Apoptotic molecules in synapse plasticity

神经元迁移是中枢神经系统发育的重要过程。迁移后，神经元到达内部颗粒细胞层并实现终末分化。小脑颗粒细胞迁移行为的位置特异性变化需要多个分子和信号系统的协调活动，如脑源性神经营养因子，生长抑素，EphB 2受体和离子通道。到目前为止，一些报告提供了直接的证据，离子通道和细胞迁移之间的联系在小脑。培养的小脑颗粒细胞具有钾（K+）电流，包括快速瞬时和延迟整流外向电流。小脑颗粒神经元中的K+通道负责决定神经元动作电位频率，控制神经元之间突触接触的强度，促进神经元静息膜电位，以及调节单个神经元的兴奋性。我们在早期的研究中发现，在低K+培养基中诱导的小脑颗粒神经元凋亡与电流和振幅的增加有关。褪黑激素（Melatonin，MT）可通过受体或非受体机制调节K+、Ca ~（2+）、Cl ~-等离子通道，保护颗粒神经元免于凋亡。MT通过相关的MT受体作为神经调质影响许多生理功能。MT水平的改变已经在几种精神和神经疾病中描述。MT的作用可归因于其与三种受体的相互作用，这三种受体现已被鉴定为MT 1 R、MT 2 R和MT 3R，并且在海马、大脑和小脑皮质以及视网膜中被表征。 在本研究中，我们研究了MT对颗粒细胞迁移的影响。 MT可增加颗粒细胞延迟整流外向K+电流幅度和迁移速度，而K+通道阻断剂TEA则可降低颗粒细胞延迟整流外向K+电流，减慢颗粒细胞迁移速度。 我们的实验揭示了存在TEA敏感的K+通道依赖的小脑颗粒细胞的迁移，并表明MT刺激颗粒细胞迁移的K+通道电流的增加，这之后cAMP信号转导。虽然离子通道和离子转运蛋白在细胞迁移中的作用的研究还处于早期阶段，但越来越多的证据表明，膜离子通道参与细胞运动。 K+通道在细胞迁移的调节中起重要作用，这表明K+通道的激活是导致细胞内K+损失的原因，这可能因此导致导致细胞迁移的内部信号转导。 用TEA孵育细胞抑制K+外向电流和抑制细胞迁移，如通过transwell分析所测量的，并引导颗粒细胞从切片培养物中的外部颗粒细胞层迁移到内部颗粒细胞层。MT对K+外向电流的增强作用是通过MT 2 R介导的。我们发现TEA对MT诱导的迁移的逆转作用是不完全的。这间接地揭示了MT对迁移的易化作用的一些机制，而不是通过调节TEA敏感的K+通道来促进迁移。在MT增强K+电流的可能机制中，cAMP是介导MT调节细胞功能的主要细胞内第二信使。在多种细胞中，MT不改变cAMP的基础水平，但抑制cAMP的细胞内积累。对于环核苷酸信号传导，cAMP信号传导充当颗粒运动的制动器。 我们发现，虽然TEA和cAMP的应用完全阻断MT诱导的K+电流增强。MT保留了对细胞迁移的部分刺激作用。这些数据表明，MT诱导的小脑颗粒细胞迁移的影响涉及一个额外的信号通路与MTR。该模型说明了参与MT对小脑颗粒细胞的神经营养作用的信号级联反应，通过MT 2 R起作用，降低细胞内cAMP并刺激细胞迁移。

Neuronal migration is an essential process in the development of the central nervous system. After migration, neurons arrive at the internal granule cell layer and achieve terminal differentiation. Position-specific changes in migratory behavior of cerebellar granule cells require coordinated activity of multiple molecules and signaling systems, such as brain derived neurotrophic factor, somatostatin, EphB2 receptor, and ion channels. To date, a few reports have provided direct evidence for the link between ion channels and cell migration in the cerebellum. Cerebellar granule cells grown in culture possess potassium (K+) currents which include fast transient and delayed rectifier outward currents. K+ channels in cerebellar granule neurons are responsible for determining the neuronal action potential frequency, controlling the strength of synaptic contacts between neurons, contributing to the neuronal resting membrane potential, and regulating excitability of individual neurons. We showed in an earlier study that apoptosis of cerebellar granule neurons induced in low K+ medium was associated with increased current and amplitudes. Melatonin (MT) can modulate ion channels, such as K+, calcium or chloride, and protect granule neurons against apoptosis by receptor or non-receptor mechanisms. MT influences a number of physiological functions by working as a neuromodulator through associated MT receptors. Alterations of the levels of MT have been described in several psychiatric and neurological disorders. The action of MT can be ascribed to its interaction with three receptors that have now been identified as MT1R, MT2R and MT3R and characterized in the hippocampus, cerebral and cerebellar cortex and retina. In the present study, we examined the effect of MT on granule cell migration. MT increased delayed rectifier outward K+ current amplitude and migration of granule cells, whereas TEA, a K+ channel blocker, decreased the delayed rectifier outward K+ current and slowed migration of the granule cells. Our experiments revealed the existence of TEA-sensitive K+ channel-dependent migration of cerebellar granule cells, and showed that MT stimulates granule cell migration by an increase in K+ channel current, which follows cAMP signal transduction. Although studies on the role of ion channels and ion transporters in cell migration are at an early stage, there is increasing evidence that membrane ion channels are involved in cell locomotion. K+ channels function substantially in the modulation of cell migration suggesting that the activation of K+ channels is responsible for the intracellular K+ loss, which might consequently cause intra-signal transduction leading to cell migration. Incubation of cells with TEA inhibited the K+ outward current and suppressed cell migration, as measured by transwell analysis, and guides the migration of granule cells from the external granule cell layer to the inner granule cell layer in slice cultures. Furthermore, the increasing effect of MT on K+ outward current on cell migration is mediated by MT2R. We found that the reversal effect of TEA on MT-induced migration is incomplete. This reveals, although indirectly, some mechanisms for the facilitory effect of MT on migration other than through modulating the TEA-sensitive K+ channel to promote migration. Among the possible mechanisms underlying the potentiation by MT of K+ currents, cAMP is a major intracellular second messenger that mediates MT regulation of cell functions. In a variety of cells, MT does not alter the basal level of cAMP but inhibits the intracellular accumulation of cAMP. For cyclic nucleotide signaling, cAMP signaling acts as a brake on granule movement. We show that although the application of TEA and cAMP completely block MT-induced K+ current potentiation. MT retains part of the stimulatory effect on cell migration. These data suggest that the MT-induced effect on cerebellar granule cell migration involves an additional signaling pathway associated with MTR. This proposed model illustrates the signaling cascade involved in the neurotrophic action of MT on cerebellar granule cells acting through MT2R, decreases intracellular cAMP and stimulates cell migration.