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Nurr1 (NR4A2) is an orphan nuclear receptor highly essential for the dopaminergic development and survival. Altered expression of Nurr1 has been suggested as a potential genetic risk factor for dopamine-related brain disorders, including schizophrenia. In support of this, recent experimental work in genetically modified mice shows that mice with a heterozygous constitutive deletion of Nurr1 show a facilitation of the development of schizophrenia-related behavioral abnormalities. However, the behavioral characterization of this Nurr1-deficient mouse model remains incomplete. This study therefore used a comprehensive behavioral test battery to evaluate schizophrenia-relevant phenotypes in Nurr1-deficient mice. We found that these mice displayed increased spontaneous locomotor activity and potentiated locomotor reaction to systemic treatment with the non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist, dizocilpine (MK-801). In addition, male but not female Nurr1-deficient mice showed significant deficits in the prepulse inhibition and prepulse-elicited reactivity. However, Nurr1 deletion did not induce overt abnormalities in other cardinal behavioral and cognitive functions known to be impaired in schizophrenia, including social interaction and recognition, spatial recognition memory or discrimination reversal learning. Our findings thus suggest that heterozygous constitutive deletion of Nurr1 results in a restricted phenotype characteristic of schizophrenia symptomatology, which primarily relates to motor activity, sensorimotor gating and responsiveness to the psychomimetic drug MK-801. This study further emphasizes a critical role of altered dopaminergic development in the precipitation of specific brain dysfunctions relevant to human psychotic disorder.

The immediate-early gene Nurr1 is a member of the inducible orphan nuclear receptor family. Nurr1 is essential to the differentiation, maturation, and maintenance of midbrain dopaminergic neurons and is expressed in different brain regions. We have reported that adult mice with reduced Nurr1 expression displayed an increase in immobility response to acute stress. These mice were also deficient in the retention of emotional memory. Thus, Nurr1 expression seems to be relevant to normal cognitive processes. To investigate the response of Nurr1 to a stress stimulus, Nurr1 mRNA expression was examined by in situ hybridization in adult mice using a depression-like behavior paradigm, the forced swim test. The Nurr1 gene was rapidly and widely up-regulated throughout the brain, including cortical areas (i.e., prefrontal cortex, primary and secondary visual cortex, primary auditory cortex, and secondary somatosensory cortex), hippocampus (dentate gyrus, CA1, CA2, and CA3), and midbrain (substantia nigra pars compacta and ventral tegmental area) at 30 min and 3 hr after the forced swim test. Dopamine content was reduced in prefrontal cortex and midbrain following swim stress. These results suggest that the increase in Nurr1 expression might be a compensatory mechanism to counteract the changes in forebrain dopamine transmission in coping with acute stress.

Both vitamin A deficiency and high doses of retinoids can result in learning and memory impairments, depression as well as decreases in cell proliferation, neurogenesis and cell survival. Physical activity enhances hippocampal neurogenesis and can also exert an antidepressant effect. Here we elucidate a putative link between running, retinoid signaling, and neurogenesis in hippocampus. Adult transgenic reporter mice designed to detect ligand-activated retinoic acid receptors (RAR) or retinoid X receptors (RXR) were used to localize the distribution of activated RAR or RXR at the single-cell level in the brain. Two months of voluntary wheel-running induced an increase in hippocampal neurogenesis as indicated by an almost two-fold increase in doublecortin-immunoreactive cells. Running activity was correlated with neurogenesis. Under basal conditions a distinct pattern of RAR-activated cells was detected in the granule cell layer of the dentate gyrus (DG), thalamus, and cerebral cortex layers 3-4 and to a lesser extent in hippocampal pyramidal cell layers CA1-CA3. Running did not change the number of RAR-activated cells in the DG. There was no correlation between running and RAR activation or between RAR activation and neurogenesis in the DG of hippocampus. Only a few scattered activated retinoid X receptors were found in the DG under basal conditions and after wheel-running, but RXR was detected in other areas such as in the hilus region of hippocampus and in layer VI of cortex cerebri. RAR agonists affect mood in humans and reduce neurogenesis, learning and memory in animal models. In our study, long-term running increased neurogenesis but did not alter RAR ligand activation in the DG in individually housed mice. Thus, our data suggest that the effects of exercise on neurogenesis and other plasticity changes in the hippocampal formation are mediated by mechanisms that do not involve retinoid receptor activation.

Vitamin A deficiency is known to be accompanied with immune deficiency and susceptibility to a wide range of infectious diseases. Experimental evidence suggests that the active metabolites of vitamin A that mediate its effects on the immune system are the retinoic acids (RA), which are ligands for the nuclear RA receptor (RAR) family. RA were previously shown both to promote proliferation and to regulate apoptosis of thymocytes. In this study we detected the age-dependent mRNA expression of retinaldehyde dehydrogenases (RALDH1 and 2), cellular RA binding protein-II and CYP26A, proteins responsible for the synthesis, nuclear transport and degradation of RA in the postnatally developing thymus. RALDH1 was located in thymic epithelial cells. However, the amount of all-trans RA in thymic homogenates was close to the detection limit, suggesting that in this tissue all-trans RA is not the main RAR-regulating product of retinol metabolism. At the same time, by measuring the induction of a RAR-responsive transgene in two independent transgenic mouse strains, we demonstrated the production of an RAR-activating ligand, which was age and RALDH dependent. Our data provide evidence for the existence of endogenous retinoid synthesis in the thymus and suggest that retinoids similar to glucocorticoids might indeed be involved in the regulation of thymic proliferation and selection processes by being present in the thymus in functionally effective amounts.

The orphan nuclear receptor Nurr1 is expressed in the developing and adult central nervous system. Previous studies have shown that Nurr1 is essential for the generation of midbrain dopamine neurons. Furthermore, Nurr1 is critical for respiratory functions associated with the brain stem. Very few Nurr1 regulated genes have been identified and it remains unclear how Nurr1 influences the function and development of neurons. To identify novel Nurr1 target genes we have searched for regulated genes in the dopaminergic MN9D cell line. These experiments identified Neuropilin-1 (Nrp1), a receptor protein involved in axon guidance and angiogenesis, as a novel Nurr1 target gene. Nrp1 expression was rapidly up-regulated by Nurr1 in MN9D cells and in situ hybridization analysis showed that Nrp1 was coexpressed with Nurr1 in the brain stem dorsal motor nucleus. Importantly, Nrp1 expression was down-regulated in this area in Nurr1 null mice. Moreover, two functional Nurr1 binding sites were identified in the Nrp1 promoter and Nurr1 was found to be recruited to these sites in MN9D cells, further supporting that Nrp1 is a direct downstream target of Nurr1. Taken together, our findings suggest that Nurr1 might influence the processes of axon guidance and/or angiogenesis via the regulation of Nrp1 expression.

Nurr1, a transcription factor belonging to the family of nuclear receptors, is expressed at high levels immediately after birth. Gene-targeted mice lacking Nurr1 fail to develop midbrain dopaminergic neurones and do not survive beyond 24 h after birth. Dopamine (DA) levels may be regulated by Nurr1, and as DA is involved in both central and peripheral respiratory control, we hypothesized that lack of Nurr1 may impair breathing and cause death by respiratory failure. We demonstrate herein that Nurr1 newborn knockout mice have a severely disturbed breathing pattern characterized by hypoventilation, numerous apnoeas and failure to increase breathing when challenged with hypoxia. In heterozygote Nurr1 mice the response to hypoxia is also altered. Furthermore, the central respiratory rhythm, generated from isolated brainstem–spinal cord preparations, exhibits impaired response to hypoxia in mice lacking Nurr1. Moreover, Nurr1 is expressed in several respiratory-related regions of the nervous system, including the nucleus of the solitary tract, the nucleus ambiguus and the dorsal motor nucleus of the vagus nerve, and in the carotid bodies. The prominent Nurr1 expression in these areas, involved in respiratory control, along with the severe respiratory phenotype, indicates that Nurr1 plays a major role in the extrauterine adaption of respiratory control and the response to hypoxia.

Retinoic acid (RA), a retinoid metabolite, acts as a gene regulator via ligand-activated transcription factors, known as retinoic acid receptors (RARs) and retinoid X receptors (RXRs), both existing in three different subtypes, alpha, beta and gamma. In the intracellular regulation of retinoids, four binding proteins have been implicated: cellular retinol binding protein (CRBP) types I and II and cellular retinoic acid binding protein (CRABP) types I and II. We have used in situ hybridization to localize mRNA species encoding CRBP- and CRABP I and II as well as all the different nuclear receptors in the developing and adult rat and mouse central nervous system (CNS), an assay to investigate the possible presence of RA, and immunohistochemistry to also analyse CRBP I- and CRABP immunoreactivity (IR). RXRbeta is found in most areas while RARalpha and -beta and RXRalpha and -gamma show much more restricted patterns of expression. RARalpha is found in cortex and hippocampus and RARbeta and RXRgamma are both highly expressed in the dopamine-innervated areas caudate/putamen, nucleus accumbens and olfactory tubercle. RARgamma could not be detected in any part of the CNS. Using an in vitro reporter assay, we found high levels of RA in the developing striatum. The caudate/putamen of the developing brain showed strong CRBP I-IR in a compartmentalized manner, while at the same time containing many evenly distributed CRABP I-IR neurons. The CRBP I- and CRABP I-IR patterns were closely paralleled by the presence of the corresponding transcripts. The specific expression pattern of retinoid-binding proteins and nuclear retinoid receptors as well as the presence of RA in striatum suggests that retinoids are important in many brain structures and emphasizes a role for retinoids in gene regulatory events in postnatal and adult striatum.

The orphan nuclear receptor Nur77 has been implicated in thymic negative selection. We studied the effect of two T cell receptor (TCR) transgenes on positive selection and Nur77 mRNA expression in thymus. DO11.10 mice, expressing a transgenic TCR specific for an ovalbumin (OVA) 323-339 peptide presented by I-Ad, were found to have an enlarged thymus with a reduced apoptotic activity, measured by flow cytometry, reduced mitochondrial membrane potential and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) techniques. In contrast, in F5 mice expressing a transgenic TCR recognizing the influenza virus nucleoprotein (NP) 366-374 peptide restricted by Db, this positive selection effect was much less pronounced. Positive thymic selection in DO11.10 TCR+ mice correlated with a reduced level of Nur77 mRNA expression shown by Northern blot. F5 mice expressed levels close to those expressed by the wild type. Both transgenic mouse strains responded with extensive cortical apoptosis, and with up-regulation of Nur77 mRNA, to injection of cognate peptides. As 9-cis-Retinoic acid (9-cis-RA) inhibits Nur77-dependent apoptosis in T cell hybridomas in vitro, mice were pretreated with the drug to investigate a similar effect in vivo. However, the drug itself, at saturating concentrations, caused extensive apoptosis in immature CD4+/CD8+ thymocytes. The result demonstrates a correlation between Nur77 expression and thymic apoptotic activity, both during positive and negative selection events.