Apical domain polarization localizes actin–myosin activity to drive ratchet-like apical constriction
TL;DR: It is found that during constriction, Rho-associated kinase (Rok) is polarized to the middle of the apical domain (medioapical cortex), separate from adherens junctions, and Twist establishes radial cell polarity of Rok/Myo-II and E-cadherin and promotes medioAPical actin assembly in mesoderm cells to stabilize cell shape fluctuations.
Abstract: Martin and colleagues analyse the mechanism underlying Twist- and Rho1-driven apical constriction of ventral furrow cells in Drosophila. They characterize the spatial localization of the Rho1 effectors Rok1 and Dia, and their effects on the actomyosin network and adherens junctions.
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TL;DR: RHO GTPases and their effectors regulate actin polymerization and actomyosin contraction at AJs during the epithelial reshaping processes, including by planar-polarized apical constriction that occurs during neural plate bending and germband extension.
Abstract: Epithelial cells display dynamic behaviours, such as rearrangement, movement and shape changes, particularly during embryonic development and in equivalent processes in adults. Accumulating evidence suggests that the remodelling of cell junctions, especially adherens junctions (AJs), has major roles in controlling these behaviours. AJs comprise cadherin adhesion receptors and cytoplasmic proteins that associate with them, including catenins and actin filaments, and exhibit various forms, such as linear or punctate. Remodelling of AJs induces epithelial reshaping in various ways, including by planar-polarized apical constriction that is driven by the contraction of AJ-associated actomyosin and that occurs during neural plate bending and germband extension. RHO GTPases and their effectors regulate actin polymerization and actomyosin contraction at AJs during the epithelial reshaping processes.
486 citations
TL;DR: The concept that cellular contractility and E-cadherin-based adhesion are functionally integrated by biomechanical feedback pathways that operate on molecular, cellular and tissue scales is discussed.
Abstract: During epithelial morphogenesis, E-cadherin adhesive junctions play an important part in mechanically coupling the contractile cortices of cells together, thereby distributing the stresses that drive cell rearrangements at both local and tissue levels. Here we discuss the concept that cellular contractility and E-cadherin-based adhesion are functionally integrated by biomechanical feedback pathways that operate on molecular, cellular and tissue scales.
437 citations
TL;DR: Understanding both the common themes and the variations in apical constriction mechanisms promises to provide insight into the mechanics that underlie tissue morphogenesis.
Abstract: Apical constriction is a cell shape change that promotes tissue remodeling in a variety of homeostatic and developmental contexts, including gastrulation in many organisms and neural tube formation in vertebrates In recent years, progress has been made towards understanding how the distinct cell biological processes that together drive apical constriction are coordinated These processes include the contraction of actin-myosin networks, which generates force, and the attachment of actin networks to cell-cell junctions, which allows forces to be transmitted between cells Different cell types regulate contractility and adhesion in unique ways, resulting in apical constriction with varying dynamics and subcellular organizations, as well as a variety of resulting tissue shape changes Understanding both the common themes and the variations in apical constriction mechanisms promises to provide insight into the mechanics that underlie tissue morphogenesis
408 citations
Cites background from "Apical domain polarization localize..."
...4D) (Mason et al., 2013)....
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...First, actin and myosin become locally recruited and/or condensed in the apical cortex simultaneously (Blanchard et al., 2010; He et al., 2010; Mason et al., 2013)....
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...First, dia mutants cause E-cadherin (Shotgun in Drosophila) to become localized across the entire apical domain rather than being restricted to the apical circumference (Mason et al., 2013)....
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...Consistent with this model, reducing Twist levels causes medioapical myosin and F-actin levels to decrease between pulses, rather than persistently accumulate (Martin et al., 2010; Mason et al., 2013)....
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...Third, disruption of actin-myosin contractility prevents associated actin reorganization and cell constriction (Martin et al., 2009; He et al., 2010; Kim and Davidson, 2011; Mason et al., 2013)....
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TL;DR: This work identifies two critical properties of MyoII dynamics that underlie stability and pulsatility: exchange kinetics governed by phosphorylation–dephosphorylation cycles of the Myo II regulatory light chain; and advection due to contraction of the motors on F-actin networks.
Abstract: Feedbacks between the dissociation and advection of myosin II result in self-organized behaviour of actomyosin networks that drives shape changes during tissue morphogenesis. Cell–cell intercalation during tissue elongation is orchestrated by actomyosin networks in which pulses of assembly and disassembly induce shape deformations that can be stabilized by a localized pool of actomyosin. What controls these mechanical ratchets has been unclear. Here Thomas Lecuit and colleagues present a potentially general model in which feedbacks between dissociation and advection of myosin II components set the frequency and amplitude of these pulses. Thus, the self-organizing properties of the actomyosin network are sufficient to define the cell shape changes that govern cell intercalation during morphogenesis. Tissue morphogenesis is orchestrated by cell shape changes. Forces required to power these changes are generated by non-muscle myosin II (MyoII) motor proteins pulling filamentous actin (F-actin). Actomyosin networks undergo cycles of assembly and disassembly (pulses)1,2 to cause cell deformations alternating with steps of stabilization to result in irreversible shape changes3,4,5,6. Although this ratchet-like behaviour operates in a variety of contexts, the underlying mechanisms remain unclear. Here we investigate the role of MyoII regulation through the conserved Rho1–Rok pathway7 during Drosophila melanogaster germband extension. This morphogenetic process is powered by cell intercalation, which involves the shrinkage of junctions in the dorsal–ventral axis (vertical junctions) followed by junction extension in the anterior–posterior axis8. While polarized flows of medial–apical MyoII pulses deform vertical junctions, MyoII enrichment on these junctions (planar polarity) stabilizes them6. We identify two critical properties of MyoII dynamics that underlie stability and pulsatility: exchange kinetics governed by phosphorylation–dephosphorylation cycles of the MyoII regulatory light chain; and advection due to contraction of the motors on F-actin networks. Spatial control over MyoII exchange kinetics establishes two stable regimes of high and low dissociation rates, resulting in MyoII planar polarity. Pulsatility emerges at intermediate dissociation rates, enabling convergent advection of MyoII and its upstream regulators Rho1 GTP, Rok and MyoII phosphatase. Notably, pulsatility is not an outcome of an upstream Rho1 pacemaker. Rather, it is a self-organized system that involves positive and negative biomechanical feedback between MyoII advection and dissociation rates.
342 citations
TL;DR: The 'full-circle' understanding of morphogenesis that is emerging, besides solving a key puzzle in biology, provides a mechanistic framework for future approaches to tissue engineering.
Abstract: A long-term aim of the life sciences is to understand how organismal shape is encoded by the genome. An important challenge is to identify mechanistic links between the genes that control cell-fate decisions and the cellular machines that generate shape, therefore closing the gap between genotype and phenotype. The logic and mechanisms that integrate these different levels of shape control are beginning to be described, and recently discovered mechanisms of cross-talk and feedback are beginning to explain the remarkable robustness of organ assembly. The 'full-circle' understanding of morphogenesis that is emerging, besides solving a key puzzle in biology, provides a mechanistic framework for future approaches to tissue engineering.
247 citations
References
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TL;DR: Pyridine derivative Y-27632 consistently suppresses Rho-induced, p160ROCK-mediated formation of stress fibres in cultured cells and dramatically corrects hypertension in several hypertensive rat models, suggesting that compounds that inhibit this process might be useful therapeutically.
Abstract: Abnormal smooth-muscle contractility may be a major cause of disease states such as hypertension, and a smooth-muscle relaxant that modulates this process would be useful therapeutically. Smooth-muscle contraction is regulated by the cytosolic Ca2+ concentration and by the Ca2+ sensitivity of myofilaments: the former activates myosin light-chain kinase and the latter is achieved partly by inhibition of myosin phosphatase. The small GTPase Rho and its target, Rho-associated kinase, participate in this latter mechanism in vitro, but their participation has not been demonstrated in intact muscles. Here we show that a pyridine derivative, Y-27632, selectively inhibits smooth-muscle contraction by inhibiting Ca2+ sensitization. We identified the Y-27632 target as a Rho-associated protein kinase, p160ROCK. Y-27632 consistently suppresses Rho-induced, p160ROCK-mediated formation of stress fibres in cultured cells and dramatically corrects hypertension in several hypertensive rat models. Our findings indicate that p160ROCK-mediated Ca2+ sensitization is involved in the pathophysiology of hypertension and suggest that compounds that inhibit this process might be useful therapeutically.
2,900 citations
TL;DR: A review briefly summarizes older studies and concentrates on recent studies on the mechanisms of action of cytochalasin and phalloidin.
Abstract: C YTOCHALASINS and phalloidins are two groups of small, naturally occurring organic molecules that bind to actin and alter its polymerization. They have been widely used to study the role of actin in biological processes and as models for actin-binding proteins. Functionally, cytochalasins resemble capping proteins, which block an end of actin filaments, nucleate polymerization, and shorten filaments. No known actin-binding protein stabilizes actin filaments as phalloidin does, but such proteins may have been missed. Cytochalasin and phalloidin have also helped to elucidate fundamental aspects of actin polymerization. This review briefly summarizes older studies and concentrates on recent v~rk on the mechanisms of action of cytochalasin and phalloidin.
1,978 citations
01 Sep 1984
TL;DR: From the distribution of the allele frequencies and the rate of discovery of new loci, it was estimated that the 61 loci represent the majority of embryonic lethal loci on the second chromosome yielding phenotypes recognizable in the larval cuticle.
Abstract: In a search for embryonic lethal mutants on the second chromosome ofDrosophila melanogaster, 5764 balanced lines isogenic for an ethyl methane sulfonate (EMS)-treatedcn bw sp chromosome were established. Of these lines, 4217 carried one or more newly induced lethal mutations corresponding to a total of 7600 lethal hits. Eggs were collected from lethal-bearing lines and unhatched embryos from the lines in which 25% or more of the embryos did not hatch (2843 lines) were dechorionated, fixed, cleared and scored under the compound microscope for abnormalities of the larval cuticle. A total of 272 mutants were isolated with phenotypes unequivocally distinguishable from wild-type embryos on the basis of the cuticular pattern. In complementation tests performed between mutants with similar phenotype, 48 loci were identified by more than one allele, the average being 5.4 alleles per locus. Complementation of all other mutants was shown by 13 mutants. Members of the complementation groups were mapped by recombination analysis. No clustering of loci with similar phenotypes was apparent. From the distribution of the allele frequencies and the rate of discovery of new loci, it was estimated that the 61 loci represent the majority of embryonic lethal loci on the second chromosome yielding phenotypes recognizable in the larval cuticle.
1,432 citations
TL;DR: A new model for apical constriction is suggested in which a cortical actin–myosin cytoskeleton functions as a developmentally controlled subcellular ratchet to reduce apical area incrementally.
Abstract: Apical constriction facilitates epithelial sheet bending and invagination during morphogenesis. Apical constriction is conventionally thought to be driven by the continuous purse-string-like contraction of a circumferential actin and non-muscle myosin-II (myosin) belt underlying adherens junctions. However, it is unclear whether other force-generating mechanisms can drive this process. Here we show, with the use of real-time imaging and quantitative image analysis of Drosophila gastrulation, that the apical constriction of ventral furrow cells is pulsed. Repeated constrictions, which are asynchronous between neighbouring cells, are interrupted by pauses in which the constricted state of the cell apex is maintained. In contrast to the purse-string model, constriction pulses are powered by actin-myosin network contractions that occur at the medial apical cortex and pull discrete adherens junction sites inwards. The transcription factors Twist and Snail differentially regulate pulsed constriction. Expression of snail initiates actin-myosin network contractions, whereas expression of twist stabilizes the constricted state of the cell apex. Our results suggest a new model for apical constriction in which a cortical actin-myosin cytoskeleton functions as a developmentally controlled subcellular ratchet to reduce apical area incrementally.
1,061 citations
TL;DR: This work addresses the mechanism by which an ordered process of cell intercalation directs polarized epithelial morphogenesis during germ-band elongation, the developmental elongation of the Drosophila embryo, and provides a general model for polarized morphogenesis in epithelial organs.
Abstract: Shaping a developing organ or embryo relies on the spatial regulation of cell division and shape. However, morphogenesis also occurs through changes in cell-neighbourhood relationships produced by intercalation. Intercalation poses a special problem in epithelia because of the adherens junctions, which maintain the integrity of the tissue. Here we address the mechanism by which an ordered process of cell intercalation directs polarized epithelial morphogenesis during germ-band elongation, the developmental elongation of the Drosophila embryo. Intercalation progresses because junctions are spatially reorganized in the plane of the epithelium following an ordered pattern of disassembly and reassembly. The planar remodelling of junctions is not driven by external forces at the tissue boundaries but depends on local forces at cell boundaries. Myosin II is specifically enriched in disassembling junctions, and its planar polarized localization and activity are required for planar junction remodelling and cell intercalation. This simple cellular mechanism provides a general model for polarized morphogenesis in epithelial organs.
928 citations