June 16, 2005
Arteries Bio-engineered from Elderly Cells
DURHAM, N.C. "“ Researchers from Duke University's Medical Center and Pratt School of Engineering have demonstrated that they can grow new human blood vessels from cells taken from patients who especially need such assistance "“ older adults with cardiovascular disease.
The researchers said the results of their latest experiments represent a "proof of principle" for an approach that could be clinically applicable within five to ten years. The first to benefit from such bio-engineered arteries, according to lead researcher Laura Niklason, M.D., Ph.D., could be older patients with cardiovascular disease who need blockages in their arteries "bypassed" but do not have their own natural vessels available.
"There is a great need for viable alternatives to our current available options for treating patients with coronary artery or peripheral arterial disease," said Niklason, an associate professor of both anesthesiology and biomedical engineering. "In this current study, we took vascular cells from four elderly men with heart disease and engineered new blood vessels.
"The ability to grow new vessels from older cells represents a crucial initial step towards growing blood vessels from a patient's own cells that can be used to treat that patient's vascular disease," she continued.
In 1999, Niklason demonstrated she could grow new blood vessels in animal models by using a novel bioreactor she developed that mimics the fetal environment. When implanted back into the animals, the bioengineered vessels functioned like natural arteries. Unlike the cells in these experiments, however, human artery cells do not possess enough life cycles to be grown into functional arteries.
In collaboration with Duke cancer researcher Christopher Counter, Ph.D., an expert in telomeres, Niklason overcame this hurdle. Every time a cell divides, the ends of its chromosomes, or telomeres, erode until they become so short that the cell receives a signal to stop growing. Counter had previously cloned the hTERT (human telomerase reverse transcriptase subunit) component of the enzyme telomerase, which stops telomeres from shortening. He showed that expression of hTERT permitted some human cells to continue to divide indefinitely, in effect making them immortal.
Using arterial cells from a two-year-old child, the Duke team reported in 2003 that when the hTERT gene was introduced into smooth muscle cells, key components of an artery, the life span of the cells was extended long enough to form arteries in the laboratory.
In the latest experiments, the researchers took cells from the saphenous vein of four men between the ages of 47 and 74 who were undergoing coronary artery bypass surgery. The saphenous vein is located in the lower leg and is often used to bypass blockages in arteries around the heart. The team isolated smooth muscle and endothelial cells, grew them in culture and treated them with hTERT.
To create the new arteries, the researchers fashioned a tube from a thin sheet of a biodegradable polymer that was 97 percent air, much like a sponge. The treated smooth muscle cells were then impregnated throughout the polymer tube. The bioreactor pulsed a vitamin and nutrient solution through and around the tube, approximating as closely as possible the conditions that would exist in nature.
Once the smooth muscle cells proliferated and filled all the spaces within the dissolving polymer scaffolding, the researchers added endothelial cells, which line the interior of blood vessels, to complete the artery. Vessels grew for up to seven weeks.
"While the resulting vessels looked like natural vessels, they were not strong enough to be implanted into humans successfully," Niklason said. "However, this issue could be solved by adding different factors to the bioreactor solution or by genetically manipulating the cells to produce more collagen, which gives structure and strength to vessels."
Additionally, she said, using telomerase expression to extend the lifespan of the vessels created from older cells proved to be an effective strategy.
Growing arteries from the patient's own cells is important, Niklason said, since the ultimate goal is to engineer arteries that will resist immunological attack. This problem is avoided by growing the arteries from the patient's own cells.
Since unchecked cellular growth is a hallmark of cancer, the researchers also conducted rigorous tests on the arteries and could not detect any signs of unwanted cellular proliferation. Before bio-engineered arteries can be implanted into humans, hTERT expression must be turned off, and arteries would then be expected to "age" like native arteries.
It is estimated that about 100,000 out of 1.4 million Americans who need small vessel grafts are unable to get them because their own or prosthetic vessels are unsuitable. While polymer vessels can be used when large vessels are required, the smaller ones tend to become clogged with clots.
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