New York State Spinal Cord Injury Research Board and the New York Academy of Sciences
Making New Connections: Latest Developments in Spinal Cord Injury Research
Posted March 24, 2008
Over the past two decades, scientists have realized that the adult brain can generate new neurons, and that the spinal cord has powerful natural repair mechanisms. Such tools for healing are actively suppressed after spinal cord injury, but can be reactivated under certain circumstances.
To date, these insights have not translated into cures, and have raised many new questions: How can we gain control over the endogenous machinery of repair? If the central nervous system has ways to fix itself, why don't surviving neurons repair themselves, and axons regrow? Why don't the treatments that have shown such impressive results in model organisms help humans?
Both excitement and frustration were heard at a conference on spinal cord injury held on January 14–16, 2008 at the Academy. Potential treatments discussed included bioengineered scaffolds, cell-based therapies, small molecules that mimic axon guidance cues, neurotrophins, antibodies, robots, brain-computer interfaces, sophisticated physical therapy regimens, and neuroprotective agents.
The Christopher and Dana Reeve Foundation site has a huge amount of research, clinical and patient information, including a description of the major areas of research in spinal cord injury, a basic tutorial on the spinal cord, and information about the North American Clinical Trials Network, a coalition of hospitals that is sharing information and standardizing acute procedures. The site also has up-to-date research news.
An interview of Jack Kessler about his role as a physician, stem-cell researcher and father to a paraplegic child can be found at the WYNC Web site; information about a documentary about him and about stem cell research is at PBS.
The Christopher and Dana Reeve Foundation also hosts the Paralysis Resource Center, with detailed information about resources, research and a guide on navigating the bureaucracies that ensure rights and provide help for people with paralysis.
Apparelyzed is a U.K.-based spinal cord injury peer support website run by a tetraplegic man.
Information about the Kineassist, another robot that can be used during physical therapy to help patients regain limb function, is at the company's Web site. Scroll down the page to find many videos demonstrating the technology.
Another conference that covered some of the same material from a different perspective was the International Clinical Trials Workshop on Spinal Cord Injury held in Vancouver in 2004. (PDF, 122 KB)
"Translating Promising Strategies for Spinal Cord Injury Therapy" is a 2003 National Institute of Neurological Disease and Stroke report discussing spinal cord injury clinical trials and the need for translational research.
The New York State Spinal Cord Injury Research Board supports research into spinal cord injury.
Christopher and Dana Reeve Foundation. Paralysis Resource Guide. (Free online order)
Liverman, CT, Altevogt, BM, Joy JE and Johnson RT, eds. Committee on Spinal Cord Injury. Spinal Cord Injury: Progress, Promise, and Priorities. National Academies Press, Washington, DC.
Reeve C. 1999. Still Me. Arrow Books, New York.
Tator CH, Benzel EC. 2001. Contemporary Management of Spinal Cord Injury: From Impact to Rehabilitation. AANS Press, Rolling Meadows, IL.
Vikhanski L. 2001. In Search of the Lost Cord: Solving the Mystery of Spinal Cord Regeneration. Joseph Henry Press, Washington, DC.
Domeniconi M, Filbin MT. 2005. Overcoming inhibitors in myelin to promote axonal regeneration. J. Neurol. Sci. 233(1-2): 43-74.
Hannila SS, Siddiq MM, Filbin MT. 2007. Therapeutic approaches to promoting axonal regeneration in the adult mammalian spinal cord. Int. Rev. Neurobiol. 77: 57-105.
Kahn LE, Lum PS, Rymer WZ, Reinkensmeyer DJ. 2006. Robot-assisted movement training for the stroke-impaired arm: Does it matter what the robot does? J. Rehabil. Res. Dev. 43: 619-630.
Scholz J, Woolf CJ. 2007. The neuropathic pain triad: neurons, immune cells and glia. Nat. Neurosci. 10: 1361-1368.
Tator CH. 2006. Review of treatment trials in human spinal cord injury: issues, difficulties, and recommendations. Neurosurgery 59: 957-982.
Wolpaw JR. 2006. The education and re-education of the spinal cord. Prog. Brain Res. 157: 261-280.
Wolpaw JR, Birbaumer N. 2006. Brain-computer interfaces for communication and control. In Selzer ME, Clarke S, Cohen S, et al, eds. Textbook of Neural Repair and Rehabilitation: Neural Repair and Plasticity. Cambridge University Press, Cambridge.
Qiu J, Cai D, Dai H, et al. 2002. Spinal axon regeneration induced by elevation of cyclic AMP. Neuron 34: 895-903.
Neumann S, Bradke F, Tessier-Lavigne M, Basbaum AI. 2002. Regeneration of sensory axons within the injured spinal cord induced by intraganglionic cAMP elevation. Neuron 34: 885-893.
Schwab ME, Bartholdi D. 1996. Degeneration and regeneration of axons in the lesioned spinal cord. Physiol. Rev. 76: 319-370. Full Text
Davies SJ, Goucher DR, Doller C, Silver J. 1999. Robust regeneration of adult sensory axons in degenerating white matter of the adult rat spinal cord. J. Neurosci. 19: 5810–5822. Full Text
Bunge MB, Pearse DD. 2003. Transplantation strategies to promote repair of the injured spinal cord. J. Rehabil. Res. Dev. 40: 55-62.
Golden KL, Pearse DD, Blits B, et al. 2007. Transduced Schwann cells promote axon growth and myelination after spinal cord injury. Exp. Neurol. 207: 203-17.
Fouad K, Schnell L, Bunge MB, et al. 2005. Combining Schwann cell bridges and olfactory-ensheathing glia grafts with chondroitinase promotes locomotor recovery after complete transection of the spinal cord. J. Neurosci. 25: 1169-1178. Full Text
Barritt AW, Davies M, Marchand F, et al. 2006. Chondroitinase ABC promotes sprouting of intact and injured spinal systems after spinal cord injury. J. Neurosci. 26: 10856-10867. Full Text
Neumann S, Woolf CJ. 1999. Regeneration of dorsal column fibers into and beyond the lesion site following adult spinal cord injury. Neuron 23: 83-91.
Biernaskie J, Sparling JS, Liu J, et al. 2007. Skin-derived precursors generate myelinating Schwann cells that promote remyelination and functional recovery after contusion spinal cord injury. J. Neurosci. 27: 9545-9559.
Campos LW, Chakrabarty S, Haque R, Martin JH. 2008. Regenerating motor bridge axons refine connections and synapse on lumbar motoneurons to bypass chronic spinal cord injury. J. Comp. Neurol. 506: 838-850.
Brus-Ramer M, Carmel JB, Chakrabarty S, Martin JH. 2007. Electrical stimulation of spared corticospinal axons augments connections with ipsilateral spinal motor circuits after injury. J. Neurosci. 27: 13793-13801.
López-Bendito G, Flames N, Ma L, et al. 2007. Robo1 and Robo2 cooperate to control the guidance of major axonal tracts in the mammalian forebrain. J. Neurosci. 27:3395-407. Full Text
Petit A, Sellers DL, Liebl DJ, et al. 2007. Adult spinal cord progenitor cells are repelled by netrin-1 in the embryonic and injured adult spinal cord. Proc. Natl. Acad. Sci. USA 104: 17837-17842.
Ma L, Tessier-Lavigne M. 2007. Dual branch-promoting and branch-repelling actions of Slit/Robo signaling on peripheral and central branches of developing sensory axons. J. Neurosci. 27: 6843-6851. Full Text
Rehabilitation and New Devices for SCI
Courtine G, Song B, Roy RR, et al. 2008. Recovery of supraspinal control of stepping via indirect propriospinal relay connections after spinal cord injury. Nat. Med. 14: 69-74.
Kubasak MD, Jindrich DL, Zhong H, et al. 2008. OEG implantation and step training enhance hindlimb-stepping ability in adult spinal transected rats. Brain 131 (Pt 1): 264-276.
Chen Y, Chen XY, Jakeman LB, et al. 2006. Operant conditioning of H-reflex can correct a locomotor abnormality after spinal cord injury in rats. J. Neurosci. 26: 12537-12543. Full Text
Davies JE, Huang C, Proschel C. 2006. Astrocytes derived from glial-restricted precursors promote spinal cord repair. J. Biol. 5: 7. Full Text
Lebkowski J, Nistor G, Bernal G, et al. 2004. Transplantation of human embryonic stem cell derived oligodendroglial progenitors for the treatment of spinal cord injury. Molecular Therapy 9: S89-S90.
Hochberg LR, Serruya MD, Friehs GM, et al. 2006. Neuronal ensemble control of prosthetic devices by a human with tetraplegia. Nature 442: 164-171.
Truccolo, W, Friehs, GM, Donoghue, JP, Hochberg, LR. 2008. Primary Motor Cortex Tuning to Intended Movement Kinematics in Humans with Tetraplegia. J. Neurosci. 28: 1163-1178. (PDF, 940 KB) Full Text
Ratan RR, Siddiq A, Smirnova N, et al. 2007. Harnessing hypoxic adaptation to prevent, treat, and repair stroke. J. Mol. Med. 85: 1331-1338. Full Text
Neumann S, Skinner K, Basbaum AI. 2005. Sustaining intrinsic growth capacity of adult neurons promotes spinal cord regeneration. Proc. Natl. Acad. Sci. USA 102: 16848-16852. Full Text
Moses Chao, PhD
Moses Chao carried out postdoctoral research in molecular biology at Columbia University with Richard Axel before joining the faculty at Cornell University Medical School in 1984. In 1998, he moved to NYU School of Medicine, where he is currently Professor of Cell Biology and Physiology & Neuroscience and Coordinator of the Molecular Neurobiology Program at the Skirball Institute. Chao is currently an Editor for the Journal of Neurobiology, Molecular and Cellular Neuroscience and Experimental Neurology. He is chair of the Board of Scientific Counselors for NICHD and was appointed chair of the New York State Spinal Cord Injury Research Board by Governor Pataki in 2002. In addition, he serves as chair of the Christopher Reeve Paralysis Foundation Scientific Advisory Board and the Scientific Advisory Board for the Glaucoma Research Foundation. He is a recipient of a Jacob Javits Neuroscience Investigator Award and a Guggenheim Fellowship.
Marie T. Filbin, PhD
Marie Filbin is a Distinguished Professor and director of the Specialized Neuroscience Research Program at Hunter College, City University of New York. She received both her BSc and PhD degrees from the University of Bath, UK. During a postdoctoral fellowship in the laboratory of Gihan Tennekoon at Johns Hopkins Medical School she began working on myelin formation at the molecular level. In 1990 she joined the Biology Department at Hunter and in 1994 made the observation that another myelin protein, MAG, was a potent inhibitor of axonal regeneration. Since then she has continued to investigate the role of MAG and myelin in general in preventing axonal regeneration after injury. More recently she devised molecular approaches to overcoming these inhibitors. Currently, she is testing these findings in animal models of spinal cord injury, as well as continuing to identify novel molecular targets for potential therapeutic intervention.
Lorne Mendell, PhD
Lorne Mendell is Distinguished Professor of Neuroscience at the State University of New York, Stony Brook. The focus of his laboratory is the physiology of neurotrophins, specifically their action in modifying well-delineated circuits in the intact and injured spinal cord, including sensory input and motor output. His laboratory is investigating the effects of neurotrophins on nociceptors and nociception in rats. In previous work the group determined that administration of the neurotrophin nerve growth factor (NGF), known to be normally upregulated in skin during inflammation, produces hyperalgesia. The group is also studying the basis for the peripheral component of this hyperalgesia. Another focus in his laboratory is the action of neurotrophins such as NT-3 and BDNF on spinal reflexes and pathways in the neonatal rat. Mendell completed his PhD at MIT, is a member of the board of governors of the State of New York Spinal Cord Injury Research Board, and is a past president of the Society of Neuroscience.
Dominick Purpura, MD
Dominick Purpura was dean of the Albert Einstein College of Medicine of Yeshiva University from 1984 until 2006 and holds the additional title of vice president for medical affairs at the University. A member of the National Academy of Sciences and its Institute of Medicine, Purpura is internationally known for studies in mental retardation that demonstrated the primary involvement of certain structural abnormalities of nerve cells in the brain. Also widely recognized for work on the origin of brain waves, developmental neurobiology and the mechanism of epilepsy, he was one of two scientists in the nation to receive the first annual National Medical Research Award of the National Health Council at a White House ceremony in September 1988. He has been the recipient of the Lifetime Achievement Award for Research from the American Epilepsy Society, and a Presidential Award from the Society of Neuroscience.
Jonathan R. Wolpaw, MD
Jonathan Wolpaw is chief of the Laboratory of Nervous System Disorders and professor of biomedical sciences at its School of Public Health. His laboratory is investigating plasticity in the spinal cord, using the H-reflex as a model for new therapeutic approach to spasticity and other forms of abnormal reflex function. He is also investigating ways of translating EEG signals as a way of controlling movement of a cursor on a computer screen. Wolpow earned his MD at Case Western Reserve University, and completed postdoctoral training at the National Institutes of Health.
Marc Tessier-Lavigne, PhD
Marc Tessier-Lavigne is senior vice president of research drug discovery at Genentech, Inc. Since completing his PhD in physiology at University of London, he has held positions at Columbia University, the University of California, San Francisco, and Stanford University, where he was Susan B. Ford Professor in the School of Humanities and Sciences. He was also a Howard Hughes Medical Institute Investigator from 1997–2003. His awards include election as a member of the National Academy of Sciences, and as a Fellow of the Royal Society of the UK and the Royal Society of Canada. At Genentech Tessier-Lavigne has been working to apply knowledge about mechanisms of brain wiring and the developmental biology of tissue growth to grapple with the problem of brain rewiring and regeneration and, more recently, to developmental and tumor angiogenesis.
Mark H. Tuszynski, MD, PhD
Mark Tuszynski is professor of neurosciences and director of the Center for Neural Repair at the University of California, San Diego. His lab is working on several approaches to spinal cord regeneration, including neurotrophin gene therapy, the use of inhibitory molecules, and the effects of BDNF on axonal growth. He also leads work on gene therapy for treating Alzheimer's disease, the relationship between estrogens and AD, and on brain plasticity. Tuszynski holds an MD from the University of Minnesota and PhD from the University of California, San Diego.
Mary Bunge, PhD
Xiang-Yang Chen, PhD
Fiona Doetsch, PhD
John Donoghue, PhD
Reggie Edgerton, PhD
Don Faber, PhD
Bob Grossman, MD
Charles Jennings, PhD
John Kessler, MD
Naomi Kleitman, PhD
Jane Lebkowski, PhD
John Martin, PhD
Ron McKay, PhD
Steve McMahon, PhD
Geoff Raisman, MD, PhD
Rajiv Ratan, MD, PhD
William Zev Rymer, MD, PhD
Martin Schwab, PhD
Derek van der Kooy, PhD
Sam Weiss, PhD
Kathleen McGowan is a freelance magazine writer specializing in science and medicine.
For researchers who focus on spinal cord injury—and for the people who are coping with these injuries—it could be the best of times. For a long time, spinal cord injury and paralysis were considered to be untreatable. But over the past two decades, scientists have realized that the adult brain can generate new neurons, and that the spinal cord has powerful natural repair mechanisms. Such tools for healing are actively suppressed after spinal cord injury, but can be reactivated under certain circumstances. These two transformative insights have brought a rush of hope, energy, and enthusiasm to the field.
Although basic research has produced revolutionary insights into spinal cord injury, many open questions remain difficult to answer.
But sometimes, the more you know, the bigger the problem seems to get. These revolutionary insights from basic research have not translated into cures, and have raised many new questions that are proving to be very difficult to answer: How can we gain control over the endogenous machinery of repair? If the central nervous system has ways to fix itself, why don't surviving neurons repair themselves, and axons regrow? Why don't the treatments that have shown such impressive results in model organisms help humans? For anyone in a hurry to see a breakthrough in spinal cord injury treatment, it could seem like the worst of times: so much promise, but few immediate successes.
Both excitement and frustration were heard at a conference held on January 14–16, 2008 at the New York Academy of Sciences. The meeting, jointly sponsored by the New York State Spinal Cord Injury Research Board (SCIRB) and the Academy, was organized by Moses Chao, professor of cell biology, physiology and neuroscience and psychiatry of New York University Medical Center, Marie Filbin, professor of biology at Hunter College, City University of New York, Lorne Mendell, SCIRB chair, and professor of neurobiology and behavior at the State of University of New York at Stony Brook, Dominick Purpura, dean emeritus and distinguished professor of neuroscience at Albert Einstein College of Medicine, Jonathan Wolpaw, an SCIRB member and chief of the Laboratory of Nervous System Disorders at the Wadsworth Center of the New York State Department of Health, and Stacie Bloom of the New York Academy of Sciences. In addition to this eBriefing, the conference will also produce a forthcoming volume of the Annals of the New York Academy of Sciences. The program was supported by the Sam Schmidt Paralysis Foundation and the Craig H. Neilsen Foundation
Given the broad physiological repercussions of spinal cord injury, the nearly 250 conference attendees and their fields of expertise were quite diverse. Participants included rehabilitation specialists, neuroscientists, stem-cell experts, physical therapists, spinal surgeons, and bioengineers. The range of the potential treatments was also broad, including bioengineered scaffolds, cell-based therapies, small molecules that mimic axon guidance cues, neurotrophins, antibodies, robots, brain-computer interfaces, sophisticated physical therapy regimens, and neuroprotective agents.
Not so simple
Spinal cord injury is, thankfully, relatively uncommon—estimates of the number of people living with such an injury in the United States range from 250,000 to 400,000. At the time of injury, patients are generally young adults (although the median age has climbed above 40 years). Eighty percent are male. In the U.S., about half of these injuries result from motor vehicle accidents; the rest are caused by falls, violence, and recreational injuries. According to data gathered as part of the North American Clinical Trials Network, about 75% are injuries to the cervical spine, 15% thoracic, and the rest in the lower spine. Patients with SCI may lose all or some of their motor control, sensory ability, and autonomic function. Spasticity and neuropathic pain are also very common.
The injury that damaged the cord is only the beginning of the problem.
The problem seems straightforward—just like a cable being snipped in two—but spinal cord injuries are actually quite complex. In the clinic, complete transection is rare: about 90% of injuries spare part of the cord. Yet half of all injuries are functionally complete, resulting in the complete loss of sensation and muscle control. In part that's because an injury to the cord is only the beginning of the problem. Within minutes, bleeding and inflammation kill additional neurons and support cells. Oligodendroctyes die and axons lose their myelin, further compromising signal strength. Later, a glial scar forms and may encapsulate a fluid-filled cyst called a syrinx, which acts as a physical barrier to regrowth. Many patients with SCI also experience other complications including infection, arrhythmia, or pulmonary problems.
Because of the variety of problems, treating these injuries can require halting the spread of damage and reducing inhibitory cues, scarring, and inflammation. Repairing the spinal cord might also require reawakening dormant nerve cells, promoting and guiding fiber growth and connectivity, and providing the cells and axons with the sustenance they need. As Jonathan Wolpaw pointed out in his introduction on the second morning, recognizing that spinal cord function is shaped by experience and that the injured spinal cord is capable of reorganization has a challenging implication. Simply restoring axonal connections between the two severed parts is not enough. In order to restore function, these connections and the spinal neurons to which they connect have to be properly tuned, or "re-educated," so that they function effectively.
Something to look forward to
The problems may be large, but many innovative treatments are already in the clinic or heading that way. Robotic physical therapists and brain-computer systems are already being used in humans. An antibody to NOGO-A, one of the growth-inhibiting compounds found in myelin, is already in clinical trials, and a stem cell-based treatment will probably begin clinical trials this year. Another treatment that may soon be ready for human trials would use autologous transplantation of Schwann cells.
Regeneration is proving to be both more tractable and more complex than it first seemed to be. These days, almost anybody can grow an axon or induce sprouting. Controlling axon migration is still a major challenge, but as work presented at this conference showed, it is certainly possible to coax axons into the lesion site, and even possible to get an axon to worm its way back out.
Whether or not stem cells are "ready for prime time" was a recurring subject of debate. Questions were raised about the potential dangers of introducing pluripotent cells into the nervous system. Several audience members raised concerns about overselling treatments, when, as Northwestern University neurobiologist Jack Kessler put it, "no life-changing treatment is on the immediate horizon." Many other cell types have been proposed for treatment in spinal cord injury, including fibroblasts, bone marrow-derived cells, Schwann cells, and olfactory ensheathing cells. So far, most have been better at promoting remyelination than restoring lost neurons, and functional recovery has been limited. Nonetheless, newer procedures to deliver these along with neurotrophic factors or scar-ablating proteins may make them more useful.
So far, even simple questions about the best acute therapy for patients have yet to be answered.
Moving new techniques into the clinic is not easy, and much conversation was devoted to identifying the right translational models and clinical standards. The tiny size of a rodent spinal cord raises questions about whether techniques used on these models will scale up. Another concern: given the diversity of injuries, without exquisite care given to choosing outcome measures, patient groups, and study design, clinical trials might not reflect the potential benefit of some new treatments. So far, even simple questions about the best acute therapy for patients—such as whether or not decompression surgery should be performed directly following injury—have yet to be answered.
The closest thing to a consensus among presenters and attendees was that combined therapies will be necessary—and will also be a challenge to test. A treatment regimen might, for example, include intensive physical therapy to encourage spontaneous rewiring of weakened tracts; treatment with precursor cells to get Schwann cells to remyelinate surviving fibers, as well as small molecules and a surgically implanted bridge to provide a highway for new axonal growth. As one participant put it, the idea isn't to talk about "cure" but "cures," requiring multiple lines of work and interdisciplinary collaborations.
"The progress in this field in the last 20 years has been phenomenal, and new therapeutic development has been creative," said keynote speaker Mark Tuszynski of the University of California at San Diego. Now, we have our work cut out for us—in terms of establishing robust translational and preclinical findings that will have the best hope of making it in the rough-and-tumble real world. "It requires continued, strong, objective preclinical research. A great deal of discovery remains to be done."
Continue reading for a detailed report on each of the conference sessions. Highlights of the meeting include the following:
- It's easier to get axons to grow into lesion sites than to get them to bridge the site and make a functional synapse.
- Inhibitory CSPGs seem to act as a "glue" in the nervous system, regulating connectivity and preventing reorganization.
- An antibody to NOGO-A, a myelin-based inhibitor of axon regeneration, has passed Phase I of clinical trials.
- Skin-derived progenitor cells have some advantages as cell-based therapeutics: they're readily accessible, amenable to autologous transplant, and controversy-free.
- Caloric restriction by way of intermittent fasting should be explored as a safe, potentially powerful adjunct therapy.
- During development, changes in the growth cone of the axon allow it to respond flexibly and be pulled or repelled toward the right destination.
- Sensory information can be used locally by the spinal cord to generate rhythmic muscle responses appropriate to stepping.
- Epidural electrical stimulation of the spinal cord, combined with appropriate training and sensory stimulation, can restore locomotor ability in rats with transected spinal cords.
- Rehabilitation robots are proliferating and may have some advantages over human therapists, but need to be fine-tuned.
- Training protocols that target specific spinal reflex pathways can induce and guide plasticity that restores more effective motor function.
- SCI is heterogeneous and relatively rare, which makes it difficult to design useful clinical trials.
- Progenitor cells, although a promising treatment for spinal cord injury, may involve the risk of tumorigenesis or promoting neuropathic pain.
- Combined therapies will probably be the most successful, since the response to spinal cord injury involves a wide range of biological mechanisms.
- Understanding the body's response to hypoxia may lead the way toward treatments for both stroke and spinal cord injury.
Despite progress in understanding spinal cord injury, the details of basic biology remain unknown. A session moderated by Lorne Mendell probed the barriers to natural regeneration, ways to co-opt the peripheral nervous system's more effective healing response, the biology of cell-based therapies, and the characteristics of the glial scar. Much of the research in this session circled back to the question of how best to reduce inhibitory signaling that stands in the way of axonal regeneration and regrowth—essentially, how to replace this anti-growth signal with a more positive message.
All roads lead to Rho
As conference organizer Marie Filbin explained, there are "quite a wide variety of inhibitors" of regeneration in the spinal cord, but all eventually converge in their signaling upon RhoA, a G-protein in the Rho family of small GTPases that is involved in molding and changing the morphology of the cell and regulating dendritic structures. Rather than take this busy and complex highway, however, her lab is looking for a bypass to block inhibition, and she described the process her group is using to identify candidates that might promote axonal regeneration.
Unlike their cousins in the central nervous system, axons in the peripheral nervous system are able to regenerate. In 1999 Simona Neumann showed that severing the axons of dorsal root ganglion cells that project to the periphery, if done before a lesion severs the second branch of the same cell that remains within the central nervous system, will cause axonal regeneration in both PNS and CNS. Such a "conditioning" lesion unfortunately does not have this magic power if it is administered following the central axotomy, so it is not a potential therapy—but it does raise many interesting questions about what, biologically, is going on.
Elevating cAMP is a blunt but effective strategy to encourage regeneration.
Cyclic AMP seems to be involved in this response, and elevating cAMP offers a blunt but effective means of encouraging regeneration that does not rely upon the Rho pathway. In previous work, Filbin's lab overcame MAG inhibition in vivo in the rat by administering the cAMP agonist dibutyryl-cAMP (db-cAMP) to the dorsal root ganglia.
However, this second messenger has broad and powerful effects on cells, and ultimately, Filbin said, "We're interested in what's downstream of cAMP" rather than in globally elevating this signaling molecule. She presented research that her group has carried out at various points along the chain of intracellular signaling: cAMP initiates upregulation of Arg1 and other genes, which in turn increase the synthesis of polyamines, which have diverse effects upon the cell, including the upregulation of p35 and consequent activation of Cdk5, which changes the cytoskeleton to permit remodeling and regeneration. Filbin discussed in greater detail her group's work on the gene Arg1, which is key in polyamine synthesis and blocking MAG inhibition, and with SLP1 (secretory leucoprotease inhibitor-1), which also circumvents the Rho pathway.
One from column A, two from column B
Mary Bunge of the Miami Project to Cure Paralysis, University of Miami Miller School of Medicine, next brought up a theme that would recur throughout the conference: the need for combination treatment. Because the spinal cord changes in so many different ways following injury, she suggested, treatment will require a range of simultaneous or sequential therapies in order to properly address the diverse deficits caused by the injury.
Because the spinal cord changes in so many ways following injury, treatment will require a range of simultaneous or sequential therapies.
Using a model that involves damage to the rat thoracic spinal cord, her group is analyzing which combinations of therapies hold the most promise, including neuroprotective strategies to dampen damaging intracellular cascades that lead to secondary tissue loss immediately after the injury, axonal regeneration through techniques like transplantation, switching off the enzymes that prevent new growth, and genetic engineering.
Bunge focused in particular on the potential therapeutic use of Schwann cells. (Olfactory ensheathing cells, which normally promote the growth of new nerve fibers in the olfactory bulb, show ability to promote regeneration and remyelination and restore function, but are less accessible at this time; while Schwann cells can be taken from the peripheral nerves in the limbs, OECs must come from the olfactory bulb inside the brain or the mucosa in the nose.) In the peripheral nervous system, these macroglial cells swaddle axons in their protective sheath of myelin and provide sustenance and support. They can also generate neurotrophic factors and are amenable to autologous transplantation. A transplant bridge or tube larded with Schwann cells will induce some of the severed ends of axons to grow and myelinate, and result in modest functional improvement.
The triple threat
Bunge ran through many examples of multiple treatments that combined Schwann cells with nerve growth factors, other cells, and treatments such as chondroitinase, which degrades a proteoglycan that inhibits growth. Growth was also achieved by grafting transduced Schwann cells so that they produce neurotrophins. These series of experiments indicate the benefits of a combination of neuroprotection, a cellular scaffold, genetically engineered cell therapy, reduction of inhibitory molecules, prevention of scar formation, and, in humans, rehabilitation and training.
Bunge said that her group is "trying to tool up" to craft an investigational new drug application to test autologous human Schwann cell treatment. Therapeutic hurdles include figuring out how to promote Schwann cell survival—right now, only 5%–20% survive transplantation—and devising a way to coax more axons to grow through and beyond the injury site. "We've encouraged growth into the implant, and now we have to think of a strategy to lure fibers beyond the implant," she said.
The glue that binds
Stephen McMahon of King's College London focused on ways to manipulate and alter the function of one set of neuronal growth and sprouting inhibitors, the chrondroitin sulphate proteoglycans (CSPGs). These extracellular matrix glycoproteins accumulate at the site of CNS injury in the glial scar and block regeneration.
"There is something very potent within the glial scar," McMahon said.
McMahon mentioned a 1999 paper describing an experiment in which adult rat sensory neurons, injected directly into a lesion of the spinal cord, promoted axonal regeneration even in degenerating tissue. Under these conditions, axons grow until they reach the glial scar, which stops them. "There is something very potent within the glial scar," he said, and fingered CSPGs as one culprit.
The CSPG family, all of which have glycosaminoglycan (GAG) sugar side chains, have "rich possibilities for diversity of actions," said McMahon. Many are expressed throughout the body, making systemic treatments that target these molecules unattractive. Nonetheless, inhibiting CSPGs sheds light on why axons have such difficulty growing back. Intrathecal treatment with chondroitinase ABC, a bacterial enzyme which nonselectively degrades the GAG chains, promotes regeneration and functional recovery after spinal cord injury, the team showed in 2002, although they also observed complications including evidence of aberrant connections.
Curiously, behavioral recovery from this treatment happens within two weeks, too quickly to be the result of regrowth. Sure enough, a subsequent paper documented sprouting of corticospinal, serotinergic, and primary afferent fibers. Functional recovery in this case appears to be the result of axonal plasticity rather than regrowth, but it seems likely that CSPGs may be involved in inhibiting both.
We don't yet understand the mechanism of CSPGs, and we don't have a good way of finding out what it might be, but this and other evidence leads McMahon to suspect that CSPGs act as a "glue" in normal nervous systems, and that they regulate connectivity in general. Dissolving them leads to functional reorganization. It's "critically important," said McMahon, that we really understand the mechanism at work before therapies are considered. One common consequence of aberrant sprouting is increased pain, and although this group did not see any behavioral evidence of allodynia in this work, the threat of this problem looms over such treatments.
A second session, moderated by Marie Filbin, turned to translational and clinical research, providing updates on the status of a range of potential treatments that are either being tested in humans or are being considered for such trials, including antibodies, cell-based therapeutics and transplantation techniques.
Getting going with NOGO
One of the most promising developments in spinal cord injury research has been identifying specific factors in the myelin that prohibit regrowth—and possible ways to shut down the inhibition. Martin Schwab of the University of Zurich and Swiss Federal Institute of Technology, working with Novartis, has developed an original solution for this problem: an antibody to NOGO-A, one of the best-known regeneration-inhibiting factors.
In small spinal lesions, fibers will grow spontaneously. When the corticospinal tract is interrupted in the mid-thoracic region of a rat, the hindlimb fibers that normally course straight through this tract will spontaneously arborize and sprout, forming new collateral connections to long propriospinal neurons in a "detour pathway." More recent data shows that other hindlimb fiber sprouts will link with interneurons belonging to forelimb circuitry. The cortex is respecified such that the hindlimb field of the motor cortex controls the forelimb, the trunk and the face. Sensory input, too, is remapped. This may be how "the cortical hardware that you have is used optimally," said Schwab.
Shutting down NOGO-A, a 200 kDa membrane protein expressed in oligodendroctyes and myelin in adults as well as in developing neurons, can elicit similar effects. in vitro, an antibody that neutralizes NOGO-A sparks neurons in monkey brain extract to start growing neurites within 14 hours. "In this soup of hundreds of proteins, neutralizing just this one makes quite a difference," said Schwab. Intact animals treated with the antibody spontaneously sprout axons from the Purkinje cells; normal animals treated with the antibody before a lesion will reach about 80% behavioral recovery in some tasks.
Schwab's group has partnered with Novartis to develop a human antibody against human NOGO-A. After in vivo proof of principle in monkeys, the treatment was extended to humans with acute injury and is now at the end of a phase I trial. About 20 ASIA-A patients have been treated so far with no observable safety problems, and the project is currently in transition to phase 2.
A jump and a SKP
Another possible treatment for spinal cord injury is to transplant support cells or pluripotent cells directly at the site of injury. Patients with SCI are already being treated all over the world with variations on this technique, but there is no agreement on which cells should be transplanted and under what conditions; safety concerns are also significant.
Wolfram Tetzlaff of the University of British Columbia and his group work with skin-derived precursor cells (SKPs), the "new kid on the block," as he called them. This approach avoids the ethical and political problems of using embryonic stem cells; they're readily available, accessible, and amenable to autologous transplantation.
Skin-derived precursor cells are "the new kid on the block," said Tetzlaff.
Cultured as SKP-derived Schwann cells, and injected into spinal contusion sites of adult rats, these cells integrate to form bridges across the lesion site, promote axonal regeneration, promote invasion of endogenous Schwann cells, and enhance remyelination. Animals show some behavioral improvement, although "functionally, the effects are modest." Ominously, animals treated with undifferentiated SKP cells showed a lower pain threshold, so clearly there is more to be understood about how these cells function in a living animal.
Tetzlaff also discussed an unconventional approach to mitigating spinal cord injury: manipulating diet. Currently, there are no evidence-based guidelines for nutritional needs after spinal cord injury, but the typical clinical recommendation is for high calorie intake following injury. He suggested that the strong and diverse effects of caloric restriction on life expectancy and disease-related mortality, however, mean that dietary regulation is worth a second look. His group investigated intermittent fasting: Rats starved every other day and allowed to eat at will on alternate days performed much better on several outcome measures after lesion than control animals —even if the dietary restriction was implemented after the injury.
The mechanism is unknown—it could be reduction of inflammation, upregulation of neurotransmitters, or some generalized effect of hormesis, but even without fully understanding the reasons why, it suggests an "excellent co-treatment," suggested Tetzlaff. The lifespan of people with SCI is dramatically reduced due to complications, and this could be a simple, cheap, and safe way to extend the healthy survival of people with these injuries.
If you've got it, use it
Two other ways to strengthen lost capability are to augment the functions of spared axons or to find a bypass around the damaged region that can reawaken motor activity.
In the developing nervous system, axons grow and find their targets based on activity, so perhaps restoring a semblance of activity in maturity might help augment spontaneous healing, connections with spinal motor circuits, and better function. In one experiment conducted by John Martin of Columbia University, rats were given a unilateral pyramidal lesion, which wiped out one hemisphere of the corticospinal tract. The intact corticospinal tract was then stimulated electrically, promoting outgrowth and strengthening connections which are normally weak on the ipsilateral side. Some motor recovery was seen in preliminary results. In humans, this could be done through transcranial stimulation.
Another, more targeted technique is to create a novel motor bridge. Here, a spinal nerve is transplanted from the muscle it normally innervates into the spinal cord caudal to the injury, in the hopes that the somatic motor axons will regenerate and form functional connections at the insertion point. So far, so good, says Martin: "This is a novel environment for these neurons to grow into, but in fact they're behaving remarkably well."
Stimulation of the bridge evokes a strong muscle response, indicating functional connections with spinal neurons. Because it requires common neurosurgical techniques and is an autologous graft, it should be relatively easy to translate to humans if it continues to look promising, and has shown some efficacy in the cat.
From attraction to repulsion
The cues and mechanisms that guide axon growth and targeting during development might be exploited to boost regeneration in the adult after injury. In the conference's first keynote address, Marc Tessier-Lavigne of Genentech described four such "canonical" guidance cues, focusing on the netrin and slit protein families. Emerging evidence indicates that growth factors may also be involved in guiding axons as they lengthen. The puzzle in understanding how corticospinal tract neurons function is that they must arrive at the right location after covering a very long distance—too long for simple chemoattraction to be the main system at work.
"There is a machine in the growth cone, and we'd like to get our hands on that machine."
The answer seems to be in the growth cone, a structure at the tip of the developing axon that explores the cellular environment and guides the growing process toward its target. The growth cone has a plastic response to various agents at work in the cord; some proteins initially function as chemoattractants, luring the growing axon toward them. Once the axon has reached this intermediate target, a switch in the biology of the growth cone makes the same compound a repellant, pushing the axon to its next destination. For the axon, "moving on involves becoming repelled as well as losing attraction," said Tessier-Lavigne, joking that this phenomenon was much like a human romance. He then discussed several papers in press that described in greater detail some of the genes that regulate this transition as axons approach the midline, cross, turn 90° to project along the floor plate of the developing spinal cord, and eventually move on. "There is a machine in the growth cone—a switching machine," he said. "We'd like to get our hands on that machine."
In the second half of his talk, Tessier-Lavigne talked about efforts to better understand the transcriptional cascade that allows peripheral neurons to regenerate—and to somehow "trigger" regrowth of central nervous system axons under the right conditions. This is the mystery Filbin has been exploring: that is, why a conditioning peripheral lesion can cause a subsequent central lesion to regenerate. Her research indicates that increased cAMP is part of the response; other research has pointed to Atf3, and Tessier-Lavigne described new evidence implicating SMAD1 as well. Tessier-Lavigne also skimmed through recent evidence on a range of other factors, concluding that this preliminary evidence on these axon guidance molecules is interesting, but "requires a lot more fleshing out."
One of the most contentious issues at the conference concerned the promise and potential danger of stem cells. A Monday afternoon panel devoted to the issue made the fault lines clear: between researchers who think that it is too early—and still too dangerous—to begin clinical trials with stem cells, and those who believe that the time has come.
Are stem cells ready for prime time?
Stem cells or other immature CNS cells, which have the innate power to promote growth or to differentiate into a wide range of cells, seem to hold the magic key to repairing spinal cord injury. They might be able to replace lost cells, remyelinate surviving axons, and, if transfected, act as factories for nerve growth factors. So far, these cells have been better at boosting remyelination than in generating new axons. That may be because they don't properly differentiate, or it may be because the injured atmosphere directs them toward the wrong cell fate—a scar formation astrocyte, say, rather than a neuron.
The ethical controversy about using embryonic stem cells may have obscured a more essential issue: whether it's going to work at all in human patients.
One unresolved question about using stem cells to treat spinal cord injury is which type of cell has the most potential. Embryonic stem cells can give rise, theoretically, to any type of cell in the body; neuronal stem cells are more restricted, but are rarer and require careful care and tending. Whether this research will translate smoothly from rodents to humans is also unknown, and the ethical controversy about using embryonic stem cells may have obscured a more essential issue: whether it's going to work at all in human patients.
"One cannot assume that the human equivalent will respond to the same signals," said panelist Sam Weiss of the University of Calgary, and although many promising results have been seen in rodents, it may turn out not to be a great therapy for spinal cord injury. Weiss remarked that it's a "steep hill to climb," although he suggested that the research was "getting to the point where we have to explore some of these things in vivo." In Parkinson's disease, translation from animals to humans has been disappointing, but in spinal cord injury the urgency is greater because there are no alternative treatments.
The goals of stem cell therapy include introducing nonspecific trophic effects and promoting remyelination by making new Schwann cells and oligodendrocytes, to the "highest-hanging fruit": actually replacing neural networks. Weiss suggested that for now, stem cells may be most useful as promoters of sprouting.
Geron Senior Vice President Jane Lebkowski discussed her company's plans to begin clinical trials in ASIA-A patients with contusion injury, using oligodendroglial progenitor cells that have been derived from human embryonic stem cells. Similar cells have shown strong results in models, but several panelists and audience members questioned the wisdom of using such cells in human patients.
From the audience, Mark Noble disputed the idea, presenting a few slides of data. His work has shown that glial-restricted precursor cells predifferentiated into astrocytes can be an effective treatment, but undifferentiated progenitor cells showed poor results. "It's important to consider whether a primary goal needs to be to build the astrocytic compartment," he said.
Safety concerns were also raised. Undifferentiated cells that are difficult to completely scrub out of a differentiated population might cause solid tumors, as has been seen already in some patients undergoing experimental cell therapy treatment. Allodynia and hyperalgesia are also a possibility; while some glial-restricted-precursor-derived predifferentiated astrocytes promote axon growth and locomotor restoration, many others generated under inflammatory signaling promote allodynia and hyperalgesia, said Noble.
"We know one of the main issues in quality of life for these patients is these syndromes," he said. "A number of us are becoming extremely concerned about giving someone who is paralyzed a neuropathic pain syndrome."
New appreciation of the electrophysiological responses of the spinal cord, and advances in computing and robotics, provide a strikingly different way of helping people with spinal cord injuries. A session chaired by Jonathan Wolpaw documented recent advances in a range of such technologies.
Who needs a brain?
After training, rats can step effectively even after complete cord transection, with simple tonic stimulation.
Understanding neural network change and adaptation seems to be another key to unlocking new treatments for spinal cord injury. Reggie Edgerton of the University of California at Los Angeles has found evidence that spontaneous recovery after SCI is often due to propriospinal relay connections, not the restoration of long descending axons from the brain. He's interested in modifying and exploiting this ability of the spinal cord to "teach" these neurons, which are clearly capable of complex activity, to adopt new patterns of activity to support standing and stepping. The plasticity of the spinal cord is "remarkable," he said.
Central pattern generation—the spinal cord's ability to coordinate motor pools to achieve flexion and extension without afferent input—is only part of the story. One overlooked problem in SCI is the formation of aberrant connections that scramble these well-coordinated rhythms and possibly contribute to spasticity. Rehabilitation training can establish some of the normal coordination, but how this actually works is unknown.
Edgerton described some of his successes using robotic assistants to train rats. After training, when rats receive simple, tonic electrical stimulation along the lumbar-sacral regions of the cord, along with the sensory stimulus of a treadmill, they can step effectively, even after complete thoracic spinal cord transection. "The peripheral sensory system is taking over what we'd think would happen supraspinally," said Edgerton. Importantly, the animal reverses direction and EMG patterns flip when the treadmill is reversed, suggesting that sensory input plays an essential role.
The human spinal cord seems to have the same potential. Working with SCI patients who've been implanted with electrodes to control pain, an Austrian team has found that tonic stimulation can produce rhythmic flexion/extension movements in subjects.
Taken together, this evidence indicates that it's possible to modulate spinal circuitry through epidural stimulation or pharmacology such that afferent signals can generate and control locomotion without supraspinal input.
Robots to the ready
"We're being inundated with robots now," began the next speaker, William Zev Rymer of the Rehabilitation Institute of Chicago. Robotic systems are widely used to train people to regain use of their limbs after spinal cord injury, but the best way to use them has not been established.
Rymer showed videos of a number of devices that can be used for training. One is the Swiss-invented Lokomat, in which a patient is hoisted onto a treadmill in a harness that offloads his weight, and his knee and hip joints are actuated by the robot. Another is the Kineassist, a refrigerator-sized device that supports the patient under the pelvis and trails the patient as he or she moves across the room. There are now "literally hundreds" of designs for upper extremity training, Rymer said.
Although these devices reduce the manpower needed to rehabilitate a patient, they are still prohibitively expensive—up to $300,000 for the Lokomat. The cost and complexity mean that they haven't been proven in large-scale randomized clinical trials, the gold standard for proof. In addition, many physical therapists and physicians are distrustful of this new technology, Rymer said.
We need to decide what we want robots to do.
Besides, we need to decide what we want these robots to do. Mechanizing the work of a physical therapist is difficult, as they are as much artists as doctors and are "not able to readily describe their interventions precisely in qualitative terms," said Rymer. Even simple questions about successful training are unanswered: Should patients be allowed to make movement errors, or be corrected? How long should the ideal training session last, and how often should it be repeated? What should be done between learning tasks, and could other activities interfere with learning?
Rymer presented "a flavor of the data" that his group has collected using the Lokomat with their SCI patients, documenting a 53% improvement in a six-minute walk, but no significant changes on other measures, such as bracing, assistive devices, and physical assistance. "Robots are helpful, but we don't have them right yet," he said. The key will be to make them more like a therapist—helping the patient when he or she needs it, but also allowing mistakes and intense effort.
Other promising inventions include a shirt with strain gauge sensors that could allow for better control of motorized wheelchairs and other such devices. But before these robotic assists will be truly helpful, the physicians and inventors who work with them will first need a better idea of what the technology should do, and also a better understanding of what the rules are for optimal recovery and rehabilitation. "We're in the model T stage," agreed Edgerton.
Even a simple, noninvasive training paradigm using reflex conditioning may help restore lost function. Xiang-Yang Chen of the Wadsworth Center of the New York State Department of Health presented evidence that reflex conditioning can be used to modify spinal reflexes. His work concerns the Hoffman or H-reflex, a reflex response under descending control from the brain that he called "the simplest motor skill." Data from animals and from patients with spinal cord injury demonstrate that training paradigms can change reflex responses, and suggest that reflex training could help refine and improve locomotion.
Long-term conditioning induces plasticity in the spinal cord, influencing GABAergic input to soleus motoneurons and motoneuron axonal conduction velocity. Rats with a unilateral transection injury will walk asymmetrically, but up-conditioning of the soleus H-reflex on that side can compensate for this problem and improve locomotor function.
A similar paradigm developed for human subjects suggests a similar change in reflex responses, although whether or not it improves locomotion is still an open question. From the audience, collaborator Richard Segal described very preliminary results with this paradigm, including a 20%–30% increase in the reflex following conditioning. At this very early stage, he said, "It looks pretty good."
Putting it all together
The final morning of the conference was devoted to a forum-style discussion hosted by the New York State Spinal Cord Injury Board, with a focus on what strategies will lead to the fastest progress for patients. It began with a provocative and also moving talk by John Kessler of Northwestern University. He spoke of his frustration, impatience, and hope as a physician, a regenerative medicine researcher, and as father—his daughter became paraplegic following a spinal cord injury. He praised the quality of the work at the conference, the great progress in understanding the biology of the injured spinal cord, and efforts to improve rigor and coordination between research agendas. But he also criticized the field of spinal cord injury research as "somewhat stalled" and "stodgy."
Kessler said it was as a physician that he felt most frustrated: "I cannot even answer a simple question," such as whether to give methylprednisolone. Other unanswered questions: What is the best animal model and why? Shouldn't clinical trials be based on more robust results than those from a single lab or model? Are primate models necessary, as Tuszynski suggested? Most important, what are the best criteria for bringing a new therapy into clinical trials? What patient group should be chosen for clinical trials and why? "I heard remarkably few tough questions asked during this meeting," he said, and concluded by warning that without greater willingness to embrace truly new technologies, we may not see the kind of progress we hope for in a decade's time.
The "boring science" is less likely to get funded, even though it is essential.
Many researchers agreed in part with what Kessler said, and others disputed his characterizations. Marie Filbin said she "disagreed strongly" that the field was stuck in a rut: "I've seen an explosion in our understanding, and that's really encouraging. As we learn more about why neurons die, perhaps we can start to address it in a more logical way." Wise Young of Rutgers University agreed: "I remember 30 years ago, when you had incomplete spinal injury and a 40% chance of walking—now you have 90%, 95%" he said. "That is progress."
Conference participants zeroed in on the particular difficulty of figuring out how to take a promising agent or therapy into clinical trials, and complained about the lack of funding. "We can bash each other for not doing this or that, but we're constricted in terms of funding," said Young. Others said that some of the most important research—the "boring science" that will, for example, determine exactly which cell type is best suited for treatment—is the least likely to get funded, because even though it is essential, it is not hypothesis-driven.
Naomi Kleitman, extramural research program director at the National Institute of Neurological Diseases and Stroke, and one of the forum moderators, said that some program money is available to help with this phase of research, but that the spinal cord injury community needs to come to consensus about how much preclinical research should be required before moving into a translational phase—and to indicate which seem to be the most promising fields for research. "Those of you here need to help us make those decisions and investments," she said. "There is still not agreement in the community on what exactly needs to be done." She and others suggested that the New York State program and others like it were a great way to fund riskier projects.
No magic bullets
Without work being replicated, and without more of this translational work, many participants were concerned that clinical trials were being launched too soon. Mark Noble, the codirector of the Center of Research Excellence, said he was "deeply concerned" about what he sees as the overselling of stem cell biology, and the rush to move these treatments into clinical trials. He reminded participants about the crash-and-burn of gene therapy, in which a few unexpected deaths froze the field for many years. Reggie Edgerton said that the field should have reasonable goals for hopes for progress in five and ten years. "We've got to get out of this syndrome of thinking there's a magic bullet," he said. Filbin suggested forming a consortium that could establish criteria for when a therapy should begin clinical trials.
A multiplicity of therapies will be required to treat these injuries effectively, and so researchers must get in the habit of collaborating.
Standardization is a major hurdle as well, in just about every area of research. Victor Arvanian of the State University of New York at Stony Brook mentioned the real-world variability in time to treatment—on average, spinal cord injury patients begin treatment one week after they enter the hospital, but timing needs to be standardized in order for clinical trials to work. Diagnosis is not where it should be either, said Martin Schwab: "In SCI today it is stone age! We don't use electrophysiology routinely, which we could have done 30 years ago." All agreed that outcome measures needed to be made both more sensitive and more specific.
The emerging consensus is that a multiplicity of therapies will be required to treat these injuries effectively, and so researchers must get in the habit of collaborating—both through formal consortia and through reaching out directly one to another. Integrating different kinds of research "always works better scientist to scientist," said one participant. Integrating work from different disciplines and crossing boundaries to work with very different disciplines such as engineering and physics, as well as within medical specialties such as rehabilitation and nursing, should also take high priority, said several other participants. "No individual lab can do this," said organizer Lorne Mendell. "My personal view is that the spinal cord injury field has reached the stage of maturity—we know lots of basic science, but we've got to put it together."
Will the ability of stem cells to encourage sprouting and remyelination ultimately be more important than neuronal replacement?
Is it too soon to begin trials in humans with stem cells?
Why do peripheral lesions promote recovery in central lesions, but only if they precede the central lesion?
How can you get axons to grow through the lesion site and on to an appropriate synaptic destination?
How do CSPGs influence cellular structure?
What is it in the glial scar that halts regrowth?
After thoracic injury, how and why do hindlimb motor cortex cells spontaneously remap to correspond to the forelimbs and face?
In models of treated spinal cord injury, why isn't there more pain and spasticity when fibers do regrow, as there is in the clinic?
Why do small CNS lesions repair themselves so well, and why doesn't this work in big injuries?
Is it necessary to test therapeutics on larger mammals such as cats or primates before moving into human trials?
For whole cell treatment, what is the best source of cell to use, and how differentiated should it be?
Can a spinal cord bypass in which nerves are transplanted to bridge the lesion site restore lost function?
How does a growing axon know where to go during development, and how can we exploit this knowledge to help with regeneration?
Rehabilitation and New Devices
How do spinal cord neurons generate and control coordinated movement in the absence of descending input?
How does learning occur after injury, and what is the best way to maximize improvement? Should errors be encouraged, corrected or ignored?
Could spasticity also be addressed with training protocols that target local pathways?
How long will implanted electrode arrays that capture signals from the motor cortex last?
For brain-computer interface devices, what are the relative advantages and disadvantages of capturing signals from the scalp, from the surface of the brain, or from within the brain?
How can we evaluate potential therapies in clinical trials when spinal cord injury is so biologically and clinically variable?
Are we using the right animal and clinical models to test treatments?
What outcome measures will better capture modest improvements?
The final session, moderated by Don Faber of the Albert Einstein College of Medicine, brought another set of options for potential therapeutics, and also launched a larger discussion about how best to meet the particular challenges of establishing clinical validity for treatments for spinal cord injury.
John Donoghue of Brown University has created brain-computer interfaces that circumvent the problem at the root of spinal cord injury: the disconnect between the fully functional brain and the outside world. His company, Cyberkinetics, has developed a "neural interface system" called BrainGate for use in tetraplegic patients with spinal cord injury, amyotrophic lateral sclerosis, and stroke that can interpret neuronal signals and use these to control external robotic or computer devices for communication and movement.
A small array of 100 microelectrodes is implanted into the surface of the motor cortex and attached to a cable that leaves the skull through a surgically implanted port. The electrodes pick up action potentials as well as field potentials, and deliver this neural information to a computer for analysis and translation either into the movement of a cursor on a computer screen, or potentially the movement of a prosthetic limb.
Using the device, a patient could control grasping with a robotic hand and even direct a wheelchair.
Four individuals have so far received implants as part of two pilot clinical trials. In one patient who had been completely immobilized and unable to speak for nine years after a brainstem stroke, the device quickly rendered signals when she was asked to imagine moving her arm. In videos of an SCI patient newly outfitted with this device, it appeared to be easy to control. Mating this system with a Norwegian-invented system that permits one-finger typing to control a computer, it was possible for a patient to control grasping with a robotic hand and even to direct a wheelchair.
Questions remain about the practicality of the system. In monkeys the device has continued to return good signals for more than three years, but how long it can possibly last is a "complex question," admitted Donoghue. The harsh environment of the body can degrade materials and hence disrupt recordings; such effects have accounted for signal loss. Once these biological impacts are better understood, the materials and manufacturing of these implants may change; the device also needs to be made fully implanted and wireless. Within five years, Donoghue hopes to modify the device so that it could directly control skeletal musculature—essentially creating a computer bypass around the site of injury.
Other signals may also give other benefits—devices might monitor activity in different parts of the brain, such as premotor cortex, or different types of signals, such as EEG. "They all give you something," said Donoghue. "It's important to acknowledge there are a number of approaches, some involving implanted and some unimplanted devices, which pretty much all need to be pursued at this point," said moderator Jonathan Wolpaw, who has been working with similar devices with signals derived from EEG.
The troubles of trials
The stubborn problem of how best to evaluate treatments for SCI emerged earlier in the conference, but Robert Grossman of the Methodist Hospital explored it most fully, suggesting that we should be able to learn from past disappointment. So far, no prospective randomized clinical trial of an SCI treatment that improved outcomes in animals has been unequivocally shown to work in humans. Methylprednisolone, for example, is routinely given in the United States in the acute setting, but never in Europe. "There's great controversy about its efficacy," Grossman said. Many therapies have failed clinical trials, raising the question, Were they truly useless, or were the studies not properly designed and executed?
There are fortunately not many spinal cord injury patients, but that makes it difficult to evaluate what works.
Grossman painted a portrait of the diversity of patients considered in data from the North American Clinical Trials Network, a coalition of nine hospitals organized by the Christopher and Dana Reeve Foundation to standardize information and protocols.
In a problem as heterogeneous and complex as spinal cord injury, simple misjudgments of dosage and timing may mean that a potentially useful therapeutic appears not to have any benefit. Other variables include the age of the patient, pre-existing conditions, underlying genetic susceptibilities, time to treatment, and the degrees of hypoxia and compression. While half of all patients have no complications, a significant number have three or more. Even standardizing therapy among different medical groups is a major challenge. Most patients undergo some kind of decompression surgery, and although it seems to be helpful, said Grossman, "it's not been possible to prove this gives a better outcome." A clinical trial, the Surgical Treatment of Acute Spinal Cord Injury Study (STACIS), is currently evaluating this treatment.
It also seems likely that multiple therapies are going to be the only way to see major improvement, since many different processes are at work. Therefore, more sensitive outcome measures must be developed and standardized in order to detect the effects of moderately beneficial treatments. Grossman made the analogy to head injury studies, in which insensitive outcome measurements masked potentially positive results.
Furthermore, the typical animal model—a young female rodent with thoracic injury caused by an impact without compression—may not be very relevant. In humans, the median age of injury is now 41, and the most common injury is to the cervical spinal cord and involves prolonged compression and a delay to treatment of up to 24 hours. The much longer distance that axons must travel in order to regenerate in the human body, as compared to that in a mouse or rat, may also make a big difference.
Flipping the homeostatic switch
Rajiv Ratan, director of the Burke/Cornell Medical Research Institute, next spoke about research efforts to cope with the diversity and heterogeneity of spinal cord injury. The work he described was conducted under the auspices of New York State Center of Research Excellence (CORE) for spinal cord injury interventions, a collaborative effort that includes several dozen researchers at Burke/Cornell, the University of Rochester, Rutgers University, and other local and national institutions.
The body has powerful protective mechanisms that kick in in the absence of oxygen to restore homeostasis and preserve tissue.
The body has powerful protective mechanisms that become active in the absence of oxygen to restore homeostasis and preserve tissue; one possible way to address the diverse problems caused by insults like spinal cord injury or stroke is to learn how to flip this switch with small molecules. Ratan mentioned HIF-1 (hypoxia-inducible factor-1), a transcription factor that can single-handedly upregulate 70 to 100 genes under hypoxic conditions. Stimulating such a process by blocking inhibition could be a powerful treatment, but a small molecule that activated such a wide variety of genes might pose major safety problems.
Instead of beginning with unknown agents, the CORE group searched existing libraries for molecules that are already clinically approved; they identified tilorone, an interferon-inducing agent that activates HIF-1. It was protective in a model of stroke, but only if given before injury, making it impractical. However, seven other compounds from this library look promising. "Agents that protect and repair the brain may have a wide therapeutic window," he said.
Ratan also mentioned other CORE projects, such as using pre-differentiated astrocytes in cell-based therapy and scaling down training robots for use with rats for a more accurate preclinical model. A biomarker study is also underway to identify SNPs that might predict recovery. Ratan emphasized what many other speakers mentioned: that combined therapies that draw from a wide range of research fields will probably be necessary to make significant progress helping spinal cord injury patients.
Why we need translation
In the closing keynote address, Mark Tuszynski talked about ways of stimulating axon growth by implanting cells modified to express various growth factors. All elicit migration of Schwann cells to the site, and some treatments can even get supraspinal axons to penetrate the lesion site, but that's where his success ends. "We can elicit growth of axons into a lesion site—the problem has been getting them out again," he said. In one model, an approach combining a post facto conditioning lesion, a scaffold, and therapy with growth factors resulted in some growth which appeared to be true regeneration. "We can get some axons out," he said, "but in lay terms, it isn't easy."
A similar set of experiments in the motor system combining another growth factor with a marrow stroma cell graft and with cAMP in the brainstem to "condition" the response caused reticulospinal axons to grow up to 1 mm beyond the graft site, although they were abnormal looking. Functional improvements followed, but Tuszynski said he suspects the effects were mostly due to reorganization, not axonal regeneration. Besides, humans need corticospinal axonal regeneration in order to have a practical therapy—so clearly "we have work left to do."
Tuszynski joined the chorus of researchers who pointed toward combined therapies as the most likely to provide clinical benefit, but emphasized doing the appropriate translational and preclinical work first. Treatments that don't show robust successes in these arenas may otherwise find "success a challenge" in the clinical arena, he said.