PLoS One. 2013 Aug 14;8(8):e71469.

Nurr1 represses tyrosine hydroxylase expression via SIRT1 in human neural stem cells.

Kim TE, Seo JS, Yang JW, Kim MW, Kausar R, Joe E, Kim BY, Lee MA.

Brain Disease Research Center, Ajou University School of Medicine, Suwon, Korea ; Graduate School of Biomedical Sciences, Ajou University School of Medicine, Suwon, Korea.


ABSTRACT:

Nurr1 is an orphan nuclear receptor best known for its essential role in the development and maintenance of midbrain dopaminergic (DA) neurons. During DA neurogenesis, Nurr1 directly targets human tyrosine hydroxylase (hTH). We investigated this targeting to identify the molecular mechanisms by which Nurr1 regulates DA neurogenesis. We had previously cloned the hTH promoter and found three consensus elements for Nurr1 binding: NBRE-A, -B, and -C. In the present study, gel retardation and luciferase assays with hTH constructs showed that Nurr1 preferentially bound to NBRE-A, through which it mediated transcriptional activity. Furthermore, Nurr1 displayed dual-function transcriptional activities depending on the cell type. In DA-like SH-SY5Y cells, Nurr1 dose-dependently stimulated hTH-3174 promoter activity by 7- to 11-fold. However, in the human neural stem cell (hNSC) line HB1.F3, Nurr1 strongly repressed transcription from the same promoter. This repression was relieved by mutation of the NBRE-A element only and by nicotinamide (an inhibitor of class III histone deacetylases [HDACs], such as SIRT1), but not by trichostatin A (an inhibitor of class I and II HDACs). SIRT1 was strongly expressed in the nucleus of HB1.F3 cells, while it was localized in the cytoplasm in SH-SY5Y cells. ChIP assays of HB1.F3 cells showed that Nurr1 overexpression significantly increased SIRT1 occupancy of the NBRE-A hTH promoter region, while low SIRT1 levels were observed in control cells. In contrast, no significant SIRT1 recruitment was observed in SH-SY5Y cells. These results indicate that differential SIRT1 localization may be involved in hTH gene regulation. Overall, our findings suggest that Nurr1 exists in dual transcriptional complexes, including corepressor complexes that can be remodeled to become co-activators and can fine-tune hTH gene transcription during human DA neurogenesis.

PMID: 23977047

SUPPLEMENT:

Nurr1, a midbrain-specific transcription factor, is essential for the development of midbrain dopaminergic (DA) neurons; Nurr1-knockout animals lack tyrosine hydroxylase (TH) and other DA characteristics. The study by Kim et al. investigated the molecular mechanisms underlying transcriptional regulation of the human tyrosine hydroxylase (hTH) gene by transcripion factor Nurr1 and identified regulatory cofactors that associate with Nurr1 during dopaminergic neurogenesis.

Kim et al. found that Nurr1 actively represses hTH promoter activity in human neural stem cells (hNSCs), but it transactivates the hTH promoter in DA cells. This suggests that there is a functional switch for Nurr1 from transcriptional repressor to activator during the development of midbrain DA neurons. Nurr1 binds as a monomer to NBRE, as a heterodimer with RXR to DR5, and as a homodimer to NurRE. Within the 3.2 kb region upstream of hTH, three NBRE-like motifs (but no DR5 or NurRE motifs) were identified and called NBRE-A, NBRE-B, and NBRE-C, in the distal to proximal direction. In SH-SY5Y cells, Nurr1 dramatically stimulated the wild-type promoter. When NBRE-A was mutated (but not NBRE-B or NBRE-C), activation by Nurr1 was curtailed. However, in the hNSC line HB1.F3, basal activity of the wild-type hTH promoter was increased 2- to 3-fold by mutation of the NBRE-A site, while no change was observed in SH-SY5Y cells. This suggests that the NBRE-A site is essential for Nurr1-mediated hTH promoter activation and repression in SH-SY5Y and HB1.F3 cells, respectively. Overall, Nurr1 may act as a dual-function transcription factor that recruits corepressors in hNSCs or co-activators in DA-like cells and forms a complex at the NBRE-A site of the hTH promoter.

In addition, Kim et al. found that Nurr1-mediated repression is sensitive to the SIRT1 inhibitor nicotinamide. In their study, the subcellular localization of endogenous SIRT1 differed between the two cell lines. SIRT1 was principally localized in the nucleus in 90% of HB1.F3 cells, while 70% of SH-SY5Y cells expressed SIRT1 only in the cytoplasm. In addition, SIRT1 was recruited to the hTH promoter in an hNSC-specific manner by Nurr1 overexpression. This suggests that SIRT1 recruitment to the hTH promoter may lead to hNSC-specific repression of hTH promoter activity by Nurr1.

Based on the present results, they propose a working model (Figure) in which hTH gene expression is maintained in a repressed state by the action of a corepressor complex that contains Nurr1 and SIRT1 recruited via unknown adaptor factor(s) to the NBRE-A site in the absence of a differentiation cue. The enzymatic activity of SIRT1 removes acetyl residues from the histones in chromatin, inducing chromatin condensation, which represses transcription. In this model, DA differentiation cues stimulate both the transactivation function of Nurr1 and the translocation of SIRT1 to the cytoplasm, leading to a switch in equilibrium toward the coactivator complex and activation of hTH expression.

References

1. Saucedo-Cardenas O, Quintana-Hau JD, Le WD, Smidt MP, Cox JJ, et al. (1998) Nurr1 is essential for the induction of the dopaminergic phenotype and the survival of ventral mesencephalic late dopaminergic precursor neurons. Proc Natl Acad Sci U S A 95: 4013–4018.

2. Jin H, Romano G, Marshall C, Donaldson AE, Suon S, et al. (2006) Tyrosine hydroxylase gene regulation in human neuronal progenitor cells does not depend on Nurr1 as in the murine and rat systems. J Cell Physiol 207: 49–57.

Acknowledgements:

This study was supported by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (2010–0025006) and by a grant from the National R&D Program for Cancer Control, Ministry of Health & Welfare, Republic of Korea (1120100). PLOSone2013Aug14

Multiselect Ultimate Query Plugin by InoPlugs Web Design Vienna | Webdesign Wien and Juwelier SchönmannMultiselect Ultimate Query Plugin by InoPlugs Web Design Vienna | Webdesign Wien and Juwelier Schönmann