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.

Germ line-inherited H3K27me3 restricts enhancer function during maternal-to-zygotic transition.

Gametes carry parental genetic material to the next generation. Stress-induced epigenetic changes in the germ line can be inherited and can have a profound impact on offspring development. However, the molecular mechanisms and consequences of transgenerational epigenetic inheritance are poorly understood. We found that Drosophila oocytes transmit the repressive histone mark H3K27me3 to their offspring. Maternal contribution of the histone methyltransferase Enhancer of zeste, the enzymatic component of Polycomb repressive complex 2, is required for active propagation of H3K27me3 during early embryogenesis. H3K27me3 in the early embryo prevents aberrant accumulation of the active histone mark H3K27ac at regulatory regions and precocious activation of lineage-specific genes at zygotic genome activation. Disruption of the germ line-inherited Polycomb epigenetic memory causes embryonic lethality that cannot be rescued by late zygotic reestablishment of H3K27me3. Thus, maternally inherited H3K27me3, propagated in the early embryo, regulates the activation of enhancers and lineage-specific genes during development.

Plasma membrane phosphoinositide balance regulates cell shape during Drosophila embryo morphogenesis.

Remodeling of cell shape during morphogenesis is driven by the coordinated expansion and contraction of specific plasma membrane domains. Loss of this coordination results in abnormal cell shape and embryonic lethality. Here, we show that plasma membrane lipid composition plays a key role in coordinating plasma membrane contraction during expansion. We found that an increase in PI(4,5)P2 levels caused premature actomyosin contraction, resulting in the formation of shortened cells. Conversely, acute depletion of PI(4,5)P2 blocked plasma membrane expansion and led to premature actomyosin disassembly. PI(4,5)P2-mediated contractility is counteracted by PI(3,4,5)P3 and the zygotic gene bottleneck, which acts by limiting myosin recruitment during plasma membrane expansion. Collectively, these data support a model in which the ratio of PI(4,5)P2/PI(3,4,5)P3 coordinates actomyosin contractility and plasma membrane expansion during tissue morphogenesis, thus ensuring proper cell shape.

Tubular endocytosis drives remodelling of the apical surface during epithelial morphogenesis in Drosophila.

During morphogenesis, remodelling of cell shape requires the expansion or contraction of plasma membrane domains. Here we identify a mechanism underlying the restructuring of the apical surface during epithelial morphogenesis in Drosophila. We show that the retraction of villous protrusions and subsequent apical plasma membrane flattening is an endocytosis-driven morphogenetic process. Quantitation of endogenously tagged GFP::Rab5 dynamics reveals a massive increase in apical endocytosis that correlates with changes in apical morphology. This increase is accompanied by the formation of tubular plasma membrane invaginations that serve as platforms for the de novo generation of Rab5-positive endosomes. We identify the Rab5-effector Rabankyrin-5 as a regulator of this pathway and demonstrate that blocking dynamin activity results in the complete inhibition of tubular endocytosis, in the disappearance of Rab5 endosomes, and in the inhibition of surface flattening. These data collectively demonstrate a requirement for endocytosis in morphogenetic remodelling during epithelial development.

Tumor suppressor properties of the ESCRT-II complex component Vps25 in Drosophila.

We have found that the Drosophila gene vps25 possesses several properties of a tumor suppressor. First, vps25 mutant cells activate Notch and Dpp receptor signaling, inducing ectopic organizers in developing eyes and limbs and consequent overproliferation of both mutant and nearby wild-type cells. Second, as the mutant cells proliferate, they lose their epithelial organization and undergo apoptosis. Strikingly, when apoptosis of mutant cells is blocked, tumor-like overgrowths are formed that are capable of metastasis. vps25 encodes a component of the ESCRT-II complex, which sorts membrane proteins into multivesicular bodies during endocytic trafficking to the lysosome. Activation of Notch and Dpp receptor signaling in mutant cells results from an endocytic blockage that causes accumulation of these receptors and other signaling components in endosomes. These results highlight the importance of endocytic trafficking in regulating signaling and epithelial organization and suggest a possible role for ESCRT components in human cancer.

Elucidation of an archaeal replication protein network to generate enhanced PCR enzymes.

Thermostable DNA polymerases are an important tool in molecular biology. To exploit the archaeal repertoire of proteins involved in DNA replication for use in PCR, we elucidated the network of proteins implicated in this process in Archaeoglobus fulgidus. To this end, we performed extensive yeast two-hybrid screens using putative archaeal replication factors as starting points. This approach yielded a protein network involving 30 proteins potentially implicated in archaeal DNA replication including several novel factors. Based on these results, we were able to improve PCR reactions catalyzed by archaeal DNA polymerases by supplementing the reaction with predicted polymerase co-factors. In this approach we concentrated on the archaeal proliferating cell nuclear antigen (PCNA) homologue. This protein is known to encircle DNA as a ring in eukaryotes, tethering other proteins to DNA. Indeed, addition of A. fulgidus PCNA resulted in marked stimulation of PCR product generation. The PCNA-binding domain was determined, and a hybrid DNA polymerase was constructed by grafting this domain onto the classical PCR enzyme from Thermus aquaticus, Taq DNA polymerase. Addition of PCNA to PCR reactions catalyzed by the fusion protein greatly stimulated product generation, most likely by tethering the enzyme to DNA. This sliding clamp-induced increase of PCR performance implies a promising novel micromechanical principle for the development of PCR enzymes with enhanced processivity.