Introduction: Prion diseases are a group of rare disorders affecting the central nervous system of animals and humans. Prion diseases occur following structural modification and accumulation of the misfolded cellular prion protein, PrP, causing spongiosis of the grey matter, reactive gliosis and neurodegeneration. Despite ongoing research into the mechanisms underlying prion disease neuropathology no cure is currently available for treatment of the disease, making it a uniformly fatal condition. Microglia are innate immune cells residing within CNS parenchyma that are highly sensitive regulators of brain homeostasis and function. Microglia undergo morphological and functional changes, adopting either pro-inflammatory and anti-inflammatory states.
Aims & Objectives: This study aims to explore and characterise microglial heterogeneity in chronic CNS neurodegeneration in prion-infected mice using a range of microglia markers: AIF1 (pan-microglia), TMEM119 (homeostatic microglia), and LGALS3 (disease-associated microglia).
Methods: Brains were analysed from mice infected with a range of distinct prion agent strains (ME7 scrapie, n=6; 79V scrapie, n=5; 22L scrapie n=6; 139A scrapie, n=6; and healthy control, n=6). Different combinations of prion agent strains were included in this study to allow comparison. Histopathological assessment of mouse brain tissue was performed using anti-AIF1, anti-TMEM119, and anti-LGALS3 antibodies to detect microglia, alongside anti-PrP to detect the abnormal accumulations of prion disease-specific PrP.
Statistical analysis: Kaplan-Meier statistical analyses were performed on the survival data and Two-way ANOVA was used to analyse the severity of disease pathogenesis. The morphology of microglia cells was analysed, generating matrices of single-cell features. Two-way ANOVA and appropriate post-hoc comparisons were performed to compare distribution of microglia cells. Significance level was set at p<0.05.
Results: My research identified the interplay between the distribution of microglial phenotypes and the infective prion strain agent. Inverse correlation was observed between TMEM119 and LGALS3 microglia subsets, highlighting differential activation states. During the early stages of prion disease, microglia predominantly displayed an anti-inflammatory, homeostatic profile (TMEM119+ microglia), as prion pathology progresses, microglia population expands and shifts towards neurotoxic, pro-inflammatory state (LGALS3+ microglia). This study found that LGALS3 expressing microglia are predominantly associated with the white matter tracts, while TMEM119 microglia are found in the grey matter areas. The regional distribution of microglia subtypes and its morphological features were determined by the infective prion strain; with grey matter-associated microglia exhibiting increased branching index compared to cells residing within myelin-rich white matter areas.
Conclusions: This study identified the regional and morphological heterogeneity of microglia in prion disease. Further work is necessary to improve our understanding of the interplay between different microglia phenotypes and their regional distributions in neurodegeneration, as it would allow the development of novel therapeutic targets.