Department of Psychiatry, Djavad Mowafaghian Centre for Brain Health.
Graduate Program in Neuroscience.
J Neurosci. 2023 May 17;43(20):3743-3763. doi: 10.1523/JNEUROSCI.1593-22.2023. Epub 2023 Mar 21.
Action potential (AP)-independent (miniature) neurotransmission occurs at all chemical synapses but remains poorly understood, particularly in pathologic contexts. Axonal endoplasmic reticulum (ER) Ca stores are thought to influence miniature neurotransmission, and aberrant ER Ca handling is implicated in progression of Huntington disease (HD). Here, we report elevated mEPSC frequencies in recordings from YAC128 mouse (HD-model) neurons (from cortical cultures and striatum-containing brain slices, both from male and female animals). Pharmacological experiments suggest that this is mediated indirectly by enhanced tonic ER Ca release. Calcium imaging, using an axon-localized sensor, revealed slow AP-independent ER Ca release waves in both YAC128 and WT cultures. These Ca waves occurred at similar frequencies in both genotypes but spread less extensively and were of lower amplitude in YAC128 axons, consistent with axonal ER Ca store depletion. Surprisingly, basal cytosolic Ca levels were lower in YAC128 boutons and YAC128 mEPSCs were less sensitive to intracellular Ca chelation. Together, these data suggest that elevated miniature glutamate release in YAC128 cultures is associated with axonal ER Ca depletion but not directly mediated by ER Ca release into the cytoplasm. In contrast to increased mEPSC frequencies, cultured YAC128 cortical neurons showed less frequent AP-dependent (spontaneous) Ca events in soma and axons, although evoked glutamate release detected by an intensity-based glutamate-sensing fluorescence reporter in brain slices was similar between genotypes. Our results indicate that axonal ER dysfunction selectively elevates miniature glutamate release from cortical terminals in HD. This, together with reduced spontaneous cortical neuron firing, may cause a shift from activity-dependent to -independent glutamate release in HD, with potential implications for fidelity and plasticity of cortical excitatory signaling. Miniature neurotransmitter release persists at all chemical neuronal synapses in the absence of action potential firing but remains poorly understood, particularly in disease states. We show enhanced miniature glutamate release from cortical neurons in the YAC128 mouse Huntington disease model. This effect is mediated by axonal ER Ca store depletion, but is not obviously due to elevated ER-to-cytosol Ca release. Conversely, YAC128 cortical pyramidal neurons fired fewer action potentials and evoked cortical glutamate release was similar between WT an YAC128 preparations, indicating axonal ER depletion selectively enhances miniature glutamate release in YAC128 mice. These results extend our understanding of action potential independent neurotransmission and highlight a potential involvement of elevated miniature glutamate release in Huntington disease pathology.
动作电位 (AP)-非依赖性 (微小) 神经递质传递发生在所有化学突触中，但仍知之甚少，尤其是在病理情况下。轴突内质网 (ER) Ca 储存被认为会影响微小神经递质传递，而 ER Ca 处理异常与亨廷顿病 (HD) 的进展有关。在这里，我们报告了 YAC128 小鼠 (HD 模型) 神经元记录中的微小兴奋性突触后电流 (mEPSC) 频率升高（来自皮质培养物和包含纹状体的脑切片，来自雄性和雌性动物）。药理学实验表明，这是通过增强 tonic ER Ca 释放间接介导的。使用轴突定位传感器的钙成像显示，YAC128 和 WT 培养物中均存在缓慢的 AP 非依赖性 ER Ca 释放波。这两种基因型的 Ca 波频率相似，但在 YAC128 轴突中传播范围较小，幅度较低，这与轴突 ER Ca 储存耗竭一致。令人惊讶的是，YAC128 末梢中的基础胞质 Ca 水平较低，并且 YAC128 mEPSC 对细胞内 Ca 螯合的敏感性较低。总的来说，这些数据表明，YAC128 培养物中升高的微小谷氨酸释放与轴突 ER Ca 耗竭有关，但不是直接由 ER Ca 释放到细胞质中介导的。与 mEPSC 频率增加相反，培养的 YAC128 皮质神经元在胞体和轴突中显示出较少的依赖动作电位的 (自发) Ca 事件，尽管在脑切片中通过基于强度的谷氨酸感应荧光报告器检测到的诱发谷氨酸释放在两种基因型之间相似。我们的结果表明，轴突 ER 功能障碍选择性地增加了 HD 皮质末梢的微小谷氨酸释放。这与皮质神经元自发放电减少一起，可能导致 HD 中从活动依赖性到非依赖性谷氨酸释放的转变，这可能对皮质兴奋性信号的保真度和可塑性产生影响。微小神经递质释放在没有动作电位放电的情况下持续存在于所有化学神经元突触中，但仍知之甚少，尤其是在疾病状态下。我们显示 YAC128 亨廷顿病模型中的皮质神经元释放的微小谷氨酸增加。这种作用是通过轴突 ER Ca 储存耗竭介导的，但显然不是由于 ER 到细胞质的 Ca 释放增加。相反，YAC128 皮质锥体神经元发射的动作电位较少，并且 WT 和 YAC128 制剂之间的诱发皮质谷氨酸释放相似，表明轴突 ER 耗竭选择性增强了 YAC128 小鼠中的微小谷氨酸释放。这些结果扩展了我们对动作电位非依赖性神经递质传递的理解，并强调了升高的微小谷氨酸释放可能参与亨廷顿病病理学。
