Distinct Neurotoxic Effects of Extracellular Tau Species in Primary Neuronal-Glial Cultures

Recent data from various experimental models support the link between extracellular tau and neurodegeneration; however, the exact mechanisms by which extracellular tau or its modified forms or aggregates cause neuronal death remain unclear. We have previously shown that exogenously applied monomers and oligomers of the longest tau isoform (2N4R) at micromolar concentrations induced microglial phagocytosis of stressed-but-viable neurons in vitro. In this study, we investigated whether extracellular phosphorylated tau(2N4R)(p-tau(2N4R)), isoform 1N4R (tau(1N4R)) and K18 peptide can induce neuronal death or loss in primary neuronal-glial cell cultures. We found that p-tau(2N4R)at 30 nM concentration induced loss of viable neurons; however, 700 nM p-tau(2N4R)caused necrosis of both neurons and microglia, and this neuronal death was partially glial cell-dependent. We also found that extracellular tau(1N4R)oligomers, but not monomers, at 3 mu M concentration caused neuronal death in mixed cell cultures: self-assembly tau(1N4R)dimers-tetramers induced neuronal necrosis and apoptosis, whereas A beta-promoted tau(1N4R)oligomers caused glial cell-dependent loss of neurons without signs of increased cell death. Monomeric and pre-aggregated tau peptide containing 4R repeats (K18) had no effect in mixed cultures, suggesting that tau neurotoxicity might be dependent on N-terminal part of the protein. Taken together, our results show that extracellular p-tau(2N4R)is the most toxic form among investigated tau species inducing loss of neurons at low nanomolar concentrations and that neurotoxicity of tau(1N4R)is dependent on its aggregation state.

Automated summary

Recent experimental models have shown a link between extracellular tau and neurodegeneration, but the exact mechanisms by which extracellular tau induces neuronal death remain unclear. This study found that extracellular phosphorylated tau(2N4R) is the most toxic form, causing neuronal loss at low nanomolar concentrations, and the neurotoxicity of tau(1N4R) depends on its aggregation state.