Regeneration game2 May 2018
For decades, nerve grafts have remained the gold standard in repairing damage to the peripheral nervous system. Artificial alternatives, however, have not enjoyed the same level of success. Professor Abhay Pandit, director of the Centre for Research in Medical Devices at the National University of Ireland, talks to Greg Noone about how an investigation into the effectiveness of grafts at the molecular level could lead to new breakthroughs in the field of nerve regeneration.
Humans are nothing if not a bundle of nerves. For all the splendid complexity of the eyes, bones, muscles, veins and arteries, it is the peripheral nervous system (PNS) – comprised of individual cells stretching up to three feet in length – that forms the basis of who people are outside the mind, guiding and informing their motions and senses. As delicate as the tangled roots that they resemble, any sudden break in an individual cell can prove highly disruptive to the way in which individuals perceive the world around them.
“Nerve injuries are quite common, especially in trauma,” explains Professor Abhay Pandit, director of the Centre for Research in Medical Devices (CÚRAM) at the National University of Ireland in Galway. Up to 3% of all trauma patients will experience an injury to the PNS, but such damage can come about in other ways. As one review article in the International Journal of Molecular Sciences recently noted, “Peripheral nerve injury is seen in surgery, anaesthesia injections, chemotherapy, and radiation for breast or head and neck tumours.”
As an expert in nerve repair, Pandit is well aware that the size of a wound is a key determinant as to whether or not regeneration is possible. “If you have a small injury, which would be about 3cm or so, nerves can grow back,” he explains. The process is slow but steady, as nerves can regenerate by up to 1mm a day, depending on their location in the body. “After a particular gap [though], the whole protective mechanism of the body takes over and, somehow, the nerves do not grow back.”
This is because, if the break in the nerve is greater than 3cm in length, the body’s priorities begin to shift.
Instead of working on repairing the damaged nerve(s), the area surrounding the injury becomes inflamed, effectively signalling the body’s intention to seal up the area before function is restored; however, this, largely prevents the nerves from healing.
In the past four decades, two main alternatives have emerged. The first involves grafting another nerve from a different part of the body to bridge the gap in the original injured nerve, otherwise known as an autograft. While considered to be the gold standard in nerve repair, it is nevertheless a flawed treatment, essentially shifting the damage caused by the injury from one part of the body to the other. The second approach – artificially coaxing nerves to regenerate of their own volition, aided by special conduit materials and growth agents – has also yielded only limited results.
Progress towards a universal nerve repair treatment has therefore been slow. Pandit, however, believes that his research at CÚRAM has enabled a much-needed breakthrough. By going back to the beginnings of modern nerve repair techniques, doctors can fully comprehend why current solutions are not functioning as well as they should be.
“We need to know why things don’t work in the first place,” says Pandit. “Understanding why they don’t work will allow us to [find] clues on what may actually work.”
Pandit’s path into nerve repair research began in his ’20s, around the time he was about to choose a PhD. It was at this point that his grandfather fell ill. “He was getting bed sores and I saw first-hand what he was struggling with,” he recalls. “I just felt like I needed to work in that space. [There] was an ad for a PhD studentship in wound healing and it just called [to] me at that point.”
His research mainly involved work at a local clinic, in the context of testing a new material, fibrin, that could be used to better treat burns victims. After that, he began collaborating with a colleague from the Mayo Clinic, who was spending a year in Ireland. “And through his engagement, I got interested in the area of the whole neural space in general.”
By then, Pandit had moved almost exclusively into wound-healing research, specifically in soft tissue, skin and cardiac repair, and designed biomaterials or “platform technologies that have a broad range of ramifications,” he says. But once there was an opportunity to engage in neural repair research, he seized it.
Pandit is the latest in a long line of researchers fascinated with the possibilities of nerve regeneration, stretching back to the exploits of the renowned physician Galen in the second century AD. Considered the father of modern surgery, he was also an inveterate boaster who once attempted to impress the High Priest of Asia by deliberately eviscerating, and then successfully operating on, an ape. On a more professional note, the physician was the first to differentiate nerves from tendons and reported successful attempts at nerve repair. Unfortunately, no evidence survives of how this was achieved – let alone whether the subject of his experiments was man or monkey.
It took four centuries before the first successful suture repair was reported, another 12 for degeneration of the PNS to be fully understood, and until the 1970s to discover that grafting nerve samples from patients was a more effective way of repairing neural damage than crudely stitching nerve stumps together under tension. Since then, autograft has remained the primary treatment for PNS damage.
Even so, the procedure’s flaws are numerous, beginning with the fact that an autograft moves the damage inflicted on the PNS from one place to another.
Furthermore, a great deal of thought needs to be given to whether the procedure is even suitable for the injury. Wounds, after all, come in all shapes and sizes, and to a wide variety of people; what is suitable for a healthy, 20-year-old male may not suit a woman in her 80s. “The size mismatch of the graft is another issue,” adds Pandit.
It is in view of these drawbacks that researchers have explored alternative pathways for nerve repair, which primarily focus on growing nerves through tubular structures – conduits – that are inserted into the affected area. In some cases, these can come from the body itself, but arteries and veins were used in the past. Today, artificial alternatives are also available.
“There are reports of using silicon tubes in the early ’80s, and then biodegradable formats, such as collagen tubes, [in] the late ’90s,” explains Pandit. Scientists then began exploring how to create structures inside those tubes to entice nerves to grow through them. “[By] adding growth factors, we did see improvements over [using] a simple conduit,” the professor adds. Even so, they never proved as effective as autografts.
This lack of progress in regenerating nerves by artificial means prompted Pandit to ask himself whether the field was missing a trick: would it not be better, he thought, to compare the mechanisms by which autografts and artificial attempts at nerve repair are healing? While there had been studies in the past about how the choice of conduit material could affect the regeneration of nerve cells, few had tried to find out what reactions were taking place between the new material and the damaged nerve at a molecular level.
So, Pandit and a team of researchers at CÚRAM took it upon themselves to do just that. Their findings, published in the Advanced Functional Materials in August 2017, showed that there are in fact “specific responses at a protein level that are modulated by a specific material itself”.
By exploring the key differences between using an autograft and collagen and polylactic-co-glycolic acid (PLGA) polymer conduits, Pandit and a team of researchers demonstrated that ‘each material selectively activates different regenerative pathways’ within the affected nerves. In other words, the damaged nerves were reacting in highly specific ways to the conduit put in place to coax it into regenerating.
“In the past, researchers would have used factors, saying, ‘Well, let’s use a nerve growth factor with a collagen conduit,’ [or] ‘Let’s use nerve growth factor with a PLGA conduit, or a polyglycolic acid (PGA) or polycaprolactone conduit’,” explains Pandit. “What I’m arguing is perhaps that’s not the case. [It’s] the conduit material is going to initiate a particular response, It may not be a nerve growth factor, [but] may be something else that you may need.”
By finding a way to supplement the expression of certain proteins, such as myelin and cholesterol, Pandit and his team argued that surgeons could preside over ‘regeneration equivalent to autograft’, and thereby ‘pave the way for incorporating future biomaterialspecific functionalities in nerve-guidance conduits’. In other words, by cracking the code of why autografts perform as well as they do, research undertaken at CÚRAM could directly lead to the healing of more severe PNS injuries.
As far as next steps go, Pandit is cautiously optimistic. He sees the main obstacle to further progress as the body’s innate response to significant trauma, namely its tendency to focus overwhelmingly on closing off the wound over and above regeneration.
“The biggest impediment right now, in my head, is this whole inflammatory response,” he states. “Perhaps we [can] start using some sort of inflammatory modulating materials in the first instance, without any further big functional changes in the conduit.”
He also considers the priorities of nerveregeneration research to be somewhat askew, and hopes to remedy this in future studies at CÚRAM. “I would say 99% of published data is in [the] non-critical gap defect,” he says, referring to the space below 3cm in which automatic nerve regeneration is possible. “Well, that doesn’t mean anything because they are going to heal anyway.”
There does seem to be some progress in this area. In February, a team at University College London managed to repair severe sciatic nerve damage in rat specimens, using artificial nerve tissue built from human stem cells. While clinical applications are likely to be a long way off, this kind of research is in line with Pandit’s ultimate ambition for the space: to think much bigger than it has done in the past.
“I want to challenge myself and the community as well to take on the critical gap, and test and find out what the impediments are,” he says. Then, perhaps, we may we see the clinical translation of such important research.