Movin' on up: the role of PtdIns(4,5)P2 in cell migration

Phosphoinositide signaling regulates the exocyst complex and polarized integrin trafficking in directionally migrating cells

Polarized delivery of signaling and adhesion molecules to the leading edge is required for directional migration of cells. Here, we describe a role for the PIP(2)-synthesizing enzyme, PIPKIγi2, in regulation of exocyst complex control of cell polarity and polarized integrin trafficking during migration. Loss of PIPKIγi2 impaired directional migration, formation of cell polarity, and integrin trafficking to the leading edge. Upon initiation of directional migration, PIPKIγi2 via PIP(2) generation controls the integration of the exocyst complex into an integrin-containing trafficking compartment that requires the talin-binding ability of PIPKIγi2, and talin for integrin recruitment to the leading edge. A PIP(2) requirement is further emphasized by inhibition of PIPKIγi2-regulated directional migration by an Exo70 mutant deficient in PIP(2) binding. These results reveal how phosphoinositide generation orchestrates polarized trafficking of integrin in coordination with talin that links integrins to the actin cytoskeleton, processes that are required for directional migration.

PI3Ks-drug targets in inflammation and cancer

Phosphoinositide 3-kinases (PI3Ks) control cell growth, proliferation, cell survival, metabolic activity, vesicular trafficking, degranulation, and migration. Through these processes, PI3Ks modulate vital physiology. When over-activated in disease, PI3K promotes tumor growth, angiogenesis, metastasis or excessive immune cell activation in inflammation, allergy and autoimmunity. This chapter will introduce molecular activation and signaling of PI3Ks, and connections to target of rapamycin (TOR) and PI3K-related protein kinases (PIKKs). The focus will be on class I PI3Ks, and extend into current developments to exploit mechanistic knowledge for therapy.

PIP kinases from the cell membrane to the nucleus

Phosphatidylinositol 4,5-bisphosphate (PIP(2)) is a membrane bound lipid molecule with capabilities to affect a wide array of signaling pathways to regulate very different cellular processes. PIP(2) is used as a precursor to generate the second messengers PIP(3), DAG and IP(3), indispensable molecules for signaling events generated by membrane receptors. However, PIP(2) can also directly regulate a vast array of proteins and is emerging as a crucial messenger with the potential to distinctly modulate biological processes critical for both normal and pathogenic cell physiology. PIP(2) directly associates with effector proteins via unique phosphoinositide binding domains, altering their localization and/or enzymatic activity. The spatial and temporal generation of PIP(2) synthesized by the phosphatidylinositol phosphate kinases (PIPKs) tightly regulates the activation of receptor signaling pathways, endocytosis and vesicle trafficking, cell polarity, focal adhesion dynamics, actin assembly and 3' mRNA processing. Here we discuss our current understanding of PIPKs in the regulation of cellular processes from the plasma membrane to the nucleus.

The phospholipase C isozymes and their regulation

The physiological effects of many extracellular neurotransmitters, hormones, growth factors, and other stimuli are mediated by receptor-promoted activation of phospholipase C (PLC) and consequential activation of inositol lipid signaling pathways. These signaling responses include the classically described conversion of phosphatidylinositol(4,5)P(2) to the Ca(2+)-mobilizing second messenger inositol(1,4,5)P(3) and the protein kinase C-activating second messenger diacylglycerol as well as alterations in membrane association or activity of many proteins that harbor phosphoinositide binding domains. The 13 mammalian PLCs elaborate a minimal catalytic core typified by PLC-d to confer multiple modes of regulation of lipase activity. PLC-b isozymes are activated by Gaq- and Gbg-subunits of heterotrimeric G proteins, and activation of PLC-g isozymes occurs through phosphorylation promoted by receptor and non-receptor tyrosine kinases. PLC-e and certain members of the PLC-b and PLC-g subclasses of isozymes are activated by direct binding of small G proteins of the Ras, Rho, and Rac subfamilies of GTPases. Recent high resolution three dimensional structures together with biochemical studies have illustrated that the X/Y linker region of the catalytic core mediates autoinhibition of most if not all PLC isozymes. Activation occurs as a consequence of removal of this autoinhibition.

Triggering actin comets versus membrane ruffles: distinctive effects of phosphoinositides on actin reorganization

A limited set of phosphoinositide membrane lipids regulate diverse cellular functions including proliferation, differentiation, and migration. We developed two techniques based on rapamycin-induced protein dimerization to rapidly change the concentration of plasma membrane phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)]. First, using a membrane-recruitable form of PI(4)P 5-kinase, we increased PI(4,5)P(2) synthesis from phosphatidylinositol 4-phosphate [PI(4)P] and found that COS-7, HeLa, and human embryonic kidney 293 cells formed bundles of motile actin filaments known as actin comets. In contrast, a second technique that increased the concentration of PI(4,5)P(2) without consuming PI(4)P induced membrane ruffles. These distinct phenotypes were mediated by dynamin-mediated vesicular trafficking and mutually inhibitory crosstalk between the small guanosine triphosphatases Rac and RhoA. Our results indicate that the effect of PI(4,5)P(2) on actin reorganization depends on the abundance of other phosphoinositides, such as PI(4)P. Thus, combinatorial regulation of phosphoinositide concentrations may contribute to the diversity of phosphoinositide functions.

Structural basis of phosphoinositide binding to kindlin-2 protein pleckstrin homology domain in regulating integrin activation

Kindlins are a subclass of FERM-containing proteins that have recently emerged as key regulators of integrin receptor activation and signaling. As compared with the conventional FERM domain, the kindlin FERM domain contains an inserted pleckstrin homology (PH) domain that recognizes membrane phosphoinositides, including phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidylinositol 3,4,5-trisphosphate (PIP3). Using NMR spectroscopy, we show that PIP3 site-specifically binds to kindlin-2 PH with substantial chemical shift changes that are much larger than PIP2. This suggests an enhanced association of kindlin-2 with membrane as mediated by PIP3 upon its conversion from PIP2 by phosphoinositide-3 kinase, a known regulator of integrin activation. We determined the NMR structure of the kindlin-2 PH domain bound to the head group of PIP3, inositol 1,3,4,5-tetraphosphate (IP4). The structure reveals a canonical PH domain fold, yet with a distinct IP4 binding pocket that appears highly conserved for the kindlin family members. Functional experiments demonstrate that although wild type kindlin-2 is capable of cooperating with integrin activator talin to induce synergistic integrin α(IIb)β(3) activation, this ability is significantly impaired for a phosphoinositide binding-defective kindlin-2 mutant. These results define a specific PIP3 recognition mode for the kindlin PH domain. Moreover, they shed light upon a mechanism as to how the PH domain mediates membrane engagement of kindlin-2 to promote its binding to integrin and cooperation with talin for regulation of integrin activation.

Synaptotagmin 1 causes phosphatidyl inositol lipid-dependent actin remodeling in cultured non-neuronal and neuronal cells

Here we demonstrate that a dramatic actin polymerizing activity caused by ectopic expression of the synaptic vesicle protein synaptotagmin 1 that results in extensive filopodia formation is due to the presence of a lysine rich sequence motif immediately at the cytoplasmic side of the transmembrane domain of the protein. This polybasic sequence interacts with anionic phospholipids in vitro, and, consequently, the actin remodeling caused by this sequence is interfered with by expression of a phosphatidyl inositol (4,5)-bisphosphate (PIP2)-targeted phosphatase, suggesting that it intervenes with the function of PIP2-binding actin control proteins. The activity drastically alters the behavior of a range of cultured cells including the neuroblastoma cell line SH-SY5Y and primary cortical mouse neurons, and, since the sequence is conserved also in synaptotagmin 2, it may reflect an important fine-tuning role for these two proteins during synaptic vesicle fusion and neurotransmitter release.

α-Syntrophin is required for the hepatocyte growth factor-induced migration of cultured myoblasts

Syntrophins are adaptor proteins that link intracellular signaling molecules to the dystrophin based scaffold. In this study, we investigated the function of syntrophins in cell migration, one of the early steps in myogenic differentiation and in regeneration of adult muscle. Hepatocyte growth factor (HGF) stimulates migration and lamellipodia formation in cultured C2 myoblasts. In the migrating cells, syntrophin concentrated in the rear-lateral region of the cell, opposite of the lamellipodia, instead of being diffusely present throughout the cytoplasm of non-migrating cells. When the expression of α-syntrophin, the major syntrophin isoform of skeletal muscle, was reduced by transfection with the α-syntrophin-specific siRNA, HGF stimulation of lamellipodia formation was prevented. Likewise, migration of myoblasts from α-syntrophin knockout mice could not be stimulated by HGF. However, HGF-induced migration was restored in myoblasts isolated from a transgenic mouse expressing α-syntrophin only in muscle cells. Treatment of C2 myoblasts with inhibitors of PI3-kinase not only reduced the rate of cell migration, but also impaired the accumulation of syntrophins in the rear-lateral region of the migrating cells. Phosphorylation of Akt was reduced in the α-syntrophin siRNA-treated C2 cells. These results suggest that α-syntrophin is required for HGF-induced migration of myoblasts and for proper PI3-kinase/Akt signaling.

PIPKIγ regulates focal adhesion dynamics and colon cancer cell invasion

Focal adhesion assembly and disassembly are essential for cell migration and cancer invasion, but the detailed molecular mechanisms regulating these processes remain to be elucidated. Phosphatidylinositol phosphate kinase type Iγ (PIPKIγ) binds talin and is required for focal adhesion formation in EGF-stimulated cells, but its role in regulating focal adhesion dynamics and cancer invasion is poorly understood. We show here that overexpression of PIPKIγ promoted focal adhesion formation, whereas cells expressing either PIPKIγ(K188,200R) or PIPKIγ(D316K), two kinase-dead mutants, had much fewer focal adhesions than those expressing WT PIPKIγ in CHO-K1 cells and HCT116 colon cancer cells. Furthermore, overexpression of PIPKIγ, but not PIPKIγ(K188,200R), resulted in an increase in both focal adhesion assembly and disassembly rates. Depletion of PIPKIγ by using shRNA strongly inhibited formation of focal adhesions in HCT116 cells. Overexpression of PIPKIγ(K188,200R) or depletion of PIPKIγ reduced the strength of HCT116 cell adhesion to fibronection and inhibited the invasive capacities of HCT116 cells. PIPKIγ depletion reduced PIP₂ levels to ∼40% of control and PIP₃ to undetectable levels, and inhibited vinculin localizing to focal adhesions. Taken together, PIPKIγ positively regulates focal adhesion dynamics and cancer invasion, most probably through PIP₂-mediated vinculin activation.

Protein 4.1R regulates cell migration and IQGAP1 recruitment to the leading edge

In red blood cells, multifunctional protein 4.1R stabilizes the spectrin-actin network and anchors it to the plasma membrane. To contribute to the characterization of functional roles of 4.1R in nonerythroid cells, we have analyzed the participation of protein 4.1R in cell migration. The distribution of endogenous 4.1R is polarized towards the leading edge of migrating cells. Exogenous 4.1R isoforms containing a complete membrane-binding domain consistently localized to plasma membrane extensions enriched in F-actin. Silencing of 4.1R caused the loss of persistence of migration in subconfluent cells and of directional migration in cells moving into a wound. Coimmunoprecipitation and pull-down assays identified the scaffold protein IQGAP1 as a partner for protein 4.1R and showed that the 4.1R membrane-binding domain is involved in binding IQGAP1. Importantly, we show that protein 4.1R is necessary for the localization of IQGAP1 to the leading edge of cells migrating into a wound, whereas IQGAP1 is not required for protein 4.1R localization. Collectively, our results indicate a crucial role for protein 4.1R in cell migration and in the recruitment of the scaffold protein IQGAP1 to the cell front.

Regulation of PIP5K activity by Arf6 and its physiological significance

The phospholipid kinase phosphatidylinositol 4-phosphate 5-kinase (PIP5K) catalyzes the phosphorylation of the membrane phospholipid phosphatidylinositol 4-phosphate to generate the pleiotropic phospholipid phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2) ]. To date, three mammalian PIP5K isozymes, α, β, and γ, and several splicing variants of the γ isozyme have been identified. These PIP5K isozymes and PIP5Kγ variants play critical roles in various cellular functions through their product PI(4,5)P(2) . The small GTPase Arf6 is one of the key activators of PIP5K. Increasing evidence suggests that PIP5K functions as a downstream effector of Arf6 to regulate a wide variety of cellular functions, such as exocytosis, endocytosis, endosomal recycling, membrane ruffle formation, immune response, and bacterial invasion. In this review, we place our focus on the recent advances in Arf6/PIP5K signaling and its linkage to cellular functions.

EZH2 regulates neuronal differentiation of mesenchymal stem cells through PIP5K1C-dependent calcium signaling

Enhancer of zeste homolog 2 (EZH2) regulates stem cells renewal, maintenance, and differentiation into different cell lineages including neuron. Changes in intracellular Ca(2+) concentration play a critical role in the differentiation of neurons. However, whether EZH2 modulates intracellular Ca(2+) signaling in regulating neuronal differentiation from human mesenchymal stem cells (hMSCs) still remains unclear. When hMSCs were treated with a Ca(2+) chelator or a PLC inhibitor to block IP(3)-mediated Ca(2+) signaling, neuronal differentiation was disrupted. EZH2 bound to the promoter region of PIP5K1C to suppress its transcription in proliferating hMSCs. Interestingly, knockdown of EZH2 enhanced the expression of PIP5K1C, which in turn increased the amount of PI(4,5)P(2), a precursor of IP(3), and resulted in increasing the intracellular Ca(2+) level, suggesting that EZH2 negatively regulates intracellular Ca(2+) through suppression of PIP5K1C. Knockdown of EZH2 also enhanced hMSCs differentiation into functional neuron both in vitro and in vivo. In contrast, knockdown of PIP5K1C significantly reduced PI(4,5)P(2) contents and intracellular Ca(2+) release in EZH2-silenced cells and resulted in the disruption of neuronal differentiation from hMSCs. Here, we provide the first evidence to demonstrate that after induction to neuronal differentiation, decreased EZH2 activates the expression of PIP5K1C to evoke intracellular Ca(2+) signaling, which leads hMSCs to differentiate into functional neuron lineage. Activation of intracellular Ca(2+) signaling by repressing or knocking down EZH2 might be a potential strategy to promote neuronal differentiation from hMSCs for application to neurological dysfunction diseases.

Comprehensive analysis of yeast surface displayed cDNA library selection outputs by exon microarray to identify novel protein-ligand interactions

Phosphatidylinositides are important signaling molecules that interact with a myriad of cellular proteins, many of which remain unidentified. We previously screened a yeast surface displayed human proteome library to identify protein fragments with affinity for the phosphatidylinositides, phosphatidylinositol-4,5-bisphosphate and phosphatidylinositol-3,4,5-trisphosphate. Much of the diversity in the screened selection outputs was represented by clones present at low frequencies, suggesting that a significant number of additional phosphatidylinositide-binding protein fragments might be present in the selection outputs. In the studies described in this report, we developed a novel cDNA library analysis method and comprehensively analyzed the polyclonal selection outputs from the phosphatidylinositol-4,5-bisphosphate and phosphatidylinositol-3,4,5-trisphosphate selections using a high-density exon microarray. In addition to the nine previously reported phosphatidylinositide-binding protein fragments, we identified 37 new phosphatidylinositide-binding candidates. Nine of 37 contain known phosphatidylinositide-binding domains, whereas the remaining 28 contain no known phosphatidylinositide-binding domain. We cloned and confirmed phosphatidylinositide binding by fluorescence-activated cell sorting for 17 of these novel candidate protein fragments. Our experiments suggest that phosphatidylinositide binding by these 17 novel protein fragments is dependent on both the inositol phosphate "headgroup" and the lipid "tail." This is in contrast with the PH domain containing fragments we tested, for which the inositol phosphate headgroup was sufficient for binding. The novel PtdIns-binding fragments come from a wide variety of proteins, including splicing factors, transcription factors, a kinase, and a polymerase. Intriguingly, 11 of the phosphatidylinositide-binding protein fragments are from nuclear proteins, including four containing homeobox domains. We found that phosphatidylinositides and double-stranded DNA oligonucleotides derived from homeobox domain target sequences compete for binding to homeobox domain-containing protein fragments, suggesting a possible mechanism for phospholipid-dependent transcriptional regulation. FACS enrichment of target-binding clones in yeast human cDNA display libraries coupled with comprehensive analysis of the selection output by DNA microarray analysis is an effective method for investigating common as well as rare protein interactions. In particular, this method is well suited for the study of small molecule/protein and drug/protein interactions.

Phosphoinositides differentially regulate bacterial uptake and Nramp1-induced resistance to Legionella infection in Dictyostelium

Membrane phosphatidylinositides recruit cytosolic proteins to regulate phagocytosis, macropinocytosis and endolysosomal vesicle maturation. Here, we describe effects of inactivation of PI3K, PTEN or PLC on Escherichia coli and Legionella pneumophila uptake by the professional phagocyte Dictyostelium discoideum. We show that L. pneumophila is engulfed by macropinocytosis, a process that is partially sensitive to PI3K inactivation, unlike phagocytosis of E. coli. Both processes are blocked by PLC inhibition. Whereas E. coli is rapidly digested, Legionella proliferates intracellularly. Proliferation is blocked by constitutively expressing Nramp1, an endolysosomal iron transporter that confers resistance against invasive bacteria. Inactivation of PI3K, but not PTEN or PLC, enhances Legionella infection and suppresses the protective effect of Nramp1 overexpression. PI3K activity is restricted to early infection and is not mediated by effects on the actin cytoskeleton; rather L. pneumophila, in contrast to E. coli, subverts phosphoinositide-sensitive fusion of Legionella-containing macropinosomes with acidic vesicles, without affecting Nramp1 recruitment. A model is presented to explain how Legionella escapes fusion with acidic vesicles and Nramp1-induced resistance to pathogens.

The FERM domain: organizing the structure and function of FAK

Focal adhesion kinase (FAK) is a scaffold and tyrosine kinase protein that binds to itself and cellular partners through its four-point-one, ezrin, radixin, moesin (FERM) domain. Recent structural work reveals that regulatory protein partners convert auto-inhibited FAK into its active state by binding to its FERM domain. Further, the identity of FAK FERM domain-interacting proteins yields clues as to how FAK coordinates diverse cellular responses, including cell adhesion, polarization, migration, survival and death, and suggests that FERM domains might mediate information transfer between the cell cortex and nucleus. Importantly, the FAK FERM domain might act as a paradigm for the actions of other FERM domain-containing proteins.

Robust patterns in the stochastic organization of filopodia

Background

Filopodia are actin-based cellular projections that have a critical role in initiating and sustaining directional migration in vertebrate cells. Filopodia are highly dynamic structures that show a rich diversity in appearance and behavior. While there are several mathematical models of filopodia initiation and growth, testing the capacity of these theoretical models in predicting empirical behavior has been hampered by a surprising shortage of quantitative data related to filopodia. Neither is it clear how quantitatively robust the cellular filopodial network is and how perturbations alter it.

Results

We have measured the length and interfilopodial separation distances of several thousand filopodia in the rodent cell line Rat2 and measured these parameters in response to genetic, chemical and physical perturbation. Our work shows that length and separation distance have a lognormal pattern distribution over their entire detection range (0.4 μm to 50 μm).

Conclusions

We find that the lognormal distribution of length and separation is robust and highly resistant to perturbation. We also find that length and separation are independent variables. Most importantly, our empirical data is not entirely in agreement with predictions made based on existing theoretical models and that filopodial size and separation are an order of magnitude larger than what existing models suggest.

Integrin-induced PIP5K1C kinase polarization regulates neutrophil polarization, directionality, and in vivo infiltration

Neutrophils are important in innate immunity and acute inflammatory responses. However, the regulation of their recruitment to sites of inflammation has not been well characterized. Here, we investigated the kinase PIP5K1C and showed that PIP5K1C deficiency impaired neutrophil recruitment because of an adhesion defect. PIP5K1C regulated the adhesion through facilitating RhoA GTPase and integrin activation by chemoattractants. Integrins could induce polarization of an isoform of PIP5K1C, PIP5K1C-90, in neutrophils through intracellular vesicle transport independently of exogenous chemoattractant. PIP5K1C-90 polarization was required for polarized RhoA activation at uropods and provided an initial directional cue for neutrophil polarization on the endothelium. Importantly, the polarization was also required for circumventing the inhibition of lamellipodium formation by RhoA so that neutrophils could form leading edges required for transendothelial migration. Because integrins are not known to regulate neutrophil polarization, our study revealed a previously underappreciated role of integrin signaling in neutrophil regulation.

Type I PIPK-alpha regulates directed cell migration by modulating Rac1 plasma membrane targeting and activation

PIPKI-α does a job other PIPKI isoforms cannot; it recruits Rac1 to the plasma membrane upon integrin activation, spatially regulating the actin-organizing GTPase during migration.

Phosphatidylinositol-4,5-bisphosphate (PI4,5P₂) is a critical regulator of cell migration, but the roles of the type I phosphatidylinositol-4-phosphate 5-kinases (PIPKIs), which synthesize PI4,5P₂, have yet to be fully defined in this process. In this study, we report that one kinase, PIPKI-α, is a novel upstream regulator of Rac1 that links activated integrins to the regulation of cell migration. We show that PIPKI-α controls integrin-induced translocation of Rac1 to the plasma membrane and thereby regulates Rac1 activation. Strikingly, this function is not shared with other PIPKI isoforms, is independent of catalytic activity, and requires physical interaction of PIPKI-α with the Rac1 polybasic domain. Consistent with its role in Rac1 activation, depletion of PIPKI-α causes pronounced defects in membrane ruffling, actin organization, and focal adhesion formation, and ultimately affects the directional persistence of migration. Thus, our study defines the role of PIPKI-α in cell migration and describes a new mechanism for the spatial regulation of Rac1 activity that is critical for cell migration.

Type I phosphatidylinositol phosphate kinase beta regulates focal adhesion disassembly by promoting beta1 integrin endocytosis

Cell migration requires the regulated disassembly of focal adhesions, but the underlying mechanisms remain poorly defined. We have previously shown that focal adhesion disassembly requires the dynamin 2- and clathrin-dependent endocytosis of ligand-activated beta1 integrins. Here, we identify type I phosphatidylinositol phosphate kinase beta (PIPKIbeta), an enzyme that generates phosphatidylinositol-4,5-bisphosphate (PI4,5P(2)), as a key regulator of this process. We found that knockdown of PIPKIbeta by RNA interference blocks the internalization of active beta1 integrins and impairs focal adhesion turnover and cell migration. These defects are caused by the failure to target the endocytic machinery, including clathrin adaptors and dynamin 2, to focal adhesion sites. As a consequence, depletion of PIPKIbeta blocks clathrin assembly at adhesion plaques and prevents complex formation between dynamin 2 and focal adhesion kinase (FAK), a critical step in focal adhesion turnover. Together, our findings identify PIPKIbeta as a novel regulator of focal adhesion disassembly and suggest that PIPKIbeta spatially regulates integrin endocytosis at adhesion sites to control cell migration.

Phosphatidylinositol 4,5-bisphosphate and PIP5-kinase Ialpha are required for invadopodia formation in human breast cancer cells

Invadopodia are ventral cell protrusions formed in invasive cancer cells. Because invadopodia have extracellular matrix (ECM) degradation activity, they are thought to function in cancer invasion. In this study, we examined the roles of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)] and PI(4,5)P(2)-producing enzymes in invadopodia formation in MDA-MB-231 human breast cancer cells. Immunofluorescence analysis showed that PI(4,5)P(2) accumulates at invadopodia on the ventral cell surface. Injection of an anti-PI(4,5)P(2) antibody inhibited invadopodia formation along with gelatin degradation activity. Sequestering of PI(4,5)P(2) by overexpression of the phospholipase C (PLC) delta1-pleckstrin homology (PH) domain, a specific probe for PI(4,5)P(2), also blocked invadopodia formation, while a mutated PLCdelta1-PH domain that lacks PI(4,5)P(2)-binding activity had no effect. Cellular PI(4,5)P(2) production is mainly mediated by type-I phosphatidylinositol 4-phosphate 5-kinase (PIP5KI) family proteins, which include PIP5KIalpha, Ibeta, and Igamma. Real-time quantitative PCR analysis showed that PIP5KIalpha is a dominant isoform expressed in MDA-MB-231 cells. Knockdown of PIP5KIalpha by small-interfering RNA (siRNA) inhibited invadopodia formation and gelatin degradation. Immunofluorescence analysis revealed that endogenous PIP5KIalpha protein localizes at invadopodia, which is corroborated by the observation that exogenously expressed green fluorescent protein (GFP)-fused PIP5KIalpha protein also accumulates at gelatin degradation sites. These results indicate that localized production of PI(4,5)P(2) by PIP5KIalpha is required for invadopodia formation and ECM degradation by human breast cancer cells.

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.

Light-induced tyrosine phosphorylation of rod outer segment membrane proteins regulate the translocation, membrane binding and activation of type II α phosphatidylinositol-5-phosphate 4-kinase

Type II phosphatidylinositol 5-phosphate 4-kinase (PIPKIIα) catalyzes the synthesis of phosphatidylinositol-4,5-bisphosphate (PI-4,5-P(2)), an essential lipid second messenger that may be involved in the regulation of phototransduction, neuroprotection, and morphogenesis in the vertebrate retina. Here we report that in rodent and transgenic frogs, the light-mediated activity and membrane binding of PIPKIIα in rod outer segments (ROS) is dependent on tyrosine phosphorylation of ROS proteins. The greater type II α PIP kinase activity in the light-adapted ROS membrane results from light-driven translocation of PIPKIIα from the rod inner segment to ROS, and subsequent binding to the ROS membrane, thus improving access of the kinase to its lipid substrates. These results indicate a novel mechanism of light regulation of the PIPKIIα activity in photoreceptors, and suggest that the greater PIPKIIα activity in light-adapted animals and the resultant accumulation of PI-4,5-P(2) within the ROS membrane may be important for the function of photoreceptor cells.

Overexpression of PIP5KL1 suppresses the growth of human cervical cancer cells in vitro and in vivo

PIP5KL1 (phosphatidylinositol-4-phosphate 5-kinase-like 1), the fourth member of PIP5Ks (phosphatidylinositol-4-phosphate 5-kinases) type I, acts as a scaffold for localization and activation of PIP5Ks, which in turn regulate numerous cellular processes. However, the role of PIP5KL1 in the development of human cancer is poorly studied. In this study, we established a stable clone of PIP5KL1 overexpressing human cervical cancer HeLa cells. RT-PCR (reverse transcription-polymerase chain reaction) and Western immunoblot analysis were performed to testify the mRNA and protein levels of PIP5KL1 in HeLa cells. The effect of PIP5KL1 overexpression on in vitro cell growth was assessed by measuring cell proliferation and migration. The athymic nude mouse model was used to examine the effects of PIP5KL1 on tumour growth in vivo. Stable transfection of PIP5KL1 induced a significant increase in expression of both mRNA and protein levels and consequent robust inhibition of proliferation (P<0.05) and migration (P<0.05) of HeLa cells. Overexpression of PIP5KL1 significantly suppressed the growth of HeLa xenograft tumours in the flanks of nude mice. Taken together, these studies indicate a functional negative correlation between elevated levels of PIP5KL1 and the development of human cervical cancer, suggesting that PIP5KL1 overexpression may suppress cervical cancer formation.

Regulation of the actin cytoskeleton-plasma membrane interplay by phosphoinositides

The plasma membrane and the underlying cortical actin cytoskeleton undergo continuous dynamic interplay that is responsible for many essential aspects of cell physiology. Polymerization of actin filaments against cellular membranes provides the force for a number of cellular processes such as migration, morphogenesis, and endocytosis. Plasma membrane phosphoinositides (especially phosphatidylinositol bis- and trisphosphates) play a central role in regulating the organization and dynamics of the actin cytoskeleton by acting as platforms for protein recruitment, by triggering signaling cascades, and by directly regulating the activities of actin-binding proteins. Furthermore, a number of actin-associated proteins, such as BAR domain proteins, are capable of directly deforming phosphoinositide-rich membranes to induce plasma membrane protrusions or invaginations. Recent studies have also provided evidence that the actin cytoskeleton-plasma membrane interactions are misregulated in a number of pathological conditions such as cancer and during pathogen invasion. Here, we summarize the wealth of knowledge on how the cortical actin cytoskeleton is regulated by phosphoinositides during various cell biological processes. We also discuss the mechanisms by which interplay between actin dynamics and certain membrane deforming proteins regulate the morphology of the plasma membrane.

Type I gamma phosphatidylinositol phosphate kinase modulates invasion and proliferation and its expression correlates with poor prognosis in breast cancer

Introduction

The loss of E-cadherin based cell-cell contacts and tumor cell migration to the vasculature and lymphatic system are hallmarks of metastasis of epithelial cancers. Type I gamma phosphatidylinositol phosphate kinase (PIPKIγ), an enzyme that generates phosphatidylinositol 4,5-bisphosphate (PI4,5P₂) a lipid messenger and precursor to many additional second messengers, was found to regulate E-cadherin cell-cell contacts and growth factor-stimulated directional cell migration, indicating that PIPKIγ regulates key steps in metastasis. Here, we assess the expression of PIPKIγ in breast cancers and have shown that expression correlated with disease progression and outcome.

Methods

Using a tissue microarray, we analyzed 438 breast carcinomas for the levels of PIPKIγ and investigated the correlation of PIPKIγ expression with patient survival via Kaplan-Meier survival analysis. Moreover, via knockdown of the expression of PIPKIγ in cultured breast cancer cells with siRNA, the roles of PIPKIγ in breast cancer migration, invasion, and proliferation were examined.

Results

Tissue microarray data shows that ~18% of the cohort immunostained showed high expression of PIPKIγ. The Kaplan-Meier survival analysis revealed a significant inverse correlation between strong PIPKIγ expression and overall patient survival. Expression of PIPKIγ correlated positively with epidermal growth factor receptor (EGFR) expression, which regulates breast cancer progression and metastasis. In cultured breast cancer cells, PIPKIγ is required for growth factor stimulated migration, invasion, and proliferation of cells.

Conclusions

The results reveal a significant correlation between PIPKIγ expression and the progression of breast cancer. This is consistent with PIPKIγ 's role in breast cancer cell migration, invasion, and proliferation.

Chemokine-induced Zap70 kinase-mediated dissociation of the Vav1-talin complex activates alpha4beta1 integrin for T cell adhesion

Lymphocyte integrins mediate cell arrest on endothelium during immune surveillance after activation by chemokine-stimulated inside-out signals. Here we show that a Vav1-talin complex in T cells is a key target for chemokine-triggered inside-out signaling leading to integrin alpha4beta1 activation. Thus, Vav1 dissociation from talin was required to generate high-affinity alpha4beta1 conformations. Assembly of the Vav1-talin complex required PtdIns(4,5)P(2), which was provided by talin-bound phosphatidylinositol phosphate kinase Igamma. Chemokine-promoted Vav1 dissociation from talin followed an initial increase in talin binding to alpha4beta1. This process was dependent on ZAP-70, which binds to and phosphorylates Vav1 in the complex, leading to further alpha4beta1 activation and cell adhesion strengthening. Moreover, Vav1-talin dissociation was needed for Rac1 activation, thus indicating that alpha4beta1 and Rac1 activation can be coupled by chemokine-stimulated ZAP-70 function. Our data suggest that Vav1 might function as a repressive adaptor of talin that must dissociate from alpha4beta1-talin complexes for efficient integrin activation.

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.

Two novel phosphatidylinositol-4-phosphate 5-kinase type Igamma splice variants expressed in human cells display distinctive cellular targeting

The generation of various phosphoinositide messenger molecules at distinct locations within the cell is mediated via the specific targeting of different isoforms and splice variants of phosphoinositide kinases. The lipid messenger PtdIns(4,5)P₂ is generated by several of these enzymes when targeted to distinct cellular compartments. Several splice variants of the type Iγ isoform of PIPK (PtdIns4P 5-kinase), which generate PtdIns(4,5)P₂, have been identified, and each splice variant is thought to serve a unique functional role within cells. Here, we have identified two novel C-terminal splice variants of PIPKIγ in human cells consisting of 700 and 707 amino acids. These two splice variants are expressed in multiple tissue types and display PIPK activity in vitro. Interestingly, both of these novel splice variants display distinct subcellular targeting. With the addition of these two new splice isoforms, there are minimally five PIPKIγ splice variants that have been identified in mammals. Therefore, we propose the use of the HUGO (Human Genome Organization) nomenclature in the naming of the splice isoforms. PIPKIγ_i4 (700 amino acids) is present in the nucleus, a targeting pattern that has not been previously observed in any PIPKIγ splice variant. PIPKIγ_i5 (707 amino acids) is targeted to intracellular vesicle-like structures, where it co-localizes with markers of several types of endosomal compartments. As occurs with other PIPKIγ splice variants, the distinctive C-terminal sequences of PIPKIγ_i4 and PIPKIγ_i5 may facilitate association with unique protein targeting factors, thereby localizing the kinases to their appropriate cellular subdomains for the site-specific generation of PtdIns(4,5)P₂.

DLC1 activation requires lipid interaction through a polybasic region preceding the RhoGAP domain

Deleted in Liver Cancer 1 (DLC1) is a GTPase-activating protein (GAP) with specificity for RhoA, RhoB, and RhoC that is frequently deleted in various tumor types. By inactivating these small GTPases, DLC1 controls actin cytoskeletal remodeling and biological processes such as cell migration and proliferation. Here we provide evidence that DLC1 binds to phosphatidylinositol-4,5-bisphosphate (PI(4,5)P(2)) through a previously unrecognized polybasic region (PBR) adjacent to its RhoGAP domain. Importantly, PI(4,5)P(2)-containing membranes are shown to stimulate DLC1 GAP activity in vitro. In living cells, a DLC1 mutant lacking an intact PBR inactivated Rho signaling less efficiently and was severely compromised in suppressing cell spreading, directed migration, and proliferation. We therefore propose that PI(4,5)P(2) is an important cofactor in DLC1 regulation in vivo and that the PBR is essential for the cellular functions of the protein.

Lowe syndrome patient fibroblasts display Ocrl1-specific cell migration defects that cannot be rescued by the homologous Inpp5b phosphatase

The Lowe syndrome (LS) is a life-threatening, developmental disease characterized by mental retardation, cataracts and renal failure. Although this human illness has been linked to defective function of the phosphatidylinositol 5-phosphatase, Ocrl1 (Oculo-Cerebro-Renal syndrome ofLowe protein1), the mechanism by which this enzyme deficiency triggers the disease is not clear. Ocrl1 is known to localize mainly to the Golgi apparatus and endosomes, however it translocates to plasma membrane ruffles upon cell stimulation with growth factors. The functional implications of this inducible translocation to the plasma membrane are presently unknown. Here we show that Ocrl1 is required for proper cell migration, spreading and fluid-phase uptake in both established cell lines and human dermal fibroblasts. We found that primary fibroblasts from two patients diagnosed with LS displayed defects in these cellular processes. Importantly, these abnormalities were suppressed by expressing wild-type Ocrl1 but not by a phosphatase-deficient mutant. Interestingly, the homologous human PI-5-phosphatase, Inpp5b, was unable to complement the Ocrl1-dependent cell migration defect. Further, Ocrl1 variants that cannot bind the endocytic adaptor AP2 or clathrin, like Inpp5b, were less apt to rescue the migration phenotype. However, no defect in membrane recruitment of AP2/clathrin or in transferrin endocytosis by patient cells was detected. Collectively, our results suggest that Ocrl1, but not Inpp5b, is involved in ruffle-mediated membrane remodeling. Our results provide new elements for understanding how Ocrl1 deficiency leads to the abnormalities associated with the LS.

FAK, PIP5KIγ and gelsolin cooperatively mediate force-induced expression of α-smooth muscle actin

During the development of pressure-induced cardiac hypertrophy, fibroblasts are activated to become myofibroblasts, which exhibit actin-cytoskeletal remodeling and express α-smooth muscle actin (SMA; encoded by ACTA2). Currently, the mechanosensing signaling pathways that regulate SMA expression are not defined. Because focal-adhesion complexes are putative mechanosensing organelles, we examined the role of focal adhesion kinase (FAK) and its interaction with gelsolin in the regulation of SMA expression. We subjected NIH3T3 cells to tensile forces (0.65 pN/μm2) by using collagen-coated magnetite beads attached to integrins. After stimulation by mechanical force, FAK and gelsolin were recruited to magnetite beads and there was increased phosphorylation of Tyr397FAK. Mechanical force enhanced SMA promoter activity by twofold; this increased activity was blocked by FAK knockdown using siRNA and by deletion of gelsolin. Force-induced nuclear translocation of MRTF-A, a transcriptional co-activator of SMA that is regulated by actin filaments, was also reduced by FAK knockdown. Phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P2], which uncaps gelsolin from actin filaments, was enriched at sites of force application. Type-I phosphatidylinositol 4-phosphate 5 kinase-γ (PIP5KIγ), which generates PtdIns(4,5)P2, associated with FAK and was required for force-mediated SMA-promoter activity and actin assembly. Catalytically inactive PIP5KIγ inhibited force-induced phosphorylation of FAK at Tyr397. These data suggest a novel pathway in which mechanosensing by FAK regulates actin assembly via gelsolin and the activity of PIP5KIγ; actin assembly in turn controls SMA expression via MRTF-A.

Excitable waves at the margin of the contact area between a cell and a substrate

In this paper, we study a new physical mechanism to generate an activator field which signals the extreme margin of the contact area between an adherent cell and the substrate. This mechanism is based on the coupling between the adhesive bridges connecting the substrate to the cytoskeleton and a cytosolic activator. Once activated by adhesion on the adhesive bridges, this activator is free to diffuse on the membrane. We propose that this activator is part of the mecano-transduction pathway which links adhesion to actin polymerization and, thus, to cellular motility. The consequences of our model are as follows: (a) the activator is localized at the rim of the contact area, (b) the adhesion is reinforced at the margin of the contact area between the cell and the substrate, (c) excitable waves of the activator can propagate along the adhesion rim.

Mutations in phosphoinositide metabolizing enzymes and human disease

Phosphoinositides are implicated in the regulation of a wide variety of cellular functions. Their importance in cellular and organismal physiology is underscored by the growing number of human diseases linked to perturbation of kinases and phosphatases that catalyze interconversion from one phosphoinositide to another. Many such enzymes are attractive targets for therapeutic interventions. Here, we review diseases linked to inheritable or somatic mutations of these enzymes.

Phosphatidic acid regulation of phosphatidylinositol 4-phosphate 5-kinases

Phosphatidic acid (PA) production by receptor-stimulated phospholipase D is believed to play an important role in the regulation of cell function. The second messenger function of PA remains to be elucidated. PA can bind and affect the activities of different enzymes and here we summarise the current status of activation of Type I phosphatidylinositol 4-phosphate 5-kinase by PA. Type 1 phosphatidylinositol 4-phosphate 5-kinase is also regulated by ARF proteins as is phospholipase D and we discuss the contributions of ARF and PA towards phosphatidylinositol(4,5)bisphosphate synthesis at the plasma membrane.

Out, in and back again: PtdIns(4,5)P(2) regulates cadherin trafficking in epithelial morphogenesis

The morphogenesis of epithelial cells in the tissue microenvironment depends on the regulation of the forces and structures that keep cells in contact with their neighbours. The formation of cell-cell contacts is integral to the establishment and maintenance of epithelial morphogenesis. In epithelial tissues, the misregulation of the signalling pathways that control epithelial polarization induces migratory and invasive cellular phenotypes. Many cellular processes influence cadherin targeting and function, including exocytosis, endocytosis and recycling. However, the localized generation of the lipid messenger PtdIns(4,5)P(2) is emerging as a fundamental signal controlling all of these processes. The PtdIns(4,5)P(2)-generating enzymes, PIPKs (phosphatidylinositol phosphate kinases) are therefore integral to these pathways. By the spatial and temporal targeting of PIPKs via the actions of its functional protein associates, PtdIns(4,5)P(2) is generated at discrete cellular locales to provide the cadherin-trafficking machinery with its required lipid messenger. In the present review, we discuss the involvement of PtdIns(4,5)P(2) and the PIPKs in the regulation of the E-cadherin (epithelial cadherin) exocytic and endocytic machinery, the modulation of actin structures at sites of adhesion, and the direction of cellular pathways which determine the fate of E-cadherin and cell-cell junctions. Recent work is also described that has defined phosphoinositide-mediated E-cadherin regulatory pathways by the use of organismal models.

HIV-1 Assembly at the Plasma Membrane: Gag Trafficking and Localization

Virus particle formation of HIV-1 is a multi-step process driven by a viral structural protein Gag. This process takes place at the plasma membrane in most cell types. However, the pathway that directs Gag to the plasma membrane has recently come under intense scrutiny because of its importance in production of progeny virions as well as virus transmission at cell-cell contacts. This review highlights recent advances in our current understanding of mechanisms that traffic and localize Gag to the plasma membrane. In addition, findings on Gag association with specific plasma membrane domains are discussed in light of potential roles in cell-to-cell transmission.

PI4P5-kinase Ialpha is required for efficient HIV-1 entry and infection of T cells

HIV-1 envelope (Env) triggers membrane fusion between the virus and the target cell. The cellular mechanism underlying this process is not well known. Phosphatidylinositol 4,5-bisphosphate (PIP(2)) is known to be important for the late steps of the HIV-1 infection cycle by promoting Gag localization to the plasma membrane during viral assembly, but it has not been implicated in early stages of HIV-1 membrane-related events. In this study, we show that binding of the initial HIV-1 Env-gp120 protein induces PIP(2) production in permissive lymphocytes through the activation of phosphatidylinositol-4-phosphate 5-kinase (PI4P5-K) Ialpha. Overexpression of wild-type PI4P5-K Ialpha increased HIV-1 Env-mediated PIP(2) production and enhanced viral replication in primary lymphocytes and CEM T cells, whereas PIP(2) production and HIV-1 infection were both severely reduced in cells overexpressing the kinase-dead mutant D227A (D/A)-PI4P5-K Ialpha. Similar results were obtained with replicative and single-cycle HIV-1 particles. HIV-1 infection was also inhibited by knockdown of endogenous expression of PI4P5-K Ialpha. These data indicate that PI4P5-K Ialpha-mediated PIP(2) production is crucial for HIV-1 entry and the early steps of infection in permissive lymphocytes.

Giant unilamellar vesicles containing phosphatidylinositol(4,5)bisphosphate: characterization and functionality

Biomimetic systems such as giant unilamellar vesicles (GUVs) are increasingly used for studying protein/lipid interactions due to their size (similar to that of cells) and to their ease of observation by light microscopy techniques. Biophysicists have begun to complexify GUVs to investigate lipid/protein interactions. In particular, composite GUVs have been designed that incorporate lipids that play important physiological roles in cellulo, such as phosphoinositides and among those the most abundant one, phosphatidylinositol(4,5)bisphosphate (PIP2). Fluorescent lipids are often used as tracers to observe GUV membranes by microscopy but they can not bring quantitative information about the insertion of unlabeled lipids. In this study, we carried out zeta-potential measurements to prove the effective incorporation of PIP2 as well as that of phosphatidylserine in the membrane of GUVs prepared by electroformation and to follow the stability of PIP2-containing GUVs. Using confocal microscopy, we found that long-chain (C16) fluorescent PIP2 analogs used as tracers (0.1% of total lipids) show a uniform distribution in the membrane whereas PIP2 antibodies show PIP2 clustering. However, the clustering effect, which is emphasized when tertiary antibodies are used in addition to secondary ones to enhance the size of the detection complex, is artifactual. We showed that divalent ions (Ca2+ and Mg2+) can induce aggregation of PIP2 in the membrane depending on their concentration. Finally, the interaction of ezrin with PIP2-containing GUVs was investigated. Using either labeled ezrin and unlabeled GUVs or both labeled ezrin and GUVs, we showed that clusters of PIP2 and proteins are formed.

Temporal profiling of changes in phosphatidylinositol 4,5-bisphosphate, inositol 1,4,5-trisphosphate and diacylglycerol allows comprehensive analysis of phospholipase C-initiated signalling in single neurons

Phosphatidylinositol 4,5-bisphosphate (PIP₂) fulfils vital signalling roles in an array of cellular processes, yet until recently it has not been possible selectively to visualize real-time changes in PIP₂ levels within living cells. Green fluorescent protein (GFP)-labelled Tubby protein (GFP-Tubby) enriches to the plasma membrane at rest and translocates to the cytosol following activation of endogenous Gα_q/11-coupled muscarinic acetylcholine receptors in both SH-SY5Y human neuroblastoma cells and primary rat hippocampal neurons. GFP-Tubby translocation is independent of changes in cytosolic inositol 1,4,5-trisphosphate and instead reports dynamic changes in levels of plasma membrane PIP₂. In contrast, enhanced GFP (eGFP)-tagged pleckstrin homology domain of phospholipase C (PLCδ1) (eGFP-PH) translocation reports increases in cytosolic inositol 1,4,5-trisphosphate. Comparison of GFP-Tubby, eGFP-PH and the eGFP-tagged C1₂ domain of protein kinase C-γ [eGFP-C1(2); to detect diacylglycerol] allowed a selective and comprehensive analysis of PLC-initiated signalling in living cells. Manipulating intracellular Ca²⁺ concentrations in the nanomolar range established that GFP-Tubby responses to a muscarinic agonist were sensitive to intracellular Ca²⁺ up to 100–200 nM in SH-SY5Y cells, demonstrating the exquisite sensitivity of agonist-mediated PLC activity within the range of physiological resting Ca²⁺ concentrations. We have also exploited GFP-Tubby selectively to visualize, for the first time, real-time changes in PIP₂ in hippocampal neurons.

Rapid turnover rate of phosphoinositides at the front of migrating MDCK cells

Phosphoinositides (PtdInss) play key roles in cell polarization and motility. With a series of biosensors based on Förster resonance energy transfer, we examined the distribution and metabolism of PtdInss and diacylglycerol (DAG) in stochastically migrating Madin-Darby canine kidney (MDCK) cells. The concentrations of phosphatidylinositol (4,5)-bisphosphate, phosphatidylinositol (3,4,5)-trisphosphate (PIP(3)), phosphatidylinositol (3,4)-bisphosphate, and DAG were higher at the plasma membrane in the front of the cell than at the plasma membrane of the rear of the cell. The difference in the concentrations of PtdInss was estimated to be less than twofold between the front and rear of the migrating MDCK cells. To decode the spatial activities of PtdIns metabolic enzymes from the obtained concentration maps of PtdInss, we developed a one-dimensional reaction diffusion model of PtdIns metabolism. In this model, the activities of phosphatidylinositol monophosphate 5-kinase, phosphatidylinositol 3-kinase, phospholipase C, and PIP(3) 5-phosphatases were higher at the plasma membrane of the front than at the plasma membrane of the rear of the cell. This result suggests that, although the difference in the steady-state level of PtdInss is less than twofold, PtdInss were more rapidly turned over at the front than the rear of the migrating MDCK cells.

Phosphatidylinositol (4,5)-bisphosphate modulates Nox5 localization via an N-terminal polybasic region

Nox5, an EF-hand-containing reactive oxygen species (ROS)-generating NADPH oxidase, contains two conserved polybasic regions: one N-terminal (PBR-N), located between the fourth EF-hand and the first transmembrane region, and one C-terminal (PBR-C), between the first and second NADPH-binding subregions. Here, we show that phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P(2)], a major phosphoinositide in plasma membrane, binds to human Nox5 causing Nox5 to localize from internal membranes to the plasma membrane. Enzymatic modulation of PtdIns(4,5)P(2) levels in intact cells altered cell surface localization of Nox5 in parallel with extracellular ROS generation. Mutations in PBR-N prevented PtdIns(4,5)P(2)-dependent localization of Nox5 to the plasma membrane and decreased extracellular ROS production. A synthetic peptide corresponding to PBR-N bound to PtdIns(4,5)P(2), but not to PtdIns, whereas mutations in the PBR-N peptide abrogated the binding to PtdIns(4,5)P(2). Arginine-197 in PBR-N was a key residue to regulate subcellular localization of Nox5 and its interaction with PtdIns(4,5)P(2). In contrast, mutation in PBR-C did not affect localization. Thus, extracellular ROS production by Nox5 is modulated by PtdIns(4,5)P(2) by localizing Nox5 to the plasma membrane.

Sequential signals toward podosome formation in NIH-src cells

Podosomes (also termed invadopodia in cancer cells) are actin-rich adhesion structures with matrix degradation activity that develop in various cell types. Despite their significant physiological importance, the molecular mechanism of podosome formation is largely unknown. In this study, we investigated the molecular mechanisms of podosome formation. The expression of various phosphoinositide-binding domains revealed that the podosomes in Src-transformed NIH3T3 (NIH-src) cells are enriched with PtdIns(3,4)P2, suggesting an important role of this phosphoinositide in podosome formation. Live-cell imaging analysis revealed that Src-expression stimulated podosome formation at focal adhesions of NIH3T3 cells after PtdIns(3,4)P2 accumulation. The adaptor protein Tks5/FISH, which is essential for podosome formation, was found to form a complex with Grb2 at adhesion sites in an Src-dependent manner. Further, it was found that N-WASP bound all SH3 domains of Tks5/FISH, which facilitated circular podosome formation. These results indicate that augmentation of the N-WASP–Arp2/3 signal was accomplished on the platform of Tks5/FISH-Grb2 complex at focal adhesions, which is stabilized by PtdIns(3,4)P2.

A voltage-sensing phosphatase, Ci-VSP, which shares sequence identity with PTEN, dephosphorylates phosphatidylinositol 4,5-bisphosphate

Phosphatidylinositol lipids play diverse physiological roles, and their concentrations are tightly regulated by various kinases and phosphatases. The enzymatic activity of Ciona intestinalis voltage sensor-containing phosphatase (Ci-VSP), recently identified as a member of the PTEN (phosphatase and tensin homolog deleted on chromosome 10) family of phosphatidylinositol phosphatases, is regulated by its own voltage-sensor domain in a voltage-dependent manner. However, a detailed mechanism of Ci-VSP regulation and its substrate specificity remain unknown. Here we determined the in vitro substrate specificity of Ci-VSP by measuring the phosphoinositide phosphatase activity of the Ci-VSP cytoplasmic phosphatase domain. Despite the high degree of identity shared between the active sites of PTEN and Ci-VSP, Ci-VSP dephosphorylates not only the PTEN substrate, phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P3], but also, unlike PTEN, phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]. Enzymatic action on PI(4,5)P2 removes the phosphate at position 5 of the inositol ring, resulting in the production of phosphatidylinositol 4-phosphate [PI(4)P]. The active site Cys-X(5)-Arg (CX(5)R) sequence of Ci-VSP differs with that of PTEN only at amino acid 365 where a glycine residue in Ci-VSP is replaced by an alanine in PTEN. Ci-VSP with a G365A mutation no longer dephosphorylates PI(4,5)P2 and is not capable of inducing depolarization-dependent rundown of a PI(4,5)P2-dependent potassium channel. These results indicate that Ci-VSP is a PI(3,4,5)P3/PI(4,5)P2 phosphatase that uniquely functions in the voltage-dependent regulation of ion channels through regulation of PI(4,5)P2 levels.

Ins and outs of ADF/cofilin activity and regulation

The actin-binding proteins of the actin-depolymerisation factor (ADF)/cofilin family were first described more than three decades ago, but research on these proteins still occupies a front role in the actin and cell migration field. Moreover, cofilin activity is implicated in the malignant, invasive properties of cancer cells. The effects of ADF/cofilins on actin dynamics are diverse and their regulation is complex. In stimulated cells, multiple signalling pathways can be initiated resulting in different activation/deactivation switches that control ADF/cofilin activity. The output of this entire regulatory system, in combination with spatial and temporal segregation of the activation mechanisms, underlies the contribution of ADF/cofilins to various cell migration/invasion phenotypes. In this framework, we describe current views on how ADF/cofilins function in migrating and invading cells.