Nobel Laureate Series What Makes Us Tick?

The existence of an internal biological clock has been known since ancient times, but the inner workings of that clock—what makes life on earth tick—remained a mystery until the three American geneticists investigated the clock’s inner workings and explained how plants, mammals, and humans adapt their circadian rhythm to synchronize with the Earth’s rotation. In the 18th century, a French astronomer Jean Jacques d’Ortous de Mairan observed how mimosa plants opened and closed their leaves in response to sunrise and sunset, even when placed in complete darkness. He concluded that the plant had its own biological mechanism— the circadian rhythm—that enabled it to respond to these fluctuations. Over 200 years later, American researchers Seymour Benzer and Ronald Konopka demonstrated how mutations in an unknown gene disrupted the circadian clock of fruit flies. They named the mutation period, but their findings did not apply to humans nor did they explain how the phenomenon came about. These studies on fruit flies formed the foundation for Hall and Rosbash’s work in the early 1980s at Brandeis University in Boston. Young, meanwhile, was working independently at Rockefeller University in New York to isolate the period gene. Hall and Rosbash discovered that PER, the protein encoded by period, accumulated during the night and degraded during the day and that it oscillated over a 24-h cycle in synchronization with the circadian rhythm. How these circadian oscillations could be generated and sustained remained unclear. The pair hypothesized that the PER protein blocked the activity of the period gene via an ‘inhibitory feedback loop’ and could thus prevent its own synthesis and thereby regulate its own level in a continuous, cyclic rhythm (Figure 1). However, in order to block the activity of the period gene, PER protein, which is produced in the cytoplasm, would have to reach the genetic material in the cell nucleus. To fully understand how PER protein builds up in the nucleus during the night, Hall and Rosbash needed to identify how it got there. In 1994, Young discovered a second clock gene, timeless, encoding the TIM protein that was required for a normal circadian rhythm. He showed that when TIM bound to PER, the two proteins were able to enter the cell nucleus where they blocked period gene activity to close the inhibitory feedback loop (Figure 2). This however, failed to identify what controlled the frequency of the oscillations until Young identified another gene, doubletime, encoding the DBT protein that delayed the accumulation of the PER protein. This explained how an oscillation is more closely adjusted to match a 24-h cycle. Together, these discoveries provided a ‘key’ by establishing the mechanistic principles which ‘unlocked’ the inner workings of the biological clock and identified how the component parts work together. These ‘fundamental brilliant studies’ were credited with solving one of the great puzzles in physiology and were judged to have ‘unravelled