New compound disrupts clock

Scientists discovered a small molecule that can lengthen the circadian rhythm in human cells more dramatically than any drug or genetic manipulation known.

Image: Wikimedia commons

Specifically, the molecule lengthened the daily cycle in human cells by more than 10 hours, and lengthened circadian rhythms in zebrafish.This proof-of-concept study, published online today (December 14) in PLoS Biology, presents the first application of a large scale chemical screen, commonly used in drug discovery, in the study of circadian rhythms. The results suggest this technique could help explore clock mechanisms and identify candidates for circadian disease therapeutics, such as the newly discovered compound. "It's like tinkering with the most elegant time piece nature constructed," said molecular biologist and clinical endocrinologist linkurl:Joe Bass;http://www.cgm.northwestern.edu/cgm/Faculty-Research/Faculty/Joseph-Bass of Northwestern University in Chicago, who was not involved in the research. "You have a molecule that can go into those minute gears and has this general effect on the ability of the entire mechanism to report correctly on time. That's kind of amazing."Circadian rhythms are oscillations in gene expression that correspond with changes in behavior, physiology, and metabolism in nearly all organisms on Earth. In addition to sleep disorders, disruptions to this internal clock have been associated with cardiovascular ailments, cancer, and metabolic diseases. Scientists have discovered many clock genes, encoding transcription factors that comprise the network that generates the rhythmic gene expression of thousands of genes involved in an organism's daily rhythms, but there remain many unknown components of the clock system.To identify other factors which may play a role in the functioning of biological clocks, molecular biologist linkurl:Steve Kay;http://biology.ucsd.edu/faculty/kay.html of the University of California, San Diego and his colleagues decided to take an unusual approach -- apply the chemical screens used by pharmaceutical companies looking for new drugs to test the effects of more than 100,000 small molecules on the clock's cycle. The researchers used a human cell line that lights up when the clock is turned on -- an assay system they had previously developed to visualize changes in clock output. Exposing the cells to each of the 120,000 uncharacterized compounds of a structurally diverse library, they watched to see which cells glowed out of synch.While a number of the compounds affected the clock's output, one in particular attracted the team's attention. It strongly lengthened the period in a dose-dependent manner. The researchers then synthesized about 40 related derivates of the compound, and found one that was even more potent, lengthening the cycle by more than 10 hours -- "far more dramatically than any single gene mutant had done," Kay told The Scientist in an email. They dubbed the molecule "longdaysin."The researchers also tested the effects of longdaysin in zebrafish in vivo and found a similar period lengthening as they had seen in the human cell cultures, confirming the compound's potent effect on the clock's cycle.Probing longdaysin's mechanism of action, the researchers identified three protein kinases that appeared responsible for its effect on the clock's cycle -- CKIδ, CKIα, and ERK2. While CKIδ was already a known component of the circadian system, CKIα and ERK2 were previously unidentified clock genes, suggesting that chemical screening may be an effective method to identify new clock components."The advantage of our chemical probe is that it hit multiple targets, revealing two new kinases involved in the clock," said Kay, who also works for Reset Therapeutics, a California-based biotech interested in identifying compounds that affect clock rhythms. "We can clearly do a variety of clock chemical screens that are likely to be very productive in revealing novel aspects of clock function."Compounds that affect the clock may also prove useful as possible therapeutics for circadian diseases, such as sleep disorders. "If you can get a faithful screen in a human cell, which is what they've done, and discover small molecules that have a quantifiable impact on some phenomenon related to the oscillations, you can take those as candidates for modulating physiological functions," said Bass, who serves as an advisor to Reset Therapeutics. "The challenge [is] to unravel now where we could use these drugs, in which tissues they might have the greatest impact, under which conditions we might want to modulate the dynamics of the oscillation."T. Hirota, et al., "High-throughput chemical screen identifies a novel potent modulator of cellular circadian rhythms and reveals CKIα as a clock regulatory kinase," PLoS Biology, 8:e1000559, 2010.
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[21st February 2008]