Human Flap Endonuclease I Is in Complex with Telomerase and Is Required for Telomerase-mediated Telomere Maintenance

Studies from budding yeast and ciliates have suggested that telomerase extension of telomeres requires the conventional DNA replication machinery, yet little is known about how DNA replication proteins regulate telomerase action in higher eukaryotic cells. Here we investigate the role of one of the DNA replication factors, flap endonuclease I (FEN1), in regulating telomerase activity in mammalian cells. FEN1 is a nuclease that plays an important role in DNA replication, repair, and recombination. We show that FEN1 is in complex with telomerase in vivo via telomeric DNA. We further demonstrate that FEN1 deficiency in mouse embryonic fibroblasts leads to an increase in telomere end-to-end fusions. In cancer cells, FEN1 deficiency induces gradual shortening of telomeres but does not alter the single-stranded G-overhangs. This is, to our knowledge, the first evidence that FEN1 and telomerase physically co-exist as a complex and that FEN1 can regulate telomerase activity at telomeres in mammalian cells

Molecular steps of G-overhang generation at human telomeres and its function in chromosome end protection

Weihang Chai et al show that generation of G-strand overhangs, which are important for telomere protection and for promoting telomerase activity, is dynamically regulated during the cell cycle, involving both DNA polymerization and processing mechanisms. Telomeric G-overhangs are required for the formation of the protective telomere structure and telomerase action. However, the mechanism controlling G-overhang generation at human telomeres is poorly understood. Here, we show that G-overhangs can undergo cell cycle-regulated changes independent of telomerase activity. G-overhangs at lagging telomeres are lengthened in S phase and then shortened in late S/G2 because of C-strand fill-in, whereas the sizes of G-overhangs at leading telomeres remain stable throughout S phase and are lengthened in G2/M. The final nucleotides at measurable C-strands are precisely defined throughout the cell cycle, indicating that C-strand resection is strictly regulated. We demonstrate that C-strand fill-in is mediated by DNA polymerase α (polα) and controlled by cyclin-dependent kinase 1 (CDK1). Inhibition of CDK1 leads to accumulation of lengthened G-overhangs and induces telomeric DNA damage response. Furthermore, depletion of hStn1 results in elongation of G-overhangs and an increase in telomeric DNA damage. Our results suggest that G-overhang generation at human telomeres is regulated by multiple tightly controlled processes and C-strand fill-in is under the control of polα and CDK1

Cross-Species Rhesus Cytomegalovirus Infection of Cynomolgus Macaques.

Cytomegaloviruses (CMV) are highly species-specific due to millennia of co-evolution and adaptation to their host, with no successful experimental cross-species infection in primates reported to date. Accordingly, full genome phylogenetic analysis of multiple new CMV field isolates derived from two closely related nonhuman primate species, Indian-origin rhesus macaques (RM) and Mauritian-origin cynomolgus macaques (MCM), revealed distinct and tight lineage clustering according to the species of origin, with MCM CMV isolates mirroring the limited genetic diversity of their primate host that underwent a population bottleneck 400 years ago. Despite the ability of Rhesus CMV (RhCMV) laboratory strain 68-1 to replicate efficiently in MCM fibroblasts and potently inhibit antigen presentation to MCM T cells in vitro, RhCMV 68-1 failed to productively infect MCM in vivo, even in the absence of host CD8+ T and NK cells. In contrast, RhCMV clone 68-1.2, genetically repaired to express the homologues of the HCMV anti-apoptosis gene UL36 and epithelial cell tropism genes UL128 and UL130 absent in 68-1, efficiently infected MCM as evidenced by the induction of transgene-specific T cells and virus shedding. Recombinant variants of RhCMV 68-1 and 68-1.2 revealed that expression of either UL36 or UL128 together with UL130 enabled productive MCM infection, indicating that multiple layers of cross-species restriction operate even between closely related hosts. Cumulatively, these results implicate cell tropism and evasion of apoptosis as critical determinants of CMV transmission across primate species barriers, and extend the macaque model of human CMV infection and immunology to MCM, a nonhuman primate species with uniquely simplified host immunogenetics

Natural Killer Cell Evasion Is Essential for Infection by Rhesus Cytomegalovirus

<div><p>The natural killer cell receptor NKG2D activates NK cells by engaging one of several ligands (NKG2DLs) belonging to either the MIC or ULBP families. Human cytomegalovirus (HCMV) UL16 and UL142 counteract this activation by retaining NKG2DLs and US18 and US20 act via lysomal degradation but the importance of NK cell evasion for infection is unknown. Since NKG2DLs are highly conserved in rhesus macaques, we characterized how NKG2DL interception by rhesus cytomegalovirus (RhCMV) impacts infection <i>in vivo</i>. Interestingly, RhCMV lacks homologs of UL16 and UL142 but instead employs Rh159, the homolog of UL148, to prevent NKG2DL surface expression. Rh159 resides in the endoplasmic reticulum and retains several NKG2DLs whereas UL148 does not interfere with NKG2DL expression. Deletion of Rh159 releases human and rhesus MIC proteins, but not ULBPs, from retention while increasing NK cell stimulation by infected cells. Importantly, RhCMV lacking Rh159 cannot infect CMV-naïve animals unless CD8+ cells, including NK cells, are depleted. However, infection can be rescued by replacing Rh159 with HCMV UL16 suggesting that Rh159 and UL16 perform similar functions <i>in vivo</i>. We therefore conclude that cytomegaloviral interference with NK cell activation is essential to establish but not to maintain chronic infection.</p></div

Deletion of Rh159 rescues intracellular transport and surface expression of MICA and MICB upon RhCMV infection.

<p><b>A)</b> Comparison of NKG2DL surface expression upon infection with RhCMV or ΔRh159. U373-NKG2DL cells were infected with RhCMV (blue) or ΔRh159 (red) (MOI = 3) for 48 h. Cell surface levels of NKG2DL or TfR were determined by flow cytometry, using specific antibodies and compared to isotype control (dotted). Depicted is NKG2DL or TfR surface expression on infected cells gated for RhCMV IE2⁺ expression. The results shown are representative of three or more independent experiments. <b>B)</b> Biosynthesis and maturation of NKG2DL in uninfected U373-NKG2DL cells or upon infection with RhCMV or ΔRh159. U373-NKG2DL cells were uninfected (NI), infected with RhCMV (WT) or ΔRh159 (MOI = 3) for 24 h, verified by light microscopy as having 100% CPE, then metabolically labeled with [35S]cysteine and [35S]methionine for 30 min prior to chasing the label for the indicated times. The indicated NKG2DLs were immunoprecipitated from cell lysates with specific mAbs. Immunoprecipitates were split and digested with EndoH (+) or mock treated (-) then analyzed by SDS-PAGE and autoradiography.</p

Cell surface expression of NKG2DLs is not affected by HCMV UL148.

<p><b>A)</b> Alignment of Rh159 and UL148. Intensity of purple shading indicates level of sequence conservation (Jalview 2 [<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1005868#ppat.1005868.ref055" target="_blank">55</a>]). <b>B</b>) MICB maturation is not affected by UL148. U373-MICB cells were transduced with AdCtrl (empty vector) (Ctrl), or AdUL148 containing a C-terminal V5 tag (UL148) at an MOI of 10 or 80, or AdRh159FL (Rh159) (MOI = 10) together with AdtTA (MOI = 2.5) for 48 h. UL148 and Rh159 expression was verified by immunoblot using anti-V5 antibody or anti-FLAG antibody, respectively, with antibodies to GAPDH providing a loading control. The mature MICB protein of 67 kDA is designated M whereas I refers to immature MICB retained in the ER by Rh159. Results shown are representative of two experiments. <b>C)</b> UL148 does not affect cell surface expression of NKG2DL. U373-NKG2DL cells were transduced with AdUL148 (blue) or AdCtrl (black) (MOI = 80) and, at 48 h, cells were harvested and analyzed for NKG2DL and TfR surface expression compared to isotype controls (dotted) by flow-cytometry. <b>D)</b> UL148 expression in C) was verified by immunoblot in each of the samples using anti-V5 antibody. GAPDH served as a loading control.</p

MICB is retained in the ER and associates with Rh159 in RhCMV infected cells.

<p><b>A)</b> Immature MICB is enriched in RhCMV-infected cells. U373-MICB cells were infected with RhCMV (MOI = 3) for 12 or 24 h or left uninfected prior to lysis in 1% NP40. Cell lysates were treated with Endoglycosidase H (E), PNGase F (P), or digestion buffer alone (-), separated by SDS-PAGE and immunoblotted with monoclonal antibodies (mAbs) to MICB or GAPDH. Mature glycosylated MICB (67kDa) is EndoH resistant (R), whereas immature MICB (I) remains EndoH sensitive with an apparent MW of 43kDa upon EndoH treatment (S) indicative of ER retention. <b>B)</b> RhCMV inhibits intracellular transport of MICB. U373-MICBs were infected with RhCMV (WT) (MOI = 3) for 24 h (visualization by light microscopy confirmed 100% cytopathic effect (CPE)) or left uninfected (NI) followed by metabolically labeling with [35S]cysteine and [35S]methionine for 30 min. The label was then chased for the indicated times (h), cells were lysed and MICB was immunoprecipitated from cell lysates using a MICB specific antibody. Precipitates were either digested with EndoH (+) or mock treated (-) followed by SDS-PAGE and autoradiography. Stars (*) indicate an EndoH-sensitive protein co-precipitating with MICB in infected cells. <b>C-D)</b> Isolation and identification of Rh159 co-immunoprecipitating with MICB. U373-MICB cells were infected with RhCMV (WT) or left non-infected (NI) as above and cells were lysed at 48 hpi. MICB was immunoprecipitated with anti–MICB mAb, the immunoprecipitates were separated by SDS-PAGE, and proteins visualized by Coomassie Blue staining. The IgG heavy chain (55kDa) is indicated. The 43kDa protein (*) was excised from the gel and digested with trypsin. <b>D)</b> Mass-spectrometric analysis by LC-MS/MS identified tryptic peptides corresponding to Rh159 (gray shaded boxes). The predicted amino acid sequence of Rh159 contains a signal sequence (italics), N-linked glycosylation sites (underlined), a C-terminal transmembrane anchor (bold), and an RXR ER retrieval motif (red). The results shown in A and B are representative of two or more independent experiments.</p

Primary infection of rhesus macaques requires evasion of NK cells by Rh159.

<p><b>A)</b> Rh159 is required for superinfection. At day 0, a RhCMV+ RM was infected subcutaneously (s.c.) with 5x10⁶ PFU of ΔRh159. The SIVgag-specific T cell response in PBMC was monitored by ICCS for CD69, TNFα and IFNγ using overlapping (by 4AA) 15mer peptide mixes. Results are shown as a percentage of total memory CD4⁺ or CD8⁺ T cells. No responses above background were measured. <b>B)</b> Rh159 is required for primary infection. Two RhCMV-naïve animals were inoculated s.c. with 5x10⁶ PFU ΔRh159 and SIVgag- and RhCMV IE-specific T cells were monitored as in A). In addition, SIVgag-specific CD8+ T cell responses to 2 MHC-E restricted (Gag69 and Gag120) and 2 MHC-II-restricted (Gag53 and Gag73) supertope peptides were quantified by flow cytometric ICS. Starting on day 63, RM were treated with anti-CD8 antibody CM-T807 to deplete CD8⁺ cells and RM were re-inoculated with 5x10⁶ PFU of ΔRh159 on day 64. <b>C)</b> The relative frequencies of CD8⁺ small CD3- lymphocytes in whole blood (WB) of each animal were monitored during CM-T807 treatment.</p

Rh159 interferes with intracellular transport of NKG2DL.

<p><b>A)</b> Association with Rh159 prevents intracellular transport of MICB. U373-MICB cells were transduced with adenovectors (MOI = 80) expressing either GFP (AdGFP) or FLAG-tagged Rh159 (AdRh159FL) under control of tetracycline-dependent transactivator provided by co-transduced AdtTA (MOI = 20). At 24 hpi cells were metabolically labeled for 30 min with [35S]cysteine + [35S]methionine. Upon chasing the label for the indicated times (h), cells were lysed and MICB was immunoprecipitated with anti–MICB mAb. Precipitates were either digested with EndoH (+) or mock treated (-) followed by SDS-PAGE and autoradiography. (S) indicates EndoH-deglycosylated proteins. <b>B)</b> Rh159 co-immunoprecipitates with MICB. U373-ULBP3 (ULBP3, left panel) or U373-MICB (MICB, right panel) cells were lysed at 48 h post-transduction with AdRh159FL (Rh159) or an adenovector expressing FLAG-tagged SVV ORF 61 (SVV61) used as a negative control. MICB and ULBP3 were immunoprecipitated with anti–MICB and anti-ULBP3 mouse and goat mAbs, respectively, then immunoblotted with mouse anti-FLAG mAb. The mouse IgG heavy chain (55kDa) is indicated (HC). Input lanes were loaded with 10% total lysate used in immunoprecipitation and immunoblotted with mAbs for FLAG and GAPDH. The results shown are representative of two independent experiments. <b>C)</b> Rh159 reduces steady state levels of MICB. U373-MICB cells were lysed at 48 h post-transduction with the indicated Ad-vectors. Lysates were digested with EndoH (+) or mock treated (-) then immunoblotted with mAbs for MICB, FLAG or GAPDH. Note that both MICB and Rh159 are EndoH sensitive consistent with ER localization. The results shown are representative of two independent experiments. <b>D-E)</b> Rh159 reduces surface expression of MICA, MICB, ULBP1 and ULBP2 but not ULBP3. U373-NKG2DL cells were transduced with AdRh159FL or AdGFP as in A) but for 48 h. Cells were then lysed and immunoblotted with mAbs for FLAG and GAPDH (<b>D</b>), or stained with antibodies specific for the indicated proteins, or isotype control (dotted) and analyzed by flow cytometry. The results shown are representative of three or more independent experiments.</p