Investigating the Function of Parental Histones During Early Embryogenesis
Creators: name: Manske, Gabriel
Publisher: name: My University
Year: 2023
Subjects: subject: CENP-A
subject: Epigenetics
subject: Centromere
subject: Spermatogenesis
subject: Oogenesis
subject: Early Embryogenesis
subject: Genetics
subject: FOS: Biological sciences
subject: Molecular, Cellular and Developmental Biology
subject: Health Sciences

description: A remarkable property of complex genomes is their ability to generate a range of cell types using an identical genetic template. How this happens has intrigued biologists and spurred recent advancements in the understanding of the epigenome. The epigenome consists of a variety of chemical modifications to DNA and DNA packaging proteins called histones. The interplay between the epigenome, noncoding RNAs, and transcription factors confers gene expression patterns in a cell, which are stably maintained in subsequent mitotic divisions. Unlike the soma, epigenetic information in germ cell precursors and the embryo are reset, but accumulating evidence suggests that epigenetic information can be passed on from generation to generation. However, our understanding of how and why certain genomic regions escape reprogramming or the nature of the molecular carriers (RNA, DNA, and histones) transmitting epigenetic information in response to an insult remains poorly understood. The role of DNA methylation and noncoding RNA has been well-established in the germline. However, less direct evidence exists to answer whether histones can serve as informational carriers across generations. Previous studies have shown that changes to histone protein levels or post-translational modifications in the germline result in developmental abnormalities in the embryo. These abnormalities have persisted for multiple generations, indicating that histone proteins from the maternal or paternal genome are likely to transmit information to offspring. Despite this exciting possibility, our ability to test whether histones can be inherited and/or instructive for development remains limited due to the lack of available mouse models that allow us to directly follow parental nucleosomes in the totipotent embryo. To overcome these limitations, we generated several transgenic mouse lines to track functionally distinct histone variants (CENP-A, H3.2, H3.3, and H4) with different protein tags (Halotag or mScarlet). These different models can be used to follow the fate of histone proteins residing in different chromatin compartments e.g., centromeres or active vs. repressed genes. Using these different genetic models in both the male and female germline, we uncovered sexually dimorphic properties of centromere regulation and dynamics during gametogenesis and embryogenesis. We also provided the first direct molecular evidence for paternal and maternal CENP-A histone inheritance in mice and uncovered a novel mechanism to equalize parental CENP-A in the early embryo. To examine whether histone inheritance extends beyond CENP-A in both parental germlines, we generated Dox-inducible transgenic lines to ectopically express tagged versions of H3.2, H3.3, and H4, since these non-centromeric H3 variants are multicopy genes distributed onto different chromosomes. In developing oocytes, we showed that tagged H3.2 and H3.3 are incorporated into chromatin and are maintained through oogenesis. Interestingly, we noticed differences in the perdurance of H3.2 and H3.3- tagged histones during the first embryonic cell cycles, possibly due to differences in H3.3 and H3.2 stability/turnover. Overall, the genetic tools created in this study have opened a new path to studying the mechanisms that regulate the inheritance of chromatin-based epigenetic regulation. This research sheds light on the role of histones in transmitting epigenetic information across generations and has important implications for understanding the molecular basis of inherited traits and diseases.