Abnormally modified
alpha-synuclein is a pathological hallmark of
Parkinson's disease and the other
alpha-synucleinopathies. Since
proteinase K (PK) treatment is known to enhance the immunoreactivity of abnormal
alpha-synuclein, we immunohistochemically examined the brain of transgenic (Tg) mice expressing human mutant A53T
alpha-synuclein using this retrieval method. PK treatment abolished the immunoreactivity of
alpha-synuclein in abnormal inclusions as well as of endogenous
alpha-synuclein in Tg mice, whereas PK-resistant
alpha-synuclein was found in the presynaptic nerve terminals, especially in the hippocampus and temporal cortex. In human
Lewy body disease, PK-resistant
alpha-synuclein was deposited in Lewy bodies and Lewy neurites, as well as in the presynapses in distinct brain regions, including the hippocampus, temporal cortex and substantia nigra. Biochemical analysis revealed that PK-resistant
alpha-synuclein was detected in the presynaptic fraction in Tg mice and human
Lewy body disease. Although PK-resistant
alpha-synuclein was found in the presynapse in Tg mice even at 1 week of age, it was not phosphorylated until at least 8 months of age. Moreover, PK-resistant
alpha-synuclein in the presynapse was not phosphorylated in human
Lewy body disease. These findings suggest that phosphorylation is not necessary to cause the conversion of soluble form to PK-resistant
alpha-synuclein. Considering that native
alpha-synuclein is a soluble
protein localized to the presynaptic terminals, our findings suggest that PK-resistant
alpha-synuclein may disturb the neurotransmission in
alpha-synucleinopathies.