Gene Regulatory Mechanisms Underlying the Spatial and Temporal Regulation of Target-Dependent Gene Expression in Drosophila Neurons.
Gene Regulatory Mechanisms Underlying the Spatial and Temporal Regulation of Target-Dependent Gene Expression in Drosophila Neurons.
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DOI：
10.1371/journal.pgen.1005754
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发表时间：
2015-12
期刊：
影响因子：
4.5
通讯作者：
Allan DW
中科院分区：
文献类型：
--
作者：
Berndt AJ;Tang JC;Ridyard MS;Lian T;Keatings K;Allan DW
神经元分化通常需要来自它们所支配的细胞的靶源信号。这些信号通常会激活神经亚型特异性基因，但基因调控机制在很大程度上仍然未知。FMRFa神经肽在果蝇Tv 4神经元中的高度限制性表达需要靶源性BMP信号传导和包括Apterous的转录因子编码。使用整合酶转基因的增强子报告，我们功能性地解剖了FMRFa的Tv 4增强子在其天然细胞环境中。我们确定了两个基本但不连续的顺式元件，一个是结合BMP激活的pMad的BMP反应元件（BMP-RE），另一个是结合Apterous的同源结构域反应元件（HD-RE）。这些顺式元件具有低活性，并且必须组合以获得Tv 4增强子活性。这种组合活动往往是一种机制，限制表达的交叉顺式元件时空活动。然而，发现HD-RE和BMP-RE顺式元件的多联体独立地产生与Tv 4增强子相同的时空表达。因此，Tv 4-增强子将两个低活性顺式元件串联组合，这两个元件从不同的输入中获得相同的输出。靶依赖性基因的激活被假定为“等待”靶接触。我们直接对此进行了测试，并且意外地发现过早的BMP活性不能诱导早期FMRFa表达;此外，我们表明BMP不敏感的HD-RE顺式元件在靶接触时被激活。这使我们揭示了核受体seven up（svp）在靶接触之前作为FMRFa诱导的阻遏物的作用。Svp通常在靶接触前立即下调，我们发现维持Svp表达可防止顺式元件激活，而减少svp基因剂量可过早激活顺式元件活性。我们的结论是，目标依赖性FMRFa基因被抑制之前，目标接触，和目标衍生的BMP信号直接激活FMRFa基因的表达，通过一个非典型的基因调控机制。神经细胞延伸出长长的突起，这些突起生长出来与它们交流的靶细胞接触。当神经细胞进行初始接触时，靶细胞将逆行信号发送回神经细胞。这种靶源信号激活并维持使神经细胞发挥功能的重要基因，例如决定神经递质类型的基因。这是一个在从果蝇到哺乳动物的神经系统中都存在的典型现象，但我们仍然不知道这些信号是如何激活基因表达的。我们现在提供有关神经细胞基因的靶依赖性信号调节的细节。我们在果蝇的Tv 4神经元中对此进行建模，该神经元需要靶源性BMP信号传导来触发FMRFa神经肽表达。我们的研究显示了BMP信号传导途径的DNA结合转录因子如何在特定的调控DNA序列处与其他转录因子整合以激活FMRFa表达，并定义了发生这种情况的非典型逻辑。我们还提供了新的见解如何靶依赖性基因的调节目标接触之前。而不是简单地等待靶依赖性激活，这些基因似乎被阻止在靶接触之前表达。这些发现与哺乳动物有关，因为靶源性BMP信号在神经细胞基因调控中的作用在脊椎动物和无脊椎动物之间是保守的。
Neuronal differentiation often requires target-derived signals from the cells they innervate. These signals typically activate neural subtype-specific genes, but the gene regulatory mechanisms remain largely unknown. Highly restricted expression of the FMRFa neuropeptide in Drosophila Tv4 neurons requires target-derived BMP signaling and a transcription factor code that includes Apterous. Using integrase transgenesis of enhancer reporters, we functionally dissected the Tv4-enhancer of FMRFa within its native cellular context. We identified two essential but discrete cis-elements, a BMP-response element (BMP-RE) that binds BMP-activated pMad, and a homeodomain-response element (HD-RE) that binds Apterous. These cis-elements have low activity and must be combined for Tv4-enhancer activity. Such combinatorial activity is often a mechanism for restricting expression to the intersection of cis-element spatiotemporal activities. However, concatemers of the HD-RE and BMP-RE cis-elements were found to independently generate the same spatiotemporal expression as the Tv4-enhancer. Thus, the Tv4-enhancer atypically combines two low-activity cis-elements that confer the same output from distinct inputs. The activation of target-dependent genes is assumed to 'wait' for target contact. We tested this directly, and unexpectedly found that premature BMP activity could not induce early FMRFa expression; also, we show that the BMP-insensitive HD-RE cis-element is activated at the time of target contact. This led us to uncover a role for the nuclear receptor, seven up (svp), as a repressor of FMRFa induction prior to target contact. Svp is normally downregulated immediately prior to target contact, and we found that maintaining Svp expression prevents cis-element activation, whereas reducing svp gene dosage prematurely activates cis-element activity. We conclude that the target-dependent FMRFa gene is repressed prior to target contact, and that target-derived BMP signaling directly activates FMRFa gene expression through an atypical gene regulatory mechanism. Nerve cells extend long processes that grow out to contact the target cells with which they communicate. When the nerve cell makes initial contact, the target cells send a retrograde signal back to the nerve cell. Such target-derived signals activate and maintain important genes that make the nerve cell functional, such as genes determining neurotransmitter type. This is a well-characterized phenomenon throughout the nervous systems of flies to mammals, but we still do not know how these signals actually activate gene expression. We now provide details regarding target-dependent signal regulation of nerve cell genes. We model this in Tv4 neurons of Drosophila melanogaster, which require target-derived BMP signaling to trigger FMRFa neuropeptide expression. Our study shows how DNA-binding transcription factors of the BMP signaling pathway integrate with other transcription factors at specific regulatory DNA sequences to activate FMRFa expression, and define the atypical logic by which this occurs. We also provide novel insight into how target-dependent genes are regulated before target contact. Instead of simply waiting for target-dependent activation, these genes seem to be blocked from being expressed prior to target contact. These findings have relevance to mammals because the role of target-derived BMP signaling in nerve cell gene regulation is conserved between vertebrates and invertebrates.