Load Adaptation of Lamellipodial Actin Networks

Abstract

Actin filaments polymerizing against membranes power endocytosis, vesicular traffic, and cell motility. In vitro reconstitution studies suggest that the structure and the dynamics of actin networks respond to mechanical forces. We demonstrate that lamellipodial actin of migrating cells responds to mechanical load when membrane tension is modulated. In a steady state, migrating cell filaments assume the canonical dendritic geometry, defined by Arp2/3-generated 70° branch points. Increased tension triggers a dense network with a broadened range of angles, whereas decreased tension causes a shift to a sparse configuration dominated by filaments growing perpendicularly to the plasma membrane. We show that these responses emerge from the geometry of branched actin: when load per filament decreases, elongation speed increases and perpendicular filaments gradually outcompete others because they polymerize the shortest distance to the membrane, where they are protected from capping. This network-intrinsic geometrical adaptation mechanism tunes protrusive force in response to mechanical load.

Ultrastructure of the actin cytoskeleton

“…Previous computer modeling studies predicted the predominant angles of ±35° relative to the direction of protrusion for branched actin filaments in lamellipodia [6,7]. This pattern corresponds to canonical geometry of branched actin networks, in which the 70° branch angle produced by the Arp2/3 complex faces the leading edge [4**,6,8]. This “slingshot” modality, however, was predicted to change to the +70°/0°/−70° (“trident”-like) distribution (Figure 1a), if branched filaments grow under too high or too low load [9] or are allowed to become longer [7].…”

“…This “slingshot” modality, however, was predicted to change to the +70°/0°/−70° (“trident”-like) distribution (Figure 1a), if branched filaments grow under too high or too low load [9] or are allowed to become longer [7]. These predictions have been experimentally validated using electron tomography of negatively stained lamellipodia [4**]. Indeed, when plasma membrane tension was experimentally increased or decreased in migrating fish keratocytes, more filaments acquired orientations around 0° and ±70°, as compared with two major peaks of ±35° at the steady state (Figure 1b).…”

“…Structural changes in branched actin networks pushing against load were studied both in cells [4**] and in vitro [5**]. Previous computer modeling studies predicted the predominant angles of ±35° relative to the direction of protrusion for branched actin filaments in lamellipodia [6,7].…”

“…Indeed, when plasma membrane tension was experimentally increased or decreased in migrating fish keratocytes, more filaments acquired orientations around 0° and ±70°, as compared with two major peaks of ±35° at the steady state (Figure 1b). Since small-scale periodic compression and relaxation of the lamellipodial network also occur without experimental interventions [4**,10], periodic rearrangements between the slingshot and trident network geometry may constantly occur at the leading edge.…”

Cell Membranes Resist Flow

“…Membrane tension affects cell migration (Gauthier et al, 2011; Houk et al, 2012; Keren et al, 2008; Mueller et al, 2017), vesicle fusion and recycling (Boulant et al, 2011; Gauthier et al, 2011; Maritzen and Haucke, 2017; Masters et al, 2013; Shillcock and Lipowsky, 2005; Shin et al, 2018; Thottacherry et al, 2017; Wen et al, 2016), the cell cycle (Stewart et al, 2011), cell signaling (Basu et al, 2016; Groves and Kuriyan, 2010; Houk et al, 2012; Huse, 2017; Romer et al, 2007), and mechanosensation (He et al, 2018; Phillips et al, 2009; Ranade et al, 2015). However, there has been controversy over the speed and degree to which localized changes in membrane tension propagate in cells (Diz-Muñoz et al, 2013).…”