Spinal cord regeneration ! A reality or Myth
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain (the medulla specifically). The brain and spinal cord together make up the central nervous system.
SPINAL CORD INJURY
A Spinal Cord Injury (SCI) is damage or trauma to the spinal cord that results in a loss or impaired function causing reduced mobility or feeling. Common causes of damage are trauma (car accident, gunshot, falls, sports injuries, etc.) or disease (Transverse Myelitis, Polio, Spina Bifida, Friedreich's Ataxia, etc.). The spinal cord does not have to be severed in order for a loss of functioning to occur. In most people with SCI, the spinal cord is intact, but the cellular damage to it results in loss of functioning. SCI is very different from back injuries such as ruptured disks, spinal stenosis or pinched nerves.
It is possible for a person to "break their back or neck" yet not sustain a spinal cord injury as long as only the bones (the vertebrae) around the spinal cord are damaged, but the spinal cord is not affected. In these cases, the person may not experience paralysis after the vertebrae are stabilized.
• Is it possible to get nerves to grow and regenerate?
• Are there factors preventing spinal nerve regrowth?
• What can be done to promote correct “connections” on both sides of the injury?
• What can be done to make it easier for nerves to grow?
• Researchers are investigating many different areas for answers to these questions on spinal cord regeneration.
Increase Growth Factor Production
Damaged nerves must first grow for regeneration to occur. The neurotrophic proteins function as “growth factors. These help prevent cell death. They also work like a "nerve fertilizer" to help neurons survive and nerves regenerate. Different pathways in the spinal cord may require particular combinations of growth factors for survival after injury
Scientists are studying several growth factors and how they can be used in treating spinal cord injury. Each growth factor has very specific target cells that it works on.
1 NT-3 (Neurotrophin 3)
2 BDNF (brain derived neurotrophic factor)
3 aFGF (acidic Fibroblast Growth Factor)
4 NGF (nerve growth factor).
Another area of research with growth factors is in spinal cord injuries that are not complete. Some fibers connecting the brain with spinal segments below the level of the injury may survive. The goal of this research is to increase the quality and strength of the nerve impulses in the surviving connections.
While nerve cells usually do not survive after axons have been severed close to the cell body, recent experiments in the rat spinal cord have shown that two trophic factors, brain-derived neurotrophic factor (BDNF) and neurotrophin 3 (NT3), can rescue nerve cells from which the axons have been recently severed.
BDNF and NT-3 growth factors are being used along with neurons “grown” in the lab from bone marrow stem cells. Researchers are analyzing their use in spinal cord injured rats to define how they may improve nerve growth.
Results from Lorne Mendell’s lab at the State University of New York, Stony Brook report that neurotrophin molecules (such as NT-3) can improve function at synapses in the spinal cord of newborn rats. The goal is to devise ways to make this finding useful in restoring function to the damaged spinal cord in adult rats and, ultimately in humans.
Although NT3 has short-term effects, BDNF can help nerve cells survive for 4 weeks or more after injury. When the trophic factors BDNF, NT3, and NT4 (neurotrophin 4) were combined with fetal tissue transplants, axons no longer stopped growing at the border of the transplant but instead greatly expanded the territory into which they projected.
The combination of transplants and trophic factors also led to an increase in c-jun, an important immediate early gene. Immediate early genes respond rapidly to many stimuli and regulate many cell functions. Interestingly, these experiments showed that axons from cells that use the neurotransmitter serotonin responded to trophic factors more vigorously than axons from cells that use other neurotransmitters. This illustrates the importance of finding the right combination of growth factors for each type of cell.
Block Inhibitory Process
One problem that occurs in regeneration is that certain factors prevent nerve cells in the central nervous system from growing. Spinal cord tissue contains certain chemicals that stop nerve regeneration. Myelin-associated neurite growth inhibitor, which is produced by oligodendrocytes, is the most important CNS growth inhibitor so far identified. Martin Schwab, a University of Zurich researcher, has identified one of these inhibitory chemicals as a protein called Noga-A. He discovered an inhibitor-neutralizing antibody (IN-1) which binds to and masks the factor from growth cones, severed axons began extending past the oligodendrocytes and reconnecting with their targets. After this treatment, rats with severed spinal cords moved more normally and partially regained their contact-placing reflexes (in which rats move their legs to support their bodies when they are placed against a surface). This process has worked in cell cultures and in animals. His researchers’ next step is to focus on preparing these antibodies for human use.
Evidence that combining some therapies may have an additive effect has prompted researchers to focus effort on finding a combination that will achieve regeneration. Some combination therapies recently tested in rats have shown exciting results.
One approach used:-
1 Neurotrophin 3, fetal cell transplants, and IN-1, the antibody to myelin-associated neurite growth inhibitor.
Rats treated with this approach showed faster and more extensive recovery after spinal cord injury than those given any single treatment alone. Researchers still need to learn if this therapy can be a general approach or if specific nerve pathways have specific requirements for growth. They also need to carefully define the time windows for effective combination treatment.
2 Another approach using nerve fiber transplantation combined with growth factors
This approach showed the first functional regeneration of completely transected rat spinal cords.
i Researchers carefully transplanted 18 pieces of peripheral nerves (one to three pieces for each of the normal nerve tracts) taken from the rats' chests to "bridge" 5-millimeter gaps at the T8 segment of rats' spinal cords.
ii To evade inhibitory proteins from oligodendrocytes, the bridges routed regenerating axons away from white matter, where they would normally grow, and into gray matter.
iii) The researchers fixed the grafts in place with a glue based on a blood-clotting factor called fibrin. The glue also contained acidic fibroblastic growth factor, or aFGF, which enhances nerve fiber development.
iv Finally, the scientists wired the vertebrae to keep the spine in place while the area healed.
After 3 weeks, rats that had received this type of graft began to recover function in their hind legs. Some of the treated rats regained some movement on both sides of their bodies, while others regained movement on only one side. The rats continued to improve gradually over the course of a year, though they never walked normally.
Rats with bridges from white matter to other white matter, rats in which the fibrin glue had no aFGF, and rats that did not receive transplants did not recover any function over time. Anatomical studies of spinal cords from rats that recovered function after this therapy showed that the nerve fibers grew into the gray matter on the opposite side of the gap. The fibers then grew at the interface between the gray matter and the white matter, an area that corresponds to the normal corticospinal tract in rats. The degree of recovery corresponded significantly to the degree of motor fiber regeneration.
Promote correct connections on both sides of injury
First, researchers must keep nerve cells alive and get them to grow. Then it is necessary for the axons to reconnect to their proper target sites. In other words, the nerves must rejoin with their companion nerve for the connection to be complete and functional. Researchers are working with different substances to guide nerve growth so nerves grow past the injury site and reconnect with the proper nerve.
• Netrins: Netrins are proteins produced in the brainstem, that "attract" nerve cells. They encourage nerve cells to migrate to and grow branches toward a "target."
• Neural Glues: Neural glues are substances that can fuse together the ends of damaged nerve axons. Scientists at the Center for Paralysis Research, Purdue University, used polyethylene glycol (PEG) in guinea pigs. This neural glue helps to partially restore nerve function immediately following spinal cord compression injury
• Fibroblasts: Fibroblast cells, commonly found in the skin, act as a "bridge" across a spinal cord lesion. Scientists genetically engineer these fibroblast cells to produce neurotrophin-3. The cells can then stimulate regrowth.
Researchers at Purdue University’s Center for Paralysis Research and Indiana University School of Medicine are using low-level electrical stimulation on paralyzed dogs. They implant a small battery pack, known as an extraspinal oscillating field stimulator (OFS), near the dog’s spine.
It sends a weak electrical signal (thousandths of a volt) to the site of injury. This helps regenerate cells and guide growth in the damaged nerves. In about a third of the cases, the dogs improved significantly. The first human clinical trial of this new treatment is now underway. Patients being entered in the study must be within 18 days from the time of their injury.
What are the effects of transplanting various cells into the injury site to promote regeneration?
Peripheral Nerve Transplants
We now know that damaged or injured peripheral nerves sometimes regenerate but cells and nerves in the spinal cord do not. Scientists are transplanting cells or pieces of peripheral nerves that produce substances that create an environment for axons to grow. This idea was first advocated by the neurologist Ramón y Cajal, about a 100 years ago. He suggested implanting cells from the peripheral nervous system, ( PNS ) into the area of a central nervous system ( CNS ) injury. Since the environment of the PNS supports axon regeneration, he believed re-creating this environment in the spinal cord might allow CNS axons to regrow after an injury. Their goal is that the transplanted nerve cells will mature and become a part of the central nervous system.
One approach for repairing spinal cords that is being tested in animals is to transplant cells and tissues into the damaged spinal cord. Ideally, this environment would also point growing nerves to the correct targets. Experiments with PNS transplants in rat models of spinal cord injury have led to axon elongation and cell body changes associated with regrowth. Transplants from the PNS also seem to reduce scarring around the injury that may impede regrowth.
One technique tested in rats is transplanting Schwann cells -- glial cells that help myelinate axons in the PNS -- into the spinal cord after injury. These transplants supported regrowth of the damaged nerves in rats with spinal cord injury. Researchers are now studying human Schwann cells to determine if this technique will work in humans.
Transplantation procedures to repair the spinal cord involve multiple steps such as:
• creating multiple peripheral nerve bridges.
• re-routing white to gray matter.
• filling grafted area with fibrin-based tissue glue.
• adding acidic fibroblast growth factor (aFGF).
• stabilizing the spine to prevent reinjury.
Fetal Central Nervous System Tissue
Another way of encouraging regeneration is to implant fetal tissue. Tissue from a growing fetus contains stem cells, progenitor cells, and many substances that support growth. Such tissue also presents fewer obstacles to growing axons. Stem cells can differentiate into several cell types, depending on the signals they receive. Transplanting them into the spinal cord may, with the right chemical signals, help them develop into neurons and supporting cells in the spinal cord, re-establishing lost circuits.
Studies in rats show that fetal transplants promote survival and regrowth of some damaged nerve cells. Transplanting fetal CNS tissue into the spinal cord of both mature and newborn rats yielded axon growth that terminated within a few millimeters of the border of the transplant. Researchers still need to learn exactly how fetal tissue transplants promote nerve regrowth. The transplants appear to "rescue" axons and provide a bridge across which regenerating axons can grow. While both adult and newborn rats regrew descending nerve fibers from the brain, the growth of descending pathways into the transplants was substantially greater in the newborns. This suggests that other changes in the maturing CNS, such as the production of inhibitory factors or a loss of certain axon guidance molecules, may influence axonal regrowth after injury.
Researchers at the Albany Medical Center, New York and Washington University in St. Louis are implanting fetal spinal cord cells from pigs into the injury site of mice and rats. Fetal neural cells from pigs are used because they grow rapidly and are “functionally identical to human fetal neural cells.”
Another step is masking (hiding) the pig proteins to prevent immediate rejection. Masking is done with antibodies. It appears this eliminates the need for long-term immune-suppressing drugs. The goal of this procedure is for these cells to produce myelin. Myelin is the substance that insulates nerve cells, giving the spinal cord the ability to heal and to send electrical signals. Some paralyzed animals regained partial use of the hind limbs after treatment. In April 2001, clinical trials with humans began. It will be months before researchers know whether these transplanted cells have resulted in any spinal cord regeneration
Certain kinds of stem cells can produce any kind of cell in the body. This means they can make replacement cells for other body parts, including spinal cord cells.
Stem cells from both rodent and human tissues are being studied. One major question is what determines whether stem cells develop into cells that help regeneration, e.g. nerve cells, or cells that make myelin, and not into cells that prevent regeneration, e.g. scar tissue. Once nerve cells can be reliably obtained from stem cells, then they must show they can grow appropriately to the type of cell lines described above.
A highly publicized human clinical trial is taking place in Israel with individuals who have a complete spinal cord injury. The treatment involves harvesting blood cells (macrophages) from the patient’s own blood. These macrophages are first treated in a lab. They are then implanted in the individual’s spinal cord to “repair” the damaged cord. Recent results report that one patient had limited restoration of leg movement. Phase I of these human trials are also ongoing in Brussels. Patients must be enrolled within weeks of injury.
Spinal cord injury research has now come of age. Because of general progress in neuroscience, as well as specific advances in spinal cord injury research, researchers can test new ideas about how changes in molecules, cells, and their complex interactions determine the outcome of spinal cord injury. Inspired by demonstrations that spinal cord nerve cells can regrow, researchers are learning to manipulate trophic factors, intrinsic growth programs, and growth inhibitors to encourage regeneration.
Overcoming spinal cord injuries will require general progress in many fields of neuroscience. The future in spinal cord regeneration seems rather promising.
Acknowledgement: The author acknowledges the contributions of Medical student Visalachi Murugesan from Melaka Manipal Medical College.