Transcriptional control of collective cell migration in the cardio-pharyngeal mesoderm of ascidian embryos
Our long-term research goal is to understand how tissue-specific gene regulatory networks control and coordinate the basic processes underlying cell behavior during morphogenesis. We study the migrating cells of the cardiogenic mesoderm in embryos of the ascidian Ciona intestinalis as a model system to investigate this problem.
Ascidians are Tunicates, the closest living relatives to vertebrates and they display a simplified but typical chordate body plan during embryonic and larval stages. Ascidian embryos offer several advantages for experimental studies of developmental gene activities and cell behavior. First, they develop with a reduced number of cells with invariant lineages that have been determined up to the pre-gastrula 110 cell stage. This allows one to visualize developmental events with a sub-cellular resolution using live microscopy. Second, extensive genomic and expression resources are readily available through user-friendly web interfaces. This facilitates functional studies utilizing microinjection of antisense morpholino oligonucleotides and an experimental tool unique to ascidians among metazoans: the simultaneous electroporation of hundreds of fertilized eggs with synthetic plasmid DNA constructs, which permits cis-regulatory analyses using reporter genes and targeted expression of recombinant and/or fluorescent proteins using tissue-specific enhancers for functional studies and fluorescent microscopy.
In ascidians, the trunk ventral cells (TVCs) migrate from the anterior part of the tail to the ventral part of the trunk in tailbud embryos and constitute the precardiac mesoderm. They originate from a single pair of blastomeres in the early embryo, called B7.5 cells. The B7.5 blastomeres give birth to the TVCs and their sister cells, the anterior tail muscles (ATMs), which differentiate into skeletal muscle and do not migrate. Previous studies showed that TVC-specific gene expression and migration require transcriptional inputs from the bHLH transcription factor Mesp, the FGF signaling pathway and the forkhead transcription factor FoxF. Because of these functional evidences, TVC migration constitutes a suitable model system to investigate the relationship between transcription regulation and directed cell migration. To this aim, we use a method combining fluorescence activated cell sorting (FACS) and microarray analysis to obtain TVC-specific whole genome transcription profiles. These data indicated that the gene regulatory network impinges on most cellular processes underlying cell migration (e.g. actin dynamics, cell-matrix adhesion, polarity and vesicle trafficking) through transcriptional regulation of subsets of the effector genes (Christiaen et al., 2008).
After this first migration, bilateral pairs of TVCs merge at the midline and divide asymmetrically along the medio-lateral axis to give birth to the (median) heart and (lateral) atrial siphon muscle (ASM) precursors. Strikingly, the four ASM precursors on each side of the heart primordium undergo a second collective migration towards the dorso-lateral atrial siphon placode in the ectoderm. We found that this second migration is regulated by an ASM-specific transcription factor, the Collier/Olf1/Ebf ortholog COE (Stolfi et al., 2010).
Our current research focuses on:
- The cellular and molecular mechanisms that establish and maintain collective cell polarization during TVC migration. We study the gene regulatory network downstream of FoxF, the regulated effector genes, the polarized cellular processes and the extrinsic signals influencing TVC polarity.
- The cellular and molecular mechanisms controlling heart vs. ASM fate specification and ASM collective migration. We study the regulation of COE in the TVC lineage. We obtained whole-genome transcription profiles for heart and ASM precursors. We use these data to compare TVC and ASM migrations and identify migration-specific effector genes and regulatory network modules.