Sizing up the cell cycle: systems and quantitative approaches in Chlamydomonas

Abstract

The unicellular green alga Chlamydomonas provides a simplified model for defining core cell cycle functions conserved in the green lineage and for understanding multiple fission, a common cell cycle variation found in many algae. Systems-level approaches including a recent groundbreaking screen for conditional lethal cell cycle mutants and genome-wide transcriptome analyses are revealing the complex relationships among cell cycle regulators and helping define roles for CDKA/CDK1 and CDKB, the latter of which is unique to the green lineage and plays a central role in mitotic regulation. Genetic screens and quantitative single-cell analyses have provided insight into cell-size control during multiple fission including the identification of a candidate `sizer' protein. Quantitative single-cell tracking and modeling are promising approaches for gaining additional insight into regulation of cellular and subcellular scaling during the Chlamydomonas cell cycle.

Figure 1

Regulation of the multiple fission cell cycle. (a) Schematic of key stages in multiple fission with cell growth (G1) followed by alternating rounds of S phase and mitosis/cytokinesis (S/M) to produce 2ⁿ daughters (four daughters pictured here), that hatch upon mitotic exit and reenter G0 or G1. Commitment is described in the main text. (b) Upper panels show a possible framework for the multiple fission cell cycle with major regulatory activities CDKA-CYCA, CDKB-CYCB, the CDC20-activated anaphase promoting complex (APC^CDC20), an E3 ubiquitin ligase which is unstable due to CDC20 itself being a APC substrate, and APC^CDH1 whose activator subunit, CDH1, is not a APC substrate [1]. Arrows indicate positive regulation, and repression bars indicate inhibition. Question marks indicate inferred relationships. Dark gray arrows pointing downward show processes that are thought to be promoted by each of the major regulators in the upper panel. (c) Cell-cycle controlled genes or groups of genes, their relative expression timing, and their CDKA or CDKB dependencies are listed in the left panel. The inferred regulatory structure of cell cycle transcription controlled by CDKs is diagrammed, with green arrows and plus signs showing positive regulation, and with red repression bars minus signs showing negative regulation. CDKA-CYCA forms a positive feedback loop via CYCA transcription and a feed forward loop that helps activate CDKB-CYCB. CDKB-CYCB activity negatively influences early cell cycle gene transcription, but this may be through indirect mechanisms.

Figure 2

Cell size control and the RBR complex. (a) Graph showing log-normal distributions of mother cells (solid lines) and their resulting daughters (dashed lines) from cultures with large (black) or small (gray) mother cells. Stochastic variation results in outliers that are larger or smaller than the idealized theoretical two-fold size range. (b) Example of stochastic behaviors of a mother cell that should divide two times to produce four daughters in the target size-range. Exiting S/M after only one division produces two large daughters, while exiting after three divisions produces eight small daughters. Note that within an individual mother cell there is little or no stochastic variation in division behavior between daughter pairs in the first or subsequent divisions, so daughter number is nearly always a power of two. (c) Schematic similar to that in Figure 1a showing the RBR complex and CDKG1 influencing size control at Commitment and during S/M. It is unknown how RBR in Chlamydomonas interfaces with the other cell cycle regulatory machinery depicted in Figure 1b. The GEX network influences entry into S/M phase, but may also operate by activating the Commitment step. (d) Adapted from [50]. CDKG1 levels scale with mother cell size and are limiting for cell division number. Scale bar depicts ratios of nuclear localized CDKG1:DNA in mother cells and its decrease in daughters with each subsequent cell division. When this ratio falls below a threshold cells exit S/M and return to a G0 or G1 state.