Paternal pachytene piRNAs are not required for fertilization, embryonic development and paternal epigenetic inheritance in mice. Mus musculus

Pachytene piRNAs are PIWI-interacting small RNAs abundantly expressed in pachytene spermatocytes and spermatids in adult mouse testes. Both MIWI and MILI-bound pachytene piRNAs have been found enriched in round spermatids. Miwi-null male mice are sterile due to spermiogenic arrest. In C. elegans, sperm-borne piRNAs appear to have an epigenetic role during fertilization and development because progeny of offspring derived from piRNA-deficient sperm display a progressive fertility loss after several generations. In mice, it remains unknown whether MIWI-bound pachytene piRNA-deficient round spermatids can produce offspring, and whether the progeny of offspring derived from MIWI-bound pachytene piRNA-deficient round spermatids also exhibit transgenerational loss of fertility. Here, we report that Miwi KO round spermatids could fertilize both wild type (WT) and Miwi KO oocytes through round spermatid microinjection (ROSI), and produce healthy and fertile offspring despite the aberrant pachytene piRNA profiles in those Miwi KO spermatids. Progeny of ROSI-derived heterozygotes, both male and female, displayed normal fertility for at least three generations when bred with either WT or Miwi KO females. Our data indicate that aberrant MIWI-bound pachytene piRNAs profiles in spermatids do not affect fertilization, early embryonic development, or fertility of the offspring, suggesting a normal pachytene piRNAs profile is not required for paternal transgenerational epigenetic inheritance in mice. Method: Round spermatids were purified from WT and Miwi KO adult testes using a mini-STA-PUT method[Methods Enzymol 1993; 225:84-113.]，the purity of round spermatids was >90% based on our previous report[ J Biol Chem 2012; 287:25173-25190.]. Small RNA was isolated from round spermatids using the mirVana RNA isolation kit (Ambion) according to the manufacturer’s instructions. RNA quality and quantity were assessed using the Agilent 2100 Bioanalyzer. Small RNA-Seq was performed on an Ion Proton sequencer (Life Technologies). Libraries were prepared using the Ion Total RNA-Seq Kit v2 (Invitrogen) with biological triplicates for WT and Miwi KO samples. Resutls:Our data indicate that aberrant MIWI-bound pachytene piRNAs profiles in spermatids do not affect fertilization, early embryonic development, or fertility of the offspring, suggesting a normal pachytene piRNAs profile is not required for paternal transgenerational epigenetic inheritance in mice. Overall design: In Miwi KO males, round spermatid production in the seminiferous tubules proceed up to step 4 (J Anat. 2000 Feb; 196(Pt 2): 217–232. ), which provides us with an opportunity to use steps 1-4 round spermatids to fertilize eggs through round spermatid microinjection (ROSI). In this study, we aimed to answer the following questions: 1) Is the pachytene piRNA profile altered in Miwi-deficient round spermatids? 2) Are the Miwi-deficient round spermatids competent for fertilization? 3) If so, are the progeny derived from ROSI offspring using Miwi-deficient round spermatids fertile? 4) Does paternal MIWI inactivation in mice phenocopy CSR-1 inactivation in C. elegans and display the “gremlin mortal” epigenetic inheritance? Here, we report that MIWI deficiency leads to altered pachytene piRNAs profiles and Miwi-deficient round spermatids can fertilize the eggs and produce normal offspring. Unlike C. elegans, the absence of paternal MIWI did not induce epigenetic “germline-mortal” phenotype in mice.