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Regrowth of nerves in the adult spinal cord

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By: Ivan Chiu

This is a summary of a paper published by a research group from Massachusetts, USA in the journal Nature Neuroscience.

Original article: Liu K, Lu Y, Lee JK, Samara R, Willenberg R, Sears-Kraxberger I, et al. PTEN deletion enhances the regenerative ability of adult corticospinal neurons. Nature neuroscience. 2010;13(9):1075–81.

There are generally two ways that nerves can reconnect after SCI. First, the injured nerves can regrow to restore the lost connections; this is known as regeneration. Second, undamaged nerves surrounding the injury can branch out and form new circuits to replace lost connections, which is referred to as plasticity or sprouting. Both processes have the potential to restore function after SCI.

One of the most important circuits in humans consists of nerves controlling voluntary movement, including fine movement of the hands. This is called the corticospinal system — achieving new growth in corticospinal nerves is one of the biggest goals of SCI research. They are some of the most important nerves, but are difficult to work with, because nerves in the brain and spinal cord are inherently reluctant to grow.

An important study in Nature Neuroscience suggests that success in corticospinal regrowth might be increasing. The researchers conducted experiments in animal models of SCI to examine the role in nerve growth of a protein named mTOR. They found that mTOR regulates nerve growth: mTOR is activated in nerves that are growing and deactivated in nerves that have stopped growing — including corticospinal nerves in the adult.

Obviously the researchers are working to reactivate the mTOR protein, but it is present in many parts of the body, and has many functions, so it is very important to restrict the effect just to the corticospinal nerves. To do this, these scientists have developed a genetic trick to activate mTOR only in corticospinal nerves.

What was the most important finding?

When mTOR was activated, corticospinal nerves showed growth of uninjured nerves, and regeneration of nerves that had been injured by cervical SCI. Some new nerves formed new connections below the site of the injury.

What are some things we need to consider?

This was a genetic study performed in an animal model. More research will need to be done — including clinical trials in people with SCI – before these findings can be translated to clinical use.

What does this mean for people with SCI?

This study is the most dramatic demonstration yet published of regeneration in the injured spinal cord. It shows that regrowth of damaged nerves can occur through the site of an injury, and that SCI researchers are making progress in achieving true regeneration.