钙蛋白酶与内毛细胞带状突触噪声损伤的相关性研究

Correlation Study of Calpain and Noise-induced Cochlear Synaptopathy in Inner Hair Cells

目的通过比较噪声暴露后基底膜不同区域内毛细胞(IHCs)带状突触损伤差异,探讨带状突触损伤易感性的相关因素。方法将28只C57BL/6J雄性小鼠随机分为噪声暴露组和对照组,每组14只。噪声暴露组小鼠给予强度103 dB SPL、频率2~20 kHz、持续2 h的宽带噪声暴露,对照组小鼠则饲养于安静环境中。噪声暴露前及噪声暴露后第一天进行ABR测试及毛细胞带状突触免疫荧光染色实验。使用全细胞膜片钳技术比较不同区域IHCs的钙离子流入。通过免疫荧光染色比较噪声暴露后的耳蜗基底膜顶回、中回、底回IHCs钙蛋白酶(Calpain)表达水平,并用蛋白质印迹实验验证钙蛋白酶对IHCs带状突触蛋白CtBP2的损伤作用。结果噪声暴露后一天,噪声暴露组在11.3、16.0、22.6、32.0 kHz的ABR阈值较对照组显著上升(均为P<0.001),中回、底回IHCs带状突触数量明显减少(P<0.05)。全细胞膜片钳实验结果表明耳蜗基底膜中回IHCs有较多的钙离子通道(P<0.01),但其单通道电流较小(P<0.01),顶回、中回IHCs钙离子通道开放率无显著差异(P>0.05)。噪声暴露后,耳蜗基底膜中回、底回IHCs的Calpain表达水平显著高于顶回(P<0.001),蛋白质印迹实验结果表明Calpain以钙离子依赖的方式降解带状突触蛋白CtBP2。结论钙蛋白酶是基底膜高频区内毛细胞带状突触噪声损伤易感的重要因素。

Objective To investigate the related factor underlying the differential vulnerability of inner hair cell(IHC)ribbon synapses to noise exposure.Methods Twenty-eight C57BL/6J mice were randomly divided into the noise exposure group(NED1)and the control group(CTR)with 14 mice in each group.The noise exposure group was exposed to bandpass noise of 2~20 kHz at 103 dB SPL for two hours while the control group was bred in a quiet environment.Before and after noise exposure,auditory brainstem response was conducted to detect the auditory function,whole-mount immunofluorescence staining was used to observe the number of inner hair cell ribbon synapses in different turns.The calcium influx of inner hair cells in the apical and middle turn using whole-cell patch clamp recording was analyzed.Furthermore,the immunofluorescence intensity of calpain in inner hair cells from different mouse cochlear turns using whole-mount immunofluorescence staining was compared.Western blotting was used to verify that CtBP2 was degraded by calpain.Results After noise exposure,the ABR threshold of 11.3,16.0,22.6,32.0 kHz increased significantly and the number of ribbon synapses of inner hair cells at the middle turn and basal turn of the cochlea decreased significantly.However,whole-cell patch clamp recordings showed that calcium ion channels were fewer but single-channel current was higher at the apical turn of the cochlea.The open probability of calcium ion channels in IHCs showed no significant difference between the apical turn and the middle turn of the cochlea.But the expression level of calpain of the inner hair cells at the middle and basal turn of the basilar membrane was significantly higher than that at the apical turn after noise exposure.The western blotting results also showed that CtBP2 was cleaved in a Ca 2+-dependent manner by calpain.Conclusion Calpain may be the main related factor that accounts for the vulnerability of ribbon synapses in inner hair cells in the high frequency region of basal membrane.