Discovery: Inter-cellular contacts control liver regeneration

Chronic liver disease is responsible for about two million deaths worldwide each year, and there is currently no cure. Researchers have discovered a cell type that controls liver regeneration by contact.

The molecular mechanisms by which adult liver cells trigger regeneration are still largely unknown

Chronic liver disease is responsible for about two million deaths worldwide each year. But there is currently no cure. Researchers from Dresden and Cambridge have now discovered a cell type that controls liver regeneration by contact.

About 29 million people in Europe suffer from chronic liver diseases such as cirrhosis or liver cancer. They are a major cause of illness and mortality, with liver disease contributing to about two million deaths worldwide each year. Currently, there is no cure and liver transplants are the only treatment for liver failure. Scientists are therefore exploring new ways to harness the regenerative capacity of the liver as an alternative to restoring its function.

It has been known since Aristotle that the human liver has the greatest regenerative capacity of any organ in the body and can regrow even after 70% amputation. This makes transplants from liver donors possible. Although the liver regenerates completely after an injury, the mechanisms that regulate how the regeneration process is activated or stopped are still largely unknown. 

Researchers at the Max Planck Institute for Molecular Cell Biology and Genetics (MPI-CBG) in Dresden, the Gurdon Institute (Cambridge, United Kingdom) and the Department of Biochemistry, University of Cambridge, have now found that a regulatory cell type - mesenchymal stem cells - can activate or stop liver regeneration. The mesenchymal cells do this through the number of contacts they make with regenerating cells (epithelial cells). This study suggests that errors in the regeneration process that can lead to cancer or chronic liver disease, are caused by an incorrect number of contacts between the two cell types. The study has been published in the peer-reviewed journal Cell Stem Cell.

Development of mini-livers

Scientists at the Max Planck Institute for Molecular Cell Biology and Genetics (Dresden, Germany), together with colleagues from the Gurdon Institute at the University of Cambridge, are researching the biological basis of liver regeneration in adults. In 2013, Meritxell Huch and Hans Clevers developed the first liver organoids - miniature liver tissues that were created from mouse liver cells in a Petri dish in the laboratory. The researchers even succeeded in transplanting the organoid into a mouse, where it could take over liver functions. In 2015, they successfully transferred this liver organoid technology to the cultivation of a human liver in a Petri dish based on human liver samples. In 2017, they developed a similar system based on human liver cancer.

A surprising, exciting observation

The two most important functional cells in the adult liver are the hepatocytes, which perform many functions in the liver, and the ductal cells, which form the network of tiny ducts through which bile is passed into the intestine. These work together with other supporting cells, such as the blood vessels or the mesenchymal cells. To build liver organoids, the research team initially used only ductal cells of the bile duct.

To improve this model and make it more similar to the real liver, PhD student Lucía Cordero-Espinoza and postdoctoral researcher Anna Dowbaj planned to build a more complex liver organoid that better mimics the cellular interactions and architecture of adult liver tissue. To do this, they added liver mesenchyme - a type of regulatory cell of connective tissue that supports the tubular structure of the bile duct. "We placed the mesenchyme cells in a petri dish next to the organoid made of ductal cells and saw that they did not touch or connect as they do in natural tissue," explained Anna Dowbaj.

The researchers turned to Florian Hollfelder at the University of Cambridge, who knew of a method to connect the cells in tiny gels where they could make contact. Anna Dowbaj and her team "were curious to see how our new and more complex organoid mimicked the architecture of the tissue in the dish, so we decided to study the behaviour of the cells and filmed them under the microscope. To our surprise, we observed a completely unexpected behaviour: The tissue (organoid) shrank when in contact with the mesenchyme cells, but grew when there was no contact. This strange behaviour was very perplexing, but it could help us to clarify why the tissue grew or stopped growing during the regeneration process."

Less is more and more is less

In a healthy liver, there are a certain number of contacts between the ductal cells and the mesenchymal cells that signal to the ductal cells not to proliferate and just stay as they are. As soon as the tissue is damaged, the mesenchyme cells reduce the number of contacts they have with the ductal cells so that they can multiply to repair the damage. From their observations, the researchers concluded that it is not the number of the two cell types, but the number of cell contacts that determines how many cells are produced to repair the damaged tissue. Too many touches by mesenchymal cells mean that fewer or no new ductal cells are produced, while fewer touches mean that more cells are produced. This regulation is very important because if there is no signal for the ductal cells to stop proliferating for tissue repair, there can be excessive proliferation, which can lead to cancer.

Meritxell Huch, study leader, summarises: "This is the first time we have been able to visualise these contacts and the first time we have proven that they exist. This has been possible thanks to our organoid systems. Even though we conducted our experiments in a petri dish outside the living body, we assume that the same process takes place in the living organism. We have been able to observe this at certain times during the regeneration process. We have not yet been able to observe this in the living organism because the technology does not exist to do so. While our study focused on ductal-mesenchymal interaction in the liver, we can imagine similar mechanisms occurring in any other system where cell numbers change dynamically, such as the lung or breast tissue. Of course, in the future we would like to produce a liver organoid with all cell types. With such an organoid, we could test drugs and see if they affect not only the regenerating cells but also their supporting environment. But for that we have to wait until the technology is available."

Lucía Cordero-Espinoza, Anna M. Dowbaj, Meritxell Huch et al.: Dynamic cell contacts between periportal mesenchyme and ductal epithelium act as a rheostat for liver cell proliferation, Cell Stem Cell, 2 August 2021.