A Translation-Activating Function of MIWI/piRNA during Mouse Spermiogenesis

Highlights

• MIWI/piRNA activates mRNA translation via imperfect base-pairing interactions

• HuR and eIF3f are required for MIWI/piRNA-mediated target mRNA activation

• piRNA system controls the translation of a subset of mRNAs in mouse spermatids

• piRNA system plays a central role in acrosome formation during spermiogenesis

Summary

Spermiogenesis is a highly orchestrated developmental process during which chromatin condensation decouples transcription from translation. Spermiogenic mRNAs are transcribed earlier and stored in a translationally inert state until needed for translation; however, it remains largely unclear how such repressed mRNAs become activated during spermiogenesis. We previously reported that the MIWI/piRNA machinery is responsible for mRNA elimination during late spermiogenesis in preparation for spermatozoa production. Here we unexpectedly discover that the same machinery is also responsible for activating translation of a subset of spermiogenic mRNAs to coordinate with morphological transformation into spermatozoa. Such action requires specific base-pairing interactions of piRNAs with target mRNAs in their 3′ UTRs, which activates translation through coupling with cis-acting AU-rich elements to nucleate the formation of a MIWI/piRNA/eIF3f/HuR super-complex in a developmental stage-specific manner. These findings reveal a critical role of the piRNA system in translation activation, which we show is functionally required for spermatid development.