A screen for modifiers of cyclin E function in Drosophila melanogaster identifies Cdk2 mutations, revealing the insignificance of putative phosphorylation sites in Cdk2

Accelerated cell cycles enable organ regeneration under developmental time constraints in the Drosophila hindgut

Individual organ development must be temporally coordinated with development of the rest of the organism. As a result, cell division cycles in a developing organ occur on a relatively fixed timescale. Despite this, many developing organs can regenerate cells lost to injury. How organs regenerate within the time constraints of organism development remains unclear. Here, we show that the developing Drosophila hindgut regenerates by accelerating the mitotic cell cycle. This process is achieved by decreasing G1 length and requires the JAK/STAT ligand unpaired-3. Mitotic capacity is then terminated by the steroid hormone ecdysone receptor and the Sox transcription factor Dichaete. These two factors converge on regulation of a hindgut-specific enhancer of fizzy-related, a negative regulator of mitotic cyclins. Our findings reveal how the cell-cycle machinery and cytokine signaling can be adapted to accomplish developmental organ regeneration.

SPEN haploinsufficiency causes a neurodevelopmental disorder overlapping proximal 1p36 deletion syndrome with an episignature of X chromosomes in females

Deletion 1p36 (del1p36) syndrome is the most common human disorder resulting from a terminal autosomal deletion. This condition is molecularly and clinically heterogeneous. Deletions involving two non-overlapping regions, known as the distal (telomeric) and proximal (centromeric) critical regions, are sufficient to cause the majority of the recurrent clinical features, although with different facial features and dysmorphisms. SPEN encodes a transcriptional repressor commonly deleted in proximal del1p36 syndrome and is located centromeric to the proximal 1p36 critical region. Here, we used clinical data from 34 individuals with truncating variants in SPEN to define a neurodevelopmental disorder presenting with features that overlap considerably with those of proximal del1p36 syndrome. The clinical profile of this disease includes developmental delay/intellectual disability, autism spectrum disorder, anxiety, aggressive behavior, attention deficit disorder, hypotonia, brain and spine anomalies, congenital heart defects, high/narrow palate, facial dysmorphisms, and obesity/increased BMI, especially in females. SPEN also emerges as a relevant gene for del1p36 syndrome by co-expression analyses. Finally, we show that haploinsufficiency of SPEN is associated with a distinctive DNA methylation episignature of the X chromosome in affected females, providing further evidence of a specific contribution of the protein to the epigenetic control of this chromosome, and a paradigm of an X chromosome-specific episignature that classifies syndromic traits. We conclude that SPEN is required for multiple developmental processes and SPEN haploinsufficiency is a major contributor to a disorder associated with deletions centromeric to the previously established 1p36 critical regions.

Drosophila Caliban mediates G1-S transition and ionizing radiation induced S phase checkpoint

Cell cycle progression is precisely regulated by diverse extrinsic and intrinsic cellular factors. Understanding the underlying mechanisms of cell cycle regulation is essential to address how normal development and tissue homeostasis are achieved. Here, we present a novel cell cycle regulator Caliban (Clbn), the Drosophila ortholog of human Serologically defined colon cancer antigen 1 (SDCCAG1) gene. We show that ionizing radiation induces expression of clbn, and over-expression of clbn blocks G1-to-S cell cycle transition in Drosophila, while flies loss of clbn have defective S phase checkpoint in response to irradiation. Mechanistically, induced expression of clbn suppressed E2F1 activity and down-regulates the DNA replication and expression of its downstream target cyclin E, a key regulator of G1-to-S transition. Meanwhile, clbn over-expression leads to upregulation of the CDK inhibitor Dacapo (Dap), and upregulated Dap is decreased when e2f1 is over-expressed. Furthermore, expression of clbn is down-regulated in cells with e2f1 over-expression or rbf1 knockdown, indicating that Clbn and E2F1 act antagonistically in mediating G1-to-S transition. Thus we provide genetic evidence that Clbn works together with E2F1 in regulating cell cycle progression, and Clbn is required for S phase cell cycle checkpoint in response to DNA damage.

The large and small SPEN family proteins stimulate axon outgrowth during neurosecretory cell remodeling in Drosophila

Split ends (SPEN) is the founding member of a well conserved family of nuclear proteins with critical functions in transcriptional regulation and the post-transcriptional processing and nuclear export of transcripts. In animals, the SPEN proteins fall into two size classes that perform either complementary or antagonistic functions in different cellular contexts. Here, we show that the two Drosophila representatives of this family, SPEN and Spenito (NITO), regulate metamorphic remodeling of the CCAP/bursicon neurosecretory cells. CCAP/bursicon cell-targeted overexpression of SPEN had no effect on the larval morphology or the pruning back of the CCAP/bursicon cell axons at the onset of metamorphosis. During the subsequent outgrowth phase of metamorphic remodeling, overexpression of either SPEN or NITO strongly inhibited axon extension, axon branching, peripheral neuropeptide accumulation, and soma growth. Cell-targeted loss-of-function alleles for both spen and nito caused similar reductions in axon outgrowth, indicating that the absolute levels of SPEN and NITO activity are critical to support the developmental plasticity of these neurons. Although nito RNAi did not affect SPEN protein levels, the phenotypes produced by SPEN overexpression were suppressed by nito RNAi. We propose that SPEN and NITO function additively or synergistically in the CCAP/bursicon neurons to regulate multiple aspects of neurite outgrowth during metamorphic remodeling.

Expression analysis of the Arabidopsis thaliana AtSpen2 gene, and its relationship with other plant genes encoding Spen proteins

Proteins of the Split ends (Spen) family are characterized by an N-terminal domain, with one or more RNA recognition motifs and a SPOC domain. In Arabidopsis thaliana, the Spen protein FPA is involved in the control of flowering time as a component of an autonomous pathway independent of photoperiod. The A. thaliana genome encodes another gene for a putative Spen protein at the locus At4g12640, herein named AtSpen2. Bioinformatics analysis of the AtSPEN2 SPOC domain revealed low sequence similarity with the FPA SPOC domain, which was markedly lower than that found in other Spen proteins from unrelated plant species. To provide experimental information about the function of AtSpen2, A. thaliana plants were transformed with gene constructs of its promoter region with uidA::gfp reporter genes; the expression was observed in vascular tissues of leaves and roots, as well as in ovules and developing embryos. There was absence of a notable phenotype in knockout and overexpressing lines, suggesting that its function in plants might be specific to certain endogenous or environmental conditions. Our results suggest that the function of Atspen2 diverged from that of fpa due in part to their different transcription expression pattern and divergence of the regulatory SPOC domain.

Spen is required for pigment cell survival during pupal development in Drosophila

Apoptosis is required during development to eliminate superfluous cells and sculpt tissues; spatial and timed control of apoptosis ensures that the necessary number of cells is eliminated at a precise time in a given tissue. The elimination of supernumerary pigment or inter-ommatidial cells (IOCs) depends on cell-cell communication and is necessary for the formation of the honeycomb-like structure of the Drosophila eye. However, the mechanisms occurring during pupal development and controlling apoptosis of superfluous IOC in space and time remain unclear. Here, we found that split-ends (spen) is required for IOC survival at the time of removal of superfluous IOCs. Loss of spen function leads to abnormal removal of IOCs by apoptosis. We show that spen is required non-autonomously in cone cells for the survival of IOCs by positively regulating the Spitz/EGFR pathway. We propose that Spen is an important survival factor that ensures spatial control of the apoptotic wave that is necessary for the correct patterning and formation of the Drosophila eye.

Biological effects of decreasing RBM15 on chronic myelogenous leukemia cells

RNA binding motif protein 15 (RBM15) was originally described as a 5' translocation partner of the MAL gene in t(1;22)(p13;q13)infant acute megakaryocytic leukemia. Although previous investigations have shown that Rbm15 has broad regulatory effects within murine hematopoiesis through modulating Notch-induced transcriptional activation, which plays key roles in leukemogenesis, it is not clear what the functions of RBM15 are in the regulation of hematological malignancies. In the present study, we show that RBM15 expression was significantly increased in patients with blast-crisis chronic myelogenous leukemia (CML) compared with chronic-phase or accelerated-phase disease by real-time reverse transcription-polymerase chain reaction (RT-PCR) assay. To further elucidate the role of RBM15 in CML, we introduced RBM15 small interfering RNA (siRNA) using pSUPER into CML cells. Fluorescence activated cell sorting (FACS), real-time RT-PCR and Western blot were used to study changes in RBM15 expression levels in transduced cells by comparing with control cells. Decreasing RBM15 levels with RNA interference could inhibit the growth and proliferation, block the cell cycle and induce apoptosis in CML cells. Knockdown of RBM15 may also act to inhibit clonogenicity and induce differentiation of CML cells along the myeloid lineage. Our studies also show that the effects of RBM15 on CML cells may be mediated, at least in part, via its effect on Notch signaling. These findings demonstrate that RBM15 does indeed play a critical role in the survival of CML cells, which may have potential application in designing molecular therapies for CML treatment.

Loss of cyclin-dependent kinase 2 (CDK2) inhibitory phosphorylation in a CDK2AF knock-in mouse causes misregulation of DNA replication and centrosome duplication

Cyclin-dependent kinase 1 (CDK1) inhibitory phosphorylation controls the onset of mitosis and is essential for the checkpoint pathways that prevent the G(2)- to M-phase transition in cells with unreplicated or damaged DNA. To address whether CDK2 inhibitory phosphorylation plays a similar role in cell cycle regulation and checkpoint responses at the start of the S phase, we constructed a mouse strain in which the two CDK2 inhibitory phosphorylation sites, threonine 14 and tyrosine 15, were changed to alanine and phenylalanine, respectively (CDK2AF). This approach showed that inhibitory phosphorylation of CDK2 had a major role in controlling cyclin E-associated kinase activity and thus both determined the timing of DNA replication in a normal cell cycle and regulated centrosome duplication. Further, DNA damage in G(1) CDK2AF cells did not downregulate cyclin E-CDK2 activity when the CDK inhibitor p21 was also knocked down. We were surprised to find that this was insufficient to cause cells to bypass the checkpoint and enter the S phase. This led to the discovery of two previously unrecognized pathways that control the activity of cyclin A at the G(1) DNA damage checkpoint and may thereby prevent S-phase entry even when cyclin E-CDK2 activity is deregulated.

Control of Drosophila endocycles by E2F and CRL4(CDT2)

Endocycles are variant cell cycles comprised of DNA Synthesis (S)- and Gap (G)- phases but lacking mitosis^1,2. Such cycles facilitate post-mitotic growth in many invertebrate and plant cells, and are so ubiquitous that they may account for up to half the world’s biomass^3,4. DNA replication in endocycling Drosophila cells is triggered by Cyclin E/Cyclin Dependent Kinase 2 (CycE/Cdk2), but this kinase must be inactivated during each G-phase to allow the assembly of pre-Replication Complexes (preRCs) for the next S-phase^5,6. How CycE/Cdk2 is periodically silenced to allow re-replication has not been established. Here, using genetic tests in parallel with computational modeling, we show that Drosophila’s endocycles are driven by a molecular oscillator in which the E2F1 transcription factor promotes CycE expression and S-phase initiation, S-phase then activates the CRL4^Cdt2 ubiquitin ligase, and this in turn mediates the destruction of E2F1⁷. We propose that it is the transient loss of E2F1 during S-phases that creates the window of low Cdk activity required for preRC formation. In support of this model over-expressed E2F1 accelerated endocycling, whereas a stabilized variant of E2F1 blocked endocycling by de-regulating target genes including CycE, as well as Cdk1 and mitotic Cyclins. Moreover, we find that altering cell growth by changing nutrition or TOR signaling impacts E2F1 translation, thereby making endocycle progression growth-dependent. Many of the regulatory interactions essential to this novel cell cycle oscillator are conserved in animals and plants^1,2,8, suggesting that elements of this mechanism act in most growth-dependent cell cycles.

Endoreplication: polyploidy with purpose

A great many cell types are necessary for the myriad capabilities of complex, multicellular organisms. One interesting aspect of this diversity of cell type is that many cells in diploid organisms are polyploid. This is called endopolyploidy and arises from cell cycles that are often characterized as "variant," but in fact are widespread throughout nature. Endopolyploidy is essential for normal development and physiology in many different organisms. Here we review how both plants and animals use variations of the cell cycle, termed collectively as endoreplication, resulting in polyploid cells that support specific aspects of development. In addition, we discuss briefly how endoreplication occurs in response to certain physiological stresses, and how it may contribute to the development of cancer. Finally, we describe the molecular mechanisms that support the onset and progression of endoreplication.

Coupling the cell cycle to development

The core machinery that drives the eukaryotic cell cycle has been thoroughly investigated over the course of the past three decades. It is only more recently, however, that light has been shed on the mechanisms by which elements of this core machinery are modulated to alter cell cycle progression during development. It has also become increasingly clear that, conversely, core cell cycle regulators can play a crucial role in developmental processes. Here, focusing on findings from Drosophila melanogaster and Caenorhabditis elegans, we review the importance of modulating the cell cycle during development and discuss how core cell cycle regulators participate in determining cell fates.

Drosophila Minus is required for cell proliferation and influences Cyclin E turnover

Turnover of cyclins plays a major role in oscillatory cyclin-dependent kinase (Cdk) activity and control of cell cycle progression. Here we present a novel cell cycle regulator, called minus, which influences Cyclin E turnover in Drosophila. minus mutants produce defects in cell proliferation, some of which are attributable to persistence of Cyclin E. Minus protein can interact physically with Cyclin E and the SCF Archipelago/Fbw7/Cdc4 ubiquitin-ligase complex. Minus does not affect dMyc, another known SCF(Ago) substrate in Drosophila. We propose that Minus contributes to cell cycle regulation in part by selectively controlling turnover of Cyclin E.

Mosaic genetic screen for suppressors of the de2f1 mutant phenotype in Drosophila

The growth suppressive function of the retinoblastoma (pRB) tumor suppressor family is largely attributed to its ability to negatively regulate the family of E2F transcriptional factors and, as a result, to repress E2F-dependent transcription. Deregulation of the pRB pathway is thought to be an obligatory event in most types of cancers. The large number of mammalian E2F proteins is one of the major obstacles that complicate their genetic analysis. In Drosophila, the E2F family consists of only two members. They are classified as an activator (dE2F1) and a repressor (dE2F2). It has been previously shown that proliferation of de2f1 mutant cells is severely reduced due to unchecked activity of the repressor dE2F2 in these cells. We report here a mosaic screen utilizing the de2f1 mutant phenotype to identify suppressors that overcome the dE2F2/RBF-dependent proliferation block. We have isolated l(3)mbt and B52, which are known to be required for dE2F2 function, as well as genes that were not previously linked to the E2F/pRB pathway such as Doa, gfzf, and CG31133. Inactivation of gfzf, Doa, or CG31133 does not relieve repression by dE2F2. We have shown that gfzf and CG31133 potentiate E2F-dependent activation and synergize with inactivation of RBF, suggesting that they may act in parallel to dE2F. Thus, our results demonstrate the efficacy of the described screening strategy for studying regulation of the dE2F/RBF pathway in vivo.

c-Myc is a target of RNA-binding motif protein 15 in the regulation of adult hematopoietic stem cell and megakaryocyte development

RNA-binding motif protein 15 (RBM15) is involved in the RBM15-megakaryoblastic leukemia 1 fusion in acute megakaryoblastic leukemia. Although Rbm15 has been reported to be required for B-cell differentiation and to inhibit myeloid and megakaryocytic expansion, it is not clear what the normal functions of Rbm15 are in the regulation of hematopoietic stem cell (HSC) and megakaryocyte development. In this study, we report that Rbm15 may function in part through regulation of expression of the proto-oncogene c-Myc. Similar to c-Myc knockout (c-Myc-KO) mice, long-term (LT) HSCs are significantly increased in Rbm15-KO mice due to an apparent LT-HSC to short-term HSC differentiation defect associated with abnormal HSC-niche interactions caused by increased N-cadherin and beta(1) integrin expression on mutant HSCs. Both serial transplantation and competitive reconstitution capabilities of Rbm15-KO LT-HSCs are greatly compromised. Rbm15-KO and c-Myc-KO mice also share related abnormalities in megakaryocyte development, with mutant progenitors producing increased, abnormally small low-ploidy megakaryocytes. Consistent with a possible functional interplay between Rbm15 and c-Myc, the megakaryocyte increase in Rbm15-KO mice could be partially reversed by ectopic c-Myc. Thus, Rbm15 appears to be required for normal HSC-niche interactions, for the ability of HSCs to contribute normally to adult hematopoiesis, and for normal megakaryocyte development; these effects of Rbm15 on hematopoiesis may be mediated at least in part by c-Myc.

The anaphase-promoting complex/cyclosome (APC/C) is required for rereplication control in endoreplication cycles

Endoreplicating cells undergo multiple rounds of DNA replication leading to polyploidy or polyteny. Oscillation of Cyclin E (CycE)-dependent kinase activity is the main driving force in Drosophila endocycles. High levels of CycE-Cdk2 activity trigger S phase, while down-regulation of CycE-Cdk2 activity is crucial to allow licensing of replication origins. In mitotic cells relicensing in S phase is prevented by Geminin. Here we show that Geminin protein oscillates in endoreplicating salivary glands of Drosophila. Geminin levels are high in S phase, but drop once DNA replication has been completed. DNA licensing is coupled to mitosis through the action of the anaphase-promoting complex/cyclosome (APC/C). We demonstrate that, even though endoreplicating cells never enter mitosis, APC/C activity is required in endoreplicating cells to mediate Geminin oscillation. Down-regulation of APC/C activity results in stabilization of Geminin protein and blocks endocycle progression. Geminin is only abundant in cells with high CycE-Cdk2 activity, suggesting that APC/C-Fzr activity is periodically inhibited by CycE-Cdk2, to prevent relicensing in S-phase cells.

dE2F2-independent rescue of proliferation in cells lacking an activator dE2F1

In Drosophila melanogaster, the loss of activator de2f1 leads to a severe reduction in cell proliferation and repression of E2F targets. To date, the only known way to rescue the proliferation block in de2f1 mutants was through the inactivation of dE2F2. This suggests that dE2F2 provides a major contribution to the de2f1 mutant phenotype. Here, we report that in mosaic animals, in addition to de2f2, the loss of a DEAD box protein Belle (Bel) also rescues proliferation of de2f1 mutant cells. Surprisingly, the rescue occurs in a dE2F2-independent manner since the loss of Bel does not relieve dE2F2-mediated repression. In the eye disc, bel mutant cells fail to undergo a G1 arrest in the morphogenetic furrow, delay photoreceptor recruitment and differentiation, and show a reduction of the transcription factor Ci155. The down-regulation of Ci155 is important since it is sufficient to partially rescue proliferation of de2f1 mutant cells. Thus, mutation of bel relieves the dE2F2-mediated cell cycle arrest in de2f1 mutant cells through a novel Ci155-dependent mechanism without functional inactivation of the dE2F2 repressor.

Wnt signaling and neural stem cells: caught in the Wnt web

Wnt proteins have now been identified as major physiological regulators of multiple aspects of stem cell biology, from self-renewal and pluripotency to precursor cell competence and terminal differentiation. Neural stem cells are the cellular building blocks of the developing nervous system and provide the basis for continued neurogenesis in the adult mammalian central nervous system. Here, we outline the most recent advances in the field about the critical factors and regulatory networks involved in Wnt signaling and discuss recent findings on how this increasingly intricate pathway contributes to the shaping of the developing and adult nervous system on the level of the neural stem cell.

Split ends antagonizes the Notch and potentiates the EGFR signaling pathways during Drosophila eye development

The Notch and Epidermal Growth Factor Receptor (EGFR) signaling pathways interact cooperatively and antagonistically to regulate many aspects of Drosophila development, including the eye. How output from these two signaling networks is fine-tuned to achieve the precise balance needed for specific inductive interactions and patterning events remains an open and important question. Previously, we reported that the gene split ends (spen) functions within or parallel to the EGFR pathway during midline glial cell development in the embryonic central nervous system. Here, we report that the cellular defects caused by loss of spen function in the developing eye imaginal disc place spen as both an antagonist of the Notch pathway and a positive contributor to EGFR signaling during retinal cell differentiation. Specifically, loss of spen results in broadened expression of Scabrous, ectopic activation of Notch signaling, and a corresponding reduction in Atonal expression at the morphogenetic furrow. Consistent with Spen's role in antagonizing Notch signaling, reduction of spen levels is sufficient to suppress Notch-dependent phenotypes. At least in part due to loss of Spen-dependent down-regulation of Notch signaling, loss of spen also dampens EGFR signaling as evidenced by reduced activity of MAP kinase (MAPK). This reduced MAPK activity in turn leads to a failure to limit expression of the EGFR pathway antagonist and the ETS-domain transcriptional repressor Yan and to a corresponding loss of cell fate specification in spen mutant ommatidia. We propose that Spen plays a role in modulating output from the Notch and EGFR pathways to ensure appropriate patterning during eye development.

The Trithorax group protein Lid is a trimethyl histone H3K4 demethylase required for dMyc-induced cell growth

The Myc oncoprotein is a potent inducer of cell growth, cell cycle progression, and apoptosis. While many direct Myc target genes have been identified, the molecular determinants of Myc's transcriptional specificity remain elusive. We have carried out a genetic screen in Drosophila and identified the Trithorax group protein Little imaginal discs (Lid) as a regulator of dMyc-induced cell growth. Lid binds to dMyc and is required for dMyc-induced expression of the growth regulatory gene Nop60B. The mammalian Lid orthologs, Rbp-2 (JARID1A) and Plu-1 (JARID1B), also bind to c-Myc, indicating that Lid-Myc function is conserved. We demonstrate that Lid is a JmjC-dependent trimethyl H3K4 demethylase in vivo and that this enzymatic activity is negatively regulated by dMyc, which binds to Lid's JmjC domain. Because Myc binding is associated with high levels of trimethylated H3K4, we propose that the Lid-dMyc complex facilitates Myc binding to, or maintenance of, this chromatin context.

Drosophila split ends homologue SHARP functions as a positive regulator of Wnt/beta-catenin/T-cell factor signaling in neoplastic transformation

Wnt ligands have pleiotropic and context-specific roles in embryogenesis and adult tissues. Among other effects, certain Wnts stabilize the beta-catenin protein, leading to the ability of beta-catenin to activate T-cell factor (TCF)-mediated transcription. Mutations resulting in constitutive beta-catenin stabilization underlie development of several human cancers. Genetic studies in Drosophila highlighted the split ends (spen) gene as a positive regulator of Wnt-dependent signaling. We have assessed the role of SHARP, a human homologue of spen, in Wnt/beta-catenin/TCF function in mammalian cells. We found that SHARP gene and protein expression is elevated in human colon and ovarian endometrioid adenocarcinomas and mouse colon adenomas and carcinomas carrying gene defects leading to beta-catenin dysregulation. When ectopically expressed, the silencing mediator for retinoid and thyroid receptors/histone deacetylase 1-associated repressor protein (SHARP) protein potently enhanced beta-catenin/TCF transcription of a model reporter gene and cellular target genes. Inhibition of endogenous SHARP function via RNA inhibitory (RNAi) approaches antagonized beta-catenin/TCF-mediated activation of target genes. The effect of SHARP on beta-catenin/TCF-regulated genes was mediated via a functional interaction between SHARP and TCF. beta-Catenin-dependent neoplastic transformation of RK3E cells was enhanced by ectopic expression of SHARP, and RNAi-mediated inhibition of endogenous SHARP in colon cancer cells inhibited their transformed growth. In toto, our findings implicate SHARP as an important positive regulator of Wnt signaling in cancers with beta-catenin dysregulation.

Identification of genes that interact with Drosophila liquid facets

We have performed mutagenesis screens of the Drosophila X chromosome and the autosomes for dominant enhancers of the rough eye resulting from overexpression of liquid facets. The liquid facets gene encodes the homolog of vertebrate endocytic Epsin, an endocytic adapter protein. In Drosophila, Liquid facets is a core component of the Notch signaling pathway required in the signaling cells for ligand endocytosis and signaling. Why ligand internalization by the signaling cells is essential for signaling is a mystery. The requirement for Liquid facets is a hint at the answer, and the genes identified in this screen provide further clues. Mutant alleles of clathrin heavy chain, Rala, split ends, and auxilin were identified as enhancers. We describe the mutant alleles and mutant phenotypes of Rala and aux. We discuss the relevance of all of these genetic interactions to the function of Liquid facets in Notch signaling.

Cyclin-dependent kinases in C. elegans

Cell division is an inherent part of organismal development, and defects in this process can lead to developmental abnormalities as well as cancerous growth. In past decades, much of the basic cell-cycle machinery has been identified, and a major challenge in coming years will be to understand the complex interplay between cell division and multicellular development. Inevitably, this requires the use of more complex multicellular model systems. The small nematode Caenorhabditis elegans is an excellent model system to study the regulation of cell division in a multicellular organism, and is poised to make important contributions to this field. The past decade has already seen a surge in cell-cycle research in C. elegans, yielding information on the function of many basic cell-cycle regulators, and making inroads into the developmental control of cell division. This review focuses on the in vivo roles of cyclin-dependent kinases in C. elegans, and highlights novel findings implicating CDKs in coupling development to cell-cycle progression.

Characterization of the split ends-like gene spenito reveals functional antagonism between SPOC family members during Drosophila eye development

The novel family of SPOC domain proteins is composed of broadly conserved nuclear factors that fall into two subclasses, termed large and small, based on protein size. Members of the large subgroup, which includes Drosophila SPEN and human SHARP, have been characterized as transcriptional corepressors acting downstream of a variety of essential cell signaling pathways, while those of the small subclass have remained largely unstudied. Since SPEN has been implicated in Drosophila eye development, and the small SPOC protein NITO is also expressed in the developing eye, we have used this context to perform a structure-function analysis of NITO and to examine the relationship between the two SPOC family subclasses. Our results demonstrate that the phenotypes obtained from overexpressing NITO share striking similarity to those associated with loss of spen. Dosage-sensitive genetic interactions further support a model of functional antagonism between NITO and SPEN during Drosophila eye development. These results suggest that large and small SPOC family proteins may have opposing functions in certain developmental contexts.

Using Drosophila melanogaster to map human cancer pathways

The development of human cancer is a multistep process, involving the cooperation of mutations in signalling, cell-cycle and cell-death pathways, as well as interactions between the tumour and the tumour microenvironment. To dissect the steps of tumorigenesis, simple animal models are needed. This article discusses the use of the genetically amenable, multicellular organism, the vinegar fly Drosophila melanogaster. In particular, recent studies have highlighted the power of D. melanogaster for examining cooperative interactions between tumour suppressors and oncogenes and for generating in vivo models of tumour development and metastasis.

Cell cycle and cell-fate determination in Drosophila neural cell lineages

"Normal" development requires a finely tuned equilibrium between cell differentiation and cell proliferation. Important issues in development include whether the cell cycle controls the cell-fate determination and whether cell identity in turn regulates cell-cycle progression. Although, these issues are of general biological relevance, stereotyped Drosophila neural lineages are particularly suited to address these questions and have provided insights into the links between cell-cycle progression and cell-fate specification.

New insights into cell cycle control from the Drosophila endocycle

During metazoan development, the organization of the cell cycle is often modified in response to developmental signals. The endocycle provides a dramatic example of this phenomenon. In the endocycle, also referred to as the endoreplicative cycle, cells undergo successive rounds of DNA replication without an intervening mitosis. Often the endocycle is used to expand the genome of a group of specialized cells that are highly biosynthetically active. In these circumstances, large polyploid cells are produced in organisms that are primarily comprised of diploid cells. However, many organisms achieve growth by increasing cell size, rather than cell number. This strategy is more generally exploited in insects and plants. For instance, in the insect Drosophila melanogaster, the majority of the larval tissues, as well as many adult tissues, enter the endocycle and become polyploid. Therefore, Drosophila has been a rich source for studies on endocycle regulation. Recent work from Drosophila is beginning to reveal how developmental signals promote the transition from the mitotic cycle to the endocycle, as well as what drives endocycle progression. In addition, studies on the endocycle have provided insight into the regulatory principles underlying the once per cell cycle replication of the genome, as well as the relationship between S phase and mitosis.

SNR1 (INI1/SNF5) mediates important cell growth functions of the Drosophila Brahma (SWI/SNF) chromatin remodeling complex

SNR1 is an essential subunit of the Drosophila Brahma (Brm) ATP-dependent chromatin remodeling complex, with counterparts in yeast (SNF5) and mammals (INI1). Increased cell growth and wing patterning defects are associated with a conditional snr1 mutant, while loss of INI1 function is directly linked with aggressive cancers, suggesting important roles in development and growth control. The Brm complex is known to function during G1 phase, where it appears to assist in restricting entry into S phase. In Drosophila, the activity of DmcycE/CDK2 is rate limiting for entry into S phase and we previously found that the Brm complex can suppress a reduced growth phenotype associated with a hypomorphic DmcycE mutant. Our results reveal that SNR1 helps mediate associations between the Brm complex and DmcycE/CDK2 both in vitro and in vivo. Further, disrupting snr1 function suppressed DmcycEJP phenotypes, and increased cell growth defects associated with the conditional snr1E1 mutant were suppressed by reducing DmcycE levels. While the snr1E1-dependent increased cell growth did not appear to be directly associated with altered expression of G1 or G2 cyclins, transcription of the G2-M regulator string/cdc25 was reduced. Thus, in addition to important functions of the Brm complex in G1-S control, the complex also appears to be important for transcription of genes required for cell cycle progression.

A genetic screen for dominant modifiers of a cyclin E hypomorphic mutation identifies novel regulators of S-phase entry in Drosophila

Cyclin E together with its kinase partner Cdk2 is a critical regulator of entry into S phase. To identify novel genes that regulate the G1- to S-phase transition within a whole animal we made use of a hypomorphic cyclin E mutation, DmcycEJP, which results in a rough eye phenotype. We screened the X and third chromosome deficiencies, tested candidate genes, and carried out a genetic screen of 55,000 EMS or X-ray-mutagenized flies for second or third chromosome mutations that dominantly modified the DmcycEJP rough eye phenotype. We have focused on the DmcycEJP suppressors, S(DmcycEJP), to identify novel negative regulators of S-phase entry. There are 18 suppressor gene groups with more than one allele and several genes that are represented by only a single allele. All S(DmcycEJP) tested suppress the DmcycEJP rough eye phenotype by increasing the number of S phases in the postmorphogenetic furrow S-phase band. By testing candidates we have identified several modifier genes from the mutagenic screen as well as from the deficiency screen. DmcycEJP suppressor genes fall into the classes of: (1) chromatin remodeling or transcription factors; (2) signaling pathways; and (3) cytoskeletal, (4) cell adhesion, and (5) cytoarchitectural tumor suppressors. The cytoarchitectural tumor suppressors include scribble, lethal-2-giant-larvae (lgl), and discs-large (dlg), loss of function of which leads to neoplastic tumors and disruption of apical-basal cell polarity. We further explored the genetic interactions of scribble with S(DmcycEJP) genes and show that hypomorphic scribble mutants exhibit genetic interactions with lgl, scab (alphaPS3-integrin--cell adhesion), phyllopod (signaling), dEB1 (microtubule-binding protein--cytoskeletal), and moira (chromatin remodeling). These interactions of the cytoarchitectural suppressor gene, scribble, with cell adhesion, signaling, cytoskeletal, and chromatin remodeling genes, suggest that these genes may act in a common pathway to negatively regulate cyclin E or S-phase entry.

Extracellular Signals Responsible for Spatially Regulated Proliferation in the Differentiating Drosophila Eye

Spatially and temporally choreographed cell cycles accompany the differentiation of the Drosophila retina. The extracellular signals that control these patterns have been identified through mosaic analysis of mutations in signal transduction pathways. All cells arrest in G1 prior to the start of neurogenesis. Arrest depends on Dpp and Hh, acting redundantly. Most cells then go through a synchronous round of cell division before fate specification and terminal cell cycle exit. Cell cycle entry is induced by Notch signaling and opposed in subsets of cells by EGF receptor activity. Unusually, Cyclin E levels are not limiting for retinal cell cycles. Rbf/E2F and the Cyclin E antagonist Dacapo are important, however. All retinal cells, including the postmitotic photoreceptor neurons, continue dividing when rbf and dacapo are mutated simultaneously. These studies identify the specific extracellular signals that pattern the retinal cell cycles and show how differentiation can be uncoupled from cell cycle exit.

Cyclin D does not provide essential Cdk4-independent functions in Drosophila

The three mammalian D-type cyclins are thought to promote progression through the G1 phase of the cell cycle as regulatory subunits of cyclin-dependent kinase 4 and 6. In addition, they have been proposed to control the activity of various transcription factors without a partner kinase. Here we describe phenotypic consequences of null mutations in Cyclin D, the single D-type cyclin gene in Drosophila. As previously observed with null mutations in the single Drosophila Cdk4 gene, these mutations do not primarily affect progression through the G1 phase. Moreover, the apparently indistinguishable phenotypes of double (CycD and Cdk4) and single mutants (CycD or Cdk4) argue against major independent functions of Cyclin D and Cdk4. The reduced cellular and organismal growth rates observed in both mutants indicate that Cyclin D-Cdk4 acts as a growth driver.

The product of the split ends gene is required for the maintenance of positional information during Drosophila development

Background

The Drosophila split ends (spen) gene encodes a large nuclear protein containing three RNP-type RNA binding motifs, and a conserved transcriptional co-repressor-interacting domain at the C-terminus. Genetic analyses indicate that spen interacts with pathways that regulate the function of Hox proteins, the response to various signaling cascades and cell cycle control. Although spen mutants affect only a small subset of morphological structures in embryos, it has been difficult to find a common theme in spen mutant structural alterations, or in the interactions of spen with known signaling pathways.

Results

By generating clones of spen mutant cells in wing imaginal discs, we show that spen function is required for the correct formation and positioning of veins and mechanosensory bristles both on the anterior wing margin and on the notum, and for the maintenance of planar polarity. Wing vein phenotypic alterations are enhanced by mutations in the crinkled (ck) gene, encoding a non-conventional myosin, and correlate with an abnormal spatial expression of Delta, an early marker of vein formation in third instar wing imaginal discs. Positioning defects were also evident in the organization of the embryonic peripheral nervous system, accompanied by abnormal E-Cadherin expression in the epidermis.

Conclusions

The data presented indicate that the role of spen is necessary to maintain the correct positioning of cells within a pre-specified domain throughout development. Its requirement for epithelial planar polarity, its interaction with ck, and the abnormal E-Cadherin expression associated with spen mutations suggest that spen exerts its function by interacting with basic cellular mechanisms required to maintain multicellular organization in metazoans. This role for spen may explain why mutations in this gene interact with the outcome of multiple signaling pathways.

New tricks for old dogs: unexpected roles for cell cycle regulators revealed using animal models

Studies in animal models have revealed many surprises regarding the importance of key cell cycle regulators during animal development and homeostasis, underscoring the plasticity and redundancy of cell cycle circuitry within a whole-animal context. Moreover, checkpoint regulators, which are not essential for viability in yeast and cultured cells, play important roles in cell cycle control during development.

A novel nuclear receptor/coregulator complex controls C. elegans lipid metabolism, larval development, and aging

Environmental cues transduced by an endocrine network converge on Caenorhabditis elegans nuclear receptor DAF-12 to mediate arrest at dauer diapause or continuous larval development. In adults, DAF-12 selects long-lived or short-lived modes. How these organismal choices are molecularly specified is unknown. Here we show that coregulator DIN-1 and DAF-12 physically and genetically interact to instruct organismal fates. Homologous to human corepressor SHARP, DIN-1 comes in long (L) and short (S) isoforms, which are nuclear localized but have distinct functions. DIN-1L has embryonic and larval developmental roles. DIN-1S, along with DAF-12, regulates lipid metabolism, larval stage-specific programs, diapause, and longevity. Epistasis experiments reveal that din-1S acts in the dauer pathways downstream of lipophilic hormone, insulin/IGF, and TGFbeta signaling, the same point as daf-12. We propose that the DIN-1S/DAF-12 complex serves as a molecular switch that implements slow life history alternatives in response to diminished hormonal signals.

SPOC: a widely distributed domain associated with cancer, apoptosis and transcription

BACKGROUND

The Split ends (Spen) family are large proteins characterised by N-terminal RNA recognition motifs (RRMs) and a conserved SPOC (Spen paralog and ortholog C-terminal) domain. The aim of this study is to characterize the family at the sequence level.

RESULTS

We describe undetected members of the Spen family in other lineages (Plasmodium and Plants) and localise SPOC in a new domain context, in a family that is common to all eukaryotes using profile-based sequence searches and structural prediction methods.

CONCLUSIONS

The widely distributed DIO (Death inducer-obliterator) family is related to cancer and apoptosis and offers new clues about SPOC domain functionality.

Differential contribution of inhibitory phosphorylation of CDC2 and CDK2 for unperturbed cell cycle control and DNA integrity checkpoints

Inhibition of cyclin-dependent kinases (CDKs) by Thr14/Tyr15 phosphorylation is critical for normal cell cycle progression and is a converging event for several cell cycle checkpoints. In this study, we compared the relative contribution of inhibitory phosphorylation for cyclin A/B1-CDC2 and cyclin A/E-CDK2 complexes. We found that inhibitory phosphorylation plays a major role in the regulation of CDC2 but only a minor role for CDK2 during the unperturbed cell cycle of HeLa cells. The relative importance of inhibitory phosphorylation of CDC2 and CDK2 may reflect their distinct cellular functions. Despite this, expression of nonphosphorylation mutants of both CDC2 and CDK2 triggered unscheduled histone H3 phosphorylation early in the cell cycle and was cytotoxic. DNA damage by a radiomimetic drug or replication block by hydroxyurea stimulated a buildup of cyclin B1 but was accompanied by an increase of inhibitory phosphorylation of CDC2. After DNA damage and replication block, all cyclin-CDK pairs that control S phase and mitosis were to different degrees inhibited by phosphorylation. Ectopic expression of nonphosphorylated CDC2 stimulated DNA replication, histone H3 phosphorylation, and cell division even after DNA damage. Similarly, a nonphosphorylation mutant of CDK2, but not CDK4, disrupted the G2 DNA damage checkpoint. Finally, CDC25A, CDC25B, a dominant-negative CHK1, but not CDC25C or a dominant-negative WEE1, stimulated histone H3 phosphorylation after DNA damage. These data suggest differential contributions for the various regulators of Thr14/Tyr15 phosphorylation in normal cell cycle and during the DNA damage checkpoint.

Cyclin-dependent kinase 2 is essential for meiosis but not for mitotic cell division in mice

We targeted the locus encoding the cyclin-dependent kinase 2 (CDK2) by homologous recombination in mouse embryonic stem (ES) cells. Embryonic fibroblasts lacking CDK2 proliferate normally and become immortal after continuous passage in culture. Elimination of a conditional Cdk2 allele in immortal cells does not have a significant effect on proliferation. Cdk2-/- mice are viable and survive for up to two years, indicating that CDK2 is also dispensable for proliferation and survival of most cell types. But CDK2 is essential for completion of prophase I during meiotic cell division in male and female germ cells, an unforeseen role for this cell cycle kinase.

Induction of string rescues the neuroblast proliferation defect in trol mutant animals

In trol mutants, neuroblasts fail to exit G1 for S phase. Increasing string expression in trol mutants rescues the number of S phase neuroblasts without an increase in M phase neuroblasts. Decreasing string expression further decreased the number of S phase neuroblasts. Coexpression of cyclin E and string did not produce additional S phase cells. Unlike cyclin E, cdk2, and cdk2AF, elevated expression of neither cyclin A, cyclin D, nor cdk1AF was able to promote S phase progression in arrested neuroblasts, indicating that String-induced activity of a Cyclin A or Cyclin D complex is unlikely to drive trol neuroblasts into S phase. Biochemical analyses revealed a rapid increase of Cyclin E-Cdk2 kinase activity to wild-type levels upon increased string expression. These results suggest that Drosophila Cdc25 may directly or indirectly increase the kinase activity of Cyclin E-Cdk2 complexes in vivo, thus driving arrested neuroblasts into cell division.

Cycling without CDK2?

Cyclin-dependent kinase 2 (CDK2) regulates diverse aspects of the mammalian cell cycle. Most cancer cells contain mutations in the pathways that control CDK2, and CDK2 activity has received much attention as a target for cancer therapy. However, a recent report demonstrating that some cancer cells can proliferate without CDK2 activity questions the essential role of CDK2 in cell-cycle control, as well as its suitability as a therapeutic target.

Splits ends is a tissue/promoter specific regulator of Wingless signaling

Wingless directs many developmental processes in Drosophila by regulating expression of specific target genes through a conserved signaling pathway. Although many nuclear factors have been implicated in mediating Wingless-induced transcription, the mechanism of how Wingless regulates different targets in different tissues remains poorly understood. We report here that the split ends gene is required for Wingless signaling in the eye, wing and leg imaginal discs. Expression of a dominant-negative version of split ends resulted in more dramatic reductions in Wingless signaling than split ends-null alleles, suggesting that it may have a redundant partner. However, removal of split ends or expression of the dominant-negative had no effect on several Wingless signaling readouts in the embryo. The expression pattern of Split ends cannot explain this tissue-specific requirement, as the protein is predominantly nuclear and present throughout embryogenesis and larval tissues. Consistent with its nuclear location, the split ends dominant-negative acts downstream of Armadillo stabilization. Our data indicate that Split ends is an important positive regulator of Wingless signaling in larval tissues. However, it has no detectable role in the embryonic Wingless pathway, suggesting that it is a tissue or promoter-specific factor.

Drosophila skpA, a component of SCF ubiquitin ligases, regulates centrosome duplication independently of cyclin E accumulation

Centrosome duplication must be coupled to the main cell cycle to ensure that each cell has precisely two centrosomes at the onset of mitosis. Supernumerary centrosomes are commonly observed in cancer cells, and may contribute to tumorigenesis. Drosophila skpA, a component of SCF ubiquitin ligases, regulates the link between the cell and centrosome cycles. Lethal skpA null mutants exhibit dramatic centrosome overduplication and additional defects in chromatin condensation, cell cycle progression and endoreduplication. Surprisingly, many mutant cells are able to organize pseudo-bipolar spindles and execute a normal anaphase in the presence of extra functional centrosomes. SkpA mutant cells accumulate higher levels of cyclin E than wildtype cells during S and G2, suggesting that elevated cdk2/cyclin E activity may account for the supernumerary centrosomes in skpA- cells. However, centrosome overduplication still occurs in skpA-; cycE- mutant animals, demonstrating that high cyclin E levels are not necessary for centrosome overduplication. These data suggest that additional SCF targets regulate the centrosome duplication pathway.

[Molecular basis of the t(1;22)(p13;q13) specific for human acute megakaryoblastic leukemia]

The t(1;22)(p13;q13) translocation is specifically associated with infant acute megakaryoblastic leukemia (M7). We have recently characterized the two genes involved in this translocation: OTT (One Two Two) and MAL (Megakaryoblastic Acute Leukemia) respectively located on chromosome 1 and 22. The t(1;22) translocation results in the fusion of these genes in all the cases studied to date. We summarize here present knowledge regarding this translocation.

Ectopic expression of the Drosophila Cdk1 inhibitory kinases, Wee1 and Myt1, interferes with the second mitotic wave and disrupts pattern formation during eye development

Wee1 kinases catalyze inhibitory phosphorylation of the mitotic regulator Cdk1, preventing mitosis during S phase and delaying it in response to DNA damage or developmental signals during G2. Unlike yeast, metazoans have two distinct Wee1-like kinases, a nuclear protein (Wee1) and a cytoplasmic protein (Myt1). We have isolated the genes encoding Drosophila Wee1 and Myt1 and are using genetic approaches to dissect their functions during normal development. Overexpression of Dwee1 or Dmyt1 during eye development generates a rough adult eye phenotype. The phenotype can be modified by altering the gene dosage of known regulators of the G2/M transition, suggesting that we could use these transgenic strains in modifier screens to identify potential regulators of Wee1 and Myt1. To confirm this idea, we tested a collection of deletions for loci that can modify the eye overexpression phenotypes and identified several loci as dominant modifiers. Mutations affecting the Delta/Notch signaling pathway strongly enhance a GMR-Dmyt1 eye phenotype but do not affect a GMR-Dwee1 eye phenotype, suggesting that Myt1 is potentially a downstream target for Notch activity during eye development. We also observed interactions with p53, which suggest that Wee1 and Myt1 activity can block apoptosis.

Drosophilap27Dacapo expression during embryogenesis is controlled by a complex regulatory region independent of cell cycle progression

dacapo encodes a CIP/KIP-type inhibitor of Cyclin E/Cdk2 complexes in Drosophila melanogaster. In the embryonic epidermis, dacapo expression starts during G2 of the final division cycle and is required for the arrest of cell cycle progression in G1 after the final mitosis. The onset of dacapo transcription is the earliest event known to be required for the epidermal cell proliferation arrest. To advance our understanding of the regulatory mechanisms that terminate cell proliferation at the appropriate stage, we have analyzed the control of dacapo transcription. We show that dacapo transcription is not coupled to cell cycle progression. It is not affected in mutants where proliferation is arrested either too early or too late. Moreover, upregulation of dacapo expression is not an obligatory event of the cell cycle exit process. During early development of the central nervous system, we cannot detect p27Dacapo during the final division cycle of ganglion mother cells, while it is expressed at later stages. The control of dacapo expression therefore varies in different stages and tissues. The dacapo regulatory region includes many independent cis-regulatory elements. The elements that control epidermal expression integrate developmental cues that time the arrest of cell proliferation.