What challenges do multicellular organisms face

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Cyanobacteria, also known as blue-green algae, are a special class of bacteria that can photosynthesize. In terms of evolution, they are ancient. Precursors appeared on Earth 2.5 billion years ago and, thanks to their ability to photosynthesize oxygen, paved the way for higher life.

Some cyanobacteria species are filamentous, multicellular organisms in which there is a certain division of labor. Some cells carry out photosynthesis, others absorb nitrogen from the air. The cyanobacteria gain energy in the form of glucose through photosynthesis and use the nitrogen to produce amino acids, the building blocks of proteins.

The cyanobacteria are faced with the problem of how the individual cells can communicate with one another and exchange substances. Cells that carry out photosynthesis have to supply their nitrogen-fixing sister cells with glucose; amino acids have to be transported in the opposite direction. For this purpose, cyanobacteria have developed special cell connections. These allow the exchange of nutrients and messenger substances across cell boundaries without the cells growing together.

Structure elucidated in a cellular context

Little was known about the detailed structure and the exact functioning of the cell connections in multicellular filamentous cyanobacteria. A group of researchers from the ETH Zurich and the University of Tübingen are now presenting in the new edition of the specialist journal "Cell" the structural details and functionality of the cell-cell connections, so-called septal connections, in the genus Anabaena in unprecedented resolution.

The researchers show that the connecting channels consist of a protein tube that can be closed with a stopper at both ends. In addition, this tube is covered with five-armed protein elements that are arranged like a camera shutter.

The channels connect the cytoplasms of the two neighboring cells and extend through the respective membranes and cell walls. The cells are separated from each other by an extremely thin gap a few nanometers wide.

“Up to now, conventional electron microscopy has not been able to clarify these details. Thanks to an extension of the cryo-electron microscopy, we have succeeded in gaining insights with previously unattainable accuracy, ”says Martin Pilhofer, professor at the Institute for Molecular Biology and Biophysics at ETH Zurich.

Pilhofer's doctoral student Gregor Weiss developed a method to prepare the cyanobacteria in such a way that the channels could be made visible using cryo-electron microscopy. To do this, Weiss “milled” the junction between two cells in frozen cyanobacteria in layers until his sample was thin enough. Without pretreatment, the spherical cells would be too thick for use in cryo-electron microscopy.

"Due to the complex structure of the connecting channels, we suspected a mechanism that opens and closes the channels," says Karl Forchhammer, Professor of Microbiology at the University of Tübingen. In fact, together with his team, he was able to demonstrate how the cells of the association communicate with one another under different stress conditions. To do this, they colored cyanobacteria chains with a fluorescent dye and then bleached individual cells with a laser. Then the researchers measured the influx of the dye from neighboring cells.

With the help of this method, the researchers were able to show that the channels actually seal when treated with chemicals or in the dark. The filigree cap structure of a channel closes like an iris diaphragm and interrupts the exchange of substances between the cells, which the scientists recognized from the different levels of fluorescence.

Closing mechanism protects the cell structure

“Such a locking mechanism protects the entire cell structure,” says Forchhammer. For example, a cell can prevent it from passing on harmful substances to its neighboring cells, which could cause the entire organism to die. The cyanobacteria can also use the channels to prevent the contents of the entire network from leaking out if individual cells are mechanically damaged.
With their study, the researchers can show that cell connections in multicellular, unrelated organisms were "invented" several times in the course of evolution and developed in parallel.

"This underlines how important it is that a multicellular organism can control the transport of goods between individual cells," says Pilhofer. With the clarification of the channel structure and function in cyanobacteria, the ETH researchers add another piece of the puzzle to the overall picture. “For us, this work is basic biological research without a focus on a possible application. Rather, the new data give us insights into the evolution of complex living beings, ”explains the ETH professor.

ETH Zurich, University of Tübingen

Original publication:

Weiss GL, Kieninger A-K, Maldener I, Forchhammer K, Pilhofer M. Structure and function of a bacterial gap junction analog. Cell, 2019, July 11th. DOI 10.1016 / j.cell.2019.05.055