Elevation of intracranial soluble amyloid-β (Aβ) levels has been implicated in the pathogenesis of Alzheimer's disease (AD). Intracellular events in neurons, which lead to memory loss in AD, however, remain elusive. Humanin (HN) is a short neuroprotective peptide abolishing Aβ neurotoxicity. Recently, we found that HN derivatives activate the Janus kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) signaling axis. We here report that an HN derivative named colivelin completely restored cognitive function in an AD model (Tg2576) by activating the JAK2/STAT3 axis. In accordance, immunofluorescence staining using a specific antibody against phospho- (p-) STAT3 revealed that p-STAT3 levels in hippocampal neurons age-dependently decreased in both AD model mice and AD patients. Intracerebroventricular administration of Aβ1-42 downregulated p-STAT3 whereas passive immunization with anti-Aβ antibody conversely restored hippocampal p-STAT3 levels in Tg2576 mice, paralleling the decrease in the brain Aβ burden. Aβ1-42 consistently modulated p-STAT3 levels in primary neurons. Pharmacological inhibition of the JAK2/STAT3 axis not only induced significant loss of spatial working memory by downregulating an acetylcholine-producing enzyme choline acetyltransferase but also desensitized the M1-type muscarinic acetylcholine receptor. Thus, we propose a novel theory accounting for memory impairment related to AD: Aβ-dependent inactivation of the JAK2/STAT3 axis causes memory loss through cholinergic dysfunction. Our findings provide not only a novel pathological hallmark in AD but also a novel target in AD therapy.
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