Lenka Roubalova

Up-regulation of adenylylcyclases I and II induced by long-term adaptation of rats to morphine fades away 20 days after morphine withdrawal.

BACKGROUND Activation of adenylyl cyclase (AC) by prolonged exposure of mammalian organism to morphine was demonstrated in previous studies of mechanism of action of this drug. However, expression level of individual AC isoforms was not analyzed in crucial cell structure, plasma membrane (PM). METHODS Rats were adapted to morphine for 10 days and sacrificed 24h (group+M10) or 20 days (+M10/-M20) after the last dose. Control animals were sacrificed in parallel with morphine-treated (groups-M10 and (-M10/-M20)). Percoll®-purified PM were isolated from brain cortex and analyzed by immunoblotting and specific radioligand binding. RESULTS ACI (ACII) was increased 8× (2.5×) in morphine-adapted rats (+M10) when compared with controls (-M10). Increase of ACI and II by long-term adaptation to increasing doses of morphine represented a specific effect as the amount of ACIII-ACIX, of prototypical PM marker, Na, K-ATPase and of trimeric G protein α and β subunits was unchanged. Increase of ACI and II was not detected in PM isolated from group (+M10/-M20). Thus, the marked increase of ACI and ACII faded away 20 days since the last dose of morphine. CONCLUSIONS We assume that the specific increase in expression level of ACI and ACII in brain cortex of morphine-adapted rats proceeds as a compensatory, homeostatic response to prolonged exposure to inhibitory drug, morphine. GENERAL SIGNIFICANCE Our findings demonstrate that the dramatic and specific change of the crucial component of the opioid receptor cascade in brain cortex, manifested as an increase in PM level of ACI and II, is reversible.

Plasma membrane cholesterol level and agonist-induced internalization of δ-opioid receptors; colocalization study with intracellular membrane markers of Rab family

Decrease of cholesterol level in plasma membrane of living HEK293 cells transiently expressing FLAG-δ-OR by β-cyclodextrin (β-CDX) resulted in a slight internalization of δ-OR. Massive internalization of δ-OR induced by specific agonist DADLE was diminished in cholesterol-depleted cells. These results suggest that agonist-induced internalization of δ-OR, which has been traditionally attributed exclusively to clathrin-mediated pathway, proceeds at least partially via membrane domains. Identification of internalized pools of FLAG-δ-OR by colocalization studies with proteins of Rab family indicated the decreased presence of receptors in early endosomes (Rab5), late endosomes and lysosomes (Rab7) and fast recycling vesicles (Rab4). Slow type of recycling (Rab11) was unchanged by cholesterol depletion. As expected, agonist-induced internalization of oxytocin receptors was totally suppressed in β-CDX-treated cells. Determination of average fluorescence lifetime of TMA-DPH, the polar derivative of hydrophobic membrane probe diphenylhexatriene, in live cells by FLIM indicated a significant alteration of the overall PM structure which may be interpreted as an increased “water-accessible space” within PM area. Data obtained by studies of HEK293 cells transiently expressing FLAG-δ-OR by “antibody feeding” method were extended by analysis of the effect of cholesterol depletion on distribution of FLAG-δ-OR in sucrose density gradients prepared from HEK293 cells stably expressing FLAG-δ-OR. Major part of FLAG-δ-OR was co-localized with plasma membrane marker Na,K-ATPase and β-CDX treatment resulted in shift of PM fragments containing both FLAG-δ-OR and Na,K-ATPase to higher density. Thus, the decrease in content of the major lipid constituent of PM resulted in increased density of resulting PM fragments.

Proteomic analysis of protein composition of rat forebrain cortex exposed to morphine for 10 days; comparison with animals exposed to morphine and subsequently nurtured for 20 days in the absence of this drug

UNLABELLED Proteomic analysis was performed in post-nuclear supernatant fraction (PNS) prepared from forebrain cortex of rats exposed to increasing doses of morphine (10-50mg/kg) for 10days and sacrificed 24h (group +M10) or 20days (group +M10/-M20) after the last dose of morphine. PNS fraction was resolved by 2D-ELFO and stained by CBB. Analysis of the difference between (+M10) and (-M10) samples of PNS by PDQuest accompanied by MALDI-TOF MS/MS indicated the significant change of 28 proteins. Importantly, the number of altered proteins was decreased to 14 after 20days of nurturing animals in the absence of morphine. This new and important finding indicating the ability of mammalian organism to return to physiological norm after removal of the drug was verified by an independent methodology - gel-free & label-free quantification and normalization procedure denominated as MaxLFQ. The 113 proteins were identified as altered by morphine in (+M10) samples when compared with (-M10) samples of PNS and this number was decreased to 19 after 20days of nurturing the animals in the absence of this drug. BIOLOGICAL SIGNIFICANCE Forebrain cortex of rats exposed to morphine for 10days is severely altered as far as the overall protein composition is involved. Depending on the method used for protein detection and quantification, 28 (MALDI-TOF MS/MS) or 113 (MaxLFQ) altered proteins were identified. Importantly, in rats sacrificed 20days after the last dose of morphine, the number of altered proteins was decreased to 14 (MALDI-TOF MS/MS) and 19 (MaxLFQ), respectively. Our data indicate the high ability of living organism to oppose the drastic, morphine-induced change of the target tissue protein composition with the aim to return to the physiological norm after complete removal of the drug.

Up-regulation of μ-, δ- and κ-opioid receptors in concanavalin A-stimulated rat spleen lymphocytes

Regulation of μ-, δ- and κ-opioid receptor protein level in spleen lymphocytes when stimulated by mitogen is not known. To answer the question whether these cells do express opioid receptor (OR) proteins, primary, fresh rat spleen lymphocytes were prepared and stimulated for 48 h with mitogenic dose of Con A. The unstimulated lymphocytes did not express μ- and δ-OR proteins in detectable amounts, however, stimulation with Con A resulted in appearance of clearly detectable immunoblot signals of both μ-OR and δ-OR. κ-OR were detected already in primary cells and increased 2.4-fold in Con A-stimulated cells. These results were supported by data obtained by flow cytometry analysis indicating a dramatic increase in number of μ-, δ- and κ-OR expressing cells after mitogen stimulation. The newly synthesized μ-, δ- and κ-OR in Con A-stimulated spleen lymphocytes were present in the cells interior and not functionally mature, at least in terms of their ability to enhance activity of trimeric G proteins determined by three different protocols of agonist-stimulated, high-affinity [35S]GTPγS binding assay. The up-regulation of μ-, δ- and κ-OR was associated with specific decrease of their cognate trimeric G proteins, Gi1α/Gi2α; the other Gα and Gβ subunits were unchanged. The level of β-arrestin-1/2 was also decreased in Con A-stimulated splenocytes. We conclude that up-regulation of OR expression level in spleen lymphocytes by Con A proceeds in conjunction with down-regulation of their intracellular signaling partners, Gi1α/Gi2α proteins and β-arrestin-1/2. These regulatory proteins are expressed in high amounts already in unstimulated cells and decreased by mitogen stimulation.

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.

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 Gi1α within δ-opioid receptor-Gi1α fusion protein, but it was also valid for PTX-sensitive G proteins of Gi/Go 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.

Induction of oxidative stress by long-term treatment of live HEK293 cells with therapeutic concentration of lithium is associated with down-regulation of δ-opioid receptor amount and function

Graphical abstract Figure. No Caption available. ABSTRACT The functional state of &dgr;‐opioid receptor signaling cascade in live cells exposed to a therapeutic concentration of lithium for a prolonged period of time (weeks) is not known because the previous studies of Li interference with OR were oriented to &mgr;‐OR only. The same applies to the analysis of the prolonged effect of Li on oxidative stress in context with &dgr;‐OR function. HEK293 cells stably expressing &dgr;‐OR were cultivated in the presence or absence of 1 mM LiCl for 7 or 21 days, homogenized and the post‐nuclear (PNS) and plasma membrane (PM) fractions prepared from all four types of cells. Level of &dgr;‐OR in PM was determined by specific radioligand [3H]DADLE binding and immunoblot assays; the functional coupling between &dgr;‐OR and G proteins was determined as DADLE‐stimulated high‐affinity [35S]GTP&ggr;S binding. In the whole cells, general oxidative stress was monitored by fluorescent dye 2′,7′‐dichlorofluorescein diacetate (DCF) and results verified by analysis of PNS and isolated PM. Generation of 4‐hydroxy‐2‐nonenal (4‐HNE)‐protein adducts and malondialdehyde (MDA) level were determined as products of lipid peroxidation. Li‐treated cells exhibited the decreased amount of &dgr;‐OR. This was evidenced by both [3H]DADLE binding and immunoblot assays. The &dgr;‐OR‐G protein coupling efficiency was diminished. Simultaneously, in Li‐treated cells, the highly increased oxidative stress measured as DCF fluorescence intensity was noticed. Importantly, this result was detected in live cells as well as PNS and PM. Accordingly, production of 4‐HNE‐protein adducts and MDA was clearly increased in Li‐treated cells. The general significance of our work lies in presentation of novel data indicating that prolonged exposure of live HEK293 cells to the therapeutic concentration of Li results in down‐regulation of &dgr;‐OR protein level and attenuation of &dgr;‐OR function in parallel with increased oxidative stress and increased level of lipid peroxidation products.