Nucleocytoplasmic Properties Shape Cell Cycle Oscillations and Transitions in Early Xenopus Development

Abstract

Cell cycle regulation is fundamental to proper development and cellular function, yet how cells maintain robust cell cycle progression while responding appropriately to developmental cues remains poorly understood. Using Xenopus laevis egg extract-based artificial cell systems, this dissertation has revealed that nucleocytoplasmic properties shape cell cycle oscillations and transitions in early Xenopus development. First, we demonstrated that embryonic cell cycle oscillations exhibited remarkable robustness across a wide range of cytoplasmic densities (0.2–1.22× RCD), with sharp but reversible transitions to arrest states beyond these boundaries. At concentrated extremes, we discovered a novel hysteretic response where different thresholds governed transitions between oscillating and arrested states depending on the system’s history. Based on these findings, we developed a mathematical model that recapitulated these observations and suggested that robustness emerged from the scaling of biochemical reaction rates with cytoplasmic density. Second, we investigated the mechanisms regulating cell cycle lengthening at the mid-blastula transition (MBT) during early Xenopus development. Through systematic manipulation of nuclear size, DNA content, and droplet size, we found that the DNA/cytoplasmic ratio, rather than absolute DNA content or N/C volume ratio, acted as the critical parameter for the timing of cell cycle lengthening at MBT. This regulation operated through Chk1-dependent pathways and functioned independently of zygotic transcription activation. Together, these findings revealed how cell cycle networks differentially responded to distinct cues across developmental phases. This work also demonstrated the power of artificial cell systems in dissecting complex cellular processes.