Microglia-mediated neuroinflammation is critical for the pathogenesis of neurodegenerative diseases, including Parkinson's disease (PD). microRNA-let-7a (miR-let-7a) targets the signal transducer and activator of transcription-3 (STAT3) and regulates microglia function. However, less is known about whether it plays a functional role in PD. In this report, by utilizing a mouse PD model induced by the overexpression of alpha-Synuclein (alpha-Syn), a pathological hallmark of PD, we found that miR-let-7a expression was downregulated, while STAT3 was synchronously activated in the substantia nigra pars compacta (SNpc). Similar results were obtained in alpha-Syn-treated BV-2 microglia cells cultured in vitro. Additionally, STAT3 was proven to be a direct target of miR-let-7a in BV-2 microglia cells, suggesting that miR-let-7a downregulation may contribute to STAT3 activation in alpha-Syn-induced mouse PD. Moreover, miR-let-7a overexpression suppressed alpha-Syn-induced BV-2 microglia cell activation and proinflammatory cytokine production, and these effects were abrogated by restoring STAT3 protein, hence establishing that miR-let-7a suppresses microglia-mediated inflammation through targeting STAT3. Lastly, miR-let-7a overexpression via injection of miR-7 mimics into mouse striatum suppressed microglia activation and reduced pro-inflammatory cytokine production, which were accompanied by relieved movement disorder and improved spatial memory deficits in alpha-Syn-induced PD mice. Altogether, these results may identify miR-let-7a as a negative regulator of microglia-elicited neuroinflammation, at least partially explaining its alleviating effects on PD symptoms. (C) 2019 Published by Elsevier Inc.