Introduction

Spinal Cord injury (SCI) is damage or trauma to the spinal cord resulting in paralysis and loss of sensation. There are ∼400 000 spinally injured patients in the United States with over 14 000 new injuries occurring each year.1 SCI imposes high physical and psychological effects not only to the individual, but also to the family and the society. Depending on the location of the injury, the symptoms of SCI may include loss of movement, sensation to feel heat, cold and touch, loss of bowel or bladder control and exaggerated reflex activities as well as pain.1

The pathophysiology of SCI is characterized by an initial primary injury followed by a secondary phase of injury in which oxidative stress is a critical component. Primary injury results immediately from the initial trauma, which includes contusion, damage to blood vessels and axonal shearing. In contrast, secondary injury is an indirect result from primary injury initiated by trauma. It occurs in hours, days and weeks following the primary injury. Secondary injury occurs not only at the site of the initial primary injury, but also results in spreading of the lesion to adjacent, otherwise uninjured tissue. Because secondary injury to spinal cord has an important role in disease progression, it is important to understand the molecular and cellular events leading to the secondary lesion of SCI.

Although the etiology and pathogenesis of SCI remain to be further elucidated, extensive studies over the last two decades have suggested that increased formation of reactive oxygen species (ROS) and the consequent oxidative stress are important events associated with SCI. The neurons and glia in the central nervous system including spinal cord are particularly prone to oxidative and electrophilic stress due to many factors, including a high content of polyunsaturated fatty acids, a high rate of oxidative metabolic activity, intense production of reactive oxygen metabolites and relatively low antioxidant capacity.2, 3 Indeed, oxidative stress is considered a hallmark of the secondary phase of injury of SCI.2, 3 Thus, alleviating oxidative stress may be an effective way of therapeutic intervention of SCI.

Methods

We searched MEDLINE (PubMed) and reference lists of the included articles published mainly between 1976 and 2011 using the following key words: SCI, oxidative stress, biomarkers, antioxidant, bioactive compounds and intervention.

Results

ROS and oxidative stress in SCI

Reactive oxygen species (ROS)

Molecular oxygen is an essential element of life, yet incomplete reduction or excitation of oxygen during aerobic metabolism generates ROS. ROS include superoxide, hydroxyl radical, singlet oxygen and hydrogen peroxide (Figure 1). Superoxide is the one-electron reduction product of molecular oxygen. If two electrons are transferred, the product is hydrogen peroxide. Transition metal ions, such as Fe2+ or Cu+ are capable of transferring a third electron to hydrogen peroxide, causing lysis of the O–O bond generating the hydroxyl radical, one of the most potent oxidants known. In phagocytes, superoxide is produced in large quantities by the enzyme nicotinamide adenine dinucleotide phosphate oxidase for use in oxygen-dependent killing mechanisms.4 Superoxide has also been implicated in the mechanisms of aging and the peroxidation of lipids. Hydrogen peroxide is not a free radical but is nonetheless a damaging species because of its ability to penetrate biological membranes. Hydrogen peroxide is toxic to cells and can lead to further free-radical generation. For example, hydrogen peroxide reacts with reduced transition metals to form the highly reactive hydroxyl radical, which readily causes damage to DNA and other biological molecules.5, 6, 7 Previous studies have demonstrated that the formation of ROS including hydroxyl radical, superoxide and hydrogen peroxide immediately after central nervous system injury may contribute to the pathogenesis of SCI.8, 9

Figure 1
figure 1

Schematic illustration of the formation of reactive oxygen species.

Full size image

Source of ROS

Production of ROS under physiological conditions is important for normal cellular redox reactions. However, excessive generation of free radicals under pathophysiological conditions such as SCI can greatly enhance the production of ROS. Mitochondrias are the major source of cellular ROS because they consume about 90% of the oxygen utilized by cells during the oxidative phosphorylation under physiological conditions. The mitochondrial dysfunction results in further increased formation of ROS. Structural and functional alterations of the mitochondria have been found in a SCI mouse model.10 SCI results in a cascade of secondary events including release of excitatory amino acids and disruption of calcium homeostasis, which may further cause the mitochondrial dysfunction and augmented ROS formation, leading to neuronal cell death.2 In addition to the mitochondria, phagocytic cells such as neutrophils and macrophages also contribute to the generation of ROS following experimental SCI.4 After traumatic SCI, phagocytic cells exhibit a marked increase in oxygen consumption and generation of superoxide from membranes-associated nicotinamide adenine dinucleotide phosphate oxidase. The enzyme complex transfers electrons from nicotinamide adenine dinucleotide phosphate oxidase at the cytosolic side of the membrane to molecular oxygen at the other side of the membrane resulting in the production of extracellular superoxide.4 Other potential biological sources of ROS associated with the pathogenesis of traumatic SCI include soluble cell constituents, cytosolic oxidases, transition metals, lysosomes, peroxisomes and endoplasmic reticulum. For example, there are many cytosolic enzymes that can generate ROS via reduction of molecular oxygen in their catalytic cycles. The most notable one is xanthine oxidase, which can directly reduce molecular oxygen to superoxide and hydrogen peroxide.11

Antioxidant defenses

ROS participate in physiological processes, such as cell signaling. However, excessive generation of ROS under pathophysiological conditions, including SCI may result in oxidative stress. To control ROS, aerobic organism has utilized several antioxidative mechanisms including enzymatic and non-enzymatic antioxidants12 (Figure 2). Non-enzyme low molecular weight antioxidant compounds include cellular glutathione, vitamins C and E, β-carotene and uric acid. The antioxidant enzymes include superoxide dismutase (SOD), catalase, glutathione reductase and glutathione peroxidase among others. SOD catalyzes the dismutation of superoxide to H2O2 in the following reaction: O2−˙+ O2−˙+ 2H+ → H2O2+O2. Mammalian cells contain three forms of SOD, MnSOD, Cu,ZnSOD and extracellular SOD. MnSOD is most abundant in the mitochondria, whereas Cn,ZnSOD predominant in the cytoplasm.13 Catalase is a major antioxidant enzyme that catalyzes the decomposition of H2O2 to H2O. Glutathione peroxidase is another major enzyme for decomposing H2O2.

Figure 2
figure 2

ROS and their detoxification by cellular antioxidants. GPx, glutathione peroxidase; GR, glutathione reductase; GSH, reduced glutathione; GSSG, oxidized form of glutathione; GST, glutathione S-transferase; G6PDH, glucose-6-phosphate dehydrogenase; 6PGDH, 6-phosphogluconate dehydrogenase; LOH, lipid alcohol; NQO1, NAD(P)H:quinone oxidoreductase 1; SOD, superoxide dismutase.

Full size image

The elimination of ROS following SCI is particularly controlled by cellular antioxidant glutathione and Cu,ZnSOD.14 Lucas, et al.14 examined the effects of glutathione treatment on lipid peroxidation after SCI in a rat contusion injury model. The results suggested that elevation of levels of glutathione by irrigation with δ-glutamylcysteine conferred significant protection against lipid peroxidation after SCI, suggesting that glutathione augmentation may be an effective strategy for curtailment of lipid peroxidation-mediated damage in SCI.14 Mutations in the gene for Cu,ZnSOD have been identified in familial amyotrophic lateral sclerosis (ALS), a disease characterized by damage and loss of neurons in both the brain and the spinal cord.15 Transgenic mice overexpressing wide-type Cu,ZnSOD gene show resistance to SCI, indicating that Cu,ZnSOD has an important role in protecting against spinal neuron death.16

NAD(P)H:quinone oxidoreductase 1 (NQO1) is another important antioxidant enzyme that has recently been demonstrated to has a protective role in electrophilic stress underlying spinal cord damage.17, 18 NQO1 catalyzes the two electron reduction of electrophilic quinone compounds, thus limiting the formation of semiquinone radicals through one electron reduction, and the subsequent generation of ROS.18, 19 NQO1 is also able to maintain the cellular levels of ubiquinol and vitamin E, two important biological antioxidants involved in the detoxification of ROS.17, 20 Therefore, the coordinated actions of a spectrum of cellular antioxidants and probably other cytoprotective proteins ensure efficient detoxification of ROS and electrophilic species that participate in the pathogenesis of SCI.

Lipid peroxidation

High levels of ROS can overwhelm the normal cellular antioxidant defenses leading to several direct and indirect health effects. Direct effects include chain of peroxidation reactions involving lipids and other macromolecules. Indirect effects include modified metabolic pathways and altered pathophysiology of the organ systems due to the oxidative damage. Oxidative stress may occur when ROS are produced faster than they can be removed by cellular defense mechanisms. The main damage to cells results from ROS-induced alteration of macromolecules such as polyunsaturated fatty acids in membrane lipids, proteins, and DNA. Lipid peroxidation is a common and dangerous type of ROS-induced cellular oxidation. It has long been recognized that much of the posttraumatic degeneration of the spinal cord following injury is caused by a secondary injury process, and a significant biochemical event is ROS-induced lipid peroxidation.2 In particular, spinal cord is highly vulnerable to oxidative injury owing to its overabundance of polyunsaturated fatty acids that are especially susceptible to peroxidation by ROS.3 The importance of ROS and lipid peroxidation in SCI is supported by many experimental and clinical studies demonstrating potential neural protective efficacy of multiple bioactive agents with antioxidant properties.2, 21, 22, 23, 24, 25, 26

Nrf2 as a central regulator of antioxidant defenses

The coordinated elevation of antioxidant enzymes is regulated through a cis-acting element called the antioxidant response element (ARE) within the regulatory region of each of the antioxidant enzyme genes27, 28 (Figure 3). Transcriptional activation mediated by the ARE is effected by the transcription factor Nrf2. Nrf2 belongs to the CNC family of b-zip transcription factors. The mechanism by which binding of Nrf2 to ARE is induced is still emerging, but likely includes contributions from the repressor of Nrf2 that normally resides in the cytosolic compartment along with the cysteine-rich chaperone Keap1. Keap 1 is an inhibitor of Nrf2 that sequesters it in the cytoplasm. Activation of Nrf2 dissociates Nrf2 from Keap1, allowing for its translocation to the nucleus, where it can interact with the ARE to activate transcription of antioxidant enzyme genes27, 28 (Figure 3). Activation of Nrf2 and Nrf2-mediated antioxidant enzyme gene expression by pro-oxidants, including H2O2 at moderate non-lethal doses suggests that Nrf2 signaling may control the adaptive response of neuronal cells to oxidative insults.29 This potential Nrf2-mediated adaptive response to oxidative stress is particularly relevant to SCI where the increased level of ROS is constantly generated for a prolonged period of time leading to progressive oxidative stress in secondary injury of SCI.18 Systematic increases in Nrf2 and ARE-driven heme oxygenase-1 in the spinal cord of SODG93A rats, an experimental model of ALS, have been reported,30, 31 which may represent an adaptive response to oxidative stress. In view of the critical involvement of ROS in SCI, upregulation of ARE/Nrf2 pathway to augment endogenous antioxidant defenses may serve as a unique approach to protective/therapeutic intervention of SCI.

Figure 3
figure 3

Nrf2 binds to the ARE and promotes transcription of antioxidant enzyme genes.

Full size image

Biomarkers of oxidative stress in SCI and their detection

Biomarkers of ROS and oxidative stress are useful for assessing the pathogenesis and progression of SCI. Biomarkers are defined as characteristics that are objectively measured and evaluated as indicators of normal biological activities, pathogenic processes or pharmacological responses to therapeutic intervention. Because the highly reactive ROS are short lived and difficult to measure directly, indirect measures are most often used to predict the amount of ROS or the extent of oxidative stress occurring in SCI. These include the measurement of glutathione, Cu,ZnSOD, malondialdehyde (MDA), acrolein and F2-isoprostanes. It is of note that each marker has its own limitation in predicting oxidation in biological systems. As such, it is recommended that at least measurement of two markers should be used.

Glutathione

Glutathione is the primary low molecular weight thiol antioxidant and co-substrate for several cellular antioxidant enzymes such as glutathione peroxidase and and glutathione transferase. It has a critical role in maintaining the intracellular oxidation–reduction (redox) balance and regulating oxidative stress-induced signaling pathways as well as detoxifying ROS and other reactive aldehydes including acrolein.32 It is well known that the central nervous system including the spinal cord has relatively low glutathione levels compared with the other organs, such as the liver, which makes the neurons of the spinal cord especially vulnerable to oxidative stress.1, 33, 34, 35 In the central nervous system, glutathione is present at high concentrations in the astrocytes, which also provide the substrate for glutathione synthesis such as cysteine and dipeptide cysteinyl-glycine to the neighboring neurons. In addition, it is reported that glutathione may also be an important regulatory neuropeptide in the central nervous system.36 The synthesis of glutathione from its constituent amino acids involves the actions of two ATP-dependent enzymes, γ-glutamylcysteine ligase (GCL) and glutathione synthase. GCL, the rate-limiting enzyme in the overall pathway, is a heterodimer composed of a catalytic (GCLC) and a modulatory (GCLM) subunit. GCLC remains all the catalytic activity and GCLM improves the catalytic efficiency. It has been demonstrated that significant decreases in glutathione occurred in spinal segments T5–T9 with the greatest decrease seen at the site of injury and immediately adjacent segments. Between 24 and 48 h, significant decreases in glutathione were also observed throughout the spinal cord from spinal segments C3 to L4, suggesting that the decrease of glutathione may be a hallmark of oxidative stress that accompanies the prominent inflammatory changes after spinal cord trauma.37

There are several chemical38 and enzymatic39 assays available for determination of glutathione. However, most of these assays lack sensitivity and specificity. Hissin and Hilf40 developed a fluorometric method for measuring glutathione. It is based on the reaction of reduced glutathione with o-phthalaldehyde to generate a highly fluorescent adduct that is specific for determination of reduced glutathione in rat tissue at pH 8.0.41 However, the original protocol developed by Hissin and Hilf requires a large amount of tissue samples. As such, it is not practical to routinely use the original protocol for glutathione determination in cultured neuronal cells especially under the conditions of using small volumes and microgram quantities of samples. We have modified the original method of Hissin and Hilf and reported a sensitive and specific assay for determining reduced glutathione status in human neuronal cells.42 The modified assay allows the use of small volumes and microgram quantities of samples, and is thus suitable for measuring reduced glutathione in a small number of cells. Given the use of a small sample volume, it also minimizes the presence of interfering substances in the samples.

Cu,ZnSOD

As mentioned before, mammalian tissues contain three forms of SOD: cytosolic Cu,ZnSOD, mitochondrial MnSOD and extracellular SOD. The presence of SOD helps dismutate superoxide immediately upon its generation and thus protects cells from superoxide-mediated oxidative damage. Several point mutations in the gene that encodes Cu,ZnSOD have been reported in some patients with familial ALS, a disease that is characterized by a loss of motor neurons in both the brain and the spinal cord.43 The cytosolic SOD activity, which is primarily Cu,ZnSOD, was reduced by 38.8% in some familial ALS patients relative to the control subjects.43, 44 Furthermore, overexpression of the Cu,ZnSOD mutants in rodents emulates clinical and pathological hallmarks of ALS through a possible mechanism of toxic gain of function.21 These results indicate a critical involvement of Cu,ZnSOD in neurodegeneration in ALS. Cellular Cu,ZnSOD activity can be measured using a commercially available kit from Trevigen, Inc. (Gaithersburg, MD, USA). This kit uses xanthine and xanthine oxidase to generate superoxide, which reacts with 2-(4-iodophenyl)-3-(4-nitrophenol)-5-phenyltetrazolium chloride to form formazan blue, which is monitored at 560 nm. The SOD activity is then determined by measuring the %inhibition of the reaction.

Malondialdehyde (MDA)

Lipid peroxidation is a well-defined mechanism of cellular damage and has been implicated in the pathogenesis of many disease processes including SCI. Lipid peroxidation is probably one of the most damaging effects of the ROS in SCI. Once a lipid radical is formed, polyunsaturated lipids can undergo self-propagating chains of peroxidation reactions. Aldehydes including MDA and the highly reactive α, β-unsaturated acrolein have been shown to be the end products of lipid peroxidation in SCI. There is increasing evidence suggesting that aldehydes may be causally involved in the pathophysiological effects associated with oxidative stress in SCI.45

Thiobarbituric acid-reacting substance assay is frequently used for detecting MDA. The standard or sample to be tested is heated with thiobarbituric acid at low pH, and MDA reacts with thiobarbituric acid to produce a MDA-thiobarbituric acid colored complex. Absorbance of the pink chromogen is measured at 532 nm or by fluorescence at 553 nm. This assay measures both free MDA and protein-bound MDA. The MDA-586 method (Oxis International, Inc., Foster City, CA, USA) is designed to assay free MDA. The assay conditions serve to minimize interference from other lipid peroxidation products, such as 4-hydroxyalkenal. The principle of the MDA-586 method is based the reaction of a chromogenic reagent, N-methyl-2-phenylindone with MDA at 45 °C. One molecule of MDA reacts with 2 molecules of N-methyl-2-phenylindone to yield a stable carbocyanine dye with maximum absorbance at 586 nm.

Acrolein

Acrolein, a major byproduct of oxidative stress and lipid peroxidation, has been implicated in the pathogenesis of SCI.3, 46 As noted earlier, acrolein is a highly reactive α, β-unsaturated aldehyde. It is produced as a byproduct of peroxidation of polyunsaturated fatty acids in cell membranes. Acrolein also occurs in the environment as a ubiquitous pollutant that is generated as a byproduct of overheated organic materials. The half-life of acrolein is estimated to be in the order of several hours, which is 100 billion times longer than that of many ROS. As such, acrolein is capable of diffusing to and injuring the otherwise healthy tissue.3, 46 Acrolein is the most potent electrophile among the unsaturated aldehydes. Acrolein-induced membrane damage may be an important pathogenic mechanism leading to cell death and functional loss in SCI.3, 46 Previous studies have demonstrated that acrolein is significantly increased following SCI, which is highly toxic to the spinal cord tissue.3, 46 As early as 4 h following SCI in vivo, acrolein was found to increase and remain elevated for at least 7 days after injury.3, 46 Notably, acrolein have been found to be elevated in the brain tissue and in the body fluids of patients with SCI.3, 46 Elevation of acrolein can deplete the cellular antioxidant glutathione as acrolein readily reacts with glutathione. This further compromises the endogenous antioxidant defenses.3, 46 Multiple studies also suggested that acrolein may represent a potential marker of oxidative stress damage to cellular constituents, especially protein targets in the spinal cord.3, 46

The presence of protein-bound acrolein could be detected by an antibody (mAb5F6) raised against the acrolein-modified proteins using immunoblot assay.47, 48 To visualized the accumulation of acrolein-modified proteins, immunohistochemical staining can be used with the same antibody against the acrolein-modified proteins.47, 48

8-Iso-prostaglandin F2α

8-Iso-prostaglandin F2α is an isoprostane that has been shown to be useful for assessing oxidative stress in spinal cord ischemia.49 It is a biologically stable end-product of the free radical-induced oxidation of arachidonic acid produced from non-cyclooxygenase and cyclooxygenase peroxidation pathways. It appears in normal plasma and urine samples and is elevated by oxidative stress. Determination of 8-iso-prostaglandin F2α has been proposed as a reliable index of oxidative stress during SCI.50

Several different techniques have been used to assay 8-iso-prostaglandin F2α. These include gas chromatography-mass spectrometry, enzyme immunoassay and radioimmunoassay. Recently, Basu50 has raised an antibody specific for 8-iso-prostaglandin F2α and developed an enzyme-linked immunosorbent assay for the measurement of 8-iso-prostaglandin F2α in biological fluids. In experimental spinal cord ischemia, a significant and immediate increase of 8-iso-prostaglandin F2α in plasma at the start and up to 60 min, and in the urine at 90–50 min was observed, indicating a consequence of oxidative injury.49 Compared with other assays, the greatest advantage of this immunoassay is that the level of 8-iso-prostaglandin F2α is sufficient to be detected in every normal biological fluid, and the limit of detection is down to 23 pmol l−1.

Antioxidant-based strategies for intervention of SCI

The involvement of ROS in the pathogenesis of SCI has prompted extensive studies on the neuroprotective effects of various compounds with antioxidative properties in animal models of SCI. It is possible that the use of certain antioxidants may slow the progression of spinal cord damage. This section begins with a description of Cu,ZnSOD in protecting against SCI followed by summarizing the major recent findings on SCI protection by some non-protein compounds with antioxidant properties.

Cu,ZnSOD

ALS is a fatal motor neuron degenerative disease characterized by a loss of motor neurons in the central nervous system including spinal cord. Among all of the familial ALS patients, 20–30% of them have point mutations in the gene that encodes Cu,ZnSOD.43 Over 100 different missense substitutions in the 153-amino acid Cu,ZnSOD have been described in individuals and kindreds affected by Cu,ZnSOD-linked familial ALS.51 Bowling et al.44 reported that cytosolic SOD activity, which was primarily Cu,ZnSOD activity, was reduced significantly in familial ALS with SOD1 mutations. Therefore, Cu,ZnSOD is likely an important therapeutic target that protects neurons from ROS damage after SCI. Utilizing Cu,ZnSOD-overexpressing transgenic rats and a mild spinal cord compression model to induce selective death of ventral horn motor neurons, Sugawara et al.52 reported that overexpression of Cu,ZnSOD reduced both the production of superoxide at an early stage after SCI and the subsequent ROS-mediated death of ventral horn motor neurons, supporting that Cu,ZnSOD is a potential therapeutic agent. However, it is important to note the overexpression of antioxidant enzymes via genetic approaches is currently not practical for the intervention of human SCI. In this context, PEP-1, a 21-residue peptide carrier, has been developed for efficiently delivering Cu,ZnSOD fusion protein into cultured neurons and injured spinal cord in vivo.22 Systemic administration of the fusion PEP-1-Cu,ZnSOD1 protein protected motor neurons from ROS damage and improved recovery after SCI.22 These findings suggest that PEP-1-Cu,ZnSOD may represent a new strategy for delivery of antioxidant enzymes for therapeutic intervention of SCI.

Non-protein compounds with antioxidant properties

Vitamin E: Vitamin E is an excellent antioxidant because it is a biological compound that is naturally present in animal and human tissues. It can cross intact cell membranes and has a long half-life and good bioavailability. Pretreatment with vitamin E has been shown to be protective in animal models of SCI.2, 53 Iwasa et al. systemically studied the effects of vitamin E on injury of the spinal cord associated with ischemia in rats.54, 55 The results showed that the motor disturbance induced by SCI was greatly reduced and spinal cord blood flow was promptly restored and remained normal after injury in the vitamin E-supplemented group. Tariq et al.56 have demonstrated that diabetic animals are more susceptible to compressive SCI as compared with non-diabetic animals most likely resulting from the higher depletion of vitamin E and glutathione in diabetic rats. Wang et al.57 reported that vitamin E feeding attenuated some of the detrimental effects of SCI on sperm functions and male accessory glands in rats, supporting a role of ROS-related events in deterioration of SCI.

Methylprednisolone: Methylprednisolone is a synthetic glucocorticoid steroid with potent anti-inflammatory activities. Methylprednisolone has been shown to also inhibit lipid peroxidation in vitro.2 Compression trauma of the cat spinal cord is known to induce a very rapid alteration in the lipid metabolism of cellular membranes leading to the secondary development of tissue ionic imbalance, ischemia, edema and inflammation.2 Studies demonstrated that pretreatment of cats with methylprednisolone (15–30 mg kg−1, i.v.) soon after blunt SCI decreased posttraumatic lipid peroxidation as measured by various biochemical indices.23, 24, 25 In addition to its ability to inhibit lipid peroxidation, methylprednisolone at high dose (30 mg kg−1, i.v.) has been shown to support energy metabolism, prevent progressive posttraumatic ischemia development, reverse intracellular calcium accumulation, ameliorate neurofilament degradation and inhibit membrane lipid hydrolysis.2 These actions could lead to an attenuation of posttraumatic neuronal degeneration after the injured spinal cord. However, when used at high dosage for extended period of time, methylprednisolone causes serious side effects, including weight gain, glaucoma, osteoporosis and psychosis. Despite the above adverse effects, methylprednisolone treatment has been shown to be beneficial in patients with SCI.

21-Aminosteroids: 21-Aminosteroids are methylprednisolone derivatives. Compared with methylprednisolone, 21-aminosteroids lack the glucocorticoid side effects that limit the clinical usefulness of high-dose methylprednisolone. The compound U-74006F is one of a series of 21-aminosteroids, which has been specifically developed for acute treatment of central nervous system trauma and ischemia due to its potent inhibition of lipid peroxidation.2, 26 U-72099E has been shown to be more potent than methylprednisolone in protecting rat brain synaptosomes from lipid peroxidation-induced inhibition of 14C-GABA uptake.2, 26 In addition, it equaled the ability of methylprednisolone to enhance the early neurological recovery of head injured mice.2 The dose-response characteristics and capability of U-74006F to promote functional recovery in cats subjected to compression trauma of the upper lumbar (L-2) spinal cord have also been evaluated.58 Results showed that over a 100-fold range of doses, U-74006F has a remarkable capacity to promote functional recovery in spinal cord injured cats. The mechanism of protective action of U-74006F against SCI is believed to involve an inhibition of oxygen radical-mediated lipid peroxidation. In vivo studies have demonstrated that U-74006F can preserve tissue vitamin E levels in central nervous system trauma.59 In a manner similar to vitamin E, U-74006F has been found to scavenge lipid peroxyl radical thereby inhibiting lipid radical chain reactions.26 U-74006F also decreased hydroxyl radical formation from Fenton reactions like due to its ability to chelate redox-active iron.2

3H-1,2-dithiole-3-thione: as noted earlier, the central nervous system is particularly vulnerable to oxidative stress due to many intrinsic factors. Notably, the central nervous system also has relatively low glutathione levels compared with the other organs such as the liver. Because the coordinated actions of glutathione and a spectrum of other cellular antioxidative enzymes are essential for efficient detoxification of ROS that participate in the pathogenesis of SCI, we propose a novel strategy for protective/therapeutic intervention of SCI through coordinated upregulation of endogenous antioxidants in the neuronal cells mediated by 3H-1,2-dithiole-3-thione (D3T). D3T is a constituent of cruciferous vegetables. We have recently reported that D3T increases multiple cellular antioxidants including glutathione and NQO1, two crucial cellular defenses against oxidative and electrophilic stress in human neuroblastoma cells (SH-SY5Y), human primary neurons and astrocytes, suggesting that D3T-mediated antioxidant induction is not cell-type specific.60, 61, 62 In addition, D3T treatment of the neuronal cells also results in a marked elevation of glutathione content in the mitochondrial compartment,62 a critical intracellular target for oxidative and electrophilic stress.63 It is important to note that the D3T upregulated endogenous defenses are accompanied by increased cellular resistance to neurocytotoxicity elicited by ROS and several well-known neurotoxins including acrolein, suggesting a potential therapeutic value in SCI.60, 61, 62

Other compounds: several epidemiological studies have demonstrated that increased consumption of antioxidant-rich fruits and vegetables is associated with reduced risk of ischemic stroke, dementia and SCI.64, 65 In this context, identification and study of the biologically active compounds may be of importance for developing strategies to inhibit, retard or reverse the pathophysiological processes underlying SCI. Beside the aforementioned agents with antioxidant properties for treating SCI, there are numerous dietary and botanical natural compounds in fruits, vegetable and plants (such as green tea compounds and herbal remedies) being actively investigated for their potential benefits in SCI. These include carotenoids and phenolic compounds. Carotenoids are ubiquitous in the plant kingdom, and as many as 1000 naturally occurring variants have been identified. Plant phenols are antioxidants by virtue of the hydrogen donating properties of the phenolic hydroxyl groups, and over 8000 plant phenols have been isolated. Dietary plants rich in these compounds include broccoli, brussel sprouts, cabbage, kale, cauliflower, carrots, onions, tomatoes, spinach, garlic and some herbal medicines. Additionally, some compounds found in these plants may improve the endogenous antioxidant defense through induction of antioxidant enzymes.

Table 1 lists some bioactive compounds derived from different sources showing benefits in protecting against oxidative stress underlying SCI. These compounds have been shown to suppress lipid peroxidation due to their antioxidant properties including scavenging ROS, chelating redox-active metal ions and inhibiting xanthine oxidase. However, the exact cell signaling pathways activated by these natural bioactive agents are still not clear, and each compound may act differently in different cells. Studies have suggested that the protective role of these compounds against oxidative stress damage is mediated, at least partially, by effects on signaling molecules including extracellular signal-regulated kinase, nuclear factor κB and Nrf2.18, 27, 28, 29, 30, 31 Nrf2 signaling appears to has a central role in regulating the induction of antioxidants by phenolic compounds.18, 27, 28, 29, 30, 31

Table 1 Other major agents shown to be protective against SCI in experimental models
Full size table

Conclusion

The central nervous system, including spinal cord is highly susceptible to free-radical-mediated damage due to its high lipid content and active oxygen metabolism. It has been demonstrated that ROS and oxidative stress have a significant role in the pathophysiology of SCI and are a hallmark of the secondary injury underlying SCI in animal models. Thus, alleviating oxidative stress of secondary injury process may represent an effective strategy for therapeutic intervention of SCI. In this regard, continued studies on bioactive compounds with ROS-scavenging capacity may lead to the development of effective antioxidant-based modalities for treating SCI in human subjects.