Role of phospholipase D2 in the agonist-induced and constitutive endocytosis of G-protein coupled receptors

Mechanical activation of TWIK-related potassium channel by nanoscopic movement and rapid second messenger signaling

Rapid conversion of force into a biological signal enables living cells to respond to mechanical forces in their environment. The force is believed to initially affect the plasma membrane and then alter the behavior of membrane proteins. Phospholipase D2 (PLD2) is a mechanosensitive enzyme that is regulated by a structured membrane-lipid site comprised of cholesterol and saturated ganglioside (GM1). Here we show stretch activation of TWIK-related K+ channel (TREK-1) is mechanically evoked by PLD2 and spatial patterning involving ordered GM1 and 4,5-bisphosphate (PIP2) clusters in mammalian cells. First, mechanical force deforms the ordered lipids, which disrupts the interaction of PLD2 with the GM1 lipids and allows a complex of TREK-1 and PLD2 to associate with PIP2 clusters. The association with PIP2 activates the enzyme, which produces the second messenger phosphatidic acid (PA) that gates the channel. Co-expression of catalytically inactive PLD2 inhibits TREK-1 stretch currents in a biological membrane. Cellular uptake of cholesterol inhibits TREK-1 currents in culture and depletion of cholesterol from astrocytes releases TREK-1 from GM1 lipids in mouse brain. Depletion of the PLD2 ortholog in flies results in hypersensitivity to mechanical force. We conclude PLD2 mechanosensitivity combines with TREK-1 ion permeability to elicit a mechanically evoked response.

Serotonin: an overlooked regulator of endocytosis and endosomal sorting?

Serotonin is a neurotransmitter and a hormone that is typically associated with regulating our mood. However, the serotonin transporter and receptors are expressed throughout the body, highlighting the much broader, systemic role of serotonin in regulating human physiology. A substantial body of data strongly implicates serotonin as a fundamental regulator of endocytosis and endocytic sorting. Serotonin has the potential to enhance endocytosis through three distinct mechanisms - serotonin signalling, serotonylation and insertion into the plasma membrane - although the interplay and relationship between these mechanisms has not yet been explored. Endocytosis is central to the cellular response to the extracellular environment, controlling receptor distribution on the plasma membrane to modulate signalling, neurotransmitter release and uptake, circulating protein and lipid cargo uptake, and amino acid internalisation for cell proliferation. Uncovering the range of cellular and physiological circumstances in which serotonin regulates endocytosis is of great interest for our understanding of how serotonin regulates mood, and also the fundamental understanding of endocytosis and its regulation throughout the body. This article has an associated Future Leader to Watch interview with the first author of the paper.

The phospholipase D inhibitor FIPI potently blocks EGF-induced calcium signaling in human breast cancer cells

BACKGROUND

Phosphotyrosine kinase (PTK)-mediated phospholipase C-γ1 (PLC-γ1) signaling plays a crucial role in the release of the universal second messenger calcium from intracellular stores, which is mandatory for several cellular processes, including cell migration. However, PLC-γ1 could also be activated in a PTK-independent manner by phospholipase D (PLD)-derived phosphatidic acid (PA). Because both higher PLD expression levels and PLD activity have also been associated with breast cancer cell invasion and migration, we wondered whether there might be a link between PLD and PLC-γ1, which was investigated in this study.

MATERIALS

MDA-MB-468-NEO (EGFR positive) and MDA-MB-468-HER2 (EGFR and HER2 positive) human breast cancer cells were used in this study. The migratory behavior of the cells in the presence of epidermal growth factor (EGF) and the PLD inhibitor 5-fluoro-2-indolyl-des-chlorohalopemide (FIPI) was analyzed using the 3D collagen matrix migration assay. Changes in cytosolic calcium levels in the presence of EGF, FIPI and Sig-1R agonists and antagonists as well as in PLD1 siRNA knockdown cells were determined by flow cytometry. Western blot analyses were performed to determine the basal expression levels and phosphorylation patterns of EGFR, HER2, AKT, MAPKp42/44, PLC-γ1 and Sig-1R.

RESULTS

The EGF-induced migration of MDA-MB-468-NEO and MDA-MB-468-HER2 cells was significantly impaired by FIPI. Likewise, FIPI also significantly abolished EGF-induced calcium release in both cell lines. However, neither the expression levels nor the phosphorylation patterns of EGFR, HER2, AKT, MAPKp42/44 and PLC-γ1 were markedly changed by FIPI. Knockdown of PLD1 expression by siRNA also significantly impaired EGF-induced calcium release in both cell lines. Targeting Sig-1R, which interacts with IP3R, with the antagonist BD1047 also abrogated EGF-induced calcium release. However, EGF-induced calcium release was also impaired if cells were treated with the Sig-1R agonists PRE084 and PPBP maleate.

CONCLUSION

In summary, blocking PLD activity with the specific inhibitor FIPI or knocking down PDL1 expression by siRNA significantly impaired EGF-induced calcium release in MDA-MB-468-NEO and MDA-MB-468-HER2 cells, likely indicating a connection between PLD activity and PLC-γ1-mediated calcium signaling. However, how PLD activity interferes with the release of calcium from intracellular stores remains unclear. Video Abstract.

Mammalian phospholipase D: Function, and therapeutics

Despite being discovered over 60 years ago, the precise role of phospholipase D (PLD) is still being elucidated. PLD enzymes catalyze the hydrolysis of the phosphodiester bond of glycerophospholipids producing phosphatidic acid and the free headgroup. PLD family members are found in organisms ranging from viruses, and bacteria to plants, and mammals. They display a range of substrate specificities, are regulated by a diverse range of molecules, and have been implicated in a broad range of cellular processes including receptor signaling, cytoskeletal regulation and membrane trafficking. Recent technological advances including: the development of PLD knockout mice, isoform-specific antibodies, and specific inhibitors are finally permitting a thorough analysis of the in vivo role of mammalian PLDs. These studies are facilitating increased recognition of PLD's role in disease states including cancers and Alzheimer's disease, offering potential as a target for therapeutic intervention.

Buprenorphine and Methadone as Opioid Maintenance Treatments for Heroin-Addicted Patients Induce Oxidative Stress in Blood

Buprenorphine and methadone are two substances widely used in the substitution treatment of patients who are addicted to opioids. Although it is known that they partly act efficiently towards this direction, there is no evidence regarding their effects on the redox status of patients, a mechanism that could potentially improve their action. Therefore, the aim of the present investigation was to examine the impact of buprenorphine and methadone, which are administered as substitutes to heroin-dependent patients on specific redox biomarkers in the blood. From the results obtained, both the buprenorphine (n = 21) and the methadone (n = 21) groups exhibited oxidative stress and compromised antioxidant defence. This was evident by the decreased glutathione (GSH) concentration and catalase activity in erythrocytes and the increased concentrations of thiobarbituric acid reactive substances (TBARS) and protein carbonyls in the plasma, while there was no significant alteration of plasma total antioxidant capacity (TAC) compared to the healthy individuals (n = 29). Furthermore, methadone revealed more severe oxidant action compared to buprenorphine. Based on relevant studies, the tested substitutes mitigate the detrimental effects of heroin on patient redox status; still it appears that they need to be boosted. Therefore, concomitant antioxidant administration could potentially enhance their beneficial action, and most probably, buprenorphine that did not induce oxidative stress in such a severe mode as methadone, on the regulation of blood redox status.

The HECT ubiquitin E3 ligase Smurf2 degrades μ-opioid receptor 1 in the ubiquitin-proteasome system in lung epithelial cells

Opioids are widely used for relieving clinical acute or chronic pain. The biological effects of opioids are through activating μ-opioid receptor 1 (MOR1). Most studies have focused on the consequences of agonist-induced MOR1 phosphorylation, ubiquitination, and internalization. Agonist-mediated MOR1 degradation, which is crucial for receptor stability and responsiveness, has not been well studied. E3 ubiquitin-protein ligase SMURF2 (Smurf2), a homolog to E6AP carboxy terminus (HECT) ubiquitin E3 ligase, has been shown to regulate MOR1 ubiquitination and internalization; however, its role in MOR1 degradation has not been studied. Here, we demonstrate that Smurf2 mediates [d-Ala²,N-MePhe⁴,Gly⁵-ol]-enkephalin (DAMGO, an agonist of MOR1)-induced MOR1 ubiquitination and degradation. DAMGO decreased MOR1 levels in the ubiquitin-proteasome system. MOR1 was modified by a Lys48-linked polyubiquitin chain. Overexpression of Smurf2 induced MOR1 ubiquitination and accelerated DAMGO-induced MOR1 degradation, whereas downregulation of Smurf2 attenuated MOR1 degradation. Furthermore, DAMGO increased lung epithelial cell migration and proliferation, and the effect was attenuated by overexpressing Smurf2. Collectively, these data unveil that Smurf2 negatively regulates MOR1 activity by reducing its stability. We also demonstrate an unrevealed biological function of MOR1 in lung epithelial cells. DAMGO-MOR1 promote cell migration and proliferation in lung epithelial cells, suggesting a potential effect of DAMGO in lung repair and remodeling after lung injury.

Oxidized LDL phagocytosis during foam cell formation in atherosclerotic plaques relies on a PLD2-CD36 functional interdependence

The uptake of cholesterol carried by low-density lipoprotein (LDL) is tightly controlled in the body. Macrophages are not well suited to counteract the cellular consequences of excess cholesterol leading to their transformation into "foam cells," an early step in vascular plaque formation. We have uncovered and characterized a novel mechanism involving phospholipase D (PLD) in foam cell formation. Utilizing bone marrow-derived macrophages from genetically PLD deficient mice, we demonstrate that PLD2 (but not PLD1)-null macrophages cannot fully phagocytose aggregated oxidized LDL (Agg-Ox-LDL), which was phenocopied with a PLD2-selective inhibitor. We also report a role for PLD2 in coupling Agg-oxLDL phagocytosis with WASP, Grb2, and Actin. Further, the clearance of LDL particles is mediated by both CD36 and PLD2, via mutual dependence on each other. In the absence of PLD2, CD36 does not engage in Agg-Ox-LDL removal and when CD36 is blocked, PLD2 cannot form protein-protein heterocomplexes with WASP or Actin. These results translated into humans using a GEO database of microarray expression data from atheroma plaques versus normal adjacent carotid tissue and observed higher values for NFkB, PLD2 (but not PLD1), WASP, and Grb2 in the atheroma plaques. Human atherectomy specimens confirmed high presence of PLD2 (mRNA and protein) as well as phospho-WASP in diseased arteries. Thus, PLD2 interacts in macrophages with Actin, Grb2, and WASP during phagocytosis of Agg-Ox-LDL in the presence of CD36 during their transformation into "foam cells." Thus, this study provides new molecular targets to counteract vascular plaque formation and atherogenesis.

Molecular Pharmacology of δ-Opioid Receptors

Opioids are among the most effective analgesics available and are the first choice in the treatment of acute severe pain. However, partial efficacy, a tendency to produce tolerance, and a host of ill-tolerated side effects make clinically available opioids less effective in the management of chronic pain syndromes. Given that most therapeutic opioids produce their actions via µ-opioid receptors (MOPrs), other targets are constantly being explored, among which δ-opioid receptors (DOPrs) are being increasingly considered as promising alternatives. This review addresses DOPrs from the perspective of cellular and molecular determinants of their pharmacological diversity. Thus, DOPr ligands are examined in terms of structural and functional variety, DOPrs' capacity to engage a multiplicity of canonical and noncanonical G protein-dependent responses is surveyed, and evidence supporting ligand-specific signaling and regulation is analyzed. Pharmacological DOPr subtypes are examined in light of the ability of DOPr to organize into multimeric arrays and to adopt multiple active conformations as well as differences in ligand kinetics. Current knowledge on DOPr targeting to the membrane is examined as a means of understanding how these receptors are especially active in chronic pain management. Insight into cellular and molecular mechanisms of pharmacological diversity should guide the rational design of more effective, longer-lasting, and better-tolerated opioid analgesics for chronic pain management.

Phosphatidic acid and neurotransmission

Lipids play a vital role in the health and functioning of neurons and interest in the physiological role of neuronal lipids is certainly increasing. One neuronal function in which neuronal lipids appears to play key roles in neurotransmission. Our understanding of the role of lipids in the synaptic vesicle cycle and neurotransmitter release is becoming increasingly more important. Much of the initial research in this area has highlighted the major roles played by the phosphoinositides (PtdIns), diacylglycerol (DAG), and phosphatidic acid (PtdOH). Of these, PtdOH has not received as much attention as the other lipids although its role and metabolism appears to be extremely important. This lipid has been shown to play a role in modulating both exocytosis and endocytosis although its precise role in either process is not well defined. The currently evidence suggest this lipid likely participates in key processes by altering membrane architecture necessary for membrane fusion, mediating the penetration of membrane proteins, serving as a precursor for other important SV cycling lipids, or activating essential enzymes. In this review, we address the sources of PtdOH, the enzymes involved in its production, the regulation of these enzymes, and its potential roles in neurotransmission in the central nervous system.

Role of phospholipase d in g-protein coupled receptor function

Prolonged agonist exposure of many G-protein coupled receptors induces a rapid receptor phosphorylation and uncoupling from G-proteins. Resensitization of these desensitized receptors requires endocytosis and subsequent dephosphorylation. Numerous studies show the involvement of phospholipid-specific phosphodiesterase phospholipase D (PLD) in the receptor endocytosis and recycling of many G-protein coupled receptors e.g., opioid, formyl or dopamine receptors. The PLD hydrolyzes the headgroup of a phospholipid, generally phosphatidylcholine (PC), to phosphatidic acid (PA) and choline and is assumed to play an important function in cell regulation and receptor trafficking. Protein kinases and GTP binding proteins of the ADP-ribosylation and Rho families regulate the two mammalian PLD isoforms 1 and 2. Mammalian and yeast PLD are also potently stimulated by phosphatidylinositol 4,5-bisphosphate. The PA product is an intracellular lipid messenger. PLD and PA activities are implicated in a wide range of physiological processes and diseases including inflammation, diabetes, oncogenesis or neurodegeneration. This review discusses the characterization, structure, and regulation of PLD in the context of membrane located G-protein coupled receptor function.

Phospholipase D is involved in the formation of Golgi associated clathrin coated vesicles in human parotid duct cells

Phospholipase D (PLD) has been implicated in many cellular functions, such as vesicle trafficking, exocytosis, differentiation, and proliferation. The aim of this study was to characterize the role of PLD in HSY cells, a human cell line originating from the intercalated duct of the parotid gland. As the function and intracellular localization of PLD varies according to cell type, initially, the intracellular localization of PLD1 and PLD2 was determined. By immunofluorescence, PLD1 and PLD2 both showed a punctate cytoplasmic distribution with extensive co-localization with TGN-46. PLD1 was also found in the nucleus, while PLD2 was associated with the plasma membrane. Treatment of cells with the primary alcohol 1-butanol inhibits the hydrolysis of phosphatidylcoline by PLD thereby suppressing phosphatidic acid (PA) production. In untreated HSY cells, there was only a slight co-localization of PLD with the clathrin coated vesicles. When HSY cells were incubated with 1-butanol the total number of clathrin coated vesicles increased, especially in the juxtanuclear region and the co-localization of PLD with the clathrin coated vesicles was augmented. Transmission electron microscopy confirmed that the number of Golgi-associated coated vesicles was greater. Treatment with 1-butanol also affected the Golgi apparatus, increasing the volume of the Golgi saccules. The decrease in PA levels after treatment with 1-butanol likewise resulted in an accumulation of enlarged lysosomes in the perinuclear region. Therefore, in HSY cells PLD appears to be involved in the formation of Golgi associated clathrin coated vesicles as well as in the structural maintenance of the Golgi apparatus.

μ opioid receptor agonist-selective regulation of interleukin-4 in T lymphocytes

Opioids are irreplaceable for the treatment of severe pain. However, opioid-induced immunomodulation affects therapies. Here we report that treatment of human T lymphocytes with the opioids fentanyl, methadone, loperamide and beta-endorphin resulted in a strong induction of the cytokine interleukin-4. In contrast, morphine and buprenorphine induced markedly and significantly lower levels of interleukin-4 mRNA and protein. These findings suggest agonist-biased μ opioid receptor signaling in T cells. In the future, better knowledge about agonist-specific immunomodulatory effects of opioids offers the possibility to select drugs for a therapy with more favorable and/or less detrimental side effects in immune cells.

Regulation of μ-opioid receptors: desensitization, phosphorylation, internalization, and tolerance

Morphine and related µ-opioid receptor (MOR) agonists remain among the most effective drugs known for acute relief of severe pain. A major problem in treating painful conditions is that tolerance limits the long-term utility of opioid agonists. Considerable effort has been expended on developing an understanding of the molecular and cellular processes that underlie acute MOR signaling, short-term receptor regulation, and the progression of events that lead to tolerance for different MOR agonists. Although great progress has been made in the past decade, many points of contention and controversy cloud the realization of this progress. This review attempts to clarify some confusion by clearly defining terms, such as desensitization and tolerance, and addressing optimal pharmacological analyses for discerning relative importance of these cellular mechanisms. Cellular and molecular mechanisms regulating MOR function by phosphorylation relative to receptor desensitization and endocytosis are comprehensively reviewed, with an emphasis on agonist-biased regulation and areas where knowledge is lacking or controversial. The implications of these mechanisms for understanding the substantial contribution of MOR signaling to opioid tolerance are then considered in detail. While some functional MOR regulatory mechanisms contributing to tolerance are clearly understood, there are large gaps in understanding the molecular processes responsible for loss of MOR function after chronic exposure to opioids. Further elucidation of the cellular mechanisms that are regulated by opioids will be necessary for the successful development of MOR-based approaches to new pain therapeutics that limit the development of tolerance.

Mechanisms of rapid opioid receptor desensitization, resensitization and tolerance in brain neurons

Agonists acting on µ-opioid receptors (MOR) are very effective analgesics but cause tolerance during long-term or repeated exposure. Intensive efforts have been made to find novel opioid agonists that are efficacious analgesics but can elude the signalling events that cause tolerance. µ-Opioid agonists differentially couple to downstream signalling mechanisms. Some agonists, such as enkephalins, D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO), methadone and sufentanyl are efficacious at mediating G-protein and effector coupling, as well as triggering MOR regulatory events that include MOR phosphorylation, β-arrestin binding, receptor endocytosis and recycling. By contrast, morphine and closely related alkaloids can mediate efficacious MOR-effector coupling but poorly trigger receptor regulation. Several models have been proposed to relate differential MOR regulation by different opioids with their propensity to cause tolerance. Most are based on dogma that β-arrestin-2 (βarr-2) binding causes MOR desensitization and/or that MOR endocytosis and recycling are required for receptor resensitization. This review will examine some of these notions in light of recent evidence establishing that MOR dephosphorylation and resensitization do not require endocytosis. Recent evidence from opioid-treated animals also suggests that impaired MOR-effector coupling is driven, at least in part, by enhanced desensitization, as well as impaired resensitization that appears to be βarr-2 dependent. Better understanding of how chronic exposure to opioids alters receptor regulatory mechanisms may facilitate the development of effective analgesics that produce limited tolerance.

The exquisite regulation of PLD2 by a wealth of interacting proteins: S6K, Grb2, Sos, WASp and Rac2 (and a surprise discovery: PLD2 is a GEF)

Phospholipase D (PLD) catalyzes the conversion of the membrane phospholipid phosphatidylcholine to choline and phosphatidic acid (PA). PLD's mission in the cell is two-fold: phospholipid turnover with maintenance of the structural integrity of cellular/intracellular membranes and cell signaling through PA and its metabolites. Precisely, through its product of the reaction, PA, PLD has been implicated in a variety of physiological cellular functions, such as intracellular protein trafficking, cytoskeletal dynamics, chemotaxis of leukocytes and cell proliferation. The catalytic (HKD) and regulatory (PH and PX) domains were studied in detail in the PLD1 isoform, but PLD2 was traditionally studied in lesser detail and much less was known about its regulation. Our laboratory has been focusing on the study of PLD2 regulation in mammalian cells. Over the past few years, we have reported, in regards to the catalytic action of PLD, that PA is a chemoattractant agent that binds to and signals inside the cell through the ribosomal S6 kinases (S6K). Regarding the regulatory domains of PLD2, we have reported the discovery of the PLD2 interaction with Grb2 via Y169 in the PX domain, and further association to Sos, which results in an increase of de novo DNA synthesis and an interaction (also with Grb2) via the adjacent residue Y179, leading to the regulation of cell ruffling, chemotaxis and phagocytosis of leukocytes. We also present the complex regulation by tyrosine phosphorylation by epidermal growth factor receptor (EGF-R), Janus Kinase 3 (JAK3) and Src and the role of phosphatases. Recently, there is evidence supporting a new level of regulation of PLD2 at the PH domain, by the discovery of CRIB domains and a Rac2-PLD2 interaction that leads to a dual (positive and negative) effect on its enzymatic activity. Lastly, we review the surprising finding of PLD2 acting as a GEF. A phospholipase such as PLD that exists already in the cell membrane that acts directly on Rac allows a quick response of the cell without intermediary signaling molecules. This provides only the latest level of PLD2 regulation in a field that promises newer and exciting advances in the next few years.

Hyperactivation of anandamide synthesis and regulation of cell-cycle progression via cannabinoid type 1 (CB1) receptors in the regenerating liver

The mammalian liver regenerates upon tissue loss, which induces quiescent hepatocytes to enter the cell cycle and undergo limited replication under the control of multiple hormones, growth factors, and cytokines. Endocannabinoids acting via cannabinoid type 1 receptors (CB(1)R) promote neural progenitor cell proliferation, and in the liver they promote lipogenesis. These findings suggest the involvement of CB(1)R in the control of liver regeneration. Here we report that mice lacking CB(1)R globally or in hepatocytes only and wild-type mice treated with a CB(1)R antagonist have a delayed proliferative response to two-thirds partial hepatectomy (PHX). In wild-type mice, PHX leads to increased hepatic expression of CB(1)R and hyperactivation of the biosynthesis of the endocannabinoid anandamide in the liver via an in vivo pathway involving conjugation of arachidonic acid and ethanolamine by fatty-acid amide hydrolase. In wild-type but not CB(1)R(-/-) mice, PHX induces robust up-regulation of key cell-cycle proteins involved in mitotic progression, including cyclin-dependent kinase 1 (Cdk1), cyclin B2, and their transcriptional regulator forkhead box protein M1 (FoxM1), as revealed by ultrahigh-throughput RNA sequencing and pathway analysis and confirmed by real-time PCR and Western blot analyses. Treatment of wild-type mice with anandamide induces similar changes mediated via activation of the PI3K/Akt pathway. We conclude that activation of hepatic CB(1)R by newly synthesized anandamide promotes liver regeneration by controlling the expression of cell-cycle regulators that drive M phase progression.

Phospholipase D in brain function and Alzheimer's disease

Alzheimer's disease is the most common neurodegenerative disorder. Although lipids are major constituents of brain, their role in Alzheimer's disease pathogenesis is poorly understood. Much attention has been given to cholesterol, but growing evidence suggests that other lipids, such as phospholipids, might play an important role in this disorder. In this review, we will summarize the evidence linking phospholipase D, a phosphatidic acid-synthesizing enzyme, to multiple aspects of normal brain function and to Alzheimer's disease. The role of phospholipase D in signaling mechanisms downstream of beta-amyloid as well as in the trafficking and processing of amyloid precursor protein will be emphasized.

Role of phospholipase D2/phosphatidic acid signal transduction in micro- and delta-opioid receptor endocytosis

We demonstrated recently that opioid-induced activation of phospholipase D2 (PLD2) enhances mu- (MOPr) and delta-opioid receptor endocytosis/recycling and thus reduces the development of opioid receptor desensitization and tolerance. However, the mechanistic basis for the PLD2-mediated induction of opioid receptor endocytosis is currently unknown. Here we show that PLD2-generated phosphatidic acid (PA) might play a key role in facilitating the endocytosis of opioid receptors. However, PLD2-derived PA is known to be further converted to diacylglycerol (DAG) by PA phosphohydrolase (PPAP2). In fact, blocking of PA phosphohydrolase activity by propranolol or PPAP2-short interfering RNA (siRNA) transfection significantly attenuated agonist-induced opioid receptor endocytosis. The primary importance of PA-derived DAG in the induction of opioid receptor endocytosis was further supported by the finding that increasing the DAG level by inhibiting the reconversion of DAG into PA with the DAG kinase inhibitor 3-[2-(4-[bis-(4-fluorophenyl)methylene]-1-piperidinyl)ethyl]-2,3-dihydro-2-thioxo-4(1H)quinazolinone (R59949) or the addition of the synthetic cell-permeable DAG analog 1,2-dioctanoyl-sn-glycerol (DOG), further increased the agonist-induced opioid receptor endocytosis. Moreover, the addition of DOG bypasses the PLD2-siRNA- or PPAP2-siRNA-mediated impairment of DAG synthesis and resulted in a restoration of agonist-induced opioid receptor internalization. Further studies established a functional link between PA-derived DAG and the activation of p38 mitogen-activated protein kinase (MAPK) and the subsequent phosphorylation of the Rab5 effector early endosome antigen 1, which has been demonstrated recently to be required for the induction of MOPr endocytosis. Taken together, our results revealed that the regulation of opioid receptor endocytosis by PLD2 involves the conversion of its product PA to DAG resulting in an activation of the p38 MAPK pathway.

A conserved cysteine residue in the third transmembrane domain is essential for homomeric 5-HT3 receptor function

The cysteine (Cys) residue at position 312 in the third transmembrane domain (M3) is conserved among 5-hydroxytryptamine type 3 (5-HT(3)) receptor subunits and many other subunits of the nicotinic acetylcholine (nACh) related Cys-loop receptor family, including most of the gamma-aminobutyric acid type A (GABA(A)) and glycine receptor subunits. To elucidate a possible role for the Cys-312 in human 5-HT(3)A receptors, we replaced it with alanine and expressed the 5-HT(3)A(C312A) mutant in HEK293 cells. The mutation resulted in an absence of 5-HT-induced whole-cell current without reducing homopentamer formation, surface expression or 5-HT binding. The 5-HT(3)A(C312A) mutant, when co-expressed with the wild-type 5-HT(3)A subunit, did not affect functional expression of receptors, suggesting that the mutant is not dominant negative. Interestingly, co-expression of 5-HT(3)A(C312A) with 5-HT(3)B led to surface expression of heteropentamers that mediated small 5-HT responses. This suggests that the Cys-312 is essential for homomeric but not heteromeric receptor gating. To further investigate the relationship between residue 312 and gating we replaced it with amino acids located at the equivalent position within other Cys-loop subunits that are either capable or incapable of forming functional homopentamers. Replacement of 5-HT(3)A Cys-312 by Gly or Leu (equivalent residues in the nACh receptor delta and gamma subunits) abolished and severely attenuated function, respectively, whereas replacement by Thr or Ser (equivalent residues in nACh receptor alpha7 and GABA(A) subunits) supported robust function. Thus, 5-HT(3)A residue 312 and equivalent polar residues in the M3 of other Cys-loop subunits are essential determinants of homopentameric gating.

Involvement of Phospholipase D 1 and 2 in the subcellular localization and activity of formyl-peptide-receptors in the human colonic cell line HT29

Epithelial cells of the alimentary tract play a central role in the mucosal host defence against pathogens and in the recognition of agonists that interact with mucosal surfaces. In particular, the formyl peptide receptor (FPR) family and their three human subtypes: FPR, formyl-peptide-receptor-like-1 (FPRL1) and FPRL2, are involved in the host defence against pathogens that mediate epithelial responses thus upregulating inflammation. To elucidate the mechanisms by which FPR function, we examined the influence of phospholipase D (PLD) 1 and 2 on the activity and signal transduction of human enterocytes cell line HT29. PLD is a key enzyme involved in secretion, endocytosis and receptor signalling. We inhibited PLD1 and 2 by small interference RNA (siRNA) and determined the activity of formyl peptide receptors using Western blotting and cAMP level measurements. We then analyzed the distribution of formyl peptide receptors FPR, FPRL1 and FPRL2 compared to a control. In this study, we demonstrated that the depletion of PLD1 and 2 resulted in a marked reduction of formyl peptide receptor activity due to inhibited extracellular-signal regulated kinases 1/2 (ERK1/2), phosphorylation and cAMP level reduction. In addition, we observed an intracellular accumulation of FPR, FPRL1 and FPRL2 as a result of receptor recycling inhibition using fluorescence microscopy. The constitutive internalization rate was unaffected. Our results support the importance of PLD1 and 2 in formyl peptide receptor function and the role of endocytosis, receptor recycling and reactivation for receptor activity.

Sustained receptor stimulation leads to sequestration of recycling endosomes in a classical protein kinase C- and phospholipase D-dependent manner

Considerable insight has been garnered on initial mechanisms of endocytosis of plasma membrane proteins and their subsequent trafficking through the endosomal compartment. It is also well established that ligand stimulation of many plasma membrane receptors leads to their internalization. However, stimulus-induced regulation of endosomal trafficking has not received much attention. In previous studies, we showed that sustained stimulation of protein kinase C (PKC) with phorbol esters led to sequestration of recycling endosomes in a juxtanuclear region. In this study, we investigated whether G-protein-coupled receptors that activate PKC exerted effects on endosomal trafficking. Stimulation of cells with serotonin (5-hydroxytryptamine (5-HT)) led to sequestration of the 5-HT receptor (5-HT2AR) into a Rab11-positive juxtanuclear compartment. This sequestration coincided with translocation of PKC as shown by confocal microscopy. Mechanistically the observed sequestration of 5-HT2AR was shown to require continuous PKC activity because it was inhibited by pretreatment with classical PKC inhibitor Gö6976 and could be reversed by posttreatment with this inhibitor. In addition, classical PKC autophosphorylation was necessary for receptor sequestration. Moreover inhibition of phospholipase D (PLD) activity and inhibition of PLD1 and PLD2 using dominant negative constructs also prevented this process. Functionally this sequestration did not affect receptor desensitization or resensitization as measured by intracellular calcium increase. However, the PKC- and PLD-dependent sequestration of receptors resulted in co-sequestration of other plasma membrane proteins and receptors as shown for epidermal growth factor receptor and protease activated receptor-1. This led to heterologous desensitization of those receptors and diverted their cellular fate by protecting them from agonist-induced degradation. Taken together, these results demonstrate a novel role for sustained receptor stimulation in regulation of intracellular trafficking, and this process requires sustained stimulation of PKC and PLD.

Phospholipase D in endocytosis and endosomal recycling pathways

The discovery that Arf GTPases, mediators of membrane traffic, activate phospholipase D (PLD) raised the possibility that Arfs could facilitate membrane traffic by altering membrane lipid composition. PLD hydrolyzes phosphatidylcholine to generate phosphatidic acid (PA), a lipid that favors membranes with negative curvature and thus can facilitate both membrane fission and fusion. This review examines studies that have reported a role for PLD in endocytosis and membrane recycling from endocytic pathways.

mu-opioid receptor-stimulated synthesis of reactive oxygen species is mediated via phospholipase D2

We have recently shown that the activation of the rat mu-opioid receptor (MOPr, also termed MOR1) by the mu-agonist [D-Ala(2), Me Phe(4), Glyol(5)]enkephalin (DAMGO) leads to an increase in phospholipase D2 (PLD2) activity and an induction of receptor endocytosis, whereas the agonist morphine which does not induce opioid receptor endocytosis fails to activate PLD2. We report here that MOPr-mediated activation of PLD2 stimulates production of reactive oxygen molecules via NADH/NADPH oxidase. Oxidative stress was measured with the fluorescent probe dichlorodihydrofluorescein diacetate and the role of PLD2 was assessed by the PLD inhibitor D-erythro-sphingosine (sphinganine) and by PLD2-small interfering RNA transfection. To determine whether NADH/NADPH oxidase contributes to opioid-induced production of reactive oxygen species, mu-agonist-stimulated cells were pre-treated with the flavoprotein inhibitor, diphenylene iodonium, or the specific NADPH oxidase inhibitor, apocynin. Our results demonstrate that receptor-internalizing agonists (like DAMGO, beta-endorphin, methadone, piritramide, fentanyl, sufentanil, and etonitazene) strongly induce NADH/NADPH-mediated ROS synthesis via PLD-dependent signaling pathways, whereas agonists that do not induce MOPr endocytosis and PLD2 activation (like morphine, buprenorphine, hydromorphone, and oxycodone) failed to activate ROS synthesis in transfected human embryonic kidney 293 cells. These findings indicate that the agonist-selective PLD2 activation plays a key role in the regulation of NADH/NADPH-mediated ROS formation by opioids.

Modulation of opioid receptor function by protein-protein interactions

Opioid receptors, MORP, DORP and KORP, belong to the family A of G protein coupled receptors (GPCR), and have been found to modulate a large number of physiological functions, including mood, stress, appetite, nociception and immune responses. Exogenously applied opioid alkaloids produce analgesia, hedonia and addiction. Addiction is linked to alterations in function and responsiveness of all three opioid receptors in the brain. Over the last few years, a large number of studies identified protein-protein interactions that play an essential role in opioid receptor function and responsiveness. Here, we summarize interactions shown to affect receptor biogenesis and trafficking, as well as those affecting signal transduction events following receptor activation. This article also examines protein interactions modulating the rate of receptor endocytosis and degradation, events that play a major role in opiate analgesia. Like several other GPCRs, opioid receptors may form homo or heterodimers. The last part of this review summarizes recent knowledge on proteins known to affect opioid receptor dimerization.

Membrane glycoprotein M6A promotes mu-opioid receptor endocytosis and facilitates receptor sorting into the recycling pathway

The interaction of mu-opioid receptor (MOPr) with the neuronal membrane glycoprotein M6a is known to facilitate MOPr endocytosis in human embryonic kidney 293 (HEK293) cells. To further study the role of M6a in the post-endocytotic sorting of MOPr, we investigated the agonist-induced co-internalization of MOPr and M6a and protein targeting after internalization in HEK293 cells that co-expressed HA-tagged MOPr and Myc-tagged M6a. We found that M6a, MOPr, and Rab 11, a marker for recycling endosomes, co-localized in endocytotic vesicles, indicating that MOPr and M6a are primarily targeted to recycling endosomes after endocytosis. Furthermore, co-expression of M6a augmented the post-endocytotic sorting of delta-opioid receptors into the recycling pathway, indicating that M6a might have a more general role in opioid receptor post-endocytotic sorting. The enhanced post-endocytotic sorting of MOPr into the recycling pathway was accompanied by a decrease in agonist-induced receptor down-regulation of M6a in co-expressing cells. We tested the physiological relevance of these findings in primary cultures of cortical neurons and found that co-expression of M6a markedly increased the translocation of MOPrs from the plasma membrane to intracellular vesicles at steady state and significantly enhanced both constitutive and agonist-induced receptor endocytosis. In conclusion, our results strongly indicate that M6a modulates MOPr endocytosis and post-endocytotic sorting and has an important role in receptor regulation.

Involvement of formyl-peptide-receptor-like-1 and phospholipase D in the internalization and signal transduction of amyloid beta 1-42 in glial cells

Recent studies suggest that the formyl-peptide-receptor-like-1 (FPRL1) plays an essential role in the inflammatory responses of host defense mechanisms and neurodegenerative disorders such as Alzheimer's disease (AD). We therefore analyzed whether amyloid beta1-42 (Abeta1-42) increased the activity of phospholipase D (PLD) via FPRL1, which is an enzyme involved in the secretion, endocytosis and receptor signaling. PLD activity was determined using a transphosphatidylation assay. The internalization of Abeta1-42 via FPRL1 was visualized using fluorescence microscopy and quantified by ELISA (Enzyme Linked Immunosorbent Assay). Determining receptor activity by extracellular-signal regulated kinases 1/2 (ERK1/2) phosphorylation and cAMP level measurement verified the Abeta1-42-induced activation of FPRL1. We were able to show that Abeta1-42 is rapidly internalized via FPRL1 in astrocytes and microglia. PLD was additionally activated by Abeta1-42 and via FPRL1 in rat glial cells. Furthermore, the ERK1/2 phosphorylation by FPRL1 agonists was dependent on the PLD product phosphatidic acid (PA). Together, these data suggest that PLD plays an important role in the regulation of Abeta1-42-induced endocytosis and FPRL1 receptor signaling.

Molecular mechanisms of PLD function in membrane traffic

The two mammalian phosphatidylcholine (PC)-selective phospholipase D (PLD) enzymes remove the choline head group from PC to produce phosphatidic acid (PA). PA stimulates phosphatidylinositol(4)phosphate 5-kinases, can function as a binding site for membrane proteins, is required for certain membrane fusion or fission events and is an important precursor for the production of diacylglycerol (DAG). Both PA and DAG are lipids that favor negatively curved membranes rather than planar bilayers and can reduce the energetic barrier to membrane fission and fusion. Recent data provide a mechanistic explanation for the role PLDs play in some aspects of membrane traffic and provide an explanation for why some membrane fusion reactions require PA and some do not. PLDs also act as guanosine triphosphatase-activating proteins for dynamin and may participate with dynamin in the process of vesicle fission.

Recycling endosomes supply cardiac pacemaker channels for regulated surface expression

AIMS

Cellular excitability is not only determined by the type but also by the number of ion channels in the plasma membrane. Recent evidence indicates that cell surface expression of cardiac pacemaker channels might be controlled beyond the level of biosynthesis by regulating their surface transport. However, neither the underlying trafficking pathways nor their molecular control have yet been investigated.

METHODS AND RESULTS

We have studied endocytic trafficking of hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channels expressed as fusions with green fluorescent protein or tagged with an extracellular haemagglutinin epitope in opossum kidney cells, dissociated rat hippocampal neurons, and ventricular cardiomyocytes. After being internalized from the plasma membrane, HCN2 and HCN4 are sorted to the Rab11-positive endocytic recycling compartment (ERC). From there, they are transported back to the cell surface depending on active phospholipase D2 (PLD2). The peptide hormone angiotensin II, which is upregulated in a number of cardiac pathologies and a known activator of PLD2, stimulates ERC trafficking of HCN4 channels. It significantly increases HCN surface expression independent of their biosynthesis.

CONCLUSION

Recycling endosomes serve as an intracellular storage compartment for the cardiac pacemaker channels HCN2 and HCN4. They are not only crucial for maintaining a homeostatic surface expression but also supply channels for rapid adaptation of their surface expression in response to extracellular stimuli.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurological disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Role of receptor internalization in opioid tolerance and dependence

Agonist-induced mu-opioid receptor (MOPr) internalization has long been suggested to contribute directly to functional receptor desensitization and opioid tolerance. In contrast, recent evidence suggests that opioid receptor internalization could in fact reduce opioid tolerance in vivo, but the mechanisms that are responsible for the internalization-mediated protection against opioid tolerance are controversely discussed. One prevailing hypothesis is, that receptor internalization leads to decreased receptor signaling and therefore to reduced associated compensatory changes in downstream signaling systems that are involved in the development of opioid tolerance. However, numerous studies have demonstrated that desensitized and internalized mu-opioid receptors are rapidly recycled to the cell surface in a reactivated state, thus counteracting receptor desensitization and opioid tolerance. Further studies revealed agonist-selective differences in the ability to induce opioid receptor internalization. Recently it has been demonstrated that the endocytotic efficacies of opioids are negatively correlated to the induced opioid tolerance. Thus, clearer understanding of the role of opioid receptor trafficking in the regulation of opioid tolerance and dependence will help in the treatment of patients suffering from chronic pain or drug dependence.

Role of receptor internalization in the agonist-induced desensitization of cannabinoid type 1 receptors

Agonist-induced internalization of G protein-coupled receptors (GPCRs) is an important mechanism for regulating signaling transduction of functional receptors at the plasma membrane. We demonstrate here that both caveolae/lipid-rafts- and clathrin-coated-pits-mediated pathways were involved in agonist-induced endocytosis of the cannabinoid type 1 receptor (CB1R) in stably transfected human embryonic kidney (HEK) 293 cells and that the internalized receptors were predominantly sorted into recycling pathway for reactivation. The treatment of CB1 receptors with the low endocytotic agonist Delta9-THC induced a faster receptor desensitization and slower resensitization than the high endocytotic agonist WIN 55,212-2. In addition, the blockade of receptor endocytosis or recycling pathway markedly enhanced agonist-induced CB1 receptor desensitization. Furthermore, co-expression of phospholipase D2, an enhancer of receptor endocytosis, reduced CB1 receptor desensitization, whereas co-expression of a phospholipase D2 negative mutant significantly increased the desensitization after WIN 55,212-2 treatment. These findings provide evidences for the importance of receptor endocytosis in counteracting CB1 receptor desensitization by facilitating receptor reactivation. Moreover, in primary cultured neurons, the low endocytotic agonist Delta9-THC or anandamide exhibited a greater desensitization of endogenous CB1 receptors than the high endocytotic agonist WIN 55,212-2, CP 55940 or 2-arachidonoyl glycerol, indicating that cannabinoids with high endocytotic efficacy might cause reduced development of cannabinoid tolerance to some kind cannabinoid-mediated effects.

Internalization of PrP106-126 by the formyl-peptide-receptor-like-1 in glial cells

Recent studies suggest that the formyl-peptide-receptor-like-1 (FPRL1) plays an essential role in inflammatory responses in the host defence mechanisms and neurodegenerative disorders. Furthermore, it may be involved in proinflammatory processes of prion diseases. However, little is known about the induction and regulation of PrP106-126-induced receptor endocytosis. We have thus analysed whether PrP106-126 increases the activity of phospholipase D (PLD) via FPRL1, an enzyme involved in the regulation of the secretion, endocytosis and receptor signalling, in glial cells. PLD activity was determined using a transphosphatidylation assay and the internalization of PrP106-126, and FPRL1 was assessed by fluorescence microscopy and quantified by ELISA. We could show that PLD is activated by PrP106-126 both in astrocytes and microglia, and moreover that PrP106-126 is rapidly internalized via FPRL1 in astrocytes and microglia cells. The determination of receptor activity by extracellular signal-regulated kinases 1/2 phosphorylation and cAMP level measurement verified the PrP106-126-induced activation of FPRL1. FPRL1-mediated PrP106-126 uptake was blocked by the receptor antagonist chenodeoxycholic acid. These studies indicate the involvement of FPRL1-mediated cellular signalling in PrP106-126-endocytosis and may allow the development of therapeutic agents interfering with prion uptake and/or PLD function, using either PLD or the FPRL1 as a possible pharmaceutical target.

Membrane glycoprotein M6a interacts with the micro-opioid receptor and facilitates receptor endocytosis and recycling

Using a yeast two-hybrid screen, the neuronal membrane glycoprotein M6a, a member of the proteolipid protein family, was identified to be associated with the mu-opioid receptor (MOPr). Bioluminescence resonance energy transfer and co-immunoprecipitation experiments confirmed that M6a interacts agonist-independently with MOPr in human embryonic kidney 293 cells co-expressing MOPr and M6a. Co-expression of MOPr with M6a, but not with M6b or DM20, exists in many brain regions, further supporting a specific interaction between MOPr and M6a. After opioid treatment M6a co-internalizes and then co-recycles with MOPr to cell surface in transfected human embryonic kidney 293 cells. Moreover, the interaction of M6a and MOPr augments constitutive and agonist-dependent internalization as well as the recycling rate of mu-opioid receptors. On the other hand, overexpression of a M6a-negative mutant prevents mu-opioid receptor endocytosis, demonstrating an essential role of M6a in receptor internalization. In addition, we demonstrated the interaction of M6a with a number of other G protein-coupled receptors (GPCRs) such as the delta-opioid receptor, cannabinoid receptor CB1, and somatostatin receptor sst2A, suggesting that M6a might play a general role in the regulation of certain GPCRs. Taken together, these data provide evidence that M6a may act as a scaffolding molecule in the regulation of GPCR endocytosis and intracellular trafficking.

The potential for phospholipase D as a new therapeutic target

Mammalian phospholipase D (PLD), a signal transduction-activated enzyme, hydrolyzes phosphatidylcholine to generate the lipid second messenger phosphatidic acid (PA) and choline. Genetic and pharmacological methods have implicated PLD and its product PA in a wide variety of cellular processes including vesicle trafficking, receptor signaling, cell proliferation and survival. Dysregulation of these cell biologic processes occurs in a diverse range of illnesses including cancer. This review summarizes PLD regulation and function and highlights its potential as a therapeutic target in disease settings.

Regulation of G protein-coupled receptor trafficking

G protein-coupled receptors (GPCRs) mediate cellular responses to diverse extracellular stimuli to play a vital role in the control of physiology and behaviour. GPCR trafficking is of fundamental importance for the regulation of GPCRs signaling. In this mini review, we will discuss some of the recent findings on the mechanisms that regulate GPCR trafficking, which include (i) large dense-core vesicle (LDCV)-associated GPCR delivery which could be a general cell biological mechanism for rapid modulation of membrane receptors in response to certain stimuli; (ii) lateral diffusion of GPCRs in the plasma membrane for rapid change of the number of neurotransmitter receptors during synaptic plasticity and (iii) constitutive internalization of GPCRs, that contributes to receptor resensitization and distribution, including axonal polarization.

Phospholipase D signaling: orchestration by PIP2 and small GTPases

Hydrolysis of phosphatidylcholine by phospholipase D (PLD) leads to the generation of the versatile lipid second messenger, phosphatidic acid (PA), which is involved in fundamental cellular processes, including membrane trafficking, actin cytoskeleton remodeling, cell proliferation and cell survival. PLD activity can be dramatically stimulated by a large number of cell surface receptors and is elaborately regulated by intracellular factors, including protein kinase C isoforms, small GTPases of the ARF, Rho and Ras families and, particularly, by the phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PIP(2)). PIP(2) is well known as substrate for the generation of second messengers by phospholipase C, but is now also understood to recruit and/or activate a variety of actin regulatory proteins, ion channels and other signaling proteins, including PLD, by direct interaction. The synthesis of PIP(2) by phosphoinositide 5-kinase (PIP5K) isoforms is tightly regulated by small GTPases and, interestingly, by PA as well, and the concerted formation of PIP(2) and PA has been shown to mediate receptor-regulated cellular events. This review highlights the regulation of PLD by membrane receptors, and describes how the close encounter of PLD and PIP5K isoforms with small GTPases permits the execution of specific cellular functions.

Ligand-specific receptor states: Implications for opiate receptor signalling and regulation

Opiate drugs produce their effects by acting upon G protein coupled receptors (GPCRs) and although they are among the most effective analgesics available, their clinical use is restricted by unwanted side effects such as tolerance, physical dependence, respiratory depression, nausea and constipation. As a class, opiates share a common profile of unwanted effects but there are also significant differences in ligand liability for producing these actions. A growing number of studies show that GPCRs may exist in multiple active states that differ in their signalling and regulatory properties and which may distinctively bind different agonists. In this review we summarize evidence supporting the existence of multiple active conformations for MORs and DORs, analyze information favouring the existence of ligand-specific receptor states and assess how ligand-selective efficacy may contribute to the production of longer lasting, better tolerated opiate analgesics.

Interaction of the mu-opioid receptor with synaptophysin influences receptor trafficking and signaling

There is increasing evidence that the signal transduction of opioid receptors is modulated by receptor-associated proteins. In the search for proteins regulating mu-opioid receptor (MOPr) endocytosis, synaptophysin was found to bind to the rat micro-opioid receptor in yeast two-hybrid assay. Coimmunoprecipitation experiments and bioluminescence resonance energy transfer assays confirmed that the micro-opioid receptor constitutively interacts with synaptophysin in human embryonic kidney 293 cells overexpressing MOPr and synaptophysin. In this study, we show that overexpression of synaptophysin enhances the micro-opioid receptor endocytosis. One explanation for the observed effects is that synaptophysin recruits dynamin to the plasma membrane, facilitating fission of clathrin-coated vesicles. This suggestion is supported by our finding that overexpression of a synaptophysin truncation mutant, which breaks the interaction between synaptophysin and dynamin, prevents agonist-mediated micro-opioid receptor endocytosis. In addition, the synaptophysin-augmented micro-opioid receptor trafficking leads to attenuated agonist-induced receptor desensitization and faster receptor resensitization. Taken together, our findings strongly suggest that synaptophysin plays an important role in the regulation of micro-opioid receptor trafficking and signaling.

Dynamic phospholipid signaling by G protein-coupled receptors

G protein-coupled receptors (GPCRs) control a variety of fundamental cellular processes by regulating phospholipid signaling pathways. Essential for signaling by a large number of receptors is the hydrolysis of the membrane phosphoinositide PIP(2) by phospholipase C (PLC) into the second messengers IP(3) and DAG. Many receptors also stimulate phospholipase D (PLD), leading to the generation of the versatile lipid, phosphatidic acid. Particular PLC and PLD isoforms take differential positions in receptor signaling and are additionally regulated by small GTPases of the Ras, Rho and ARF families. It is now recognized that the PLC substrate, PIP(2), has signaling capacity by itself and can, by direct interaction, affect the activity and subcellular localization of PLD and several other proteins. As expected, the synthesis of PIP(2) by phosphoinositide 5-kinases is tightly regulated as well. In this review, we present an overview of how these signaling pathways are governed by GPCRs, explain the molecular basis for the spatially and temporally organized, highly dynamic quality of phospholipid signaling, and point to the functional connection of the pathways.

Analysis of a missense variant of the human N-formyl peptide receptor that is associated with agonist-independent β-arrestin association and indices of inflammation

Formyl-Met-Leu-Phe (fMLP) is a potent chemoattractant molecule released from both bacteria and damaged mitochondria that activates fMLP receptors (FPR) leading to neutrophil chemotaxis, degranulation and superoxide production. A common missense single nucleotide polymorphism in the human FPR1 gene at nucleotide c.32C>T results in the amino-acid substitution, p.I11T, in the FPR1 extracellular amino-terminus. The minor (c.32T) allele frequencies were 0.25, 0.27, 0.25, 0.15 and 0.14 in healthy Caucasian, African, East Indian, Chinese and Native Canadian individuals, respectively. In subjects homozygous for the p.T11 allele, we find elevated serum concentrations of C-reactive protein, increased absolute counts of blood leukocytes and neutrophils, and erythrocyte sedimentation rates. When expressed in HEK 293 and RBL-2H3 cells a substantial proportion of FPR1 p.I11T variant is retained intracellularly and agonist-independent internalization of the FPR1 p.I11T variant, but not the wild-type FPR1, is constitutively associated with beta-arrestin2-GFP in vesicles. Moreover, basal N-acetyl-D-glucosaminidase release is increased in primary neutrophils isolated from subjects either heterozygous or homozygous for the FPR1 p.T11 allele. Taken together, the data suggest an increased receptor activity and phenotypic expression of increased inflammatory indices in subjects with the p.T11 allele.