Biological complexity: intercellular transport controls (Introduction)

by David Turell , Wednesday, April 07, 2021, 21:49 (1326 days ago) @ David Turell

Where three cells meet:

https://phys.org/news/2021-04-junctions-cells-gateways-substances.html

"Special connections between neighboring cells ensure that these cellular barriers are, on the one hand, stable and tight—thus protecting the body and organs against pathogens—while, on the other hand, they remain permeable to specific substances or migrating cells. This is how the cells allow dissolved nutrients to be transported into organs, and how cells of the immune system are able to migrate from the blood across the blood vessel wall into inflamed tissue.

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"The researchers found that, at the points where three cells meet, the epithelial cells loosen their connections in a controlled manner and the yolk proteins are transported to the egg cell through the resulting gaps.

"At places where only two cells connect, the connections are maintained, thus keeping the tissue integrity intact.

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"The scientists found that the epithelial cells sequentially removed four different adhesion proteins from their membranes. "This process takes several hours, and only when the last protein is gone do the cell junctions open," explains biologist Jone Isasti-Sanchez, who is the first author of the study and a doctoral candidate in the Integrated Research Training Group within the Collaborative Research Center 1348 "Dynamic Cellular Interfaces" at the University of Münster. After the junctions have opened, the uptake of yolk proteins into the egg proceeds over about 16 hours and, subsequently, the process reverses—the intercellular spaces close again.

"The researchers demonstrated that the cells open their contact sites by taking up adhesion proteins from the surface into the cell interior, using a basic cellular process called endocytosis. An important new finding is that endocytosis seems to take place to a greater extent at those points where three cells meet, and, as a result, the cell junctions only open up at these points. Where only two cells meet, the contact is maintained. "The fact that this process occurs selectively at the three-cell contact-points and, moreover, in such an orderly fashion, is probably important for preventing the tissue from falling apart," says Stefan Luschnig. In addition, he adds that the process presumably has to take place in a very controlled manner because the opening of gates in a tissue comes with the risk that pathogens will enter.

"In their experiments, the scientists also genetically increased or reduced the amount of the adhesion protein E-cadherin and were able to show that the amount of this protein determines how wide the intercellular spaces open. In addition, they found that the mechanical tension in the cytoskeleton plays a key role in the process. This tension is generated by a structure consisting of the proteins actin and myosin, which encircles the cell periphery and is able to contract, similar to a rubber band. When the researchers increased the activity of myosin in the cell, the cytoskeleton contracted more, which prevented the cell junctions from opening."

Comment: Another tightly controlled mechanism that can only develop by design.