Membrane raft microdomains mediate front-rear polarity in migrating cells

A feedback loop between lamellipodial extension and HGF-ERK signaling specifies leader cells during collective cell migration

Upon the initiation of collective cell migration, the cells at the free edge are specified as leader cells; however, the mechanism underlying the leader cell specification remains elusive. Here, we show that lamellipodial extension after the release from mechanical confinement causes sustained extracellular signal-regulated kinase (ERK) activation and underlies the leader cell specification. Live-imaging of Madin-Darby canine kidney (MDCK) cells and mouse epidermis through the use of Förster resonance energy transfer (FRET)-based biosensors showed that leader cells exhibit sustained ERK activation in a hepatocyte growth factor (HGF)-dependent manner. Meanwhile, follower cells exhibit oscillatory ERK activation waves in an epidermal growth factor (EGF) signaling-dependent manner. Lamellipodial extension at the free edge increases the cellular sensitivity to HGF. The HGF-dependent ERK activation, in turn, promotes lamellipodial extension, thereby forming a positive feedback loop between cell extension and ERK activation and specifying the cells at the free edge as the leader cells. Our findings show that the integration of physical and biochemical cues underlies the leader cell specification during collective cell migration.

Lipid raft involvement in signal transduction in cancer cell survival, cell death and metastasis

Lipid rafts are cholesterol‐ and sphingolipid‐enriched specialized membrane domains within the plasma membrane. Lipid rafts regulate the density and activity of signal receptors by compartmentalizing them, promoting signalling cascades that play important roles in the survival, death and metastasis of cancer cells. In this review, we emphasize the current concept initially postulated by F. Mollinedo and C. Gajate on the importance of lipid rafts in cancer survival, death and metastasis by describing representative signalling pathways, including the IGF system and the PI3K/AKT, Fas/CD95, VEGF/VEGFR2 and CD44 signalling pathways, and we also discuss the concept of CASMER (cluster of apoptotic signalling molecule‐enriched rafts), coined, originally introduced and further advanced by F. Mollinedo and C. Gajate in the period 2005–2010. Then, we summarize relevant research progress and suggest that lipid rafts play important roles in the survival, death and metastasis of cancer cells, making them promising targets for cancer therapy.

Sodium Transporters in Human Health and Disease

Sodium (Na+) electrochemical gradients established by Na+/K+ ATPase activity drives the transport of ions, minerals, and sugars in both excitable and non-excitable cells. Na+-dependent transporters can move these solutes in the same direction (cotransport) or in opposite directions (exchanger) across both the apical and basolateral plasma membranes of polarized epithelia. In addition to maintaining physiological homeostasis of these solutes, increases and decreases in sodium may also initiate, directly or indirectly, signaling cascades that regulate a variety of intracellular post-translational events. In this review, we will describe how the Na+/K+ ATPase maintains a Na+ gradient utilized by multiple sodium-dependent transport mechanisms to regulate glucose uptake, excitatory neurotransmitters, calcium signaling, acid-base balance, salt-wasting disorders, fluid volume, and magnesium transport. We will discuss how several Na+-dependent cotransporters and Na+-dependent exchangers have significant roles in human health and disease. Finally, we will discuss how each of these Na+-dependent transport mechanisms have either been shown or have the potential to use Na+ in a secondary role as a signaling molecule.

Wnt-controlled sphingolipids modulate Anthrax Toxin Receptor palmitoylation to regulate oriented mitosis in zebrafish

Oriented cell division is a fundamental mechanism to control asymmetric stem cell division, neural tube elongation and body axis extension, among other processes. During zebrafish gastrulation, when the body axis extends, dorsal epiblast cells display divisions that are robustly oriented along the animal-vegetal embryonic axis. Here, we use a combination of lipidomics, metabolic tracer analysis and quantitative image analysis to show that sphingolipids mediate spindle positioning during oriented division of epiblast cells. We identify the Wnt signaling as a regulator of sphingolipid synthesis that mediates the activity of serine palmitoyltransferase (SPT), the first and rate-limiting enzyme in sphingolipid production. Sphingolipids determine the palmitoylation state of the Anthrax receptor, which then positions the mitotic spindle of dividing epiblast cells. Our data show how Wnt signaling mediates sphingolipid-dependent oriented division and how sphingolipids determine Anthrax receptor palmitoylation, which ultimately controls the activation of Diaphanous to mediate spindle rotation and oriented mitosis.

During development, oriented cell division is important to proper body axis extension. Here, the authors show that sphingolipids are required to direct spindle rotation and oriented mitosis via Anthrax receptor palmitoylation in zebrafish gastrulation.

Direct monitoring of drug-induced mechanical response of individual cells by atomic force microscopy

Mechanical characteristics of individual cells play a vital role in many biological processes and are considered as indicators of the cells' states. Disturbances including methyl-β-cyclodextrin (MβCD) and cytochalasin D (cytoD) are known to significantly affect the state of cells, but little is known about the real-time response of single cells to these drugs in their physiological condition. Here, nanoindentation-based atomic force microscopy (AFM) was used to measure the elasticity of human embryonic kidney cells in the presence and absence of these pharmaceuticals. The results showed that depletion of cholesterol in the plasma membrane with MβCD resulted in cell stiffening whereas depolymerization of the actin cytoskeleton by cytoD resulted in cell softening. Using AFM for real-time measurements, we observed that cells mechanically responded right after these drugs were added. In more detail, the cell´s elasticity suddenly increased with increasing instability upon cholesterol extraction while it is rapidly decreased without changing cellular stability upon depolymerizing actin cytoskeleton. These results demonstrated that actin cytoskeleton and cholesterol contributed differently to the cell mechanical characteristics.

The Effect of Transmembrane Protein Shape on Surrounding Lipid Domain Formation by Wetting

Signal transduction through cellular membranes requires the highly specific and coordinated work of specialized proteins. Proper functioning of these proteins is provided by an interplay between them and the lipid environment. Liquid-ordered lipid domains are believed to be important players here, however, it is still unclear whether conditions for a phase separation required for lipid domain formation exist in cellular membranes. Moreover, membrane leaflets are compositionally asymmetric, that could be an obstacle for the formation of symmetric domains spanning the lipid bilayer. We theoretically show that the presence of protein in the membrane leads to the formation of a stable liquid-ordered lipid phase around it by the mechanism of protein wetting by lipids, even in the absence of conditions necessary for the global phase separation in the membrane. Moreover, we show that protein shape plays a crucial role in this process, and protein conformational rearrangement can lead to changes in the size and characteristics of surrounding lipid domains.

Hypoxia Downregulates LPP3 and Promotes the Spatial Segregation of ATX and LPP1 During Cancer Cell Invasion

Hypoxia is a common characteristic of advanced solid tumors and a potent driver of tumor invasion and metastasis. Recent evidence suggests the involvement of autotaxin (ATX) and lysophosphatidic acid receptors (LPARs) in cancer cell invasion promoted by the hypoxic tumor microenvironment; however, the transcriptional and/or spatiotemporal control of this process remain unexplored. Herein, we investigated whether hypoxia promotes cell invasion by affecting the main enzymes involved in its production (ATX) and degradation (lipid phosphate phosphatases, LPP1 and LPP3). We report that hypoxia not only modulates the expression levels of lysophosphatidic acid (LPA) regulatory enzymes but also induces their significant spatial segregation in a variety of cancers. While LPP3 expression was downregulated by hypoxia, ATX and LPP1 were asymmetrically redistributed to the leading edge and to the trailing edge, respectively. This was associated with the opposing roles of ATX and LPPs in cell invasion. The regulated expression and compartmentalization of these enzymes of opposing function can provide an effective way to control the generation of an LPA gradient that drives cellular invasion and migration in the hypoxic zones of tumors.

Increasing ergosterol levels delays formin-dependent assembly of F-actin cables and disrupts division plane positioning in fission yeast

In most eukaryotes, cytokinesis is mediated by the constriction of a contractile acto-myosin ring (CR), which promotes the ingression of the cleavage furrow. Many components of the CR interact with plasma membrane lipids suggesting that lipids may regulate CR assembly and function. Although there is clear evidence that phosphoinositides play an important role in cytokinesis, much less is known about the role of sterols in this process. Here, we studied how sterols influence division plane positioning and CR assembly in fission yeast. We show that increasing ergosterol levels in the plasma membrane blocks the assembly of F-actin cables from cytokinetic precursor nodes, preventing their compaction into a ring. Abnormal F-actin cables form after a delay, leading to randomly placed septa. Since the formin Cdc12 was detected on cytokinetic precursors and the phenotype can be partially rescued by inhibiting the Arp2/3 complex, which competes with formins for F-actin nucleation, we propose that ergosterol may inhibit formin dependent assembly of F-actin cables from cytokinetic precursors.

Lipid pathway deregulation in advanced prostate cancer

The link between prostate cancer (PC) development and lipid metabolism is well established, with AR intimately involved in a number of lipogenic processes involving SREBP1, PPARG, FASN, ACC, ACLY and SCD1. Recently, there is growing evidence implicating the role of obesity and peri-prostatic adipose tissue (PPAT) in PC aggressiveness and related mortality, suggesting the importance of lipid pathways in both localised and disseminated disease. A number of promising agents are in development to target the lipogenic axis in PC, and the likelihood is that these agents will form part of combination drug strategies, with targeting of multiple metabolic pathways (e.g. FASN and CPT1), or in combination with AR pathway inhibitors (SCD1 and AR).

pH sensing by lipids in membranes: The fundamentals of pH-driven migration, polarization and deformations of lipid bilayer assemblies

Most biological molecules contain acido-basic groups that modulate their structure and interactions. A consequence is that pH gradients, local heterogeneities and dynamic variations are used by cells and organisms to drive or regulate specific biological functions including energetic metabolism, vesicular traffic, migration and spatial patterning of tissues in development. While the direct or regulatory role of pH in protein function is well documented, the role of hydrogen and hydroxyl ions in modulating the properties of lipid assemblies such as bilayer membranes is only beginning to be understood. Here, we review approaches using artificial lipid vesicles that have been instrumental in providing an understanding of the influence of pH gradients and local variations on membrane vectorial motional processes: migration, membrane curvature effects promoting global or local deformations, crowding generation by segregative polarization processes. In the case of pH induced local deformations, an extensive theoretical framework is given and an application to a specific biological issue, namely the structure and stability of mitochondrial cristae, is described. This article is part of a Special Issue entitled: Emergence of Complex Behavior in Biomembranes edited by Marjorie Longo.

Line Activity of Ganglioside GM1 Regulates the Raft Size Distribution in a Cholesterol-Dependent Manner

Liquid-ordered lipid domains, also called rafts, are assumed to be important players in different cellular processes, mainly signal transduction and membrane trafficking. They are thicker than the disordered part of the membrane and are thought to form to compensate for the hydrophobic mismatch between transmembrane proteins and the lipid environment. Despite the existence of such structures in vivo still being an open question, they are observed in model systems of multicomponent lipid bilayers. Moreover, the predictions obtained from model experiments allow the explanation of different physiological processes possibly involving rafts. Here we present the results of the study of the regulation of raft size distribution by ganglioside GM1. Combining atomic force microscopy with theoretical considerations based on the theory of membrane elasticity, we predict that this glycolipid should change the line tension of raft boundaries in two different ways, mainly depending on the cholesterol content. These results explain the shedding of gangliosides from the surface of tumor cells and the following ganglioside-induced apoptosis of T-lymphocytes in a raft-dependent manner. Moreover, the generality of the model allows the prediction of the line activity of different membrane components based on their molecular geometry.

Chemokine Receptor Signaling and the Hallmarks of Cancer

The chemokines are a family of chemotactic cytokines that mediate their activity by acting on seven-transmembrane-spanning G protein-coupled receptors. Both the ability of the chemokines and their receptors to form homo- and heterodimers and the promiscuity of the chemokine-chemokine receptor interaction endow this protein family with enormous signaling plasticity and complexity that are not fully understood at present. Chemokines were initially identified as essential regulators of homeostatic and inflammatory trafficking of innate and adaptive leucocytes from lymphoid organs to tissues. Chemokines also mediate the host response to cancer. Nevertheless, chemokine function in this response is not limited to regulating leucocyte infiltration into the tumor microenvironment. It is now known that chemokines and their receptors influence most-if not all-hallmark processes of cancer; they act on both neoplastic and untransformed cells in the tumor microenvironment, including fibroblasts, endothelial cells (blood and lymphatic), bone marrow-derived stem cells, and, obviously, infiltrating leucocytes. This review begins with an overview of chemokine and chemokine receptor structure, to better define how chemokines affect the proliferation, survival, stemness, and metastatic potential of neoplastic cells. We also examine the main mechanisms by which chemokines regulate tumor angiogenesis and immune cell infiltration, emphasizing the pro- and antitumorigenic activity of this protein superfamily in these interrelated processes.

Mapping out the intricate relationship of the HIV envelope protein and the membrane environment

The HIV gp160 envelope fusion protein is situated in the viral membrane and mediates virus entry into its host cell. Increasing evidence suggests that virtually all parts of the HIV envelope are structurally and functionally dependent on membranes. Protein-lipid interactions and membrane properties influence the dynamics of a manifold of gp160 biological activities such as membrane fusion, immune suppression and gp160 incorporation into virions during HIV budding and assembly. In the following we will summarize our current understanding of this interdependence between membrane interaction, structural conformation and functionality of the different gp160 domains. This article is part of a Special Issue entitled: Lipid order/lipid defects and lipid-control of protein activity edited by Dirk Schneider.

Lowered Expression of Tumor Suppressor Candidate MYO1C Stimulates Cell Proliferation, Suppresses Cell Adhesion and Activates AKT

Myosin-1C (MYO1C) is a tumor suppressor candidate located in a region of recurrent losses distal to TP53. Myo1c can tightly and specifically bind to PIP2, the substrate of Phosphoinositide 3-kinase (PI3K), and to Rictor, suggesting a role for MYO1C in the PI3K pathway. This study was designed to examine MYO1C expression status in a panel of well-stratified endometrial carcinomas as well as to assess the biological significance of MYO1C as a tumor suppressor in vitro. We found a significant correlation between the tumor stage and lowered expression of MYO1C in endometrial carcinoma samples. In cell transfection experiments, we found a negative correlation between MYO1C expression and cell proliferation, and MYO1C silencing resulted in diminished cell migration and adhesion. Cells expressing excess of MYO1C had low basal level of phosphorylated protein kinase B (PKB, a.k.a. AKT) and cells with knocked down MYO1C expression showed a quicker phosphorylated AKT (pAKT) response in reaction to serum stimulation. Taken together the present study gives further evidence for tumor suppressor activity of MYO1C and suggests MYO1C mediates its tumor suppressor function through inhibition of PI3K pathway and its involvement in loss of contact inhibition.

Critical role of CAV1/caveolin-1 in cell stress responses in human breast cancer cells via modulation of lysosomal function and autophagy

CAV1 (caveolin 1, caveolae protein, 22kDa) is well known as a principal scaffolding protein of caveolae, a specialized plasma membrane structure. Relatively, the caveolae-independent function of CAV1 is less studied. Autophagy is a process known to involve various membrane structures, including autophagosomes, lysosomes, and autolysosomes for degradation of intracellular proteins and organelles. Currently, the function of CAV1 in autophagy remains largely elusive. In this study, we demonstrate for the first time that CAV1 deficiency promotes both basal and inducible autophagy. Interestingly, the promoting effect was found mainly in the late stage of autophagy via enhancing lysosomal function and autophagosome-lysosome fusion. Notably, the regulatory function of CAV1 in lysosome and autophagy was found to be caveolae-independent, and acts through lipid rafts. Furthermore, the elevated autophagy level induced by CAV1 deficiency serves as a cell survival mechanism under starvation. Importantly, downregulation of CAV1 and enhanced autophagy level were observed in human breast cancer cells and tissues. Taken together, our data reveal a novel function of CAV1 and lipid rafts in breast cancer development via modulation of lysosomal function and autophagy.

Low Density Lipoprotein Receptor Related Proteins as Regulators of Neural Stem and Progenitor Cell Function

The central nervous system (CNS) is a highly organised structure. Many signalling systems work in concert to ensure that neural stem cells are appropriately directed to generate progenitor cells, which in turn mature into functional cell types including projection neurons, interneurons, astrocytes, and oligodendrocytes. Herein we explore the role of the low density lipoprotein (LDL) receptor family, in particular family members LRP1 and LRP2, in regulating the behaviour of neural stem and progenitor cells during development and adulthood. The ability of LRP1 and LRP2 to bind a diverse and extensive range of ligands, regulate ligand endocytosis, recruit nonreceptor tyrosine kinases for direct signal transduction and signal in conjunction with other receptors, enables them to modulate many crucial neural cell functions.

The effect of LDL particles on the behaviour of epithelial noncancer and cancer cell lines after in vitro induced injury

Cell spreading capability and cell proliferation are the major processes in wound healing of injured epithelia as well as in tumour progression. The effect of low-density lipoprotein (LDL) particles as a major extracellular source of cholesterol was evaluated in the re-epithelisation assay of in vitro induced injury. We selected two noncancer cell lines with different dependence on LDL concentrations, the kidney epithelial cells (MDCK) with higher dependence and keratinocytes (HaCaT) with lower dependence on LDL, and three cancer cell lines originating from epithelial cells: A549 (alveolar), CaCo-2 (intestinal) and RT4 (urothelial). All cells were incubated in a control medium, in an LDL-enriched medium or in an LDL-deficient medium. The LDL-enriched medium stimulated cell spreading of MDCK cells which, together with increased proliferation of these cells, resulted in an enhanced re-epithelisation of in vitro induced injury. LDL deficiency caused lower cell spreading which resulted in a decreased re-epithelisation despite the higher proliferation of MDCK cells in this medium. The re-epithelisation of keratinocytes (HaCaT) was not affected by altered LDL concentrations. In cancer cell lines A549, CaCo-2 and RT4, wide heterogeneity regarding cell proliferation and spreading capability was observed after treatment with different LDL concentrations. LDL had no influence on actin filament and tight junction distribution in any of the tested cell lines. The cholesterol content of all cell types, except for CaCo-2 cells, proved to be independent of the LDL level. Further research of the beneficial effects of LDL is needed to prove LDL as a safe enhancer of epithelial wound healing.

Herpes simplex virus enhances chemokine function through modulation of receptor trafficking and oligomerization

Glycoprotein G (gG) from herpes simplex virus 1 and 2 (HSV-1 and HSV-2, important human neurotropic pathogens) is the first viral chemokine-binding protein found to potentiate chemokine function. Here we show that gG attaches to cell surface glycosaminoglycans and induces lipid raft clustering, increasing the incorporation of CXCR4 receptors into these microdomains. gG induces conformational rearrangements in CXCR4 homodimers and changes their intracellular partners, leading to sustained, functional chemokine/receptor complexes at the surface. This results in increased chemotaxis dependent on the cholesterol content of the plasma membrane and receptor association to Src-kinases and phosphatidylinositol-3-kinase signalling pathways, but independent of clathrin-mediated endocytosis. Furthermore, using electron microscopy, we show that such enhanced functionality is associated with the accumulation of low-order CXCR4 nanoclusters. Our results provide insights into basic mechanisms of chemokine receptor function and into a viral strategy of immune modulation.

Signaling switch of the urotensin II vasosactive peptide GPCR: prototypic chemotaxic mechanism in glioma

Multiform glioblastomas (GBM) are the most frequent and aggressive primary brain tumors in adults. The poor prognosis is due to neo-angiogenesis and cellular invasion, processes that require complex chemotaxic mechanisms involving motility, migration and adhesion. Understanding these different cellular events implies identifying receptors and transduction pathways that lead to and promote either migration or adhesion. Here we establish that glioma express the vasoactive peptide urotensin II (UII) and its receptor UT and that UT-mediated signaling cascades are involved in glioma cell migration and adhesion. Components of the urotensinergic systems, UII and UT, are widely expressed in patient-derived GBM tissue sections, glioma cell lines and fresh biopsy explants. Interestingly, gradient concentrations of UII produced chemoattracting migratory/motility effects in glioma as well as HEK293 cells expressing human UT. These effects mainly involved the G13/Rho/rho kinase pathway while partially requiring Gi/o/PI3K components. In contrast, we observed that homogeneous concentrations of UII drastically blocked cell motility and stimulated cell-matrix adhesions through a UT/Gi/o signaling cascade, partially involving phosphatidylinositol-3 kinase. Finally, we provide evidence that, in glioma cells, homogeneous concentration of UII allowed translocation of Gα13 to the UT receptor at the plasma membrane and increased actin stress fibers, lamellipodia formation and vinculin-stained focal adhesions. UII also provoked a re-localization of UT precoupled to Gαi in filipodia and initiated integrin-stained focal points. Altogether, these findings suggest that UT behaves as a chemotaxic receptor, relaying a signaling switch between directional migration and cell adhesion under gradient or homogeneous concentrations, thereby redefining sequential mechanisms affecting tumor cells during glioma invasion. Taken together, our results allow us to propose a model in order to improve the design of compounds that demonstrate signaling bias for therapies that target specifically the Gi/o signaling pathway.

Stromal cell-derived factor-1 (SDF-1)/CXCR4 axis enhances cellular invasion in ovarian carcinoma cells via integrin β1 and β3 expressions

Accumulating evidence has showed that stromal cell-derived factor-1 (SDF-1/CXCR4 axis played important roles in cancer metastases, but the detailed function in ovarian cancer is still largely unknown. In the present study, we determined the location of CXCR4 and lipid rafts, a specialized structure on cell membrane, in ovarian cancer tissues and ovarian cancer cell line SKOV3 cells by immunofluorescence. To analyze the role of SDF-1/CXCR4 and lipid rafts in tumor cell migration and invasion, Transwell assay and wound healing assay were also performed. Cytoflowmetry was carried out to determine the participation of integrins. Our data showed that CXCR4 and GM1 (marker of lipid rafts) were expressed in both ovarian cancer tissue and SKOV3 cells, and SDF-1 promoted the invasion and migration of SKOV3 cells, which was mediated by complete lipid rafts. Further studies uncovered that SDF-1 upregulated the expression of integrin β1 and β3, two molecules closely related with cancer metastasis. These results indicated that SDF-1 might promote the invasion and metastasis of ovarian cancer by regulating these two integrin molecules.

Lipid rafts direct macrophage motility in the tissue microenvironment

Infiltrating leukocytes are exposed to a wide range of tissue elasticities. While we know the effects of substrate elasticity on acute inflammation via the study of neutrophil migration, we do not know its effects on leukocytes that direct chronic inflammatory events. Here, we studied morphology and motility of macrophages, the innate immune cells that orchestrate acute and chronic inflammation, on polyacrylamide hydrogels that mimicked a wide range of tissue elasticities. As expected, we found that macrophage spreading area increased as substrate elasticity increased. Unexpectedly, we found that morphology did not inversely correlate with motility. In fact, velocity of steady-state macrophages remained unaffected by substrate elasticity, while velocity of biologically stimulated macrophages was limited on stiff substrates. We also found that the lack of motility on stiff substrates was due to a lack of lipid rafts on the leading edge of the macrophages. This study implicates lipid rafts in the mechanosensory mechanism of innate immune cell infiltration.

Brain delivery of proteins via their fatty acid and block copolymer modifications

It is well known that hydrophobic small molecules penetrate cell membranes better than hydrophilic molecules. Amphiphilic molecules that dissolve both in lipid and aqueous phases are best suited for membrane transport. Transport of biomacromolecules across physiological barriers, e.g. the blood-brain barrier, is greatly complicated by the unique structure and function of such barriers. Two decades ago we adopted a simple philosophy that to increase protein delivery to the brain one needs to modify this protein with hydrophobic moieties. With this general idea we began modifying proteins (antibodies, enzymes, hormones, etc.) with either hydrophobic fatty acid residues or amphiphilic block copolymer moieties, such as poy(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (pluronics or poloxamers) and more recently, poly(2-oxasolines). This simple approach has resulted in impressive successes in CNS drug delivery. We present a retrospective overview of these works initiated in the Soviet Union in 1980s, and then continued in the United States and other countries. Notably some of the early findings were later corroborated by brain pharmacokinetic data. Industrial development of several drug candidates employing these strategies has followed. Overall modification by hydrophobic fatty acids residues or amphiphilic block copolymers represents a promising and relatively safe strategy to deliver proteins to the brain.

Tritrichomonas foetus displays classical detergent-resistant membrane microdomains on its cell surface

Tritrichomonas foetus is a serious veterinary parasite that causes bovine trichomoniasis, a sexually transmitted disease that results in reproductive failure and considerable economic losses in areas that practice natural breeding. T. foetus is an extracellular parasite, which initially adheres to and infects the urogenital tract using a diverse array of surface glycoconjugates, including adhesins and extracellular matrix-binding molecules. However, the cellular mechanisms by which T. foetus colonizes mucosal surfaces and causes tissue damage are not well defined. Several studies have demonstrated the involvement of pathogen or host lipid rafts in cellular events that occur during pathogenesis, including adhesion, invasion and evasion of the immune response. In this study, we demonstrate that detergent-resistant membranes are present in the plasma membrane of T. foetus. We further demonstrate that microdomains are cholesterol-enriched and contain ganglioside GM1-like molecules. In addition, we demonstrate that lipid microdomains do not participate in T. foetus adhesion to host cells. However, the use of agents that disrupt and disorganize the plasma membrane indicated the involvement of the T. foetus lipid microdomains, in cell division and in the formation of endoflagellar forms. Our results suggest that trophozoites and endoflagellar forms present a different plasma membrane organization.

A novel mechanism of regulating breast cancer cell migration via palmitoylation-dependent alterations in the lipid raft affiliation of CD44

INTRODUCTION

Most breast cancer-related deaths result from metastasis, a process involving dynamic regulation of tumour cell adhesion and migration. The adhesion protein CD44, a key regulator of cell migration, is enriched in cholesterol-enriched membrane microdomains termed lipid rafts. We recently reported that raft affiliation of CD44 negatively regulates interactions with its migratory binding partner ezrin. Since raft affiliation is regulated by post-translational modifications including palmitoylation, we sought to establish the contribution of CD44 palmitoylation and lipid raft affiliation to cell migration.

METHODS

Recovery of CD44 and its binding partners from raft versus non-raft membrane microdomains was profiled in non-migrating and migrating breast cancer cell lines. Site-directed mutagenesis was used to introduce single or double point mutations into both CD44 palmitoylation sites (Cys286 and Cys295), whereupon the implications for lipid raft recovery, phenotype, ezrin co-precipitation and migratory behaviour was assessed. Finally CD44 palmitoylation status and lipid raft affiliation was assessed in primary cultures from a small panel of breast cancer patients.

RESULTS

CD44 raft affiliation was increased during migration of non-invasive breast cell lines, but decreased during migration of highly-invasive breast cells. The latter was paralleled by increased CD44 recovery in non-raft fractions, and exclusive non-raft recovery of its binding partners. Point mutation of CD44 palmitoylation sites reduced CD44 raft affiliation in invasive MDA-MB-231 cells, increased CD44-ezrin co-precipitation and accordingly enhanced cell migration. Expression of palmitoylation-impaired (raft-excluded) CD44 mutants in non-invasive MCF-10a cells was sufficient to reversibly induce the phenotypic appearance of epithelial-to-mesenchymal transition and to increase cell motility. Interestingly, cell migration was associated with temporal reductions in CD44 palmitoylation in wild-type breast cells. Finally, the relevance of these findings is underscored by the fact that levels of palmitoylated CD44 were lower in primary cultures from invasive ductal carcinomas relative to non-tumour tissue, while CD44 co-localisation with a lipid raft marker was less in invasive ductal carcinoma relative to ductal carcinoma in situ cultures.

CONCLUSION

Our results support a novel mechanism whereby CD44 palmitoylation and consequent lipid raft affiliation inversely regulate breast cancer cell migration, and may act as a new therapeutic target in breast cancer metastasis.

Sterol dynamics during endocytic trafficking in Arabidopsis

Sterols are lipids found in membranes of eukaryotic cells. Functions of sterols have been demonstrated for various cellular processes including endocytic trafficking in animal, fungal, and plant cells. The ability to visualize sterols at the subcellular level is crucial to understand sterol distribution and function during endocytic trafficking. In plant cells, the polyene antibiotic filipin is the most extensively used tool for the specific detection of fluorescently labeled 3-β-hydroxysterols in situ. Filipin can to some extent be used to track sterol internalization in live cells, but this application is limited, due to the inhibitory effects filipin exerts on sterol-dependent endocytosis. Nevertheless, filipin-sterol labeling can be performed on aldehyde-fixed cells which allows for sterol detection in endocytic compartments. This approach can combine studies correlating sterol distribution with experimental manipulations of endocytic trafficking pathways. Here, we describe step-by-step protocols and troubleshooting for procedures on live and fixed cells to visualize sterols during endocytic trafficking. We also provide a detailed discussion of advantages and limitations of both methods. Moreover, we illustrate the use of the endocytic recycling inhibitor brefeldin A and a genetically modified version of one of its target molecules for studying endocytic sterol trafficking.

Insights into the mechanism for dictating polarity in migrating T-cells

This review is focused on mechanisms of chemokine-induced polarization of T-lymphocytes. Polarization involves, starting from spherical cells, formation of a morphologically and functionally different rear (uropod) and front (leading edge). This polarization is required for efficient random and directed T-cell migration. The addressed topics concern the specific location of cell organelles and of receptors, signaling molecules, and cytoskeletal proteins in chemokine-stimulated polarized T-cells. In chemokine-stimulated, polarized T-cells, specific proteins, signaling molecules and organelles show enrichment either in the rear, the midzone, or the front; different from the random location in spherical resting cells. Possible mechanisms involved in this asymmetric location will be discussed. A major topic is also the functional role of proteins and cell organelles in T-cell polarization and migration. Specifically, the roles of adhesion and chemokine receptors, cytoskeletal proteins, signaling molecules, scaffolding proteins, and membrane microdomains in these processes will be discussed. The polarity which is established during contact formation of T-cells with antigen-presenting cells is not discussed in detail.

Roles of ion transport in control of cell motility

Cell motility is an essential feature of life. It is essential for reproduction, propagation, embryonic development, and healing processes such as wound closure and a successful immune defense. If out of control, cell motility can become life-threatening as, for example, in metastasis or autoimmune diseases. Regardless of whether ciliary/flagellar or amoeboid movement, controlled motility always requires a concerted action of ion channels and transporters, cytoskeletal elements, and signaling cascades. Ion transport across the plasma membrane contributes to cell motility by affecting the membrane potential and voltage-sensitive ion channels, by inducing local volume changes with the help of aquaporins and by modulating cytosolic Ca(2+) and H(+) concentrations. Voltage-sensitive ion channels serve as voltage detectors in electric fields thus enabling galvanotaxis; local swelling facilitates the outgrowth of protrusions at the leading edge while local shrinkage accompanies the retraction of the cell rear; the cytosolic Ca(2+) concentration exerts its main effect on cytoskeletal dynamics via motor proteins such as myosin or dynein; and both, the intracellular and the extracellular H(+) concentration modulate cell migration and adhesion by tuning the activity of enzymes and signaling molecules in the cytosol as well as the activation state of adhesion molecules at the cell surface. In addition to the actual process of ion transport, both, channels and transporters contribute to cell migration by being part of focal adhesion complexes and/or physically interacting with components of the cytoskeleton. The present article provides an overview of how the numerous ion-transport mechanisms contribute to the various modes of cell motility.

Thrombomodulin functions as a plasminogen receptor to modulate angiogenesis

Urokinase-type plasminogen activator (uPA) activates plasminogen (Plg) through a major pericellular proteolytic system involved in cell migration and angiogenesis; however, the Plg receptor that participates in uPA-mediated Plg activation has not yet been identified. In this study, we demonstrated that thrombomodulin (TM), a type I transmembrane glycoprotein, is a novel Plg receptor that plays a role in pericellular proteolysis and cell migration. Plg activation at the cell surface and the extent of its cell migration- and invasion-promoting effect are cellular TM expression dependent. Direct binding of Plg and the recombinant TM extracellular domain, with a KD of 0.1-0.3 μM, was determined through surface plasmon resonance analysis. Colocalization of TM, Plg, and the uPA receptor within plasma membrane lipid rafts, at the leading edge of migrating endothelial cells, was demonstrated and was also shown to overlap with areas of major pericellular proteolysis. Moreover, the roles of TM and Plg in neoangiogenesis were demonstrated in vivo through the skin wound-healing model. In conclusion, we propose that TM is a novel Plg receptor that regulates uPA/uPA receptor-mediated Plg activation and pericellular proteolysis within lipid rafts at the leading edge of migrating cells during angiogenesis.

Ductal barriers in mammary epithelium

Tissue barriers play an integral role in the biology and pathobiology of mammary ductal epithelium. In normal breast physiology, tight and adherens junctions undergo dynamic changes in permeability in response to hormonal and other stimuli, while several of their proteins are directly involved in mammary tumorigenesis. This review describes first the structure of mammary ductal epithelial barriers and their role in normal mammary development, examining the cyclical changes in response to puberty, pregnancy, lactation and involution. It then examines the role of adherens and tight junctions and the participation of their constituent proteins in mammary tumorigenic functions such as migration, invasion and metastasis. Finally, it discusses the potential of these adhesion proteins as both prognostic biomarkers and potential therapeutic targets in breast cancer.

Plasma membrane signaling in HIV-1 infection

Plasma membrane is a multifunctional structure that acts as the initial barrier against infection by intracellular pathogens. The productive HIV-1 infection depends upon the initial interaction of virus and host plasma membrane. Immune cells such as CD4+ T cells and macrophages contain essential cell surface receptors and molecules such as CD4, CXCR4, CCR5 and lipid raft components that facilitate HIV-1 entry. From plasma membrane HIV-1 activates signaling pathways that prepare the grounds for viral replication. Through viral proteins HIV-1 hijacks host plasma membrane receptors such as Fas, TNFRs and DR4/DR5, which results in immune evasion and apoptosis both in infected and uninfected bystander cells. These events are hallmark in HIV-1 pathogenesis that leads towards AIDS. The interplay between HIV-1 and plasma membrane signaling has much to offer in terms of viral fitness and pathogenicity, and a better understanding of this interplay may lead to development of new therapeutic approaches. This article is part of a Special Issue entitled: Viral Membrane Proteins - Channels for Cellular Networking.

Detection of CD133 (prominin-1) in a human hepatoblastoma cell line (HuH-6 clone 5)

We examined CD133 distribution in a human hepatoblastoma cell line (HuH-6 clone 5). We directly observed the cultured cells on a pressure-resistant thin film (silicon nitride thin film) in a buffer solution by using the newly developed atmospheric scanning electron microscope (ASEM), which features an open sample dish with a silicon nitride thin film window at its base, through which the scanning electron microscope beam scans samples in solution, from below. The ASEM enabled observation of the ventral cell surface, which could not be observed using standard SEM. However, observation of the dorsal cell surface was difficult with the ASEM. Therefore, we developed a new method to observe the dorsal side of cells by using Aclar® plastic film. In this method, cells are cultured on Aclar plastic film and the dorsal side of cells is in contact with the thin silicon nitride film of the ASEM dish. A preliminary study using the ASEM showed that CD133 was mainly localized in membrane ruffles in the peripheral regions of the cell. Standard transmission electron microscopy and scanning electron microscopy revealed that CD133 was preferentially concentrated in a complex structure comprising filopodia and the leading edge of lamellipodia. We also observed co-localization of CD133 with F-actin. An antibody against CD133 decreased cell migration. Methyl-β-cyclodextrin treatment decreased cell adhesion as well as lamellipodium and filopodium formation. A decrease in the cholesterol level may perturb CD133 membrane localization. The results suggest that CD133 membrane localization plays a role in tumor cell adhesion and migration.

Endocytosis of gene delivery vectors: from clathrin-dependent to lipid raft-mediated endocytosis

The ideal nonviral vector delivers its nucleic acid cargo to a specific intracellular target. Vectors enter cells mainly through endocytosis and are distributed to various intracellular organelles. Recent advances in microscopy, lipidomics, and proteomics confirm that the cell membrane is composed of clusters of lipids, organized in the form of lipid raft domains, together with non-raft domains that comprise a generally disordered lipid milieu. The binding of a nonviral vector to either region can determine the pathway for its endocytic uptake and subsequent intracellular itinerary. Given this model of the cell membrane structure, endocytic pathways should be reclassified in relation to lipid rafts. In this review, we attempt to assess the currently recognized endocytic pathways in mammalian cells. The endocytic pathways are classified in relation to the membrane regions that make up the primary endocytic vesicles. This review covers the well-recognized clathrin-mediated endocytosis (CME), phagocytosis, and macropinocytosis in addition to the less addressed pathways that take place in lipid rafts. These include caveolae-mediated, flotillin-dependent, GTPase regulator associated with focal adhesion kinase-1 (GRAF1)-dependent, adenosine diphosphate-ribosylation factor 6 (Arf6)-dependent, and RhoA-dependent endocytic pathways. We summarize the regulators associated with each uptake pathway and methods for interfering with these regulators are discussed. The fate of endocytic vesicles resulting from each endocytic uptake pathway is highlighted.

Hydroxy-terminated conjugated polymer nanoparticles have near-unity bright fraction and reveal cholesterol-dependence of IGF1R nanodomains

Fluorescent nanoparticles have enabled many discoveries regarding how molecular machines function. Quantum dots have been the dominant class of fluorescent nanoparticles but suffer from blinking and from a substantial dark fraction--particles where the fluorescence is never seen--complicating any analysis of biological function. Nanoparticles composed of conjugated fluorescent polymers (Pdots) have recently been shown to have high brightness and no blinking. Here we develop a robust and efficient means to measure the dark fraction of Pdots, conjugating Atto dyes to the nanoparticles and testing fluorescence colocalization of dye and Pdot puncta. This established that the Pdots we generated had minimal dark fraction: ∼3%. The application of nanoparticles in biological environments is highly sensitive to surface functionalization. For Pdots we found that passivation with uncharged hydroxy-terminated polyethylene glycol caused a dramatic reduction in nonspecific cell binding and aggregation compared to a charged coating. Using carbonyl di-imidazole the hydroxy-Pdots were functionalized efficiently with streptavidin for high stability targeting, allowing specific labeling of mammalian cells. Type I insulin-like growth factor receptor (IGF1R) regulates cell survival and development, with roles in aging, heart disease, and cancer. We used hydroxy-Pdots to track the dynamics of IGF1R on a breast cancer cell-line, determining the diffusion characteristics and showing cholesterol-containing membrane nanodomains were important for receptor mobility at the plasma membrane. The near-unity bright fraction and low nonspecific binding of hydroxy-Pdots, combined with Pdot photostability and lack of blinking, provides many advantages for investigations at the single molecule level.

Chemical genetics of acetyl-CoA carboxylases

Chemical genetic studies on acetyl-CoA carboxylases (ACCs), rate-limiting enzymes in long chain fatty acid biosynthesis, have greatly advanced the understanding of their biochemistry and molecular biology and promoted the use of ACCs as targets for herbicides in agriculture and for development of drugs for diabetes, obesity and cancers. In mammals, ACCs have both biotin carboxylase (BC) and carboxyltransferase (CT) activity, catalyzing carboxylation of acetyl-CoA to malonyl-CoA. Several classes of small chemicals modulate ACC activity, including cellular metabolites, natural compounds, and chemically synthesized products. This article reviews chemical genetic studies of ACCs and the use of ACCs for targeted therapy of cancers.

Role of ion channels and transporters in cell migration

Cell motility is central to tissue homeostasis in health and disease, and there is hardly any cell in the body that is not motile at a given point in its life cycle. Important physiological processes intimately related to the ability of the respective cells to migrate include embryogenesis, immune defense, angiogenesis, and wound healing. On the other side, migration is associated with life-threatening pathologies such as tumor metastases and atherosclerosis. Research from the last ≈ 15 years revealed that ion channels and transporters are indispensable components of the cellular migration apparatus. After presenting general principles by which transport proteins affect cell migration, we will discuss systematically the role of channels and transporters involved in cell migration.

Functional roles for myosin 1c in cellular signaling pathways

Cellular signaling pathways underlie the transfer of information throughout the cell and to adjoining cells and so govern most critical cellular functions. Increasing evidence points to the molecular motor myosin 1c as a prominent player in many signaling cascades, from the integrin-dependent signaling involved in cell migration to the signaling events underlying insulin resistance. Myosin 1c functions on these pathways both via an important role in regulating lipid raft recycling and also via direct involvement in signaling cascades. This review provides an overview of the functional involvement of myosin 1c in cellular signaling and discusses the possible potential for myosin 1c as a target for drug-based treatments for human diseases.

Ceramide function in the brain: when a slight tilt is enough

Ceramide, the precursor of all complex sphingolipids, is a potent signaling molecule that mediates key events of cellular pathophysiology. In the nervous system, the sphingolipid metabolism has an important impact. Neurons are polarized cells and their normal functions, such as neuronal connectivity and synaptic transmission, rely on selective trafficking of molecules across plasma membrane. Sphingolipids are abundant on neural cellular membranes and represent potent regulators of brain homeostasis. Ceramide intracellular levels are fine-tuned and alteration of the sphingolipid-ceramide profile contributes to the development of age-related, neurological and neuroinflammatory diseases. The purpose of this review is to guide the reader towards a better understanding of the sphingolipid-ceramide pathway system. First, ceramide biology is presented including structure, physical properties and metabolism. Second, we describe the function of ceramide as a lipid second messenger in cell physiology. Finally, we highlight the relevance of sphingolipids and ceramide in the progression of different neurodegenerative diseases.

Segregative clustering of Lo and Ld membrane microdomains induced by local pH gradients in GM1-containing giant vesicles: a lipid model for cellular polarization

Several cell polarization processes are coupled to local pH gradients at the membrane surface. We have investigated the involvement of a lipid-mediated effect in such coupling. The influence of lateral pH gradients along the membrane surface on lipid microdomain dynamics in giant unilamellar vesicles containing phosphatidylcholine, sphingomyelin, cholesterol, and the ganglioside GM1 was studied. Lo/Ld phase separation was generated by photosensitization. A lateral pH gradient was established along the external membrane surface by acid local microinjection. The gradient promotes the segregation of microdomains: Lo domains within an Ld phase move toward the higher pH side, whereas Ld domains within an Lo phase move toward the lower pH side. This results in a polarization of the vesicle membrane into Lo and Ld phases poles in the axis of the proton source. A secondary effect is inward tubulation in the Ld phase. None of these processes occurs without GM1 or with the analog asialo-GM1. These are therefore related to the acidic character of the GM1 headgroup. LAURDAN fluorescence experiments on large unilamellar vesicles indicated that, with GM1, an increase in lipid packing occurs with decreasing pH, attributed to the lowering of repulsion between GM1 molecules. Packing increase is much higher for Ld phase vesicles than for Lo phase vesicles. It is proposed that the driving forces for domain vectorial segregative clustering and vesicle polarization are related to such differences in packing variations with pH decrease between the Lo and Ld phases. Such pH-driven domain clustering might play a role in cellular membrane polarization processes in which local lateral pH gradients are known to be important, such as migrating cells and epithelial cells.

A role for ion channels in glioma cell invasion

Many cells, including neuronal and glial progenitor cells, stem cells and microglial cells, have the capacity to move through the extracellular spaces of the developing and mature brain. This is particularly pronounced in astrocyte-derived tumors, gliomas, which diffusely infiltrate the normal brain. Although a significant body of literature exists regarding signals that are involved in the guidance of cells and their processes, little attention has been paid to cell-shape and cell-volume changes of migratory cells. However, extracellular spaces in the brain are very narrow and represent a major obstacle that requires cells to dynamically regulate their volume. Recent studies in glioma cells show that this involves the secretion of Cl(-) and K(+) with water. Pharmacological inhibition of Cl(-) channels impairs their ability to migrate and limits tumor progression in experimental tumor models. One Cl(-)-channel inhibitor, chlorotoxin, is currently in Phase II clinical trials to treat malignant glioma. This article reviews our current knowledge of cell-volume changes and the role of ion channels during the migration of glioma cells. It also discusses evidence that supports the importance of channel-mediated cell-volume changes in the migration of immature neurons and progenitor cells during development. New unpublished data is presented, which demonstrates that Cl(-) and K(+) channels involved in cell shrinkage localize to lipid-raft domains on the invadipodia of glioma cells and that their presence might be regulated by trafficking of these proteins in and out of lipid rafts.

HaCaT keratinocytes exhibit a cholesterol and plasma membrane viscosity gradient during directed migration

Keratinocyte migration plays an important role in cutaneous wound healing by supporting the process of reepithelialisation. During directional migration cells develop a polarised shape with an asymmetric distribution of a variety of signalling molecules in their plasma membrane. Here, we investigated front-to-back differences of the physical properties of the plasma membrane of migrating keratinocyte-like HaCaT cells. Using FRAP and fluorescence lifetime analysis, both under TIR illumination, we demonstrate a reduced viscosity of the plasma membrane in the lamellipodia of migrating HaCaT cells compared with the cell rears. This asymmetry is most likely caused by a reduced cholesterol content of the lamellipodia as demonstrated by filipin staining. siRNA-mediated silencing of the cholesterol transporter ABCA1, which is known to redistribute cholesterol from rafts to non-raft regions, as well as pharmacological inhibition of this transporter with glibenclamide, strongly diminished the viscosity gradient of the plasma membrane. In addition, HaCaT cell migration was inhibited by glibenclamide treatment. These data suggest a preferential role of non-raft cholesterol in the establishment of the asymmetric plasma membrane viscosity.

The TR (i)P to Ca²⁺ signaling just got STIMy: an update on STIM1 activated TRPC channels

Calcium is a ubiquitous signaling molecule, indispensable for cellular metabolism of organisms from unicellular life forms to higher eukaryotes. The biological function of most eukaryotic cells is uniquely regulated by changes in cytosolic calcium, which is largely achieved by the universal phenomenon of store-operated calcium entry (SOCE). The canonical TRPs and Orai channels have been described as the molecular components of the store-operated calcium channels (SOCC). Importantly, the ER calcium-sensor STIM1 has been shown to initiate SOCE via gating of SOCC. Since the discovery of STIM1, as the critical regulator of SOCE, there has been a flurry of observations suggesting its obligatory role in regulating TRPC and Orai channel function. Considerable effort has been made to identify the molecular details as how STIM1 activates SOCC. In this context, findings as of yet has substantially enriched our understanding on, the modus operandi of SOCE, the distinct cellular locales that organize STIM1-SOCC complexes, and the physiological outcomes entailing STIM1-activated SOCE. In this review we discuss TRPC channels and provide an update on their functional regulation by STIM1.

The role of lipid rafts in cancer cell adhesion and migration

Lipid rafts are cholesterol-enriched microdomains of the cell membrane and possess a highly dynamic nature. They have been involved in various cellular functions including the regulation of cell adhesion and membrane signaling through proteins within lipid rafts. The dynamic features of the cancer cell surface may modulate the malignant phenotype of cancer, including adhesion disorders and aggressive phenotypes of migration and invasion. Recently, it was demonstrated that lipid rafts play critical roles in cancer cell adhesion and migration. This article summarizes the important roles of lipid rafts in cancer cell adhesion and migration, with a focus on the current state of knowledge. This article will improve the understanding of cancer progression and lead to the development of novel targets for cancer therapy.

Function of membrane rafts in viral lifecycles and host cellular response

Membrane rafts are small (10–200 nm) sterol- and sphingolipid-enriched domains that compartmentalize cellular processes. Membrane rafts play an important role in viral infection cycles and viral virulence. Viruses are divided into four main classes, enveloped DNA virus, enveloped RNA virus, nonenveloped DNA virus, and nonenveloped RNA virus. General virus infection cycle is also classified into two sections, the early stage (entry process) and the late stage (assembly, budding, and release processes of virus particles). In the viral cycle, membrane rafts act as a scaffold of many cellular signal transductions, which are associated with symptoms caused by viral infections. In this paper, we describe the functions of membrane rafts in viral lifecycles and host cellular response according to each virus classification, each stage of the virus lifecycle, and each virus-induced signal transduction.