Instituto de Investigaciones en Ingeniería Genética y Biología Molecular, Consejo Nacional de Investigaciones Científicas y Técnicas, C1428ADN Buenos Aires, Argentina.
Proc Natl Acad Sci U S A. 2011 Sep 13;108(37):15270-5. doi: 10.1073/pnas.1104997108. Epub 2011 Aug 29.
The proopiomelanocortin gene (POMC) is expressed in a group of neurons present in the arcuate nucleus of the hypothalamus. Neuron-specific POMC expression in mammals is conveyed by two distal enhancers, named nPE1 and nPE2. Previous transgenic mouse studies showed that nPE1 and nPE2 independently drive reporter gene expression to POMC neurons. Here, we investigated the evolutionary mechanisms that shaped not one but two neuron-specific POMC enhancers and tested whether nPE1 and nPE2 drive identical or complementary spatiotemporal expression patterns. Sequence comparison among representative genomes of most vertebrate classes and mammalian orders showed that nPE1 is a placental novelty. Using in silico paleogenomics we found that nPE1 originated from the exaptation of a mammalian-apparent LTR retrotransposon sometime between the metatherian/eutherian split (147 Mya) and the placental mammal radiation (≈ 90 Mya). Thus, the evolutionary origin of nPE1 differs, in kind and time, from that previously demonstrated for nPE2, which was exapted from a CORE-short interspersed nucleotide element (SINE) retroposon before the origin of prototherians, 166 Mya. Transgenic mice expressing the fluorescent markers tomato and EGFP driven by nPE1 or nPE2, respectively, demonstrated coexpression of both reporter genes along the entire arcuate nucleus. The onset of reporter gene expression guided by nPE1 and nPE2 was also identical and coincidental with the onset of Pomc expression in the presumptive mouse diencephalon. Thus, the independent exaptation of two unrelated retroposons into functional analogs regulating neuronal POMC expression constitutes an authentic example of convergent molecular evolution of cell-specific enhancers.
阿黑皮素原基因（POMC）在位于下丘脑弓状核的一组神经元中表达。哺乳动物中神经元特异性的 POMC 表达由两个远端增强子（nPE1 和 nPE2）介导。先前的转基因小鼠研究表明，nPE1 和 nPE2 独立地驱动报告基因在 POMC 神经元中的表达。在这里，我们研究了塑造不仅仅一个而是两个神经元特异性 POMC 增强子的进化机制，并测试了 nPE1 和 nPE2 是否驱动相同或互补的时空表达模式。在大多数脊椎动物类群和哺乳动物目中的代表基因组之间的序列比较表明，nPE1 是胎盘类的新事物。通过计算机古基因组学，我们发现 nPE1 是从哺乳动物出现的 LTR 反转录转座子的适应性进化而来的，时间在有袋类/真兽类的分化（147 Mya）和胎盘哺乳动物辐射（≈90 Mya）之间。因此，nPE1 的进化起源在性质和时间上与之前证明的 nPE2 不同，nPE2 是从 CORE 短散在核苷酸元件（SINE）反转座子进化而来的，时间在原兽类起源之前，即 166 Mya。分别由 nPE1 和 nPE2 驱动的表达荧光标记番茄和 EGFP 的转基因小鼠显示，这两个报告基因沿着整个弓状核共表达。由 nPE1 和 nPE2 指导的报告基因表达的起始也相同，与假定的小鼠间脑的 Pomc 表达起始同时发生。因此，两个不相关的反转座子独立地适应为调节神经元 POMC 表达的功能类似物，构成了细胞特异性增强子的趋同分子进化的真实范例。
