Department of Developmental Biology, Hope Center for Neurological Disorders, and Department of Genetics, Washington University School of Medicine, St. Louis, Missouri 63110.
J Neurosci. 2013 Nov 6;33(45):17863-73. doi: 10.1523/JNEUROSCI.2196-13.2013.
The MAPKKK dual leucine zipper-containing kinase (DLK, Wallenda in Drosophila) is an evolutionarily conserved component of the axonal injury response pathway. After nerve injury, DLK promotes degeneration of distal axons and regeneration of proximal axons. This dual role in coordinating degeneration and regeneration suggests that DLK may be a sensor of axon injury, and so understanding how DLK is activated is important. Two mechanisms are known to activate DLK. First, increasing the levels of DLK via overexpression or loss of the PHR ubiquitin ligases that target DLK activate DLK signaling. Second, in Caenorhabditis elegans, a calcium-dependent mechanism, can activate DLK. Here we describe a new mechanism that activates DLK in Drosophila: loss of the spectraplakin short stop (shot). In a genetic screen for mutants with defective neuromuscular junction development, we identify a hypomorphic allele of shot that displays synaptic terminal overgrowth and a precocious regenerative response to nerve injury. We demonstrate that both phenotypes are the result of overactivation of the DLK signaling pathway. We further show that, unlike mutations in the PHR ligase Highwire, loss of function of shot activates DLK without a concomitant increase in the levels of DLK. As a spectraplakin, Shot binds to both actin and microtubules and promotes cytoskeletal stability. The DLK pathway is also activated by downregulation of the TCP1 chaperonin complex, whose normal function is to promote cytoskeletal stability. These findings support the model that DLK is activated by cytoskeletal instability, which is a shared feature of both spectraplakin mutants and injured axons.
MAPKKK 双亮氨酸拉链激酶（DLK，果蝇中的 Wallenda）是轴突损伤反应途径中进化保守的组成部分。在神经损伤后，DLK 促进远端轴突的退化和近端轴突的再生。这种在协调退化和再生方面的双重作用表明，DLK 可能是轴突损伤的传感器，因此了解 DLK 如何被激活是很重要的。有两种机制已知可以激活 DLK。首先，通过过表达或失去靶向 DLK 的 PHR 泛素连接酶来增加 DLK 的水平，从而激活 DLK 信号。其次，在秀丽隐杆线虫中，一种钙依赖性机制可以激活 DLK。在这里，我们描述了一种在果蝇中激活 DLK 的新机制：缺失 spectrins 斑疹短停（shot）。在一个用于研究神经肌肉接头发育缺陷的突变体的遗传筛选中，我们发现了 shot 的一个功能减弱等位基因，其表现为突触末端过度生长和对神经损伤的早熟再生反应。我们证明了这两种表型都是由于 DLK 信号通路的过度激活。我们进一步表明，与 PHR 连接酶 Highwire 的突变不同，shot 的功能丧失会激活 DLK，而不会同时增加 DLK 的水平。作为一种 spectrins，Shot 与肌动蛋白和微管结合，并促进细胞骨架的稳定性。DLK 途径也被 TCP1 伴侣蛋白复合物的下调激活，其正常功能是促进细胞骨架的稳定性。这些发现支持了这样一种模型，即 DLK 被细胞骨架的不稳定性激活，这是 spectrins 突变体和受伤轴突的共同特征。
