Mitochondrial apoptotic cell death and moderate superoxide generation upon selective activation of non-desensitizing AMPA receptors in hippocampal cultures

The role of calpains in traumatic brain injury

AIM

This article attempts to provide a framework that will help to illustrate the roles of calpains in the process of traumatic brain injury (TBI).

METHOD

This review provides meaningful points about the essential role of calpains in the neuropathological changes that follow TBI, identifies useful biomarkers of calpain activation and states the important roles of calpain in the treatment of TBI.

RESULTS

Neuronal calpains can be activated within hours or even minutes following contusive or diffuse brain trauma in animals. It has been suggested that they are early mediators of neuronal damage. Trauma can produce sustained calpain activation. In turn, this may result in axonal degeneration and neuronal death in models of TBI. Calpains can cleave cytoskeletal proteins into stable proteolytic fragments that have been widely used as biomarkers of the activation of calpain. The inhibition of calpains can reduce the functional and behavioural deficits by ameliorating axonal pathology and reducing cell deaths in animal models of TBI.

CONCLUSION

This review concentrates on the current understanding of the role of calpains in neuropathology that has been induced by TBI and the significance of calpains as a therapeutic target for the treatment of primary and secondary injuries that are associated with brain trauma.

FK506 ameliorates cell death features in Huntington's disease striatal cell models

Huntington's disease (HD) is a genetic neurodegenerative disorder characterized by striatal neurodegeneration, involving apoptosis. FK506, an inhibitor of calcineurin (or protein phosphatase 3, formerly known as protein phosphatase 2B), has shown neuroprotective effects in several cellular and animal models of HD. In the present study, we show the protective effects of FK506 in two striatal HD models, primary rat striatal neurons treated with 3-nitropropionic acid (3-NP) and immortalized striatal STHdh cells derived from HD knock-in mice expressing normal (STHdh(7/7)) or full-length mutant huntingtin (FL-mHtt) with 111 glutamines (STHdh(111/111)), under basal conditions and after exposure to 3-NP or staurosporine (STS). In rat striatal neurons, FK506 abolished 3-NP-induced increase in caspase-3 activation, DNA fragmentation/condensation and necrosis. Nevertheless, in STHdh(111/111) cells under basal conditions, FK506 did not prevent, in a significant manner, the release of cytochrome c and apoptosis inducing factor (AIF) from mitochondria, or alter Bax/Bcl-2 ratio, but significantly reverted caspase-3 activation. In STHdh(111/111) cells treated with 0.3mM 3-NP or 25 nM STS, linked to high necrosis, exposure to FK506 exerted no significant effects on caspase-3 activation. However, treatment of STHdh(111/111) cells exposed to 10nM STS with FK506 effectively prevented cell death by apoptosis and moderate necrosis. The results suggest that FK506 may be neuroprotective against apoptosis and necrosis under mild cell death stimulus in the presence of FLmHtt.

Methamphetamine- and trauma-induced brain injuries: comparative cellular and molecular neurobiological substrates

The use of methamphetamine (METH) is a growing public health problem, because its abuse is associated with long-term biochemical and structural effects on the human brain. Neurodegeneration is often observed in humans, because of mechanical injuries (e.g., traumatic brain injury [TBI]) and ischemic damage (strokes). In this review, we discuss recent findings documenting the fact that the psychostimulant drug METH can cause neuronal damage in several brain regions. The accumulated evidence from our laboratories and those of other investigators indicates that acute administration of METH leads to activation of calpain and caspase proteolytic systems. These systems are also involved in causing neuronal damage secondary to traumatic and ischemic brain injuries. Protease activation is accompanied by proteolysis of endogenous neuronal structural proteins (alphaII-spectrin protein and microtubule-associated protein-tau), evidenced by the appearance of their breakdown products after these injuries. When taken together, these observations suggest that METH exposure, like TBI, can cause substantial damage to the brain by causing both apoptotic and necrotic cell death in the brains of METH addicts who use large doses of the drug during their lifetimes. Finally, because METH abuse is accompanied by functional and structural changes in the brain similar to those in TBI, METH addicts might experience greater benefit if their treatment involved greater emphasis on rehabilitation in conjunction with potential neuroprotective pharmacological agents such as calpain and caspase inhibitors similar to those used in TBI.

Elevated serum superoxide dismutase and thiobarbituric acid reactive substances in different phases of bipolar disorder and in schizophrenia

There is an increasing body of evidence suggesting that oxidative stress may play a role in the pathophysiology of both schizophrenia (SZ) and bipolar disorder (BD).

METHODS

We compared the antioxidant enzyme, serum superoxide dismutase (SOD) and the lipid peroxidation product, thiobarbituric acid reactive substances (TBARS) as assessed in depressed (N=21), manic (N=32) and euthymic (N=31) bipolar patients, and in chronically medicated patients with schizophrenia (N=97), all fulfilling DSM-IV diagnostic criteria, and a group of healthy controls (N=32).

RESULTS

Serum SOD (U/mg protein) activity was significantly increased (p<0.001) in manic (7.44+/-3.88) and depressed (6.12+/-4.64) BD patients and SZ (9.48+/-4.51) when compared to either controls (1.81+/-0.63) or euthymic (2.75+/-1.09) BD patients. TBARS (mol/L) levels were significantly higher in the SZ group (4.95+/-1.56, p=0.016), bipolar euthymic (6.36+/-1.46, p<0.001), bipolar manic (7.54+/-1.74, p<0.001), and bipolar depressed patients (5.28+/-1.54, p=0.028) compared to controls (3.96+/-1.51).

DISCUSSION

Our findings show increased SOD activity in SZ, as well as in depressed and manic bipolar patients, but not in euthymic BD subjects. This suggests a dysregulation in oxidative defenses in both disorders. It is likely that such changes reflect state changes in bipolar disorder. It is possible that this is a compensatory response to the oxidative stress that occurs in the acute phase of bipolar episodes. TBARS results show increases in lipid peroxidation in mania. TBARS levels in SZ and in euthymic as well as depressed individuals with BD were higher than in controls. This suggests persistent increases in SZ, which may reflect ongoing symptomatology or treatment, and a state dependent gradient in BD, with greatest oxidative stress in mania. These data support oxidative biology as both a key component of the pathophysiology of both BD and SZ, and the use of agents that modulate oxidative biology as a promising avenue for intervention in both disorders.

Thrombin-induced oxidative stress contributes to the death of hippocampal neurons: role of neuronal NADPH oxidase

The present study investigated whether thrombin can induce the production of reactive oxygen species (ROS) through activation of neuronal NADPH oxidase and whether this contributes to oxidative damage and consequently to neurodegeneration. Immunocytochemical and biochemical evidence demonstrated that, in neuron-enriched hippocampal cultures, thrombin induces neurodegeneration in a dose-dependent manner. In parallel, ROS production was evident as assessed by analyzing DCF and hydroethidine. Real-time PCR analysis, at various time points after thrombin treatment, also demonstrated that expression of NADPH oxidase subunits (p47(phox) and p67(phox)) occurs. In addition, Western blot analysis and double-label immunocytochemistry showed an up-regulation in the expression of cytosolic components (Rac 1 and p67(phox)), the translocation of cytosolic proteins (p47(phox) and p67(phox)) to the membrane, and the localization of gp91(phox) or p47(phox) expression in hippocampal neurons of cultures and CA1 layer. The thrombin-induced ROS production, protein oxidation, and loss of cultured hippocampal neurons were partially attenuated by an NADPH oxidase inhibitor and/or by several antioxidants. Collectively, the present study is the first to demonstrate that, in cultured hippocampal neurons, thrombin-induced neurotoxicity is, at least in part, caused by neuronal NADPH oxidase-mediated oxidative stress. This strongly suggests that thrombin can act as an endogenous neurotoxin, and inhibitors of thrombin and/or antioxidants can be useful agents for treating oxidative stress-mediated hippocampal neurodegenerative diseases, such as Alzheimer's disease.

Searching for unique endophenotypes for schizophrenia and bipolar disorder within neural circuits and their molecular regulatory mechanisms

The endophenotype is a construct that has utility for the study of postmortem brains from patients with psychotic disorders. By identifying networks of genes that show changes in expression within specific neuronal populations implicated in the pathophysiology of schizophrenia and bipolar disorder, it may be possible to move toward understanding these disorders at the cellular and molecular levels. The ultimate goal is to characterize their respective underlying genotypes.

Changes in calcium dynamics following the reversal of the sodium-calcium exchanger have a key role in AMPA receptor-mediated neurodegeneration via calpain activation in hippocampal neurons

Proteolytic cleavage of the Na(+)/Ca(2+) exchanger (NCX) by calpains impairs calcium homeostasis, leading to a delayed calcium overload and excitotoxic cell death. However, it is not known whether reversal of the exchanger contributes to activate calpains and trigger neuronal death. We investigated the role of the reversal of the NCX in Ca(2+) dynamics, calpain activation and cell viability, in alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor-stimulated hippocampal neurons. Selective overactivation of AMPA receptors caused the reversal of the NCX, which accounted for approximately 30% of the rise in intracellular free calcium concentration ([Ca(2+)](i)). The NCX reverse-mode inhibitor, 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea (KB-R7943), partially inhibited the initial increase in [Ca(2+)](i), and prevented a delayed increase in [Ca(2+)](i). In parallel, overactivation of AMPA receptors strongly activated calpains and led to the proteolysis of NCX3. KB-R7943 prevented calpain activation, cleavage of NCX3 and was neuroprotective. Silencing of NCX3 reduced Ca(2+) uptake, calpain activation and was neuroprotective. Our data show for the first time that NCX reversal is an early event following AMPA receptor stimulation and is linked to the activation of calpains. Since calpain activation subsequently inactivates NCX, causing a secondary Ca(2+) entry, NCX may be viewed as a new suicide substrate operating in a Ca(2+)-dependent loop that triggers cell death and as a target for neuroprotection.

Susceptibility of hippocampal neurons to Abeta peptide toxicity is associated with perturbation of Ca2+ homeostasis

Neuritic dystrophy, loss of synapses and neuronal death in the cerebral cortex and hippocampus are hallmarks of Alzheimer's disease. The aim of the present study was to investigate the differential susceptibility of cortical and hippocampal neurons to amyloid-beta (Abeta)-induced toxicity. For that, we have used primary neuronal cultures prepared from rat brain cortex and hippocampus which were treated with the synthetic peptides Abeta25-35 or Abeta1-40. Abeta-induced apoptotic cell death was analyzed by determining caspase-3-like activity. Neuritic dystrophy was evaluated by cobalt staining and MAP2 immunoreactivity. Perturbation of Ca(2+) homeostasis caused by exposure to Abeta was evaluated by determining basal cytosolic calcium levels in the whole neuronal population and by single cell calcium imaging under basal and KCl-depolarization conditions. Finally, levels of GluR2 subunit of glutamate AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionate) receptors were quantified by western blotting. Our results demonstrated that hippocampal neurons in culture are more susceptible than cortical neurons to Abeta-induced apoptosis and also that this mechanism involves the perturbation of Ca(2+) homeostasis. Accordingly, the exposure of hippocampal neurons to Abeta peptides decreases the protein levels of the GluR2 subunit of glutamate AMPA receptors that may be associated with a significant rise of cytosolic Ca(2+) concentration, leading to dendritic dystrophy and activation of apoptotic neuronal death.

Peroxiredoxins in the central nervous system

Oxidative stress is considered one of the causative pathomechanisms of nervous system diseases such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, stroke and excitotoxicity. The basal expression of six different peroxiredoxin (Prx) isozymes show distinct distribution profiles in different brain regions and different cell types. PrxI and VI are expressed in glial cells but not in neurons; while PrxII, III, IV and V are expressed in neurons. Various diseases or models show altered expression levels of these isozymes, such as by upregulation of PrxI, II and VI and downregulation of PrxIII. Thioredoxin (Trx)I mRNA is distributed widely in the rat brain. This distribution pattern may reflect the specific functions of these isozymes. Recently, the neuroprotective roles of Prx III and V against ibotenate-induced-excitotoxicity were reported by two independent groups. Adenovirus transduction of PrxIII eliminated protein nitration and prevented gliosis caused by direct infusion of ibotenate. Systemic administration of recombinant PrxV diminished brain lesions in animals treated with ibotenate. In this chapter, we review the causative mechanisms of oxidative stress in neurodegenerative diseases, as well as describe the basal and disease-induced changes in Prxs/Trxs/Trx reductases expression levels and neuroprotective roles of Trxs and Prxs as demonstrated in overexpression models.

Complement-dependent apoptosis and inflammatory gene changes in murine lupus cerebritis

The role of complement activation in the brains of MRL/lpr lupus mice was determined using the potent C3 convertase inhibitor, CR1-related y (Crry), administered both as an overexpressing Crry transgene and as Crry-Ig. Prominent deposition of complement proteins C3 and C9 in brains of MRL/lpr mice was indicative of complement activation and was significantly reduced by Crry. Apoptosis was determined in brain using different independent measures of apoptosis, including TUNEL staining, DNA laddering, and caspase-3 activity, all of which were markedly increased in lupus mice and could be blocked by inhibiting complement with Crry. Complement activation releases inflammatory mediators that can induce apoptosis. The mRNA for potentially proinflammatory proteins such as TNFR1, inducible NO synthase, and ICAM-1 were up-regulated in brains of lupus mice. Crry prevented the increased expression of these inflammatory molecules, indicating that the changes were complement dependent. Furthermore, microarray analysis revealed complement-dependent up-regulation of glutamate receptor (AMPA-GluR) expression in lupus brains, which was also validated for AMPA-GluR1 mRNA and protein. Our results clearly demonstrate that apoptosis is a prominent feature in lupus brains. Complement activation products either directly and/or indirectly through TNFR1, ICAM-1, inducible NO synthase, and AMPA-GluR, all of which were altered in MRL/lpr mouse brains, have the potential to induce such apoptosis. These findings present the exciting possibility that complement inhibition is a therapeutic option for lupus cerebritis.

Mitochondrial dysfunction in choline deficiency-induced apoptosis in cultured rat hepatocytes

Our recent studies have demonstrated that generation of ROS is associated with choline deficiency (CD)-induced apoptosis in CWSV-1 cells, an immortalized rat hepatocyte that becomes tumorigenic by stepwise culturing in decreasing levels of choline. In the present study, we investigated the effect of CD on loss of mitochondrial membrane potential (MMP), using the JC-1 probe by FASCAN assay. Our data demonstrate that MMP in CD-cultured cells was decreased in a time- and dose-dependent manner and that significant disruption occurred at 24 h, relative to high choline (HC, 70 microM) cultured cells. In order to investigate further the relationship among the CD-induced ROS, MMP collapse, and apoptosis, we examined the effects of different inhibitors on ROS production, MMP disruption, and apoptosis in CD or HC-cultured CWSV-1 cells. These data indicate that the disruption of MMP is an upstream event in CD-induced apoptosis, and mitochondrial dysfunction plays a key role in mediating CD-induced apoptosis in CWSV-1 cells.

Early calpain-mediated proteolysis following AMPA receptor activation compromises neuronal survival in cultured hippocampal neurons

In this work, we investigated the involvement of calpains in the neurotoxicity induced by short-term exposure to kainate (KA) in non-desensitizing conditions of AMPA receptor activation (cyclothiazide present, CTZ), in cultured rat hippocampal neurons. The calpain inhibitor MDL28170 had a protective effect in cultures treated with KA plus CTZ (p < 0.01), preventing the decrease in MTT reduction caused by exposure to KA (p < 0.001). Caspase inhibition by ZVAD-fmk was not neuroprotective against the toxic effect of KA. At 1 h after treatment, we could already observe significantly increased calpain activity, which was prevented by MDL 28170 and NBQX. Western blot analysis of calpain substrates, GluR1, neuronal nitric oxide synthase (nNOS) and nonerythroid spectrin (fodrin), showed a time-dependent and MDL 28170-sensitive proteolysis of these proteins. This effect was due to calpains, but not caspases, since ZVAD-fmk was ineffective in preventing proteolytic events. Breakdown products of fodrin (BDPs) were detected as early as 15 min after exposure to KA. Overall, these results show early activation of calpains following activation of AMPA receptors as well as compromise of neuronal survival, likely due to proteolytic events that affect proteins involved in neuronal signaling.

Nitric oxide inhibits complex I following AMPA receptor activation via peroxynitrite

We investigated the role of nitric oxide (NO) on mitochondrial complexes activity, following short-term non-desensitizing activation of AMPA receptors with kainate (KA) plus cyclothiazide (CTZ), in cultured rat hippocampal neurons. In these conditions, we observed a decrease in the activity of mitochondrial complexes I, II/III, and IV. A selective neuronal nitric oxide synthase inhibitor, 7-Nitroindazole, prevented the decrease in the activity of mitochondrial complex I, but not for the other complexes. Exposure to KA plus CTZ also increased cyclic GMP levels significantly, and led to increased levels of 3-nitrotyrosine, a biomarker for peroxynitrite production. Taken together, our results suggest that non-desensitizing activation of AMPA receptors causes inhibition of mitochondrial complex I via peroxynitrite.

The role of iron and copper molecules in the neuronal vulnerability of locus coeruleus and substantia nigra during aging

In this study, a comparative analysis of metal-related neuronal vulnerability was performed in two brainstem nuclei, the locus coeruleus (LC) and substantia nigra (SN), known targets of the etiological noxae in Parkinson's disease and related disorders. LC and SN pars compacta neurons both degenerate in Parkinson's disease and other Parkinsonisms; however, LC neurons are comparatively less affected and with a variable degree of involvement. In this study, iron, copper, and their major molecular forms like ferritins, ceruloplasmin, neuromelanin (NM), manganese-superoxide dismutase (SOD), and copper/zinc-SOD were measured in LC and SN of normal subjects at different ages. Iron content in LC was much lower than that in SN, and the ratio heavy-chain ferritin/iron in LC was higher than in the SN. The NM concentration was similar in LC and SN, but the iron content in NM of LC was much lower than SN. In both regions, heavy- and light-chain ferritins were present only in glia and were not detectable in neurons. These data suggest that in LC neurons, the iron mobilization and toxicity is lower than that in SN and is efficiently buffered by NM. The bigger damage occurring in SN could be related to the higher content of iron. Ferritins accomplish the same function of buffering iron in glial cells. Ceruloplasmin levels were similar in LC and SN, but copper was higher in LC. However, the copper content in NM of LC was higher than that of SN, indicating a higher copper mobilization in LC neurons. Manganese-SOD and copper/zinc-SOD had similar age trend in LC and SN. These results may explain at least one of the reasons underlying lower vulnerability of LC compared to SN in Parkinsonian syndromes.

Neuroprotective effect of H. perforatum extracts on beta-amyloid-induced neurotoxicity

In the present study we assessed the neuroprotective role of a Hypericum perforatum ethanolic extract and obtained fractions in amyloid-beta peptide (Abeta)((25-35))-induced cell death in rat cultured hippocampal neurons. Lipid peroxidation was used as a marker of oxidative stress by following the formation of TBARS in rat cortical synaptosomes, after incubation with ascorbate/Fe2+, alone or in the presence of EC97 effective concentrations of H. perforatum fractions. Induced lipid peroxidation was significantly inhibited by fractions containing flavonol glycosides, flavonol and biflavone aglycones, and by a fraction containing several phenols, mainly chlorogenic acid-type phenolics (21%, 77% and 98%, respectively). Lipid peroxidation evaluated after incubation with 25 microM Abeta(25-35), was significantly inhibited by H. perforatum extract. Cell viability was assessed by use of the Syto-13/PI assay. The total ethanolic extract (TE) and fractions containing flavonol glycosides, flavonol and biflavone aglycones, reduced Abeta(25-35)-induced cell death (65%, 58% and 59%, respectively). These results were further supported by morphological analysis of cells stained with cresyl violet. Peptide beta-amyloid(25-35) induced a decrease in cell volume, chromatin condensation and nuclear fragmentation, alterations not evident in the presence of the TE and fractions containing hypericins (hypericin concentration = 11.02 microM), or fractions containing flavonoids (quercetin concentration = 21.13 microM). Dendritic lesion, an evidence of neurodegeneration, was observed by neuronal staining with cobalt following insult with Abeta(25-35), but prevented after exposure to the peptide plus the fractions referred above. The results of the present paper suggest that H. perforatum extracts may be endowed with neuroprotective compounds able to prevent Abeta(25-35)-induced toxicity.

Neuroprotective properties of Valeriana officinalis extracts

Valeriana officinalis have been used in traditional medicine for its sedative, hypnotic, and anticonvulsant effects. There are several reports in the literature supporting a GABAergic mechanism of action for valerian. The rationale of the present work is based on the concept that by decreasing neuronal network excitability valerian consumption may contribute to neuroprotection. The aim of our investigation was to evaluate the neuroprotective effects of V. officinalis against the toxicity induced by amyloid beta peptide 25-35 Abeta(25-35). Cultured rat hippocampal neurons were exposed to Abeta(25-35) (25 microM) for 24-48 h, after which morphological and biochemical properties were evaluated. The neuronal injury evoked by Abeta, which includes a decrease in cell reducing capacity and associated neuronal degeneration, was prevented by valerian extract. Analysis of intracellular free calcium (Ca(2+)i) indicated that the neuroprotective mechanisms may involve the inhibition of excess influx of Ca2+ following neuronal injury. Moreover, membrane peroxidation in rat hippocampal synaptosomes was evaluated, and our data indicate that valerian extract partially inhibited ascorbate/iron-induced peroxidation. In conclusion we show evidence that the signalling pathways involving Ca(2+)i and the redox state of the cells may play a central role in the neuroprotective properties of V. officinalis extract against Abeta toxicity. The novelty of the findings of the present work, indicating neuroprotective properties of valerian against Abeta toxicity may, at the long-term, contribute to introduction of a new relevant use of valerian alcoholic extract to prevent neuronal degeneration in aging or neurodegenerative disorders.

Mitochondrial dysfunction and reactive oxygen species in excitotoxicity and apoptosis: implications for the pathogenesis of neurodegenerative diseases

In recent years we have witnessed a major interest in the study of the role of mitochondria, not only as ATP producers through oxidative phosphorylation but also as regulators of intracellular Ca2+ homeostasis and endogenous producers of reactive oxygen species (ROS). Interestingly, the mitochondria have been also implicated as central executioners of cell death. Increased mitochondrial Ca2+ overload as a result of excitotoxicity has been associated with the generation of superoxide and may induce the release of proapoptotic mitochondrial proteins, proceeding through DNA fragmentation/condensation and culminating in cell demise by apoptosis and/or necrosis. In addition, these processes have been implicated in the pathogenesis of many neurodegenerative diseases, which share several features of cell death: selective brain areas undergo neurodegeneration, involving mitochondrial dysfunction (mitochondrial complexes are affected), loss of intracellular Ca2+ homeostasis, excitotoxicity, and the extracellular or intracellular accumulation of insoluble protein aggregates in the brain.