Researchers have had a breakthrough in the central questions about #the origin of life. A collaboration of physicists and biologists in Germany has found a simple mechanism that might have enabled liquid droplets to evolve into living #cells in early Earth’s primordial soup.
How did these first cells arise from primitive precursors? What were those precursors, dubbed “#protocells,” and how did they come alive? How could something as complex as a membrane start to self-replicate and proliferate, allowing #evolution to act on it?
New work by David Zwicker along with the Max Planck Institute for the Physics of Complex Systems and the Max Planck Institute of Molecular Cell Biology and Genetics, both in Dresden, may have an answer. The scientists studied the physics of “chemically active” droplets, which cycle chemicals in and out of the surrounding fluid, and discovered these droplets tend to grow to cell size and divide, just like cells, as well as move across cells via diffusion. This active droplet behavior is different from more familiar tendencies of oil droplets in water, which glom together into bigger and bigger droplets without dividing.
Frank Jülicher, a biophysicist in Dresden and co-author of the new paper said, It makes it more plausible that there could have been spontaneous emergence of life from nonliving soup.
Is it possible that these “chemically active” droplets are responsible for the evolution of modern life for everything from amoeba to zebras? Experiments are underway to observe the growth and division of active droplets made of synthetic polymers that are modeled after the droplets found in living cells.
Frank and his team are not the only ones interested in this topic. David Dreamer, a biochemist at the University of California, Santa Cruz, and a longtime champion of the membrane-first hypothesis, admits that while droplet division is interesting, he contends its relevance to the origin of life remains to be seen. Dreamer believes that the multistep process by which modern cells divide is far different from the simplicity of “chemically charged” droplets.
Princeton University’s biophysicist, Clifford Brangwynne, who was part of the Dresden-based study, explained that it would not be surprising if these were vestiges of evolutionary history. Like mitochondria, organelles that have their own #DNA came from ancient bacteria that infected cells and developed a symbiotic relationship with them: “The condensed liquid phases that we see in living cells might reflect, in a similar sense, a sort of fossil record of the driving forces that helped set up cells in the first place,” he said.
The demystification of droplets began in 2009 when Brangwynne and collaborators analyzed the nature of little dots known as “#P granules” in #C. elegans germline cells. It was during the observation in which “P granules” divide into sperm and egg cells that the researchers noticed that they grow, shrink, and move across the cells via diffusion. After Brangwynne and team reported in Science Magazine that they showed that P granules exhibit liquid-like behaviors, including fusion, dripping, and wetting, it prompted a wave of activity as other subcellular structures were also identified as droplets.
When germline cells in the roundworm C. elegans divide, P granules, shown in green, condense in the daughter cell that will become a viable sperm or egg and dissolve in the other daughter cell.
Drawing back from Oparin’s 1924 protocell theory, Brangwynne and Tony Hyman, head of the Dresden biology lab where the initial experiments took place, quickly made the connection to the droplets.
Brangwynne and Hyman wrote an essay reflecting on the life of A.I. Oparin in 2012 titled, The Origin of Life. In it they wrote, Oparin belongs in the pantheon of the twentieth century’s greatest scientists for providing a foundation for understanding early molecular evolution. Like the ancient mitochondrial organisms found in each of our cells, intracellular #RNA droplets could reflect a still more ancient lineage in the assembly of complex cellular structure. Oparin’s coacervates may still be alive and well, safe within our cells, like flies in life’s evolving amber.
One of Oparin’s most famous hypotheses was that lightning strikes or geothermal activity on early Earth could have triggered the synthesis of organic macromolecules necessary for life. British scientist John Haldane made the same assumption. This became known as The Oparin-Haldane hypothesis which suggests that life arose gradually from inorganic molecules, with “building blocks” like amino acids forming first and then combining to make complex polymers. Later the first evidence that organic molecules needed for life could be formed from inorganic components was successfully proven with The Miller-Urey experiment. A far less popular idea of Oparin, in part because he had no clue as to how the droplets might have reproduced, thereby enabling evolution, was that liquid aggregates of these macromolecules might have served as protocell. The Dresden group studying P granules didn’t know either.
In order to unravel the physics of centrosomes, organelles involved in animal cell division that also seemed to behave like droplets, Jülicher assigned his new student, Zwicker, the new task in light of their discovery. One of Zwicker’s ideas was to model the centrosomes as “out-of-equilibrium” systems that are chemically active, continuously cycling constituent proteins into and out of the surrounding liquid cytoplasm. In order for Zwicker’s model to work, he created 2 different #chemical states for the proteins. The proteins that dissolved in the surrounding liquid were in state A and the insoluble proteins, aggregating inside a droplet, state B. Zwicker noticed that proteins in state B randomly switch to state A, and flowed out of the droplet. Zwicker’s model also showed that a reverse reaction could be triggered by an energy source causing a protein in state A to overcome a chemical barrier and transform into state B; when this insoluble protein bumps into a droplet, it slinks easily inside, like a raindrop in a puddle. These results showed that molecules flow in and out of an active droplet as long as there is an energy source. “In the context of early Earth, sunlight would be the driving force,” Jülicher said.
The discovery of this chemical influx and efflux by Zwicker showed that that they will exactly counterbalance each other when an active droplet reaches a certain volume, which causes the droplet to stop growing. In Zwicker’s simulations typical droplets grew to tens or hundreds of microns across depending on their properties — the scale of cells.
Zwicker was even more surprised to find out that although the droplets have a stable size, they are unstable with respect to shape: When a surplus of B molecules enters a droplet on one part of its surface, causing it to bulge a slightly in that direction, the extra surface area from the bulging further accelerates the droplet’s growth as more molecules can diffuse inside. The droplet elongates further and pinches in at the middle, which has low surface area. Eventually, it splits into a pair of droplets, which then grow to the characteristic size. When Jülicher saw simulations of Zwicker’s equations, “he immediately jumped on it and said, ‘That looks very much like division,’” Zwicker said. “And then this whole protocell idea emerged quickly.”
Over the next 3 years Zwicker, Jülicher and their collaborators, Rabea Seyboldt, Christoph Weber and Tony Hyman extended Oparin’s vision,tirelessly developing their theory. “If you just think about droplets like Oparin did, then it’s not clear how evolution could act on these droplets,” Zwicker said. “For evolution, you have to make copies of yourself with slight modifications, and then natural selection decides how things get more complex.”
Jülicher began meeting with Dora Tang, head of a biology lab at the Max Planck Institute of Molecular Cell Biology and Genetics, last spring in order to come up with ideas to observe active-droplet division in action.
One of the interesting things Tang’s lab does is manufacture artificial cells made of polymers, lipids and proteins that resemble biochemical molecules. Tang and her team over the next few months will monitor the liquid droplets made of polymers that are physically similar to the proteins in P granules and centrosomes to see if there’s any behavior of division. A joint effort with Hyman’s lab is to try to observe centrosomes or other biological droplets dividing, and to determine if they utilize the mechanism identified in the paper by Zwicker and colleagues, over the next few months. “That would be a big deal,” said Giomi, the Leiden biophysicist.
Membrane-first proponent, David Deamer, upon reading the new paper, recalled having once observed something like the predicted behavior in hydrocarbon droplets he had extracted from a meteorite. He noticed they began moving and dividing when he illuminated the droplets in near-ultra violet light. Even though Deamer sent footage of the phenomenon to Jülicher, he still isn’t convinced of the effect’s significance. “There is no obvious way for the mechanism of division they reported to evolve into the complex process by which living cells actually divide,” he said.
Tang, along with other researchers disagrees with Deamer’s statement. One of the points made by Tang was that the droplets could have easily developed the ability to transfer genetic information, sharing a batch of protein-coding RNA or DNA into equal loads for their daughter cells, once the droplets started to divide. Evolution would favor the behavior of increased droplet division, if the genetic material was coded for useful proteins. Ruled by the law of increasing entropy and fueled by sunlight, the complexity of protocells would have increased.
The crusts of lipids, that usually the interface between the droplets and the surrounding liquid are naturally collected by the droplets. It’s entirely possible that these genes may have started coding for these membranes to develop some sort of protection. Jülicher and colleagues argue that this could also explain the membrane first hypothesis. Deamer, while noting he would call the protocells as the first droplets that had membranes, stated, “I can go along with that.”
What will determine how vigorous and relevant the predicted droplet division mechanism really is will be the outcome of future experiments. The goal will be to find chemicals with the right two states required (A and B) to give credit to the theory of life coming from nonlife.
Jülicher, barring in mind that the Droplets require a lot of chemical material to spontaneously arise or “nucleate,” and there’s no real way of knowing how so many of the right complex macromolecules could have accumulated in the primordial soup to make it happen as it did- in Jülicher’s opinion, the most amazing part of the whole process was not that droplets turned into cells, but that the first droplet — our globule ancestor — formed to begin with.
“It’s a very rare event. You have to wait a long time for it to happen,” he said. “And once it happens, then the next things happen more easily, and more systematically.”
Nature 491, 524–525 (22 November 2012)
Science 26 Jun 2009:
Vol. 324, Issue 5935, pp. 1729-1732