Enzyme activation appears key in helping internal clock tell night from day

August 18, 2004

CHAMPAIGN, Ill. -- Feel like time is repeating itself and won't move on? It could be your internal clock is backpedaling because your PKG-II is out of whack.

That scenario was played out at the University of Illinois at Urbana-Champaign in experiments at a molecular level deep within the brain of rats where, like in all mammals, the primary circadian clock is located. The clock is a dynamic biological process with a near-24-hour cycle. PKG-II is an enzyme, a protein that triggers biochemical reactions.

Reporting in the Aug. 19 issue of the journal Neuron, the Illinois scientists say that the activation of PKG-II may be the critical control point that tells our biological clock to proceed by putting night behind and journeying into a new day.

In their research, the scientists blocked the phosphorylation action of PKG-II during the normal cycling of the clock. By doing so, they disrupted the key activity of this kinase enzyme that adds phosphates to proteins, a signaling mechanism necessary for information transfer in cells.

The internal clock produces the 24-hour rhythm in the brain and all cells. The cycle consists of an automatically regulated loop of transcription and translation of special clock genes, whose products act as "gears" of the clockwork. Correct cycling is vital to the oscillation of metabolism and behaviors, which happen during sleep and wakefulness.

"Without PKG-II, the clock behaves as if locked in a dynamic loop that encompasses the biochemical state of late night," the researchers wrote. "By potentially interacting with CLOCK (a critical protein of the central clockwork) during its phosphorylation, PKG-II may influence core clock components to signal the completion of nighttime processes and permit transit to the daytime domain. Thus, clock-controlled activation of PKG-II may serve as a critical checkpoint of temporal state at the night-to-day transition, which would align with dawn in the solar cycle."

In other words, they say, such inhibition of PKG-II leads to "significant phase delay of circadian rhythm." The clock is literally forced back in time to repeat the necessary sequence of protein communications before it can advance.

"These findings suggest that 'check-point' regulatory processes like those in the cell-division cycle, which generates two cells from one, are similar in the brain clock," said Martha U. Gillette, a professor in the department of cell and structural biology in the U. of I. College of Medicine at Urbana-Champaign.

The research was done in her laboratory.
-end-
In addition to Gillette, five other researchers were involved in the study: Shelley A. Tischkau, a professor of veterinary biosciences in the College of Veterinary Medicine; postdoctoral researchers Jennifer W. Mitchell, Jessica W. Barnes and Jeffrey A. Barnes; and doctoral student Laura A. Pace.

The research was supported by six different Public Health Service grants to Gillette, Jessica and Jeffrey Barnes, Tischkau and Mitchell, and a grant to Tischkau from the Molecular and Endocrine Pharmacology Program of the UI campus Governor's Venture Technology Fund.

University of Illinois at Urbana-Champaign

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