A combination Alzheimer's therapy targeting BACE1 and neprilysin in 5XFAD transgenic mice

Background: Accumulating evidence indicates that partial inhibition of beta-site APP-cleaving enzyme 1 (BACE1), which initiates amyloid-beta (A beta) production, mitigates Alzheimer's disease (AD)-like pathologies and memory deficits in a battery of transgenic mouse models. However, our previous investigations suggest that therapeutic BACE1 suppression may be beneficial only if targeted on earlier stages of AD and encounter dramatic reductions in efficacy during disease progression. This study was designed to test the possibility that a combination approach, aimed at inhibiting BACE1 and boosting neprilysin (a major A beta-degrading enzyme) activities, may be able to mechanistically overcome the limited efficacy of anti-A beta therapy in advanced AD. Results: After crossbreeding between BACE1 heterozygous knockout (BACE1(+/-)), neprilysin transgenic (NEP) and 5XFAD mice, we analyzed the resultant mice at 12 months of age when 5XFAD controls showed robust amyloid-beta (A beta) accumulation and elevation of BACE1 expression (similar to 2 folds). Although haploinsufficiency lowered BACE1 expression by similar to 50% in concordance with reduction in gene copy number, profound beta-amyloidosis, memory deficits and cholinergic neuron death were no longer rescued in BACE1(+/-) . 5XFAD mice concomitant with their persistently upregulated BACE1 (i.e., equivalent to wild-type control levels). Notably, neprilysin overexpres.ion not only prevented A beta accumulation but also suppressed the translation initiation factor eIF2 alpha-associated elevation of BACE1 and lowered levels of the beta-secretase-cleaved C-terminal fragment of APP (C99) in NEP . 5XFAD mice. Interestingly, these markers for beta-amyloidogenesis in BACE1(+/-) . NEP . 5XFAD mice were further reduced to the levels reflecting a combination of single BACE1 allele ablation and the abolishment of translational BACE1 upregulation. However, since neprilysin overexpression was striking (similar to 8-fold relative to wild-type controls), memory impairments, cholinergic neuronal loss and beta-amyloidosis were similarly prevented in NEP . 5XFAD and BACE1(+/-) . NEP . 5XFAD mice. Conclusions: Our findings indicate that robust overexpression of neprilysin is sufficient to ameliorate AD-like phenotypes in aged 5XFAD mice. We also found that A beta-degrading effects of overexpressed neprilysin can block deleterious BACE1-elevating mechanisms that accelerate A beta production, warranting further study to test whether interventions moderately activating neprilysin may be useful for boosting the limited efficacy of therapeutic BACE1 inhibition in treating AD with established A beta pathology.