The role of insulin and neurotrophic factor signaling in brain aging and Alzheimer's Disease

Although increased lifespan is associated with reduced insulin signaling, insulin signaling is essential for neuronal development and survival. Insulin resistance is central to Type II diabetes and is also implicated in the pathogenesis of Alzheimer's Disease (AD). This has prompted ongoing clinical trials in AD patients to test the efficacy of improving insulin - like signaling with dietary omega-3 fatty acids or insulin - sensitizing drugs as well as exercise regimens. Here we review the role of insulin signaling in brain aging and AD, concluding that the signaling pathways downstream to neurotrophic and insulin signaling are defective and coincident with aberrant phosphorylation and translocation of key components, notably AKT and GSK3 beta, but also rac > PAK signaling. These responses are likely to contribute to defects in synaptic plasticity, learning and memory. Both oligomers of beta-amyloid (which are elevated in the AD brain) and pro-inflammatory cytokines (which are elevated in the aged or AD brain) can be used to mimic the trophic factor/insulin resistance observed in AD, but details on other factors and mechanisms contributing to this resistance remain elusive. A better understanding of the precise mechanisms underlying alterations in the insulin/neurotrophic factor signal transduction pathways should aid the search for better AD therapeutic and prevention strategies. (c) 2006 Elsevier Inc. All rights reserved.