Synaptic mechanism of A118G OPRM1 Gene Variants In Human Neurons

Abstract

Drug use disorders (DUDs) related to prescription opioids (narcotic pain medications) have a huge socioeconomical impact; however, the molecular and cellular mechanism of how opioids function in human is largely unknown. The Single Nucleotide Polymorphism (SNP) rs1799971 (OPRM1 A118G), produces a non-synonymous amino acid substitution, replacing Asparagine (N40) with Aspartate (D40), in the μ-opioid receptor (MOR), and is strongly associated with Drug use disorders (DUDs) and alcoholism. The impact of these genetic variants on neural function, including synaptic transmission and its molecular/cellular underpinnings, has not been established in humans. To lay the foundation for addressing these important and outstanding questions, we are using the induced neuronal (iN) cell technology that we have pioneered to generate human neurons carrying homozygous N40 or D40 MORs. Specifically, we have generated multiple induced pluripotent stem (iPS) cell lines carrying either homozygous MOR N40 or MOR D40 alleles from human subjects. Additionally, in order to mitigate the possible confounding issues related to the genetic background variations among these subjects, we also used CRISPR/Cas9 mediated gene targeting to create two isogenic pairs of human pluripotent stem cell lines: one introducing homozygous D40 alleles into a human embryonic stem (ES) cell line and the other converting one of the D40 homozygous iPS cell lines to homozygous N40. Using human neurons as a model system, our compelling preliminary data suggest that: 1) activation of both N40 and D40 MORs by agonist DAMGO in human neurons suppresses inhibitory synaptic release and the D40 MORs show stronger inhibitory effects; 2) exposure of human neurons to DAMGO for 1 day leads to loss of the DAMGO-induced suppression of synaptic release, possibly due to internalization of MORs or altered intracellular signaling; and 3) D40 human neurons show defective recovery from the loss of MOR regulation on synaptic transmission induced by prolonged agonist exposure. The objective of this proposal is to determine the functional impact and the underlying cellular/molecular mechanisms of OPRM1 allelic variants. The central hypothesis is that the N40 to D40 substitution leads to aberrant MOR expression and/or downstream signaling, resulting in altered cellular responses. The proposed research is innovative, because we will combine recent developments in stem cell biology with synaptic physiology to directly probe the impact of OPRM1 gene variants on synaptic function in human neurons. Our expectations are that cellular and synaptic function is altered in neurons carrying the MOR D40 variant. This study will provide fundamental biological insight into the regulatory functions of the MOR in human neurons.