Epigenomics of Post-testicular Sperm Maturation

Beyond the haploid genome, mammalian sperm carry a payload of epigenetic information with the potential to modulate offspring phenotype. Morphologically mature sperm exit the testes, but cannot swim or interact with the oocyte without extensive remodeling during epididymal transit; this includes modifications to the lipid composition of the sperm membrane, gain of necessary proteins, and a dramatic shift in sperm RNA content. Epididymal maturation has also been linked to changes in the sperm methylome suggesting that the epididymis might play a broader role in shaping the sperm epigenome. First, we characterized the genome-wide methylation landscape in seven germ cell populations from throughout the male reproductive tract. Our data emphasize the stability of cytosine methylation in mammalian sperm, and identify a surprising, albeit transient, period during which sperm are associated with extracellular DNA. Second, given our interest in the small RNA repertoire of sperm we set out to address known bias in sequencing protocols by comparing several small RNA cloning protocols. We found a protocol recently developed by Kathleen Collins’ lab (OTTR) to be superior to commercially available kits in providing an accurate representation of tRNA fragment levels as compared to Northern blotting. These results not only provide a more accurate representation of tRNA fragments, but also more complexity than previously seen allowing us to reassess the true sperm small RNA content. Taken together, these results provide significant insight into the mechanisms and factors modulating sperm epigenomics during post-testicular sperm maturation