How Dysfunctional Microglia/Astrocyte Signaling Leads to Age-Associated Neuroinflammation and Cognitive Impairment

• Main Author: O'Neil, Shane Mitchell
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01.01.2021
• Subjects: Aging; Cognitive psychology; Developmental psychology; Immunology; Neurosciences
• ISBN: 9798363524950

Biological aging is a stressor causing several forms of dysfunction at the cellular, tissue, and organismal levels. As inflammatory signaling in the central nervous system is critical for normal physiological and behavioral responses to infection, the dysfunctional neuro-immune communication resulting from normal aging leads to heightened risk of mortality and co-morbidity of depression or dementia after peripheral infection. Moreover, elderly individuals suffering from an infection often experience mood disorders and cognitive dysfunction. Indeed, rodent studies have shown this altered neuroinflammatory state is primarily facilitated by dysfunctional signaling from microglia and astrocytes in the brain.Microglia, the innate immune cells of the central nervous system, develop a pro-inflammatory, "primed" profile with age. Once activated by an inflammatory stimulus, these primed cells produce an exaggerated and prolonged neuroinflammatory response mediated by cytokines and chemokines. To test the hypothesis that this primed phenotype is the result of the accumulation of cellular damage through aging, I pharmacologically forced the turnover of microglia in the brain with a colony-stimulating factor 1 receptor (CSF1R) antagonist. While this approach showed microglia in the aged brain are capable of rapidly and efficiently repopulating after cessation of CSF1R antagonism, indicating these cells are senescent in the classical sense, the repopulated cells still exhibited a pro-inflammatory response to peripheral immune challenge. Moreover, soluble signals from aged brain tissue conferred a primed phenotype to neonatal microglia ex vivo, potentiating their transcriptional response to lipopolysaccharide. Taken together, these data show age-associated priming likely results from microglia-extrinsic signals within the aged brain.Interestingly, forced turnover of microglia exerted beneficial effects on neurons within the aged brain. While microglia showed no effect of forced turnover on phagocytosis or reactive oxygen species ex vivo, in vivo transcription of Cd21 and Cd68, genes associated with dysfunctional phagocytosis in the aged brain, were normalized to adult levels by forced turnover of microglia within the aged hippocampus. Moreover, this forced turnover improved spatial memory in aged mice, indicating a neuronal effect of microglial repopulation, but not depletion. Taken together, these data implicate dysfunctional synaptic pruning in the aged brain may mediate hippocampal spatial memory deficits, and these deficits can be attenuated by the restoration of microglial phagocytic capacity in the aged brain.Finally, I used single-cell RNA-sequencing to characterize the cell-, age-, and time-specific transcriptional response to peripheral immune challenge with lipopolysaccharide and how this signaling is influenced by aging. Here, I determined that, while microglia exhibit a pro-inflammatory innate immune phenotype termed "inflammaging," astrocytes in the aged brain are immunosenescent. These astrocytes fail to respond to microglial inflammatory stimuli, thus lacking the transforming growth factor (TGF)-β and cholesterol signaling required for attenuation of microglial inflammation. Moreover, while microglia in the aged brain express increased levels of the anti-inflammatory cytokine interleukin (IL)-10, the driver of astrocytic TGFβ production, astrocytes express decreased levels of the IL-10 receptor and, therefore, fail to attenuate microglial inflammation. To confirm this, I generated an astrocyte-specific IL-10 receptor knock-out mouse line and challenged these mice with peripheral lipopolysaccharide to activate the innate immune system. As expected, mice lacking the astrocytic IL-10 receptor displayed an exaggerated and prolonged behavioral and molecular neuroinflammatory response to this peripheral immune challenge. Taken together, these data show astrocyte immunosenescence, including IL-10 receptor dysfunction and decreased cholesterol biosynthesis, drives the pro-inflammatory microglial profile observed with aging, thereby making elderly individuals more susceptible to the cognitive and neurodegenerative effects of excessive neuroinflammatory signaling.