Biogenesis and function of tRNA fragments during sperm maturation and fertilization in mammals

Parental dietary conditions can influence the metabolic traits of offspring. In mice, paternal consumption of low protein diet alters cholesterol and lipid metabolism of progeny. Here, we examine RNA species expressed in male reproductive tissues of mice. Protein restriction leads to altered levels of multiple small RNAs in mature sperm, as well as throughout the male reproductive tract, with decreased levels of let-7 family members and increased levels of 5’ fragments of tRNA-Gly isoacceptors. Intriguingly, tRNA fragments are scarce in the testis, but their levels increase in sperm during posttesticular maturation in the epididymis. We find that epididymosomes – extracellular vesicles which fuse with sperm during epididymal transit – exhibit RNA payloads closely matching those of mature sperm, and can deliver tRNA fragments to immature sperm in vitro both in mouse and in bull. Finally, we show that tRNA-Gly-GCC fragments play a role in repressing genes associated with the endogenous retroelement MERVL, both in ES cells and in preimplantation embryos. Our results shed light on small RNA biogenesis during post-testicular sperm maturation, and link tRNA fragments to regulation of endogenous retroelements active in the early embryo.