Both Chromosome Decondensation and Condensation Are Dependent on DNA Replication in C. elegans Embryos

Abstract

During cell division, chromatin alternates between a condensed state to facilitate chromosome segregation and a decondensed form when DNA replicates. In most tissues, S phase and mitosis are separated by defined G1 and G2 gap phases, but early embryogenesis involves rapid oscillations between replication and mitosis. Using Caenorhabditis elegans embryos as a model system, we show that chromosome condensation and condensin II concentration on chromosomal axes require replicated DNA. In addition, we found that, during late telophase, replication initiates on condensed chromosomes and promotes the rapid decondensation of the chromatin. Upon replication initiation, the CDC-45-MCM-GINS (CMG) DNA helicase drives the release of condensin I complexes from chromatin and the activation or displacement of inactive MCM-2-7 complexes, which together with the nucleoporin MEL-28/ELYS tethers condensed chromatin to the nuclear envelope, thereby promoting chromatin decondensation. Our results show how, in an early embryo, the chromosome-condensation cycle is functionally linked with DNA replication.

Graphical abstract

Figure 1

C. elegans Chromatin Dynamics (A) Chromatin dynamics in relation to the nuclear envelope cycle. Images taken from a time-lapse sequence of an embryo expressing GFP-NPP-9 (top row) and mCherry-H2B (bottom row) progressing from the second meiotic division to the first embryonic cell cycle. The timeline below indicates the duration of cell-cycle stages. (B) Chromatin dynamics and MCM-2–7. Images from wild-type and mcm-7 RNAi embryos expressing GFP-MCM-3 (top rows) and mCherry-H2B (bottom rows). (C) EdU incorporation during the first embryonic S phase. Permeabilized embryos pulsed with EdU from meiosis II to the first embryonic mitosis. EdU uptake (top images) and DNA (bottom images) determined during the first mitosis. (D) Quantification of EdU incorporation for ten embryos. Error bars represent SD. Scale bars represent 5 μm (A and B) and 20 μm (C). See Movies S1 and S2 for embryos shown in (A) and (B).

Figure 2

Chromatin Condensation Requires Duplicated DNA (A and G) EdU incorporation assayed during the first S phase. (A) Permeabilized embryos pulsed with EdU for 30 min and stained at mitosis for EdU (left images) and DNA (right images, Hoechst). (B) Visualization of chromosome condensation during the first mitosis. Images taken from time-lapse sequences of wild-type, cdt-1, mcm-7, cdc-45, rpa-1, pcn-1, rnr-1, and smc-4 RNAi embryos expressing GFP-H2B. (C) Visualization of chromosome condensation during prophase. Wild-type, cdt-1, mcm-7, cdc-45, rpa-1, rnr-1, and smc-4 RNAi embryos stained for phospho-H3S10 (red), nuclear pores (mAb414, green), and DNA (blue). Chromatin imaged with a higher magnification is shown on the left. (D) Visualization of the condensin II subunit KLE-2. Images taken from time-lapse sequences of wild-type and cdc-45 RNAi embryos expressing GFP-KLE-2 (top images) and mCherry-H2B (bottom images). (E and F) Visualization of HCP-6. Wild-type and cdc-45 RNAi embryos stained for HCP-6 (red), nuclear pores (mAb414 green) and DNA (blue). Magnified images of nuclei are shown on the left. (E) Late prophase. (F) Metaphase. (G) Quantification of EdU incorporation for ten embryos prepared as in (A). (H) Quantification of GFP-H2B on chromatin during mitosis from five embryos prepared as in (B). (I) Duration between the meeting of male and female nuclei and nuclear envelope breakdown (NEB) from five embryos prepared as in (B). Scale bars represent 20 μm (A) and 5 μm (B–E). Error bars represent SD. See Movies S3 and S4 for embryos shown in (B) and in (D).

Figure 3

Graded Depletion of Chromatin Bound MCM-2–7 (A) Images taken from time-lapse sequences upon mcm-7 RNAi titration in embryos expressing GFP-MCM-3 (upper images) and mCherry-H2B (lower images) during prophase (upper panel) and anaphase of mitosis (lower panel). The percentage of mcm-7 RNAi expressing bacteria relative to bacteria containing an empty vector is shown. (B) Quantification of average chromatin bound GFP-MCM-3 1 min after anaphase onset (n = 3). (C) Average time elapsing between the meeting of nuclei and NEB (black bars) and the number of mitoses with anaphase bridges (white bars) (n = 3). Stronger staining of the mCherry-H2B reporter in the female chromatin as compared to the male chromatin, allowed to separately score anaphase bridges on female and male chromatins. Embryos with anaphase bridges on female and male chromatin scored 1, while embryos with anaphase bridges only on male chromatin scored 0.5. Scale bars are 5 μm. Error bars represent SD.

Figure 4

DNA Replication Initiates on Condensed Chromosomes (A) Timing of DNA replication initiation. Projections of 15 z sections spaced by 0.5 μm (left panel) showing EdU (left image) and DNA (Hoechst, right image). Permeabilized embryos pulsed for 10 min with EdU were stained. Red arrow indicates the female nucleus. Yellow arrow indicated the male nucleus. Yellow asterisks indicate polar bodies. Magnification of three Z-sections (right panel) showing EdU (top images) and DNA (bottom images) of the female nucleus. (B and C) GFP-CDC-45 binds chromatin during decondensation. Images taken from time-lapse sequences of embryos expressing GFP-CDC-45 (upper images) and mCherry-H2B (lower images) during chromatin decondensation of the female nucleus. (B) Wild-type. (C) mcm-7 RNAi. (D) Permeabilized embryos pulsed for 5 min with EdU and stained during anaphase of the first mitosis (left panel) and S phase of the second cell cycle (right panel). Top images are EdU, and bottom images are Hoechst. (E and F) Images taken from time-lapse sequences of embryos expressing GFP-CDC-45 (upper images) and mCherry-H2B (lower images) during chromatin decondensation at the second cell cycle. (E) Wild-type. (F) mcm-7 RNAi. (G) GFP-CDC-45 chromatin binding requires nuclear envelope formation. Images of wild-type (upper panel) and npp-8 RNAi (lower panel) embryos expressing GFP-CDC-45 (upper images) and mCherry-H2B (lower images) during meiosis II and S phase. (H) Chromatin decondensation requires CDC-45. Images of wild-type (upper panel) and cdc-45 RNAi (lower panel) embryos expressing GFP-MCM-3 (upper images) and mCherry-H2B (lower images) during meiosis II and S phase. Scale bars represent 20 μm (A) and 5 μm (B–H). Time is expressed in minutes and seconds relative to anaphase onset.

Figure 5

CMG Helicase Promotes the Rapid Chromatin Decondensation (A) Images taken from time-lapse sequences of wild-type, cdc-45, mcm-7, cdt-1, and rpa-1 RNAi embryos expressing GFP-NPP-9 (upper images) and mCherry-H2B (lower images) during the first embryonic S phase. (B) Images of wild-type, cdc-45, mcm-7, cdt-1, and rpa-1 RNAi embryos during early S phase. Embryos were stained for phospho-H3S10 (red), nuclear pore (mAb414, green), and DNA (blue). Magnifications of the female and male nuclei are shown on the left. Scale bars represent 5 μm (A) and 20 μm (B). See Movie S5 for embryos shown in (A).

Figure 6

CMG Helicase Triggers Condensin Release from Chromatin (A) Images from wild-type, cdc-45, and mcm-7 RNAi embryos expressing GFP-CAPG-1 (top images) and mCherry-H2B (bottom images) during the first embryonic S phase. (B) Images from cdc-45 single, cdc-45; smc-4 double, mcm-7 single, mcm-7; smc-4 double and smc-4 single RNAi embryos expressing GFP-CAPG-1 (top images) and mCherry-H2B (bottom images) during the first embryonic S phase. (C and D) Quantification of mCherry-H2B (C) or GFP-CAPG-1 (D) chromatin enrichment at the first mitosis averaged from five embryos. Error bars represent SD. See Movie S6 for embryos shown in (B).

Figure 7

Inactive MCM-2–7 and MEL-28 Prevent the Rapid Chromatin Decondensation (A) GFP-MCM-3 binding to chromatin before and during the licensing period. Images taken from time-lapse sequences of wild-type, cdc-45, rpa-1, and rnr-1 RNAi embryos expressing GFP-MCM-3 (left images) and mCherry-H2B (right images) during metaphase (left panel) and anaphase (right panel) of the first embryonic cell cycle. (B) Images from a cdc-45 RNAi embryo expressing GFP-MCM-3 and mCherry-H2B during the first embryonic cell cycle. The female DNA (red arrows) was photobleached during early S phase (left panel). GFP-MCM-3 gets imported in nuclei (middle panel). Recovery of GFP-MCM-3 was determined during metaphase (right panel). Male chromatin (yellow arrows) served as “unbleached control.” (C) mcm-7 inactivation partially rescues the cdc-45 decondensation phenotype. Images taken from time-lapse sequences upon double inactivation of cdc-45; mcm-7 and cdc-45; par-2 in embryos expressing GFP-NPP-9 (upper images) and mCherry-H2B (lower images) during the first embryonic S phase. (D) mel-28 inactivation rescue the cdc-45 decondensation phenotype. Images of wild-type, mel-28, cdc-45, and double cdc-45; mel-28 RNAi embryos during early S phase. Embryos were stained for MEL-28 (red), nuclear pore (mAb414, green), and DNA (blue). Magnifications of the female and male nuclei are shown on the left. (E) Images from cdc-45 and double cdc-45; mel-28 RNAi embryos expressing GFP-NPP-9 (top rows) and mCherry-H2B (bottom rows) during the first embryonic S phase. (F) Model for the effect of DNA replication on chromatin structure (see Discussion). Scale bars represent 5 μm (A and B, right panel, and C and E) and 20 μm (B, left panel, and D). Time is expressed in minutes and seconds relative to anaphase II onset. See Movies S7, S8, S9, and S10 for embryos shown in (A)–(C) and (E).