twins

DEVELOPMENTAL BIOLOGY

Embryonic to pupal period

There is a maternal contribution of transcripts to the embryo. Transcripts for both isoforms are expressed simultaneously but in variable amounts in all tisues examined. The level of maternal transcripts remains very high throughout the syncytial stage of embryogenesis and then, around cellularization, decreases rapidly to below the detection. At full germband extension (stage 10) zygotic transcripts can be detected in the neuroblasts and the migrating gonads. This expression is more clearly seen after germband retraction. In the later stages of embryogenesis, the expression can be detected not only in the nervous system and the gonads, but also in the hindgut, the anal pads and the Malpighian tubules. There is a uniform distribution of both types of transcripts in discs and in the testes of late pupae, and lower transcript levels in the larval brain, where they appear restricted to regions of cell proliferation in optic lobes (Mayer-Jaekel, 1993).

Adult

In ovarioles, there is a high level of expression in nurse cells of the developing egg chamber, and transcripts are detectable starting from around stage 6 in oogenesis. Upon oocyte maturation, the transcripts are transported into the egg cell, and high transcript levels can be detected in the unfertilized egg.

Effects of Mutation or Deletion

Mutations in the twins locus causes mitotic abnormalities during early embryogenesis and late larval development in an allele-specific manner. Homozygotes have small brains and die during late pupal stages. These individuals probably survive to late developmental stages due to the presence of a maternally provided wild-type product. There is a range of mitotic abnormalities during metaphase and anaphase in third-instar larval neuroblasts. The abnormal metaphase figures are characterized by excessive chromosome condensation, low level of polyploidy, and in some cases, the presence of irregular chromatid condenstion. These abnormal phenotypes are probably not due to the absence of functional spindles, since anaphases can be easily found in these brains. However, most anaphases appear abnormal. They are characterized by the presence of stretched chromatids, which extend all the way between the poles, and/or lagging chromatids, which are left in the mid-zone between the two poles. Some anaphases also show a variable degree of chromosome condensation. This mutation causes an increase in the mitotic index. The data suggest that the twins mutation causes a delay in the initiation of anaphase, which results in the large number of metaphase figures with condensed chromosomes (Gomes, 1993).

Mutation of twins causes a pattern duplication in Drosophila imaginal discs. Inactivation of twins induces the formation of extra wing blade anlagen in the posterior compartment. The duplication is mirror symmetrical, and the line of symmetry does not correspond to any of the known compartment borders. The wing duplication in twins is associated with partial losses of engrailed expression outside the wing pouches. It is thought that the proximal domain of the posterior compartment is missing, with the missing region corresponding to ventral structures of the notum (Uemura, 1993).

Partial loss-of-function mutations in twins alter cell fate lineage in peripheral nervous system mechanoreceptor development. Hypomorphic twins mutations do not plock division of the sensory organ precursor, but most likely lresult in production of accessory (shaft and socket) cells at the expense of neural and glial cells. Almost all duplicated sockets are physically connected or fused to one another. No identifiable mitotic arrest of sensory organ precursors is found. The other two subunits of phosphatase are expressed at normal levels in twins mutants. A similar transformation is seen in musashi and numb, and the opposite transformation is seen in tramtrack mutants. Homozygous twins alleles produce a slightly rough eye phenotype, suggesting a transformation of non-neuronal cone cells to R7 photoreceptor cells (Shiomi, 1994). (Note: also see below, Wassarman, 1996).

Genetic evidence suggests that protein phosphatase regulates Ras mediated photoreceptor development in Drosophila. Using transgenic flies expressing constitutively activated Ras1 or Raf proteins that function independently of upstream signaling events, it has been shown that a reduction in the dose of the gene encoding the catalytic subunit of PP2A stimulates signaling from Ras1 but impairs signaling from Raf. The dominant Ras1 transgene results in a transformation of non-neuronal cone cells to R7 photoreceptor cells producing a visibly rough eye. Mutants in the catalytic subunit enhance the rough eye phenotype, causing a 48% increase in the number of supernumary R7 cells. Combining mutant PP2A catalytic subunit with constitutively activated ras results in a 30% decrease in the number of R7 cells per ommatidium. Germ-line clones of PP2A catalytic subunit block oogenesis suggesting a role for PP2A in oogenesis. What are the substrates for PP2A in the Ras1 pathway? Phosphorylation of Raf at different sites can either activate or inhibit its kinase activity, making it a possible substrate for negative or positive regulation by PP2A. Ksr kinase is also a potential target for negative regulation by PP2A; it appears to function between Ras1 and Raf and contains four consensus phosphorylation sites for MAPK (see Drosophila rolled), but it is not known whether phosphorylation of Ksr modulates its activity. PP2A has been shown to dephosphorylate and inactivate both MEK and MAPK in vitro, but PP2A alleles do not interact with MEK or MAPK (Wassarman, 1996).

Progression through mitosis requires the ubiquitin-mediated proteolysis of several regulatory proteins. A large multisubunit complex known as the anaphase-promoting complex or cyclosome (APC/C) plays a key role as an E3 ubiquitin-protein ligase in this process. The APC/C adds chains of ubiquitin to substrate proteins, targeting them for proteolysis by the 26S proteasome. The gene makos (mks) encodes the Drosophila counterpart of the Cdc27 subunit of the anaphase promoting complex (APC/C). Neuroblasts from third-larval-instar mks mutants arrest mitosis in a metaphase-like state but show some separation of sister chromatids. In contrast to metaphase-checkpoint-arrested cells, such mutant neuroblasts contain elevated levels not only of cyclin B but also of cyclin A. Mutations in mks enhance the reduced ability of hypomorphic polo mutant alleles to recruit and/or maintain the centrosomal antigens gamma-tubulin and CP190 at the spindle poles. Absence of the MPM2 epitope from the spindle poles in such double mutants suggests Polo kinase is not fully activated at this location. Thus, it appears that spindle pole functions of Polo kinase require the degradation of early mitotic targets of the APC/C, such as cyclin A, or other specific proteins. The metaphase-like arrest of mks mutants cannot be overcome by mutations in the spindle integrity checkpoint gene bub1, confirming this surveillance pathway has to operate through the APC/C. However, mutations in the twins/aar gene, which encodes the 55kDa regulatory subunit of PP2A, do suppress the mks metaphase arrest and so permit an alternative means of initiating anaphase. Thus the APC/C might normally be required to inactivate wild-type twins/aar gene product (Deak, 2003).

The metaphase-like arrest of mks cells cannot be overcome by the bub1 mutation. This is consistent with known functions of the APC/C downstream of the spindle integrity checkpoint. However, the ability of mutants in the aar/twins gene to overcome the metaphase arrest of mks is suggestive of an alternate mechanism for regulating the transition. In fact, the aar/twins mutant appears to be totally epistatic to (functioning downstram of) mks. Thus, the mks aar/twins double mutant shows a similar proportion of anaphase figures to the aar/twins mutant alone, and this is higher than the frequency of anaphases seen in wild-type cells. These observations cast some light on the possible multiple functions of the regulatory subunit of PP2A encoded by aar/twins in regulating anaphase and mitotic exit. It suggests that APC/C function might normally be required to inactivate the wild-type 55 kDa PP2A subunit that, in turn, negatively regulates sister chromatid separation. Thus, in the absence of aar/twins function, this aspect of APC/C involvement would not be required for anaphase, thus accounting for the epistasis of aar/twins to mks. The mutant aar/twins phenotype that then develops is akin to that observed following the expression of non-degradable forms of cyclin B, in which mitosis proceeds into anaphase. This outcome would be reinforced by a failure to exit mitosis as a result of the reduced ability of aar/twins mutants to dephosphorylate substrates of Cdk1 (Deak, 2003).

The anaphases in aar/twins and in the double mutant are highly abnormal, indicating that the checkpoint pathway that monitors chromosome alignment at metaphase and works through regulation of the APC/C is being circumvented. Consequently, there are many bridging and lagging chromatids in both aar/twins and mks aar/twins anaphase figures. This phenotype bears a striking resemblance to that seen in mutants of the CDC55 gene of budding yeast that encodes the orthologous regulatory subunit of PP2A. Cells with a cdc55 mutation have also been shown to leave mitosis without B-type cyclin destruction, in this case apparently owing to inhibitory tyrosine phosphorylation. However, it is also postulated in budding yeast that Cdc55p function is required for the kinetochore/spindle checkpoint. Such cdc55 mutants are sensitive to nocodazole and, in contrast to the situation for Drosophila cells, cdc55 mutations do not overcome the arrest imposed by mutation in an APC/C protein, in this case Cdc23p. Nevertheless, the abnormal morphology of cdc55 mutants and their conditional lethality is suppressed by a cdc28F19 mutation that encodes a variant kinase not susceptible to inhibitory phosphorylation. By contrast, nocodazole sensitivity cannot be suppressed by cdc28F19. This suggests that, in yeast, Cdc55p might have a checkpoint role that is independent of Cdc28/Cdk1 and a second role in regulating Cdc28 phosphorylation (Deak, 2003).

At the present time, it is not possible to account for how the APC/C might regulate the function of the 55kDa PP2A subunit although one possibility is through its direct proteolysis. It appears that this regulatory subunit of PP2A must participate in regulating the metaphase-anaphase transition, in controlling the activity of Cdk1 and in dephosphorylating Cdk1 substrates. It therefore remains a question of considerable future interest to determine exactly how these activities are coordinated (Deak, 2003).

Twins regulates Armadillo levels in response to Wg/Wnt signal

Protein Phosphatase 2A (PP2A) has a heterotrimeric-subunit structure, consisting of a core dimer of ~36 kDa catalytic and ~65 kDa scaffold subunits complexed to a third variable regulatory subunit. Several studies have implicated PP2A in Wg/Wnt signaling. However, reports on the precise nature of the PP2A role in Wg/Wnt pathway in different organisms are conflicting. twins (tws), which codes for the B/PR55 regulatory subunit of PP2A in Drosophila, is shown to be a positive regulator of Wg/Wnt signaling. In tws^- wing discs both short- and long-range targets of Wingless morphogen are downregulated. Analyses of tws^- mitotic clones suggest that requirement of Tws in Wingless pathway is cell-autonomous. Epistatic genetic studies indicate that Tws functions downstream of Dishevelled and upstream of Sgg and Armadillo. These results suggest that Tws is required for the stabilization of Armadillo/ß-catenin in response to Wg/Wnt signaling. Interestingly, overexpression of an otherwise normal Tws protein induces dominant-negative phenotypes. The conflicting reports on the role of PP2A in Wg/Wnt signaling could be due to the dominant-negative effect caused by the overexpression of one of the subunits (Bajpai, 2004).

Results of these studies show that Twins is involved in modifying Wg signaling. Partial to complete downregulation of short- (Ct and Sca) and long-range (Dll and Vg) targets of Wg pathway is observed in tws^- background. The downregulation of Wg signaling in wing discs is reflected in adult phenotypes, such as serrated wing margin in mitotic clones of tws. Loss-of-Wg phenotypes (induced by the overexpression of DN-TCF/pan or Sgg or Cad^intra) are enhanced in tws heterozygous mutant background. In addition, mutation in tws suppresses the phenotypes induced by Dsh, a positive component of Wg signaling. Finally, some of the phenotypes induced by the overexpression of Tws are characteristic of gain-of-Wg phenotypes. These results suggest that Tws functions as a positive regulator of Wg signaling (Bajpai, 2004).

Overexpression of otherwise normal Tws protein induces dominant-negative phenotypes. The dominant-negative phenotype is unlikely to be neomorphic or antimorphic, since UAS-Tws rescues tws alleles (at the levels of both Wingless-dependent and independent developmental events) and also induces gain-of-Wg phenotypes. The dominant-negative phenotype is probably due to imbalance in the relative amounts of the three subunits in the heterotrimeric complex, proper formation of which is obligatory for PP2A function. Thus, the conflicting reports on the role of PP2A in Wnt signaling could be due to the dominant negative effect caused by the overexpression of one of the subunits (Bajpai, 2004).

In tws mutant background, cytoplasmic Arm levels are downregulated. Even overexpressed Arm is degraded in tws^- background. Furthermore, loss of tws had no effect on the degradation-resistant form of Arm, which suggests that Tws functions upstream of Arm to mediate Wg signaling. These results could not be confirmed directly by Western blotting, since only a very small fraction (such as DV cells) of wing disc shows changes in Arm levels in response to Wg signaling. Nevertheless, results presented in this report suggest that stabilization of the cytoplasmic form of Arm by Wg signaling is dependent on Tws function (Bajpai, 2004).

A dominant-negative form of Sgg/GSK-3ß is able to rescue tws^- phenotype at the level of Dll expression. However, overexpression of Dsh failed to rescue Dll expression in tws^- discs, suggesting that Tws functions downstream of Dsh and upstream of Sgg to stabilize cytoplasmic Arm in response to Wg signaling. Preliminary results presented here suggest that function of Tws in Wg pathway is inactivation of Sgg. Normally, overexpressed APC sequesters Arm only in those cells in which Sgg activity is downregulated. In other cells, APC participates in Arm-degradation machinery. In tws^- wing discs, overexpressed APC fails to sequester Arm in DV cells, suggesting that loss of tws results in upregulation of Sgg activity. However, it has been reported that PR/B56epsilon functions upstream of Dsh to regulate Wnt signaling in Xenopus embryos. The PR/B56epsilon homolog in Drosophila is widerborst (with 80% identity at the protein level), which is involved in the determination of planar cell polarity. widerborst is also known to be functional upstream of Dsh, but not in the canonical Wg/Wnt pathway. Although Tws homologs in other organisms have not been well characterized, the current studies are consistent with a role for PP2A in dephosphorylation of Axin (Bajpai, 2004).

The next question regards the substrate of PP2A function in the Wg pathway. In mammalian cells, Axin is dephosphorylated in response to Wnt signaling. Furthermore, dephosphorylated Axin binds ß-catenin less efficiently than the phosphorylated form. Thus, dephosphorylation of Axin would free ß-catenin from the degradation machinery. Thus, Tws may function by inhibiting the activity of Axin, which acts a scaffold protein to bring Sgg and Arm to close proximity. Further biochemical work is in progress to determine phosphorylated status of Arm in tws^- background and to determine if Tws directly binds to Sgg or Axin or both (Bajpai, 2004).

Multiple protein phosphatases are required for mitosis in Drosophila

Approximately one-third of the Drosophila kinome has been ascribed some cell-cycle function. However, little is known about which of its 117 protein phosphatases (PPs) or subunits have counteracting roles. This study investigated mitotic roles of PPs through systematic RNAi. It was found that G2-M progression requires Puckered, the JNK MAP-kinase inhibitory phosphatase and PP2C in addition to string (Cdc25). Strong mitotic arrest and chromosome congression failure occurs after Pp1-87B downregulation. Chromosome alignment and segregation defects also occurs after knockdown of PP1-Flapwing, not previously thought to have a mitotic role. Reduction of several nonreceptor tyrosine phosphatases produced spindle and chromosome behavior defects, and for corkscrew, premature chromatid separation. RNAi of the dual-specificity phosphatase, Myotubularin, or the related Sbf 'antiphosphatase' resulted in aberrant mitotic chromosome behavior. Finally, for PP2A, knockdown of the catalytic or A subunits led to bipolar monoastral spindles, knockdown of the Twins B subunit led to bridged and lagging chromosomes, and knockdown of the B' Widerborst subunit led to scattering of all mitotic chromosomes. Widerborst is associated with MEI-S332 (Shugoshin) and is required for its kinetochore localization. This study has identified cell-cycle roles for 22 of 117 Drosophila PPs. Involvement of several PPs in G2 suggests multiple points for its regulation. Major mitotic roles are played by PP1 with tyrosine PPs and Myotubularin-related PPs having significant roles in regulating chromosome behavior. Finally, depending upon its regulatory subunits, PP2A regulates spindle bipolarity, kinetochore function, and progression into anaphase. Discovery of several novel cell-cycle PPs identifies a need for further studies of protein dephosphorylation (Chen, 2007).

P2A is a heterotrimeric serine/threonine phosphatase composed of invariant catalytic (C) and structural (A) subunits together with one member of a family of B regulatory subunits thought to direct the AC core to different substrates. The Drosophila gene for the catalytic subunit of type 2A protein serine/threonine phosphatase (PP2A) is known as microtubule star (mts) because mutant embryos show multiple individual centrosomes with disorganized astral arrays of microtubules. In agreement with this mutant phenotype, it was found that S2 cells depleted for Mts (PP2A-C) displayed aberrant elongated arrays of microtubules with a high proportion (5- to 10-fold increase over the control) of bipolar monoastral spindles or monopolar spindles emanating from a single centrosomal mass. This phenotype is also consistent with the observations in Xenopus egg extracts where mitotic microtubules grow longer and bipolar spindles can not be assembled after inhibition of PP2A by low concentrations of okadaic acid (OA). It is speculated that the monopolar spindle phenotype after mts dsRNA treatment is a consequence of the spindle collapse rather than a failure in centrosome duplication or separation because most of the RNAi-treated cells showed well-separated centrosomes during prophase. In support of this view, spindle bipolarity can be rescued by restoration of microtubule dynamics in OA-treated Xenopus egg extracts (Chen, 2007).

In Drosophila, as in many other eukaryotes, mitosis-specific phosphorylation of histone H3 requires Aurora B activity, but the identity of the opposing phosphatase remains unclear. Because P-H3 (Ser 10) levels were used for monitoring the mitotic index in this analysis, it is possible that a high mitotic index observed after RNAi for PPs may also reflect a defect in dephosphorylating P-H3 in the absence of PPs upon mitotic exit. The phosphorylation state of this histone was therefore studied after RNAi for PPs that displayed a significant increase in the mitotic index in the screen. The immunostaining of control cells showed that P-H3 signals began to decrease at early telophase and then disappeared completely at late telophase. After RNAi knockdown of Mts (PP2A-C) or Pp1-87B, however, the majority of mitotic cells were arrested at prometaphase, but late telophase figures could occassionally be found showing an abnormal accumulation of P-H3 on decondensed chromosomes. To better assess the effect of depletion of these two PPs on P-H3 dephosphorylation, the spindle-assembly checkpoint was inactivated by simultaneously knocking down BubR1. It was found that this rescued the prometaphase arrest of cells simultaneously depleted for Mts or Pp1-87B; this allowed a study of telophase cells. P-H3 was present in the majority of such telophase cells compared to control cells, indicating that both PPs are required for P-H3 dephosphorylation. These results are in accordance with previous studies showing that reduction of PP1 activity can partially suppress defects in the mitotic histone H3 phosphorylation in yeast and C. elegans (Chen, 2007).

Downregulation of Pp2A-29B, the structural A subunit, revealed almost identical aberrant phenotypes to those observed after mts (PP2A-C) RNAi. Consistently, knockdown of Pp2A-29B (PP2A-A) led to a reduction of the protein level of Mts (PP2A-C) (Chen, 2007).

The Drosophila genome contains 4 B-type PP2A regulatory subunits, twins/tws/aar (B sub-type), widerborst/wdb (B' sub-type), Pp2A-B' (B' sub-type), and Pp2A-B" (B" sub-type), but mitotic defects have so far only been reported for mutants of tws. Consistent with the phenotype of tws mutants, it was observed that RNAi for this gene led to an increased proportion of anaphase figures showing lagging chromosomes and chromosome bridges (Chen, 2007).

In metazoans, the B' regulatory subunits of PP2A have evolved into two related subclasses with conserved central regions and diverged amino and carboxy termini. The protein encoded by widerborst (wdb) is more closely related to the human α and ɛ subunits (79%–80% identity) than to the β, γ, or δ subunits (69%–75% identity). Whereas RNAi for tws led to lagging chromosomes, wdb RNAi led to dramatic scattering of chromosomes throughout the spindle. Whether this dramatic effect of wdb RNAi on chromosome segregation reflected any particular subcellular localization of this regulatory subunit was examined. To this end, a GFP-tagged Wdb was expressed in S2 cells. During interphase and prophase, Wdb::GFP partially colocalized with the centromeric marker CID (CENP-A). After spindle formation, Wdb::GFP was found adjacent and external to the centromeres. Although less pronounced, this distribution remained during chromosome segregation at anaphase. Because MEI-S332 (Drosophila Shugoshin) is a dynamic centromeric marker, its distribution was examined in wdb RNAi cells. In control cells, MEI-S332 localized in a band between each pair of the centromeres at metaphase. After downregulation of wdb, however, greatly reduced MEI-S332 staining was found on the metaphase chromosome. In contrast, depletion of MEI-S332 by RNAi did not affect the normal localization of the Wdb B' PP2A subunit. Thus, it is concluded that the Wdb B' subunit is required for correct localization of MEI-S332 but not vice versa. Whether the two proteins existed in the same complex was examined. To address this, a Protein A (PrA)-tagged form of MEI-S332 was expressed in S2 cells to purify potential protein complexes and identify its components by mass spectrometry. The catalytic C (Mts), the structural A (PP2A-29B), and the regulatory B' (Wdb) and B (Tws) subunits of PP2A were identified associated with MEI-S332. Three recent studies also identified PP2A complexed to the B' subunit bound to Shugoshin (Sgo) in human and yeast cells, where they are thought to protect centromeric cohesin subunits from phosphorylation that would promote premature sister-chromatid separation. As with the archetypal family member, Drosophila MEI-S332, the Shugoshins function primarily to protect sister chromatids from separation in the first meiotic division but are also present in mitotic divisions. Consistent with these observations in Drosophila S2 cells, it has been found that depletion of PP2A in human cells led to premature dissociation of Shugoshin 1 (Sgo1) from the kinetochore and loss of mitotic centromere cohesion. The finding of Shugoshin complexed to PP2A/B' in yeast and human, and now in Drosophila, points to a highly evolutionally conserved role for this particular PP2A heterotrimer in regulating sister-chromatid cohesion. Interestingly, Tws B regulatory subunit was also recovered associated with MEI-S332. How this subunit of PP2A might function with MEI-S332 should be the subject of future investigations (Chen, 2007).

Only a moderatedly elevated mitotic index (by approximately 10%) was observed after downregulation of the second Drosophila B' regulatory subunit (Pp2A-B'/B56-1). However, when this second B' subunit was simultaneously knocked down with Wdb, this led to similar phenotypes seen in Mts (PP2A-C) or Pp2A-29B (PP2A-A)-depleted cells. Western-blot analysis showed that the Mts (PP2A-C) level decreased after simultaneous knockdown of both B' subunits, suggesting that this phenotype could be partially due to the loss of PP2A catalytic subunit, although the possibility that the two B' subunits share partially redundant mitotic functions cannot be excluded (Chen, 2007).

Cell-cycle kinases represent a large family of enzymes governing the cell division cycle. It is therefore not surprising that a considerable number of counteracting cell-cycle phosphatases (19% of the genes for tested) were identified in the current study. In addition to finding all the well-known PPs required for cell-cycle progression in Drosophila (Mts, Tws, String, Pp4-19C, and Pp1-87B), the Drosophila counterparts of some eight PPs implicated in cell-cycle functions were identified from studies on other organisms together with six PPs for which novel cell-cycle roles were ascribe. These results were validated by confirming the observed phenotypes with a second nonoverlapping dsRNA. In two cases (flw and csw), their mitotic roles were confirmed through the analysis of phenotypes in mutant larval neuroblasts. The RNAi phenotypes of catalytic subunits were evaluated by observing similar phenotypes after downregulation of the corresponding regulatory subunits (e.g., Pp4-19C and PPP4R2r, Mts/PP2A-C and Pp2A-29B/PP2A-A, and simultaneous RNAi of the two PP2A-B' regulatory subunits). Although a recent large-scale RNAi screen based solely on flow cytometry in Drosophila S2 cells identified many regulators of the cell cycle, cell size, and cell death, this study showed a very low degree of overlap with the cureent analysis (only six), reflecting the need for more sensitive small-scale screens that can examine the functional requirements of assayed proteins in greater detail. These results have provided novel insights into the cell-cycle functions of the Drosophila PPs, and it is likely that, in many cases, these functions have been conserved in other metazoans including humans. This study should guide future work aimed at elucidating the significance and mechanisms of the balanced activities of PKs and PPs in regulating the cell division cycle. The challenge ahead will be to match up the functions of the PPs that were identified with their corresponding counteracting PKs and to identify their common key substrates (Chen, 2007).

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date revised: 10 April 2008

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