Stein Ivar S, Gray John A, Zito Karen
Center for Neuroscience and.
Center for Neuroscience and Department of Neurology, University of California Davis, Davis, California 95618.
J Neurosci. 2015 Sep 2;35(35):12303-8. doi: 10.1523/JNEUROSCI.4289-14.2015.
The elimination of dendritic spine synapses is a critical step in the refinement of neuronal circuits during development of the cerebral cortex. Several studies have shown that activity-induced shrinkage and retraction of dendritic spines depend on activation of the NMDA-type glutamate receptor (NMDAR), which leads to influx of extracellular calcium ions and activation of calcium-dependent phosphatases that modify regulators of the spine cytoskeleton, suggesting that influx of extracellular calcium ions drives spine shrinkage. Intriguingly, a recent report revealed a novel non-ionotropic function of the NMDAR in the regulation of synaptic strength, which relies on glutamate binding but is independent of ion flux through the receptor (Nabavi et al., 2013). Here, we tested whether non-ionotropic NMDAR signaling could also play a role in driving structural plasticity of dendritic spines. Using two-photon glutamate uncaging and time-lapse imaging of rat hippocampal CA1 neurons, we show that low-frequency glutamatergic stimulation results in shrinkage of dendritic spines even in the presence of the NMDAR d-serine/glycine binding site antagonist 7-chlorokynurenic acid (7CK), which fully blocks NMDAR-mediated currents and Ca(2+) transients. Notably, application of 7CK or MK-801 also converts spine enlargement resulting from a high-frequency uncaging stimulus into spine shrinkage, demonstrating that strong Ca(2+) influx through the NMDAR normally overcomes a non-ionotropic shrinkage signal to drive spine growth. Our results support a model in which NMDAR signaling, independent of ion flux, drives structural shrinkage at spiny synapses.
Dendritic spine elimination is vital for the refinement of neural circuits during development and has been linked to improvements in behavioral performance in the adult. Spine shrinkage and elimination have been widely accepted to depend on Ca(2+) influx through NMDA-type glutamate receptors (NMDARs) in conjunction with long-term depression (LTD) of synaptic strength. Here, we use two-photon glutamate uncaging and time-lapse imaging to show that non-ionotropic NMDAR signaling can drive shrinkage of dendritic spines, independent of NMDAR-mediated Ca(2+) influx. Signaling through p38 MAPK was required for this activity-dependent spine shrinkage. Our results provide fundamental new insights into the signaling mechanisms that support experience-dependent changes in brain structure.
在大脑皮质发育过程中，树突棘突触的消除是神经元回路精细化的关键步骤。多项研究表明，活动诱导的树突棘收缩和回缩依赖于NMDA型谷氨酸受体（NMDAR）的激活，这会导致细胞外钙离子内流以及钙依赖性磷酸酶的激活，这些酶会修饰棘突细胞骨架的调节因子，这表明细胞外钙离子内流驱动了棘突收缩。有趣的是，最近的一份报告揭示了NMDAR在调节突触强度方面的一种新的非离子型功能，该功能依赖于谷氨酸结合，但与通过受体的离子通量无关（纳巴维等人，2013年）。在这里，我们测试了非离子型NMDAR信号传导是否也能在驱动树突棘的结构可塑性中发挥作用。通过对大鼠海马CA1神经元进行双光子谷氨酸解笼和延时成像，我们发现即使存在NMDAR的d - 丝氨酸/甘氨酸结合位点拮抗剂7 - 氯犬尿氨酸（7CK），低频谷氨酸能刺激也会导致树突棘收缩，7CK能完全阻断NMDAR介导的电流和Ca(2+)瞬变。值得注意的是，应用7CK或MK - 801也会将高频解笼刺激引起的棘突增大转变为棘突收缩，这表明通过NMDAR的强烈Ca(2+)内流通常会克服非离子型收缩信号来驱动棘突生长。我们的结果支持了一种模型，即NMDAR信号传导独立于离子通量，驱动棘突触处的结构收缩。
树突棘消除对于发育过程中神经回路的精细化至关重要，并且与成年个体行为表现的改善有关。棘突收缩和消除已被广泛认为依赖于通过NMDA型谷氨酸受体（NMDARs）的Ca(2+)内流以及突触强度的长时程抑制（LTD）。在这里，我们使用双光子谷氨酸解笼和延时成像来表明，非离子型NMDAR信号传导可以驱动树突棘收缩，独立于NMDAR介导的Ca(2+)内流。这种活动依赖性棘突收缩需要通过p38丝裂原活化蛋白激酶进行信号传导。我们的结果为支持大脑结构中经验依赖性变化的信号传导机制提供了重要的新见解。
