The role of human microglia and microglial LRRK2 in tau pathogenesis

Abstract

AbstractBackgroundThe G2019S mutation in leucine‐rich repeat kinase 2 (LRRK2) is the most common genetic determinant of Parkinson’s disease (PD), reported in both familial and sporadic PD cases. Even though tau‐positive pathology features prominently in the G2019S PD patients, the role of LRRK2 in tau pathogenesis remains poorly understood. Human microglia – the chief brain‐resident phagocytes of the innate immune system ‐ become one of the highest LRR2K expressing cells upon IFN‐y stimulation. Moreover, microglia have been recently implicated in increasing pathological tau transmission by secreting exosome‐bound, seeding‐competent tau into the extracellular space. Using human iPS‐macrophages, a simple but physiologically relevant and genetically tractable model of human microglia, we set out to define the role of microglial LRRK2 in uptake, degradation, and secretion of disease‐associated tau protein.MethodWe have developed a quantitative phagolysosomal proteolysis assay based on dequenching of DQ‐BSA fluorogenic substrate attached to silica microspheres, allowing us to investigate cell‐autonomous phagocytic and degradative capacity of G2019S, LRRK2 KO and isogenic control iPS‐macrophages. In parallel, we have optimised purification and in vitro aggregation of endotoxin‐free, human recombinant 2N4R tau to study tau‐induced phagolysosomal proteolysis deficits in the said iPS‐macrophages.ResultWe have found that human iPS‐macrophages readily internalize extracellular tau monomer and fibrils, and LRRK2 is required for macrophage uptake of tau monomer. Degradation of internalized tau is concentration and incubation‐length dependent regardless of genotype. However, G2019S macrophages show deficits in degradation of internalized tau fibrils. Moreover, non‐degraded tau fibrils present within G2019S cell‐ lysates 24h post incubation retain seeding competency and induce aggregation of naïve tau monomer in K11 tau RT‐QuIC assay, unlike tau fibrils internalized by LRRK2 KO macrophages.ConclusionLRRK2 expressed by iPS‐macrophages is important for monomeric tau entry, degradation, and seeding capacity of internalized tau fibrils. Future studies will focus on elucidating the molecular mechanisms behind the LRRK2 and tau interaction.