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. 2017 Mar 2;65(5):818-831.e5.
doi: 10.1016/j.molcel.2017.01.015. Epub 2017 Feb 16.

Nek7 Protects Telomeres From Oxidative DNA Damage by Phosphorylation and Stabilization of TRF1

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Free PMC article

Nek7 Protects Telomeres From Oxidative DNA Damage by Phosphorylation and Stabilization of TRF1

Rong Tan et al. Mol Cell. .
Free PMC article

Abstract

Telomeric repeat binding factor 1 (TRF1) is essential to the maintenance of telomere chromatin structure and integrity. However, how telomere integrity is maintained, especially in response to damage, remains poorly understood. Here, we identify Nek7, a member of the Never in Mitosis Gene A (NIMA) kinase family, as a regulator of telomere integrity. Nek7 is recruited to telomeres and stabilizes TRF1 at telomeres after damage in an ATM activation-dependent manner. Nek7 deficiency leads to telomere aberrations, long-lasting γH2AX and 53BP1 foci, and augmented cell death upon oxidative telomeric DNA damage. Mechanistically, Nek7 interacts with and phosphorylates TRF1 on Ser114, which prevents TRF1 from binding to Fbx4, an Skp1-Cul1-F box E3 ligase subunit, thereby alleviating proteasomal degradation of TRF1, leading to a stable association of TRF1 with Tin2 to form a shelterin complex. Our data reveal a mechanism of efficient protection of telomeres from damage through Nek7-dependent stabilization of TRF1.

Keywords: KillerRed; Nek7; TRF1; oxidative DNA damage; phosphorylation; telomere.

Conflict of interest statement

AUTHOR CONTRIBUTIONS

R.T. designed and performed the major experiments. Q.W., H.S., S.H., J.X., J.W., X.Z., N.A.Y., and Y.J. assisted with experiments. Y.J., T.E.S., S.N., M.L., A.S.L., B.S., and L.L. helped design the experiments and discussed and aided in planning the entire research theme. All authors were involved in manuscript preparation.

Figures

Figure 1
Figure 1. Nek7 Is Recruited to Telomeres and Protects Cells from Telomeric DNA Damage
(A) Schematic illustration of the KillerRed-TRF1-induced oxidative DNA damage at sites of telomeres. (B) Recruitment of Nek7 to telomeres in the presence or absence of telomeric DNA damage in U2OS cells. The percentages of cells with positive Nek7 foci (≥5) at telomeres are shown. The scale bars represent 2 μm. Error bars represent ± SEM (n = 3, ***p < 0.001). (C) Recruitment of Nek7 to telomeres in the presence or absence of telomeric DNA damage in HeLa 1.3 cells. (D) Telomere aberrations (arrows) in control (shCtrl) and Nek7 knockdown (shNek7) HeLa cells. Telomeric DNA was stained by Tel C-Cy3 (red) and total DNA by DAPI (blue). The percentage of sister telomere loss (a) and sister telomere association (b) are shown. Error bars represent ± SEM (n = 3, 4,500 chromosomes, **p < 0.01). (E) Telomere and chromosomal aberrations in shNek7-treated HeLa cells after KR-TRF1-induced oxidative telomeric DNA damage (shNek7+KR-TRF1). (a) Fragile telomeres; (b1 and b2) intrachromosomal telomeric insertions. The table shows quantification of telomere aberrations in cells. (F) Colony formation assays for shCtrl- or shNek7-U2OS cells transiently expressing KR-TRF1. Nek7 expression levels are shown (insert). (G) Recruitment of Nek7 under treatment of ATM inhibitor. U2OS cells were either untreated (control) or treated with ATM inhibitor KU55933 (10 μM) for 1 hr before being exposed to light for 1 hr. Error bars represent ± SEM (n = 3). See also Figures S1 and S2.
Figure 2
Figure 2. Nek7 Stabilizes TRF1 upon Oxidative Telomeric DNA Damage
(A) Percentage of γH2AX TIF in siCtrl- or siNek7-U2OS cells expressing KR-TRF1. Cells were exposed to light for 10 min and then recovered in the dark for indicated time periods. Error bars represent ± SEM (n = 3). (B) Percentage of 53BP1 TIF in siCtrl- or siNek7-U2OS cells co-expressing KR-TRF1 and GFP-53BP1 were analyzed. (C) Immunoblotting to detect γH2AX expression in shCtrl- or shNek7-HeLa cells stably expressing KR-TRF1. Relative expression of γH2AX was normalized to KR-TRF1. (D) Expression of TRF1, TRF2, Pot1, Tin2 in shCtrl, shNek7 knockdown Flp-in T-REX KR-TRF1 293 cells and shNek7 knockdown cells rescued by Flag-Nek7. Cells had light-induced telomere damage. (E) Expression of TRF1 and KR-TRF1 in shCtrl and two Nek7 shRNA knockdown Flp-in T-REX KR-TRF1 293 cell lines was determined. (F) The Myc-TRF1 expression level in HeLa cells with increasing Flag-Nek7 expression was determined. Tubulin expression was used as a loading control. (G) The design for Cas9-CRISPR KO cells (top). Genotyping results and colony formation assays are shown (bottom). (H) Expression of TRF1, TRF2, Pot1, and Tin2 in WT and two Flp-in T-REX 293 KR-TRF1 Nek7 KO cell lines. See also Figures S2 and S3.
Figure 3
Figure 3. Stabilization of TRF1 by Nek7 Occurs at the Post-translational Level
(A) TRF1 mRNA level in control or Flag-Nek7 transfected HeLa cells was measured by real-time PCR. Error bars represent ± SEM (n = 3, p = 0.23). (B) Relative TRF1 expression in shCtrl and shNek7 knockdown Flp-in T-REX KR-TRF1 293 cells was analyzed. Cells were treated with CHX (100 mg/ml) for the indicated time periods, in the absence or presence of 15 mM MG132, and expression was determined. Error bars represent ± SEM (n = 3, *p < 0.05). (C) Myc-TRF1 expression in control (Flag-empty vector) or Flag-Nek7 transfected HeLa cells was analyzed. (D) shNek7 knockdown HeLa cells co-transfected with KR-TRF2, Myc-TRF1, and Flag-Nek7 were either untreated, or treated with CHX or an ATM inhibitor alone, or together as indicated. Cells were then exposed to light for an additional 1 hr before being analyzed for Myc-TRF1 expression. Error bars represent ± SEM (n = 3). See also Figure S4.
Figure 4
Figure 4. Nek7 Is Recruited and Interacts with TRF1 at Telomeres in Response to Oxidative Telomeric DNA Damage
(A) Schematic illustration of a bimolecular fluorescence complementation (BiFC) assay. (B) Interaction of TRF1 and Nek7 using a BiFC assay in HeLa 1.3 cells. The nuclei were stained with DAPI (blue). (C) The interaction of TRF1 and Nek7 in U2OS cells at telomeres. Telomeres were stained by PNA Tel C-Cy3 (red). (D) Co-immunoprecipitation of Myc-TRF1 and Flag-Nek7 in HeLa cells after telomeric DNA damage. (E) TRF1 binds to endogenous Nek7 in HeLa cells. (F) Inhibition of ATM prevents TRF1 and Nek7 interaction in Flp-in TREX KR-TRF1 293 cells. See also Figure S5.
Figure 5
Figure 5. Nek7 Catalytic Activity Is Required to Phosphorylate and Stabilize TRF1
(A) The recruitment of Nek7 mutants to telomere damage sites. (B) Co-immunoprecipitation of TRF1 with Nek7 mutants. (C) Myc-TRF1 was co-transfected with Nek7 mutants (Y97A or KM) in Nek7-KD HeLa cells, and its expression determined by IB. The relative Myc-TRF1 expression is shown. (D) In vitro kinase assay for Nek7 activity. Recombinant GST and GST-TRF1 were purified and used as substrates. Flag-Nek7 was immunoprecipitated from shNek7 Flp-in T-REX KR-TRF1 293 cells and used as the enzyme for the kinase assay. (E) In vitro kinase assay for Nek7 on TRF1 with or without telomeric DNA damage. GST-TRF1 and MBP were used as substrates for the Flag-Nek7 or Flag-Nek7 KM mutant. (F) Colony formation assays for shNek7 U2OS cells co-expressing KR-TRF1 and an shRNA-resistant Flag-Nek7 or an Flag-Nek7 KM mutant. Error bars represent ± SEM (n = 3).
Figure 6
Figure 6. Nek7-Mediated TRF1 S114 Phosphorylation Prevents TRF1 Interaction with Fbx4, Leading to an Increased Interaction with Tin2
(A) TRF1 phosphor peptides identified by a liquid chromatography-tandem mass spectrometry (LC/MS) analysis. The ratio of S114 phosphorylation between the Nek7 and Nek7 KM samples are shown. Error bars represent ± SEM (n = 5). (B) Surface representation of S114 phosphorylated TRF1 with Fbx4 (left), or non-phosphorylated TRF1 with Fbx4 (right). Green ribbons represent TRF1 structure. Blue indicates positive charges, and red indicates negative charges. Enlarged images from the interacting surface are shown. (C) Phosphor-mimicry TRF1 S114D mutation disrupts the TRF1 interaction with Fbx4. (D) The Fbx4 interaction with TRF1 or TRF1 S114A mutant in the presence of Nek7 or Nek7(KM) expression after DNA damage. shTRF1 and shNek7 Flp-in TREX 293 cells were co-transfected KR-TRF2, Flag-Fbx4 with plasmids as indicated. The relative Fbx4 binding to TRF1 was quantified (n = 4). Error bars represent ± SEM (n = 4). (E) The interaction of Flag-Tin2 with Myc-TRF1 or Myc-TRF1 mutants (S114A or S114D). shTRF1 stably expressing Flp-in TREX 293 cells was co-transfected with Flag-Tin2, Myc-TRF1, or Myc-TRF S114A or S114D mutants as indicated. (F) Interaction of TRF1 with Tin2 in HeLa cells with or without Nek7 knockdown in the presence of telomeric DNA damage. Flag-control vector or an shRNA-resistant Flag-Nek7 plasmid was co-transfected with Myc-TRF1 and HA-Tin2 in shNek7 HeLa cells as indicated. The relative TRF1 binding to Tin2 was quantified (n = 3). Error bars represent ± SEM (n = 3). (G) A model illustrating Nek7 regulation of TRF1 and Fbx4 interaction. See also Figure S6.
Figure 7
Figure 7. Nek7-Mediated TRF1 Phosphorylation Prevents TRF1 Ubiquitination and Proteasome-Targeted Protein Degradation
(A) Relative expression of Myc-TRF1mutants (S114A or S114D) in HeLa cells were analyzed. See also Figure 3B. (B) Wild-type Myc-TRF1 and Myc-TRF1 S114A expression levels in HeLa cells with increased Flag-Nek7 expression were determined by IB. (C) Ubiquitination of TRF1 in the absence or presence of telomeric DNA damage. Myc-ubiquitin (Myc-Ub) and KR-TRF1 plasmids were co-transfected into shCtrl-treated or shNek7-2-treated HeLa cells. (D). Colony formation assays for shTRF1 and shNek7 double-knockdown U2OS cells co-expressing KR-TRF2 and an shRNA-resistant Myc-TRF1 or an Myc-TRF1 S114D. Error bars represent ± SEM (n = 3). (E) Model of how Nek7 protects telomere integrity after DNA damage.

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