The amyloid precursor protein (APR) is sequentially cleaved by BACE1 and PS/y-secretase to yield A(3. APLP2 undergoes similar processing, resulting in the production of an Ap-like peptide. We have recently identified a third substrate for BACE1 and PS/y-secretase activities, the (32-subunit ((32) of the voltage-gated sodium channel (Nav1). Nav1s are almost exclusively responsible for the rising phase of action potentials. Our preliminary data indicate that BACE1-/y-secretase-mediated processing of (32 regulates sodium channel function and both mRNA and protein levels of a NaJ a-subunit, Nav1.1, in neuronal cells and BACE1 transgenic mice. In AD patient brains with elevated BACE1 levels, we also found elevated (32 C-terminal fragments and Nav1.1 levels. Interestingly, increased incidence of epileptic seizures has been associated with AD, more frequently with familial forms AD caused by mutations in PS1. Mutations in both a- and (3- subunits of the Najs have been linked to epileptic symptoms, induced by both decreased and increased sodium channel activity resulting in an imbalance in sodium channel function. Therefore, altered BACE1/ysecretase- mediated cleavages may result in two separate AD-associated pathogenic events, changes in A(3 generation (toxic amyloid formation) and sodium channel malfunction (epileptic seizures). To define the factors regulating these cleavages, here we propose to address the hypothesis that novel binding proteins regulate the processing and metabolism of (32 and these may be shared by APP and/or APLP2. In these studies, we will primarily characterize (32 processing and metabolism since this protein is not as well characterized as APP. We will take advantage of APLP2 as a third BACE1/y-secretase substrate.
In Specific Aim 1 we identify novel binding partners for (32 and assess their functional significance in proteolytic processing of (32. If these interacting proteins are shared by APP or APLP2, we will also study whether they affect processing APP or APLP2 including A(3 generation.
Specific Aim 2 will characterize metabolic pathways of (32 regulating sodium channel function, and whether these are shared by APP and APLP2. Since all three proteins share localization to lipid rafts, we will analyze these in addition to all major subcellular compartments for the presence of BACE1/y-secretase substrates. We will assess the effect of newly identified binding partners on metabolic pathways of (32 and on sodium channel function in cell-based assays and in vivo. The effects of seizure-associated FAD mutations in PS1 on sodium channel levels and function will also be determined in cells and in vivo. Results from these experiments will uncover novel processing or metabolic pathways that may lead to therapeutic strategies aimed at reducing A(3 generation as well as normalizing membrane excitability in AD patients with epileptic symptoms. Lay language: We propose to understand aspects of how amyloid is formed in the brains of Alzheimer's patients and why some patients develop seizures. This information can help find therapies for Alzheimer's disease.
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