Constraints on cosmic string tension imposed by the limit on the stochastic gravitational wave background from the European Pulsar Timing Array

We investigate the constraints that can be placed on the cosmic string tension by using the current Pulsar Timing Array limits on the stochastic gravitational wave background (SGWB). We have developed a code to compute the spectrum of gravitational waves (GWs) based on the widely accepted one-scale model. In its simplest form the one-scale model allows one to vary: (i) the string tension, G\mu/c^2; (ii) the size of cosmic string loops relative to the horizon at birth, \alpha; (iii) the spectral index of the emission spectrum, q; (iv) the cut-off in the emission spectrum, n_*; and (v) the intercommutation probability, p. The amplitude and slope of the spectrum in the nHz frequency range is very sensitive to these unknown parameters. We have also investigated the impact of more complicated scenarios with multiple initial loop sizes, in particular the 2-\alpha models proposed in the literature and a log-normal distribution for \alpha. We have computed the constraint on G\mu/c^2 due to the limit on a SGWB imposed by data from the European Pulsar Timing Array. Taking into account all the possible uncertainties in the parameters we find a conservative upper limit of G\mu/c^2<5.3x 10^{-7} which typically occurs when the loop production scale is close to the gravitational backreaction scale, \alpha\approx\Gamma G\mu/c^2. Stronger limits are possible for specific values of the parameters which typically correspond to the extremal cases \alpha\ll \Gamma G\mu/c^2 and \alpha\gg \Gamma G\mu/c^2. This limit is less stringent than the previously published limits which are based on cusp emission, an approach which does not necessarily model all the possible uncertainties. We discuss the prospects for lowering this limit by two orders of magnitude, or even a detection of the SGWB, in the very near future in the context of the Large European Array for Pulsars and the Square Kilometre Array.Comment: 24 pages, 14 figures, accepted for publication in Physical Review D. Minor corrections and additional comments - updated to match the published versio

The Downregulation of GFI1 by the EZH2-NDY1/KDM2B-JARID2 Axis and by Human Cytomegalovirus (HCMV) Associated Factors Allows the Activation of the HCMV Major IE Promoter and the Transition to Productive Infection

<div><p>Earlier studies had suggested that epigenetic mechanisms play an important role in the control of human cytomegalovirus (HCMV) infection. Here we show that productive HCMV infection is indeed under the control of histone H3K27 trimethylation. The histone H3K27 methyltransferase EZH2, and its regulators JARID2 and NDY1/KDM2B repress GFI1, a transcriptional repressor of the major immediate-early promoter (MIEP) of HCMV. Knocking down EZH2, NDY1/KDM2B or JARID2 relieves the repression and results in the upregulation of GFI1. During infection, the incoming HCMV rapidly downregulates the GFI1 mRNA and protein in both wild-type cells and in cells in which EZH2, NDY1/KDM2B or JARID2 were knocked down. However, since the pre-infection levels of GFI1 in the latter cells are significantly higher, the virus fails to downregulate it to levels permissive for MIEP activation and viral infection. Following the EZH2-NDY1/KDM2B-JARID2-independent downregulation of GFI1 in the early stages of infection, the virus also initiates an EZH2-NDY1/ΚDM2Β-JARID2-dependent program that represses GFI1 throughout the infection cycle. The EZH2 knockdown also delays histone H3K27 trimethylation in the immediate early region of HCMV, which is accompanied by a drop in H3K4 trimethylation that may contribute to the shEZH2-mediated repression of the major immediate early HCMV promoter. These data show that HCMV uses multiple mechanisms to allow the activation of the HCMV MIEP and to prevent cellular mechanisms from blocking the HCMV replication program.</p></div

GFI1 mRNA and protein are degraded rapidly during HCMV infection and their degradation is followed by upregulation of EZH2, JARID2 and NDY1/KDM2B and downregulation of JMJD3.

<p><b>A</b>. The GFI1 mRNA levels in lysates of HFFs harvested at the indicated time points after HCMV infection were monitored by real time RT-PCR. <b>B</b>. The GFI1 protein levels in the same cells were monitored by western blotting. <b>C</b>. HCMV infection promotes the degradation of GFI1 at the RNA level. HFFs were mock-infected or infected with HCMV after treatment with Actinomycin D (5 µg/ml). GFI1 mRNA levels in cell lysates harvested at the indicated time points after exposure to the virus were measured by real time RT-PCR. <b>D</b>. Proteasomal inhibition blocks the rapid downregulation of GFI1 in HCMV-infected cells. HFFs were mock-infected or infected with HCMV after pretreatment with MG132 (10 µM). GFI1 protein levels in cell lysates harvested at the indicated time points after exposure to the virus were measured by western blotting. <b>E</b>. GFI1 protein levels in HFFs transduced with the indicated constructs and harvested before, and 2 hours after HCMV infection, were measured by western blotting. <b>F</b>. The expression of EZH2, NDY1/KDM2B, JARID2 and JMJD3 was measured by real time RT-PCR in lysates of HFFs, harvested at the indicated time points after HCMV infection.</p

Lentiviral and retroviral constructs used.

<p>Lentiviral and retroviral constructs used.</p

Primer sets used in Real-Time PCR.

<p>Primer sets used in Real-Time PCR.</p

Primers set used for ChIP analysis.

<p>Primers set used for ChIP analysis.</p

NDY1/KDM2B, EZH2 and H3K27 tri-methylation are required for immediate-early gene transcription.

<p><b>A and B</b>. HFFs were lentivirally or retrovirally transduced with the indicated constructs and they were subsequently infected with HCMV (MOI 0.5). The cells were fixed 5 hours later and the percentage of IE1-expressing cells was measured by FACS analysis. The bars show the percentage of IE1-positive cells (mean ± SD). <b>C</b>. Comparison of IE1 expression in HCMV-infected HFFs, transduced with pLKO.1, pLKO.1-shEZH2 or pLKO.1-shNDY1/KDM2B, prior to the infection. Cells were infected with HCMV (MOI 0.5). Western blots of cell lysates harvested at the indicated time points, were probed with anti-IE1 or anti-actin (loading control) antibodies. <b>D</b>. HFFs were infected with HCMV (MOI 0.5 PFU/cell) before and after a 30 minute pretreatment with the EZH2 inhibitor DZNep. The Western blotting shows the expression of EZH2 and IE1 in untreated and DZNep-pretreated cells at 24 hours from the start of the infection. <b>E</b>. HFFs were infected with HCMV (MOI 0.5 PFU/cell) before and after a 30-minute pretreatment with the EZH2 inhibitor DZNep. The infected cells were monitored by light microscopy 5 days later. In addition, they were stained for IE1 and counterstained with DAPI at 5 hours post-infection, and they were analyzed by epifluoerescence microscopy. Bar = 100 µm. <b>F</b>. The progeny virus harvested from the DZNep-treated and untreated cells as in D, 5 days after infection, was titrated by standard viral plaque assays. The bars show the viral titers (mean ± SD).</p

Infection by HCMV depends on the downregulation of GFI1, a repressor of immediate-early gene transcription.

<p>Model summarizing the data on the interaction between the virus and the host. (Panel A) This panel describes the infection of wild type HFFs. The incoming virus rapidly degrades GFI1 to allow the activation of the MIEP of HCMV, and viral infection. In addition, virus infection alters the expression of NDY1/KDM2B, EZH2, JARID2 and JMJD3. The solid lines from these molecules to GFI1 indicate that they actively repress GFI1 both before and after infection, although due to HCMV-induced changes in their expression, the repression is enhanced after infection. (Panel B, Left) The repression of GFI1 in uninfected cells was blocked by the knockdown of NDY1/KDM2B, EZH2 or JARID2 and by the overexpression of JMJD3, resulting in significant up-regulation of GFI1 (dotted lines). (Panel B, Right) describes the infection of HFFs in the left side of panel B. The virus continues to degrade GFI1. However, the degradation of GFI1 by the virus is insufficient to downregulate it to levels that allow the activation of the MIEP and viral infection.</p