μ-Opioid receptor 6-transmembrane isoform: A potential therapeutic target for new effective opioids

Abstract

The μ-opioid receptor (MOR) is the primary target for opioid analgesics. MOR induces analgesia through the inhibition of second messenger pathways and the modulation of ion channels activity. Nevertheless, cellular excitation has also been demonstrated, and proposed to mediate reduction of therapeutic efficacy and opioid-induced hyperalgesia upon prolonged exposure to opioids. In this mini-perspective, we review the recently identified, functional MOR isoform subclass, which consists of six transmembrane helices (6 TM) and may play an important role in MOR signaling. There is evidence that 6 TM MOR signals through very different cellular pathways and may mediate excitatory cellular effects rather than the classic inhibitory effects produced by the stimulation of the major (7 TM) isoform. Therefore, the development of 6 TM and 7 TM MOR selective compounds represents a new and exciting opportunity to better understand the mechanisms of action and the pharmacodynamic properties of a new class of opioids.

Keywords: 6TM MOR isoform; Drug discovery; Review; μ-Opioid receptor.

Figure 1. Schematic structure of human OPRM1 spliced variants

(A) All predicted and cloned exons of human OPRM1 gene are in accordance with the NCBI database, UCSC genome browser GRCh37/hg19. Exons and introns are shown by vertical and horizontal boxes, respectively. Solid boxes represent constitutive exons coding for transmembrane dolmens 2-7. (B) All cloned alternatively spliced variants of OPRM1 coding for 7TM receptor isoforms. (C) All cloned alternatively spliced variants of OPRM1 coding for 6TM receptor isoforms, (p) partial sequence, (*) exons with variable exon/intron boundaries. Start codons of MOR isoforms are schematically indicated by lower vertical bars, while stop codons are indicated by upper vertical bars. Maximal sizes of human exons (for lower panel) are shown in parentheses (nt): exon 11 (206), exon 1 (580), exon T (117), exon 14 (105), exon 13 (1200), exon 2 (353), exon 3 (521), exon Y (109), exon 16 (314), exon X (1271), exon 17 (128), exon 5 (1013), exon 4 (304), exon 18 (412), exon 6 (124), exon 7 (89), exon 9 (393).

Figure 2

Computer model of 7TM and 6TM MOR. (A) The experimentally validated structure model of 7TM MOR. The protein is shown in cartoon representation with rainbow color for N-(blue) to C-terminal (red). The extra-cellular loop is white with a weak propensity to form β-strands. (B) The initial 6TM model with the N-terminal 100 residues excised from the 7TM model. (C) The surface of 6TM model structure colored according to the local electrostatic potential. Blue and red colors correspond to positively and negatively charged surfaces, respectively, and white color corresponds the neutral hydrophobic ones.

Figure 3

Cellular localization of 7TM or 6TM MOR isoforms shows. Confocal images of HEK-293 cells expressing 7TM or 6TM MOR: A) 7TM MOR localizes on the cell surface, B) 6TM isoform is mainly localized in intracellular compartments. HEK-293 cells were transiently transfected with plasmids encoding MYC-tagged 7TM or FLAG-tagged 6TM. After forty-eight hours cells were fixed and stained with either Anti-MYC-FITC Antibody or Anti-DYKDDDDK Tag Antibody (Alexa Fluor 488 conjugate); DNA was counterstained with Hoechst 33342. Scale bar: 10 μm.