Death of cortical and striatal neurons induced by mitochondrial defect involves differential molecular mechanisms

Sodium hydrogen sulfide upregulates cystathionine β-synthase and protects striatum against 3-nitropropionic acid-induced neurotoxicity in rats

OBJECTIVES

Hydrogen sulfide (H2S) is a neuromodulator that plays a protective role in multiple neurodegenerative diseases including Alzheimer's (AD) and Parkinson's (PD). However, the precise mechanisms underlying its effects against Huntington's disease (HD) are still questioned.This study aimed to examine the neuroprotective effects of sodium hydrogen sulfide (NaHS; H2S donor) against 3-nitropropionic acid (3NP)-induced HD like pathology in rats. Methods: Male Wistar rats were randomly allocated into four groups; (1) normal control receiving saline; (2) NaHS control receiving (0.5 mg/kg/day, i.p.) for 14 days; (3,4) receiving 3NP (10 mg/kg/day, i.p.) for 14 days, with NaHS 30 min later in group 4.

KEY FINDINGS

NaHS improved cognitive and locomotor deficits induced by 3NP as confirmed by the striatal histopathological findings. These former events were biochemically supported by the increment in cystathionine β-synthase (CBS) gene expression, reduction of glutamate (Glu), dopamine (DA), malondialdehyde (MDA), tumour necrosis factor-alpha (TNF-α), cytochrome-c, cleaved caspase-3 and pc-FOS indicating antioxidant, anti-inflammatory as well as anti-apoptotic effects. Furthermore, NaHS pretreatment improved cholinergic dysfunction and increased brain-derived neurotropic factor (BDNF) and nuclear factor erythroid-2-related factor 2 (Nrf2).

CONCLUSIONS

These findings suggest that appropriate protection with H2S donors might represent a novel approach to slow down HD-like symptoms.

Inhibition of mitochondrial complex II in neuronal cells triggers unique pathways culminating in autophagy with implications for neurodegeneration

Mitochondrial dysfunction and neurodegeneration underlie movement disorders such as Parkinson’s disease, Huntington’s disease and Manganism among others. As a corollary, inhibition of mitochondrial complex I (CI) and complex II (CII) by toxins 1-methyl-4-phenylpyridinium (MPP⁺) and 3-nitropropionic acid (3-NPA) respectively, induced degenerative changes noted in such neurodegenerative diseases. We aimed to unravel the down-stream pathways associated with CII inhibition and compared with CI inhibition and the Manganese (Mn) neurotoxicity. Genome-wide transcriptomics of N27 neuronal cells exposed to 3-NPA, compared with MPP⁺ and Mn revealed varied transcriptomic profile. Along with mitochondrial and synaptic pathways, Autophagy was the predominant pathway differentially regulated in the 3-NPA model with implications for neuronal survival. This pathway was unique to 3-NPA, as substantiated by in silico modelling of the three toxins. Morphological and biochemical validation of autophagy markers in the cell model of 3-NPA revealed incomplete autophagy mediated by mechanistic Target of Rapamycin Complex 2 (mTORC2) pathway. Interestingly, Brain Derived Neurotrophic Factor (BDNF), which was elevated in the 3-NPA model could confer neuroprotection against 3-NPA. We propose that, different downstream events are activated upon neurotoxin-dependent CII inhibition compared to other neurotoxins, with implications for movement disorders and regulation of autophagy could potentially offer neuroprotection.

Mitochondrial impairment induced by 3-nitropropionic acid is enhanced by endogenous metalloprotease activity inhibition in cultured rat striatal neurons

Metalloproteases from the metzincin family mediate molecule processing at the cell membrane termed ectodomain shedding (ES). This mechanism enables the generation of intracellular and extracellular fragments from cell membrane molecules that exert additional functions involved in cell processes including cell death, beyond those of full length molecules. Micotoxin 3-nitropropionic acid (3-NP) induces striatal neuronal degeneration in vivo and in vitro through mitochondrial complex II inhibition. In this study, we hypothesized that metalloproteases regulate mitochondrial activity in cultured rat striatal neurons undergoing degeneration. To test this idea, striatal neuronal cultures characterized by NeuN and GAD-67 expression were treated with 3-NP together with the metalloprotease inhibitor GM6001 and their mitochondrial activity was evaluated by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay. Our results showed that metalloprotease inhibition potentiated mitochondrial activity impairment induced by 3-NP whereas the inhibitor alone had no effect. These results indicate that metalloproteases regulate and promote mitochondrial functionality in striatal neurons undergoing degeneration induced by 3-NP. Since NMDA receptor is involved in the excitotoxic neuronal death triggered by 3-NP and is known to undergo ES, we analyzed NMDAR subunit NR1 phenotypic distribution by immunofluorescence. 3-NP and GM6001 induced abnormal perinuclear NR1 accumulation that was not observed with 3-NP or GM6001 alone. This observation suggests that metalloproteases are involved in NR1 cellular reorganization induced by 3-NP, and that their inhibition results in abnormal NR1 distribution. Together results indicate that endogenous metalloproteases are activated during striatal neurodegeneration induced by 3-NP eliciting an adaptative or compensatory response that protects mitochondrial functionality.

3-Nitropropionic acid induces autophagy by forming mitochondrial permeability transition pores rather than activating the mitochondrial fission pathway

BACKGROUND AND PURPOSE

Huntington's disease is a neurodegenerative process associated with mitochondrial alterations. Inhibitors of the electron-transport channel complex II, such as 3-nitropropionic acid (3NP), are used to study the molecular and cellular pathways involved in this disease. We studied the effect of 3NP on mitochondrial morphology and its involvement in macrophagy.

EXPERIMENTAL APPROACH

Pharmacological and biochemical methods were used to characterize the effects of 3NP on autophagy and mitochondrial morphology. SH-SY5Y cells were transfected with GFP-LC3, GFP-Drp1 or GFP-Bax to ascertain their role and intracellular localization after 3NP treatment using confocal microscopy.

KEY RESULTS

Untreated SH-SY5Y cells presented a long, tubular and filamentous net of mitochondria. After 3NP (5 mM) treatment, mitochondria became shorter and rounder. 3NP induced formation of mitochondrial permeability transition pores, both in cell cultures and in isolated liver mitochondria, and this process was inhibited by cyclosporin A. Participation of the mitochondrial fission pathway was excluded because 3NP did not induce translocation of the dynamin-related protein 1 (Drp1) to the mitochondria. The Drp1 inhibitor Mdivi-1 did not affect the observed changes in mitochondrial morphology. Finally, scavengers of reactive oxygen species failed to prevent mitochondrial alterations, while cyclosporin A, but not Mdivi-1, prevented the generation of ROS.

CONCLUSIONS AND IMPLICATIONS

There was a direct correlation between formation of mitochondrial permeability transition pores and autophagy induced by 3NP treatment. Activation of autophagy preceded the apoptotic process and was mediated, at least partly, by formation of reactive oxygen species and mitochondrial permeability transition pores.

Expression pattern of ataxia telangiectasia mutated (ATM), p53, Akt, and glycogen synthase kinase-3β in the striatum of rats treated with 3-nitropropionic acid

3-Nitropropionic acid (3-NPA) is a mitochondrial toxin used in the laboratory to replicate neurodegenerative conditions that are accompanied by degeneration of the caudate-putamen. 3-NPA induces depletion in ATP production, reactive oxygen species production, and secondary excitotoxicity mediated by activation of N-methyl-D-aspartate receptors that culminates in the triggering of cell death mechanisms, including apoptosis. We here examined by immunohistochemical methods whether cellular expression of phospho(Ser1981) -ataxia telangiectasia mutated (ATM), phospho(Ser15) -p53, phospho(Ser473) -Akt, and phospho(Ser9) -glycogen synthase kinase-3β (GSK3β), which are key signal molecules that play a critical role in regulating cellular processes related to cell survival and demise, were involved in the striatal neurodegeneration in the brains of rats treated with 3-NPA. Our results indicate that the toxin induced the activation of ATM and p53 only in astrocytes, and a role for these proteins in neuronal degeneration was ruled out. On the other hand, striatal neurons lost the active form of Akt as soon as they began to appear pyknotic, indicating impairment of the PI3K/Akt/GSK3 pathway in their degenerative process. The inactive form of GSK3β was detected extensively, mainly in the rim of the striatal lesions around degenerating neurons, which could be attributed to a cell death or cell survival response.

c-Jun-dependent sulfiredoxin induction mediates BDNF protection against mitochondrial inhibition in rat cortical neurons

In current study, we tested the hypothesis that c-Jun-dependent sulfiredoxin expression mediates protective effects of brain-derived neurotrophic factor (BDNF) against neurotoxicity induced by 3-nitropropionic acid (3-NP), a mitochondrial complex II inhibitor, in primary rat cortical cultures. We found that BDNF-dependent c-Jun expression and nuclear translocation required prior phosphorylation of extracellular signal-regulated kinase (ERK)1/2, but not Akt. BDNF also transiently activated the expression of sulfiredoxin, an ATP-dependent antioxidant enzyme, at both mRNA and protein levels. Furthermore, both c-Jun siRNA and ERK1/2 inhibitor PD98059 suppressed BDNF-induced sulfiredoxin expression. Finally, PD98059, c-Jun siRNA, and sulfiredoxin siRNA all abrogated BDNF-mediated 3-NP resistance. Together, these results established a signaling cascade of "BDNF → ERK1/2-Pi → c-Jun → sulfiredoxin → 3-NP resistance". We therefore conclude that c-Jun-induced sulfiredoxin mediates the BDNF-dependent neuroprotective effects against 3-NP toxicity in primary rat cortical neurons, at least in part.

Brain region-specific vulnerability of astrocytes in response to 3-nitropropionic acid is mediated by cytochrome c oxidase isoform expression

Brain region specificity is a feature characteristic of neurodegenerative disorders, such as Huntington's disease (HD). We have studied the brain region-specific vulnerability of striatal compared with cortical and mesencephalic astrocytes treated with 3-nitropropionic acid (NPA), an in vitro model of HD. Mitochondrial dysfunction is involved in neurodegenerative processes. We have previously demonstrated a causal relationship between NPA-induced transcription of the cytochrome c oxidase (COX) subunit IV isoform (cox4i2) and increased oxidative stress leading to higher rates of necrotic cell death in striatal astrocytes by the application of a small interfering RNA knockdown system. Here, we have investigated the correlation of COX IV-2 protein expression with intracellular ATP content, mitochondrial peroxide production, and viability of astrocytes from three different brain regions. In cortical and mesencephalic astrocytes, NPA caused an elevation of cox4i2 transcription as in striatal astroglia. However, increased COX IV-2 and decreased COX IV-1 protein expression levels have been observed only in striatal astrocytes. In agreement with our hypothesis, Striatal astrocytes showed the highest levels of peroxide production and necrotic cell death rates compared with cortical and mesencephalic astroglia. Thus, we suggest that the higher vulnerability of astrocytes from the striatum in our in vitro model of HD is, at least in part, based on brain region-specific differences of the COX IV-2/COX IV-1 protein ratios and accompanied elevated oxidative stress.

Uncoupling oxidative/energy metabolism with low sub chronic doses of 3-nitropropionic acid or iodoacetate in vivo produces striatal cell damage

A variety of evidence suggests that the failure of cellular metabolism is one of the underlying causes of neurodegenerative diseases. For example, the inhibition of mitochondrial function produces a pattern of cellular pathology in the striatum that resembles that seen in Huntington's disease. However, neurons can also generate ATP through the glycolytic pathway. Recent work has suggested a direct interaction between mutated huntingtin and a key enzyme in the glycolytic pathway, glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Yet little work has been gone into examination of the cellular pathology that results from the inhibition of this alternative energy source. Therefore, the aim of the present study is to characterize the cellular pathology that results in the striatum of mice after treatment with a toxin (iodoacete, IOA) that compromises anaerobic metabolism. This striatal pathology is compared to that produced by a widely studied blocker of mitochondrial function (3-nitropropionic acid, 3-NP). We found that low doses of either toxin resulted in significant pathology in the mouse striatum. Signs of apoptosis were observed in both experimental groups, although apoptosis triggered by IOA treatment was independent from caspase-3 activation. Importantly, each toxin appears to produce cellular damage through distinct mechanisms; only 3-NP generated clear evidence of oxidative stress as well as inhibition of endogenous antioxidants. Understanding the distinct pathological fingerprints of cell loss produced by blockade of oxidative and anaerobic metabolisms may give us insights into neurodegenerative diseases.

3-Nitropropionic acid as a tool to study the mechanisms involved in Huntington's disease: past, present and future

Huntington's disease (HD) is an inheritable autosomal-dominant disorder whose causal mechanisms remain unknown. Experimental models have begun to uncover these pathways, thus helping to understand the mechanisms implicated and allowing for the characterization of potential targets for new therapeutic strategies. 3-Nitropropionic acid is known to produce in animals behavioural, biochemical and morphologic changes similar to those occurring in HD. For this reason, this phenotypic model is gaining attention as a valuable tool to mimick this disorder and further developing new therapies. In this review, we will focus on the past and present research of this molecule, to finally bring a perspective on what will be next in this promising field of study.

Complex II inhibition by 3-NP causes mitochondrial fragmentation and neuronal cell death via an NMDA- and ROS-dependent pathway

Mitochondrial respiratory complex II inhibition plays a central role in Huntington's disease (HD). Remarkably, 3-NP, a complex II inhibitor, recapitulates HD-like symptoms. Furthermore, decreases in mitochondrial fusion or increases in mitochondrial fission have been implicated in neurodegenerative diseases. However, the relationship between mitochondrial energy defects and mitochondrial dynamics has never been explored in detail. In addition, the mechanism of neuronal cell death by complex II inhibition remains unclear. Here, we tested the temporal and spatial relationship between energy decline, impairment of mitochondrial dynamics, and neuronal cell death in response to 3-NP using quantitative fluorescence time-lapse microscopy and cortical neurons. 3-NP caused an immediate drop in ATP. This event corresponded with a mild rise in reactive oxygen species (ROS), but mitochondrial morphology remained unaltered. Unexpectedly, several hours after this initial phase, a second dramatic rise in ROS occurred, associated with profound mitochondrial fission characterized by the conversion of filamentous to punctate mitochondria and neuronal cell death. Glutamate receptor antagonist AP5 abolishes the second peak in ROS, mitochondrial fission, and cell death. Thus, secondary excitotoxicity, mediated by glutamate receptor activation of the NMDA subtype, and consequent oxidative and nitrosative stress cause mitochondrial fission, rather than energy deficits per se. These results improve our understanding of the cellular mechanisms underlying HD pathogenesis.

Calpain-mediated signaling mechanisms in neuronal injury and neurodegeneration

Calpain is a ubiquitous calcium-sensitive protease that is essential for normal physiologic neuronal function. However, alterations in calcium homeostasis lead to persistent, pathologic activation of calpain in a number of neurodegenerative diseases. Pathologic activation of calpain results in the cleavage of a number of neuronal substrates that negatively affect neuronal structure and function, leading to inhibition of essential neuronal survival mechanisms. In this review, we examine the mechanistic underpinnings of calcium dysregulation resulting in calpain activation in the acute neurodegenerative diseases such as cerebral ischemia and in the chronic neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, Huntington's disease, multiple sclerosis, prion-related encephalopathy, and amylotrophic lateral sclerosis. The premise of this paper is that analysis of the signaling and transcriptional consequences of calpain-mediated cleavage of its various substrates for any neurodegenerative disease can be extrapolated to all of the neurodegenerative diseases vulnerable to calcium dysregulation.

Dopamine determines the vulnerability of striatal neurons to the N-terminal fragment of mutant huntingtin through the regulation of mitochondrial complex II

In neurodegenerative disorders associated with primary or secondary mitochondrial defects such as Huntington's disease (HD), cells of the striatum are particularly vulnerable to cell death, although the mechanisms by which this cell death is induced are unclear. Dopamine, found in high concentrations in the striatum, may play a role in striatal cell death. We show that in primary striatal cultures, dopamine increases the toxicity of an N-terminal fragment of mutated huntingtin (Htt-171-82Q). Mitochondrial complex II protein (mCII) levels are reduced in HD striatum, indicating that this protein may be important for dopamine-mediated striatal cell death. We found that dopamine enhances the toxicity of the selective mCII inhibitor, 3-nitropropionic acid. We also demonstrated that dopamine doses that are insufficient to produce cell loss regulate mCII expression at the mRNA, protein and catalytic activity level. We also show that dopamine-induced down-regulation of mCII levels can be blocked by several dopamine D2 receptor antagonists. Sustained overexpression of mCII subunits using lentiviral vectors abrogated the effects of dopamine, both by high dopamine concentrations alone and neuronal death induced by low dopamine concentrations together with Htt-171-82Q. This novel pathway links dopamine signaling and regulation of mCII activity and could play a key role in oxidative energy metabolism and explain the vulnerability of the striatum in neurodegenerative diseases.

BH3-only proteins Bid and Bim(EL) are differentially involved in neuronal dysfunction in mouse models of Huntington's disease

Apoptosis, a cell death mechanism regulated by Bcl-2 family members, has been proposed as one of the mechanisms leading to neuronal loss in Huntington's disease (HD). Here we examined the regulation of Bcl-2 family proteins in three different mouse models of HD with exon 1 mutant huntingtin: the R6/1, the R6/1:BDNF+/-, and the Tet/HD94 in which the huntingtin transgene is controlled by the tetracycline-inducible system. Our results disclosed an increase in the levels of the BH3-only proteins Bid and Bim(EL) in the striatum of HD mouse models that was different depending on the stage of the disease. At 16 weeks of age, Bid was similarly enhanced in the striatum of R6/1 and R6/1:BDNF+/- mice, whereas Bim(EL) protein levels were enhanced only in R6/1:BDNF+/- mice. In contrast, at later stages of the disease, both genotypes displayed increased levels of Bid and Bim(EL) proteins. Furthermore, Bax, Bak, Bad, Bcl-2, and Bcl-x(L) proteins were not modified in any of the points analyzed. We next explored the potential reversibility of this phenomenon by analyzing conditional Tet/HD94 mice. Constitutive expression of the transgene resulted in increased levels of Bid and Bim(EL) proteins, and only the Bid protein returned to wild-type levels 5 months after mutant huntingtin shutdown. In conclusion, our results show that enhanced Bid protein levels represent an early mechanism linked to the continuous expression of mutant huntingtin that, together with enhanced Bim(EL), may be a reporter of the progress and severity of neuronal dysfunction.

Mitochondrial NAD+-linked State 3 respiration and complex-I activity are compromised in the cerebral cortex of 3-nitropropionic acid-induced rat model of Huntington's disease

Mitochondrial complex-I dysfunction has been observed in patients of Huntington's disease (HD). We assessed whether such a defect is present in the 3-nitropropionic acid (3-NP) model of HD. Rats treated with 3-NP (10-20 mg/kg i.p., for 4 days) exhibited weight loss, gait abnormalities, and striatal lesions with increased glial fibrillary acidic protein immunostaining on fifth and ninth days, while increase in striatal dopamine and loss of tyrosine hydroxylase immunoreactivity were observed on fifth day following treatment. We report for the first time a dose-dependent reduction in complex-I activity in the cerebral cortex when analyzed spectrophotometrically and by blue native-polyacrylamide gel electrophoresis following 3-NP treatment. The citrate synthase normalized activities of mitochondrial complex-I, -II, -(I + III) and -IV were decreased in the cortex of 3-NP treated rats. In addition, succinate driven State 3 respiration was also significantly inhibited in vivo and in the isolated mitochondria. These findings taken together with the observation of a significant decrease in vivo but not in vitro of State 3 respiration with NAD(+)-linked substrates, suggest complex-I dysfunction in addition to irreversible inhibition of complex-II and succinate dehydrogenase activity as a contributing factor in 3-NP-induced cortico-striatal lesion.

Cannabidiol reduced the striatal atrophy caused 3-nitropropionic acid in vivo by mechanisms independent of the activation of cannabinoid, vanilloid TRPV1 and adenosine A2A receptors

The neuroprotective potential of cannabinoids has been examined in rats with striatal lesions caused by 3-nitropropionic acic (3NP), an inhibitor of mitochondrial complex II. We used the CB1 agonist arachidonyl-2-chloroethylamide (ACEA), the CB2 agonist HU-308, and cannabidiol (CBD), an antioxidant phytocannabinoid with negligible affinity for cannabinoid receptors. The administration of 3NP reduced GABA contents and also mRNA levels for several markers of striatal GABAergic projection neurons, including proenkephalin (PENK), substance P (SP) and neuronal-specific enolase (NSE). We also found reductions in mRNA levels for superoxide dismutase-1 (SOD-1) and -2 (SOD-2), which indicated that 3NP reduced the endogenous antioxidant defences. The administration of CBD, but not ACEA or HU-308, completely reversed 3NP-induced reductions in GABA contents and mRNA levels for SP, NSE and SOD-2, and partially attenuated those found in SOD-1 and PENK. This indicates that CBD is neuroprotective but acted preferentially on striatal neurons that project to the substantia nigra. The effects of CBD were not reversed by the CB1 receptor antagonist SR141716. The same happened with the TRPV1 receptor antagonist capsazepine, in concordance with the observation that capsaicin, a TRPV1 receptor agonist, failed to reproduce the CBD effects. The effects of CBD were also independent of adenosine signalling as they were not attenuated by the adenosine A2A receptor antagonist MSX-3. In summary, this study demonstrates that CBD provides neuroprotection against 3NP-induced striatal damage, which may be relevant for Huntington's disease, a disorder characterized by the preferential loss of striatal projection neurons. This capability seems to be based exclusively on the antioxidant properties of CBD.

Cannabinoids and neuroprotection in basal ganglia disorders

Cannabinoids have been proposed as clinically promising neuroprotective molecules, as they are capable to reduce excitotoxicity, calcium influx, and oxidative injury. They are also able to decrease inflammation by acting on glial processes that regulate neuronal survival and to restore blood supply to injured area by reducing the vasoconstriction produced by several endothelium-derived factors. Through one or more of these processes, cannabinoids may provide neuroprotection in different neurodegenerative disorders including Parkinson's disease and Huntington's chorea, two chronic diseases that are originated as a consequence of the degeneration of specific nuclei of basal ganglia, resulting in a deterioration of the control of movement. Both diseases have been still scarcely explored at the clinical level for a possible application of cannabinoids to delay the progressive degeneration of the basal ganglia. However, the preclinical evidence seems to be solid and promising. There are two key mechanisms involved in the neuroprotection by cannabinoids in experimental models of these two disorders: first, a cannabinoid receptor-independent mechanism aimed at producing a decrease in the oxidative injury and second, an induction/upregulation of cannabinoid CB2 receptors, mainly in reactive microglia, that is capable to regulate the influence of these glial cells on neuronal homeostasis. Considering the relevance of these preclinical data and the lack of efficient neuroprotective strategies in both disorders, we urge the development of further studies that allow that the promising expectatives generated for these molecules progress from the present preclinical evidence till a real clinical application.

3-Nitropropionic acid activates calpain/cdk5 pathway in rat striatum

3-Nitropropionic acid (3-NP) is a neurotoxin that inhibits mitochondrial complex II and is used in an experimental model of Huntington's disease. Treatment of rats with 3-NP 30mgkg(-1) i.p. once a day for 5 days induced an increase in calpain activation in rat striatum, measured by the formation of 145kDa fragment of alpha-spectrin breakdown and by an increase in enzymatic calpain activity. In this neurotoxic model, Western blot studies revealed that calpain activity increase was followed by changes in cyclin-dependent kinase 5 (cdk5) and its activator p25. Our results indicated, after 10 days of treatment with 3-NP, a decrease in myocyte enhancer factor phosphorylation, a neuronal prosurvival factor. Thus, a decrease in its expression indicates a new potential mechanism of neuronal cell death mediated by the neurotoxin 3-NP. Accordingly, in our study we demonstrated in rat striatum the activation of the calpain/cdk5/p25 pathway in the 3-NP model. Previous studies have linked the deregulation of cdk5 with neurodegenerative diseases, such as Alzheimer's and Parkinson's. We suggest that calpain/cdk5 activation could also be a common pathway activated in other neurodegenerative diseases, which is liable to be targeted.

Mitochondrial membrane permeabilization in cell death

Irrespective of the morphological features of end-stage cell death (that may be apoptotic, necrotic, autophagic, or mitotic), mitochondrial membrane permeabilization (MMP) is frequently the decisive event that delimits the frontier between survival and death. Thus mitochondrial membranes constitute the battleground on which opposing signals combat to seal the cell's fate. Local players that determine the propensity to MMP include the pro- and antiapoptotic members of the Bcl-2 family, proteins from the mitochondrialpermeability transition pore complex, as well as a plethora of interacting partners including mitochondrial lipids. Intermediate metabolites, redox processes, sphingolipids, ion gradients, transcription factors, as well as kinases and phosphatases link lethal and vital signals emanating from distinct subcellular compartments to mitochondria. Thus mitochondria integrate a variety of proapoptotic signals. Once MMP has been induced, it causes the release of catabolic hydrolases and activators of such enzymes (including those of caspases) from mitochondria. These catabolic enzymes as well as the cessation of the bioenergetic and redox functions of mitochondria finally lead to cell death, meaning that mitochondria coordinate the late stage of cellular demise. Pathological cell death induced by ischemia/reperfusion, intoxication with xenobiotics, neurodegenerative diseases, or viral infection also relies on MMP as a critical event. The inhibition of MMP constitutes an important strategy for the pharmaceutical prevention of unwarranted cell death. Conversely, induction of MMP in tumor cells constitutes the goal of anticancer chemotherapy.

Mitochondrial-dependent Ca2+ handling in Huntington's disease striatal cells: effect of histone deacetylase inhibitors

Evidence suggests that neuronal dysfunction in Huntington's disease (HD) striatum involves deficits in mitochondrial function and in Ca2+ handling. However, the relationship between mitochondria and Ca2+ handling has been incompletely studied in intact HD striatal cells. Treatment with histone deacetylase (HDAC) inhibitors reduces cell death in HD models, but the effects of this promising therapy on cellular function are mostly unknown. Here, we use real-time functional imaging of intracellular Ca2+ and mitochondrial membrane potential to explore the role of in situ HD mitochondria in Ca2+ handling. Immortalized striatal (STHdh) cells and striatal neurons from transgenic mice, expressing full-length mutant huntingtin (Htt), were used to model HD. We show that (1) active glycolysis in STHdh cells occludes the mitochondrial role in Ca2+ handling as well as the effects of mitochondrial inhibitors, (2) STHdh cells and striatal neurons in the absence of glycolysis are critically dependent on oxidative phosphorylation for energy-dependent Ca2+ handling, (3) expression of full-length mutant Htt is associated with deficits in mitochondrial-dependent Ca2+ handling that can be ameliorated by treatment with HDAC inhibitors (treatment with trichostatin A or sodium butyrate decreases the proportion of STHdh cells losing Ca2+ homeostasis after Ca2+-ionophore challenging, and accelerates the restoration of intracellular Ca2+ in striatal neurons challenged with NMDA), and (4) neurons with different response patterns to NMDA receptor activation exhibit different average somatic areas and are differentially affected by treatment with HDAC inhibitors, suggesting subpopulation or functional state specificity. These findings indicate that neuroprotection induced by HDAC inhibitors involves more efficient Ca2+ handling, thus improving the neuronal ability to cope with excitotoxic stimuli.

Memantine reduces striatal cell death with decreasing calpain level in 3-nitropropionic model of Huntington's disease

Huntington's disease has an increase in the activated calpain, which is enhanced by the NMDA receptor activation. We investigated the neuroprotective effect of memantine in 3-nitropropionic acid (3NP)-induced striatal degeneration model. Either memantine (20 mg/kg/day) or PBS was intraperitoneally administered for five days with 3NP continuous infusion. In the memantine-treated group, the striatal lesion volume, the number of TUNEL+ cells, and Fluoro-Jade C+ degenerating neurons were all decreased. Memantine increased Bcl-xl and decreased Bax level. Memantine also exerted an inhibitory effect on the micro-calpain level and decreased the huntingtin proteolytic fragments. Those rats treated with memantine showed less degree of weight loss at 5 days. Subsequently, memantine was found to have neuroprotective effects and save striatal cells with decreasing calpain levels in the 3NP model of Huntington's disease.

Delayed minocycline treatment reduces long-term functional deficits and histological injury in a rodent model of focal ischemia

The absence of effective treatments for stroke presents a critical need for novel strategies that can reduce ischemic injury. Neuroinflammation following focal ischemia induces secondary injury in the region surrounding the insult, thus anti-inflammatory agents are potential neuroprotectants. Minocycline is one such agent possessing neuroprotective properties, however many studies examining minocycline after ischemia have used minimal delays between ischemia and treatment, short survival periods, and lack measures of functional outcome. Such studies do not distinguish whether minocycline provides sustained protection or merely delays cell death. This study was designed to address some of these concerns. Male Sprague-Dawley rats were treated with multiple doses of minocycline (45 mg/kg i.p.) or vehicle beginning 2.5 h after endothelin-1-induced focal ischemia. Measures of forelimb asymmetry and skilled reaching (staircase test) were used to determine functional outcome 7, 15 and 28 days after ischemia. Long-term functional assessment indicates that minocycline provides limited benefit in the staircase test, but confers long-term benefit in the forelimb asymmetry test. Subcortical and whole hemisphere infarct volumes were reduced by 41 and 39% respectively in minocycline-treated animals. Further analysis revealed that minocycline attenuated long-term white matter damage adjacent to the striatal injury core, which correlated with sustained functional benefits. This study indicates that delayed minocycline treatment improves long-term functional outcome which is linked to protection of both white and gray matter.

Brain mitochondrial defects amplify intracellular [Ca2+] rise and neurodegeneration but not Ca2+entry during NMDA receptor activation

According to the "indirect" excitotoxicity hypothesis, mitochondrial defects increase Ca2+ entry into neurons by rendering NMDA-R hypersensitive to glutamate. We tested this hypothesis by investigating in the rat striatum and cultured striatal cells how partial mitochondrial complex II inhibition produced by 3-nitropropionic acid (3NP) modifies the toxicity of the NMDA-R agonist quinolinate (QA). We showed that nontoxic 3NP treatment, leading to partial inhibition of complex II activity, greatly exacerbated striatal degeneration produced by slightly toxic QA treatment through an "all-or-nothing" process. The potentiation of QA-induced cell death by 3NP was associated with increased calpain activity and massive calpain-mediated cleavage of several postsynaptic proteins, suggesting major neuronal Ca2+ deregulation in the striatum. However, Ca2+ anomalies probably do not result from NMDA-R hypersensitivity. Indeed, brain imaging experiments using [(18)F]fluorodeoxyglucose indirectly showed that 3NP did not increase QA-induced ionic perturbations at the striatal glutamatergic synapses in vivo. Consistent with this, the exacerbation of QA toxicity by 3NP was not related to an increase in the QA-induced entry of 45Ca2+ into striatal neurons. The present results demonstrate that the potentiation of NMDA-R-mediated excitotoxicity by mitochondrial defects involves primarily intracellular Ca2+ deregulation, in the absence of NMDA-R hypersensitivity.

Memantine attenuates staurosporine-induced activation of caspase-3 and LDH release in mouse primary neuronal cultures

Developmental aspects of pro- and antiapoptotic action of some NMDA receptor antagonists in the central nervous system have been postulated. In order to further elucidate this problem, we investigated effect of memantine, an uncompetitive NMDA receptor antagonist and staurosporine alone and in combination on caspase-3 activity and lactate dehydrogenase (LDH) release in primary hippocampal, neocortical and striatal cell cultures on 7 and 12 days in vitro. The data showed that the vulnerability of neuronal cells to induction of caspase-3 activity by staurosporine was higher on 7 DIV than on 12 DIV, whereas staurosporine-mediated LDH release increased with days in vitro in striatal culture only. A specific inhibitor of caspase-3, AcDEVDCHO (60 microM), completely abolished the effect of staurosporine on this enzyme's activity, but only partially attenuated staurosporine-induced LDH release in hippocampal cells. Memantine alone (0.05-2.0 microM) did not induce any cytotoxic effect but attenuated the staurosporine-induced caspase-3 activity and LDH release in hippocampal cultured neurons on each investigated day in vitro. In striatal culture, memantine had a moderate inhibitory effect on staurosporine-evoked LDH release only on 7 DIV with no significant influence on caspase-3 activity. As for neocortical cultures, memantine partially inhibited staurosporine-induced neuronal injury only on 7 DIV. These data showed that the induction of caspase-3 activity by staurosporine was more profound in immature cells, however, the staurosporine neurotoxicity, as reflected by LDH release, only partially depended on caspase-3 activation and stage of cell development. Furthermore, memantine attenuated staurosporine-induced apoptosis more efficiently in hippocampal cultures than in neocortical and striatal ones, which points to tissue specificity of effects of this neuroprotectant.

3-Nitropropionic acid: a mitochondrial toxin to uncover physiopathological mechanisms underlying striatal degeneration in Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder caused by a mutation in the gene encoding Huntingtin. The mechanisms underlying the preferential degeneration of the striatum, the most striking neuropathological change in HD, are unknown. Of those probably involved, mitochondrial defects might play an important role. The behavioural and anatomical similarities found between HD and models using the mitochondrial toxin 3-nitropropionic acid (3NP) in rats and primates support this hypothesis. Here, we discuss the recently identified mechanisms of 3NP-induced striatal degeneration. Two types of important factor have been identified. The first are the 'executioner' components that have direct roles in cell death, such as c-Jun N-terminal kinase and Ca2+-activated protease calpains. The second are 'environmental' factors, such as glutamate, dopamine and adenosine, which modulate the striatal degeneration induced by 3NP. Interestingly, these recent studies support the hypothesis that 3NP and mutated Huntingtin have certain mechanisms of toxicity in common, suggesting that the use of 3NP might give new insights into the pathogenesis of HD. Research on 3NP provides additional proof that the neurochemical environment of a given neurone can determine its preferential vulnerability in neurodegenerative diseases.

The therapeutic role of creatine in Huntington's disease

Huntington's disease (HD) is an autosomal dominant and fatal neurological disorder characterized by a clinical triad of progressive choreiform movements, psychiatric symptoms, and cognitive decline. HD is caused by an expanded trinucleotide CAG repeat in the gene coding for the protein huntingtin. No proven treatment to prevent the onset or to delay the progression of HD currently exists. While a direct causative pathway from the gene mutation to the selective neostriatal neurodegeneration remains unclear, it has been hypothesized that interactions of the mutant huntingtin protein or its fragments may result in a number of interrelated pathogenic mechanisms triggering a cascade of molecular events that lead to the untimely neuronal death observed in HD. One putative pathological mechanism reported to play a prominent role in the pathogenesis of HD is mitochondrial dysfunction and the subsequent reduction of cellular energy. Indeed, if mitochondrial impairment and reduced energy stores play roles in the neuronal loss in HD, then a therapeutic strategy that buffers intracellular energy levels may ameliorate the neurodegenerative process. Sustained ATP levels may have both direct and indirect importance in ameliorating the severity of many of the pathogenic mechanisms associated with HD. Creatine, a guanidino compound produced endogenously and acquired exogenously through diet, is a critical component in maintaining much needed cellular energy. As such, creatine is one of a number of ergogens that may provide a relatively safe and immediately available therapeutic strategy to HD patients that may be the cornerstone of a combined treatment necessary to delay the relentless progression of HD.

Neuroprotective effect of zVAD against the neurotoxin 3-nitropropionic acid involves inhibition of calpain

The contribution of calpains and caspases to cell death has been widely studied using pharmacological inhibitors. Among them, the caspase inhibitor N-benzyloxycarbonyl-valyl-alanyl-aspartyl-fluoromethylketone (zVAD) has been used as a specific caspase inhibitor in nearly 1000 published studies. However, several studies showed that zVAD also behaves as a calpain inhibitor in peripheral cells. The effects of zVAD as a calpain inhibitor have never been assessed in neurodegeneration models. We examined here whether zVAD could reduce neurodegeneration in Huntington's disease models using the mitochondrial inhibitor 3-nitropropionic acid (3NP). In these models, 3NP toxicity has been shown to require calpain activation. In rats, intra-cerebro-ventricular infusion of zVAD significantly reduced 3NP-induced striatal degeneration, and decreased the 3NP-induced activation of calpain and calpain-dependent cleavage of fodrin. zVAD (100 microM) also blocked 3NP-induced death of cultured striatal neurons. In vitro, zVAD inhibited purified mu-calpain with high affinity (IC50=10 nM). The present data demonstrate that zVAD protects neurons against 3NP through calpain inhibition. This suggests that, in certain models of neuronal death where zVAD showed protective effects, caspases but also calpains may be involved.

Caspase-dependent and -independent neuronal death: two distinct pathways to neuronal injury

Caspases are cysteine proteases that mediate apoptotic death in a variety of cellular systems, including neurons. Caspases are activated through extrinsic or intrinsic pathways. The latter is used by most neurons in most situations. In this pathway, release of mitochondrial cytochrome c into the cytoplasm induces formation of the apoptosome, which leads to the activation of caspase 9 and subsequently other caspases. Recent data demonstrate that when caspase activation is inhibited at or downstream of the apoptosome, neurons undergo a delayed, caspase-independent death. Furthermore, there are instances, most notably following excitotoxic injury and calcium overload, in which the direct cell death pathway elicited differs from classical apoptosis. The molecular and biochemical features of such caspase-independent, nonapoptotic forms of neuronal death are just beginning to be elucidated, but alterations at the level of the mitochondria and noncaspase proteases play significant roles. Mitochondrial alterations in caspase-independent death may include energy depletion, generation of free radicals, opening of the permeability transition pore, and release of cytotoxic proteins, such as apoptosis-inducing factor. The particular mechanisms employed can be context dependent. In disease states, in which a combination of apoptotic and nonapoptotic death occurs, therapeutic strategies need to take into account both caspase-dependent and -independent pathways.

Minocycline in phenotypic models of Huntington's disease

Minocycline has been shown to be neuroprotective in various models of neurodegenerative diseases. However, its potential in Huntington's disease (HD) models characterized by calpain-dependent degeneration and inflammation has not been investigated. Here, we have tested minocycline in phenotypic models of HD using 3-nitropropionic acid (3NP) intoxication and quinolinic acid (QA) injections. In the 3NP rat model, where the development of striatal lesions involves calpain, we found that minocycline was not protective, although it attenuated the development of inflammation induced after the onset of striatal degeneration. The lack of minocycline activity on calpain-dependent cell death was also confirmed in vitro using primary striatal cells. Conversely, we found that minocycline reduced lesions and inflammation induced by QA. In cultured cells, minocycline protected against mutated huntingtin and staurosporine, stimulations known to promote caspase-dependent cell death. Altogether, these data suggested that, in HD, minocycline may counteract the development of caspase-dependent neurodegeneration, inflammation, but not calpain-dependent neuronal death.

Increased Alix (apoptosis-linked gene-2 interacting protein X) immunoreactivity in the degenerating striatum of rats chronically treated by 3-nitropropionic acid

Chronic intoxication by 3-nitropropionic acid in the Lewis rat reproduces many features reminiscent of Huntington's disease including behavioural alterations and cortico-striatal degeneration. In particular, in this model, striatal degeneration is accompanied by calpain activation as found in the human disease. The present study was undertaken to determine whether the expression of Alix (apoptosis linked gene-2 interacting protein), a widespread protein involved in neuronal death, would be modified in the striatum and cortex of 3NP-treated rats. The results clearly show that Alix immunoreactivity is increased in the neuronal cell bodies of the lateral striatum, where degeneration is massive. The medial striatum and the cortex that lack neurodegeneration remain only weakly labelled. This is further evidence suggesting an involvement of Alix in the events driving neuronal death.

Acute effects of pulsed microwaves and 3-nitropropionic acid on neuronal ultrastructure in the rat caudate-putamen

Ultrastructure of the medium sized "spiny" neuron in rat dorsal-lateral caudate-putamen was assessed after administration of 3-nitropropionic acid (3-NP) and exposure to pulsed microwaves. Sprague-Dawley male rats were given two daily intraperitoneal doses of 0 or 10 mg/kg 3-NP and 1.5 h after each dose were exposed to microwave radiation at a whole body averaged specific absorption rate (SAR) of 0 (sham exposure), 0.6, or 6 W/kg for 30 min. Microwave exposure consisted of 1.25 GHz radiation delivered as 5.9 micros pulses with repetition frequency 10 Hz. Tissue samples taken 2-3 h after the second sham or microwave exposure showed no injury with light microscope methods. Blinded qualitative assessment of ultrastructure of randomly selected neurons from the same samples did reveal differences. Subsequent detailed, quantitative measurements showed that, when followed by sham exposure, administration of 3-NP significantly increased endoplasmic reticulum (ER) intracisternal width, ER area density, and nuclear envelope thickness. Microwave exposure at 6 W/kg alone also significantly increased these measures. Exposure of 3-NP treated animals at 6 W/kg significantly increased effects of 3-NP on ultrastructure. Although exposure at 0.6 W/kg alone did not affect ultrastructure measures, exposure of 3-NP treated animals at 0.6 W/kg reduced the effects of 3-NP. We concluded that 3-NP changed neuronal ultrastructure and that the microwave exposures used here changed neuronal ultrastructure in ways that depended on microwave SAR and neuron metabolic status. The apparent cancellation of 3-NP induced changes by exposure to pulsed microwaves at 0.6 W/kg indicated the possibility that such exposure can protect against the effects of mitochondrial toxins on the nervous system.