(623f) Intracellular Cell Cycle Model of the Embryonic Stem Cell G1-S Transit During Self-Renewal and Differentiation

Embryonic stem cells (ESC) exhibit numerous unique traits which allow them to maintain self-renewal and differentiation potential. The cell cycle is one important trait showing vastly different behavior between ESC and adult cells. In particular, the G1-S checkpoint, a main regulator maintaining proper growth of somatic cells, is structured differently in ESC. Because ESC are characterized by a shortened G1 phase compared to mature cells, this checkpoint must give limited hindrance to transit to S phase while also being adaptable enough to allow lengthening of the G1 phase during differentiation . Although these unique cell cycle traits are central to ESC, limited work has been done in mechanistically describing this checkpoint during the transition from undifferentiated to mature cells. While mechanistic information for this complex, non-linear behavior would be difficult to obtain with a purely experimental approach, mathematical modeling can aid in obtaining a more comprehensive understanding of the system.  In the current work, we develop a mathematical model to describe the unique stem cell behavior during G1-S transit and during differentiation. 

Molecular dynamics are a major factor in governing cycle phase transitions in individual cells. Hence, an intracellular protein network model was developed to describe hESC at the single cell level. To develop our single-cell model, we employ a system of ordinary differential equations (ODE) to describe molecular dynamics during the G1 phase and G1-S transition. The system of ODE consist of 17 state variables representing key cell cycle molecules governing cell cycle dynamics and hESC cycle behavior, including cyclins, CDK inhibitors (CKI), and Rb/E2F. A model of adult G1-S transit was used as a base model, with parameter estimation and structure modifications included to capture characteristics/signatures unique to the hESC system. A major inclusion to the model is miRNA, with multiple miRNA complexes being combined and associated with inhibition of cyclin D and CKI.  

Our ODE model is able to capture the unique hESC G1-S transition, including shortened G1 time, which gradually lengthens, but does not arrest, upon differentiation. hESC molecular dynamics are appropriately captured, including low levels of CKI and cyclin D. Inclusion of miRNA regulation is necessary to capture the system’s unique behavior. miRNA levels remain elevated during pluripotency and decrease upon differentiation (as shown by miRNA qPCR data), leading to increased cyclin D and CKI. These latter two molecules perform antagonistic roles (promoting and inhibiting G1-S transition, respectively), thereby leading to a “balancing act” during differentiation which helps G1 lengthen but not arrest. At a critical threshold, the CKI remain elevated, leading to growth arrest and terminal differentiation. 

The developed ODE model can accurately describe hESC cycle behavior at the single-cell level. This model of the G1-S restriction point can mechanistically explain cell cycle observations made at the population level. These non-linear molecular dynamics of the ESC cell cycle cannot be fully comprehended without computational support, and it has been shown herein that mathematical modeling of the system extracts more information than would be possible with an experimental approach alone.