Adenosine receptors and membrane microdomains

Low Density Lipoprotein Cholesterol Decreases the Expression of Adenosine A2A Receptor and Lipid Rafts-Protein Flotillin-1: Insights on Cardiovascular Risk of Hypercholesterolemia

High blood levels of low-density lipoprotein (LDL)-cholesterol (LDL-C) are associated with atherosclerosis, mainly by promoting foam cell accumulation in vessels. As cholesterol is an essential component of cell plasma membranes and a regulator of several signaling pathways, LDL-C excess may have wider cardiovascular toxicity. We examined, in untreated hypercholesterolemia (HC) patients, selected regardless of the cause of LDL-C accumulation, and in healthy participants (HP), the expression of the adenosine A2A receptor (A2AR), an anti-inflammatory and vasodilatory protein with cholesterol-dependent modulation, and Flotillin-1, protein marker of cholesterol-enriched plasma membrane domains. Blood cardiovascular risk and inflammatory biomarkers were measured. A2AR and Flotillin-1 expression in peripheral blood mononuclear cells (PBMC) was lower in patients compared to HP and negatively correlated to LDL-C blood levels. No other differences were observed between the two groups apart from transferrin and ferritin concentrations. A2AR and Flotillin-1 proteins levels were positively correlated in the whole study population. Incubation of HP PBMCs with LDL-C caused a similar reduction in A2AR and Flotillin-1 expression. We suggest that LDL-C affects A2AR expression by impacting cholesterol-enriched membrane microdomains. Our results provide new insights into the molecular mechanisms underlying cholesterol toxicity, and may have important clinical implication for assessment and treatment of cardiovascular risk in HC.

Rethinking purinergic concepts and updating the emerging role of P2X7 and P2X4 in amyotrophic lateral sclerosis

The topic of the present review regards the ubiquitous and phylogenetically most ancient prototype of intercellular signaling, the one mediated by extracellular nucleosides and nucleotides, bearing a strong influence on pathophysiological processes in the nervous system. Not by chance, purine and pyrimidine molecules are the most prevalent and ubiquitous chemical messengers in the animal and plant kingdoms, operating through a large plethora of purinergic metabolizing enzymes, P1 and P2 receptors, nucleoside and nucleotide channels and transporters. Because ectonucleotidases degrade the agonists of P2 receptors while simultaneously generate the agonists for P1 receptors, and because several agonists, or antagonists, simultaneously bind and activate, or inhibit, more than one receptor subtype, it follows that an all-inclusive "purinergic network" perspective should be better considered when looking at purinergic actions. This becomes particularly crucial during pathological conditions as for instance amyotrophic lateral sclerosis, where the contribution of purinergic signaling has been demonstrated to differ according to each target cell phenotype and stage of disease progression. Here we will present some newly updated results about P2X7 and P2X4 as the most thoroughly investigated P2 receptors in amyotrophic lateral sclerosis, being aware that the comprehension of their actions is still in progress, and that the purinergic rationale for studying this disease must be however wide-ranging and all-inclusive. This article is part of the Special Issue on 'Purinergic Signaling: 50 years'.

Molecular Simulations and Drug Discovery of Adenosine Receptors

G protein-coupled receptors (GPCRs) represent the largest family of human membrane proteins. Four subtypes of adenosine receptors (ARs), the A1AR, A2AAR, A2BAR and A3AR, each with a unique pharmacological profile and distribution within the tissues in the human body, mediate many physiological functions and serve as critical drug targets for treating numerous human diseases including cancer, neuropathic pain, cardiac ischemia, stroke and diabetes. The A1AR and A3AR preferentially couple to the Gi/o proteins, while the A2AAR and A2BAR prefer coupling to the Gs proteins. Adenosine receptors were the first subclass of GPCRs that had experimental structures determined in complex with distinct G proteins. Here, we will review recent studies in molecular simulations and computer-aided drug discovery of the adenosine receptors and also highlight their future research opportunities.

Smooth-talking T cells leave asthma breathless

The plasma membrane channel PANX1 mediates release of bio-active adenine nucleotides; however, its function in immune cells is unknown. In this issue of Immunity, Medina et al. show that PANX1 mediates adenosine-dependent communication between regulatory and effector CD4⁺ T cells during allergic airway inflammation.

Clustering of adenosine A2B receptors with ectonucleotidases in caveolin-rich lipid rafts underlies immunomodulation by Leishmania amazonensis

Extracellular adenosine plays important roles in modulating the immune responses. We have previously demonstrated that infection of dendritic cells (DC) by Leishmania amazonensis leads to increased expression of CD39 and CD73 and to the selective activation of the low affinity A_2B receptors (A_2B R), which contributes to DC inhibition, without involvement of the high affinity A_2A R. To understand this apparent paradox, we now characterized the alterations of both adenosine receptors in infected cells. With this aim, bone marrow-derived DC from C57BL/6J mice were infected with metacyclic promastigotes of L. amazonensis. Fluorescence microscopy revealed that L. amazonensis infection stimulates the recruitment of A_2B R, but not of A_2A R, to the surface of infected DC, without altering the amount of mRNA or the total A_2B R density, an effect dependent on lipophosphoglycan (LPG). Log-phase promastigotes or axenic amastigotes of L. amazonensis do not stimulate A_2B R recruitment. A_2B R clusters are localized in caveolin-rich lipid rafts and the disruption of these membrane domains impairs A_2B R recruitment and activation. More importantly, our results show that A_2B R co-localize with CD39 and CD73 forming a "purinergic cluster" that allows for the production of extracellular adenosine in close proximity with these receptors. We conclude that A_2B R activation by locally produced adenosine constitutes an elegant and powerful evasion mechanism used by L. amazonensis to down-modulate the DC activation.

Molecular Dynamics Simulations of Adenosine Receptors: Advances, Applications and Trends

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Adenosine receptors (ARs) are transmembrane proteins that belong to the G protein-coupled receptors (GPCRs) superfamily and mediate the biological functions of adenosine. To date, four AR subtypes are known, namely A1, A2A, A2B and A3 that exhibit different signaling pathways, tissue localization, and mechanisms of activation. Moreover, the widespread ARs and their implication in numerous physiological and pathophysiological conditions had made them pivotal therapeutic targets for developing clinically effective agents.

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The crystallographic success in identifying the 3D crystal structures of A2A and A1 ARs has dramatically enriched our understanding of their structural and functional properties such as ligand binding and signal transduction. This, in turn, has provided a structural basis for a larger contribution of computational methods, particularly molecular dynamics (MD) simulations, toward further investigation of their molecular properties and designing bioactive ligands with therapeutic potential. MD simulation has been proved to be an invaluable tool in investigating ARs and providing answers to some critical questions. For example, MD has been applied in studying ARs in terms of ligand-receptor interactions, molecular recognition, allosteric modulations, dimerization, and mechanisms of activation, collectively aiding in the design of subtype selective ligands.

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In this review, we focused on the advances and different applications of MD simulations utilized to study the structural and functional aspects of ARs that can foster the structure-based design of drug candidates. In addition, relevant literature was briefly discussed which establishes a starting point for future advances in the field of drug discovery to this pivotal group of drug targets.

Reduced level of docosahexaenoic acid shifts GPCR neuroreceptors to less ordered membrane regions

G protein-coupled receptors (GPCRs) control cellular signaling and responses. Many of these GPCRs are modulated by cholesterol and polyunsaturated fatty acids (PUFAs) which have been shown to co-exist with saturated lipids in ordered membrane domains. However, the lipid compositions of such domains extracted from the brain cortex tissue of individuals suffering from GPCR-associated neurological disorders show drastically lowered levels of PUFAs. Here, using free energy techniques and multiscale simulations of numerous membrane proteins, we show that the presence of the PUFA DHA helps helical multi-pass proteins such as GPCRs partition into ordered membrane domains. The mechanism is based on hybrid lipids, whose PUFA chains coat the rough protein surface, while the saturated chains face the raft environment, thus minimizing perturbations therein. Our findings suggest that the reduction of GPCR partitioning to their native ordered environments due to PUFA depletion might affect the function of these receptors in numerous neurodegenerative diseases, where the membrane PUFA levels in the brain are decreased. We hope that this work inspires experimental studies on the connection between membrane PUFA levels and GPCR signaling.

Our current picture of cellular membranes depicts them as laterally heterogeneous sheets of lipids crowded with membrane proteins. These proteins often require a specific lipid environment to efficiently perform their functions. Certain neuroreceptor proteins are regulated by membrane cholesterol that is considered to be enriched in ordered membrane domains. In the brain, these very same domains also contain a fair amount of polyunsaturated fatty acids (PUFAs) that have also been discovered to interact favorably with many receptor proteins. However, certain neurological diseases—associated with the inadequate functioning of the neuroreceptors—seem to result in the decrease of brain PUFA levels. We hypothesized that this decrease in PUFA levels somehow inhibits receptor partitioning to cholesterol-rich domains, which could further compromise their function. We verified our hypothesis by an extensive set of computer simulations. They demonstrated that the PUFA–receptor interaction indeed leads to favorable partitioning of the receptors in the cholesterol-rich ordered domains. Moreover, the underlying mechanism based on the shielding of the rough protein surface by the PUFAs seems to be exclusive for multi-helical protein structures, of which neuroreceptors are a prime example.

Targeting Adenosine Receptor Signaling in Cancer Immunotherapy

The immune system plays a major role in the surveillance and control of malignant cells, with the presence of tumor infiltrating lymphocytes (TILs) correlating with better patient prognosis in multiple tumor types. The development of ‘checkpoint blockade’ and adoptive cellular therapy has revolutionized the landscape of cancer treatment and highlights the potential of utilizing the patient’s own immune system to eradicate cancer. One mechanism of tumor-mediated immunosuppression that has gained attention as a potential therapeutic target is the purinergic signaling axis, whereby the production of the purine nucleoside adenosine in the tumor microenvironment can potently suppress T and NK cell function. The production of extracellular adenosine is mediated by the cell surface ectoenzymes CD73, CD39, and CD38 and therapeutic agents have been developed to target these as well as the downstream adenosine receptors (A₁R, A_2AR, A_2BR, A₃R) to enhance anti-tumor immune responses. This review will discuss the role of adenosine and adenosine receptor signaling in tumor and immune cells with a focus on their cell-specific function and their potential as targets in cancer immunotherapy.

Role of membrane cholesterol in differential sensitivity of muscarinic receptor subtypes to persistently bound xanomeline

Xanomeline (3-(Hexyloxy)-4-(1-methyl-1,2,5,6-tetrahydropyridin-3-yl)-1,2,5-thiadiazole) is a muscarinic agonist that is considered to be functionally selective for the M₁/M₄ receptor subtypes. Part of xanomeline binding is resistant to washing. Wash-resistant xanomeline activates muscarinic receptors persistently, except for the M₅ subtype. Mutation of leucine 6.46 to isoleucine at M₁ or M₄ receptors abolished persistent activation by wash-resistant xanomeline. Reciprocal mutation of isoleucine 6.46 to leucine at the M₅ receptor made it sensitive to activation by wash-resistant xanomeline. Lowering of membrane cholesterol made M₁ and M₄ mutants and M₅ wild type receptors sensitive to activation by wash-resistant xanomeline. Molecular docking revealed a cholesterol binding site in the groove between transmembrane helices 6 and 7. Molecular dynamics showed that interaction of cholesterol with this binding site attenuates receptor activation. We hypothesize that differences in cholesterol binding to this site between muscarinic receptor subtypes may constitute the basis for xanomeline apparent functional selectivity and may have notable therapeutic implications. Differences in receptor-membrane interactions, rather than in agonist-receptor interactions, represent a novel possibility to achieve pharmacological selectivity. Our findings may be applicable to other G protein coupled receptors.

Is the adenosine A2B 'biased' receptor a valuable target for the treatment of pulmonary arterial hypertension?

Pulmonary arterial hypertension (PAH) is a maladaptive disorder characterized by increased pulmonary vascular resistance leading to right ventricular failure and death. Adenosine released by injured tissues, such as the lung and heart, influences tissue remodeling through the activation of adenosine receptors. Evidence regarding activation of the low-affinity A_2BAR by adenosine points towards pivotal roles of this receptor in processes associated with both acute and chronic lung diseases. Conflicting results exist concerning the beneficial or detrimental roles of the A_2B 'biased' receptor in right ventricular failure secondary to PAH. In this review, we discuss the pros and cons of manipulating A_2BARs as a putative therapeutic target in PAH.

Ubiquitination mediates Kv1.3 endocytosis as a mechanism for protein kinase C-dependent modulation

The voltage-dependent potassium channel Kv1.3 plays essential physiological functions in the immune system. Kv1.3, regulating the membrane potential, facilitates downstream Ca²⁺ -dependent pathways and becomes concentrated in specific membrane microdomains that serve as signaling platforms. Increased and/or delocalized expression of the channel is observed at the onset of several autoimmune diseases. In this work, we show that adenosine (ADO), which is a potent endogenous modulator, stimulates PKC, thereby causing immunosuppression. PKC activation triggers down-regulation of Kv1.3 by inducing a clathrin-mediated endocytic event that targets the channel to lysosomal-degradative compartments. Therefore, the abundance of Kv1.3 at the cell surface decreases, which is clearly compatible with an effective anti-inflammatory response. This mechanism requires ubiquitination of Kv1.3, catalyzed by the E3 ubiquitin-ligase Nedd4-2. Postsynaptic density protein 95 (PSD-95), a member of the MAGUK family, recruits Kv1.3 into lipid-raft microdomains and protects the channel against ubiquitination and endocytosis. Therefore, the Kv1.3/PSD-95 association fine-tunes the anti-inflammatory response in leukocytes. Because Kv1.3 is a promising multi-therapeutic target against human pathologies, our results have physiological relevance. In addition, this work elucidates the ADO-dependent PKC-mediated molecular mechanism that triggers immunomodulation by targeting Kv1.3 in leukocytes.

Increased activity of TNAP compensates for reduced adenosine production and promotes ectopic calcification in the genetic disease ACDC

ACDC (arterial calcification due to deficiency of CD73) is an autosomal recessive disease resulting from loss-of-function mutations in NT5E, which encodes CD73, a 5'-ectonucleotidase that converts extracellular adenosine monophosphate to adenosine. ACDC patients display progressive calcification of lower extremity arteries, causing limb ischemia. Tissue-nonspecific alkaline phosphatase (TNAP), which converts pyrophosphate (PPi) to inorganic phosphate (Pi), and extracellular purine metabolism play important roles in other inherited forms of vascular calcification. Compared to cells from healthy subjects, induced pluripotent stem cell-derived mesenchymal stromal cells (iMSCs) from ACDC patients displayed accelerated calcification and increased TNAP activity when cultured under conditions that promote osteogenesis. TNAP activity generated adenosine in iMSCs derived from ACDC patients but not in iMSCs from control subjects, which have CD73. In response to osteogenic stimulation, ACDC patient-derived iMSCs had decreased amounts of the TNAP substrate PPi, an inhibitor of extracellular matrix calcification, and exhibited increased activation of AKT, mechanistic target of rapamycin (mTOR), and the 70-kDa ribosomal protein S6 kinase (p70S6K), a pathway that promotes calcification. In vivo, teratomas derived from ACDC patient cells showed extensive calcification and increased TNAP activity. Treating mice bearing these teratomas with an A2b adenosine receptor agonist, the mTOR inhibitor rapamycin, or the bisphosphonate etidronate reduced calcification. These results show that an increase of TNAP activity in ACDC contributes to ectopic calcification by disrupting the extracellular balance of PPi and Pi and identify potential therapeutic targets for ACDC.

Connectome and molecular pharmacological differences in the dopaminergic system in restless legs syndrome (RLS): plastic changes and neuroadaptations that may contribute to augmentation

Restless legs syndrome (RLS) is primarily treated with levodopa and dopaminergics that target the inhibitory dopamine receptor subtypes D3 and D2. The initial success of this therapy led to the idea of a hypodopaminergic state as the mechanism underlying RLS. However, multiple lines of evidence suggest that this simplified concept of a reduced dopamine function as the basis of RLS is incomplete. Moreover, long-term medication with the D2/D3 agonists leads to a reversal of the initial benefits of dopamine agonists and augmentation, which is a worsening of symptoms under therapy. The recent findings on the state of the dopamine system in RLS that support the notion that a dysfunction in the dopamine system may in fact induce a hyperdopaminergic state are summarized. On the basis of these data, the concept of a dynamic nature of the dopamine effects in a circadian context is presented. The possible interactions of cell adhesion molecules expressed by the dopaminergic systems and their possible effects on RLS and augmentation are discussed. Genome-wide association studies (GWAS) indicate a significantly increased risk for RLS in populations with genomic variants of the cell adhesion molecule receptor type protein tyrosine phosphatase D (PTPRD), and PTPRD is abundantly expressed by dopamine neurons. PTPRD may play a role in the reconfiguration of neural circuits, including shaping the interplay of G protein-coupled receptor (GPCR) homomers and heteromers that mediate dopaminergic modulation. Recent animal model data support the concept that interactions between functionally distinct dopamine receptor subtypes can reshape behavioral outcomes and change with normal aging. Additionally, long-term activation of one dopamine receptor subtype can increase the receptor expression of a different receptor subtype with opposite modulatory actions. Such dopamine receptor interactions at both spinal and supraspinal levels appear to play important roles in RLS. In addition, these interactions can extend to the adenosine A₁ and A_2A receptors, which are also prominently expressed in the striatum. Interactions between adenosine and dopamine receptors and dopaminergic cell adhesion molecules, including PTPRD, may provide new pharmacological targets for treating RLS. In summary, new treatment options for RLS that include recovery from augmentation will have to consider dynamic changes in the dopamine system that occur during the circadian cycle, plastic changes that can develop as a function of treatment or with aging, changes in the connectome based on alterations in cell adhesion molecules, and receptor interactions that may extend beyond the dopamine system itself.

A2A adenosine receptor modulates drug efflux transporter P-glycoprotein at the blood-brain barrier

The blood-brain barrier (BBB) protects the brain from toxic substances within the peripheral circulation. It maintains brain homeostasis and is a hurdle for drug delivery to the CNS to treat neurodegenerative diseases, including Alzheimer's disease and brain tumors. The drug efflux transporter P-glycoprotein (P-gp) is highly expressed on brain endothelial cells and blocks the entry of most drugs delivered to the brain. Here, we show that activation of the A2A adenosine receptor (AR) with an FDA-approved A2A AR agonist (Lexiscan) rapidly and potently decreased P-gp expression and function in a time-dependent and reversible manner. We demonstrate that downmodulation of P-gp expression and function coincided with chemotherapeutic drug accumulation in brains of WT mice and in primary mouse and human brain endothelial cells, which serve as in vitro BBB models. Lexiscan also potently downregulated the expression of BCRP1, an efflux transporter that is highly expressed in the CNS vasculature and other tissues. Finally, we determined that multiple pathways, including MMP9 cleavage and ubiquitinylation, mediated P-gp downmodulation. Based on these data, we propose that A2A AR activation on BBB endothelial cells offers a therapeutic window that can be fine-tuned for drug delivery to the brain and has potential as a CNS drug-delivery technology.

Changes in Purines Concentration in the Cerebrospinal Fluid of Pregnant Women Experiencing Pain During Active Labor

Labor pain has been reported as a severe pain and can be considered as a model of acute visceral pain. It is well known that extracellular purines have an important role in pain signaling in the central nervous system. This study analyzes the relationship between extracellular purines and pain perception during active labor. A prospective observational study was performed. Cerebrospinal fluid (CSF) levels of the purines and their metabolites were compared between women at term pregnancy with labor pain (n = 49) and without labor pain (Caesarian section; n = 47). Control groups (healthy men and women without chronic or acute pain-n = 40 and 32, respectively) were also investigated. The CSF levels of adenosine were significantly lower in the labor pain group (P = 0.026) and negatively correlated with pain intensity measured by a visual analogue scale (r = -0.48, P = 0.0005). Interestingly, CSF levels of uric acid were significantly higher in healthy men as compared to women. Additionally, pregnant women showed increased CSF levels of ADP, GDP, adenosine and guanosine and reduced CSF levels of AMP, GTP, and uric acid as compared to non-pregnant women (P < 0.05). These findings suggest that purines, in special the nucleoside adenosine, are associated with pregnancy and labor pain.

New perspectives in signaling mediated by receptors coupled to stimulatory G protein: the emerging significance of cAMP efflux and extracellular cAMP-adenosine pathway

G protein-coupled receptors (GPCRs) linked to stimulatory G (Gs) proteins (GsPCRs) mediate increases in intracellular cyclic AMP as consequence of activation of nine adenylyl cyclases , which differ considerably in their cellular distribution and activation mechanisms. Once produced, cyclic AMP may act via distinct intracellular signaling effectors such as protein kinase A and the exchange proteins activated by cAMP (Epacs). More recently, attention has been focused on the efflux of cAMP through a specific transport system named multidrug resistance proteins that belongs to the ATP-binding cassette transporter superfamily. Outside the cell, cAMP is metabolized into adenosine, which is able to activate four distinct subtypes of adenosine receptors, members of the GPCR family: A₁, A_2A, A_2B, and A₃. Taking into account that this phenomenon occurs in numerous cell types, as consequence of GsPCR activation and increment in intracellular cAMP levels, in this review, we will discuss the impact of cAMP efflux and the extracellular cAMP-adenosine pathway on the regulation of GsPCR-induced cell response.

Adenosine inhibits renin release from juxtaglomerular cells via an A1 receptor-TRPC-mediated pathway

Renin is synthesized and released from juxtaglomerular (JG) cells. Adenosine inhibits renin release via an adenosine A1 receptor (A1R) calcium-mediated pathway. How this occurs is unknown. In cardiomyocytes, adenosine increases intracellular calcium via transient receptor potential canonical (TRPC) channels. We hypothesized that adenosine inhibits renin release via A1R activation, opening TRPC channels. However, higher concentrations of adenosine may stimulate renin release through A2R activation. Using primary cultures of isolated mouse JG cells, immunolabeling demonstrated renin and A1R in JG cells, but not A2R subtypes, although RT-PCR indicated the presence of mRNA of both A2AR and A2BR. Incubating JG cells with increasing concentrations of adenosine decreased renin release. Different concentrations of the adenosine receptor agonist N-ethylcarboxamide adenosine (NECA) did not change renin. Activating A1R with 0.5 μM N6-cyclohexyladenosine (CHA) decreased basal renin release from 0.22 ± 0.05 to 0.14 ± 0.03 μg of angiotensin I generated per milliliter of sample per hour of incubation (AngI/ml/mg prot) (P < 0.03), and higher concentrations also inhibited renin. Reducing extracellular calcium with EGTA increased renin release (0.35 ± 0.08 μg AngI/ml/mg prot; P < 0.01), and blocked renin inhibition by CHA (0.28 ± 0.06 μg AngI/ml/mg prot; P < 0. 005 vs. CHA alone). The intracellular calcium chelator BAPTA-AM increased renin release by 55%, and blocked the inhibitory effect of CHA. Repeating these experiments in JG cells from A1R knockout mice using CHA or NECA demonstrated no effect on renin release. However, RT-PCR showed mRNA from TRPC isoforms 3 and 6 in isolated JG cells. Adding the TRPC blocker SKF-96365 reversed CHA-mediated inhibition of renin release. Thus A1R activation results in a calcium-dependent inhibition of renin release via TRPC-mediated calcium entry, but A2 receptors do not regulate renin release.

Interaction of δ and κ opioid receptors with adenosine A1 receptors mediates cardioprotection by remote ischemic preconditioning

Multiple initiatives are underway to harness the clinical benefits of remote ischemic preconditioning (rIPC) based on applying non-invasive, brief, intermittent limb ischemia/reperfusion using an external occluder. However, little is known about how rIPC induces protection in cardiomyocytes, particularly through G-protein coupled receptors. In these studies, we determined the role of opioid and adenosine receptors and their functional interactions in rIPC cardioprotection. In freshly isolated cardiomyocytes subjected to 45-min simulated ischemia followed by 60-min simulated reperfusion, we examined the ability of plasma dialysate (derived from blood obtained from rabbits remotely preconditioned by application of brief cycles of hind limb ischemia/reperfusion, rIPC dialysate) to protect cells against necrosis. rIPC dialysate and selective activation of either δ-opioid receptors or κ-opioid receptors significantly reduced the % of dead cells after simulated ischemia and simulated reperfusion. Inhibition of adenosine A1 receptors, but not adenosine A3 receptors, blocked the protection by rIPC dialysate, δ-opioid receptor and κ-opioid receptor activation. In HEK293 cells expressing either hemagglutinin A-tagged δ-opioid receptors or hemagglutinin A-tagged κ-opioid receptors, selective immunoprecipitation of adenosine A1 receptors pulled down both δ-opioid and κ-opioid receptors. This molecular association of adenosine A1 receptors with δ-opioid and κ-opioid receptors was confirmed by reverse pull-down assays. These findings strongly suggest that rIPC cardioprotection requires the activation of δ-opioid and κ-opioid receptors and relies on these receptors functionally interacting with adenosine A1 receptors.

Lipid rafts, synaptic transmission and plasticity: impact in age-related neurodegenerative diseases

The synapse is a crowded area. In the last years, the concept that proteins can be organized in different membrane domains according to their structure has emerged. Cholesterol-rich membrane domains, or lipid rafts, form an organized portion of the membrane that is thought to concentrate signaling molecules. Accumulating evidence has shown that both the pre-synaptic and post-synaptic sites are highly enriched in lipid rafts, which are likely to organize and maintain synaptic proteins in their precise localization. Here we review recent studies highlighting the importance of lipid rafts for synaptic function and plasticity, as well as their relevance for age or disease-related cognitive impairment. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Do annexins participate in lipid messenger mediated intracellular signaling? A question revisited

Annexins are physiologically important proteins that play a role in calcium buffering but also influence membrane structure, participate in Ca²⁺-dependent membrane repair events and in remodelling of the cytoskeleton. Thirty years ago several peptides isolated from lung perfusates, peritoneal leukocytes, neutrophiles and renal cells were proven inhibitory to the activity of phospholipase A₂. Those peptides were found to derive from structurally related proteins: annexins AnxA1 and AnxA2. These findings raised the question whether annexins may participate in regulation of the production of lipid second messengers and, therefore, modulate numerous lipid mediated signaling pathways in the cell. Recent advances in the field of annexins made also with the use of knock-out animal models revealed that these proteins are indeed important constituents of specific signaling pathways. In this review we provide evidence supporting the hypothesis that annexins, as membrane-binding proteins and organizers of the membrane lateral heterogeneity, may participate in lipid mediated signaling pathways by affecting the distribution and activity of lipid metabolizing enzymes (most of the reports point to phospholipase A₂) and of protein kinases regulating activity of these enzymes. Moreover, some experimental data suggest that annexins may directly interact with lipid metabolizing enzymes and, in a calcium-dependent or independent manner, with some of their substrates and products. On the basis of these observations, many investigators suggest that annexins are capable of linking Ca²⁺, redox and lipid signaling to coordinate vital cellular responses to the environmental stimuli.

Annexins as organizers of cholesterol- and sphingomyelin-enriched membrane microdomains in Niemann-Pick type C disease

Growing evidence suggests that membrane microdomains enriched in cholesterol and sphingomyelin are sites for numerous cellular processes, including signaling, vesicular transport, interaction with pathogens, and viral infection, etc. Recently some members of the annexin family of conserved calcium and membrane-binding proteins have been recognized as cholesterol-interacting molecules and suggested to play a role in the formation, stabilization, and dynamics of membrane microdomains to affect membrane lateral organization and to attract other proteins and signaling molecules onto their territory. Furthermore, annexins were implicated in the interactions between cytosolic and membrane molecules, in the turnover and storage of cholesterol and in various signaling pathways. In this review, we focus on the mechanisms of interaction of annexins with lipid microdomains and the role of annexins in membrane microdomains dynamics including possible participation of the domain-associated forms of annexins in the etiology of human lysosomal storage disease called Niemann-Pick type C disease, related to the abnormal storage of cholesterol in the lysosome-like intracellular compartment. The involvement of annexins and cholesterol/sphingomyelin-enriched membrane microdomains in other pathologies including cardiac dysfunctions, neurodegenerative diseases, obesity, diabetes mellitus, and cancer is likely, but is not supported by substantial experimental observations, and therefore awaits further clarification.