AsianScientist (Oct. 4, 2017) – Even as the clock on the wall ticks away the hours of the day, an internal rhythm governs the diverse biological processes that take place in our bodies. Known as the circadian rhythm, this exquisitely precise and complex process of cellular timekeeping is conserved in species ranging from plants to flies to animals, affecting all aspects of their behavior and health. But how does a living cell synchronize its activities with the 24-hour cycle of the Earth?
This was the question that the recipients of the Nobel Prize in Physiology or Medicine 2017, Professors Jeffrey C. Hall, Michael Rosbash and Michael W. Young, sought to answer. In 1984, Hall and Rosbash demonstrated that a protein called PER, short for ‘period’, accumulated in fruit flies during the night but was broken down in the day. They detailed the mechanism of how PER suppressed its own production once it was expressed at high levels in a cell, forming the basis of a self-regulating, cyclical rhythm.
A decade later, Young identified another two components of the circadian rhythm, proteins called TIM, short for ‘timeless,’ and DBT which stands for ‘doubletime’. Both genes work to carefully orchestrate the cycling of PER, defining its oscillation time in cells.
Today, the circadian rhythm is known to be intricately involved in sleep and feeding patterns, hormone circulation, blood pressure, body temperature and a whole host of other bodily functions and behaviors. Over the years, Asian Scientist Magazine has highlighted various studies on the circadian rhythm; here are five of the most interesting.
- It’s controlled by light switches
- It’s linked to mood swings
- It affects female fertility
- It regulates appetite
- It controls when the rooster crows
Intuitively, for the circadian rhythm to be pegged to the cycles of day and night, light sensing must be part of the system. In a study of green algae, Chlamydomonas reinhardtii, scientists demonstrated that the circadian clock is controlled by at least two pathways that respond to different colors of light. They were able identify the gene, CSL, in the algae that was responsible for this color-specific response, noting that the gene has equivalent counterparts in plants and animals. Their work highlights the possibility that our circadian clock can even be fine-tuned by the wavelengths of light we are exposed to.
Insufficient sleep might result in a foul mood in the morning, but the root cause of mood swings could actually be the disruption of circadian rhythm. Scientists showed that mutant mice with behavioral deficits and other brain features consistent with bipolar disorder also exhibited abnormal fluctuations in circadian clock genes—the expression levels of these genes go up and down in tandem with mood-change-like behaviors in the mice. Therefore, the maintenance of a healthy circadian rhythm can have far-reaching consequences on mental wellbeing.
Getting pregnant becomes more difficult with age, and this has often been attributed to disruptions in the menstrual cycle. However, researchers have demonstrated that the circadian rhythm also has role to play in influencing fertility and reproductive function. Importantly, by varying the periodicity of the light-dark cycle, they were able to reverse age-related fertility decline in female mice. These experiments suggest that shift work and jet-lag may impact fertility in women, and correcting the circadian rhythm may help to boost reproductive success.
In light of the obesity epidemic, research into eating behavior has received increasing attention. Scientists have described how the circadian clock in nerve cells of the stomach is responsible for setting the threshold of fullness. During the day, the nerves in the stomach are less responsive to stretch, meaning that more food can be ingested before the sensation of fullness sets in. At night, however, the gut nerves are more sensitive, signaling fullness even when smaller amounts of food are consumed. Shift work that disrupts sleep cycles may therefore have important implications on predisposition to obesity.
Roosters have been known to announce the start of the day for centuries, but only recently have scientists figured out how they are timing their calls. The key challenge was to distinguish whether rooster crowing occurs in response to external stimuli or is under the control of a biological clock. By altering the duration of light exposure experienced by the roosters, scientists revealed that the roosters’ circadian clock not only drives predawn crowing, but also controls crowing in response to external stimuli.
Copyright: Asian Scientist Magazine; Photo: Shutterstock.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.