Huntington’s disease (HD) is an inherited neurodegenerative disorder caused by the abnormal expansion of CAG repeats in the huntingtin (HTT) gene, which leads to progressive loss of neurons starting in the striatum and cortex. One possible mechanism for this selective loss of neurons in the early stage of HD is altered neurotransmission at synapses. Despite the recent finding that presynaptic terminals play an important role in HD, neurotransmitter release at synapses in HD remains poorly understood. Here, we measured synaptic vesicle release in real time at single presynaptic terminals during electrical field stimulation. We found the increase in synaptic vesicle release at presynaptic terminals in primary cortical neurons in a knock-in mouse model of HD (zQ175). We also found the increase in Ca2+ influx at presynaptic terminals in HD neurons during the electrical stimulation. Consistent with increased Ca2+-dependent neurotransmission in HD neurons, the increase in vesicle release and Ca2+ influx was rescued with Ca2+ chelators or by blocking N-type voltage-gated Ca2+ channels, suggesting N-type voltage-gated Ca2+ channels play an important role in HD. Taken together, our results suggest that the increased synaptic vesicles release due to increased Ca2+ influx at presynaptic terminals in cortical neurons contributes to the selective neurodegeneration of these neurons in early HD and provide a possible therapeutic target.

Huntington’s disease (HD) is a dominantly inherited neurodegenerative disease caused by an increase in CAG repeats in the Huntingtin gene (HTT). The striatum is one of the most vulnerable brain regions in HD, and altered delivery of BDNF to the striatum is believed to underlie this high vulnerability. However, the delivery of BDNF to the striatum in HD remains poorly understood. Here, we used real-time imaging to visualize release of BDNF from cortical neurons cultured alone or co-cultured with striatal neurons. BDNF release was significantly decreased in the cortical neurons of zQ175 mice (a knock-in model of HD), and total internal reflection fluorescence microscopy revealed several release patterns of single BDNF-containing vesicles, with distinct kinetics and prevalence, in co-cultured cortical HD neurons. Notably, a smaller proportion of single BDNF-containing vesicles underwent full release in HD neurons than in wild-type neurons. This decreased release of BDNF in cortical neurons might lead to decreased BDNF levels in the striatum because the striatum receives BDNF mainly from the cortex. In addition, we observed a decrease in the total travel length and speed of BDNF-containing vesicles in HD neurons, indicating altered transport of these vesicles in HD. Our findings suggest a potential mechanism for the vulnerability of striatal neurons in HD and offer new insights into the pathogenic mechanisms underlying the degeneration of neurons in HD.

Store-operated Ca 2+ entry (SOCE) is a major Ca 2+ influx pathway that is controlled by the ER Ca 2+ sensor STIM1. Abnormal activation of STIM1 directly influences Ca 2+ influx, resulting in severe diseases such as Stormorken syndrome. The inactivation domain of STIM1 (IDstim) has been identified as being essential for Ca 2+-dependent inactivation of STIM1 (CDI) after SOCE occurs. However, it is unknown whether IDstim is involved in keeping STIM1 inactive before CDI. Herein, we show that IDstim helps STIM1 keep inactive through intramolecular binding with the coiled-coil domain. Between IDstim and the coiled-coil domain, we found a short conserved linker whose extension or mutation leads to the constitutive activation of STIM1. We have demonstrated that IDstim needs the coiled-coil domain 1 (CC1) to inhibit the Ca 2+ release-activated Ca 2+ (CRAC) activation domain (CAD) activity and binds to a CC1-CAD fragment. Serial deletion of CC1 revealed that CC1α1 is a co-inhibitory domain of IDstim. CC1α1 deletion or leucine mutation, which abolishes the closed conformation, impaired the inhibitory effect and binding of IDstim. These results suggest that IDstim cooperates with CC1α1 to help STIM1 keep inactive under resting conditions.