In textbook illustrations of migrating cells, actomyosin contractility is typically depicted as the contraction force necessary for cell body retraction. This dogma has been transformed by the molecular clutch model, which acknowledges that actomyosin traction forces also generate and transmit biomechanical signals at the leading edge, enabling cells to sense and shape their migratory path in mechanically complex environments. To fulfill these complementary functions, the actomyosin system assembles a gradient of contractile energy along the front‐rear axis of (...) migratory cells. Here, we highlight the hierarchic assembly and self‐regulatory network structure of the actomyosin system and explain how the kinetics of different nonmuscle myosin II (NM II) paralogs synergize during contractile force generation. Our aim is to emphasize how protrusion formation, cell adhesion, contraction, and retraction are spatiotemporally integrated during different modes of migration, including chemotaxis and durotaxis. Finally, we hypothesize how different NM II paralogs might tune aspects of migration in vivo, highlighting future research directions. (shrink)

The remodeling of biological membranes is crucial for a vast number of cellular activities and is an inherently multiscale process in both time and space. Seminal work has provided important insights into nanometer‐scale membrane deformations, and highlighted the remarkable variation and complexity in the underlying molecular machineries and mechanisms. However, how membranes are remodeled at the micron‐scale, particularly in vivo, remains poorly understood. Here, we discuss how using regulated exocytosis of large (1.5–2.0 μm) membrane‐bound secretory granules in the salivary gland (...) of live mice as a model system, has provided evidence for the importance of the actomyosin cytoskeleton in micron‐scale membrane remodeling in physiological conditions. We highlight some of these advances, and present mechanistic hypotheses for how the various biochemical and biophysical properties of distinct actomyosin networks may drive this process. (shrink)

Actomyosin (actin‐myosin II complex)‐mediated contractile forces are central to the generation of multifaceted uni‐ and multi‐cellular material properties and dynamics such as cell division, migration, and tissue morphogenesis. In the present article, we summarize our recent researches addressing molecular mechanisms that ensure actomyosin‐mediated directional cell–cell junction remodeling, either shortening or extension, driving cell rearrangement for epithelial morphogenesis. Genetic perturbation clarified two points concerning cell–cell junction remodeling: an inhibitory mechanism against negative feedback in which actomyosin contractile forces, which (...) are well known to induce cell–cell junction shortening, can concomitantly alter actin dynamics, oppositely leading to perturbation of the shortening; and tricellular junctions as a point that organizes extension of new cell–cell junctions after shortening. These findings highlight the notion that cells develop underpinning mechanisms to transform the multi‐tasking property of actomyosin contractile forces into specific and proper cellular dynamics in space and time. (shrink)

Release of the ATP hydrolysis product ortophosphate (Pi) from the active site of myosin is central in chemo‐mechanical energy transduction and closely associated with the main force‐generating structural change, the power‐stroke. Despite intense investigations, the relative timing between Pi‐release and the power‐stroke remains poorly understood. This hampers in depth understanding of force production by myosin in health and disease and our understanding of myosin‐active drugs. Since the 1990s and up to today, models that incorporate the Pi‐release either distinctly before or (...) after the power‐stroke, in unbranched kinetic schemes, have dominated the literature. However, in recent years, alternative models have emerged to explain apparently contradictory findings. Here, we first compare and critically analyze three influential alternative models proposed previously. These are either characterized by a branched kinetic scheme or by partial uncoupling of Pi‐release and the power‐stroke. Finally, we suggest critical tests of the models aiming for a unified picture. (shrink)

Rho GTPases are critically important and are centrally positioned regulators of the actomyosin cytoskeleton. By influencing the organization and architecture of the cytoskeleton, Rho proteins play prominent roles in many cellular processes including adhesion, migration, intra‐cellular transportation, and proliferation. The most important method of Rho GTPase regulation is via the GTPase cycle; however, post‐translational modifications (PTMs) also play critical roles in Rho protein regulation. Relative to other PTMs such as lipidation or phosphorylation that have been extensively characterized, protein oxidation (...) is a regulatory PTM that has been poorly studied. Protein oxidation primarily occurs from the reaction of reactive oxygen species (ROS), such as hydrogen peroxide (H2O2), with amino acid side chain thiols on cysteine (Cys) and methionine (Met) residues. The versatile redox modifications of cysteine residues exemplify their integral role in cell signalling processes. Here we review prominent members of the Rho GTPase family and discuss how lipidation, phosphorylation, and oxidation on conserved cysteine residues affects their regulation and function. (shrink)

Axons are the longest cellular structure reaching over a meter in the case of human motor axons. They have a relatively small diameter and contain several cytoskeletal elements that mediate both material and information exchange within neurons. Recently, a novel type of axonal plasticity, termed axonal radial contractility, has been unveiled. It is represented by dynamic and transient diameter changes of the axon shaft to accommodate the passages of large organelles. Mechanisms underpinning this plasticity are not fully understood. Here, we (...) first summarised recent evidence of the functional relevance for axon radial contractility, then discussed the underlying structural basis, reviewing nanoscopic evidence of the subtle changes. Two models are proposed to explain how actomyosin rings are organised. Possible roles of non‐muscle myosin II (NM‐II) in axon degeneration are discussed. Finally, we discuss the concept of periodic functional nanodomains, which could sense extracellular cues and coordinate the axonal responses. Also see the video abstract here: https://youtu.be/ojCnrJ8RCRc. (shrink)

Rnd3/RhoE has two distinct functions, regulating the actin cytoskeleton and cell proliferation. This might explain why its expression is often altered in cancer and by multiple stimuli during development and disease. Rnd3 together with its relatives Rnd1 and Rnd2 are atypical members of the Rho GTPase family in that they do not hydrolyse GTP. Rnd3 and Rnd1 both antagonise RhoA/ROCK‐mediated actomyosin contractility, thereby regulating cell migration, smooth muscle contractility and neurite extension. In addition, Rnd3 has been shown to have (...) a separate role in inhibiting cell cycle progression by reducing translation of cell cycle regulators, including cyclin D1 and Myc. We propose that Rnd3 could act as a tumour suppressor to limit proliferation, but when mutations bypass this activity of Rnd3, it can promote cancer invasion through its effects in the actin cytoskeleton. (shrink)

The change in shape of the C. elegans embryo from an ovoid ball of cells into a worm-shaped larva is driven by three events within the cells of the hypodermis (epidermis): (1) intercalation of two rows of dorsal cells, (2) enclosure of the ventral surface by hypodermis, and (3) elongation of the embryo. While the behavior of the hypodermal cells involved in each of these processes differs dramatically, it is clear that F-actin and microtubules have essential functions in each of (...) these processes, whereas contraction of actomyosin structures appears to be involved specifically in elongation. Molecular analysis of these processes is revealing components specific to C. elegans as well as components found in other systems. Since C. elegans hypodermal cells demonstrate dramatically different behaviors during intercalation, enclosure and elongation, the study of cytoskeletal dynamics in these processes may reveal both unique and conserved activities during distinct epithelial morphogenetic movements. BioEssays 23:12–23, 2001. © 2001 John Wiley & Sons, Inc. (shrink)

Cytokinesis in animal cells is accomplished by the active constriction of the equatorial regions of a cell by an actomyosin‐containing contractile ring. The mitotic apparatus specifies the position and orientation of the furrow such that the mitotic spindle is always bisected. Global cortical contractions occur in the cortex of a cell prior to cytokinesis that are independent of the presence of the mitotic apparatus. It was proposed some years ago that the asters of the mitotic apparatus could act to (...) relax the preformed cortical tension in their vicinity. This would produce a differential in tension between the equatorial regions and the adjacent regions of the cortex so that the equatorial regions would contract, forming a cleavage furrow. It can be shown that, as it stands, this theory cannot explain cleavage. However, if cortical contractile elements are assumed to be laterally mobile in the plane of the cortex, then the astral relaxation theory can account for many of the aspects of cleavage, including the formation of the contractile ring. Similar schemes may account for the behaviour of the lamellapodia of motile cells. (shrink)

The retinal pigment epithelium (RPE) is a specialized epithelium at the back of the eye that carries out a variety of functions essential for visual health. Recent studies have advanced our molecular understanding of one of the major functions of the RPE; phagocytosis of spent photoreceptor outer segments (POS). Notably, a mechanical link, formed between apical integrins bound to extracellular POS and the intracellular actomyosin cytoskeleton, is proposed to drive the internalization of POS. The process may involve a “nibbling” (...) action, as an initial step, to sever outer segment tips. These insights have led us to hypothesize an “integrin adhesome‐like” network, atypically assembled at apical membrane RPE‐POS contacts. I propose that this hypothetical network orchestrates the complex membrane remodeling events required for particle internalization. Therefore, its analysis and characterization will likely lead to a more comprehensive understanding of the molecular mechanisms that control POS phagocytosis. (shrink)

The convergence of morphogenesis into viable organisms under variable conditions suggests closed‐loop dynamics involving multiscale functional feedback. We develop the idea that morphogenesis is based on synergy between mechanical and bio‐signaling processes, spanning all levels of organization: molecular, cellular, tissue, up to the whole organism. This synergy provides feedback within and between all levels of organization, to close the loop between the dynamics of the morphogenesis process and its robust functional outcome. Hydra offer a powerful platform to explore this direction, (...) thanks to their simple body plan, extraordinary regeneration capabilities, and the accessibility and flexibility of their tissues. Our recent experiments show that structural inheritance of the actomyosin organization directs body‐axis formation during Hydra regeneration. Morphogenesis is then stabilized through dynamic cytoskeletal reorganization induced by the inherited structure. The observed cytoskeletal stability and reorganization capabilities suggest that mechanical feedback integrates with biochemical processes to establish viable patterns and ensure canalization. (shrink)

During morphogenesis, tissues undergo extensive remodeling to get their final shape. Such precise sculpting requires the application of forces generated within cells by the cytoskeleton and transmission of these forces through adhesion molecules within and between neighboring cells. Within individual cells, microtubules together with actomyosin filaments and intermediate filaments form the composite cytoskeleton that controls cell mechanics during tissue rearrangements. While studies have established the importance of actin‐based mechanical forces that are coupled via intercellular junctions, relatively little is known (...) about the contribution of other cytoskeletal components such as microtubules to cell mechanics during morphogenesis. In this review the focus is on recent findings, highlighting the direct mechanical role of microtubules beyond its well‐established role in trafficking and signaling during tissue formation. (shrink)