Axonal ER Ca²⁺Release Enhances Miniature, but Reduces Activity-Dependent Glutamate Release in a Huntington Disease Model

Abstract

Huntington's disease (HD) is a fatal, inherited neurodegenerative disorder caused by a CAG repeat expansion in the huntingtin (HTT) gene. Altered cortical glutamate release onto striatal spiny projection neurons (SPN) is thought to contribute to pathological synaptic dysfunction in HD; however, the direction of this change is controversial and underlying mechanisms uncertain.We used the YAC128 mouse model of Huntington's disease, expressing the full-length genomic human HTT DNA with 128 repeats, to investigate this question. Here we show that in YAC128 cortical-striatal co-cultures, individual cortical terminals release vesicles onto SPNs at an increased rate compared to wild-type (WT) when action potentials (AP) are blocked with tetrodotoxin (TTX), whereas the release rate is higher in WT than YAC128 cultures when APs are intact. Further, we report increased frequency of miniature excitatory postsynaptic currents (mEPSCs) during the stage of rapid synaptogenesis in vitro for YAC128 cortical cultures, which can be explained by increased spontaneous Ca 2+ released from presynaptic endoplasmic reticulum (ER) stores. Experiments in which axonal bouton Ca 2+ was directly imaged (in TTX) with a synapsin-tagged GCaMP construct revealed higher cytosolic Ca 2+ concentrations and more frequent axonal Ca 2+ waves in YAC128 cultures. Conversely, in the absence of TTX, axonal boutons showed more frequent, larger amplitude Ca 2+ events in WT cortical cultures.Releasing ER Ca 2+ into the cytoplasm with low dose ryanodine, which we believe mimics YAC128 presynaptic Ca 2+ conditions, reduced the amplitude of these AP-dependent Ca 2+ events in WT but not YAC128 boutons. Low dose ryanodine similarly reduced the amplitude of cortical-evoked glutamate release in the striatum of WT, but not YAC128 mouse-derived brain slices. Together, our results demonstrate a shift toward increased spontaneous, and reduced

Presynaptic store-operated Ca2+ entry drives excitatory spontaneous neurotransmission and augments endoplasmic reticulum stress

“…As a consequence, the sustained increase in spontaneous glutamate release observed during chronic ER stress may have a critical impact on network firing properties and information processing, contributing to the deterioration of neural circuits. A recent report found increased axonal Ca 2+ waves and augmented spontaneous glutamate release in a mouse model of Huntington's disease, proposing presynaptic release properties as a relevant component in the etiology of neurotransmission deficits (Mackay et al, 2020). Accordingly, blocking postsynaptic NMDA receptors reverts the increase in ER-stress-related signaling molecules and excitatory neurotransmission (using the same chronic ER stress model used here; Nosyreva and Kavalali, 2010), indicating that defects in glutamatergic neurotransmission can propagate through the neuronal network via spontaneous neurotransmission.…”