N-WASP generates a buzz at membranes on the move

Branching and capping determine the force-velocity relationships of branching actin networks

A branching actin network is the major engine that drives cell motility. A measure of the effectiveness of an engine is the velocity the engine is able to produce at a given resistance-the force-velocity relationship. Concave force-velocity relationships consist of a force-insensitive region, indicative of an adaptive response. In contrast, convex force-velocity relationships would reflect a passive response. Even in in vitro experiments, branching actin networks can exhibit both concave and convex force-velocity curves. However, the exact mechanism that can explain both force-velocity curves is not yet known. We carried out an agent-based stochastic simulation to explore such a mechanism. We discovered an emergent behavior of a branching actin network: Upon resistance, it remodels itself by increasing the number of filaments growing in contact with the load. The remodeling is favored by branching events and limited by capping. The force-velocity relationship hinges on the relative time-scale between the intrinsic kinetics of the branching actin network and the loading. Shortly after encountering resistance (∼seconds), the force-velocity relationship of the actin network is always convex, as it does not have enough time to remodel itself. A concave force-velocity relationship requires network remodeling at longer time-scales (∼tens of seconds to minutes) and the faster branching event relative to capping. Furthermore, our model explains the observed hysteresis in the force-velocity relationship of actin networks. Our model thus establishes a unified mechanism that can account for both convex and concave force-velocity relationships observed in branching actin networks.

Actin filament elasticity and retrograde flow shape the force-velocity relation of motile cells

Cells migrate through a crowded environment during processes such as metastasis or wound healing, and must generate and withstand substantial forces. The cellular motility responses to environmental forces are represented by their force-velocity relation, which has been measured for fish keratocytes but remains unexplained. Even pN opposing forces slow down lamellipodium motion by three orders of magnitude. At larger opposing forces, the retrograde flow of the actin network accelerates until it compensates for polymerization, and cell motion stalls. Subsequently, the lamellipodium adapts to the stalled state. We present a mechanism quantitatively explaining the cell's force-velocity relation and its changes upon application of drugs that hinder actin polymerization or actomyosin-based contractility. Elastic properties of filaments, close to the lamellipodium leading edge, and retrograde flow shape the force-velocity relation. To our knowledge, our results shed new light on how these migratory responses are regulated, and on the mechanics and structure of the lamellipodium.

Modeling morphodynamic phenotypes and dynamic regimes of cell motion

Many cellular processes and signaling pathways converge onto cell morphology and cell motion, which share important components. The mechanisms used for propulsion could also be responsible for shape changes, if they are capable of generating the rich observed variety of dynamic regimes. Additionally, the analysis of cell shape changes in space and time promises insight into the state of the cytoskeleton and signaling pathways controlling it. While this has been obvious for some time by now, little effort has been made to systematically and quantitatively explore this source of information. First pioneering experimental work revealed morphodynamic phenotypes which can be associated with dynamic regimes like oscillations and excitability. Here, we review the current state of modeling of morphodynamic phenotypes, the experimental results and discuss the ideas on the mechanisms driving shape changes which are suggested by modeling.

Modeling of protrusion phenotypes driven by the actin-membrane interaction

We propose a mathematical model for simulating the leading-edge dynamics of a migrating cell from the interplay among elastic properties, architecture of the actin cytoskeleton, and the mechanics of the membrane. Our approach is based on the description of the length and attachment dynamics of actin filaments in the lamellipodium network. It is used to determine the total force exerted on the membrane at each position along the leading edge and at each time step. The model reproduces the marked state switches in protrusion morphodynamics found experimentally between epithelial cells in control conditions and cells expressing constitutively active Rac, a signaling molecule involved in the regulation of lamellipodium network assembly. The model also suggests a mechanistic explanation of experimental distortions in protrusion morphodynamics induced by deregulation of Arp2/3 and cofilin activity.

Activation of EGFR on monocytes is required for human cytomegalovirus entry and mediates cellular motility

Human cytomegalovirus (HCMV) rapidly induces a mobile and functionally unique proinflammatory monocyte following infection that is proposed to mediate viral spread. The cellular pathways used by HCMV to initiate these biological changes remain unknown. Here, we document the expression of the epidermal growth factor receptor (EGFR) on the surface of human peripheral blood monocytes but not on other blood leukocyte populations. Inhibition of EGFR signaling abrogated viral entry into monocytes, indicating that EGFR can serve as a cellular tropism receptor. Moreover, HCMV-activated EGFR was required for the induction of monocyte motility and transendothelial migration, two biological events required for monocyte extravasation into peripheral tissue, and thus viral spread. Transcriptome analysis revealed that HCMV-mediated EGFR signaling up-regulated neural Wiskott-Aldrich syndrome protein (N-WASP), an actin nucleator whose expression and function are normally limited in leukocytes. Knockdown of N-WASP expression blocked HCMV-induced but not phorbol 12-myristate 13-acetate (PMA)-induced monocyte motility, suggesting that a switch to and/or the distinct use of a new actin nucleator controlling motility occurs during HCMV infection of monocytes. Together, these data provide evidence that EGFR plays an essential role in the immunopathobiology of HCMV by mediating viral entry into monocytes and stimulating the aberrant biological activity that promotes hematogenous dissemination.

Dynamic regimes and bifurcations in a model of actin-based motility

Propulsion by actin polymerization is widely used in cell motility. Here, we investigate a model of the brush range of an actin gel close to a propelled object, describing the force generation and the dynamics of the propagation velocity. We find transitions between stable steady states and relaxation oscillations when the attachment rate of actin filaments to the obstacle is varied. The oscillations set in at small values of the attachment rate via a homoclinic bifurcation. A second transition from a stable steady state to relaxation oscillations, found for higher values of the attachment rate, occurs via a supercritical Hopf bifurcation. The behavior of the model near the second transition is similar that of a system undergoing a canard explosion. Consequently, we observe excitable dynamics also. The model further exhibits bistability between stationary states or stationary states and limit cycles. Therefore, the brush of actin filament ends appears to have a much richer dynamics than was assumed until now.