Actin disassembly clock determines shape and speed of lamellipodial fragments

Abstract

A central challenge in motility research is to quantitatively understand how numerous molecular building blocks self-organize to achieve coherent shape and movement on cellular scales. A classic example of such self-organization is lamellipodial motility in which forward translocation is driven by a treadmilling actin network. Actin polymerization has been shown to be mechanically restrained by membrane tension in the lamellipodium. However, it remains unclear how membrane tension is determined, what is responsible for retraction and shaping of the rear boundary, and overall how actin-driven protrusion at the front is coordinated with retraction at the rear. To answer these questions, we utilize lamellipodial fragments from fish epithelial keratocytes which lack a cell body but retain the ability to crawl. The absence of the voluminous cell body in fragments simplifies the relation between lamellipodial geometry and cytoskeletal dynamics. We find that shape and speed are highly correlated over time within individual fragments, whereby faster crawling is accompanied by larger front-to-rear lamellipodial length. Furthermore, we find that the actin network density decays exponentially from front-to-rear indicating a constant net disassembly rate. These findings lead us to a simple hypothesis of a disassembly clock mechanism in which rear position is determined by where the actin network has disassembled enough for membrane tension to crush it and haul it forward. This model allows us to directly relate membrane tension with actin assembly and disassembly dynamics and elucidate the role of the cell membrane as a global mechanical regulator which coordinates protrusion and retraction.cell motility | keratocyte fragments U nderstanding the large-scale coordination of molecular processes into coherent behavior at the cellular level is one of the central challenges in cell biology. Actin-based motility involves numerous molecular players with complex interactions (1) that span a wide-range of scales from the molecular level to the cellular one. Despite substantial progress in characterizing the molecular details involved (2), we still do not understand the remarkable self-organization of these molecular components into a moving cell. The complex interplay between biochemical reactions and biophysical forces plays a central role in this selforganization. In particular, the mechanical feedback between the cell membrane and the dynamic actin network has been shown to have a substantial effect on cell protrusion (3, 4), and a significant role in coordinating protrusion over cellular scales (5, 6). However, quantitative understanding of the coupling between the cell membrane and the motility machinery is still lacking; it is unknown what determines membrane tension and how membrane tension is related to the protrusion and retraction dynamics. Elucidating this dynamic interplay is essential for understanding how overall cell morphology and movement emerge from the underlying molecular processes.Fish epithelial keratocytes are...

Crawling cell locomotion revisited

“…According to the common physical rules, the force required for the adhesion decomposition or detachment would increase with growing velocity of the cell movement, which would mean a larger membrane tension in faster-moving cells. The same tension/velocity relationship resulting in the observed increase in the cell front-to-rear distance is one of the implications of the Ofer et al (2) hypothesis. Hence, the mechanism of the cell rear retraction based on the adhesion decomposition-detachment seems to result in a phenomenological behavior of the moving cell similar to that observed…”

“…Specifically, for the fish keratocytes, the major adhesions have been visualized in the lobes of the cell, which, apparently, have to be dealt with to enable the keratocyte movement (11,12). It is plausible that the detachment or disintegrationrecycling of the cell adhesions requires application of a force, similarly to the mechanism of the actin network decomposition assumed by Ofer et al (2). Moreover, this force can come from the membrane tension directly or through actin filaments.…”

“…The convincing mathematical modeling and experimentation by Ofer et al (2) leave no doubts that the suggested mechanism for the actin network decomposition at the cell rear plays an essential role in the retraction phase of the cell motion. However, taking into account the variability of mechanisms in cells of different kinds and the relative simplicity of keratocyte fragments as a model for a whole cell, the important remaining questions are whether the process of crashing the remnants of the actin network is always the only or the major one enabling the advancing of the cell rear.…”

“…The latter faces the remnants of the actin network and, because of the tension, pushes on them. According to the hypothesis by Ofer et al (2), the pushing membrane crashes the residual actin filaments. Fig.…”

“…Does the work by Ofer et al (2) provide us with an understanding sufficient for the design of an artificial system that could mimic cell crawling? Obviously, actin polymerization and depolymerization are needed, whereas, according to this work, the myosin contractile activity is redundant (2) and can be skipped.…”