Mechanisms of synapse dysfunction in Alzheimer's disease

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DESCRIPTION (provided by applicant): A prominent theory about Alzheimer's disease (AD) proposes that early cognitive deficits are due to subtle alterations in synaptic transmission, but specific AD-related changes in synaptic transmission are not well understood. In order to better understand the role of synaptic deficits during the early stages of AD, we must study the effects of AD-related proteins on synaptic transmission in a mammalian central nervous system preparation. Two proteins that have been strongly implicated in AD-related synaptic dysfunction are amyloid precursor protein (APP) and presenilin. We have recently shown that overexpression of APP depresses synaptic transmission through both pre- and postsynaptic mechanisms, and that this depression depends on production of amyloid beta peptide (A?). It remains to be determined which specific isoform of A? (A?40 or A?42) is the relevant ligand, and which surface receptors (if any) is responsible mediating its effects. Presenilin is a critical component of ?-secretase, an enzyme required for A? production. Presenilin is also known to influence storage and release of calcium from internal stores. Changes in the levels of intracellular calcium are a critical signal for many pathways inside the cell, including signals that tell neurons how much neurotransmitter to release when they fire an action potential. Thus, changes in presenilin levels or function could affect synaptic transmission by altering either A? production or intracellular calcium levels. Our long-term objective is to develop a model system that will allow us to investigate the molecules and signaling pathways that are responsible for synaptic dysfunction underlying cognitive deficits associated with AD. We will focus initially on PS1 and APP. Specific Aim 1 of this proposal is to identify the role of wild-type PS1 in synaptic transmission and test the hypothesis that expression of Familial AD-linked mutant PS1 alters synaptic transmission. Specific Aim 2a is to determine whether elevated levels of secreted A?42 depress transmission at excitatory synapses, and whether either A240 or the caspase cleavage-resistant mutant APPD664A can reduce this depression. Specific Aim 2b is to identify the role of nicotinic acetylcholine receptors, NMDA receptors, group I metabotropic glutamate receptors, and insulin receptors in APP-mediated depression of synaptic transmission. Our experimental strategy is to use electrophysiological and optical imaging techniques to identify specific changes in neurotransmission produced by virally-mediated overexpression of wild-type and mutant forms of presenilin, APP, and APP-cleavage products in cultured mouse hippocampal neurons. Our lab has extensive experience studying the effects of virally-mediated overexpression of a variety of proteins on synaptic transmission in cultured hippocampal neurons, and is, therefore, in an excellent position to exploit this system to identify the effects of AD-related proteins on neurotransmission. These studies will provide molecular targets for novel therapies to improve cognitive function and delay further neurodegeneration in patients with early Alzheimer's disease. Alzheimer's disease is the most common cause of cognitive deficits in the aged, and is thought to begin with synaptic dysfunction. Understanding the cellular and molecular mechanisms underlying this synaptic dysfunction will provide new targets for therapeutic treatments to relieve symptoms, and slow or perhaps even stop disease progression.