A Genetic Link between Morphogenesis and Cell Division during Formation of the Ventral Furrow in Drosophila

Abstract

Stages in development with rapid transitions between mitosis and morphogenesis may require specific mechanisms to coordinate cell shape change. Here we describe a novel mitotic inhibitor that acts during Drosophila gastrulation to counteract String/Cdc25, specifically in the cells that invaginate to form the mesoderm. We have identified two genes, frühstart and tribbles, that are required for this ventral inhibition. tribbles encodes a kinase-related protein whose RNA, however, is also present outside of the ventral region. Effective inhibition of mitosis in the cells of the ventral furrow depends on the transcription factor Snail that triggers the ventral cell shape changes. When overexpressed in a microinjection assay, Tribbles directly inhibits mitosis. We propose that Frühstart and Tribbles form a link between the morphogenetic movements and mitotic control.

Developmental roles of tribbles protein family members

“…Some data indicate that the Trib domain does not contribute to function: (1) mutations in the Trib domain of fly Tribbles did not affect its ability to block cell proliferation in the embryo (Grosshans and Wieschaus, 2000), (2) Trib3 binding to CHOP was unaffected by a K to R catalytic site mutant (Ohoka et al, 2005), and (3) the Trib2 K/R mutation had no effect on the protein's pro-apoptotic functions (Lin et al, 2007).…”

“…While published work on mouse knockouts of individual isoforms reveal no notable developmental defects, reduction of Xenopus Xtrb2 does; thus, it is likely that double and triple Trib mouse knockouts will reveal informative phenotypes. Fly trbl mutant alleles exhibit variable defects in mesoderm internalization yet result in weakly fertile escaper adults (Grosshans and Wieschaus, 2000) leading to the conclusion that trbl has a nonessential role to confer precision on a rapidly unfolding developmental program (reviewed in Johnston, 2000;Leptin, 2005). In support of this notion, Trbl turnover of String protein during mesoderm invagination is augmented by activity of the Held-out wing (How(l)) protein, which binds and degrades string RNA (Nabel-Rosen et al, 2005).…”

“…In flies, a mutation in the Trib domain retains wild-type activity to block cell division, which would indicate this domain does not catalyze phosphate transfer. However, this mutation made was in lysine K268 of the proposed catalytic loop (Grosshans and Wieschaus, 2000) rather than the highly conserved K266, where catalytic null mutations are typically made. When the corresponding mutation was made in the catalytic loop of the Trib domain of Trib3, its function was abrogated (Naiki et al, 2007).…”

“…Two similarly designed deficiency screens identified lesions in the trbl gene that caused a failure of gastrulation (Fig. 1A,B) due to overproliferation of mesodermal cells (Grosshans and Wieschaus, 2000;Seher and Leptin, 2000). During normal gastrulation, the ventral mesoderm arrests at G2 of the cell cycle and then undergoes ingression and dorsal migration to pattern muscle and other tissue derivatives.…”

“…First, double mutants for trbl and string invaginate normally, suggesting that the main role of Trbl at this stage is to block String phosphatase activity (Grosshans and Wieschaus, 2000). And second, while injection of wild-type Trbl is sufficient to block excess cell divisions, injection into snail mutant embryos does not, suggesting that snail activates expression of a Trbl co-activator, whose identity and mechanism of activity remains undetermined (Grosshans and Wieschaus, 2000). Notably, during mouse gastrulation cell proliferation is regulated by Snail, and knockdown of Trib3 in tumor cells leads to down-regulation of Twist and Snail, increased E-cadherin and inhibition of migration (Hua et al, 2011).…”