Human Stem Cell Research: Stepping It Up A Notch

Researchers at Johns Hopkins have discovered that the Notch protein helps human embryonic stem cells “decide” their own fate, a finding which may eventually be useful in programming cells for the development of stem cell therapies. Their results are reported in the May 2008 issue of Cell Stem Cell.

Human embryonic stem cells (hESCs) receive signals from neighboring cells instructing them either to grow more of themselves or become other cell types, including the three types that make up the developing embryo or type that becomes the placenta. Researchers are just beginning to understand the many signals involved in committing hESCs to different fates.

“If you can understand the mechanisms involved in lineage commitment, basically you can open the door to a lot of things,” says Xiaobing Yu, M.D., a research associate at the Institute for Cell Engineering at Johns Hopkins. Such an understanding would help scientists program hESCs to replace cells that patients lose as a result of injury or disease.

Yu and his colleagues have made one initial step towards this goal by clarifying the often disputed fate-determining role of one protein found on the surface of hESCs—Notch.

In an effort to figure out Notch’s role in determining stem cell fate, the researchers first grew hESCs in a laboratory culture dish lined with a feeding layer of embryonic mouse cells, which encourage hESCs to replicate. They examined the cells and found very few stem cells contained active Notch. When the researchers chemically prevented the protein from being turned on, they found that a greater number of hESCs self-renewed and made more of themselves suggesting that Notch is required for differentiation and the lack thereof helps maintain cells in an undifferentiated state.

The researchers then prodded hESCs to differentiate. While Notch stayed inactive in self-renewing hESCs, its activity spiked in cells that started differentiating, suggesting that Notch somehow was involved in cell differentiation. The question was: Does the protein cause differentiation or is it turned on as a result?

In order to figure this out, the researchers stimulated hESCs to differentiate and in the same cells prevented Notch from being turned on. Without it, these cells grew into cells that become the placenta, suggesting to the researchers that early placental development does not require Notch. Without Notch, these cells did not ever grow into any of the three cell types that make up a developing human embryo. Therefore, it must be required for differentiation into embryonic cell types, the researchers concluded.

“We are interested in coaxing human embryonic stem cells into blood stem cells to develop therapies for patients unable to maintain blood cell levels,” says Linzhao Cheng, Ph.D., an associate professor of gynecology and obstetrics, medicine and oncology and a member of the Johns Hopkins Institute for Cell Engineering.

To see if Notch enables hESCs to become blood precursor cells, the researchers turned on Notch in hESCs that were stimulated to differentiate and found that some of the cells did indeed grow into blood precursor cells.

Encouraged by their results and the potential for developing blood disease therapies, Yu emphasizes that hESCs have tremendous potential and “can become all cell types if they’re provoked to differentiate randomly. In the case of treating blood diseases, it’s important to be able to ensure that the stem cells become only blood cells, not nerve or liver cells.”

Says Yu, “We are still many steps away from clinical application, but learning more and more each day.”

This work was funded by the National Institutes of Health and the Johns Hopkins Institute for Cell Engineering.

Authors on the paper are Xiaobing Yu, Jizhong Zou, Zhaohui Ye, Holly Hammond, Guibin Chen, Akinori Tokunaga, Prashant Mali, Curt Civin, Nicholas Gaiano, and Linzhao Cheng, all of Johns Hopkins, and Yue-Ming Li of Memorial Sloan Kettering Cancer Center.