Interacting protomer domains in CB₁R-5-HT_2AR heteromers and heteromer disruption by TM interference peptides.

<p>In (A), HEK-293T cells expressing CB₁R and 5-HT_2AR were treated for 4 h with vehicle (left panel) or 4 μM of CB₁R TM 7, TM 5, or TM 6 interference peptides before performing proximity ligation assays. Confocal microscopy images (superimposed sections) are shown in which heteromers appear as green spots in cells treated with vehicle and with TM 7 interference peptide, but not in cells treated with TM 5 or TM 6 interference peptides. In all cases, cell nuclei were stained with DAPI (blue). Scale bars = 20 μm. In (B–D), HEK-293T cells expressing CB₁R and 5-HT_2AR were preincubated for 20 min with rimonabant (1 μM, RIM) or MDL 100,907 (300 nM, MDL) before stimulation for 10 min (B) or 5 min (C, D) with the CB₁R agonist WIN 55,212–2 (100 nM), the 5-HT_2AR agonist DOI (100 nM), or both in the presence (B) or absence (C, D) of 0.5 μM forskolin. In (B), cAMP production was determined. Values represent mean ± SEM of <i>n</i> = 3–9 and are expressed as the percentage of the cAMP produced in forskolin-treated cells. Quantification of phosphorylated ERK 1/2 (C) or Akt (D) was determined by western blot. Values, expressed as a percentage of basal (nontreated cells), were mean ± SEM of <i>n</i> = 3–6. One-way ANOVA followed by a Bonferroni post hoc tests showed a significant effect over forskolin’s effects alone in each condition (B) or over basal (C, D) (* <i>p</i> < 0.05, ** <i>p</i> < 0.01, *** <i>p</i> <0.001) or of the antagonist plus agonist treatment over the agonist treatment (# <i>p</i> < 0.05, ## <i>p</i> < 0.01, ### <i>p</i> < 0.001).</p

5-HT_2AR and CB₁R form heteromers in transfected cells.

<p>In (A), BRET saturation experiments were performed in HEK-293T cells transfected with 0.025 μg of 5-HT_2AR-Rluc cDNA and increasing amounts of CB₁R-YFP cDNA (0.05 μg to 1.5 μg, black curve), with 0.5 μg of dopamine D₁R-Rluc cDNA and increasing amounts of CB₁R-YFP cDNA (0.5 μg to 6 μg, yellow line), or with 0.025 μg of 5-HT_2AR-Rluc cDNA and increasing amounts of adenosine A₁R-YFP cDNA (0.05 μg to 1.5 μg, red line). The relative amount of BRET is given as a function of 100 x the ratio between the fluorescence of the acceptor (YFP) and the luciferase activity of the donor (Rluc). BRET is expressed as milli BRET units (mBU) and is given as the mean ± standard deviation (SD) of 3–6 experiments grouped as a function of the amount of BRET acceptor. In (B), a schematic representation of fluorescence complementation experiments is depicted in the left panel showing that fluorescence only appears after the YFP Venus hemiprotein complementation due to the proximity of two receptors fused to hemi-YFP Venus proteins (cYFP or nYFP). In the right panel, fluorescence at 530 nm was detected in HEK-293T cells transfected with different amounts of cDNA corresponding to both 5-HT_2AR-cYFP and CB₁R-nYFP (equal amount for each construct), but not in negative controls in which cells were transfected with cDNA corresponding to 5-HT_2AR-cYFP and the noninteracting adenosine A₁ receptor-nYFP or CB₁R-nYFP and the noninteracting dopamine D₁ receptor-cYFP. One-way ANOVA followed by a Dunnett’s multiple comparison test showed a significant fluorescence over basal values in HEK-293T cells (** <i>p</i> < 0.01, *** <i>p</i> < 0.001). In (C), PLAs were performed in HEK-293T cells expressing CB₁R and 5-HT_2AR. Confocal microscopy images (superimposed sections) are shown in which heteromers appear as green spots. In all cases, cell nuclei were stained with DAPI (blue). Scale bars = 20 μm. In (D), PLAs were performed in nontransfected HEK-293T cells, cells transiently transfected with 0.5 μg of CB₁R or 5-HT_2AR cDNA (negative controls, white columns), or with increasing amounts of CB₁R and 5-HT_2AR cDNA (black columns). In each case, the ratio between the number of green spots and the number of cells showing spots (ratio r) was calculated. One-way ANOVA followed by a Dunnett’s multiple comparison test showed a significant PLA staining over nontranfected cells (*** <i>p</i> < 0.001).</p

Differential expression of CB₁R-5-HT_2AR heteromers in the brain detected by heteromer signaling.

<p>Slices from the hippocampus (A), caudate-putamen (B), cortex (C), and nucleus accumbens (D) of WT mice (white bars) and 5-HT_2AR KO mice (black bars) were preincubated or not with CB₁R antagonist rimonabant (1 μM, RIM) or the 5-HT_2AR antagonist MDL 100,907 (300 nM, MDL) for 20 min before the addition of the CB₁R agonist WIN 55,212–2 (1 μM, WIN), the 5-HT_2AR agonist DOI (1 μM), or both for an additional incubation period of 10 min. ERK 1/2 phosphorylation was determined by western blot. Immunoreactive bands from three to seven slices obtained from ten WT or KO animals were quantified for each condition. Values represent mean ± SEM of the percentage of phosphorylation relative to basal levels found in untreated slices. No significant differences were obtained between the basal levels of the WT and the KO mice. One-way ANOVA followed by Bonferroni post hoc tests showed a significant (* <i>p</i> < 0.05, ** <i>p</i> < 0.01, *** <i>p</i> < 0.001) effect over basal or of the antagonist plus agonist treatment over the agonist treatment (# <i>p</i> < 0.05, ## <i>p</i> < 0.01).</p

Proposed functional properties of CB₁R-5-HT_2AR heteromers.

<p>In (A), agonist binding to CB₁R (blue) or 5-HT_2AR (light green) triggers the conformational changes of TMs 5 and 6, opening the intracellular cavity for Gi and Gq binding, respectively. In (B), the formation of the CB₁R-5-HT_2AR heteromer makes both receptors signal via Gi. In (C), rimonabant binding to CB₁R or MDL 100,907 to 5-HT_2AR stabilizes the closed conformation of the receptor, facilitating heterodimerization via TMs 5 and 6 as in the crystal structure of the μ-opioid receptor. In this assembly, both protomers are locked in the closed conformation since the opening of TMs 5 and 6 for G-protein binding is not feasible (see <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002194#pbio.1002194.s008" target="_blank">S7 Fig</a>). Bidirectional cross antagonism is due to the fact that antagonist binding to any protomer must, in addition to its common role in a monomeric signaling unit, disrupt this very stable four-helix association. (D) In agreement, bidirectional cross antagonism is abrogated following treatment with TM 5 or TM 6 interference peptides (dark blue), which disrupt the heteromer structure.</p

5-HT_2AR mediates THC-induced amnesic- and anxiolytic-like effects.

<p>(A) The administration of THC (3 and 10 mg/kg) induced memory impairments in the novel object recognition test in WT mice as compared to vehicle (VEH) treatment (<i>n</i> = 8–11), and this effect was significantly abrogated in 5-HT_2AR KO mice at the dose of 3 mg/kg, but not at the dose of 10 mg/kg. (B) The anxiolytic effects of THC (0.3 mg/kg) observed in WT mice tested in the elevated plus maze were blocked in 5-HT_2AR KO mice (<i>n</i> = 9–12). (C) The increase in social interaction induced by THC (0.3 mg/kg) in WT mice was abolished in 5-HT_2AR KO mice (<i>n</i> = 5–7). (D) Neuronal firing of representative dorsal raphe (DR) neurons before and after THC administration (1 and 10 nM) in WT (upper panel) and 5-HT_2AR KO (lower panel) animals. (E) A challenge with THC (1 nM) reduced the percent change in firing rate of DR neurons from WT mice, and this effect was blunted in DR neurons from 5-HT_2AR KO mice. No significant differences between genotypes were observed following a challenge with THC at 10 nM (<i>n</i> = 7–14). (F) The basal firing rate of DR neurons was similar in WT and 5-HT_2AR KO animals (<i>n</i> = 20–24). (G–I) Withdrawal symptoms, including paw tremor (G), sniffing (H), and global withdrawal score (GWS) (I), were reduced in 5-HT_2AR KO mice when compared to WT mice. Data are mean + standard error of the mean (SEM). * <i>p</i> < 0.05, ** <i>p</i> < 0.01, and *** <i>p</i> < 0.001 versus VEH; # <i>p</i> < 0.05 versus WT mice. The statistical analyses used and their corresponding F and <i>p</i>-values are shown in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002194#pbio.1002194.s013" target="_blank">S1 Table</a>.</p

Prevention of THC-induced amnesic and anxiolytic-like effects by pharmacological blockade of 5-HT_2AR or by CB₁R-5-HT_2AR heteromer disruption with TM interference peptides.

<p>The amnesic effects of THC (3 mg/kg) observed in C57BL/6J mice in the novel object recognition test were abrogated by pretreatment with the 5-HT_2AR antagonist, MDL 100,907 (0.01 mg/kg) (A) and by pretreatment with TM 5 and TM 6, but not TM 7, interference peptides (0.2 μg/ 2μl ICV) (B) (<i>n</i> = 5–9). The anxiolytic effects of THC (0.3 mg/kg) observed in the elevated plus maze in C57BL/6J mice were blocked by pretreatment with the 5-HT_2AR antagonist, MDL 100,907 (0.01 mg/kg) (C) and by pretreatment with TM 5 and TM 6, but not TM 7, interference peptides (0.2 μg/ 2μl ICV) (D) (<i>n</i> = 4–11). The data represent mean + SEM. * <i>p</i> < 0.05, ** <i>p</i> < 0.01, *** <i>p</i> < 0.001 versus vehicle, # <i>p</i> < 0.05 versus THC-treated mice. In (E), PLA performed in hippocampal CA3, striatal (caudate-putamen), and cortical (somatomotor layers 1, 2, and 3) slices from mice treated with VEH, TM 6, and TM 7 interference peptides (0.2 μg/ 2μl ICV). Confocal microscopy images (superimposed sections) are shown in which heteromers appear as green spots in VEH and TM 7-treated mice, but not in mice treated with TM 6 interference peptides. In all cases, cell nuclei were stained with DAPI (blue). Scale bars = 20 μm. In (F), the number of cells containing one or more green spots is expressed as the percentage of the total number of cells (blue nucleus) in the hippocampus, striatum, and cortex (top to bottom). Data (percentage of positive cells) are the mean ± SEM of counts in 8–12 different fields. *** <i>p</i> < 0.001 versus vehicle-treated mice. Pretreatment with TM 5, TM 6, or TM 7 peptides (0.2 μg/ 2μl ICV) had no significant effects on hypolocomotion (G), hypothermia (H), or analgesia (I) induced by THC (10 mg/kg) in C57BL/6J mice. The statistical analyses used and their corresponding F and <i>p</i>-values are shown in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002194#pbio.1002194.s013" target="_blank">S1 Table</a>.</p

Differential expression of CB₁R-5-HT_2AR heteromers in the brain detected by in situ PLAs.

<p>In (A), PLAs were performed using slices of mouse hippocampus CA3, caudate-putamen (striatum), cortex (somatomotor layers 1, 2, and 3) or nucleus accumbens (NaC). Confocal microscopy images (superimposed sections) are shown in which heteromers appear as green spots in WT mice, but not in 5-HT_2AR KO or CB₁R KO mice in the hippocampus, caudate-putamen, and cortex. Any staining was observed in the nucleus accumbens of either WT or KO animals. In all cases, cell nuclei were stained with DAPI (blue). Scale bars = 20 μm. In (B), the number of cells containing one or more green spots is expressed as the percentage of the total number of cells (blue nucleus) in the hippocampus, striatum, cortex, and nucleus accumbens of WT (white bars), 5-HT_2AR KO (black bars), or CB₁R KO (grey bars) mice. Data (percentage of positive cells) are the mean ± SEM of counts in 4–9 different fields (see experimental procedures). Student’s <i>t</i> test showed a significant effect over 5-HT_2AR KO or over CB₁R KO mice in each condition (*** <i>p</i> < 0.001).</p

5-HT_2AR does not mediate THC-induced hypolocomotion, hypothermia, analgesia, anxiogenic-like behavior, or the reinforcing properties of WIN 55,212–2.

<p>(A) Locomotor activity (% of baseline) was dose-dependently reduced in both WT and 5-HT_2AR KO mice treated with THC (0.3, 1, 3, and 10 mg/kg) as compared to vehicle (VEH) administration (<i>n</i> = 5–15). (B) THC induced hypothermia at the dose of 10 mg/kg to a similar extent in WT and 5-HT_2AR KO mice (<i>n</i> = 13–24). (C) In the tail-immersion test, percent analgesia was dose-dependently increased by THC (1, 3, and 10 mg/kg) in both WT and 5-HT_2AR KO mice (<i>n</i> = 10–16). In the hot-plate test, the percent of analgesia as calculated from the latency to paw-licking (D) and to jumping behavior (E) was similar in WT and KO animals treated with THC (1, 3, and 10 mg/kg) as compared to VEH administration (<i>n</i> = 7–16). (F) No significant differences between genotypes were observed in anxiogenic-like behavior induced by THC (3 mg/kg) (<i>n</i> = 6–10). (G) Both WT and 5-HT_2AR KO mice acquired WIN 55,212–2 self-administration behavior and responded equally for this drug during the 12 d of training (<i>n</i> = 12–15). Data are mean + SEM. * <i>p</i> < 0.05, ** <i>p</i> < 0.01, and *** <i>p</i> < 0.001 versus VEH-treated animals. The statistical analyses used and their corresponding F and <i>p</i>-values are shown in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002194#pbio.1002194.s013" target="_blank">S1 Table</a>.</p