Self-organization of a propulsive actin network as an evolutionary process

Abstract

The leading edge of motile cells is propelled by polymerization of actin filaments according to a dendritic nucleation͞array treadmilling mechanism. However, little attention has been given to the origin and maintenance of the dendritic array. Here we develop and test a population-kinetics model that explains the organization of actin filaments in terms of the reproduction of dendritic units. The life cycle of an actin filament consists of dendritic nucleation on another filament (birth), elongation by addition of actin subunits and, finally, termination of filament growth by capping protein (death). The regularity of branch angle between daughter and mother filaments endows filaments with heredity of their orientation. Fluctuations of branch angle that become fixed in the actin network create errors of orientation (mutations) that may be inherited. In our model, birth and death rates depend on filament orientation, which then becomes a selectable trait. Differential reproduction and elimination of filaments, or natural selection, leads to the evolution of a filament pattern with a characteristic distribution of filament orientations. We develop a procedure based on the Radon transform for quantitatively analyzing actin networks in situ and show that the experimental results are in agreement with the distribution of filament orientations predicted by our model. We conclude that the propulsive actin network can be understood as a self-organizing supramolecular ensemble shaped by the evolution of dendritic lineages through natural selection of their orientation.A fundamental issue beyond genomics and proteomics is the origin of large-scale order in biomolecular ensembles. An important example of such an ensemble is the network of actin filaments responsible for the protrusive activity of motile cells. Actin filaments are intrinsically polar, with one end (the barbed end) favored for growth and the other end (the pointed end) favored for shortening (1). Actin polymerized in vitro forms populations of randomly organized filaments. In contrast, actin filaments at the leading edge of cells form branched arrays in a characteristic criss-cross pattern with filament barbed ends uniformly facing forward (2). How does this dendritic pattern arise? Why do the barbed ends face forward? Why are rearwardfacing branches not seen? These questions prompted an inquiry into the organizing principles of the network.The most general organizing principle in biology is natural selection. Higher-level biological systems will be shaped by natural selection whenever Darwin's conditions of variation and heritability of fitness are satisfied, regardless of the nature of the entities being selected (3). Not only speciation (4), but also development of immune and neural networks (5) and communication (6, 7), has been studied in an evolutionary framework. The question we consider here is whether self-organization by means of natural selection is a central mechanism for shaping the intracellular architecture of the motility machinery. Model and...

Polarization and Movement of Keratocytes: A Multiscale Modelling Approach

“…However, because we discretized angles into six possible orientations, our resolution limits our ability to study the precise details of the preferred orientation. (For example, we cannot explain the dominant angular preference around ±30 • ; but see Mogilner and Oster (1996a); Maly and Borisy (2001). )…”

“…The development of this correct orientation can be understood as follows: Only the barbed ends pointing in the right direction will stay inside the protected region (with low capping and high nucleation) when they extend. All others will be left outside, to be capped and eliminated; see Maly and Borisy (2001) for a similar conclusion. Moreover, since daughter side-branches are formed at angles of ±60 • , some of these still point in the right direction by "inheritance".…”

“…The filament orientations dictate the local direction of extension of the cell. In keratocytes, most filaments near the leading edge point at angles within ±70 • of the axis of protrusion (Maly and Borisy, 2001;, resulting in forward translocation. Importantly, we do not impose a pre-determined cell front or back, nor do we make any ad hoc assumptions about local polarity information within the cell to guide the assembly of the network: rather, our purpose is to show that the observed orientation of the network can emerge as a natural consequence of the mechanism of protrusion.…”

Single Molecule Kinetic Analysis of Actin Filament Capping

“…These experiments led to the widely accepted view that PPIs not only inhibit association of CP with barbed ends but also dissociate CP from actin filaments. These two effects are postulated to bias the growth of actin filament branches toward the inside of the plasma membrane at the leading edge (11,12). Steric interference would be the simplest way for PPIs to block interaction of CP with filament ends.…”

A quantitative measure for alterations in the actin cytoskeleton investigated with automated high‐throughput microscopy

“…For the lamellipodium, the rapidly polymerizing actin network used by many cell types for propelling the cell forward, such an approach has already been adopted by different groups (17,37,38). Here, we introduce a similar approach for the actin cytoskeleton of stationary adherent cells.…”