The β-Secretase-Derived C-Terminal Fragment of βAPP, C99, But Not Aβ, Is a Key Contributor to Early Intraneuronal Lesions in Triple-Transgenic Mouse Hippocampus

Triple-transgenic mice (3xTgAD) overexpressing Swedish-mutated beta-amyloid precursor protein (beta APP(swe)), P310L-Tau (Tau(P301L)), and physiological levels of M146V-presenilin-1 (PS1(M146V)) display extracellular amyloid-beta peptides (A beta) deposits and Tau tangles. More disputed is the observation that these mice accumulate intraneuronal A beta that has been linked to synaptic dysfunction and cognitive deficits. Here, we provide immunohistological, genetic, and pharmacological evidences for early, age-dependent, and hippocampus-specific accumulation of the beta-secretase-derived beta APP fragment C99 that is observed from 3 months of age and enhanced by pharmacological blockade of gamma-secretase. Notably, intracellular A beta is only detectable several months later and appears, as is the case of C99, in enlarged cathepsin B-positive structures, while extracellular A beta deposits are detected similar to 12 months of age and beyond. Early C99 production occurs mainly in the CA1/subicular interchange area of the hippocampus corresponding to the first region exhibiting plaques and tangles in old mice. Furthermore, the comparison of 3xTgAD mice with double-transgenic mice bearing the beta APP(swe) and Tau(P301L) mutations but expressing endogenous PS1 (2xTgAD) demonstrate that C99 accumulation is not accounted for by a loss of function triggered by PS1 mutation that would have prevented C99 secondary cleavage by gamma-secretase. Together, our work identifies C99 as the earliest beta APP catabolite and main contributor to the intracellular beta APP-related immunoreactivity in 3xTgAD mice, suggesting its implication as an initiator of the neurodegenerative process and cognitive alterations taking place in this mouse model.