Delta-mu opioid receptor heteromers in chronic pain pathophysiology

Morphine and other opioids form the cornerstone of modern day pain management. Yet, chronic opioid use carries several negative side effects, such as tolerance, dependence, respiratory depression and constipation, limiting its prescription and thus complicating treatment options. Studies focused on the receptors involved in opioid pharmacology, suggest that these proteins can interact with each other to form complexes (heteromers) that can differ in their function from the non-complexed (monomers) receptors. Recently, I was part of a team that showed that DOR-MOR heteromers may counteract MOR-mediated analgesia. In another study I showed that alcohol exposure reveals functional DOR-MOR heteromers in the spinal cord that play a role in analgesia. Here we propose using two novel techniques, developed by us, to study the receptor heteromers in better detail in hopes of finding ways to disrupt the protein-protein interactions and learn more about the exact function of the heteromer in the process. The techniques used rely on receptor fusion proteins for functional molecular complementation of a fluorescent protein and functional complementation through receptor fusion with a special chimeric g-protein. Single amino acid mutations will be produced of residues that are sharing the heteromer contact interface, to identify those amino acids that are crucial for the formation of stable complexes. The information gathered here will then facilitate the development of computer models of the receptor heteromer interface and will enable us to perform rationale-based drug design for protein-protein interaction inhibitors. The protein-protein interaction inhibitors will be administered alongside opioids in rodent animal models to determine their effect on opioid induced analgesia. These inhibitors are invaluable tools, as they allow us to study the development of opioid tolerance, dependence and other compensatory changes such as opioid-induced hyperalgesia. Moreover, the techniques used here will function as a proof of principle for other receptor heteromers involved in chronic pain conditions. 

For this project we collaborate with Drs. Markus Lill, Val Watts, Greg Hockerman and Chang-Deng Hu at Purdue University

Relevant lab publications

Milan-Lobo L, Enquist JE, Van Rijn RM, Whistler JL, anti-analgesic effect of the mu/delta opioid receptor-heteromer revealed by ligand-biased antagonism, 2013, PLOS One, 8(3): e58362, PMC3598907

Harvey JH, Van Rijn RM, Whistler JL, High-throughput screening of GPCR heteromer-specific ligands using a FLIPR calcium mobilization assay, 2013, Humana Press, Springer, Methods Mol Biol, 995, Chemical Neurobiology: 43-54., 

Van Rijn RM, Harvey JH, Brissett DI, DeFriel JN, Whistler JL, Novel screening assay for the selective detection of G protein-coupled receptor heteromer signaling, J Pharmacol Exp Ther, 334(1):179-188, PMC3533407 

Brissett DI, Whistler JL, Van Rijn RM, Contribution of mu and delta opioid receptors to the pharmacological profile of kappa opioid receptor subtypes, Eur. J Pain, 2012, 16 (3): 327-337, PMC4086711

Van Rijn RM, Brissett DI, Whistler JL, Emergence of functional spinal delta-mu opioid receptors heteromers after chronic ethanol exposure, Biol Psychiatry, 2012, 71:232-238, PMC4086708

Van Rijn RM, Whistler JL, Waldhoer M, Opioid receptor heteromer-specific trafficking and pharmacology, Curr Opin Pharmacol, 2010, 10:73-79, PMC2900797

Van Rijn RM, Whistler JL, The only way is up: preventing opioid tolerance by promoting cell surface expression of MOR-DOR heterodimers? Mol. Interv, 2008, 8 (6): 277-280