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

Triple-transgenic mice (3xTgAD) overexpressing Swedish-mutated β-amyloiḍprecursoṛprotein (βAPPswe), P310L-Tau (TauP301L) and physiological levels of M146V-presenilin-1 (PS1M146V) display extracellular amyloid-β peptides (Aβ) deposits and Tau tangles. More disputed is the observation that these mice accumulate intraneuronal Aβ 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 β-secretase-derived βAPP fragment C99 that is observed in 3 month-old mice and enhanced by pharmacological blockade of γ-secretase. Notably, intracellular Aβ is only detectable several months later and appears, as is the case for C99, in enlarged cathepsin B-positive structures, while extracellular Aβ deposits are detected around 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. The examination of two other mice models harboring mutated βAPP but endogenous wild type PS1 and Tau protein (TgCRND8 or Tg2576) indicate that C99 levels are largely higher in all animal models than in their respective control mice. Furthermore, the comparison of 3xTgAD mice with double transgenic mice bearing the βAPPswe and TauP301L mutations but expressing endogenous PS1 (2xTgAD) demonstrate that C99 accumulation could not be accounted for by a loss of function triggered by PS1 mutation that would have prevented C99 secondary cleavage by γ-secretase. Altogether, our work identifies C99 as the earliest βAPP catabolite and main contributor to the intracellular βAPP-related immunoreactivity in 3xTgAD mice, suggesting its implication as an initiator of the neurodegenerative process and cognitive alterations taking place in this mice model.