STUDIES ON NEURITE OUTGROWTH AND RECEPTOR PHOSPHORYLATION FOLLOWING KAPPA OPIOID RECEPTOR ACTIVATION

Abstract

Kappa opioid receptor (KOPR) is involved in many physiological functions and pharmacological responses such as analgesia, anti-pruritic effect, sedation, motor incoordination and aversion (Simonin et al., 1998; Liu-Chen, 2004). The cellular mechanisms following activation of KOPR involve in part Gi/o protein-dependent pathways (Law et al., 2000). Following KOPR activation, the receptor is phosphorylated and arrestins are recruited. Arrestins mediate agonist-dependent KOPR desensitization, internalization and down-regulation (Liu-Chen, 2004). In recent years, arrestins were found to initiate arrestin-dependent downstream signaling. Thus, agonist-promoted KOPR phosphorylation plays a pivotal role in KOPR regulation and signaling. Previous studies from our lab showed that in Chinese hamster ovary (CHO) cells stably transfected with the human KOPR (hKOPR), U50,488H induced phosphorylation (Li et al., 2002a); however, sites of phosphorylation were not determined. Using LC-MS/MS, our lab recently identified four residues (S356, T357, T363 and S369) to be the sites of U50,488H-promoted phosphorylation in the mouse KOPR (mKOPR) stably expressed in N2A cells (Chen et al., 2016). Antibodies were generated against phosphopeptides and purified and three antibodies were found to have high specificity for the mKOPR phosphorylated at S356/T357, T363 and S369, respectively (Chen et al., 2016). Our lab previously showed that while U50,488H promoted robust hKOPR phosphorylation and internalization, etorphine induced little phosphorylation and internalization, although both were potent full agonists in enhancing [35S]GTPγS (Li et al., 2002a; Zhang et al., 2002; Li et al., 2003). Etorphine caused lower levels of KOPR phosphorylation at all the four residues than U50,488H by immunoblotting with the phospho-specific antibodies (Chen et al., 2016). Using the SILAC (stable isotope labeling by amino acids in cell culture) approach, we have found that compared to etorphine, U50,488H promoted higher levels of single phosphorylation at T363 and S369 and double phosphorylation at T363+S369 and T357+S369 as well as triple phosphorylation at S356+T357+S369 (Chen et al., 2016). These results indicate that an above-threshold phosphorylation is required for KOPR internalization. It has been reported that KOPR is involved in neuronal differentiation and neurogenesis. In the first chapter, I focused on whether there are differences in the mechanisms underlying neurite outgrowth induced by U50,488H and etorphine. In the chapter 2, mechanisms of KOPR phosphorylation were characterized in detail using phospho-specific KOPR antibodies. Protein kinase C was found, for the first time, to be involved in agonist-promoted KOPR phosphorylation. The roles of PKC in behavioral effects induced by KOPR agonists in mice were examined. For the chapter 1, in Neuro2a mouse neuroblastoma cells stably transfected with the hKOPR (N2A-3HA-hKOPR), U50,488H robustly induced neurite outgrowth, but etorphine caused outgrowth to a much lower extent. G protein-dependent pathway was found to be involved in the actions of both agonists, but β-arrestin-dependent pathway was not. Inhibition of ERK1/2 phosphorylation decreased neurite outgrowth promoted by both agonists, indicating the roles of MAP kinase cascades in KOPR agonist-induced neuritogenesis. In contrast, β-arrestin2, 14-3-3ζ, GEC1 and Rap1 are not involved in U50,488H- or etorphine-promoted neurite outgrowth. Thus, the two agonists appear to share the same signaling pathways and the difference between two agonists is likely due to the lower efficacy of etorphine. For the chapter 2, U50,488H caused phosphorylation of the mKOPR at S356, T357, T363 and S369 in N2A cells stably transfected with FmK6H (FmK6H-N2A cells). NorBNI abolished U50,488H-induced KOPR phosphorylation at all four residues. GRKs (GRKs2, 3, 5 and 6) and PKCs were involved in U50,488H-mediated KOPR phosphorylation. In addition, PKC also participated in agonist-independent KOPR phosphorylation. This is the first time that PKC was shown to be involved in agonist-induced KOPR phosphorylation. We found that U50,488H caused KOPR phosphorylation at T363 and S369 in the mouse brain and PKC participated in phosphorylation of S369, but not T363, by using the PKC inhibitor chelerythrine (CHL). Thus, we further characterized effects of PKC inhibition on KOPR-mediated behaviors in CD1 mice. PKC was involved in KOPR-mediated sedation, motor incoordination and conditioned place aversion, but not analgesia and anti-scratching effect in mice. Studies in this thesis revealed the mechanisms of KOPR-mediated neurite outgrowth and KOPR-mediated phosphorylation and the involvement of PKC in KOPR-mediated pharmacological effects in vivo. These studies push the frontier of molecular pharmacology of the KOPR, which may be useful for development of KOPR agonists for therapeutic use.