It is well known that exposure to daylight keeps our body clock in check. But what impact does meal timing have? A new study published in Cell helps answer this question and provides new insights on how cells keep a circadian rhythm. The study also has important implications for shift workers and travelers wanting to avoid jet lag.
Anyone who has flown across time zones or pulled an all-nighter will know the powerful side effects of a shift in circadian rhythm, such as a feeling of “grogginess” and messed up sleeping patterns.
While it is difficult to separate a shift in circadian rhythm from other confounding factors (e.g. shift workers are more likely than non-shift workers to be obese), studies suggest that a disrupted body clock can contribute to the development of disease. According to the NHS, shift workers are more likely to report poor health.
In light of this, a collaborative effort was directed at investigating the circadian rhythm mechanisms in more detail. John O’Neill, molecular biologist and principal investigator at the MRC, explains:
“Our main question is, how do individual cells keep time? Because, that’s fascinating, right? In order for their internal clock to be of any use, it needs to be able to be synchronized with the outside world.”
The work led to fresh insights on the intricate control of how cells keep time, which has important implications for shift workers and anyone wanting to avoid jet lag.
The research was published today in Cell and was carried out by researchers from the MRC Laboratory of Molecular Biology (LMB) in Cambridge and the University of Manchester.
An age-old question about circadian rhythmsHow mammalian circadian rhythms sense and entrain to light is well-studied: photoreceptors in the retina relay light cues to the suprachiasmic nucleus in the hypothalamus, which integrates these signals and relays them to cells. These signals are partly communicated to cells via regular daily fluctuations in adrenal glucocorticoids – cortisol rises in the morning and primes us for wakefulness.
Whilst the role of daylight hours in circadian time-keeping has been well-characterized, the effect of meal timing is less well understood.
“It’s also been known that the time when we eat communicates timing information to cells throughout the body, and the mechanism there has just not been understood. We pursued this because we thought we had a unique insight into how this might happen.”
Guided by “radical research” from the ‘70sA key component of the findings stemmed from work in the 1970s by Seymour Benzer and Ron Konopka. The pair worked with fruit flies (Drosphila melanogaster) as their model organism and hypothesized that the timing of sleep and wakefulness they saw in the fruit flies might have a genetic basis.
O’Neill explained that this was a completely radical notion at the time, as nobody believed that something as complex as the organization of the day and behavior could have a single gene (or set of genes) that was relevant to understanding the behavior.
Benzer and Konopka threw a spanner in the works when they isolated three types of mutant flies that had either a long or short circadian period, or no circadian rhythm at all, and attributed the difference to mutations in a particular gene.
“That was the really pivotal discovery that there were a number of genes, and people like to call them clock genes, just because it's a short kind of metaphor for describing these activities. But there's a small number of genes in the genome of all animals that we share, a common set of transcriptional mechanisms that seems to be really quite important for coordinating the 24-hour organization of physiology and cellular function, and even something as complex as the timing of sleep versus wake."
A new role for insulinPrior to this study, there was a line of thinking that a systemic signal results from our feeding cycle, to entrain the phase of every cell in the body, and that this signal worked independently of cells in the suprachiasmic nucleus in the hypothalamus.
Until now, this nutrient-sensing signal was a mystery. The group reasoned that this feeding entrainment signal would have to meet a few criteria:
- The signal would indicate information about feeding
- It would have receptors with a very wide distribution
- The signal would elicit changes in clock protein levels across different cell types
There had been a few candidates with some evidence to suggest that they played a role (ghrelin, glucose, glucagon), but only in particular cell types. In contrast, insulin was looking to be a strong candidate for directing all cells, says O’Neill:
“The critical thing about the insulin receptor, and also the IGF-1 receptor is that those receptors are expressed in every single cell. The additional observation that it was able to increase the activity of the clock protein period, in every cell type that we tested, as well as in a whole mouse was a really strong piece of evidence that this was the feeding signal that communicates time of feeding to clocks throughout the body.”
Indeed, insulin was shown to reset circadian clocks in vitro and in vivo, by increasing the synthesis of PERIOD proteins (controlled by the Period “clock genes”).
The study was carried out in mice and in cell cultures of different types (mouse-derived fibroblasts, cortical neurons, organotypic liver and kidney slices, and intestinal organoids).
The nitty gritty: insulin-PER signaling requires the perfect trifectaHow does an increase in insulin lead to this increase in the PER protein?
The researchers established that by driving mTOR signaling, insulin leads to an increase in the PER protein.
But, important questions remained, as complexes containing mTOR play really important roles in many different signaling pathways.
How does the cell distinguish different types of mTOR signaling? How could there be any selectivity? How could mTOR be specifically relaying timing information as well as the host of other jobs that it does?
The circadian entrainment of cells via insulin works through a very clever and coordinated mechanism. For PER levels to be increased by insulin, three things must happen simultaneously:
- Activation of MTOR
- Inhibition of an enzyme called PTEN, which opposes signaling through the insulin receptor
- Reduction in the level of micro-interfering RNAs that would normally destabilize the mRNA of the PERIOD gene
This coordinated detection allows the cell to discriminate this particular type of mTOR-dependent signal from the vast number of other signals that are relayed.
The relative timing of cues from food and light is importantPERIOD genes have a cyclical rhythm in their expression, and it seems that having a high amplitude of this daily rhythm is important for our health. O’Neill explains:
“We see in people with neurodegenerative disorders, or even aged individuals, a progressive decrease in the amplitude of clock gene expression rhythms, which seems to associate very well with an increased predisposition and increase propensity towards a number of diseases.
It's almost as if to be healthy, all of the clocks throughout the body in every individual cell, they have to be synchronized with each other, and then they have to be synchronized with the external world. And as that synchrony decreases, and as the amplitude of the rhythm in individual cells decreases, we seem to be more and more susceptible to disease.”
The researchers established, using mouse models and cell culture studies, that the relative timing of light and feeding signals were really important.
Exposure to a critical level of cortisol (or corticosterone, in mice) which acts as an internal representation of the external light/dark cycle, needs to occur approximately four hours before the insulin signal, in order to get the highest amplitude in PER gene activity.
O’Neill explains: “that's because cortisol turns on the transcription of PER, but then insulin increases the translation of PER - so they have to occur in that order.”
On the other hand, if this occurs in the wrong order (i.e. the feeding signal occurs before cortisol), then there will be a lower amplitude in clock gene rhythm.
Implications for shift workers and travelersThese findings suggest ways that we can theoretically, maximize the amplitude of our PERIOD genes – and adapt more quickly to new shift work or a new time zone.
O’Neill acknowledges that shift workers have been aware for a long time that light exposure makes a difference.
“What we’re saying is that it’s the relative timing of when you see lights and have a meal, that is the thing that will help you reset as quickly as possible. And that’s also going to be true for jet lag.”
If this work was to be extrapolated to maximize the amplitude of our clock genes, these would be the recommendations:
Flying across time zones: Try not to see any bright lights, and try not to eat for about 12 hours before you have breakfast at the time of your destination’s morning. Even if you’re still flying, try to eat your breakfast and see light when morning greets your destination.
Implications for shift workers: Similarly, O’Neill recommends avoiding bright lights and food during the rest phase: “Let's say that I've gone on to the night shift. The day before, I would try to avoid seeing any bright lights, I would starve myself so that the point at which I started my shift, that's when I would ensure that I was seeing bright lights and have the first meal of the day. Then I'd have lunch at maybe midnight, still seeing bright lights. And there may be a meal just before dawn making sure that when I go to bed, I'm not seeing bright lights, I’ve got blackout curtains and I'm not snacking during the time when I should be resting.”
The authors acknowledge that they did not distinguish the effect of insulin from IGF-1 in the study. There is known partial redundancy between insulin and IGF-1 – only when they blocked both receptors, did they abolish the effect of feeding time on circadian rhythms.
Furthermore, they also don’t discount that other hormones or proteins may contribute to food entrainment.
The fresh insights on circadian rhythm are bound to tempt travelers and shift workers to give the approach a go. In fact, O’Neill has tried it himself, when he flew from the UK to Korea, and reported that he didn’t experience any jet lag and adjusted almost immediately: “Obviously, the placebo effect is very strong. I was expecting it to work and it worked amazingly! That’s completely anecdotal and useless from a scientific point of view. But we would be very interested to find collaborators to do human trials. We’ve really gone about as far as we can with the mouse experiments now.”
Crosby, P., Hamnett, R., Putker, M., Hoyle, N. P., Reed, M., Karam, C. J., Maywood, E. S., Stangherlin, A., Chesham, J. E., Hayter, E. A., Rosenbrier-Ribereiro, L., Newham., P., Clevers, H., Bechtold, D. A., O'Neill, J. S. (2019). Insulin/IGF-1 drives PERIOD synthesis to entrain circadian rhythms with feeding time. Cell 177: 1-14