IL-6 Mediated Degeneration of Forebrain GABAergic Interneurons and Cognitive Impairment in Aged Mice through Activation of Neuronal NADPH Oxidase Laura L. Dugan1,2*, Sameh S. Ali1, Grigoriy Shekhtman1, Amanda J. Roberts3, Jacinta Lucero1, Kevin L. Quick1, M. Margarita Behrens1 1 Division of Geriatric Medicine, Department of Medicine, University of California San Diego, La Jolla, California, United States of America, 2 Department of Neurosciences, University of California San Diego, La Jolla, California, United States of America, 3 Mouse Behavioral Assessment Core, The Scripps Research Institute, La Jolla, California, United States of America Abstract Background: Multiple studies have shown that plasma levels of the pro-inflammatory cytokine interleukin-6 (IL-6) are elevated in patients with important and prevalent adverse health conditions, including atherosclerosis, diabetes, obesity, obstructive sleep apnea, hypertension, and frailty. Higher plasma levels of IL-6, in turn, increase the risk of many conditions associated with aging including age-related cognitive decline. However, the mechanisms underlying this association between IL-6 and cognitive vulnerability remain unclear. Methods and Findings: We investigated the role of IL-6 in brain aging in young (4 mo) and aged (24 mo) wild-type C57BL6 and genetically-matched IL-62/2 mice, and determined that IL-6 was necessary and sufficient for increased neuronal expression of the superoxide-producing immune enzyme, NADPH-oxidase, and this was mediated by non-canonical NFkB signaling. Furthermore, superoxide production by NADPH-oxidase was directly responsible for age-related loss of parvalbumin (PV)-expressing GABAergic interneurons, neurons essential for normal information processing, encoding, and retrieval in hippocampus and cortex. Targeted deletion of IL-6 or elimination of superoxide by chronic treatment with a superoxide-dismutase mimetic prevented age-related loss of PV-interneurons and reversed age-related cognitive deficits on three standard tests of spatial learning and recall. Conclusions: Present results indicate that IL-6 mediates age-related loss of critical PV-expressing GABAergic interneurons through increased neuronal NADPH-oxidase-derived superoxide production, and that rescue of these interneurons preserves cognitive performance in aging mice, suggesting that elevated peripheral IL-6 levels may be directly and mechanistically linked to long-lasting cognitive deficits in even normal older individuals. Further, because PV-interneurons are also selectively affected by commonly used anesthetic agents and drugs, our findings imply that IL-6 levels may predict adverse CNS effects in older patients exposed to these compounds through specific derangements in inhibitory interneurons, and that therapies directed at lowering IL-6 may have cognitive benefits clinically. Citation: Dugan LL, Ali SS, Shekhtman G, Roberts AJ, Lucero J, et al. (2009) IL-6 Mediated Degeneration of Forebrain GABAergic Interneurons and Cognitive Impairment in Aged Mice through Activation of Neuronal NADPH Oxidase. PLoS ONE 4(5): e5518. doi:10.1371/journal.pone.0005518 Editor: Christoph Kleinschnitz, Julius-Maximilians-Universita¨t W u¨rzburg, Germany Received March 3, 2009; Accepted April 17, 2009; Published May 13, 2009 Copyright: 2009 Dugan et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by NIA AG030320, the Larry L. Hillblom Foundation, and a Hartford Beeson Scholar Collaborative Award (LLD), and by an NIH Blueprint Grant NS057096 (AJR). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: ladugan@ucsd.edu Introduction Multiple studies have found that circulating markers of inflammation, including interleukin-6 (IL-6), are increased in older individuals[1–3] and are strongly associated with enhanced risk of disease and disability[3]. Longitudinal studies on centenar- ians and long-lived human populations indicate that longevity is inversely correlated with plasma levels of IL-6[2,4], and conversely, that higher plasma levels of this interleukin increase the risk of many conditions associated with aging[5], including age-related cognitive decline[3,6–8]. In recent epidemiologic studies, peripheral IL-6 levels showed an inverse correlation with cognitive function in aged subjects[3,6], with chronic elevation of peripheral IL-6 associated with mild cognitive deficits[7], even in apparently healthy community-living older adults[8]. Elevated circulating IL-6 also enhances risk of stroke and dementia in older individuals[9] and has been shown to predict future cognitive decline[6]. Despite this body of literature, the specific mechanisms which link peripheral IL-6 and central nervous system impairment are still largely unknown. In the periphery, IL-6 is well characterized as a pleiotropic cytokine with trophic effects on T-cells and other immune cells (reviewed in [2,3,10]). IL-6 also acts as an important pro- inflammatory cytokine and key mediator of the acute phase response, including induction of C-reactive protein (CRP). Canonical IL-6 signaling involves binding of IL-6 to its cognate receptor (IL-6R)[11] to cause dimerization of two gp130 proteins, which in turn activate intracellular signaling and transcriptional PLoS ONE | www.plosone.org 1 May 2009 | Volume 4 | Issue 5 | e5518