Orthogonal lipid sensors identify transbilayer asymmetry of plasma membrane cholesterol

Composition dependence of cholesterol flip-flop rates in physiological mixtures

Unbiased, all-atom simulations of mixtures representative of the inner and outer leaflets of a mammalian red blood cell and a synaptic vesicle reveal many cholesterol flip-flop events over the 5 μsec duration of the simulations. Enough events are observed for a direct estimate of the flip-flop rate. Slower rates are found in more ordered membranes, and faster rates in more disordered membranes, consistent with earlier reports in the literature. However, the rates found here are neither as fast as the fastest nor as slow as the slowest rates obtained by previous simulations. The difference likely stems from the compositions studied here, which unlike previous work include exclusively lipids with differing acyl chains, as observed in mammalian lipidomes.

Photostable and Orthogonal Solvatochromic Fluorophores for Simultaneous In Situ Quantification of Multiple Cellular Signaling Molecules

Ratiometric fluorescence sensors are powerful tools for direct quantification of diverse biological analytes. To overcome a shortage of solvatochromic fluorophores crucial for in situ ratiometric imaging of biological targets, we prepared and characterized a small library of modular fluorophores with diverse spectral properties. Among them, WCB and WCR showed excellent spectral properties, including high photostability, brightness, and solvatochromism, and are ideally suited for dual ratiometric imaging due to their spectral orthogonality. By conjugating WCB and WCR with protein-based lipid sensors, we were able to achieve robust simultaneous in situ quantitative imaging of two metabolically linked signaling lipids, phosphatidylinositol-4,5-bisphosphate and phosphatidylinositol-3,4,5-trisphosphate in live cells. This study shows that any combination of signaling molecules can be simultaneously quantified in a spatiotemporally resolved manner by ratiometric imaging with finely tuned solvatochromic fluorophores.

Interactions of dietary polyphenols with epithelial lipids: advances from membrane and cell models in the study of polyphenol absorption, transport and delivery to the epithelium

Currently, diet-related diseases such as diabetes, obesity, hypertension, and cardiovascular diseases account for 70% of all global deaths. To counteract the rising prevalence of non-communicable diseases governments are investing in persuasive educational campaigns toward the ingestion of fresh fruits and vegetables. The intake of dietary polyphenols abundant in Mediterranean and Nordic-type diets holds great potential as nutritional strategies in the management of diet-related diseases. However, the successful implementation of healthy nutritional strategies relies on a pleasant sensory perception in the mouth able to persuade consumers to adopt polyphenol-rich diets and on a deeper understanding on the chemical modifications, that affect not only their chemical properties but also their physical interaction with epithelial lipids and in turn their permeability, location within the lipid bilayer, toxicity and biological activity, and fate during absorption at the gastro-intestinal epithelium, transport in circulation and delivery to the endothelium. In this paper, we review the current knowledge on the interactions between polyphenols and their metabolites with membrane lipids in artificial membranes and epithelial cell models (oral, stomach, gut and endothelium) and the findings from polyphenol-lipid interactions to physiological processes such as oral taste perception, gastrointestinal absorption and endothelial health. Finally, we discuss the limitations and challenges associated with the current experimental approaches in membrane and cell model studies and the potential of polyphenol-rich diets in the quest for personalized nutritional strategies ("personalized nutrition") to assist in the prevention, treatment, and management of non-communicable diseases in an increasingly aged population.

Do ABC transporters regulate plasma membrane organization?

The plasma membrane (PM) spatiotemporal organization is one of the major factors controlling cell signaling and whole-cell homeostasis. The PM lipids, including cholesterol, determine the physicochemical properties of the membrane bilayer and thus play a crucial role in all membrane-dependent cellular processes. It is known that lipid content and distribution in the PM are not random, and their transversal and lateral organization is highly controlled. Mainly sphingolipid- and cholesterol-rich lipid nanodomains, historically referred to as rafts, are extremely dynamic “hot spots” of the PM controlling the function of many cell surface proteins and receptors. In the first part of this review, we will focus on the recent advances of PM investigation and the current PM concept. In the second part, we will discuss the importance of several classes of ABC transporters whose substrates are lipids for the PM organization and dynamics. Finally, we will briefly present the significance of lipid ABC transporters for immune responses.

Impact of Plasma Membrane Domains on IgG Fc Receptor Function

Lipid cell membranes not only represent the physical boundaries of cells. They also actively participate in many cellular processes. This contribution is facilitated by highly complex mixtures of different lipids and incorporation of various membrane proteins. One group of membrane-associated receptors are Fc receptors (FcRs). These cell-surface receptors are crucial for the activity of most immune cells as they bind immunoglobulins such as immunoglobulin G (IgG). Based on distinct mechanisms of IgG binding, two classes of Fc receptors are now recognized: the canonical type I FcγRs and select C-type lectin receptors newly referred to as type II FcRs. Upon IgG immune complex induced cross-linking, these receptors are known to induce a multitude of cellular effector responses in a cell-type dependent manner, including internalization, antigen processing, and presentation as well as production of cytokines. The response is also determined by specific intracellular signaling domains, allowing FcRs to either positively or negatively modulate immune cell activity. Expression of cell-type specific combinations and numbers of receptors therefore ultimately sets a threshold for induction of effector responses. Mechanistically, receptor cross-linking and localization to lipid rafts, i.e., organized membrane microdomains enriched in intracellular signaling proteins, were proposed as major determinants of initial FcR activation. Given that immune cell membranes might also vary in their lipid compositions, it is reasonable to speculate, that the cell membrane and especially lipid rafts serve as an additional regulator of FcR activity. In this article, we aim to summarize the current knowledge on the interplay of lipid rafts and IgG binding FcRs with a focus on the plasma membrane composition and receptor localization in immune cells, the proposed mechanisms underlying this localization and consequences for FcR function with respect to their immunoregulatory capacity.

Plasma membranes are asymmetric in lipid unsaturation, packing and protein shape

A fundamental feature of cellular plasma membranes (PMs) is an asymmetric lipid distribution between the bilayer leaflets. However, neither the detailed, comprehensive compositions of individual PM leaflets nor how these contribute to structural membrane asymmetries have been defined. We report the distinct lipidomes and biophysical properties of both monolayers in living mammalian PMs. Phospholipid unsaturation is dramatically asymmetric, with the cytoplasmic leaflet being approximately twofold more unsaturated than the exoplasmic leaflet. Atomistic simulations and spectroscopy of leaflet-selective fluorescent probes reveal that the outer PM leaflet is more packed and less diffusive than the inner leaflet, with this biophysical asymmetry maintained in the endocytic system. The structural asymmetry of the PM is reflected in the asymmetric structures of protein transmembrane domains. These structural asymmetries are conserved throughout Eukaryota, suggesting fundamental cellular design principles.

A direct fluorometric activity assay for lipid kinases and phosphatases

Lipid kinases and phosphatases play key roles in cell signaling and regulation, are implicated in many human diseases, and are thus attractive targets for drug development. Currently, no direct in vitro activity assay is available for these important enzymes, which hampers mechanistic studies as well as high-throughput screening of small molecule modulators. Here, we report a highly sensitive and quantitative assay employing a ratiometric fluorescence sensor that directly and specifically monitors the real-time concentration change of a single lipid species. Because of its modular design, the assay system can be applied to a wide variety of lipid kinases and phosphatases, including class I phosphoinositide 3-kinase (PI3K) and phosphatase and tensin homolog (PTEN). When applied to PI3K, the assay provided detailed mechanistic information about the product inhibition and substrate acyl-chain selectivity of PI3K and enabled rapid evaluation of small molecule inhibitors. We also used this assay to quantitatively determine the substrate specificity of PTEN, providing new insight into its physiological function. In summary, we have developed a fluorescence-based real-time assay for PI3K and PTEN that we anticipate could be adapted to measure the activities of other lipid kinases and phosphatases with high sensitivity and accuracy.

Astrocytes in rapid ketamine antidepressant action

Ketamine, a general anaesthetic and psychotomimetic drug, exerts rapid, potent and long-lasting antidepressant effect, albeit the cellular and molecular mechanisms of this action are yet to be discovered. Besides targeting neuronal NMDARs fundamental for synaptic transmission, ketamine affects the function of astroglia the key homeostatic cells of the central nervous system that contribute to pathophysiology of psychiatric diseases including depression. Here we review studies revealing that (sub)anaesthetic doses of ketamine elevate intracellular cAMP concentration ([cAMP]_i) in astrocytes, attenuate stimulus-evoked astrocyte calcium signalling, which regulates exocytotic secretion of gliosignalling molecules, and stabilize the vesicle fusion pore in a narrow configuration possibly hindering cargo discharge or vesicle recycling. Next we discuss how ketamine affects astroglial capacity to control extracellular K⁺ by reducing cytoplasmic mobility of vesicles delivering the inward rectifying potassium channel (Kir4.1) to the plasmalemma. Modified astroglial K⁺ buffering impacts upon neuronal excitability as demonstrated in the lateral habenula rat model of depression. Finally, we highlight the recent discovery that ketamine rapidly redistributes cholesterol in the plasmalemma of astrocytes, but not in fibroblasts nor in neuronal cells. This alteration of membrane structure may modulate a host of processes that synergistically contribute to ketamine's rapid and prominent antidepressant action.

Classifying Cell Types with DNA-Encoded Ligand-Receptor Interactions on the Cell Membrane

Clustering, endocytosis, and intracellular transport of molecules on the cell membrane are critically dependent on the type of cells. However, the membrane-associated redistribution of molecules has not been exploited to realize cell classification for diagnostic purposes. Here, we develop a set of DNA-encoded artificial receptors and ligands to monitor the cell membrane redistribution. In this system, a cholesterol-modified single-stranded DNA strand serves as the receptor localized on the membrane, and a tetrahedral DNA framework (TDF) nanostructure with a complementary overhang serves as the ligand. The DNA-encoded receptor-ligand interaction is highly orthogonal, mimicking the dynamics of natural receptors and ligands on cells. We demonstrate that the dynamics of membrane redistribution can be resolved by the dual-color fluorescent patterns of the receptor-ligand interactions in a single image, which can be exploited to classify cell lines with high fidelity. This DNA-encoded method thus holds great promise for cell typing and diagnosis.

Plasma membranes are asymmetric in lipid unsaturation, packing and protein shape

A fundamental feature of cellular plasma membranes (PM) is asymmetric lipid distribution between the bilayer leaflets. However, neither the detailed, comprehensive compositions of individual PM leaflets, nor how these contribute to structural membrane asymmetries have been defined. We report the distinct lipidomes and biophysical properties of both monolayers in living mammalian PMs. Phospholipid unsaturation is dramatically asymmetric, with the cytoplasmic leaflet being ~2-fold more unsaturated than the exoplasmic. Atomistic simulations and spectroscopy of leaflet-selective fluorescent probes reveal that the outer PM leaflet is more packed and less diffusive than the inner leaflet, with this biophysical asymmetry maintained in the endocytic system. The structural asymmetry of the PM is reflected in asymmetric structures of protein transmembrane domains (TMD). These structural asymmetries are conserved throughout Eukaryota, suggesting fundamental cellular design principles.

The ins and outs of lipid rafts: functions in intracellular cholesterol homeostasis, microparticles, and cell membranes: Thematic Review Series: Biology of Lipid Rafts

Cellular membranes are not homogenous mixtures of proteins; rather, they are segregated into microdomains on the basis of preferential association between specific lipids and proteins. These microdomains, called lipid rafts, are well known for their role in receptor signaling on the plasma membrane (PM) and are essential to such cellular functions as signal transduction and spatial organization of the PM. A number of disease states, including atherosclerosis and other cardiovascular disorders, may be caused by dysfunctional maintenance of lipid rafts. Lipid rafts do not occur only in the PM but also have been found in intracellular membranes and extracellular vesicles (EVs). Here, we focus on discussing newly discovered functions of lipid rafts and microdomains in intracellular membranes, including lipid and protein trafficking from the ER, Golgi bodies, and endosomes to the PM, and we examine lipid raft involvement in the production and composition of EVs. Because lipid rafts are small and transient, visualization remains challenging. Future work with advanced techniques will continue to expand our knowledge about the roles of lipid rafts in cellular functioning.

Comparative lipidomics of 5-Fluorouracil-sensitive and -resistant colorectal cancer cells reveals altered sphingomyelin and ceramide controlled by acid sphingomyelinase (SMPD1)

5-Fluorouracil (5-FU) is a chemotherapeutic drug widely used to treat colorectal cancer. 5-FU is known to gradually lose its efficacy in treating colorectal cancer following the acquisition of resistance. We investigated the mechanism of 5-FU resistance using comprehensive lipidomic approaches. We performed lipidomic analysis on 5-FU–resistant (DLD-1/5-FU) and -sensitive (DLD-1) colorectal cancer cells using MALDI-MS and LC-MRM-MS. In particular, sphingomyelin (SM) species were significantly up-regulated in 5-FU–resistant cells in MALDI-TOF analysis. Further, we quantified sphingolipids including SM and Ceramide (Cer) using Multiple Reaction Monitoring (MRM), as they play a vital role in drug resistance. We found that 5-FU resistance in DLD-1/5-FU colorectal cancer cells was mainly associated with SM increase and Cer decrease, which are controlled by acid sphingomyelinase (SMPD1). In addition, reduction of SMPD1 expression was confirmed by LC-MRM-MS analysis and the effect of SMPD1 in drug resistance was assessed by treating DLD-1 cells with siRNA-SMPD1. Furthermore, clinical colorectal cancer data set analysis showed that down-regulation of SMPD1 was associated with resistance to chemotherapy regimens that include 5-FU. Thus, from our study, we propose that SM/Cer and SMPD1 are new potential target molecules for therapeutic strategies to overcome 5-FU resistance.

Cholesterol asymmetry at the tip of filopodia during cell adhesion

During adhesion, cells develop filopodia to facilitate the attachment to the extracellular matrix. The small guanosine triphosphate (GTP)-binding protein, Cdc42, plays a central role in the formation of filopodia. It has been reported that Cdc42 activity is regulated by cholesterol (Chol). We examined Chol distribution in filopodia using Chol-binding domain 4 (D4) fragment of bacterial toxin, perfringolysin O that senses high membrane concentration of Chol. Our results indicate that fluorescent D4 was enriched at the tip of the outer leaflet of filopodia in the initiation phase of cell adhesion. This enrichment was accompanied by a defect of D4 labeling in the inner leaflet. Steady phase adhered cell experiment indicated that both Cdc42 and ATP-binding cassette transporter, ABCA1, were involved in the binding of D4 to the cell surface. Depletion of Chol activated Cdc42. Our results suggest that asymmetric distribution of Chol at the tip of filopodia induces activation of Cdc42, and thus, facilitates filopodia formation.

The LDL-Receptor and its Molecular Properties: From Theory to Novel Biochemical and Pharmacological Approaches in Reducing LDL-cholesterol

BACKGROUND

The Low-Density Lipoprotein (LDL) Receptor (LDL-R) is a transmembrane protein playing a crucial role in effective lipid homeostasis. Various therapeutic agents have been used in the management of dyslipidemias, however, the outcome of therapeutic target is debated.

OBJECTIVE

The aim of this review is to summarize and fully understand the current concept regarding LDL-R and its molecular properties, metabolic pathway, factors affecting LDL-R activity and all available pharmacological interventions. Additionally, non-lipid related properties of LDL-R are also referred.

METHODS

Literature from the PubMed database was extracted to identify papers between 1984 to 2017 regarding LDL-R and therapeutic agents on dyslipidemia management.

RESULTS

We analyzed basic data regarding agents associated with LDL-R (Sterol Regulating Element-Binding Proteins - SREBPs, Protein ARH, IDOL, Thyroid Hormones, Haematologic Disorders, Protein convertase subtilisin kexintype 9 - PCSK-9, ApoC-III) as well as non-lipid related properties of LDL-R, while all relevant (common and novel) pharmacological interventions (statins, fibrates, cholesterol absorption inhibitors, bile acid sequestrants and PCSK- 9) are also referred.

CONCLUSION

LDL-R and its molecular properties are involved in lipid homeostasis, so potentially sets the therapeutic goals in cardiovascular patients, which is usually debated. Further research is needed in order to fully understand its properties, as well as to find the potential pharmacological interventions that could be beneficial in cholesterol homeostasis and various morbidities in order to reach the most appropriate therapeutic goal.

AdipoRon Attenuates Wnt Signaling by Reducing Cholesterol-Dependent Plasma Membrane Rigidity

The increasing prevalence of adult and adolescent obesity and its associated risk of colorectal cancer reinforces the urgent need to elucidate the underlying mechanisms contributing to the promotion of colon cancer in obese individuals. Adiponectin is an adipose tissue-derived adipokine, whose levels are reduced during obesity. Both epidemiological and preclinical data indicate that adiponectin suppresses colon tumorigenesis. We have previously demonstrated that both adiponectin and AdipoRon, a small-molecule adiponectin receptor agonist, suppress colon cancer risk in part by reducing the number of Lgr5+ stem cells in mouse colonic organoids. However, the mechanism by which the adiponectin signaling pathway attenuates colon cancer risk remains to be addressed. Here, we have hypothesized that adiponectin signaling supports colonic stem cell maintenance through modulation of the biophysical properties of the plasma membrane (PM). Specifically, we investigated the effects of adiponectin receptor activation by AdipoRon on the biophysical perturbations linked to the attenuation of Wnt-driven signaling and cell proliferation as determined by LEF luciferase reporter assay and colonic organoid proliferation, respectively. Using physicochemical sensitive dyes, Di-4-ANEPPDHQ and C-laurdan, we demonstrated that AdipoRon decreased the rigidity of the colonic cell PM. The decrease in membrane rigidity was associated with a reduction in PM free cholesterol levels and the intracellular accumulation of free cholesterol in lysosomes. These results suggest that adiponectin signaling plays a role in modulating cellular cholesterol homeostasis, PM biophysical properties, and Wnt-driven signaling. These findings are noteworthy because they may in part explain how obesity drives colon cancer progression.

27-Hydroxycholesterol Impairs Plasma Membrane Lipid Raft Signaling as Evidenced by Inhibition of IL6-JAK-STAT3 Signaling in Prostate Cancer Cells

We recently reported that restoring the CYP27A1-27hydroxycholesterol axis had antitumor properties. Thus, we sought to determine the mechanism by which 27HC exerts its anti-prostate cancer effects. As cholesterol is a major component of membrane microdomains known as lipid rafts, which localize receptors and facilitate cellular signaling, we hypothesized 27HC would impair lipid rafts, using the IL6-JAK-STAT3 axis as a model given its prominent role in prostate cancer. As revealed by single molecule imaging of DU145 prostate cancer cells, 27HC treatment significantly reduced detected cholesterol density on the plasma membranes. Further, 27HC treatment of constitutively active STAT3 DU145 prostate cancer cells reduced STAT3 activation and slowed tumor growth in vitro and in vivo. 27HC also blocked IL6-mediated STAT3 phosphorylation in nonconstitutively active STAT3 cells. Mechanistically, 27HC reduced STAT3 homodimerization, nuclear translocation, and decreased STAT3 DNA occupancy at target gene promoters. Combined treatment with 27HC and STAT3 targeting molecules had additive and synergistic effects on proliferation and migration, respectively. Hallmark IL6-JAK-STAT gene signatures positively correlated with CYP27A1 gene expression in a large set of human metastatic castrate-resistant prostate cancers and in an aggressive prostate cancer subtype. This suggests STAT3 activation may be a resistance mechanism for aggressive prostate cancers that retain CYP27A1 expression. In summary, our study establishes a key mechanism by which 27HC inhibits prostate cancer by disrupting lipid rafts and blocking STAT3 activation. IMPLICATIONS: Collectively, these data show that modulation of intracellular cholesterol by 27HC can inhibit IL6-JAK-STAT signaling and may synergize with STAT3-targeted compounds.

More than one way to bind to cholesterol: atypical variants of membrane-binding domain of perfringolysin O selected by ribosome display

Herein, we report a high-throughput approach for the selection of peripheral protein domains that bind specifically to cholesterol in lipid membranes. We discovered variants of perfringolysin O, with non-conserved amino acid substitutions at regions crucial for cholesterol recognition, demonstrating an unprecedented amino acid sequence variability with binding ability for cholesterol. The developed approach provides an effective platform for a comprehensive study of protein lipid interactions.

By using developed ribosomal display, we discovered variants of perfringolysin O, a pore forming toxin from bacteria Clostridium perfringens, with non-conserved amino acid substitutions at regions crucial for cholesterol recognition.

Modulation of Transient Receptor Potential C Channel Activity by Cholesterol

Changes of cholesterol level in the plasma membrane of cells have been shown to modulate ion channel function. The proposed mechanisms underlying these modulations include association of cholesterol to a single binding site at a single channel conformation, association to a highly flexible cholesterol binding site adopting multiple poses, and perturbation of lipid rafts. These perturbations have been shown to induce reversible targeting of mammalian transient receptor potential C (TRPC) channels to the cholesterol-rich membrane environment of lipid rafts. Thus, the observed inhibition of TRPC channels by methyl-β-cyclodextrin (MβCD), which induces cholesterol efflux from the plasma membrane, may result from disruption of lipid rafts. This perturbation was also shown to disrupt multimolecular signaling complexes containing TRPC channels. The Drosophila TRP and TRP-like (TRPL) channels belong to the TRPC channel subfamily. When the Drosophila TRPL channel was expressed in S2 or HEK293 cells and perfused with MβCD, the TRPL current was abolished in less than 100 s, fitting well the fast kinetic phase of cholesterol sequestration experiments in cells. It was thus suggested that the fast kinetics of TRPL channel suppression by MβCD arise from disruption of lipid rafts. Accordingly, lipid raft perturbation by cholesterol sequestration could give clues to the function of lipid environment in TRPC channel activity and its mechanism.

C-terminal of ABCA1 separately regulates cholesterol floppase activity and cholesterol efflux activity

ATP-Binding Cassette A1 (ABCA1) is a key lipid transporter for cholesterol homeostasis. We recently reported that ABCA1 not only exports excess cholesterol in an apoA-I dependent manner, but that it also flops cholesterol from the inner to the outer leaflet of the plasma membrane. However, the relationship between these two activities of ABCA1 is still unclear. In this study, we analyzed the subcellular localization of ABCA1 by using a newly generated monoclonal antibody against its extracellular domain and the functions of eleven chimera proteins, in which the C-terminal domain of ABCA1 was replaced with those of the other ABCA subfamily members. We identified two motifs important for the functions of ABCA1. Three periodically repeated leucine residues were necessary for the cholesterol floppase activity but not the cholesterol efflux activity, while a VFVNFA motif was essential for both activities of ABCA1. These results suggest that the C-terminal of ABCA1 separately regulates the cholesterol floppase activity and the cholesterol efflux activity.

Organelle contacts: Sub-organelle zones to facilitate rapid and accurate inter-organelle trafficking of lipids

When one person wants to communicate securely with another, he/she should contact the other person directly. This rule applies not only to human society, but also to the intracellular micro-society. In the past two decades, it has become increasingly clear that the sub-organelle regions called membrane contact sites (MCSs) are pivotal for inter-organelle transport of lipids in cells, as highlighted in the thematic review series "Interorganelle trafficking of lipids" held in Traffic in 2014-2015. In this commentary, we will describe how the currently prevailing model for lipid trafficking at MCSs was generated, and comment on three important issues that have not been explored: (a1) the principles guiding the generation of an asymmetrical inter-organelle flow of lipids in cells, (b2) the advantages in lipid trafficking at organelle contacts, and (c3) the dynamic network of inter-organelle lipid trafficking.

Tetraspanin CD82 interaction with cholesterol promotes extracellular vesicle-mediated release of ezrin to inhibit tumour cell movement

Tumour metastasis suppressor KAI1/CD82 inhibits tumour cell movement. As a transmembrane protein, tetraspanin CD82 bridges the interactions between membrane microdomains of lipid rafts and tetraspanin-enriched microdomains (TEMs). In this study, we found that CD82 and other tetraspanins contain cholesterol recognition/interaction amino-acid consensus (CRAC) sequences in their transmembrane domains and revealed that cholesterol binding of CD82 determines its interaction with lipid rafts but not with TEMs. Functionally, CD82 needs cholesterol binding to inhibit solitary migration, collective migration, invasion and infiltrative outgrowth of tumour cells. Importantly, CD82–cholesterol/–lipid raft interaction not only promotes extracellular release of lipid raft components such as cholesterol and gangliosides but also facilitates extracellular vesicle (EV)–mediated release of ezrin–radixin–moesin (ERM) protein Ezrin. Since ERM proteins link actin cytoskeleton to the plasma membrane, we show for the first time that cell movement can be regulated by EV-mediated releases, which disengage the plasma membrane from cytoskeleton and then impair cell movement. Our findings also conceptualize that interactions between membrane domains, in this case converge of lipid rafts and TEMs by CD82, can change cell movement. Moreover, CD82 coalescences with both lipid rafts and TEMs are essential for its inhibition of tumour cell movement and for its enhancement of EV release. Finally, our study underpins that tetraspanins as a superfamily of functionally versatile molecules are cholesterol-binding proteins.

Abbreviations:Ab: antibody; CBM: cholesterol-binding motif; CCM: cholesterol consensus motif; CRAC/CARC: cholesterol recognition or interaction amino-acid consensus; CTxB: cholera toxin B subunit; ECM: extracellular matrix; ERM: ezrin, radixin and moesin; EV: extracellular vesicles; FBS: foetal bovine serum; mAb: monoclonal antibody; MST: microscale thermophoresis; pAb: polyclonal antibody; and TEM: tetraspanin-enriched microdomain

Membrane Structure-Function Insights from Asymmetric Lipid Vesicles

The lipid bilayer, together with embedded proteins, is the central structure in biomembranes. While artificial lipid bilayers are useful to model natural membranes, they are generally symmetric, with the same membrane lipid composition in each lipid monolayer (leaflet). In contrast, natural membranes are often asymmetric, with different lipids in each leaflet. To prepare asymmetric lipid vesicles, we developed cyclodextrin-catalyzed phospholipid exchange procedures. The basic method is that an excess of vesicles with one set of lipids (the donor vesicles) is mixed with a second set of vesicles (acceptor vesicles) with a different set of lipids. Cyclodextrin is introduced into the external aqueous solution, so that lipids in the outer leaflet of the vesicles bind to it and are shuttled between the vesicles. At equilibrium, the lipids in the outer leaflet of the acceptor vesicles are replaced by those from the donor vesicles. The exchanged acceptor vesicles are then isolated. Asymmetric vesicles are versatile in terms of vesicle sizes and lipid compositions that can be prepared. Measuring asymmetry is often difficult. A variety of assays can be used to measure the extent of asymmetry, but most are specific for one particular membrane lipid type or class, and there are none that can be used in all situations. Studies using asymmetric vesicles have begun to explore how asymmetry influences lipid movement across the bilayer, the formation of ordered lipid domains, coupling between the physical properties in each leaflet, and membrane protein conformation. Lipid domain formation stands out as one of the most important properties in which asymmetry is likely to be crucial. Lipid bilayers can exist in both liquidlike and solid/ordered-like states depending on lipid structure, and in lipid vesicles with a mixture of lipids highly ordered and disordered domains can coexist. However, until very recently, such studies only had been carried out in symmetric artificial membranes. Whether ordered domains (often called lipid rafts) and disordered lipid domains coexist in asymmetric cell membranes remains controversial partly because lipids favoring the formation of an ordered state are largely restricted to the leaflet facing the external environment. Studies using asymmetric vesicles have recently shown that each leaflet can influence the physical behavior of the other, i.e., that the domain forming properties in each leaflet tend to be coupled, with consequences highly dependent upon the details of lipid structure. Future studies investigating the dependence of coupling and properties upon the details of lipid composition should clarify the potential of natural membranes to form lipid domains. In addition, we recently extended the exchange method to living mammalian cells, using exchange to efficiently replace virtually the entire phospholipid and sphingolipid population of the plasma membrane outer leaflet with exogenous lipids without harming cells. This should allow detailed studies of the functional impact of lipid structure, asymmetry, domain organization, and interactions with membrane proteins in living cells.

Astrocyte Specific Remodeling of Plasmalemmal Cholesterol Composition by Ketamine Indicates a New Mechanism of Antidepressant Action

Ketamine is an antidepressant with rapid therapeutic onset and long-lasting effect, although the underlying mechanism(s) remain unknown. Using FRET-based nanosensors we found that ketamine increases [cAMP]_i in astrocytes. Membrane capacitance recordings, however, reveal fundamentally distinct mechanisms of effects of ketamine and [cAMP]_i on vesicular secretion: a rise in [cAMP]_i facilitated, whereas ketamine inhibited exocytosis. By directly monitoring cholesterol-rich membrane domains with a fluorescently tagged cholesterol-specific membrane binding domain (D4) of toxin perfringolysin O, we demonstrated that ketamine induced cholesterol redistribution in the plasmalemma in astrocytes, but neither in fibroblasts nor in PC 12 cells. This novel mechanism posits that ketamine affects density and distribution of cholesterol in the astrocytic plasmalemma, consequently modulating a host of processes that may contribute to ketamine's rapid antidepressant action.

Reciprocal modulation between amyloid precursor protein and synaptic membrane cholesterol revealed by live cell imaging

The amyloid precursor protein (APP) has been extensively studied because of its association with Alzheimer's disease (AD). However, APP distribution across different subcellular membrane compartments and its function in neurons remains unclear. We generated an APP fusion protein with a pH-sensitive green fluorescent protein at its ectodomain and a pH-insensitive blue fluorescent protein at its cytosolic domain and used it to measure APP's distribution, subcellular trafficking, and cleavage in live neurons. This reporter, closely resembling endogenous APP, revealed only a limited correlation between synaptic activities and APP trafficking. However, the synaptic surface fraction of APP was increased by a reduction in membrane cholesterol levels, a phenomenon that involves APP's cholesterol-binding motif. Mutations at or near binding sites not only reduced both the surface fraction of APP and membrane cholesterol levels in a dominant negative manner, but also increased synaptic vulnerability to moderate membrane cholesterol reduction. Our results reveal reciprocal modulation of APP and membrane cholesterol levels at synaptic boutons.

Cholesterol-Dependent Bending Energy Is Important in Cholesterol Distribution of the Plasma Membrane

We consider the plasma membrane that contains a cholesterol molar fraction of 0.4 and ask how that cholesterol is distributed between the two leaves. Because of the rapid flip-flop of cholesterol between leaves, we assume that its distribution is determined by the equality of its chemical potentials in the two leaves. When we consider only the contributions of entropy and interactions to the cholesterol chemical potential in our model system, we find, not surprisingly, that the cholesterol is mostly in the outer leaf because of the strong attraction between cholesterol and sphingomyelin (SM), which is predominantly in that leaf. We find 72% there. We then include the contribution from the bending energy in each leaf that must be overcome to join the leaves in a flat bilayer. The product of bending modulus and spontaneous curvature is obtained from simulation. We find that the addition of cholesterol to the outer leaf reduces the spontaneous curvature, which is initially positive, until it passes through zero when the molar fraction of cholesterol in the outer leaf is 0.28. Additional cholesterol is driven toward the inner leaf by the sphingomyelin phosphatidylcholine mixture. This is resisted by the bending energy contribution to the inner leaf. We find, again by simulation, that the addition of cholesterol monotonically increases the magnitude of the spontaneous curvature of the inner leaf, which is negative. This increases its bending energy. We conclude that, as a result of these competing effects, the percentage of cholesterol in the outer leaf is reduced to ∼63 ± 6%.

Intrinsic Curvature-Mediated Transbilayer Coupling in Asymmetric Lipid Vesicles

We measured the effect of intrinsic lipid curvature, J0, on structural properties of asymmetric vesicles made of palmitoyl-oleoyl-phosphatidylethanolamine (POPE; J0<0) and palmitoyl-oleoyl-phosphatidylcholine (POPC; J0∼0). Electron microscopy and dynamic light scattering were used to determine vesicle size and morphology, and x-ray and neutron scattering, combined with calorimetric experiments and solution NMR, yielded insights into leaflet-specific lipid packing and melting processes. Below the lipid melting temperature we observed strong interleaflet coupling in asymmetric vesicles with POPE inner bilayer leaflets and outer leaflets enriched in POPC. This lipid arrangement manifested itself by lipids melting cooperatively in both leaflets, and a rearrangement of lipid packing in both monolayers. On the other hand, no coupling was observed in vesicles with POPC inner bilayer leaflets and outer leaflets enriched in POPE. In this case, the leaflets melted independently and did not affect each other’s acyl chain packing. Furthermore, we found no evidence for transbilayer structural coupling above the melting temperature of either sample preparation. Our results are consistent with the energetically preferred location of POPE residing in the inner leaflet, where it also resides in natural membranes, most likely causing the coupling of both leaflets. The loss of this coupling in the fluid bilayers is most likely the result of entropic contributions.

Multiscale Simulations of Biological Membranes: The Challenge To Understand Biological Phenomena in a Living Substance

Biological membranes are tricky to investigate. They are complex in terms of molecular composition and structure, functional over a wide range of time scales, and characterized by nonequilibrium conditions. Because of all of these features, simulations are a great technique to study biomembrane behavior. A significant part of the functional processes in biological membranes takes place at the molecular level; thus computer simulations are the method of choice to explore how their properties emerge from specific molecular features and how the interplay among the numerous molecules gives rise to function over spatial and time scales larger than the molecular ones. In this review, we focus on this broad theme. We discuss the current state-of-the-art of biomembrane simulations that, until now, have largely focused on a rather narrow picture of the complexity of the membranes. Given this, we also discuss the challenges that we should unravel in the foreseeable future. Numerous features such as the actin-cytoskeleton network, the glycocalyx network, and nonequilibrium transport under ATP-driven conditions have so far received very little attention; however, the potential of simulations to solve them would be exceptionally high. A major milestone for this research would be that one day we could say that computer simulations genuinely research biological membranes, not just lipid bilayers.

The Phase Behavior and Organization of Sphingomyelin/Cholesterol Membranes: A Deuterium NMR Study

We have studied the dependence of the phase and domain characteristics of sphingomyelin (SM)/cholesterol model membranes on sterol content and temperature using deuterium nuclear magnetic resonance. NMR spectra of N-palmitoyl(D31)-D-erythro-sphingosylphosphorylcholine (PSM-d31) were taken for temperatures from 25 to 70°C and cholesterol concentrations of 0-40%. Analogous experiments were performed using 1-palmitoyl,2-palmitoyl(D31)-sn-glycero-3-phosphocholine (DPPC-d31)/cholesterol membranes to carefully compare the data obtained using palmitoyl chains that have similar "kinked" conformations. The constructed phase diagrams exhibit both solid-ordered (so) + liquid-ordered (lo) and liquid-disordered (ld) + lo phase-coexistence regions with a clear three-phase line. Macroscopic (micron-sized) coexistence of ld and lo phases was not observed; instead, line-broadening in the ld+lo region was characterized by intermediate exchange of lipids between the two types of domains. The length scales associated with the domains were estimated to be 75-150 nm for PSM-d31/cholesterol and DPPC-d31/cholesterol model membranes.

Functional link between plasma membrane spatiotemporal dynamics, cancer biology, and dietary membrane-altering agents

The cell plasma membrane serves as a nexus integrating extra- and intracellular components, which together enable many of the fundamental cellular signaling processes that sustain life. In order to perform this key function, plasma membrane components assemble into well-defined domains exhibiting distinct biochemical and biophysical properties that modulate various signaling events. Dysregulation of these highly dynamic membrane domains can promote oncogenic signaling. Recently, it has been demonstrated that select membrane-targeted dietary bioactives (MTDBs) have the ability to remodel plasma membrane domains and subsequently reduce cancer risk. In this review, we focus on the importance of plasma membrane domain structural and signaling functionalities as well as how loss of membrane homeostasis can drive aberrant signaling. Additionally, we discuss the intricacies associated with the investigation of these membrane domain features and their associations with cancer biology. Lastly, we describe the current literature focusing on MTDBs, including mechanisms of chemoprevention and therapeutics in order to establish a functional link between these membrane-altering biomolecules, tuning of plasma membrane hierarchal organization, and their implications in cancer prevention.

Genetically encoded lipid biosensors

Lipids convey both structural and functional properties to eukaryotic membranes. Understanding the basic lipid composition and the dynamics of these important molecules, in the context of cellular membranes, can shed light on signaling, metabolism, trafficking, and even membrane identity. The development of genetically encoded lipid biosensors has allowed for the visualization of specific lipids inside individual, living cells. However, a number of caveats and considerations have emerged with the overexpression of these biosensors. In this Technical Perspective, we provide a current list of available genetically encoded lipid biosensors, together with criteria that determine their veracity. We also provide some suggestions for the optimal utilization of these biosensors when both designing experiments and interpreting results.

Effect of sterol structure on ordered membrane domain (raft) stability in symmetric and asymmetric vesicles

Sterol structure influences liquid ordered domains in membranes, and the dependence of biological functions on sterol structure can help identify processes dependent on ordered domains. In this study we compared the effect of sterol structure on ordered domain formation in symmetric vesicles composed of mixtures of sphingomyelin, 1, 2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and cholesterol, and in asymmetric vesicles in which sphingomyelin was introduced into the outer leaflet of vesicles composed of DOPC and cholesterol. In most cases, sterol behavior was similar in symmetric and asymmetric vesicles, with ordered domains most strongly stabilized by 7-dehydrocholesterol (7DHC) and cholesterol, stabilized to a moderate degree by lanosterol, epicholesterol and desmosterol, and very little if at all by 4-cholesten-3-one. However, in asymmetric vesicles desmosterol stabilized ordered domain almost as well as cholesterol, and to a much greater degree than epicholesterol, so that the ability to support ordered domains decreased in the order 7-DHC > cholesterol > desmosterol > lanosterol > epicholesterol > 4-cholesten-3-one. This contrasts with values for intermediate stabilizing sterols in symmetric vesicles in which the ranking was cholesterol > lanosterol ~ desmosterol ~ epicholesterol or prior studies in which the ranking was cholesterol ~ epicholesterol > lanosterol ~ desmosterol. The reasons for these differences are discussed. Based on these results, we re-evaluated our prior studies in cells and conclude that endocytosis levels and bacterial uptake are even more closely correlated with the ability of sterols to form ordered domains than previously thought, and do not necessarily require that a sterol have a 3β-OH group.

Changes in the asymmetric distribution of cholesterol in the plasma membrane influence streptolysin O pore formation

ATP-binding cassette A1 (ABCA1) plays a key role in generating high-density lipoprotein (HDL) and preventing atherosclerosis. ABCA1 exports cholesterol and phospholipid to apolipoprotein A-I (apoA-I) in serum to generate HDL. We found that streptolysin O (SLO), a cholesterol-dependent pore-forming toxin, barely formed pores in ABCA1-expressing cells, even in the absence of apoA-I. Neither cholesterol content in cell membranes nor the amount of SLO bound to cells was affected by ABCA1. On the other hand, binding of the D4 domain of perfringolysin O (PFO) to ABCA1-expressing cells increased, suggesting that the amount of cholesterol in the outer leaflet of the plasma membrane (PM) increased and that the cholesterol dependences of these two toxins differ. Addition of cholesterol to the PM by the MβCD-cholesterol complex dramatically restored SLO pore formation in ABCA1-expressing cells. Therefore, exogenous expression of ABCA1 causes reduction in the cholesterol level in the inner leaflet, thereby suppressing SLO pore formation.

Cholesterol Interaction Sites on the Transmembrane Domain of the Hedgehog Signal Transducer and Class F G Protein-Coupled Receptor Smoothened

Transduction of Hedgehog signals across the plasma membrane is facilitated by the class F G-protein-coupled-receptor (GPCR) Smoothened (SMO). Recent studies suggest that SMO is modulated via interactions of its transmembrane (TM) domain with cholesterol. We apply molecular dynamics simulations of SMO embedded in cholesterol containing lipid bilayers, revealing a direct interaction of cholesterol with the TM domain at regions distinct from those observed in class A GPCRs. In particular the extracellular tips of helices TM2 and TM3 form a well-defined cholesterol interaction site. Potential of mean force calculations yield a free energy landscape for cholesterol binding. Alongside analysis of equilibrium cholesterol occupancy, this reveals the existence of a dynamic "greasy patch" interaction with the TM domain of SMO, which may be compared with previously identified lipid interaction sites on other membrane proteins. These predictions provide molecular-level insights into cholesterol interactions with a class F GPCR, suggesting potential druggable sites.

Cholesterol Flip-Flop in Heterogeneous Membranes

Cholesterol is the most abundant molecule in the plasma membrane of mammals. Its distribution across the two membrane leaflets is critical for understanding how cells work. Cholesterol trans-bilayer motion (flip-flop) is a key process influencing its distribution in membranes. Despite extensive investigations, the rate of cholesterol flip-flop and its dependence on the lateral heterogeneity of membranes remain uncertain. In this work, we used atomistic molecular dynamics simulations to sample spontaneous cholesterol flip-flop events in a DPPC:DOPC:cholesterol mixture with heterogeneous lateral distribution of lipids. In addition to an overall flip-flop rate at the time scale of sub-milliseconds, we identified a significant impact of local environment on flip-flop rate. We discuss the atomistic details of the flip-flop events observed in our simulations.

Cholesterol-Dependent Gating Effects on Ion Channels

Biomembranes separate a live cell from its environment and keep it in an off-equilibrium, steady state. They contain both phospholipids and nonphospholipids, depending on whether there are phosphate groups in the headgroup regions. Cholesterol (CHOL) is one type of nonphospholipids, and one of the most abundant lipid molecules in humans. Its content in plasma membranes and intracellular membranes varies and is tightly regulated. Voltage-gated ion channels are universally present in every cell and are fairly diversified in the eukaryotic domain of life. Our lipid-dependent gating hypothesis postulates that the controlled switch of the voltage-sensor domains (VSDs) in a voltage-gated potassium (Kv) channel between the "down" and the "up" state (gating) is sensitive to the ratio of phospholipids:nonphospholipids in the annular layer around the channel. High CHOL content is found to exert strong inhibitory effects on Kv channels. Such effects have been observed in in vitro membranes, cultured cells, and animal models for cholesterol metabolic defects. Thermodynamic analysis of the CHOL-dependent gating suggests that the inhibitory effects of CHOL result from collective interactions between annular CHOL molecules and the channel, which appear to be a more generic principle behind the CHOL effects on other ion channels and transporters. We will review the recent progress in the CHOL-dependent gating of voltage-gated ion channels, discuss the current technical limitations, and then expand briefly the learned principles to other ion channels that are known to be sensitive to the CHOL-channel interactions.

Rapid diffusion of cholesterol along polyunsaturated membranes via deep dives

Cholesterol regulates the function of membrane proteins either via direct or membrane-mediated effects. Therefore, its ready availability is crucial for many protein-governed cellular processes. Recent studies suggest that cholesterol can partition to the core of polyunsaturated membranes, where cholesterol binding sites of many membrane proteins are also located. This core region is characterized by a lower viscosity. Therefore, we hypothesized that cholesterol partitioning into the membrane interior increases the rate of its diffusion in polyunsaturated membrane environments. We studied the behavior of cholesterol in membranes with increasing level of lipid chain unsaturation using a combination of atomistic and coarse-grained molecular dynamics simulations. Our simulations suggest a strong correlation between entropy-driven enhanced cholesterol partitioning to the membrane core and its faster lateral diffusion, which indicates that the less viscous membrane core indeed provides an efficient means for cholesterol movement in polyunsaturated membrane environments.

Cholesterol Effects on the Physical Properties of Lipid Membranes Viewed by Solid-state NMR Spectroscopy

In this chapter, we review the physical properties of lipid/cholesterol mixtures involving studies of model membranes using solid-state NMR spectroscopy. The approach allows one to quantify the average membrane structure, fluctuations, and elastic deformation upon cholesterol interaction. Emphasis is placed on understanding the membrane structural deformation and emergent fluctuations at an atomistic level. Lineshape measurements using solid-state NMR spectroscopy give equilibrium structural properties, while relaxation time measurements study the molecular dynamics over a wide timescale range. The equilibrium properties of glycerophospholipids, sphingolipids, and their binary and tertiary mixtures with cholesterol are accessible. Nonideal mixing of cholesterol with other lipids explains the occurrence of liquid-ordered domains. The entropic loss upon addition of cholesterol to sphingolipids is less than for glycerophospholipids, and may drive formation of lipid rafts. The functional dependence of ²H NMR spin-lattice relaxation (R _1Z) rates on segmental order parameters (S _CD) for lipid membranes is indicative of emergent viscoelastic properties. Addition of cholesterol shows stiffening of the bilayer relative to the pure lipids and this effect is diminished for lanosterol. Opposite influences of cholesterol and detergents on collective dynamics and elasticity at an atomistic scale can potentially affect lipid raft formation in cellular membranes.

Intracellular and Plasma Membrane Cholesterol Labeling and Quantification Using Filipin and GFP-D4

Cholesterol, a major component of biological membranes, is rapidly trafficked and unevenly distributed between organelles. Anomalies of intracellular cholesterol distribution are the hallmark of a number of lysosomal lipid storage disorders. A major methodological obstacle for studying cholesterol trafficking is tracing this molecule in situ. The use of fluorescent probes that specifically bind cholesterol allows the visualization and imaging of cellular cholesterol. Here, we describe a series of assays optimized for quantifying free cholesterol in cell populations and at the single cell level, both at the plasma membrane and inside cells. These methods use two fluorescent probes: the D4 fragment of perfringolysin O fused to GFP (GFP-D4) and the polyene macrolide filipin. First, we report a robust method for quantifying plasma membrane cholesterol by flow cytometry using the GFP-D4 probe. Second, to optically distinguish and quantify intracellular cholesterol accumulation, we have adapted the classical filipin cholesterol staining protocol. Indeed, we observed that treatment of living cells with methyl-β-cyclodextrin, a chemical known to extract cholesterol from the plasma membrane, improves the visualization of the intracellular cholesterol pool with filipin. To complement these staining procedures, we developed an image analysis protocol based on image segmentation to quantify, in a robust manner, intracellular cholesterol stained with filipin. Thus, this chapter is a guideline for cellular cholesterol staining and signal quantification.

Plant lipids: Key players of plasma membrane organization and function

The plasma membrane (PM) is the biological membrane that separates the interior of all cells from the outside. The PM is constituted of a huge diversity of proteins and lipids. In this review, we will update the diversity of molecular species of lipids found in plant PM. We will further discuss how lipids govern global properties of the plant PM, explaining that plant lipids are unevenly distributed and are able to organize PM in domains. From that observation, it emerges a complex picture showing a spatial and multiscale segregation of PM components. Finally, we will discuss how lipids are key players in the function of PM in plants, with a particular focus on plant-microbe interaction, transport and hormone signaling, abiotic stress responses, plasmodesmata function. The last chapter is dedicated to the methods that the plant membrane biology community needs to develop to get a comprehensive understanding of membrane organization in plants.

Cell migration is negatively modulated by ABCA1

Temporal and spatial changes of membrane lipid distribution in the plasma membrane are thought to be important for various cellular functions. ATP-Binding Cassette A1 (ABCA1) is a key lipid transporter for the generation of high density lipoprotein. Recently, we reported that ABCA1 maintains an asymmetric distribution of cholesterol in the plasma membrane. Here we report that ABCA1 suppresses cell migration by modulating signal pathways. ABCA1 knockdown in mouse embryonic fibroblasts accelerated cell migration and increased activation of Rac1 and its localization to detergent-resistant membranes. Phosphorylation of MEK and ERK also increased. Inhibition of Rac1 or MEK-ERK signals suppressed cell migration in ABCA1 knockdown cells. Because our experimental conditions for cell migration did not contain cholesterol or lipid acceptors for ABCA1, cellular cholesterol content was not changed. These data suggest that ABCA1 modulates cell migration via Rac1 and MEK-ERK signaling by altering lipid distribution in the plasma membrane.

Structural Basis for Cholesterol Transport-like Activity of the Hedgehog Receptor Patched

Hedgehog protein signals mediate tissue patterning and maintenance by binding to and inactivating their common receptor Patched, a 12-transmembrane protein that otherwise would suppress the activity of the 7-transmembrane protein Smoothened. Loss of Patched function, the most common cause of basal cell carcinoma, permits unregulated activation of Smoothened and of the Hedgehog pathway. A cryo-EM structure of the Patched protein reveals striking transmembrane domain similarities to prokaryotic RND transporters. A central hydrophobic conduit with cholesterol-like contents courses through the extracellular domain and resembles that used by other RND proteins to transport substrates, suggesting Patched activity in cholesterol transport. Cholesterol activity in the inner leaflet of the plasma membrane is reduced by PTCH1 expression but rapidly restored by Hedgehog stimulation, suggesting that PTCH1 regulates Smoothened by controlling cholesterol availability.

Comment on 'Orthogonal lipid sensors identify transbilayer asymmetry of plasma membrane cholesterol'

The plasma membrane in mammalian cells is rich in cholesterol, but how the cholesterol is partitioned between the two leaflets of the plasma membrane remains a matter of debate. Recently, Liu et al. used domain 4 (D4) of perfringolysin O as a cholesterol sensor to argue that cholesterol is mostly in the exofacial leaflet (Liu et al., 2017). This conclusion was made by interpreting D4 binding in live cells using in vitro calibrations with liposomes. However, liposomes may be unfaithful in mimicking the plasma membrane, as we demonstrate here. Also, D4 binding is highly sensitive to the presence of cytosolic proteins. In addition, we find that a D4 variant, which requires >35 mol% cholesterol to bind to liposomes in vitro, does in fact bind to the cytoplasmic leaflet of the plasma membrane in a cholesterol-dependent manner. Thus, we believe, based on the current evidence, that it is unlikely that there is a significantly higher proportion of cholesterol in the exofacial leaflet of the plasma membrane compared to the cytosolic leaflet.

Divide and Rule: Plant Plasma Membrane Organization

Since the publication of the fluid mosaic as a relevant model for biological membranes, accumulating evidence has revealed the outstanding complexity of the composition and organization of the plant plasma membrane (PM). Powerful new methodologies have uncovered the remarkable multiscale and multicomponent heterogeneity of PM subcompartmentalization, and this is emerging as a general trait with different features and properties. It is now evident that the dynamics of such a complex organization are intrinsically related to signaling pathways that regulate key physiological processes. Listing and linking recent progress in precisely qualifying these heterogeneities will help to draw an integrated picture of the plant PM. Understanding the key principles governing such a complex dynamic organization will contribute to deciphering the crucial role of the PM in cell physiology.

Quantitative Lipid Imaging Reveals a New Signaling Function of Phosphatidylinositol-3,4-Bisphophate: Isoform- and Site-Specific Activation of Akt

Activation of class I phosphatidylinositol 3-kinase (PI3K) leads to formation of phosphatidylinositol-3,4,5-trisphophate (PIP₃) and phosphatidylinositol-3,4-bisphophate (PI34P₂), which spatiotemporally coordinate and regulate a myriad of cellular processes. By simultaneous quantitative imaging of PIP₃ and PI34P₂ in live cells, we here show that they have a distinctively different spatiotemporal distribution and history in response to growth factor stimulation, which allows them to selectively induce the membrane recruitment and activation of Akt isoforms. PI34P₂ selectively activates Akt2 at both the plasma membrane and early endosomes, whereas PIP₃ selectively stimulates Akt1 and Akt3 exclusively at the plasma membrane. These spatiotemporally distinct activation patterns of Akt isoforms provide a mechanism for their differential regulation of downstream signaling molecules. Collectively, our studies show that different spatiotemporal dynamics of PIP₃ and PI34P₂ and their ability to selectively activate key signaling proteins allow them to mediate class I PI3K signaling pathways in a spatiotemporally specific manner.

Cholesterol-dependent cytolysins: from water-soluble state to membrane pore

The cholesterol-dependent cytolysins (CDCs) are a family of bacterial toxins that are important virulence factors for a number of pathogenic Gram-positive bacterial species. CDCs are secreted as soluble, stable monomeric proteins that bind specifically to cholesterol-rich cell membranes, where they assemble into well-defined ring-shaped complexes of around 40 monomers. The complex then undergoes a concerted structural change, driving a large pore through the membrane, potentially lysing the target cell. Understanding the details of this process as the protein transitions from a discrete monomer to a complex, membrane-spanning protein machine is an ongoing challenge. While many of the details have been revealed, there are still questions that remain unanswered. In this review, we present an overview of some of the key features of the structure and function of the CDCs, including the structure of the secreted monomers, the process of interaction with target membranes, and the transition from bound monomers to complete pores. Future directions in CDC research and the potential of CDCs as research tools will also be discussed.

Structural snapshot of the cholesterol-transport ATP-binding cassette proteins 1

The ATP-binding cassette (ABC) proteins play critical roles in maintaining lipid and sterol homeostasis in higher eukaryotes. In humans, several subfamily-A and -G members function as cholesterol transporters across the cellular membranes. Deficiencies of these ABC proteins can cause dyslipidemia that is associated with health conditions such as atherosclerosis, diabetes, fatty liver disease, and neurodegeneration. The physiological roles of ABC cholesterol transporters have been implicated in mediating cholesterol efflux for reverse cholesterol transport and in maintaining membrane integrity for cell survival. The precise role of these ABC transporters in cells remains elusive, and little is known about the sterol-transport mechanism. The membrane constituents of ABC transporters have been postulated to play a key role in determining the transport substrates and the translocation mechanisms via the transmembrane domains. Recent breakthroughs in determining high-resolution structures of the human sterol transporter ABCG5/G8 and its functional homologs have shed light on new structural features of ABC transporters, providing a more relevant framework for mechanistic analysis of cholesterol-transport ABC proteins. This minireview outlines what is known about ABCG cholesterol transporters, addresses key structural features in the putative sterol translocation pathway on the transmembrane domains, and concludes by proposing a mechanistic model of ABC cholesterol transporters.