Signaling of neuronal cell death by the p75NTR neurotrophin receptor

The neurotrophin receptor (p75NTR) is best known for mediating tropic support by participating in the formation of high-affinity nerve growth factor (NGF) receptor complexes with trkA, however, p75NTR more recently has been shown to act as a bona fide death-signaling receptor, which can signal independently of trkA. This article discusses the evidence for an active role of p75NTR in neuronal cell death and the mechanisms controlling this process, including roles for Bcl-2 family members, the c-jun stress kinase JNK, the transcription factor nuclear factor kappa B (NF kappa B), and caspases.

Role of neurotrophin receptor p75NTR in mediating neuronal cell death following injury

1. The neurotrophin receptor p75NTR has been shown to mediate neuronal cell death after nerve injury. 2. Down-regulation of p75NTR by antisense oligonucleotides is able to inhibit both sensory and motor neuron death and this treatment is more effective than treatment with growth factors. 3. p75NTR induces cell death by a unique death signalling pathway involving transcription factors (nuclear factor kappa B and c-jun), Bcl-2 family members and caspases.

Chopper, a new death domain of the p75 neurotrophin receptor that mediates rapid neuronal cell death

The cytoplasmic juxtamembrane region of the p75 neurotrophin receptor (p75(NTR)) has been found to be necessary and sufficient to initiate neural cell death. The region was named Chopper to distinguish it from CD95-like death domains. A 29-amino acid peptide corresponding to the Chopper region induced caspase- and calpain-mediated death in a variety of neural and nonneural cell types and was not inhibited by signaling through Trk (unlike killing by full-length p75(NTR)). Chopper triggered cell death only when bound to the plasma membrane by a lipid anchor, whereas non-anchored Chopper acted in a dominant-negative manner, blocking p75(NTR)-mediated death both in vitro and in vivo. Removal of the ectodomain of p75(NTR) increased the potency of Chopper activity, suggesting that it regulates the association of Chopper with downstream signaling proteins.

A rescue factor abolishing neuronal cell death by a wide spectrum of familial Alzheimer's disease genes and Aβ

Through functional expression screening, we identified a gene, designated Humanin (HN) cDNA, which encodes a short polypeptide and abolishes death of neuronal cells caused by multiple different types of familial Alzheimer's disease genes and by Aβ amyloid, without effect on death by Q79 or superoxide dismutase-1 mutants. Transfected HN cDNA was transcribed to the corresponding polypeptide and then was secreted into the cultured medium. The rescue action clearly depended on the primary structure of HN. This polypeptide would serve as a molecular clue for the development of new therapeutics for Alzheimer's disease targeting neuroprotection.

Hydrogen peroxide-mediated neuronal cell death induced by an endogenous neurotoxin, 3-hydroxykynurenine.

3-Hydroxykynurenine (3-HK) is a tryptophan metabolite whose level in the brain is markedly elevated under several pathological conditions, including Huntington disease and human immunodeficiency virus infection. Here we demonstrate that micromolar concentrations (1-100 microM) of 3-HK cause cell death in primary neuronal cultures prepared from rat striatum. The neurotoxicity of 3-HK was blocked by catalase and desferrioxamine but not by superoxide dismutase, indicating that the generation of hydrogen peroxide and hydroxyl radical is involved in the toxicity. Measurement of peroxide levels revealed that 3-HK caused intracellular accumulation of peroxide, which was largely attenuated by application of catalase. The peroxide accumulation and cell death caused by 1-10 microM 3-HK were also blocked by pretreatment with allopurinol or oxypurinol, suggesting that endogenous xanthine oxidase activity is involved in exacerbation of 3-HK neurotoxicity. Furthermore, NADPH diaphorase-containing neurons were spared from toxicity of these concentrations of 3-HK, a finding reminiscent of the pathological characteristics of several neurodegenerative disorders such as Huntington disease. These results suggest that 3-HK at pathologically relevant concentrations renders neuronal cells subject to oxidative stress leading to cell death, and therefore that this endogenous compound should be regarded as an important factor in pathogenesis of neurodegenerative disorders.

bcl-x is expressed in embryonic and postnatal neural tissues and functions to prevent neuronal cell death.

Previous studies have implicated the bcl-2 protooncogene as a potential regulator of neuronal survival. However, mice lacking functional bcl-2 exhibited normal development and maintenance of the central nervous system (CNS). Since bcl-2 appears dispensable for neuronal survival, we have examined the expression and function of bcl-x, another member of the bcl-2 family of death regulatory genes. Bcl-2 is expressed in neuronal tissues during embryonic development but is down-regulated in the adult CNS. In contrast, Bcl-xL expression is retained in neurons of the adult CNS. Two different forms of bcl-x mRNA and their corresponding products, Bcl-xL and Bcl-x beta, were expressed in embryonic and adult neurons of the CNS. Microinjection of bcl-xL and bcl-x beta cDNAs into primary sympathetic neurons inhibited their death induced by nerve growth factor withdrawal. Thus, Bcl-x proteins appear to play an important role in the regulation of neuronal survival in the adult CNS.

Activation of the peroxisome proliferator-activated receptor-alpha enhances cell death in cultured cerebellar granule cells

Peroxisome proliferator-activated receptor-alpha (PPAR alpha) is a member of the steroid hormone receptor superfamily. In rodents, PPAR alpha. alters genes involved in cell cycle regulation in hepatocytes. Some of these genes are implicated in neuronal cell death. Therefore, in this study, we examined the toxicological consequence of PPAR alpha activation in rat primary cultures of cerebellar granule neurons. Our studies demonstrated the presence of PPAR alpha mRNA in cultures by reverse transcriptase-polymerase chain reaction. After 10 days in vitro, cerebellar granule neuron cultures were incubated with the selective PPAR alpha activator 4-chloro-6-(2,3-xylidino)2-pyrimidinylthioacetic acid (Wy-14,643). The inherent toxicity of Wy-14,643 and the effect of PPAR alpha activation following toxic stimuli were assessed. In these studies, neurotoxicity was induced through reduction of extracellular [KCl] from 25 mM to 5.36 mM. We observed no inherent toxicity of Wy-1 4,643 (24 hr) in cultured cerebellar granule cells. However, after reduction of [KCl], cerebellar granule cell cultures incubated with Wy-14,643 showed significantly greater toxicity than controls. These results suggest a posssible role for PPAR(x in augmentation of cerebellar granule neuronal death after toxic stimuli. (C) 2001 Wiley-Liss, Inc.

Phosphatidylcholine-specific phospholipase C regulates glutamate-induced nerve cell death

Phosphatidylcholine-specific phospholipase C (PC-PLC) is a necessary intermediate in transducing apoptotic signals for tumor necrosis factor and Fas/Apo-1 ligands in nonneuronal cells. The data presented here show that PC-PLC also is required in oxidative glutamate-induced programmed cell death of both immature cortical neurons and a hippocampal nerve cell line, HT22. In oxidative glutamate toxicity, which is distinct from excitotoxicity, glutamate interferes with cystine uptake by blocking the cystine/glutamate antiporter, indirectly causing a depletion of intracellular glutathione. A PC-PLC inhibitor blocks oxidative glutamate toxicity, and exogenous PC-PLC potentiates glutamate toxicity. The inhibition of PC-PLC uncouples the cystine uptake from glutamate inhibition, allowing the maintenance of glutathione synthesis and cell viability. These data suggest that PC-PLC modulates neuronal cell death through a mechanism that is distinct from that involved in nonneuronal apoptosis.

bcl-2 transgene expression can protect neurons against developmental and induced cell death.

The bcl-2 protooncogene, which protects various cell types from apoptotic cell death, is expressed in the developing and adult nervous system. To explore its role in regulation of neuronal cell death, we generated transgenic mice expressing Bcl-2 under the control of the neuron-specific enolase promoter, which forced expression uniquely in neurons. Sensory neurons isolated from dorsal root ganglia of newborn mice normally require nerve growth factor for their survival in culture, but those from the bcl-2 transgenic mice showed enhanced survival in its absence. Furthermore, apoptotic death of motor neurons after axotomy of the sciatic nerve was inhibited in these mice. The number of neurons in two neuronal populations from the central and peripheral nervous system was increased by 30%, indicating that Bcl-2 expression can protect neurons from cell death during development. The generation of these transgenic mice suggests that Bcl-2 may play an important role in survival of neurons both during development and throughout adult life.

Microarray analysis of expression of cell death-associated genes in rat spinal cord cells exposed to cyclic tensile stresses in vitro

The application of mechanical insults to the spinal cord results in profound cellular and molecular changes, including the induction of neuronal cell death and altered gene expression profiles. Previous studies have described alterations in gene expression following spinal cord injury, but the specificity of this response to mechanical stimuli is difficult to investigate in vivo. Therefore, we have investigated the effect of cyclic tensile stresses on cultured spinal cord cells from E15 Sprague-Dawley rats, using the FX3000 Flexercell Strain Unit. We examined cell morphology and viability over a 72 hour time course. Microarray analysis of gene expression was performed using the Affymetrix GeneChip System, where categorization of identified genes was performed using the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) systems. Changes in expression of 12 genes were validated with quantitative real-time reverse transcription polymerase chain reaction (RT-PCR).

Activation of ERK1/2, JNK and PKB by hydrogen peroxide in human SH-SY5Y neuroblastoma cells:role of ERK1/2 in H2O2-induced cell death

Reactive oxygen species including H2O2 activate an array of intracellular signalling cascades that are closely associated with cell death and cell survival pathways. The human neuroblastoma SH-SY5Y cell line is widely used as model cell system for studying neuronal cell death induced by oxidative stress. However, at present very little is known about the signalling pathways activated by H2O2 in SH-SY5Y cells. Therefore, in this study we have investigated the effect of H2(O2 on extracellular signal-regulated kinase 1/2 (ERK1/2), c-Jun N-terminal kinase (JNK), p38 mitogen-activated protein kinase (p38 MAPK) and protein kinase B (PKB) activation in undifferentiated and differentiated SH-SY5Y cells. H2O2 stimulated time and concentration increases in ERK1/2, JNK and PKB phosphorylation in undifferentiated and differentiated SH-SY5Y cells. No increases in p38 MAPK phosphorylation were observed following H2O2 treatment. The phosphatidylinositol 3-kinase (PI-3K) inhibitors wortmannin and LY 294002 ((2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one) inhibited H2O2-induced increases in ERK1/2 and PKB phosphorylation. Furthermore, H2O2-mediated increases in ERK1/2 activation were sensitive to the MAPK kinase 1 (MEK1) inhibitor PD 98059 (2'-amino-3'-methoxyflavone), whereas JNK responses were blocked by the JNK inhibitor SP 600125 (anthra[1-9-cd]pyrazol-6(2H)-one). Treatment of SH-SY5Y cells with H2O2 (1 mM; 16 h) significantly increased the release of lactate dehydrogenase (LDH) into the culture medium indicative of a decrease in cell viability. Pre-treatment with wortmannin, SP 600125 or SB 203580 (4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole; p38 MAPK inhibitor) had no effect on H2O2-induced LDH release from undifferentiated or differentiated SH-SY5Y cells. In contrast, PD 98059 and LY 294002 significantly decreased H2O2-induced cell death in both undifferentiated and differentiated SH-SY5Y cells. In conclusion, we have shown that H2O2 stimulates robust increases in ERK1/2, JNK and PKB in undifferentiated and differentiated SH-SY5Y cells. Furthermore, the data presented clearly suggest that inhibition of the ERK1/2 pathway protects SH-SY5Y cells from H2O2-induced cell death.

Neuronal death after perinatal cerebral hypoxia-ischemia: Focus on autophagy-mediated cell death.

Neonatal hypoxic-ischemic encephalopathy is a critical cerebral event occurring around birth with high mortality and neurological morbidity associated with long-term invalidating sequelae. In view of the great clinical importance of this condition and the lack of very efficacious neuroprotective strategies, it is urgent to better understand the different cell death mechanisms involved with the ultimate aim of developing new therapeutic approaches. The morphological features of three different cell death types can be observed in models of perinatal cerebral hypoxia-ischemia: necrotic, apoptotic and autophagic cell death. They may be combined in the same dying neuron. In the present review, we discuss the different cell death mechanisms involved in neonatal cerebral hypoxia-ischemia with a special focus on how autophagy may be involved in neuronal death, based: (1) on experimental models of perinatal hypoxia-ischemia and stroke, and (2) on the brains of human neonates who suffered from neonatal hypoxia-ischemia.

Glyceraldehyde-3-phosphate dehydrogenase: Nuclear translocation participates in neuronal and nonneuronal cell death

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) protein levels increase in particulate fractions in association with cell death in HEK293 cells, S49 cells, primary thymocytes, PC12 cells, and primary cerebral cortical neuronal cultures. Subcellular fractionation and immunocytochemistry reveal that this increase primarily reflects nuclear translocation. Nuclear GAPDH is tightly bound, resisting extraction by DNase or salt treatment. Treating primary thymocytes, PC12 cells, and primary cortical neurons with antisense but not sense oligonucleotides to GAPDH prevents cell death. Because cell-death-associated nuclear translocation of GAPDH and antisense protection occur in multiple neuronal and nonneuronal systems, we propose that GAPDH is a general mediator of cell death and uses nuclear translocation as a signaling mechanism.