Cell-sized liposomes reveal how actomyosin cortical tension drives shape change

Abstract

Animal cells actively generate contractile stress in the actin cortex, a thin actin network beneath the cell membrane, to facilitate shape changes during processes like cytokinesis and motility. On the microscopic scale, this stress is generated by myosin molecular motors, which bind to actin cytoskeletal filaments and use chemical energy to exert pulling forces. To decipher the physical basis for the regulation of cell shape changes, here, we use a cell-like system with a cortex anchored to the outside or inside of a liposome membrane. This system enables us to dissect the interplay between motor pulling forces, cortex-membrane anchoring, and network connectivity. We show that cortices on the outside of liposomes either spontaneously rupture and relax built-up mechanical stress by peeling away around the liposome or actively compress and crush the liposome. The decision between peeling and crushing depends on the cortical tension determined by the amount of motors and also on the connectivity of the cortex and its attachment to the membrane. Membrane anchoring strongly affects the morphology of cortex contraction inside liposomes: cortices contract inward when weakly attached, whereas they contract toward the membrane when strongly attached. We propose a physical model based on a balance of active tension and mechanical resistance to rupture. Our findings show how membrane attachment and network connectivity are able to regulate actin cortex remodeling and membrane-shape changes for cell polarization.

Keywords: active gels; actomyosin contractility; biomimetism; soft condensed matter.

Fig. 1.

Liposomes with a biomimetic actin-myosin cortex with two distinct geometries: outside geometry (Upper) and inside geometry (Lower). (A and D) Scheme of a liposome with actin filaments attached to the outside or inside by actin–membrane linkers. (B and C) Phase contrast images (a, a1, and a2) and actin fluorescence images (b, b1, and b2) of liposomes containing 1% biotinylated lipids in the absence (B) or in the presence (C) of myosin. (E and F) Confocal image of fluorescent encapsulated actin in low attachement condition (E, a) and in High attachment condition (E, b) in the absence or in the presence (F) of myosin. (Scale bars: 5 µm unless otherwise indicated.)

Fig. 2.

Effect of cortex cross-linking and actin–membrane attachment on cortex contraction in the inside geometry. Confocal images of actin (green) and myosin (red) inside liposomes in the absence of cross-linkers (i and ii) or in the presence of cross-linkers and either strong attachment (iii) or weak attachment (iv). (Scale bars: 5 µm.) Plots are the distance of the myosin cluster from the center of the liposome (D_myo) normalized by the liposome radius, R_liposome, for different liposomes (n = 9 for strong attachment and n = 37 for weak attachment). Box plots show the mean value (small square), maximum and minimum values (crosses), and 5th and 95th percentiles (whiskers).

Fig. 3.

Outside geometry: kinetics of vesicle crushing and cortex peeling. Images taken by phase contrast (A and C, Left) and epifluorescence (A and C, Right). Time lapse images (A and B) of contractile liposomes and kymograph along the blue line in A and along the liposome contour (D). (Scale bars: A and C, 10 µm; B and D, 5 µm.) Conditions were as follows: 1% biotinylated lipids and 20 nM myosin. (E) Characteristic times: duration of contraction, t_CC − t_AM, for crushing (triangles) and peeling (circles) (Left); time where actin starts to move, t_AM (Right). Open symbols indicate weak attachment conditions; solid symbols indicate strong attachment conditions. All symbols are spaced for clarity.

Fig. 4.

Effect of membrane anchoring and actin cortex cross-linking on cortex contraction in the outside geometry. (A and B) Percentage of liposomes crushing (black bar), peeling (red bar), undetermined cases (green bar), or noncontracting. Data are from at least three independent experiments, with ∼100 liposomes in each case. (B and C) Weak attachment condition (W) and strong attachment condition (S). (A) Varying level of cortex connectivity (200 nM myosin in all cases in weak attachment condition). (B) Effect of myosin concentration and attachment condition on contraction outcome. (C) Maximum measured contraction time, of all contracting cases (crushing and peeling) as a function of myosin concentration.

Fig. 5.

Effect of geometry and actin–membrane anchoring on myosin-driven cortex contraction. In the outside geometry (Upper), myosin motors build up cortical tension, τ . If τ exceeds the critical tension, τ_C, for network rupture, the cortex will elastically retract, resulting in cortex peeling toward one side of the liposome. If the cortex is strongly anchored or strongly cross-linked, the tension remains subcritical and the motors will instead actively compress and crush the liposome. In the inside geometry (Lower), motors contract strongly anchored cortices toward the membrane, whereas they contract and detach weakly anchored cortices away from the membrane.