Please can you give a brief overview of the circadian rhythm?
The human circadian rhythm is an internal “clock” that controls numerous physiological processes in the body.
The circadian rhythm is affected by many different stimuli—such as sleep and light, which are the most broadly appreciated ways—but also eating— all of which can modulate, or change, important processes in our bodies such as temperature, production of hormones or other signalling small molecules, cellular regeneration, and others.
How much is currently known about the physiological processes that underlie the circadian rhythm in humans?
It has become clear that the circadian rhythm is an important physiological control mechanism in our bodies, not only in terms of regulating entire organs, such as the brain, but studies have also shown that the circadian rhythm persists in (and can be controlled to affect) individual cells within our body.
And more recently there have been significant advances in identifying proteins and receptors that are involved in regulating the human circadian rhythm. Some of the proteins, such as the REV-ERB receptors, appear to be druggable, indicating we may be able to design drugs to alter the circadian rhythm.
What potential benefits would the ability to alter the circadian rhythm have?
There are likely many benefits that could arise from designing a drug to alter the circadian rhythm. It is possible such drugs could cure sleeping disorders, for example, or to extend wakefulness in cases where it may be appropriate or necessary to do so.
In addition, dysfunction in the circadian rhythm is implicated in numerous diseases. As one example, people who work the night shift have increased risk for a number of diseases, including substance abuse, depression, obesity, cardiovascular issues, cancer, as well as family/home problems. Drugs targeting the circadian rhythm may help to treat these types of issues and others.
It was recently announced that you had discovered a pair of compounds that could potentially alter the circadian rhythm. What are these compounds and how did you discover them?
These are compounds that bind to and affect the function of a class of receptors called the REV-ERBs. These compounds are similar to a natural porphyrin ligand found in the body, heme.
Heme, which contains an iron metal center, is the natural ligand for the REV-ERB receptors. We were curious to know if REV-ERBs could bind to a heme-like molecule with a different metal center that is not normally found in the body.
In our study published in the Journal of Biological Chemistry, we show that two additional porphyrin analogs of heme, which contain cobalt or zinc instead of iron, can bind to REV-ERBs similar to heme—but interestingly they have different functional effects on REV-ERB activity.
How are REV-ERBs normally regulated and why do cobalt protoporphyrin IX (CoPP) and zinc protoporphyrin IX (ZnPP) have a different effect on REV-ERBs?
The natural ligand for REV-ERBs in the cell is heme, and it is thought that when heme binds to the REV-ERBs it activates its ability to function.
Our study indeed found that changing the metal center of the porphyrin scaffold of heme—called protoporphyrin IX—from iron to either cobalt (CoPP) or zinc (ZnPP) inhibited the function of REV-ERB.
To determine why CoPP and ZnPP inhibits REV-ERB function, we studied how the compounds bind to REV-ERB on the atomic level. Interestingly, or perhaps unfortunately, the results were in fact quite similar to other published studies on heme-bound REV-ERB. This indicates to us that there are likely more complex functional mechanisms at play here as opposed to the compound simply binding to REV-ERB.
In fact, other studies have shown that REV-ERB function can be affected by small molecule gases in our body—such as molecular oxygen (O2), carbon monoxide (CO) and nitric oxide (NO)—binding to the heme/REV-ERB complex. Our data on CoPP and ZnPP indicate that changing the metal center might affect the ability of these small molecule gases to bind to REV-ERB.
How could the compounds you discovered be used to uncover new therapeutics for diabetes and obesity?
There is great interest in designing “synthetic” compounds that can alter the circadian rhythm and optimizing them for use as therapeutics in humans. Unfortunately, many compounds that are developed synthetically through medicinal chemistry approaches can be toxic in the body.
Once nice feature about CoPP and ZnPP is that they are derived from a natural product commonly found in the body, heme—which is a natural ligand for the REV-ERBs. In fact, prior studies have demonstrated that CoPP has functional effects “in vivo” (in mice), including anti-obesity activity.
What impact do you think your results will have and what are the next steps in your research?
In other studies, we have designed “synthetic” REV-ERB compounds that do alter the circadian rhythm. However, all of these “synthetic” compounds are REV-ERB agonists (or activators). Thus these new compounds, CoPP and ZnPP, which are REV-ERB antagonists (inhibitors), may provide a unique tool to determine how different classes of potential, future REV-ERB drugs (activators vs. inhibitors) can alter the circadian rhythm.
In addition to working with other scientists to determine their functional effects in vivo, we are also working to study the molecular details of why changing the metal center of the natural ligand heme can cause an opposite effect on REV-ERB activity.
How far do you think we will be able to alter the circadian rhythm in the future? What will be the main limiting factors?
Although we have come a long way in understanding how the circadian rhythm is controlled, there is still a lot we do not know. Certainly a dysfunctional circadian rhythm can cause disease, and drugs may be a viable treatment, but there are always risks to using a drug to treat any disorder.
In addition, the major challenge for any drug development project is optimizing the drugs for use in humans. Although a compound may show beneficial effects in controlled laboratory settings or in animal models, one cannot easily predict if the same compound can be tolerated in humans and it can take years to work around this bottleneck.
Where can readers find more information?
Our study was published in the Journal of Biological Chemistry. The text can be found at the journal website:
https://www.jbc.org/
And at the US National Library of Medicine “PubMed” website:
http://www.ncbi.nlm.nih.gov/pubmed/24872411
More information about the work of the Kojetin lab can be found at the following website: http://www.scripps.edu/kojetin/
About Dr. Doug Kojetin
Dr. Douglas Kojetin is currently an Associate Professor in the Department of Molecular Therapeutics at The Scripps Research Institute in Jupiter, Florida, USA.
Research Dr. Kojetin’s laboratory focuses on a class of human receptors called “nuclear receptors”, which are important transcription factors that are the targets of a significant number of clinically used drugs.
Their research is focused on understanding the molecular and atomic details of how natural small molecules, currently available drugs, and new synthetic compounds bind to nuclear receptors to affect their function. Research in the Kojetin laboratory is currently supported by the National Institutes of Health (NIH).
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