Epigenetics can be defined as non-genetic changes that are transmitted through cell-divisions. Epigenetic patterns of histone modifications contribute to the maintenance of tissue-specific gene expression, but little is known about how such patterns are initially established during early embryo development. We investigate how the three germlayers mesoderm, neuroectoderm, and dorsal etoderm come to differ in their epigenetic patterns in response to the Dorsal morphogen.

The early Drosophila embryo is patterned along the anterior-posterior and dorsal-ventral axes by transcription of developmental control genes in different parts of the embryo. Dorsal-ventral patterning is controlled by an intra-nuclear concentration gradient of Dorsal, a Rel-family transcription factor related to NF-kappaB. Over 50 Dorsal target genes are known, and this gene regulatory network constitutes one of the best understood in the development of any animal. Dorsal enters ventral nuclei at high levels in response to signaling by the transmembrane receptor Toll. Target genes such as twist (twi) and snail (sna) with low-affinity bindning sites are turned on in ventral, presumptive mesodermal cells where Dorsal concentration is highest.  In lateral regions of the embryo, cells destined to become neuroectoderm activate short-gastrulation (sog), brinker (brk) and rhomboid (rho) in response to low levels of Dorsal. The Snail repressor prevents expression of these neuroectodermal genes in the mesoderm. Snail genes encode zinc-finger transcription factors that are fundamental for metazoan embryonic development in processes such as gastrulation and epithelial-to-mesenchymal transition (EMT), converting static epithelial cells into motile and invasive mesenchymal cells with stem cell properties. Finally, when Dorsal sites are positioned in proximity to AT-rich binding sites, Dorsal is converted to a repressor and thereby prevents expression of dorsal ectoderm targets such as tolloid (tld), decapentaplegic (dpp) and zerknüllt (zen) in lateral and ventral parts of the embryo.

Epigenetic landscapes
The Dorsal protein forms a gradient along the ventral-dorsal axis to specify mesoderm, neurogenic ectoderm, and dorsal ectoderm. High Dorsal concentration activates genes in the mesoderm, whereas intermediate Dorsal concentration activates neurogenic ectoderm genes that are repressed in the mesoderm by Snail. Dorsal can be converted to a repressor to prevent expression of dorsal ectoderm targets in mesoderm and neurogenic ectoderm. In Toll10B mutants, the entire embryo is converted to mesoderm, Tollrm9/rm10 mutant embryos become neurogenic ectoderm, and in gd7 embryos all cells are turned into dorsal ectoderm.
 
 

Investigation of dorsal-ventral patterning in Drosophila provides a unique opportunity to study cell specification, since mutants in the Toll signaling pathway can be used to generate embryos with a homogenous population of cell types (see Fig. 1). In embryos derived from gd7 mutant mothers, Dorsal fails to enter the nucleus and the entire embryo is transformed into dorsal ectoderm. In embryos derived from Tollrm9/rm10 mothers, all cells become neurogenic ectoderm since Dorsal uniformly enters nuclei at an intermediate concentration. Toll10B mothers produce embryos where Dorsal enters all nuclei at high concentration thereby transforming them into mesoderm. Chromatin extracts from these mutant embryos circumvent the problem of a mixed population of cell types, but enable investigation of histone modifications and regulator binding to endogenous loci under conditions that the different cell types normally experience during development. These tools allow us to study mechanisms of tissue-specific gene expression in vivo, in a manner that is exclusive to the early Drosophila embryo.

By characterizing the interplay of the two evolutionarily conserved transcription factors, the NFkappaB protein Dorsal and the zinc-finger repressor Snail, with the CBP co-regulator, we determine how patterns of histone modifications are established at Dorsal-target genes (Boija and Mannervik 2016). We will also identify factors that recruit Polycomb proteins to epigenetically silence transcription in the dorsal ectoderm.

 

References

Boija A and Mannervik M. (2016) Initiation of diverse epigenetic states during nuclear programming of the Drosophila body plan. Proc Natl Acad Sci U S A. (31):8735-40