Gene and chromatin (de)regulation during spermiogenesis: actors & impact on the progeny – CHROMATOZOA

Mammalian male gametes are produced in the testis by series of sequential events generally divided in three steps: proliferation of spermatogonial stem cells, meiosis and differentiation of haploid germ cells, known as spermatids, into spermatozoa. This last step (called spermiogenesis) occurs after meiosis and involves profound morphological and functional changes that are essential for male fertility: elongation of the cell, cytoplasm reduction and acquisition of new structures such as the acrosome (required for penetration of the oocyte) and the flagellum (that enables sperm motility) Importantly, during this step, the spermatid nucleus is compacted to ~one fifth of its original size and spermatid chromatin is extensively remodeled to achieve optimal compaction. Spermatid chromatin remodeling requires the incorporation of multiple histone variants and post translational modifications of histone residues; eventually, most histones are replaced by proteins that are smaller and more basic than histones: the protamines. These steps are essential to properly compact the paternal genome and preserve its integrity until fecundation. The regulation of this process requires a genetic program unique to postmeiotic male germ cells, with ~3000 genes only expressed at that time.
The aims of the present proposal are i) to study gene regulation and chromatin remodeling during spermiogenesis, and ii) to investigate the consequences on the embryo when this process is abnormal.
We have a mouse model presenting gene deregulation associated with abnormal sperm chromatin remodeling and compaction, high incidence of sperm DNA damage and male infertility. Our recent work on this mouse model has led to the identification of several interesting pathways which are apparently important for gene and chromatin regulation in spermatids; yet, nothing is known of their role during sperm differentiation. This is what we will investigate by molecular and cell biology approaches, and large scale genomic analyses, making use of existing models and developing new ones. In parallel, we will use single-cell transcriptome analyses to characterize changes in gene expression throughout the differentiation of wild-type mouse spermatids. This innovative approach should produce datasets which will be useful to the present project and to scientists of the field. Those two complementary approaches (gene-candidate orientated and global) should improve our knowledge of the gene network and pathways relevant to gene and chromatin regulation during spermiogenesis.
Finally, using our mouse model, we will investigate the consequences of gene deregulation/abnormal chromatin remodeling during spermiogenesis on embryo development and genome integrity. Indeed, several studies, including ours, have shown that abnormally compacted sperm resulting from a defective chromatin remodeling during spermiogenesis display high levels of DNA damage. The use of those sperm in assisted reproductive technologies (ART), such as ICSI (Intra-cytoplasmic sperm injection), could have negative consequences on embryo development and health of the progeny.
All in all, the present proposal should address questions relevant to both fundamental and clinical research. The project will be coordinated by a junior researcher who is developing her own research team at the Cochin institute. It will benefit from the coordinator and members of the team’s expertise in cellular and molecular biology, epigenetics, developmental and reproductive biology, as well as from the use of state-of-the-art techniques, such as ICSI, next generation sequencing and single cell analyses. Obtaining ANRJCJC funds will be essential for the coordinator to reach full independency; the data she will generate with the present proposal will be a pre-requisite to apply for international grants and collaborative grants, such as ERC, and ANR-PRC/I.