The cleaved-Caspase-3 antibody is a marker of Caspase-9-like DRONC activity in Drosophila

doi:10.1038/cdd.2009.185

Cell Death Differ. Author manuscript; available in PMC 2010 September 1.

Published in final edited form as:

Cell Death Differ. 2010 March; 17(3): 534–539.

Published online 2009 December 4. doi: 10.1038/cdd.2009.185

PMCID: PMC2822068

NIHMSID: NIHMS157068

The cleaved-Caspase-3 antibody is a marker of Caspase-9-like DRONC activity in Drosophila

Yun Fan and Andreas Bergmann^*

The University of Texas MD Anderson Cancer Center Department of Biochemistry & Molecular Biology Graduate Program in Genes & Development 1515 Holcombe Boulevard – Unit 1000 Houston, TX 77030 USA

^*Author for correspondence +1 713 834 6294 (phone) +1 713 834 6291 (fax) Email: abergman/at/mdanderson.org

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Abstract

The cleaved-Caspase-3 antibody is a popular tool in apoptosis research in Drosophila. Because the antibody was raised against cleaved human Caspase-3, it was assumed that it detects cleaved DRICE and DCP-1, Caspase-3-like effector caspases in Drosophila. However, as shown here strong immunoreactivity persists in apoptotic models doubly mutant for drICE and dcp-1. In contrast, mutants of the apoptosome components DRONC (Caspase-9-like) and ARK (Apaf-1 related) do not label with cleaved-Caspase-3 antibody. By peptide blocking experiments and further genetic studies, we provide evidence that the cleaved-Caspase-3 antibody recognizes multiple proteins including DCP-1 and likely DRICE, but also at least one additional unknown protein, all of which require DRONC for epitope exposure. The unknown substrate may be involved in non-apoptotic functions of DRONC. Because the cleaved-Caspase-3 antibody not only detects cleaved Caspase-3-like proteins in Drosophila, but also other proteins in a DRONC-dependent manner, it is more accurate to consider the cleaved-Caspase-3 antibody as a marker for DRONC activity, rather than effector caspase activity.

Introduction

The cleaved-Caspase-3 (Asp175) Antibody (referred to as cleaved-Caspase-3 antibody) from Cell Signaling Technology is a polyclonal antibody obtained in Rabbit that was raised against a peptide in the large subunit of the human effector caspase, Caspase-3, amino-terminal to Asp175.¹ The antibody does not detect unprocessed Caspase-3. However, after proteolytic cleavage between Asp175 and Ser176, separating the large and small subunits leading to activation of Caspase-3, the epitope is exposed and can be detected by cleaved-Caspase-3 antibody, making the antibody a marker for cleaved and thus active Caspase-3 in dying cells. A similar antibody, termed CM1, has previously been described;²^,³ however, the CM1 antibody is no longer available and is not subject of this analysis.

Apoptosis in Drosophila is under control of the pro-apoptotic genes reaper, hid and grim.⁴^-⁶ (reviewed in ⁷) The products of these genes trigger apoptosis through induction of proteolytic degradation of inhibitor of apoptosis proteins (IAPs), most notably DIAP1,⁸^-¹¹ to activate a caspase program. Of the seven caspase genes in Drosophila,⁷ only the putative initiator caspase DRONC which is most similar to mammalian Caspase-9, and the Caspase-3-like effector caspases DRICE and DCP-1 have been implicated in developmental apoptosis.¹²^-¹⁸ After release from DIAP1 inhibition, DRONC becomes part of the apoptosome through interaction with ARK, also known as HAC-1 and D-APAF-1, the APAF-1-related gene in Drosophila.¹⁹^-²¹ The apoptosome cleaves and activates the effector caspases DCP-1 and DRICE.

The cleaved-Caspase-3 antibody has become a very popular tool for detection of dying cells in Drosophila (see for example references ³^,¹³^,²²^-²⁷). Because it has been raised against an epitope of human Caspase-3, it was proposed that the antibody would cross-react with cleaved Caspase-3-like effector caspases DRICE and DCP-1 in Drosophila.³ However, this has never been rigorously tested. Preciously, it was shown that the antibody does not loose its immunoreactivity in drICE single mutants¹⁵ suggesting that it is not specific for cleaved DRICE. However, partial redundancy with DCP-1 (ref. ¹³) may account for this result. Here, we show that the cleaved-Caspase-3 antibody still labels cells induced to die which are double mutant for defined null alleles of dcp-1 and drICE. In contrast, the antibody requires the activity of the apoptosome components DRONC and ARK for immunoreactivity. Subsequent analysis demonstrates that the cleaved-Caspase-3 antibody detects at least one additional putative DRONC substrate which may be involved in non-apoptotic processes. Because the cleaved-Caspase-3 antibody is not entirely specific for cleaved DRICE and DCP-1, but requires DRONC for its immunoreactivity, we propose that it is more accurate to refer to this antibody as a marker of DRONC activity in Drosophila.

Results

Labeling by the cleaved-Caspase-3 antibody persists in dcp-1 drICE double mutants

To evaluate the specificity of the cleaved-Caspase-3 antibody, we analyzed eye imaginal discs from third instar larvae. In wild-type eye imaginal discs, the antibody detects a few dying cells scattered throughout the disc (Figure 1a). Surprisingly, in eye discs doubly mutant for the null alleles dcp-1^Prev and drICE^Δ1 (ref. ¹⁴^,¹⁵), the cleaved-Caspase-3 antibody still labels cells (Figure 1b). Labeling in dcp-1^Prev drICE^Δ1 double mutants occurs in clusters (Figure 1b), similar to what has been observed previously when cell death was blocked by expression of the Caspase-3 inhibitor P35.³ These observations would suggest that the cleaved-Caspase-3 antibody still detects an epitope in the absence of the Caspase-3-like proteins DCP-1 and DRICE. Nevertheless, because apoptosis at this stage does not occur in a defined pattern, we were uncertain about the specificity of these labeling signals.

Figure 1

The cleaved-Caspase-3 antibody is a marker for DRONC activity

Therefore, we used GMR-hid transgenes, a well characterized apoptotic model,⁵ to further examine the specificity of the cleaved-Caspase-3 antibody. Through GMR-driven expression of the pro-apoptotic gene hid specifically in the posterior half of the developing eye, GMR-hid transgenes induce apoptosis in two distinct waves as shown by cleaved-Caspase-3 antibody and TUNEL labeling²⁸ (Figure 1c,d). To evaluate the specificity of the cleaved-Caspase-3 antibody, we examined GMR-hid eye discs which were doubly mutant for dcp-1^Prev and drICE^Δ1. Consistent with the expectation, loss of dcp-1^Prev and drICE^Δ1 completely abrogates TUNEL-positive apoptosis in GMR-hid discs (Figure 1f). Surprisingly, however, dcp-1^Prev drICE^Δ1 double mutant GMR-hid eye discs still showed strong immunoreactivity with cleaved-Caspase-3 antibody (Figure 1e). Thus, the cleaved-Caspase-3 antibody does not or not only detect DRICE and/or DCP-1. We do note, though, that the labeling appearance of the cleaved-Caspase-3 antibody changes in the absence of DCP-1 and DRICE (compare Figure 1c and 1e). The labeling signal is no longer restricted to two distinct waves (Figure 1c), but rather fills the entire posterior compartment of the eye disc and is confined to interommatidial cells (Figure 1e). A similar change of labeling pattern has been reported for CM1 antibody labeling upon expression of the caspase inhibitor P35.³ This change of the labeling pattern is likely due to the fact that cells in dcp-1^Prev drICE^Δ1 double mutant GMR-hid eye discs do not die (Figure 1f) and thus maintain the epitope detected by cleaved-Caspase-3 antibody. Importantly, however, this analysis demonstrates that the cleaved-Caspase-3 antibody still detects an epitope in the absence of the Caspase-3-like proteins DCP-1 and DRICE.

Immunoreactivity of the cleaved-Caspase-3 antibody depends on the apoptosome components DRONC and ARK

There are two possibilities why the cleaved-Caspase-3 antibody labels dcp-1 drICE double mutant cells, although they are not apoptotic. First, the antibody may not detect an apoptotic epitope; or second, the antibody may detect an apoptotic epitope generated upstream or in parallel to DCP-1 and DRICE. To distinguish between these possibilities, we examined GMR-hid eye discs mutant for the apoptosome components DRONC and ARK, which act upstream of DRICE and DCP-1. In dronc and ark mutant GMR-hid eye discs, both TUNEL and cleaved-Caspase-3 antibody labelings are blocked (Figure 1g,h; Figure 3a,b). These data confirm that the cleaved-Caspase-3 antibody indeed detects an apoptotic epitope in Drosophila. Furthermore, because it fails to detect the apoptotic epitope in dronc and ark mutants, but not in dcp-1 drice double mutants, it is more accurate to consider the cleaved-Caspase-3 antibody as a marker for DRONC activity, rather than effector caspase activity, in dying Drosophila cells.

Figure 3

Expression of a ΔN-dcp-1 transgene in ark mutant clones partially restores cleaved-Caspase-3 immunoreactivity

The tripeptide ETD is the apoptotic epitope detected by cleaved-Caspase-3 antibody

The epitope detected by the cleaved-Caspase-3 antibody depends on DRONC activity. It may be possible that the antibody directly recognizes activated DRONC. Alternatively, it is also possible that the cleaved-Caspase-3 antibody detects an epitope generated through cleavage of an unknown substrate by active DRONC, independently of DRICE and DCP-1.

To distinguish between these two possibilities, we aligned the residues from the catalytic Cysteine (Cys163) to the cleavage site at Asp175 of human Caspase-3 (Caspase-3 peptide) with the corresponding regions of the Drosophila caspases (Figure 2a; see also ref. ³). The most C-terminal residues of the Caspase-3 peptide, ETD, are conserved in DRICE and DCP-1 (Figure 2a). Similar to Caspase-3, this is the cleavage site for activation of at least DRICE,²⁹ and possibly DCP-1. Interestingly, the N-terminal two-third of the Caspase-3 peptide bears highest similarity to DRONC: six out of nine residues are conserved (Figure 2a). This part of the Caspase-3 peptide is less well conserved in DRICE, DCP-1 and the remaining Drosophila caspases. Although cleavage between the large and small subunits of DRONC is not necessary for its activity,³⁰^,³¹ and may not occur in vivo, we considered the possibility that antibodies directed against the N-terminal part of the Caspase-3 peptide may directly detect active DRONC in dying cells.

Figure 2

A peptide containing ETD blocks cleaved-Caspase-3 immunoreactivity

We used blocking peptides to evaluate which epitopes of the Caspase-3 peptide are detected by the cleaved-Caspase-3 antibody in dying Drosophila cells. The sequences of the blocking peptides are shown in Figure 2b and underlined in Figure 2a. Blocking peptide A (TETD) is derived from DRICE and DCP-1 and blocking peptide B (CRGDEYDLG) corresponds to the region of highest similarity in DRONC (Figure 2a,b). Peptide C is a control peptide corresponding to the residues immediately adjacent to peptide B in DRONC (Figure 2a,b). Peptide D is another control peptide which is very similar to peptide A and is derived from the prodomain of DRONC at position 113. If the prodomain of DRONC is cleaved at this site, it will expose ESD at its C-terminus which is very similar to the C-terminus of the Caspase-3 peptide (ETD, peptide A) and thus may be detected by the cleaved-Caspase-3 antibody.

The blocking peptides were mixed with the cleaved-Caspase-3 antibody sixty minutes before incubation with the eye imaginal discs. The results of the blocking experiments are summarized in Figure 2b, and for peptides A and B shown in Figure 2c-f. Peptide A is sufficient to block the entire immunoreactivity of the cleaved-Caspase-3 antibody in GMR-hid and in dcp-1 drICE double mutant GMR-hid eye discs (Figure 2c,d). In contrast, peptide B does not abrogate immunoreactivity of the antibody in these discs (Figure 2e,f). The control peptides C and D also fail to block cleaved-Caspase-3 immunoreactivity (Figure 2b; data not shown).

These data demonstrate that the cleaved-Caspase-3 antibody specifically detects the epitope ETD in apoptotic cells. Among the Drosophila caspases, this epitope is only present in DRICE and DCP-1, making it very likely that the antibody does indeed detect these effector caspases. In contrast, the fact that the DRONC-derived peptides B, C and D fail to block immunoreactivity suggest that it is unlikely that the antibody directly detects active DRONC. Therefore, because the cleaved-Caspase-3 antibody does not lose immunoreactivity in dcp-1 drICE double mutants (Figure 1e), it detects at least one other protein which exposes the ETD epitope in a DRONC-dependent manner.

Activation of DCP-1 independently of ARK restores cleaved-Caspase-3 immunoreactivity

The analysis presented in Figure 2 suggests, but does not prove, that the cleaved-Caspase-3 antibody does indeed detect cleaved DCP-1 and DRICE. To directly test this possibility, we expressed a GMR-ΔN-dcp-1 transgene in ark mutant background. ΔN-dcp-1 lacks the N-terminal prodomain of DCP-1. It is thought that prodomain-depleted ΔN-DCP-1 readily promotes autoprocessing, and consistently expression under GMR control induces an eye ablation phenotype.³² This eye ablation phenotype is caused by induction of apoptosis (Figure 3c,d). As mentioned above, ark mutant clones in GMR-hid background fail to induce TUNEL-positive apoptosis (Figure 3b’) and the cleaved-Caspase-3 antibody does not have any immunoreactivity in ark clones (Figure 3a’) suggesting that neither DCP-1 nor DRICE nor the unknown DRONC substrates are cleaved in ark mutant clones. Therefore, we tested whether expression of ΔN-Dcp-1 in the absence of apoptosome activity, i.e. in ark clones, can restore cleaved-Caspase-3 antibody labeling. Compared to wild-type tissue (marked by GFP in Figure 3c,c’), the number of cleaved-Caspase-3 positive cells is strongly reduced in ark mutant clones (Figure 3c,c’) suggesting that activation of ΔN-DCP-1 is at least partially dependent on the apoptosome. However, about 50% of ark mutant clones (n=32) in GMR-ΔN-Dcp-1 background contain cleaved-Caspase-3 immunoreactive cells (arrows in Figure 3c, c’). This is unlikely to be a labeling artifact, because these cells also are TUNEL-positive (Figure 3d, d’). Therefore, because DRONC is inactive in ark mutant background suggesting that the cleaved-Caspase-3 labeling signal is not caused by the unknown DRONC substrate, this analysis demonstrates that the cleaved-Caspase-3 antibody does indeed detect cleaved DCP-1. It is also possible that the cleaved-Caspase-3 antibody detects cleaved DRICE in this experiment, because DCP-1 can proteolytically process DRICE, at least in vitro.²⁹^,³² A similar analysis with DRICE could not be performed because GMR-drICE and GMR-ΔN-drICE do not cause an eye ablation phenotype.³² Nevertheless, whether DCP-1 cleaves DRICE in vivo or not, given the sequence similarity of DCP-1 and DRICE at the C-terminus of the large subunit and the peptide blocking data of Figure 2, it suggests that the antibody may also detect cleaved DRICE.

Discussion

The findings of this study are summarized in Figure 4. In addition to DCP-1 and likely DRICE, the cleaved-Caspase-3 antibody also recognizes at least one, perhaps even several additional proteins containing the ETD epitope. Because exposure of this epitope is dependent on apoptosome activity, it is very likely that the protein is a substrate of DRONC. However, cleavage of this protein by DRONC is not sufficient to induce apoptosis because dcp-1 drICE double mutants in which this epitope is present, are not apoptotic (Figure 1e,f). Consistently, this protein is unlikely to be a caspase because none of the remaining Drosophila caspases carries the ETD epitope (Figure 2a).

Figure 4

Specificity of the cleaved-Caspase-3 antibody

Nevertheless, these findings are interesting from a different point of view. In addition to induction of apoptosis, DRONC and caspases in general also have non-apoptotic functions (reviewed in ³³). For example, apoptosis-induced compensatory proliferation is dependent on a non-apoptotic function of DRONC.¹⁶^,³⁴^,³⁵ It is currently unknown how DRONC is mediating this response. Identification of DRONC substrates involved in non-apoptotic functions may be very helpful in elucidating the mechanism by which DRONC exerts its non-apoptotic functions. Unfortunately, database searches using ETD as query to identify potential non-apoptotic DRONC substrates were not successful. Other approaches such as immunoprecipitation using the cleaved-Caspase-3 antibody will be necessary to identify this substrate.

This study also demonstrates that caution should be taken about the specificity of antibodies. As shown here in Drosophila, it is likely that the cleaved-Caspase-3 antibody not only detects cleaved Caspase-3 in mammals, but also additional proteins containing the ETD epitope. In Drosophila, exposure of this epitope is apoptosome-dependent and thus the antibody is a useful reagent for detecting apoptosis. But in other model organisms this may not be the case and may result in mis-interpretations.

In summary, we demonstrated in this paper that the cleaved-Caspase-3 antibody not only detects active effector caspases DRICE and DCP-1 in apoptotic cells in Drosophila, but also additional potential DRONC substrates which may be involved in non-apoptotic processes. Therefore, we propose that it is most useful to interpret the labeling information obtained with the cleaved-Caspase-3 antibody as activity of the Caspase-9-like initiator caspase DRONC in Drosophila.

Materials and methods

The cleaved-Caspase-3 (Asp175) Antibody was purchased from Cell Signaling Technology (Catalog #9661).¹ GMR-hid, GMR-ΔN-dcp-1, dronc^I24, ark^G8, drice^Δ1 and dcp-1^Prev are described elsewhere.⁵^,¹²^,¹⁴^,¹⁵^,³²^,³⁶ For details about making ark clones, see Srivastava et al. (ref. ³⁶). Immunohistochemistry and TUNEL labeling was done as described.²²^,²⁸^,³⁷ Photographs were taken by confocal microscopy.

Blocking peptides A, B, C, and D were obtained from ProImmune Inc. They were dissolved in a small amount of 0.1% ammonium bicarbonate and diluted to a final concentration of 1mg/ml with sterile, distilled water according to manufacturers instructions. 1 ul of the peptide solutions was mixed with 1ul of cleaved-Caspase-3 antibody in labeling solution for sixty minutes at room temperature before incubation with the eye discs.

Acknowledgement

We would like to thank Kim McCall, Hermann Steller and Bruce Hay for fly stocks. This work was supported by the NIH (GM068016, GM081543, GM074977) and The Welch Foundation (G-1496).

Abbreviations


APAF-1	Apoptosis Peptidase Activating Protein-1

ARK	Apoptosis Peptidase Activating Protein-1-related killer

D-APAF-1	Drosophila Apoptosis Peptidase Activating Protein-1

DCP-1	Death Caspase-1

ΔN	Delta N-terminus

DIAP-1	Drosophila Inhibitor of Apoptosis Protein 1

DRICE	Drosophila Interleukin-converting enzyme

DRONC	Drosophila Nedd2-like Caspase

ETD	Glu-Thr-Asp

GMR	Glass multimer reporter

HAC-1	Homolog of APAF-1 and Ced4-1

IAP	Inhibitor of Apoptosis Protein

TUNEL	terminal deoxynucleotidyl transferase mediated dUTP-nick end labeling

References

1. See Cell Signaling Technology website. http://www.cellsignal.com/pdf/9661.pdf.

2. Srinivasan A, Roth KA, Sayers RO, Shindler KS, Wong AM, Fritz LC, et al. In situ immunodetection of activated caspase-3 in apoptotic neurons in the developing nervous system. Cell Death Differ. 1998;5:1004–1016. [PubMed]

3. Yu SY, Yoo SJ, Yang L, Zapata C, Srinivasan A, Hay BA, et al. A pathway of signals regulating effector and initiator caspases in the developing Drosophila eye. Development. 2002;129:3269–3278. [PubMed]

4. White K, Grether ME, Abrams JM, Young L, Farrell K, Steller H. Genetic control of programmed cell death in Drosophila. Science. 1994;264:677–683. [PubMed]

5. Grether ME, Abrams JM, Agapite J, White K, Steller H. The head involution defective gene of Drosophila melanogaster functions in programmed cell death. Genes Dev. 1995;9:1694–1708. [PubMed]

6. Chen P, Nordstrom W, Gish B, Abrams JM. grim, a novel cell death gene in Drosophila. Genes Dev. 1996;10:1773–1782. [PubMed]

7. Xu D, Woodfield SE, Lee TV, Fan Y, Antonio C, Bergmann A. Genetic control of programmed cell death (apoptosis) in Drosophila. Fly (Austin) 2009;3 [PMC free article] [PubMed]

8. Ryoo HD, Bergmann A, Gonen H, Ciechanover A, Steller H. Regulation of Drosophila IAP1 degradation and apoptosis by reaper and ubcD1. Nat Cell Biol. 2002;4:432–438. [PubMed]

9. Holley CL, Olson MR, Colon-Ramos DA, Kornbluth S. Reaper eliminates IAP proteins through stimulated IAP degradation and generalized translational inhibition. Nat Cell Biol. 2002;4:439–444. [PMC free article] [PubMed]

10. Hays R, Wickline L, Cagan R. Morgue mediates apoptosis in the Drosophila melanogaster retina by promoting degradation of DIAP1. Nat Cell Biol. 2002;4:425–431. [PubMed]

11. Yoo SJ, Huh JR, Muro I, Yu H, Wang L, Wang SL, et al. Hid, Rpr and Grim negatively regulate DIAP1 levels through distinct mechanisms. Nat Cell Biol. 2002;4:416–424. [PubMed]

12. Xu D, Li Y, Arcaro M, Lackey M, Bergmann A. The CARD-carrying caspase Dronc is essential for most, but not all, developmental cell death in Drosophila. Development. 2005;132:2125–2134. [PMC free article] [PubMed]

13. Xu D, Wang Y, Willecke R, Chen Z, Ding T, Bergmann A. The effector caspases drICE and dcp-1 have partially overlapping functions in the apoptotic pathway in Drosophila. Cell Death Differ. 2006;13:1697–1706. [PMC free article] [PubMed]

14. Laundrie B, Peterson JS, Baum JS, Chang JC, Fileppo D, Thompson SR, et al. Germline cell death is inhibited by P-element insertions disrupting the dcp-1/pita nested gene pair in Drosophila. Genetics. 2003;165:1881–1888. [PubMed]

15. Muro I, Berry DL, Huh JR, Chen CH, Huang H, Yoo SJ, et al. The Drosophila caspase Ice is important for many apoptotic cell deaths and for spermatid individualization, a nonapoptotic process. Development. 2006;133:3305–3315. [PubMed]

16. Kondo S, Senoo-Matsuda N, Hiromi Y, Miura M. DRONC coordinates cell death and compensatory proliferation. Mol Cell Biol. 2006;26:7258–7268. [PMC free article] [PubMed]

17. Chew SK, Akdemir F, Chen P, Lu WJ, Mills K, Daish T, et al. The apical caspase dronc governs programmed and unprogrammed cell death in Drosophila. Dev Cell. 2004;7:897–907. [PubMed]

18. Daish TJ, Mills K, Kumar S. Drosophila caspase DRONC is required for specific developmental cell death pathways and stress-induced apoptosis. Dev Cell. 2004;7:909–915. [PubMed]

19. Kanuka H, Sawamoto K, Inohara N, Matsuno K, Okano H, Miura M. Control of the cell death pathway by Dapaf-1, a Drosophila Apaf-1/CED-4-related caspase activator. Mol Cell. 1999;4:757–769. [PubMed]

20. Rodriguez A, Oliver H, Zou H, Chen P, Wang X, Abrams JM. Dark is a Drosophila homologue of Apaf-1/CED-4 and functions in an evolutionarily conserved death pathway. Nat Cell Biol. 1999;1:272–279. [PubMed]

21. Zhou L, Song Z, Tittel J, Steller H. HAC-1, a Drosophila homolog of APAF-1 and CED-4 functions in developmental and radiation-induced apoptosis. Mol Cell. 1999;4:745–755. [PubMed]

22. Fan Y, Bergmann A. Distinct mechanisms of apoptosis-induced compensatory proliferation in proliferating and differentiating tissues in the Drosophila eye. Dev Cell. 2008;14:399–410. [PMC free article] [PubMed]

23. Shapiro PJ, Hsu HH, Jung H, Robbins ES, Ryoo HD. Regulation of the Drosophila apoptosome through feedback inhibition. Nat Cell Biol. 2008;10:1440–1446. [PMC free article] [PubMed]

24. Peterson JS, Barkett M, McCall K. Stage-specific regulation of caspase activity in drosophila oogenesis. Dev Biol. 2003;260:113–123. [PubMed]

25. Arama E, Bader M, Srivastava M, Bergmann A, Steller H. The two Drosophila cytochrome C proteins can function in both respiration and caspase activation. EMBO J. 2006;25:232–243. [PubMed]

26. Baum JS, Arama E, Steller H, McCall K. The Drosophila caspases Strica and Dronc function redundantly in programmed cell death during oogenesis. Cell Death Differ. 2007;14:1508–1517. [PubMed]

27. Reis T, Edgar BA. Negative regulation of dE2F1 by cyclin-dependent kinases controls cell cycle timing. Cell. 2004;117:253–264. [PubMed]

28. McCall K, Peterson JS. Detection of apoptosis in Drosophila. Methods Mol Biol. 2004;282:191–205. [PubMed]

29. Hawkins CJ, Yoo SJ, Peterson EP, Wang SL, Vernooy SY, Hay BA. The Drosophila caspase DRONC cleaves following glutamate or aspartate and is regulated by DIAP1, HID, and GRIM. J Biol Chem. 2000;275:27084–27093. [PubMed]

30. Dorstyn L, Kumar S. A biochemical analysis of the activation of the Drosophila caspase DRONC. Cell Death Differ. 2008;15:461–470. [PubMed]

31. Snipas SJ, Drag M, Stennicke HR, Salvesen GS. Activation mechanism and substrate specificity of the Drosophila initiator caspase DRONC. Cell Death Differ. 2008;15:938–945. [PubMed]

32. Song Z, Guan B, Bergman A, Nicholson DW, Thornberry NA, Peterson EP, et al. Biochemical and genetic interactions between Drosophila caspases and the proapoptotic genes rpr, hid, and grim. Mol Cell Biol. 2000;20:2907–2914. [PMC free article] [PubMed]

33. Kuranaga E, Miura M. Nonapoptotic functions of caspases: caspases as regulatory molecules for immunity and cell-fate determination. Trends Cell Biol. 2007;17:135–144. [PubMed]

34. Huh JR, Guo M, Hay BA. Compensatory proliferation induced by cell death in the Drosophila wing disc requires activity of the apical cell death caspase Dronc in a nonapoptotic role. Curr Biol. 2004;14:1262–1266. [PubMed]

35. Fan Y, Bergmann A. Apoptosis-induced compensatory proliferation. The Cell is dead. Long live the Cell! Trends Cell Biol. 2008;18:467–473. [PMC free article] [PubMed]

36. Srivastava M, Scherr H, Lackey M, Xu D, Chen Z, Lu J, et al. ARK, the Apaf-1 related killer in Drosophila, requires diverse domains for its apoptotic activity. Cell Death Differ. 2007;14:92–102. [PMC free article] [PubMed]

37. Arama E, Steller H. Detection of apoptosis by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling and acridine orange in Drosophila embryos and adult male gonads. Nat Protoc. 2006;1:1725–1731. [PubMed]