Trafficking and Signaling of Opioid Receptor Heterodimers

Many G protein coupled receptors (GPCRs) have been shown to dimerize/oligomerize. In some cases receptor oligomerization is essential for receptor function, i.e. for the GABAB (Pin et al., 2004), metabotropic glutamate (Kniazeff et al., 2004), taste receptors (Nelson et al., 2002) and rhodopsin (Filipek et al., 2004), which form heterodimers/oligomeric structures in native disc membranes (Fotiadis et al., 2004). In other cases, oligomerization plays a modulatory role. In particular, dimerization of opioid receptors has been shown to alter opioid ligand properties and affect receptor trafficking in cell culture model systems (Jordan and Devi, 1999; George et al., 2000) and in vivo (He et al., 2002). Furthermore, the phenotypes of opioid receptor knock-out mice hint that receptor heterodimerization may have functional consequences (Zhu et al., 1999). Several pharmacologically-defined subtypes of opioid receptor have been described, although only a single gene has been identified for each receptor. There are many explanations for this finding, one possibility is that heterodimerization of the opioid receptors alters the pharmacology to produce unique opioid receptor subtypes. Our assumptions are as follows: (i) opioid receptors exhibit pharmacological properties in vivo that have not been recapitulated in heterologous systems when only a single receptor is expressed suggesting that the receptors are modified in some way in vivo; (ii) opioid receptors could form unique targets for heterodimer selective agents. These targets are expressed in a tissue-selective manner providing a "proof-of-concept" for the in vivo relevance of opioid receptor heterodimers; (iii) the trafficking of the opioid receptors is critical for regulating responsiveness to opioid drugs. How receptor heterodimerization affects the processes of endocytosis and post-endocytic sorting has not been examined. We will use a combination of in vitro techniques to address this question. In the first specific aim of this proposal we will examine whether heterodimerization can create unique opioid receptor complexes and whether the opioid receptor subtypes could be a consequence of receptor heterodimerization. In the second aim, we will examine whether the trafficking of the opioid receptors, either endocytic or postendocytic, is influenced by receptor heterodimerization. I believe the laboratory of Dr. Whistler is uniquely poised to address these questions. They have built a "toolbox" of techniques and resources, including cell lines and knock-out mice that can be used to address whether opioid receptor heterodimers form, have altered signaling and trafficking properties and whether they are altered during drug use and thus contribute to opiate tolerance and dependence. In addition, it is possible that certain opioid receptor heterodimers are expressed in a tissue-selective manner, a gender-specific manner, or even in a disease state-specific manner like for example in chronic pain states or following prolonged use of opiate drugs.