Genes Affecting Cell Competition in Drosophila

Abstract

ADULT Drosophila grow to a consistent size and proportion. One way in which tissue size is regulated is through the “cell competition” that can occur when growth is perturbed. In the imaginal discs, which give rise to much of the external tissues of the adult fly, cell competition coordinates growth and apoptosis and is required for consistent size regulation (de la Covaet al. 2004).

Cell competition was first described by Morata and Ripoll (1975) while studying the growth parameters of Minute mutations (M). Many Minutes are now known to correspond to mutations in ribosomal protein genes (Lambertsson 1998). Homozygosity for M mutations is cell lethal. Heterozygous M/+ cells have a reduced rate of cell division (Morata and Ripoll 1975), and while M/+ flies grow to a size and shape similar to wild-type flies, they take longer to do so (Lindsley and Zimm 1992).

In mosaic compartments containing both M/+ and wild-type cells, the M/+ cells are disproportionately eliminated from the developing tissue and may not contribute to the adult animal, even though a wholly M/+ animal would be viable (Morata and Ripoll 1975). At the same time, growth of the wild-type cells is correspondingly enhanced, sometimes leading the entire compartment to be constructed from just these cells (Simpson 1979; Simpson and Morata 1981). These reciprocal growth effects in mosaic compartments define cell competition and indicate that the growth rates of cells are moderated in response to that of their neighbors. Cell competition has also been described in mesodermal compartments as well as in imaginal discs, and between cells differing in myc gene expression as well as in ribosome complement (Lawrence 1982; de la Covaet al. 2004; Moreno and Basler 2004). Recent evidence suggests that cell competition occurs during repopulation of rat liver (Oertelet al. 2006).

It has been proposed that, in the Drosophila wing primordium, cells compete for the extracellular signaling molecule Dpp (Morenoet al. 2002). Dpp signaling is proposed to repress the expression of the transcription factor Brinker and thereby prevent Jun N-terminal kinase (JNK)-mediated apoptosis. This model is based on the findings that cell competition correlates with and can be corrected by Dpp signaling (Morenoet al. 2002; Moreno and Basler 2004). When M/+ cells are introduced into wing discs by mitotic recombination, these cells and their descendants exhibit reduced Dpp signaling, elevate JNK activity, and are lost by apoptosis. Such competed M/+ clones can be protected by elevated Dpp signaling [achieved by mutating brinker (brk)] or reduced JNK signaling (achieved by mutating the JunKK hemipterous) (Morenoet al. 2002). Similarly, in the case of competition between cells with differing doses of myc gene expression, overexpression of activated Dpp receptors can rescue the cells with a lower myc dose (Moreno and Basler 2004).

This model was based on studies of the X-linked genes brk and hemipterous, exploiting a translocation T(1,2)sc ^{to obtain circumstances where FLP-mediated mitotic recombination of the X chromosome uncovered heterozygosity for the second chromosome} M(2)60E locus in one class of somatically recombinant cells (Morenoet al. 2002). Because it focused on candidate genes located on the X chromosome, it is uncertain how many other genes may be required for cell competition and whether novel pathways might also be involved. In addition, a complete version of the model would explain how reduced translational capacity interferes with competition for Dpp, how reduced Dpp signaling activates JNK, and how JNK activity promotes cell death. Furthermore, Dpp availability is expected to differ even among wild-type cells, depending on their distance from the Dpp source, so the survival response to Dpp signaling must be calibrated in some way to explain why differing Dpp levels do not induce cell death during normal development. Thus it is likely that other genes and pathways that have not yet been identified by the candidate gene approach are involved in cell competition.

Another interesting observation is that stimulating cell growth by overexpression of Myc turns such cells into “supercompetitors” that can eliminate nearby wild-type cells. Other methods of activating cellular growth, such as overexpression of the phosophoinositide 3-kinase Dp110 or of cyclin D/Cdk4, do not cause supercompetition (de la Covaet al. 2004; Moreno and Basler 2004). It remains to be determined how many types of growth perturbation induce cell competition and what distinguishes them from growth pathways that do not affect competition.

Both to test the existing model and to identify other genes and pathways involved in cell competition, we performed a genetic screen for autosomal mutations that protect M/+ cells from cell competition. One would predict that mutations in autosomally located, negative regulators of the Dpp pathway, or in positive components of the JNK pathway, would protect M/+ cells from cell competition. The results indicate a complex relationship among cell competition, Dpp signaling, and JNK activity. In addition, we identify a hyperplastic tumor-suppressor pathway and novel cell death genes that are related to cell competition. Not all hyperplastic mutations rescued M/+ clones, confirming that cell competition reflects specific growth perturbations and identifying some of the components.

Mutations in the following genes were also tested and found not to rescue M/+ cells in the adult eye: msn, jun, RhoA, bsk, lgl, lgd, scrib, ds, fz, gug, fj.

Acknowledgments