Amnioserosa cell constriction but not epidermal actin cable tension autonomously drives dorsal closure

Abstract

Tissue morphogenesis requires coordination of multiple force-producing components. During dorsal closure in fly embryogenesis, an epidermis opening closes. A tensioned epidermal actin/MyosinII cable, which surrounds the opening, produces a force that is thought to combine with another MyosinII force mediating apical constriction of the amnioserosa cells that fill the opening. A model proposing that each force could autonomously drive dorsal closure was recently challenged by a model in which the two forces combine in a ratchet mechanism. Acute force elimination via selective MyosinII depletion in one or the other tissue shows that the amnioserosa tissue autonomously drives dorsal closure while the actin/MyosinII cable cannot. These findings exclude both previous models, although a contribution of the ratchet mechanism at dorsal closure onset remains likely. This shifts the current view of dorsal closure being a combinatorial force-component system to a single tissue-driven closure event.

Myosin II is not required for Drosophila tracheal branch elongation and cell intercalation

“…S4B). These results are similar to the ones obtained by the striped expression of a dominant-negative version of Rho1, which is a positive upstream regulator of MyoII (Jacinto et al, 2002), and corroborate two more recent studies using either loss-of-function mutants or deGradFP to disrupt the leading edge actomyosin cable (Ducuing and Vincent, 2016;Pasakarnis et al, 2016). Finally, knockdown of Sqh-GFP in stripes resulted in embryonic lethality in male embryos, while female embryos, which carry a wild-type sqh copy, gave rise to viable progeny ( Fig.…”

“…S3B, arrowheads). This observation was similar to what was previously reported in the epidermis (Pasakarnis et al, 2016) and the wing imaginal discs (Caussinus et al, 2012). Live imaging of the F-actin reporter Lifeact-Ruby revealed that Sqh-GFP knockdown males formed thicker and longer filipodia at the leading edge than the control embryos (compare Fig.…”

“…Given the prominent role of MyoII during epithelial morphogenesis, its presence in the tracheal system throughout development and its clear role in tracheal placode invagination (Nishimura et al, 2007) and tracheal fusion (Kato et al, 2016), we decided to further investigate the function of MyoII during SCI in the tracheal system. To overcome prior limitations due to MyoII maternal contribution and pleiotropic roles in morphogenesis and cytokinesis, we used a nanobody-based approach that can interfere with MyoII activity in a time-and tissue-specific manner (Caussinus et al, 2012;Pasakarnis et al, 2016). Our results show that, in the absence of actomyosin contractibility, tip cell migration and stalk cell intercalation occur normally.…”

“…However, F-actin was not enriched at the bright Sqh-GFP 'inclusion bodies' (Fig. S3B, asterisk; see also Pasakarnis et al, 2016).…”

“…deGradFP allows for the efficient degradation of GFPfusion proteins and can be used to phenocopy loss-of-function mutations (Blattner et al, 2016;Caussinus et al, 2012;Lee et al, 2016;Nagarkar-Jaiswal et al, 2015;Pasakarnis et al, 2016). Here, we used a null mutant for sqh (Jordan and Karess, 1997) rescued by a Sqh-GFP transgene (sqhAX3; sqh-Sqh-GFP) (Royou et al, 2004).…”