Epigenetic regulation of histone eviction in spermatogenesis

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ABSTRACT Over 15% of couples in the United States suffer from infertility. Half of these cases are due to male factors such as low sperm count, abnormal sperm morphology and motility, or unexplained poor embryogenesis. Mammalian sperm exhibit a highly compacted nuclear and chromatin structure with only 5- 15% of the normal cellular histone complement. This transition occurs when nucleosome-associated histones within post-meiotic male germ cells are acetylated by histone acetyltransferases to open the structure to, in turn, allow subsequent eviction and replacement with protamines - leading to the profound compaction in mature sperm. Abnormal histone retention in sperm can lead to infertility and poor embryogenesis, thus implicating epigenetic regulation of the male gamete as a critical factor in reproduction. My preliminary data depict dynamic changes in histone post-translational modifications, such as acetylation, during spermatogenesis. My findings further demonstrate that pre-meiotic conditional deletion of the histone acetyltransferase Gcn5 results in abnormal sperm with increased histone retention. The goal of the proposed study is to investigate the functional and mechanistic role of histone acetylation in nucleosome eviction in post- meiotic male germ cells, and to determine whether contribution of excess retained paternal histones affects embryonic transcription, developmental programming and embryogenesis. To conduct these experiments I will use the conditional mouse model to functionally perturb histone acetylation through ablation of the histone acetyltransferase Gcn5 in pre-meiotic male germ cells (Gcn5cKO). In specific aim 1, I will use the Gcn5cKO mice to determine the mechanisms through which loss of histone acetylation leads to altered chromatin remodeling and excess histone retention in mature sperm, and how the consequences of reduced histone acetylation affects gene transcription during spermatogenesis. In specific aim 2, I will examine fertility of, and early embryonic development and transcription, resulting from the Gcn5cKO male mice to investigate the effect of excess paternal histones and associated altered paternal chromatin accessibility. To our knowledge, this will be the first investigation to conditionally ablate a histone acetyltransferase in pre-meiotic germ cells in the testes and investigate the functional consequences. As a result of these experiments we expect to further our understanding of the link between acetylation of histone residues and histone eviction and determine if transmission of excess paternal histones affects offspring development. Furthermore, as mouse and human spermatogenesis and sperm function are highly conserved, the findings of these experiments will be an invaluable source of information for understanding, diagnosing, and treating male infertility.