Analysis and reconstitution of the genetic cascade controlling early mesoderm morphogenesis in the Drosophila embryo

Abstract

To understand how transcription factors direct developmental events, it is necessary to know their target or 'effector' genes whose products mediate the downstream cell biological events. Whereas loss of a single target may partially or fully recapitulate the phenotype of loss of the transcription factor, this does not mean that this target is the only direct mediator. For a complete understanding of the pathway it is necessary to identify the full set of targets that together are sufficient to carry out the programme initiated by the transcription factor, which has not yet been attempted for any pathway. In the case of the transcriptional activator Twist, which acts at the top of the mesodermal developmental cascade in Drosophila, two targets, Snail and Fog, are known to be necessary for the first morphogenetic event, the orderly invagination of the mesoderm. We use a system of reconstituting loss of Twist function by transgenes expressing Snail and Fog independently of Twist to analyse the sufficiency of these factors-a loss of function assay for additional gene functions to assess what further functions might be needed downstream of Twist. Confirming and extending previous studies, we show that Snail plays an essential role, allowing basic cell shape changes to take place. Fog and at least two other genes are needed to accelerate and coordinate shape changes. Furthermore, this study represents the first step in the systematic reconstruction of the morphogenetic programme downstream of Twist.

A conserved role for Snail as a potentiator of active transcription

“…This might be an indication that misregulation of a larger set of both repressed and activated genes leads to the failure in furrow formation. This is also consistent with the fact that simply reducing the level of Snail by half leads to a delay in gastrulation (Seher et al 2007).…”

“…Twist and Dorsal cooperate to activate mesodermal gene expression, while Snail promotes mesoderm development by repressing ectodermal genes within the mesodermal domain and establishes a sharp border between the mesoderm and mesectoderm (for review, see Chopra and Levine 2009). Although loss of either Twist or Snail function results in a failure of mesoderm formation (Leptin and Grunewald 1990), Snail is sufficient to promote the first steps of ventral furrow invagination (Ip et al 1994;Seher et al 2007). Snail therefore has an independent role in promoting mesoderm formation, but how this is achieved remains unclear.…”

Assessing the critical period for Rho kinase activity during Drosophila ventral furrow formation

“…When fog is expressed in a twi mutant background, VFF completes . However, when fog is ectopically expressed in sna or sna, twi mutants, no cell shape changes are observed and a ventral furrow is not formed (Morize et al, 1998;Seher et al, 2007). This suggests that Snadriven nuclear migration and random apical constriction is a prerequisite for Fog-mediated concerted apical constriction to occur.…”

“…Furthermore, embryos lacking Twi activity still undergo some shape changes, even though they do not form a ventral furrow. These shape changes can most likely be attributed to low levels of Sna that are present from a transient source of Twi-independent early transcription, as double mutants have no evidence of cellshape change (Leptin and Grunewald, 1990;Kosman et al, 1991;Leptin, 1991;Sweeton et al, 1991;Ip et al, 1994;Seher et al, 2007). Thus, in twi mutant embryos that do not express downstream targets such as Fog and T48, myosin II is not apically activated, but some shape changes are still observed .…”

Regulation of the Rac GTPase pathway by the multifunctional Rho GEF Pebble is essential for mesoderm migration in theDrosophilagastrula

“…Internalization is controlled by targets of the zygotically active transcription factors Twist (Twi) and Snail (Sna) (Leptin and Roth, 1994;Leptin, 1999;Seher et al, 2007). Cell signalling through the secreted glycoprotein Folded Gastrulation and the transmembrane protein T48 are both implicated in local activation of Rho1 at the apical cell cortex of invaginating mesoderm cells (Costa et al, 1994;Leptin and Roth, 1994;Barrett et al, 1997;Kolsch et al, 2007).…”