Synaptic dysfunction and abnormal processing of amyloid precursor protein (APP) are early pathological features in Alzheimer\'s disease (AD).Recently,noncoding RNAs such as microRNAs (miRNAs) and circular RNAs (circRNAs) have been reported to contribute to the pathogenesis of AD.We found an age-dependent elevation of miR-138 in APP/PS1 (presenilin-1) mice.MiR-138 inhibited the expression of ADAM10 [a disintegrin and metalloproteinase domain-containing protein 10],promoted amyloid beta (Aβ) production,and induced synaptic and learning/memory deficits in APP/PS1 mice,while its suppression alleviated the AD-like phenotype in these mice.Overexpression of sirtuin 1 (Sirt1),a target of miR-138,ameliorated the miR-138-induced inhibition of ADAM10 and elevation of Aβ in vitro.The circRNA HDAC9 (circHDAC9) was predicted to contain a miR-138 binding site in several databases.Its expression was inversely correlated with miR-138 in both Aβ-oligomertreated N2a cells and APP/PS1 mice,and it co-localized with miR-138 in the cytoplasm of N2a cells.CircHDAC9 acted as a miR-138 sponge,decreasing miR-138 expression,and reversing the Sirt1 suppression and excessive Aβ production induced by miR-138 in vitro.Moreover,circHDAC9 was decreased in the serum of both AD patients and individuals with mild cognitive impairment.These results suggest that the circHDAC9/miR-138/Sirtl pathway mediates synaptic function and APP processing in AD,providing a potential therapeutic target for its treatment.