Cholesterol reduction by methyl-beta-cyclodextrin attenuates the delta opioid receptor-mediated signaling in neuronal cells but enhances it in non-neuronal cells

Paternal morphine exposure enhances morphine self-administration and induces region-specific neural adaptations in reward-related brain regions of male offspring

Background

A growing body of preclinical studies report that preconceptional experiences can have a profound and long-lasting impact on adult offspring behavior and physiology. However, less is known about paternal drug exposure and its effects on reward sensitivity in the next generation.

Methods

Adult male rats self-administered morphine for 65 days; controls received saline. Sires were bred to drug-naïve dams to produce first-generation (F1) offspring. Morphine, cocaine, and nicotine self-administration were measured in adult F1 progeny. Molecular correlates of addiction-like behaviors were measured in reward-related brain regions of drug naïve F1 offspring.

Results

Male, but not female offspring produced by morphine-exposed sires exhibited dose-dependent increased morphine self-administration and increased motivation to earn morphine infusions under a progressive ratio schedule of reinforcement. This phenotype was drug-specific as self-administration of cocaine, nicotine, and sucrose were not altered by paternal morphine history. The male offspring of morphine-exposed sires also had increased expression of mu-opioid receptors in the ventral tegmental area but not in the nucleus accumbens.

Conclusions

Paternal morphine exposure increased morphine addiction-like behavioral vulnerability in male but not female progeny. This phenotype is likely driven by long-lasting neural adaptations within the reward neural brain pathways.

Cyclodextrins: Only Pharmaceutical Excipients or Full-Fledged Drug Candidates?

Cyclodextrins, representing a versatile family of cyclic oligosaccharides, have extensive pharmaceutical applications due to their unique truncated cone-shaped structure with a hydrophilic outer surface and a hydrophobic cavity, which enables them to form non-covalent host-guest inclusion complexes in pharmaceutical formulations to enhance the solubility, stability and bioavailability of numerous drug molecules. As a result, cyclodextrins are mostly considered as inert carriers during their medical application, while their ability to interact not only with small molecules but also with lipids and proteins is largely neglected. By forming inclusion complexes with cholesterol, cyclodextrins deplete cholesterol from cellular membranes and thereby influence protein function indirectly through alterations in biophysical properties and lateral heterogeneity of bilayers. In this review, we summarize the general chemical principles of direct cyclodextrin-protein interactions and highlight, through relevant examples, how these interactions can modify protein functions in vivo, which, despite their huge potential, have been completely unexploited in therapy so far. Finally, we give a brief overview of disorders such as Niemann-Pick type C disease, atherosclerosis, Alzheimer's and Parkinson's disease, in which cyclodextrins already have or could have the potential to be active therapeutic agents due to their cholesterol-complexing or direct protein-targeting properties.

Post-translational Modifications of Opioid Receptors

Post-translational modifications (PTMs) are key events in signal transduction since they affect protein function by regulating their abundance and/or activity. PTMs involve the covalent attachment of functional groups to specific amino acids. Since they tend to be generally reversible, PTMs serve as regulators of signal transduction pathways. G-protein-coupled receptors (GPCRs) are major signaling proteins that undergo multiple types of PTMs. In this Review, we focus on the opioid receptors, members of GPCR family A, and highlight recent advances in the field that have underscored the importance of PTMs in the functional regulation of these receptors. Since opioid receptor activity plays a central role in the development of tolerance and addiction to morphine and other drugs of abuse, understanding the molecular mechanisms regulating receptor activity is of fundamental importance.

Location and Cell-Type-Specific Bias of Metabotropic Glutamate Receptor, mGlu5, Negative Allosteric Modulators

Emerging data indicate that G-protein coupled receptor (GPCR) signaling is determined by not only the agonist and a given receptor but also a variety of cell-type-specific factors that can influence a receptor's response. For example, the metabotropic glutamate receptor, mGlu₅, which is implicated in a number of neuropsychiatric disorders such as depression, anxiety, and autism, also signals from inside the cell which leads to sustained Ca²⁺ mobilization versus rapid transient responses. Because mGlu₅ is an important drug target, many negative allosteric modulators (NAMs) have been generated to modulate its activity. Here we show that NAMs such as AFQ056, AZD2066, and RG7090 elicit very different end points when tested in postnatal neuronal cultures expressing endogenous mGlu₅ receptors. For example, AFQ056 fails to block intracellular mGlu₅-mediated Ca²⁺ increases whereas RG7090 is very effective. These differences are not due to differential receptor levels, since about the same number of mGlu₅ receptors are present on neurons from the cortex, hippocampus, and striatum based on pharmacological, biochemical, and molecular data. Moreover, biotinylation studies reveal that more than 90% of the receptor is intracellular in these neurons. Taken together, these data indicate that the tested NAMs exhibit both location-dependent and cell type specific bias for mGlu₅-mediated Ca²⁺ mobilization which may affect clinical outcomes.

Determination of δ-opioid receptor molecules mobility in living cells plasma membrane by novel method of FRAP analysis

Fluorescence recovery after photobleaching (FRAP) is the preferred method for analyzing the lateral mobility of fluorescently-tagged proteins in the plasma membranes (PMs) of live cells. FRAP experiments are described as being easy to perform; however, the analysis of the acquired data can be difficult. The evaluation procedure must be properly combined with the imaging setup of the confocal microscope to provide unbiased results. With the aim of increasing the accuracy of determining the diffusion coefficient (D) and mobile fraction (M_f) of PM proteins, we developed a novel method for FRAP analysis in the equatorial plane of the cell. This method is based on the calculation of photobleaching characteristics, derived from the light intensity profile and optical parameters of the confocal microscope, and on the model of fluorescent molecule diffusion in PM regions outside of the focal plane. Furthermore, cell movement artifacts in the FRAP data are ameliorated by using a region of interest, which is not fixed but instead moves adaptively in coordination with the movement of cells. When this method was used to determine the mobility of the δ-opioid receptor-eYFP in HEK293 cells, a highly significant decrease in receptor mobility was detected in cholesterol-depleted cells. This decrease was fully reversible by the replenishment of cholesterol levels. Our results demonstrate the crucial role played by cholesterol in the dynamic organization of δ-opioid receptors in the PM under in vivo conditions. Our method may be applied for the determination of the D and M_f values of other PM proteins.

A Critical Analysis of Molecular Mechanisms Underlying Membrane Cholesterol Sensitivity of GPCRs

G protein-coupled receptors (GPCRs) are the largest and a diverse family of proteins involved in signal transduction across biological membranes. GPCRs mediate a wide range of physiological processes and have emerged as major targets for the development of novel drug candidates in all clinical areas. Since GPCRs are integral membrane proteins, regulation of their organization, dynamics, and function by membrane lipids, in particular membrane cholesterol, has emerged as an exciting area of research. Cholesterol sensitivity of GPCRs could be due to direct interaction of cholesterol with the receptor (specific effect). Alternately, GPCR function could be influenced by the effect of cholesterol on membrane physical properties (general effect). In this review, we critically analyze the specific and general mechanisms of the modulation of GPCR function by membrane cholesterol, taking examples from representative GPCRs. While evidence for both the proposed mechanisms exists, there appears to be no clear-cut distinction between these two mechanisms, and a combination of these mechanisms cannot be ruled out in many cases. We conclude that classifying the mechanism underlying cholesterol sensitivity of GPCR function merely into these two mutually exclusive classes could be somewhat arbitrary. A more holistic approach could be suitable for analyzing GPCR-cholesterol interaction.

Is preoperative hypocholesterolemia a risk factor for severe postoperative pain? Analysis of 1,944 patients after laparoscopic colorectal cancer surgery

Purpose

This study aimed to identify the effect of preoperative serum total cholesterol on postoperative pain outcome in patients who had undergone laparoscopic colorectal cancer surgery.

Methods

We retrospectively reviewed the medical records of patients diagnosed with colorectal cancer who had undergone laparoscopic colorectal surgery from January 1, 2011, to June 30, 2017, to identify the relationship of total cholesterol levels within a month prior to surgery with the numeric rating scale (NRS) scores and total opioid consumption on postoperative days (PODs) 0–2.

Results

We included 1,944 patients. No significant correlations were observed between total cholesterol and the NRS (POD 0), NRS (POD 1), and oral morphine equivalents (PODs 0–2) (P>0.05). There was no significant difference between the low (<160 mg/dL), medium (160–199 mg/dL), and high (≥200 mg/dL) groups in NRS scores on PODs 0, 1, or 2 (P>0.05). Furthermore, there was no significant association in multivariate linear regression analysis for postoperative opioid consumption according to preoperative serum total cholesterol level (coefficient 0.08, 95% CI −0.01 to 0.18, P=0.81).

Conclusion

This study showed that there was no meaningful association between preoperative total cholesterol level and postoperative pain outcome after laparoscopic colorectal cancer surgery.

Association of Preoperative Serum Total Cholesterol Level with Postoperative Pain Outcomes after Laparoscopic Surgery for Gastric Cancer

BACKGROUND

Opioid usage is reportedly increased in patients with advanced cancer and low total cholesterol (TC). The aim of this study was to determine the effects of preoperative serum TC levels on postoperative opioid usage in patients undergoing laparoscopic gastric cancer surgery.

METHODS

We retrospectively analyzed the medical records of patients with gastric cancer who underwent laparoscopic gastrectomy at our institution between January 1, 2011 and July 31, 2017. We investigated the correlation between TC levels in the month before surgery and numeric rating scale (NRS) scores and opioid consumption on postoperative days (PODs) 0 to 3. The patients were stratified according to preoperative TC level (< 160 mg/dL, low; 160 to 199 mg/dL, medium; ≥ 200 mg/dL, high).

RESULTS

TC and NRS scores (PODs 0, 1) for 1,919 eligible patients showed weak but significant positive coefficients (0.074 and 0.098 on POD 0 and POD 1, respectively, P < 0.01). After adjusting for confounders, there were no significant differences in postoperative NRS scores on PODs 0, 1, 2, or 3 or in oral morphine equivalents on PODs 0 to 3 according to TC level.

CONCLUSION

Preoperative serum TC levels have no effect on pain outcomes in the 3 days following laparoscopic gastric surgery.

Delta Opioid Receptors and Cardioprotection

The opioid receptor family, with associated endogenous ligands, has numerous roles throughout the body. Moreover, the delta opioid receptor (DORs) has various integrated roles within the physiological systems, including the cardiovascular system. While DORs are important modulators of cardiovascular autonomic balance, they are well-established contributors to cardioprotective mechanisms. Both endogenous and exogenous opioids acting upon DORs have roles in myocardial hibernation and protection against ischaemia-reperfusion (I-R) injury. Downstream signalling mechanisms governing protective responses alternate, depending on the timing and duration of DOR activation. The following review describes models and mechanisms of DOR-mediated cardioprotection, the impact of co-morbidities and challenges for clinical translation.

Determination of μ-, δ- and κ-opioid receptors in forebrain cortex of rats exposed to morphine for 10 days: Comparison with animals after 20 days of morphine withdrawal

BACKGROUND

Chronic exposure of mammalian organism to morphine results in adaption to persistent high opioid tone through homeostatic adjustments. Our previous results indicated that in the frontal brain cortex (FBC) of rats exposed to morphine for 10 days, such a compensatory adjustment was detected as large up-regulation of adenylylcyclases I (8-fold) and II (2.5-fold). The other isoforms of AC (III-IX) were unchanged. Importantly, the increase of ACI and ACII was reversible as it disappeared after 20 days of morphine withdrawal. Changes of down-stream signaling molecules such as G proteins and adenylylcyclases should respond to and be preceded by primary changes proceeding at receptor level. Therefore in our present work, we addressed the problem of reversibility of the long-term morphine effects on μ-, δ- and κ-OR protein levels in FBC.

METHODS

Rats were exposed to increasing doses of morphine (10-40 mg/kg) for 10 days and sacrificed either 24 h (group +M10) or 20 days (group +M10/-M20) after the last dose of morphine in parallel with control animals (groups -M10 and -M10/-M20). Post-nuclear supernatant (PNS) fraction was prepared from forebrain cortex, resolved by 1D-SDS-PAGE under non-dissociated (-DTT) and dissociated (+DTT) conditions, and analyzed for the content of μ-, δ- and κ-OR by immunoblotting with C- and N-terminus oriented antibodies.

RESULTS

Significant down-regulation of δ-OR form exhibiting Mw ≈ 60 kDa was detected in PNS prepared from both (+M10) and (+M10/-M20) rats. However, the total immunoblot signals of μ-, δ- and κ-OR, respectively, were unchanged. Plasma membrane marker Na, K-ATPase, actin and GAPDH were unaffected by morphine in both types of PNS. Membrane-domain marker caveolin-1 and cholesterol level increased in (+M10) rats and this increase was reversed back to control level in (+M10/-M20) rats.

CONCLUSIONS

In FBC, prolonged exposure of rats to morphine results in minor (δ-OR) or no change (μ- and κ-OR) of opioid receptor content. The reversible increases of caveolin-1 and cholesterol levels suggest participation of membrane domains in compensatory responses during opioid withdrawal.

GENERAL SIGNIFICANCE

Analysis of reversibility of morphine effect on mammalian brain.

Membrane cholesterol access into a G-protein-coupled receptor

Cholesterol is a key component of cell membranes with a proven modulatory role on the function and ligand-binding properties of G-protein-coupled receptors (GPCRs). Crystal structures of prototypical GPCRs such as the adenosine A_2A receptor (A_2AR) have confirmed that cholesterol finds stable binding sites at the receptor surface suggesting an allosteric role of this lipid. Here we combine experimental and computational approaches to show that cholesterol can spontaneously enter the A_2AR-binding pocket from the membrane milieu using the same portal gate previously suggested for opsin ligands. We confirm the presence of cholesterol inside the receptor by chemical modification of the A_2AR interior in a biotinylation assay. Overall, we show that cholesterol's impact on A_2AR-binding affinity goes beyond pure allosteric modulation and unveils a new interaction mode between cholesterol and the A_2AR that could potentially apply to other GPCRs.

G-protein-coupled receptors trigger several signalling pathways and their activity was proposed to be allosteric modulated by cholesterol. Here the authors use molecular dynamics simulations and ligand binding assays to show that membrane cholesterol can bind to adenosine A_2A receptor orthosteric site.

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.

Lipid Microdomains in Synapse Formation

Membrane lipid rafts (i.e., cholesterol/sphingolipids domains) exhibit functional roles in both healthy and pathological states of the nervous system. However, due to their highly dynamic nature, it remains a challenge to characterize the fundamental aspects of lipid rafts that are important for specific neuronal processes. An experimental approach is presented here that allows for the interfacing of living neurons with an experimentally accessible model membrane where lipid order in cellular rafts can be reproducibly mimicked. It is demonstrated that coexisting lipid microdomains in model membranes can regulate axonal guidance and establish stable presynaptic contacts when interfaced with neurons in vitro. Experimental evidence is provided where specific functional groups and lateral organizations are favored by neurons in establishing synaptic connections. The model membrane platform presented in this work provides an accessible and direct means to investigate how lipid rafts regulate synapse formation. This experimental platform can similarly be extended to explore a variety of other cellular events where lipid lateral organization is believed to be important.

Opioid doses required for pain management in lung cancer patients with different cholesterol levels: negative correlation between opioid doses and cholesterol levels

BACKGROUND

Pain management has been considered as significant contributor to broad quality-of-life improvement for cancer patients. Modulating serum cholesterol levels affects analgesia abilities of opioids, important pain killer for cancer patients, in mice system. Thus the correlation between opioids usages and cholesterol levels were investigated in human patients with lung cancer.

METHODS

Medical records of 282 patients were selected with following criteria, 1) signed inform consent, 2) full medical records on total serum cholesterol levels and opioid administration, 3) opioid-naïve, 4) not received/receiving cancer-related or cholesterol lowering treatment, 5) pain level at level 5-8. The patients were divided into different groups basing on their gender and cholesterol levels. Since different opioids, morphine, oxycodone, and fentanyl, were all administrated at fixed low dose initially and increased gradually only if pain was not controlled, the percentages of patients in each group who did not respond to the initial doses of opioids and required higher doses for pain management were determined and compared.

RESULTS

Patients with relative low cholesterol levels have larger percentage (11 out of 28 in female and 31 out of 71 in male) to not respond to the initial dose of opioids than those with high cholesterol levels (0 out of 258 in female and 8 out of 74 in male). Similar differences were obtained when patients with different opioids were analyzed separately. After converting the doses of different opioids to equivalent doses of oxycodone, significant correlation between opioid usages and cholesterol levels was also observed.

CONCLUSIONS

Therefore, more attention should be taken to those cancer patients with low cholesterol levels because they may require higher doses of opioids as pain killer.

Allosteric regulation of G protein-coupled receptor activity by phospholipids

Lipids are emerging as key regulators of membrane protein structure and activity. These effects can be attributed either to the modification of bilayer properties (thickness, curvature and surface tension) or to the binding of specific lipids to the protein surface. For G protein-coupled receptors (GPCRs), the effects of phospholipids on receptor structure and activity remain poorly understood. Here we reconstituted purified β2-adrenergic receptor (β2R) in high-density lipoparticles to systematically characterize the effect of biologically relevant phospholipids on receptor activity. We observed that the lipid headgroup type affected ligand binding (agonist and antagonist) and receptor activation. Specifically, phosphatidylgycerol markedly favored agonist binding and facilitated receptor activation, whereas phosphatidylethanolamine favored antagonist binding and stabilized the inactive state of the receptor. We then showed that these effects could be recapitulated with detergent-solubilized lipids, demonstrating that the functional modulation occurred in the absence of a bilayer. Our data suggest that phospholipids act as direct allosteric modulators of GPCR activity.

High Efficacy but Low Potency of δ-Opioid Receptor-G Protein Coupling in Brij-58-Treated, Low-Density Plasma Membrane Fragments

Principal Findings

HEK293 cells stably expressing PTX-insensitive δ-opioid receptor-Gi1α (C³⁵¹I) fusion protein were homogenized, treated with low concentrations of non-ionic detergent Brij-58 at 0°C and fractionated by flotation in sucrose density gradient. In optimum range of detergent concentrations (0.025–0.05% w/v), Brij-58-treated, low-density membranes exhibited 2-3-fold higher efficacy of DADLE-stimulated, high-affinity [³²P]GTPase and [³⁵S]GTPγS binding than membranes of the same density prepared in the absence of detergent. The potency of agonist DADLE response was significantly decreased. At high detergent concentrations (>0.1%), the functional coupling between δ-opioid receptors and G proteins was completely diminished. The same detergent effects were measured in plasma membranes isolated from PTX-treated cells. Therefore, the effect of Brij-58 on δ-opioid receptor-G protein coupling was not restricted to the covalently bound G_i1α within δ-opioid receptor-G_i1α fusion protein, but it was also valid for PTX-sensitive G proteins of G_i/G_o family endogenously expressed in HEK293 cells. Characterization of the direct effect of Brij-58 on the hydrophobic interior of isolated plasma membranes by steady-state anisotropy of diphenylhexatriene (DPH) fluorescence indicated a marked increase of membrane fluidity. The time-resolved analysis of decay of DPH fluorescence by the “wobble in cone” model of DPH motion in the membrane indicated that the exposure to the increasing concentrations of Brij-58 led to a decreased order and higher motional freedom of the dye.

Summary

Limited perturbation of plasma membrane integrity by low concentrations of non-ionic detergent Brij-58 results in alteration of δ-OR-G protein coupling. Maximum G protein-response to agonist stimulation (efficacy) is increased; affinity of response (potency) is decreased. The total degradation plasma membrane structure at high detergent concentrations results in diminution of functional coupling between δ-opioid receptors and G proteins.

Opioid receptors and cardioprotection - 'opioidergic conditioning' of the heart

Ischaemic heart disease (IHD) remains a major cause of morbidity/mortality globally, firmly established in Westernized or 'developed' countries and rising in prevalence in developing nations. Thus, cardioprotective therapies to limit myocardial damage with associated ischaemia-reperfusion (I-R), during infarction or surgical ischaemia, is a very important, although still elusive, clinical goal. The opioid receptor system, encompassing the δ (vas deferens), κ (ketocyclazocine) and μ (morphine) opioid receptors and their endogenous opioid ligands (endorphins, dynorphins, enkephalins), appears as a logical candidate for such exploitation. This regulatory system may orchestrate organism and organ responses to stress, induces mammalian hibernation and associated metabolic protection, triggers powerful adaptive stress resistance in response to ischaemia/hypoxia (preconditioning), and mediates cardiac benefit stemming from physical activity. In addition to direct myocardial actions, central opioid receptor signalling may also enhance the ability of the heart to withstand I-R injury. The δ- and κ-opioid receptors are strongly implicated in cardioprotection across models and species (including anti-infarct and anti-arrhythmic actions), with mixed evidence for μ opioid receptor-dependent protection in animal and human tissues. A small number of clinical trials have provided evidence of cardiac benefit from morphine or remifentanil in cardiopulmonary bypass or coronary angioplasty patients, although further trials of subtype-specific opioid receptor agonists are needed. The precise roles and utility of this GPCR family in healthy and diseased human myocardium, and in mediating central and peripheral survival responses, warrant further investigation, as do the putative negative influences of ageing, IHD co-morbidities, and relevant drugs on opioid receptor signalling and protective responses.

Opioid receptors and cardioprotection - 'opioidergic conditioning' of the heart

Ischaemic heart disease (IHD) remains a major cause of morbidity/mortality globally, firmly established in Westernized or 'developed' countries and rising in prevalence in developing nations. Thus, cardioprotective therapies to limit myocardial damage with associated ischaemia-reperfusion (I-R), during infarction or surgical ischaemia, is a very important, although still elusive, clinical goal. The opioid receptor system, encompassing the δ (vas deferens), κ (ketocyclazocine) and μ (morphine) opioid receptors and their endogenous opioid ligands (endorphins, dynorphins, enkephalins), appears as a logical candidate for such exploitation. This regulatory system may orchestrate organism and organ responses to stress, induces mammalian hibernation and associated metabolic protection, triggers powerful adaptive stress resistance in response to ischaemia/hypoxia (preconditioning), and mediates cardiac benefit stemming from physical activity. In addition to direct myocardial actions, central opioid receptor signalling may also enhance the ability of the heart to withstand I-R injury. The δ- and κ-opioid receptors are strongly implicated in cardioprotection across models and species (including anti-infarct and anti-arrhythmic actions), with mixed evidence for μ opioid receptor-dependent protection in animal and human tissues. A small number of clinical trials have provided evidence of cardiac benefit from morphine or remifentanil in cardiopulmonary bypass or coronary angioplasty patients, although further trials of subtype-specific opioid receptor agonists are needed. The precise roles and utility of this GPCR family in healthy and diseased human myocardium, and in mediating central and peripheral survival responses, warrant further investigation, as do the putative negative influences of ageing, IHD co-morbidities, and relevant drugs on opioid receptor signalling and protective responses.

Lipid correlates of attentional impulsivity in first episode mania: results from an Indian population

BACKGROUND

Attentional/cognitive impulsivity has been demonstrated as being associated with an increased risk for suicide and other self-harming behaviors, along with a more severe course in patients with bipolar disorder. That an alteration of the various serum lipid fractions might be associated with increased impulsivity has been proposed in the past, but evidences are ambiguous and mainly based on western population data.

OBJECTIVE

The present study was aimed to analyze the attentional impulsivity and various serum lipid fractions in bipolar patients, from an Indian perspective.

MATERIALS AND METHODS

At presentation, 60 drug free/naïve first episode Mania patients were rated on the Barratt impulsiveness scale-version 11 and Young Mania Rating Scale; body mass index (BMI) was calculated and blood samples were analyzed for total cholesterol (TC), high density lipoproteins, low density lipoproteins and very low density lipoproteins (VLDL), triglycerides (TG) and apolipoproteins A1 and B.

RESULTS

The analysis revealed statistically significant negative correlation and inverse linear relationship between TC, TG, VLDL and BMI with attentional impulsivity.

CONCLUSION

The present study adds to the growing literature on a complex relationship between lipid fractions and attentional impulsivity. The findings present interesting insights into the possible substrates of human behavior at biochemical levels. The implications are many, including a need to introspect regarding the promotion of weight loss and cholesterol reduction programs in constitutionally vulnerable population.

Sarcolemmal cholesterol and caveolin-3 dependence of cardiac function, ischemic tolerance, and opioidergic cardioprotection

Cholesterol-rich caveolar microdomains and associated caveolins influence sarcolemmal ion channel and receptor function and protective stress signaling. However, the importance of membrane cholesterol content to cardiovascular function and myocardial responses to ischemia-reperfusion (I/R) and cardioprotective stimuli are unclear. We assessed the effects of graded cholesterol depletion with methyl-β-cyclodextrin (MβCD) and lifelong knockout (KO) or overexpression (OE) of caveolin-3 (Cav-3) on cardiac function, I/R tolerance, and opioid receptor (OR)-mediated protection. Langendorff-perfused hearts from young male C57Bl/6 mice were untreated or treated with 0.02-1.0 mM MβCD for 25 min to deplete membrane cholesterol and disrupt caveolae. Hearts were subjected to 25-min ischemia/45-min reperfusion, and the cardioprotective effects of morphine applied either acutely or chronically [sustained ligand-activated preconditioning (SLP)] were assessed. MβCD concentration dependently reduced normoxic contractile function and postischemic outcomes in association with graded (10-30%) reductions in sarcolemmal cholesterol. Cardioprotection with acute morphine was abolished with ≥20 μM MβCD, whereas SLP was more robust and only inhibited with ≥200 μM MβCD. Deletion of Cav-3 also reduced, whereas Cav-3 OE improved, myocardial I/R tolerance. Protection via SLP remained equally effective in Cav-3 KO mice and was additive with innate protection arising with Cav-3 OE. These data reveal the membrane cholesterol dependence of normoxic myocardial and coronary function, I/R tolerance, and OR-mediated cardioprotection in murine hearts (all declining with cholesterol depletion). In contrast, baseline function appears insensitive to Cav-3, whereas cardiac I/R tolerance parallels Cav-3 expression. Novel SLP appears unique, being less sensitive to cholesterol depletion than acute OR protection and arising independently of Cav-3 expression.

HFE gene variants, iron, and lipids: a novel connection in Alzheimer's disease

Iron accumulation and associated oxidative stress in the brain have been consistently found in several neurodegenerative diseases. Multiple genetic studies have been undertaken to try to identify a cause of neurodegenerative diseases but direct connections have been rare. In the iron field, variants in the HFE gene that give rise to a protein involved in cellular iron regulation, are associated with iron accumulation in multiple organs including the brain. There is also substantial epidemiological, genetic, and molecular evidence of disruption of cholesterol homeostasis in several neurodegenerative diseases, in particular Alzheimer's disease (AD). Despite the efforts that have been made to identify factors that can trigger the pathological events associated with neurodegenerative diseases they remain mostly unknown. Because molecular phenotypes such as oxidative stress, synaptic failure, neuronal loss, and cognitive decline, characteristics associated with AD, have been shown to result from disruption of a number of pathways, one can easily argue that the phenotype seen may not arise from a linear sequence of events. Therefore, a multi-targeted approach is needed to understand a complex disorder like AD. This can be achieved only when knowledge about interactions between the different pathways and the potential influence of environmental factors on them becomes available. Toward this end, this review discusses what is known about the roles and interactions of iron and cholesterol in neurodegenerative diseases. It highlights the effects of gene variants of HFE (H63D- and C282Y-HFE) on iron and cholesterol metabolism and how they may contribute to understanding the etiology of complex neurodegenerative diseases.

High- and low-affinity sites for sodium in δ-OR-Gi1α (Cys (351)-Ile (351)) fusion protein stably expressed in HEK293 cells; functional significance and correlation with biophysical state of plasma membrane

The effect of sodium, potassium, and lithium on δ-opioid receptor ligand binding parameters and coupling with the cognate G proteins was compared in model HEK293 cell line stably expressing PTX-insensitive δ-OR-Gi1α (Cys(351)-Ile(351)) fusion protein. Agonist [(3)H]DADLE binding was decreased in the order Na(+) ≫ Li(+) > K(+) > (+)NMDG. When plotted as a function of increasing NaCl concentrations, the binding was best-fitted with a two-phase exponential decay considering two Na(+)-responsive sites (r (2) = 0.99). High-affinity Na(+)-sites were characterized by Kd = 7.9 mM and represented 25 % of the basal level determined in the absence of ions. The remaining 75 % represented the low-affinity sites (Kd = 463 mM). Inhibition of [(3)H]DADLE binding by lithium, potassium, and (+)-NMDG proceeded in low-affinity manner only. Surprisingly, the affinity/potency of DADLE-stimulated [(35)S]GTPγS binding was increased in a reverse order: Na(+) < K(+) < Li(+). This result was demonstrated in PTX-treated as well as PTX-untreated cells. Therefore, it is not restricted to Gi1α(Cys(351)-Ile(351)) within the δ-OR-Gi1α fusion protein, but is also valid for stimulation of endogenous G proteins of Gi/Go family in HEK293 cells. Biophysical studies of interaction of ions with polar head-group region of lipids using Laurdan generalized polarization indicated the low-affinity type of interaction only proceeding in the order: Cs(+) < K(+) < Na(+) < Li(+). The results are discussed in terms of interaction of Na(+), K(+) and Li(+) with the high- and low-affinity sites located in water-accessible part of δ-OR binding pocket. We also consider the role of negatively charged Cl(-), Br(-), and I(-) counter anions in inhibition of both [(3)H]DADLE and [(35)S]GTPγS binding.

Opioid-Receptor (OR) Signaling Cascades in Rat Cerebral Cortex and Model Cell Lines: the Role of Plasma Membrane Structure

Large number of extracellular signals is received by plasma membrane receptors which, upon activation, transduce information into the target cell interior via trimeric G-proteins (GPCRs) and induce activation or inhibition of adenylyl cyclase enzyme activity (AC). Receptors for opioid drugs such as morphine (μ-OR, δ-OR and κ-OR) belong to rhodopsin family of GPCRs. Our recent results indicated a specific up-regulation of AC I (8-fold) and AC II (2.5-fold) in plasma membranes (PM) isolated from rat brain cortex exposed to increasing doses of morphine (10-50 mg/kg) for 10 days. Increase of ACI and ACII represented the specific effect as the amount of ACIII-ACIX, prototypical PM marker Na, K-ATPase and trimeric G-protein α and β subunits was unchanged. The up-regulation of ACI and ACII faded away after 20 days since the last dose of morphine. Proteomic analysis of these PM indicated that the brain cortex of morphine-treated animals cannot be regarded as being adapted to this drug because significant up-regulation of proteins functionally related to oxidative stress and alteration of brain energy metabolism occurred. The number of δ-OR was increased 2-fold and their sensitivity to monovalent cations was altered. Characterization of δ-OR-G-protein coupling in model HEK293 cell line indicated high ability of lithium to support affinity of δ-OR response to agonist stimulation. Our studies of PM structure and function in context with desensitization of GPCRs action were extended by data indicating participation of cholesterol-enriched membrane domains in agonist-specific internalization of δ-OR. In HEK293 cells stably expressing δ-OR-Gi1α fusion protein, depletion of PM cholesterol was associated with the decrease in affinity of G-protein response to agonist stimulation, whereas maximum response was unchanged. Hydrophobic interior of isolated PM became more “fluid”, chaotically organized and accessible to water molecules. Validity of this conclusion was supported by the analysis of an immediate PM environment of cholesterol molecules in living δ-OR-Gi1α-HEK293 cells by fluorescent probes 22- and 25-NBD-cholesterol. The alteration of plasma membrane structure by cholesterol depletion made the membrane more hydrated. Understanding of the positive and negative feedback regulatory loops among different OR-initiated signaling cascades (µ-, δ-, and κ-OR) is crucial for understanding of the long-term mechanisms of drug addiction as the decrease in functional activity of µ-OR may be compensated by increase of δ-OR and/or κ-OR signaling.

Interaction of methionine-enkephalins with raft-forming lipids: monolayers and BAM experiments

Enkephalins (Tyr-Gly-Gly-Phe-Met/Leu) are opioid peptides with proven antinociceptive action in organism. They interact with opioid receptors belonging to G-protein coupled receptor superfamily. It is known that these receptors are located preferably in membrane rafts composed mainly of sphingomyelin (Sm), cholesterol (Cho), and phosphatidylcholine. In the present work, using Langmuir's monolayer technique in combination with Wilhelmy's method for measuring the surface pressure, the interaction of synthetic methionine-enkephalin and its amidated derivative with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), Sm, and Cho, as well as with their double and triple mixtures, was studied. From the pressure/area isotherms measured, the compressional moduli of the lipids and lipid-peptide monolayers were determined. Our results showed that the addition of the synthetic enkephalins to the monolayers studied led to change in the lipid monolayers characteristics, which was more evident in enkephalinamide case. In addition, using Brewster angle microscopy (BAM), the surface morphology of the lipid monolayers, before and after the injection of both enkephalins, was determined. The BAM images showed an increase in surface density of the mixed surface lipids/enkephalins films, especially with double and triple component lipid mixtures. This effect was more pronounced for the enkephalinamide as well. These observations showed that there was an interaction between the peptides and the raft-forming lipids, which was stronger for the amidated peptide, suggesting a difference in folding of both enkephalins. Our research demonstrates the potential of lipid monolayers for elegant and simple membrane models to study lipid-peptide interactions at the plane of biomembranes.

The membrane protein LeuT in micellar systems: aggregation dynamics and detergent binding to the S2 site

Structural and functional properties of integral membrane proteins are often studied in detergent micellar environments (proteomicelles), but how such proteomicelles form and organize is not well understood. This makes it difficult to evaluate the relationship between the properties of the proteins measured in such a detergent-solubilized form and under native conditions. To obtain mechanistic information about this relationship for the leucine transporter (LeuT), a prokaryotic homologue of the mammalian neurotransmitter/sodium symporters (NSSs), we studied the properties of proteomicelles formed by n-dodecyl-β,D-maltopyranoside (DDM) detergent. Extensive atomistic molecular dynamics simulations of different protein/detergent/water number ratios revealed the formation of a proteomicelle characterized by a constant-sized shell of detergents surrounding LeuT protecting its transmembrane segments from unfavorable hydrophobic/hydrophilic exposure. Regardless of the DDM content in the simulated system, this shell consisted of a constant number of DDM molecules (∼120 measured at a 4 Å cutoff distance from LeuT). In contrast, the overall number of DDMs in the proteomicelle (aggregation number) was found to depend on the detergent concentration, reaching a saturation value of 226±17 DDMs in the highest concentration regime simulated. Remarkably, we found that at high detergent-to-protein ratios we observed two independent ways of DDM penetration into LeuT, both leading to a positioning of the DDM molecule in the second substrate (S2) binding site of LeuT. Consonant with several recent experimental studies demonstrating changes in functional properties of membrane proteins due to detergent, our findings highlight how the environment in which the membrane proteins are examined may affect the outcome and interpretation of their mechanistic features.

PKA and ERK1/2 are involved in dopamine D₁ receptor-induced heterologous desensitization of the δ opioid receptor

AIMS

Chronic administration of cocaine attenuates delta opioid receptor (DOPR) signaling in the striatum and the desensitization is mediated by the indirect actions of cocaine on dopamine D1 receptors (D1R). In addition, DOPR and D1R co-exist in some rat striatal neurons. In the present study, we examined the underlying mechanism of DOPR desensitization by D1R activation.

MAIN METHODS

NG 108-15 cells stably expressing HA-rat D1 receptor (HA-D1R) and Chinese hamster ovary (CHO) cells stably expressing both FLAG-mouse DOPR (FLAG-DOPR) and HA-D1R were used as the cell models. Receptor binding, [(35)S]GTPγS binding, receptor phosphorylation and western blot were conducted to examine DOPR affinity, expression, internalization, downregulation, desensitization, phosphorylation and phosphorylated ERK1/2.

KEY FINDINGS

Pretreatment with either the DOPR agonist DPDPE or the D1R agonist SKF-82958 for 30min attenuated DPDPE-stimulated [(35)S]GTPγS binding to G proteins, demonstrating homologous and heterologous desensitization of the DOPR, respectively. SKF-82958 pretreatment did not affect the level of DOPR or affinity of DOPR antagonist or agonists, nor did it induce phosphorylation, internalization or down-regulation of the DOPR in the CHO-FLAG-DOPR/HA-D1R cells. Pretreatment of cells with inhibitors of PKA, MEK1 and PI3K, but not PKC, attenuated SKF-82958-induced desensitization of the DOPR. The D1R agonist SKF-82958 enhanced phosphorylation of ERK1/2, and pretreatment with inhibitors of MEK1 and PI3K, but not PKA and PKC, reduced the effect. These results indicate that activation of ERK1/2 and/or PKA, but not PKC, is involved in D1 receptor-induced heterologous desensitization of the DOPR.

SIGNIFICANCE

This study provides possible mechanisms underlying D1R activation-induced DOPR desensitization.

Methods for the study of dopamine receptors within lipid rafts of kidney cells

There is increasing evidence that G protein-coupled receptor (GPCR) signaling is regulated in lipid raft microdomains. GPCRs and GPCR-signaling molecules, including G proteins and protein kinases, have been reported to compartmentalize in these microdomains. Dopamine D(1)-like receptors (D(1)R and D(5)R) belong to a family of GPCRs that are important in the regulation of renal function. These receptors are not only localized and regulated in caveolae that contains caveolin-1 but are also distributed in non--caveolar lipid rafts which do not contain caveolin-1. This chapter describes detergent- and non-detergent-based methods to obtain lipid raft fractions from renal proximal tubule cells.

Sensitivity of cholecystokinin receptors to membrane cholesterol content

Cholesterol represents a structurally and functionally important component of the eukaryotic cell membrane, where it increases lipid order, affects permeability, and influences the lateral mobility and conformation of membrane proteins. Several G protein-coupled receptors have been shown to be affected by the cholesterol content of the membrane, with functional impact on their ligand binding and signal transduction characteristics. The effects of cholesterol can be mediated directly by specific molecular interactions with the receptor and/or indirectly by altering the physical properties of the membrane. This review focuses on the importance and differential effects of membrane cholesterol on the activity of cholecystokinin (CCK) receptors. The type 1 CCK receptor is quite sensitive to its cholesterol environment, while the type 2 CCK receptor is not. The possible structural basis for this differential impact is explored and the implications of pathological states, such as metabolic syndrome, in which membrane cholesterol may be increased and CCK1R function may be abnormal are discussed. This is believed to have substantial potential importance for the development of drugs targeting the CCK receptor.

GPCR stabilization using the bicelle-like architecture of mixed sterol-detergent micelles

The biophysical characterization of purified membrane proteins typically requires detergent mediated extraction from native lipid membrane environments. In the case of human G protein-coupled receptors (GPCRs), this process has been complicated by their conformational heterogeneity and the general lack of understanding the composition and interactions within the diverse human cellular membrane environment. Several successful GPCR structure determination efforts have shown that the addition of cholesterol analogs is often critical for maintaining protein stability. We have identified sterols that substantially increase the stability of the NOP receptor (ORL-1), a member of the opioid GPCR family, in a mixed micelle environment. Using dynamic light scattering and small-angle X-ray scattering, we have determined that the most thermal stabilizing sterol, cholesteryl hemisuccinate, induces the formation of a bicelle-like micelle architecture when mixed with dodecyl maltoside detergent. Together with mutagenesis studies and recent GPCR structures, our results provide indications that stabilization is attained through a combination of specific sterol binding to GPCRs and modulation of micelle morphology.

Neuronal glycoprotein M6a induces filopodia formation via association with cholesterol-rich lipid rafts

A neuronal integral membrane glycoprotein M6a has been suggested to be involved in a number of biological processes, including neuronal remodeling and differentiation, trafficking of mu-opioid receptors, and Ca(2+) transportation. Moreover, pathological situations such as chronic stress in animals and depression in humans have been associated with alterations in M6a sequence and expression. The mechanism of action of M6a is essentially unknown. In this work, we analyze the relevance of M6a distribution in plasma membrane, namely its lipid microdomain targeting, for its biological function in filopodia formation. We demonstrate that M6a is localized in membrane microdomains compatible with lipid rafts in cultured rat hippocampal neurons. Removal of cholesterol from neuronal membranes with methyl-β-cyclodextrin decreases M6a-induced filopodia formation, an effect that is reversed by the addition of cholesterol. Inhibition of Src kinases and MAPK prevents filopodia formation in M6a-over-expressing neurons. Src-deficient SYF cells over-expressing M6a fail to promote filopodia formation. Taken together, our findings reveal that the association of M6a with lipid rafts is important for its role in filopodia formation and Src and MAPK kinases participate in M6a signal propagation.

Inhibitory effect of ganglioside on mastoparan-induced cytotoxicity and degranulation in lipid raft of connective tissue type mast cell

Antihistamine, the most important drug for Hymenoptera stinging, cannot attenuate cytotoxicity and mast cell direct activation by mastoparan that is the most abundant polypeptides in the venoms of social wasps. The aim of this study was to investigate whether gangliosides inhibit the effect of mastoparan on mast cells activation. The degranulation and cytotoxicity in canine cutaneous mastocytoma cells (CM-MC) were done by measurement of β-hexosaminidase release and MTT assay. Lipid raft was isolated with discontinuous sucrose gradient centrifuge for the analysis of distribution of Gα(q) and Gα(i) protein by western blotting. We found that mastoparan induced the degranulation in (CM-MC) via direct activation of Gα(i) and Gα(q) with a decrease in their amount in lipid raft. Ganglioside G(D1a) (disialoganglioside) and G(M1) (monosialoganglioside) strongly reduced the degranulation and cytotoxicity through stabilizing the structure of lipid raft domain. In addition, mastoparan generated intracellular reactive oxygen species (ROS) independently from cytotoxicity, through arachidonic cascade but not G-protein activations. Crude wasp venom showed cytotoxicity and induction of the release from CM-MC, which were potently reduced by gangliosides. We show here that mastoparan activates both Gα(i) and Gα(q) protein and that the exogenous ganglioside G(D1a) and G(M1) inhibit the degranulation and cytotoxicity through stabilizing lipid raft. Gangliosides have potentials to be therapeutic tool or clinical prophylaxis for wasp stinging.

2-hydroxypropyl-beta-cyclodextrin raises hearing threshold in normal cats and in cats with Niemann-Pick type C disease

2-hydroxypropyl-beta-cyclodextrin (HPbetaCD) is a promising experimental therapy for Niemann-Pick type C disease that improved intracellular cholesterol transport, substantially reduced neurodegeneration and hepatic disease, and increased lifespan in npc1 mice. On the basis of favorable treatment outcome in mice, HPbetaCD is being evaluated as a therapy in children with Niemann-Pick type C (NPC) disease. We evaluated the efficacy of HPbetaCD in the feline model of NPC disease and recognized a dose-dependent increase in hearing threshold associated with therapy as determined by brain stem auditory evoked response (BAER) testing. To further assess the effect of HPbetaCD on hearing threshold, normal cats were administered the drug s.c. at either 4000 mg/kg or 8000 mg/kg body weight, or intrathecally at a dose of 4000 mg/kg brain weight. HPbetaCD caused a significant increase in hearing threshold following one dose of 8000 mg/kg s.c. or 120 mg intrathecally, and the effect was maintained for at least 12 weeks. Repeated weekly s.c. administration of 4000 mg/kg HPbetaCD resulted in a similar increase in hearing threshold. These studies are the first to describe a specific negative effect of HPbetaCD on the auditory system and suggest the need for auditory testing in patients receiving similar doses of HPbetaCD.

Cholesterol and affective morbidity

Depression and mania have been linked with low cholesterol though there has been limited prospective study of cholesterol and subsequent course of affective illness. We studied the relationship between fasting total cholesterol and subsequent depressive and manic symptoms. A total of 131 participants from a prospective cohort study were identified as having had a fasting total cholesterol evaluation at intake. Participants were predominantly inpatients at index visit and were followed for a median of 20 and up to 25 years. Cholesterol was modeled with age, gender, and index use of a mood stabilizer in linear regression to assess its influence on subsequent depressive symptom burden in participants with unipolar disorder as well as depressive and manic symptom burden in participants with bipolar disorder. Among bipolar participants (N=65), low cholesterol predicted a higher proportion of follow-up weeks with manic, but not depressive symptoms. Cholesterol did not appear to predict depressive symptom burden among participants with unipolar depression (N=66). Lower cholesterol levels may predispose individuals with bipolar disorder to a greater burden of manic symptomatology and may provide some insight into the underlying neurobiology.

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

Cholesterol is an essential component of higher eukaryotic membranes and plays a crucial role in membrane organization, dynamics and function. The G-protein coupled receptors (GPCRs) are the largest class of molecules involved in signal transduction across membranes, and represent major targets in the development of novel drug candidates in all clinical areas. Membrane cholesterol has been reported to have a modulatory role in the function of a number of GPCRs. Two possible mechanisms have been previously suggested by which membrane cholesterol could influence the structure and function of GPCRs (i) through a direct/specific interaction with GPCRs, or (ii) through an indirect way by altering membrane physical properties in which the receptor is embedded, or due to a combination of both. Recently reported crystal structures of GPCRs have shown structural evidence of cholesterol binding sites. Against this backdrop, we recently proposed a novel mechanism by which membrane cholesterol could affect structure and function of GPCRs. According to our hypothesis, cholesterol binding sites in GPCRs could represent 'nonannular' binding sites. Interestingly, previous work from our laboratory has demonstrated that membrane cholesterol is required for the function of the serotonin(1A) receptor (a representative GPCR), which could be due to specific interaction of the receptor with cholesterol. Based on these results, we envisage that there could be specific/nonannular cholesterol binding site(s) in the serotonin(1A) receptor. We have analyzed putative cholesterol binding sites from protein databases in the serotonin(1A) receptor. Our analysis shows that cholesterol binding sites are inherent characteristic features of serotonin(1A) receptors and are conserved through natural evolution. Progress in deciphering molecular details of the GPCR-cholesterol interaction in the membrane would lead to better insight into our overall understanding of GPCR function in health and disease, thereby enhancing our ability to design better therapeutic strategies to combat diseases related to malfunctioning of GPCRs.

Cholesterol-protein interaction: methods and cholesterol reporter molecules

Cholesterol is a major constituent of the plasma membrane in eukaryotic cells. It regulates the physical state of the phospholipid bilayer and is crucially involved in the formation of membrane microdomains. Cholesterol also affects the activity of several membrane proteins, and is the precursor for steroid hormones and bile acids. Here, methods are described that are used to explore the binding and/or interaction of proteins to cholesterol. For this purpose, a variety of cholesterol probes bearing radio-, spin-, photoaffinity- or fluorescent labels are currently available. Examples of proven cholesterol binding molecules are polyene compounds, cholesterol-dependent cytolysins, enzymes accepting cholesterol as substrate, and proteins with cholesterol binding motifs. Main topics of this report are the localization of candidate membrane proteins in cholesterol-rich microdomains, the issue of specificity of cholesterol- protein interactions, and applications of the various cholesterol probes for these studies.

Down-regulation of c-Cbl by morphine accounts for persistent ERK1/2 signaling in delta-opioid receptor-expressing HEK293 cells

Opioids display ligand-specific differences in the time course of ERK1/2 signaling. Whereas full agonists, like etorphine, induce only transient activation of ERK1/2, the partial agonist morphine mediates persistent stimulation of mitogenic signaling. Here we report that in stably delta-opioid receptor (DOR)-expressing HEK293 (HEK/DOR) cells, the transient nature of etorphine-induced ERK1/2 signaling is due to desensitization of epidermal growth factor (EGF) receptor-mediated activation of the Ras/Raf-1/ERK1/2 cascade. Desensitization of ERK1/2 activity by etorphine is associated with down-regulation of EGF receptors, an effect mediated by the ubiquitin ligase c-Cbl. In contrast, chronic morphine treatment failed to desensitize EGF receptors, resulting in unimpeded ERK1/2 signaling. The failure of morphine to desensitize ERK1/2 signaling is mediated by persistent activation of c-Src, which induces degradation of c-Cbl. The role of c-Src in opioid-specific ERK1/2 signaling is further demonstrated by pretreatment of the cells with PP2 and SKI-I as well as overexpression of a dominant negative c-Src mutant (c-Src(dn)) or a c-Src-resistant c-Cbl mutant (CblY3F), both of which facilitate desensitization of ERK1/2 signaling by morphine. Conversely, overexpression of c-Src as well as down-regulation of c-Cbl by small interfering RNA results in persistent etorphine-induced stimulation of ERK1/2 activity. Subcellular fractionation experiments finally attributed the ability of morphine to persistently activate c-Src to its redistribution from Triton X-100-insensitive membrane rafts to DOR and EGF receptor containing high density membrane compartments implicated in ERK1/2 signaling. These results demonstrate that agonist-specific differences in the temporal and spatial pattern of c-Src activation determine the kinetics of DOR-mediated regulation of ERK1/2 signaling.

Kappa and delta opioid receptor signaling is augmented in the failing heart

The opioidergic system, an endogenous stress pathway, modulates cardiac function. Furthermore, opioid peptide and receptor expression is altered in a number of cardiac pathologies. However, whether the response of myocardial opioid receptor signaling is altered in heart failure progression is currently unknown. Elucidating possible alterations in and effects of opioidergic signaling in the failing myocardium is of critical importance as opioids are commonly used for pain management, including in patients at risk for cardiovascular disease. A hamster model of cardiomyopathy and heart failure (Bio14.6) was used to investigate cardiac opioidergic signaling in heart failure development. This study found an augmented negative inotropic and lusitropic response to administration of agonists selective for the kappa opioid receptor and delta opioid receptor in the failing heart that was mediated by a pertussis toxin-sensitive G-protein. The augmented decrease in cardiac function was manifested by increased inhibition of cAMP accumulation and the amplitude of the systolic Ca(2+) transient. Furthermore, increased depression of cardiac function and of two important second messengers, cAMP and intracellular Ca(2+), were independent of changes in cardiac opioid peptide or receptor expression. Thus, the cardiomyopathy-induced failing heart experiences increased cardiac depressant effects following opioid receptor stimulation which could exacerbate diminished cardiac function in end-stage heart failure. As cardiac function is already depressed in heart failure patients, administration of opioids could exacerbate the degree of cardiac dysfunction and worsen disease progression.

Membrane cholesterol depletion enhances ligand binding function of human serotonin1A receptors in neuronal cells

Membrane lipid composition of cells in the nervous system is unique and displays remarkable diversity. Cholesterol metabolism and homeostasis in the central nervous system and their role in neuronal function represent important determinants in neuropathogenesis. The serotonin(1A) receptor is an important member of the G-protein coupled receptor superfamily, and is involved in a variety of cognitive, behavioral, and developmental functions. We report here, for the first time, that the ligand binding function of human serotonin(1A) receptors exhibits an increase in membranes isolated from cholesterol-depleted neuronal cells. Our results gain pharmacological significance in view of the recently described structural evidence of specific cholesterol binding site(s) in GPCRs, and could be useful in designing better therapeutic strategies for neurodegenerative diseases associated with GPCRs.