Adenosine A 2A receptor (A 2A R)-dopamine D 2 receptor (D 2 R) heteromers are key modulators of striatal neuronal function. It has been suggested that the psychostimulant effects of caffeine depend on its ability to block an allosteric modulation within the A 2A R-D 2 R heteromer, by which adenosine decreases the affinity and intrinsic efficacy of dopamine at the D 2 R. We describe novel unsuspected allosteric mechanisms within the heteromer by which not only A 2A R agonists, but also A 2A R antagonists, decrease the affinity and intrinsic efficacy of D 2 R agonists and the affinity of D 2 R antagonists. Strikingly, these allosteric modulations disappear on agonist and antagonist coadministration. This can be explained by a model that considers A 2A R-D 2 R heteromers as heterotetramers, constituted by A 2A R and D 2 R homodimers, as demonstrated by experiments with bioluminescence resonance energy transfer and bimolecular fluorescence and bioluminescence com- Most evidence indicates that G protein-coupled receptors (GPCRs) form homodimers and heteromers. Homodimers seem to be a predominant species, and oligomeric entities can be viewed as multiples of dimers (1). It has been proposed that GPCR heteromers are constituted mainly by heteromers of homodimers (1, 2). Allosteric mechanisms determine a multiplicity of unique pharmacologic properties of GPCR homodimers and heteromers (1, 3). First, binding of a ligand to one of the receptors in the heteromer can modify the affinity of ligands for the other receptor (1, 3, 4). The most widely reproduced allosteric modulation of ligand-binding properties in a GPCR heteromer is the ability of adenosine A 2A receptor (A 2A R) agonists to decrease the affinity of dopamine D 2 receptor (D 2 R) agonists in the A 2A R-D 2 R heteromer (5). A 2A R-D 2 R heteromers have been revealed both in transfected cells (6, 7), striatal neurons in culture (6,8) and in situ, in mammalian striatum (9, 10), where they play an important role in the modulation of GABAergic striatopallidal neuronal function (9, 11).In addition to ligand-binding properties, unique properties for each GPCR oligomer emerge in relation to the varying intrinsic efficacy of ligands for different signaling pathways (1-3). Intrinsic efficacy refers to the power of the agonist to induce a functional response, independent of its affinity for the receptor. Thus, allosteric modulation of an agonist can potentially involve changes in affinity and/or intrinsic efficacy (1, 3). This principle can be observed in the A 2A R-D 2 R heteromer, where a decrease in D 2 R agonist affinity cannot alone explain the ability of an A 2A R agonist to abolish the decreased excitability of GABAergic striatopallidal neurons induced by high concentration of a D 2 R agonist (9), which should overcome the decrease in affinity. Furthermore, a differential effect of allosteric modulations of different agonist-mediated signaling responses (i.e., functional selectivity) can occur within GPCR heteromers (1, 2, 8 It has been hypothesized that the allos...

Identification of higher-order oligomers in the plasma membrane is essential to decode the properties of molecular networks controlling intercellular communication. We combined bioluminescence resonance energy transfer (BRET) and fluorescence resonance energy transfer (FRET) in a technique called sequential BRET-FRET (SRET) that permits identification of heteromers formed by three different proteins. In SRET, the oxidation of a Renilla luciferase (Rluc) substrate by an Rluc fusion protein triggers acceptor excitation of a second fusion protein by BRET and subsequent FRET to a third fusion protein. We describe two variations of SRET that use different Rluc substrates with appropriately paired acceptor fluorescent proteins. Using SRET, we identified complexes of cannabinoid CB(1), dopamine D(2) and adenosine A(2A) receptors in living cells. SRET is an invaluable technique to identify heteromeric complexes of more than two neurotransmitter receptors, which will allow us to better understand how signals are integrated at the molecular level.

It is well known that cocaine blocks the dopamine transporter. This mechanism should lead to a general increase in dopaminergic neurotransmission, and yet dopamine D 1 receptors (D 1 Rs) play a more significant role in the behavioral effects of cocaine than the other dopamine receptor subtypes. Cocaine also binds to σ-1 receptors, the physiological role of which is largely unknown. In the present study, D 1 R and σ 1 R were found to heteromerize in transfected cells, where cocaine robustly potentiated D 1 R-mediated adenylyl cyclase activation, induced MAPK activation per se and counteracted MAPK activation induced by D 1 R stimulation in a dopamine transporterindependent and σ 1 R-dependent manner. Some of these effects were also demonstrated in murine striatal slices and were absent in σ 1 R KO mice, providing evidence for the existence of σ 1 R-D 1 R heteromers in the brain. Therefore, these results provide a molecular explanation for which D 1 R plays a more significant role in the behavioral effects of cocaine, through σ 1 R-D 1 R heteromerization, and provide a unique perspective toward understanding the molecular basis of cocaine addiction.receptor heteromer | drug addiction A key molecular mechanism contributing to the development of addiction by drugs of abuse consist of the increase of the extracellular levels of dopamine in the striatum, particularly in its ventral portion, the nucleus accumbens (1, 2). Cocaine causes a rapid and strong increase in striatal extracellular dopamine by its ability to bind with high affinity to the dopamine transporter (DAT) and to inhibit its function (3-5). In the striatum, dopamine signaling is mediated mainly by dopamine D 1 and D 2 receptors (D 1 Rs and D 2 Rs, respectively), which are mostly segregated in two phenotypically different subtypes of GABAergic medium-sized spiny neurons (MSNs) (6). Activation of D 1 Rs is an absolute requirement for the induction of many of the cellular and behavioral responses to cocaine, as deduced from studies performed in D 1 R KO mice and from experiments with transgenic mice in which D 1 R-or D 2 R-expressing MSNs are visualized by the expression of fluorescent proteins (7-11).The σ-1 receptor, originally proposed as a subtype of opioid receptors, is now considered to be a nonopioid receptor with two transmembrane domains, one extracellular loop and cytosolic N and C termini (12). The σ 1 R is highly expressed in the brain, including the striatum, and its association with neurons is well established (12, 13). However, its biological function and even its main endogenous neurotransmitter remain enigmatic (12). Cocaine interacts with σ 1 Rs at pharmacologically relevant concentrations (12,14). In fact, reducing brain σ 1 R levels with antisense oligonucleotides attenuates the convulsive and locomotor stimulant actions of cocaine (15, 16), and σ 1 R antagonists mitigate the actions of cocaine in animal models (12,14). A recent study showed that σ 1 R agonists not only potentiate the reinforcing effects of cocaine, but they may be self...

Activation of cannabinoid CB1 receptors (CB1R) by delta9-tetrahydrocannabinol (THC) produces a variety of negative effects with major consequences in cannabis users that constitute important drawbacks for the use of cannabinoids as therapeutic agents. For this reason, there is a tremendous medical interest in harnessing the beneficial effects of THC. Behavioral studies carried out in mice lacking 5-HT2A receptors (5-HT2AR) revealed a remarkable 5-HT2AR-dependent dissociation in the beneficial antinociceptive effects of THC and its detrimental amnesic properties. We found that specific effects of THC such as memory deficits, anxiolytic-like effects, and social interaction are under the control of 5-HT2AR, but its acute hypolocomotor, hypothermic, anxiogenic, and antinociceptive effects are not. In biochemical studies, we show that CB1R and 5-HT2AR form heteromers that are expressed and functionally active in specific brain regions involved in memory impairment. Remarkably, our functional data shows that costimulation of both receptors by agonists reduces cell signaling, antagonist binding to one receptor blocks signaling of the interacting receptor, and heteromer formation leads to a switch in G-protein coupling for 5-HT2AR from Gq to Gi proteins. Synthetic peptides with the sequence of transmembrane helices 5 and 6 of CB1R, fused to a cell-penetrating peptide, were able to disrupt receptor heteromerization in vivo, leading to a selective abrogation of memory impairments caused by exposure to THC. These data reveal a novel molecular mechanism for the functional interaction between CB1R and 5-HT2AR mediating cognitive impairment. CB1R-5-HT2AR heteromers are thus good targets to dissociate the cognitive deficits induced by THC from its beneficial antinociceptive properties.

RNA interference by short interfering RNA (siRNA) holds promise to attenuate production of specific target proteins but is challenging in practice owing to the barriers for its efficient intracellular delivery. We have synthesized a tri-block co-polymeric system, poly(amidoamine) dendrimer (generation 4)-poly(ethylene glycol)-1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (G(4)-D-PEG-2K-DOPE). G(4)-PAMAM dendrimer was utilized as a cationic source for efficient siRNA condensation; DOPE provided optimum hydrophobicity and compatible cellular interaction for enhanced cell penetration; PEG rendered flexibility to the G(4)-D for easy accessibility of siRNA for condensation; PEG-DOPE system provided stable micellization in a mixed micellar system. G(4)-D-PEG-2K-DOPE was incorporated into the self-assembled PEG-5K-PE micelles at a 1:1 molar ratio. Our results demonstrate that the modified dendrimer, G(4)-D-PEG-2K-DOPE and the micellar nanocarrier form stable polyplexes with siRNA, shows excellent serum stability and a significantly higher cellular uptake of siRNA that results in target protein down-regulation when compared to the G(4)-PAMAM-dendrimer. Moreover, the mixed micellar system showed efficient micellization and higher drug (doxorubicin) loading efficiency. The G(4)-D-PEG-2K-DOPE has the higher efficacy for siRNA delivery, whereas G(4)-D-PEG-2K-DOPE/PEG-5K-PE micelles appear to be a promising carrier for drug/siRNA co-delivery, especially useful for the treatment of multidrug resistant cancers.

G protein-coupled receptors (GPCRs), G proteins and adenylyl cyclase (AC) comprise one of the most studied transmembrane cell signaling pathways. However, it is unknown whether the ligand-dependent interactions between these signaling molecules are based on random collisions or the rearrangement of pre-coupled elements in a macromolecular complex. Furthermore, it remains controversial whether a GPCR homodimer coupled to a single heterotrimeric G protein constitutes a common functional unit. Using a peptide-based approach, we here report evidence for the existence of functional pre-coupled complexes of heteromers of adenosine A2A receptor and dopamine D2 receptor homodimers coupled to their cognate Gs and Gi proteins and to subtype 5 AC. We also demonstrate that this macromolecular complex provides the necessary frame for the canonical Gs-Gi interactions at the AC level, sustaining the ability of a Gi-coupled GPCR to counteract AC activation mediated by a Gs-coupled GPCR.

The striatum is the main input structure of the basal ganglia, which are subcortical structures involved in the processing of information related to the performance and learning of complex motor acts. It is widely accepted that dopamine receptor subtypes, which are fundamental for motor control and are implicated in numerous neuropsychiatric disorders, are largely segregated in the two subtypes of medium spiny neurons (MSNs), 4 the most populated neuronal type in the striatum. Dopamine D 2 receptors (D 2 Rs) are mostly localized in the striatopallidal MSNs, which express the peptide enkephalin and which gives rise to the indirect striatal efferent pathway, whereas dopamine D 1 receptors (D 1 Rs) are mostly expressed by the striatonigral MSNs, which express substance P and dynorphin and constitute the direct striatal efferent pathway (1, 2). Dopaminergic drugs activate the ERK transduction pathway, which is involved in basic physiological processes and in synaptic plasticity (3). In the dopamine-depleted striatum, ERK signaling is implicated in the development of L-DOPA-induced dyskinesia. Thus, in dopamine-denervated mice, L-DOPA activates ERK signaling specifically in D 1 Rs containing striatonigral MSNs but not in D 2 Rs containing striatopallidal MSNs (4). This regulation may result in ERKdependent changes in striatal plasticity leading to dyskinesia.Histamine is an important regulatory transmitter in the nervous system involved in the sleep/wake cycle, attention, memory, and other functions. Four histamine receptor types (H 1 R-H 4 R) have been cloned. H 3 Rs are expressed in abundance in the brain and high densities are particularly found in the striatum (5-7). H 3 Rs were first identified as autoreceptors (8), but they were later found to act as heteroreceptors (9). * This study was supported by Grants SAF2008-00146, SAF2008-03229-E, SAF2009-07276, SAF2006-08240, and SAF2009-12510

A low molecular weight polyethyleneimine (PEI 1.8 kDa) was modified with dioleoylphosphatidylethanolamine (PE) to form the PEI-PE conjugate investigated as a transfection vector. The optimized PEI-PE/pDNA complexes at an N/P ratio of 16 had a particle size of 225 nm, a surface charge of +31 mV, and protected the pDNA from the action of DNase I. The PEI-PE conjugate had a critical micelle concentration (CMC) of about 34 μg/ml and exhibited no toxicity compared to a high molecular weight PEI (PEI 25 kDa) as tested with B16-F10 melanoma cells. The B16-F10 cells transfected with PEI-PE/pEGFP complexes showed protein expression levels higher than with PEI-1.8 or PEI-25 vectors. Complexes prepared with YOYO 1-labeled pEGFP confirmed the enhanced delivery of the plasmid with PEI-PE compared to PEI-1.8 and PEI-25. The PEI-PE/pDNA complexes were also mixed with various amounts of micelle-forming material, polyethylene glycol (PEG)-PE to improve biocompatibility. The resulting particles exhibited a neutral surface charge, resistance to salt-induced aggregation, and good transfection activity in the presence of serum in complete media. The use of the low-pH-degradable PEG-hydrazone-PE produced particles with transfection activity sensitive to changes in pH consistent with the relatively acidic tumor environment.