Towards Creating Blood Vessels

Scientists find a better way to make more of the cells that make blood vessels.

An efficient new method of making endothelial cells, which give rise to blood vessels, could prove a huge boost for tissue engineering and regenerative medicine. By first finding a way to effectively tag endothelial cells, researchers at Weill Cornell Medical College developed a simple way to increase production of these cells by more than 30-fold. The cells might one day by used to create blood vessels in engineered tissue or administered to patients directly to repair injury after heart attack or stroke, resupplying blood to damaged organs.

Boosting blood vessels: Exposing human embryologic stem cells to a small inhibitor molecule dramatically boosts production of endothelial cells (green), which form the inner lining of blood vessels. The bottom image shows a culture of cells exposed to the inhibitor, while the cells in the top image were not.

“[Eventually], we want to be able to inject slurries of these cells into people who have suffered heart attacks, and allow those tissues to recuperate by renewed blood flow,” says Daylon James, assistant research professor in the department of reproductive medicine at Weill Cornell Medical College, who led the research.

Our bodies house billions of endothelial cells, which line the interior of blood vessels. This vast network is responsible for maintaining vascular health, controlling blood pressure, managing clotting, and giving rise to new blood vessels. While researchers already knew how to turn embryonic stem cells into endothelial cells, the challenge has been a matter of commitment and scale. Once stem cells turn into endothelial cells, it’s difficult to make them stay that way. Getting endothelial cells to expand to numbers great enough to engineer functional artificial blood vessels has been another major roadblock.

While previous methods produced about 0.2 endothelial cells for every embryonic stem cell, James and his colleagues have found a much more efficient way to make committed endothelial cells. The new technique yields seven endothelial cells for every stem cell, according to research published in the advanced online edition of the journal Nature Biotechnology. When these differentiated cells were injected into mice, they formed tiny, capillary-like structures.

To create the new method, the team first developed a way of identifying endothelial cells among cultures of differentiating embryonic stem cells. James recognized a gene called VE-cadherin that only appears in endothelial cells, making it an ideal marker. He then genetically engineered a green fluorescent protein to turn on in embryonic stem cells only when VE-cadherin is expressed, signaling in real time that the stem cell has differentiated into an endothelial cell.

Researchers then looked for molecular triggers that ramp up endothelial differentiation and production. Shahin Rafii, a professor of medicine at Weill Cornell Medical College, and a Howard Hughes Medical Institute investigator, carefully selected various drugs and small molecules, and tested their effects in cultures of embryonic stem cells. Rafii found one candidate in particular that significantly boosted the yield of differentiated, committed endothelial cells.

This molecule, which goes by the technical label SB431542, is known to inhibit TGF-beta, a protein involved in cell differentiation and proliferation. Researchers found they were able to get the highest yield of endothelial cells when they first allowed TGF-beta to act uninterrupted, then introduced the inhibitor molecule to turn off TGF-beta at just the right time. Properly timed exposure to the TGF-beta inhibitor boosted production of endothelial cells 36-fold.

That increase could have real clinical significance, says Joseph Wu, assistant professor of medicine and radiology at Stanford University School of Medicine. “This new protocol is a significant advance, and a very good amplification process, because in order to translate therapy to humans and animals, you have to scale up the numbers,” says Wu, who was not involved in the research. Heart grafts to treat cardiovascular disease, for example, would likely require 20 million to 50 million cells, he says.

The researchers now plan to determine whether the cells can actually restore blood flow to damaged tissue by injecting them into injured animals. The group is also working to grow engineered endothelial cells into functional blood vessels in vitro, on three-dimensional scaffolds that simulate the real conditions in the body. Sina Rabbany, a bioengineering professor at Hofstra University, is designing polymeric scaffolds, and growing endothelial cells on grafts of smooth muscle cells.

“Almost every body part requires a vascular supply, and endothelial cells are the building blocks of that supply,” says Rabbany. “So now that we can make these cells, how can we make them grow in a 3-D setting and make conduits that carry blood, and can provide oxygen and nutrients to other cells? [Because] whether you want to grow a pancreas for diabetes, or help treat Parkinson’s disease, every cell in the human body is next to an endothelial cell.”

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