Distribution of gelsolin and phosphoinositol 4,5-bisphosphate in lamellipodia during EGF-induced motility

Gelsolin Modulates Platelet Dense Granule Secretion and Hemostasis via the Actin Cytoskeleton

BACKGROUND AND OBJECTIVE

 The mechanisms underlying platelet granule release are not fully understood. The actin cytoskeleton serves as the platelet's structural framework that is remodeled upon platelet activation. Gelsolin is a calcium-dependent protein that severs and caps existing actin filaments although its role in modulating platelet granule exocytosis is unknown.

METHODS

 The hemostatic function of wild-type (WT) and gelsolin null (Gsn-/- ) mice was measured ex vivo by rotational thromboelastometry analysis of whole blood. Platelets were purified from WT and Gsn-/- mouse blood and activated with thrombin. Platelet aggregation was assessed by light-transmission aggregometry. Clot retraction was measured to assess outside-in integrin signaling. Adenosine triphosphate (ATP) release and surface P-selectin were measured as markers of dense- and α-granule secretion, respectively.

RESULTS

 The kinetics of agonist-induced aggregation, clot retraction, and ATP release were accelerated in Gsn-/- platelets relative to WT. However, levels of surface P-selectin were diminished in Gsn-/- platelets. ATP release was also accelerated in WT platelets pretreated with the actin-depolymerizing drug cytochalasin D, thus mimicking the kinetics observed in Gsn-/- platelets. Conversely, ATP release kinetics were normalized in Gsn-/- platelets treated with the actin polymerization agonist jasplakinolide. Rab27b and Munc13-4 are vesicle-priming proteins known to promote dense granule secretion. Co-immunoprecipitation indicates that the association between Rab27b and Munc13-4 is enhanced in Gsn-/- platelets.

CONCLUSIONS

 Gelsolin regulates the kinetics of hemostasis by modulating the platelet's actin cytoskeleton and the protein machinery of dense granule exocytosis.

Podocalyxin in Normal Tissue and Epithelial Cancer

Podocalyxin (PODXL), a glycosylated cell surface sialomucin of the CD34 family, is normally expressed in kidney podocytes, vascular endothelial cells, hematopoietic progenitors, mesothelium, as well as a subset of neurons. In the kidney, PODXL functions primarily as an antiadhesive molecule in podocyte epithelial cells, regulating adhesion and cell morphology, and playing an essential role in the development and function of the organ. Outside the kidney, PODXL plays subtle roles in tissue remodelling and development. Furthermore, many cancers, especially those that originated from the epithelium, have been reported to overexpress PODXL. Collective evidence suggests that PODXL overexpression is linked to poor prognosis, more aggressive tumour progression, unfavourable treatment outcomes, and possibly chemoresistance. This review summarises our current knowledge of PODXL in normal tissue function and epithelial cancer, with a particular focus on its underlying roles in cancer metastasis, likely involvement in chemoresistance, and potential use as a diagnostic and prognostic biomarker.

Articular Chondrocyte Phenotype Regulation through the Cytoskeleton and the Signaling Processes That Originate from or Converge on the Cytoskeleton: Towards a Novel Understanding of the Intersection between Actin Dynamics and Chondrogenic Function

Numerous studies have assembled a complex picture, in which extracellular stimuli and intracellular signaling pathways modulate the chondrocyte phenotype. Because many diseases are mechanobiology-related, this review asked to what extent phenotype regulators control chondrocyte function through the cytoskeleton and cytoskeleton-regulating signaling processes. Such information would generate leverage for advanced articular cartilage repair. Serial passaging, pro-inflammatory cytokine signaling (TNF-α, IL-1α, IL-1β, IL-6, and IL-8), growth factors (TGF-α), and osteoarthritis not only induce dedifferentiation but also converge on RhoA/ROCK/Rac1/mDia1/mDia2/Cdc42 to promote actin polymerization/crosslinking for stress fiber (SF) formation. SF formation takes center stage in phenotype control, as both SF formation and SOX9 phosphorylation for COL2 expression are ROCK activity-dependent. Explaining how it is molecularly possible that dedifferentiation induces low COL2 expression but high SF formation, this review theorized that, in chondrocyte SOX9, phosphorylation by ROCK might effectively be sidelined in favor of other SF-promoting ROCK substrates, based on a differential ROCK affinity. In turn, actin depolymerization for redifferentiation would “free-up” ROCK to increase COL2 expression. Moreover, the actin cytoskeleton regulates COL1 expression, modulates COL2/aggrecan fragment generation, and mediates a fibrogenic/catabolic expression profile, highlighting that actin dynamics-regulating processes decisively control the chondrocyte phenotype. This suggests modulating the balance between actin polymerization/depolymerization for therapeutically controlling the chondrocyte phenotype.

ATP competes with PIP2 for binding to gelsolin

Gelsolin is a severing and capping protein that targets filamentous actin and regulates filament lengths near plasma membranes, contributing to cell movement and plasma membrane morphology. Gelsolin binds to the plasma membrane via phosphatidylinositol 4,5-bisphosphate (PIP2) in a state that cannot cap F-actin, and gelsolin-capped actin filaments are uncapped by PIP2 leading to filament elongation. The process by which gelsolin is removed from PIP2 at the plasma membrane is currently unknown. Gelsolin also binds ATP with unknown function. Here we characterize the role of ATP on PIP2-gelsolin complex dynamics. Fluorophore-labeled PIP2 and ATP were used to study their interactions with gelsolin using steady-state fluorescence anisotropy, and Alexa488-labeled gelsolin was utilized to reconstitute the regulation of gelsolin binding to PIP2-containing phospholipid vesicles by ATP. Under physiological salt conditions ATP competes with PIP2 for binding to gelsolin, while calcium causes the release of ATP from gelsolin. These data suggest a cycle for gelsolin activity. Firstly, calcium activates ATP-bound gelsolin allowing it to sever and cap F-actin. Secondly, PIP2-binding removes the gelsolin cap from F-actin at low calcium levels, leading to filament elongation. Finally, ATP competes with PIP2 to release the calcium-free ATP-bound gelsolin, allowing it to undergo a further round of severing.

Gelsolin-Cu/ZnSOD interaction alters intracellular reactive oxygen species levels to promote cancer cell invasion

The actin-binding protein, gelsolin, is a well known regulator of cancer cell invasion. However, the mechanisms by which gelsolin promotes invasion are not well established. As reactive oxygen species (ROS) have been shown to promote cancer cell invasion, we investigated on the hypothesis that gelsolin-induced changes in ROS levels may mediate the invasive capacity of colon cancer cells.

Herein, we show that increased gelsolin enhances the invasive capacity of colon cancer cells, and this is mediated via gelsolin's effects in elevating intracellular superoxide (O₂^.-) levels. We also provide evidence for a novel physical interaction between gelsolin and Cu/ZnSOD, that inhibits the enzymatic activity of Cu/ZnSOD, thereby resulting in a sustained elevation of intracellular O₂^.-. Using microarray data of human colorectal cancer tissues from Gene Omnibus, we found that gelsolin gene expression positively correlates with urokinase plasminogen activator (uPA), an important matrix-degrading protease invovled in cancer invasion. Consistent with the in vivo evidence, we show that increased levels of O₂^.- induced by gelsolin overexpression triggers the secretion of uPA. We further observed reduction in invasion and intracellular O₂^.- levels in colon cancer cells, as a consequence of gelsolin knockdown using two different siRNAs. In these cells, concurrent repression of Cu/ZnSOD restored intracellular O₂^.- levels and rescued invasive capacity.

Our study therefore identified gelsolin as a novel regulator of intracellular O₂^.- in cancer cells via interacting with Cu/ZnSOD and inhibiting its enzymatic activity. Taken together, these findings provide insight into a novel function of gelsolin in promoting tumor invasion by directly impacting the cellular redox milieu.

Podocalyxin-like protein, linked to poor prognosis of pancreatic cancers, promotes cell invasion by binding to gelsolin

The cell‐adhesion glycoprotein PODXL is associated with an aggressive tumor phenotype in several forms of cancer. Here, we report that high PODXL expression was an independent predictor of worse overall survival of pancreatic cancer patients, and that PODXL promoted pancreatic cancer cell motility and invasion by physically binding to the cytoskeletal protein gelsolin. Suppression of PODXL or gelsolin decreased membrane protrusions with abundant peripheral actin structures, and in turn inhibited cell motility and invasion. Transfection of a PODXL‐rescue construct renewed the expression of gelsolin bound to peripheral actin structures in cell protrusions, and abrogated the decreased cell protrusions caused by the knockdown of PODXL. Furthermore, transfection of a PODXL‐rescue construct into pancreatic cancer cells in which both PODXL and gelsolin were suppressed failed to increase the formation of the protrusions. Thus, PODXL enhances motility and invasiveness through an increase in gelsolin–actin interactions in cell protrusions.

Tandem phosphorylation within an intrinsically disordered region regulates ACTN4 function

Phosphorylated residues occur preferentially in the intrinsically disordered regions of eukaryotic proteins. In the disordered amino-terminal region of human α-actinin-4 (ACTN4), Tyr(4) and Tyr(31) are phosphorylated in cells stimulated with epidermal growth factor (EGF), and a mutant with phosphorylation-mimicking mutations of both tyrosines exhibits reduced interaction with actin in vitro. Cleavage of ACTN4 by m-calpain, a protease that in motile cells is predominantly activated at the rear, removes the Tyr(4) site. We found that introducing a phosphomimetic mutation at only Tyr(31) was sufficient to inhibit the interaction with actin in vitro. However, molecular dynamics simulations predicted that Tyr(31) is mostly buried and that phosphorylation of Tyr(4) would increase the solvent exposure and thus kinase accessibility of Tyr(31). In fibroblast cells, EGF stimulation increased tyrosine phosphorylation of a mutant form of ACTN4 with a phosphorylation-mimicking residue at Tyr(4), whereas a truncated mutant representing the product of m-calpain cleavage exhibited EGF-stimulated tyrosine phosphorylation at a background amount similar to that observed for a double phosphomimetic mutant of Tyr(4) and Tyr(31). We also found that inhibition of the receptor tyrosine kinases of the TAM family, such as AXL, blocked EGF-stimulated tyrosine phosphorylation of ACTN4. Mathematical modeling predicted that the kinetics of phosphorylation at Tyr(31) can be dictated by the kinase affinity for Tyr(4). This study suggests that tandem-site phosphorylation within intrinsically disordered regions provides a mechanism for a site to function as a switch to reveal a nearby function-regulating site.

Synaptojanin 2 is a druggable mediator of metastasis and the gene is overexpressed and amplified in breast cancer

Amplified HER2, which encodes a member of the epidermal growth factor receptor (EGFR) family, is a target of effective therapies against breast cancer. In search for similarly targetable genomic aberrations, we identified copy number gains in SYNJ2, which encodes the 5'-inositol lipid phosphatase synaptojanin 2, as well as overexpression in a small fraction of human breast tumors. Copy gain and overexpression correlated with shorter patient survival and a low abundance of the tumor suppressor microRNA miR-31. SYNJ2 promoted cell migration and invasion in culture and lung metastasis of breast tumor xenografts in mice. Knocking down SYNJ2 impaired the endocytic recycling of EGFR and the formation of cellular lamellipodia and invadopodia. Screening compound libraries identified SYNJ2-specific inhibitors that prevented cell migration but did not affect the related neural protein SYNJ1, suggesting that SYNJ2 is a potentially druggable target to block cancer cell migration.

The actin-binding protein profilin is required for germline stem cell maintenance and germ cell enclosure by somatic cyst cells

Specialized microenvironments, or niches, provide signaling cues that regulate stem cell behavior. In the Drosophila testis, the JAK-STAT signaling pathway regulates germline stem cell (GSC) attachment to the apical hub and somatic cyst stem cell (CySC) identity. Here, we demonstrate that chickadee, the Drosophila gene that encodes profilin, is required cell autonomously to maintain GSCs, possibly facilitating localization or maintenance of E-cadherin to the GSC-hub cell interface. Germline specific overexpression of Adenomatous Polyposis Coli 2 (APC2) rescued GSC loss in chic hypomorphs, suggesting an additive role of APC2 and F-actin in maintaining the adherens junctions that anchor GSCs to the niche. In addition, loss of chic function in the soma resulted in failure of somatic cyst cells to maintain germ cell enclosure and overproliferation of transit-amplifying spermatogonia.

Gelsolin activity controls efficient early HIV-1 infection

Background

HIV-1 entry into target lymphocytes requires the activity of actin adaptors that stabilize and reorganize cortical F-actin, like moesin and filamin-A. These alterations are necessary for the redistribution of CD4-CXCR4/CCR5 to one pole of the cell, a process that increases the probability of HIV-1 Envelope (Env)-CD4/co-receptor interactions and that generates the tension at the plasma membrane necessary to potentiate fusion pore formation, thereby favouring early HIV-1 infection. However, it remains unclear whether the dynamic processing of F-actin and the amount of cortical actin available during the initial virus-cell contact are required to such events.

Results

Here we show that gelsolin restructures cortical F-actin during HIV-1 Env-gp120-mediated signalling, without affecting cell-surface expression of receptors or viral co-receptor signalling. Remarkably, efficient HIV-1 Env-mediated membrane fusion and infection of permissive lymphocytes were impaired when gelsolin was either overexpressed or silenced, which led to a loss or gain of cortical actin, respectively. Indeed, HIV-1 Env-gp120-induced F-actin reorganization and viral receptor capping were impaired under these experimental conditions. Moreover, gelsolin knockdown promoted HIV-1 Env-gp120-mediated aberrant pseudopodia formation. These perturbed-actin events are responsible for the inhibition of early HIV-1 infection.

Conclusions

For the first time we provide evidence that through its severing of cortical actin, and by controlling the amount of actin available for reorganization during HIV-1 Env-mediated viral fusion, entry and infection, gelsolin can constitute a barrier that restricts HIV-1 infection of CD4+ lymphocytes in a pre-fusion step. These findings provide important insights into the complex molecular and actin-associated dynamics events that underlie early viral infection. Thus, we propose that gelsolin is a new factor that can limit HIV-1 infection acting at a pre-fusion step, and accordingly, cell-signals that regulate gelsolin expression and/or its actin-severing activity may be crucial to combat HIV-1 infection.

The carboxyl tail of alpha-actinin-4 regulates its susceptibility to m-calpain and thus functions in cell migration and spreading

Alpha-actinin-4 links the cytoskeleton to sites of adhesion and has been shown to be modulated to enable cell migration. Such focal adhesions must be labile to accomplish migration, with this detachment occurring at least in part via m-calpain activation (Glading et al., 2001, 2002; Xie et al., 1998). In this study, we report that alpha-actinin-4 is initially cleaved by m-calpain between tyrosine 13 and glycine. Removal of the first 13 amino acids does not affect alpha-actinin-4 binding to actin filaments and its localization within fibroblasts but drives cell migration with less persistence. Binding of phosphoinositides PI(4,5)P2, PI(3,4,5)P3 and PI(3,4)P2 to alpha-actinin-4, as well as binding of alpha-actinin-4 to actin filaments all inhibit m-calpain cleavage of ACTN4 between tyrosine 13 and glycine 14. Interestingly, the carboxyl terminus of alpha-actinin-4 including its calcium binding motifs, is inhibitory for a secondary cleavage of alpha-actinin-4 between lysine 283 and valine 284. The minimal length of inhibitory domain is mapped to the last 11 amino acids of alpha-actinin-4. The C-terminal tail of alpha-actinin-4 is essential for maintaining its normal actin binding activity and localization within cytoplasm and also its colocalization with actin in the lamellipodia of locomoting fibroblasts. Live cell imaging reveals that the 1-890 fragment fails to rescue neither the basal or growth factor-stimulated migration nor the revert the spread area of fibroblasts to the level of NR6WT. These findings suggest that the C-terminal tail of alpha-actinin-4 is essential for its function in cell migration and adhesion to substratum.

Gelsolin induces colorectal tumor cell invasion via modulation of the urokinase-type plasminogen activator cascade

Gelsolin is a cytoskeletal protein which participates in actin filament dynamics and promotes cell motility and plasticity. Although initially regarded as a tumor suppressor, gelsolin expression in certain tumors correlates with poor prognosis and therapy-resistance. In vitro, gelsolin has anti-apoptotic and pro-migratory functions and is critical for invasion of some types of tumor cells. We found that gelsolin was highly expressed at tumor borders infiltrating into adjacent liver tissues, as examined by immunohistochemistry. Although gelsolin contributes to lamellipodia formation in migrating cells, the mechanisms by which it induces tumor invasion are unclear. Gelsolin's influence on the invasive activity of colorectal cancer cells was investigated using overexpression and small interfering RNA knockdown. We show that gelsolin is required for invasion of colorectal cancer cells through matrigel. Microarray analysis and quantitative PCR indicate that gelsolin overexpression induces the upregulation of invasion-promoting genes in colorectal cancer cells, including the matrix-degrading urokinase-type plasminogen activator (uPA). Conversely, gelsolin knockdown reduces uPA levels, as well as uPA secretion. The enhanced invasiveness of gelsolin-overexpressing cells was attenuated by treatment with function-blocking antibodies to either uPA or its receptor uPAR, indicating that uPA/uPAR activity is crucial for gelsolin-dependent invasion. In summary, our data reveals novel functions of gelsolin in colorectal tumor cell invasion through its modulation of the uPA/uPAR cascade, with potentially important roles in colorectal tumor dissemination to metastatic sites.

Non-steroidal anti-inflammatory drugs inhibit calpain activity and membrane localization of calpain 2 protease

Non-steroidal anti-inflammatory drugs (NSAIDs) are used frequently worldwide for the alleviation of pain despite their capacity to cause adverse gastrointestinal (GI) side effects. GI toxicity, once thought to be the result of non-specific inhibition of cyclooxegenase (COX) enzymes, is now hypothesized to have multiple other causes that are COX independent. In particular, NSAIDs inhibit intestinal epithelial restitution, the process by which barrier function in intestinal mucosa is restored at sites of epithelial wounds within hours through cell spreading and migration. Accordingly, recent evidence indicates that the expression of calpain proteases, which play a key role in cell migration, is decreased by NSAIDs that inhibit cell migration in intestinal epithelial cells (IEC). Here, we examine the effect of NSAIDs on calpain activity and membrane expression in IEC-6 cells. Indomethacin, NS-398, and SC-560 inhibited calpain activity and decreased expression of calpain 2 in total membrane fractions and in plasma membranes involved in cell attachment to the substrate. Additionally, we demonstrated that inhibition of calpain activity by NSAIDs or ALLM, a calpain inhibitor, limits cell migration and in vitro wound healing of IEC-6 cells. Our results indicate that NSAIDs may inhibit cell migration by decreasing calpain activity and membrane-associated expression of calpain 2. Our results provide valuable insight into the mechanisms behind NSAID-induced GI toxicity and provide a potential pathway through which these negative side effects can be avoided in future members of the NSAID class.

m-Calpain activation is regulated by its membrane localization and by its binding to phosphatidylinositol 4,5-bisphosphate

m-Calpain plays a critical role in cell migration enabling rear de-adhesion of adherent cells by cleaving structural components of the adhesion plaques. Growth factors and chemokines regulate keratinocyte, fibroblast, and endothelial cell migration by modulating m-calpain activity. Growth factor receptors activate m-calpain secondary to phosphorylation on serine 50 by ERK. Concurrently, activated m-calpain is localized to its inner membrane milieu by binding to phosphatidylinositol 4,5-bisphosphate (PIP(2)). Opposing this, CXCR3 ligands inhibit cell migration by blocking m-calpain activity secondary to a PKA-mediated phosphorylation in the C2-like domain. The failure of m-calpain activation in the absence of PIP(2) points to a key regulatory role, although whether this PIP(2)-mediated membrane localization is regulatory for m-calpain activity or merely serves as a docking site for ERK phosphorylation is uncertain. Herein, we report the effects of two CXCR3 ligands, CXCL11/IP-9/I-TAC and CXCL10/IP-10, on the EGF- and VEGF-induced redistribution of m-calpain in human fibroblasts and endothelial cells. The two chemokines block the tail retraction and, thus, the migration within minutes, preventing and reverting growth factor-induced relocalization of m-calpain to the plasma membrane of the cells. PKA phosphorylation of m-calpain blocks the binding of the protease to PIP(2). Unexpectedly, we found that this was due to membrane anchorage itself and not merely serine 50 phosphorylation, as the farnesylation-induced anchorage of m-calpain triggers a strong activation of this protease, leading notably to an increased cell death. Moreover, the ERK and PKA phosphorylations have no effect on this membrane-anchored m-calpain. However, the presence of PIP(2) is still required for the activation of the anchored m-calpain. In conclusion, we describe a novel mechanism of m-calpain activation by interaction with the plasma membrane and PIP(2) specifically, this phosphoinositide acting as a cofactor for the enzyme. The phosphorylation of m-calpain by ERK and PKA by growth factors and chemokines, respectively, act in cells to regulate the enzyme only indirectly by controlling its redistribution.

BCL2 inhibits cell adhesion, spreading, and motility by enhancing actin polymerization

BCL2 is best known as a multifunctional anti-apoptotic protein. However, little is known about its role in cell-adhesive and motility events. Here, we show that BCL2 may play a role in the regulation of cell adhesion, spreading, and motility. When BCL2 was overexpressed in cultured murine and human cell lines, cell spreading, adhesion, and motility were impaired. Consistent with these results, the loss of Bcl2 resulted in higher motility observed in Bcl2-null mouse embryonic fibroblast (MEF) cells compared to wild type. The mechanism of BCL2 regulation of cell adhesion and motility may involve formation of a complex containing BCL2, actin, and gelsolin, which appears to functionally decrease the severing activity of gelsolin. We have observed that the lysate from MCF-7 and NIH3T3 cells that overexpressed BCL2 enhanced actin polymerization in cell-free in vitro assays. Confocal immunofluorescent localization of BCL2 and F-actin during spreading consistently showed that increased expression of BCL2 resulted in increased F-actin polymerization. Thus, the formation of BCL2 and gelsolin complexes (which possibly contain other proteins) appears to play a critical role in the regulation of cell adhesion and migration. Given the established correlation of cell motility with cancer metastasis, this result may explain why the expression of BCL2 in some tumor cell types reduces the potential for metastasis and is associated with improved patient prognosis.

The Flightless I homolog, fli-1, regulates anterior/posterior polarity, asymmetric cell division and ovulation during Caenorhabditis elegans development

Flightless I (Fli I) is an evolutionarily conserved member of the gelsolin family, containing actin-binding and severing activity in vitro. The physiological function of Fli I during animal development remains largely undefined. In this study, we reveal a key role of the Caenorhabditis elegans Fli I homolog, fli-1, in specifying asymmetric cell division and in establishing anterior-posterior polarity in the zygote. The fli-1 gene also regulates the cytokinesis of somatic cells and the development of germline and interacts with the phosphoinositol-signaling pathway in the regulation of ovulation. The fli-1 reporter gene shows that the localization of FLI-1 coincides with actin-rich regions and that the actin cytoskeleton is impaired in many tissues in the fli-1 mutants. Furthermore, the function of fli-1 in C. elegans can be functionally substituted by the Drosophila Fli I. Our studies demonstrate that fli-1 plays an important role in regulating the actin-dependent events during C. elegans development.

Type I gamma phosphatidylinositol phosphate kinase is required for EGF-stimulated directional cell migration

Phosphatidylinositol 4,5-bisphosphate (PI4,5P₂) modulates a plethora of cytoskeletal interactions that control the dynamics of actin assembly and, ultimately, cell migration. We show that the type Iγ phosphatidylinositol phosphate kinase 661 (PIPKIγ661), an enzyme that generates PI4,5P₂, is required for growth factor but not G protein–coupled receptor–stimulated directional migration. By generating PI4,5P₂ and regulating talin assembly, PIPKIγ661 modulates nascent adhesion formation at the leading edge to facilitate cell migration. The epidermal growth factor (EGF) receptor directly phosphorylates PIPKIγ661 at tyrosine 634, and this event is required for EGF-induced migration. This phosphorylation regulates the interaction between PIPKIγ661 and phospholipase Cγ1 (PLCγ1, an enzyme previously shown to be involved in the regulation of EGF-stimulated migration). Our results suggest that phosphorylation events regulating specific PIPKIγ661 interactions are required for growth factor–induced migration. These interactions in turn define the spatial and temporal generation of PI4,5P₂ and derived messengers required for directional migration.

EphA4 signaling regulates phospholipase Cgamma1 activation, cofilin membrane association, and dendritic spine morphology

Specialized postsynaptic structures known as dendritic spines are the primary sites of glutamatergic innervation at synapses of the CNS. Previous studies have shown that spines rapidly remodel their actin cytoskeleton to modify their shape and this has been associated with changes in synaptic physiology. However, the receptors and signaling intermediates that restructure the actin network in spines are only beginning to be identified. We reported previously that the EphA4 receptor tyrosine kinase regulates spine morphology. However, the signaling pathways downstream of EphA4 that induce spine retraction on ephrin ligand binding remain poorly understood. Here, we demonstrate that ephrin stimulation of EphA4 leads to the recruitment and activation of phospholipase Cgamma1 (PLCgamma1) in heterologous cells and in hippocampal slices. This interaction occurs through an Src homology 2 domain of PLCgamma1 and requires the EphA4 juxtamembrane tyrosines. In the brain, PLCgamma1 is found in multiple compartments of synaptosomes and is readily found in postsynaptic density fractions. Consistent with this, PLC activity is required for the maintenance of spine morphology and ephrin-induced spine retraction. Remarkably, EphA4 and PLC activity modulate the association of the actin depolymerizing/severing factor cofilin with the plasma membrane. Because cofilin has been implicated previously in the structural plasticity of spines, this signaling may enable cofilin to depolymerize actin filaments and restructure spines at sites of ephrin-EphA4 contact.

Mechanism of depolymerization and severing of actin filaments and its significance in cytoskeletal dynamics

The actin cytoskeleton is one of the major structural components of the cell. It often undergoes rapid reorganization and plays crucial roles in a number of dynamic cellular processes, including cell migration, cytokinesis, membrane trafficking, and morphogenesis. Actin monomers are polymerized into filaments under physiological conditions, but spontaneous depolymerization is too slow to maintain the fast actin filament dynamics observed in vivo. Gelsolin, actin-depolymerizing factor (ADF)/cofilin, and several other actin-severing/depolymerizing proteins can enhance disassembly of actin filaments and promote reorganization of the actin cytoskeleton. This review presents advances as well as a historical overview of studies on the biochemical activities and cellular functions of actin-severing/depolymerizing proteins.

Dynamic phospholipid signaling by G protein-coupled receptors

G protein-coupled receptors (GPCRs) control a variety of fundamental cellular processes by regulating phospholipid signaling pathways. Essential for signaling by a large number of receptors is the hydrolysis of the membrane phosphoinositide PIP(2) by phospholipase C (PLC) into the second messengers IP(3) and DAG. Many receptors also stimulate phospholipase D (PLD), leading to the generation of the versatile lipid, phosphatidic acid. Particular PLC and PLD isoforms take differential positions in receptor signaling and are additionally regulated by small GTPases of the Ras, Rho and ARF families. It is now recognized that the PLC substrate, PIP(2), has signaling capacity by itself and can, by direct interaction, affect the activity and subcellular localization of PLD and several other proteins. As expected, the synthesis of PIP(2) by phosphoinositide 5-kinases is tightly regulated as well. In this review, we present an overview of how these signaling pathways are governed by GPCRs, explain the molecular basis for the spatially and temporally organized, highly dynamic quality of phospholipid signaling, and point to the functional connection of the pathways.

Silencing profilin-1 inhibits endothelial cell proliferation, migration and cord morphogenesis

Expression of several actin-binding proteins including profilin-1 is up-regulated during capillary morphogenesis of endothelial cells, the biological significance of which remains unknown. Specifically, we hypothesized that profilin-1 is important for endothelial migration and proliferation. In this study, we suppressed profilin-1 expression in human umbilical vein endothelial cells by RNA-interference. Gene silencing of profilin-1 led to significant reduction in the formation of actin filaments and focal adhesions. Loss of profilin-1 expression was also associated with reduced dynamics of cell-cell adhesion. Data from both wound-healing experiments and time-lapse imaging of individual cells showed inhibition of cell migration when profilin-1 expression was suppressed. Cells lacking profilin-1 exhibited defects in membrane protrusion, both in terms of its magnitude and directional persistence. Furthermore, loss of profilin-1 expression inhibited cell growth without compromising cell survival, at least in the short-term, thus suggesting that profilin-1 also plays an important role in endothelial proliferation as hypothesized. Finally, silencing profilin-1 expression suppressed matrigel-induced early cord morphogenesis of endothelial cells. Taken together, our data suggest that profilin-1 may play important role in biological events that involve endothelial proliferation, migration and morphogenesis.

Interaction of phospholipase C-gamma1 with villin regulates epithelial cell migration

Tyrosine-phosphorylated villin regulates actin dynamics, cell morphology, and cell migration. Previously, we identified four tyrosine phosphorylation sites in the amino-terminal domain of villin. In this study we report six new sites in the carboxyl-terminal region of the villin core. With this study we document all phosphorylatable tyrosine residues in villin and map them to functions of villin. In this study, we identify for the first time the functional relevance of the carboxyl-terminal domains of the villin core. Expression of the carboxyl-terminal phosphorylation site mutant, as well as the villin truncation mutant S1-S3, inhibited cell migration in HeLa and Madin-Darby canine kidney Tet-Off cells, confirming the role of the carboxyl-terminal phosphorylation sites in villin-induced cell migration. The carboxyl-terminal phosphorylation sites were found to be critical for the interaction of villin with its ligand phospholipase C-gamma1 and for its localization to the developing lamellipodia in a motile cell. The results presented here elucidate the molecular basis for tyrosine-phosphorylated villin-induced changes in cell motility.

Spatial localization of m-calpain to the plasma membrane by phosphoinositide biphosphate binding during epidermal growth factor receptor-mediated activation

Calpain activity is required for de-adhesion of the cell body and rear to enable productive locomotion of adherent cells during wound repair and tumor invasion. Growth factors activate m-calpain (calpain 2, CAPN2) via ERK/mitogen-activated protein kinases, but only when these kinases are localized to the plasma membrane. We thus hypothesized that m-calpain is activated by epidermal growth factor (EGF) only when it is juxtaposed to the plasma membrane secondary to specific docking. Osmotic disruption of NR6 fibroblasts expressing the EGF receptor demonstrated m-calpain being complexed with the substratum-adherent membrane with this increasing in an EGF-dependent manner. m-Calpain colocalized with phosphoinositide biphosphate (PIP(2)) with exogenous phospholipase C removal of phosphoinositides, specifically, PI(4,5)P(2) but not PI(4)P(1) or PIP(3), releasing the bound m-calpain. Downregulation of phosphoinositide production by 1-butanol resulted in diminished PIP(2) in the plasma membrane and eliminated EGF-induced calpain activation. This PIP(2)-binding capacity resided in domain III of calpain, which presents a putative C2-like domain. This active conformation of this domain appears to be partially masked in the holoenzyme as both activation of m-calpain by phosphorylation at serine 50 and expression of constitutively active phosphorylation mimic glutamic acid-increased m-calpain binding to the membrane, consistent with blockade of this cascade diminishing membrane association. Importantly, we found that m-calpain was enriched toward the rear of locomoting cells, which was more pronounced in the plasma membrane footprints; EGF further enhanced this enrichment, in line with earlier reports of loss of PIP(2) in lamellipodia of motile cells. These data support a model of m-calpain binding to PIP(2) concurrent with and likely to enable ERK activation and provides a mechanism by which cell de-adhesion is directed to the cell body and tail as phospholipase C-gamma hydrolyzes PIP(2) in the protruding lamellipodia.

Nuclear actin-binding proteins as modulators of gene transcription

Dynamic transformations in the organization of the cellular microfilament system are the driving force behind fundamental biological processes such as cellular motility, cytokinesis, wound healing and secretion. Eukaryotic cells express a plethora of actin-binding proteins (ABPs) allowing cells to control the organization of the actin cytoskeleton in a flexible manner. These structural proteins were, not surprisingly, originally described as (major) constituents of the cytoplasm. However, in recent years, there has been a steady flow of reports detailing not only translocation of ABPs into and out of the nucleus but also describing their role in the nuclear compartment. This review focuses on recent developments pertaining to nucleocytoplasmic transport of ABPs, including their mode of translocation and nuclear function. In particular, evidence that structurally and functionally unrelated cytoplasmic ABPs regulate transcription activation by various nuclear (steroid hormone) receptors is steadily accruing. Furthermore, the recent finding that actin is a necessary component of the RNA polymerase II-containing preinitiation complex opens up new opportunities for nuclear ABPs in gene transcription regulation.

Cofilin takes the lead

Cofilin has emerged as a key regulator of actin dynamics at the leading edge of motile cells. Through its actin-severing activity, it creates new actin barbed ends for polymerization and also depolymerizes old actin filaments. Its function is tightly regulated in the cell. Spatially, its activity is restricted by other actin-binding proteins, such as tropomyosin, which compete for accessibility of actin filament populations in different regions of the cell. At the molecular level, it is regulated by phosphorylation, pH and phosphatidylinositol (4,5)-bisphosphate binding downstream of signaling cascades. In addition, it also appears to be regulated by interactions with 14-3-3zeta and cyclase-associated protein. In vivo, cofilin acts synergistically with the Arp2/3 complex to amplify local actin polymerization responses upon cell stimulation, which gives it a central role in setting the direction of motility in crawling cells.

Calpain proteases in cell adhesion and motility

Cell adhesion and its role during cell spreading and motility are central to normal development and homeostasis, including its effects on immune response and wound repair and tissue regeneration. Disruption of cell adhesion impacts not only the healing process but promotes tumor invasion and metastasis. A family of intracellular, limited proteases, the calpains, has recently been shown to be a key molecular control point in attachment of cells to the surrounding matrix. Herein, the two main and ubiquitously expressed calpain isoforms will be introduced as to their modes of regulation and the current status of research will be discussed as to how these calpains might function in the biophysical process of adhesion and biological cellular responses of spreading and motility.

Regulation and cellular roles of phosphoinositide 5-kinases

The membrane phospholipid, phosphatidylinositol 4,5-bisphosphate (PIP(2)), plays a critical role in various, apparently very different cellular processes. As precursor for second messengers generated by phospholipase C isoforms and class I phosphoinositide 3-kinases, PIP(2) is indispensable for cellular signaling by membrane receptors. In addition, PIP(2) directly affects the localization and activity of many cellular proteins via specific interaction with unique phosphoinositide-binding domains and thereby regulates actin cytoskeletal dynamics, vesicle trafficking, ion channel activity, gene expression and cell survival. The activity and subcellular localization of phosphatidylinositol 4-phosphate 5-kinase (PIP5K) isoforms, which catalyze the formation of PIP(2), are actively regulated by membrane receptors, by phosphorylation and by small GTPases of the Rho and ARF families. Spatially and temporally organized regulation of PIP(2) synthesis by PIP5K enables dynamic and versatile PIP(2) signaling and represents an important link in the execution of cellular tasks by Rho and ARF GTPases.

The co-workers of actin filaments: from cell structures to signals

Cells have various surface architectures, which allow them to carry out different specialized functions. Actin microfilaments that are associated with the plasma membrane are important for generating these cell-surface specializations, and also provide the driving force for remodelling cell morphology and triggering new cell behaviour when the environment is modified. This phenomenon is achieved through a tight coupling between cell structure and signal transduction, a process that is modulated by the regulation of actin-binding proteins.

Lamellipodial versus filopodial mode of the actin nanomachinery: pivotal role of the filament barbed end

Understanding how a particular cell type expresses the lamellipodial or filopodial form of the actin machinery is essential to understanding a cell's functional interactions. To determine how a cell "chooses" among these alternative modes of "molecular hardware," we tested the role of key proteins that affect actin filament barbed ends. Depletion of capping protein (CP) by short hairpin RNA (shRNA) caused loss of lamellipodia and explosive formation of filopodia. The knockdown phenotype was rescued by a CP mutant refractory to shRNA, but not by another barbed-end capper, gelsolin, demonstrating that the phenotype was specific for CP. In Ena/VASP deficient cells, CP depletion resulted in ruffling instead of filopodia. We propose a model for selection of lamellipodial versus filopodial organization in which CP is a negative regulator of filopodia formation and Ena/VASP has recruiting/activating functions downstream of actin filament elongation in addition to its previously suggested anticapping and antibranching activities.

DU145 human prostate carcinoma invasiveness is modulated by urokinase receptor (uPAR) downstream of epidermal growth factor receptor (EGFR) signaling

Tumor cell motility and invasion have been linked to upregulated signaling from both the epidermal growth factor receptor (EGFR) and that for urokinase-type plasminogen activator (uPAR). However, we do not know whether these events are interdependent or unrelated, despite the obvious diagnostic and therapeutic implications. Gene microarray analyses have suggested that EGFR signaling via phospholipase C-gamma (PLCgamma) induces uPAR transcription. We utilized two sublines of the DU145 human prostate carcinoma cell line that are genetically engineered to differentially activate the EGFR/PLCgamma cascade and are variously invasive in vitro and in vivo. uPAR protein levels in these cells were found to be dependent on PLC signaling, pharmacologic inhibition of PLC signaling reduced uPAR expression. To determine whether uPAR was a required element in EGFR-mediated invasion, we stably expressed uPAR cDNA in either sense or antisense orientation in the two DU145 sublines. Interestingly, uPA production was modulated in parallel, although to a lesser degree, with uPAR in these sublines. Antisense to uPAR significantly restricted invasion of the highly invasive DU145 WT cells through Matrigel and reduced aggressiveness of tumors in nude mice. Up-regulation of uPAR significantly increased the invasiveness of the moderately invasive DU145 parental (DU145 P) cells through Matrigel, but this increased invasiveness was not seen in mice. uPA activity appears to contribute to invasiveness at least through Matrigel, as antibody to uPA or amiloride limited the transmigration. These results support a model of tumor invasion promoted by autocrine EGFR signaling involving reinforcing altered gene expression, of uPAR at least, that further induces cell motility. Herein, a number of key molecules whose expression levels are interrelated, including both EGFR and uPAR, are required but none are sufficient in the absence of other keys molecules in promoting tumor progression.

Epidermal growth factor-stimulated intestinal epithelial cell migration requires Src family kinase-dependent p38 MAPK signaling

Members of the epidermal growth factor (EGF) family of ligands and their receptors regulate migration and growth of intestinal epithelial cells. However, our understanding of the signal transduction pathways determining these responses is incomplete. In this study we tested the hypothesis that p38 is required for EGF-stimulated intestinal epithelial monolayer restitution. EGF-stimulated migration in a wound closure model required continuous presence of ligand for several hours for maximal response, suggesting a requirement for sustained signal transduction pathway activation. In this regard, prolonged exposure of cells to EGF activated p38 for up to 5 h. Furthermore genetic or pharmacological blockade of p38 signaling inhibited the ability of EGF to accelerate wound closure. Interestingly p38 inhibition was associated with increased EGF-stimulated ERK1/ERK2 phosphorylation and cell proliferation, suggesting that p38 regulates the balance of proliferation/migration signaling in response to EGF receptor activity. Activation of p38 in intestinal epithelial cells through EGF receptor was abolished by blockade of Src family tyrosine kinase signaling but not inhibition of phosphatidylinositol 3-kinase or protein kinase C. Taken together, these data suggest that Src family kinase-dependent p38 activation is a key component of a signaling switch routing EGF-stimulated responses to epithelial cell migration/restitution rather than proliferation during wound closure.

Structure of the N-terminal half of gelsolin bound to actin: roles in severing, apoptosis and FAF

The actin filament-severing functionality of gelsolin resides in its N-terminal three domains (G1-G3). We have determined the structure of this fragment in complex with an actin monomer. The structure reveals the dramatic domain rearrangements that activate G1-G3, which include the replacement of interdomain interactions observed in the inactive, calcium-free protein by new contacts to actin, and by a novel G2-G3 interface. Together, these conformational changes are critical for actin filament severing, and we suggest that their absence leads to the disease Finnish-type familial amyloidosis. Furthermore, we propose that association with actin drives the calcium-independent activation of isolated G1-G3 during apoptosis, and that a similar mechanism operates to activate native gelsolin at micromolar levels of calcium. This is the first structure of a filament-binding protein bound to actin and it sets stringent, high-resolution limitations on the arrangement of actin protomers within the filament.

Proteinase-activated receptor-2 stimulates prostaglandin production in keratinocytes: analysis of prostaglandin receptors on human melanocytes and effects of PGE2 and PGF2alpha on melanocyte dendricity

Prostaglandins (PG) are key mediators of diverse functions in the skin and several reports suggest that PG mediate post-inflammatory pigmentary changes through modulation of melanocyte dendricity and melanin synthesis. The proteinase-activated receptor 2 (PAR-2) is important for skin pigmentation because activation of keratinocyte PAR-2 stimulates uptake of melanosomes through phagocytosis in a Rho-dependent manner. In this report, we show that activation of keratinocyte PAR-2 stimulates release of PGE(2) and PGF(2alpha) and that PGE(2) and PGF(2alpha) act as paracrine factors that stimulate melanocyte dendricity. We characterized the expression of the EP and FP receptors in human melanocytes and show that human melanocytes express EP1 and EP3, and the FP receptor, but not EP2 and EP4. Treatment of melanocytes with EP1 and EP3 receptor agonists resulted in increased melanocyte dendricity, indicating that both EP1 and EP3 receptor signaling contribute to PGE(2)-mediated melanocyte dendricity. Certain EP3 receptor subtypes have been shown to increase adenosine 3',5'-cyclic monophosphate (cAMP) through coupling to Gs, whereas EP1 is known to couple to Gq to activate phospholipase C with elevation in Ca(2+). The cAMP/protein kinase A system is known to modulate melanocyte dendrite formation through modulation of Rac and Rho activity. Neither PGF(2alpha) or PGE(2) elevated cAMP in human melanocytes showing that dendricity observed in response to PGE(2) and PGF(2alpha) is cAMP-independent. Our data suggest that PAR-2 mediates cutaneous pigmentation both through increased uptake of melanosomes by keratinocytes, as well as by release of PGE(2) and PGF(2alpha) that stimulate melanocyte dendricity through EP1, EP3, and FP receptors.

Epidermal growth factor activates m-calpain (calpain II), at least in part, by extracellular signal-regulated kinase-mediated phosphorylation

How m-calpain is activated in cells has challenged investigators because in vitro activation requires near-millimolar calcium. Previously, we demonstrated that m-calpain activation by growth factors requires extracellular signal-regulated kinase (ERK); this enables tail deadhesion and allows productive motility. We now show that ERK directly phosphorylates and activates m-calpain both in vitro and in vivo. We identified serine 50 as required for epidermal growth factor (EGF)-induced calpain activation in vitro and in vivo. Replacing the serine with alanine limits activation by EGF and subsequent cell deadhesion and motility. A construct with the serine converted to glutamic acid displays constitutive activity in vivo; expression of an estrogen receptor fusion construct produces a tamoxifen-sensitive enzyme. Interestingly, EGF-induced m-calpain activation occurs in the absence of increased intracellular calcium levels; EGF triggers calpain even in the presence of intracellular calcium chelators and in calcium-free media. These data provide evidence that m-calpain can be activated through the ERK cascade via direct phosphorylation and that this activation may occur in the absence of cytosolic calcium fluxes.

Epidermal Growth Factor Induces Fibroblast Contractility and Motility via a Protein Kinase C δ-dependent Pathway

Myosin-based cell contractile force is considered to be a critical process in cell motility. However, for epidermal growth factor (EGF)-induced fibroblast migration, molecular links between EGF receptor (EGFR) activation and force generation have not been clarified. Herein, we demonstrate that EGF stimulation increases myosin light chain (MLC) phosphorylation, a marker for contractile force, concomitant with protein kinase C (PKC) activity in mouse fibroblasts expressing human EGFR constructs. Interestingly, PKCdelta is the most strongly phosphorylated isoform, and the preferential PKCdelta inhibitor rottlerin largely prevented EGF-induced phosphorylation of PKC substrates and MARCKS. The pathway through which EGFR activates PKCdelta is suggested by the fact that the MEK-1 inhibitor U0126 and the phosphatidylinositol 3-kinase inhibitor LY294002 had no effect on PKCdelta activation, whereas lack of PLCgamma signaling resulted in delayed PKCdelta activation. EGF-enhanced MLC phosphorylation was prevented by a specific MLC kinase inhibitor ML-7 and the PKC inhibitors chelerythrine chloride and rottlerin. Further indicating that PKCdelta is required, a dominant-negative PKCdelta construct or RNAi-mediated PKCdelta depletion also prevented MLC phosphorylation. In the absence of PLC signaling, MLC phosphorylation and cell force generation were delayed similarly to PKCdelta activation. All of the interventions that blocked PKCdelta activation or MLC phosphorylation abrogated EGF-induced cell contractile force generation and motility. Our results suggest that PKCdelta activation is responsible for a major part of EGF-induced fibroblast contractile force generation. Hence, we identify here a new pathway helping to govern cell motility, with PLC signaling playing a role in activation of PKCdelta to promote the acute phase of EGF-induced MLC activation.

Membrane cholesterol, lateral mobility, and the phosphatidylinositol 4,5-bisphosphate-dependent organization of cell actin

Responses to cholesterol depletion are often taken as evidence of a role for lipid rafts in cell function. Here, we show that depletion of cell cholesterol has global effects on cell and plasma membrane architecture and function. The lateral mobility of membrane proteins is reduced when cell cholesterol is chronically or acutely depleted. The change in mobility is a consequence of the reorganization of the cell actin. Binding of a GFP-tagged pleckstrin homology domain specific for phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] to the plasma membrane is reduced after cholesterol depletion. This result implies that the reorganization of cytoskeleton depends on the loss or redistribution of plasma membrane PI(4,5)P2. Consistent with this observation, agents that sequester plasma membrane PI(4,5)P2 mimic the effects of cholesterol depletion on actin organization and on lateral mobility.

Association of villin with phosphatidylinositol 4,5-bisphosphate regulates the actin cytoskeleton

Villin, an epithelial cell actin-binding protein, severs actin in vitro and in vivo. Previous studies report that phosphatidylinositol 4,5-bisphosphate (PIP(2)) regulates actin severing by villin, presumably by interaction with villin. However, direct association of villin with PIP(2) has never been characterized. In this report, we presented mutational analysis to identify the PIP(2)-binding sites in villin. Villin (human) binds PIP(2) with a K(d) of 39.5 microm, a stoichiometry of 3.3, and a Hill coefficient of 1. We generated deletion mutants of villin lacking putative PIP(2)-binding sites and examined the impact of these mutations on PIP(2) binding and actin dynamics. Our analysis revealed the presence of three PIP(2)-binding sites, two in the amino-terminal core and one in the carboxyl-terminal headpiece of human villin. Synthetic peptides analogous with these sites confirmed the binding domains. Circular dichroism and quenching of intrinsic tryptophan fluorescence revealed a significant conformational change in these peptides ensuing in their association with PIP(2). By using site-directed mutagenesis (arginine 138 to alanine), we demonstrated the presence of an identical F-actin and PIP(2)-binding site in the capping and severing domain of villin. In contrast, the mutants lysine 822 and 824 to alanine demonstrated the presence of an overlapping F-actin and PIP(2)-binding site in the actin cross-linking domain of villin. Consistent with this observation, association of villin with PIP(2) inhibited the actin capping and severing functions of villin and enhanced the actin bundling function of villin. Our studies revealed that structural changes induced by association with PIP(2) could regulate the actin-modifying functions of villin. This study provided biochemical proof of the functional significance of villin association with PIP(2) and identified the molecular mechanisms involved in the regulation of actin dynamics by villin and PIP(2).

Directional motility induced by epidermal growth factor requires Cdc42

Cell motility is actuated by a host of intracellular signaling cascades that result in movement of the cell in one direction, even without an external gradient. Phospholipase C-gamma (PLCgamma) has been shown to be important for growth factor-induced lamellipodial protrusion at the front of the cell while Cdc42 has been implicated in both filopodium formation at the leading edge and control of polarity of migrating cells. We asked whether these asymmetries in effector molecules may be linked. When we overexpressed either constitutively active, dominant negative, or GFP-tagged Cdc42, wild-type NR6 fibroblasts lost directionality, as expected. On epidermal growth factor (EGF) exposure these cells produced multiple, transient protrusions in every direction; these extensions failed to result in productive motility. GFP-tagged Cdc42 appeared transiently at edges of newly formed protrusions in EGF-stimulated cells while they moved haphazardly. While PLCgamma is distributed throughout the cell, the ratio of active, tyrosyl-phosphorylated PLCgamma was increased at the leading edge, where phosphatidylinositol (4,5)-bisphosphate (PIP(2)) hydrolysis is concentrated. This co-localization of activities may be due to Cdc42 directing PLCgamma to the cell front, as PLCgamma associated with Cdc42 in an EGF-dependent manner. We conclude that Cdc42 controls cell polarity, likely in part, through its binding to active PLCgamma.

Phosphoinositide regulation of the actin cytoskeleton

Phosphoinositides [PPIs, which collectively refer to phosphorylated derivatives of phosphatidylinositol (PI)] have a pivotal role as precursors to important second messengers and as bona fide signaling and scaffold targeting molecules. This review focuses on recent advances that elucidate how PPIs, particularly PI(4,5)P2 (PIP2), directly regulate the actin cytoskeleton in vivo by modulating the activity and targeting of actin regulatory proteins. The role of PIP2 in stimulating actin polymerization and in establishing cytoskeleton-plasma membrane linkages is emphasized. In addition, the review presents tantalizing evidence that suggests how binding of selected cytoskeletal proteins to membrane PPIs may promote PPI clustering into raft lipid microdomains, alter their accessibility to other proteins, and even distort the bilayer conformation. These actions have profound implications for many other PPI-regulated membrane functions that are beginning to be uncovered, and they suggest how PPIs can mediate crosstalk between the actin cytoskeleton and an expanding spectrum of essential cellular functions.