RIKEN and others unravel 20-year-old mystery of how left-right asymmetry in organ building is determined at an early stage by mechanical forces

Organs function properly when they are asymmetrically positioned, for example when the heart is on the left and the liver to the right. Whether the signals that determine these differences between the left and right sides of the body are a chemical or a mechanical stimulus has been a mystery for 20 years. A joint research group led by Team Leader Hiroshi Hamada, Takanobu A. Katoh, Team Leader Yasushi Okada and Team Leader Atsuko Iwane at the RIKEN Center for Biosystems Dynamics Research, Assistant Professor Toshihiro Omori and Professor Takuji Ishikawa of the School of Engineering, Tohoku University, and Professor Takayuki Nishizaka of the Department of Physics, Faculty of Science, Gakushuin University, has revealed that the signals in mammalian development that determine the differences between the left and right sides of the body are controlled by mechanical forces early in the development process. The group's findings were published in the online edition of Science.

Mammalian embryos are initially symmetrical, but asymmetry begins when a stream of dimples called "nodes," which form transiently on the ventral side of the early embryo, activates signals that determine the left side of the organism. Specifically, the researchers found that in the nodes, the dynamic cilia create a leftward flow by rotational movement and are switched through signals to immobile cilia, which act as sensors. However, the length of immobile cilia is as small as five micrometers, making detailed analysis difficult, so two theories were put forward: one that the cilia sense chemical substances transported at the node, and the other that they sense the force of the flow.

In their research, the researchers developed a proprietary optical microscopy system. The microscope contained a built-in life-support system, allowing the shape of immobile cilia to be observed at high resolution while irradiating the nodes of a live mouse embryo with high-precision ultraviolet light. It also has optical tweezers to manipulate immobile cilia. First, they visualized immobile cilia with fluorescent proteins and then photographed the differences in the three-dimensional shape of immobile cilia with a high-resolution microscope, both with nodal flow and with nodal flow stopped by disrupting the molecular motors of the dynamic cilia with UV light. The results show that the nodal flow causes the immobile cilia on the left side of the node to bend ventrally and the immobile cilia on the right side to bend dorsally.

Next, they trapped microbeads with optical tweezers and measured the signal that determined the left side while bending the immobile cilia artificially. They found that signals determining the left side were only activated when immobile cilia were bent ventrally. In other words, they realized that immobile cilia are activated by mechanical forces.

"We didn't understand the function of force in developmental biology," said Katoh. "I specialize in microscopy, while Dr. Nishizaka can manipulate genes, and Dr. Omori is a mathematical analyst. I think the discovery from this research was due to the work of researchers from different disciplines working together."

Journal Information
Publication: Science
Title: Immotile cilia mechanically sense the direction of fluid flow for left-right determination
DOI: 10.1126/science.abq8148