Nanoscale imaging of pT217‐tau in aged rhesus macaque: Trans‐synaptic propagation and seeding of tau pathology in entorhinal cortex

Background Tau pathology in Alzheimer’s Disease (AD) targets higher cortical circuits, with evidence that phosphorylated tau propagates from the entorhinal cortex (ERC) to “seed” pathology throughout the neuronal network. Recent discoveries indicate that tau phosphorylated at threonine 217 (pT217‐tau) can be captured in cerebrospinal fluid (CSF) and plasma as an early biomarker of ensuing disease. However, the role of pT217‐tau in brain tau pathology is unknown, especially as soluble tau species are dephosphorylated postmortem in humans. Rhesus macaques naturally develop the same qualitative pattern and sequence of tau and amyloid pathology, with neurofibrillary tangles comprised of paired helical filaments, identical to human AD. Perfusion fixation of monkey tissue preserves phosphorylation state and allows imaging of molecular location and interactions with nanometer resolution not possible in humans due to postmortem degradation. The current study examined the ultrastructural localization of pT217‐tau in layer II ERC of the aged rhesus macaques, focusing on potential evidence of propagation between neurons, and exposure to the extracellular space. Method We used immunohistochemistry paired with high spatial‐resolution immunoelectron microscopy (immunoEM) in aged rhesus macaques (18‐31 years) to localize pT217‐tau in the stellate cell islands in ERC layer II, which show the earliest signatures of tau pathology in AD. Result pT217‐tau immunolabeling was predominantly observed in postsynaptic compartments in macaque ERC layer II. pT217‐tau accumulated on the calcium‐storing smooth endoplasmic reticulum spine apparatus near axospinous asymmetric glutamatergic synapses in dendritic spines. We observed extensive, trans‐synaptic pT217‐tau trafficking between interconnected neurons within omega‐shaped bodies in ERC layer II, specifically near excitatory, but not inhibitory synapses. Within dendritic shafts, pT217‐tau aggregated on microtubules often in concordance with autophagic vacuoles indicative of neurite dystrophy. Conclusion pT217‐tau accumulates in ERC layer II subcompartments known to be the earliest to show pathology in humans. The data provide the first evidence of pT217‐tau trafficking between neurons to “seed” tau pathology in higher brain circuits, potentially interfacing with the extracellular space to become readily accessible and captured in CSF and blood as a robust AD biomarker. Illuminating patterns of neurodegeneration with pT217‐tau could