Project 3 of the Johns Hopkins Alzheimer's Disease Research Center (ADRC) is titled "Neuronal activity-dependent secretion of AB and immediate early genes". This project focuses on the activity-regulated gene termed Arc/Arg3.1, in order to identify mechanisms that are critical for secretion of the amyloid-beta peptide (AB) from neurons. Arc is an immediate early gene that is transcriptionally induced in response to forms of neuronal activity that underlie learning and memory. The overarching goal of the study is to examine the molecular mechanisms that underlie activity-dependent secretion of AB and to examine their potential impact on synaptic dysfunction in Alzheimer's disease (AD). There are four specific aims: (1) Aim 1: To examine the cellular basis of Arc-dependent AB secretion. We will test the hypothesis that Arc enhances the processing of amyloid precursor protein (APP) by gamma secretase in recycling endosomes. We will examine the model that Arc enhances AB generation by recruiting gamma secretase, either from the plasma membrane or intracellular endosomes, to endosomes that traffic APP from the plasma membrane. (2) Aim 2: To examine the hypothesis that Arc binding to presenilin 1 (PS-1) is essential for Arc-dependent AB generation. We will define regions of Arc and PS-1 that are necessary and sufficient for binding. As part of this analysis, we will identify peptides that can selectively block their interaction and we will test if these peptides can interrupt activity-dependent generation of AB in primary neuronal cultures. (3) Aim 3: To examine the hypothesis that Arc contributes to AB generation and plaque deposition in vivo. These studies will monitor the age and gender dependence of soluble and insoluble AB40/42 and plaque deposition in transgenic APPswe/PS1AE9/Arc+/+ mice versus APPswe/PS1AE9/Arc-/- mice. (4) Aim 4: To examine expression of Arc and associated proteins in the brains of cognitively normal and cognitively impaired subjects. Tissue samples will be obtained through the Neuropathology Core (Core D) of the ADRC. Preliminary studies indicate that Arc protein is up-regulated in brains of patients of AD. We will determine if this is consistent across a larger group of subjects and assess the association with plaques and with dementia severity.
Understanding the mechanisms that influence the connections between brain cells and the accumulation of Alzheimer's pathology (amyloid plaques and neurofibrillary tangles) in the brains of some elderly subjects is of crucial importance in developing strategies to combat Alzheimer's disease, a neurodegenerative disorder which affects over 6 million Americans.
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