“Researchers have so far failed to clarify these details with conventional electron microscopy. By extending cryo-electron microscopy, we were able to gain a degree of precision never before achieved,” says Professor Martin Pilhofer from the Institute of Molecular Biology and Biophysics at ETH Zurich.
Gregor Weiss, Pilhofer’s doctoral student, developed a process of preparing the cyanobacteria in such a way that the channels could be visualised via cryo-electron microscopy. Using frozen cyanobacteria, Weiss “milled” the junction between two cells, layer by layer, until his sample was thin enough. Without this pre-processing, the spherical cells would have been too thick for cryo-electron microscopy.
Mechanism to prevent leaking
“Due to the complex structure of the connecting channels, we suspected there was a mechanism to open and close them,” said Karl Forchhammer, Professor for Microbiology at the University of Tübingen. He and his team were in fact able to show how the cells of the complex communicate with each other under different stress conditions. They stained cyanobacteria chains with a fluorescent dye and then bleached individual cells with a laser. The researchers then measured the influx of the dye from neighbouring cells.
Using this method, they were able to show that the channels actually close when treated with chemicals or in the dark. The filigree cap structure of a channel closes like an iris and interrupts the exchange of substances between the cells; the researchers recognised this phenomenon through the varying degree of fluorescence they observed.
“This closing mechanism protects the entire multicellular organism,” Forchhammer says. For example, it can prevent a cell from passing on harmful substances to its neighbouring cells, which could destroy the whole organism. The cyanobacteria can also use the channels to prevent the cell contents of the entire network from leaking out if individual cells are mechanically damaged.
Conserved structures
With their study, the researchers are able to show that in the course of evolution, multicellular organisms of different lineages repeatedly and independently “invented” cell junctions. “It emphasises just how important it is for a multicellular organism to be able to monitor the transport of substances between its individual cells,” Pilhofer says. By elucidating the channel structure and function in cyanobacteria, the ETH researchers are adding another piece to the puzzle. “As far as we are concerned, this is fundamental biological research, without focusing on any potential application. The new data rather gives us a greater understanding of the evolution of complex life forms,” the ETH professor explains.
