Skoltech scientists and their colleagues from Germany and the USA have discovered new structures in liver cells responsible for the formation and regulation of the lumen between hepatocytes.
The authors of the study, published in the Journal of Cell Biology, also identified a protein required to form these previously unknown bulkhead-like bulkheads. The human body has many surfaces covered with epithelial cells. In this case, in the vessels or the intestine, the cells are turned into the canal by the so-called apical sections of the membrane, from which the inner surface of the “tube” is formed.
However, hepatocytes - the most common liver cells - behave differently in this sense: they form cavities in pairs, uniting only with cells in the immediate vicinity. The result is an extensive three-dimensional network of very narrow gaps. Until now, it was not clear what is behind this feature of hepatocytes.
In this case, liver cells of a different type, cholangiocytes, form channels of a much larger diameter and do this in the same way as ordinary epithelial cells. And although scientists suspected that the shape of the lumen between hepatocytes and the formation of networks by them was explained by deterministic local mechanical interaction between cells, this hypothesis was previously general in nature and was not supported by experimental data.
Scientists from the Max Planck Institute of Molecular Cell Biology and Genetics, led by Marino Zerial, together with a group of Skoltech researchers led by Associate Professor Timofey Zatsepin and specialists from other organizations, discovered on the apical surface of the lumen between hepatocytes outgrowths that form structures inside the canal that resemble bulkheads - stiffening ribs in the ship's hull. Examining them with an electron microscope, scientists have shown that the narrowness of the lumen and the complexity of the network of bile ducts are due precisely to the presence of these structures.
Both structures - both the man-made bulkheads between the sides of the ship, and their natural counterpart in the gap between the hepatocytes - serve to ensure the rigidity of the "structure". Only the bulkheads divide the hull of the ship into compartments, while the apical bulkheads keep the channel continuous. To ensure that the bridges were not an artifact introduced by the cell culture observation procedure, the team examined the mouse embryonic liver using an electron microscope. This in vivo experiment confirmed the presence of structures similar to the bulkheads of the vessel in the forming lumen in the liver of the embryo.
It was also unambiguously shown that the bridges do not divide the channel lumen into separate compartments. The same structures were found in the liver of an adult mouse. In addition, an in vivo experiment made it possible to eliminate confusion between webs and microvilli, a previously studied formation of hepatocytes on the cell membrane.
The researchers looked at a number of proteins that may be involved in bridging. The team focused on the Rab35 protein, which had never before been associated with the structure of the lumen between hepatocytes. Using electron microscopy and 3D modeling, the scientists showed that when the Rab35 gene expression is turned off, no bridges are formed between hepatocytes, and the lumen is the same as between cholangiocytes.
“It is important to note that hepatocyte and cholangiocyte have a common precursor - hepatoblast, so this observation allows us to identify Rab35 as a direct participant in the events.We know that the Rab35 protein does not directly control the formation of bridges,”says Timofey Zatsepin.
- It is known as a carrier and, apparently, is responsible for the transport of some complex or complexes of proteins inside the cell, which, in turn, lead to the formation of bridges. We are now looking for these complexes - we want to fully study the mechanism that makes the liver so different from other organs."
The researchers speculate that the jumpers they discovered are important for further research with possible medical applications. “These structures are very interesting and beautiful in themselves. Apical bridges meet quasi-periodically at a distance from each other equal to the diameter of the lumen, which outwardly resembles the load-bearing elements of engineering structures, explains Timofey Zatsepin. "We plan to use the opportunity to regulate this mechanism to study the functioning of the liver and its regeneration in conditions of obesity and liver fibrosis."
The study was carried out with the participation of specialists from the Skolkovo Institute of Science and Technology (Skoltech), the Max Planck Institute for Molecular Cell Biology and Genetics, the Max Planck Institute of Molecular Genetics, the Christian Albrecht University of Keele and the Friedrich Schiller University of Jena (Germany), as well as the Moscow State University named after M.V. Lomonosov (Russia) and Nelson Laboratories LLC (USA).