Cytoskeletal dynamics and supracellular organisation of cell shape fluctuations during dorsal closure

Abstract

SUMMARYFluctuations in the shape of amnioserosa (AS) cells during Drosophila dorsal closure (DC) provide an ideal system with which to understand contractile epithelia, both in terms of the cellular mechanisms and how tissue behaviour emerges from the activity of individual cells. Using quantitative image analysis we show that apical shape fluctuations are driven by the medial cytoskeleton, with periodic foci of contractile myosin and actin travelling across cell apices. Shape changes were mostly anisotropic and neighbouring cells were often, but transiently, organised into strings with parallel deformations. During the early stages of DC, shape fluctuations with long cycle lengths produced no net tissue contraction. Cycle lengths shortened with the onset of net tissue contraction, followed by a damping of fluctuation amplitude. Eventually, fluctuations became undetectable as AS cells contracted rapidly. These transitions were accompanied by an increase in apical myosin, both at cell-cell junctions and medially, the latter ultimately forming a coherent, but still dynamic, sheet across cells. Mutants with increased myosin activity or actin polymerisation exhibited precocious cell contraction through changes in the subcellular localisation of myosin. thickveins mutant embryos, which exhibited defects in the actin cable at the leading edge, showed similar timings of fluctuation damping to the wild type, suggesting that damping is an autonomous property of the AS. Our results suggest that cell shape fluctuations are a property of cells with low and increasing levels of apical myosin, and that medial and junctional myosin populations combine to contract AS cell apices and drive DC.

Active multistage coarsening of actin networks driven by myosin motors

“…In contrast, actomyosin aggregates appear in cells from diverse tissues and organisms as transient structures that coalesce into larger arrays that exert contractile forces. Examples include the formation of contractile rings driving cytokinesis (3,4) and wound healing (23), and the formation of contractile networks driving deformation of epithelial cell layers in developing embryos (24)(25)(26) and polarizing cortical flows (5,6). Our findings suggest that the formation and subsequent coalescence of myosin foci in vivo can emerge spontaneously from physical interactions between actin filaments and motors.…”

“…Wounded cells display similar phenomena, with accumulation of myosin foci at the wound border and fusion of these foci into a tight ring capable of constriction (23). Collective shape changes in epithelial cell layers also seem to be driven by the transient formation of myosin spots that subsequently fuse (24)(25)(26)(27).…”

Apical constriction: themes and variations on a cellular mechanism driving morphogenesis

“…Some cell types can dynamically change the apparent strength of coupling between actin-myosin networks and AJs, suggesting the presence of a molecular 'clutch' at AJs that modulates the ability of the actin-myosin engine to elicit cell shape change (Roh-Johnson et al, 2012). Furthermore, some cell types undergo apical cell shape fluctuations, and the speed of apical constriction can vary dramatically, depending on the extent to which the apical domain relaxes after decreasing in area, suggesting a regulated cellular component that serves as a 'ratchet' to tune the dynamics of apical constriction (Martin et al, 2009;Solon et al, 2009;Blanchard et al, 2010). Thus, although the cellular machinery required for apical constriction appears to be the same for various cell types, the organization and dynamics of this cellular machinery can vary.…”

Apical constriction is driven by a pulsatile apical myosin network in delaminating Drosophila neuroblasts

“…This nonjunctional myosin movement on the cell apical surface was termed medial myosin pulses, and each pulse coincides with a rapid apical area constriction phase (Martin et al, 2009;Mason et al, 2013). The amnioserosa cells also assemble a dynamic medial actomyosin network that drives apical cell shape fluctuations during the Drosophila dorsal closure process (Blanchard et al, 2010;David et al, 2010). Moreover, the salivary gland placode cells also undergo apical constriction driven by a medial actomyosin network in Drosophila embryos (Booth et al, 2014;Röper, 2012).…”