Hendricks Michael, Jesuthasan Suresh
Temasek Life Sciences Laboratory, The National University of Singapore, Singapore 117604.
J Neurosci. 2009 May 20;29(20):6593-8. doi: 10.1523/JNEUROSCI.1115-09.2009.
Axonal growth cones use intermediate targets to navigate in the developing nervous system. Encountering these sites is correlated with growth cone pausing. PHR (Phr1, Esrom, Highwire, RPM-1) is a large neuronal ubiquitin ligase that interacts with multiple signaling pathways. Mouse and zebrafish phr mutants have highly penetrant axon pathfinding defects at intermediate targets. Mouse phr mutants contain excessive microtubules in the growth cone, which has been attributed to upregulation of DLK/p38 signaling. Here, we ask whether this pathway and microtubule misregulation are indeed linked to guidance errors in the vertebrate brain, using the zebrafish. By live imaging, we show that loops form when microtubules retract without depolymerizing. JNK, but not p38, phosphorylation is increased in mutant growth cones. However microtubule looping cannot be suppressed by inhibiting JNK. The phr microtubule defect can be phenocopied by taxol, while microtubule destabilization in vitro using nocodazole prevents loop formation. Acute disruption in vivo with nocodazole suppresses the intermediate target guidance defect. Given that microtubule looping is associated with growth cone pausing, we propose that microtubule disassembly, mediated by PHR, is essential for exiting the paused state at intermediate targets.
轴突生长锥利用中间靶点在发育中的神经系统中导航。遇到这些位点与生长锥暂停相关。PHR（Phr1、Esrom、Highwire、RPM-1）是一种大型神经元泛素连接酶，它与多种信号通路相互作用。小鼠和斑马鱼的phr突变体在中间靶点处有高度显性的轴突导向缺陷。小鼠phr突变体的生长锥中含有过多的微管，这被归因于DLK/p38信号通路的上调。在这里，我们利用斑马鱼来探究这条信号通路和微管调控异常是否确实与脊椎动物大脑中的导向错误有关。通过实时成像，我们发现当微管收缩而不解聚时会形成环。在突变体生长锥中，JNK的磷酸化增加，而p38的磷酸化没有增加。然而，抑制JNK并不能抑制微管环的形成。紫杉醇可以模拟phr的微管缺陷，而使用诺考达唑在体外使微管不稳定则可防止环的形成。在体内用诺考达唑进行急性破坏可抑制中间靶点导向缺陷。鉴于微管环的形成与生长锥暂停相关，我们提出由PHR介导的微管解聚对于在中间靶点处退出暂停状态至关重要。
