Caspase-8 is an effector in apoptotic death of dopaminergic neurons in Parkinson's disease, but pathway inhibition results in neuronal necrosis

Structural basis for executioner caspase recognition of P5 position in substrates

Caspase-3, -6 and -7 cleave many proteins at specific sites to induce apoptosis. Their recognition of the P5 position in substrates has been investigated by kinetics, modeling and crystallography. Caspase-3 and -6 recognize P5 in pentapeptides as shown by enzyme activity data and interactions observed in the crystal structure of caspase-3/LDESD and in a model for caspase-6. In caspase-3 the P5 main-chain was anchored by interactions with Ser209 in loop-3 and the P5 Leu side-chain interacted with Phe250 and Phe252 in loop-4 consistent with 50% increased hydrolysis of LDEVD relative to DEVD. Caspase-6 formed similar interactions and showed a preference for polar P5 in QDEVD likely due to interactions with polar Lys265 and hydrophobic Phe263 in loop-4. Caspase-7 exhibited no preference for P5 residue in agreement with the absence of P5 interactions in the caspase-7/LDESD crystal structure. Initiator caspase-8, with Pro in the P5-anchoring position and no loop-4, had only 20% activity on tested pentapeptides relative to DEVD. Therefore, caspases-3 and -6 bind P5 using critical loop-3 anchoring Ser/Thr and loop-4 side-chain interactions, while caspase-7 and -8 lack P5-binding residues.

Effects of calpain inhibition on dopaminergic markers and motor function following intrastriatal 6-hydroxydopamine administration in rats

The neurotoxin 6-hydroxydopamine has been widely used to model aspects of Parkinson's disease in rodents, but the mechanisms underlying toxin-induced dopaminergic degeneration and functional impairment have not been fully elucidated. The main aim of the present study was to assess a possible role for calpains in neurochemical and behavioral deficits following unilateral infusion of intrastriatal 6-hydroxydopamine in adult rats. Toxin administration produced a profound dopaminergic denervation, as indicated by a 90-95% reduction in dopamine transporter radiolabeling measured in the caudate-putamen at 2 weeks post-lesion. Treatment with 6-hydroxydopamine also resulted in calpain activation in both caudate-putamen and substantia nigra, as measured by the appearance of calpain-specific spectrin breakdown products. Calpain activation peaked at 24 h after 6-hydroxydopamine infusion and remained elevated at later time points. In contrast, caspase-3-mediated spectrin cleavage subsided within 48 h in both brain areas. In a subsequent experiment, calpain inhibition was achieved by intrastriatal infusion of an adenovirus expressing the endogenous calpain inhibitor, calpastatin. Calpastatin delivery abolished the lesion-induced calpain-mediated spectrin cleavage and alleviated forelimb asymmetries resulting from unilateral intrastriatal 6-hydroxydopamine. Unexpectedly, dopamine transporter and tyrosine hydroxylase labeling revealed significant neuroprotection, not in the nigrostriatal pathway but rather in the ventral tegmental area. These findings support a role for calpain activation in 6-hydroxydopamine-induced degeneration of dopaminergic neurons. However, after near-total dopaminergic depletion, the primary benefit of calpain inhibition may not occur within the nigrostriatal dopaminergic pathway itself.

Caspases as therapeutic targets

The development of small molecules to modulate caspase activity offers a novel therapeutic strategy in the treatment of apoptosis-related and inflammatory diseases. Caspases are key mediators of apoptosis and inflammation; deregulation of their activation or expression can lead to the development of conditions such as neurodegenerative and autoinflammatory disorders. This review details the different caspase-associated disorders while focusing on caspase-1 inhibition as a potential therapeutic strategy. Problems facing the development of effective and safe caspase therapeutics will also be addressed.

Neuronal cell death in neurodegenerative diseases: recurring themes around protein handling

Neuronal cell death plays a role in many chronic neurodegenerative diseases with the loss of particular subsets of neurons. The loss of the neurons occurs during a period of many years, which can make the mode(s) of cell death and the initiating factors difficult to determine. In vitro and in vivo models have proved invaluable in this regard, yielding insight into cell death pathways. This review describes the main mechanisms of neuronal cell death, particularly apoptosis, necrosis, excitotoxicity and autophagic cell death, and their role in neurodegenerative diseases such as ischaemia, Alzheimer's, Parkinson's and Huntington's diseases. Crosstalk between these death mechanisms is also discussed. The link between cell death and protein mishandling, including misfolded proteins, impairment of protein degradation, protein aggregation is described and finally, some pro-survival strategies are discussed.

MPP+-induced neuronal death in rats involves tyrosine 33 phosphorylation of WW domain-containing oxidoreductase WOX1

WW domain-containing oxidoreductase (named WWOX, FOR or WOX1) is a pro-apoptotic protein and tumor suppressor. Animals treated with dopaminergic neurotoxin 1-methyl-4-phenyl-pyridinium (MPP+) develop Parkinson's disease (PD)-like symptoms. Here we investigated whether WOX1 is involved in MPP+-induced neurodegeneration. Upon insult with MPP+ in rat brains, WOX1 protein was upregulated and phosphorylated at Tyr33 (or activated) in the injured neurons in the striatum and cortex ipsilaterally to intoxication, as determined by immunohistochemistry and Western blotting. Also, WOX1 was present in the condensed nuclei and damaged mitochondria of degenerative neurons, as revealed by transmission immunoelectron microscopy. Time-lapse microscopy revealed that MPP+ induced membrane blebbing and shrinkage of neuroblastoma SK-N-SH cells. Dominant-negative WOX1, a potent inhibitor of Tyr33 phosphorylation, abolished this event, indicating a critical role of the phosphorylation in apoptosis. c-Jun N-terminal kinase (JNK1) is known to bind and counteract the apoptotic function of WOX1. Suppression of JNK1 function by a dominant-negative spontaneously induced WOX1 activation. WOX1 physically interacted with JNK1 in SK-N-SH cells and rat brain extracts. MPP+ rapidly increased the binding, followed by dissociation, which is probably needed for WOX1 to exert apoptosis. We synthesized a short Tyr33-phosphorylated WOX1 peptide (11 amino acid residues). Interestingly, this peptide blocked MPP+-induced neuronal death in the rat brains, whereas non-phospho-WOX1 peptide had no effect. Together, activated WOX1 plays an essential role in the MPP+-induced neuronal death. Our synthetic phospho-WOX1 peptide prevents neuronal death, suggestive of its therapeutic potential in mitigating the symptoms of PD.

ATP-sensitive potassium channels: novel potential roles in Parkinson's disease

The ATP-sensitive potassium (K(ATP)) channels which extensively distribute in diverse tissues (e.g. vascular smooth muscle, cardiac cells, and pancreas) are well-established for characteristics like vasodilatation, myocardial protection against ischemia, and insulin secretion. The aim of this review is to get insight into the novel roles of K(ATP) channels in Parkinson's disease (PD), with consideration of the specificities K(ATP) channels in the central nervous system (CNS), such as the control of neuronal excitability, action potential, mitochondrial function and neurotransmitter release.

Dopamine induces TNFalpha and TNF-R1 expression in SH-SY5Y human neuroblastoma cells

Cytotoxic concentrations of dopamine (100-500 microM DA) induce expression of tumour necrosis factor receptor-1 (TNF-R1) and tumour necrosis factor-alpha (TNFalpha) in SH-SY5Y human neuroblastoma cells. TNFalpha expression is dose-dependent and can also be detected after 6-hydroxydopamine (6-OHDA) or 1-methyl-4-phenylpyridinium iodide (MPP) treatment. The expression of TNF-R1 is also dose-dependent, but was not observed in 6-OHDA or MPP-treatment. Cells not expressing TNF-R1 were insensitive to TNFalpha, whereas those treated with DA showed a further decrease in viability when subsequently treated with TNFalpha. Thus, DA treatment confers sensitivity to TNFalpha. The decrease of cell viability caused by DA was in part prevented by neutralizing TNFalpha with anti-TNFalpha. As TNF-R1 is increased in substantia nigra of Parkinsonian brains, we suggest that nonvesiculated DA might also play a role in inducing TNF-R1 expression and predispose the neuron to the action of cytokines released in a microglia-mediated inflammatory response.

Pedicularioside A from Buddleia lindleyana inhibits cell death induced by 1-methyl-4-phenylpyridinium ions (MPP+) in primary cultures of rat mesencephalic neurons

Parkinson's disease is characterized by the progressive degeneration of midbrain dopaminergic neurons. Buddleia lindleyana is a traditional Chinese herb, commonly called Zui Yu Cao. The purification and identification of pedicularioside A and other phenylethanoid glycosides from this plant have been reported. However, their neuroprotective effects on the 1-methyl-4-phenylpyridinium ion (MPP(+))-induced death of rat mesencephalic neuron primary cultures and the precise mechanism of this protection remains unclear. We used the 3-(4, 5-dimethylthiozol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) assay for cellular growth to examine the effects of five phenylethanoid glycosides isolated from B. lindleyana, including pedicularioside A, leucosceptoside A, isoacteoside, acteoside, and arenariside, on the viability of mesencephalic neurons treated with MPP(+). Of the compounds tested, pedicularioside A exhibited the greatest degree of protection from MPP(+)-induced cell death. We also observed a marked increase in the number of tyrosine hydroxylase immunoreactive neurons. Pedicularioside A inhibited expression of the caspase-3 gene and cleavage of poly (ADP-ribose) polymerase (PARP) in cultures exposed to MPP(+). Our results suggest that pedicularioside A has a neuroprotective effect to improve the survival of mesencephalic neurons (dopaminergic neurons and non-dopaminergic neurons). The mode of action appears to be the inhibition of caspase-3 gene expression, thereby protecting mesencephalic neurons from MPP(+)-induced cell death.

Plasticity of S2-S4 specificity pockets of executioner caspase-7 revealed by structural and kinetic analysis

Many protein substrates of caspases are cleaved at noncanonical sites in comparison to the recognition motifs reported for the three caspase subgroups. To provide insight into the specificity and aid in the design of drugs to control cell death, crystal structures of caspase-7 were determined in complexes with six peptide analogs (Ac-DMQD-Cho, Ac-DQMD-Cho, Ac-DNLD-Cho, Ac-IEPD-Cho, Ac-ESMD-Cho, Ac-WEHD-Cho) that span the major recognition motifs of the three subgroups. The crystal structures show that the S2 pocket of caspase-7 can accommodate diverse residues. Glu is not required at the P3 position because Ac-DMQD-Cho, Ac-DQMD-Cho and Ac-DNLD-Cho with varied P3 residues are almost as potent as the canonical Ac-DEVD-Cho. P4 Asp was present in the better inhibitors of caspase-7. However, the S4 pocket of executioner caspase-7 has alternate regions for binding of small branched aliphatic or polar residues similar to those of initiator caspase-8. The observed plasticity of the caspase subsites agrees very well with the reported cleavage of many proteins at noncanonical sites. The results imply that factors other than the P4-P1 sequence, such as exosites, contribute to the in vivo substrate specificity of caspases. The novel peptide binding site identified on the molecular surface of the current structures is suggested to be an exosite of caspase-7. These results should be considered in the design of selective small molecule inhibitors of this pharmacologically important protease.

Clinical trials for neuroprotection in Parkinson??s disease: overcoming angst and futility?

PURPOSE OF REVIEW

To summarize recently published results of neuroprotection trials for Parkinson's disease, and discuss them in the context of evolving concepts in clinical study design and animal models.

RECENT FINDINGS

Despite compelling preclinical evidence from laboratory models suggesting potential neuroprotective benefits, the antioxidant, antiapoptotic, antiexcitotoxic, immunomodulatory and neurotrophic agents studied to date have not shown clear benefit in human studies. The futility study design, an alternative approach focused on efficiently excluding less promising compounds, has been adopted recently to investigate four candidate neuroprotectants. A delayed-start trial design has also been introduced in a study of the monoamine oxidase inhibitor rasagiline, demonstrating a possible neuroprotective effect as well as its clear symptomatic benefit. In parallel with these clinical innovations, preclinical research initiatives are identifying new animal models that more closely resemble the clinical course and pathology of Parkinson's disease.

SUMMARY

Angst over disappointing results of neuroprotection trials in Parkinson's disease has engendered efforts to refine animal models at one end of the therapeutics pipeline, and to optimize clinical trial design at the other. Building on new insights into the genetics, epidemiology and pathogenesis of Parkinson's disease, these recent improvements in 'translational infrastructure' will enhance the prospects of achieving the critical goal of slowing the progression of disability.

Apoptotic neuronal death in Parkinson's disease: Involvement of nitric oxide

Apoptosis of nigral dopaminergic neurons by various mechanisms is an emerging phenomenon involved in the degeneration of dopaminergic neurons in Parkinson's disease (PD). Both extrinsic and intrinsic pathways seems to be involved in death of nigral neurons, intrinsic pathway however, seems to be more important due to the energy crisis. Apoptosis by intrinsic pathway is executed by several initiators and effector caspases, which have been found activated in PD patients, experimental models as well as in neuronal cultures. Nitric oxide (NO) seems to be a central molecule due to its ability to modulate both pro and antiapoptotic phenomenon. The review focuses on the diverse extrinsic and intrinsic factors, signaling pathways and their modulation by NO leading to the death of dopaminergic neurons.

A comparative study of proteasomal inhibition and apoptosis induced in N27 mesencephalic cells by dopamine and MG132

Dopamine (DA) and its metabolites have been implicated in the pathogenesis of Parkinson's disease. DA can produce reactive-oxygen species and DA-derived quinones such as aminochrome can induce proteasomal inhibition. We therefore examined the ability of DA and MG132 to induce apoptosis and proteasomal inhibition in N27 rat dopaminergic cells. DA (0-500 micromol/L, 0-24 h) and MG132 (0-5 micromol/L, 0-24 h) treated N27 cells resulted in time- and concentration-dependent apoptosis. To better define DA and MG132-induced apoptosis, the activation of initiator caspases 2 and caspase 9 and the executioner caspase 3 was investigated. Activation of caspase 2, caspase 9, and caspase 3 occurred early and prior to cell death. In addition, N-acetylcysteine (NAC) blocked DA but not MG132-induced apoptosis and mitochondrial membrane potential loss. NAC can react with both reactive-oxygen and quinoid metabolites and its inhibitory activity suggests a role for reactive species in DA-induced apoptosis. Proteasomal inhibition was detected after DA treatment in N27 cells which occurred prior to cell death and was abrogated by NAC. Our results implicate DA-derived reactive species in proteasomal inhibition and caspase-dependent apoptosis in N27 cells. The ability of endogenous DA-derived metabolites to induce proteasomal inhibition and apoptosis may contribute to the selective loss of dopaminergic neurons in Parkinson's disease.

Apoptotic Mode of Cell Death in Substantia Nigra Following Intranigral Infusion of the Parkinsonian Neurotoxin, MPP+ in Sprague-Dawley Rats: Cellular, Molecular and Ultrastructural Evidences

The potent parkinsonian neurotoxin 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP) is known to cause dopaminergic neurodegeneration in nigrostriatal system. In the present study we investigated the nuclear morphology of cells in the substantia nigra pars compacta (SNpc) region of rats following unilateral intranigral infusion of the active metabolite, 1-methyl-4-phenyl pyridinium ion (MPP(+)), which resulted in a dose-dependent and prolonged dopamine depletion in the ipsilateral striatum. There appeared a substantial loss of tyrosine hydroxylase immunoreactive neurons in the SNpc that received the neurotoxin. Specific nuclear staining with Hoechst 33342 or acridine orange revealed bright pyknotic, shrunken, distorted nuclei and condensed chromatin with perinuclear nucleolus respectively following visualization with the former and latter dyes in the ipsilateral SNpc, as compared to the round, intact nuclei and centrally positioned nucleolus in the contralateral side. Ultrastructural details of the nucleus under transmission electron microscope confirmed distorted nuclear organization with shrunken or condensed nuclei and disrupted nuclear membrane. These features are typical of nucleus undergoing apoptosis, and suggest that MPP(+) causes dopaminergic neuronal death through an apoptotic mode. Typical laddering pattern of genomic DNA isolated from the ipsilateral SN in agarose gel electrophoresis conclusively established apoptosis following intranigral administration of MPP(+) in rats.

Multifunctional tellurium molecule protects and restores dopaminergic neurons in Parkinson's disease models

In Parkinson's disease (PD) dopaminergic neurons in the substantia nigra (SN) become dysfunctional and many ultimately die. We report that the tellurium immunomodulating compound ammonium trichloro(dioxoethylene-O,O'-)tellurate (AS101) protects dopaminergic neurons and improves motor function in animal models of PD. It is effective when administered systemically or by direct infusion into the brain. Multifunctional activities of AS101 were identified in this study. These were mainly due to the peculiar Tellur(IV)-thiol chemistry of the compound, which enabled the compound to interact with cysteine residues on both inflammatory and apoptotic caspases, resulting in their inactivation. Conversely, its interaction with a key cysteine residue on p21(ras), led to its activation, an obligatory activity for AS101-induced neuronal differentiation. Furthermore, AS101 inhibited IL-10, resulting in up-regulation of GDNF in the SN. This was associated with activation of the neuroprotective kinases Akt and mitogen-activated protein kinases, and up-regulation of the antiapoptotic protein Bcl-2. Inhibition of caspase-1 and caspase-3 activities were associated with decreased neuronal death and inhibition of IL-1beta. We suggest that, because multiple mechanisms are involved in the dysfunction and death of neurons in PD, use of a multifunctional compound, exerting antiapoptotic, anti-inflammatory, and neurotrophic-inducing capabilities may be potentially efficacious for the treatment of PD.

Neuroprotective effects of echinacoside in the mouse MPTP model of Parkinson's disease

In the present study, we investigated the neuroprotective effects of echinacoside, a phenylethanoid glycoside extracted from the medicinal Chinese herb Cistanches salsa, against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced dopaminergic toxicity. We confirmed that exposure to MPTP in mice leads to permanent behavioral deficits and depletion of dopamine and its metabolites. When administered prior to MPTP, echinacoside reduced behavioral deficits, increased striatal dopamine and dopamine metabolite levels, reduced cell death, and led to a marked increase in tyrosine hydroxylase expression relative to mice treated with MPTP alone. In addition, pre-treatment with echinacoside significantly reduced caspase-3 and caspase-8 activation in 1-methyl-4-phenylpyridinium (MPP(+))-induced apoptosis in cerebellar granule neurons. Taken together, these findings suggest that echinacoside improves the behavioral and neurochemical outcomes in MPTP mice model of Parkinson's disease and inhibits caspase-3 and caspase-8 activation in cerebellar granule neurons, making the compound an attractive candidate treatment for various neurodegenerative disorders, including Parkinson's disease.

Rifampicin protects PC12 cells against MPP+-induced apoptosis and inhibits the expression of an alpha-Synuclein multimer

The potential cytoprotective effects of the anti-leprosy antibiotic rifampicin were investigated in rat pheochromocytoma (PC12) cells prior to intoxication with 1-Methyl-4-phenyl pyridinium (MPP(+)). MPP(+) induced both apoptotic and necrotic cell death, and increased the expression of a 57 kDa species of alpha-Synuclein. This species of alpha-Synuclein is larger than the monomer, and is therefore an oligomer or an aggregated form of the protein. Rifampicin significantly increased survival of these catecholaminergic cells in a concentration-dependent manner. The expression of the higher molecular mass alpha-Synuclein was increased by MPP(+) exposure, and its expression was inversely related to cell survival in the rifampicin-treated cells. Importantly, rifampicin suppressed apoptosis almost completely, without shifting the death cascade to necrosis, which is a problem that has been reported with caspase inhibitors of apoptosis (Hartmann, A., Troadec, J.D., Hunot, S., Kikly, K., Faucheux, B.A., Mouatt-Prigent, A., Ruberg, M. Agid, Y., Hirsch, E.C., 2001. Caspase-8 is an effector in apoptotic death of dopaminergic neurons in Parkinson's disease, but pathway inhibition results in neuronal necrosis. J. Neurosci. 21, 2247-2255). These results suggest that rifampicin improves survival of catecholamine- and alpha-Synuclein-containing cells, which degenerate in Parkinson's disease (PD), and thus may be therapeutic in this disease.

Intrastriatal infusion of the Parkinsonian neurotoxin, MPP+, induces damage of striatal cell nuclei in Sprague–Dawley rats

The potent Parkinsonian neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine is known to destroy dopaminergic neurons of the basal ganglia. Its neurotoxically active metabolite, 1-methyl-4-phenyl pyridinium (MPP(+)), has been examined in the present study to verify whether administration of the neurotoxin that depletes about 70% of the striatal dopamine (DA) can cause damage to nuclear components of the cells at the terminal region, the striatum. Unilateral intrastriatal infusion of MPP(+) (100 and 200 nmol in 4 microl saline) caused a dose-dependent depletion of striatal DA (69 and 92%, respectively), as measured employing HPLC electrochemistry. It also resulted in the loss of tyrosine hydroxylase (TH) immunoreactivity in the striatum and in the perikarya at substantia nigra pars compacta (SNpc) and acetylcholinesterase histoenzymological staining in the striatum. Specific nuclear staining employing Hoechst 33342 and acridine orange revealed distorted and spindle shaped nuclei, and perinuclear positioning of nucleolus, respectively, for the former and latter dyes in several of the cell populations in the ipsilateral striatum compared to the contralateral side. Existence of a widened lateral ventricle at the side that received the neurotoxin, as well as denser cellular population, as compared to the contralateral side under transmission electron microscope evidenced general shrinkage of the striatum. Extensive damage of the nuclei was visible in the cell bodies in the treated side. These results demonstrate non-specific damage extending to the cellular groups including cholinergic neurons in addition to dopaminergic neurons in the striatum to intrastriatal administration of the Parkinsonian neurotoxin, MPP(+).

Divergent Mechanisms of Paraquat, MPP+, and Rotenone Toxicity: Oxidation of Thioredoxin and Caspase-3 Activation

Paraquat, N-methyl-4-phenyl-1,2,3,6 tetrahydropyridine, and rotenone have been shown to reproduce several features of Parkinson's disease in animal and cell culture models. Although these chemicals are known to perturb dopamine homeostasis and induce dopaminergic cell death, their molecular mechanisms of action are not well defined. We have previously shown that paraquat does not require functional dopamine transporter and does not inhibit mitochondrial complex I in order to mediate its toxic action (Richardson et al., 2005). In this study, we show that paraquat specifically oxidized the cytosolic form of thioredoxin and activated Jun N-terminal kinase (JNK), followed by caspase-3 activation. Conversely, 1-methyl-4-phenylpyridinium (MPP(+)) and rotenone oxidized the mitochondrial form of thioredoxin but did not activate JNK-mitogen-activated protein kinase and caspase-3. Loading cells with exogenous dopamine did not exacerbate the toxicity of any of these compounds. These data suggest that oxidative modification of cytosolic proteins is critical to paraquat toxicity, while oxidation of mitochondrial proteins is important for MPP(+) and rotenone toxicity. In addition, intracellular dopamine does not seem to exacerbate the toxicity of these dopaminergic neurotoxicants in this model.

Optimization of the Tet-On system for inducible expression of RAGE

We have optimized a two-plasmid Tet-On system, the regulatory plasmid and the response plasmid, to produce tightly controlled inducible expression of the gene RAGE in cell-culture models. Two sets of plasmids were constructed: set 1 (universal; for broad range of cell types) and set 2 (neuron specific). For the response plasmid, the gene RAGE was cloned in pIRES2-EGFP plasmid (Clontech) and the CMV promoter replaced with TREtight (modified seven copies of Tet-operon fused with CMVm promoter). For the regulatory plasmid, rtTA (reverse tetracycline transactivator) was placed under either the CMV promoter or the cell-specific promoter neuronal specific enolase. Both plasmids have the mammalian selection marker neomycine; the EGFP reporter gene is only in the response plasmid and IRES is between the gene and EGFP. Following induction with doxycycline, cells expressing RAGE showed neomycine resistance and green fluorescence (EGFP). Our system has been tested in two different cell lines and showed negligible basal leakiness, high induction of the gene RAGE (142-fold), dose-dependent response to doxycycline, and strict cell-type specificity. This system is highly suitable for cell-specific expression of any gene of interest in primary cultures and mixed cell populations.

Recent clinical failures in Parkinson's disease with apoptosis inhibitors underline the need for a paradigm shift in drug discovery for neurodegenerative diseases

Understanding the mechanisms of neuronal death in concert with the identification of drugable molecular targets key to this process has held great promise for the development of novel chemical entities (NCEs) to halt neurodegenerative disease progression. Two key targets involved in the apoptotic process identified over the past decade include the mixed lineage kinase (MLK) family and glyceraldehyde phosphate dehydrogenase (GAPDH). Two NCEs, CEP-1347 and TCH346, directed against these respective targets have progressed to the clinic. For each, robust neuroprotective activity was demonstrated in multiple in vitro and in vivo models of neuronal cell death, but neither NCE proved effective Parkinson's disease (PD) patients. These recent clinical failures require a reassessment of both the relevance of apoptosis to neurodegenerative disease etiology and the available animal models used to prioritize NCEs for advancement to the clinic in this area.

Acute intranigral homocysteine administration produces stereotypic behavioral changes and striatal dopamine depletion in Sprague–Dawley rats

Homocysteine has been considered a major risk factor for cardiovascular diseases, and patients with hyperhomocystinemia exhibit neurological and psychological abnormalities. Elevated level of this molecule in the blood of Parkinson's disease patients receiving long-term l-DOPA therapy prompted us to investigate whether homocysteine is neurotoxic to the nigrostriatal dopaminergic system in Sprague-Dawley rats. Animals infused unilaterally with different doses of homocysteine (0.25-1 micromol in 1 microl) intranigrally exhibited significant and dose-dependent decrease in dopamine levels in the ipsilateral striatum as assayed employing an HPLC coupled with electrochemical detector, 19 days post-infusion. While 3,4-dihydroxyphenylacetic acid level in the striatum showed a dose-dependent decrease, homovanillic acid was found to be inhibited only for the highest dose. Amphetamine administration in these animals on the 14th day caused stereotypic turning behavior ipsilateral to the side of infusion. Apomorphine challenge on the 16th day elicited stereotypic contralateral circling behavior. Neurotransmitter levels in the serotonergic perikarya or terminals were unaltered 19 days following intraraphe infusion of homocysteine, which suggested the specificity of its action to dopaminergic neurons. These results indicate nigrostriatal lesions similar to that observed following intranigral infusion of the dopaminergic neurotoxin, 6-hydroxydopamine and suggest its closeness to the parkinsonian animal model. Furthermore, these findings provide evidence for the neurotoxic nature of homocysteine to dopaminergic neurons and suggest that elevated level of this molecule in parkinsonian patients may be conducive to accelerate the progression of the disease.

Anti-apoptotic gene therapy in Parkinson's disease

Apoptosis, whether caspase-dependent or caspase-independent, has been implicated as one of the important mechanisms leading to the death of dopaminergic neurons in the substantia nigra of Parkinson's disease patients. Major advances of our understanding of apoptosis have been achieved in studies of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity in mice and monkeys and 6-hydroxydopamine (6-OHDA) toxicity in rats and monkeys. The use of viral vectors to either express anti-apoptotic proteins or to downregulate pro-apoptotic proteins has the major advantage of addressing selective molecular targets, bypassing the blood-brain-barrier to specifically target the nigrostriatal pathway by their stereotaxic application and by the choice of the appropriate virus and promotor. Used thus far have been virus-mediated overexpression of inhibitor of apoptosis proteins, inhibitors of the c-jun-N-terminal kinase (JNK) pathway, inhibitors of calpains and dominant negative inhibitors of the protease activating factor (APAF)-1 and cdk5. Most studies implicate the endogenous, mitochondrial pathway in the apoptosis of dopaminergic neurons. The results suggest that only an inhibition of this pathway upstream of caspase activation will also result in the protection of nigrostriatal dopaminergic terminals and behavioral benefit, whereas an inhibition of caspases alone may not be sufficient to prevent the degeneration of terminals, although it may promote the survival of neuronal cell bodies for some time.

Neuroprotection by rasagiline: a new therapeutic approach to Parkinson's disease?

Neuronal death in Parkinson's disease (PD) may originate from the reciprocal interactions of a restricted number of conditions, such as mitochondrial defects, oxidative stress and protein mishandling, which would favor a state of apoptotic cell death in the nigrostriatal pathway. The search for pharmacological treatments able to counteract the nigrostriatal degeneration, possibly by interfering with these phenomena, has recently raised considerable interest in rasagiline [R(+)-N-propargyl-1-aminoindan], a potent, selective, and irreversible inhibitor of monoamine oxidase B (MAO-B). Rasagiline, like selegiline, is a propargylamine, but is approximately 10 times more potent. Unlike selegiline, rasagiline is not metabolized to amphetamine and/or methamphetamine and is devoid of sympathomimetic activity. Numerous experimental studies, conducted both in vitro and in vivo, have shown that rasagiline possesses significant protective properties on neuronal populations. The pro-survival effects of the drug appear to be linked to its propargyl moiety, rather than to the inhibitory effect on MAO-B. Rasagiline's major metabolite, aminoindan--which possesses intrinsic neuroprotective activity--may also contribute to the beneficial effects of the parent compound. Rasagiline has been recently evaluated in early PD patients, with results that are consistent with slowing the progression of the disease. Therefore, the neuroprotective activity shown by the drug under experimental conditions may be reflected in the clinic, thus providing new perspectives for the treatment of PD.

Epilepsy and apoptosis pathways

Epilepsy is a common, chronic neurologic disorder characterized by recurrent unprovoked seizures. Experimental modeling and clinical neuroimaging of patients has shown that certain seizures are capable of causing neuronal death. Such brain injury may contribute to epileptogenesis, impairments in cognitive function or the epilepsy phenotype. Research into cell death after seizures has identified the induction of the molecular machinery of apoptosis. Here, the authors review the clinical and experimental evidence for apoptotic cell death pathway function in the wake of seizure activity. We summarize work showing intrinsic (mitochondrial) and extrinsic (death receptor) apoptotic pathway function after seizures, activation of the caspase and Bcl-2 families of cell death modulators and the acute and chronic neuropathologic impact of intervening in these molecular cascades. Finally, we describe evolving data on nonlethal roles for these proteins in neuronal restructuring and cell excitability that have implications for shaping the epilepsy phenotype. This review highlights the work to date on apoptosis pathway signaling during seizure-induced neuronal death and epileptogenesis, and speculates on how emerging roles in brain remodeling and excitability have enriched the number of therapeutic strategies for protection against seizure-damage and epileptogenesis.

Increased plasma levels of TNF-α but not of IL1-β in MPTP-treated monkeys one year after the MPTP administration

The cause of Parkinson's disease remains unknown although some evidence suggests that an inflammatory reaction, mediated by cytokines such as TNF-alpha and IL-1beta, is related with dopaminergic degeneration in the brain. In the present work we measured the plasma levels of TNF-alpha and IL-1beta in parkinsonian monkeys one year after MPTP administration. TNF-alpha levels were seen to have increased in parkinsonian monkeys reflecting the clinical symptoms observed, while IL-1beta levels remained unchanged. These results suggest that TNF-alpha plays a role in sustaining of dopaminergic degeneration in chronic parkinsonism.

CHOP/GADD153 is a mediator of apoptotic death in substantia nigra dopamine neurons in an in vivo neurotoxin model of parkinsonism

There is increasing evidence that neuron death in neurodegenerative diseases, such as Parkinson's disease, is due to the activation of programmed cell death. However, the upstream mediators of cell death remain largely unknown. One approach to the identification of upstream mediators is to perform gene expression analysis in disease models. Such analyses, performed in tissue culture models induced by neurotoxins, have identified up-regulation of CHOP/GADD153, a transcription factor implicated in apoptosis due to endoplasmic reticulum stress or oxidative injury. To evaluate the disease-related significance of these findings, we have examined the expression of CHOP/GADD153 in neurotoxin models of parkinsonism in living animals. Nuclear expression of CHOP protein is observed in developmental and adult models of dopamine neuron death induced by intrastriatal injection of 6-hydroxydopamine (6OHDA) and in models induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). CHOP is a mediator of neuron death in the adult 60HDA model because a null mutation results in a reduction in apoptosis. In the chronic MPTP model, however, while CHOP is robustly expressed, the null mutation does not protect from the loss of neurons. We conclude that the role of CHOP depends on the nature of the toxic stimulus. For 6OHDA, an oxidative metabolite of dopamine, it is a mediator of apoptotic death.

Mammalian initiator apoptotic caspases

Caspases are a conserved family of cysteine proteases. They play diverse roles in inflammatory responses and apoptotic pathways. Among the caspases is a subgroup whose primary function is to initiate apoptosis. Within their long prodomains, caspases-2, -9 and -12 contain a caspase activation and recruitment domain while caspases-8 and -10 bear death effector domains. Activation follows the recruitment of the procaspase molecule via the prodomain to a high molecular mass complex. Despite sharing some common features, other aspects of the biochemistry, substrate specificity, regulation and signaling mechanisms differ between initiator apoptotic caspases. Defects in expression or activity of these caspases are related to certain pathological conditions including neurodegenerative disorders, autoimmune diseases and cancer.

Apoptosis inducing factor mediates caspase-independent 1-methyl-4-phenylpyridinium toxicity in dopaminergic cells

Parkinson's disease is a debilitating neurodegenerative disease characterized by loss of midbrain dopaminergic neurons. These neurons are particularly sensitive to the neurotoxin 1-methyl-4-phenylpyridinium (MPP+), the active metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), which causes parkinsonian syndromes in humans, monkeys and rodents. Although apoptotic cell death has been implicated in MPTP/MPP+ toxicity, several recent studies have challenged the role of caspase-dependent apoptosis in dopaminergic neurons. Using the midbrain-derived MN9D dopaminergic cell line, we found that MPP+ treatment resulted in an active form of cell death that could not be prevented by caspase inhibitors or over-expression of a dominant negative inhibitor of apoptotic protease activating factor 1/caspase-9. Apoptosis inducing factor (AIF) is a mitochondrial protein that may mediate caspase-independent forms of regulated cell death following its translocation to the nucleus. We found that MPP+ treatment elicited nuclear translocation of AIF accompanied by large-scale DNA fragmentation. To establish the role of AIF in MPP+ toxicity, we constructed a DNA vector encoding a short hairpin sequence targeted against AIF. Reduction of AIF expression by RNA interference inhibited large-scale DNA fragmentation and conferred significant protection against MPP+ toxicity. Studies of primary mouse midbrain cultures further supported a role for AIF in caspase-independent cell death in MPP+-treated dopaminergic neurons.

Identification of midkine as a mediator for intercellular transfer of drug resistance

Resistance to cytotoxic agents is a major limitation for their clinical use to treat human cancers. Tumors become resistant to chemotherapy when a subset of cells undergoes molecular changes leading to overexpression of drug transport proteins, alterations in drug-target interactions or reduced ability to commit apoptosis. However, such changes may not be sufficient to explain why both resistant and nonresistant cells survive drug's action in tumors that ultimately become drug resistant. We hypothesized that, in such tumors, a cytoprotective relationship may exist between drug-resistant and neighboring drug-sensitive cells. The present study addresses the possibility that drug-resistant cells secrete in their culture medium factors able to protect sensitive cells from drug toxicity. A survival molecule, midkine, was identified by cDNA array to be expressed only in drug-resistant cells. Midkine-enriched fractions obtained by affinity chromatography exert a significant cytoprotective effect against doxorubicin in the wild-type drug-sensitive cells. Moreover, transfection of these cells with the midkine gene caused a decreased response to doxorubicin. The underlying mechanism of this cytoprotection appeared to imply activation of the Akt pathway and inhibition of drug-induced proliferation arrest as well as apoptotic cell death. These findings provide evidence for the existence of intercellular cytoprotective signals such as the one mediated by midkine, originating from cells with acquired drug resistance to protect neighboring drug-sensitive cells and thus contribute to development of resistance to chemotherapy.

Neuroinflammatory processes in Parkinson's disease

In Parkinson's disease (PD), post-mortem examination reveals a loss of dopaminergic (DA) neurons in the substantia nigra (SN) associated with a massive astrogliosis and the presence of activated microglial cells. Similarly, microglial activation has also been reported to be associated with the loss of DA neurons in animal models of PD induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), rotenone, annonacine and lipopolysaccharide (LPS). Recent evidence suggests that the disease may progress even when the initial cause of neuronal degeneration has disappeared, raising the possibility that toxic substances released by glial cells could be involved in the propagation of neuronal degeneration. Inhibition of the glial reaction and the inflammatory processes may thus represent a therapeutic target to reduce neuronal degeneration in PD.

Granulocyte colony-stimulating factor is not protective against selective dopaminergic cell death in vitro

In the present study, we evaluated the potential neuroprotective effect of granulocyte colony-stimulating factor (G-CSF), a hematopoietic growth factor in two different culture models in which dopaminergic (DA) neurons die selectively: first, in a culture model in which death of DA neurons occurs spontaneously and second, in a toxin-based paradigm, the in vitro 1-methyl-4-phenylpyridinium model of PD. In neither of the two models, a treatment with G-CSF, could prevent or halt the progressive neurodegeneration. However, we cannot rule out that G-CSF might exert neuroprotective or even deleterious effects in in vivo models of PD, based on the significant increase in the number of microglial cells observed after G-CSF treatment.

Acute intranigral infusion of rotenone in rats causes progressive biochemical lesions in the striatum similar to Parkinson's disease

We examined in Sprague-Dawley rats whether intranigral administration of complex-I inhibitor, rotenone, produces biochemical lesions in the striatum similar to those observed in Parkinson's disease (PD). Unilateral stereotaxic infusion of rotenone (2-12 mug in 1 mul) into substantia nigra (SN) pars compacta caused significant inhibition of complex-I activity and increased production of hydroxyl radicals in vivo as measured employing spectrophotometric and HPLC-electrochemical procedures, respectively. It also caused a significant time- and dose-dependent reduction of dopamine level, but not serotonin, in the ipsilateral striatum when assayed using an HPLC electrochemical method. This effect was found to be progressive for 90 days. A dose-dependent decrease in nigral glutathione level, as measured fluorimetrically, was also observed to be progressive till 90th day. A significant decrease in tyrosine hydroxylase immunoreactivity in the striatum (73 +/- 8.4% as assessed by densitometric studies) or in SN ipsilateral to the side of infusion suggested nigrostriatal neuronal degeneration. A dose of rotenone (6 microg in 1 microl) that caused 55% striatal dopamine depletion when infused into the SN failed to affect serotonin levels in the terminal regions when infused into the nucleus raphe dorsalis, indicating rotenone's specificity of action towards dopaminergic neurons. Our findings suggest that unilateral infusion of rotenone reproduces neurochemical and neuropathological features of hemiparkinsonism in rats and indicate an active involvement of oxidative stress in rotenone-induced nigrostriatal neurodegeneration. The present study also demonstrates more sensitivity of dopaminergic neurons towards rotenone and establishes mitochondrial complex-I damage as one of the major contributory components of neurodegeneration in PD. The progressive nature of pathology in this model closely mimics idiopathic PD, and absence of mortality warrants the use of this model in drug discovery programs.

The role of α-synuclein in neurodegenerative diseases

Alpha-synuclein is a 140 amino acid neuronal protein that has been associated with several neurodegenerative diseases. A point mutation in the gene coding for the alpha-synuclein protein was the first discovery linking this protein to a rare familial form of Parkinson's disease (PD). Subsequently, other mutations in the alpha-synuclein gene have been identified in familial PD. The aggregated proteinaceous inclusions called Lewy bodies found in PD and cortical Lewy body dementia (LBD) were discovered to be predominantly alpha-synuclein. Aberrant aggregation of alpha-synuclein has been detected in an increasing number of neurodegenerative diseases, collectively known as synucleopathies. Alpha-synuclein exists physiologically in both soluble and membrane-bound states, in unstructured and alpha-helical conformations, respectively. The physiological function of alpha-synuclein appears to require its translocation between these subcellular compartments and interconversion between the 2 conformations. Abnormal processing of alpha-synuclein is predicted to lead to pathological changes in its binding properties and function. In this review, genetic and environmental risk factors for alpha-synuclein pathology are described. Various mechanisms for in vitro and in vivo alpha-synuclein aggregation and neurotoxicity are summarized, and their relevance to neuropathology is explored.

Cytokine production of activated microglia and decrease in neurotrophic factors of neurons in the hippocampus of Lewy body disease brains

Dementia is a frequent complication of Parkinson's disease (PD) and usually occurs late in the protracted course of the illness. We have already reported numerous MHC class II-positive microglia in the hippocampus in PD patients, and that this phenomenon may be responsible for functional changes in the neurons and the cognitive decline in PD patients. In this study, we have investigated the distribution of activated microglia and the immunohistochemical and the mRNA expression of several cytokines and neurotrophic factors of the hippocampus in PD and dementia with Lewy bodies (DLB). The brains from five cases of PD and five cases of DLB that were clinically and neuropathologically diagnosed, and those from four normal controls (NC) were evaluated by immunohistochemistry using anti-HLA-DP, -DQ, -DR (CR3/43), anti-alpha-synuclein, anti-brain-derived neurotrophic factor (BDNF), and anti-glial fibrillary acidic protein antibodies. In addition, the mRNA expressions of cytokines (IL-1alpha, IL-1beta, TNF-alpha, IL-6, TGF-beta) and neurotrophic factors (BDNF, GDNF, NGF, NT-3) of these brains were evaluated by the reverse transcription-PCR method. MHC class II-positive microglia were distributed diffusely in the hippocampus of PD and DLB brains. Although the cytoplasm of pyramidal and granular cells of the hippocampus in NC brains was strongly stained by anti-BDNF antibodies, it was only weakly stained in PD and DLB brains. The mRNA expression of IL-6 was significantly increased in the hippocampus of PD and DLB brains, and that of BDNF was significantly decreased in the hippocampus of DLB brains. The increased number of activated microglia and the production of neurotrophic cytokines such as IL-6, together with the decreased expression of the neurotrophic factors of neurons in the hippocampus of PD and DLB brains, may be related to functional cellular changes associated with dementia.

Role of activated astrocytes in neuronal damage: potential links to HIV-1-associated dementia

HIV-1-associated dementia (HAD) is an important complication of HIV-1 infection. Reactive astrogliosis is a key pathological feature in HAD brains and in other central nervous system (CNS) diseases. Activated astroglia may play a critical role in CNS inflammatory diseases such as HAD. In order to test the hypothesis that activated astrocytes cause neuronal injury, we stimulated primary human fetal astrocytes with HAD-relevant pro-inflammatory cytokine IL-1beta. IL-1beta-activated astrocytes induced apoptosis and significant changes in metabolic activity in primary human neurons. An FITC-conjugated pan-caspase inhibitor peptide FITC-VAD-FMK was used for confirming caspase activation in neurons. IL-1beta activation enhanced the expression of death protein FasL in astrocytes, suggesting that FasL is one of the potential factors responsible for neurotoxicity observed in HAD and other CNS diseases involving glial inflammation. Our data presented here add to the developing picture of role of activated glia in HAD pathogenesis.

Reduction in endogenous parkin levels renders glial cells sensitive to both caspase-dependent and caspase-independent cell death

Mutations in the parkin gene give rise to a familial form of Parkinson's disease, autosomal recessive juvenile Parkinsonism (AR-JP). Although the exact mechanisms are unclear, it is thought that these 'loss-of-function' mutations contribute to the pathological process by interfering with parkin's E3 ubiquitin ligase activity. In order to mimic the in vivo loss-of-function, we produced tet-inducible glial cell lines that, in the presence of doxycycline, were able either to under- or to over-express the parkin protein. Using this cell-culture system, we found that the induced alteration of parkin levels in glial cell lines caused different responses compared with their un-induced counterparts under conditions of stress (staurosporine, hydrogen peroxide and dopamine). In particular, reduction in the levels of parkin within the transfected cells rendered them more susceptible to both apoptotic and necrotic cell death. Interestingly, blocking the cell death pathway with caspase inhibitors rescued the cells under-expressing parkin from only some of the stress-induced death. These findings implicate a pathogenic role of glial cells in the pathogenesis of AR-JP caused by mutations in the parkin gene.

Neuronal pathology in Parkinson?s disease

Parkinson's disease (PD) is characterized by the progressive loss of dopaminergic neurons in the substantia nigra leading to the major clinical and pharmacological abnormalities of PD. In order to establish causal or protective treatments for PD, it is necessary to identify the cascade of deleterious events that lead to the dysfunction and death of dopaminergic neurons. Based on genetic, neuropathological, and biochemical data in patients and experimental animal models, dysfunction of the ubiquitin-proteasome pathway, protein aggregation, mitochondrial dysfunction, oxidative stress, activation of the c-Jun N-terminal kinase pathway, and inflammation have all been identified as important pathways leading to excitotoxic and apoptotic death of dopaminergic neurons. Toxin-based and genetically engineered animal models allow (1) the study of the significance of these aspects and their interaction with each other and (2) the development of causal treatments to stop disease progression.

Cellular pathology of Parkinson?s disease: astrocytes, microglia and inflammation

Parkinson's disease (PD) is a frequent neurological disorder of the basal ganglia, which is characterized by the progressive loss of dopaminergic neurons mainly in the substantia nigra pars compacta (SNpc). Inflammatory processes have been shown to be associated with the pathogenesis of PD. Activated microglia, as well as to a lesser extent reactive astrocytes, are found in the area associated with cell loss, possibly contributing to the inflammatory process by the release of pro-inflammatory prostaglandins or cytokines. Further deleterious factors released by activated microglia or astrocytes are reactive oxygen species. On the other hand, they may mediate neuroprotective properties by the release of trophic factors or the uptake of glutamate. In this review, we will discuss the different aspects of activated glial cells and potential mechanisms that mediate or protect against cell loss in PD.

Immunofluorescent labeling of increased calpain expression and neuronal death in the spinal cord of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated mice

Parkinson's disease (PD) is a movement disorder characterized by rigidity, tremor, and bradykinesia, originating from degeneration of dopaminergic neurons in the substantia nigra (SN), retrorubral area, and locus ceoruleus (LC). Calpain has been implicated in the pathophysiology of neurodegenerative diseases. Since the spinal cord (SC) and brain are integrally connected and calpain is involved in cell death and mitochondrial dysfunction, we hypothesized that SC neurons are also affected in PD. In order to examine this hypothesis, we examined both brain and SC from mice treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). To identify cells expressing calpain, double immunofluorescent labeling was performed with antibodies specific for calpain and a cell type (OX-42, GFAP, or NeuN). Combined terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) and double immunofluorescent labeling were used to identify death of specific cells in the central nervous system (CNS). There was an increase in calpain expression in microglia, astrocytes, and neurons in the SC of MPTP-treated mice at 1 and 7 days, as compared to controls. TUNEL-positive neurons in the SC and SN showed apoptotic characteristics. These results demonstrated that neuronal death occurred not only in SN but also in the SC of MPTP-treated mice and has provided evidence for a possible calpain-mediated SC neuronal death in MPTP-induced parkinsonism in mice.

Interrupting apoptosis in neurodegenerative disease: potential for effective therapy?

Current treatment options for neurodegenerative diseases are limited and mainly affect only the symptoms of disease. Because of the unknown and probably multiple causes of these diseases, they cannot be readily targeted. However, it has been established that apoptosis contributes to neuronal loss in most neurodegenerative diseases. A possible treatment option is to interrupt the signaling networks that link neuronal damage to apoptotic degradation in neurodegeneration. The viability of this option depends upon the extent to which apoptosis accounts for neuron loss, whether or not interruption of apoptosis signaling results in recovery of neurological function and whether or not there are significant downsides to targeting apoptosis. Several compounds acting at different sites in known apoptotic signaling networks are currently in development and a few are in clinical trial. If an apoptosis-targeted compound succeeds in slowing or halting neurological dysfunction in one or more neurodegenerative diseases, a new era in the treatment of neurodegenerative diseases will begin.

Effects of 6-hydroxydopamine on primary cultures of substantia nigra: specific damage to dopamine neurons and the impact of glial cell line-derived neurotrophic factor

6-Hydroxydopamine (6-OHDA)-induced loss of dopamine (DA) neurons has served to produce an animal model of DA neuron loss in Parkinson's disease. We report here the use of 6-OHDA to produce an in vitro model of this phenomena using dissociated cultures prepared from neonatal rat mesencephalon. Cultures were exposed to 6-OHDA (40-100 microm, 15 min) in an antioxidant medium, and DA and GABA neurons evaluated by immunocytochemistry. 6-OHDA induced morphological and biochemical signs of cell death in DA neurons within 3 h, followed by loss of tyrosine hydroxylase immunoreactive neurons within 2 days. In substantia nigra (SN) cultures, DA neurons were much more affected by 6-OHDA than were GABA neurons. In contrast, DA neurons from the ventral tegmental area were only lost at higher, non-specific concentrations of 6-OHDA. The effects of 6-OHDA on nigral DA neurons were blocked by inhibitors of high affinity DA transport and by z-DEVD-fmk (150 microm), a caspase inhibitor. Glial cell line-derived neurotrophic factor (GDNF) treatment reduced TUNEL labeling 3 h after 6-OHDA exposure, but did not prevent loss of DA neurons at 48 h. Thus, 6-OHDA can selectively destroy DA neurons in post-natal cultures of SN, acting at least in part by initiating caspase-dependent apoptosis, and this effect can be attenuated early but not late by GDNF.

Phosphorylation of p38 MAPK Induced by Oxidative Stress Is Linked to Activation of Both Caspase-8- and -9-mediated Apoptotic Pathways in Dopaminergic Neurons

We evaluated the contribution of p38 mitogen-activated protein kinase and the events upstream/downstream of p38 leading to dopaminergic neuronal death. We utilized MN9D cells and primary cultures of mesencephalic neurons treated with 6-hydroxydopamine. Phosphorylation of p38 preceded apoptosis and was sustained in 6-hydroxydopamine-treated MN9D cells. Co-treatment with PD169316 (an inhibitor of p38) or expression of a dominant negative p38 was neuroprotective in death induced by 6-hydroxydopamine. The superoxide dismutase mimetic and the nitric oxide chelator blocked 6-hydroxydopamine-induced phosphorylation of p38, suggesting a role for superoxide anion and nitric oxide in eliciting a neurotoxic signal by activating p38. Following 6-hydroxydopamine treatment, inhibition of p38 prevented both caspase-8- and -9-mediated apoptotic pathways as well as generation of truncated Bid. Consequently, 6-hydroxydopamine-induced cell death was rescued by blockading activation of caspase-8 and -9. In primary cultures of mesencephalic neurons, the phosphorylation of p38 similarly appeared in tyrosine hydroxylase-positive, dopaminergic neurons after 6-hydroxydopamine treatment. This neurotoxin-induced phosphorylation of p38 was inhibited in the presence of superoxide dismutase mimetic or nitric oxide chelator. Co-treatment with PD169316 deterred 6-hydroxydopamine-induced loss of dopaminergic neurons and activation of caspase-3 in these neurons. Furthermore, inhibition of caspase-8 and -9 significantly rescued 6-hydroxydopamine-induced loss of dopaminergic neurons. Taken together, our data suggest that superoxide anion and nitric oxide induced by 6-hydroxydopamine initiate the p38 signal pathway leading to activation of both mitochondrial and extramitochondrial apoptotic pathways in our culture models of Parkinson's disease.

Caspase inhibition switches the mode of cell death induced by cyanide by enhancing reactive oxygen species generation and PARP-1 activation

Execution of apoptosis can involve activation of the caspase family of proteases. Recent studies show that caspase inhibition can switch the morphology of cell death from apoptotic to necrotic without altering the level of death among cell populations. In the present study, the effect of caspase inhibition on cortical (CX) cell death induced by cyanide was investigated. In primary cultured CX cells exposed to cyanide (400 microM), death was primarily apoptotic as indicated by positive TUNEL staining. Reactive oxygen species (ROS) generation and subsequent caspase activation mediated the apoptosis. Inhibition of the caspase cascade with zVAD-fmk switched the apoptotic response to necrotic cell death, as assessed by increased cellular efflux of LDH and propidium iodide uptake by the cells. The change in death mode was accompanied by a marked increase in poly (ADP-ribose) polymerase-1 (PARP-1) activity, reactive oxygen species (ROS) generation, a reduction in the mitochondrial membrane potential (Delta psi(m)), and reduced cellular ATP. Prior treatment of cells with 3-aminobenzamide (3-AB), a PARP-1 inhibitor, prevented the cells from undergoing necrosis and preserved intracellular ATP levels. These findings indicate that apoptosis and necrosis share common initiation pathways and caspase inhibition can switch the apoptotic response to necrosis. Inhibition of PARP-1 preserves cellular ATP levels and in turn blocks execution of the necrotic death pathway.

Senescence-initiated Reversal of Drug Resistance

The present study was undertaken to verify whether induction of senescence could be sufficient to reverse drug resistance and, if so, to determine the underlying mechanism(s). Our findings indicated that cotreatment of drug-resistant neuroblastoma cells with doxorubicin, at sublethal concentrations, in combination with the pan-caspase inhibitor, Q-VD-OPH, elicited a strong reduction of cell viability that occurred in a caspase-independent manner. This was accompanied by the appearance of a senescence phenotype, as evidenced by increased p21/WAF1 expression and senescence-associated beta-galactosidase activity. Experiments using specific inhibitors of major cellular proteases other than caspases have shown that inhibition of cathepsin L, but not proteasome or cathepsin B, was responsible for the senescence-initiated reversal of drug resistance. This phenomenon appeared to be general because it was valid for other drugs and drug-resistant cell lines. A nonchemical approach, through cell transfection with cathepsin L small interfering RNA, also strongly reversed drug resistance. Further investigation of the underlying mechanism revealed that cathepsin L inhibition resulted in the alteration of intracellular drug distribution. In addition, in vitro experiments have demonstrated that p21/WAF1 is a substrate for cathepsin L, suggesting that inhibition of this enzyme may result in p21/WAF1 stabilization and its increased accumulation. All together, these findings suggest that cathepsin L inhibition in drug-resistant cells facilitates induction of senescence and reversal of drug resistance. This may represent the basis for a novel function of cathepsin L as a cell survival molecule responsible for initiation of resistance to chemotherapy.