A STRIKING animated logo adorns the Web site of the Miami Project to Cure Paralysis, a leading research centre on the repair of spinal injuries at the University of Miami. The familiar symbol of a person in a wheelchair, usually seen on ramps and parking spaces, leaps to its feet more than once every second, for as long as you can bear to watch. It鈥檚 neat, memorable, and it expresses the excitement of a science that is gathering pace in the full glare of the media. But the image is jumping a little ahead of reality. About 20 years after scientists first began to promise that shattered spines could be healed, a few rats have been cured, but for people, paralysis is still paralysis.
If, on the other hand, you accept that the pace of real research into such a tricky problem must be more modest, then scientists have come a long way even in the past five years. They now have sophisticated tools at their disposal, including tailor-made cells cultured from clones of primitive stem cells-cells that can turn into any cell in the body. Some who once doubted that repairs could ever succeed are now preparing to test experimental therapies on people. And, although few would underwrite Christopher Reeve鈥檚 vow that he will walk by 2005, many now believe that there will be treatments to restore some degree of control and sensation within the foreseeable future. 鈥淚 am conservative, but I do think that the spinal cord is fixable,鈥� says Paul Reier, at the University of Florida in Gainesville, the first researcher to test experimental spinal-repair techniques on people.
First, you need to know what you鈥檙e up against, say most researchers. The central nervous system (CNS) in mammals, made up of the brain and spinal cord, is among the most complex structures found in living organisms. So simple-minded approaches to sticking it back together after a messy crush or a cut are doomed to fail, even if they look promising in the tidier conditions of a laboratory experiment. 鈥淓volution has honed the CNS over millions and millions of years,鈥� says Geoffrey Raisman, a neurobiologist at the National Institute for Medical Research in London. 鈥淚t is not about to yield to a sledgehammer.鈥� Indeed, says Raisman, it is only by developing a healthy respect for the complexity of the system, and by working with its own inherent capacity for repair, that researchers are likely to make progress.
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Any biology textbook will tell you that, whereas the tissues of the peripheral nervous system will regenerate after injury, fully developed CNS tissue will not. In fact, this is an oversimplification, as neurobiologists have known for about a century. Neural fibres in the spinal cord do, after all, sprout when they are cut. But the sprouting fibres travel only short distances, whereas some repairs, say a cut in the neck or lumbar region of the cord, might need neurons to grow half a metre. And-crucially-they fail to form the necessary connections to restore lost function.
Some scientists think that neurons fail to form new connections because they are inherently unable to grow more than a few tenths of a millimetre. Some say that conditions in the spinal cord are hostile to new growth or that scar tissue at the injury site forms an impenetrable barrier. For others, the problem is the destruction of the tracts along which the fibres run, rather than anything to do with the neurons. As befits their different views, scientists have chosen many different approaches, but most of them try to exploit those parts of the nervous system that are more plastic than the adult CNS.
The will to grow
The first of these ideas-that neurons lack the will to grow-is rapidly falling out of favour. But most researchers agree that the CNS needs help to regenerate. Clifford Woolf and others at the Massachusetts General Hospital have shown that certain fibres in the adult CNS can be 鈥渟witched鈥� from minimal growth to a much more vigorous growing mode, simply by cutting through a part of the cell that projects into the peripheral nervous system. Woolf thinks that this cut may activate certain genes that encourage growth, and hopes to find these genetic switches (快猫短视频, 5 June, p 3).
Reier helps the cells out in a different way, borrowing from the CNS of the fetus, which readily regenerates after cutting. In studies in rats and cats, Reier and his colleagues implanted tissue from the fetal CNS into the injured spinal cords of adult animals. The transplanted fetal tissue contained neurons and glia-the supporting cells that surround the cell bodies and nerve fibres. The severed adult fibres grow and the transplanted cells integrate well into the surrounding spinal tissue.
But, while Reier is certain that fetal tissue experiments have transformed scientists鈥� understanding of the way the CNS develops, he doesn鈥檛 think fetal tissue will ever be widely used in therapy, not just because of ethical doubts, but also because it is difficult to get fetal tissue at the right stage of development.
Nevertheless, he has moved ahead with his first human trials of an experimental therapy for a form of spinal injury. Sometimes a fluid-filled cyst forms where the spine has been crushed or severed. This cyst tends to get bigger and create further damage-a condition known as post-traumatic progressive syringomyelia. Reier鈥檚 team has transplanted human fetal CNS tissue into the spines of seven people with the condition over the past two years. He still has to announce his formal results. 鈥淏ut in some of these individuals we do see changes that represent degrees of improvement,鈥� says Reier. Whether the improvements are a direct effect of the implants or whether there are other causes, however, is not yet clear.
Reier intended his implant to bridge the break and relay impulses along the spinal cord. At the University of Massachusetts, Worcester, Charles Vacanti is working on the same principle but with a different technique that attracted widespread media attention after he described his unpublished findings at a meeting in London in July. Vacanti, already famous for engineering cartilage in the shape of an ear and grafting this onto a mouse鈥檚 back, made a polymer scaffold and seeded it with cultured immature nerve cells. He implanted this scaffold into paralysed rats鈥� spinal cords and, he says, some have regained near-normal movements. However, other scientists are reserving judgement until they have seen more details of the work.
In a different approach, a team led by Fred Gage at the Salk Institute for Biological Sciences in San Diego is trying to persuade injured neurons to grow farther and faster. They transplant skin cells called fibroblasts into the injury site. These fibroblasts have been genetically modified to express growth factors such as neurotrophin-3 (NT3) and brain-derived neurotrophic factor (BDNF). These growth factors seem to trigger the growth of new fibres, and they also help to stimulate the production of myelin, a fatty coating that wraps around nerve fibres and prevents impulses from short-circuiting.
Philip Horner, who leads the group working on spinal injuries in Gage鈥檚 laboratories, says he is 鈥渃autiously optimistic鈥� that the approach will coax neural fibres to grow the required distances. But he admits that there are still problems with ensuring that the growth factors switch off when and where they are not needed, and that they do not trigger destructive immune responses.
Held back
At the University of Zurich, in Switzerland, meanwhile, Martin Schwab has a completely different approach. He was the first to suggest, in the mid-1980s, that the regeneration problem was due to factors secreted in the CNS that were holding the neurons back. Schwab鈥檚 team went on to find and purify the culprits: proteins known as neurite inhibitory factors, such as NI-250, which is secreted by myelin. NI-250 binds to the surface of the growing neural fibres in a similar way in both rats and people. Once it has docked, it triggers a cascade of reactions that ultimately stop production of actin, an essential component of the cell skeleton. The skeleton collapses and forward growth is halted.
Schwab reasoned that, if you could somehow restrain the inhibitory proteins, neural fibres might regrow more easily. In animals at least, his hunch seems to have been right. Rats with a cut in their corticospinal tract-the largest and best defined pathway of fibres connecting the cortex with the motor machinery of the spinal cord-were given a monoclonal antibody, IN-1, that blocks NI-250. Within 12 weeks, a proportion of the severed fibres had grown again, for 鈥渟everal centimetres鈥�, says Schwab. More importantly, the animals regained functions they had lost with the injury. 鈥淭heir walking patterns improved tremendously,鈥� he says. He has now moved on to test the antibody in monkeys. 鈥淚f we don鈥檛 stumble, we鈥檒l be at the level of clinical trials in the not-too-distant future,鈥� he says.
This summer, Schwab reported at a conference in Jerusalem that he had cloned the gene responsible for NI-250, now re-named NOGO-A, paving the way for more sophisticated therapies. But the move from rats into monkeys and ultimately people is not simple, as Schwab is keen to stress. One worry is that severed sensory neurons treated with the antibody might form inappropriate connections so that ordinary touch feels like pain. This condition, known as neurogenic pain, is all too familiar to one in five paraplegic people (see 快猫短视频, 9 March 1996, p 27). In Schwab鈥檚 rats, so far, it doesn鈥檛 seem to be a problem. Nevertheless, the worry remains.
Another concern is how to put the brakes back on once the damaged fibres have regenerated. In Schwab鈥檚 technique the pumps or cell implants that secrete the antibody in rats can be removed or destroyed after a set period, but developing an equivalent technology that will work in people is complicated. And there鈥檚 a third problem-how to prevent the risk of an immune reaction to the antibody, which at the moment contains mouse components. If this happened, immune cells could attack and destroy perfectly healthy spinal tissue, leaving the person worse off than before. Schwab鈥檚 team is now working on an all-human antibody, but it won鈥檛 be ready tomorrow.
While Schwab acknowledges all these potential problems, he is readier than most to talk about treating spinal injuries in the near future. 鈥淢any people feel that our approach is the closest to the clinic, and probably the most solid,鈥� he says. His peers recognise how important his work is, but some warn that it is still early days. 鈥淭his is not a one-molecule situation,鈥� says one. 鈥淚 think that the idea that infusions of an antibody alone can overcome chronic injury is too simple,鈥� says Naomi Kleitman at the Miami Project. Schwab readily admits that there is still a long way to go.
For Jerry Silver, a neurobiologist at Case Western Reserve University in Ohio, there鈥檚 another explanation for the hostility of the environment surrounding the sprouting neuron. He believes that the principal problem is scarring, blocking the path of new fibres. One type of glial cell, astrocytes, gather at the injury site and release substances known as proteoglycans which, he believes, are critical in stopping fibres in their tracks. He finds that, in mice, microtransplants of adult neurons can grow through an old injury scar, if they are helped on their way by gently using a microscopic needle that causes only minimal damage. The fibres grow several centimetres and form new connections, he says. 鈥淭his is strongly compelling evidence that the glial scar is the problem,鈥� he says.
For Raisman鈥檚 group in London and Mary Bunge鈥檚 group at the Miami Project, meanwhile, there are strong clues that it is more important to rebuild the pathways damaged by an injury than to help the neurons to grow. Raisman has closely studied the structures that form when the corticospinal tract is damaged. Several different types of glial cell arrange themselves in an intricate, regular pattern (see Diagram) that is highly complex and looks completely different, he says, from anything seen in the uncut tract. There are astrocytes, which migrate to injury sites and seem to inhibit the growth of neurons; oligodendrocytes, cells that lay down myelin; and microglia, cousins of macrophages, the defence cells that clean up unwanted debris in peripheral tissues. There are also endothelial cells, which normally line blood vessels. 鈥淚t鈥檚 a whole interactive community,鈥� says Raisman. How can such a complex system be persuaded to help new fibres grow and connect?
In an attempt to coax some new connections, both teams initially tried implanting Schwann cells-the peripheral nervous system鈥檚 form of the glial cell-into the injured spinal cord of rats. To their delight, the Schwann cells migrated along the existing pathways and into the injury site and formed myelin sheaths. But, encouraging as the findings were, the sprouting neurons did not grow right across a damaged area or re-connect with the severed fibres beyond.FIG-mg21994801.jpg
So Raisman turned his attention to another population of rat glial cells, which connect the nerve endings in the lining of the nose with the brain. These cells are known as olfactory ensheathing glial cells and are found only in the olfactory bulb. Because the nerves in the lining of the nose are constantly being replaced, the ensheathing cells must constantly forge new connections between them and the CNS. For Raisman, the idea was: 鈥淭hese cells are track-layers. They are clever: why not use them?鈥�
His hunch paid off. When he transplanted cells into a small injury site in the rat corticospinal tract they formed a perfect bridge. Cut fibres grew along the tracks that they laid, into the injury site and out the other side, and formed new connections. Animals treated with these transplants regained movements they had lost. The Miami team came up with similar results, with bigger injuries. They encouraged further regeneration when they added growth factors at the same time.
Both teams hope to try out the technique in people one day. Their labs, and some others, have already identified the equivalent cells in humans, although no one yet knows whether it will work as well as in the rat. But, stresses Raisman, there is a major practical problem: for a typical spinal injury in a person, the amount of tissue needed for transplant would be much greater than for a neat laboratory experiment. For that reason, he and others are keenly interested in the possibility of using stem cells.
These cells can be 鈥渢rained鈥� to become any type of CNS cell, such as a neuron or an oligodendrocyte, by creating the right conditions when the cells are growing in culture. The difficulty is persuading them to keep their new identities once they are transplanted into the spinal injury site, rather than revert to more hostile cell types, such as astrocytes. Horner鈥檚 group are working on the cells in the hope that they could one day form the basis of large-scale therapy. But, as Raisman stresses: 鈥淭his is not imminent鈥�.
While neurobiologists may agree about few things, almost all of them believe that the next generation of therapies will incorporate several strategies at once-for example, transplants and growth factors, or antibodies to inhibit brake proteins together with chemical triggers to switch on growth-promoting genes. For the immediate future, however, a less high-tech approach is causing some excitement. Several teams, including those at the Miami Project, are working on the idea of using intensive physical therapy to strengthen and hone newly formed connections in the CNS in people with spinal injuries.
Their optimism is based on the findings of a few clinical researchers, some of whose patients have shown strong 鈥渨alking鈥� reflexes even though they cannot feel or control their movements. With intensive exercise, these patients regained some sensation. It seems as if new connections-sometimes in unexpected pathways-have formed and stabilised, restoring communication between legs and brain. These findings suggest that, in some people at least, regular use of certain circuits may stimulate neurons to grow and form new connections. There is still much to do, but at least two leading laboratories are watching closely. 鈥淚 think it is very promising,鈥� says Kleitman.
But like other researchers, Kleitman stresses that there is more to treatment than achieving the obvious and glamorous goal of walking again. For some patients, a major objective of treatment might be, for example, simply to regain bladder control. Ultimately, success means individuals with better lives-something that may not, after all, be measured accurately by the number of dumped wheelchairs.