Damage to a neuron's axon, or nerve fiber - the 'long tail' that sends signals from the cell body - is a key element of spinal cord injury.
A team of researchers at Boston Children's Hospital in Massachusetts have developed a "recipe" for a mixture of molecules and tested its therapeutic potential in mice with spinal cord injury (SCI) or stroke.
Stroke is the leading cause of paralysis in the United States, accounting for over a third of the 5.4 million people who are living with different forms of paralysis.
SCI comes a close second, as 27 percent of all cases of paralysis are caused by SCI, and 17,000 new cases of SCI occur every year.
After a patient has an SCI or a stroke, the axons in the brain's cortex and along the spinal cord become damaged. A neuron is comprised of a cell body and two extensions: the dendrite and the axon, which looks like a long chord that sends signals from the main cell.
In the new study, the researchers - led by Zhigang He, Ph.D., of Boston Children's Hospital and Harvard Medical School - administered a molecule mix to mice in the hope that it would restore their axons. The findings were published in the journal Neuron.
Testing the molecular mix in mice
He and colleagues started out from a previous study they had collaborated on with scientists at Harvard.
In this research, they found that combining a growth hormone secreted by the liver called "insulin-like growth factor 1" (IGF1) with a protein called osteopontin (OPN) improved vision in optically injured rodents by regenerating the axons of their optical nerve.
OPN has been shown to be involved in the inflammation and degeneration of the nervous system, playing a key role in neurodegenerative diseases such as multiple sclerosis (MS), Parkinson's disease, ans Alzheimer's disease.
In the mouse model of SCI, He and team examined two groups of mice: a group that received the molecular mix after having the injury, and a control group that did not.
In the former group, the researchers injected the mice with the mix of IGF1 and OPN 1 day after the rodents had the SCI.
In the stroke model, the treated mice received the mix 3 days after injury.
The researchers tested the mice's motor abilities, including their fine motor skills, by examining their ability to walk on a horizontal ladder with unevenly spaced rungs.
Treated mice recover fine motor skills
The researchers found that, compared with the control group, the treated mice showed drastic improvement in their fine motor skills.
In the untreated control group, motor function was gradually and partially restored after injury due to the natural regrowth of axons.
The mice regained a lot of their motor function, but remained significantly impaired in their fine motor skills, as revealed by the irregular ladder test.
Treated mice, however, made far fewer errors on this test; in fact, at week 12 after the injury, the mice made errors only 46 percent of the time. By contrast, the control group had an error rate of 70 percent.
Next, the researchers wanted to test if adding 4-aminopyridine-3-methanol - a potassium channel blocker known to improve axonal conduction in patients with MS - would improve the results even further.
After adding this third ingredient, the error rates in treated mice further decreased to 30 percent. Healthy mice made mistakes 20 percent of the time, so the treated mice fared very well by comparison.
"In our lab, for the first time, we have a treatment that allowed the spinal cord injury and the stroke model to regain functional recovery."
Zhigang He, Ph.D.
To see whether these results were due to a "resprouting" of axons, the researchers also examined spinal cord sections of the mice.
"We saw what we expected - axon sprouting in [the] spinal cord," says He. "But we also found something unexpected - increased axon sprouting in the subcortical area."
He and colleagues performed further tests where they genetically engineered mice to lack axons in the corticospinal tract (CST) in the spinal cord.
Further assessments of the mice's fine motor skills revealed that improvements in post-injury error rates decreased significantly in these mice that lacked CST axons.
Therefore, this suggests the recovery achieved by the therapeutic mix did not depend so much on the axons' regrowth in subcortical areas, but on the regeneration of axons in the CST.
So, the "functional outcomes" of the subcortical axons that were found to have resprouted "remain to be tested," the lead investigator says.
Ultimately, as a next step stemming from this research, He and colleagues plan to test the molecular mix in human clinical trials.