Whether your heart winds up in the right place may be determined as early as the first hour of your life in the womb.
That’s the way it is in frogs, Harvard scientists have just discovered, and researchers all over the world believe that frogs and humans develop in a similar way. Experiments show, for example, that some of the same mechanisms put the hearts of both creatures on the left side. The proteins responsible for shifting around a frog embryo’s heart, gut, gall bladder, and other organs are also found in abundance in human embryos.
“Our research shows the same protein family, known as 14-3-3, plays important roles across the three kingdoms of living things, fungi, plants, and animals,” says Michael Levin, a biologist at the Harvard School of Dental Medicine and the Forsyth Institute in Boston. “Our latest findings provide strong evidence that the determination of right-left asymmetry in vertebrates, possibly including humans, occurs at a much earlier time than previously believed.”
Levin and two colleagues from the Netherlands describe their newest experiments in the Oct. 20 issue of the journal Development. The work originates from a casual contact Levin had during a conference where he spoke on how tiny electrical voltages in the bodies of living creatures determine the architectures of their internal organs. A member of the audience, a plant biologist, suggested he try a substance called fusicoccin, which controls the behavior of plant cells by interacting with the ubiquitous 14-3-3 proteins in those cells.
Levin looked into it and found that fusicoccin comes not from plants but from a fungus that infects them. The substance combines with a 14-3-3 protein and activates a so-called ion pump. It was not known at this time whether fusicoccin also affects animal cells, so Levin tried putting fusicoccin into early frog embryos. By this means, he discovered he could juggle the position of the developing croakers’ heart, stomach, and gall bladder. A close look at the embryo revealed that asymmetry occurs as soon as a fertilized egg begins to grow, on the very first division of the egg into two cells. When that happens, the new left and right cells acquire different amounts of 14-3-3 protein.
“This shows that the same cell-control mechanism has been conserved during the evolution of creatures from fungi to plants to animals, and, possibly, to humans,” Levin comments.
The flow of life
Of course, that scenario is hotly debated. What goes on in frogs and other creatures may not go on in humans. And what goes on in humans and other mammals hasn’t always been found to play a role in frogs, fish, and other animals studied in the laboratory. To the latter, Levin replies, “My guess is that we probably haven’t looked hard enough.”
Whether in fungus, frog, or presidential candidate, Levin sees the following scenario. A substance such as fusicoccin combines with a 14-3-3 protein within an hour after an egg has been fertilized. The union turns on an ion pump. Such pumps move electrically charged particles in and out of minuscule channels in the baglike membrane that holds the cell’s contents. In the frog experiments, Levin’s group followed the positively charged nuclei of a hydrogen atom. Other ions carry negative charges, and their movements set up electric fields in body fluids.
It is ion flow that makes it possible for muscles, hearts, kidneys, brains, and other organs to operate. Without it there’s no thinking, no movement, no metabolism, no life. The 2003 Nobel Prize in chemistry was awarded to Roderick MacKinnon and Peter Agre for discoveries of how ions and water get in and out of cells.
In frogs, if the ion pump in the right half of the first cleavage is activated, the heart loops to the left side. When Levin used fusicoccin or added extra amounts 14-3-3 protein to switch on ion pumps on both sides, the amphibians’ organs were randomized. The heart appeared as often on the right side as on the left.
Occasionally, humans are born with reversed hearts. More rarely, all organs flip, creating a mirror image of the usual arrangement. If all the blood vessels and other plumbing connect properly, people with inverse organs suffer no unusual medical problems. Some of them may not even know about the switch until they have X-rays.
This raises the question of how ion pumps get from one side of an embryo to the other in the first place. Levin speculates that they are moved by motor proteins sliding along tracks formed by the delicate skeleton that gives a cell its shape. This cellular transportation system is known to shuttle other types of biological passengers around cells. “We are working on what physically moves the pumps, or the subunits from which they are assembled, from one side to the other,” Levin notes.
“Establishing ion flow is not the first step in left-right asymmetry,” he continues, “but we’ve now got the beginning boxed in to the first hour of development. If we back things up to where ion pumps get into place, we’ll have the initial step that everyone is looking for.”
New limbs and mad cows
Such fundamental research is not without future medical applications. One of them bears on learning how arms, legs, and other tissues damaged by trauma or birth defects might be regenerated. “Learning what role ion flow plays in controlling body patterns is a first step in understanding how to grow a new leg or heart,” Levin comments.
Ion flows also play a key but unclear role in development of cancer. “Tumor cells show different ion fluxes than normal cells,” Levin points out. “These differences may reveal what causes tumor cells to multiply without normal controls. The work we are now doing is relevant to solving this problem.”
It is also relevant to misshapen proteins called prions, which appear to be culprits in mad cow disease and its human equivalent, Jakob-Creutzfeldt syndrome. Prions boast a bizarre shape. When they enter the brain, they cause native proteins to adopt the same shape, a condition that destroys brain tissue.
Normal brains possess an asymmetry not unlike right-left handedness. For example, in most people language ability is located on the left side. That’s even true when other organs are reversed. “No one understands the link between brain and organ asymmetry,” Levin admits. “But we believe that the new mechanisms we have discovered will eventually shed new light on this mystery.”