The fluorescent ligand bTVBT2 reveals increased p-tau uptake by retinal microglia in Alzheimer's disease patients and AppNL-F/NL-F mice

Background Amyloid beta (A beta) deposits and hyperphosphorylated tau (p-tau) accumulation have been identified in the retina of Alzheimer's disease (AD) patients and transgenic AD mice. Previous studies have shown that retinal microglia engulf A beta, but this property decreases in AD patients. Whether retinal microglia also take up p-tau and if this event is affected in AD is yet not described. In the current study, we use the p-tau-specific thiophene-based ligand bTVBT2 to investigate the relationship between disease progression and p-tau uptake by microglia in the retina of AD patients and AppNL-F/NL-F knock-in mice, an AD mouse model known to demonstrate extracellular A beta plaques and dystrophic neurites in the brain from 6 months of age.Methods Evaluation of bTVBT2 specificity and its presence within microglia was assessed by immunofluorescent staining of hippocampal sections and flat-mount retina samples from non-demented controls, AD patients, 3-, 9-, and 12-month-old AppNL-F/NL-F knock-in mice and 12- and 18-month-old wild type (WT) mice. We used ImageJ to analyze the amount of bTVBT2 inside Iba1-positive microglia. Co-localization between the ligand and p-tau variant Ser396/Ser404 (PHF-1), A beta, phosphorylated TAR DNA binding protein 43 (pTDP-43), and islet amyloid polypeptide (IAPP) in the brain and retina was analyzed using confocal imaging.Results Confocal imaging analysis showed that bTVBT2 binds to PHF-1- and AT8-positive aggregates inside retinal microglia, and not to A beta, pTDP-43, or IAPP. The density of bTVBT2-positive microglia was higher in cases with a high A beta load compared to those with a low A beta load. This density correlated with the neurofibrillary tangle load in the brain, but not with retinal levels of high molecular weight (aggregated) A beta 40 or A beta 42. Analysis of AppNL-F/NL-F knock-in mouse retina further showed that 50% of microglia in 3-month-old AppNL-F/NL-F knock-in mice contained bTVBT2. The percentage significantly increased in 9- and 12-month-old mice.Conclusion Our study suggests that the microglial capability to uptake p-tau in the retina persists and intensifies with AD progression. These results also highlight bTVBT2 as a ligand of interest in future monitoring of retinal AD pathology.