Ludwig Institute for Cancer Research

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Receptor Transcription Factors

Nurr1

The Nurr family of nuclear receptors is critical for cell growth and apoptosis, and its members are constitutively activated in several cell types. The Nurr1 receptor is essential for the development of midbrain dopamine cells that degenerate in Parkinson’s disease, and may be critical in other neurological disorders. Thus Nurr1 is a target for the development of drugs for both cancer and/or neurological conditions.

Introduction

Nuclear receptors (NRs) are fascinating proteins for several different reasons. First, they are excellent targets for understanding how genes are regulated since small molecule ligands can be used to switch these transcription factors between active and inactive states. Second, classical NR signaling pathways, e.g. steroid hormone and retinoid receptors, influence many biological pathways important in development and adult physiology. Many cancers have been shown to be ‘hormone-responsive’, and therapies that mimic hormones, but block the hormone NRs, have been shown to inhibit tumor growth. Finally, there are a number of NRs called ‘orphan receptors’, which lack identified ligands but have crucial roles in development and disease. Nurr1 is one of these orphan receptors.

The Nurr Family and Function

Nurr1 belongs to a highly related subgroup of receptors known as the Nurr family, which also includes Nur77 and Nor1. A first indication that Nurr family members are unusual NRs comes from the realization that they are products of immediate early genes, meaning that they are some of the first proteins produced after cell stimulation by various stimuli, including growth factors. Thus Nurr receptors seem to function as nuclear mediators of signaling pathways modulating cellular processes driven by signals from receptors on the cell surface. Interestingly, LICR investigators from the Stockholm Branch found that Nurr1 may also able to regulate cell surface receptors in turn, with the receptor tyrosine kinase Ret deregulated in Nurr1-deficient mice⁽¹⁾.

Importantly, Nurr1 is essential in the development of midbrain dopamine (DA) cells in the central nervous system. LICR investigators from the Stockholm Branch found that Nurr1 plays a critical role in the maturation, migration, innervation and survival of midbrain progenitor DA cells⁽²⁾. Nurr family members play pivotal roles in cell growth and apoptosis in DA cell development. While Nur77 promotes apoptosis in certain contexts, Nurr1 seems to play a role in protection against apoptosis, particularly in neurons⁽³⁾. This DA cell development may be mediated by the cyclin-dependent kinase inhibitor, p57Kip2, which interacts direct with Nurr1⁽⁴⁾. The LICR team also showed that Nurr1 regulates DA synthesis and storage in adult DA neurons⁽⁵⁾. These neurons are clinically very interesting since they degenerate in Parkinson’s disease, and play important roles in other disorders including schizophrenia. Understanding the process of DA cell development may facilitate efforts aimed at engineering stem cells for cell replacement in Parkinson’s patients

Ligand Binding

For many years there were major efforts invested in finding ligands that can bind and activate Nurr receptors. Although the Stockholm Branch team found that Nurr receptors appeared to be constitutively activated in many cell types⁽⁶⁾, ligands that bound and activated Nurr1 remained unidentified. However highly unexpected findings from the team later explained the elusiveness of Nurr1 ligands. Previous structural studies illustrated how ligands bind in a hydrophobic cavity situated within a highly conserved ligand binding domain (LBD) of NRs. In a collaborative effort, the LICR team solved the X-ray crystal structure of the Nurr1 LBD, and found that although it is folded much the same way as in other NRs, the space that is normally occupied by ligands is entirely filled by hydrophobic amino acid side chains in Nurr1 (see Figure)⁽⁷⁾. Hence, Nurr1 lacks the capacity for ligand binding. This important finding provides the first evidence that not all NRs will function as ligand binding receptors. Indeed, sequence homology analyses suggest that some other Nurr family members might also prove to be incapable of ligand-binding, and that they too should be defined as ligand-independent transcription factors.

Nurr Activation

Nurr1 has two ‘activation functions’ (AFs), with AF1 located in the N-terminal region and AF2 located in the C-terminal region of the protein. Characterization of these AFs by LICR investigators from the Stockholm Branch has shown that each operates in highly cell-type specific fashion to activate in the absence of endogenous ligands and as a point of influence by other signaling pathways for AF2 and AF1, respectively^(6,8).

Despite the lack of a ligand binding cavity in Nurr1, the Stockholm Branch team has demonstrated that Nurr1 is central in ligand-regulated processes in an entirely unexpected way. Nurr1 can form heterodimers with an archetypal NR, known as RXR, a promiscuous partner of many other NRs including retinoid and thyroid hormone receptors. In this capacity, RXR functions as an obligatory and essential partner required for efficient binding to the promoter DNA of genes transcribed by these receptors. Although RXR can be defined as an essential cofactor in NR-mediated signaling, it can also bind its own ligands, such as the retinoid metabolite 9-cis retinoic acid, and activate transcription in a ligand-dependent manner. LICR investigators identified a second endogenous RXR ligand, the omega-3 polyunsaturated fatty acid docosahexaenoic acid⁽⁹⁾, and thus showed that endogenous lipophilic molecules can also activate RXR.

The Stockholm Branch investigators have now found that Nurr1-RXR heterodimers are activated by endogenous RXR ligands in the embryonic central nervous system⁽¹⁰⁾. Importantly, these heterodimers promote the survival of neurons via a mechanism that is dependent on Nurr1, RXR, and ligands that can activate RXR. These results probably explain the basis for Nurr1’s essential role in promoting the survival of neurons in the adult mouse and human brain. The results provide an unexpected function for Nurr1 as a critical silent partner in signaling events that depend on RXR ligands. The team characterized the binding of Nurr1 and RXR and identified unique structural and functional properties of these heterodimers⁽¹¹⁾. These heterodimers are potential targets in the treatment of neurodegenerative disease.

References

1. Wallen A.A., Castro D.S., Zetterstrom R.H., Karlen M., Olson L., Ericson J., and Perlmann T. Orphan nuclear receptor Nurr1 is essential for Ret expression in midbrain dopamine neurons and in the brain stem. Mol.Cell Neurosci. (2001) 18(6):649-663.
2. Wallen A., Zetterstrom R.H., Solomin L., Arvidsson M., Olson L., and Perlmann T. Fate of mesencephalic AHD2-expressing dopamine progenitor cells in NURR1 mutant mice. Exp.Cell Res. (1999) 253(2):737-746.
3. Castro, D.S., Hermanson, E., Joseph, B., Wallén, Å., Aarnisalo, P., Heller, A. and Perlmann, T. Induction of cell cycle arrest and morphological differentiation by Nurr1 and retinoids in dopamine MN9D cells. Journal of Biological Chemistry 276:43277-43284, 2001.
4. Joseph, B, Wallén-Mackenzie, Å, Benoit, G., Murata, T., Joodmardi, E., Okret S and Perlmann, T. p57kip2 cooperates with Nurr1 in developing dopamine cells. Proceedings of the National Academy of Sciences U.S.A. 100:51619-151624, 2003.
5. Hermanson E., Joseph B., Castro D., Lindqvist E., Aarnisalo P., Wallen A., Benoit G., Hengerer B., Olson L., and Perlmann T. Nurr1 regulates dopamine synthesis and storage in MN9D dopamine cells. Exp.Cell Res (2003) 288(2):324-334.
6. Castro D.S., Arvidsson M., Bondesson B.M., and Perlmann T. Activity of the Nurr1 carboxyl-terminal domain depends on cell type and integrity of the activation function 2. J.Biol.Chem. (1999) 274(52):37483-37490.
7. Wang, Z., Benoit, G., Liu, J., Prasad, S., Aarnisalo, P., Liu, X., Xu, H., Walker, P.C. and Perlmann, T. Structure and function of Nurr1 reveals a novel class of ligand-independent nuclear receptors. Nature 423:555-560, 2003.
8. Nordzell M., Aarnisalo P., Benoit G., Castro D.S., and Perlmann T. Defining an N-terminal activation domain of the orphan nuclear receptor Nurr1. Biochem.Biophys.Res.Commun. (2004) 313(1):205-211.
9. de Urquiza A.M., Liu S., Sjoberg M., Zetterstrom R.H., Griffiths W., Sjovall J., and Perlmann T. Docosahexaenoic acid, a ligand for the retinoid X receptor in mouse brain. Science (2000) 290(5499):2140-2144.
10. Wallén-Macenzie, Å., Mata de Urquiza, A., Petterson, S., Rodriguez, F.J., Friling, S., Wagner, J., Ordentlich, P., Lengqvist, J., Heyman, R.A., Arenas, E. and Perlmann, T. Nurr1-RXR heterodimers mediate RXR ligand-induced signaling in neuronal cells. Genes & Development. 17:3036-3047, 2003.
11. Aarnisalo P., Kim C.H., Lee J.W., and Perlmann T. Defining requirements for heterodimerization between the retinoid X receptor and the orphan nuclear receptor Nurr1. J.Biol.Chem. (2002) 277(38):35118-35123.