chromatin; nucleosome; epigenetics; spermatogenesis; fertility
Tomizawa Shin-ichi, Kobayashi Yuki, Shirakawa Takayuki, Watanabe Kumiko, Mizoguchi Keita, Hoshi Ikue, Nakajima Kuniko, Nakabayashi Jun, Singh Sukhdeep, Dahl Andreas, Alexopoulou Dimitra, Seki Masahide, Suzuki Yutaka, Royo Hélène, Peters Antoine H. F. M., Anastassiadis Konstantinos, Stewart A. Francis, Ohbo Kazuyuki (2018), Kmt2b conveys monovalent and bivalent H3K4me3 in mouse spermatogonial stem cells at germline and embryonic promoters, in Development
, 145(23), dev169102-dev169102.
In the life cycle of mammalian organisms, fertilization represents the moment at which genetic information of two individuals is united to give rise to a new individual. A major question in biology is whether and/or to what extent mature gametes carry information beyond the DNA sequence that is important for the development of offspring. Over recent years, numerous studies involving mammalian systems provided some evidence for inter- and/or transgenerational inheritance of phenotypic traits in response to external environmental cues, such as altered nutrition, chemical pollutants, and endocrine disruptors. Alterations in DNA methylation, composition of small RNAs and/or chromatin states have been implicated in the inheritance of such traits, yet, clear mechanistic insights are rather limiting. To elucidate molecular mechanisms involved in such acquired forms of epigenetic inheritance, we aim to dissect (a) the fundamental mechanisms involved in defining chromatin states in mature gametes in unchallenged conditions, and (b) whether such states are instructive in specifying embryonic development. In this proposal, we focus on the development of male germ cells. Following an almost eight week lasting developmental process millions of highly differentiated haploid spermatozoa are day-to-day generated from, originally, a few spermatogonial stem cells. Upon leaving the spermatogonial stem cell pool, differentiating spermatogonia undergo first multiple rounds of replication and thenceforth enter into meiosis to undergo meiotic recombination and haploidisation. During the following spermiogenesis phase, haploid spermatids dramatically remodel their chromatin structure and undertake a major cellular morphogenesis process resulting in the generation of highly compacted and motile sperm.We and others have previously shown that nucleosomes, the basic building units of chromatin, remain present at specific sequences in mature sperm of mouse and men. Ongoing experiments indicate that H3 proteins in nucleosomes “change their identity” by the exchange of canonical replication dependent H3.1 and H3.2 proteins by a testis specific isoform called H3t as well as by the H3.3 variant histone. H3t is conserved among mammals and harbors a few amino acid substitutions relative to H3.1. H3t deficient animals display major defects in the proliferative differentiating spermatogoia and reduced H3.3 levels cause spermatogenic arrest at different stages, pending on residual levels. In this proposal, we combine mouse transgenesis, molecular genetic, epigenomic, and biochemical approaches to understand the function of the H3t and H3.3 histone variants, and to test the role of specific amino acid residues and their post-translational modifications in male germ cell development. We further focus on the mechanisms underlying global transcriptional silencing occurring at the end of spermatogenesis. We propose to perform proteomic analyses of chromatin isolated from developing spermatids to identify novel factors involved in nuclear condensation and nucleosome eviction versus retention. Finally, we aim to determine the localization of nucleosomes in individual spermatozoa and assess their relevance for intergenerational transmission of epigenetic information.We believe that this research will enhance our understanding of the regulatory role of chromatin in male gamete generation and transmission of epigenetic information to the next generation.