DN dominant-negative Cdk cyclin-dependent kinase hr hours IR ionizing radiation MBT midblastula transition pf post-fertilization wt wild-type
Early Xenopus laevis embryos possess cell cycles that do not arrest at checkpoints in response to damaged DNA. At the midblastula transition (MBT), embryos with damaged DNA undergo apoptosis. After the MBT, DNA damage triggers cell cycle arrest rather than apoptosis. The transition from checkpoint-unregulated to checkpoint-regulated cycles makes Xenopus embryos compelling for studying mechanisms regulating response to genomic damage. The DNA damage checkpoint is mediated by the Chk2/Cds1 kinase. Conflicting evidence implicates Chk2 as an inhibitor or promoter of apoptosis. To better understand the developmental function of Chk2, we expressed wild-type (wt) and dominant-negative (DN) Chk2 in Xenopus embryos. Wt-Chk2 created a pre-MBT checkpoint due to degradation of Cdc25A and phosphorylation of cyclin-dependent kinases. Embryos expressing DN-Chk2 developed normally until gastrulation and then underwent apoptosis. Conversely, low doses of wt-Chk2 blocked radiation-induced apoptosis. Therefore, Chk2 operates at a switch between cell cycle arrest or apoptosis in response to genomic assaults. Developmental Dynamics 233:1359–1365, 2005. © 2005 Wiley-Liss, Inc.
Checkpoints that arrest the cell cycle in response to DNA damage are conserved features among eukaryotes. Mutations incapacitating these checkpoints are frequent in cancers (Bartek and Lukas, 2003). Early embryonic cell cycles of Xenopus laevis lack checkpoints (Newport and Dasso, 1989; Anderson et al., 1997; Hensey and Gautier, 1997). Following fertilization, the embryo begins twelve rapid cleavage cycles that alternate between DNA replication and mitosis without cell growth, gap phases, or checkpoints (Newport and Dasso, 1989; Anderson et al., 1997; Hensey and Gautier, 1997). Completion of the twelfth cleavage marks the MBT when transcription initiates, cells become motile, an apoptotic program is functional, and cell cycles lengthen as they acquire the gap phases characteristic of somatic cell cycles (Newport and Kirschner, 1982a; Frederick and Andrews, 1994). Furthermore, checkpoints become operative after the MBT (Newport and Dasso, 1989). The transition from checkpoint-unregulated to checkpoint-regulated cycles makes Xenopus embryos an excellent system for studying the molecular mechanisms that regulate cell cycle arrest and apoptosis in response to damaged DNA.
In eukaryotes, Chk1 and Chk2 kinases are central components of signaling networks activated in response to damaged and unreplicated DNA. Activation of Chk1 or Chk2 can promote cell cycle arrest (via phosphorylation of Cdc25A, Cdc25C, and p53), repair (via phosphorylation of BRCA1), and apoptosis (via phosphorylation of p53, E2F1, and PML) (Novak et al., 2002; Ahn et al., 2004). What determines whether cells respond to DNA damage by arresting the cell cycle or committing apoptosis is not well understood.
We have shown previously that Chk1 is required for Xenopus embryos to survive beyond gastrulation, even in the absence of explicitly damaged DNA (Carter and Sible, 2003). Likewise, Chk1 is essential for early development of mice (Liu et al., 2000; Takai et al., 2000) In contrast, Chk2 is not essential in mice (Takai et al., 2002) but has been implicated as both a promoter and inhibitor of apoptosis (Hirao et al., 2002; Jack et al., 2002; Peters et al., 2002; Takai et al., 2002; Yang et al., 2002; Stevens et al., 2003; Wu and Chen, 2003; Brodsky et al., 2004; Castedo et al., 2004; Hong and Stambrook, 2004; Jack et al., 2004; Rogoff et al., 2004).
With the goal of better understanding the developmental function and regulation of Chk2 in the remodeling cell cycle of the Xenopus embryo, we overexpressed wt- or DN-Chk2 during early development. Our results indicate that Chk2 functions to inhibit developmentally regulated and DNA damage-induced apoptosis, suggesting that both Chk1 and Chk2 kinases operate at developmental switches that determine whether cells arrest or die in response to genomic assaults.
Prior to the MBT, Xenopus embryos do not arrest cell cycles in response to damaged or unreplicated DNA (Newport and Dasso, 1989; Anderson et al., 1997; Hensey and Gautier, 1997) even though Chk1 and Chk2 kinases are expressed (Nakajo et al., 1999; Kappas et al., 2000; Gotoh et al., 2001). Chk1 does not become activated by unreplicated DNA until the MBT or by ionizing radiation (IR) until early gastrulation (Kappas et al., 2000), correlating with the time at which embryos arrest cell cycles in response to unreplicated or damaged DNA. However, overexpression of Chk1 triggers degradation of Cdc25A, tyrosine phosphorylation of Cdks, and cell cycle arrest in pre-MBT embryos (Kappas et al., 2000; Shimuta et al., 2002; Petrus et al., 2004). When embryos are treated at the MBT with agents that block DNA replication, Chk2 likewise is activated (Gotoh et al., 2001).
To determine whether pre-MBT cell cycles respond to exogenous Chk2, mRNA encoding FLAG-tagged wild-type Chk2 (wt-Chk2) or FLAG-tagged luciferase (as a control) was microinjected into one-cell stage embryos. Western analysis with a FLAG antibody confirmed expression of exogenous proteins (Fig. 1A). Embryos expressing exogenous wt-Chk2 displayed a dose-dependent delay of cleavage cycles. Embryos injected with 2 ng wt-Chk2 mRNA exhibited slower cleavage cycles compared to controls. At 4.5 hr post-fertilization, control embryos had reached developmental Stage 6.5, corresponding to 64 cells (Nieuwkoop and Faber, 1975), whereas embryos expressing wt-Chk2 were approximately one cycle delayed (Stage 6 = 32 cells; Fig. 1B). At 8 hr pf, embryos expressing wt-Chk2 completed the MBT and remained delayed by approximately one cell cycle compared to controls (Fig. 1B). These embryos proceeded to gastrulate and survived through neurulation (not shown; see Fig. 3B). Embryos injected with a higher dose of 8–10 ng wt-Chk2 mRNA arrested cell cycles at the 2–4 cell stage, whereas control embryos injected with the same dose of luciferase mRNA divided normally (Fig. 1C).
To determine whether the cell cycle arrest induced by wt-Chk2 resulted from engagement of known checkpoint events, embryos expressing luciferase or wt-Chk2 were assayed for tyrosine phosphorylation of Cdks by Western analysis with a phosphoCdk antibody. In metazoans, Chk2 inactivates Cdc25 phosphatases, resulting in Cdks in their tyrosine-phosphorylated, inactive state (Bartek et al., 2001; McGowan, 2002). In untreated embryos, periodic tyrosine phosphorylation of Cdks occurs but is low until after the MBT (Ferrell et al., 1991; Hartley et al., 1996; Kim et al., 1999). Likewise, the level of tyrosine phosphorylation in control embryos was low until after the MBT (Fig. 1D; 7–9 hr pf). In contrast, Cdks were phosphorylated on tyrosine well before the MBT, by 3 hr pf in embryos expressing exogenous wt-Chk2, indicating that the cell cycle delay resulted from reduced activity of Cdks. Therefore, pre-MBT embryos, which do not arrest cleavage cycles in response to damaged DNA (Anderson et al., 1997; Hensey and Gautier, 1997), can arrest the cell cycle in response to exogenous wt-Chk2. These data fit an emerging model that embryos possess the necessary components of checkpoint signaling pathways, but require threshold amounts of DNA to effectively transduce a signal resulting in cell cycle arrest (Kappas et al., 2000; Conn et al., 2004).
The extent of tyrosine phosphorylation in these embryos expressing wt-Chk2 was approximately the same as in control embryos at 9 hr post-fertilization (Fig. 1D), when cell cycles are asynchronous, averaging >200 min in length (Howe et al., 1995). Since the embryos expressing this dose of wt-Chk2 exhibited only modestly delayed cleavage cycles (Fig. 1B), additional changes at the MBT (such as changes in the pool of cyclin mRNAs (Audic et al., 2002) and the expression of the Cdk inhibitor, Xic1 (Su et al., 1995), are likely to contribute to the longer cell cycles at the MBT.
Another event of the MBT that may contribute to cell cycle lengthening is the degradation of Cdc25A, leading to enhanced tyrosine phosphorylation of Cdks (Kim et al., 1999). To determine whether expression of exogenous wt-Chk2 triggered premature degradation of Cdc25A, embryos were assayed for steady-state levels of Cdc25A (Fig. 1E). As shown in Figure 1D, Cdc25A content was decreased in embryos expressing exogenous wt-Chk2 compared to control embryos. These data provide further evidence that exogenous Chk2 disrupts cleavage cycles by perturbing known cell cycle checkpoint signaling pathways, leading to decreased activity of Cdks.
Although pre-MBT embryos do not engage checkpoints in response to damaged DNA, Chk1 and Chk2 are expressed throughout early development (Nakajo et al., 1999; Kappas et al., 2000; Gotoh et al., 2001). Chk1 is transiently activated at the MBT (Shimuta et al., 2002) and is required for embryonic survival beyond gastrulation (Shimuta et al., 2002; Carter and Sible, 2003). To determine whether Chk2 is required for early development, one-cell stage embryos were microinjected with either 8 ng luciferase or catalytically inactive DN-Chk2 mRNA, and development was observed. Both embryos expressing luciferase or DN-Chk2 developed normally through the MBT. During gastrulation, ∼90% of embryos (52/58) expressing DN-Chk2 appeared abnormal with loss of organization and cell-cell attachment, whereas all embryos expressing luciferase appeared normal. At neurulation, embryos expressing luciferase continued to develop normally, whereas the embryos expressing DN-Chk2 had lost attachments and appeared abnormal (Fig. 2A). The morphology of embryos expressing Chk2 resembled that of embryos that had undergone apoptosis in response to IR (Anderson et al., 1997; Hensey and Gautier, 1997), unreplicated DNA (Carter and Sible, 2003), and expression of DN-Chk1 (Carter and Sible, 2003).
To determine whether DN-Chk2 induced apoptosis, albino embryos expressing luciferase or DN-Chk2 were collected at gastrulation and subjected to whole-mount TUNEL assays, which detect fragmented DNA, characteristic of apoptotic cells (Sible et al., 1997). Approximately 90% (50/55) of embryos expressing DN-Chk2 exhibited a positive TUNEL reaction indicated by punctate, deep purple staining. By comparison, ∼2% (1/52) of embryos expressing luciferase exhibited positive TUNEL staining (Fig. 2B).
As a biochemical assay of apoptosis, embryos expressing luciferase or DN-Chk2 were collected at the times indicated and assayed for caspase activity by the cleavage of PARP protein (Carter and Sible, 2003). Embryos expressing DN-Chk2 were positive for caspase activity beginning at gastrulation (Fig. 2C), indicating that these embryos had undergone apoptosis. Therefore, Xenopus embryos may require Chk2 function in order to survive beyond the MBT.
Previously, Shimuta et al. (2002) observed death in embryos expressing DN-Chk1 but not DN-Chk2. The death of embryos expressing DN-Chk1 corresponded with a delayed increase in tyrosine phosphorylation of Cdks and degradation of Cdc25A and could be mimicked by expressing nondegradable Cdc25A (Shimuta et al., 2002). Because different mutations were generated to create the dominant-negative constructs, the proteins expressed by their group may have functioned differently in the embryo or may have been expressed at lower levels. To test whether our DN-Chk1 and DN-Chk2 created the same effects on cell cycle remodeling at the MBT, we compared the effects of each (Fig. 2D). Our data indicate that both DN-Chk1 and DN-Chk2 delay the increase in tyrosine phosphorylation of Cdks and degradation of Cdc25A, which normally begins at the MBT. Therefore, expression of our DN-Chk2 construct disrupts cell cycle remodeling at the MBT and triggers a program of apoptosis.
Embryos exposed before the MBT to a variety of assaults to the genome, including IR, develop normally through the MBT and then die by apoptosis during early gastrulation (Anderson et al., 1997; Hensey and Gautier, 1997; Sible et al., 1997; Stack and Newport, 1997). When embryos are exposed to these same assaults after the MBT, they are resistant to apoptosis (Anderson et al., 1997; Hensey and Gautier, 1997). To see how susceptibility to apoptosis correlated with activation of Chk2, embryos were exposed to IR before (3 hr pf) or after (14 hr pf) the MBT, then collected at several times points and analyzed by Western blotting for activation of Chk2, as determined by a shift in electrophoretic mobility, indicative of the activating phosphorylation. When exposed to 30 or 60 Gy IR before the MBT, no shift in Chk2 mobility was observed after 1 or 4 hr, suggesting that little or no Chk2 was activated in these embryos, which ultimately died by apoptosis during early gastrulation (Fig. 3A). In contrast, when embryos were irradiated after the MBT, a fraction of the Chk2 was shifted 1 or 2 hr later. The amount of shifted Chk2 was greater in embryos treated with 60 Gy versus 30 Gy IR, indicating a dose-dependent effect (Fig. 3A). These experiments indicate an inverse correlation between activation of Chk2 and susceptibility to apoptosis in embryos.
Based on these results as well as our observation that DN-Chk2 triggers apoptosis during gastrulation of Xenopus embryos (Fig. 2), we hypothesized that wt-Chk2 could play a dominant role in blocking apoptosis. To test this hypothesis, one-cell stage embryos were microinjected with 2 ng luciferase or wt-Chk2 mRNA, then exposed to 30 Gy IR before the MBT, at 3.5 hr pf. All embryos (20/20) expressing luciferase exhibited abnormal morphology, consistent with apoptosis, at gastrulation (Fig. 3B). In contrast, only 10% of embryos (2/20) expressing exogenous wt-Chk2 exhibited abnormal morphology at gastrulation, and 25–30% completed gastrulation and formed a neural plate (Fig. 3B). Therefore, irradiated embryos expressing exogenous Chk2 developed normally, well beyond the developmental stage at which irradiated embryos usually, die by apoptosis. These data indicate that Chk2 inhibits IR-induced apoptosis in the early Xenopus embryo.
We provide evidence that Chk2 inhibits apoptosis during early development of Xenopus laevis. However, other studies have indicated that Chk2 promotes apoptosis (Ahn et al., 2004). Mice lacking the Chk2 gene are viable, but deficient in IR-induced apoptosis (Hirao et al., 2002; Jack et al., 2002, 2004; Takai et al., 2002). In the Drosophila eye, Chk2 promotes the induction of apoptosis in response to DNA damage (Peters et al., 2002). In both cases, Chk2 promotes apoptosis via phosphorylation of p53. In human breast carcinoma cells, Chk2 promotes DNA damage-induced apoptosis by phosphorylating the transcription factor E2F-1 (Stevens et al., 2003), and in lymphoma cells by phosphorylating the scaffolding protein PML (Yang et al., 2002). Thus, in response to DNA damage, Chk2 can phosphorylate at least three targets to promote apoptosis (Ahn et al., 2004).
In contrast, other studies have implicated Chk2 as an inhibitor of apoptosis. Mouse embryonic stem cells are deficient in a G1 checkpoint and highly susceptible to IR-induced apoptosis. Ectopic expression of Chk2 in these cells confers protection from IR-induced apoptosis. This protection may be p53-independent, since the p53 target, p21, is not induced (Hong and Stambrook, 2004). Expression of DN-Chk2 in human heterokarya and HCT116 colon carcinoma cells promotes apoptosis in response to mitotic catastrophe (Castedo et al., 2004). Likewise, we show both that ectopic wt-Chk2 blocked IR-induced apoptosis (Fig. 3) and DN-Chk2 promoted apoptosis (Fig. 2) in the early Xenopus embryo, implicating Chk2 as an inhibitor of apoptosis.
Our studies may shed light upon the apparent paradox regarding the relationship between Chk2 and apoptosis. Early embryos contain p53, but development is not noticeably affected by expression of either wt- or DN-p53 until well past gastrulation (Wallingford et al., 1997). Furthermore, p53 cannot function as a transcription factor before the MBT because transcription is silent (Newport and Kirschner, 1982b). Therefore, early embryos provide a context in which the p53 signaling pathway is not fully functional, and may explain why Chk2 does not promote apoptosis. On the other hand, we have shown that Chk2 and Chk1 are capable of activating cell cycle checkpoints in the early embryos by triggering tyrosine phosphorylation of cyclin-dependent kinases (Fig. 1; Kappas et al., 2000). By causing cell cycle delay in response to IR, Chk2 may allow for repair and may also promote repair pathways through its interaction with proteins such as BRCA1 and Mus81 (McGowan, 2002). Since many tumors lack functional p53, it will be important to determine whether Chk2 truly blocks apoptosis in a p53-negative background, in order to decide when drugs that target Chk2 are appropriate chemotherapeutics.
Interestingly, induction of apoptosis by DN-Chk2 did not require exposure to IR or any other overt assault to the embryo. At this stage, we do not know why Xenopus embryos require Chk2 and Chk1 (Shimuta et al., 2002; Carter and Sible, 2003) during early development, but we suggest two possibilities: (1) inhibition of Chk2 during the 12 rapid cycles, which lack checkpoints before the MBT inevitably results in DNA damage that triggers apoptosis post-MBT, or (2) the lengthening of cell cycles at the MBT requires functional Chk2 and/or Chk1 (Shimuta et al., 2002; Carter and Sible, 2003). As we explore these possibilities in the Xenopus embryo, we will build a better understanding of how checkpoint controls are coordinated with other aspects of metazoan development.
Eggs from wild-type and albino Xenopus laevis (Xenopus Express and Nasco) were fertilized and maintained as described (Kappas et al., 2000). Embryos were staged according to Nieuwkoop and Faber (1975). In some experiments, embryos were microinjected at the one-cell stage with indicated concentrations of wt-Chk2, DN-Chk2, or luciferase mRNA dissolved in 25–30 nL TE buffer (10 mM Tris, pH 8.0, 1 mM EDTA). At the indicated stages, some embryos were subjected to IR emitted from a TFI Mini Shot X-ray machine. Embryos were visualized with an Olympus SZX12 stereo microscope and photographed with an Olympus DP10 digital camera.
Plasmid constructions encoding FLAG-tagged, wild-type (wt), and dominant-negative (DN) Xenopus Chk2 were generated by subcloning cDNAs donated by Dr. William G. Dunphy, into the pSP64polyA vector (Promega, Madison, WI). In DN-Chk2, asparagine 324 was mutationally altered to alanine (Guo and Dunphy, 2000). The plasmids were linearized and used as templates to produce polyadenylated mRNA encoding FLAG-tagged wt-Chk2 and DN-Chk2 using the Ambion SP6 mMessage mMachine in vitro transcription kit. Luciferase mRNA was produced as described (Kappas et al., 2000).
Embryos were lysed in EB buffer (Kappas et al., 2000). Samples were then resolved by SDS-PAGE and Western blotting for FLAG, PARP, and phospho-Cdk was performed as described previously (Kappas et al., 2000; Carter and Sible, 2003). Western blotting of Chk2 was performed with a polyclonal rabbit antibody generated against recombinant Xenopus Chk2/Cds1, provided by Dr. Paul Mueller (McSherry and Mueller, 2004), and Western blotting of Cdc25A was performed with a polyclonal rabbit antibody provided by Dr. James Maller (Kim et al., 1999).
Whole-mount TUNEL assays and caspase assays were performed as described (Carter and Sible, 2003).
This research was supported by a grant from the National Institutes of Health (R01 GM59688) to J.C.S. We thank Paul Mueller and Troy McSherry for the Chk2 antibodies and for very helpful comments on this manuscript. We thank William Dunphy for the Chk2 cDNA and James Maller for the Cdc25A antibodies.