Heteromerization of GPR55 and cannabinoid CB2 receptors modulates signalling.

BACKGROUND AND PURPOSE: Heteromerization of GPCRs is key to the integration of extracellular signals and the subsequent cell response via several mechanisms including heteromer-selective ligand binding, trafficking and/or downstream signalling. As the lysophosphatidylinositol GPCR 55 (GPR55) has been shown to affect the function of the cannabinoid receptor subtype 2 (CB2 receptor) in human neutrophils, we investigated the possible heteromerization of CB2 receptors with GPR55. EXPERIMENTAL APPROACH: The direct interaction of human GPR55 and CB2 receptors heterologously expressed in HEK293 cells was assessed by co-immunoprecipitation and bioluminescence resonance energy transfer assays. The effect of cross-talk on signalling was investigated at downstream levels by label-free real-time methods (Epic dynamic mass redistribution and CellKey impedance assays), ERK1/2-MAPK activation and gene reporter assays. KEY RESULTS: GPR55 and CB2 receptors co-localized on the surface of HEK293 cells, co-precipitated in membrane extracts and formed heteromers in living HEK293 cells. Whereas heteromerization led to a reduction in GPR55-mediated activation of transcription factors (nuclear factor of activated T-cells, NF-κB and cAMP response element), ERK1/2-MAPK activation was potentiated in the presence of CB2 receptors. CB2 receptor-mediated signalling was also affected by co-expression with GPR55. Label-free assays confirmed cross-talk between the two receptors. CONCLUSIONS AND IMPLICATIONS: Heteromers, unique signalling units, form in HEK293 cells expressing GPR55 and CB2 receptors. The signalling by agonists of either receptor was governed (i) by the presence or absence of the partner receptors (with the consequent formation of heteromers) and (ii) by the activation state of the partner receptor.

CB1 and GPR55 receptors are co-expressed and form heteromers in rat and monkey striatum.

Heteromerization of G-protein-coupled receptors is an important event as they integrate the actions of extracellular signals to give heteromer-selective ligand binding and signaling, opening new avenues in the development of potential drug targets in pharmacotherapy. The aim of the present paper was to check for cannabinoid CB1-GPR55 receptor heteromers in the central nervous system (CNS), specifically in striatum. First, a direct interaction was demonstrated in cells transfected with the cDNA for the human version of the receptors, using bioluminescence resonance energy transfer and in situ proximity ligation assays (PLA). In the heterologous system, a biochemical fingerprint consisting on cross-antagonism in ERK1/2 phosphorylation was detected. The cross-antagonism was also observed on GPR55-mediated NFAT activation. Direct identification of GPR55 receptors in striatum is here demonstrated in rat brain slices using a specific agonist. Moreover, the heteromer fingerprint was identified in these rat slices by ERK1/2 phosphorylation assays whereas PLA assays showed results consistent with receptor-receptor interactions in both caudate and putamen nuclei of a non-human primate. The results indicate not only that GPR55 is expressed in striatum but also that CB1 and GPR55 receptors form heteromers in this specific CNS region.

Defining the mechanism of action of 4-phenylbutyrate to develop a small-molecule-based therapy for Alzheimer's disease.

4-phenylbutyrate (PBA) is a small molecule that restores cognitive deficits in animal models of Alzheimer's disease (AD). Although the molecular basis of the cognitive benefits of PBA remains unknown, a multi-modal/multi-target mechanism has been proposed. Putative targets of this drug are different from those of drugs that are now used in clinical trials. As PBA is already administered to patients with congenital defects affecting enzymes in the urea cycle, it can be rapidly tested in AD clinical trials. However, the main drawback to its therapeutic use is the high dosage required (up to 15 g/day). Thus, deciphering the precise mechanism(s) of action of this drug may enable novel drugs with similar therapeutic effects to PBA to be developed that can be used at more manageable doses.

Sildenafil restores cognitive function without affecting ß-amyloid burden in a mouse model of Alzheimer's disease.

BACKGROUND AND PURPOSE: Inhibitors of phosphodiesterase 5 (PDE5) affect signalling pathways by elevating cGMP, which is a second messenger involved in processes of neuroplasticity. In the present study, the effects of the PDE5 inhibitor, sildenafil, on the pathological features of Alzheimer's disease and on memory-related behaviour were investigated. EXPERIMENTAL APPROACH: Sildenafil was administered to the Tg2576 transgenic mouse model of Alzheimer's disease and to age-matched negative littermates (controls). Memory function was analysed using the Morris water maze test and fear conditioning tasks. Biochemical analyses were performed in brain lysates from animals treated with saline or with sildenafil. KEY RESULTS: Treatment of aged Tg2576 animals with sildenafil completely reversed their cognitive impairment. Such changes were accompanied in the hippocampus by a reduction of tau hyperphosphorylation and a decrease in the activity of glycogen synthase kinase 3ß (GSK3ß) and of cyclin-dependent kinase 5 (CDK5) (p25/p35 ratio). Moreover, sildenafil also increased levels of brain-derived neurotrophic factor (BDNF) and the activity-regulated cytoskeletal-associated protein (Arc) in the hippocampus without any detectable modification of brain amyloid burden. CONCLUSIONS AND IMPLICATIONS: Sildenafil improved cognitive functions in Tg2576 mice and the effect was not related to changes in the amyloid burden. These data further strengthen the potential of sildenafil as a therapeutic agent for Alzheimer's disease.