Vascular Progenitors Made from Induced Pluripotent Stem Cells Repair Blood Vessels in the Eye Regardless of the Site of Injection

Johns Hopkins University medical researchers have reported the derivation of human induced-pluripotent stem cells (iPSCs) that can repair damaged retinal vascular tissue in mice. These stem cells, which were derived from human umbilical cord-blood cells and reprogrammed into an embryonic-like state, were derived without the conventional use of viruses, which can damage genes and initiate cancers. This safer method of growing the cells has drawn increased support among scientists, they say, and paves the way for a stem cell bank of cord-blood derived iPSCs to advance regenerative medical research.

In a report published Jan. 20 in the journal Circulation, Johns Hopkins University stem cell biologist Elias Zambidis and his colleagues described laboratory experiments with these non-viral, human retinal iPSCs, that were created generated using the virus-free method Zambidis first reported in 2011.

“We began with stem cells taken from cord-blood, which have fewer acquired mutations and little, if any, epigenetic memory, which cells accumulate as time goes on,” says Zambidis, associate professor of oncology and pediatrics at the Johns Hopkins Institute for Cell Engineering and the Kimmel Cancer Center. The scientists converted these cells to a status last experienced when they were part of six-day-old embryos.

Instead of using viruses to deliver a gene package to the cells to turn on processes that convert the cells back to stem cell states, Zambidis and his team used plasmids, which are rings of DNA that replicate briefly inside cells and then are degraded and disappear.

Next, the scientists identified and isolated high-quality, multipotent, vascular stem cells that resulted from the differentiation of these iPSC that can differentiate into the types of blood vessel-rich tissues that can repair retinas and other human tissues as well. They identified these cells by looking for cell surface proteins called CD31 and CD146. Zambidis says that they were able to create twice as many well-functioning vascular stem cells as compared with iPSCs made with other methods, and, “more importantly these cells engrafted and integrated into functioning blood vessels in damaged mouse retina.”

Working with Gerard Lutty, Ph.D., and his team at Johns Hopkins’ Wilmer Eye Institute, Zambidis’ team injected these newly iPSC-derived vascular progenitors into mice with damaged retinas (the light-sensitive part of the eyeball). The cells were injected into the eye, the sinus cavity near the eye or into a tail vein. When Zamdibis and his colleagues took images of the mouse retinas, they found that the iPSC-derived vascular progenitors, regardless of injection location, engrafted and repaired blood vessel structures in the retina.

“The blood vessels enlarged like a balloon in each of the locations where the iPSCs engrafted,” says Zambidis. Their vascular progenitors made from cord blood-derived iPSCs compared very well with the ability of vascular progenitors derived from fibroblast-derived iPSCs to repair retinal damage.

Zambidis says that he has plans to conduct additional experiments in diabetic rats, whose conditions more closely resemble human vascular damage to the retina than the mouse model used for the current study, he says.

With mounting requests from other laboratories, Zambidis says he frequently shares his cord blood-derived iPSC with other scientists. “The popular belief that iPSCs therapies need to be specific to individual patients may not be the case,” says Zambidis. He points to recent success of partially matched bone marrow transplants in humans, shown to be as effective as fully matched transplants.

“Support is growing for building a large bank of iPSCs that scientists around the world can access,” says Zambidis, although large resources and intense quality-control would be needed for such a feat. However, Japanese scientists led by stem-cell pioneer Shinya Yamanaka are doing exactly that, he says, creating a bank of stem cells derived from cord-blood samples from Japanese blood banks.

Synthetic Matrices that Induce Stem Cell-Mediated Bone Formation

Biomimetic matrices resemble living structures even though they are made from synthetic materials. Researchers in the laboratory of Shyni Varghese at the UC San Diego Jacobs School of Engineering have used calcium phosphate to direct mesenchymal stem cells to form bone. In doing so, Varghese and his colleagues have identified a surprising pathway from biomaterials to bone.

Varghese and his colleagues think that their work may point out new targets for treating bone defects, such as major fractures, and bone metabolic disorders such as osteoporosis.

The first goal of this research was to use materials to build something that looked like bone. This way, stem cells harvested from bone marrow (the squishy stuff inside our bones) could sense the presence of bone and differentiate into osteoblasts, the cells in our bodies that build bone.

“We knew for years that calcium phosphate-based materials promote osteogenic differentiation of stem cells, but none of use knew why.” said Varghese. “As engineers, we want to build something that is reproducible and consistent, so we need to know how building factors contribute to this end.”

Varghese and co-workers discovered that phosphate ions dissolved from calcium phosphate-based materials and these stray phosphate ions are taken up by the stem cells and used for the production of adenosine triphosphate or ATP. ATP is the energy currency of the cell, and it is the way cells store energy in a form that is readily usable for powering other reactions.

In stem cells, the generation of ATP eventually increases the intracellular concentration of the ATP breakdown product adenosine, and adenosine signals to stem cells to differentiate into osteoblasts and make bone.

Varghese said that she was surprised that “the biomaterials were connected to metabolic pathways. And we didn’t know how these metabolic pathways could influence stem cells,” and their commitment to bone formation.

These results also explain another clinical observation. Plastic surgeons have been using fat-based stem cells for eyelid lifts, breast augmentation, and other types of reconstructive surgeries. In once case, a plastic surgeon injected a dermal filler that contained calcium hydroxyapatite with the fat-based stem cells into a woman’s eyelid to provide an eye lift. However, the stem cells formed bone, and the poor lady’s lid painfully clicked every time she blinked and she had to have surgery to remove the ectopic bone. These results from Varghese’s laboratory explains why these fat-based stem cells formed bone in this case, and great care should be taken to never use such fillers in fat-based transplantation procedures.