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Deficits in synaptic function, particularly through NMDA receptors (NMDARs), are linked to late-stage cognitive impairments in Alzheimer's disease (AD). At earlier disease stages, however, there is evidence for altered endoplasmic reticulum (ER) calcium signaling in human cases and in neurons from AD mouse models. Despite the fundamental importance of calcium to synaptic function, neither the extent of ER calcium dysregulation in dendrites nor its interaction with synaptic function in AD pathophysiology is known. Identifying the mechanisms underlying early synaptic calcium dysregulation in AD pathogenesis is likely a key component to understanding, and thereby preventing, the synapse loss and downstream cognitive impairments. Using two-photon calcium imaging, flash photolysis of caged glutamate, and patch-clamp electrophysiology in cortical brain slices, we examined interactions between synaptically and ER-evoked calcium release at glutamatergic synapses in young AD transgenic mice. We found increased ryanodine receptor-evoked calcium signals within dendritic spine heads, dendritic processes, and the soma of pyramidal neurons from 3xTg-AD and TAS/TPM AD mice relative to NonTg controls. In addition, synaptically evoked postsynaptic calcium responses were larger in the AD strains, as were calcium signals generated from NMDAR activation. However, calcium responses triggered by back-propagating action potentials were not different. Concurrent activation of ryanodine receptors (RyRs) with either synaptic or NMDAR stimulation generated a supra-additive calcium response in the AD strains, suggesting an aberrant calcium-induced calcium release (CICR) effect within spines and dendrites. We propose that presenilin-linked disruptions in RyR signaling and subsequent CICR via NMDAR-mediated calcium influx alters synaptic function and serves as an early pathogenic factor in AD.
Goussakov et al. (Wed,) studied this question.