Cannabinoid CB1 receptor-interacting proteins: novel targets for central nervous system drug discovery?

Abstract

The main pharmacological effects of marijuana, as well as synthetic and endogenous cannabinoids, are mediated through G-protein-coupled receptors (GPCRs), including CB(1) and CB(2) receptors. The CB(1) receptor is the major cannabinoid receptor in the central nervous system and has gained increasing interest as a target for drug discovery for treatment of nausea, cachexia, obesity, pain, spasticity, neurodegenerative diseases and mood and substance abuse disorders. Evidence has accumulated to suggest that CB(1) receptors, like other GPCRs, interact with and are regulated by several other proteins beyond the established role of heterotrimeric G-proteins. These proteins, which include the GPCR kinases, beta-arrestins, GPCR-associated sorting proteins, factor associated with neutral sphingomyelinase, other GPCRs (heterodimerization) and the novel cannabinoid receptor-interacting proteins: CRIP(1a/b), are thought to play important roles in the regulation of intracellular trafficking, desensitization, down-regulation, signal transduction and constitutive activity of CB(1) receptors. This review examines CB(1) receptor-interacting proteins, including heterotrimeric G-proteins, but with particular emphasis on non-G-protein entities, that might comprise the CB(1) receptosomal complex. The evidence for direct interaction with CB(1) receptors and potential functional roles of these interacting proteins is discussed, as are future directions and challenges in this field with an emphasis on the possibility of eventually targeting these proteins for drug discovery.

Figure 1

G-protein-coupled receptor (GPCR)-mediated G-protein activation. In the inactive state, G-proteins exist in the form of an αβγ heterotrimer, with the Gα subunit bound to GDP. Upon receptor activation, either by the binding of agonist or constitutively, the receptor changes to an active conformation (green), thereby activating G-proteins by promoting the exchange of GDP for GTP. The Gα-GTP and Gβγ dimer functionally dissociate from one another and the receptor and are free to modulate downstream effectors. The cycle concludes when the GTPase activity of the Gα subunit hydrolyses GTP to GDP, allowing the Gα subunit to return to its resting confirmation and reassociate with Gβγ.

Figure 2

G-protein-coupled receptor (GPCR) desensitization, internalization and down-regulation. Upon activation of the GPCR, GPCR kinase (GRK) phosphorylates the receptor, generally on C-terminal Ser/Thr residues. Once phosphorylated, β-arrestin can bind to the GPCR, desensitizing the receptor and causing the receptor to internalize via clathrin-coated pits. Once internalized, GPCRs may be recycled back to the cell surface following dephosphorylation in acidified endosomal compartments. Alternatively, GPCRs can be trafficked to lysosomes and degraded (down-regulation), a process that is facilitated by GPCR-associated sorting protein (GASP)1.

Figure 3

Schematic summary of CB₁ receptor-interacting proteins. CB₁ receptors can be bound by cannabinoid receptor-interacting protein (CRIP)_1a (or CRIP_1b in primates) on their distal C-terminus, which might stabilize the receptors in an inactive state. Once activated, such as by the binding of an agonist, CB₁ receptors can activate G_i/o-proteins in many cells types and could also activate FAN (factor associated with neutral sphingomyelinase) in a G-protein-independent manner in astrocytes. Activated CB₁ receptors also might become a substrate for G-protein-coupled receptor kinase (GRK)-mediated phosphorylation, presumably in the C-terminus. GRK-phosphorylated CB₁ receptors could recruit β-arrestin, thereby undergoing desensitization and clathrin-dependent internalization, followed in some cell types by G-protein-coupled receptor-associated sorting protein (GASP)1-mediated lysosmal degradation. CB₁ receptors might in some cells types form heterodimers with other GPCRs, such as dopamine D₂, adenosine A_2A, µ-, δ- or κ-opioid, or orexin-1, which could have numerous effects on their signalling and intracellular trafficking.