Myosin light chain kinase regulates cell polarization independently of membrane tension or Rho kinase

From actin waves to mechanism and back: How theory aids biological understanding

Actin dynamics in cell motility, division, and phagocytosis is regulated by complex factors with multiple feedback loops, often leading to emergent dynamic patterns in the form of propagating waves of actin polymerization activity that are poorly understood. Many in the actin wave community have attempted to discern the underlying mechanisms using experiments and/or mathematical models and theory. Here, we survey methods and hypotheses for actin waves based on signaling networks, mechano-chemical effects, and transport characteristics, with examples drawn from Dictyostelium discoideum, human neutrophils, Caenorhabditis elegans, and Xenopus laevis oocytes. While experimentalists focus on the details of molecular components, theorists pose a central question of universality: Are there generic, model-independent, underlying principles, or just boundless cell-specific details? We argue that mathematical methods are equally important for understanding the emergence, evolution, and persistence of actin waves and conclude with a few challenges for future studies.

Next-generation plasmids for transgenesis in zebrafish and beyond

Transgenesis is an essential technique for any genetic model. Tol2-based transgenesis paired with Gateway-compatible vector collections has transformed zebrafish transgenesis with an accessible modular system. Here, we establish several next-generation transgenesis tools for zebrafish and other species to expand and enhance transgenic applications. To facilitate gene regulatory element testing, we generated Gateway middle entry vectors harboring the small mouse beta-globin minimal promoter coupled to several fluorophores, CreERT2 and Gal4. To extend the color spectrum for transgenic applications, we established middle entry vectors encoding the bright, blue-fluorescent protein mCerulean and mApple as an alternative red fluorophore. We present a series of p2A peptide-based 3' vectors with different fluorophores and subcellular localizations to co-label cells expressing proteins of interest. Finally, we established Tol2 destination vectors carrying the zebrafish exorh promoter driving different fluorophores as a pineal gland-specific transgenesis marker that is active before hatching and through adulthood. exorh-based reporters and transgenesis markers also drive specific pineal gland expression in the eye-less cavefish (Astyanax). Together, our vectors provide versatile reagents for transgenesis applications in zebrafish, cavefish and other models.

Confined keratocytes mimic in vivo migration and reveal volume-speed relationship

Fish basal epidermal cells, known as keratocytes, are well-suited for cell migration studies. In vitro, isolated keratocytes adopt a stereotyped shape with a large fan-shaped lamellipodium and a nearly spherical cell body. However, in their native in vivo environment, these cells adopt a significantly different shape during their rapid migration toward wounds. Within the epidermis, keratocytes experience two-dimensional (2D) confinement between the outer epidermal cell layer and the basement membrane; these two deformable surfaces constrain keratocyte cell bodies to be flatter in vivo than in isolation. In vivo keratocytes also exhibit a relative elongation of the front-to-back axis and substantially more lamellipodial ruffling, as compared to isolated cells. We have explored the effects of 2D confinement, separated from other in vivo environmental cues, by overlaying isolated cells with an agarose hydrogel with occasional spacers, or with a ceiling made of polydimethylsiloxane (PDMS) elastomer. Under these conditions, isolated keratocytes more closely resemble the in vivo migratory shape phenotype, displaying a flatter apical-basal axis and a longer front-to-back axis than unconfined keratocytes. We propose that 2D confinement contributes to multiple dimensions of in vivo keratocyte shape determination. Further analysis demonstrates that confinement causes a synchronous 20% decrease in both cell speed and volume. Interestingly, we were able to replicate the 20% decrease in speed using a sorbitol hypertonic shock to shrink the cell volume, which did not affect other aspects of cell shape. Collectively, our results suggest that environmentally imposed changes in cell volume may influence cell migration speed, potentially by perturbing physical properties of the cytoplasm.

Leading edge maintenance in migrating cells is an emergent property of branched actin network growth

Animal cell migration is predominantly driven by the coordinated, yet stochastic, polymerization of thousands of nanometer-scale actin filaments across micron-scale cell leading edges. It remains unclear how such inherently noisy processes generate robust cellular behavior. We employed high-speed imaging of migrating neutrophil-like HL-60 cells to explore the fine-scale shape fluctuations that emerge and relax throughout the process of leading edge maintenance. We then developed a minimal stochastic model of the leading edge that reproduces this stable relaxation behavior. Remarkably, we find lamellipodial stability naturally emerges from the interplay between branched actin network growth and leading edge shape – with no additional feedback required – based on a synergy between membrane-proximal branching and lateral spreading of filaments. These results thus demonstrate a novel biological noise-suppression mechanism based entirely on system geometry. Furthermore, our model suggests that the Arp2/3-mediated ~70–80° branching angle optimally smooths lamellipodial shape, addressing its long-mysterious conservation from protists to mammals.

In every human cell, there are tens of millions of proteins which work together to control everything from the cell’s shape to its behavior. One of the most abundant proteins is actin, which organizes itself into filaments that mechanically support the cell and help it to move.

These filaments are very dynamic, with individual actin molecules constantly being added or removed. This allows the cell to build large structures with distinct shapes and properties. Many motile cells, for example, have a structure called a lamellipodium which protrudes at their ‘leading edge’ and pushes them forward. The lamellipodium has a very robust shape that does not vary much between different cell types, or change significantly as cells migrate. But how the tens of thousands of actin molecules inside the lamellipodium organize themselves into this large, stable structure is not fully understood.

To investigate, Garner and Theriot used high-speed video microscopy to track the shape of human cells cultured in the laboratory. As the cells crawled along a glass surface, their leading edge undulated like strings being plucked on a guitar. A computer simulation showed that these ripples can be caused by filaments randomly adding and removing actin molecules.

While these random movements could destabilize the structure of the leading edge, the simulation suggests that another aspect of actin filament growth smooths out any fluctuations in the lamellipodium’s shape. Actin networks in the lamellipodium have a branched configuration, with new strands emerging off each other at an angle like branches in a tree. Garner and Theriot found that the specific angle in which new filaments are added smooths out the lamellipodium’s shape, which may explain why this geometry has persisted throughout evolution.

These findings suggest that the way in which actin filaments join together helps to maintain the shape of large cellular structures. In the future, scientists could use this design principle to build molecular machines that can self-organize into microstructures. These engineered constructs could be used to modulate the activity of living cells that have been damaged by disease.

β-Hydroxybutyrate impairs neutrophil migration distance through activation of a protein kinase C and myosin light chain 2 signaling pathway in ketotic cows

Ketosis in dairy cows often occurs in the peripartal period and is accompanied by immune dysfunction. High concentrations of β-hydroxybutyrate (BHB) in peripheral blood during ketosis are closely related to the impairment of polymorphonuclear neutrophil (PMN) chemotaxis and contribute to immune dysfunction. The specific effect of BHB on PMN chemotaxis in dairy cows and the underlying molecular mechanisms are unclear. Here, 30 multiparous cows (within 3 wk postpartum) classified based on serum BHB as control (n = 15, BHB <0.6 mM) or clinically ketotic (n = 15, BHB >3.0 mM) were used. Blood samples were collected before feeding, and the isolated PMN were treated with platelet-activating factor for 0.5 h to activate their migration. Scanning electron microscopy revealed a longer tail in PMN of ketotic cows. In addition, the phosphorylation and transcription levels of myosin light chain 2 (MLC2) increased in PMN of ketotic cows. Polymorphonuclear neutrophils from control dairy cows were incubated with 3.0 mM BHB for different times in vitro, and 6 h was selected as the proper duration of BHB stimulation according to its inhibition effect on PMN migration using an under-agarose PMN chemotaxis model. Similarly, BHB stimulation in vitro resulted in inhibition of migration distance and deviation of migration direction of PMN, as well as a longer tail in morphology in the scanning electron microscope data, suggesting that BHB-induced PMN migration inhibition may be mediated by impairing the trailing edge contraction. To confirm this hypothesis, sotrastaurin (Sotra)-a specific inhibitor of protein kinase C (PKC), which is the core regulator of cell contraction-was used with or without BHB treatment in vitro. Sotra was pretreated 0.5 h before BHB treatment. Accordingly, BHB treatment increased the phosphorylation level of PKC and MLC2, the protein abundance of RhoA and rho-kinase 1 (ROCK1), and the mRNA abundance of PRKCA, MYL2, RHOA, and ROCK1 in PMN. In contrast, these effects of BHB on PMN were dampened by Sotra. As demonstrated by immunofluorescence experiments in vitro, the BHB-induced inhibition of trailing edge contraction of PMN was relieved by Sotra. In addition, Sotra also dampened the effects of BHB on PMN migration in vitro. Furthermore, as verified by in vivo experiments, compared with the control cows, both abundance and activation of PKC signaling were enhanced in PMN of ketotic cows. Overall, the present study revealed that high concentrations of blood BHB impaired PMN migration distance through inhibition of the trailing edge contraction, mediated by enhancing the activation of PKC-MLC2 signaling. These findings help explain the dysfunctional immune state in ketotic cows and provide information on the pathogenesis of infectious diseases secondary to ketosis.

Comparative mapping of crawling-cell morphodynamics in deep learning-based feature space

Navigation of fast migrating cells such as amoeba Dictyostelium and immune cells are tightly associated with their morphologies that range from steady polarized forms that support high directionality to those more complex and variable when making frequent turns. Model simulations are essential for quantitative understanding of these features and their origins, however systematic comparisons with real data are underdeveloped. Here, by employing deep-learning-based feature extraction combined with phase-field modeling framework, we show that a low dimensional feature space for 2D migrating cell morphologies obtained from the shape stereotype of keratocytes, Dictyostelium and neutrophils can be fully mapped by an interlinked signaling network of cell-polarization and protrusion dynamics. Our analysis links the data-driven shape analysis to the underlying causalities by identifying key parameters critical for migratory morphologies both normal and aberrant under genetic and pharmacological perturbations. The results underscore the importance of deciphering self-organizing states and their interplay when characterizing morphological phenotypes.

Migratory cells that move by crawling do so by extending and retracting their plasma membrane. When and where these events take place determine the cell shape, and this is directly linked to the movement patterns. Understanding how the highly plastic and interconvertible morphologies appear from their underlying dynamics remains a challenge partly because their inherent complexity makes quantitatively comparison against the outputs of mathematical models difficult. To this end, we employed machine-learning based classification to extract features that characterize the basic migrating morphologies. The obtained features were then used to compare real cell data with outputs of a conceptual model that we introduced which describes coupling via feedback between local protrusive dynamics and polarity. The feature mapping showed that the model successfully recapitulates the shape dynamics that were not covered by previous related models and also hints at the critical parameters underlying state transitions. The ability of the present approach to compare model outputs with real cell data systematically and objectively is important as it allows outputs of future mathematical models to be quantitatively tested in an accessible and common reference frame.

Phosphorylation-dependent subcellular redistribution of small myosin light chain kinase

BACKGROUND

Myosin light chain kinase (MLCK) is a Ca²⁺-calmodulin-dependent enzyme dedicated to phosphorylate and activate myosin II to provide force for various motile processes. In smooth muscle cells and many other cells, small MLCK (S-MLCK) is a major isoform. S-MLCK is an actomyosin-binding protein firmly attached to contractile machinery in smooth muscle cells. Still, it can leave this location and contribute to other cellular processes. However, molecular mechanisms for switching the S-MLCK subcellular localization have not been described.

METHODS

Site-directed mutagenesis and in vitro protein phosphorylation were used to study functional roles of discrete in-vivo phosphorylated residues within the S-MLCK actin-binding domain. In vitro co-sedimentation analysis was applied to study the interaction of recombinant S-MLCK actin-binding fragment with filamentous actin. Subcellular distribution of phosphomimicking S-MLCK mutants was studied by fluorescent microscopy and differential cell extraction.

RESULTS

Phosphorylation of S-MLCK actin-binding domain at Ser25 and/or Thr56 by proline-directed protein kinases or phosphomimicking these posttranslational modifications alters S-MLCK binding to actin filaments both in vitro and in cells, and induces S-MLCK subcellular translocation with no effect on the enzyme catalytic properties.

CONCLUSIONS

Phosphorylation of the amino terminal actin-binding domain of S-MLCK renders differential subcellular targeting of the enzyme and may, thereby, contribute to a variety of context-dependent responses of S-MLCK to cellular and tissue stimuli.

GENERAL SIGNIFICANCE

S-MLCK physiological function can potentially be modulated via phosphorylation of its actin recognition domain, a regulation distinct from the catalytic and calmodulin regulatory domains.

Elastic wrinkling of keratocyte lamellipodia driven by myosin-induced contractile stress

During actin-based cell migration, the actin cytoskeleton in the lamellipodium both generates and responds to force, which has functional consequences for the ability of the cell to extend protrusions. However, the material properties of the lamellipodial actin network and its response to stress on the timescale of motility are incompletely understood. Here, we describe a dynamic wrinkling phenotype in the lamellipodium of fish keratocytes, in which the actin sheet buckles upward away from the ventral membrane of the cell, forming a periodic pattern of wrinkles perpendicular to the cell's leading edge. Cells maintain an approximately constant wrinkle wavelength over time despite new wrinkle formation and the lateral movement of wrinkles in the cell frame of reference, suggesting that cells have a preferred or characteristic wrinkle wavelength. Generation of wrinkles is dependent upon myosin contractility, and their wavelength scales directly with the density of the actin network and inversely with cell adhesion. These results are consistent with a simple physical model for wrinkling in an elastic sheet under compression and suggest that the lamellipodial cytoskeleton behaves as an elastic material on the timescale of cell migration despite rapid actin turnover.

Kindlin-2 promotes rear focal adhesion disassembly and directional persistence during cell migration

Cell migration involves front-to-rear asymmetric focal adhesion (FA) dynamics, which facilitates trailing edge detachment and directional persistence. Here, we show that kindlin-2 is crucial for FA sliding and disassembly in migrating cells. Loss of kindlin-2 markedly reduced FA number and selectively impaired rear FA sliding and disassembly, resulting in defective rear retraction and reduced directional persistence during cell migration. Kindlin-2-deficient cells failed to develop serum-induced actomyosin-dependent tension at FAs. At the molecular level, kindlin-2 directly interacted with myosin light chain kinase (MYLK, hereafter referred to as MLCK), which was enhanced in response to serum stimulation. Serum deprivation inhibited rear FA disassembly, which was released in response to serum stimulation. Overexpression of the MLCK-binding kindlin-2 F0F1 fragment (amino acid residues 1-167), which inhibits the interaction of endogenous kindlin-2 with MLCK, phenocopied kindlin-2 deficiency-induced migration defects. Inhibition of MLCK, like loss of kindlin-2, also impaired trailing-edge detachment, rear FA disassembly and directional persistence. These results suggest a role of kindlin-2 in promoting actomyosin contractility at FAs, leading to increased rear FA sliding and disassembly, and directional persistence during cell migration.

Osmolarity-independent electrical cues guide rapid response to injury in zebrafish epidermis

The ability of epithelial tissues to heal after injury is essential for animal life, yet the mechanisms by which epithelial cells sense tissue damage are incompletely understood. In aquatic organisms such as zebrafish, osmotic shock following injury is believed to be an early and potent activator of a wound response. We find that, in addition to sensing osmolarity, basal skin cells in zebrafish larvae are also sensitive to changes in the particular ionic composition of their surroundings after wounding, specifically the concentration of sodium chloride in the immediate vicinity of the wound. This sodium chloride-specific wound detection mechanism is independent of cell swelling, and instead is suggestive of a mechanism by which cells sense changes in the transepithelial electrical potential generated by the transport of sodium and chloride ions across the skin. Consistent with this hypothesis, we show that electric fields directly applied within the skin are sufficient to initiate actin polarization and migration of basal cells in their native epithelial context in vivo, even overriding endogenous wound signaling. This suggests that, in order to mount a robust wound response, skin cells respond to both osmotic and electrical perturbations arising from tissue injury.

Dynamics of a cell motility model near the sharp interface limit

Phase-field models have recently had great success in describing the dynamic morphologies and motility of eukaryotic cells. In this work we investigate the minimal phase-field model introduced in Berlyand et al. (2017). Rigorous analysis of its sharp interface limit dynamics was completed in Mizuhara et al. (2016) and Mizuhara et al. (2019), where it was observed that persistent cell motion was not stable. In this work we numerically study the pre-limiting phase-field model near the sharp interface limit, to better understand this lack of persistent motion. We find that immobile, persistent, and rotating states are all exhibited in this minimal model, and investigate the loss of persistent motion in the sharp interface limit.

CompuCell3D Simulations Reproduce Mesenchymal Cell Migration on Flat Substrates

Mesenchymal cell crawling is a critical process in normal development, in tissue function, and in many diseases. Quantitatively predictive numerical simulations of cell crawling thus have multiple scientific, medical, and technological applications. However, we still lack a low-computational-cost approach to simulate mesenchymal three-dimensional (3D) cell crawling. Here, we develop a computationally tractable 3D model (implemented as a simulation in the CompuCell3D simulation environment) of mesenchymal cells crawling on a two-dimensional substrate. The Fürth equation, the usual characterization of mean-squared displacement (MSD) curves for migrating cells, describes a motion in which, for increasing time intervals, cell movement transitions from a ballistic to a diffusive regime. Recent experiments have shown that for very short time intervals, cells exhibit an additional fast diffusive regime. Our simulations' MSD curves reproduce the three experimentally observed temporal regimes, with fast diffusion for short time intervals, slow diffusion for long time intervals, and intermediate time -interval-ballistic motion. The resulting parameterization of the trajectories for both experiments and simulations allows the definition of time- and length scales that translate between computational and laboratory units. Rescaling by these scales allows direct quantitative comparisons among MSD curves and between velocity autocorrelation functions from experiments and simulations. Although our simulations replicate experimentally observed spontaneous symmetry breaking, short-timescale diffusive motion, and spontaneous cell-motion reorientation, their computational cost is low, allowing their use in multiscale virtual-tissue simulations. Comparisons between experimental and simulated cell motion support the hypothesis that short-time actomyosin dynamics affects longer-time cell motility. The success of the base cell-migration simulation model suggests its future application in more complex situations, including chemotaxis, migration through complex 3D matrices, and collective cell motion.

Bridging the gap between single-cell migration and collective dynamics

Motivated by the wealth of experimental data recently available, we present a cellular-automaton-based modeling framework focussing on high-level cell functions and their concerted effect on cellular migration patterns. Specifically, we formulate a coarse-grained description of cell polarity through self-regulated actin organization and its response to mechanical cues. Furthermore, we address the impact of cell adhesion on collective migration in cell cohorts. The model faithfully reproduces typical cell shapes and movements down to the level of single cells, yet allows for the efficient simulation of confluent tissues. In confined circular geometries, we find that specific properties of individual cells (polarizability; contractility) influence the emerging collective motion of small cell cohorts. Finally, we study the properties of expanding cellular monolayers (front morphology; stress and velocity distributions) at the level of extended tissues.

A Tale of Two States: Normal and Transformed, With and Without Rigidity Sensing

For many years, major differences in morphology, motility, and mechanical characteristics have been observed between transformed cancer and normal cells. In this review, we consider these differences as linked to different states of normal and transformed cells that involve distinct mechanosensing and motility pathways. There is a strong correlation between repeated tissue healing and/or inflammation and the probability of cancer, both of which involve growth in adult tissues. Many factors are likely needed to enable growth, including the loss of rigidity sensing, but recent evidence indicates that microRNAs have important roles in causing the depletion of growth-suppressing proteins. One microRNA, miR-21, is overexpressed in many different tissues during both healing and cancer. Normal cells can become transformed by the depletion of cytoskeletal proteins that results in the loss of mechanosensing, particularly rigidity sensing. Conversely, the transformed state can be reversed by the expression of cytoskeletal proteins-without direct alteration of hormone receptor levels. In this review, we consider the different stereotypical forms of motility and mechanosensory systems. A major difference between normal and transformed cells involves a sensitivity of transformed cells to mechanical perturbations. Thus, understanding the different mechanical characteristics of transformed cells may enable new approaches to treating wound healing and cancer.

Apical-basal polarity inhibits epithelial-mesenchymal transition and tumour metastasis by PAR-complex-mediated SNAI1 degradation

Loss of apical-basal polarity and activation of epithelial-mesenchymal transition (EMT) both contribute to carcinoma progression and metastasis. Here, we report that apical-basal polarity inhibits EMT to suppress metastatic dissemination. Using mouse and human epithelial three-dimensional organoid cultures, we show that the PAR-atypical protein kinase C (aPKC) polarity complex inhibits EMT and invasion by promoting degradation of the SNAIL family protein SNAI1. Under intact apical-basal polarity, aPKC kinases phosphorylate S249 of SNAI1, which leads to protein degradation. Loss of apical-basal polarity prevents aPKC-mediated SNAI1 phosphorylation and stabilizes the SNAI1 protein to promote EMT and invasion. In human breast tumour xenografts, inhibition of the PAR-complex-mediated SNAI1 degradation mechanism promotes tumour invasion and metastasis. Analyses of human breast tissue samples reveal negative correlations between PAR3 and SNAI1 protein levels. Our results demonstrate that apical-basal polarity functions as a critical checkpoint of EMT to precisely control epithelial-mesenchymal plasticity during tumour metastasis.

Steps in Mechanotransduction Pathways that Control Cell Morphology

It is increasingly clear that mechanotransduction pathways play important roles in regulating fundamental cellular functions. Of the basic mechanical functions, the determination of cellular morphology is critical. Cells typically use many mechanosensitive steps and different cell states to achieve a polarized shape through repeated testing of the microenvironment. Indeed, morphology is determined by the microenvironment through periodic activation of motility, mechanotesting, and mechanoresponse functions by hormones, internal clocks, and receptor tyrosine kinases. Patterned substrates and controlled environments with defined rigidities limit the range of cell behavior and influence cell state decisions and are thus very useful for studying these steps. The recently defined rigidity sensing process provides a good example of how cells repeatedly test their microenvironment and is also linked to cancer. In general, aberrant extracellular matrix mechanosensing is associated with numerous conditions, including cardiovascular disease, aging, and fibrosis, that correlate with changes in tissue morphology and matrix composition. Hence, detailed descriptions of the steps involved in sensing and responding to the microenvironment are needed to better understand both the mechanisms of tissue homeostasis and the pathomechanisms of human disease.

A Salutary Role of Reactive Oxygen Species in Intercellular Tunnel-Mediated Communication

The reactive oxygen species, generally labeled toxic due to high reactivity without target specificity, are gradually uncovered as signaling molecules involved in a myriad of biological processes. But one important feature of ROS roles in macromolecule movement has not caught attention until recent studies with technique advance and design elegance have shed lights on ROS signaling for intercellular and interorganelle communication. This review begins with the discussions of genetic and chemical studies on the regulation of symplastic dye movement through intercellular tunnels in plants (plasmodesmata), and focuses on the ROS regulatory mechanisms concerning macromolecule movement including small RNA-mediated gene silencing movement and protein shuttling between cells. Given the premise that intercellular tunnels (bridges) in mammalian cells are the key physical structures to sustain intercellular communication, movement of macromolecules and signals is efficiently facilitated by ROS-induced membrane protrusions formation, which is analogously applied to the interorganelle communication in plant cells. Although ROS regulatory differences between plant and mammalian cells exist, the basis for ROS-triggered conduit formation underlies a unifying conservative theme in multicellular organisms. These mechanisms may represent the evolutionary advances that have enabled multicellularity to gain the ability to generate and utilize ROS to govern material exchanges between individual cells in oxygenated environment.

Load Adaptation of Lamellipodial Actin Networks

Actin filaments polymerizing against membranes power endocytosis, vesicular traffic, and cell motility. In vitro reconstitution studies suggest that the structure and the dynamics of actin networks respond to mechanical forces. We demonstrate that lamellipodial actin of migrating cells responds to mechanical load when membrane tension is modulated. In a steady state, migrating cell filaments assume the canonical dendritic geometry, defined by Arp2/3-generated 70° branch points. Increased tension triggers a dense network with a broadened range of angles, whereas decreased tension causes a shift to a sparse configuration dominated by filaments growing perpendicularly to the plasma membrane. We show that these responses emerge from the geometry of branched actin: when load per filament decreases, elongation speed increases and perpendicular filaments gradually outcompete others because they polymerize the shortest distance to the membrane, where they are protected from capping. This network-intrinsic geometrical adaptation mechanism tunes protrusive force in response to mechanical load.

The shift in GH3 cell shape and cell motility is dependent on MLCK and ROCK

Cytoskeletal organization, actin-myosin contractility and the cell membrane together regulate cell morphology in response to the cell environment, wherein the extracellular matrix (ECM) is an indispensable component. Plasticity in cell shape enables cells to adapt their migration mode to their surroundings. GH3 endocrine cells respond to different ECM proteins, acquiring different morphologies: a rounded on collagen I-III (C I-III) and an elongated on collagen IV (C IV). However, the identities of the molecules that participate in these responses remain unknown. Considering that actin-myosin contractility is crucial to maintaining cell shape, we analyzed the participation of MLCK and ROCK in the acquisition of cell shape, the generation of cellular tension and the cell motility mode. We found that a rounded shape with high cortical tension depends on MLCK and ROCK, whereas in cells with an elongated shape, MLCK is the primary protein responsible for cell spreading. Further, in cells with a slow and directionally persistent motility, MLCK predominates, while rapid and erratic movement is ROCK-dependent. This behavior also correlates with GTPase activation. Cells on C I-III exhibited higher Rho-GTPase activity than cells on C IV and vice versa with Rac-GTPase activity, showing a plastic response of GH3 cells to their environment, leading to the generation of different cytoskeleton and membrane organizations and resulting in two movement strategies, rounded and fibroblastoid-like.

Myosin Light Chain Kinase MYLK1: Anatomy, Interactions, Functions, and Regulation

This review discusses and summarizes the results of molecular and cellular investigations of myosin light chain kinase (MLCK, MYLK1), the key regulator of cell motility. The structure and regulation of a complex mylk1 gene and the domain organization of its products is presented. The interactions of the mylk1 gene protein products with other proteins and posttranslational modifications of the mylk1 gene protein products are reviewed, which altogether might determine the role and place of MLCK in physiological and pathological reactions of cells and entire organisms. Translational potential of MLCK as a drug target is evaluated.

Adhesion-Dependent Wave Generation in Crawling Cells

Dynamic actin networks are excitable. In migrating cells, feedback loops can amplify stochastic fluctuations in actin dynamics, often resulting in traveling waves of protrusion. The precise contributions of various molecular and mechanical interactions to wave generation have been difficult to disentangle, in part due to complex cellular morphodynamics. Here we used a relatively simple cell type-the fish epithelial keratocyte-to define a set of mechanochemical feedback loops underlying actin network excitability and wave generation. Although keratocytes are normally characterized by the persistent protrusion of a broad leading edge, increasing cell-substrate adhesion strength results in waving protrusion of a short leading edge. We show that protrusion waves are due to fluctuations in actin polymerization rates and that overexpression of VASP, an actin anti-capping protein that promotes actin polymerization, switches highly adherent keratocytes from waving to persistent protrusion. Moreover, VASP localizes both to adhesion complexes and to the leading edge. Based on these results, we developed a mathematical model for protrusion waves in which local depletion of VASP from the leading edge by adhesions-along with lateral propagation of protrusion due to the branched architecture of the actin network and negative mechanical feedback from the cell membrane-results in regular protrusion waves. Consistent with our model simulations, we show that VASP localization at the leading edge oscillates, with VASP leading-edge enrichment greatest just prior to protrusion initiation. We propose that the mechanochemical feedbacks underlying wave generation in keratocytes may constitute a general module for establishing excitable actin dynamics in other cellular contexts.

Engulfed cadherin fingers are polarized junctional structures between collectively migrating endothelial cells

The development and maintenance of tissues requires collective cell movement, during which neighbouring cells coordinate the polarity of their migration machineries. Here, we ask how polarity signals are transmitted from one cell to another across symmetrical cadherin junctions, during collective migration. We demonstrate that collectively migrating endothelial cells have polarized VE-cadherin-rich membrane protrusions, 'cadherin fingers', which leading cells extend from their rear and follower cells engulf at their front, thereby generating opposite membrane curvatures and asymmetric recruitment of curvature-sensing proteins. In follower cells, engulfment of cadherin fingers occurs along with the formation of a lamellipodia-like zone with low actomyosin contractility, and requires VE-cadherin/catenin complexes and Arp2/3-driven actin polymerization. Lateral accumulation of cadherin fingers in follower cells precedes turning, and increased actomyosin contractility can initiate cadherin finger extension as well as engulfment by a neighbouring cell, to promote follower behaviour. We propose that cadherin fingers serve as guidance cues that direct collective cell migration.

Membrane Tension Acts Through PLD2 and mTORC2 to Limit Actin Network Assembly During Neutrophil Migration

For efficient polarity and migration, cells need to regulate the magnitude and spatial distribution of actin assembly. This process is coordinated by reciprocal interactions between the actin cytoskeleton and mechanical forces. Actin polymerization-based protrusion increases tension in the plasma membrane, which in turn acts as a long-range inhibitor of actin assembly. These interactions form a negative feedback circuit that limits the magnitude of membrane tension in neutrophils and prevents expansion of the existing front and the formation of secondary fronts. It has been suggested that the plasma membrane directly inhibits actin assembly by serving as a physical barrier that opposes protrusion. Here we show that efficient control of actin polymerization-based protrusion requires an additional mechanosensory feedback cascade that indirectly links membrane tension with actin assembly. Specifically, elevated membrane tension acts through phospholipase D2 (PLD2) and the mammalian target of rapamycin complex 2 (mTORC2) to limit actin nucleation. In the absence of this pathway, neutrophils exhibit larger leading edges, higher membrane tension, and profoundly defective chemotaxis. Mathematical modeling suggests roles for both the direct (mechanical) and indirect (biochemical via PLD2 and mTORC2) feedback loops in organizing cell polarity and motility—the indirect loop is better suited to enable competition between fronts, whereas the direct loop helps spatially organize actin nucleation for efficient leading edge formation and cell movement. This circuit is essential for polarity, motility, and the control of membrane tension.

A mechanosensory biochemical cascade involving phospholipase D2 and mTORC2 coordinates physical forces and cytoskeletal rearrangements to allow efficient polarization and migration of neutrophils.

How cells regulate the size and number of their protrusions for efficient polarity and motility is a fundamental question in cell biology. We recently found that immune cells known as neutrophils use physical forces to regulate this process. Actin polymerization-based protrusion stretches the plasma membrane, and this increased membrane tension acts as a long-range inhibitor of actin-based protrusions elsewhere in the cell. Here we investigate how membrane tension limits protrusion. We demonstrate that the magnitude of actin network assembly in neutrophils is determined by a mechanosensory biochemical cascade that converts increases in membrane tension into decreases in protrusion. Specifically, we show that increasing plasma membrane tension acts through a pathway containing the phospholipase D2 (PLD2) and the mammalian target of rapamycin complex 2 (mTORC2) to limit actin network assembly. Without this negative feedback pathway, neutrophils exhibit larger leading edges, higher membrane tension, and profoundly defective chemotaxis. Mathematical modeling indicates that this feedback circuit is a favorable topology to enable competition between protrusions during neutrophil polarization. Our work shows how biochemical signals, physical forces, and the cytoskeleton can collaborate to generate large-scale cellular organization.

Time series modeling of live-cell shape dynamics for image-based phenotypic profiling

Live-cell imaging can be used to capture spatio-temporal aspects of cellular responses that are not accessible to fixed-cell imaging. As the use of live-cell imaging continues to increase, new computational procedures are needed to characterize and classify the temporal dynamics of individual cells. For this purpose, here we present the general experimental-computational framework SAPHIRE (Stochastic Annotation of Phenotypic Individual-cell Responses) to characterize phenotypic cellular responses from time series imaging datasets. Hidden Markov modeling is used to infer and annotate morphological state and state-switching properties from image-derived cell shape measurements. Time series modeling is performed on each cell individually, making the approach broadly useful for analyzing asynchronous cell populations. Two-color fluorescent cells simultaneously expressing actin and nuclear reporters enabled us to profile temporal changes in cell shape following pharmacological inhibition of cytoskeleton-regulatory signaling pathways. Results are compared with existing approaches conventionally applied to fixed-cell imaging datasets, and indicate that time series modeling captures heterogeneous dynamic cellular responses that can improve drug classification and offer additional important insight into mechanisms of drug action. The software is available at http://saphire-hcs.org.

The mechanisms of spatial and temporal patterning of cell-edge dynamics

Adherent cells migrate and change their shape by means of protrusion and retraction at their edges. When and where these activities occur defines the shape of the cell and the way it moves. Despite a great deal of knowledge about the structural organization, components, and biochemical reactions involved in protrusion and retraction, the origins of their spatial and temporal patterns are still poorly understood. Chemical signaling circuitry is believed to be an important source of patterning, but recent studies highlighted mechanisms based on physical forces, motion, and mechanical feedback.

Competition for actin between two distinct F-actin networks defines a bistable switch for cell polarization

Symmetry-breaking polarization enables functional plasticity of cells and tissues and is yet not well understood. Here we show that epithelial cells, hard-wired to maintain a static morphology and to preserve tissue organization, can spontaneously switch to a migratory polarized phenotype upon relaxation of the actomyosin cytoskeleton. We find that myosin-II engages actin in the formation of cortical actomyosin bundles and thus makes it unavailable for deployment in the process of dendritic growth normally driving cell motility. At low contractility regimes epithelial cells polarize in a front-back manner due to emergence of actin retrograde flows powered by dendritic polymerization of actin. Coupled to cell movement, the flows transport myosin-II from the front to the back of the cell, where the motor locally “locks” actin in contractile bundles. This polarization mechanism could be employed by embryonic and cancer epithelial cells in microenvironments where high contractility-driven cell motion is inefficient.

Disentangling Membrane Dynamics and Cell Migration; Differential Influences of F-actin and Cell-Matrix Adhesions

Cell migration is heavily interconnected with plasma membrane protrusion and retraction (collectively termed “membrane dynamics”). This makes it difficult to distinguish regulatory mechanisms that differentially influence migration and membrane dynamics. Yet such distinctions may be valuable given evidence that cancer cell invasion in 3D may be better predicted by 2D membrane dynamics than by 2D cell migration, implying a degree of functional independence between these processes. Here, we applied multi-scale single cell imaging and a systematic statistical approach to disentangle regulatory associations underlying either migration or membrane dynamics. This revealed preferential correlations between membrane dynamics and F-actin features, contrasting with an enrichment of links between cell migration and adhesion complex properties. These correlative linkages were often non-linear and therefore context-dependent, strengthening or weakening with spontaneous heterogeneity in cell behavior. More broadly, we observed that slow moving cells tend to increase in area, while fast moving cells tend to shrink, and that the size of dynamic membrane domains is independent of cell area. Overall, we define macromolecular features preferentially associated with either cell migration or membrane dynamics, enabling more specific interrogation and targeting of these processes in future.