The replacements

Strokes kill brain cells. Can Gary Steinberg cajole new cells to swap in?

Brian Cronin


Five years after his debilitating stroke, Bill Parent lay on an operating table at Stanford while doctors threaded a thin needle deep into his brain and flooded the stroke-damaged area with millions of immature neurons. Parent left the hospital the next day and soon began a rehabilitation program designed to spark the experimental new cells into action.  

And so Parent, 63, became one of the pioneers of neural transplantation, the process of rebuilding the brain with cells, such as embryonic stem cells or immature brain cells, that could become new neurons. It’s an emerging field that is thought to be the next frontier in the treatment of a wide range of neurologic problems, says Gary Steinberg, MD, PhD, the Lacroute-Hearst Professor and chair of neurosurgery.

“I’m convinced that this is the future not just for stroke but for chronic neurological disease,” such as spinal cord injuries, Parkinson’s disease and Alzheimer’s, Steinberg says. Even if the cells themselves can’t replace the neurons on their own, it’s conceivable they could serve as vehicles to deliver healing proteins, such as growth factors, to the site of a stroke or other brain injury, he says.

Today patients who suffer strokes have few options for treatment, all of which have to be administered within a narrow window of time. When a stroke hits, a clot or a ruptured blood vessel prevents blood from flowing to the brain, and oxygen-starved brain cells quickly start to die. There’s a stretch of up to eight hours in which doctors can intervene to prevent some of the damage, using the drug tPA (tissue plasminogen activator) or a newly approved retrieval device that can be fed through a catheter to trap and remove a clot. Mechanisms to cool the brain, such as cooling blankets or cooling catheters, also have been found to help forestall some damage, Steinberg notes.

The cells we’re talking about

Cancer-derived cells: Scientists derived this line of cells more than two decades ago from a patient’s metastatic lung tumor. Layton BioScience Inc. used a patented process to transform the tumor cells into non-cancerous, non-dividing immature neurons.

Human neural fetal stem cells: StemCells Inc., a company founded by Stanford’s Irving Weissman, MD, first reported isolating these cells from human fetal tissue in December 2000. The cells are in an early stage of development and have the ability to form various types of brain cells. The company now grows the cells in bulk and distributes them to researchers studying brain injuries.

Human embryonic stem cells: These cells are derived from the inner cell mass of a blastocyst, or very early stage embryo, and are able to generate all differentiated cell types in the body.

But once the critical window has passed, the damage is done, and the brain, unlike other organs such as the skin or the liver, has limited ability to repair itself. Of the roughly 750,000 Americans who suffer stroke each year, about a third become severely and permanently debilitated. Stroke is now the leading cause of disability in this country. In theory, these incapacitated patients could benefit greatly from neural transplantation, which could be done years after the stroke to help them regain some of their lost faculties, such as the ability to move, walk, see, speak, swallow or think clearly.

“That is what people are dreaming of — restoring brain function with new cells,” says neurosurgeon Raphael Guzman, MD, a postdoctoral scholar in Steinberg’s lab.The hope is that we could actually find something for these patients who have severe neurologic deficits and no treatment option.”

Since 2000, Steinberg’s team has been doing ground-breaking experiments, testing ways to transplant potentially healing cells both in humans and in rodents. His lab is among a handful in the world engaged in this work. Scientists elsewhere have done neural cell transplants in patients with Parkinson’s disease, with mixed results. Steinberg nonetheless remains optimistic, saying it’s a question of determining which cell types are most likely to work and the best way to manipulate them for treating both Parkinson’s and stroke.

Recently, he and colleagues at the University of Pittsburgh published results of the first clinical trial involving transplants of laboratory-grown cells into the brains of stroke patients. The trial involved 18 men and women who had suffered strokes in the basal ganglia, an egg-shaped structure deep in the brain that helps control movement and is a common site for strokes. All of the patients had been partially paralyzed as a result of strokes they’d suffered one to six years before the trial. Four patients served as controls, participating in rehab alone; the researchers covered the patients’ heads with caps so they couldn’t tell the surgery patients from the controls.

Bill Parent, a Fresno, Calif., resident, was only too happy to volunteer for the trial. After 30 years of working as a physician’s assistant, he had decided in the mid-1990s to go to medical school and become a full-fledged doctor. He was midway through training in the Caribbean when one day in 1996, he was heading home from class with some friends and suddenly found himself unable to speak. His medical-student companions immediately realized he was in the throes of a stroke; Parent could hear them talking about what was happening, but he couldn’t respond.

The event left him partially paralyzed on his right side. Initially he couldn’t walk or speak at all. Through rehabilitation, he began to use his immobilized limbs again; he learned to walk, though unsteadily, and to raise and lower his damaged right arm, and he regained about 90 percent of his speech. But he wanted to do more.

Angela Wyant
  Gary Steinberg, MD, PhD, believes cell transplants are the wave of the future for curing brain diseases.

He learned about Steinberg’s trial, got on the waiting list and finally came to Stanford for the experimental treatment in May 2001. Steinberg was motivated to try the new approach in part because of his frustration in seeing so many of his stroke patients suffer under the strain of severe disability, he says. He was encouraged by previous experiments done elsewhere, in which rats with strokes were able to fully recover their ability to perform on basic motor tests, such as walking on a beam, within a month after receiving the experimental cells, which formed new neurons and new connections.

The cells used in the trial were developed by Layton BioScience Inc. in Atherton, Calif. The company’s process treats human tumor cells with retinoic acid, transforming them into non-dividing, immature neurons. The cells were purified for use in humans and have never been shown to cause tumors in humans or animals, Steinberg says. The trial was sponsored by the company.

For the test, Parent was fitted in a standard stereotactic surgery frame used in neurosurgeries to help guide doctors to specific targets in the brain. Steinberg and his colleagues drilled a small hole in Parent’s skull and then threaded a spaghetti-sized needle down into his basal ganglia. They then pumped 5 million cells, suspended in a cloudy liquid to keep them alive, into 25 different spots in the damaged structure, hoping some would take up residence as new, functioning neurons. Half of the 14 patients got this treatment, while the other half were infused with 10 million cells.

The entire procedure took less than three hours. Parent was awake the whole time and sensed nothing, he says. After he left the hospital, he enrolled in a rehab program designed to exercise the treated part of the brain. Rehabilitation is expected to be a key part of the process, as cells that don’t get a good workout aren’t likely to form new connections with their surrounding counterparts, Guzman notes.

“For the brain to build up synapses and eventually circuitry, it needs impulse, and the impulse is helped with rehab,” Guzman says. 

Parent’s results were dramatic. Within two months, he was able to move his thumb for the first time since his stroke and to hold 10 pounds or more of groceries with his once-paralyzed right arm. “I’ve got good function down to my elbow and limited function after that. And that was after the surgery. I didn’t have that before. Now I feel I can do almost anything,” says Parent. He still lacks the fine motor skill to suture a patient, walks with a limp and has a barely perceptible speech impediment. But he’s doing so well he’s back full-time at his job treating patients at a clinic for Native Americans.

The trial wasn’t designed to show efficacy, but the researchers did find that patients who received the cells and completed the rehab program had modest improvement in their ability to make simple movements with their hands and fingers, compared with the four control patients who did rehab alone, Steinberg says. The patients who received the cells also showed some significant improvement on memory tests compared with controls and had better day-to-day functioning, though the numbers on the functional tests were not statistically significant, he adds.

“Some of the patients showed meaningful improvement, but there weren’t enough patients to prove it was the cells and not the physical therapy,” Steinberg says. The group of patients receiving 5 million cells scored slightly better than controls on some tests, but again there were too few patients to draw any conclusions.

Angela Wyant
  Bill Parent’s right arm regained strength lost because of his stroke. Did nerve cells injected into his brain do the trick?

He was encouraged by the results in that they showed this type of therapy is safe. Stroke patients could be vulnerable to having a needle stuck in their brain, so the fact that patients suffered no harm was reassuring, he says. One patient died of unrelated causes 27 months after the procedure, and an autopsy showed that the experimental cells survived and remained viable in the brain, Steinberg says. The results were published in July in the Journal of Neurosurgery. The researchers are planning future clinical trials of transplants for stroke patients but are determining what kinds of cells would be best to use, he says.

In the meantime, Steinberg says it’s important to continue to do more work in the lab, as what researchers learn from testing in laboratory animals will help them design future trials in people. The lab work could help answer key questions, such as how the cells behave inside a stroke-damaged brain, as well as when and where best to target the cells.

To get some of those answers, Tonya Bliss, PhD, a senior research associate in Steinberg’s lab, has done experiments in rats using the same cells, as well as cells derived from human fetal brain tissue. She used a fine needle to infuse the cells into the cortex of rats that had lost a major part of the brain to stroke. She later sacrificed the animals and looked at the results. With the cancer-derived cells — the same ones used in the clinical trial — she was surprised to find that many survived around the area of the stroke, where massive inflammation and cell death typically create a very hostile environment.

“There was an amazing survival of these cells and they were very close to the lesion,” Bliss says. Under the microscope, the researchers observed the round cell bodies with thin, hair-like structures extending from them — their axons and dendrites reaching out into the surrounding brain.

With the human neural fetal cells, supplied by StemCells Inc. in Palo Alto, Bliss and her colleagues observed equally interesting activity. The cells, which have the potential to differentiate into various brain cell types, moved en masse toward the site of the stroke damage and survived there for months. In rats without strokes, the cells hardly moved at all.

“We saw this massive migration,” Bliss says. “It’s clear they’re receiving some signals that draw them to the stroke, but it’s not clear what that is, or what they are doing there. If we could make this more efficient, get more cells moving to the site of damage, it could be a potentially better therapy.” The results of that study were published in August 2004 in the Proceedings of the National Academy of Sciences.

Neither study proved that the rats had better function with the cells, as the animals didn’t perform any better on movement tests. The researchers speculate that the stroke damage was just too great for the cells to make a difference.

“It might be a stretch to ask the cells to replace the dead neurons or really restore the function of a dead spot that large,” says Dave Schaal, PhD, a senior research scholar in neurosurgery who works with Steinberg. “But if they move toward the injury, they may be able to deposit protective factors to limit the damage. So they might be useful even if they don’t reconstruct the dead tissue.”

Bliss is now trying different approaches — testing whether depositing the cells in a different part of the brain, such as the striatum, would work better or whether transplanting cells earlier would prevent more cells from dying. “There’s potential, but a long way to go before we realize this potential,” Bliss says.

One of the difficulties now in using the therapy in people is that there’s no good way to track the cells in a living person’s brain and prove they even survive. Guzman, the neuroscience fellow, is working on a technique using micron-sized particles of iron oxide, a harmless substance that’s FDA-approved for human use, to tag transplanted neural stem cells in rats and watch the movement of the cells on an MRI scan. This technique could be important to future clinical trials, he says.

“We’ll actually have a way to follow these cells as they move around the brain,” Guzman notes. “Combining this with positron emission tomography, which gives us information on the metabolic characteristics of the transplant, we’ll know where the cells go and where they’re active.”

Neuroscientist Benjamin Hoehn, PhD, another member of the group, is also looking at ways to dampen the inflammation that accompanies stroke, as inflammation may kill some transplanted cells and blunt the body’s natural healing abilities. When a stroke occurs in rats, there’s a natural movement of beneficial cells from a central part of the brain called the subventricular zone to the area that’s been injured, he notes. Hoehn has found that he can keep more of these cells alive and for a longer time simply by giving the injured rats chow that’s been mixed with the common anti-inflammatory drug indomethacin.

Ultimately, Hoehn and his colleagues envision a day when doctors could inject stem cells into the arms of patients in the throes of a stroke and watch the labeled cells via MRI as they make their way to the brain to help limit the damage.

“If this works, we can engineer the cells to deliver all kinds of proteins — growth factors, antioxidants — all these things that are known to help protect against stroke,” Hoehn says. “It could help save some cells that are there. That would be beautiful.”

Steinberg, who is about to begin animal studies with human embryonic stem cells, says he believes stem cell therapy has myriad potential applications — if not as a stand-alone therapy, as a tool to deliver valuable proteins to the brain. For instance, researchers in Boston recently reported using neural stem cells as a delivery system for gene therapy to treat brain tumors. Stem cells also could be used to deliver antibodies for immune therapy or growth factors that would help neurons survive and develop in the face of stroke, Steinberg says.

Bill Parent, for his part, says he’s ready for the next step. When the next clinical trial begins, he’ll be the first in line.

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