Modulation of Chemokine Receptor Function by Cholesterol: New Prospects for Pharmacological Intervention

Structural insights into CXCR4 modulation and oligomerization

Activation of the chemokine receptor CXCR4 by its chemokine ligand CXCL12 regulates diverse cellular processes. Previously reported crystal structures of CXCR4 revealed the architecture of an inactive, homodimeric receptor. However, many structural aspects of CXCR4 remain poorly understood. Here, we use cryo-electron microscopy to investigate various modes of human CXCR4 regulation. CXCL12 activates CXCR4 by inserting its N terminus deep into the CXCR4 orthosteric pocket. The binding of US Food and Drug Administration-approved antagonist AMD3100 is stabilized by electrostatic interactions with acidic residues in the seven-transmembrane-helix bundle. A potent antibody blocker, REGN7663, binds across the extracellular face of CXCR4 and inserts its complementarity-determining region H3 loop into the orthosteric pocket. Trimeric and tetrameric structures of CXCR4 reveal modes of G-protein-coupled receptor oligomerization. We show that CXCR4 adopts distinct subunit conformations in trimeric and tetrameric assemblies, highlighting how oligomerization could allosterically regulate chemokine receptor function.

Cholesterol Biases the Conformational Landscape of the Chemokine Receptor CCR3: A MAS SSNMR-Filtered Molecular Dynamics Study

Cholesterol directs the pathway of ligand-induced G protein-coupled receptor (GPCR) signal transduction. The GPCR C-C motif chemokine receptor 3 (CCR3) is the principal chemotactic receptor for eosinophils, with roles in cancer metastasis and autoinflammatory conditions. Recently, we discovered a direct correlation between bilayer cholesterol and increased agonist-triggered CCR3 signal transduction. However, the allosteric molecular mechanism escalating ligand affinity and G protein coupling is unknown. To study cholesterol-guided CCR3 conformational selection, we implement comparative, objective measurement of protein architectures by scoring shifts (COMPASS) to grade model structures from molecular dynamics simulations. In this workflow, we scored predicted chemical shifts against 2-dimensional solid-state NMR 13C-13C correlation spectra of U-15N,13C-CCR3 samples prepared with and without cholesterol. Our analysis of trajectory model structures uncovers that cholesterol induces site-specific conformational restraint of extracellular loop (ECL) 2 and conserved motion in transmembrane helices and ECL3 not observed in simulations of bilayers with only phosphatidylcholine lipids. PyLipID analysis implicates direct cholesterol agency in CCR3 conformational selection and dynamics. Residue-residue contact scoring shows that cholesterol biases the conformational selection of the orthosteric pocket involving Y411.39, Y1133.32, and E2877.39. Lastly, we observe contact remodeling in activation pathway residues centered on the initial transmission switch, Na+ pocket, and R3.50 in the DRY motif. Our observations have unique implications for understanding of CCR3 ligand recognition and specificity and provide mechanistic insight into how cholesterol functions as an allosteric regulator of CCR3 signal transduction.

Heparin Specifically Interacts with Basic BBXB Motifs of the Chemokine CCL21 to Define CCR7 Signaling

Chemokines are critically involved in controlling directed leukocyte migration. Spatiotemporal secretion together with local retention processes establish and maintain local chemokine gradients that guide directional cell migration. Extracellular matrix proteins, particularly glycosaminoglycans (GAGs), locally retain chemokines through electrochemical interactions. The two chemokines CCL19 and CCL21 guide CCR7-expressing leukocytes, such as antigen-bearing dendritic cells and T lymphocytes, to draining lymph nodes to initiate adaptive immune responses. CCL21-in contrast to CCL19-is characterized by a unique extended C-terminus composed of highly charged residues to facilitate interactions with GAGs. Notably, both chemokines can trigger common, but also ligand-biased signaling through the same receptor. The underlying molecular mechanism of ligand-biased CCR7 signaling is poorly understood. Using a series of naturally occurring chemokine variants in combination with newly designed site-specific chemokine mutants, we herein assessed CCR7 signaling, as well as GAG interactions. We demonstrate that the charged chemokine C-terminus does not fully confer CCL21-biased CCR7 signaling. Besides the positively charged C-terminus, CCL21 also possesses specific BBXB motifs comprising basic amino acids. We show that CCL21 variants where individual BBXB motifs are mutated retain their capability to trigger G-protein-dependent CCR7 signaling, but lose their ability to interact with heparin. Moreover, we show that heparin specifically interacts with CCL21, but not with CCL19, and thereby competes with ligand-binding to CCR7 and prevents signaling. Hence, we provide evidence that soluble heparin, but not the other GAGs, complexes with CCL21 to define CCR7 signaling in a ligand-dependent manner.

CD28 and chemokine receptors: Signalling amplifiers at the immunological synapse

T cells are master regulators of the immune response tuning, among others, B cells, macrophages and NK cells. To exert their functions requiring high sensibility and specificity, T cells need to integrate different stimuli from the surrounding microenvironment. A finely tuned signalling compartmentalization orchestrated in dynamic platforms is an essential requirement for the proper and efficient response of these cells to distinct triggers. During years, several studies have depicted the pivotal role of the cytoskeleton and lipid microdomains in controlling signalling compartmentalization during T cell activation and functions. Here, we discuss mechanisms responsible for signalling amplification and compartmentalization in T cell activation, focusing on the role of CD28, chemokine receptors and the actin cytoskeleton. We also take into account the detrimental effect of mutations carried by distinct signalling proteins giving rise to syndromes characterized by defects in T cell functionality.

Sphingomyelin Depletion Inhibits CXCR4 Dynamics and CXCL12-Mediated Directed Cell Migration in Human T Cells

Sphingolipids, ceramides and cholesterol are integral components of cellular membranes, and they also play important roles in signal transduction by regulating the dynamics of membrane receptors through their effects on membrane fluidity. Here, we combined biochemical and functional assays with single-particle tracking analysis of diffusion in the plasma membrane to demonstrate that the local lipid environment regulates CXCR4 organization and function and modulates chemokine-triggered directed cell migration. Prolonged treatment of T cells with bacterial sphingomyelinase promoted the complete and sustained breakdown of sphingomyelins and the accumulation of the corresponding ceramides, which altered both membrane fluidity and CXCR4 nanoclustering and dynamics. Under these conditions CXCR4 retained some CXCL12-mediated signaling activity but failed to promote efficient directed cell migration. Our data underscore a critical role for the local lipid composition at the cell membrane in regulating the lateral mobility of chemokine receptors, and their ability to dynamically increase receptor density at the leading edge to promote efficient cell migration.

Shifting CCR7 towards Its Monomeric Form Augments CCL19 Binding and Uptake

The chemokine receptor CCR7, together with its ligands, is responsible for the migration and positioning of adaptive immune cells, and hence critical for launching adaptive immune responses. CCR7 is also induced on certain cancer cells and contributes to metastasis formation. Thus, CCR7 expression and signalling must be tightly regulated for proper function. CCR7, like many other members of the G-protein coupled receptor superfamily, can form homodimers and oligomers. Notably, danger signals associated with pathogen encounter promote oligomerisation of CCR7 and is considered as one layer of regulating its function. Here, we assessed the dimerisation of human CCR7 and several single point mutations using split-luciferase complementation assays. We demonstrate that dimerisation-defective CCR7 mutants can be transported to the cell surface and elicit normal chemokine-driven G-protein activation. By contrast, we discovered that CCR7 mutants whose expression are shifted towards monomers significantly augment their capacities to bind and internalise fluorescently labelled CCL19. Modeling of the receptor suggests that dimerisation-defective CCR7 mutants render the extracellular loops more flexible and less structured, such that the chemokine recognition site located in the binding pocket might become more accessible to its ligand. Overall, we provide new insights into how the dimerisation state of CCR7 affects CCL19 binding and receptor trafficking.

Mechanistic Understanding from Molecular Dynamics in Pharmaceutical Research 2: Lipid Membrane in Drug Design

We review the use of molecular dynamics (MD) simulation as a drug design tool in the context of the role that the lipid membrane can play in drug action, i.e., the interaction between candidate drug molecules and lipid membranes. In the standard "lock and key" paradigm, only the interaction between the drug and a specific active site of a specific protein is considered; the environment in which the drug acts is, from a biophysical perspective, far more complex than this. The possible mechanisms though which a drug can be designed to tinker with physiological processes are significantly broader than merely fitting to a single active site of a single protein. In this paper, we focus on the role of the lipid membrane, arguably the most important element outside the proteins themselves, as a case study. We discuss work that has been carried out, using MD simulation, concerning the transfection of drugs through membranes that act as biological barriers in the path of the drugs, the behavior of drug molecules within membranes, how their collective behavior can affect the structure and properties of the membrane and, finally, the role lipid membranes, to which the vast majority of drug target proteins are associated, can play in mediating the interaction between drug and target protein. This review paper is the second in a two-part series covering MD simulation as a tool in pharmaceutical research; both are designed as pedagogical review papers aimed at both pharmaceutical scientists interested in exploring how the tool of MD simulation can be applied to their research and computational scientists interested in exploring the possibility of a pharmaceutical context for their research.

Cholesterol Is a Dose-Dependent Positive Allosteric Modulator of CCR3 Ligand Affinity and G Protein Coupling

Cholesterol as an allosteric modulator of G protein-coupled receptor (GPCR) function is well documented. This quintessential mammalian lipid facilitates receptor–ligand interactions and multimerization states. Functionally, this introduces a complicated mechanism for the homeostatic modulation of GPCR signaling. Chemokine receptors are Class A GPCRs responsible for immune cell trafficking through the binding of endogenous peptide ligands. CCR3 is a CC motif chemokine receptor expressed by eosinophils and basophils. It traffics these cells by transducing the signal stimulated by the CC motif chemokine primary messengers 11, 24, and 26. These behaviors are close to the human immunoresponse. Thus, CCR3 is implicated in cancer metastasis and inflammatory conditions. However, there is a paucity of experimental evidence linking the functional states of CCR3 to the molecular mechanisms of cholesterol–receptor cooperativity. In this vein, we present a means to combine codon harmonization and a maltose-binding protein fusion tag to produce CCR3 from E. coli. This technique yields ∼2.6 mg of functional GPCR per liter of minimal media. We leveraged this protein production capability to investigate the effects of cholesterol on CCR3 function in vitro. We found that affinity for the endogenous ligand CCL11 increases in a dose-dependent manner with cholesterol concentration in both styrene:maleic acid lipid particles (SMALPs) and proteoliposomes. This heightened receptor activation directly translates to increased signal transduction as measured by the GTPase activity of the bound G-protein α inhibitory subunit 3 (Gα_i3). This work represents a critical step forward in understanding the role of cholesterol-GPCR allostery in regulation of signal transduction.

Multiple Targets for Oxysterols in Their Regulation of the Immune System

Oxysterols, or cholesterol oxidation products, are naturally occurring lipids which regulate the physiology of cells, including those of the immune system. In contrast to effects that are mediated through nuclear receptors or by epigenetic mechanism, which take tens of minutes to occur, changes in the activities of cell-surface receptors caused by oxysterols can be extremely rapid, often taking place within subsecond timescales. Such cell-surface receptor effects of oxysterols allow for the regulation of fast cellular processes, such as motility, secretion and endocytosis. These cellular processes play critical roles in both the innate and adaptive immune systems. This review will survey the two broad classes of cell-surface receptors for oxysterols (G-protein coupled receptors (GPCRs) and ion channels), the mechanisms by which cholesterol oxidation products act on them, and their presence and functions in the different cell types of the immune system. Overall, this review will highlight the potential of oxysterols, synthetic derivatives and their receptors for physiological and therapeutic modulation of the immune system.

In Silico Identification of Cholesterol Binding Motifs in the Chemokine Receptor CCR3

CC motif chemokine receptor 3 (CCR3) is a Class A G protein-coupled receptor (GPCR) mainly responsible for the cellular trafficking of eosinophils. As such, it plays key roles in inflammatory conditions, such as asthma and arthritis, and the metastasis of many deadly forms of cancer. However, little is known about how CCR3 functionally interacts with its bilayer environment. Here, we investigate cholesterol binding sites in silico through Coarse-Grained Molecular Dynamics (MD) and Pylipid analysis using an extensively validated homology model based on the crystal structure of CCR5. These simulations identified several cholesterol binding sites containing Cholesterol Recognition/Interaction Amino Acid Consensus motif (CRAC) and its inversion CARC motifs in CCR3. One such site, a CARC site in TM1, in conjunction with aliphatic residues in TM7, emerged as a candidate for future investigation based on the cholesterol residency time within the binding pocket. This site forms the core of a cholesterol binding site previously observed in computational studies of CCR2 and CCR5. Most importantly, these cholesterol binding sites are conserved in other chemokine receptors and may provide clues to cholesterol regulation mechanisms in this subfamily of Class A GPCRs.

C-C motif chemokine receptor 2 as a novel intermediate in the ovulatory cascade

Expression of immune function genes within follicle cells has been reported in ovaries from many species. Recent work from our laboratory showed a direct effect of the monocyte chemoattractant protein 1/C-C motif chemokine receptor 2 system within the feline cumulus oocyte complex, by increasing the mRNA levels of key genes involved in the ovulatory cascade in vitro. Studies were designed to evaluate if C-C motif chemokine receptor 2 acts as a novel mediator of the ovulatory cascade in vitro. Therefore, feline cumulus oocyte complexes were cultured in the presence or absence of a highly selective C-C motif chemokine receptor 2 antagonist together with known inducers of cumulus-oocyte expansion and/or oocyte maturation to assess mRNA expression of key genes related to periovulatory events in other species as well as oocyte maturation. Also, the effects of recombinant monocyte chemoattractant protein 1 on spontaneous or gonadotrophin-induced oocyte maturation were assessed. This is an in vitro system using isolated cumulus oocyte complexes from feline ovaries. The present study reveals the modulation of several key ovulatory genes by a highly selective C-C motif chemokine receptor 2 antagonist. However, this antagonist was not enough to block the oocyte maturation induced by gonadotropins or amphiregulin. Nonetheless, recombinant monocyte chemoattractant protein 1 had a significant effect on spontaneous oocyte maturation, increasing the percentage of metaphase II stage oocytes in comparison to the control. This is the first study in any species to establish C-C motif chemokine receptor 2 as a mediator of some actions of the mid-cycle gonadotrophin surge.

Latest update on chemokine receptors as therapeutic targets

The chemokine system plays a fundamental role in a diverse range of physiological processes, such as homeostasis and immune responses. Dysregulation in the chemokine system has been linked to inflammatory diseases and cancer, which renders chemokine receptors to be considered as therapeutic targets. In the past two decades, around 45 drugs targeting chemokine receptors have been developed, yet only three are clinically approved. The challenging factors include the limited understanding of aberrant chemokine signalling in malignant diseases, high redundancy of the chemokine system, differences between cell types and non-specific binding of the chemokine receptor antagonists due to the broad ligand-binding pockets. In recent years, emerging studies attempt to characterise the chemokine ligand–receptor interactions and the downstream signalling protein–protein interactions, aiming to fine tuning to the promiscuous interplay of the chemokine system for the development of precision medicine. This review will outline the updates on the mechanistic insights in the chemokine system and propose some potential strategies in the future development of targeted therapy.

Structural insights into the activation of chemokine receptor CXCR2

The C-X-C motif chemokine CXCL8 (interleukin-8, IL-8) and its receptor chemokine receptor 2 (CXCR2) mediate neutrophil migration during cell development and inflammatory responses and thus are related to numerous inflammatory diseases and cancers. We have determined the cryo-electron microscopy structure of CXCL8 bound CXCR2 coupled to G_i protein, as well as the crystal structure of inactive CXCR2 in complex with a designed allosteric antagonist. These results reveal the binding modes between CXCL8 and CXCR2, CXCR2 and G protein, and the detailed binding pattern of the allosteric antagonist, 00767013. Further structural analysis of the inactive- and active- states of CXCR2 reveals the unique shallow-pocket activation mechanism of C-X-C chemokine receptors and promotes our understanding on how a G protein-coupled receptor (GPCR) is activated by an endogenous protein molecule. In addition, the cholesterol molecule is observed in the activated CXCR2 structure, providing the structural basis of the potential allosteric modulation role of cholesterol in chemokine receptors.

Direct interaction of HIV gp120 with neuronal CXCR4 and CCR5 receptors induces cofilin-actin rod pathology via a cellular prion protein- and NOX-dependent mechanism

Nearly 50% of individuals with long-term HIV infection are affected by the onset of progressive HIV-associated neurocognitive disorders (HAND). HIV infiltrates the central nervous system (CNS) early during primary infection where it establishes persistent infection in microglia (resident macrophages) and astrocytes that in turn release inflammatory cytokines, small neurotoxic mediators, and viral proteins. While the molecular mechanisms underlying pathology in HAND remain poorly understood, synaptodendritic damage has emerged as a hallmark of HIV infection of the CNS. Here, we report that the HIV viral envelope glycoprotein gp120 induces the formation of aberrant, rod-shaped cofilin-actin inclusions (rods) in cultured mouse hippocampal neurons via a signaling pathway common to other neurodegenerative stimuli including oligomeric, soluble amyloid-β and proinflammatory cytokines. Previous studies showed that synaptic function is impaired preferentially in the distal proximity of rods within dendrites. Our studies demonstrate gp120 binding to either chemokine co-receptor CCR5 or CXCR4 is capable of inducing rod formation, and signaling through this pathway requires active NADPH oxidase presumably through the formation of superoxide (O2-) and the expression of cellular prion protein (PrPC). These findings link gp120-mediated oxidative stress to the generation of rods, which may underlie early synaptic dysfunction observed in HAND.

Allosteric Modulation of GPCRs of Class A by Cholesterol

G-protein coupled receptors (GPCRs) are membrane proteins that convey extracellular signals to the cellular milieu. They represent a target for more than 30% of currently marketed drugs. Here we review the effects of membrane cholesterol on the function of GPCRs of Class A. We review both the specific effects of cholesterol mediated via its direct high-affinity binding to the receptor and non-specific effects mediated by cholesterol-induced changes in the properties of the membrane. Cholesterol binds to many GPCRs at both canonical and non-canonical binding sites. It allosterically affects ligand binding to and activation of GPCRs. Additionally, it changes the oligomerization state of GPCRs. In this review, we consider a perspective of the potential for the development of new therapies that are targeted at manipulating the level of membrane cholesterol or modulating cholesterol binding sites on to GPCRs.

Cholesterol as a modulator of cannabinoid receptor CB2 signaling

Signaling through integral membrane G protein-coupled receptors (GPCRs) is influenced by lipid composition of cell membranes. By using novel high affinity ligands of human cannabinoid receptor CB₂, we demonstrate that cholesterol increases basal activation levels of the receptor and alters the pharmacological categorization of these ligands. Our results revealed that (2-(6-chloro-2-((2,2,3,3-tetramethylcyclopropane-1-carbonyl)imino)benzo[d]thiazol-3(2H)-yl)ethyl acetate ligand (MRI-2646) acts as a partial agonist of CB₂ in membranes devoid of cholesterol and as a neutral antagonist or a partial inverse agonist in cholesterol-containing membranes. The differential effects of a specific ligand on activation of CB₂ in different types of membranes may have implications for screening of drug candidates in a search of modulators of GPCR activity. MD simulation suggests that cholesterol exerts an allosteric effect on the intracellular regions of the receptor that interact with the G-protein complex thereby altering the recruitment of G protein.

Mechanisms underlying the production of chemokine CXCL11 in the reaction of renal tubular epithelial cells with CD4+ and CD8+ T cells

AIM

To study the release mechanism of C-X-C motif chemokine 11 (CXCL11) and other chemokines after the co-cultivation of CD4⁺ and CD8⁺ T cells with the renal tubular epithelial cells (RTEC) in the process of allograft renal transplantation rejection.

METHODS

The Human CD4⁺, CD8⁺ T cells were obtained from the blood of volunteers and kidney transplantation (Ktx) patients, and co-cultured with renal tubular epithelial cells (RTEC) in vitro. RT-PCR was run for detecting the mRNA transcription of CXCL11, IFN-induced protein of 10 (CXCL10), and IL-6 in cells after RTEC was stimulated with IFN-γ or co-cultured with CD4⁺ and CD8⁺ T cells. The concentration of CXCL11, CXCL10 and IL-6 in the culture medium was detected by Multiplex Assay after RTEC was stimulated with IFN-γ or co-cultured with CD4⁺ and CD8⁺ T cells. IFN-γ receptor antibody was used for interfering with the above reaction and the blocking effect was observed. Western blot was used for protein expression analysis. Finally, we applied renal biopsies from kidney transplantation patients with and without rejection to verify the results of the above experiments by using RT-PCR and Western blot.

RESULTS

The mRNA expression of CXCL11 and CXCL10 were significantly increased after RTEC was stimulated with IFN-γ or co-cultured with CD4⁺ and CD8⁺ T cells. Multiplex Assay showed that the concentration of CXCL11 and CXCL10 in the supernatant were significantly increased in a time-dependence fashion after stimulation RTEC by IFN-γ. Anti-IFN-γ receptor1 (anti-IFN-γR1) antibody could reduce the production of CXCL11 and CXCL10 in this situation. The concentration of CXCL11 and CXCL11 in the supernatant was significantly increased with a time-dependent effect after the co-culture of CD4⁺ and CD8⁺ T cells with RTEC. The anti-IFN-γR1 blocked this effect. Our study showed that the expression levels of CXCL11 and CXCL10 were upgraded in the biopsies of patients with renal transplant rejection comparatively to pre-transplant biopsies, both at mRNA and protein levels.

CONCLUSIONS

RTEC and T cells can stimulate each other during the acute rejection of allogeneic kidney transplantation and secret CXCL11，CXCL10 and other chemokines. IFN-γ plays a key role in this process.

Cholesterol Constrains the Antigenic Configuration of the Membrane-Proximal Neutralizing HIV-1 Epitope

The envelope glycoprotein (Env) enables HIV-1 cell entry through fusion of host-cell and viral membranes induced by the transmembrane subunit gp41. Antibodies targeting the C-terminal sequence of the membrane-proximal external region (C-MPER) block the fusogenic activity of gp41 and achieve neutralization of divergent HIV-1 strains and isolates. Thus, recreating the structure that generates broadly neutralizing C-MPER antibodies during infection is a major goal in HIV vaccine development. Here, we have reconstituted a peptide termed CpreTM-TMD in a membrane environment. This peptide contains the C-MPER epitope and the minimum TMD residues required for the anchorage of the Env glycoprotein to the viral membrane. In addition, we have used antibody 10E8 variants to gauge the antigenic configuration attained by CpreTM-TMD as a function of the membrane cholesterol content, a functional determinant of the HIV envelope and liposome-based vaccines. Differential binding of the 10E8 variants and the trend of the IgG responses recovered from rabbits immunized with liposome-peptide formulations, suggested that cholesterol may restrict 10E8 accessibility to the C-MPER epitope. Our data ruled out the destabilization of the lipid bilayer architecture in CpreTM-TMD-containing membranes, and pointed to the perturbation of the helical conformation by lipid packing as the cause of the antigenic configuration loss induced by cholesterol. Overall, our results provide additional insights into the structural basis of the Env complex anchoring to membranes, and suggest new approaches to the design of effective immunogens directed against the near pan-neutralizing HIV-1 epitope C-MPER.

CCR5 deficiency/CCR5Δ32: resistant to HIV infection at the cost of curtailed CD4+ T cell memory responses

Orchestrated trafficking and activation by pathogen-derived peptides define the ability of CD4+ T helper cells to contribute to an effective adaptive immunity. In this issue of The EMBO Journal, Martín-Leal et al show that the inflammatory chemokine receptor CCR5, well known for its role in cell migration and HIV infection, regulates ceramide synthesis and TCR nanoclustering to promote memory CD4+ T cell activation.

Structural basis of CXC chemokine receptor 2 activation and signalling

Chemokines and their receptors mediate cell migration, which influences multiple fundamental biological processes and disease conditions such as inflammation and cancer¹. Although ample effort has been invested into the structural investigation of the chemokine receptors and receptor-chemokine recognition^2-4, less is known about endogenous chemokine-induced receptor activation and G-protein coupling. Here we present the cryo-electron microscopy structures of interleukin-8 (IL-8, also known as CXCL8)-activated human CXC chemokine receptor 2 (CXCR2) in complex with G_i protein, along with a crystal structure of CXCR2 bound to a designed allosteric antagonist. Our results reveal a unique shallow mode of binding between CXCL8 and CXCR2, and also show the interactions between CXCR2 and G_i protein. Further structural analysis of the inactive and active states of CXCR2 reveals a distinct activation process and the competitive small-molecule antagonism of chemokine receptors. In addition, our results provide insights into how a G-protein-coupled receptor is activated by an endogenous protein molecule, which will assist in the rational development of therapeutics that target the chemokine system for better pharmacological profiles.

Membrane Compartmentalization and Scaffold Proteins in Leukocyte Migration

Leukocyte migration across vessels into and within peripheral and lymphoid tissues is essential for host defense against invading pathogens. Leukocytes are specialized in sensing a variety of guidance cues and to integrate environmental stimuli to navigate in a timely and spatially controlled manner. These extracellular signals must be transmitted across the leukocyte’s plasma membrane in a way that intracellular signaling cascades enable directional cell movement. Therefore, the composition of the membrane in concert with proteins that influence the compartmentalization of the plasma membrane or contribute to delineate intracellular signaling molecules are key in controlling leukocyte navigation. This becomes evident by the fact that mislocalization of membrane proteins is known to deleteriously affect cellular functions that may cause diseases. In this review we summarize recent advances made in the understanding of how membrane cholesterol levels modulate chemokine receptor signaling and hence leukocyte trafficking. Moreover, we provide an overview on the role of membrane scaffold proteins, particularly tetraspanins, flotillins/reggies, and caveolins in controlling leukocyte migration both in vitro and in vivo.

Brahma Related Gene 1 (Brg1) Regulates Cellular Cholesterol Synthesis by Acting as a Co-factor for SREBP2

Hepatocyte is a hub for cholesterol metabolism. Augmented synthesis of cholesterol in the liver is associated with hypercholesterolemia and contributes to the pathogenesis of a host of cardiovascular and metabolic diseases. Sterol response element binding protein 2 (SREBP2) regulates hepatic cholesterol metabolism by activating the transcription of rate-limiting enzymes in the cholesterol biosynthesis pathway. The underlying epigenetic mechanism is not well understood. We report here that mice with hepatocyte-specific knockout (CKO) of Brg1, a chromatin remodeling protein, exhibit reduced levels of hepatic cholesterol compared to the wild type (WT) littermates when placed on a high-fact diet (HFD) or a methionine-and-choline-deficient diet (MCD). Down-regulation of cholesterol levels as a result of BRG1 deficiency was accompanied by attenuation of cholesterogenic gene transcription. Likewise, BRG1 knockdown in hepatocytes markedly suppressed the induction of cholesterogenic genes by lipid depletion formulas. Brg1 interacted with SREBP2 and was recruited by SREBP2 to the cholesterogenic gene promoters. Reciprocally, Brg1 deficiency dampened the occupancies of SREBP2 on target promoters likely through modulating H3K9 methylation on the cholesterogenic gene promoters. Mechanistically, Brg1 recruited the H3K9 methyltransferase KDM3A to co-regulate pro-cholesterogenic transcription. KDM3A silencing dampened the cholesterogenic response in hepatocytes equivalent to Brg1 deficiency. In conclusion, our data demonstrate a novel epigenetic pathway that contributes to SREBP2-dependent cholesterol synthesis in hepatocytes.

The multilayered complexity of the chemokine receptor system

The chemokines receptor family are membrane-expressed class A-specific seven-transmembrane receptors linked to G proteins. Through interaction with the corresponding ligands, the chemokines, they induce a wide variety of cellular responses including cell polarization, movement, immune and inflammatory responses, as well as the prevention of HIV-1 infection. Like a Russian matryoshka doll, the chemokine receptor system is more complex than initially envisaged. This review focuses on the mechanisms that contribute to this dazzling complexity and how they modulate the signaling events triggered by chemokines. The chemokines and their receptors exist as monomers, dimers and oligomers, their expression pattern is highly regulated, and the ligands can bind distinct receptors with similar affinities. The use of novel imaging-based technologies, particularly real-time imaging modalities, has shed new light on the very dynamic conformations that chemokine receptors adopt depending on the cellular context, and that affect chemokine-mediated responses. This complex scenario presents both challenging and exciting opportunities for drug discovery.

Vulnerable Plaque, Characteristics, Detection, and Potential Therapies

Plaque development and rupture are hallmarks of atherosclerotic vascular disease. Despite current therapeutic developments, there is an unmet necessity in the prevention of atherosclerotic vascular disease. It remains a challenge to determine at an early stage if atherosclerotic plaque will become unstable and vulnerable. The arrival of molecular imaging is receiving more attention, considering it allows for a better understanding of the biology of human plaque and vulnerabilities. Various plaque therapies with common goals have been tested in high-risk patients with cardiovascular disease. In this work, the process of plaque instability, along with current technologies for sensing and predicting high-risk plaques, is debated. Updates on potential novel therapeutic approaches are also summarized.

Engineering of Nanobodies Recognizing the Human Chemokine Receptor CCR7

The chemokine receptor CCR7 plays a pivotal role in health and disease. In particular, CCR7 controls homing of antigen-bearing dendritic cells and T cells to lymph nodes, where adaptive immune responses are initiated. However, CCR7 also guides T cells to inflamed synovium and thereby contributes to rheumatoid arthritis and promotes cancer cell migration and metastasis formation. Nanobodies have recently emerged as versatile tools to study G-protein-coupled receptor functions and are being tested in diagnostics and therapeutics. In this study, we designed a strategy to engineer novel nanobodies recognizing human CCR7. We generated a nanobody library based on a solved crystal structure of the nanobody Nb80 recognizing the β2-adrenergic receptor (β2AR) and by specifically randomizing two segments within complementarity determining region 1 (CDR1) and CDR3 of Nb80 known to interact with β2AR. We fused the nanobody library to one half of split-YFP in order to identify individual nanobody clones interacting with CCR7 fused to the other half of split-YFP using bimolecular fluorescence complementation. We present three novel nanobodies, termed Nb1, Nb5, and Nb38, that recognize human CCR7 without interfering with G-protein-coupling and downstream signaling. Moreover, we were able to follow CCR7 trafficking upon CCL19 triggering using Nb1, Nb5, and Nb38.

The neutral sphingomyelinase 2 in T cell receptor signaling and polarity

By hydrolyzing its substrate sphingomyelin at the cytosolic leaflet of cellular membranes, the neutral sphingomyelinase 2 (NSM2) generates microdomains which serve as docking sites for signaling proteins and thereby, functions to regulate signal relay. This has been particularly studied in cellular stress responses while the regulatory role of this enzyme in the immune cell compartment has only recently emerged. In T cells, phenotypic polarization by co-ordinated cytoskeletal remodeling is central to motility and interaction with endothelial or antigen-presenting cells during tissue recruitment or immune synapse formation, respectively. This review highlights studies adressing the role of NSM2 in T cell polarity in which the enzyme plays a major role in regulating cytoskeletal dynamics.

Identification of Cosalane as an Inhibitor of Human and Murine CC-Chemokine Receptor 7 Signaling via a High-Throughput Screen

CC-chemokine receptor 7 (CCR7) is a G protein-coupled receptor expressed on a variety of immune cells. CCR7 plays a critical role in the migration of lymphocytes into secondary lymphoid tissues. CCR7 expression, however, has been linked to numerous disease states. Due to its therapeutic relevance and absence of available CCR7 inhibitors, we undertook a high-throughput screen (HTS) to identify small-molecule antagonists of the receptor. Here, we describe a robust HTS approach using a commercially available β-galactosidase enzyme fragment complementation system and confirmatory transwell chemotaxis assays. This work resulted in the identification of several compounds with activity against CCR7. The most potent of these was subsequently determined to be cosalane, a cholesterol derivative previously designed as a therapeutic for human immunodeficiency virus. Cosalane inhibited both human and murine CCR7 in response to both CCL19 and CCL21 agonists at physiologic concentrations. Furthermore, cosalane produced durable inhibition of the receptor following a cellular incubation period with subsequent washout. Overall, our work describes the development of an HTS-compatible assay, completion of a large HTS campaign, and demonstration for the first time that cosalane is a validated CCR7 antagonist. These efforts could pave the way for new approaches to address CCR7-associated disease processes.

Remodeling our concept of chemokine receptor function: From monomers to oligomers

The chemokines direct leukocyte recruitment in both homeostatic and inflammatory conditions, and are therefore critical for immune reactions. By binding to members of the class A G protein-coupled receptors, the chemokines play an essential role in numerous physiological and pathological processes. In the last quarter century, the field has accumulated much information regarding the implications of these molecules in different immune processes, as well as mechanistic insight into the signaling events activated through their binding to their receptors. Here, we will focus on chemokine receptors and how new methodological approaches have underscored the role of their conformations in chemokine functions. Advances in biophysical-based techniques show that chemokines and their receptors act in very complex networks and therefore should not be considered isolated entities. In this regard, the chemokine receptors can form homo- and heterodimers as well as oligomers at the cell surface. These findings are changing our view as to how chemokines influence cell biology, identify partners that regulate chemokine function, and open new avenues for therapeutic intervention.

Closely related, yet unique: Distinct homo- and heterodimerization patterns of G protein coupled chemokine receptors and their fine-tuning by cholesterol

Chemokine receptors, a subclass of G protein coupled receptors (GPCRs), play essential roles in the human immune system, they are involved in cancer metastasis as well as in HIV-infection. A plethora of studies show that homo- and heterodimers or even higher order oligomers of the chemokine receptors CXCR4, CCR5, and CCR2 modulate receptor function. In addition, membrane cholesterol affects chemokine receptor activity. However, structural information about homo- and heterodimers formed by chemokine receptors and their interplay with cholesterol is limited. Here, we report homo- and heterodimer configurations of the chemokine receptors CXCR4, CCR5, and CCR2 at atomistic detail, as obtained from thousands of molecular dynamics simulations. The observed homodimerization patterns were similar for the closely related CC chemokine receptors, yet they differed significantly between the CC receptors and CXCR4. Despite their high sequence identity, cholesterol modulated the CC homodimer interfaces in a subtype-specific manner. Chemokine receptor heterodimers display distinct dimerization patterns for CXCR4/CCR5 and CXCR4/CCR2. Furthermore, associations between CXCR4 and CCR5 reveal an increased cholesterol-sensitivity as compared to CXCR4/CCR2 heterodimerization patterns. This work provides a first comprehensive structural overview over the complex interaction network between chemokine receptors and indicates how heterodimerization and the interaction with the membrane environment diversifies the function of closely related GPCRs.

Myocyte membrane and microdomain modifications in diabetes: determinants of ischemic tolerance and cardioprotection

Cardiovascular disease, predominantly ischemic heart disease (IHD), is the leading cause of death in diabetes mellitus (DM). In addition to eliciting cardiomyopathy, DM induces a ‘wicked triumvirate’: (i) increasing the risk and incidence of IHD and myocardial ischemia; (ii) decreasing myocardial tolerance to ischemia–reperfusion (I–R) injury; and (iii) inhibiting or eliminating responses to cardioprotective stimuli. Changes in ischemic tolerance and cardioprotective signaling may contribute to substantially higher mortality and morbidity following ischemic insult in DM patients. Among the diverse mechanisms implicated in diabetic impairment of ischemic tolerance and cardioprotection, changes in sarcolemmal makeup may play an overarching role and are considered in detail in the current review. Observations predominantly in animal models reveal DM-dependent changes in membrane lipid composition (cholesterol and triglyceride accumulation, fatty acid saturation vs. reduced desaturation, phospholipid remodeling) that contribute to modulation of caveolar domains, gap junctions and T-tubules. These modifications influence sarcolemmal biophysical properties, receptor and phospholipid signaling, ion channel and transporter functions, contributing to contractile and electrophysiological dysfunction, cardiomyopathy, ischemic intolerance and suppression of protective signaling. A better understanding of these sarcolemmal abnormalities in types I and II DM (T1DM, T2DM) can inform approaches to limiting cardiomyopathy, associated IHD and their consequences. Key knowledge gaps include details of sarcolemmal changes in models of T2DM, temporal patterns of lipid, microdomain and T-tubule changes during disease development, and the precise impacts of these diverse sarcolemmal modifications. Importantly, exercise, dietary, pharmacological and gene approaches have potential for improving sarcolemmal makeup, and thus myocyte function and stress-resistance in this ubiquitous metabolic disorder.

The Activity of the Neutral Sphingomyelinase Is Important in T Cell Recruitment and Directional Migration

Breakdown of sphingomyelin as catalyzed by the activity of sphingomyelinases profoundly affects biophysical properties of cellular membranes which is particularly important with regard to compartmentalization of surface receptors and their signaling relay. As it is activated both upon TCR ligation and co-stimulation in a spatiotemporally controlled manner, the neutral sphingomyelinase (NSM) has proven to be important in T cell activation, where it appears to play a particularly important role in cytoskeletal reorganization and cell polarization. Because these are important parameters in directional T cell migration and motility in tissues, we analyzed the role of the NSM in these processes. Pharmacological inhibition of NSM interfered with early lymph node homing of T cells in vivo indicating that the enzyme impacts on endothelial adhesion, transendothelial migration, sensing of chemokine gradients or, at a cellular level, acquisition of a polarized phenotype. NSM inhibition reduced adhesion of T cells to TNF-α/IFN-γ activated, but not resting endothelial cells, most likely via inhibiting high-affinity LFA-1 clustering. NSM activity proved to be highly important in directional T cell motility in response to SDF1-α, indicating that their ability to sense and translate chemokine gradients might be NSM dependent. In fact, pharmacological or genetic NSM ablation interfered with T cell polarization both at an overall morphological level and redistribution of CXCR4 and pERM proteins on endothelial cells or fibronectin, as well as with F-actin polymerization in response to SDF1-α stimulation, indicating that efficient directional perception and signaling relay depend on NSM activity. Altogether, these data support a central role of the NSM in T cell recruitment and migration both under homeostatic and inflamed conditions by regulating polarized redistribution of receptors and their coupling to the cytoskeleton.