The Molecular Metronome
June 24, 2013: More than 2000 years ago, the Greek Admiral Androsthenes of Tasos sat next to a tamarind tree and noticed something interesting: its leaves moved in sync with the daylight, spreading out during the day and dropping low against the branches at night.
Historians now pinpoint this observation, noted in the Admiral’s journal as he traveled along the Persian Gulf Coast, as the first written account of the circadian clock—an organism’s internal biological timing device. But despite a spattering of mentions over the next two millennia, few scientists studied the mechanisms behind this process. In fact, the term “circadian clock” wasn’t coined until the 1950s.
Scientists now know, however, about the importance of the circadian clock. But while it appears to be intertwined in the evolutionary histories of nearly every species on Earth, the inner-workings of this clock are complex and not completely understood by scientists.
To add to that complexity, recent studies identified a link between circadian clocks and overall metabolic health. But while the discovery of this link is intriguing, many questions remain as to the underlying molecular mechanisms that govern it.
Now, scientists at the Gladstone Institutes have discovered how a single protein not only receives direct instructions from the body’s circadian clock, but also regulates a series of fundamental circadian and metabolic processes. This discovery, described in the latest issue of the Journal of Neuroscience, is the first to provide a molecular missing ‘link’ between circadian clocks and metabolism—and offer clues as to how disrupting the circadian clock can throw this system out of whack.
Keeping in Sync
Virtually every organism alive today—from bacteria to humans—has a circadian clock; a biological timing mechanism that oscillates with a period of about 24 hours and is coordinated with the cycle of day and night. And while it runs independent of external cues (even animals that live in total darkness have a circadian clock), it is influenced by the rhythms of light, temperature and food availability. But studies have also shown that circadian clocks are essential for metabolic health.
“Important metabolic functions are heavily influenced by circadian clocks, which is why activities such as chronic night-shift work—which can cause a misalignment of the clock—increases one’s risk for disease,” said neuroscientist Katerina Akassoglou, PhD, who led the Gladstone study. “For example, chronic dysfunctions of the circadian clock can increase one’s risk for metabolic diseases such as obesity and type 2 diabetes, autoimmune diseases such as multiple sclerosis and even some forms of cancer.”
These initial studies suggested that an array of molecular processes need to come together to forge the connection between the circadian clock and metabolism. So in this study, Gladstone scientists delved deeper. And they found a very important protein at the heart of that connection: the p75 neurotrophin receptor (or p75NTR).
The Rhythm of p75NTR
Originally, p75NTR was only thought to be active in the nervous system. Later studies found it to be active throughout the body, suggesting that there was more to this protein than originally thought. Just last year, Dr. Akassoglou and her team discovered that p75NTR actually serves to regulate glucose levels in the blood—an important metabolic process.
“Since we had already established a link between p75NTR and metabolic processes,” recalls lead author Bernat Baeza-Raja, PhD, “we thought that there might also be a link between that very protein and the circadian clock.”
The team focused on two genes called Clock and Bmal1. The proteins generated from these so-called “circadian regulator genes,” and others like them, are known to control the body’s circadian clock. In individual cells, the team saw for the first time that these two proteins bound directly to the gene that codes for the p75NTR, a process that then initiated p75NTR production.
But perhaps even more important than how p75NTR was produced was when. The team found that p75NTR production, like the circadian clock genes themselves, oscillated in sync with the cells’ natural circadian rhythm. Experiments in mouse models corroborated these results.
And when the team genetically modified a group of mice so that they lacked the circadian Clock gene, everything else fell out of sync. The circadian oscillation of p75NTR production was disrupted, and p75NTR levels dropped. In turn, when they tested mice that lacked expression of p75NTR, they found that the regular oscillations of other circadian genes in the brain and the liver were disrupted, as well as genes known to regulate glucose and lipid metabolism.
“The finding that a loss of p75NTR affected circadian and metabolic systems is strong evidence that this protein is intricately tied to both,” said diabetes and endocrinology expert Alan Saltiel, PhD, director of the Life Sciences Institute at the University of Michigan, who was not involved in the study. “It will be fascinating to see what additional insight Dr. Akassoglou and her team will uncover.”
Indeed, what scientists know now about circadian rhythms has come a long way since Androsthenes wrote about the tamarind tree, but there is still much to do in order to translate discovery into therapies that fight metabolic and autoimmune disease. But Dr. Akassoglou is optimistic that they are on the right track.
“We’ve uncovered one such ‘molecular metronome’ in p75NTR, and there are almost certainly more to discover,” explained Dr. Akassoglou. “But with each new discovery, we are that much closer not only in understanding the complexities of the circadian clock, but also to devising therapeutic strategies that can be used to treat the variety of metabolic and autoimmune diseases linked to circadian clock dysfunction."