Do the laundry? Check. Buy fabric softener? Got it. Write that next manuscript? Working on it…
We’ve all got a lot on our to-do lists these days, but it’s nothing compared to the checklist that developing drosophila embryos have to contend with. On top of coordinating histone methylation marks, they’ve got to prepare for a surge in zygotic gene activation (ZGA) that occurs around the 14th cell division.
All the proteins and transcripts that a zygote needs before ZGA are given by the oocyte, and a new study from the lab of Asifa Akhtar (Max Planck Institute of Immunobiology and Epigenetics, Germany) suggests that fly mothers pass down acetylated histones to their offspring as well.
The talented team focused on H4K16ac, a mark that is deposited in the zygotic genome by the male-specific lethal (MSL) complex. After confirming that H4K16ac is present in fly oocytes but not in spermatids, when measured with immunofluorescence, and that knocking down components of the MSL with small RNA interference (RNAi) decreases oocyte acetylation, they examined zygotes post-fertilization and found that:
- Unlike other acetylated histones, H4K16ac is present in the maternal pronucleus and is deposited de novo onto paternal chromosomes after fertilization
- H4K16ac is also present during early embryogenesis in mice, before the start of embryonic gene transcription
- 7000 promoters are H4K16ac-positive before ZGA, according to ChIP-seq, and most remain positive throughout development
- RNAi depletion of members of the MSL in maternal cells reduces H4K16ac in embryos, and decreases pre-ZGA chromatin accessibility, as measured by ATAC-seq, especially at binding sites for developmental regulator proteins
- In terms of 3D chromatin conformation, early embryos that lack maternal H4K16ac have weaker separation between topologically associated domains (TADs), more long-range contacts outside of TADs, and decreased separation between active and inactive chromatin domains, as measured by Hi-C
- Even though chromatin architecture is restored in late stage knockdown embryos, they still have gene expression deficits as measured by RNA-seq
- Genes that are normally primed for activation by maternal H4K16ac have decreased RNA Pol II recruitment according to ChIP-seq, and decreased expression later in development
- 80% of embryos (including 100% of male embryos) from mothers with germline genetic deletion of the MOF acetyltransferase, a member of the MSL complex, fail to survive
- Introducing a MOF transgene, at any point during embryogenesis, fails to rescue the lethality phenotype
- Although it increases the amount of other histone acetylation marks, RNAi of HDACs along with MOF fails to rescue the decrease in H4K16ac in embryos
Finally, since H4K16ac is important for X chromosome dosage compensation, the skillful scientists tasked themselves with understanding how maternal loss of MSL impacts sex chromosomes, via single-embryo ChIP-seq and RNA-seq. They found that without maternal MOF, male embryos are don’t accumulate H4K16ac at X-chromosome genes that are normally upregulated and they’re are unable to compensate with MOF that they express later on.
First author Maria Samata shares, “H4K16ac has an instructive function in the germline and is indispensable for embryonic development later on. It is almost like the mother leaves sticky notes with instructions on where to find the food or who to call in an emergency and so on, when the child is home alone for the first time. “
Add these exciting results to your to-read list with the original article in Cell, July 2020.