Synaptic density and tau pathology in Alzheimer’s disease: a dual role in susceptibility and degeneration

Fibrillar tau is a defining hallmark of Alzheimer's disease, gradually accumulating and spreading throughout the brain. The synapse plays a key role in this process-both by facilitating the spread of tau between neurons and by serving as a direct target of tau-induced neurotoxicity. However, few studies in humans have explored synaptic density both as a facilitating factor and as a target of tau pathology. In this dual PET tracer study, we used 18F-SynVesT-1 synaptic vesicle glycoprotein 2A (SV2A) PET to assess synaptic density and 18F-MK6240 tau-PET to assess fibrillar tau in 59 amyloid-PET-positive patients and 25 amyloid-PET-negative cognitively normal individuals from the Chinese Preclinical Alzheimer's Disease Study (CPAS). Spatial correlation analyses revealed that brain regions with higher SV2A tracer uptake in cognitively normal individuals exhibited higher 18F-MK6240 uptake in Alzheimer's disease. These findings were independently replicated in the Alzheimer's Disease Neuroimaging Initiative (ADNI), where normal SV2A-PET maps from CPAS were correlated with tau-PET data from 372 Aβ+ participants, including longitudinal follow-up in 204 cases. In both cohorts, regions with higher normal synaptic density showed greater tau burden, and in ADNI, higher synaptic density in normal brain also predicted faster tau accumulation over time. Gene-set enrichment analyses of transcriptomic data mapped onto regional tau-PET uptake further showed that areas of high 18F-MK6240 uptake were enriched in genes encoding synapse-related proteins. Together, these findings suggest that synapse-rich regions are especially prone to tau accumulation. We also investigated the downstream impact of tau on synaptic integrity. Notably, regions with higher 18F-MK6240 uptake showed reduced SV2A tracer uptake, indicating synaptic loss, not only in the same regions but also in areas connected to those with high tau. Stronger loss of SV2A tracer uptake was observed in the regions with stronger resting-state functional connectivity to epicenters of high 18F-MK6240 uptake in the Alzheimer's disease group. The connectivity-dependent synaptic loss could not be fully explained by tau levels in the connected target regions but was found to be partially mediated by them. These findings suggest that tau pathology contributes to synaptic loss both locally and in distant, connected brain regions. Overall, our results highlight the central importance of the synapse in Alzheimer's disease: synapses appear to both facilitate fibrillar tau accumulation and become impaired through its toxic effects. Understanding the synapse's role in tau pathology may open new avenues for therapeutic interventions aimed at slowing down tau progression and neurodegeneration.