Wiskott-Aldrich syndrome protein regulates podosomes in primary human macrophages

[Formation, organization and function of invadosomes in cell motility and tumor invasion]

Invadosome is an umbrella term used to describe a family of cellular structures including podosomes and invadopodia. They serve as contact zones between the cell plasma membrane and extracellular matrix, contributing to matrix remodeling by locally enriched proteolytic enzymes. Invadosomes, which are actin-dependent, are implicated in cellular processes promoting adhesion, migration, and invasion. Invadosomes, which exist in various cell types, play crucial roles in physiological phenomena such as vascularization and bone resorption. Invadosomes are also implicated in pathological processes such as matrix tissue remodeling during metastatic tumor cell invasion. This review summarizes basic information and recent advances about mechanisms underlying podosome and invadopodia formation, their organization and function.

Get a grip: Podosomes as potential players in phagocytosis

Podosomes have been known for several decades as micron-sized, F-actin-rich structures that play a pivotal role in cell migration and invasion, as they are able to mediate both cell-matrix attachment as well as extracellular matrix degradation. Particularly in monocytic cells, podosomes have been shown to fulfill a variety of additional functions such as sensing of substrate rigidity and topography, or cell-cell fusion. Increasing evidence now points to the involvement of podosome-like structures also during phagocytosis by immune cells such as macrophages, dendritic cells, and neutrophils. Here, we compare the different cell models and experimental set ups where "phagocytic podosomes" have been described. We also discuss the composition and architecture of these structures, their potential involvement in mechanosensing and particle disruption, as well as the pros and cons for addressing them as bona fide podosomes.

Monogenic inflammatory bowel disease-genetic variants, functional mechanisms and personalised medicine in clinical practice

Over 100 genes are associated with monogenic forms of inflammatory bowel disease (IBD). These genes affect the epithelial barrier function, innate and adaptive immunity in the intestine, and immune tolerance. We provide an overview of newly discovered monogenic IBD genes and illustrate how a recently proposed taxonomy model can integrate phenotypes and shared pathways. We discuss how functional understanding of genetic disorders and clinical genomics supports personalised medicine for patients with monogenic IBD.

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.

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.

Genome editing for primary immunodeficiencies: A therapeutic perspective on Wiskott-Aldrich syndrome

Primary immunodeficiency diseases (PIDs) are a group of rare inherited disorders affecting the immune system that can be conventionally treated with allogeneic hematopoietic stem cell transplantation and with experimental autologous gene therapy. With both approaches still facing important challenges, gene editing has recently emerged as a potential valuable alternative for the treatment of genetic disorders and within a relatively short period from its initial development, has already entered some landmark clinical trials aimed at tackling several life-threatening diseases. In this review, we discuss the progress made towards the development of gene editing-based therapeutic strategies for PIDs with a special focus on Wiskott - Aldrich syndrome and outline their main challenges as well as future directions with respect to already established treatments.

Actin nano-architecture of phagocytic podosomes

Podosomes are actin-enriched adhesion structures important for multiple cellular processes, including migration, bone remodeling, and phagocytosis. Here, we characterize the structure and organization of phagocytic podosomes using interferometric photoactivated localization microscopy, a super-resolution microscopy technique capable of 15-20 nm resolution, together with structured illumination microscopy and localization-based super-resolution microscopy. Phagocytic podosomes are observed during frustrated phagocytosis, a model in which cells attempt to engulf micropatterned IgG antibodies. For circular patterns, this results in regular arrays of podosomes with well-defined geometry. Using persistent homology, we develop a pipeline for semi-automatic identification and measurement of podosome features. These studies reveal an hourglass shape of the podosome actin core, a protruding knob at the bottom of the core, and two actin networks extending from the core. Additionally, the distributions of paxillin, talin, myosin II, α-actinin, cortactin, and microtubules relative to actin are characterized.

Building the phagocytic cup on an actin scaffold

Cells ingest large particles, such as bacteria, viruses, or apoptotic cells, via the process of phagocytosis, which involves formation of an actin-rich structure known as the phagocytic cup. Phagocytic cup assembly and closure results from a concerted action of phagocytic receptors, regulators of actin polymerization, and myosin motors. Recent studies using advanced imaging approaches and biophysical techniques have revealed new information regarding phagocytic cup architecture, regulation of actin assembly, and the distribution, direction, and magnitude of the forces produced by the cytoskeletal elements that form the cup. These findings provide insights into the mechanisms leading to the assembly, expansion, and closure of phagocytic cups. The new data show that engulfment and internalization of phagocytic targets rely on several distinct yet complementary mechanisms that support the robust uptake of foreign objects and may be precisely tailored to the demands of specific phagocytic pathways.

Wiskott-Aldrich syndrome protein forms nuclear condensates and regulates alternative splicing

The diverse functions of WASP, the deficiency of which causes Wiskott-Aldrich syndrome (WAS), remain poorly defined. We generated three isogenic WAS models using patient induced pluripotent stem cells and genome editing. These models recapitulated WAS phenotypes and revealed that WASP deficiency causes an upregulation of numerous RNA splicing factors and widespread altered splicing. Loss of WASP binding to splicing factor gene promoters frequently leads to aberrant epigenetic activation. WASP interacts with dozens of nuclear speckle constituents and constrains SRSF2 mobility. Using an optogenetic system, we showed that WASP forms phase-separated condensates that encompasses SRSF2, nascent RNA and active Pol II. The role of WASP in gene body condensates is corroborated by ChIPseq and RIPseq. Together our data reveal that WASP is a nexus regulator of RNA splicing that controls the transcription of splicing factors epigenetically and the dynamics of the splicing machinery through liquid-liquid phase separation.

Wasl is crucial to maintain microglial core activities during glioblastoma initiation stages

Microglia actively promotes the growth of high-grade gliomas. Within the glioma microenvironment an amoeboid microglial morphology has been observed, however the underlying causes and the related impact on microglia functions and their tumor promoting activities is unclear. Using the advantages of the larval zebrafish model, we identified the underlying mechanism and show that microglial morphology and functions are already impaired during glioma initiation stages. The presence of pre-neoplastic HRasV12 expressing cells induces an amoeboid morphology of microglia, increases microglial numbers and decreases their motility and phagocytic activity. RNA sequencing analysis revealed lower expression levels of the actin nucleation promoting factor wasla in microglia. Importantly, a microglia specific rescue of wasla expression restores microglial morphology and functions. This results in increased phagocytosis of pre-neoplastic cells and slows down tumor progression. In conclusion, we identified a mechanism that de-activates core microglial functions within the emerging glioma microenvironment. Restoration of this mechanism might provide a way to impair glioma growth.

WASP family proteins: Molecular mechanisms and implications in human disease

Proteins of the Wiskott-Aldrich syndrome protein (WASP) family play a central role in regulating actin cytoskeletal dynamics in a wide range of cellular processes. Genetic mutations or misregulation of these proteins are tightly associated with many diseases. The WASP-family proteins act by transmitting various upstream signals to their conserved WH2-Central-Acidic (WCA) peptide sequence at the C-terminus, which in turn binds to the Arp2/3 complex to stimulate the formation of branched actin networks at membranes. Despite this common feature, the regulatory mechanisms and cellular functions of distinct WASP-family proteins are very different. Here, we summarize and clarify our current understanding of WASP-family proteins and how disruption of their functions is related to human disease.

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.

Essential Oil from Eucalyptus globulus (Labill.) Activates Complement Receptor-Mediated Phagocytosis and Stimulates Podosome Formation in Human Monocyte-Derived Macrophages

Eucalyptus essential oil and its major constituent eucalyptol are extensively employed in the cosmetic, food, and pharmaceutical industries and their clinical use has recently expanded worldwide as an adjuvant in the treatment of infective and inflammatory diseases. We previously demonstrated that essential oil from Eucalyptus globulus (Labill.) (EO) stimulates in vitro the phagocytic activity of human monocyte-derived macrophages and counteracts the myelotoxicity induced by the chemotherapeutic 5-fluorouracil in immunocompetent rats. Here we characterize some mechanistic aspects underlying the immunostimulatory ability exerted by EO on macrophages. The internalization of fluorescent beads, fluorescent zymosan BioParticles, or apoptotic cancer cells was evaluated by confocal microscopy. Pro-inflammatory cytokine and chemokine release was determined by flow cytometry using the BD cytometric bead array. Receptor involvement in EO-stimulated phagocytosis was assessed using complement- or IgG-opsonized zymosan particles. The localization and expression of podosome components was analyzed by confocal microscopy and western blot. The main results demonstrated that: EO-induced activation of a macrophage is ascribable to its major component eucalyptol, as recently demonstrated for other cells of innate immunity; EO implements pathogen internalization and clearance by stimulating the complement receptor-mediated phagocytosis; EO stimulates podosome formation and increases the expression of podosome components. These results confirm that EO extract is a potent activator of innate cell-mediated immunity and thereby increase the scientific evidence supporting an additional property of this plant extract besides the known antiseptic and anti-inflammatory properties.

The Actin Regulators Involved in the Function and Related Diseases of Lymphocytes

Actin is an important cytoskeletal protein involved in signal transduction, cell structure and motility. Actin regulators include actin-monomer-binding proteins, Wiskott-Aldrich syndrome (WAS) family of proteins, nucleation proteins, actin filament polymerases and severing proteins. This group of proteins regulate the dynamic changes in actin assembly/disassembly, thus playing an important role in cell motility, intracellular transport, cell division and other basic cellular activities. Lymphocytes are important components of the human immune system, consisting of T-lymphocytes (T cells), B-lymphocytes (B cells) and natural killer cells (NK cells). Lymphocytes are indispensable for both innate and adaptive immunity and cannot function normally without various actin regulators. In this review, we first briefly introduce the structure and fundamental functions of a variety of well-known and newly discovered actin regulators, then we highlight the role of actin regulators in T cell, B cell and NK cell, and finally provide a landscape of various diseases associated with them. This review provides new directions in exploring actin regulators and promotes more precise and effective treatments for related diseases.

The circle of life: Phases of podosome formation, turnover and reemergence

Podosomes are highly dynamic actin-rich structures in a variety of cell types, especially monocytic cells. They fulfill multiple functions such as adhesion, mechanosensing, or extracellular matrix degradation, thus allowing cells to detect and respond to a changing environment. These abilities are based on an intricate architecture that enables podosomes to sense mechanical properties of their substratum and to transduce them intracellularly in order to generate an appropriate cellular response. These processes are enabled through the tightly orchestrated interplay of more than 300 different components that are dynamically recruited during podosome formation and turnover. In this review, we discuss the different phases of the podosome life cycle and the current knowledge on regulatory factors that impact on the genesis, activity, dissolution and reemergence of podosomes. We also highlight mechanoregulatory processes that become important during these different stages, on the level of individual podosomes, and also at podosome sub- and superstructures.

Single-Molecule Force Imaging Reveals That Podosome Formation Requires No Extracellular Integrin-Ligand Tensions or Interactions

Podosomes are integrin-mediated cell adhesion units involved in many cellular and physiological processes. Integrins likely transmit tensions critical for podosome functions, but such force remains poorly characterized. DNA-based tension sensors are powerful in visualizing integrin tensions but subject to degradation by podosomes which ubiquitously recruit DNase. Here, using a DNase-resistant tension sensor based on a DNA/PNA (peptide nucleic acid) duplex, we imaged podosomal integrin tensions (PIT) in the adhesion rings of podosomes on solid substrates with single molecular tension sensitivity. PIT was shown to be generated by both actomyosin contractility and actin polymerization in podosomes. Importantly, by monitoring PIT and podosome structure in parallel, we showed that extracellular integrin-ligand tensions, despite being critical for the formation of focal adhesions, are dispensable for podosome formation, as PIT reduction or elimination has an insignificant impact on structure formation and FAK (focal adhesion kinase) phosphorylation in podosomes. We further verified that even integrin-ligand interaction is dispensable for podosome formation, as macrophages form podosomes normally on passivated surfaces that block integrin-ligand interaction but support macrophage adhesion through electrostatic adsorption or Fc receptor-immunoglobin G interaction. In contrast, focal adhesions are unable to form on these passivated surfaces.

ARPC1B binds WASP to control actin polymerization and curtail tonic signaling in B cells

Immune cells exhibit low-level, constitutive signaling at rest (tonic signaling). Such tonic signals are required for fundamental processes, including the survival of B lymphocytes, but when they are elevated by genetic or environmental causes, they can lead to autoimmunity. Events that control ongoing signal transduction are, therefore, tightly regulated by submembrane cytoskeletal polymers like F-actin. The actin-binding proteins that underpin the process, however, are poorly described. By investigating patients with ARPC1B deficiency, we report that ARPC1B-containing ARP2/3 complexes are stimulated by Wiskott Aldrich Syndrome protein (WASP) to nucleate the branched actin networks that control tonic signaling from the B cell receptor (BCR). Despite an upregulation of ARPC1A, ARPC1B-deficient cells were not capable of WASP-mediated nucleation by ARP2/3, and this caused the loss of WASP-dependent structures, including podosomes in macrophages and lamellipodia in B cells. In the B cell compartment, ARPC1B deficiency also led to weakening of the cortical F-actin cytoskeleton that normally curtails the diffusion of BCRs and ultimately resulted in increased tonic lipid signaling, oscillatory calcium release from the endoplasmic reticulum (ER), and phosphorylated Akt. These events contributed to skewing the threshold for B cell activation in response to microbial-associated molecular patterns (MAMPs). Thus, ARPC1B is critical for ARP2/3 complexes to control steady-state signaling of immune cells.

Molecular Tuning of Actin Dynamics in Leukocyte Migration as Revealed by Immune-Related Actinopathies

Motility is a crucial activity of immune cells allowing them to patrol tissues as they differentiate, sample or exchange information, and execute their effector functions. Although all immune cells are highly migratory, each subset is endowed with very distinct motility patterns in accordance with functional specification. Furthermore individual immune cell subsets adapt their motility behaviour to the surrounding tissue environment. This review focuses on how the generation and adaptation of diversified motility patterns in immune cells is sustained by actin cytoskeleton dynamics. In particular, we review the knowledge gained through the study of inborn errors of immunity (IEI) related to actin defects. Such pathologies are unique models that help us to uncover the contribution of individual actin regulators to the migration of immune cells in the context of their development and function.

Nectin stabilization at adherens junctions is counteracted by Rab5a-dependent endocytosis

Cell-cell junctions undergo constant remodeling, which is crucial for the control of vascular integrity. Indeed, transport of junctional components such as cadherins is understood in increasing depth. However, little is known about the respective pathways regulating localization of nectin at cell-cell junctions. Here, we performed an siRNA-based screen of vesicle regulators of the RabGTPase family, leading to the identification of a novel role for Rab5a in the endocytosis nectin-2 at adherens junctions of primary human endothelial cells (HUVEC). Confocal microscopy experiments revealed disordered nectin-2 localization at adherens junctions upon Rab5a depletion. In addition, internalized nectin-2 was shown to prominently localize to Rab5a-positive vesicles in both fixed and living cells. As shown previously, nectin-2 stabilization at junctions is achieved via drebrin-dependent coupling to the subcortical actin cytoskeleton. Consistently, depletion of drebrin in this study leads to enhanced internalization of nectin-2 from junctions. Strikingly, simultaneous silencing of Rab5a and drebrin restored the junctional localization of nectin-2, pointing to Rab5a as counteracting the drebrin-dependent stabilization of nectin-2 at adherens junctions. This mechanism could be further validated by transendothelial resistance measurements. Collectively, our results identify Rab5a as a key player in the endocytosis of nectin-2 and thus in the regulation of adherens junction integrity in primary human endothelial cells.

Small GTPases all over invadosomes

Cell invasion is associated with numerous patho-physiologic states including cell development and metastatic dissemination. This process couples the activation of cell motility with the capacity to degrade the extracellular matrix, thereby permitting cells to pass through basal membranes. Invasion is sustained by the actions of invadosomes, an ensemble of subcellular structures with high functional homology. Invadosomes are 3D acto-adhesive structures that can also mediate local extracellular matrix degradation through the controlled delivery of proteases. Intracellular RHO GTPases play a central role in the regulation of invadosomes where their complex interplay regulates multiple invadosome functions. This review aims to provide an overview of the synergistic activities of the small GTPases in invadosome biology. This broad-based review also reinforces the importance of the spatiotemporal regulation of small GTPases and the impact of this process on invadosome dynamics.

Baculovirus actin-rearrangement-inducing factor ARIF-1 induces the formation of dynamic invadosome clusters

The baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV), a pathogen of lepidopteran insects, has a striking dependence on the host cell actin cytoskeleton. During the delayed-early stage of infection, AcMNPV was shown to induce the accumulation of actin at the cortex of infected cells. However, the dynamics and molecular mechanism of cortical actin assembly remained unknown. Here, we show that AcMNPV induces dynamic cortical clusters of dot-like actin structures that mediate degradation of the underlying extracellular matrix and therefore function similarly to clusters of invadosomes in mammalian cells. Furthermore, we find that the AcMNPV protein actin-rearrangement-inducing factor-1 (ARIF-1), which was previously shown to be necessary and sufficient for cortical actin assembly and efficient viral infection in insect hosts, is both necessary and sufficient for invadosome formation. We mapped the sequences within the C-terminal cytoplasmic region of ARIF-1 that are required for invadosome formation and identified individual tyrosine and proline residues that are required for organizing these structures. Additionally, we found that ARIF-1 and the invadosome-associated proteins cortactin and the Arp2/3 complex localize to invadosomes and Arp2/3 complex is required for their formation. These ARIF-1-induced invadosomes may be important for the function of ARIF-1 in systemic virus spread.

Loss of Hem1 disrupts macrophage function and impacts migration, phagocytosis, and integrin-mediated adhesion

Hematopoietic-specific protein 1 (Hem1) is an essential subunit of the WAVE regulatory complex (WRC) in immune cells. WRC is crucial for Arp2/3 complex activation and the protrusion of branched actin filament networks. Moreover, Hem1 loss of function in immune cells causes autoimmune diseases in humans. Here, we show that genetic removal of Hem1 in macrophages diminishes frequency and efficacy of phagocytosis as well as phagocytic cup formation in addition to defects in lamellipodial protrusion and migration. Moreover, Hem1-null macrophages displayed strong defects in cell adhesion despite unaltered podosome formation and concomitant extracellular matrix degradation. Specifically, dynamics of both adhesion and de-adhesion as well as concomitant phosphorylation of paxillin and focal adhesion kinase (FAK) were significantly compromised. Accordingly, disruption of WRC function in non-hematopoietic cells coincided with both defects in adhesion turnover and altered FAK and paxillin phosphorylation. Consistently, platelets exhibited reduced adhesion and diminished integrin αIIbβ3 activation upon WRC removal. Interestingly, adhesion phenotypes, but not lamellipodia formation, were partially rescued by small molecule activation of FAK. A full rescue of the phenotype, including lamellipodia formation, required not only the presence of WRCs but also their binding to and activation by Rac. Collectively, our results uncover that WRC impacts on integrin-dependent processes in a FAK-dependent manner, controlling formation and dismantling of adhesions, relevant for properly grabbing onto extracellular surfaces and particles during cell edge expansion, like in migration or phagocytosis.

Focal adhesion dynamics in cellular function and disease

Acting as a bridge between the cytoskeleton of the cell and the extra cellular matrix (ECM), the cell-ECM adhesions with integrins at their core, play a major role in cell signalling to direct mechanotransduction, cell migration, cell cycle progression, proliferation, differentiation, growth and repair. Biochemically, these adhesions are composed of diverse, yet an organised group of structural proteins, receptors, adaptors, various enzymes including protein kinases, phosphatases, GTPases, proteases, etc. as well as scaffolding molecules. The major integrin adhesion complexes (IACs) characterised are focal adhesions (FAs), invadosomes (podosomes and invadopodia), hemidesmosomes (HDs) and reticular adhesions (RAs). The varied composition and regulation of the IACs and their signalling, apart from being an integral part of normal cell survival, has been shown to be of paramount importance in various developmental and pathological processes. This review per-illustrates the recent advancements in the research of IACs, their crucial roles in normal as well as diseased states. We have also touched on few of the various methods that have been developed over the years to visualise IACs, measure the forces they exert and study their signalling and molecular composition. Having such pertinent roles in the context of various pathologies, these IACs need to be understood and studied to develop therapeutical targets. We have given an update to the studies done in recent years and described various techniques which have been applied to study these structures, thereby, providing context in furthering research with respect to IAC targeted therapeutics.

Ubiquitous membrane-bound DNase activity in podosomes and invadopodia

Podosomes and invadopodia, collectively termed invadosomes, are adhesive and degradative membrane structures formed in many types of cells and are well known for recruiting various proteases. However, another major class of degradative enzymes, deoxyribonuclease (DNase), remains unconfirmed and not studied in invadosomes. Here, using surface-immobilized nuclease sensor (SNS), we demonstrated that invadosomes recruit DNase to their core regions, which degrade extracellular double-stranded DNA. We further identified the DNase as GPI-anchored membrane-bound DNase X. DNase recruitment is ubiquitous and consistent in invadosomes of all tested cell types. DNase activity exhibits within a minute after actin nucleation, functioning concomitantly with protease in podosomes but preceding it in invadopodia. We further showed that macrophages form DNase-active podosome rosettes surrounding bacteria or micropatterned antigen islets, and the podosomes directly degrade bacterial DNA on a surface, exhibiting an apparent immunological function. Overall, this work reports DNase in invadosomes for the first time, suggesting a richer arsenal of degradative enzymes in invadosomes than known before.

Durotaxis: the mechanical control of directed cell migration

Directed cell migration is essential for cells to efficiently migrate in physiological and pathological processes. While migrating in their native environment, cells interact with multiple types of cues, such as mechanical and chemical signals. The role of chemical guidance via chemotaxis has been studied in the past, the understanding of mechanical guidance of cell migration via durotaxis remained unclear until very recently. Nonetheless, durotaxis has become a topic of intensive research and several advances have been made in the study of mechanically guided cell migration across multiple fields. Thus, in this article we provide a state of the art about durotaxis by discussing in silico, in vitro and in vivo data. We also present insights on the general mechanisms by which cells sense, transduce and respond to environmental mechanics, to then contextualize these mechanisms in the process of durotaxis and explain how cells bias their migration in anisotropic substrates. Furthermore, we discuss what is known about durotaxis in vivo and we comment on how haptotaxis could arise from integrating durotaxis and chemotaxis in native environments.

Macrophage 3D migration: A potential therapeutic target for inflammation and deleterious progression in diseases

Macrophages are heterogeneous cells that have different physiological functions, such as chemotaxis, phagocytosis, endocytosis, and secretion of various factors. All physiological functions of macrophages are integral to homeostasis, immune defense and tissue repair. However, in several diseases, macrophages are recruited from the blood towards inflammatory sites. This process is called macrophage migration, which promotes deleterious disease progression. Macrophage migration is a key player in many inflammatory diseases, autoimmune diseases and cancers because it contributes to the accumulation of proinflammatory factors, the destruction of tissues and the development of tumors. Therefore, macrophage migration is proposed to be a potential therapeutic target. Macrophages migrate between two-dimensional (2D) and three-dimensional (3D) environments, implying that distinct migratory features and mechanisms are involved. Compared with the 2D migration of macrophages, 3D migration involves more complex variations in cellular morphology and dynamics. The structure of the extracellular matrix, a key factor, is modified in diseases that influence macrophage 3D migration. Macrophage 3D migration relates to disease pathology. Research that focuses on macrophage 3D migration is an emerging field and was reviewed in this article to indicate the molecular and cellular mechanisms of macrophage migration in 3D environments and to provide potential targets for controlling disease progression associated with this migration.

Targeting the cytoskeleton against metastatic dissemination

Cancer is a pathology characterized by a loss or a perturbation of a number of typical features of normal cell behaviour. Indeed, the acquisition of an inappropriate migratory and invasive phenotype has been reported to be one of the hallmarks of cancer. The cytoskeleton is a complex dynamic network of highly ordered interlinking filaments playing a key role in the control of fundamental cellular processes, like cell shape maintenance, motility, division and intracellular transport. Moreover, deregulation of this complex machinery contributes to cancer progression and malignancy, enabling cells to acquire an invasive and metastatic phenotype. Metastasis accounts for 90% of death from patients affected by solid tumours, while an efficient prevention and suppression of metastatic disease still remains elusive. This results in the lack of effective therapeutic options currently available for patients with advanced disease. In this context, the cytoskeleton with its regulatory and structural proteins emerges as a novel and highly effective target to be exploited for a substantial therapeutic effort toward the development of specific anti-metastatic drugs. Here we provide an overview of the role of cytoskeleton components and interacting proteins in cancer metastasis with a special focus on small molecule compounds interfering with the actin cytoskeleton organization and function. The emerging involvement of microtubules and intermediate filaments in cancer metastasis is also reviewed.

PAK4 Kinase Activity Plays a Crucial Role in the Podosome Ring of Myeloid Cells

p21-Activated kinase 4 (PAK4), a serine/threonine kinase, is purported to localize to podosomes: transient adhesive structures that degrade the extracellular matrix to facilitate rapid myeloid cell migration. We find that treatment of transforming growth factor β (TGF-β)-differentiated monocytic (THP-1) cells with a PAK4-targeted inhibitor significantly reduces podosome formation and induces the formation of focal adhesions. This switch in adhesions confers a diminution of matrix degradation and reduced cell migration. Furthermore, reduced PAK4 expression causes a significant reduction in podosome number that cannot be rescued by kinase-dead PAK4, supporting a kinase-dependent role. Concomitant with PAK4 depletion, phosphorylation of Akt is perturbed, whereas a specific phospho-Akt signal is detected within the podosomes. Using superresolution analysis, we find that PAK4 specifically localizes in the podosome ring, nearer to the actin core than other ring proteins. We propose PAK4 kinase activity intersects with the Akt pathway at the podosome ring:core interface to drive regulation of macrophage podosome turnover.

Targeted gene correction of human hematopoietic stem cells for the treatment of Wiskott - Aldrich Syndrome

Wiskott-Aldrich syndrome (WAS) is an X-linked primary immunodeficiency with severe platelet abnormalities and complex immunodeficiency. Although clinical gene therapy approaches using lentiviral vectors have produced encouraging results, full immune and platelet reconstitution is not always achieved. Here we show that a CRISPR/Cas9-based genome editing strategy allows the precise correction of WAS mutations in up to 60% of human hematopoietic stem and progenitor cells (HSPCs), without impairing cell viability and differentiation potential. Delivery of the editing reagents to WAS HSPCs led to full rescue of WASp expression and correction of functional defects in myeloid and lymphoid cells. Primary and secondary transplantation of corrected WAS HSPCs into immunodeficient mice showed persistence of edited cells for up to 26 weeks and efficient targeting of long-term repopulating stem cells. Finally, no major genotoxicity was associated with the gene editing process, paving the way for an alternative, yet highly efficient and safe therapy.

Cytoskeletal regulation of dendritic cells: An intricate balance between migration and presentation for tumor therapy

Dendritic cells (DCs) are the main players in many approaches for cancer therapy. The idea with DC tumor therapy is to promote activation of tumor infiltrating cytotoxic T cells that kill tumor cells. This requires that DCs take up tumor Ag and present peptides on MHC class I molecules in a process called cross-presentation. For this process to be efficient, DCs have to migrate to the tumor draining lymph node and there activate the machinery for cross-presentation. In this review, we will discuss recent progress in understanding the role of actin regulators for control of DC migration and Ag presentation. The potential to target actin regulators for better DC-based tumor therapy will also be discussed.

Estimation of non-null SNP effect size distributions enables the detection of enriched genes underlying complex traits

Traditional univariate genome-wide association studies generate false positives and negatives due to difficulties distinguishing associated variants from variants with spurious nonzero effects that do not directly influence the trait. Recent efforts have been directed at identifying genes or signaling pathways enriched for mutations in quantitative traits or case-control studies, but these can be computationally costly and hampered by strict model assumptions. Here, we present gene-ε, a new approach for identifying statistical associations between sets of variants and quantitative traits. Our key insight is that enrichment studies on the gene-level are improved when we reformulate the genome-wide SNP-level null hypothesis to identify spurious small-to-intermediate SNP effects and classify them as non-causal. gene-ε efficiently identifies enriched genes under a variety of simulated genetic architectures, achieving greater than a 90% true positive rate at 1% false positive rate for polygenic traits. Lastly, we apply gene-ε to summary statistics derived from six quantitative traits using European-ancestry individuals in the UK Biobank, and identify enriched genes that are in biologically relevant pathways.

Abl-mediated PI3K activation regulates macrophage podosome formation

Podosomes play crucial roles in macrophage adhesion and migration. Wiskott-Aldrich syndrome protein (WASP; also known as WAS)-mediated actin polymerization is one of the key events initiating podosome formation. Nevertheless, membrane signals to trigger WASP activation at macrophage podosomes remain unclear. Here, we show that phosphatidylinositol (3,4,5)-trisphosphate [PI(3,4,5)P3] lipids are enriched at the podosome and stably recruit WASP rather than the WASP-5KE mutant. Phosphatidylinositol 4,5-bisphosphate 3-kinase catalytic subunit β (PIK3CB) is spatially located at the podosome core. Inhibition of PIK3CB and overexpression of phosphatase and tensin homolog (PTEN) impede F-actin polymerization of the podosome. PIK3CB activation is regulated by Abl1 and Src family kinases. At the podosome core, Src and Hck promote the phosphorylation of Tyr488 in the consensus Y-x-x-M motif of Abl1, which enables the association of phosphoinositide 3-kinase (PI3K) regulatory subunits. Knockdown of Abl1 rather than Abl2 suppresses the PI3K/Akt pathway, regardless of Src and Hck activities. Reintroduction of wild-type Abl1 rather than the Abl1-Y488F mutant rescues PI3KR1 recruitment and PI3K activation. When PIK3CB, Abl1 or Src/Hck is suppressed, macrophage podosome formation, matrix degradation and chemotactic migration are inhibited. Thus, Src/Hck-mediated phosphorylation of Abl1 Tyr488 triggers PIK3CB-dependent PI(3,4,5)P3 production and orchestrates the assembly and function of macrophage podosomes.

Poji: a Fiji-based tool for analysis of podosomes and associated proteins

Podosomes are actin-based adhesion and invasion structures in a variety of cell types, with podosome-forming cells displaying up to several hundreds of these structures. Podosome number, distribution and composition can be affected by experimental treatments or during regular turnover, necessitating a tool that is able to detect even subtle differences in podosomal properties. Here, we present a Fiji-based macro code termed 'Poji' ('podosome analysis by Fiji'), which serves as an easy-to-use tool to characterize a variety of cellular and podosomal parameters, including area, fluorescence intensity, relative enrichment of associated proteins and radial podosome intensity profiles. This tool should be useful to gain more detailed insight into the regulation, architecture and functions of podosomes. Moreover, we show that Poji is easily adaptable for the analysis of invadopodia and associated extracellular matrix degradation, and likely also of other micron-size punctate structures. This article describes the workflow of the Poji macro, presents several examples of its applications, and also points out limitations, as well as respective solutions, and adaptable features to streamline the analysis.This article has an associated First Person interview with the first author of the paper.

Genetic engineering of Hoxb8-immortalized hematopoietic progenitors - a potent tool to study macrophage tissue migration

Tumor-associated macrophages (TAMs) are detrimental in most cancers. Controlling their recruitment is thus potentially therapeutic. We previously found that TAMs perform protease-dependent mesenchymal migration in cancer, while macrophages perform amoeboid migration in other tissues. Inhibition of mesenchymal migration correlates with decreased TAM infiltration and tumor growth, providing rationale for a new cancer immunotherapy specifically targeting TAM motility. To identify new effectors of mesenchymal migration, we produced ER-Hoxb8-immortalized hematopoietic progenitors (cells with estrogen receptor-regulated Hoxb8 expression), which show unlimited proliferative ability in the presence of estrogen. The functionality of macrophages differentiated from ER-Hoxb8 progenitors was compared to bone marrow-derived macrophages (BMDMs). They polarized into M1- and M2-orientated macrophages, generated reactive oxygen species (ROS), ingested particles, formed podosomes, degraded the extracellular matrix, adopted amoeboid and mesenchymal migration in 3D, and infiltrated tumor explants ex vivo using mesenchymal migration. We also used the CRISPR/Cas9 system to disrupt gene expression of a known effector of mesenchymal migration, WASP (also known as WAS), to provide a proof of concept. We observed impaired podosome formation and mesenchymal migration capacity, thus recapitulating the phenotype of BMDM isolated from Wasp-knockout mice. Thus, we validate the use of ER-Hoxb8-immortalized macrophages as a potent tool to investigate macrophage functionalities.

Cause and effect of microenvironmental acidosis on bone metastases

Skeletal involvement is a frequent and troublesome complication in advanced cancers. In the process of tumor cells homing to the skeleton to form bone metastases (BM), different mechanisms allow tumor cells to interact with cells of the bone microenvironment and seed in the bone tissue. Among these, tumor acidosis has been directly associated with tumor invasion and aggressiveness in several types of cancer although it has been less explored in the context of BM. In bone, the association of local acidosis and cancer invasiveness is even more important for tumor expansion since the extracellular matrix is formed by both organic and hard inorganic matrices and bone cells are used to sense protons and adapt or react to a low pH to maintain tissue homeostasis. In the BM microenvironment, increased concentration of protons may derive not only from glycolytic tumor cells but also from tumor-induced osteoclasts, the bone-resorbing cells, and may influence the progression or symptoms of BM in many different ways, by directly enhancing cancer cell motility and aggressiveness, or by modulating the functions of bone cells versus a pro-tumorigenic phenotype, or by inducing bone pain. In this review, we will describe and discuss the cause of acidosis in BM, its role in BM microenvironment, and which are the final effectors that may be targeted to treat metastatic patients.

miR-149 Suppresses Breast Cancer Metastasis by Blocking Paracrine Interactions with Macrophages

Paracrine activation of cells contained in the tumor microenvironment promotes tumor progression and metastasis. In breast cancer, malignant cells recruit and educate macrophages into a M2 tumor-promoting phenotype that supports the metastatic spread of cancer cells. Here, we show that miR-149 functions as a metastasis-suppressing microRNA in breast cancer cells by limiting colony-stimulating factor-1 (CSF1)-dependent recruitment and M2 polarization of macrophages. In lymph node-positive, triple-negative breast cancer (TNBC) tissues, low miR-149 expression correlated with macrophage infiltration and reduced patient survival. By directly targeting CSF1, miR-149 expression in TNBC cell lines (MDA-MB-231 and BT-549) inhibited the recruitment of human monocytic THP-1 cells and primary human macrophages. Furthermore, in macrophages cocultured with MDA-MB-231 cells expressing miR-149, epidermal growth factor (EGF) and amphiregulin expression levels were strongly reduced, resulting in reduced EGF receptor activation in the cancer cells. In vivo, lung metastases developing from orthotopic MDA-MB-231 tumors were reduced by 75% by miR-149 expression, and this was associated with impaired M2 macrophage infiltration of the primary tumors. These data suggest that miR-149 downregulation functionally contributes to breast tumor progression by recruiting macrophages to the tumor and facilitating CSF1 and EGF receptor cross-talk between cancer cells and macrophages. SIGNIFICANCE: These findings contribute to the understanding of tumor-stroma interactions by showing that miR-149 downregulation in TNBC enhances reciprocal growth factor signaling between macrophages and cancer cells, which promotes tumor progression and metastasis. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/6/1330/F1.large.jpg.

Probing the mechanical landscape – new insights into podosome architecture and mechanics

Podosomes are dynamic adhesion structures formed constitutively by macrophages, dendritic cells and osteoclasts and transiently in a wide variety of cells, such as endothelial cells and megakaryocytes. They mediate numerous functions, including cell–matrix adhesion, extracellular matrix degradation, mechanosensing and cell migration. Podosomes present as micron-sized F-actin cores surrounded by an adhesive ring of integrins and integrin–actin linkers, such as talin and vinculin. In this Review, we highlight recent research that has considerably advanced our understanding of the complex architecture–function relationship of podosomes by demonstrating that the podosome ring actually consists of discontinuous nano-clusters and that the actin network in between podosomes comprises two subsets of unbranched actin filaments, lateral and dorsal podosome-connecting filaments. These lateral and dorsal podosome-connecting filaments connect the core and ring of individual podosomes and adjacent podosomes, respectively. We also highlight recent insights into the podosome cap as a novel regulatory module of actomyosin-based contractility. We propose that these newly identified features are instrumental for the ability of podosomes to generate protrusion forces and to mechanically probe their environment. Furthermore, these new results point to an increasing complexity of podosome architecture and have led to our current view of podosomes as autonomous force generators that drive cell migration.

Excitable networks controlling cell migration during development and disease

The directed movements of individual, groups, or sheets of cells at specific times in particular locations bring about form and complexity to developing organisms. Cells move by extending protrusions, such as macropinosomes, pseudopods, lamellipods, filopods, or blebs. Although many of the cytoskeletal components within these structures are known, less is known about the mechanisms that determine their location, number, and characteristics. Recent evidence suggests that control may be exerted by a signal transduction excitable network whose components and activities, including Ras, PI3K, TorC2, and phosphoinositides, self-organize on the plasma membrane and propagate in waves. The waves drive the various types of protrusions, which in turn, determine the modes of cell migration. Acute perturbations at specific points in the network produce abrupt shifts in protrusion type, including transitions from pseudopods to filopods or lamellipods. These observations have also contributed to a delineation of the signal transduction network, including candidate fast positive and delayed negative feedback loops. The network contains many oncogenes and tumor suppressors, and other molecules which have recently been implicated in developmental and metabolic abnormalities. Thus, the concept of signal transduction network excitability in cell migration can be used to understand disease states and morphological changes occurring in development.

DNA mechanotechnology reveals that integrin receptors apply pN forces in podosomes on fluid substrates

Podosomes are ubiquitous cellular structures important to diverse processes including cell invasion, migration, bone resorption, and immune surveillance. Structurally, podosomes consist of a protrusive actin core surrounded by adhesion proteins. Although podosome protrusion forces have been quantified, the magnitude, spatial distribution, and orientation of the opposing tensile forces remain poorly characterized. Here we use DNA nanotechnology to create probes that measure and manipulate podosome tensile forces with molecular piconewton (pN) resolution. Specifically, Molecular Tension-Fluorescence Lifetime Imaging Microscopy (MT-FLIM) produces maps of the cellular adhesive landscape, revealing ring-like tensile forces surrounding podosome cores. Photocleavable adhesion ligands, breakable DNA force probes, and pharmacological inhibition demonstrate local mechanical coupling between integrin tension and actin protrusion. Thus, podosomes use pN integrin forces to sense and respond to substrate mechanics. This work deepens our understanding of podosome mechanotransduction and contributes tools that are widely applicable for studying receptor mechanics at dynamic interfaces.

Epithelial-to-mesenchymal transition and invadopodia markers in breast cancer: Lumican a key regulator

A great hallmark of breast cancer is the absence or presence of estrogen receptors ERα and ERβ, with a dominant role in cell proliferation, differentiation and cancer progression. Both receptors are related with Epithelial-to-Mesenchymal Transition (EMT) since there is a relation between ERs and extracellular matrix (ECM) macromolecules expression, and therefore, cell-cell and cell-ECM interactions. The endocrine resistance of ERα endows epithelial cells with increased aggressiveness and induces cell proliferation, resulting into a mesenchymal phenotype and an EMT status. ERα signaling may affect the transcriptional factors which govern EMT. Knockdown or silencing of ERα and ERβ in MCF-7 and MDA-MB-231 breast cancer cells respectively, provoked pivotal changes in phenotype, cellular functions, mRNA and protein levels of EMT markers, and consequently the EMT status. Mesenchymal cells owe their migratory and invasive properties to invadopodia, while in epithelial cells, lamellipodia and filopodia are mostly observed. Invadopodia, are actin-rich protrusions of plasma membrane, promoting proteolytic degradation of ECM and tumor invasion. Cortactin and MMP-14 govern the formation and principal functions of invadopodia. In vitro experiments proved that lumican inhibits cortactin and MMP-14 expression, alters the formation of lamellipodia and transforms mesenchymal cells into epithelial-like. Conclusively, lumican may inhibit or even reverse the several metastatic features that EMT endows in breast cancer cells. Therefore, a lumican-based anti-cancer therapy which will pharmacologically target and inhibit EMT might be interesting to be developed.

Forces and constraints controlling podosome assembly and disassembly

Podosomes are a singular category of integrin-mediated adhesions important in the processes of cell migration, matrix degradation and cancer cell invasion. Despite a wealth of biochemical studies, the effects of mechanical forces on podosome integrity and dynamics are poorly understood. Here, we show that podosomes are highly sensitive to two groups of physical factors. First, we describe the process of podosome disassembly induced by activation of myosin-IIA filament assembly. Next, we find that podosome integrity and dynamics depends upon membrane tension and can be experimentally perturbed by osmotic swelling and deoxycholate treatment. We have also found that podosomes can be disrupted in a reversible manner by single or cyclic radial stretching of the substratum. We show that disruption of podosomes induced by osmotic swelling is independent of myosin-II filaments. The inhibition of the membrane sculpting protein, dynamin-II, but not clathrin, resulted in activation of myosin-IIA filament formation and disruption of podosomes. The effect of dynamin-II inhibition on podosomes was, however, independent of myosin-II filaments. Moreover, formation of organized arrays of podosomes in response to microtopographic cues (the ridges with triangular profile) was not accompanied by reorganization of myosin-II filaments. Thus, mechanical elements such as myosin-II filaments and factors affecting membrane tension/sculpting independently modulate podosome formation and dynamics, underlying a versatile response of these adhesion structures to intracellular and extracellular cues. This article is part of a discussion meeting issue 'Forces in cancer: interdisciplinary approaches in tumour mechanobiology'.

An actin-based protrusion originating from a podosome-enriched region initiates macrophage fusion

Macrophage fusion resulting in the formation of multinucleated giant cells occurs in a variety of chronic inflammatory diseases, yet the mechanism responsible for initiating this process is unknown. Here, we used live cell imaging to show that actin-based protrusions at the leading edge initiate macrophage fusion. Phase-contrast video microscopy demonstrated that in the majority of events, short protrusions (∼3 µm) between two closely apposed cells initiated fusion, but occasionally we observed long protrusions (∼12 µm). Using macrophages isolated from LifeAct mice and imaging with lattice light sheet microscopy, we further found that fusion-competent protrusions formed at sites enriched in podosomes. Inducing fusion in mixed populations of GFP- and mRFP-LifeAct macrophages showed rapid spatial overlap between GFP and RFP signal at the site of fusion. Cytochalasin B strongly reduced fusion and when rare fusion events occurred, protrusions were not observed. Fusion of macrophages deficient in Wiskott-Aldrich syndrome protein and Cdc42, key molecules involved in the formation of actin-based protrusions and podosomes, was also impaired both in vitro and in vivo. Finally, inhibiting the activity of the Arp2/3 complex decreased fusion and podosome formation. Together these data suggest that an actin-based protrusion formed at the leading edge initiates macrophage fusion.

Ubiquitination and Long Non-coding RNAs Regulate Actin Cytoskeleton Regulators in Cancer Progression

Actin filaments are a major component of the cytoskeleton in eukaryotic cells and play an important role in cancer metastasis. Dynamics and reorganization of actin filaments are regulated by numerous regulators, including Rho GTPases, PAKs (p21-activated kinases), ROCKs (Rho-associated coiled-coil containing kinases), LIMKs (LIM domain kinases), and SSH1 (slingshot family protein phosphate 1). Ubiquitination, as a ubiquitous post-transcriptional modification, deceases protein levels of actin cytoskeleton regulatory factors and thereby modulates the actin cytoskeleton. There is increasing evidence showing cytoskeleton regulation by long noncoding RNAs (lncRNAs) in cancer metastasis. However, which E3 ligases are activated for the ubiquitination of actin-cytoskeleton regulators involved in tumor metastasis remains to be fully elucidated. Moreover, it is not clear how lncRNAs influence the expression of actin cytoskeleton regulators. Here, we summarize physiological and pathological mechanisms of lncRNAs and ubiquitination control mediators of actin cytoskeleton regulators which that are involved in tumorigenesis and tumor progression. Finally, we briefly discuss crosstalk between ubiquitination and lncRNA control mediators of actin-cytoskeleton regulators in cancer.

Scattered podosomes and podosomes associated with the sealing zone architecture in cultured osteoclasts revealed by cell shearing, quick freezing, and platinum-replica electron microscopy

Osteoclasts (OCs) can adhere to a variety of substrate surfaces by highly dynamic actin-based cytoskeletal structures termed podosomes. This tight attachment is established by a sealing zone (SZ), which is made of interconnected individual podosomes. Compared with scattered podosomes in various cell types, the architecture of the SZ is still unclear. Especially, ultrastructural studies on the details of the cytoskeletal structure of an OC have been challenging, because the high density of filaments in their podosomes obscure visualization of individual filaments. Therefore, to study this organization in more exact detail, we employed shearing open combined with replica electron microscopy. The present study provides several new details of the podosome and SZ structure, which were previously unrecognized: (a) the SZ consists of recognizable podosomes with a dense actin network of interpodosomal regions characterized by multiple layers of crossing, branching and anastomosing actin filament networks; (b) the Arp2/3 complex is distributed throughout the actin network of podosomes and SZ, indicating that actin polymerization is concentrated at these regions; (c) a close spatial relationship between the podosome and the dorsal membrane; and (d) a network of membranous organelles in close proximity to the podosomes in the SZ. Taken together, the present study reveals that a more complicated interpodosomal actin network among neighboring individual podosomes, which is more complicated than previously thought, appears to form the SZ. Indeed, individual podosomes are not an isolated structural unit from other organelles; and, in turn, their dynamism might affect the surrounding interpodosomal cytoskeletons, membranous organelles, and plasma membrane.

Integrins: Moonlighting Proteins in Invadosome Formation

Invadopodia are actin-rich protrusions developed by transformed cells in 2D/3D environments that are implicated in extracellular matrix (ECM) remodeling and degradation. These structures have an undoubted association with cancer invasion and metastasis because invadopodium formation in vivo is a key step for intra/extravasation of tumor cells. Invadopodia are closely related to other actin-rich structures known as podosomes, which are typical structures of normal cells necessary for different physiological processes during development and organogenesis. Invadopodia and podosomes are included in the general term 'invadosomes,' as they both appear as actin puncta on plasma membranes next to extracellular matrix metalloproteinases, although organization, regulation, and function are slightly different. Integrins are transmembrane proteins implicated in cell-cell and cell-matrix interactions and other important processes such as molecular signaling, mechano-transduction, and cell functions, e.g., adhesion, migration, or invasion. It is noteworthy that integrin expression is altered in many tumors, and other pathologies such as cardiovascular or immune dysfunctions. Over the last few years, growing evidence has suggested a role of integrins in the formation of invadopodia. However, their implication in invadopodia formation and adhesion to the ECM is still not well known. This review focuses on the role of integrins in invadopodium formation and provides a general overview of the involvement of these proteins in the mechanisms of metastasis, taking into account classic research through to the latest and most advanced work in the field.

PLD-dependent phosphatidic acid microdomains are signaling platforms for podosome formation

Local membrane phospholipid enrichment serves as docking platform for signaling proteins involved in many processes including cell adhesion and migration. Tissue-resident dendritic cells (DCs) assemble actomyosin-based structures called podosomes, which mediate adhesion and degradation of extracellular matrix for migration and antigen sampling. Recent evidence suggested the involvement of phospholipase D (PLD) and its product phosphatidic acid (PA) in podosome formation, but the spatiotemporal control of this process is poorly characterized. Here we determined the role of PLD1 and PLD2 isoforms in regulating podosome formation and dynamics in human primary DCs by combining PLD pharmacological inhibition with a fluorescent PA sensor and fluorescence microscopy. We found that ongoing PLD2 activity is required for the maintenance of podosomes, whereas both PLD1 and PLD2 control the early stages of podosome assembly. Furthermore, we captured the formation of PA microdomains accumulating at the membrane cytoplasmic leaflet of living DCs, in dynamic coordination with nascent podosome actin cores. Finally, we show that both PLD1 and PLD2 activity are important for podosome-mediated matrix degradation. Our results provide novel insight into the isoform-specific spatiotemporal regulation of PLD activity and further our understanding of the role of cell membrane phospholipids in controlling localized actin polymerization and cell protrusion.

[Variations on the theme of podosomes, context matters]

Podosomes are actin-based microdomains connecting the cell with its extracellular matrix. Contractile actin-myosin cables assemble them into a network that constitutes a versatile cellular superstructure. Discovered and extensively described in in vitro conditions, podosomes now appear as major actors of specific physiological processes. They share common characteristics but their morphology and their effect on cell functioning can only be apprehended in specific cellular contexts. We focus here on three cellular processes involving podosomes and discuss their properties in context.