A study of the molecular mechanism of selective synapse formation using Drosophila as a model system

本研究旨在以果蝇神经肌肉连接为模型系统，阐明选择性突触形成的分子机制。为此，我们采用错表达筛选来寻找在所有肌肉中异位表达会改变神经肌肉特异性的基因。将大约500个独立的uas插入系（GS系，Toba et al, Genetics, 1999）与24B-GAL4系杂交，筛选F1三龄幼虫在运动神经投射方面的非自主表型。虽然一些细胞系表现出细胞自主表型，其中肌肉分化明显受损，但一些细胞系在运动神经的引导和/或突触发生方面表现出特异性缺陷。后一种表型的负责转录本随后被RT-PCR克隆并测序。其中两条系位于netrin B和任性基因中，这两种基因先前已被证明在该系统中起轴突引导线索的作用，从而表明我们筛选的有效性。通过对其余细胞系的表征，我们成功地确定了在所有肌肉中都诱导了一种新的表达，通常支配肌肉12的运动神经元在邻近肌肉13上形成了异位突触。这些运动神经元的靶特异性在功能丧失突变体中也发生了改变。在胚胎发育过程中，在中枢神经系统和外周细胞的一个亚群中检测到fendmrna。这些结果表明，残端残端残端残端残端残端残端残端残端残端残端是参与神经肌肉特异性的新型轴突引导分子。

The aim of this research was to elucidate the molecular mechanism of selective synapse formation by using the Drosophila neuromuscular connectivity as a model system. For this end, we adopted misexpression screening to search for genes whose ectopic expression in all muscles alters neuromuscular specificity.Approximately 500 independent UAS-insertion lines (GS lines, Toba et al., Genetics, 1999) were crossed to 24B-GAL4 line and F1 third instar larvae were screened for non-autonomous phenotypes in motor nerve projection. While some of the lines showed cell autonomous phenotypes in which muscle differentiation is apparently impaired, several displayed specific defects in the guidance and/or synaptogenesis of motor nerves. Responsible transcripts for the latter phenotypes were then cloned by RT-PCR and were sequenced. Two of the lines were found to be in the netrin B and capricious genes, which had been previously shown to function as axon guidance cues in this system, thus indicating the effectiveness of our screening.By characterizing the remaining lines, we succeeded in identifying a novel expression was induced in all muscles, motoneurons that normally innervate muscle 12 formed ectopic synapses on a neighboring muscle 13. The target specificity of these motoneurons was also altered in the loss-of-function mutant of fend. During embryonic development, fend mRNA was detected in a subset of cells in the central nervoussystem and in the periphery. These results suggest that FEND is a novel axon guidance molecule involved in neuromuscular specificity.