There’s a season for everything, King Solomon reportedly said: a time to be born and a time to die, a time for war and a time for peace. Modern science says there’s also the best time to wake up – at 6 am, when metabolism starts running; the best time for love – at 8 am, when sex hormones are about to peak; the best time to see a dentist – at 2 pm, when sensitivity to pain drops; and the worst time to drive – at 2 am, when the body is geared for its deepest sleep.
Such peaks and valleys are determined by internal biological “clocks” that operate in approximately 24-hour cycles known as circadian, from Latin,
circa diem, meaning “approximately a day.” These clocks regulate the daily fluctuations in heartbeat, blood pressure, kidney function, body temperature, sleeping and waking, sensory perception and the secretion of many hormones. The body’s master circadian clock, residing in the brain, synchronizes a multitude of peripheral clocks present not only in every organ in the body but in every single cell. “Our body is a collection of myriad microscopic clocks that all ‘tick’ in unison,” says
Dr. Gad Asher, who recently joined the Weizmann Institute’s Biological Chemistry Department. “Moreover, amazingly enough, these clocks function perfectly well even when the cells are cultured in a laboratory dish.”
A major aim of Asher’s new lab at the Institute is to find out why we need circadian clocks and how they function. Though science does not yet have an answer to these questions, it’s obvious that the clocks are vital. For one, studies suggest that shift workers tend to have a higher incidence of cancer, diabetes and obesity, as well as signs of accelerated aging, apparently because their circadian clocks are disrupted by work patterns that don’t follow the regular day-and-night cycle. Another indication of the clocks’ importance is their survival throughout millions of years of evolution, from plants and bacteria to humans: Such survival generally means that a mechanism is important enough for nature to keep. Finally, as many as 15 percent of all genes have circadian rhythms. “That’s an incredibly high number – it means that various processes in our bodies run entirely differently at different times of the day,” says Asher.
One striking example of such a process was discovered by accident, as often happens in science, in the lab of Prof. Ueli Schibler at the University of Geneva. Several years before Asher joined that lab to conduct his postdoctoral research, Schibler’s team had discovered that a protein called DBP could be purified in unusually high amounts from liver cells. But after the scientists had already published these findings in a prestigious journal, something seemed to go terribly wrong. When they repeated the same experiments, they suddenly couldn’t purify the protein at all. After a brief spell of panic, the quandary was resolved. It turned out that the original experiments had been performed by a student who used to come to the lab at 6 am, as was the custom in his Swiss peasant family. In contrast, the follow-up experiments were conducted by an American student who, needing a rest after late-night partying, used to start work at 2 pm. It so happened that the liver protein under study was regulated by the circadian clock: Its early-morning production was a hundred times higher than in the afternoon.
This fascinating discovery gave rise to an entire series of in-depth studies of circadian clocks in Schibler’s Geneva lab. “Geneva is a world leader in all types of clock – not just the Swiss cuckoo clocks, but circadian clocks as well,” says Asher.
Among the major questions addressed by Asher’s current studies is how circadian clocks are regulated. It’s known that the master clock in the brain is reset every day by light-dark cycles. As for the clocks in peripheral organs, the major cues for their synchronization are feeding times; it therefore seems that peripheral clocks are extremely sensitive to the metabolic state of cells. While working in Prof. Schibler’s lab, Asher discovered that a protein called SIRT1, which plays a central role in cellular metabolism, also controls the circadian clocks’ function. This discovery, according to the journal Cell, suggests that the SIRT1 might be the “missing link” between metabolism and circadian clocks. In his lab at Weizmann, Asher now intends to expand his studies into the relationship between metabolic factors and circadian mechanisms.
Asher’s ultimate goal is to clarify how circadian clocks work at the molecular and cellular levels, and how the central clock in the brain synchronizes peripheral clocks in other organs. His studies are aimed at answering fundamental biological questions that are related to a wide variety of physiological and pathological conditions – from jetlag and sleep disturbances to cancer, diabetes, obesity and aging.
Devoted to science
Born in Ramat Gan, Gad Asher started out by studying mathematics as part of a special program at Tel Aviv University to promote excellence, but then switched to medicine, earning his M.D. degree with distinction in 1998. Already as a medical student, he was fascinated by biological research, spending three summer internships at the Weizmann Institute. While working as a resident internist at the Tel Aviv Sourasky Medical Center, he began doctoral studies in the Weizmann Institute’s Department of Molecular Genetics under the guidance of Prof. Yosef Shaul. Upon earning his Ph.D. degree in 2006, he decided to devote himself entirely to science. After four years of postdoctoral research at the University of Geneva, Switzerland, he joined the Weizmann faculty as a Senior Scientist in May 2011. Outside the lab, he plays classical piano and enjoys bicycle racing and mountain climbing.
Dr. Gad Asher's research is supported by the estate of Dorothy Geller; Ms. Rudolfine Steindling, Austria; and the Willner Family Leadership Institute.
Rejecting Arsenate
Not long ago, some unassuming bacteria found themselves at the center of a scientific controversy: A group claimed that these microorganisms, which live in an environment that is rich in the arsenic-based compound arsenate, could take up that arsenate and use it – instead of the phosphate on which all known life on Earth depends. The claim, since disproved, raised another question: How do organisms living with arsenate pick and choose the right substance?
Chemically, arsenate is nearly indistinguishable from phosphate. Prof. Dan Tawfik of the Biological Chemistry Department says: “Phosphate forms highly stable bonds in DNA and other key biological compounds, while bonds to arsenate are quickly broken. But how does a microorganism surrounded by arsenate distinguish between two molecules that are almost the same size and have identical shapes and ionic properties?”
To investigate, Tawfik, postdoctoral fellow Dr. Mikael Elias, Ph.D. student Alon Wellner and lab assistant Korina Goldin, in collaboration with Tobias Erb and Julia Vorholt of ETH Zurich, looked at a protein in bacteria that takes up phosphate. This protein, called PBP (short for phosphate binding protein), sits near the bacteria’s outer membrane, where it latches onto phosphates and passes them on to pumps that transport them into the cell.
In research that recently appeared in Nature, the team compared the activity of several different PBPs – some from bacteria like E. coli that are sensitive to arsenate and others, like those from the arsenic-rich environment, which are tolerant of the chemical. While the PBPs in the ordinary bacterium were about 500 times more likely to bind phosphate over arsenate, in the arsenic-tolerant bacterium that factor jumped to around 5000. In other words, to cope with their toxic environment, the bacteria evolved a mechanism of extreme selectivity to ensure their supply of phosphate while keeping the arsenate out.
Elias then compared phosphate and arsenate binding by crystallizing PBPs along with one of the two compounds. But the initial comparison suggested that when arsenate bound to the protein, it did so in just the same way as phosphate. Elias suspected that the key might lie in a single, highly unusual bond between a hydrogen atom in the protein and the molecule. This bond had been previously noted but ignored, as phosphate binding occurred with or without it.
To see the difference, the team had to stretch the limits of crystallization technology, getting the resolution to less than one angstrom – fine enough to identify individual hydrogen atoms and compare their bonds. Only then were they able to identify a single disparity: The angles of that unusual hydrogen bond were different. Inside a tight cavity within the PBP structure, phosphate binds at a “textbook angle,” according to Elias. The slightly larger arsenate molecule, on the other hand, gets pushed up against the hydrogen and bonds at unnatural, distorted angles. Tawfik thinks that the angle is likely to lead to repulsion between the molecule and other atoms in the cavity, preventing the PBP from passing arsenate into the cell’s interior.
Tawfik: “These findings may go beyond the solving of a biological mystery. Because phosphates are scarce in many environments, there is quite a bit of interest in understanding how this crucial resource is taken up by organisms. This first observation of a PBP discrimination mechanism is an exciting demonstration of the exquisite fine tuning that enables proteins to distinguish between two nearly-identical molecules.”