Embryonic Stem Cells License a High Level of Dormant Origins to Protect the Genome against Replication Stress

Abstract

Maintaining genomic integrity during DNA replication is essential for stem cells. DNA replication origins are licensed by the MCM2-7 complexes, with most of them remaining dormant. Dormant origins (DOs) rescue replication fork stalling in S phase and ensure genome integrity. However, it is not known whether DOs exist and play important roles in any stem cell type. Here, we show that embryonic stem cells (ESCs) contain more DOs than tissue stem/progenitor cells such as neural stem/progenitor cells (NSPCs). Partial depletion of DOs does not affect ESC self-renewal but impairs their differentiation, including toward the neural lineage. However, reduction of DOs in NSPCs impairs their self-renewal due to accumulation of DNA damage and apoptosis. Furthermore, mice with reduced DOs show abnormal neurogenesis and semi-embryonic lethality. Our results reveal that ESCs are equipped with more DOs to better protect against replicative stress than tissue-specific stem/progenitor cells.

Figure 1

ESCs Possess More DOs Than NSPCs (A and B) DNA fiber assay on mouse ESCs (CCE strain). For exclusion of artifacts arising from fork-to-fork fusion, cells were pulsed with BrdU for 10 min in the absence of HU and 20 min in the presence of HU to achieve similar replication fork length. (A) Examples of a DNA fiber containing a replicon cluster of four BrdU-labeled forks are shown. (B) Distribution of the mean intra-cluster fork spacing from >50 replicon clusters is shown. Overall fork spacing ± SEM is indicated in the chart. (C–G) Comparisons between CCE cells derived from the 129/Sv mice and NSPCs from the E13.5 129/Sv embryo brains. (C) Immunoblotting of chromatin-bound MCM proteins with H3 as a loading control for quantification is shown. (D) Quantification of chromatin-bound MCM2 in G1-phase cells and cell-cycle distribution by FACS are shown. (E) 2D projection confocal and SIM images of chromatin-bound MCM2, MCM3, and MCM7 in G1 phase cells are shown. (F) Quantification of chromatin-bound MCM foci number and average focus volume imaged by SIM are shown. Error bars represent SEM of three independent experiments. (G) DNA fiber analysis of NSPCs and ESCs is shown. Cells were incubated with 100 μM HU for 4 hr before BrdU pulse. Overall fork spacing ± SEM from >50 replicon clusters is indicated. p values are from two-tailed t test.

Figure 2

Reducing DOs Impairs ESC Differentiation (A) CCE cell proliferation at 48–96 hr after transfection with a serial dilution of Mcm5 siRNA. SC, scrambled siRNA. ∼90% transfection efficiency is achieved. (B) Immunoblotting of total cell lysate at 72 hr after Mcm5 siRNA transfection. Fifteen picomoles Mcm5 siRNA knocked down MCM5 and had no effect on cell growth or DNA replication; hence, it was used for further analysis. (C) TUNEL assay of ESCs after treatment with 500 μM HU or 0.075 μg/ml aphidicolin (Aph) for 48 hr. Fifteen picomoles Mcm5 or scrambled siRNA was transfected into the cells 72 hr prior to the HU and Aph treatment. (D–K) Assays on Mcm4^C/C ESCs (C1 and C2) and wild-type Mcm4^+/+ ESCs (W1 and W2). (D) Cell proliferation rate analyzed over 72 hr is shown. (E) Overlay of OCT4 or SOX2 immunofluorescence images with DIC images is shown. OCT4- or SOX2-negative cells, larger than ESCs, are mostly MEF contamination in the ESC culture. (F) TUNEL assay of ESCs after treatment with HU or Aph for 48 hr is shown. (G and H) DIC images and immunofluorescence of NESTIN, respectively, of NSPCs at 96 hr after induced differentiation from ESCs are shown. (I) qRT-PCR analysis of Nestin and Sox1 expression during induced NSPC differentiation from ESCs is shown. (J) qRT-PCR data of the expression of three germ layer markers from days 1 to 13 during embryoid body (EB) differentiation from ESCs are shown. (K) DIC images of EBs at day 13 after induced differentiation from ESCs are shown. (L) Frequency and average weight of teratomas generated from the wild-type (W1–W4) and Mcm4^C/C (C1–C4) ESCs. Error bars in (A), (D), (F), (I), and (J) all represent SEM of three independent experiments. See also Figures S1 and S2.

Figure 3

Reducing DOs Impairs the Differentiation of NSPCs (A–C) Analysis of the NSPC differentiation from the Mcm4^+/+ (W1 and W2) and Mcm4^C/C (C1 and C2) ESCs. (A) Immunoblot of the NSPC total lysate is shown. (B) TUNEL assay on NSPCs at 96 hr after induced differentiation is shown. (C) qRT-PCR of Nestin and Sox1 expression in NSPCs is shown. Treatment with caffeine (4 mM) or Z-VAD-FMK (40 μM) started at 48 hr after induction, and NSPCs were harvested at 96 hr for analysis. (D–I) Analysis of neurospheres clonally derived from NSPCs isolated from the E13.5 mouse forebrain. (D) Neurospheres were passaged every 6 days to give a new round of clonogenic assay. Number and growth rate of neurospheres were measured by counting the neurospheres and the total number of cells at each passage. Error bars represent SEM from four independent experiments and each experiment containing five embryos of each genotype. (E) Representative images of neurospheres at fifth passage are shown. (F) DNA fiber analysis is shown. Cells were treated with 100 μM HU for 4 hr before analysis. Overall average fork spacing ± SEM from >50 replicon clusters is shown. p values are from two-tailed t test. (G) Cell-cycle analysis of neurospheres at fifth passage by FACS after pyronin Y and DAPI staining is shown. Note G2-M blockage of the cells in the Mcm4^C/C neurospheres. Two-tailed t test: non-significant (ns); p < 0.005 (^∗∗). (H) Immunofluorescence quantifying the percentage of γH2AX or 53BP1 positive cells in neurospheres is shown. (I) Immunoblot of total cell lysate of neurospheres is shown. Error bars in (B), (C), (G), and (H) all represent SEM of three independent experiments. See also Figure S3.

Figure 4

Reducing DOs Impairs Embryonic Neurogenesis and Affects Embryonic Viability (A and B) Coronal sections of the Mcm4^+/+ and Mcm4^C/C embryonic forebrain. (A) Phase contrast views are shown. Red lines show width of ventral forebrain (VF) and thickness of cortex (Cx). (B) Quantification of forebrain size is shown. (C) DAPI staining and measurement of the E15.5 cortex: ventricular/sub-ventricular zone (VZ/SVZ); intermediate zone (IZ); and cortical plate (CP). (D) Cortex coronal sections with immunolabeling of VZ/SVZ stem/progenitor cells (PAX6⁺), intermediate progenitor cells (TBR2⁺), and early born, deep-layer cortical neurons (TBR1⁺). (E) Immunolabeling of phospho-H3 and cleaved-CASPASE 3 (c-CASP3) cells on cortex coronal sections. lv, lateral ventricles. Error bars represent SEM of three independent experiments comprising in total five Mcm4^+/+ and four Mcm4^C/C embryos at E13.5 and seven Mcm4^+/+ and seven Mcm4^C/C embryos at E15.5. Two-tailed t test: ns; p < 0.05 (^∗); p < 0.005 (^∗∗); p < 0.0005 (^∗∗∗). (F) Model. Black and orange colors indicate the conditions of the wild-type and the partial depletion of DOs (as in the Mcm4^C/C mice), respectively. In the wild-type ESCs, DOs initiate back-up replication forks to rescue fork stalling and maintain genome integrity. ESCs possess a greater number of DOs than NSPCs. Upon reduction of DO, there is a further reduction of DOs in the NSPCs, likely reaching the threshold required to rescue the endogenous fork stalling during DNA replication. As a result, DNA damage is accumulated and cell death incurs, eventually impairing NSPC proliferation and differentiation. This explains the neurogenic defect in the Mcm4^C/C embryos, which could contribute to the semi-embryonic lethality of the Mcm4^C/C mice. See also Figure S4.