MAEL-dependent selective processing of pachytene piRNA precursors and translation of spermiogenic mRNAs

Piwi-interacting small RNAs (piRNAs) of fetal prospermatogonia of mice have been strongly implicated in transposon control. In contrast, little is known about biogenesis and function of abundant piRNAs from adult testes expressed in late spermatocytes and round spermatids. These so-called "pachytene" piRNAs are processed from long non-coding piRNA precursors and have no defined RNA targets in the transcriptome even though their binding partner Piwi, MIWI, is essential for spermiogenesis and fertility. Here we report that 129SvJae mice lacking Maelstrom (MAEL), a conserved piRNA pathway protein, exhibit spermiogenic arrest with defects in acrosome and flagellum formation. Further analysis revealed MAEL association with RNPs containing MIWI, TDRD6, and processed intermediates of pachytene piRNA precursors of various length. Loss of MAEL causes a 10-fold drop in pachytene piRNA levels but an increase in piRNAs from abundantly expressed mRNAs. These results suggest a MAEL-dependent mechanism for the selective processing of pachytene piRNA precursor into piRNAs. Strikingly, ribosome profiling of Mael-null testes revealed that reduced piRNA production is accompanied by reduced translation of over 800 spermiogenic mRNAs including those encoding acrosome and flagellum proteins. In light of recent reports of piRNA-independent protection of translationally repressed mRNPs by MIWI and piRNA-dependent turnover of MIWI, we propose that pachytene piRNAs function by controlling the availably of MIWI for the translational repression of spermiogenic mRNAs. Overall design: piRNA sequencing, RNA immunoprecipitation, and expression measurements (RNA-Seq and ribosome profiling) in wild-type and Mael -/- testes

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Castañeda J, Genzor P, van der Heijden GW, Sarkeshik A, Yates JR 3rd, Ingolia NT, Bortvin A