An RNAi Screen for Genes Required for Growth of Drosophila Wing Tissue

Cell division and tissue growth must be coordinated with development. Defects in these processes are the basis for a number of diseases, including developmental malformations and cancer. We have conducted an unbiased RNAi screen for genes that are required for growth in the Drosophila wing, using GAL4-inducible short hairpin RNA (shRNA) fly strains made by the Drosophila RNAi Screening Center. shRNA expression down the center of the larval wing disc using dpp-GAL4, and the central region of the adult wing was then scored for tissue growth and wing hair morphology. Out of 4,753 shRNA crosses that survived to adulthood, 18 had impaired wing growth. FlyBase and the new Alliance of Genome Resources knowledgebases were used to determine the known or predicted functions of these genes and the association of their human orthologs with disease. The function of eight of the genes identified has not been previously defined in Drosophila The genes identified included those with known or predicted functions in cell cycle, chromosome segregation, morphogenesis, metabolism, steroid processing, transcription, and translation. All but one of the genes are similar to those in humans, and many are associated with disease. Knockdown of lin-52, a subunit of the Myb-MuvB transcription factor, or βNACtes6, a gene involved in protein folding and trafficking, resulted in a switch from cell proliferation to an endoreplication growth program through which wing tissue grew by an increase in cell size (hypertrophy). It is anticipated that further analysis of the genes that we have identified will reveal new mechanisms that regulate tissue growth during development

The PI3K/Akt1 pathway enhances steady-state levels of FANCL

Fanconi anemia hematopoietic stem cells display poor self-renewal capacity when subjected to a variety of cellular stress. This phenotype raises the question of whether the Fanconi anemia proteins are stabilized or recruited as part of a stress response and protect against stem cell loss. Here we provide evidence that FANCL, the E3 ubiquitin ligase of the Fanconi anemia pathway, is constitutively targeted for degradation by the proteasome. We confirm biochemically that FANCL is polyubiquitinated with Lys-48-linked chains. Evaluation of a series of N-terminal-deletion mutants showed that FANCL's E2-like fold may direct ubiquitination. In addition, our studies showed that FANCL is stabilized in a complex with axin1 when glycogen synthase kinase-3β is overexpressed. This result leads us to investigate the potential regulation of FANCL by upstream signaling pathways known to regulate glycogen synthase kinase-3β. We report that constitutively active, myristoylated-Akt increases FANCL protein level by reducing polyubiquitination of FANCL. Two-dimensional PAGE analysis shows that acidic forms of FANCL, some of which are phospho-FANCL, are not subject to polyubiquitination. These results indicate that a signal transduction pathway involved in self-renewal and survival of hematopoietic stem cells also functions to stabilize FANCL and suggests that FANCL participates directly in support of stem cell function

An RNAi Screen for Genes Required for Growth of<i>Drosophila</i>Wing Tissue

AbstractCell division and tissue growth must be coordinated with development. Defects in these processes are the basis for a number of diseases, including developmental malformations and cancer. We have conducted an unbiased RNAi screen for genes that are required for growth in the Drosophila wing, using GAL4-inducible short hairpin RNA (shRNA) fly strains made by the Drosophila RNAi Screening Center. shRNA expression down the center of the larval wing disc using dpp-GAL4, and the central region of the adult wing was then scored for tissue growth and wing hair morphology. Out of 4,753 shRNA crosses that survived to adulthood, 18 had impaired wing growth. FlyBase and the new Alliance of Genome Resources knowledgebases were used to determine the known or predicted functions of these genes and the association of their human orthologs with disease. The function of eight of the genes identified has not been previously defined in Drosophila. The genes identified included those with known or predicted functions in cell cycle, chromosome segregation, morphogenesis, metabolism, steroid processing, transcription, and translation. All but one of the genes are similar to those in humans, and many are associated with disease. Knockdown of lin-52, a subunit of the Myb-MuvB transcription factor, or βNACtes6, a gene involved in protein folding and trafficking, resulted in a switch from cell proliferation to an endoreplication growth program through which wing tissue grew by an increase in cell size (hypertrophy). It is anticipated that further analysis of the genes that we have identified will reveal new mechanisms that regulate tissue growth during development.</jats:p

Regulation of FANCL by Glycogen Synthase Kinase-3beta Links the Fanconi anemia pathway to Self Renewal and Survival Signals

Abstract Abstract 1263 The molecular basis for how a Fanconi anemia (FA) genetic background contributes to hematopoietic stem cell defects and hypoplastic organ development remains poorly understood. Protein modification by ubiquitination is a mechanism that diversifies the function and regulation of proteins. In light of this, we focus on the dysfunction of FANCL, the E3 ubiquitin ligase of the FA pathway, as a key molecular defect in Fanconi anemia. Here we report our studies investigating mechanisms of post-translational regulation of FANCL. We view these mechanisms as potential targets to augment the function of the FA core complex and correct hematopoietic stem cell defects. We provide evidence that FANCL is exquisitely regulated by ubiquitin-proteosome degradation. Ligase-inactive mutants (FANCL-C307A and -W341G) are less sensitive to this regulation, suggesting a role for auto-ubiquitination in directing lysine-48 polyubiquitination. This constitutive negative regulation of FANCL is partially reversed with an ATP-competitive glycogen synthase kinase-3beta (GSK-3beta) inhibitor. GSK-3beta is a serine/threonine kinase that phosphorylates proteins and marks them for ubiquitin-mediated proteolysis. Mitogenic and survival pathways, including Ras/MAPK and PI3K/Akt, negatively regulate GSK-3beta by serine-9 phosphorylation. We show that the regulation of FANCL by GSK-3beta is likely direct because FANCL and GSK-3beta co-immunoprecipitate in cell lysates and as GST-fusion proteins. To define the biochemical mechanisms of FANCL regulation, we generated N-terminal deletion mutants of FANCL and we show that the regulation of FANCL is dictated by a region at the N-terminus (aa1-78). Mutational analysis of FANCL (lysine to arginine) in this N-terminus region does not affect the overall protein level or ubiquitination of FANCL, suggesting that FANCL may be targeted for degradation by phosphorylation and/or in a complex with other proteins. The potential biological relevance of our findings, that FANCL is regulated by GSK-3beta is revealed in studies overexpressing constitutively active, myristoylated-Akt. This experimental condition increases FANCL protein levels and suggests a role for FANCL as a downstream effector of PI3K/Akt signaling. In turn, FANCL likely regulates non-canonical targets that alter the transcriptome profile favoring self-renewal and survival of hematopoietic stem cells. We recently published our studies identifying beta-catenin as one such downstream target (Blood 2012 Jul 12;120:323). Suppression of FANCL expression severely disrupts Wnt/beta-catenin signaling and expression of downstream Wnt-responsive targets MYC and CCND1. We also identified that GSK3B gene expression is approximately 5-fold higher in Fancc-deficient hematopoietic stem cells exposed to TNF-alpha compared to untreated cells or to wildtype cells with or without TNF-alpha. Our current studies show that inhibition of GSK-3beta preserves the number of murine Fancc-deficient hematopoietic stem cells exposed to TNF-alpha compared with no GSK-3beta inhibition. Taken together, we have accumulated evidence suggesting that GSK-3beta is a promising molecular target to improve the self-renewal and survival of FA hematopoietic stem cells. Disclosures: No relevant conflicts of interest to declare. </jats:sec

A Cyclin A-Myb-MuvB-Aurora B network regulates the choice between mitotic cycles and polyploid endoreplication cycles.

Endoreplication is a cell cycle variant that entails cell growth and periodic genome duplication without cell division, and results in large, polyploid cells. Cells switch from mitotic cycles to endoreplication cycles during development, and also in response to conditional stimuli during wound healing, regeneration, aging, and cancer. In this study, we use integrated approaches in Drosophila to determine how mitotic cycles are remodeled into endoreplication cycles, and how similar this remodeling is between induced and developmental endoreplicating cells (iECs and devECs). Our evidence suggests that Cyclin A / CDK directly activates the Myb-MuvB (MMB) complex to induce transcription of a battery of genes required for mitosis, and that repression of CDK activity dampens this MMB mitotic transcriptome to promote endoreplication in both iECs and devECs. iECs and devECs differed, however, in that devECs had reduced expression of E2F1-dependent genes that function in S phase, whereas repression of the MMB transcriptome in iECs was sufficient to induce endoreplication without a reduction in S phase gene expression. Among the MMB regulated genes, knockdown of AurB protein and other subunits of the chromosomal passenger complex (CPC) induced endoreplication, as did knockdown of CPC-regulated cytokinetic, but not kinetochore, proteins. Together, our results indicate that the status of a CycA-Myb-MuvB-AurB network determines the decision to commit to mitosis or switch to endoreplication in both iECs and devECs, and suggest that regulation of different steps of this network may explain the known diversity of polyploid cycle types in development and disease

FANCL Ubiquitinates Beta-Catenin and Enhances Its Nuclear Function

Abstract Abstract 1335 Fanconi anemia (FA) is associated with a hereditary predisposition to bone marrow failure. The proteins encoded by the FANC genes are primarily involved in DNA repair responses through the formation of a large, multisubunit complex that has E3 ubiquitin ligase activity (Annual Review of Genetics 2009;43:223). FA hematopoietic stem cells display defective stem cell properties and limited replicative potential. However, the molecular basis for how a FA genetic background contributes to those defects remains poorly understood. Here we provide evidence that FANCL, which has E3 ubiquitin ligase activity, enhances beta-catenin activity (Figure A) and protein expression. Beta-catenin is a nuclear effector of canonical Wnt signaling. The Wnt/beta-catenin pathway is active in normal hematopoietic stem cells in the native bone marrow environment and disruption of this signaling pathway results in defective hematopoietic stem cells (Nature 2003;423:409). To test whether FANCL positively regulates beta-catenin through its ubiquitination activity, we performed cell-based ubiquitination assays. We show that FANCL functionally ubiquitinates beta-catenin (Figure B) and that ubiquitin chain extension can occur via non-lysine-48 ubiquitin linkages. Accumulating evidence reveal diverse, non-proteolytic biological roles for proteins modified by atypical ubiquitin chains (EMBO Reports 2008;9:536). Our data suggests that FANCL may enhance the protein function of beta-catenin via ubiquitination with atypical ubiquitin chains. Importantly, we demonstrate that suppression of FANCL expression in human CD34+ cord blood stem cells reduces beta-catenin expression (Figure C) and multilineage progenitor expansion. These results demonstrate a role for the FA pathway in regulating Wnt/beta-catenin signaling. Therefore, diminished Wnt/beta-catenin signaling may be an important underlying molecular defect in FA hematopoietic stem cells leading to their accelerated loss. A, LEF-TCF-luciferase reporter assay showing increasing beta-catenin activity in 293FT cells with increasing FANCL expression compared with vector-control (VC) (n=4). B, Immunoprecipitation of beta-catenin in cells transfected with vector-control or FANCL and probed for hemagglutinin (HA)-tagged ubiquitin shows increased ubiquitinated forms of beta-catenin with FANCL expression (n=4). C, shRNA suppression of FANCL expression in CD34+ cord blood stem cells results in decreased beta-catenin expression compared with a scramble control (Scr) by immunofluorescence analysis (three different shRNA constructs, n=3 for each construct). Disclosures: No relevant conflicts of interest to declare. </jats:sec

Model: The CycA—Myb—AurB network regulates the choice between cell cycle programs.

Depicted are two alternative cell cycle programs, the mitotic cycle (left), and endoreplication cycle (right, yellow). During mitotic cycles, CycA / CDK1 activates the Myb-MuvB (MMB) to induce transcription of multiple genes with mitotic functions (“mitotic” genes). Among these are the subunits of the chromosome passenger complex (CPC), which phosphorylates multiple targets to regulate chromosome condensation, kinetochore-microtubule (KT-MT) attachment, the spindle assembly checkpoint (SAC), and cytokinesis. Our findings suggest that CycA / CDK1, MMB, and the CPC are key nodes of this mitotic network whose repression promotes a transition to endoreplication in both iECs and devECs. See text for further details.</p

Summary of follicle cell clone phenotypes.

Summary of follicle cell clone phenotypes.</p

Both iECs and devECs have reduced expression of Myb target genes that function at multiple steps of mitosis and cytokinesis.

(A, A’) Volcano plots of RNA-Seq results for differentially expressed (DE) genes in CycA dsRNA iECs (A) and Myb dsRNA iECs (A’) each relative to GFP dsRNA control cells (N = 3 biological replicates). Vertical red and green dotted lines indicate thresholds for log2 fold change (≤ -0.5 and ≥+0.5) in iECs and horizontal red line the FDR adjusted p-value = 0.05. Blue dots represent genes that fulfill both of these criteria. See also S1 and S2 Tables. (B) Venn diagrams comparing the overlap of DE genes in CycA dsRNA and Myb dsRNA relative to control GFP dsRNA cells. See also S3 Table. (C) Gene Ontology (GO) analysis of genes downregulated in iECs and devECs indicate an enrichment for Myb target genes that are required for mitosis. Shown is a network analysis with GO biological process categories in blue and downregulated genes in green. See also S3A Fig and S4 Table. (D) Volcano plot of RNA-Seq results for DE genes in endocycling cells from early 3rd instar larval salivary glands relative to mitotic cycling larval brains and imaginal discs from the same animals (N = 3 biological replicates). Vertical red and green dotted lines indicate thresholds for log2 fold change (≤ -0.5 and ≥+0.5) and horizontal red line the FDR adjusted p-value = 0.05. Blue dots represent genes that fulfill both of these criteria. See also S4 and S5 Tables. (E) Venn diagrams showing the overlap of DE genes in iECs with DE genes in salivary gland (SG) devECs (the latter relative to mitotic brains and discs). See also S3B Fig and S5 Table. (F) Meta-analysis of 111 E2F1-dependent genes (Dimova categories A+B+C) [61] in iECs and SG devECs. The first three bars represent the number of E2F1-dependent genes that were increased, decreased, or unchanged relative to mitotic cycling controls for each endoreplicating cell type (total 111 E2F1-dependent genes). The other bars represent E2F1-dependent genes whose expression relative to mitotic cycling cells was similar among pairs or all three endoreplicating cell types. See also S6 Table.</p