GABA activation of the M-current

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This Alzheimer’s disease (AD) supplement proposal is based on two recent discoveries from the Abbott lab. The first discovery, which forms the basis of the parent R01, is that KCNQ2/3 voltage-gated potassium (Kv) channels are directly activated by γ-aminobutyric acid (GABA), the primary inhibitory neurotransmitter in vertebrate CNS, with a sensitivity comparable to that of the most sensitive canonical GABAA receptors. This newly described form of regulation is potentially highly influential because KCNQ2/3 channels are the primary molecular correlate of neuronal M-current, a powerful inhibitory force in mammalian brain. The second discovery is our more recent finding that the Alzheimer’s precursor protein (APP) cleavage product, C99, both bears similarities to the KCNE family of Kv channel regulatory subunits, and itself regulates multiple Kv channels, including those of the KCNQ family. Specifically, we found that C99 co-localizes with KCNQ2/3 in vivo, forms physical complexes with KCNQ2/3 in vitro, and inhibits KCNQ2/3 function in vitro. We now intend to study the relevance to AD of C99 regulation of KCNQ2/3 channels in vivo, as a logical extension of the parent R01, bringing an AD perspective to it. We propose two Specific Aims directed towards a fuller understanding of the novel potential role in AD pathology of C99, APP and potentially other APP cleavage product inhibition of neuronal KCNQ channels. We plan to utilize the Kcnq3 and Kcnq5 GABA binding-site mutant mouse lines we developed for the parent R01, in conjunction with the well-characterized 3xTg-AD mouse model that mimics many features of human AD. In Aim 1, we will test the hypothesis that neuronal M-current is inhibited in the 3xTg-AD mouse brain. This hypothesis is based on our in vitro data, that suggest increased neuronal concentrations of C99, and potentially other APP cleavage products, will inhibit neuronal M-current and alter its gating kinetics in vivo. We will also determine whether 3xTg-AD mouse brains have altered GABA regulation of M-current, linking this aim tightly with the parent R01 but with an AD focus that was absent from the parent R01. In Aim 2, we will test the hypothesis that disrupted GABA regulation of M-current exacerbates behavioral changes in AD. We will quantify behavioral changes in Kcnq3-W265L and Kcnq5-W270L GABA binding-site mutant mice bred for the parent R01, in both a wild-type and 3xTg-AD background, and compare them to homozygous 3xTg-AD mice, to determine if disruption of GABA modulation of KCNQ channels exacerbates AD pathogenesis. The overall goals of this AD supplement are to (i) determine if C99 accumulation in early AD correlates with downregulation of M-current, as suggested by our in vitro studies and (ii) determine how this affects neuronal firing; (iii) identify whether preventing GABA modulation of KCNQ channels exacerbates neuronal and M-current firing defects, and behavioral abnormalities, in AD. The studies in this supplement have the potential to identify novel therapeutic directions for cognitive dysfunction in AD.