Circadian Clock Regulates Cell Division Cycles

Researchers at the University of Cincinnati (UC) have identified key molecules linking circadian rhythms and cell division cycles, providing insights that could influence disease treatments and drug delivery.

The researchers in the UC College of Medicine Department of Molecular and Cellular Physiology, led by Christian Hong, PhD, published their findings in the journal PNAS (Proceedings of the National Academy of Sciences).

"Our work helps us understand the connection between the cell cycle and the circadian clock,” says Hong, an assistant professor in the molecular and cellular physiology department who collaborated with an international team of researchers on the project.

---

The circadian rhythm, often referred to as the biological clock, is a cycle of biological activity based on a 24-hour period and generated by an internal clock synchronized to light and dark cycles — and other external cues.

"Everything has a schedule, and we are interested in understanding these schedules at a molecular level. We also wanted to know the components that connect two different oscillators (the circadian clock and cell division, or mitosis).

"Cell divisions happen during a certain time of day, and are molecularly regulated by circadian rhythms.”

Christian Hong, PhD, University of Cincinnati College of Medicine Department of Molecular and Cellular Physiology

---

Using the thread-like fungi Neurospora crassa, researchers tested the amount of coupling between the cell cycle and the circadian clock using mathematic models they generated. Using these models, they were able to demonstrate that the same mechanism is constant in Neurospora that is also constant in mammals and results in circadian clock regulated mitotic cycles. These results are more than likely due to the similarities in gene structure - serine/threonine protein kinase-29 (STK-29) in the Neurospora fungi, and the mammalian WEE1 kinase DNA sequence.

Additionally, the researchers conducted phase-shift experiments in which they transferred Neurospora into constant darkness, then administered 90-minute pulses of white fluorescent light at specific points in time to induce phase-shift.

"We were able to show that when we phase-shift the circadian clock, we also observe phase-shifting in cell cycle components,” Hong says.

---

Building on experimentally validated mathematic models created from Neurospora, researchers will be able to make predictions not only in other strains of Neurospora, but in mammalian cells as well.

---

As Hong puts it, "This discovery will serve as a stepping stone for further investigations to uncover conserved principles of coupled mechanisms between the cell cycle and circadian rhythms.”

Significance
Circadian rhythms provide temporal information to other cellular processes, such as metabolism. We investigate the coupling between the cell cycle and the circadian clock using mathematical modeling and experimentally validate model-driven predictions with a model filamentous fungus, Neurospora crassa. We demonstrate a conserved coupling mechanism between the cell cycle and the circadian clock in Neurospora as in mammals, which results in circadian clock-gated mitotic cycles. Furthermore, we observe circadian clock-dependent phase shifts of G1 and G2 cyclins, which may alter the timing of divisions. Our work has large implications for the general understanding of the connection between the cell cycle and the circadian clock.

Abstract
The cell cycle and the circadian clock communicate with each other, resulting in circadian-gated cell division cycles. Alterations in this network may lead to diseases such as cancer. Therefore, it is critical to identify molecular components that connect these two oscillators. However, molecular mechanisms between the clock and the cell cycle remain largely unknown. A model filamentous fungus, Neurospora crassa, is a multinucleate system used to elucidate molecular mechanisms of circadian rhythms, but not used to investigate the molecular coupling between these two oscillators. In this report, we show that a conserved coupling between the circadian clock and the cell cycle exists via serine/threonine protein kinase-29 (STK-29), the Neurospora homolog of mammalian WEE1 kinase. Based on this finding, we established a mathematical model that predicts circadian oscillations of cell cycle components and circadian clock-dependent synchronized nuclear divisions. We experimentally demonstrate that G1 and G2 cyclins, CLN-1 and CLB-1, respectively, oscillate in a circadian manner with bioluminescence reporters. The oscillations of clb-1 and stk-29 gene expression are abolished in a circadian arrhythmic frqko mutant. Additionally, we show the light-induced phase shifts of a core circadian component, frq, as well as the gene expression of the cell cycle components clb-1 and stk-29, which may alter the timing of divisions. We then used a histone hH1-GFP reporter to observe nuclear divisions over time, and show that a large number of nuclear divisions occur in the evening. Our findings demonstrate the circadian clock-dependent molecular dynamics of cell cycle components that result in synchronized nuclear divisions in Neurospora.

Author contributions: C.I.H. and A.C.-N. designed research; J.Z., M.B., L.L., K.J., H.L., L.F.L., A.G., H.S.C., and W.J.B. performed research; J.Z. performed mathematical modeling; C.I.H., J.Z., and A.C.-N. analyzed data; and C.I.H., J.Z., L.F.L., W.J.B., and A.C.-N. wrote the paper.

The authors declare no conflict of interest.

Funding for Hong’s research was provided by a four-year, $3.7 million grant from the Defense Advanced Research Projects Agency (DARPA), an agency of the U.S. Department of Defense. He also received startup funds from UC’s molecular and cellular physiology department.

Return to top of page