Volume 6, Issue 43 April 2009
Habitat for Human Stem Cells
David Schaffer is also codirector of the Berkeley Stem Cell Center.
With the power to transform into any tissue, stem cells hold great promise for the future of medicine. Scientists hope someday to be able to transplant stem cells into the body to repair injured nerve cells, regenerate diseased organs, and replace defective tissues. But stem cells are also prima donnas. They're tricky to keep alive in the lab, difficult to multiply, and must be coaxed with a series of largely unknown cues before they will differentiate on demand.
Once stem cells are removed from their habitat within the body, "they lose the signals they need to mature and repair tissues," says David Schaffer, a Berkeley professor of chemical engineering. "To control their behavior, we need to learn the way in which their natural microenvironment, or niche, feeds signals to cells to regulate their behavior."
Schaffer is identifying the elements that constitute good stem cell habitat. By examining the roles of small molecules, proteins, mechanical stressors, and other factors, he hopes to learn how the body controls stem cell behavior. He is now developing synthetic ways to mimic these factors, to speed the day when stem cell therapies are a trusted part of modern medicine.
The adult brain adds new neurons throughout human life. Here, stem cells in the adult hippocampus (blue) divide and differentiate into mature neurons (green), whose function is supported by neighboring astrocytes (red). Image credit: David Schaffer and Karen Lai, Schaffer Lab
The environments in which stem cells live are rich indeed. Resting atop a cushion of membrane proteins, stem cells are also surrounded by a matrix of neighboring cells and bathed in a mixture of proteins and other small molecules. Each communicates developmental messages via the cell's surface receptors.
"It sounds like a complex recipe to reconstitute outside the body, but we've come a long way in trying to reassemble niches from elemental pieces to create environments that can really regulate cell behavior," Schaffer says.
For example, like Goldilocks, stem cells care about surface hardness or softness. Schaffer found that by growing neural stem cells atop surfaces of varying stiffness, he could alter their fate. Those raised on soft surfaces developed into neurons. Those raised on hard surfaces matured into supporting glial cells. And cells on surfaces of intermediate rigidity multiplied while still in their immature state.
Neural stem cells isolated from the adult hippocampus can be grown in cell culture and differentiated into the three major cell types of the nervous system, including astrocytes (red) and oligodendrocytes (green). Nuclei are labeled blue. Image credit: Linda Hinh and David Schaffer, Schaffer Lab
But exterior cues tell only half the story. The rest occurs inside stem cells themselves. Environmental inputs such as surface hardness get filtered through elaborate networks of surface receptors and signaling pathways. This processed message is ultimately what turns genes on and off in the nucleus, and then guides cells into making important decisions like continuing to divide, or converting into specialized cell types that can potentially be used to treat disease.
Schaffer is untangling stem cell signaling networks to write a truly complete stem cell control manual. Using a combination of laboratory experiments and computer models, he is refining ideas of how certain signals affect molecular pathways and gene transcription. "We view these networks as nothing more than computational devices. They take in information, compute or process it, and the cell switches behavior as a result," he says.
Schaffer is now applying what he knows about stem cell behavior to engineer artificial environments for them in the laboratory. At present, most human embryonic stem cells are cultured on a commercial cocktail of hundreds of proteins derived from mouse tumor cells. But this concoction is hardly suited for medical use, Schaffer says. "Its contents are poorly defined, so it raises concerns for reproducibility, safety, virus infection and other issues that will be important to consider when using stem cells in patients."
Viruses can be used to deliver genes to the adult brain, revealing gene function in the nervous system and neural stem cells. Here, a lentivirus carrying the gene for green fluorescent protein (GFP) has been injected into the adult brain. Nuclei are blue. Image credit: Julie Yu, Schaffer Lab
To address this problem, Schaffer and fellow Berkeley engineering professor Kevin Healy are developing a 100-percent synthetic, chemically defined medium for embryonic stem cell growth. Instead of using complex whole proteins that can only be manufactured by live cells, they plan to use only snippets in direct contact with cell receptors. These pieces are small enough to manufacture and can be attached to an artificial surface. Their artificial matrix would be cheaper and easier to produce on a large scale for clinical use, and could keep the costs of future stem cell treatments affordable.
So far, this effort has identified a handful of protein bits that stimulate critical stem cell receptors. The present version of his matrix can keep human embryonic stem cells dividing for several weeks at a time. Future versions might help guide stem cells to differentiate into specific cell types.
"We know stem cells have all these incredible properties. The challenge is now to control that process better to turn stem cells into powerful, affordable and viable therapies," Schaffer says.