The business of turning stem cells into specific endothelial cells is now a fairly mature field of science, but bringing that knowledge into clinical use requires being able to do this quickly and on a large scale. With the help of green fluorescent protein markers, a group of scientists from a number of US institutions identified a compound that may increase the efficiency of producing endothelial cells 40-fold.
Daylon James, a scientist in Rafii’s laboratory, engineered a new line of human embryonic stem cells that produce green fluorescent protein when they become vascular endothelial cells. The label allowed the scientists to rapidly and easily track when the stem cells morphed into this cell type. The researchers bathed the stem cells in a variety of different small molecules and looked for those that resulted in more green cells. They found the most green —and hence vascular endothelial – cells when the stem cells were exposed to a compound that blocks TGF-beta, a growth factor that helps control cell specialization.
Blocking TGF-beta at just the right time during cell culturing dramatically increased the number of vascular endothelial cells produced. Previously, the researchers needed to start with five stem cells for every endothelial cell they hoped to generate. But with the new method, starting with five stem cells gave Rafii and his colleagues 40 endothelial cells. “We’ve turned the ratio around,” says James.
Most importantly, the cells work. The researchers grew human vascular endothelial cells and injected them into mice. After a week, the new “humanized” cells had assimilated into the mouse circulatory system. The cells still glowed green, allowing the team to pick out the injected cells. In addition, a molecule that sticks to the walls of working blood vessels also stuck to the green cells, suggesting that the new cells functioned normally.
Using a small molecule inhibitor of TGF-beta circumvents some problems of testing in humans. Other methods for generating endothelial cells from embryonic stem cells have required factors derived from animals, and because of safety concerns, cells produced in this way are not suited for clinical application. Rafii’s approach, however, avoids the use of animal-derived factors, making it appealing for therapeutic blood vessel formation in patients.
Here’s a 3D rendering of vessels being formed from newly created endothelial cells:
Video: Embryonic stem cell -derived endothelial cells connecting to one another to form primitive vascular tubules…
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Study abstract in Nature Biotechnology: Expansion and maintenance of human embryonic stem cell-derived endothelial cells by TGFβ inhibition is Id1 dependent