"Diabetic retinopathy is the leading cause of vision loss in working-age Americans," observed clinical ophthalmologist and cell/molecular biologist Martin Friedlander at the Scripps Research Institute in La Jolla, Calif. "The prevalence of diabetes is 16 percent to 17 percent of the U.S. population," he pointed out.
"Of those diabetics under 55, virtually every one of them will have some kind of a diabetic retinopathy, after 20 years of the disease. Their current rate of some vision loss from the disease counts about 40,000 Americans per year. And 12 million to 15 million Americans over the age of 65," Friedlander added, "will acquire macular degeneration, a different retinal disorder. Ten percent to 15 percent of them will suffer profound loss of central vision. They don't go blind. They don't lose peripheral vision, just reading vision. As the population ages, we see a lot more patients with this problem."
Friedlander is senior author of an article in the forthcoming September 2002 issue of Nature Medicine, which published his paper online July 29, 2002. Its title: "Bone marrow-derived stem cells target retinal astrocytes and can promote or inhibit retinal angiogenesis."
"From my perspective," he told BioWorld Today, "what's exciting about the work we report is that it now suggests we can use adult-derived bone marrow cells - not controversial embryonic stem cells, not viruses, not some strange hybrid cells. We use normal syngeneic [immune-matched] bone marrow cells to treat illnesses of retinal vasculature. And that's exciting because the vast majority of diseases that cause catastrophic loss of vision are the result of abnormal angiogenesis."
Activated Astrocytes Implicated In Damaged Retinas
Friedlander sums up his article's findings in three major points:
"Point 1 is that we could take mouse stem cells from an adult bone marrow, and identify a population of endothelial progenitor cells, which selectively targeted activated astrocytes. These," he explained, "are a type of glial cell or connective tissue cell of the central nervous system. Just as connective tissue cells are involved in wound healing and inflammation in the periphery of the body, astrocytes or other neuroglial cells are involved in wound healing, inflammation or angiogenesis in the CNS. And the retina, being a part of the CNS, is loaded with glial cells or astrocytes. So we injected adult bone marrow-derived stem cells into the eyes of mice, selectively targeted to activated astrocytes.
"Point 2," he continued: "If blood vessels are destined to degenerate because of some underlying disease or genetic disorder, as they do in the rd/rd mouse - one of the animal models of ocular disease we report in our paper - we inject these bone marrow-derived stem cells into their eyes at a point just at, or prior to, the degeneration of the blood vessels. These stem cells then targeted the sites where the vessels should form, and completely rescued and stabilized a degenerating vasculature.
"Point 3," Friedlander went on: "We then used a neonatal mouse model of angiogenesis - the blood vessel-proliferating process that feeds oxygen-rich blood to tumors and prenatal fetal development. In this case we studied normal developmental angiogenesis - but nonetheless hypoxia-determined angiogenesis - which is similar to the things we see happening in a variety of retinal vascular diseases. When we then isolated these stem cells, they incorporated themselves into the developing vessels. And if we previously transfected those stem cells with a potent anti-angiogenic enzyme called T2-tryptophranyl-tRNA, we could inhibit the formation of those blood vessels. This was basically cell-based anti-angiogenesis therapy. It's likely the first time someone has done it in the eye.
"Moreover," Friedlander declared, "this is a whole new way of treating retinal diseases. Currently there are anti-angiogenic clinical trials for such things as VEGF [blood-vessel growth factor] antagonists, anti-angiogenic steroids, integrin antagonists, PKC [protein kinase C] antagonists - all directed at killing new blood vessels in nascent human tumors. And what happens in the eye is misguided response to hypoxia that elicits formation of new blood vessels, which are very fragile, rupture, bleed, leak fluid and wreak havoc with the vision of these patients. Now we have several ongoing clinical trials in ophthalmology, testing the efficacy of anti-angiogenesis in treating neovascular eye diseases - like macular degeneration and diabetic retinopathy."
Mouse retinal stem cells come in two contradictory persuasions or lineages - positive or negative. "We took bone marrow from normal adult mice," Friedlander recounted, "and collected a whole bunch of cells. Then we sorted those cells and separated them out based upon their pluripotent ability to differentiate to hematopoietic [blood-forming] cells - that is, lymphocytes, platelets and erythrocytes. These we called lineage-plus - positive."
Negative Stem Cell Lines Yield Positive Results
"But what interested us more were hematopoietic lineage-minus cells - the ones that don't develop into lymphocytes or erythrocytes. This fraction presumably contains endothelial progenitor cells, that is, blood vessel stem cells. We knew this population had some cells that would differentiate into blood vessels, when put in the appropriate environment. And we don't see this in the lineage-positive cells."
The co-authors tested these lineage-plus and -minus stem cells in vivo in their rd/rd mice. "This mouse," Friedlander said, "incurs profound retinal degeneration by one month after birth. The photoreceptors in the back part of its retina, which does the seeing, are all gone. In addition, they lose the two deep layers of retinal blood vessels. That's the model we used for reconstructing their vasculature with these stem cells. We treated them a couple of days after birth, then compared one eye - into which we had injected an appropriate lineage-positive stem cell population - with the other eye, which received the inappropriate lineage-negative stem cell population. The positive looked like a typical rd/rd - no photoreceptors. If we injected their retinas with lineage-minus stem cells, they actually got some photoreceptors back, and much of their missing vessels."
Looking ahead, Friedlander remarked, "Now we'd like to know if those vessels we rescued in those rd/rd mice, and the photoreceptors that appeared to appear, were functional. Not the vessels, but: Can those mice see? That is, did we restore vision in mice treated with stem cells? If so, this can have applications in human patients with acquired retinal degenerations. Clinical trials will come," he concluded, "when we know the treatment is safe and efficacious - or perhaps we'll design drugs with which to treat them."