Circadian Rhythm and Cell Cycle: two Synchronized Processes

Circadian rhythms are biological processes found in most living organisms, displaying a roughly 24-hour period, responding primarily to light darkness cycles in an organism’s environment. At the cellular level, the circa-24h rhythmicity is generated by a molecular clock based on a transcription-translation feedback network. The circadian clock temporally orchestrates many aspects of cellular physiology and consists of a cell-autonomous and self-sustained oscillator with a period of about 24 hours. In conditions where cells proliferate, the cell division cycle can be also considered as an oscillator. Since both processes run with similar periods in several mammalian cells, it is reasonable to expect that interactions between these two cycles may cause synchronization. Many studies reported evidences of interactions between the circadian and the cell cycles in different organisms. In particular, it appeared that in several systems, specific cell cycle phases occur in distinct temporal windows rather than being randomly distributed in time. These findings led to the concept of circadian gating of the cell cycle, through which the circadian clock can favor or forbid cell cycle transitions at specific circadian phases. However, it was also reported the converse, namely an effect of cell division on the circadian oscillator. Even though interactions between the circadian clock and the cell cycle have been identified in both directions, the dynamical consequences and the directionality of the coupling at the single-cell level were not extensively investigated. In order to better characterize the potential synchronization in mammalian cells, we estimated the mutual interactions between circadian clock and cell cycle in NIH3T3 mouse fibroblasts by the use of time-lapse fluorescent microscopy in combination with statistical analysis and mathematical modeling. NIH3T3 cells, harboring a fluorescent reporter under the control of the circadian Rev-Erbα gene promoter, were imaged for several days allowing the simultaneous detection of circadian oscillations and time of divisions. The analysis of thousands of circadian cycles in dividing cells indicated that both oscillators are synchronized, with cell divisions occurring about 5 h before the peak of the circadian Rev-Erbα -reporter. In order to further analyse the observed interaction between the circadian clock and the cell cycle we tested several perturbations such as different serum concentrations, different temperatures, treatment with pharmacological compounds and shRNAmediated knockdown of circadian regulators. Surprisingly, this showed that circadian rhythm and cell cycle remain synchronized over the wide range of conditions probed. Our data combined with stochastic modeling showed that this synchronization state reflects an unexpected predominant influence of the cell cycle on the circadian oscillator, and did not support the leading hypothesis about a