Axonal ER Ca21 Release Selectively Enhances Activity-Independent Glutamate Release in a Huntington Disease Model

Abstract

Action potential (AP)-independent (miniature) neurotransmission occurs at all chemical synapses but remains poorly understood, particularly in pathologic contexts. Axonal endoplasmic reticulum (ER) Ca21 stores are thought to influence miniature neurotransmission, and aberrant ER Ca21 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 Ca21 release. Calcium imaging, using an axon-localized sensor, revealed slow AP-independent ER Ca21 release waves in both YAC128 and WT cultures. These Ca21 waves occurred at similar frequencies in both genotypes but spread less extensively and were of lower amplitude in YAC128 axons, consistent with axonal ER Ca21 store depletion. Surprisingly, basal cytosolic Ca21 levels were lower in YAC128 boutons and YAC128 mEPSCs were less sensitive to intracellular Ca21 chelation. Together, these data suggest that elevated miniature glutamate release in YAC128 cultures is associated with axonal ER Ca21 depletion but not directly mediated by ER Ca21 release into the cytoplasm. In contrast to increased mEPSC frequencies, cultured YAC128 cortical neurons showed less frequent AP-dependent (spontaneous) Ca21 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.