Microglial Activation in Alzheimer's Disease

Abstract

Alzheimer's disease (AD) is the most prevalent type of dementia and characterized by the deposition of extracellular amyloid-beta (Aβ) and tau hyper-phosphorylation. Over the past decade, neuroinflammation has emerged as an additional pathological component. In the brain, microglia, representing the brain major innate immune cells, play an important role during AD. Once activated, microglia show substantial changes in their morphology, characterized by a retraction of cell processes and concomitant increase of soma volume. Systemic inflammation is known to increase the risk for cognitive decline in humans including AD, however the mechanism remains elusive. In the first part of this dissertation, microglial changes upon a transient peripheral immune challenge in the context of aging and AD are assessed in vivo, using 2-photon laser scanning microscopy (2PLSM). CX3CR1-EGFP-positive microglia were monitored at 2 and 10 days post immune challenge by lipopolysaccharide (LPS). Microglia exhibited a significant reduction of branches and the area covered by those at 2 days after LPS, a phenomenon that had been resolved at 10 days. Importantly, morphology changes were concomitant to changes in the inflammatory response. Transient systemic inflammation reduced microglial phagocytic clearance of Aβ in APP/PS1 mice, increasing amyloid deposition. Importantly, NACHT-,LRR- and pyrin (PYD)-domain-containing protein 3 (NLRP3) inflammasome deficiency blocked many of the observed microglial changes upon peripheral immune challenge, including alterations of microglial morphology and amyloid pathology. NLRP3 inhibition may thus represent a novel therapeutic strategy for brain protection during systemic inflammation. <br /> In patients with AD, deposition of amyloid-β is accompanied by activation of the innate immune system, particularly microglial NLRP3 inflammasome activation. Activation of the NLRP3 inflammasome involves the recruitment of the adaptor protein apoptosis-associated speck-like protein containing CARD (ASC). This process ultimately is leading to the formation of an ASC speck. The spreading of pathology within and between brain areas is a hallmark of neurodegenerative disorders. Although neuroinflammation is currently accepted as a hallmark of AD, its contribution to the spreading of the pathology has not been investigated. In the second part of this dissertation the involvement of microglial activation in spreading of Aβ pathology is investigated. Here it was found that ASC specks released by microglia bind rapidly to Aβ in AD patients and AD murine models. This release increased the formation of Aβ oligomers and aggregates, acting as an inflammation-driven cross-seed for Aβ pathology. Together these results support the hypothesis that inflammasome activation is connected to seeding and spreading of amyloid pathology. <br /> One of the features of AD is the degeneration of the Locus Ceruleus (LC) neurons, early in the disease. The LC is the major source of noradrenaline (NA) in the brain. Besides its role as a neurotransmitter, NA has been found to be a potent immunosuppressor acting through β-adrenoreceptors expressed by microglia cells. The strong correlation between LC degeneration, NA depletion and severity of AD in patients has prompted multiple studies of the contribution of LC dysfunction to AD progression and neuroinflammation through the use of animal models. However, current approaches to study LC degeneration rely on the use of pharmacological toxins or by using genetic knockout. These approaches carry the risk of misinterpretation due to confounding factors including modulation or loss of peripheral NA. Additionally, these approaches do not reflect the progressive nature of LC degeneration in the context of AD. Based on these observations, the third part of this dissertation it is based on the establishment of an inducible model that allows transient silencing of LC activity in response to optogenetic modulation in transgenic mice. The results presented in this part of the dissertation showed that LC silencing was successfully achieved. Additionally, simultaneous LC optogenetic modulation and in vivo imaging revealed that microglia was transiently activated by LC silencing. These results indicate that optogenetic modulation is suitable tool to study LC degeneration and highlight the importance of NA in the microglial activation process. <br /> In summary, this dissertation focuses on the role of microglial activation in the context of AD. The first part, particularly bring light to how microglial activation triggered by systemic inflammation influences the progression of AD in a NLRP3-dependent manner. In the second part, it is uncovered how microglial activation contributes to the spreading of AD pathology. Finally, the third part of this dissertation highlights the importance of LC degeneration regulating microglial activation in the context of AD. In conclusion, the work presented here shows that microglial activation is not only a consequence but also a cause for AD.