Upregulated Ca²⁺ Release from the Endoplasmic Reticulum Leads to Impaired Presynaptic Function in Familial...

Neurotransmitter release from presynaptic terminals is primarily regulated by rapid Ca²⁺ influx through membrane-resident voltage-gated Ca²⁺ channels (VGCCs). Moreover, accumulating evidence indicates that the endoplasmic reticulum (ER) is extensively present in axonal terminals of neurons and plays a modulatory role in synaptic transmission by regulating Ca²⁺ levels. Familial Alzheimer's disease (FAD) is marked by enhanced Ca²⁺ release from the ER and downregulation of Ca²⁺ buffering proteins. However, the precise consequence of impaired Ca²⁺ signaling within the vicinity of VGCCs (active zone (AZ)) on exocytosis is poorly understood. Here, we perform in silico experiments of intracellular Ca²⁺ signaling and exocytosis in a detailed biophysical model of hippocampal synapses to investigate the effect of aberrant Ca²⁺ signaling on neurotransmitter release in FAD. Our model predicts that enhanced Ca²⁺ release from the ER increases the probability of neurotransmitter release in FAD. Moreover, over very short timescales (30-60 ms), the model exhibits activity-dependent and enhanced short-term plasticity in FAD, indicating neuronal hyperactivity-a hallmark of the disease. Similar to previous observations in AD animal models, our model reveals that during prolonged stimulation (~450 ms), pathological Ca²⁺ signaling increases depression and desynchronization with stimulus, causing affected synapses to operate unreliably. Overall, our work provides direct evidence in support of a crucial role played by altered Ca²⁺ homeostasis mediated by intracellular stores in FAD.