Inheritance of H3K9 methylation regulates genome architecture in Drosophila early embryos.

Constitutive heterochromatin is essential for transcriptional silencing and genome integrity. The establishment of constitutive heterochromatin in early embryos and its role in early fruitfly development are unknown. Lysine 9 trimethylation of histone H3 (H3K9me3) and recruitment of its epigenetic reader, heterochromatin protein 1a (HP1a), are hallmarks of constitutive heterochromatin. Here, we show that H3K9me3 is transmitted from the maternal germline to the next generation. Maternally inherited H3K9me3, and the histone methyltransferases (HMT) depositing it, are required for the organization of constitutive heterochromatin: early embryos lacking H3K9 methylation display de-condensation of pericentromeric regions, centromere-centromere de-clustering, mitotic defects, and nuclear shape irregularities, resulting in embryo lethality. Unexpectedly, quantitative CUT&Tag and 4D microscopy measurements of HP1a coupled with biophysical modeling revealed that H3K9me2/3 is largely dispensable for HP1a recruitment. Instead, the main function of H3K9me2/3 at this developmental stage is to drive HP1a clustering and subsequent heterochromatin compaction. Our results show that HP1a binding to constitutive heterochromatin in the absence of H3K9me2/3 is not sufficient to promote proper embryo development and heterochromatin formation. The loss of H3K9 HMTs and H3K9 methylation alters genome organization and hinders embryonic development.

The Dpp/TGFß-dependent corepressor Schnurri protects epithelial cells from JNK-induced apoptosis in drosophila embryos.

Jun N-terminal kinase (JNK) often mediates apoptosis in response to cellular stress. However, during normal development, JNK signaling controls a variety of live cell behaviors, such as during dorsal closure in Drosophila embryos. During this process, the latent proapoptotic activity of JNK becomes apparent following Dpp signaling suppression, which leads to JNK-dependent transcriptional activation of the proapoptotic gene reaper. Dpp signaling also protects cells from JNK-dependent apoptosis caused by epithelial disruption. We find that repression of reaper transcription by Dpp is mediated by Schnurri. Moreover, reporter gene analysis shows that a transcriptional regulatory module comprising AP-1 and Schnurri binding sites located upstream of reaper integrate the activities of JNK and Dpp. This arrangement allows JNK to control a migratory behavior without triggering apoptosis. Dpp plays a dual role during dorsal closure. It cooperates with JNK in stimulating cell migration and also prevents JNK from inducing apoptosis.