A Novel Alternatively Spliced Isoform of the Mu-Opioid Receptor: Functional Antagonism

Abstract

BackgroundOpioids are the most widely used analgesics for the treatment of clinical pain. They produce their therapeutic effects by binding to μ-opioid receptors (MORs), which are 7 transmembrane domain (7TM) G-protein-coupled receptors (GPCRs), and inhibiting cellular activity. However, the analgesic efficacy of opioids is compromised by side-effects such as analgesic tolerance, dependence and opioid-induced hyperalgesia (OIH). In contrast to opioid analgesia these side effects are associated with cellular excitation. Several hypotheses have been advanced to explain these phenomena, yet the molecular mechanisms underlying tolerance and OIH remain poorly understood.ResultsWe recently discovered a new human alternatively spliced isoform of MOR (MOR1K) that is missing the N-terminal extracellular and first transmembrane domains, resulting in a 6TM GPCR variant. To characterize the pattern of cellular transduction pathways activated by this human MOR1K isoform, we conducted a series of pharmacological and molecular experiments. Results show that stimulation of MOR1K with morphine leads to excitatory cellular effects. In contrast to stimulation of MOR1, stimulation of MOR1K leads to increased Ca2+ levels as well as increased nitric oxide (NO) release. Immunoprecipitation experiments further reveal that unlike MOR1, which couples to the inhibitory Gαi/o complex, MOR1K couples to the stimulatory Gαs complex.ConclusionThe major MOR1 and the alternative MOR1K isoforms mediate opposite cellular effects in response to morphine, with MOR1K driving excitatory processes. These findings warrant further investigations that examine animal and human MORK1 expression and function following chronic exposure to opioids, which may identify MOR1K as a novel target for the development of new clinically effective classes of opioids that have high analgesic efficacy with diminished ability to produce tolerance, OIH, and other unwanted side-effects.

Interactive HIV-1 Tat and Morphine-Induced Synaptodendritic Injury Is Triggered through Focal Disruptions in Na+ Influx, Mitochondrial Instability, and Ca2+ Overload

“…Morphine-dependent exacerbation of Tat neurotoxicity has been attributed to direct actions on glia . Although morphine typically acts in an inhibitory manner, an excitatory MOR-1K splice variant (Gris et al, 2010) has been described in human astroglia (Dever et al, 2014). In that study, MOR-1K was not found to be expressed by neurons, although the human neurons sampled were from undefined brain regions and did not appear to include striatal neurons.…”

Mu Opioids and Their Receptors: Evolution of a Concept

“…Although strongly suggestive, the complexity of the mu receptor MOR-1 gene and its splice variants (section VIII) raises a number of issues. With dozens of splice variants identified in mice, rats, and humans (Bare et al, 1994;Du et al, 1997;Pan et al, 1998bPan et al, , 1999Pan et al, , 2000Pan et al, , 2001Pan et al, , 2003Pan et al, , 2005aPasternak et al, 2004;Doyle et al, 2007b;Xu et al, 2009Xu et al, , 2011Gris et al, 2010), it is not yet clear which one(s) dimerize with delta receptors and how this affects their pharmacology.…”

“…Originally identified in mice and located approximately 30 kb upstream of exon 1 (Pan et al, 2001), exon 11, and its splice variants have been identified in an additional eight mammalian OPRM1 genes by homologous cloning or bioinformatic searches, including rat and human (Pan et al, 2001Xu et al, 2006Xu et al, , 2009Gris et al, 2010) (section VIII). Exon 11 contains its own promoter that is distinct from the promoter responsible for generating the exon 1 variants.…”

Inhibition of DOR prevents remifentanil induced postoperative hyperalgesia through regulating the trafficking and function of spinal NMDA receptors in vivo and in vitro

“…Furthermore, membrane trafficking of NMDA receptors with NR2B subunit have been shown in remifentanil induced hyperalgesia through glycogen synthase kinase-3␤ (GSK-3␤) pathway (Li et al, 2013;Yuan et al, 2013). Some studies indicated that NMDA receptor function would be enhanced after chronic morphine exposure as well as 4, 6 or 8 nM remifentanil infusion (Gris et al, 2010;Hang et al, 2011;Zhao and Joo, 2008). Therefore, the expression level, membrane trafficking and functional changes of NMDA receptors are potential mechanisms of OIH.…”