Constitutive activation of the delta opioid receptor by mutations in transmembrane domains III and VII

We have investigated whether transmembrane amino acid residues Asp128 (domain III), Tyr129 (domain III) [corrected], and Tyr308 (domain VII) in the mouse delta opioid receptor play a role in receptor activation. To do so, we have used a [35S]GTPgammaS (where GTPgammaS is guanosine 5'-3-O-(thio)triphosphate) binding assay to quantify the activation of recombinant receptors transiently expressed in COS cells and compared functional responses of D128N, D128A, Y129F, Y129A, and Y308F point-mutated receptors to that of the wild-type receptor. In the absence of ligand, [35S]GTPgammaS binding was increased for every mutant receptor under study (1.6-2.6-fold), suggesting that all mutations are able to enhance constitutive activity at the receptor. In support of this finding, the inverse agonist N,N-diallyl-Tyr-Aib-Aib-Phe-Leu (where Aib represents alpha-aminobutyric acid) efficiently reduced basal [35S]GTPgammaS binding in the mutated receptor preparations. The potent agonist BW373U86 stimulated [35S]GTPgammaS binding above basal levels with similar (D128N, Y129F, and Y129A) or markedly increased (Y308F) efficacy compared with wild-type receptor. BW373U86 potency was maintained or increased. In conclusion, our results demonstrate that the mutations under study increase functional activity of the receptor. Three-dimensional modeling suggests that Asp128 (III) and Tyr308 (VII) interact with each other and that Tyr129 (III) undergoes H bonding with His278 (VI). Thus, Asp128, Tyr129, and Tyr308 may be involved in a network of interhelical bonds, which contributes to maintain the delta receptor under an inactive conformation. We suggest that the mutations weaken helix-helix interactions and generate a receptor state that favors the active conformation and/or interacts with heterotrimeric G proteins more effectively.

Detection of opioid receptor mRNA by RT-PCR reveals alternative splicing for the delta- and kappa-opioid receptors

The three mu-, delta- and kappa-opioid receptors have recently been cloned and characterized at the molecular level. Our analysis of opioid receptor transcripts by RT-PCR revealed two PCR products derived from delta and kappa mRNAs with size higher than expected from the known cDNA sequences. DNA sequencing showed additional nucleotides inserted between the known splice sites, indicating the possible existence of alternative splicing pathways for delta and kappa receptors. The novel delta-opioid receptor transcript is expressed in mouse brain and contains a 243 bp insertion. This additional sequence is located at the splice junction between the first and second coding exons and is encoded by a single exon located 9 kb upstream exon 2 in the mDOR gene. The other alternative transcript occurs in human monocytic and T lymphocytic cell lines and encodes a novel form of the kappa-opioid receptor. The PCR product presents a 23 bp deletion at the 3' end of exon 2 followed by a 246 bp insertion found between exons 2 and 3. In the hKOR gene, this insertion is encoded by two DNA segments. One of them is located 0.4 kb downstream exon 2 while the second is flanking exon 3 on the 5' side. Both novel putative delta and kappa exons present in-frame stop codons that would lead to truncated receptor proteins. A possible functional or regulatory role of these shorter proteins in opioid function remains to be determined.