Rapid Cell Type-Dependent Uptake of 0N4R TAU Monomer is not Solely Heparin Sulfate Proteoglycan Dependen

Abstract

The brains of Alzheimer's patients typically show a distribution of extracellular amyloid plaques, composed of Aβ and neurofibrillary tangles, composed of postranslationally-modified and abnormally aggregated tau protein. The mechanism underlying the initial conversion and spread of pathological tau aggregates remains unknown. Recent work suggests that tau species can be released into the extracellular space and be subsequently endocytosed via negatively-charged heparan sulfate proteoglycans (HSPGs) available on the cell surface. Here, we produced monomeric 0N4R tau in both nonphosphorylated and phosphorylated states via recombinant expression in E. coli and purified for testing in cell culture. Using live-cell confocal microscopy and flow cytometry, we show that soluble 0N4R monomer can be rapidly endocytosed by multiple cell types and that this uptake is independent of phosphorylation state. Furthermore, we demonstrate that HSPGs enhance, but are not essential, for cellular endocytosis of both phosphorylated and non-phosphorylated tau. Current studies are focused on the impact of isoform type and degree of aggregation on uptake rate.