Activation of the caspase-3 apoptotic cascade in traumatic spinal cord injury

Systemic Administration of 17β-Estradiol Reduces Apoptotic Cell Death and Improves Functional Recovery following Traumatic Spinal Cord Injury in Rats

Recent evidence indicates that estrogen exerts neuroprotective effects in both brain injury and neurodegenerative diseases. We examined the protective effect of estrogen on functional recovery after spinal cord injury (SCI) in rats. 17beta-estradiol (3, 100, or 300 microg/kg) was administered intravenously 1-2 h prior to injury (pre-treatment), and animals were then subjected to a mild, weight-drop spinal cord contusion injury. Estradiol treatment significantly improved hind limb motor function as determined by the Basso-Beattie-Bresnahan (BBB) locomotor open field behavioral rating test. Fifteen to 30 days after SCI, BBB scores were significantly higher in estradiol-treated (100 microg/kg) rats when compared to vehicle-treated rats. Morphological analysis showed that lesion sizes increased progressively in either vehicle-treated or 17beta-estradiol-treated spinal cords. However, in response to treatment with 17beta-estradiol, the lesion size was significantly reduced 18-28 days after SCI when compared to vehicle-treated controls. Terminal deoxynucleotidyl transferase-mediated UTP nickend labeling (TUNEL) staining and DNA gel electrophoresis revealed that apoptotic cell death peaked 24-48 h after injury. Also, SCI induced a marked increase in activated caspase-3 in the spinal cord, evident by 4 h after injury. However, administration of 17beta-estradiol significantly reduced the SCI-induced increase in apoptotic cell death and caspase-3 activity after SCI. Furthermore, 17beta-estradiol significantly increased expression of the anti-apoptotic genes, bcl-2 and bcl-x, after SCI while expression of the pro-apoptotic genes, bad and bax, was not affected by drug treatment. Finally, intravenous administration of 17beta-estradiol (100 microg/kg) immediately after injury (post-treatment) also significantly improved hind limb motor function 19-30 days after SCI compared to vehicle-treated controls. These data suggest that after SCI, 17 beta-estradiol treatment improved functional recovery in the injured rat, in part, by reducing apoptotic cell death.

Minocycline inhibits contusion-triggered mitochondrial cytochrome c release and mitigates functional deficits after spinal cord injury

We investigated whether permeability transition-mediated release of mitochondrial cytochrome c is a potential therapeutic target for treating acute spinal cord injury (SCI). Based on previous reports, minocycline, a second-generation tetracycline, exerts neuroprotection partially by inhibiting mitochondrial cytochrome c release and reactive microgliosis. We first evaluated cytochrome c release at the injury epicenter after a T10 contusive SCI in rats. Cytochrome c release peaked at approximately 4-8 h postinjury. A dose-response study generated a safe pharmacological regimen that enabled i.p. minocycline to significantly lower cytosolic cytochrome c at the epicenter 4 h after SCI. In the long-term study, i.p. minocycline (90 mg/kg administered 1 h after SCI followed by 45 mg/kg administered every 12 h for 5 days) markedly enhanced long-term hind limb locomotion relative to that of controls. Coordinated motor function and hind limb reflex recoveries also were improved significantly. Histopathology suggested that minocycline treatment alleviated later-phase tissue loss, with significant sparing of white matter and ventral horn motoneurons at levels adjacent to the epicenter. Furthermore, glial fibrillary acidic protein and 2',3' cyclic nucleotide 3' phosphodiesterase immunocytochemistry showed an evident reduction in astrogliosis and enhanced survival of oligodendrocytes. Therefore, release of mitochondrial cytochrome c is an important secondary injury mechanism in SCI. Drugs with multifaceted effects in antagonizing this process and microgliosis may protect a proportion of spinal cord tissue that is clinically significant for functional recovery. Minocycline, with its proven clinical safety, capability to cross the blood-brain barrier, and demonstrated efficacy during a clinically relevant therapeutic window, may become an effective therapy for acute SCI.

Minocycline reduces cell death and improves functional recovery after traumatic spinal cord injury in the rat

We examined the effects of minocycline, an anti-inflammatory drug, on functional recovery following spinal cord injury (SCI). Rats received a mild, weight-drop contusion injury to the spinal cord and were treated with the vehicle or minocycline at a dose of 90 mg/kg immediately after SCI and then twice at a dose of 45 mg/kg every 12 h. Injecting minocycline after SCI improved hind limb motor function as determined by the Basso-Beattie-Bresnahan (BBB) locomotor open field behavioral rating test. Twenty four to 38 days after SCI, BBB scores were significantly higher in minocycline-treated rats as compared with those in vehicle-treated rats. Morphological analysis showed that lesion size increased progressively in both vehicle-treated and minocycline-treated spinal cords. However, in response to treatment with minocycline, the lesion size was significantly reduced at 21-38 days after SCI when compared to the vehicle control. Minocycline treatment significantly reduced the number of terminal deoxynucleotidyl transferase (TdT)-mediated deoxyuridine triphosphate-biotin nick end labeling (TUNEL)-positive cells 24 h after SCI as compared to that of the vehicle control. DNA gel electrophoresis also revealed a marked decrease in DNA laddering in response to treatment with minocycline. In addition, minocycline treatment significantly reduced the specific caspase-3 activity after SCI as compared to that of vehicle control. Furthermore, RT-PCR analyses revealed that minocycline treatment increased expression of interleukin-10 mRNA but decreased tumor necrosis factor-alpha expression. These data suggest that, after SCI, minocycline treatment modulated expression of cytokines, attenuated cell death and the size of lesions, and improved functional recovery in the injured rat. This approach may provide a therapeutic intervention enabling us to reduce cell death and improve functional recovery after SCI.

Mitochondrial permeability transition in CNS trauma: Cause or effect of neuronal cell death?

Experimental traumatic brain injury (TBI) and spinal cord injury (SCI) result in a rapid and significant necrosis of neuronal tissue at the site of injury. In the ensuing hours and days, secondary injury exacerbates the primary damage, resulting in significant neurologic dysfunction. It is believed that alterations in excitatory amino acids (EAA), increased reactive oxygen species (ROS), and the disruption of Ca(2+) homeostasis are major factors contributing to the ensuing neuropathology. Mitochondria serve as the powerhouse of the cell by maintaining ratios of ATP:ADP that thermodynamically favor the hydrolysis of ATP to ADP + P(i), yet a byproduct of this process is the generation of ROS. Proton-pumping by components of the electron transport system (ETS) generates a membrane potential (DeltaPsi) that can then be used to phosphorylate ADP or sequester Ca(2+) out of the cytosol into the mitochondrial matrix. This allows mitochondria to act as cellular Ca(2+) sinks and to be in phase with changes in cytosolic Ca(2+) levels. Under extreme loads of Ca(2+), however, opening of the mitochondrial permeability transition pore (mPTP) results in the extrusion of mitochondrial Ca(2+) and other high- and low-molecular weight components. This catastrophic event discharges DeltaPsi and uncouples the ETS from ATP production. Cyclosporin A (CsA), a potent immunosuppressive drug, inhibits mitochondrial permeability transition (mPT) by binding to matrix cyclophilin D and blocking its binding to the adenine nucleotide translocator. Peripherally administered CsA attenuates mitochondrial dysfunction and neuronal damage in an experimental rodent model of TBI, in a dose-dependent manner. The underlying mechanism of neuroprotection afforded by CsA is most likely via interaction with the mPTP because the immunosuppressant FK506, which has no effect on the mPT, was not neuroprotective. When CsA was administrated after experimental SCI at the same dosage and regimen used TBI paradigms, however, it had no beneficial neuroprotective effects. This review takes a comprehensive and critical look at the evidence supporting the role for mPT in central nervous system (CNS) trauma and highlights the differential responses of CNS mitochondria to mPT induction and the implications this has for therapeutically targeting the mPT in TBI and SCI.

Radiosensitizer sanazole (AK-2123) enhances gamma-radiation-induced apoptosis in murine fibrosarcoma

Sanazole (AK-2123) (N-2'-methoxy ethyl)-2-(3"-nitro-1"-triazolyl)acetamide, which has completed phase III clinical trials as a radiosensitizer, enhanced gamma-radiation induced apoptosis in murine fibrosarcoma upon i.p. administration at 40 mg/kg body weight one hour prior to irradiation. A microscopic examination of Giemsa-May-Grunwald stained cells has shown a higher frequency of condensed nuclei and fragmented nuclei in the tumor cells. The administration of sanazole to tumor-bearing animals enhanced the radiation-induced internucleosomal fragmentation in the nuclear genome of tumor cells. Higher levels of caspase-3 activity were also observed in the cell extracts of tumours from AK-2123 administered mice. Exposure to gamma-radiation of AK-2123-treated mouse further enhanced the caspase-3 activity, indicating the induction of apoptosis. The radiation sensitization property of sanazole was discernible by comparing the relative tumor diameter following irradiation after i.p. administration of AK-2123 and irradiation alone; it was higher during the first few days followed by the treatment.

A decade of caspases

Caspases are a family of cysteine proteases that play important roles in regulating apoptosis. A decade of research has generated a wealth of information on the signal transduction pathways mediated by caspases, the distinct functions of individual caspases and the mechanisms by which caspases mediate apoptosis and a variety of physiological and pathological processes.

Autonomic and motor neuron death is progressive and parallel in a lumbosacral ventral root avulsion model of cauda equina injury

Injuries to the cauda equina of the spinal cord result in autonomic and motor neuron dysfunction. We developed a rodent lumbosacral ventral root avulsion injury model of cauda equina injury to investigate the lesion effect in the spinal cord. We studied the retrograde effects of a unilateral L5-S2 ventral root avulsion on efferent preganglionic parasympathetic neurons (PPNs) and pelvic motoneurons in the L6 and S1 segments at 1, 2, 4, and 6 weeks postoperatively in the adult male rat. We used Fluoro-Gold-prelabeling techniques, immunohistochemistry, and quantitative stereologic analysis to show an injury-induced progressive and parallel death of PPNs and motoneurons. At 6 weeks after injury, only 22% of PPNs and 16% of motoneurons remained. Furthermore, of the neurons that survived at 6 weeks, the soma volume was reduced by 25% in PPNs and 50% in motoneurons. Choline acetyltransferase (ChAT) protein was expressed in only 30% of PPNs, but 80% of motoneurons remaining at 1 week postoperatively, suggesting early differential effects between these two neuronal types. However, all remaining PPNs and motoneurons were ChAT positive at 4 weeks postoperatively. Nuclear condensation and cleaved caspase-3 were detected in axotomized PPNs and motoneurons, suggesting apoptosis as a contributing mechanism of the neural death. We conclude that lumbosacral ventral root avulsions progressively deplete autonomic and motor neurons. The findings suggest that early neuroprotection will be an important consideration in future attempts of treating acute cauda equina injuries.

Molecular biology of cervical myelopathy and spinal cord injury: role of oligodendrocyte apoptosis

BACKGROUND CONTEXT

Rational design of treatment strategies for cervical myelopathy and spinal cord injury requires a working knowledge of the molecular biology underlying these pathological processes. The cellular process of apoptosis is an important component of tissue and organ development as well as the natural response to disease and injury. Recent studies have convincingly demonstrated that apoptosis also plays a pivotal role in numerous pathological processes, contributing to the adverse effects of various diseases and traumatic conditions. A growing body of evidence has implicated apoptosis as a key determinant of the extent of neurological damage and dysfunction after acute spinal cord injury and in chronic cervical myelopathy.

PURPOSE

To provide clinicians and research investigators interested in spinal cord injury and myelopathy with a practical and up-to-date basic science review of cellular apoptosis in the context of spinal cord pathology.

STUDY DESIGN/SETTING

A review of recently published or presented data from molecular biological, animal model and human clinical studies.

METHODS

A computer-based comprehensive review of the English-language scientific and medical literature was performed in order to identify relevant publications with emphasis given to more recent studies.

RESULTS

Investigation into the role of apoptosis in spinal cord injury and myelopathy has drawn the interest of an increasing number of researchers and has yielded a substantial amount of new information.

CONCLUSIONS

Apoptosis is a fundamental biological process that contributes to preservation of health as well as development of disease. There is now strong evidence to support a significant role for apoptosis in secondary injury mechanisms after acute spinal cord injury as well in the progressive neurological deficits observed in such conditions as spondylotic cervical myelopathy.

Expression of pro-inflammatory cytokine and caspase genes promotes neuronal apoptosis in pontine reticular formation after spinal cord transection

We identified apoptotic neurons in pontine reticular formation (PRF), the origin of pontine reticulospinal fibers, in adult Sprague-Dawley rats after complete spinal cord transection (SCT) at T8 level. SCT also increased the expression in PRF of tumor necrosis factor-alpha (TNF-alpha), interleukin (IL)-1beta, IL-6, caspase-1, or caspase-3 mRNA. This was followed by an augmented expression of activated caspase-3 protein, an increase in caspase-3 activity, and expression of a cleaved fragment of poly(ADP-ribose) polymerase (PARP), a proteolytic substrate of the activated caspase-3. Microinjection bilaterally into the PRF of an antiserum against TNF-alpha attenuated the expression of IL-6 mRNA and up-regulation of caspase-3 mRNA, and a caspase-3 inhibitor, DEVD-CHO, suppressed the augmentation in activated caspase-3 or cleaved PARP expression after SCT. Both treatments also reduced the number of SCT-induced apoptotic PRF neurons. We conclude that PRF neurons in adult mammalian brain may actively degrade themselves after SCT through apoptosis, via signaling processes that involve activation of proinflammatory cytokine genes and the intracellular caspase-3 pathway.

Enhanced oligodendrocyte survival after spinal cord injury in Bax-deficient mice and mice with delayed Wallerian degeneration

Mechanisms of oligodendrocyte death after spinal cord injury (SCI) were evaluated by T9 cord level hemisection in wild-type mice (C57BL/6J and Bax+/+ mice), Wlds mice in which severed axons remain viable for 2 weeks, and mice deficient in the proapoptotic protein Bax (Bax-/-). In the lateral white-matter tracts, substantial oligodendrocyte death was evident in the ipsilateral white matter 3-7 mm rostral and caudal to the hemisection site 8 d after injury. Ultrastructural analysis and expression of anti-activated caspase-3 characterized the ongoing oligodendrocyte death at 8 d as primarily apoptotic. Oligodendrocytes were selectively preserved in Wlds mice compared with C57BL/6J mice at 8 d after injury, when severed axons remained viable as verified by antereograde labeling of the lateral vestibular spinal tract. However, 30 d after injury when the severed axons in Wlds animals were already degenerated, the oligodendrocytes preserved at 8 d were lost, and numbers were then equivalent to control C57BL/6J mice. In contrast, oligodendrocyte death was prevented at both time points in Bax-/- mice. When cultured oligodendrocytes were exposed to staurosporine or cyclosporin A, drugs known to stimulate apoptosis in oligodendrocytes, those from Bax-/- mice but not from Bax+/+ or Bax+/- mice were resistant to the apoptotic death. In contrast, the three groups were equally vulnerable to excitotoxic necrosis death induced by kainate. On the basis of these data, we hypothesize that the Wallerian degeneration of white matter axons that follows SCI removes axonal support and induces apoptotic death in oligodendrocytes by triggering Bax expression.

Temporal and spatial distribution of activated caspase-3 after subdural kainic acid infusions in rat spinal cord

The molecular events initiating apoptosis following traumatic spinal cord injury (SCI) remain poorly understood. Soon after injury, the spinal cord is exposed to numerous secondary insults, including elevated levels of glutamate, that contribute to cell dysfunction and death. In the present study, we attempted to mimic the actions of glutamate by subdural infusion of the selective glutamate receptor agonist, kainic acid, into the uninjured rat spinal cord. Immunohistochemical colocalization studies revealed that activated caspase-3 was present in ventral horn motor neurons at 24 hours, but not 4 hours or 96 hours, following kainic acid treatment. However, at no time point examined was there evidence of significant neuronal loss. Kainic acid resulted in caspase-3 activation in several glial cell populations at all time points examined, with the most pronounced effect occurring at 24 hours following infusion. In particular, caspase-3 activation was observed in a significant number of oligodendroglia in the dorsal and ventral funiculi, and there was a pronounced loss of oligodendroglia at 96 hours following treatment. The results of these experiments indicate a role for glutamate as a mediator of oligodendroglial apoptosis in traumatic SCI. In addition, understanding the apoptotic signaling events activated by glutamate will be important for developing therapies targeting this cell death process.

Post-traumatic inflammation following spinal cord injury

Inflammatory reaction following a spinal cord injury (SCI) contributes substantially to secondary effects, with both beneficial and devastating effects. This review summarizes the current knowledge concerning the structural features (vascular, cellular, and biochemical events) of SCI and gives an overview of the regulation of post-traumatic inflammation.

Caspase inhibition attenuates transection-induced oligodendrocyte apoptosis in the developing chick spinal cord

A developmental model of spinal cord injury in the embryonic chick was specifically developed to characterize the involvement of caspases in injury-induced oligodendrocyte apoptosis remote from the lesion and the ability of caspase inhibitors to attenuate this process. Developmental apoptosis in the cervical spinal cord increased within the white matter between embryonic days 13 and 18, the period of myelination of this region. Spinal cord transection during this period induced a rapid increase in apoptotic cells in the ventral and lateral white matter over several millimeters caudal to the injury. Immunostaining identified large numbers of these cells as oligodendrocytes. Catalytic activity assays and immunostaining demonstrated caspase-3-like but not caspase-1-like activity to be involved in this apoptotic response. In vivo application of specific caspase inhibitors significantly attenuated transection-induced apoptosis. Thus, we describe a developmental period during which spinal oligodendrocytes exhibited a heightened, caspase-dependent sensitivity to transection-induced apoptosis that is attenuated by caspase inhibition.

Peroxynitrite generated in the rat spinal cord induces apoptotic cell death and activates caspase-3

We previously demonstrated that the peroxynitrite concentration increases after impact spinal cord injury. This study tests whether spinal cord injury-elevated peroxynitrite induces apoptotic cell death. Peroxynitrite was generated at the concentration and duration produced by spinal cord injury by administering S-morpholinosydnonimine through a microdialysis fiber into the gray matter of the rat spinal cord. Fragmented DNA was visualized by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end-labeling. Transferase-mediated deoxyuridine triphosphate-biotin nick end-labeling-positive neurons were quantitated by counting the transferase-mediated deoxyuridine triphosphate-biotin nick end-labeling and neuron-specific enolase double-stained neurons along the fiber track in the sections removed at 6, 12, 24 and 48 h post-peroxynitrite exposure. Peroxynitrite significantly increased transferase-mediated deoxyuridine triphosphate-biotin nick end-labeling-positive neurons at all time points examined (P< or =0.001) compared with artificial cerebrospinal fluid controls (Two-way analysis of variance followed by Tukey test), peaking at 24 h post-exposure. Electron microscopic observation of characteristic features of apoptosis confirmed peroxynitrite-induced neuronal apoptosis. Total transferase-mediated deoxyuridine triphosphate-biotin nick end-labeling-positive cells were counted in areas near and 0.2 mm away from the fiber track. The counts both peaked at 24 h with no significant difference between the two areas. However, at 6 and 12 h post-exposure the counts were significantly higher near than away from the fiber track (P=0.03 and P=0.007 respectively, paired t test). Immunohistochemical staining indicates caspase-3 was activated by peroxynitrite; this activation peaked at 6 h post-exposure, suggesting that activation of caspase-3 might be an early event in the apoptotic cell death cascade. We conclude that 1) peroxynitrite generated in the cord at the level produced by spinal cord injury induces neuronal apoptosis, indicating a role for peroxynitrite in secondary spinal cord injury; 2) caspase activation might be involved in peroxynitrite-induced neuronal apoptosis; 3) therefore removal of peroxynitrite should reduce secondary cell death after spinal cord injury.

Primary cortical motor neurons undergo apoptosis after axotomizing spinal cord injury

Spinal cord injury (SCI) results in loss of voluntary motor control followed by incomplete recovery, which is partly mediated by the descending corticospinal tract (CST). This system is an important target for therapeutic repair strategies after SCI; however, the question of whether apoptotic cell death occurs in these axotomized neurons remains unanswered. In this study, adult (150-175 g) male Sprague-Dawley rats underwent T9 transection of the dorsal funiculus, which axotomizes the dorsal CST, and introduction of the retrograde tracer Fluoro-Gold into the lesion site. Primary motor cortex (M1) was then examined for evidence of apoptosis weekly for 4 weeks after injury. Axotomized pyramidal cells, identified by retrograde transport of Fluoro-Gold, were found in M1 (57.5 +/- 9.6/median section, 6127 +/- 292 total), and a significant proportion were terminal deoxynucleotidyl transferase (TdT) -mediated deoxyuridine triphosphate (dUTP)-rhodamine nick end labeling (TUNEL) -positive at 1 week after injury (39.3 +/- 5.6%), compared with animals undergoing sham surgery (1.2 +/- 1.4%). At 2-4 weeks, fewer cells were Fluoro-Gold-positive (24.6 +/- 65.06 to 25.3 +/- 6.4/median section, 2338 +/- 233 to 2393 +/- 124 total), of which very few were TUNEL-positive. In TUNEL-positive cells, Hoechst 33342 staining revealed nuclear morphology consistent with apoptosis, chromatin condensation, and formation of apoptotic bodies. Fluoro-Gold-positive cells showed increased caspase-3 and Bax immunoreactivity. Hematoxylin and eosin staining revealed similar nuclear changes and dystrophic cells. Internucleosomal DNA fragmentation was detected by gel electrophoresis at the 1-week time point. Lesioned animals not receiving Fluoro-Gold exhibited the same markers of apoptosis. These results document, for the first time, features of apoptotic cell death in a proportion of axotomized cortical motor neurons after SCI, suggesting that protection from apoptosis may be a prerequisite for regenerative approaches to SCI.

Calpain in the pathophysiology of spinal cord injury: neuroprotection with calpain inhibitors

Spinal cord injury (SCI) evokes an increase in intracellular free Ca(2+) level resulting in activation of calpain, a Ca(2+)-dependent cysteine protease, which cleaves many cytoskeletal and myelin proteins. Calpain is widely expressed in the central nervous system (CNS) and regulated by calpastatin, an endogenous calpain-specific inhibitor. Calpastatin degraded by overactivation of calpain after SCI may lose its regulatory efficiency. Evidence accumulated over the years indicates that uncontrolled calpain activity mediates the degradation of many cytoskeletal and membrane proteins in the course of neuronal death and contributes to the pathophysiology of SCI. Cleavage of the key cytoskeletal and membrane proteins by calpain is an irreversible process that perturbs the integrity and stability of CNS cells leading to cell death. Calpain in conjunction with caspases, most notably caspase-3, can cause apoptosis of the CNS cells following trauma. Aberrant Ca(2+) homeostasis following SCI inevitably activates calpain, which has been shown to play a crucial role in the pathophysiology of SCI. Therefore, calpain appears to be a potential therapeutic target in SCI. Substantial research effort has been focused upon the development of highly specific inhibitors of calpain and caspase-3 for therapeutic applications. Administration of cell permeable and specific inhibitors of calpain and caspase-3 in experimental animal models of SCI has provided significant neuroprotection, raising the hope that humans suffering from SCI may be treated with these inhibitors in the near future.

Caspase-3 mediates hippocampal apoptosis in pneumococcal meningitis

Bacterial meningitis causes neuronal apoptosis in the hippocampal dentate gyrus, which is associated with learning and memory impairments after cured disease. The execution of the apoptotic program involves pathways that converge on activation of caspase-3, which is required for morphological changes associated with apoptosis. Here, the time course and the role of caspase-3 in neuronal apoptosis was assessed in an infant rat model of pneumococcal meningitis. During clinically asymptotic meningitis (0-12 h after infection), only minor apoptotic damage to the dentate gyrus was observed, while the acute phase (18-24 h) was characterized by a massive increase of apoptotic cells, which peaked at 36 h. In the subacute phase of the disease (36-72 h), the number of apoptotic cells decreased to control levels. Enzymatic caspase-3 activity was significantly increased in hippocampal tissue of infected animals compared to controls at 22 h. The activated enzyme was localized to immature cells of the dentate gyrus, and in vivo activity was evidenced by cleavage of the amyloid-beta precursor protein. Intracisternal administration of the caspase-3-specific inhibitor Ac-DEVD-CHO significantly reduced apoptosis in the hippocampal dentate gyrus. In contrast to a study where the decrease of hippocampal apoptosis after administration of a pan-caspase inhibitor was due to downmodulation of the inflammatory response, our data demonstrate that specific inhibition of caspase-3 did not affect inflammation assessed by TNF-alpha and IL-1beta concentrations in the cerebrospinal fluid space. Taken together, the present results identify caspase-3 as a key effector of neuronal apoptosis in pneumococcal meningitis.

Mitochondria in cell death: novel targets for neuroprotection and cardioprotection

Post-mitotic neurons and heart muscle cells undergo apoptotic cell death in a variety of acute and chronic degenerative diseases. The intrinsic pathway of apoptosis involves the permeabilization of mitochondrial membranes, which leads to the release of protease and nuclease activators, and to bioenergetic failure. Mitochondrial permeabilization is induced by a variety of pathologically relevant second messengers, including reactive oxygen species, calcium, stress kinases and pro-apoptotic members of the Bcl-2 family. Several pharmacological agents act on mitochondria to prevent the permeabilization of their membranes, thereby inhibiting apoptosis. Such agents include inhibitors of the permeability transition pore complex (in particular ligands of cyclophilin D), openers of mitochondrial ATP-sensitive or Ca(2+)-activated K(+) channels, and proteins from the Bcl-2 family engineered to cross the plasma membrane. In addition, manipulations that modulate the expression or activity of mitochondrial uncoupling proteins can prevent the death of post-mitotic cells. Such agents hold promise for use in clinical neuroprotection and cardioprotection.

Glutamate enhances caspase-3 immunoreactivity in cultured spinal cord neurons of newborn rats

The role of glutamate in the mechanism of spinal neuron death is not fully understood. With addition of glutamate to primary culture of 11-day-old rat spinal cord, the number of caspase-3 positive small neurons of the dorsal horn greatly increased at 6-24 h in contrast to the case with vehicle. The addition of glutamate made caspase-3 immunoreactivity stronger in the cytoplasm of large motor neurons in the ventral horn. The present results show that excessive amount of glutamate enhances apoptotic pathway through caspase-3 in cultured spinal neurons of newborn rat.

Gene profiling in spinal cord injury shows role of cell cycle in neuronal death

Spinal cord injury causes secondary biochemical changes leading to neuronal cell death. To clarify the molecular basis of this delayed injury, we subjected rats to spinal cord injury and identified gene expression patterns by high-density oligonucleotide arrays (8,800 genes studied) at 30 minutes, 4 hours, 24 hours, or 7 days after injury (total of 26 U34A profiles). Detailed analyses were limited to 4,300 genes consistently expressed above background. Temporal clustering showed rapid expression of immediate early genes (30 minutes), followed by genes associated with inflammation, oxidative stress, DNA damage, and cell cycle (4 and 24 hours). Functional clustering showed a novel pattern of cell cycle mRNAs at 4 and 24 hours after trauma. Quantitative reverse transcription polymerase chain reaction verified mRNA changes in this group, which included gadd45a, c-myc, cyclin D1 and cdk4, pcna, cyclin G, Rb, and E2F5. Changes in their protein products were quantified by Western blot, and cell-specific expression was determined by immunocytochemistry. Cell cycle proteins showed an increased expression 24 hours after injury and were, in part, colocalized in neurons showing morphological evidence of apoptosis. These findings suggest that cell cycle-related genes, induced after spinal cord injury, are involved in neuronal damage and subsequent cell death.

Caspase inhibitors as anti-inflammatory and antiapoptotic agents

The striking efficacy of Z-VAD-fmk in the various animal models presented above may reflect its ability to inhibit multiple enzymes including caspases. In accord with this, more selective, reversible inhibitors usually show low efficacy in multifactorial models such as ischaemia, but may offer some protection against NMDA-induced excitotoxicity and hepatitis. Importantly, caspase inhibitors may exhibit significant activity in vivo even when they are applied post insult. As far as the CNS is concerned, the first systemically active inhibitors have emerged. Functional recovery could be achieved in some ischaemia models, but long-term protection by caspase inhibitors is still being questioned. Recent developments in drug design enabled the first caspase inhibitors to enter the clinic. Although initially directed towards peripheral indications such as rheumatoid arthritis, caspase inhibitors will no doubt eventually be used to target CNS disorders. For this purpose the peptidic character of current inhibitors will have to be further reduced. Small molecule, nonpeptidic caspase inhibitors, which have appeared recently, indicate that this goal can be accomplished. Unfortunately, many fundamental questions still remain to be addressed. In particular, the necessary spectrum of inhibitory activity required to achieve the desired effect needs to be determined. There is also a safety aspect associated with prolonged administration. Therefore, the next therapeutic areas for broader-range caspase inhibitors are likely to involve acute treatment. Recent results with synergistic effects between MK-801 and caspase inhibitors in ischaemia suggest that caspase inhibitors may need to be used in conjunction with other drugs. It can be expected that, in the near future, research on caspases and their inhibitors will remain a rapidly developing area of biology and medicinal chemistry. More time, however, may be needed for the first caspase inhibitors to appear on the market.

Increased production of tumor necrosis factor-alpha induces apoptosis after traumatic spinal cord injury in rats

We showed previously that, after spinal cord injury (SCI), tumor necrosis factor-alpha (TNF-alpha) may serve as an external signal, initiating apoptosis in neurons and oligodendrocytes. To further characterize the apoptotic cascade initiated by TNF-alpha after SCI, we examined the expression of TNF-alpha, inducible nitric oxide (NO) synthase (iNOS), and the level of NO after SCI. Western blots and reverse transcription polymerase chain reactions showed an early upregulation of TNF-alpha after injury. A peak TNF-alpha expression was observed within 1 h of injury. By 4 h after injury, the expression of iNOS and the level of NO were markedly increased in the injured spinal cord. Terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end labeling (TUNEL)-positive cells were also first observed in the lesioned area 4 h after SCI. The largest number of TUNEL-positive cells was observed between 24-48 h after SCI. Injecting a neutralizing antibody against TNF-alpha into the lesion site after injury significantly reduced the expression of iNOS, the level of NO and the number of TUNEL-positive cells in the injured spinal cord. Injecting the NOS inhibitors, N(G)-monomethyl-L-arginine monoacetate and S-methylisothiourea sulfate, or an NO scavenger, carboxy-PTIO, into the lesion site also significantly reduced the level of NO and the degree of DNA laddering in the injured spinal cord. These data suggest that after SCI, apoptosis induced by TNF-alpha may be mediated in part by NO via upregulation of iNOS, induced in response to TNF-alpha.

Effects of intrathecal administration of a cell permeant caspase inhibitor, boc-D-fluoromethylketone (BDFMK), on behavioral deficits following spinal cord ischemia: a dose-response analysis

Caspase inhibition has been proposed as a target to attenuate ischemia-induced neurodegeneration and behavioral dysfunction. The present study evaluated the pharmacological effects of a single dose of an irreversible cell permeant general (nonselective) caspase inhibitor, Boc-D-fluoromethylketone (BDFMK) administered intrathecally (i.t.) in a reversible spinal cord ischemia model (RSCIM). Quantal analysis indicated that the P(50) (represents the duration of ischemia that produces permanent paraplegia in 50% of the animals in a group) of the control group was 25.08+/-4.71 min. Using the RSCIM, neuroprotection is observed if a drug significantly prolongs the P(50) compared to the control group. The P(50) values for the BDFMK-treated groups were 27.21+/-2.62, 27.28+/-3.29 and 28.98+/-2.32 min, for the three dose groups studied. There were no statistically significant changes when measured 18 or 48 h following ischemia. Biochemical analysis of cell extracts from the caudal lumbar spinal cord indicated that there was increased production of the 120-kDa fragment of fodrin suggesting enhanced caspase-3 activity, an increase that was reduced by i.t. BDFMK administration. Moreover, in caudal lumbar spinal cord extracts from a set of paraplegic rabbits (25-50 min occlusion), we measured a 32-42% decrease of caspase-3 activity in BDFMK-treated rabbits. The present study shows that i.t. administration of BDFMK reduced caspase-3 activity, but the inhibition did not translate into a significant behavioral improvement. Our results suggest that administration of a single dose of the caspase inhibitor BDFMK is insufficient to attenuate ischemia-induced behavioral deficits following aortic occlusion.

Cell permeable ROS scavengers, Tiron and Tempol, rescue PC12 cell death caused by pyrogallol or hypoxia/reoxygenation

The role of superoxide anion (O(2)*-) in neuronal cell injury induced by reactive oxygen species (ROS) was examined in PC12 cells using pyrogallol (1,2,3-benzenetrior), a donor to release O(2)*-. Pyrogallol induced PC12 cell death at concentrations, which evidently increased intracellular O(2)*-, as assessed by O(2)(*-)-sensitive fluorescent precursor hydroethidine (HEt). Caspase inhibitors, Z-VAD-FMK and Z-Asp-CH(2)-DCB, failed to protect cells from injury caused by elevation of intracellular O(2)*-, although these inhibitors had effects on hypoxia- or hydrogen peroxide (H(2)O(2))-induced PC12 cell death. Two known O(2)*- scavengers, Tiron (4,5-dihydroxy-1,3-benzenedisulfonic acid) and Tempol (4-hydroxy-2,2,6,6-tetramethylpiperydine-1-oxyl) rescued PC12 cells from pyrogallol-induced cell death. Hypoxia/reoxygenation injury of PC12 cells was also blocked by Tiron and Tempol. Further understanding of the underlying mechanism of the protective effects of these radical scavengers reducing intracellular O(2)*- on neuronal cell death may lead to development of new therapeutic treatments for hypoxic/ischemic brain injury.

Caspase activation in neuronal and glial apoptosis following spinal cord injury in mice

The involvement of caspases in apoptosis after spinal cord injury (SCI) was investigated in adult mouse spinal cord after contusion. Sections of spinal cord were processed for staining 7 days after SCI with the fluorescent dye Hoechst 33342, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end-labeling (TUNEL), and immunostaining with an antibody (CM1) recognizing activated caspase-3. Caspase-3- and caspase-8-like enzyme activities were measured colorimetrically at 8 hours to 7 days after SCI using the specific substrates Asp-Glu-Val-Asp-p-nitroanilide and Ile-Glu-Thr-Asp-p-nitroanilide, respectively. Hoechst 33342 staining showed small, bright areas in fragmented nuclei. Double labeling with TUNEL plus immunostaining with cell type-specific markers identified TUNEL-positive neurons stained by anti-neuronal nuclear protein/neurons antibody, and TUNEL-positive oligodendrocytes stained by anti-cyclic nucleotide 3'-phosphohydrolase antibody. Double labeling with CM1 and cell-type specific markers similarly identified CM1-positive neurons and oligodendrocytes. Caspase-8-like enzyme activity was increased significantly on days 3 and 7 (p < 0.01), whereas caspase-3-like activity increased on day 7 (p < 0.01). Intraventricular injection of a nonspecific tetrapeptide caspase inhibitor or a specific tetrapeptide inhibitor of caspase-3 just after SCI reduced enzyme activity at 7 days. Apoptotic cells were identified with TUNEL staining in both neurons and oligodendrocytes in mice after SCI, which also showed activated caspase-3. Increased caspase-3- and caspase-8-like activity was detected in the injured spinal cord on days 3 and 7. Caspase protease activities may be involved in delayed neuronal and glial apoptosis after SCI.

Decreased neural damage after spinal cord injury in tPA-deficient mice

Tissue plasminogen activator (tPA) is a serine protease that converts plasminogen to plasmin. It plays an important role in the nervous system, including the processes of neuronal migration, neurite outgrowth, and neuronal plasticity. tPA has also been suggested to have a role in several neuropathological conditions, such as cerebral ischemia, seizures, and demyelinating diseases. To investigate the role of tPA in spinal cord injury, wild-type mice and mice with homozygous tPA deficiency (tPA(-/-) mice) were subjected to spinal cord contusion and the differences of hindlimb function, electrophysiological changes, and histopathological changes were assessed for 6 weeks. Functional recovery was greater in tPA(-/-) mice than in wild-type mice throughout the observation period. The time course of myoelectric motor-evoked potentials supported the hindlimb functional findings. Histological examination showed that injured areas were smaller in tPA(-/-) mice than wild-type mice on Luxol fast blue staining or myelin basic protein and neurofilament protein immunostaining at 6 weeks after contusion. Electron microscopy showed that the white matter was better preserved in tPA(-/-) mice than in wild-type mice. The expression of tPA protein was widespread on the first day after contusion and this expression was detected for at least a week. Activation of microglia/macrophages and apoptotic cell death were significantly reduced in tPA(-/-) mice after contusion. This study shows that neural damage is decreased in tPA(-/-) mice after spinal cord injury. Suppression of tPA production may help to decrease secondary injury after spinal cord contusion.

Dynorphin A (1–17) induces apoptosis in striatal neurons in vitro through α-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate receptor-mediated cytochrome C release and caspase-3 activation

Dynorphin A (1-17), an endogenous opioid neuropeptide, can have pathophysiological consequences at high concentrations through actions involving glutamate receptors. Despite evidence of excitotoxicity, the basic mechanisms underlying dynorphin-induced cell death have not been explored. To address this question, we examined the role of caspase-dependent apoptotic events in mediating dynorphin A (1-17) toxicity in embryonic mouse striatal neuron cultures. In addition, the role of opioid and/or glutamate receptors were assessed pharmacologically using dizocilpine maleate (MK(+)801), a non-equilibrium N-methyl-D-aspartate (NMDA) antagonist; 6-cyano-7-nitroquinoxaline-2,3-dione, a competitive alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate antagonist; or (-)-naloxone, a general opioid antagonist. The results show that dynorphin A (1-17) (>or=10 nM) caused concentration-dependent increases in caspase-3 activity that were accompanied by mitochondrial release of cytochrome c and the subsequent death of cultured mouse striatal neurons. Moreover, dynorphin A-induced neurotoxicity and caspase-3 activation were significantly attenuated by the cell permeable caspase inhibitor, caspase-3 inhibitor-II (z-DEVD-FMK), further suggesting an apoptotic cascade involving caspase-3. AMPA/kainate receptor blockade significantly attenuated dynorphin A-induced cytochrome c release and/or caspase-3 activity, while NMDA or opioid receptor blockade typically failed to prevent the apoptotic response. Last, dynorphin-induced caspase-3 activation was mimicked by the ampakine CX546 [1-(1,4-benzodioxan-6-ylcarbonyl)piperidine], which suggests that the activation of AMPA receptor subunits may be sufficient to mediate toxicity in striatal neurons. These findings provide novel evidence that dynorphin-induced striatal neurotoxicity is mediated by a caspase-dependent apoptotic mechanism that largely involves AMPA/kainate receptors.

Cell death in models of spinal cord injury

Current treatments for acute spinal cord injury are based on animal models of human spinal cord injury (SCI). These models have shown that the initial traumatic injury to cord tissue is followed by a long period of secondary injury that includes a number of cellular and biochemical cascades. These secondary injury processes are potential targets for therapies. Continued refinement of rat and mouse models of SCI, along with more detailed analyses of the biology of the lesion in these models, points to both necrotic and apoptotic mechanisms of cell death after SCI. In this chapter, we review recent evidence for long-term apoptotic death of oligodendrocytes in long tracts undergoing Wallerian degeneration following SCI. This process appears to be related closely to activation of microglial cells. It is has been thought that microglial cells might be the source of cytotoxic cytokines, such as tumor necrosis factor-alpha (TNF-alpha), that kill oligodendrocytes. However, more recent evidence in vivo suggests that TNF-alpha by itself may not induce necrosis or apoptosis in oligodendrocytes. We review data that suggests other possible pathways for apoptosis, such as the neurotrophin receptor p75 which is expressed in both neurons and oligodendrocytes after SCI in rats and mice. In addition, it appears that microglial activation and TNF-alpha may be important in acute SCI. Ninety minutes after a moderate contusion lesion, microglia are activated and surround dying neurons. In an 'atraumatic' model of SCI, we have now shown that TNF-alpha appears to greatly potentiate cell death mediated by glutamate receptors. These studies emphasize that multiple mechanisms and interactions contribute to secondary injury after SCI. Continued study of both contusion models and other new approaches to studying these mechanisms will be needed to maximize strategies for acute and chronic therapies, and for neural repair.

Overexpression of SOD1 protects vulnerable motor neurons after spinal cord injury by attenuating mitochondrial cytochrome c release

Defective Cu,Zn-superoxide dismutase (SOD1) is responsible for some types of amyotrophic lateral sclerosis, and ventral horn motor neurons (VMN) have been shown to die through a mitochondria-dependent apoptotic pathway after chronic exposure to high levels of reactive oxygen species (ROS). VMN are also selectively vulnerable to mild spinal cord injury (SCI); however, the involvement of SOD1, ROS, and apoptosis in their death has not been clarified. Mild compression SCI was induced in SOD1-overexpressing transgenic rats and wild-type littermates. Superoxide production, mitochondrial release of cytochrome c, and activation of caspase-9 were examined, and apoptotic DNA injury was also characterized. In the wild-type animals, increased superoxide production, mitochondrial release of cytochrome c, and cleaved caspase-9 were observed exclusively in VMN after SCI. Subsequently, a majority of VMN (75%) selectively underwent delayed apoptotic cell death. Transgenic animals showed less superoxide production, mitochondrial cytochrome c release, and caspase-9 activation, resulting in death of only 45% of the VMN. These results suggest that the ROS-initiated mitochondrial signaling pathway possibly plays a pivotal role in apoptotic VMN death after SCI and that increased levels of SOD1 in VMN reduce oxidative stress, thereby attenuating the activation of the pathway and delayed cell death.

Caspase activation and neuroprotection in caspase-3- deficient mice after in vivo cerebral ischemia and in vitro oxygen glucose deprivation

Caspase-3 is a major cell death effector protease in the adult and neonatal nervous system. We found a greater number and higher density of cells in the cortex of caspase-3(-/-) adult mice, consistent with a defect in developmental cell death. Caspase-3(-/-) mice were also more resistant to ischemic stress both in vivo and in vitro. After 2 h of ischemia and 48 h of reperfusion, cortical infarct volume was reduced by 55%, and the density of terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling-positive cells was decreased by 36% compared with wild type. When subjected to oxygen-glucose deprivation (2 h), cortical neurons cultured from mice deficient in caspase-3 expression were also more resistant to cell death by 59%. Mutant brains showed caspase-specific poly(ADP-ribose) polymerase cleavage product (85-kDa fragment) in vivo and in vitro, suggesting redundant mechanisms and persistence of caspase-mediated cell death. In the present study, we found that caspase-8 mediated poly(ADP-ribose) polymerase cleavage in caspase-3(-/-) neurons in vivo and in vitro. In addition, mutant neurons showed no evidence of compensatory activation by caspase-6 or caspase-7 after ischemia. Taken together, these data extend the pharmacological evidence supporting an important role for caspase-3 and caspase-8 as cell death mediators in mammalian cortex and indicate the potential advantages of targeting more than a single caspase family member to treat ischemic cell injury.

A role for caspase-1 and -3 in the pathology of experimental allergic encephalomyelitis : inflammation versus degeneration

Axonal loss, already present in the acute and first relapse phases of experimental allergic encephalomyelitis (EAE) in the ABH mouse, only becomes apparent in the third relapse in the interleukin-12 model of relapsing EAE in the Lewis rat. Caspase-1 immunostaining in the spinal cord of Lewis rats was mainly localized to inflammatory cuffs with the greatest proportion of active caspase-1-positive cells detected during the first and second relapses, correlating with enzyme activity and protein on Western blots. However, in the spinal cord of ABH mice during acute EAE, caspase-1 immunostaining was localized both on inflammatory and neuronal cells, again correlating with enzyme activity and protein production. In contrast, caspase-3 expression in the spinal cord of Lewis rats did not increase significantly until the third relapse when inflammatory and neuronal cells and axons became positive in line with a significant increase in caspase activity. In ABH mice active caspase-3 was already immunolocalized on axons and apoptotic neurons in the spinal cord during the acute stage of EAE. Because caspase-3 is a downstream cell death signal it may be possible to reduce apoptosis by selectively blocking caspase-3 and therefore provide a therapeutic target for EAE and potentially, multiple sclerosis.

ProNGF induces p75-mediated death of oligodendrocytes following spinal cord injury

The neurotrophin receptor p75 is induced by various injuries to the nervous system, but its role after injury has remained unclear. Here, we report that p75 is required for the death of oligodendrocytes following spinal cord injury, and its action is mediated mainly by proNGF. Oligodendrocytes undergoing apoptosis expressed p75, and the absence of p75 resulted in a decrease in the number of apoptotic oligodendrocytes and increased survival of oligodendrocytes. ProNGF is likely responsible for activating p75 in vivo, since the proNGF from the injured spinal cord induced apoptosis among p75(+/+), but not among p75(-/-), oligodendrocytes in culture, and its action was blocked by proNGF-specific antibody. Together, these data suggest that the role of proNGF is to eliminate damaged cells by activating the apoptotic machinery of p75 after injury.

Multiple caspases are activated after traumatic brain injury: evidence for involvement in functional outcome

Caspase-3 is a cysteine protease that is strongly implicated in neuronal apoptosis. Activation of caspase-3 may be induced by at least two major initiator pathways: a caspase-8-mediated pathway activated through cell surface death receptors (extrinsic pathway), and a caspase-9-mediated pathway activated by signals from the mitochondria that lead to formation of an apoptosomal complex (intrinsic pathway). In the present studies, we compare the activation of caspases-3, -8, and -9 after lateral fluid-percussion traumatic brain injury (TBI) in rats. Immunoblot analysis identified cleaved forms of caspases-3 and -9, but not caspase-8, at 1, 12, and 48 h after injury. Immunocytochemistry specific for cleaved caspases-3 and -9 revealed their expression primarily in neurons. These caspases were also frequently localized in TUNEL-positive cells, some of which demonstrated morphological features of apoptosis. However, caspases-3 and -9 were also found in neurons that were not TUNEL-positive, and other TUNEL-positive cells did not show activated caspases. In contrast to caspases-3 or -9, caspase-8 expression was only minimally changed by injury. An increase in expression of this caspase was undetectable by immunoblotting methods, and appeared as positive immunostaining restricted to a few cells within the injured cortex. Treatment with the pan-caspase inhibitor z-VAD-fmk at 15 min after TBI improved performance on motor and spatial learning tests. These data suggest that several caspases may be involved in the pathophysiology of TBI and that pan-caspase inhibition strategies may improve neurological outcomes.

Microarray analysis suggests the involvement of proteasomes, lysosomes, and matrix metalloproteinases in the response of motor neurons to root avulsion

We used microarray analysis of RNA expression from punch samples from ventral horn of spinal cord to identify alterations in gene expression in motor neurons 3 days after proximal spinal root avulsion, a traumatic injury that results in the death of 80% of the motor neurons. This analysis identified the anticipated increases in expression of genes coding for proteins involved in the apoptosis cascades and abortive cell cycle re-entry, as well as decreases in expression of genes coding for proteins related to neuronal functional activity, including groups of genes related to energy metabolism, transporter proteins, ion channels, and receptors. It was also found that cathepsins, metalloproteinases, and proteasome-related protein products were highly up-regulated in motor neurons following axotomy. Each of these products represent pathways that have been implicated in other models of neuronal damage, but which have not previously been described as a response to axotomy.

Cytochrome c release and caspase activation after traumatic brain injury

Experimental traumatic brain injury (TBI) results in a rapid and significant necrosis of cortical tissue at the site of injury. In the ensuing hours and days, secondary injury exacerbates the primary damage resulting in significant neurological dysfunction. The identification of cell death pathways that mediate this secondary traumatic injury have not been elucidated, however recent studies have implicated a role for apoptosis in the neuropathology of traumatic brain injury. The present study utilized a controlled cortical impact model of brain injury to assess the involvement of apoptotic pathways: release of cytochrome c from mitochondria and the activation of caspase-1- and caspase-3-like proteases in the injured cortex at 6, 12 and 24 h post-injury. Collectively, these results demonstrate cytochrome c release from mitochondria and its redistribution into the cytosol occurs in a time-dependent manner following TBI. The release of cytochrome c is accompanied by a time-dependent increase in caspase-3-like protease activity with no apparent increase in caspase-1-like activity. However, pretreatment with a general caspase inhibitor had no significant effect on the amount of cortical damage observed at 7 days post-injury. Our data suggest that several pro-apoptotic events occur following TBI, however the translocation of cytochrome c itself and/or other events upstream of caspase activation/inhibition may be sufficient to induce neuronal cell death.

Neuronal apoptosis associated with morphine tolerance: evidence for an opioid-induced neurotoxic mechanism

Tolerance to the analgesic effect of an opioid is a pharmacological phenomenon that occurs after its prolonged administration. Activation of the NMDA receptor (NMDAR) has been implicated in the cellular mechanisms of opioid tolerance. However, activation of NMDARs can lead to neurotoxicity under many circumstances. Here we demonstrate that spinal neuronal apoptosis was induced in rats made tolerant to morphine administered through intrathecal boluses or continuous infusion. The apoptotic cells were predominantly located in the superficial spinal cord dorsal horn, and most apoptotic cells also expressed glutamic acid decarboxylase, a key enzyme for the synthesis of the inhibitory neurotransmitter GABA. Consistently, increased nociceptive sensitivity to heat stimulation was observed in these same rats. Mechanistically, the spinal glutamatergic activity modulated morphine-induced neuronal apoptosis, because pharmacological perturbation of the spinal glutamate transporter activity or coadministration of morphine with the NMDAR antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate affected both morphine tolerance and neuronal apoptosis. At the intracellular level, prolonged morphine administration resulted in an upregulation of the proapoptotic caspase-3 and Bax proteins but a downregulation of the antiapoptotic Bcl-2 protein in the spinal cord dorsal horn. Furthermore, coadministration with morphine of N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone (a pan-caspase inhibitor) or acetyl-aspartyl-glutamyl-valyl-aspart-1-aldehyde (a relatively selective caspase-3 inhibitor) blocked morphine-induced neuronal apoptosis. Blockade of the spinal caspase-like activity also partially prevented morphine tolerance and the associated increase in nociceptive sensitivity. These results indicate an opioid-induced neurotoxic consequence regulated by the NMDAR-caspase pathway, a mechanism that may have clinical implications in opioid therapy and substance abuse.

Expression and differential processing of caspases 6 and 7 in relation to specific epileptiform EEG patterns following limbic seizures

The caspase family of cell death proteases has been implicated in the mechanism of neuronal death following seizures. We investigated the expression and processing of caspases 6 and 7, putative executioner caspases. Brief limbic seizures were evoked by intraamygdala kainic acid to elicit unilateral death of target hippocampal CA3 neurons in the rat. Seizures rapidly induced cleavage of constitutively expressed caspase-6, followed by elevated VEIDase activity and the proteolysis of lamin A. Neuronal caspase-6 immunoreactivity was markedly upregulated within cortex and hippocampus in relation to bursts of polyspike paroxysmal discharges. In contrast, while caspase-7 expression also increased within cortical and hippocampal neuronal populations in response to the same seizure patterns, caspase-7 was not proteolytically activated. These data highlight differences in expression and activation of caspases 6 and 7 in response to identifiable seizure patterns, focusing potential therapeutic targets for neuroprotection in epilepsy.

Platelet-activating factor induces cell death in cultured astrocytes and oligodendrocytes: involvement of caspase-3

The biologically active lipid metabolite, platelet-activating factor (PAF), is thought to contribute to inflammatory processes and tissue damage in a variety of central nervous system (CNS) injuries. In previous studies, we found that after contusion spinal cord injury, treatment with a PAF antagonist led to significantly increased white matter tissue sparing as well as decreased mRNA levels for pro-inflammatory cytokines. Some studies suggest that PAF can also have toxic effects on neurons in vitro. Few studies, however, have examined the effects of PAF on glial cells of the CNS. In the present study, the potential for PAF to act as a toxin to cultured astrocytes was examined. Also investigated were the effects of PAF on oligodendrocytes at two different stages of development. Treatment with 0.02-2 microM PAF for 72 h resulted in significant levels of cell death in both cell types (P < 0.05), an effect that was blocked by the PAF receptor antagonists, WEB 2170 and BN 52021. To investigate PAF-induced glial cell death further, we looked for activation of the enzyme, caspase-3, which can be indicative of apoptosis. Immunocytochemistry demonstrated that PAF at all concentrations caused activation of caspase-3 at 24, 48, and 72 h after treatment in both cell types. Caspase-3-dependent cell death was further confirmed using knockout mice (-/-) deficient in the caspase-3 gene. Toxicity was lost when astrocytes (-/-) were exposed to 0.02-2 microM PAF (P < 0.01). Oligodendrocytes (-/-) were not susceptible to toxicity at 2 microM PAF (P < 0.001). The results demonstrate that the pro-inflammatory molecule, PAF, induces cell death in cultured CNS glial cells and that this effect is, in part, dependent on caspase-3 activation.

Activation of caspase-3 during degeneration of the outer nuclear layer in the rd mouse retina

Caspase activation has been implicated in apoptosis, and the nature of the apoptotic stimulus determines the specific caspase activation during apoptosis. In this study, we examined the activation of caspase-3 during photoreceptor degeneration in the rd mouse, which has a mutation on a gene encoding cyclic GMP phosphodiesterase. The outer nuclear layer of the rd mouse retina was observed using light and electron microscopy. The progress of degeneration was determined chronologically and correlated with the activation of caspase-3 and the fragmentation of poly-ADP-ribose polymerase. Additionally, the active form of caspase-3 was detected during photoreceptor degeneration in the outer nuclear layer of the rd mouse. The chronological observation of the caspase-3 activation pattern correlates with the pattern of photoreceptor degeneration. As a result of this study, we present here our findings that caspase-3 was activated in photoreceptor cells of the rd mouse.

The effect of neonatal nerve injury on the expression of heat shock proteins in developing rat motoneurones

The expression of the heat shock proteins hsp27 and hsp70 was examined in the spinal cord and sciatic nerves of developing rats. Using immunohistochemistry, we found that hsp27 is present in many motoneurones at birth. With development, the intensity of staining increases, reaching adult levels by 21 days, when all sciatic motoneurones express hsp27. In the sciatic nerve, hsp27 is strongly expressed throughout postnatal development. In contrast, hsp70 immunoreactivity in motoneurones and the sciatic nerve is weak at birth and does not change with development. The expression of heat shock proteins has been shown to increase in cells under conditions of stress, where they have beneficial effects on cell survival. The effect of neonatal nerve injury on hsp27 and hsp70 expression was also examined in this study. Four days after injury, staining for hsp27 increases in motoneurones, whereas hsp70 does not change. However, there is a significant increase in hsp70 staining in glial cells surrounding the injured motor pool, predominantly in astrocytes. Since neonatal nerve injury induces apoptotic motoneurone death, we also studied the co-expression of hsp27 with markers of apoptosis. No hsp27-positive motoneurones were found to be apoptotic, as assessed by both TUNEL and caspase-3 immunoreactivity. Therefore, it is possible that the upregulation of hsp27 observed in injured motoneurones may play a role in protecting motoneurones from apoptotic cell death following nerve injury.

Non-surgical management of spinal cord injury

Spinal cord injury remains a devastating neurological condition with limited therapeutic opportunities. Since decompressive surgery and high-dose methylprednisolone have limited utility for most patients, spinal cord injury clearly represents a major medical challenge. Experimental evidence has suggested that secondary cellular injury processes may be a realistic target for therapeutic intervention with the goal of inhibiting the progression of detrimental changes that normally follows traumatic injury to the cord. Preventing or reducing this delayed cellular injury may alone improve neurological recovery or facilitate future regenerative approaches to the injured cord. This review summarises recent advances in the development of pharmacological agents targeting the acute phase of spinal cord injury as well as potential strategies to facilitate regeneration of the spinal cord.

Mitogen-activated protein kinase inhibition in traumatic brain injury: in vitro and in vivo effects

The authors provide the first in vitro and in vivo evidence that perturbations in mitogen-activated protein kinase (MAPK) signal-transduction pathways are involved in the pathophysiology of traumatic brain injury. In primary rat cortical cultures, mechanical trauma induced a rapid and selective phosphorylation of the extracellular signal-regulated kinase (ERK) and p38 kinase, whereas there was no detectable change in the c-jun N-terminal kinase (JNK) pathway. Treatment with PD98059, which inhibits MAPK/ERK 1/2, the upstream activator of ERK, significantly increased cell survival in vitro. The p38 kinase and JNK inhibitor SB203580 had no protective effect. Similar results were obtained in vivo using a controlled cortical impact model of traumatic injury in mouse brain. Rapid and selective upregulation occurred in ERK and p38 pathways with no detectable changes in JNK. Confocal immunohistochemistry showed that phospho-ERK colocalized with the neuronal nuclei marker but not the astrocytic marker glial fibrillary acidic protein. Inhibition of the ERK pathway with PD98059 resulted in a significant reduction of cortical lesion volumes 7 days after trauma. The p38 kinase and JNK inhibitor SB203580 had no detectable beneficial effect. These data indicate that critical perturbations in MAPK pathways mediate cerebral damage after acute injury, and further suggest that ERK is a novel therapeutic target in traumatic brain injury.

Current developments in spinal cord injury research

BACKGROUND CONTEXT

Recent advances in neuroscience have opened the door for hope toward prevention and cure of the devastating effects of spinal cord injury (SCI).

PURPOSE

To highlight the current understanding of traumatic SCI mechanisms, provide information regarding state-of-the-art care for the acute spinal cord-injured patient, and explore future treatments aimed at neural preservation and reconstruction.

STUDY DESIGN/SETTING

A selective overview of the literature pertaining to the neuropathophysiology of traumatic SCI is provided with an emphasis on pharmacotherapies and posttraumatic experimental strategies aimed at improved neuropreservation and late neuroregenerative repair.

METHODS

One hundred fifty-four peer-reviewed basic science and clinical articles pertaining to SCI were reviewed. Articles cited were chosen based on the relative merits and contribution to the current understanding of SCI neuropathophysiology, neuroregeneration, and clinical SCI treatment patterns.

RESULTS

A better understanding of the pathophysiology and early treatment for the spinal cord-injured patient has led to a continued decrease in mortality, decreased acute hospitalization and complication rates, and more rapid rehabilitation and re-entry into society. Progressive neural injury results from a combination of secondary injury mechanisms, including ischemia, biochemical alterations, apoptosis, excitotoxicity, calpain proteases, neurotransmitter accumulation, lipid peroxidation/free radical injury, and inflammatory responses. Experimental studies suggest that the final posttraumatic neurologic deficit is not only a result of the initial impaction forces but rather a combination of these forces and secondary time-dependent events that follow shortly after the initial impact.

CONCLUSIONS

Experimental studies continue to provide a better understanding of the complex interaction of pathophysiologic events after traumatic SCI. Future approaches will involve strategies aimed at blocking the multiple mechanisms of progressive central nervous system injury and promoting neuroregeneration.

Prolonged survival and decreased abnormal movements in transgenic model of Huntington disease, with administration of the transglutaminase inhibitor cystamine

An expanded polyglutamine domain in huntingtin underlies the pathogenic events in Huntington disease (HD), characterized by chorea, dementia and severe weight loss, culminating in death. Transglutaminase (TGase) may be critical in the pathogenesis, via cross-linking huntingtin. Administration of the TGase competitive inhibitor, cystamine, to transgenic mice expressing exon 1 of huntingtin containing an expanded polyglutamine repeat, altered the course of their HD-like disease. Cystamine given intraperitoneally entered brain where it inhibited TGase activity. When treatment began after the appearance of abnormal movements, cystamine extended survival, reduced associated tremor and abnormal movements and ameliorated weight loss. Treatment did not influence the appearance or frequency of neuronal nuclear inclusions. Unexpectedly, cystamine treatment increased transcription of one of the two genes shown to be neuroprotective for polyglutamine toxicity in Drosophila, dnaj (also known as HDJ1 and Hsp40 in humans and mice, respectively). Inhibition of TGase provides a new treatment strategy for HD and other polyglutamine diseases.

Induction of caspase-activated deoxyribonuclease activity after focal cerebral ischemia and reperfusion

Deoxyribonucleic acid fragmentation at nucleosomal junctions is a hallmark of neuronal apoptosis in ischemic brain injury, for which the mechanism is not fully understood. Using the in vitro cell-free apoptosis assay, the authors found that caspase-3-dependent deoxyribonuclease activity caused internucleosomal DNA fragmentation in brain-cell extracts in a rat model of transient focal ischemia. This in vitro deoxyribonuclease activity was completely inhibited by purified inhibitor of caspase-activated deoxyribonuclease protein, the specific endogenous inhibitor of caspase-activated deoxyribonuclease, or by caspase-activated deoxyribonuclease immunodepletion. The induction of the deoxyribonuclease activity was correlated with caspase-3 activation and caspase-3-mediated degradation of inhibitor of caspase-activated deoxyribonuclease. Furthermore, inhibiting caspase-3-like protease activity prevented the endogenous induction of internucleosomal DNA fragmentation in the ischemic brain. These results suggest that caspase-3-dependent caspase-activated deoxyribonuclease activity plays an important role in mediating DNA fragmentation after focal ischemia.

Epidemiology, demographics, and pathophysiology of acute spinal cord injury

Spinal cord injury occurs through various countries throughout the world with an annual incidence of 15 to 40 cases per million, with the causes of these injuries ranging from motor vehicle accidents and community violence to recreational activities and workplace-related injuries. Survival has improved along with a greater appreciation of patterns of presentation, survival, and complications. Despite much work having been done, the only treatment to date known to ameliorate neurologic dysfunction that occurs at or below the level of neurologic injury has been intravenous methylprednisolone therapy. Much research over the past 30 to 40 years has focused on elucidating the mechanisms of spinal cord injury, with the complex pathophysiologic processes slowly being unraveled. With a greater understanding of both primary and secondary mechanisms of injury, the roles of calcium, free radicals, sodium, excitatory amino acids, vascular mediators, and apoptosis have been elucidated. This review examines the epidemiology, demographics, and pathophysiology of acute spinal cord injury.

Bcl-xL expression after contusion to the rat spinal cord

After contusion-derived spinal cord injury, (SCI) there is localized tissue disruption and energy failure that results in early necrosis and delayed apoptosis, events that contribute to chronic central pain in a majority of patients. We assessed the extent of contusion-induced apoptosis of neurons in a known central pain-signaling pathway, the spinothalamic tract (STT), which may be a contributor to SCI-induced pain. We observed the loss of STT cells and localized increase of DNA fragmentation and cytoplasmic histone-DNA complexes, which suggested potential apoptotic changes among STT neurons after SCI. We also showed SCI-associated changes in the expression of the antiapoptotic protein Bcl-xL, especially among STT cells, consistent with the hypothesis that Bcl-xL regulates the extent of apoptosis after SCI. Apoptosis in the injured spinal cord correlated well with prompt decreases in Bcl-xL protein levels and Bcl-xL/Bax protein ratios at the contusion site. We interpret these results as evidence that regulation of Bcl-xL may play a role in neural sparing after spinal injury and pain-signaling function.

Caspase-3 activation and DNA fragmentation in primary hippocampal neurons following glutamate excitotoxicity

Excitotoxic glutamate CNS stimulation can result in neuronal cell death. Contributing mechanisms and markers of cell death are the activation of caspase-3 and DNA fragmentation. It remains to be resolved to which extent both cellular reactions overlap and/or indicate different processes of neurodegeneration. In this study, mixed neuronal cultures from newborn mice pubs (0-24 h) were stimulated with glutamate, and the co-localization of active caspase-3 and DNA fragmentation was investigated by immunocytochemistry and the TUNEL nick-end labelling. In untreated cultures, 8% scattered neurons (marked by MAP-2) displayed activated caspase-3 at different morphological stages of degeneration. TUNEL staining was detected in 5% of cell nuclei including GFAP-positive astrocytes. However, co-localization of active caspase-3 with TUNEL was less than 2%. After glutamate stimulation (125 microM), the majority of neurons was dying between 12 and 24 h. The absolute number of active caspase-3 neurons increased only moderately but in relation of surviving neurons after 24 h from 8 to 36% (125 microM), to 53% (250 microM) or to 32% (500 microM). TUNEL staining also increased after 24 h following glutamate treatment to 37% but the co-localization with active caspase-3 remained at the basal low level of 2%. In our system, glutamate-mediated excitotoxicity effects the DNA fragmentation and caspase-3 activation. Co-localization of both parameters, however, is very poor. Active caspase-3 in the absence of TUNEL indicates a dynamic degenerative process, whereas TUNEL marks the end stage of severe irreversible cell damage regardless to the origin of the cell.