Programming of the paternal nucleus for embryonic development

Abstract

Historically, sperm has been considered merely as a carrier of genetic material at fertilisation. However, it is known that sperm supports embryonic development better than other cell types, suggesting that it might also have additional important, non-genetic contributions to embryonic development. The work described in this dissertation focuses on identifying the molecular determinants of developmental programming of sperm.

First, the development of embryos derived from sperm and spermatids, immature precursors of sperm was compared. Sperm-derived embryos developed significantly better than spermatid-derived embryos. Further research aiming to identify the reasons for the developmental advantage of sperm led to the identification of proteins that are present specifically in sperm and not in spermatids.

Moreover, egg factors which are preferentially incorporated into the sperm, but not into the spermatid chromatin were identified with the use of egg extracts, suggesting that the chromatin of sperm could be programmed to interact with the components of the egg.

Subsequently, the reasons for developmental failure of spermatid-derived embryos were investigated. By comparing the sperm with spermatids it was shown that the programming of sperm to support efficient development is linked to its special ability to regulate expression of developmentally-important embryonic genes, and not to its ability to support DNA replication or rRNA production. Further characterisation of the sperm and spermatid chromatin with the use of genome-wide sequencing allowed me to link the correct regulation of gene expression in the embryo with a certain combination of epigenetic marks in the sperm, but not in the spermatid chromatin.

Finally, it is shown that enzymatic removal of epigenetic modifications at fertilisation leads to misregulation of gene expression. This therefore suggests that epigenetic information contained in parental genomes at fertilisation is required for a proper regulation of embryonic transcription.

My results support the hypothesis that the sperm is not only a carrier of genetic material, but also provides the embryo with epigenetic information for regulation of transcription after fertilisation.

I believe that these findings advance our current understanding of the nature and mechanisms of sperm programming for embryonic development, and are important contributions to the emerging field of transgenerational inheritance of epigenetic traits in general.