Ying-Xian Pan, MD, PhD

The mu opioid receptor has a special place within the opioid receptor family, mediating the actions of morphine and most clinically used analgesics, as well as drugs of abuse such as heroin. Although pharmacological studies initially characterized several mu receptor subtypes, only a single copy of the OPRM1 gene has been identified across species. How does a single OPRM1 gene generate these multiple mu opioid receptor subtypes? How does a single OPRM1 gene mediate different mu opioid responses seen in various animal models and individual humans? Alternative pre-mRNA splicing of the OPRM1 gene is a hypothesis that can account for multiple mu opioid receptor subtypes, and the individualized mu opioid responses. Driven by this hypothesis, most of our previous and current work has focused on alternative splicing of the OPRM1 gene and its promoter. We have discovered a multitude of alternatively spliced variants and promoters of the OPRM1 gene in rodent and human. Using a combination of molecular biology, biochemistry, pharmacology, and mouse genetics, we have investigated the functional relevance of these splice variants and alternative promoters. 

One of our major projects is to characterize the exon 11 promoter, an alternative promoter of the OPRM1 gene, and its relationship to the exon 1 promoter. The exon 11 promoter controls expression of a number of exon 11–associated splice variants. Our recent studies used an exon 11 knockout mouse model to demonstrate that certain exon 11–associated splice variants mediate a specific subset of mu opioids, such as heroin and morphine-6β-glucuronide, as well as kappa₃-like ligands. Our goal is to identify potential cis-acting elements and trans-acting factors corresponding to region- and cell-specific promoter activity by using a variety of in vitro and in vivo approaches, such as chromatin immunoprecipitation, DNA affinity purification coupled with geLC-MS/MS, and transgenic and double knockout/knockin mouse models.

Another central project is exploring the mechanisms underlying alternative splicing of the OPRM1 gene. Our efforts have focused on region- and cell-specific regulation, the relationship of such regulation to different behavioral responses in mouse and human, and identification of genetic influences, such as single nucleotide polymorphisms (SNPs), on modulating alternative splicing. Using gene-targeting mouse models, we aim to explore in vivo functions of C-terminal splice variants generated through 3’end splicing.