The Forkhead Transcription Factor FOXO3a Controls Microglial Inflammatory Activation and Eventual Apoptotic Injury through Caspase 3

Yan   Chen   Shang; Zhao   Zhong   Chong; Jinling      Hou; Kenneth      Maiese

Abstract

Memory loss and cognitive failure are increasingly being identified as potential risks with the recognized increase in life expectancy of the general population. As a result, the development of novel therapeutic strategies for disorders such as Alzheimers disease have garnered increased attention. The etiologies that can lead to Alzheimers disease are extremely varied, but a number of therapeutic options are directed against amyloid-β peptide and inflammatory cell regulation to prevent or halt progressive cognitive loss. In particular, inflammatory microglial cells may have disparate functions that in some scenarios lead to disability through the removal of functional neurovascular cells and in other circumstances foster tissue repair. Given the significance microglial cells hold for neurodegenerative disorders, we therefore examined the function that amyloid (Aβ1-42) has upon the microglial cell line EOC 2 and identified a novel role for the forkhead transcription factor FoxO3a and caspase 3. Here we show that Aβ1-42 leads to progressive injury and apoptotic cell loss in microglial cells that involves both early phosphatidylserine (PS) externalization and late genomic DNA fragmentation over a 24 hour course. Prior to these injury programs, Aβ1-42 results in the activation and proliferation of microglia as demonstrated by increased proliferating cell nuclear antigen (PCNA) expression and bromodeoxyuridine (BrdU) uptake. Both apoptotic injury as well as the prior activation and proliferation of microglial cells relies upon the presence of FoxO3a, since specific gene silencing of FoxO3a promotes microglial cell protection and prevents the early activation and proliferation of these cells. Furthermore, Aβ1-42 exposure maintained FoxO3a in an unphosphorylated “active” state and facilitated the cellular trafficking of FoxO3a from the cytoplasm to the cell nucleus to potentially lead to “pro-apoptotic” programs by this transcription factor. One apoptotic program in particular appears to involve the activation of caspase 3, since loss of FoxO3a through gene silencing prevents the induction of caspase 3 activity by Aβ1-42.

Keywords: Alzheimer's disease, amyloid, apoptosis, bromodeoxyuridine, caspases, forkhead transcription factors, FoxO3a, inflammation, microglia, oxidative stress