CSF1R inhibitor abrogates tau propagation exacerbated in APPNL‐G‐F knock‐in mice but enhances fibrillar beta‐amyloidosis and dystrophic neurite formation in the brain

Background Microglia are the primary innate immune cells in the brain. They can phagocytose apoptotic neurons and dystrophic neurites containing aggregated and phosphorylated tau (p‐tau) in tauopathies, and possibly spread pathological tau via extracellular vesicles (EVs) release. Proteinopathic stress like amyloid plaque accumulation can transform microglia into a neurodegenerative phenotype (MGnD), possessing enhanced phagocytic and exocytotic functions. Thus, MGnD could exacerbate tau spreading throughout the brain. To determine the role of MGnD on tau propagation, we employed microglia depletion in adeno‐associated virus (AAV)‐mediated tau propagation mouse models. Method C57BL/6 (WT) and APPNL‐G‐F mice were fed with a CSF1R inhibitor (PLX5622) or control chow for one month before and after stereotaxic injections of AAV2/6‐SYN1‐P301Ltau expressing P301L tau mutant into the medial entorhinal cortex (MEC) at 5 months of age. Propagation of tau to the dentate granular cells of the hippocampus, amyloid plaque formation, and association of microglia and p‐tau with plaques were assessed by immunohistochemistry after one month. MGnD and homeostatic microglia were separately isolated from APPNL‐G‐F mouse brains by FACS and evaluated for expression of EV marker molecules. Result FACS data showed higher percentage of Clec7a (MGnD marker)‐positive microglia (20.8%) in APPNL‐G‐F compared to WT. Immunohistochemistry of APPNL‐G‐F mouse brains revealed strong positivity of Clec7a+ microglia surrounding both amyloid plaques and plaque‐associated p‐tau. APPNL‐G‐F mice exhibited approximately a 10‐fold increase in tau propagation compared to WT mice. Strikingly, PLX5622 treatment, which depleted ∼ 99% of microglia, showed 74 and 87% reduction of tau propagation in WT and APPNL‐G‐F groups, respectively. Contrarily, PLX5622 treatment increased intensity of plaque associated p‐tau along with increased size and number of amyloid plaques in the APPNL‐G‐F mice, suggesting their regulation by MGnD. Gene expression of EV markers, CD9 and CD63, was upregulated in Clec7a+ microglia compared to Clec7– microglia isolated from APPNL‐G‐F mice, suggesting enhanced microglial EV synthesis. This was further supported by the significant colocalization of Tsg101, an exosome marker, in Clec7a+ MGnD. Conclusion Tau propagation may be influenced by engulfment of pathological tau seeds by microglia and secretion through EVs, which is exacerbated in MGnD in APPNL‐G‐F mice and mitigated by PLX5