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

Quantitative FRET Microscopy Reveals a Crucial Role of Cytoskeleton in Promoting PI(4,5)P2 Confinement

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) is an essential plasma membrane component involved in several cellular functions, including membrane trafficking and cytoskeleton organization. This function multiplicity is partially achieved through a dynamic spatiotemporal organization of PI(4,5)P₂ within the membrane. Here, we use a Förster resonance energy transfer (FRET) approach to quantitatively assess the extent of PI(4,5)P₂ confinement within the plasma membrane. This methodology relies on the rigorous evaluation of the dependence of absolute FRET efficiencies between pleckstrin homology domains (PH_PLCδ) fused with fluorescent proteins and their average fluorescence intensity at the membrane. PI(4,5)P₂ is found to be significantly compartmentalized at the plasma membrane of HeLa cells, and these clusters are not cholesterol-dependent, suggesting that membrane rafts are not involved in the formation of these nanodomains. On the other hand, upon inhibition of actin polymerization, compartmentalization of PI(4,5)P₂ is almost entirely eliminated, showing that the cytoskeleton network is the critical component responsible for the formation of nanoscale PI(4,5)P₂ domains in HeLa cells.

Phosphoinositide Signaling and Mechanotransduction in Cardiovascular Biology and Disease

Phosphoinositides, which are membrane-bound phospholipids, are critical signaling molecules located at the interface between the extracellular matrix, cell membrane, and cytoskeleton. Phosphoinositides are essential regulators of many biological and cellular processes, including but not limited to cell migration, proliferation, survival, and differentiation, as well as cytoskeletal rearrangements and actin dynamics. Over the years, a multitude of studies have uniquely implicated phosphoinositide signaling as being crucial in cardiovascular biology and a dominant force in the development of cardiovascular disease and its progression. Independently, the cellular transduction of mechanical forces or mechanotransduction in cardiovascular cells is widely accepted to be critical to their homeostasis and can drive aberrant cellular phenotypes and resultant cardiovascular disease. Given the versatility and diversity of phosphoinositide signaling in the cardiovascular system and the dominant regulation of cardiovascular cell functions by mechanotransduction, the molecular mechanistic overlap and extent to which these two major signaling modalities converge in cardiovascular cells remain unclear. In this review, we discuss and synthesize recent findings that rightfully connect phosphoinositide signaling to cellular mechanotransduction in the context of cardiovascular biology and disease, and we specifically focus on phosphatidylinositol-4,5-phosphate, phosphatidylinositol-4-phosphate 5-kinase, phosphatidylinositol-3,4,5-phosphate, and phosphatidylinositol 3-kinase. Throughout the review, we discuss how specific phosphoinositide subspecies have been shown to mediate biomechanically sensitive cytoskeletal remodeling in cardiovascular cells. Additionally, we discuss the direct interaction of phosphoinositides with mechanically sensitive membrane-bound ion channels in response to mechanical stimuli. Furthermore, we explore the role of phosphoinositide subspecies in association with critical downstream effectors of mechanical signaling in cardiovascular biology and disease.

Membrane Order Is a Key Regulator of Divalent Cation-Induced Clustering of PI(3,5)P2 and PI(4,5)P2

Although the evidence for the presence of functionally important nanosized phosphorylated phosphoinositide (PIP)-rich domains within cellular membranes has accumulated, very limited information is available regarding the structural determinants for compartmentalization of these phospholipids. Here, we used a combination of fluorescence spectroscopy and microscopy techniques to characterize differences in divalent cation-induced clustering of PI(4,5)P₂ and PI(3,5)P₂. Through these methodologies we were able to detect differences in divalent cation-induced clustering efficiency and cluster size. Ca²⁺-induced PI(4,5)P₂ clusters are shown to be significantly larger than the ones observed for PI(3,5)P₂. Clustering of PI(4,5)P₂ is also detected at physiological concentrations of Mg²⁺, suggesting that in cellular membranes, these molecules are constitutively driven to clustering by the high intracellular concentration of divalent cations. Importantly, it is shown that lipid membrane order is a key factor in the regulation of clustering for both PIP isoforms, with a major impact on cluster sizes. Clustered PI(4,5)P₂ and PI(3,5)P₂ are observed to present considerably higher affinity for more ordered lipid phases than the monomeric species or than PI(4)P, possibly reflecting a more general tendency of clustered lipids for insertion into ordered domains. These results support a model for the description of the lateral organization of PIPs in cellular membranes, where both divalent cation interaction and membrane order are key modulators defining the lateral organization of these lipids.

Single-molecule Super-resolution Imaging of Phosphatidylinositol 4,5-bisphosphate in the Plasma Membrane with Novel Fluorescent Probes

Phosphoinositides in the cell membrane are signaling lipids with multiple cellular functions. Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) is a determinant phosphoinositide of the plasma membrane (PM), and it is required to modulate ion channels, actin dynamics, exocytosis, endocytosis, intracellular signaling, and many other cellular processes. However, the spatial organization of PI(4,5)P2 in the PM is controversial, and its nanoscale distribution is poorly understood due to the technical limitations of research approaches. Here by utilizing single molecule localization microscopy and the Pleckstrin Homology (PH) domain based dual color fluorescent probes, we describe a novel method to visualize the nanoscale distribution of PI(4,5)P2 in the PM in fixed membrane sheets as well as live cells.

Nanoscale Landscape of Phosphoinositides Revealed by Specific Pleckstrin Homology (PH) Domains Using Single-molecule Superresolution Imaging in the Plasma Membrane

Both phosphatidylinositol 4-phosphate (PI4P) and phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) are independent plasma membrane (PM) determinant lipids that are essential for multiple cellular functions. However, their nanoscale spatial organization in the PM remains elusive. Using single-molecule superresolution microscopy and new photoactivatable fluorescence probes on the basis of pleckstrin homology domains that specifically recognize phosphatidylinositides in insulin-secreting INS-1 cells, we report that the PI(4,5)P2 probes exhibited a remarkably uniform distribution in the major regions of the PM, with some sparse PI(4,5)P2-enriched membrane patches/domains of diverse sizes (383 ± 14 nm on average). Quantitative analysis revealed a modest concentration gradient that was much less steep than previously thought, and no densely packed PI(4,5)P2 nanodomains were observed. Live-cell superresolution imaging further demonstrated the dynamic structural changes of those domains in the flat PM and membrane protrusions. PI4P and phosphatidylinositol (3,4,5)-trisphosphate (PI(3,4,5)P3) showed similar spatial distributions as PI(4,5)P2. These data reveal the nanoscale landscape of key inositol phospholipids in the native PM and imply a framework for local cellular signaling and lipid-protein interactions at a nanometer scale.

Control of lipid organization and actin assembly during clathrin-mediated endocytosis by the cytoplasmic tail of the rhomboid protein Rbd2

Clathrin-mediated endocytosis (CME) requires precise regulation of the actin cytoskeleton. The yeast rhomboid protein Rbd2 controls the timing of actin polymerization during CME through its cytoplasmic tail and a PtdIns(4,5)P2-dependent mechanism.

Clathrin-mediated endocytosis (CME) is facilitated by a precisely regulated burst of actin assembly. PtdIns(4,5)P2 is an important signaling lipid with conserved roles in CME and actin assembly regulation. Rhomboid family multipass transmembrane proteins regulate diverse cellular processes; however, rhomboid-mediated CME regulation has not been described. We report that yeast lacking the rhomboid protein Rbd2 exhibit accelerated endocytic-site dynamics and premature actin assembly during CME through a PtdIns(4,5)P2-dependent mechanism. Combined genetic and biochemical studies showed that the cytoplasmic tail of Rbd2 binds directly to PtdIns(4,5)P2 and is sufficient for Rbd2's role in actin regulation. Analysis of an Rbd2 mutant with diminished PtdIns(4,5)P2-binding capacity indicates that this interaction is necessary for the temporal regulation of actin assembly during CME. The cytoplasmic tail of Rbd2 appears to modulate PtdIns(4,5)P2 distribution on the cell cortex. The syndapin-like F-BAR protein Bzz1 functions in a pathway with Rbd2 to control the timing of type 1 myosin recruitment and actin polymerization onset during CME. This work reveals that the previously unstudied rhomboid protein Rbd2 functions in vivo at the nexus of three highly conserved processes: lipid regulation, endocytic regulation, and cytoskeletal function.

Phosphatidylinositol-4,5-bisphosphate enhances anionic lipid demixing by the C2 domain of PKCα

The C2 domain of PKCα (C2α) induces fluorescence self-quenching of NBD-PS in the presence of Ca²⁺, which is interpreted as the demixing of phosphatidylserine from a mixture of this phospholipid with phosphatidylcholine. Self-quenching of NBD-PS was considerably increased when phosphatidylinositol-4,5-bisphosphate (PIP₂) was present in the membrane. When PIP₂ was the labeled phospholipid, in the form of TopFluor-PIP₂, fluorescence self-quenching induced by the C2 domain was also observed, but this was dependent on the presence of phosphatidylserine. An independent indication of the phospholipid demixing effect given by the C2α domain was obtained by using ²H-NMR, since a shift of the transition temperature of deuterated phosphatidylcholine was observed as a consequence of the addition of the C2α domain, but only in the presence of PIP₂. The demixing induced by the C2α domain may have a physiological significance since it means that the binding of PKCα to membranes is accompanied by the formation of domains enriched in activating lipids, like phosphatidylserine and PIP₂. The formation of these domains may enhance the activation of the enzyme when it binds to membranes containing phosphatidylserine and PIP₂.

Microscopic methods to observe the distribution of lipids in the cellular membrane

Membrane lipids not only provide the structural framework of cellular membranes but also influence protein functions in several different ways. In comparison to proteins, however, relatively little is known about distribution of membrane lipids because of the insufficiency of microscopic methods. The difficulty in studying lipid distribution results from several factors, including their unresponsiveness to chemical fixation, fast translational movement, small molecular size, and high packing density. In this Current Topic, we consider the major microscopic methods and discuss whether and to what degree of precision these methods can reveal membrane lipid distribution in situ. We highlight two fixation methods, chemical and physical, and compare the theoretical limitations to their spatial resolution. Recognizing the strengths and weaknesses of each method should help researchers interpret their microscopic results and increase our understanding of the physiological functions of lipids.

Myosin-1c regulates the dynamic stability of E-cadherin-based cell-cell contacts in polarized Madin-Darby canine kidney cells

Myo1c knockdown causes defects in E-cadherin localization, E-cadherin binding, and cell–cell contact of Madin–Darby canine kidney cells. Expression of wild-type Myo1c, but not motor-dead mutants or those unable to bind membrane, reverses the phenotype, evidence that Myo1c modulates the assembly/maintenance of adherens junctions.

Cooperation between cadherins and the actin cytoskeleton controls the formation and maintenance of cell–cell adhesions in epithelia. We find that the molecular motor protein myosin-1c (Myo1c) regulates the dynamic stability of E-cadherin–based cell–cell contacts. In Myo1c-depleted Madin–Darby canine kidney cells, E-cadherin localization was dis­organized and lateral membranes appeared less vertical with convoluted edges versus control cells. In polarized monolayers, Myo1c-knockdown (KD) cells were more sensitive to reduced calcium concentration. Myo1c separated in the same plasma membrane fractions as E-cadherin, and Myo1c KD caused a significant reduction in the amount of E-cadherin recovered in one peak fraction. Expression of green fluorescent protein (GFP)–Myo1c mutants revealed that the phosphatidylinositol-4,5-bisphosphate–binding site is necessary for its localization to cell–cell adhesions, and fluorescence recovery after photobleaching assays with GFP-Myo1c mutants revealed that motor function was important for Myo1c dynamics at these sites. At 18°C, which inhibits vesicle recycling, Myo1c-KD cells accumulated more E-cadherin–positive vesicles in their cytoplasm, suggesting that Myo1c affects E-cadherin endocytosis. Studies with photoactivatable GFP–E-cadherin showed that Myo1c KD reduced the stability of E-cadherin at cell–cell adhesions. We conclude that Myo1c stabilizes E-cadherin at adherens junctions in polarized epithelial cells and that the motor function and ability of Myo1c to bind membrane are critical.

Phosphatidylinositol 4,5-bisphosphate decreases the concentration of Ca2+, phosphatidylserine and diacylglycerol required for protein kinase C α to reach maximum activity

The C2 domain of PKCα possesses two different binding sites, one for Ca(2+) and phosphatidylserine and a second one that binds PIP2 with very high affinity. The enzymatic activity of PKCα was studied by activating it with large unilamellar lipid vesicles, varying the concentration of Ca(2+) and the contents of dioleylglycerol (DOG), phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphadidylserine (POPS) in these model membranes. The results showed that PIP2 increased the Vmax of PKCα and, when the PIP2 concentration was 5 mol% of the total lipid in the membrane, the addition of 2 mol% of DOG did not increase the activity. In addition PIP2 decreases K0.5 of Ca(2+) more than 3-fold, that of DOG almost 5-fold and that of POPS by a half. The K0.5 values of PIP2 amounted to only 0.11 µM in the presence of DOG and 0.39 in its absence, which is within the expected physiological range for the inner monolayer of a mammalian plasma membrane. As a consequence, PKCα may be expected to operate near its maximum capacity even in the absence of a cell signal producing diacylglycerol. Nevertheless, we have shown that the presence of DOG may also help, since the K0.5 for PIP2 notably decreases in its presence. Taken together, these results underline the great importance of PIP2 in the activation of PKCα and demonstrate that in its presence, the most important cell signal for triggering the activity of this enzyme is the increase in the concentration of cytoplasmic Ca(2+).

The functions of anionic phospholipids during clathrin-mediated endocytosis site initiation and vesicle formation

Anionic phospholipids PI(4,5)P2 and phosphatidylserine (PS) are enriched in the cytosolic leaflet of the plasma membrane where endocytic sites form. In this study, we investigated the roles of PI(4,5)P2 and PS in clathrin-mediated endocytosis (CME) site initiation and vesicle formation in Saccharomyces cerevisiae. Live-cell imaging of endocytic protein dynamics in an mss4(ts) mutant, which has severely reduced PI(4,5)P2 levels, revealed that PI(4,5)P2 is required for endocytic membrane invagination but is less important for endocytic site initiation. We also demonstrated that, in various deletion mutants of genes encoding components of the Rcy1-Ypt31/32 GTPase pathway, endocytic proteins dynamically assemble not only on the plasma membrane but also on intracellular membrane compartments, which are likely derived from early endosomes. In rcy1Δ cells, fluorescent biosensors indicated that PI(4,5)P2 only localized to the plasma membrane while PS localized to both the plasma membrane and intracellular membranes. Furthermore, we found that polarized endocytic patch establishment is defective in the PS-deficient cho1Δ mutant. We propose that PS is important for directing endocytic proteins to the plasma membrane and that PI(4,5)P2 is required to facilitate endocytic membrane invagination.

Caveolin-1: role in cell signaling

Caveolins (Cavs) are integrated plasma membrane proteins that are complex signaling regulators with numerous partners and whose activity is highly dependent on cellular context. Cavs are both positive and negative regulators of cell signaling in and/or out of caveolae, invaginated lipid raft domains whose formation is caveolin expression dependent. Caveolins and rafts have been implicated in membrane compartmentalization; proteins and lipids accumulate in these membrane microdomains where they transmit fast, amplified and specific signaling cascades. The concept of plasma membrane organization within functional rafts is still in exploration and sometimes questioned. In this chapter, we discuss the opposing functions of caveolin in cell signaling regulation focusing on the role of caveolin both as a promoter and inhibitor of different signaling pathways and on the impact of membrane domain localization on caveolin functionality in cell proliferation, survival, apoptosis and migration.

Lipids of plant membrane rafts

Lipids tend to organize in mono or bilayer phases in a hydrophilic environment. While they have long been thought to be incapable of coherent lateral segregation, it is now clear that spontaneous assembly of these compounds can confer microdomain organization beyond spontaneous fluidity. Membrane raft microdomains have the ability to influence spatiotemporal organization of protein complexes, thereby allowing regulation of cellular processes. In this review, we aim at summarizing briefly: (i) the history of raft discovery in animals and plants, (ii) the main findings about structural and signalling plant lipids involved in raft segregation, (iii) imaging of plant membrane domains, and their biochemical purification through detergent-insoluble membranes, as well as the existing debate on the topic. We also discuss the potential involvement of rafts in the regulation of plant physiological processes, and further discuss the prospects of future research into plant membrane rafts.

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.

Rab38 targets to lamellar bodies and normalizes their sizes in lung alveolar type II epithelial cells

Rab38 is a rat Hermansky-Pudlak syndrome gene that plays an important role in surfactant homeostasis in alveolar type II (ATII) pneumocytes. We examined Rab38 function in regulating lamellar body (LB) morphology in ATII cells. Quantitative electron microscopy revealed that LBs in ATII cells were ∼77% larger in Rab38-null fawn-hooded hypertension (FHH) than control Sprague-Dawley (SD) rats. Rab38 protein expression was restricted in lung epithelial cells but was not found in primary endothelial cells. In SD ATII cells, Rab38 protein level gradually declined during 5 days in culture. Importantly, endogenous Rab38 was present in LB fractions purified from SD rat lungs, and transiently expressed enhanced green fluorescent protein (EGFP)-tagged Rab38 labeled only the limiting membranes of a subpopulation (∼30%) of LBs in cultured ATII cells. This selective targeting was abolished by point mutations to EGFP-Rab38 and was not shared by Rab7 and Rab4b, which also function in the ATII cells. Using confocal microscopy, we established a method for quantitative evaluation of the enlarged LB phenotype temporally preserved in cultured FHH ATII cells. A direct causal relationship was established when the enlarged LB phenotype was reserved and then rescued by transiently reexpressed EGFP-Rab38 in cultured FHH ATII cells. This rescuing effect was associated with dynamic EGFP-Rab38 targeting to and on LB limiting membranes. We conclude that Rab38 plays an indispensible role in maintaining LB morphology and surfactant homeostasis in ATII pneumocytes.

The Pathobiology of Neisseria gonorrhoeae Lower Female Genital Tract Infection

Infection and disease associated with Neisseria gonorrhoeae, the gonococcus, continue to be a global health problem. Asymptomatic and subclinical gonococcal infections occur at a high frequency in females; thus, the true incidence of N. gonorrhoeae infections are presumed to be severely underestimated. Inherent to this asymptomatic/subclinical diseased state is the continued prevalence of this organism within the general population, as well as the medical, economic, and social burden equated with the observed chronic, disease sequelae. As infections of the lower female genital tract (i.e., the uterine cervix) commonly result in subclinical disease, it follows that the pathobiology of cervical gonorrhea would differ from that observed for other sites of infection. In this regard, the potential responses to infection that are generated by the female reproductive tract mucosa are unique in that they are governed, in part, by cyclic fluctuations in steroid hormone levels. The lower female genital tract has the further distinction of being able to functionally discriminate between resident commensal microbiota and transient pathogens. The expression of functionally active complement receptor 3 by the lower, but not the upper, female genital tract mucosa; together with data indicating that gonococcal adherence to and invasion of primary cervical epithelial cells and tissue are predominately aided by this surface-expressed host molecule; provide one explanation for asymptomatic/subclinical gonococcal cervicitis. However, co-evolution of the gonococcus with its sole human host has endowed this organism with variable survival strategies that not only aid these bacteria in successfully evasion of immune detection and function but also enhance cervical colonization and cellular invasion. To this end, we herein summarize current knowledge pertaining to the pathobiology of gonococcal infection of the human cervix.

Localization of myosin 1b to actin protrusions requires phosphoinositide binding

Myosin 1b (Myo1b), a class I myosin, is a widely expressed, single-headed, actin-associated molecular motor. Transient kinetic and single-molecule studies indicate that it is kinetically slow and responds to tension. Localization and subcellular fractionation studies indicate that Myo1b associates with the plasma membrane and certain subcellular organelles such as endosomes and lysosomes. Whether Myo1b directly associates with membranes is unknown. We demonstrate here that full-length rat Myo1b binds specifically and with high affinity to phosphatidylinositol 4,5-bisphosphate (PIP(2)) and phosphatidylinositol 3,4,5-triphosphate (PIP(3)), two phosphoinositides that play important roles in cell signaling. Binding is not Ca(2+)-dependent and does not involve the calmodulin-binding IQ region in the neck domain of Myo1b. Furthermore, the binding site is contained entirely within the C-terminal tail region, which contains a putative pleckstrin homology domain. Single mutations in the putative pleckstrin homology domain abolish binding of the tail domain of Myo1b to PIP(2) and PIP(3) in vitro. These same mutations alter the distribution of Myc-tagged Myo1b at membrane protrusions in HeLa cells where PIP(2) localizes. In addition, we found that motor activity is required for Myo1b localization in filopodia. These results suggest that binding of Myo1b to phosphoinositides plays an important role in vivo by regulating localization to actin-enriched membrane projections.

Polyphosphoinositides are enriched in plant membrane rafts and form microdomains in the plasma membrane

In this article, we analyzed the lipid composition of detergent-insoluble membranes (DIMs) purified from tobacco (Nicotiana tabacum) plasma membrane (PM), focusing on polyphosphoinositides, lipids known to be involved in various signal transduction events. Polyphosphoinositides were enriched in DIMs compared with whole PM, whereas all structural phospholipids were largely depleted from this fraction. Fatty acid composition analyses suggest that enrichment of polyphosphoinositides in DIMs is accompanied by their association with more saturated fatty acids. Using an immunogold-electron microscopy strategy, we were able to visualize domains of phosphatidylinositol 4,5-bisphosphate in the plane of the PM, with 60% of the epitope found in clusters of approximately 25 nm in diameter and 40% randomly distributed at the surface of the PM. Interestingly, the phosphatidylinositol 4,5-bisphosphate cluster formation was not significantly sensitive to sterol depletion induced by methyl-beta-cyclodextrin. Finally, we measured the activities of various enzymes of polyphosphoinositide metabolism in DIMs and PM and showed that these activities are present in the DIM fraction but not enriched. The putative role of plant membrane rafts as signaling membrane domains or membrane-docking platforms is discussed.

Differential regulation of phospholipase C-beta2 activity and membrane interaction by Galphaq, Gbeta1gamma2, and Rac2

We combined fluorescence recovery after photobleaching (FRAP) beam-size analysis with biochemical assays to investigate the mechanisms of membrane recruitment and activation of phospholipase C-beta(2) (PLCbeta(2)) by G protein alpha(q) and betagamma dimers. We show that activation by alpha(q) and betagamma differ from activation by Rac2 and from each other. Stimulation by alpha(q) enhanced the plasma membrane association of PLCbeta(2), but not of PLCbeta(2)Delta, which lacks the alpha(q)-interacting region. Although alpha(q) resembled Rac2 in increasing the contribution of exchange to the FRAP of PLCbeta(2) and in enhancing its membrane association, the latter effect was weaker than with Rac2. Moreover, the membrane recruitment of PLCbeta(2) by alpha(q) occurred by enhancing PLCbeta(2) association with fast-diffusing (lipid-like) membrane components, whereas stimulation by Rac2 led to interactions with slow diffusing membrane sites. On the other hand, activation by betagamma shifted the FRAP of PLCbeta(2) and PLCbeta(2)Delta to pure lateral diffusion 3- to 5-fold faster than lipids, suggesting surfing-like diffusion along the membrane. We propose that these different modes of PLCbeta(2) membrane recruitment may accommodate contrasting functional needs to hydrolyze phosphatidylinositol 4,5-bisphosphate (PtdInsP(2)) in localized versus dispersed populations. PLCbeta(2) activation by Rac2, which leads to slow lateral diffusion and much faster exchange, recruits PLCbeta(2) to act locally on PtdInsP(2) at specific domains. Activation by alpha(q) leads to lipid-like diffusion of PLCbeta(2) accompanied by exchange, enabling the sampling of larger, yet limited, areas prior to dissociation. Finally, activation by betagamma recruits PLCbeta(2) to the membrane by transient interactions, leading to fast "surfing" diffusion along the membrane, sampling large regions for dispersed PtdInsP(2) populations.

Novel role of cPLA(2)alpha in membrane and actin dynamics

Actin-directed processes such as membrane ruffling and cell migration are regulated by specific signal transduction pathways that become activated by growth factor receptors. The same signaling pathways that lead to modifications in actin dynamics also activate cPLA(2)alpha. Moreover, arachidonic acid, the product of cPLA(2)alpha activity, is involved in regulation of actin dynamics. Therefore, it was investigated whether cPLA(2)alpha plays a role in actin dynamics, more specifically during growth factor-induced membrane ruffling and cell migration. Upon stimulation of ruffling and cell migration by growth factors, endogenous cPLA(2)alpha and its active phosphorylated form were shown to relocate at protrusions of the cell membrane involved in actin and membrane dynamics. Inhibition of cPLA(2)alpha activity with specific inhibitors blocked growth factor-induced membrane and actin dynamics, suggesting an important role for cPLA(2)alpha in these processes.

Arabidopsis Rab-E GTPases exhibit a novel interaction with a plasma-membrane phosphatidylinositol-4-phosphate 5-kinase

Rab GTPases of the Arabidopsis Rab-E subclass are related to mammalian Rab8 and are implicated in membrane trafficking from the Golgi to the plasma membrane. Using a yeast two-hybrid assay, Arabidopsis phosphatidylinositol-4-phosphate 5-kinase 2 (PtdIns(4)P 5-kinase 2; also known as PIP5K2), was shown to interact with all five members of the Rab-E subclass but not with other Rab subclasses residing at the Golgi or trans-Golgi network. Interactions in yeast and in vitro were strongest with RAB-E1d[Q74L] and weakest with the RAB-E1d[S29N] suggesting that PIP5K2 interacts with the GTP-bound form. PIP5K2 exhibited kinase activity towards phosphatidylinositol phosphates with a free 5-hydroxyl group, consistent with PtdIns(4)P 5-kinase activity and this activity was stimulated by Rab binding. Rab-E proteins interacted with PIP5K2 via its membrane occupancy and recognition nexus (MORN) domain which is missing from animal and fungal PtdIns(4)P 5-kinases. In plant cells, GFP:PIP5K2 accumulated at the plasma membrane and caused YFP:RAB-E1d to relocate there from its usual position at the Golgi. GFP:PIP5K2 was rapidly turned over by proteasomal activity in planta, and overexpression of YFP:PIP5K2 caused pleiotropic growth abnormalities in transgenic Arabidopsis. We propose that plant cells exhibit a novel interaction in which PIP5K2 binds GTP-bound Rab-E proteins, which may stimulate temporally or spatially localized PtdIns(4,5)P(2) production at the plasma membrane.

Calcium-dependent lateral organization in phosphatidylinositol 4,5-bisphosphate (PIP2)- and cholesterol-containing monolayers

Biological membrane function, in part, depends upon the local regulation of lipid composition. The spatial heterogeneity of membrane lipids has been extensively explored in the context of cholesterol and phospholipid acyl-chain-dependent domain formation, but the effects of lipid head groups and soluble factors in lateral lipid organization are less clear. In this contribution, the effects of divalent calcium ions on domain formation in monolayers containing phosphatidylinositol 4,5-bisphosphate (PIP2), a polyanionic, multifunctional lipid of the cytosolic leaflet of the plasma bilayer, are reported. In binary monolayers of PIP2 mixed with zwitterionic lipids, calcium induced a rapid, PIP2-dependent surface pressure drop, with the concomitant formation of laterally segregated, PIP2-rich domains. The effect was dependent upon head-group multivalency, because lowered pH suppressed the surface-pressure effect and domain formation. In accordance with previous observations, inclusion of cholesterol in lipid mixtures induced coexistence of two liquid phases. Phase separation strongly segregated PIP2 to the cholesterol-poor phase, suggesting a role for cholesterol-dependent lipid demixing in regulating PIP2 localization and local concentration. Similar to binary mixtures, subphase calcium induced contraction of ternary cholesterol-containing monolayers; however, in these mixtures, calcium induced an unexpected, PIP2- and multivalency-dependent decrease in the miscibility phase transition surface pressure, resulting in rapid dissolution of the domains. This result emphasizes the likely critical role of subphase factors and lipid head-group specificity in the formation and stability of cholesterol-dependent domains in cellular plasma membranes.

Kinetics of the interaction of myo1c with phosphoinositides

myo1c is a single-headed myosin that dynamically links membranes to the actin cytoskeleton. A putative pleckstrin homology domain has been identified in the myo1c tail that binds phosphoinositides and soluble inositol phosphates with high affinity. However, the kinetics of association and dissociation and the influence of phospholipid composition on the kinetics have not been determined. Stopped-flow spectroscopy was used to measure the binding and dissociation of a recombinant myo1c construct containing the tail and regulatory domains (myo1c(IQ-tail)) to and from 100-nm diameter large unilamellar vesicles (LUVs). We found the time course of association of myo1c(IQ-tail) with LUVs containing 2% phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) followed a two-exponential time course, and the rate of the predominant fast phase depended linearly upon the total lipid concentration. The apparent second-order rate constant was approximately diffusion-limited. Increasing the molar ratio of anionic phospholipid by adding phosphatidylserine, additional PtdIns(4,5)P(2), or by situating PtdIns(4,5)P(2) in a more physiologically relevant lipid background increased the apparent association rate constant less than 2-fold. myo1c(IQ-tail) dissociated from PtdIns(4,5)P(2) at a slower rate (2.0 s(-1)) than the pleckstrin homology domain of phospholipase C-delta (13 s(-1)). The presence of additional anionic phospholipid reduced the myo1c(IQ-tail) dissociation rate constant >50-fold but marginally changed the dissociation rate of phospholipase C-delta, suggesting that additional electrostatic interactions in myo1c(IQ-tail) help to stabilize binding. Remarkably, high concentrations of soluble inositol phosphates induce dissociation of myo1c(IQ-tail) from LUVs, suggesting that phosphoinositides are able to bind to and dissociate from myo1c(IQ-tail) as it remains bound to the membrane.

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

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

F-BAR-containing adaptor CIP4 localizes to early endosomes and regulates Epidermal Growth Factor Receptor trafficking and downregulation

Cdc42-Interacting Protein-4 (CIP4) family adaptors have been implicated in promoting Epidermal Growth Factor Receptor (EGFR) internalization, however, their unique or overlapping functions remain unclear. Here, we show that although CIP4 was not required for early events in clathrin-mediated endocytosis of EGFR, CIP4 localizes to vesicles containing EGFR and Rab5. Furthermore, expression of constitutively active Rab5 led to accumulation of CIP4 and the related adaptor Toca-1 in giant endosomes. Using a mutagenesis approach, we show that localization of CIP4 to endosomes is mediated in part via the curved phosphoinositide-binding face of the CIP4 F-BAR domain. Downregulation of CIP4 in A431 epidermoid carcinoma cells by RNA interference led to elevated EGFR levels, compared to control cells. Although surface expression of EGFR was not affected by CIP4 silencing, EGF-induced transit of EGFR from EEA1-positive endosomes to lysosomes was reduced compared to control cells. This correlated with more robust activation of ERK kinase and entry to S phase in CIP4-depleted A431 cells, compared to control cells. The combined silencing of CIP4 and Toca-1 was more effective in driving cells into S phase, suggesting a partial redundancy in their functions. Overall, our results implicate CIP4 and Toca-1 in regulating late events in EGFR trafficking from endosomes that serves to limit sustained ERK activation within the endosomal compartment.

A distinct pool of phosphatidylinositol 4,5-bisphosphate in caveolae revealed by a nanoscale labeling technique

Multiple functionally independent pools of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)] have been postulated to occur in the cell membrane, but the existing techniques lack sufficient resolution to unequivocally confirm their presence. To analyze the distribution of PI(4,5)P(2) at the nanoscale, we developed an electron microscopic technique that probes the freeze-fractured membrane preparation by the pleckstrin homology domain of phospholipase C-delta1. This method does not require chemical fixation or expression of artificial probes, it is applicable to any cell in vivo and in vitro, and it can define the PI(4,5)P(2) distribution quantitatively. By using this method, we found that PI(4,5)P(2) is highly concentrated at the rim of caveolae both in cultured fibroblasts and mouse smooth muscle cells in vivo. PI(4,5)P(2) was also enriched in the coated pit, but only a low level of clustering was observed in the flat undifferentiated membrane. When cells were treated with angiotensin II, the PI(4,5)P(2) level in the undifferentiated membrane decreased to 37.9% within 10 sec and then returned to the initial level. Notably, the PI(4,5)P(2) level in caveolae showed a slower but more drastic change and decreased to 20.6% at 40 sec, whereas the PI(4,5)P(2) level in the coated pit was relatively constant and decreased only to 70.2% at 10 sec. These results show the presence of distinct PI(4,5)P(2) pools in the cell membrane and suggest a unique role for caveolae in phosphoinositide signaling.

Phosphatidylinositol metabolism and membrane fusion

Membrane fusion underlies many cellular events, including secretion, exocytosis, endocytosis, organelle reconstitution, transport from endoplasmic reticulum to Golgi and nuclear envelope formation. A large number of investigations into membrane fusion indicate various roles for individual members of the phosphoinositide class of membrane lipids. We first review the phosphoinositides as membrane recognition sites and their regulatory functions in membrane fusion. We then consider how modulation of phosphoinositides and their products may affect the structure and dynamics of natural membranes facilitating fusion. These diverse roles underscore the importance of these phospholipids in the fusion of biological membranes.

Reversible binding and rapid diffusion of proteins in complex with inositol lipids serves to coordinate free movement with spatial information

Polyphosphoinositol lipids convey spatial information partly by their interactions with cellular proteins within defined domains. However, these interactions are prevented when the lipids' head groups are masked by the recruitment of cytosolic effector proteins, whereas these effectors must also have sufficient mobility to maximize functional interactions. To investigate quantitatively how these conflicting functional needs are optimized, we used different fluorescence recovery after photobleaching techniques to investigate inositol lipid–effector protein kinetics in terms of the real-time dissociation from, and diffusion within, the plasma membrane. We find that the protein–lipid complexes retain a relatively rapid (∼0.1–1 µm²/s) diffusion coefficient in the membrane, likely dominated by protein–protein interactions, but the limited time scale (seconds) of these complexes, dictated principally by lipid–protein interactions, limits their range of action to a few microns. Moreover, our data reveal that GAP1^IP4BP, a protein that binds PtdIns(4,5)P₂ and PtdIns(3,4,5)P₃ in vitro with similar affinity, is able to “read” PtdIns(3,4,5)P₃ signals in terms of an elongated residence time at the membrane.

Contributions of F-BAR and SH2 domains of Fes protein tyrosine kinase for coupling to the FcepsilonRI pathway in mast cells

This study investigates the roles of Fer-CIP4 homology (FCH)-Bin/amphiphysin/Rvs (F-BAR) and SH2 domains of Fes protein tyrosine kinase in regulating its activation and signaling downstream of the high-affinity immunoglobulin G (IgE) receptor (FcepsilonRI) in mast cells. Homology modeling of the Fes F-BAR domain revealed conservation of some basic residues implicated in phosphoinositide binding (R113/K114). The Fes F-BAR can bind phosphoinositides and induce tubulation of liposomes in vitro. Mutation of R113/K114 to uncharged residues (RK/QQ) caused a significant reduction in phosphoinositide binding in vitro and a more diffuse cytoplasmic localization in transfected COS-7 cells. RBL-2H3 mast cells expressing full-length Fes carrying the RK/QQ mutation show defects in FcepsilonRI-induced Fes tyrosine phosphorylation and degranulation compared to cells expressing wild-type Fes. This correlated with reduced localization to Lyn kinase-containing membrane fractions for the RK/QQ mutant compared to wild-type Fes in mast cells. The Fes SH2 domain also contributes to Fes signaling in mast cells, via interactions with the phosphorylated FcepsilonRI beta chain and the actin regulatory protein HS1. We show that Fes phosphorylates C-terminal tyrosine residues in HS1 implicated in actin stabilization. Thus, coordinated actions of the F-BAR and SH2 domains of Fes allow for coupling to FcepsilonRI signaling and potential regulation the actin reorganization in mast cells.

Phosphoinositides in insulin action on GLUT4 dynamics: not just PtdIns(3,4,5)P3

Accumulated evidence over the last several years indicates that insulin regulates multiple steps in the overall translocation of GLUT4 vesicles to the fat/muscle cell surface, including formation of an intracellular storage pool of GLUT4 vesicles, its movement to the proximity of the cell surface, and the subsequent docking/fusion with the plasma membrane. Insulin-stimulated formation of phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P(3); and in some cases, of its catabolite PtdIns(3,4)P(2)] plays a pivotal role in this process. PtdIns(3,4,5)P(3) is synthesized by the activated wortmannin-sensitive class IA phosphoinositide (PI) 3-kinase and controls the rate-limiting cell surface terminal stages of the GLUT4 journey. However, recent research is consistent with the conclusion that signals by each of the remaining five PIs, i.e., PtdIns(3)P, PtdIns(4)P, PtdIns(5)P, PtdIns(3,5)P(2), and PtdIns(4,5)P(2), may act in concert with that of PtdIns(3,4,5)P(3) in integrating the insulin receptor-issued signals with GLUT4 surface translocation and glucose transport activation. This review summarizes the experimental evidence supporting the complementary function of these PIs in insulin responsiveness of fat and muscle cells, with particular reference to mechanistic insights and functional significance in the regulation of overall GLUT4 vesicle dynamics.

Compartmentalization of phosphatidylinositol 4,5-bisphosphate signaling evidenced using targeted phosphatases

Phosphatidylinositol 4,5-bisphosphate (PIP(2)) is a prevalent phosphoinositide in cell membranes, with important functions in cell signaling and activation. A large fraction of PIP(2) associates with the detergent-resistant membrane "raft" fraction, but the functional significance of this association remains controversial. To measure the properties of raft and nonraft PIP(2) in cell signaling, we targeted the PIP(2)-specific phosphatase Inp54p to either the raft or nonraft membrane fraction using minimal membrane anchors. Interestingly, we observed that targeting Inp54p to the nonraft fraction resulted in an enrichment of raft-associated PIP(2) and striking changes in cell morphology, including a wortmannin-sensitive increase in cell filopodia and cell spreading. In contrast, raft-targeted Inp54p depleted the raft pool of PIP(2) and produced smooth T cells void of membrane ruffling and filopodia. Furthermore, raft-targeted Inp54p inhibited capping in T cells stimulated by cross-linking the T cell receptor, but without affecting the T cell receptor-dependent Ca(2+) flux. Altogether, these results provide evidence of compartmentalization of PIP(2)-dependent signaling in cell membranes such as predicted by the membrane raft model.

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

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

Combined electrostatics and hydrogen bonding determine intermolecular interactions between polyphosphoinositides

Membrane lipids are active contributors to cell function as key mediators in signaling pathways controlling cell functions including inflammation, apoptosis, migration, and proliferation. Recent work on multimolecular lipid structures suggests a critical role for lipid organization in regulating the function of both lipids and proteins. Of particular interest in this context are the polyphosphoinositides (PPI's), especially phosphatidylinositol (4,5) bisphosphate (PIP 2). The cellular functions of PIP 2 are numerous but the organization of PIP 2 in the inner leaflet of the plasma membrane, as well as the factors controlling targeting of PIP 2 to specific proteins, remains poorly understood. To analyze the organization of PIP 2 in a simplified planar system, we used Langmuir monolayers to study the effects of subphase conditions on monolayers of purified naturally derived PIP 2 and other anionic or zwitterionic phospholipids. We report a significant molecular area expanding effect of subphase monovalent salts on PIP 2 at biologically relevant surface densities. This effect is shown to be specific to PIP 2 and independent of subphase pH. Chaotropic agents (e.g., salts, trehalose, urea, temperature) that disrupt water structure and the ability of water to mediate intermolecular hydrogen bonding also specifically expanded PIP 2 monolayers. These results suggest a combination of water-mediated hydrogen bonding and headgroup repulsion in determining the organization of PIP 2, and may contribute to an explanation for the unique functionality of PIP 2 compared to other anionic phospholipids.

Structural basis and mechanism of autoregulation in 3-phosphoinositide-dependent Grp1 family Arf GTPase exchange factors

Arf GTPases regulate membrane trafficking and actin dynamics. Grp1, ARNO, and Cytohesin-1 comprise a family of phosphoinositide-dependent Arf GTPase exchange factors with a Sec7-pleckstrin homology (PH) domain tandem. Here, we report that the exchange activity of the Sec7 domain is potently autoinhibited by conserved elements proximal to the PH domain. The crystal structure of the Grp1 Sec7-PH tandem reveals a pseudosubstrate mechanism of autoinhibition in which the linker region between domains and a C-terminal amphipathic helix physically block the docking sites for the switch regions of Arf GTPases. Mutations within either element result in partial or complete activation. Critical determinants of autoinhibition also contribute to insulin-stimulated plasma membrane recruitment. Autoinhibition can be largely reversed by binding of active Arf6 to Grp1 and by phosphorylation of tandem PKC sites in Cytohesin-1. These observations suggest that Grp1 family GEFs are autoregulated by mechanisms that depend on plasma membrane recruitment for activation.

Regulation of the epithelial sodium channel by phosphatidylinositides: experiments, implications, and speculations

Recent studies suggest that the activity of epithelial sodium channels (ENaC) is increased by phosphatidylinositides, especially phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) and phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)). Stimulation of phospholipase C by either adenosine triphosphate (ATP)-activation of purinergic P2Y receptors or epidermal growth factor (EGF)-activation of EGF receptors reduces membrane PI(4,5)P(2), and consequently decreases ENaC activity. Since ATP and EGF may be trapped in cysts formed by the distal tubule, it is possible that ENaC inhibition induced by ATP and EGF facilitates cyst formation in polycystic kidney diseases (PKD). However, some results suggest that ENaC activity is increased in PKD. In contrast to P2Y and EGF receptors, stimulation of insulin-like growth factor-1 (IGF-1) receptor by aldosterone or insulin produces PI(3,4,5)P(3), and consequently increases ENaC activity. The acute effect of aldosterone on ENaC activity through PI(3,4,5)P(3) possibly accounts for the initial feedback for blood volume recovery after hypovolemic hypotension. PI(4,5)P(2) and PI(3,4,5)P(3), respectively, interacts with the N terminus of beta-ENaC and the C terminus of gamma-ENaC. However, whether ENaC selectively binds to PI(4,5)P(2) and PI(3,4,5)P(3) over other anionic phospholipids remains unclear.

"Actin"g on GLUT4: membrane & cytoskeletal components of insulin action

The dissection of mechanisms that regulate glucose transport by insulin has revealed an intricate network of signaling molecules scattered from the insulin receptor to the intracellular glucose transporter GLUT4. It is also appreciated that some insulin receptor signals jaunt in different directions to regulate events essential for the efficient redistribution of GLUT4 to the plasma membrane. Moreover key assists in the process appear to be arranged by membrane lipids and cytoskeletal proteins. Following current considerations of insulin signals regulating GLUT4, this review will focus on in vitro and in vivo evidence that supports an essential role for phosphoinositides and actin filaments in the control of glucose transport. The discussion will visit recent cell culture, whole animal, and human data highlighting membrane and cytoskeletal aspects of insulin resistance.

Insulin stimulates membrane fusion and GLUT4 accumulation in clathrin coats on adipocyte plasma membranes

Total internal reflection fluorescence (TIRF) microscopy reveals highly mobile structures containing enhanced green fluorescent protein-tagged glucose transporter 4 (GLUT4) within a zone about 100 nm beneath the plasma membrane of 3T3-L1 adipocytes. We developed a computer program (Fusion Assistant) that enables direct analysis of the docking/fusion kinetics of hundreds of exocytic fusion events. Insulin stimulation increases the fusion frequency of exocytic GLUT4 vesicles by approximately 4-fold, increasing GLUT4 content in the plasma membrane. Remarkably, insulin signaling modulates the kinetics of the fusion process, decreasing the vesicle tethering/docking duration prior to membrane fusion. In contrast, the kinetics of GLUT4 molecules spreading out in the plasma membrane from exocytic fusion sites is unchanged by insulin. As GLUT4 accumulates in the plasma membrane, it is also immobilized in punctate structures on the cell surface. A previous report suggested these structures are exocytic fusion sites (Lizunov et al., J. Cell Biol. 169:481-489, 2005). However, two-color TIRF microscopy using fluorescent proteins fused to clathrin light chain or GLUT4 reveals these structures are clathrin-coated patches. Taken together, these data show that insulin signaling accelerates the transition from docking of GLUT4-containing vesicles to their fusion with the plasma membrane and promotes GLUT4 accumulation in clathrin-based endocytic structures on the plasma membrane.

Plasma membrane sterol distribution resembles the surface topography of living cells

Cholesterol is an important constituent of cellular membranes. It has been suggested that cholesterol segregates into sterol-rich and -poor domains in the plasma membrane, although clear evidence for this is lacking. By fluorescence imaging of the natural sterol dehydroergosterol (DHE), the lateral sterol distribution has been visualized in living cells. The spatial labeling pattern of DHE coincided with surface structures such as ruffles, microvilli, and filopodia with correlation lengths in the range of 0.8-2.5 microm. DHE staining of branched tubules and of nanotubes connecting two cells was detected. Dynamics of DHE in folded and plane membrane regions was comparable as determined by fluorescence recovery after photobleaching. DHE colocalized with fluid membrane-preferring phospholipids in surface structures and at sites of cell attachment as well as in the cleavage furrow of dividing cells, but it was not particularly enriched in those regions. Fluorescent sterol showed homogeneous staining in membrane blebs induced by F-actin disruption. Cross-linking the ganglioside GM1--a putative raft marker--did not affect the cell surface distribution of DHE. The results suggest that spatial heterogeneities of plasma membrane staining of DHE resolvable by light microscopy reflect the cell surface topography but not phase-separated sterol domains in the bilayer plane.

Dynamic phospholipid signaling by G protein-coupled receptors

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

Myo1c binds phosphoinositides through a putative pleckstrin homology domain

Myo1c is a member of the myosin superfamily that binds phosphatidylinositol-4,5-bisphosphate (PIP(2)), links the actin cytoskeleton to cellular membranes and plays roles in mechano-signal transduction and membrane trafficking. We located and characterized two distinct membrane binding sites within the regulatory and tail domains of this myosin. By sequence, secondary structure, and ab initio computational analyses, we identified a phosphoinositide binding site in the tail to be a putative pleckstrin homology (PH) domain. Point mutations of residues known to be essential for polyphosphoinositide binding in previously characterized PH domains inhibit myo1c binding to PIP(2) in vitro, disrupt in vivo membrane binding, and disrupt cellular localization. The extended sequence of this binding site is conserved within other myosin-I isoforms, suggesting they contain this putative PH domain. We also characterized a previously identified membrane binding site within the IQ motifs in the regulatory domain. This region is not phosphoinositide specific, but it binds anionic phospholipids in a calcium-dependent manner. However, this site is not essential for in vivo membrane binding.

Phosphatidylinositol 3-phosphate [PtdIns3P] is generated at the plasma membrane by an inositol polyphosphate 5-phosphatase: endogenous PtdIns3P can promote GLUT4 translocation to the plasma membrane

Exogenous delivery of carrier-linked phosphatidylinositol 3-phosphate [PtdIns(3)P] to adipocytes promotes the trafficking, but not the insertion, of the glucose transporter GLUT4 into the plasma membrane. However, it is yet to be demonstrated if endogenous PtdIns(3)P regulates GLUT4 trafficking and, in addition, the metabolic pathways mediating plasma membrane PtdIns(3)P synthesis are uncharacterized. In unstimulated 3T3-L1 adipocytes, conditions under which PtdIns(3,4,5)P3 was not synthesized, ectopic expression of wild-type, but not catalytically inactive 72-kDa inositol polyphosphate 5-phosphatase (72-5ptase), generated PtdIns(3)P at the plasma membrane. Immunoprecipitated 72-5ptase from adipocytes hydrolyzed PtdIns(3,5)P2, forming PtdIns(3)P. Overexpression of the 72-5ptase was used to functionally dissect the role of endogenous PtdIns(3)P in GLUT4 translocation and/or plasma membrane insertion. In unstimulated adipocytes wild type, but not catalytically inactive, 72-5ptase, promoted GLUT4 translocation and insertion into the plasma membrane but not glucose uptake. Overexpression of FLAG-2xFYVE/Hrs, which binds and sequesters PtdIns(3)P, blocked 72-5ptase-induced GLUT4 translocation. Actin monomer binding, using latrunculin A treatment, also blocked 72-5ptase-stimulated GLUT4 translocation. 72-5ptase expression promoted GLUT4 trafficking via a Rab11-dependent pathway but not by Rab5-mediated endocytosis. Therefore, endogenous PtdIns(3)P at the plasma membrane promotes GLUT4 translocation.

Neisseria gonorrhoeae PLD directly interacts with Akt kinase upon infection of primary, human, cervical epithelial cells

Neisseria gonorrhoeae secrets a phospholipase D (NgPLD), which augments complement receptor 3 (CR3)-mediated invasion of cervical epithelial cells. To elucidate the signalling pathways triggered with gonococcus CR3-engagement and the putative function of NgPLD in these events, we analysed the contribution of the phosphoinositide-Akt pathway to cervical infection. Our data indicated that Akt plays a critical role in cervical infection. Inhibition of myosin light chain kinase, PtdIns(4,5)P2, and Akt functions resulted in decreased gonococcus invasion of primary, human, cervical epithelial cells as well as Akt kinase activity. Akt activity was similarly impaired when cervical cells were challenged with NgPLD-mutant gonococci. Conversely, the PI3-kinase inhibitor, LY294002, enhanced gonococcal invasion of, and Akt activity within, primary cervical cells. We demonstrated that NgPLD directly binds to the Akt PH domain and can compete with a natural Akt ligand, PtdIns(3,4,5)P3, for Akt binding. Collectively, our data suggested that NgPLD augments gonococcus invasion of cervical epithelia by interacting with Akt kinase in a PI3-kinase-independent manner, which results in subversion of normal cervical cell signalling.

Big gulps: specialized membrane domains for rapid receptor-mediated endocytosis

Eukaryotic cells are well known to use an elaborate, clathrin-based endocytic machinery to internalize ligand-receptor complexes. Although the number of components identified as being used during this essential process has increased substantially over the past decade, an appreciation for how receptor-mediated endocytosis is organized at a broader, cellular scale is still mostly undefined. Here, some new insights into how cells cluster and organize the endocytic machinery at defined cytoplasmic domains to increase the speed and efficiency of ligand-receptor internalization are presented.

Tuba stimulates intracellular N-WASP-dependent actin assembly

Tuba is a multidomain scaffolding protein that links cytoskeletal dynamics and membrane trafficking pathways. The N-terminus of Tuba binds dynamin1, and the C-terminus contains domains that can interact with signaling pathways and cytoskeletal regulatory elements. We investigated Tuba localization, distribution and function in B16 melanoma cells. Tuba overexpression stimulated dorsal ruffles that occurred independently of dynamin function. Tuba expression induced actin-driven motility of small puncta that required the C-terminal SH3, GEF and BAR domains. Additionally, Tuba was recruited to lipid vesicles generated by overexpression of phosphatidylinositol-4-phosphate 5-kinase type Ialpha (PIP5Kalpha), localizing prominently to the head of the comets and at lower levels along the actin tail. We propose that Tuba facilitates dorsal ruffling of melanoma cells through direct interaction with actin-regulatory proteins and the recruitment of signaling molecules to lipid microdomains for the coordinated assembly of a cytoskeletal network. Knockdown of Tuba by RNA interference (RNAi) attenuated PIP5Kalpha-generated comet formation and the invasive behavior of B16 cells, implying that Tuba function is required for certain aspects of these processes. These results suggest first that Tuba-stimulated dorsal ruffling might represent a novel mechanism for the coordination of N-WASP-dependent cytoskeletal rearrangements and second that Tuba function is implicated in motility processes.

Insulin receptor signals regulating GLUT4 translocation and actin dynamics

In skeletal muscle and adipose tissue, insulin-stimulated glucose uptake is dependent upon translocation of the insulin-responsive glucose transporter GLUT4 from intracellular storage compartments to the plasma membrane. This insulin-induced redistribution of GLUT4 protein is achieved through a series of highly organized membrane trafficking events, orchestrated by insulin receptor signals. Recently, several key molecules linking insulin receptor signals and membrane trafficking have been identified, and emerging evidence supports the importance of subcellular compartmentalization of signaling components at the right time and in the right place. In addition, the translocation of GLUT4 in adipocytes requires insulin stimulation of dynamic actin remodeling at the inner surface of the plasma membrane (cortical actin) and in the perinuclear region. This results from at least two independent insulin receptor signals, one leading to the activation of phosphatidylinositol (PI) 3-kinase and the other to the activation of the Rho family small GTP-binding protein TC10. Thus, both spatial and temporal regulations of actin dynamics, both beneath the plasma membrane and around endomembranes, by insulin receptor signals are also involved in the process of GLUT4 translocation.