Directional reorientation of migrating neutrophils is limited by suppression of receptor input signaling at the cell rear through myosin II activity

The amoeboid migration of monocytes in confining channels requires the local remodeling of the cortical actin cytoskeleton by cofilin-1

Within the bloodstream, monocytes must traverse the microvasculature to prevent leukostasis, which is the entrapment of monocytes within the confines of the microvasculature. Using the model cell line, THP-1, and VCAM-1 coated channels to simulate the microvasculature surface, we demonstrate that monocytes predominantly adopt an amoeboid phenotype, which is characterized by the formation of blebs. As opposed to cortical actin flow in leader blebs, cell movement is correlated with myosin contraction at the cell rear. It was previously documented that cofilin-1 promotes cortical actin turnover at leader bleb necks in melanoma cells. In monocytes, our data suggest that cofilin-1 promotes the local upregulation of myosin contractility through actin cytoskeleton remodeling. In support of this concept, cofilin-1 is found to localize to a single cell edge. Moreover, the widespread upregulation of myosin contractility was found to inhibit migration. Thus, monocytes within the microvasculature may avoid entrapment by adopting an amoeboid mode of migration.

Principles of organelle positioning in motile and non-motile cells

Cells are equipped with asymmetrically localised and functionally specialised components, including cytoskeletal structures and organelles. Positioning these components to specific intracellular locations in an asymmetric manner is critical for their functionality and affects processes like immune responses, tissue maintenance, muscle functionality, and neurobiology. Here, we provide an overview of strategies to actively move, position, and anchor organelles to specific locations. By conceptualizing the cytoskeletal forces and the organelle-to-cytoskeleton connectivity, we present a framework of active positioning of both membrane-enclosed and membrane-less organelles. Using this framework, we discuss how different principles of force generation and organelle anchorage are utilised by different cells, such as mesenchymal and amoeboid cells, and how the microenvironment influences the plasticity of organelle positioning. Given that motile cells face the challenge of coordinating the positioning of their content with cellular motion, we particularly focus on principles of organelle positioning during migration. In this context, we discuss novel findings on organelle positioning by anchorage-independent mechanisms and their advantages and disadvantages in motile as well as stationary cells.

Cooperative polarization of MCAM/CD146 and ERM family proteins in melanoma

The WRAMP structure is a protein network associated with tail-end actomyosin contractility, membrane retraction, and directional persistence during cell migration. A marker of WRAMP structures is melanoma cell adhesion molecule (MCAM) which dynamically polarizes to the cell rear. However, factors that mediate MCAM polarization are still unknown. In this study, BioID using MCAM as bait identifies the ERM family proteins, moesin, ezrin, and radixin, as WRAMP structure components. We also present a novel image analysis pipeline, Protein Polarity by Percentile ("3P"), which classifies protein polarization using machine learning and facilitates quantitative analysis. Using 3P, we find that depletion of moesin, and to a lesser extent ezrin, decreases the proportion of cells with polarized MCAM. Furthermore, although copolarized MCAM and ERM proteins show high spatial overlap, 3P identifies subpopulations with ERM proteins closer to the cell periphery. Live-cell imaging confirms that MCAM and ERM protein polarization is tightly coordinated, but ERM proteins enrich at the cell edge first. Finally, deletion of a juxtamembrane segment in MCAM previously shown to promote ERM protein interactions impedes MCAM polarization. Our findings highlight the requirement for ERM proteins in recruitment of MCAM to WRAMP structures and an advanced computational tool to characterize protein polarization.

An Intriguing Structural Modification in Neutrophil Migration Across Blood Vessels to Inflammatory Sites: Progress in the Core Mechanisms

The role and function of neutrophils are well known, but we still have incomplete understanding of the mechanisms by which neutrophils migrate from blood vessels to inflammatory sites. Neutrophil migration is a complex process that involves several distinct steps. To resist the blood flow and maintain their rolling, neutrophils employ tether and sling formation. They also polarize and form pseudopods and uropods, guided by hierarchical chemotactic agents that enable precise directional movement. Meanwhile, chemotactic agents secreted by neutrophils, such as CXCL1, CXCL8, LTB4, and C5a, can recruit more neutrophils and amplify their response. In the context of diapedesis neutrophils traverse the endothelial cells via two pathways: the transmigratory cup and the lateral border recycling department. These structures aid in overcoming the narrow pore size of the endothelial barrier, resulting in more efficient transmembrane migration. Interestingly, neutrophils exhibit a preference for the paracellular pathway over the transcellular pathway, likely due to the former's lower resistance. In this review, we will delve into the intricate process of neutrophil migration by focusing on critical structures that underpins this process.

Complementary Cytoskeletal Feedback Loops Control Signal Transduction Excitability and Cell Polarity

To move through complex environments, cells must constantly integrate chemical and mechanical cues. Signaling networks, such as those comprising Ras and PI3K, transmit chemical cues to the cytoskeleton, but the cytoskeleton must also relay mechanical information back to those signaling systems. Using novel synthetic tools to acutely control specific elements of the cytoskeleton in Dictyostelium and neutrophils, we delineate feedback mechanisms that alter the signaling network and promote front- or back-states of the cell membrane and cortex. First, increasing branched actin assembly increases Ras/PI3K activation while reducing polymeric actin levels overall decreases activation. Second, reducing myosin II assembly immediately increases Ras/PI3K activation and sensitivity to chemotactic stimuli. Third, inhibiting branched actin alone increases cortical actin assembly and strongly blocks Ras/PI3K activation. This effect is mitigated by reducing filamentous actin levels and in cells lacking myosin II. Finally, increasing actin crosslinking with a controllable activator of cytoskeletal regulator RacE leads to a large decrease in Ras activation both globally and locally. Curiously, RacE activation can trigger cell spreading and protrusion with no detectable activation of branched actin nucleators. Taken together with legacy data that Ras/PI3K promotes branched actin assembly and myosin II disassembly, our results define front- and back-promoting positive feedback loops. We propose that these loops play a crucial role in establishing cell polarity and mediating signal integration by controlling the excitable state of the signal transduction networks in respective regions of the membrane and cortex. This interplay enables cells to navigate intricate topologies like tissues containing other cells, the extracellular matrix, and fluids.

Microfluidic devices for precise measurements of cell directionality reveal a role for glutamine during cell migration

Cancer cells that migrate from tumors into surrounding tissues are responsible for cancer dissemination through the body. Microfluidic devices have been instrumental in discovering unexpected features of cancer cell migration, including the migration in self-generated gradients and the contributions of cell-cell contact during collective migration. Here, we design microfluidic channels with five successive bifurcations to characterize the directionality of cancer cell migration with high precision. We uncover an unexpected role for glutamine in epithelial cancer cell orientation, which could be replaced by alfa-keto glutarate but not glucose.

Adaptive pathfinding by nucleokinesis during amoeboid migration

Motile cells encounter microenvironments with locally heterogeneous mechanochemical composition. Individual compositional parameters, such as chemokines and extracellular matrix pore sizes, are well known to provide guidance cues for pathfinding. However, motile cells face diverse cues at the same time, raising the question of how they respond to multiple and potentially competing signals on their paths. Here, we reveal that amoeboid cells require nuclear repositioning, termed nucleokinesis, for adaptive pathfinding in heterogeneous mechanochemical micro-environments. Using mammalian immune cells and the amoeba Dictyostelium discoideum, we discover that frequent, rapid and long-distance nucleokinesis is a basic component of amoeboid pathfinding, enabling cells to reorientate quickly between locally competing cues. Amoeboid nucleokinesis comprises a two-step polarity switch and is driven by myosin-II forces that readjust the nuclear to the cellular path. Impaired nucleokinesis distorts path adaptions and causes cellular arrest in the microenvironment. Our findings establish that nucleokinesis is required for amoeboid cell navigation. Given that many immune cells, amoebae, and some cancer cells utilize an amoeboid migration strategy, these results suggest that nucleokinesis underlies cellular navigation during unicellular biology, immunity, and disease.

A dynamic partitioning mechanism polarizes membrane protein distribution

The plasma membrane is widely regarded as the hub of the numerous signal transduction activities. Yet, the fundamental biophysical mechanisms that spatiotemporally compartmentalize different classes of membrane proteins remain unclear. Using multimodal live-cell imaging, here we first show that several lipid-anchored membrane proteins are consistently depleted from the membrane regions where the Ras/PI3K/Akt/F-actin network is activated. The dynamic polarization of these proteins does not depend upon the F-actin-based cytoskeletal structures, recurring shuttling between membrane and cytosol, or directed vesicular trafficking. Photoconversion microscopy and single-molecule measurements demonstrate that these lipid-anchored molecules have substantially dissimilar diffusion profiles in different regions of the membrane which enable their selective segregation. When these diffusion coefficients are incorporated into an excitable network-based stochastic reaction-diffusion model, simulations reveal that the altered affinity mediated selective partitioning is sufficient to drive familiar propagating wave patterns. Furthermore, normally uniform integral and lipid-anchored membrane proteins partition successfully when membrane domain-specific peptides are optogenetically recruited to them. We propose "dynamic partitioning" as a new mechanism that can account for large-scale compartmentalization of a wide array of lipid-anchored and integral membrane proteins during various physiological processes where membrane polarizes.

Whole-genome screens reveal regulators of differentiation state and context-dependent migration in human neutrophils

Neutrophils are the most abundant leukocyte in humans and provide a critical early line of defense as part of our innate immune system. We perform a comprehensive, genome-wide assessment of the molecular factors critical to proliferation, differentiation, and cell migration in a neutrophil-like cell line. Through the development of multiple migration screen strategies, we specifically probe directed (chemotaxis), undirected (chemokinesis), and 3D amoeboid cell migration in these fast-moving cells. We identify a role for mTORC1 signaling in cell differentiation, which influences neutrophil abundance, survival, and migratory behavior. Across our individual migration screens, we identify genes involved in adhesion-dependent and adhesion-independent cell migration, protein trafficking, and regulation of the actomyosin cytoskeleton. This genome-wide screening strategy, therefore, provides an invaluable approach to the study of neutrophils and provides a resource that will inform future studies of cell migration in these and other rapidly migrating cells.

Local negative feedback of Rac activity at the leading edge underlies a pilot pseudopod-like program for amoeboid cell guidance

To migrate efficiently, neutrophils must polarize their cytoskeletal regulators along a single axis of motion. This polarization process is thought to be mediated through local positive feedback that amplifies leading edge signals and global negative feedback that enables sites of positive feedback to compete for dominance. Though this two-component model efficiently establishes cell polarity, it has potential limitations, including a tendency to "lock" onto a particular direction, limiting the ability of cells to reorient. We use spatially defined optogenetic control of a leading edge organizer (PI3K) to probe how neutrophil-like HL-60 cells balance "decisiveness" needed to polarize in a single direction with the flexibility needed to respond to new cues. Underlying this balancing act is a local Rac inhibition process that destabilizes the leading edge to promote exploration. We show that this local inhibition enables cells to process input signal dynamics, linking front stability and orientation to local temporal increases in input signals.

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.

Actuation of single downstream nodes in growth factor network steers immune cell migration

Ras signaling is typically associated with cell growth, but not direct regulation of motility or polarity. By optogenetically targeting different nodes in the Ras/PI3K/Akt network in differentiated human HL-60 neutrophils, we abruptly altered protrusive activity, bypassing the chemoattractant receptor/G-protein network. First, global recruitment of active KRas4B/HRas isoforms or a RasGEF, RasGRP4, immediately increased spreading and random motility. Second, activating Ras at the cell rear generated new protrusions, reversed pre-existing polarity, and steered sustained migration in neutrophils or murine RAW 264.7 macrophages. Third, recruiting a RasGAP, RASAL3, to cell fronts extinguished protrusions and changed migration direction. Remarkably, persistent RASAL3 recruitment at stable fronts abrogated directed migration in three different chemoattractant gradients. Fourth, local recruitment of the Ras-mTORC2 effector, Akt, in neutrophils or Dictyostelium amoebae generated new protrusions and rearranged pre-existing polarity. Overall, these optogenetic effects were mTORC2-dependent but relatively independent of PI3K. Thus, receptor-independent, local activations of classical growth-control pathways directly control actin assembly, cell shape, and migration modes.

Optogenetic modulation of guanine nucleotide exchange factors of Ras superfamily proteins directly controls cell shape and movement

In this article, we provide detailed protocols on using optogenetic dimerizers to acutely perturb activities of guanine nucleotide exchange factors (GEFs) specific to Ras, Rac or Rho small GTPases of the migratory networks in various mammalian and amoeba cell lines. These GEFs are crucial components of signal transduction networks which link upstream G-protein coupled receptors to downstream cytoskeletal components and help cells migrate through their dynamic microenvironment. Conventional approaches to perturb and examine these signaling and cytoskeletal networks, such as gene knockout or overexpression, are protracted which allows networks to readjust through gene expression changes. Moreover, these tools lack spatial resolution to probe the effects of local network activations. To overcome these challenges, blue light-inducible cryptochrome- and LOV domain-based dimerization systems have been recently developed to control signaling or cytoskeletal events in a spatiotemporally precise manner. We illustrate that, within minutes of global membrane recruitment of full-length GEFs or their catalytic domains only, widespread increases or decreases in F-actin rich protrusions and cell size occur, depending on the particular node in the networks targeted. Additionally, we demonstrate localized GEF recruitment as a robust assay system to study local network activation-driven changes in polarity and directed migration. Altogether, these optical tools confirmed GEFs of Ras superfamily GTPases as regulators of cell shape, actin dynamics, and polarity. Furthermore, this optogenetic toolbox may be exploited in perturbing complex signaling interactions in varied physiological contexts including mammalian embryogenesis.

Leading edge competition promotes context-dependent responses to receptor inputs to resolve directional dilemmas in neutrophil migration

Maintaining persistent migration in complex environments is critical for neutrophils to reach infection sites. Neutrophils avoid getting trapped, even when obstacles split their front into multiple leading edges. How they re-establish polarity to move productively while incorporating receptor inputs under such conditions remains unclear. Here, we challenge chemotaxing HL60 neutrophil-like cells with symmetric bifurcating microfluidic channels to probe cell-intrinsic processes during the resolution of competing fronts. Using supervised statistical learning, we demonstrate that cells commit to one leading edge late in the process, rather than amplifying structural asymmetries or early fluctuations. Using optogenetic tools, we show that receptor inputs only bias the decision similarly late, once mechanical stretching begins to weaken each front. Finally, a retracting edge commits to retraction, with ROCK limiting sensitivity to receptor inputs until the retraction completes. Collectively, our results suggest that cell edges locally adopt highly stable protrusion/retraction programs that are modulated by mechanical feedback.

Cell size and actin architecture determine force generation in optogenetically activated cells

Adherent cells use actomyosin contractility to generate mechanical force and to sense the physical properties of their environment, with dramatic consequences for migration, division, differentiation, and fate. However, the organization of the actomyosin system within cells is highly variable, with its assembly and function being controlled by small GTPases from the Rho family. To understand better how activation of these regulators translates into cell-scale force generation in the context of different physical environments, here we combine recent advances in non-neuronal optogenetics with micropatterning and traction force microscopy on soft elastic substrates. We find that, after whole-cell RhoA activation by the CRY2/CIBN optogenetic system with a short pulse of 100 ms, single cells contract on a minute timescale in proportion to their original traction force, before returning to their original tension setpoint with near perfect precision, on a longer timescale of several minutes. To decouple the biochemical and mechanical elements of this response, we introduce a mathematical model that is parametrized by fits to the dynamics of the substrate deformation energy. We find that the RhoA response builds up quickly on a timescale of 20 s, but decays slowly on a timescale of 50 s. The larger the cells and the more polarized their actin cytoskeleton, the more substrate deformation energy is generated. RhoA activation starts to saturate if optogenetic pulse length exceeds 50 ms, revealing the intrinsic limits of biochemical activation. Together our results suggest that adherent cells establish tensional homeostasis by the RhoA system, but that the setpoint and the dynamics around it are strongly determined by cell size and the architecture of the actin cytoskeleton, which both are controlled by the extracellular environment.

A dynamic partitioning mechanism polarizes membrane protein distribution

The plasma membrane is widely regarded as the hub of the signal transduction network activities that drives numerous physiological responses, including cell polarity and migration. Yet, the symmetry breaking process in the membrane, that leads to dynamic compartmentalization of different proteins, remains poorly understood. Using multimodal live-cell imaging, here we first show that multiple endogenous and synthetic lipid-anchored proteins, despite maintaining stable tight association with the inner leaflet of the plasma membrane, were unexpectedly depleted from the membrane domains where the signaling network was spontaneously activated such as in the new protrusions as well as within the propagating ventral waves. Although their asymmetric patterns resembled those of standard peripheral "back" proteins such as PTEN, unlike the latter, these lipidated proteins did not dissociate from the membrane upon global receptor activation. Our experiments not only discounted the possibility of recurrent reversible translocation from membrane to cytosol as it occurs for weakly bound peripheral membrane proteins, but also ruled out the necessity of directed vesicular trafficking and cytoskeletal supramolecular structure-based restrictions in driving these dynamic symmetry breaking events. Selective photoconversion-based protein tracking assays suggested that these asymmetric patterns instead originate from the inherent ability of these membrane proteins to "dynamically partition" into distinct domains within the plane of the membrane. Consistently, single-molecule measurements showed that these lipid-anchored molecules have substantially dissimilar diffusion profiles in different regions of the membrane. When these profiles were incorporated into an excitable network-based stochastic reaction-diffusion model of the system, simulations revealed that our proposed "dynamic partitioning" mechanism is sufficient to give rise to familiar asymmetric propagating wave patterns. Moreover, we demonstrated that normally uniform integral and lipid-anchored membrane proteins in Dictyostelium and mammalian neutrophil cells can be induced to partition spatiotemporally to form polarized patterns, by optogenetically recruiting membrane domain-specific peptides to these proteins. Together, our results indicate "dynamic partitioning" as a new mechanism of plasma membrane organization, that can account for large-scale compartmentalization of a wide array of lipid-anchored and integral membrane proteins in different physiological processes.

Cell Repolarization: A Bifurcation Study of Spatio-Temporal Perturbations of Polar Cells

The intrinsic polarity of migrating cells is regulated by spatial distributions of protein activity. Those proteins (Rho-family GTPases, such as Rac and Rho) redistribute in response to stimuli, determining the cell front and back. Reaction-diffusion equations with mass conservation and positive feedback have been used to explain initial polarization of a cell. However, the sensitivity of a polar cell to a reversal stimulus has not yet been fully understood. We carry out a PDE bifurcation analysis of two polarity models to investigate routes to repolarization: (1) a single-GTPase ("wave-pinning") model and (2) a mutually antagonistic Rac-Rho model. We find distinct routes to reversal in (1) vs. (2). We show numerical simulations of full PDE solutions for the RD equations, demonstrating agreement with predictions of the bifurcation results. Finally, we show that simulations of the polarity models in deforming 1D model cells are consistent with biological experiments.

Quantifying the Probing and Selection of Microenvironmental Pores by Motile Immune Cells

Immune cells are constantly on the move through multicellular organisms to explore and respond to pathogens and other harmful insults. While moving, immune cells efficiently traverse microenvironments composed of tissue cells and extracellular fibers, which together form complex environments of various porosity, stiffness, topography, and chemical composition. In this protocol we describe experimental procedures to investigate immune cell migration through microenvironments of heterogeneous porosity. In particular, we describe micro-channels, micro-pillars, and collagen networks as cell migration paths with alternative pore size choices. Employing micro-channels or micro-pillars that divide at junctions into alternative paths with initially differentially sized pores allows us to precisely (1) measure the cellular translocation time through these porous path junctions, (2) quantify the cellular preference for individual pore sizes, and (3) image cellular components like the nucleus and the cytoskeleton. This reductionistic experimental setup thus can elucidate how immune cells perform decisions in complex microenvironments of various porosity like the interstitium. The setup further allows investigation of the underlying forces of cellular squeezing and the consequences of cellular deformation on the integrity of the cell and its organelles. As a complementary approach that does not require any micro-engineering expertise, we describe the usage of three-dimensional collagen networks with different pore sizes. Whereas we here focus on dendritic cells as a model for motile immune cells, the described protocols are versatile as they are also applicable for other immune cell types like neutrophils and non-immune cell types such as mesenchymal and cancer cells. In summary, we here describe protocols to identify the mechanisms and principles of cellular probing, decision making, and squeezing during cellular movement through microenvironments of heterogeneous porosity. © 2022 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Immune cell migration in micro-channels and micro-pillars with defined pore sizes Support Protocol 1: Epoxy replica of generated and/or published micro-structures Support Protocol 2: Dendritic cell differentiation Basic Protocol 2: Immune cell migration in 3D collagen networks of variable pore sizes.

Optogenetic control of receptors reveals distinct roles for actin- and Cdc42-dependent negative signals in chemotactic signal processing

During chemotaxis, neutrophils use cell surface G Protein Coupled Receptors to detect chemoattractant gradients. The downstream signaling system is wired with multiple feedback loops that amplify weak inputs and promote spatial separation of cell front and rear activities. Positive feedback could promote rapid signal spreading, yet information from the receptors is transmitted with high spatial fidelity, enabling detection of small differences in chemoattractant concentration across the cell. How the signal transduction network achieves signal amplification while preserving spatial information remains unclear. The GTPase Cdc42 is a cell-front polarity coordinator that is predictive of cell turning, suggesting an important role in spatial processing. Here we directly measure information flow from receptors to Cdc42 by pairing zebrafish parapinopsina, an optogenetic G Protein Coupled Receptor with reversible ON/OFF control, with a spectrally compatible red/far red Cdc42 Fluorescence Resonance Energy Transfer biosensor. Using this toolkit, we show that positive and negative signals downstream of G proteins shape a rapid, dose-dependent Cdc42 response. Furthermore, F-actin and Cdc42 itself provide two distinct negative signals that limit the duration and spatial spread of Cdc42 activation, maintaining output signals local to the originating receptors.