Abstract
Alzheimer's disease (AD) is a secondary tauopathy characterized by the accumulation of aggregated amyloid-beta (Aβ) peptides as extracellular neuritic and diffuse plaques, alongside hyperphosphorylated tau aggregates forming intracellular neurofibrillary tangles (NFTs). Although NFT pathology is strongly linked to neurodegeneration and cognitive decline in AD, growing evidence suggests that soluble "pretangle" aggregates of tau, formed prior to NFT development, are the primary neurotoxic species driving disease progression and cognitive impairment. We quantified pretangle tau moieties in postmortem tissues of the default mode network (DMN), a resting state connectome that mediates episodic memory and falters early in AD, to determine the association between DMN tau pathology and antemortem cognitive status. To accomplish this, we assayed postmortem fixed and frozen tissue from frontal cortex (FC), posterior cingulate cortex (PCC), and precuneus (PreC) DMN hubs obtained from participants of the Rush Religious Orders Study (RROS). Immunohistochemical and biochemical quantification was performed using site-specific tau antibodies recognizing pathological pretangle epitopes pS422 (phosphorylation at tau residue serine 422), TOC1 (tau oligomers), and TNT2 (N-terminal tau misfolding), as well as the mid-stage NFT marker TauC3 (C-terminal tau truncation). Pretangle tau profiles included neuropil threads and neuronal inclusions as early as Braak stage III, with significant increases from Braak stage IV to V across the DMN hubs. There was a step-wise increase in the pretangle markers in subjects who died with no cognitive impairment (NCI) to mild cognitive impairment (MCI) to AD. The increase in pretangle tau pathology accrual correlated with poorer antemortem neuropsychological tests, including episodic memory, semantic memory, and a global cognitive z score calculated for each RROS participant. Pretangle tau accrual also correlated with postmortem neuropathological diagnostic measures of AD severity. In addition, we found that the PCC displayed greater tau pathology compared to the PreC, suggesting selective vulnerability within DMN hubs. Collectively, our findings are a first of its kind cellular and protein quantitative analysis to demonstrate neurotoxic pre-tangle tau accumulation within the DMN connectome that highly correlated with cognitive decline in MCI and AD, underscoring pretangle tau as a potential early driver of disease progression and a target for therapeutic intervention. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s44477-026-00019-y.