The Steroid/thyroid Hormone Receptor Superfamily

中枢神经系统多巴胺主要分布于黑质纹状体、中边缘和中皮质系统。黑质纹状体系统是由位于黑质致密部并支配纹状体的神经元细胞体组成的。中边缘系统和中皮层系统由位于腹侧被盖区和支配边缘区的神经元胞体组成，分别包括伏隔核和皮层。多巴胺神经传递的改变与许多神经系统疾病有关，包括帕金森病、精神分裂症、注意缺陷多动障碍和药物成瘾。核孤儿受体Nurr1对中脑神经元的终末分化至关重要。在小鼠中，通过同源重组破坏Nurr1基因，阻止多巴胺神经元特异性蛋白的表达，从而完全抑制神经元递质多巴胺的合成，从而抑制多巴胺神经元前体的发育。最近，我们使用了DNA微阵列技术和来自野生型和Nurr1基因敲除小鼠腹侧被盖区中脑的探针。我们已经确定了一些表达受Nurr1影响的基因。这表明Nurr1可以作为特定基因表达的抑制因子和诱导剂。使用独立的方法（实时聚合酶链反应和原位杂交），我们证实了Nurr1靶基因表达的变化。鸟苷三磷酸环水解酶（GTPCH）是四氢生物蝶呤（BH4）合成的限速酶，BH4是甲状腺氨酸羟化酶（TH）活性的重要组成部分。BH4由GTP依次由GTPCH和另外两种酶合成。在大多数情况下，BH4处于亚饱和水平，因此BH4可用性的任何变化，无论是通过改变GTPCH活性还是表达水平，都会影响BH4，从而影响TH活性。通过对野生型和Nurr1-null小鼠腹侧标记区制备的rna进行比较微阵列分析，发现Nurr1-null幼鼠GTPCH mRNA大量减少，从而导致BH4含量减少。因此，GTPCH可能是多巴胺生物合成中第一个调节前额叶皮层和伏隔核神经传递的酶。此外，最近在精神分裂症患者中发现了Nurr1基因的突变。虽然Nurr1对中脑神经元的分化至关重要，但其在成熟多巴胺神经元中的作用尚未确定。野生和杂合th免疫反应神经元的密度在这两个基因型不同的组之间没有显著差异。这表明在体内，多巴胺神经元不受Nurr1基因型的影响。为了研究Nurr1在体外多巴胺神经元存活中的作用，我们从Nurr1杂合型和野生型仔鼠中培养出出生后的中皮质神经元。产后培养显示，仅Nurr1杂合幼崽的TH免疫反应神经元在培养1天后的存活率显著降低。此外，神经突的总长度也比对照组短。在体外实验中，Nurr1杂合TH免疫反应神经元的存活程度和神经突生长减少的差异，以及在体内观察到Nurr1野生基因型和杂合基因型中TH免疫反应神经元的强度和密度没有显著差异，表明在“环境侮辱”（在培养中分散和生长）和遗传易感（丢失一个Nurr1等位基因）下，Nurr1杂合神经元处于显著劣势。这种情况可能类似于帕金森氏症，遗传因素和环境毒素的结合加速了多巴胺神经元的死亡。

The majority of central nervous system dopamine is located in the nigrostriatal, mesolimbic and mesocortical systems. The nigrostriatal system is comprised of neuron cell bodies located in the substantia nigra pars compacta and innervating the striatum. The mesolimbic and mesocortical system consists of neuron cell bodies in the ventral tegmental area and innervating the limbic areas, including the nucleus accumbens and the cortex, respectively. Alterations in dopamine neurotransmission have been implicated in a number of neurological conditions including Parkinson's disease, schizophrenia, attention deficit hyperactivity disorder, and drug addiction. The nuclear orphan receptor Nurr1 is essential for the terminal differentiation of midbrain neurons as we and others showed. The arrest of dopamine neuron precursors in development, by disruption of the Nurr1 gene by homologous recombination in mice, prevents expression of dopamine neuron specific proteins leading to the complete inhibition of neuron transmitter dopamine synthesis. Recently, we have used DNA microarray technology and probes derived from the midbrain of the ventral tegmental area of wild type and Nurr1 knockout mice. We have identified some genes whose expression is affected by Nurr1. It appears that Nurr1 can function as a repressor and inducer of the expression of specific genes. Using independent methods (real time polymerase chain reaction and in situ hybridization), we confirmed changes in the expression of Nurr1 target genes. Guanosine triphosphate cyclohydrolase (GTPCH) is the rate-limiting enzyme in the synthesis of tetra-hydrobiopterin (BH4) which is a very important component of thyrosine hydroxyase (TH) activity. BH4 is synthesized from GTP sequentially by GTPCH and two other enzymes. In most situations, BH4 is at a subsaturating level so that any changes in BH4 availability, either by alteration in GTPCH activity or expression level, affect BH4 and consequently TH activity. Using comparative microarray analysis of RNAs from wild type and Nurr1-null mice prepared from the ventral tagmental area has shown a large decrease in GTPCH mRNA in Nurr1 null pups, which led to concomitant reduction in BH4 content. Thus, GTPCH would be the first enzyme in dopamine biosynthesis that appear to regulate neurotransmission in the prefrontal cortex and nucleus accumbens. Furthermore, mutations in the Nurr1 gene have been recently identified in schizophrenic patients. Although Nurr1 is essential for differentiation of mesencephalic neurons, its role in mature dopamine neurons has not been determined. No significant difference in the density of wild and heterozygous TH-immunoreactive neurons was found between these two genotypically different groups. This indicates that in vivo, dopamine neurons are not affected by the Nurr1 genotype. In order to investigate the role of Nurr1 in survival of dopamine neurons in vitro postnatal mesocortical neurons were cultured from Nurr1 heterozygous and wild type littermates. Postnatal cultures revealed a significant reduction only in the survival of TH immunoreactive neurons derived from Nurr1 heterozygous pups as early as 1 day in culture. Furthermore, the total length of neurites was less than that seen in the controls. The differential degree of survival and reduced neurite growth of the Nurr1 heterozygous TH immunoreactive neurons in vitro and the observation that in vivo there is no substantial differnce in the intensity and density of TH immunoreactive neurons in the Nurr1 wild and heterozygous genotypes, demonstrate that under "environmental insult" (dispersion and growth in culture) and genetic susceptibility (loss of one Nurr1 allele) the Nurr1 heterozygous neurons are at significant disadvantage. This scenario might be similar to that proposed for Parkinson's disorder when a combination of genetic factors and environmental toxins accelerate dopamine neuron death.