Justin Chong, Biological Sciences, Summer 2021
Figure 1: Ribbon diagram of chromatin wrapped around a nucleosome. (Source: Wikimedia Commons)
Embryonic development begins when a sperm cell fertilizes an egg, and they fuse together, forming a totipotent cell, which can give rise to the embryo and the placenta (Mitalipov & Wolf 2009). The totipotent cell transitions into a pluripotent cell, which gives rise to all germ cells in the embryo; this transition is mediated by chromatin reassembly and histone binding and modification (Lu et al. 2016). Nucleosomes are octamers of histones that chromatin wraps around to become compacted and form chromosomes. These canonical nucleosomes, which are constructed in a DNA replication-dependent manner, are composed of the canonical H2A, H2B, H3, and H4 histones, but the histone variant H3.3 can be deposited in a replication-independent manner, meaning that histones are deposited onto the DNA using conserved assembly complexes not contingent upon DNA replication (Elsaesser et al. 2010).
Previous studies have noted the importance of H3.3 in chromatin reprogramming, male pronucleus formation, and pericentric heterochromatin assembly (Santenard et al. 2010). This study by Guo, Liu, and Zhou sought to elucidate how the kinetics of H3.3 in GV (germinal vesicle) oocytes and early embryos reflects the chromatin compaction status of parental genomes and plays an essential role in mouse embryogenesis. GV oocytes and early embryos were collected from transgenic mice that had H3.3 tagged with a green fluorescent protein and H2B tagged with a red fluorescent protein to compare the histone variant of interest, H3.3, against a canonical histone, H2B (Guo et al. 2021). Fluorescence recovery after photobleaching (FRAP)—which works by exposing a certain section of a cell to intense light to permanently disable fluorophores to assess the kinetics of a molecule of interest—was used to measure the mobility of these histones in the male and female parental genomes throughout different stages of embryogenesis.
The results showed that maternal H3.3 mobility is highly correlated with chromatin compaction status, as the incorporation of newly synthesized H3.3 into the genome increases overall H3.3 mobility and chromatin decompaction, resulting in increased transcription levels. When the sperm and egg first fuse together, female chromatin is packed by histones while male chromatin is packed by protamine, a type of protein that allows for denser DNA packaging in sperm cells (Adenot et al. 1991). The protamine is gradually replaced by the maternal histones as the male chromatin becomes repacked.
During this maternal-to-embryonic transition, H3.3 incorporation and mobility are significantly higher in the male pronucleus compared to the female pronucleus. This higher mobility is linearly correlated to chromatin compaction, as more H3.3 mobility allows for higher chromatin mobility. This may be due to the fact that H3.3 can form temporary, unstable structures with H4 that lead to specific higher-order chromatin structures and different transcriptome profiles (Macfarlan et al. 2012). H3.3 is also known to antagonize bulk histone H1, which is a non-canonical histone that sits on top of packed chromatin to stabilize the chromatin structure (Wen et al. 2014). By antagonizing H1, H3.3 maintains the decompation state of the male parental genome during early embryogenesis.
Overall, the differential mobility of histone variant H3.3 between the male and parental genomes after fusion of the male sperm and female oocyte plays a critical role in the compaction of the parental chromatin. This indubitably influences the accessibility of the chromatin to the transcriptional machinery and reorganizes chromatin on a higher level. However, the interactions between H3.3 and other histones and histone variants must be explored further to illuminate the mechanism by which H3.3 mobility and incorporation influence embryogenesis as a whole.
References
Adenot, P. G. et al. (1991). Dynamics of paternal chromatin changes in live one-cell mouse embryo after natural fertilization. Molecular Reproduction and Development 28(1): 23-24. https://doi.org/10.1002/mrd.1080280105.
Elsaesser, S. J., Goldberg, A. D., & Allis, C. D. (2010). New functions for an old variant: no substitute for histone H3.3. Current Opinion in Genetics and Development 20(2): 110-117. https://doi.org/10.1016/j.gde.2010.01.003.
Guo, S. et al. (2021). H3.3 kinetics predicts the chromatin compaction status of parental genomes in early embryos. Reproductive Biology and Endocrinology 19, 87. https://doi.org/10.1186/s12958-021-00776-3.
Lu, F. et al. (2016). Establishing chromatin regulatory landscape during mouse preimplantation development. Cell 165(6): 1375-1388. https://doi.org/10.1016/j.cell.2016.05.050.
Macfarlan, T. S. et al. (2012). Embryonic stem cell potency fluctuates with endogenous retrovirus activity. Nature 487: 57-63. https://doi.org/10.1038/nature11244.
Mitalipov, S. & Wolf, D. (2009). Totipotency, pluripotency, and nuclear reprogramming. Advances in Biochemical Engineering/Biotechnology 114: 185-199. https://doi.org/10.1007/10_2008_45.
Santenard, A. et al. (2010). Heterochromatin formation in the mouse embryo requires critical residues of the histone variant H3.3. Nature Cell Biology 12: 853-862. https://doi.org/10.1038/ncb2089.
Wen, D. et al. (2014). Histone variant H3.3 is an essential maternal factor for oocyte reprogramming. PNAS 111(20): 7325-7330. https://doi.org/10.1073/pnas.1406389111.