Research
Update on Spinal Cord Injury Regeneration Research Mark Tuszynski, M.D., Ph.D. Director, Center for Neural Repair University of California, San Diego
Achieving regeneration of the spinal cord is a difficult challenge. Yet the pace and amount of research is strong, and the last few years have given rise to several breakthroughs in animal research that yield real promise for the development of human therapies. This article will review recent research in the field of spinal cord injury regeneration, and will try to provide a realistic assessment of how and when some of this research may find its way to human clinical trials.
Why doesn't the spinal cord regenerate?
When a nerve in the arm or leg (a "peripheral nerve") is injured, it often regenerates successfully. Yet the spinal cord does not regenerate. In the last ten years, scientists have made significant progress in understanding how peripheral nerves successfully regenerate, and this has yielded important clues for generating strategies to promote spinal cord regeneration
Peripheral nerves regenerate for at least five reasons:
- Growth-stimulating proteins, called "nervous system growth factors", are produced by support cells in injured peripheral nerves. Growth factors are not produced in sufficient quantities, at the appropriate time, or in the appropriate location, in the injured spinal cord.
- "Bridges" are produced in the peripheral nerve after injury to support the regeneration of injured connections (called "axons"). In the injured spinal cord, bridges are not produced after injury. Instead, the site of the injury becomes filled with either fluid or a scar that does not support the re-growth of injured connections (axons).
The peripheral nerve is sub-divided into individual compartments or tubular structures, unlike the spinal cord. Thus, when the peripheral nerve is injured, connections can be guided back to their natural targets by these tubes. Such guidance tubes are not present in the spinal cord. - Not only does the spinal cord lack growth factors, bridges, and guidance, but substances are present in the normal, intact spinal cord that actively inhibit the re-growth of connections. These inhibitors probably help to maintain the complicated organization of the normal, intact spinal cord. However, after injury these inhibitors "inadvertently" impede regeneration. Inhibitors to regeneration are not present in peripheral nerve.
- Finally, injured cells from peripheral nerves appear to make proteins that assist in regeneration that are not necessarily produced by cells in the brain and spinal cord. Thus, the intrinsic capacity of cells in the spinal cord to regenerate may be less that than of cells that give rise to peripheral nerves.
Research to Promote Spinal Cord Regeneration The above 5 factors that appear to support successful growth of peripheral nerves suggest clear strategies for stimulating regeneration of the spinal cord. These strategies can be grouped into 5 categories:
- Providing growth factors to the injured spinal cord.
- Placing "bridges" for axon growth in the injured spinal cord.
- Providing guidance to injured axons.
- Neutralizing inhibitory substances in the spinal cord.
- Stimulating the production of proteins within cells that increase growth.
- Growth Factors, Bridges and Gene Therapy
In 1994, our team of researchers at the University of California – San Diego in collaboration with colleagues at the Salk Institute began experiments in which growth factors were delivered to sites of spinal cord injury in rats. To deliver growth factors precisely to the sites of injury, we used techniques of genetic engineering. Cells were taken from the skin of injured adult rats and were genetically engineered to produce large quantities of nervous system growth factors. These growth factor-producing, genetically modified cells were then put into a collagen gel to make a "bridge", and were placed into sites of spinal cord injury. As a result, injured connections (axons) of the spinal cord extensively grew into the growth factor-producing bridges. In 1997, we found that this approach could result in partial functional recovery in spinal cord-injured rats. In subsequent years, we and other scientists tested the growth-promoting effects of several different types of nervous system growth factors. We now have a good idea regarding which growth factors can elicit the best degree of growth of different classes of connections (axons) in the spinal cord.
These experiments have not yet led to clinical trials for two reasons: 1) because until very recently we lacked a clinically practical "bridge" in which to implant these genetically engineered cells in humans with spinal cord injury, and 2) because connections (axons) grew into, but not beyond, the spinal cord injury site. We now believe that a clinically practical bridge may be available. Further, in the last year an experimental method has been developed that could potentially stimulate connections (axons) to grow beyond the injury site. We and others are currently testing these recent advances in rats with spinal cord injuries. In addition, experiments are beginning in larger animals to determine whether these experimental strategies will be practical for testing in humans. - Other Bridges
Peripheral Nerve Bridges: Perhaps the first modern success in promoting regeneration of the injured spinal cord was reported by Sam David and Albert Aguayo in Canada in 1981. They placed small pieces of peripheral nerves into the injured spinal cord, and found that spinal cord axons could use the peripheral nerve as a bridge to support the long growth of small numbers of axons. This approach is still being utilized in experimental studies today, with various modifications. For example, combinations of peripheral nerve bridges soaked in growth factors have been studied in rats with spinal cord injury. Based on the reported success with this approach in rat experiments conducted in Sweden in 1996, several humans with spinal cord injury in Brazil and Taiwan underwent an attempt at spinal cord repair using peripheral nerve bridges over the last several years. Doctors in Brazil reported that their patients did not improve. Unfortunately, reliable reports regarding patient outcomes have not been forthcoming from the studies in Taiwan . In fact, several groups of scientists attempted unsuccessfully to repeat in rats the experimental findings of the Swedish researchers, leaving this branch of research with some uncertainty.
Fetal Spinal Cord Bridges: Beginning as far back as the early 1900's, scientists attempted to place fetal spinal cord tissue into the injured adult rat spinal cord to promote regeneration. Generally, most experiments found modest if any benefit from this experimental approach. Notwithstanding the mixed success of fetal grafting, researchers at the University of Florida - Gainesville began human clinical trials of fetal spinal cord transplantation a few years ago. The first patients to undergo this experimental therapy had an additional condition that occurs in approximately 10% of people with spinal cord injury: "syringomyelia", or a progressive enlargement of the original cyst-like lesion cavity in the spinal cord. To date, the researchers in Florida have not reported dramatic success with their clinical trial of fetal grafting, although the studies are still in progress. Very recently, scientists at Georgetown University combined fetal grafting with growth factor treatment in rats with spinal cord injury, and reported partial functional recovery. These experiments are continuing.
Stem Cells: Stem cells are early-stage, "undifferentiated" cells of the body that that have the potential to become various types of mature cells in the body. Stem cells can be obtained from fetuses or, in fact, from adult mammals (including humans). In spinal cord injury, stem cells offer the potential to constitute bridges at sites of injury over which injured connections (axons) could extend. Stem cells might also replace injured host cells to hypothetically directly restore the function of lost cells. Beginning in 1999, researchers at Washington University in St. Louis reported that transplants of stem cells to rats with spinal cord injury could partially restore function, perhaps by stimulating the activity of spared but non-functional connections in the injured spinal cord. A number of different research groups are actively studying the potential of stem cells for spinal cord injury, and indeed clinical trials of stem cells are planned in St. Louis . This research is still in early stages in animals; whether stem cells will live up to their "potential" will be discovered in continued, active research
Schwann Cells: Schwann cells are support cells of the peripheral nerve which contribute substantially to repair after injury. Several researchers, including scientists at The Miami Project, Yale, and in our laboratory at UCSD, have shown that transplants of Schwann cells to the spinal cord can support the re-growth of connections (axons) after spinal cord injury. Based on the growth and repair-inducing properties of Schwann cells, they are currently being tested in a clinical trial in patients with multiple sclerosis at Yale University . In the clinical trial, Schwann cells are obtained from the patients themselves after nerve biopsies, and are implanted into the brain into sites of multiple sclerosis-related demyelination (loss of nerve insulation). Schwann cells remain a topic of active study in spinal cord injury, and clinical trials of transplants of Schwann cells in patients with spinal cord injury may begin in the next several years. Unfortunately, functional recovery is only modest in rats that receive transplants of Schwann cells after spinal cord injury, and additional animal research is required to amplify the regeneration-promoting effects of Schwann cell transplants. Combining Schwann cells with growth factors may ultimately be more useful than transplants of Schwann cells alone.
Olfactory Ensheathing Cells: Olfactory ensheathing cells are a cell type that is present in the only part of the adult brain that naturally regenerates: the olfactory (smell) system. In this odor-detecting part of the brain, olfactory ensheathing cells help to guide connections (axons) to their appropriate targets. Beginning in 1997, two different sets of scientists reported that transplants of olfactory ensheathing cells to the injured spinal cord promote moderate regeneration and functional recovery. Other scientists are attempting to repeat the success of these first experiments. Whether olfactory ensheathing cells truly have the potential to promote regeneration after spinal cord injury remains to be established, and these cells are a subject of current active study.
Synthetic Guidance Channels: In addition to the biological guidance channels described above, several researchers are examining whether synthetic substances such as polymers and other "matrix" materials, possibly combined with regeneration-promoting substances such as growth factors, can be used to promote spinal cord regeneration. To date, only modest success has resulted from these approaches. However, rapid and impressive advances in chemistry, biochemistry and biomedical engineering make this an intriguing and potentially important field that could yield significant progress in the future. - Providing Guidance to Injured Axons
Most researchers in the field of spinal cord injury believe that axons that successfully bridge the gap of a spinal cord injury will restore function to at least a partial degree, without requiring a substantial degree of additional "guidance" to a target. This belief is based upon the observation that other areas of the brain "re-wire" themselves after injury in a manner that can restore at least some function – for example, after a stroke. In addition, when axons bridge an experimental injury site in the brain, they appear at times to be able to locate generally correct targets when given several different choices.
Will this optimism regarding guidance prove to be correct? At present, we do not know. Most regeneration research continues to focus on stimulating axons to grow, and relegates guidance to a topic of secondary importance. We hope that this faith is not mis-guided. - Neutralizing Inhibitors
Several different types of natural substances exist in the spinal cord that actively block the regrowth of connections after injury. Some of these inhibitors to growth are found on cells that insulate connections (axons) in the spinal cord ("myelin-associated inhibitors"), and some are found in spaces between cells ("inhibitory extracellular matrix molecules"). Researchers in Switzerland , Canada , Ohio and UC-Irvine and other locations have investigated potential means of neutralizing these inhibitory substances to enhance spinal cord regeneration. To date, modest degrees of functional recovery have been reported after these experimental strategies in rats. To be optimally useful, however, it is likely that inhibitor neutralization approaches will need to be combined with growth-enhancing strategies described above. - Stimulating the Production of Proteins Within Cells that Increase Growth
One question that the research of Sam David and Albert Aguayo answered in 1981 was whether connections (axons) of the adult spinal cord could grow at all: clearly, they could. But are cells of the brain and spinal cord equally intrinsically capable of growing connections as, for example, cells that make up the peripheral nerve? The answer to this more subtle question appears to be "no". Research for a number of years, however, has shown that a spinal cord nerve can be "primed", or "pre-conditioned", to enhance its regeneration to a level more similar to that of a peripheral nerve. The process of priming appears to consist of increasing the levels of certain proteins (or, turning "on" the genes that make proteins) in cells that subseaquently lead to enhanced growth. Much research is currently being devoted to this topic in the spinal cord injury field. Scientists are attempting to discover which genes and proteins are the most important for increasing the capacity of cells to regenerate, and then using techniques such as gene therapy to introduce those genes into injured cells.
One type of protein that is of particular interest in potentially increasing regeneration is an energy-related protein called "cyclic AMP". When present in high levels in cells, this protein appears to increase the capacity of a cell to regenerate.
Summary
Hopefully it is evident from the above that a great deal of new knowledge related to spinal cord regeneration has been gained in the last several years. Indeed, several initial approaches have begun clinical trials, although it is likely that the most promising approaches are yet to come.
What can reasonably be expected to emerge from clinical trials that will attempt to promote spinal cord regeneration? In my opinion, upcoming clinical trials will initiate a process of generating small improvements in function over just one or two spinal segments. For example, a person with a clinical C5-level injury may regain some C6-level function; this means that an individual who has spared biceps function and a wrist that is fixed in partial flexion may, after treatment, achieve movement of the wrist. Or an individual with a C7 level injury, who can move the wrist but not the fingers, may regain some finger control. Although this may not sound like much at first, such regeneration could significantly improve quality of life. Further, these early efforts will lead to modifications and refinements in our experimental treatments that may subsequently restore more significant levels of function. Realistically, this is likely to be the pattern of our first attempts to promote regeneration in people with chronic, stable injuries.
Yet for those who were injured several years ago, this "realism" sounds dramatically different from the "realism" that they were confronted with at the time of their initial injury. This is realism wrapped in optimism. This is a realism that relays a different vision: that regeneration will be possible, that it will be implemented clinically within a likely time frame of years rather than decades, and that there is hope beyond this for further improvement.
This hope is bred from the continuation of high quality research. One means through which everyone can help this cause is to support research into spinal cord injury regeneration. Here in California , through the efforts of a number of dedicated individuals, the California legislature passed the Roman-Reed Bill in the year 2000. The Roman-Reed bill provided funding of $1 million per year for spinal cord research in California . In the year 2001 (only), that amount was supplemented by an additional $1 million. And we can make a difference at the local level as well: here in San Diego , the Tony Mezzadri Surf Contest raised $50,000 in 2001 to support regeneration research. The efforts of individuals, foundations and charities in raising funds for regeneration all makes a difference, resulting in more knowledge and more discoveries that are essential in generating promising, worthwhile and sensible regeneration strategies.