On retinoid receptors, Nurr1 and related transcription factors in the CNS

Abstract

It has long been known that retinoids (vitamin A derivatives) are required for normal development and growth; in excess however metabolites such as all-trans retinoic acid are potent teratogens. Retinoids exert their effects through transcription factors known as the retinoic acid receptors (RARam -ß and -y) and retinoid X receptors (RXRa, -ß and -y). Intracellularly, two types of binding proteins also appear to be involved in retinoid homeostasis, cellular retinol binding protein I and II (CRBP I and II) and cellular retinoic acid binding protein I and II (CRABP I and II). RARs and RXRs belong to the nuclear receptor superfamily which also includes receptors for the steroid and thyroid hormones, vitamin D3 and several other small lipophilic molecules. Moreover, the superfamily includes a large group of receptors which lack identified ligands (termed "orphan receptors"). Nurr1, NGFI-B and Nor1 are closely related members of this latter group of proteins.

This study was undertaken in order to explore the roles of retinoids and the Nurr1/NGFI-B/Norl subfamily of orphan receptors in the developing and adult CNS. The expression patterns of CRBPs and CRABPs and RARs and RXRs in the postnatal and adult CNS were investigated using immunocytochemistry and in situ hybridization histochemistry (ISH). In addition, an in vitro reporter assay was used to detect retinoids in striatal tissue. It was found that many of the retinoid binding proteins and receptors are present in the CNS. Striatum, a part of the basal ganglia, contains all necessary components for retinoid signaling. Both CRBP I and CRABP I as well as RARß RXRß and RXRy are highly expressed within this region. In addition, it has also been possible to detect retinoids in striatal tissue. The role of Nurr1, NGFI-B and Nor1 has been explored. In particular, these studies have emphasized the role of NuTrl. All three members have previously been shown to interact with DNA as monomers, but Nurr1 and NGFI-B have also been shown to heterodimerize with RXR, thereby defining a distinct pathway for retinoid signaling. The most recently cloned member, Nor1, was shown to lack the ability to heterodimerize with RXR. To begin to understand the function of these orphan receptors, ISH was used to localize the mRNA expression patterns in both developing and adult tissues. All three members are highly expressed in the CNS; a striking observation was that Nurr1 mRNA was present in the developing ventral midbrain as well as in the mature dopamine (DA) neurons of the substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA).

To further investigate the function of Nurr1, a targeted mutation was introduced into the Nurr1 locus. Mice lacking Nurr1 are born alive but die within the first two days after birth. The mice were found to completely lack DA neurons in the ventral midbrain as demonstrated by the absence of several markers, such as the rate limiting enzyme in DA synthesis, tyrosine hydroxylase, a postulated retinoic acid converting enzyme, AHD2, and the DA transporter in SNC, the VTA as well as in the retrorubral field. In addition, high-pressure liquid chromatography was used to demonstrate the absence of DA itself in the major target area for the DAergic midbrain neurons, striatum. The role of Nurrl during early development of the ventral midbrain has been further studied and the relation to other factors postulated to be important for DA cell generation such as sonic hedgehog explored. Since survival could not be prolonged by administration of either the DA precursor L-DOPA, the DA receptor agonist apomorphine or DA itself despite apparently normal DA receptor expression, it is hypothesized that the mice do not die from lack of DA. Interestingly, brains of newborn and two months old heterozygous, apparently healthy, mice contained less DA than wild type littermates, indicating a function for Nurr1 not only in the generation of the DA cells, but also for maturation and maintenance of a normal DAergic phenotype. These studies demonstrate that retinoids most likely are important signaling molecules in the postnatal and adult CNS and establish Nurr1 as a key component for proper development of DA midbrain neurons, known to degenerate in patients with Parkinson's disease.