Invadopodia are actin-based membrane protrusions formed at contact sites between invasive tumor cells and the extracellular matrix with matrix proteolytic activity. Actin regulatory proteins participate in invadopodia formation, whereas matrix degradation requires metalloproteinases (MMPs) targeted to invadopodia. In this study, we show that the vesicle-tethering exocyst complex is required for matrix proteolysis and invasion of breast carcinoma cells. We demonstrate that the exocyst subunits Sec3 and Sec8 interact with the polarity protein IQGAP1 and that this interaction is triggered by active Cdc42 and RhoA, which are essential for matrix degradation. Interaction between IQGAP1 and the exocyst is necessary for invadopodia activity because enhancement of matrix degradation induced by the expression of IQGAP1 is lost upon deletion of the exocyst-binding site. We further show that the exocyst and IQGAP1 are required for the accumulation of cell surface membrane type 1 MMP at invadopodia. Based on these results, we propose that invadopodia function in tumor cells relies on the coordination of cytoskeletal assembly and exocytosis downstream of Rho guanosine triphosphatases.
WASH and exocyst promote pericellular matrix degradation and tumor cell invasion by enabling localized exocytosis of MT1-MMP from late endosomes.
Remodeling of the extracellular matrix by carcinoma cells during metastatic dissemination requires formation of actin-based protrusions of the plasma membrane called invadopodia, where the trans-membrane type 1 matrix metalloproteinase (MT1-MMP) accumulates. Here, we describe an interaction between the exocyst complex and the endosomal Arp2/3 activator Wiskott-Aldrich syndrome protein and Scar homolog (WASH) on MT1-MMP–containing late endosomes in invasive breast carcinoma cells. We found that WASH and exocyst are required for matrix degradation by an exocytic mechanism that involves tubular connections between MT1-MMP–positive late endosomes and the plasma membrane in contact with the matrix. This ensures focal delivery of MT1-MMP and supports pericellular matrix degradation and tumor cell invasion into different pathologically relevant matrix environments. Our data suggest a general mechanism used by tumor cells to breach the basement membrane and for invasive migration through fibrous collagen-enriched tissues surrounding the tumor.
β1 integrin is a major regulator of invadopodium maturation. Studies reveal that β1 integrin–mediated adhesion is a key upstream switch that induces Arg-dependent cortactin phosphorylation, actin polymerization, and MMP recruitment to invadopodia for extracellular matrix degradation.
β1 integrin has been shown to promote metastasis in a number of tumor models, including breast, ovarian, pancreatic, and skin cancer; however, the mechanism by which it does so is poorly understood. Invasive membrane protrusions called invadopodia are believed to facilitate extracellular matrix degradation and intravasation during metastasis. Previous work showed that β1 integrin localizes to invadopodia, but its role in regulating invadopodial function has not been well characterized. We find that β1 integrin is required for the formation of mature, degradation-competent invadopodia in both two- and three-dimensional matrices but is dispensable for invadopodium precursor formation in metastatic human breast cancer cells. β1 integrin is activated during invadopodium precursor maturation, and forced β1 integrin activation enhances the rate of invadopodial matrix proteolysis. Furthermore, β1 integrin interacts with the tyrosine kinase Arg and stimulates Arg-dependent phosphorylation of cortactin on tyrosine 421. Silencing β1 integrin with small interfering RNA completely abrogates Arg-dependent cortactin phosphorylation and cofilin-dependent barbed-end formation at invadopodia, leading to a significant decrease in the number and stability of mature invadopodia. These results describe a fundamental role for β1 integrin in controlling actin polymerization–dependent invadopodial maturation and matrix degradation in metastatic tumor cells.
The exocyst is a multi-protein complex essential for exocytosis and plasma membrane remodeling. The assembly of the exocyst complex mediates the tethering of post-Golgi secretory vesicles to the plasma membrane prior to fusion. Elucidating the mechanisms regulating exocyst assembly is important for the understanding of exocytosis. Here we show that the exocyst component Exo70 is a direct substrate of the Extracellular signal-Regulated Kinases 1/2 (ERK1/2). ERK1/2 phosphorylation enhances the binding of Exo70 to other exocyst components and promotes the assembly the exocyst complex in response to epidermal growth factor (EGF) signaling. We further demonstrate that ERK1/2 regulates exocytosis as blocking ERK1/2 signaling by a chemical inhibitor or the expression of an Exo70 mutant defective in ERK1/2 phosphorylation inhibited exocytosis. In tumor cells, blocking Exo70 phosphorylation inhibits matrix metalloproteinase secretion and invadopodia formation. ERK1/2 phosphorylation of Exo70 may thus coordinate exocytosis with other cellular events in response to growth factor signaling.
ERK1/2; exocyst; Exo70; vesicle tethering; exocytosis; EGF; invadopodia
The invasiveness of cells is correlated with the presence of dynamic actin-rich membrane structures called invadopodia, which are membrane protrusions that are associated with localized polymerization of sub-membrane actin filaments. Similar to focal adhesions and podosomes, invadopodia are cell matrix adhesion sites. Indeed, invadopodia share several features with podosomes, but whether they are distinct structures is still a matter of debate. Invadopodia are built upon an N-WASP-dependent branched actin network, and the Rho GTPase Cdc42 is involved in inducing invadopodial-membrane protrusion, which is mediated by actin filaments that are organized in bundles to form an actin core. Actin-core formation is thought to be an early step in invadopodium assembly, and the actin core is perpendicular to the extracellular matrix and the plasma membrane; this contrasts with the tangential orientation of actin stress fibers anchored to focal adhesions. In this Commentary, we attempt to summarize recent insights into the actin dynamics of invadopodia and podosomes, and the forces that are transmitted through these invasive structures. Although the mechanisms underlying force-dependent regulation of invadopodia and podosomes are largely unknown compared with those of focal adhesions, these structures do exhibit mechanosensitivity. Actin dynamics and associated forces might be key elements in discriminating between invadopodia, podosomes and focal adhesions. Targeting actin regulatory molecules that specifically promote invadopodium formation is an attractive strategy against cancer-cell invasion.
Actins; Animals; Cell Adhesion; Cell Movement; Cell-Matrix Junctions; Extracellular Matrix; Focal Adhesions; Humans; Integrins; Models, Biological; Podosomes; invadopodia; invasion; cancer; osteoporosis
Invadopodia are actin-dependent organelles that function in the invasion and remodeling of the extracellular matrix (ECM) by tumor cells. Cortactin, a regulator of the Arp2/3 complex, is of particular importance in invadopodia function. While most of the focus has been on the possible role of cortactin in actin assembly for direct formation of actin-rich invadopodia puncta, our recent data suggest that the primary role of cortactin in invadopodia is to promote protease secretion. In this manuscript, we review our previous work and present new data showing that cortactin is essential for both the localization of key invadopodia matrix metalloproteinases (MMPs) to actin-positive puncta at the cell-ECM interface and for ECM degradation induced by overexpression of MT1-MMP-GFP. Based on these data and results from the literature, we propose potential mechanisms by which cortactin may link vesicular trafficking and dynamic branched actin assembly to regulate protease secretion for invadopodia-associated ECM degradation.
Cortactin; Invadopodia; Matrix metalloproteinase; Protease; Membrane trafficking; Vesicle
Invadopodia are specialized actin-rich protrusions of metastatic tumor and transformed cells with crucial functions in ECM degradation and invasion. Although early electron microscopy studies described invadopodia as long filament-like protrusions of the cell membrane adherent to the matrix, fluorescence microscopy studies have focused on invadopodia as actin-cortactin aggregates localized to areas of ECM degradation. The absence of a clear conceptual integration of these two descriptions of invadopodial structure has impeded understanding of the regulatory mechanisms that govern invadopodia. To determine the relationship between the membrane filaments identified by electron microscopy and the actin-cortactin aggregates of invadopodia, we applied rapid live-cell high-resolution TIRF microscopy to examine cell membrane dynamics at the cortactin core of the invadopodia of human carcinoma cells. We found that cortactin docking to the cell membrane adherent to 2D fibronectin matrix initiates invadopodium assembly associated with the formation of a invadopodial membrane process that extends from a ventral cell membrane lacuna toward the ECM. The tip of the invadopodial process flattens as it interacts with the 2D matrix, and it undergoes constant rapid ruffling and dynamic formation of filament-like protrusions as the invadopodium matures. To describe this newly discovered dynamic relationship between the actin-cortactin core and invadopodial membranes, we propose a model of the invadopodial complex. Using TIRF microscopy, we also established that – in striking contrast to the invadopodium – membrane at the podosome of a macrophage fails to form any process- or filament-like membrane protrusions. Thus, the undulation and ruffling of the invadopodial membrane together with the formation of dynamic filament-like extensions from the invadopodial cortactin core defines invadopodia as invasive superstructures that are distinct from the podosomes.
invadopodia; podosomes; cortactin; focal adhesions; invasion
Our recent studies implicated key and distinct roles for the highly related RalA and RalB small GTPases (82% sequence identity) in pancreatic ductal adenocarcinoma (PDAC) tumorigenesis and invasive and metastatic growth, respectively. How RalB may promote PDAC invasion and metastasis has not been determined. In light of known Ral effector functions in regulation of actin organization and secretion, we addressed a possible role for RalB in formation of invadopodia, actin-rich membrane protrusions that contribute to tissue invasion and matrix remodeling. We determined that a majority of KRAS mutant PDAC cell lines exhibited invadopodia and that expression of activated K-Ras is both necessary and sufficient for invadopodium formation. Invadopodium formation was not dependent on the canonical Raf-MEK-ERK effector pathway and was instead dependent on the Ral effector pathway. However, this process was more dependent on RalB than on RalA. Surprisingly, RalB-mediated invadopodium formation was dependent on RalBP1/RLIP76 but not Sec5 and Exo84 exocyst effector function. Unexpectedly, the requirement for RalBP1 was independent of its best known function as a GTPase-activating protein for Rho small GTPases. Instead, disruption of the ATPase function of RalBP1 impaired invadopodium formation. Our results identify a novel RalB-mediated biochemical and signaling mechanism for invadopodium formation.
Tumor cells use actin-rich protrusions called invadopodia to degrade extracellular matrix (ECM) and invade tissues; related structures, termed podosomes, are sites of dynamic ECM interaction. We show here that supervillin (SV), a peripheral membrane protein that binds F-actin and myosin II, reorganizes the actin cytoskeleton and potentiates invadopodial function. Overexpressed SV induces redistribution of lamellipodial cortactin and lamellipodin/RAPH1/PREL1 away from the cell periphery to internal sites and concomitantly increases the numbers of F-actin punctae. Most punctae are highly dynamic and colocalize with the podosome/invadopodial proteins, cortactin, Tks5, and cdc42. Cortactin binds SV sequences in vitro and contributes to the formation of enhanced green fluorescent protein (EGFP)-SV induced punctae. SV localizes to the cores of Src-generated podosomes in COS-7 cells and with invadopodia in MDA-MB-231 cells. EGFP-SV overexpression increases average numbers of ECM holes per cell; RNA interference-mediated knockdown of SV decreases these numbers. Although SV knockdown alone has no effect, simultaneous down-regulation of SV and the closely related protein gelsolin reduces invasion through ECM. Together, our results show that SV is a component of podosomes and invadopodia and that SV plays a role in invadopodial function, perhaps as a mediator of cortactin localization, activation state, and/or dynamics of metalloproteinases at the ventral cell surface.
Invadopodia or invasive feet, which are actin-rich membrane protrusions with matrix degradation activity formed by invasive cancer cells, are a key determinant in the malignant invasive progression of tumors and represent an important target for cancer therapies. In this work, we presented a microfluidic 3D culture device with continuous supplement of fresh media via a syringe pump. The device mimicked tumor microenvironment in vivo and could be used to assay invadopodia formation and to study the mechanism of human lung cancer invasion. With this device, we investigated the effects of epidermal growth factor (EGF) and matrix metalloproteinase (MMP) inhibitor, GM6001 on invadopodia formation by human non-small cell lung cancer cell line A549 in 3D matrix model. This device was composed of three units that were capable of achieving the assays on one control group and two experimental groups' cells, which were simultaneously pretreated with EGF or GM6001 in parallel. Immunofluorescence analysis of invadopodia formation and extracellular matrix degradation was conducted using confocal imaging system. We observed that EGF promoted invadopodia formation by A549 cells in 3D matrix and that GM6001 inhibited the process. These results demonstrated that epidermal growth factor receptor (EGFR) signaling played a significant role in invadopodia formation and related ECM degradation activity. Meanwhile, it was suggested that MMP inhibitor (GM6001) might be a powerful therapeutic agent targeting invadopodia formation in tumor invasion. This work clearly demonstrated that the microfluidic-based 3D culture device provided an applicable platform for elucidating the mechanism of cancer invasion and could be used in testing other anti-invasion agents.
Invasion and metastasis are aggressive cancer phenotypes that are highly related to the ability of cancer cells to degrade extracellular matrix (ECM). At the cellular level, specialized actin-rich structures called invadopodia mediate focal matrix degradation by serving as exocytic sites for ECM-degrading proteinases. Adhesion signaling is likely to be a critical regulatory input to invadopodia, but the mechanism and location of such adhesion signaling events are poorly understood. Here, we report that adhesion rings surround invadopodia shortly after formation and correlate strongly with invadopodium activity on a cell-by-cell basis. By contrast, there was little correlation of focal adhesion number or size with cellular invadopodium activity. Prevention of adhesion ring formation by inhibition of RGD-binding integrins or knockdown (KD) of integrin-linked kinase (ILK) reduced the number of ECM-degrading invadopodia and reduced recruitment of IQGAP to invadopodium actin puncta. Furthermore, live cell imaging revealed that the rate of extracellular MT1-MMP accumulation at invadopodia was greatly reduced in both integrin-inhibited and ILK-KD cells. Conversely, KD of MT1-MMP reduced invadopodium activity and dynamics but not the number of adhesion-ringed invadopodia. These results suggest a model in which adhesion rings are recruited to invadopodia shortly after formation and promote invadopodium maturation by enhancing proteinase secretion. Since adhesion rings are a defining characteristic of podosomes, similar structures formed by normal cells, our data also suggest further similarities between invadopodia and podosomes.
Invadopodia; Adhesion rings; MT1-MMP; ILK; Integrin; Invasion
Invasive carcinoma cells form actin-rich matrix-degrading protrusions called invadopodia. These structures resemble podosomes produced by some normal cells and play a crucial role in extracellular matrix remodeling. In cancer, formation of invadopodia is strongly associated with invasive potential. Although deregulated signals from the receptor tyrosine kinase Met (also known as hepatocyte growth factor are linked to cancer metastasis and poor prognosis, its role in invadopodia formation is not known. Here we show that stimulation of breast cancer cells with the ligand for Met, hepatocyte growth factor, promotes invadopodia formation, and in aggressive gastric tumor cells where Met is amplified, invadopodia formation is dependent on Met activity. Using both GRB2-associated-binding protein 1 (Gab1)-null fibroblasts and specific knockdown of Gab1 in tumor cells we show that Met-mediated invadopodia formation and cell invasion requires the scaffold protein Gab1. By a structure–function approach, we demonstrate that two proline-rich motifs (P4/5) within Gab1 are essential for invadopodia formation. We identify the actin regulatory protein, cortactin, as a direct interaction partner for Gab1 and show that a Gab1–cortactin interaction is dependent on the SH3 domain of cortactin and the integrity of the P4/5 region of Gab1. Both cortactin and Gab1 localize to invadopodia rosettes in Met-transformed cells and the specific uncoupling of cortactin from Gab1 abrogates invadopodia biogenesis and cell invasion downstream from the Met receptor tyrosine kinase. Met localizes to invadopodia along with cortactin and promotes phosphorylation of cortactin. These findings provide insights into the molecular mechanisms of invadopodia formation and identify Gab1 as a scaffold protein involved in this process.
Invadopodia; Met RTK; Gab1; Cortactin; Matrix remodeling; Cell invasion
Exocyst-positive organelle (EXPO) is a double-membrane organelle mediating unconventional protein secretion in plants. The Arabidopsis exocyst subunit AtExo70E2 is essential for the recruitment of other exocyst subunits to EXPO and plays a key role in EXPO formation in both plant and animal cells.
In contrast to a single copy of Exo70 in yeast and mammals, the Arabidopsis genome contains 23 paralogues of Exo70 (AtExo70). Using AtExo70E2 and its GFP fusion as probes, we recently identified a novel double-membrane organelle termed exocyst-positive organelle (EXPO) that mediates an unconventional protein secretion in plant cells. Here we further demonstrate that AtExo70E2 is essential for exocyst subunit recruitment and for EXPO formation in both plants and animals. By performing transient expression in Arabidopsis protoplasts, we established that a number of exocyst subunits (especially the members of the Sec family) are unable to be recruited to EXPO in the absence of AtExo70E2. The paralogue AtExo70A1 is unable to substitute for AtExo70E2 in this regard. Fluorescence resonance energy transfer assay and bimolecular fluorescence complementation analyses confirm the interaction between AtExo70E2 and Sec6 and Sec10. AtExo70E2, but not its yeast counterpart, is also capable of inducing EXPO formation in an animal cell line (HEK293A cells). Electron microscopy confirms the presence of double-membraned, EXPO-like structures in HEK293A cells expressing AtExo70E2. Inversely, neither human nor yeast Exo70 homologues cause the formation of EXPO in Arabidopsis protoplasts. These results point to a specific and crucial role for AtExo70E2 in EXPO formation.
The exocyst complex localizes to distinct foci at the plasma membrane of Arabidopsis thaliana cells. Their localization at the plasma membrane is insensitive to BFA treatment but is decreased in an exocyst-subunit mutant. In turn, exocyst-subunit mutants show decreased exocytosis.
The exocyst complex, an effector of Rho and Rab GTPases, is believed to function as an exocytotic vesicle tether at the plasma membrane before soluble N-ethylmaleimide–sensitive factor attachment protein receptor (SNARE) complex formation. Exocyst subunits localize to secretory-active regions of the plasma membrane, exemplified by the outer domain of Arabidopsis root epidermal cells. Using variable-angle epifluorescence microscopy, we visualized the dynamics of exocyst subunits at this domain. The subunits colocalized in defined foci at the plasma membrane, distinct from endocytic sites. Exocyst foci were independent of cytoskeleton, although prolonged actin disruption led to changes in exocyst localization. Exocyst foci partially overlapped with vesicles visualized by VAMP721 v-SNARE, but the majority of the foci represent sites without vesicles, as indicated by electron microscopy and drug treatments, supporting the concept of the exocyst functioning as a dynamic particle. We observed a decrease of SEC6–green fluorescent protein foci in an exo70A1 exocyst mutant. Finally, we documented decreased VAMP721 trafficking to the plasma membrane in exo70A1 and exo84b mutants. Our data support the concept that the exocyst-complex subunits dynamically dock and undock at the plasma membrane to create sites primed for vesicle tethering.
Directional cell migration requires the coordination of actin assembly and membrane remodeling. The exocyst is an octameric protein complex essential for exocytosis and plasma membrane remodeling [1,2]. A component of the exocyst, Exo70, directly interacts with the Arp2/3 complex, a core nucleating factor for the generation of branched actin networks for cell morphogenesis and migration [3-9]. Using in vitro actin polymerization assay and time-lapse TIRF microscopy, we found Exo70 functions as a kinetic activator of the Arp2/3 complex that promotes actin filament nucleation and branching. We further found that the effect of Exo70 on actin is mediated by promoting the interaction of Arp2/3 complex with WAVE2, a member of the N-WASP/WAVE family of nucleation promoting factors (NPFs). At the cellular level, the stimulatory effect of Exo70 on Arp2/3 is required for lamellipodia formation and maintaining directional persistence of cell migration. Our findings provide a novel mechanism for regulating actin polymerization and branching for effective membrane protrusion during cell morphogenesis and migration.
Extracellular matrix (ECM) degradation is a critical process in tumor cell invasion and requires matrix degrading protrusions called invadopodia. The Na+/H+ exchanger (NHE1) has recently been shown to be fundamental in the regulation of invadopodia actin cytoskeleton dynamics and activity. However, the structural link between the invadopodia cytoskeleton and NHE1 is still unknown. A candidate could be ezrin, a linker between the NHE1 and the actin cytoskeleton known to play a pivotal role in invasion and metastasis. However, the mechanistic basis for its role remains unknown. Here, we demonstrate that ezrin phosphorylated at T567 is highly overexpressed in the membrane of human breast tumors and positively associated with invasive growth and HER2 overexpression. Further, in the metastatic cell line, MDA-MB-231, p-ezrin was almost exclusively expressed in invadopodia lipid rafts where it co-localized in a functional complex with NHE1, EGFR, ß1-integrin and phosphorylated-NHERF1. Manipulation by mutation of ezrins T567 phosphorylation state and/or PIP2 binding capacity or of NHE1s binding to ezrin or PIP2 demonstrated that p-ezrin expression and binding to PIP2 are required for invadopodia-mediated ECM degradation and invasion and identified NHE1 as the membrane protein that p-ezrin regulates to induce invadopodia formation and activity.
RhoGTPases have been implicated in the regulation of cancer metastasis. Invasive carcinoma cells form invadopodia, F-actin-rich matrix degrading protrusions that are thought to be important for tumor cell invasion and intravasation. Regulation of actin dynamics at invadopodial protrusions is crucial to drive invasion. This process requires the severing activity of cofilin to generate actin-free barbed ends. Previous work demonstrates that cofilin’s severing activity is tightly regulated through multiple mechanisms including regulation of cofilin serine phosphorylation by Rho GTPases. However, it is not known which Rho GTPase is involved in regulating cofilin’s phosphorylation status at invadopodia.
We show here, for the first time, how RhoC activation is controlled at invadopodia and how this activation regulates cofilin phosphorylation to control cofilin’s generation of actin-free barbed ends. Live-cell imaging of fluorescent RhoC biosensor reveals that RhoC activity is spatially confined to areas surrounding invadopodia. This spatiotemporal restriction of RhoC activity is controlled by “spatially distinct regulatory elements” that confines RhoC activation within this compartment. p190RhoGEF localizes around invadopodia to activate RhoC, while p190RhoGAP localizes inside invadopodia to deactivate the GTPase within the structure. RhoC activation enhances cofilin phosphorylation outside invadopodia.
These results show how RhoC activity is spatially regulated at invadopodia by p190RhoGEF and p190RhoGAP. RhoC activation in areas surrounding invadopodia restricts cofilin activity to within the invadopodium core resulting in a focused invadopodial protrusion. This mechanism likely enhances tumor cell invasion during metastasis.
Metastasis; Invadopodia; RhoC; p190RhoGEF; p190RhoGAP; Cofilin; Cofilin phosphorylation; tumor invasion
Invadopodia are matrix-degrading membrane protrusions in invasive carcinoma cells. The mechanisms regulating invadopodium assembly and maturation are not understood. We have dissected the stages of invadopodium assembly and maturation and show that invadopodia use cortactin phosphorylation as a master switch during these processes. In particular, cortactin phosphorylation was found to regulate cofilin and Arp2/3 complex–dependent actin polymerization. Cortactin directly binds cofilin and inhibits its severing activity. Cortactin phosphorylation is required to release this inhibition so cofilin can sever actin filaments to create barbed ends at invadopodia to support Arp2/3-dependent actin polymerization. After barbed end formation, cortactin is dephosphorylated, which blocks cofilin severing activity thereby stabilizing invadopodia. These findings identify novel mechanisms for actin polymerization in the invadopodia of metastatic carcinoma cells and define four distinct stages of invadopodium assembly and maturation consisting of invadopodium precursor formation, actin polymerization, stabilization, and matrix degradation.
Talin binds directly to moesin in vitro and recruits a moesin–NHE-1 complex to invadopodia to promote tumor cell invasion.
Invadopodia are actin-rich protrusions that degrade the extracellular matrix and are required for stromal invasion, intravasation, and metastasis. The role of the focal adhesion protein talin in regulating these structures is not known. Here, we demonstrate that talin is required for invadopodial matrix degradation and three-dimensional extracellular matrix invasion in metastatic breast cancer cells. The sodium/hydrogen exchanger 1 (NHE-1) is linked to the cytoskeleton by ezrin/radixin/moesin family proteins and is known to regulate invadopodium-mediated matrix degradation. We show that the talin C terminus binds directly to the moesin band 4.1 ERM (FERM) domain to recruit a moesin–NHE-1 complex to invadopodia. Silencing talin resulted in a decrease in cytosolic pH at invadopodia and blocked cofilin-dependent actin polymerization, leading to impaired invadopodium stability and matrix degradation. Furthermore, talin is required for mammary tumor cell motility, intravasation, and spontaneous lung metastasis in vivo. Thus, our findings provide a novel understanding of how intracellular pH is regulated and a molecular mechanism by which talin enhances tumor cell invasion and metastasis.
The Neurospora crassa exocyst presents two distinct localization patterns. EXO-70 and -84 colocalize with a region of the Spitzenkörper occupied by secretory macrovesicles. In contrast, SEC-3, -5, -6, -8, and -15 localize distinctively at the apical plasma membrane.
Fungal hyphae are among the most highly polarized cells. Hyphal polarized growth is supported by tip-directed transport of secretory vesicles, which accumulate temporarily in a stratified manner in an apical vesicle cluster, the Spitzenkörper. The exocyst complex is required for tethering of secretory vesicles to the apical plasma membrane. We determined that the presence of an octameric exocyst complex is required for the formation of a functional Spitzenkörper and maintenance of regular hyphal growth in Neurospora crassa. Two distinct localization patterns of exocyst subunits at the hyphal tip suggest the dynamic formation of two assemblies. The EXO-70/EXO-84 subunits are found at the peripheral part of the Spitzenkörper, which partially coincides with the outer macrovesicular layer, whereas exocyst components SEC-5, -6, -8, and -15 form a delimited crescent at the apical plasma membrane. Localization of SEC-6 and EXO-70 to the plasma membrane and the Spitzenkörper, respectively, depends on actin and microtubule cytoskeletons. The apical region of exocyst-mediated vesicle fusion, elucidated by the plasma membrane–associated exocyst subunits, indicates the presence of an exocytotic gradient with a tip-high maximum that dissipates gradually toward the subapex, confirming the earlier predictions of the vesicle supply center model for hyphal morphogenesis.
Fascin is an actin bundling protein involved in filopodia assembly and cancer invasion and metastasis of multiple epithelial cancer types. Fascin forms stable actin bundles with slow dissociation kinetics in vitro  and is regulated by phosphorylation of serine 39 by protein kinase C (PKC) . Cancer cells use invasive finger-like protrusions termed invadopodia to invade into and degrade extracellular matrix. Invadopodia have highly dynamic actin that is assembled by both Arp2/3 complex and formins [3, 4]; they also contain components of membrane trafficking machinery such as dynamin and cortactin  and have been compared with focal adhesions and podosomes [6, 7]. We show that fascin is an integral component of invadopodia and that it is important for the stability of actin in invadopodia. The phosphorylation state of fascin at S39, a PKC site, contributes to its regulation at invadopodia. We further implicate fascin in invasive migration into collagen I-Matrigel gels and particularly in cell types that use an elongated mesenchymal type of motility in 3D. We provide a potential molecular mechanism for how fascin increases the invasiveness of cancer cells and we compare invadopodia with invasive filopod-like structures in 3D.
Adhesion of epithelial cells to basement membranes (BM) occurs through 2 major structures: actin-associated focal contacts and keratin-associated hemidesmosomes, both of which form on laminin-332 (Ln-332). In epithelial-derived cancer cells, additional actin-linked structures with putative adhesive properties, invadopodia, are frequently present and mediate BM degradation. A recent study proposed that BM invasion requires a proper combination of focal contacts and invadopodia for invading cells to gain traction through degraded BM, and suggested that these structures may compete for common molecular components such as Src kinase. In this study, we tested the role of the Ln-332 in regulating invadopodia in 804G rat bladder carcinoma cells, a cell line that secretes Ln-332 and forms all 3 types of adhesions. Expression of shRNA to Ln-332 γ2 chain (γ2-kd) led to increased numbers of invadopodia and enhanced extracellular matrix degradation. Replating γ2-kd cells on Ln-332 or collagen-I fully recovered cell spreading and inhibition of invadopodia. Inhibition of α3 or β1, but not α6 or β4, phenocopied the effect of γ2-kd, suggesting that α3β1-mediated focal contacts, rather than a6β4-mediated hemidesmosome pathways, intersect with invadopodia regulation. γ2-kd cells exhibited alterations in focal contact-type structures and in activation of focal adhesion kinase (FAK) and Src kinase. Inhibition of FAK also increased invadopodia number, which was reversible with Src inhibition. These data are consistent with a model whereby actin-based adhesions can limit the availability of active Src that is capable of invadopodia initiation and identifies Ln-332-β1 interactions as a potent upstream regulator that limits cell invasion.
invadopodia; integrin; laminin-332
Invadopodia are actin-rich membrane protrusions with a matrix degradation activity formed by invasive cancer cells. We have studied the molecular mechanisms of invadopodium formation in metastatic carcinoma cells. Epidermal growth factor (EGF) receptor kinase inhibitors blocked invadopodium formation in the presence of serum, and EGF stimulation of serum-starved cells induced invadopodium formation. RNA interference and dominant-negative mutant expression analyses revealed that neural WASP (N-WASP), Arp2/3 complex, and their upstream regulators, Nck1, Cdc42, and WIP, are necessary for invadopodium formation. Time-lapse analysis revealed that invadopodia are formed de novo at the cell periphery and their lifetime varies from minutes to several hours. Invadopodia with short lifetimes are motile, whereas long-lived invadopodia tend to be stationary. Interestingly, suppression of cofilin expression by RNA interference inhibited the formation of long-lived invadopodia, resulting in formation of only short-lived invadopodia with less matrix degradation activity. These results indicate that EGF receptor signaling regulates invadopodium formation through the N-WASP–Arp2/3 pathway and cofilin is necessary for the stabilization and maturation of invadopodia.
Localized F-actin disassembly by ADF/cofilin drives invadopodial membrane recycling through endolysosomes, which promotes efficient cell transmigration through the basement membrane.
Invadopodia are protrusive, F-actin–driven membrane structures that are thought to mediate basement membrane transmigration during development and tumor dissemination. An understanding of the mechanisms regulating invadopodia has been hindered by the difficulty of examining these dynamic structures in native environments. Using an RNAi screen and live-cell imaging of anchor cell (AC) invasion in Caenorhabditis elegans, we have identified UNC-60A (ADF/cofilin) as an essential regulator of invadopodia. UNC-60A localizes to AC invadopodia, and its loss resulted in a dramatic slowing of F-actin dynamics and an inability to breach basement membrane. Optical highlighting indicated that UNC-60A disassembles actin filaments at invadopodia. Surprisingly, loss of unc-60a led to the accumulation of invadopodial membrane and associated components within the endolysosomal compartment. Photobleaching experiments revealed that during normal invasion the invadopodial membrane undergoes rapid recycling through the endolysosome. Together, these results identify the invadopodial membrane as a specialized compartment whose recycling to form dynamic, functional invadopodia is dependent on localized F-actin disassembly by ADF/cofilin.
Genetic analyses in zebrafish identify a novel physical signaling mechanism that drives formation of invadopodia-like structures and promotes cell invasion in vivo.
The signals that initiate cell invasion are not well understood, but there is increasing evidence that extracellular physical signals play an important role. Here we show that epithelial cell invasion in the intestine of zebrafish meltdown (mlt) mutants arises in response to unregulated contractile tone in the surrounding smooth muscle cell layer. Physical signaling in mlt drives formation of membrane protrusions within the epithelium that resemble invadopodia, matrix-degrading protrusions present in invasive cancer cells. Knockdown of Tks5, a Src substrate that is required for invadopodia formation in mammalian cells blocked formation of the protrusions and rescued invasion in mlt. Activation of Src-signaling induced invadopodia-like protrusions in wild type epithelial cells, however the cells did not migrate into the tissue stroma, thus indicating that the protrusions were required but not sufficient for invasion in this in vivo model. Transcriptional profiling experiments showed that genes responsive to reactive oxygen species (ROS) were upregulated in mlt larvae. ROS generators induced invadopodia-like protrusions and invasion in heterozygous mlt larvae but had no effect in wild type larvae. Co-activation of oncogenic Ras and Wnt signaling enhanced the responsiveness of mlt heterozygotes to the ROS generators. These findings present the first direct evidence that invadopodia play a role in tissue cell invasion in vivo. In addition, they identify an inducible physical signaling pathway sensitive to redox and oncogenic signaling that can drive this process.
The epithelial cells lining the digestive tract are separated from the connective tissue stroma by a thin layer of extracellular matrix called the basement membrane. During cell invasion, as occurs during cancer metastasis, epithelial cells breach the basement membrane and invade the tissue stroma. The proteases used by invasive cells to degrade basement membrane in vitro are localized in specialized plasma membrane protrusions known as invadopodia. It is not known, however, whether invadopodia are required for cell invasion in vivo or what triggers their formation. Here, we show that epithelial cells in the intestine of the zebrafish mutant meltdown form invadopodia-like protrusions and invade the tissue stroma in response to unregulated contractile tone in the surrounding smooth muscle layer. The invadopodia-like protrusions that form in response to this physical signal are required for epithelial cell invasion in this in vivo model, and they can be induced when unregulated smooth muscle contraction is induced by oxidative stress. These findings provide the first direct evidence that invadopodia play a role in tissue cell invasion in vivo and identify a novel inducible physical signaling mechanism that can drive this process.