As the United States’ and the world’s population ages, the number of people enduring a silent procession of cell death mounts. For millions, those unchecked deaths mean the gradual fading of the world around them.
These are individuals suffering from age-related macular degeneration (AMD). The disease disrupts the functioning of the retina, the area at the back of the eye that is responsible for receiving light through photoreceptor cells, which convert the light to electrical impulses and pass the impulses along a cell network to the optic nerve and on to the brain.
The disruption occurs in two ways for the estimated 11 million people in the United States who have AMD. About 10 percent have the “wet” form, caused by abnormal growth of new blood vessels in the retina and the retinal pigment epithelium (RPE), a covering of cells that nourishes and cleans the retina. The vast majority suffer from geographic atrophy, or “dry” age-related macular degeneration. In this form, cells die in the RPE, depriving the photoreceptors of the support they need to live. The cell atrophy and death diminishes vision and eventually leads to irreversible blindness.
Injections can help to slow the loss of the wet variety of AMD. For now there is no treatment for the dry form, but researchers and clinicians at the University of Colorado School of Medicine’s Department of Ophthalmology are working to change that. It’s a massive scientific, philanthropic and administrative undertaking that springs from humble Petri dishes.
Eyes on the prize
The vessels hold retinal organoids: mini-retinas created from human induced pluripotent stem cells (iPSCs). These marvels of medical engineering begin as adult blood or skin cells, then revert, through laboratory reprogramming, to their embryonic beginnings. With that, they can be reprogrammed again and guided in labs on the UCHealth Anschutz Medical Campus to begin new cellular lives as retinas with active photoreceptor cells. They provide a window to the workings of the retina – and the hope one day of a retinal transplant for those with dry age-related macular degeneration.
The “3D retinal organoids,” as they are called, are “fully organized tissue” with photoreceptor cells “capable of sensing light just the way the photoreceptor cells in the human eye do,” said Dr. Valeria Canto-Soler, director of the 3DRet Laboratory. Canto-Soler’s lab is an integral part of the Ocular Stem Cell and Regeneration Program, aka CellSight, a team of researchers targeting eye disease and real-world methods for attacking it.
Canto-Soler’s understanding of retinal organoids is unique: She led a team at the Johns Hopkins Wilmer Eye Institute that in 2014 was the first to produce mini-retinas using iPSCs that develop fully over five to six months, just as they do in humans. That fact underscores her ultimate research motivation.
“In my mind, it was always the goal to develop a system that could be transferred to the clinic,” Canto-Soler said. “It had to be straightforward and related to human diseases and potential therapies for human patients.”
Stem cells a promising path to retinal transplant
The most tantalizing therapeutic prospect is using the lab-grown cells as a retinal patch to replace cells lost in patients with dry age-related macular degeneration, said Dr. Naresh Mandava, chair of the Department of Ophthalmology at CU.
“For patients with the atrophic form of AMD, where you just lose cells, we have no treatment,” Mandava said. “All of the medications that have been tested so far have failed. The promise of stem cell therapy is to rejuvenate the retina so that if functions as it did years before the cells began to degenerate.”
The approach of Canto-Soler and her team to retinal transplant has several strengths, Mandava said. Because a transplant would use a patient’s own cells, providers might not need immunosuppressant drugs to fight rejection, although Canto-Soler cautions that that must be validated.
“Even though the cells are your own, you are intervening and disrupting the environment,” she said. “That could trigger an immune response.”
The therapy also avoids the ethical dilemmas associated with using embryonic stem cells, Mandava said. Most importantly, the work at CU focuses on transplanting both photoreceptor and RPE cells. An upcoming trial will use RPE cells grown from reprogrammed stem cells for transplant, but will not use reprogrammed photoreceptor cells.
A successful retinal transplant hinges on transplanting both layers of cells, Mandava maintained.
“I can’t think of any other way we are going to be able to take someone who has lost those cells and restore their vision,” he said.
Stem cell research for age-related macular degeneration
The promise of using stem cells to combat age-related macular degeneration – and the demand for it – was sufficient to launch an ambitious fundraising effort in 2014 at CU that resulted in successfully recruiting Canto-Soler and creating CellSight. The initiative raised $10 million, half from a Gates Frontiers Fund challenge grant and the remainder from donors, notably the Solich Fund, the Lyda Hill Foundation and Sue Anschutz-Rodgers, for whom the UCHealth Eye Center is now named.
Mandava noted that the effort succeeded in large part on an existing infrastructure necessary to support successful stem cell research for age-related macular degeneration, notably the Gates Center for Regenerative Medicine, which was established in 2006 by the children of Charles Gates, who was stricken by AMD. If all goes as planned, the Gates Center will be responsible for producing clinical-grade retinal cells under rigorous procedures that ensure patient safety.
The Gates Center is a crucial piece of the stem cell research for age-related macular degeneration program, but it relies on the underpinnings of basic science research and discovery and clinical care – and collaboration between the two, said Dr. Mark Petrash, professor and vice chair of research for the Department of Ophthalmology. He noted that over the past decade, the number of basic research scientists in the department rose from one to 11. By this summer, researchers will be working on five National Institutes of Health grants totaling $7.5 million, Petrash said. There was none a decade ago, he added.
“We have created a culture where basic scientists interact with clinicians,” he said. “Clinicians are now doing basic research studies and basic scientists are working with clinicians. That’s where the magic happens.”
Even with all these pieces in place and a department dotted with researchers and clinicians familiar with bringing new therapies to market, the road to a retinal transplant on humans is a long one. Canto-Soler said the team has a prototype of a retinal patch for transplant that is ready to be tested in mini pigs within the next few months. But even if those early efforts are successful, there are many questions that clinical trials in humans will have to answer.
Manufacturing mass quantities of retinal cells at the Gates Center will be a technical challenge, but “feasible,” Canto-Soler said. Mandava said Dr. Marc Mathias is working on surgical tools that will be used to deliver a roughly 3-millimeter-wide patch of cells and implant them in the back of the retina. A prototype should be ready in the next three to six months, Mandava said.
A tougher question to answer: What happens when the cells go into the eye?
“The biggest challenge will be getting transplanted cells to consistently integrate into the retina of a patient, become functional and restore [vision],” Canto-Soler said. It’s not enough that the cells simply survive; they will have to establish all the connections with other cells and nerves that nature supplied before disease struck.
Without those connections, the transplanted cells probably will die, Canto-Soler said. “But even if they don’t die and they sit there for a while, they are not going to restore vision anyway because they are not talking to the rest of cells,” she added.
The cell reconnection question will require much investigation, Petrash acknowledged. But even if retinal cell transplant works in humans, another challenge awaits: how to scale cell-manufacturing technology to meet the needs of millions of people around the world with the dry form of AMD while accounting for their individual genetic and biologic differences.
“These cells aren’t widgets,” Petrash said. “If we’re going to take cells from you, you’re going to be bringing across your life history in those cells. They are going to behave a little differently than your wife’s or your children’s cells. How can we drill down through all that variability and find the common elements in these cells so we can manufacture tissue for a mass market?”
Some of the answers may come from work pursued by Canto-Soler’s colleagues in the CellSight program. The Ocular Development and Translational Technologies Laboratory team, headed by Dr. Natalia Vergara, is using mini-retinas as a platform to investigate the pathology that produces AMD and a springboard to developing drugs that could slow the progression of the disease or, as Mandava pointed out, keep transplanted cells from degenerating.
“We may also find that certain drugs work better on certain cells or that some drugs work better in certain people,” Mandava said.
The Laboratory of Developmental Genetics, led by Dr. Joseph Brzezinski, meanwhile, is delving into the inner workings of photoreceptor cells. These cells are composed of rods, which handle vision in low light, and cones, which respond to higher light and make color vision possible. It turns out that rods vastly outnumber cones, but cones are also the cells lost primarily in age-related macular degeneration, Petrash said. Brezezinski’s team is working to understand how genes control cell development in the retina. With that understanding, researchers could grow cone-rich mini-retinas that could, in turn, serve as tailor-made retinal patches that meet the needs of individual patients, Petrash said.
Returning to the question of determining whether or not a retinal patch has “plugged in” to the rest of the cellular environment, the Laboratory of Advanced Ophthalmic Imaging is developing methods of peering into the inner workings of the eye. The work, initiated by Dr. Omid Masihzadeh, who recently moved his imaging expertise to the computing industry, aims to provide noninvasive technology to help clinicians monitor the cellular integration after a transplant in real time.
“All of these teams’ work integrates with each other,” Petrash said. “Together they can accomplish much more than any one of them could accomplish by themselves.”
Keeping it real
Amidst all of the promise and hope their work is generating, Petrash, Canto-Soler and Mandava each offered cautionary notes, particularly regarding stem cell therapy, which is widely and very often misleadingly touted as a miracle cure for a multitude of ailments. For example, Mandava said, ubiquitous ads that trumpet “stem cell therapies” for bone and joint injury and disease may have anti-inflammatory properties, but there is no evidence that they regenerate tissue.
The fate of retinal transplants will be determined by thousands of hours of painstaking work that has been and will be done, said Canto-Soler.
“We are not curing blindness,” she said, “and we are years away from knowing if that is even possible.”
Still, Canto-Soler expresses optimism for the future and is satisfied with the pace of progress in the year and a half since she arrived at CU.
“We are moving forward steadily,” she said. “We have assembled a team that brings expertise in all areas of retinal transplant.”