Cannabinoid receptor interacting protein (CRIP1a) attenuates CB₁R signaling in neuronal cells

CB₁ cannabinoid receptors (CB₁R) are one of the most abundantly expressed G protein coupled receptors (GPCR) in the CNS and regulate diverse neuronal functions. The identification of GPCR interacting proteins has provided additional insight into the fine-tuning and regulation of numerous GPCRs. The cannabinoid receptor interacting protein 1a (CRIP1a) binds to the distal carboxy terminus of CB₁R, and has been shown to alter CB₁R-mediated neuronal function [1]. The mechanisms by which CRIP1a regulates CB₁R activity have not yet been identified; therefore the focus of this investigation is to examine the cellular effects of CRIP1a on CB₁R signaling using neuronal N18TG2 cells stably transfected with CRIP1a over-expressing and CRIP1a knockdown constructs. Modulation of endogenous CRIP1a expression did not alter total levels of CB₁R, ERK, or forskolin-activated adenylyl cyclase activity. When compared to WT cells, CRIP1a over-expression reduced basal phosphoERK levels, whereas depletion of CRIP1a augmented basal phosphoERK levels. Stimulation of phosphoERK by the CB₁R agonists WIN55212-2, CP55940 or methanandamide was unaltered in CRIP1a over-expressing clones compared with WT. However, CRIP1a knockdown clones exhibited enhanced ERK phosphorylation efficacy in response to CP55940. In addition, CRIP1a knockdown clones displayed a leftward shift in CP55940-mediated inhibition of forskolin-stimulated cAMP accumulation. CB₁R-mediated G_i3 and G_o activation by CP99540 was attenuated by CRIP1a over-expression, but robustly enhanced in cells depleted of CRIP1a. Conversely, CP55940-mediated G_i1 and G_i2 activation was significant enhanced in cells over-expressing CRIP1a, but not in cells deficient of CRIP1a. These studies suggest a mechanism by which endogenous levels of CRIP1a modulate CB₁R-mediated signal transduction by facilitating a G_i/o protein subtype preference for G_i1 and G_i2, accompanied by an overall suppression of G-protein-mediated signaling in neuronal cells.