Editor's Note: This is part one of a two-part series on stroke research. Part two will run in Wednesday's issue.
Teasing apart the intricate wiring of brain circuitry takes a lot of time and patience on the part of those involved, but knowing the details of the wiring diagram is not necessary for manipulating it to achieve clinical benefits.
The effectiveness of cell transplants in motor diseases like Parkinson's, Huntington's and stroke often is based more on the blanket secretion of neurotransmitters and trophic factors in the host brain, rather than their functional integration into neural microcircuits. Now, in the October 2004 issue of Stroke, researchers from the Medical College of Georgia and the University of South Florida report that intravenous injection of human embryonic cord blood cells can reduce stroke damage in an animal model without those cells ever being grafted into the brain at all.
Previous preclinical and clinical research has shown that transplanting cells directly into the brain can improve motor function after stroke, Huntington's and Parkinson's disease. Usually, such improvements are correlated with the long-term survival of the transplanted neurons, but in rare cases in which improved motor functions have been seen after transplantation without demonstrable transplant grafts, such improvements have been dismissed as placebo effects.
In a paper titled "CNS Entry Of Peripherally Injected Umbilical Cord Blood Cells Is Not Required For Neuroprotection In Stroke," Cesario Borlongan, associate professor of neurology at the Medical College of Georgia and lead author of the study, and his colleagues present evidence suggesting that improvements in motor functions are indeed possible without viable cell grafts in the brain.
A Mixed Bag Of Mannitol, Cord Blood Cells
Borlongan told BioWorld Today that the cells used in those studies were not a pure population of stem cells, but "the mononuclear fraction of human umbilical cord blood," which contains both stem cells and other cord blood cell types.
The scientists found that peripherally injected cord blood cells can effectively reduce both the anatomical area and the behavioral deficits of induced stroke, but only when co-administered with a second agent: mannitol, a carbohydrate which can be found in the manna plant, but for public consumption it is produced by a process known as catalytic hydrogenation of corn.
Mannitol is "already an accepted clinical product," Borlongan said. Mannitol functions by relaxing the blood-brain barrier. It is used to reduce brain swelling and can help deliver chemotherapy agents to the brain.
The researchers induced stroke in rats by blocking the middle cerebral artery, treated them with various substances, and later tested the animals' motor functions and learning and memory skills. In supporting anatomical studies, the size of the infarcted brain area also was determined. Intravenous injection of labeled cord blood cells and mannitol together during the stroke induction reduced the area of brain damage by as much as 40 percent and improved both motor and memory capabilities, compared to animals receiving either no treatment or either substance alone.
Hey, Where Did That Cell Go?
However, when the scientists labeled the cord blood cells and investigated their fate after injection, they found that the cells did not enter the brain. Borlongan said, "the initial results were quite discouraging because we couldn't detect a single [cord blood] cell in the brain," using either the fluorescent label or antibodies specific to human cells, though they did find cord blood cells grafted onto some peripheral organs.
Given the strong behavioral results in the rats, the scientists decided to test whether the cells might secrete growth factors that could explain their findings. They found elevated levels of several growth factors one day after cord blood treatment; for one growth factor, glial-derived neurotrophic factor (or GDNF) those elevated levels persisted over several days. When the cord blood cells were treated with antibodies that prevented them from secreting growth factors, both the elevation of growth factors and the behavioral and anatomical benefits of cord blood cell treatment were blocked.
Borlongan draws the overall conclusion that "in the acute setting, you may not need cells to survive in the brain, as long as the therapeutic molecules they secrete are available."
The researchers hope that intravenous delivery "will circumvent the traumatic injury associated with direct transplantation," Borlongan said. That also would mean that the therapy can be initiated very soon after stroke. Cord blood cells given peripherally longer after a stroke have been shown to reduce behavioral deficits at later times. The addition of mannitol allows the quick initiation of effective cord blood therapy.
"With a stroke patient, it would be nearly impossible to get them into the stereotactic apparatus [the positioning device allowing accurate intracranial delivery of cells during neurosurgery] right afterwards. We really want a strategy that can be applied in the clinic, and there's much less trauma associated with peripheral IV injection of the cells," Borlongan said.
Asked whether he believes those faster improvements will lead to better long-term outcomes as well, Borlongan replied that investigating such long-term outcomes was "the next step" for the research team. They also are conducting analogous experiments in a nonhuman primate animal model.