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

Transcriptome-wide identification of development related genes and pathways in Tribolium castaneum

The growth and development in Tribolium castaneum were poorly understood at the transcriptome level. Currently, we identified 15,756, 9941 and 10,080 differentially expressed transcripts between late eggs VS early larvae, late larvae VS early pupae, and late pupae VS early adults of T. castaneum by RNA-seq, which was confirmed by qRT-PCR analysis on nine genes expression. Functional enrichment analysis indicated that DNA replication, cell cycle and insect hormone biosynthesis significantly enriched differentially expressed genes. The transcription of DNA replication and cell cycle genes decreased after hatching but increased after pupation. The juvenile hormone (JH) and ecdysteroid biosynthesis genes decreased after hatching, and the JH degradation genes were stimulated after pupation and eclosion while the ecdysteroid degradation gene CYP18A1 decreased after pupation. Silencing CYP18A1 elevated the titer of ecdysteroids and caused developmental arrest at the late larval stage. This study promotes the understanding of insect growth and development.

Ubiquitin signaling in the control of centriole duplication

The centrosome plays an essential role in maintaining genetic stability, ciliogenesis and cell polarisation. The core of the centrosome is made up of two centrioles that duplicate precisely once during every cell cycle to generate two centrosomes that are required for bipolar spindle assembly and chromosome segregation. Abundance of centriole proteins at optimal levels and their recruitment to the centrosome are tightly regulated in time and space in order to restrict aberrant duplication of centrioles, a phenomenon that is observed in many cancers. Recent advances have conclusively shown that dedicated ubiquitin ligase-dependent protein degradation machineries are involved in governing centriole duplication. These studies revealed intricate mechanistic insights into how the ubiquitin ligases target different centriole proteins. In certain cases, a specific ubiquitin ligase targets a number of substrate proteins that co-regulate centriole assembly, prompting the possibility that substrate-targeting occurs during formation of the sub-centriolar structures. There are also instances where a specific centriole duplication protein is targeted by several ubiquitin ligases at different stages of the cell cycle, suggesting synchronised actions. Recent evidence also indicated a direct association of E3 ubiquitin ligase with the centrioles, supporting the notion that substrate-targeting occurs in the organelle itself. In this review, we highlight these advances by underlining the mechanisms of how different ubiquitin ligase machineries control centriole duplication and discuss our views on their coordination.

SCF-Fbxo42 promotes synaptonemal complex assembly by downregulating PP2A-B56

Meiosis creates genetic diversity by recombination and segregation of chromosomes. The synaptonemal complex assembles during meiotic prophase I and assists faithful exchanges between homologous chromosomes, but how its assembly/disassembly is regulated remains to be understood. Here, we report how two major posttranslational modifications, phosphorylation and ubiquitination, cooperate to promote synaptonemal complex assembly. We found that the ubiquitin ligase complex SCF is important for assembly and maintenance of the synaptonemal complex in Drosophila female meiosis. This function of SCF is mediated by two substrate-recognizing F-box proteins, Slmb/βTrcp and Fbxo42. SCF-Fbxo42 down-regulates the phosphatase subunit PP2A-B56, which is important for synaptonemal complex assembly and maintenance.

Mutant p53 drives the loss of heterozygosity by the upregulation of Nek2 in breast cancer cells

BACKGROUND

Mutations in one allele of the TP53 gene in early stages are frequently followed by the loss of the remaining wild-type p53 (wtp53) allele (p53LOH) during tumor progression. Despite the strong notion of p53LOH as a critical step in tumor progression, its oncogenic outcomes that facilitate the selective pressure for p53LOH occurrence were not elucidated.

METHODS

Using MMTV;ErbB2 mouse model of breast cancer carrying heterozygous R172H p53 mutation, we identified a novel gain-of-function (GOF) activity of mutant p53 (mutp53): the exacerbated loss of wtp53 allele in response to γ-irradiation.

RESULTS

As consequences of p53LOH in mutp53 heterozygous cells, we observed profound stabilization of mutp53 protein, the loss of p21 expression, the abrogation of G2/M checkpoint, chromosomal instability, centrosome amplification, and transcriptional upregulation of mitotic kinase Nek2 (a member of Never in Mitosis (NIMA) Kinases family) involved in the regulation of centrosome function. To avoid the mitotic catastrophe in the absence of G2/M checkpoint, cells with centrosome amplification adapt Nek2-mediated centrosomes clustering as pro-survival mutp53 GOF mechanism enabling unrestricted proliferation and clonal expansion of cells with p53LOH. Thus, the clonal dominance of mutp53 cells with p53LOH may represent the mechanism of irradiation-induced p53LOH. We show that pharmacological and genetic ablation of Nek2 decreases centrosome clustering and viability of specifically mutp53 cells with p53LOH.

CONCLUSION

In a heterogeneous tumor population, Nek2 inhibition may alter the selective pressure for p53LOH by contraction of the mutp53 population with p53LOH, thus, preventing the outgrowth of genetically unstable, more aggressive cells.

Drosophila Skp1 Homologue SkpA Plays a Neuroprotective Role in Adult Brain

Skp1, a component of the ubiquitin E3 ligases, was found to be decreased in the brains of sporadic Parkinson's disease (PD) patients, and its overexpression prevented death of murine neurons in culture. Here we expose the neuroprotective role of the Drosophila skp1 homolog, skpA, in the adult brain. Neuronal knockdown of skpA leads to accumulation of ubiquitinated protein aggregates and loss of dopaminergic neurons accompanied by motor dysfunction and reduced lifespan. Conversely, neuronal overexpression of skpA reduces aggregate load, improves age-related motor decline, and prolongs lifespan. Moreover, SkpA rescues neurodegeneration in a Drosophila model of PD. We also show that a Drosophila homolog of FBXO7, the F Box protein, Nutcracker (Ntc), works in the same pathway with SkpA. However, skpA overexpression rescues ntc knockdown phenotype, suggesting that SkpA interacts with additional F box proteins in the adult brain neurons. Collectively, our study discloses Skp1/SkpA as a potential therapeutic target in neurodegenerative diseases.

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SkpA-mediated protein degradation is required for normal function of the adult brain

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SkpA overexpression rescues neurodegeneration in α-synuclein-induced fly PD model

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SkpA and Ntc work in the same pathway of protein degradation in adult brain neurons

Apoptotic Cell Clearance in Drosophila melanogaster

The swift clearance of apoptotic cells (ACs) (efferocytosis) by phagocytes is a critical event during development of all multicellular organisms. It is achieved through phagocytosis by professional or amateur phagocytes. Failure in this process can lead to the development of inflammatory autoimmune or neurodegenerative diseases. AC clearance has been conserved throughout evolution, although many details in its mechanisms remain to be explored. It has been studied in the context of mammalian macrophages, and in the nematode Caenorhabditis elegans, which lacks “professional” phagocytes such as macrophages, but in which other cell types can engulf apoptotic corpses. In Drosophila melanogaster, ACs are engulfed by macrophages, glial, and epithelial cells. Drosophila macrophages perform similar functions to those of mammalian macrophages. They are professional phagocytes that participate in phagocytosis of ACs and pathogens. Study of AC clearance in Drosophila has identified some key elements, like the receptors Croquemort and Draper, promoting Drosophila as a suitable model to genetically dissect this process. In this review, we survey recent works of AC clearance pathways in Drosophila, and discuss the physiological outcomes and consequences of this process.

Autophagy controls centrosome number by degrading Cep63

Centrosome number is associated with the chromosome segregation and genomic stability. The ubiquitin-proteasome system is considered to be the main regulator of centrosome number. However, here we show that autophagy also regulates the number of centrosomes. Autophagy-deficient cells carry extra centrosomes. The autophagic regulation of centrosome number is dependent on a centrosomal protein of 63 (Cep63) given that cells lacking autophagy contain multiple Cep63 dots that are engulfed and digested by autophagy in wild-type cells, and that the upregulation of Cep63 increases centrosome number. Cep63 is recruited to autophagosomes via interaction with p62, a molecule crucial for selective autophagy. In vivo, hematopoietic cells from autophagy-deficient and p62^-/- mice also contained multiple centrosomes. These results indicate that autophagy controls centrosome number by degrading Cep63.

KIFC1: a promising chemotherapy target for cancer treatment?

The kinesin motor KIFC1 has been suggested as a potential chemotherapy target due to its critical role in clustering of the multiple centrosomes found in cancer cells. In this regard, KIFC1 seems to be non-essential in normal somatic cells which usually possess only two centrosomes. Moreover, KIFC1 is also found to initiatively drive tumor malignancy and metastasis by stabilizing a certain degree of genetic instability, delaying cell cycle and protecting cancer cell surviving signals. However, that KIFC1 also plays roles in other specific cell types complicates the question of whether it is a promising chemotherapy target for cancer treatment. For example, KIFC1 is found functionally significant in vesicular and organelle trafficking, spermiogenesis, oocyte development, embryo gestation and double-strand DNA transportation. In this review we summarize a recent collection of information so as to provide a generalized picture of ideas and mechanisms against and in favor of KIFC1 as a chemotherapy target. And we also drew the conclusion that KIFC1 is a promising chemotherapy target for some types of cancers, because the side-effects of inhibiting KIFC1 mentioned in this review are theoretically easy to avoid, while KIFC1 is functionally indispensable during mitosis and malignancy of multi-centrosome cancer cells. Further investigations of how KIFC1 is regulated throughout the mitosis in cancer cells are needed for the understanding of the pathways where KIFC1 is involved and for further exploitation of indirect KIFC1 inhibitors.

The E3 ligase Ubr3 regulates Usher syndrome and MYH9 disorder proteins in the auditory organs of Drosophila and mammals

Myosins play essential roles in the development and function of auditory organs and multiple myosin genes are associated with hereditary forms of deafness. Using a forward genetic screen in Drosophila, we identified an E3 ligase, Ubr3, as an essential gene for auditory organ development. Ubr3 negatively regulates the mono-ubiquitination of non-muscle Myosin II, a protein associated with hearing loss in humans. The mono-ubiquitination of Myosin II promotes its physical interaction with Myosin VIIa, a protein responsible for Usher syndrome type IB. We show that ubr3 mutants phenocopy pathogenic variants of Myosin II and that Ubr3 interacts genetically and physically with three Usher syndrome proteins. The interactions between Myosin VIIa and Myosin IIa are conserved in the mammalian cochlea and in human retinal pigment epithelium cells. Our work reveals a novel mechanism that regulates protein complexes affected in two forms of syndromic deafness and suggests a molecular function for Myosin IIa in auditory organs.

SkpA restrains synaptic terminal growth during development and promotes axonal degeneration following injury

The Wallenda (Wnd)/dual leucine zipper kinase (DLK)-Jnk pathway is an evolutionarily conserved MAPK signaling pathway that functions during neuronal development and following axonal injury. Improper pathway activation causes defects in axonal guidance and synaptic growth, whereas loss-of-function mutations in pathway components impairs axonal regeneration and degeneration after injury. Regulation of this pathway is in part through the E3 ubiquitin ligase Highwire (Hiw), which targets Wnd/DLK for degradation to limit MAPK signaling. To explore mechanisms controlling Wnd/DLK signaling, we performed a large-scale genetic screen in Drosophila to identify negative regulators of the pathway. Here we describe the identification and characterization of SkpA, a core component of SCF E3 ubiquitin ligases. Mutants in SkpA display synaptic overgrowth and an increase in Jnk signaling, similar to hiw mutants. The combination of hypomorphic alleles of SkpA and hiw leads to enhanced synaptic growth. Mutants in the Wnd-Jnk pathway suppress the overgrowth of SkpA mutants demonstrating that the synaptic overgrowth is due to increased Jnk signaling. These findings support the model that SkpA and the E3 ligase Hiw function as part of an SCF-like complex that attenuates Wnd/DLK signaling. In addition, SkpA, like Hiw, is required for synaptic and axonal responses to injury. Synapses in SkpA mutants are more stable following genetic or traumatic axonal injury, and axon loss is delayed in SkpA mutants after nerve crush. As in highwire mutants, this axonal protection requires Nmnat. Hence, SkpA is a novel negative regulator of the Wnd-Jnk pathway that functions with Hiw to regulate both synaptic development and axonal maintenance.

Everything in moderation: Proteolytic regulation of centrosome duplication

The presence of too many or too few centrosomes at mitosis can disrupt the timely formation of a bipolar spindle and may lead to aneuploidy and cancer. Strict control of centrosome duplication is therefore crucial. Centrosome duplication must occur once per cell cycle and the number of new centrioles made must be tightly controlled. The importance of protein degradation for the orderly progression of the cell cycle has long been recognized, but until recently the role of proteolysis in the regulation of centrosome duplication had not been appreciated. Recent evidence suggests that restricting protein levels so that a single new centriole is built next to each pre-existing centriole is one way in which centrosome duplication is controlled. Here we discuss our recent finding that the SCF ubiquitin ligase complex regulates centrosome duplication in C. elegans in the larger context of the proteolytic regulation of centrosome duplication.

A Cullin1-based SCF E3 ubiquitin ligase targets the InR/PI3K/TOR pathway to regulate neuronal pruning

Pruning that selectively eliminates unnecessary axons/dendrites is crucial for sculpting the nervous system during development. During Drosophila metamorphosis, dendrite arborization neurons, ddaCs, selectively prune their larval dendrites in response to the steroid hormone ecdysone, whereas mushroom body γ neurons specifically eliminate their axon branches within dorsal and medial lobes. However, it is unknown which E3 ligase directs these two modes of pruning. Here, we identified a conserved SCF E3 ubiquitin ligase that plays a critical role in pruning of both ddaC dendrites and mushroom body γ axons. The SCF E3 ligase consists of four core components Cullin1/Roc1a/SkpA/Slimb and promotes ddaC dendrite pruning downstream of EcR-B1 and Sox14, but independently of Mical. Moreover, we demonstrate that the Cullin1-based E3 ligase facilitates ddaC dendrite pruning primarily through inactivation of the InR/PI3K/TOR pathway. We show that the F-box protein Slimb forms a complex with Akt, an activator of the InR/PI3K/TOR pathway, and promotes Akt ubiquitination. Activation of the InR/PI3K/TOR pathway is sufficient to inhibit ddaC dendrite pruning. Thus, our findings provide a novel link between the E3 ligase and the InR/PI3K/TOR pathway during dendrite pruning.

Ter94 ATPase complex targets k11-linked ubiquitinated ci to proteasomes for partial degradation

The Cubitus interruptus (Ci)/Gli family of transcription factors can be degraded either completely or partially from a full-length form (Ci155/Gli(FL)) to a truncated repressor (Ci75/Gli(R)) by proteasomes to mediate Hedgehog (Hh) signaling. The mechanism by which proteasomes distinguish ubiquitinated Ci/Gli to carry out complete versus partial degradation is not known. Here, we show that Ter94 ATPase and its mammalian counterpart, p97, are involved in processing Ci and Gli3 into Ci75 and Gli3(R), respectively. Ter94 regulates the partial degradation of ubiquitinated Ci by Cul1-Slimb-based E3 ligase through its adaptors Ufd1-like and dNpl4. We demonstrate that Cul1-Slimb-based E3 ligase, but not Cul3-Rdx-based E3 ligase, modifies Ci by efficient addition of K11-linked ubiquitin chains. Ter94(Ufd1-like/dNpl4) complex interacts directly with Cul1-Slimb, and, intriguingly, it prefers K11-linked ubiquitinated Ci. Thus, Ter94 ATPase and K11-linked ubiquitination in Ci contribute to the selectivity by proteasomes for partial degradation.

Never tear us apart--the importance of centrosome clustering

The presence of more than two centrosomes (centrosome amplification) at the onset of mitosis has long been associated with multipolar spindle formation, and with the generation of genetic instability. However, in recent years, several studies have shown that a process termed 'centrosome clustering' actively contributes to bipolar division by promoting the gathering of extra centrosomes in two main poles. In this Commentary, we describe the main proteins that are involved in centriole duplication and discuss how centrosome amplification can be generated both in vitro and in vivo. We then summarize what is currently known about the processes that contribute to bipolar spindle formation when extra centrosomes are present, and which forces contribute to this process. Finally, we discuss how extra centrosomes might contribute to tumorigenesis, giving emphasis to the role of centrosome amplification in promoting genetic instability.

Show me your license, please: deregulation of centriole duplication mechanisms that promote amplification

Centrosomes are organelles involved in generating and organizing the interphase microtubule cytoskeleton, mitotic spindles and cilia. At the centrosome core are a pair of centrioles, structures that act as the duplicating elements of this organelle. Centrioles function to recruit and organize pericentriolar material which nucleates microtubules. While centrioles are relatively simple in construction, the mechanics of centriole biogenesis remain an important yet poorly understood process. More mysterious still are the regulatory mechanisms that oversee centriole assembly. The fidelity of centriole duplication is critical as defects in either the assembly or number of centrioles promote aneuploidy, primary microcephaly, birth defects, ciliopathies and tumorigenesis. In addition, some pathogens employ mechanisms to promote centriole overduplication to the detriment of the host cell. This review summarizes our current understanding of this important topic, highlighting the need for further study if new therapeutics are to be developed to treat diseases arising from defects of centrosome duplication.

KIFC1 is essential for bipolar spindle formation and genomic stability in the primary human fibroblast IMR-90 cell

Kinesin family member C1 (KIFC1) is the only member of the minus-end-directed kinesin-14 family in human cells. In cancer cells, KIFC1 plays an essential role in bipolar spindle formation by clustering the multiple poles during mitosis. However, it has not been clearly demonstrated whether KIFC1 also functions to mediate bipolar spindle formation and to maintain genomic stability in normal cells. In this study, by using human primary lung fibroblast IMR-90 cells, we showed that KIFC1 knock-down with lentiviral KIFC1 shRNA induced 17% of cells with multiple microtubule organizing centers (MTOCs) and delayed cyclin A degradation for more than 2 hr in early mitosis. However, these cells eventually carried out mitosis, resulting in 24% of cells with lagging chromosomes and 9% of cells with micronuclei after mitosis. Karyotyping of KIFC1-depleted IMR-90 cells demonstrated that cells with various abnormal numbers of chromosomes are produced. When IMR-90 cells treated with KIFC1 or the control shRNA for 60 hr were compared, 20% less cells were observed in KIFC1-depleted cells without an obvious immediate cell death. As reported for Mad2 depletion in IMR-90 cells, KIFC1-depleted IMR-90 cells showed typical features of senescence, like senescence-associated (SA) β-galactosidase expression, when incubated 6 days or more. However, IMR-90 cells knocked down with both KIFC1 and Mad2 underwent apoptosis, suggesting that KIFC1 and Mad2 likely function in different pathways during mitosis. Taken together, we suggest that KIFC1 plays an essential role for bipolar MTOC formation and maintaining chromosomal stability in the mitosis of human primary fibroblast IMR-90.

The SKP1-like gene family of Arabidopsis exhibits a high degree of differential gene expression and gene product interaction during development

The Arabidopsis thaliana genome encodes several families of polypeptides that are known or predicted to participate in the formation of the SCF-class of E3-ubiquitin ligase complexes. One such gene family encodes the Skp1-like class of polypeptide subunits, where 21 genes have been identified and are known to be expressed in Arabidopsis. Phylogenetic analysis based on deduced polypeptide sequence organizes the family of ASK proteins into 7 clades. The complexity of the ASK gene family, together with the close structural similarity among its members raises the prospect of significant functional redundancy among select paralogs. We have assessed the potential for functional redundancy within the ASK gene family by analyzing an expanded set of criteria that define redundancy with higher resolution. The criteria used include quantitative expression of locus-specific transcripts using qRT-PCR, assessment of the sub-cellular localization of individual ASK:YFP auto-fluorescent fusion proteins expressed in vivo as well as the in planta assessment of individual ASK-F-Box protein interactions using bimolecular fluorescent complementation techniques in combination with confocal imagery in live cells. The results indicate significant functional divergence of steady state transcript abundance and protein-protein interaction specificity involving ASK proteins in a pattern that is poorly predicted by sequence-based phylogeny. The information emerging from this and related studies will prove important for defining the functional intersection of expression, localization and gene product interaction that better predicts the formation of discrete SCF complexes, as a prelude to investigating their molecular mode of action.

Expression and putative function of innate immunity genes under in situ conditions in the symbiotic hydrothermal vent tubeworm Ridgeia piscesae

The relationships between hydrothermal vent tubeworms and sulfide-oxidizing bacteria have served as model associations for understanding chemoautotrophy and endosymbiosis. Numerous studies have focused on the physiological and biochemical adaptations that enable these symbioses to sustain some of the highest recorded carbon fixation rates ever measured. However, far fewer studies have explored the molecular mechanisms underlying the regulation of host and symbiont interactions, specifically those mediated by the innate immune system of the host. To that end, we conducted a series of studies where we maintained the tubeworm, Ridgeia piscesae, in high-pressure aquaria and examined global and quantitative changes in gene expression via high-throughput transcriptomics and quantitative real-time PCR (qPCR). We analyzed over 32,000 full-length expressed sequence tags as well as 26 Mb of transcript sequences from the trophosome (the organ that houses the endosymbiotic bacteria) and the plume (the gas exchange organ in contact with the free-living microbial community). R. piscesae maintained under conditions that promote chemoautotrophy expressed a number of putative cell signaling and innate immunity genes, including pattern recognition receptors (PRRs), often associated with recognizing microbe-associated molecular patterns (MAMPs). Eighteen genes involved with innate immunity, cell signaling, cell stress and metabolite exchange were further analyzed using qPCR. PRRs, including five peptidoglycan recognition proteins and a Toll-like receptor, were expressed significantly higher in the trophosome compared to the plume. Although PRRs are often associated with mediating host responses to infection by pathogens, the differences in expression between the plume and trophosome also implicate similar mechanisms of microbial recognition in interactions between the host and symbiont. We posit that regulation of this association involves a molecular "dialogue" between the partners that includes interactions between the host's innate immune system and the symbiont.

The C. elegans F-box proteins LIN-23 and SEL-10 antagonize centrosome duplication by regulating ZYG-1 levels

The correct segregation of DNA during cell division requires formation of a bipolar spindle, organized at each pole by a centrosome. The regulation of centrosome duplication such that each mitotic cell has exactly two centrosomes is therefore of central importance to cell division. Deregulation of centrosome duplication causes the appearance of supernumerary centrosomes, which are a hallmark of many cancer cells and can contribute to tumorigenesis. Overexpression of the kinase Plk4, which is required for centrosome duplication, causes the formation of extra centrosomes, and aberrant Plk4 expression levels are associated with cancer. Data from Drosophila and human cells show that Plk4 levels are regulated by the SCF ubiquitin ligase and proteasomal degradation. Recognition of Plk4 by the SCF complex is mediated by the F-box protein Slimb/βTrCP. We show that levels of the C. elegans Plk4 homolog ZYG-1 are elevated by impairing proteasome or SCF function, indicating that ZYG-1 is regulated by a conserved mechanism. In C. elegans, similar to Drosophila and humans, we find that the Slimb/βTrCP homolog LIN-23 regulates ZYG-1 levels. In addition, we show that a second F-box protein, SEL-10, also contributes to ZYG-1 regulation. Co-depletion of LIN-23 and SEL-10 suggests these proteins function cooperatively. Because SEL-10 is the homolog of human FBW7, which is frequently mutated in cancer, our findings have implications for understanding tumorigenesis.

The HIV-1 Vpu protein induces apoptosis in Drosophila via activation of JNK signaling

The genome of the human immunodeficiency virus type 1 (HIV-1) encodes the canonical retroviral proteins, as well as additional accessory proteins that enhance the expression of viral genes, the infectivity of the virus and the production of virions. The accessory Viral Protein U (Vpu), in particular, enhances viral particle production, while also promoting apoptosis of HIV-infected human T lymphocytes. Some Vpu effects rely on its interaction with the ubiquitin-proteasome protein degradation system, but the mechanisms responsible for its pro-apoptotic effects in vivo are complex and remain largely to be elucidated.We took advantage of the Drosophila model to study the effects of Vpu activity in vivo. Expression of Vpu in the developing Drosophila wing provoked tissue loss due to caspase-dependent apoptosis. Moreover, Vpu induced expression of the pro-apoptotic gene reaper, known to down-regulate Inhibitor of Apoptosis Proteins (IAPs) which are caspase-antagonizing E3 ubiquitin ligases. Indeed, Vpu also reduced accumulation of Drosophila IAP1 (DIAP1). Though our results demonstrate a physical interaction between Vpu and the proteasome-addressing SLIMB/β-TrCP protein, as in mammals, both SLIMB/βTrCP-dependent and -independent Vpu effects were observed in the Drosophila wing. Lastly, the pro-apoptotic effect of Vpu in this tissue was abrogated upon inactivation of the c-Jun N-terminal Kinase (JNK) pathway. Our results in the fly thus provide the first functional evidence linking Vpu pro-apoptotic effects to activation of the conserved JNK pathway.

Defining components of the ß-catenin destruction complex and exploring its regulation and mechanisms of action during development

Background

A subset of signaling pathways play exceptionally important roles in embryonic and post-embryonic development, and mis-regulation of these pathways occurs in most human cancers. One such pathway is the Wnt pathway. The primary mechanism keeping Wnt signaling off in the absence of ligand is regulated proteasomal destruction of the canonical Wnt effector ßcatenin (or its fly homolog Armadillo). A substantial body of evidence indicates that SCF^βTrCP mediates βcat destruction, however, an essential role for Roc1 has not been demonstrated in this process, as would be predicted. In addition, other E3 ligases have also been proposed to destroy βcat, suggesting that βcat destruction may be regulated differently in different tissues.

Methodology/Principal Findings

Here we used cultured Drosophila cells, human colon cancer cells, and Drosophila embryos and larvae to explore the machinery that targets Armadillo for destruction. Using RNAi in Drosophila S2 cells to examine which SCF components are essential for Armadillo destruction, we find that Roc1/Roc1a is essential for regulating Armadillo stability, and that in these cells the only F-box protein playing a detectable role is Slimb. Second, we find that while embryonic and larval Drosophila tissues use the same destruction complex proteins, the response of these tissues to destruction complex inactivation differs, with Armadillo levels more elevated in embryos. We provide evidence consistent with the possibility that this is due to differences in armadillo mRNA levels. Third, we find that there is no correlation between the ability of different APC2 mutant proteins to negatively regulate Armadillo levels, and their recently described function in positively-regulating Wnt signaling. Finally, we demonstrate that APC proteins lacking the N-terminal Armadillo-repeat domain cannot restore Armadillo destruction but retain residual function in negatively-regulating Wnt signaling.

Conclusions/Significance

We use these data to refine our model for how Wnt signaling is regulated during normal development.

Cks85A and Skp2 interact to maintain diploidy and promote growth in Drosophila

The Cks or Suc1 proteins are highly conserved small proteins that play remarkably diverse roles in the cell cycle. All Cks homologues have the ability to associate with Cyclin dependent kinases (Cdks) and in many cases this interaction has been shown to be important for function. Here we characterize the null and RNAi knockdown phenotype of the Drosophila Cks1 (Cks85A) gene. Cks85A is essential for viability in Drosophila. Cks85A null animals have reduced overall growth and this correlates with reduced ploidy and impaired DNA replication in endoreplicating cells. Interestingly, Cks85A is also required for the maintenance of diploidy in mitotically cycling cells. The requirement for Cks85A in growth is similar to that of the mammalian Cks1, which was found to interact with the SCF(Skp2) ubiquitin ligase. We identified the Drosophila Skp2 gene and generated null alleles. Comparison of these mutants to null mutants for Cks85A reveals a remarkably similar dual requirement in growth and in maintenance of diploidy. We find that Cks85A interacts directly with the SCF(Skp2) ubiquitin ligase and genetic evidence indicates that this is its major molecular function. The closely related Cks30A cannot interact with the SCF(Skp2) and cannot functionally compensate for loss of Cks85A. We also find that the critical growth promoting and diploidy maintaining functions of Cks85A and Skp2 are independent of known SCF(Skp2) substrates, p27 and Cdt1, indicating that other critical substrates remain to be identified.

Genotoxicity of all-trans retinoic acid (ATRA) and its steroidal analogue EA-4 in human lymphocytes and mouse cells in vitro

The aim of our study is to: (a) investigate whether ATRA and its steroidal analogue EA-4 enhance micronucleation in human lymphocytes and mouse cells in vitro and clarify the micronucleation mechanism by FISH and CREST analysis respectively, and (b) analyze their effect on spindle organization by immunofluorescence of β- and γ-tubulin in mouse cells. We found that they: (a) induce micronucleation mainly via chromosome breakage and chromosome delay in a lesser extent, (b) disturb microtubule network, chromosome orientation and centrosome duplication/separation, (c) accumulate cell cycle at ana-telophases, which exert micronucleation, multiple γ-tubulin signals, nucleoplasmic bridges and multinucleation, and (d) generate multinucleated and multimicronucleated interphase cells.

A genome-wide RNAi screen identifies core components of the G₂-M DNA damage checkpoint

The DNA damage checkpoint, the first pathway known to be activated in response to DNA damage, is a mechanism by which the cell cycle is temporarily arrested to allow DNA repair. The checkpoint pathway transmits signals from the sites of DNA damage to the cell cycle machinery through the evolutionarily conserved ATM (ataxia telangiectasia mutated) and ATR (ATM- and Rad3-related) kinase cascades. We conducted a genome-wide RNAi (RNA interference) screen in Drosophila cells to identify previously unknown genes and pathways required for the G₂-M checkpoint induced by DNA double-strand breaks (DSBs). Our large-scale analysis provided a systems-level view of the G₂-M checkpoint and revealed the coordinated actions of particular classes of proteins, which include those involved in DNA repair, DNA replication, cell cycle control, chromatin regulation, and RNA processing. Further, from the screen and in vivo analysis, we identified previously unrecognized roles of two DNA damage response genes, mus101 and mus312. Our results suggest that the DNA replication preinitiation complex, which includes MUS101, and the MUS312-containing nuclease complexes, which are important for DSB repair, also function in the G₂-M checkpoint. Our results provide insight into the diverse mechanisms that link DNA damage and the checkpoint signaling pathway.

From zero to many: control of centriole number in development and disease

Centrioles are essential for the formation of microtubule-derived structures, including cilia, flagella and centrosomes. These structures are involved in a variety of functions, from cell motility to division. In most dividing animal cells, centriole formation is coupled to the chromosome cycle. However, this is not the case in certain specialized divisions, such as meiosis, and in some differentiating cells. For example, oocytes loose their centrioles upon differentiation, whereas multiciliated epithelial cells make several of those structures after they exit the cell cycle. Aberrations of centriole number are seen in many cancer cells. Recent studies began to shed light on the molecular control of centriole number, its variations in development, and how centriole number changes in human disease. Here we review the recent developments in this field.

FM19G11: A new modulator of HIF that links mTOR activation with the DNA damage checkpoint pathways

The network consisting of mTOR and p53 pathways is crucial to understanding a wide variety of physiological and pathological events, including cancer and aging. In addition, the HIF1alpha protein, a downstream target of mTOR, is a hallmark of different tumor types and was the desired strategy of many drug discovery efforts. Here we present the novel chemical entity FM19G11, a new modulator of HIF1alpha expression, which was used as a molecular tool to dissect and further characterize the cross-talk between these signaling cascades in human colon carcinoma cell lines. To our knowledge, FM19G11 is the first drug that triggers a DNA damage response (DDR) associated with G(1)/S-phase arrest in a p53-dependent manner, due to rapid hyper-activation of the growth signaling pathway through mTOR. Assessment of colonies demonstrated that FM19G11 decreases the clonogenicity of HT29, HCT116/p53(+/+) and HCT116/p53(-/-) cells. Moreover, FM19G11 causes significant lower colony growth in soft agar of p53-proficient human colon cancer cells. Consequently, p53 sensitizes human colon cancer cells to FM19G11 by significant reduction of their viability, lessening their colony formation capability and shrinking their anchorage-independent growth. Cell signaling studies served to assign a new mode of action to FM19G11, whose tumor-suppressant activity compromises the survival of functional p53 malignant cells.

Hsp90-Sgt1 and Skp1 target human Mis12 complexes to ensure efficient formation of kinetochore-microtubule binding sites

The Hsp90–Sgt1 chaperone and the ubiquitin ligase subunit Skp1 regulate the assembly and turnover of the kinetochore complex Mis12.

The formation of functional kinetochores requires the accurate assembly of a large number of protein complexes. The Hsp90–Sgt1 chaperone complex is important for this process; however, its targets are not conserved and its exact contribution to kinetochore assembly is unclear. Here, we show that human Hsp90–Sgt1 interacts with the Mis12 complex, a so-called keystone complex required to assemble a large fraction of the kinetochore. Inhibition of Hsp90 or Sgt1 destabilizes the Mis12 complex and delays proper chromosome alignment due to inefficient formation of microtubule-binding sites. Interestingly, coinhibition of Sgt1 and the SCF subunit, Skp1, increases Mis12 complexes at kinetochores and restores timely chromosome alignment but forms less-robust microtubule-binding sites. We propose that a balance of Mis12 complex assembly and turnover is required for the efficient and accurate assembly of kinetochore–microtubule binding sites. These findings support a novel role for Hsp90–Sgt1 chaperones in ensuring the fidelity of multiprotein complex assembly.

[Centrosomes, mitotic spindle and cancer: find the odd one out!]

Centrosomes are essential protagonists during cell division through microtubule nucleation and spindle formation which are key to the harmonious distribution of sister chromatids in the two daughter cells. However, during the past decade, a wealth of new observations has extended their role beyond mitosis, particularly in the asymmetrical partition of cell fate determinants. Remarkably, asymmetric centrosome inheritance per se, through the segregation of differently aged mother -centrioles, seems to regulate the differential behaviour of daughter cells, in part through asynchronous expression of primary cilia, governing the response to environmental signals. It is thus understandable why any quantitative or qualitative dysfunction of centrioles contributes to genomic -instability and thus -tumorigenesis.

Linking H3K79 trimethylation to Wnt signaling through a novel Dot1-containing complex (DotCom)

Epigenetic modifications of chromatin play an important role in the regulation of gene expression. KMT4/Dot1 is a conserved histone methyltransferase capable of methylating chromatin on Lys79 of histone H3 (H3K79). Here we report the identification of a multisubunit Dot1 complex (DotCom), which includes several of the mixed lineage leukemia (MLL) partners in leukemia such as ENL, AF9/MLLT3, AF17/MLLT6, and AF10/MLLT10, as well as the known Wnt pathway modifiers TRRAP, Skp1, and beta-catenin. We demonstrated that the human DotCom is indeed capable of trimethylating H3K79 and, given the association of beta-catenin, Skp1, and TRRAP, we investigated, and found, a role for Dot1 in Wnt/Wingless signaling in an in vivo model system. Knockdown of Dot1 in Drosophila results in decreased expression of a subset of Wingless target genes. Furthermore, the loss of expression for the Drosophila homologs of the Dot1-associated proteins involved in the regulation of H3K79 shows a similar reduction in expression of these Wingless targets. From yeast to human, specific trimethylation of H3K79 by Dot1 requires the monoubiquitination of histone H2B by the Rad6/Bre1 complex. Here, we demonstrate that depletion of Bre1, the E3 ligase required for H2B monoubiquitination, leads specifically to reduced bulk H3K79 trimethylation levels and a reduction in expression of many Wingless targets. Overall, our study describes for the first time the components of DotCom and links the specific regulation of H3K79 trimethylation by Dot1 and its associated factors to the Wnt/Wingless signaling pathway.

Nek2 targets the mitotic checkpoint proteins Mad2 and Cdc20: a mechanism for aneuploidy in cancer

In mitosis, the duplicated chromosomes are separated and equally distributed to progeny cells under the guidance of the spindle, a dynamic microtubule network. Previous studies revealed a mitotic checkpoint that prevents segregation of the chromosomes until all of the chromosomes are properly attached to microtubules through the kinetochores. A variety of kinetochore-localized proteins, including Mad2 and Cdc20, have been implicated in controlling the mitotic checkpoint. Here we report that both Mad2 and Cdc20 can physically associate with Nek2, a serine/threonine kinase implicated in centrosome functions. We show that, similar to Nek2, the endogenous Cdc20 protein can be detected in the centrosome and the spindle poles. Both Cdc20 and Mad2 can be phosphorylated by Nek2. Moreover, our studies demonstrate that overexpression of Nek2 enhances the ability of Mad2 to induce a delay in mitosis. These observations indicate that Nek2 may act upon the Mad2-Cdc20 protein complex and play a critical role in regulating the mitotic checkpoint protein complex. We propose that overexpression of Nek2 may promote aneuploidy by disrupting the control of the mitotic checkpoint.

The SCF Slimb ubiquitin ligase regulates Plk4/Sak levels to block centriole reduplication

Restricting centriole duplication to once per cell cycle is critical for chromosome segregation and genomic stability, but the mechanisms underlying this block to reduplication are unclear. Genetic analyses have suggested an involvement for Skp/Cullin/F box (SCF)-class ubiquitin ligases in this process. In this study, we describe a mechanism to prevent centriole reduplication in Drosophila melanogaster whereby the SCF E3 ubiquitin ligase in complex with the F-box protein Slimb mediates proteolytic degradation of the centrosomal regulatory kinase Plk4. We identified SCF(Slimb) as a regulator of centriole duplication via an RNA interference (RNAi) screen of Cullin-based ubiquitin ligases. We found that Plk4 binds to Slimb and is an SCF(Slimb) target. Both Slimb and Plk4 localize to centrioles, with Plk4 levels highest at mitosis and absent during S phase. Using a Plk4 Slimb-binding mutant and Slimb RNAi, we show that Slimb regulates Plk4 localization to centrioles during interphase, thus regulating centriole number and ensuring the block to centriole reduplication.

The SCF/Slimb ubiquitin ligase limits centrosome amplification through degradation of SAK/PLK4

Centrioles are essential for the formation of microtubule-derived structures, including cilia and centrosomes. Abnormalities in centrosome number and structure occur in many cancers and are associated with genomic instability. In most dividing animal cells, centriole formation is coordinated with DNA replication and is highly regulated such that only one daughter centriole forms close to each mother centriole. Centriole formation is triggered and dependent on a conserved kinase, SAK/PLK4. Downregulation and overexpression of SAK/PLK4 is associated with cancer in humans, mice, and flies. Here we show that centrosome amplification is normally inhibited by degradation of SAK/PK4 degradation, mediated by the SCF/Slimb ubiquitin ligase. This complex physically interacts with SAK/PLK4, and in its absence, SAK/PLK4 accumulates, leading to the striking formation of multiple daughter centrioles surrounding each mother. This interaction is mediated via a conserved Slimb binding motif in SAK/PLK4, mutations of which leads to centrosome amplification. This regulation is likely to be conserved, because knockout of the ortholog of Slimb, beta-Trcp1 in mice, also leads to centrosome amplification. Because the SCF/beta-Trcp complex plays an important role in cell-cycle progression, our results lead to new understanding of the control of centrosome number and how it may go awry in human disease.

Multiple centrosomes: together they stand, divided they fall

Cells with extra centrosomes rely entirely on centrosome clustering mechanisms to assemble a bipolar spindle and to divide in a bipolar fashion. To identify the pathways involved in suppression of multipolarity, Kwon, Godinho, and colleagues (pp. 2189-2203) have set up a genome-wide screen in Drosophila S2 cells. Surprisingly, they found that efficient clustering requires a large number of proteins associated with a variety of cellular functions.

Functional characterization of the Drosophila Hmt4-20/Suv4-20 histone methyltransferase

Di- and trimethylation of histone H4 lysine20 (H4K20) are thought to play an important role in controlling gene expression in vertebrates and in Drosophila. By inducing a null mutation in Drosophila Suv4-20, we show that it encodes the histone H4 lysine20 di- and trimethyltransferase. In Suv4-20 mutants, the H4K20 di- and trimethyl marks are strongly reduced or absent, and the monomethyl mark is significantly increased. We find that even with this biochemical function, Suv4-20 is not required for survival and does not control position-effect variegation (PEV).

Centrosome amplification can initiate tumorigenesis in flies

Centrosome amplification is a common feature of many cancer cells, and it has been previously proposed that centrosome amplification can drive genetic instability and so tumorigenesis. To test this hypothesis, we generated Drosophila lines that have extra centrosomes in approximately 60% of their somatic cells. Many cells with extra centrosomes initially form multipolar spindles, but these spindles ultimately become bipolar. This requires a delay in mitosis that is mediated by the spindle assembly checkpoint (SAC). As a result of this delay, there is no dramatic increase in genetic instability in flies with extra centrosomes, and these flies maintain a stable diploid genome over many generations. The asymmetric division of the larval neural stem cells, however, is compromised in the presence of extra centrosomes, and larval brain cells with extra centrosomes can generate metastatic tumors when transplanted into the abdomens of wild-type hosts. Thus, centrosome amplification can initiate tumorigenesis in flies.

Downregulation of vertebrate Tel (ETV6) and Drosophila Yan is facilitated by an evolutionarily conserved mechanism of F-box-mediated ubiquitination

The vertebrate Ets transcriptional repressor Tel (ETV6) and its invertebrate orthologue, Yan, are both indispensable for development, and they orchestrate cell growth and differentiation by binding to DNA, thus inhibiting gene expression. To trigger cell differentiation, these barriers to transcriptional activation must be relieved, and it is established that posttranslational modifications, such as phosphorylation and sumoylation, can specifically impair the repressive functions of Tel and Yan and are crucial for modulating their transcriptional activity. To date, however, relatively little is known about the control of Tel and Yan protein degradation. In recent years, there has been a concentrated effort to assign functions to the large number of F-box proteins encoded by both vertebrate and invertebrate genomes. Here, we report the identification and characterization of a previously unreported, evolutionarily conserved F-box protein named Fbl6. We isolated both human and Drosophila melanogaster fbl6 cDNA and show that the encoded Fbl6 protein binds to both Tel and Yan via their SAM domains. We demonstrate that both Tel and Yan are ubiquitinated, a process which is stimulated by Fbl6 and leads to proteasomal degradation. We recently established that the sumoylation of Tel on lysine 11 negatively regulates its repressive function and that the sumoylation of Tel monomers, but not that of Tel oligomers, may sensitize Tel for proteasomal degradation. Here, we found that Fbl6 regulates Tel/Yan protein stability and allows appropriate spatiotemporal control of gene expression by these repressors.

Coupling metabolite flux to transcriptomics: insights into the molecular mechanisms underlying primary productivity by the hydrothermal vent tubeworm Ridgeia piscesae

Deep-sea hydrothermal vents host highly productive ecosystems. Many of these communities are dominated by vestimentiferan tubeworms that house endosymbiotic chemoautotrophic bacteria that provide the hosts with their primary nutritional needs. Rates of carbon fixation by these symbioses are also among the highest recorded. Despite the breadth of physiological and biochemical research on these associations, the underlying molecular mechanisms that regulate host and symbiont metabolite flux and carbon fixation are largely unknown. Here we present metabolite flux and transcriptomics data from shipboard high-pressure respirometry experiments in which we maintained Ridgeia piscesae tubeworms at conditions comparable to those in situ. Host trophosome was used for cDNA library construction and sequencing. Of the 19,132 clones sequenced, 10,684 represented unique expressed sequence tags (ESTs). The highest proportions of genes are involved with translation, ribosomal structure and biogenesis, cellular processing, and signal transduction. There was moderate representation of genes involved in metabolite exchange and acid-base regulation. These data represent the first concomitant surveys of metabolite flux rates and gene expression for a chemoautotrophic symbiosis during net autotrophy, and they suggest that-in the case of Ridgeia piscesae-host-symbiont interactions such as cell cycle regulation may play a significant role in maintaining physiological poise during high productivity.

A microtubule interactome: complexes with roles in cell cycle and mitosis

The microtubule (MT) cytoskeleton is required for many aspects of cell function, including the transport of intracellular materials, the maintenance of cell polarity, and the regulation of mitosis. These functions are coordinated by MT-associated proteins (MAPs), which work in concert with each other, binding MTs and altering their properties. We have used a MT cosedimentation assay, combined with 1D and 2D PAGE and mass spectrometry, to identify over 250 MAPs from early Drosophila embryos. We have taken two complementary approaches to analyse the cellular function of novel MAPs isolated using this approach. First, we have carried out an RNA interference (RNAi) screen, identifying 21 previously uncharacterised genes involved in MT organisation. Second, we have undertaken a bioinformatics analysis based on binary protein interaction data to produce putative interaction networks of MAPs. By combining both approaches, we have identified and validated MAP complexes with potentially important roles in cell cycle regulation and mitosis. This study therefore demonstrates that biologically relevant data can be harvested using such a multidisciplinary approach, and identifies new MAPs, many of which appear to be important in cell division.

The DRE/DREF transcriptional regulatory system: a master key for cell proliferation

The coordinate expression of many cell proliferation-related genes is required for the cellular shift from the resting state into the proliferating state. One regulatory factor involved in this process, the transcription regulatory factor named DREF (DNA replication-related element-binding factor) was discovered in Drosophila and later found to have orthologues in other species including human. Drosophila DREF is a homo-dimer of a polypeptide of 709 amino acid residues, and shares about 22% identity in its amino acid sequence with the human homolog of 694 amino acid residues. The Drosophila DREF homo-dimer binds specifically to the DRE sequence (5'-TATCGATA) in the promoters of many DNA replication/ cell proliferation-related genes to activate their transcription, and the N-terminal region of DREF carries a domain for specific DRE-binding and homo-dimer formation. Ectopic expression of DREF in eye imaginal discs induces abnormal DNA synthesis, apoptosis and failure to differentiate. Conversely, expression of the dominant negative N-terminal region in larval salivary glands reduces endo-replication. Furthermore, RNA interference-mediated knockdown of DREF in vivo demonstrated its requirement for normal progression through the cell cycle and consequently for growth of imaginal discs and the endoreplicating organs. Both Drosophila and human DREF's interact genetically and physically with regulatory factors related to chromatin structures, suggesting that DREF activates the expression of proliferation-related genes through modification of the 3-D conformation of DNA. A search of the Drosophila genome database identified about 150 genes carrying DRE sequences in their promoter regions, many of which are related to reactions required for cell proliferation such as DNA replication, transcriptional regulation, cell cycle regulation, growth signal transduction and protein metabolism. Thus, DREF appears to be a master key-like factor for cell proliferation. Several differentiation-related transcription factors containing homeodomains down-regulate the function or expression of DREF by distinct mechanisms, suggesting a differentiation-coupled repression of cell proliferation via the DRE/DREF system.

Preventing the degradation of mps1 at centrosomes is sufficient to cause centrosome reduplication in human cells

Supernumerary centrosomes promote the assembly of abnormal mitotic spindles in many human tumors. In human cells, overexpression of the cyclin-dependent kinase (Cdk)2 partner cyclin A during a prolonged S phase produces extra centrosomes, called centrosome reduplication. Cdk2 activity protects the Mps1 protein kinase from proteasome-mediated degradation, and we demonstrate here that Mps1 mediates cyclin A-dependent centrosome reduplication. Overexpression of cyclin A or a brief proteasome inhibition increases the centrosomal levels of Mps1, whereas depletion of Cdk2 leads to the proteasome-dependent loss of Mps1 from centrosomes only. When a Cdk2 phosphorylation site within Mps1 (T468) is mutated to alanine, Mps1 cannot accumulate at centrosomes or participate in centrosome duplication. In contrast, phosphomimetic mutations at T468 or deletion of the region surrounding T468 prevent the proteasome-dependent removal of Mps1 from centrosomes in the absence of Cdk2 activity. Moreover, cyclin A-dependent centrosome reduplication requires Mps1, and these stabilizing Mps1 mutations cause centrosome reduplication, bypassing cyclin A. Together, our data demonstrate that the region surrounding T468 contains a motif that regulates the accumulation of Mps1 at centrosomes. We suggest that phosphorylation of T468 attenuates the degradation of Mps1 at centrosomes and that preventing this degradation is necessary and sufficient to cause centrosome reduplication in human cells.

Centrosome biogenesis and function: centrosomics brings new understanding

Centrosomes, which were first described in the late 19th century, are found in most animal cells and undergo duplication once every cell cycle so that their number remains stable, like the genetic material of a cell. However, their function and regulation have remained elusive and controversial. Only recently has some understanding of these fundamental aspects of centrosome function and biogenesis been gained through the concerted application of genomics and proteomics, which we term 'centrosomics'. The identification of new molecules has highlighted the evolutionary conservation of centrosome function and provided a conceptual framework for understanding centrosome behaviour and how it can go awry in human disease.

Identification of the Drosophila skpA gene as a novel target of the transcription factor DREF

SKPa is component of a Drosophila SCF complex that functions in combination with the ubiquitin-conjugating enzyme UbcD1. skpA null mutation results in centrosome overduplication, unusual chromatin condensation, defective endoreduplication and cell-cycle progression. While the molecular mechanisms that regulate expression of the skpA gene are poorly understood, the DNA replication-related element (DRE) and the DRE-binding factor (DREF) play important roles in regulating proliferation-related genes in Drosophila and DRE (5'-TATCGATA) and DRE-like (5'-CATCGATT) sequences were here found to be involved in skpA promoter activity. Thus both luciferase transient expression assays in cultured Drosophila S2 cells using skpA promoter-luciferase fusion plasmids and anti-lacZ immunostaining of various tissues from transgenic third instar larvae carrying the skpA promoter-lacZ fusion genes provided supportive evidence. Furthermore, anti-SKPa immunostaining of eye imaginal discs from flies overexpressing DREF showed ectopic expression of protein in the region posterior to the morphogenetic furrow where DREF is overexpressed. Knockdown of DREF in some tissues where SKPa distribution is well known almost completely abrogated the skpA gene expression. These findings, taken together, indicate that the Drosophila skpA gene is a novel target of the transcription factor DREF.

Melanotic mutants in Drosophila: pathways and phenotypes

Mutations in >30 genes that regulate different pathways and developmental processes are reported to cause a melanotic phenotype in larvae. The observed melanotic masses were generally linked to the hemocyte-mediated immune response. To investigate whether all black masses are associated with the cellular immune response, we characterized melanotic masses from mutants in 14 genes. We found that the melanotic masses can be subdivided into melanotic nodules engaging the hemocyte-mediated encapsulation and into melanizations that are not encapsulated by hemocytes. With rare exception, the encapsulation is carried out by lamellocytes. Encapsulated nodules are found in the hemocoel or in association with the lymph gland, while melanizations are located in the gut, salivary gland, and tracheae. In cactus mutants we found an additional kind of melanized mass containing various tissues. The development of these tissue agglomerates is dependent on the function of the dorsal gene. Our results show that the phenotype of each mutant not only reflects its connection to a particular genetic pathway but also points to the tissue-specific role of the individual gene.

RNA silencing of S-phase kinase-interacting protein 2 inhibits proliferation and centrosome amplification in lung cancer cells

The S-phase kinase-associated protein-2 (SKP2) plays a key role in ubiquitin-mediated proteolysis, which results in the progression of cells from a quiescence to proliferative state. SKP2 is overexpressed in a variety of tumors. In this study, we used small interfering RNAs (siRNAs) to inhibit the SKP2 expression in lung cancer cells and thereby investigate the role of SKP2 in lung tumorigenesis. Three lung cancer cell lines were transfected with siRNAs targeted against SKP2. SKP2-siRNAs specifically and efficiently reduced the levels of the SKP2 protein by 90% 48 h after transfection in all cell lines. In the A549 and H1792 cells, p27 expression was increased and the increase was inversely proportional to the level of SKP2; cell proliferation was reduced to 12 and 28%, respectively; apoptosis was increased to 36 and 30%, respectively; 36 and 28% of cells accumulated in the sub-G1 phase, respectively; and the population of cells in the G1 phase was decreased to 37 and 41%, respectively. In addition, the SKP2-depleted A549 and H1792 cells showed decreased levels of cyclin E/CDK2. Correspondingly, only 4 and 6% of the treated A549 and H1792 cells had multiple centrosomes, respectively, compared with 43 and 46% of the control cells, respectively. These results imply that SKP2 plays an oncogenic role in lung cancer and that SKP2 silencing may be useful in the treatment of lung cancer.

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.

A hitchhiker's guide to the cullin ubiquitin ligases: SCF and its kin

The SCF (Skp1-Cullin-F-box) E3 ubiquitin ligase family was discovered through genetic requirements for cell cycle progression in budding yeast. In these multisubunit enzymes, an invariant core complex, composed of the Skp1 linker protein, the Cdc53/Cul1 scaffold protein and the Rbx1/Roc1/Hrt1 RING domain protein, engages one of a suite of substrate adaptors called F-box proteins that in turn recruit substrates for ubiquitination by an associated E2 enzyme. The cullin-RING domain-adaptor architecture has diversified through evolution, such that in total many hundreds of distinct SCF and SCF-like complexes enable degradation of myriad substrates. Substrate recognition by adaptors often depends on posttranslational modification of the substrate, which thus places substrate stability under dynamic regulation by intracellular signaling events. SCF complexes control cell proliferation through degradation of critical regulators such as cyclins, CDK inhibitors and transcription factors. A plethora of other processes in development and disease are controlled by other SCF-like complexes, including those based on Cul2-SOCS-box adaptor protein and Cul3-BTB domain adaptor protein combinations. Recent structural insights into SCF-like complexes have begun to illuminate aspects of substrate recognition and catalytic reaction mechanisms.

Targeted disruption of Drosophila Roc1b reveals functional differences in the Roc subunit of Cullin-dependent E3 ubiquitin ligases

Cullin-dependent ubiquitin ligases regulate a variety of cellular and developmental processes by recruiting specific proteins for ubiquitin-mediated degradation. Cullin proteins form a scaffold for two functional modules: a catalytic module comprised of a small RING domain protein Roc1/Rbx1 and a ubiquitin-conjugating enzyme (E2), and a substrate recruitment module containing one or more proteins that bind to and bring the substrate in proximity to the catalytic module. Here, we present evidence that the three Drosophila Roc proteins are not functionally equivalent. Mutation of Roc1a causes lethality that cannot be rescued by expression of Roc1b or Roc2 by using the Roc1a promoter. Roc1a mutant cells hyperaccumulate Cubitus interruptus, a transcription factor that mediates Hedgehog signaling. This phenotype is not rescued by expression of Roc2 and only partially by expression of Roc1b. Targeted disruption of Roc1b causes male sterility that is partially rescued by expression of Roc1a by using the Roc1b promoter, but not by similar expression of Roc2. These data indicate that Roc proteins play nonredundant roles during development. Coimmunoprecipitation followed by Western or mass spectrometric analysis indicate that the three Roc proteins preferentially bind certain Cullins, providing a possible explanation for the distinct biological activities of each Drosophila Roc/Rbx.

The arithmetic of centrosome biogenesis

How do cells regulate centrosome number? A canonical duplication cycle generates two centrosomes from one in most proliferating cells. Centrioles are key to this process, and molecules such as centrins, SAS-4 and ZYG-1 govern daughter centriole formation. Cdk2 activity probably couples centrosome duplication with the S phase, and a licensing mechanism appears to limit centrosome duplication to once per cell cycle. However, such mechanisms must be altered in some cells – for example, spermatocytes – in which centrosome duplication and DNA replication are uncoupled. There are also alternative pathways of centrosome biogenesis. For example, one centrosome is reconstituted from two gametes at fertilization; in this case, the most common strategy involves differential contributions of centrioles and pericentriolar material (PCM) from each gamete. Furthermore, centrioles can sometimes form de novo from no apparent template. This occurs, for instance, in the early mouse embryo and in parthenogenetic species and might rely on a pre-existing seed that resides within PCM but is not visible by ultrastructural analysis.