WASp triggers mechanosensitive actin patches to facilitate immune cell migration in dense tissues

Actin-driven nanotopography promotes stable integrin adhesion formation in developing tissue

Morphogenesis requires building stable macromolecular structures from highly dynamic proteins. Muscles are anchored by long-lasting integrin adhesions to resist contractile force. However, the mechanisms governing integrin diffusion, immobilization, and activation within developing tissues remain elusive. Here, we show that actin polymerization-driven membrane protrusions form nanotopographies that enable strong adhesion at Drosophila muscle attachment sites (MASs). Super-resolution microscopy reveals that integrins assemble adhesive belts around Arp2/3-dependent actin protrusions, forming invadosome-like structures with membrane nanotopographies. Single protein tracking shows that, during MAS development, integrins become immobile and confined within diffusion traps formed by the membrane nanotopographies. Actin filaments also display restricted motion and confinement, indicating strong mechanical connection with integrins. Using isolated muscle cells, we show that substrate nanotopography, rather than rigidity, drives adhesion maturation by regulating actin protrusion, integrin diffusion and immobilization. These results thus demonstrate that actin-polymerization-driven membrane protrusions are essential for the formation of strong integrin adhesions sites in the developing embryo, and highlight the important contribution of geometry to morphogenesis.

T cells use focal adhesions to pull themselves through confined environments

Immune cells are highly dynamic and able to migrate through environments with diverse biochemical and mechanical compositions. Their migration has classically been defined as amoeboid under the assumption that it is integrin independent. Here, we show that activated primary Th1 T cells require both confinement and extracellular matrix proteins to migrate efficiently. This migration is mediated through small and dynamic focal adhesions that are composed of the same proteins associated with canonical mesenchymal cell focal adhesions, such as integrins, talin, and vinculin. These focal adhesions, furthermore, localize to sites of contractile traction stresses, enabling T cells to pull themselves through confined spaces. Finally, we show that Th1 T cells preferentially follow tracks of other T cells, suggesting that these adhesions modify the extracellular matrix to provide additional environmental guidance cues. These results demonstrate not only that the boundaries between amoeboid and mesenchymal migration modes are ambiguous, but that integrin-mediated focal adhesions play a key role in T cell motility.

Alveolar macrophage function is impaired following inhalation of berry e-cigarette vapor

In the lower respiratory tract, the alveolar spaces are divided from the bloodstream and the external environment by only a few microns of interstitial tissue. Alveolar macrophages (AMs) defend this delicate mucosal surface from invading infections by regularly patrolling the site. AMs have three behavior modalities to achieve this goal: extending cell protrusions to probe and sample surrounding areas, squeezing the whole cell body between alveoli, and patrolling by moving the cell body around each alveolus. In this study, we found Rho GTPase, cell division control protein 42 (CDC42) expression significantly decreased after berry-flavored e-cigarette (e-cig) exposure. This shifted AM behavior from squeezing to probing. Changes in AM behavior led to a reduction in the clearance of inhaled bacteria, Pseudomonas aeruginosa. These findings shed light on pathways involved in AM migration and highlight the harmful impact of e-cig vaping on AM function.

Hem1 inborn errors of immunity: waving goodbye to coordinated immunity in mice and humans

Inborn errors of immunity (IEI) are a group of diseases in humans that typically present as increased susceptibility to infections, autoimmunity, hyperinflammation, allergy, and in some cases malignancy. Among newly identified genes linked to IEIs include 3 independent reports of 9 individuals from 7 independent kindreds with severe primary immunodeficiency disease (PID) and autoimmunity due to loss-of-function mutations in the NCKAP1L gene encoding Hematopoietic protein 1 (HEM1). HEM1 is a hematopoietic cell specific component of the WASp family verprolin homologous (WAVE) regulatory complex (WRC), which acts downstream of multiple immune receptors to stimulate actin nucleation and polymerization of filamentous actin (F-actin). The polymerization and branching of F-actin is critical for creating force-generating cytoskeletal structures which drive most active cellular processes including migration, adhesion, immune synapse formation, and phagocytosis. Branched actin networks at the cell cortex have also been implicated in acting as a barrier to regulate inappropriate vesicle (e.g. cytokine) secretion and spontaneous antigen receptor crosslinking. Given the importance of the actin cytoskeleton in most or all hematopoietic cells, it is not surprising that HEM1 deficient children present with a complex clinical picture that involves overlapping features of immunodeficiency and autoimmunity. In this review, we will provide an overview of what is known about the molecular and cellular functions of HEM1 and the WRC in immune and other cells. We will describe the common clinicopathological features and immunophenotypes of HEM1 deficiency in humans and provide detailed comparative descriptions of what has been learned about Hem1 disruption using constitutive and immune cell-specific mouse knockout models. Finally, we discuss future perspectives and important areas for investigation regarding HEM1 and the WRC.

Cell shape sensing licenses dendritic cells for homeostatic migration to lymph nodes

Immune cells experience large cell shape changes during environmental patrolling because of the physical constraints that they encounter while migrating through tissues. These cells can adapt to such deformation events using dedicated shape-sensing pathways. However, how shape sensing affects immune cell function is mostly unknown. Here, we identify a shape-sensing mechanism that increases the expression of the chemokine receptor CCR7 and guides dendritic cell migration from peripheral tissues to lymph nodes at steady state. This mechanism relies on the lipid metabolism enzyme cPLA2, requires nuclear envelope tensioning and is finely tuned by the ARP2/3 actin nucleation complex. We also show that this shape-sensing axis reprograms dendritic cell transcription by activating an IKKβ-NF-κB-dependent pathway known to control their tolerogenic potential. These results indicate that cell shape changes experienced by immune cells can define their migratory behavior and immunoregulatory properties and reveal a contribution of the physical properties of tissues to adaptive immunity.

Membrane curvature catalyzes actin nucleation through nano-scale condensation of N-WASP-FBP17

Actin remodeling is spatiotemporally regulated by surface topographical cues on the membrane for signaling across diverse biological processes. Yet, the mechanism dynamic membrane curvature prompts quick actin cytoskeletal changes in signaling remain elusive. Leveraging the precision of nanolithography to control membrane curvature, we reconstructed catalytic reactions from the detection of nano-scale curvature by sensing molecules to the initiation of actin polymerization, which is challenging to study quantitatively in living cells. We show that this process occurs via topographical signal-triggered condensation and activation of the actin nucleation-promoting factor (NPF), Neuronal Wiskott-Aldrich Syndrome protein (N-WASP), which is orchestrated by curvature-sensing BAR-domain protein FBP17. Such N-WASP activation is fine-tuned by optimizing FBP17 to N-WASP stoichiometry over different curvature radii, allowing a curvature-guided macromolecular assembly pattern for polymerizing actin network locally. Our findings shed light on the intricate relationship between changes in curvature and actin remodeling via spatiotemporal regulation of NPF/BAR complex condensation.

Atypical cofilin signaling drives dendritic cell migration through the extracellular matrix via nuclear deformation

To mount an adaptive immune response, dendritic cells must migrate to lymph nodes to present antigens to T cells. Critical to 3D migration is the nucleus, which is the size-limiting barrier for migration through the extracellular matrix. Here, we show that inflammatory activation of dendritic cells leads to the nucleus becoming spherically deformed and enables dendritic cells to overcome the typical 2- to 3-μm diameter limit for 3D migration through gaps in the extracellular matrix. We show that the nuclear shape change is partially attained through reduced cell adhesion, whereas improved 3D migration is achieved through reprogramming of the actin cytoskeleton. Specifically, our data point to a model whereby the phosphorylation of cofilin-1 at serine 41 drives the assembly of a cofilin-actomyosin ring proximal to the nucleus and enhances migration through 3D collagen gels. In summary, these data describe signaling events through which dendritic cells deform their nucleus and enhance their migratory capacity.

Mechanosensing regulates tissue repair program in macrophages

Tissue-resident macrophages play important roles in tissue homeostasis and repair. However, how macrophages monitor and maintain tissue integrity is not well understood. The extracellular matrix (ECM) is a key structural and organizational component of all tissues. Here, we find that macrophages sense the mechanical properties of the ECM to regulate a specific tissue repair program. We show that macrophage mechanosensing is mediated by cytoskeletal remodeling and can be performed in three-dimensional environments through a noncanonical, integrin-independent mechanism analogous to amoeboid migration. We find that these cytoskeletal dynamics also integrate biochemical signaling by colony-stimulating factor 1 and ultimately regulate chromatin accessibility to control the mechanosensitive gene expression program. This study identifies an "amoeboid" mode of ECM mechanosensing through which macrophages may regulate tissue repair and fibrosis.

Evidence and therapeutic implications of biomechanically regulated immunosurveillance in cancer and other diseases

Disease progression is usually accompanied by changes in the biochemical composition of cells and tissues and their biophysical properties. For instance, hallmarks of cancer include the stiffening of tissues caused by extracellular matrix remodelling and the softening of individual cancer cells. In this context, accumulating evidence has shown that immune cells sense and respond to mechanical signals from the environment. However, the mechanisms regulating these mechanical aspects of immune surveillance remain partially understood. The growing appreciation for the 'mechano-immunology' field has urged researchers to investigate how immune cells sense and respond to mechanical cues in various disease settings, paving the way for the development of novel engineering strategies that aim at mechanically modulating and potentiating immune cells for enhanced immunotherapies. Recent pioneer developments in this direction have laid the foundations for leveraging 'mechanical immunoengineering' strategies to treat various diseases. This Review first outlines the mechanical changes occurring during pathological progression in several diseases, including cancer, fibrosis and infection. We next highlight the mechanosensitive nature of immune cells and how mechanical forces govern the immune responses in different diseases. Finally, we discuss how targeting the biomechanical features of the disease milieu and immune cells is a promising strategy for manipulating therapeutic outcomes.

Medioapical contractile pulses coordinated between cells regulate Drosophila eye morphogenesis

Lattice cells (LCs) in the developing Drosophila retina change shape before attaining final form. Previously, we showed that repeated contraction and expansion of apical cell contacts affect these dynamics. Here, we describe another factor, the assembly of a Rho1-dependent medioapical actomyosin ring formed by nodes linked by filaments that contract the apical cell area. Cell area contraction alternates with relaxation, generating pulsatile changes in cell area that exert force on neighboring LCs. Moreover, Rho1 signaling is sensitive to mechanical changes, becoming active when tension decreases and cells expand, while the negative regulator RhoGAP71E accumulates when tension increases and cells contract. This results in cycles of cell area contraction and relaxation that are reciprocally synchronized between adjacent LCs. Thus, mechanically sensitive Rho1 signaling controls pulsatile medioapical actomyosin contraction and coordinates cell behavior across the epithelium. Disrupting the kinetics of pulsing can lead to developmental errors, suggesting this process controls cell shape and tissue integrity during epithelial morphogenesis of the retina.

Extracellular matrix dynamics: A key regulator of cell migration across length-scales and systems

The interactions between cells and their surrounding extracellular matrix (ECM) are dynamic and play critical roles in cell migration during development, health, and diseases. Recent advances have highlighted the complexity and diversity of ECM compositions, or "matrisomes", of tissues resulting in ECMs of different physical, mechanical, and biochemical properties. Investigating the effects of these properties on cell-ECM interactions in the context of cell migration have led to a better understanding of the principles underlying tissue morphogenesis, wound healing, immune response, or cancer metastasis. These new insights into the interplay between ECM dynamics and cell migration can lead to the identification of unique opportunities for therapeutic interventions.

Mechanobiology of myeloid cells

Tissue-resident myeloid cells sense and transduce mechanical signals such as stiffness, stretch and compression. In the past two years, our understanding of the mechanosensitive signalling pathways in myeloid cells has significantly expanded. Moreover, it is increasingly clear which mechanical signals induce myeloid cells towards a pro- or anti-inflammatory phenotype. This is especially relevant in the context of altered matrix mechanics in immune-related pathologies or in the response to implanted biomaterials. A detailed understanding of myeloid cell mechanosensing may eventually lead to more effective cell-based immunotherapies for cancer, the development of mechanically inspired therapies to target fibrosis, and the engineering of safer implants. This review covers these recent advances in the emerging field of mechanoimmunology of myeloid cells.

Dynamic Multiplex Tissue Imaging in Inflammation Research

Inflammation is a highly dynamic process with immune cells that continuously interact with each other and parenchymal components as they migrate through tissue. The dynamic cellular responses and interaction patterns are a function of the complex tissue environment that cannot be fully reconstructed ex vivo, making it necessary to assess cell dynamics and changing spatial patterning in vivo. These dynamics often play out deep within tissues, requiring the optical focus to be placed far below the surface of an opaque organ. With the emergence of commercially available two-photon excitation lasers that can be combined with existing imaging systems, new avenues for imaging deep tissues over long periods of time have become available. We discuss a selected subset of studies illustrating how two-photon microscopy (2PM) has helped to relate the dynamics of immune cells to their in situ function and to understand the molecular patterns that govern their behavior in vivo. We also review some key practical aspects of 2PM methods and point out issues that can confound the results, so that readers can better evaluate the reliability of conclusions drawn using this technology.

Guidelines for mouse and human DC functional assays

This article is part of the Dendritic Cell Guidelines article series, which provides a collection of state-of-the-art protocols for the preparation, phenotype analysis by flow cytometry, generation, fluorescence microscopy, and functional characterization of mouse and human dendritic cells (DC) from lymphoid organs and various non-lymphoid tissues. Recent studies have provided evidence for an increasing number of phenotypically distinct conventional DC (cDC) subsets that on one hand exhibit a certain functional plasticity, but on the other hand are characterized by their tissue- and context-dependent functional specialization. Here, we describe a selection of assays for the functional characterization of mouse and human cDC. The first two protocols illustrate analysis of cDC endocytosis and metabolism, followed by guidelines for transcriptomic and proteomic characterization of cDC populations. Then, a larger group of assays describes the characterization of cDC migration in vitro, ex vivo, and in vivo. The final guidelines measure cDC inflammasome and antigen (cross)-presentation activity. While all protocols were written by experienced scientists who routinely use them in their work, this article was also peer-reviewed by leading experts and approved by all co-authors, making it an essential resource for basic and clinical DC immunologists.

A mechanosensitive caveolae-invadosome interplay drives matrix remodelling for cancer cell invasion

Invadosomes and caveolae are mechanosensitive structures that are implicated in metastasis. Here, we describe a unique juxtaposition of caveola clusters and matrix degradative invadosomes at contact sites between the plasma membrane of cancer cells and constricting fibrils both in 2D and 3D type I collagen matrix environments. Preferential association between caveolae and straight segments of the fibrils, and between invadosomes and bent segments of the fibrils, was observed along with matrix remodelling. Caveola recruitment precedes and is required for invadosome formation and activity. Reciprocally, invadosome disruption results in the accumulation of fibril-associated caveolae. Moreover, caveolae and the collagen receptor β1 integrin co-localize at contact sites with the fibrils, and integrins control caveola recruitment to fibrils. In turn, caveolae mediate the clearance of β1 integrin and collagen uptake in an invadosome-dependent and collagen-cleavage-dependent mechanism. Our data reveal a reciprocal interplay between caveolae and invadosomes that coordinates adhesion to and proteolytic remodelling of confining fibrils to support tumour cell dissemination.

WAVE2 Regulates Actin-Dependent Processes Induced by the B Cell Antigen Receptor and Integrins

B cell antigen receptor (BCR) signaling induces actin cytoskeleton remodeling by stimulating actin severing, actin polymerization, and the nucleation of branched actin networks via the Arp2/3 complex. This enables B cells to spread on antigen-bearing surfaces in order to increase antigen encounters and to form an immune synapse (IS) when interacting with antigen-presenting cells (APCs). Although the WASp, N-WASp, and WAVE nucleation-promoting factors activate the Arp2/3 complex, the role of WAVE2 in B cells has not been directly assessed. We now show that both WAVE2 and the Arp2/3 complex localize to the peripheral ring of branched F-actin when B cells spread on immobilized anti-Ig antibodies. The siRNA-mediated depletion of WAVE2 reduced and delayed B cell spreading on immobilized anti-Ig, and this was associated with a thinner peripheral F-actin ring and reduced actin retrograde flow compared to control cells. Depleting WAVE2 also impaired integrin-mediated B cell spreading on fibronectin and the LFA-1-induced formation of actomyosin arcs. Actin retrograde flow amplifies BCR signaling at the IS, and we found that depleting WAVE2 reduced microcluster-based BCR signaling and signal amplification at the IS, as well as B cell activation in response to antigen-bearing cells. Hence, WAVE2 contributes to multiple actin-dependent processes in B lymphocytes.

T cells Use Focal Adhesions to Pull Themselves Through Confined Environments

Immune cells are highly dynamic and able to migrate through environments with diverse biochemical and mechanical composition. Their migration has classically been defined as amoeboid under the assumption that it is integrin-independent. Here we show that activated primary Th1 T cells require both confinement and extracellular matrix protein to migrate efficiently. This migration is mediated through small and dynamic focal adhesions that are composed of the same proteins associated with canonical mesenchymal focal adhesions, such as integrins, talin, and vinculin. These focal adhesions, furthermore, localize to sites of contractile traction stresses, enabling T cells to pull themselves through confined spaces. Finally, we show that Th1 T cell preferentially follows tracks of other T cells, suggesting that these adhesions are modifying the extracellular matrix to provide additional environmental guidance cues. These results demonstrate not only that the boundaries between amoeboid and mesenchymal migration modes are ambiguous, but that integrin-mediated adhesions play a key role in T cell motility.

Degenerative and Regenerative Actin Cytoskeleton Rearrangements, Cell Death, and Paradoxical Proliferation in the Gills of Pearl Gourami (Trichogaster leerii) Exposed to Suspended Soot Microparticles

The effect is studied of water-suspended soot microparticles on the actin cytoskeleton, apoptosis, and proliferation in the gill epithelium of pearl gourami. To this end, the fish are kept in aquariums with 0.005 g/L of soot for 5 and 14 days. Laser confocal microscopy is used to find that at the analyzed times of exposure to the pollutant zones appear in the gill epithelium, where the actin framework of adhesion belts dissociates and F-actin either forms clumps or concentrates perinuclearly. It is shown that the exposure to soot microparticles enhances apoptosis. On day 5, suppression of the proliferation of cells occurs, but the proliferation increases to the control values on day 14. Such a paradoxical increase in proliferation may be a compensatory process, maintaining the necessary level of gill function under the exposure to toxic soot. This process may occur until the gills' recovery reserve is exhausted. In general, soot microparticles cause profound changes in the actin cytoskeleton in gill cells, greatly enhance cell death, and influence cell proliferation as described. Together, these processes may cause gill dysfunction and affect the viability of fish.

Synchronization in collectively moving inanimate and living active matter

Whether one considers swarming insects, flocking birds, or bacterial colonies, collective motion arises from the coordination of individuals and entails the adjustment of their respective velocities. In particular, in close confinements, such as those encountered by dense cell populations during development or regeneration, collective migration can only arise coordinately. Yet, how individuals unify their velocities is often not understood. Focusing on a finite number of cells in circular confinements, we identify waves of polymerizing actin that function as a pacemaker governing the speed of individual cells. We show that the onset of collective motion coincides with the synchronization of the wave nucleation frequencies across the population. Employing a simpler and more readily accessible mechanical model system of active spheres, we identify the synchronization of the individuals' internal oscillators as one of the essential requirements to reach the corresponding collective state. The mechanical 'toy' experiment illustrates that the global synchronous state is achieved by nearest neighbor coupling. We suggest by analogy that local coupling and the synchronization of actin waves are essential for the emergent, self-organized motion of cell collectives.

Cell-matrix and cell-cell interaction mechanics in guiding migration

Physical properties of tissue are increasingly recognised as major regulatory cues affecting cell behaviours, particularly cell migration. While these properties of the extracellular matrix have been extensively discussed, the contribution from the cellular components that make up the tissue are still poorly appreciated. In this mini-review, we will discuss two major physical components: stiffness and topology with a stronger focus on cell-cell interactions and how these can impact cell migration.

The extracellular matrix - immune microenvironment crosstalk in cancer therapy: Challenges and opportunities

Targeting the tumour immune microenvironment (TIME) by cancer immunotherapy has led to improved patient outcomes. However, response to these treatments is heterogeneous and cancer-type dependant. The therapeutic activity of classical cancer therapies such as chemotherapy, radiotherapy, and surgical oncology is modulated by alterations of the TIME. A major regulator of immune cell function and resistance to both immune and classical therapies is the extracellular matrix (ECM). Concurrently, cancer therapies reshape the TIME as well as the ECM, causing both pro- and anti-tumour responses. Accordingly, the TIME-ECM crosstalk presents attractive opportunities to improve therapy outcomes. Here, we review the molecular crosstalk between the TIME and the ECM in cancer and its implications in cancer progression and clinical intervention. Additionally, we discuss examples and future directions of ECM and TIME co-targeting in combination with oncological therapies including surgery, chemotherapy, and radiotherapy.

Mechanosensory feedback loops during chronic inflammation

Epithelial tissues are crucial to maintaining healthy organization and compartmentalization in various organs and act as a first line of defense against infection in barrier organs such as the skin, lungs and intestine. Disruption or injury to these barriers can lead to infiltration of resident or foreign microbes, initiating local inflammation. One often overlooked aspect of this response is local changes in tissue mechanics during inflammation. In this mini-review, we summarize known molecular mechanisms linking disruption of epithelial barrier function to mechanical changes in epithelial tissues. We consider direct mechanisms, such as changes in the secretion of extracellular matrix (ECM)-modulating enzymes by immune cells as well as indirect mechanisms including local activation of fibroblasts. We discuss how these mechanical changes can modulate local immune cell activity and inflammation and perturb epithelial homeostasis, further dysregulating epithelial barrier function. We propose that this two-way relationship between loss of barrier function and altered tissue mechanics can lead to a positive feedback loop that further perpetuates inflammation. We discuss this cycle in the context of several chronic inflammatory diseases, including inflammatory bowel disease (IBD), liver disease and cancer, and we present the modulation of tissue mechanics as a new framework for combating chronic inflammation.

Cell-extracellular matrix mechanotransduction in 3D

Mechanical properties of extracellular matrices (ECMs) regulate essential cell behaviours, including differentiation, migration and proliferation, through mechanotransduction. Studies of cell-ECM mechanotransduction have largely focused on cells cultured in 2D, on top of elastic substrates with a range of stiffnesses. However, cells often interact with ECMs in vivo in a 3D context, and cell-ECM interactions and mechanisms of mechanotransduction in 3D can differ from those in 2D. The ECM exhibits various structural features as well as complex mechanical properties. In 3D, mechanical confinement by the surrounding ECM restricts changes in cell volume and cell shape but allows cells to generate force on the matrix by extending protrusions and regulating cell volume as well as through actomyosin-based contractility. Furthermore, cell-matrix interactions are dynamic owing to matrix remodelling. Accordingly, ECM stiffness, viscoelasticity and degradability often play a critical role in regulating cell behaviours in 3D. Mechanisms of 3D mechanotransduction include traditional integrin-mediated pathways that sense mechanical properties and more recently described mechanosensitive ion channel-mediated pathways that sense 3D confinement, with both converging on the nucleus for downstream control of transcription and phenotype. Mechanotransduction is involved in tissues from development to cancer and is being increasingly harnessed towards mechanotherapy. Here we discuss recent progress in our understanding of cell-ECM mechanotransduction in 3D.

Medioapical contractile pulses coordinated between cells regulate Drosophila eye morphogenesis

Lattice cells (LCs) in the developing Drosophila retina constantly move and change shape before attaining final forms. Previously we showed that repeated contraction and expansion of apical cell contacts affect these dynamics. Here we describe a second contributing factor, the assembly of a medioapical actomyosin ring composed of nodes linked by filaments that attract each other, fuse, and contract the LCs' apical area. This medioapical actomyosin network is dependent on Rho1 and its known effectors. Apical cell area contraction alternates with relaxation, generating pulsatile changes in apical cell area. Strikingly, cycles of contraction and relaxation of cell area are reciprocally synchronized between adjacent LCs. Further, in a genetic screen, we identified RhoGEF2 as an activator of these Rho1 functions and RhoGAP71E/C-GAP as an inhibitor. Thus, Rho1 signaling regulates pulsatile medioapical actomyosin contraction exerting force on neighboring cells, coordinating cell behavior across the epithelium. This ultimately serves to control cell shape and maintain tissue integrity during epithelial morphogenesis of the retina.

Negatively curved cellular membranes promote BAIAP2 signaling hub assembly

Plasma membrane deformations are associated with curvature-dependent protein enrichment that contributes to a wide array of cellular functions. While the spatio-temporal protein dynamics at membrane indentations is well characterized, relatively little is known about protein kinetics at outwardly deforming membrane sites. This is in part due to the lack of high throughput approaches to systematically probe the curvature-dependence of protein-membrane interactions. Here, we developed a nanopatterned array for multiplexed analysis of protein dynamics at negatively curved cellular membranes. Taking advantage of this robust and versatile platform, we explored how membrane shape influences the prototypic negative curvature sensing protein BAIAP2 and its effector proteins. We find assembly of multi-protein signaling hubs and increased actin polymerization at outwardly deformed membrane sections, indicative of curvature-dependent BAIAP2 activation. Collectively, this study presents technical and conceptual advancements towards a quantitative understanding of spatio-temporal protein dynamics at negatively curved membranes.

A machine learning approach to discover migration modes and transition dynamics of heterogeneous dendritic cells

Dendritic cell (DC) migration is crucial for mounting immune responses. Immature DCs (imDCs) reportedly sense infections, while mature DCs (mDCs) move quickly to lymph nodes to deliver antigens to T cells. However, their highly heterogeneous and complex innate motility remains elusive. Here, we used an unsupervised machine learning (ML) approach to analyze long-term, two-dimensional migration trajectories of Granulocyte-macrophage colony-stimulating factor (GMCSF)-derived bone marrow-derived DCs (BMDCs). We discovered three migratory modes independent of the cell state: slow-diffusive (SD), slow-persistent (SP), and fast-persistent (FP). Remarkably, imDCs more frequently changed their modes, predominantly following a unicyclic SD→FP→SP→SD transition, whereas mDCs showed no transition directionality. We report that DC migration exhibits a history-dependent mode transition and maturation-dependent motility changes are emergent properties of the dynamic switching of the three migratory modes. Our ML-based investigation provides new insights into studying complex cellular migratory behavior.

Soft topographical patterns trigger a stiffness-dependent cellular response to contact guidance

Topographical patterns are a powerful tool to study directional migration. Grooved substrates have been extensively used as in vitro models of aligned extracellular matrix fibers because they induce cell elongation, alignment, and migration through a phenomenon known as contact guidance. This process, which involves the orientation of focal adhesions, F-actin, and microtubule cytoskeleton along the direction of the grooves, has been primarily studied on hard materials of non-physiological stiffness. But how it unfolds when the stiffness of the grooves varies within the physiological range is less known. Here we show that substrate stiffness modulates the cellular response to topographical contact guidance. We find that for fibroblasts, while focal adhesions and actin respond to topography independently of the stiffness, microtubules show a stiffness-dependent response that regulates contact guidance. On the other hand, both clusters and single breast carcinoma epithelial cells display stiffness-dependent contact guidance, leading to more directional and efficient migration when increasing substrate stiffness. These results suggest that both matrix stiffening and alignment of extracellular matrix fibers cooperate during directional cell migration, and that the outcome differs between cell types depending on how they organize their cytoskeletons.

Understanding immunoactinopathies: A decade of research on WAS gene defects

Immunoactinopathies caused by mutations in actin-related proteins are a growing group of inborn errors of immunity (IEI). Immunoactinopathies are caused by a dysregulated actin cytoskeleton and affect hematopoietic cells especially because of their unique capacity to survey the body for invading pathogens and altered self, such as cancer cells. These cell motility and cell-to-cell interaction properties depend on the dynamic nature of the actin cytoskeleton. Wiskott-Aldrich syndrome (WAS) is the archetypical immunoactinopathy and the first described. WAS is caused by loss-of-function and gain-of-function mutations in the actin regulator WASp, uniquely expressed in hematopoietic cells. Mutations in WAS cause a profound disturbance of actin cytoskeleton regulation of hematopoietic cells. Studies during the last 10 years have shed light on the specific effects on different hematopoietic cells, revealing that they are not affected equally by mutations in the WAS gene. Moreover, the mechanistic understanding of how WASp controls nuclear and cytoplasmatic activities may help to find therapeutic alternatives according to the site of the mutation and clinical phenotypes. In this review, we summarize recent findings that have added to the complexity and increased our understanding of WAS-related diseases and immunoactinopathies.

Deciphering actin remodelling in immune cells through the prism of actin-related inborn errors of immunity

Actin cytoskeleton remodelling drives cell motility, cell to cell contacts, as well as membrane and organelle dynamics. Those cellular activities operate at a particularly high pace in immune cells since these cells migrate through various tissues, interact with multiple cellular partners, ingest microorganisms and secrete effector molecules. The central and multifaceted role of actin cytoskeleton remodelling in sustaining immune cell tasks in humans is highlighted by rare inborn errors of immunity due to mutations in genes encoding proximal and distal actin regulators. In line with the specificity of some of the actin-based processes at work in immune cells, the expression of some of the affected genes, such as WAS, ARPC1B and HEM1 is restricted to the hematopoietic compartment. Exploration of these natural deficiencies highlights the fact that the molecular control of actin remodelling is tuned distinctly in the various subsets of myeloid and lymphoid immune cells and sustains different networks associated with a vast array of specialized tasks. Furthermore, defects in individual actin remodelling proteins are usually associated with partial cellular impairments highlighting the plasticity of actin cytoskeleton remodelling. This review covers the roles of disease-associated actin regulators in promoting the actin-based processes of immune cells. It focuses on the specific molecular function of those regulators across various immune cell subsets and in response to different stimuli. Given the fact that numerous immune-related actin defects have only been characterized recently, we further discuss the challenges lying ahead to decipher the underlying patho-mechanisms.

Coordinated efforts of different actin filament populations are needed for optimal cell wound repair

Cells are subjected to a barrage of daily insults that often lead to their cortices being ripped open and requiring immediate repair. An important component of the cell's repair response is the formation of an actomyosin ring at the wound periphery to mediate its closure. Here we show that inhibition of myosin or the linear actin nucleation factors Diaphanous and/or dishevelled associated activator of morphogenesis results in a disrupted contractile apparatus and delayed wound closure. We also show that the branched actin nucleators WASp and SCAR function nonredundantly as scaffolds to assemble and maintain this contractile actomyosin cable. Removing branched actin leads to the formation of smaller circular actin-myosin structures at the cell cortex and to slow wound closure. Removing linear and branched actin simultaneously results in failed wound closure. Surprisingly, removal of branched actin and myosin results in the formation of parallel linear F-actin filaments that undergo a chiral swirling movement to close the wound, uncovering a new mechanism of cell wound closure. Taken together, we demonstrate the roles of different actin substructures that are required for optimal actomyosin ring formation and the extraordinary resilience of the cell to undergo wound repair when it is unable to form different subsets of these substructures.

Mechanisms and roles of podosomes and invadopodia

Cell invasion into the surrounding extracellular matrix or across tissue boundaries and endothelial barriers occurs in both physiological and pathological scenarios such as immune surveillance or cancer metastasis. Podosomes and invadopodia, collectively called 'invadosomes', are actin-based structures that drive the proteolytic invasion of cells, by forming highly regulated platforms for the localized release of lytic enzymes that degrade the matrix. Recent advances in high-resolution microscopy techniques, in vivo imaging and high-throughput analyses have led to considerable progress in understanding mechanisms of invadosomes, revealing the intricate inner architecture of these structures, as well as their growing repertoire of functions that extends well beyond matrix degradation. In this Review, we discuss the known functions, architecture and regulatory mechanisms of podosomes and invadopodia. In particular, we describe the molecular mechanisms of localized actin turnover and microtubule-based cargo delivery, with a special focus on matrix-lytic enzymes that enable proteolytic invasion. Finally, we point out topics that should become important in the invadosome field in the future.

Engineering Hydrogels for Modulation of Dendritic Cell Function

Dendritic cells (DCs), the most potent antigen-presenting cells, are necessary for the effective activation of naïve T cells. DCs encounter numerous microenvironments with different biophysical properties, such as stiffness and viscoelasticity. Considering the emerging importance of mechanical cues for DC function, it is essential to understand the impacts of these cues on DC function in a physiological or pathological context. Engineered hydrogels have gained interest for the exploration of the impacts of biophysical matrix cues on DC functions, owing to their extracellular-matrix-mimetic properties, such as high water content, a sponge-like pore structure, and tunable mechanical properties. In this review, the introduction of gelation mechanisms of hydrogels is first summarized. Then, recent advances in the substantial effects of developing hydrogels on DC function are highlighted, and the potential molecular mechanisms are subsequently discussed. Finally, persisting questions and future perspectives are presented.

Primordial germ cells adjust their protrusion type while migrating in different tissue contexts in vivo

In both physiological processes and disease contexts, migrating cells have the ability to adapt to conditions in their environment. As an in vivo model for this process, we use zebrafish primordial germ cells that migrate throughout the developing embryo. When migrating within an ectodermal environment, the germ cells form fewer and smaller blebs when compared with their behavior within mesodermal environment. We find that cortical tension of neighboring cells is a parameter that affects blebbing frequency. Interestingly, the change in blebbing activity is accompanied by the formation of more actin-rich protrusions. These alterations in cell behavior that correlate with changes in RhoA activity could allow the cells to maintain dynamic motility parameters, such as migration speed and track straightness, in different settings. In addition, we find that the polarity of the cells can be affected by stiff structures positioned in their migration path This article has an associated 'The people behind the papers' interview.

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.

Motor neurons use push-pull signals to direct vascular remodeling critical for their connectivity

The nervous system requires metabolites and oxygen supplied by the neurovascular network, but this necessitates close apposition of neurons and endothelial cells. We find motor neurons attract vessels with long-range VEGF signaling, but endothelial cells in the axonal pathway are an obstacle for establishing connections with muscles. It is unclear how this paradoxical interference from heterotypic neurovascular contacts is averted. Through a mouse mutagenesis screen, we show that Plexin-D1 receptor is required in endothelial cells for development of neuromuscular connectivity. Motor neurons release Sema3C to elicit short-range repulsion via Plexin-D1, thus displacing endothelial cells that obstruct axon growth. When this signaling pathway is disrupted, epaxial motor neurons are blocked from reaching their muscle targets and concomitantly vascular patterning in the spinal cord is altered. Thus, an integrative system of opposing push-pull cues ensures detrimental axon-endothelial encounters are avoided while enabling vascularization within the nervous system and along peripheral nerves.

Biochemical and mechanical regulation of actin dynamics

Polymerization of actin filaments against membranes produces force for numerous cellular processes, such as migration, morphogenesis, endocytosis, phagocytosis and organelle dynamics. Consequently, aberrant actin cytoskeleton dynamics are linked to various diseases, including cancer, as well as immunological and neurological disorders. Understanding how actin filaments generate forces in cells, how force production is regulated by the interplay between actin-binding proteins and how the actin-regulatory machinery responds to mechanical load are at the heart of many cellular, developmental and pathological processes. During the past few years, our understanding of the mechanisms controlling actin filament assembly and disassembly has evolved substantially. It has also become evident that the activities of key actin-binding proteins are not regulated solely by biochemical signalling pathways, as mechanical regulation is critical for these proteins. Indeed, the architecture and dynamics of the actin cytoskeleton are directly tuned by mechanical load. Here we discuss the general mechanisms by which key actin regulators, often in synergy with each other, control actin filament assembly, disassembly, and monomer recycling. By using an updated view of actin dynamics as a framework, we discuss how the mechanics and geometry of actin networks control actin-binding proteins, and how this translates into force production in endocytosis and mesenchymal cell migration.

Microfluidic Lab-on-a-Chip for Studies of Cell Migration under Spatial Confinement

Understanding cell migration is a key step in unraveling many physiological phenomena and predicting several pathologies, such as cancer metastasis. In particular, confinement has been proven to be a key factor in the cellular migration strategy choice. As our insight in the field improves, new tools are needed in order to empower biologists’ analysis capabilities. In this framework, microfluidic devices have been used to engineer the mechanical and spatial stimuli and to investigate cellular migration response in a more controlled way. In this work, we will review the existing technologies employed in the realization of microfluidic cellular migration assays, namely the soft lithography of PDMS and hydrogels and femtosecond laser micromachining. We will give an overview of the state of the art of these devices, focusing on the different geometrical configurations that have been exploited to study specific aspects of cellular migration. Our scope is to highlight the advantages and possibilities given by each approach and to envisage the future developments in in vitro migration studies under spatial confinement in microfluidic devices.

Actin Retrograde Flow Regulated by the Wiskott–Aldrich Syndrome Protein Drives the Natural Killer Cell Response

Understanding the crosstalk between natural killer (NK) cells and the tumor microenvironment (TME) has enhanced the potential of exploiting the interplay between activation and inhibition of NK cells for immunotherapy. This interaction is crucial for understanding how tumor cells escape NK cell immune surveillance. NK cell dysfunction is regulated by two molecular mechanisms, downregulated activating receptor ligand expression on the tumor cells, and upregulated inhibitory signals delivered to NK cells. Recent studies demonstrated the role of mechanotransduction in modulating NK cell responses in the TME. The immunological synapse represents a functional interface between the NK cell and its target, regulated by Actin Retrograde Flow (ARF), which drives the adhesion molecules and receptors toward the central zone of the immunological synapse (IS). Here, we further characterize the role of ARF in controlling the immune response of NK cells, using CRISPR/cas9-mediated Wiskott–Aldrich Syndrome protein (WASp) gene silencing of NK cells. We demonstrate that WASp regulates ARF velocity, affecting the conformation and function of the key NK inhibitory regulator, SH2-domain containing protein tyrosine phosphatase-1 (SHP-1), and consequently, the NK cell response. Our results demonstrate the potential of modulating the biophysical and intracellular regulation of NK activation as a promising approach for improving immunotherapy.

The Actin Cytoskeleton Responds to Inflammatory Cues and Alters Macrophage Activation

Much remains to be learned about the molecular mechanisms underlying a class of human disorders called actinopathies. These genetic disorders are characterized by loss-of-function mutations in actin-associated proteins that affect immune cells, leading to human immunopathology. However, much remains to be learned about how cytoskeletal dysregulation promotes immunological dysfunction. The current study reveals that the macrophage actin cytoskeleton responds to LPS/IFNγ stimulation in a biphasic manner that involves cellular contraction followed by cellular spreading. Myosin II inhibition by blebbistatin blocks the initial contraction phase and lowers iNOS protein levels and nitric oxide secretion. Conversely, conditional deletion of Arp2/3 complex in macrophages attenuates spreading and increases nitric oxide secretion. However, iNOS transcription is not altered by loss of myosin II or Arp2/3 function, suggesting post-transcriptional regulation of iNOS by the cytoskeleton. Consistent with this idea, proteasome inhibition reverses the effects of blebbistatin and rescues iNOS protein levels. Arp2/3-deficient macrophages demonstrate two additional phenotypes: defective MHCII surface localization, and depressed secretion of the T cell chemokine CCL22. These data suggest that interplay between myosin II and Arp2/3 influences macrophage activity, and potentially impacts adaptive-innate immune coordination. Disrupting this balance could have detrimental impacts, particularly in the context of Arp2/3-associated actinopathies.

Integrin Regulated Autoimmune Disorders: Understanding the Role of Mechanical Force in Autoimmunity

The pathophysiology of autoimmune disorders is multifactorial, where immune cell migration, adhesion, and lymphocyte activation play crucial roles in its progression. These immune processes are majorly regulated by adhesion molecules at cell–extracellular matrix (ECM) and cell–cell junctions. Integrin, a transmembrane focal adhesion protein, plays an indispensable role in these immune cell mechanisms. Notably, integrin is regulated by mechanical force and exhibit bidirectional force transmission from both the ECM and cytosol, regulating the immune processes. Recently, integrin mechanosensitivity has been reported in different immune cell processes; however, the underlying mechanics of these integrin-mediated mechanical processes in autoimmunity still remains elusive. In this review, we have discussed how integrin-mediated mechanotransduction could be a linchpin factor in the causation and progression of autoimmune disorders. We have provided an insight into how tissue stiffness exhibits a positive correlation with the autoimmune diseases’ prevalence. This provides a plausible connection between mechanical load and autoimmunity. Overall, gaining insight into the role of mechanical force in diverse immune cell processes and their dysregulation during autoimmune disorders will open a new horizon to understand this physiological anomaly.

WASP stings into matrix to lead immune cell migration

WASP is a remodeler of the actin cytoskeleton, but its mechanistic contribution to neutrophil migration is unclear. In this issue, Brunetti et al. (2021. J. Cell Biol.https://doi.org/10.1083/jcb.202104046) show that WASP is recruited to substrate-induced membrane deformations near the cell front, where it induces Arp2/3 complex-mediated local actin assembly to direct migration.