δ-Secretase-cleaved Tau stimulates Aβ production via upregulating STAT1-BACE1 signaling in Alzheimer’s disease

δ-Secretase, an age-dependent asparagine protease, cleaves both amyloid precursor protein (APP) and Tau and is required for amyloid plaque and neurofibrillary tangle pathologies in Alzheimer’s disease (AD). However, whether δ-secretase activation is sufficient to trigger AD pathogenesis remains unknown. Here we show that the fragments of δ-secretase-cleavage, APP (586–695) and Tau(1–368), additively drive AD pathogenesis and cognitive dysfunctions. Tau(1–368) strongly augments BACE1 expression and Aβ generation in the presence of APP. The Tau(1–368) fragment is more robust than full-length Tau in binding active STAT1, a BACE1 transcription factor, and promotes its nuclear translocation, upregulating BACE1 and Aβ production. Notably, Aβ-activated SGK1 or JAK2 kinase phosphorylates STAT1 and induces its association with Tau(1–368). Inhibition of these kinases diminishes stimulatory effect of Tau(1–368). Knockout of STAT1 abolishes AD pathologies induced by δ-secretase-generated APP and Tau fragments. Thus, we show that Tau may not only be a downstream effector of Aβ in the amyloid hypothesis, but also act as a driving force for Aβ, when cleaved by δ-secretase.