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. 2016 Aug 22;38(4):399-412.
doi: 10.1016/j.devcel.2016.07.023.

Waves of Cdk1 Activity in S Phase Synchronize the Cell Cycle in Drosophila Embryos

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Free PMC article

Waves of Cdk1 Activity in S Phase Synchronize the Cell Cycle in Drosophila Embryos

Victoria E Deneke et al. Dev Cell. .
Free PMC article

Abstract

Embryos of most metazoans undergo rapid and synchronous cell cycles following fertilization. While diffusion is too slow for synchronization of mitosis across large spatial scales, waves of Cdk1 activity represent a possible process of synchronization. However, the mechanisms regulating Cdk1 waves during embryonic development remain poorly understood. Using biosensors of Cdk1 and Chk1 activities, we dissect the regulation of Cdk1 waves in the Drosophila syncytial blastoderm. We show that Cdk1 waves are not controlled by the mitotic switch but by a double-negative feedback between Cdk1 and Chk1. Using mathematical modeling and surgical ligations, we demonstrate a fundamental distinction between S phase Cdk1 waves, which propagate as active trigger waves in an excitable medium, and mitotic Cdk1 waves, which propagate as passive phase waves. Our findings show that in Drosophila embryos, Cdk1 positive feedback serves primarily to ensure the rapid onset of mitosis, while wave propagation is regulated by S phase events.

Figures

Figure 1
Figure 1. Cdk1 waves drive mitotic waves
(A) Time lapse series depicting the propagation of a mitotic wave in an embryo with RFP-tagged histones. (B) Schematic view of the Cdk1 FRET biosensor composed of two fluorophores, YPet and mCerulean, which are linked by a Cdk1 phosphorylation site from cyclin B1 (S126) and the polo box domain of Plk1. Upon phosphorylation of the Cdk1-specific phosphorylation site, the sensor undergoes a conformational change that results in increased FRET efficiency. (C) Emission ratio of FRET sensor averaged across one embryo shows clear oscillations of Cdk1 activity, which increases upon mitotic entry and remains low during early interphase 14, when the levels of active Cdk1 are uniformly low. Red line, S126A mutant; red dotted line, average anaphase entry time. (D) Cdk1 traveling waves can be visualized by plotting the activity profiles as a function of space and time. First, images are divided into different regions along the anterior-posterior axis of the embryo (colored boxes). Activity profiles for each region are then calculated and plotted as a function of time and space, which allows for the visualization of the wave front (dotted line). (E) Heat-map of Cdk1 activity over time and along the anterior-posterior (AP)-axis of an embryo. White circles indicate wave origins at cycle 13, dotted line indicates the mitotic entry front and the black line indicates the anaphase wave front. (F) Cdk1 activity profiles for different positions along the AP-axis of an embryo in cell cycle 13. (G) The speed of the wave is estimated by computing the time elapsed for the wave to travel a given distance along the embryo: the inverse slope of a linear fit yields the speed. (H) Mitotic wave speed as a function of Cdk1 wave speed. Dotted line, identity line; solid line, best-fit curve. Scale-bars, 10μm. Error bars, 95% confidence interval (CI); a.u., arbitrary units. See also Figure S1 and Movies S1 and S2.
Figure 2
Figure 2. Cdk1 activity during S-phase is predictive of mitotic and Cdk1 wave speed
(A) Cdk1 wave speed as a function of developmental cycles. Red line, mean; gray box, 95% CI; red box, 1 standard deviation (SD). (B) Normalized fluorescence intensity profile of YFP-tagged Cdk1 before and after photobleaching. Inset, calculated diffusion coefficient per cell cycle. Error bars, standard error of the mean (s.e.m.) (C) Emission ratio of Cdk1 sensor for cycle 13 displays biphasic behavior. Red dotted line corresponds to Cdk1 activation rate during S-phase (kS) and blue dotted line corresponds to Cdk1 activation rate during mitosis (kM). Completion of S-phase was determined through the disappearance of RFP-tagged PCNA foci, see Supplemental Information for details. (D) Average Cdk1 activation rate per cell cycle. Red bars, S-phase Cdk1 activation rate (kS); blue bars, mitotic Cdk1 activation rate (kM). Error bars, s.e.m. (E) Nuclear and cytoplasmic Cdk1 activities were calculated by segmenting nuclei with an intensity threshold and averaging intensity inside and outside nuclear mask, respectively. Dotted line, line with a slope given by the ratio of nuclear Cdk1 levels to cytoplasmic Cdk1 levels, measured using embryos expressing Cdk1-YFP; solid line, best-fit curve. Error bars, 95% CI. (F) Log-log plot of Cdk1 speed versus S-phase Cdk1 activation rate. Solid line, best-fit curve; dotted line, Luther’s formula. Error bars, 95% CI. For all graphs, *, p<0.05; **, p<0.001; ***, p<0.0001; ns, not significant. See also Figure S2.
Figure 3
Figure 3. Cdk1 waves are dependent on the Chk1/Wee1 pathway and on DNA content
(A-B) Emission ratio of Cdk1 sensor for wildtype embryos (black line), chk1 chk2 embryos (A, green line), and wee1 embryos (B, red line). (C-D) Average Cdk1 activation rate per cell cycle for wildtype (black), chk1 chk2 (C, green) embryos, and wee1 embryos (D, red). Error bars, s.e.m. (E) Log-log plot of Cdk1 speed versus S-phase Cdk1 activation rate for wildtype (black), chk1 chk2 (green), and wee1 (red) embryos. Error bars, 95% CI. (F) S-phase Cdk1 activation rate as a function of DNA content. Inset, cell cycle period, marked from anaphase start time of one cycle to the next, as a function of DNA content. 100% indicates the DNA content of wild type embryos at cycle 13. Error bars, s.e.m. For all graphs, ***, p<0.0001; ns, not significant.
Figure 4
Figure 4. A Cdk1/Chk1 double negative feedback controls Cdk1 waves
(A) Schematic view of Chk1 localization sensor. The cytoplasmic to nuclear ratio provides a readout of Chk1 activity. (B) Cytoplasmic to nuclear intensity ratio of the Chk1 sensor in wild type (black), mutant sensor (S216A; red line), and chk1 chk2 mutants (blue line) for cycles 11–13. In order to compare cell cycles of similar durations, we used cycle 15 (instead of cycle 13) for chk1 chk2 mutants. Gray shaded box represents mitosis, when the absence of nuclear envelope precludes a reliable measure of the C/N ratio. (C) C/N ratio for a cycle 12 wild type and an embryo injected with Chk1-CA mRNA. (D) Average Cdk1 and Chk1 activities at cycle 13 measured in two different embryos. Dotted line, completion of S-phase. (E) Heat-map of Chk1 activity over time and along the anterior-posterior (AP)-axis of an embryo. Black line, Chk1 inactivation wave front. (F) Time of entry into mitosis (TM) and anaphase start time (TA) as a function of the time of completion of S-phase (TS) (black line) and Chk1 inactivation (red line). Slopes: 1.0±0.1. See also Figure S3 and Movie S3.
Figure 5
Figure 5. Mitotic switch ensures rapid activation of Cdk1 during mitosis but does not regulate S-phase Cdk1 activation rate
(A) Emission ratio of Cdk1 sensor for wildtype embryos (black line) and twine-3A wee1-9A embryos (red line). (B) Average Cdk1 activation rate per cell cycle for wildtype (black) and twine-3A wee1-9A (red) embryos. Error bars, s.e.m., ***, p<0.0001; ns, not significant. (C) Log-log plot of Cdk1 speed versus S-phase Cdk1 activation rate for wildtype (black) and twine-3A wee1-9A (red) embryos. Error bars, 95% CI. Inset, Cdk1 wave speed of wildtype (black) and twine-3A wee1-9A (red) embryos at cycle 13. Error bars, s.e.m. See also Figure S4 and Movie S4.
Figure 6
Figure 6. A mathematical model of Cdk1 activity captures the scaling of the speed with S-phase activity
(A) Top: phase plane diagram depicting stable fixed points (filled circles) and unstable fixed points (open circles). Bottom: schematic view of the Cdk1 network. (B) Cdk1 (solid line) and Chk1 (dotted line) activity profiles for cycles 10–13, as obtained from the mathematical model described in the main text and Supplemental Information. (C) Phase plane analysis for different time points in cycle 11 (left) and 13 (right). The open circles denote unstable fixed points and filled circle denote stable fixed points. Notice that for both cycles bistability is observed transiently and for early time points, plots extend beyond cyclin-Cdk1 concentration. (D) Log-log plot of Cdk1 speed versus S-phase Cdk1 activation rate for experimental data (black) and simulated data (red). (E) Log-log plot of Cdk1 speed versus S-phase Cdk1 activation rate for simulations of the wild type (black) and simulations of the twe3A wee19A mutants (purple). (F) Plot of the effective bistable potential appearing in the model (calculated at the time when Cdk1 transitions from the low to the high state). Arrows indicate the kinetic barrier. All error bars, 95% CI. See also Figure S5.
Figure 7
Figure 7. Mitotic waves are kinematic phase waves
(A) Schematic of the difference between trigger waves and phase waves. Trigger waves involve the transport or diffusion (red line) of material from one neighboring region to another, and depend on positive feedback loops (black circular arrow), which ensure the rapid production of the diffusing species. Phase waves are kinematic phase waves that reflect a delay in timing between neighboring regions (bottom panel). (B) The difference between a trigger wave and a phase wave becomes evident when an impermeable barrier is introduced. A trigger wave is blocked when a barrier is present (top panel), whereas a phase wave is unaffected (bottom panel). (C) Schematic of our prediction that synchronization of mitosis should be disrupted if an impermeable barrier is introduced during S-phase (prior to the S-phase Cdk1 trigger wave) and be unaffected if a barrier is introduced during M-phase. (D-E) Heat-map of Cdk1 activity through time and across the anterior-posterior (AP)-axis of an embryo ligated during S-phase (D) or M-phase (E), respectively. Gray box, ligation barrier; dotted line, prophase wave front; solid line, anaphase wave front. The continuity in (E) of the lines on the two sides of the barrier demonstrates that mitotic waves are indeed phase waves. (F) Anaphase delay (computed as the difference in anaphase time on the two sides of the barrier) caused by the introduction of a barrier in S-phase or M-phase. Error bars, s.e.m. See also Figure S6 and Movies S5 and S6.

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