β-amyloid peptide at sublethal concentrations downregulates brain-derived neurotrophic factor functions in cultured cortical neurons

The accumulation of beta-amyloid (Abeta) is one of the etiological factors in Alzheimer's disease ( AD). It has been assumed that the underlying mechanism involves a critical role of Abeta-induced neurodegeneration. However, low levels of Abeta, such as will accumulate during the course of the disease, may interfere with neuronal function via mechanisms other than those involving neurodegeneration. We have been testing, therefore, the hypothesis that Abeta at levels insufficient to cause degeneration ( sublethal) may interfere with critical signal transduction processes. In cultured cortical neurons Abeta at sublethal concentrations interferes with the brain-derived neurotrophic factor (BDNF)-induced activation of the Ras-mitogen-activated protein kinase/extracellular signal-regulated protein kinase (ERK) and phosphatidylinositol 3-kinase (PI3-K)/Akt pathways. The effect of sublethal Abeta(1-42) on BDNF signaling results in the suppression of the activation of critical transcription factor cAMP response element-binding protein and Elk-1 and cAMP response element-mediated and serum response element-mediated transcription. The site of interference with the Ras/ERK and PI3-K/Akt signaling is downstream of the TrkB receptor and involves docking proteins insulin receptor substrate-1 and Shc, which convey receptor activation to the downstream effectors. The functional consequences of Abeta interference with signaling are robust, causing increased vulnerability of neurons, abrogating BDNF protection against DNA damage- and trophic deprivation-induced apoptosis. These new findings suggest that Abetaengenders a dysfunctional encoding state in neurons and may initiate and/or contribute to cognitive deficit at an early stage of AD before or along with neuronal degeneration.