Angiogenesis is refueling ischemic hearts, brains and other blood-starved bodily tissues. Scads of researchers are currently trying stem cell and gene therapy strategies to promote angiogenesis in the tissues and organs where it's needed. It's a dicey business.Veins, arteries, capillaries, venules and arterioles consist of long, slender, branching tubes made of tight-junction endothelial cells inside smooth-muscle cells. Jump-starting these concentric cylinders are two of the high-powered molecules – vascular endothelial growth factor (VEGF), which fabricates the seamless inner tube, and platelet-derived growth factor (PDGF), manufacturer of the outer, muscular integument.
"VEGF is clearly a very powerful drug," said biomedical engineer David Mooney at the University of Michigan (Ann Arbor, Michigan), "but the consistent observation is that while it forms a lot of new blood vessels, they tend to be immature; they're leaky, and often regress following removal of the growth-factor stimulus." He said VEGF "is believed to have a very important role in the initial stages of angiogenesis, particularly in activating endothelial cells in pre-existing blood vessels to start to digest their matrix, migrate, and proliferate toward the source of the molecule, then forming those immature vessels."
Mooney added that PDGF "is thought to be important in the subsequent angiogenic stages, in which those immature vessels comprised of endothelial-cell tubes, can recruit smooth-muscle cells, pericytes, to wrap around them. Then the interaction of those two cell types — endothelia and pericytes – will lead to expression of PDGF and formation of the extracellular matrix, coating the endothelial cells with a dense network of mature smooth muscle."
Mooney is senior author of a paper in the November issue of Nature Biotechnology. He and his co-authors designed and created an implantable plastic scaffolding, seeded with both growth factors, configured for optimal dosing.
"I think their physical form would be very different for different applications in biology and medicine," Mooney said. "We went into using a particular physical form which is based on a porous sponge made of PLG - poly(lactide-co-glycolide). It makes a well-defined model system of the space in which we're forming the blood vessels and the tissue. I could see this particular physical form being useful in a variety of applications." For example, he said, it could be used as a heart patch in cardiac ischemia, or for treating diabetic ulcers, where such a scaffold would serve as a template for laying new tissue over the open lesion.
"There are other potentially important physical forms or systems using this concept," he added. "For example, coronary artery disease. Instead of implanting a sponge like this, you might inject two different populations of polymer particles, one that controlled the first drug, VEGF, which would be released quickly, and the second containing PDGF, released more slowly."
Mooney noted that the researchers did two in vivo experiments to demonstrate the concept. "The first was very simple; we took our materials, made scaffolds, and implanted them under the skin of rats. Then we documented that in that particular site they formed new blood vessels and that we could drive their maturation with the sequential growth factor delivery."
The second model, he said, "was the non-obese diabetic mouse, which has impaired wound healing and impaired ability to form vasculature. It's a fairly standard model for ischemic tissue in the angiogenesis arena. In that animal, what we did was to mimic the ischemic tissue that you might see in a diabetic patient ... We put our scaffolds in the skeletal muscles surrounding the artery and vein. Then we examined how much peripheral circulation we could generate – how many blood vessels would come into existence to bypass those severed vessels, and carry blood down to lower in the limb." The result: "We had a significant increase in the number of new blood vessels we formed, as well as their maturity – measured by their association with the smooth-muscle cells and pericytes."
Mooney noted that his lab does "a lot of tissue regeneration work, so we are planning to use this system in conjunction with transplantation of cell types that we're interested in regenerating – specifically bone and liver." He said the researchers also are looking to test this in a coronary heart model, "something in which we do not have particular expertise." He said his lab pretty much stops at the rat level. "We're not a surgical lab, so we're looking to collaborate with other groups that can do things in larger, more relevant animal models." The university has a patent pending on Mooney's angiogenic invention, and he has been interacting with Curis (Cambridge, Massachusetts) on some of these studies.
His bottom-line point: "It's not just the identity of these growth factor molecules ... it's how we use them. We need to pay attention to how we deliver them, and whether we need to do so as single molecules or combinations of molecules." That concept isn't just about coronary artery disease, Mooney said. "It isn't just angiogenesis. I think it's equally valid in bone regeneration, neural regeneration and a variety of other angiogenesis situations."