Alterations in retinal tau phosphorylation in Alzheimer’s disease patients identified by mass spectrometry

Most neurodegenerative diseases, including Alzheimer’s disease (AD) and multiple sclerosis (MS), are associated with abnormal phosphorylation of tau (p-tau) in the brain. Immunostaining studies have revealed an accumulation of p-tau in the AD retina and suggested it may reflect tau pathology in the brain. To validate these findings and further investigate the relationship between retinal and brain tau pathology, we used mass spectrometry to measure p-tau peptides in matched retinal and hippocampal samples from non-demented controls (NC, n=8), AD (n=12), and MS (n=4). We analysed the differences in p-tau levels between diagnoses and explored how retinal p-tau variants correlate with hippocampal p-tau and neuropathological changes. Our results revealed peaks corresponding to tau peptides phosphorylated at T181, S199/S202, T231, S396 + T403/S404, and T403/S404. These p-tau peptides were also detected in the hippocampal samples, along with additional p-tau peptides such as T217 and S262. Total tau phosphorylation and phosphorylation at S199/S202 and T231 were significantly higher in the retina of AD cases compared to NC. These two peptides, along with peptides phosphorylated at S396+T403/S404 and T403/S404, were also higher in cases with high amyloid-beta (Aβ) Braak stages compared to those with low Aβ Braak stages. Higher Aβ Braak stages were also associated with higher mass spectrometry peak intensities of peptides phosphorylated at S199/S202 and S396+T403/S404. Additionally, retinal p-tau peptides at S396+T403/S404 and T403/S404 correlated with neurofibrillary tangle (NFT) Braak stages, and p-tau peptides S396+T403/S404 in the retina were linked to corresponding phosphorylation in the CA1 region. These findings underscore the connection between retinal and brain tau pathology and highlight the potential of retinal tau as a biomarker for AD diagnosis and monitoring while also deepening our understanding of tauopathies in both the retina and brain.