The Role of ASC Specks as Essential Cofactor in Amyloid β-induced Pathophysiology
Creators: name: Schuler, Katharina Olivia
Publisher: name: Universitäts- und Landesbibliothek Bonn
Year: 2024
Subjects: subject: Alzheimer
subject: Amyloid Plaque
subject: ASC Specks
subject: Mikroglia
subject: Mausmodell der Alzheimer Erkrankung
subject: Microglia
subject: Murine Model of Alzheimer
subject: ddc:610

description: Histopathologically and molecularly, AD is characterized by the deposition of extracellular amyloid β (Aβ) fibrils and plaques as well as intraneuronal aggregation of neurofibrillary tau tangles in the brain. Recent studies suggest a pivotal role of neuroinflammation, particularly activated innate immunity signalling, in AD pathophysiology. At a molecular level this is initiated by the formation of supramolecular complexes called inflammasomes. The NACHT-, LRR-, and pyrin domain (PYD)-containing protein 3 (NLRP3) inflammasome complex is formed by NLRP3, caspase-1 and apoptosis-associated speck-like protein containing a CARD (ASC). Recently, the NLRP3 inflammasome has been implicated as central regulator of Aβ-induced pathology of AD. In AD, amyloid begins to accumulate in the hippocampal area and afterwards spreads throughout the entire brain following specific trajectories. In experimental models the injection of synthetic Aβ into the hippocampi of <em>wildtype</em> mice failed to seed and therefore did not lead to spreading of amyloid with the subsequent development of AD pathologies. From this observation it was concluded that additional factor(s) are required to trigger Aβ-seeding and inflammation-dependent spreading events. Recent work from our group has described a central role of ASC in NLRP3 activation in microglial cells <em>in vitro</em>. The aim of this thesis was to investigate <em>in vivo</em> whether ASC aggregates, called ASC specks, are necessary and sufficient co-factors to induce seeding and spreading of synthetic Aβ injected in murine brains. <br> Towards this end, <em>wildtype</em> (C57/BL6 strain) and transgenic APP/PS1<sup>wt/tg</sup> mice were treated by stereotactic intrahippocampal injections of synthetic Aβ, ASC, or co-injection of ASC and synthetic Aβ. As positive control we injected APP/PS1<sup>wt/tg</sup> brain homogenate. Readout systems for the presence or absence of seeding and spreading were histopathological and biochemical analyses of the hippocampi three months after the injections. By fluorescence immunohistochemistry we found a statistically significant increase of Aβ signals in <em>wildtype</em> animals co-injected with ASC and synthetic Aβ. Only upon co-injection of Aβ and ASC, but not upon injection of Aβ or ASC alone, we observed the formation of amyloid composites. These aggregates differed morphologically from the classical amyloid plaques developing in APP/PS1<sup>wt/tg</sup> mice. However, co-injection of ASC and Aβ fully reproduced the phenotype induced by intrahippocampal injection of brain homogenates from APP/PS1<sup>wt/tg</sup> mice as demonstrated by automated image analyses. These “pre-plaques” most likely represent an early stage of plaque formation in AD pathophysiology. Microglial cells were recruited to the injection sites independently of the respective treatment group. In biochemical assays we could orthogonally confirm the enhancement of Aβ and recombinant ASC in hippocampal cell homogenates, but without statistical significance due to limitations in sensitivity of biochemical assays. <br> In summary, we have demonstrated a role of ASC specks as essential co-factor in early Aβ-induced AD pathophysiology <em>in vivo</em>. This supports a potential relevance of ASC as therapeutic target for novel approaches to treatment of this devastating neurodegenerative disease.