Epididymal acquired sperm microRNAs modify post-fertilization embryonic gene expression
Sperm small RNAs have emerged as important non-genetic contributors to embryogenesis and offspring health. A subset of sperm small RNAs are thought to be acquired during epididymal transit. However, the transfer of RNAs from the somatic epididymis to the sperm has been questioned, and the identity of the specific small RNAs transferred remains unclear. Here, we employ Cre/Lox genetics to generate germline- and epididymal-specific Dgcr8 conditional knockout mice to investigate the dynamics of sperm microRNAs and their function in the early embryo. Interestingly, sperm from germline specific Dgcr8 knockout males restored the levels of 58 of the 98 (59%) miRNAs that were lost in testicular sperm during epididymal transit. Conversely, sperm from epididymal Dgcr8 knockouts displayed a 5-fold reduction in 25 miRNAs. This substantial loss of epididymal miRNAs in sperm was accompanied by transcriptomic changes in the embryo which was rescued by microinjection of epididymal miRNAs. These findings ultimately demonstrate the acquisition of miRNAs by sperm during epididymal transit and their regulation of post-fertilization embryonic gene expression. Overall design: To examine the impact of ablation of the miRNA pathway  on male fertility and the regulation of gene expression in the male reproductive tract we sequenced the small RNA fraction of testicular and cauda sperm from 2 Dgcr8 conditional knockout mice and their respective controls and performed RNA-seq on epididymal tissue from these mice. Additionally, in examining the impact of altered sperm miRNA levels on gene expression in the early embryo we sequenced single embryos  (4-cell and morula stage) fertilized by control and mutant male mice.