Genetic Modifier Screens on Hairless Gain-of-Function Phenotypes Reveal Genes Involved in Cell Differentiation, Cell Growth and Apoptosis in Drosophila Melanogaster
Müller, Dominik, Kugler, Sabrina J., Preiss, Anette, Maier, Dieter, Nagel, Anja C., Genetics
Overexpression of Hairless (H) causes a remarkable degree of tissue loss and apoptosis during imaginal development. H functions as antagonist in the Notch-signaling pathway in Drosophila, and the link to growth and apoptosis is poorly understood. To further our insight into II-mediated apoptosis, we performed two large-scale screens for modifiers of a small rough eye phenotype caused by H overexpression. Both loss- and gain-of-function screens revealed known and new genetic interactors representing diverse cellular functions. Many of them did not cause eye phenotypes on their own, emphasizing a specific genetic interaction with H. As expected, we also identified components of different signaling pathways supposed to be involved in the regulation of cell growth and cell death. Accordingly, some of them also acted as modifiers of proapoptotic genes, suggesting a more general involvement in the regulation of apoptosis. Overall, these screens highlight the importance of H and the Notch pathway in mediating cell death in response to developmental and environmental cues and emphasize their role in maintaining developmental cellular homeostasis.
APOPTOSIS or programmed cell death is crucial to the correct development of all multicellular organisms. Moreover, dysfunction of apoptosis has been linked to pathologies such as cancer and neurodegeneration (THOMPSON 1995; VAUX and KORSMEYER 1999). The core mediators of apoptosis are members of the caspase family of cysteine proteases, triggering, when activated, the distinct cellular changes observed in dying cells (HENGARTNER 2000). Inhibitor of apoptosis proteins (IAPs) directly inhibit caspase activity and promote their ubiquitination and subsequent degradation (PALAGA and OSBORNE 2002). A delicate balance between factors that activate and those that inhibit caspase activity determines cell death or survival. In Drosophila, induction of apoptosis relies on the function of at least one of the closely linked proapoptotic genes reaper (rpr), head involution defective (hid), and grim (BANGS and WHITE 2000; VERNOOY et al. 2000). Recently it was shown that the proapoptotic activity of the encoded proteins is caused by their ability to target IAPs for ubiquitin-mediated degradation as well as by a generalized inhibition of translation (reviewed in PALAGA and OSBORNE 2002). Although rpr, hid, and grim may induce apoptosis through similar mechanisms, they are differentially expressed and therefore not redundant. For example, hid and rpr but not grim are expressed in nonneural tissues doomed to die during metamorphosis (JiANG et al. 1997). In contrast, rpr and grim eliminate supernumerary cells in the central nervous system, whereas hid acts on midline glia of the embryo (GRETHER et al. 1995; CHEN et al. 1996; ROBINOW et al. 1997). These differences can be explained by a different regulation of these genes. It has been shown that rpr is a target of the tumor suppressor p53 and the ecdysone receptor signaling pathway (BRODSKY et al. 2000; JIANG et al. 2000; OLLMANN et al. 2000). Two independent studies revealed that MAPK signaling negatively regulates the function and expression of hid (BERGMANN et al. 1998; KURADA and WHITE 1998). These observations indicate that genes that regulate developmental decisions, such as members of the EGFR or ecdysone receptor pathways, also influence the apoptotic machinery (reviewed in BANGS and WHITE 2000). Therefore it was not unexpected that the Notch-signaling pathway regulates apoptosis as well. The Notch pathway is highly conserved and promotes cell fate decisions through local cell-cell interactions (reviewed in ARTAVANIS-TSAKONAS et al. 1999; SCHWEISGUTH 2004). Originally it was associated with lateral inhibition processes, e.g., during embryonic central nervous system development. Later it was shown to regulate a vast array of patterning processes and cell fate decisions (reviewed in ARTAVANIS-TSAKONAS et al. 1999). More recently, the Notch-signaling pathway has been implicated in growth control in the developing eye and wing of Drosophila as an activation of Notch is linked to tissue overgrowth and the development of cancer (Go et al. …