BALTIMORE - Reports of paralyzed rats made to walk come and go, all claiming to be new and different. That said, this one is based on coaxing mouse embryonic stem cells into forming functional motor neuron circuits that extend to skeletal muscle, and it may well be different.
BALTIMORE, June 22 ? Coaxing mouse embryonic stem cells into forming functional motor neuron circuits that extend to skeletal muscle has restored some walking ability to paralyzed adult rats.
In a proof-of-concept study that's sure to fan the flames of the stem cell debate, neurologist Douglas Kerr, M.D., Ph.D., and colleagues at Johns Hopkins here showed that 11 of 15 adult rats with virally-induced paralyzed hind limbs could walk again following injection into their spinal cords of treated mouse embryonic stem cells.
"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," Dr. Kerr and colleagues reported in an early release from the June 26 online edition of Annals of Neurology.
"It's a remarkable advance that can help us understand how stem cells can begin to fulfill their great promise," said Elias A. Zerhouni, M.D., 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." A component of NIH funded the research.
Others have shown that stem cells can halt spinal motor neuron degeneration and restore function in animals with spinal cord injury, or in models for amyotrophic laterals sclerosis, but not before have newly generated neurons formed functional connections in adults mammals, the researchers asserted.
In a previous study, researchers from the University of California at Irvine showed that animals with paralyzing spinal injuries had significant evidence or remyelination of spinal nerves and recovery of some locomotor function if they were treated with human embryonic stem cell-derived oligodendrocyte progenitor cells within seven days of an injury.
But in the same study, animals that received the progenitor cells 10 months after an injury had no evidence of remyelination or restoration of walking ability.
In the current study, Dr. Kerr and colleagues sought ways to overcome the problems of myelin inhibition of axon growth that have hampered earlier attempts. To do this the authors treated some of the stem cells with dibutryl cyclic AMP (dbcAMP), an agent that guides cells toward differentiation, and injected some of the animals with rolipram, a neuroprotectant.
The stem cells were delivered into the spinal cords of the adult rats 28 days after they had become paralyzed following infection with the selective ventral motor neuron depleting virus Neuroadapted Sindbis virus (NSV).
In one group of animals, the authors also injected into sciatic nerves glial cell derived neurotrophic factor (GDNF) as an attractant to help guide the nurtured embryonic cells into forming functional neuronal pathways, and they gave the animals cyclosporine to prevent rejection of the transplanted embryonic stem cells.
The investigators reported that only those rats treated with the "cocktail" of dbcAMP, rolipram, and GDNF had weight gain at six months, suggesting that that they were more mobile and better able to reach their food than litter mates. Hind limb grip strength also improved only in animals in this group, compared with others who received either stem cells alone or stem cells plus neuroprotectants but not the growth factor.
In all, 11 of the 15 animals that received the full panel had significant but incomplete recovery from paralysis. The animals recovered enough muscle strength to bear weight and step with the previously paralyzed hind leg on the side treated with GDNF, but not the contralateral side.
"We conclude from these studies GDNF acts as a focal attractive cue for embryonic stem cell-derived motor axons and that when co-administered with dbcAMP and rolipram facilitates the establishment of neuromuscular junctions between transplant and host resulting in physiologic and behavioral recovery," the investigators wrote.
"This research represents significant progress," said David Owens, Ph.D., of the National Institute for Neurological Disorders and Stroke, which funded the research. "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."
"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," said Dr. Kerr. "To overcome the loss of function, we need to actually replace lost neurons."
Although the results of this early study are encouraging and offer hope for a future method for treating spinal cord injury and demyelinating diseases such as ALS, the technique has not been tested in large animals or in humans, and it's unclear whether human embryonic stem cells will respond in a fashion similar to that of the mouse cells when treated and put into a similar environment, Dr. Kerr said.
He and his colleagues are working to develop a pig model of motor neuron degeneration, he added.