Inverse and Postponed Impacts of Extracellular Tau PHF on Astrocytes and Neurons' Mitochondrial Function

Background: Tauopathies encompass a spectrum of neurodegenerative disorders which are marked by the pathological aggregation of tau protein into paired helical filaments (PHF-tau), neurofibrillary tangles (NFTs) and Glial-fibrillary tangles (GFTs). These aggregates impair cellular, mitochondrial, and synaptic functions. The emergence of extracellular tau (ePHF-tau), featuring a myriad of isoforms and phosphorylation states, presents a challenge in comprehending its nuanced effects on neural cells, particularly concerning synaptic and mitochondrial integrity. Methods: We studied the impact of ePHF-tau (2N4R) on different states and ages of primary cultures of rat neuroglia. Using confocal microscopy and proteomic analysis of synaptosomes, we studied the impact of ePHF-tau on neurite and synapse number. We monitored mitochondrial responses in neurons and astrocytes over 72 hours using advanced fluorescence microscopy for dynamic, high-throughput analysis. Results: Treatment with ePHF-tau has a strong effect on the neurites of immature neurons, but its toxicity is negligible when the neurons are more mature. At the mature stage of their development, we observed a substantial increase in the density of the PSD-95/vGlut1 zone in neurite, suggesting altered synaptic connectivity and ePHF-tau excitotoxicity. Proteomics revealed significant changes in mitochondrial protein in synaptosomes following exposure to ePHF-tau. In the neuronal compartment, real-time imaging revealed rapid and persistent mitochondrial dysfunction, increased ATP production, and reduced mitochondrial turnover. In contrast, we observed increased mitochondrial turnover and filamentation after treatment in the astrocyte processes, indicating cell-specific adaptive responses to ePHF-tau. Conclusions: This study sheds light on the intricate effects of extracellular tau aggregates on neuronal and astrocytic mitochondrial populations, highlighting how tau pathology can lead to mitochondrial disturbances and synaptic alterations. By delineating the differential responses of neurons and astrocytes to ePHF-tau, our findings pave the way for developing targeted therapeutic interventions to mitigate the detrimental impacts of tau aggregates in neurodegenerative diseases. ### Competing Interest Statement The authors have declared no competing interest.