The PAR complex regulates pulsed actomyosin contractions during amnioserosa apical constriction in Drosophila

Abstract

SUMMARYApical constriction is a major mechanism underlying tissue internalization during development. This cell constriction typically requires actomyosin contractility. Thus, understanding apical constriction requires characterization of the mechanics and regulation of actomyosin assemblies. We have analyzed the relationship between myosin and the polarity regulators Par-6, aPKC and Bazooka (Par-3) (the PAR complex) during amnioserosa apical constriction at Drosophila dorsal closure. The PAR complex and myosin accumulate at the apical surface domain of amnioserosa cells at dorsal closure, the PAR complex forming a patch of puncta and myosin forming an associated network. Genetic interactions indicate that the PAR complex supports myosin activity during dorsal closure, as well as during other steps of embryogenesis. We find that actomyosin contractility in amnioserosa cells is based on the repeated assembly and disassembly of apical actomyosin networks, with each assembly event driving constriction of the apical domain. As the networks assemble they translocate across the apical patch of PAR proteins, which persist at the apical domain. Through loss-and gain-of-function studies, we find that different PAR complex components regulate distinct phases of the actomyosin assembly/disassembly cycle: Bazooka promotes the duration of actomyosin pulses and Par-6/aPKC promotes the lull time between pulses. These results identify the mechanics of actomyosin contractility that drive amnioserosa apical constriction and how specific steps of the contractile mechanism are regulated by the PAR complex.

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

“…Recent work shows that the formation of transient actomyosin networks correlates with the apical constriction of AS cells (David et al, 2010). The results are compatible with the frequency data measured in our study.…”

Mechanochemical actuators of embryonic epithelial contractility

“…This timescale for this contraction response is comparable to the timescales reported previously for mechanical or biochemical signal transmission. Whereas a few seconds or milliseconds have been reported in some systems for the timescale of the mechanical and chemical signal transmission (14), this timescale was estimated at a singlecell level and not in a multicellular integrated tissue, which is what our work presents as shown in literature on the timescale for mechanical stimulus transmission of the cycle length of actomyosin fluctuation ranges from 1 to 5 min in the Drosophila (20,21), and Xenopus gastrula tissues (22), as well as other induced contractions in Xenopus (16). Additionally, we find the signal transmission distance--the distance between the edge of the ATP stream and the intersection of the negative and positive strains on the strain maps--of the contractile response always extends beyond the regions exposed directly to the ATP stream.…”

Phosphoinositides in Cell Architecture

“…Of particular note, PTEN recruits NM-II to the cortex in response to application of force in Dictyostelium (Pramanik et al 2009). Additionally, Baz and PAR-6/ aPKC regulate distinct phases of the myosin assembly disassembly cycle during amnioserosa apical constriction during Drosophila dorsal closure (David et al 2010). Although, it is possible that apical constriction alone may not be sufficient to induce sheet folding because integration of both circumapical and lateral contraction of the endoderm was required to drive early ascidian gastrulation (Sherrard et al 2010).…”

“…Although, it is possible that apical constriction alone may not be sufficient to induce sheet folding because integration of both circumapical and lateral contraction of the endoderm was required to drive early ascidian gastrulation (Sherrard et al 2010). Because NM-II can be recruited to the cortex by tension and by protein interactions the involvement of polarity proteins in apical constriction is particularly germane (Choi and Sokol 2009;Hava et al 2009;David et al 2010). Polarity protein involvement in apical constriction suggests that mechanical tension is integrated by modifying the properties of the plasma membrane and, by extension, the PI composition of the cell surface.…”