Chiba A, Hing H, Cash S, Keshishian H
Department of Biology, Yale University, New Haven, Connecticut 06511.
J Neurosci. 1993 Feb;13(2):714-32. doi: 10.1523/JNEUROSCI.13-02-00714.1993.
In Drosophila embryos, each motoneuron is accurately matched to one or more singly identifiable muscle fibers. In this article we altered the number and pattern of the embryonic muscle fibers using genetic, heat shock, and laser ablation methods to test whether motoneuron growth cones are able to recognize specific targets. The choices made by two motoneurons were assayed using both intracellular dye fills and immunocytochemistry. The motoneurons RP1 and RP3 have nearly identical central and peripheral axonal trajectories. However, RP3 innervates the two most ventral longitudinal muscle fibers, 7 and 6, while RP1 grows past these fibers to innervate only muscle fiber 13. In rhomboid mutants muscle fiber 7 does not develop. Despite the loss of one of its targets, RP3 faithfully innervated the remaining muscle fiber 6 in over 80% of the observed cases. Furthermore, neuron RP1 accurately innervated muscle fiber 13, although it traversed one fiber fewer to reach it. Laser ablation of muscle fiber 7 confirmed the target choices shown by the motoneurons. In numb mutants, multiple muscle fibers, including 7, 13, and 12, fail to develop. This allowed us to test whether fibers distal to the target are involved in muscle fiber recognition, possibly by halting the growth cone advance. In mutant embryos, RP3 innervated muscle fiber 6 at the same frequency regardless of the absence of the distal muscle fiber 13. By contrast, RP1, which had lost its target entirely, frequently failed to innervate any muscle fiber during the period examined. Finally, muscle fiber 13 can be duplicated in wild-type embryos by means of a brief heat pulse during myogenesis. Presented with two targets, RP1 innervated both fibers in each case examined, while RP3 synapsed with muscle fibers 7 and 6 normally. Neuron-specific antibodies revealed that the embryonic growth cone choices were not transient, but persisted into the larval neuromuscular projections. These results indicate that each motoneuron growth cone has a primary target preference, which is retained even when the numbers of the muscle fibers, and therefore their relative positions, are altered. We therefore suggest that synaptic recognition by Drosophila motoneuron growth cones relies on unique features of the individual muscle fibers.
在果蝇胚胎中，每个运动神经元都精确地与一根或多根可单独识别的肌纤维相匹配。在本文中，我们使用基因、热休克和激光消融方法改变胚胎肌纤维的数量和模式，以测试运动神经元生长锥是否能够识别特定靶点。使用细胞内染料填充和免疫细胞化学方法检测了两个运动神经元的选择。运动神经元RP1和RP3具有几乎相同的中枢和外周轴突轨迹。然而，RP3支配两条最腹侧的纵向肌纤维，即7号和6号纤维，而RP1越过这些纤维，仅支配13号肌纤维。在菱形突变体中，7号肌纤维不发育。尽管失去了一个靶点，但在超过80%的观察案例中，RP3仍忠实地支配了剩余的6号肌纤维。此外，神经元RP1准确地支配了13号肌纤维，尽管它到达该纤维时少穿过了一根纤维。激光消融7号肌纤维证实了运动神经元所显示的靶点选择。在麻木突变体中，包括7号、13号和12号在内的多根肌纤维未能发育。这使我们能够测试靶点远端的纤维是否参与肌纤维识别，可能是通过阻止生长锥前进来实现。在突变胚胎中，无论远端13号肌纤维是否缺失，RP3支配6号肌纤维的频率相同。相比之下，完全失去靶点的RP1在检查期间经常无法支配任何肌纤维。最后，在野生型胚胎中，通过在肌生成过程中进行短暂的热脉冲，可以使13号肌纤维加倍。面对两个靶点，在每个检查的案例中，RP1都支配了两根纤维，而RP3则正常地与7号和6号肌纤维形成突触。神经元特异性抗体显示，胚胎生长锥的选择并非短暂的，而是持续到幼虫的神经肌肉投射阶段。这些结果表明，每个运动神经元生长锥都有一个主要的靶点偏好，即使肌纤维数量及其相对位置发生改变，这种偏好仍会保留。因此，我们认为果蝇运动神经元生长锥的突触识别依赖于单个肌纤维的独特特征。
