The miR-317 functions as a negative regulator of Toll immune response and influences Drosophila survival.

The miR-317 has been revealed to involve in the reproductive response and the larval ovary morphogenesis of Drosophila. However, whether the miR-317 can also regulate Drosophila innate immune responses, which remains unclear to date. Here we have verified that miR-317 can directly target the 3'UTR of Dif-Rc to down-regulate the expression levels of AMP Drs to negatively control Drosophila Toll immune response in vivo and vitro. Specially, the Dif is an important transcription factor of Toll pathway with four transcripts (Dif-Ra, Dif-Rb, Dif-Rc and Dif-Rd). Our results show that miR-317 only targets to Dif-Rc, but not Dif-Ra/b/d, implying that miRNAs can regulate different isoforms of an alternative splicing gene to fine tune immune responses and maintain homeostasis in post-transcriptional level. Furthermore, we have demonstrated that the miR-317 sponge can restore the expression levels of Drs and Dif-Rc at mRNA and protein levels. Remarkably, during Gram-positive bacterial infection, the overexpressed miR-317 flies have poor survival outcome, whereas the knockout miR-317 flies have favorable survival compared to the control group, respectively, suggesting that the miR-317 might play a key role in Drosophila survival. Taken together, our current works not only reveal an innate immune function and a novel regulation pattern of miR-317, but also provide a new insight into the underlying molecular mechanisms of immunity disorder influencing on Drosophila survival.