Next the video focuses on a cluster of small spheres grouped together. While most mind their own business, one sphere has a sudden hunger attack and begins to consume its nearest neighbor. That being only an appetizer, it proceeds to canabilize the entire neighborhood, one by one, satisfied only when its food supply is exhausted. Having grown larger in direct proportion to what it has eaten, it appears to have overdone it. Its membrane begins to disappear, and in a few seconds it has completely self-destructed.
Yet another larger sphere contains a smaller sphere within it. The larger sphere abruptly spits out the smaller one, as if giving birth.
Birth, growth, death. These are characteristics of living systems. And yet these spheres, called "vesicles," are not biological, but are man-made collections of chemicals.
For the past three years, Menger, Emory's Candler Professor of Organic Chemistry, has been manufacturing giant vesicles - hollow spherical objects composed of a lipid shell arranged in a double layer of lipid molecules - by using a lipid called DDAB (didodecyldimethylammonium bromide). Although the absolute thickness of the vesicles is sub-microscopic, the shells diffract light, making them visible under a light microscope. Menger is using the man-made vesicles to gain insight into how living cells might function.
In past years Menger, along with dozens of other organic chemists, has studied tiny, man-made sub-microscopic vesicles because they exhibited some of the characteristics of living systems. "We used various techniques like fluorescence, but our conclusions were always indirect," he said. "We needed to get the vesicles large enough to see." Since very little had been done with giant vesicles, Menger felt they were ripe for the plucking. He knows of no other laboratory in the world currently pursuing this line of work.
Making the vesicles sounds as easy as a second-grade science experiment. The lipid is immersed in water and begins to grow spaghetti-like formations that pinch off into spheres. Vesicles can be made with a variety of lipids, including natural ones like lecithin, but Menger's lab has had the most success with the synthetic DDAB. Unilamellar vesicles, composed of just one shell, are easier to study than the multi-lamellar variety, which are more onion-like. After they grow a crop of new vesicles, the chemists irritate them with chemicals, probes, heat or cold to see whether they will act like living cells. In many ways they do. "People who have seen our video think they are looking at living systems," reported Menger, "and yet it's just a simple chemical." Surfactants, or soap-like chemicals like octyl glucoside, weaken vesicle membranes, allowing smaller vesicles to pass through. Then, the membrane quickly heals itself. Steroids, such as the bile salt cholate, used by the human digestive system to solubilize fat, stimulates a vesicle to eat other vesicles until it grows larger then finally disappears. Vesicle fusion can be induced by adding sodium acetate.
Of particular interest to Menger is the behavior of cell membranes, which are critical to many biological processes. For example, membranes are damaged in burn injuries. Drug delivery depends on drugs passing through membranes to get into the blood stream. Cancer cells, when they metastasize, burrow their way through membranes, get into the blood stream or lymphatic system and travel to a different site. In gene therapy, genes must pass through membranes.
"We want to understand the process of molecules diffusing from the outside of a cell to the inside, or the outside of a vesicle to the inside," explained Menger. "In our laboratory we are seeing processes that have never been observed before. For example, when we physically damage a vesicle, we can see it heal. We can inject a fluorescent dye into a vesicle and measure how fast a substance diffuses through a membrane.
Menger's vesicles are unlike living cells in that they are not self-replicating. They cannot yet divide in two, grow to a certain size, and divide again. In some cases vesicles can grow bigger; in other cases big vesicles "give birth" to smaller ones. So far, however, all these things don't happen simultaneously.
Menger and his colleagues, Kurt Gabrielson, who completed his Ph.D. at Emory last year, and current graduate student Stephen Lee, have had to manufacture and learn to use equipment found more often in cytology labs or in vitro fertilization labs than in chemistry labs. Micromanipulators and micropipettes are necessary for handling the cell-like vesicles. New computer software allows the images to be rearranged, cut, enhanced, stored and printed with precision.
"The big difference in what we are doing and in what cytologists do is that they are working with living systems and we are working with chemical systems," said Menger. "The trouble with living cells is that they are so complicated you don't always know what you're working with. Here we have defined the system and simplified the whole thing."
Soon the chemists will be able to study how their vesicles interact with living cells. Postdoctoral student Lillian Daniels, an expert in cell growth, will be joining Menger's lab to begin culturing cells. "We want to find out things like whether an amoeba will eat a vesicle, and whether we can promote vesicle entry into a cell," said Menger. "This is the borderline where chemistry and biology meet."
Menger's work has been supported for the past three years by a recently renewed grant from the National Institutes of Health. The German chemical journal, Angewandt Chemie, will publish a long article summarizing his work, which is largely unknown to other chemists, later this year.
-- Holly Korschun