Minocha Shilpi, Boll Werner, Noll Markus
Institute of Molecular Life Sciences, University of Zürich, Zürich, Switzerland.
PLoS One. 2017 Apr 25;12(4):e0176002. doi: 10.1371/journal.pone.0176002. eCollection 2017.
The paired box gene Pox neuro (Poxn) is expressed in two bilaterally symmetric neuronal clusters of the developing adult Drosophila brain, a protocerebral dorsal cluster (DC) and a deutocerebral ventral cluster (VC). We show that all cells that express Poxn in the developing brain are postmitotic neurons. During embryogenesis, the DC and VC consist of only 20 and 12 neurons that express Poxn, designated embryonic Poxn-neurons. The number of Poxn-neurons increases only during the third larval instar, when the DC and VC increase dramatically to about 242 and 109 Poxn-neurons, respectively, virtually all of which survive to the adult stage, while no new Poxn-neurons are added during metamorphosis. Although the vast majority of Poxn-neurons express Poxn only during third instar, about half of them are born by the end of embryogenesis, as demonstrated by the absence of BrdU incorporation during larval stages. At late third instar, embryonic Poxn-neurons, which begin to express Poxn during embryogenesis, can be easily distinguished from embryonic-born and larval-born Poxn-neurons, which begin to express Poxn only during third instar, (i) by the absence of Pros, (ii) their overt differentiation of axons and neurites, and (iii) the strikingly larger diameter of their cell bodies still apparent in the adult brain. The embryonic Poxn-neurons are primary neurons that lay out the pioneering tracts for the secondary Poxn-neurons, which differentiate projections and axons that follow those of the primary neurons during metamorphosis. The DC and the VC participate only in two neuropils of the adult brain. The DC forms most, if not all, of the neurons that connect the bulb (lateral triangle) with the ellipsoid body, a prominent neuropil of the central complex, while the VC forms most of the ventral projection neurons of the antennal lobe, which connect it ipsilaterally to the lateral horn, bypassing the mushroom bodies. In addition, Poxn-neurons of the VC are ventral local interneurons of the antennal lobe. In the absence of Poxn protein in the developing brain, embryonic Poxn-neurons stall their projections and cannot find their proper target neuropils, the bulb and ellipsoid body in the case of the DC, or the antennal lobe and lateral horn in the case of the VC, whereby the absence of the ellipsoid body neuropil is particularly striking. Poxn is thus crucial for pathfinding both in the DC and VC. Additional implications of our results are discussed.
配对盒基因Pox神经（Poxn）在发育中的成年果蝇大脑的两个双侧对称神经元簇中表达，即原脑背侧簇（DC）和中脑腹侧簇（VC）。我们发现，发育中的大脑中所有表达Poxn的细胞都是有丝分裂后的神经元。在胚胎发育过程中，DC和VC仅由20个和12个表达Poxn的神经元组成，称为胚胎Poxn神经元。Poxn神经元的数量仅在第三龄幼虫期增加，此时DC和VC分别急剧增加到约242个和109个Poxn神经元，几乎所有这些神经元都存活到成年期，而在变态过程中没有添加新的Poxn神经元。尽管绝大多数Poxn神经元仅在第三龄期表达Poxn，但其中约一半在胚胎发育结束时就已产生，这一点可通过幼虫期不存在BrdU掺入来证明。在第三龄后期，在胚胎发育期间开始表达Poxn的胚胎Poxn神经元，可通过以下方式与仅在第三龄期开始表达Poxn的胚胎期产生和幼虫期产生的Poxn神经元轻松区分开来：（i）不存在Pros，（ii）它们的轴突和神经突明显分化，以及（iii）它们的细胞体直径在成体大脑中仍然明显更大。胚胎Poxn神经元是初级神经元，为次级Poxn神经元铺设先驱通路，次级Poxn神经元在变态过程中分化出跟随初级神经元的投射和轴突。DC和VC仅参与成年大脑的两个神经纤维网。DC形成了连接球状体（侧三角）与椭球体（中央复合体的一个突出神经纤维网）的大多数（如果不是全部）神经元，而VC形成了触角叶的大多数腹侧投射神经元，这些神经元绕过蘑菇体，同侧连接触角叶和侧角。此外，VC的Poxn神经元是触角叶的腹侧局部中间神经元。在发育中的大脑中缺乏Poxn蛋白的情况下，胚胎Poxn神经元的投射停滞，无法找到其合适的目标神经纤维网，对于DC来说是球状体和椭球体，对于VC来说是触角叶和侧角，因此椭球体神经纤维网的缺失尤为明显。因此，Poxn对于DC和VC中的路径寻找至关重要。我们还讨论了研究结果的其他意义。
