Cannabinoid Receptor Interacting Protein (CRIP1a) affects CB1 signaling

Abstract

Cannabinoid receptor interacting protein 1a (CRIP1a) binds to the last nine amino acids of the cannabinoid (CB1) receptor. Receptor binding studies using human embryonic kidney cells transfected with the CB1 receptor (CB1‐HEK) found that co‐transfection of these cells with CRIP1a (CB1‐HEK‐CRIP1a) did not affect CB1 receptor expression. [35S]GTPγ S binding studies examined the efficacy of various agonists to activate G‐proteins in CB1‐HEK cells with and without CRIP1a transfection. CRIP1a decreased maximal agonist efficacy (Emax) values for the cannabinoids WIN 55,212‐2 (WIN) and CP 55,940 (CP). Emax values for methananamide, levonantradol, and Δ9‐tetrahydrocanabinol (THC) were unaffected. Inhibition of basal CB1 receptor activity by the inverse agonist SR141716A (SR1) was significantly decreased in the presence of CRIP1a. EC50 values of all ligands tested were unaffected. Effects of CRIP1a on CB1 receptor downregulation was examined by incubating CB1‐HEK cells (±CRIP1a co‐transfection) with various cannabinoids and measuring receptor levels via [3H]SR1 saturation binding. CRIP1a inhibited THC‐induced downregulation, but not WIN‐induced downregulation. Downstream signaling studies found no effect of CRIP1a on modulation of adenyly cyclase or MAP kinase activity by maximally‐effective agonist concentrations. A trend toward a decrease in the inverse agonist effects of SR1 was observed. These results suggest that CRIP1a modulates CB1 signaling during acute receptor activation and modulates CB1 receptor downregulation in a ligand‐specific manner. (Funding: NIH, NIDA grant DA‐05274)