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Logo of hhmipaAbout Author manuscriptsSubmit a manuscriptHHMI Howard Hughes Medical Institute; Author Manuscript; Accepted for publication in peer reviewed journal
Dev Biol. Author manuscript; available in PMC Aug 1, 2011.
Published in final edited form as:
Dev Biol. Feb 1, 2011; 350(1): 139–151.
Published online Dec 9, 2010. doi:  10.1016/j.ydbio.2010.11.036
PMCID: PMC3038240
HHMIMSID: HHMIMS267702

Regulation of Hippo signaling by Jun kinase signaling during compensatory cell proliferation and regeneration, and in neoplastic tumors

Abstract

When cells undergo apoptosis, they can stimulate the proliferation of nearby cells, a process referred to as compensatory cell proliferation. The stimulation of proliferation in response to tissue damage or removal is also central to epimorphic regeneration. The Hippo signaling pathway has emerged as an important regulator of growth during normal development and oncogenesis from Drosophila to humans. Here we show that induction of apoptosis in the Drosophila wing imaginal disc stimulates activation of the Hippo pathway transcription factor Yorkie in surviving and nearby cells, and that Yorkie is required for the ability of the wing to regenerate after genetic ablation of the wing primordia. Induction of apoptosis activates Yorkie through the Jun kinase pathway, and direct activation of Jun kinase signaling also promotes Yorkie activation in the wing disc. We also show that depletion of neoplastic tumor suppressor genes, including lethal giant larvae and discs large, or activation of aPKC, activates Yorkie through Jun kinase signaling, and that Jun kinase activation is necessary, but not sufficient, for the disruption of apical-basal polarity associated with loss of lethal giant larvae. Our observations identify Jnk signaling as a modulator of Hippo pathway activity in wing imaginal discs, and implicate Yorkie activation in compensatory cell proliferation and disc regeneration.

Keywords: Hippo, Yorkie, growth, regeneration, Jun kinase

INTRODUCTION

When cells in a tissue are damaged, proliferation of neighboring cells can be induced, enabling tissue repair. This phenomena is central to epimorphic regeneration, which enables the regrowth and replacement of body parts after injury or amputation. The capacity of tissues to undergo epimorphic regeneration has been known for centuries and exists throughout the metazoa, although it varies between organisms, organs, and developmental stages. An important insight into epimorphic regeneration was provided by the observation that when cells initiate apoptosis, they produce mitogenic signals, thereby stimulating the proliferation of neighboring cells. This process, termed compensatory cell proliferation, was first characterized in the developing imaginal discs of Drosophila, but similar phenomena occur in other systems (reviewed in Bergantinos et al., 2010b; Fan and Bergmann, 2008).

Compensatory cell proliferation has been observed in Drosophila imaginal discs upon induction of cell death by X-irradiation, by expression of pro-apoptotic genes, or by mutation of the anti-apoptotic gene thread (Diap1) (Haynie and Bryant, 1977; Huh et al., 2004; Perez-Garijo et al., 2004; Ryoo et al., 2004). Compensatory cell proliferation is associated with the induction of signaling molecules that have been linked to the promotion of cell proliferation, including Wingless (Wg) and Decapentaplegic (Dpp) (Huh et al., 2004; Perez-Garijo et al., 2004; Ryoo et al., 2004). Another common and essential feature of compensatory cell proliferation is the activation of Jun-kinase (Jnk) signaling (Fan and Bergmann, 2008; Perez-Garijo et al., 2009; Ryoo et al., 2004). Jnk signaling is a MAPK signaling pathway regulated by diverse cellular stresses, including irradiation, reactive oxygen species, infection, aging, disruption of cell polarity, cytoskeletal changes, and induction of apoptosis (reviewed in Bogoyevitch et al., 2010; Igaki, 2009; Karin and Gallagher, 2005). Jnk signaling has distinct outcomes in different contexts. It is crucial for morphogenesis during embryogenesis and wound healing (Martin and Parkhurst, 2004), and has an important pro-apoptotic function (Igaki, 2009; Kanda and Miura, 2004). However, when apoptosis is blocked, Jnk signaling can promote cell proliferation (Hariharan and Bilder, 2006; Igaki et al., 2006; McEwen and Peifer, 2005; Ryoo et al., 2004).

In addition to its role in compensatory cell proliferation, Jnk signaling has been linked to proliferative and metastatic features of tumors associated with disruptions of apical-basal polarity in epithelial cells (Igaki et al., 2006; Uhlirova and Bohmann, 2006). Genes that, when mutated, result in over-proliferation coupled to loss of normal tissue architecture are classified in Drosophila as neoplastic tumor suppressors (reviewed in Hariharan and Bilder, 2006). Three of the best studied neoplastic tumor suppressors, lethal giant larvae (lgl), discs large (dlg) and scribbled (scrib), form a junctional complex that contributes to apical-basal polarity in epithelial cells (Bilder et al., 2000). Their effects on growth are complex. When an entire disc is mutant for one of these genes, it can overgrow and form a tumorous mass of unpolarized cells (Agrawal et al., 1995; Bilder et al., 2000). However, when clones of cells mutant for these genes are induced by mitotic recombination in wing discs, they generally fail to survive, and are eliminated by Jnk-dependent apoptosis. But if combined with other oncogenic mutations, such as expression of Myc or activated-Ras or Notch, clones of cells mutant for lgl, dlg or scrib can survive and form large tumors that are prone to metastasis; the growth and metastasis of these tumors also depends on Jnk signaling (Brumby and Richardson, 2003; Froldi et al., 2010; Igaki et al., 2006; Igaki et al., 2009; Pagliarini and Xu, 2003).

The Hippo pathway controls growth during normal development, and its dysregulation is associated with oncogenesis (reviewed in Reddy and Irvine, 2008; Zhao et al., 2010). Hippo signaling is mediated by a transcriptional co-activator protein, Yorkie (Yki) (reviewed in Oh and Irvine, 2010). When Hippo signaling is active, Yki is kept inactive, retained in the cytoplasm through the action of upstream tumor suppressor genes in the Hippo pathway. The key, direct repressor of Yki activity is the kinase Warts (Wts), which phosphorylates Yki (Huang et al., 2005). In the absence of Wts, unphosphorylated Yki accumulates in the nucleus (Dong et al., 2007; Oh and Irvine, 2008), and in conjunction with DNA-binding proteins, regulates the transcription of downstream genes. Recently, mutation of lgl, or activation of aPKC, were reported to result in activation of Yki, and Yki was functionally linked to over-proliferation phenotypes in these genotypes (Grzeschik et al., 2010; Menéndez et al., 2010). The mechanism by which Yki becomes activated by these manipulations is not known, although it was suggested that it might involve mis-localization of Hippo and a Hippo-interacting protein, dRassf.

Here, we characterize the regulation and role of the Hippo pathway in compensatory cell proliferation and regeneration. Damage to the epithelial cells of the Drosophila wing imaginal disc by expression of pro-apoptotic genes results in activation of Yki. This Yki activation is mediated by the Jnk signaling pathway. We further determined that disruption of apical-basal polarity by depletion of neoplastic tumor suppressor genes, or activation of aPKC, activates Yki through Jnk signaling, and that Yki is required for wing disc regeneration after genetic ablation of the wing primordia. Our results identify Jnk signaling as a mechanism for regulating Hippo pathway activity in wing imaginal discs, and establish a fundamental role for Hippo signaling in regenerative responses to tissue damage.

MATERIALS AND METHODS

Drosophila genetics

Stocks used included ex-lacZ en-Gal4 UAS-GFP/CyO; UAS-dcr2/TM6b, UAS-lgl-RNAi (VDRC 51249), UAS-dlg-RNAi (VDRC 41136), UAS-bsk-RNAi (VDRC 104569), UAS-yki-RNAi (VDRC 104532), UAS-wg-RNAi (VDRC 104579), rn-Gal4 UAS-egr tub-Gal80ts/TM6b,Gal80 (Smith-Bolton et al., 2009), rn-Gal4 UAS-rpr tub-Gal80ts/TM6b,Gal80 (Smith-Bolton et al., 2009), puc-lacZ[A251.1F3] ry/TM3 (Bloomington 11173), UAS-myc:wts.2, FRT42D ykiB5/CyO,Act-GFP, UAS-puc, salPE-Gal4 UAS-GFP/CyO; Dronc129 FRT2A/TM6b (Perez-Garijo et al., 2009), UAS-hep.CA/CyO; Dronc129 FRT2A/TM6b (Perez-Garijo et al., 2009), UAS-wts:myc[2–2](gift of Tian Xu), UAS-GFP[T-2](Bloomington 1521), UAS-aPKC:CAAX (Lee et al., 2006), UAS-yki:V5 (Oh and Irvine, 2009), tub-Gal80ts/TM6B, tub>CD2>Gal4 UAS-CD8:GFP/CyO; tub-Gal80ts/TM6B (Buttitta et al., 2007), UAS-hep.CA (Bloomington 6406), UAS-rpr[14] (Bloomington 5824), rn-lacZ89, ry+, ry506/TM3, Sb (St Pierre et al., 2002), UAS-egr (Moreno et al., 2002), UAS-lacZ (Brand and Perrimon, 1993). The specificity of lgl RNAi has been described previously (Grzeschik et al., 2010). The specificity yki RNAi was confirmed by rescue using a UAS-yki line to over-express Yki. Because we lacked a direct test for bsk RNAi specificity, all experiments with bsk RNAi were repeated using UAS-puc, which gave similar results.

Larvae from crosses of rn-Gal4 UAS-egr tub-Gal80ts/TM6b,Gal80 or rn-Gal4 UAS-rpr tub-Gal80ts/TM6b,Gal80 were kept at 18°C for 8 days and shifted to 30°C for 30h to induce cell death. After cell death induction, larvae were either dissected or put back to 18°C for 24h, 48h, 72h recovery. To make rpr or hep.CA clones, y w hs-flp; UAS-rpr/CyO,GFP or y w hs-flp; UAS-hep.CA/CyO,GFP flies were crossed to tub>CD2>Gal4 UAS-CD8:GFP/CyO; tub-Gal80ts/TM6B. Larvae were maintained at 25°C for 3 days and clones were induced by heat shock at 38°C for 10 min. Larvae were then kept at 18°C for 3 days to let clones grow larger, and then cell death was induced by temperature shift to 30°C for 12–14h.

To evaluate the effect of ykiB5/+ on development timing, rn-Gal4 UAS-egr tub-Gal80ts/TM6b,Gal80 females were crossed to Oregon-R or FRT42D ykiB5/CyO,Act-GFP males. Eggs were collected at 25°C in 8h intervals on grape juice plates covered with yeast, and subsequently kept at 18°C. The dates of larvae hatching and pupa formation were recorded. For adult wings, larvae were maintained at 18°C after tissue damage until eclosion. Wings were mounted in Gary's Magic Mountant, and measured using NIH ImageJ software.

Immunofluorescent staining

Wing discs of third instar larvae were fixed in 4% paraformaldehyde and stained as described previously. Primary antibodies used include rabbit anti-Yki (1:400), mouse anti-Wg (1:800, 4D4, Developmental Studies Hybridoma Bank (DSHB), mouse anti-DLG (1:400, 4F3, DSHB), rat anti-Fat (1:800) (Feng and Irvine, 2009), goat anti-β-gal (1:1000, Biogenesis), rabbit anti-GFP (1:400, Molecular Probes), rat anti-DE-cadherin (1:450, DCAD2, DSHB), rabbit anti-active Caspase 3 (1:200, Asp175, Cell Signaling Technology), rabbit anti-phospho-JNK (1:100), mouse anti-Myc (1:400, 9E10,Babco), mouse anti-Nubbin (1:100, gift from Steve Cohen). EdU labelling was performed using Click-it™ EdU Alexa Fluor Imaging Kit (Molecular Probes). Images were captured on a Leica TCS SP5 confocal microscope.

RESULTS

Activation of Yki adjacent to apoptotic cells

To investigate the potential involvement of Hippo signaling in compensatory cell proliferation and regeneration, we examined the sub-cellular localization of Yki in wing imaginal discs after localized induction of cell death. In one approach, we adopted a system developed by Smith-Bolton et al (2009) for analysis of regenerative growth in imaginal discs, which involves expressing pro-apoptotic genes throughout the wing primordia of the developing disc under the control of rotund (rn-Gal), and then controlling the timing of expression using a temperature-sensitive repressor of Gal4 (Gal80ts). The pro-apoptotic genes expressed were reaper (rpr), an inhibitor of the Drosophila apoptosis inhibitor Diap1 (Goyal et al., 2000; Wang et al., 1999; White et al., 1994), or eiger (egr), a Drosophila TNFα that is a ligand for the Jnk pathway (Igaki et al., 2002; Kanda and Miura, 2004; Moreno et al., 2002).

Normally, Yki is predominantly cytoplasmic within imaginal disc cells (Fig 1A). However, if Hippo signaling is impaired, then Yki can be detected in the nucleus (Dong et al., 2007; Oh and Irvine, 2008). When rpr or egr were expressed for 30 h under rn-Gal4 control, most of the wing primordia was ablated, although a small, irregular region of rn-Gal4 expression persists, which includes both dying cells in which relatively stable marker proteins (ß-galactosidase or GFP) are still detectable, and some cells that appear viable (Fig. 1B,C and data not shown)(Smith-Bolton et al., 2009). Wing discs in which wing pouch cells have been ablated by expression of rpr or egr exhibit a striking re-localization of Yki to the nucleus, both in cells adjacent to the rn-Gal4 domain, as well as among surviving cells within the rn-Gal4 domain (Fig. 1B,C). This re-localization implies that ablation of cells and/or induction of apoptosis results in a strong, local activation of Yki. Consistent with this inference, a downstream target of Yki, expanded (ex) (Hamaratoglu et al., 2006), was upregulated within cells exhibiting nuclear Yki (Fig. 1D).

Fig. 1
Yki activation induced by expression of pro-apoptotic genes

To investigate whether the re-localization of Yki was effected through an influence on Hippo signaling, we took advantage of the observations that over-expression of wild-type Wts has relatively little effect on wing growth in wild-type flies, but can suppress the over-growth phenotypes associated with mutation of upstream tumor suppressors that activate Yki, including fat, ex, and dco3 (Feng and Irvine, 2007, 2009). Over-expression of Wts similarly reduced the influence of rpr and egr on Yki localization, as strong nuclear Yki was no longer detected (Fig 1E, F). This suggests that the influence of rpr and egr on Yki is mediated through an effect on Hippo signaling. Wts expression not only reduced the activation of Yki within the rn-Gal4 domain, but also in neighboring cells, which implies that the non-autonomous activation of Yki is dependent upon Yki activation within rpr or egr-expressing cells. This inhibition of nuclear Yki by Wts over-expression was not due to inhibition of cell death, because the wing primordia was still ablated in these animals, and nuclei that, based on their small size and basal location appeared apoptotic, were still detected (not shown).

Apoptosis activates Yki through the Jnk pathway

Jnk signaling is an important regulator of both cell death and compensatory cell proliferation (Igaki et al., 2002; Kanda and Miura, 2004; Moreno et al., 2002; Perez-Garijo et al., 2009; Ryoo et al., 2004). Egr activates Jnk signaling through its receptor Wengen (Kauppila et al., 2003), and inhibition of Jnk signaling suppresses Egr-overexpression phenotypes (Igaki et al., 2002; Moreno et al., 2002). We confirmed activation of Jnk signaling in rn-Gal4 UAS-egr discs by examining a downstream target of the Jnk pathway (puc-lacZ) (Figs 2A, S1A). Notably, Caspase activation can also result in Jnk activation (Igaki, 2009), and consistent with this we observed induction of puc-lacZ in rn-Gal4 UAS-rpr discs (Fig 2B). The observations that both Egr and Rpr induce Jnk pathway activation, together with its role in compensatory cell proliferation (McEwen and Peifer, 2005; Perez-Garijo et al., 2009; Ryoo et al., 2004), suggested that activation of Yki upon expressing pro-apoptotic genes might be due to Jnk activation. To test this hypothesis, we inhibited Jnk activation by expressing the Jnk phosphatase puckered (puc). Overexpression of Puc blocked Egr-induced apoptosis, and inhibited Yki activation (Figs 2C, S2B). Rpr-induced apoptosis isn't blocked by Jnk inactivation, but over-expression of Puc inhibits Rpr-induced compensatory cell proliferation (Ryoo et al., 2004). When puc was co-expressed with rpr, the developing wing pouch was still ablated (Fig 2D), and apoptotic nuclei were still detected (not shown), but Yki activation was dramatically reduced (Figs 2D, S2C). Although low levels of Yki activation (Figs 2C,S2B), or rare nuclei with nuclear Yki (Figs 2D, S2C), could be detected when Puc was over-expressed, this could reflect differences in the extent and pattern of activation versus inhibition of Jnk, and our experiments establish that Jnk activation makes a crucial contribution to the activation of Yki in and adjacent to Egr- and Rpr-expressing cells.

Fig. 2
Jnk activates Yki

Expression of Egr induces non-autonomous activation of Yki, but Egr is a secreted ligand for the Jnk pathway. To assess the consequences of autonomous pathway activation, we compared expression of an activated form of the Jnk kinase hemipterous (Hep.CA) (Adachi-Yamada et al., 1999), with expression of Egr. For these experiments we used an enhancer from the spalt gene (sal.PE-Gal4) (Fig. 2E), previously used to express hep.CA in studies of compensatory proliferation (Perez-Garijo et al., 2009). Most cells expressing hep.CA or egr undergo apoptosis (Figs 2F,G, S1B). Nonetheless, nuclear localization of Yki was detected within and adjacent to surviving cells (Figs 2F,G, S2D,F). When egr was expressed, a non-autonomous effect on Yki activation, in some cases extending for several cells, was clearly evident (Fig 2G). When hep.CA was expressed, the non-autonomous effect was more limited, but still detectable in some cells immediately adjacent to hep.CA-expressing cells (Figs 2F, S2F). Because analysis of egr- or hep.CA-expressing discs is complicated by cell death and tissue folding, we further confirmed the existence of non-autonomous Yki activation by hep.CA in vertical sections through the disc epithelia (Fig. 2F”,G”), and in clones of cells expressing hep.CA or rpr (Fig. S1D,E).

Activation of Jnk signaling has diverse effects, but one major consequence is activation of caspases and induction of apoptosis (Igaki, 2009). Induction of apoptosis, and more specifically activation of the initiator caspase Dronc, can induce compensatory proliferation in neighboring cells (Huh et al., 2004). To investigate whether caspase activation and apoptosis contribute to the activation of Yki by Jnk, we expressed hep.CA or egr in animals mutant for a hypomorphic allele of Dronc, Dronc129. It has been reported that when compensatory cell proliferation is induced by expression of pro-apoptotic genes, including rpr, hid, or hep.CA, while apoptosis is reduced by Dronc mutation, that overgrowth of wing tissue and induction of Wg and Dpp expression can still occur, even though caspase activation is reduced (Kondo et al., 2006; Perez-Garijo et al., 2009; Wells et al., 2006). In our experiments, staining for cleaved caspase 3 (Cas3) confirmed that the number of apoptotic cells upon Jnk activation was reduced in Dronc129 discs (Fig S1C). However, activation of Yki was not diminished (Figs 2H,I, S2E,G). In fact, many more cells with nuclear Yki were detected, both within and adjacent to the sal.PE expression domain, presumably because Dronc mutation inhibits the apoptosis of cells with activated Jnk. These observations indicate that Jnk signaling does not require Dronc activation or apoptosis in order to promote Yki activation.

We also took advantage of the decreased apoptosis in Dronc129 discs to further investigate the linkage between Jnk and Hpo signaling. Hep.CA-mediated Yki activation could be efficiently repressed either by over-expression of Wts, or by over-expression of Hpo (Figs 2J,K).

Regulation of Yki during wing disc regeneration

The observation that Yki is activated after tissue damage, together with its importance for growth control, suggests that Yki activation could contribute to wing disc regeneration. To examine Yki activity during regeneration, we monitored Yki localization during recovery after conditional induction of Egr. rn-expressing cells were ablated by expressing Egr under rn-Gal4 control for 30h, then Egr expression was removed by shift back to the non-permissive temperature for Gal80ts, and the discs were allowed to recover. After 24 h of recovery, strong Yki activation remained detectable (Fig. 3B). Substantial Yki activation was also detected 48 h after removal of Egr expression (Fig. 3C), but by 72 h Yki localization appeared almost normal (Fig. 3D). This Yki localization profile correlates with active proliferation and regeneration of the wing disc after genetic ablation of rn-Gal4 expressing cells (Smith-Bolton et al., 2009).

Fig 3
Yki activation during wing regeneration

Wg expression is also upregulated during regeneration, and Wg has been functionally linked to disc regeneration (Smith-Bolton et al., 2009). As both Wg and Yki can be associated with promotion of cell proliferation, we explored the relationship between them during regeneration. Activated Yki and upregulated Wg were mostly overlapping in discs damaged by Rpr or Egr expression (Fig 3A and data not shown). However, during regeneration, Wg expression appeared to recover more quickly than Yki localization, as Wg expression was almost normal by 48 h, at which time significant nuclear Yki was still detected (Fig. 3C). This difference suggests that Yki and Wg are regulated independently. Nonetheless, since activated Yki and upregulated Wg initially overlapped (Fig. 3A), and inactivation of the Hippo pathway can normally upregulate Wg in the proximal wing disc (Cho et al., 2006; Cho and Irvine, 2004), we investigated whether upregulated Wg in regenerative cells might be due to activation of Yki. However, neither reduction of Yki protein by RNAi, nor reduction of Yki activity by Wts over-expression, blocked Wg expression (Fig. 3F,G). We also observed that wg RNAi substantially decreased Wg expression, but didn't visibly decrease nuclear Yki (Fig. 3E). These observations imply that Yki activation and Wg expression are induced in parallel by activation of Jnk during regeneration, rather than being dependent upon one another.

Yki is required for wing regeneration after tissue damage

The above results identify a correlation between Yki activation and epimorphic regeneration of the wing disc after elimination of cells by induction of apoptosis. Hippo signaling is very sensitive to the dose of Yki (Bennett and Harvey, 2006; Silva et al., 2006; Willecke et al., 2006). Thus, to determine whether the activation of Yki functionally contributes to regeneration after genetic ablation of the wing pouch, we induced a 30h pulse of Egr expression at mid-third instar in the rn-Gal4 domain, and then allowed animals to recover and develop into adult flies. Because there is some variability in the efficiency of regeneration in this type of experiment (Smith-Bolton et al., 2009), regenerated wings were measured and then classified according their percentage of normal wing size. Under our experimental conditions, when the wing primordia was ablated by expression of Egr, complete regeneration (to >80% wild-type wing size) was observed in 1/4 of animals, partial regeneration (20–80% normal wing size) was observed in 2/5 of animals, and minimal regeneration (<20% normal wing size) was observed in 1/3 of animals (Fig. 3H,J,K). When the same experiment was performed in animals heterozygous for yki, then none of the wings fully regenerated, and 90% of animals exhibited only minimal regeneration (Fig. 3H,M). Since heterozygosity for yki did not affect the rate of development or the extent of initial ablation (Fig. S3), this impairment of wing disc regeneration indicates that the elevated activity of Yki observed after induction of tissue damage is functionally required for efficient regeneration. Co-expression of Wts had even stronger effects, as 100% of the animals failed to exhibit significant regeneration (Fig. 3H).

Activation of Yki in neoplastic tumors

The results described above establish that Jnk signaling can influence Yki activity, and identify a role for this activation of Yki in wing regeneration after tissue damage. The loss of normal epithelial architecture associated with mutation of members of the neoplastic Lgl-Dlg-Scrib complex constitutes another type of epithelial cell damage. Indeed, Jnk signaling is activated in neoplastic tumor suppressor clones (Brumby and Richardson, 2003; Igaki et al., 2006; Uhlirova and Bohmann, 2006). Thus, we investigated the relationship between activation of Jnk, activation of Yki, and tumorous overgrowths associated with cells in which the Lgl-Dlg-Scrib complex was disrupted. To investigate this in a system that would facilitate genetic manipulations, and to avoid the necessity of combining lgl loss-of-function with oncogenic mutations that could complicate the analysis, we depleted lgl from large, contiguous domains of cells using RNAi by expressing UAS-hairpin transgenes under the control of broadly expressed Gal4 lines, and in conjunction with UAS-dcr2, which enhances the effectiveness of RNAi (Dietzl et al., 2007).

Depletion of lgl in the posterior half of the wing disc, under en-Gal4 control, resulted in overgrowth of the posterior compartment (Figs 4A,C,E) and loss of apical-basal polarity (5C,D,I,J). Thus, in this situation lgl RNAi phenocopied lgl mutant discs, or lgl mutant clones co-expressed with oncogenes (Brumby and Richardson, 2003; Igaki et al., 2006; Uhlirova and Bohmann, 2006). The overgrowth was apparent from the increased size of the posterior compartment, which often became multilayered, distorted and folded (Fig 5C,D,I,J), and was also reflected in elevated cell proliferation detected by EdU labeling (Fig 4E). The loss of polarity was apparent from the rounded morphology and multilayered appearance of cells (Fig. 5D,J). In addition, proteins that normally have discrete localizations within the cells, such as Fat or Dlg, instead became mis-localized such that they were equally distributed around the plasma membrane (Fig. 5C,D,I,J).

Fig 4
Yki activation by depletion of neoplastic tumor suppressors
Fig 5
Jnk is required for disruption of polarity upon lgl depletion

To investigate whether the overgrowth of these lgl-depleted cells was related to effects on Hippo signaling, we examined the sub-cellular distribution of Yki. A strong re-localization of Yki, from predominantly cytoplasmic to predominantly nuclear, was evident in posterior cells (Fig. 4A,C). A similar effect on Yki localization was induced by RNAi for dlg (Fig 4B), which indicates that this influence on Hippo signaling is not specific to lgl, but rather appears to be a property of the Lgl-Dlg-Scrib complex. To confirm that the relocalization of Yki to the nucleus is associated with Yki activation, we examined ex-lacZ; it was strongly upregulated in posterior cells of en-Gal4 UAS-RNAi-lgl wing discs (Fig 4C).

To establish the relevance of this Yki activation to the tumorous overgrowth of lgl-depleted cells, we examined the consequences of simultaneous knockdown of both yki and lgl. Co-overexpression of yki-RNAi with lgl-RNAi blocked the overgrowth of the posterior compartment, and yielded discs in which the P compartment was reduced in size (Fig 4G), and posterior nuclei appeared apoptotic (not shown), which indicates that Yki is required for the growth and survival of lgl RNAi cells. To examine the relationship of lgl-induced overgrowths to Hippo signaling under milder conditions of reduced Yki activity, we over-expressed Wts. Using a UAS-wts line that only modestly reduced growth of the P compartment on its own (Fig. 4J), we observed a strong suppression of the lgl RNAi over-proliferation phenotype (Fig 4F), and also suppressed the influence of lgl on Yki localization and activity (Fig. 4I), resulting in relatively normal looking wing discs. These observations confirm that lgl acts as a tumor suppressor in the wing disc by limiting activation of Yki, and further suggest that the influence of lgl on Hippo signaling occurs at or upstream of Wts.

Lgl acts through Jnk signaling to regulate Hippo signaling in the wing

lgl depletion is associated with Jnk activation (Figs 5Q, ,6A,6A, S3D) (Igaki et al., 2006; Uhlirova and Bohmann, 2006). Both Jnk activation and Yki activation were largely confined to lgl depleted cells, but occasionally non-autonomous activation was observed (Figs 6A, S4D). This is consistent with the reported detection of non-autonomous Jnk activation adjacent to scrib mutant cells in eye discs (Igaki et al., 2006). The general correspondance between Jnk activation and Yki activation in discs with lgl depletion suggested they could be functionally linked. To directly examine this, we blocked Jnk activation by RNAi-mediated depletion of the Drosophila Jnk bsk, or by over-expression of the Jnk phosphatase Puc. Both of these approaches suppressed the overgrowth of lgl RNAi, the nuclear localization of Yki, the upregulation of downstream target genes, and the elevation of EdU labeling (Figs 6D,E,S4A). Thus, the influence of lgl on Hippo signaling and growth in the wing disc is dependent upon Jnk signaling.

Fig 6
lgl Depletion activates Yki through Jnk

Strikingly, expression of puc or bsk RNAi also appeared to rescue the cell polarity defect of lgl RNAi (Figs 6D, S4A). This was confirmed by the observation that the apical localization of Dlg and Fat proteins was restored (Fig. 5E,F,K,L). Thus, the loss of apical-basal polarity in lgl depleted wing cells depends on activation of the Jnk pathway. This observation, together with the general correspondance between polarity disruption and Jnk activation in lgl RNAi discs (Fig. 5Q), led us to investigate whether Jnk activation could be sufficient for polarity disruption. However, while some tissue folding was induced by Jnk activation, both Fat and Dlg maintained their normal apical localization in wing discs expressing hep.CA under sal-Gal4 control (Fig. 5M–P).

In a recent study of lgl mutant clones in eye discs, activation of Yki was observed and attributed at least in part to activation of aPKC (Grzeschik et al., 2010). Thus, we investigated the relationships among aPKC activation, Jnk activation, and Yki activation in wing discs. Expression of an activated form of aPKC, aPKCCAAX (Lee et al., 2006), induced a modest activation of Yki in the wing disc (Fig. 7A). Using the puc-lacZ reporter, we observed that expression of aPKCCAAX is also associated with Jnk activation (Fig. 7C). Moreover, the puc-lacZ insertion, which is also a puc allele, dominantly enhanced the Yki activation associated with aPKCCAAX expression (Fig. 7C). Conversely, reducing Jnk activation by expression of Puc reduced Yki activation (Fig. 7B). Together, these observations indicate that in wing discs, aPKC activates Jnk, and activation of Yki by aPKC is a Jnk-dependent process.

Fig 7
aPKC activates Yki through Jnk

Jnk activation has diverse effects in different contexts. Our observation that Yki activation in the wing is Jnk-dependent in neoplastic tumor suppressors and in response to tissue damage is consistent with oncogenic effects of Jnk that have been identified in neoplastic tumor suppressors and during regeneration and compensatory cell proliferation. However, under other conditions, Jnk acts as a tumor suppressor by promoting apoptosis. Indeed, in eye discs, mutation of scrib in clones can promote Jnk-dependent apoptosis, and blocking Jnk by expression of a dominant negative isoform promotes the proliferation of scrib mutant clones (Brumby and Richardson, 2003). To directly compare the influence of Jnk activation on Yki activation in different Drosophila organs, we analyzed multiple imaginal discs from larvae in which clones of cells expressing hep.CA had been induced. Because these discs were isolated from the same animals, the intensity and duration of Hep activation (induced by first making Flp-out clones expressing tub-Gal4 and tub-Gal80ts, allowing clones to grow for three days, and then inducing expression of UAS-hep.CA for 14 h by temperature shift) should be identical. Strong induction of Yki was observed in wing and haltere discs within and adjacent Hep.CA-expressing clones (Fig. S1E,F). By contrast, in leg discs, strong nuclear Yki was not detected, but instead we observed a more even distribution of Yki between nucleus and cytoplasm (Fig. S1G), indicative of a modest level of Yki activation. Finally, in eye discs, most clones were not associated with any visible change in Yki localization (Fig. S1H).

DISCUSSION

Participation of Yki in regeneration and compensatory cell proliferation

Many tissues have the capacity respond to the removal or death of cells by increasing proliferation of the remaining cells. In Drosophila, this phenomena has been characterized both in the context of imaginal disc regeneration and compensatory cell proliferation. Our studies implicate the Hippo signaling pathway as a key player in these proliferative responses to tissue damage. After genetically ablating the wing primordia by inducing apoptosis, we observed that Yki becomes activated to high levels in surrounding cells, based on its nuclear abundance and induction of a downstream target of Yki transcriptional activity. Moreover, high level Yki activation is crucial for wing disc regeneration, as even modest reduction of Yki levels, to a degree that has only minor effects on normal wing development, severely impaired wing disc regeneration. While it was known that Yki is required for wing growth during development (Huang et al., 2005), our observations establish that Yki is also required for wing growth during regeneration, and moreover that regeneration requires higher levels of Yki activation than during normal development.

Regulation of Yki by Jnk signaling

Our studies identify Jnk activation as a promoter of Yki activity in the wing disc. Most aspects of imaginal disc development, including imaginal disc growth, normally do not require Jnk signaling (Igaki, 2009). By contrast, Jnk signaling is both necessary and sufficient for Yki activation in response to wing damage. Jnk signaling has previously been linked to compensatory cell proliferation and regeneration in imaginal discs (Bergantinos et al., 2010a; Mattila et al., 2005; Perez-Garijo et al., 2009; Ryoo et al., 2004), and we can now ascribe at least part of that requirement to activation of Yki. However, Jnk signaling also promotes the expression of other mitogens, including Wg, which were linked to regeneration and proliferative responses to apoptosis (Perez-Garijo et al., 2004; Ryoo et al., 2004; Smith-Bolton et al., 2009). Wg and Yki are not required for each other's expression, suggesting that they are regulated and act in parallel to influence cell proliferation after tissue damage. The mechanism by which Jnk activation induces Yki activation is not yet known. The observation that it could be suppressed by over-expression of Wts or Hpo suggests that it might impinge on Hippo signaling at or upstream of Hpo and Wts, but the possibility that Jnk-dependent Yki regulation occurs in parallel to these Hippo pathway components can not be excluded (Fig. 8). The high level of nuclear Yki localization is striking by contrast with the more modest effects of upstream tumor suppressors in the Hippo pathway (Dong et al., 2007; Oh and Irvine, 2008), which suggests that Jnk might regulate Yki through a distinct mechanism, or simultaneously affect multiple upstream regulators.

Fig 8
Diverse inputs and outputs of Jnk signaling

We detected strong Yki activation within the wing and haltere discs in response to Jnk activation, but weaker or non-existent effects in leg or eye discs. Jnk activation has previously been linked to oncogenic effects of neoplastic tumor suppressors in eye discs (Brumby and Richardson, 2003; Igaki et al., 2006; Uhlirova and Bohmann, 2006), and it is possible that Yki activation might be induced in eye discs if a distinct Jnk activation regime were employed. Nonetheless, since we employed identical conditions in both wing and eye discs, isolating them from the same animals, our studies emphasize the importantance of context-dependence for Yki activation by Jnk. A link between Jnk activation and Yki activation is not limited to the wing however, as a connection between these pathways was recently discovered in the adult intestine, where damage to intestinal epithelial cells, and activation of Jnk, can activate also Yki (Staley and Irvine, 2010).

There was a general correspondence between activation of Jnk and activation of Yki under multiple experimental conditions, including expression of Rpr, direct activation of Jnk signaling by Egr or Hep.CA, and depletion of lgl. Some experiments, most notably direct activation of Jnk by Hep.CA, revealed a non-autonomous effect on Yki, which could imply that the influence of Jnk on Yki activity is indirect. Although the basis for this non-autonomous effect is not yet known, we favor the hypothesis that it is actually also mediated through Jnk signaling, as it has been reported that Jnk activation can propagate from cell to cell in the wing disc (Wu et al., 2010). Consistent with this possibility, we observed that a non-autonomous activation of Jnk adjacent to lgl depleted cells was blocked by depletion of bsk solely within the lgl RNAi cells. Conversely, alternative signals previously implicated in compensatory cell proliferation do not appear to be good candidates for mediating Yki activation, as we found that Wg is not required for Yki activation in regenerating discs, and prior studies did not detect a direct influence of Dpp pathway activity on Yki activation (Rogulja et al., 2008).

Activation of Yki adjacent to Egr- or Rpr-expressing cells was also reduced by over-expression of Wts. This might reflect an influence of Yki on signaling from these cells, but because expression of Wts inhibits Yki activity, and activated Yki promotes expression of an inhibitor of apoptosis (Diap1), it is also possible that this effect could be explained simply by Wts over-expression resulting in reduction or more rapid elimination of Egr- or Rpr-expressing cells; the reduced survival of these cells would then limit their ability to signal to neighbors.

Although Jnk has been implicated in compensatory cell proliferation and regeneration (Bergantinos et al., 2010a; Mattila et al., 2005; Ryoo et al., 2004), it is better known for its ability to promote apoptosis (Igaki et al., 2002; Kanda and Miura, 2004; Moreno et al., 2002). The dual, opposing roles of Jnk signaling as a promoter of apoptosis and a promoter of cell proliferation raise the question of how one of these distinct downstream outcomes becomes favored in cells with Jnk activation (Fig. 8). Given the links between Jnk activation and human diseases, including cancer (Bogoyevitch et al., 2010; Karin and Gallagher, 2005), defining mechanisms that influence this is an important question, and our identification of the role of Yki activation in Jnk-mediated proliferation and wing regeneration should facilitate future investigations into how the balance between proliferation or apoptosis downstream of Jnk is regulated.

Regulation of Yki activity by neoplastic tumor suppressors

Hippo signaling is regulated by proteins that exhibit discrete localization at the subapical membrane, e.g. Fat, Ex, and Merlin (Reddy and Irvine, 2008). The observation that disruption of apical-basal polarity is associated with disruption of Hippo signaling underscores the importance of this localization to normal pathway regulation. Our observations, together with other recent studies (Grzeschik et al., 2010; Menéndez et al., 2010), establish that Hippo signaling is inhibited by neoplastic tumor suppressor mutations, resulting in Yki activation, and that this activation of Yki is required for the tumorous overgrowths associated with these mutations.

Although our results agree with these recent studies in linking lgl to Hippo signaling, there are some notable differences. Grzeschik et al. (2010) examined lgl mutant clones in the eye imaginal disc, under conditions where cells retained apical-basal polarity, whereas we examined wing imaginal discs, where apical-basal polarity was lost. Intriguingly we found that conditions associated with activation of Yki by Jnk in the wing disc were not sufficient to activate Yki in the eye disc. This observation, together with the discovery that loss of polarity in lgl depleted wing cells requires Jnk activation, suggests as a possible explanation for why lgl null mutant clones retain apical-basal polarity in eye discs, that eye disc cells have a distinct, and apparently reduced, sensitivity to Jnk activation as compared to wing disc cells.

We also identified distinct processes linked to Yki activation in the absence of lgl. Grzeschik et al. (2010) reported an effect of lgl on Hpo protein localization. In wing discs, we do not detect the discrete apical localization of Hpo reported by Grzeschik et al. (2010) in their studies of eye discs. Thus, their proposed mechanism, involving activation of Yki via mis-localization of Hpo and dRassf, might not be relevant to the wing. By contrast, we identified an essential role for Jnk signaling in regulating Yki activation in lgl-depleted cells in the wing. Because we did not detect an effect of direct Jnk activation on Yki in eye discs, it is possible that Lgl can act through multiple pathways to influence Yki, including a Jnk-dependent pathway that is crucial in the wing disc, and a Jnk-independent pathway that is crucial in the eye disc. Grzeschik et al. (2010) also linked the influence of lgl in the eye disc to its antagonistic relationship with aPKC. Our observation that the influence of aPKC in the wing depends on Jnk activation is consistent with an Lgl-aPKC link, and identifies a role for Jnk activation in the oncogeneic effects of aPKC.

Our observation that the loss of polarity in lgl RNAi discs is dependent upon Jnk signaling was unexpected, but a related observation was recently reported by Zhu et al. (2010). These results suggest that the established role of the Lgl-Dlg-Scrib complex in maintaining epithelial polarity (Bilder et al., 2000; Bilder et al., 2003) depends in part on repressing Jnk activity. However, since Jnk activation on its own was not sufficient to disrupt polarity, multiple polarity complexes might need to be disturbed in order for wing cells to lose apical-basal polarity, including both Lgl and additional, Jnk-regulated polarity complexes.

The discovery of the role of Jnk signaling in Yki activation provides a common molecular mechanism for the overgrowths observed in conjunction with mutations of neoplastic tumor suppressors, and those associated with compensatory cell proliferation, because in both cases a proliferative response is mediated through Jnk-dependent activation of Yki. Although the molecular basis for the linkage of these two pathways is not understood yet, it operates in multiple Drosophila organs, and thus appears to establish a novel regulatory input into Hippo signaling that is of particular importance in abnormal or damaged tissues. Moreover, Jnk activation has also been observed in conjunction with regeneration of disc fragments after surgical wounding (Mattila et al., 2005), and thus its participation in regeneration is not limited to paradigms involving induction of apoptosis. It is also noteworthy that under conditions of widespread lgl depletion (ie lgl mutant or lgl RNAi), and consequent Jnk activation, the balance between induction of apoptosis and induction of cell proliferation is shifted towards a proliferative response. By contrast, in the wing disc clones of cells mutant for lgl fail to survive, unless oncogenic co-factors are co-expressed (Brumby and Richardson, 2003; Froldi et al., 2010; Igaki et al., 2006; Igaki et al., 2009; Menéndez et al., 2010; Pagliarini and Xu, 2003). The loss of lgl mutant clones in wing discs was recently attributed to cell competition (Menéndez et al., 2010). Together, these observations suggest that the choice between proliferative versus apoptotic responses to Jnk activation can be influenced by the Jnk activation status of neighboring cells.

Supplementary Material

01

ACKNOWLEDGEMENTS

We thank E. Bach, K. Basler, C. Doe, B. Edgar, I. Hariharan, G. Morata, T. Xu, the Developmental Studies Hybridoma Bank, and the the Bloomington stock center for antibodies and Drosophila stocks. This research was supported by the Howard Hughes Medical Institute.

Footnotes

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