Pathogenic consequences of altered insulin signaling in the diabetic brain.

Nathan S. Kline Institute For Psych Res, Orangeburg NY

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Abstract

Our proposal's scientific premise is that diabetes mellitus (DM) patients are at an increased risk of developing Alzheimer's disease (AD) and that AD itself is associated with brain insulin resistance. We focus on the insulin- signaling pathway as an important mediator of AD-risk in the diabetic brain. Building upon our findings in a primate model of Type 1 DM and preliminary findings from mouse models and cultured neuronal cells, our hypothesis is that insulin dysregulation in DM results in alterations within insulin-signaling kinase cascades that predispose the brain to AD pathobiology. We have shown that increased tau phosphorylation occurs in many brain regions including the hippocampus as a result of DM, whereas the A?-degrading protease neprilysin (NEP) is downregulated selectively within the hippocampus, superimposing in this vulnerable brain region two events: increased A? and altered tau phosphorylation-that contribute to AD. Thus, NEP is a previously unappreciated insulin-responsive protease in the hippocampus. Using quantitative proteomics, we now have preliminary findings showing alterations in the expression/phosphorylation-state of multiple proteins in the diabetic hippocampus, including the non-receptor tyrosine-kinase ABL1. ABL1 is an appealing clinical target: FDA-approved ABL1 inhibitors are used to treat a myeloid leukemia resulting from constitutive ABL1 overactivity, and clinical trials are underway to determine whether autophagy-promoting effects of ABL1 inhibition are beneficial in AD. In support of this resubmission, we now show that ABL1 inhibition can positively regulate NEP expression. Aim 1 of our proposal focuses on the molecular effectors of the insulin-signaling cascade in vulnerable and relatively spared brain regions and neuron subtypes to test the idea that NEP expression is regionally regulated in response to insulin while also examining events that lead to alterations in tau phosphorylation. Our analyses include proteomics and transcriptomics to examine insulin-pathway signaling differences at the cellular and regional levels, using in vivo and in vitro models. Aim 1 will also directly manipulate the insulin-signaling pathway in the brains of mice, examining regional vulnerability of genes and proteins implicated in AD pathobiology, as well as behavioral endpoints. Aim 2 builds upon preliminary proteomic analysis of the hippocampus in diabetic monkeys that has identified ABL1 as a kinase that can be targeted to reduce AD-risk. We will test whether NEP expression, increased A? levels, and tau phosphorylation can be rescued in the diabetic brain using FDA-approved ABL1 inhibitors. This project will enhance our understanding of insulin signaling in the brain by defining novel regional and cellular responses to insulin, particularly for gene and protein alterations that impact AD-risk. Our proposed studies have translational significance, as our findings may identify novel therapeutic targets in the diabetic brain that can be regulated by repurposing existing drugs, reducing AD-risk in diabetics.