Using Embryonic Stem Cells to Awaken Latent Motor Nerve Repair
Jul 1, 2006 - 5:22:37 PM
In a dramatic display of stem cells potential for healing, a team of Johns Hopkins scientists reports that theyve engineered new, completed, fully-working motor neuron circuits -- neurons stretching from spinal cord to target muscles -- in paralyzed adult animals.
The research, in which mouse embryonic stem (ES) cells were injected into rats whose virus-damaged spinal cords model nerve disease, shows that such cells can be made to re-trace complex pathways of nerve development long shut off in adult mammals, the researchers say.
This is proof of the principle that we can recapture what happens in early stages of motor neuron development and use that to repair damaged nervous systems, says Douglas Kerr, M.D., Ph.D., a neurologist who led the Hopkins team.
Its a remarkable advance that can help us understand how stem cells can begin to fulfill their great promise, says Elias A. Zerhouni, director of the National Institutes of Health. Demonstrating restoration of function is an important step forward, though we still have a great distance to go.
The researchers created what amounts to a cookbook recipe to restore lost nerve function, Kerr explains. The approach could one day repair damage from such diseases as ALS (Lou Gehrigs disease), multiple sclerosis or transverse myelitis or from traumatic spinal cord injury, the researchers say. With small adjustments keyed to differences in nervous system targets, Kerr says, the approach may also apply to patients with Parkinson's or Huntingtons disease.
In a report on the study, to be released online June 26 in the Annals of Neurology, the Hopkins team says 11 of the 15 treated rats gained significant, though partial, recovery from paralysis after losing motor neurons to an aggressive infection with Sindbis virus -- one that, in rodents, specifically targets motor neurons and kills them. The animals recovered enough muscle strength to bear weight and step with the previously paralyzed hind leg.
Kerr likens the approach to electrical repair. Paralysis is like turning on a light switch and the light doesnt go on. The connectivity is messed up but you dont know where. Weve asked stem cells to go where needed to fix the circuit.
For a brief period after a nerve dies, it leaves behind whats essentially an empty shell, with some scaffolding and non-nerve substances remaining. But with ES injections at the right time and place, and by adding the right cues, weve learned to restore the biological memory for growing neurons, which is clearly still in place, he added.
The motor circuit engineering combines recent discoveries on stem cell differentiation, a growing understanding of early development of the nervous system, and insights into behavior of the nervous system in traumatic injury, Kerr notes.
As adults, our cells no longer respond to early developmental cues because those cues are usually gone, says Kerr. Thats why we dont recover well from severe injuries. But thats what we believe we have changed. We asked what was there when motor neurons were born, and specifically what let motor neurons extend outward. Then we tried to bring that environment back, in the presence of adaptable, receptive stem cells.
In the study, Kerrs team first pre-treated cultures of mouse embryonic stem cells with growth factors that both increase survival and prompt specialization into motor neurons. Adding retinoic acid and sonic hedgehog protein -- agents that direct cells in the first weeks of life to assume the proper places in the spinal cord -- readied the conditioned ES cells for the motor neuron circuit that starts in the spinal cord. Then, stem cells were fed into the paralyzed rats spinal cords.
Extending new motor neurons in an adult nervous system, however, meant overcoming hurdles. One involved myelin, the fatty material that insulates mature motor neurons. Like the coating on electrical wire, myelin prevents weakening of the traveling electrical impulse and lets it continue long distances. In humans, the myelinated sciatic nerve, for example, exits the spinal cord and extends to the leg muscles it activates, carrying impulses several feet.
Once laid down, however, myelin inhibits further nerve growth -- natures way to discourage excessive wiring in the nervous system. We had to overcome inhibition from myelin lingering in the dead nerve pathways, Kerr explains. Two recently-developed agents, rolipram and dbcAMP enabled that.
The assorted treatments let the new motor neurons survive, grow through the spinal cord and extend slightly into the outlying nervous system. A second hurdle remained in getting the neurons to skeletal muscle targets.
As suggested by earlier work by team member Ahmet Hoke on repair in the outlying, peripheral nervous system, the researchers applied GDNF, a powerful stimulator of neuron growth, to the remains of the newly-dead sciatic nerve at a point near its former leg muscle contacts. GDNF attracted the extending motor neurons, luring them to the muscles. To ensure a continuous supply of GDNF, the researchers relied on injected fetal mouse neural stem cells, a known source of the molecule.
Of some 4,100 new motor neurons created in the spinal cord, roughly 200 exited the cord and 120 reached skeletal muscle, forming typical nerve-muscle junctions, with appropriate, typical chemical markers. Microscopically, the neurons and their muscle associations appear identical to natural ones in healthy animals.
Fifty of the new neurons were found to carry electrical impulses. (Because such testing is time and labor intensive, only a small area of leg muscle was assayed. The improved ability of treated rats, however, suggests more functional neurons are likely.) The rats gained weight, were more mobile in their cages and measures of muscle strength increased.
Animals treated without even one component of the cocktail experienced no such recovery. Novel ways of tracing the neurons back to their source assured the scientists that they indeed had come from the injected stem cells, not from lingering host neurons.
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