LICR NewsLink

March 2004

Nurr1 - an unusual member of nuclear hormone receptors

Thomas Perlmann
Ludwig Institute for Cancer Research, Stockholm Branch

Nuclear receptors (NRs) are ligand-regulated transcription factors that bind steroid hormones, thyroid hormone, retinoids and other small and lipophilic signaling molecules. These receptors are fascinating proteins for several different reasons: First, they are excellent tools 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. Accordingly, their significance in disease, including cancer, is critical. Third, NRs include a large number of related but less well characterized orphan receptors lacking identified ligands. The existence of these proteins is intriguing and suggests that additional unexplored NR-mediated signaling pathways remain to be characterized.

Nurr1 is one of the orphan members of the NR family that has been intensely investigated at the Stockholm Branch of the LICR. It belongs to a highly related subgroup of receptors (here referred to as the Nurr-family) also including 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 and are upregulated by various stimuli such as growth factors. Moreover, these receptors are also constitutively active in many cell types, even in the absence of added exogenous ligands. Thus, Nurr-receptors seem to function as nuclear mediators of signaling pathways modulating cellular processes via the cell surface. We have shown that Nurr1 is critical in the central nervous system where it is essential in the development of midbrain dopamine cells (1). These neurons are clinically very interesting since they degenerate in Parkinson’s disease and play critical roles in other disorders including schizophrenia. Importantly, understanding the process of dopamine cell development will facilitate efforts aimed at engineering stem cells for cell replacement in Parkinson’s patients (see e.g. reference 2).

Additional data from us and others have shown that Nurr-family members also play pivotal roles in cell growth and apoptosis (3, 4). For example, Orla Conneely and collaborators at Baylor College in Houston have reported that Nur77/Nor1 double knock-out mice develop a severe myeloid leukemia and die only a few weeks after birth (Nuclear Receptors: Orphan Brothers, Keystone Symposium, February 28 - March 4, 2004). Moreover, while Nur77 promotes apoptosis in certain contexts, Nurr1 seems to play a role in neuroprotection.

Obviously, these functions make Nurr-family members prime candidates as drug targets, and major efforts have been invested in finding ligands that can bind and activate these receptors. However, ligands for Nurr1 have so far remained unidentified. Results from our lab have now explained the reason for these difficulties and provided some highly unexpected findings. Previous structural studies have nicely illustrated how ligands bind in a hydrophobic cavity situated within a highly conserved ligand binding domain of NRs. In a collaborative effort we have now solved the X-ray crystal structure of the Nurr1 ligand binding domain (5). The results show that the Nurr1 ligand binding domain is folded much the same way as in other NRs. However, the space that is normally occupied by ligands in other NRs is entirely filled by hydrophobic amino acid side chains in Nurr1 (see figure). Hence, Nurr1 lacks capacity for ligand binding and providing the first evidence that not all NRs will function as ligand binding receptors. Indeed, sequence homology suggests that also other Nurr-family members lack capacity for ligand-binding and that they too should be defined as ligand-independent transcription factors. However, despite the lack of a ligand binding cavity in Nurr1, another new study from our lab has demonstrated that Nurr1 is central in ligand-regulated processes in an entirely unexpected way.

Nurr1 can form heterodimers with RXR, another NR that probably represents the most central member of the NR family. Accordingly, RXR is a promiscuous heterodimer partner of many nuclear receptors including e.g. retinoid and thyroid hormone receptors. In this capacity RXR functions as an obligatory and essential partner that is required for efficient DNA binding of these receptors. RXR can thus be defined as an essential cofactor in NR-mediated signaling; however, it can also bind its own ligands and activate transcription in a ligand-dependent manner. Natural RXR ligands include the retinoid metabolite 9-cis retinoic acid. A few years back we were able to identify yet another endogenous RXR ligand, the omega-3 polyunsaturated fatty acid docosahexaenoic acid (6). Clearly, endogenous lipophilic molecules can activate RXR, but evidence showing that RXR is functioning as a liganded NR in vivo is lacking. Importantly, RXR heterodimer partners that promote RXR signaling have not been previously identified.

From our structural studies we could assume that Nurr1 lacks the ability to bind any cognate endogenous ligands, so we speculated that one of its functions might be to serve as silent non-ligand binding partner in RXR-mediated signaling. This is exactly what has been demonstrated from our new studies in which we found that Nurr1-RXR heterodimers are activated by endogenous RXR ligands in the embryonic central nervous system (7). Importantly, these heterodimers promote the survival of neurons via a mechanism that strictly depends on Nurr1, RXR and ligands that can activate RXR. These results likely explain the basis for other observations indicating that Nurr1 is essential for promoting the survival of neurons in the adult mouse and human brain. The results provide an unexpected function of Nurr1 as a silent partner in signaling events depending on ligands activating RXR. Curiously, such a role as a silent partner of RXR resembles how RXR functions in other heterodimers.

Clearly, Nurr receptors are unusual non-ligand binding members of the NR family that can function both alone and in heterodimers with RXR. In future research it will be critical to investigate the significance of the interaction with RXR and how important Nurr-family members are for the RXR signaling pathway. It will also be critical to determine if RXR-Nurr-mediated signaling is more generally important for the ability of Nurr-family members to influence growth, differentiation and cell death. Finally, it will be exciting to explore if this signaling pathway can be pharmacologically manipulated and if such manipulation may prove significant in new therapies of e.g. neurodegenerative disease and cancer. Undoubtedly, these and other efforts at the Stockholm Branch will continue to keep us very busy.

 

1. Zetterström, R.H., Solomin, L., Jansson, L., Hoffer, B.J., Olson, L., and Perlmann, T. (1997) "Dopamine neuron agenesis in Nurr1-deficient mice" Science, 276, 248-250
2. Wagner, J., Åkerud, P., Castro, D., Holm, P.C., Snyder, E. Y., Perlmann, T. and Arenas, E. (1999) “Type 1 astrocytes induce a midbrain dopaminergic phenotype in Nurr1-overexpressing neural stem cells” Nature Biotech, 17, 653-659
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” (2001) J. Biol. Chem. 276, 43277-43284J
4. Joseph, B, Wallén-Mackenzie, Å, Benoit, G., Murata, T., Joodmardi, E., Okret S and Perlmann, T. (2003) “p57kip2 cooperates with Nurr1 in developing dopamine cells.” Proc Natl Acad. Sci., USA, 100, 51619-151624
5. Wang, Z., Benoit, G., Liu, J., Prasad, S., Aarnisalo, P., Liu, X., Xu, H., Walker, P.C. and Perlmann, T. (2003) “Structure and function of Nurr1 reveals a novel class of ligand-independent nuclear receptors.” Nature, 423: 555-560
6. Mata de Urquiza, A, Liu, S., Sjöberg, M., Zetterström, R., Griffiths, W., Sjövall, J. and Perlmann, T. (2000) “Docosahexaenoic acid, a ligand for the retinoid X receptor in the mouse brain” Science, 290, 2140-2144
7. 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. (2003) “Nurr1-RXR heterodimers mediate RXR ligand-induced signaling in neuronal cells” Genes Dev, 17, 3036-3047