The Steroid/thyroid Hormone Receptor Superfamily

1.多巴胺神经传递的改变与许多神经疾病有关，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。似乎Nurr1既可以作为特定基因表达的抑制因子，也可以作为特定基因表达的诱导因子。通过对野生型和Nurr1缺失小鼠的RNA进行比较微阵列分析，从腹侧被盖区制备的RNA显示，Nurr1缺失幼鼠的鸟苷三磷酸环水解酶(GTPCH)mRNA大幅下降，从而导致BH4含量的降低。基因芯片分析显示，在12.5天大的Nurr1基因缺失的胚胎和新生儿的腹侧被盖区，GTPCH的表达降低了70%。虽然野生型小鼠的GTPCH mRNA水平在胚胎12.5至出生期间显著升高，但在未出生的小鼠中未见这种变化。启动子缺失分析表明，Nurr1在没有NGF1-B反应元件的情况下激活GTPCH转录。 2.我们还决定研究Nurr1是否参与了其他类型细胞的分化。由于全球肥胖症的流行，了解体重调节机制是一个非常重要的目标。最近，我们发现，当以正常或高脂肪饮食喂养而食物摄入量没有显著差异时，Nurr1基因缺失杂合子小鼠的脂肪质量是野生型小鼠的两倍。通过激素诱导，由Nurr1基因缺失的小鼠产生的小鼠胚胎成纤维细胞(MEF)显示出更高的脂肪细胞分化率，根据油滴积累的估计，与野生型小鼠制备的MEF相比。利用逆转录病毒感染MEF-Nurr1缺失型，当Nurr1结构性过表达时，成脂率与观察到的MEF野生型相当。与这些结果一致的是，成脂C/EBPα和PPAR-γ的分子标志物水平在没有Nurr1的情况下分别增加，在Nurr1存在的情况下分别降低。这些结果表明，Nurr1在体内和体外都是脂肪生成的抑制因子，可能在与肥胖相关的脂肪生成分化中发挥作用。我们最近的研究结果表明，在脂肪形成的早期阶段，Nurr1的mRNA水平升高。因此，Nurr1反过来可以调节已知的成脂抑制因子的表达。据报道，前脂肪细胞因子1(Pref-1)在前脂肪细胞中高表达，而在分化后的脂肪细胞中几乎不表达。我们发现，在成脂过程中，在Nurr1过表达的情况下，Pref-1的水平显著升高，可能导致脂肪细胞分化的抑制。

1. 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 either as a repressor or inducer of the expression of specific genes. Using comparative microarray analysis of RNAs from wild type and Nurr1-null mice prepared from the ventral tegmental area has shown a large decrease in guanosine triphosphate cyclohydrolase (GTPCH) mRNA in Nurr1-null pups, which led to concomitant reduction in BH4 content. Microarray analysis showed 70% reduction in GTPCH expression in the ventral tegmental area of both 12.5-day old Nurr1-null embryos and neonates. Although levels of GTPCH mRNA increased significantly between E12.5 and birth in wild type mice, no such change was seen in the null neonates. The promoter deletional analyses revealed that Nurr1 activates GTPCH transcription in the absence of NGF1-B response element. 2. We have also decided to investigate whether Nurr1 is involved in differentiation of other cell types. Understanding mechanisms involved in regulation of body weight is a very important goal due to the worldwide obesity epidemic. Recently, we discovered that Nurr1-null heterozygous mice have twice as much fat mass when compared to wild type mice when fed with either normal or high fat diet without significant differences in food intake. By hormonal induction, mouse embryonic fibroblasts (MEF) generated from Nurr1-null mice have shown increased rate of adipocyte differentiation as estimated by accumulation of oil droplets in contrast to MEF prepared from wild type littermates. When Nurr1 was constitutively overexpressed, using retroviral infection in MEF-Nurr1-null, the rate of adipogenesis was comparable to that observed in MEF wild type. In agreement with these results, the level of molecular markers of adipogenesis C/EBP alpha and PPAR gamma was increased in the absence and decreased in the presence of Nurr1, respectively. These results suggest that Nurr1 functions as a repressor of adipogenesis in vivo and in vitro and might play a role in adipogenesis differentiation linked to obesity. Our recent results indicate that the level of Nurr1 mRNA in early stages of adipogenesis is increased. Thus, that in turn, Nurr1 can regulate expression of known repressors of adipogensis. It has been reported that the preadipocyte factor 1, (Pref-1), is highly expressed in preadipocytes and is nearly absent in differentiated adipocytes. We have found that during adipogenesis, in the presence of Nurr1 overexpression, the level of Pref-1 is substantially elevated, possibly leading to inhibition of adipocyte differentiation.