RPS6-p70-protein kinase

DEVELOPMENTAL BIOLOGY

The S6k gene gives rise to two major transcripts that are expressed throughout development, suggesting a continuous requirement for this enzyme. Two major transcripts of 3.0 and 5.0 kb are found at abundant steady-state levels in whole animal poly(A)⁺ or total RNA. Two smaller transcripts (<3.0 kb) are expressed at lower levels at all tested developmental stages. All of these transcripts hybridize with probes that include the catalytic domain, C-terminal domain and the 3' UTR. Only the two major transcripts are detected with 5'-UTR probes common to class 1 and class 2 S6k cDNAs that also share a complete ORF. These 5'-UTR probes did not detect the smaller transcripts, suggesting that they have different N-terminal sequences. The smaller S6k transcripts are the predominant forms found in cultured air8 blood cells, which are derived from RpS6^air8 mutant larvae suggesting that the S6k gene gives rise to tissue-specific expression patterns. An alternative explanation is that the S6k transcripts expressed in air8 cultured cells are influenced by the RpS6^air8 mutation (Watson, 1996).

Effects of Mutation

The adaptation of growth in response to nutritional changes is essential for the proper development of all organisms. The identification of the Drosophila homolog of the target of rapamycin (TOR), a candidate effector for nutritional sensing, is described. Genetic and biochemical analyses indicate that dTOR impinges on the insulin signaling pathway by autonomously affecting growth through modulating the activity of Drosophila S6k. However, in contrast to other components in the insulin signaling pathway, partial loss of dTOR function preferentially reduces growth of the endoreplicating tissues. These results are consistent with dTOR residing on a parallel amino acid sensing pathway (Oldham, 2000).

A tissue-specific genetic screen has been carried out for recessive mutations affecting cell growth and proliferation in the Drosophila compound eye. In this screen, genetically mosaic flies are generated in which the eye and head capsule are homozygous for a randomly induced mutation, while the rest of the body and the germ line are heterozygous and, thus, phenotypically wild type. Remarkably, mosaic flies containing a mutation in a growth-promoting gene have eye and head structures that are strongly reduced in size relative to their wild-type sized heterozygous bodies and are termed pinhead flies. Two EMS-induced pinhead mutations (2L1 and 2L19) map to chromosomal position 34A, where the Drosophila homolog of TOR (dTOR) is located. These mutations fail to complement two lethal P-element insertions, EP(2)2353 and l(2)k17004, located 262 and 211 bp, respectively, upstream of the putative translation start site of dTOR. The dTOR genomic region spans ~10 kb and is composed of seven exons (7.4 kb cDNA) encoding a 2480-amino acid protein with a predicted M_w of 282 kD. The dTOR protein exhibits 35%, 38%, 35%, and 41% overall amino acid identity to yeast TOR1, TOR2, C. elegans TOR, and Arabidopsis thaliana TOR, respectively. The overall identity with mTOR is significantly higher (56%) and is especially conserved in the kinase and FRB domains (74% and 77%). Sequence analysis of DNA from flies heterozygous for the EMS-induced dTOR mutations reveal two nucleotide substitutions. The lesion of dTOR^2L1 results in a change of a proline to a leucine at amino acid position 2303 (P2303L). The location of this mutation within a highly conserved region of the kinase domain implies that the kinase activity of dTOR is critical for its function, as has been shown for the yeast TORs. In contrast, the lesion in dTOR^2L19 is an arginine changed to a nonsense mutation at amino acid residue 248 (R248Stop), giving rise to a stop codon. This mutation would be predicted to result in a short, truncated protein and should thus be a complete loss-of-function mutation (Oldham, 2000).

The phenotypes associated with the complete loss of dTOR function are remarkably similar to phenotypes associated with mutations in the Insulin-like receptor (Inr) pathway. (1) Strong dTOR mutants arrest development at a similar stage as do strong mutants in the Inr pathway or amino acid-starved larvae with little detectable imaginal tissue. (2) dTOR mutant clones have a significant proliferative disadvantage similar to Inr pathway mutant clones. Clones of dTOR null mutant cells, although severely affected, are not cell lethal. Similarly, in most mammalian cell types, rapamycin decreases but does not abolish cell growth, except for IL2-mediated T-cell proliferation. (3) The strict autonomous control of cell growth without disturbing the specification and differentiation is also seen with the dTOR mutants. Indeed, loss of dTOR function in clones of homozygous mutant cells in the adult eye show that only the mutant cells, as exemplified by the dark, circular rhabdomeres, are severely reduced in size. Analysis of imaginal wing disc cells at the end of the third larval instar by fluorescence-activated cell sorting (FACS), confirms that cells from the weak heteroallelic combination, dTOR^2L1/dTOR^l(2)k17004, are smaller than those of wild type. The effect on cell size is more pronounced in G₁ than G₂, consistent with mTOR function having a predominante role on cell growth during G₁ (Oldham, 2000).

Despite this observation, there is no apparent difference between the distribution of dTOR mutant and wild-type cells within each phase of the cell cycle. Although the similarities of the loss-of-function mutant phenotypes of dTOR and other components of the Inr pathway are consistent with a model in which dTOR acts downstream of dPI3K, the analysis of partial loss-of-function dTOR mutants and the biochemical analysis of Drosophila S6K activity indicates a more complex relationship between dTOR and the Inr pathway (Oldham, 2000).

Genetic interactions between dTOR mutants and other mutations in the insulin pathway were examined. Drosophila Pten encodes a negative effector of insulin signaling, and eyes and heads lacking Pten function are significantly larger than wild-type heads. Removal of dTOR function strongly reduces the size of Pten mutant heads, suggesting that dTOR is required for the increased growth generated by the loss of Pten function (Oldham, 2000).

In vertebrates, S6K activity is blocked by rapamycin, an inhibitor of TOR. Therefore, Drosophila S6K activity was examined in immunoprecipitates of extracts from larvae mutant for dTOR, S6K, chico, and larvae treated with rapamycin or deprived of amino acids. A severe reduction in the phosphorylation of ribosomal protein S6 was observed in extracts from strong dTOR^2L1/dTOR^2L19 mutant larvae. This was not caused by a reduction in Drosophila S6k protein as shown by Western blotting of these extracts. In addition, the S6k protein is up-regulated in the dTOR mutant larvae and amino acid-starved larvae. In all cases, Western blot analysis has shown equivalent amounts of initiation factor 4E (eIF-4E). S6k activity is not detected in S6k^l-1 null mutants and is severely reduced when wild-type larvae are starved for amino acids or treated with rapamycin. Higher doses of rapamycin blocks development during early larval stages, leading to lethality. Analysis of the weak dTOR^2L1/dTOR^l(2)k17004 or dTOR^2L1/dTOR^EP(2)2353 heteroallelic combinations also reveal a reduction in S6k activity in the third larval instar and an up-regulation of the protein as compared with wild-type flies. The surprising fact that dTOR mutants and amino acid starvation result in an up-regulation of S6k levels suggests that dTOR and amino acids may negatively control the protein levels of S6k. Unexpectedly, S6k activity as well as protein levels are unaffected in chico mutants. It may be that Inr does not signal to S6k or that S6k resides on a parallel pathway that bifurcates upstream of Chico. In support of the latter possibility, Inr has been shown to genetically interact with PI3K independently of Chico, presumably through docking sites for the p60 adaptor of PI3K in the Inr C-terminal tail. This result suggests that there is a S6k independent pathway for growth control and that the reduced Inr-mediated PI3K signaling in a chico mutant is sufficient for S6k activation (Oldham, 2000).

The biochemical differences between the ability of Chico and dTOR to activate S6k argue for a more complex relationship between the Inr pathway and dTOR. Given the low number of pharate adults, the weights of dTOR, S6k^l-1, and chico mutants were compared at an early pupal stage. The weight of the dTOR mutant pupae is more similar to S6k than to chico mutant pupae. Thus, in the absence of S6k function or the presence of reduced dTOR levels, cellular growth rates are diminished but larvae pupariate at a larger size as a result of a longer developmental delay. Importantly, S6k mutant flies have cells that are smaller but of the normal number. However, in chico mutants, pupariation is initiated at a much smaller size. The result is that chico mutants emerge after only a 2-d delay and are smaller than dTOR and S6k mutants because of fewer and smaller cells. Therefore, while insulin signaling controls cell size and cell number, S6k primarily controls cell size. It will be of interest to know whether dTOR is also limited to controlling only cell size (Oldham, 2000).

Larvae are composed of mitotic cells, largely represented by the imaginal discs, and of endoreplicating tissues, which form larval structures like the gut, fat body, and salivary glands. An increase in DNA ploidy of larval cells is required for the ~200-fold increase in mass obtained by the larvae during the 5-d period between the completion of embryogenesis and the beginning of pupation. During starvation, larvae sacrifice their endoreplicating tissue to maintain the growth and proliferation of the mitotic cells that are required to form the reproductive adult. Furthermore, S6k activity is reduced in starved larvae and dTOR mutants. These observations prompted an analysis of the mitotic and endoreplicating tissues of dTOR, S6k, and chico mutant larvae just before pupariation. Strong dTOR and PI3K mutants, as well as amino acid-starved larvae, are incapable of growth and have barely detectable imaginal and endoreplicative tissues. Surprisingly, the wing discs of the weak dTOR heteroallelic combination are of approximately equivalent size to that of wild-type larvae, whereas those of S6k^l-1 mutants are reduced. However, the amount of endoreplicating tissue in the dTOR mutant as compared to wild-type larvae is severely decreased. This is clearly demonstrated by comparing the salivary glands of dTOR mutant and wild-type larvae. In contrast, the size of endoreplicating tissue and imaginal discs in S6k null mutants as well as chico null mutants is reduced to approximately the same extent. Staining of the salivary glands with DAPI and phalloidin reveals that the size of the nuclei and, thus, the degree of endoreplication is severely reduced in S6k, chico, and dTOR mutants. The difference in size between dTOR and S6k mutant salivary glands is largely caused by a very pronounced reduction in cytoplasmic volume in dTOR mutants. The nuclear to cytoplasmic ratio is higher in dTOR salivary glands than in y w, S6k, or chico mutant salivary glands. Thus, it appears that partial loss of dTOR function permits the growth of imaginal tissue to wild-type size, while endoreplicating tissue is disproportionally reduced, a phenotype distinct from S6k mutants. Consistent with this finding, the lethality of the different dTOR mutants could not be rescued by constitutive expression of a S6K1 variant, D₃E-E₃₈₉, which exhibits high basal activity in the absence of mitogens under the control of the alpha-tubulin promoter, which rescues all aspects of the S6k^l-1 null phenotype. Therefore, S6k-independent processes must contribute to the weak dTOR phenotype (Oldham, 2000).

The effect of rapamycin and amino acids on translation in mammals is mediated through the S6Ks and the 4E-BPs. Unlike the other elements in the PI3K signaling pathway, absence of amino acids blocks both S6K activation and 4E-BP phosphorylation. Indeed, a mutant of S6K1, lacking a portion of both its amino and carboxyl termini, is resistant to rapamycin but still sensitive to the fungal metabolite wortmannin, an inhibitor of PI3K. This suggests that the PI3K-dependent signal to S6K activation does not involve TOR. This same mutant is also unaffected by amino acid withdrawal, consistent with the role of mTOR as an amino acid checkpoint in S6K activation. Although there is some controversy concerning the ability of mitogens to activate mTOR, the in vitro activity of mTOR from cultured cells toward either itself, S6K1, or 4E-BP1 is unaffected by mitogens. Thus, mTOR may act as a permissive signal that primes 4E-BP phosphorylation and S6K activation by the PI3K signaling pathway if amino acids, and possibly other nutrients, are at sufficient levels. Likewise, in Drosophila larvae, amino acids are necessary, but not sufficient, for imaginal disc and endoreplicating tissue proliferation, compatible with dTOR acting in a parallel pathway involved in amino acid sensing. The fact that chico mutant larvae have normal levels of S6k activity and that the dTOR larval phenotypes with respect to the imaginal discs and endoreplicating tissues are so distinct compared with other mutants in the Inr pathway, supports the possibility that dTOR is not responsive to insulin signaling (Oldham, 2000 and references therein).

It is well established in yeast that TOR is an important mediator of nutrient limitation, and it has been proposed that TOR acts as an amino acid effector to coordinate the response of yeast to different nutritional conditions (Barbet, 1996). Indeed, the similarities between dTOR mutant larvae and larvae deprived of amino acids are striking. Therefore, it is likely that dTOR also functions as an amino acid sensor in multicellular organisms. The fact that yeast and Arabidopsis do not have an insulin system suggests that TOR may be an ancestral and widespread nutritional sensor. To provide additional levels of control, it may have been integrated into the insulin system later to respond to different modes of nutrient deprivation with different developmental responses (Oldham, 2000 and references therein).

Insulin treatment of Drosophila Kc 167 cells induces the multiple phosphorylation of a Drosophila ribosomal protein, as judged by its decreased electrophoretic mobility on two-dimensional polyacrylamide gels. The extent to which insulin induces this response is potentiated by cycloheximide and blocked by pretreatment with rapamycin. Isolation and mass spectrometric analysis have revealed that the multiply phosphorylated protein is the larger of two Drosophila melanogaster orthologs of mammalian 40S ribosomal protein S6, termed here DS6A. Proteolytic cleavage of DS6A (derived from stimulated Kc 167 cells), with the endoproteinase Lys-C releases a number of peptides, one of which contains all the putative phosphorylation sites. Conversion of phosphoserines to dehydroalanines with Ba(OH)(2) shows that the sites of phosphorylation reside at the carboxy terminus of DS6A. The sites of phosphorylation have been identified by Edman degradation after conversion of the phosphoserine residues to S-ethylcysteine as Ser(233), Ser(235), Ser(239), Ser(242), and Ser(245). Phosphopeptide mapping of individual phosphoderivatives, isolated from two-dimensional polyacrylamide gels, indicate that DS6A phosphorylation, in analogy to mammalian S6 phosphorylation, appears to proceed in an ordered fashion (Oldham, 2000 and references therein).

Precise body and organ sizes in the adult animal are ensured by a range of signaling pathways. Rheb (Ras homolog enriched in brain), a novel, highly conserved member of the Ras superfamily of G-proteins, promotes cell growth. Overexpression of Rheb in the developing fly causes dramatic overgrowth of multiple tissues: in the wing, this is due to an increase in cell size; in cultured cells, Rheb overexpression results in accumulation of cells in S phase and an increase in cell size. Rheb is required in the whole organism for viability (growth) and for the growth of individual cells. Inhibition of Rheb activity in cultured cells results in their arrest in G1 and a reduction in size. These data demonstrate that Rheb is required for both cell growth (increase in mass) and cell cycle progression; one explanation for this dual role would be that Rheb promotes cell cycle progression by affecting cell growth. Consistent with this interpretation, flies with reduced Rheb activity are hypersensitive to rapamycin, an inhibitor of the growth regulator target of rapamycin (TOR), a kinase required for growth factor-dependent phosphorylation of ribosomal S6 kinase (S6K). In cultured cells, the effect of overexpressing Rheb was blocked by the addition of rapamycin. These results imply that Rheb is involved in TOR signaling (Patel, 2003). Additional studies show that Rheb functions downstream of the tumor suppressors Tsc1 (tuberous sclerosis 1)-Tsc2, with Tsc2 functioning as a GAP for Rheb (Saucedo, 2003; Zhang, 2003), and that a major effector of Rheb function in controlling growth is, in fact, ribosomal S6 kinase (Stocker, 2003). It is still not clear, however, how Rheb signals to TOR (Zhang, 2003).

Given the similarities between Rheb and mutants in the InR and TOR signalling pathways, it is conceivable that Rheb represents a novel component of one of these growth control pathways. To test this possibility, a detailed epistasis analysis was performed. Examined first was whether the negative regulators of InR and TOR signalling (PTEN and Tsc1-Tsc2, respectively) could counteract the effects of Rheb overexpression. All overexpression experiments were performed in the eye using the GMR-Gal4 driver line. Expression of either PTEN or Tsc1-Tsc2 alone results in a very similar size reduction of the ommatidia when compared with control ommatidia. However, whereas expression of PTEN has no influence on the increase in ommatidial size caused by Rheb overexpression, co-expression of Tsc1-Tsc2 results in ommatidia of approximately wild-type size, indicating that the activities of Rheb and Tsc1-Tsc2 can counteract each other. Next, the enlarged ommatidia phenotype of GMR-Rheb was assayed in a number of mutant backgrounds. Reducing the activity of Drosophila protein kinase B (PKB) has no effect on ommatidial size. Similar results were obtained with hypomorphic mutations in InR and Dp110, respectively. In contrast, ommatidial size is dominantly reduced by a mutation in TOR (TOR^2L1), and a suppression to wild-type size is observed in a S6K mutant background. Thus, the Rheb overexpression phenotype is dependent on TOR and S6K function, but is independent of InR signal strength. Finally, the behaviors of Rheb PTEN and Rheb Tsc1 double mutants were examined. The phenotypic consequences were assayed in mosaic animals using the ey-Flp method. As expected, the Rheb PTEN double-mutant tissue clearly displays a Rheb phenotype. The Rheb Tsc1 mutant tissue also resembles Rheb single mutants, indicating that Rheb is epistatic over (functions downstream of) Tsc1 (Stocker, 2003).

Regulation of lifespan in Drosophila by modulation of genes in the TOR signaling pathway

In many species, reducing nutrient intake without causing malnutrition extends lifespan. Like DR (dietary restriction), modulation of genes in the insulin-signaling pathway, known to alter nutrient sensing, has been shown to extend lifespan in various species. In Drosophila, the target of rapamycin (TOR) and the insulin pathways have emerged as major regulators of growth and size. Hence, the role of TOR pathway genes in regulating lifespan has been examined by using Drosophila. Inhibition of TOR signaling pathway by alteration of the expression of genes in this nutrient-sensing pathway, which is conserved from yeast to human, extends lifespan in a manner that may overlap with known effects of dietary restriction on longevity. In Drosophila, TSC1 and TSC2/Gigas (tuberous sclerosis complex genes 1 and 2) act together to inhibit TOR (target of rapamycin), which mediates a signaling pathway that couples amino acid availability to S6 kinase, translation initiation, and growth. Overexpression of dTsc1, dTsc2, or dominant-negative forms of dTOR or dS6K all cause lifespan extension. Modulation of expression in the fat is sufficient for the lifespan-extension effects. The lifespan extensions are dependent on nutritional condition, suggesting a possible link between the TOR pathway and dietary restriction (Kapahi, 2004).

The Drosophila homologs of human Tsc1 (Hamartin) and Tsc2 (tuberin) function in vivo as a complex that controls growth and size in a cell-autonomous manner. To examine their role in regulating lifespan, dTsc1 and dTsc2 were overexpressed through the ubiquitously expressed driver, daughterless (da-GAL4). Overexpression in transgenic flies carrying UAS constructs containing dTsc1 or dTsc2 extends mean lifespan at 29°C by 14% and 12%, respectively. Since GAL4 enhancer traps generally yield stronger effects at 29°C, most of the experiments were performed at that temperature (Kapahi, 2004).

dTsc1 and dTsc2 physically interact with dTOR, which is conserved from yeast to human as a nutrient sensor. Loss of dTsc1 in Drosophila eye leads to an increase in cell size, provided that dTOR is present. Surprisingly, however, dTOR overexpression causes a reduction in cell size, a phenotype similar to dTOR loss-of-function mutations, perhaps due to titration of cofactors required for TOR signaling. The effect of dTOR on lifespan was examined by using three UAS. One carries the full-length wild-type TOR gene. The second carries FRB, the 11 kDa FKBP12-rapamycin binding domain, which has been shown to prevent S phase entry when injected into human osteosarcoma cells. The third carries TED (toxic effector domain), containing the 754 amino acid central region, which inhibits cell growth and arrests cells in G1 when overexpressed in yeast (Hennig, 2002). Ubiquitous overexpression with the da-GAL4 driver of UAS-dTOR^FRB led to a mean lifespan increase at 29°C of 24%. However, overexpression of UAS -dTOR^WT or UAS-dTOR^TED prevented eclosion to adulthood (Kapahi, 2004).

S6 kinase activation upon phosphorylation has been implicated in mediating the downstream effects of TOR on translation initiation in flies and mammals. S6 kinase phosphorylation of ribosomal protein S6 is accompanied by upregulation of a class of mRNAs containing an oligopyrimidine tract at their transcriptional start site termed 5′TOP (Thomas, 2002). Some 200 genes, most of which encode components of the translational apparatus including ribosomal proteins and elongation factors, have this sequence and can account for about 20% of total cellular mRNA. Flies carrying homozygous mutations in dS6K show a developmental delay and a reduction in body size. The stimulation of dS6K phosphorylation by dTOR is abrogated when dTsc1 and dTsc2 are overexpressed. Furthermore, flies with reduced dTSC1 show increased dS6 kinase activation, and genetic reduction of S6 kinase level can rescue the lethality caused by loss of function of dTsc1 (Kapahi, 2004).

The role of S6 kinase in regulating lifespan was examined by using dominant-negative and constitutively active constructs. The dominant-negative effect was achieved by replacing the conserved lysine in the ATP binding site by glutamine (UAS-dS6K^KQ), which causes cell-size reduction. The constitutively active form was generated by replacing the phosphorylation sites of S6 kinase by acidic amino acids (UAS-dS6K^STDETE), causing an autonomous cell size increase. By using da-GAL4 to drive ubiquitous overexpression of the dominant-negative form, a mean lifespan increase of 22% at 29°C was observed. Conversely, overexpression of the constutively active form of S6 kinase caused a mean lifespan decrease of 34% at 29°C. Overexpression of dTsc2 and dTOR^FRB was also tested at 25°C and led to a 20% and 26% increase in mean lifespan increase, respectively (Kapahi, 2004).

To determine which tissues are responsible for the lifespan extension, various GAL4 drivers with specific GAL4 expression pattern were employed to overexpress dTsc2 via a UAS promoter. Overexpression in the eye by using the driver gmr-GAL4 or in the nervous system by using appl-GAL4 did not extend lifespan. In contrast, by using the drivers 24B-GAL4 and PO188-GAL4, enhancer traps that are predominantly expressed in the muscle and fat, results in mean lifespan extensions of 27% and 37%, respectively, at 29°C. The fat-specific drivers DJ634-GAL4 and PO163-GAL4, when used to overexpress dTsc2, also led to a mean lifespan extension of 22% and 31%, respectively, at 29°C. Using DJ634-GAL4 to overexpress the dominant-negative form of TOR (UAS-dTOR^FRB) or of S6 kinase (UAS-UAS-dS6K^KQ) also led to mean lifespan increases of 30% and 29%, respectively, at 29°C. These results indicate that manipulation of the TSC, TOR, and S6 kinase genes in the fat tissue is sufficient for their lifespan extension effects in Drosophila (Kapahi, 2004).

Amino acids have been shown to activate dS6k via TOR, an effect that can be abrogated in the presence of increased levels of dTsc1 and dTsc2. Since nutrients in the diet can modulate lifespan and because the TOR pathway is a critical mediator of nutrient signaling, it was asked whether the observed lifespan-extension effects are dependent on nutrient conditions. This was tested with overexpression of dTsc2 by using the ubiquitously expressing da-GAL4 driver. Flies were allowed to develop to adulthood under standard laboratory food and then maintained on specially prepared food containing various concentrations of yeast extract. At high concentrations of yeast extract, which may be regarded as the opposite of dietary restriction, the lifespan of control flies (da-GAL4/+) is severely reduced. However, overexpression of dTsc2 protects the fly from the deleterious effects of rich food, as if mimicking the effect of dietary restriction. Similar results were observed by overexpression of the dominant-negative form of S6 kinase (Kapahi, 2004).

Recent evidence from Drosophila suggests that signaling through TSC is both parallel to and interacting with the insulin pathway. This is supported by the finding that heterozygosity of dTsc1 or dTsc2 is sufficient to rescue the lethality of loss-of-function dInR mutants. However, the finding that loss-of-function mutations of dTsc1 and dPTEN, a phosphatase that negatively regulates the insulin-signaling pathway, cause cell autonomous and additive increases in cell size suggests that they may be in parallel pathways. Furthermore, in Drosophila, dPTEN loss of function, which leads to an increase in cell size, is only slightly suppressible by loss of function of dFOXO, a fly homolog of C. elegans daf-16. However, the increase in cell size resulting from dTsc1 is enhanced by dFOXO loss of function. Interestingly, unlike long-lived daf-2 mutants, the lifespan extension due to TOR deficiency in C. elegans is not suppressible by a daf-16 mutation. However, the TOR mutant animals do not further extend lifespan in a daf-2 background, leading to the possibility that TOR may be acting downstream or separately from daf-16 to exert its lifespan effects (Kapahi, 2004).

Lifespan extension has been linked with other phenotypes, including stress resistance, metabolic rate, lipid level, reproductive capacity, and body size. The long-lived strains described above with their respective controls for resistance to starvation were compared but no significant differences were found. Similarly, no significant differences were observed for weight and lipid content among these strains. It may be that lifespan extension can be produced by mild modulation of these genes, whereas effects on other phenotypes require severe perturbations. While lifespan extension is observed by using the da-GAL4 driver to overexpress dTsc1 or dTsc2 alone, simultaneous overexpression of dTsc1 and dTsc2 prevented eclosion to adulthood. Similarly, no change in size is observed if dTsc1 or dTsc2 alone are overexpressed in the eye, but a cell-autonomous decrease in size is seen when both are overexpressed simultaneously. Lifespan extension by chico is semidominant, but its effect on body size is recessive. Dominant effects on lifespan are observed with the genes Inr, EcR, Indy, and Rpd3, but their effects on lifespan can be uncoupled from other phenotypes such as fecundity, stress resistance, or lipid accumulation (Kapahi, 2004).

In humans, mutations in TSC1 and TSC2 lead to tuberous sclerosis, a common disorder characterized by the presence of benign tumors in various tissues, with some having large cells. DR in mice has been shown to protect against age-related tumorigenesis. These results suggest a link between lifespan extension by DR and the activities of genes in the TOR pathway. Hence, it is conceivable that the protective effects of DR on tumorigenesis and age-related decline might come from inhibition of such nutrient-responsive pathways (Kapahi, 2004).

These results show that upregulation of dTsc2 in the fat is sufficient for lifespan extension effects in Drosophila. Reduction of daf-2 levels in the C. elegans nervous system has been shown to be sufficient for lifespan extension. However, the lifespan extensions due to mutations in the insulin pathway or germline ablation in C. elegans are dependent on daf-16 activity in the intestine, the fat storage tissue in C. elegans. In Drosophila, the fat body has been proposed to modulate insulin signaling in peripheral tissues by secretion of dALS (acid-labile subunit), which, in mammals, forms a ternary complex with insulin-like growth factor, leading to an extension of the half-life of its ligand. Recently, mice with FIRKO (fat-specific insulin receptor knockout) have been shown to live 18% longer than controls . Hence, it is possible that secondary endocrine signals downstream of the insulin and TOR signaling pathways are released from the fat, and these affect the rate of aging in other tissues. Juvenile hormone and ecdysone are two such endocrine signals that have been implicated in regulating lifespan in conjunction with the insulin pathway in Drosophila (Kapahi, 2004).

Role and regulation of starvation-induced autophagy in the Drosophila fat body

In response to starvation, eukaryotic cells recover nutrients through autophagy, a lysosomal-mediated process of cytoplasmic degradation. Autophagy is known to be inhibited by TOR signaling, but the mechanisms of autophagy regulation and its role in TOR-mediated cell growth are unclear. Signaling through TOR and its upstream regulators PI3K and Rheb is necessary and sufficient to suppress starvation-induced autophagy in the Drosophila fat body. In contrast, TOR's downstream effector S6K promotes rather than suppresses autophagy, suggesting S6K downregulation may limit autophagy during extended starvation. Despite the catabolic potential of autophagy, disruption of conserved components of the autophagic machinery, including ATG1 and ATG5, does not restore growth to TOR mutant cells. Instead, inhibition of autophagy enhances TOR mutant phenotypes, including reduced cell size, growth rate, and survival. Thus, in cells lacking TOR, autophagy plays a protective role that is dominant over its potential role as a growth suppressor (Scott, 2004).

Autophagy likely evolved in single-cell eukaryotes to provide an energy and nutrient source allowing temporary survival of starvation. In yeast, Tor1 and Tor2 act as direct links between nutrient conditions and cell metabolism. These proteins sense nutritional status by an unknown mechanism, and effect a variety of starvation responses including changes in transcriptional and translational programs, nutrient import, protein and mRNA stability, cell cycle arrest, and induction of autophagy. Autophagy thus occurs in the context of a comprehensive reorganization of cellular activities aimed at surviving low nutrient levels (Scott, 2004).

In multicellular organisms, TOR is thought to have retained its role as a nutrient sensor but has also adopted new functions in regulating and responding to growth factor signaling pathways and developmental programs. Thus in a variety of signaling, developmental, and disease contexts, TOR activity can be regulated independently of nutritional conditions. In these cases, autophagy may be induced in response to downregulation of TOR despite the presence of abundant nutrients and may potentially play an important role in suppressing cell growth rather than promoting survival. Identification of the tumor suppressors PTEN, and TSC1 and TSC2 as positive regulators of autophagy provides correlative evidence supporting such a role for autophagy in growth control. Alternatively, since TOR activity is required for proper expression and localization of a number of nutrient transporters, inactivation of TOR may lead to reduced intracellular nutrient levels, and autophagy may therefore be required under these conditions to provide the nutrients and energy necessary for normal cell metabolism and survival (Scott, 2004).

The results presented here provide genetic evidence that under conditions of low TOR signaling, autophagy functions primarily to promote normal cell function and survival, rather than to suppress cell growth. This conclusion is based on the finding that genetic disruption of autophagy does not restore growth to cells lacking TOR, but instead exacerbates multiple TOR mutant phenotypes. It is important to note that mutations in TOR do not disrupt larval feeding, and thus disruption of autophagy is detrimental in TOR mutants despite the presence of ample extracellular nutrients. The finding that autophagy is critical in cells lacking TOR further supports earlier studies suggesting that inactivation of TOR causes defects in nutrient import, resulting in an intracellular state of pseudo-starvation (Scott, 2004).

Can the further reduction in growth of TOR mutant cells upon disruption of autophagy be reconciled with the potential catabolic effects of autophagy? TOR regulates the bidirectional flow of nutrients between protein synthesis and degradation through effects on nutrient import, autophagy, and ribosome biogenesis. When TOR is inactivated, rates of nutrient import and protein synthesis decrease, resulting in a commensurate reduction in mass accumulation and cell growth. In addition, autophagy is induced to maintain intracellular nutrient and energy levels sufficient for normal cell metabolism. When autophagy is experimentally inhibited in cells lacking TOR, this reserve source of nutrients is blocked, leading to a further decrease in energy levels, protein synthesis, and growth. It is noted that autophagy may have additional functions in cells with depressed TOR signaling, including recycling of organelles damaged by the absence of TOR activity, or selective degradation of cell growth regulators, analogous to the regulatory roles of ubiquitin-mediated degradation (Scott, 2004).

Autophagy is required for normal developmental responses to inactivation of insulin/PI3K signaling in the nematode C. elegans. In response to starvation or disruption of insulin/PI3K signaling, C. elegans larvae enter a dormant state called the dauer. Autophagy has been observed in C. elegans larvae undergoing dauer formation: disruption of a number of ATG homologs interfers with normal dauer morphogenesis. Importantly, simultaneous disruption of insulin/PI3K signaling and autophagy genes results in lethality, similar to the results presented in this study. Thus despite significant differences in developmental strategies for surviving nutrient deprivation, autophagy plays an essential role in the starvation responses of yeast, flies, and worms (Scott, 2004).

The prevailing view that S6K acts to suppress autophagy was founded on correlations between induction of autophagy and dephosphorylation of rpS6 in response to amino acid deprivation or rapamycin treatment. However, the genetic data presented in this study argue strongly against a role for S6K in suppressing autophagy: unlike other positive components of the TOR pathway, null mutations in S6K do not induce autophagy in fed animals. It is suggested that the observed correlation between S6K activity and suppression of autophagy is due to common but independent regulation of S6K and autophagy by TOR. Thus, autophagy suppression and S6K-dependent functions such as ribosome biogenesis represent distinct outputs of TOR signaling (Scott, 2004).

How might TOR signal to the autophagic machinery, if not through S6K? In yeast, this is accomplished in part through regulation of Atg1 kinase activity and ATG8 gene expression (Kamada, 2000 and Kirisako, 1999). The demonstration of a role for Drosophila ATG1 and ATG8 homologs [see TG8a (CG32672) and ATG8b (CG12334)] in starvation-induced autophagy, and the genetic interaction observed between ATG1 and TOR, are consistent with a related mode of regulation in higher eukaryotes. However, it is noted that other components of the yeast Atg1 complex such as Atg17 and Atg13, whose phosphorylation state is rapamycin sensitive, do not have clear homologs in metazoans, indicating that differences in regulation of autophagy by TOR are likely (Scott, 2004).

In addition to excluding a role for S6K in suppression of autophagy, these results reveal a positive role for S6K in induction of autophagy. S6K may promote autophagy directly, through activation of the autophagy machinery, or indirectly through its effects on protein synthesis. The latter possibility is consistent with previous reports that protein synthesis is required for expansion and maturation of autophagosomes. Interestingly, despite being required for autophagy, S6K is downregulated under conditions that induce it, including chronic starvation and TOR inactivation. Consistent with this, it was found that lysotracker staining is significantly weaker in chronically starved animals or in TOR mutants than in wild-type animals starved 3-4 hr. Furthermore, expression of constitutively activated S6K has no effect in wild-type, but restores lysotracker staining in TOR mutants to levels similar to those of acutely starved wild-type animals. It is suggested that downregulation of S6K may limit rates of autophagy under conditions of extended starvation or TOR inactivation and that this may protect cells from the potentially damaging effects of unrestrained autophagy (Scott, 2004).

Co-culture and conditioned media experiments have shown that the Drosophila fat body is a source of diffusible mitogens. The fat body has also been shown to act as a nutrient sensor through a TOR-dependent mechanism and to regulate organismal growth through effects on insulin/PI3K signaling. The results in this study extend these findings by showing that this endocrine response is accompanied by the regulated release of nutrients through autophagic degradation of fat body cytoplasm. Preventing this reallocation of resources, either through constitutive activation of PI3K or through inactivation of ATG genes, results in profound nutrient sensitivity. Thus, in response to nutrient limitation, the fat body is capable of simultaneously restricting growth of peripheral tissues through downregulation of insulin/PI3K signaling and providing these tissues with a buffering source of nutrients necessary for survival through autophagy (Scott, 2004).

Regulation of hunger-driven behaviors by neural ribosomal S6 kinase in Drosophila

Hunger elicits diverse, yet coordinated, adaptive responses across species, but the underlying signaling mechanism remains poorly understood. This study reports on the function and mechanism of the Drosophila insulin-like system in the central regulation of different hunger-driven behaviors. Overexpression of Drosophila insulin-like peptides (DILPs) in the nervous system of fasted larvae suppresses the hunger-driven increase of ingestion rate and intake of nonpreferred foods (e.g., a less accessible solid food). Moreover, up-regulation of Drosophila p70/S6 kinase activity in DILP neurons leads to attenuated hunger response by fasted larvae, whereas its down-regulation triggered fed larvae to display motivated foraging and feeding. Finally, evidence is provided that neural regulation of food preference but not ingestion rate may involve direct signaling by DILPs to neurons expressing neuropeptide F receptor 1, a receptor for neuropeptide Y-like neuropeptide F. This study reveals a prominent role of neural Drosophila p70/S6 kinase in the modulation of hunger response by insulin-like and neuropeptide Y-like signaling pathways (Wu, 2006).

The relatively simple Drosophila larva offers a genetically tractable model to define and characterize different neuronal signaling pathways that constitute a complete central feeding apparatus. Younger third-instar larvae forage actively and use their mouth hooks for food intake. Larvae normally feed on liquid food, and their food ingestion can be quantified by measuring the contraction rate of the mouth hooks. This study examined how food deprivation affects larval feeding response to a liquid (e.g., 10% glucose-agar paste) and less accessible solid food (e.g., 10% glucose agar blocks). To extract embedded glucose from the solid food, larvae have to pulverize the food by scraping agar surface with mouth hooks. Unless stated otherwise, synchronized third-instar larvae (74 h after egg laying) were used for the assays (Wu, 2006).

When fed ad libitum, normal larvae (w¹¹¹⁸) display significant feeding activity in the liquid food with an average mouth-hook contraction frequency of ~30 times in a 30-s test period; in contrast, these larvae declined the solid food. However, larvae withheld from food (on a wet tissue) for 40 or 120 min display increased intake of both liquid and solid foods. For example, larvae fasted for 120 min show a 100% and >500% increase in mouth-hook contraction rate in liquid and solid food, respectively. Thus, deprivation not only enhances feeding rate in a graded fashion, but also triggers motivated foraging on the less accessible food normally rejected by fed larvae. In addition, larvae display virtually identical feeding responses to liquid and solid foods containing 10% glucose, apple juice, or 10% glucose/yeast under deprived and nondeprived conditions. Therefore, these paradigms appear to provide a general assessment of larval feeding response (Wu, 2006).

dS6K is a cell-autonomous effector of nutrient-sensing pathways. This study investigated a possible role of neural dS6K in coupling peripheral physiological hunger signals and neuronal activities critical for hunger-driven behaviors. The transcripts of dilp1, dilp2, dilp3, and dilp5 are predominantly expressed in two small clusters of medial neurosecretory cells that project to the ring gland, the fly heart, and the brain lobes. A gal4 driver containing a 2-kb fragment from the dilp2 promoter (dilp2-gal4) was generated that directs the specific expression of a GFP reporter in those cells. Using dilp2-gal4, two transgenes, UAS-dS6K^DN, encoding a dominant negative, and UAS-dS6K^ACT, a constitutively active form of dS6K, were expressed. When fed ad libitum, control larvae (w x UAS-dS6K^DN or UAS-dS6K^ACT) behave like w larvae. However, dilp2-gal4 x UAS-dS6K^DN larvae displayed a 50% increase in the rate of liquid-food intake and significant feeding of the solid food. Conversely, fasted larvae overexpressing dS6K activity (dilp2-gal4 x UAS-dS6K^ACT) showed attenuated feeding response to both liquid and solid foods. These findings reveal that dS6K in DILP neurons mediates hunger regulation of approaching/consumptive behaviors, controlling both quality and quantity of food for ingestion. The body size and the developmental rate of all four groups of larvae were measured, and no significant differences were detected (Wu, 2006).

DILPs act as neurohormones in Drosophila larvae. Down-regulation of dS6K activity in DILP neurons may reduce DILP release, thereby promoting increased food intake that is normally triggered only by hunger. A corollary of this interpretation is that overproduction of DILPs in the nervous system should interfere with hunger response by deprived animals. To test this idea, a neural-specific elav-gal4 driver was used to direct dilp expression in the larval nervous system. Three UAS-dilp lines (UAS-dilp2, UAS-dilp3, and UAS-dilp4) were chosen for the analysis. The elav-gal4 x UAS-dilp2 and UAS-dilp4 larvae displayed normal feeding response when fed ad libitum. However, the same larvae fasted for 120 min displayed significantly attenuated feeding rates, similar to those of dilp2-gal4 x UAS-dS6K^ACT larvae. For example, the comparative analysis of the elav-gal4 x UAS-GFP control and elav-gal4 x UAS-dilp2 and UAS-dilp4 experimental larvae showed that the latter were ~30% and 33–45% lower in the ingestion rate of the liquid and solid food, respectively; surprisingly, elav-gal4 x UAS-dilp3 and UAS-GFP larvae showed virtually identical feeding responses. Therefore, DILP2 and DILP4 negatively regulate hunger-driven feeding activities. Taken together, these results suggest that a high level of dS6K activity in DILP neurons may suppress hunger response by reducing DILP release (Wu, 2006).

Attempts were made to delineate the signaling mechanism that couples the dS6K activity in DILP neurons with its broad impact on hunger-driven feeding activities. A previous study showed that fasted larvae ablated of NPF or its receptor (NPFR1) neurons are deficient in motivated feeding of the less-preferred solid food but normal in feeding of richer liquid food. It was of interest to enquire whether the NPF/NPFR1 neuronal pathway might be one of the downstream effectors of the DILP pathway. To test this hypothesis, the function of three components of the dInR signaling pathway were analyzed in NPFR1 neurons: dInR, phosphatidylinositol 3,4,5-trisphosphate 3-phosphatase (dPTEN), and phosphatidylinositol 3-kinase (dPI3K). Five different transgenes were used: UAS-dInR^ACT and UAS-dInR^DN encode a constitutively active and a dominant-negative form of dInR, respectively; UAS-Dp110 and UAS-dPI3K^DN encode a catalytic subunit and a dominant-negative form of dPI3K, respectively; and UAS-dPTEN encodes a functional enzyme. When fed ad libitum, npfr1-gal4 x UAS-dInR^DN, UAS-dPTEN, or UAS-dPI3K^DN larvae display hyperactive feeding of the solid food, similar to w larvae deprived for 40 min. In contrast, fasted larvae overexpressing dInR or dPI3K (npfr1-gal4 x UAS-dInR^ACT or UAS-Dp110) display attenuated feeding response to the solid food. Importantly, larvae with up- or down-regulated dInR signaling in NPFR1 neurons do not exhibit significant changes in the intake rate of the richer liquid food relative to the paired controls. Taken together, these findings suggest that the dInR pathway negatively regulates the activity of NPFR1 neuron and mediates the DILP-regulated change in food preference but not ingestion rate. Furthermore, the results suggest that NPFR1 neurons are the direct targets of DILPs (Wu, 2006).

A possible role of dS6K in hunger regulation of the functioning of NPFR1 neurons was evaluated, by expressing UAS-dS6K^DN and UAS-dS6K^ACT using npfr1-gal4. When fed ad libitum, npfr1-gal4 x UAS-dS6K^DN larvae display hyperactive feeding of the solid food, similar to npfr1-gal4 x UAS-dInR^DN larvae. However, these larvae, unlike dilp2-gal4 x UAS-dS6K^DN animals, display no increases in the ingestion rate of the richer liquid food. Conversely, fasted larvae overexpressing dS6K (npfr1-gal4 x UAS-dS6K^ACT) display attenuated feeding response to the solid food. These findings suggest that dS6K also negatively regulates the activity of NPFR1 neurons in food preference, but does not mediate the regulation of feeding rate by DILP signaling (Wu, 2006).

The food response was evaluated of the solid and liquid food by larvae overexpressing an npfr1 cDNA under the control of an npfr1-gal4 driver. In the presence of the liquid food, both experimental (npfr1-gal4 x UAS-npfr1) and control larvae (e.g., npfr1-gal4 x UAS-ANF-GFP), fed or fasted, show similar intake rates and comparable increases in feeding response to hunger. However, when forced to feed on the solid food, fed experimental larvae exhibit significant intake of the solid food (30 times per 30 s), whereas fed controls rejected the same food. Thus, NPFR1 overexpression selectively promotes change in food preference without increasing ingestion rate. It was also observed that the feeding responses of NPFR1-overexpressing larvae and controls fasted for 120 min were indistinguishable. Thus, the effect of NPFR1 overexpression on food preference is detectable only in fed or mildly fasted larvae, suggesting hunger-activated NPFR1 signaling approaches a plateau in severely fasted animals (Wu, 2006).

npfr1 activity was selectively knocked down by expressing npfr1 dsRNA in the nervous system. The UAS-npfr1^dsRNA lines were previously used to functionally disrupt npfr1 activity. It was found that 120-min fasted larvae expressing npfr1 dsRNA in NPFR1 or the nervous system (npfr1-gal4, elav-gal4, or appl-gal4 x UAS-npfr1^dsRNA) were deficient in motivated feeding of the solid but not liquid food. In contrast, all control larvae, including those expressing npfr1^dsRNA in muscle cells (MHC82-gal4 x UAS-npfr1^dsRNA), showed normal feeding responses. These results indicate that neural NPFR1 mediates hunger regulation of food selection (Wu, 2006).

A potential problem of the previous transgenic studies is that NPF/NPFR1 signaling is likely to be disrupted in a relatively early stage of larval development. Conceivably, the NPF/NPFR1 neuronal pathway could be essential for ad libitum or hunger-driven feeding of richer liquid foods, but such an activity might be masked by some yet-unidentified compensatory mechanism triggered by its early loss. To test this idea, attempts were made to disrupt NPF/NPFR1 neuronal signaling in a temporally controlled manner by expressing a temperature-sensitive allele of shibire (shi^ts1) driven by npf-gal4 or npfr1-gal4. The shi^ts1 allele encodes a semidominant-negative form of dynamin that blocks neurotransmitter release at a restrictive temperature (>29°C). At the permissive temperature of 23°C, 120-min-fasted experimental larvae (npf-gal4 and npfr1-gal4 x UAS-shi^ts1) and paired controls (y w x UAS-shi^ts1 and npf-gal4 and npfr1-gal4 x w¹¹¹⁸) displayed normal feeding responses to both liquid and solid foods. However, if larvae were incubated at 30°C for 15 min, controls still displayed normal feeding activities, whereas the experimental larvae showed attenuated feeding response to the solid but not liquid food. Therefore, there was no detectable developmental or physiological compensation for the loss of NPF signaling in Drosophila larvae. These results also suggest that the NPF/NPFR1 neuronal pathway is acutely required to initiate and maintain larval hunger response. The foraging activity of the experimental larvae was completely restored when the assay temperature was reduced to 23°C, suggesting that the NPF system can modulate the intensity and duration of feeding response (Wu, 2006).

This study has shown that dS6K regulates different, yet coordinated, behaviors controlling quantitative and qualitative aspects of hunger-adaptive food response. Evidence is provided that dS6K mediates hunger regulation of two opposing insulin- and NPY-like signaling activities, dynamically modifying larval food preference and feeding rate based on the nutritional state. For example, hunger stimuli may cause a reduction of dS6K activity in DILP neurons, resulting in the suppression of DILP signaling that negatively regulates a downstream NPF/NPFR1-dependent and another NPF-independent neuronal pathway. The DILP/NPFR1 neuronal pathway selectively mediates hunger-adaptive change in food preference, possibly by overriding the high threshold of food acceptance set by a separate default pathway, enabling hungry animals to be receptive to less preferred foods. The NPF/NPFR1-independent pathway promotes a general increase in the ingestion rate of preferred/less preferred foods, enabling animals to compete effectively for limited food sources. This study also implicates the presence of a separate default pathway for mediating the selective intake of preferred foods (baseline feeding) in larvae fed ad libitum. This default pathway may be largely insensitive to DILP or NPF signaling, because overexpression of dS6K, DILPs, or NPFR1 in nondeprived larvae does not affect ad libitum feeding in the liquid food. It is suggested that the conserved S6K pathway may be critical for regulating behavioral adaptation to hunger in diverse organisms, including humans, and its components are potential drug targets for appetite control (Wu, 2006).

The functional differences of DILP1–7 have not been reported previously. In this study, dilp2, dilp3, and dilp4 were shown to be functionally distinct. DILP2 and DILP3 both are produced in the same medial neurosecretory cells. However, unlike DILP2, DILP3 is apparently not involved in suppressing deprivation-motivated feeding. It is still unclear whether the differential activities of DILP2 and DILP3 reflect their structural divergence or are caused by the presence of yet-unidentified dInR isoforms. DILP4 is not expressed within the two medial clusters of DILP neurons. Under acute deprivation, the level of dilp4 transcripts showed a 5-fold reduction in the larval CNS. Thus, it is possible that DILP4 may play a localized role in promoting feeding response inside the CNS (Wu, 2006).

Feeding is a reward-seeking behavior, and deprivation strengthens the reinforcing effect (reward value) of food. These studies suggest a previously uncharacterized role of the DILP/dInR signaling pathway in regulating an animal's perception of food quality. The DILP/NPF neural network may regulate an animal's incentive to acquire lower-quality foods by modifying the reward circuit. This hypothesis is interesting in light of the findings that foods and abused substances may act on the same reward circuit, and highly palatable foods can reduce drug-seeking behaviors. It is also possible that the DILP/NPF system might represent a specialized neural circuit that positively alters the reward value of lesser-quality foods. Conceivably, a better understanding of the action of this signaling system may provide fresh insights into neural mechanisms for controlling eating and drug-seeking behaviors (Wu, 2006).

Given its prominent role in behavioral adaptation to hunger, the insulin/NPY-like neural network is likely of primary importance to animal evolution. In addition, insulin and NPY family molecules have been found in a wide range of animals from humans to worms. Therefore, the insulin/NPY-like network may be a useful model for studying comparatively how diverse animals have evolved distinct ways of adapting an ancestral neural system to suit their respective lifestyles (Wu, 2006).

S6k is identified in a screen for genes that function in leg disc regeneration in Drosophila

Many diverse animal species regenerate parts of an organ or tissue after injury. However, the molecules responsible for the regenerative growth remain largely unknown. The screen reported in this study aimed to identify genes that function in regeneration and the transdetermination events closely associated with imaginal disc regeneration using Drosophila melanogaster. A collection of 97 recessive lethal P-lacZ enhancer trap lines were screened for two primary criteria: first, the ability to dominantly modify wg-induced leg-to-wing transdetermination and second, for the activation or repression of the lacZ reporter gene in the blastema during disc regeneration. Of the 97 P-lacZ lines, six genes (Krüppelhomolog- 1, rpd3, jing, combgap, Aly and S6 kinase) were identified that met both criteria. Five of these genes suppress, while one enhances, leg-to-wing transdetermination and therefore affects disc regeneration. Two of the genes, jing and rpd3, function in concert with chromatin remodeling proteins of the Polycomb Group (PcG) and trithorax Group (trxG) genes during Drosophila development, thus linking chromatin remodeling with the process of regeneration (McClure, 2008).

There are three different mechanisms that organisms use to re-grow and replace lost or damaged body parts, and often, more than one mechanism can function within different tissues of the same organism. Muscle and bone, for example, repair themselves by activating a resident stem cell population, while the liver regenerates by compensatory proliferation of normally quiescent differentiated cells. Appendage/fin regeneration in lower vertebrates occurs by a process termed epimorphic regeneration, which proceeds in three distinct stages: (1) wound healing and migration of the surrounding epithelial cells to form the wound epidermis, (2) formation of the regeneration blastema -- a mass of undifferentiated and proliferating cells of mesenchymal origin and (3) regenerative outgrowth and pattern re-formation. Whether these diverse modes of regeneration share a common molecular and genetic basis is not known (McClure, 2008).

Regeneration in the Drosophila imaginal discs, the primordia of the adult fly appendages, closely parallels epimorphic limb/fin regeneration in lower vertebrates. Cells in the imaginal discs are rigidly determined to form specific adult structures (e.g., legs and wings) by the third larval instar. If the discs are fragmented at this time and cultured in vivo, they will regenerate. Disc regeneration begins 12 h after wounding, when transient heterotypic contacts are made between peripodial (squamous epithelium) and columnar cells (disc proper) near the cut edges of the wound. These initial contacts involve microvilli-like extensions and provide temporary wound closure. Then, approximately 24 h after wounding, homotypic cell contacts (between columnar or between squamous cells) are made involving the close apposition of cell membranes and cellular bridges, which eventually (48 h after wounding) restore the physical continuity of the disc. Before and during wound healing, cell division is randomly distributed throughout the disc. However, once completed (36-48 h after wounding), division is observed only in cells near the wound site. These cells are known as the regeneration blastema. Thus, like appendage regeneration in lower vertebrates, disc regeneration involves wound healing followed by blastema formation (McClure, 2008).

Blastema cells are responsible for the regeneration and repatterning of the entire missing disc fragment. Thus, these cells exhibit remarkable developmental plasticity. For example, in anterior- only leg disc fragments, some blastema cells will switch to posterior identity and establish a novel posterior compartment in the regenerate. This anterior/posterior conversion occurs during heterotypic wound healing, when hedgehog (hh)- expressing peripodial cells induce ectopic engrailed (en) expression in the apposing anterior columnar cells. In addition, the disc blastema, like its vertebrate counterpart, is able to form a normal regenerate (complete leg disc and adult leg) when isolated from the remaining disc fragment. Regenerative plasticity is also observed when a few blastema cells switch fate to that of another disc type (e.g., leg-to-wing), in a phenomenon known as transdetermination. Transdetermination events are closely associated with regenerative disc growth. Clonal analysis, for example, has shown that blastema cells first regenerate the missing disc structures, and only then, are they competent to transdetermine (McClure, 2008).

Little is known about how the regeneration blastema forms in the fragmented leg disc, although ectopic Wingless (Wg/Wnt1) expression is detected along the cut site, both prior to and during blastema formation. Wg is a developmental signal in many different tissues and animals; in flies Wg patterns all of the imaginal discs, functioning as both a morphogen and mitogen to regulate disc cell fate and growth. In lower vertebrates, Wnt ligands are key regulators of blastema formation during epimorphic regeneration. Thus, activation of Wg within the disc blastema is potentially important for regeneration. This idea is consistent with the observation that ubiquitous expression of wg during the second or third larval instars, in unfragmented leg discs, is sufficient to induce a regeneration blastema in the proximodorsal region of the disc, known as the weak point. Moreover, ubiquitous expression of wg mimics the pattern deviations associated with leg disc fragmentation and subsequent regeneration, including the duplication of ventral with concomitant loss of dorsal pattern elements and leg-to-wing transdetermination events. Thus, leg disc regeneration can be examined using two experimental protocols: fragmentation or ubiquitous wg expression. However, it is important to point out that only fragmentation-induced regeneration involves wound healing (McClure, 2008).

Precisely which molecules and signaling pathways are required for the process of regeneration remain poorly understood, partly because the organisms historically used to study regeneration (e.g., newts and salamanders) have been refractory to genetics and molecular manipulations. Recently, however, the use of new genetic techniques together with 'regeneration' model systems -- such as planarians, hydra and zebrafish have given researchers the opportunity to examine the mechanisms of regeneration and to identify the genes, proteins and signaling pathways that regulate different regenerative processes. For example, a large scale RNAi-based screen was performed to survey gene function in planarian tissue homeostasis and regeneration. Out of ~1000 genes examined, RNAi knock-down of 240 displayed regeneration-related phenotypes, including defects in wound healing, blastema formation and blastema cell differentiation. Despite these studies, however, it remains unclear whether regeneration requires only the modulation of genes expressed at the time of injury, the reactivation of earlier developmental genes and/or signaling pathways, or the activation of novel genes specific to the process of regeneration. Thus, a major interest in the field of regenerative biology is the identification of gene products that regulate blastema formation, blastema growth and regenerative cellular plasticity. A genetic screen, using wg-induced leg disc regeneration, aimed at identifying genes that regulate cellular plasticity and regeneration using Drosophila was carried out prothoracic leg discs. A collection of 97 recessive lethal P-element lacZ (PZ) insertion lines were screened for ectopic lacZ expression during wg-induced leg disc regeneration, and six genes were identified that function in wg-induced leg disc regeneration, including genes with functional ties to Wg signaling as well as chromatin remodeling proteins (McClure, 2008).

This study consisted of an enhancer trap screen designed to identify genes with changed gene expression during leg disc regeneration as well as required for regenerative proliferation and growth. The screen identified 19 genes that when heterozygous mutant (PZ/+), dominantly modify wg-induced leg-to-wing transdetermination, which serves as a functional assay for disc regeneration. Of the 19 genes, 37% are transcription factors or involved in transcriptional regulation (tai, Krh1, ken, jing, combgap (cg), rpd3 and Aly), 21% function in cell cycle regulation and growth (oho23B, S6k, polo and cycA), 10.5% play a role in protein secretion (Secβ61 and Syx13), and 31% are of other or unknown function [l(3)01629, CG30947, l(2)00248, l(3)05203, l(3)01344, Nup154]. The identification of transcription factors as the most frequent class of genes that modify wg-induced leg disc regeneration was similarly observed in a DNA microarray screen designed to identify genes enriched in leg disc cells that transdetermine to wing (Klebes, 2005). Together, these findings strongly suggest that transcription factors and their downstream targets play a prominent role in disc cell plasticity (McClure, 2008).

Using lacZ expression analyses, together with whole mount in situ hybridization experiments, the expression patterns of the 19 genes that modified wg-induced leg-to-wing transdetermination were verified. This analysis identified several different expression patterns upon wg-induced regeneration, including a loss of gene expression, ubiquitous expression and genes with expression limited to the regeneration blastema. Such observations indicate that a complex change of gene expression, both negative and positive, mediates the process of epimorphic regeneration. Six (jing, Alyi cg, rpd3, Kr-h1 and S6k) of the 19 modifiers displayed expression limited to the regeneration blastema, indicating that novel markers of regeneration and transdetermination have been identified. The blastema-specific expression patterns of jing, Aly, cg, Kr-h1, rpd3 and S6k raised the intriguing possibility that these genes may be functionally involved in the formation, cell proliferation or maintenance of the blastema during disc regeneration. Indeed, upon ubiquitous wg expression jing/+ animals rarely formed a regeneration blastema, indicating that two wild-type copies of jing are required for the initiation of the regenerative process. In contrast, Aly/+ and cg/+ animals formed a normal blastema, but only after a one-day delay. Therefore, two wild-type copies of the Aly and cg genes are required for the proper timing of regeneration. In addition, it was found that the frequency of blastema formation was reduced in rpd3/+ animals, implicating this gene in the process of regeneration. Interestingly, heterozygous mutations in all four of these genes (jing, Aly, cg and rpd3) strongly suppress wg-induced leg-to-wing transdetermination. It is speculated that the transdetermination frequency declines in these mutant animals because the initiation and/or timing of blastema formation is delayed. This idea is consistent with all previous work which has shown that blastema cells are only competent to transdetermine after they have regenerated the missing disc structures. Heterozygous mutations in Kr-h1 and S6k did not significantly alter the formation of the wg-induced regeneration blastema, however, these genes did affect regeneration-induced transdetermination. Such results suggest that Kr-h1 and S6k specifically function to modulate the cell fate changes that occur as a consequence of regeneration (McClure, 2008).

Investigations into the molecular basis of transdetermination have shown that inputs from the Wg, Decapentapelagic (Dpp) and Hedgehog (Hh) signaling pathways activate key selector genes out of their normal developmental context, such as ectopic Vg activation in the leg disc, which then drives cell-fate switches. Several of the genes identified in this screen have functional ties to Wg, Dpp and Hh signaling pathways. For example, Cg is a zinc-finger transcription factor that is required for proper transcriptional regulation of the Hh signaling effector gene Cubitus interruptus (Ci). In cg mutant wing and leg discs, Ci expression is lowered in the anterior compartment, resulting in the ectopic activation of wg and dpp and significant disc overgrowth. Another gene identified in this screen -- ken, functions in concert with Dpp to direct the development of the Drosophila terminalia. Further characterizations of whether these genes and other modifiers of transdetermination and regeneration affect Wg, Dpp and Hh expression and/or signaling may shed light on the regulation of regeneration and regeneration-induced proliferation and cell fate plasticity (McClure, 2008).

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date revised: 30 July 2008

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