Enzymatic reconstitution of brain membrane and membrane opiate receptors

Membrane functional organisation and dynamic of mu-opioid receptors

The activation and signalling activity of the membrane mu-opioid receptor (MOP-R) involve interactions among the receptor, G-proteins, effectors and many other membrane or cytosolic proteins. Decades of investigation have led to identification of the main biochemical processes, but the mechanisms governing the successive protein-protein interactions have yet to be established. We will need to unravel the dynamic membrane organisation of this complex and multifaceted molecular machinery if we are to understand these mechanisms. Here, we review and discuss advances in our understanding of the signalling mechanism of MOP-R resulting from biochemical or biophysical studies of the organisation of this receptor in the plasma membrane.

Cholesterol content drives distinct pharmacological behaviours of micro-opioid receptor in different microdomains of the CHO plasma membrane

Cholesterol in the plasma membrane of eukaryotic cells contributes to modulating the functions and signalling pathways of numerous transmembrane proteins, including G protein Coupled Receptors (GPCRs). We have previously shown that the function of the human micro-opioid receptor (hMOR) expressed in Saccharomyces cerevisiae is modulated by sterols including cholesterol. Here, we investigated the effects of cholesterol content on hMOR pharmacology and on hMOR partitioning in cholesterol-poor and -rich domains in eukaryotic mammalian cells (CHO). We show that cholesterol is required for the stabilization of a receptor conformation with high agonist affinity and for triggering G-protein activation after agonist binding to the receptor. Biochemical analysis of untreated and cholesterol-depleted membranes in cells expressing hMOR indicated that the receptor is only present in cholesterol poor domains, in the basal state. After agonist binding to untreated CHO membranes, two distinct populations of receptor were found in cholesterol-rich and -poor domains. Cholesterol depletion or treatment of CHO membranes with the G-protein-decoupling agent GppNHp prevented the redistribution, indicating that receptor activated states localized into cholesterol-rich domains. Pharmacological data and biochemical analysis indicate that distinct activated conformations of hMOR exist in CHO plasma membrane and correspond to microdomains differing by thickness and proportions of lipid components, including cholesterol.

Inhibitory mechanism of opioid binding to receptor by phospholipase A2

1. The inhibition of [3H]naloxone binding to the opioid receptor upon short-term incubation with phospholipase A2 (Plase A2) was abolished by treatment with BSA, but not after long-term incubation. 2. In contrast to the restorative effect of BSA on the strong inhibition occurring with Plase A2, BSA only partially abolished the inhibitory effect of arachidonic acid or lysophosphatides, even though the degree of inhibition was slight. 3. These results suggest that Plase A2 products only become associated with hydrophobic receptor sites or with phospholipids near to the receptor, thus reversibly inhibiting opioid binding, and that irreversible inhibition occurs through release of the phospholipid necessary for receptor binding.

Ligand/receptor interactions--the influence of the microenvironment on macroscopic properties. Electrostatic interactions with the membrane phase

The heterogeneous environment in which ligand/receptor interactions occur often leads to complex binding behaviour. We consider here the ligand/membrane interaction, emphasizing the possibilities of electrostatic modulation of the overall binding characteristics. The binding of Substance P to neutral or negatively charged planar lipid bilayers was monitored using the capacitance minimization technique. The electrostatic attraction to the charged bilayer potentiates the interaction by more than two orders of magnitude and leads to a nonlinearity in the Scatchard plot of bound vs. bulk concentrations. The Boltzmann accumulation factor, along with the direct measurement of the surface potential, provides an easy explanation of the effect. The general importance of electrostatic accumulation (or repulsion) at surfaces is discussed and the concept applied to examples from the literature.

Lipid requirement for mu opioid receptor binding

We have previously shown that a partially purified mu opioid receptor from bovine brain requires lipids to exhibit full binding activity. In the present report, we have determined the specificity of this lipid requirement. Lipids active in this regard were found always to contain an acidic head group and a fatty acid with two or more double bonds. Free, polyunsaturated fatty acids were also able to confer high binding activity on the partially purified opioid receptor. The possible roles lipids play in opioid binding are discussed in light of these data.

Reconstitution of affinity-purified dopamine D2 receptor binding activities by specific lipids

The role of lipids in maintaining ligand binding properties of affinity-purified bovine striatal dopamine D2 receptor was investigated in detail. The receptor, purified on a haloperidol-linked Sepharose CL6B affinity column, exhibited low [3H]spiroperidol binding unless reconstituted with soybean phospholipids. In order to understand the role of individual phospholipids in maintaining the receptor binding activity, the purified preparation was reconstituted separately with individual phospholipids and assayed for [3H]spiroperidol binding. Except for phosphatidylcholine and phosphatidylethanolamine, that respectively restored 30 and 20% binding as compared to that obtained with soybean lipids, reconstitution with other lipids had very little effect. When various combinations of phospholipids were used for reconstitution, a phosphatidylcholine and phosphatidylserine mixture seemed to almost fully restore the receptor binding. A mixture of phosphatidylcholine and phosphatidylethanolamine was as effective as phosphatidylcholine alone in reconstituting ligand binding; however, when phosphatidylserine was also included in the mixture, there was a pronounced increase in binding (about 2-fold compared to the soybean lipids and about 6-fold compared to the phosphatidylcholine-phosphatidylethanolamine mixture). Substitution of other phospholipids or cholesterol for phosphatidylserine in phosphatidylcholine and phosphatidylethanolamine mixture had little effect. Maximal reconstitution of [3H]spiroperidol binding was obtained with phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine mixture (2:2:1, w/w) at a concentration of 0.5 mg/ml. The reconstituted receptor exhibited high affinity binding for [3H]spiroperidol which was comparable to that obtained with membrane or solubilized preparations. Various dopaminergic antagonists and agonists showed appropriate order of potency for the reconstituted receptor. The presently described reconstitution data suggest a role of specific phospholipids in preserving the binding properties of dopamine D2 receptor and should prove useful in studies on functional reconstitution of the receptor.