Ito Yuri, Oinuma Izumi, Katoh Hironori, Kaibuchi Kozo, Negishi Manabu
Laboratory of Molecular Neurobiology, Graduate School of Biostudies, Kyoto University, Kyoto 606-8501, Japan.
EMBO Rep. 2006 Jul;7(7):704-9. doi: 10.1038/sj.embor.7400737. Epub 2006 Jun 16.
Plexins are receptors for the axonal guidance molecules known as semaphorins, and the semaphorin 4D (Sema4D) receptor plexin-B1 induces repulsive responses by functioning as an R-Ras GTPase-activating protein (GAP). Here we characterized the downstream signalling of plexin-B1-mediated R-Ras GAP activity, inducing growth cone collapse. Sema4D suppressed R-Ras activity in hippocampal neurons, in parallel with dephosphorylation of Akt and activation of glycogen synthase kinase (GSK)-3beta. Ectopic expression of the constitutively active mutant of Akt or treatment with GSK-3 inhibitors suppressed the Sema4D-induced growth cone collapse. Constitutive activation of phosphatidylinositol-3-OH kinase (PI(3)K), an upstream kinase of Akt and GSK-3beta, also blocked the growth cone collapse. The R-Ras GAP activity was necessary for plexin-B1-induced dephosphorylation of Akt and activation of GSK-3beta and was also required for phosphorylation of a downstream kinase of GSK-3beta, collapsin response mediator protein-2. Plexin-A1 also induced dephosphorylation of Akt and GSK-3beta through its R-Ras GAP activity. We conclude that plexin-B1 inactivates PI(3)K and dephosphorylates Akt and GSK-3beta through R-Ras GAP activity, inducing growth cone collapse.
丛状蛋白是轴突导向分子（即信号素）的受体，信号素4D（Sema4D）受体丛状蛋白-B1作为一种R-Ras GTP酶激活蛋白（GAP）发挥作用，从而诱导排斥反应。在此，我们对丛状蛋白-B1介导的R-Ras GAP活性的下游信号进行了表征，该活性可诱导生长锥塌陷。Sema4D抑制海马神经元中的R-Ras活性，同时伴随着Akt的去磷酸化和糖原合酶激酶（GSK）-3β的激活。Akt组成型活性突变体的异位表达或用GSK-3抑制剂处理可抑制Sema4D诱导的生长锥塌陷。磷脂酰肌醇-3-OH激酶（PI(3)K）（Akt和GSK-3β的上游激酶）的组成型激活也可阻止生长锥塌陷。R-Ras GAP活性对于丛状蛋白-B1诱导的Akt去磷酸化和GSK-3β激活是必需的，对于GSK-3β的下游激酶——塌陷反应介导蛋白-2的磷酸化也是必需的。丛状蛋白-A1也通过其R-Ras GAP活性诱导Akt和GSK-3β的去磷酸化。我们得出结论，丛状蛋白-B1通过R-Ras GAP活性使PI(3)K失活，并使Akt和GSK-3β去磷酸化，从而诱导生长锥塌陷。
