The combination of mathematical modeling and experiments identifies the difference in the molecular clockwork of master and slave clock neurons in Drosophila. This solves the long-standing mystery of the molecular mechanisms responsible for the circadian (~24-hour) clock’s paradoxical properties of robustness (strong rhythms) and plasticity (flexible adaptation).

How Our Daily Rhythm is Regulated

From tiny fruit flies to humans, all animals on Earth maintain their daily rhythm using their internal circadian clock. The circadian clock enables organisms to undergo rhythmic changes in their behavior and physiology based on a 24-hour circadian cycle. For example, our own biological clock instructs our brain to release melatonin, a sleep-promoting hormone, at night. The discovery of the molecular mechanism of the circadian clock was awarded the 2017 Nobel Prize in Physiology or Medicine. According to our current knowledge, there is no central clock responsible for our circadian cycles. Instead, this mechanism functions in a hierarchical network in which there is a “master clock” and “slave oscillators.”

The master clock receives various input signals from the environment, such as light. The master then controls the slave oscillator, which regulates various outputs such as sleep, nutrition, and metabolism. Despite the different functions of pacemaker neurons, they are known to share common molecular mechanisms that are well preserved in all life forms. For example, interconnected systems of multiple transcriptional-translational feedback loops (TTFLs) consisting of nuclear clock proteins have been studied in detail in fruit flies.

Master Clock and Slave Clock Function via Different Molecular Mechanisms

However, there is still much to learn about our own biological clock. The hierarchically organized nature of master and slave clock neurons leads to the widespread assumption that they have identical molecular clockworks. At the same time, the different roles they play in regulating body rhythms raises the question of whether they may function with different molecular clockworks. Led by Prof. KIM Jae Kyoung and KIM Eun Young, researchers at the Institute for Basic Science (IBS) and Ajou University have used a combination of mathematical and experimental approaches using fruit flies to answer this question. The team found that the master clock and slave clock function via different molecular mechanisms.

A circadian rhythm protein called PER is produced in both the master and slave neurons of fruit flies and is degraded at different rates depending on the time of day. Previously, the team had found that master clock neurons (sLN_vs) and slave clock neurons (DN1_ps) in wild-type and Clk-Δ mutants of Drosophila have different PER profiles. This suggested that there may be a difference in the molecular clock mechanisms between master and slave clock neurons.

However, due to the complexity of the molecular clockwork, it was difficult to identify the cause of these differences. The team therefore developed a mathematical model describing the molecular clockwork of the master and slave clocks. All possible molecular differences between the master and slave clock neurons were then systematically investigated using computer simulations. The model predicted that PER is produced more efficiently in the master clock and then degraded more quickly than in the slave clock neurons. This prediction was subsequently confirmed by follow-up experiments in animals.

When the Circadian Clock Loses its Robustness and Flexibility, Circadian Sleep Disorders Can Occur

Why, then, do master clock neurons have such different molecular properties from slave clock neurons? To answer this question, the research team again combined mathematical model simulations with experiments. They found that the faster synthesis of PER in master clock neurons enables them to generate synchronized rhythms with high amplitude. The generation of such a strong rhythm with high amplitude is crucial for sending clear signals to the slave clock neurons. However, such strong rhythms are generally unfavorable when it comes to adapting to environmental changes. These include natural causes such as different daylight hours in summer and winter, as well as more extreme artificial cases such as jet lag after international travel.

Thanks to the special property of master clock neurons, they can undergo phase dispersion when the normal light-dark cycle is disrupted, causing PER levels to drop dramatically. The master clock neurons can then easily adapt to the new daily cycle. The plasticity of our master clock explains why we can quickly adapt to the new time zone after international flights after only a short jet lag phase. It is hoped that the results of this study will have clinical implications for the treatment of various disorders that affect our circadian rhythm in the future. Lead researcher Kim explained, “When the circadian clock loses its robustness and flexibility, circadian sleep disorders can occur. Since this study identifies the molecular mechanism that generates the robustness and flexibility of the circadian clock, it may facilitate the identification of the cause and treatment strategy for circadian sleep disorders.”