Monitoring lipid anchor organization in cell membranes by PIE-FCCS

Biomolecular condensates tunes immune signaling at the Host-Pathogen interface

Membraneless organelles participate in diverse spatiotemporal regulation of cellular signal transduction by recruiting necessary signaling factors. During host-pathogen interactions, the plasma membrane (PM) at the interface between the plant and microbes serves as a central platform for forming multicomponent immune signaling hubs. The macromolecular condensation of the immune complex and regulators is important in regulating immune signaling outputs regarding strength, timing, and crosstalk between signaling pathways. This review discusses mechanisms that regulate specific and crosstalk of plant immune signal transduction pathways through macromolecular assembly and condensation.

Polymeric biomaterial-inspired cell surface modulation for the development of novel anticancer therapeutics

Immune cell-based therapies are a rapidly emerging class of new medicines that directly treat and prevent targeted cancer. However multiple biological barriers impede the activity of live immune cells, and therefore necessitate the use of surface-modified immune cells for cancer prevention. Synthetic and/or natural biomaterials represent the leading approach for immune cell surface modulation. Different types of biomaterials can be applied to cell surface membranes through hydrophobic insertion, layer-by-layer attachment, and covalent conjugations to acquire surface modification in mammalian cells. These biomaterials generate reciprocity to enable cell-cell interactions. In this review, we highlight the different biomaterials (lipidic and polymeric)-based advanced applications for cell-surface modulation, a few cell recognition moieties, and how their interplay in cell-cell interaction. We discuss the cancer-killing efficacy of NK cells, followed by their surface engineering for cancer treatment. Ultimately, this review connects biomaterials and biologically active NK cells that play key roles in cancer immunotherapy applications.

Imaging Membrane Order and Dynamic Interactions in Living Cells with a DNA Zipper Probe

The cell membrane is a dynamic and heterogeneous structure composed of distinct sub-compartments. Within these compartments, preferential interactions occur among various lipids and proteins. Currently, it is still challenging to image these short-lived membrane complexes, especially in living cells. In this work, we present a DNA-based probe, termed "DNA Zipper", which allows the membrane order and pattern of transient interactions to be imaged in living cells using standard fluorescence microscopes. By fine-tuning the length and binding affinity of DNA duplex, these probes can precisely extend the duration of membrane lipid interactions via dynamic DNA hybridization. The correlation between membrane order and the activation of T-cell receptor signaling has also been studied. These programmable DNA probes function after a brief cell incubation, which can be easily adapted to study lipid interactions and membrane order during different membrane signaling events.

Regulation of local GTP availability controls RAC1 activity and cell invasion

Physiological changes in GTP levels in live cells have never been considered a regulatory step of RAC1 activation because intracellular GTP concentration (determined by chromatography or mass spectrometry) was shown to be substantially higher than the in vitro RAC1 GTP dissociation constant (RAC1-GTP Kd). Here, by combining genetically encoded GTP biosensors and a RAC1 activity biosensor, we demonstrated that GTP levels fluctuating around RAC1-GTP Kd correlated with changes in RAC1 activity in live cells. Furthermore, RAC1 co-localized in protrusions of invading cells with several guanylate metabolism enzymes, including rate-limiting inosine monophosphate dehydrogenase 2 (IMPDH2), which was partially due to direct RAC1-IMPDH2 interaction. Substitution of endogenous IMPDH2 with IMPDH2 mutants incapable of binding RAC1 did not affect total intracellular GTP levels but suppressed RAC1 activity. Targeting IMPDH2 away from the plasma membrane did not alter total intracellular GTP pools but decreased GTP levels in cell protrusions, RAC1 activity, and cell invasion. These data provide a mechanism of regulation of RAC1 activity by local GTP pools in live cells.

Changes in intracellular GTP levels are not considered as a regulatory event in RAC1 activation in live cells since total GTP levels are substantially higher than the RAC1 GTP dissociation constant determined in vitro. Here, the authors demonstrate that the availability of free GTP in live cells controls the activity of RAC1 and cell invasion.

Membrane anchoring facilitates colocalization of enzymes in plant cytochrome P450 redox systems

Plant metabolism depends on cascade reactions mediated by dynamic enzyme assemblies known as metabolons. In this context, the cytochrome P450 (P450) superfamily catalyze key reactions underpinning the unique diversity of bioactive compounds. In contrast to their soluble bacterial counterparts, eukaryotic P450s are anchored to the endoplasmic reticulum membrane and serve as metabolon nucleation sites. Hence, membrane anchoring appears to play a pivotal role in the evolution of complex biosynthetic pathways. Here, a model membrane assay enabled characterization of membrane anchor dynamics by single molecule microscopy. As a model system, we reconstituted the membrane anchor of cytochrome P450 oxidoreductase (POR), the ubiquitous electron donor to all microsomal P450s. The transmembrane segment in the membrane anchor of POR is relatively conserved, corroborating its functional importance. We observe dynamic colocalization of the POR anchors in our assay suggesting that membrane anchoring might promote intermolecular interactions and in this way impact assembly of metabolic multienzyme complexes.

A Decade of Advances in Single-Cell Nanocoating for Mammalian Cells

Strategic advances in the single-cell nanocoating of mammalian cells have noticeably been made during the last decade, and many potential applications have been demonstrated. Various cell-coating strategies have been proposed via adaptation of reported methods in the surface sciences and/or materials identification that ensure the sustainability of labile mammalian cells during chemical manipulation. Here an overview of the methodological development and potential applications to the healthcare sector in the nanocoating of mammalian cells made during the last decade is provided. The materials used for the nanocoating are categorized into polymers, hydrogels, polyphenolic compounds, nanoparticles, and minerals, and the corresponding strategies are described under the given set of materials. It also suggests, as a future direction, the creation of the cytospace system that is hierarchically composed of the physically separated but mutually interacting cellular hybrids.

Integrin intra-heterodimer affinity inversely correlates with integrin activatability

Integrins are heterodimeric cell surface receptors composed of an α and β subunit that mediate cell adhesion to extracellular matrix proteins such as fibronectin. We previously studied integrin α5β1 activation during zebrafish somitogenesis, and in the present study, we characterize the integrin αV fibronectin receptors. Integrins are activated via a conformational change, and we perform single-molecule biophysical measurements of both integrin activation via fluorescence resonance energy transfer (FRET)-fluorescence lifetime imaging microscopy (FLIM) and integrin intra-heterodimer stability via fluorescence cross-correlation spectroscopy (FCCS) in living embryos. We find that integrin heterodimers that exhibit robust cell surface expression, including αVβ3, αVβ5, and αVβ6, are never activated in this in vivo context, even in the presence of fibronectin matrix. In contrast, activatable integrins, such as integrin αVβ1, and alleles of αVβ3, αVβ5, αVβ6 that are biased to the active conformation exhibit poor cell surface expression and have a higher intra-heterodimer dissociation constant (KD). These observations suggest that a weak integrin intra-heterodimer affinity decreases integrin cell surface stability and increases integrin activatability.

Current Methods for Detecting Cell Membrane Transient Interactions

Short-lived cell membrane complexes play a key role in regulating cell signaling and communication. Many of these complexes are formed based on low-affinity and transient interactions among various lipids and proteins. New techniques have emerged to study these previously overlooked membrane transient interactions. Exciting functions of these transient interactions have been discovered in cellular events such as immune signaling, host-pathogen interactions, and diseases such as cancer. In this review, we have summarized current experimental methods that allow us to detect and analyze short-lived cell membrane protein-protein, lipid-protein, and lipid-lipid interactions. These methods can provide useful information about the strengths, kinetics, and/or spatial patterns of membrane transient interactions. However, each method also has its own limitations. We hope this review can be used as a guideline to help the audience to choose proper approaches for studying membrane transient interactions in different membrane trafficking and cell signaling events.

Clustering of the ζ-Chain Can Initiate T Cell Receptor Signaling

T cell activation is initiated when ligand binding to the T cell receptor (TCR) triggers intracellular phosphorylation of the TCR-CD3 complex. However, it remains unknown how biophysical properties of TCR engagement result in biochemical phosphorylation events. Here, we constructed an optogenetic tool that induces spatial clustering of ζ-chain in a light controlled manner. We showed that spatial clustering of the ζ-chain intracellular tail alone was sufficient to initialize T cell triggering including phosphorylation of ζ-chain, Zap70, PLCγ, ERK and initiated Ca²⁺ flux. In reconstituted COS-7 cells, only Lck expression was required to initiate ζ-chain phosphorylation upon ζ-chain clustering, which leads to the recruitment of tandem SH2 domain of Zap70 from cell cytosol to the newly formed ζ-chain clusters at the plasma membrane. Taken together, our data demonstrated the biophysical relevance of receptor clustering in TCR signaling.

Probing Colocalization of N-Ras and K-Ras4B Lipoproteins in Model Biomembranes

Signaling of N-Ras and K-Ras4B proteins depends strongly on their correct localization in the cell membrane. In vivo studies suggest that intermolecular interactions foster the self-association of both N-Ras and K-Ras4B and the formation of nanoclusters in the cell membrane. As sites for effector binding, nanocluster formation is thought to be essential for effective signal transmission of both N-Ras and K-Ras4B. To shed more light on the spatial arrangement and mechanism underlying the proposed cross-talk between spatially segregated Ras proteins, the simultaneous localization of N-Ras and K-Ras4B and their effect on the lateral organization of a heterogeneous model biomembrane has been studied by using AFM and FRET methodology. It is shown that, owing to the different natures of their membrane anchor systems, N-Ras and K-Ras4B not only avoid assembly in bulk solution and do not colocalize, but rather form individual nanoclusters that diffuse independently in the fluid membrane plane.

Quantifying membrane protein oligomerization with fluorescence cross-correlation spectroscopy

Fluorescence cross-correlation spectroscopy (FCCS) is an advanced fluorescence technique that can quantify protein-protein interactions in vivo. Due to the dynamic, heterogeneous nature of the membrane, special considerations must be made to interpret FCCS data accurately. In this study, we describe a method to quantify the oligomerization of membrane proteins tagged with two commonly used fluorescent probes, mCherry (mCH) and enhanced green (eGFP) fluorescent proteins. A mathematical model is described that relates the relative cross-correlation value (f_c) to the degree of oligomerization. This treatment accounts for mismatch in the confocal volumes, combinatoric effects of using two fluorescent probes, and the presence of non-fluorescent probes. Using this model, we calculate a ladder of f_c values which can be used to determine the oligomer state of membrane proteins from live-cell experimental data. Additionally, a probabilistic mathematical simulation is described to resolve the affinity of different dimeric and oligomeric protein controls.

Membrane localization and dynamics of geranylgeranylated Rab5 hypervariable region

The small GTPase Rab5 is a key regulator of endosomal trafficking processes and a marker for the early endosome. The C-terminal hypervariable region (HVR) of Rab5 is post-translationally modified at residues Cys²¹² and Cys²¹³ to accommodate two geranylgeranyl anchors (C20 carbon chain length) in order to associate Rab5 with the membrane. The structural role of the HVR regarding protein-early endosome membrane recruitment is not resolved due to its high degree of flexibility and lack of crystallographic information. Here, full-atomistic and coarse-grained molecular dynamics simulations of the truncated Rab5 HVR^206-215 in three model membranes of increasing complexity (pure phospholipid bilayer, ternary membrane with cholesterol, six-component early endosome) were performed. Specific electrostatic interactions between the HVR^206-215 Arg²⁰⁹ residue and the phosphate group of the inositol ring of PI(3)P were detected. This shows that PI(3)P acts as a first contact site of protein recruitment to the early endosome. The free energy change of HVR^206-215 extraction from the bilayer was largest for the physiological negatively charged membrane. 5μs coarse-grained simulations revealed an active recruitment of PI(3)P to the HVR^206-215 supporting the formation of Rab5- and PI(3)P enriched signaling platforms.

Dual-Functional Lipid Coating for the Nanopillar-Based Capture of Circulating Tumor Cells with High Purity and Efficiency

Clinical studies of circulating tumor cells (CTC) have stringent demands for high capture purity and high capture efficiency. Nanostructured surfaces have been shown to significantly increase the capture efficiency yet suffer from low capture purity. Here we introduce a dual-functional lipid coating on nanostructured surfaces. The lipid coating serves both as an effective passivation layer that helps prevent nonspecific cell adhesion and as a functionalized layer for antibody-based specific cell capture. In addition, the fluidity of lipid bilayers enables antibody clustering that enhances the cell-surface interaction for efficient cell capture. As a result, the lipid-coating method helps promote both the capture efficiency and capture purity of nanostructure-based CTC capture.

Plasma membrane organization and dynamics is probe and cell line dependent

The action and interaction of membrane receptor proteins take place within the plasma membrane. The plasma membrane, however, is not a passive matrix. It rather takes an active role and regulates receptor distribution and function by its composition and the interaction of its lipid components with embedded and surrounding proteins. Furthermore, it is not a homogenous fluid but contains lipid and protein domains of various sizes and characteristic lifetimes which are important in regulating receptor function and signaling. The precise lateral organization of the plasma membrane, the differences between the inner and outer leaflet, and the influence of the cytoskeleton are still debated. Furthermore, there is a lack of comparisons of the organization and dynamics of the plasma membrane of different cell types. Therefore, we used four different specific membrane markers to test the lateral organization, the differences between the inner and outer membrane leaflet, and the influence of the cytoskeleton of up to five different cell lines, including Chinese hamster ovary (CHO-K1), Human cervical carcinoma (HeLa), neuroblastoma (SH-SY5Y), fibroblast (WI-38) and rat basophilic leukemia (RBL-2H3) cells by Imaging Total Internal Reflection (ITIR)-Fluorescence Correlation Spectroscopy (FCS). We measure diffusion in the temperature range of 298-310K to measure the Arrhenius activation energy (E_Arr) of diffusion and apply the FCS diffusion law to obtain information on the spatial organization of the probe molecules on the various cell membranes. Our results show clear differences of the FCS diffusion law and E_Arr for the different probes in dependence of their localization. These differences are similar in the outer and inner leaflet of the membrane. However, these values can differ significantly between different cell lines raising the question how molecular plasma membrane events measured in different cell lines can be compared. This article is part of a Special Issue entitled: Interactions between membrane receptors in cellular membranes edited by Kalina Hristova.

Class A Plexins Are Organized as Preformed Inactive Dimers on the Cell Surface

Plexins are single-pass transmembrane receptors that bind the axon guidance molecules semaphorins. Single-pass transmembrane proteins are an important class of receptors that display a wide variety of activation mechanisms, often involving ligand-dependent dimerization or conformational changes. Resolving the activation mechanism and dimerization state of these receptors is extremely challenging, especially in a live-cell environment. Here, we report on the dimerization state of PlexinA4 and its response to activation by semaphorin binding. Semaphorins are dimeric molecules that activate plexin by binding two copies of plexin simultaneously and inducing formation of a specific active dimer of plexin. An open question is whether there are preexisting plexin dimers that could act as autoinhibitory complexes. We address these questions with pulsed interleaved excitation fluorescence cross-correlation spectroscopy (PIE-FCCS). PIE-FCCS is a two-color fluorescence microscopy method that is directly sensitive to protein dimerization in a live-cell environment. With PIE-FCCS, we show that inactive PlexinA4 is dimerized in the live-cell plasma membrane. By comparing the cross correlation of full-length PlexinA4 to control proteins and plexin mutants, we show that dimerization of inactive PlexinA4 requires the Sema domain, but not the cytoplasmic domain. Ligand stimulation with Sema6A does not change the degree of cross correlation, indicating that plexin activation does not lead to higher-order oligomerization. Together, the results suggest that semaphorin activates plexin by disrupting an inhibitory plexin dimer and inducing the active dimer.

Sensitive and Specific Biomimetic Lipid Coated Microfluidics to Isolate Viable Circulating Tumor Cells and Microemboli for Cancer Detection

Here we presented a simple and effective membrane mimetic microfluidic device with antibody conjugated supported lipid bilayer (SLB) "smart coating" to capture viable circulating tumor cells (CTCs) and circulating tumor microemboli (CTM) directly from whole blood of all stage clinical cancer patients. The non-covalently bound SLB was able to promote dynamic clustering of lipid-tethered antibodies to CTC antigens and minimized non-specific blood cells retention through its non-fouling nature. A gentle flow further flushed away loosely-bound blood cells to achieve high purity of CTCs, and a stream of air foam injected disintegrate the SLB assemblies to release intact and viable CTCs from the chip. Human blood spiked cancer cell line test showed the ~95% overall efficiency to recover both CTCs and CTMs. Live/dead assay showed that at least 86% of recovered cells maintain viability. By using 2 mL of peripheral blood, the CTCs and CTMs counts of 63 healthy and colorectal cancer donors were positively correlated with the cancer progression. In summary, a simple and effective strategy utilizing biomimetic principle was developed to retrieve viable CTCs for enumeration, molecular analysis, as well as ex vivo culture over weeks. Due to the high sensitivity and specificity, it is the first time to show the high detection rates and quantity of CTCs in non-metastatic cancer patients. This work offers the values in both early cancer detection and prognosis of CTC and provides an accurate non-invasive strategy for routine clinical investigation on CTCs.

With or without rafts? Alternative views on cell membranes

The fundamental mechanisms of protein and lipid organization at the plasma membrane have continued to engage researchers for decades. Among proposed models, one idea has been particularly successful which assumes that sterol-dependent nanoscopic phases of different lipid chain order compartmentalize proteins, thereby modulating protein functionality. This model of membrane rafts has sustainably sparked the fields of membrane biophysics and biology, and shifted membrane lipids into the spotlight of research; by now, rafts have become an integral part of our terminology to describe a variety of cell biological processes. But is the evidence clear enough to continue supporting a theoretical concept which has resisted direct proof by observation for nearly twenty years? In this essay, we revisit findings that gave rise to and substantiated the raft hypothesis, discuss its impact on recent studies, and present alternative mechanisms to account for plasma membrane heterogeneity.

Detection of rhodopsin dimerization in situ by PIE-FCCS, a time-resolved fluorescence spectroscopy

Rhodopsin self-associates in the plasma membrane. At low concentrations, the interactions are consistent with a monomer-dimer equilibrium (Comar et al., J Am Chem Soc 136(23):8342-8349, 2014). At high concentrations in native tissue, higher-order clusters have been observed (Fotiadis et al., Nature 421:127-128, 2003). The physiological role of rhodopsin dimerization is still being investigated, but it is clear that a quantitative assessment is essential to determining the function of rhodopsin clusters in vision. To quantify rhodopsin interactions, I will outline the theory and methodology of a specialized time-resolved fluorescence spectroscopy for measuring membrane protein-protein interactions called pulsed-interleaved excitation fluorescence cross-correlation spectroscopy (PIE-FCCS). The strength of this technique is its ability to quantify rhodopsin interactions in situ (i.e., a live cell plasma membrane). There are two reasons for restricting the scope to live cell membranes. First, the compositional heterogeneity of the plasma membrane creates a complex milieu with thousands of lipid, protein, and carbohydrate species. This makes it difficult to infer quaternary interactions from detergent solubilized samples or construct a model phospholipid bilayer that recapitulates all of the interactions present in native membranes. Second, organizational structure and dynamics is a key feature of the plasma membrane, and fixation techniques like formaldehyde cross-linking and vitrification will modulate the interactions. PIE-FCCS is based on two-color fluorescence imaging with time-correlated single-photon counting (TCSPC) (Becker et al., Rev Sci Instrum 70:1835-1841, 1999). By time-tagging every detected photon, the data can be analyzed as a fluorescence intensity distribution, fluorescence lifetime histogram, or fluorescence (cross-)correlation spectra (FCS/FCCS) (Becker, Advanced time-correlated single-photon counting techniques, Springer, Berlin, 2005). These analysis tools can then be used to quantify protein concentration, mobility, clustering, and Förster resonance energy transfer (FRET). In this paper I will focus on PIE-FCCS, which interleaves two wavelength excitation events in time so that the effects of spectral cross-talk and FRET can be isolated. In this way it is possible to characterize monomer-dimer-oligomer equilibria with high accuracy (Müller et al., Biophys J 89:3508-3522, 2005). Currently, PIE-FCCS requires a customized equipment configuration that will be described below. There is an excellent protocol that outlines traditional FCCS on a commercially available instrument (Bacia and Schwille, Nat Protoc 2:2842-2856, 2007). The PIE-FCCS approach is a relatively recent advance in FCCS that has been used in live cell assays to quantify lipid-anchored protein clustering (Triffo et al., J Am Chem Soc 134:10833-10842, 2012), epidermal growth factor receptor dimerization (Endres et al., Cell 152:543-556, 2013), and recently the dimerization of opsin (Comar et al., J Am Chem Soc 136(23):8342-8349, 2014). This paper will outline the theory and instrumentation requirements for PIE-FCCS, as well as the data collection and analysis process.

Current approaches to studying membrane organization

The local structure and composition of the outer membrane of an animal cell are important factors in the control of many membrane processes and mechanisms. These include signaling, sorting, and exo- and endocytic processes that are occurring all the time in a living cell. Paradoxically, not only are the local structure and composition of the membrane matters of much debate and discussion, the mechanisms that govern its genesis remain highly controversial. Here, we discuss a swathe of new technological advances that may be applied to understand the local structure and composition of the membrane of a living cell from the molecular scale to the scale of the whole membrane.

Diffusion of GPI-anchored proteins is influenced by the activity of dynamic cortical actin

Membrane proteins that couple to cortical actin show temperature-independent diffusion. The loss of this coupling and perturbation of cortical actomyosin dynamics render the diffusion temperature dependent. The findings suggest that active fluctuations arising from dynamic actin filaments at the cortex can drive diffusion on the cell membrane.

Molecular diffusion at the surface of living cells is believed to be predominantly driven by thermal kicks. However, there is growing evidence that certain cell surface molecules are driven by the fluctuating dynamics of cortical cytoskeleton. Using fluorescence correlation spectroscopy, we measure the diffusion coefficient of a variety of cell surface molecules over a temperature range of 24–37°C. Exogenously incorporated fluorescent lipids with short acyl chains exhibit the expected increase of diffusion coefficient over this temperature range. In contrast, we find that GPI-anchored proteins exhibit temperature-independent diffusion over this range and revert to temperature-dependent diffusion on cell membrane blebs, in cells depleted of cholesterol, and upon acute perturbation of actin dynamics and myosin activity. A model transmembrane protein with a cytosolic actin-binding domain also exhibits the temperature-independent behavior, directly implicating the role of cortical actin. We show that diffusion of GPI-anchored proteins also becomes temperature dependent when the filamentous dynamic actin nucleator formin is inhibited. However, changes in cortical actin mesh size or perturbation of branched actin nucleator Arp2/3 do not affect this behavior. Thus cell surface diffusion of GPI-anchored proteins and transmembrane proteins that associate with actin is driven by active fluctuations of dynamic cortical actin filaments in addition to thermal fluctuations, consistent with expectations from an “active actin-membrane composite” cell surface.

E-cadherin junction formation involves an active kinetic nucleation process

Epithelial (E)-cadherin-mediated cell-cell junctions play important roles in the development and maintenance of tissue structure in multicellular organisms. E-cadherin adhesion is thus a key element of the cellular microenvironment that provides both mechanical and biochemical signaling inputs. Here, we report in vitro reconstitution of junction-like structures between native E-cadherin in living cells and the extracellular domain of E-cadherin (E-cad-ECD) in a supported membrane. Junction formation in this hybrid live cell-supported membrane configuration requires both active processes within the living cell and a supported membrane with low E-cad-ECD mobility. The hybrid junctions recruit α-catenin and exhibit remodeled cortical actin. Observations suggest that the initial stages of junction formation in this hybrid system depend on the trans but not the cis interactions between E-cadherin molecules, and proceed via a nucleation process in which protrusion and retraction of filopodia play a key role.

Cross-Scale Integrin Regulation Organizes ECM and Tissue Topology

The diverse morphologies of animal tissues are underlain by different configurations of adherent cells and extracellular matrix (ECM). Here, we elucidate a cross-scale mechanism for tissue assembly and ECM remodeling involving Cadherin 2, the ECM protein Fibronectin, and its receptor Integrin α5. Fluorescence cross-correlation spectroscopy within the zebrafish paraxial mesoderm mesenchyme reveals a physical association between Integrin α5 on adjacent cell membranes. This Integrin-Integrin complex correlates with conformationally inactive Integrin. Cadherin 2 stabilizes both the Integrin association and inactive Integrin conformation. Thus, Integrin repression within the adherent mesenchymal interior of the tissue biases Fibronectin fibrillogenesis to the tissue surface lacking cell-cell adhesions. Along nascent somite boundaries, Cadherin 2 levels decrease, becoming anti-correlated with levels of Integrin α5. Simultaneously, Integrin α5 clusters and adopts the active conformation and then commences ECM assembly. This cross-scale regulation of Integrin activation organizes a stereotypic pattern of ECM necessary for vertebrate body elongation and segmentation.

Live cell plasma membranes do not exhibit a miscibility phase transition over a wide range of temperatures

Lipid/cholesterol mixtures derived from cell membranes as well as their synthetic reconstitutions exhibit well-defined miscibility phase transitions and critical phenomena near physiological temperatures. This suggests that lipid/cholesterol-mediated phase separation plays a role in the organization of live cell membranes. However, macroscopic lipid-phase separation is not generally observed in cell membranes, and the degree to which properties of isolated lipid mixtures are preserved in the cell membrane remain unknown. A fundamental property of phase transitions is that the variation of tagged particle diffusion with temperature exhibits an abrupt change as the system passes through the transition, even when the two phases are distributed in a nanometer-scale emulsion. We support this using a variety of Monte Carlo and atomistic simulations on model lipid membrane systems. However, temperature-dependent fluorescence correlation spectroscopy of labeled lipids and membrane-anchored proteins in live cell membranes shows a consistently smooth increase in the diffusion coefficient as a function of temperature. We find no evidence of a discrete miscibility phase transition throughout a wide range of temperatures: 14-37 °C. This contrasts the behavior of giant plasma membrane vesicles (GPMVs) blebbed from the same cells, which do exhibit phase transitions and macroscopic phase separation. Fluorescence lifetime analysis of a DiI probe in both cases reveals a significant environmental difference between the live cell and the GPMV. Taken together, these data suggest the live cell membrane may avoid the miscibility phase transition inherent to its lipid constituents by actively regulating physical parameters, such as tension, in the membrane.

The structural role of cholesterol in cell membranes: from condensed bilayers to lipid rafts

CONSPECTUS: Defining the two-dimensional structure of cell membranes represents one of the most daunting challenges currently facing chemists, biochemists, and biophysicists. In particular, the time-averaged lateral organization of the lipids and proteins that make up these natural enclosures has yet to be established. As the classic Singer-Nicolson model of cell membranes has evolved over the past 40 years, special attention has focused on the structural role played by cholesterol, a key component that represents ca. 30% of the total lipids that are present. Despite extensive studies with model membranes, two fundamental issues have remained a mystery: (i) the mechanism by which cholesterol condenses low-melting lipids by uncoiling their acyl chains and (ii) the thermodynamics of the interaction between cholesterol and high- and low-melting lipids. The latter bears directly on one of the most popular notions in modern cell biology, that is, the lipid raft hypothesis, whereby cholesterol is thought to combine with high-melting lipids to form "lipid rafts" that float in a "sea" of low-melting lipids. In this Account, we first describe a chemical approach that we have developed in our laboratories that has allowed us to quantify the interactions between exchangeable mimics of cholesterol and low- and high-melting lipids in model membranes. In essence, this "nearest-neighbor recognition" (NNR) method involves the synthesis of dimeric forms of these lipids that contain a disulfide moiety as a linker. By means of thiolate-disulfide interchange reactions, equilibrium mixtures of dimers are then formed. These exchange reactions are initiated either by adding dithiothreitol to a liposomal dispersion to generate a small amount of thiol monomer or by including a small amount of thiol monomer in the liposomes at pH 5.0 and then raising the pH to 7.4. We then show how such NNR measurements have allowed us to distinguish between two very different mechanisms that have been proposed for cholesterol's condensing effect: (i) an umbrella mechanism in which the acyl chains and cholesterol become more tightly packed as cholesterol content increases because they share limited space under phospholipid headgroups and (ii) a template mechanism whereby cholesterol functions as a planar hydrophobic template at the membrane surface, thereby maximizing hydrophobic interactions and the hydrophobic effect. Specifically, our NNR experiments rule out the umbrella mechanism and provide strong support for the template mechanism. Similar NNR measurements have also allowed us to address the question of whether the interactions between low-melting kinked phospholipids and cholesterol can play a significant role in the formation of lipid rafts. Specifically, these NNR measurements have led to our discovery of a new physical principle in the lipids and membranes area that must be operating in biological membranes, that is, a "push-pull" mechanism, whereby cholesterol is pushed away from low-melting phospholipids and pulled toward high-melting lipids. Thus, to the extent that lipid rafts play a role in the functioning of cell membranes, low-melting phospholipids must be active participants.

Detecting protein association at the T cell plasma membrane

At the moment, many models on T cell signaling rely on results obtained via rather indirect methodologies, which makes direct comparison and conclusions to the in vivo situation difficult. Recently, a variety of new imaging methods were developed, which have the potential to directly shed light onto the mysteries of protein association at the T cell membrane. While the new modalities are extremely promising, for a broad readership it may be difficult to judge the results, since technological shortcomings are not always obvious. In this review article, we put key questions on the mechanism of protein interactions in the T cell plasma membrane into relation with techniques that allow to address such questions. We discuss applicability of the techniques, their strengths and weaknesses. This article is part of a Special Issue entitled: Nanoscale membrane organisation and signalling.

Efficient elusion of viable adhesive cells from a microfluidic system by air foam

We developed a new method for releasing viable cells from affinity-based microfluidic devices. The lumen of a microchannel with a U-shape and user-designed microstructures was coated with supported lipid bilayers functionalized by epithelial cell adhesion molecule antibodies to capture circulating epithelial cells of influx solution. After the capturing process, air foam was introduced into channels for releasing target cells and then carrying them to a small area of membrane. The results show that when the air foam is driven at linear velocity of 4.2 mm/s for more than 20 min or at linear velocity of 8.4 mm/s for more than 10 min, the cell releasing efficiency approaches 100%. This flow-induced shear stress is much less than the physiological level (15 dyn/cm(2)), which is necessary to maintain the intactness of released cells. Combining the design of microstructures of the microfluidic system, the cell recovery on the membrane exceeds 90%. Importantly, we demonstrate that the cells released by air foam are viable and could be cultured in vitro. This novel method for releasing cells could power the microfluidic platform for isolating and identifying circulating tumor cells.

Molecular kinetics. Ras activation by SOS: allosteric regulation by altered fluctuation dynamics

Activation of the small guanosine triphosphatase H-Ras by the exchange factor Son of Sevenless (SOS) is an important hub for signal transduction. Multiple layers of regulation, through protein and membrane interactions, govern activity of SOS. We characterized the specific activity of individual SOS molecules catalyzing nucleotide exchange in H-Ras. Single-molecule kinetic traces revealed that SOS samples a broad distribution of turnover rates through stochastic fluctuations between distinct, long-lived (more than 100 seconds), functional states. The expected allosteric activation of SOS by Ras-guanosine triphosphate (GTP) was conspicuously absent in the mean rate. However, fluctuations into highly active states were modulated by Ras-GTP. This reveals a mechanism in which functional output may be determined by the dynamical spectrum of rates sampled by a small number of enzymes, rather than the ensemble average.

Time-resolved fluorescence spectroscopy measures clustering and mobility of a G protein-coupled receptor opsin in live cell membranes

Determining membrane protein quaternary structure is extremely challenging, especially in live cell membranes. We measured the oligomerization of opsin, a prototypical G protein-coupled receptor with pulsed-interleaved excitation fluorescence cross-correlation spectroscopy (PIE-FCCS). Individual cell measurements revealed that opsin is predominantly organized into dimeric clusters. At low concentrations, we observed that the population of oligomers increased linearly with the square of the individual monomer populations. This finding supports a monomer-dimer equilibrium and provides an experimental measurement of the equilibrium constant.

Spatial organization of EphA2 at the cell-cell interface modulates trans-endocytosis of ephrinA1

EphA2 is a receptor tyrosine kinase (RTK) that is sensitive to spatial and mechanical aspects of the cell's microenvironment. Misregulation of EphA2 occurs in many aggressive cancers. Although its juxtacrine signaling geometry (EphA2's cognate ligand ephrinA1 is expressed on the surface of an apposing cell) provides a mechanism by which the receptor may experience extracellular forces, this also renders the system challenging to decode. By depositing living cells on synthetic supported lipid membranes displaying ephrinA1, we have reconstituted key features of the juxtacrine EphA2-ephrinA1 signaling system while maintaining the ability to perturb the spatial and mechanical properties of the membrane-cell interface with precision. In addition, we developed a trans-endocytosis assay to monitor internalization of ephrinA1 from a supported membrane into the apposing cell using a quantitative three-dimensional fluorescence microscopy assay. Using this experimental platform to mimic a cell-cell junction, we found that the signaling complex is not efficiently internalized when lateral reorganization at the membrane-cell contact sites is physically hindered. This suggests that EphA2-ephrinA1 trans-endocytosis is sensitive to the mechanical properties of a cell's microenvironment and may have implications in physical aspects of tumor biology.

Recent applications of fluorescence correlation spectroscopy in live systems

Fluorescence correlation spectroscopy (FCS) is a widely used technique in biophysics and has helped address many questions in the life sciences. It provides important advantages compared to other fluorescence and biophysical methods. Its single molecule sensitivity allows measuring proteins within biological samples at physiological concentrations without the need of overexpression. It provides quantitative data on concentrations, diffusion coefficients, molecular transport and interactions even in live organisms. And its reliance on simple fluorescence intensity and its fluctuations makes it widely applicable. In this review we focus on applications of FCS in live samples, with an emphasis on work in the last 5 years, in the hope to provide an overview of the present capabilities of FCS to address biologically relevant questions.

H-Ras forms dimers on membrane surfaces via a protein-protein interface

The lipid-anchored small GTPase Ras is an important signaling node in mammalian cells. A number of observations suggest that Ras is laterally organized within the cell membrane, and this may play a regulatory role in its activation. Lipid anchors composed of palmitoyl and farnesyl moieties in H-, N-, and K-Ras are widely suspected to be responsible for guiding protein organization in membranes. Here, we report that H-Ras forms a dimer on membrane surfaces through a protein-protein binding interface. A Y64A point mutation in the switch II region, known to prevent Son of sevenless and PI3K effector interactions, abolishes dimer formation. This suggests that the switch II region, near the nucleotide binding cleft, is either part of, or allosterically coupled to, the dimer interface. By tethering H-Ras to bilayers via a membrane-miscible lipid tail, we show that dimer formation is mediated by protein interactions and does not require lipid anchor clustering. We quantitatively characterize H-Ras dimerization in supported membranes using a combination of fluorescence correlation spectroscopy, photon counting histogram analysis, time-resolved fluorescence anisotropy, single-molecule tracking, and step photobleaching analysis. The 2D dimerization Kd is measured to be ∼1 × 10(3) molecules/µm(2), and no higher-order oligomers were observed. Dimerization only occurs on the membrane surface; H-Ras is strictly monomeric at comparable densities in solution. Analysis of a number of H-Ras constructs, including key changes to the lipidation pattern of the hypervariable region, suggest that dimerization is a general property of native H-Ras on membrane surfaces.

DNA-mediated assembly of protein heterodimers on membrane surfaces

We present a method based on self-assembling oligonucleotides to anchor proteins to a supported membrane surface. This anchoring method allows control of the surface density of multiple proteins. By incorporating additional recognition sequences into the DNA linkers, defined heterodimers can be produced upon the addition of a heterospecific DNA cross-linking strand. Characterization by fluorescence cross-correlation spectroscopy (FCCS) confirmed lateral mobility and the formation of specific heterodimers. We further demonstrate that proteins linked in this manner as either monomers or dimers can form functional interfaces with living cells.

A critical survey of methods to detect plasma membrane rafts

The plasma membrane is still one of the enigmatic cellular structures. Although the microscopic structure is getting clearer, not much is known about the organization at the nanometre level. Experimental difficulties have precluded unambiguous approaches, making the current picture rather fuzzy. In consequence, a variety of different membrane models has been proposed over the years, on the basis of different experimental strategies. Recent data obtained via high-resolution single-molecule microscopy shed new light on the existing hypotheses. We thus think it is a good time for reviewing the consistency of the existing models with the new data. In this paper, we summarize the available models in ten propositions, each of which is discussed critically with respect to the applied technologies and the strengths and weaknesses of the approaches. Our aim is to provide the reader with a sound basis for his own assessment. We close this chapter by exposing our picture of the membrane organization at the nanoscale.

Sorting of lipidated peptides in fluid bilayers: a molecular-level investigation

Nearest-neighbor recognition (NNR) measurements have been made for two lipidated forms of GlyCys, interacting with analogues of cholesterol and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in the liquid-ordered (l(o)) and liquid-disordered (l(d)) phases. Interaction free energies that have been determined from these measurements have been used in Monte Carlo simulations to quantify the distribution of the peptides between liquid-ordered and liquid-disordered regions. These simulations have shown that significant differences in the lipid chains have a very weak influence on the partitioning of the peptide between these two phases. They have also revealed an insensitivity of the peptide partition coefficient, K(p), to the size of the l(o) and l(d) domains that are present. In a broader context, these findings strongly suggest that the sorting of peripheral proteins in cellular membranes via differential lipidation may be more subtle than previously thought.