Modulation of OPRM1 Alternative Splicing by Morphine and HIV–1 Nef

Clinically used opioids, such as morphine, activate the mu opioid receptor (MOR) encoded by Opioid Receptor Mu 1 (OPRM1) gene. Examination of the opioid receptor genes showed that the human OPRM1 pre–mRNA undergoes extensive alternative splicing events and capable of expressing 21 isoforms. However, characterization of OPRM1 signaling is generalized, and only one isoform (MOR–1) has been extensively studied. Compounding this issue is the increasing significance of intravenous drug abuse in HIV neuropathogenesis. Here, we investigated the molecular impact of morphine and HIV–1 on regulation of OPRM1 pre–mRNA splicing in in vitro and in vivo models. Our results suggested that morphine treatment specifically induces the alternative splicing of MOR–1X isoform among the other isoforms analyzed in neuronal cells. Interestingly, alternative splicing and expression of MOR–1X isoform was also induced in postmortem brain tissues obtained from people with HIV (PWH). Additionally, treatment of control rats with morphine induced alternative splicing of MOR–1X in the brain regions involved in the reward pathways. More interestingly, HIV–1 transgenic (HIV–1Tg) rats, showed an additive induction of MOR–1X isoform with the exposure to morphine. To further assess the possible role of HIV secretory proteins in alternative splicing of OPRM1 gene, we analyzed the impact of HIV–1 Tat, gp120 and Nef proteins on alternative splicing of MOR–1X isoform. While the Tat and gp120 had no visible effects, treatment of neurons with Nef induced MOR–1X alternative splicing that was comparable to treatment with morphine. Altogether, our results suggest that HIV–1 may alter MOR isoform expression with Nef protein by amplifying the rate of MOR–1X alternative splicing induced by morphine. Graphical abstract