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Loss of putzig Activity Results in Apoptosis during Wing Imaginal Development in Drosophila

Loss of putzig Activity Results in Apoptosis during Wing Imaginal Development in Drosophila

  • Mirjam Zimmermann, 
  • Sabrina J. Kugler, 
  • Adriana Schulz, 
  • Anja C. Nagel
PLOS
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Abstract

The Drosophila gene putzig (pzg) encodes a nuclear protein that is an integral component of the Trf2/Dref complex involved in the transcription of proliferation-related genes. Moreover, Pzg is found in a complex together with the nucleosome remodeling factor NURF, where it promotes Notch target gene activation. Here we show that downregulation of pzg activity in the developing wing imaginal discs induces an apoptotic response, accompanied by the induction of the pro-apoptotic gene reaper, repression of Drosophila inhibitor of apoptosis protein accumulation and the activation of the caspases Drice, Caspase3 and Dcp1. As a further consequence ‘Apoptosis induced Proliferation’ (AiP) and ‘Apoptosis induced Apoptosis’ (AiA) are triggered. As expected, the activity of the stress kinase Jun N-terminal kinase (JNK), proposed to mediate both processes, is ectopically induced in response to pzg loss. In addition, the expression of the mitogen wingless (wg) but not of decapentaplegic (dpp) is observed. We present evidence that downregulation of Notch activates Dcp1 caspase and JNK signaling, however, neither induces ectopic wg nor dpp expression. In contrast, the consequences of Dref-RNAi were largely indistinguishable from pzg-RNAi with regard to apoptosis induction. Moreover, overexpression of Dref ameliorated the downregulation of pzg compatible with the notion that the two are required together to maintain cell and tissue homeostasis in Drosophila.

Introduction

Cellular and tissue homeostasis describes a complex process ensuring the survival and correct development of an organism. Apoptosis, the major form of programmed cell death, contributes to tissue homeostasis by eliminating aberrant, surplus or malignant cells during 'normal' development and in response to stress induced conditions. This safeguarding system demands a fine-tuned control as an unregulated apoptosis has been connected to various human diseases including cancer (reviewed in [14]). Research in several model organisms including Drosophila has expanded our knowledge on the molecular mechanisms underlying the well conserved apoptotic execution program in metazoans: Under normal conditions, cell survival is guaranteed by the Inhibitors of Apoptosis Proteins (IAP, DIAP1 in Drosophila), which bind and inhibit caspases, the key executing enzymes of apoptosis (reviewed in [46]). In contrast, if cell death is triggered, e.g. under cellular stress conditions, pro-apoptotic gene activity is induced in Drosophila: Pro-apoptotic gene products include the DIAP1-antagonists Hid, Rpr and Grim, which themselves mediate the ubiquitin dependent degradation of DIAP1 thereby enabling caspases to provoke the death of the cell [713].

Although diverse stress signals provoke a strong apoptotic answer in the respective tissue, organisms can often compensate this cell loss allowing them to survive with no or only minor consequences on the final tissue or body size. This intriguing fact has been designated Apoptosis-induced Proliferation (AiP) and describes the striking property of dying cells to stimulate proliferation of adjacent surviving cells, and therefore tissue regeneration (reviewed in [1417]). The model system Drosophila with its sophisticated genetic methods offers the opportunity to study the mechanisms underlying the communication between damaged cells and the surrounding tissues. Here, cell death was experimentally induced in cells of larval imaginal discs and, as a consequence, the ectopic induction of mitogens like wingless (wg), decapentaplegic (dpp) or hedgehog (hh) was often observed in the apoptotic cells [1821]. These mitogens, the key players in the regulation of morphogenesis and growth in the course of Drosophila development [22,23], provide a well-founded explanation for the compensatory proliferation of neighboring cells. However, since the genuine dying cells are often rapidly eliminated, much of our current knowledge on AiP was deduced from a special type of apoptotic cells referred to as 'undead' cells. These cells are experimentally obtained by provoking cell death while expressing the caspase inhibitor p35 at the same time. p35 specifically blocks the function of the effector caspases Drice and Dcp-1 without affecting the activity of the initiator caspase Dronc [19,24,25]. Therefore, these cells are trapped in the execution of cell death, but fail to complete the process due to the blocked function of effector caspases [18,19,26]. This experimental approach led to the conclusion that the Jun N-terminal kinase (JNK) acts as a central player of AiP in the dying cells. A robust activity of this stress kinase is associated with diverse aspects of tissue regeneration, including the expression of the aforementioned mitogens and the delay of larval development that keeps the animal in the growth phase ([2728]; reviewed in [29]). Moreover, as dying cells lose their epithelial integrity, JNK-signaling enforces the restitution of an intact epithelium including the formation of actin cables and filopodia in accordance with its well-defined role in the healing of epidermal wounds [3033].

Intriguingly, JNK-signaling activity is not only associated with emanating proliferative signals from the apoptotic cells, but also with the non-autonomous induction of secondary cell death at a considerable distance from the primary cell death source. This phenomenon was recently termed Apoptosis induced Apoptosis (AiA) and might be the mechanistic explanation for the systemic cell death occurring both in normal development of metazoans as well as in some human pathologies like e.g. neurodegenerative disorders ([34]; reviewed in [35]). Obviously, tissue homeostasis is coordinated by a cross-regulatory relationship of widespread signaling molecules keeping proliferation and apoptosis in a balanced ratio.

The DNA replication-related element-binding factor (Dref)-complex as well as the Notch (N) signaling pathway are both suitable candidates for being part of such a regulatory network, as they are known to govern numerous developmental processes including cell proliferation and apoptosis (reviewed in [3638]). Dref acts as a transcription factor in Drosophila and is proposed to regulate the expression of a multitude of genes required for cell proliferation such as cell cycle regulators, growth factors or DNA replication factors (reviewed in [37]). A similar pleiotropic influence is mediated by the highly conserved N signaling pathway, being reiteratively used during the development of a variety of tissues in higher eumetazoa. Depending on the cellular context, N can either promote or inhibit growth processes emphasizing the importance of a tight and fine-tuned regulation of the signaling cascade (reviewed in [38,39]). In Drosophila, both signaling networks depend on putzig (pzg), an essential positive regulator of the Dref and N signaling cascades [40,41]. Pzg is an integral component of the Trf2/Dref protein complex that regulates proliferation-related genes [40]. Moreover, Pzg acts Dref independently and promotes N target gene activation via the Nucleosome remodeling factor (Nurf), implying a Pzg-mediated epigenetic influence on N target gene activation [41].

Here, we show that reduction of pzg activity during larval wing development results in the induction of genuine dying cells which initiate AiP mechanisms and enable AiA. As expected, ectopic JNK-signaling activity is induced autonomously and non-autonomously, likely to mediate the systemic response. This spectrum of consequences is not mimicked by a downregulation of N receptor activity: Though apoptosis and proliferation are induced, the latter is not mediated by the ectopic induction of wg in the genuine dying cells, unlike in the pzg depleted cells. In contrast, a downregulation of Dref activity does not only provoke apoptosis but also AiP, mediated by the induction of wg, similar to the effects observed after pzg knockdown. We conclude that Pzg is fundamental for the fine-tuned homeostasis of cell survival and proliferation via its influence on important signaling networks during the development of Drosophila.

Materials and Methods

Fly stocks, genetics and work

The following fly stocks were used:

Gal4/UAS lines.

EP-pzg (EP-756; Exelixis stock collection, USA). Gal4-lines: en-Gal4 UAS-GFP [42]; en-Gal4 UAS-GFP UAS-pzg-RNAi/CyO [40]; Gmr-Gal4 [43]; Gmr-grim/TM3Sb [9]; Gmr-hid/CyO [8]; Gmr-rpr/TM6B [44]; Ombmd65-Gal4/FM7 [45]. UAS-lines: UAS-Dref [46]; gift from D. Bohmann); UAS-Dref-RNAi (BL31941); UAS-lacZ (BL8529); UAS-H-RNAi [47]; UAS-N-RNAi (BL7078); UAS-p35 (BL6298); UAS-pzg-RNAi [40]; UAS-pzg-RNAi (VDRC v25542).

Reporter-strains.

dpp-lacZ [48]; puc-lacZ [49]; rpr-lacZ [50].

Flies and crosses were raised on standard fly food supplemented with fresh yeast paste at 25°C. Crosses with UAS-RNAi lines were cultured at 29°C to ensure strong RNAi induction.

Antibody staining and documentation with confocal microscopy

Antibody staining on wing imaginal discs was done according to Müller et al. [51]. The following antibodies were used: guinea-pig anti-Hairless (H) (1:500) [52]; guinea-pig anti-Pzg (1:500) [40]; mouse anti-Arm (1:50; DSHB); mouse anti-beta Galactosidase (1:50; DSHB); mouse anti-Notch (ICN) (1:25; DSHB); mouse anti-Wg (1:50; DSHB) all obtained from the Developmental Studies Hybridoma Bank, Department of Biological Science, University of Iowa City, IA 52242, USA; mouse anti-DIAP1 (1:400) [13]; rabbit anti-activated Caspase 3 (1:200; Cell Signaling, Germany); rabbit anti-activated Dcp-1 (1:200; Cell Signaling, Germany), rabbit anti-activated Drice (1:250 [13]; rabbit anti-GFP (1:100; Santa Cruz, USA); rabbit anti-Phospho-Histone H3 (PH3) (1:50; Cell Signaling, Germany); rat anti-Dilp8 (1:500 [53]; gift from P. Léopold, Nice, France). Secondary antibodies coupled to fluorescein, Cy3 or Cy5 were purchased from Jackson Laboratories (Dianova, Germany). Dissected tissues were mounted in Vectashield (Vector Laboratories, USA).

For labeling cells undergoing DNA synthesis, the Click-iT EdU Alexa Fluor 488 Imaging Kit (Invitrogen, Eugene, Orgeon, USA) was used. After dissection in ice-cold PBS, larval tissues were incubated for two hours at room temperature in M3 insect medium including 10 mM EdU stock solution. A 4% para-formaldehyde fixative was added for 25 minutes, removed, and washed at least four times with PBS, followed by a 20 minutes incubation with PBX (PBS with 0.3% Triton X-100) and two washing steps for 10 minutes each. The Click-iT reaction cocktail was added containing 443.5 μl H2O, 21.5 μl 10X Click-iT reaction buffer, 10 μl CuSO4, 0.5 μl Alexa Fluor Azide and 25 μl reaction buffer additive, and the discs were incubated for 30 minutes in the dark. After removal of the reaction cocktail solution, the tissues were washed with PBS two times, prepared and embedded in Vectashield (Vector Laboratories, USA).

Confocal images were acquired with a Zeiss Axioskop linked to a Bio-Rad MRC1024 scanhead by using Bio-Rad Laser Sharp 3.1 software.

Documentation of adult eyes and statistical quantification of eye size

Adult eyes of females were documented with an ES120 camera (Optronics, Goleta CA, USA) using Pixera viewfinder software version 2.0. For quantification, eye size of five females each was measured with Image J and eye area was calculated. Statistical significance of probes was determined according to Student's T-test (http://www.physics.csbsju.edu/stats/t-test.html) and p-value was scaled accordingly: p>0.05 (not significant, n.s.); p<0.05 (weakly significant; *); p<0.01 (significant; **); p<0.001 (highly significant; ***).

Results

Pzg genetically interacts with pro-apoptotic genes and is required for cell survival

Knock down of pzg gene activity by pzg-RNAi induction during larval development results in tissue size reduction (Fig 1A–1C). Previously, we have shown that pzg-RNAi effects are negatively correlated with cell cycle progression, thereby influencing cell proliferation and cell growth [40]. In order to test whether tissue loss and size reduction might also be a consequence of the induction of programmed cell death, we performed genetic interaction assays with the well defined apoptosis-inducing factors hid, rpr and grim, whose transcriptional activation is an essential step in the execution of most apoptotic events in the development of Drosophila [5457] (reviewed in [58]). It is well established that ectopic expression of pro-apoptotic genes in the developing eye imaginal disc using the Gmr promoter causes cell killing and therefore conspicuously small eyes in the adults (Fig 1D–1F). Eye size reduction was considerably suppressed by additional expression of Pzg (Fig 1G–1I and S1 Fig), whereas it was enhanced by pzg-RNAi induction within the affected tissue (Fig 1J–1L and S1 Fig).

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Fig 1. pzg genetically interacts with pro-apoptotic genes.

(A-C) Control eyes after overexpression of lacZ and pzg, as well as pzg-RNAi induction. (D-F) Overexpression of hid, rpr and grim during eye development causes small and rough eyes in the adults. (G-I) Overexpression of pzg ameliorates the small eye size resulting from ectopically expressed pro-apoptotic genes, whereas depletion of pzg activity by RNAi enhances the phenotypes (J-L). Genotypes analyzed: Gmr-Gal4/+; UAS-lacZ/+. Gmr-Gal4/+; EP-pzg/+. Gmr-Gal4/+; UAS-pzg-RNAi/+. Gmr-hid/Gmr-Gal4; UAS-lacZ/+. Gmr-Gal4/+; Gmr-rpr or Gmr-grim/ UAS-lacZ. Gmr-hid/Gmr-Gal4; EP-pzg/+. Gmr-Gal4/+; Gmr-rpr or Gmr-grim/EP-pzg. Gmr-hid/Gmr-Gal4; UAS-pzg-RNAi/+. Gmr-Gal4/+; Gmr-rpr or Gmr-grim/ UAS-pzg-RNAi.

https://doi.org/10.1371/journal.pone.0124652.g001

To address this phenomenon in more detail, we examined the expression of several components of the cell death machinery in wing imaginal discs upon pzg depletion. We have chosen the wing disc tissue for our analyses, as there the endogenous level of apoptosis is comparatively low [18,59]. Thus higher levels of apoptosis can be attributed to a reduced pzg activity.

Pzg activity was downregulated with the UAS-pzg-RNAi shown before to efficiently reduce Pzg protein levels [40]. Pzg-RNAi induction either in the posterior compartment (using en-Gal4) or in a more central area (using omb-Gal4) of the wing disc triggered the execution of the apoptotic program: Pro-apoptotic gene activity, visualized by the activation of a rpr-lacZ reporter (Fig 2A-A''' and S2A-A'' Fig), a reduced level of the anti-apoptotic protein DIAP1 (Fig 2B-B''' and S2B-B'' Fig) and finally the accumulation of activated initiator and effector caspases Drice, Caspase-3 and Dcp-1 were observed in the pzg mutant area of the wing disc (Fig 2C-E''' and S2C-E'' Fig). To exclude, however, off-target effects, a second independent RNAi-line (VDRC v25542) was included in the analysis of apoptosis induction and gave the same overall results. These data show that an impaired pzg activity leads to an increase in apoptosis.

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Fig 2. pzg depletion autonomously triggers the apoptotic signaling cascade in wing imaginal discs.

RNAi mediated depletion of pzg was induced in the posterior part of the wing disc using en-Gal4. Rpr-lacZ, DIAP1 and caspase activity (in red) was monitored as indicated. (A-A''') A strong activation of the pro-apoptotic gene rpr (arrows) as well as the Drosophila activated caspases Driceact (C-C''', arrow), Caspase 3act (D-D''', arrow) and Dcp-1act (E-E''', arrows) is detected in the posterior half of the disc, whereas DIAP1 protein level is reduced (B-B''', repressive arrow). (A-A''') en-Gal4 UAS-GFP UAS-pzg-RNAi/+; rpr-lacZ/+. (B-E''') en-Gal4 UAS-GFP UAS-pzg-RNAi/+. Pzg protein is shown in blue (anti-Pzg, A-E and A'''- E'''); GFP in green (en-Gal4 GFP) marks the posterior compartment (A-E and A''-E''). Posterior is to the right and dorsal up. The antero-posterior compartment boundary is marked with a dashed line. Scale bars: 100 μm.

https://doi.org/10.1371/journal.pone.0124652.g002

pzg silencing triggers JNK-mediated stress responses

A key factor known to convey apoptosis in Drosophila and mammals alike is the c-Jun N-terminal kinase (JNK) pathway (reviewed in [60,61]). To measure JNK signaling activity in pzg mutant cells, we made use of the JNK downstream effector puckered (puc) and monitored the expression of a puc-lacZ enhancer trap line [62]. In wild type third instar wing discs, puc-lacZ expression is detected in the small stalk region attaching the disc to the larval epidermis [62]. Knock down of pzg robustly activated the puc-lacZ reporter gene within the depleted area and weakly in adjacent regions notably in older wing discs (Fig 3A-B'' and S2F-F'' Fig), consistent with the induction of JNK-mediated cell death. Great experimental insights were gained in the recent years demonstrating that JNK-mediated cell death in Drosophila is crucial for eliminating aberrant cells, thereby ensuring further development and morphogenesis (reviewed in [16,29,61]). This includes also the induction of a developmental delay and inhibition of metamorphosis, allowing the larva to compensate growth deficits and repair injured tissue. The gene encoding Dilp8, a member of the insulin-relaxin peptide family, was found to be upregulated in response to JNK signaling and is thought to delay metamorphosis by inhibiting ecdysone biosynthesis [53,63]. Tracing Dilp8 in wing discs, where pzg-RNAi was induced in the posterior half, revealed no obvious accumulation in younger discs (approximately 96 h AEL; Fig 3C-C''), whereas Dilp8 was highly enriched in later stages (approximately 120 h AEL; Fig 3D-D'').

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Fig 3. pzg depletion results in an inappropriate JNK-signaling activation.

pzg was downregulated in the posterior part of the wing disc (Pzg protein is shown in blue). (A-B'') puc-lacZ activity is induced in pzg-RNAi mutant cells reflecting JNK-signaling activity (red in A-A', B-B', arrows) (en-Gal4 UAS-GFP UAS-pzg-RNAi/+; puc-lacZ/+). In addition, especially in late third instar larval discs, a non-autonomous activity can be detected in the anterior compartment (open arrows in A', B'). (C-F'') Further consequences of JNK-mediated developmental apoptosis induction can be observed: (C-D'') Dilp8 protein is secreted in the pzg depleted cells, however not before late third larval instar (red in D, D', arrow). (E-E'') In pzg-RNAi mutant cells, Arm protein accumulation is disturbed (red, repressive arrow). (F-F'') pzg mutant cells penetrate into the anterior compartment while retaining their posterior identity (green, arrows in F, F'). (C-F'') en-Gal4 UAS-GFP UAS-pzg-RNAi/+. The A/P compartment boundary is marked with a dotted line. Scale bars: 100 μm.

https://doi.org/10.1371/journal.pone.0124652.g003

Interestingly, the older pzg mutant wing discs appeared more crumpled and folded than the younger ones (compare Fig 3A and 3C with 3B and 3D). This phenomenon is reminiscent of regenerative growth during larval development experimentally induced with non-surgical tissue damage, e.g. pro-apoptotic gene activity ([6465]; reviewed in [66]). Moreover it has been shown that cell-adhesion is reduced by the caspase dependent cleavage of Armadillo (Arm) [67,68]. Indeed, we observed a strong reduction of membrane-associated Armadillo (Arm, beta-Catenin) in the pzg silenced area of the wing disc (Fig 3E-E''). In a wild type wing disc, the antero-posterior compartment border is straight and defined. In contrast we observed ‘finger-like’ structures notably in older pzg-silenced discs (Fig 3F-F''). Apparently, pzg-depletion enabled the mutant cells to invade the anterior compartment while still retaining posterior identity (Fig 3F-F'').

Loss of pzg activity results in 'genuine' dying cells and consequently AiP

Genetic studies in Drosophila have shown that different mechanisms are triggered in apoptotic cells leading to an increase in proliferation and cell division rates of adjacent cells, a process which is referred to as Apoptosis induced Proliferation (AiP) (reviewed in [14,17,69,70]). In order to investigate the consequences of pzg depletion with regard to AiP, we monitored the proliferation rates in wing discs, where pzg-RNAi was induced in the posterior compartment. As pzg was shown to be required for the activation of cell cycle related genes [40], the autonomous decrease of actively dividing cells upon pzg depletion compared with controls was expected (Fig 4A-B'''). In addition we noted an elevated number of cells undergoing DNA synthesis (EdU labeled) as well of mitotic cells (PH3 labeled) abutting the pzg-RNAi mutant territory at the anterior, implying the induction of a non-autonomous cell division response (Fig 4A-B''').

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Fig 4. pzg mutant cells show characteristics of genuine dying cells and induce AiP.

pzg was downregulated in the posterior part of the wing disc. Pzg protein is shown in blue (C, C''), the posterior compartment is marked in green with GFP (A-D; A''-D''; A'''-B'''). (A-A''') DNA-synthesis visualized with EdU-labeling (red) is amplified anteriorly along the A/P compartment boundary (arrows) upon pzg depletion (A-A') compared to en-Gal4 UAS-GFP control (A'''), whereas a loss can be observed within the posterior compartment (open arrow, A-A'). (B-B''') Cell division was visualized with anti-PH3 (red). Compared to the control (B'''), many cells in the posterior domain lost this marker upon pzg depletion (arrowhead, B-B'), whereas a strip of cells anterior to the A/P boundary shows stronger PH3 labeling (arrows, B-B'). (C-C'') Expression of Wg protein was monitored (red): it is interrupted at the dorso-ventral boundary in response to pzg-RNAi depletion (arrowhead in C'), whereas ectopic induction of Wg is observed in the posterior compartment outside the normal Wg expression domain (arrows, C'). (D-D'') dpp-lacZ expression (red) is unchanged by pzg depletion in the posterior compartment. Genotypes: (A'''-B''') en-Gal4 UAS-GFP/+. (A-C'') en-Gal4 UAS-GFP UAS-pzg-RNAi/+. (D-D'') en-Gal4 UAS-GFP UAS-pzg-RNAi/+; dpp-lacZ/+. Wing discs are oriented posterior rightwards and dorsally upwards. The A/P compartment boundary is marked with a dotted line. Scale bars represent: (A, B, D) 100 μm, (C) 50 μm.

https://doi.org/10.1371/journal.pone.0124652.g004

Apoptotic cells ectopically activate morphogenetic genes like wingless (wg) or decapentaplegic (dpp), responsible for the proliferation stimulus in directly adjacent cells [18,19]. To test if this is also a response to pzg loss, we firstly analyzed the distribution of Wg protein in wing discs, where pzg-RNAi was induced in the posterior compartment. At later stages (approximately 120 h AEL) we observed an ectopic expression of Wg in areas of the posterior compartment (Fig 4C', arrows), whereas the wild type expression along the dorso-ventral boundary in the posterior compartment of the disc was interrupted (Fig 4C', arrowhead). As wg is a well known target of N signaling, a reduction of wild type Wg accumulation along the dorso-ventral boundary was expected [40]. Intriguingly, no ectopic induction of dpp was observed in pzg silenced cells, inferred from the normal expression levels of a dpp-lacZ reporter along the antero-posterior compartment boundary (Fig 4D-D''). This demonstrates that pzg mutant cells behave like 'genuine' apoptotic cells with respect to the induction of ectopic wg, and that they induce AiP without the involvement of ectopic dpp activity.

Loss of pzg activity is accompanied by Apoptosis induced Apoptosis (AiA)

Interestingly, cells doomed to die not only trigger AiP but also induce non-autonomous secondary apoptosis, abbreviated AiA (Apoptosis induced Apoptosis) [34]. To examine if such an effect is also observed in pzg-RNAi mutant cells we followed the activation of Caspase3 and Dcp1 in late third instar larval wing discs. Under these conditions, a strong caspase activity was observed in the anterior half of the disc, either visualized by staining for cleaved Caspase-3 or Dcp-1 (Fig 5A-A'' and 5C-C''). The co-expression of the baculovirus caspase inhibitor p35 in the pzg-RNAi mutant cells strongly enhanced this effect and triggered tumorous overgrowth of the wing discs and non autonomous AiA (Fig 5B-B'' and 5D-D''). This can be explained by the induction of the apoptotic machinery but the prevention of execution through inhibition of effector caspases by p35, resulting in so-called 'undead' cells [18,19,26,71]. Altogether, these data indicate that loss of pzg activity results in apoptosis followed by AiP and AiA.

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Fig 5. pzg-RNAi induces AiA in late larval stages.

(A-D'') Strong caspase activity, either visualized with anti-Caspase 3act (red in A-A', B-B') or anti-Dcp-1act (red in C-C', D-D'), can be detected non-autonomously in late larval wing disc in the anterior compartment (arrowheads in A-D'). Preventing cell death execution with p35 amplifies the overgrowth effect (asterisks in B'') but still induces AiA in the anterior half of the disc (B-B'', D-D'', arrowheads). Genotypes: (A-A'') and (C-C'') en-Gal4 UAS-GFP UAS-pzg-RNAi/+. (B-B'') and (D-D'') UAS-p35; en-Gal4 UAS-GFP UAS-pzg-RNAi/+. The A/P compartment boundary is marked with a dotted line. Scale bars: 100 μm.

https://doi.org/10.1371/journal.pone.0124652.g005

Apoptotic consequences of N depletion in wing discs

As Pzg was shown to be required for efficient N signaling to occur, we asked whether the observed apoptotic outcome after pzg-RNAi induction might be the consequence of an impaired N signaling activity [40,41]. It is well established that a reduction in N signaling activity during imaginal development is correlated with tissue loss and apoptosis [51,72]. Therefore, we induced N-RNAi in the posterior compartment of the wing disc to compare the effects with those obtained after pzg depletion. To this end, we used the same N-RNAi line that has been shown by others to provide RNAi-mediated knock-down of N activity in wing imaginal discs [38,73]. We observed an accumulation of activated Dcp-1 caspase autonomously in N mutant cells and also non-autonomously in the anterior compartment, indicating that a downregulation of N signaling contributes to AiA (Fig 6A-B''). Moreover, a robust induction of puc-lacZ was detected in both compartments, indicating that JNK-mediated activity was induced as well (Fig 6C-C''). In contrast, cell proliferation, visualized with EdU labeling, was different from pzg-RNAi depleted cells: Cells within the N-RNAi depleted compartment were still able to cycle through the cell cycle concluded from EdU incorporation (Fig 6D-D''). Based on the modest reduction of EdU signals, less cells however, appeared to enter the S-phase. Moreover, AiP was only weakly observed as EdU labeled cells abutting the N-deficient area appeared only more densely spaced (Fig 6D-D''), and no ectopic induction of the mitogens Wg (Fig 6E-E'') or dpp-lacZ (Fig 6F-F'') was detected.

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Fig 6. Apoptotic consequences of Notch depletion in wing imaginal discs.

N-RNAi was induced in the posterior compartment of the wing disc, N protein is shown in blue (A, B, D and A'', B'', D''), the posterior compartment is marked in green with GFP (A-F and C'', E'', F''). (A-B'') Caspase activity, visualized with anti-Dcp-1act (red in A, A' and B, B') is detectable in early (96 h AEL) and late third instar wing discs (120 h AEL) autonomously (arrows in A, A' and B, B') and non-autonomously (open arrows in A, A' and B, B'). The effect of AiA is enhanced in later phases of development (B, B'). (Genotype: UAS-N-RNAi; en-Gal4 UAS-GFP/+). (C-C'') JNK-signaling readout visualized with puc-lacZ expression is seen in both compartments (red in C, C' arrows; UAS-N-RNAi; en-Gal4 UAS-GFP/+; puc-lacZ/+). The wild type expression of puc-lacZ at the stalk region is marked with an asterisk (C, C'). (D-D'') Cells within S-phase are labeled with EdU (red); a minor autonomous reduction and weak increase in most central cells abutting the A/P compartment boundary is observed (arrow in D, D'; genotype as in A). (E-E'') The expression of Wg is lost at the dorso-ventral boundary in the posterior compartment (red in E, E', repressive arrow; genotype as in A). (F-F'') dpp expression is not affected by downregulation of N in the posterior compartment (red in F, F'; UAS-N-RNAi; en-Gal4 UAS-GFP/+; dpp-lacZ/+). The A/P compartment boundary is marked with a dotted line. Scale bars: 100 μm.

https://doi.org/10.1371/journal.pone.0124652.g006

In order to investigate, whether an upregulation of N signaling activity could ameliorate the apoptotic consequences of pzg loss, we initially sought to overexpress the activated form of the N receptor in pzg-RNAi depleted cells. However probably due to the immensely hyper-activated N response, we failed to obtain any viable larvae of this genotype. A more gentle method to increase N signaling activity is to reduce the activity of the general antagonist Hairless (H), e.g. by inducing a H-RNAi construct proven to reduce H activity in the fly [47]. Reduction of H activity in pzg-RNAi mutant cells of the wing disc did not considerably abrogate the pzg-RNAi induced apoptosis defects (S3 Fig). These data suggest that the cell death resulting from loss of pzg is not primarily triggered by an inappropriate N signaling activity.

Apoptotic consequences of Dref depletion in wing discs

Apart from its role in N target gene activation, Pzg is important for cell proliferation as a member of the Trf2/Dref complex in Drosophila [40,74]. The transcription factor Dref regulates the expression of many proliferation-related genes, and has been described as a master key factor for cell proliferation (reviewed in [37]). We hence examined the consequences of Dref-RNAi depletion on apoptosis, AiP and AiA. To this end the UAS-Dref-RNAi line was used that has been shown before to specifically target Dref activity [75]. Inducing Dref-RNAi in the posterior half of the wing disc indeed triggered a strong apoptotic response, including robust Dcp1 caspase activation in the anterior half of late third instar wing discs (appr. 120 h AEL), indicative of AiA (Fig 7A-B''). Moreover, JNK signaling, i.e. puc-lacZ, was induced autonomously in the affected compartment, and to a lesser degree also in the anterior compartment of the wing disc (Fig 7C-C''). As expected, reduction of Dref activity was correlated with impaired cell proliferation as demonstrated by reduced EdU labeling in these areas (Fig 7D-D''). Proliferation rates were, however, significantly increased in the tissue abutting the Dref mutant cells (Fig 7D-D''). Moreover, similar to the effects of pzg-RNAi depletion, ectopic Wg induction was detected in the Dref-RNAi mutant tissue (Fig 7E-E''), whereas dpp activity appeared unaffected (Fig 7F-F''). As expected from our earlier work, Wg expression along the dorso-ventral boundary was not affected by the depletion of Dref, in support of the notion that the effects of Pzg on N regulation are Dref independent [40, 41]. Overall, induction of apoptosis with all its consequences is very similar upon the depletion of either Dref or Pzg during larval wing development. We therefore asked whether an increase of Dref activity might ameliorate the apoptotic consequences of pzg depletion. To this end we overexpressed UAS-pzg-RNAi together with UAS-Dref in the posterior wing disc compartment. To account for possible titration effects of Gal4 by the extra UAS-dose, we compared the effects of Dref overexpression with those obtained in a pzg-RNAi plus UAS-lacZ background. We observed a weaker apoptotic response in the UAS-lacZ background, (Fig 8). However, a concomitant increase of Dref activity ameliorated the pzg-RNAi proliferation defects much more robustly and strongly attenuated the induction of activated Dcp-1 caspase (Fig 8). Therefore, although Pzg and Dref are part of a large multi-subunit complex, increasing the amount of Dref protein helps to counterbalance the apoptotic effects resulting from pzg loss.

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Fig 7. Loss of Dref activity entails cell cycle arrest, cell death and AiP.

Dref was downregulated in the posterior compartment of the wing disc, which is marked in green with GFP (A-F; A''-F''). (A-B'') Dref-RNAi induction in the posterior compartment of the wing disc is correlated with an autonomous (arrow) and non-autonomous (open arrow) cell death induction, monitored with anti-Dcp1act (red in A, A' and B, B'). Although AiA can be detected already in early third instar wing discs (96 h AEL, A' open arrow), the amount of non-autonomous cell death is strongly increased in later stages (120 h AEL, B' open arrows). (C-C'') puc-lacZ is ectopically induced in both compartments (red in C, C'; closed and open arrows). Asterisk highlights the wild type expression in the stalk region of the disc. (D-D'') Replication is disturbed autonomously after Dref-RNAi depletion, visualized with EdU staining (red in D, D', repressive arrow), whereas enhanced proliferation is induced anteriorly (open arrow). (E-F'') Ectopic induction of Wg (red in E, E') can be detected in the Dref-depleted compartment (arrows in E, E'), whereas dpp-lacZ expression appears unchanged (red in F-F'). Genotypes: (A-B'') and (D-E'') en-Gal4 UAS-GFP/+; UAS-Dref-RNAi/+. (C-C'') en-Gal4 UAS-GFP/+; UAS-Dref-RNAi/puc-lacZ. (F-F'') en-Gal4 UAS-GFP/+; UAS-Dref-RNAi/dpp-lacZ. Posterior is right, dorsal upwards. The dashed line marks the A/P compartment boundary. Scale bars: 100 μm.

https://doi.org/10.1371/journal.pone.0124652.g007

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Fig 8. Dref overexpression ameliorates pzg-RNAi apoptosis defects.

Comparison of the apoptotic consequences of pzg depletion in the presence (A-E’) or absence (B-F’) of ectopic Dref expression. (A-D') Compared to the control (B,B',D,D'), overexpression of Dref in pzg-RNAi depleted cells ameliorates the cell cycle arrest defect (A,A',C,C'), visualized either with EdU (S-phase, red in A-B') or anti-PH3 (M-phase, red in C-D'). Repression of cell division by pzg depletion (repressive arrow in B', D'), is absent (C') or less pronounced in the Dref overexpression background (open repressive arrow in A'). Activation of Dcp-1 (anti-Dcp-1act, red in E-F') upon pzg downregulation (arrow in F'), is much weaker when Dref is overexpressed (open arrow in E'). Genotypes: (A, A'; C, C'; E, E') en-Gal4 UAS-GFP UAS-pzg-RNAi/+; UAS-Dref/+. (B, B'; D, D'; F, F') en-Gal4 UAS-GFP UAS-pzg-RNAi / UAS-lacZ. Posterior is right and dorsal up. The A/P compartment boundary is marked with a dashed line. Scale bars: 100 μm.

https://doi.org/10.1371/journal.pone.0124652.g008

Discussion

In this work we show that loss of pzg induces apoptosis, including the autonomous activation of rpr and of various Caspases, of JNK and of Dilp-8 expression. Moreover, we observed non-autonomous apoptosis induced proliferation (AiP) including the induction of Wingless as well as apoptosis induced apoptosis (AiA). Apparently, pzg is required for the survival of the cell which in its absence undergoes apoptosis with all its consequences.

Pzg is a nuclear protein found in different multimeric complexes including the Trf2/Dref complex and the NURF complex. The Pzg/NURF complex has been implied in the epigenetic regulation of N and EcR target genes [41,76], whereas Dref is involved in the regulation of replication and proliferation related genes (reviewed in [37]). Both, downregulation of N as well as of Dref induced apoptosis. Whereas the former was expected [51,72], the latter was not since overexpression rather than downregulation of Dref was associated with apoptosis so far [77,78], presumably because Dref overexpression is sufficient to drive terminally differentiated cells into a new cell cycle [78]. Very similar to pzg RNAi, downregulation of Dref not only resulted in autonomous JNK- and Caspase activation, but also induced ectopic expression of Wingless, which was not observed as a consequence of N depletion. Moreover, unlike the activation of N signaling activity, overexpression of Dref was sufficient to ameliorate the effects of pzg induced apoptosis. Overall we conclude that the apoptosis induced by a depletion of pzg is primarily triggered by a disturbance of the Pzg/Trf2/Dref complex. In this case, at least part of the apoptotic consequences may result from a dysregulatin of Pzg/Dref target genes involved in cell survival or cell death. For example, the failure to activate anti-apoptotic factors in the pzg- and Dref-RNAi mutant background might directly induce apoptosis. Amongst the plethora of Dref target genes is the Drosophila proto-oncogene raf [79]. Indeed it was shown that overexpression of Dref stimulates MAPK signaling activity [46]. MAPK, however, is an important negative regulator of pro-apoptotic gene activity in Drosophila [55,80], easily explaining the pro-apoptotic effects of a Dref loss. Moreover, inspection of the promoter region of the Drosophila inhibitor of apoptosis (DIAP1) gene reveals the presence of several potential DRE sites (S4 Fig). Hence, Dref may support cell survival by activating DIAP1 under normal circumstances. Future will show whether Dref acts together with Pzg, i.e. whether Pzg binds to the promoters of cell death regulators as well. Dref, however, not only stimulates the expression of anti-apoptotic or survival factors, but it also promotes the transcriptional activation of the apoptosis inducer dmp53 [81]. Similar to the apparent disparate role of Dref in the regulation of apoptosis, Dref not only promotes cell proliferation by the transcriptional activation of proliferation related genes like Pol alpha or PCNA, but Dref also inhibits proliferation as different members of the Hippo signaling pathway are Dref target genes as well [82,83].

Alternatively to a direct role of Dref and Pzg in the transcriptional regulation of genes involved in cell survival, cell death may arise as a consequence of conflicting signals that result from a collective dysregulation of the numerous Dref target genes. Depletion of pzg or Dref presumably affects a wide range of Dref-target genes. Dref target sequences have been found in more than 200 Drosophila genes, including replication and proliferation related genes, genes involved in growth, development and differentiation as well as components of protein biosynthesis or RNA binding proteins (reviewed in [37]; [84]). Therefore, it appears likely that crippling pzg activity, impairs the Dref-mediated cellular homeostasis, and hence the balance between survival and death decisions.

Recent molecular studies in Drosophila demonstrated that Pzg protein can be specifically detected at the telomeres of the chromosomes [85]. In Drosophila, the maintenance of telomeres is realized by a repeated transposition of retrotransposons instead of the telomerase-dependent extension of other eukaryotes (reviewed in [86,87]). Despite this difference, mutations that cause dysfunctions of the telomeres give rise to chromosome- and DNA-damage in all eukaryotes, and consequently result in apoptosis and in an increased lethality ([88,89]; reviewed in [90]). Pzg mutant animals were shown to suffer from moderate telomere instability, inferred from a significant increase in the in vivo incidence of telomere fusions in anaphase neuroblasts. Telomere fusions were attributed to major chromatin changes causing altered transcriptional activity of the retrotransposon Het-A due to the loss of pzg activity [91]. In accordance with a functional relationship between Pzg and Dref in this developmental context, Dref mutants show similar alterations in retrotransposon expression [92]. Moreover, specific Dref target sequences were identified in the promoters of several retrotransposons in Drosophila, implying a direct regulatory function of Dref on the retrotransposon activity and telomere elongation [92]. Therefore, apoptosis induction observed in pzg and Dref mutant tissues might involve excessive retrotransposon activity, destroying the fine-tuned genomic stability.

Supporting Information

S1 Fig. Quantification of eye sizes.

Eye size of flies was determined in five females of each combination shown in Fig 1. Average eye area is shown in each column. The ordinate shows the percentage of eye area relative to the respective control (left column each, light grey, 100%). Error bars denote standard deviation. ***p<0.001; **p<0.01; *p<0.05; ns: not significant according to Student's T-test.

https://doi.org/10.1371/journal.pone.0124652.s001

(DOC)

S2 Fig. Induction of pzg-RNAi with omb-Gal4 provokes cell death.

pzg-RNAi application in the most central part of the wing disc with omb-Gal4 induces rpr-lacZ (red in A, A', arrows), activated Drice (red in C, C', arrows), activated Caspase 3 (red in D, D', arrow), Dcp-1act (red in E, E', arrows) and puc-lacZ (red in F, F', arrows). In contrast, the level of the anti-apoptotic protein DIAP1 is reduced (red in B, B', repressive arrows). (A-A''') omb-Gal4; UAS-pzg-RNAi/+; rpr-lacZ/+, (B-E'') omb-Gal4; UAS-pzg-RNAi/+, (F-F'') omb-Gal4; UAS-pzg-RNAi/+; puc-lacZ/+. Anti-Putzig staining is shown in green. Posterior is right and dorsal up. The affected area is outlined. Scale bars: 100 μm.

https://doi.org/10.1371/journal.pone.0124652.s002

(DOC)

S3 Fig. Reduced H activity still induces apoptotic effects in pzg-RNAi mutant cells.

Reducing the activity of the N repressor Hairless (H) formally enhances N activity but does not rescue the apoptotic consequences observed in pzg-RNAi mutant cells. (A-B'') Autonomous induction of Dcp-1act (red in A, A', arrow) can be detected in wing discs app. 96 h AEL, whereas additional non autonomous Dcp-1act activity is provoked in later stages (B, B', open arrows). (C-D'') Cell cycle progression is still autonomously impeded in pzg-RNAi depleted cells (cells in S-phase marked with EdU-labeling red in C, C', repressive arrow and cells in M-phase depicted with anti-PH3, red in D, D' repressive arrow). Enhanced proliferation in cells directly abutting the posterior compartment is still observed (open arrows in C' and D'). Anti-H staining is shown in blue (A'', B'', C'', D'') depicting loss of H protein by induction of H-RNAi. Posterior is right and dorsal up. The dashed line assigns the A/P compartment boundary. Scale bars: 100 μm.

https://doi.org/10.1371/journal.pone.0124652.s003

(DOC)

S4 Fig. Potential DRE sites in Diap1.

According to flybase (R6.03; FB2014_06, released November 12th, 2014), there are 6 strongly supported transcripts Diap1 RA-RF, transcribed from 5 different promoters (http://flybase.org/cgi-bin/gbrowse2/dmel/?Search=1;name=FBgn0260635). Potential DRE sites are marked with arrows and listed below. Transcript RB starts only 363 bp downstream of RF; they may share the DRE sites. No DRE sites were found in the proximity of the RA/RE transcription start. Dref regulation of RC appears less likely due to sequence divergence and distance of DRE.

https://doi.org/10.1371/journal.pone.0124652.s004

(DOC)

Acknowledgments

We thank D. Bohmann, P. Léopold, M. Yamaguchi, the Bloomington and Vienna stock centers and the Developmental Studies Hybridoma Bank in Iowa for fly strains and antibodies. We thank Nicole Anderle for performing some of the apoptosis staining. We are indebted to A. Preiss for fruitful discussions and critically reading of the manuscript.

Author Contributions

Conceived and designed the experiments: ACN SJK. Performed the experiments: MZ SJK AS ACN. Analyzed the data: MZ SJK ACN. Contributed reagents/materials/analysis tools: MZ SJK AS ACN. Wrote the paper: ACN.

References

  1. 1. Thompson CB. Apoptosis in the pathogenesis and treatment of disease. Science 1995; 267: 1456–1462. pmid:7878464
  2. 2. Jacobson MD. Apoptosis: Bcl-2-related proteins get connected. Curr Biol. 1997; 7: R277–R281. pmid:9115386
  3. 3. Hanahan D, Weinberg RA. The hallmarks of cancer. Cell 2000; 100: 57–70. pmid:10647931
  4. 4. McIlwain DR, Berger T, Mak TW. Caspase functions in cell death and disease. Cold Spring Harb Perspect Biol. 2013; 5:a008656. pmid:23545416
  5. 5. Kumar S. Caspase function in programmed cell death. Cell Death Differ. 2007; 14: 32–43. pmid:17082813
  6. 6. Berthelet J, Dubrez L. Regulation of apoptosis by inhibitors of apoptosis (IAPs). Cells 2013; 2: 163–187. pmid:24709650
  7. 7. 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. pmid:8171319
  8. 8. Grether ME, Abrams JM, Aqapite J, White K, Steller H. The head involution defective gene of Drosophila melanogaster functions in programmed cell death. Genes Dev. 1995; 9: 1694–1708. pmid:7622034
  9. 9. Chen P, Nordstrom W, Gish B, Abrams JM. grim, a novel cell death gene in Drosophila. Genes Dev. 1996; 10: 1773–1782. pmid:8698237
  10. 10. Wang SL, Hawkins CJ, Yoo SJ, Müller HA, Hay BA. The Drosophila caspase DIAP1 is essential for cell survival and is negatively regulated by HID. Cell 1999; 98: 453–463. pmid:10481910
  11. 11. Goyal L, McCall K, Agapite J, Hartwieg E, Steller H. Induction of apoptosis by Drosophila reaper, hid and grim through inhibition of IAP function. EMBO J. 2000; 19: 589–597. pmid:10675328
  12. 12. Ryoo HD, Bergmann A, Gonen H, Ciechanover A, Steller H. Regulation of Drosophila IAP1 degradation and apoptosis by reaper and ubcD1. Nature Cell Biol. 2002; 4: 432–438. pmid:12021769
  13. 13. 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. Nature Cell Biol. 2002; 6: 416–424. pmid:12021767
  14. 14. Fan Y, Bergmann A. Apoptosis-induced compensatory proliferation. The cell is dead: Long live the cell! Trends Cell Biol. 2008; 18: 467–473. pmid:18774295
  15. 15. Bergmann A, Steller H. Apoptosis, stem cells, and tissue regeneration. Sci Signal 2010; 3(145):re8. pmid:20978240
  16. 16. Morata G, Shlevkov E, Pérez-Garijo A. Mitogenic signaling from apoptotic cells in Drosophila. Develop Growth Differ. 2011; 53: 168–176. pmid:21338343
  17. 17. Ryoo HD, Bergmann A. The role of apoptosis-induced proliferation for regeneration and cancer. Cold Spring Harb Perspect Biol. 2012; 4: a008797. pmid:22855725
  18. 18. Pérez-Garijo A, Martín FA, Morata G. Caspase inhibition during apoptosis causes abnormal signalling and developmental aberrations in Drosophila. Development 2004; 131: 5591–5598. pmid:15496444
  19. 19. Ryoo HD, Gorenc T, Steller H. Apoptotic cells can induce compensatory proliferation through the JNK and the Wingless signaling pathways. Dev Cell 2004; 7: 491–501. pmid:15469838
  20. 20. Vidal M, Cagan RL. Drosophila models for cancer research. Curr Opin Genet Dev. 2006; 16: 10–16. pmid:16359857
  21. 21. Fan Y, Wang S, Hernandez J, Betul Venigun V, Hertlein G, Fogarty CE et al. Genetic models of apoptosis-induced proliferation decipher activation of JNK and identify a requirement of EGFR signaling for tissue regenerative responses in Drosophila. PLOS Genet. 2014; 10: e1004131. pmid:24497843
  22. 22. Lawrence P, Struhl G. Morphogens, compartments, and pattern: lessons from Drosophila? Cell 1996; 85: 951–961. pmid:8674123
  23. 23. Tabata T, Takei Y. Morphogens, their identification and regulation. Development 2004; 131: 703–712. pmid:14757636
  24. 24. Clem RJ, Fechheimer M, Miller MK. Prevention of apoptosis by a baculovirus gene during infection of insect cells. Science 1991; 254: 1388–1390. pmid:1962198
  25. 25. Kondo S, Senoo-Matsuda N, Hiromi Y, Miura M. DRONC coordinates cell death and compensatory proliferation. Mol Cell Biol. 2006; 26: 7258–7268. pmid:16980627
  26. 26. 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. pmid:15268856
  27. 27. Pérez-Garijo A, Shlevkov E, Morata G. The role of Dpp and Wg in compensatory proliferation and in the formation of hyperplastic overgrowths caused by apoptotic cells in the Drosophila wing disc. Development 2009; 136: 1169–1177. pmid:19244279
  28. 28. Bergantiños C, Corominas M, Serras F. Cell death-induced regeneration in wing discs requires JNK signaling. Development 2010; 137: 1169–1179. pmid:20215351
  29. 29. Worley MI, Setiawan L, Hariharan IK. Regeneration and transdetermination in Drosophila imaginal discs. Annu Rev Genet. 2012; 46: 289–310. pmid:22934642
  30. 30. Bosch M, Serras F, Martin-Blanco F, Baguna J. JNK signaling pathway required for wound healing in regenerating Drosophila wing imaginal discs. Dev Biol. 2005; 280: 73–86. pmid:15766749
  31. 31. Mattila J, Omelyanchuck L, Kyttala S, Turunen H, Nokkala S. Role of Jun N-terminal kinase (JNK) signaling in the wound healing and regeneration of a Drosophila melanogaster wing imaginal disc. Int J Dev Biol. 2005; 49: 391–399. pmid:15968584
  32. 32. Iagki T, Pagliarini RA, Xu T. Loss of cell polarity drives tumor growth and invasion through JNK activation in Drosophila. Curr Biol. 2006; 16: 1139–1146. pmid:16753569
  33. 33. You H, Padmashali RM, Ranganathan A, Lei P, Girnius N, Davis RJ et al. JNK regulates compliance-induced adherens junctions formation in epithelial cells and tissues. J Cell Sci. 2013; 126: 2718–2729. pmid:23591817
  34. 34. Pérez-Garijo A, Fuchs Y, Steller H. Apoptotic cells can induce non-autonomous apoptosis through the TNF pathway. eLife 2013; 2: e01004. pmid:24066226
  35. 35. Morata G, Herrera SC. Eiger triggers death from afar. eLife 2013; 2:e01388. pmid:24069529
  36. 36. Bray SJ. Notch signaling: a simple pathway becomes complex. Nat Rev Mol Cell Biol. 2006; 7: 678–689. pmid:16921404
  37. 37. Matsukage A, Hirose F, Yoo MA, Yamaguchi M. The DRE/DREF transcriptional regulatory system: a master key for cell proliferation. Biochim Biophys Acta 2008; 1779: 81–90. pmid:18155677
  38. 38. Djiane A, Krejci A, Bernard F, Fexova S, Millen K, Bray SJ. Dissecting the mechanisms of Notch induced hyperplasia. EMBO J. 2013; 32: 60–71. pmid:23232763
  39. 39. Dominguez M. Oncogenic programmes and Notch activity: An 'organized crime'? Semin Cell Dev Biol. 2014; 28: 78–85. pmid:24780858
  40. 40. Kugler SJ, Nagel AC. putzig is required for cell proliferation and regulates Notch activity in Drosophila. Mol Biol Cell. 2007; 18: 3733–3740. pmid:17634285
  41. 41. Kugler SJ, Nagel AC. A novel Pzg-NURF complex regulates Notch target gene activity. Mol Biol Cell. 2010; 21: 3443–3448. pmid:20685964
  42. 42. Neufeld TP, Edgar BA. Connections between growth and the cell cycle. Curr Opin Cell Biol. 1998; 10: 784–790. pmid:9914170
  43. 43. Hay BA, Maile R, Rubin GM. P element insertion-dependent gene activation in the Drosophila eye. Proc Natl Acad Sci USA 1997; 10: 5195–5200. pmid:9144214
  44. 44. White K, Tahaoglu E, Steller H. Cell killing by the Drosophila gene reaper. Science 1996; 271: 805–807. pmid:8628996
  45. 45. Lecuit T, Brook WJ, Ng M, Celleja M, Sun H, Cohen SM. Two distinct mechanisms for long-range patterning by Decapentaplegic in the Drosophila wing. Nature 1996; 381: 387–393. pmid:8632795
  46. 46. Yoshida H, Kwon E, Hirose F, Otsuki K, Yamada M, Yamaguchi M. DREF is required for EGFR signaling during Drosophila wing development. Genes Cells 2004; 9: 935–944. pmid:15461664
  47. 47. Nagel AC, Krejci A, Tenin G, Bravo-Patiño A, Bray S, Maier D et al. Hairless-mediated repression of Notch target genes requires the combined activity of Groucho and CtBP corepressors. Mol Cell Biol. 2005; 25: 10433–10441. pmid:16287856
  48. 48. Blackman RK, Sanicola M, Raftery LA, Gillevet T, Gelbart WM. An extensive 3' cis-regulatory region directs the imaginal disk expression of decapentaplegic, a member of the TGF-beta family in Drosophila. Development 1991; 111: 657–666. pmid:1908769
  49. 49. Ring JM, Martinez-Arias A. puckered, a gene involved in position-specific cell differentiation in the dorsal epidermis of the Drosophila larva. Dev Suppl. 1993: 251–259. pmid:8049480
  50. 50. Sogame N, Kim M, Abrams JM. Drosophila p53 preserves genomic stability by regulating cell death. Proc Natl Acad Sci USA 2003; 100: 4696–4701. pmid:12672954
  51. 51. Müller D, Kugler SJ, Preiss A, Maier D, Nagel AC. Genetic modifier screens on Hairless gain-of-function phenotypes reveal genes involved in cell differentiation, cell growth and apoptosis in Drosophila melanogaster. Genetics 2005; 171: 1137–1152. pmid:16118195
  52. 52. Maier D, Nagel AC, Preiss A. Two isoforms of the Notch antagonist Hairless are produced by differential translation initiation. Proc Natl Acad Sci USA 2002; 99: 15480–15485. pmid:12422020
  53. 53. Colombani J, Andersen DS, Léopold P. Secreted peptide Dilp8 coordinates Drosophila tissue growth with developmental timing. Science 2012; 336: 582–585. pmid:22556251
  54. 54. McCall K, Steller H. Facing death in the fly: genetic analysis of apoptosis in Drosophila. Trends Genet. 1997; 13: 222–226. pmid:9196327
  55. 55. Bergmann A, Agapite J, McCall K, Steller H. The Drosophila gene hid is a direct molecular target of Ras-dependent survival signaling. Cell 1998; 95: 331–341. pmid:9814704
  56. 56. Abrams JM. An emerging blueprint for apoptosis in Drosophila. Trends Cell Biol. 1999; 9: 435–440. pmid:10511707
  57. 57. Martin SJ. Destabilizing influences in apoptosis: sowing the seeds of IAP destruction. Cell 2002; 109: 793–796. pmid:12110175
  58. 58. Richardson H, Kumar S. Death to flies: Drosophila as a model system to study programmed cell death. J Immunol Methods 2002; 265: 21–38. pmid:12072176
  59. 59. Milán M, Campuzano S, Garcia-Bellido A. Developmental parameters of cell death in the wing disc of Drosophila. Proc Natl Acad Sci USA 1997; 94: 5691–5696. pmid:9159134
  60. 60. Dhanasekaran DN, Reddy EP. JNK signaling in apoptosis. Oncogene 2008; 27: 6245–6251. pmid:18931691
  61. 61. Igaki T. Correcting developmental errors by apoptosis: lessons from Drosophila JNK signaling. Apoptosis 2009; 14: 1021–1028. pmid:19466550
  62. 62. Martin-Blanco E, Gampel A, Ring J, Virdee K, Kirov N, Tolkovsky AM et al. puckered encodes a phosphatase that mediates a feedback loop regulating JNK activity during dorsal closure in Drosophila. Genes Dev. 1998; 12: 557–570. pmid:9472024
  63. 63. Garelli A, Gontijo AM, Miguela V, Caparros E, Dominguez M. Imaginal discs secrete insulin-like peptide 8 to mediate plasticity of growth and maturation. Science 2012; 336: 579–582. pmid:22556250
  64. 64. Smith-Bolton RK, Worley MI, Kanda H, Hariharan IK. Regenerative growth in Drosophila imaginal discs is regulated by Wingless and Myc. Dev Cell 2009; 16: 797–809. pmid:19531351
  65. 65. Herrera SC, Martin R, Morata G. Tissue homeostasis in the wing disc of Drosophila melanogaster: immediate response to massive damage during development. PLOS Genet. 2013; 9: e1003446. pmid:23633961
  66. 66. Kashio S, Obata F, Miura M. Interplay of cell proliferation and cell death in Drosophila tissue regeneration. Develop Growth Differ. 2014; 56: 368–375. pmid:24819984
  67. 67. Brancolini C, Lazarevic D, Rodriguez J, Schneider C. 2005 Dismanteling cell-cell contacts during apoptosis is coupled to a caspase-dependent proteolytic cleavage of beta-catenin. J Cell Biol. 2005; 139: 759–771.
  68. 68. Kessler T, Müller AJ. Cleavage of Armadillo/beta catenin by the Caspase DrICE in Drosophila apoptotic epithelial cells. BMC Dev Biol. 2009; 9: 15. pmid:19232093
  69. 69. Martín F, Peréz-Garijo A, Morata G. Apoptosis in Drosophila: compensatory proliferation and undead cells. Int J Dev Biol. 2009; 53: 1341–1347. pmid:19247932
  70. 70. Mollereau B, Pérez-Garijo A, Bergmann A, Miura N, Gerlitz O, Ryoo HD et al. Compensatory proliferation and apoptosis-induced proliferation: a need for clarification. Cell Death Differ. 2013; 20: 181. pmid:22722336
  71. 71. Hay BA, Wolff T, Rubin GM. Expression of baculovirus p35 prevents cell death in Drosophila. Development 1994; 120: 2121–2129. pmid:7925015
  72. 72. Protzer CE, Wech I, Nagel AC. Hairless induces cell death by downregulation of EGFR signaling activity. J Cell Sci. 2008; 121: 3167–3176. pmid:18765565
  73. 73. Djiane A, Zaessinger S, Babaoglan AB, Bray SJ. Notch inhibits yorkie activity in Drosophila wing discs. PLOS One 2014; 9: e106211. pmid:25157415
  74. 74. Hochheimer A, Zhou S, Zheng S, Holmes MC, Tjian R. TRF2 associates with DREF and directs promoter-selective gene expression in Drosophila. Nature 2002; 420: 439–445. pmid:12459787
  75. 75. Iyer EPR, Iyer SC, Sullivan L, Wang D, Meduri R, Graybeal LL et al. Functional genomic analyses of two morphologically distinct classes of Drosophila sensory neurons: Post-mitotic roles of transcription factors in dendritic patterning. PLOS One 2013; 8: e72434. pmid:23977298
  76. 76. Kugler SJ, Gehring E-M, Wallkamm V, Krüger V, Nagel AC. The Putzig-NURF nucleosome remodeling complex is required for ecdysone receptor signaling and innate immunity in Drosophila melanogaster. Genetics 2011; 188: 127–139. pmid:21385730
  77. 77. Hirose F, Ohshima N, Shiraki M, Inoue YH, Taguchi O, Nishi Y et al. Ectopic expression of DREF induces DNA synthesis, apoptosis, and unusual morphogenesis in the Drosophila eye imaginal disc: possible interaction with Polycomb and trithorax group proteins. Mol Cell Biol 2001; 21: 7231–7242. pmid:11585906
  78. 78. Hyun J, Jasper H, Bohmann D. DREF is required for efficient growth and cell cycle progression in Drosophila imaginal discs. Mol Cell Biol. 2005; 25: 5590–5598. pmid:15964814
  79. 79. Ryu JR, Choi TY, Kwon EJ, Lee WH, Nishida Y, Hayashi Y et al. Transcriptional regulation of the Drosophila raf proto-oncogene by the DNA replication-related element (DRE)/DRE-binding factor (DREF) system. Nucleic Acids Res. 1997; 25: 794–799. pmid:9016631
  80. 80. Kurada P, White K. Ras promotes cell survival in Drosophila by downregulating hid expression. Cell 1998; 95: 319–329. pmid:9814703
  81. 81. Trong-Tue N, Thao DT, Yamaguchi M. Role of DREF in transcriptional regulation of the Drosophila p53 gene. Oncogene 2010; 29: 2060–9. pmid:20101238
  82. 82. Fujiwara S, Ida H, Yoshioka Y, Yoshida H, Yamaguchi M. The warts gene as a novel target of the Drosophila DRE/DREF transcription pathway. Am J Cancer Res. 2012; 2: 36–44. pmid:22206044
  83. 83. Vo N, Horii T, Yanai H, Yoshida H, Yamaguchi M. The Hippo pathway as a target of the Drosophila DRE/DREF transcriptional regulatory pathway. Sci Rep. 2014; 4:7196. pmid:25424907
  84. 84. Ohler U, Liao G-C, Niemann H, Rubin GM. Computational analysis of core promoters in the Drosophila genome. Genome Biology 2002; 3 (12): research0087.1–0087.12.
  85. 85. Andreyeva EN, Belyaeva ES, Semeshin VF, Pokholkova GV, Zhimulev IF. Three distinct chromatin domains in telomere ends of polytene chromosomes in Drosophila melanogaster Tel mutants. J Cell Sci. 2005; 118: 5465–5477. pmid:16278293
  86. 86. Stewart SA, Weinberg RA. Telomeres: cancer to human aging. Ann Rev Cell Dev Biol. 2006; 22: 531–557.
  87. 87. Mason JM, Frydrychova RC, Biessmann H. Drosophila telomeres: an exception providing new insights. BioEssays 2008; 30: 25–37. pmid:18081009
  88. 88. Ahmad K, Golic KG. Telomere loss in somatic cells of Drosophila causes cell cycle arrest and apoptosis. Genetics 1999; 151: 1041–1051. pmid:10049921
  89. 89. Titen SWA, Golic KG. Telomere loss provokes multiple pathways to apoptosis and produces genomic instability in Drosophila melanogaster. Genetics 2008; 180: 1821–1832. pmid:18845846
  90. 90. Nigg EA. Genome instability in cancer development In: Advances in experimental Medicine and Biology. Springer, New York. 2005
  91. 91. Silva-Sousa R, López-Panadès E, Piñeyro D, Casacuberta E. The chromosomal proteins JIL-1 and Z4/Putzig regulate the telomeric chromatin in Drosophila melanogaster. PLOS Genet. 2012; 8: e1003153. pmid:23271984
  92. 92. Silva-Sousa R, Varela MD, Casacuberta E. The Putzig partners Dref, Trf2 and Ken are involved in the regulation of the Drosophila telomere retrotransposons, Het-A and TART. Mob DNA 2013; 4: 18 pmid:23822164