Abstract
The calcium (Ca2+) hypothesis proposes that Ca2+ dyshomeostasis underlies Alzheimer’s disease (AD) pathophysiology. Ryanodine receptors type II (RyR2) are key intracellular Ca2+-release channels highly expressed in CA1 hippocampal neurons that regulate neuronal Ca2+ homeostasis. The formation of RyR2 clusters and functionally coupled groups of clusters (calcium release units; CRUs) regulate RyR2-mediated Ca2+ release. Changes in RyR2 activity influences learning, memory, and neuronal excitability, whilst pathological RyR2 Ca2+ release impairs synaptic transmission, leading to AD-like symptoms. We wanted to investigate the organization of RyR2 clusters and CRUs in an AD murine model.
Direct stochastic optical reconstruction microscopy (dSTORM) was used to assess nanoscale properties of RyR2 clusters and CRUs in CA1 hippocampal neurons of male and female APPswe/PS1dE9 (AD) mice and wild-type (WT) littermates at 7-9, 12-15, 19-21, and 31-33 weeks of age. We wanted to assess these changes pre (7-21 weeks) and during cognitive impairment (31-33 weeks). At 7-9 and 12-15 RyR2 cluster size was unchanged. However, RyR2 cluster size was reduced in AD animals at 19-21 weeks (P=0.0005) and 31-33 weeks (P=0.0005). In AD, there are fewer clusters per CRU at 7-9 weeks (P=0.0106), 12-15 weeks (P=0.0140), 31-33 weeks (P=0.0232), but is unaltered at 19-21 weeks compared to WT animals. These changes suggest that in AD pathology, clusters remodel differently, increasing the risk of pathological RyR2-mediated Ca2+ release that diminishes with age. Collectively, this suggests Ca2+ dyshomeostasis, exhibited between 7-21 weeks of age, precedes the cognitive impairment, exhibited at 31-33 weeks, observed in this AD-like mouse model. Therefore, targeting RyR2 clusters could be a new therapeutic to slow the progression of AD. By targeting RyR2 clusters, this has the potential to re-establish Ca2+ homeostasis within neurons that can reduce the risk of seizure onset in AD patients, but also prevent the disease from progressing further. Additionally, understanding timepoints at which Ca2+ homeostasis is altered can provide insight to how we can treat different stages of the disease and its clinical manifestations.