In early development, how do cells know to put the right spacing between ribs, fingers and toes? How do they communicate with each other to form symmetrical and repeated patterns such as zebra stripes or leopard spots?
For the first time, UCLA researchers have recreated the ability of mammalian cells to self-organize, forming evenly spaced patterns in a test tube. Published in the June 22, 2004 issue of the Proceedings of the National Academy of Sciences, the findings may help improve methods for regenerating tissue, controlling birth defects and developing new treatments for specific diseases.
"Just as a marching band needs direction from a conductor to line up in formation on a football field, cells also need guidance to form patterns -- but until now we didn't know how they were communicating or receiving direction," said Alan Garfinkel, Ph.D., first author and professor of physiology and cardiology at the David Geffen School of Medicine at UCLA.
"Previously it was a bit magical how cells knew exactly how far apart to space ribs or tiger stripes," said Dr. Linda L. Demer, senior investigator, Guthman Professor of Medicine and Physiology, and vice chair for cardiovascular and vascular medicine at the David Geffen School of Medicine at UCLA. "We now know that it's orchestrated by specific proteins produced by cells that disperse at different rates and interfere with one another. These interactions can be described in mathematical formulas dictating how cells organize into specific, evenly spaced patterns."
Demer notes that similar mechanisms may explain how an embryo creates structures in evenly spaced patterns in early development or how certain diseases may trigger cells to create lesions in specific patterns.
Researchers grew stem cells from adult bovine arteries and found that they produce intricate, lace-like patterns in culture dishes. Such patterns are known to be created in nature by a process called reaction-diffusion discovered by Alan Turing, the mathematician famous for his role in breaking the Nazi code during World War II. He showed that patterns required interaction between an activating protein that draws cells together (activator) and another protein that stops them from coming together (inhibitor). The inhibitor protein must diffuse or disperse more rapidly than the activator. The result creates areas where cells pile up separated by empty spaces. The exact patterns depend on the strength and speed of the two proteins.
The UCLA researchers knew the likely activator protein was BMP-2; it was produced by the cells and caused cells to draw together. One of the researchers, Dr. Kristina Bostrom, had recently discovered a new inhibitor of BMP-2, an unusually small protein known as MGP. The investigators theorized that interference between these two proteins was the source of the patterns. To test this idea, collaborator Dr. Danny Petrasek from the California Institute of Technology generated computer simulations of the expected interactions. He predicted that adding MGP to the cell culture would change the pattern from stripes to spots. Without knowing his result, Bostrom added MGP to the cells and found that they indeed produced spots instead of stripes.
"Using the mathematical formula based on Turing's concepts, we were able to recreate the classic stripe or spot patterns seen throughout nature – such as in a zebra's stripes or leopard's spots," said Garfinkel.
Garfinkel adds that many parts of the body are based on patterns: Stripe patterns are used to generate fingers, ribs and toes, while branching patterns generate vessels, lungs and nerves, and spot patterns produce the organization of hair follicles, vertebrae and teeth. The type of structure formed depends upon the types and amounts of the proteins and cells involved.
To be sure that the proteins were controlling the patterns produced by cells, the researchers added the drug warfarin, which blocks MGP. The result was a double-striped pattern, also predicted by the simulation. This may help explain the known association of warfarin with birth defects.
"The abnormal cell pattern resulting from adding warfarin, may give researchers some insight into how birth defects develop," said Garfinkel.
The next step, Garfinkel added, is to generate more complex patterns by adjusting the ratios of the two proteins BMP-2 and MGP. Such control would be useful for tissue engineering architecture – producing replacement tissue in desired shapes and patterns.
Demer also notes that the research may offer a greater understanding of how artery cells calcify and turn to bone in atherosclerotic heart disease.
"Our ability to recreate cell patterns may ultimately help us learn how to better control them, leading to new ways to treat certain conditions like heart disease," said Demer.