Amyloid-β(25-35) Modulates the Expression of GirK and KCNQ Channel Genes in the Hippocampus.

Abstract

During early stages of Alzheimer's disease (AD), synaptic dysfunction induced by toxic amyloid-β (Aβ) is present before the accumulation of histopathological hallmarks of the disease. This scenario produces impaired functioning of neuronal networks, altered patterns of synchronous activity and severe functional deficits mainly due to hyperexcitability of hippocampal networks. The molecular mechanisms underlying these alterations remain unclear but functional evidence, shown by our laboratory and others, points to the involvement of receptors/channels which modulate neuronal excitability, playing a pivotal role in early Aβ-induced AD pathogenesis. In particular, two potassium channels that control neuronal excitability, G protein-coupled activated inwardly-rectifying potassium channel (GirK), and voltage-gated K channel (KCNQ), have been recently linked to Aβ pathophysiology in the hippocampus. Specifically, by using Aβ25-35, we previously found that GirK conductance is greatly decreased in the hippocampus, and similar effects have also been reported on KCNQ conductance. Thus, in the present study, our goal was to determine the effect of Aβ on the transcriptional expression pattern of 17 genes encoding neurotransmitter receptors and associated channels which maintain excitatory-inhibitory neurotransmission balance in hippocampal circuits, with special focus in potassium channels. For this purpose, we designed a systematic and reliable procedure to analyze mRNA expression by reverse transcriptase quantitative polymerase chain reaction (RT-qPCR) in hippocampal rat slices incubated with Aβ25-35. We found that: 1) Aβ down-regulated mRNA expression of ionotropic GluN1 and metabotropic mGlu1 glutamate receptor subunits as previously reported in other AD models; 2) Aβ also reduced gene expression levels of GirK2, 3, and 4 subunits, and KCNQ2 and 3 subunits, but did not change expression levels of its associated GABAB and M1 receptors, respectively. Our results provide evidence that Aβ can modulate the expression of these channels which could affect the hippocampal activity balance underlying learning and memory processes impaired in AD.