Simionato Elena, Kerner Pierre, Dray Nicolas, Le Gouar Martine, Ledent Valérie, Arendt Detlev, Vervoort Michel
Evolution et Développement des métazoaires, Centre de Génétique Moléculaire-UPR 2167 CNRS, 1, av. de terrasse, 91198 Gif-sur-Yvette, France.
BMC Evol Biol. 2008 Jun 9;8:170. doi: 10.1186/1471-2148-8-170.
Functional studies in model organisms, such as vertebrates and Drosophila, have shown that basic Helix-loop-Helix (bHLH) proteins have important roles in different steps of neurogenesis, from the acquisition of neural fate to the differentiation into specific neural cell types. However, these studies highlighted many differences in the expression and function of orthologous bHLH proteins during neural development between vertebrates and Drosophila. To understand how the functions of neural bHLH genes have evolved among bilaterians, we have performed a detailed study of bHLH genes during nervous system development in the polychaete annelid, Platynereis dumerilii, an organism which is evolutionary distant from both Drosophila and vertebrates.
We have studied Platynereis orthologs of the most important vertebrate neural bHLH genes, i.e. achaete-scute, neurogenin, atonal, olig, and NeuroD genes, the latter two being genes absent of the Drosophila genome. We observed that all these genes have specific expression patterns during nervous system formation in Platynereis. Our data suggest that in Platynereis, like in vertebrates but unlike Drosophila, (i) neurogenin is the main proneural gene for the formation of the trunk central nervous system, (ii) achaete-scute and olig genes are involved in neural subtype specification in the central nervous system, in particular in the specification of the serotonergic phenotype. In addition, we found that the Platynereis NeuroD gene has a broad and early neuroectodermal expression, which is completely different from the neuronal expression of vertebrate NeuroD genes.
Our analysis suggests that the Platynereis bHLH genes have both proneural and neuronal specification functions, in a way more akin to the vertebrate situation than to that of Drosophila. We conclude that these features are ancestral to bilaterians and have been conserved in the vertebrates and annelids lineages, but have diverged in the evolutionary lineage leading to Drosophila.
在诸如脊椎动物和果蝇等模式生物中的功能研究表明，碱性螺旋-环-螺旋（bHLH）蛋白在神经发生的不同阶段发挥着重要作用，从获得神经命运到分化为特定的神经细胞类型。然而，这些研究突出了脊椎动物和果蝇在神经发育过程中直系同源bHLH蛋白的表达和功能存在许多差异。为了了解神经bHLH基因的功能在两侧对称动物中是如何进化的，我们对多毛纲环节动物杜氏阔沙蚕神经系统发育过程中的bHLH基因进行了详细研究，杜氏阔沙蚕是一种在进化上与果蝇和脊椎动物都相距甚远的生物。
我们研究了最重要的脊椎动物神经bHLH基因的杜氏阔沙蚕直系同源基因，即无刚毛-缺刻基因、神经生成素、无调基因、少突细胞转录因子基因和神经分化因子基因，后两个基因在果蝇基因组中不存在。我们观察到所有这些基因在杜氏阔沙蚕神经系统形成过程中都有特定的表达模式。我们的数据表明，在杜氏阔沙蚕中，与脊椎动物一样但与果蝇不同的是，（i）神经生成素是躯干中枢神经系统形成的主要神经源性基因，（ii）无刚毛-缺刻基因和少突细胞转录因子基因参与中枢神经系统中神经亚型的特化，特别是5-羟色胺能表型的特化。此外，我们发现杜氏阔沙蚕神经分化因子基因具有广泛且早期的神经外胚层表达，这与脊椎动物神经分化因子基因的神经元表达完全不同。
我们的分析表明，杜氏阔沙蚕bHLH基因具有神经源性和神经元特化功能，其方式与脊椎动物的情况比与果蝇的情况更为相似。我们得出结论，这些特征是两侧对称动物的祖先特征，在脊椎动物和环节动物谱系中得以保留，但在导致果蝇的进化谱系中发生了分歧。
