Buried deep inside our skulls, in the midline crevice betweenthe two brain hemispheres, nestles the pea-size, pine cone-shaped pineal gland. The brain hugs it on all sides, but thepineal is not a part of the brain. What it is and what it doeshave been anybody's guess for centuries.
Since daily and seasonal biological rhythms were first definedon a scientific basis, scientists have vaguely described thepineal as a component of the "biological clock" that controlssuch cyclic phenomena as sleep and sex drives. But untilbiology got down to the molecular level, in the past decade ortwo, the biological clock ticked away inside a tightly lockedblack box.
It opened a crack last week when Nature carried a paper titled"Adrenergic signals direct rhythmic expression oftranscriptional repressor CREM in the pineal gland."
But the so-called biological clock remains almost as inscrutableas the mechanical clock's precursor did to one king in ancientGreece. He found his favorite philosopher on the beach, staringat a stick in the sand. The sage explained to his monarch thatthe shadow cast by the sun on this vertical pole grew longerand shorter from dawn to noon to dusk, enabling him to tell thetime of day. "Amazing," exclaimed the ruler, reaching out hishand. "I must have that stick!"
Two millennia later the famous French philosopher ReneDescartes described the pineal not as a clock, but as the body's"third eye." This was a shrewd guess in light of the gland'scircadian responsiveness to the light-dark cycles of day-nightand summer-winter. Now, developmental biologists think thepineal's tissue seems to be derived from embryonic tissues thatresemble those of the eye.
The principal author of the Nature paper is Paolo Sassone-Corsi,research director of the eukaryotic molecular geneticslaboratory of the French National Center for Scientific Researchin Strasbourg.
He and his colleagues have traced and elucidated some of thekey periodic promoters and repressors of gene expression inthe hyper-complex swings of the molecular pendulum thatgovern the bio-clock's movements from light source to pineal totarget tissue.
"Every living form on this planet is directly under the influenceof the seasons and day-night shifts," Sassones-Corsi said. "Thatworks through the light that gets to the retina, which starts aseries of physiological and neuroendocrine processes."
When the mammalian retina perceives light or darkness, ittransmits the message to a neuronal switch point in the opticnerve complex behind the eyes called the suprachiasmaticnucleus. From here, the signal goes to the pineal gland, whichsynchronizes biological rhythms.
Sassone-Corsi told BioWorld that a cyclic, rhythmic release ofadrenergic stimuli from the suprachiasmatic nucleus activatesthe cyclic adenosine 3',5'-cyclic phosphate (cAMP) pathway.
"Cyclic AMP," he explained, "is a 'third messenger,' known toactivate a cascade of gene expression events in the cell." Headded that 95 percent of the pineal consists of cells,pinealocytes, "that are very strongly sensitive to cAMP."
When Sassone-Corsi and his colleagues looked at a gene calledCREM (cAMP-responsive element modulator) that they hadcloned in the pineal gland, "we saw a dramatic change in itsexpression between day and night."
CREM is one of many master genes in the system that encodeproteins that by a cascade of complex molecular eventsregulate the transcription of gene expression. "These proteinscan be both activators and repressors of gene transcription,"the Strasbourg researcher said. "This gene in the pineal gland isexpressed very, very strongly at night, almost not at all duringthe day."
To reach this determination, the group set up rats in a dailyregimen of 12 hours light, 12 hours darkness. At progressiveintervals, they removed serial brain sections fromconsecutively sacrificed animals. They performed in situhybridization of these cerebral slices with an antisense CREM-specific riboprobe. The hybridization signals obtained were asdifferent as night and day.
The researchers call the form of protein that CREM synthesizesduring night "ICER" (inducible cAMP early repressor). "ICER,which acts only as a DNA binder, is a very strong repressor ofcAMP-induced transcription," said Sassones-Corsi. "It'sproduced because there is an internal promoter, a regulatoryregion, inside one intron in the gene that has a promoter andmakes this isoform," he said.
"cAMP itself induces this promoter, as our paper reports. Andthis circadian rhythm in gene expression arises in thesuprachiasmatic nucleus when it releases the adrenergicstimulus to activate the cAMP pathway," said Sassones-Corsi."We believe that with this report in Nature, we somehow shednew light on the story," he added.
"Today, ICER is probably the closest we can get in molecularterms to the real biological clock," Sassones-Corsi said. This isbecause ICER may be directly related to production of thepineal hormone, melatonin. Melatonin is able to influence manycircadian rhythm processes, such as sleep cycles, gonadalfunction, lactation, growth and stress responses. Its synthesisincreases dramatically at night and falls off by day.
"The way the hormone works is still kind of obscure," he noted."Its receptor has not been characterized or cloned."
Where does his team's work go from here? By creatingtransgenic (or knockout) mice, he hopes to alter the animals'circadian-clock rhythms and so identify the downstream geneskept silent by the likes of his ICER nocturnal repressor gene. "Ifwe eventually get to understand rodents very well," heobserved, "we'll get to understand humans."
When that time comes, Sassones-Corsi concluded, "we will beable to understand how to block, for instance, jet lag, contributeto the suspended animation of astronauts en route to Mars, orallow people not to sleep and still go on feeling fine."
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.