October 19, 2021

I, Science

The science magazine of Imperial College

Circadian rhythms run the biological world like clockwork. Madeleine Hurry explains how they work, from the genes in a bacterium to the human brain

“you are probably less safe driving between 4am and 6am than when you are legally drunk”

The Latin words circa diem translate to ‘about a day’. Speaking of old things, circadian rhythms are pretty ancient. They are a result of life that evolved on a planet that rotates over a period of 24 hours. Being able to anticipate daily environmental fluctuations is remarkably useful, hence these biological clocks are virtually ubiquitous. The clock components are fairly basic: simply a handful of proteins and some ATP (the cellular energy transfer molecule) can suffice. But disrupting them causes complications ranging from jet-lag to cancer. Circadian rhythms are therefore fascinating on a molecular, medical and evolutionary level.

The discovery of circadian rhythms

LinnaeusWeddingPortraitBiology is dynamic; even seemingly motionless plants change during the course of the day. Linnaeus, the father of taxonomy, demonstrated this in 1751 when he designed his ‘flower clock’. If different plants with variations in the opening and closing times of their flower buds were planted in a circle, they could theoretically tell you the time!

But it wasn’t until the 1970s and the genetic revolution that this research really took off.

The first circadian genes were discovered in fruit flies by Seymour Benzer and his student Ron Konopka. They controlled when flies emerged from their pupal cases and were named ‘Period’.

In 1997 a second gene was identified in mice, aptly named ‘Clock’. Clock mutants exhibit variability in sleep-wake cycles. A wild-type mouse kept in a dark cage has a circadian cycle slightly shorter than 24 hours. Animals with one copy of the mutated Clock gene have abnormally long cycles and mutants with two abnormal copies of the Clock gene have asynchronous cycles, even in normal lightdark environments.

How do you build a circadian clock?

This is a representation of the transcriptome of the cyanobacterium Cyanothece sp. ATCC 51146 under 12-hour light/dark cycles. Expression levels at 7AM (1 hour of light) are shown with highest expression being red and lowest being blue. Expression levels at 7AM (1 hour of light) are shown with highest expression being red and lowest being blue. The cyclic nature of the diurnal system is revealed by a simple analysis of the data.

The basic components of the body’s biological clock vary across the tree of life. Whilst the proteins used to build the clock vary, the logic of the architecture is conserved. Fundamentally, it requires an oscillatory mechanism that follows a 24-hour period.

The simplest biological oscillator exists in the cyanobacterium Synechococcus elongates, the core of which involves only three proteins: KaiA, KaiB and KaiC. Here, KaiC becomes increasingly phosphorylated then dephosphorylated over a period of 24 hours. These proteins will even continue to function as a basic circadian clock in vitro in the presence of ATP and magnesium.

In mammals, the core oscillator unit of circadian rhythm is slightly more complex. The transcription factors (proteins which help gene expression) CLOCK and BMAL pair up and promote transcription of the genes Per and Cry. PER and CRY proteins join together and travel back to the nucleus to suppress the transcription of Clock and Bmal. This happens slowly, producing the 24-hour cycle. Other genes governed by circadian rhythms are activated or suppressed as the levels of CLOCK and BMAL rise and fall. This may be noticeable, like the secretion of a hormone telling you it’s time to wake up, or go completely undetected, such as increased DNA repair in the late afternoon.

This oscillatory mechanism can free-run by itself, but cell-cell signalling is required to synchronise the rhythm with external environmental cues, such as light.

Circadian rhythms and the brain

Let’s take a giant evolutionary leap from single-celled prokaryotes to mammals and discuss how our circadian rhythm is organised anatomically.

When we see things, light enters the visual pathway through our pupils. This signal is transduced by photoreceptors in rod and cone cells, causing retinal neurons to fire, which finally forms an image interpreted by our brain. But another light pathway exists, one that entrains your biological clock. This time the light signal is picked up by special cells known as intrinsically photosensitive Retinal Ganglion Cells (ipRGCs), which contain the photopigment melanopsin.

The discovery of ipRGCs was made by Professor Russell Foster, former Chair of Molecular Neurosciences at Imperial, and his team. This was a contested but momentous discovery, as it was previously thought that no photosensitive cells other than rods and cones existed. The ipRGCs stimulate neurons in the suprachiasmic nucleus (SCN) in the brain. The SCN is a tiny structure within the hypothalamus. But it is the master regulator for circadian rhythm because it synchronises the rest of the cells in the body with the 24-hour light-dark cycle.

Interestingly, some non-mammalian vertebrates, such as frogs and lizards, possess a light-sensitive organ found on the tops of the heads. This is called a parietal eye, also known as the ‘third eye’. In mammals this organ is absent, but the analogous structure is the pineal gland, which produces melatonin, the hormone responsible for sleep patterns.

What implications do circadian rhythms have?

circadian clock vaXzine 5160061979_89e5f878d4_oPerhaps you just can’t seem to get to sleep before midnight. Meanwhile your friend can’t sleep past 7.30am, even on weekends. The reason is, of course, your personal circadian rhythm.

Whilst healthy people can generally adapt their routine when required, those who aren’t may have a circadian rhythm sleep disorder (CRSD), so cannot adapt their cycle to conventional social hours. This causes disrupted or inadequate sleep, which can lead to depression, weight gain, and various other diseases. CRSDs can be caused extrinsically, by shift work or jet lag, or intrinsically, by a mutation in a circadian gene.

You can feel the effects of an asynchronous circadian rhythm when you suffer from jet lag. A common plague of the transmeridian traveller, jet lag is caused by an abrupt change in time zones. This means your circadian rhythm is out of sync with your new location, causing sleep disturbance, irritability, headaches and digestive problems.

Both physical and cognitive performance is paired with circadian rhythm. For example, you can swim three seconds faster at 8pm than 6am simply due to the balance of circadian-controlled hormones in your body. Similarly but more seriously, melatonin and other sleep hormones which are released at night reduce your cognitive ability, so much so that you are probably less safe driving between the hours of 4am and 6am than when you are legally drunk!

Metabolism and circadian rhythm are also interlinked, so lack of sleep causes hormonal imbalances. Leptin, responsible for fullness, and ghrelin, which stimulates hunger, exist in equilibrium. But sleep disruption will tip the equilibrium towards ghrelin and increase its production by 28%. In real terms, this means that people who get less than five hours sleep per night have a 50% likelihood of being obese! Also related to obesity, the circadian clock gene BMAL1 has been associated with type 2 diabetes and hypertension.

circadian rhythm aging oregon 14485093188_c5e94b7948_bMany genes involved in regulating circadian rhythm have been implicated in psychological diseases. For example, SNAP25, which is associated with schizophrenia in humans, causes disrupted sleep cycles and hormone release when mutated in mice. Similarly, ADHD and sleep disorders are often found together, although it is not yet known if there is a causal relationship between the two.

Circadian rhythms also need to be considered in pharmacology. The efficiency of nucleotide excision repair (a form of DNA repair) peaks in the late afternoon, after most UV-related DNA damage has accumulated. The synthesis of the proteins required for DNA repair are controlled by the circadian clock. Therefore, timing the administration of chemotherapy to coincide with this peak could increase the efficacy of the drug.

Conversely, studies have also shown that certain drugs can be more toxic at different times of day. If a group of mice are given cyclophosphamide (a chemotherapeutic agent) at dusk, there is 20% mortality. Give them the drug at dawn and all of them will die.

Ultimately, circadian rhythms alter your body’s biochemistry causing profound effects. Next time you feel like falling asleep in your 9am lecture, consider that it is due to the action of some evolutionarily ancient genes!

Madeleine Hurry is studying for an MRes in Experimental Neuroscience

Images: Carl Linnaeus, WikipediaTranscriptome of the Cyanobacterium Cyanothece by Pacific Northwest National LaboratoryCircadian Rhythm – Human Biological Clock by vaXzine; Shifting rhythms by Oregon State University;