Nerve-Insulating Substance, Myelin, May Lead to Functional Improvements in Animals with Spinal Cord Injury
Combining partially differentiated stem cells with gene therapy can promote the growth of new “insulation” around nerve fibers in the damaged spinal cords of rats, a new study shows.
The treatment, which mimics the activity of two nerve growth factors, also improves the animals’ motor function and electrical conduction from the brain to the leg muscles. The finding may eventually lead to new ways of treating spinal cord injury in humans. The study was funded in part by the National Institute of Neurological Disorders and Stroke (NINDS).
The new study provides the best demonstration to date that producing a nerve-insulating substance called myelin can lead to functional improvements in animals with spinal cord injury. Previous studies have shown that the loss of myelin around nerve fibers contributes to the impaired function after a spinal cord injury. However, until now it has not been clear whether promoting new myelin growth in the spinal cord can reverse this damage, says Scott R. Whittemore, Ph.D., of the University of Louisville in Kentucky, who led the new study. “Many other investigators have suggested that remyelination is a possible approach to repair the spinal cord, but this is the first study to show unequivocally that it works,” says Dr. Whittemore. “It is a proof of principle.” Although the finding is promising, much work remains before such a technique could be used in humans. The study appears in the July 27, 2005, issue of the Journal of Neuroscience.
In the study, the researchers took cells called special cells called glial-restricted precursors from the spinal cords of embryonic rats. These precursor cells develop from stem cells and are specialized so that they can form only two kinds of cells: astrocytes, which help support neurons and influence their activity, and oligodendrocytes, which produce myelin. The scientists used a modified virus to insert genes for marker proteins that make the cells visible. Some cells also received a gene called D15A. This gene produces a protein with activity similar to growth factors called neurotrophin 3 (NT3) and brain-derived neurotrophic factor (BDNF). Both NT3 and BDNF help myelin-producing cells (oligodendrocytes) develop and survive.
Dr. Whittemore and his colleagues injected the treated precursor cells into the spinal cords of rats with a type of spinal injury called a contusion, which is caused by an impact to the spinal cord. Other groups of spinal cord-injured rats received just precursor cells, D15A gene therapy, or other treatments that were used for comparison. The rats were evaluated weekly for 6 weeks after the treatment using a behavioral test called the Basso-Beattie-Bresnahan scale, which measures characteristics such as weight support, joint movements, and coordination. The researchers also used an electrical current test in which they put a magnetic stimulator on the skull and measured whether the resulting electrical current was transmitted to a muscle in one of the hind legs.
Most of the rats treated with the combination of precursor cells and gene therapy improved significantly on both tests, the researchers found. The combination therapy led to an improvement in the rats’ ability to walk and about a 10 percent improvement on the electrical current test. Rats that received the other treatments did not improve significantly, and untreated rats did not have any electrical activity that passed through the damaged spinal cord. Studies of the damaged spinal cord tissue after the combined treatment showed that many of the transplanted cells survived and migrated within the cord and that about 30 percent of them developed into myelin-producing oligodendrocytes.
“The key word here is ‘combination.’ This is one of a series of new studies showing that a combination of therapies is needed for successful spinal repair, in this case, specialized cells and growth factors. The experiments also used a combination of outcomes — physiology, behavior, and anatomy — to point clearly at myelination as the cause for improved function,” says Naomi Kleitman, Ph.D., the NINDS program director for the grants that funded this work. “The study also is a good example of strong collaboration between two spinal cord injury research centers, one at the University of Louisville and the other at the University of Miami in Florida.”
The researchers are now investigating ways to improve this type of therapy with additional genetic modifications to the transplanted cells, and they plan to test similar techniques that start with undifferentiated embryonic stem (ES) cells instead of glial-restricted precursor cells. ES cells would be better for human studies than glial-restricted precursors because ES cells can be more readily obtained, Dr. Whittemore says.