Paxillin phosphorylation controls invadopodia/podosomes spatiotemporal organization

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.

The Role of Paxillin Aberrant Expression in Cancer and Its Potential as a Target for Cancer Therapy

Paxillin is a multi-domain adaptor protein. As an important member of focal adhesion (FA) and a participant in regulating cell movement, paxillin plays an important role in physiological processes such as nervous system development, embryonic development, and vascular development. However, increasing evidence suggests that paxillin is aberrantly expressed in many cancers. Many scholars have also recognized that the abnormal expression of paxillin is related to the prognosis, metastases, invasion, survival, angiogenesis, and other aspects of malignant tumors, suggesting that paxillin may be a potential cancer therapeutic target. Therefore, the study of how aberrant paxillin expression affects the process of tumorigenesis and metastasis will help to develop more efficacious antitumor drugs. Herein, we review the structure of paxillin and its function and expression in tumors, paying special attention to the multifaceted effects of paxillin on tumors, the mechanism of tumorigenesis and progression, and its potential role in tumor therapy. We also hope to provide a reference for the clinical prognosis and development of new tumor therapeutic targets.

Protein dynamics at invadopodia control invasion-migration transitions in melanoma cells

Cell invasion is a highly complex process that requires the coordination of cell migration and degradation of the extracellular matrix. In melanoma cells, as in many highly invasive cancer cell types these processes are driven by the regulated formation of adhesives structures such as focal adhesions and invasive structures like invadopodia. Structurally, focal adhesion and invadopodia are quite distinct, yet they share many protein constituents. However, quantitative understanding of the interaction of invadopodia with focal adhesion is lacking, and how invadopodia turn-over is associated with invasion-migration transition cycles remains unknown. In this study, we investigated the role of Pyk2, cortactin and Tks5 in invadopodia turnover and their relation with focal adhesions. We found that active Pyk2 and cortactin are localised at both focal adhesions and invadopodia. At invadopodia, localisation of active Pyk2 is correlated with ECM degradation. During invadopodia disassembly, Pyk2 and cortactin but not Tks5 are often relocated at nearby nascent adhesions. We also show that during ECM degradation, cell migration is reduced which is likely related to the sharing of common molecules within the two structures. Finally, we found that the dual FAK/Pyk2 inhibitor PF-431396 inhibits both focal adhesion and invadopodia activities thereby reducing both migration and ECM degradation.

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.

ARHGAP17 regulates the spatiotemporal activity of Cdc42 at invadopodia

Invadopodia formation is regulated by Rho GTPases. However, the molecular mechanisms that control Rho GTPase signaling at invadopodia remain poorly understood. Here, we have identified ARHGAP17, a Cdc42-specific RhoGAP, as a key regulator of invadopodia in breast cancer cells and characterized a novel ARHGAP17-mediated signaling pathway that controls the spatiotemporal activity of Cdc42 during invadopodia turnover. Our results show that during invadopodia assembly, ARHGAP17 localizes to the invadopodia ring and restricts the activity of Cdc42 to the invadopodia core, where it promotes invadopodia growth. Invadopodia disassembly starts when ARHGAP17 translocates from the invadopodia ring to the core, in a process that is mediated by its interaction with the Cdc42 effector CIP4. Once at the core, ARHGAP17 inactivates Cdc42 to promote invadopodia disassembly. Our results in invadopodia provide new insights into the coordinated transition between the activation and inactivation of Rho GTPases.

Rho GTPase signalling networks in cancer cell transendothelial migration

Rho GTPases are small signalling G-proteins that are central regulators of cytoskeleton dynamics, and thereby regulate many cellular processes, including the shape, adhesion and migration of cells. As such, Rho GTPases are also essential for the invasive behaviour of cancer cells, and thus involved in several steps of the metastatic cascade, including the extravasation of cancer cells. Extravasation, the process by which cancer cells leave the circulation by transmigrating through the endothelium that lines capillary walls, is an essential step for metastasis towards distant organs. During extravasation, Rho GTPase signalling networks not only regulate the transmigration of cancer cells but also regulate the interactions between cancer and endothelial cells and are involved in the disruption of the endothelial barrier function, ultimately allowing cancer cells to extravasate into the underlying tissue and potentially form metastases. Thus, targeting Rho GTPase signalling networks in cancer may be an effective approach to inhibit extravasation and metastasis. In this review, the complex process of cancer cell extravasation will be discussed in detail. Additionally, the roles and regulation of Rho GTPase signalling networks during cancer cell extravasation will be discussed, both from a cancer cell and endothelial cell point of view.

The Helicobacter pylori type IV secretion system upregulates epithelial cortactin expression by a CagA- and JNK-dependent pathway

Cortactin represents an important actin-binding factor, which controls actin-cytoskeletal remodelling in host cells. In this way, cortactin has been shown to exhibit crucial functions both for cell movement and tumour cell invasion. In addition, the cortactin gene cttn is amplified in various cancer types of humans. Helicobacter pylori is the causative agent of multiple gastric diseases and represents a significant risk factor for the development of gastric adenocarcinoma. It has been repeatedly shown that H. pylori manipulates cancer-related signal transduction events in infected gastric epithelial cells such as the phosphorylation status of cortactin. In fact, H. pylori modifies the activity of cortactin's binding partners to stimulate changes in the actin-cytoskeleton, cell adhesion and motility. Here we show that H. pylori infection of cultured AGS and Caco-2 cells for 24-48 hr leads to the overexpression of cortactin by 2-3 fold at the protein level. We demonstrate that this activity requires the integrity of the type IV secretion system (T4SS) encoded by the cag pathogenicity island (cagPAI) as well as the translocated effector protein CagA. We further show that ectopic expression of CagA is sufficient to stimulate cortactin overexpression. Furthermore, phosphorylation of CagA at the EPIYA-repeat region is not required, suggesting that this CagA activity proceeds in a phosphorylation-independent fashion. Inhibitor studies further demonstrate that the involved signalling pathway comprises the mitogen-activated protein kinase JNK (c-Jun N-terminal kinase), but not ERK1/2 or p38. Taken together, using H. pylori as a model system, this study discovered a previously unrecognised cortactin activation cascade by a microbial pathogen. We suggest that H. pylori targets cortactin to manipulate the cellular architecture and epithelial barrier functions that can impact gastric cancer development. TAKE AWAYS: Helicobacter pylori infection induces overexpression of cortactin at the protein level Cortactin upregulation requires the T4SS and effector protein CagA Ectopic expression of CagA is sufficient to stimulate cortactin overexpression Overexpression of cortactin proceeds CagA phosphorylation-independent The involved host cell signalling pathway comprises the MAP kinase JNK.

Rab40-Cullin5 complex regulates EPLIN and actin cytoskeleton dynamics during cell migration

Rab40b is a SOCS box–containing protein that regulates the secretion of MMPs to facilitate extracellular matrix remodeling during cell migration. Here, we show that Rab40b interacts with Cullin5 via the Rab40b SOCS domain. We demonstrate that loss of Rab40b–Cullin5 binding decreases cell motility and invasive potential and show that defective cell migration and invasion stem from alteration to the actin cytoskeleton, leading to decreased invadopodia formation, decreased actin dynamics at the leading edge, and an increase in stress fibers. We also show that these stress fibers anchor at less dynamic, more stable focal adhesions. Mechanistically, changes in the cytoskeleton and focal adhesion dynamics are mediated in part by EPLIN, which we demonstrate to be a binding partner of Rab40b and a target for Rab40b–Cullin5-dependent localized ubiquitylation and degradation. Thus, we propose a model where Rab40b–Cullin5-dependent ubiquitylation regulates EPLIN localization to promote cell migration and invasion by altering focal adhesion and cytoskeletal dynamics.

Ubiquitous membrane-bound DNase activity in podosomes and invadopodia

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

Endothelin-1 drives invadopodia and interaction with mesothelial cells through ILK

Cancer cells use actin-based membrane protrusions, invadopodia, to degrade stroma and invade. In serous ovarian cancer (SOC), the endothelin A receptor (ETAR) drives invadopodia by a not fully explored coordinated function of β-arrestin1 (β-arr1). Here, we report that β-arr1 links the integrin-linked kinase (ILK)/βPIX complex to activate Rac3 GTPase, acting as a central node in the adhesion-based extracellular matrix (ECM) sensing and degradation. Downstream, Rac3 phosphorylates PAK1 and cofilin and promotes invadopodium-dependent ECM proteolysis and invasion. Furthermore, ETAR/ILK/Rac3 signaling supports the communication between cancer and mesothelial cells, favoring SOC cell adhesion and transmigration. In vivo, ambrisentan, an ETAR antagonist, inhibits the adhesion and spreading of tumor cells to intraperitoneal organs, and invadopodium marker expression. As prognostic factors, high EDNRA/ILK expression correlates with poor SOC clinical outcome. These findings provide a framework for the ET-1R/β-arr1 pathway as an integrator of ILK/Rac3-dependent adhesive and proteolytic signaling to invadopodia, favoring cancer/stroma interactions and metastatic behavior.

EB1 Restricts Breast Cancer Cell Invadopodia Formation and Matrix Proteolysis via FAK

Regulation of microtubule dynamics by plus-end tracking proteins (+TIPs) plays an essential role in cancer cell migration. However, the role of +TIPs in cancer cell invasion has been poorly addressed. Invadopodia, actin-rich protrusions specialized in extracellular matrix degradation, are essential for cancer cell invasion and metastasis, the leading cause of death in breast cancer. We, therefore, investigated the role of the End Binding protein, EB1, a major hub of the +TIP network, in invadopodia functions. EB1 silencing increased matrix degradation by breast cancer cells. This was recapitulated by depletion of two additional +TIPs and EB1 partners, APC and ACF7, but not by the knockdown of other +TIPs, such as CLASP1/2 or CLIP170. The knockdown of Focal Adhesion Kinase (FAK) was previously proposed to similarly promote invadopodia formation as a consequence of a switch of the Src kinase from focal adhesions to invadopodia. Interestingly, EB1-, APC-, or ACF7-depleted cells had decreased expression/activation of FAK. Remarkably, overexpression of wild type FAK, but not of FAK mutated to prevent Src recruitment, prevented the increased degradative activity induced by EB1 depletion. Overall, we propose that EB1 restricts invadopodia formation through the control of FAK and, consequently, the spatial regulation of Src activity.

Invadopodia: A potential target for pancreatic cancer therapy

Dissemination of cancer cells is an intricate multistep process that represents the most deadly aspect of cancer. Cancer cells form F-actin-rich protrusions known as invadopodia to invade surrounding tissues, blood vessels and lymphatics. A number of studies have demonstrated the significant roles of invadopodia in cancer. Therefore, the specific cells and molecules involved in invadopodia activity can provide as therapeutic targets. In this review, we included a thorough overview of studies in invadopodia and discussed their functions in cancer metastasis. We then presented the specific cells and molecules involved in invadopodia activity in pancreatic cancer and analyzed their suitability to be effective therapeutic targets. Currently, drugs targeting invadopodia and relevant clinical trials are negligible. Here, we highlighted the significance of potential drugs and discussed future obstacles in implementing clinical trials. This review presents a new perspective on invadopodia-induced pancreatic cancer metastasis and may prosper the development of targeted therapeutics against pancreatic cancer.

Multilayer platform to model the bioactivity of hyaluronic acid in gastric cancer

Hyaluronic acid (HA) has a key role in cancer progression. The HA's molecular weight (M_w) is altered in this pathological state: increased concentration of shorter fragments due to the overexpressed hyaluronidases and ROS. Aiming to mimic this microenvironment, we developed a Layer-by-Layer (LbL) platform presenting HA of different M_ws, namely 6.4, 752 and 1500 kDa, to study the influence of HA M_w on the formation of focal adhesion sites (FAs), and the involvement of paxillin and CD44 in this process. High paxillin expression and formation of FAs, via CD44, is observed for MKN45 cells seeded on LbLs presenting HA 6.4 kDa, with the activation of the ERK1/2 pathway, responsible for cell motility and tumour progression. In contrast, activation of p38 pathway, usually related with cancer latency, is observed for cells seeded on LbLs with high M_w HA, i.e. 1500 kDa. Overall, we demonstrate the suitability of the developed platform to study cancer invasiveness.

Paxillin family of focal adhesion adaptor proteins and regulation of cancer cell invasion

The paxillin family of proteins, including paxillin, Hic-5, and leupaxin, are focal adhesion adaptor/scaffolding proteins which localize to cell-matrix adhesions and are important in cell adhesion and migration of both normal and cancer cells. Historically, the role of these proteins in regulating the actin cytoskeleton through focal adhesion-mediated signaling has been well documented. However, studies in recent years have revealed additional functions in modulating the microtubule and intermediate filament cytoskeletons to affect diverse processes including cell polarization, vesicle trafficking and mechanosignaling. Expression of paxillin family proteins in stromal cells is also important in regulating tumor cell migration and invasion through non-cell autonomous effects on the extracellular matrix. Both paxillin and Hic-5 can also influence gene expression through a variety of mechanisms, while their own expression is frequently dysregulated in various cancers. Accordingly, these proteins may serve as valuable targets for novel diagnostic and treatment approaches in cancer.

Kindlin-3 stokes the life span of podosomes

Macrophages and other cell types can form podosomes, highly dynamic adhesive structures that mediate the interaction of these cells with the surrounding matrix. In this issue, Klapproth et al. (2019. J. Cell Biol. https://doi.org/10.1083/jcb.201903109) find that kindlin-3 regulates podosome stability by recruiting leupaxin, with concomitant effects on PTP-PEST phosphatase activity and paxillin phosphorylation.

A kindlin-3-leupaxin-paxillin signaling pathway regulates podosome stability

Kindlin-3 regulates podosome stability by recruiting leupaxin to podosomes, which in turn controls PTP-PEST activity and paxillin phosphorylation. Kindlin-3 deficiency allows formation of initial adhesion patches containing talin, vinculin, and paxillin, whereas paxillin family proteins are dispensable for podosome formation.

Binding of kindlins to integrins is required for integrin activation, stable ligand binding, and subsequent intracellular signaling. How hematopoietic kindlin-3 contributes to the assembly and stability of the adhesion complex is not known. Here we report that kindlin-3 recruits leupaxin into podosomes and thereby regulates paxillin phosphorylation and podosome turnover. We demonstrate that the activity of the protein tyrosine phosphatase PTP-PEST, which controls paxillin phosphorylation, requires leupaxin. In contrast, despite sharing the same binding mode with leupaxin, paxillin recruitment into podosomes is kindlin-3 independent. Instead, we found paxillin together with talin and vinculin in initial adhesion patches of kindlin-3–null cells. Surprisingly, despite its presence in these early adhesion patches, podosomes can form in the absence of paxillin or any paxillin member. In conclusion, our findings show that kindlin-3 not only activates and clusters integrins into podosomes but also regulates their lifetime by recruiting leupaxin, which controls PTP-PEST activity and thereby paxillin phosphorylation and downstream signaling.

Regulators of the RAS-ERK pathway as therapeutic targets in thyroid cancer

Thyroid cancer is mostly an ERK-driven carcinoma, as up to 70% of thyroid carcinomas are caused by mutations that activate the RAS/ERK mitogenic signaling pathway. The incidence of thyroid cancer has been steadily increasing for the last four decades; yet, there is still no effective treatment for advanced thyroid carcinomas. Current research efforts are focused on impairing ERK signaling with small-molecule inhibitors, mainly at the level of BRAF and MEK. However, despite initial promising results in animal models, the clinical success of these inhibitors has been limited by the emergence of tumor resistance and relapse. The RAS/ERK pathway is an extremely complex signaling cascade with multiple points of control, offering many potential therapeutic targets: from the modulatory proteins regulating the activation state of RAS proteins to the scaffolding proteins of the pathway that provide spatial specificity to the signals, and finally, the negative feedbacks and phosphatases responsible for inactivating the pathway. The aim of this review is to give an overview of the biology of RAS/ERK regulators in human cancer highlighting relevant information on thyroid cancer and future areas of research.

Hic-5 regulates Src-induced invadopodia rosette formation and organization

Fibroblasts transformed by the proto-oncogene Src form individual invadopodia that can spontaneously self-organize into large matrix-degrading superstructures called rosettes. However, the mechanisms by which the invadopodia can spatiotemporally reorganize their architecture is not well understood. Here, we show that Hic-5, a close relative of the scaffold protein paxillin, is essential for the formation and organization of rosettes in active Src-transfected NIH3T3 fibroblasts and cancer-associated fibroblasts. Live cell imaging, combined with domain-mapping analysis of Hic-5, identified critical motifs as well as phosphorylation sites that are required for the formation and dynamics of rosettes. Using pharmacological inhibition and mutant expression, we show that FAK kinase activity, along with its proximity to and potential interaction with the LD2,3 motifs of Hic-5, is necessary for rosette formation. Invadopodia dynamics and their coalescence into rosettes were also dependent on Rac1, formin, and myosin II activity. Superresolution microscopy revealed the presence of formin FHOD1 and INF2-mediated unbranched radial F-actin fibers emanating from invadopodia and rosettes, which may facilitate rosette formation. Collectively, our data highlight a novel role for Hic-5 in orchestrating the organization of invadopodia into higher-order rosettes, which may promote the localized matrix degradation necessary for tumor cell invasion.

Invadopodia are chemosensing protrusions that guide cancer cell extravasation to promote brain tropism in metastasis

Invadopodia are cell protrusions that mediate cancer cell extravasation but the microenvironmental cues and signaling factors that induce invadopodia formation during extravasation remain unclear. Using intravital imaging and loss of function experiments, we determined invadopodia contain receptors involved in chemotaxis, namely GABA receptor and EGFR. These chemotaxis capabilities are mediated in part by PAK1 which controls invadopodia responsiveness to ligands such as GABA and EGF via assembly, stability, and turnover of invadopodia in vivo. PAK1 knockdown rendered cells unresponsive to chemotactic stimuli present in the stroma, resulting in dramatically lower rates of cancer cell extravasation and metastatic colony formation compared to stimulated cancer cells. In an experimental mouse model of brain metastasis, inhibition of PAK1 significantly reduced overall tumor burden and reduced the average size of brain metastases. In summary, invadopodia contain chemotaxis receptors that can respond to microenvironmental cues to guide cancer cell extravasation, and when PAK1 is depleted, brain tropism of metastatic breast cancer cells is significantly reduced, blocking secondary colony growth at sites otherwise permissive for metastatic outgrowth.

Conditioned medium from rat dental pulp reduces the number of osteoclasts via attenuation of adhesiveness in osteoclast precursors

Dental pulp is known to play crucial roles in homeostasis of teeth and periodontal tissue. Although resorption of bone around the roots of nonvital teeth is occasionally observed in clinical practice, little is known about the role of dental pulp in osteoclastogenesis. Here we evaluated the effects of conditioned medium (CM) from rat dental pulp on osteoclastogenesis. It was found that the CM reduced the number of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated osteoclasts, but did not alter the mRNA levels of nuclear factor of activated T-cells, cytoplasmic 1 and TRAP. To further understand the mechanism behind these results, we evaluated the effects of CM on osteoclast precursors and found that the CM removed cell processes, resulting in a significant reduction in the number of attached cells and an increase in the number of freely floating cells. Furthermore, the CM suppressed the mRNA levels of focal adhesion kinase and paxillin, which are involved in cell adhesiveness and spreading. Collectively, the present results show that CM from dental pulp serves as an inhibitor of osteoclastogenesis by reducing the number and adhesiveness of osteoclast precursors, suggesting novel therapeutic applicability for osteoporosis.

Plasma gelsolin is associated with hip BMD in Chinese postmenopausal women

Gelsolin (GSN) protein, expressed in circulating monocytes, was previously reported to be associated with osteoporosis in both Chinese and Caucasian women. This study aims to test if plasma GSN protein level is associated with hip bone mineral density (BMD) in Chinese population. Based on two study Groups containing 6,308 old Chinese, we adopted extreme sampling scheme and selected 3 independent samples (Subgroups 1-3) for discovery, replication, and validation purposes. We tested plasma GSN concentration, and analyzed whether plasma GSN level differs between subjects with extremely low vs. high hip BMD. In Group 1 (N = 1,860), the plasma GSN level increased in the female with low BMD, which was discovered in the Subgroup 1 (N = 42, p = 0.093) and replicated in the Subgroup 2 (N = 39, p = 0.095). With more extreme sampling for the Subgroup 3 from the Group 2 (N = 4,448), the difference of plasma GSN level in the female with low BMD vs. high BMD is more significant (N = 45, p = 0.037). After the subjects were pooled from Subgroups 2 and 3, the difference in plasma GSN between low and high BMD subjects became even more significant (p = 0.016). The plasma GSN level was negatively correlated with total hip BMD (r = -0.26, p = 0.033). We concluded that plasma GSN was associated with hip BMD in Chinese postmenopausal women and plasma GSN might be a potential risk biomarker for osteoporosis.

The Sealing Zone in Osteoclasts: A Self-Organized Structure on the Bone

Osteoclasts form a specialized cell-matrix adhesion structure, known as the "sealing zone", during bone resorption. The sealing zone is a dynamic actin-rich structure that defines the resorption area of the bone. The detailed dynamics and fine structure of the sealing zone have been elusive. Osteoclasts plated on glass do not form a sealing zone, but generate a separate supra-molecular structure called the "podosome belt". Podosomes are integrin-based adhesion complexes involved in matrix adhesion, cell migration, matrix degradation, and mechanosensing. Invadopodia, podosome-like protrusions in cancer cells, are involved in cell invasion into other tissues by promoting matrix degradation. Both podosomes and invadopodia exhibit actin pattern transitions during maturation. We previously found that Arp2/3-dependent actin flow occurs in all observed assembly patterns of podosomes in osteoclasts on glass. It is known that the actin wave in Dictyostelium cells exhibits a similar pattern transition in its evolution. Because of significant advances in our understanding regarding the mechanism of podosomes/invadopodia formation over the last decade, we revisited the structure and function of the sealing zone in this review, highlighting the possible involvement of self-organized actin waves in the organogenesis of the sealing zone.

Cortactin function in invadopodia

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

Symmetrical retrograde actin flow in the actin fusion structure is involved in osteoclast fusion

The aim of this study was to elucidate the role of the zipper-like structure (ZLS), a podosome-related structure that transiently appears at the cell contact zone, in osteoclast fusion. Live-cell imaging of osteoclasts derived from RAW264.7 cells transfected with EGFP-actin revealed consistent symmetrical retrograde actin flow in the ZLS, but not in the podosome cluster, the podosome ring or the podosome belt. Confocal imaging showed that the distributions of F-actin, vinculin, paxillin and zyxin in the ZLS were different from those in the podosome belt. Thick actin filament bundles running outside the ZLS appeared to recruit non-muscle myosin IIA. The F-actin-rich domain of the ZLS contained actin-related protein 2/3 complex (Arp2/3). Inhibition of Arp2/3 activity disorganized the ZLS, disrupted actin flow, deteriorated cell-cell adhesion and inhibited osteoclast hypermultinucleation. In contrast, ML-7, an inhibitor of myosin light chain kinase, had little effect on the structure of ZLS and promoted osteoclast hypermultinucleation. These results reveal a link between actin flow in the ZLS and osteoclast fusion. Osteoclast fusion was promoted by branched actin elongation and negatively regulated by actomyosin contraction.

Summary: Multinucleated osteoclasts form a podosome-derived fusion structure during cell fusion. Juxtaposition of fusion partner cells is probably maintained via force generated by symmetrical retrograde actin flow in the fusion structure.

Roles of paxillin family members in adhesion and ECM degradation coupling at invadosomes

The exact functions of all paxillin family members in mechanosensing and adhesion at invadosomes are unclear. Petropoulos et al. show that redundant and specific activities of paxillin and Hic-5 can couple original adhesion and ECM degradation in invadosomes.

Invadosomes are acto-adhesive structures able to both bind the extracellular matrix (ECM) and digest it. Paxillin family members—paxillin, Hic-5, and leupaxin—are implicated in mechanosensing and turnover of adhesion sites, but the contribution of each paxillin family protein to invadosome activities is unclear. We use genetic approaches to show that paxillin and Hic-5 have both redundant and distinctive functions in invadosome formation. The essential function of paxillin-like activity is based on the coordinated activity of LD motifs and LIM domains, which support invadosome assembly and morphology, respectively. However, paxillin preferentially regulates invadosome assembly, whereas Hic-5 regulates the coupling between ECM degradation and acto-adhesive functions. Mass spectrometry analysis revealed new partners that are important for paxillin and Hic-5 specificities: paxillin regulates the acto-adhesive machinery through janus kinase 1 (JAK1), whereas Hic-5 controls ECM degradation via IQGAP1. Integrating the redundancy and specificities of paxillin and Hic-5 in a functional complex provides insights into the coupling between the acto-adhesive and ECM-degradative machineries in invadosomes.

A RhoG-mediated signaling pathway that modulates invadopodia dynamics in breast cancer cells

One of the hallmarks of cancer is the ability of tumor cells to invade surrounding tissues and metastasize. During metastasis, cancer cells degrade the extracellular matrix, which acts as a physical barrier, by developing specialized actin-rich membrane protrusion structures called invadopodia. The formation of invadopodia is regulated by Rho GTPases, a family of proteins that regulates the actin cytoskeleton. Here, we describe a novel role for RhoG in the regulation of invadopodia disassembly in human breast cancer cells. Our results show that RhoG and Rac1 have independent and opposite roles in the regulation of invadopodia dynamics. We also show that SGEF (also known as ARHGEF26) is the exchange factor responsible for the activation of RhoG during invadopodia disassembly. When the expression of either RhoG or SGEF is silenced, invadopodia are more stable and have a longer lifetime than in control cells. Our findings also demonstrate that RhoG and SGEF modulate the phosphorylation of paxillin, which plays a key role during invadopodia disassembly. In summary, we have identified a novel signaling pathway involving SGEF, RhoG and paxillin phosphorylation, which functions in the regulation of invadopodia disassembly in breast cancer cells.

Significance of kinase activity in the dynamic invadosome

Invadosomes are actin rich protrusive structures that facilitate invasive migration in multiple cell types. Comprised of invadopodia and podosomes, these highly dynamic structures adhere to and degrade the extracellular matrix, and are also thought to play a role in mechanosensing. Many extracellular signals have been implicated in invadosome stimulation, activating complex signalling cascades to drive the formation, activity and turnover of invadosomes. While the structural components of invadosomes have been well studied, the regulation of invadosome dynamics is still poorly understood. Protein kinases are essential to this regulation, affecting all stages of invadosome dynamics and allowing tight spatiotemporal control of their activity. Invadosome organisation and function have been linked to pathophysiological states such as cancer invasion and metastasis; therapeutic targeting of invadosome regulatory components is thus warranted. In this review, we discuss the involvement of kinase signalling in every stage of the invadosome life cycle and evaluate its significance.

Augmentation of invadopodia formation in temozolomide-resistant or adopted glioma is regulated by c-Jun terminal kinase-paxillin axis

Temozolomide (TMZ) is one of the few effective anticancer agents against gliomas. However, acquisition of TMZ resistance or adaptation by gliomas is currently a crucial problem, especially increased invasiveness which is critical for the determination of clinical prognosis. This study investigated the molecular regulatory mechanisms of TMZ resistance in gliomas involved in invasiveness, particularly invadopodia formation, a molecular complex formed at the invasive front to cause extracellular matrix degradation during cellular local invasion. The TMZ-resistant clone of the U343 MG human glioma cell line (U343-R cells) was established. U343-R cells demonstrated higher invadopodia formation compared with U343 cells without TMZ resistance (U343-Con cells). Immunoblot analysis of DNA damage-related mitogen-activated protein kinase signals found increased phosphorylation of c-Jun terminal kinase (JNK) and higher activation of its downstream signaling in U343-R cells compared with U343-Con cells. Treatment of U343-R cells with specific inhibitors of JNK or siRNA targeting JNK suppressed up-regulation of invadopodia formation. In addition, paxillin, one of the known JNK effectors which is phosphorylated and affects cell migration, was phosphorylated at serine 178 in JNK activity-dependent manner. Expression of paxillin with mutation of the serine 178 phosphorylation site in U343-R cells blocked invadopodia formation. The present findings suggest that increased formation of invadopodia in U343-R cells is mediated by hyperactivation of JNK-paxillin signaling, and both JNK and paxillin might become targets of novel therapies against TMZ-resistant gliomas.

Integrin-mediated adhesion as self-sustained waves of enzymatic activation

Integrin receptors mediate interaction between the cellular actin-cytoskeleton and extracellular matrix. Based on their activation properties, we propose a reaction-diffusion model where the kinetics of the two-state receptors is modulated by their lipidic environment. This environment serves as an activator variable, while a second variable plays the role of a scaffold protein and controls the self-sustained activation of the receptors. Due to receptor diffusion which couples dynamically the activator and the inhibitor, our model connects major classes of reaction diffusion systems for excitable media. Spot and rosette solutions, characterized by receptor clustering into localized static or dynamic structures, are organized into a phase diagram. It is shown that diffusion and kinetics of receptors determines the dynamics and the stability of these structures. We discuss this model as a precursor model for cell signaling in the context of podosomes forming actoadhesive metastructures, and we study how generic signaling defects influence their organization.

Thymosin β4 induces proliferation, invasion, and epithelial-to-mesenchymal transition of oral squamous cell carcinoma

The epithelial-to-mesenchymal transition (EMT) plays a vital role in carcinogenesis, invasion, and metastasis of many epithelial tumors including oral squamous cell carcinoma (OSCC), a common malignancy of the head and neck. However, the functional role of the actin-sequestering protein thymosin β4 (Tβ4) in the EMT in OSCCs remains unclear. Thus, we investigated whether overexpression of Tβ4 could induce in vitro tumorigenesis such as cell proliferation and anchorage independency and an EMT-like phenotype in OSCCs. Also, we examined whether it affects invasiveness and cell motility-associated signaling molecules. Tβ4-overexpressing OSCCs, SCC-15_Tβ4 and SCC-25_Tβ4, enhanced cell proliferation and colony formation. In addition, we observed that Tβ4 overexpression induced an EMT-like phenotype, accompanied by a decrease in expression of the epithelial cell marker E-cadherin and an increase in expression of mesenchymal cell markers vimentin and N-cadherin. Also, the expression level of Twist1, an EMT-inducing transcription factor, was significantly enhanced in SCC-15_Tβ4 and SCC-25_Tβ4 cells. Tβ4 overexpression augmented in vitro invasion and MMP-2 activity and enhanced the phosphorylation of paxillin and cortactin and expression of LIMK1. Taken together, these results suggest that Tβ4 overexpression could be one of the causes of tumorigenesis and progression in OSCCs. Further investigation on the Tβ4 molecule would encourage the development of specific targets for cancer treatment.

Ezrin regulates focal adhesion and invadopodia dynamics by altering calpain activity to promote breast cancer cell invasion

Ezrin regulates proper focal adhesion and invadopodia turnover by regulating calpain-1, in part by directing its proteolytic activity toward key substrates talin, FAK, and cortactin. Ezrin-deficient tumor cells show reduced lung seeding and colonization in vivo but not primary tumor growth, thus implicating ezrin as a metastasis-associated protein.

Up-regulation of the cytoskeleton linker protein ezrin frequently occurs in aggressive cancer types and is closely linked with metastatic progression. However, the underlying molecular mechanisms detailing how ezrin is involved in the invasive and metastatic phenotype remain unclear. Here we report a novel function of ezrin in regulating focal adhesion (FA) and invadopodia dynamics, two key processes required for efficient invasion to occur. We show that depletion of ezrin expression in invasive breast cancer cells impairs both FA and invadopodia turnover. We also demonstrate that ezrin-depleted cells display reduced calpain-mediated cleavage of the FA and invadopodia-associated proteins talin, focal adhesion kinase (FAK), and cortactin and reduced calpain-1–specific membrane localization, suggesting a requirement for ezrin in maintaining proper localization and activity of calpain-1. Furthermore, we show that ezrin is required for cell directionality, early lung seeding, and distant organ colonization but not primary tumor growth. Collectively our results unveil a novel mechanism by which ezrin regulates breast cancer cell invasion and metastasis.

Coupling between acto-adhesive machinery and ECM degradation in invadosomes

Invadosomes have two main functions represented by their actin-rich and adhesive components and their polarized secretory pathways controlling the delivery of metalloproteases necessary to degrade extracellular matrix (ECM). Invadosomes include invadopodia and podosomes, which have subtle differences in molecular composition, dynamics, and structure. These differences could reflect different stages of invadosome maturation. This review will outline current knowledge on the coupling between the acto-adhesive machinery and the ECM degradation activity in invadosome diversity. Multiple works support that these two functions are not automatically linked but seem to be finely regulated to allow different functions of invadosomes. We will explore the paradigmatic aspect of invadosomes, which are able to interact with ECM to degrade it so as to better control their own dynamics. Understanding the fine-tuning between these two functions could serve to understand the link between the different types of invadosomes from invadopodia to podosomes.

A key role for PTP1B in dendritic cell maturation, migration, and T cell activation

Dendritic cells (DC) are the major antigen-presenting cells bridging innate and adaptive immunity, a function they perform by converting quiescent DC to active, mature DC with the capacity to activate naïve T cells. They do this by migrating from the tissues to the T cell area of the secondary lymphoid tissues. Here, we demonstrate that myeloid cell-specific genetic deletion of PTP1B (LysM PTP1B) leads to defects in lipopolysaccharide-driven bone marrow-derived DC (BMDC) activation associated with increased levels of phosphorylated Stat3. We show that myeloid cell-specific PTP1B deletion also causes decreased migratory capacity of epidermal DC, as well as reduced CCR7 expression and chemotaxis to CCL19 by BMDC. PTP1B deficiency in BMDC also impairs their migration in vivo. Further, immature LysM PTP1B BMDC display fewer podosomes, increased levels of phosphorylated Src at tyrosine 527, and loss of Src localization to podosome puncta. In co-culture with T cells, LysM PTP1B BMDC establish fewer and shorter contacts than control BMDC. Finally, LysM PTP1B BMDC fail to present antigen to T cells as efficiently as control BMDC. These data provide first evidence for a key regulatory role for PTP1B in mediating a central DC function of initiating adaptive immune responses in response to innate immune cell activation.

αII-spectrin regulates invadosome stability and extracellular matrix degradation

Invadosomes are actin-rich adhesion structures involved in tissue invasion and extracellular matrix (ECM) remodelling. αII-Spectrin, an ubiquitous scaffolding component of the membrane skeleton and a partner of actin regulators (ABI1, VASP and WASL), accumulates highly and specifically in the invadosomes of multiple cell types, such as mouse embryonic fibroblasts (MEFs) expressing SrcY527F, the constitutively active form of Src or activated HMEC-1 endothelial cells. FRAP and live-imaging analysis revealed that αII-spectrin is a highly dynamic component of invadosomes as actin present in the structures core. Knockdown of αII-spectrin expression destabilizes invadosomes and reduces the ability of the remaining invadosomes to digest the ECM and to promote invasion. The ECM degradation defect observed in spectrin-depleted-cells is associated with highly dynamic and unstable invadosome rings. Moreover, FRAP measurement showed the specific involvement of αII-spectrin in the regulation of the mobile/immobile β3-integrin ratio in invadosomes. Our findings suggest that spectrin could regulate invadosome function and maturation by modulating integrin mobility in the membrane, allowing the normal processes of adhesion, invasion and matrix degradation. Altogether, these data highlight a new function for spectrins in the stability of invadosomes and the coupling between actin regulation and ECM degradation.

Working together: spatial synchrony in the force and actin dynamics of podosome first neighbors

Podosomes are mechanosensitive adhesion cell structures that are capable of applying protrusive forces onto the extracellular environment. We have recently developed a method dedicated to the evaluation of the nanoscale forces that podosomes generate to protrude into the extracellular matrix. It consists in measuring by atomic force microscopy (AFM) the nanometer deformations produced by macrophages on a compliant Formvar membrane and has been called protrusion force microscopy (PFM). Here we perform time-lapse PFM experiments and investigate spatial correlations of force dynamics between podosome pairs. We use an automated procedure based on finite element simulations that extends the analysis of PFM experimental data to take into account podosome architecture and organization. We show that protrusion force varies in a synchronous manner for podosome first neighbors, a result that correlates with phase synchrony of core F-actin temporal oscillations. This dynamic spatial coordination between podosomes suggests a short-range interaction that regulates their mechanical activity.

Visualization of structural organization of ventral membranes of sheared-open resorbing osteoclasts attached to apatite pellets

Osteoclasts are highly polarized cells from both morphological and functional points of view. Using quick-freeze, rotary-replication methods combined with cell-shearing, we clarified the variability of cytoplasmic surface of the polarized membranes of osteoclasts seeded on apatite. As to the organization of actin filaments and clathrin sheets, we confirmed almost the same ventral membrane specializations of osteoclasts on apatite as seen on glass plates. The organized actin filaments and membrane-associated particles supported the ruffled border membranes. Inside the actin sealing zone, membrane specializations were not always occupied with the ruffled border but also with other types of membranes. Some osteoclasts formed an actin ring but lacked the ruffled border projections. We report a unique and distinctive membrane modification of apatite-attached osteoclasts, i.e., the presence of dense aggregates of membrane-associated particles and related structures not found in the osteoclasts seeded on glass plates. Actin filament polarity in the podosomes was determined by decoration with myosin S1. The actin filament polarity within podosome appears to be oriented predominantly with its barbed ends toward the core, whereas the interconnecting F-actin appears to be mixed oriented. Two different types of clathrin plaques displayed different distributions: clathrin-dependent endocytosis was observed in the ruffled border regions, whereas flat clathrin sheets were found in the leading edge of lamellipodia and near podosomes. The clathrin sheets adhered to the apatite surface tightly on the ventral membranes overlaying the resorption lacunae. All these membrane specializations as mentioned above may indicate the functional variability of osteoclasts seeded on apatite.

Arene ruthenium(ii) complex, a potent inhibitor against proliferation, migration and invasion of breast cancer cells, reduces stress fibers, focal adhesions and invadopodia

Effective chemotherapy drugs for cancer that would inhibit tumor growth and suppress metastasis are currently lacking. In this study, a series of arene ruthenium complexes, [(η6-arene)Ru(H2iip)Cl]Cl (arene = p-cymene, RAWQ03; CH3C6H5, RAWQ04; and C6H6, RAWQ11), were synthesized and their inhibitory activity against tumor cells were evaluated. The results showed that the complex RAWQ11 inhibited the growth of MDA-MB-231 breast cancer cells by inducing S-phase arrest, which is closely related to the inhibition of cell mitosis-mediated cell nucleus damage. Further studies showed that RAWQ11 can inhibit the invasion and metastasis of MDA-MB-231 cells. The morphology of MDA-MB-231 cells changed, the number of focal adhesions decreased, and the stress fibers de-polymerized upon dealing with the complex RAWQ11. The FITC-gelatin assay confirmed that the formation of invadopodia in MDA-MB-231 cells was significantly blocked by RAWQ11. Furthermore, RAWQ11 can block the AKT signal pathway by upregulating the PTEN expression through binding and downregulating miR-21. These results demonstrated that this type of arene ruthenium(ii) complex can block the invadopodia formation by regulating the PTEN/AKT signal pathway mediated by miR-21 to inhibit the invasion and metastasis of breast cancer cells. Therefore, this complex can be used as a potential dual functional agent to inhibit the growth and metastasis of tumor cells.

A crucial role for β2 integrins in podosome formation, dynamics and Toll-like-receptor-signaled disassembly in dendritic cells

The dynamic properties of podosomes, their ability to degrade the underlying matrix and their modulation by Toll-like receptor (TLR) signaling in dendritic cells (DCs) suggests they have an important role in migration. Integrins are thought to participate in formation and dynamics of podosomes but the multiplicity of integrins in podosomes has made this difficult to assess. We report that murine DCs that lack β2 integrins fail to form podosomes. Re-expression of β2 integrins restored podosomes but not when the membrane proximal or distal NPxF motifs, or when an intervening triplet of threonine residues were mutated. We show that β2 integrins are remarkably long-lived in podosome clusters and form a persistent framework that hosts multiple actin-core-formation events at the same or adjacent sites. When β2 integrin amino acid residues 745 or 756 were mutated from Ser to Ala, podosomes became resistant to dissolution mediated through TLR signaling. TLR signaling did not detectably modulate phosphorylation at these sites but mutation of either residue to phospho-mimetic Asp increased β2 integrin turnover in podosomes, indicating that phosphorylation at one or both sites establishes permissive conditions for TLR-signaled podosome disassembly.

Vinculin binding angle in podosomes revealed by high resolution microscopy

Podosomes are highly dynamic actin-rich adhesive structures formed predominantly by cells of the monocytic lineage, which degrade the extracellular matrix. They consist of a core of F-actin and actin-regulating proteins, surrounded by a ring of adhesion-associated proteins such as vinculin. We have characterised the structure of podosomes in macrophages, particularly the structure of the ring, using three super-resolution fluorescence microscopy techniques: stimulated emission depletion microscopy, structured illumination microscopy and localisation microscopy. Rather than being round, as previously assumed, we found the vinculin ring to be created from relatively straight strands of vinculin, resulting in a distinctly polygonal shape. The strands bind preferentially at angles between 116° and 135°. Furthermore, adjacent vinculin strands are observed nucleating at the corners of the podosomes, suggesting a mechanism for podosome growth.

Src-dependent Tks5 phosphorylation regulates invadopodia-associated invasion in prostate cancer cells

BACKGROUND

The Src tyrosine kinase substrate and adaptor protein Tks5 had previously been implicated in the invasive phenotype of normal and transformed cell types via regulation of cytoskeletal structures called podosomes/invadopodia. The role of Src-Tks5 signaling in invasive prostate cancer, however, had not been previously evaluated.

METHODS

We measured the relative expression of Tks5 in normal (n = 20) and cancerous (n = 184, from 92 patients) prostate tissue specimens by immunohistochemistry using a commercially available tumor microarray. We also manipulated the expression and activity of wild-type and mutant Src and Tks5 constructs in the LNCaP and PC-3 prostate cancer cell lines in order to ascertain the role of Src-Tks5 signaling in invadopodia development, matrix-remodeling activity, motility, and invasion.

RESULTS

Our studies demonstrated that Src was activated and Tks5 upregulated in high Gleason score prostate tumor specimens and in invasive prostate cancer cell lines. Remarkably, overexpression of Tks5 in LNCaP cells was sufficient to induce invadopodia formation and associated matrix degradation. This Tks5-dependent increase in invasive behavior further depended on Src tyrosine kinase activity and the phosphorylation of Tks5 at tyrosine residues 557 and 619. In PC-3 cells we demonstrated that Tks5 phosphorylation at these sites was necessary and sufficient for invadopodia-associated matrix degradation and invasion.

CONCLUSIONS

Our results suggest a general role for Src-Tks5 signaling in prostate tumor progression and the utility of Tks5 as a marker protein for the staging of this disease.

Myosin 1e is a component of the invadosome core that contributes to regulation of invadosome dynamics

Myosin 1e (myo1e) is an actin-based motor protein that has been implicated in cell adhesion and migration. We examined the role of myo1e in invadosomes, actin-rich adhesion structures that are important for degradation and invasion of the extracellular matrix. RSV-transformed BHK-21 cells, which readily form invadosomes and invadosome rosettes, were used as the experimental model. Myo1e localization to the actin-rich core of invadosomes required the proline-rich Tail Homology 2 (TH2) domain. During invadosome rosette expansion, we observed myo1e recruitment to newly forming invadosomes via Tail Homology 1 (TH1)-dependent interactions with the plasma membrane, where it preceded actin and paxillin. Dominant-negative inhibition of myo1e resulted in mislocalized invadosome formation, usually at the center of the rosette. We propose that TH2 domain of myo1e provides the key signal for localization to invadosomes, while TH1 domain interactions facilitate myo1e targeting to the plasma membrane-proximal locations within the rosettes. Myo1e may then act as a scaffold, linking the plasma membrane with the actin cytoskeleton and helping direct new invadosome formation to the periphery of the rosette.

Soft matrix is a natural stimulator for cellular invasiveness

ECM softness (low stiffness comparable to soft tissues) alone is sufficient to prevent cell-to-cell adherens junction formation, up-regulate MMP secretion, promote MMP activity, and induce invadosome-like protrusion formation. Such findings suggest that cell invasion in vivo is a spontaneous cell behavior in response to ECM stiffness.

Directional mesenchymal cell invasion in vivo is understood to be a stimulated event and to be regulated by cytokines, chemokines, and types of extracellular matrix (ECM). Instead, by focusing on the cellular response to ECM stiffness, we found that soft ECM (low stiffness) itself is sufficient to prevent stable cell-to-cell adherens junction formation, up-regulate matrix metalloproteinase (MMP) secretion, promote MMP activity, and induce invadosome-like protrusion (ILP) formation. Consistently, similar ILP formation was also detected in a three-dimensional directional invasion assay in soft matrix. Primary human fibroblasts spontaneously form ILPs in a very narrow range of ECM stiffness (0.1–0.4 kPa), and such ILP formation is Src family kinase dependent. In contrast, spontaneous ILP formation in malignant cancer cells and fibrosarcoma cells occurs across a much wider range of ECM stiffness, and these tumor cell ILPs are also more prominent at lower stiffness. These findings suggest that ECM softness is a natural stimulator for cellular invasiveness.

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.

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.

Filopodia as sensors

Filopodia are sensors on both excitable and non-excitable cells. The sensing function is well documented in neurons and blood vessels of adult animals and is obvious during dorsal closure in embryonic development. Nerve cells extend neurites in a bidirectional fashion with growth cones at the tips where filopodia are concentrated. Their sensing of environmental cues underpins the axon's ability to "guide," bypassing non-target cells and moving toward the target to be innervated. This review focuses on the role of filopodia structure and dynamics in the detection of environmental cues, including both the extracellular matrix (ECM) and the surfaces of neighboring cells. Other protrusions including the stereocilia of the inner ear and epididymus, the invertebrate Type I mechanosensors, and the elongated processes connecting osteocytes, share certain principles of organization with the filopodia. Actin bundles, which may be inside or outside of the excitable cell, function to transduce stress from physical perturbations into ion signals. There are different ways of detecting such perturbations. Osteocyte processes contain an actin core and are physically anchored on an extracellular structure by integrins. Some Type I mechanosensors have bridge proteins that anchor microtubules to the membrane, but bundles of actin in accessory cells exert stress on this complex. Hair cells of the inner ear rely on attachments between the actin-based protrusions to activate ion channels, which then transduce signals to afferent neurons. In adherent filopodia, the focal contacts (FCs) integrated with ECM proteins through integrins may regulate integrin-coupled ion channels to achieve signal transduction. Issues that are not understood include the role of Ca(2+) influx in filopodia dynamics and how integrins coordinate or gate signals arising from perturbation of channels by environmental cues.

Signaling inputs to invadopodia and podosomes

Remodeling of extracellular matrix (ECM) is a fundamental cell property that allows cells to alter their microenvironment and move through tissues. Invadopodia and podosomes are subcellular actin-rich structures that are specialized for matrix degradation and are formed by cancer and normal cells, respectively. Although initial studies focused on defining the core machinery of these two structures, recent studies have identified inputs from both growth factor and adhesion signaling as crucial for invasive activity. This Commentary will outline the current knowledge on the upstream signaling inputs to invadopodia and podosomes and their role in governing distinct stages of these invasive structures. We discuss invadopodia and podosomes as adhesion structures and highlight new data showing that invadopodia-associated adhesion rings promote the maturation of already-formed invadopodia. We present a model in which growth factor stimulation leads to phosphoinositide 3-kinase (PI3K) activity and formation of invadopodia, whereas adhesion signaling promotes exocytosis of proteinases at invadopodia.

Hypoxia-induced invadopodia formation: a role for β-PIX

During tumour progression, oxygen tension in the microenvironment surrounding tumour cells is reduced, resulting in hypoxia. It is well established that cancer cells resist the negative effects of hypoxia by inducing angiogenesis predominantly via the activity of transcription factor hypoxia-inducible factor-1 (HIF-1). However, more recently HIF-1α has also been linked to increased invasive potential, although the molecular mechanisms remain to be defined. Invasive cancer cells are thought to employ membrane protrusions, termed invadopodia, to achieve matrix degradation. While many invadopodia components have been identified, signalling pathways that link extracellular stimuli to invadopodia formation remain largely unknown. Indeed, the relationship between invadopodia formation and HIF-1α has not been explored. We now report that HIF-1α is a driver of invadopodia formation. Furthermore, we have identified an important, direct and novel link between the Rho family activator β-PIX, HIF-1α and invadopodia formation. Indeed, we find that β-PIX expression is essential for invadopodia formation. In conclusion, we identify a new HIF-1α mechanistic pathway and suggest that β-PIX is a novel downstream signalling mediator during invadopodia formation.