Diet induced neuroinflammation and cognitive decline
2017-12
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Diet induced neuroinflammation and cognitive decline
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2017-12
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The prevalence of obesity in the U.S. is over 50%, and midlife obesity is a clinical risk factor for cognitive impairment and the onset of neurodegenerative diseases such as Alzheimer’s disease (1-3). Inflammation of the brain (neuroinflammation), a state associated with progressive neuronal loss, is heightened in cognitive decline and obesity (4-7). Consumption of high fat diets (HFD), specifically those high in the saturated fatty acid palmitic acid (C16:0; PA), exacerbate neuroinflammation, neurodegeneration, and cognitive impairment (8-16). The research presented within this thesis seeks to define the role of microglia in the context of obesity and cognition. The central hypothesis of my thesis is that high fat diets induce microglial activation resulting in altered immunometabolic response, neuroinflammation, and subsequent cognitive decline. While neuroinflammation normally increases with age, risk of neuroinflammation and cognitive impairment is exacerbated by obesogenic diets (14). The findings from this work will provide a deeper understanding of diet-induced neuroinflammation, and will facilitate the development of novel therapeutics for cognitive disorders. This research has focused on utilizing basic science approaches to understand the effects of high fat diets on the central nervous system. Highlighted below are the major findings from my dissertation research. 1) Orexin A-induced neuroprotection: Excess intake of dietary PA increases the risk for developing obesity (17, 18). PA is known to induce neuronal cell death in the hypothalamus, a region of the brain important in regulating feeding behaviors (11, 19-21). One potential target to prevent this is orexin A, a hypothalamic signaling protein important in promoting obesity resistance that also has neuroprotective properties (22). I hypothesized that orexin A would protect against PA-induced hypothalamic cell death. To test this, I evaluated the response of hypothalamic neurons to orexin A and PA. I demonstrated that orexin A decreases PA-induced programmed cell death and stabilizes expression of the pro-survival gene Bcl-2. I also demonstrated that orexin A protects against PA-induced damage to the mitochondria (measured via changes in reactive oxygen species (ROS) and mitochondrial respiration). These data support that orexin A protects against PA-induced hypothalamic cell death. 2) Orexin A signaling in PA-activated microglia: Obesity is associated with chronic low-grade inflammation, characterized by increased circulating pro-inflammatory signals and immune cell activation (8, 11, 23). Microglia are highly responsive to changes throughout the brain, and communication between neurons and microglia depends in part on pro- or anti-inflammatory secreted signals (cytokines and chemokines). Furthermore, PA promotes microglia to release pro-inflammatory signaling cascades (9, 24-26). My next goal was to determine how PA and orexin A treatment influences microglial secretion and activation states. To test this, I exposed microglial cells to orexin A and PA and measured changes in secreted signals. I demonstrated that orexin A treatment reduces PA-induced upregulation of pro-inflammatory markers and increases anti-inflammatory markers in microglia. Next, I sought to determine if the factors secreted by the activated microglia influenced neuronal survival. To test this, I filtered the microglial cell culture media to remove excess orexin A and PA while retaining the secreted cytokines, exposed neurons to this filtered supernatant, and determined neuronal cell death. I found that neurons exposed to media from orexin A-treated microglia have increased cell survival compared to those treated with media from PA-activated microglia. This result demonstrated that microglia exposed to orexin secreted protective signals that protected neurons, whereas microglia exposed to PA alone secreted harmful cytokines that resulted in neuronal cell death. My findings are the first to demonstrate that orexin A modulates PA-activated microglial cells. 3) Loss of orexin and high fat diet increases cognitive impairment: Obesity is recognized as a risk factor for development of cognitive disorders such as Alzheimer’s disease (3). Moreover, deficiencies in orexin signaling have been linked to neurodegenerative diseases. My overall hypothesis was that reduced orexin signaling will increase diet-induced cognitive decline through a microglial-mediated pathway. To test this, I used wild type (WT) mice or a mouse model of orexin loss to determine differences in a cognitive task. I found that mice lacking orexin showed significant impairments in cognition vs. WT mice. Next, to determine the effects of HFD on microglia and cognition, mice were placed on a HFD or remained on normal chow, and the cognitive task was retested at 2 and 4 weeks. I demonstrated that cognition was impaired and microglial activation was increased in mice lacking orexin given a HFD vs. WT mice. Collectively, my results show that orexin loss impairs cognition, and that HFD accelerate cognitive deficits and the onset of neuroinflammation in orexin-deficient mice. 4) Fatty acid binding protein 4-uncoupling protein 2 axis in modulating microglia and cognition: Fatty acid binding proteins (FABP) are lipid chaperones regulating metabolic and inflammatory pathways in response to fatty acids (27, 28). To further define a mechanism for diet-induced microglial activation and cognitive decline, I sought to determine if the FABP4-UCP2 (uncoupling protein 2) axis is involved in neuroinflammation. I hypothesized that inhibition of microglial FABP4 would upregulate UCP2 and attenuate PA-induced inflammation. To test this, I measured hypothalamic gene expression changes in WT mice and mice lacking FABP4 (AKO mice) fed a HFD. I found hypothalamic tissue from AKO mice exhibit increased UCP2 expression and reduced pro-inflammatory makers compared to WT mice. Next, I pharmacologically inhibited FABP4 in microglia and demonstrated increased UCP2 expression and reduced PA-induced pro-inflammatory response and ROS production. Further, this effect is negated in microglia lacking UCP2, indicating the FABP4-UCP2 axis is pivotal in obesity-induced neuroinflammation. Finally, to determine if the FABP4-UCP2 axis was involved in attenuating diet-induced cognitive decline, WT and AKO mice were fed a HFD for 12 weeks and tested in a panel of cognitive tasks. I found that mice maintained on a HFD had reduced locomotor activity. Further, WT mice maintained on HFD had impaired memory, and AKO had attenuated HFD-induced memory impairment. Collectively, these results indicate that the FABP4-UCP2 axis is a link between HFD, neuroinflammation, and cognitive impairment.
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University of Minnesota Ph.D. dissertation.December 2017. Major: Nutrition. Advisor: Tammy Butterick. 1 computer file (PDF); xii, 143 pages.
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