Spine and Spinal Cord Surgery

Spring 2001
Volume 12, Number 1

 Next Article  

W. Dalton Dietrich, PhD

Injury to the spinal cord initiates a cascade of events that ultimately lead to cell death. Experimental and clinical studies have identified distinct biochemical pathways that are activated in both the acute and subacute injury state. Clarification of dominant injury mechanisms is extremely important from the therapeutic standpoint in that pharmacological strategies can be directed toward specific injury processes. In addition, there is a growing interest in the therapeutic potential of utilizing cell transplantation strategies for therapy in spinal cord injury (SCI). The replacement of lost cell populations, as well as the delivery of genes or proteins to enhance axonal regeneration, is an exciting research direction. The purpose of this chapter will be to review neuroprotective strategies that are currently being utilized in the laboratory as well as briefly summarize several transplantation strategies directed toward a cure for Paralysis following SCI.

Neuroprotection following spinal cord injury. In acutely injured spinal cord injury (SCI) patients, especially those having incomplete lesions, neuroprotective strategies have the potential of limiting secondary injury mechanisms. Although methylprednisolone treatment has been reported to benefit a subpopulation of SCI patients,1 new therapies need to be developed and tested. Glutamate, the major endogenous excitotory neurotransmitter of the central nervous system (CNS), has been shown to mediate pathological processes in many injury models, including SCI. With the use of intracerebral microdialysis, sampling of the extracellular space has shown massive release of glutamate as well as other neurotransmitters during and following injury.2 In recent studies, Wrathall and colleagues3 have reported that treatment with NBQX, a highly selective antagonist of the non-N-methyl-D-aspartate excitotory amino acid receptor, reduces histopathological and functional deficits following traumatic SCI. Likewise, convincing support for the importance of free radicals and lipid peroxidation in SCI models has been derived from studies reporting that oxygen radical scavengers or the use of inhibitors of lipid peroxidation can limit neuronal damage and improve outcome.4 In a model of TBI, the novel inhibitor of lipid peroxidation, LY341122, was reported to improve histopathological outcome only when the agent was given in the early post-injury period.5 Thus, when evaluating the potential use of agents that block excitotoxic or free radical- mediated damage, questions regarding the therapeutic window for these strategies must be addressed. Obviously, if the window of opportunity has passed prior to the patient’s arrival in the emergency room, these pathomechanisms may not be appropriate targets for therapeutic strategies.

A fundamental question regarding pathophysiology of SCI is the nature of cell death: is it necrosis or apoptosis? Necrosis is characterized by the early compromise of cell membrane integrity leading to a loss of ionic homeostasis and prominent cell swelling. Apoptosis reflects the activation of intrinsic genetic programs leading to the endonuclease cleavage of DNA and eventual death of the cell.6 While death of neurons induced by ischemia or trauma has been classically considered to be necrosis, growing evidence is suggestive that some cells undergo apoptosis following SCI.6, 7 Indeed, recent experimental data have suggested that the death of oligodendrocytes following traumatic SCI involves activation of apoptotic pathways.7 This research direction is particularly important because the potential for developing anti-apoptotic strategies to target late occurring cell death after SCI is an exciting possibility. For example, the anti-apoptotic gene, Bcl-2, has been reported to inhibit neuronal death induced by glutamate. The potential use of gene therapy to introduce protective genes into the host to enhance cellular neuroprotective mechanisms or antagonize cytotoxic products associated with apoptosis is an exciting research direction.

An emerging strategy for the treatment of SCI is directed toward inflammatory events. Experimental studies have indicated roles of several inflammatory molecules in the pathophysiology, including tumor necrosis factor (TNFa) and interleukin-I, as well as various chemokines.8 In this regard, recent experimental data have demonstrated that the administration of the potent anti-inflammatory cytokine, IL-10, improves both histopathological and locomotive function following traumatic SCI in rats.8 In that study, IL-10 treatment significantly decreased overall contusion volume, preserved the white matter tracts, and improved behavioral recovery as assessed by the Basso, Bresnahan and Beattie (BBB) open-field test. In terms of therapeutic windows for anti-inflammatory strategies, current investigations are determining how long after SCI IL-10 can be given to promote improved outcome.

Mild to moderate hypothermia has been shown to be neuroprotective in many experimental models of CNS injury.9 Local as well as systemic hypothermia has been used by various laboratories to prevent energy failure, reduce histopathological damage, diminish free radical activity, and elevated levels of glutamate following injury. Recent studies have evaluated the relationship between systemic and epidural temperature after SCI and the effects of moderate systemic hypothermia on histopathological and locomotive outcome following traumatic SCI in rats. In one study, post-traumatic hypothermia (32-33°C) initiated 30 minutes after injury for a 4-hour period significantly protected against locomotive deficits and reduced the area of tissue damage.10 In this regard, recent data also indicate that post-traumatic hypothermia following SCI reduces polymorphonuclear leukocyte accumulation.11 In a recent study, post-traumatic hypothermia significantly reduced myeloperoxidase (MPO) activity (an enzyme for neutrophil infusion) compared with normothermic animals. Thus, a potential mechanism by which hypothermia improves outcome following SCI is by attenuating post-traumatic inflammation. Whether mild systemic or local hypothermia can be used clinically to inhibit the detrimental effects of post-injury hyperthermia and/or protect the spinal cord from secondary injury merits further investigation.

Cellular transplantation following SCI. There is an unprecedented sense of enthusiasm within the scientific community that one day therapeutic strategies will be developed to treat paralysis following SCI. The replacement of lost cell populations by cell transplantation strategies as well as utilizing helper cells to deliver genes or proteins to enhance axonal regeneration is currently being investigated in many laboratories throughout the world.12 Indeed, several types of intraspinal transplants have effectively treated experimental models of human diseases. The peripheral nervous system provides an appropriate environment for axonal regeneration. David and Aguayo 13 first demonstrated that segments of sciatic nerve could be used as bridges between the medulla and lower cervical or thoracic spinal cord. In that study, evidence was presented that axons from nerve cells in the injured spinal cord and brainstem could elongate for long distances when the CNS’ glial environment was replaced by peripheral nerve. Axonal elongation is thought to be due to the presence of Schwann cells (SCs) that produce neurotrophic factors and synthesize and secrete elements of extracellular matrix, including laminin and cell adhesion molecules. Indeed, the use of guidance channels lined with SC’s has been successfully utilized to support axonal regeneration following transection of the adult spinal cord.14 Most recently, the axonal growth-promoting properties of adult olfactory bulb ensheathing glia (EG) and SC-filled guidance channels have been utilized to bridge spinal cord stumps and to enhance regeneration into the host spinal cord.15 Supraspinal serotonergic axons were reported to cross the transection gap through the bridges and elongate in white and periaqueductal gray. Long distance regeneration (at least 2.5 cm) of injured ascending propriospinal axons was also observed in the rostral spinal cord. Taken together, this study and others indicate that EG appear to provide injured spinal axons with factors for long-distance regeneration. Successful regeneration may, therefore, be attained eventually by using a combination of “helper cell” transplantation strategies following adult CNS injury.

The infusion of various neurotrophins to enhance axonal regeneration following SCI has also been utilized in various experimental models. Grill, et al,16 determined the effects of transgenic cellular delivery of neurotrophin-3 (NT-3) on the morphological and functional disturbances following SCI in rats. In that study, experimental subjects received grafts of fibroblasts genetically modified to produce NT-3. Importantly, the local cellular delivery of NT-3 in this model of SCI led to regrowth of corticospinal tracts and improved functional recovery at one and 3 months post-injury. Thus, the use of genetically engineered cells to locally deliver various neurotrophins represents an important approach to enhancing axonal regeneration.

Neuronal progenitors isolated from the adult CNS may one day be used to replace populations of neurons, astrocytes, and oligodendrocytes destroyed following injury. Recently, the consequences of transplanting embryonic stem cells into the injured rat spinal cord on recovery of function has been assessed. McDonald and colleagues,17 transplanted neuro-differentiated mouse embryonic stem cells into the injured spinal cord to determine the fate of these cells, as well as to determine whether the transplant procedure led to behavioral recovery. Histological analysis demonstrated that the transplanted cells survived and differentiated into astrocytes, oligodendrocytes, and neurons. Importantly, gait analysis showed improved hind- limb, weight support, and coordination in rats transplanted with stem cells, compared with controls. Ongoing research continues to investigate novel sources of stem cells that one day may be isolated from SCI subjects prior to transplantation procedures.

Although inflammatory processes have been implicated in the pathophysiology of neuronal cell injury following SCI,8 recent data indicate that activated macrophages may also enhance functional recovery. Rapalino and colleagues18 have reported that blood-borne macrophages stimulated with segments of rat peripheral sciatic nerve and transplanted into the lesion site lead to improved the electrophysiological, morphological, and behavioral outcome after SC transection, compared with non-treated animals. Based on these findings, it appears that therapeutic strategies to attenuate inflammatory responses after SCI may have to target early but not later occurring inflammatory processes. Indeed, the inflammatory cascade appears to be extremely complicated, and more experimental work is required to assess what specific inflammatory processes need to be attenuated or promoted after SCI.

Conclusion. Future directions. In terms of both acute neuroprotection and cellular transplantation strategies for the treatment of spinal cord dysfunction, great progress has been made in the last several years. In the area of neuroprotection, future investigations will target combination therapy where multiple strategies may be used to promote more complete protection and recovery of function following SCI. One exciting direction involves the use of mild hypothermia plus pharmacotherapy. Indeed, recent studies in cerebral ischemia found that mild hypothermia plus IL-10 protected the brain better than either therapy alone.9 In addition, better therapies must be developed to target white matter pathology which obviously plays an extremely important role in the functional consequences of SCI.

Recent breakthrough in the cell biology of CNS injury have demonstrated the regenerative capacity of the adult spinal cord to recover function under the right circumstances. Many laboratories throughout the world have reported novel strategies that appear to be successful in converting non-permissive environments for regeneration into permissive ones. Future studies will continue to investigate combination therapies to target axonal regeneration that may also include neuroprotective strategies to protect cellular transplants and enhance growth. It is conceivable that the use of multiple helper cells in addition to the administration of neuroprotective agents, neurotrophic factors and antibodies that target inhibitory proteins will be necessary to one day successfully regenerate the spinal cord. The correct combination of treatments in both the acute and chronic injury setting continues to be an exciting area of investigation in the field of SCI.

Acknowledgments. I would like to thank The Miami Project faculty and fellows for helpful discussions and Charlaine Rowlette for word processing and editorial assistance.

Address comments and questions to:

W. Dalton Dietrich, PhD
The Miami Project to Cure Paralysis
University of Miami School of Medicine
P.O. Box 016960 (R-48)
Miami, FL 33101

 Next Article