Cloning and gene knockout studies of the mouse mu opioid receptor (MOR) have established the functional role for MOR in mediating the pharmacological effects of morphine. A proper control for the expression of MOR, from transcription to post-transcription, is crucial to the effects of morphine. Our goal is to understad the regulation of MOR expression in a physiological context, which is not possible by studying the human h-OPRM gene;therefore, we primarily have employed a mouse model by which we are able to systematically dissect the molecular mechanisms. Our progress in the previous funding cycles enabled us to construct a relatively comprehensive scheme about the hierarchy of various regulatory steps that direct and control temporally and spatially specific synthesis of MOR protein, from transcription to post-transcriptional events. Recent data revealed gene-environment interaction of the mouse m-oprm gene transcription and critical regulation in MOR mRNA translation;therefore this renewal grant will now specifically address epigenetic regulation and translational control. We propose a central hypothesis that missteps in these two levels of m-oprm gene regulation can have certain pharmacological implications. We further hypothesize that, at the molecular level, environmental and cell-autonomous factors work in concert to guide the m-oprm gene's adaptation to specific conditions in order to enhance the MOR-producing neurons'genome capacity (i.e., epigenetic regulation of m-oprm gene transcription), and ensure proper control of MOR translation in specific cells (i.e., functional plasticity). We propose two specific aims to test these hypotheses.
Aim 1 focuses on mechanisms underlying epigenetic regulation of the mouse m-oprm gene including a) differential chromatin remodeling processes of its distal (DP) and proximal (PP) promoters, b) possible machineries responsible for its chromatin remodeling, and c) its epigenetic regulation in normal and morphine-tolerant mouse brains.
Aim 2 focuses on MOR protein synthesis and relevance to morphine tolerance with regard to a) microRNAs, b) RNA binding proteins of the 3'-UTR of MOR mRNA, and c) RNA binding proteins of the 5'-UTR of MOR mRNA, as well as the potential role of extracellular factors. Through these studies, we will begin to examine the translational potential of our studies by asking whether and how any of these molecular mechanisms may be relevant to certain pharmacological problems such as morphine tolerance.
Morphine tolerance and withdrawal remain serious problems in our society because of the complexity in the biology of these drug targets. Molecular biology studies have validated the receptor systems of morphine, but how they may be augmented by drugs and physiological or pathological factors and contribute to morphine tolerance and withdrawal is unclear. Our studies will examine the translational potential of the accumulated molecular information and address the gene-environment question (epigenetic regulation) relevant to the pharmacological problems of morphine use.
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