The cortactin-binding domain of WIP is essential for podosome formation and extracellular matrix degradation by murine dendritic cells

Indoxyl sulfate- and P-cresol-induced monocyte adhesion and migration is mediated by integrin-linked kinase-dependent podosome formation

Cardiovascular disease is an important cause of death in patients with chronic kidney disease (CKD). Protein-bound uremic toxins, such as p-cresyl and indoxyl sulfate (IS), are poorly removed during hemodialysis, leading to vascular endothelial dysfunction and leukocyte extravasation. These processes can be related to dynamic adhesion structures called podosomes. Several studies have indicated the role of integrin-linked kinase (ILK) in the accumulation of integrin-associated proteins in podosomes. Here, we investigated the involvement of ILK and podosome formation in the adhesion and extravasation of monocytes under p-cresol (pc) and IS exposure. Incubation of THP-1 human monocyte cells with these toxins upregulated ILK kinase activity. Together, both toxins increased cell adhesion, podosome formation, extracellular matrix degradation, and migration of THP-1 cells, whereas ILK depletion with specific small interfering RNAs suppressed these processes. Interestingly, F-actin colocalized with cortactin in podosome cores, while ILK was colocalized in podosome rings under toxin stimulation. Podosome Wiskott-Aldrich syndrome protein (WASP)-interacting protein (WIP) and AKT protein depletion demonstrated that monocyte adhesion depends on podosome formation and that the ILK/AKT signaling pathway is involved in these processes. Ex vivo experiments showed that both toxins induced adhesion and podosome formation in leukocytes from wild-type mice, whereas these effects were not observed in leukocytes of conditional ILK-knockdown animals. In summary, under pc and IS stimulation, monocytes increase podosome formation and transmigratory capacity through an ILK/AKT signaling pathway-dependent mechanism, which could lead to vascular injury. Therefore, ILK could be a potential therapeutic target for the treatment of vascular damage associated with CKD.

An enzyme involved in organizing structural proteins into protrusions of the cell membrane helps facilitate the movement of white blood cells that occurs in chronic kidney disease and can lead to cardiovascular damage. Laura Calleros from the University of Alcalá, Madrid, Spain, and colleagues showed how toxic metabolites that build up in the bloodstream as kidneys fail induce the activity of an enzyme called integrin-linked kinase (ILK). This enzyme in turn spurs white blood cells to form protrusions called podosomes that make the cells more mobile, enabling them to move out of blood vessels and into surrounding tissues where they can cause inflammation -mediated injury. ILK inactivation reversed these effects. Therapies that target the enzyme could therefore help limit the cardiovascular complications of chronic kidney disease.

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.

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.

The Disordered Cellular Multi-Tasker WIP and Its Protein-Protein Interactions: A Structural View

WASp-interacting protein (WIP), a regulator of actin cytoskeleton assembly and remodeling, is a cellular multi-tasker and a key member of a network of protein-protein interactions, with significant impact on health and disease. Here, we attempt to complement the well-established understanding of WIP function from cell biology studies, summarized in several reviews, with a structural description of WIP interactions, highlighting works that present a molecular view of WIP's protein-protein interactions. This provides a deeper understanding of the mechanisms by which WIP mediates its biological functions. The fully disordered WIP also serves as an intriguing example of how intrinsically disordered proteins (IDPs) exert their function. WIP consists of consecutive small functional domains and motifs that interact with a host of cellular partners, with a striking preponderance of proline-rich motif capable of interactions with several well-recognized binding partners; indeed, over 30% of the WIP primary structure are proline residues. We focus on the binding motifs and binding interfaces of three important WIP segments, the actin-binding N-terminal domain, the central domain that binds SH3 domains of various interaction partners, and the WASp-binding C-terminal domain. Beyond the obvious importance of a more fundamental understanding of the biology of this central cellular player, this approach carries an immediate and highly beneficial effect on drug-design efforts targeting WIP and its binding partners. These factors make the value of such structural studies, challenging as they are, readily apparent.

Primary immunodeficiencies caused by mutations in actin regulatory proteins

The identification of patients with monogenic gene defects have illuminated the function of different proteins in the immune system, including proteins that regulate the actin cytoskeleton. Many of these actin regulatory proteins are exclusively expressed in leukocytes and regulate the formation and branching of actin filaments. Their absence or abnormal function leads to defects in immune cell shape, cellular projections, migration, and signaling. Through the study of patients' mutations and generation of mouse models that recapitulate the patients' phenotypes, our laboratory and others have gained a better understanding of the role these proteins play in cell biology and the underlying pathogenesis of immunodeficiencies and immune dysregulatory syndromes.

A C-Terminal Fragment of Chlorotoxin Retains Bioactivity and Inhibits Cell Migration

Chlorotoxin was originally isolated from the venom of the Israeli scorpion Leiurus quinquestriatus, and has potential as a tumor imaging agent based on its selective binding to tumor cells. Several targets have been suggested for chlorotoxin including voltage-gated chloride channels, and it has been shown to have anti-angiogenic activity and inhibit cell migration. The structure of chlorotoxin is stabilized by four disulfide bonds and contains β-sheet and helical structure. Interestingly, the reduced form has previously been shown to inhibit cell migration to the same extent as the wild type, but structural analysis indicates that the reduced form of the peptide does not maintain the native secondary structure and appears unstructured in solution. This lack of structure suggests that a short stretch of amino acids might be responsible for the bioactivity. To explore this hypothesis, we have synthesized fragments of chlorotoxin without disulfide bonds. As expected for such small peptides, NMR analysis indicated that the peptides were unstructured in solution. However, the peptide corresponding to the eight C-terminal residues inhibited cell migration, in contrast to the other fragments. Our results suggest that the C-terminal region plays a critical role in the bioactivity of chlorotoxin.

Specialized Roles for Actin in Osteoclasts: Unanswered Questions and Therapeutic Opportunities

Osteoclasts are cells of the hematopoietic lineage that are specialized to resorb bone. In osteoclasts, the actin cytoskeleton engages in at least two unusual activities that are required for resorption. First, microfilaments form a dynamic and structurally elaborate actin ring. Second, microfilaments bind vacuolar H⁺-ATPase (V-ATPase) and are involved in forming the V-ATPase-rich ruffled plasma membrane. The current review examines these two specialized functions with emphasis on the identification of new therapeutic opportunities. The actin ring is composed of substructures called podosomes that are interwoven to form a cohesive superstructure. Studies examining the regulation of the formation of actin rings and its constituent proteins are reviewed. Areas where there are gaps in the knowledge are highlighted. Microfilaments directly interact with the V-ATPase through an actin binding site in the B2-subunit of V-ATPase. This binding interaction is required for ruffled membrane formation. Recent studies show that an inhibitor of the interaction blocks bone resorption in pre-clinical animal models, including a model of post-menopausal osteoporosis. Because the unusual actin-based resorption complex is unique to osteoclasts and essential for bone resorption, it is likely that deeper understanding of its underlying mechanisms will lead to new approaches to treat bone disease.

WIP/ITSN1 complex is involved in cellular vesicle trafficking and formation of filopodia-like protrusions

WIP (WASP interacting protein) together with N-WASP (neural Wiskott-Aldrich syndrome protein) regulates actin polymerization that is crucial for invadopodia and filopodia formation. Recently, we reported the WIP interaction with ITSN1 which is highly implicated in endo-/exocytosis, apoptosis, mitogenic signaling and cytoskeleton rearrangements. Here we demonstrate that the WIP/ITSN1 complex is involved in the transferrin receptor recycling and partially co-localizes with a marker of the fast recycling endosomes, RAB4. Moreover, ITSN1 recruits WIP to RAB4-positive vesicles upon overexpression. Our data indicate that WIP enhances the interaction of N-WASP with ITSN1 and promotes ITSN1/β-actin association. Moreover, the WIP/ITSN1-L complex facilitates formation of filopodia-like protrusions in MCF-7 cells. Thus, WIP/ITSN1 complex is involved in the cellular vesicle trafficking and actin-dependent membrane processes.

WIP-YAP/TAZ as A New Pro-Oncogenic Pathway in Glioma

Wild-type p53 (wtp53) is described as a tumour suppressor gene, and mutations in p53 occur in many human cancers. Indeed, in high-grade malignant glioma, numerous molecular genetics studies have established central roles of RTK-PI3K-PTEN and ARF-MDM2-p53 INK4a-RB pathways in promoting oncogenic capacity. Deregulation of these signalling pathways, among others, drives changes in the glial/stem cell state and environment that permit autonomous growth. The initially transformed cell may undergo subsequent modifications, acquiring a more complete tumour-initiating phenotype responsible for disease advancement to stages that are more aggressive. We recently established that the oncogenic activity of mutant p53 (mtp53) is driven by the actin cytoskeleton-associated protein WIP (WASP-interacting protein), correlated with tumour growth, and more importantly that both proteins are responsible for the tumour-initiating cell phenotype. We reported that WIP knockdown in mtp53-expressing glioblastoma greatly reduced proliferation and growth capacity of cancer stem cell (CSC)-like cells and decreased CSC-like markers, such as hyaluronic acid receptor (CD44), prominin-1 (CD133), yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ). We thus propose a new CSC signalling pathway downstream of mtp53 in which Akt regulates WIP and controls YAP/TAZ stability. WIP drives a mechanism that stimulates growth signals, promoting YAP/TAZ and β-catenin stability in a Hippo-independent fashion, which allows cells to coordinate processes such as proliferation, stemness and invasiveness, which are key factors in cancer progression. Based on this multistep tumourigenic model, it is tantalizing to propose that WIP inhibitors may be applied as an effective anti-cancer therapy.

Epigenetically upregulated WIPF1 plays a major role in BRAF V600E-promoted papillary thyroid cancer aggressiveness

How the BRAF V600E mutation promotes the pathogenesis and aggressiveness of papillary thyroid cancer (PTC) is not completely understood. Here we explored a novel mechanism involving WASP interacting protein family member 1 (WIPF1). In PTC tumors, compared with the wild-type BRAF, BRAF V600E was associated with over-expression and hypomethylation of the WIPF1 gene. In thyroid cancer cell lines with wild-type BRAF, WIPF1 expression was robustly upregulated upon introduced expression of BRAF V600E (P=0.03) whereas the opposite was seen upon BRAF knockdown or treatment with BRAF V600E or MEK inhibitors in cells harboring BRAF V600E. Methylation of a functionally critical region of the WIPF1 promoter was decreased by expressing BRAF V600E in cells harboring the wild-type BRAF and increased by BRAF knockdown or treatment with BRAF V600E or MEK inhibitors in cells harboring BRAF V600E mutation. Under-expression and hypermethylation of WIPF1 induced by stable BRAF knockdown was reversed by DNA demethylating agent 5′-azadeoxycytidine. Knockdown of WIPF1 robustly inhibited anchorage-independent colony formation, migration, and invasion of thyroid cancer cells and suppressed xenograft thyroid cancer tumor growth and vascular invasion, mimicking the effects of BRAF knockdown. In human PTC tumors, WIPF1 expression was associated with extrathyroidal invasion (P=0.01) and lymph node metastasis (P=2.64E-05). In summary, BRAF V600E-activated MAP kinase pathway causes hypomethylation and overexpression of WIPF1; WIPF1 then functions like an oncoprotein to robustly promote aggressive cellular and tumor behaviors of PTC. This represents a novel mechanism in BRAF V600E-promoted PTC aggressiveness and identifies WIPF1 as a novel therapeutic target for thyroid cancer.

Cortactin function in invadopodia

Actin remodeling plays an essential role in diverse cellular processes such as cell motility, vesicle trafficking or cytokinesis. The scaffold protein and actin nucleation promoting factor Cortactin is present in virtually all actin-based structures, participating in the formation of branched actin networks. It has been involved in the control of endocytosis, and vesicle trafficking, axon guidance and organization, as well as adhesion, migration and invasion. To migrate and invade through three-dimensional environments, cells have developed specialized actin-based structures called invadosomes, a generic term to designate invadopodia and podosomes. Cortactin has emerged as a critical regulator of invadosome formation, function and disassembly. Underscoring this role, Cortactin is frequently overexpressed in several types of invasive cancers. Herein we will review the roles played by Cortactin in these specific invasive structures.

Invadosomes are coming: new insights into function and disease relevance

Invadopodia and podosomes are discrete, actin-based molecular protrusions that form in cancer cells and normal cells, respectively, in response to diverse signaling pathways and extracellular matrix cues. Although they participate in a host of different cellular processes, they share a common functional theme of controlling pericellular proteolytic activity, which sets them apart from other structures that function in migration and adhesion, including focal adhesions, lamellipodia, and filopodia. In this review, we highlight research that explores the function of these complex structures, including roles for podosomes in embryonic and postnatal development, in angiogenesis and remodeling of the vasculature, in maturation of the postsynaptic membrane, in antigen sampling and recognition, and in cell-cell fusion mechanisms, as well as the involvement of invadopodia at multiple steps of the metastatic cascade, and how all of this may apply in the treatment of human disease states. Finally, we explore recent research that implicates a novel role for exosomes and microvesicles in invadopodia-dependent and invadopodia-independent mechanisms of invasion, respectively.

Persistent Morbillivirus Infection Leads to Altered Cortactin Distribution in Histiocytic Sarcoma Cells with Decreased Cellular Migration Capacity

Histiocytic sarcomas represent rare but fatal neoplasms in humans. Based on the absence of a commercially available human histiocytic sarcoma cell line the frequently affected dog displays a suitable translational model. Canine distemper virus, closely related to measles virus, is a highly promising candidate for oncolytic virotherapy. Therapeutic failures in patients are mostly associated with tumour invasion and metastasis often induced by misdirected cytoskeletal protein activities. Thus, the impact of persistent canine distemper virus infection on the cytoskeletal protein cortactin, which is frequently overexpressed in human cancers with poor prognosis, was investigated in vitro in a canine histiocytic sarcoma cell line (DH82). Though phagocytic activity, proliferation and apoptotic rate were unaltered, a significantly reduced migration activity compared to controls (6 hours and 1 day after seeding) accompanied by a decreased number of cortactin mRNA transcripts (1 day) was detected. Furthermore, persistently canine distemper virus infected DH82 cells showed a predominant diffuse intracytoplasmic cortactin distribution at 6 hours and 1 day compared to controls with a prominent membranous expression pattern (p ≤ 0.05). Summarized, persistent canine distemper virus infection induces reduced tumour cell migration associated with an altered intracellular cortactin distribution, indicating cytoskeletal changes as one of the major pathways of virus-associated inhibition of tumour spread.

WIP and WICH/WIRE co-ordinately control invadopodium formation and maturation in human breast cancer cell invasion

Cancer cells form actin-rich degradative protrusions (invasive pseudopods and invadopodia), which allows their efficient dispersal during metastasis. Using biochemical and advanced imaging approaches, we demonstrate that the N-WASP-interactors WIP and WICH/WIRE play non-redundant roles in cancer cell invasion. WIP interacts with N-WASP and cortactin and is essential for invadopodium assembly, whereas WICH/WIRE regulates N-WASP activation to control invadopodium maturation and degradative activity. Our data also show that Nck interaction with WIP and WICH/WIRE modulates invadopodium maturation; changes in WIP and WICH/WIRE levels induce differential distribution of Nck. We show that WIP can replace WICH/WIRE functions and that elevated WIP levels correlate with high invasiveness. These findings identify a role for WICH/WIRE in invasiveness and highlight WIP as a hub for signaling molecule recruitment during invadopodium generation and cancer progression, as well as a potential diagnostic biomarker and an optimal target for therapeutic approaches.

Human Immunodeficiencies Related to Defective APC/T Cell Interaction

The primary event for initiating adaptive immune responses is the encounter between T lymphocytes and antigen presenting cells (APCs) in the T cell area of secondary lymphoid organs and the formation of highly organized intercellular junctions referred to as immune synapses (IS). In vivo live-cell imaging of APC-T cell interactions combined to functional studies unveiled that T cell fate is dictated, in large part, by the stability of the initial contact. Immune cell interaction is equally important during delivery of T cell help to B cells and for the killing of target cells by cytotoxic T cells and NK cells. The critical role of contact dynamics and synapse stability on the immune response is well illustrated by human immune deficiencies in which disease pathogenesis is linked to altered adhesion or defective cross-talk between the synaptic partners. The Wiskott-Aldrich syndrome (WAS) is a severe primary immunodeficiency caused by mutations in the Wiskott-Aldrich syndrome protein (WASp), a scaffold that promotes actin polymerization and links TCR stimulation to T cell activation. Absence or mutations in WASp affects intercellular APC-T cell communications by interfering with multiple mechanisms on both sides of the IS. The warts, hypogammaglobulinemia, infections, and myelokathexis (WHIM) syndrome is caused by mutations in CXCR4, a chemokine receptor that in mutant form leads to impairment of APC-T cell interactions. Present evidences suggest that other recently characterized primary immune deficiencies caused by mutation in genes linked to actin cytoskeletal reorganization, such as WIP and DOCK8, may also depend on altered synapse stability. Here, we will discuss in details the mechanisms of disturbed APC-T cell interactions in WAS and WHIM. Moreover, we will summarize the evidence pointing to a compromised conjugate formation in WIP, DOCK8, and X-linked lymphoproliferative syndrome.

Podosome organization drives osteoclast-mediated bone resorption

Osteoclasts are the cells responsible for physiological bone resorption. A specific organization of their most prominent cytoskeletal structures, podosomes, is crucial for the degradation of mineralized bone matrix. Each podosome is constituted of an F-actin-enriched central core surrounded by a loose F-actin network, called the podosome cloud. In addition to intrinsic actin dynamics, podosomes are defined by their adhesion to the extracellular matrix, mainly via core-linking CD44 and cloud-linking integrins. These properties allow podosomes to collectively evolve into different patterns implicated in migration and bone resorption. Indeed, to resorb bone, osteoclasts polarize, actively secrete protons, and proteases into the resorption pit where these molecules are confined by a podosome-containing sealing zone. Here, we review recent advancements on podosome structure and regulatory pathways in osteoclasts. We also discuss the distinct functions of different podosome patterns during the lifespan of a single osteoclast.

Intersectin adaptor proteins are associated with actin-regulating protein WIP in invadopodia

Invasive cancer cells form actin-rich membrane protrusions called invadopodia that degrade extracellular matrix and facilitate cell invasion and metastasis. WIP (WASP-interacting protein) together with N-WASP (neural Wiskott-Aldrich syndrome protein) are localized in invadopodia and play a crucial role in their formation. Here we show that WIP interacts with endocytic adaptor proteins of the intersectin (ITSN) family, ITSN1 and ITSN2. The interaction is mediated by the SH3 domains of ITSNs and the middle part of the WIP proline-rich motifs. We have also demonstrated that ITSN1, WIP and N-WASP can form a complex in cells. Endogenous ITSN1 and ITSN2 are located in invasive protrusions of MDA-MB-231 breast cancer cell line. Moreover, data from immunofluorescent analysis revealed co-localization of ITSN1 and WIP at sites of invadopodia formation and in clathrin-coated pits. Together, these findings provide insights into the molecular mechanisms of invadopodia formation and identify ITSNs as scaffold proteins involved in this process.

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.

Tyrosine phosphorylation of WIP releases bound WASP and impairs podosome assembly in macrophages

Podosomes are integrin-containing adhesion structures commonly found in migrating leukocytes of the monocytic lineage. The actin cytoskeletal organisation of podosomes is based on a WASP- and Arp2/3-mediated mechanism. WASP also associates with a second protein, WIP (also known as WIPF1), and they co-localise in podosome cores. Here, we report for the first time that WIP can be phosphorylated on tyrosine residues and that tyrosine phosphorylation of WIP is a trigger for release of WASP from the WIP-WASP complex. Using a knockdown approach together with expression of WIP phosphomimics, we show that in the absence of WIP-WASP binding, cellular WASP is rapidly degraded, leading to disruption of podosomes and a failure of cells to degrade an underlying matrix. In the absence of tyrosine phosphorylation, the WIP-WASP complex remains intact and podosome lifetimes are extended. A screen of candidate kinases and inhibitor-based assays identified Bruton's tyrosine kinase (Btk) as a regulator of WIP tyrosine phosphorylation. We conclude that tyrosine phosphorylation of WIP is a crucial regulator of WASP stability and function as an actin-nucleation-promoting factor.

WIP is necessary for matrix invasion by breast cancer cells

Actin filament assembly and reorganisation during cell migration and invasion into extracellular matrices is a well-documented phenomenon. Among actin-binding proteins regulating its polymerisation, the members of the WASP (Wiskott Aldrich Syndrome Protein) family are generally thought to play the most significant role in supporting cell invasiveness. In situ, cytosolic N-WASP (neural WASP) is associated with a partner protein termed WIP (WASP Interacting Protein) that is bound to the N-terminal domain of N-WASP. Despite much effort, rather little is known about the role of WIP in regulating N-WASP and consequent actin-filament assembly. Even less is known about the function of WIP within the specialised cell adhesion and attachment structures known as podosomes and invadopodia. In particular, whilst the interaction of WIP with known participants in the development and maturation of invadopodia such as N-WASP, the Arp2/3 complex and cortactin has been described, little is known concerning the direct contribution of WIP to invadopodia and its potential role as a regulator of cancer cell invasion. In this report, we use 2D and 3D culture systems to describe the role played by WIP in modulating the morphology and invasiveness of metastatic breast cancer cells in vitro, as well as its effect on the process of mesenchymal-epithelial transition (MET) seen in these cells. We demonstrate that WIP is necessary for invadopodium formation and matrix degradation by basal breast cancer cells, but not sufficient to induce invasiveness in luminal cells.

WIP: more than a WASp-interacting protein

WIP plays an important role in the remodeling of the actin cytoskeleton, which controls cellular activation, proliferation, and function. WIP regulates actin polymerization by linking the actin machinery to signaling cascades. WIP binding to WASp and to its homolog, N-WASp, which are central activators of the actin-nucleating complex Arp2/3, regulates their cellular distribution, function, and stability. By binding to WASp, WIP protects it from degradation and thus, is crucial for WASp retention. Indeed, most mutations that result in WAS, an X-linked immunodeficiency caused by defective/absent WASp activity, are located in the WIP-binding region of WASp. In addition, by binding directly to actin, WIP promotes the formation and stabilization of actin filaments. WASp-independent activities of WIP constitute a new research frontier and are discussed extensively in this article. Here, we review the current information on WIP in human and mouse systems, focusing on its associated proteins, its molecular-regulatory mechanisms, and its role as a key regulator of actin-based processes in the immune system.

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.

Stratifying fascin and cortactin function in invadopodium formation using inhibitory nanobodies and targeted subcellular delocalization

Invadopodia are actin-rich protrusions arising through the orchestrated regulation of precursor assembly, stabilization, and maturation, endowing cancer cells with invasive properties. Using nanobodies (antigen-binding domains of Camelid heavy-chain antibodies) as perturbators of intracellular functions and/or protein domains at the level of the endogenous protein, we examined the specific contribution of fascin and cortactin during invadopodium formation in MDA-MB-231 breast and PC-3 prostate cancer cells. A nanobody (K(d)~35 nM, 1:1 stoichiometry) that disrupts fascin F-actin bundling emphasizes the importance of stable actin bundles in invadopodium array organization and turnover, matrix degradation, and cancer cell invasion. Cortactin-SH3 dependent WIP recruitment toward the plasma membrane was specifically inhibited by a cortactin nanobody (K(d)~75 nM, 1:1 stoichiometry). This functional domain is shown to be important for formation of properly organized invadopodia, MMP-9 secretion, matrix degradation, and cancer cell invasion. Notably, using a subcellular delocalization strategy to trigger protein loss of function, we uncovered a fascin-bundling-independent role in MMP-9 secretion. Hence, we demonstrate that nanobodies enable high resolution protein function mapping in cells.

L-plastin nanobodies perturb matrix degradation, podosome formation, stability and lifetime in THP-1 macrophages

Podosomes are cellular structures acting as degradation ‘hot-spots’ in monocytic cells. They appear as dot-like structures at the ventral cell surface, enriched in F-actin and actin regulators, including gelsolin and L-plastin. Gelsolin is an ubiquitous severing and capping protein, whereas L-plastin is a leukocyte-specific actin bundling protein. The presence of the capping protein CapG in podosomes has not yet been investigated. We used an innovative approach to investigate the role of these proteins in macrophage podosomes by means of nanobodies or Camelid single domain antibodies. Nanobodies directed against distinct domains of gelsolin, L-plastin or CapG were stably expressed in macrophage-like THP-1 cells. CapG was not enriched in podosomes. Gelsolin nanobodies had no effect on podosome formation or function but proved very effective in tracing distinct gelsolin populations. One gelsolin nanobody specifically targets actin-bound gelsolin and was effectively enriched in podosomes. A gelsolin nanobody that blocks gelsolin-G-actin interaction was not enriched in podosomes demonstrating that the calcium-activated and actin-bound conformation of gelsolin is a constituent of podosomes. THP-1 cells expressing inhibitory L-plastin nanobodies were hampered in their ability to form stable podosomes. Nanobodies did not perturb Ser5 phosphorylation of L-plastin although phosphorylated L-plastin was highly enriched in podosomes. Furthermore, nanobody-induced inhibition of L-plastin function gave rise to an irregular and unstable actin turnover of podosomes, resulting in diminished degradation of the underlying matrix. Altogether these results indicate that L-plastin is indispensable for podosome formation and function in macrophages.

Podosomes in adhesion, migration, mechanosensing and matrix remodeling

Cells use various actin-based motile structures to allow them to move across and through matrix of varying density and composition. Podosomes are actin cytoskeletal structures that form in motile cells and that mediate adhesion to substrate, migration, and other specialized functions such as transmigration through cell and matrix barriers. The podosome is a unique and interesting entity, which appears in the light microscope as an individual punctum, but is linked to other podosomes like a node on a network of the underlying cytoskeleton. Here, we discuss the signals that control podosome assembly and dynamics in different cell types and the actin organising proteins that regulate both the inner actin core and integrin-rich surrounding ring structures. We review the structure and composition of podosomes and also their functions in various cell types of both myeloid and endothelial lineage. We also discuss the emerging idea that podosomes can sense matrix stiffness and enable cells to respond to their environment.

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.

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.

WIP: WASP-interacting proteins at invadopodia and podosomes

Regulated cell invasion resulting from migratory and matrix-degrading events is an essential step in physiological processes such as the inflammatory response and tissue repair. Cell invasion is also thought to be a critical parameter in pathological conditions such as cancer metastasis. The migration of normal and cancer cells is largely driven by the actin cytoskeleton, which controls cell shape, adhesion and contractility. Podosomes and invadopodia are actin-rich protrusions that drive invasion in normal and cancer cells. These structures protrude from the basal region of the cell facing the extracellular matrix, where they adhere to and degrade the matrix, thus facilitating invasive migration. WASP (Wiskott-Aldrich syndrome protein) and WIP (WASP-interacting protein) localise to the actin rich core of podosomes and play a critical role in their formation. More recently, studies performed on microarray data sets from cancer patients of several tumour categories show a strong correlation between reduced WIP expression and improved prognosis. In this article, we identify endogenous WIP at the distal tips of cancer cell invasive protrusions and we summarise recent advances in the study of the roles of WIP- and WASP-protein families during migration and invasion of normal and cancer cells related to podosome and invadopodium generation.

WIP remodeling actin behind the scenes: how WIP reshapes immune and other functions

Actin polymerization is a fundamental cellular process regulating immune cell functions and the immune response. The Wiskott-Aldrich syndrome protein (WASp) is an actin nucleation promoting factor, which is exclusively expressed in hematopoietic cells, where it plays a key regulatory role in cytoskeletal dynamics. WASp interacting protein (WIP) was first discovered as the binding partner of WASp, through the use of the yeast two hybrid system. WIP was later identified as a chaperone of WASp, necessary for its stability. Mutations occurring at the WASp homology 1 domain (WH1), which serves as the WIP binding site, were found to cause the Wiskott-Aldrich syndrome (WAS) and X-linked thrombocytopenia (XLT). WAS manifests as an immune deficiency characterized by eczema, thrombocytopenia, recurrent infections, and hematopoietic malignancies, demonstrating the importance of WIP for WASp complex formation and for a proper immune response. WIP deficiency was found to lead to different abnormalities in the activity of various lymphocytes, suggesting differential cell-dependent roles for WIP. Additionally, WIP deficiency causes cellular abnormalities not found in WASp-deficient cells, indicating that WIP fulfills roles beyond stabilizing WASp. Indeed, WIP was shown to interact with various binding partners, including the signaling proteins Nck, CrkL and cortactin. Recent studies have demonstrated that WIP also takes part in non immune cellular processes such as cancer invasion and metastasis, in addition to cell subversion by intracellular pathogens. Understanding of numerous functions of WIP can enhance our current understanding of activation and function of immune and other cell types.

Src-mediated phosphorylation of mammalian Abp1 (DBNL) regulates podosome rosette formation in transformed fibroblasts

Podosomes are dynamic actin-based structures that mediate adhesion to the extracellular matrix and localize matrix degradation to facilitate cell motility and invasion. Drebrin-like protein (DBNL), which is homologous to yeast mAbp1 and is therefore known as mammalian actin-binding protein 1 (mAbp1), has been implicated in receptor-mediated endocytosis, vesicle recycling and dorsal ruffle formation. However, it is not known whether mAbp1 regulates podosome formation or cell invasion. In this study, we found that mAbp1 localizes to podosomes and is necessary for the formation of podosome rosettes in Src-transformed fibroblasts. Despite their structural similarity, mAbp1 and cortactin play distinct roles in podosome regulation. Cortactin was necessary for the formation of podosome dots, whereas mAbp1 was necessary for the formation of organized podosome rosettes in Src-transformed cells. We identified specific Src phosphorylation sites, Tyr337 and Tyr347 of mAbp1, which mediate the formation of podosome rosettes and degradation of the ECM. In contrast to dorsal ruffles, the interaction of mAbp1 with WASP-interacting protein (WIP) was not necessary for the formation of podosome rosettes. Finally, we showed that depletion of mAbp1 increased invasive cell migration, suggesting that mAbp1 differentially regulates matrix degradation and cell invasion. Collectively, our findings identify a role for mAbp1 in podosome rosette formation and cell invasion downstream of Src.

Mass spectrometric analysis identifies a cortactin-RCC2/TD60 interaction in mitotic cells

Cortactin is an F-actin binding protein that functions as a scaffold to regulate Arp2/3 mediated actin polymerization in lamellipodia and invadopodia formation as well as functioning in cell migration and endocytosis of many different cell types. In light of the fact that regulated actin polymerization is critical for many cellular processes we launched a search for novel cortactin interactions with cellular proteins that might indicate heretofore undescribed biological activities supported by cortactin. Using a modified stable isotope labeling in cell culture (SILAC) approach in HEK293 cells and Flag-tagged cortactin (F-cortactin) as bait, we identified a limited set of cortactin interactions including several proteins which have not previously been identified as cortactin associated proteins. Among these were serine/threonine-protein phosphatase 2A subunit beta (PP2A-beta) and RCC2/TD60, a Rac guanine nucleotide exchange factor (GEF) required for completion of mitosis and cytokinesis. The interaction between cortactin and RCC2/TD60 was verified in cell lysates immunoprecitated with anti-RCC2/TD60 antibody. Furthermore, cortactin was localized by immunofluorescence in the equatorial plane of dividing HeLa cells in the region where RCC2/TD60 has previously been localized thus providing support for a complex containing cortactin and RCC2/TD60 complex that may play a functional role in cells undergoing mitosis.

Hematopoietic lineage cell-specific protein 1 functions in concert with the Wiskott-Aldrich syndrome protein to promote podosome array organization and chemotaxis in dendritic cells

Dendritic cells (DCs) are professional APCs that reside in peripheral tissues and survey the body for pathogens. Upon activation by inflammatory signals, DCs undergo a maturation process and migrate to lymphoid organs, where they present pathogen-derived Ags to T cells. DC migration depends on tight regulation of the actin cytoskeleton to permit rapid adaptation to environmental cues. We investigated the role of hematopoietic lineage cell-specific protein 1 (HS1), the hematopoietic homolog of cortactin, in regulating the actin cytoskeleton of murine DCs. HS1 localized to lamellipodial protrusions and podosomes, actin-rich structures associated with adhesion and migration. DCs from HS1(-/-) mice showed aberrant lamellipodial dynamics. Moreover, although these cells formed recognizable podosomes, their podosome arrays were loosely packed and improperly localized within the cell. HS1 interacts with Wiskott-Aldrich syndrome protein (WASp), another key actin-regulatory protein, through mutual binding to WASp-interacting protein. Comparative analysis of DCs deficient for HS1, WASp or both proteins revealed unique roles for these proteins in regulating podosomes with WASp being essential for podosome formation and with HS1 ensuring efficient array organization. WASp recruitment to podosome cores was independent of HS1, whereas HS1 recruitment required Src homology 3 domain-dependent interactions with the WASp/WASp-interacting protein heterodimer. In migration assays, the phenotypes of HS1- and WASp-deficient DCs were related, but distinct. WASp(-/y) DCs migrating in a chemokine gradient showed a large decrease in velocity and diminished directional persistence. In contrast, HS1(-/-) DCs migrated faster than wild-type cells, but directional persistence was significantly reduced. These studies show that HS1 functions in concert with WASp to fine-tune DC cytoarchitecture and direct cell migration.