P38 Modulates Internalization of Opioid Receptors

Abstract

The μ opioid receptors (MORs) have strong analgesic properties. Internalization of MORs by endocytosis is a critical event modulating receptor signaling, and Macé et al. have elucidated a new mechanism for control of this process. Treatment of cells expressing exogenous MOR with a receptor agonist caused activation of the mitogen-activated protein kinase (MAPK) p38, and pharmacological inhibition of p38 activity inhibited receptor internalization. Mouse embryonic fibroblasts (MEFs) express endogenous MOR, and p38α —/— MEFs showed no detectable internalization of the receptor in response to an agonist. Furthermore, expression of a constitutively activated MAPK kinase that activates p38 promoted internalization of MOR. The small GTPase (guanosine triphosphatase) Rab5 is an important regulator of receptor endocytosis, and inhibition of Rab5 function by expression of a dominant negative mutant or expression of a GAP (GTPase activating protein) inhibited MOR endocytosis under conditions where p38 was active. The authors therefore searched known Rab5 effectors as possible substrates for p38 and identified EEA1 and Rabanosyn-5 , two proteins that function in Rab5-dependent docking and fusion of endosomes. Studies with mutant proteins with alterations to prevent or mimic phosphorylation indicated that p38α-induced phosphorylation is important for recruitment of both proteins to the membrane. The EEA1 mutant that mimicked phosphorylation partially restored endocytosis of MOR in p38α —/— cells. Rab5 function was already known to be regulated by association with GDI (guanine nucleotide dissociation inhibitor) and GEF (guanine nucleotide exchange factor) proteins. The new results demonstrate still further regulation through the p38 MAPK pathway and should contribute to improved understanding of the medically useful analgesic effects of morphine and also support efforts to discover how unwanted side effects and tolerance of the drug can be avoided. G. Macé, M. Miaczynska, M. Zerial, A. R Nebreda, Phosphorylation of EEA1 by p38 MAP kinase regulates μ opioid receptor endocytosis. EMBO 24 , 3235-3246 (2005). [Abstract] [Full Text]