For the first time, stem cell researchers have succeeded in getting transplanted nerve cells to establish connections with other neurons to improve movement in paralyzed rats. The success is thanks to neurons cultivated from embryonic stem cells and the right "cocktail" of ingredients to guide cell development. Earlier experiments have improved movement in paralyzed rodents, but the gains have been from indirect contributions by the cells and not from the generation of new neurons.
The team, led by neurologist Douglas Kerr, cultivated rat embyronic stem cells in a dish until they developed into neural precursors. The researchers then injected 60,000 of the cells into the spinal columns of rats that had been paralyzed by the injection of a virus. The rats were then divided into eight treatment groups of 15 animals each to test slightly differing combinations of chemicals that would allow the precursor cells to take on their desired roles.
Co-author Jeffrey Rothstein explains that for a precursor cell to become functional, it first has to differentiate into a motor neuron. Then it has to develop an axon that can penetrate "this huge wall of white matter," the outside covering of the spinal column. Finally, that axon has to grow a long way to make connections with neurons in leg muscle.
Comparing slightly different cocktail combinations, the researchers found that three chemicals were needed to coax the neurons to make the necessary transitions. Daily injections of two of them, dibutryl cyclic AMP and Rolipram, were given to help neurons overcome factors that inhibit their ability to break through the spinal cord. The third, a powerful growth factor called GDNF, was later injected into sciatic nerves to lure the axons to reach out to peripheral muscles.
The cells that make the grade are still rare: Only about 20% of those injected into the animals developed into motor neurons. Of those, less than 1% made it out of the spinal column. Only a few hundred of the original 60,000 eventually made it all the way to form new neuromuscular junctions, says Rothstein. But that was enough to bring about partial but significant recovery of function in 11 of 15 rats in the group with the winning cocktail, the scientists report in a paper published online in Neurology on 26 June. The scientists now plan to try the experiment using pigs, injecting them with neural precursors developed from human embyronic stem cells.
This is a "very exciting paper," says University of Wisconsin stem cell researcher Clive Svendsen. "It's really stimulated everybody in the field." He says the success of the experiment underscores the importance of having human embryonic stem cells available for research. "Of all the stem cells it's really only the embryonic stem cells that can make motor neurons."