Sequential Activation of Apical and Basolateral Contractility Drives Ascidian Endoderm Invagination

Abstract

SUMMARY Background Epithelial invagination is a fundamental morphogenetic behavior that transforms a flat cell sheet into a pit or groove. Previous studies of invagination have focused on the role of actomyosin-dependent apical contraction; other mechanisms remain largely unexplored. Results We combined experimental and computational approaches to identify a two-step mechanism for endoderm invagination during ascidian gastrulation. During Step 1, which immediately precedes invagination, endoderm cells constrict their apices due to Rho/Rhokinase-dependent apical enrichment of 1P–myosin. Our data suggest that endoderm invagination itself occurs during Step 2, without further apical shrinkage, via a novel mechanism we call collared rounding: Rho/Rho-kinase-independent lateral enrichment of 1P–myosin drives apico-basal shortening, while Rho/Rho-kinase-dependent enrichment of 1P and 2P myosin in circumapical collars is required to prevent apical expansion and for deep invagination. Simulations show that boundary-specific tension values consistent with these distributions of active myosin can explain the cell shape changes observed during invagination both in normal embryos and in embryos treated with pharmacological inhibitors of either Rho-kinase or Myosin II ATPase. Indeed, we find that the balance of strong circumapical and basolateral tension is the only mechanism based on differential cortical tension that can explain ascidian endoderm invagination. Finally, simulations suggest that mesectoderm cells resist endoderm shape changes during both steps and we confirm this prediction experimentally. Conclusions Our findings suggest that early ascidian gastrulation is driven by the coordinated apposition of circumapical and lateral endoderm contraction, working against a resisting mesectoderm. We propose that similar mechanisms may operate during other invaginations.

Phosphoinositides in Cell Architecture

“…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). 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).…”

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

“…Apical constriction of populations of cells that maintain cell-cell adhesion can bend and fold epithelial tissues, in some cases transforming flat epithelial sheets into threedimensional structures, such as tubes (Lewis, 1947;Hardin and Keller, 1988;Alvarez and Navascués, 1990;Kam et al, 1991;Sweeton et al, 1991;Wallingford et al, 2013). In these cases, changes in the geometry and mechanics of constituent cells appear to impact global tissue shape directly (Odell et al, 1981;Escudero et al, 2007;Sherrard et al, 2010;Eiraku et al, 2011). Apical constriction of individual cells can contribute to cell ingression from epithelial tissues, sometimes as a step in an epithelialmesenchymal transition (EMT) (Anstrom, 1992;Nance and Priess, 2002;Harrell and Goldstein, 2011;Williams et al, 2012).…”

Whole population cell analysis of a landmark-rich mammalian epithelium reveals multiple elongation mechanisms

“…Bao et al, 2006) and ascidian (e.g. Sherrard et al, 2010). Most of these studies have tracked cells over time, relating cell movements to morphogenesis, but very few have associated changes in cell shape with tissue morphogenesis.…”