(680f) Circadian Gating of the Mammalian Cell Cycle Restriction Point: A Mathematical Analysis

Reliable decision making triggered by dynamic extracellular signals is crucial for proper functioning of mammalian cells. Exploring network motifs of such decision making systems is necessary for understanding fundamental design principles, analyzing system dynamics and revealing possible system vulnerabilities. Control of cell cycle entry under circadian humoral epithelial growth factor (EGF) fluctuations plays a critical role in development, stem cell and progenitor renewal, wound recovery and carcinogenesis. Among the most studied epithelial growth factor receptors (EGFRs), EGF receptor 1 (EGFR1) and human EGF receptor 2 (HER2) initiate the cell cycle, sustain cell survival and contribute to various types of cancers when overexpressed or improperly regulated. Over the last several decades, numerous experimental and modeling studies illustrating EGFR signaling mechanisms and downstream events at the molecular level have been reported. Once thought to be highly entangled, EGFR signaling networks are now known to be spatially and temporally separated into tunable modules.

Humoral variations of EGF levels are sensed by the EGFR system though EGF/EGFR binding. Phosphorylation of EGFR homo- and hetero-dimers occurs within minutes, and the resulting signal rapidly moves forward towards the restriction point (R-point) control machinery. The R-point is underlaid by the bi-stable Rb-E2F switch, a mutual inhibitory lock between the growth-suppressing retinoblastoma protein (Rb) and growth-promoting E2F transcription factors. If the R-point is successfully transitioned, the cell becomes committed to entering the cell cycle and retraction of growth factor has no effect on cell fate. Circadian control of cell cycle entry via EGFRs and other growth factor receptor signaling pathways is crucial for functional coordination of organs and suppression of cancer development. The temporal distribution of cell cycle phases shows strong, tissue specific preferences for specific circadian time windows. Carcinogenesis is often characterized by cell cycle progression gradually losing its synchronization to circadian rhythms. Understanding how circadian signals coordinate cell cycle phases to specific circadian times has potential applications in chronotherapy, chemotherapy and growth factor therapy for cancer treatment. However, in silico studies focusing on the effects of external growth factor signals on circadian control of the R-point have not yet been reported.

The goal of this study was to computationally investigate circadian gating of the R-point by coupling an existing molecular model of the core circadian clock to a new, minimal model of the EGFR-ERK signaling system and the R-point switch. Our model was based on the hypothesis that the core clock regulates the global EGF level and coordinates local EGF secretion in different tissues such that the resultant rhythmic EGF fluctuations regulate entry into the cell cycle via EGFR signaling. We established an information flow based minimal model to systematically analyze regulation mechanisms, circadian gating of the R-point under subtle, physiological circadian EGF fluctuations, and the effects of naive (fast and quickly attenuated) and endosomatic (slow and sustained) signaling pathways on system robustness. Our goal was not to develop a detailed molecular model but rather to emphasize essential system elements and to reveal design principles of the growth factor receptor and R-point switch gating machinery. More generally, we systematically studied for the first time a robust decision making system termed the Differential Adaptation Decision Gate (DADG) element. A novel feature of this study was that we combined differential signal adaptation and fast/slow dual signaling pathways with decision gates and switches to reveal fundamental design principles of cell cycle control and circadian gating of cell cycle entry.