Regulation of a opioid receptor chaperone protein, RTP4, by morphine.

Signaling by classic analgesics such as morphine is governed primarily by the relative abundance of opioid receptors at the cell surface and this is regulated by receptor delivery to, and retrieval from, the plasma membrane. While retrieval mechanisms such as receptor endocytosis have been extensively investigated, fewer studies have explored mechanisms of receptor maturation and delivery to the plasma membrane. A previous study implicated receptor chaperone proteins (RTPs) in the latter process. Since not much is known about regulation of RTP expression, we initiated studies examining the effect of chronic morphine administration on the levels of RTPs in the brain. Among the four RTPs, we detect selective and region-specific changes in RTP4 expression; RTP4 mRNA is significantly upregulated in the hypothalamus compared to other brain regions. We examined whether increased RTP4 expression impacted receptor protein levels and find a significant increase in the abundance of some (Mu opioid receptors, MOPr) but not all (delta opioid, CB1 cannabinoid or D2 dopamine) receptors in hypothalamic membranes from animals chronically treated with morphine. Next, we used a cell culture system to show that RTP4 expression is necessary and sufficient for regulating opioid receptor abundance at the cell surface. Interestingly, selective MOPr-mediated increase in RTP4 expression leads to increases in cell surface levels of MOPr-DOPr heteromers and this increase is significantly attenuated by RTP4 siRNA. Together, these results suggest that RTP4 expression is regulated by chronic morphine administration and this, in turn, regulates opioid receptor cell surface levels and function.