Age-related changes of intracellular Aβ in cynomolgus monkey brains

To confirm the intracellular accumulation of amyloid beta-protein (A beta), we carefully performed immunohistochemistry using brains of cynomolgus monkeys of various ages. Cortical neurones and their large neurites were immunostained with antibodies against A beta in young monkey brains. In aged monkey brains, intracellular A beta localized within cortical neurones; no clear association was found between the presence of intracellular A beta and senile plaques (SPs). Interestingly, we did not observe A beta-immunoreactive cortical neurones in brains fixed with neutral buffered formalin. Western blot analyses of microsomal and nerve ending fractions derived from the brains of young to aged monkeys revealed that intracellular A beta generation changed with age. In the microsomal fraction, the amount of A beta 42 significantly increased in brains from older monkeys (> 30 years of age), and the amount of A beta 43 significantly decreased with age in the microsomal fraction. The amount of A beta 40 remained the same regardless of age. Biochemical analyses also showed that intracellular levels of each of these A beta molecules significantly increased with age in nerve ending fractions. As we previously observed that a similar accumulation of presenilin1, beta-amyloid precursor protein (APP) and APP C-terminal fragment cleaved by beta-secretase in the nerve ending fractions obtained from brains with SPs, the accumulation of intracellular A beta in this fraction may be closely related to formation of spontaneous SPs with age. Taken together, these results suggest that intensive investigation of age-related changes in the nerve ending will contribute to a better understanding of the pathogenesis of age-related neurodegenerative disorders such as sporadic Alzheimer's disease.