Calmodulin Activation by Calcium Transients in the Postsynaptic Density of Dendritic Spines

The entry of calcium into dendritic spines can trigger a sequence of biochemical reactions that begins with the activation of calmodulin (CaM) and ends with long-term changes to synaptic strengths. The degree of activation of CaM can depend on highly local elevations in the concentration of calcium and the duration of transient increases in calcium concentration. Accurate measurement of these local changes in calcium is difficult because the spaces are so small and the numbers of molecules are so low. We have therefore developed a Monte Carlo model of intracellular calcium dynamics within the spine that included calcium binding proteins, calcium transporters and ion channels activated by voltage and glutamate binding. The model reproduced optical recordings using calcium indicator dyes and showed that without the dye the free intracellular calcium concentration transient was much higher than predicted from the fluorescent signal. Excitatory postsynaptic potentials induced large, long-lasting calcium gradients across the postsynaptic density, which activated CaM. When glutamate was released at the synapse 10 ms before an action potential occurred, simulating activity patterns that strengthen hippocampal synapses, the calcium gradient and activation of CaM in the postsynaptic density were much greater than when the order was reversed, a condition that decreases synaptic strengths, suggesting a possible mechanism underlying the induction of long-term changes in synaptic strength. The spatial and temporal mechanisms for selectivity in CaM activation demonstrated here could be used in other signaling pathways.

钙进入树突棘可触发一系列生化反应，该反应以钙调蛋白（calmodulin, CaM）的激活为起始，最终以突触强度的长期改变告终。钙调蛋白的激活程度可取决于钙浓度的极高局部升高幅度，以及钙浓度瞬时升高的持续时长。由于该空间尺度极小且分子数量极低，精准测量钙的这类局部变化颇具难度。为此，我们构建了树突棘内细胞内钙动力学的蒙特卡洛（Monte Carlo）模型，该模型纳入了钙结合蛋白、钙转运体，以及受电压激活和谷氨酸结合激活的离子通道。该模型复现了基于钙指示剂染料的光学记录结果，并揭示：在未使用染料的情况下，细胞内游离钙的瞬时浓度远高于从荧光信号推算出的数值。兴奋性突触后电位可在突触后致密区引发大范围且持续时间长久的钙梯度，进而激活钙调蛋白。当谷氨酸在动作电位发生前10毫秒于突触处释放时（该实验条件模拟了增强海马突触的活动模式），突触后致密区的钙梯度与钙调蛋白激活程度，远高于动作电位先于谷氨酸释放的反向实验条件（后者会削弱突触强度），这提示了介导突触强度长期改变的潜在机制。本研究揭示的钙调蛋白激活选择性的时空机制，可应用于其他信号通路当中。