Sex combs on midleg

DEVELOPMENTAL BIOLOGY

Embryonic

In agreement with genetic studies that establish a maternal contribution to Scm function (Breen, 1986), SCM mRNA is expressed in ovaries. The highest levels are seen in 0- to 2-hour embryos, presumably reflecting the maternal product. SCM mRNA is present at lower levels during later embryonic and larval stages, and a modest increase is seen during the pupal stage. This temporal profile resembles that seen for most other PcG products, which are expressed maternally and then continuously during subsequent development. Since the size of the mRNA detected is approximately 4 kb, the 4.1 kb and 3.8 kb cDNAs represent full-length or nearly full-length products. Although this Northern analysis fails to resolve multiple mRNA species in the 4 kb size range, the relative mobilities of the hybridizing species are consistent with maternal expression of both the 4.1 and 3.8 kb mRNA forms and zygotic expression of primarily the 4.1 kb form. The spatial distribution of SCM mRNA in embryos was determined by whole-mount in situ hybridization. In 9-hour-old embryos SCM mRNA is distributed throughout the embryo. The uniform pattern persists until about 12 hours of development, after which expression is more concentrated in the central nervous system (CNS). This parallels the time when homeotic gene expression also becomes enriched in the CNS compared to other tissues. The even distribution of SCM mRNA along the A-P axis and its concentration in the embryonic CNS resemble the expression patterns of other PcG products (Bornemann, 1996).

Effects of Mutation or Deletion

Animals doubly mutant for two different PcG genes often show phenotypes more extreme than either single mutant alone. It has been suggested that this phenotypic enhancement results from PcG protein complexes that are more severely impaired by simultaneous reduction or alteration of multiple components. Having defined the molecular lesions and relative strengths of many Scm mutations, it was of interest to test the Scm alleles for interactions with other PcG mutations. In particular, a comparison was made of genetic interactions exhibited by mutations affecting different parts of SCM protein (Bornemann, 1998).

Animals were generated that were doubly heterozygous for each of the Scm alleles and for a lethal allele of Polycomb (Pc³). Transheterozygous Pc³/Scm adults display more severe homeotic phenotypes than Pc³/+ adults, and this enhancement is seen with all Scm alleles tested. These phenotypes include transformations of wing to haltere, antenna to leg, second and third leg to first leg, and fourth abdominal segment to fifth. However, three Scm alleles, Su(z)302, R5-13, and ET50, produce much stronger interactions with Pc³ than do other Scm alleles. These three are the only Scm alleles to cause partial lethality in combination with Pc³. The transheterozygous progeny classes for Su(z)302, R5-13, and ET50 are reduced to about one-third that expected for full viability. In contrast, the Scm null alleles H1 and M56 are fully viable with Pc³. The surviving Su(z)302, R5-13, and ET50 transheterozygous progeny also exhibit more severe homeotic phenotypes than do other Pc³/Scm combinations. A marked male sex bias was observed among these survivors. In the most severe case, only about 5% of the surviving Pc³/Scm^Su(z)302 progeny were female. Similarly, partial lethality and a male sex bias were seen with the reciprocal crosses consisting of Su(z)302, R5-13, or ET50 females mated to Pc³ males. These interactions likely result from the Scm lesions rather than mutations at other loci because these three Scm alleles produce similar phenotypic effects and were isolated independently on different genetic backgrounds. Each of the three alleles maps to the first mbt repeat in SCM protein. Thus, Su(z)302, R5-13, and ET50 are hypomorphic mutations based upon their behavior as homozygotes, yet they interact with Pc³ more strongly than do null Scm alleles (Bornemann, 1998).

It was of interest to see if the strong Su(z)302, R5-13, and ET50 interactions reflect a general property of these mbt repeat alleles. In particular, since Pc³ is an antimorphic allele, Pc³ might show unusual interaction patterns with Scm alleles. To address this, genetic interactions of Scm alleles with mutations in other PcG genes and with other mutant alleles of the Pc gene were examined. Interaction of several Scm alleles was found with Sce^D1, an allele of the Sex combs extra (Sce) gene. This is the single existing allele of the uncloned Sce gene; it is homozygous lethal and its precise nature has not been characterized. As with Pc³, there is partial lethality and male sex bias among Sce^D1/Scm transheterozygotes when the Scm mbt repeat alleles are used. In the most extreme example, only 1 out of 60 surviving Sce^D1/Scm^ET50 adults was female. Once again, transheterozygotes with the Scm null alleles H1 and M56 are fully viable and produce progeny in the expected sex ratio (Bornemann, 1998).

The ph⁴⁰⁹ mutation disrupts one of the two tandem copies of the polyhomeotic gene located on the X chromosome. Although ph⁴⁰⁹ is a hypomorphic, homozygous viable allele, it is lethal or near-lethal in combination with other PcG mutations, including alleles of Scm. To assess ph interaction, ph⁴⁰⁹ females were crossed to males bearing Scm mutations and viability was scored among the double mutant male progeny. All Scm alleles cause partial lethality among ph⁴⁰⁹; Scm/+ males. However, Su(z)302, R5-13, and ET50 show much higher lethality in this combination than do H1 and M56 (Bornemann, 1998).

Finally, a subset of the Scm alleles was tested for interaction with additional Pc alleles, Pc² and Pc^XT109. Pc² is a frameshift near the C terminus that produces detectable PC protein of about the normal size. Pc^XT109 is associated with a 2-kb deletion, and PC protein is not detected in Pc^XT109 mutant embryos. Interactions with Pc² are similar to interactions with Pc³; the three mbt repeat alleles produce partial lethality in combination with Pc², whereas Scm^H1/Pc² and Scm^M56/Pc² animals are fully viable. The Scm mutations, as a group, show less severe enhancement with Pc^XT109 than with Pc² or Pc³. However, the same trend is observed; combinations with the three mbt repeat alleles produce more severe wing-to-haltere and extra sex comb transformations compared to the H1 and M56 null alleles. Thus, in tests for genetic enhancement employing different PcG genes and different Pc alleles, three mbt repeat alleles of Scm consistently produce the strongest interactions (Bornemann, 1998).

Mutations in a subset of PcG genes, including Scm, have been shown to modify eye color in flies bearing the zeste¹ (z¹) mutation. Indeed, the Scm^Su(z)302 allele was isolated as a dominant suppressor of z¹. The Scm^XF24 null allele and a deficiency for the Scm locus are also dominant suppressors of z¹ eye color, although their effects are weaker than seen with Su(z)302. To further this analysis, other Scm alleles were tested in combination with the z¹ w^is tester chromosome. Except for Su(z)302, heterozygosity for each Scm allele converts eye color in z¹ w^is males from light-orange to red-orange. Su(z)302 is unique in producing much stronger suppression, which is manifested by dark red eye color in this combination. Since Su(z)302 is a missense mutation in mbt repeat 1, this difference may reflect a role for mbt repeats in molecular interactions that contribute to zeste¹ suppression (Bornemann, 1998).

Mbt repeat mutants produce stable SCM protein that associates with normal chromosomal target loci. The Su(z)302, R5-13, and ET50 molecular lesions suggest that these strongly-interacting alleles encode altered SCM proteins that are essentially full-length. To test for production of these mutant proteins, extracts were prepared from Scm mutant pupae ; Western blots with anti-SCM antibody were performed. Homozygous Su(z)302, homozygous R5-13, and hemizygous ET50 pupae express full-length versions of SCM protein at levels similar to wild type. Since these mutant pupae are derived from heterozygous Scm mutant mothers, it is conceivable that the signals could reflect maternal protein that has perdured to pupal stages. However, the increase in wild-type SCM levels during development from larval to pupal stages, indicates that a substantial portion of pupal SCM is newly synthesized protein. In addition, reduced signal was reproducibly seen with extracts from homozygous M36 mutant pupae; this indicates that maternal product, if present, does not compromise detection of reduced zygotic SCM levels at this stage. Thus, the Su(z)302, R5-13, and ET50 mutants accumulate SCM proteins at levels comparable to wild type. To assess whether these SCM mutant proteins associate with target sites in vivo, anti-SCM antibodies were used to stain polytene chromosomes from larvae homozygous for the Su(z)302 or R5-13 alleles. Five sites of wild-type SCM protein accumulation in the bithorax complex (BX-C) are apparent. These sites still accumulate SCM protein encoded by the Su(z)302 and R5-13 alleles. The number of staining sites per genome and the signal intensities were similar for these two mutants and wild type. The same result is obtained if both the maternal and zygotic contributions consist of Su(z)302 mutant SCM protein. Thus, the signals are not due to perdurance of wild-type maternal SCM and must reflect the chromosome-binding properties of the mutant protein (Bornemann, 1998).

Su(z)302 mutant SCM protein is sufficient for embryonic but not pupal development. The homeotic gene misexpression and embryonic lethality seen with Scm null alleles show that SCM protein is required during embryogenesis. In contrast, expression of Abdominal-A protein in homozygous mutant Scm^Su(z)302 embryos appears normal, and these animals survive most of pupal development to die as pharate adults with homeotic phenotypes. Thus, Su(z)302 mutant protein appears defective in homeotic repression in pupae, and not in embryos. Whether Su(z)302 protein is also defective in embryonic processes is unclear because maternally provided, wild-type SCM product is present in homozygous Su(z)302 embryos. To address this, the dominant female sterile technique and FLP recombination system were used to generate Su(z)302 mutant embryos from Su(z)302 germline clone mothers. Embryos that express solely the Su(z)302 form of SCM protein from maternal and zygotic sources still show normal patterns of Abd-A expression. Like Su(z)302 homozygotes from Su(z)302/+ mothers, these animals survive to pupal stages. It is concluded that Su(z)302 mutant protein provides sufficient Scm function for apparently normal embryogenesis but that it is compromised in requirements for pupal development (Bornemann, 1998).

The fact that four Scm alleles cause alterations within the first mbt repeat shows that this domain contributes to Scm function in vivo. Several of these hypomorphic alleles produce wild-type levels of Scm protein that associates with normal sites in polytene chromosomes. This suggests that mbt repeats are not crucial for targeting SCM protein to specific loci but, rather, that they provide a biochemical activity required at the normal location in chromatin. The repeats could provide catalytic activity or a protein interaction surface needed to localize or bind another partner protein. The conservation of the two mbt repeats with 69% identity in a mouse SCM homolog (F. Randazzo, personal communication to Bornemann, 1999) implies that their biochemical role is key for Scm function. Thus, the SPM domain and the mbt repeats are two distinct functional domains in SCM protein (Bornemann, 1998).

Two genes known to control the determination of segmental identity in Drosophila melanogaster are polycomb and antennapedia. To identify additional genes involved in the determination of segmental identity, dominant modifers (both suppressors and enhancers) of polycomb and/or antennapedia mutations have been isolated. Sixty-four such modifier mutations have been recovered and mapped to 18 complementation groups. All of the mutations identify genes necessary for viability of the zygote. Six of the 18 genes that were identified by mutations that interact with polycomb and/or antennapedia have been previously characterized as homoeotic genes [i.e., Sex combs reduced (Scr), Brista (Ba), trithorax (trx), Polycomb (Pc), Polycomblike (Pcl) , and Sex combs on midleg (Scm)]. Mutations in several of the additional loci identified here have also been shown to have homoeotic phenotypes (Kennison, 1988).

The Polycomb (Pc) group genes encode repressors that restrict expression of homeotic genes to precise domains along the anterior-posterior (A-P) axis. Germ-line transformation constructs were used, containing portions of the bxd, iab-2, or iab-3 regulatory regions of the bithorax complex (BX-C), that are controlled by Pc group products in embryos. There are multiple BX-C elements that mediate Pc group control, are called Pc group response elements (PREs), and they can work when removed from the normal BX-C context. These constructs are each regulated by the genes Polycomb (Pc), extra sex combs (esc), Enhancer of zeste (E[z]), polyhomeotic (ph), Polycomb-like (Pcl), Sex combs on midleg (Scm), Posterior sex combs (Psc) , and Additional sex combs (Asx), consistent with multiple Pc group products acting together. Depending upon context, a PRE from the iab-3 region can restrict expression in different A-P positions. Thus, PREs are not specialized for particular parasegments, suggesting that Pc group products do not directly specify A-P boundaries of homeotic expression. Instead, the results support the idea that Pc group products provide stable memory or imprinting of boundaries that are initially specified by gap and pair-rule regulators (Simon, 1993).

The Polycomb (Pc) group genes of Drosophila are negative regulators of homeotic genes, but individual loci have pleiotropic phenotypes. It has been suggested that Pc group genes might form a regulatory hierarchy, or might be members of a multimeric complex that obeys the law of mass action. Recently, it was shown that Polyhomeotic (Ph) immunoprecipitates in a multimeric complex that includes Pc. Duplications of ph are shown to suppress homeotic transformations of Pc and Pcl, supporting a mass-action model for Pc group function. ph alleles were crossed to all members of the Polycomb group, and to E(Pc) and Su(z)2 to look for synergistic effects. Extragenic noncomplementation was observed between ph503 and Pc, Psc1 and Su(z)2(1) in females, and between ph409 and Sce1, ScmD1 and E(z)1 mutations in males, suggesting that these gene products might interact directly with Ph. Males hemizygous for a temperature-sensitive allele, ph2, are lethal when heterozygous with mutants in Asx, Pc, Pcl, Psc, Sce and Scm, and with E(Pc) and Su(z)2. Mutations in trithorax group genes are not able to suppress the lethality of ph2/Y;Psc1/+ males. ph2 is not lethal with extra sex combs, E(z), super sex combs (sxc) or l(4)102EFc heterozygotes, but cause earlier lethality in embryos homozygous for E(z), sxc and l(4)102EFc. However, ph503 does not enhance homeotic phenotypes of esc heterozygotes derived from homozygous esc- mothers. The embryonic phenotypes of ph2 embryos were examined that were lethal when heterozygous or homozygous for other mutations. Based on this phenotypic analysis, it has been suggested that ph may perform different functions in conjunction with differing subsets of Pc group genes (Cheng, 1994).

The Polycomb (Pc) group of genes are required for maintenance of cell determination in Drosophila melanogaster. At least 11 Pc group genes have been described and there may be up to 40; all are required for normal regulation of homeotic genes, but as a group, their phenotypes are rather diverse. It has been suggested that the products of Pc group genes might be members of a heteromeric complex that acts to regulate the chromatin structure of target loci. The phenotypes of adult flies heterozygous for every pairwise combination of Pc group genes were examined in an attempt to subdivide the Pc group functionally. The results support the idea that Additional sex combs (Asx), Pc, Polycomblike (Pcl), Posterior sex combs (Psc), Sex combs on midleg (Scm), and Sex combs extra (Sce) have similar functions in some imaginal tissues. Genetic interactions are shown among extra sex combs (esc) and Asx, Enhancer of Pc, Pcl, Enhancer of zeste [E(z)], and super sex combs and the idea that most Pc group genes function independently of esc has been reassessed. Most duplications of Pc group genes neither exhibit anterior transformations nor suppress the extra sex comb phenotype of Pc group mutations, suggesting that not all Pc group genes behave as predicted by the mass-action model. Surprisingly, duplications of E(z) have been found to enhance homeotic phenotypes of esc mutants. Flies with increasing doses of esc+ exhibit anterior transformations, but these are not enhanced by mutations in trithorax group genes. The results are discussed with respect to current models of Pc group function (Campbell, 1995).

The proteins encoded by two groups of conserved genes, the Polycomb and trithorax groups, have been proposed to maintain, at the level of chromatin structure, the expression pattern of homeotic genes during Drosophila development. To identify new members of the trithorax group, a collection of deficiencies were screened for intergenic noncomplementation with a mutation in ash1, a trithorax group gene. Five of the noncomplementing deletions uncover genes previously classified as members of the Polycomb group. This evidence suggests that there are actually three groups of genes that maintain the expression pattern of homeotic genes during Drosophila development. The products of the third group appear to be required to maintain chromatin in both transcriptionally inactive and active states. Six of the noncomplementing deficiencies uncover previously unidentified trithorax group genes. One of these deficiencies removes 25D2-3 to 26B2-5. Within this region, there are two allelic, lethal, P-insertion mutations that identify one of these new trithorax group genes. The gene has been called little imaginal discs based on the phenotype of mutant larvae. The protein encoded by the little imaginal discs gene is the Drosophila homolog of human retinoblastoma binding protein 2 (Gilde, 2000).

When heterozygous, trithorax mutations cause either no transformations or an extremely low frequency of transformations of the third thoracic segment to the second segment. However, when homozygous, trithorax mutations cause transformations of the first and third thoracic segments to the second segment and anterior transformations of the abdominal segments. Other genes in which mutations cause similar phenotypes have been classified as members of the trithorax group. Trithorax group genes have been identified by several approaches. Two of the trithorax group genes, ash1 and ash2, were identified as pupal lethal mutations that disrupt imaginal disc development. Most of the other trithorax group genes were identified in a genetic screen for dominant suppressors of the adult phenotypes of dominant Polycomb or Antennapedia mutations. Like mutations in Polycomb group genes, mutations in trithorax group genes show intergenic noncomplementation, i.e., heterozygosis for recessive mutations in two different trithorax group genes can cause an adult mutant phenotype. The phenotype can include partial transformations of the first and third thoracic segments to the second thoracic segment and partial anterior transformations of the abdominal segments. The similar phenotypes of mutations in trithorax group genes and their intergenic noncomplementation has suggested that the products of these genes also act via multimeric protein complexes. Indeed, a 2-MD complex has been detected in embryos that contains the products of the trithorax group genes, brahma. However, this complex does not contain the products of the trithorax group gene ash1, which is in a different 2-MD complex, which also contains the product of the trithorax gene, nor does this complex contain the product of ash2, which is in a 0.5-MD complex. Taking advantage of the phenomenon of intergenic noncomplementation, a large fraction of the Drosophila genome was screened to look for new trithorax group genes. Females heterozygous for an ash1 mutation were crossed to males heterozygous for one of 133 deficiencies and the progeny doubly heterozygous for the ash1 mutation and the deficiency for homeotic transformations were examined. In this way regions of the genome with candidate trithorax group genes were identified (Gilde, 2000).

Six of the deficiencies uncovered genes that were previously classified in the Polycomb group. They were so classified, because they either enhanced the Polycomb mutant phenotype or caused a phenotype like Polycomb mutants. This result was quite unexpected because the antagonism between trithorax and Polycomb group genes suggested that loss of function of Polycomb group genes should suppress trithorax mutant phenotypes, while these deficiencies showed an enhancement of trithorax group mutant phenotypes. Nevertheless it is likely that the Polycomb group genes uncovered by these deficiencies are responsible for the observed intergenic noncomplementation with ash1^RE418. It was thought possible that the observed intergenic noncomplementation is specific for ash1 mutations rather than general for mutations in trithorax group genes. This possibility was excluded for four of the five genes by showing that E(Pc)¹, Psc¹, Su(z)2¹, Asx^XF23, Asx³, and Asx¹³ also show intergenic noncomplementation with trx^b11 and/or brm² and increase the penetrance of two different double mutants: ash1^{VF101 trxb11} and brm² trx^e2. It has also been reported that Asx mutations show intergenic noncomplementation with mutations in trithorax group genes. In some of these cases, the different mutant alleles tested give inconsistent results. For example, both Scm^D1 and Scm^m56 show intergenic noncomplementation with ash1^VV183 and enhance the phenotype of the ash1^VF101 trx^b11 double mutant, whereas Scm³⁰² does not enhance the phenotype of ash1^VV183 and suppresses the phenotype of ash1^VF101 trx^b11. It is supposed that this difference is due to differences in the specific alterations of the Scm protein caused by these mutations (Gilde, 2000).

Until now the antagonism of function between the products of Polycomb group genes and trithorax group genes has been demonstrated unidirectionally by the suppression of Polycomb group mutant phenotypes by mutations in trithorax group genes. Advantage was taken of the intergenic noncomplementation of mutations in trithorax group genes to assay suppression of trithorax group mutant phenotypes by mutations in genes previously classified as Polycomb group genes. Among ash1^VF101;trx^b11 and brm²;trx^e2 double heterozygotes, 52% and 35%, respectively, of adult flies express transformations of the third thoracic segment to the second thoracic segment. Most mutations in seven of the genes that have been classified as members of the Polycomb group (Polycomb, polyhomeotic, pleiohomeotic, Polycomb-like, multi sex combs, extra sex combs, and Super sex combs) suppress the penetrance of these transformations, in both of these double heterozygotes. Moreover, most mutations in these genes do not show intergenic noncomplementation with mutations in any of the three trithorax group genes that have been tested. It is suggested that these genes represent the Polycomb group defined here as genes in which loss-of-function mutations enhance the dominant phenotype caused by Polycomb mutations and suppress the phenotype caused by heterozygosity for double mutations in trithorax group genes such as ash1^VF101;trx^b11 and brm²;trx^e2 (Gilde, 2000).

The zeste (z) gene encodes a transcription factor that binds DNA in a sequence-specific manner. The z¹ mutation causes reduced white gene transcription. Mutations in three genes identified as dominant modifiers of the zeste-white interaction, Enhancer of zeste, Suppressor of zeste-2, and Sex comb on midleg, can also cause phenotypes like mutations in Polycomb group genes. Here it is shown that mutations in these three genes also behave as mutations in trithorax group genes: they show intergenic noncomplementation with mutations in trithorax group genes and/or increase the penetrance of ash1^VF101;trx^b11 and/or brm²;trx^e2 heterozygotes. Moreover, mutations in three other genes identified as suppressors of the zeste-white interaction, Suppressor of zeste-4, Suppressor of zeste-6, and Suppressor of zeste-7, may show intergenic noncomplementation with mutations in trithorax group genes and/or increase the penetrance of ash1^VF101;trx^b11 heterozygotes. The biochemical mechanism by which mutations in these genes modify the zeste-white interaction is not known. However, it is thought to be significant that many of the genes identified as Suppressors of zeste behave as if they are both trithorax and Polycomb group genes; that Enhancer of Polycomb is a suppressor of zeste, and that sex combs extra is an enhancer of zeste (Gilde, 2000).

It is proposed that the six genes (previously classified as Polycomb group genes) belong in a distinct group; in these genes, loss-of-function or antimorphic mutations show intergenic noncomplementation with mutations in trithorax group genes and increase the penetrance caused by double heterozygosis of mutations in trithorax group genes. It is proposed that this group be called the ETP (Enhancers of trithorax and Polycomb mutations) group. Loss-of-function mutations in this group of genes not only enhance the dominant phenotype caused by Polycomb mutations, as do mutations in Polycomb group genes but they also enhance the phenotype caused by heterozygosity for double mutations in trithorax group genes, such as ash1^VF101;trx^b11 and brm²;trx^e2, as do mutations in trithorax group genes (Gilde, 2000).

Mutations in many of the genes that have been classified in the ETP group lead to ectopic expression of homeotic genes in embryos. It has been inferred from such results that the normal function of the products of these genes is to repress transcription. However, a recent study of the consequences of mutations in one of these genes, Enhancer of zeste, demonstrated both ectopic expression and loss of expression of the same homeotic genes. That study was made possible by the availability of a strong temperature-sensitive allele. Without such alleles it would be very difficult to directly assay other members of the group for loss of homeotic gene expression. Nevertheless, the enhancement of the phenotype of mutations in both Polycomb and trithorax group genes by loss-of-function mutations in genes of the ETP group is interpreted as an indication that the products of these genes are required for both activation and repression of transcription. It has been proposed that the product of the zeste gene itself is also involved in both activation and repression of transcription. Little information is available on the biochemical mechanism of action of any of these genes. There is evidence of a multimeric protein complex containing the products of the Polycomb group genes, Polycomb and Polyhomeotic, and of three different complexes containing the products of the trithorax group genes, brahma, ash1, and ash2. One way of rationalizing how mutations in the ETP group of genes could behave as both Polycomb and trithorax group mutations would be to suggest that the products of the ETP genes are components of complexes required for both repression and activation. Perhaps they are responsible for the structure of these complexes or different protein variants encoded by these genes are components of different complexes. Although Polycomb and trithorax group genes were first identified in Drosophila, homologous genes exist in mammals. Until now, most interpretations of the functions of the products of such genes have been based on the idea that the products of Polycomb group genes repress gene transcription and the products of trithorax group genes activate gene transcription. The data presented here together with earlier data suggest that some of the genes previously classified as Polycomb group genes and at least some of the genes identified as suppressors or enhancers of zeste belong to a group of genes whose products play a role in both the repression and activation of gene transcription. These data will require new interpretations of the functions of such genes (Gilde, 2000).

Characterization of the grappa gene, a Drosophila histone H3 lysine 79 methyltransferase that interacts genetically with Sex combs on midleg

grappa (gpp) is the Drosophila ortholog of the Saccharomyces cerevisiae gene Dot1, a histone methyltransferase that modifies the lysine (K)79 residue of histone H3. gpp is an essential gene identified in a genetic screen for dominant suppressors of pairing-dependent silencing, a Polycomb-group (Pc-G)-mediated silencing mechanism necessary for the maintenance phase of Bithorax complex (BX-C) expression. Surprisingly, gpp mutants not only exhibit Pc-G phenotypes, but also display phenotypes characteristic of trithorax-group mutants. gpp dominantly enhances the phenotypic effects of mutations in Sex combs extra, Polycomblike, Sex combs on the midleg, and Polycomb. Mutations in gpp also disrupt telomeric silencing but do not affect centric heterochromatin. These apparent contradictory phenotypes may result from loss of gpp activity in mutants at sites of both active and inactive chromatin domains. Unlike the early histone H3 K4 and K9 methylation patterns, the appearance of methylated K79 during embryogenesis coincides with the maintenance phase of BX-C expression, suggesting that there is a unique role for this chromatin modification in development (Shanower, 2005).

Regulation of cellular plasticity in Drosophila imaginal disc cells by the Polycomb group, trithorax group and lama genes

Drosophila imaginal disc cells can switch fates by transdetermining from one determined state to another. The expression profiles of cells induced by ectopic Wingless expression to transdetermine from leg to wing were examined by dissecting transdetermined cells and hybridizing probes generated by linear RNA amplification to DNA microarrays. Changes in expression levels implicated a number of genes: lamina ancestor, CG12534 (a gene orthologous to mouse augmenter of liver regeneration), Notch pathway members, and the Polycomb and trithorax groups of chromatin regulators. Functional tests revealed that transdetermination was significantly affected in mutants for lama and seven different PcG and trxG genes. These results validate the described methods for expression profiling as a way to analyze developmental programs, and they show that modifications to chromatin structure are key to changes in cell fate. These findings are likely to be relevant to the mechanisms that lead to disease when homologs of Wingless are expressed at abnormal levels and to the manifestation of pluripotency of stem cells (Klebes, 2005).

When prothoracic (1st) leg discs are fragmented and cultivated in vivo, cells in a proximodorsal region known as the 'weak point' can switch fate and transdetermine. These 'weak point' cells give rise to cuticular wing structures. The leg-to-wing switch is regulated, in part, by the expression of the vestigial (vg) gene, which encodes a transcriptional activator that is a key regulator of wing development. vg is not expressed during normal leg development, but it is expressed during normal wing development and in 'weak point' cells that transdetermine from leg to wing. Activation of vg gene expression marks leg-to-wing transdetermination (Klebes, 2005).

Sustained proliferation appears to be a prerequisite for fate change, and conditions that stimulate growth increase the frequency and enlarge the area of transdetermined tissue. Transdetermination was discovered when fragments of discs were allowed to grow for an extensive period of in vivo culture. More recently, ways to express Wg ectopically have been used to stimulate cell division and cell cycle changes in 'weak point' cells (Sustar, 2005), and have been shown to induce transdetermination very efficiently. Experiments were performed to characterize the genes involved in or responsible for transdetermination that is induced by ectopic Wg. Focus was placed on leg-to-wing transdetermination because it is well characterized, it can be efficiently induced and it can be monitored by the expression of a real-time GFP reporter. These attributes make it possible to isolate transdetermining cells as a group distinct from dorsal leg cells, which regenerate, and ventral leg cells in the same disc, which do not regenerate; and, in this work, to directly define their expression profiles. This analysis identified unique expression properties for each of these cell populations. It also identified a number of genes whose change in expression levels may be significant to understanding transdetermination and the factors that influence developmental plasticity. One is lamina ancestor (lama), whose expression correlates with undifferentiated cells and is shown to control the area of transdetermination. Another has sequence similarity to the mammalian augmenter of liver regeneration (Alr; Gfer -- Mouse Genome Informatics), which controls regenerative capacity in the liver and is upregulated in mammalian stem cells. Fifteen regulators of chromatin structure [e.g. members of the Polycomb group (PcG) and trithorax group (trxG)] are differentially regulated in transdetermining cells, and mutants in seven of these genes have significant effects on transdetermination. These studies identify two types of functions that transdetermination requires -- functions that promote an undifferentiated cell state and functions that re-set chromatin structure (Klebes, 2005).

The importance of chromatin structure to the transcriptional state of determined cells makes it reasonable to assume that re-programming cells to different fates entails reorganization of the Polycomb group (PcG) and trithorax group (trxG) protein complexes that bind to regulatory elements. Although altering the distribution of proteins that mediate chromatin states for transcriptional repression and activation need not involve changes in the levels of expression of the PcG and trxG proteins, the array hybridization data was examined to determine if they do. The PcG Suppressor of zeste 2 [Su(z)2] gene had a median fold repression of 2.1 in eight TD to D_Wg/V_Wg comparisons, but the cut-off settings did not detect significant enrichment or repression of most of the other PcG or trxG protein genes with either clustering analysis or the method of ranking median ratios. Since criteria for assigning biological significance to levels of change are purely subjective, the transdetermination expression data was re-analyzed to identify genes whose median ratio changes within a 95% confidence level. Fourteen percent of the genes satisfied these conditions. Among these genes, 15/32 PcG and trxG genes (47%) had such statistically significant changes. Identification of these 15 genes with differential expression suggests that transdetermination may be correlated with large-scale remodeling of chromatin structure (Klebes, 2005).

To test if the small but statistically significant changes in the expression of PcG and trxG genes are indicative of a functional role in determination, discs from wild-type, Polycomb (Pc), Enhancer of Polycomb [E(Pc)], Sex comb on midleg (Scm), Enhancer of zeste [E(z)], Su(z)2, brahma (brm) and osa (osa) larvae were examined. The level of Wg induction was adjested to reduce the frequency of transdetermination and both frequency of transdetermination and area of transdetermined cells was determined. The frequency of leg discs expressing vg increased significantly in E(z), Pc, E(Pc), brm and osa mutants, and the frequency of leg to wing transdetermination in adult cuticle increased in Scm, E(z), Pc, E(Pc) and osa mutants. Remarkably, Su(z)2 heterozygous discs had no vg expression, suggesting that the loss of Su(z)2 function limits vg expression (Klebes, 2005).

Members of the PcG and trxG are known to act as heteromeric complexes by binding to cellular memory modules (CMMs). The functional tests demonstrate that mutant alleles for members of both groups have the same functional consequence (they increase transdetermination frequency). The findings are consistent with recent observations that the traditional view of PcG members as repressors and trxG factors as activators might be an oversimplification, and that a more complex interplay of a varying composition of PcG and trxG proteins takes place at individual CMMs. Furthermore the opposing effects of Pc and Su(z)2 functions are consistent with the proposal that Su(z)2 is one of a subset of PcG genes that is required to activate as well as to suppress gene expression. In addition to measuring the frequency of transdetermination, the relative area of vg expression was examined in the various PcG and trxG heterozyogous mutant discs. The relative area decreased in E(Pc), brm and osa mutant discs, despite the increased frequency of transdetermination in these mutants. There is no evidence to explain these contrasting effects, but the roles in transdetermination of seven PcG and trxG genes that were identified by these results support the proposition that the transcriptional state of determined cells is implemented through the controls imposed by the regulators of chromatin structure (Klebes, 2005).

The determined states that direct cells to particular fates or lineages can be remarkably stable and can persist after many cell divisions in alien environments, but they are not immune to change. In Drosophila, three experimental systems have provided opportunities to investigate the mechanisms that lead to switches of determined states. These are: (1) the classic homeotic mutants; (2) the PcG and trxG mutants that affect the capacity of cells to maintain homeotic gene expression, and (3) transdetermination. During normal development, the homeotic genes are expressed in spatially restricted regions, and cells that lose (or gain) homeotic gene function presumably change the transcriptional profiles characteristic of the particular body part. In the work reported here, techniques of micro-dissection, RNA amplification and array hybridization were used to monitor the transcription profiles of cells in normal leg and wing imaginal discs, in leg disc cells that regenerate and in cells that transdetermine from leg to wing. The results validate the idea that changing determined states involves global changes in gene expression. They also identify genes whose function may be unrelated to the specific fates of the cells characterized, but instead may correlate with developmental plasticity (Klebes, 2005).

Overlap between the transcriptional profiles in the wing and transdetermination lists (15 genes) and with genes in subcluster IV (high expression in wing discs) is extensive. The overlap is sufficient to indicate that the TD leg disc cells have changed to a wing-like program of development, but interestingly, not all wing-specific genes are activated in the TD cells. The reasons could be related to the incomplete inventory of wing structures produced (only ventral wing) or to the altered state of the TD cells. During normal development, vg expression is activated in the embryo and continues through the 3rd instar. Although the regulatory sequences responsible for activation in the embryo have not been identified, in 2nd instar wing discs, vg expression is dependent upon the vgBE enhancer, and in 3rd instar wing discs expression is dependent upon the vgQE enhancer. Expression of vg in TD cells depends on activation by the vgBE enhancer, indicating that cells that respond to Wg-induction do not revert to an embryonic state. Recent studies of the cell cycle characteristics of TD cells support this conclusion (Sustar, 2005), but the role of the vgBE enhancer in TD cells and the incomplete inventory of 'wing-specific genes' in their expression profile probably indicates as well the stage at which the TD cells were analyzed: they were not equivalent to the cells of late 3rd instar wing discs (Klebes, 2005).

Investigations into the molecular basis of transdetermination have led to a model in which inputs from the Wg, Dpp and Hh signaling pathways alter the chromatin state of key selector genes to activate the transdetermination pathway. The analyses were limited to a period 2-3 days after the cells switched fate, because several cell doublings were necessary to produce sufficient numbers of marked TD cells. As a consequence, these studies did not analyze the initial stages. Despite this technical limitation, this study identified several genes that are interesting novel markers of transdetermination (e.g., ap, CG12534, CG14059 and CG4914), as well as several genes that function in the transdetermination process (e.g., lama and the PcG genes). The results from transcriptional profiling add significant detail to a general model proposed for transdetermination (Klebes, 2005).

(1) It is reported that ectopic wg expression results in statistically significant changes in the expression of 15 PcG and trxG genes. Moreover, although the magnitudes of these changes were very small for most of these genes, functional assays with seven of these genes revealed remarkably large effects on the metrics used to monitor transdetermination -- the fraction of discs with TD cells, the proportion of disc epithelium that TD cells represent, and the fraction of adult legs with wing cuticle. These effects strongly implicate PcG and trxG genes in the process of transdetermination and suggest that the changes in determined states manifested by transdetermination are either driven by or are enabled by changes in chromatin structure. This conclusion is consistent with the demonstrated roles of PcG and trxG genes in the self-renewing capacity of mouse hematopoietic stem cells, in Wg signaling and in the maintenance of determined states. The results now show that the PcG and trxG functions are also crucial to pluripotency in imaginal disc cells, namely that pluripotency by 'weak point' cells is dependent upon precisely regulated levels of PcG and trxG proteins, and is exquisitely sensitive to reductions in gene dose (Klebes, 2005).

The data do not suggest how the PcG and trxG genes affect transdetermination, but several possible mechanisms deserve consideration. A recent study (Sustar, 2005) reported that transdetermination correlates with an extension of the S phase of the cell cycle. Several proteins involved in cell cycle regulation physically associate with PcG and trxG proteins, and Brahma, one of the proteins that affects the metrics of transdetermination, has been shown to dissociate from chromatin in late S-phase and to reassociate in G1. It is possible that changes in the S-phase of TD cells are a consequence of changes in PcG/trxG protein composition (Klebes, 2005).

Another generic explanation is that transdetermination is dependent or sensitive to expression of specific targets of PcG and trxG genes. Among the 167 Pc/Trx response elements (PRE/TREs) predicted to exist in the Drosophila genome, one is in direct proximity to the vg gene. It is possible that upregulation of vg in TD cells is mediated through this element. Another factor may be the contribution of targets of Wg signaling, since targets of Wg signaling have been shown to be upregulated in osa and brm mutants. These are among a number of likely possible targets, and identifying the sites at which the PcG and trxG proteins function will be necessary if an understand is to be gained of how transdetermination is regulated. Importantly, understanding the roles of such targets and establishing whether these roles are direct will be essential to rationalize how expression levels of individual PcG and trxG genes correlate with the effects of PcG and trxG mutants on transdetermination (Klebes, 2005).

(2) The requirement for lama suggests that proliferation of TD cells involves functions that suppress differentiation. lama expression has been correlated with neural and glial progenitors prior to, but not after, differentiation, and it is observed that lama is expressed in imaginal progenitor cells and in early but not late 3rd instar discs. lama expression is re-activated in leg cells that transdetermine. The upregulation of unpaired in TD cells may be relevant in this context, since the JAK/STAT pathway functions to suppress differentiation and to promote self-renewal of stem cells in the Drosophila testis. It is suggested that it has a similar role in TD cells (Klebes, 2005).

(3) A role for Notch is implied by the expression profiles of several Notch pathway genes. Notch may contribute directly to transdetermination through the activation of the vgBE enhancer [which has a binding site for Su(H)] and of similarly configured sequences that were found to be present in the regulatory regions of 45 other TD genes. It will be important to test whether Notch signaling is required to activate these co-expressed genes, and if it is, to learn what cell-cell interactions and 'community effects' regulate activation of the Notch pathway in TD cells (Klebes, 2005).

(4) The upregulation in TD cells of many genes involved in growth and division, and the identification of DNA replication element (DRE) sites in the regulatory region of many of these genes supports the observation that TD cells become re-programmed after passing through a novel proliferative state (Sustar, 2005), and suggests that this change is in part implemented through DRE-dependent regulation (Klebes, 2005).

There was an interesting correlation between transdetermination induced by Wg mis-expression and the role of Wg/Wnt signaling for stem cells. Wg/Wnt signaling functions as a mitogen and maintains both somatic and germline stem cells in the Drosophila ovary, and mammalian hematopoetic stem cells. Although the 'weak point' cells in the Drosophila leg disc might lack the self-renewing capacity that characterizes stem cells, they respond to Wg mis-expression by manifesting a latent potential for growth and transdetermination. It seems likely that many of the genes are conserved that are involved in regulating stem cells and that lead to disease states when relevant regulatory networks lose their effectiveness (Klebes, 2005).

The prevalence of transcription factors among the genes whose relative expression levels differed most in the tissue comparisons was intriguing. It is commonly assumed that transcription factors function catalytically and that they greatly amplify the production of their targets, so the expectation was that the targets of tissue-specific transcription factors would have the highest degree of tissue-specific expression. In these studies, tissue-specific expression of 15 transcription factors among the 40 top-ranking genes in the wing and leg data sets (38%) is consistent with the large number of differentially expressed genes in these tissues, but these rankings suggest that the targets of these transcription factors are expressed at lower relative levels than the transcription factors that regulate their expression. One possible explanation is that the targets are expressed in both wing and leg disc cells, but the transcription factors that regulate them are not. This would imply that the importance of position-specific regulation lies with the regulator, not the level of expression of the target. Another possibility is that these transcription factors do not act catalytically to amplify the levels of their targets, or do so very inefficiently and require a high concentration of transcription factor to regulate the production of a small number of transcripts. Further analysis will be required to distinguish between these or other explanations, but it is noted that the prevalence of transcription factors in such data sets is neither unique to wing-leg comparisons nor universal (Klebes, 2005).

Suppression of Polycomb group proteins by JNK signalling induces transdetermination in Drosophila imaginal discs

During the regeneration of Drosophila imaginal discs, cellular identities can switch fate in a process known as transdetermination. For leg-to-wing transdetermination, the underlying mechanism involves morphogens such as Wingless that, when activated outside their normal context, induce ectopic expression of the wing-specific selector gene vestigial. Polycomb group (PcG) proteins maintain cellular fates by controlling the expression patterns of homeotic genes and other developmental regulators. Transdetermination events are coupled to PcG regulation. The frequency of transdetermination is enhanced in PcG mutant flies. Downregulation of PcG function, as monitored by the reactivation of a silent PcG-regulated reporter gene, is observed in transdetermined cells. This downregulation is directly controlled by the Jun amino-terminal kinase (JNK) signalling pathway, which is activated in cells undergoing regeneration. Accordingly, transdetermination frequency is reduced in a JNK mutant background. This regulatory interaction also occurs in mammalian cells, indicating that the role of this signalling cascade in remodelling cellular fates may be conserved (Lee, 2005).

Imaginal discs are clusters of cells that form the precursors of adult cuticular structures. On fragmentation and cultivation, a disc structure regenerates and forms the appendage, such as a leg or a wing, for which it was initially determined. Transdetermination events occur at sites of regeneration and on the ectopic expression of morphogens. Misregulation of PcG function causes homeotic transformations that are often phenocopied in transdetermination events. To determine whether PcG proteins are involved in transdetermination events, leg-to-wing fate changes were induced by ectopically overexpressing wingless (wg) and visualizing the transdetermined tissue by staining for ectopic expression of vestigial (vg) coupled to a lacZ reporter gene. In this assay, wild-type discs showed a transdetermination rate of 2.3% (2/87), whereas Polycomb (Pc) and Sex combs on midleg (Scm) heterozygous mutant discs showed an increased transdetermination rate of 17.7% (11/62) and 11.0% (10/91), respectively (Lee, 2005).

Polycomb group genes are required for neural stem cell survival in postembryonic neurogenesis of Drosophila

Genes of the Polycomb group (PcG) are part of a cellular memory system that maintains appropriate inactive states of Hox gene expression in Drosophila. This study investigates the role of PcG genes in postembryonic development of the Drosophila CNS. Mosaic-based MARCM techniques were used to analyze the role of these genes in the persistent larval neuroblasts and progeny of the central brain and thoracic ganglia. Proliferation in postembryonic neuroblast clones is dramatically reduced in the absence of Polycomb, Sex combs extra, Sex combs on midleg, Enhancer of zeste or Suppressor of zeste 12. The proliferation defects in these PcG mutants are due to the loss of neuroblasts by apoptosis in the mutant clones. Mutation of PcG genes in postembryonic lineages results in the ectopic expression of posterior Hox genes, and experimentally induced misexpression of posterior Hox genes, which in the wild type causes neuroblast death, mimics the PcG loss-of-function phenotype. Significantly, full restoration of wild-type-like properties in the PcG mutant lineages is achieved by blocking apoptosis in the neuroblast clones. These findings indicate that loss of PcG genes leads to aberrant derepression of posterior Hox gene expression in postembryonic neuroblasts, which causes neuroblast death and termination of proliferation in the mutant clones. These findings demonstrate that PcG genes are essential for normal neuroblast survival in the postembryonic CNS of Drosophila. Moreover, together with data on mammalian PcG genes, they imply that repression of aberrant reactivation of Hox genes may be a general and evolutionarily conserved role for PcG genes in CNS development (Bello, 2007).

Genetic analysis indicates that the PcG genes Sce, Scm, Pc, E(z) and Su(z)12 are required for postembryonic neurogenesis in the central brain and thoracic ganglia of Drosophila. In the absence of any one of these genes, several mutant phenotypes are observed in the third-instar CNS: (1) neural proliferation is dramatically reduced and only small numbers of cells are found in neuroblast clones; (2) proliferating postembryonic neuroblasts are absent in most of the mutant clones due to apoptosis; (3) posterior Hox genes are ectopically expressed in the postembryonic neuroblast lineages. It is hypothesized that these phenotypes are causally related, in that loss of PcG genes leads to ectopic Hox gene expression in postembryonic neuroblasts resulting in their premature cell death and, thereby, in drastically reduced neuroblast lineage size. Strong support for this hypothesis is provided by the fact that the mutant lineages proliferate normally if apoptosis is blocked. Corollary support for this notion is provided by the fact that Psc-Su(z)2 mutant clones, which do not show ectopic Hox gene expression, are consistently wild-type-like in size and presence of neuroblast (Bello, 2007).

Numerous genes are required for the continued mitotic activity of neuroblasts during postembryonic life. These findings provide the first demonstration that PcG genes are essential for neuroblast survival and proliferation in the postembryonic CNS. Previous work on PcG gene action during embryonic neurogenesis has demonstrated that the derepression of posterior Hox genes in PcG mutants leads to a change in the segmental determination of neuroblasts and their lineage, but not to their mitotic arrest and death. Thus, the effects of PcG gene loss on neurogenesis are context-dependent and differ during embryonic development as compared with postembryonic development. This is underscored in recent work which indicates that the PcG gene ph is essential for maintaining neuronal identity and diversity during metamorphosis (Bello, 2007).

In postembryonic development of the Drosophila CNS, a remarkable link exists between neuroblast survival and Hox gene expression. In the ventral ganglia, a neuroblast-specific pulse of abd-A during the third instar provides the cue for cell death, which limits the number of progeny produced per neuroblast. These data indicate that this mechanism, which in the wild type relates Hox gene expression to the clone size of neural stem cells, also operates in PcG mutants and is responsible for the PcG mutant phenotypes. Indeed, a general function of PcG genes in postembryonic neurogenesis may be to prevent the premature and widespread operation of this mechanism for temporal regulation of neurogenesis through termination of neuroblast life. It is noteworthy that the Hox gene-dependent activation of apoptosis within the CNS is selective for the neuroblast and does not occur when Hox genes are derepressed in neurons, either during normal development or in misexpression experiments. This explains why the neurons in PcG mutant clones, which were generated before the induction of neuroblast cell death, continue to survive despite the presence of ectopic Hox gene derepression (Bello, 2007).

These data indicate that loss of specific PcG genes in larval neuroblasts leads to ectopic Hox gene expression that is sufficient to cause neuroblast cell death. However, the PcG proteins may also contribute to neuroblast survival by repressing other unidentified target genes which, when derepressed, might result in premature death of postembryonic neuroblasts. Indeed, although deregulation of Hox gene expression is one of the hallmarks of PcG phenotypes in Drosophila, a diverse set of other target genes, including genes involved in cell cycle regulation, are controlled by PcG genes (Bello, 2007).

Interesting parallels to these findings on the role of PcG genes in neural proliferation come from studies of mammalian PcG genes, specifically of the Bmi1 gene. Bmi1 mutant mice develop ataxia, seizures and tremors in early postnatal life, and display a significant reduction in overall brain size, which is particularly severe in the granular and molecular layers of the cerebellum. Strikingly, Bmi1-deficent mice become depleted of cerebellar neural stem cells postnataly, indicating an in vivo requirement for Bmi1 in neural stem cell renewal. Bmi1 deficiency leads to increased expression of the cell cycle regulators p16Ink-4a and p19Arf (both now known as Cdkn2a - Mouse Genome Informatics), and the neurogenesis defect in the mutant mice can be partially rescued by further deleting p16Ink4a. This suggests that one way in which Bmi1 promotes the maintenance of adult stem cells is by repressing the p16Ink4a pathway. However, it is also likely that Hox gene repression through Bmi1 is involved in this process, given that loss of Bmi1 has been shown to cause a deregulation of posterior Hox gene expression in neural stem cells in vitro. Moreover, a direct molecular link between Bmi1 and Hox gene regulation has recently been discovered in mammalian development, in that the promyelocytic leukemia zinc finger (Plzf; Zbtb16 -- Mouse Genome Informatics) protein directly binds Bmi1 and recruits PcG proteins in the HoxD cluster (Bello, 2007 and references therein).

In Drosophila, the homologs of the mammalian Bmi1 gene are the PcG genes Psc and Su(z)2. Psc and Su(z)2 encode very similar proteins and are partially redundant in function, but both genes are eliminated in a deletion in the Psc-Su(z)2 line. Rather surprisingly, mutational loss of Psc-Su(z)2 does not lead to ectopic Hox gene derepression and, in consequence, does not appear to affect neuronal proliferation in the postembryonic CNS of Drosophila. This is in stark contrast to the other five PcG genes investigated, which do play important roles in proliferation control by preventing ectopic Hox gene expression and cell death in postembryonic neuroblasts. The discrepancy between murine Bmi1 and Drosophila Psc-Su(z)2 function in neuronal proliferation suggests that although a general role of PcG genes in neuronal proliferation control may be conserved between mammals and flies, conservation of gene action may not always be retained at the level of individual PcG homologs (Bello, 2007).

In terms of overall development, it is clear that one and the same PcG gene can have very different functions depending on the developmental context in which it acts. For example, as mentioned above, during embryonic neurogenesis the Drosophila Pc gene acts in tagmata-specific differentiation of neuroblasts, in contrast to its role in postembryonic neurogenesis. Moreover, in postembryonic development of imaginal discs, deletions in the Drosophila Psc-Su(z)2 genes have been shown to result in cellular hyperproliferation, which contrasts with the lack-of-proliferation phenotype of Psc-Su(z)2 mutants in postembryonic development of the CNS. Similarly, in the mouse, the Bmi1 gene has been implicated in tumor progression in mantle cell lymphoma, colorectal cancer, liver carcinomas and non-small cell lung cancer, in addition to its role in nervous system development. Nevertheless, in all of the Drosophila and mammalian phenotypes mentioned, deregulation of Hox gene expression appears to be one of the conserved and thus unifying features of PcG gene functional loss (Bello, 2007).

Polycomb group genes are required to maintain a binary fate choice in the Drosophila eye

Identifying the mechanisms by which cells remain irreversibly committed to their fates is a critical step toward understanding and being able to manipulate development and homeostasis. Polycomb group (PcG) proteins are chromatin modifiers that maintain transcriptional silencing, and loss of PcG genes causes widespread derepression of many developmentally important genes. However, because of their broad effects, the degree to which PcG proteins are used at specific fate choice points has not been tested. To understand how fate choices are maintained, R7 photoreceptor neuron development has been examined in the fly eye. R1, R6, and R7 neurons are recruited from a pool of equivalent precursors. In order to adopt the R7 fate, these precursors make three binary choices. They: (1) adopt a neuronal fate, as a consequence of high receptor tyrosine kinase (RTK) activity (they would otherwise become non-neuronal support cells); (2) fail to express Seven-up (Svp), as a consequence of Notch (N) activation (they would otherwise express Svp and become R1/R6 neurons); and (3) fail to express Senseless (Sens), as a parallel consequence of N activation (they would otherwise express Sens and become R8 neurons in the absence of Svp). PcG genes were removed specifically from post-mitotic R1/R6/R7 precursors, allowing these genes' roles to be probed in the three binary fate choices that R1/R6/R7 precursors face when differentiating as R7s. This study shows that loss of the PcG genes Sce, Scm, or Pc specifically affects one of the three binary fate choices that R7 precursors must make: mutant R7s derepress Sens and adopt R8 fate characteristics. This fate transformation occurs independently of the PcG genes' canonical role in repressing Hox genes. While N initially establishes Sens repression in R7s, this study shows that N is not required to keep Sens off, nor do these PcG genes act downstream of N. Instead, the PcG genes act independently of N to maintain Sens repression in R1/R6/R7 precursors that adopt the R7 fate. It is concluded that cells can use PcG genes specifically to maintain a subset of their binary fate choices (Finley, 2015).

Identifying the mechanisms by which cells remain irreversibly committed to their fates is a critical step toward understanding and being able to manipulate development and homeostasis. Polycomb group (PcG) proteins are chromatin modifiers that maintain transcriptional silencing, and loss of PcG genes causes widespread derepression of many developmentally important genes. However, because of their broad effects, the degree to which PcG proteins are used at specific fate choice points has not been tested. To understand how fate choices are maintained, R7 photoreceptor neuron development has been examined in the fly eye . R1, R6, and R7 neurons are recruited from a pool of equivalent precursors. In order to adopt the R7 fate, these precursors make three binary choices. They: (1) adopt a neuronal fate, as a consequence of high receptor tyrosine kinase (RTK) activity (they would otherwise become non-neuronal support cells); (2) fail to express Seven-up (Svp) , as a consequence of Notch (N) activation (they would otherwise express Svp and become R1/R6 neurons); and (3) fail to express Senseless (Sens) , as a parallel consequence of N activation (they would otherwise express Sens and become R8 neurons in the absence of Svp). PcG genes were removed specifically from post-mitotic R1/R6/R7 precursors, allowing these genes' roles to be probed in the three binary fate choices that R1/R6/R7 precursors face when differentiating as R7s. This study shows that loss of the PcG genes Sce , Scm , or Pc specifically affects one of the three binary fate choices that R7 precursors must make: mutant R7s derepress Sens and adopt R8 fate characteristics. This fate transformation occurs independently of the PcG genes' canonical role in repressing Hox genes. While N initially establishes Sens repression in R7s, this study shows that N is not required to keep Sens off, nor do these PcG genes act downstream of N. Instead, the PcG genes act independently of N to maintain Sens repression in R1/R6/R7 precursors that adopt the R7 fate. It is concluded that cells can use PcG genes specifically to maintain a subset of their binary fate choices (Finley, 2015).

The GMR-FLP/MARCM system allowed allowed the removal of Sce and Scm function specifically from post-mitotic R1/R6/R7 precursors, allowing these genes' roles to be probed in the limited number of binary fate choices that R1/R6/R7 precursors face. In order to adopt the R7 fate, these precursors must choose to: (1) become neurons in response to high RTK activity-they would otherwise become non-neuronal cells; (2) fail to express Svp in response to N activity-they would otherwise become R1/R6s; and (3) fail to express Sens in response to N activity-they would otherwise become R8s. Loss of Sce or Scm from R7s specifically was found to compromises maintenance of the last of these choices. By contrast, no evidence was found that PcG genes maintain either of the other two choices. Sce mutant R7s were examined throughout larval and pupal development and none were found none misexpressed Svp, nor were Sce or Scm mutant R7s that displayed other R1/R6 characteristics found, such as large rhabdomeres positioned at the periphery of the ommatidium or expression of the R1-R6-specific rhodopsin Rh1. While loss of the Abelson kinase was recently shown to cause R neurons to lose expression of the neuronal marker Elav and switch to a non-neuronal pigment cell fate, this study found that Sce and Scm mutant R1/R6s and R7s maintain expression of Elav and the photoreceptor-specific protein Chaoptin, indicating that their commitment to a neuronal fate is also independent of PcG gene function. It is concluded that R7s use Sce and Scm to maintain repression of one but not all alternative binary fate choices (Finley, 2015).

The Sens-encoding region is bound by Pc in Drosophila embryos and by Sce in Drosophila larvae , suggesting that Sens is directly regulated by these proteins in at least some cell types. However, because of the technical difficulty in isolating sufficient quantities of chromatin specifically from R7 cells, it was not possible to determine whether PcG proteins bind the Sens locus in R7s. It remains possible, therefore, that Sce, Scm, and Pc maintain Sens repression indirectly in R7s-however, the evidence suggests that they do so independently of their canonical role in repressing Hox genes (Finley, 2015).

Considerable differences were observed in the strengths of the R7 defects caused by loss of Sce, Scm, Pc, or Psc. One possibility is that these proteins do not contribute equally to PRC1's gene-silencing ability. Indeed, the fly genome contains a second Psc-related gene that plays a redundant role with Psc in some cells, possibly accounting for the lack of defect in Psc mutant R7s. Alternatively, the different wild-type PcG proteins may perdure to different degrees within the mutant R7 clones (the cells that divide to generate the mutant R1/R6/R7 precursors contain a wild-type copy of the mutant gene). Attempts were made, but it was not possible to measure the time course of Sce and Scm protein levels in Sce and Scm mutant R7s, respectively, to test their perdurance directly. However, this thought that perdurance is likely, as this study found that Gal80 perdures until early pupal development within GMR-FLP/MARCM-induced R7 clones (Finley, 2015).

Sce and Scm were found to be required to maintain Sens repression in R7s generated either in the presence or absence of N activity. What might be regulating the deployment of Sce and Scm in these cells? One possibility is that Sce and Scm repress Sens in R1/R6/R7 precursors by default, since these cells never normally express Sens. However, it was found that neither Sce nor Scm is required to maintain the repression of Sens that is established by Svp. Alternatively, Sce and Scm may be deployed to repress Sens as part of a cell's initial commitment to the R7 fate. As mentioned above, wild-type Sce or Scm protein is likely to perdure in newly created homozygous Sce or Scm mutant R7s, respectively, leaving open the possibility that these genes are required not only for the maintenance but also for the establishment of the R7 fate. Previous work showed that the NF-YC subunit of the heterotrimeric transcription factor nuclear factor Y (NF-Y) is also required to maintain Sens repression in R7s. Like the PcG proteins, NF-YC is broadly expressed in all photoreceptor neurons and is not sufficient to cause R7s to adopt R8 fates, indicating that NF-YC is not responsible for the specific role of PcG proteins in R7s. However, the resemblance between the R7 defects caused by loss of Sce, Scm, and NF-YC suggests that NF-Y may participate in PRC1 function. In support of this possibility, loss of the NF-YA subunit from Caenorhabditis elegans also causes defects similar to those caused by loss of the PcG gene sop-2, including derepression of the Hox gene egl-5 (Finley, 2015).

PcG proteins have been shown to silence many regulators of development in addition to their canonical Hox targets, suggesting that PcG proteins are likely to play broad roles in maintaining cell fate commitments. However, whether PcG proteins are used to maintain specific binary fate choices as cells differentiate is unclear. In fact, the opposite is true during stem cell differentiation, when the repression of terminal differentiation genes by PcG proteins must instead be relieved. This paper has have identified a role for PRC1-associated PcG proteins in maintaining a specific binary fate choice made during adoption of the R7 fate-a choice that does not involve Hox gene regulation or misregulation. The same PRC1-associated proteins are not required to maintain two other binary fate choices that R7s must make. It is concluded that PcG genes are indeed used to maintain some though not all binary fate choices (Finley, 2015).

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