Molecular Mechanisms of a Fundamental Receptor Trafficking Pathway: Cannabinoid Receptor 2 Cell Surface Delivery

Abstract

Cannabinoid Receptor 2 (CB2) is abundantly expressed on immune tissues and cells and mediates cannabis-induced immunosuppression. Due to its unique immunological distribution, CB2 is not associated with the psychoactive effects that result from cannabinoid receptor 1 (CB1) activation, and is a promising therapeutic target for modulating inflammation. Synthetic pathway trafficking of G-protein coupled receptors (GPCRs) is an aspect of GPCR intracellular trafficking that is not well-characterised, and to date, no studies have focused on this aspect of CB2 intracellular trafficking. Accordingly, this thesis sought to begin to investigate the molecular mechanisms underlying CB2 cell surface delivery. A di-lysine (KK) motif residing in the C-terminal tail of CB2 was identified to be important for basal cell surface delivery, mutation of which resulted in near-abolished cell surface expression of this ‘KK mutant’ receptor, and further characterisation suggested this mutant was likely trapped intracellularly. Despite this, surface expression was evident following agonist stimulation, a surprising finding which was consistent with unpublished data from the Receptor Signalling Laboratory for CB2 wild-type (wt) suggesting that CB2 agonists can act as pharmacological chaperones, stabilising the folded receptor conformation and allowing it to be delivered to the cell surface. To further explore this KK motif–driven and agonist-induced cell surface delivery, confocal microscopy was utilised to assess the colocalisation between CB2 and fluorescent subcellular markers under basal conditions and in response to chronic agonist stimulation. An intensity-dependent, pixel-based quantitative analysis method was developed and used. Findings suggested that the endoplasmic reticulum (ER) is likely to be the most important organelle for controlling KK motif-mediated cell surface delivery, and that agonist stimulation allows a proportion of both CB2 wt and KK mutant to progress through the ER continuing toward the cell surface, though the KK mutant appears to (at least transiently) accumulate in the Golgi. This thesis provides an initial insight into understanding the molecular mechanisms underlying the synthetic pathway trafficking of CB2, which once fully elucidated, gives rise to the potential for novel therapeutic intervention.