Central nervous system (CNS) myelin is produced by oligodendrocytes, which are glial cells that arise from oligodendrocyte progenitor cells (OPCs). By published transcriptome analysis, the orphan nuclear receptor Nur77 (nr4a1 gene) is highly expressed by OPCs, and downregulated as newly formed oligodendrocytes become myelinating oligodendrocytes. Nur77 is a nuclear receptor, and it also has nontranscriptional roles in cell growth and survival, which are largely determined by its subcellular localization. Phosphorylation or protein interactions such as with other nuclear receptors may control the localization and thus the function of the receptor. Nur77 is thought to bind directly to DNA as monomers or dimers, but can also bind with other nuclear receptors, including retinoid X receptors (RXRs), which are known modulators of OPC differentiation and may be critical transcriptional regulators of myelination. It has also been shown that Akt and ERK2, which are master regulators of myelination, phosphorylate the receptor. The current proposal will establish 1) the role of Nur77 in oligodendrocytes, and 2) which pathways/binding partners regulate Nur77 function in oligodendrocytes. The first specific aim will test the hypothesis that Nur77 downregulation is necessary for proper differentiation of OPCs to myelinating oligodendrocytes. First, we will characterize the expression profile of Nur77 in oligodendrocyte lineage cells during differentiation in primary rat OPC cultures and during myelination in embryonic zebrafish. Next, using pharmacology, over/under expression studies and CrispR/Cas-9 mediated genomic editing, we will test the role of Nur77 as a regulator of OPC maturation/differentiation, using oligodendrocyte-specific transgenic reporter fish and live imaging. In the second specific aim, we will test the hypothesis that Akt and/or ERK1/2 signaling increases Nur77 phosphorylation, to promote Nur77 interaction with RXRs and nuclear localization of Nur77/RXRs heterodimers during OL lineage progression. Here, we will use zebrafish and rodent cell culture models to test 1) if AKT and/or ERK1/2 signaling control Nur77 phosphorylation and localization during OPC differentiation; and 2) whether formation of heterodimers with RXRs mediates the impact of Nur77 on OPCs and myelination.
The current proposal investigates the role of a novel nuclear receptor, Nur77, also known as Nr4a1, in oligodendrocyte development. The hypotheses to be tested are 1) that Nur77 is expressed in progenitor cells, but must be downregulated for myelination, and 2) that Nur77 is regulated by Akt, Erk1/2 signaling and/or RXR heterodimerization.