rolipram under the skin before and after the transplants. Rolipram, a drug approved to treat depression, helps to counteract axon-inhibiting signals from myelin. Some animals also received transplants of neural stem cells that secreted the nerve growth factor GDNF into the sciatic nerve (the sciatic nerve extends from the spine down the back of the hind leg). GDNF causes axons to grow toward it.

Three months after the transplants, the investigators examined the rats for signs that the stem cell-derived neurons had survived and integrated with the nervous system. The rats that had received the full cocktail of treatments — transplanted motor neurons, rolipram, dbcAMP, and GDNF-secreting neural stem cells in the sciatic nerve — had several hundred transplant-derived axons extending into the peripheral nervous system, more than in any other group. The axons in these animals reached all the way to the gastrocnemius muscle in the lower leg and formed functional connections, called synapses, with the muscle. The rats showed an increase in the number of functioning motor neurons and an approximately 50 percent improvement in hind limb grip strength by 4 months after transplantation. In contrast, none of the rats given other combinations of treatments recovered lost function.

"We found that we needed a combination of all of the treatments in order to restore function," Dr. Kerr says.

Follow-up experiments with GDNF treatment on only one side of the body showed that, by 6 months after treatment, 75 percent of rats given the full combination of treatments regained the ability to bear weight on the GDNF-treated limbs and to take steps and push away with the foot on that side of the body.

"This research represents significant progress," says David Owens, Ph.D., the NINDS program director for the grant that funded the work. "It is a convergence of embryonic stem cell research with other areas of research that we've funded, including work that uses combination therapies such as rolipram and dbcAMP, growth factors, and cells to facilitate the repair of the injured spinal cord.”

Previous studies have shown that stem cells can halt spinal motor neuron degeneration and restore function in animals with spinal cord injury or ALS. However, this study is the first to show that transplanted neurons can form functional connections with the adult mammalian nervous system, the researchers say. They used both electrophysiological and behavioral studies to verify that the recovery was due to connections between the peripheral nervous system and the transplanted neurons.

"We’ve previously shown that stem cells can protect at-risk neurons, but in ongoing neurodegenerative diseases, there is a very small window of time to do so. After that, there is nothing left to protect," says Dr. Kerr. "To overcome the loss of function, we need to actually replace lost neurons."

While these results are promising, much work remains before a similar strategy could be tried in humans, Dr. Kerr says. The therapy must first be tested in larger animals to determine if the nerves can reconnect over longer distances and to make sure the treatments are safe. There currently is no large-animal model for motor neuron degeneration, so Dr. Kerr's group is working to develop a pig model. Researchers also need to test human embryonic stem cells to learn if they will work in the same way as the mouse cells. It has only recently become possible to grow motor neurons from human embryonic stem cells, Dr. Kerr adds. However, if the future studies go well, this type of therapy might eventually be useful for spinal muscular atrophy, ALS, and other motor neuron diseases.

NINDS is a component of the National Institutes of Health (NIH) within the Department of Health and Human Services and is the nation’s primary supporter of biomedical research on the brain and nervous system. The NINDS mission is to reduce the burden of neurological disease. Go to http://www.ninds.nih.gov/ for more information.

The National Institutes of Health (NIH) — The Nation's Medical Research Agency — includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. It is the primary federal agency for conducting and supporting basic, clinical and translational medical research, and it investigates the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit www.nih.gov.