Basic Science and Pathogenesis

Alzheimer's disease (AD) is prevalent in people ≥ 65-year-olds, which counts up to ∼75% dementia cases worldwide. AD patient is characterized by progressive cognitive impairment, the accumulation of beta-amyloid (Aβ) plaques and neurofibrillary hyperphosphorylated tau (pTau) tangles. Molecular mechanisms of most AD cases are not fully understood. We recently identified poly-Glycine-Arginine containing (polyGR+) aggregates that frequently accumulate in AD autopsy brains. We also identified interrupted GGGAGA repeat expansions (exp) in CASP8 that produces polyGR+ protein accumulated in brain tissue from expansion AD carriers. Specific CASP8-GGGAGA^exp variants are associated with increased AD risk. Previous studies suggested that impairment of autophagy-lysosomal pathway, which maintains cellular protein homeostasis, play important role in AD pathogenesis. In this study, we examine if polyGR+ aggregates are associated with known AD pathological hallmarks (Aβ, pTau). We also study if polyGR+ aggregates are linked with disruption of autophagy-lysosomal pathway.

We performed immunohistochemical staining for polyGR, Aβ and pTau using the hippocampal sections from 133 AD cases, 30 controls, and 15 primary age-related tauopathy (PART) cases. We studied the levels of polyGR+ aggregate staining and Aβ and pTau deposition levels in the hippocampal regions. Double immunofluorescence (IF) staining of polyGR and autophagy-lysosomal markers (p62 and LC3B) was performed to study if polyGR co-aggregates with p62 and LC3B in AD autopsy brains.

Consistent to our previous study, frequent a-polyGR positive aggregates are found in ∼50% AD autopsy brains but not in controls and PART cases (p <0.0001). PolyGR+ signal accumulates as cytoplasmic or nuclear puncta in CA regions, subiculum and presubiculum regions in the hippocampus of AD autopsy brains. We detect positive correlations of polyGR+ staining with Aβ (R²=0.2544, p = 0.0007) and pTau (R² =0.2822, p = 0.0003), which is stronger than the correlation of Aβ and pTau in the hippocampus (R²=0.1047, p = 0.0366). PolyGR+ aggregates co-localize with p62 and LC3B staining in polyGR+ AD brains.

Our data demonstrates that polyGR+ aggregates are an important proteinopathological feature in AD autopsy brains. Additionally, our data suggests that co-localization of autophagy and proteasome markers p62 and LC3B with polyGR+ aggregates could lead to dysregulation of autophagy-lysosomal pathway, which contributes to disease pathogenesis.