Early presynaptic and postsynaptic calcium signaling abnormalities mask underlying synaptic depression in presymptomatic Alzheimer's disease mice

Naringin Enhances CaMKII Activity and Improves Long-Term Memory in a Mouse Model of Alzheimer's Disease

The Amyloid-β (Aβ)-induced impairment of hippocampal synaptic plasticity is an underlying mechanism of memory loss in the early stages of Alzheimer’s disease (AD) in human and mouse models. The inhibition of the calcium/calmodulin-dependent protein kinase II (CaMKII) autophosphorylation plays an important role in long-term memory. In this study, we isolated naringin from Pomelo peel (a Citrus species) and studied its effect on long-term memory in the APPswe/PS1dE9 transgenic mouse model of AD. Three-month-old APPswe/PS1dE9 transgenic mice were randomly assigned to a vehicle group, two naringin (either 50 or 100 mg/kg body weight/day) groups, or an Aricept (2 mg/kg body weight/day) group. After 16 weeks of treatment, we observed that treatment with naringin (100 mg/kg body weight/day) enhanced the autophosphorylation of CaMKII, increased the phosphorylation of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic (AMPA) receptor at a CaMKII-dependent site and improved long-term learning and memory ability. These findings suggest that the increase in CaMKII activity may be one of the mechanisms by which naringin improves long-term cognitive function in the APPswe/PS1dE9 transgenic mouse model of AD.

β amyloid peptide plaques fail to alter evoked neuronal calcium signals in APP/PS1 Alzheimer's disease mice

Alzheimer's disease (AD) is a multifactorial disorder of unknown etiology. Mechanistically, beta amyloid peptides (Aβ) and elevated Ca(2+) have been implicated as proximal and likely interactive features of the disease process. We tested the hypothesis that proximity to Aβ plaque might exacerbate activity-dependent neuronal Ca(2+) signaling in hippocampal pyramidal neurons from APPSWE/PS1M146V mice. Using combined approaches of whole cell patch clamp recording and 2-photon imaging of neuronal Ca(2+) signals with thioflavin-S plaque labeling in hippocampal slices, we found no correlation between thioflavin-S labeled Aβ plaque proximity and Ca(2+) responses triggered by ryanodine receptor (RyR) activation or action potentials in either dendrites or somata of AD mice, regardless of age. Baseline and RyR-stimulated spontaneous excitatory postsynaptic potentials also showed little difference in relation to Aβ plaque proximity. Consistent with previous studies, RyR-evoked Ca(2+) release in APPSWE/PS1M146V mice was greater than in nontransgenic controls. Within the soma, RyR-evoked Ca(2+) release was elevated in older APPSWE/PS1M146V mice compared with younger APPSWE/PS1M146V mice, but was still independent of plaque proximity. The results indicate that early Ca(2+) signaling disruptions can become yet more severe with age through mechanisms independent of Aβ plaques, suggesting that alternative pathogenic mechanisms might contribute to AD-associated dysfunction.

Stabilizing ER Ca2+ channel function as an early preventative strategy for Alzheimer's disease

Alzheimer’s disease (AD) is a devastating neurodegenerative condition with no known cure. While current therapies target late-stage amyloid formation and cholinergic tone, to date, these strategies have proven ineffective at preventing disease progression. The reasons for this may be varied, and could reflect late intervention, or, that earlier pathogenic mechanisms have been overlooked and permitted to accelerate the disease process. One such example would include synaptic pathology, the disease component strongly associated with cognitive impairment. Dysregulated Ca²⁺ homeostasis may be one of the critical factors driving synaptic dysfunction. One of the earliest pathophysiological indicators in mutant presenilin (PS) AD mice is increased intracellular Ca²⁺ signaling, predominantly through the ER-localized inositol triphosphate (IP₃) and ryanodine receptors (RyR). In particular, the RyR-mediated Ca²⁺ upregulation within synaptic compartments is associated with altered synaptic homeostasis and network depression at early (presymptomatic) AD stages. Here, we offer an alternative approach to AD therapeutics by stabilizing early pathogenic mechanisms associated with synaptic abnormalities. We targeted the RyR as a means to prevent disease progression, and sub-chronically treated AD mouse models (4-weeks) with a novel formulation of the RyR inhibitor, dantrolene. Using 2-photon Ca²⁺ imaging and patch clamp recordings, we demonstrate that dantrolene treatment fully normalizes ER Ca²⁺ signaling within somatic and dendritic compartments in early and later-stage AD mice in hippocampal slices. Additionally, the elevated RyR2 levels in AD mice are restored to control levels with dantrolene treatment, as are synaptic transmission and synaptic plasticity. Aβ deposition within the cortex and hippocampus is also reduced in dantrolene-treated AD mice. In this study, we highlight the pivotal role of Ca²⁺ aberrations in AD, and propose a novel strategy to preserve synaptic function, and thereby cognitive function, in early AD patients.

Age-dependent alterations in the presynaptic active zone in a Drosophila model of Alzheimer's disease

The accumulation of beta amyloid (Aβ) can cause synaptic impairments, but the characteristics and mechanisms of the synaptic impairment induced by the accumulation of Aβ in Alzheimer's disease (AD) remain unclear. In identified single neurons in a newly developed Drosophila AD model, in which Aβ accumulates intraneuronally, we found an age-dependent reduction in the synaptic vesicle release probability that was associated with a decrease in the density of presynaptic calcium channel clusters and an increase in the presynaptic and postsynaptic contact length. Moreover, these alterations occurred in the absence of presynaptic bouton loss. In addition, we found that Aβ expression also produced an age-dependent decrease in the amount of Bruchpilot (Brp), which plays an important role in controlling Ca(2+) channel clustering and synaptic vesicle release in the presynaptic active zone. Our study indicates that the chronic accumulation of intraneuronal Aβ can induce functional and structural changes in the presynaptic active zone prior to a loss of presynaptic buttons in the same neuron.

In vivo hippocampal microdialysis reveals impairment of NMDA receptor-cGMP signaling in APP(SW) and APP(SW)/PS1(L166P) Alzheimer's transgenic mice

Transgenic (Tg) mice overexpressing human amyloid precursor protein (APP) mutants reproduce features of early Alzheimer's disease (AD) including memory deficit, presence of β-amyloid (Aβ) oligomers, and age-associated formation of amyloid deposits. In this study we used hippocampal microdialysis to characterize the signaling of N-methyl-d-aspartic acid receptors (NMDA-Rs) in awake and behaving AD Tg mice. The NMDA-R signaling is central to hippocampal synaptic plasticity underlying memory formation and several lines of evidence implicate the role of Aβ oligomers in effecting NMDA-R dysfunction. CA1 NMDA-Rs were stimulated by NMDA infused through reverse microdialysis while changes in the cyclic guanosine monophosphate (cGMP) concentration in the brain interstitial fluid (ISF) were used to determine NMDA-Rs responsiveness. While 4 months old wild type C57BL/6 mice mounted robust cGMP response to the NMDA challenge, the same stimulus failed to significantly change the cGMP level in 4 and 15 months old APP(SW) and 4 months old APP(SW)/PS1(L166P) Tg mice, which were all on C57BL/6 background. Lack of response to NMDA in AD Tg mice occurred in the absence of changes in expression levels of several synaptic proteins including synaptophysin, NR1 NMDA-R subunit and postsynaptic density protein 95, which indicates lack of profound synaptic degeneration. Aβ oligomers were detected in all three AD Tg mice groups and their concentration in the hippocampus ranged from 40.5±3.6ng/g in 4 months old APP(SW) mice to 60.8±15.9ng/g in 4 months old APP(SW)/PS1(L166P) mice. Four months old APP(SW) mice had no Aβ amyloid plaques, while the other two AD Tg mice groups showed evidence of incipient Aβ amyloid plaque formation. Our studies describes a novel approach useful to study the function of NMDA-Rs in awake and behaving AD Tg mice and demonstrate impairment of NMDA-R response in the presence of endogenously formed Aβ oligomers but predating onset of Aβ amyloidosis.