Cell type- and tau lesion-specific accumulation of phospho-Ser65 ubiquitin in primary tauopathies

Defective mitochondrial quality control is increasingly implicated in tau-mediated neurodegeneration, but how distinct tau lesions in different cell types interact with PINK1-PRKN-mediated mitophagy remains unclear. Phosphorylated ubiquitin at serine 65 (pS65-Ub) is the joint product of PINK1-PRKN signaling. Significant deposition of pS65-Ub has been previously demonstrated in autopsy brains from patients with Lewy body disease, Alzheimer disease, and MAPT mutation-associated frontotemporal lobar degeneration, where increased pS65-Ub signal was particularly strongly associated with early neuronal tau pathology. We here expanded the analyses to 92 postmortem human brains comparing neuropathologically normal controls and cases of primary age-related tauopathy (PART), Pick’s disease (PiD), progressive supranuclear palsy (PSP), and corticobasal degeneration (CBD). Using pS65-Ub and pS202-tau immunohistochemistry in selectively vulnerable regions, deep learning-based quantification of tau lesion subtypes, and single-cell co-localization analyses, we found considerable disease-, cell type-, and tau inclusion-specific differences in pS202-tau and pS65-Ub accumulation. Although pS202-tau burden was elevated across all primary tauopathies in their respective vulnerable regions, the extent of pathology and predominant tau inclusion types varied markedly among diseases. pS65-Ub positive cells were most abundant in the hippocampus of PART and PiD but were sparse in the primary motor cortex of PSP and especially in CBD, despite extensive tau pathology. In both PART and PiD, pS65-Ub primarily accumulated in neurons containing tau inclusions, with significantly greater intracellular burden and more frequent large vacuolar structures in PART. The density of pS65-Ub/tau double-positive cells strongly correlated with tau tangle burden in PART but not with Pick body burden in PiD. Together, our findings revealed marked heterogeneity in the distribution, density, intracellular abundance, and morphology of pS65-Ub deposits across affected brain regions in primary tauopathies, highlighting pS65-Ub as a potential marker for differential diagnosis of neuronal tau-associated disorders in tauopathies.