The study findings appear in the June 15 issue of the journal Blood.
Blood vessel formation, or angiogenesis, is an integral part of normal organ development and function. It also contributes to abnormal conditions, particularly tumor formation and growth.
Angiogenesis begins with the establishment of an intricately branched rudimentary network called the vascular plexus, which is assembled from blood vessel precursor cells. This is followed by increased cell division of specific cells, endothelial cells that make up the lining of blood vessels.
These cells then sprout and migrate away from the parent vessel, and the sprouts ultimately connect with each other, allowing the vessel network to expand. This process is called sprouting angiogenesis.
"It is very important to understand the sprouting process, because it occurs any time there is angiogenesis, whether for helpful reasons, such as wound healing, or in the context of pathology, such as cancer," said Dr. Victoria L. Bautch, who is a member of the School of Medicine's Carolina Cardiovascular Biology Center and a professor of biology at the university. Angiogenesis is coordinated by the actions of a number of proteins, and one of the most critical regulators of this process is the protein Vascular Endothelial Growth Factor-A, or VEGFA, said Bautch. Sprouting angiogenesis occurs as a result of the interactions of VEGFA with two cell receptor molecules, VEGFR1 (also called flt-1) and VEGFR2 (also called flk-1), she added.
While flk-1 is thought to promote endothelial cell division, the exact functions of flt-1 are poorly understood and have been difficult to uncover until now, said Bautch.
Research by Bautch's group reveals for the first time that flt-1 positively controls sprouting by regulating endothelial cell migration.
UNC co-authors postdoctoral researcher Joseph Kearney and graduate student Nicholas Kappas measured the efficiency of vessel formation using mouse embryonic stem cells genetically engineered to lack the flt-1 gene and then induced to become endothelial cells.
Mutant and normal embryonic stem cells were additionally engineered to express the green fluorescent protein. This "marker" allows fluorescence microscopy to visualize living cells.
The experiment enabled the researchers to analyze the dynamics of vessel formation in real time by performing time-lapse imaging of live endothelial cells. Using this method they demonstrated that blood vessels made from cells lacking the flt gene are defective in sprouting and that these sprouts migrate less quickly. These findings may have implications for future therapies.
"For instance, coronary heart disease, which is commonly treated by bypass surgery, requires reconstruction of blood vessels using veins from other parts of the body," said Bautch. "Diabetes is another pathological condition associated with loss of circulation in the limbs and extremities."
The goal of angiogenic therapy in these situations is to restore circulation non-surgically.
"There have been attempts to induce blood vessel formation by manipulating the VEGF molecular pathway. Most of the time you don't get functional vessels, but a set of dilated vessels that haven't made the right connections," said Bautch.
"We, along with others, are now beginning to unravel the complexity of this pathway. We think the flt-1 receptor actually regulates the amount of VEGFA required for proper vessel formation. So having the right amount of VEGF at the right spot and in the right context is critical," she added. Department of biology co-authors, along with Bautch, Kearney and Kappas, were Catharina Ellerstrom and Frank DiPaola.
This study was supported by grants from the National Institutes of Health.
By Subhashini Chandrasekharan
UNC School of Medicine
Note: Contact Bautch at 919-966-6797 or bautch@med.unc.edu.
Photo note: To view time-lapse images (in minutes) of blood vessel sprout formation, click on: http://www.unc.edu/news/newsserv/pics/research/blood_vessel_sproutings.jpg.
School of Medicine contact: Les Lang, 919-843-9687 or llang@med.unc.edu.
Journal
Blood