The biased signaling has been extensively studied in the original mu opioid receptor (MOR-1), particularly through G protein and β-arrestin2 signaling pathways. The concept that the G protein pathway is often linked to the therapeutic effect of the drug, while the β-arrestin pathway is associated to the side effects has been proposed to develop biased analgesic compounds with limited side-effects associated with traditional opiates. The mu opioid receptor gene, OPRM1, undergoes extensive alternative pre-mRNA splicing, generating multiple splice variants or isoforms that are conserved from rodent to human. One type of the Oprm1 splice variants are the full-length 7 transmembrane (7TM) C-terminal splice variants, which have identical receptor structures including entire binding pocket, but contain a different intracellular C-terminal tail resulted from 3′ alternative splicing. Increasing evidence suggest that these full-length 7TM C-terminal variants play important roles in mu opioid pharmacology, raising questions regarding biased signaling at these multiple C-terminal variants. In the present study, we investigated the effect of different C-terminal variants on mu agonist-induced G protein coupling, β-arrestin2 recruitment, and ultimately, signaling bias. We found that mu agonists produced marked differences in G protein activation and β-arrestin2 recruitment among various C-terminal variants, leading to biased signaling at various level. Particularly, MOR-1O, an exon 7-associated variant, showed greater β-arrestin2 bias for most mu agonists than MOR-1, an exon 4-associated variant. Biased signaling of G protein-coupled receptors has been defined by evidences that different agonists can produce divergent signaling transduction pathways through a single receptor. Our findings that a single mu agonist can induce differential signaling through multiple 7TM splice variants provide a new perspective on biased signaling at least for Oprm1, which perhaps is important for our understanding of the complex mu opioid actions in vivo where all the 7TM splice variants co-exist.

偏向信号已在原始 mu 阿片受体 (MOR-1) 中得到广泛研究，特别是通过 G 蛋白和 β-arrestin2 信号通路。G 蛋白途径通常与药物的治疗效果相关，而 β-arrestin 途径与副作用相关的概念已被提出，以开发与传统阿片类药物相关的副作用有限的偏向镇痛化合物。mu阿片受体基因OPRM1经历广泛的可变前mRNA剪接，产生从啮齿动物到人类保守的多个剪接变体或同种型。Oprm1的一种剪接变体是全长 7 跨膜 (7TM) C 末端剪接变体，其具有相同的受体结构，包括整个结合口袋，但包含由 3' 可变剪接产生的不同的细胞内 C 末端尾部。越来越多的证据表明，这些全长 7TM C 末端变体在 mu 阿片类药物药理学中发挥重要作用，从而引发了有关这些多个 C 末端变体的偏向信号传导的问题。在本研究中，我们研究了不同 C 末端变体对 mu 激动剂诱导的 G 蛋白偶联、β-arrestin2 募集以及最终信号偏倚的影响。我们发现 mu 激动剂在各种 C 末端变体中 G 蛋白活化和 β-arrestin2 募集产生显着差异，导致不同水平的偏向信号传导。特别是，MOR-1O是一种与外显子 7 相关的变体，对大多数 mu 激动剂显示出比外显子 4 相关变体 MOR-1 更大的 β-arrestin2 偏倚。G 蛋白偶联受体的偏向信号传导已通过不同激动剂可通过单一受体产生不同信号转导通路的证据来定义。我们的研究结果表明，单个 mu 激动剂可以通过多个 7TM 剪接变体诱导差异信号传导，这为至少对Oprm1的偏向信号传导提供了新的视角，这对于我们理解体内复杂的 mu 阿片类药物作用可能很重要，其中所有 7TM 剪接变体共同作用-存在。