The Steroid/thyroid Hormone Receptor Superfamily

中枢神经系统的多巴胺主要分布在黑质纹状体、中脑边缘和中皮质系统。黑质纹状体系统由分布于黑质致密部并支配纹状体的神经元细胞体组成。中边缘系统和中皮质系统分别由腹侧被盖区和边缘区域的神经元胞体组成，边缘区域包括伏隔核和皮质。多巴胺神经传递的改变与许多神经系统疾病有关，包括帕金森氏症、精神分裂症、注意力缺陷多动障碍和药物成瘾。正如我们和其他人所表明的那样，核孤儿受体Nurr1对于中脑神经元的末端分化是必不可少的。在小鼠中，通过同源重组破坏Nurr1基因，阻止发育中的多巴胺神经元前体，阻止多巴胺神经元特异性蛋白的表达，导致神经递质多巴胺合成的完全抑制。我们已经确定了一些表达受Nurr1影响的基因。研究表明，Nurr1可以作为特定基因表达的抑制因子和诱导因子。鸟苷三磷酸环水解酶(GTPCH)是四氢生物蝶呤(BH4)合成过程中的限速酶，是甲状腺羟基酶(TH)活性的重要组成部分。对野生型和Nurr1缺失小鼠腹侧标记区RNA的比较微阵列分析表明，Nurr1缺失幼鼠的GTPCH mRNA大幅减少，导致BH4含量随之减少。因此，GTPCH可能是多巴胺生物合成中第一种似乎调节前额叶皮质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。尽管Nurr1对中脑神经元的分化是必不可少的，但它在成熟的多巴胺神经元中的作用尚未确定。野生型和杂合型TH免疫反应神经元密度在两组间无显著差异。为了探讨Nurr1在体外培养的多巴胺神经元存活中的作用，我们从Nurr1杂合子和野生型仔鼠中分离培养了新生中皮质神经元。出生后培养显示，仅在培养1天时，来自Nurr1杂合子幼鼠的TH免疫反应阳性神经元的存活率显著降低。此外，神经突起的总长度比对照组要短。Nurr1杂合TH免疫反应神经元在体外的存活分化程度和突起生长减少，以及在体内观察到野生型和杂合型Nurr1 TH免疫反应神经元的强度和密度没有显著差异，表明在“环境伤害”(培养中分散和生长)和遗传易感性(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为帕金森氏症提出的方案，即遗传因素和环境毒素的组合加速多巴胺神经元死亡。我们还研究了神经母细胞瘤和神经胶质瘤细胞系中与cAMP诱导Nurr1相关的信号通路。腺苷环化酶激活剂Forskolin通过CREB激活诱导两种细胞中Nurr1的表达。然而，诱导Nurr1表达的信号通路似乎是细胞特异性的。最近，我们研究了Nurr1在成骨细胞分化中的作用。我们的结果表明，利用Nurr1 siRNA降低Nurr1的表达，会影响成骨细胞分化标志基因、骨钙素、1型胶原的表达和碱性磷酸酶的活性。此外，在来自Nurr1缺失小鼠的原代培养的小鼠颅骨成骨细胞中，这些基因的表达减少。因此，我们的结果表明Nurr1在成骨细胞分化中起重要作用。

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. 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. 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. 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. 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 difference 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. We have also investigated the signaling pathway related to induction of Nurr1 by cAMP in neuroblastoma and glioma cell lines. Nurr1 expression was induce by forskolin, an adenylate cyclase activator, via activation by CREB in both cell lines. However, the signaling pathway leading to induction of Nurr1 expression appears to be cell specific. Recently, we have studied the role of Nurr1 in osteoblast differentiation. Our results revealed that reduced Nurr1 expression, using Nurr1 siRNA, affected the expression of osteoblast differentiation marker genes; osteocalcin, type one collagen and the activity of alkaline phosphatase. In addition, the expression of those genes was decreased in primary cultured mouse calvarial osteoblasts derived from Nurr1 null mice. Thus, our results suggest that Nurr1 is important for osteoblast differentiation.