Although persistent neurological sequelae in long COVID are reportedly well associated with neuroinflammation, the underlying regulatory mechanisms remain poorly characterized. Previously, we identified dysregulated microRNA expression, including let-7a-5p, in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike S1-stimulated human microglial cells by RNA sequencing; however, how let-7a-5p regulates the S1-mediated neuroinflammatory processes remains undetermined. In the present study, we examined the functional role of let-7a-5p in alleviating S1-induced microglial inflammation in the CHME3 cell line as well as in human monocyte-derived microglia (MDMi) using a loss- and gain-of-function approach. Functional inhibition of let-7a-5p resulted in mitigating S1-induced inflammatory cytokine release and markers of pyroptosis. Mechanistically, we established SHIP-1 as a direct target of let-7a-5p using luciferase reporter assay validation. Interestingly, we noted upregulated expression of the TLR3 gene alongside TLR2/4 in S1-stimulated microglia. Although we could not establish exactly how TLR3 is stimulated in S1-induced neuroinflammatory processes, using siRNA-mediated inhibition and a pharmacological inhibitor in both CHME3 cells and MDMi, our study certainly provides evidence of TLR3 involvement during S1-induced microglial inflammation, which needs further investigation. Together, these in vitro findings demonstrate that the let-7a-5p/SHIP-1 axis regulates S1-induced inflammatory cascades in CHME3 and MDMi cells, providing mechanistic insight into its role in SARS-CoV-2-associated neuroinflammation and warranting further validation in appropriate in vivo/organoid models.
