Morphine-induced MOR-1X and ASF/SF2 Expressions Are Independent of Transcriptional Regulation: Implications for MOR-1X Signaling

Abstract

Recently, multiple μ-opioid receptor (MOR) isoforms have been identified that originate from a single gene, OPRM1; however, both their regulation and their functional significance are poorly characterized. The objectives of this study were to decipher, first, the regulation of alternatively spliced μ-opioid receptor isoforms and the spliceosome components that determine splicing specificity and, second, the signaling pathways utilized by particular isoforms both constitutively and following agonist binding. Our studies demonstrated that the expression of a particular splice variant, MOR-1X, was up-regulated by morphine, and this coincided with an increase in the essential splicing factor ASF/SF2. Structural comparison of this isoform to the prototypical variant MOR-1 revealed that the unique distal portion of the C-terminal domain contains additional phosphorylation sites, while functional comparison found distinct signaling differences, particularly in the ERK and p90 RSK pathways. Additionally, MOR-1X expression significantly reduced Bax expression and mitochondrial dehydrogenase activity, suggesting a unique functional consequence for MOR-1X specific signaling. Collectively, these findings suggest that alternative splicing of the MOR is altered by exogenous opioids, such as morphine, and that individual isoforms, such as MOR-1X, mediate unique signal transduction with distinct functional consequence. Furthermore, we have identified for the first time a potential mechanism that involves the essential splicing factor ASF/SF2 through which morphine regulates splicing specificity of the MOR encoding gene, OPRM1.

Novel Roles of Non-Coding RNAs in Opioid Signaling and Cardioprotection

“…The 3′ UTRs of MOR, KOR, and DOR mRNAs contain multiple regulatory elements that are able to modulate their translational efficiency, mRNA stability, and transport [ 118 , 119 ]. Evidence indicates that opioids affect MOR expression through alternative splicing and changes in translational efficiency via miRNAs rather than altering its transcription [ 120 , 121 ]. The let-7 family of miRNAs, which include miR-23b-5p, miR-212/132, miR-16-5p, miR-339-3p, and miR-103/107, have all been identified as miRNAs that are able to bind to MOR 3′ UTR to post-transcriptionally repress its expression.…”