Temperature Compensation in Circadian Clock Explained by Fruit Fly Mutation

Scientists have made a significant discovery about the inner clock that regulates the sleep-wake cycle in organisms, known as the circadian clock. The team, led by Professor Ralf Stanewsky from the University of Münster in Germany, in collaboration with researchers at Dalhousie University in Canada and the University of Mainz in Germany, has found a point mutation in the fruit fly Drosophila melanogaster that causes the circadian clock to lengthen in response to higher temperatures.

Normally, the circadian clock runs for 24 hours, but in the presence of the perI530A mutation, the clock runs for 29 hours at a temperature of 29 degrees Celsius. This lengthening of the clock period also affects the activity of the period gene in the clock neurons of the brain.

The mutation studied by the team affects a nuclear export signal (NES), which is also found in the period genes of mammals and plays a role in transporting proteins out of the cell nucleus. It was previously unknown what the function of this export was. The current study, published in the journal Current Biology, shows that the mutation leads to an extended retention of the PERIOD protein in the cell nucleus of central clock neurons, only at higher temperatures.

To investigate this phenomenon, the scientists used fruit fly mutants with a modification in the period gene, which they produced using CRISPR/Cas9 mutagenesis and homologous recombination. These mutants were then tested to see if their sleep-wake cycle and activity differed depending on the ambient temperature.

The researchers used various methods to visualize the clock genes and their activity in the brain neurons, including a new technique called Locally Activatable BioLuminescence (LABL) developed in collaboration with researchers in Canada. This method allows for the measurement of rhythmic gene expression in clock neurons in living flies.

The team found that the mutant protein had normal fluctuations and chemical changes at colder temperatures, but at warm temperatures, there were reduced chemical changes and decreased protein activity. The researchers also observed that the mutation protein accumulated in the nucleus of clock cells at higher temperatures, suggesting that the normal protein is exported from the nucleus at warmer temperatures.

The discovery of the role of the nuclear export signal in temperature compensation provides an important piece in the puzzle of understanding why the inner clock runs in an almost unchanging way despite fluctuations in temperatures, a phenomenon known as temperature compensation. Understanding the mechanisms behind this process could have significant implications for studying sleep and circadian rhythms in humans and other organisms.