α-Synuclein-positive structures in cases with sporadic Alzheimer’s disease: morphology and its relationship to tau aggregation

Alzheimer’s disease (AD) and Parkinson’s disease share common clinical and pathological features. In this study, we examined the relationship between AD pathology and α-synuclein aggregation. The frequency and distribution of α-synuclein-positive structures were systematically investigated in 27 cases with sporadic AD by α-synuclein immuno-histochemistry. Thirteen (48.2%) of 27 cases had various α-synuclein-positive structures as well as Lewy bodies. The frequency and density of senile plaques and neurofibrillary tangles were not significantly different between cases with α-synuclein structures and those without. α-Synuclein-positive structures were found most frequently in the amygdala. The α-synuclein-positive inclusions that are different from Lewy bodies were observed at the highest rate in the hippocampus. The discovery of α-synuclein as the constituent of Lewy bodies facilitated the detection of Lewy-related structures even in AD cases with widespread and numerous neurofibrillary tangles. α-Synuclein-positive inclusions except for Lewy bodies are exposed, and the distribution of them indicates that Lewy body formation may be influenced by the degree of tau aggregation. This study also supports the suggestion that cases with AD pathology can be classified into two groups according to the existence or absence of α-synuclein aggregation.