically using micropatterned surfaces (Reymann et al., 2010). These actin templates were used to evaluate the response of oriented actin structures to myosin-induced contractility. We followed in real time, actin filament growth, myosin motor protein localization, myosin-induced contraction, filament network deformation and disassembly of the micro-patterned actin structures.

We demonstrate how actin organization can control myosin-dependent contraction versus disassembly of subcellular structures. We show that all actin architectures are not equally sensitive to myosin-induced actin network reorganization.  Rather, filaments are selected for contraction or disassembly according to their geometric orientations. We establish how the proportion of oriented actin filaments could regulate the scalability of the contraction process. We define this general principal by an “orientation selection” mechanism that reveals how the overall organization and dynamics of the actin cytoskeleton influences the consequences of myosin contractility. This “orientation selection” during myosin-induced contraction explains how a constantly changing complex system such as a cytoskeleton network assembly can reach global equilibrium.

Reference

Actin network architecture can determine myosin motor activity. Reymann AC, Boujemaa-Paterski R, Martiel JL, Guérin C, Cao W, Chin HF, De La Cruz EM, Théry M, Blanchoin L. Science. 2012 336 (6086):1310-4.

Pubmed link

Other references

Nucleation geometry governs ordered actin networks structures. Reymann AC, Martiel JL, Cambier T, Blanchoin L, Boujemaa-Paterski R, Théry M. Nat. Mater. 2010; 9(10): 827-32.

Pubmed link