May 1
Dartmouth Medical School
Contact: Hali Wickner; (603) 650-1520; [email protected]
Promising Clockwork Clues Found
Dartmouth Medical School geneticists decoding the biological clocks that pace
the daily activities of plants and animals have discovered new clues to what
makes cells tick. Their discoveries, reported May 2 in the magazines Science
and Cell , are expected to advance understanding of how both light and
temperature regulate the circadian rhythm, the 24-hour cycle most organisms --
from plants to people -- share.
"Biological clocks are the cellular basis of the commonly known
circadian rhythms that determine many of our body's functions, including when
we go to sleep and wake up. Slow resetting by the clock is the underlying
cause of jet lag, and clock malfunction has been linked to seasonal affective
disorder and various sleep and mental disorders," explain researchers Jay
Dunlap, PhD, professor of biochemistry, and Jennifer Loros, PhD, associate
professor of biochemistry.
Their findings published in the May 2 issue of Science suggest a link
in the evolutionary spectrum from light perception to time keeping that paves
the way for detailing the gears of the modern clock in many organisms.
Another set of results, published in back-to-back papers in the May 2
issue of Cell, will help researchers unravel how biological clocks are
assembled and how they keep time over the wide range of environmental
conditions living things encounter.
Their work is supported by the National Institutes of Health (National
Institute of General Medical Sciences and the National Institute of Mental
Health), the National Science Foundation, and the U.S. Air Force.
Though they dictate diverse functions, all circadian clocks share
common characteristics, including their 24-hour light-dark cycle. Visible
light, along with temperature, is a major cue for the internal circadian
rhythms that time behavior and metabolism in most plants and animals. Given
the ubiquity, similarity and cellular basis of these biological clocks,
findings in lower forms are likely to apply to humans.
Research by Loros and Dunlap, who for years have pieced together clocks
in one of the best-known model systems, has helped explain how light resets all
biological clocks and how the clockwork is built. Exploring cellular
timekeepers that tell the bread mold, Neurospora, when to send out spores, the
investigators were the first to demonstrate how light resets the circadian
rhythm.
The Science findings, with Susan Crosthwaite, postdoctoral fellow,
detail the actions of two clock proteins, White Collar-1 and White Collar-2,
which regulate light responses. The researchers found that the two white collar
proteins are also essential to the circadian clock, or oscillator, and that
they work in the dark without light stimulation.
"Previous activities ascribed to the proteins were associated only with
light signals, so their the involvement time keeping came as a complete
surprise, "said Dunlap.
The dual function of the proteins opens an evolutionary window on the
origin of biological timers from primitive proteins that rely on light.
"This provides a clear connection between molecules involved in light
perception and in circadian rhythmicity in an organism, and strongly suggests
an evolutionary link connecting ancient proteins involved in photoreception
with modern proteins required for the assembly of circadian clocks in organisms
ranging from fungi through mammals," Loros said.
Results reported in Cell explain some fundamental properties common
to all biological clocks: their response to ambient temperature changes.
"Ambient temperature directly determines the presence or absence of
rhythmicity in plants and animals," notes Dunlap, "and abrupt changes in
temperature reset the clock very efficiently".
The white collar proteins discussed in Science are similar to others
involved either in light responses in plants or separately in time keeping in
insects. The sequence of amino acids comprising the proteins indicates ties to
photoreception proteins in bacteria and plants and also to time keeping
proteins in the fruit fly, Drosophila, evidence that all biological clocks may
share common components, say the researchers.
The clockwork cycle is an intricate loop where products feed back to
shut off activity, based on light relays. If clocks operated solely on a
negative feedback delay, they would wind down quickly. The white collar
proteins, particularly White Collar-2, are proteins with a known biochemical
function that are involved in the timekeeping loop; they provide the positive
feedback in the loop that was anticipated, but not previously found.
White Collar-1 is a clock-associated protein, required for sustained
rhythmicity in the dark, but outside the feedback loop. White Collar-2,
however, appears to be a clock component required to complete the loop. Unlike
other clock components only associated with keeping time, White Collar-2 plays
a distinct role in light response and is also necessary to operate the clock in
constant darkness.
All organisms have exhibited a correlation between the ability to
respond to light and the ability to keep time, and vertebrate tissues that can
respond to light also have internal biological clocks.
In Cell, Dunlap and Loros, with postdoctoral fellows Norman Garceau
and Yi Liu, report studies of the Frequency (FRQ) protein, a central cog in the
Neurospora biological clock. The researchers found that ambient temperature
determines both how much FRQ protein is made, and also the form; two forms of
the protein arise from the single frequency (frq) gene. The organism has
adapted to make the two different forms of the FRQ protein to control rhythm
over a wide range of temperatures.
Ambient temperature regulates the ratio of the two forms in a way that
expands the temperature range over which the clock will keep time. This
explains in large part why organisms lose their ability to keep time at low
temperatures.
"It's an aspect of biological timing that is common to a wide variety
of living things," notes Loros.
The first paper describes how a single gene, frq, gives rise to the two
forms of FRQ that are subsequently modified within cells in a time-of-day
specific manner as a part of the operation of the clock. Eventually the
products feed back to shut off the activity of their own gene. The feedback
cycle thus generated is the molecular basis of the circadian clock in
Neurospora, and similar clocks are believed to function in most, if not all,
higher organisms, including people.
The second paper shows how ambient temperature regulates the ratio of
the two forms of FRQ in a specific way to expand the temperature range over
which the clock will function.
-DMS-
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