1. Scaling of Embryos and Gene Expression Dynamics

We aim to expand our understanding of the Dorsal-Ventral (DV) patterning gene-regulatory network (GRN) acting in the early Drosophila embryo: a developmental system, which uses morphogens to support patterning and undergoes rapid development. Our goal is to integrate spatiotemporal information into the DV patterning GRN with the objective of obtaining additional insights into the roles of transcription factor and target gene dynamics (see Reeves, Trisnadi et al., Dev Cell 2012). In particular, we are interested in why some target genes appear ‘plastic’, with levels changing constantly both upwards and downwards; whereas others exhibit more of a ‘ratchet’ effect in that levels continue to steadily increase. Furthermore, we have found that the size of the DV axis can change as much as 20% due to naturally occurring variation. Some patterns change accordingly, they ‘scale’, whereas other patterns remain constant (see Garcia et al., Dev Bio 2013). How is robust development of embryos supported in the face of such natural variability in embryo size? Why do genes exhibit different dynamics, and how does this impact developmental progression?

2. Coordinate action of cis-regulatory modules

Many genes are pervasively expressed throughout development and exhibit changes of expression in a stage-specific manner. It is appreciated that different cis-regulatory modules (CRMs) act to control dynamic expression; however, not much is known about how CRM order of action is regulated. Using the Drosophila embryo as a model system, we have the exceptional opportunity to investigate how CRMs support spatiotemporally-regulated gene expression during the animal’s developmental course. Current experiments focus on understanding how CRM order of action is controlled. The brinker gene locus is particularly well-suited to provide insight into the regulation of temporal gene expression. Our preliminary data show that the sequence just upstream of the brk minimal promoter contributes to sequential, coordinate action of CRMs controlling expression in the early embryo (Dunipace et al., Dev Cell 2013). However, how this promoter proximal sequence as well as chromatin conformation supports timing of CRMs acting in series is not yet understood, and will be investigated. In addition, a necessary technical advance for analysis of dynamic developmental systems is analysis of chromatin conformation on a cell by cell basis, which will support studies of when and how particular CRMs interact with the promoter with temporal and spatial resolution. To this end, we are developing various technologies to acquire this information. We are also looking broadly at the regulation of genes in time and how the action of CRMs is regulated (Dunipace et al., Dev. 2011).

3. Fibroblast growth factor signaling

Fibroblast growth factor (FGF) signaling impacts a number of different cellular functions important for supporting embryonic development. FGF ligands are polypeptide growth factors that bind to cell surface fibroblast growth factor receptors (FGFRs). These receptor ligands trigger tyrosine kinase activity associated with the intracellular domains of their receptors, and thereby elicit signaling responses within cells. Both ligands and receptors exhibit diverse and dynamic patterns of expression that support directional signaling across epithelial-mesenchymal boundaries. In early embryos, FGF signaling controls mesoderm induction and patterning, cell growth, migration, and differentiation; while later functions include organ formation and maintenance, neuronal differentiation and survival, wound healing, and malignant transformation. In vertebrates, nevertheless, the system remains quite complex with over 120 FGF-FGFR combinations possible. Moreover, up to five FGF ligands may function concurrently to activate a single receptor in an apparently redundant fashion, making dissection of individual activities linked to particular ligands quite challenging. Studies in Drosophila have yielded valuable insights into the functions of many signaling pathways during development, as this system is amenable to molecular and genetic techniques as well as to live in vivo imaging. Therefore, Drosophila, with only three FGF-FGFR combinations acting, offers a simpler system to study FGF signaling. We have found that FGF ligand choice, levels, and cleavage-state can all affect FGFR-dependent outputs (Kadam et al., Dev 2009; McMahon et al., Dev 2010; Tulin and Stathopoulos, BMC Dev Bio 2010). Moreover our results demonstrate that FGF ligands that act concurrently to activate the same receptor are not redundant. FGF ligands fulfill distinct roles in the Drosophila embryo (Kadam et al., Dev 2009; McMahon et al., Dev 2010). In future directions, we are interested in answering the following questions: How are FGF ligands different and how is their activity regulated? How does FGF signaling regulate cell movement? Is there a link between FGF signaling and regulation of cell adhesion?

4. Collective migration of groups of cells

Cell migration is a very influential process during embryonic development as it results in rearrangement of cells from one part of the embryo to another, effectively controlling cell-cell interactions to drive cell differentiation and organogenesis. The shape of most complex organ systems arises from the directed migration of cohesive groups of cells. Therefore, cell migration must be regulated temporally and spatially for organisms to develop properly. The overlying goal of our research objective is to provide insight into how cells within a migrating groups sense their environment and how this contributes to their collective movement. We study the movement of mesoderm cells during gastrulation (McMahon et al., Science 2008; McMahon et al., Dev 2010) and at later stages mesoderm cells which are precursors of longitudinal muscles. Caudal visceral mesoderm (CVM) cells exhibit directed cell migration as two distinct groups on either side of the body, from the posterior-most position of the embryo toward the anterior. The cells undergo the longest migration in all of Drosophila embryogenesis, but little is understood about how they are directed along their course (Kadam et al., Dev. 2012). The migration ensues over six hours and is necessary to position CVM cells along the entire length of the developing gut. At the end of their migration, CVM cells fuse with fusion-competent myoblasts to form the longitudinal muscles which ensheath the gut. We are developing an imaging strategy to describe the behavior of these two mesoderm cell migrations, and plan to using an imaging approach to analyze mutants isolated in several screens. In addition, we are developing new approach for creating mutant clones and studying coordinate cell migration using light-activated molecules.

©2014 Stathopoulos Lab, Caltech Division of Biology & Biological Engineering.         
Design by N Trisnadi. Last updated 12 Feb 2014.