Morphine Regulates Expression ofμ-Opioid Receptor MOR-1A, an Intron-Retention Carboxyl Terminal Splice Variant of theμ-Opioid Receptor (OPRM1) Gene via miR-103/miR-107

Abstract

The m-opioid receptor (MOR-1) gene OPRM1 undergoes extensive alternative splicing, generating an array of splice variants. Of these variants, MOR-1A, an intron-retention carboxyl terminal splice variant identical to MOR-1 except for the terminal intracellular tail encoded by exon 3b, is quite abundant and conserved from rodent to humans. Increasing evidence indicates that miroRNAs (miRNAs) regulate MOR-1 expression and that m agonists such as morphine modulate miRNA expression. However, little is known about miRNA regulation of the OPRM1 splice variants. Using 39-rapid amplification cDNA end and Northern blot analyses, we identified the complete 39-untranslated region (39-UTR) for both mouse and human MOR-1A and their conserved polyadenylation site, and defined the role the 39-UTR in mRNA stability using a luciferase reporter assay. Computer models predicted a conserved miR-103/107 targeting site in the 39-UTR of both mouse and human MOR-1A. The functional relevance of miR-103/107 in regulating expression of MOR-1A protein through the consensus miR-103/107 binding sites in the 39-UTR was established by using mutagenesis and a miR-107 inhibitor in transfected human embryonic kidney 293 cells and Be (2)C cells that endogenously express human MOR-1A. Chronic morphine treatment significantly upregulated miR-103 and miR-107 levels, leading to downregulation of polyribosome-associated MOR-1A in both Be(2)C cells and the striatum of a morphinetolerant mouse, providing a new perspective on understanding the roles of miRNAs and OPRM1 splice variants in modulating the complex actions of morphine in animals and humans.

OPRM1 Methylation Contributes to Opioid Tolerance in Cancer Patients

“…31 In addition, opioid tolerance developed in these mice after insertion of morphine pellets, consistent with studies that showed chronic opioid administration led to opioid tolerance via mu-opioid receptor downregulation on neurons. 14,27 Our finding that these 2 collective mechanisms, persistent pain and chronic opioid administration, lead to mu-opioid receptor downregulation on neurons in our mouse model is consistent with previous studies demonstrating that these are 2 well known mechanisms of mu-opioid receptor downregulation. 17 We attempted to reverse the effect of mu-opioid receptor downregulation through adenoviral gene therapy.…”

“…This finding was consistent with our previous publication 31 in which we showed that although cancer pain tends to dampen mu-opioid receptor expression on the associated sensory neurons, mu-opioid receptor expression in these neurons is maintained in the presence of cancers that are transduced with Ad-OPRM1 to re-express mu-opioid receptors. Furthermore, we showed that morphine treatment, which was generally thought to dampen mu-opioid receptor expression in central nervous system tissue, 14 did not dampen mu-opioid receptor expression in neurons from mice transduced with Ad-OPRM1. In other words, Ad-OPRM1 treatment rescued mu-opioid expression on neurons.…”

Identification of Circulating miRNAs Differentially Regulated by Opioid Treatment

“…miR-181a-5p was found to be upregulated by morphine treatment in mouse hippocampal progenitor cell lineages, suggesting its potential role in neurogenesis [26] and drug addiction [27]. miR-103a-3p was also reported to be upregulated following chronic exposure to morphine, which is known to downregulate MOR [28]. On the other hand, oxycodone treatment resulted in the upregulation of two miRNAs, namely, miR-339-3p and miR-23a-3p (Table 1), and miR-339-3p was reported to inhibit MOR function [29].…”

Interactions of HIV and Drugs of Abuse

“…In addition, MOR-1 (exon 1), MOR-1A, MOR-1X, and MOR-1K splice variants appear to differ across CNS cell types (Dever et al, 2012; Dever et al, 2014). Evolving evidence suggests that there are significant functional differences among MOR splice variants (Pan et al, 2005; Majumdar et al, 2011; Dever et al, 2012; Xu et al, 2014; Lu, Xu, Xu, Pasternak, & Pan, 2014). Thus far, the findings are limited to neural cells isolated from a relative small number of individuals, and from imprecisely defined brain regions.…”