The natural absence of RPA1N domain did not impair Leishmania amazonensis RPA-1 participation in DNA damage response and telomere protection.

We have previously shown that the subunit 1 of Leishmania amazonensis RPA (LaRPA-1) alone binds the G-rich telomeric strand and is structurally different from other RPA-1. It is analogous to telomere end-binding proteins described in model eukaryotes whose homologues were not identified in the protozoan's genome. Here we show that LaRPA-1 is involved with damage response and telomere protection although it lacks the RPA1N domain involved with the binding with multiple checkpoint proteins. We induced DNA double-strand breaks (DSBs) in Leishmania using phleomycin. Damage was confirmed by TUNEL-positive nuclei and triggered a G1/S cell cycle arrest that was accompanied by nuclear accumulation of LaRPA-1 and RAD51 in the S phase of hydroxyurea-synchronized parasites. DSBs also increased the levels of RAD51 in non-synchronized parasites and of LaRPA-1 and RAD51 in the S phase of synchronized cells. More LaRPA-1 appeared immunoprecipitating telomeres in vivo and associated in a complex containing RAD51, although this interaction needs more investigation. RAD51 apparently co-localized with few telomeric clusters but it did not immunoprecipitate telomeric DNA. These findings suggest that LaRPA-1 and RAD51 work together in response to DNA DSBs and at telomeres, upon damage, LaRPA-1 works probably to prevent loss of single-stranded DNA and to assume a capping function.

Quercetin decreases inflammatory response and increases insulin action in skeletal muscle of ob/ob mice and in L6 myotubes

Quercetin is a potent anti-inflammatory flavonoid, but its capacity to modulate insulin sensitivity in obese insulin resistant conditions is unknown. This study investigated the effect of quercetin treatment upon insulin sensitivity of ob/ob mice and its potential molecular mechanisms. Obese ob/ob mice were treated with quercetin for 10 weeks, and L6 myotubes were treated with either palmitate or tumor necrosis factor-alpha (TNF alpha) plus quercetin. Cells and muscles were processed for analysis of glucose transporter 4 (GLUT4), TNF alpha and inducible nitric oxide synthase (iNOS) expression, and c-Jun N-terminal kinase (JNK) and inhibitor of nuclear factor-kappa B (NF-kappa B) kinase (I kappa K) phosphorylation. Myotubes were assayed for glucose uptake and NF-kappa B translocation. Chromatin immunoprecipitation assessed NF-kappa B binding to GLUT4 promoter. Quercetin treatment improved whole body insulin sensitivity by increasing GLUT4 expression and decreasing JNK phosphorylation, and TNF alpha and iNOS expression in skeletal muscle. Quercetin suppressed palmitate-induced upregulation of TNF alpha and iNOS and restored normal levels of GLUT4 in myotubes. In parallel, quercetin suppressed TNF alpha-induced reduction of glucose uptake in myotubes. Nuclear accumulation of NF-kappa B in myotubes and binding of NF-kappa B to GLUT4 promoter in muscles of ob/ob mice were also reduced by quercetin. We demonstrated that quercetin decreased the inflammatory status in skeletal muscle of obese mice and in L6 myotubes. This effect was followed by increased muscle GLUT4, with parallel improvement of insulin sensitivity. These results point out quercetin as a putative strategy to manage inflammatory-related insulin resistance. (C) 2012 Elsevier B.V. All rights reserved.

Ein räumliches Orientierungsgedächtnis im Zentralkomplex von Drosophila melanogaster und die spezifische Rolle von ellipsoid-body-open

In der vorliegenden Arbeit wurden zwei Strukturmutationslinien von Drosophila melanogaster, grf und ebo, hinsichtlich ihres Lauf- und Orientierungsverhaltens im Buridanschen sowie im Detour-Paradigma untersucht. Als Kernthema der Arbeit entwickelte sich rasch die molekulare Analyse von ebo in Bezug auf das räumliche Orientierungsgedächtnis, da ebo-mutante Fliegen Letzteres nicht zeigen. Durch Wiederherstellen der EBO-Funktion kann der Verhaltensphänotyp der ebo-Mutante in jeder Ringneuronengruppe des Ellipsoidkörpers gerettet werden, jedoch nicht der Strukturdefekt. Zudem wird zur Ausbildung des Orientierungsgedächtnisses EBO nicht während der Entwicklung, sondern akut benötigt. Aufgrund der Tatsache, dass ebo für das nukleäre Protein Exportin6 codiert, und selbiges für den Export von Aktin-Profilin-Komplexen aus dem Zellkern verantwortlich ist (STÜVEN ET AL., 2003), zeigen ebo-Tiere nukleäre Aktin-Akkumulationen sowohl während der Entwicklung in Speicheldrüsen als auch im adulten Gehirn, was mittels Expression eines Actin::GFP-Fusionsproteins gezeigt wurde. Die genetischen Interaktionsexperimente zeigen, dass der anatomische Defekt von ebo durch eine reduzierte Aktin-Polymerisation erfolgt, für den Verhaltensphänotyp jedoch die Aktin-Anreicherung in den Zellkernen von Ringneuronen des Ellipsoidkörpers ursächlich ist. Die erstaunliche Redundanz der Ringneurone in Bezug auf die Rettung des Verhaltensphänotyps legt nahe, dass diffusible Faktoren eine wichtige Rolle für die Ausbildung eines Orientierungsgedächtnisses spielen. Bezüglich dieser Hypothese konnte nachgeweisen werden, dass durch FMRFamid-RNAi in R2- und R4-Ringneuronen des Ellipsoidkörpers das Orientierungsgedächtnis zerstört wird. Eine daraufhin durchgeführte Antikörperfärbung gegen pro-FMRFa in wildtypischen und ebo-mutanten Gehirnen ergab jedoch keine Verschiedenheit die Menge oder Lokalisation betreffend. Die bei ebo vorhandene Anreicherung von Aktin im Zellkern bewirkt, dass die Aktin-Monomere im Nucleus an den Cofaktor dMRTF (Mrtf) binden und diesen somit inaktivieren. Dadurch kann der Transkriptionsfaktor dSRF (bs) nicht mehr durch dMRTF aktiviert werden, was den Orientierungsgedächtnis-Verlust bewirkt. Da es jedoch unwahrscheinlich ist, dass ein Gedächtnis, welches nur wenige Sekunden andauert, von Transkriptionsregulation abhängt, könnte dSRF auch die Genexpression von Molekülen, die schnelle Veränderungen synaptischer Transmission der Ringneurone vermitteln, modulieren. Für die Zukunft wäre es demnach von enormer Bedeutung, weitere Zielgene von dSRF aufzuklären und zu analysieren.

Stress-induced targeting of molecular chaperones in the yeast Saccharomyces cerevisiae

The eukaryotic stress response is an essential mechanism that helps protect cells from a variety of environmental stresses. Cell death can result if cells are not able to properly adapt and protect themselves against adverse stress conditions. Failure to properly deal with stress has implications in human diseases including neurodegenerative disorders and distinct cancers, emphasizing the importance of understanding the eukaryotic stress response in detail. As part of this response, expression of a battery of heat shock proteins (HSP) is induced, which act as molecular chaperones to assist in the repair or triage of unfolded proteins. The 90-kDa HSP (Hsp90) operates in the context of a multi-chaperone complex to promote the maturation of nuclear and cytoplasmic clients. I have discovered that Hsp90 and the co-chaperone Sba1 accumulate in the nucleus of quiescent Saccharomyces cerevisiae cells in a karyopherin-dependent manner. I isolated nuclear accumulation- defective HSP82 mutant alleles to probe the nature of this targeting event and identified a mutant with a single amino acid substitution (I578F) sufficient to prevent nuclear accumulation of Hsp90 in quiescent cells. Diploid hsp82-I578F cells exhibited pronounced defects in spore wall construction and maturation, resulting in catastrophic sporulation. The mislocalization and sporulation phenotypes were shared by another previously identified HSP82 mutant allele, further linking localization to Hsp90 functional status. Pharmacological inhibition of Hsp90 with macbecin in sporulating diploid cells also blocked spore formation, underscoring the importance of this chaperone in this developmental program. The yeast molecular chaperone Hsp104 is a member of the Hsp100 superfamily of AAA+ ATPases. Unlike the Hsp90 family of chaperones, Hsp104 is not restricted to a specific set of client proteins, but rather assists in reactivating stress-denatured proteins by solubilizing protein aggregates. I have discovered that Hsp104, along with the Hsp70 chaperone, Ssa1, and the sHSP Hsp26 accumulate into RNA processing bodies (P- bodies) and stress granules, sites of mRNA metabolism. I found that Hsp104 recruits both Ssa1 and Hsp26 to P-bodies and that these three chaperones are required for stress granule formation. These findings suggest a possible role for chaperones in mRNA metabolism by aiding in the assembly, disassembly or conversion of these enigmatic mRNP complexes. Taken together, the work presented in this dissertation serves to better understand the eukaryotic stress response by illustrating the importance of subcellular-chaperone localization in key biological processes.

The role of BCL-2 in regulation of apoptosis susceptibility by modulation of glutathione

The role of oxidative stress and apoptosis has recently been recognized as an important determinant in the development of a variety of diseases known to man. The oncogene BCL-2 is known to regulate sensitivity to induction of apoptosis and appears to function in an antioxidant pathway by regulating glutathione. We have investigated various steps in the oxidative stress cascade to determine possible sites of action for BCL-2. The fluorescent probes H2DCFDA, dihydroethidium and cis-parinaric acid were used to quantitate generation of peroxides, superoxide and lipid peroxidation, respectively. While each of these agents was able to detect substantial increases in oxidative stress following exposure of cells to ionizing radiation, there was no significant difference between cells expressing high or low levels of BCL-2. Investigation of mitochondrial dysfunction during apoptosis revealed a possible site of bcl-2 intervention, but, analysis of kinetic events occurring during apoptosis suggested that the observed effect is not in the direct apoptotic effector pathway. When glutathione was studied, localization to the nucleus was observed in cells overexpressing BCL-2 that did not occur in cells lacking BCL-2. Additionally, nuclear accumulation of glutathione was sufficient to block granzyme b-mediated nuclear DNA fragmentation, poly (ADP-ribose) polymerase cleavage and caspase activity suggesting that nuclear accumulation of glutathione via a bcl-2 dependent process is functionally relevant to suppression of apoptosis. Thus, a model system emerges where BCL-2 is able to regulate a cell's ability to prevent apoptosis by modifying the cell's antioxidant systems at the organelle level to compensate for oxidative stresses placed upon it. ^

The low abundance of U7 snRNA is partly determined by its Sm binding site.

In transient expression studies after DNA transfection of HeLa cells, the mouse U7 gene produces only approximately 30% of the RNA produced by a mouse U1b gene. This difference persists even when the transfected genes have all their 5' and 3' flanking sequences exchanged suggesting a post-transcriptional effect. When the special U7 Sm binding site is mutated to a consensus derived from the major snRNAs (Sm-opt), the U7 RNA level increases 4- to 5-fold, whereas no RNA is detected from a U7 gene with a non-functional Sm binding site (Sm-mut). Moreover, U1b genes with the U7 Sm binding site yield reduced RNA levels. The Sm-opt site also alters the cellular behaviour of the corresponding U7 snRNA. It accumulates to a higher level in the nucleus than wild type U7 RNA, and is better immunoprecipitable with anti-Sm antibodies. Injection experiments in Xenopus oocytes indicate that the U7 genes with either Sm-opt or Sm-mut sites produce similar amounts of RNA as wild type U7, but that they differ in opposing ways in the processing of precursors to mature size U7 snRNA and in nuclear accumulation. However, in reconstitution experiments using Xenopus oocytes, we show that U7 Sm-opt RNA, despite its efficient nuclear accumulation, is not active in 3' processing of histone pre-mRNA, whereas wild type U7 RNA is assembled into functional snRNPs, which correctly process histone pre-mRNA substrate. This suggests a functional importance of the special U7 Sm sequence.

Activities and response to DNA damage of latent and active sequence-specific DNA binding forms of mouse p53

The mouse p53 protein generated by alternative splicing (p53as) has amino acid substitutions at its C terminus that result in constitutively active sequence-specific DNA binding (active form), whereas p53 protein itself binds inefficiently (latent form) unless activated by C-terminal modification. Exogenous p53as expression activated transcription of reporter plasmids containing p53 binding sequences and inhibited growth of mouse and human cells lacking functional endogenous p53. Inducible p53as in stably transfected p53 null fibroblasts increased p21WAF1/Cip-1/Sdi and decreased bcl-2 protein steady-state levels. Endogenous p53as and p53 proteins differed in response to cellular DNA damage. p53 protein was induced transiently in normal keratinocytes and fibroblasts whereas p53as protein accumulation was sustained in parallel with induction of p21WAF1/Cip-1/Sdi protein and mRNA, in support of p53as transcriptional activity. Endogenous p53 and p53as proteins in epidermal tumor cells responded to DNA damage with different kinetics of nuclear accumulation and efficiencies of binding to a p53 consensus DNA sequence. A model is proposed in which C-terminally distinct p53 protein forms specialize in functions, with latent p53 forms primarily for rapid non-sequence-specific binding to sites of DNA damage and active p53 forms for sustained regulation of transcription and growth.

Enhancement of DNA repair in human skin cells by thymidine dinucleotides: Evidence for a p53-mediated mammalian SOS response

Thymidine dinucleotide (pTpT) stimulates melanogenesis in mammalian pigment cells and intact skin, mimicking the effects of UV irradiation and UV-mimetic DNA damage. Here it is shown that, in addition to tanning, pTpT induces a second photoprotective response, enhanced repair of UV-induced DNA damage. This enhanced repair results in a 2-fold increase in expression of a UV-damaged chloramphenicol acetyltransferase expression vector transfected into pTpT-treated skin fibroblasts and keratinocytes, compared with diluent-treated cells. Direct measurement of thymine dimers and (6–4) photoproducts by immunoassay demonstrates faster repair of both of these UV-induced photoproducts in pTpT-treated fibroblasts. This enhanced repair capacity also improves cell survival and colony-forming ability after irradiation. These effects of pTpT are accomplished, at least in part, by the up-regulation of a set of genes involved in DNA repair (ERCC3 and GADD45) and cell cycle inhibition (SDI1). At least two of these genes (GADD45 and SDI1) are known to be transcriptionally regulated by the p53 tumor suppressor protein. Here we show that pTpT activates p53, leading to nuclear accumulation of this protein, and also increases the specific binding of this transcription factor to its DNA consensus sequence.

β-Catenin Can Be Transported into the Nucleus in a Ran-unassisted Manner

The nuclear accumulation of β-catenin plays an important role in the Wingless/Wnt signaling pathway. This study describes an examination of the nuclear import of β-catenin in living mammalian cells and in vitro semi-intact cells. When injected into the cell cytoplasm, β-catenin rapidly migrated into the nucleus in a temperature-dependent and wheat germ agglutinin–sensitive manner. In the cell-free import assay, β-catenin rapidly migrates into the nucleus without the exogenous addition of cytosol, Ran, or ATP/GTP. Cytoplasmic injection of mutant Ran defective in its GTP hydrolysis did not prevent β-catenin import. Studies using tsBN2, a temperature-sensitive mutant cell line that possesses a point mutation in the RCC1 gene, showed that the import of β-catenin is insensitive to nuclear Ran-GTP depletion. These results show that β-catenin possesses the ability to constitutively translocate through the nuclear pores in a manner similar to importin β in a Ran-unassisted manner. We further showed that β-catenin also rapidly exits the nucleus in homokaryons, suggesting that the regulation of nuclear levels of β-catenin involves both nuclear import and export of this molecule.

Active Nucleocytoplasmic Shuttling Required for Function and Regulation of Stress-Activated Kinase Spc1/StyI in Fission Yeast

Transcriptional induction of many stress-response genes is dependent on stress-induced nuclear accumulation of stress-activated protein kinases (SAPKs). In the fission yeast Schizosaccharomyces pombe, nuclear accumulation of the SAPK Spc1 (also known as StyI) requires activating phosphorylation catalyzed by the SAPK kinase Wis1; however, it is unknown whether the localization of Spc1 is regulated by nuclear transport factors. Herein are reported studies that show that Spc1 localization is regulated by active transport mechanisms during osmotic stress. Nuclear import of Spc1 requires Pim1, a homologue of the guanine nucleotide exchange factor RCC1 that is essential for nucleocytoplasmic shuttling of proteins. Nuclear export of Spc1 is regulated by the export factor Crm1. An Spc1–Crm1 complex forms as Spc1 is exported from the nucleus. Wis1 and the tyrosine phosphatases Pyp1 and Pyp2 that inactivate Spc1 are excluded from the nucleus by a Crm1-independent mechanism; hence the nuclear import of Spc1 leads to transient isolation from its regulatory proteins. Thus, active nucleocytoplasmic shuttling is required for both the function and regulation of Spc1 during the osmotic shock response.

Nucleocytoplasmic translocation of Stat1 is regulated by a leucine-rich export signal in the coiled-coil domain

Signal transducer and activator of transcription (Stat) proteins are latent transcription factors that reside in the cytoplasm before activation. On cytokine-induced tyrosine phosphorylation, these molecules dimerize and accumulate transiently in the nucleus. No specific signals mediating these processes have been identified to date. In this report, we examine the nuclear export of Stat1. We find that treatment of cells with the export inhibitor leptomycin B does not affect steady-state localization of Stat1 but impedes nuclear export after IFNγ-induced nuclear accumulation. We identify a conserved leucine-rich helical segment in the coiled-coil domain of Stat1, which is responsible for the efficient nuclear export of this protein. Mutation of two hallmark leucines within this segment greatly attenuate the back transport of Stat1 in the cytoplasm. When fused to a carrier protein, the Stat1 export sequence can mediate nuclear export after intranuclear microinjection. We show that prolonging the nuclear presence of Stat1 by inhibiting nuclear export reduces the transcriptional response to stimulation with IFNγ. These data suggest that Stats are actively exported from the nucleus via several separate pathways and link this activity to transcriptional activation.

The junction-associated protein, zonula occludens-1, localizes to the nucleus before the maturation and during the remodeling of cell-cell contacts.

The junction-associated protein zonula occludens-1 (ZO-1) is a member of a family of membrane-associated guanylate kinase homologues thought to be important in signal transduction at sites of cell-cell contact. We present evidence that under certain conditions of cell growth, ZO-1 can be detected in the nucleus. Two different antibodies against distinct portions of the ZO-1 polypeptide reveal nuclear staining in subconfluent, but not confluent, cell cultures. An exogenously expressed, epitope-tagged ZO-1 can also be detected in the nuclei of transfected cells. Nuclear accumulation can be stimulated at sites of wounding in cultured epithelial cells, and immunoperoxidase detection of ZO-1 in tissue sections of intestinal epithelial cells reveals nuclear labeling only along the outer tip of the villus. These results suggest that the nuclear localization of ZO-1 is inversely related to the extent and/or maturity of cell contact. Since cell-cell contacts are specialized sites for signaling pathways implicated in growth and differentiation, we suggest that the nuclear accumulation of ZO-1 may be relevant for its suggested role in membrane-associated guanylate kinase homologue signal transduction.

Visualization of glucocorticoid receptor translocation and intranuclear organization in living cells with a green fluorescent protein chimera.

A highly fluorescent mutant form of the green fluorescent protein (GFP) has been fused to the rat glucocorticoid receptor (GR). When GFP-GR is expressed in living mouse cells, it is competent for normal transactivation of the GR-responsive mouse mammary tumor virus promoter. The unliganded GFP-GR resides in the cytoplasm and translocates to the nucleus in a hormone-dependent manner with ligand specificity similar to that of the native GR receptor. Due to the resistance of the mutant GFP to photobleaching, the translocation process can be studied by time-lapse video microscopy. Confocal laser scanning microscopy showed nuclear accumulation in a discrete series of foci, excluding nucleoli. Complete receptor translocation is induced with RU486 (a ligand with little agonist activity), although concentration into nuclear foci is not observed. This reproducible pattern of transactivation-competent GR reveals a previously undescribed intranuclear architecture of GR target sites.

Anterior axis duplication in Xenopus induced by the over-expression of the cadherin-binding protein plakoglobin.

Plakoglobin interacts with both classical and desmosomal cadherins. It is closely related to Drosophila aramadillo (arm) gene product; arm acts in the wingless (wg)-signaling pathway to establish segment polarity. In Xenopus, homologs of wg--i.e., wnts, can produce anterior axis duplications by inducing dorsal mesoderm. Studies in Drosophila suggest that wnt acts by increasing the level of cytoplasmic armadillo protein (arm). To test whether simply increasing the level of plakoglobin mimics the effects of exogenous wnts in Xenopus, we injected fertilized eggs with RNA encoding an epitope-tagged form of plakoglobin; this induced both early radial gastrulation and anterior axis duplication. Exogenous plakoglobin accumulates in the nuclei of embryonic cells. Plakoglobin binds to the tail domain of the desmosomal cadherin desmoglein 1. When RNA encoding the tail domain of desmoglein was coinjected with plakoglobin RNA, both the dorsalizing effect and nuclear accumulation of plakoglobin were suppressed. Mutational analysis indicates that the central arm repeat region of plakoglobin is sufficient to induce axis duplication and that this polypeptide accumulates in the nuclei of embryonic cells. These data show that increased plakoglobin levels can, by themselves, generate the intracellular signals involved in the specification of dorsal mesoderm.

Hypoxia induces chondrocyte-specific gene expression in mesenchymal cells in association with transcriptional activation of Sox9

Endochondral bone is formed during an avascular period in an environment of low oxygen. Under these conditions, pluripotential mesenchymal stromal cells preferentially differentiate into chondrocytes and form cartilage. In this study, we investigated the hypothesis that oxygen tension modulates bone mesenchymal cell fate by altering the expression of genes that function to promote chondrogenesis. Microarray of RNA samples from ST2 cells revealed significant changes in 728 array elements (P < 0.01) in response to hypoxia. Real-time PCR on these RNA samples, and separate samples from C3H10T1/2 cells, revealed hypoxia-induced changes in the expression of additional genes known to be expressed by chondrocytes including Sox9 and its downstream targets aggrecan and Col2a. These changes were accompanied by the accumulation of mucopolysacharide as detected by alcian blue staining. To investigate the mechanisms responsible for upregulation of Sox9 by hypoxia, we determined the effect of hypoxia on HIF-1 alpha levels and Sox9 promoter activity in ST2 cells. Hypoxia increased nuclear accumulation of HIF-1 alpha and activated the Sox9 promoter. The ability of hypoxia to transactivate the Sox9 promoter was virtually abolished by deletion of HIF-1 alpha consensus sites within the proximal promoter. These findings suggest that hypoxia promotes the differentiation of mesenchymal cells along a chondrocyte pathway in part by activating Sox-9 via a HIF-1 alpha-dependent mechanism. (c) 2005 Elsevier Inc. All rights reserved.