“It spreads in this fashion and never slows down, never peters out,” Ferrell said. “It doesn’t get any lower in amplitude because every step of the way it’s generating its own impetus by converting more inactive molecules to active molecules, until apoptosis has spread to every nook and cranny of the cell.”
To see how death takes over a single cell, Cheng and Ferrell used Xenopus frog eggs. One egg is a single cell, and as cells go, these are enormous, making them a prime candidate to observe how death spreads from one end of the cell to the other, which can be done with the naked eye.
To start, the two scientists took fluid from the egg and inserted it into Teflon tubes, which were several millimeters long, and initiated apoptosis through a molecular “death signal.” By using a fluorescent technique linked to the activation of apoptosis, Ferrell and Cheng could watch as the bright green glow moved its way down the tube at a constant speed, indicating that apoptosis was spreading via trigger waves, as opposed to some other more rudimentary mechanism, such as diffusion, which slows down as it moves.
The question was, did apoptosis also spread like that in cells as they naturally occur?
Turning to fluorescence microscopy here proved more difficult, as intact frog eggs are quite opaque. However, Cheng and Ferrell noticed that when frog eggs die, a sort of ripple of pigmentation occurs at the egg’s surface. The scientists saw that during death, a dark ripple moved like a curved line across the egg at a constant speed from one side to the other. The speed of this surface wave, which was constant and did not slow down, tipped them off to trigger waves here too. So to further confirm, they analyzed individual dying eggs: Every egg that had undergone this surface wave contained activated caspase, whereas the eggs that had not yet undergone the waves did not ¾ more evidence that trigger waves propagate cell death in an intact cell too.
A wave of trigger waves
So far, apoptosis is the only form of cell death in which trigger waves have been identified, but Ferrell is investigating other processes in biology to see if the continual waves might play a role.
Now, they’re looking into whether trigger waves might be responsible for how our innate immune response spreads from cell to cell. Viruses spread from cell to cell through trigger waves, so it makes sense that our initial line of immune defense might employ the same tactic.
“We have all this information on proteins and genes in all sorts of organisms, and we’re trying to understand what the recurring themes are,” Ferrell said. “We show that long-range communication can be accomplished by trigger waves, which depend on things like positive feedback loops, thresholds and spatial coupling mechanisms. These ingredients are present all over the place in biological regulation. Now we want to know where else trigger waves are found.”
The study was funded by the National Institutes of Health (grants R01GM110564 and P50GM107615).