Inhibition of NADPH oxidase promotes alternative and anti–inflammatory microglial activation during neuroinflammation

NADPH oxidase is a multi-subunit enzyme complex responsible for the production of both extracellular and intracellular reactive oxygen species (ROS) by phagocytic cells including microglia. NADPH oxidase is comprised of cytoplasmic subunits (p47phox, p67phox, p40phox, and Rac2), which upon phosphorylation by specific kinases can form a complex and translocate to the membrane to dock with the membrane subunits (gp91phox and p22phox) (Babior 1999). NADPH oxidase expression is up-regulated in Alzheimer’s disease (AD) (Shimohama et al. 2000; Bruce-Keller et al. 2010) and is an essential component of microglia-mediated amyloid neurotoxicity (Qin et al. 2002, 2004). Microglia are ubiquitously distributed throughout the brain and function as resident macrophages. They are highly dynamic and constantly carry out homeostatic surveillance to sense and respond to CNS abnormalities (Nimmerjahn et al. 2005). Upon detection of alterations in brain homeostasis, microglia undergo morphological and functional changes, referred to as microglial activation. Microglia are found surrounding amyloid plaques in the brain of AD patients and in transgenic AD mouse models (McGeer et al. 1987; Wyss-Coray 2006). However, whether activated microglia play detrimental or beneficial roles in AD remains to be elucidated. In the last few years, much attention has been focused on the functional states of microglia rather than generalized activation by determining generic markers. Like peripheral macrophages, microglia are functionally polarized into different activation phenotypes during neuroinflammation. In the classical activation state, pro-inflammatory cytokines and ROS induce tissue damage and pathogen destruction, whereas the anti-inflammatory cytokine IL-4 induces an alternative activation state, characterized by the expression of arginase 1 (Arg1), Found in Inflammatory Zone 1 (Fizz1), mannose receptor 1 (Mrc1), and chitinase 3-like 3 (Chi3l3/Ym1), which dampen the inflammatory response and promote tissue repair and healing response (Martinez et al. 2008; Colton 2009). Recently, an age-dependent switch in the microglial phenotype from an alternative to a classical activation state has been reported in a transgenic mouse model of AD (Jimenez et al. 2008). Similarly, young and aged mice challenged with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine exhibit age-related microglia activation and neurodegeneration (Sugama et al. 2003). At present, it is not clear why microglial activation significantly differs between young and aged mice, and how it is regulated in the aged brain or during the progression of neurodegenerative diseases. Thus, a better understanding of the mechanisms and functional significance of microglial activation state may provide novel therapeutic anti-inflammatory approaches. In this study, we show that gene deletion or pharmacological inhibition of NADPH oxidase drives microglial phenotype from a classical to an alternative activation state. Finally, we link NADPH oxidase-dependent effects on microglial activation state to the imbalance between markers of classical versus alternative microglial phenotype in the brain of AD patients.