Of course, this 24-hour clock is more than just a social construct. It is engrained into our very cells. I am talking about circadian rhythms.
For example, wakefulness, body temperature, the gut microbiome, and some hormone levels all cycle in a periodic manner. Now, part of the definition of “circadian” is that these patterns are entrainable, meaning they can be adjusted to environmental stimuli (aside, these stimuli are gloriously called “zeitgebers”). Light and food consumption are two examples of environmental stimuli that can have this type of influence. Based on these inputs, “central oscillators” then control the rhythms that exist in peripheral tissues.
What is incredible is that even if these central control mechanisms are removed, our tissues and the cells that make them up can have “cell autonomous signals” – meaning, the cells themselves have a self-generated circadian rhythm, driven by periodic feedback loops in expression of certain genes. What an incredible feat of evolution!
And while it is not totally clear why these patterns developed, what does seem to be true is that they can be clinically relevant. For example, the Van Laake group out of the University Medical Center Utrecht points out that there is a higher incidence of cardiac events (heart attacks, arrhythmias, etc.) just as a person is waking up vs. other times of day.
This has motivated their research to try to model circadian rhythms in cardiac cells in vitro. They specifically wondered whether embryonic stem (ES) cells had an intrinsic clock and, if not, at what point this clock developed during differentiation into cardiomyocytes (CM) (or heart cells).
First, they looked at the expression of genes known to be the master controllers of circadian expression loops, so called “clock” genes. They found that, in general, multiple clock genes were expressed in ES cells, but the expression greatly increased by D15 (immature CMs) and D30 (mature CMs) of differentiation.
To then look at whether these genes fluctuated in expression, the CMs were synchronized using dexamethasone and analyzed for the classic anti-phasic expression of two different clock proteins (PER2 and BMAL1), which represents a functional molecular circadian clock (see image). Since they used cell lines that glow in proportion to the level of gene expression, they could literally watch changes in PER2 and BMAL1 over the next two days. While immature (D15) CMs and undifferentiated ES cells did not exhibit these functional clock patterns, they were witnessed in D30 mature cardiomyocytes, suggesting that this is a behaviour that is learned during development. It also appears to be quite robust, as it lasted in cells cultured out to 45 days.
The investigators then looked at which genes were controlled by clock genes. Over 700 transcripts were found to oscillate across the day in D30 CMs. After investigating whether these transcripts were related in a functional way, they discovered that they were enriched for genes involved in cardiac development and the stress response. These changes in expression of stress response genes occurred to a relevant degree, as the CMs had a differential ability to handle the cardio-toxic effects of doxorubicin treatment across the day when this drug was added to culture.
So what does this cell-based clock mean in the grand scheme of things?
Well if our central oscillators keep cardiac clocks synchronized (all cells the same), it may suggest we are better able to handle potential pharmaceutical stressors at different times of day (like doxorubicin), which could lead to smarter treatment schedules. And as the authors point out, given scientists are starting to use engineered heart tissues to test a variety of different compounds, they might consider factoring in the “time zone” of the heart tissue, to get a more clinically relevant prediction of how their drug may affect patients (and when to administer it).
At the end of the day, if our goal with engineering tissues is to be biomimetic, then we should remember that people are creatures of habit. And, somehow, due to incredible biology, this also pertains to their cells.