Abstract
Dysfunctional regulation of the type 2 ryanodine receptor (RyR2) can result in increased open time of this channel and spontaneous calcium release (SCR) from the endoplasmic reticulum, leading to increased neuronal excitability. This process has been linked to both seizures and Alzheimer’s disease. Protein kinase CK2 (CK2) has been identified as a novel regulator of RyR2 function in the heart. In the heart, reduced phosphorylation of RyR2 by CK2 increased SCR, while hyperphosphorylation showed protection against SCR. As CK2 phosphorylation of RyR2 modifies SCR, this study investigated if CK2 phosphorylation of RyR2 controls the excitability of hippocampal CA1 pyramidal neurons.
Mice expressing RyR2 mutations that either mimicked hyperphosphorylation of RyR2 by CK2 (+CK2) or prevented phosphorylation of RyR2 by CK2 (-CK2) were used to prepare brain slices containing the CA1. Through ex vivo calcium imaging using neuronally expressed GCaMP6f, the spontaneous activity of CA1 neurons in these two genotypes was compared to wildtype controls; wildtype (7 animals, 21 slices); +CK2 (10 animals, 35 slices); -CK2 (8 animals, 25 slices). The occurrence of activity and cumulative duration of calcium transients were used to characterise excitability in both basal conditions and with a generalised increase in excitability through application of 4-aminopyradine.
The +CK2 neurons showed an increase in occurrence of activity; the mean (±SEM) percentage of active neurons was 52 ± 3.5% for +CK2 and 38 ± 4.3% for wildtype (two-way ANOVA, P <0.05). No difference in occurrence of activity was seen in -CK2 neurons. When comparing the cumulative duration of calcium transients there was no difference between genotypes. Overall, results suggest that hyperphosphorylation of RyR2 by CK2 increases neuronal excitability and may present a novel mechanism of neuronal hyperexcitability. This also prompts further consideration of the differences between regulation of RyR2 in neurons and the heart.