Protein markers of inflammation and ageing in a European multi-centre dementia cohort

Abstract

Inflammation plays a key role in Alzheimerʼs disease (AD) progression. Microglia in the AD brain are exposed to pathological amyloid β (Aβ42) and tau peptide aggregates, resulting in the NLRP3 inflammasome activation within these cells. Activated microglia release pro-inflammatory cytokines IL-1β and IL-18 resulting in the acceleration of neuronal death in AD brain. Besides AD, an ageing brain also underlies neuroinflammation due to senescent cells that generate reactive oxygen species and NLPR3 activation. Hence, aging and AD brain have inflammatory byproducts and factors within the brain parenchyma, which might be detected in the cerebrospinal fluid (CSF) as a biomarker. <br /> In this study, the primary aim was to measure the pre-selected 15 biomarkers namely YKL40, MIF, Tyro3, Axl, TREM2, VCAM1, ICAM1, TNFR1, TNFR2, C1q, C3, C4, Factor B, Factor H, and CRP in the CSF from different European dementia cohorts (DELCODE and F.ACE). It was shown that most of the biomarkers were significantly increased in the tau positive (T+) subjects, implying that tau pathology plays a crucial role in inflammatory byproduct release in the CSF in two independent cohorts. Applying these results in the PREADAPT data analysis workflow showed that subjects with elevated soluble TAM receptors, Tyro3 and Axl, in their CSF had larger cortical volume and were more stable in cognition at follow-up (Brosseron et. al., 2022). <br /> In order to determine the functional relevance of our clinical findings, an in vitro setup was used that consists of three human monocytic leukemia (THP-1) cell lines: Wild-type (Control), Tyro3-overexpressing (Tyro3OE), and Axl-overexpressing (AxlOE). Since TAM receptors facilitate phagocytosis, it was hypothesized that their overexpression in THP-1 cells might enhance tau and Aβ42 phagocytosis. There was a significant increase in Aβ42 phagocytosis by the Tyro3OE cells. Also, Aβ42 phagocytosis was reduced when the control THP-1 cells were pre-treated with tau, but this effect was absent in Tyro3OE cells. It was speculated that this beneficial role of Tyro3OE might be supported by an anti-inflammatory effect. The NLRP3 inflammasome activity was examined in these cells using two models: the AD microenvironment model (tau + Aβ42) and classical NLRP3 inflammasome model (LPS + Nigericin). A significant reduction in IL-1β release was found in the Tyro3OE when compared with control THP-1 cells. This was supported by a reduced IL-1β mRNA expression in Tyro3OE cells. STAT1 phosphorylation in Tyro3OE cells was significantly increased in western blot analysis, which when inhibited by JAK1/2 inhibitor Ruxolitinib, partially restored IL-1β release in Tyro3OE. Hence, STAT1 phosphorylation is key for Tyro3OE-mediated immunosuppression in models of AD. Lastly, novel biomarkers MerTK receptor and the TAM ligands Gas6, Protein S were also found to be elevated in the tau positive subjects in the DELCODE cohort. <br /> In summary, these results suggest that subjects with elevated TAM receptors, also have increased ligands Gas6 and Protein S which activate the TAM signalling in their brain. Activated TAM signalling leads to the Tyro3-STAT1-IL-1β pathway that was described in order to suppress inflammation and enhance Aβ42 phagocytosis. It is proposed that subjects with increased Tyro3 in their CSF might employ this mechanism that confers subtle protection against AD. Results from this thesis show promising effects for Tyro3 overexpression and activation that could disarm inflammatory cells against not only NLPR3, but also AIM2, NLRP1, and NLRC4-mediated IL-1β release and henceforth ameliorate pathogenesis in AD. Future studies are required to verify these beneficial functions of Tyro3 in AD mouse models.