With a genetic tweak, scientists have created an unlimited supply of a type of nerve cells found in the spinal cord and have been able to use the cells to partially repair damaged spinal cords in lab animals.

While application of the discovery to humans is still years away, being able to generate such a limitless supply of the specialized nerve cells has long been a goal toward treating many neurological diseases.

"This work is the culmination of six years of work, and it will be many more years before an approach like this can be tried in human patients. But the promise is extraordinary," said Dr. Steven Goldman, a professor of neurology at the University of Rochester Medical Center in New York.

His team reports online today and in the March issue of Nature Biotechnology that it was able to create the unique cells by introducing a gene called telomerase, which allows stem cells to live indefinitely, into more specialized "progenitor" cells.

In normal development, progenitor cells turn into specific types of spinal cells. But because they lack the ability to continuously divide, they can only form a few generations of the nerve cells. Due to this limitation, scientists have been unable to produce enough of the cells to have an impact on spinal cords damaged by injury or disease.

But with the addition of telomerase just at the point where the progenitors have committed to making a specific type of cell, the spinal progenitor cells were able to churn out new neurons indefinitely.

"The progenitor cells are immortalized at a stage when they only give rise to the type of neuron we want, thus becoming an ongoing source," said Goldman, whose work was supported by Project ALS and the Christopher Reeve Paralysis Foundation.

Goldman's team propagated the cells for more than two years, the longest anyone has ever maintained a line of progenitor cells.

Using some of those neurons, a group of Goldman's colleagues led by Dr. Maiken Nedergaard, a professor of neurosurgery, injected the cells into rats in which small sections of spinal cords had been damaged. The cells replaced the damaged part of the spinal cord with new nerve cells. But after about a month, the cells in the animals stopped proliferating, as neurons in the spinal cord normally do.

The researchers also were pleased to find that the telomerase-enhanced cells did not show any inclination to grow tumors.

Spinal cords are made up of several types of neurons, so the group is now creating and working with other cells that would create the types of neurons needed to completely repair spinal nerves.

Treating some neurological conditions may actually require a steady supply of just one specific type of nerve cell. A patient with Parkinson's disease may only have to replace neurons that manufacture dopamine, for instance, or a patient with multiple sclerosis may only require restoration of cells that produce myelin.

More information is available on the Web site: www.nature.com.