Elucidating the role of opioid receptor heteromers as targets for analgesic drug design

Abstract

Opioid receptors are class A members of the G protein coupled-receptor (GPCR) superfamily. There is high amino acid homology (~60%) within the opioid receptor family that constitutes a group of four receptor types: MOP (mu), DOP (delta), KOP (kappa), and NOP (nociceptin, orphanin FQ, ORL1). Opioid receptors are present in the central nervous system and peripherally, including immune cells, and are believed to function as neuromodulators or immunomodulators. Morphine is among the best known clinically employed analgesics that activate opioid receptors. Although the concept of opioid receptor dimers was proposed nearly 30 years ago, classical models of GPCRs, including opioid receptors, were generally based on the assumption that they are organized and function as monomers. However, in view of burgeoning evidence for the existence of heteromeric GPCRs, that includes at least twelve different heteromeric opioid receptors in cultured cells, it seems likely that constitutive oligomerization of GPCRs may be the general rule rather than the exception. Among the many heteromers reported in the opioid receptor family are mu/kappa, kappa/delta and mu/delta. However, the in vivo physiological and behavioral relevance for the proposed heteromers have not yet been rigorously established. The greatest drawback in studying the signaling and trafficking properties of heteromers pertains to the lack of selective ligands targeting opioid heteromers. It is, therefore, necessary to first develop tools that can be used as probes to address the shortcomings. Hence, we evaluated standard opioids agonists and antagonists, novel ligands synthesized in our lab, and clinically used opioid analgesics to establish a ligand selectivity profile that takes into account the existence of heteromers. Utilizing mu-delta agonist/antagonist bivalent ligands we have provided direct evidence for bridging of opioid receptor heteromers using immunofluorescent methods. Moreover, we performed studies to elucidate how the individual protomers constituting a heteromer modulate the trafficking and functional properties of each other. The results have painted an intriguing picture suggesting that the effects are dependent on both protomer composition and the ligands used. Those studies have given us valuable information on the role of opioid heteromers in physiology, and as unique targets in drug discovery.