Pre-synaptic C-terminal truncated tau is released from cortical synapses in Alzheimer's disease

The microtubule-associated protein tau has primarily been associated with axonal location and function; however, recent work shows tau release from neurons and suggests an important role for tau in synaptic plasticity. In our study, we measured synaptic levels of total tau using synaptosomes prepared from cryopreserved human postmortem Alzheimer's disease (AD) and control samples. Flow cytometry data show that a majority of synaptic terminals are highly immunolabeled with the total tau antibody (HT7) in both AD and control samples. Immunoblots of synaptosomal fractions reveal increases in a 20kDa tau fragment and in tau dimers in AD synapses, and terminal-specific antibodies show that in many synaptosome samples tau lacks a C-terminus. Flow cytometry experiments to quantify the extent of C-terminal truncation reveal that only 15-25% of synaptosomes are positive for intact C-terminal tau. Potassium-induced depolarization demonstrates release of tau and tau fragments from pre-synaptic terminals, with increased release from AD compared to control samples. This study indicates that tau is normally highly localized to synaptic terminals in cortex where it is well-positioned to affect synaptic plasticity. Tau cleavage may facilitate tau aggregation as well as tau secretion and propagation of tau pathology from the pre-synaptic compartment in AD.