Combat Injuries: Regenerating the Nerves
[ Sound familiar? = Bio-Engineered Synthetic nervous system -- MC ]
When it comes to research on regenerating nerve cells Mayo Clinic is taking point for the U.S. military. The research team is part of a national consortium aimed at restoring mobility to severely injured American combat veterans. The solution involves special growth factors, dissolving polymers, and stem cells to reconnect and restore feeling to the nervous system.
If there is any kindness at all in war it is derived from the push it gives medical science to explore novel ways of treating wounded service men and women. This impetus in part is integral to a new medical emphasis known as regenerative medicine. Mayo Clinic researchers are in the vanguard of this field with collaborative efforts leading to innovations to aid in the regeneration of peripheral nerves and bone that has been damaged in warfare.
The good news about Iraq and Afghanistan war injuries is that well over 90 percent of wounded service men and women survive. In contrast, only a quarter of wounded soldiers in Korea, Vietnam and the Gulf War died from their injuries. The majority of injuries in the Iraq and Afghanistan conflicts affect the arms and legs because improved Kevlar body armor has lessened damage to the head, chest and abdomen. The armor was tested for use on arms and legs, but soldiers rejected it, as it reduced their mobility and quick access to their weapons.
The bad news?
“They survive with really serious injuries, which can be life-changing,” says Michael Yaszemski, M.D., Ph.D., a brigadier general in the U.S. Air Force Reserve, who has deployed to Iraq and served as deputy commander of the theater hospital at Balad Air Base, north of Bagdad. Dr. Yaszemski is also a Mayo Clinic orthopedic surgeon and biomedical engineer. Dr. Yaszemski and collaborator Anthony Windebank, M.D., Mayo neurologist and molecular neuroscientist, are co-directors for nerve injury research in AFIRM — the Armed Forces Institute of Regenerative Medicine. AFIRM was created and funded by the Army, Navy and the National Institutes of Health to focus on new treatments for war wounded. The consortium of 16 institutions has been granted $85 million over five years to achieve its goals.
Dr. Yaszemski has personally cared for patients in the field that he is now trying to help in the lab. “In using synthetic polymer scaffolds as our core, we are engineering new tissue where it doesn’t exist. It’s not exactly the same process in place when we were in the womb, but we want to get to the same end point to offer a better life for our wounded and for anyone suffering traumatic nerve and bone injury.”
The pair has collaborated at Mayo for five years on nerve and spinal cord regeneration and has successfully regenerated peripheral nerves in the spinal cord of laboratory rats. They are a couple of important regulatory steps away from testing their system on humans in clinical trials.
Scaffolds and Stem Cells
The system is comprised of synthetic polymer scaffolds that deliver cells and nerve growth factors to severed peripheral nerves. The goal is to recreate or regenerate the nerves so patients’ functions and feelings will be restored. It entails several Mayo Clinic innovations, including Dr. Yaszemski’s development of the co-polymer, polycaprolactone fumarate, two compatible polymers never brought together before, to serve as the scaffold. It involves novel efforts to use adult stem cells from a patient’s own adipose (fat) tissues. And it brings to bear the application of microfabrication techniques, such as stereolithography — powerful lasers that form new polymers, which are the foundation of the system’s architecture. The scaffold is an analog of the naturally occurring extra cellular matrix, Dr. Yaszemski explains.
Two close up views of the synthetic polymer scaffolds, with image (top) of myelinated nerve fibers that have grown in the scaffolds.
“This is connective tissue that provides the platform upon which cells, including stem cells, signaling molecules and other components such as nerve growth factors ‘do their thing,’ if you will, that will stimulate nerves to regrow,” he says. “To build new tissue, we need to provide that foundation which functions to let cells that will make the natural tissue anchor to it.”
Peripheral nerves are sorts of electrical connections between the brain and spinal cord that make muscles work and feelings report from all over the body, Dr. Windebank explains. The nerve is a very long cell; the cell body with its nucleus for motor nerves that control muscle is in the spinal cord; if it’s a sensory nerve, the cell body is beside the spinal cord. The very long extension of the cell is called the axon. When severed, the part of the axon in continuity with the cell body stays there, but the part beyond the cut — going into the hand, for example — degenerates completely, along with the myelin, the insulating material around it. To reestablish function, the nerve ending at the cut has to regrow the entire length back to the target in the hand.
The team is seeking approval from the Food and Drug Administration and Mayo Clinic Institutional Review Board for human clinical trials, which will involve defined injuries in which the gap between the nerve cell body and the damaged axon is not so far to bridge.
“The goal of our research is to come up with new strategies to build bridges in the peripheral nervous system,” Dr. Windebank says. “The synthetic polymer scaffolds are the basis of those bridges.”
Single lumen or tubule (A) and (B) multiple lumen biodegradable nerve scaffolds. Frame C shows the flexibility of a nerve guidance scaffold; it mimics the properties of a normal peripheral nerve.
The researchers are creating an environment whereby the synthetic polymer scaffolds support cells, recognition and signaling molecules and nerve growth factors to generate new nerve tissue over a period of weeks to months and then harmlessly biodegrades in the body after delivering their goods.
Their system encourages a nerve to grow so it gets longer and extends to its target. If this is done naturally, a patient runs the risk of the nerve growing into a neuroma, which Dr. Windebank describes as resembling a knotted piece of wool, wrapped in circles, and very painful, one of the reasons people endure much pain after amputations.
“There is a guidance component of the system, with the scaffold providing the physical guidance and the growth factors giving directional guidance,” Dr. Windebank says. “There is a whole series of nerve growth factors well-known in their developmental roles, and one of the things we’re doing is to supply these factors to the regenerating nerves in a controlled way. We can essentially impregnate the polymers with the nerve growth factors and then control how they’re released.”
Scaffolds in regenerative medicine also are made of minerals, primarily ceramics, and metals, mainly titanium and tantalum. The Mayo researchers prefer synthetic polymers in this application because they offer better control over the chemistry and mechanical properties and flexibility compared with mineral or metal counterparts.
“We can change polymer chemistry if we feel that will be beneficial to do the job that we want done,” Dr. Yaszemski says. “For example, if we want a bone cell to attach, there are certain sequences of peptides that those bone cells recognize. We can modify the surface chemically and attach recognition molecules. Bone cells would see this and recognize it as ‘home’.”
To create polycaprolactone fumarate, polyester similar to the material in a sports jacket, the researchers employed stereolithography, an engineering technique used to make microchip computer components. It allows the chemicals to operate in a watery environment and function as a cross-linking polymer, rather than independent molecules, which is their natural tendency. A pair of powerful lasers shone on the solution brings about this polymerization.
The Yaszemski-Windebank team has developed a method of procuring a patient’s own stem cells from adipose tissue. It very recently received FDA approval.
“You make a small skin incision, take out a small piece of fat, and we can make as many stem cells as we need,” Dr. Windebank says.
Available also is a recent technique that allows them to take adult stem cells and engender them with the properties of embryonic stem cells. It was developed in other parts of the world and the country, but “it’s now in-house here,” he adds. (Editor’s note: see “iPS Cells (http://discoverysedge.mayo.edu/ips-regenerative-medicine/)
“Our overall goal in this endeavor is to improve the lives and functions of our patients,” Dr. Windebank says. “We’re focused on helping service personnel with terrible injuries, but what will come out of this will have applications for anyone with a serious limb injury from whatever misadventure.”
Dr. Yaszemski notes that Mayo Clinic discoveries in polymer chemistry and basic cell biology are now enabling a coalition of people with expertise in a wide range of technologies applicable to regenerative medicine.
“On the AFIRM project alone, we have Mayo Clinic collaborators in various surgical specialties, immunology, molecular neuroscience, physiology and biomedical engineering,” he says. “That’s consistent with the Mayo Clinic philosophy of forming teams of individuals with their own expertise. We expect a much better result than if one person were trying to figure everything out.”
— Tony Fitzpatrick, December 2009