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15 November 2012

Study sheds light on genetic ‘clock’ in embryonic cells

As they develop, vertebrate embryos form vertebrae in a sequential, time-controlled way. This process of body segmentation is controlled by a kind of ‘clock’, regulated by the oscillating activity of certain genes within embryonic cells.

A new international cross-disciplinary collaboration between physicists and molecular genetics researchers advances scientists’ understanding of this crucial biological timing system. The study, co-authored by McGill University Professor Paul François and Ohio State University Professor Sharon L. Amacher and published in Developmental Cell, sheds light on the clock mechanism by providing the first real-time, visual evidence of how it operates at the level of individual cells.

While previous scientific studies have examined the oscillation phenomenon in the tissue of mouse embryos, the McGill and Ohio State researchers were able to observe and analyse it in single cells. To do so, they genetically modified zebra fish – a freshwater fish whose body is nearly transparent during early development, making its anatomy easy to observe. The researchers used a fluorescent marker in the transgenic fish and developed software tools to monitor the concentration of a certain “cyclic” protein, whose production rises and falls with the oscillating expression of the molecular clock genes.

It is known that cells communicate with neighbouring cells through a messaging system known as the Notch signalling pathway. In their experiments with the zebra fish, the researchers cut off this inter-cellular communication network – enabling them to see how that would affect the oscillation pattern in individual cells and their neighbours.

These experiments revealed that cyclic protein concentrations in individual cells of the zebra fish continued to rise and fall, indicating that they continued to oscillate. With the inter-cellular signalling pathway blocked, however, the oscillations were no longer synchronized among neighbouring cells. The cellular clocks were still ticking, in other words, but not in unison. This finding confirms that the Notch pathway serves to coordinate timing among cells – a crucial role, since the cells must act in concert in order to form vertebrae.

By observing normal zebra fish embryos, the researchers were also able to show that cells desynchronize their oscillations while performing cellular division, and then later resynchronize with their neighbours as they proceed collectively to form vertebrae.

“In humans, defects in Notch signalling are associated with congenital developmental disorders called spondylocostal dysostosis, which are typified by scoliosis and trunk dwarfism caused by malformed ribs and vertebrae,” Amacher notes. “Studies such as ours may provide insight into potential therapies for human disease. It is likely that many cells in our bodies – stem cells, cancer cells – have similar molecular oscillators that regulate response to environmental signals. By unravelling such molecular clocks, we can understand how to modify them and thus change the number of oscillating cells that respond to differentiating signals, providing tremendous insight for studies in stem cell and cancer biology and tissue engineering.”

“The formation of the vertebral column is very important, because everything follows from that” in the development of vertebrates, François adds. A physicist, he developed the computer tools used to analyse video footage of the zebra fish embryos. Francois’s research focuses on the modelling of physical properties of gene networks and their evolution – a field that has emerged at the nexus of biology and physics in recent years, following sequencing of the human genome and rapid growth in scientists’ understanding of the processes inside cells.

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