Discover, September 30, 1998
In the early 1950s, when biologist John Tyler Bonner was just beginning his career at Princeton, he was startled one day to receive a message from Albert Einstein, who worked at the nearby Institute for Advanced Study. Einstein wanted Bonner to come to his office and show him a movie.
The film that Einstein was so eager to see starred an amoeba named Dictyostelium. Normally, this single-celled organism goes about its quiet business of hunting down, engulfing, and digesting bacteria that live in soil. After gorging itself sufficiently, Dictyostelium divides in two, and the new pair go their separate, bacteria-devouring ways.
But if the thousands of Dictyostelium in a stamp-size plot of soil should eat their surroundings clean, they do something exceptional, and this is what Bonner had captured on film.
Rather than crawling around randomly, the amoebas start streaming toward one another in inwardly pulsing ripples. As many as 100,000 converge in a swirling mound. And then, remarkably, the mound itself begins to act as if it were the organism. It stretches out into a bullet-shaped slug the size of a sand grain and begins to move. It slithers up toward the surface of the soil, probes specks of dirt, and turns around when it hits a dead end. Its movements are slow-it would need a day to travel an inch-but in a stop-action film, such as the one that Bonner showed Einstein in his darkened office, the deliberateness of the movements eerily evoke an it rather than a they.
After several hours, the Dictyostelium slug goes through another change. The back end catches up with the tip, and the slug turns into a blob. The blob stretches upward a second time, and now some amoebas produce rigid bundles of cellulose. They die in the process, but their sacrifice allows the blob to become a slender stalk. Perched atop the stalk is a globe, bulging with living amoebas, each of which covers itself in a cellulose coat and becomes a dormant spore. In this form the colony will wait until something–a drop of rainwater, a passing worm, the foot of a bird-picks up the spores and takes them to a bacteria-rich place where they can emerge from their shells and start their lives over.
Einstein was mesmerized by Bonner’s movie, and when it was over, he burst into questions. “He asked, ‘What’s happening here?'” remembers Bonner. “‘Why are they doing this? How are they doing this? What are the big problems?’ It was all questions. He unfortunately didn’t tell me how to solve them.”
And unfortunately, Bonner didn’t have much to offer Einstein: this strange amoeba had been isolated for the first time by biologist Ken Raper in 1933; for many years thereafter the only other scientist who studied it was Bonner. But if Einstein were alive today, it’s safe to say that he would have been pleased with science’s progress on the Dictyostelium front. Hundreds of scientists around the world are now slaving over the organism. Some are engaged in a multimillion dollar project to sequence its entire genome; others are unveiling the interactions of proteins that let the amoebas respond to external signals and perform their part in the Dictyostelium dance. It’s a dance, they’re finding, that’s uncannily like the one our own cells perform-and it might therefore tell us something about our origins.
“I would love to have that conversation with Einstein again today,” says Bonner.
Unlike bacteria and other microbes, Dictyostelium has a structure that’s similar in fundamental ways to our own cells.
While bacteria let their DNA float loose within their cell walls, for example, Dictyostelium houses its DNA in a nucleus, as we do. Such nucleated organisms are known as eukaryotes, and they descend from a common ancestor that lived perhaps 2 billion years ago. That first eukaryote gave rise to many lineages of single-celled creatures, ranging from disease-causing protozoans to photosynthesizing algae.
For perhaps a billion years, eukaryotes remained single-celled. But then one lineage began branching out into new, multicellular forms. These were the ancestors of today’s plants, fungi, and animals. Becoming multicellular was a huge achievement. Individual cells had to start working together in new ways, to differentiate into specialized kinds of tissues, to stick together and communicate across great distances. And whereas reproduction used to require simply dividing into two, now it involved an embryo developing from a single cell.
Originally biologists placed Dictyostelium among the “slime molds,” a term that many Dictyostelium experts resent, given that it’s neither slimy nor a mold. But the conventional wisdom was that Dictyostelium is only distantly related to truly multicellular organisms, that it evolved its version of multicellularity very much on its own. All the same, precisely because of its eerie similarity to us many-celled beings, over the past 20 years or so Dictyostelium has become a sort of eukaryote lab rat. Its strange mode of development offers biologists a way to discover what’s going on in our own bodies.
Often, biologists can figure out how genes and the proteins they make work by finding cases in which they go awry. A creature with a mutant gene may produce an ill-shaped protein that causes an entire cascade of chemical reactions to come to a screeching halt. The consequence is that the creature fails to develop a tail or accidentally sprouts an extra finger. But it’s hard, slow work looking for these sorts of mutations in an animal like a mouse or a fruit fly. “The way these proteins are identified in animal cells is basically by luck,” says Adam Kuspa, a molecular geneticist at Baylor College of Medicine in Houston. “You have to rely on diseases, overexpressions of the proteins, and so on. It’s a hit-or-miss proposition.” Making matters worse, mutations to genes that build embryos often kill an animal while it’s still just a ball of cells.
Dictyostelium, however, is also a eukaryote that goes through a cycle of development-but its cycle can be far more easily tracked than that of a mouse or a fruit fly. Just as important, it’s one that can withstand many more mutations. To grow these amoebas, all you have to do is spread a thin blanket of any sort of bacteria on a petri dish and then inoculate the dish with some Dictyostelium spores or cells. That’s it. The spores swell with water and split open. The emerging amoebas rove their neighborhood, eating bacteria, dividing into more amoebas, and otherwise having a grand time. After a couple of days you can see a spot turning clear with your naked eye, as the Dictyostelium extirpate the bacteria. The younger Dictyostelium at the edge of the home base move out into new territory and continue to divide, but eventually they strip these regions bare too. A clear space expands across the dish, and within it the barely visible slugs appear. In time you see that where the slugs once were, a miniature forest of globe-tipped stalks has grown.
Mutations can push a strain of Dictyostelium off this normal path. It may be unable to aggregate into a slug, for example, in which case the starving amoebas turn into scattered individual spores. Yet unlike an animal with a lethal mutation, at the end of the process the mutant Dictyostelium remain alive and capable of reproducing. Kuspa and his fellow researchers have invented ways to riddle Dictyostelium’s DNA with mutations and then look for the cases in which development is obviously derailed. Some of the amoebas go through the whole process of forming a stalk in half the normal time. Some sprout strange long tendrils from their globes, others form clusters of fruits from single stalks, and some sprout a dozen stalks at once but can’t manage to form a single globe of spores on any of them.
Once these mutants turn up, the biologists can use methods developed by Kuspa to quickly find which genes have been mutated. “In the 1980s you’d get a gene every year or two,” explains Kuspa, “and a whole lab would do that. Now, in six months, one of my grad students analyzed 25 genes in a screen of one particular mutation.” As a result, a torrent of information on Dictyostelium is now coming in, and biologists are forming a rapidly improving picture of how the organism goes through its metamorphosis.
A healthy Dictyostelium is always sniffing its surrounding, gauging how many bacteria are in its neighborhood by the molecules they release. Somehow, perhaps by measuring the levels of the bacterial molecules and the molecules released by its fellow Dictyostelium, it can decide when conditions are starting to deteriorate. Long before its food has actually run out, the amoeba starts gearing up for an aggregation.
A few scattered cells among the individuals begin emitting pulses of a common signaling molecule called cAMP. When neighboring amoebas pick up the signal, they crawl for a short time toward the source. They in turn also send out CAMP pulses, attracting other Dictyostelium farther away. In this way, all the Dictyostelium work together to create expanding waves of cAMP that guide them into a single mound.
When the amoebas get close to one another, there is so much cAMP saturating their receptors-the docking stations for the molecule on the cell’s surface that the on-off pattern of the early pulses switches to a continuous on signal. That change helps trigger a mechanism that controls the later stages of Dictyostelium’s development. Scattered throughout the mound, 15 percent of the cells now take the initial steps that will ultimately turn them into part of the stalk, while the remaining 85 percent are already preparing for their future as spores.
How they decide this isn’t yet clear but it’s more or less a lottery: cells that divide just before joining the mound are apparently destined to die as they build the stalk, while more mature cells become spores in the globe. Ironically, although the spores ultimately survive, it is the doomed pre-stalk cells that take control of the aggregation. They rise to the top of the mound and push it upward so that when the newly created slug topples over, they form the tip.
These cells guide the slug toward the surface of the soil by heading toward heat and light. They form a sensitive thermometer-a slug can steer itself toward a heat source when its tip is only a thousandth of a degree warmer than its tail.
They also form an eye of sorts: when light passes through the individual amoebas, they act like a lens, focusing the light onto the far side of the slug. If a slug veers off course, the side in shadow thus actually receives more light than the one facing the sun. By turning so that the light is more balanced, the slug gets itself back on track.
The amoebas are held together inside the slug by a sheath of cellulose and other proteins, and as they travel together, they jostle past one another. Eventually the cells in the tip settle into distinct slices, from front to back. In each slice a unique combination of genes switch on, start making proteins, and presumably, direct the amoebas’ behavior. As the tip of the slug now rises up again, the cells at the very end of the tip stream down through the colony. They become the stalk. Meanwhile, the pre-spore cells go up in the other direction. Some of the slices in the pre-stalk region ultimately turn into cups that support the top and underside of the ball of spores. Another slice forms a disk at the bottom of the stalk that anchors it to a speck of soil.
By teasing out the genes that determine these migrations and transformations, biologists have answered some important questions about the basic workings of the eukaryotic cell. In all eukaryotes, for example, certain chemical signals make their way from the cell’s outer membrane to the nucleus through what are called G protein-linked receptors. There are many types of these molecular docking stations, and you can find them at work throughout our bodies. The molecules in a whiff of perfume lock into G protein-linked receptors in our noses and send signals of smell to our brains; photons bouncing off the pages of this magazine use G protein-linked receptors in your retinas to transmit visual data. The same receptors carry the fight-or-flight signals of an adrenaline rush, and they even keep our hearts beating.
As vital as this pathway is to life, parts of it still remain a mystery. Biologists know that a signaling molecule approaching a cell docks on a particular receptor in the membrane and in the process changes the receptor’s shape. That change attracts proteins, the G proteins, floating in the cell’s membrane. A complex chain of protein reactions ultimately carries the signal all the way to the cell’s nucleus, where the DNA responds by producing new proteins.
In Dictyostelium, researchers are quickly discovering new proteins that help carry G protein signals. Kuspa is confident that these new proteins will soon be found in our own cells also, and he suspects that some important medical discoveries will come out of the discovery. In Alzheimer’s disease, for example, it seems that faulty neurons suffer from a defective G protein-linked pathway. “It’s conceivable that it’s defective because it’s missing a protein biologists don’t know about yet,” says Kuspa. And it’s possible that they will find that protein in Dictyostelium first, rather than in human beings.
To this extent, research on Dictyustelium today is fulfilling the original hopes of the biologists who began studying it decades ago. But it is also surprising them with evidence that suggests tp us than anyone had imagined. In case after case, the proteins found working together in Dictostelium are the same as the ones doing the same tasks in multicellular organisms. In some cases, only animals and Dictyostelium have these proteins in common.
One set of such proteins, known as STATs, are responsible for many signals critical to development in human embryos; in our adult life they continue to carry information in immune cells. Last year British researchers discovered that Dictyostelium uses STAT proteins to relay its signals as well.
The proteins that let Dictyostelium move are also remarkably like ones in some animal cells. Since the late 1970s Peter Devreotes at Johns Hopkins has been studying how Dictyostelium mobilize and converge on the source of a cAMP signal. He and his colleagues have reconstructed much of the complex protein sequence that ultimately triggers the cellular skeleton to rearrange itself, extending a footlike blob toward the alarm’s source. They have shown that, protein for protein, much of the process is an uncanny match for the way that certain immune cells behave. They can even get a Dictyostelium skeleton to obey immune cell signaling proteins, and vice versa.
These immune cells, known as phagocytes, are the most primitive part of our bodily defense. When an infection is inflamed, the phagocytes sense chemicals released by bacteria and cells and begin crawling toward the source-nearly identical to the way Dictyostelium heads for cAMP. And when the phagocyte gets to the infection, it phagocytizes–or eats–the bacteria, much as Dictyostelium does in soil.
With all these similarities, some biologists think Dictyostelium may be a closer relative to animals and fungi than plants are. At the very least, they think Dictyostelium will turn out to reside in the same tiny corner of eukaryote diversity along with plants, animals, and fungi-right in the thick of the multicellular action.
There are skeptics, though. Richard Kessin, a Dictyostelium expert at Columbia University, points out that while some proteins in animals and Dictyostelium cells may look and function in the same way, we have been able to compare them with only a few other eukaryotes, such as yeast (a fungus) and a handful of plants. And no matter where Dictyostelium ends up in the tree of life, he thinks its brand of multicellularity is fundamentally different from our own. After all, we animals got our start from single-celled eggs that divided and then differentiated, with the result that all our cells have identical sets of DNA. Dictyostelium, on the other hand, develops as a genetically mixed collective. “We develop and then we grow; Dicty grow and then they develop. It’s as if you went to Mars and found an utterly different kind of development,” Kessin says.
He speculates that Dictyostelium evolved its current way of life to survive in the rough-and-tumble world of forest soils. “It’s a desperate existence; they live in a spotty desert where they eat, run out of food, and spore over and over again.” The biggest threat to Dictyostelium is being devoured by tiny nematode worms. As Kessin points out, a slug isn’t the only form into which Dictyostelium can develop.
Individual Dictyostelium can turn into isolated cysts, wringing out the water in their interiors and covering themselves in a tough cellulose coating. Another, stranger transformation happens when Dictyostelium have sex. Two amoebas will sometimes fuse together as if they were a sperm and an egg, mixing their genes. They begin sending out cAMP to their neighbors, which dutifully come running to the alarm. But rather than joining together into a slug, these amoebas get a rude surprise: the mating Dictyostelium swallow them up like bacteria and digest them. As more and more neighboring Dictyostelium are devoured, the cell grows to an enormous size and coats itself with cellulose. Once the cell has absorbed hundreds of amoebas, it turns dormant, waiting for the right conditions of moisture and temperature to germinate. When it does, it begins dividing into thousands of smaller clones, all sharing the combined genes of the original mating pair. This new generation of Dictyostelium burst out of the cellulose jacket and go their own ways.
Kessin suggests that Dictyostelium’s evolution is in part the story of better and better ways to evade nematodes. They first evolved individual cysts, which gave them a chance to pass through a nematode’s gut without being digested. A giant cell is an even better defense because it may get to be so big that nematodes can’t even begin to swallow it in the first place. Of course, the disadvantages to the giant cell are obvious: “It’s a slaughter,” says Kessin. By forming into a toughcoated slug that eventually produces spores, Dictostelium can survive in far higher numbers.
Kessin’s scenario has the attraction of being a step-by-step evolutionary buildup. “Nothing springs from the head of Zeus fully formed-they had to evolve in some order,” he says. To form a cyst, Dictyostelium needs to make cellulose. To form a giant cell, it needs both cellulose and the caMP signaling system. An even more complex system is necessary to form a slug. Thus what looks like the primary value of the slug helping Dictyostelium get to fresh soil may have been a side benefit to an adaptation evolved to avoid being eaten.
To Kessin, Dictyostelium has a Martian development-a radical alternative built independently from the basic tool kit of eukaryote proteins. But to others, such as Devreotes, we may be seeing hints of our own origins when we look at Dictyostelium crawling around a petri dish. Let’s say that the many detailed similarities that biologists such as Devreotes have discovered between Dictyostelium and our immune cells really are a sign of a close common heritage. “One of the thoughts I’ve had,” Devreotes says, “is that the similarity of the free-living cells of Dicty to our own immune cells has something to do with the origin of multicellularity. First you have single-celled eukaryotes that move around phagocytizing, and they divide. But then some of them get together and realize that if some cells differentiate-if they make something like the filter part of the sponge, say, and the filter part sticks up and the bacteria that flow by get caught on it and go down-then the phagocytic cells have an easier time of it.
“That’s good, and so they keep that. Maybe that’s how what we call the immune system came about: it’s a remnant of the cells that first came together to create multicellularity. Then you get adaptation. Some things turn into a Dicty structure, some turn into a sponge, and you go from there. So what if we turn things around? Maybe the organism exists to feed the immune system in a sense.” As weird as Dictyostelium may look, Devreotes is suggesting, we may all carry a little of it inside us.
Still, Devreotes accepts the difficulty that critics such as Kessin have pointed out: “What we have is a failure to understand where the egg comes from,” he says. How could all the cells in a multicellular body, obeying the individualistic laws of natural selection, agree that only a handful of cells would be the ones that would carry on to the next generation? And how did Dictyostelium end up on another path? When it comes to these sorts of puzzles, we are still with Einstein, sitting in the dark, watching movies, and firing off questions.
Copyright 1998 Discover Magazine. Reprinted with permission.