Neuronal Ca2ϩ signaling through inositol triphosphate receptors (IP 3 R) and ryanodine receptors (RyRs) must be tightly regulated to maintain cell viability, both acutely and over a lifetime. Exaggerated intracellular Ca 2ϩ levels have been associated with expression of Alzheimer's disease (AD) mutations in young mice, but little is known of Ca 2ϩ dysregulations during normal and pathological aging processes. Here, we used electrophysiological recordings with two-photon imaging to study Ca 2ϩ signaling in nontransgenic (NonTg) and several AD mouse models (PS1 KI , 3xTg-AD, and APP Swe Tau P301L ) at young (6 week), adult (6 months), and old (18 months) ages. At all ages, the PS1 KI and 3xTg-AD mice displayed exaggerated endoplasmic reticulum (ER) Ca 2ϩ signals relative to NonTg mice. The PS1 mutation was the predominant "calciopathic" factor, because responses in 3xTg-AD mice were similar to PS1 KI mice , and APP Swe Tau P301L mice were not different from controls. In addition, we uncovered powerful signaling interactions and differences between IP 3 R-and RyR-mediated Ca 2ϩ components in NonTg and AD mice. In NonTg mice, RyR contributed modestly to IP 3 -evoked Ca 2ϩ , whereas the exaggerated signals in 3xTg-AD and PS1 KI mice resulted primarily from enhanced RyR-Ca 2ϩ release and were associated with increased RyR expression across all ages. Moreover, IP 3 -evoked membrane hyperpolarizations in AD mice were even greater than expected from exaggerated Ca 2ϩ signals, suggesting increased coupling efficiency between cytosolic [Ca 2ϩ ] and K ϩ channel regulation. We conclude that lifelong ER Ca 2ϩ disruptions in AD are related to a modulation of RyR signaling associated with PS1 mutations and represent a discrete "calciumopathy," not merely an acceleration of normal aging.

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Presenilin mutations result in exaggerated endoplasmic reticulum (ER) calcium release in cellular and animal models of Alzheimer's disease (AD). In this study, we examined whether dysregulated ER calcium release in young 3xTg-AD neurons alters synaptic transmission and plasticity mechanisms before the onset of histopathology and cognitive deficits. Using electrophysiological recordings and two-photon calcium imaging in young (6 -8 weeks old) 3xTg-AD and non-transgenic (NonTg) hippocampal slices, we show a marked increase in ryanodine receptor (RyR)-evoked calcium release within synapse-dense regions of CA1 pyramidal neurons. In addition, we uncovered a deviant contribution of presynaptic and postsynaptic ryanodine receptor-sensitive calcium stores to synaptic transmission and plasticity in 3xTg-AD mice that is not present in NonTg mice. As a possible underlying mechanism, the RyR2 isoform was found to be selectively increased more than fivefold in the hippocampus of 3xTg-AD mice relative to the NonTg controls. These novel findings demonstrate that 3xTg-AD CA1 neurons at presymptomatic ages operate under an aberrant, yet seemingly functional, calcium signaling and synaptic transmission system long before AD histopathology onset. These early signaling alterations may underlie the later synaptic breakdown and cognitive deficits characteristic of later stage AD.

Disruptions in intracellularsignals may result from increased store filling and not from increased flux through additional IP 3 -gated channels. Even in young animals, PS1 mutations have profound effects on neuronal Ca 2ϩ and electrical signaling: cumulatively, these disruptions may contribute to the long-term pathophysiology of AD.

Alzheimer's disease (AD) is a fatal neurodegenerative disorder that has no known cure, nor is there a clear mechanistic understanding of the disease process itself. Although amyloid plaques, neurofibrillary tangles, and cognitive decline are late-stage markers of the disease, it is unclear how they are initially generated, and if they represent a cause, effect, or end phase in the pathology process. Recent studies in AD models have identified marked dysregulations in calcium signaling and related downstream pathways, which occur long before the diagnostic histopathological or cognitive changes. Under normal conditions, intracellular calcium signals are coupled to effectors that maintain a healthy physiological state. Consequently, sustained up-regulation of calcium may have pathophysiological consequences. Indeed, upon reviewing the current body of literature, increased calcium levels are functionally linked to the major features and risk factors of AD: ApoE4 expression, presenilin and APP mutations, beta amyloid plaques, hyperphosphorylation of tau, apoptosis, and synaptic dysfunction. In turn, the histopathological features of AD, once formed, are capable of further increasing calcium levels, leading to a rapid feed-forward acceleration once the disease process has taken hold. The views proposed here consider that AD pathogenesis reflects long-term calcium dysregulations that ultimately serve an enabling role in the disease process. Therefore, "Calcinists" do not necessarily reject betaAptist or Tauist doctrine, but rather believe that their genesis is associated with earlier calcium signaling dysregulations.

The mechanisms underlying ryanodine receptor (RyR) dysfunction associated with Alzheimer disease (AD) are still not well understood. Here, we show that neuronal RyR2 channels undergo post-translational remodeling (PKA phosphorylation, oxidation, and nitrosylation) in brains of AD patients, and in two murine models of AD (3 × Tg-AD, APP /PS1). RyR2 is depleted of calstabin2 (KFBP12.6) in the channel complex, resulting in endoplasmic reticular (ER) calcium (Ca) leak. RyR-mediated ER Ca leak activates Ca-dependent signaling pathways, contributing to AD pathogenesis. Pharmacological (using a novel RyR stabilizing drug Rycal) or genetic rescue of the RyR2-mediated intracellular Ca leak improved synaptic plasticity, normalized behavioral and cognitive functions and reduced Aβ load. Genetically altered mice with congenitally leaky RyR2 exhibited premature and severe defects in synaptic plasticity, behavior and cognitive function. These data provide a mechanism underlying leaky RyR2 channels, which could be considered as potential AD therapeutic targets.

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 ERevoked 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.

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