The polarization defect of Wiskott-Aldrich syndrome macrophages is linked to dislocalization of the Arp2/3 complex

Disulfidptosis: a novel cell death modality induced by actin cytoskeleton collapse and a promising target for cancer therapeutics

Disulfidptosis is a novel discovered form of programmed cell death (PCD) that diverges from apoptosis, necroptosis, ferroptosis, and cuproptosis, stemming from disulfide stress-induced cytoskeletal collapse. In cancer cells exhibiting heightened expression of the solute carrier family 7 member 11 (SLC7A11), excessive cystine importation and reduction will deplete nicotinamide adenine dinucleotide phosphate (NADPH) under glucose deprivation, followed by an increase in intracellular disulfide stress and aberrant disulfide bond formation within actin networks, ultimately culminating in cytoskeletal collapse and disulfidptosis. Disulfidptosis involves crucial physiological processes in eukaryotic cells, such as cystine and glucose uptake, NADPH metabolism, and actin dynamics. The Rac1-WRC pathway-mediated actin polymerization is also implicated in this cell death due to its contribution to disulfide bond formation. However, the precise mechanisms underlying disulfidptosis and its role in tumors are not well understood. This is probably due to the multifaceted functionalities of SLC7A11 within cells and the complexities of the downstream pathways driving disulfidptosis. This review describes the critical roles of SLC7A11 in cells and summarizes recent research advancements in the potential pathways of disulfidptosis. Moreover, the less-studied aspects of this newly discovered cell death process are highlighted to stimulate further investigations in this field.

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

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

Get a grip: Podosomes as potential players in phagocytosis

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

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.

Novel Roles of MT1-MMP and MMP-2: Beyond the Extracellular Milieu

Matrix metalloproteinases (MMPs) are critical enzymes involved in a variety of cellular processes. MMPs are well known for their ability to degrade the extracellular matrix (ECM) and their extracellular role in cell migration. Recently, more research has been conducted on investigating novel subcellular localizations of MMPs and their intracellular roles at their respective locations. In this review article, we focus on the subcellular localization and novel intracellular roles of two closely related MMPs: membrane-type-1 matrix metalloproteinase (MT1-MMP) and matrix metalloproteinase-2 (MMP-2). Although MT1-MMP is commonly known to localize on the cell surface, the protease also localizes to the cytoplasm, caveolae, Golgi, cytoskeleton, centrosome, and nucleus. At these subcellular locations, MT1-MMP functions in cell migration, macrophage metabolism, invadopodia development, spindle formation and gene expression, respectively. Similar to MT1-MMP, MMP-2 localizes to the caveolae, mitochondria, cytoskeleton, nucleus and nucleolus and functions in calcium regulation, contractile dysfunction, gene expression and ribosomal RNA transcription. Our particular interest lies in the roles MMP-2 and MT1-MMP serve within the nucleus, as they may provide critical insights into cancer epigenetics and tumor migration and invasion. We suggest that targeting nuclear MT1-MMP or MMP-2 to reduce or halt cell proliferation and migration may lead to the development of new therapies for cancer and other diseases.

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

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

Autoimmunity in Wiskott-Aldrich Syndrome: Updated Perspectives

Wiskott-Aldrich syndrome (WAS) is an uncommon X-linked combined-immunodeficiency disorder characterized by a triad of thrombocytopenia, eczema, and immunodeficiency. Patients with WAS are also predisposed to autoimmunity and malignancy. Autoimmune manifestations have been reported in 26%-72% of patients with WAS. Autoimmunity is an independent predictor of poor prognosis and predisposes to malignancy. Development of autoimmunity is also an early pointer of the need for hematopoietic stem-cell transplantation. In this manuscript, we have collated the published data and present a narrative review on autoimmune manifestations in WAS. A summary of currently proposed immunopathogenic mechanisms and genetic variants associated with development of autoimmunity in WAS is also included.

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

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

Wiskott Aldrich Syndrome: A Multi-Institutional Experience From India

Background

Wiskott Aldrich syndrome (WAS) is characterized by bleeding manifestations, recurrent infections, eczema, autoimmunity, and malignancy. Over the last decade, improved awareness and better in-house diagnostic facilities at several centers in India has resulted in increased recognition of WAS. This study reports collated data across major primary immunodeficiency diseases (PID) centers in India that are involved in care of children with WAS and highlights the varied clinical presentations, genetic profile, and outcomes of patients in India.

Methods

Request to share data was sent to multiple centers in India that are involved in care and management of patients with PID. Six centers provided requisite data that were compiled and analyzed.

Results

In this multi-institutional cohort, clinical details of 108 patients who had a provisional diagnosis of WAS were received. Of these, 95 patients with 'definite WAS' were included Fourteen patients were classified as XLT and 81 patients as WAS. Median age at onset of symptoms of patients was 3 months (IQR 1.6, 6.0 months) and median age at diagnosis was 12 months (IQR 6,48 months). Clinical profile included bleeding episodes (92.6%), infections (84.2%), eczema (78.9%), various autoimmune manifestations (40%), and malignancy (2.1%). DNA analysis revealed 47 variants in 67 cases. Nonsense and missense variants were the most common (28.4% each), followed by small deletions (19.4%), and splice site defects (16.4%). We also report 24 novel variants, most of these being frameshift and nonsense mutations resulting in premature termination of protein synthesis. Prophylactic intravenous immunoglobulin (IVIg) was initiated in 52 patients (54.7%). Hematopoietic stem cell transplantation (HSCT) was carried out in 25 patients (26.3%). Of those transplanted, disease-free survival was seen in 15 patients (60%). Transplant related mortality was 36%. Outcome details were available for 89 patients. Of these, 37% had died till the time of this analysis. Median duration of follow-up was 36 months (range 2 weeks- 12 years; IQR 16.2 months- 70 months).

Conclusions

We report the first nationwide cohort of patients with WAS from India. Bleeding episodes and infections are common manifestations. Mortality continues to be high as curative therapy is not accessible to most of our patients.

Influence of external forces on actin-dependent T cell protrusions during immune synapse formation

BACKGROUND INFORMATION

We have previously observed that in response to antigenic activation, T cells produce actin-rich protrusions that generate forces involved in T cell activation. These forces are influenced by the mechanical properties of antigen-presenting cells (APCs). However, how external forces, which can be produced by APCs, influence the dynamic of the actin protrusion remains unknown. In this study, we quantitatively characterised the effects of external forces in the dynamic of the protrusion grown by activated T cells.

RESULTS

Using a micropipette force probe, we applied controlled compressive or pulling forces on primary T lymphocytes activated by an antibody-covered microbead, and measured the effects of these forces on the protrusion generated by T lymphocytes. We found that the application of compressive forces slightly decreased the length, the time at which the protrusion stops growing and retracts and the velocity of the protrusion formation, whereas pulling forces strongly increased these parameters. In both cases, the applied forces did not alter the time required for the T cells to start growing the protrusion (delay). Exploring the molecular events controlling the dynamic of the protrusion, we showed that inhibition of the Arp2/3 complex impaired the dynamic of the protrusion by reducing both its maximum length and its growth speed and increasing the delay to start growing. Finally, T cells developed similar protrusions in more physiological conditions, that is, when activated by an APC instead of an activating microbead.

CONCLUSIONS

Our results suggest that the formation of the force-generating protrusion by T cells is set by an intracellular constant time and that its dynamic is sensitive to external forces. They also show that actin assembly mediated by actin-related protein Arp2/3 complex is involved in the formation and dynamic of the protrusion.

SIGNIFICANCE

Actin-rich protrusions developed by T cells are sensory organelles that serve as actuators of immune surveillance. Our study shows that forces experienced by this organelle modify their dynamic suggesting that they might modify immune responses. Moreover, the quantitative aspects of our analysis should help to get insight into the molecular mechanisms involved in the formation of the protrusion.

Transition of podosomes into zipper-like structures in macrophage-derived multinucleated giant cells

Macrophage fusion resulting in the formation of multinucleated giant cells (MGCs) is a multistage process that requires many adhesion-dependent steps and involves the rearrangement of the actin cytoskeleton. The diversity of actin-based structures and their role in macrophage fusion is poorly understood. In this study, we revealed hitherto unrecognized actin-based zipper-like structures (ZLSs) that arise between MGCs formed on the surface of implanted biomaterials. We established an in vitro model for the induction of these structures in mouse macrophages undergoing IL-4–mediated fusion. Using this model, we show that over time MGCs develop cell–cell contacts containing ZLSs. Live-cell imaging using macrophages isolated from mRFP- or eGFP-LifeAct mice demonstrated that ZLSs are dynamic formations undergoing continuous assembly and disassembly and that podosomes are precursors of these structures. Immunostaining experiments showed that vinculin, talin, integrin α_Mβ₂, and other components of podosomes are present in ZLSs. Macrophages deficient in WASp or Cdc42, two key molecules involved in actin core organization in podosomes, as well as cells treated with the inhibitors of the Arp2/3 complex, failed to form ZLSs. Furthermore, E-cadherin and nectin-2 were found between adjoining membranes, suggesting that the transition of podosomes into ZLSs is induced by bridging plasma membranes by junctional proteins.

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

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

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

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

Coupling of β2 integrins to actin by a mechanosensitive molecular clutch drives complement receptor-mediated phagocytosis

αMβ2 integrin (complement receptor 3) is a major receptor for phagocytosis in macrophages. In other contexts, integrins’ activities and functions are mechanically linked to actin dynamics through focal adhesions (FAs). We asked whether mechanical coupling of αMβ2 integrin to the actin cytoskeleton mediates phagocytosis. We found that particle internalization was driven by formation of Arp2/3 and formin-dependent actin protrusions that wrapped around the particle. Focal complex-like adhesions formed in the phagocytic cup that contained β2 integrins, FA proteins and tyrosine kinases. Perturbation of talin and Syk demonstrated that a talin-dependent link between integrin and actin and Syk-mediated recruitment of vinculin enable force transmission to target particles and promote phagocytosis. Altering target mechanical properties demonstrated more efficient phagocytosis of stiffer targets. Thus, macrophages use tyrosine kinase signaling to build a mechanosensitive, talin- and vinculin-mediated, FA-like molecular clutch, which couples integrins to cytoskeletal forces to drive particle engulfment.

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.

Regulation and related mechanism of GSN mRNA level by hnRNPK in lung adenocarcinoma cells

Gelsolin (GSN) is an actin filament-capping protein that plays a key role in cell migration. Here we show that heterogeneous nuclear ribonucleoprotein K (hnRNPK) regulates GSN expression level by binding to the 3'-untranslated region (3'UTR) of GSN mRNA in non-small cell lung cancers (NSCLC) H1299 cells which are highly metastatic and express high level of GSN. We found that hnRNPK overexpression increased the mRNA and protein level of GSN, whereas hnRNPK knockdown by siRNA decreased the mRNA and protein level of GSN in both H1299 and A549 cells, indicating a positive role of hnRNPK in the regulation of GSN expression. Furthermore, hnRNPK knockdown affected the migration ability of H1299 and A549 cells which could be rescued by ectopic expression of GSN in those cells. Conversely, GSN knockdown in hnRNPK-overexpressing cells could abort the stimulatory effect of hnRNPK on the cell migration. These results suggest that hnRNPK function in the regulation of cell migration is GSN-dependent. Taken together, these data unveiled a new mechanism of regulation of the GSN expression by hnRNPK and provides new clues for the discovery of new anti-metastatic therapy.

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

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

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

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

[Variations on the theme of podosomes, context matters]

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

Variations on the theme of podosomes: A matter of context

Extensive in vitro studies have described podosomes as actin-based structures at the plasma membrane, connecting the cell with its extracellular matrix and endowed with multiple capabilities. Contractile actin-myosin cables assemble them into a network that constitutes a multifaceted cellular superstructure taking different forms - with common characteristics - but manifesting different properties depending on the context of study. Their morphology and their role in cell functioning and behavior are therefore now apprehended in in vivo or in vitro situations relevant to physiological processes. We focus here on three of them, namely: macrophage migration, antigen presentation by dendritic cells and endothelial cell sprouting during angiogenesis to highlight the characteristics of podosomes and their functioning shaped by the microenvironment.

A fruitful tree: developing the dendritic nucleation model of actin-based cell motility

A fundamental question in cell biology concerns how cells move, and this has been the subject of intense research for decades. In the 1990s, a major leap forward was made in our understanding of cell motility, with the proposal of the dendritic nucleation model. This essay describes the events leading to the development of the model, including findings from many laboratories and scientific disciplines. The story is an excellent example of the scientific process in action, with the combination of multiple perspectives leading to robust conclusions.

Actin assembly mechanisms at a glance

The actin cytoskeleton and associated motor proteins provide the driving forces for establishing the astonishing morphological diversity and dynamics of mammalian cells. Aside from functions in protruding and contracting cell membranes for motility, differentiation or cell division, the actin cytoskeleton provides forces to shape and move intracellular membranes of organelles and vesicles. To establish the many different actin assembly functions required in time and space, actin nucleators are targeted to specific subcellular compartments, thereby restricting the generation of specific actin filament structures to those sites. Recent research has revealed that targeting and activation of actin filament nucleators, elongators and myosin motors are tightly coordinated by conserved protein complexes to orchestrate force generation. In this Cell Science at a Glance article and the accompanying poster, we summarize and discuss the current knowledge on the corresponding protein complexes and their modes of action in actin nucleation, elongation and force generation.

Metalloproteinase MT1-MMP islets act as memory devices for podosome reemergence

The authors find that matrix metalloproteinase MT1-MMP is enriched at the plasma membrane of macrophage podosomes, where it persists beyond podosome lifetime and, through binding to the subcortical actin cytoskeleton, forms subcellular signposts that facilitate podosome reformation.

Podosomes are dynamic cell adhesions that are also sites of extracellular matrix degradation, through recruitment of matrix-lytic enzymes, particularly of matrix metalloproteinases. Using total internal reflection fluorescence microscopy, we show that the membrane-bound metalloproteinase MT1-MMP is enriched not only at podosomes but also at distinct “islets” embedded in the plasma membrane of primary human macrophages. MT1-MMP islets become apparent upon podosome dissolution and persist beyond podosome lifetime. Importantly, the majority of MT1-MMP islets are reused as sites of podosome reemergence. siRNA-mediated knockdown and recomplementation analyses show that islet formation is based on the cytoplasmic tail of MT1-MMP and its ability to bind the subcortical actin cytoskeleton. Collectively, our data reveal a previously unrecognized phase in the podosome life cycle and identify a structural function of MT1-MMP that is independent of its proteolytic activity. MT1-MMP islets thus act as cellular memory devices that enable efficient and localized reformation of podosomes, ensuring coordinated matrix degradation and invasion.

Proactive for invasion: Reuse of matrix metalloproteinase for structural memory

Migratory cells translocate membrane type-1 matrix metalloproteinase (MT1-MMP) to podosomes or invadosomes to break extracellular matrix barriers. In this issue, El Azzouzi et al. (2016. J. Cell. Biol.http://dx.doi.org/10.1083/jcb.201510043) describe an unexpected function for the MT1-MMP cytoplasmic domain in imprinting spatial memory for podosome reformation via assembly in membrane islets.

Induction of Macrophage Function in Human THP-1 Cells Is Associated with Rewiring of MAPK Signaling and Activation of MAP3K7 (TAK1) Protein Kinase

Macrophages represent the primary human host response to pathogen infection and link the immediate defense to the adaptive immune system. Mature tissue macrophages convert from circulating monocyte precursor cells by terminal differentiation in a process that is not fully understood. Here, we analyzed the protein kinases of the human monocytic cell line THP-1 before and after induction of macrophage differentiation by using kinomics and phosphoproteomics. When comparing the macrophage-like state with the monocytic precursor, 50% of the kinome was altered in expression and even 71% of covered kinase phosphorylation sites were affected. Kinome rearrangements are for example characterized by a shift of overrepresented cyclin-dependent kinases associated with cell cycle control in monocytes to calmodulin-dependent kinases and kinases involved in proinflammatory signaling. Eventually, we show that monocyte-to-macrophage differentiation is associated with major rewiring of mitogen-activated protein kinase signaling networks and demonstrate that protein kinase MAP3K7 (TAK1) acts as the key signaling hub in bacterial killing, chemokine production and differentiation. Our study proves the fundamental role of protein kinases and cellular signaling as major drivers of macrophage differentiation and function. The finding that MAP3K7 is central to macrophage function suggests MAP3K7 and its networking partners as promising targets in host-directed therapy for macrophage-associated disease.

A dominant gain-of-function mutation in universal tyrosine kinase SRC causes thrombocytopenia, myelofibrosis, bleeding, and bone pathologies

The Src family kinase (SFK) member SRC is a major target in drug development because it is activated in many human cancers, yet deleterious SRC germline mutations have not been reported. We used genome sequencing and Human Phenotype Ontology patient coding to identify a gain-of-function mutation in SRC causing thrombocytopenia, myelofibrosis, bleeding, and bone pathologies in nine cases. Modeling of the E527K substitution predicts loss of SRC's self-inhibitory capacity, which we confirmed with in vitro studies showing increased SRC kinase activity and enhanced Tyr(419) phosphorylation in COS-7 cells overexpressing E527K SRC. The active form of SRC predominates in patients' platelets, resulting in enhanced overall tyrosine phosphorylation. Patients with myelofibrosis have hypercellular bone marrow with trilineage dysplasia, and their stem cells grown in vitro form more myeloid and megakaryocyte (MK) colonies than control cells. These MKs generate platelets that are dysmorphic, low in number, highly variable in size, and have a paucity of α-granules. Overactive SRC in patient-derived MKs causes a reduction in proplatelet formation, which can be rescued by SRC kinase inhibition. Stem cells transduced with lentiviral E527K SRC form MKs with a similar defect and enhanced tyrosine phosphorylation levels. Patient-derived and E527K-transduced MKs show Y419 SRC-positive stained podosomes that induce altered actin organization. Expression of mutated src in zebrafish recapitulates patients' blood and bone phenotypes. Similar studies of platelets and MKs may reveal the mechanism underlying the severe bleeding frequently observed in cancer patients treated with next-generation SFK inhibitors.

Dose-dependent effects of prostaglandin E2 in macrophage adhesion and migration

Macrophage migration to the focus of infection is a hallmark of the innate immune response. Macrophage spreading, adhesion, and migration through the extracellular matrix require dynamic remodeling of the actin cytoskeleton associated to integrin clustering in podosomes and focal adhesions. Here, we show that prostaglandin E2 (PGE2 ), the main prostaglandin produced by macrophages during inflammation, promote the distinctive dose-dependent formation of podosomes or focal adhesions in macrophages. Low concentrations of PGE2 increased p110γ PI3K expression, phosphorylation of actin-related protein 2, and formation of podosomes, which enhanced macrophage migration in response to chemokines. However, high doses of PGE2 increased phosphorylation of paxillin and focal adhesion kinase, the expression of serine/threonine protein kinase 1, and promoted focal adhesion formation and macrophage adhesion, reducing macrophage chemotaxis. In summary, we describe the dual role of PGE2 as a promoter of macrophage chemotaxis and adhesion, proposing a new model of macrophage migration to the inflammatory focus in the presence of a gradient of PGE2 .

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.

Podosomes in space: macrophage migration and matrix degradation in 2D and 3D settings

Migration of macrophages is a key process for a variety of physiological functions, such as pathogen clearance or tissue homeostasis. However, it can also be part of pathological scenarios, as in the case of tumor-associated macrophages. This review presents an overview of the different migration modes macrophages can adopt, depending on the physical and chemical properties of specific environments, and the constraints they impose upon cells. We discuss the importance of these environmental and also of cellular parameters, as well as their relative impact on macrophage migration and on the formation of matrix-lytic podosomes in 2D and 3D. Moreover, we present an overview of routinely used and also newly developed assays for the study of macrophage migration in both 2D and 3D contexts, their respective advantages and limitations, and also their potential to reliably mimic in vivo situations.

Control of polarized assembly of actin filaments in cell motility

Actin cytoskeleton remodeling, which drives changes in cell shape and motility, is orchestrated by a coordinated control of polarized assembly of actin filaments. Signal responsive, membrane-bound protein machineries initiate and regulate polarized growth of actin filaments by mediating transient links with their barbed ends, which elongate from polymerizable actin monomers. The barbed end of an actin filament thus stands out as a hotspot of regulation of filament assembly. It is the target of both soluble and membrane-bound agonists as well as antagonists of filament assembly. Here, we review the molecular mechanisms by which various regulators of actin dynamics bind, synergize or compete at filament barbed ends. Two proteins can compete for the barbed end via a mutually exclusive binding scheme. Alternatively, two regulators acting individually at barbed ends may be bound together transiently to terminal actin subunits at barbed ends, leading to the displacement of one by the other. The kinetics of these reactions is a key in understanding how filament length and membrane-filament linkage are controlled. It is also essential for understanding how force is produced to shape membranes by mechano-sensitive, processive barbed end tracking machineries like formins and by WASP-Arp2/3 branched filament arrays. A combination of biochemical and biophysical approaches, including bulk solution assembly measurements using pyrenyl-actin fluorescence, single filament dynamics, single molecule fluorescence imaging and reconstituted self-organized filament assemblies, have provided mechanistic insight into the role of actin polymerization in motile processes.

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.

Podosomes revealed by advanced bioimaging: what did we learn?

Podosomes are micrometer-sized, circular adhesions formed by cells such as osteoclasts, macrophages, dendritic cells, and endothelial cells. Because of their small size and the lack of methods to visualize individual proteins and protein complexes, podosomes have long been considered a simple two-module structure with a protrusive actin core and a surrounding adhesive ring composed of integrins and cytoskeletal adaptor proteins such as vinculin and talin. In the past decade, the applications of fluorescence based techniques that circumvent the diffraction limit of conventional light microscopy took a major leap forward. Podosomes have been imaged by a variety of these super-resolution methods, and in this concise review we discuss how these super-resolution data have increased our understanding of the podosome ultra-structure and function.

Microtubule acetylation regulates dynamics of KIF1C-powered vesicles and contact of microtubule plus ends with podosomes

Microtubule dynamics are important for a variety of key cellular functions such as intracellular trafficking, adjustment of the cell surface proteome, or adhesion structure turnover. In the current study, we investigate the effects of altered microtubule acetylation levels on the subcellular distribution of kinesins and actin cytoskeletal architecture in primary human macrophages. Microtubule acetylation was altered by overexpression or siRNA-induced depletion of the acetylase MEC-17, or by blocking α-tubulin deacetylation by addition of the inhibitor tubacin. We show that microtubule acetylation influences the subcellular distribution of vesicles associated with the kinesin KIF1C, as well as their directionality, velocity and run length. Moreover, tubulin acetylation alters the targeting frequency of microtubule plus ends on podosomes and influences the number of podosomes per cell and thus the matrix-degrading capacity of macrophages. Collectively, our results point to α-tubulin acetylation as an important modification that impacts on kinesin vesicle dynamics, actin cytoskeletal architecture and cellular function of macrophages.

The matricellular protein CCN5 regulates podosome function via interaction with integrin αvβ 3

CCN proteins play crucial roles in cell motility, matrix turnover, and proliferation. In particular, CCN5 plays a role in cell motility and proliferation in several cell types; however, no functional binding proteins for CCN5 have been identified. In this study we report that CCN5 binds to the cell surface receptor integrin αvβ3 in vascular smooth muscle cells. Furthermore, this interaction takes place in podosomes, organelles known to degrade matrix and mediate motility. We show that CCN5 regulates the ability of podosomes to degrade matrix, but does not affect podosome formation. The level of CCN5 present in a podosome negatively correlates with its ability to degrade matrix. Conversely, knockdown of CCN5 greatly enhances the matrix-degrading ability of podosomes. These findings suggest that the antimotility effects of CCN5 may be mediated through the direct interaction of CCN5 and integrin αvβ3 in podosomes and the concomitant suppression of matrix degradation that is required for cell migration.

Essential function of dynamin in the invasive properties and actin architecture of v-Src induced podosomes/invadosomes

The large GTPase dynamin plays a key role in endocytosis but is also localized at numerous actin rich sites. We investigated dynamin functions at podosomes/invadosomes, actin-based cellular adhesion structures implicated in tissue invasion. Podosomes/invadosomes are constituted of long F-actin bundles perpendicular to the substratum (actin cores), connected to randomly arranged F-actin fibers parallel to the substratum (actin cloud). We show here that dynamin depletion in v-Src-transformed fibroblasts triggers a massive disorganization of podosomes/invadosomes (isolated or in rosettes), with a corresponding inhibition of their invasive properties. The action of dynamin at podosomes/invadosomes requires a functional full-length protein, suggesting that the effects of dynamin at these sites and in membrane remodelling during endocytosis are mediated by similar mechanisms. In order to determine direct effect of dynamin depletion on invadosome, an optogenetic approach based on the photosensitizer KillerRed was developed. Acute dynamin photo-inactivation leads to a very rapid disorganization of invadosome without affecting focal adhesions. Dynamin therefore is a key regulator of the architecture of actin in podosomes/invadosomes.

Processive acceleration of actin barbed-end assembly by N-WASP

Clustered N-WASP binds directly to actin-filament barbed ends and can either slow individual filament growth or processively accelerate the assembly of bundled actin filaments. This novel Arp2/3-independent mechanism of N-WASP likely plays a role in invadopodia and podosome formation, in which both N-WASP and actin filaments are tightly clustered.

Neuronal Wiskott–Aldrich syndrome protein (N-WASP)–activated actin polymerization drives extension of invadopodia and podosomes into the basement layer. In addition to activating Arp2/3, N-WASP binds actin-filament barbed ends, and both N-WASP and barbed ends are tightly clustered in these invasive structures. We use nanofibers coated with N-WASP WWCA domains as model cell surfaces and single-actin-filament imaging to determine how clustered N-WASP affects Arp2/3-independent barbed-end assembly. Individual barbed ends captured by WWCA domains grow at or below their diffusion-limited assembly rate. At high filament densities, however, overlapping filaments form buckles between their nanofiber tethers and myosin attachment points. These buckles grew ∼3.4-fold faster than the diffusion-limited rate of unattached barbed ends. N-WASP constructs with and without the native polyproline (PP) region show similar rate enhancements in the absence of profilin, but profilin slows barbed-end acceleration from constructs containing the PP region. Increasing Mg²⁺ to enhance filament bundling increases the frequency of filament buckle formation, consistent with a requirement of accelerated assembly on barbed-end bundling. We propose that this novel N-WASP assembly activity provides an Arp2/3-independent force that drives nascent filament bundles into the basement layer during cell invasion.

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.

Mechanistic insights into the regulation of circular dorsal ruffle formation

Growth factor stimulations induce dynamic changes in the cytoskeleton beneath the plasma membrane. Among them is the formation of membrane ruffles organized in a circular array, called 'circular dorsal ruffles' (CDRs). Physiological functions of CDRs include downregulation of cell growth by desensitizing the signalling from growth factor receptors as well as rearrangement of adhesion sites at the onset of cell migration. For the formation of CDRs, not only the activators of actin polymerization, such as N-WASP and the Arp2/3-complex, but also membrane deforming proteins with BAR/F-BAR domains are necessary. Small GTPases are also involved in the formation of CDRs by controlling intracellular trafficking through endosomes. Moreover, recent analyses of another circular cytoskeletal structure, podosome rosettes, have revealed common molecular features shared with CDRs. Among them, the roles of PI3-kinase and phosphoinositide 5-phosphatase may hold the key to the induction of these circular structures.

Podosomes in migrating microglia: components and matrix degradation

Background

To perform their functions during development and after central nervous system injury, the brain’s immune cells (microglia) must migrate through dense neuropil and extracellular matrix (ECM), but it is not known how they degrade the ECM. In several cancer cell lines and peripheral cells, small multi-molecular complexes (invadopodia in cancer cells, podosomes in nontumor cells) can both adhere to and dissolve the ECM. Podosomes are tiny multi-molecular structures (0.4 to 1 μm) with a core, rich in F-actin and its regulatory molecules, surrounded by a ring containing adhesion and structural proteins.

Methods

Using rat microglia, we performed several functional assays: live cell imaging for chemokinesis, degradation of the ECM component, fibronectin, and chemotactic invasion through Matrigel™, a basement membrane type of ECM. Fluorescent markers were used with high-resolution microscopy to identify podosomes and their components.

Results

The fan-shaped lamella at the leading edge of migrating microglia contained a large F-actin-rich superstructure composed of many tiny (<1 μm) punctae that were adjacent to the substrate, as expected for cell–matrix contact points. This superstructure (which we call a podonut) was restricted to cells with lamellae, and conversely almost every lamella contained a podonut. Each podonut comprised hundreds of podosomes, which could also be seen individually adjacent to the podonut. Microglial podosomes contained hallmark components of these structures previously seen in several cell types: the plaque protein talin in the ring, and F-actin and actin-related protein (Arp) 2 in the core. In microglia, podosomes were also enriched in phosphotyrosine residues and three tyrosine-kinase-regulated proteins: tyrosine kinase substrate with five Src homology 3 domains (Tks5), phosphorylated caveolin-1, and Nox1 (nicotinamide adenine dinucleotide phosphate oxidase 1). When microglia expressed podonuts, they were able to degrade the ECM components, fibronectin, and Matrigel™.

Conclusion

The discovery of functional podosomes in microglia has broad implications, because migration of these innate immune cells is crucial in the developing brain, after damage, and in disease states involving inflammation and matrix remodeling. Based on the roles of invadosomes in peripheral tissues, we propose that microglia use these complex structures to adhere to and degrade the ECM for efficient migration.

Immunological responses and actin dynamics in macrophages are controlled by N-cofilin but are independent from ADF

Dynamic changes in the actin cytoskeleton are essential for immune cell function and a number of immune deficiencies have been linked to mutations, which disturb the actin cytoskeleton. In macrophages and dendritic cells, actin remodelling is critical for motility, phagocytosis and antigen presentation, however the actin binding proteins, which control antigen presentation have been poorly characterized. Here we dissect the specific roles of the family of ADF/cofilin F-actin depolymerizing factors in macrophages and in local immune responses.

Macrophage migration, cell polarization and antigen presentation to T-cells require n-cofilin mediated F-actin remodelling. Using a conditional mouse model, we show that n-cofilin also controls MHC class II-dependent antigen presentation. Other cellular processes such as phagocytosis and antigen processing were found to be independent of n-cofilin. Our data identify n-cofilin as a novel regulator of antigen presentation, while ADF on the other hand is dispensable for macrophage motility and antigen presentation.

The BAR Domain Superfamily Proteins from Subcellular Structures to Human Diseases

Eukaryotic cells have complicated membrane systems. The outermost plasma membrane contains various substructures, such as invaginations and protrusions, which are involved in endocytosis and cell migration. Moreover, the intracellular membrane compartments, such as autophagosomes and endosomes, are essential for cellular viability. The Bin-Amphiphysin-Rvs167 (BAR) domain superfamily proteins are important players in membrane remodeling through their structurally determined membrane binding surfaces. A variety of BAR domain superfamily proteins exist, and each family member appears to be involved in the formation of certain subcellular structures or intracellular membrane compartments. Most of the BAR domain superfamily proteins contain SH3 domains, which bind to the membrane scission molecule, dynamin, as well as the actin regulatory WASP/WAVE proteins and several signal transduction molecules, providing possible links between the membrane and the cytoskeleton or other machineries. In this review, we summarize the current information about each BAR superfamily protein with an SH3 domain(s). The involvement of BAR domain superfamily proteins in various diseases is also discussed.

Supervillin couples myosin-dependent contractility to podosomes and enables their turnover

Podosomes are actin-rich adhesion and invasion structures. Especially in macrophages, podosomes exist in two subpopulations, large precursors at the cell periphery and smaller podosomes (successors) in the cell interior. To date, the mechanisms that differentially regulate these subpopulations are largely unknown. Here, we show that the membrane-associated protein supervillin localizes preferentially to successor podosomes and becomes enriched at precursors immediately before their dissolution. Consistently, podosome numbers are inversely correlated with supervillin protein levels. Using deletion constructs, we find that the myosin II regulatory N-terminus of supervillin [SV(1-174)] is crucial for these effects. Phosphorylated myosin light chain (pMLC) localizes at supervillin-positive podosomes, and time-lapse analyses show that enrichment of GFP-supervillin at podosomes coincides with their coupling to contractile myosin-IIA-positive cables. We also show that supervillin binds only to activated myosin IIA, and a dysregulated N-terminal construct [SV(1-830)] enhances pMLC levels at podosomes. Thus, preferential recruitment of supervillin to podosome subpopulations might both require and induce actomyosin contractility. Using siRNA and pharmacological inhibition, we demonstrate that supervillin and myosin IIA cooperate to regulate podosome lifetime, podosomal matrix degradation and cell polarization. In sum, we show here that podosome subpopulations differ in their molecular composition and identify supervillin, in cooperation with myosin IIA, as a crucial factor in the regulation of podosome turnover and function.

Drosophila cellular immunity: a story of migration and adhesion

Research during the past 15 years has led to significant breakthroughs, providing evidence of a high degree of similarity between insect and mammalian innate immune responses, both humoural and cellular, and highlighting Drosophila melanogaster as a model system for studying the evolution of innate immunity. In a manner similar to cells of the mammalian monocyte and macrophage lineage, Drosophila immunosurveillance cells (haemocytes) have a number of roles. For example, they respond to wound signals, are involved in wound healing and contribute to the coagulation response. Moreover, they participate in the phagocytosis and encapsulation of invading pathogens, are involved in the removal of apoptotic bodies and produce components of the extracellular matrix. There are several reasons for using the Drosophila cellular immune response as a model to understand cell signalling during adhesion and migration in vivo: many genes involved in the regulation of Drosophila haematopoiesis and cellular immunity have been maintained across taxonomic groups ranging from flies to humans, many aspects of Drosophila and mammalian innate immunity seem to be conserved, and Drosophila is a simplified and well-studied genetic model system. In the present Commentary, we will discuss what is known about cellular adhesion and migration in the Drosophila cellular immune response, during both embryonic and larval development, and where possible compare it with related mechanisms in vertebrates.

The genetic profiling of preferentially expressed genes in murine splenic CD8α+ dendritic cells

In the murine splenocytes, CD8α+ dendritic cells (abbreviated as 8+DC) and CD8α- dendritic cells (abbreviated as 8-DC) are identified with some vague features for each of them. 8+DCs but not 8-DCs cross-prime cytotoxic T cells in vivo. We aim to distinguish the two subtypes of DC based on gene expression profiling. Suppressive subtractive hybridization was undertaken to get differentially expressed genes from such subtracted cDNA library specific to 8+DC. A total of 114 sequences from the subtracted cDNA library specific to 8+DC library were analyzed. Most of them are known proteins, but some of them were novel, either totally novel genes or homologs to known genes, but with novel exon. About 55 probably novel exons were discovered, and 11 exons had longer length than those in gene bank. The clones 12, 44, 79, and 110 have no match with known sequences in gene bank. Then, semi-quantitative PCR was done to compare the expression of the enriched sequences between 8+DC and 8-DC. About 14 genes are differentially expressed in 8+DC. Therefore, SSH is an effective method to clone differentially expressed genes for 8+DC compared to 8-DC.

Degrading devices: invadosomes in proteolytic cell invasion

Podosomes and invadopodia, collectively known as invadosomes, are cell-matrix contacts in a variety of cell types, such as monocytic cells or cancer cells, that have to cross tissue barriers. Both structures share an actin-rich core, which distinguishes them from other matrix contacts, and are regulated by a multitude of signaling pathways including RhoGTPases, kinases, actin-associated proteins, and microtubule-dependent transport. Invadosomes recruit and secrete proteinases and are thus able to lyse extracellular matrix components. They are therefore considered to be potential key structures in proteolytic cell invasion in both physiological and pathological settings. This review provides an overview of the field, with special focus on current developments such as intracellular transport processes, ultrastructural analysis, the possible involvement of invadosomes in disease, and the tentative identification of invadosomes in 3D environments and in vivo.