Ligand-induced partitioning of human CXCR1 chemokine receptors with lipid raft microenvironments facilitates G-protein-dependent signaling

The Regulation of Neutrophil Migration in Patients with Sepsis: The Complexity of the Molecular Mechanisms and Their Modulation in Sepsis and the Heterogeneity of Sepsis Patients

Sepsis is defined as life-threatening organ dysfunction caused by a dysregulated host response to infection. Common causes include gram-negative and gram-positive bacteria as well as fungi. Neutrophils are among the first cells to arrive at an infection site where they function as important effector cells of the innate immune system and as regulators of the host immune response. The regulation of neutrophil migration is therefore important both for the infection-directed host response and for the development of organ dysfunctions in sepsis. Downregulation of CXCR4/CXCL12 stimulates neutrophil migration from the bone marrow. This is followed by transmigration/extravasation across the endothelial cell barrier at the infection site; this process is directed by adhesion molecules and various chemotactic gradients created by chemotactic cytokines, lipid mediators, bacterial peptides, and peptides from damaged cells. These mechanisms of neutrophil migration are modulated by sepsis, leading to reduced neutrophil migration and even reversed migration that contributes to distant organ failure. The sepsis-induced modulation seems to differ between neutrophil subsets. Furthermore, sepsis patients should be regarded as heterogeneous because neutrophil migration will possibly be further modulated by the infecting microorganisms, antimicrobial treatment, patient age/frailty/sex, other diseases (e.g., hematological malignancies and stem cell transplantation), and the metabolic status. The present review describes molecular mechanisms involved in the regulation of neutrophil migration; how these mechanisms are altered during sepsis; and how bacteria/fungi, antimicrobial treatment, and aging/frailty/comorbidity influence the regulation of neutrophil migration.

Leronlimab, a humanized monoclonal antibody to CCR5, blocks breast cancer cellular metastasis and enhances cell death induced by DNA damaging chemotherapy

Background

Triple-negative breast cancer (BCa) (TNBC) is a deadly form of human BCa with limited treatment options and poor prognosis. In our prior analysis of over 2200 breast cancer samples, the G protein-coupled receptor CCR5 was expressed in > 95% of TNBC samples. A humanized monoclonal antibody to CCR5 (leronlimab), used in the treatment of HIV-infected patients, has shown minimal side effects in large patient populations.

Methods

A humanized monoclonal antibody to CCR5, leronlimab, was used for the first time in tissue culture and in mice to determine binding characteristics to human breast cancer cells, intracellular signaling, and impact on (i) metastasis prevention and (ii) impact on established metastasis.

Results

Herein, leronlimab was shown to bind CCR5 in multiple breast cancer cell lines. Binding of leronlimab to CCR5 reduced ligand-induced Ca^+ 2 signaling, invasion of TNBC into Matrigel, and transwell migration. Leronlimab enhanced the BCa cell killing of the BCa chemotherapy reagent, doxorubicin. In xenografts conducted with Nu/Nu mice, leronlimab reduced lung metastasis of the TNBC cell line, MB-MDA-231, by > 98% at 6 weeks. Treatment with leronlimab reduced the metastatic tumor burden of established TNBC lung metastasis.

Conclusions

The safety profile of leronlimab, together with strong preclinical evidence to both prevent and reduce established breast cancer metastasis herein, suggests studies of clinical efficacy may be warranted.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13058-021-01391-1.

A sensitive S-Trap-based approach to the analysis of T cell lipid raft proteome

The analysis of T cell lipid raft proteome is challenging due to the highly dynamic nature of rafts and the hydrophobic character of raft-resident proteins. We explored an innovative strategy for bottom-up lipid raftomics based on suspension-trapping (S-Trap) sample preparation. Mouse T cells were prepared from splenocytes by negative immunoselection, and rafts were isolated by a detergent-free method and OptiPrep gradient ultracentrifugation. Microdomains enriched in flotillin-1, LAT, and cholesterol were subjected to proteomic analysis through an optimized protocol based on S-Trap and high pH fractionation, followed by nano-LC-MS/MS. Using this method, we identified 2,680 proteins in the raft-rich fraction and established a database of 894 T cell raft proteins. We then performed a differential analysis on the raft-rich fraction from nonstimulated versus anti-CD3/CD28 T cell receptor (TCR)-stimulated T cells. Our results revealed 42 proteins present in one condition and absent in the other. For the first time, we performed a proteomic analysis on rafts from ex vivo T cells obtained from individual mice, before and after TCR activation. This work demonstrates that the proposed method utilizing an S-Trap-based approach for sample preparation increases the specificity and sensitivity of lipid raftomics.

The membrane-associated form of cyclin D1 enhances cellular invasion

The essential G₁-cyclin, CCND1, is a collaborative nuclear oncogene that is frequently overexpressed in cancer. D-type cyclins bind and activate CDK4 and CDK6 thereby contributing to G₁–S cell-cycle progression. In addition to the nucleus, herein cyclin D1 was also located in the cytoplasmic membrane. In contrast with the nuclear-localized form of cyclin D1 (cyclin D1^NL), the cytoplasmic membrane-localized form of cyclin D1 (cyclin D1^MEM) induced transwell migration and the velocity of cellular migration. The cyclin D1^MEM was sufficient to induce G₁–S cell-cycle progression, cellular proliferation, and colony formation. The cyclin D1^MEM was sufficient to induce phosphorylation of the serine threonine kinase Akt (Ser473) and augmented extranuclear localized 17β-estradiol dendrimer conjugate (EDC)-mediated phosphorylation of Akt (Ser473). These studies suggest distinct subcellular compartments of cell cycle proteins may convey distinct functions.

Mechanobiology predicts raft formations triggered by ligand-receptor activity across the cell membrane

Clustering of ligand-binding receptors of different types on thickened isles of the cell membrane, namely lipid rafts, is an experimentally observed phenomenon. Although its influence on cell's response is deeply investigated, the role of the coupling between mechanical processes and multiphysics involving the active receptors and the surrounding lipid membrane during ligand-binding has not yet been understood. Specifically, the focus of this work is on G-protein-coupled receptors (GPCRs), the widest group of transmembrane proteins in animals, which regulate specific cell processes through chemical signalling pathways involving a synergistic balance between the cyclic Adenosine Monophosphate (cAMP) produced by active GPCRs in the intracellular environment and its efflux, mediated by the Multidrug Resistance Proteins (MRPs) transporters. This paper develops a multiphysics approach based on the interplay among energetics, multiscale geometrical changes and mass balance of species, i.e. active GPCRs and MRPs, including diffusion and kinetics of binding and unbinding. Because the obtained energy depends upon both the kinematics and the changes of species densities, balance of mass and of linear momentum are coupled and govern the space-time evolution of the cell membrane. The mechanobiology involving remodelling and change of lipid ordering of the cell membrane allows to predict dynamics of transporters and active receptors -in full agreement with experimentally observed cAMP levels- and how the latter trigger rafts formation and cluster on such sites. Within the current scientific debate on Severe Acute Respiratory Syndrome CoronaVirus 2 (SARS-CoV-2) and on the basis of the ascertained fact that lipid rafts often serve as an entry port for viruses, it is felt that approaches accounting for strong coupling among mechanobiological aspects could even turn helpful in better understanding membrane-mediated phenomena such as COVID-19 virus-cell interaction.

G-protein-coupled formyl peptide receptors play a dual role in neutrophil chemotaxis and bacterial phagocytosis

A dogma of innate immunity is that neutrophils use G-protein–coupled receptors (GPCRs) for chemoattractant to chase bacteria through chemotaxis and then use phagocytic receptors coupled with tyrosine kinases to destroy opsonized bacteria via phagocytosis. Our current work showed that G-protein–coupled formyl peptide receptors (FPRs) directly mediate neutrophil phagocytosis. Mouse neutrophils lacking formyl peptide receptors (Fpr1/2^–/–) are defective in the phagocytosis of Escherichia coli and the chemoattractant N-formyl-Met-Leu-Phe (fMLP)-coated beads. fMLP immobilized onto the surface of a bead interacts with FPRs, which trigger a Ca²⁺ response and induce actin polymerization to form a phagocytic cup for engulfment of the bead. This chemoattractant GPCR/Gi signaling works independently of phagocytic receptor/tyrosine kinase signaling to promote phagocytosis. Thus, in addition to phagocytic receptor-mediated phagocytosis, neutrophils also utilize the chemoattractant GPCR/Gi signaling to mediate phagocytosis to fight against invading bacteria.

Cannabinoid Receptor 2 Signalling Bias Elicited by 2,4,6-Trisubstituted 1,3,5-Triazines

Cannabinoid receptor 2 (CB2) is predominantly distributed in immune tissues and cells and is a promising therapeutic target for modulating inflammation. In this study we designed and synthesised a series of 2,4,6-trisubstituted 1,3,5-triazines with piperazinylalkyl or 1,2-diethoxyethane (PEG2) chains as CB2 agonists, all of which were predicted to be considerably more polar than typical cannabinoid ligands. In this series, we found that triazines containing an adamantanyl group were conducive to CB2 binding whereas those with a cyclopentyl group were not. Although the covalent attachment of a PEG2 linker to the adamantyl triazines resulted in a decrease in binding affinity, some of the ligands produced very interesting hCB2 signalling profiles. Six compounds with notable hCB2 orthosteric binding were functionally characterised in three pathways; internalisation, cyclic adenosine monophosphate (cAMP) and ERK phosphorylation (pERK). These were predominantly confirmed to be hCB2 agonists, and upon comparison to a reference ligand (CP 55,940), four compounds exhibited signalling bias. Triazines 14 (UOSD017) and 15 were biased towards internalisation over cAMP and pERK, and 7 was biased away from pERK activation relative to cAMP and internalisation. Intriguingly, the triazine with an amino-PEG2-piperazinyl linker (13 [UOSD008]) was identified to be a mixed agonist/inverse agonist, exhibiting apparent neutral antagonism in the internalisation pathway, transient inverse agonism in the cAMP pathway and weak partial agonism in the pERK pathway. Both the cAMP and pERK signalling were pertussis toxin (PTX) sensitive, implying that 13 is acting as both a weak agonist and inverse agonist at CB2 via Gαi/o. Compound 10 (UOSD015) acted as a balanced high intrinsic efficacy agonist with the potential to produce greater hCB2-mediated efficacy than reference ligand CP 55,940. As 10 includes a Boc-protected PEG2 moiety it is also a promising candidate for further modification, for example with a secondary reporter or fluorophore. The highest affinity compound in this set of relatively polar hCB2 ligands was compound 16, which acted as a slightly partial balanced agonist in comparison with CP 55,940. The ligands characterised here may therefore exhibit unique functional properties in vivo and have the potential to be valuable in the future development of CB2-directed therapeutics.

Pancreatic islet inflammation: an emerging role for chemokines

Both type 1 and type 2 diabetes exhibit features of inflammation associated with alterations in pancreatic islet function and mass. These immunological disruptions, if unresolved, contribute to the overall pathogenesis of disease onset. This review presents the emerging role of pancreatic islet chemokine production as a critical factor regulating immune cell entry into pancreatic tissue as well as an important facilitator of changes in tissue resident leukocyte activity. Signaling through two specific chemokine receptors (i.e., CXCR2 and CXCR3) is presented to illustrate key points regarding ligand-mediated regulation of innate and adaptive immune cell responses. The prospective roles of chemokine ligands and their corresponding chemokine receptors to influence the onset and progression of autoimmune- and obesity-associated forms of diabetes are discussed.

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.

Clostridium difficile binary toxin CDT induces clustering of the lipolysis-stimulated lipoprotein receptor into lipid rafts

Clostridium difficile is the leading cause of antibiotics-associated diarrhea and pseudomembranous colitis. Hypervirulent C. difficile strains produce the binary actin-ADP-ribosylating toxin CDT (C. difficile transferase), in addition to the Rho-glucosylating toxins A and B. We recently identified the lipolysis-stimulated lipoprotein receptor (LSR) as the host receptor that mediates uptake of CDT into target cells. Here we investigated in H1-HeLa cells, which ectopically express LSR, the influence of CDT on the plasma membrane distribution of the receptor. We found by fluorescence microscopy that the binding component of CDT (CDTb) induces clustering of LSR into subcompartments of the plasma membrane. Detergent extraction of cells treated with CDTb, followed by sucrose gradient fractionation, uncovered accumulation of LSR in detergent-resistant membranes (DRMs) that contained typical marker proteins of lipid rafts. Membrane cholesterol depletion with methyl-β-cyclodextrin inhibited the association of LSR with DRMs upon addition of CDTb. The receptor-binding domain of CDTb also triggered LSR clustering into DRMs. CDTb-triggered clustering of LSR into DRMs could be confirmed in Caco-2 cells. Our data suggest that CDT forces its receptor to cluster into lipid rafts and that oligomerization of the B component might enhance but is not essential for this process.

C. difficile binary toxin CDT is a member of the iota-like, actin ADP-ribosylating toxin family. The mechanism that mediates endocytic uptake of these toxins still remains elusive. Previous studies highlighted the importance of lipid rafts for oligomerization of the binding component of these toxins and for cell entry. Recently, the host cell receptor for this toxin family, namely, the lipolysis-stimulated lipoprotein receptor (LSR), has been identified. Our study now demonstrates that the binding component of CDT (CDTb) induces clustering of LSR into lipid rafts. Importantly, LSR clustering is efficiently induced also by the receptor-binding domain of CDTb, suggesting that oligomerization of the B component of CDT is not the main trigger of this process. The current work extends our knowledge on the cooperative play between iota-like toxins and their receptor.

Heterodimerization with the prostacyclin receptor triggers thromboxane receptor relocation to lipid rafts

OBJECTIVE

Prostacyclin and thromboxane mediate opposing cardiovascular actions through receptors termed IP and TP, respectively. When dimerized with IP, the TP shifts to IP-like function. IP localizes to cholesterol-enriched membrane rafts, but TP and IPTP heterodimer localization is not defined. We examined these receptors' membrane localization and the role of rafts in receptor function.

METHODS AND RESULTS

Microdomain distribution of IP, TP, and IPTP heterodimers was examined in COS-7 cells by measuring energy transfer from renilla luciferase-fused receptors to fluorescently labeled rafts. IP raft association was confirmed. TP was raft excluded, but redistributed to rafts upon dimerization with IP. Signaling of the IP and IPTP heterodimer, but not TP alone, was suppressed after raft disruption by cholesterol depletion. Cholesterol enrichment also selectively suppressed IP and IPTP function. Native IP and IPTP signaling in smooth muscle cells and macrophages were similarly sensitive to cholesterol manipulation, whereas macrophages from hypercholesterolemic mice displayed suppressed IP and IPTP function.

CONCLUSIONS

IP and TP function within distinct microdomains. Raft incorporation of TP in the IPTP heterodimer likely facilitates its signaling shift. We speculate that changes in IP and IPTP signaling after perturbation of membrane cholesterol may contribute to cardiovascular disease associated with hypercholesterolemia.

Leu128(3.43) (l128) and Val247(6.40) (V247) of CXCR1 are critical amino acid residues for g protein coupling and receptor activation

CXCR1, a classic GPCR that binds IL-8, plays a key role in neutrophil activation and migration by activating phospholipase C (PLC)β through Gα₁₅ and Gα_i which generates diacylglycerol and inositol phosphates (IPs). In this study, two conserved amino acid residues of CXCR1 on the transmembrane domain (TM) 3 and TM6, Leu128^3.43 (L128) and Val247^6.40 (V247), respectively, were selectively substituted with other amino acids to investigate the role of these conserved residues in CXCR1 activation. Although two selective mutants on Leu128, Leu128Ala (L128A) and Leu128Arg (L128R), demonstrated high binding affinity to IL-8, they were not capable of coupling to G proteins and consequently lost the functional response of the receptors. By contrast, among the four mutants at residue Val247 (TM6.40), replacing Val247 with Ala (V247A) and Asn (V247N) led to constitutive activation of mutant receptors when cotransfected with Gα₁₅. The V247N mutant also constitutively activated the Gα_i protein. These results indicate that L128 on TM3.43 is involved in G protein coupling and receptor activation but is unimportant for ligand binding. On the other hand, V247 on TM6.40 plays a critical role in maintaining the receptor in the inactive state, and the substitution of V247 impaired the receptor constraint and stabilized an active conformation. Functionally, there was an increase in chemotaxis in response to IL-8 in cells expressing V247A and V247N. Our findings indicate that Leu128^3.43 and Val247^6.40 are critical for G protein coupling and activation of signaling effectors, providing a valuable insight into the mechanism of CXCR1 activation.

The interaction with H-2D(d) in cis is associated with a conformational change in the Ly49A NK cell receptor

Mouse natural killer (NK) cells express Ly49 family receptors that recognize major histocompatibility complex class I (MHC-I) molecules. By interacting with MHC-I molecules expressed on other cells (in trans), inhibitory Ly49 receptors prevent the NK cell-mediated killing of normal cells. In addition, some Ly49 receptors have the unusual property to also interact with MHC-I molecules expressed by the NK cell itself (in cis). cis Binding sequesters a significant fraction of the NK cells' Ly49 receptors, reducing the number of receptors available for trans binding. This lowers the threshold at which NK cell activation exceeds inhibition rendering NK cells more sensitive. It is unclear how Ly49 receptors can bind MHC-I in trans and in cis using the same binding site. We have proposed that this is mediated by two distinct conformations of Ly49 receptors. Here we have tested this model by inferring the distance between the ligand-binding domain of Ly49A and the cell membrane using fluorescence resonance energy transfer (FRET). Consistent with the concept, reducing the distance between the ligand-binding domain of Ly49A and the cell membrane, by shortening the Ly49A stalk, resulted in a substantially increased FRET. The co-expression of cognate MHC-I ligand reduced FRET derived from Ly49A variants with a shortened stalk, indicating that cis association alters FRET. Indeed, FRET improved when cis complexes were disrupted using acid-mediated destruction of MHC-I complexes. These data provide direct evidence that the interaction with MHC-I in cis is associated with a conformational change in the Ly49A receptor on the surface of live cells. The novel FRET based approach may be generally applicable to study conformational changes in cell surface receptors.

Temporal proteomics profiling of lipid rafts in CCR6-activated T cells reveals the integration of actin cytoskeleton dynamics

Chemokines orchestrate leukocyte migration toward sites of inflammation and infection and target leukocytes via chemokine receptors such as CCR6, a subfamily of the seven-transmembrane G-protein-coupled receptors. Lipid rafts are cholesterol and sphingolipid-enriched liquid-ordered membrane microdomains thought to serve as scaffolding platforms for signal transduction. To globally understand the dynamic changes of proteins within lipid rafts upon CCR6 activation in T cells, we quantitatively analyzed the time-dependent changes of lipid raft proteome using our recently reported membrane proteomics strategy combining gel-assisted digestion, iTRAQ labeling and LC-MS/MS. To our knowledge, the error-free identification of 852 proteins represents the first data set of the raft proteome in T cells upon chemokine receptor activation, including 354 previously annotated raft proteins and 85 dynamically recruited proteins that are potential raft-associated proteins. The temporal profiles revealed that many proteins involved in the actin cytoskeleton rearrangement are actively recruited into lipid rafts upon CCR6 activation. We further confirmed the proteomics results by Western blotting and used small interfering RNA-mediated knockdown to evaluate their roles upon CCR6 activation. In sum, we employed quantitative proteomic strategy to analyze raft proteome and identified many molecules actively involved in the control of actin assembly and disassembly regulating CCR6 activation-induced cell migration.

Cloning, expression and functional characterization of chicken CCR6 and its ligand CCL20

Chemokines are key molecules that drive migration of lymphoid and myeloid cells toward organs in basal as well as inflammatory conditions. By recruiting immature dendritic cells to the mucosal surfaces, CCL20 acts in the very early events leading to the development of a specific immune response. In order to characterize dendritic cells in birds and better understand their role in the initiation of immune responses against pathogens of economic as well as human health relevance, we have cloned and expressed chicken CCL20 (chCCL20) and its specific receptor chCCR6. chCCL20 has 51% identity (60% similarity) with human CCL20, while the chicken receptor and its human counterpart display nearly 55% identity (and up to 70% similarity). chCCL20 and its specific receptor chCCR6 mRNAs are mainly expressed in bone marrow, secondary lymphoid organs and in the mucosal surfaces, in particular lungs and intestine. Both receptor and chemokine are functionally active when expressed as genuine or tagged proteins in mammalian expression systems, that is chCCR6 is mainly located at the cell surface within lipid rafts like its human counterpart. And secondly, both human and chicken chemokines were able to drive the migration of either chicken or human CCR6-transfected cells.

Lipid raft-mediated regulation of G-protein coupled receptor signaling by ligands which influence receptor dimerization: a computational study

G-protein coupled receptors (GPCRs) are the largest family of cell surface receptors; they activate heterotrimeric G-proteins in response to ligand stimulation. Although many GPCRs have been shown to form homo- and/or heterodimers on the cell membrane, the purpose of this dimerization is not known. Recent research has shown that receptor dimerization may have a role in organization of receptors on the cell surface. In addition, microdomains on the cell membrane termed lipid rafts have been shown to play a role in GPCR localization. Using a combination of stochastic (Monte Carlo) and deterministic modeling, we propose a novel mechanism for lipid raft partitioning of GPCRs based on reversible dimerization of receptors and then demonstrate that such localization can affect GPCR signaling. Modeling results are consistent with a variety of experimental data indicating that lipid rafts have a role in amplification or attenuation of G-protein signaling. Thus our work suggests a new mechanism by which dimerization-inducing or inhibiting characteristics of ligands can influence GPCR signaling by controlling receptor organization on the cell membrane.

CCR5 signalling, but not DARC or D6 regulatory, chemokine receptors are targeted by herpesvirus U83A chemokine which delays receptor internalisation via diversion to a caveolin-linked pathway

Background

Herpesviruses have evolved chemokines and chemokine receptors, which modulate the recruitment of human leukocytes during the inflammatory response to infection. Early post-infection, human herpesvirus 6A (HHV-6A) infected cells express the chemokine receptor U51A and chemokine U83A which have complementary effects in subverting the CC-chemokine family thereby controlling anti-viral leukocyte recruitment. Here we show that, to potentiate this activity, the viral chemokine can also avoid clearance by scavenger chemokine receptors, DARC and D6, which normally regulate an inflammatory response. Conversely, U83A delays internalisation of its signalling target receptor CCR5 with diversion to caveolin rich membrane domains. This mechanism can redirect displaced human chemokines to DARC and D6 for clearance of the anti-viral inflammatory response, leaving the viral chemokine unchecked.

Methods

Cell models for competitive binding assays were established using radiolabeled human chemokines and cold U83A on CCR5, DARC or D6 expressing cells. Flow cytometry was used to assess specific chemotaxis of CCR5 bearing cells to U83A, and internalisation of CCR5 specific chemokine CCL4 after stimulation with U83A. Internalisation analyses were supported by confocal microscopy of internalisation and co-localisation of CCR5 with caveosome marker caveolin-1, after virus or human chemokine stimulation.

Results

U83A displaced efficiently human chemokines from CCR5, with a high affinity of 0.01nM, but not from DARC or D6. Signalling via CCR5 resulted in specific chemoattraction of primary human leukocytes bearing CCR5. However, U83A effective binding and signalling to CCR5 resulted in delayed internalisation and recycling up to 2 hours in the absence of continual re-stimulation. This resulted in diversion to a delayed caveolin-linked pathway rather than the rapid clathrin mediated endocytosis previously shown with human chemokines CCL3 or CCL4.

Conclusion

U83A diverts human chemokines from signalling, but not regulatory or scavenger, receptors facilitating their clearance, while occupying signalling receptors at the cell surface. This can enhance virus specific inflammation, facilitating dissemination to replication sensitive leukocytes while evading clearance; this has implications for linked neuro-inflammatory pathologies.

Glycine transporter 1 associates with cholesterol-rich membrane raft microdomains

Membrane rafts, the highly-ordered, cholesterol-rich microdomains of the plasma membrane play important roles in cellular functions. In this study, GLYT1-CFP and GLYT2-CFP were constructed, followed by investigation of whether the tagged transporters associate with a fluorescence probe that labels membrane rafts (DilC16) by using Fluorescence Resonance Energy Transfer. A close association was observed between DiIC16 and GLYT1-CFP, but not for GLYT2-CFP. The glycine transport ability of GLYT1 is also highly dependent on the integrity of this area. Together, the results suggest that GLYT1 and membrane rafts are co-localized in the membrane, and that this influences the rate of glycine transport.

Monitoring dynamic GPCR signaling events using fluorescence microscopy, FRET imaging, and single-molecule imaging

How a eukaryotic cell translates a small concentration difference of a chemoattractant across the length of its surface into highly polarized intracellular responses is a fundamental question in chemotaxis. Chemoattractants are detected by G-protein-coupled receptors (GPCRs). Binding of chemoattractants to GPCRs induces the dissociation of heterotrimeric G-proteins into G alpha and G betagamma subunits, which in turn, activate downstream signaling networks. To fully understand the molecular mechanisms of chemotaxis, it is essential to quantitatively measure the dynamic changes of chemoattractant concentrations around cells, activation of heterotrimeric G-proteins, and the mobility of GPCR and G-protein subunits in the cell membrane. Here, we outline fluorescence imaging methods including Förster resonance energy transfer (FRET) imaging and a single-molecule analysis that allow us to measure the dynamic properties of GPCR signaling in single live cells.

Regulation of T cell receptor activation by dynamic membrane binding of the CD3epsilon cytoplasmic tyrosine-based motif

Many immune system receptors signal through cytoplasmic tyrosine-based motifs (ITAMs), but how receptor ligation results in ITAM phosphorylation remains unknown. Live-cell imaging studies showed a close interaction of the CD3epsilon cytoplasmic domain of the T cell receptor (TCR) with the plasma membrane through fluorescence resonance energy transfer between a C-terminal fluorescent protein and a membrane fluorophore. Electrostatic interactions between basic CD3epsilon residues and acidic phospholipids enriched in the inner leaflet of the plasma membrane were required for binding. The nuclear magnetic resonance structure of the lipid-bound state of this cytoplasmic domain revealed deep insertion of the two key tyrosines into the hydrophobic core of the lipid bilayer. Receptor ligation thus needs to result in unbinding of the CD3epsilon ITAM from the membrane to render these tyrosines accessible to Src kinases. Sequestration of key tyrosines into the lipid bilayer represents a previously unrecognized mechanism for control of receptor activation.

How chemokines invite leukocytes to dance

A prominent activity of the chemokine system is the regulation of leukocyte trafficking. Here we summarize recent findings on the initial steps in chemokine receptor-induced signal transduction in leukocytes. In particular, we discuss the potential influences of the formation of oligomers of ligand and receptor and of coupling between chemokine signals and regulators of the cytoskeleton, such as small GTPases.

Fluorescence resonance energy transfer imaging reveals that chemokine-binding modulates heterodimers of CXCR4 and CCR5 receptors

Background

Dimerization has emerged as an important feature of chemokine G-protein-coupled receptors. CXCR4 and CCR5 regulate leukocyte chemotaxis and also serve as a co-receptor for HIV entry. Both receptors are recruited to the immunological synapse during T-cell activation. However, it is not clear whether they form heterodimers and whether ligand binding modulates the dimer formation.

Methodology/Principal Findings

Using a sensitive Fluorescence Resonance Energy Transfer (FRET) imaging method, we investigated the formation of CCR5 and CXCR4 heterodimers on the plasma membrane of live cells. We found that CCR5 and CXCR4 exist as constitutive heterodimers and ligands of CCR5 and CXCR4 promote different conformational changes within these preexisting heterodimers. Ligands of CCR5, in contrast to a ligand of CXCR4, induced a clear increase in FRET efficiency, indicating that selective ligands promote and stabilize a distinct conformation of the heterodimers. We also found that mutations at C-terminus of CCR5 reduced its ability to form heterodimers with CXCR4. In addition, ligands induce different conformational transitions of heterodimers of CXCR4 and CCR5 or CCR5^STA and CCR5^Δ4.

Conclusions/Significance

Taken together, our data suggest a model in which CXCR4 and CCR5 spontaneously form heterodimers and ligand-binding to CXCR4 or CCR5 causes different conformational changes affecting heterodimerization, indicating the complexity of regulation of dimerization/function of these chemokine receptors by ligand binding.

Imaging techniques: new insights into chemokine/chemokine receptor biology at the immune system

Our current knowledge of molecular and cellular responses in vivo is based mainly on event reconstruction from time-freeze observations. Conventional biochemical and genetic methods consider the cell as an individual entity and ligand/receptor pairs as isolated systems. In addition, the data refer to the average behavior of a pool of cells and/or receptors removed from their real-life context. The use of new technologies, particularly real-time imaging approaches, is showing us that biological responses are highly dynamic and extremely dependent on the context in which they take place, and therefore much more diverse than initially envisaged. This review focuses on the mechanistic insights that new imaging techniques, such as those based on resonance energy transfer and two-photon microscopy, contribute to our understanding of how receptors work within a single cell, and how cells work within a tissue. Cell movement is a complex and regulated process; it has a key role in embryogenesis, organogenesis, wound-healing and tumor invasion. Nonetheless, it is in immune system homeostasis and response that cell movement becomes essential. For this reason, immunology is being radically transformed and enriched by these new approaches. We will discuss the use of these techniques for studying chemokine/chemokine receptors and their role in the immune system function, and comment on the potential contribution to the design of therapeutic strategies.

Cholesterol-dependent separation of the beta2-adrenergic receptor from its partners determines signaling efficacy: insight into nanoscale organization of signal transduction

Determining the role of lipid raft nanodomains in G protein-coupled receptor signaling remains fraught by the lack of assays directly monitoring rafts in native membranes. We thus combined extensive biochemical and pharmacological approaches to a nanoscale strategy based on bioluminescence resonance energy transfer (BRET) to assess the spatial and functional influence of cholesterol-rich liquid-ordered lipid nanodomains on beta2 adrenergic receptor (beta2AR) signaling. The data revealed that whereas beta2AR did not partition within liquid-ordered lipid phase, a pool of G protein and adenylyl cyclase (AC) were sequestered in these domains. Destabilization of the liquid-ordered phase by cholesterol depletion led to a lateral redistribution of Galphas and AC that favored interactions between the receptor and its signaling partners as assessed by BRET. This resulted in an increased basal and agonist-promoted beta2AR-stimulated cAMP production that was partially dampened as a result of constitutive protein kinase A-dependent phosphorylation and desensitization of the receptor. This restraining influence of nanodomains on beta2AR signaling was further substantiated by showing that liquid-ordered lipid phase stabilization using caveolin overexpression or increasing membrane cholesterol amount led to an inhibition of beta2AR-associated signaling. Given the emerging concept that clustering of receptors and effectors into signaling platforms contributes to the efficacy and selectivity of signal transduction, our results support a model whereby cholesterol-promoted liquid-ordered lipid phase-embedding Gs and AC allows their lateral separation from the receptor, thus restraining the basal activity and controlling responsiveness of beta2AR signaling machinery within larger signaling platforms.

Slamming the DOR on chemokine receptor signaling: heterodimerization silences ligand-occupied CXCR4 and delta-opioid receptors

Dimerization has emerged as a common mechanism for regulating the function of G protein-coupled receptors (GPCR). Among these are chemokine receptors, which detect various chemokines and regulate a range of physiological process, including immune cell trafficking, cancer cell migration, and neuronal patterning. Homo- and heterodimerization in response to chemokine binding has been shown to be required for the initiation or alteration of signaling by a number of chemokine receptors. In this issue of the European Journal of Immunology, a new study indicates that the formation of heterodimers of chemokine receptor CXCR4 and the delta-opioid receptor (DOR) prevents each of them from actively signaling, suggesting a novel mechanism for silencing GPCR function.

Tether and trap: regulation of membrane-raft dynamics by actin-binding proteins

The existence of plasma-membrane-raft microdomains has been widely debated during the past few years. However, it is clear that during lymphocyte stimulation a lipid-based reorganization occurs at the plasma membrane, with markers of the membrane rafts being selectively recruited to key active regions of the cell. Recent reports have demonstrated that membrane-raft dynamics are controlled by proteins that are linked to the actin cytoskeleton and have suggested a new model for the plasma membrane based on protein-lipid interactions. This new and dynamic view of the plasma membrane may improve our understanding of the complex process leading to cell polarization during lymphocyte migration and activation.

Functional membrane diffusion of G-protein coupled receptors

G-protein-coupled receptor function involves interactions between the receptor, G-proteins and effectors in the cell plasma membrane. The main biochemical processes have been individually identified but the mechanisms governing the successive protein-protein interactions of this complex multi-molecular machinery have yet to be established. We discuss advances in understanding the functional dynamics of the receptor resulting from diffusion measurements, and in the context of the plasma membrane organization.

CXCL12-induced partitioning of flotillin-1 with lipid rafts plays a role in CXCR4 function

Lipid rafts play an important role in signal integration and in the cellular activation of a number of cytokine and growth factor receptors. It has recently been demonstrated that flotillin proteins are recruited to lipid raft microdomains upon cellular activation and play a role in neural cell regeneration, receptor signaling and lymphocyte activation. However, little is known about the relevance of the flotillin proteins during T cell responses to chemoattractant stimulation. To this end, cytoplasmic and lipid raft fractions from human T cells were analyzed for flotillin protein redistribution prior to and after CXCL12 stimulation. Flotillin-1, but not flotillin-2, redistributes to lipid rafts upon CXCR4 ligation. Moreover, in CXCL12-treated T cells, flotillin-1 also associates with several raft proteins including LAT, CD48 and CD11a but not Lck. In addition, an increase in CXCR4 association with flotillin-1 in lipid rafts was observed after chemokine treatment. RNAi technology was also utilized to inhibit the expression of flotillin-1, resulting in an inhibition of CXCL12-mediated signaling, function and CXCR4 recruitment into lipid rafts. Together, these data suggest that the increased association of cellular flotillin-1 with lipid raft microdomains during chemokine exposure may play an important role in chemokine receptor signaling and receptor partitioning with lipid rafts.

The involvement of lipid rafts in epidermal growth factor-induced chemotaxis of breast cancer cells

Metastasis is the major cause of morbidity and mortality in cancer. Recent studies reveal a role of chemotaxis in cancer cell metastasis. Epidermal growth factor receptors (EGFR) have potent chemotactic effects on human breast cancer cells. Lipid rafts, organized microdomain on plasma membranes, regulate the activation of many membrane receptors. In the current study, we investigated the role of lipid rafts in EGFR-mediated cancer cell chemotaxis. Our confocal microscopy results suggested that EGFR co-localized with GM1-positive rafts. Disrupting rafts with methyl-beta-cyclodextrin (mbetaCD) inhibited EGF-induced chemotaxis of human breast cancer cells. Supplementation with cholesterol reversed the inhibitory effects. Pretreatment with mbetaCD also impaired directional migration of cells in an in vitro "wound healing" assay, EGF-induced cell adhesion, actin polymerization, Akt phosphorylation and protein kinase Czeta (PKCzeta) translocation. Taken together, our study indicated that integrity of lipid rafts was critical in EGF-induced chemotaxis of human breast cancer cells.

Establishment and maintenance of cell polarity during leukocyte chemotaxis

The term polarity refers to the differential distribution of the macromolecular elements of a cell, resulting in its asymmetry in function, shape and/or content. Polarity is a fundamental property of all metazoan cells in at least some stages, and is pivotal to processes such as epithelial differentiation (apical/basal polarity), coordinated cell activity within the plane of a tissue (planar cell polarity), asymmetric cell division, and cell migration. In the last case, an apparently symmetric cell responds to directional cues provided by chemoattractants, creating a polarity axis that runs from the cell anterior, or leading edge, in which actin polymerization takes place, to the cell posterior (termed uropod in leukocytes), in which acto-myosin contraction occurs. Here we will review some of the molecular mechanisms through which chemoattractants break cell symmetry to trigger directed migration, focusing on cells of the immune system. We briefly highlight some common or apparently contradictory pathways reported as important for polarity in other cells, as this suggests conserved or cell type-specific mechanisms in eukaryotic cell chemotaxis.

The lipid raft proteins flotillins/reggies interact with Galphaq and are involved in Gq-mediated p38 mitogen-activated protein kinase activation through tyrosine kinase

The heterotrimeric G protein alpha q subunit (Galphaq) mediates a variety of cell functions by activating the effector molecule phospholipase Cbeta. Galphaq activity is regulated by G protein betagamma subunits, G protein-coupled receptors, RGS proteins, and Ric-8. In this study, we identified the lipid raft resident proteins, flotillin-1/reggie-2 and flotillin-2/reggie-1, as Galphaq-binding proteins. The interactions of Galphaq and flotillins were independent of the nucleotide-binding state of Galphaq, and the N-terminal portion of flotillins was critical for the interaction. A short interfering RNA-mediated knockdown of flotillins, particularly flotillin-2, attenuated the UTP-induced activation of p38 mitogen-activated protein kinase (MAPK) but not that of ERK1/2. The activation of p38 MAPK was inhibited by the Src family tyrosine kinase inhibitor PP2 and the cholesterol-depleting agent methyl-beta-cyclodextrin, which is generally used for the disruption of lipid rafts. In contrast, the activation of ERK1/2 was not inhibited by these compounds. These lines of evidence suggested that a Gq-coupled receptor activates specifically p38 MAPK through lipid rafts and Src kinase activation, in which flotillins positively modulate the Gq signaling.

Selective localization of recognition complexes for leukotriene B4 and formyl-Met-Leu-Phe within lipid raft microdomains of human polymorphonuclear neutrophils

Neutrophilic polymorphonuclear leukocytes contain glycosphingolipid- and cholesterol-enriched lipid raft microdomains within the plasma membrane. Although there is evidence that lipid rafts function as signaling platforms for CXCR chemokine receptors, their role in recognition systems for other chemotaxins such as leukotriene B4 (LTB4) and fMLP is unknown. To address this question, human neutrophils were extracted with 1% Brij-58 and fractionated on sucrose gradients. B leukotriene receptor-1 (BLT-1), the primary LTB4 receptor, partitioned to low density fractions, co-isolating with the lipid raft marker, flotillin-1. By contrast, formyl peptide receptor (FPR), the primary fMLP receptor, partitioned to high density fractions, co-isolating with a non-raft marker, Cdc42. This pattern was preserved after the cells were stimulated with LTB4 or fMLP. Fluorescence resonance energy transfer (FRET) was performed to confirm the proximity of BLT-1 and FPR with these markers. FRET was detected between BLT1 and flotillin-1 but not Cdc42, whereas FRET was detected between FPR and Cdc42, but not flotillin-1. Pretreating neutrophils with methyl-beta-cyclodextrin, a lipid raft-disrupting agent, suppressed intracellular Ca(2+) mobilization and ERK1/2 phosphorylation in response to LTB4 but had no effect on either of these responses to fMLP. We conclude that BLT-1 is physically located within lipid raft microdomains of human neutrophils and that disrupting lipid raft integrity suppresses LTB4-induced activation. By contrast, FPR is not associated with lipid rafts, and fMLP-induced signaling does not require lipid raft integrity. These findings highlight the complexity of chemotaxin signaling pathways and offer one mechanism by which neutrophils may spatially organize chemotaxin signaling within the plasma membrane.

Physical association between neuropeptide FF and micro-opioid receptors as a possible molecular basis for anti-opioid activity

Neuropeptide FF (NPFF) modulates the opioid system by exerting functional anti-opioid activity on neurons, the mechanism of which is unknown. By using a model of SH-SY5Y cells, we recently postulated that anti-opioid activity likely takes place upstream from the signaling cascade, suggesting that NPFF receptors could block opioid receptors by physical interaction. In the present study, fluorescence techniques were used to monitor the physical association and the dynamic of NPFF2 and micro-opioid (MOP) receptors tagged with variants of the green fluorescent protein. Importantly, cyan fluorescent protein-tagged NPFF2 receptors retained their capacity to antagonize opioid receptors. Fluorescence resonance energy transfer (FRET) and coimmunoprecipitation studies indicate that NPFF and MOP receptors are close enough to generate a basal FRET signal. The opioid agonist Tyr-D-Ala-Gly-NMe-Phe-Gly-ol disrupts by 20-30% this FRET signal, mainly because it concomitantly induces 40% internalization of receptors. In contrast, the NPFF analog 1DMe significantly increases by 10-15% the basal FRET signal, suggesting an association between both receptors. In addition, 1DMe reduces, by half, MOP receptor internalization, indicating that, besides a functional blockade of opioid receptors, the NPFF analog also inhibits their internalization. Finally, as a first report showing the modulation of the mobility of a G-protein-coupled receptor by another one, fluorescence recovery after photobleaching analysis reveals that 1DMe modifies the lateral diffusion of MOP receptors in the cell membrane, changing them from a confined to a freely diffusing state. By promoting NPFF-MOP receptor heteromerization, 1DMe could disrupt the domain organization of MOP receptors in the membrane, resulting in a reduction of opioid response.

Signaling complexes associated with the type I gonadotropin-releasing hormone (GnRH) receptor: colocalization of extracellularly regulated kinase 2 and GnRH receptor within membrane rafts

Our previous work demonstrated that the type I GnRH receptor (GnRHR) resides exclusively and constitutively within membrane rafts in alphaT3-1 gonadotropes and that this association was necessary for the ability of the receptor to couple to the ERK signaling pathway. G(alphaq), c-raf, and calmodulin have also been shown to reside in this compartment, implicating a raft-associated multiprotein signaling complex as a functional link between the GnRHR and ERK signaling. In the studies reported here, we used subcellular fractionation and coimmunoprecipitation to analyze the behavior of ERKs with respect to this putative signaling platform. ERK 2 associated partially and constitutively with low-density membranes both in alphaT3-1 cells and in whole mouse pituitary. Cholesterol depletion of alphaT3-1 cells reversibly blocked the association of both the GnRHR and ERKs with low-density membranes and uncoupled the ability of GnRH to activate ERK. Analysis of the kinetics of recovery of ERK inducibility after cholesterol normalization supported the conclusion that reestablishment of the association of the GnRHR and ERKs with the membrane raft compartment was not sufficient for reconstitution of signaling activity. In alphaT3-1 cells, the GnRHR and ERK2 coimmunoprecipitated from low-density membrane fractions prepared either in the presence or absence of detergent. The GnRHR also partitioned into low-density, detergent-resistant membrane fractions in mouse pituitary and coimmunoprecipitated with ERK2 from these fractions. Collectively, these data support a model in which coupling of the GnRHR to the ERK pathway in gonadotropes involves the assembly of a multiprotein signaling complex in association with specialized microdomains of the plasma membrane.

HIV gp120-induced interaction between CD4 and CCR5 requires cholesterol-rich microenvironments revealed by live cell fluorescence resonance energy transfer imaging

Binding of the human immunodeficiency virus (HIV) envelope gp120 glycoprotein to CD4 and CCR5 receptors on the plasma membrane initiates the viral entry process. Although plasma membrane cholesterol plays an important role in HIV entry, its modulating effect on the viral entry process is unclear. Using fluorescence resonance energy transfer imaging, we have provided evidence here that CD4 and CCR5 localize in different microenvironments on the surface of resting cells. Binding of the third variable region V3-containing gp120 core to CD4 and CCR5 induced association between these receptors, which could be directly monitored by fluorescence resonance energy transfer on the plasma membrane of live cells. Depletion of cholesterol from the plasma membrane abolished the gp120 core-induced associations between CD4 and CCR5, and reloading cholesterol restored the associations in live cells. Our studies suggest that, during the first step of the HIV entry process, gp120 binding alters the microenvironments of unbound CD4 and CCR5, with plasma membrane cholesterol required for the formation of the HIV entry complex.

Lipid rafts in lymphocyte activation and migration

Functional polarization of leukocytes is a requisite to accomplish immune function. Immune synapse formation or chemotaxis requires asymmetric redistribution of membrane receptors, signaling molecules and the actin cytoskeleton. There is increasing evidence that compartmentalization of the plasma membrane into distinct lipid microdomains is pivotal in establishing and maintaining leukocyte polarity. Specific rafts assemble into large-scale domains to create plasma membrane asymmetries at specific cell locations, thus coordinating temporally and spatially cell signaling in these processes. In this review we discuss the roles of lipid rafts as organizers of T lymphocyte polarity during cell activation and migration.

Adenylyl cyclase superactivation induced by long-term treatment with opioid agonist is dependent on receptor localized within lipid rafts and is independent of receptor internalization

Long-term opioid agonist treatment results in adenylyl cyclase superactivation. A recent "RAVE" theory implicates a direct correlation between the ability of agonist to induce receptor internalization and the magnitude of adenylyl cyclase superactivation. We decided to test such a theory by examining the adenylyl cyclase superactivation after long-term activation of mu-opioid receptor (MOR) in an EcR293 cell model. We examined the magnitudes of adenylyl cyclase superactivation in the presence of naloxone after long-term treatment with morphine, etorphine, and methadone, three agonists reported to have differential activities in promoting MOR internalization. It can be shown that the magnitudes of adenylyl cyclase superactivation after treating with these three agonists, although different, were dependent on MOR density. Blunting MOR internalization with the dominant-negative mutant of dynamin, K44E, did not alter the magnitude of either morphine- or etorphine-induced adenylyl cyclase superactivation. In the presence of diprenorphine, the magnitude of adenylyl cyclase superactivation after etorphine treatment was identical to that observed with morphine. It could be demonstrated further that adenylyl cyclase superactivation is dependent on the cell surface-located MOR. Sucrose gradient fractionation demonstrated the colocalization of MOR and adenylyl cyclase V/VI with caveolin-1, a marker for lipid rafts. After long-term agonist treatment, the majority of MOR remained at the lipid rafts. Methyl-beta-cyclodextrin (MbetaCD) completely blunted the adenylyl cyclase superactivation and agonist-induced receptor internalization. These MbetaCD actions were reversed by incubating the cells with cholesterol. Thus, the adenylyl cyclase superactivation is not dependent on agonist-induced receptor internalization. Rather, the location of MOR at lipid rafts is an absolute requirement for the observed adenylyl cyclase superactivation.

Differential impact of intracellular carboxyl terminal domains on lipid raft localization of the murine gonadotropin-releasing hormone receptor

The mammalian type I GNRH receptor (GNRHR) is unique among G protein-coupled receptors (GPCRs) because of the absence of an intracellular C-terminus. Previously, we have found that the murine GNRHR is constitutively localized to low-density membrane microdomains termed lipid rafts. As such, association of the GNRHR with lipid rafts may reflect both a loss (C-terminus) and a gain (raft association address) of structural characteristics. To address this, we fused either the full-length C-terminus from the nonraft-associated LH receptor (LHCGR; GNRHR-LF) or a truncated (t631) LHCGR C-terminus to the GNRHR. These chimeric receptors are trafficked to the plasma membrane, bind ligand, and display increased agonist-induced receptor internalization, but they do not partition into lipid rafts. Thus, a heterologous C-terminus from a nonraft-associated GPCR redirects localization of the GNRHR to nonraft domains. In contrast to the murine GNRHR, the catfish GNRHR (cfGNRHR) possesses an intracellular C-terminus. We found that the cfGNRHR was localized to lipid rafts and that the cfGNRHR C-terminus did not alter raft localization of the mammalian receptor. Consistent with placement in different lipid microenvironments within the plasma membrane, fluorescence recovery after photobleaching revealed different lateral diffusion phenotypes of the raft-associated GNRHR and cfGNRHR versus the nonraft-associated GNRHR-LF fusion protein. We conclude that whereas an intracellular C-terminus is capable of redirecting the GNRHR to nonraft compartments, this is not a generalized feature of GPCR C-terminal tails. Thus, constitutive raft localization of the GNRHR is not simply a result of the loss of an intracellular C-terminus.

In vivo interaction between RGS4 and calmodulin visualized with FRET techniques: possible involvement of lipid raft

Regulators of G-protein signaling (RGS) are a family of proteins which accelerate intrinsic GTP-hydrolysis on heterotrimeric G-protein-alpha-subunits. Although it has been suggested that the function of RGS4 is reciprocally regulated by competitive binding of the membrane phospholipid, phosphatidylinositol-3,4,5,-trisphosphate(PtdIns(3,4,5)P(3)), and Ca(2+)/calmodulin (CaM), it remains to be shown that these interactions occur in vivo. Here, using fluorescence resonance energy transfer (FRET) techniques, we show that an elevation of intracellular Ca(2+) concentration by ionomycin increased the FRET efficiency from ECFP (a variant of cyan fluorescent protein)-labeled calmodulin to Venus (a variant of yellow fluorescent protein)-labeled RGS4. The increase in FRET efficiency was greatly attenuated by pre-treating the cells with methyl-beta-cyclodextrin, which depletes membrane cholesterol and thus disrupts lipid rafts. These results provide the first demonstration of a Ca(2+)-dependent interaction between RGS4 and CaM in vivo and show that association in lipid rafts of the plasma membrane might be involved in this physiological regulation of RGS proteins.