Senile plaques in Alzheimer’s disease arise from Cathepsin D and Aβ-enriched proteinaceous mixtures out of hemolysis and vascular degeneration

Abstract

AbstractThe senile plaque is a prominent pathological hallmark of Alzheimer’s disease (AD), yet the mechanism governing senile plaque generation remains intensively debated. Many researchers believed that senile plaques are derived from neuronal cells; however, there is also strong evidence that senile plaques are linked to cerebral microhemorrhage. We analyzed major neural markers, blood vessel and blood markers on AD brain sections with immunohistochemistry, TUNEL assay, fluorescence imaging and histochemical staining. We found little co-expression between neural markers and plaque Aβ while abundant co-expression between blood markers such as HBA, HbA1C or ApoE and plaque Aβ. Senile plaques additionally colocalized with a characteristic blue autofluorescence, which also prominently existed in red blood cells. Moreover, platelets were found along blood vessel walls and in plaques, suggesting that platelets also contribute to senile plaque formation. Senile plaque formation was intrinsically linked to vascular degeneration as showed by ColIV, ACTA2 and LRP1 immunostaining. We found that Aβ distributes unevenly among red cells or in the blood vessels, enriching in domains of red cell damage or vascular damage, and with the presence of luminal Cathepsin D expression. In addition, the senile plaques have a constitutive Cathepsin D expression overlapping with Aβ, HBA and HbA1C, reiterating a senile plaque connection with hemolysis. With Rhodanine staining, we further proved that there is widespread hemolysis in AD brain tissues, which concurs with Aβ aggregation, Cathepsin D and ApoE expression. Moreover, we provided the biochemical evidence that Aβ directly interacts with hemoglobin (Hb) and form protease-sensitive complexes in an in vitro polymerization assay. In summary, our data suggested that senile plaques arise from Cathepsin D and Aβ-enriched proteinaceous mixtures out of hemolysis and vascular degeneration. In addition, hemoglobin could be a primary physiological target of Aβ in hemolysis and in senile plaques.