November 25, 1999
Shedding Light on Circadian Rhythms
Several teams of scientists have found what may be the missing
molecular link between sunlight and the circadian clocks in both
mammals and fruit flies.
Circadian rhythms, the patterns of activity that occur on a 24-hour
cycle, are important biological regulators in virtually every living
creature. In mammals, the internal circadian clock resides in the
brain, and sunlight is the cue that rewinds this clock daily.
Researchers have now found a biochemical pathway that senses blue
light, and thereby connects the sun to molecular components of the
circadian clock, says Joseph Takahashi, an
HHMI investigator at Northwestern University.
In an article published in the November 20, 1998, issue of
Science, Takahashi and Aziz Sancar of the University of North
Carolina School of Medicine in Chapel Hill report that the protein
cryptochrome 2 serves as a transducer by which light drives the
molecular machinery that generates circadian rhythm. Cryptochromes are
light-reactive pigments found in the eye and in plants.
Cryptochrome proteins 1 and 2 (CRY1 and CRY2), which were first
discovered in plants, trigger plant growth by responding to light in
the blue to ultraviolet part of the spectrum. Another article in the
same issue of Science explains how CRY1 and CRY 2 help regulate
circadian rhythm in plants.
In the May 26, 1998, issue of Proceedings of the National Academy
of Sciences, Yasuhide Miyamoto and Sancar showed that cells in the
mouse and human retina express genes similar to those that code for
plant cryptochromes. Takahashi, Sancar and their colleagues sought to
find whether these genes were part of the eye's circadian light-sensing
apparatus. There was ample circumstantial evidence: They knew that the
same retinal cells that contain CRY1 and CRY2 are connected directly to
the suprachiasmatic nucleus (SCN), a group of neurons deep in the mouse
brain. The SCN has long been thought to be a crucial circadian
pacemaker in mammals. They also knew that expression of the Cry1
gene within the SCN follows a circadian pattern.
In the mouse retina, however, CRY2 is the predominant cryptochrome
protein. To determine CRY2's role in the light-induced setting of
circadian rhythms, Takahashiís and Sancarís research
teams created mice that lacked the Cry2 gene. The researchers
then used these mutant mice to measure whether the absence of CRY2
affected expression of another gene, called mPer1, that is known
to oscillate in a circadian manner.
In mice that lacked Cry2, light produced less than half the
normal boost in mPer1 expression within the SCN. The scientists
reasoned that the residual mPer1 expression may be caused by a
second, as yet undiscovered, transducer that also conveys light signals
to the SCN. The role of CRY1 in the circadian system, say the
investigators, is still unclear.
Further experiments showed that eliminating Cry2 also
lengthened the circadian cycle by about one hour when the mice were
kept in total darkness. This effect was not seen in wild-type mice that
have Cry2.
This latter finding, say the investigators, suggests that CRY2 is
not only the molecular link between sunlight and the circadian clock,
but that it might also play a subtle role in the clock mechanism as
well. "While it is clear that CRY2 represents a light transducer for
the circadian system, we cannot think linearly—when this protein
is missing, so is a part of the clock itself," says Sancar.
Similar findings have also come from two new studies of cryptochrome
proteins in the fruit fly Drosophila. In two articles published
in the November 25, 1998, issue of Cell, HHMI investigator Michael Rosbash,
Jeffrey Hall and colleagues at Brandeis University, and Stephen Kay at
The Scripps Research Institute, confirm that cryptochrome is an
important in bringing light into the circadian system. In addition,
Rosbash says that his laboratory's work shows that without
cryptochrome, "the circadian system is largely, though not entirely,
dead."
Rosbash and his colleagues first characterized the fruit fly gene,
cry, which is similar to cryptochrome genes in plants and mice.
They found that the transcription of cry is regulated in a
circadian fashion, and is closely tied to the Drosophila
circadian system genes, period, timeless, clock, and cycle.
Next, Rosbash and his colleagues created strains of Drosophila with
mutations that disabled the cryptochrome gene. The resulting flies
lacked the normal daily oscillations in the proteins Period and
Timelesss.
Despite the dramatic impact that the mutation had on circadian
biochemistry, the overall circadian locomotor activity rhythms of these
mutant flies was surprisingly normal. But the addition of a second
mutation that leaves the fly eyes unresponsive to light created a
mutant fly that exhibited no behavioral rhythms.
According to Rosbash, these studies show that the role of
cryptochrome in circadian function in fruit flies "is simple and
uncomplicated, and it lies at the top of the pathway." If cryptochrome
does not get light, says Rosbash, the circadian system simply does not
work correctly. But for certain cells there may be a second input from
the eyes which apparently does not use the same cryptochrome protein
for its circadian light perception.
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