Spatial and temporal control of cofilin activity is required for directional sensing during chemotaxis

Functions of cofilin in cell locomotion and invasion

Recently, a consensus has emerged that cofilin severing activity can generate free actin filament ends that are accessible for F-actin polymerization and depolymerization without changing the rate of G-actin association and dissociation at either filament end. The structural basis of actin filament severing by cofilin is now better understood. These results have been integrated with recently discovered mechanisms for cofilin activation in migrating cells, which led to new models for cofilin function that provide insights into how cofilin regulation determines the temporal and spatial control of cell behaviour.

Spatial regulation of RhoC activity defines protrusion formation in migrating cells

Protrusion formation is the first step that precedes cell movement of motile cells. Spatial control of actin polymerization is necessary to achieve directional protrusion during cell migration. Here we show that the spatial coordinators p190RhoGEF and p190RhoGAP regulate actin polymerization during leading edge protrusions by regulating the actin barbed end distribution and amplitude. The distribution of RhoC activity and proper balance of cofilin activation achieved by p190RhoGEF and p190RhoGAP determines the direction of final protrusive activity. These findings provide a new insight into the dynamic plasticity in the amplitude and distribution of barbed ends, which can be modulated by fine-tuning RhoC activity by upstream GEFs and GAPs for directed cell motility.

ARK5 promotes glioma cell invasion, and its elevated expression is correlated with poor clinical outcome

Poor prognosis of malignant gliomas is primarily attributed to their highly invasive nature. Despite the identification of new biomarkers and molecular targets for the improvement of the diagnosis, prognosis and treatment of glioma, the overall prognosis of the disease remains poor. This study is the first to show the significant upregulation of ARK5 paraffin-embedded archival glioma biopsies compared with those in adjacent non-cancerous brain tissues. Statistical analysis suggests that the upregulation of ARK5 correlates with the World Health Organization grade of glioma (P<0.001) and that patients with a high ARK5 level exhibit shorter survival time (P<0.01). In addition, ARK5 can promote glioma cell invasion by regulating cytoskeleton rearrangement and matrix metalloproteinase activation. ARK5 knockdown was found to reduce brain invasion in a glioma xenograft mouse model. Our results strongly suggest that ARK5 represents a novel and valuable biomarker to aid in the prediction of patient prognosis and is a potential therapeutic target against glioma.

ErbB2-dependent chemotaxis requires microtubule capture and stabilization coordinated by distinct signaling pathways

Activation of the ErbB2 receptor tyrosine kinase stimulates breast cancer cell migration. Cell migration is a complex process that requires the synchronized reorganization of numerous subcellular structures including cell-to-matrix adhesions, the actin cytoskeleton and microtubules. How the multiple signaling pathways triggered by ErbB2 coordinate, in time and space, the various processes involved in cell motility, is poorly defined. We investigated the mechanism whereby ErbB2 controls microtubules and chemotaxis. We report that activation of ErbB2 increased both cell velocity and directed migration. Impairment of the Cdc42 and RhoA GTPases, but not of Rac1, prevented the chemotactic response. RhoA is a key component of the Memo/ACF7 pathway whereby ErbB2 controls microtubule capture at the leading edge. Upon Memo or ACF7 depletion, microtubules failed to reach the leading edge and cells lost their ability to follow the chemotactic gradient. Constitutive ACF7 targeting to the membrane in Memo-depleted cells reestablished directed migration. ErbB2-mediated activation of phospholipase C gamma (PLCγ) also contributed to cell guidance. We further showed that PLCγ signaling, via classical protein kinases C, and Memo signaling converged towards a single pathway controlling the microtubule capture complex. Finally, inhibiting the PI3K/Akt pathway did not affect microtubule capture, but disturbed microtubule stability, which also resulted in defective chemotaxis. PI3K/Akt-dependent stabilization of microtubules involved repression of GSK3 activity on the one hand and inhibition of the microtubule destabilizing protein, Stathmin, on the other hand. Thus, ErbB2 triggers distinct and complementary pathways that tightly coordinate microtubule capture and microtubule stability to control chemotaxis.

Neuregulin mediates F-actin-driven cell migration through inhibition of protein kinase D1 via Rac1 protein

Neuregulin (NRG; heregulin) is overexpressed in ∼30% of breast cancers and mediates various processes involved in tumor progression, including tumor cell migration and invasion. Here, we show that NRG mediates its effects on tumor cell migration via PKD1. Downstream of RhoA, PKD1 can prevent directed cell migration through phosphorylation of its substrate SSH1L. NRG exerts its inhibitory effects on PKD1 through Rac1/NADPH oxidase, leading to decreased PKD1 activation loop phosphorylation and decreased activity toward SSH1L. The consequence of PKD1 inhibition by NRG is decreased binding of 14-3-3 to SSH1L, localization of SSH1L to F-actin at the leading edge, and increased cofilin activity, resulting in increased reorganization of the actin cytoskeleton and cell motility. Our data provide a mechanism through which the Rho GTPase Rac1 cross-talks with PKD1 signaling pathways to facilitate directed cell migration.

Cofilin overexpression affects actin cytoskeleton organization and migration of human colon adenocarcinoma cells

The dynamic reorganization of actin cytoskeleton is regulated by a large number of actin-binding proteins. Among them, the interaction of ADF/cofilin with monomeric and filamentous actin is very important, since it severs actin filaments. It also positively influences actin treadmilling. The activity of ADF/cofilin is reversibly regulated by phosphorylation and dephosphorylation at Ser-3, with the phosphorylated form (P-cofilin) being inactive. Here, we studied the effects of overexpression of cofilin and two cofilin variants in the human colon adenocarcinoma LS180 cell line. We have generated the LS180 cells expressing three different cofilin variants: WT (wild type), Ser 3 Ala (S3A) (constitutively active) or Ser 3 Asp (S3D) (constitutively inactive cofilin). The cells expressing WT cofilin were characterized by abundant cell spreading and colocalization of cofilin with the submembranous F-actin. Similar effects were observed in cells expressing S3A cofilin. In contrast, LS180 cells expressing S3D cofilin remained longitudinal in morphology and cofilin was equally distributed within the cell body. Furthermore, the migration ability of LS180 cells expressing different cofilin mutants was analyzed. In comparison to control cells, we have noticed a significant, approximately fourfold increase in the migration factor value of cells overexpressing WT type cofilin. The overexpression of S3D cofilin resulted in an almost complete inhibition of cell motility. The estimation of actin pool in the cytosol of LS180 cells expressing S3A cofilin has shown a significantly lower level of total actin in reference to control cells. The opposite effect was observed in LS180 cells overexpressing S3D cofilin. In summary, the results of our experiments indicate that phosphorylation "status" of cofilin is a factor affecting the actin cytoskeleton organization and migration abilities of colon adenocarcinoma LS180 cells.

Stimulus-dependent phosphorylation of profilin-1 in angiogenesis

Angiogenesis, the formation of new blood vessels, is fundamental to development and post-injury tissue repair. Vascular endothelial growth factor (VEGF)-A guides and enhances endothelial cell migration to initiate angiogenesis. Profilin-1 (Pfn-1) is an actin-binding protein that enhances actin filament formation and cell migration, but stimulus-dependent regulation of Pfn-1 has not been observed. Here, we show that VEGF-A-inducible phosphorylation of Pfn-1 at Tyr 129 is critical for endothelial cell migration and angiogenesis. Chemotactic activation of VEGF receptor kinase-2 (VEGFR2) and Src induces Pfn-1 phosphorylation in the cell leading edge, promoting Pfn-1 binding to actin and actin polymerization. Conditional endothelial knock-in of phosphorylation-deficient Pfn1(Y129F) in mice reveals that Pfn-1 phosphorylation is critical for angiogenesis in response to wounding and ischaemic injury, but not for developmental angiogenesis. Thus, VEGFR2/Src-mediated phosphorylation of Pfn-1 bypasses canonical, multistep intracellular signalling events to initiate endothelial cell migration and angiogenesis, and might serve as a selective therapeutic target for anti-angiogenic therapy.

C(16)-Ceramide-induced F-actin regulation stimulates mouse embryonic stem cell migration: involvement of N-WASP/Cdc42/Arp2/3 complex and cofilin-1/α-actinin

Ceramide, a major structural element in the cellular membrane, is a key regulatory factor in various cellular behaviors that are dependent on ceramide-induced association of specific proteins. However, molecular mechanisms that regulate ceramide-induced embryonic stem cell (ESC) migration are still not well understood. Thus, we investigated the effect of ceramide on migration and its related signal pathways in mouse ESCs. Among ceramide species with different fatty acid chain lengths, C(16)-Cer increased migration of mouse ESCs in a dose- (≥1μM) and time-dependent (≥8h) manners, as determined by the cell migration assay. C(16)-Cer (10μM) increased protein-kinase C (PKC) phosphorylation. Subsequently, C(16)-Cer increased focal adhesion kinase (FAK) and Paxillin phosphorylation, which were inhibited by PKC inhibitor Bisindolylmaleimide I (1μM). When we examined for the downstream signaling molecules, C(16)-Cer activated small G protein (Cdc42) and increased the formation of complex with Neural Wiskott-Aldrich Syndrome Protein (N-WASP)/Cdc42/Actin-Related Protein 2/3 (Arp2/3). This complex formation was disrupted by FAK- and Paxillin-specific siRNAs. Furthermore, C(16)-Cer-induced increase of filamentous actin (F-actin) expression was inhibited by Cdc42-, N-WASP-, and Arp2/3-specific siRNAs, respectively. Indeed, C(16)-Cer increased cofilin-1/F-actin interaction or F-actin/α-actinin-1 and α-actinin-4 interactions in the cytoskeleton compartment, which was reversed by Cdc42-specific siRNA. Finally, C(16)-Cer-induced increase of cell migration was inhibited by knocking down each signal pathway-related molecules with siRNA or inhibitors. In conclusion, C(16)-Cer enhances mouse ESC migration through the regulation of PKC and FAK/Paxillin-dependent N-WASP/Cdc42/Arp2/3 complex formation as well as through promoting the interaction between cofilin-1 or α-actinin-1/-4 and F-actin.

Role of the cofilin activity cycle in astrocytoma migration and invasion

The cofilin pathway plays a central role in the regulation of actin polymerization and the formation of cell membrane protrusions that are essential for cell migration. Overexpression of cofilin has been linked to the aggressiveness of a variety of different cancers. In these cancers, the phosphorylation of cofilin at Ser3 is a key regulatory mechanism modulating cofilin activity. The activation status of cofilin has been directly linked to tumor invasion. Accordingly, in this study, we examined the expression of cofilin and its activation status in astrocytoma cell lines and astrocytic tumors. We show that cofilin expression was increased and correlated with increasing grade malignant astrocytoma. In addition, both cofilin and LIMK had elevated expression in astrocytoma cell lines. Knockdown of cofilin by siRNA altered astrocytoma cell morphology and inhibited astrocytoma migration and invasion. Conversely, overexpression of a cofilin phosphorylation mutant in an in vivo intracranial xenograft model resulted in a more highly invasive phenotype than those xenographs expressing wild-type cofilin. Animals harboring astrocytomas stably expressing the cofilin phosphorylation mutant (cofilin-S3A) demonstrated marked local invasiveness and spread across the corpus callosum to the contralateral hemisphere in all animals. Taken together, these data indicate that the cofilin activity pathway may represent a novel therapeutic target to diminish the invasion of these highly malignant tumors.

The actin-severing protein cofilin is downstream of neuregulin signaling and is essential for Schwann cell myelination

Myelination is a complex process requiring coordination of directional motility and an increase in glial cell size to generate a multilamellar myelin sheath. Regulation of actin dynamics during myelination is poorly understood. However, it is known that myelin thickness is related to the abundance of neuregulin-1 (NRG1) expressed on the axon surface. Here we identify cofilin1, an actin depolymerizing and severing protein, as a downstream target of NRG1 signaling in rat Schwann cells (SCs). In isolated SCs, NRG1 promotes dephosphorylation of cofilin1 and its upstream regulators, LIM kinase (LIMK) and Slingshot-1 phosphatase (SSH1), leading to cofilin1 activation and recruitment to the leading edge of the plasma membrane. These changes are associated with rapid membrane expansion yielding a 35-50% increase in SC size within 30 min. Cofilin1-deficient SCs increase phosphorylation of ErbB2, ERK, focal adhesion kinase, and paxillin in response to NRG1, but fail to increase in size possibly due to stabilization of unusually long focal adhesions. Cofilin1-deficient SCs cocultured with sensory neurons do not myelinate. Ultrastructural analysis reveals that they unsuccessfully segregate or engage axons and form only patchy basal lamina. After 48 h of coculturing with neurons, cofilin1-deficient SCs do not align or elongate on axons and often form adhesions with the underlying substrate. This study identifies cofilin1 and its upstream regulators, LIMK and SSH1, as end targets of a NRG1 signaling pathway and demonstrates that cofilin1 is necessary for dynamic changes in the cytoskeleton needed for axon engagement and myelination by SCs.

Cadherin-6B stimulates an epithelial mesenchymal transition and the delamination of cells from the neural ectoderm via LIMK/cofilin mediated non-canonical BMP receptor signaling

We previously provided evidence that cadherin-6B induces de-epithelialization of the neural crest prior to delamination and is required for the overall epithelial mesenchymal transition (EMT). Furthermore, de-epithelialization induced by cadherin-6B was found to be mediated by BMP receptor signaling independent of BMP. We now find that de-epithelialization is mediated by non-canonical BMP signaling through the BMP type II receptor (BMPRII) and not by canonical Smad dependent signaling through BMP Type I receptor. The LIM kinase/cofilin pathway mediates non-canonical BMPRII induced de-epithelialization, in response to either cadherin-6B or BMP. LIMK1 induces de-epithelialization in the neural tube and dominant negative LIMK1 decreases de-epithelialization induced by either cadherin-6B or BMP. Cofilin is the major known LIMK1 target and a S3A phosphorylation deficient mutated cofilin inhibits de-epithelialization induced by cadherin-6B as well as LIMK1. Importantly, LIMK1 as well as cadherin-6B can trigger ectopic delamination when co-expressed with the competence factor SOX9, showing that this cadherin-6B stimulated signaling pathway can mediate the full EMT in the appropriate context. These findings suggest that the de-epithelialization step of the neural crest EMT by cadherin-6B/BMPRII involves regulation of actin dynamics via LIMK/cofilin.

Quantitative analysis of gradient sensing: towards building predictive models of chemotaxis in cancer

Chemotaxis of tumor cells in response to a gradient of extracellular ligand is an important step in cancer metastasis. The heterogeneity of chemotactic responses in cancer has not been widely addressed by experimental or mathematical modeling techniques. However, recent advancements in chemoattractant presentation, fluorescent-based signaling probes, and phenotypic analysis paradigms provide rich sources for building data-driven relational models that describe tumor cell chemotaxis in response to a wide variety of stimuli. Here we present gradient sensing, and the resulting chemotactic behavior, in a 'cue-signal-response' framework and suggest methods for utilizing recently reported experimental methods in data-driven modeling ventures.

The chemotactic defect in wiskott-Aldrich syndrome macrophages is due to the reduced persistence of directional protrusions

Wiskott-Aldrich syndrome protein (WASp) is an actin nucleation promoting factor that is required for macrophages to directionally migrate towards various chemoattractants. The chemotaxis defect of WASp-deficient cells and its activation by Cdc42 in vivo suggest that WASp plays a role in directional sensing, however, its precise role in macrophage chemotaxis is still unclear. Using shRNA-mediated downregulation of WASp in the murine monocyte/macrophage cell line RAW/LR5 (shWASp), we found that WASp was responsible for the initial wave of actin polymerization in response to global stimulation with CSF-1, which in Dictyostelium discoideum amoebae and carcinoma cells has been correlated with the ability to migrate towards chemoattractants. Real-time monitoring of shWASp cells, as well as WASp^−/− bone marrow-derived macrophages (BMMs), in response to a CSF-1 gradient revealed that the protrusions from WASp-deficient cells were directional, showing intact directional sensing. However, the protrusions from WASp-deficient cells demonstrated reduced persistence compared to their respective control shRNA and wild-type cells. Further examination showed that tyrosine phosphorylation of WASp was required for both the first wave of actin polymerization following global CSF-1 stimulation and proper directional responses towards CSF-1. Importantly, the PI3K, Rac1 and WAVE2 proteins were incorporated normally in CSF-1 – elicited protrusions in the absence of WASp, suggesting that membrane protrusion driven by the WAVE2 complex signaling is intact. Collectively, these results suggest that WASp and its phosphorylation play critical roles in coordinating the actin cytoskeleton rearrangements necessary for the persistence of protrusions required for directional migration of macrophages towards CSF-1.

Phosphoinositides in chemotaxis

Phosphatidylinositol lipids generated through the action of phosphinositide 3-kinase (PI3K) are key mediators of a wide array of biological responses. In particular, their role in the regulation of cell migration has been extensively studied and extends to amoeboid as well as mesenchymal migration. Through the emergence of fluorescent probes that target PI3K products as well as the use of specific inhibitors and knockout technologies, the spatio-temporal distribution of PI3K products in chemotaxing cells has been shown to represent a key anterior polarity signal that targets downstream effectors to actin polymerization. In addition, through intricate cross-talk networks PI3K products have been shown to regulate signals that control posterior effectors. Yet, in more complex environments or in conditions where chemoattractant gradients are steep, a variety of cell types can still chemotax in the absence of PI3K signals. Indeed, parallel signal transduction pathways have been shown to coordinately regulate cell polarity and directed movement. In this chapter, we will review the current role PI3K products play in the regulation of directed cell migration in various cell types, highlight the importance of mathematical modeling in the study of chemotaxis, and end with a brief overview of other signaling cascades known to also regulate chemotaxis.

Actin depolymerization-based force retracts the cell rear in polarizing and migrating cells

In migrating cells, the relative importance of myosin II contractility for cell rear retraction varies [1-12]. However, in myosin II-inhibited polarizing cells, actin organization is compromised [13-18]; thus it remains unclear whether myosin II is simply required for correct actin arrangement or also directly drives rear retraction [9]. Ascaris sperm cells lack actin and associated motors, and depolymerization of major sperm protein is instead thought to pull the cell rear forward [19, 20]. Opposing views exist on whether actin could also have this function [19, 20] and has not been directly experimentally sought. We probe function at high temporal resolution in polarizing fibroblasts that establish migration by forming the cell rear first [9, 15, 21]. We show that in cells with correctly organized actin, that actin filament depolymerization directly drives retraction of the rear margin to polarize cells and spatially accounts for most cell rear retraction during established migration. Myosin II contractility is required early, to form aligned actin bundles that are needed for polarization, and also later to maintain bundle length that ensures directed protrusion at the cell front. Our data imply a new mechanism: actin depolymerization-based force retracts the cell rear to polarize cells with no direct contribution from myosin II contractility.

A temporal model of cofilin regulation and the early peak of actin barbed ends in invasive tumor cells

Cofilin is an important regulator of actin polymerization, cell migration, and chemotaxis. Recent experimental data on mammary carcinoma cells reveal that stimulation by epidermal growth factor (EGF) generates a pool of active cofilin that results in a peak of actin filament barbed ends on the timescale of 1 min. Here, we present results of a mathematical model for the dynamics of cofilin and its transition between several pools in response to EGF stimulation. We describe the interactions of phospholipase C, membrane lipids (PIP(2)), and cofilin bound to PIP(2) and to F-actin, as well as diffusible cofilin in active G-actin-monomer-bound or phosphorylated states. We consider a simplified representation in which the thin cell edge (lamellipod) and the cell interior are represented by two compartments that are linked by diffusion. We demonstrate that a high basal level of active cofilin stored by binding to PIP(2), as well as the highly enriched local milieu of F-actin at the cell edge, is essential to capture the EGF-induced barbed-end amplification observed experimentally.

Protein kinase D regulates cofilin activity through p21-activated kinase 4

Dynamic reorganization of the actin cytoskeleton at the leading edge is required for directed cell migration. Cofilin, a small actin-binding protein with F-actin severing activities, is a key enzyme initiating such actin remodeling processes. Cofilin activity is tightly regulated by phosphorylation and dephosphorylation events that are mediated by LIM kinase (LIMK) and the phosphatase slingshot (SSH), respectively. Protein kinase D (PKD) is a serine/threonine kinase that inhibits actin-driven directed cell migration by phosphorylation and inactivation of SSH. Here, we show that PKD can also regulate LIMK through direct phosphorylation and activation of its upstream kinase p21-activated kinase 4 (PAK4). Therefore, active PKD increases the net amount of phosphorylated inactive cofilin in cells through both pathways. The regulation of cofilin activity at multiple levels may explain the inhibitory effects of PKD on barbed end formation as well as on directed cell migration.

Chemotaxis in cancer

Chemotaxis of tumour cells and stromal cells in the surrounding microenvironment is an essential component of tumour dissemination during progression and metastasis. This Review summarizes how chemotaxis directs the different behaviours of tumour cells and stromal cells in vivo, how molecular pathways regulate chemotaxis in tumour cells and how chemotaxis choreographs cell behaviour to shape the tumour microenvironment and to determine metastatic spread. The central importance of chemotaxis in cancer progression is highlighted by discussion of the use of chemotaxis as a prognostic marker, a treatment end point and a target of therapeutic intervention.

New approaches to targeting the actin cytoskeleton for chemotherapy

The actin cytoskeleton is indispensable for normal cellular function. In particular, several actin-based structures coordinate cellular motility, a process hijacked by tumor cells in order to facilitate their propagation to distant sites. The actin cytoskeleton, therefore, represents a point for chemotherapeutic intervention. The challenge in disrupting the actin cytoskeleton is in preserving actin-driven contraction of cardiac and skeletal muscle. By targeting actin-binding proteins with altered expression in malignancy, it may be possible to achieve tumor-specific toxicity. A number of actin-binding proteins act cooperatively and synergistically to regulate actin structures required for motility. The actin cytoskeleton is characterized by a significant degree of plasticity. Targeting specific actin-binding proteins for chemotherapy will only be successful if no other compensatory mechanisms exist.

Computational analysis of the spatiotemporal coordination of polarized PI3K and Rac1 activities in micro-patterned live cells

Polarized molecular activities play important roles in guiding the cell toward persistent and directional migration. In this study, the polarized distributions of the activities of phosphatidylinositol 3-kinase (PI3K) and the Rac1 small GTPase were monitored using chimeric fluorescent proteins (FPs) in cells constrained on micro-patterned strips, with one end connecting to a neighboring cell (junction end) and the other end free of cell-cell contact (free end). The recorded spatiotemporal dynamics of the fluorescent intensity from different cells was scaled into a uniform coordinate system and applied to compute the molecular activity landscapes in space and time. The results revealed different polarization patterns of PI3K and Rac1 activity induced by the growth factor stimulation. The maximal intensity of different FPs, and the edge position and velocity at the free end were further quantified to analyze their correlation and decipher the underlying signaling sequence. The results suggest that the initiation of the edge extension occurred before the activation of PI3K, which led to a stable extension of the free end followed by the Rac1 activation. Therefore, the results support a concerted coordination of sequential signaling events and edge dynamics, underscoring the important roles played by PI3K activity at the free end in regulating the stable lamellipodia extension and cell migration. Meanwhile, the quantification methods and accompanying software developed can provide a convenient and powerful computational analysis platform for the study of spatiotemporal molecular distribution and hierarchy in live cells based on fluorescence images.

A novel spatiotemporal RhoC activation pathway locally regulates cofilin activity at invadopodia

BACKGROUND

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.

RESULTS

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 confine RhoC activation within this compartment. p190RhoGEF localizes around invadopodia to activate RhoC, whereas p190RhoGAP localizes inside invadopodia to deactivate the GTPase within the structure. RhoC activation enhances cofilin phosphorylation outside invadopodia.

CONCLUSION

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.

Slit-2 repels the migration of olfactory ensheathing cells by triggering Ca2+-dependent cofilin activation and RhoA inhibition

Olfactory ensheathing cells (OECs) migrate from the olfactory epithelium towards the olfactory bulb during development. However, the guidance mechanism for OEC migration remains a mystery. Here we show that migrating OECs expressed the receptor of the repulsive guidance factor Slit-2. A gradient of Slit-2 in front of cultured OECs first caused the collapse of the leading front, then the reversal of cell migration. These Slit-2 effects depended on the Ca(2+) release from internal stores through inositol (1,4,5)-triphosphate receptor channels. Interestingly, in response to Slit-2 stimulation, collapse of the leading front required the activation of the F-actin severing protein cofilin in a Ca(2+)-dependent manner, whereas the subsequent reversal of the soma migration depended on the reversal of RhoA activity across the cell. Finally, the Slit-2-induced repulsion of cell migration was fully mimicked by co-application of inhibitors of F-actin polymerization and RhoA kinase. Our findings revealed Slit-2 as a repulsive guidance factor for OEC migration and an unexpected link between Ca(2+) and cofilin signaling during Slit-2-triggered repulsion.

Intersectin1-s is involved in migration and invasion of human glioma cells

Malignant gliomas have a tendency to invade diffusely into surrounding healthy brain tissues, thereby precluding their successful surgical removal. Intersectin1 (ITSN1) as a molecular linker in the central nervous system is well known as an important regulator of endocytosis and exocytosis. ITSN1 has two isoforms: ITSN1-l and ITSN1-s. In this study, we show that siRNA-mediated down regulation of ITSN1-s inhibited migration and invasion of glioma cells. In addition, we demonstrate the possible mechanisms by which ITSN1-s functions in migration and invasion. Several key proteins, including cofilin, LIMK, PAK, FAK, integrin β1, and MMP-9, which are critical for cells migration and invasion, were probably involved in ITSN1-s signaling pathways. These results suggest that ITSN1-s contributes to glioma cells migration and invasion by regulating the formation of cytoskeleton, influencing adhesion and increasing expression of MMP-9. Our results indicate that ITSN1-s is a critical factor in gliomas invasion and identify that ITSN1-s is a new potentially antiinvasion target for therapeutic intervention in gliomas.

Cancer cell invasion is enhanced by applied mechanical stimulation

Metastatic cells migrate from the site of the primary tumor, through the stroma, into the blood and lymphatic vessels, finally colonizing various other tissues to form secondary tumors. Numerous studies have been done to identify the stimuli that drive the metastatic cascade. This has led to the identification of multiple biochemical signals that promote metastasis. However, information on the role of mechanical factors in cancer metastasis has been limited to the affect of compliance. Interestingly, the tumor microenvironment is rich in many cell types including highly contractile cells that are responsible for extensive remodeling and production of the dense extracellular matrix surrounding the cancerous tissue. We hypothesize that the mechanical forces produced by remodeling activities of cells in the tumor microenvironment contribute to the invasion efficiency of metastatic cells. We have discovered a significant difference in the extent of invasion in mechanically stimulated verses non-stimulated cell culture environments. Furthermore, this mechanically enhanced invasion is dependent upon substrate protein composition, and influenced by topography. Finally, we have found that the protein cofilin is needed to sense the mechanical stimuli that enhances invasion. We conclude that other types of mechanical signals in the tumor microenvironment, besides the rigidity, can enhance the invasive abilities of cancer cells in vitro. We further propose that in vivo, non-cancerous cells located within the tumor micro-environment may be capable of providing the necessary mechanical stimulus during the remodeling of the extracellular matrix surrounding the tumor.

Targeting cytoskeleton reorganisation as antimetastatic treatment

Metastatic relapse is responsible for 90% of cancer-related deaths. The process of distant spreading is a cascade of events that is regulated in a highly complex manner; one cellular phenomenon underlying all the events is cytoskeletal reorganisation. Despite the fact that the ability to leave the primary site and establish a viable mass in a distant site is a hallmark of cancer, targeting cytoskeletal reorganisation is an emerging field. In this review we describe the key signalling pathways controlling cytoskeletal reorganisation and the current targeted therapies against the "druggable" nodes. Finally, we discuss potential implications of trial design that can play a role in detecting the specific activity of this drug class.

Regulation of cofilin phosphorylation and asymmetry in collective cell migration during morphogenesis

During Drosophila oogenesis, two actin dynamics regulators, cofilin and Rac, are required for the collective migration of a coherent cluster of cells called border cells. Cell culture data have shown that Rac and cofilin are both essential for lamellipodium formation, but Rac signaling results in phosphorylation and hence inactivation of cofilin. So it remains unclear whether cofilin phosphorylation plays a promoting or inhibitory role during cell migration. We show here that cofilin is required for F-actin turnover and lamellipodial protrusion in the border cells. Interestingly, reducing the dosage of cofilin by half or expressing a phospho-mimetic mutant form, S3E, partially rescues the migration and protrusion defects of Rac-deficient border cells. Moreover, cofilin exhibits moderate accumulation in border cells at the migratory front of the cluster, whereas phospho-cofilin has a robust and uniform distribution pattern in all the outer border cells. Blocking or overactivating Rac signaling in border cells greatly reduces or increases cofilin phosphorylation, respectively, and each abolishes cell migration. Furthermore, Rac may signal through Pak and LIMK to result in uniform phosphorylation of cofilin in all the outer border cells, whereas the guidance receptor Pvr (PDGF/VEGF receptor) mediates the asymmetric localization of cofilin in the cluster but does not affect its phosphorylation. Our study provides one of the first models of how cofilin functions and is regulated in the collective migration of a group of cells in vivo.

Peculiarities of proliferation and differentiation of cambial and daughter cells of epidermal-dermal morphofunctional zone in normal epithelium and in cancer

The cambial and daughter cells of normal epithelium function in the morphofunctional zone consisting of two subunits with 12 cambial cells in each. Daughter cells are differentiated in an electrical field created by 12 pairs of maternal and daughter cells, products of division of cambial cells located in the same subunit. The differentiation requires relaxation of the cortex of daughter cells via expression of SH3 domain of Src kinase by dermal daughter cells, which leads to a decrease in activity of RhoA in epidermal cells, their stretching, and activation of SH2 domain of Src responsible for differentiation. Reduction of the number of cambial cells to 6 and, consequently, weakening of electrical field produced by them to a threshold value corresponding to very weak stretching of daughter epithelial cells results in a decrease in SH2 domain expression in these cells and its kinase contribution in Src. This leads to an increase in RhoA relative to Src, enhances cell contraction, impairs formation of stress fibrils and focal contacts, reduces cell flattening, and increases cell mobility. The decrease in the number of microtubules, intermediate filaments, and stress-fibrils changes the major cell axis direction, which, in turn, sharply reduces nucleus stretching and leads to impaired chromosome looping out near the centromeres and telomeres; the cells acquires signs of an epitheliocyte and a fibroblast, protein transcription is impaired, and daughter cells are transformed into malignant cell.

Loss of cofilin 1 disturbs actin dynamics, adhesion between enveloping and deep cell layers and cell movements during gastrulation in zebrafish

During gastrulation, cohesive migration drives associated cell layers to the completion of epiboly in zebrafish. The association of different layers relies on E-cadherin based cellular junctions, whose stability can be affected by actin turnover. Here, we examined the effect of malfunctioning actin turnover on the epibolic movement by knocking down an actin depolymerizing factor, cofilin 1, using antisense morpholino oligos (MO). Knockdown of cfl1 interfered with epibolic movement of deep cell layer (DEL) but not in the enveloping layer (EVL) and the defect could be specifically rescued by overexpression of cfl1. It appeared that the uncoordinated movements of DEL and EVL were regulated by the differential expression of cfl1 in the DEL, but not EVL as shown by in situ hybridization. The dissociation of DEL and EVL was further evident by the loss of adhesion between layers by using transmission electronic and confocal microscopy analyses. cfl1 morphants also exhibited abnormal convergent extension, cellular migration and actin filaments, but not involution of hypoblast. The cfl1 MO-induced cell migration defect was found to be cell-autonomous in cell transplantation assays. These results suggest that proper actin turnover mediated by Cfl1 is essential for adhesion between DEL and EVL and cell movements during gastrulation in zebrafish.

Imaging the dynamics of intracellular protein translocation by photoconversion of phamret-cybr/ROM

Cybr/Reduced On-random Motile (ROM) is a scaffold protein, containing a postsynaptic density protein-95/discs-large/ZO-1 (PDZ) domain, a LEU region and a PDZ domain binding region at the C-terminus. In the immune system, Cybr/ROM was found to localize in vesicles and at the plasma membrane, through interactions with cytohesin-1. In this investigation, we reported Cybr/ROM as occurring in vesicles, the cytoplasm and at membrane ruffles of H1299 lung cancer cells. Its localization at the ruffles was dependent on intact actin structures as indicated by latrunculin A treatment, which abrogated ruffle formation and staining of Cybr/ROM at the cells' periphery. Transfection of truncation mutants consisting of either the PDZ or LEU domain showed that the LEU domain of ROM was localized to membrane ruffles, vesicles and the cytoplasm, whereas, the PDZ domain localized to the membrane ruffles and cytoplasm only. There was therefore, domain/molecular segregation of Cybr/ROM in different cellular compartments. Cybr/ROM was subcloned into a plasmid carrying the photoactivation-mediated resonance energy transfer (Phamret) protein. The photoconversion experiments demonstrated the diffusion of ROM from the cytoplasm to the membrane ruffling sites and conversely from membrane ruffles to the cytoplasm. Large variances in the transport velocity of Cybr/ROM in the cytoplasm suggested that its movements were facilitated by other mechanisms in addition to diffusion.

Differing contributions of LIMK and ROCK to TGFβ-induced transcription, motility and invasion

The ability of transforming growth factor β (TGFβ) to induce epithelial-mesenchymal transition (EMT) is mediated by SMAD-dependent and SMAD-independent pathways such as the activation of Rho GTPase signalling. Upon activation, GTP-bound Rho stimulates the ROCK kinases, which in turn phosphorylate numerous substrates including the LIM kinases (LIMK). The net result of ROCK activation is increased actin-myosin contractile force generation, with a contribution from LIMK-induced actin filament stabilisation. In this study, we made use of siRNA-mediated knockdown and selective inhibitors to determine the contributions of ROCK and LIMK to TGFβ-induced responses. We find that both ROCK and LIMK are required for TGFβ stimulation of serum-response factor (SRF) transcriptional activity and actin stress fibre formation during EMT. In contrast, although LIMK inhibition had little effect on cell motility in scratch wound and Transwell migration assays, ROCK inhibition actually promoted TGFβ-induced cell motility by helping individual cells to break free from the epithelial sheet. Furthermore, we demonstrate that selective inhibition of LIMK, but not ROCK, effectively blocked TGFβ driven invasion through a layer of matrigel extracellular matrix protein. These results indicate that the roles of LIMK and ROCK in the Rho signalling pathway downstream of TGFβ are not identical and suggest that LIMK represents an attractive therapeutic target in TGFβ driven organ fibrosis and metastatic cancer spread.

CFL1 expression levels as a prognostic and drug resistance marker in nonsmall cell lung cancer

BACKGROUND

Nonsmall cell lung cancer (NSCLC) is the major determinant of overall cancer mortality worldwide. Despite progress in molecular research, current treatments offer limited benefits. Because NSCLC generates early metastasis, and this behavior requires great cell motility, herein the authors assessed the potential value of CFL1 gene (main member of the invasion/metastasis pathway) as a prognostic and predictive NSCLC biomarker.

METHODS

Metadata analysis of tumor tissue microarray was applied to examine expression of CFL1 in archival lung cancer samples from 111 patients, and its clinicopathologic significance was investigated. The robustness of the finding was validated using another independent data set. Finally, the authors assayed in vitro the role of CFL1 levels in tumor invasiveness and drug resistance using 6 human NSCLC cell lines with different basal degrees of CFL1 gene expression.

RESULTS

CFL1 levels in biopsies discriminate between good and bad prognosis at early tumor stages (IA, IB, and IIA/B), where high CFL1 levels are correlated with lower overall survival rate (P<.0001). Biomarker performance was further analyzed by immunohistochemistry, hazard ratio (P<.001), and receiver-operating characteristic curve (area=0.787; P<.001). High CFL1 mRNA levels and protein content are positively correlated with cellular invasiveness (determined by Matrigel Invasion Chamber System) and resistance (2-fold increase in drug 50% growth inhibition dose) against a list of 22 alkylating agents. Hierarchical clustering analysis of the CFL1 gene network had the same robustness for stratified NSCLC patients.

CONCLUSIONS

This study indicates that the CFL1 gene and its functional gene network can be used as prognostic biomarkers for NSCLC and could also guide chemotherapeutic interventions.

Expression of actin-interacting protein 1 suppresses impaired chemotaxis of Dictyostelium cells lacking the Na+-H+ exchanger NHE1

Dictyostelium cells lacking the intracellular pH regulator NHE1 have defective chemotaxis. A modifier screen and reconstitution studies show expression of recombinant actin interacting protein 1 (Aip1) suppresses the Ddnhe1-phenotype. Aip1 promotes cofilin-dependent actin remodeling, which is likely a major determinant in pH-dependent chemotaxis.

Increased intracellular pH is an evolutionarily conserved signal necessary for directed cell migration. We reported previously that in Dictyostelium cells lacking H⁺ efflux by a Na⁺-H⁺ exchanger (NHE; Ddnhe1⁻), chemotaxis is impaired and the assembly of filamentous actin (F-actin) is attenuated. We now describe a modifier screen that reveals the C-terminal fragment of actin-interacting protein 1 (Aip1) enhances the chemotaxis defect of Ddnhe1⁻ cells but has no effect in wild-type Ax2 cells. However, expression of full-length Aip1 mostly suppresses chemotaxis defects of Ddnhe1⁻ cells and restores F-actin assembly. Aip1 functions to promote cofilin-dependent actin remodeling, and we found that although full-length Aip1 binds cofilin and F-actin, the C-terminal fragment binds cofilin but not F-actin. Because pH-dependent cofilin activity is attenuated in mammalian cells lacking H⁺ efflux by NHE1, our current data suggest that full-length Aip1 facilitates F-actin assembly when cofilin activity is limited. We predict the C-terminus of Aip1 enhances defective chemotaxis of Ddnhe1⁻ cells by sequestering the limited amount of active cofilin without promoting F-actin assembly. Our findings indicate a cooperative role of Aip1 and cofilin in pH-dependent cell migration, and they suggest defective chemotaxis in Ddnhe1⁻ cells is determined primarily by loss of cofilin-dependent actin dynamics.

Activation of ADF/cofilin mediates attractive growth cone turning toward nerve growth factor and netrin-1

Proper neural circuitry requires that growth cones, motile tips of extending axons, respond to molecular guidance cues expressed in the developing organism. However, it is unclear how guidance cues modify the cytoskeleton to guide growth cone pathfinding. Here, we show acute treatment with two attractive guidance cues, nerve growth factor (NGF) and netrin-1, for embryonic dorsal root ganglion and temporal retinal neurons, respectively, results in increased growth cone membrane protrusion, actin polymerization, and filamentous actin (F-actin). ADF/cofilin (AC) family proteins facilitate F-actin dynamics, and we found the inactive phosphorylated form of AC is decreased in NGF- or netrin-1-treated growth cones. Directly increasing AC activity mimics addition of NGF or netrin-1 to increase growth cone protrusion and F-actin levels. Extracellular gradients of NGF, netrin-1, and a cell-permeable AC elicit attractive growth cone turning and increased F-actin barbed ends, F-actin accumulation, and active AC in growth cone regions proximal to the gradient source. Reducing AC activity blunts turning responses to NGF and netrin. Our results suggest that gradients of NGF and netrin-1 locally activate AC to promote actin polymerization and subsequent growth cone turning toward the side containing higher AC activity.

beta-Arrestins scaffold cofilin with chronophin to direct localized actin filament severing and membrane protrusions downstream of protease-activated receptor-2

Protease-activated receptor-2 (PAR-2) mediates pro-inflammatory signals in a number of organs, including enhancing leukocyte recruitment to sites of injury and infection. At the cellular level, PAR-2 promotes activation of the actin filament-severing protein cofilin, which is crucial for the reorganization of the actin cytoskeleton and chemotaxis. These responses require the scaffolding functions of beta-arrestins; however, the mechanism by which beta-arrestins spatially regulate cofilin activity and the role of this pathway in primary cells has not been investigated. Here, using size-exclusion chromatography and co-immunoprecipitation, we demonstrate that PAR-2 promotes the formation of a complex containing beta-arrestins, cofilin, and chronophin (CIN) in primary leukocytes and cultured cells. Both association of cofilin with CIN and cell migration are inhibited in leukocytes from beta-arrestin-2(-/-) mice. We show that, in response to PAR-2 activation, beta-arrestins scaffold cofilin with its upstream activator CIN, to facilitate the localized generation of free actin barbed ends, leading to membrane protrusion. These studies suggest that a major role of beta-arrestins in chemotaxis is to spatially regulate cofilin activity to facilitate the formation of a leading edge, and that this pathway may be important for PAR-2-stimulated immune cell migration.

Phosphoinositide signalling in cancer: beyond PI3K and PTEN

There are numerous studies that suggest multiple links between the cellular phosphoinositide system and cancer. As key roles in cancer have been established for PI3K and PTEN - enzymes that regulate the levels of phosphatidylinositol-3,4,5-trisphosphate - compounds targeting this pathway are entering the clinic at a rapid pace. Several other phosphoinositide-modifying enzymes, including phosphoinositide kinases, phosphatases and phospholipase C enzymes, have been implicated in the generation and progression of tumours. Studies of these enzymes are providing new insights into the mechanisms and the extent of their involvement in cancer, highlighting new potential targets for therapeutic intervention.

Pseudopodial actin dynamics control epithelial-mesenchymal transition in metastatic cancer cells

A key cellular process associated with the invasive or metastatic program in many cancers is the transformation of epithelial cells toward a mesenchymal state, a process called epithelial to mesenchymal transition or EMT. Actin-dependent protrusion of cell pseudopodia is a critical element of mesenchymal cell migration and therefore of cancer metastasis. However, whether EMT occurs in human cancers and, in particular, whether it is a prerequisite for tumor cell invasion and metastasis, remains a subject of debate. Microarray and proteomic analysis of actin-rich pseudopodia from six metastatic human tumor cell lines identified 384 mRNAs and 64 proteins common to the pseudopodia of six metastatic human tumor cell lines of various cancer origins leading to the characterization of 19 common pseudopod-specific proteins. Four of these (AHNAK, septin-9, eIF4E, and S100A11) are shown to be essential for pseudopod protrusion and tumor cell migration and invasion. Knockdown of each of these proteins in metastatic cells resulted in reduced actin cytoskeleton dynamics and induction of mesenchymal-epithelial transition (MET) that could be prevented by the stabilization of the actin cytoskeleton. Actin-dependent pseudopodial protrusion and tumor cell migration are therefore determinants of EMT. Protein regulators of pseudopodial actin dynamics may represent unique molecular targets to induce MET and thereby inhibit the metastatic potential of tumor cells.

The cofilin activity cycle in lamellipodia and invadopodia

The actin severing protein cofilin is essential for directed cell migration and chemotaxis, in many cell types and is also important for tumor cell invasion during metastasis. Through its severing activity, cofilin increases the number of free barbed ends to initiate actin polymerization for actin-based protrusion in two distinct subcellular compartments in invasive tumor cells: lamellipodia and invadopodia. Cofilin severing activity is tightly regulated and multiple mechanisms are utilized to regulate cofilin activity. In this prospect, we have grouped the primary on/off regulation into two broad categories, both of which are important for inhibiting cofilin from binding to F-actin or G-actin: (1) Blocking cofilin activity by the binding of cofilin to either PI(4,5)P(2) at lamellipodia, or cortactin at invadopodia. (2) Blocking cofilin's ability to bind to actin via serine phosphorylation. Although the literature suggests that these cofilin regulatory mechanisms may be cell-type dependent, we propose the existence of a common cofilin activity cycle in which both operate. In this common cycle, the mechanism used to initiate cofilin activity is determined by the starting point in the cycle in a given subcellular compartment.

Microtubules regulate migratory polarity through Rho/ROCK signaling in T cells

Background

Migrating leukocytes normally have a polarized morphology with an actin-rich lamellipodium at the front and a uropod at the rear. Microtubules (MTs) are required for persistent migration and chemotaxis, but how they affect cell polarity is not known.

Methodology/Principal Findings

Here we report that T cells treated with nocodazole to disrupt MTs are unable to form a stable uropod or lamellipodium, and instead often move by membrane blebbing with reduced migratory persistence. However, uropod-localized receptors and ezrin/radixin/moesin proteins still cluster in nocodazole-treated cells, indicating that MTs are required specifically for uropod stability. Nocodazole stimulates RhoA activity, and inhibition of the RhoA target ROCK allows nocodazole-treated cells to re-establish lamellipodia and uropods and persistent migratory polarity. ROCK inhibition decreases nocodazole-induced membrane blebbing and stabilizes MTs. The myosin inhibitor blebbistatin also stabilizes MTs, indicating that RhoA/ROCK act through myosin II to destabilize MTs.

Conclusions/Significance

Our results indicate that RhoA/ROCK signaling normally contributes to migration by affecting both actomyosin contractility and MT stability. We propose that regulation of MT stability and RhoA/ROCK activity is a mechanism to alter T-cell migratory behavior from lamellipodium-based persistent migration to bleb-based migration with frequent turning.

DGCR6L, a novel PAK4 interaction protein, regulates PAK4-mediated migration of human gastric cancer cell via LIMK1

Overexpression, genetic amplification and mutations of p21-activated kinase 4 (PAK4) were found in a variety of human cancers. PAK4 regulated actin cytoskeleton reorganization by phosphorylating LIMK1 and promoted cancer cells migration. Using yeast two-hybrid screen, we identified a novel PAK4 binding protein, DGCR6L, which was associated with cancer cell metastasis. We confirmed PAK4 binding to the DGCR6L specifically by GST pull-down assay, and found an association between endogenous PAK4 and DGCR6L by immunoprecipitation in mammalian cells. Furthermore, L115 of DGCR6L was the critical amino acid to bind 466-572aa in the very C-terminus of PAK4. Importantly, DGCR6L was required for the formation of PAK4-DGCR6L-beta-actin complex. Overexpressed DGCR6L promoted migration of AGS cells mediated by PAK4, whereas knock-down of DGCR6L markedly inhibited the migration of those cells. Moreover, DGCR6L (L115V), which did not bind to PAK4, lost the ability to promote AGS cells migration. DGCR6L colocalized with PAK4 or F-actin and enhanced the phosphorylation level of LIMK1 and cofilin in a dose dependent manner. Taken together, our results demonstrated that DGCR6L, a novel PAK4 interacting protein, regulated PAK4-mediated migration of human gastric cancer cells via LIMK1.

Unbalancing the phosphatidylinositol-4,5-bisphosphate-cofilin interaction impairs cell steering

Cofilin is a key player in actin dynamics during cell migration. Its activity is regulated by (de)phosphorylation, pH, and binding to phosphatidylinositol-4,5-bisphosphate [PI(4,5)P(2)]. Here, we here use a human cofilin-1 (D122K) mutant with increased binding affinity for PI(4,5)P(2) and slower release from the plasma membrane to study the role of the PI(4,5)P(2)-cofilin interaction in migrating cells. In fibroblasts in a background of endogenous cofilin, D122K cofilin expression negatively affects cell turning frequency. In carcinoma cells with down-regulated endogenous cofilin, D122K cofilin neither rescues the drastic morphological defects nor restores the effects in cell turning capacity, unlike what has been reported for wild-type cofilin. In cofilin knockdown cells, D122K cofilin expression promotes outgrowth of an existing lamellipod in response to epidermal growth factor (EGF) but does not result in initiation of new lamellipodia. This indicates that, next to phospho- and pH regulation, the normal release kinetics of cofilin from PI(4,5)P(2) is crucial as a local activation switch for lamellipodia initiation and as a signal for migrating cells to change direction in response to external stimuli. Our results demonstrate that the PI(4,5)P(2) regulatory mechanism, that is governed by EGF-dependent phospholipase C activation, is a determinant for the spatial and temporal control of cofilin activation required for lamellipodia initiation.

Adaptive-control model for neutrophil orientation in the direction of chemical gradients

Neutrophils have a remarkable ability to detect the direction of chemoattractant gradients and move directionally in response to bacterial infections and tissue injuries. For their role in health and disease, neutrophils have been extensively studied, and many of the molecules involved in the signaling mechanisms of gradient detection and chemotaxis have been identified. However, the cellular-scale mechanisms of gradient sensing and directional neutrophil migration have been more elusive, and existent models provide only limited insight into these processes. Here, we propose a what we believe is a novel adaptive-control model for the initiation of cell polarization in response to gradients. In this model, the neutrophils first sample the environment by extending protrusions in random directions and subsequently adapt their sensitivity depending on localized, temporal changes in stimulation levels. Our results suggest that microtubules may play a critical role in integrating all the sensing events from the cellular periphery through their redistribution inside the neutrophils, and may also be involved in modulating local signaling. An unexpected finding was that model neutrophils exhibit significant randomness in timing and directionality of activation, comparable to our experimental observations in microfluidic devices. Moreover, their responses are robust against alterations of the rate and amplitude of the signaling reactions, and for a broad range in chemoattractant concentrations and spatial gradients.

Reduction of Akt2 inhibits migration and invasion of glioma cells

Malignant gliomas have a tendency to invade diffusely into surrounding healthy brain tissues, thereby precluding their successful surgical removal. The serine/threonine kinase Akt2 is well known as an important regulator of cell survival and growth. In this study, we show that siRNA-mediated depletion of Akt2 inhibited migration and invasion of glioma cells. In addition, we demonstrate the mechanisms by which Akt2 functions to promote cell migration and invasion. Phosphorylation of cofilin, a critical step of actin polymerization, and phosphorylation of Girdin, essential for the integrity of the actin cytoskeleton and cell migration, were impaired. Furthermore, epidermal growth factor-induced ACAP1 phosphorylation and integrin beta1 phosphorylation were also blocked, consistent with defects in adhesion. Thus, Akt2 regulates both cell adhesion and cytoskeleton rearrangement during migration. Decreased MMP-9 expression in Akt2 knocked-down glioma cells was subsequently confirmed by Western blotting, consistent with the decreased invasion in vitro and in vivo. These results suggest that Akt2 contributes to glioma cells migration and invasion by regulating the formation of cytoskeleton, influencing adhesion and increasing expression of MMP-9. Our immunohistochemistry results by using human gliomas tissue sections also indicated that Akt2 expression was closely related with the malignancy of gliomas. This is coincident with our in vivo and in vitro results from cell lines. All of these results indicate that Akt2 is a critical factor in gliomas invasion. This study identifies that Akt2 is a potentially antiinvasion target for therapeutic intervention in gliomas.

N-WASP and cortactin are involved in invadopodium-dependent chemotaxis to EGF in breast tumor cells

Metastatic mammary carcinoma cells, which have previously been observed to form mature, matrix degrading invadopodia on a thick ECM matrix, are able to form invadopodia with similar characteristics on glass without previously applied matrix. They form in response to epidermal growth factor (EGF), and contain the usual invadopodium core proteins N-WASP, Arp2/3, cortactin, cofilin, and F-actin. The study of invadopodia on glass allows for higher resolution analysis including the use of total internal reflection microscopy and analysis of their relationship to other cell motility events, in particular, lamellipodium extension and chemotaxis toward an EGF gradient. Invadopodium formation on glass requires N-WASP and cortactin but not microtubules. In a gradient of EGF more invadopodia form on the side of the cells facing the source of EGF. In addition, depletion of N-WASP or cortactin, which blocks invadopodium fromation, inhibits chemotaxis of cells towards EGF. This appears to be a localized defect in chemotaxis since depletion of N-WASP or cortactin via siRNA had no effect on lamellipodium protrusion or barbed end generation at the lamellipodium's leading edge. Since chemotaxis to EGF by breast tumor cells is involved in metastasis, inhibiting N-WASP activity in breast tumor cells might prevent metastasis of tumor cells while not affecting chemotaxis-dependent innate immunity which depends on WASp function in macrophages.

Simulating sharp gradients for short-term, Ca(2+) transients and long-term chemotaxis in cancer cells

Rapid progress in the elucidation of candidate genes and proteins that play a role in disease processes such as cancer has been possible with widespread use of genomics and proteomics in the last ten years. It is becoming important to adapt the knowledge gained from mass analytical techniques to visual techniques that enable spatial-temporal discernment of molecular events. This is significant, particularly for the study of pathways that regulate dynamic processes such as cell migration or early events associated with differentiation such as Ca(2+) signaling. This paper describes the use of techniques that create sharp growth factor gradients suitable for local activation of cell surface receptors. The methods involve retardation of the direct flow to create steep gradients at the surface plane where cells are grown. These methods are shown to be suitable for rapid biological assays such as Ca(2+) transients that occur within 1 min of receptor activation, demonstrating that the speed and level of Ca(2+) transients are related to gradient strength. A microfabricated chamber is also determined to be suitable for longer-term analyses such as cancer cell chemotaxis.

Protein kinase D1 regulates cofilin-mediated F-actin reorganization and cell motility through slingshot

Dynamic actin remodelling processes at the leading edge of migrating tumour cells are concerted events controlled by a fine-tuned temporal and spatial interplay of kinases and phosphatases. Actin severing is regulated by actin depolymerizing factor (ADF)/cofilin, which regulates stimulus-induced lamellipodia protrusion and directed cell motility. Cofilin is activated by dephosphorylation through phosphatases of the slingshot (SSH) family. SSH activity is strongly increased by its binding to filamentous actin (F-actin); however, other upstream regulators remain unknown. Here we show that in response to RhoA activation, protein kinase D1 (PKD1) phosphorylates the SSH enzyme SSH1L at a serine residue located in its actin-binding motif. This generates a 14-3-3-binding motif and blocks the localization of SSH1L to F-actin-rich structures in the lamellipodium by sequestering it in the cytoplasm. Consequently, expression of constitutively active PKD1 in invasive tumour cells enhanced the phosphorylation of cofilin and effectively blocked the formation of free actin-filament barbed ends and directed cell migration.

A common cofilin activity cycle in invasive tumor cells and inflammatory cells

In many cell types, the formation of membrane protrusions and directional migration depend on the spatial and temporal regulation of the actin-binding protein cofilin. Cofilin, which is important for the regulation of actin-polymerization initiation, increases the number of actin free barbed ends through three mechanisms: its intrinsic actin-nucleation activity; binding and severing of existing actin filaments; and recycling actin monomers from old filaments to new ones through its actin-depolymerization activity. The increase in free barbed ends that is caused by cofilin initiates new actin polymerization, which can be amplified by the actin-nucleating ARP2/3 complex. Interestingly, different cell systems seem to have different mechanisms of activating cofilin. The initial activation of cofilin in mammary breast tumors is dependent on PLCgamma, whereas cofilin activation in neutrophils is additionally dependent on dephosphorylation, which is promoted through Rac2 signaling. Although the literature seems to be confusing and inconsistent, we propose that all of the data can be explained by a single activity-cycle model. In this Opinion, we give an overview of cofilin activation in both tumor cells and inflammatory cells, and demonstrate how the differences in cofilin activation that are observed in various cell types can be explained by different starting points in this single common activity cycle.