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

The ER tether VAPA is required for proper cell motility and anchors ER-PM contact sites to focal adhesions

Cell motility processes highly depend on the membrane distribution of Phosphoinositides, giving rise to cytoskeleton reshaping and membrane trafficking events. Membrane contact sites serve as platforms for direct lipid exchange and calcium fluxes between two organelles. Here, we show that VAPA, an ER transmembrane contact site tether, plays a crucial role during cell motility. CaCo2 adenocarcinoma epithelial cells depleted for VAPA exhibit several collective and individual motility defects, disorganized actin cytoskeleton and altered protrusive activity. During migration, VAPA is required for the maintenance of PI(4)P and PI(4,5)P2 levels at the plasma membrane, but not for PI(4)P homeostasis in the Golgi and endosomal compartments. Importantly, we show that VAPA regulates the dynamics of focal adhesions (FA) through its MSP domain, is essential to stabilize and anchor ventral ER-PM contact sites to FA, and mediates microtubule-dependent FA disassembly. To conclude, our results reveal unknown functions for VAPA-mediated membrane contact sites during cell motility and provide a dynamic picture of ER-PM contact sites connection with FA mediated by VAPA.

A novel homeostatic mechanism tunes PI(4,5)P2-dependent signaling at the plasma membrane

The lipid molecule phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P2] controls all aspects of plasma membrane (PM) function in animal cells, from its selective permeability to the attachment of the cytoskeleton. Although disruption of PI(4,5)P2 is associated with a wide range of diseases, it remains unclear how cells sense and maintain PI(4,5)P2 levels to support various cell functions. Here, we show that the PIP4K family of enzymes, which synthesize PI(4,5)P2 via a minor pathway, also function as sensors of tonic PI(4,5)P2 levels. PIP4Ks are recruited to the PM by elevated PI(4,5)P2 levels, where they inhibit the major PI(4,5)P2-synthesizing PIP5Ks. Perturbation of this simple homeostatic mechanism reveals differential sensitivity of PI(4,5)P2-dependent signaling to elevated PI(4,5)P2 levels. These findings reveal that a subset of PI(4,5)P2-driven functions might drive disease associated with disrupted PI(4,5)P2 homeostasis.

The role of PIP5K1A in cancer development and progression

Malignant tumors are formed via a pathological process of uncontrolled cell division that seriously endangers human physical and mental health. The PI3K/AKT signaling pathway plays an important role in the occurrence and development of various cancers. As a lipid kinase, PIP5K1A acts on the upstream of the PI3K/AKT signaling pathway and has a variety of biological functions, including cell differentiation, cell migration, and sperm development. An increasing number of studies have shown that the overexpression of PIP5K1A promotes the growth, invasion, and migration of cancer cells, and the inhibition of PIP5K1A can effectively hinder tumor progression. These findings imply that PIP5K1A are potential markers and targets for cancers. The aim of this study was to systemically review the structure and function of PIP5K1A, the relationship between PIP5K1A and tumors and the potential therapeutic value of PIP5K1A inhibitors in cancer. PIP5K1A affects the occurrence and progression of many tumors and will provide new strategies for cancer diagnosis and treatment.

Study and detection of potential markers for predicting metastasis into lymph nodes in prostate cancer

Hormone-refractory prostate carcinoma has a different cell surface protein profile than hormone-sensitive prostate carcinoma, which provides migration ability and interactions with organs/tissues. Detection and association of these proteins with lymph node metastasis via lymphadenectomy might be beneficial for patients. Gene expression analysis in hormone-refractory and hormone-sensitive commercial cancer cell lines was performed and, after co-cultivation with osteoblasts or endothelial cells, knockdown experiments followed to validate potential biomarkers. "Myeloid-associated differentiation markers, myosin 1b and phosphatidylinositol-4-phosphate-5-kinase type 1 alpha are implicated in metastasis", their knockdown altered the expression of key regulators of endothelial-mesenchymal transition, invasion, motility and migration. In primary prostate tumors, these genes could be an indicator for future metastasis into lymph nodes.

Phosphatidylinositol-4-phosphate 5-kinase type 1α attenuates Aβ production by promoting non-amyloidogenic processing of amyloid precursor protein

Alzheimer's disease (AD) is characterized by a chronic decline in cognitive function and is pathologically typified by cerebral deposition of amyloid-β peptide (Aβ). The production of Aβ is mediated by sequential proteolysis of amyloid precursor protein (APP) by β- and γ-secretases, and has been implicated as the essential determinant of AD pathology. Previous studies have demonstrated that the level of phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] in the membrane may potentially modulate Aβ production. Given that PI(4,5)P2 is produced by type 1 phosphatidylinositol-4-phosphate 5-kinases (PIP5Ks), we sought to determine whether the level of PIP5K type Iα (PIP5K1A) can affect production of Aβ by modulating the lipid composition of the membrane. Using a HEK-derived cell line that constitutively expresses yellow fluorescent protein-tagged APP (APP-YFP), we demonstrated that overexpression of PIP5K1A results in significant enhancement of non-amyloidogenic APP processing and a concomitant suppression of the amyloidogenic pathway, leading to a marked decrease in secreted Aβ. Consistently, cells overexpressing PIP5K1A exhibited a significant redistribution of APP-YFP from endosomal compartments to the cell surface. Our findings suggest that PIP5K1A may play a critical role in governing Aβ production by modulating membrane distribution of APP, and as such, the pathway may be a valuable therapeutic target for AD.

Stochastic geometry sensing and polarization in a lipid kinase-phosphatase competitive reaction

Phosphorylation reactions, driven by competing kinases and phosphatases, are central elements of cellular signal transduction. We reconstituted a native eukaryotic lipid kinase-phosphatase reaction that drives the interconversion of phosphatidylinositol-4-phosphate [PI(4)P] and phosphatidylinositol-4,5-phosphate [PI(4,5)P2] on membrane surfaces. This system exhibited bistability and formed spatial composition patterns on supported membranes. In smaller confined regions of membrane, rapid diffusion ensures the system remains spatially homogeneous, but the final outcome-a predominantly PI(4)P or PI(4,5)P2 membrane composition-was governed by the size of the reaction environment. In larger confined regions, interplay between the reactions, diffusion, and confinement created a variety of differentially patterned states, including polarization. Experiments and kinetic modeling reveal how these geometric confinement effects arise from a mechanism based on stochastic fluctuations in the copy number of membrane-bound kinases and phosphatases. The underlying requirements for such behavior are unexpectedly simple and likely to occur in natural biological signaling systems.

Membrane Recognition and Binding by the Phosphatidylinositol Phosphate Kinase PIP5K1A: A Multiscale Simulation Study

Phosphatidylinositol phosphates (PIPs) are lipid signaling molecules that play key roles in many cellular processes. PIP5K1A kinase catalyzes phosphorylation of PI4P to form PIP₂, which in turn interacts with membrane and membrane-associated proteins. We explore the mechanism of membrane binding by the PIP5K1A kinase using a multiscale molecular dynamics approach. Coarse-grained simulations show binding of monomeric PIP5K1A to a model cell membrane containing PI4P. PIP5K1A did not bind to zwitterionic or anionic membranes lacking PIP molecules. Initial encounter of kinase and bilayer was followed by reorientation to enable productive binding to the PI4P-containing membrane. The simulations suggest that unstructured regions may be important for the preferred orientation for membrane binding. Atomistic simulations indicated that the dimeric kinase could not bind to the membrane via both active sites at the same time, suggesting a conformational change in the protein and/or bilayer distortion may be needed for dual-site binding to occur.

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PIP5K1A kinase interacts with PIP-containing membranes via its activation loop

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PIP5K1A does not bind to zwitterionic or anionic membranes lacking PIP molecules

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Initial encounter of protein and bilayer is followed by reorientation and binding

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Dimeric PIP5K1A binds with membrane contacts via only one catalytic site at a time

Interrogating the protein interactomes of RAS isoforms identifies PIP5K1A as a KRAS-specific vulnerability

In human cancers, oncogenic mutations commonly occur in the RAS genes KRAS, NRAS, or HRAS, but there are no clinical RAS inhibitors. Mutations are more prevalent in KRAS, possibly suggesting a unique oncogenic activity mediated by KRAS-specific interaction partners, which might be targeted. Here, we determine the specific protein interactomes of each RAS isoform by BirA proximity-dependent biotin identification. The combined interactomes are screened by CRISPR-Cas9 loss-of-function assays for proteins required for oncogenic KRAS-dependent, NRAS-dependent, or HRAS-dependent proliferation and censored for druggable proteins. Using this strategy, we identify phosphatidylinositol phosphate kinase PIP5K1A as a KRAS-specific interactor and show that PIP5K1A binds to a unique region in KRAS. Furthermore, PIP5K1A depletion specifically reduces oncogenic KRAS signaling and proliferation, and sensitizes pancreatic cancer cell lines to a MAPK inhibitor. These results suggest PIP5K1A as a potential target in KRAS signaling for the treatment of KRAS-mutant cancers.

RAS isoforms are frequently mutated in cancer but their inhibition remains challenging. By comparing the protein interactomes of the highly similar isoforms HRAS, NRAS and KRAS, the authors here identify PIP5K1A as a KRAS-specific interactor and a target to inhibit KRAS-driven cell growth.

Spatiotemporal Control of Lipid Conversion, Actin-Based Mechanical Forces, and Curvature Sensors during Clathrin/AP-1-Coated Vesicle Biogenesis

Clathrin/adaptor protein-1-coated carriers connect the secretory and the endocytic pathways. Carrier biogenesis relies on distinct protein networks changing membrane shape at the trans-Golgi network, each regulating coat assembly, F-actin-based mechanical forces, or the biophysical properties of lipid bilayers. How these different hubs are spatiotemporally coordinated remains largely unknown. Using in vitro reconstitution systems, quantitative proteomics, and lipidomics, as well as in vivo cell-based assays, we characterize the protein networks controlling membrane lipid composition, membrane shape, and carrier scission. These include PIP5K1A and phospholipase C-beta 3 controlling the conversion of PI[4]P into diacylglycerol. PIP5K1A binding to RAC1 provides a link to F-actin-based mechanical forces needed to tubulate membranes. Tubular membranes then recruit the BAR-domain-containing arfaptin-1/2 guiding carrier scission. These findings provide a framework for synchronizing the chemical/biophysical properties of lipid bilayers, F-actin-based mechanical forces, and the activity of proteins sensing membrane shape during clathrin/adaptor protein-1-coated carrier biogenesis.

Downregulation of exocyst Sec10 accelerates kidney tubule cell recovery through enhanced cell migration

Migration of surviving kidney tubule cells after sub-lethal injury, for example ischemia/reperfusion (I/R), plays a critical role in recovery. Exocytosis is known to be involved in cell migration, and a key component in exocytosis is the highly-conserved eight-protein exocyst complex. We investigated the expression of a central exocyst complex member, Sec10, in kidneys following I/R injury, as well as the role of Sec10 in wound healing following scratch injury of cultured Madin-Darby canine kidney (MDCK) cells. Sec10 overexpression and knockdown (KD) in MDCK cells were used to investigate the speed of wound healing and the mechanisms underlying recovery. In mice, Sec10 decreased after I/R injury, and increased during the recovery period. In cell culture, Sec10 OE inhibited ruffle formation and wound healing, while Sec10 KD accelerated it. Sec10 OE cells had higher amounts of diacylglycerol kinase (DGK) gamma at the leading edge than did control cells. A DGK inhibitor reversed the inhibition of wound healing and ruffle formation in Sec10 OE cells. Conclusively, downregulation of Sec10 following I/R injury appears to accelerate recovery of kidney tubule cells through activated ruffle formation and enhanced cell migration.

GTPases Rac1 and Ras Signaling from Endosomes

The endocytic compartment is not only the functional continuity of the plasma membrane but consists of a diverse collection of intracellular heterogeneous complex structures that transport, amplify, sustain, and/or sort signaling molecules. Over the years, it has become evident that early, late, and recycling endosomes represent an interconnected vesicular-tubular network able to form signaling platforms that dynamically and efficiently translate extracellular signals into biological outcome. Cell activation, differentiation, migration, death, and survival are some of the endpoints of endosomal signaling. Hence, to understand the role of the endosomal system in signal transduction in space and time, it is therefore necessary to dissect and identify the plethora of decoders that are operational in the different steps along the endocytic pathway. In this chapter, we focus on the regulation of spatiotemporal signaling in cells, considering endosomes as central platforms, in which several small GTPases proteins of the Ras superfamily, in particular Ras and Rac1, actively participate to control cellular processes like proliferation and cell mobility.

Characterization of Novel Molecular Mechanisms Favoring Rac1 Membrane Translocation

The Rac1 GTPase plays key roles in cytoskeletal organization, cell motility and a variety of physiological and disease-linked responses. Wild type Rac1 signaling entails dissociation of the GTPase from cytosolic Rac1-Rho GDP dissociation inhibitor (GDI) complexes, translocation to membranes, activation by exchange factors, effector binding, and activation of downstream signaling cascades. Out of those steps, membrane translocation is the less understood. Using transfections of a expression cDNA library in cells expressing a Rac1 bioreporter, we previously identified a cytoskeletal feedback loop nucleated by the F-actin binding protein coronin 1A (Coro1A) that promotes Rac1 translocation to the plasma membrane by facilitating the Pak-dependent dissociation of Rac1-Rho GDI complexes. This screening identified other potential regulators of this process, including WDR26, basigin, and TMEM8A. Here, we show that WDR26 promotes Rac1 translocation following a Coro1A-like and Coro1A-dependent mechanism. By contrast, basigin and TMEM8A stabilize Rac1 at the plasma membrane by inhibiting the internalization of caveolin-rich membrane subdomains. This latter pathway is F-actin-dependent but Coro1A-, Pak- and Rho GDI-independent.

Every day I'm rufflin': Calcium sensing and actin dynamics in the growth factor-independent membrane ruffling of professional phagocytes

Professional phagocytes continuously extend dynamic, actin-driven membrane protrusions. These protrusions, often referred to as membrane ruffles, serve a critical role in the essential phagocyte processes of macropinocytosis and phagocytosis. Small GTPases, such as RAC1/2, spatially and temporally regulate membrane ruffle formation. We have recently shown that extracellular calcium regulates the elaboration of membrane ruffles primarily through the synthesis of phosphatidic acid (PtdOH) at the plasma membrane. RAC1/2 guanine nucleotide exchange factors harbouring polybasic stretches are recruited by PtdOH to sites of ruffle formation. Here we discuss our findings and offer perspectives on how the regulation of dynamic actin structures at the plasma membrane by small GTPases is a critical component of phagocyte function.

F-actin waves, actin cortex disassembly and focal exocytosis driven by actin-phosphoinositide positive feedback

Actin polymerization is controlled by the phosphoinositide composition of the plasma membrane. However, the molecular mechanisms underlying the spatiotemporal regulation of actin network organization over extended length scales are still unclear. To observe phosphoinositide-dependent cytoskeletal dynamics we combined the model system of frustrated phagocytosis, total internal reflection microscopy and manipulation of the buffer tonicity. We found that macrophages interacting with IgG-coated glass substrates formed circular F-actin waves on their ventral surface enclosing a region of plasma membrane devoid of cortical actin. Plasma membrane free of actin cortex was strongly depleted of PI(4,5)P2 , but enriched in PI(3,4)P2 and displayed a fivefold increase in exocytosis. Wave formation could be promoted by application of a hypotonic shock. The actin waves were characteristic of a bistable wavefront at the boundary between the regions of membrane containing and lacking cortical actin. Phosphoinositide modifiers and RhoGTPase activities dramatically redistributed with respect to the wavefronts, which often exhibited spatial oscillations. Perturbation of either lipid or actin cytoskeleton-related pathways led to rapid loss of both the polarized lipid distribution and the wavefront. As waves travelled over the plasma membrane, wavefront actin was seen to rapidly polymerize and depolymerize at pre-existing clusters of FcγRIIA, coincident with rapid changes in lipid composition. Thus the potential of receptors to support rapid F-actin polymerization appears to depend acutely on the local concentrations of multiple lipid species. We propose that interdependence through positive feedback from the cytoskeleton to lipid modifiers leads to coordinated local cortex remodeling, focal exocytosis, and organizes extended actin networks.

siRNA Screen Identifies Trafficking Host Factors that Modulate Alphavirus Infection

Little is known about the repertoire of cellular factors involved in the replication of pathogenic alphaviruses. To uncover molecular regulators of alphavirus infection, and to identify candidate drug targets, we performed a high-content imaging-based siRNA screen. We revealed an actin-remodeling pathway involving Rac1, PIP5K1- α, and Arp3, as essential for infection by pathogenic alphaviruses. Infection causes cellular actin rearrangements into large bundles of actin filaments termed actin foci. Actin foci are generated late in infection concomitantly with alphavirus envelope (E2) expression and are dependent on the activities of Rac1 and Arp3. E2 associates with actin in alphavirus-infected cells and co-localizes with Rac1-PIP5K1-α along actin filaments in the context of actin foci. Finally, Rac1, Arp3, and actin polymerization inhibitors interfere with E2 trafficking from the trans-Golgi network to the cell surface, suggesting a plausible model in which transport of E2 to the cell surface is mediated via Rac1- and Arp3-dependent actin remodeling.

Resolution of structure of PIP5K1A reveals molecular mechanism for its regulation by dimerization and dishevelled

Type I phosphatidylinositol phosphate kinase (PIP5K1) phosphorylates the head group of phosphatidylinositol 4-phosphate (PtdIns4P) to generate PtdIns4,5P₂, which plays important roles in a wide range of cellular functions including Wnt signalling. However, the lack of its structural information has hindered the understanding of its regulation. Here we report the crystal structure of the catalytic domain of zebrafish PIP5K1A at 3.3 Å resolution. This molecule forms a side-to-side dimer. Mutagenesis study of PIP5K1A reveals two adjacent interfaces for the dimerization and interaction with the DIX domain of the Wnt signalling molecule dishevelled. Although these interfaces are located distally to the catalytic/substrate-binding site, binding to these interfaces either through dimerization or the interaction with DIX stimulates PIP5K1 catalytic activity. DIX binding additionally enhances PIP5K1 substrate binding. Thus, this study elucidates regulatory mechanisms for this lipid kinase and provides a paradigm for the understanding of PIP5K1 regulation by their interacting molecules.

Type I phosphatidylinositol phosphate kinase is an important component of many cellular pathways, including Wnt signalling. Here the authors report the crystal structure of the zebrafish protein along with in vitro assays that help to elucidate the regulation and function of this kinase.

PIP kinases define PI4,5P₂signaling specificity by association with effectors

Phosphatidylinositol 4,5-bisphosphate (PI4,5P₂) is an essential lipid messenger with roles in all eukaryotes and most aspects of human physiology. By controlling the targeting and activity of its effectors, PI4,5P₂modulates processes, such as cell migration, vesicular trafficking, cellular morphogenesis, signaling and gene expression. In cells, PI4,5P₂has a much higher concentration than other phosphoinositide species and its total content is largely unchanged in response to extracellular stimuli. The discovery of a vast array of PI4,5P₂ binding proteins is consistent with data showing that the majority of cellular PI4,5P₂is sequestered. This supports a mechanism where PI4,5P₂functions as a localized and highly specific messenger. Further support of this mechanism comes from the de novo synthesis of PI4,5P₂which is often linked with PIP kinase interaction with PI4,5P₂effectors and is a mechanism to define specificity of PI4,5P₂signaling. The association of PI4,5P₂-generating enzymes with PI4,5P₂effectors regulate effector function both temporally and spatially in cells. In this review, the PI4,5P₂effectors whose functions are tightly regulated by associations with PI4,5P₂-generating enzymes will be discussed. This article is part of a Special Issue entitled Phosphoinositides.

Structural and mechanical functions of integrins

Integrins are ubiquitously expressed cell surface receptors that play a critical role in regulating the interaction between a cell and its microenvironment to control cell fate. These molecules are regulated either via their expression on the cell surface or through a unique bidirectional signalling mechanism. However, integrins are just the tip of the adhesome iceberg, initiating the assembly of a large range of adaptor and signalling proteins that mediate the structural and signalling functions of integrin. In this review, we summarise the structure of integrins and mechanisms by which integrin activation is controlled. The different adhesion structures formed by integrins are discussed, as well as the mechanical and structural roles integrins play during cell migration. As the function of integrin signalling can be quite varied based on cell type and context, an in depth understanding of these processes will aid our understanding of aberrant adhesion and migration, which is often associated with human pathologies such as cancer.

Spatial and temporal control of Rho GTPase functions

Rho family GTPases control almost every aspect of cell physiology and, since their discovery, a wealth of knowledge has accumulated about their biochemical regulation and function. However, each Rho GTPase distributes between multiple cellular compartments, even within the same cell, where they are controlled by multiple regulators and signal to multiple effectors. Thus, major questions about spatial and temporal aspects of regulation remain unanswered. In particular, what are the nano-scale dynamics for their activation, membrane targeting, diffusion, effector activation and GTPase inactivation? How do these mechanisms differ in the different cellular compartments where Rho GTPases function? Addressing these complex aspects of Rho GTPase biology will significantly advance our understanding of the spatial and temporal control of cellular functions.

IQGAP1 is a novel phosphatidylinositol 4,5 bisphosphate effector in regulation of directional cell migration

Phosphatidylinositol 4,5 bisphosphate (PIP₂) is a key lipid messenger for regulation of cell migration. PIP₂ modulates many effectors, but the specificity of PIP₂ signalling can be defined by interactions of PIP₂-generating enzymes with PIP₂ effectors. Here, we show that type Iγ phosphatidylinositol 4-phosphate 5-kinase (PIPKIγ) interacts with the cytoskeleton regulator, IQGAP1, and modulates IQGAP1 function in migration. We reveal that PIPKIγ is required for IQGAP1 recruitment to the leading edge membrane in response to integrin or growth factor receptor activation. Moreover, IQGAP1 is a PIP₂ effector that directly binds PIP₂ through a polybasic motif and PIP₂ binding activates IQGAP1, facilitating actin polymerization. IQGAP1 mutants that lack PIPKIγ or PIP₂ binding lose the ability to control directional cell migration. Collectively, these data reveal a synergy between PIPKIγ and IQGAP1 in the control of cell migration.

mRNA encoding WAVE-Arp2/3-associated proteins is co-localized with foci of active protein synthesis at the leading edge of MRC5 fibroblasts during cell migration

During cell spreading, mammalian cells migrate using lamellipodia formed from a large dense branched actin network which produces the protrusive force required for leading edge advancement. The formation of lamellipodia is a dynamic process and is dependent on a variety of protein cofactors that mediate their local regulation, structural characteristics and dynamics. In the present study, we show that mRNAs encoding some structural and regulatory components of the WAVE [WASP (Wiskott-Aldrich syndrome protein) verprolin homologous] complex are localized to the leading edge of the cell and associated with sites of active translation. Furthermore, we demonstrate that steady-state levels of ArpC2 and Rac1 proteins increase at the leading edge during cell spreading, suggesting that localized protein synthesis has a pivotal role in controlling cell spreading and migration.

Phosphatidylinositol 4,5-bisphosphate: targeted production and signaling

Phosphatidylinositol 4,5-bisphosphate (PI4,5P(2)) is a key lipid signaling molecule that regulates a vast array of biological activities. PI4,5P(2) can act directly as a messenger or can be utilized as a precursor to generate other messengers: inositol trisphosphate, diacylglycerol, or phosphatidylinositol 3,4,5-trisphosphate. PI4,5P(2) interacts with hundreds of different effector proteins. The enormous diversity of PI4,5P(2) effector proteins and the spatio-temporal control of PI4,5P(2) generation allow PI4,5P(2) signaling to control a broad spectrum of cellular functions. PI4,5P(2) is synthesized by phosphatidylinositol phosphate kinases (PIPKs). The array of PIPKs in cells enables their targeting to specific subcellular compartments through interactions with targeting factors that are often PI4,5P(2) effectors. These interactions are a mechanism to define spatial and temporal PI4,5P(2) synthesis and the specificity of PI4,5P(2) signaling. In turn, the regulation of PI4,5P(2) effectors at specific cellular compartments has implications for understanding how PI4,5P(2) controls cellular processes and its role in diseases.

ErbB/integrin signaling interactions in regulation of myocardial cell-cell and cell-matrix interactions

Neuregulin (Nrg)/ErbB and integrin signaling pathways are critical for the normal function of the embryonic and adult heart. Both systems activate several downstream signaling pathways, with different physiological outputs: cell survival, fibrosis, excitation-contraction coupling, myofilament structure, cell-cell and cell-matrix interaction. Activation of ErbB2 by Nrg1β in cardiomycytes or its overexpression in cancer cells induces phosphorylation of FAK (Focal Adhesion Kinase) at specific sites with modulation of survival, invasion and cell-cell contacts. FAK is also a critical mediator of integrin receptors, converting extracellular matrix alterations into intracellular signaling. Systemic FAK deletion is lethal and is associated with left ventricular non-compaction whereas cardiac restriction in adult hearts is well tolerated. Nevertheless, these hearts are more susceptible to stress conditions like trans-aortic constriction, hypertrophy, and ischemic injury. As FAK is both downstream and specifically activated by integrins and Nrg-1β, here we will explore the role of FAK in the heart as a protective factor and as possible mediator of the crosstalk between the ErbB and Integrin receptors. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Cardiac Pathways of Differentiation, Metabolism and Contraction.

Arsenic trioxide induces abnormal mitotic spindles through a PIP4KIIγ/Rho pathway

Arsenite-induced spindle abnormalities result in mitotic cell apoptosis in several cancer cell lines, but how arsenite induces these effects is not known. Evidence to date has revealed that arsenite activates Rho guanosine triphosphatases (GTPases). Because Rho GTPases regulate spindle orientation, chromosome congression, and cytokinesis, we therefore examined the involvement of Rho GTPases and their modulators in arsenite-induced mitotic abnormalities. We demonstrated that arsenic trioxide (ATO) disrupted the positioning of bipolar mitotic spindles and induced centrosome and spindle abnormalities. ATO increased the level of the active guanosine triphosphate-bound form of Rho. Inhibition of Rho-associated protein kinases (ROCKs) by Y-27632 ameliorated ATO-induced spindle defects, mitotic arrest, and cell death. These results indicate that ATO may induce spindle abnormalities and mitotic cell death through a Rho/ROCK pathway. In addition, screening of a human kinase and phosphatase shRNA library to select genes that mediate ATO induction of spindle abnormalities resulted in the identification of phosphatidylinositol-5-phosphate 4-kinase type-2 gamma (PIP4KIIγ), a phosphatidylinositol 4,5-biphosphate (PIP2) synthesis enzyme that belongs to the phosphatidylinositol phosphate kinase (PIPK) family. Sequestration of PIP2 by ectopic overexpression of the pleckstrin homology domain of phospholipase C-δ1 protected cells from ATO-induced cell death. Furthermore, depletion of PIP4KIIγ, but not other isoforms of the PIPK family, not only reduced Rho GTPase activation in ATO-treated cells but also alleviated ATO-induced spindle defects, mitotic arrest, and mitotic cell apoptosis. Thus, our results imply that ATO induces abnormalities in mitotic spindles through a PIP4KIIγ/Rho pathway, leading to apoptosis of mitotic cells.

Integrin adhesion and force coupling are independently regulated by localized PtdIns(4,5)2 synthesis

The 90-kDa isoform of the lipid kinase PIP kinase Type I γ (PIPKIγ) localizes to focal adhesions (FAs), where it provides a local source of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)). Although PtdIns(4,5)P(2) regulates the function of several FA-associated molecules, the role of the FA-specific pool of PtdIns(4,5)P(2) is not known. We report that the genetic ablation of PIPKIγ specifically from FAs results in defective integrin-mediated adhesion and force coupling. Adhesion defects in cells deficient in FAPtdIns(4,5)P(2) synthesis are corrected within minutes while integrin-actin force coupling remains defective over a longer period. Talin and vinculin, but not kindlin, are less efficiently recruited to new adhesions in these cells. These data demonstrate that the specific depletion of PtdIns(4,5)P(2) from FAs temporally separates integrin-ligand binding from integrin-actin force coupling by regulating talin and vinculin recruitment. Furthermore, it suggests that force coupling relies heavily on locally generated PtdIns(4,5)P(2) rather than bulk membrane PtdIns(4,5)P(2).

Phosphatidylinositol 4, 5 bisphosphate and the actin cytoskeleton

Dynamic changes in PM PIP(2) have been implicated in the regulation of many processes that are dependent on actin polymerization and remodeling. PIP(2) is synthesized primarily by the type I phosphatidylinositol 4 phosphate 5 kinases (PIP5Ks), and there are three major isoforms, called a, b and g. There is emerging evidence that these PIP5Ks have unique as well as overlapping functions. This review will focus on the isoform-specific roles of individual PIP5K as they relate to the regulation of the actin cytoskeleton. We will review recent advances that establish PIP(2) as a critical regulator of actin polymerization and cytoskeleton/membrane linkages, and show how binding of cytoskeletal proteins to membrane PIP(2) might alter lateral or transverse movement of lipids to affect raft formation or lipid asymmetry. The mechanisms for specifying localized increase in PIP(2) to regulate dynamic actin remodeling will also be discussed.

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.

Structure and function of focal adhesions

Integrin-dependent cell adhesions come in different shapes and serve in different cell types for tasks ranging from cell-adhesion, migration, and the remodeling of the extracellular matrix to the formation and stabilization of immunological and chemical synapses. A major challenge consists in the identification of adhesion-specific as well as common regulatory mechanisms, motivating the need for a deeper analysis of protein-protein interactions in the context of intact focal adhesions. Specifically, it is critical to understand how small differences in binding of integrins to extracellular ligands and/or cytoplasmic adapter proteins affect the assembly and function of an entire focal adhesion. By using the talin-integrin pair as a starting point, I would like to discuss how specific protein-protein and protein-lipid interactions can control the behavior and function of focal adhesions. By responding to chemical and mechanical cues several allosterically regulated proteins create a dynamic multifunctional protein network that provides both adhesion to the extracellular matrix as well as intracellular signaling in response to mechanical changes in the cellular environment.

Phosphatidylinositol (4,5) bisphosphate controls T cell activation by regulating T cell rigidity and organization

Here we investigate the role of Phosphatidylinositol (4,5) bisphosphate (PIP(2)) in the physiological activation of primary murine T cells by antigen presenting cells (APC) by addressing two principal challenges in PIP(2) biology. First, PIP(2) is a regulator of cytoskeletal dynamics and a substrate for second messenger generation. The relative importance of these two processes needs to be determined. Second, PIP(2) is turned over by multiple biosynthetic and metabolizing enzymes. The joint effect of these enzymes on PIP(2) distributions needs to be determined with resolution in time and space. We found that T cells express four isoforms of the principal PIP(2)-generating enzyme phosphatidylinositol 4-phosphate 5-kinase (PIP5K) with distinct spatial and temporal characteristics. In the context of a larger systems analysis of T cell signaling, these data identify the T cell/APC interface and the T cell distal pole as sites of differential PIP(2) turnover. Overexpression of different PIP5K isoforms, as corroborated by knock down and PIP(2) blockade, yielded an increase in PIP(2) levels combined with isoform-specific changes in the spatiotemporal distributions of accessible PIP(2). It rigidified the T cell, likely by impairing the inactivation of Ezrin Moesin Radixin, delayed and diminished the clustering of the T cell receptor at the cellular interface, reduced the efficiency of T cell proximal signaling and IL-2 secretion. These effects were consistently more severe for distal PIP5K isoforms. Thus spatially constrained cytoskeletal roles of PIP(2) in the control of T cell rigidity and spatiotemporal organization dominate the effects of PIP(2) on T cell activation.

Molecular architecture and function of matrix adhesions

Cell adhesions mediate important bidirectional interactions between cells and the extracellular matrix. They provide an interactive interface between the extracellular chemical and physical environment and the cellular scaffolding and signaling machinery. This dynamic, reciprocal regulation of intracellular processes and the matrix is mediated by membrane receptors such as the integrins, as well as many other components that comprise the adhesome. Adhesome constituents assemble themselves into different types of cell adhesion structures that vary in molecular complexity and change over time. These cell adhesions play crucial roles in cell migration, proliferation, and determination of cell fate.

MicroRNA-142-3p, a new regulator of RAC1, suppresses the migration and invasion of hepatocellular carcinoma cells

RAC1 regulates a diverse array of cellular events including migration and invasion. MicroRNAs (miRNAs) have a key role in the regulation of gene expression. In this study, we demonstrated that microRNA-142-3p (miR-142-3p) acted as a negative regulator of human RAC1. Overexpression of miR-142-3p decreased RAC1 mRNA and protein levels. Moreover, the overexpression of miR-142-3p suppressed, while blocking of miR-142-3p increased colony formation, migration and invasion in hepatocellular carcinoma (HCC) cell lines (QGY-7703 and SMMC-7721). RAC1 overexpression without the 3'untranslated region abolished the effect of miR-142-3p in the QGY-7703 and SMMC-7721 cells. These results demonstrated that miR-142-3p directly and negatively regulates RAC1 in HCC cells, which highlights the importance of miRNAs in tumorigenesis.

Integrins: versatile receptors controlling melanocyte adhesion, migration and proliferation

From the onset of melanocyte specification from the neural crest, throughout their migration during embryogenesis and until they reside in their niche in the basal keratinocyte layer, melanocytes interact in dynamic ways with the extracellular environment of the growing embryo. To recognize and to adhere to their environment, melanocytes depend on heterodimeric cell surface receptors of the family of integrins. In addition to the control of adhesive interactions between melanocytes and the extracellular matrix scaffold secreted by fibroblasts and keratinocytes, the integrin receptors allow cells also to sense the mechanical condition of the extracellular environment, responding by intracellular signaling, triggering cell survival, proliferation or migration events. In this review, we summarize the recently emerged concepts that explain integrin-dependent adhesion and how this adhesion system interfaces with integrin-dependent signaling events. The gained information will help to understand melanocyte behavior in pathological situations such as melanoma growth and metastasis formation.