Buried deep inside our skulls, in the midline crevice between the two brain hemispheres, nestles the pea-size, pine cone- shaped pineal gland. The brain hugs it on all sides, but the pineal is not a part of the brain. What it is and what it does have been anybody's guess for centuries.
Since daily and seasonal biological rhythms were first defined on a scientific basis, scientists have vaguely described the pineal as a component of the "biological clock" that controls such cyclic phenomena as sleep and sex drives. But until biology got down to the molecular level, in the past decade or two, the biological clock ticked away inside a tightly locked black box.
It opened a crack last week when Nature carried a paper titled "Adrenergic signals direct rhythmic expression of transcriptional repressor CREM in the pineal gland."
But the so-called biological clock remains almost as inscrutable as the mechanical clock's precursor did to one king in ancient Greece. He found his favorite philosopher on the beach, staring at a stick in the sand. The sage explained to his monarch that the shadow cast by the sun on this vertical pole grew longer and shorter from dawn to noon to dusk, enabling him to tell the time of day. "Amazing," exclaimed the ruler, reaching out his hand. "I must have that stick!"
Two millennia later the famous French philosopher Rene Descartes described the pineal not as a clock, but as the body's "third eye." This was a shrewd guess in light of the gland's circadian responsiveness to the light-dark cycles of day-night and summer-winter. Now, developmental biologists think the pineal's tissue seems to be derived from embryonic tissues that resemble those of the eye.
The principal author of the Nature paper is Paolo Sassone-Corsi, research director of the eukaryotic molecular genetics laboratory of the French National Center for Scientific Research in Strasbourg.
He and his colleagues have traced and elucidated some of the key periodic promoters and repressors of gene expression in the hyper-complex swings of the molecular pendulum that govern the bio-clock's movements from light source to pineal to target tissue.
"Every living form on this planet is directly under the influence of the seasons and day-night shifts," Sassones-Corsi said. "That works through the light that gets to the retina, which starts a series of physiological and neuroendocrine processes."
When the mammalian retina perceives light or darkness, it transmits the message to a neuronal switch point in the optic nerve complex behind the eyes called the suprachiasmatic nucleus. From here, the signal goes to the pineal gland, which synchronizes biological rhythms.
Sassone-Corsi told BioWorld that a cyclic, rhythmic release of adrenergic stimuli from the suprachiasmatic nucleus activates the cyclic adenosine 3',5'-cyclic phosphate (cAMP) pathway.
"Cyclic AMP," he explained, "is a 'third messenger,' known to activate a cascade of gene expression events in the cell." He added 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 called CREM (cAMP-responsive element modulator) that they had cloned in the pineal gland, "we saw a dramatic change in its expression between day and night."
CREM is one of many master genes in the system that encode proteins that by a cascade of complex molecular events regulate the transcription of gene expression. "These proteins can be both activators and repressors of gene transcription," the Strasbourg researcher said. "This gene in the pineal gland is expressed very, very strongly at night, almost not at all during the day."
To reach this determination, the group set up rats in a daily regimen of 12 hours light, 12 hours darkness. At progressive intervals, they removed serial brain sections from consecutively sacrificed animals. They performed in situ hybridization of these cerebral slices with an antisense CREM- specific riboprobe. The hybridization signals obtained were as different as night and day.
The researchers call the form of protein that CREM synthesizes during night "ICER" (inducible cAMP early repressor). "ICER, which acts only as a DNA binder, is a very strong repressor of cAMP-induced transcription," said Sassones-Corsi. "It's produced because there is an internal promoter, a regulatory region, inside one intron in the gene that has a promoter and makes this isoform," he said.
"cAMP itself induces this promoter, as our paper reports. And this circadian rhythm in gene expression arises in the suprachiasmatic nucleus when it releases the adrenergic stimulus to activate the cAMP pathway," said Sassones-Corsi. "We believe that with this report in Nature, we somehow shed new light on the story," he added.
"Today, ICER is probably the closest we can get in molecular terms to the real biological clock," Sassones-Corsi said. This is because ICER may be directly related to production of the pineal hormone, melatonin. Melatonin is able to influence many circadian rhythm processes, such as sleep cycles, gonadal function, lactation, growth and stress responses. Its synthesis increases 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 creating transgenic (or knockout) mice, he hopes to alter the animals' circadian-clock rhythms and so identify the downstream genes kept silent by the likes of his ICER nocturnal repressor gene. "If we eventually get to understand rodents very well," he observed, "we'll get to understand humans."
When that time comes, Sassones-Corsi concluded, "we will be able to understand how to block, for instance, jet lag, contribute to the suspended animation of astronauts en route to Mars, or allow people not to sleep and still go on feeling fine."
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.