Human oxytocin receptors in cholesterol-rich vs. cholesterol-poor microdomains of the plasma membrane

Beta-subunit of cardiac Na+-K+-ATPase dictates the concentration of the functional enzyme in caveolae

Previous studies showed the presence of a significant fraction of Na(+)-K(+)-ATPase alpha-subunits in cardiac myocyte caveolae, suggesting the caveolar interactions of Na(+)-K(+)-ATPase with its signaling partners. Because both alpha- and beta-subunits are required for ATPase activity, to clarify the status of the pumping function of caveolar Na(+)-K(+)-ATPase, we have examined the relative distribution of two major subunit isoforms (alpha(1) and beta(1)) in caveolar and noncaveolar membranes of adult rat cardiac myocytes. When cell lysates treated with high salt (Na(2)CO(3) or KCl) concentrations were fractionated by a standard density gradient procedure, the resulting light caveolar membranes contained 30-40% of alpha(1)-subunits and 80-90% of beta(1)-subunits. Use of Na(2)CO(3) was shown to inactivate Na(+)-K(+)-ATPase; however, caveolar membranes obtained by the KCl procedure were not denatured and contained approximately 75% of total myocyte Na(+)-K(+)-ATPase activity. Sealed isolated caveolae exhibited active Na(+) transport. Confocal microscopy supported the presence of alpha,beta-subunits in caveolae, and immunoprecipitation showed the association of the subunits with caveolin oligomers. The findings indicate that cardiac caveolar inpocketings are the primary portals for active Na(+)-K(+) fluxes, and the sites where the pumping and signaling functions of Na(+)-K(+)-ATPase are integrated. Preferential concentration of beta(1)-subunit in caveolae was cell specific; it was also noted in neonatal cardiac myocytes but not in fibroblasts and A7r5 cells. Uneven distributions of alpha(1) and beta(1) in early and late endosomes of myocytes suggested different internalization routes of two subunits as a source of selective localization of active Na(+)-K(+)-ATPase in cardiac caveolae.

Role of cholesterol in the function and organization of G-protein coupled receptors

Cholesterol is an essential component of eukaryotic membranes and plays a crucial role in membrane organization, dynamics and function. The modulatory role of cholesterol in the function of a number of membrane proteins is well established. This effect has been proposed to occur either due to a specific molecular interaction between cholesterol and membrane proteins or due to alterations in the membrane physical properties induced by the presence of cholesterol. The contemporary view regarding heterogeneity in cholesterol distribution in membrane domains that sequester certain types of membrane proteins while excluding others has further contributed to its significance in membrane protein function. The seven transmembrane domain G-protein coupled receptors (GPCRs) are among the largest protein families in mammals and represent approximately 2% of the total proteins coded by the human genome. Signal transduction events mediated by this class of proteins are the primary means by which cells communicate with and respond to their external environment. GPCRs therefore represent major targets for the development of novel drug candidates in all clinical areas. In view of their importance in cellular signaling, the interaction of cholesterol with such receptors represents an important determinant in functional studies of such receptors. This review focuses on the effect of cholesterol on the membrane organization and function of GPCRs from a variety of sources, with an emphasis on the more contemporary role of cholesterol in maintaining a domain-like organization of such receptors on the cell surface. Importantly, the recently reported role of cholesterol in the function and organization of the neuronal serotonin(1A) receptor, a representative of the GPCR family which is present endogenously in the hippocampal region of the brain, will be highlighted.

Antidepressants and antipsychotic drugs colocalize with 5-HT3 receptors in raft-like domains

Despite different chemical structure and pharmacodynamic signaling pathways, a variety of antidepressants and antipsychotics inhibit ion fluxes through 5-HT3 receptors in a noncompetitive manner with the exception of the known competitive antagonists mirtazapine and clozapine. To further investigate the mechanisms underlying the noncompetitive inhibition of the serotonin-evoked cation current, we quantified the concentrations of different types of antidepressants and antipsychotics in fractions of sucrose flotation gradients isolated from HEK293 (human embryonic kidney 293) cells stably transfected with the 5-HT3A receptor and of N1E-115 neuroblastoma cells in relation to the localization of the 5-HT3 receptor protein within the cell membrane. Western blots revealed a localization of the 5-HT3 receptor protein exclusively in the low buoyant density (LBD) fractions compatible with a localization within raft-like domains. Also, the antidepressants desipramine, fluoxetine, and reboxetine and the antipsychotics fluphenazine, haloperidol, and clozapine were markedly enriched in LBD fractions, whereas no accumulation occurs for mirtazapine, carbamazepine, moclobemide, and risperidone. The concentrations of psychopharmacological drugs within LBD fractions was strongly associated with their inhibitory potency against serotonin-induced cation currents. The noncompetitive antagonism of antidepressants at the 5-HT3 receptor was not conferred by an enhancement of receptor internalization as shown by immunofluorescence studies, assessment of receptor density in clathrin-coated vesicles, and electrophysiological recordings after coexpression of a dominant-negative mutant of dynamin I, which inhibits receptor internalization. In conclusion, enrichment of antidepressants and antipsychotics in raft-like domains within the cell membrane appears to be crucial for their antagonistic effects at ligand-gated ion channels such as 5-HT3 receptors.

Oxytocin--its role in male reproduction and new potential therapeutic uses

Oxytocin (OT) is traditionally thought of as a "female" neurohypophysis hormone due to its role in parturition and milk ejection. However, OT is recognized as having endocrine and paracrine roles in male reproduction. At ejaculation, a burst of OT is released from the neurohypophysis into the systemic circulation and stimulates contractions of the reproductive tract aiding sperm release. There is conclusive evidence that OT is synthesized within the mammalian testis, epididymis and prostate and the presence of OT receptors (OTRs) through the reproductive tract supports a local action for this peptide. OT has a paracrine role in stimulating contractility of the seminiferous tubules, epididymis and the prostate gland. Interestingly, OT has also been shown to modulate androgen levels in these tissues via stimulation of the conversion of testosterone to dihydrotestostone (DHT) by 5alpha-reductase. The elucidation of OT's role in male reproduction has suggested a number of potential therapeutic uses for this hormone. Exogenous administration of OT has, in some cases, been shown to increase the numbers of ejaculated sperm, possibly by stimulating contractions of the reproductive tract and thus aiding sperm passage. Within the prostate, OT has been shown to affect gland growth both directly and via its interaction with androgen metabolism. Prostate pathologies due to unregulated cell proliferation/growth, such as benign prostatic hyperplasia and cancer, are unfortunately very common and few effective treatments are available. Greater understanding of paracrine growth mediators, such as OT, is likely to provide new mechanisms for treating such pathologies.

The neuropeptide PACAP promotes ?‐secretase pathway for processing Alzheimer amyloid precursor protein

The neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) has neurotrophic as well as anti-apoptotic properties and is involved in learning and memory processes. Its specific G protein-coupled receptor PAC1 is expressed in several central nervous system (CNS) regions, including the hippocampal formation. Here we examined the effect of PAC1 receptor activation on alpha-secretase cleavage of the amyloid precursor protein (APP) and the production of secreted APP (APPsalpha). Stimulation of endogenously expressed PAC1 receptors with PACAP in human neuroblastoma cells increased APPsalpha secretion, which was completely inhibited by the PAC1 receptor specific antagonist PACAP-(6-38). In HEK cells stably overexpressing functional PAC1 receptors, PACAP-27 and PACAP-38 strongly stimulated alpha-secretase cleavage of APP. The PACAP-induced APPsalpha production was dose dependent and saturable. This increase of alpha-secretase activity was completely abolished by hydroxamate-based metalloproteinase inhibitors, including a preferential ADAM 10 inhibitor. By using several specific protein kinase inhibitors, we show that the MAP-kinase pathway [including extracellular-regulated kinase (ERK) 1 and ERK2] and phosphatidylinositol 3-kinase mediate the PACAP-induced alpha-secretase activation. Our findings provide evidence for a role of the neuropeptide PACAP in stimulation of the nonamyloidogenic pathway, which might be related to its neuroprotective properties.

Exceptional total and functional yields of the human adenosine (A2a) receptor expressed in the yeast Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae was used to express a medically relevant G-protein coupled receptor (GPCR), the human adenosine (A2a) receptor, with a C-terminal green fluorescent protein (GFP) fusion tag. In prior studies, we established an expression system for A2a-GFP. Here, we quantified the total A2a-GFP expression levels by correlating GFP levels as detected by fluorescence and densitometry to A2a-GFP molecules overexpressed in the system. We also quantified A2a-GFP functional levels by classical radioligand binding assays. Approximately, 120,000 functional A2a-GFP molecules per cell were present on the plasma membrane as determined by radioligand binding. Using whole cell GFP fluorescence, 340,000 A2a-GFP molecules per cell were detected; approximately 70% of those molecules were plasma membrane localized, as determined by using confocal microscopy analysis. These results show that a significant portion of the total expressed protein is functional. In addition, the quick and inexpensive whole cell fluorescence appears to provide a good approximation of functional receptor numbers for this case. Importantly, the amount of functionally expressed A2a-GFP per culture ( approximately 4 mg/L) is among the highest reported for any GPCR in any expression system.

Decreases in yeast expression yields of the human adenosine A2a receptor are a result of translational or post-translational events

The human adenosine receptor (A2a), a G-protein-coupled receptor (GPCR), was C-terminally tagged with the green fluorescent protein (GFP) and expressed in the yeast Saccharomyces cerevisiae to gain an understanding of the expression limitations of this medically relevant class of membrane proteins. The A2a-GFP protein was able to bind adenosine analogs indicating that the GFP tag did not alter the ligand binding activity of the receptor. A screen based on whole cell fluorescence was developed and a library of clones with various gene copy numbers was screened via flow cytometry to isolate clones with the highest protein expression levels. All clones studied exhibited a decrease in the net A2a-GFP protein production rate over time as determined by whole cell fluorescence, Western blotting, confocal microscopy, and ligand binding. Quantitative PCR showed that A2a-GFP mRNA levels remained relatively high even as the protein production rate decreased. A cycloheximide chase experiment showed that the mature protein was stable over time and was not significantly degraded. Taken together, these results suggest that heterologous expression of GPCRs is limited by a translational or post-translational bottleneck that is unique from expression limitations seen for soluble proteins.

Ligand modulation of lateral segregation of a G-protein-coupled receptor into lipid microdomains in sphingomyelin/phosphatidylcholine solid-supported bilayers

A growing body of evidence supports the idea that the plasma membrane bilayer is characterized by a laterally inhomogeneous mixture of lipids, having an organized structure in which lipid molecules segregate into small domains or patches. Such microdomains are characterized by high contents of sphingolipids that form thicker liquid-ordered regions that are resistant to extraction with nonionic detergents. The existence of lipid lateral segregation has been demonstrated in both model and biological membranes, although its role in protein sorting and membrane function still remains unclear. In these studies, plasmon-waveguide resonance (PWR) spectroscopy was employed to investigate the properties of microdomains in a model system consisting of a solid-supported lipid bilayer composed of a 1:1 mixture of palmitoyloleoylphosphatidylcholine (POPC) and brain sphingomyelin (SM), and their influence on the partitioning and functioning of the human delta opioid receptor (hDOR), a G-protein coupled receptor (GPCR). Resonance signals corresponding to two microdomains (POPC-rich and SM-rich) were observed in such bilayers, and the sorting of the receptor into each domain was highly dependent on the type of ligand that was bound. When no ligand was bound, the receptor was incorporated preferentially into the POPC-rich domain; when an agonist or antagonist was bound, the receptor was incorporated preferentially into the SM-rich component, although with a 2-fold greater propensity for this microdomain in the case of the agonist. Binding of G-protein to the agonist-bound receptor in the SM-rich domain occurred with a 30-fold higher affinity than binding to the receptor in the PC-rich domain. The binding of the agonist to an unliganded receptor in the bilayer produced receptor trafficking from the PC-rich to the SM-rich component. Since the SM-rich domain is thicker than the PC-rich domain, and previous studies with the hDOR have shown that the receptor is elongated upon agonist activation, we propose that hydrophobic matching between the receptor and the lipid is a driving force for receptor trafficking to the SM-rich component.

Increased cholesterol decreases uterine activity: functional effects of cholesterol alteration in pregnant rat myometrium

Uterine quiescence is essential for successful pregnancy. Cholesterol and triglycerides are markedly increased in pregnancy. Cholesterol is enriched in microdomains of the plasma membrane known as rafts and caveolae. Both lipid rafts and caveolae have been implicated in cellular signaling cascades. The purpose of this work was to investigate whether manipulation of cholesterol content alters uterine contractility. Late pregnancy (19–21 days) rats were humanely euthanized and strips of longitudinal myometrium were then dissected. Force and Ca2+ measurements were simultaneously recorded and cholesterol increased by the addition of 5 mg/ml cholesterol or 0.25 mg/ml low-density lipoproteins (LDLs) or reduced by 2% methyl-β-cyclodextrin (MCD) or 2 U/ml cholesterol oxidase addition to the perfusate. Both LDLs and cholesterol profoundly inhibited spontaneous uterine force production and associated Ca2+ transients; frequency, amplitude, and duration of contraction were all significantly reduced compared with preceding control contractions. Force and Ca2+ were also reduced by cholesterol when 1 nM oxytocin was used to stimulate the myometrium. Uterine activity was significantly increased by cholesterol extraction with MCD or cholesterol oxidase treatment. Electron microscopy confirmed the lipid raft disrupting effect of MCD, as formerly electron microscopy-visible caveolae in the myometrial cell membrane all but disappeared after MCD treatment. These data show that uterine smooth muscle cell cholesterol content is critically important for functional activity. A novel finding of our study is that cholesterol is inhibitory for force generation. It may be one of the mechanisms operating to maintain uterine quiescence throughout gestation and may also contribute to difficulties in labor suffered by obese women.

CHAPSTEROL. A novel cholesterol-based detergent

Design, synthesis and characterization of CHAPSTEROL, a novel cholesterol-based detergent developed for functional solubilization of cholesterol-dependent membrane proteins are described. To validate CHAPSTEROL, we employed the oxytocin receptor, a G protein-coupled receptor requiring cholesterol for its high-affinity binding state. Using the photoactivatable cholesterol analogue [3H]6,6-azocholestan-3beta-ol[3alphaH], we demonstrate that solubilization by CHAPSTEROL leads to an enrichment of cholesterol-binding proteins whereas the widely used bile acid derivative CHAPSO leads to a significant depletion of cholesterol-binding proteins. Similar to Triton X-100 and CHAPS, CHAPSTEROL maintains the localization of caveolin as well as cholesterol and sphingomyelin to lipid rafts, i.e. detergent-insoluble microdomains of the plasma membrane. The data suggest that CHAPSTEROL is an appropriate detergent for the solubilization of cholesterol-dependent membrane proteins and isolation of rafts.

Structure and cholesterol domain dynamics of an enriched caveolae/raft isolate

Despite the importance of cholesterol in the formation and function of caveolar microdomains in plasma membranes, almost nothing is known regarding the structural properties, cholesterol dynamics or intracellular factors affecting caveolar cholesterol dynamics. A non-detergent method was employed to isolate caveolae/raft domains from purified plasma membranes of murine fibroblasts. A series of fluorescent lipid probe molecules or a fluorescent cholesterol analogue, dehydroergosterol, were then incorporated into the caveolae/raft domains to show that: (i) fluorescence polarization of the multiple probe molecules [diphenylhexatriene analogues, DiI18 (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate), parinaric acids and NBD-stearic acid [12-(N-methyl)-N-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-octadecanoic acid] indicated that acyl chains in caveolae/raft domains were significantly less 'fluid' (i.e. more rigid) and the transbilayer 'fluidity gradient' was 4.4-fold greater than in plasma membranes; (ii) although sterol was more ordered in caveolae/raft domains than plasma membranes, spontaneous sterol transfer from caveolae/raft domains was faster (initial rate, 32%; half-time, t(1/2), 57%) than from the plasma membrane; (iii) although kinetic analysis showed similar proportions of exchangeable and non-exchangeable sterol pools in caveolae/raft domains and plasma membranes, addition of SCP-2 (sterol carrier protein-2) 1.3-fold more selectively increased sterol transfer from caveolae/raft domains by decreasing the t(1/2) (50%) and increasing the initial rate (5-fold); (iv) SCP-2 was also 2-fold more selective in decreasing the amount of non-exchangeable sterol in caveolae/raft domains compared with plasma membranes, such that nearly 80% of caveolar/raft sterol became exchangeable. In summary, although caveolae/raft lipids were less fluid than those of plasma membranes, sterol domains in caveolae/rafts were more spontaneously exchangeable and more affected by SCP-2 than those of the bulk plasma membranes. Thus caveolae/raft domains isolated without the use of detergents display unique structure, cholesterol domain kinetics and responsiveness to SCP-2 as compared with the parent plasma membrane.

Antipsychotic drugs antagonize human serotonin type 3 receptor currents in a noncompetitive manner

The serotonin type 3 (5-HT(3)) receptor is the only ligand-gated ion channel receptor for serotonin (5-HT). 5-HT(3) receptors play an important role in modulating the inhibitory action of dopamine in mesocorticolimbic brain regions. Neuroleptic drugs are commonly thought to exert their psychopharmacological action mainly through dopamine and serotonin type 2 (5-HT(2)) receptors. Except for clozapine, a direct pharmacological interaction of neuroleptics with 5-HT(3) receptors has not yet been described. Using the concentration-clamp technique, we investigated the effects of flupentixol, various phenothiazines, haloperidol, clozapine and risperidone on Na(+)-inward currents through 5-HT(3) receptors stably expressed in human embryonic kidney 293 cells, and through endogenous 5-HT(3) receptors of murine N1E-115 neuroblastoma cells. In addition, we studied their effects on Ca(2+) influx, measured as a change in intracellular Ca(2+) concentrations ([Ca(2+)](i)). All neuroleptic drugs, but not risperidone, antagonized Na(+)- and Ca(2+)-inward currents evoked by 5-HT (10 microM for 2 s and 1 microM, respectively) in a voltage-independent manner. Only clozapine was a competitive antagonist, while all other compounds turned out to be noncompetitive. Fluphenazine and haloperidol affected membrane anisotropy at concentrations below their IC(50) values, indicating that a change in membrane anisotropy might contribute to their antagonistic effect at the 5-HT(3) receptor. Only structure analogues of flupentixol and fluphenazine with a lipophilic side chain were potent antagonists against 5-HT-evoked Na(+) and Ca(2+) currents. Since 5-HT(3) receptors modulate mesolimbic and mesocortical dopaminergic activity, the functional antagonism of neuroleptics at 5-HT(3) receptors may contribute to their antipsychotic efficacy and may constitute a not yet recognized pharmacological principle of these drugs.

Antidepressants are functional antagonists at the serotonin type 3 (5-HT3) receptor

Antidepressants are commonly supposed to enhance serotonergic and/or noradrenergic neurotransmission by inhibition of neurotransmitter reuptake through binding to the respective neurotransmitter transporters or through inhibition of the monoamine oxidase. Using the concentration-clamp technique and measurements of intracellular Ca2+, we demonstrate that different classes of antidepressants act as functional antagonists at the human 5-HT3A receptor stably expressed in HEK 293 cells and at endogenous 5-HT3 receptors of rat hippocampal neurons and N1E-115 neuroblastoma cells. The tricyclic antidepressants desipramine, imipramine, and trimipramine, the serotonin reuptake inhibitor fluoxetine, the norepinephrine reuptake inhibitor reboxetine, and the noradrenergic and specific serotonergic antidepressant mirtazapine effectively reduced the serotonin-induced Na(+)- and Ca(2)(+)-currents in a dose-dependent fashion. This effect was voltage-independent and, with the exception of mirtazapine, noncompetitive. Desipramine, imipramine, trimipramine, and fluoxetine also accelerated receptor desensitization. Moclobemide and carbamazepine had no effect on the serotonin-induced cation current. By analyzing analogues of desipramine and carbamazepine, we found that a basic propylamine side chain increases the antagonistic potency of tricyclic compounds, whereas it is abolished by an uncharged carboxamide group. The antagonistic effects of antidepressants at the 5-HT3 receptor did not correlate with their effects on membrane fluidity. In conclusion, structurally different types of antidepressants modulate the function of this ligand-gated ion channel. This may represent a yet unrecognized pharmacological principle of antidepressants.

Glutamate-binding affinity of Drosophila metabotropic glutamate receptor is modulated by association with lipid rafts

Metabotropic glutamate receptors (mGluRs) are responsible for the effects of glutamate in slow synaptic transmission, and are implicated in the regulation of many processes in the CNS. Recently, we have reported the expression and purification of a mGluR from Drosophila melanogaster (DmGluRA), a homologue of mammalian group II mGluRs. We have shown that ligand binding to reconstituted DmGluRA requires the presence of ergosterol in the liposomes [Eroglu, C., Cronet, P., Panneels, V., Beaufils, P. & Sinning, I. (2002) EMBO Rep. 3, 491-496]. Here we demonstrate that the receptor exists in different affinity states for glutamate, depending on the membrane composition. The receptor is in a high-affinity state when associated with sterol-rich lipid microdomains (rafts), and in a low-affinity state out of rafts. Enrichment of the membranes with cholesterol shifts the receptor into the high-affinity state, and induces its association with rafts. The receptor was crosslinked to photocholesterol. Our data suggest that sterol-rich lipid rafts act as positive allosteric regulators of DmGluRA.

Lipid rafts and HIV pathogenesis: virion-associated cholesterol is required for fusion and infection of susceptible cells

We have shown that HIV budding occurs at cholesterol-rich membrane microdomains called lipid rafts (Nguyen and Hildreth, J Virol 2000;74:3264-3272). This observation prompted us to examine the role in HIV entry of cholesterol in the membrane of cells. We recently reported that host cell cholesterol is required for HIV infection (Liao et al., AIDS Res Hum Retroviruses 2001;17:1009-1019). In the present study we examined the role of virion-associated cholesterol in HIV infection by modulating the cholesterol content of virions and infected cells with 2-hydoxypropyl-beta-cyclodextrin (beta-cyclodextrin). Our results show that removal of cholesterol from the membrane of HIV-infected cells dramatically lowered virus release and that virions released from cholesterol-depleted cells are minimally infectious. Exposure of infectious HIV particles to beta-cyclodextrin resulted in a dose-dependent inactivation of the virus. In both cases, the effect was attributable to loss of cholesterol and could be reversed by replenishing cholesterol. beta-Cyclodextrin-treated, noninfectious HIV retained its ability to bind cells. Western blot, p24 core ELISA, and reverse transcription assays indicated that virions remained intact after treatment with beta-cyclodextrin at concentrations that abolished infectivity. Electron microscopy revealed that beta-cyclodextrin-treated HIV had a morphology very similar to that of untreated virus. R18 fluorescence dequenching studies showed that beta-cyclodextrin-treated HIV did not fuse to the membrane of susceptible cells. Dequenching was restored by replenishing virion-associated cholesterol. The results indicate that cholesterol in HIV particles is strictly required for fusion and infectivity. These observations in combination with those of past studies indicate beta-cyclodextrin to be an excellent candidate for use as a chemical barrier for AIDS prophylaxis.

CD4 dependence of gp120IIIB-CXCR4 interaction is cell-type specific

The HIV-1 envelope protein gp120IIIB is selective for the CXCR4 chemokine receptor and has been shown to induce apoptosis in neurons both in vivo and in vitro. We examined the ability of gp120IIIB to signal through the rat CXCR4 (rCXCR4) receptor and its dependence on the presence of the human CD4 (hCD4) protein in a number of cell systems. SDF-1alpha potently inhibited N-type Ca channels in cultured HEK293 cells expressing both the Ca channel subunits and rCXCR4 receptors. However, gp120IIIB was ineffective in producing either Ca channel inhibition or in blocking the effects of SDF-1alpha. However, when hCD4 was coexpressed with rCXCR4 and Ca channel subunits, gp120IIIB also produced Ca channel inhibition. Similarly, in PC12 cells transfected with the rCXCR4, SDF-1alpha produced mobilization of intracellular Ca, while gp120IIIB was only effective when hCD4 was coexpressed. SDF-1alpha induced endocytosis of Yellow Fluorescent Protein (YFP)-tagged rCXCR4 expressed in PC12 cells, as did gp120IIIB, an effect which was enhanced by hCD4 coexpression. When tagged rCXCR4 was expressed in F-11 cells or in rat DRG neurons, SDF-1alpha produced extensive receptor endocytosis. However, the ability of gp120IIIB to produce endocytosis was dependent on the coexpression of hCD4. Our results demonstrate that the degree of hCD4 dependence of the agonist effects of gp120IIIB at the rCXCR4 receptor is cell-type specific.

Transport of plasma membrane-derived cholesterol and the function of Niemann-Pick C1 Protein

To visualize the intracellular transport of plasma membrane-derived cholesterol under physiological and pathophysiological conditions, a novel fluorescent cholesterol analog, 6-dansyl cholestanol (DChol), has been synthesized. We present several lines of evidence that DChol mimics cholesterol. The cholesterol probe could be efficiently incorporated into the plasma membrane via cyclodextrin-donor complexes. The itinerary of DChol from the plasma membrane to the cell was studied to determine its dependence on the function of Niemann-Pick C1 (NPC) protein. In all cells, DChol moved from the plasma membrane to the endoplasmic reticulum. Its further transport to the Golgi complex was observed but with marked differences among various cell lines. DChol was finally transported to small (approximately 0.5 microm diameter) lipid droplets, a process that required functional acyl-CoA:cholesterol acyltransferase. In human NPC fibroblasts, NPC-like cells, or in cells mimicking the NPC phenotype, DChol was found in enlarged (>1 microm diameter) droplets. When the NPC-phenotype was corrected by transfection with NPC1, DChol was again found in small-sized droplets. Our data show that NPC1 has an essential role in the distribution of plasma membrane-derived cholesterol by maintaining the small size of cholesterol-containing lipid droplets in the cell.

Cholesterol and steroid hormones: modulators of oxytocin receptor function

The function and physiological regulation of the oxytocin-receptor system is strongly steroid-dependent. This is, unexpectedly, only partially reflected by the promoter sequences in the oxytocin receptor and favors the idea that posttranscriptional mechanisms may also play a significant role for the physiological regulation of the oxytocin-receptor system. Our data indicate that cholesterol acts as an allosteric modulator of the oxytocin receptor and stabilizes both membrane-associated and solubilized OT receptors in a high-affinity state for agonists and antagonists. Moreover, high-affinity OT receptors are 2-fold enriched in cholesterol-rich plasma membrane domains in HEK293 fibroblasts stably expressing the human OT receptor. Biochemical data suggest a direct and cooperative molecular interaction of cholesterol molecules with OT receptors. To localize the cholesterol interacting domain of the oxytocin receptor the C-terminal part including the last two transmembrane domains have been exchanged by the corresponding sequences of the cholecystokinin type B receptor, which is functionally not dependent on cholesterol. Concerning its ligand-binding behavior this chimeric receptor protein showed the same dependence on cholesterol and its analogues as the wild type oxytocin receptor. From mutagenesis experiments and studies with receptor chimera between the OTR and cholecystokinin type B receptor, we conclude that a major part of the cholesterol interacting domain may be localized in the first part of the oxytocin receptor, possibly in a domain nearby the agonist binding site. Progesterone is considered to be essential to maintain the uterine quiescence. High concentrations of progesterone (> 10 microM) attenuate or block the signaling of several GPCRs, including the OT receptor via a fast, reversible and non-genomic pathway. Progesterone is known to inhibit both cholesterol biosynthesis and the intracellular trafficking of cholesterol. We therefore test the hypothesis that progesterone affects the signal transduction and subdomain localization of receptors via its influence on cholesterol trafficking. Since cholesterol-rich subdomains (rafts) are considered to be organization centers for cellular signal transduction, changes of the level or distribution of cholesterol may have profound effects on receptor-mediated signaling in general. Using fluorescence recovery after photobleaching (FRAP) measurements with GFP-tagged oxytocin receptors the influence of steroids on the mobility and distribution of the oxytocin receptor in the plasma membrane was analyzed. Progesterone had no effect on the lateral mobility of the oxytocin receptor, but it led to marked inhibition of cellular motility such as vesicle trafficking and movements of filopodia. Non-genomic effects of progesterone and estradiol with respect to receptor signaling as well as the influence of cholesterol on signal transduction will be discussed in more detail.

Green fluorescent protein as a reporter of human mu-opioid receptor overexpression and localization in the methylotrophic yeast Pichia pastoris

The human mu-opioid receptor (HuMOR) was fused in its N-terminus end to the green fluorescent protein (GFP) or/and to the c-myc and six histidines tags in its C-terminus end, and expressed in the methylotrophic yeast Pichia pastoris. Neither the C- nor the N-terminal tagging of the receptor does modify its pharmacological properties as compared to the untagged receptor. Expression levels of fusion receptors determined by GFP fluorescence measurements strongly correlates with the number of sites expressed per cell detected through saturation studies (Bmax value), thus showing that GFP is an efficient and reliable reporter of the HuMOR functional expression. The N- and C-terminus tags have allowed to show that the entire molecule is overexpressed. They have permitted in-situ localization experiments using fluorescence and electron microscopy techniques and have shown a dense intracellular labelling. Above all, the quantification of expression levels made possible through fluorescence intensity analysis, have revealed that huge amounts of receptor are produced that could not be detected through classical binding experiments: for a Bmax value of 1 pmol mg(-1) of receptor determined through binding studies, 16 pmol were found in membrane preparations using fluorescence and 100 pmol in whole cells. These results should be very useful for large-scale production and structural biology of HuMOR, and other G-protein coupled receptors (GPCRs).

Cholesterol as stabilizer of the oxytocin receptor

The function of the oxytocin receptor system is strongly dependent on steroids as demonstrated by several physiological studies. One key element of this dependence on steroids may be the interaction of cholesterol and the oxytocin receptor. In this study, we show that cholesterol stabilizes the solubilized human oxytocin receptor against thermal inactivation and proteolytic degradation. In the absence of additional cholesterol, the soluble receptor inactivates within minutes. Maximal stabilization of the oxytocin receptor requires a continuous supply with cholesterol from a cholesterol-rich environment. A structure-activity analysis of various cholesterol analogues and their effect on the thermal stability of the oxytocin receptor showed that the stabilizing function of cholesterol was highly specific. The structural requirements of a potent stabilizing steroid are very similar to those necessary to support the high-affinity state of the receptor. Moreover, in the presence of cholesterol, the oxytocin receptor is significantly more stable against alterations of pH value (pH 4-12). The results show that cholesterol acts as a general stabilizer of the oxytocin receptor.

Human immunodeficiency virus type 1 uses lipid raft-colocalized CD4 and chemokine receptors for productive entry into CD4(+) T cells

In this report, we describe a crucial role of lipid raft-colocalized receptors in the entry of human immunodeficiency virus type 1 (HIV-1) into CD4(+) T cells. We show that biochemically isolated detergent-resistant fractions have characteristics of lipid rafts. Lipid raft integrity was required for productive HIV-1 entry as determined by (i) semiquantitative PCR analysis and (ii) single-cycle infectivity assay using HIV-1 expressing the luciferase reporter gene and pseudotyped with HIV-1 HXB2 envelope or vesicular stomatitis virus envelope glycoprotein (VSV-G). Depletion of plasma membrane cholesterol with methyl-beta-cyclodextrin (MbetaCD) relocalized raft-resident markers to a nonraft environment but did not significantly change the surface expression of HIV-1 receptors. MbetaCD treatment inhibited productive infection of HIV-1 by 95% as determined by luciferase activity in cells infected with HXB2 envelope-pseudotyped virus. In contrast, infection with VSV-G-pseudotyped virus, which enters the cells through an endocytic pathway, was not suppressed. Biochemical fractionation and confocal imaging of HIV-1 receptor distribution in live cells demonstrated that CD4, CCR5, and CXCR4 colocalized with raft-resident markers, ganglioside GM1, and glycosylphosphatidylinositol-anchored CD48. While confocal microscopy analysis revealed that HIV-1 receptors localized most likely to the same lipid microdomains, sucrose gradient analysis of the receptor localization showed that, in contrast to CD4 and CCR5, CXCR4 was associated preferentially with the nonraft membrane fraction. The binding of HIV-1 envelope gp120 to lipid rafts in the presence, but not in the absence, of cholesterol strongly supports our hypothesis that raft-colocalized receptors are directly involved in virus entry. Dramatic changes in lipid raft and HIV-1 receptor redistribution were observed upon binding of HIV-1 NL4-3 to PM1 T cells. Colocalization of CCR5 with GM1 and gp120 upon engagement of CD4 and CXCR4 by HIV-1 further supports our observation that HIV-1 receptors localize to the same lipid rafts in PM1 T cells.

Localization of the human oxytocin receptor in caveolin-1 enriched domains turns the receptor-mediated inhibition of cell growth into a proliferative response

In this study, we investigated the functional role of the localization of human OTR in caveolin-1 enriched membrane domains. Biochemical fractionation of MDCK cells stably expressing the WT OTR-GFP indicated that only minor quantities of receptor are partitioned in caveolin-1 enriched domains. However, when fused to caveolin-2, the OTR protein proved to be exclusively localized in caveolin-1 enriched fractions, where it bound the agonist with increased affinity and efficiently coupled to Galpha(q/11). Interestingly, the chimeric protein was unable to undergo agonist-induced internalization and remained confined to the plasma membrane even after prolonged agonist exposure (120 min). A striking difference in receptor stimulation was observed when the OT-induced effect on cell proliferation was analysed: stimulation of the human WT OTR inhibited cell growth, whereas the chimeric protein had a proliferative effect. These data indicate that the localization of human OTR in caveolin-1 enriched microdomains radically alters its regulatory effects on cell growth; the fraction of OTR residing in caveolar structures may therefore play a crucial role in regulating cell proliferation.

The effect of cholesterol in a liposomal Muc1 vaccine

A liposomal Muc1 mucin vaccine for treatment of adenocarcinomas was formulated by incorporating a synthetic Muc1 mucin-based lipopeptide and Lipid A into a DPPC/cholesterol bilayer. Vaccination of mice with the liposomal formulation produced a peptide-specific immune response dependent on the cholesterol content. The response occurred at a threshold of 20-23 mol% cholesterol, and was optimal at cholesterol levels of > or =30 mol%. To understand this cholesterol dependency, we studied the effect of cholesterol on the liposomal bilayer and surface properties. Freeze-fracture electron microscopy showed a unique surface texture that was codependent upon cholesterol (> or =20 mol%) and lipopeptide content. Fluorescence anisotropy measurements exhibited a significant decrease in the rotational motion of 1,6-diphenyl-1,3,5-hexatriene in formulations containing >20 mol% cholesterol and only in the presence of the lipopeptide. At 20 mol% cholesterol and with lipopeptide, DSC showed a significant increase in the main phase transition of the DPPC bilayers, while Raman spectroscopy indicated a more ordered arrangement of DPPC molecules compared to control liposomes containing DPPC/cholesterol alone. Taken together, the data suggest the presence of lipopeptide-rich microdomains at and above a threshold of 20 mol% cholesterol that may play a role in the induction of a peptide-specific immunological response.

Lipid rafts and HIV pathogenesis: host membrane cholesterol is required for infection by HIV type 1

In a previous study we showed that budding of HIV-1 particles occurs at highly specialized membrane microdomains known as lipid rafts. These microdomains are characterized by a distinct lipid composition that includes high concentrations of cholesterol, sphingolipids, and glycolipids. Since cholesterol is known to play a key role in the entry of some other viruses, our observation of HIV budding from lipid rafts led us to investigate the role in HIV-1 entry of cholesterol and lipid rafts in the plasma membrane of susceptible cells. We have used 2-OH-propyl-beta-cyclodextrin (beta-cyclodextrin) to deplete cellular cholesterol and disperse lipid rafts. Our results show that removal of cellular cholesterol rendered primary cells and cell lines highly resistant to HIV-1-mediated syncytium formation and to infection by both CXCR4- and CCR5-specific viruses. beta-Cyclodextrin treatment of cells partially reduced HIV-1 binding, while rendering chemokine receptors highly sensitive to antibody-mediated internalization. There was no effect on CD4 expression. All of the above-described effects were readily reversed by incubating cholesterol-depleted cells with low concentrations of cholesterol-loaded beta-cyclodextrin to restore cholesterol levels. Cholesterol depletion made cells resistant to SDF-1-induced binding to ICAM-1 through LFA-1. Since LFA-1 contributes significantly to cell binding by HIV-1, this latter effect may have contributed to the observed reduction in HIV-1 binding to cells after treatment with beta-cyclodextrin. Our results indicate that cholesterol may be critical to the HIV-1 coreceptor function of chemokine receptors and is required for infection of cells by HIV-1.

The oxytocin receptor system: structure, function, and regulation

The neurohypophysial peptide oxytocin (OT) and OT-like hormones facilitate reproduction in all vertebrates at several levels. The major site of OT gene expression is the magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei. In response to a variety of stimuli such as suckling, parturition, or certain kinds of stress, the processed OT peptide is released from the posterior pituitary into the systemic circulation. Such stimuli also lead to an intranuclear release of OT. Moreover, oxytocinergic neurons display widespread projections throughout the central nervous system. However, OT is also synthesized in peripheral tissues, e.g., uterus, placenta, amnion, corpus luteum, testis, and heart. The OT receptor is a typical class I G protein-coupled receptor that is primarily coupled via G(q) proteins to phospholipase C-beta. The high-affinity receptor state requires both Mg(2+) and cholesterol, which probably function as allosteric modulators. The agonist-binding region of the receptor has been characterized by mutagenesis and molecular modeling and is different from the antagonist binding site. The function and physiological regulation of the OT system is strongly steroid dependent. However, this is, unexpectedly, only partially reflected by the promoter sequences in the OT receptor gene. The classical actions of OT are stimulation of uterine smooth muscle contraction during labor and milk ejection during lactation. While the essential role of OT for the milk let-down reflex has been confirmed in OT-deficient mice, OT's role in parturition is obviously more complex. Before the onset of labor, uterine sensitivity to OT markedly increases concomitant with a strong upregulation of OT receptors in the myometrium and, to a lesser extent, in the decidua where OT stimulates the release of PGF(2 alpha). Experiments with transgenic mice suggest that OT acts as a luteotrophic hormone opposing the luteolytic action of PGF(2 alpha). Thus, to initiate labor, it might be essential to generate sufficient PGF(2 alpha) to overcome the luteotrophic action of OT in late gestation. OT also plays an important role in many other reproduction-related functions, such as control of the estrous cycle length, follicle luteinization in the ovary, and ovarian steroidogenesis. In the male, OT is a potent stimulator of spontaneous erections in rats and is involved in ejaculation. OT receptors have also been identified in other tissues, including the kidney, heart, thymus, pancreas, and adipocytes. For example, in the rat, OT is a cardiovascular hormone acting in concert with atrial natriuretic peptide to induce natriuresis and kaliuresis. The central actions of OT range from the modulation of the neuroendocrine reflexes to the establishment of complex social and bonding behaviors related to the reproduction and care of the offspring. OT exerts potent antistress effects that may facilitate pair bonds. Overall, the regulation by gonadal and adrenal steroids is one of the most remarkable features of the OT system and is, unfortunately, the least understood. One has to conclude that the physiological regulation of the OT system will remain puzzling as long as the molecular mechanisms of genomic and nongenomic actions of steroids have not been clarified.