H3K9me-Independent Gene Silencing in Fission Yeast Heterochromatin by Clr5 and Histone Deacetylases

Nucleosomes in heterochromatic regions bear histone modifications that distinguish them from euchromatic nucleosomes. Among those, histone H3 lysine 9 methylation (H3K9me) and hypoacetylation have been evolutionarily conserved and are found in both multicellular eukaryotes and single-cell model organisms such as fission yeast. In spite of numerous studies, the relative contributions of the various heterochromatic histone marks to the properties of heterochromatin remain largely undefined. Here, we report that silencing of the fission yeast mating-type cassettes, which are located in a well-characterized heterochromatic region, is hardly affected in cells lacking the H3K9 methyltransferase Clr4. We document the existence of a pathway parallel to H3K9me ensuring gene repression in the absence of Clr4 and identify a silencing factor central to this pathway, Clr5. We find that Clr5 controls gene expression at multiple chromosomal locations in addition to affecting the mating-type region. The histone deacetylase Clr6 acts in the same pathway as Clr5, at least for its effects in the mating-type region, and on a subset of other targets, notably a region recently found to be prone to neo-centromere formation. The genomic targets of Clr5 also include Ste11, a master regulator of sexual differentiation. Hence Clr5, like the multi-functional Atf1 transcription factor which also modulates chromatin structure in the mating-type region, controls sexual differentiation and genome integrity at several levels. Globally, our results point to histone deacetylases as prominent repressors of gene expression in fission yeast heterochromatin. These deacetylases can act in concert with, or independently of, the widely studied H3K9me mark to influence gene silencing at heterochromatic loci

Hyphal Tip-Associated Localization of Cdc42 Is F-Actin Dependent in Candida albicans

The rho-type GTPase Cdc42 is important for the establishment and maintenance of eukaryotic cell polarity. To examine whether Cdc42 is regulated during the yeast-to-hypha transition in Candida albicans, we constructed a green fluorescence protein (GFP)-Cdc42 fusion under the ACT1 promoter and observed its localization in live C. albicans cells. As in Saccharomyces cerevisiae, GFP-Cdc42 was observed around the entire periphery of the cell. In yeast-form cells of C. albicans, it clustered to the tips and sides of small buds as well as to the mother-daughter neck region of large-budded cells. Upon hyphal induction, GFP-Cdc42 clustered to the site of hyphal evagination and remained at the tips of the hyphae. This temporal and spatial localization of Cdc42 suggests that its activity is regulated during the yeast-to-hypha transition. In addition to the accumulation at the hyphal tip, GFP-Cdc42 was also seen as a band within the hyphal tube in cells that had undergone nuclear separation. With the F-actin-assembly inhibitor latrunculin A, we found that GFP-Cdc42 accumulation at the bud site in yeast-form cells is F-actin independent, whereas GFP-Cdc42 accumulation at the hyphal tip requires F-actin. Furthermore, disruption of the F-actin cytoskeleton impaired the transcriptional induction of hypha-specific genes. Therefore, hypha formation resembles mating in Saccharomyces cerevisiae in that both require F-actin for GFP-Cdc42 localization and efficient signaling

Tumor Suppressor Genes within Common Fragile Sites Are Active Players in the DNA Damage Response.

The role of common fragile sites (CFSs) in cancer remains controversial. Two main views dominate the discussion: one suggests that CFS loci are hotspots of genomic instability leading to inactivation of genes encoded within them, while the other view proposes that CFSs are functional units and that loss of the encoded genes confers selective pressure, leading to cancer development. The latter view is supported by emerging evidence showing that expression of a given CFS is associated with genome integrity and that inactivation of CFS-resident tumor suppressor genes leads to dysregulation of the DNA damage response (DDR) and increased genomic instability. These two viewpoints of CFS function are not mutually exclusive but rather coexist; when breaks at CFSs are not repaired accurately, this can lead to deletions by which cells acquire growth advantage because of loss of tumor suppressor activities. Here, we review recent advances linking some CFS gene products with the DDR, genomic instability, and carcinogenesis and discuss how their inactivation might represent a selective advantage for cancer cells

A G(1) Cyclin Is Necessary for Maintenance of Filamentous Growth in Candida albicans

Candida albicans undergoes a dramatic morphological transition in response to various growth conditions. This ability to switch from a yeast form to a hyphal form is required for its pathogenicity. The intractability of Candida to traditional genetic approaches has hampered the study of the molecular mechanism governing this developmental switch. Our approach is to use the more genetically tractable yeast Saccharomyces cerevisiae to yield clues about the molecular control of filamentation for further studies in Candida. G(1) cyclins Cln1 and Cln2 have been implicated in the control of morphogenesis in S. cerevisiae. We show that C. albicans CLN1 (CaCLN1) has the same cell cycle-specific expression pattern as CLN1 and CLN2 of S. cerevisiae. To investigate whether G(1) cyclins are similarly involved in the regulation of cell morphogenesis during the yeast-to-hypha transition of C. albicans, we mutated CaCLN1. Cacln1/Cacln1 cells were found to be slower than wild-type cells in cell cycle progression. The Cacln1/Cacln1 mutants were also defective in hyphal colony formation on several solid media. Furthermore, while mutant strains developed germ tubes under several hypha-inducing conditions, they were unable to maintain the hyphal growth mode in a synthetic hypha-inducing liquid medium and were deficient in the expression of hypha-specific genes in this medium. Our results suggest that CaCln1 may coordinately regulate hyphal development with signal transduction pathways in response to various environmental cues

Hierarchical model of cancer progression links between replication stress, DDR, CFS-Gs, and genomic instability.

<p>CFSs and their gene product are affected by replication stress. While CFSs (for example, FRA3B, FRA16D, and FRA6E) are prone to DSBs, CFS-Ps (FHIT, WWOX, PARK2) modulate DNA damage repair and/or apoptosis. Upon extensive damage, breaks within CFSs appears as deletions. Cells with deletions within CFS-Gs are positively selected because of their roles in DDR, leading to genomic instability and a mutator phenotype. Additional mutations in classical recessive genes (i.e., <i>Trp53</i>) would release the antitumor barriers (apoptosis and senescence), leading to cancer progression.</p

The sensitivity of CFS tumor-suppressor genes to DSBs as compared to their deletions in cancers.

<p><b>A</b>. DSBs occur at CFS-Gs but not at classical tumor suppressors in HeLa cells treated with aphidicolin and neocarzinostatin (NCS) by BLESS (re-analyzed from [<a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1006436#pgen.1006436.ref044" target="_blank">44</a>]). <b>B</b>. Chromosome-wide mapping of DSBs in HeLa cells treated with aphidicolin (brown) and NCS (grey) by BLESS [<a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1006436#pgen.1006436.ref044" target="_blank">44</a>] compared to the genomic deletion profile in 746 cancer cell lines [<a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1006436#pgen.1006436.ref013" target="_blank">13</a>]. Homozygous (red) and hemizygous (blue) deletion. The rectangles mark loci of the CFS-Gs.</p

Most frequently deleted genomic loci in 8,822 epithelial cancer samples from 25 cancer types.

<p>Data contains copy number data from non-TCGA projects [<a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1006436#pgen.1006436.ref014" target="_blank">14</a>] and data generated by the Broad Institute TCGA Genome Characterization Center of the GISTIC portal. The blue bars (left <i>y-</i>axis) are Q-values (–Log10), representing that the gene inside the locus significantly enriched for deletion because of selective pressures. Highly deleted genes imply their role as tumor suppressors. The red dots (right <i>y</i>-axis) are the number of genes inside the peak. Numbers under CFS genes emphasize that these are unique within their peak.</p

Tumor suppressive functions of WWOX.

<p>WWOX interacts with signaling molecules in the cytoplasm and prevents their entry to the nucleus to modulate their tumorigenic pathways. Upon stress such as ionizing radiation (IR), ultraviolet radiation (UV), or maybe even neuronal injury, WWOX enters the nucleus and promotes DSB repair. In the absence of WWOX, DSB repair is impaired and WWOX-interacting molecules enter the nucleus and induce their tumorigenic signals.</p

Temporal and Spatial Control of HGC1 Expression Results in Hgc1 Localization to the Apical Cells of Hyphae in Candida albicans

The human fungal pathogen Candida albicans can undergo a morphological transition from a unicellular yeast growth form to a multicellular hyphal growth form. During hyphal growth, cell division is asymmetric. Only the apical cell divides, whereas subapical cells remain in G(1), and cell surface growth is highly restricted to the tip of the apical cell. Hgc1, a hypha-specific, G(1) cyclin-like protein, is essential for hyphal development. Here, we report, using indirect immunofluorescence, that Hgc1 is preferentially localized to the dividing apical cells of hyphae. Hgc1 protein is rapidly degraded in a cell cycle-independent manner, and the protein turnover likely occurs in both the apical and the subapical cells of hyphae. In addition to rapid protein turnover, the HGC1 transcript is also dynamically regulated during cell cycle progression in hyphal growth. It is induced upon germ tube formation in early G(1); the transcript level is reduced during the G(1)/S transition and peaks again around the G(2)/M phase in the subsequent cell cycles. Transcription from the HGC1 promoter is essential for its apical cell localization, as Hgc1 no longer exhibits preferential apical localization when expressed under the MAL2 promoter. Using fluorescence in situ hybridization, the HGC1 transcript is detected only in the apical cells of hyphae, suggesting that HGC1 is transcribed in the apical cell. Therefore, the preferential localization of Hgc1 to the apical cells of hyphae results from the dynamic temporal and spatial control of HGC1 expression

Endurance exercise diverts the balance between Th17 cells and regulatory T cells.

Endurance, marathon-type exertion is known to induce adverse changes in the immune system. Increased airway hyper-responsiveness and airway inflammation are well documented in endurance athletes and endurance exercise is considered a major risk factor for asthma in elite athletes. Yet, the mechanisms underlying this phenomenon are still to be deduced. We studied the effect of strenuous endurance exercise (marathon and half-ironman triathlon) on CD4+ lymphocyte sub-populations and on the balance between effector and regulatory CD4+ lymphocytes in the peripheral blood of trained athletes, Endurance exercise induced a significant increase in Th17 cells and a sustained decrease in peripheral blood regulatory T cells (Tregs). While interleukin (IL)-2 levels remained undetectable, post-race serum IL-6 and transforming growth factor (TGF) β levels were significantly elevated. Treg levels in sedentary controls' decreased in vitro after incubation with athletes' post-exercise serum, an effect that was attenuated by supplements of IL-2 or anti IL-6 neutralizing antibodies. Our data suggest that exercise-induced changes in serum cytokine levels promote alterations in Tregs and Th17 cell populations, which may divert the subtle balance in the immune system towards inflammation. This may explain allergic and autoimmune phenomena previously reported in endurance athletes and contribute to our understanding of exercise-related asthma