Anatomical characterization of pT217‐tau in aged rhesus macaque association cortices: Relevance for trans‐synaptic propagation in sporadic Alzheimer’s Disease

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 affecting the dorsolateral prefrontal cortex (dlPFC) with advanced neurodegeneration. Recent discoveries indicate that phosphorylated tau (pT217‐tau) can be captured in 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 fixed 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 ERC and dlPFC 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 (18‐33 years) to localize pT217‐tau in ERC layer II, which show the earliest signatures of tau pathology in AD and dlPFC layer III critical for working memory. Result pT217‐tau immunolabeling was predominantly observed in postsynaptic compartments and 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 and endosomes, 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 and dlPFC 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 neurodegene