Cholesterol depletion delocalizes phosphatidylinositol bisphosphate and inhibits hormone-stimulated phosphatidylinositol turnover

Does plasma membrane transbilayer asymmetry coupled to lipid nanodomains drive fast kinetics of FGF2 membrane translocation into the extracellular space?

Fibroblast Growth Factor 2 (FGF2) is a potent mitogen secreted from mammalian cells through an unconventional secretory pathway. This process is mediated by direct translocation of FGF2 across the plasma membrane into the extracellular space. It requires several components that are asymmetrically distributed between the two leaflets of the plasma membrane. At the inner plasma membrane leaflet, FGF2 undergoes sequential interactions with the Na,K-ATPase, Tec kinase, and the phosphoinositide PI(4,5)P2. While the Na,K-ATPase, and Tec kinase are auxiliary factors, interactions of FGF2 with PI(4,5)P2 trigger the core mechanism of FGF2 membrane translocation, inducing FGF2-oligomerization-dependent formation of lipidic membrane pores. At the outer plasma membrane leaflet, membrane-inserted FGF2 oligomers are captured and disassembled by Glypican-1 (GPC1), resulting in translocation of FGF2 to the cell surface. In a cellular context, a single FGF2 membrane translocation event occurs within 200 milliseconds. In contrast, in an in vitro system, which uses a fully reconstituted liposomal inside-out system with FGF2 added from the outside and luminal encapsulation of high-affinity heparin molecules, FGF2 membrane translocation takes several minutes. Here, we hypothesize that the observed difference is, at least in part, due to the asymmetrical membrane lipid distribution and the spatial organization of the FGF2 translocation machinery in native plasma membranes. We suggest that the molecular machinery mediating FGF2 membrane translocation assembles in ordered nanodomains, characterized by sphingomyelin (SM), cholesterol and phosphoinositide PI(4,5)P2 coupled together. The transbilayer asymmetry of these lipids likely plays a crucial role in regulating the thermodynamics and kinetics of FGF2-induced membrane pore formation. Therefore, succeeding in reconstituting the FGF2 translocation machinery in artificial membranes with an asymmetric transbilayer distribution of SM, PI(4,5)P2 and other membrane lipids may reveal a direct impact on pore-opening kinetics. Similarly, disrupting lipid asymmetry in cells may significantly impact FGF2 secretion rates, a finding that would underscore the importance of the spatial organization of lipids in membrane dynamics. Testing this hypothesis may advance our understanding of how membrane asymmetry and ordered lipid nanodomains regulate critical biological processes, such as the unconventional secretion of FGF2.

Two cooperative lipid binding sites within the pleckstrin homology domain are necessary for AKT binding and stabilization to the plasma membrane

Almost four decades after the identification of the AKT protein and understanding of its role in cancer, barriers remain in the translation of AKT inhibitors for clinical applications. Here, we provide new molecular insight into the first step of AKT activation where AKT binds to the plasma membrane and its orientation is stabilized in a bilayer with lateral heterogeneity (Lo-Ld phase coexistence). We have applied molecular dynamic simulations and molecular and cell biology approaches, and demonstrate that AKT recruitment to the membrane requires a second binding site in the AKT pleckstrin homology (PH) domain that acts cooperatively with the known canonical binding site. Given the precision with which we have identified the protein-lipid interactions, the study offers new directions for AKT-targeted therapy and for testing small molecules to target these specific amino acid-PIP molecular bonds.

SR-BI regulates the synergistic mast cell response by modulating the plasma membrane-associated cholesterol pool

The high-affinity IgE receptor FcεRI is the mast cell (MC) receptor responsible for the involvement of MCs in IgE-associated allergic disorders. Activation of the FcεRI is achieved via crosslinking by multivalent antigen (Ag) recognized by IgE resulting in degranulation and proinflammatory cytokine production. In comparison to the T- and B-cell receptor complexes, for which several co-receptors orchestrating the initial signaling events have been described, information is scarce about FcεRI-associated proteins. Additionally, it is unclear how FcεRI signaling synergizes with input from other receptors and how regulators affect this synergistic response. We found that the HDL receptor SR-BI (gene name: Scarb1/SCARB1) is expressed in MCs, functionally associates with FcεRI, and regulates the plasma membrane cholesterol content in cholesterol-rich plasma membrane nanodomains. This impacted the activation of MCs upon co-stimulation of the FcεRI with receptors known to synergize with FcεRI signaling. Amongst them, we investigated the co-activation of the FcεRI with the receptor tyrosine kinase KIT, the IL-33 receptor, and GPCRs activated by adenosine or PGE2. Scarb1-deficient bone marrow-derived MCs showed reduced cytokine secretion upon co-stimulation conditions suggesting a role for plasma membrane-associated cholesterol regulating respective MC activation. Mimicking Scarb1 deficiency by cholesterol depletion employing MβCD, we identified PKB and PLCγ1 as cholesterol-sensitive proteins downstream of FcεRI activation in bone marrow-derived MCs. When MCs were co-stimulated with stem cell factor (SCF) and Ag, PLCγ1 activation was boosted, which could be mitigated by cholesterol depletion and SR-BI inhibition. Similarly, SR-BI inhibition attenuated the synergistic response to PGE2 and anti-IgE in the human ROSAKIT WT MC line, suggesting that SR-BI is a crucial regulator of synergistic MC activation.

Lipid Raft Integrity and Cellular Cholesterol Homeostasis Are Critical for SARS-CoV-2 Entry into Cells

Lipid rafts in cell plasma membranes play a critical role in the life cycle of many viruses. However, the involvement of membrane cholesterol-rich lipid rafts in the entry of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) into target cells is not well known. In this study, we investigated whether the presence of cholesterol-rich microdomains is required for the entry of SARS-CoV-2 into host cells. Our results show that depletion of cholesterol in the rafts by methyl-beta-cyclodextrin (MβCD) treatment impaired the expression of the cell surface receptor angiotensin-converting enzyme 2 (ACE2), resulting in a significant increase in SARS-CoV-2 entry into cells. The effects exerted by MβCD could be substantially reversed by exogenous cholesterol replenishment. In contrast, disturbance of intracellular cholesterol homeostasis by statins or siRNA knockdown of key genes involved in the cholesterol biosynthesis and transport pathways reduced SARS-CoV-2 entry into cells. Our study also reveals that SREBP2-mediated cholesterol biosynthesis is involved in the process of SARS-CoV-2 entry in target cells. These results suggest that the host membrane cholesterol-enriched lipid rafts and cellular cholesterol homeostasis are essential for SARS-CoV-2 entry into cells. Pharmacological manipulation of intracellular cholesterol might provide new therapeutic strategies to alleviate SARS-CoV-2 entry into cells.

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 (PHPLCδ) 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.

Compartmentalization of phosphatidylinositol 4,5-bisphosphate metabolism into plasma membrane liquid-ordered/raft domains

Possible segregation of plasma membrane (PM) phosphoinositide metabolism in membrane lipid domains is not fully understood. We exploited two differently lipidated peptide sequences, L10 and S15, to mark liquid-ordered, cholesterol-rich (L_o) and liquid-disordered, cholesterol-poor (L_d) domains of the PM, often called raft and nonraft domains, respectively. Imaging of the fluorescent labels verified that L10 segregated into cholesterol-rich L_o phases of cooled giant plasma-membrane vesicles (GPMVs), whereas S15 and the dye FAST DiI cosegregated into cholesterol-poor L_d phases. The fluorescent protein markers were used as Förster resonance energy transfer (FRET) pairs in intact cells. An increase of homologous FRET between L10 probes showed that depleting membrane cholesterol shrank L_o domains and enlarged L_d domains, whereas a decrease of L10 FRET showed that adding more cholesterol enlarged L_o and shrank L_d Heterologous FRET signals between the lipid domain probes and phosphoinositide marker proteins suggested that phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P ₂] and phosphatidylinositol 4-phosphate (PtdIns4P) are present in both L_o and L_d domains. In kinetic analysis, muscarinic-receptor-activated phospholipase C (PLC) depleted PtdIns(4,5)P ₂ and PtdIns4P more rapidly and produced diacylglycerol (DAG) more rapidly in L_o than in L_d Further, PtdIns(4,5)P ₂ was restored more rapidly in L_o than in L_d Thus destruction and restoration of PtdIns(4,5)P ₂ are faster in L_o than in L_d This suggests that L_o is enriched with both the receptor G protein/PLC pathway and the PtdIns/PI4-kinase/PtdIns4P pathway. The significant kinetic differences of lipid depletion and restoration also mean that exchange of lipids between these domains is much slower than free diffusion predicts.

Cholesterol Stimulates the Transient Receptor Potential Melastatin 4 Channel in mpkCCDc14 Cells

We have shown that cholesterol regulates the activity of ion channels in mouse cortical collecting duct (CCD) mpkCCDc14 cells and that the transient receptor potential melastatin 4 (TRPM4) channel is expressed in these cells. However, whether TRPM4 channel is regulated by cholesterol remains unclear. Here, we performed inside-out patch-clamp experiments and found that inhibition of cholesterol biosynthesis by lovastatin significantly decreased, whereas enrichment of cholesterol with exogenous cholesterol significantly increased, TRPM4 channel open probability (Po) by regulating its sensitivity to Ca2+ in mpkCCDc14 cells. In addition, inside-out patch-clamp data show that acute depletion of cholesterol in the membrane inner leaflet by methyl-β-cyclodextrin (MβCD) significantly reduced TRPM4 Po, which was reversed by exogenous cholesterol. Moreover, immunofluorescence microscopy, Western blot, cell-surface biotinylation, and patch clamp analysis show that neither inhibition of intracellular cholesterol biosynthesis with lovastatin nor application of exogenous cholesterol had effect on TRPM4 channel protein abundance in the plasma membrane of mpkCCDc14 cells. Sucrose density gradient centrifugation studies demonstrate that TRPM4 was mainly located in cholesterol-rich lipid rafts. Lipid-protein overlay experiments show that TRPM4 directly interacted with several anionic phospholipids, including PI(4,5)P2. Depletion of PI(4,5)P2 with either wortmannin or PGE2 abrogated the stimulatory effects of exogenous cholesterol on TRPM4 activity, whereas exogenous PI(4,5)P2 (diC8-PI(4,5)P2, a water-soluble analog) increased the effects. These results suggest that cholesterol stimulates TRPM4 via a PI(4,5)P2-dependent mechanism.

PI(4,5)P2 Clustering and Its Impact on Biological Functions

Located at the inner leaflet of the plasma membrane (PM), phosphatidyl-inositol 4,5-bisphosphate [PI(4,5)P₂] composes only 1-2 mol% of total PM lipids. With its synthesis and turnover both spatially and temporally regulated, PI(4,5)P₂ recruits and interacts with hundreds of cellular proteins to support a broad spectrum of cellular functions. Several factors contribute to the versatile and dynamic distribution of PI(4,5)P₂ in membranes. Physiological multivalent cations such as Ca²⁺ and Mg²⁺ can bridge between PI(4,5)P₂ headgroups, forming nanoscopic PI(4,5)P₂-cation clusters. The distinct lipid environment surrounding PI(4,5)P₂ affects the degree of PI(4,5)P₂ clustering. In addition, diverse cellular proteins interacting with PI(4,5)P₂ can further regulate PI(4,5)P₂ lateral distribution and accessibility. This review summarizes the current understanding of PI(4,5)P₂ behavior in both cells and model membranes, with emphasis on both multivalent cation- and protein-induced PI(4,5)P₂ clustering. Understanding the nature of spatially separated pools of PI(4,5)P₂ is fundamental to cell biology.

Modulation of the Cholesterol-Dependent Activity of Macrophages IC-21 by CRAC Peptides with Substituted Motif-Forming Amino Acids

The activity of many membrane proteins, such as receptors, ionic channels, transporters, and enzymes, is cholesterol dependent; however, mechanisms of the cholesterol-dependent regulation of protein functions remain obscure. Recent studies suggest that membrane proteins can directly interact with cholesterol owing to the presence of the cholesterol-recognizing amino-acid consensus (CRAC) motifs. One of the ways to verify and further develop this notion is a design of CRAC-containing peptides and investigation of their effects on cholesterol-dependent cell functions. Previously we showed that a newly constructed peptide RTKLWEMLVELGNMDKAVKLWRKLKR (peptide P4) containing two CRAC motifs modulates cholesterol-dependent interactions of cultured macrophages IC-21 with 2-μm particles. In this work, in order to clarify the role of CRAC-forming amino acids, we employed the same experimental system to test the activity of peptides closely related to P4 but with modified CRAC motifs. We found that peptide STKLSEMLSELGNMDKASKLSRKLSR (Mut2) analogous to P4, except that all CRAC-forming amino acids (V, W, K/R) were substituted by serine, did not produce any effect in the concentration range 0.5-50 μM corresponding to the range of the P4 activity. Neither was effective peptide RTKLSEMLVELGNMDKAVKLSRKLKR (Mut3), in which only aromatic amino acids (W) of the CRAC motifs were substituted. Peptide STKLWEMLVELGNMDKAVKLWRKLSR (Mut4), in which only cationic amino acids (R/K) in the CRAC motifs were changed, produced almost the same effect as that of peptide P4 with a bell-shape dose-response curve. At low concentrations (1-4 μM) Mut4 notably increased the number of beads per cell, at higher concentrations this parameter diminished, and at 50 μM Mut4 produced a robust toxic effect. Finally, peptide EWGMAVLWERNRKLKKDLKVLKMLRT (Mut1) composed of the same amino acid residues as P4 but in a random order ("scramble") and possessing one CRAC motif, different from that in P4, produced a moderate stimulation at 4-10 μM but was not toxic at 50 μM. As in the case of peptide P4, the effects of Mut4 and Mut1 depended on the cholesterol content in the cell membrane: after the incubation of cells with cholesterol-extracting agent methyl-β-cyclodextrin stimulatory effects produced by Mut4 and Mut1 at low doses were suppressed. Our results indicate that CRAC motifs play an important role in the mechanisms of the peptide-induced modulations of cholesterol-dependent cell functions in the experimental system used and that of the three motif-forming amino acids, critical is the presence of the aromatic amino acid (W). Further research is required to comprehend the molecular mechanisms of interactions of CRAC-containing peptides with cell membrane components that lead to modulation of cell functions. We anticipate that CRAC-containing peptides may provide a basis for the development of new tools for directed regulation of the activity of target cholesterol-dependent membrane proteins and for the design of new antimicrobial and immunomodulating drugs in particular.

Lateral heterogeneity and domain formation in cellular membranes

As early as the development of the fluid mosaic model for cellular membranes, researchers began observing the telltale signs of lateral heterogeneity. Over the decades this has led to the development of the lipid raft hypothesis and the ensuing controversy that has unfolded, as a result. Here, we review the physical concepts behind domain formation in lipid membranes, both of their structural and dynamic origins. This, then leads into a discussion of coarse-grained, phenomenological approaches that describe the wide range of phases associated with lipid lateral heterogeneity. We use these physical concepts to describe the interaction between raft-lipid species, such as long-chain saturated lipids, sphingomyelin, and cholesterol, and non-raft forming lipids, such as those with short acyl chains or unsaturated fatty acids. While debate has persisted on the biological relevance of lipid domains, recent research, described here, continues to identify biological roles for rafts and new experimental approaches have revealed the existence of lipid domains in living systems. Given the recent progress on both the biological and structural aspects of raft formation, the research area of membrane lateral heterogeneity will not only expand, but will continue to produce exciting results.

Phosphatidylinositol(4,5)bisphosphate: diverse functions at the plasma membrane

Phosphatidylinositol(4,5) bisphosphate (PI(4,5)P2) has become a major focus in biochemistry, cell biology and physiology owing to its diverse functions at the plasma membrane. As a result, the functions of PI(4,5)P2 can be explored in two separate and distinct roles - as a substrate for phospholipase C (PLC) and phosphoinositide 3-kinase (PI3K) and as a primary messenger, each having unique properties. Thus PI(4,5)P2 makes contributions in both signal transduction and cellular processes including actin cytoskeleton dynamics, membrane dynamics and ion channel regulation. Signalling through plasma membrane G-protein coupled receptors (GPCRs), receptor tyrosine kinases (RTKs) and immune receptors all use PI(4,5)P2 as a substrate to make second messengers. Activation of PI3K generates PI(3,4,5)P3 (phosphatidylinositol(3,4,5)trisphosphate), a lipid that recruits a plethora of proteins with pleckstrin homology (PH) domains to the plasma membrane to regulate multiple aspects of cellular function. In contrast, PLC activation results in the hydrolysis of PI(4,5)P2 to generate the second messengers, diacylglycerol (DAG), an activator of protein kinase C and inositol(1,4,5)trisphosphate (IP3/I(1,4,5)P3) which facilitates an increase in intracellular Ca2+. Decreases in PI(4,5)P2 by PLC also impact on functions that are dependent on the intact lipid and therefore endocytosis, actin dynamics and ion channel regulation are subject to control. Spatial organisation of PI(4,5)P2 in nanodomains at the membrane allows for these multiple processes to occur concurrently.

The Cell Biology of Tau Secretion

The progressive accumulation and spread of misfolded tau protein in the nervous system is the hallmark of tauopathies, progressive neurodegenerative diseases with only symptomatic treatments available. A growing body of evidence suggests that spreading of tau pathology can occur via cell-to-cell transfer involving secretion and internalization of pathological forms of tau protein followed by templated misfolding of normal tau in recipient cells. Several studies have addressed the cell biological mechanisms of tau secretion. It now appears that instead of a single mechanism, cells can secrete tau via three coexisting pathways: (1) translocation through the plasma membrane; (2) membranous organelles-based secretion; and (3) ectosomal shedding. The relative importance of these pathways in the secretion of normal and pathological tau is still elusive, though. Moreover, glial cells contribute to tau propagation, and the involvement of different cell types, as well as different secretion pathways, complicates the understanding of prion-like propagation of tauopathy. One of the important regulators of tau secretion in neuronal activity, but its mechanistic connection to tau secretion remains unclear and may involve all three secretion pathways of tau. This review article summarizes recent advancements in the field of tau secretion with an emphasis on cell biological aspects of the secretion process and discusses the role of neuronal activity and glial cells in the spread of pathological forms of tau.

Structure and Lateral Organization of Phosphatidylinositol 4,5-bisphosphate

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) is a minor but ubiquitous component of the inner leaflet of the plasma membrane of eukaryotic cells. However, due to its particular complex biophysical properties, it stands out from its neighboring lipids as one of the most important regulators of membrane-associated signaling events. Despite its very low steady-state concentration, PI(4,5)P₂ is able to engage in a multitude of simultaneous cellular functions that are temporally and spatially regulated through the presence of localized transient pools of PI(4,5)P₂ in the membrane. These pools are crucial for the recruitment, activation, and organization of signaling proteins and consequent regulation of downstream signaling. The present review showcases some of the most important PI(4,5)P2 molecular and biophysical properties as well as their impact on its membrane dynamics, lateral organization, and interactions with other biochemical partners.

Roles of Membrane Domains in Integrin-Mediated Cell Adhesion

The composition and organization of the plasma membrane play important functional and regulatory roles in integrin signaling, which direct many physiological and pathological processes, such as development, wound healing, immunity, thrombosis, and cancer metastasis. Membranes are comprised of regions that are thick or thin owing to spontaneous partitioning of long-chain saturated lipids from short-chain polyunsaturated lipids into domains defined as ordered and liquid-disorder domains, respectively. Liquid-ordered domains are typically 100 nm in diameter and sometimes referred to as lipid rafts. We posit that integrin β senses membrane thickness and that mechanical force on the membrane regulates integrin activation through membrane thinning. This review examines what we know about the nature and mechanism of the interaction of integrins with the plasma membrane and its effects on regulating integrins and its binding partners.

A multiscale biophysical model for the recruitment of actin nucleating proteins at the membrane interface

The dynamics and organization of the actin cytoskeleton are crucial to many cellular events such as motility, polarization, cell shaping, and cell division. The intracellular and extracellular signaling associated with this cytoskeletal network is communicated through cell membranes. Hence the organization of membrane macromolecules and actin filament assembly are highly interdependent. Although the actin-membrane linkage is known to happen through many routes, the major class of interactions is through the direct interaction of actin-binding proteins with the lipid class containing poly-phosphatidylinositols (PPIs). Among the PPIs, phosphatidylinositol bisphosphate (PI(4,5)P₂) acts as a significant factor controlling actin polymerization in the proximity of the membrane by binding to actin-associated proteins. The molecular interactions between these actin-binding proteins and the membrane lipids remain elusive. Here, using molecular modeling, analytical theory, and experimental methods, we investigate the binding of three different actin-binding proteins, mDia2, NWASP, and gelsolin, to membranes containing PI(4,5)P₂ lipids. We perform molecular dynamics simulations on the protein-bilayer system and analyze the membrane binding in the form of hydrogen bonds and salt bridges at various PI(4,5)P₂ and cholesterol concentrations. Our experimental study with PI(4,5)P₂-containing large unilamellar vesicles mimics the computational experiments. Using the multivalencies of the proteins obtained in molecular simulations and the cooperative binding mechanisms of the proteins, we also propose a multivalent binding model that predicts the actin filament distributions at various PI(4,5)P₂ and protein concentrations.

Intracellular cholesterol stimulates ENaC by interacting with phosphatidylinositol‑4,5‑bisphosphate and mediates cyclosporine A-induced hypertension

We have previously shown that blockade of ATP-binding cassette transporter A1 (ABCA1) with cyclosporine A (CsA) stimulates the epithelial sodium channel (ENaC) in cultured distal nephron cells. Here we show that CsA elevated systolic blood pressure in both wild-type and apolipoprotein E (ApoE) knockout (KO) mice to a similar level. The elevated systolic blood pressure was completely reversed by inhibition of cholesterol (Cho) synthesis with lovastatin. Inside-out patch-clamp data show that intracellular Cho stimulated ENaC in cultured distal nephron cells by interacting with phosphatidylinositol‑4,5‑bisphosphate (PIP2), an ENaC activator. Confocal microscopy data show that both α‑ENaC and PIP2 were localized in microvilli via a Cho-dependent mechanism. Deletion of membrane Cho reduced the levels of γ‑ENaC in the apical membrane. Reduced ABCA1 expression and elevated intracellular Cho were observed in old mice, compared to young mice. In parallel, cell-attached patch-clamp data from the split-open cortical collecting ducts (CCD) show that ENaC activity was significantly increased in old mice. These data suggest that elevation of intracellular Cho due to blockade of ABCA1 stimulates ENaC, which may contribute to CsA-induced hypertension. This study also implies that reduced ABCA1 expression may mediate age-related hypertension by increasing ENaC activity via elevation of intracellular Cho.

Amphipathic CRAC-Containing Peptides Derived from the Influenza Virus A M1 Protein Modulate Cholesterol-Dependent Activity of Cultured IC-21 Macrophages

Entry of many viral and bacterial pathogens into host cells depends on cholesterol and/or cholesterol-enriched domains (lipid rafts) in the cell membrane. Earlier, we showed that influenza virus A matrix protein M1 contains amphipathic α-helices with exposed cholesterol-recognizing amino acid consensus (CRAC) motifs. In order to test possible functional activity of these motifs, we studied the effects of three synthetic peptides corresponding to the CRAC-containing α-helices of the viral M1 protein on the phagocytic activity of cultured mouse IC-21 macrophages. The following peptides were used: LEVLMEWLKTR (M1 α-helix 3, a.a. 39-49; further referred to as peptide 1), NNMDKAVKLYRKLK (M1 α-helix 6, a.a. 91-105; peptide 2), and GLKNDLLENLQAYQKR (M1 α-helix 13, a.a. 228-243; peptide 3). We found that all three peptides modulated interactions of IC-21 macrophages with non-opsonized 2-µm target particles. The greatest effect was demonstrated by peptide 2: in the presence of 35 μM peptide 2, the phagocytic index of IC-21 macrophages exceeded the control value by 60%; 10-11 mM methyl-β-cyclodextrin abolished this effect. Peptides 1 and 3 exerted weak inhibitory effect in a narrow concentration range of 5-10 μM. The dose-response curves could be approximated by a sum of two (stimulatory and inhibitory) components with different Hill coefficients, suggesting existence of at least two peptide-binding sites with different affinities on the cell surface. CD spectroscopy confirmed that the peptides exhibit structural flexibility in solutions. Altogether, our data indicate that amphipathic CRAC-containing peptides derived from the viral M1 protein modulate lipid raft-dependent processes in IC-21 macrophages.

Cholesterol and vitamin E-conjugated PEGylated polymeric micelles for efficient delivery and enhanced anticancer activity of curcumin: evaluation in 2D monolayers and 3D spheroids

A newly synthesized PEGylated cholesterol/α-tocopheryl succinate (α-TOS) linked polymer (CV) was self-assembled and loaded with curcumin to form a micellar system (C-CVM). The tri-functionalized amphiphilic polymer was constituted of hydrophobic cholesterol and α-TOS connected to hydrophilic PEG via a lysine linker. The synthesized polymer and the micelles were characterized by ¹H NMR, DLS, zeta potentiometer, TEM, CMC determination and hemolysis studies. CVM displayed low CMC value of 15 µM with extent of hemolysis as less than 4%. The stable C-CVM with optimum % drug loading (14.2 ± 0.24) displayed Z average of 175.8 ± 0.68 nm with PDI (0.248 ± 0.075) and released curcumin in sustained manner in the in vitro drug release study. C-CVM demonstrated dose-dependent cellular uptake and cytotoxicity in murine melanoma, B16F10 and human breast cancer, MDA-MB-231 cell lines. CV exhibited marked reversal of drug resistance as indicated by significantly higher retention of P-glycoprotein substrate, rhodamine-123 in the resistant B16F10 cell line compared to standard P-glycoprotein inhibitor, verapamil. C-CVM demonstrated significantly higher spheroidal growth inhibition compared to C-PPM. The results provide strong evidence for CVM as promising drug delivery system and confirm the potential of C-CVM as chemotherapy in cancer.

Phosphatidylinositol 4,5-bisphosphate, cholesterol, and fatty acids modulate the calcium-activated chloride channel TMEM16A (ANO1)

The TMEM16A-mediated Ca²⁺-activated Cl^- current drives several important physiological functions. Membrane lipids regulate ion channels and transporters but their influence on members of the TMEM16 family is poorly understood. Here we have studied the regulation of TMEM16A by phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2), cholesterol, and fatty acids using patch clamp, biochemistry and fluorescence microscopy. We found that depletion of membrane PI(4,5)P2 causes a decline in TMEM16A current that is independent of cytoskeleton, but is partially prevented by removing intracellular Ca²⁺. On the other hand, supplying PI(4,5)P2 to inside-out patches attenuated channel rundown and/or partially rescued activity after channel rundown. Also, depletion (with methyl-β-cyclodextrin M-βCD) or restoration (with M-βCD+cholesterol) of membrane cholesterol slows down the current decay observed after reduction of PI(4,5)P2. Neither depletion nor restoration of cholesterol change PI(4,5)P2 content. However, M-βCD alone transiently increases TMEM16A activity and dampens rundown whereas M-βCD+cholesterol increases channel rundown. Thus, PI(4,5)P2 is required for TMEM16A function while cholesterol directly and indirectly via a PI(4,5)P2-independent mechanism regulate channel function. Stearic, arachidonic, oleic, docosahexaenoic, and eicosapentaenoic fatty acids as well as methyl stearate inhibit TMEM16A in a dose- and voltage-dependent manner. Phosphatidylserine, a phospholipid whose hydrocarbon tails contain stearic and oleic acids also inhibits TMEM16A. Finally, we show that TMEM16A remains in the plasma membrane after treatment with M-βCD, M-βCD+cholesterol, oleic, or docosahexaenoic acids. Thus, we propose that lipids and fatty acids regulate TMEM16A channels through a membrane-delimited protein-lipid interaction.

Myocyte membrane and microdomain modifications in diabetes: determinants of ischemic tolerance and cardioprotection

Cardiovascular disease, predominantly ischemic heart disease (IHD), is the leading cause of death in diabetes mellitus (DM). In addition to eliciting cardiomyopathy, DM induces a ‘wicked triumvirate’: (i) increasing the risk and incidence of IHD and myocardial ischemia; (ii) decreasing myocardial tolerance to ischemia–reperfusion (I–R) injury; and (iii) inhibiting or eliminating responses to cardioprotective stimuli. Changes in ischemic tolerance and cardioprotective signaling may contribute to substantially higher mortality and morbidity following ischemic insult in DM patients. Among the diverse mechanisms implicated in diabetic impairment of ischemic tolerance and cardioprotection, changes in sarcolemmal makeup may play an overarching role and are considered in detail in the current review. Observations predominantly in animal models reveal DM-dependent changes in membrane lipid composition (cholesterol and triglyceride accumulation, fatty acid saturation vs. reduced desaturation, phospholipid remodeling) that contribute to modulation of caveolar domains, gap junctions and T-tubules. These modifications influence sarcolemmal biophysical properties, receptor and phospholipid signaling, ion channel and transporter functions, contributing to contractile and electrophysiological dysfunction, cardiomyopathy, ischemic intolerance and suppression of protective signaling. A better understanding of these sarcolemmal abnormalities in types I and II DM (T1DM, T2DM) can inform approaches to limiting cardiomyopathy, associated IHD and their consequences. Key knowledge gaps include details of sarcolemmal changes in models of T2DM, temporal patterns of lipid, microdomain and T-tubule changes during disease development, and the precise impacts of these diverse sarcolemmal modifications. Importantly, exercise, dietary, pharmacological and gene approaches have potential for improving sarcolemmal makeup, and thus myocyte function and stress-resistance in this ubiquitous metabolic disorder.

Membrane Rafts in T Cell Activation: A Spotlight on CD28 Costimulation

Spatiotemporal compartmentalization of signaling pathways and second messengers is pivotal for cell biology and membrane rafts are, therefore, required for several lymphocyte functions. On the other hand, T cells have the specific necessity of tuning signaling amplification depending on the context in which the antigen is presented. In this review, we discuss of membrane rafts in the context of T cell signaling, focusing on CD28-mediated costimulation.

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.

Plasma membrane phosphatidylinositol 4-phosphate and 4,5-bisphosphate determine the distribution and function of K-Ras4B but not H-Ras proteins

Plasma membrane (PM) localization of Ras proteins is crucial for transmitting signals upon mitogen stimulation. Post-translational lipid modification of Ras proteins plays an important role in their recruitment to the PM. Electrostatic interactions between negatively charged PM phospholipids and basic amino acids found in K-Ras4B (K-Ras) but not in H-Ras are important for permanent K-Ras localization to the PM. Here, we investigated how acute depletion of negatively charged PM polyphosphoinositides (PPIns) from the PM alters the intracellular distribution and activity of K- and H-Ras proteins. PPIns depletion from the PM was achieved either by agonist-induced activation of phospholipase C β or with a rapamycin-inducible system in which various phosphatidylinositol phosphatases were recruited to the PM. Redistribution of the two Ras proteins was monitored with confocal microscopy or with a recently developed bioluminescence resonance energy transfer-based approach involving fusion of the Ras C-terminal targeting sequences or the entire Ras proteins to Venus fluorescent protein. We found that PM PPIns depletion caused rapid translocation of K-Ras but not H-Ras from the PM to the Golgi. PM depletion of either phosphatidylinositol 4-phosphate (PtdIns4P) or PtdIns(4,5)P₂ but not PtdIns(3,4,5)P₃ was sufficient to evoke K-Ras translocation. This effect was diminished by deltarasin, an inhibitor of the Ras-phosphodiesterase interaction, or by simultaneous depletion of the Golgi PtdIns4P. The PPIns depletion decreased incorporation of [³H]leucine in K-Ras-expressing cells, suggesting that Golgi-localized K-Ras is not as signaling-competent as its PM-bound form. We conclude that PPIns in the PM are important regulators of K-Ras-mediated signals.

The Epidermal Growth Factor Receptor Forms Location-Dependent Complexes in Resting Cells

The epidermal growth factor receptor (EGFR) is a prototypical receptor tyrosine kinase involved in cell growth and proliferation and associated with various cancers. It is commonly assumed that after activation by binding of epidermal growth factor to the extracellular domain it dimerizes, followed by autophosphorylation of tyrosine residues at the intracellular domain. However, its oligomerization state before activation is controversial. In the absence of ligands, EGFR has been found in various, inconsistent amounts of monomeric, inactive dimeric, and oligomeric forms. In addition, evidence suggests that the active conformation is not a simple dimer but contains higher oligomers. As experiments in the past have been conducted at different conditions, we investigate here the influence of cell lines (HEK293, COS-7, and CHO-K1), temperature (room temperature and 37°C), and membrane localization on the quantitation of preformed dimers using SW-FCCS, DC-FCCS, quasi PIE-FCCS, and imaging FCCS. While measurement modality, temperature, and localization on upper or lower membranes have only a limited influence on the dimerization amount observed, the cell line and location to periphery versus center of the cell can change dimerization results significantly. The observed dimerization amount is strongly dependent on the expression level of endogenous EGFR in a cell line and shows a strong cell-to-cell variability even within the same cell line. In addition, using imaging FCCS, we find that dimers have a tendency to be found at the periphery of cells compared to central positions.

Fission yeast myosin I facilitates PI(4,5)P2-mediated anchoring of cytoplasmic dynein to the cortex

Several key processes in the cell, such as vesicle transport and spindle positioning, are mediated by the motor protein cytoplasmic dynein, which produces force on the microtubule. For the functions that require movement of the centrosome and the associated nuclear material, dynein needs to have a stable attachment at the cell cortex. In fission yeast, Mcp5 is the anchor protein of dynein and is required for the oscillations of the horsetail nucleus during meiotic prophase. Although the role of Mcp5 in anchoring dynein to the cortex has been identified, it is unknown how Mcp5 associates with the membrane as well as the importance of the underlying attachment to the nuclear oscillations. Here, we set out to quantify Mcp5 organization and identify the binding partner of Mcp5 at the membrane. We used confocal and total internal reflection fluorescence microscopy to count the number of Mcp5 foci and the number of Mcp5 molecules in an individual focus. Further, we quantified the localization pattern of Mcp5 in fission yeast zygotes and show by perturbation of phosphatidylinositol 4-phosphate 5-kinase that Mcp5 binds to phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂]. Remarkably, we discovered that the myosin I protein in fission yeast, Myo1, which is required for organization of sterol-rich domains in the cell membrane, facilitates the localization of Mcp5 and that of cytoplasmic dynein on the membrane. Finally, we demonstrate that Myo1-facilitated association of Mcp5 and dynein to the membrane determines the dynamics of nuclear oscillations and, in essence, dynein activity.

Piecing it together: Unraveling the elusive structure-function relationship in single-pass membrane receptors

The challenge of crystallizing single-pass plasma membrane receptors has remained an obstacle to understanding the structural mechanisms that connect extracellular ligand binding to cytosolic activation. For example, the complex interplay between receptor oligomerization and conformational dynamics has been, historically, only inferred from static structures of isolated receptor domains. A fundamental challenge in the field of membrane receptor biology, then, has been to integrate experimentally observable dynamics of full-length receptors (e.g. diffusion and conformational flexibility) into static structural models of the disparate domains. In certain receptor families, e.g. the ErbB receptors, structures have led somewhat linearly to a putative model of activation. In other families, e.g. the tumor necrosis factor (TNF) receptors, structures have produced divergent hypothetical mechanisms of activation and transduction. Here, we discuss in detail these and other related receptors, with the goal of illuminating the current challenges and opportunities in building comprehensive models of single-pass receptor activation. The deepening understanding of these receptors has recently been accelerated by new experimental and computational tools that offer orthogonal perspectives on both structure and dynamics. As such, this review aims to contextualize those technological developments as we highlight the elegant and complex conformational communication between receptor domains. This article is part of a Special Issue entitled: Interactions between membrane receptors in cellular membranes edited by Kalina Hristova.

Short-Lived Cages Restrict Protein Diffusion in the Plasma Membrane

The plasma membrane is a heterogeneous environment characterized by anomalous diffusion and the presence of microdomains that are molecularly distinct from the bulk membrane. Using single particle tracking of the C-type lectin CD93, we have identified for the first time the transient trapping of transmembrane proteins in cage-like microdomains which restrict protein diffusion. These cages are stabilized by actin-dependent confinement regions, but are separate structures with sizes and lifespans uncorrelated to those of the underlying actin corral. These membrane cages require cholesterol for their strength and stability, with cholesterol depletion decreasing both. Despite this, cages are much larger in size and are longer lived than lipid rafts, suggesting instead that cholesterol-dependent effects on membrane fluidity or molecular packing play a role in cage formation. This diffusional compartment in the plasma membrane has characteristics of both a diffusional barrier and a membrane microdomain, with a size and lifespan intermediate between short-lived microdomains such as lipid rafts and long-lasting diffusional barriers created by the actin cytoskeleton.

Cholesterol-rich membrane microdomains modulate Wnt/β-catenin morphogen gradient during Xenopus development

Wnt/β-catenin has been described as crucial for dorsal-ventral and antero-posterior patterning, playing multiple roles at different stages of development. Cholesterol-rich membrane microdomains (CRMMs), cholesterol- and sphingolipid-enriched domains of the plasma membrane, are known as platforms for signaling pathways. Although we have demonstrated the importance of the CRMMs for head development, how they participate in prechordal plate formation and embryo axis patterning remains an open question. Moreover, the participation of the CRMMs in the Wnt/β-catenin signaling pathway activity in vivo is unclear, particularly during embryonic development. In this study, we demonstrated that CRMMs disruption by methyl-beta-cyclodextrin (MβCD) potentiates the activation of the Wnt/β-catenin signaling pathway in vitro and in vivo during embryonic development, causing head defects by expanding the Wnt expression domain. Furthermore, we also found that the action of CRMMs depends on the microenvironmental context because it also works in conjunction with dkk1, when dkk1 is overexpressed. Thus, we propose CRMMs as a further mechanism of prechordal plate protection against the Wnt signals secreted by posterolateral cells, complementing the action of secreted antagonists.

Sarcolemmal dependence of cardiac protection and stress-resistance: roles in aged or diseased hearts

Disruption of the sarcolemmal membrane is a defining feature of oncotic death in cardiac ischaemia-reperfusion (I-R), and its molecular makeup not only fundamentally governs this process but also affects multiple determinants of both myocardial I-R injury and responsiveness to cardioprotective stimuli. Beyond the influences of membrane lipids on the cytoprotective (and death) receptors intimately embedded within this bilayer, myocardial ionic homeostasis, substrate metabolism, intercellular communication and electrical conduction are all sensitive to sarcolemmal makeup, and critical to outcomes from I-R. As will be outlined in this review, these crucial sarcolemmal dependencies may underlie not only the negative effects of age and common co-morbidities on myocardial ischaemic tolerance but also the on-going challenge of implementing efficacious cardioprotection in patients suffering accidental or surgically induced I-R. We review evidence for the involvement of sarcolemmal makeup changes in the impairment of stress-resistance and cardioprotection observed with ageing and highly prevalent co-morbid conditions including diabetes and hypercholesterolaemia. A greater understanding of membrane changes with age/disease, and the inter-dependences of ischaemic tolerance and cardioprotection on sarcolemmal makeup, can facilitate the development of strategies to preserve membrane integrity and cell viability, and advance the challenging goal of implementing efficacious 'cardioprotection' in clinically relevant patient cohorts. Linked Articles This article is part of a themed section on Molecular Pharmacology of G Protein-Coupled Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.20/issuetoc.

Control of diverse subcellular processes by a single multi-functional lipid phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]

Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] is a multi-functional lipid that regulates several essential subcellular processes in eukaryotic cells. In addition to its well-established function as a substrate for receptor-activated signalling at the plasma membrane (PM), it is now recognized that distinct PI(4,5)P2 pools are present at other organelle membranes. However, a long-standing question that remains unresolved is the mechanism by which a single lipid species, with an invariant functional head group, delivers numerous functions without loss of fidelity. In the present review, we summarize studies that have examined the molecular processes that shape the repertoire of PI(4,5)P2 pools in diverse eukaryotes. Collectively, these studies indicate a conserved role for lipid kinase isoforms in generating functionally distinct pools of PI(4,5)P2 in diverse metazoan species. The sophistication underlying the regulation of multiple functions by PI(4,5)P2 is also shaped by mechanisms that regulate its availability to enzymes involved in its metabolism as well as molecular processes that control its diffusion at nanoscales in the PM. Collectively, these mechanisms ensure the specificity of PI(4,5)P2 mediated signalling at eukaryotic membranes.

Phosphatidylinositol 4-Phosphate 5-Kinases in the Regulation of T Cell Activation

Phosphatidylinositol 4,5-biphosphate kinases (PIP5Ks) are critical regulators of T cell activation being the main enzymes involved in the synthesis of phosphatidylinositol 4,5-biphosphate (PIP2). PIP2 is indeed a pivotal regulator of the actin cytoskeleton, thus controlling T cell polarization and migration, stable adhesion to antigen-presenting cells, spatial organization of the immunological synapse, and co-stimulation. Moreover, PIP2 also serves as a precursor for the second messengers inositol triphosphate, diacylglycerol, and phosphatidylinositol 3,4,5-triphosphate, which are essential for the activation of signaling pathways regulating cytokine production, cell cycle progression, survival, metabolism, and differentiation. Here, we discuss the impact of PIP5Ks on several T lymphocyte functions with a specific focus on the role of CD28 co-stimulation in PIP5K compartimentalization and activation.

Microbial sphingomyelinase induces RhoA-mediated reorganization of the apical brush border membrane and is protective against invasion

Both commensal and pathogenic microbes that colonize the GI tract can synthesize and secrete spingomyelinase enzymes that cleave membrane sphingomyelin, leaving the ceramide component intact in the cell membrane. This study examines how this reaction affects the structure and function of host enterocytes and mucosal defense.

The apical brush border membrane (BBM) of intestinal epithelial cells forms a highly structured and dynamic environmental interface that serves to regulate cellular physiology and block invasion by intestinal microbes and their products. How the BBM dynamically responds to pathogenic and commensal bacterial signals can define intestinal homeostasis and immune function. We previously found that in model intestinal epithelium, the conversion of apical membrane sphingomyelin to ceramide by exogenous bacterial sphingomyelinase (SMase) protected against the endocytosis and toxicity of cholera toxin. Here we elucidate a mechanism of action by showing that SMase induces a dramatic, reversible, RhoA-dependent alteration of the apical cortical F-actin network. Accumulation of apical membrane ceramide is necessary and sufficient to induce the actin phenotype, and this coincides with altered membrane structure and augmented innate immune function as evidenced by resistance to invasion by Salmonella.

Phosphatidylinositol 4-Phosphate 5-Kinase β Controls Recruitment of Lipid Rafts into the Immunological Synapse

Phosphatidylinositol 4,5-biphosphate (PIP2) is critical for T lymphocyte activation serving as a substrate for the generation of second messengers and the remodeling of actin cytoskeleton necessary for the clustering of lipid rafts, TCR, and costimulatory receptors toward the T:APC interface. Spatiotemporal analysis of PIP2 synthesis in T lymphocytes suggested that distinct isoforms of the main PIP2-generating enzyme, phosphatidylinositol 4-phosphate 5-kinase (PIP5K), play a differential role on the basis of their distinct localization. In this study, we analyze the contribution of PIP5Kβ to T cell activation and show that CD28 induces the recruitment of PIP5Kβ to the immunological synapse, where it regulates filamin A and lipid raft accumulation, as well as T cell activation, in a nonredundant manner. Finally, we found that Vav1 and the C-terminal 83 aa of PIP5Kβ are pivotal for the PIP5Kβ regulatory functions in response to CD28 stimulation.

Modeling the effects of cyclodextrin on intracellular membrane vesicles from Cos-7 cells prepared by sonication and carbonate treatment

Cholesterol has important functions in the organization of membrane structure and this may be mediated via the formation of cholesterol-rich, liquid-ordered membrane microdomains often referred to as lipid rafts. Methyl-beta-cyclodextrin (cyclodextrin) is commonly used in cell biology studies to extract cholesterol and therefore disrupt lipid rafts. However, in this study we reassessed this experimental strategy and investigated the effects of cyclodextrin on the physical properties of sonicated and carbonate-treated intracellular membrane vesicles isolated from Cos-7 fibroblasts. We treated these membranes, which mainly originate from the trans-Golgi network and endosomes, with cyclodextrin and measured the effects on their equilibrium buoyant density, protein content, represented by the palmitoylated protein phosphatidylinositol 4-kinase type IIα, and cholesterol. Despite the reduction in mass stemming from cholesterol removal, the vesicles became denser, indicating a possible large volumetric decrease, and this was confirmed by measurements of hydrodynamic vesicle size. Subsequent mathematical analyses demonstrated that only half of this change in membrane size was attributable to cholesterol loss. Hence, the non-selective desorption properties of cyclodextrin are also involved in membrane size and density changes. These findings may have implications for preceding studies that interpreted cyclodextrin-induced changes to membrane biochemistry in the context of lipid raft disruption without taking into account our finding that cyclodextrin treatment also reduces membrane size.

Dissecting the molecular properties of prokaryotic flotillins

Flotillins are universally conserved proteins that are present in all kingdoms of life. Recently it was demonstrated that the B. subtilis flotillin YuaG (FloT) has a direct influence on membrane domain formation by orchestrating lipid domains. Thereby it allocates a proper environment for diverse cellular machineries. YuaG creates platforms for signal transduction, processes crucial for biofilm formation, sporulation, competence, secretion, and others. Even though, flotillins are an emerging topic of research in the field of microbiology little is known about the molecular architecture of prokaryotic flotillins. All flotillins share common structural elements and are tethered to the membrane N’- terminally, followed by a so called PHB domain and a flotillin domain. We show here that prokaryotic flotillins are, similarly to eukaryotic flotillins, tethered to the membrane via a hairpin loop. Further it is demonstrated by sedimentation assays that B. subtilis flotillins do not bind to the membrane via their PHB domain contrary to eukaryotic flotillins. Size exclusion chromatography experiments, blue native PAGE and cross linking experiments revealed that B. subtilis YuaG can oligomerize into large clusters via the PHB domain. This illustrates an important difference in the setup of prokaryotic flotillins compared to the organization of eukaryotic flotillins.

A dPIP5K dependent pool of phosphatidylinositol 4,5 bisphosphate (PIP2) is required for G-protein coupled signal transduction in Drosophila photoreceptors

Multiple PIP₂ dependent molecular processes including receptor activated phospholipase C activity occur at the neuronal plasma membranes, yet levels of this lipid at the plasma membrane are remarkably stable. Although the existence of unique pools of PIP₂ supporting these events has been proposed, the mechanism by which they are generated is unclear. In Drosophila photoreceptors, the hydrolysis of PIP₂ by G-protein coupled phospholipase C activity is essential for sensory transduction of photons. We identify dPIP5K as an enzyme essential for PIP₂ re-synthesis in photoreceptors. Loss of dPIP5K causes profound defects in the electrical response to light and light-induced PIP₂ dynamics at the photoreceptor membrane. Overexpression of dPIP5K was able to accelerate the rate of PIP₂ synthesis following light induced PIP₂ depletion. Other PIP₂ dependent processes such as endocytosis and cytoskeletal function were unaffected in photoreceptors lacking dPIP5K function. These results provide evidence for the existence of a unique dPIP5K dependent pool of PIP₂ required for normal Drosophila phototransduction. Our results define the existence of multiple pools of PIP₂ in photoreceptors generated by distinct lipid kinases and supporting specific molecular processes at neuronal membranes.

PIP₂ has been implicated in multiple functions at the plasma membrane. Some of these require its hydrolysis by receptor-activated phospholipase C, whereas others, such as membrane transport and cytoskeletal function, involve the interaction of the intact lipid with cellular proteins. The mechanistic basis underlying the segregation of these two classes of PIP₂ dependent functions is unknown; it has been postulated that this might involve unique pools of PIP₂ generated by distinct phosphoinsoitide kinases. We have studied this question in Drosophila photoreceptors, a model system where sensory transduction requires robust phospholipase C mediated PIP₂ hydrolysis. We find that the activity of phosphatidylinositol-4-phosphate 5 kinase encoded by dPIP5K is required to support normal sensory transduction and PIP₂ dynamics in photoreceptors. Remarkably, non-PLC dependent functions of PIP₂, such as vesicular transport and the actin cytoskeleton, were unaffected in dPIP5K mutants. Thus, dPIP5K supports a pool of PIP₂ that is readily available to PLC, but has no role in sustaining other non-PLC mediated PIP₂ dependent processes. These findings support the existence of at least two non-overlapping pools of PIP₂ at the plasma membrane, and provide a platform for future studies of PIP₂ regulation at the plasma membrane.

Dynamic shaping of cellular membranes by phospholipids and membrane-deforming proteins

All cellular compartments are separated from the external environment by a membrane, which consists of a lipid bilayer. Subcellular structures, including clathrin-coated pits, caveolae, filopodia, lamellipodia, podosomes, and other intracellular membrane systems, are molded into their specific submicron-scale shapes through various mechanisms. Cells construct their micro-structures on plasma membrane and execute vital functions for life, such as cell migration, cell division, endocytosis, exocytosis, and cytoskeletal regulation. The plasma membrane, rich in anionic phospholipids, utilizes the electrostatic nature of the lipids, specifically the phosphoinositides, to form interactions with cytosolic proteins. These cytosolic proteins have three modes of interaction: 1) electrostatic interaction through unstructured polycationic regions, 2) through structured phosphoinositide-specific binding domains, and 3) through structured domains that bind the membrane without specificity for particular phospholipid. Among the structured domains, there are several that have membrane-deforming activity, which is essential for the formation of concave or convex membrane curvature. These domains include the amphipathic helix, which deforms the membrane by hemi-insertion of the helix with both hydrophobic and electrostatic interactions, and/or the BAR domain superfamily, known to use their positively charged, curved structural surface to deform membranes. Below the membrane, actin filaments support the micro-structures through interactions with several BAR proteins as well as other scaffold proteins, resulting in outward and inward membrane micro-structure formation. Here, we describe the characteristics of phospholipids, and the mechanisms utilized by phosphoinositides to regulate cellular events. We then summarize the precise mechanisms underlying the construction of membrane micro-structures and their involvements in physiological and pathological processes.

Lovastatin-Induced Phosphatidylinositol-4-Phosphate 5-Kinase Diffusion from Microvilli Stimulates ROMK Channels

We recently showed that lovastatin attenuates cyclosporin A (CsA)-induced damage of cortical collecting duct (CCD) principal cells by reducing intracellular cholesterol. Previous studies showed that, in cell expression models or artificial membranes, exogenous cholesterol directly inhibits inward rectifier potassium channels, including Kir1.1 (Kcnj1; the gene locus for renal outer medullary K(+) [ROMK1] channels). Therefore, we hypothesized that lovastatin might stimulate ROMK1 by reducing cholesterol in CCD cells. Western blots showed that mpkCCDc14 cells express ROMK1 channels with molecular masses that approximate the molecular masses of ROMK1 in renal tubules detected before and after treatment with DTT. Confocal microscopy showed that ROMK1 channels were not in the microvilli, where cholesterol-rich lipid rafts are located, but rather, the planar regions of the apical membrane of mpkCCDc14 cells. Furthermore, phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2], an activator of ROMK channels, was detected mainly in the microvilli under resting conditions along with the kinase responsible for PI(4,5)P2 synthesis, phosphatidylinositol-4-phosphate 5-kinase, type I γ [PI(4)P5K I γ], which may explain the low basal open probability and increased sensitivity to tetraethylammonium observed here for this channel. Notably, lovastatin induced PI(4)P5K I γ diffusion into planar regions and elevated PI(4,5)P2 and ROMK1 open probability in these regions through a cholesterol-associated mechanism. However, exogenous cholesterol alone did not induce these effects. These results suggest that lovastatin stimulates ROMK1 channels, at least in part, by inducing PI(4,5)P2 synthesis in planar regions of the renal CCD cell apical membrane, suggesting that lovastatin could reduce cyclosporin-induced nephropathy and associated hyperkalemia.

PI(4,5)P₂-binding effector proteins for vesicle exocytosis

PI(4,5)P₂participates directly in priming and possibly in fusion steps of Ca²⁺-triggered vesicle exocytosis. High concentration nanodomains of PI(4,5)P₂reside on the plasma membrane of neuroendocrine cells. A subset of vesicles that co-localize with PI(4,5)P₂ domains appear to undergo preferential exocytosis in stimulated cells. PI(4,5)P₂directly regulates vesicle exocytosis by recruiting and activating PI(4,5)P₂-binding proteins that regulate SNARE protein function including CAPS, Munc13-1/2, synaptotagmin-1, and other C2 domain-containing proteins. These PI(4,5)P₂effector proteins are coincidence detectors that engage in multiple interactions at vesicle exocytic sites. The SNARE protein syntaxin-1 also binds to PI(4,5)P₂, which promotes clustering, but an activating role for PI(4,5)P₂in syntaxin-1 function remains to be fully characterized. Similar principles underlie polarized constitutive vesicle fusion mediated in part by the PI(4,5)P₂-binding subunits of the exocyst complex (Sec3, Exo70). Overall, focal vesicle exocytosis occurs at sites landmarked by PI(4,5)P2, which serves to recruit and/or activate multifunctional PI(4,5)P₂-binding proteins. This article is part of a Special Issue entitled Phosphoinositides.

Interplay Between Lipid Modulators of Kir2 Channels: Cholesterol and PIP2

We have shown earlier that Kir2 channels are suppressed by the elevation of membrane cholesterol. Moreover, it is also well known that activation of Kir channels is critically dependent on a regulatory phospholipid, phosphatidylinositol-4,5-bisphosphate (PIP2). In this study we examined the cross-talk between cholesterol and PIP2 in the regulation of Kir2 channels. The strength of Kir2-PIP2 interactions was assessed by acute sequestering of PIP2 with neomycin dialyzed into cells through a patch pipette while simultaneously recording whole cell currents. Consistent with a reduction in PIP2 levels, dialysis of neomycin resulted in a decrease in Kir2.1 and Kir2.3 current amplitudes (current rundown), however, this effect was significantly delayed by cholesterol depletion for both types of channels suggesting that cholesterol depletion strengthens the interaction between Kir2 channels and PIP2. Furthermore, mutation of Kir2.1 that renders the channels' cholesterol insensitive abrogated cholesterol depletion-induced delay in the current rundown whereas reverse mutation in Kir2.3 has the opposite effect. These observations provide further support for the functional cross-talk between cholesterol and PIP2 in regulating Kir2 channels. Consistent with these observations, there is a significant structural overlap between cytosolic residues that are critical for the sensitivity of Kir2 channels to the two lipid modulators but based on recent studies, there is little or no overlap between cholesterol and PIP2 binding sites. Taken together, these observations suggest that cholesterol and PIP2 regulate the channels through distinct binding sites but that the signals generated by the binding of the two modulators converge.

PI3 kinase enzymology on fluid lipid bilayers

We report the use of fluid lipid bilayer membrane as a model platform to study the influence of the bilayer microenvironment and composition on the enzymology in membrane. As a model system we determined the enzyme kinetics on membranes for the transformation of bilayers containing phosphoinositol(4,5)-bisphosphate (PI(4,5)P2) to phosphoinositol(3,4,5)-trisphosphate (PI(3,4,5)P3) by the enzyme phosphoinositol-3-kinase (PI3K) using radiolabeled ATP. The activity of the enzyme was monitored as a function of the radioactivity incorporated within the bilayer. The transformation of PI(4,5)P2 to PI(3,4,5)P3 was determined using a mass strip assay. The fluidity of the bilayer was confirmed by Fluorescence Recovery After Photobleaching (FRAP) experiments. Kinetic simulations were performed based on Langmuir adsorption and Michaelis-Menton kinetics equations to generate the rate constants for the enzymatic reaction. The effect of cholesterol on the enzyme kinetics was studied by doping the bilayer with 1% cholesterol. This leads to significant reduction in reaction rate due to change in membrane microenvironment. This strategy provides a method to study the enzymology of various kinases and phosphatases occurring at the membrane and also how these reactions are affected by the membrane composition and surface microenvironment.

The role of the Niemann-Pick disease, type C1 protein in adipocyte insulin action

The Niemann-Pick disease, type C1 (NPC1) gene encodes a transmembrane protein involved in cholesterol efflux from the lysosome. SNPs within NPC1 have been associated with obesity and type 2 diabetes, and mice heterozygous or null for NPC1 are insulin resistant. However, the molecular mechanism underpinning this association is currently undefined. This study aimed to investigate the effects of inhibiting NPC1 function on insulin action in adipocytes. Both pharmacological and genetic inhibition of NPC1 impaired insulin action. This impairment was evident at the level of insulin signalling and insulin-mediated glucose transport in the short term and decreased GLUT4 expression due to reduced liver X receptor (LXR) transcriptional activity in the long-term. These data show that cholesterol homeostasis through NPC1 plays a crucial role in maintaining insulin action at multiple levels in adipocytes.

Fixation-induced cell blebbing on spread cells inversely correlates with phosphatidylinositol 4,5-bisphosphate level in the plasma membrane

•

Protein- but not lipid-stabilizing fixatives induce cell blebbing of spread cells.

•

Asymmetric distribution of fixation-induced blebs coincides with that of PIP₂.

•

Fixation less readily induces blebbing on spread cells with elevated PIP₂ levels.

•

Fixation more readily induces blebbing on spread cells with lower PIP₂ levels.

•

Disruption of lipid rafts enhances fixation-induced blebbing of spread cells.

While most attention has been focused on physiologically generated blebs, the molecular mechanisms for fixation-induced cell blebbing are less investigated. We show that protein-fixing (e.g. aldehydes and picric acid) but not lipid-stabilizing (e.g. OsO₄ and KMnO₄) fixatives induce blebbing on spread cells. We also show that aldehyde fixation may induce the loss or delocalization of phosphatidylinositol 4,5-bisphosphate (PIP₂) in the plasma membrane and that the asymmetric distribution of fixation-induced blebs on spread/migrating cells coincides with that of PIP₂ on the cells prefixed by lipid-stabilizing fixatives (e.g., OsO₄). Moreover, fixation induces blebbing less readily on PIP₂-elevated spread cells but more readily on PIP₂-lowered or lipid raft-disrupted spread cells. Our data suggest that fixation-induced lowering of PIP₂ level at cytoskeleton-attaching membrane sites causes bleb formation via local breakdown of the membrane–cytoskeleton coupling.

Counterion-mediated cluster formation by polyphosphoinositides

Polyphosphoinositides (PPI) and in particular PI(4,5)P2, are among the most highly charged molecules in cell membranes, are important in many cellular signaling pathways, and are frequently targeted by peripheral polybasic proteins for anchoring through electrostatic interactions. Such interactions between PIP2 and proteins containing polybasic stretches depend on the physical state and the lateral distribution of PIP2 within the inner leaflet of the cell's lipid bilayer. The physical and chemical properties of PIP2 such as pH-dependent changes in headgroup ionization and area per molecule as determined by experiments together with molecular simulations that predict headgroup conformations at various ionization states have revealed the electrostatic properties and phase behavior of PIP2-containing membranes. This review focuses on recent experimental and computational developments in defining the physical chemistry of PIP2 and its interactions with counterions. Ca(2+)-induced changes in PIP2 charge, conformation, and lateral structure within the membrane are documented by numerous experimental and computational studies. A simplified electrostatic model successfully predicts the Ca(2+)-driven formation of PIP2 clusters but cannot account for the different effects of Ca(2+) and Mg(2+) on PIP2-containing membranes. A more recent computational study is able to see the difference between Ca(2+) and Mg(2+) binding to PIP2 in the absence of a membrane and without cluster formation. Spectroscopic studies suggest that divalent cation- and multivalent polyamine-induced changes in the PIP2 lateral distribution in model membrane are also different, and not simply related to the net charge of the counterion. Among these differences is the capacity of Ca(2+) but not other polycations to induce nm scale clusters of PIP2 in fluid membranes. Recent super resolution optical studies show that PIP2 forms nanoclusters in the inner leaflet of a plasma membrane with a similar size distribution as those induced by Ca(2+) in model membranes. The mechanisms by which PIP2 forms nanoclusters and other structures inside a cell remain to be determined, but the unique electrostatic properties of PIP2 and its interactions with multivalent counterions might have particular physiological relevance.

Simvastatin inhibits glucose metabolism and legumain activity in human myotubes

Simvastatin, a HMG-CoA reductase inhibitor, is prescribed worldwide to patients with hypercholesterolemia. Although simvastatin is well tolerated, side effects like myotoxicity are reported. The mechanism for statin-induced myotoxicity is still poorly understood. Reports have suggested impaired mitochondrial dysfunction as a contributor to the observed myotoxicity. In this regard, we wanted to study the effects of simvastatin on glucose metabolism and the activity of legumain, a cysteine protease. Legumain, being the only known asparaginyl endopeptidase, has caspase-like properties and is described to be involved in apoptosis. Recent evidences indicate a regulatory role of both glucose and statins on cysteine proteases in monocytes. Satellite cells were isolated from the Musculus obliquus internus abdominis of healthy human donors, proliferated and differentiated into polynuclear myotubes. Simvastatin with or without mevalonolactone, farnesyl pyrophosphate or geranylgeranyl pyrophosphate were introduced on day 5 of differentiation. After 48 h, cells were either harvested for immunoblotting, ELISA, cell viability assay, confocal imaging or enzyme activity analysis, or placed in a fuel handling system with [¹⁴C]glucose or [³H]deoxyglucose for uptake and oxidation studies. A dose-dependent decrease in both glucose uptake and oxidation were observed in mature myotubes after exposure to simvastatin in concentrations not influencing cell viability. In addition, simvastatin caused a decrease in maturation and activity of legumain. Dysregulation of glucose metabolism and decreased legumain activity by simvastatin points out new knowledge about the effects of statins on skeletal muscle, and may contribute to the understanding of the myotoxicity observed by statins.