New research from the University of North Carolina at Chapel Hill has identified two genes that play key roles in regulating blood vessel development.
The research appears in two reports published in the August 15 issue of Molecular and Cellular Biology, a professional journal. Dr. Cam Patterson, professor of medicine and director of the Carolina Cardiovascular Biology Center and a member of the UNC Lineberger Comprehensive Cancer Center, led both studies.
Both research papers focus on angiogenesis, the molecular program by which endothelial cells lining blood vessels develop or differentiate from their precursor stem cells.
"I think of endothelial cells as the 'intelligent cells' of blood vessels," Patterson said. "They are communicators between the blood vessel wall and bloodstream. They are the cells that determine what a blood vessel does. For example, during angiogenesis, when new blood vessels are being formed, it's the endothelial cells that determine where they go and how big they get."
And as communicators, Patterson added, endothelial cells also help determine what passes through the blood-brain barrier, through the endothelium and into the brain, and what does not.
"One study in this issue of the journal sheds important new light on the molecular process that prevents a particular cell type from overrunning the developing embryo," Patterson said. "The findings from this study also offer tantalizing possibilities for new treatments aimed at putting the brakes on blood vessel development in tumors and other disorders having important vascular growth components, such as diabetes."
The second paper focuses on what activates the endothelial cell program, and it reports having found a possible answer in a single protein, a known transcription factor that has never been characterized functionally.
"This has really been a 'holy grail' finding for us," Patterson said. "No other research group has found a single transcription factor that by itself is both necessary and sufficient to activate the endothelial cell program." For the first study, co-authors were Patterson and School of Medicine research colleagues Drs. Martin Moser, Olav Binder, Yaxu Wu, Julius Aitsebaomo, Rongqin Ren, Victoria Bautch and Frank L. Conlon. Dr. Christoph Bode of Freiburg University in Germany also served as co-author. The study team discovered a gene they called BMPER for BMP-binding endothelial precursor-derived regulator. The molecule was found using "a sophisticated molecular approach to separate out endothelial cell precursors from non-endothelial cell cells in a stem-cell model," Patterson said.
In a series of experiments, the researchers demonstrated that only endothelial cells and their precursors express the BMPER gene. Endothelial cells secrete the protein as they differentiate, issuing a molecular "stop" order to inhibit further differentiation.
"This was shown very clearly when we used stem cells to generate endothelial cells and then added BMPER. We found that BMPER inhibited the whole process," Patterson said.
In the second study, co-authors Wu, Moser, Bautch and Patterson looked at the gene flk1, a molecular marker for early endothelial cell precursors. "Flk1 is important because it's a receptor for vascular endothelial growth factor, which is an angiogenic factor," Patterson said.
"But it's also important to us because it's the first gene that gets turned on in endothelial progenitor cells. So we wanted to know what transcription factors turn on flk1 and are those factors themselves sufficient enough to turn on the whole endothelial cell gene program - that is, can you use those factors to take a precursor cell and turn it into an endothelial cell?"
The study team used a screening procedure (yeast-1 hybrid screen) in which a piece of DNA was employed as a kind of bait to screen a library containing a variety of transcription factors.
"And the protein we pulled out with the bait is called HOXB5, a transcription factor that's known but that has never been functionally characterized," Patterson said.
The researchers then asked whether HOXB5 would increase expression of flk1 by binding to genetic regulatory elements in the gene. "And indeed that was the case in our in vitro studies," Patterson said. "But the really important findings came when we over-expressed HOXB5 in stem cells. We used a stem cell model developed a few years ago here at UNC and found we could double or triple the number of flk1-positive cells that were produced from stem cells.
"But most importantly, if we look at actual vessel formation in stem cell cultures, we found vessel formation is hugely increased." Thus, the findings indicate that simply over-expressing HOXB5 by itself not only increases expression of the regulatory protein, but also increases the number of endothelial cells that will form from the precursors.
"We're especially excited about the possibility that we can use this transcription factor to create renewable populations of endothelial cell precursors. I think this will be very important, as it would be analogous to hematopoietic (blood cell-forming) stem cells," Patterson said.
"And if we can create an analogous endothelial stem cell line, we can use that for gene therapy applications, for example, as a regenerative therapy for aged blood vessels.
"The therapeutic potentials for this research are many."
A grant from the National Heart, Lung and Blood Institute, a component of the National Institutes of Health, supported this research.