The immune system alters mammalian circadian clock
gene expression, perhaps explaining why fatigue accompanies some infections and autoimmune diseases, according to a study
in this week's Proceedings of the National Academy of Sciences
. The authors found that a protein involved in mammalian immunity interferes with circadian clock gene expression and promotes sleep
"The study certainly provides a framework within which to address the question of how immunomodulators affect both the timing and the quantity or quality of sleep," said Mark Opp
of the University of Michigan in Ann Arbor, who was not involved in the study.
The immune response
to microbial infection activates pro-inflammatory molecules such as tumor necrosis factor alpha (TNF-α). Previous work has suggested that TNF-α causes lethargy and fatigue in people with cancer
, rheumatoid arthritis
, and sleep apnea
. Animal studies have also shown that TNF-α enhances sleep
, but the mechanism has remained unknown.
To see if TNF-α directly regulates circadian oscillations, researchers led by Gionata Cavadini of University Hospital Zurich in Switzerland analyzed TNF-α's effects on expression of circadian clock
"There's very little known about how [immune response] is related to circadian rhythms," said James Krueger
of Washington State University in Pullman, who was not involved in the work.
The central circadian oscillator in mammals lies in the suprachiasmatic nucleus (SCN) of the hypothalamus, but other body cells also contain peripheral oscillators that keep their own rhythms. Interacting loops
of gene transcription and translation form a negative feedback system that generates the 24-hour mammalian clock.
The researchers first found that, in culture, TNF-α suppresses the expression of three circadian clock genes called Period
, and the expression of three transcription factors they controlled by clock genes. TNF-α only affected the genes' expression levels; it did not affect the times at which they were expressed.
has shown that activation of Period
genes as well as the transcription factors depends on a specific DNA sequence - called an E-box - lying upstream of these genes. Transcription factors bind to the E-box sequence to initiate these genes' transcription.
When Cavadini and her colleagues mutated the E-box sequence, administering TNF-α did not suppress transcription. Conversely, TNF-α did not activate circadian clock genes whose expression does not depend on an E-box sequence.
Their results suggest that TNF-α affects only clock genes that contain an E-box and that the reduced activity of these genes affects sleep regulation, co-author Thomas Birchler, also of University Hospital Zurich, told The Scientist
in an Email.
When the researchers tested the effects of TNF-α in live mice, they found that animals treated with the cytokine kept normal cycles -- sleeping during the day and becoming active at night - but they moved less and rested more during their active periods than did untreated mice. These results paralleled in vitro
findings showing that TNF-α changes clock genes' expression levels but not timing, the authors say. Also, as in the in vitro
studies, TNF-α down-regulated expression of Period
and the three clock-controlled transcription factors in the liver.
It's possible that compensating effects by other clock-related genes is keeping the circadian period normal in mice treated with TNF-α, senior author Adriano Fontana
of University Hospital Zurich told The Scientist
in an Email. "This may explain why TNF-α, which influences the expression of several clock genes, does not result in a shift of the circadian clock."
Another possibility, however, is that TNF-α is affecting not the central circadian pacemaker in the SCN, but only peripheral oscillators in the liver or other tissues, according to Vincent Cassone
of Texas A&M University in College Station, who wasn't involved in the study. If Period
gene expression is suppressed, "you should get a change in [circadian] period," he said. Since the authors don't see that, Cassone suspects that they've found "purely a direct effect on output and peripheral oscillations, which I think is kind of interesting."
The authors believe that TNF-α is affecting SCN rhythms, because they found a slight reduction of one of the clock-controlled transcription factors in the SCN of TNF-α-treated mice. However, they didn't look at expression of the other clock genes.
According to Krueger, the study's findings fit in well with previous findings about immune response effects on sleep. However, he said, "I don't think TNF's effects on sleep are entirely dependent on activating or deactivating the circadian rhythm genes." TNF-α also controls expression of many other genes, Krueger said, and some of these likely influence sleep in other ways.
From an evolutionary point of view, increased sleep during illness may help animals to fight off infections or may keep sick animals from socializing and spreading an infection to others, Fontana said. It's possible that increased sleep is merely a byproduct of illness, Opp said, but many researchers believe that "the way we sleep when we are sick facilitates recovery."
Melissa Lee Phillips
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