Background and PurposeReceptor activity‐modifying proteins (RAMPs) and melanocortin receptor accessory proteins (MRAPs) modulate expression and signalling of calcitonin and melanocortin GPCRs. Interactions with other GPCRs have also been reported. The cannabinoid receptors, CB1 and CB2, and two putative cannabinoid receptors, GPR18 and GPR55, exhibit substantial intracellular expression and there are discrepancies in ligand responsiveness between studies. We investigated whether interactions with RAMPs or MRAPs could explain these phenomena.Experimental ApproachReceptors and accessory proteins were co‐expressed in HEK‐293 cells. Selected receptors were studied at basal expression levels and also with enhanced expression produced by incorporation of a preprolactin signal sequence/peptide (pplss). Cell surface and total expression of receptors and accessory proteins were quantified using immunocytochemistry. Signalling was measured using cAMP (CAMYEL) and G protein dissociation (TRUPATH Gα13) biosensors.Key ResultsMRAP2 enhanced surface and total expression of GPR18. Pplss‐GPR18 increased detection of cell surface MRAP2. MRAP1α and MRAP2 reduced GPR55 surface and total expression, correlating with reduced constitutive, but not agonist‐induced, signalling. GPR55, pplss‐CB1 and CB2 reduced detection of MRAP1α at the cell surface. Pplss‐CB1 agonist potency was reduced by MRAP2 in Gα13 but not cAMP assays, consistent with MRAP2 reducing pplss‐CB1 expression. Some cannabinoid receptors increased RAMP2 or RAMP3 total expression without influencing surface expression.Conclusions and ImplicationsMutual influences on expression and/or function for specific accessory protein‐receptor pairings raises the strong potential for physiological and disease‐relevant consequences. Sequestration and/or hetero‐oligomerisation of cannabinoid receptors with accessory proteins is a possible novel mechanism for receptor crosstalk.

Cannabinoid CB2 receptor agonists are in development as therapeutic agents, including for immune modulation and pain relief. Despite promising results in rodent preclinical studies, efficacy in human clinical trials has been marginal to date. Fundamental differences in ligand engagement and signalling responses between the human CB2 receptor and preclinical model species orthologues may contribute to mismatches in functional outcomes. This is a tangible possibility for the CB2 receptor in that there is a relatively large degree of primary amino acid sequence divergence between human and rodent. Here, we summarise CB2 receptor gene and protein structure, assess comparative molecular pharmacology between CB2 receptor orthologues, and review the current status of preclinical to clinical translation for drugs targeted at the CB2 receptor, focusing on comparisons between human, mouse and rat receptors. We hope that raising wider awareness of, and proposing strategies to address, this additional challenge in drug development will assist in ongoing efforts toward successful therapeutic translation of drugs targeted at the CB2 receptor.