Everolimus rescues the early learning and memory deficits and ameliorates the AD-like pathology in the 3xTg-AD mice

Introduction Overwhelming evidence shows a primary role for the mammalian target of rapamycin (mTOR) signaling in the pathogenesis of Alzheimer's disease (AD). To investigate the relation between amyloid-β peptide (Aβ) and mTOR, we injected the synthetic analogue of rapamycin, everolimus, into the cerebroventricular space of a triple transgenic mouse model of AD (3×Tg-AD), which develops age-dependent Aβ and tau accumulation associated with cognitive decline. In particular, 6-month-old 3×Tg-AD mice and age-matched wild-type littermates (Non-Tg) were used. At this age, the 3×Tg-AD mice show early intraneuronal Aβ accumulation and tau mislocalization, which correlate with the onset of cognitive decline. The mTOR enzymatic activity and the levels of phosphorylated p70S6K, a downstream target of mTOR, were significantly increased in the 3×Tg-AD mice compared to control mice. Methods: Everolimus was delivered continuously into the brain by osmotic pump (Alzet 1002) for 4 weeks at 2 mg/kg/day. To determine the effects of everolimus on short- and long-term memory, 3×Tg-AD and Non-Tg mice treated either with everolimus or with vehicle (artificial cerebrospinal fluid) were tested using three independent behavioural paradigms: the novel object recognition test, the inhibitory avoidance and the spatial version of the Morris water maze. Mice were also tested for the depressive-like phenotype by the forced swimming test and the tail suspension test. The expression levels of the main neuropathological hallmarks of AD, in particular APP, Aβ and hyperphosphorylated tau, were evaluated in frontal cortex and hippocampus of vehicle- and everolimus-treated 3×Tg-AD mice by western blotting and immunohistochemical experiments. Results: Overall, our data indicate that everolimus infusion rescued the early learning and memory deficits and increased the immobility time in the 3×Tg-AD. Western blot analysis of the cortical and hippocampal homogenates have shown that centrally administered everolimus decreased the expression levels of APP and Aβ oligomers. The phosphorylation levels of tau on AT8 and AT180 sites were not significantly reduced both in the frontal cortex and in the hippocampus, although a decreasing trend of AT8 positive tau was observed in everolimus-treated 3×Tg-AD mice in both brain regions. The Aβ reduction was also confirmed by immunohistochemical analysis. Conclusion All together, the results of our study indicate that early Aβ, but not tau pathology, in 3×Tg-AD mice was significantly decreased after brain infusion with everolimus. Moreover, the depressive-like phenotype showed by 3×Tg-AD mice was reverted by the everolimus infusion. In conclusion, we show that autophagy induction via everolimus may represent a valid therapeutic strategy in AD when administered early in the disease progression.