Subcellular functions of tau mediate repair response and synaptic homeostasis in injury.

Injury responses in terminally differentiated cells such as neurons are tightly regulated by pathways aiding homeostatic maintenance. Cancer patients subjected to neuronal injury in brain radiation experience cognitive declines similar to those seen in primary neurodegenerative diseases. Numerous studies have investigated the effect of radiation in proliferating cells of the brain, yet the impact in differentiated, post-mitotic neurons, especially the structural and functional alterations remain largely elusive. We identified that microtubule-associated tau is a critical player in neuronal injury response via compartmentalized functions in both repair-centric and synaptic regulatory pathways. Ionizing radiation-induced injury acutely induces an increase in phosphorylated tau in the nucleus where it directly interacts with histone 2AX (H2AX), a DNA damage repair (DDR) marker. Loss of tau significantly reduced H2AX phosphorylation after irradiation, indicating that tau may play an important role in the neuronal DDR response. We also observed that loss of tau increases eukaryotic elongation factor levels, a positive regulator of protein translation after irradiation. This initial response cascades into a significant increase in synaptic proteins, resulting in disrupted homeostasis. Downstream, the novel object recognition test showed a decrease in learning and memory in tau-knockout mice after irradiation, and electroencephalographic activity contained increased delta and theta band oscillations, often seen in dementia patients. Our findings demonstrate tau's previously undefined, multifunctional role in acute responses to injury, ranging from DDR response in the nucleus to synaptic function within neurons. Such knowledge is vital to develop therapeutic strategies targeting neuronal injury in cognitive decline for at risk and vulnerable populations.