Flying to a halt: Drosophila Aven arrests the cell cycle.

DNA damage can trigger cell cycle arrest and repair but can also trigger cell death. The integration of signals that decide cell fate in this scenario remains unclear. One molecular candidate linking life and death is human Aven. Human Aven was initially identified as a binding partner of the anti-apoptotic Bcl-2 family protein Bcl-xL and can cooperate with Bcl-2-related proteins to promote cell survival.1,2 Aven binds separately to Apaf1, which oligomerizes during apoptosis to form the caspase-activating apoptosome.1 Although the Bcl-2 homolog in worms (CED-9) binds directly to the worm ortholog of Apaf1 (CED-4), this does not occur in mammals.3 Therefore, Aven, which is apparently absent in C. elegans, is the only direct molecular link between mammalian Bcl-2 proteins and the caspase activation machinery (Fig. 1). The first clue that Aven was also involved in regulating the cell cycle was uncovered when Chau and colleagues4 performed a yeast two-hybrid screen that identified the kinase domain of ATM (ataxia telangiectasia mutated) as a binding partner of Aven. The breakthrough that linked the functions of Aven and ATM came from studying Aven in Xenopus egg extracts. This versatile in vitro model system revealed that human Aven regulates cell cycle progression at G2/M.4 In fact, Aven is critical for ATM activation of the G2/M checkpoint in response to DNA damage, indicating that Aven is a novel upstream regulator of ATM. Mapping a functional NES (nuclear export sequence) in human Aven further supports a nuclear role for Aven despite its predominant cytoplasmic localization.1,5 Thus, Aven appears to have yet another role as a molecular link, this time between DNA damage and the cell cycle (Fig. 1). The possibility that Aven also links cell cycle checkpoints to cell survival is supported by a recent report showing that human Aven stabilizes the Bcl-xL protein, and that the interaction between Aven with Bcl-xL decreases with UV-induced DNA damage.6 Figure 1 Human (h) versus Drosophila (d) Aven. Irusta and colleagues7 have recently identified the Drosophila ortholog of human Aven. Although this single-copy gene shares only about 15–20% overall sequence identity between flies and mammals, the evidence indicates that dAven may share both the cell cycle and pro-survival functions of human Aven.7 RNAi knockdown of dAven results in lethality, consistent with widespread expression of dAven in early embryos. Thus, Aven appears to be essential in both mammals and Drosophila. Furthermore, by knocking down dAven in adult fly tissues, the authors identified a cell cycle phenotype where dAvendeficiency causes enlargement of the fat body.7 This may reflect a defect in G2/M arrest because the enlarged fat body appears to be populated with small cells (increased density of nuclei) that eventually undergo mitotic arrest. It is intriguing that dAven is prominently expressed in embryonic tissues (e.g., salivary gland and malpighian tubules) and in adult tissues (e.g., ovary) that undergo endocycling, where DNA replication occurs without cell division.8 Interestingly, ATM-dependent apoptosis, which results from DNA damage that occurs during inappropriate re-replication, is suppressed in endocycling cells.9 These tantalizing correlations raise the possibility that Aven could be involved in checkpoint control and in cell fate responses to re-replication versus endocycling. There are also some perplexing differences between human and dAven. Irusta et al.7 did not detect an interaction between dAven and the Drosophila orthologs of Apaf1 (Dark) or Bcl-2 family members (Buffy and Debcl). It is also not known whether dAven responds to DNA damage or if dAven is phosphorylated on S/TQ sites by Drosophila ATM/R, analogous to human Aven.4 While human Aven contains four SQ sites, at least two of which are phosphorylated by ATM, these sites are not conserved in dAven. However, a survey reveals that most all sequenced species of Drosophila have one or both conserved TQ sites in Aven. The Drosophila salivary gland is already an important model system for studying cell death, which occurs by both apoptosis and autophagy and requires growth arrest to do so.10 The discovery of the Drosophila ortholog for human Aven could greatly spur the sluggish pace towards understanding its critical functions.