Body clocks not all created equal
Study of mouse with short circadian cycle lends insight into what drives daily rhythms
By Deborah Wormser
UT Southwestern Medical Center researchers say their study of a mouse with a short circadian cycle has provided new insight into how the body’s cellular clock operates.
“In mammals, the circadian clock regulates the daily ebb and flow of behavior, physiology, and metabolism,” said Dr. Joseph Takahashi, Chair of Neuroscience and senior author of the study published Feb. 28 in Cell. Dr. Takahashi is also a Howard Hughes Medical Institute (HHMI) Investigator.
The study’s lead author is Dr. Seung-Hee Yoo, Instructor of Neuroscience.
Circadian clocks, which are located in cells throughout the body, govern daily cycles and can affect a person’s ability to get a good night’s sleep or bounce back from jet lag. They can even determine the best time to take some medicines.
The mammalian Clock gene, which Dr. Takahashi discovered in 1997, regulates circadian rhythms by encoding the CLOCK transcription factor, a protein that binds to other genes and controls whether they become active. The CLOCK and BMAL1 transcription factors form the CLOCK:BMAL1 complex in the nucleus of cells that is at the heart of the cellular clock.
The transcription factors activate the circadian timekeeper by binding soon after the start of the circadian day (wake time in people; rest time in mice, which are nocturnal). The repressor proteins, most notably Period (PER) and Cryptochrome (CRY), inhibit the transcription factors and turn off their genes during the circadian night and then become degraded so that the process can begin again in a self-regulating transcriptional ebb and flow through the normal 24-hour cycle, Dr. Takahashi explained.
“Although significant progress has been made in understanding the majority of the components of the circadian clock in mammals, very little is known about the molecular mechanisms that define the periodicity or rate of the circadian clock,” Dr. Takahashi said.
In this study, the researchers identified a mouse mutation, Past-time, that is associated with a circadian period of just under 23 hours. It is caused by a missense mutation in the Fbxl21 gene, which is instrumental in tagging other proteins for degradation.
The Fbxl21 gene and the Fbxl3 gene are related to each other and, as such, would be expected to have an additive effect on one another. That means the FBXL21 protein would be expected to add to the effects of the FBXL3 protein, which was known to degrade the CRY repressor protein inside the cell’s nucleus. But that’s not what happened, Dr. Takahashi said.
“Our genetic interaction experiments in mice showed that FBXL21 counteracts FBXL3 to counterbalance the degradation of CRY. The mechanism by which FBXL21 protects CRY is quite unusual and novel for a protein that tags another protein for degradation,” Dr. Takahashi said. “FBXL21 interacts (binds) more strongly with CRY than does FBXL3, and as a consequence can protect CRY from degradation,” he explained.
The researchers also found that the reason for the unexpected activity of FBXL21 seems to be location, location, location: The distribution of FBXL21 and FBXL3 are strikingly different, he said.
“FBXL3 is found almost exclusively in the nucleus, while FBXL21 is distributed in both nucleus and cytoplasm. FBXL21 plays a dual role: protecting CRY from FBXL3 degradation in the nucleus and promoting CRY degradation in the cytoplasm.
“Countless models have placed the location of FBXL3 activity in the cytoplasm, which is incorrect in retrospect. Clearly, a revision in our thinking is in order. This study reveals a prominent and sensitive role of the nucleus in regulating circadian period and the speed of the clock in mammals,” he said.
UT Southwestern study co-authors included Dr. Jennifer Mohawk, a research specialist in the HHMI; Dr. Yongli Shan, a postdoctoral researcher in Neuroscience; Dr. Seong Kwon Huh, a former research technician now attending Texas A&M University; Izabela Kornblum, a laboratory manager in the HHMI; Dr. Vivek Kumar, Instructor of Neuroscience; Dr. Nobuya Koike, a senior research associate in Neuroscience; Dr. Ming Xu, Instructor of Molecular Biology; Dr. Xinran Liu, a former Assistant Professor of Neuroscience and Director of the Neuroscience Morphology and Imaging Core at UTSW now at Yale University School of Medicine; Dr. Zhijian “James” Chen, Professor of Molecular Biology and in the Center for the Genetics of Host Defense; and Dr. Carla Green, Professor of Neuroscience.
Other researchers participating in the study included Dr. Sandra Siepka and Dr. Hee-Kyung Hong of Northwestern University, Dr. Justin Nussbaum of the University of Virginia, and Dr. Zheng Chen of UT Health Science Center at Houston.
The study was funded by the National Institutes of Health, the HHMI, the Welch Foundation, and the American Heart Association.
Dr. Chen holds the George L. MacGregor Distinguished Chair in Biomedical Science.
Dr. Green is a Distinguished Scholar in Neuroscience.
Dr. Takahashi holds the Loyd B. Sands Distinguished Chair in Neuroscience.