Intracellular trafficking of cannabinoid receptor 1: a thorough characterisation and investigation into the role of the intracellular pool

Abstract

Cannabinoid Receptor 1 (CB1), an abundant G-protein coupled receptor (GPCR) in the central nervous system, is currently of significant interest as a therapeutic target. The cellular control of receptor trafficking is intimately linked with drug effects, however in comparison with other GPCRs, the study of CB1 trafficking is in its infancy. Although the existing literature suggests CB1 should be classified as a “dual-fate” receptor, some conflicting evidence exists as to the conditions under which CB1 recycles or degrades. Of particular interest is the widely noted intracellular pool which has been speculated to form part of a constitutive internalisation and recycling pathway. This study performs a detailed quantification of CB1 trafficking in four cell lines, one of which expresses CB1 endogenously. A novel high-throughput immunocytochemistry-based approach is applied to quantitatively measure receptor trafficking. An important advance on previous studies is the use of a proteolytic method to directly quantitate intracellular receptors. Contrary to previous reports, the data suggests that CB1 does not recycle following constitutive or agonist-induced internalisation but instead exhibits a primarily degradative phenotype. Evidence is obtained through antibody “live-feeding” protocols and the effects of protein synthesis inhibitors, among other approaches. In addition, the data suggests that the intracellular pool does not traffic to the cell surface and therefore does not contribute to CB1 signalling via classical paradigms. The effects of Rab GTPase dominantly-acting positive and negative mutants on basal CB1 localisation corroborate these results. The findings of this thesis have significant implications for the interpretation of CB1 biochemical studies and call for a revision of the currently held theories of CB1 intracellular trafficking. The study provides a foundation for further mechanistic studies and may impact the design and application of cannabinoid therapeutics.