FoxO3a Governs Early Microglial Proliferation and Employs Mitochondrial Depolarization with Caspase 3, 8, and 9 Cleavage During Oxidant Induced Apoptosis

Microglia of the central nervous system have a dual role in the ability to influence the survival of neighboring cells. During inflammatory cell activation, microglia can lead to the disposal of toxic cellular products and permit tissue regeneration, but microglia also may lead to cellular destruction with phagocytic removal. For these reasons, it is essential to elucidate not only the underlying pathways that control microglial activation and proliferation, but also the factors that determine microglial survival. In this regard, we investigated in the EOC 2 microglial cell line with an oxygen-glucose deprivation (OGD) injury model of oxidative stress the role of the O class forkhead transcription factor FoxO3a that in some scenarios is closely linked to immune system function. We demonstrate that FoxO3a is a necessary element in the control of early and late apoptotic injury programs that involve membrane phosphatidylserine externalization and nuclear DNA degradation, since transient knockdown of FoxO3a in microglia preserves cellular survival 24 hours following OGD exposure. However, prior to the onset of apoptotic injury, FoxO3a facilitates the activation and proliferation of microglia as early as 3 hours following OGD exposure that occurs in conjunction with the trafficking of the unphosphorylated and active post-translational form of FoxO3a from the cytoplasm to the cell nucleus. FoxO3a also can modulate apoptotic mitochondrial signal transduction pathways in microglia, since transient knockdown of FoxO3a prevents mitochondrial membrane depolarization as well as the release of cytochrome c during OGD. Control of this apoptotic cascade also extends to progressive caspase activation as early as 1 hour following OGD exposure. The presence of FoxO3a is necessary for the expression of cleaved ( active) caspase 3, 8, and 9, since loss of FoxO3a abrogates the induction of caspase activity. Interestingly, elimination of FoxO3a reduced caspase 9 activity to a lesser extent than that noted with caspase 3 and 8 activities, suggesting that FoxO3a in relation to caspase 9 may be more reliant upon other signal transduction pathways potentially independent from caspase 3 and 8.