August 29, 2011
In Cell Culture, Like Real Estate, The Neighborhood Matters
Ever since scientists first began growing human cells in lab dishes in 1952, they have focused on improving the chemical soup that feeds the cells and helps regulate their growth. But surfaces also matter, says Laura Kiessling, a professor of chemistry at the University of Wisconsin-Madison, who observes that living cells are normally in contact with each other and with a structure called the extracellular matrix, not just with the dissolved chemicals in their surroundings.
"Soluble factors are important, but cells normally interact with the extracellular matrix and with neighboring cells, and these have not been considered in most efforts to refine growth conditions," says Kiessling. "We wanted to know, can we replace the neighboring cells and extracellular matrix with synthetics?"
Creating a more precise system for growing cells offers both theoretical and practical advantages, Kiessling says. First, it would reduce uncertainty in experiments by simplifying conditions. Second, it would remove the risk of biological contamination like viruses, so the cultured cells could be used in medicine. Third, new surfaces that improve the control over cell growth and development could facilitate the formation of artificial tissues, which are complex assemblies of different cell types.
In a talk on Aug. 28 to the annual meeting of the American Chemical Society in Denver, Kiessling outlined two areas of progress from her lab at UW-Madison. One series of experiments used a lab dish decorated with molecules called peptides to amplify the response of cells to a growth factor called transforming growth factor beta (TGF-beta). TGF-beta can affect healing, cell division and transformation into a more specialized cell or into a tumor cell, Kiessling notes, so TGF-beta can be helpful or harmful in different situations.
After screening thousands of potential anchors, Kiessling, graduate student Joe Klim, former graduate student Lingyin Li and colleagues found a peptide that would safely hold the cells in place and simultaneously make them extremely sensitive to TGF-beta made by the cells themselves. "TGF-beta receptors have to assemble into a complex before TGF-beta sends its signal to the cell," says Kiessling. "We made surfaces that organized the receptors so they were especially sensitive to this growth factor when the cells were bound to the surface, and so the growth factor affects the cell at incredibly low levels, levels we cannot detect."
The secret, she says, lies in the preparation: The manufactured surface primes the cells to respond to tiny amounts of growth factor. "It's like the surface acts as an amplifier to allow the cells to sense the presence of the growth factor."
When they are grown on surfaces without the peptides, the cells used in the experiment are like skin cells, but when they are on the peptide surface, they detect the growth factor and transform into muscle-like cells. "That shows the power of this approach," says Kiessling. "We have a way to make cells do one thing if they are attached to this surface and another thing if they are not."
A second series of experiments concerned human embryonic stem cells – the versatile cells have the potential to form any cell type in the body. Since these cells were first identified in 1998 by James Thomson of UW-Madison, their therapeutic potential has remained tantalizingly difficult to reach, partly because they have been grown with substances derived from mice that could contain viruses or other pathogens.
While scientists have refined the liquid portion of the environment needed to grow and transform embryonic stem cells, the solid portion has received less attention. "Human embryonic stem cells need not only a defined medium, but also a defined substrate," Kiessling says. "Historically, the field has relied on mouse embryonic feeder cells and various mixtures of proteins isolated from mice, which contain who knows what in the way of viruses or other infectious particles."
Others have experimented with growing human embryonic stem cells on artificial surfaces, she says, "but there are some advantages to the surface we have found." She now has to move the defined peptides from the gold backing that she presently use to the polystyrene Petri dishes that are common in cell culture.
There are even hints that surfaces can also control the differentiation of stem cells, Kiessling adds.
"Our work highlights the fact that we can use a patterned surface itself to instruct the cells, which could be really useful for growing cells on a larger scale and differentiating them under defined conditions," Kiessling says. "Patterned surfaces are found throughout our tissues. Hair, eyes, brain, everywhere we have organized tissues, so if we want to grow different types of cells in ordered arrays to build up a tissue, we need the cells to be organized. We are not close to building a complicated tissue, but the first step is to localize specific types of cells where we want them. We've only scratched the surface of our ability to regulate and instruct cell growth and transformation using elaborately structured surfaces."
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