RyR2-Mediated Ca2+ Release and Mitochondrial ROS Generation Partake in the Synaptic Dysfunction Caused by Amyloid β Peptide Oligomers

Abstract

Amyloid β peptide oligomers (AβOs), toxic aggregates with pivotal roles in Alzheimer’s disease, trigger persistent and low magnitude Ca2+ signals in neurons. We reported previously that these Ca2+ signals, which arise from Ca2+ entry and subsequent amplification by Ca2+ release through ryanodine receptor (RyR) channels, promote mitochondrial network fragmentation and reduce RyR2 expression. Here, we examined if AβOs, by inducing redox sensitive RyR-mediated Ca2+ release, stimulate mitochondrial Ca2+-uptake, ROS generation and mitochondrial fragmentation, and also investigated the effects of the antioxidant N-acetyl cysteine (NAC) and the mitochondrial antioxidant EUK-134 on AβOs-induced mitochondrial dysfunction. In addition, we studied the contribution of the RyR2 isoform to AβOs-induced Ca2+ release, mitochondrial Ca2+ uptake and fragmentation. We show here that inhibition of NADPH oxidase type-2 prevented the emergence of RyR-mediated cytoplasmic Ca2+ signals induced by AβOs in primary hippocampal neurons. Treatment with AβOs promoted mitochondrial Ca2+ uptake and increased mitochondrial superoxide and hydrogen peroxide levels; ryanodine, at concentrations that suppress RyR activity, prevented these responses. The antioxidants NAC and EUK-134 impeded the mitochondrial ROS increase induced by AβOs. Additionally, EUK-134 prevented the mitochondrial fragmentation induced by AβOs, as previously reported for NAC and ryanodine. These findings show that both antioxidants, NAC and EUK-134, prevented the Ca2+-mediated noxious effects of AβOs on mitochondrial function. Our results also indicate that Ca2+ release mediated by the RyR2 isoform causes the deleterious effects of AβOs on mitochondrial function. Knockdown of RyR2 with antisense oligonucleotides reduced by about 50% RyR2 mRNA and protein levels in primary hippocampal neurons, decreased by 40% Ca2+ release induced by the RyR agonist 4-chloro-m-cresol, and significantly reduced the cytoplasmic and mitochondrial Ca2+ signals and the mitochondrial fragmentation induced by AβOs. Based on our results, we propose that AβOs-induced Ca2+ entry and ROS generation jointly stimulate RyR2 activity, causing mitochondrial Ca2+ overload and fragmentation in a feed forward injurious cycle. The present novel findings highlight the specific participation of RyR2-mediated Ca2+ release on AβOs-induced mitochondrial malfunction.

Alzheimer’s Disease-Like Pathology Triggered by Porphyromonas gingivalis in Wild Type Rats Is Serotype Dependent

“…Since our experimental model consists of wild type rats injected with P. gingivalis and examined for the presence of brain Ab amyloid 55 days after bacterial injection, we did not expect yet to detect the presence of accumulated Ab in plaques. Nevertheless, our protocol did detect monomeric forms as well as soluble oligomeric conformations of Ab, which are pivotal to trigger Tau hyperphosphorylation, and neuronal dysfunction, leading to cognitive deficits in AD pathology (39,(86)(87)(88)(89)(90)(91). We show that periodontal infections with the encapsulated serotypes K1 or K2 of P. gingivalis induced neuroinflammation, astrogliosis, cognitive decline, and histopathological signs of AD in the hippocampus, as compared with the less virulent K4 strain and GPA.…”

Dysregulation of Intracellular Calcium Signaling in Alzheimer's Disease

“…Studies performed by San-Martin et al demonstrated that Ab oligomers promote RyanR2-mediated Ca 2+ release, mitochondrial Ca 2+ entry, ROS generation, and fragmentation of the mitochondrial structural network. It was further shown that RyanR2 knockdown as well as usage of antioxidants reduces Ca 2+mediated noxious effects of Ab oligomers on mitochondrial function (101,102). Some AD models demonstrated intracellular Ab accumulation, which may also take part in ER calcium signaling destabilization (75).…”

Ca2+homeostasis dysregulation in Alzheimer's disease: a focus on plasma membrane and cell organelles

“…It was reported that the open probability of the RyR channel is elevated with Ab42 exposure in planar lipid bilayers (147). In the studies carried out by SanMartín et al (148), the Ab-induced Ca 2+ efflux through RyR in hippocampal neurons is prevented by the inhibition of NADPH oxidase, which suggested that oxidative stress may underlie the mechanism. Ab oligomers are likely to increase the expression of RyR, especially RyR3 (149), the most abundant isoform in brain tissue.…”