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

Abstract

Abstract Background: Amyloid beta (Ab) 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 Ab, but this property decrease 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 microglia p-tau uptake in the retina of AD patients and AppNL-F/NL-F knock-in mice, an AD mouse model known to demonstrate extracellular Ab plaques and dystrophic neurites in the brain from at 6 months of age. Methods: Evaluation of bTVBT2 specificity and presence of bTVBT2 inside microglia was performed by immunoflourescently stain hippocampi sections and whole mount samples of retina from non-demented controls (NC), AD patients, 3-, 9- and 12-months-old AppNL-F/NL-F knock-in mice and 12-month-old wild type (WT) mice. The amount of bTVBT2 inside Iba-1-positive microglia was analyzed using ImageJ and co-localization between the ligand and p-tau variant Ser396/Ser404 (PHF-1), Ab, phosphorylated TAR DNA binding protein 43 (pTDP-43) and islet amyloid polypeptide (IAPP) in brain and retina was analyzed using confocal imaging. Results: Confocal imaging analysis showed that bTVBT2 binds to PHF-1-positive aggregates inside retinal microglia, and not to Ab, pTDP-43 or IAPP. The density of bTVBT2 positive microglia was higher in cases with high Ab load patients compared to cases with low Ab load and correlated with neurofibrillary tangle load in brain, but not with retinal levels of high molecular weight (aggregates) Ab40 or Ab42. Analysis of AppNL-F/NL-F knock-in mouse retina further showed that 50% of microglia in 3-months-old AppNL-F/NL-F knock-in mice contained bTVBT2, and that the percentage was significantly increased in 9- and 12-months-old mice. Conclusion: Our study suggests that microglial capability to take up p-tau in retina remains and enhances along with AD progression. These results also highlight bTVBT2 as a ligand of interest in future monitoring of retinal AD pathology.