A critical process during thymic development of the T cell repertoire is the induction of self-tolerance. Tolerance in developing T cells is highly dependent on medullary thymic epithelial cells (mTEC) and mTEC development in turn requires signals from mature single positive (SP) thymocytes, a bidirectional relationship termed thymus crosstalk. We show that CD28-CD80/86 and CD40-CD40L costimulatory interactions, which mediate negative selection and self-tolerance, upregulate expression of LTα, LTβ and RANK in the thymus and are necessary for medullary development. Combined absence of CD28-CD80/86 and CD40-CD40L results in profound deficiency in mTEC development comparable to that observed in the absence of SP thymocytes. This requirement for costimulatory signaling is maintained even in a TCR transgenic model of high affinity TCR-ligand interactions. CD4 thymocytes maturing in the altered thymic epithelial environment of CD40/CD80/86 KO mice are highly autoreactive in vitro and are lethal in congenic adoptive transfer in vivo, demonstrating a critical role for these costimulatory pathways in self-tolerance as well as thymic epithelial development. These findings demonstrate that cooperativity between CD28-CD80/86 and CD40-CD40L pathways is required for normal medullary epithelium and for maintenance of self-tolerance in thymocyte development.
The DNA damage response (DDR) occurs in the context of chromatin structure, and architectural features of chromatin contribute to DNA damage signaling and repair. While the role of chromatin decondensation in the DDR is established, we show here that chromatin condensation is integral to DDR signaling. We find that upon DNA damage, chromatin regions transiently expand before undergoing extensive compaction. Using a protein-chromatin tethering system to create defined chromatin domains, we show that interference with chromatin condensation results in failure to fully activate DDR. Conversely, forced induction of local chromatin condensation promotes ATM- and ATR-dependent activation of upstream DDR signaling in a break-independent manner. Finally, while persistent chromatin compaction enhanced upstream DDR signaling from irradiation-induced breaks, it reduced recovery and survival after damage. Our results demonstrate that chromatin condensation is sufficient for activation of DDR signaling and is an integral part of physiological DDR signaling.
In contrast to BRCA1, CtIP has indispensable roles in promoting resection and embryonic development.
Homologous recombination (HR) is initiated by DNA end resection, a process in which stretches of single-strand DNA (ssDNA) are generated and used for homology search. Factors implicated in resection include nucleases MRE11, EXO1, and DNA2, which process DNA ends into 3′ ssDNA overhangs; helicases such as BLM, which unwind DNA; and other proteins such as BRCA1 and CtIP whose functions remain unclear. CDK-mediated phosphorylation of CtIP on T847 is required to promote resection, whereas CDK-dependent phosphorylation of CtIP-S327 is required for interaction with BRCA1. Here, we provide evidence that CtIP functions independently of BRCA1 in promoting DSB end resection. First, using mouse models expressing S327A or T847A mutant CtIP as a sole species, and B cells deficient in CtIP, we show that loss of the CtIP-BRCA1 interaction does not detectably affect resection, maintenance of genomic stability or viability, whereas T847 is essential for these functions. Second, although loss of 53BP1 rescues the embryonic lethality and HR defects in BRCA1-deficient mice, it does not restore viability or genome integrity in CtIP−/− mice. Third, the increased resection afforded by loss of 53BP1 and the rescue of BRCA1-deficiency depend on CtIP but not EXO1. Finally, the sensitivity of BRCA1-deficient cells to poly ADP ribose polymerase (PARP) inhibition is partially rescued by the phospho-mimicking mutant CtIP (CtIP-T847E). Thus, in contrast to BRCA1, CtIP has indispensable roles in promoting resection and embryonic development.
Subcutaneous tumors induce the accumulation of myeloid derived suppressor cells (MDSC) not only in blood and spleens, but also in livers of these animals. Unexpectedly, we observed a moderate increase in serum transaminases in mice with EL4 subcutaneous tumors, which prompted us to study the relationship of hepatic MDSC accumulation and liver injury. MDSC were the predominant immune cell population expanding in livers of all subcutaneous tumor models investigated (RIL175, B16, EL4, CT26 and BNL), while liver injury was only observed in EL4 and B16 tumor-bearing mice. Elimination of hepatic MDSC in EL4 tumor-bearing mice using low dose 5-fluorouracil (5-FU) treatment reversed transaminase elevation and adoptive transfer of hepatic MDSC from B16 tumor-bearing mice caused transaminase elevation indicating a direct MDSC mediated effect. Surprisingly, hepatic MDSC from B16 tumor-bearing mice partially lost their damage-inducing potency when transferred into mice bearing non damage-inducing RIL175 tumors. Furthermore, MDSC expansion and MDSC-mediated liver injury further increased with growing tumor burden and was associated with different cytokines including GM-CSF, VEGF, interleukin-6, CCL2 and KC, depending on the tumor model used. In contrast to previous findings, which have implicated MDSC only in protection from T cell-mediated hepatitis, we show that tumor-induced hepatic MDSC themselves can cause moderate liver damage.
Constitutive activation of NF-κB is a hallmark of the activated B cell-like (ABC) subtype of diffuse large B cell lymphoma (DLBCL), owing to upstream signals from the B cell receptor (BCR) and MyD88 pathways. The linear polyubiquitin chain assembly complex (LUBAC) attaches linear polyubiquitin chains to IκB kinase γ, a necessary event in some pathways that engage NF-κB. Two germ line polymorphisms affecting the LUBAC subunit RNF31 are rare among healthy individuals (~1%) but enriched in ABC DLBCL (7.8%). These polymorphisms alter RNF31 α helices that mediate binding to the LUBAC subunit RBCK1, thereby increasing RNF31-RBCK1 association, LUBAC enzymatic activity, and NF-κB engagement. In the BCR pathway, LUBAC associates with the CARD11/MALT1/BCL10 adapter complex and is required for ABC DLBCL viability. A stapled RNF31 α-helical peptide based on the ABC DLBCL-associated Q622L polymorphism inhibited RFN31-RBCK1 binding, decreased NF-κB and killed ABC DLBCL cells, credentialing this protein-protein interface as a therapeutic target.
We performed a loss-of-function, RNA interference screen to define therapeutic targets in multiple myeloma, a genetically diverse plasma cell malignancy. Unexpectedly, we discovered that all myeloma lines require caspase-10 for survival, irrespective of their genetic abnormalities. The transcription factor IRF4 induces both caspase-10 and its associated protein cFLIPL in myeloma, generating a protease that does not induce apoptosis but rather blocks an autophagy-dependent cell death pathway. Caspase-10 inhibits autophagy by cleaving the BCL2-interacting protein BCLAF1, itself a strong inducer of autophagy that acts by displacing beclin-1 from BCL2. While myeloma cells require a basal level of autophagy for survival, caspase-10 tempers this response to avoid cell death. Drugs that disrupt this vital balance may have therapeutic potential in myeloma.
The DNA damage response (DDR) protein 53BP1 protects DNA ends from excessive resection in G1, and thereby favors repair by non-homologous end joining (NHEJ) as opposed to homologous recombination (HR). During S phase, BRCA1 antagonizes 53BP1 to promote HR. The pro-NHEJ and anti-recombinase functions of 53BP1 are mediated in part by RIF1, the only known factor that requires 53BP1 phosphorylation for its recruitment to double strand breaks (DSBs). Here we show that a 53BP1 phospho-mutant 53BP18A, comprising alanine substitutions of the 8 most N-terminal S/TQ phosphorylation sites, mimics 53BP1 deficiency by restoring genome stability in BRCA1 deficient cells yet behaves like wild-type 53BP1 with respect to immunoglobulin class switch recombination (CSR). 53BP18A recruits RIF1 but fails to recruit the DDR protein PTIP to DSBs, and disruption of PTIP phenocopies 53BP18A. We conclude that 53BP1 promotes productive CSR and suppresses mutagenic DNA repair through distinct phospho-dependent interactions with RIF1 and PTIP.
Gold nanoparticles (AuNPs) with core sizes below 2 nm and compact ligand shells constitute versatile platforms for the development of novel reagents in nanomedicine. Due to their ultrasmall size, these AuNPs are especially attractive in applications requiring delivery to crowded intracellular spaces in the cytosol and nucleus. For eventual use in vivo, ultrasmall AuNPs should ideally be monodisperse, since small variations in size may affect how they interact with cells and behave in the body. Here we report the synthesis of ultrasmall, uniform 144-atom AuNPs protected by p-mercaptobenzoic acid (Au144(pMBA)60) followed by ligand exchange with glutathione (GSH). Quantitative scanning transmission electron microscopy (STEM) reveals that the resulting GSH-coated AuNPs (Au(GSH)) have a uniform mass distribution with cores that contain 134 gold atoms on average. Particle size dispersity is analyzed by analytical ultracentrifugation, giving a narrow distribution of apparent hydrodynamic diameter of 4.0 ± 0.6 nm. To evaluate the nanoparticles' intracellular fate, the cell penetrating peptide TAT is attached non-covalently to Au(GSH), which is confirmed by fluorescence quenching and isothermal titration calorimetry. HeLa cells are then incubated with both Au(GSH) and the Au(GSH)-TAT complex, and imaged without silver enhancement of the AuNPs in unstained thin sections by STEM. This imaging approach enables unbiased detection and quantification of individual ultrasmall nanoparticles and aggregates in the cytoplasm and nucleus of the cells.
gold nanoparticle; nanomedicine; cellular uptake; STEM; cell penetrating peptide
Polyadenylate-binding protein cytoplasmic 1 (PABPC1) is a cytoplasmic-nuclear shuttling protein important for protein translation initiation and both RNA processing and stability. We report that PABPC1 forms a complex with the Kaposi's sarcoma-associated herpesvirus (KSHV) ORF57 protein, which allows ORF57 to interact with a 9-nucleotide (nt) core element of KSHV polyadenylated nuclear (PAN) RNA, a viral long noncoding RNA (lncRNA), and increase PAN stability. The N-terminal RNA recognition motifs (RRMs) of PABPC1 are necessary for the direct interaction with ORF57. During KSHV lytic infection, the expression of viral ORF57 leads to a substantial decrease in overall PABPC1 expression, along with a shift in the cellular distribution of the remaining PABPC1 to the nucleus. Interestingly, PABPC1 and ORF57 have opposing functions in modulating PAN steady-state accumulation. The suppressive effect of PABPC1 specific to PAN expression is alleviated by small interfering RNA knockdown of PABPC1 or by overexpression of ORF57. Conversely, ectopic PABPC1 reduces ORF57 steady-state protein levels and induces aberrant polyadenylation of PAN and thereby indirectly inhibits ORF57-mediated PAN accumulation. However, E1B-AP5 (heterogeneous nuclear ribonucleoprotein U-like 1), which interacts with a region outside the 9-nt core to stimulate PAN expression, does not interact or even colocalize with ORF57. Unlike PABPC1, the nuclear distribution of E1B-AP5 remains unchanged by viral lytic infection or overexpression of ORF57. Together, these data indicate that PABPC1 is an important cellular target of viral ORF57 to directly upregulate PAN accumulation during viral lytic infection, and the ability of host PABPC1 to disrupt ORF57 expression is a strategic host counterbalancing mechanism.
Multiple sclerosis (MS) and its mouse model, experimental autoimmune encephalomyelitis (EAE), are inflammatory disorders of the central nervous system (CNS). The function of platelets in inflammatory and autoimmune pathologies is thus far poorly defined.
Here we addressed the role of platelets in mediating CNS inflammation in EAE.
We found that platelets were present in human MS lesions as well as in the CNS of mice subjected to EAE but not in the CNS from control non-diseased mice. Platelet depletion at the effector-inflammatory phase of EAE in mice resulted in significantly ameliorated disease development and progression. EAE suppression upon platelet depletion was associated with reduced recruitment of leukocytes to the inflamed CNS, as assessed by intravital microscopy, and with a blunted inflammatory response. The platelet-specific receptor glycoprotein Ib alpha (GPIbα) promotes both platelet adhesion as well as inflammatory actions of platelets, and, targeting of GPIbα attenuated EAE in mice. Moreover, targeting another platelet adhesion receptor, glycoprotein IIb/IIIa (GPIIb/IIIa) also reduced EAE severity in mice.
Thus, platelets contribute to the pathogenesis of EAE by promoting CNS inflammation. Targeting platelets may therefore represent an important new therapeutic approach for MS treatment.
Platelets; EAE; inflammation; autoimmune disease
Translocations involving the T cell receptor alpha/delta (TCRα/δ) chain locus, which bring oncogenes in the proximity of the TCRα enhancer, are one of the hallmark features of human T cell malignancies from ataxia telangiectasia (AT) and non-AT patients. These lesions are frequently generated by the fusion of DNA breaks at the TCRα/δ locus to a disperse region centromeric of the immunoglobulin heavy chain (IgH) locus. Aberrant VDJ joining accounts for TCRα/δ associated DNA cleavage, but the molecular mechanism that leads to generation of the “oncogene partner” DNA break is unclear. Here we show that in ATM deficient primary mouse T cells, IgH/TCRα/δ fusions arise at a remarkably similar frequency as in human AT lymphocytes. Recombinase-activating gene (RAG) is responsible for both TCRα/δ as well as IgH associated breaks on chromosome 12 (Chr12), which are subject to varying degrees of chromosomal degradation. We suggest a new model for how oncogenic translocations can arise from two non-concerted physiological DSBs.
translocations; T cell leukemia; V(D)J recombination; ATM
V(D)J recombination and class switch recombination employ overlapping but distinct non-homologous end-joining (NHEJ) pathways to repair DNA double strand break (DSB) intermediates. 53BP1 is a DNA damage response protein that is rapidly recruited to sites of chromosomal DSBs, where it appears to function in a subset of ataxia-telangiectasia mutated (ATM) kinase, H2AX- and MDC1- dependent events1,2. A 53BP1 dependent end joining pathway has been described that is dispensable for V(D)J recombination but essential for class-switch recombination CSR3, 4. Here, we report a previously unrecognized defect in the joining phase of V(D)J recombination in 53BP1 deficient lymphocytes distinct from that found in classical NHEJ-, H2AX-, MDC1- and Atm-deficient mice. Absence of 53BP1 leads to impairment of distal V-DJ joining with extensive degradation of un-repaired coding ends and episomal signal joint reintegration at V(D)J junctions. This results in apoptosis, loss of T-cell receptor alpha locus integrity and lymphopenia. Further impairment of the apoptotic checkpoint causes propagation of lymphocytes bearing antigen receptor breaks. These data suggest a more general role for 53BP1 in maintaining genomic stability during long range joining of DNA breaks.
Kaposi sarcoma-associated herpesvirus (KSHV) encodes a viral interleukin 6 (vIL6) that mimics many activities of human IL6 (hIL6). Both vIL6 and hIL6 play important roles in stimulating the proliferation of tumors caused by KSHV. Here, we provide evidence that a miRNA pathway is involved in regulation of vIL6 and hIL6 expression through binding sites in their open reading frames (ORF). We show a direct repression of vIL6 by hsa-miR-1293 and hIL6 by hsa-miR-608. The repression of vIL6 by miR-1293 was reversed by disruption of the vIL6 miR-1293 seed match through the introduction of point mutations. In addition, expression of vIL6 or hIL6 in KSHV-infected cells could be enhanced by transfection of the respective miRNA inhibitors. In situ hybridization of human lymph node sections revealed that miR-1293 is primarily expressed in the germinal center, but is deficient in the mantle zone of lymph nodes where the expression of vIL6 is often found in patients with KSHV-associated multicentric Castleman’s disease, providing evidence of an anatomic correlation. Together, our study indicates that IL6 expression can be regulated by miRNA interactions in its ORF and provides evidence for the role of these interactions in the pathogenesis of KSHV-associated diseases.
Viral IL6; IL6; miRNA; Kaposi sarcoma-associated herpesvirus; germinal center; gene expression; post-transcriptional regulation
Chromosome band 9p24 is frequently amplified in primary mediastinal B-cell lymphoma (PMBL) and Hodgkin lymphoma (HL). To identify oncogenes in this amplicon, we screened an RNA interference library targeting amplicon genes and thereby identified JAK2 and the histone demethylase JMJD2C as essential genes in these lymphomas. Inhibition of JAK2 and JMJD2C cooperated in killing these lymphomas by decreasing tyrosine 41 phosphorylation and increasing lysine 9 trimethylation of histone H3, promoting heterochromatin formation. MYC, a major target of JAK2-mediated histone phosphorylation, was silenced following JAK2 and JMJD2C inhibition, with a corresponding increase in repressive chromatin. Hence, JAK2 and JMJD2C cooperatively remodel the PMBL and HL epigenome, offering a mechanistic rationale for the development of JAK2 and JMJD2C inhibitors in these diseases.
Kaposi's sarcoma-associated herpesvirus (KSHV) lytic infection increases the expression of viral and human interleukin-6 (vIL-6 and hIL-6, respectively), an important factor for cell growth and pathogenesis. Here, we report genome-wide analysis of viral RNA targets of KSHV ORF57 by a novel UV-cross-linking and immunoprecipitation (CLIP) assay. We identified 11 viral transcripts as putative ORF57 targets and demonstrate that vIL-6 mRNA is an authentic target of ORF57. Disrupting the ORF57 gene in the KSHV genome leads to inefficient expression of vIL-6. With transient transfection, the expression of vIL-6 could be enhanced greatly in the presence of ORF57 in a dose-dependent manner. We found that the open reading frame (ORF) region of vIL-6 RNA contains an MRE (MTA [ORF57]-responsive element) composed of two motifs, MRE-A and MRE-B, and binding of ORF57 to these two motifs stabilizes vIL-6 RNA and promotes vIL-6 translation. We demonstrate that vIL-6 MRE-B bears an miR-1293 binding site and that, mechanistically, ORF57 competes with miR-1293 for the same binding site to interact with vIL-6 RNA, thereby preventing vIL-6 RNA from association with the miR-1293-specified RNA-induced silencing complex (RISC). Consistent with this, ORF57 also interacts with an miR-608 binding site in the hIL-6 ORF and prevents miR-608 repression of hIL-6. Collectively, our results identify a novel function of ORF57 in being responsible for stabilization of viral and human IL-6 RNAs and the corresponding enhancement of RNA translation. In addition, our data provide the first evidence that a tumor virus may use a viral protein to interfere with microRNA (miRNA)-mediated repression of an miRNA target to induce cell proliferation and tumorigenesis during virus infection.
Kaposi's sarcoma-associated herpesvirus (KSHV) encodes ORF57, which promotes the accumulation of specific KSHV mRNA targets, including ORF59 mRNA. We report that the cellular export NXF1 cofactors RBM15 and OTT3 participate in ORF57-enhanced expression of KSHV ORF59. We also found that ectopic expression of RBM15 or OTT3 augments ORF59 production in the absence of ORF57. While RBM15 promotes the accumulation of ORF59 RNA predominantly in the nucleus compared to the levels in the cytoplasm, we found that ORF57 shifted the nucleocytoplasmic balance by increasing ORF59 RNA accumulation in the cytoplasm more than in the nucleus. By promoting the accumulation of cytoplasmic ORF59 RNA, ORF57 offsets the nuclear RNA accumulation mediated by RBM15 by preventing nuclear ORF59 RNA from hyperpolyadenylation. ORF57 interacts directly with the RBM15 C-terminal portion containing the SPOC domain to reduce RBM15 binding to ORF59 RNA. Although ORF57 homologs Epstein-Barr virus (EBV) EB2, herpes simplex virus (HSV) ICP27, varicella-zoster virus (VZV) IE4/ORF4, and cytomegalovirus (CMV) UL69 also interact with RBM15 and OTT3, EBV EB2, which also promotes ORF59 expression, does not function like KSHV ORF57 to efficiently prevent RBM15-mediated nuclear accumulation of ORF59 RNA and RBM15's association with polyadenylated RNAs. Collectively, our data provide novel insight elucidating a molecular mechanism by which ORF57 promotes the expression of viral intronless genes.
Biofilm formation is an important component of vascular catheter infections caused by Candida albicans. Little is known about the interactions between human phagocytes and antifungal agents on Candida biofilms.
Materials and Methods
The interactions of C. albicans biofilms with human phagocytes alone and in combination with anidulafungin or voriconazole were investigated and compared with their corresponding planktonic counterparts using an in vitro biofilm model with clinical intravascular and green fluorescent protein (GFP) expressing strains. Phagocyte- and antifungal agent-mediated damages were determined by 2,3-bis[2-methoxy-4-nitro-5-sulfophenyl]2H-tetrazolium-5-carboxanilide assay and structural effects visualized by confocal microscopy. Oxidative burst was evaluated by flow cytometric measurement of dihydrorhodamine (DHR)-123 oxidation and cytokine release measured by EIA.
Phagocytes alone or in combination with antifungal agents induced less damage against biofilms as compared to planktonic cells. However, additive effects occurred between phagocytes and anidulafungin against Candida biofilms. Confocal microscopy demonstrated absence of phagocytosis within biofilms, but marked destruction caused by anidulafungin and phagocytes. Anidulafungin but not voriconazole elicited a TNF-α release from phagocytes compared with untreated biofilms.
Candida albicans within biofilms are more resistant to phagocytic host defenses but are susceptible to additive effects between phagocytes and an echinocandin.
Polymorphonuclear leukocyte; monocyte; Candida albicans; biofilm; voriconazole; anidulafungin; confocal laser scanning microscopy; cytokines; oxidative burst
Translocations involving the T cell receptor alpha/delta (TCRα/δ) chain locus, which bring oncogenes in the proximity of the TCRα enhancer, are one of the hallmark features of human T cell malignancies from ataxia telangiectasia (AT) and non-AT patients. These lesions are frequently generated by the fusion of DNA breaks at the TCRα/δ locus to a disperse region centromeric of the immunoglobulin heavy chain (IgH) locus. Aberrant VDJ joining accounts for TCRα/δ associated DNA cleavage, but the molecular mechanism that leads to generation of the "oncogene partner" DNA break is unclear. Here we show that in ATM deficient primary mouse T cells, IgH/TCRα/δ fusions arise at a remarkably similar frequency as in human AT lymphocytes. Recombinase-activating gene (RAG) is responsible for both TCRα/δ as well as IgH associated breaks on chromosome 12 (Chr12), which are subject to varying degrees of chromosomal degradation. We suggest a new model for how oncogenic translocations can arise from two non-concerted physiological DSBs.
translocations; T cell leukemia; V(D)J recombination; ATM
In mammals, ten Toll like receptors (TLR) recognize conserved pathogen associated molecular patterns (PAMP), resulting in the induction of inflammatory innate immune responses. One of these, TLR9, is activated intracellularly by bacterial DNA and synthetic oligodeoxynucleotides (ODN) containing unmethylated CpG dinucleotides. Following treatment with CpG ODN, TLR9 is found in LAMP-1 positive lysosomes and we asked which intracellular compartment contains TLR9 prior to CpG exposure. Surprisingly, we found by microscopy and supporting biochemical evidence that both transfected and endogenously expressed human TLR9 is retained in the endoplasmic reticulum (ER). By contrast, human TLR4 trafficked to the cell surface, indicating that ER retention is not a property common to all TLRs. Since TLR9 is observed in endocytic vesicles following exposure to CpG ODN, our data indicate that a special mechanism must exist for translocating TLR9 to the signaling compartments that contain the CpG DNA.
Human; B lymphocytes; Toll-Like Receptors
Mechanisms controlling the disassembly of ezrin/radixin/moesin (ERM) proteins, which link the cytoskeleton to the plasma membrane, are incompletely understood. In lymphocytes, chemokine (e.g., SDF-1) stimulation inactivates ERM proteins, causing their release from the plasma membrane and dephosphorylation. SDF-1–mediated inactivation of ERM proteins is blocked by phospholipase C (PLC) inhibitors. Conversely, reduction of phosphatidylinositol 4,5-bisphosphate (PIP2) levels by activation of PLC, expression of active PLC mutants, or acute targeting of phosphoinositide 5-phosphatase to the plasma membrane promotes release and dephosphorylation of moesin and ezrin. Although expression of phosphomimetic moesin (T558D) or ezrin (T567D) mutants enhances membrane association, activation of PLC still relocalizes them to the cytosol. Similarly, in vitro binding of ERM proteins to the cytoplasmic tail of CD44 is also dependent on PIP2. These results demonstrate a new role of PLCs in rapid cytoskeletal remodeling and an additional key role of PIP2 in ERM protein biology, namely hydrolysis-mediated ERM inactivation.
The viral early-to-late switch of papillomavirus infection is tightly linked to keratinocyte differentiation and is mediated in part by alternative mRNA splicing. Here, we report that SRp20, a cellular splicing factor, controls the early-to-late switch via interactions with A/C-rich RNA elements. An A/C-rich SE4 element regulates the selection of a bovine papillomavirus type 1 (BPV-1) late-specific splice site, and binding of SRp20 to SE4 suppresses this selection. Expression of late BPV-1 L1 or human papillomavirus (HPV) L1, the major capsid protein, inversely correlates with SRp20 levels in the terminally differentiated keratinocytes. In HPV type 16, a similar SRp20-interacting element also controls the viral early-to-late switch. Keratinocytes in raft cultures, which support L1 expression, make considerably less SRp20 than keratinocytes in monolayer cultures, which do not support L1 expression. Conversely, abundant SRp20 in cancer cells or undifferentiated keratinocytes is important for the expression of the viral early E6 and E7 by promoting the expression of cellular transcription factor SP1 for transactivation of viral early promoters.
Kaposi's sarcoma-associated herpesvirus (KSHV) ORF57 facilitates the expression of both intronless viral ORF59 genes and intron-containing viral K8 and K8.1 genes (V. Majerciak, N. Pripuzova, J. P. McCoy, S. J. Gao, and Z. M. Zheng, J. Virol. 81:1062-1071, 2007). In this study, we showed that disruption of ORF57 in a KSHV genome led to increased accumulation of ORF50 and K8 pre-mRNAs and reduced expression of ORF50 and K-bZIP proteins but had no effect on latency-associated nuclear antigen (LANA). Cotransfection of ORF57 and K8β cDNA, which retains a suboptimal intron of K8 pre-mRNA due to alternative splicing, promoted RNA splicing of K8β and production of K8α (K-bZIP). Although Epstein-Barr virus EB2, a closely related homolog of ORF57, had a similar activity in the cotransfection assays, herpes simplex virus type 1 ICP27 was inactive. This enhancement of RNA splicing by ORF57 correlates with the intact N-terminal nuclear localization signal motifs of ORF57 and takes place in the absence of other viral proteins. In activated KSHV-infected B cells, KSHV ORF57 partially colocalizes with splicing factors in nuclear speckles and assembles into spliceosomal complexes in association with low-abundance viral ORF50 and K8 pre-mRNAs and essential splicing components. The association of ORF57 with snRNAs occurs by ORF57-Sm protein interaction. We also found that ORF57 binds K8β pre-mRNAs in vitro in the presence of nuclear extracts. Collectively our data indicate that KSHV ORF57 functions as a novel splicing factor in the spliceosome-mediated splicing of viral RNA transcripts.
Double-strand break (DSB) damage in yeast and mammalian cells induces the rapid ATM (ataxia telangiectasia mutated)/ATR (ataxia telangiectasia and Rad3 related)-dependent phosphorylation of histone H2AX (γ-H2AX). In budding yeast, a single endonuclease-induced DSB triggers γ-H2AX modification of 50 kb on either side of the DSB. The extent of γ-H2AX spreading does not depend on the chromosomal sequences. DNA resection after DSB formation causes the slow, progressive loss of γ-H2AX from single-stranded DNA and, after several hours, the Mec1 (ATR)-dependent spreading of γ-H2AX to more distant regions. Heterochromatic sequences are only weakly modified upon insertion of a 3-kb silent HMR locus into a γ-H2AX–covered region. The presence of heterochromatin does not stop the phosphorylation of chromatin more distant from the DSB. In mouse embryo fibroblasts, γ-H2AX distribution shows that γ-H2AX foci increase in size as chromatin becomes more accessible. In yeast, we see a high level of constitutive γ-H2AX in telomere regions in the absence of any exogenous DNA damage, suggesting that yeast chromosome ends are transiently detected as DSBs.
Since the tyrosine phosphatase SHP-1 plays a major role in regulating T-cell signaling, we investigated regulation thereof by Ser/Thr phosphorylation. We found that TCR stimulation induced fast (≤1min) and transient phosphorylation of SHP-1 S591 in both Jurkat and human peripheral blood T-cells (PBT). Phosphorylation of S591 in T-cells could be mediated artificially by a constitutive active PKC-theta construct, but the dose dependence of inhibition by PKC inhibitors indicated that PKCs were not the relevant basophilic kinase in the physiologic response. S591 phosphorylation inhibited phosphatase function since a S591D mutant had lower activity than the S591A mutant. Additional evidence that S591 phosphorylation alters SHP-1 function was provided by studies of Jurkat cells stably expressing SHP-1 wildtype or mutants. In those cells, S591D mutation reduced the capacity of transfected SHP-1 to inhibit TCR-induced phosphorylation of PLC-γ1. Interestingly, SHP-1 Y536 phosphorylation (previously shown to augment phosphatase activity) was also induced in PBT by TCR signal but at a much later time compared to S591 (~30 min). S591 phosphorylation also altered cellular distribution of SHP-1 because: 1) SHP-1 in lipid rafts and a sheared membrane fraction was hypo-phosphorylated; 2) In stably transfected Jurkat cell lines, S591D mutant protein had reduced presence in both lipid raft and the sheared membrane fraction; 3) S591 phosphorylation prevented nuclear localization of a C-terminal GFP tagged SHP-1 construct. Our studies also shed light on an additional mechanism regulating SHP-1 nuclear localization, namely conformational autoinhibition. These findings highlight elegant regulation of SHP-1 by sequential phosporylation of serine then tyrosine.
T lymphocyte; signal transduction; lipid raft; nuclear localization
The chromosomal instability syndromes Nijmegen breakage syndrome (NBS) and ataxia telangiectasia (AT) share many overlapping phenotypes, including cancer predisposition, radiation sensitivity, cell-cycle checkpoint defects, immunodeficiency, and gonadal dysfunction. The NBS protein Nbs1 is not only a downstream target of AT mutated (ATM) kinase but also acts upstream, promoting optimal ATM activation, ATM recruitment to breaks, and ATM accessibility to substrates. By reconstituting Nbs1 knockout mice with bacterial artificial chromosomes, we have assessed the contribution of distinct regions of Nbs1 to the ATM-dependent DNA damage response. We find that T cell and oocyte development, as well as DNA damage-induced G2/M and S phase checkpoint arrest and radiation survival are dependent on the N-terminal forkhead-associated domain, but not on the principal residues phosphorylated by ATM (S278 and S343) or on the evolutionarily conserved C-terminal region of Nbs1. However, the C-terminal region regulates irradiation-induced apoptosis. These studies provide insight into the complex interplay between Nbs1 and ATM in the DNA damage response.