Macropinocytosis of aggregated amyloid-beta and tau requires Arf6 and the RhoGTPases Rac1, Cdc42 and RhoA

The neuron-to-neuron transfer of amyloid-beta (Aβ) and tau aggregates have been proposed to underlie the propagation of protein aggregation in Alzheimers disease (AD) and contributing to progressive neurodegeneration. Several studies have provided evidence that aggregates of Aβ and tau are taken up into neuronal cells and neurons from the extracellular environment, where they contribute to the propagation of Aβ and tau aggregation in AD through seeding their aggregation. As a result, attention has been placed on determining the cellular mechanisms that contribute to this uptake, with the hopes that targeting these mechanisms could halt the progression of AD by preventing aggregate transfer. Previous studies have demonstrated the uptake of Aβ and tau aggregates through the endocytic process called macropinocytosis. The activity of several GTPases has been demonstrated to regulate macropinocytosis, including Arf6 and the RhoGTPases Rac1, Cdc42 and RhoA. Here, we examined the uptake of Aβ42 oligomers and tau fibrils by macropinocytosis in neurons and the role of Arf6, Rac1, Cdc42 and RhoA activity in the macropinocytosis of these aggregates. In this study, we demonstrated that extracellular Aβ42 oligomers and tau fibrils are taken up by iPSC-derived neurons and delivered directly to LAMP1- labeled lysosomes through macropinocytosis. This was demonstrated by reduced uptake in response to treatment with the macropinocytosis inhibitor EIPA, but not in response to the clathrin inhibitor Pitstop2. The uptake of these aggregates by macropinocytosis was significantly reduced by the inhibition of the GTPases Arf6, Rac1, Cdc42 and RhoA. Further, we also demonstrated that accumulation of extracellular Aβ42 oligomers and tau fibrils results in an accumulation of cytoplasmic tau within aggregate-containing lysosomes. Together these results provide evidence that the GTPases Arf6, Rac1, Cdc42 and RhoA play a role in the neuronal uptake of Abeta and tau aggregates by macropinocytosis and identifies new molecular targets to explore preventing the neuron-to-neuron transfer of these aggregates in AD.