Medical Research Council Centre for Developmental Neurobiology, King's College London, London SE1 1UL, United Kingdom.
J Neurosci. 2013 Apr 17;33(16):6950-63. doi: 10.1523/JNEUROSCI.0277-13.2013.
The axon initial segment (AIS) is a specialized neuronal subcompartment located at the beginning of the axon that is crucially involved in both the generation of action potentials and the regulation of neuronal polarity. We recently showed that prolonged neuronal depolarization produces a distal shift of the entire AIS structure away from the cell body, a change associated with a decrease in neuronal excitability. Here, we used dissociated rat hippocampal cultures, with a major focus on the dentate granule cell (DGC) population, to explore the signaling pathways underlying activity-dependent relocation of the AIS. First, a pharmacological screen of voltage-gated calcium channels (VGCCs) showed that AIS relocation is triggered by activation of L-type Cav1 VGCCs with negligible contribution from any other VGCC subtypes. Additional pharmacological analysis revealed that downstream signaling events are mediated by the calcium-sensitive phosphatase calcineurin; inhibition of calcineurin with either FK506 or cyclosporin A totally abolished both depolarization- and optogenetically-induced activity-dependent AIS relocation. Furthermore, calcineurin activation is sufficient for AIS plasticity, because expression of a constitutively active form of the phosphatase resulted in relocation of the AIS of DGCs without a depolarizing stimulus. Finally, we assessed the role of calcineurin in other forms of depolarization-induced plasticity. Neither membrane resistance changes nor spine density changes were affected by FK506 treatment, suggesting that calcineurin acts via a separate pathway to modulate AIS plasticity. Together, these results emphasize calcineurin as a vital player in the regulation of intrinsic plasticity as governed by the AIS.
轴突起始段（AIS）是位于轴突起始处的一种特化神经元亚区，对于动作电位的产生和神经元极性的调节都至关重要。我们最近发现，神经元持续去极化会导致整个 AIS 结构从细胞体向远端迁移，这种变化与神经元兴奋性的降低有关。在这里，我们使用分离的大鼠海马培养物，主要集中在齿状颗粒细胞（DGC）群体，来探讨与 AIS 活动依赖性重定位相关的信号通路。首先，对电压门控钙通道（VGCC）的药理学筛选表明，AIS 重定位是由 L 型 Cav1 VGCC 的激活触发的，而其他任何 VGCC 亚型的贡献可以忽略不计。进一步的药理学分析表明，下游信号事件由钙敏感磷酸酶钙调神经磷酸酶介导；用 FK506 或环孢菌素 A 抑制钙调神经磷酸酶完全消除了去极化和光遗传学诱导的活性依赖性 AIS 重定位。此外，钙调神经磷酸酶的激活足以导致 AIS 可塑性，因为磷酸酶的组成性激活形式的表达导致 DGC 的 AIS 无需去极化刺激即可发生迁移。最后，我们评估了钙调神经磷酸酶在其他形式的去极化诱导可塑性中的作用。FK506 处理并不影响膜电阻变化或棘突密度变化，这表明钙调神经磷酸酶通过独立的途径调节 AIS 可塑性。总之，这些结果强调了钙调神经磷酸酶作为调节 AIS 固有可塑性的重要参与者。
