When nerve cells become damaged through a severe injury such as spinal cord damage or a stroke, the body’s natural defences go to work to limit the damage. This forms what is known as a glial scar, which protects against further injury but can impede long-term nerve repair. Scientists have now redesigned an enzyme found in nature that selectively untangles some of the scarring in a way that promotes regrowth of the nerve cells, opening up new pathways in the development of treatments for these types of conditions.
The breakthrough comes from a team of engineers from the University of Toronto and the University of Michigan, but can be traced back a couple of decades to when scientists discovered an interesting function of an enzyme called chondroitinase ABC, which is produced in nature by bacteria. A glial scar consists of tightly woven cells and biochemicals that come together around the damaged nerve, and it was found that chondroitinase ABC could selectively degrade some of these building blocks in useful ways.
So much so that follow-up studies demonstrated how the enzyme could promote regrowth of nerve cells, with some experiments even demonstrating how it could lead injured animals to recover some of their lost functions. But scientists kept running into the same roadblock, with the enzyme proving too unstable for the job at hand.
“It’s stable enough for the environment that the bacteria live in, but inside the body it is very fragile,” says Shoichet. “It aggregates, or clumps together, which causes it to lose activity. This happens faster at body temperature than at room temperature. It is also difficult to deliver chondroitinase ABC because it is susceptible to chemical degradation and shear forces typically used in formulations.”
Some of the more innovative attempts to overcome this hurdle have involved wrapping the enzyme in biocompatible polymers or fixing it to nanoparticles as ways of shoring up the stability once inside the body. Shoichet and her team believe they have now come up with a solution, using biochemistry to break the enzyme down and rebuild it into a more stable structure.
“The idea was probably a little crazy, because just like in nature, a single bad mutation can wreck the structure,” says Mathew O’Meara, from the University of Michigan, co-lead author of the paper. “There are more than 1,000 links in the chain that forms this enzyme, and for each link you have 20 amino acids to choose from. There are too many choices to simulate them all.”
The team sought the help of computer algorithms that enabled them to determine the correct building blocks to form a re-engineered form of chondroitinase ABC. Lab tests showed this version, with a total of 37 amino acid substitutions, to be more stable and active than the wild version.
“The wild type chondroitinase ABC loses most of its activity within 24 hours, whereas our re-engineered enzyme is active for seven days,” says Marian Hettiaratchi, the other co-lead author of the paper. “This is a huge difference. Our improved enzyme is expected to even more effectively degrade the glial scar than the version commonly used by other research groups."
While this is a potential breakthrough for treating nerve damage caused by spinal cord injuries and strokes, the team has so far only demonstrated the enzyme’s capabilities in early lab experiments. The next steps will involve progressing to environments where the wild version of the enzyme has been studied previously, to better understand its potential as a therapeutic tool.
“When we started this project, we were advised not to try as it would be like looking for a needle in a haystack,” says Shoichet. “Having found that needle, we are investigating this form of the enzyme in our models of stroke and spinal cord injury to better understand its potential as a therapeutic, either alone or in combination with other strategies.”
The research was published in the journal Nature Neuroscience.
Source: University of Toronto