Anatomy 101: Potential Treatments
Topics:
Immune System Modulation
The body's natural response to spinal cord injury is not ideal for recovery of function. Indeed, there is good evidence that the body's immune response results in even more damage following the trauma. After an injury, there is a molecule, called IP-10, that recruits certain immune cells into the injury site. These first responders are the clean up crew. They are the cells that kill bacteria and remove debris from the injury. Unfortunately, they also kill neurons and other cells that are in shock or damaged but could survive.
One approach to preventing the damage caused by the immune system is to prevent destructive immune cells from entering the injury site. UCI researcher and Center Associate Tom Lane found that the IP-10 molecule brings killer T cells and macrophages into the injury site in a multiple sclerosis model. Center researcher Hans Keirstead has found that IP-10 is also the molecule that recruits these destructive cells into a spinal cord injury.
Working together, Keirstead and Lane have found that if you block IP-10, you can prevent a large amount of the secondary damage, that is the tissue destruction inflicted by the injured individual's own body. This treatment needs to be given immediately after the injury, within hours of injury.
This research has resulted in the production of a human anti-body that blocks IP-10. Even more exciting is the finding that when human cells are placed in a petri dish, the human anti-body neutralizes IP-10 and stops the recruitment of killer T cells and macrophages, in short shutting down the damaging parts of the immune response. This experiment was done with all human cells and is very encouraging for future treatments.
Medarex Inc. now owns this therapy and is taking it to clinical trial in immune system mediated bowel disease. While not in SCI, the Medarex trial will provide important safety date that will allow for the translation to Sci more rapidly. The Reeve-Irvine Research Center is planning an SCI specific clinical trial as well.
Methylprednisolone
High-dose methylprednisolone is the only FDA approved treatment for acute SCI in the U.S. Methylprednisolone is a glucocorticoid that has been used in the acute treatment of SCI for 20 years. Based on the observation that the use of this drug could produce improved function in individuals suffering from brain edema (a build up of fluid that causes swelling), neurosurgeons had used methylprednisolone on an empirical basis in an attempt to treat edema following spinal cord trauma. Research from the late 1970's and 1980's culminated in the first randomized placebo-controlled large-scale trial of acute SCI treatment to yield positive results. Patients whose methylprednisolone therapy was began within 8 hours of injury showed improvements in motor and sensory scores when compared with placebo (sugar pill) counterparts at 6 weeks, 6 months, and 1 year after injury. The use of high-dose methylprednisolone has become widespread in the United States for the treatment of acute SCI.
High-dose methylprednisolone has become controversial in recent years with questions of efficacy versus the side effects.
Early Surgical Decompression and Stabilization
Decompression surgery after SCI is done to relieve pressure on the
spinal cord, pressure that likey causes additional tissue damage.
The extent of disability resulting from a traumatic SCI is dependent not only upon the neurologic damage, but also on the quality of acute surgical and rehabilitative care. A study in 1999 by The Surgical Treatment for Acute Spinal Cord Injury Study group, or STASCIS, revealed inconsistencies in whether individuals had surgery after SCI, and this they did, when they had the surgery. About one third of the cases sampled did not have surgery, while of the two thirds of people who had surgery after injury, timing varied from less than 24 hours to more than 5 days. These results spurred the group to undertake a prospective, multi-center study in North America to determine the effectiveness of early surgical decompression and stabilization procedures for reducing the possibility of further damage to the spinal cord. Specifically, they are comparing acute decompression of the cervical spinal cord (C3-T1) within 8 hours of acute SCI with the more standard treatment of decompression after 24 hours of medical stabilization, and improvements in clinical neurological outcome are measured. This study is currently underway.
Macrophage Therapy
There is a hot debate among researchers right now as to whether macrophages are good or bad following spinal cord trauma. Some think that they are helpful and their role should be supported after injury, while others think they are destructive and should be blocked following injury.
Based on work from Michal Schwartz, Ph.D., at the Weizmann Institute in Israel, the company Proneuron has launched the ProCord trial, an international, multi-center, randomized-controlled, Phase II clinical trial for complete SCI. In this study, macrophages are taken from the blood of patients, activated in the laboratory and then injected into the injury site to increase the number of macrophages available for cellular clean up. Proneuron believes that these macrophages will help heal and support regeneration, ultimately leading to improved function after spinal cord injury.
On the other hand, several Reeve-Irvine Research Center researchers, including Drs. Keirstead and Lane and Dr. Aileen Anderson, and others in the SCI research community are significantly slowing the immune response and blocking macrophages and killer T cells, from entering the injury site, with the goal of significantly decreasing secondary damage. Strong evidence from animal models shows that when macrophages and T cells are slowed or blocked, secondary damage is decreased and recovery after injury is improved.
Which approach for dealing with macrophages in the spinal cord is the right one? It may be that different treatment techniques result in different immune responses or different molecule profiles for the cells that move into the injury site. There are also time course differences. The ProCord therapy is given within 3 weeks of injury, while Keirstead's therapy is given within hours. There are conflicting views and quite a bit of data that seems contradictory. It's a story that will unfold with continued research.
Inhibitory Molecules
Nogo
As a baby, your body had to grow
the connections that make up your
nervous system. Once the right
connections are made, an off switch is needed to stop
the developmental growth.
Myelin is the insulation that wraps around the axon of a nerve cell and allows for electrical messages to be sent to other nerve cells. Over 15 years ago, Martin Schwab's group in Zurich discovered that the myelin sheath of axons was inhibitory for axon regeneration following injury. That is, myelin gives off molecular stop signals that prevent damaged nerve cells from regrowing. Schwab's group went on to identify a number of inhibitory molecules including one they called "Nogo". Since that time, Schwab's group and others have developed several strategies to try to eliminate the inhibitory action of Nogo so as to improve regeneration, but most scientists assumed that the best strategy would be to find some way to remove Nogo entirely.
Nogo is a protein molecule that is encoded by a gene, and one of the great achievements in science is the ability to create animals (specifically mice) in which particular genes have been deleted. Several researchers, including Dr. Steward at the Reeve-Irvine Research Center, have created mice in which the gene encoding Nogo had been deleted to see if axon regeneration is improved after spinal cord injury. Research is underway to find therapies for humans that block Nogo.
Rho
Like Nogo, Rho also prevents regeneration after injury, however Rho has also been implicated in increased death of neurons and other central nervous system cells. Research by Dr. Lisa McKerracher, McGill University in Montreal Canada, has shown that blocking Rho improves movement and sensation after spinal injury. BioAxone has begun a Phase I/IIa clinical trail using Cethrin ®, which inhibits Rho and improves regeneration and cell survival in animal models.
The BioAxone clinical trial involves the application of single dose of Cethrin ® to the spinal cord during the surgery that often follows a spinal injury. Patients are being recruited from multiple sites in Canada and the United States.
Cell Replacement Strategies
Olfactory Ensheathing Glia
Olfactory neurons, which allow mammals to sense smells, are one of the very few central nervous system (CNS) nerves that regenerate normally. This regeneration is aided by cells called olfactory ensheathing glia or OEGs. Several studies have found that OEGs can help regeneration after spinal cord injury when used as a combination therapy. For example, Mary Bunge, Ph.D., of the Miami Project, has demonstrated that implanting Schwann cells (myelin makers outside the CNS) to act as a bridge through the injury site together with OEGs to help regenerating axons move through and beyond this bridge into healthy spinal cord above or below the injury results in improved locomotor abilities. The combination of 2 different implants holds great promise and may eventually become part of new clinical treatments for spinal cord injury.
Neural Derived Stem Cells
Reeve-Irvine Research Center researchers Brian Cummings, Aileen Anderson and colleagues have used human neural stem cells to successfully regenerate damaged spinal cord tissue and improve mobility in mice.
In their study, Drs. Cummings and Anderson injected human fetal derived neural stem cells into mice with limited mobility due to spinal cord injuries. These transplanted stem cells turned into new oligodendrocytes that restored myelin around damaged mouse axons. Transplanted cells also became new neurons that formed synaptic connections with mouse neurons.
Mice that received human neural stem cells nine days after spinal cord injury showed improvements in walking ability compared to mice that received either no cells or a control transplant of human fibroblast cells (which cannot become nervous system cells). Further experiments showed behavioral improvements after either moderate or more severe injuries, with the treated mice being able to step using the hind paws and coordinate stepping between paws whereas control mice were uncoordinated.
Moreover, the cells survived and improved walking ability for at least four months after transplantation. Sixteen weeks after transplantation, the engrafted human cells were killed using diphtheria toxin (which is only toxic to the human cells, not the mouse). This procedure abolished the improvements in walking, suggesting that the human neural stem cells were the vital catalysts for the maintained mobility.
The human neural stem cells are different from embryonic stem cells in that they are multipotent and will only become nervous system tissue, that is neurons, astrocytes, and oligodendrocytes. These cells are not directed in anyway before transplantation, however they become the cells around them after transplantation. If there are myelin makers around them, that is what they become, while if there are neurons around them that is what they become. Moreover, extensive research has shown that these cells do not become tumors. Drs. Cummings and Anderson's work was published in the prestigious Proceedings of the National Academy of Science
Human Embryonic Stem Cells
Dr. Hans Keirstead's group at the Reeve-Irvine Research Center, working in collaboration with Geron, Inc., has published two papers that bring us closer to being able to use human embryonic stem cells to repair the injured spinal cord. The first paper describes a technique to produce high purity populations of "oligodendrocytes" (the myelin forming cells of the central nervous system) from human embryonic stem cells, and shows that these cells are actually capable of regenerating myelin in the spinal cord. The second paper describes a "proof of principle" experiment in which the oligodendrocytes were transplanted into rats with spinal cord injuries.
Myelin is critical for signal transmission in the nervous system because it creates the essential insulation around axons (like the insulation on an electrical cord). One of the factors that contribute to the loss of function after spinal cord injury is demyelination and death of oligodendrocytes, leaving axons without their critical insulation. Thus, a potential therapy involves replacing oligodendrocytes that could regenerate myelin.
Keirstead's group has been working to find ways to cause human embryonic stem cells to develop into oligodendrocytes. Embryonic stem cells can become any cell type in the body, and their promise lies in the ability to tailor-make cellular treatments for different diseases and disorders, heart muscle for heart disease, pancreas cells for diabetes, or nervous system cells for spinal cord injury. Stem cells are relatively new on the research scene; it was only in 1998 that the techniques were developed to isolate stem cells from humans, and we have a lot to learn about how to make the cells develop in the ways that will be essential for therapeutic application.
To use embryonic stem cells for therapy, it is critical to devise ways to cause them to turn into particular cell types; otherwise, they will become many different types of cells. For example, if un-differentiated stem cells are transplanted into the brain or spinal cord, they may become a teratoma, a tumor made of many different cells like bone, muscle, and hair. So, to be useful for therapy, embryonic stem cells must be "restricted" to differentiate into the desired cell types. That is, they must be told what specific cell type to turn into as they mature.
Dr. Keirstead's group developed a way to make high purity oligodendrocytes from human embryonic stem cells, by treating the cells with mixtures of growth factors, and then growing the cells under very specific conditions. Reported in the peer-reviewed journal Glia, this is the first time that a high purity population of any cell type has been developed from human embryonic stem cells. In the culture or petri dish, 98% of the stem cells become oligodendrocytes. The remaining 2% is still a concern, however, because even a single cell that is still "un-restricted" has the potential to develop into a tumor. Keirstead and colleagues found that most of the remaining 2%, were neural progenitor cells, but still, a small number could still be "un-restricted". They showed that the oligodendrocytes restored myelin when transplanted into mice that have a mutation that causes them to lack myelin, and did not produce tumors within the testing period of several weeks. Experiments to ensure that the cells are safe over longer time periods are ongoing. This work shows that stem cell derived oligodendrocytes can survive in a living organism and do what they are supposed to do (that is, form a myelin sheath around axons).
In the second paper, published in the Journal of Neuroscience, Dr. Keirstead and colleagues transplanted the oligodendrocyte precursors into rats with a partial spinal cord injury, and tested the animals walking ability. By testing the cells in a partial injury paradigm, there was a better chance of seeing any improvement that might occur. Control (un-treated) animals with this type of injury could walk, often in a coordinated fashion, but sometimes stumbled and placed their feet in abnormal positions. . Rats that received stem cell transplants 7 days after an injury showed significant improvements in walking ability, however. The treated were nearly always coordinated while walking, and usually placed their feet in the correct position while stepping, although they were still somewhat impaired in comparison to normal animals. It remains to be seen whether the transplants can improve function after more serious injuries.
Unfortunately, when the same treatment was tried in animals 10 months after injury, there was no recovery in the transplanted animals as compared to animals that did not receive the cells. Significant effort is being put into understanding why this treatment failed at a chronic time point. Preliminary data suggests it might be due to the fact that a different type of glial cell (an astrocyte) wraps around the de-myelinated axons at longer post-injury intervals, preventing re-myelination by the transplanted oligodendrocytes. Work is underway to try to overcome this problem.
Sometimes non-scientists wonder why it takes so long to publish findings. The reason is that important papers undergo a high level of scrutiny during the review process. For example, this latest study was published in the Journal of Neuroscience, which is the flagship journal of our field. The first time the paper was submitted, the reviewers wanted additional analyses to further document the key findings. Eventually, this paper underwent 3 rounds of review and revision before it was finally accepted. This is the kind of careful process that the best journals undertake to ensure that scientific reports are as complete as possible.
Together, the two papers demonstrate that human embryonic stem cells can be turned into a clinically useful cell type and that when oligodendrocyte progenitor cells are transplanted into animals with injury, the cells move to areas where they are needed, they make myelin, and the result is a significant improvement in walking ability, at least at an early time point post injury. These results are important steps on the path to spinal cord injury therapy.
