PI(4,5)P(2) regulation of surface membrane traffic

PIP2 signaling in lipid domains: a critical re-evaluation

Microdomains such as rafts are considered as scaffolds for phosphatidylinositol (4,5) bisphosphate (PIP2) signaling, enabling PIP2 to selectively regulate different processes in the cell. Enrichment of PIP2 in microdomains was based on cholesterol-depletion and detergent-extraction studies. Here we show that two distinct phospholipase C-coupled receptors (those for neurokinin A and endothelin) share the same, homogeneously distributed PIP2 pool at the plasma membrane, even though the neurokinin A receptor is localized to microdomains and is cholesterol dependent in its PIP2 signaling whereas the endothelin receptor is not. Our experiments further indicate that detergent treatment causes PIP2 clustering and that cholesterol depletion interferes with basal, ligand-independent recycling of the neurokinin A receptor, thereby providing alternative explanations for the enrichment of PIP2 in detergent-insoluble membrane fractions and for the cholesterol dependency of PIP2 breakdown, respectively.

Phosphatidylinositol phosphate kinase type I gamma regulates dynamics of large dense-core vesicle fusion

Phosphatidylinositol-4,5-bisphosphate was proposed to be an important regulator of large dense-core vesicle exocytosis from neuroendocrine tissues. Here, we have examined the kinetics of secretion in chromaffin cells from mice lacking phosphatidylinositol phosphate kinase type I gamma, the major neuronal phosphatidylinositol-4-phosphate 5-kinase. Absence of this enzyme caused a reduction of the readily releasable vesicle pool and its refilling rate, with a small increase in morphologically docked vesicles, indicating a defect in vesicle priming. Furthermore, amperometry revealed a delay in fusion pore expansion. These results provide direct genetic evidence for a key role of phosphatidylinositol-4,5-bisphosphate synthesis in the regulation of large dense-core vesicle fusion dynamics.

PI(4,5)P2-dependent microdomain assemblies capture microtubules to promote and control leading edge motility

The lipid second messenger PI(4,5)P₂ modulates actin dynamics, and its local accumulation at plasmalemmal microdomains (rafts) might mediate regulation of protrusive motility. However, how PI(4,5)P₂-rich rafts regulate surface motility is not well understood. Here, we show that upon signals promoting cell surface motility, PI(4,5)P₂ directs the assembly of dynamic raft-rich plasmalemmal patches, which promote and sustain protrusive motility. The accumulation of PI(4,5)P₂ at rafts, together with Cdc42, promotes patch assembly through N-WASP. The patches exhibit locally regulated PI(4,5)P₂ turnover and reduced diffusion-mediated exchange with their environment. Patches capture microtubules (MTs) through patch IQGAP1, to stabilize MTs at the leading edge. Captured MTs in turn deliver PKA to patches to promote patch clustering through further PI(4,5)P₂ accumulation in response to cAMP. Patch clustering restricts, spatially confines, and polarizes protrusive motility. Thus, PI(4,5)P₂-dependent raft-rich patches enhance local signaling for motility, and their assembly into clusters is regulated through captured MTs and PKA, coupling local regulation of motility to cell polarity, and organization.

The Activation of Exocytotic Sites by the Formation of Phosphatidylinositol 4,5-Bisphosphate Microdomains at Syntaxin Clusters

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) is a minor component of the lipid bilayer but plays an important role in various cellular functions, including exocytosis and endocytosis. Recently, PI(4,5)P2 was shown to form microdomains in the plasma membrane. In this study, we investigated the relationship between the spatial organization of PI(4,5)P2 microdomains and exocytotic machineries in clonal rat pheochromocytoma PC12 cells. Both PI(4,5)P2 and syntaxin, a soluble N-ethylmaleimide-sensitive factor attachment protein receptor protein essential for exocytosis, exhibited punctate clusters in isolated plasma membranes. The number of PI(4,5)P2 microdomains colocalizing with syntaxin clusters and large dense core vesicles (LDCVs) was decreased after catecholamine release. Alternatively, the expression of type I phosphatidylinositol-4-phosphate 5-kinase (PIP5KI) increased the number of PI(4,5)P2 microdomains at syntaxin clusters with docked LDCVs and enhanced exocytotic activity, possibly by increasing the number of release sites. About half of the PI(4,5)P2 microdomains were not colocalized with Thy-1, a specific marker of lipid rafts, and the colocalization of transfected PIP5KI with syntaxin clusters was observed. These results suggest that the formation of PI(4,5)P2 microdomains at syntaxin clusters with docked LDCVs is essential for Ca2+-dependent exocytosis.

The pleckstrin homology domain proteins Slm1 and Slm2 are required for actin cytoskeleton organization in yeast and bind phosphatidylinositol-4,5-bisphosphate and TORC2

Phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P(2)] is a key second messenger that regulates actin and membrane dynamics, as well as other cellular processes. Many of the effects of PtdIns(4,5)P(2) are mediated by binding to effector proteins that contain a pleckstrin homology (PH) domain. Here, we identify two novel effectors of PtdIns(4,5)P(2) in the budding yeast Saccharomyces cerevisiae: the PH domain containing protein Slm1 and its homolog Slm2. Slm1 and Slm2 serve redundant roles essential for cell growth and actin cytoskeleton polarization. Slm1 and Slm2 bind PtdIns(4,5)P(2) through their PH domains. In addition, Slm1 and Slm2 physically interact with Avo2 and Bit61, two components of the TORC2 signaling complex, which mediates Tor2 signaling to the actin cytoskeleton. Together, these interactions coordinately regulate Slm1 targeting to the plasma membrane. Our results thus identify two novel effectors of PtdIns(4,5)P(2) regulating cell growth and actin organization and suggest that Slm1 and Slm2 integrate inputs from the PtdIns(4,5)P(2) and TORC2 to modulate polarized actin assembly and growth.

Lipid regulation of the synaptic vesicle cycle

Membrane vesicle cycling is orchestrated through the combined actions of proteins and lipids. At neuronal synapses, this orchestration must meet the stringent demands of speed, fidelity and sustainability of the synaptic vesicle cycle that mediates neurotransmission. Historically, the lion's share of the attention has been focused on the proteins that are involved in this cycle; but, in recent years, it has become clear that the previously unheralded plasma membrane and vesicle lipids are also key regulators of this cycle. This article reviews recent insights into the roles of lipid-modifying enzymes and lipids in the acute modulation of neurotransmission.

A Polybasic Motif Allows N-WASP to Act as a Sensor of PIP2 Density

Phosphatidylinositol 4,5-bisphosphate (PIP(2)) activates the actin regulatory protein N-WASP by binding to a short polybasic region involved in N-WASP autoinhibition. Here, we show that unlike canonical lipid binding modules, such as PH domains, this polybasic motif binds PIP(2) in a multivalent, cooperative manner. As a result, PIP(2) activation of N-WASP-mediated actin polymerization in vitro and in extracts is ultrasensitive: above a certain threshold, N-WASP responds in a switch-like manner to a small increase in the density of PIP(2) (Hill coefficient n(H) = approximately 20). We show that the sharpness of the PIP(2) activation threshold can be tuned by varying the length of the polybasic motif. This sharp activation threshold may help suppress N-WASP activation by quiescent PIP(2) levels yet leave it poised for activation upon subtle, signaling-induced perturbations in PIP(2) distribution.

Effects of thyroxine and 1-methyl, 2-mercaptoimidazol on phosphoinositides synthesis in rat liver

BACKGROUND

Phosphoinositides mediate one of the intracellular signal transduction pathways and produce a class of second messengers that are involved in the action of hormones and neurotransmitters on target cells. Thyroid hormones are well known regulators of lipid metabolism and modulators of signal transduction in cells. However, little is known about phosphoinositides cycle regulation by thyroid hormones. The present paper deals with phosphoinositides synthesis de novo and acylation in liver at different thyroid status of rats.

RESULTS

The experiments were performed in either the rat liver or hepatocytes of 90- and 720-day-old rats. Myo-[3H]inositol, [14C]CH3COONa, [14C]oleic and [3H]arachidonic acids were used to investigate the phosphatidylinositol (PtdIns), phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate (PtdInsP2) synthesis. 1-methyl, 2-mercaptoimidazol-induced hypothyroidism was associated with the decrease of myo-[3H]inositol and [3H]arachidonic acids incorporation into liver phosphoinositides and total phospholipids, respectively. The thyroxine (L-T4) injection to hypothyroid animals increased the hormones contents in blood serum and PtdInsP2 synthesis de novo as well as [3H]arachidonic acids incorporation into the PtdIns and PtdInsP2. Under the hormone action, the [14C]oleic acid incorporation into PtdIns reduced in the liver of hypothyroid animals. A single injection of L-T4 to the euthyroid [14C]CH3COONa-pre-treated animals or addition of the hormone to a culture medium of hepatocytes was accompanied by the rapid prominent increase in the levels of the newly synthesized PtdIns and PtdInsP2 and in the mass of phosphatidic acid in the liver or the cells.

CONCLUSIONS

The data obtained have demonstrated that thyroid hormones are of vital importance in the regulation of arachidonate-containing phosphoinositides metabolism in the liver. The drug-induced malfunction of thyroid gland noticeably changed the phosphoinositides synthesis de novo. The L-T4 injection to the animals was followed by the time-dependent increase of polyphosphoinositide synthesis in the liver. The both long-term and short-term hormone effects on the newly synthesized PtdInsP2 have been determined.

Interaction of Sla2p's ANTH domain with PtdIns(4,5)P2 is important for actin-dependent endocytic internalization

A variety of studies have implicated the lipid PtdIns(4,5)P2 in endocytic internalization, but how this lipid mediates its effects is not known. The AP180 N-terminal homology (ANTH) domain is a PtdIns(4,5)P2-binding module found in several proteins that participate in receptor-mediated endocytosis. One such protein is yeast Sla2p, a highly conserved actin-binding protein essential for actin organization and endocytic internalization. To better understand how PtdIns(4,5)P2 binding regulates actin-dependent endocytosis, we investigated the functions of Sla2p's ANTH domain. A liposome-binding assay revealed that Sla2p binds to PtdIns(4,5)P2 specifically through its ANTH domain and identified specific lysine residues required for this interaction. Mutants of Sla2p deficient in PtdIns(4,5)P2 binding showed significant defects in cell growth, actin organization, and endocytic internalization. These defects could be rescued by increasing PtdIns(4,5)P2 levels in vivo. Strikingly, mutant Sla2p defective in PtdIns(4,5)P2 binding localized with the endocytic machinery at the cell cortex, establishing that the ANTH-PtdIns(4,5)P2 interaction is not necessary for this association. In contrast, multicolor real-time fluorescence microscopy and particle-tracking analysis demonstrated that PtdIns(4,5)P2 binding is required during endocytic internalization. These results demonstrate that the interaction of Sla2p's ANTH domain with PtdIns(4,5)P2 plays a key role in regulation of the dynamics of actin-dependent endocytic internalization.

The MARCKS family of phospholipid binding proteins: regulation of phospholipase D and other cellular components

Myristoylated alanine-rich C kinase substrate (MARCKS) and MARCKS-related protein (MRP) are essential proteins that are implicated in coordination of membrane-cytoskeletal signalling events, such as cell adhesion, migration, secretion, and phagocytosis in a variety of cell types. The most prominent structural feature of MARCKS and MRP is a central basic effector domain (ED) that binds F-actin, Ca2+-calmodulin, and acidic phospholipids; phosphorylation of key serine residues within the ED by protein kinase C (PKC) prevents the above interactions. While the precise roles of MARCKS and MRP have not been established, recent attention has focussed on the high affinity of the MARCKS ED for phosphatidylinositol 4,5-bisphosphate (PIP2), and a model has emerged in which calmodulin- or PKC-mediated regulation of these proteins at specific membrane sites could in turn control spatial availability of PIP2. The present review summarizes recent progress in this area and discusses how the above model might explain a role for MARCKS and MRP in activation of phospholipase D and other PIP2-dependent cellular processes.

Protein–lipid interactions in membrane trafficking at the Golgi complex

The integrated interplay between proteins and lipids drives many key cellular processes, such as signal transduction, cytoskeleton remodelling and membrane trafficking. The last of these, membrane trafficking, has the Golgi complex as its central station. Not only does this organelle orchestrates the biosynthesis, transport and intracellular distribution of many proteins and lipids, but also its own function and structure is dictated by intimate functional and physical relationships between protein-based and lipid-based machineries. These machineries are involved in the control of the fundamental events that govern membrane traffic, such as in the budding, fission and fusion of transport intermediates, in the regulation of the shape and geometry of the Golgi membranes themselves, and, finally, in the generation of "signals" that can have local actions in the secretory system, or that may affect other cellular systems. Lipid-protein interactions rely on the abilities of certain protein domains to recognize specific lipids. These interactions are mediated, in particular, through the headgroups of the phospholipids, although a few of these protein domains are able to specifically interact with the phospholipid acyl chains. Recent evidence also indicates that some proteins and/or protein domains are more sensitive to the physical environment of the membrane bilayer (such as its curvature) than to its chemical composition.

Phosphatidylinositol-4,5-bisphosphate-rich plasma membrane patches organize active zones of endocytosis and ruffling in cultured adipocytes

A major regulator of endocytosis and cortical F-actin is thought to be phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P2] present in plasma membranes. Here we report that in 3T3-L1 adipocytes, clathrin-coated membrane retrieval and dense concentrations of polymerized actin occur in restricted zones of high endocytic activity. Ultrafast-acquisition and superresolution deconvolution microscopy of cultured adipocytes expressing an enhanced green fluorescent protein- or enhanced cyan fluorescent protein (ECFP)-tagged phospholipase Cdelta1 (PLCdelta1) pleckstrin homology (PH) domain reveals that these zones spatially coincide with large-scale PtdIns(4,5)P2-rich plasma membrane patches (PRMPs). PRMPs exhibit lateral dimensions exceeding several micrometers, are relatively stationary, and display extensive local membrane folding that concentrates PtdIns(4,5)P2 in three-dimensional space. In addition, a higher concentration of PtdIns(4,5)P2 in the membranes of PRMPs than in other regions of the plasma membrane can be detected by quantitative fluorescence microscopy. Vesicular structures containing both clathrin heavy chains and PtdIns(4,5)P2 are revealed immediately beneath PRMPs, as is dense F actin. Blockade of PtdIns(4,5)P2 function in PRMPs by high expression of the ECFP-tagged PLCdelta1 PH domain inhibits transferrin endocytosis and reduces the abundance of cortical F-actin. Membrane ruffles induced by the expression of unconventional myosin 1c were also found to localize at PRMPs. These results are consistent with the hypothesis that PRMPs organize active PtdIns(4,5)P2 signaling zones in the adipocyte plasma membrane that in turn control regulators of endocytosis, actin dynamics, and membrane ruffling.

Phosphatidylinositol (4,5) bisphosphate regulates HIV-1 Gag targeting to the plasma membrane

A critical early event in the HIV type 1 (HIV-1) particle assembly pathway is the targeting of the Gag protein to the site of virus assembly. In many cell types, assembly takes place predominantly at the plasma membrane. Cellular factors that regulate Gag targeting remain undefined. The phosphoinositide phosphatidylinositol (4,5) bisphosphate [PI(4,5)P2] controls the plasma membrane localization of a number of cellular proteins. To explore the possibility that this lipid may be involved in Gag targeting and virus particle production, we overexpressed phosphoinositide 5-phosphatase IV, an enzyme that depletes cellular PI(4,5)P2, or overexpressed a constitutively active form of Arf6 (Arf6/Q67L), which induces the formation of PI(4,5)P2-enriched endosomal structures. Both approaches severely reduced virus production. Upon 5-phosphatase IV overexpression, Gag was no longer localized on the plasma membrane but instead was retargeted to late endosomes. Strikingly, in cells expressing Arf6/Q67L, Gag was redirected to the PI(4,5)P2-enriched vesicles and HIV-1 virions budded into these vesicles. These results demonstrate that PI(4,5)P2 plays a key role in Gag targeting to the plasma membrane and thus serves as a cellular determinant of HIV-1 particle production.

Regulation and cellular roles of phosphoinositide 5-kinases

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

Impaired PtdIns(4,5)P2 synthesis in nerve terminals produces defects in synaptic vesicle trafficking

Phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2) has an important function in cell regulation both as a precursor of second messenger molecules and by means of its direct interactions with cytosolic and membrane proteins. Biochemical studies have suggested a role for PtdIns(4,5)P2 in clathrin coat dynamics, and defects in its dephosphorylation at the synapse produce an accumulation of coated endocytic intermediates. However, the involvement of PtdIns(4,5)P2 in synaptic vesicle exocytosis remains unclear. Here, we show that decreased levels of PtdIns(4,5)P2 in the brain and an impairment of its depolarization-dependent synthesis in nerve terminals lead to early postnatal lethality and synaptic defects in mice. These include decreased frequency of miniature currents, enhanced synaptic depression, a smaller readily releasable pool of vesicles, delayed endocytosis and slower recycling kinetics. Our results demonstrate a critical role for PtdIns(4,5)P2 synthesis in the regulation of multiple steps of the synaptic vesicle cycle.

Membrane position of a basic aromatic peptide that sequesters phosphatidylinositol 4,5 bisphosphate determined by site-directed spin labeling and high-resolution NMR

The membrane interactions and position of a positively charged and highly aromatic peptide derived from a secretory carrier membrane protein (SCAMP) are examined using magnetic resonance spectroscopy and several biochemical methods. This peptide (SCAMP-E) is shown to bind to membranes containing phosphatidylinositol 4,5-bisphosphate, PI(4,5)P2, and sequester PI(4,5)P2 within the plane of the membrane. Site-directed spin labeling of the SCAMP-E peptide indicates that the position and structure of membrane bound SCAMP-E are not altered by the presence of PI(4,5)P2, and that the peptide backbone is positioned within the lipid interface below the level of the lipid phosphates. A second approach using high-resolution NMR was used to generate a model for SCAMP-E bound to bicelles. This approach combined oxygen enhancements of nuclear relaxation with a computational method to dock the SCAMP-E peptide at the lipid interface. The model for SCAMP generated by NMR is consistent with the results of site-directed spin labeling and places the peptide backbone in the bilayer interfacial region and the aromatic side chains within the lipid hydrocarbon region. The charged side chains of SCAMP-E lie well within the interface with two arginine residues lying deeper than a plane defined by the position of the lipid phosphates. These data suggest that SCAMP-E interacts with PI(4,5)P2 through an electrostatic mechanism that does not involve specific lipid-peptide contacts. This interaction may be facilitated by the position of the positively charged side chains on SCAMP-E within a low-dielectric region of the bilayer interface.

Phospholipase C delta-type consists of three isozymes: bovine PLCdelta2 is a homologue of human/mouse PLCdelta4

To date, 12 phospholipase C (PLC) isozymes have been identified in mammals, and they are divided into five classes, beta-, gamma-, delta-, epsilon-, and zeta-type. PLCdelta-type is reported to be composed of four isozymes, PLCdelta1-delta4. Here we report that a screening for mouse PLCdelta2 from a BAC library with primers that amplify a specific region of bovine PLCdelta2 resulted in isolation of one clone containing the mouse PLCdelta4 gene. Furthermore, a database search revealed that there is only one gene corresponding to PLCdelta2 and PLCdelta4 in the mouse and human genomes, indicating that bovine PLCdelta2 is a homologue of human and mouse PLCdelta4. However, PLCdelta2 Western blot analysis with a widely used commercial anti-PLCdelta2 antibody showed an expression pattern distinct from that of PLCdelta4 in wild-type mice. In addition, an 80-kDa band, which was recognized by antibody against PLCdelta2, was smaller than an 85-kDa band detected by anti-PLCdelta4 antibody, and the 80-kDa band was detectable in lysates of brain, testis, and spleen from PLCdelta4-deficient mice. We also found that immunoprecipitates from brain lysates with this PLCdelta2 antibody contained no PLC activity. From these data, we conclude that bovine PLCdelta2 is a homologue of human and mouse PLCdelta4, and that three isozymes (delta1, delta3, and delta4) exist in the PLCdelta family.

CAPS Acts at a Prefusion Step in Dense-Core Vesicle Exocytosis as a PIP2 Binding Protein

CAPS-1 is required for Ca2+-triggered fusion of dense-core vesicles with the plasma membrane, but its site of action and mechanism are unknown. We analyzed the kinetics of Ca2+-triggered exocytosis reconstituted in permeable PC12 cells. CAPS-1 increased the initial rate of Ca2+-triggered vesicle exocytosis by acting at a rate-limiting, Ca2+-dependent prefusion step. CAPS-1 activity depended upon prior ATP-dependent priming during which PIP2 synthesis occurs. CAPS-1 activity and binding to the plasma membrane depended upon PIP2. Ca2+ was ineffective in triggering vesicle fusion in the absence of CAPS-1 but instead promoted desensitization to CAPS-1 resulting from decreased plasma membrane PIP2. We conclude that CAPS-1 functions following ATP-dependent priming as a PIP2 binding protein to enhance Ca2+-dependent DCV exocytosis. Essential prefusion steps in dense-core vesicle exocytosis involve sequential ATP-dependent synthesis of PIP2 and the subsequent PIP2-dependent action of CAPS-1. Regulation of PIP2 levels and CAPS-1 activity would control the secretion of neuropeptides and monoaminergic transmitters.

A novel neuronal-specific splice variant of Type I phosphatidylinositol 4-phosphate 5-kinase isoform gamma

Type I PIPkins (phosphatidylinositol 4-phosphate 5-kinases) are the enzymes that catalyse the major cellular route of synthesis of PtdIns(4,5) P2, and three isoforms (alpha, beta and gamma) with several splice variants have been found to date. In the present paper, we describe the discovery of a novel splice variant of the gamma isoform, which we call PIPkin Igammac, and which is characterized by the inclusion of a 26-amino-acid insert near the C-terminus. Its transcript appears to be selectively expressed in brain, where it locates in the neurons of restricted regions, such as cerebellum, hippocampus, cortex and olfactory bulb, as indicated by in situ hybridization studies. Overexpression of two different catalytically inactive constructs of PIPkin Igammac in rat cerebellar granule cells causes a progressive loss of their neuronal processes, whereas equivalent kinase-dead versions of PIPkin Igammaa did not induce any such effect, suggesting the possible existence of a specific PtdIns(4,5) P2 pool synthesized by PIPkin Igammac, which is involved in the maintenance of some neuronal cellular processes.

Lipid Raft-Associated GTPase Signaling Controls Morphology and CD8+T Cell Stimulatory Capacity of Human Dendritic Cells

Their eponymous morphology and unique ability to activate naive T cells are hallmark features of dendritic cells (DCs). Specific properties of the actin cytoskeleton may define both characteristics. In search for regulators that coordinate DC phenotype and function, we observed strongly increased expression of the actin-remodeling GTPases Cdc42 and Rac1 during DC development from human stem cells. Cdc42 and Rac1 are constitutively active in immature DCs, and their activity is further up-regulated by maturational stimuli such as LPS or CD40L. Activation of Rac1 is associated with its rapid recruitment into lipid rafts. Cdc42 is not recruited into rafts, but readily activated by raft-associated moieties. The functional interplay of rafts, GTPases, and cortical actin is further shown by GTPase activation and actin remodeling after pharmacological disruption of lipid rafts and by the loss of the actin-based DC morphology by transfection of dominant-negative Cdc42 and Rac1. Both Cdc42 and Rac1 also control the transport of essential immunostimulatory molecules to the DC surface. Transfection with dominant-negative GTPases led to reduced surface expression of MHC class I and CD86. Consecutively, DCs display a reduced stimulatory capacity for CD8(+) T cells, whereas MHC class II-dependent stimulation of CD4(+) T cells remains unperturbed. We conclude that Cdc42 and Rac1 signaling controls DC morphology and conditions DCs for efficient CD8(+) T cell stimulation.

Phosphoinositides in Constitutive Membrane Traffic

Proteins that make, consume, and bind to phosphoinositides are important for constitutive membrane traffic. Different phosphoinositides are concentrated in different parts of the central vacuolar pathway, with phosphatidylinositol 4-phosphate predominate on Golgi, phosphatidylinositol 4,5-bisphosphate predominate at the plasma membrane, phosphatidylinositol 3-phosphate the major phosphoinositide on early endosomes, and phosphatidylinositol 3,5-bisphosphate found on late endocytic organelles. This spatial segregation may be the mechanism by which the direction of membrane traffic is controlled. Phosphoinositides increase the affinity of membranes for peripheral membrane proteins that function for sorting protein cargo or for the docking and fusion of transport vesicles. This implies that constitutive membrane traffic may be regulated by the mechanisms that control the activity of the enzymes that produce and consume phosphoinositides. Although the lipid kinases and phosphatases that function in constitutive membrane traffic are beginning to be identified, their regulation is poorly understood.

Annexin 2 is a phosphatidylinositol (4,5)-bisphosphate binding protein recruited to actin assembly sites at cellular membranes

Annexin 2 is a Ca(2+)-regulated membrane protein and an F-actin-binding protein enriched at actin assembly sites both, on the plasma membrane and on endosomal vesicles. Here, we identify annexin 2 as a phosphatidylinositol (4,5)-bisphosphate (PtdIns(4,5)P(2))-interacting protein, thereby explaining this specific membrane association. Using the pleckstrin-homology (PH) domain of phospholipase Cdelta1 fused to yellow fluorescent protein as a marker for PtdIns(4,5)P(2), we show that annexin 2 and its ligand p11 (S100A10) are targeted to sites of PtdIns(4,5)P(2) enrichment where F-actin accumulates. At the plasma membrane, adhesion of pedestal-forming enteropathogenic Escherichia coli induces a recruitment of 1-phosphatidylinositol-4-phosphate 5-kinase (PtdIns4P 5-kinase) and an enrichment of PtdIns(4,5)P(2) and annexin 2-p11 at sites of bacterial adhesion. Induction of PtdIns(4,5)P(2)-enriched ruffles and PtdIns(4,5)P(2)-positive, actin-coated vacuoles by Arf6-mediated activation of PtdIns4P 5-kinase also leads to a concomitant accumulation of the annexin 2-p11 complex and the PH domain. Binding studies with immobilized phosphoinositides and phosphoinositide-containing liposomes reveal that the purified annexin 2-p11 complex directly and specifically binds to PtdIns(4,5)P(2) with an affinity comparable to that of the PH domain of phospholipase Cdelta1. Experiments using individual subunits identify annexin 2 as the PtdIns(4,5)P(2)-binding entity. Thus, the direct interaction of annexin 2 with PtdIns(4,5)P(2) is a means of specifically recruiting the annexin 2-p11 complex to sites of membrane-associated actin assembly.

PI-loting membrane traffic

Phosphoinositides (PIs) undergo phosphorylation/dephosphorylation cycles through organelle-specific PI kinases and PI phosphatases that lead to distinct subcellular distributions of the individual PI species. Specific PIs control the correct timing and location of many trafficking events. Their ultimate mode of action is not always well defined, but it includes localized recruitment of transport machinery, allosteric regulation of PI-binding proteins and changes in the physical properties of the membrane.

Phospholipase D1 Regulates Secretagogue-stimulated Insulin Release in Pancreatic β-Cells

Phospholipase D (PLD) has been strongly implicated in the regulation of Golgi trafficking as well as endocytosis and exocytosis. Our aim was to investigate the role of PLD in regulating the biphasic exocytosis of insulin from pancreatic beta-cells that is essential for mammalian glucose homeostasis. We observed that PLD activity in MIN6 pancreatic beta-cells is closely coupled to secretion. Cellular PLD activity was increased in response to a variety of secretagogues including the nutrient glucose and the cholinergic receptor agonist carbamoylcholine. Conversely, pharmacological or hormonal inhibition of stimulated secretion reduced PLD activity. Most importantly, blockade of PLD-catalyzed phosphatidic acid formation using butan-1-ol inhibited insulin secretion in both MIN6 cells and isolated pancreatic islets. It was further established that PLD activity was required for both the first and the second phase of glucose-stimulated insulin release, suggesting a role in the very distal steps of exocytosis, beyond granule recruitment into a readily releasable pool. Visualization of granules using green fluorescent protein-phogrin confirmed a requirement for PLD prior to granule fusion with the plasma membrane. PLD1 was shown to be the predominant isoform in MIN6 cells, and it was located at least partially on insulin granules. Overexpression of wild-type or a dominant negative catalytically inactive mutant of PLD1 augmented or inhibited secretagogue-stimulated secretion, respectively. The results suggest that phosphatidic acid formation on the granule membrane by PLD1 is essential for the regulated secretion of insulin from pancreatic beta-cells.

Essential role for the myotubularin-related phosphatase Ymr1p and the synaptojanin-like phosphatases Sjl2p and Sjl3p in regulation of phosphatidylinositol 3-phosphate in yeast

The requirement of Vps34p, the sole phosphatidylinositol (PI) 3-kinase in Saccharomyces cerevisiae, for protein sorting to the vacuole in yeast has exemplified the essential role for phosphoinositides, phosphorylated derivatives of PI, in membrane trafficking. To better understand mechanisms that regulate PI 3-phosphate [PI(3)P]-mediated signaling, the role of the yeast myotubularin-related PI(3)P phosphatase Ymr1p was investigated. We found that Ymr1p and the synaptojanin-like phosphatase Sjl3p function as key regulators of the localization and levels of PI(3)P. Our data indicated that the ymr1Delta sjl3Delta double mutant aberrantly accumulated PI(3)P and demonstrated a steady-state redistribution of this lipid that leads to enrichment on the vacuolar membrane. This resulted in vacuole protein sorting defects, vacuolar fragmentation, and the misregulation of PI(3)P-specific effectors. Triple deletion of YMR1, SJL2, and SJL3 was lethal, suggesting an essential requirement for phosphatase-mediated PI(3)P regulation. Consistent with this, growth was restored to a ymr1Delta sjl2Delta sjl3Delta triple mutant by a PI(3)P-targeted Sac1p domain chimera (GFP-Sac1DeltaC-FYVE(EEA1)) that returned PI(3)P to levels comparable with wild-type cells. Together, this study demonstrated that Ymr1p, a myotubularin phosphatase family member, functions in the control of PI(3)P-dependent signaling and the maintenance of endosomal system integrity. In addition, this work defined an essential overlapping role for lipid phosphatases in the regulation of 3' phosphoinositides in yeast.

Neomycin Prevents the Wortmannin Inhibition of Insulin-stimulated Glut4 Translocation and Glucose Transport in 3T3-L1 Adipocytes

Insulin stimulates the movement of the facilitative glucose transporter glucose transporter-4 (Glut4) from an intracellular compartment to the plasma membrane in adipocytes and muscle cells, resulting in an increased rate of glucose uptake. Insulin-stimulated Glut4 translocation and glucose transport are abolished by wortmannin, a specific inhibitor of phosphatidylinositol 3'-kinase (PI3K). Here, we demonstrate that neomycin, a drug that masks the cellular substrate of PI3K, phosphatidylinositol 4,5-bisphosphate (PIP), prevents wortmannin inhibition of insulin-stimulated (2)Glut4 translocation and glucose transport without activating protein kinase B, a downstream effector of PI3K. These results suggest that PIP(2) may have an important regulatory function in insulin-stimulated Glut4 translocation and glucose transport.

Phosphatidylinositol 4,5-bisphosphate regulates adipocyte actin dynamics and GLUT4 vesicle recycling

To investigate the potential role of phosphatidylinositol 4, 5-bisphosphate (PI(4,5)P2) in the regulation of actin polymerization and GLUT4 translocation, the type I phosphatidylinositol 4-phosphate 5-kinases (PIP5Ks) were expressed in 3T3L1 adipocytes. In preadipocytes (fibroblasts) PIP5K expression promoted actin polymerization on membrane-bound vesicles to form motile actin comets. In contrast, expression of PIP5K in differentiated 3T3L1 adipocytes resulted in the formation of enlarged vacuole-like structures coated with F-actin, cortactin, dynamin, and N-WASP. Treatment with either latrunculin B (an inhibitor for actin polymerization) or Clostridium difficile toxin B (a general Rho family inhibitor) resulted in a relatively slower disappearance of coated F-actin from these vacuoles, but the vacuoles themselves remained unaffected. Functionally, the increased PI(4,5)P2 levels resulted in an inhibition of transferrin receptor and GLUT4 endocytosis and a slow accumulation of these proteins in the PI(4,5)P2-enriched vacuoles along with the non-clathrin-derived endosome marker (caveolin) and the AP-2 adaptor complex. However, these structures were devoid of early endosome markers (EEA1, clathrin) and the biosynthetic membrane secretory machinery markers p115 (Golgi) and syntaxin 6 (trans-Golgi Network). Taken together, these data demonstrate that PI(4,5)P2 has distinct morphologic and functional properties depending upon specific cell context. In adipocytes, altered PI(4,5)P2 metabolism has marked effects on GLUT4 endocytosis and intracellular vesicle trafficking due to the derangement of actin dynamics.

A computational model for the electrostatic sequestration of PI(4,5)P2 by membrane-adsorbed basic peptides

The multivalent acidic phospholipid phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) plays a key role in many biological processes. Recent studies show that unstructured clusters of basic residues from a number of peripheral proteins can laterally sequester PI(4,5)P2 in membranes. Specifically, experiments suggest that the basic effector domain of the myristoylated alanine-rich C kinase substrate (MARCKS), or a peptide corresponding to this domain, MARCKS(151-175), sequesters several PI(4,5)P2 and that this sequestration is due to nonspecific electrostatic interactions. Here, we use the finite difference Poisson-Boltzmann method to test this hypothesis by calculating the electrostatic free energy of lateral sequestration of PI(4,5)P2 by membrane-adsorbed basic peptides: Lys-7, Lys-13, and FA-MARCKS(151-175), a peptide based on MARCKS(151-175). In agreement with experiments, we find that the electrostatic free energy becomes more favorable when: 1), Lys-13 and FA-MARCKS(151-175) sequester several PI(4,5)P2; 2), the linear charge density of the basic peptide increases; 3), the mol percent monovalent acidic lipid in the membrane decreases; and 4), the ionic strength of the solution decreases. In addition, the electrostatic sequestration free energy is in excess of the entropic penalty associated with localizing PI(4,5)P2. Our calculations, thus, provide a structural and quantitative description of the observed interaction of PI(4,5)P2 with membrane-adsorbed basic sequences.

Electrostatic sequestration of PIP2 on phospholipid membranes by basic/aromatic regions of proteins

The basic effector domain of myristoylated alanine-rich C kinase substrate (MARCKS), a major protein kinase C substrate, binds electrostatically to acidic lipids on the inner leaflet of the plasma membrane; interaction with Ca2+/calmodulin or protein kinase C phosphorylation reverses this binding. Our working hypothesis is that the effector domain of MARCKS reversibly sequesters a significant fraction of the L-alpha-phosphatidyl-D-myo-inositol 4,5-bisphosphate (PIP2) on the plasma membrane. To test this, we utilize three techniques that measure the ability of a peptide corresponding to its effector domain, MARCKS(151-175), to sequester PIP2 in model membranes containing physiologically relevant fractions (15-30%) of the monovalent acidic lipid phosphatidylserine. First, we measure fluorescence resonance energy transfer from Bodipy-TMR-PIP2 to Texas Red MARCKS(151-175) adsorbed to large unilamellar vesicles. Second, we detect quenching of Bodipy-TMR-PIP2 in large unilamellar vesicles when unlabeled MARCKS(151-175) binds to vesicles. Third, we identify line broadening in the electron paramagnetic resonance spectra of spin-labeled PIP2 as unlabeled MARCKS(151-175) adsorbs to vesicles. Theoretical calculations (applying the Poisson-Boltzmann relation to atomic models of the peptide and bilayer) and experimental results (fluorescence resonance energy transfer and quenching at different salt concentrations) suggest that nonspecific electrostatic interactions produce this sequestration. Finally, we show that the PLC-delta1-catalyzed hydrolysis of PIP2, but not binding of its PH domain to PIP2, decreases markedly as MARCKS(151-175) sequesters most of the PIP2.

Phosphatidylinositol 4,5-bisphosphate promotes both [3H]-noradrenaline and [14C]-glutamate exocytosis from nerve endings

Inhibition of phosphatidylinositol 4-phosphate (PI4P) and phosphatidylinositol 4,5-bisphosphate (PI4,5P(2)) synthesis by phenylarsine oxide (PAO) inhibits both [3H]-noradrenaline ([3H]-NA) and [14C]-glutamate ([14C]-glu) exocytosis from streptolysin-O (SLO)-perforated synaptosomes. When PI4,5P(2) is blocked by an antibody or chelated by neomycin, neurotransmitter exocytosis again is inhibited. Also, when phosphoinositide (PI) synthesis is indirectly decreased by shunting phosphatidic acid (PA) synthesis into phosphatidylbutanol production, both [14C]-glutamate and [3H]-noradrenaline exocytosis are inhibited. All of these results indicate that PI4,5P(2) is necessary for exocytosis of both synaptic vesicles (SVs) and dense core vesicles (DCVs).

Ca2+/calmodulin transfers the membrane-proximal lipid-binding domain of the v-SNARE synaptobrevin from cis to trans bilayers

Soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor (SNARE) protein interactions at the synaptic vesicle/plasma membrane interface play an essential role in neurotransmitter release. The membrane-proximal region (amino acids 77-90) of the v-SNARE vesicle-associated membrane protein 2 (VAMP 2, synaptobrevin) binds acidic phospholipids or Ca(2+)/calmodulin in a mutually exclusive manner, processes that are required for Ca(2+)-dependent exocytosis. To address the mechanisms involved, we asked whether this region of VAMP can interact with cis (outer vesicle leaflet) and/or trans (inner plasma membrane leaflet) lipids. To evaluate cis lipid binding, recombinant VAMP was reconstituted into liposomes and accessibility to site-directed antibodies was probed by surface plasmon resonance. Data indicated that the membrane-proximal domain of VAMP dips into the cis lipid bilayer, sequestering epitopes between the tetanus toxin cleavage site and the membrane anchor. These epitopes were unmasked by VAMP double mutation W89A, W90A, which abolishes lipid interactions. To evaluate trans lipid binding, VAMP was reconstituted in cis liposomes, which were then immobilized on beads. The ability of VAMP to capture protein-free (3)H-labeled trans liposomes was then measured. When cis lipid interactions were eliminated by omitting negatively charged lipids, trans lipid binding to VAMP was revealed. In contrast, when cis and trans liposomes both contained acidic headgroups (i.e., approximating physiological conditions), cis lipid interactions totally occluded trans lipid binding. In these conditions Ca(2+)/calmodulin displaced cis inhibition, transferring the lipid-binding domain of VAMP from the cis to the trans bilayer. Our results suggest that calmodulin acts as a unidirectional Ca(2+)-activated shuttle that docks the juxtamembrane portion of the v-SNARE in the target membrane to prepare fusion.

Sensitization of Epidermal Growth Factor-induced Signaling by Bradykinin Is Mediated by c-Src

Communication between receptor tyrosine kinase (RTK)- and G protein-coupled receptor (GPCR)-mediated signaling systems has received increasing attention in recent years. Here, we report that activation of G protein-coupled bradykinin B2 receptor induces an up-regulation of cellular responses mediated by epidermal growth factor receptor (EGFR) and provide essential mechanistic characteristics of this sensitization process. EGF, which failed to evoke detectable amount of calcium increase and neurotransmitter release when administrated alone in primary cultures of rat adrenal chromaffin cells and PC12 cells, became capable of inducing these responses specifically after bradykinin pretreatment. Both EGFR and non-receptor tyrosine kinase p60Src, whose kinase activities were required in the sensitization, were found to be enriched in cholesterol-rich lipid rafts. Bradykinin caused activation of p60Src and Src-dependent phosphorylation of the EGFR on Tyr-845 in lipid rafts, as well as recruitment of phospholipase C (PLC) gamma1 to the rafts. Depletion of cholesterol by methyl-beta-cyclodextrin disrupted the raft localization of EGFR and Src, as well as bradykinin-induced translocation of PLCgamma1. Furthermore, sensitization, which was impaired by cholesterol depletion, was restored by repletion of cholesterol. Therefore, we suggest that lipid rafts are essential participants in the regulation of receptor-mediated signal transduction and cross-talk via organizing signaling complexes in membrane microdomains.

Clathrin-dependent endocytosis

The process by which clathrin-coated vesicles are produced involves interactions of multifunctional adaptor proteins with the plasma membrane, as well as with clathrin and several accessory proteins and phosphoinositides. Here we review recent findings highlighting new insights into mechanisms underlying clathrin-dependent endocytosis.

Membrane/cytoskeleton communication

Accumulations of particular lipids in ordered arrays in the membrane (termed microdomains or lipid rafts) can attract proteins with specific targeting domains. Both the lipid and protein components of rafts communicate with the cytoskeleton directly thereby regulating cellular responses. Recent evidence implicating phosphoinositide 1,5 bisphosphate (PIP2) in cytoskeletal regulation shows that agonist sensitive regulation of PIP2 homoeostasis occurs specifically rafts, which appear to provide a major structural substrate for its function. The crucial role of PIP2 in generating cytoskeletal responses is chiefly achieved by regulating proteins that control actin dynamics directly. Many of these regulatory proteins are also specifically enriched in rafts either directly (by insertion into the lipid bilayer via acetylation motifs), or indirectly via interactions with other raft components. The notion that rafts form membrane platforms or modules that mediate signaling responses has been most extensively demonstrated in the immune synapse (IS) of T cells, a complex assemblage of rafts that integrates signaling cascades originating from the simultaneous activation of a wide variety of receptors. The IS is essential for both the amplification and maintenance of T-cell activation, and its assembly at the antigen presenting site depends on the interactions between rafts and the actin cytoskeleton that regulates coalescence of smaller raft components into the larger IS complex. Likewise the neuron, which represents the most highly polarized cell in the body, utilizes the regulation of actin dynamics in response to a plethora of extracellular signals to control axon pathfinding thereby sculpting nervous system cytoarchitecture with utmost precision. It is now becoming clear, that as in the T-cell, lipid rafts in the growing axon can assemble into highly specific, yet malleable and dynamic, signaling modules that regulate actin dynamics in a fashion that is also PIP2-dependent and that utilizes both familiar and novel regulatory mechanisms. It seems clear that raft mediated cytoskeletal regulation represents a highly conserved mechanism to integrate cellular responses to diverse signals.

Regulation of Endocytosis by Phosphatidylinositol 4,5-Bisphosphate and ENTH Proteins

Clathrin-mediated endocytosis starts by a recruitment of endocytic proteins to the plasma membrane to induce invagination of lipid bilayer and subsequent vesicule formation. The recruitment of these components requires PtdIns(4,5)P2, a phosphoinositide on the plasma membrane. Although it is well known that the synthesis as well as the disruption of this lipid is important, recent studies have revealed the indispensable roles of direct interaction between PtdIns(4,5)P2 and the endocytic machinery. The ENTH domain is a newly found PtdIns(4,5)P2 binding unit conserved among endocytic proteins like epsins, AP180, and the Hip1/Sla2 family. This review focuses on the essential roles of PtdIns(4,5)P2 and its specific binding partner, the ENTH domain, in clathrin-mediated endocytosis.

Activation of type I phosphatidylinositol 4-phosphate 5-kinase isoforms by the Rho GTPases, RhoA, Rac1, and Cdc42

Type I phosphatidylinositol 4-phosphate 5-kinase (PIP5K) catalyzes the formation of the phospholipid, phosphatidylinositol 4,5-bisphosphate (PIP(2)), which is implicated in many cellular processes. The Rho GTPases, RhoA and Rac1, have been shown previously to activate PIP5K and to bind PIP5K. Three type I PIP5K isoforms (Ialpha,Ibeta, and Igamma) have been identified; however, it is unclear whether these isoforms are differentially or even sequentially regulated by Rho GTPases. Here we show that RhoA and Rac1, as well as Cdc42, but not the Ras-like GTPases, RalA and Rap1A, markedly stimulate PIP(2) synthesis by all three PIP5K isoforms expressed in human embryonic kidney 293 cells, both in vitro and in vivo. RhoA-stimulated PIP(2) synthesis by the PIP5K isoforms was mediated by the RhoA effector, Rho-kinase. Stimulation of PIP5K isoforms by Rac1 and Cdc42 was apparently independent of and additive with RhoA- and Rho-kinase, as shown by studies with C3 transferase and Rho-kinase mutants. RhoA, and to a lesser extent Rac1, but not Cdc42, interacted in a nucleotide-independent form with all three PIP5K isoforms. Binding of PIP5K isoforms to GTP-bound, but not GDP-bound, RhoA could be displaced by Rho-kinase, suggesting a direct and constitutive PIP5K-Rho GTPase binding, which, however, does not trigger PIP5K activation. In summary, our findings indicate that synthesis of PIP(2) by the three PIP5K isoforms is controlled by RhoA, acting via Rho-kinase, as well as Rac1 and Cdc42, implicating that regulation of PIP(2) synthesis has a central position in signaling by these three Rho GTPases.

Membrane recognition and targeting by lipid-binding domains

Modular domains that recognize and target intracellular membranes play a critical role in the assembly, localization, and function of signaling and trafficking complexes in eukaryotic cells. Large domain families, including PH, FYVE, PX, PHD, and C2 domains, combine specific, nonspecific, and multivalent interactions to achieve selective membrane targeting. Despite structural and functional diversity, general features of lipid recognition are evident in the various membrane-targeting mechanisms.

Impairing actin filament or syndapin functions promotes accumulation of clathrin-coated vesicles at the apical plasma membrane of acinar epithelial cells

In this article, we investigate the contributions of actin filaments and accessory proteins to apical clathrin-mediated endocytosis in primary rabbit lacrimal acini. Confocal fluorescence and electron microscopy revealed that cytochalasin D promoted apical accumulation of clathrin, alpha-adaptin, dynamin, and F-actin and increased the amounts of coated pits and vesicles at the apical plasma membrane. Sorbitol density gradient analysis of membrane compartments showed that cytochalasin D increased [14C]dextran association with apical membranes from stimulated acini, consistent with functional inhibition of apical endocytosis. Recombinant syndapin SH3 domains interacted with lacrimal acinar dynamin, neuronal Wiskott-Aldrich Syndrome protein (N-WASP), and synaptojanin; their introduction by electroporation elicited remarkable accumulation of clathrin, accessory proteins, and coated pits at the apical plasma membrane. These SH3 domains also significantly (p </= 0.05) increased F-actin, with substantial colocalization of dynamin and N-WASP with the additional filaments. Coelectroporation with the VCA domain of N-WASP blocked the increase in F-actin and reversed the morphological changes indicative of impaired apical endocytosis. We suggest that transient modulation of actin polymerization by syndapins through activation of the Arp2/3 complex via N-WASP coordinates dynamin-mediated vesicle fission at the apical plasma membrane of acinar epithelia. Trapping of assembled F-actin intermediates during this process by cytochalasin D or syndapin SH3 domains impairs endocytosis.

Regulation of exocytosis in adrenal chromaffin cells: focus on ARF and Rho GTPases

Neurons and neuroendocrine cells release transmitters and hormones by exocytosis, a highly regulated process in which secretory vesicles or granules fuse with the plasma membrane to release their contents in response to a calcium trigger. Several stages have been recognized in exocytosis. After recruitment and docking at the plasma membrane, vesicles/granules enter a priming step, which is then followed by the fusion process. Cortical actin remodelling accompanies the exocytotic reaction, but the links between actin dynamics and trafficking events remain poorly understood. Here, we review the action of Rho and ADP-ribosylation factor (ARF) GTPases within the exocytotic pathway in adrenal chromaffin cells. Rho proteins are well known for their pivotal role in regulating the actin cytoskeleton. ARFs were originally identified as regulators of vesicle transport within cells. The possible interplay between these two families of GTPases and their downstream effectors provides novel insights into the mechanisms that govern exocytosis.

Crystal structures of the Dab homology domains of mouse disabled 1 and 2

Disabled (Dab) 1 and 2 are mammalian homologues of Drosophila DAB. Dab1 is a key cytoplasmic mediator in Reelin signaling that controls cell positioning in the developing central nervous system, whereas Dab2 is an adapter protein that plays a role in endocytosis. DAB family proteins possess an amino-terminal DAB homology (DH) domain that is similar to the phosphotyrosine binding/phosphotyrosine interaction (PTB/PI) domain. We have solved the structures of the DH domains of Dab2 (Dab2-DH) and Dab1 (Dab1-DH) in three different ligand forms, ligand-free Dab2-DH, the binary complex of Dab2-DH with the Asn-Pro-X-Tyr (NPXY) peptide of amyloid precursor protein (APP), and the ternary complex of Dab1-DH with the APP peptide and inositol 1,4,5-trisphosphate (Ins-1,4,5-P3, the head group of phosphatidylinositol-4,5-diphosphate (PtdIns-4,5-P2)). The similarity of these structures suggests that the rigid Dab DH domain maintains two independent pockets for binding of the APP/lipoprotein receptors and phosphoinositides. Mutagenesis confirmed the structural determinants specific for the NPXY sequence and PtdIns-4,5-P2 binding. NMR spectroscopy confirmed that the DH domain binds to Ins-1,4,5-P3 independent of the NPXY peptides. These findings suggest that simultaneous interaction of the rigid DH domain with the NPXY sequence and PtdIns-4,5-P2 plays a role in the attachment of Dab proteins to the APP/lipoprotein receptors and phosphoinositide-rich membranes.

Lipid raft localization of cell surface E-selectin is required for ligation-induced activation of phospholipase C gamma

E-selectin, an endothelial cell surface adhesion receptor for leukocytes, also acts as a signaling receptor. Upon multivalent ligation, E-selectin transduces outside-in signals into the endothelium leading to changes in intracellular Ca(2+) concentration and activation of the mitogen-activated protein kinase signaling pathway. In addition, following leukocyte engagement, E-selectin associates via its cytoplasmic domain with components of the actin cytoskeleton and undergoes alterations in phosphorylation state that result in changes in gene expression. In this study, we show that E-selectin is localized in cholesterol-rich lipid rafts at the cell surface, and that upon ligation E-selectin clusters and redistributes in the plasma membrane colocalizing with a fraction of caveolin-1-containing rafts. In addition, we demonstrate that leukocyte adhesion via E-selectin results in association with and activation of phospholipase Cgamma (PLCgamma). Moreover, we show that disruption of lipid rafts with the cholesterol-depleting drug methyl-beta-cyclodextrin disrupts the raft localization of E-selectin as well as the ligation-induced association of E-selectin with PLCgamma, and subsequent tyrosine phosphorylation of PLCgamma. In contrast, cholesterol depletion has no effect on E-selectin-dependent mitogen-activated protein kinase activation. Thus, these findings demonstrate that the presence of E-selectin in lipid rafts is necessary for its association with, and activation of, PLCgamma, and suggest that this subcellular localization of E-selectin is related to its signaling function(s) during leukocyte-endothelial interactions.

Retrograde regulation of synaptic vesicle endocytosis and recycling

Sustained release of neurotransmitter depends upon the recycling of synaptic vesicles. Until now, it has been assumed that vesicle recycling is regulated by signals from the presynaptic bouton alone, but results from rat hippocampal neurons reported here indicate that this need not be the case. Fluorescence imaging and pharmacological analysis show that a nitric oxide (NO) signal generated postsynaptically can regulate endocytosis and at least one later step in synaptic vesicle recycling. The proposed retrograde pathway involves an NMDA receptor (NMDAR)-dependent postsynaptic production of NO, diffusion of NO to a presynaptic site, and a cGMP-dependent increase in presynaptic phosphatidylinositol 4,5-biphosphate (PIP2). These results indicate that the regulation of synaptic vesicle recycling may integrate a much broader range of neural activity signals than previously recognized, including postsynaptic depolarization and the activation of NMDARs at both immediate and nearby postsynaptic active zones.

Location is everything: lipid rafts and immune cell signaling

The cells of both the adaptive and innate immune systems express a dizzying array of receptors that transduce and integrate an enormous amount of information about the environment that allows the cells to mount effective immune responses. Over the past several years, significant advances have been made in elucidating the molecular details of signal cascades initiated by the engagement of immune cell receptors by their ligands. Recent evidence indicates that immune receptors and components of their signaling cascades are spatially organized and that this spatial organization plays a central role in the initiation and regulation of signaling. A key organizing element for signaling receptors appears to be cholesterol- and sphingolipid-rich plasma membrane microdomains termed lipid rafts. Research into the molecular basis of the spatial segregation and organization of signaling receptors provided by rafts is adding fundamentally to our understanding of the initiation and prolongation of signals in the immune system.

Phosphatidylinositol phosphate 5-kinase Ibeta recruits AP-2 to the plasma membrane and regulates rates of constitutive endocytosis

Overexpression of phosphatidylinositol phosphate 5-kinase (PIP5KI) isoforms alpha, beta, or gamma in CV-1 cells increased phosphatidylinositol 4,5-bisphosphate (PIP2) levels by 35, 180, and 0%, respectively. Endocytosis of transferrin receptors, association of AP-2 proteins with membranes, and the number of clathrin-coated pits at the plasma membrane increased when PIP2 increased. When expression of PIP5KIbeta was inhibited with small interference RNA in HeLa cells, expression of PIP5KIalpha was also reduced slightly, but PIP5KIgamma expression was increased. PIP2 levels and internalization of transferrin receptors dropped 50% in these cells; thus, PIP5KIgamma could not compensate for loss of PIP5KIbeta. When expression of PIP5KIalpha was reduced, expression of both PIP5KIbeta and PIP5KIgamma increased and PIP2 levels did not change. A similar increase of PIP5KIalpha and PIP5KIbeta occurred when PIP5KIgamma was inhibited. These results indicate that constitutive endocytosis in CV-1 and HeLa cells requires (and may be regulated by) PIP2 produced primarily by PIP5KIbeta.

Unique targeting of cytosolic phospholipase A2 to plasma membranes mediated by the NADPH oxidase in phagocytes

Cytosolic phospholipase A2 (cPLA2)-generated arachidonic acid (AA) has been shown to be an essential requirement for the activation of NADPH oxidase, in addition to its being the major enzyme involved in the formation of eicosanoid at the nuclear membranes. The mechanism by which cPLA2 regulates NADPH oxidase activity is not known, particularly since the NADPH oxidase complex is localized in the plasma membranes of stimulated cells. The present study is the first to demonstrate that upon stimulation cPLA2 is transiently recruited to the plasma membranes by a functional NADPH oxidase in neutrophils and in granulocyte-like PLB-985 cells. Coimmunoprecipitation experiments and double labeling immunofluorescence analysis demonstrated the unique colocalization of cPLA2 and the NADPH oxidase in plasma membranes of stimulated cells, in correlation with the kinetic burst of superoxide production. A specific affinity in vitro binding was detected between GST-p47phox or GST-p67phox and cPLA2 in lysates of stimulated cells. The association between these two enzymes provides the molecular basis for AA released by cPLA2 to activate the assembled NADPH oxidase. The ability of cPLA2 to regulate two different functions in the same cells (superoxide generation and eicosanoid production) is achieved by a novel dual subcellular localization of cPLA2 to different targets.

PIKfyve controls fluid phase endocytosis but not recycling/degradation of endocytosed receptors or sorting of procathepsin D by regulating multivesicular body morphogenesis

The mammalian phosphatidylinositol (PtdIns) 5-P/PtdIns 3,5-P2-producing kinase PIKfyve has been implicated in maintaining endomembrane homeostasis in mammalian cells. To address the role of PIKfyve in trafficking processes, we examined the functioning of the biosynthetic, endocytic, and recycling pathways in stable human embryonic kidney 293 cell lines inducibly expressing the wild-type or kinase-defective dominant-negative form. PIKfyveWT or PIKfyveK1831E expression did not affect the processing and lysosomal targeting of newly synthesized procathepsin D. Likewise the rates of transferrin uptake/recycling or epidermal growth factor receptor degradation were not altered upon expression of either protein. In contrast, PIKfyveK1831E but not PIKfyveWT expression markedly impaired the late uptake of fluid phase marker horseradish peroxidase. Inspection of the organelle morphology by confocal microscopy with specific markers in COS cells transiently expressing PIKfyveK1831E showed the Golgi apparatus, end lysosomes, and the recycling compartment indistinguishable from nontransfected cells, despite the dramatic PIKfyveK1831E-induced endomembrane vacuolation. In contrast, we observed a striking effect on the late endocytic compartment, marked by disruption of the dextran-labeled perinuclear endosomal compartment and formation of dispersed enlarged vesicles. Electron microscopy identified the cytoplasmic vacuoles in the PIKfyveK1831E-expressing human embryonic kidney 293 cells as enlarged multivesicular body-like structures with substantially lower number of internal vesicles and membrane whorls. Together, these data indicate that PIKfyve selectively regulates the sorting and traffic of peripheral endosomes containing lysosomaly directed fluid phase cargo through controlling the morphogenesis and function of multivesicular bodies.

Arf and its many interactors

Arf GTP-binding proteins regulate membrane traffic and actin remodeling. Similar to other GTP-binding proteins, a complex of Arf-GTP with an effector protein mediates Arf function. Arf interacts with at least three qualitatively different types of effectors. First, it interacts with structural proteins, the vesicle coat proteins. The second type of effector is lipid-metabolizing enzymes, and the third comprises those proteins that bind to Arf-GTP but whose biochemical or biological functions are not yet clearly defined. Arf interacts with two other families of proteins, the exchange factors and the GTPase-activating proteins. Recent work examining the functional relationships among the diverse Arf interactors has led to reconsideration of the prevailing paradigms for Arf action.

Synapsin I-associated phosphatidylinositol 3-kinase mediates synaptic vesicle delivery to the readily releasable pool

Maintaining synaptic transmission requires replenishment of docked synaptic vesicles within the readily releasable pool (RRP) from synaptic vesicle clusters in the synapsin-bound reserve pool. We show that synapsin forms a complex with phosphatidylinositol 3-kinase (PI 3-kinase) in intact nerve terminals and that synapsin-associated kinase activity increases on depolarization. Disruption of either PI 3-kinase activity or its interaction with synapsin inhibited replenishment of the RRP, but did not affect exocytosis from the RRP. Thus we conclude that a synapsin-associated PI 3-kinase activity plays a role in synaptic vesicle delivery to the RRP. This also suggests that PI 3-kinase contributes to the maintenance of synaptic transmission during periods of high activity, indicating a possible role in synaptic plasticity.