Dopamine neurons of the substantia nigra and ventral tegmental area regulate movement and affective behavior and degenerate in Parkinson's disease. The orphan nuclear receptor Nurr1 was shown to be expressed in developing dopamine neurons before the appearance of known phenotypic markers for these cells. Mice lacking Nurr1 failed to generate midbrain dopaminergic neurons, were hypoactive, and died soon after birth. Nurr1 expression continued into adulthood, and brains of heterozygous animals, otherwise apparently healthy, contained reduced dopamine levels. These results suggest that putative Nurr1 ligands may be useful for treatment of Parkinson's disease and other disorders of midbrain dopamine circuitry.

No abstract

Despite its long history, the central effects of progressive depletion of vitamin A in adult mice has not been previously described. An examination of vitamin-deprived animals revealed a progressive and ultimately profound impairment of hippocampal CA1 longterm potentiation and a virtual abolishment of long-term depression. Importantly, these losses are fully reversible by dietary vitamin A replenishment in vivo or direct application of all transretinoic acid to acute hippocampal slices. We find retinoid responsive transgenes to be highly active in the hippocampus, and by using dissected explants, we show the hippocampus to be a site of robust synthesis of bioactive retinoids. In aggregate, these results demonstrate that vitamin A and its active derivatives function as essential competence factors for long-term synaptic plasticity within the adult brain, and suggest that key genes required for long-term potentiation and long-term depression are retinoid dependent. These data suggest a major mental consequence for the hundreds of millions of adults and children who are vitamin A deficient.V itamin A and its derivatives (the retinoids) activate signaling pathways necessary for development, differentiation, and homeostasis of several tissues, including the nervous system (1, 2). Known impairments caused by the lack of dietary vitamin A include blindness, infertility, embryonic malformations, and compromised immunity. Vitamin A deficiency (VAD) is currently a risk for over 100 million children in over 75 countries, and results in nearly 3.2 million associated childhood deaths annually (refs. 3 and 4, and http:͞͞www.unicef.org͞sowc98).The biological effects of retinoids are mediated by retinoid receptors, a subgroup of the nuclear receptor superfamily. The retinoid receptor family includes the retinoic acid receptors (RARs; ␣, ␤, and ␥), which bind all trans-retinoic acid and 9-cis retinoic acid, and the retinoid X receptors (RXRs; ␣, ␤, and ␥), which bind 9-cis retinoic acid only. RAR͞RXR heterodimers, and to some extent RXR homodimers, act as transcription factors by binding to retinoid response elements in the promoters of target genes and activating gene expression in the presence of ligand (1, 4, 5). Multiple combinations of RAR͞RXR heterodimers are possible, depending on the overlapping expression of receptor subtypes within tissues (1).Each RAR and RXR exhibits a specific expression pattern in the adult central nervous system (CNS), distinct from that found in the developing nervous system (2, 6-10), indicating that, in addition to the modulation of neuronal development during embryogenesis, retinoids are likely to regulate activities in the mature brain. Supporting this notion, evidence suggests that RAR␤, RXR␤, and RXR␥ modulate locomotor behavior by regulating the expression of dopamine receptors in the adult striatum (11). Additionally, retinoic acid production is required in distinct regions of the adult songbird brain for song maturation, a learned behavior (12). Moreover, we recently found that RAR␤ ϪϪ ...

NGFI-B, Nurr1, and Nor1 are three closely related orphan members of the steroid/thyroid hormone receptor superfamily. These receptors can bind to DNA as monomers and exhibit constitutive transcriptional activity. Moreover, two of the receptors, NGFI-B and Nurr1, have previously been shown to form heterodimers with the retinoid X receptor (RXR). Such heterodimers as well as complexes formed between RXR and the all-trans retinoic acid receptor bind to DNA response elements composed of direct repeats spaced by five nucleotides (DR5). However, whereas retinoic acid receptor can inhibit ligand-dependent RXR activation, NGFI-B and Nurr1 allow efficient RXR activation through DR5 elements and thus define a distinct pathway for vitamin A signaling. In this study we demonstrate that the most recently identified member of the subfamily, Nor1, shows similar monomer DNA-binding and constitutive transactivation properties as NGFI-B and Nurr1. In contrast, however, Nor1 is unable to promote RXR signaling due to its inability to form heterodimers with RXR. To begin to understand the physiological implications of these functional differences we used in situ hybridization to compare the distribution of Nor1, NGFI-B, and Nurr1 messenger RNAs during different developmental stages. The receptors are expressed in both distinct and overlapping patterns, predominantly in the central nervous system. Notably, Nurr1 is expressed in the prenatal ventral midbrain in a region that gives rise to dopaminergic neurons. Nor1 is also expressed during embryonic development, and all three receptors show a complex distribution in the postnatal brain. Furthermore, Nor1 colocalizes with NGFI-B in the adrenal glands and thymus, two tissues in which NGFI-B has been suggested to be functionally important. These data may indicate redundancy between members of the NGFI-B/Nurr1/Nor1 subfamily and could explain why no phenotypic disturbances have yet been found in mice in which the NGFI-B gene has been inactivated.

Retinoids regulate gene expression through the action of retinoic acid receptors (RARs) and retinoid-X receptors (RXRs), which both belong to the family of nuclear hormone receptors. Retinoids are of fundamental importance during development, but it has been difficult to assess the distribution of ligand-activated receptors in vivo. This is particularly the case for RXR, which is a critical unliganded auxiliary protein for several nuclear receptors, including RAR, but its ligand-activated role in vivo remains uncertain. Here we describe an assay in transgenic mice, based on the expression of an effector fusion protein linking the ligand-binding domain of either RXR or RAR to the yeast Gal4 DNA-binding domain, and the in situ detection of ligand-activated effector proteins by using an inducible transgenic lacZ reporter gene. We detect receptor activation in the spinal cord in a pattern that indicates that the receptor functions in the maturation of limb-innervating motor neurons. Our results reveal a specific activation pattern of Gal4-RXR which indicates that RXR is a critical bona fide receptor in the developing spinal cord.

In the absence of the viral regulatory protein Rev, the human immunodeficiency virus type 1 gaglpol and env mRNAs are inefficiently expressed, since nucleocytoplasmic transport, stability, and polysomal loading are impaired. It has been suggested that splicing is necessary for Rev function and that the low expression of the unspliced and intermediate spliced mRNAs in the absence of Rev is associated with specific splice sites. Previous studies identified distinct RNA elements within the gagipol region responsible for low expression that are not associated with splice sites. Here we study the determinants for Rev dependence of the authentic env mRNA. We demonstrate that upon removal of all the utilized splice sites, the env mRNA is still Rev dependent and Rev responsive for expression in human cells. We have identified several regions within the env mRNA that inhibit expression of a gag-env hybrid mRNA. Elimination of one of these elements, located within the

We have analyzed the action of the Rev and Tev proteins of human immunodeficiency virus type 1 (HIV-1) and of the Rex protein of human T-cell leukemia virus type I (HTLV-I) on a series of Rev-responsive element (RRE) mutants. The minimum continuous RRE region necessary and sufficient for Rev function was determined to be 204 nucleotides. Interestingly, this region was not sufficient for Tev or Rex function. These proteins require additional sequences, which may stabilize the structure of the RRE or may contain additional sequence-specific elements. Internal RRE deletions revealed that the targets for Rev and Rex can be separated, since mutants responding to Rev and not Rex and vice versa were identified. Tev was active on both types of mutants, suggesting that it has a more relaxed specificity than do both Rev and Rex proteins. Although Rev and Rex targets within the RRE appear to be distinct, the trans-dominant mutant RevBL prevents the RRE interaction with Rex. RevBL cannot inhibit the function of Rex on RRE deletions that lack the Rev-responsive portion. These results indicate the presence of distinct sites within the RRE for interaction with these proteins. The binding sites for the different proteins do not function independently and may interfere with one another. Mutations affecting the RRE may change the accessibility and binding characteristics of the different binding sites.

The Rex protein of human T-ceil leukemia virus type I (HTLV-I) was expressed in bacteria and partially purified. Rex was shown to bind in vitro specifically to an RNA sequence located in the 3' long terminal repeat of HTLV-I, named Rex-responsive element (RXRE). Rex also bound in vitro to the human immunodeficiency virus type 1 (HIV-1) Rev-responsive element (RRE), while purified HIV-1 Rev protein did not bind to the RXRE. The binding results obtained in vitro are therefore in agreement with the nonreciprocal function of Rev and Rex in vivo. Rex binds specifically to both RRE and RXRE and activates expression in both HIV-1 and HTLV-I, while Rev binds to RRE and activates only HIV-1. Binding of Rex to RRE deletion mutants previously shown to lack either the Rev-responsive or the Rex-responsive portion suggested preferential binding of Rex to a distinct target within the RRE. These results demonstrated that Rex, like Rev, acts by binding to a specific RNA target.Analysis of the genomic sequences and functional properties of lentiviruses, oncoretroviruses of the human T-cell leukemia virus (HTLV) family, and spumaviruses revealed that they express additional proteins, possess a complex splicing program, and, more importantly, regulate their own expression via viral factors. We have proposed the term complex retroviruses to distinguish these retroviruses from those that do not regulate their own expression (1, 2). HTLV and human immunodeficiency virus (HIV) are complex retroviruses that regulate expression of their structural proteins by specific viral factors called Rex and Rev, respectively (for recent reviews, see refs. 3-5). Both Rex and Rev are essential for viral replication.Rev acts via a unique sequence named the Rev-responsive element (RRE) (6, 7) or CAR (8), located in the env region of HIV (6-13). The RRE is present only in the full-length and intermediate-sized mRNAs, which include those encoding Gag, Gag-Pol, Vif, Vpr, and Env proteins (14,15). In contrast, due to splicing, the RRE is absent from the small multiply spliced mRNAs encoding regulatory proteins such as Tat, Rev, and Nef (16). Rev affects the transport of RRE-containing RNAs from the nucleus to the cytoplasm (6, 10-12, 17) and increases their half-lives (6). This results in higher levels of RRE-containing RNAs in the cytoplasm that are translated efficiently into structural viral proteins (6, 18) (D. M. Benko, B.K.F., J. Harrison, and G.N.P., unpublished data).The sequence necessary for Rex function is named the Rex-responsive element (RXRE) and is located in the 3' long terminal repeat of all members of the HTLV family (19-23); due to its location, RXRE is present in all viral mRNAs. The presence ofRex is essential for the accumulation of unspliced and singly spliced cytoplasmic viral mRNAs and for the production of the viral structural proteins (19,20,22,(24)(25)(26)(27). The expression of the doubly spliced mRNAs encoding Rex and Tax is independent of Rex; nevertheless, these mRNAs contain RXRE.The cis-acting RNA sequences RRE and...