Regulation of podosome dynamics by WASp phosphorylation: implication in matrix degradation and chemotaxis in macrophages

Podosome Nucleation Is Facilitated by Multivalent Interactions between Syk and ITAM-containing Membrane Complexes

Immune cells survey their microenvironment by forming dynamic cellular protrusions that enable chemotaxis, contacts with other cells, and phagocytosis. Podosomes are a unique type of protrusion structured by an adhesive ring of active integrins that surround an F-actin-rich core harboring degradative proteases. Although the features of podosomes, once-established, have been well defined, the steps that lead to podosome formation remain poorly understood by comparison. In this study, we report that spleen tyrosine kinase (Syk) is a critical regulator of podosome formation. Deletion of Syk or targeting its kinase activity eliminated the ability for murine macrophages to form podosomes. We found that the kinase activity of Syk was important for the phosphorylation of its substrates, HS1 and Pyk2, both of which regulate podosome formation. Additionally, before podosomes form, we report that the tandem Src homology 2 domains of Syk afforded multivalent clustering of ITAM-containing adaptors that associated with integrins to structure platforms that initiate podosomes. We therefore propose that Syk has a dual role in regulating podosomes: first, by facilitating the assembly of multivalent signaling hubs that nucleate their formation and second, by sustaining tyrosine kinase activity of the podosomes once they form against their substrates. In cells expressing recently identified gain-of-function variants of SYK, podosomes were dysregulated. These results implicate SYK in the (patho)physiological functions of podosomes in macrophages.

Tau aggregates improve the purinergic receptor P2Y12-associated podosome rearrangements in microglial cells

Alzheimer's disease (AD) is a progressive neurodegenerative disease that is associated with protein misfolding, plaque accumulation, neuronal dysfunction, synaptic loss, and cognitive decline. The pathological cascade of AD includes the intracellular Tau hyperphosphorylation and its subsequent aggregation, extracellular Amyloid-β plaque formation and microglia-mediated neuroinflammation. The extracellular release of aggregated Tau is sensed by surveilling microglia through the involvement of various cell surface receptors. Among all, purinergic P2Y12R signaling is involved in microglial chemotaxis towards the damaged neurons. Microglial migration is highly linked with membrane-associated actin remodeling leading to the phagocytosis of extracellular Tau species. Here, we studied the formation of various actin structures such as podosome, lamellipodia and filopodia, in response to extracellular Tau monomers and aggregates. Microglial podosomes are colocalized with actin nucleator protein WASP, Arp2 and TKS5 adaptor protein during Tau-mediated migration. Moreover, the P2Y12 receptors were associated with F-actin-rich podosome structures, which signify the potential of Tau aggregates in microglial chemotaxis through the involvement of actin remodeling.

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.

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.

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.

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.

A proteogenomic portrait of lung squamous cell carcinoma

Lung squamous cell carcinoma (LSCC) remains a leading cause of cancer death with few therapeutic options. We characterized the proteogenomic landscape of LSCC, providing a deeper exposition of LSCC biology with potential therapeutic implications. We identify NSD3 as an alternative driver in FGFR1-amplified tumors and low-p63 tumors overexpressing the therapeutic target survivin. SOX2 is considered undruggable, but our analyses provide rationale for exploring chromatin modifiers such as LSD1 and EZH2 to target SOX2-overexpressing tumors. Our data support complex regulation of metabolic pathways by crosstalk between post-translational modifications including ubiquitylation. Numerous immune-related proteogenomic observations suggest directions for further investigation. Proteogenomic dissection of CDKN2A mutations argue for more nuanced assessment of RB1 protein expression and phosphorylation before declaring CDK4/6 inhibition unsuccessful. Finally, triangulation between LSCC, LUAD, and HNSCC identified both unique and common therapeutic vulnerabilities. These observations and proteogenomics data resources may guide research into the biology and treatment of LSCC.

Probing invadosomes: technologies for the analysis of invadosomes

Invadosomes are protrusive and mechanosensitive actin devices critical for cell migration, invasion, and extracellular matrix remodeling. The dynamic, proteolytic, and protrusive natures of invadosomes have made these structures fascinating and attracted many scientists to develop new technologies for their analysis. With these exciting methodologies, many biochemical and biophysical properties of invadosomes have been well characterized and appreciated, and those discoveries elegantly explained the biological and pathological effects of invadosomes in human health and diseases. In this review, we focus on these commonly used or newly developed methods for invadosome analysis and effort to reason some discrepancies among those assays. Finally, we explore the opposite regulatory mechanisms among invadosomes and focal adhesions, another actin-rich adhesive structures, and speculate a potential rule for their switch.

Mechanobiology of Macrophages: How Physical Factors Coregulate Macrophage Plasticity and Phagocytosis

In addition to their early-recognized functions in host defense and the clearance of apoptotic cell debris, macrophages play vital roles in tissue development, homeostasis, and repair. If misregulated, they steer the progression of many inflammatory diseases. Much progress has been made in understanding the mechanisms underlying macrophage signaling, transcriptomics, and proteomics, under physiological and pathological conditions. Yet, the detailed mechanisms that tune circulating monocytes/macrophages and tissue-resident macrophage polarization, differentiation, specification, and their functional plasticity remain elusive. We review how physical factors affect macrophage phenotype and function, including how they hunt for particles and pathogens, as well as the implications for phagocytosis, autophagy, and polarization from proinflammatory to prohealing phenotype. We further discuss how this knowledge can be harnessed in regenerative medicine and for the design of new drugs and immune-modulatory drug delivery systems, biomaterials, and tissue scaffolds.

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.

Colony-Stimulating Factor 1 Receptor (CSF1R) Activates AKT/mTOR Signaling and Promotes T-Cell Lymphoma Viability

PURPOSE

Peripheral T-cell lymphomas are clinically aggressive and usually fatal, as few complete or durable remissions are achieved with currently available therapies. Recent evidence supports a critical role for lymphoma-associated macrophages during T-cell lymphoma progression, but the specific signals involved in the cross-talk between malignant T cells and their microenvironment are poorly understood. Colony-stimulator factor 1 receptor (CSF1R, CD115) is required for the homeostatic survival of tissue-resident macrophages. Interestingly, its aberrant expression has been reported in a subset of tumors. In this article, we evaluated its expression and oncogenic role in T-cell lymphomas.

EXPERIMENTAL DESIGN

Loss-of-function studies, including pharmacologic inhibition with a clinically available tyrosine kinase inhibitor, pexidartinib, were performed in multiple in vitro and in vivo models. In addition, proteomic and genomic screenings were performed to discover signaling pathways that are activated downstream of CSF1R signaling.

RESULTS

We observed that CSF1R is aberrantly expressed in many T-cell lymphomas, including a significant number of peripheral and cutaneous T-cell lymphomas. Colony-stimulating factor 1 (CSF1), in an autocrine or paracrine-dependent manner, leads to CSF1R autophosphorylation and activation in malignant T cells. Furthermore, CSF1R signaling was associated with significant changes in gene expression and in the phosphoproteome, implicating PI3K/AKT/mTOR in CSF1R-mediated T-cell lymphoma growth. We also demonstrated that inhibition of CSF1R in vivo and in vitro models is associated with decreased T-cell lymphoma growth.

CONCLUSIONS

Collectively, these findings implicate CSF1R in T-cell lymphomagenesis and have significant therapeutic implications.

Interrogating Synaptic Architecture: Approaches for Labeling Organelles and Cytoskeleton Components

Synaptic transmission has been studied for decades, as a fundamental step in brain function. The structure of the synapse, and its changes during activity, turned out to be key aspects not only in the transfer of information between neurons, but also in cognitive processes such as learning and memory. The overall synaptic morphology has traditionally been studied by electron microscopy, which enables the visualization of synaptic structure in great detail. The changes in the organization of easily identified structures, such as the presynaptic active zone, or the postsynaptic density, are optimally studied via electron microscopy. However, few reliable methods are available for labeling individual organelles or protein complexes in electron microscopy. For such targets one typically relies either on combination of electron and fluorescence microscopy, or on super-resolution fluorescence microscopy. This review focuses on approaches and techniques used to specifically reveal synaptic organelles and protein complexes, such as cytoskeletal assemblies. We place the strongest emphasis on methods detecting the targets of interest by affinity binding, and we discuss the advantages and limitations of each method.

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.

Tunneling nanotubes, a novel mode of tumor cell-macrophage communication in tumor cell invasion

The interaction between tumor cells and macrophages is crucial in promoting tumor invasion and metastasis. In this study, we examined a novel mechanism of intercellular communication, namely membranous actin-based tunneling nanotubes (TNTs), that occurs between macrophages and tumor cells in the promotion of macrophage-dependent tumor cell invasion. The presence of heterotypic TNTs between macrophages and tumor cells induced invasive tumor cell morphology, which was dependent on EGF-EGFR signaling. Furthermore, reduction of a protein involved in TNT formation, M-Sec (TNFAIP2), in macrophages inhibited tumor cell elongation, blocked the ability of tumor cells to invade in 3D and reduced macrophage-dependent long-distance tumor cell streaming in vitro Using an in vivo zebrafish model that recreates macrophage-mediated tumor cell invasion, we observed TNT-mediated macrophage-dependent tumor cell invasion, distant metastatic foci and areas of metastatic spread. Overall, our studies support a role for TNTs as a novel means of interaction between tumor cells and macrophages that leads to tumor progression and metastasis.

The Role of Rho-GTPases and actin polymerization during Macrophage Tunneling Nanotube Biogenesis

Macrophage interactions with other cells, either locally or at distances, are imperative in both normal and pathological conditions. While soluble means of communication can transmit signals between different cells, it does not account for all long distance macrophage interactions. Recently described tunneling nanotubes (TNTs) are membranous channels that connect cells together and allow for transfer of signals, vesicles, and organelles. However, very little is known about the mechanism by which these structures are formed. Here we investigated the signaling pathways involved in TNT formation by macrophages using multiple imaging techniques including super-resolution microscopy (3D-SIM) and live-cell imaging including the use of FRET-based Rho GTPase biosensors. We found that formation of TNTs required the activity and differential localization of Cdc42 and Rac1. The downstream Rho GTPase effectors mediating actin polymerization through Arp2/3 nucleation, Wiskott-Aldrich syndrome protein (WASP) and WASP family verprolin-homologous 2 (WAVE2) proteins are also important, and both pathways act together during TNT biogenesis. Finally, TNT function as measured by transfer of cellular material between cells was reduced following depletion of a single factor demonstrating the importance of these factors in TNTs. Given that the characterization of TNT formation is still unclear in the field; this study provides new insights and would enhance the understanding of TNT formation towards investigating new markers.

PAPA, PASH and PAPASH Syndromes: Pathophysiology, Presentation and Treatment

Pyoderma gangrenosum (PG) is a neutrophilic dermatosis usually manifesting as skin ulcers with undermined erythematous-violaceous borders. It may be isolated, associated with systemic conditions or occurring in the context of autoinflammatory syndromes such as PAPA (pyogenic arthritis, PG and acne), PASH (PG, acne and suppurative hidradenitis) or PAPASH (pyogenic arthritis, acne, PG and suppurative hidradenitis). From a physiopathological point of view, all these conditions share common mechanisms consisting of over-activation of the innate immune system leading to increased production of the interleukin (IL)-1 family and 'sterile' neutrophil-rich cutaneous inflammation. From a genetic point of view, a number of mutations affecting the proteins of the inflammasome complex (the molecular platform responsible for triggering autoinflammation) or the proteins that regulate inflammasome function have been described in these disorders. As these debilitating entities are all associated with the over-expression of IL-1 and tumour necrosis factor (TNF)-α, biological drugs specifically targeting these cytokines are currently the most effective treatments but, given the emerging role of IL-17 in the pathogenesis of these syndromes, IL-17 antagonists may represent the future management of these conditions.

WASP and SCAR are evolutionarily conserved in actin-filled pseudopod-based motility

Eukaryotic cells use diverse cellular mechanisms to crawl through complex environments. Fritz-Laylin et al. define α-motility as a mode of migration associated with dynamic, actin-filled pseudopods and show that WASP and SCAR constitute an evolutionarily conserved genetic signature of α-motility.

Diverse eukaryotic cells crawl through complex environments using distinct modes of migration. To understand the underlying mechanisms and their evolutionary relationships, we must define each mode and identify its phenotypic and molecular markers. In this study, we focus on a widely dispersed migration mode characterized by dynamic actin-filled pseudopods that we call “α-motility.” Mining genomic data reveals a clear trend: only organisms with both WASP and SCAR/WAVE—activators of branched actin assembly—make actin-filled pseudopods. Although SCAR has been shown to drive pseudopod formation, WASP’s role in this process is controversial. We hypothesize that these genes collectively represent a genetic signature of α-motility because both are used for pseudopod formation. WASP depletion from human neutrophils confirms that both proteins are involved in explosive actin polymerization, pseudopod formation, and cell migration. WASP and WAVE also colocalize to dynamic signaling structures. Moreover, retention of WASP together with SCAR correctly predicts α-motility in disease-causing chytrid fungi, which we show crawl at >30 µm/min with actin-filled pseudopods. By focusing on one migration mode in many eukaryotes, we identify a genetic marker of pseudopod formation, the morphological feature of α-motility, providing evidence for a widely distributed mode of cell crawling with a single evolutionary origin.

Pyoderma gangrenosum and its syndromic forms: evidence for a link with autoinflammation

Pyoderma gangrenosum is a rare inflammatory neutrophilic dermatosis manifesting as painful ulcers with violaceous, undermined borders on the lower extremities. It may occur in the context of classic syndromes like PAPA (pyogenic arthritis, pyoderma gangrenosum and acne) and SAPHO (synovitis, acne, pustulosis, hyperostosis, osteitis), as well as in a recently described entity named PASH (pyoderma gangrenosum, acne and suppurative hidradenitis). Pyoderma gangrenosum has recently been included within the spectrum of autoinflammatory diseases, which are characterized by recurrent episodes of sterile inflammation, without circulating autoantibodies and autoreactive T cells. In PAPA syndrome, different mutations involving the PSTPIP1 gene, via an increased binding affinity to pyrin, induce the assembly of inflammasomes. These are molecular platforms involved in the activation of caspase 1, a protease that cleaves inactive prointerleukin (pro-IL)-1β to its active isoform IL-1β. The overproduction of IL-1β triggers the release of a number of proinflammatory cytokines and chemokines, which are responsible for the recruitment and activation of neutrophils, leading to neutrophil-mediated inflammation. In SAPHO syndrome, the activation of the PSTPIP2 inflammasome has been suggested to play a role in inducing the dysfunction of the innate immune system. Patients with PASH have recently been reported to present alterations of genes involved in well-known autoinflammatory diseases, such as PSTPIP1, MEFV, NOD2 and NLRP3. Pyoderma gangrenosum and its syndromic forms can be regarded as a single clinicopathological spectrum in the context of autoinflammation.

Super-resolution imaging for monitoring cytoskeleton dynamics

The cytoskeleton is a key cellular structure that is important in the control of cellular movement, structure, and sensing. To successfully image the individual cytoskeleton components, high resolution and super-resolution fluorescence imaging methods are needed. This review covers the three basic cytoskeletal elements and the relative benefits and drawbacks of fixed versus live cell imaging before moving on to recent studies using high resolution and super-resolution techniques. The techniques covered include the near-diffraction limited imaging methods of confocal microscopy and TIRF microscopy and the super-resolution fluorescence imaging methods of STORM, PALM, and STED.

Cortical actin and the plasma membrane: inextricably intertwined

The plasma membrane serves as a barrier, separating the cell from its external environment. Simultaneously it acts as a site for information transduction, entry of nutrients, receptor signaling, and adapts to the shape of the cell. This requires local control of organization at multiple scales in this heterogeneous fluid lipid bilayer with a plethora of proteins and a closely juxtaposed dynamic cortical cytoskeleton. New membrane models highlight the influence of the underlying cortical actin on the diffusion of membrane components. Myosin motors as well as proteins that remodel actin filaments have additionally been implicated in defining the organization of many membrane constituents. Here we provide a perspective of the intimate relationship of the membrane lipid matrix and the underlying cytoskeleton.

Action and Traction: Cytoskeletal Control of Receptor Triggering at the Immunological Synapse

It is well known that F-actin dynamics drive the micron-scale cell shape changes required for migration and immunological synapse (IS) formation. In addition, recent evidence points to a more intimate role for the actin cytoskeleton in promoting T cell activation. Mechanotransduction, the conversion of mechanical input into intracellular biochemical changes, is thought to play a critical role in several aspects of immunoreceptor triggering and downstream signal transduction. Multiple molecules associated with signaling events at the IS have been shown to respond to physical force, including the TCR, costimulatory molecules, adhesion molecules, and several downstream adapters. In at least some cases, it is clear that the relevant forces are exerted by dynamics of the T cell actomyosin cytoskeleton. Interestingly, there is evidence that the cytoskeleton of the antigen-presenting cell also plays an active role in T cell activation, by countering the molecular forces exerted by the T cell at the IS. Since actin polymerization is itself driven by TCR and costimulatory signaling pathways, a complex relationship exists between actin dynamics and receptor activation. This review will focus on recent advances in our understanding of the mechanosensitive aspects of T cell activation, paying specific attention to how F-actin-directed forces applied from both sides of the IS fit into current models of receptor triggering and activation.

Feel the force: Podosomes in mechanosensing

Cells interact with their environment through highly localized contact structures. Podosomes represent a subgroup of cell-matrix contacts, which is especially prominent in cells of the monocytic lineage such as monocytes, macrophages and dendritic cells, but also in a variety of other cell types. Comparable to other adhesion structures, podosomes feature a complex architecture, which forms the basis for their extensive repertoire of sensory and effector functions. These functions are mainly linked to interactions with the extracellular matrix and comprise well known properties such as cell-matrix adhesion and extracellular matrix degradation. A more recent discovery is the ability of podosomes to act as mechanosensory devices, by detecting rigidity and topography of the substratum. In this review, we focus especially on the molecular events involved in mechanosensing by podosomes, the structural elements of podosomes that enable this function, as well as the intra- and extracellular signals generated downstream of podosome mechanosensing.

Calmodulin inhibition regulates morphological and functional changes related to the actin cytoskeleton in pure microglial cells

The roles of calmodulin (CaM), a multifunctional intracellular calcium receptor protein, as concerns selected morphological and functional characteristics of pure microglial cells derived from mixed primary cultures from embryonal forebrains of rats, were investigated through use of the CaM antagonists calmidazolium (CALMID) and trifluoperazine (TFP). The intracellular localization of the CaM protein relative to phalloidin, a bicyclic heptapeptide that binds only to filamentous actin, and the ionized calcium-binding adaptor molecule 1 (Iba1), a microglia-specific actin-binding protein, was determined by immunocytochemistry, with quantitative analysis by immunoblotting. In unchallenged and untreated (control) microglia, high concentrations of CaM protein were found mainly perinuclearly in ameboid microglia, while the cell cortex had a smaller CaM content that diminished progressively deeper into the branches in the ramified microglia. The amounts and intracellular distributions of both Iba1 and CaM proteins were altered after lipopolysaccharide (LPS) challenge in activated microglia. CALMID and TFP exerted different, sometimes opposing, effects on many morphological, cytoskeletal and functional characteristics of the microglial cells. They affected the CaM and Iba1 protein expressions and their intracellular localizations differently, inhibited cell proliferation, viability and fluid-phase phagocytosis to different degrees both in unchallenged and in LPS-treated (immunologically challenged) cells, and differentially affected the reorganization of the actin cytoskeleton in the microglial cell cortex, influencing lamellipodia, filopodia and podosome formation. In summary, these CaM antagonists altered different aspects of filamentous actin-based cell morphology and related functions with variable efficacy, which could be important in deciphering the roles of CaM in regulating microglial functions in health and disease.

HIV-1 Infection of T Lymphocytes and Macrophages Affects Their Migration via Nef

The human immunodeficiency virus (HIV-1) disseminates in the body and is found in several organs and tissues. Although HIV-1 mainly targets both CD4⁺ T lymphocytes and macrophages, it has contrasting effects between these cell populations. HIV-1 infection namely reduces the viability of CD4⁺ T cells, whereas infected macrophages are long-lived. In addition, the migration of T cells is reduced by the infection, whereas HIV-1 differentially modulates the migration modes of macrophages. In 2-dimensions (2D) assays, infected macrophages are less motile compared to the control counterparts. In 3D environments, macrophages use two migration modes that are dependent on the matrix architecture: amoeboid and mesenchymal migration. HIV-1-infected macrophages exhibit a reduced amoeboid migration but an enhanced mesenchymal migration, via the viral protein Nef. Indeed, the mesenchymal migration involves podosomes, and Nef stabilizes these cell structures through the activation of the tyrosine kinase Hck, which in turn phosphorylates the Wiskott–Aldrich syndrome protein (WASP). WASP is a key player in actin remodeling and cell migration. The reprogramed motility of infected macrophages observed in vitro correlates in vivo with enhanced macrophage infiltration in experimental tumors in Nef-transgenic mice compared to control mice. In conclusion, HIV infection of host target cells modifies their migration capacity; we infer that HIV-1 enhances virus spreading in confined environments by reducing T cells migration, and facilitates virus dissemination into different organs and tissues of the human body by enhancing macrophage mesenchymal migration.

Thymosin α1 modifies podosome architecture and promptly stimulates the expression of podosomal markers in mature macrophages

BACKGROUND AND AIMS

The immunomodulatory activity of thymosin α1 (Tα1) on innate immunity has been extensively described, but its mechanism of action is not completely understood. We explored the possibility that Tα1-stimulation could affect the formation of podosomes, the highly dynamic, actin-rich, adhesion structures involved in macrophage adhesion/chemotaxis.

METHODS

The following methods were used: optical and scanning electron microscopy for analyzing morphology of human monocyte-derived macrophages (MDMs); time-lapse imaging for visualizing the time-dependent modifications induced at early times by Tα1 treatment; confocal microscopy and Western blot for analyzing localization and expression of podosome components; and Matrigel Migration Assay and zymography for testing MDM invasive ability and metalloproteinase secretion.

RESULTS

We obtained data to support that Tα1 could affect MDM motility, invasion and chemotaxis by promptly stimulating assembly and disassembly of podosomal structures. At very early times after its addition to cell culture medium and within 1 h of treatment, Tα1 induces modifications in MDM morphology and in podosomal components that are suggestive of increased podosome turnover.

CONCLUSIONS

Since impairment of podosome formation leads to reduced innate immunity and is associated with several immunodeficiency disorders, we confirm the validity of Tα1 as a potent activator of innate immunity and suggest possible new clinical application of this thymic peptide.

Actin foci facilitate activation of the phospholipase C-γ in primary T lymphocytes via the WASP pathway

Wiscott Aldrich Syndrome protein (WASP) deficiency results in defects in calcium ion signaling, cytoskeletal regulation, gene transcription and overall T cell activation. The activation of WASP constitutes a key pathway for actin filament nucleation. Yet, when WASP function is eliminated there is negligible effect on actin polymerization at the immunological synapse, leading to gaps in our understanding of the events connecting WASP and calcium ion signaling. Here, we identify a fraction of total synaptic F-actin selectively generated by WASP in the form of distinct F-actin ‘foci’. These foci are polymerized de novo as a result of the T cell receptor (TCR) proximal tyrosine kinase cascade, and facilitate distal signaling events including PLCγ1 activation and subsequent cytoplasmic calcium ion elevation. We conclude that WASP generates a dynamic F-actin architecture in the context of the immunological synapse, which then amplifies the downstream signals required for an optimal immune response.

DOI:http://dx.doi.org/10.7554/eLife.04953.001

The immune system is made up of several types of cells that protect the body against infection and disease. Immune cells such as T cells survey the body and when receptors on their surface encounter infected cells, the receptors activate the T cell by triggering a signaling pathway.

The early stages of T cell receptor signaling lead to the formation of a cell–cell contact zone called the immunological synapse. Filaments of a protein called F-actin—which are continuously assembled and taken apart—make versatile networks and help the immunological synapse to form. F-actin filaments have crucial roles in the later stages of T cell receptor signaling as well, but how they contribute to this is not clear. Whether it is the same F-actin network that participates both in synapse formation and the late stages of T cell receptor signaling, and if so, then by what mechanism, remains unknown.

The answers came from examining the function of a protein named Wiscott-Aldrich Syndrome Protein (WASP), which forms an F-actin network at the synapse. Loss of WASP is known to result in the X-linked Wiscott-Aldrich Syndrome immunodeficiency and bleeding disorder in humans. Although T cells missing WASP can construct immunological synapses, and these synapses do have normal levels of F-actin and early T cell receptor signaling, they still fail to respond to infected cells properly.

Kumari et al. analyzed the detailed structure and dynamics of actin filament networks at immunological synapses of normal and WASP-deficient T cells. Normally, cells had visible foci of newly polymerized F-actin directly above T cell receptor clusters in the immunological synapses, but these foci were not seen in the cells lacking WASP. Kumari et al. found that the F-actin foci facilitate the later stages of the signaling that activates the T cells; this signaling was lacking in WASP-deficient cells.

Altogether, Kumari et al. show that WASP-generated F-actin foci at immunological synapses bridge the early and later stages of T cell receptor signaling, effectively generating an optimal immune response against infected cells. Further work will now be needed to understand whether there are other F-actin substructures that play specialized roles in T cell signaling, and if foci play a related role in other cell types known to be affected in Wiscott-Aldrich Syndrome immunodeficiency.

DOI:http://dx.doi.org/10.7554/eLife.04953.002

Fascin actin bundling controls podosome turnover and disassembly while cortactin is involved in podosome assembly by its SH3 domain in THP-1 macrophages and dendritic cells

Podosomes are dynamic degrading devices present in myeloid cells among other cell types. They consist of an actin core with associated regulators, surrounded by an adhesive ring. Both fascin and cortactin are known constituents but the role of fascin actin bundling is still unclear and cortactin research rather focuses on its homologue hematopoietic lineage cell-specific protein-1 (HS1). A fascin nanobody (FASNb5) that inhibits actin bundling and a cortactin nanobody (CORNb2) specifically targeting its Src-homology 3 (SH3) domain were used as unique tools to study the function of these regulators in podosome dynamics in both THP-1 macrophages and dendritic cells (DC). Upon intracellular FASNb5 expression, the few podosomes present were aberrantly stable, long-living and large, suggesting a role for fascin actin bundling in podosome turnover and disassembly. Fascin modulates this by balancing the equilibrium between branched and bundled actin networks. In the presence of CORNb2, the few podosomes formed show disrupted structures but their dynamics were unaffected. This suggests a role of the cortactin SH3 domain in podosome assembly. Remarkably, both nanobody-induced podosome-losses were compensated for by focal adhesion structures. Furthermore, matrix degradation capacities were altered and migratory phenotypes were lost. In conclusion, the cortactin SH3 domain contributes to podosome assembly while fascin actin bundling is a master regulator of podosome disassembly in THP-1 macrophages and DC.

Tools of the trade: podosomes as multipurpose organelles of monocytic cells

Podosomes are adhesion and invasion structures that are particularly prominent in cells of the monocytic lineage such as macrophages, dendritic cells, and osteoclasts. They are multifunctional organelles that combine several key abilities required for cell migration and invasion. The podosome repertoire includes well-established functions such as cell-substrate adhesion, and extracellular matrix degradation, recently discovered abilities such as rigidity and topology sensing as well as antigen sampling, and also more speculative functions such as cell protrusion stabilization and transmigration. Collectively, podosomes not only enable dynamic interactions of cells with their surroundings, they also gather information about the pericellular environment, and are actively involved in its reshaping. This review presents an overview of the current knowledge on podosome composition, architecture, and regulation. We focus in particular on the growing list of podosome functions and discuss the specific properties of podosomes in macrophages, dendritic cells, and osteoclasts. Moreover, this article highlights podosome-related intracellular transport processes, the formation of podosomes in 3D environments as well as potentially podosome-associated diseases involving monocytic cells.

HIV-1 reprograms the migration of macrophages

Macrophages are motile leukocytes, targeted by HIV-1, thought to play a critical role in host dissemination of the virus. However, whether infection impacts their migration capacity remains unknown. We show that 2-dimensional migration and the 3-dimensional (3D) amoeboid migration mode of HIV-1-infected human monocyte-derived macrophages were inhibited, whereas the 3D mesenchymal migration was enhanced. The viral protein Nef was necessary and sufficient for all HIV-1-mediated effects on migration. In Nef transgenic mice, tissue infiltration of macrophages was increased in a tumor model and in several tissues at steady state, suggesting a dominant role for mesenchymal migration in vivo. The mesenchymal motility involves matrix proteolysis and podosomes, cell structures constitutive of monocyte-derived cells. Focusing on the mechanisms used by HIV-1 Nef to control the mesenchymal migration, we show that the stability, size, and proteolytic function of podosomes are increased via the phagocyte-specific kinase Hck and Wiskott-Aldrich syndrome protein (WASP), 2 major regulators of podosomes. In conclusion, HIV-1 reprograms macrophage migration, which likely explains macrophage accumulation in several patient tissues, which is a key step for virus spreading and pathogenesis. Moreover, Nef points out podosomes and the Hck/WASP signaling pathway as good candidates to control tissue infiltration of macrophages, a detrimental phenomenon in several diseases.

Protrusion force microscopy reveals oscillatory force generation and mechanosensing activity of human macrophage podosomes

Podosomes are adhesion structures formed in monocyte-derived cells. They are F-actin-rich columns perpendicular to the substrate surrounded by a ring of integrins. Here, to measure podosome protrusive forces, we designed an innovative experimental setup named protrusion force microscopy (PFM), which consists in measuring by atomic force microscopy the deformation induced by living cells onto a compliant Formvar sheet. By quantifying the heights of protrusions made by podosomes onto Formvar sheets, we estimate that a single podosome generates a protrusion force that increases with the stiffness of the substratum, which is a hallmark of mechanosensing activity. We show that the protrusive force generated at podosomes oscillates with a constant period and requires combined actomyosin contraction and actin polymerization. Finally, we elaborate a model to explain the mechanical and oscillatory activities of podosomes. Thus, PFM shows that podosomes are mechanosensing cell structures exerting a protrusive force.

Reaching for far-flung antigen: How solid-core podosomes of dendritic cells transform into protrusive structures

We recently identified a novel role for podosomes in antigen sampling. Podosomes are dynamic cellular structures that consist of point-like concentrations of actin surrounded by integrins and adaptor proteins such as vinculin and talin. Podosomes establish cellular contact with the extracellular matrix (ECM) and facilitate cell migration via ECM degradation. In our recent paper, we studied podosomes of human dendritic cells (DCs), major antigen presenting cells (APC) that take-up, process, and present foreign antigen to naive T-cells. We employed gelatin-impregnated porous polycarbonate filters to demonstrate that the mechanosensitive podosomes of DCs selectively localize to regions of low-physical resistance such as the filter pores. After degradation of the gelatin, podosomes increasingly protrude into the lumen of these pores. These protrusive podosome-derived structures contain several endocytic and early endosomal markers such as clathrin, Rab5, and VAMP3, and, surprisingly, also contain C-type lectins, a type of pathogen recognition receptors (PRRs). Finally, we performed functional uptake experiments to demonstrate that these PRRs facilitate uptake of antigen from the opposite side of the filter. Our data provide mechanistic insight in how dendritic cells sample for antigen across epithelial barriers for instance from the lumen of the lung and gut.

Two-dimensional motility of a macrophage cell line on microcontact-printed fibronectin

The ability of macrophages to migrate to sites of infection and inflammation is critical for their role in the innate immune response. Macrophage cell lines have made it possible to study the roles of individual proteins responsible for migration using molecular biology, but it has not been possible to reliably elicit the motility of macrophage cell lines in two dimensions. In the past, measurements of the motility of macrophage cell lines have been largely limited to transwell assays which provide limited quantitative information on motility and limited ability to visualize cell morphology. We used microcontact printing to create polydimethylsiloxane (PDMS) surfaces functionalized with fibronectin that otherwise support little macrophage adhesion. We used these surfaces to measure macrophage migration in two dimensions and found that these cells migrate efficiently in a uniform field of colony-stimulating factor-1, CSF-1. Knockdown of Cdc42 led to a nonstatistically significant reduction in motility, whereas chemical inhibition of PI3K activity led to a complete loss of motility. Inhibition of the RhoA kinase, ROCK, did not abolish the motility of these cells but caused a quantitative change in motility, reducing motility significantly on high concentrations of fibronectin but not on low concentrations. This study illustrates the importance of studying cell motility on well controlled materials to better understand the exact roles of specific proteins on cell migration. © 2014 Wiley Periodicals, Inc.

Inhibition of osteoclast bone resorption activity through osteoprotegerin-induced damage of the sealing zone

Bone remodeling is dependent on the dynamic equilibrium between osteoclast-mediated bone resorption and osteoblast-mediated osteogenesis. The sealing zone is an osteoclast-specific cytoskeletal structure, the integrity of which is critical for osteoclast-mediated bone resorption. To date, studies have focused mainly on the osteoprotegerin (OPG)‑induced inhibition of osteoclast differentiation through the OPG/receptor activator of the nuclear factor kappa-B ligand (RANKL)/RANK system, which affects the bone resorption of osteoclasts. However, the effects of OPG on the sealing zone have not been reported to date. In this study, the formation of the sealing zone was observed by Hoffman modulation contrast (HMC) microscopy and confocal laser scanning microscopy. The effects of OPG on the existing sealing zone and osteoclast-mediated bone resorption activity, as well as the regulatory role of genes involved in the formation of the sealing zone were examined by immunofluorescence staining, HMC microscopy, quantitative reverse transcription polymerase chain reaction (RT-qPCR), western blot analysis and scanning electron microscopy. The sealing zone was formed on day 5, with belt-like protuberances at the cell edge and scattered distribution of cell nuclei, but no filopodia. The sealing zone was intact in the untreated control group. However, defects in the sealing zone were observed in the OPG-treated group (20 ng/ml) and the structure was absent in the groups treated with 40 and 80 ng/ml OPG. The podosomes showed a scattered or clustered distribution between the basal surface of the osteoclasts and the well surface. Furthermore, resorption lacunae were not detected in the 20 ng/ml OPG-treated group, indicating the loss of osteoclast-mediated bone resorption activity. Treatment with OPG resulted in a significant decrease in the expression of Arhgef8/Net1 and DOCK5 Rho guanine nucleotide exchange factors (RhoGEFs), 10 of 18 RhoGTPases (RhoA, RhoB, cdc42v1, cdc42v2, RhoU/Wrch1, RhoF/Rif, Rac2, RhoG, Rnd1 and RhoBTB1), ROCK1 and ROCK2. In conclusion, podosome distribution was affected by the OPG-induced inhibition of the expression of genes in the RhoGTPase signaling pathway. This resulted in damage to or destruction of the sealing zone, thus inhibiting osteoclast-mediated bone resorption activity.

Triple-color FRET analysis reveals conformational changes in the WIP-WASp actin-regulating complex

Wiskott-Aldrich syndrome protein (WASp) is a key regulator of the actin cytoskeletal machinery. Binding of WASp-interacting protein (WIP) to WASp modulates WASp activity and protects it from degradation. Formation of the WIP-WASp complex is crucial for the adaptive immune response. We found that WIP and WASp interacted in cells through two distinct molecular interfaces. One interaction occurred between the WASp-homology-1 (WH1) domain of WASp and the carboxyl-terminal domain of WIP that depended on the phosphorylation status of WIP, which is phosphorylated by protein kinase C θ (PKCθ) in response to T cell receptor activation. The other interaction occurred between the verprolin homology, central hydrophobic region, and acidic region (VCA) domain of WASp and the amino-terminal domain of WIP. This latter interaction required actin, because it was inhibited by latrunculin A, which sequesters actin monomers. With triple-color fluorescence resonance energy transfer (3FRET) technology, we demonstrated that the WASp activation mechanism involved dissociation of the first interaction, while leaving the second interaction intact. This conformation exposed the ubiquitylation site on WASp, leading to degradation of WASp. Together, these data suggest that the activation and degradation of WASp are delicately balanced and depend on the phosphorylation state of WIP. Our molecular analysis of the WIP-WASp interaction provides insight into the regulation of actin-dependent processes.

CSF-1 receptor signaling in myeloid cells

The CSF-1 receptor (CSF-1R) is activated by the homodimeric growth factors colony-stimulating factor-1 (CSF-1) and interleukin-34 (IL-34). It plays important roles in development and in innate immunity by regulating the development of most tissue macrophages and osteoclasts, of Langerhans cells of the skin, of Paneth cells of the small intestine, and of brain microglia. It also regulates the differentiation of neural progenitor cells and controls functions of oocytes and trophoblastic cells in the female reproductive tract. Owing to this broad tissue expression pattern, it plays a central role in neoplastic, inflammatory, and neurological diseases. In this review we summarize the evolution, structure, and regulation of expression of the CSF-1R gene. We discuss the structures of CSF-1, IL-34, and the CSF-1R and the mechanism of ligand binding to and activation of the receptor. We further describe the pathways regulating macrophage survival, proliferation, differentiation, and chemotaxis downstream from the CSF-1R.

Generation of membrane structures during phagocytosis and chemotaxis of macrophages: role and regulation of the actin cytoskeleton

Macrophages are best known for their protective search and destroy functions against invading microorganisms. These processes are commonly known as chemotaxis and phagocytosis. Both of these processes require actin cytoskeletal remodeling to produce distinct F-actin-rich membrane structures called lamellipodia and phagocytic cups. This review will focus on the mechanisms by which macrophages regulate actin polymerization through initial receptor signaling and subsequent Arp2/3 activation by nucleation-promoting factors like the WASP/WAVE family, followed by remodeling of actin networks to produce these very distinct structures.

A new genetically encoded single-chain biosensor for Cdc42 based on FRET, useful for live-cell imaging

Cdc42 is critical in a myriad of cellular morphogenic processes, requiring precisely regulated activation dynamics to affect specific cellular events. To facilitate direct observations of Cdc42 activation in live cells, we developed and validated a new biosensor of Cdc42 activation. The biosensor is genetically encoded, of single-chain design and capable of correctly localizing to membrane compartments as well as interacting with its upstream regulators including the guanine nucleotide dissociation inhibitor. We characterized this new biosensor in motile mouse embryonic fibroblasts and observed robust activation dynamics at leading edge protrusions, similar to those previously observed for endogenous Cdc42 using the organic dye-based biosensor system. We then extended our validations and observations of Cdc42 activity to macrophages, and show that this new biosensor is able to detect differential activation patterns during phagocytosis and cytokine stimulation. Furthermore, we observe for the first time, a highly transient and localized activation of Cdc42 during podosome formation in macrophages, which was previously hypothesized but never directly visualized.

Integrin linked kinase (ILK) regulates podosome maturation and stability in dendritic cells

Podosomes are integrin-based adhesions fundamental for stabilisation of the leading lamellae in migrating dendritic cells (DCs) and for extracellular matrix (ECM) degradation. We have previously shown that soluble factors and chemokines such as SDF 1-a trigger podosome initiation whereas integrin ligands promote podosome maturation and stability in DCs. The exact intracellular signalling pathways that regulate the sequential organisation of podosomal components in response to extracellular cues remain largely undetermined. The Wiskott Aldrich Syndrome Protein (WASP) mediates actin polymerisation and the initial recruitment of integrins and associated proteins in a circular configuration surrounding the core of filamentous actin (F-actin) during podosome initiation. We have now identified integrin linked kinase (ILK) surrounding the podosomal actin core. We report that DC polarisation in response to chemokines and the assembly of actin cores during podosome initiation require PI3K-dependent clustering of the Wiskott Aldrich Syndrome Protein (WASP) in puncta independently of ILK. ILK is essential for the clustering of integrins and associated proteins leading to podosome maturation and stability that are required for degradation of the subjacent extracellular matrix and the invasive motility of DCs across connective tissue barriers.

We conclude that WASP regulates DCs polarisation for migration and initiation of actin polymerisation downstream of PI3K in nascent podosomes. Subsequently, ILK mediates the accumulation of integrin-associated proteins during podosome maturation and stability for efficient degradation of the subjacent ECM during the invasive migration of DCs.

Tyrosine phosphorylation of Wiskott-Aldrich syndrome protein (WASP) by Hck regulates macrophage function

We have shown previously that tyrosine phosphorylation of Wiskott-Aldrich syndrome protein (WASP) is important for diverse macrophage functions including phagocytosis, chemotaxis, podosome dynamics, and matrix degradation. However, the specific tyrosine kinase mediating WASP phosphorylation is still unclear. Here, we provide evidence that Hck, which is predominantly expressed in leukocytes, can tyrosine phosphorylate WASP and regulates WASP-mediated macrophage functions. We demonstrate that tyrosine phosphorylation of WASP in response to stimulation with CX3CL1 or via Fcγ receptor ligation were severely reduced in Hck(-/-) bone marrow-derived macrophages (BMMs) or in RAW/LR5 macrophages in which Hck expression was silenced using RNA-mediated interference (Hck shRNA). Consistent with reduced WASP tyrosine phosphorylation, phagocytosis, chemotaxis, and matrix degradation are reduced in Hck(-/-) BMMs or Hck shRNA cells. In particular, WASP phosphorylation was primarily mediated by the p61 isoform of Hck. Our studies also show that Hck and WASP are required for passage through a dense three-dimensional matrix and transendothelial migration, suggesting that tyrosine phosphorylation of WASP by Hck may play a role in tissue infiltration of macrophages. Consistent with a role for this pathway in invasion, WASP(-/-) BMMs do not invade into tumor spheroids with the same efficiency as WT BMMs and cells expressing phospho-deficient WASP have reduced ability to promote carcinoma cell invasion. Altogether, our results indicate that tyrosine phosphorylation of WASP by Hck is required for proper macrophage functions.

Podosomes of dendritic cells facilitate antigen sampling

Dendritic cells sample the environment for antigens and play an important role in establishing the link between innate and acquired immunity. Dendritic cells contain mechanosensitive adhesive structures called podosomes that consist of an actin-rich core surrounded by integrins, adaptor proteins and actin network filaments. They facilitate cell migration via localized degradation of extracellular matrix. Here, we show that podosomes of human dendritic cells locate to spots of low physical resistance in the substrate (soft spots) where they can evolve into protrusive structures. Pathogen recognition receptors locate to these protrusive structures where they can trigger localized antigen uptake, processing and presentation to activate T-cells. Our data demonstrate a novel role in antigen sampling for the podosomes of dendritic cells.

The F-BAR protein PSTPIP1 controls extracellular matrix degradation and filopodia formation in macrophages

PSTPIP1 is a cytoskeletal adaptor and F-BAR protein that has been implicated in autoinflammatory disease, most notably in the PAPA syndrome: pyogenic sterile arthritis, pyoderma gangrenosum, and acne. However, the mechanism by which PSTPIP1 regulates the actin cytoskeleton and contributes to disease pathogenesis remains elusive. Here, we show that endogenous PSTPIP1 negatively regulates macrophage podosome organization and matrix degradation. We identify a novel PSTPIP1-R405C mutation in a patient presenting with aggressive pyoderma gangrenosum. Identification of this mutation reveals that PSTPIP1 regulates the balance of podosomes and filopodia in macrophages. The PSTPIP1-R405C mutation is in the SRC homology 3 (SH3) domain and impairs Wiskott-Aldrich syndrome protein (WASP) binding, but it does not affect interaction with protein-tyrosine phosphatase (PTP)-PEST. Accordingly, WASP inhibition reverses the elevated F-actin content, filopodia formation, and matrix degradation induced by PSTPIP1-R405C. Our results uncover a novel role for PSTPIP1 and WASP in orchestrating different types of actin-based protrusions. Our findings implicate the cytoskeletal regulatory functions of PSTPIP1 in the pathogenesis of pyoderma gangrenosum and suggest that the cytoskeleton is a rational target for therapeutic intervention in autoinflammatory disease.

Spire-1 contributes to the invadosome and its associated invasive properties

Cancer cells have an increased ability to squeeze through extracellular matrix gaps that they create by promoting proteolysis of its components. Major sites of degradation are specialized micro-domains in the plasma membrane collectively named invadosomes where the Arp2/3 complex and formin proteins cooperate to spatio-temporally control actin nucleation and the folding of a dynamic F-actin core. At invadosomes, proper coupling of exo-endocytosis allows polarized delivery of proteases that facilitate degradation of ECM and disruption of the cellular barrier. We investigated the contribution of the actin nucleator Spire-1 to invadosome structure and function, using Src-activated cells and cancer cells. We found that Spire-1 is specifically recruited at invadosomes and is part of a multi-molecular complex containing Src kinase, the formin mDia1 and actin. Spire-1 interacts with the Rab3A GTPase, a key player in the regulation of exocytosis that is present at invadosomes. Finally, over- and under-expression of Spire-1 resulted in cells with an increased or decreased potential for matrix degradation, respectively, therefore suggesting a functional interplay of Spire-1 with both actin nucleation and vesicular trafficking that might impact on cell invasive and metastatic behavior.

Wiskott-Aldrich syndrome protein regulates leukocyte-dependent breast cancer metastasis

A paracrine interaction between epidermal growth factor (EGF)-secreting tumor-associated macrophages (TAMs) and colony-stimulating factor 1 (CSF-1)-secreting breast carcinoma cells promotes invasion and metastasis. Here, we show that mice deficient in the hematopoietic-cell-specific Wiskott-Aldrich syndrome protein (WASp) are unable to support TAM-dependent carcinoma cell invasion and metastasis in both orthotopic and transgenic models of mammary tumorigenesis. Motility and invasion defects of tumor cells were recapitulated ex vivo upon coculture with WASp(-/-) macrophages. Mechanistically, WASp is required for macrophages to migrate toward CSF-1-producing carcinoma cells, as well as for the release of EGF through metalloprotease-dependent shedding of EGF from the cell surface of macrophages. Our findings suggest that WASp acts to support both the migration of TAMs and the production of EGF, which in concert promote breast tumor metastasis.

Uniform total internal reflection fluorescence illumination enables live cell fluorescence resonance energy transfer microscopy

Fluorescence resonance energy transfer (FRET) microscopy is a powerful technique to quantify dynamic protein-protein interactions in live cells. Total internal reflection fluorescence (TIRF) microscopy can selectively excite molecules within about 150 nm of the glass-cell interface. Recently, these two approaches were combined to enable high-resolution FRET imaging on the adherent surface of living cells. Here, we show that interference fringing of the coherent laser excitation used in TIRF creates lateral heterogeneities that impair quantitative TIRF-FRET measurements. We overcome this limitation by using a two-dimensional scan head to rotate laser beams for donor and acceptor excitation around the back focal plane of a high numerical aperture objective. By setting different radii for the circles traced out by each laser in the back focal plane, the penetration depth was corrected for different wavelengths. These modifications quell spatial variations in illumination and permit calibration for quantitative TIRF-FRET microscopy. The capability of TIRF-FRET was demonstrated by imaging assembled cyan and yellow fluorescent protein-tagged HIV-Gag molecules in single virions on the surfaces of living cells. These interactions are shown to be distinct from crowding of HIV-Gag in lipid rafts.

Megakaryocytes assemble podosomes that degrade matrix and protrude through basement membrane

Megakaryocytes give rise to platelets via extension of proplatelet arms, which are released through the vascular sinusoids into the bloodstream. Megakaryocytes and their precursors undergo varying interactions with the extracellular environment in the bone marrow during their maturation and positioning in the vascular niche. We demonstrate that podosomes are abundant in primary murine megakaryocytes adherent on multiple extracellular matrix substrates, including native basement membrane. Megakaryocyte podosome lifetime and density, but not podosome size, are dependent on the type of matrix, with podosome lifetime dramatically increased on collagen fibers compared with fibrinogen. Podosome stability and dynamics depend on actin cytoskeletal dynamics but not matrix metalloproteases. However, podosomes degrade matrix and appear to be important for megakaryocytes to extend protrusions across a native basement membrane. We thus demonstrate for the first time a fundamental requirement for podosomes in megakaryocyte process extension across a basement membrane, and our results suggest that podosomes may have a role in proplatelet arm extension or penetration of basement membrane.

Deficiencies of the lipid-signaling enzymes phospholipase D1 and D2 alter cytoskeletal organization, macrophage phagocytosis, and cytokine-stimulated neutrophil recruitment

Cell migration and phagocytosis ensue from extracellular-initiated signaling cascades that orchestrate dynamic reorganization of the actin cytoskeleton. The reorganization is mediated by effector proteins recruited to the site of activity by locally-generated lipid second messengers. Phosphatidic acid (PA), a membrane phospholipid generated by multiple enzyme families including Phospholipase D (PLD), has been proposed to function in this role. Here, we show that macrophages prepared from mice lacking either of the classical PLD isoforms PLD1 or PLD2, or wild-type macrophages whose PLD activity has been pharmacologically inhibited, display isoform-specific actin cytoskeleton abnormalities that likely underlie decreases observed in phagocytic capacity. Unexpectedly, PA continued to be detected on the phagosome in the absence of either isoform and even when all PLD activity was eliminated. However, a disorganized phagocytic cup was observed as visualized by imaging PA, F-actin, Rac1, an organizer of the F-actin network, and DOCK2, a Rac1 activator, suggesting that PLD-mediated PA production during phagocytosis is specifically critical for the integrity of the process. The abnormal F-actin reorganization additionally impacted neutrophil migration and extravasation from the vasculature into interstitial tissues. Although both PLD1 and PLD2 were important in these processes, we also observed isoform-specific functions. PLD1-driven processes in particular were observed to be critical in transmigration of macrophages exiting the vasculature during immune responses such as those seen in acute pancreatitis or irritant-induced skin vascularization.