Plasma membrane phospholipid asymmetry

A Breast Cancer Stem Active Cobalt(III)-Cyclam Complex Containing Flufenamic Acid with Immunogenic Potential

The cytotoxic and immunogenic-activating properties of a cobalt(III)-cyclam complex bearing the non-steroidal anti-inflammatory drug, flufenamic acid is reported within the context of anti-cancer stem cell (CSC) drug discovery. The cobalt(III)-cyclam complex 1 displays sub-micromolar potency towards breast CSCs grown in monolayers, 24-fold and 31-fold greater than salinomycin (an established anti-breast CSC agent) and cisplatin (an anticancer metallopharmaceutical), respectively. Strikingly, the cobalt(III)-cyclam complex 1 is 69-fold and 50-fold more potent than salinomycin and cisplatin towards three-dimensionally cultured breast CSC mammospheres. Mechanistic studies reveal that 1 induces DNA damage, inhibits cyclooxygenase-2 expression, and prompts caspase-dependent apoptosis. Breast CSCs treated with 1 exhibit damage-associated molecular patterns characteristic of immunogenic cell death and are phagocytosed by macrophages. As far as we are aware, 1 is the first cobalt complex of any oxidation state or geometry to display both cytotoxic and immunogenic-activating effects on breast CSCs.

The Bulk Breast Cancer Cell and Breast Cancer Stem Cell Activity of Binuclear Copper(II)-Phenanthroline Complexes

Mononuclear copper(II)-phenanthroline complexes have been widely investigated as anticancer agents whereas multinuclear copper(II)-phenanthroline complexes are underexplored. Here the synthesis and characterisation of two new binuclear copper(II)-phenanthroline complexes 1 and 2 is reported, comprising of 2,9-dimethyl-1,10-phenanthroline or 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, terminal chloride ligands, and bridging chloride or hydroxide ligands. The binuclear copper(II) complex containing 2,9-dimethyl-1,10-phenanthroline 1 displays nanomolar toxicity towards bulk breast cancer cells and breast cancer stem cells (CSCs) grown in monolayers, >50-fold greater than cisplatin (an anticancer metallodrug) and salinomycin (a gold-standard anti-CSC agent). Spectacularly, 1 exhibits >100-fold greater potency toward three-dimensionally cultured mammospheres than cisplatin and salinomycin. Mechanistic studies show that 1 evokes breast CSC apoptosis by elevating intracellular reactive oxygen species levels and damaging genomic DNA (possibly by an oxidative mechanism). To the best of our knowledge, this is the first study to probe the anti-breast CSC properties of binuclear copper(II)-phenanthroline complexes.

Permeation of a Homologous Series of NBD-Labeled Fatty Amines through Lipid Bilayers: A Molecular Dynamics Study

Permeation through biomembranes is ubiquitous for drugs to reach their active sites. Asymmetry of the cell plasma membrane (PM) has been described as having an important role in this process. Here we describe the interaction of a homologous series of 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-labeled amphiphiles (NBD-Cn, n = 4 to 16) with lipid bilayers of different compositions (1-palmitoyl, 2-oleoyl-sn-glycero-3-phosphocholine (POPC):cholesterol (1:1) and palmitoylated sphingomyelin (SpM):cholesterol (6:4)), including an asymmetric bilayer. Both unrestrained and umbrella sampling (US) simulations (at varying distances to the bilayer center) were carried out. The free energy profile of NBD-Cn at different depths in the membrane was obtained from the US simulations. The behavior of the amphiphiles during the permeation process was described regarding their orientation, chain elongation, and H-bonding to lipid and water molecules. Permeability coefficients were also calculated for the different amphiphiles of the series, using the inhomogeneous solubility-diffusion model (ISDM). Quantitative agreement with values obtained from kinetic modeling of the permeation process could not be obtained. However, for the longer, and more hydrophobic amphiphiles, the variation trend along the homologous series was qualitatively better matched by the ISDM when the equilibrium location of each amphiphile was taken as reference (ΔG = 0), compared to the usual choice of bulk water.

Encapsulation and Delivery of an Osteosarcoma Stem Cell Active Gallium(III)-Diflunisal Complex Using Polymeric Micelles

Here we report the encapsulation of an osteosarcoma stem cell (OSC) potent gallium(III)-diflunisal complex 1 into polymeric nanoparticles, and its delivery into osteosarcoma cells. At the optimum feed (20 %, 1 NP²⁰ ), nanoparticle encapsulation of 1 enhances potency towards bulk osteosarcoma cells and OSCs (cultured in monolayer and three-dimensional systems). Strikingly, the nanoparticle formulation exhibits up to 5645-fold greater potency towards OSCs than frontline anti-osteosarcoma drugs, doxorubicin and cisplatin. The nanoparticle formulation evokes a similar mechanism of action as the payload, which bodes well for future translation. Specifically, the nanoparticle formulation induces nuclear DNA damage, cyclooxygenase-2 downregulation, and caspase-dependent apoptosis. To the best of our knowledge, this is the first study to demonstrate that polymeric nanoparticles can be used to effectively deliver an OSC-active metal complex into osteosarcoma cells.

The anti-breast cancer stem cell properties of gold(i)-non-steroidal anti-inflammatory drug complexes

The anti-breast cancer stem cell (CSC) properties of a series of gold(i) complexes comprising various non-steroidal anti-inflammatory drugs (NSAIDs) and triphenylphosphine 1-8 are reported. The most effective gold(i)-NSAID complex 1, containing indomethacin, exhibits greater potency for breast CSCs than bulk breast cancer cells (up to 80-fold). Furthermore, 1 reduces mammosphere viability to a better extent than a panel of clinically used breast cancer drugs and salinomycin, an established anti-breast CSC agent. Mechanistic studies suggest 1-induced breast CSC death results from breast CSC entry, cytoplasm localisation, an increase in intracellular reactive oxygen species levels, cyclooxygenase-2 downregulation and inhibition, and apoptosis. Remarkably, 1 also significantly inhibits tumour growth in a murine metastatic triple-negative breast cancer model. To the best of our knowledge, 1 is the first gold complex of any geometry or oxidation state to demonstrate anti-breast CSC properties.

The Osteosarcoma Stem Cell Activity of a Gallium(III)-Phenanthroline Complex Appended to Salicylate

We report the synthesis, characterisation, and anti-osteosarcoma properties of a gallium(III) complex (1) comprising of two 1,10-phenanthroline ligands and salicylate, a non-steroidal anti-inflammatory drug. The gallium(III) complex 1 displays micromolar potency towards bulk osteosarcoma cells and osteosarcoma stem cells (OSCs). Notably, the gallium(III) complex 1 exhibits significantly higher toxicity towards OSCs grown in monolayer and three-dimensional cultures than cisplatin, a frontline anti-osteosarcoma drug. Nuclei isolation and immunoblotting studies show that the gallium(III) complex 1 enters osteosarcoma cell nuclei and induces DNA damage. Flow cytometry and cytotoxicity studies (in the presence of prostaglandin E2) indicate that the gallium(III) complex 1 downregulates cyclooxygenase-2 (COX-2) expression and kills osteosarcoma cells in a COX-2-dependent manner. Further, the mode of osteosarcoma cell death evoked by the gallium(III) complex 1 is characterised as caspase-dependent apoptosis.

Bilayer compositional asymmetry influences the nanoscopic to macroscopic phase domain size transition

The eukaryotic plasma membrane (PM) exhibits lipid mixing heterogeneities known as lipid rafts. These lipid rafts, the result of liquid-liquid phase separation, can be modeled by coexisting liquid ordered (Lo) and liquid disordered (Ld) domains. Four-lipid component systems with a high-melting lipid, a nanodomain-inducing low-melting lipid, a macrodomain-inducing low-melting lipid, and cholesterol (chol) can give rise to domains of different sizes. These four-component systems have been characterized in experiments, yet there are few studies that model the asymmetric distribution of lipids actually found in the PM. We used molecular dynamics (MD) simulations to analyze the transition from nanoscopic to macroscopic domains in symmetric and in asymmetric model membranes. Using coarse-grained MD simulations, we found that asymmetry promotes macroscopic domain growth in a case where symmetric systems exhibit nanoscopic domains. Also, macroscopic domain formation in symmetric systems is highly dependent on registration of like phases in the cytoplasmic and exoplasmic leaflets. Using united-atom MD simulations, we found that symmetric Lo domains are only slightly more ordered than asymmetric Lo domains. We also found that large Lo domains in our asymmetric systems induce a slight chain ordering in the apposed cytoplasmic regions. The chol fractions of phase-separated Lo and Ld domains of the exoplasmic leaflet were unchanged whether the system was symmetric or asymmetric.

Alzheimer's Disease as a Membrane Disorder: Spatial Cross-Talk Among Beta-Amyloid Peptides, Nicotinic Acetylcholine Receptors and Lipid Rafts

Biological membranes show lateral and transverse asymmetric lipid distribution. Cholesterol (Chol) localizes in both hemilayers, but in the external one it is mostly condensed in lipid-ordered microdomains (raft domains), together with saturated phosphatidyl lipids and sphingolipids (including sphingomyelin and glycosphingolipids). Membrane asymmetries induce special membrane biophysical properties and behave as signals for several physiological and/or pathological processes. Alzheimer’s disease (AD) is associated with a perturbation in different membrane properties. Amyloid-β (Aβ) plaques and neurofibrillary tangles of tau protein together with neuroinflammation and neurodegeneration are the most characteristic cellular changes observed in this disease. The extracellular presence of Aβ peptides forming senile plaques, together with soluble oligomeric species of Aβ, are considered the major cause of the synaptic dysfunction of AD. The association between Aβ peptide and membrane lipids has been extensively studied. It has been postulated that Chol content and Chol distribution condition Aβ production and posterior accumulation in membranes and, hence, cell dysfunction. Several lines of evidence suggest that Aβ partitions in the cell membrane accumulate mostly in raft domains, the site where the cleavage of the precursor AβPP by β- and γ- secretase is also thought to occur. The main consequence of the pathogenesis of AD is the disruption of the cholinergic pathways in the cerebral cortex and in the basal forebrain. In parallel, the nicotinic acetylcholine receptor has been extensively linked to membrane properties. Since its transmembrane domain exhibits extensive contacts with the surrounding lipids, the acetylcholine receptor function is conditioned by its lipid microenvironment. The nicotinic acetylcholine receptor is present in high-density clusters in the cell membrane where it localizes mainly in lipid-ordered domains. Perturbations of sphingomyelin or cholesterol composition alter acetylcholine receptor location. Therefore, Aβ processing, Aβ partitioning, and acetylcholine receptor location and function can be manipulated by changes in membrane lipid biophysics. Understanding these mechanisms should provide insights into new therapeutic strategies for prevention and/or treatment of AD. Here, we discuss the implications of lipid-protein interactions at the cell membrane level in AD.

Role of Cell Surface Lipids and Thiol-Disulphide Exchange Pathways in Regulating the Encryption and Decryption of Tissue Factor

Tissue factor (TF), a transmembrane glycoprotein, is the cellular receptor of the coagulation factors VII (FVII) and VIIa (FVIIa). The formation of TF-FVIIa complex triggers the initiation of the blood coagulation pathway. TF plays an essential role in haemostasis, but an aberrant expression of TF activity contributes to thrombotic disorders. In health, TF pro-coagulant activity on cells is controlled tightly to allow sufficient coagulant activity to achieve haemostasis but not to cause thrombosis. It is achieved largely by selective localization of TF in the body and encryption of TF at the cell surface. A vast majority of TF on resting cells exists in an encrypted state with minimal pro-coagulant activity but becomes pro-thrombotic following cell injury or activation. At present, the mechanisms that are responsible for TF encryption and activation (decryption) are not entirely clear, but recent studies provide important mechanistic insights into these processes. To date, externalization of phosphatidylserine to the outer leaflet and thiol-disulphide exchange pathways that either turn on and off the allosteric disulphide bond in TF are shown to play a major role in regulating TF pro-coagulant activity on cell surfaces. Recent studies showed that sphingomyelin, a major phospholipid in the outer leaflet of plasma membrane, plays a critical role in the encryption of TF in resting cells. The present review provides an overview of recent literature on the above-described mechanisms of TF encryption and decryption with a particular emphasis on our recent findings.

Functional role of highly conserved residues of the N-terminal tail and first transmembrane segment of a P4-ATPase

The P4 family of P-type ATPases (P4-ATPases) plays an important role in maintaining phospholipid asymmetry in eukaryotic cell membranes. Leishmania miltefosine transporter (LMT) is a plasma membrane (PM) P4-ATPase that catalyses translocation into the parasite of the leishmanicidal drug miltefosine as well as phosphatidylcholine and phosphatidylethanolamine analogues. In the present study, we analysed the role, in LMT, of a series of highly conserved amino acids previously undescribed in the N-terminal region of P4-ATPases. Seven residues were identified and, according to an LMT structural model, five were located in the cytosolic N-terminal tail (Asn58, Ile60, Lys64, Tyr65 and Phe70) and the other two (Pro72 and Phe79) in the first transmembrane segment (TM1). Alanine-scanning mutagenesis analysis showed that N58A, Y65A and F79A mutations caused a considerable reduction in the LMT translocase activity. These mutations did not affect protein expression levels. We generated additional mutations in these three residues to assess the influence of the conservation degree on LMT translocase activity. Some of these mutations reduced expression levels without affecting the interaction between LMT and its CDC50 subunit, LRos3. Conserved and non-conserved mutations in the invariant residue Asn58 drastically reduced the translocase activity. Consequently, Asn58 may be necessary to achieve optimal catalytic LMT activity as previously described for the potentially equivalent Asn39 of the sarco/endoplasmic reticulum Ca2+-ATPase isoform 1a (SERCA1a). Additionally, conservation of a hydrophobic residue at position 79 is crucial for LMT stability.

Triterpenoid modulates the salt tolerance of lanosterol synthase deficient Saccharomyces cerevisiae, GIL77

This study examined the effect of triterpenoid on the salt tolerance of lanosterol synthase deficient yeast mutant GIL77. The expression of the triterpenoid synthase gene under GAL1 promoter in GIL77 increased the triterpenoid concentration of both whole cell and plasma membrane fractions. Without the induction of the genes, the growth curve of BgbAS or RsM1 transformant depicted patterns similar to control cells in both the presence and absence of salt with growth inhibition at 500 mM NaCl. The induction of BgbAS and RsM1 gene expression slightly repressed growth compared with control cells in the absence of NaCl. The growth of GIL77 was significantly suppressed by the expression of BgbAS or RsM1 under salinity conditions. Of the triterpenoid synthase genes, BgbAS rather than RsM1 was found to strongly inhibit the growth of GIL77 cells under salt stressed conditions. The expression of the triterpenoid synthase gene in GIL77 also influenced their tolerance to other abiotic stresses. In contrast to the endogenous synthesis, the exogenous supply of triterpenoid in the culture medium appeared to occur in the plasma membrane fraction and enhanced the salt tolerance of GIL77. This study thus discussed the physiological significance of triterpenoid in relation to its possible role in modulating salt tolerance.

Quantitative contributions of processes by which polyanion drugs reduce intracellular bioavailability and transfection efficiency of cationic siRNA lipoplex

RNA Interference (RNAi) is a potentially useful tool to correct the detrimental effects of faulty genes; several RNAi are undergoing clinical evaluation in various diseases. The present study identified the relative contributions of three mechanisms by which polyanion drugs reduced the gene silencing activity of Lipoplex, a complex of small interfering RNA (siRNA) and cationic liposomes. The study used a siRNA against the chemoresistance gene survivin and two model polyanion drugs (suramin, heparin). Products of Lipoplex destabilization were separated, identified, and/or quantified using ultrafiltration, gel electrophoresis, and RT-qPCR (quantitative reverse transcription polymerase chain reaction). Cell binding and endocytosis of fluorescence-labeled Lipoplex and the amount of siRNA at its site of action RISC (RNA-induced silencing complex) were evaluated using endocytosis markers, confocal microscopy, quantitative image analysis, immunoprecipitation, and RT-qPCR. The results show suramin and heparin exerted multiple concentration-dependent effects. First, these agents altered several Lipoplex properties (i.e., reduced particle size, changed surface charge, modified composition of protein biocorona). Second, both caused Lipoplex destabilization to release double- and single-strand siRNA and/or smaller siRNA-lipid complexes with reduced siRNA cargo. Third, both prevented the cell surface binding and internalization of Lipoplex, diminished the siRNA concentration in RISC, and retarded the mRNA knockdown. Suramin and heparin yielded qualitatively and quantitatively different results. Analysis of the experimental results of suramin using quantitative pharmacology (QP) modeling indicated the major cause of gene silencing activity loss depended on drug concentration, changing from inhibition of endocytosis at lower concentration (accounting for 60% loss at ~9μM) to inhibition of cell surface binding and loss of siRNA cargo at higher concentrations (accounting for 64% and 27%, respectively, at 70μM). In summary, the present study demonstrates the complex and dynamic interactions between polyanions and Lipoplex, and the use of QP modeling to delineate the contributions of three mechanisms to the eventual loss of gene silencing activity.

Synthesis of Dipyridyl Ketone Isonicotinoyl Hydrazone Copper(II) Complex: Structure, Anticancer Activity and Anticancer Mechanism

In an effort to better understand the biological efficacy of the tridentate aroyl hydrazone Cu(II) complexes, the Cu(II) complex of di-2-pyridyl ketone isonicotinoyl hydrazone ligand (HL), {[Cu(L)(H₂O)]·H₂O·NO₃}_n (C1) was synthesized and characterized. Single crystal X-ray study reveals that complex C1 forms 1D zigzag chains in solid state. In water, the hydrolysis of the 1D zigzag chains was observed, and finally formation of monomeric species. In vitro studies revealed that complex C1 showed significantly more anticancer activity than the ligand alone. Investigation of the anticancer mechanisms of C1, confirmed that the Cu(II) complex exhibit a strong capacity to promote productions of reactive oxygen species (ROS), leading to caspase-dependent apoptotic cell death.

Integrating TRPV1 Receptor Function with Capsaicin Psychophysics

Capsaicin is a naturally occurring vanilloid that causes a hot, pungent sensation in the human oral cavity. This trigeminal stimulus activates TRPV1 receptors and stimulates an influx of cations into sensory cells. TRPV1 receptors function as homotetramers that also respond to heat, proinflammatory substances, lipoxygenase products, resiniferatoxin, endocannabinoids, protons, and peptide toxins. Kinase-mediated phosphorylation of TRPV1 leads to increased sensitivity to both chemical and thermal stimuli. In contrast, desensitization occurs via a calcium-dependent mechanism that results in receptor dephosphorylation. Human psychophysical studies have shown that capsaicin is detected at nanomole amounts and causes desensitization in the oral cavity. Psychophysical studies further indicate that desensitization can be temporarily reversed in the oral cavity if stimulation with capsaicin is resumed at short interstimulus intervals. Pretreatment of lingual epithelium with capsaicin modulates the perception of several primary taste qualities. Also, sweet taste stimuli may decrease the intensity of capsaicin perception in the oral cavity. In addition, capsaicin perception and hedonic responses may be modified by diet. Psychophysical studies with capsaicin are consistent with recent findings that have identified TRPV1 channel modulation by phosphorylation and interactions with membrane inositol phospholipids. Future studies will further clarify the importance of capsaicin and its receptor in human health and nutrition.

A Pt(IV) Pro-drug Preferentially Targets Indoleamine-2,3-dioxygenase, Providing Enhanced Ovarian Cancer Immuno-Chemotherapy

Expression of indoleamine-2,3-dioxygenase (IDO), an immunosuppressive enzyme in human tumors, leads to immune evasion and tumor tolerance. IDO is therefore a tumor immunotherapeutic target, and several IDO inhibitors are currently undergoing clinical trials. IDO inhibitors can enhance the efficacy of common cancer chemotherapeutics. Here we investigate Pt(IV)-(D)-1-methyltryptophan conjugates 1 and 2 for combined immunomodulation and DNA cross-link-triggered apoptosis for cancer "immuno-chemotherapy". Compound 2 effectively kills hormone-dependent, cisplatin-resistant human ovarian cancer cells, inhibiting IDO by transcriptional deregulation of the autocrine-signaling loop IDO-AHR-IL6, which blocks kynurenine production and promotes T-cell proliferation. Additionally, 1 and 2 display low toxicity in mice and are stable in blood. To our knowledge, this construct is the first Pt drug candidate with immune checkpoint blockade properties.

Myelin basic protein induces neuron-specific toxicity by directly damaging the neuronal plasma membrane

The central nervous system (CNS) insults may cause massive demyelination and lead to the release of myelin-associated proteins including its major component myelin basic protein (MBP). MBP is reported to induce glial activation but its effect on neurons is still little known. Here we found that MBP specifically bound to the extracellular surface of the neuronal plasma membrane and induced neurotoxicity in vitro. This effect of MBP on neurons was basicity-dependent because the binding was blocked by acidic lipids and competed by other basic proteins. Further studies revealed that MBP induced damage to neuronal membrane integrity and function by depolarizing the resting membrane potential, increasing the permeability to cations and other molecules, and decreasing the membrane fluidity. At last, artificial liposome vesicle assay showed that MBP directly disturbed acidic lipid bilayer and resulted in increased membrane permeability. These results revealed that MBP induces neurotoxicity through its direct interaction with acidic components on the extracellular surface of neuronal membrane, which may suggest a possible contribution of MBP to the pathogenesis in the CNS disorders with myelin damage.

Milk sphingomyelin domains in biomimetic membranes and the role of cholesterol: morphology and nanomechanical properties investigated using AFM and force spectroscopy

Milk sphingomyelin (MSM) and cholesterol segregate into domains in the outer bilayer membrane surrounding milk fat globules. To elucidate the morphology and mechanical properties of theses domains, supported lipid bilayers with controlled molar proportions of MSM, dioleoylphosphatidylcholine (DOPC) and cholesterol were produced in buffer mimicking conditions of the milk aqueous phase. Atomic force microscopy imaging showed that (i) for T < 35 °C MSM segregated in gel phase domains protruding above the fluid phase, (ii) the addition of 20 mol % cholesterol resulted in smaller and more elongated l(o) phase domains than in equimolar MSM/DOPC membranes, (iii) the MSM/cholesterol-enriched l(o) phase domains were less salient than the MSM gel phase domains. Force spectroscopy measurements furthermore showed that cholesterol reduced the resistance of MSM/DOPC membrane to perforation. The results are discussed with respect to the effect of cholesterol on the biophysical properties of lipid membranes. The combination of AFM imaging and force mapping provides unprecedented insight into the structural and mechanical properties of milk lipid membranes, and opens perspectives for investigation of the functional properties of MSM domains during milk fat processing or digestion.

A dual-targeting, p53-independent, apoptosis-inducing platinum(II) anticancer complex, [Pt(BDI(QQ))]Cl

The therapeutic index and cellular mechanism of action of [Pt(BDI(QQ))]Cl, a monocationic, square-planar platinum(II) complex, are reported. [Pt(BDI(QQ))]Cl was used to treat several cell lines, including wild type and cisplatin-resistant ovarian carcinoma cells (A2780 and A2780CP70) and non-proliferating lung carcinoma cells (A549). [Pt(BDI(QQ))]Cl selectively kills cancer cells over healthy cells and exhibits no cross-resistance with cisplatin. The mechanism of cell killing was established through detailed cell-based assays. [Pt(BDI(QQ))]Cl exhibits dual-threat capabilities, targeting nuclear DNA and mitochondria simultaneously. [Pt(BDI(QQ))]Cl induces DNA damage, leading to p53 enrichment, mitochondrial membrane potential depolarisation, and caspase-mediated apoptosis. [Pt(BDI(QQ))]Cl also accumulates in the mitochondria, resulting in direct mitochondrial damage. Flow cytometric studies demonstrated that [Pt(BDI(QQ))]Cl has no significant effect on cell cycle progression. Remarkably, p53-status is a not a determinant of [Pt(BDI(QQ))]Cl activity. In p53-null cells, [Pt(BDI(QQ))]Cl induces cell death through mitochondrial dysfunction. Cancers with p53-null status could therefore be targeted using [Pt(BDI(QQ))]Cl.

Update of the 1972 Singer-Nicolson Fluid-Mosaic Model of Membrane Structure

The Fluid-Mosaic Membrane Model of cell membrane structure was based on thermodynamic principals and the available data on component lateral mobility within the membrane plane [Singer SJ, Nicolson GL. The Fluid Mosaic Model of the structure of cell membranes. Science 1972; 175: 720-731]. After more than forty years the model remains relevant for describing the basic nano-scale structures of a variety of biological membranes. More recent information, however, has shown the importance of specialized membrane domains, such as lipid rafts and protein complexes, in describing the macrostructure and dynamics of biological membranes. In addition, membrane-associated cytoskeletal structures and extracellular matrix also play roles in limiting the mobility and range of motion of membrane components and add new layers of complexity and hierarchy to the original model. An updated Fluid-Mosaic Membrane Model is described, where more emphasis has been placed on the mosaic nature of cellular membranes where protein and lipid components are more crowded and limited in their movements in the membrane plane by lipid-lipid, protein-protein and lipid-protein interactions as well as cell-matrix, cell-cell and cytoskeletal interactions. These interactions are important in restraining membrane components and maintaining the unique mosaic organization of cell membranes into functional, dynamic domains.

Plasma membrane--cortical cytoskeleton interactions: a cell biology approach with biophysical considerations

From a biophysical standpoint, the interface between the cell membrane and the cytoskeleton is an intriguing site where a "two-dimensional fluid" interacts with an exceedingly complex three-dimensional protein meshwork. The membrane is a key regulator of the cytoskeleton, which not only provides docking sites for cytoskeletal elements through transmembrane proteins, lipid binding-based, and electrostatic interactions, but also serves as the source of the signaling events and molecules that control cytoskeletal organization and remolding. Conversely, the cytoskeleton is a key determinant of the biophysical and biochemical properties of the membrane, including its shape, tension, movement, composition, as well as the mobility, partitioning, and recycling of its constituents. From a cell biological standpoint, the membrane-cytoskeleton interplay underlies--as a central executor and/or regulator--a multitude of complex processes including chemical and mechanical signal transduction, motility/migration, endo-/exo-/phagocytosis, and other forms of membrane traffic, cell-cell, and cell-matrix adhesion. The aim of this article is to provide an overview of the tight structural and functional coupling between the membrane and the cytoskeleton. As biophysical approaches, both theoretical and experimental, proved to be instrumental for our understanding of the membrane/cytoskeleton interplay, this review will "oscillate" between the cell biological phenomena and the corresponding biophysical principles and considerations. After describing the types of connections between the membrane and the cytoskeleton, we will focus on a few key physical parameters and processes (force generation, curvature, tension, and surface charge) and will discuss how these contribute to a variety of fundamental cell biological functions.

Bidentate ligands on osmium(VI) nitrido complexes control intracellular targeting and cell death pathways

The cellular response evoked by antiproliferating osmium(VI) nitrido compounds of general formula OsN(N^N)Cl3 (N^N = 2,2'-bipyridine 1, 1,10-phenanthroline 2, 3,4,7,8-tetramethyl-1,10-phenanthroline 3, or 4,7-diphenyl-1,10-phenanthroline 4) can be tuned by subtle ligand modifications. Complex 2 induces DNA damage, resulting in activation of the p53 pathway, cell cycle arrest at the G2/M phase, and caspase-dependent apoptotic cell death. In contrast, 4 evokes endoplasmic reticulum (ER) stress leading to the upregulation of proteins of the unfolded protein response pathway, increase in ER size, and p53-independent apoptotic cell death. To the best of our knowledge, 4 is the first osmium compound to induce ER stress in cancer cells.

Factor VII and protein C are phosphatidic acid-binding proteins

Seven proteins in the human blood clotting cascade bind, via their GLA (γ-carboxyglutamate-rich) domains, to membranes containing exposed phosphatidylserine (PS), although with membrane binding affinities that vary by 3 orders of magnitude. Here we employed nanodiscs of defined phospholipid composition to quantify the phospholipid binding specificities of these seven clotting proteins. All bound preferentially to nanobilayers in which PS headgroups contained l-serine versus d-serine. Surprisingly, however, nanobilayers containing phosphatidic acid (PA) bound substantially more of two of these proteins, factor VIIa and activated protein C, than did equivalent bilayers containing PS. Consistent with this finding, liposomes containing PA supported higher proteolytic activity by factor VIIa and activated protein C toward their natural substrates (factors X and Va, respectively) than did PS-containing liposomes. Moreover, treating activated human platelets with phospholipase D enhanced the rates of factor X activation by factor VIIa in the presence of soluble tissue factor. We hypothesize that factor VII and protein C bind preferentially to the monoester phosphate of PA because of its accessibility and higher negative charge compared with the diester phosphates of most other phospholipids. We further found that phosphatidylinositol 4-phosphate, which contains a monoester phosphate attached to its myo-inositol headgroup, also supported enhanced enzymatic activity of factor VIIa and activated protein C. We conclude that factor VII and protein C bind preferentially to monoester phosphates, which may have implications for the function of these proteases in vivo.

Molecular determinants that regulate plasma membrane association of HIV-1 Gag

Human immunodeficiency virus type 1 assembly is a multistep process that occurs at the plasma membrane (PM). Targeting and binding of Gag to the PM are the first steps in this assembly process and are mediated by the matrix domain of Gag. This review highlights our current knowledge on viral and cellular determinants that affect specific interactions between Gag and the PM. We will discuss potential mechanisms by which the matrix domain might integrate three regulatory components, myristate, phosphatidylinositol-(4,5)-bisphosphate, and RNA, to ensure that human immunodeficiency virus type 1 assembly occurs at the PM.

Cell cholesterol homeostasis: mediation by active cholesterol

Recent evidence suggests that the major pathways mediating cell cholesterol homeostasis respond to a common signal: active membrane cholesterol. Active cholesterol is the fraction that exceeds the complexing capacity of the polar bilayer lipids. Increments in plasma membrane cholesterol exceeding this threshold have an elevated chemical activity (escape tendency) and redistribute via diverse transport proteins to both circulating plasma lipoproteins and intracellular organelles. Active cholesterol thereby prompts several feedback responses. It is the substrate for its own esterification and for the synthesis of regulatory side-chain oxysterols. It also stimulates manifold pathways that down-regulate the biosynthesis, curtail the ingestion and increase the export of cholesterol. Thus, the abundance of cell cholesterol is tightly coupled to that of its polar lipid partners through active cholesterol.

Pivotal Advance: Phospholipids determine net membrane surface charge resulting in differential localization of active Rac1 and Rac2

In this investigation, we used primary murine neutrophils to demonstrate that local changes in membrane phospholipid composition alter the net cytoplasmic membrane surface charge, which results in selective recruitment of Rac1 or Rac2 based on the net charge of their respective C-terminal domains. Murine neutrophils undergoing chemotaxis or carrying out phagocytosis were transfected with K-ras4B-derived membrane charge biosensors and lipid markers, which allowed us to simultaneously monitor the levels of PIP(2), PIP(3), and PS and net membrane charge of the newly developing phagosome membrane and plasma membrane. Our results indicate that the combination of PIP(2), PIP(3), and PS generates a high negative charge (-8) at the plasma membrane of actin-rich pseudopods, where active Rac1 preferentially localizes during phagosome formation. The lipid metabolism that occurs during phagosome maturation results in the localized depletion of PIP(2), PIP(3), and partial decrease in PS. This creates a moderately negative net charge that correlates with the localization of active Rac2. Conversely, the accumulation of PIP(3) at the leading-edge membrane during chemotaxis generates a polarized accumulation of negative charges that recruits Rac1. These results provide evidence that alterations in membrane lipid composition and inner-membrane surface charge are important elements for the recruitment of differentially charged proteins and localization of signaling pathways during phagocytosis and chemotaxis in neutrophils.

Surface charge: a key determinant of protein localization and function

Electrostatic charge at the membrane surface has emerged as a crucial determinant of the localization and activation of many proteins containing polycationic domains in their amino acid sequence. The spatiotemporal regulation of surface charge, as well as the downstream effects of dysregulation of surface charge, may have a significant impact on many of the signaling molecules important to cancer biology such as K-ras. Cancer Res; 70(4); 1277-80.

Domain coupling in asymmetric lipid bilayers

Biological membranes are heterogeneous assemblies of lipids, proteins, and cholesterol that are organized as asymmetric bimolecular leaflets of lipids with embedded proteins. Modulated by the concentration of cholesterol lipids and proteins may segregate into two or more liquid phases with different physical properties that can coexist in the same membrane. In this review, we summarize recent advances on how this situation can be recreated in a supported bilayer format and how this system has been used to demonstrate the induction of ordered lipid domains in lipid compositions that are typical for the inner leaflet by lipid compositions that are typical for the outer leaflet of mammalian plasma membranes. Proteins are shown to differentially target such induced inner leaflet domains.

The liquid-ordered phase in membranes

A range of physiological processes has been imputed to lateral domain formation in biological membranes. However the molecular mechanisms of these functions and the details of how domain structures mediate these processes remain largely speculative. That domains exist in biomembranes and can be modeled in relatively simple lipid systems has contributed to our understanding of the principles governing phase behaviour in membranes. A presentation of these principles is the subject of this review. The condensing effect of sterols on phospholipids spread as monomolecular films at the air-water interface is described in terms of the dependence of the effect on sterol and phospholipid structure. The thermodynamics of sphingomyelin-cholesterol interactions are considered from calorimetric, densitometry and equilibrium cholesterol exchange measurements. Biophysical characterisation of the structure of liquid-ordered phase and its relationship with liquid-disordered phase is described from spectroscopic and X-ray scattering studies. Finally, the properties of liquid-ordered phase in the context of membrane physiology and permeability barrier properties are considered.

Dissociation of Rac1(GDP).RhoGDI complexes by the cooperative action of anionic liposomes containing phosphatidylinositol 3,4,5-trisphosphate, Rac guanine nucleotide exchange factor, and GTP

Rac plays a pivotal role in the assembly of the superoxide-generating NADPH oxidase of phagocytes. In resting cells, Rac is found in the cytosol in complex with Rho GDP dissociation inhibitor (RhoGDI). NADPH oxidase assembly involves dissociation of the Rac.RhoGDI complex and translocation of Rac to the membrane. We reported that liposomes containing high concentrations of monovalent anionic phospholipids cause Rac.RhoGDI complex dissociation ( Ugolev, Y., Molshanski-Mor, S., Weinbaum, C., and Pick, E. (2006) J. Biol. Chem. 281, 19204-19219 ). We now designed an in vitro model mimicking membrane phospholipid remodeling during phagocyte stimulation in vivo. We showed that liposomes of "resting cell membrane" composition (less than 20 mol % monovalent anionic phospholipids), supplemented with 1 mol % of polyvalent anionic phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) in conjunction with constitutively active forms of the guanine nucleotide exchange factors (GEFs) for Rac, Trio, or Tiam1 and a non-hydrolyzable GTP analogue, cause dissociation of Rac1(GDP).RhoGDI complexes, GDP to GTP exchange on Rac1, and binding of Rac1(GTP) to the liposomes. Complexes were not dissociated in the absence of GEF and GTP, and optimal dissociation required the presence of PtdIns(3,4,5)P(3) in the liposomes. Dissociation of Rac1(GDP).RhoGDI complexes was correlated with the affinity of particular GEF constructs, via the N-terminal pleckstrin homology domain, for PtdIns(3,4,5)P(3) and involved GEF-mediated GDP to GTP exchange on Rac1. Phagocyte membranes enriched in PtdIns(3,4,5)P(3) responded by NADPH oxidase activation upon exposure in vitro to Rac1(GDP).RhoGDI complexes, p67(phox), GTP, and Rac GEF constructs with affinity for PtdIns(3,4,5)P(3) at a level superior to that of native membranes.

Cholesterol homeostasis and the escape tendency (activity) of plasma membrane cholesterol

We review evidence that sterols can form stoichiometric complexes with certain bilayer phospholipids, and sphingomyelin in particular. These complexes appear to be the basis for the formation of condensed and ordered liquid phases, (micro)domains and/or rafts in both artificial and biological membranes. The sterol content of a membrane can exceed the complexing capacity of its phospholipids. The excess, uncomplexed membrane sterol molecules have a relatively high escape tendency, also referred to as fugacity or chemical activity (and, here, simply activity). Cholesterol is also activated when certain membrane intercalating amphipaths displace it from the phospholipid complexes. Active cholesterol projects from the bilayer and is therefore highly susceptible to attack by cholesterol oxidase. Similarly, active cholesterol rapidly exits the plasma membrane to extracellular acceptors such as cyclodextrin and high-density lipoproteins. For the same reason, the pool of cholesterol in the ER (endoplasmic reticulum) increases sharply when cell surface cholesterol is incremented above the physiological set-point; i.e., equivalence with the complexing phospholipids. As a result, the escape tendency of the excess cholesterol not only returns the plasma membrane bilayer to its set-point but also serves as a feedback signal to intracellular homeostatic elements to down-regulate cholesterol accretion.

Lipid signaling and the modulation of surface charge during phagocytosis

Phagocytosis is an important component of innate and adaptive immunity. The formation of phagosomes and the subsequent maturation that capacitates them for pathogen elimination and antigen presentation are complex processes that involve signal transduction, cytoskeletal reorganization, and membrane remodeling. Lipids are increasingly appreciated to play a crucial role in these events. Sphingolipids, cholesterol, and glycerophospholipids, notably the phosphoinositides, are required for the segregation of signaling microdomains and for the generation of second messengers. They are also instrumental in the remodeling of the actin cytoskeleton and in directing membrane traffic. They accomplish these feats by congregating into liquid-ordered domains, by generating active metabolites that activate receptors, and by recruiting and anchoring specific protein ligands to the membrane, often altering their conformation and catalytic activity. A less appreciated role of acidic phospholipids is their contribution to the negative surface charge of the inner leaflet of the plasmalemma. The unique negativity of the inner aspect of the plasma membrane serves to attract and anchor key signaling and effector molecules that are required to initiate phagosome formation. Conversely, the loss of charge that accompanies phospholipid metabolism as phagosomes seal facilitates the dissociation of proteins and the termination of signaling and cytoskeleton assembly. In this manner, lipids provide a binary electrostatic switch to control phagocytosis.

Scrambling of phospholipids activates red cell membrane cholesterol

Cholesterol is predicted to associate more strongly with the outer than the inner leaflet of plasma membrane bilayers based on the relative in vitro affinities of their phospholipids. Complex formation with the high-affinity species (especially saturated sphingomyelins) is said to reduce the chemical activity (escape potential or fugacity) of the sterol. We therefore tested the hypothesis that scrambling the sidedness of plasma membrane phospholipids of intact cells will increase the chemical activity of outer surface cholesterol. Upon activating the plasma membrane scramblase in intact human red cells by introducing ionomycin to raise cytoplasmic Ca++, phosphatidylserine became exposed and, concomitantly, the chemical activity of exofacial cholesterol was increased. (This was gauged by its susceptibility to cholesterol oxidase and its rate of transfer to cyclodextrin.) Similar behavior was observed in human fibroblasts. Two other treatments known to activate cell surface cholesterol (namely, exposure to glutaraldehyde and to low-ionic-strength buffer) also brought phosphatidylserine to the cell surface but by a Ca++-independent mechanism. Given that phospholipid scrambling is important in blood coagulation and apoptosis, the concomitant activation of cell surface cholesterol could contribute to these and other pathophysiological signaling processes.

Fluorescence microscopy to study domains in supported lipid bilayers

Fluorescence microscopy of model membranes is a powerful tool for identifying the nature and extent of coexisting phases in biologically relevant lipid mixtures. Planar supported bilayers offer the advantage over spherical model membranes in that both overall composition and lipid asymmetry can be controlled. In addition, the membrane can be easily accessed by perfusion of soluble components. Here, the necessary techniques for reconstituting bilayers of complex composition and phase behavior in planar systems are outlined. Effective methods for the formation of both symmetric and asymmetric bilayers are described. Considerations that must be taken into account in choosing suitable lipid compositions, effective fluorescent lipid dyes, and adequate microscope setups are discussed.

Transbilayer effects of raft-like lipid domains in asymmetric planar bilayers measured by single molecule tracking

Cell membranes have complex lipid compositions, including an asymmetric distribution of phospholipids between the opposing leaflets of the bilayer. Although it has been demonstrated that the lipid composition of the outer leaflet of the plasma membrane is sufficient for the formation of raft-like liquid-ordered (l(o)) phase domains, the influence that such domains may have on the lipids and proteins of the inner leaflet remains unknown. We used tethered polymer supports and a combined Langmuir-Blodgett/vesicle fusion (LB/VF) technique to build asymmetric planar bilayers that mimic plasma membrane asymmetry in many ways. We show that directly supported LB monolayers containing cholesterol-rich l(o) phases are inherently unstable when exposed to water or vesicle suspensions. However, tethering the LB monolayer to the solid support with the lipid-anchored polymer 1,2-dimyristoyl phophatidylethanolamine-N-[poly(ethylene glycol)-triethoxysilane] significantly improves stability and allows for the formation of complex planar-supported bilayers that retain >90% asymmetry for 1-2 h. We developed a single molecule tracking (SPT) system for the study of lipid diffusion in asymmetric bilayers with coexisting liquid phases. SPT allowed us to study in detail the diffusion of individual lipids inside, outside, or directly opposed to l(o) phase domains. We show here that l(o) phase domains in one monolayer of an asymmetric bilayer do not induce the formation of domains in the opposite leaflet when this leaflet is composed of palmitoyl-oleoyl phosphatidylcholine and cholesterol but do induce domains when this leaflet is composed of porcine brain phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and cholesterol. The diffusion of lipids is similar in l(o) and liquid-disordered phase domains and is not affected by transbilayer coupling, indicating that lateral and transverse lipid interactions that give rise to the domain structure are weak in the biological lipid mixtures that were employed in this work.

Toward oxidative lipidomics of cell signaling

Involvement and role of oxidatively modified lipids become increasingly evident in different signaling pathways, although specific mechanisms remain to be explored. The current forum focuses on several aspects of signaling by oxidatively modified lipid molecules during apoptosis and implications of different oxidized phospholipids in elimination of apoptotic cells and regulation of inflammatory response. Studies of signaling mechanisms by oxidatively modified lipids represent a rapidly expanding field of oxidative lipidomics, a new and exciting research focus at the interface of traditional lipid/membrane biochemistry and biophysics with free radical chemistry and cell biology.

Membrane Lipid Homeostasis

The lipid matrix of biological membranes is composed of a complex mixture of polar lipids. It has been estimated that more than 600 distinct molecular species of lipid are constituents of biological membranes. This rather remarkable feature raises the questions of why such complexity is required when barrier properties and many protein functions can be reconstituted with relatively simple lipid systems. Secondly, the molecular species composition of morphologically distinct membranes appears to be preserved within fairly narrow limits. The biochemical mechanism(s) responsible for this homeostasis are not fully understood. This review examines the origin of membrane lipid complexity, the methods that are currently employed to measure and detect lipid molecular species and the biochemical reactions associated with the turnover of membrane lipids in resting and stimulated cells.

Appetizing rancidity of apoptotic cells for macrophages: oxidation, externalization, and recognition of phosphatidylserine

Programmed cell death (apoptosis) functions as a mechanism to eliminate unwanted or irreparably damaged cells ultimately leading to their orderly phagocytosis in the absence of calamitous inflammatory responses. Recent studies have demonstrated that the generation of free radical intermediates and subsequent oxidative stress are implicated as part of the apoptotic execution process. Oxidative stress may simply be an unavoidable yet trivial byproduct of the apoptotic machinery; alternatively, intermediates or products of oxidative stress may act as essential signals for the execution of the apoptotic program. This review is focused on the specific role of oxidative stress in apoptotic signaling, which is realized via phosphatidylserine-dependent pathways leading to recognition of apoptotic cells and their effective clearance. In particular, the mechanisms involved in selective phosphatidylserine oxidation in the plasma membrane during apoptosis and its association with disturbances of phospholipid asymmetry leading to phosphatidylserine externalization and recognition by macrophage receptors are at the center of our discussion. The putative importance of this oxidative phosphatidylserine signaling in lung physiology and disease are also discussed.