Is p75NGFR involved in developmental neural cell death?

The concept of intrinsic versus extrinsic apoptosis

Regulated cell death is a vital and dynamic process in multicellular organisms that maintains tissue homeostasis and eliminates potentially dangerous cells. Apoptosis, one of the better-known forms of regulated cell death, is activated when cell-surface death receptors like Fas are engaged by their ligands (the extrinsic pathway) or when BCL-2-family pro-apoptotic proteins cause the permeabilization of the mitochondrial outer membrane (the intrinsic pathway). Both the intrinsic and extrinsic pathways of apoptosis lead to the activation of a family of proteases, the caspases, which are responsible for the final cell demise in the so-called execution phase of apoptosis. In this review, I will first discuss the most common types of regulated cell death on a morphological basis. I will then consider in detail the molecular pathways of intrinsic and extrinsic apoptosis, discussing how they are activated in response to specific stimuli and are sometimes overlapping. In-depth knowledge of the cellular mechanisms of apoptosis is becoming more and more important not only in the field of cellular and molecular biology but also for its translational potential in several pathologies, including neurodegeneration and cancer.

A single-nucleotide polymorphism in lnc-LAMC2-1:1 interferes with its interaction with miR-128 to alter the expression of deleted in colorectal cancer and its effect on the survival rate of subjects with ovarian cancer

This study aimed to identify the association between lnc-LAMC2-1:1 polymorphism rs2147578 and the recurrence of ovary cancer, as well as to study the underlying mechanism of rs2147578 in ovary cancer. Real-time polymerase chain reaction, Western blot analysis, immunohistochemistry, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, Logrank test, and Kaplan-Meier analysis were carried out to explore the role of rs2147578 in ovary cancer. No obvious difference was observed concerning all clinical characteristics among 90 patients genotyped as CC (N = 28), CG (N = 38), and GG (N = 24) in their rs2147578 polymorphism. In addition, the subjects carrying the CC genotype had longer recurrence-free survival time and showed a lower level of malignancy compared with those carrying CG and GG genotypes. Lnc-LAMC2-1:1 and miR-128 were lowly expressed in the CC group, while deleted in colorectal cancer (DCC) was highly expressed in the CC group. Furthermore, DCC was identified as a target gene of miR-128, and miR-128 mimics decreased the luciferase activity of cells cotransfected with wild-type DCC 3'-untranslated region. Lnc-LAMC2:1-1 directly targeted and affected miR-128 expression, and the G allele in lnc-LAMC2-1:1 rs2147578 upregulated miR-128 expression. Transfection with a miR-128 precursor evidently downregulated the expression of lnc-LAMC2-1:1, miR-128, and DCC expression, but did not affect the expression of ABCC5 and body mass index. Finally, miR-128 precursor promoted cell proliferation and inhibited cell apoptosis. Compared with lnc-LAMC2-1:1 rs2147578C allele, the G allele increases the risk of ovarian cancer by reducing the binding between lnc-LAMC2-1:1 and miR-128-3p, which in turn further decreases the expression of DCC and inhibits cell apoptosis.

REVIEW ■ : Keeping Neurons Alive: The Molecular Control of Apoptosis (Part I

Apoptosis is a form of cellular suicide in which the cell activates an intrinsic program to bring about its own demise. Recognized for years as the mechanism by which developing cells are lost naturally, it has become apparent recently that this same process may play an important role in many acute and chronic diseases in which neural cell death occurs, such as stroke and Alzheimer's disease. This growing recognition suggests that a knowledge of the gene products controlling this process may lead to improved treatments for some disease states, as well as to improved understanding of neuronal development, physiology, and pathophysiology. Some controls with important roles in neural apoptosis have been identified, and these controls, as well as their putative mechanisms of action, are described in this article. NEUROSCIENTIST 2:181-190, 1996

Role of p75NTR in NMDAR-mediated excitotoxic brain injury in neonatal mice

BACKGROUND

Perinatal brain injury in preterm infants is a major cause of neurological handicap. The role of the neurotrophin receptor p75 (p75(NTR)) in the pathogenesis and repair of neonatal excitotoxic brain injury is unknown. Depending on a complex interplay of neurotrophin signalling, p75(NTR) can, in addition to its trophic function, also induce apoptosis.

HYPOTHESIS

We hypothesised that excitotoxicity increases p75(NTR) expression and p75(NTR) knockout (KO) mice have a significantly smaller lesion size upon excitotoxicity as compared to wild-type (WT) mice.

METHODS

We used an established animal model of neonatal excitotoxic brain injury mimicking several key aspects of human preterm brain damage. We subjected five-day-old WT and KO mice to excitotoxic injury by means of a single intracranial ibotenate injection (N-methyl-D-aspartate receptor agonist, NMDAR) into one brain hemisphere. Lesion size, number of activated caspase-3- and apoptosis-inducing factor (AIF)-positive cells were determined as outcome parameters. Gender analyses were taken into account retrospectively.

RESULTS

NMDAR-mediated excitotoxicity induced an upregulation of p75(NTR) expression in the peri-lesion area. Lesion size was significantly increased in female KO as compared to male KO animals. Knockout of p75(NTR) reduced the number of activated caspase-3 but not AIF-positive cells after NMDAR-mediated excitotoxic injury independently of gender.

CONCLUSION

Since NMDAR-mediated excitotoxic brain injury induced p75(NTR) expression and caspase-3-activated apoptosis in p75(NTR) KO animals was decreased, we conclude that activation of p75(NTR) contributes to NMDAR-mediated apoptosis in the neonatal brain. An increase in lesion size in female animals after excitotoxic brain injury suggests that in females p75(NTR) seems to play a dual role.

Neurodegeneration in Alzheimer's disease: caspases and synaptic element interdependence

Extensive genetic, biochemical, and histological evidence has implicated the amyloid-β peptide (Aβ) in Alzheimer's disease pathogenesis, and several mechanisms have been suggested, such as metal binding, reactive oxygen species production, and membrane pore formation. However, recent evidence argues for an additional role for signaling mediated by the amyloid precursor protein, APP, in part via the caspase cleavage of APP at aspartate 664. Here we review the effects and implications of this cleavage event, and propose a model of Alzheimer's disease that focuses on the critical nature of this cleavage and its downstream effects.

Neural cell death is induced by neutralizing antibody to nerve growth factor: an in vivo study

The central nervous system (CNS) of vertebrates originates from neuroepithelial cells located within the embryonic neural tube. Coincidental with the processes of proliferation, migration and differentiation in the developing CNS, cell death is also a major phenomenon during normal development. The investigation of neural cell death in development has focused on the role of target-derived survival factors such as nerve growth factor (NGF). In this study, the effects of anti-NGF antibody on neural cell death in the cerebral cortex have been investigated. Injection of anti-NGF antibody into the cisterna magnum of mouse pups increased the number of neural cell deaths and resulted in thinning of the cerebral cortex compared with a control group. It is concluded that endogenous NGF is essential for cortical cell survival in the cerebral cortex of the newborn mouse. Moreover, this method may be applied to the other factors and different CNS regions, allowing identification of molecules and signals involved in neural cell survival.

Production of nerve growth factor by β-amyloid-stimulated astrocytes induces p75NTR-dependent tau hyperphosphorylation in cultured hippocampal neurons

Reactive astrocytes surround amyloid depositions and degenerating neurons in Alzheimer's disease (AD). It has been previously shown that beta-amyloid peptide induces inflammatory-like responses in astrocytes, leading to neuronal pathology. Reactive astrocytes up-regulate nerve growth factor (NGF), which can modulate neuronal survival by signaling through TrkA or p75 neurotrophin receptor (p75NTR). Here, we analyzed whether soluble Abeta peptide 25-35 (Abeta) stimulated astrocytic NGF expression, modulating the survival of cultured embryonic hippocampal neurons. Hippocampal astrocytes incubated with Abeta up-regulated NGF expression and release to the culture medium. Abeta-stimulated astrocytes increased tau phosphorylation and reduced the survival of cocultured hippocampal neurons. Neuronal death and tau phosphorylation were reproduced by conditioned media from Abeta-stimulated astrocytes and prevented by caspase inhibitors or blocking antibodies to NGF or p75NTR. Moreover, exogenous NGF was sufficient to induce tau hyperphosphorylation and death of hippocampal neurons, a phenomenon that was potentiated by a low steady-state concentration of nitric oxide. Our findings show that Abeta-activated astrocytes potently stimulate NGF secretion, which in turn causes the death of p75-expressing hippocampal neurons, through a mechanism regulated by nitric oxide. These results suggest a potential role for astrocyte-derived NGF in the progression of AD.

Receptors that mediate cellular dependence

Cells depend for their survival on stimulation by trophic factors and other prosurvival signals, the withdrawal of which induces apoptosis, both via the loss of antiapoptotic signaling and the activation of proapoptotic signaling via specific receptors. These receptors, dubbed dependence receptors, activate apoptotic pathways following the withdrawal of trophic factors and other supportive stimuli. Such receptors may feature in developmental cell death, carcinogenesis (including metastasis), neurodegeneration, and possibly subapoptotic events such as neurite retraction and somal atrophy. Mechanistic studies of dependence receptors suggest that these receptors form ligand-dependent complexes that include specific caspases. Complex formation in the absence of ligand leads to caspase activation by a mechanism that is typically dependent on caspase cleavage of the receptor itself, releasing proapoptotic peptides. Cellular dependence receptors, considered in the aggregate, may thus form a system of molecular integration, analogous to the electrical integration system provided by dendritic arbors in the nervous system.

Ethanol potentiates HIV-1 gp120-induced apoptosis in human neurons via both the death receptor and NMDA receptor pathways

Neuronal loss is a hallmark of AIDS dementia syndromes. Human immunodeficiency virus type I (HIV-1)-specific proteins may induce neuronal apoptosis, but the signal transduction of HIV-1 gp120-induced, direct neuronal apoptosis remains unclear. Ethanol (EtOH) is considered to be an environmental co-factor in AIDS development. However, whether EtOH abuse in patients with AIDS increases neuronal dysfunction is still uncertain. Using pure, differentiated, and post-mitotic NT2.N-derived human neurons, we investigated the mechanisms of HIV-1 and/or EtOH-related direct neuronal injury and the molecular interactions between HIV-1-specific proteins and EtOH. It was demonstrated that NT2.N neurons were susceptible to HIV-1 Bal (R5-tropic strain) gp120-induced direct cell death. Of importance, EtOH induced cell death in human neurons in a clinically-relevant dose range and EtOH strongly potentiated HIV-1 gp120-induced neuronal injury at low and moderate concentrations. Furthermore, this potentiation of neurotoxicity could be blocked by N-methyl-D-aspartate (NMDA) receptor subunit 2B (NR2B) antagonists. We analyzed human genomic profiles in these human neurons, using Affymetrix genomics technology, to elucidate the apoptotic pathways involved in HIV-1- and EtOH-related neurodegeneration. Our findings indicated significant over-expression of selected apoptosis functional genes. Significant up-regulation of TRAF5 gene expression may play an essential role in triggering potentiation by EtOH of HIV-1 gp120-induced neuronal apoptosis at early stages of interaction. These studies suggested that two primary apoptotic pathways, death receptor (extrinsic) and NMDA receptor (intrinsic)-related programmed cell-death pathways, are both involved in the potentiation by EtOH of HIV-1 gp120-induced direct human neuronal death. Thus, these data suggest rationally-designed, molecular targets for potential anti-HIV-1 neuroprotection.

p75NTR-mediated signaling promotes the survival of myoblasts and influences muscle strength

During muscle development, the p75(NTR) is expressed transiently on myoblasts. The temporal expression pattern of the receptor raises the possibility that the receptor is influencing muscle development. To test this hypothesis, p75(NTR)-deficient mutant mice were tested for muscle strength by using a standard wire gripe strength test and were found to have significantly decreased strength relative to that of normal mice. When normal mybolasts were examined in vivo for expression of NGF receptors, p75(NTR) was detected on myoblasts but the high affinity NGF receptor, trk A, was not co-expressed with p75(NTR). In vitro, proliferating C2C12 and primary myoblasts co-expressed the p75(NTR) and MyoD, but immunofluorescent analysis of primary myoblasts and RT-PCR analysis of C2C12 mRNA revealed that myoblasts were devoid of trk A. In contrast to the cell death functions that characterize the p75(NTR) in neurons, p75(NTR)-positive primary and C2C12 myoblasts did not differentiate or undergo apoptosis in response to neurotrophins. Rather, myoblasts survived and even proliferated when grown at subconfluent densities in the presence of the neurotrophins. Furthermore, when myoblasts treated with NGF were lysed and immunoprecipitated with antibodies against phosphorylated I-kappaB and AKT, the cells contained increased levels of both phospho-proteins, both of which promote cell survival. By contrast, neurotrophin-treated myoblasts did not induce phosphorylation of Map Kinase p42/44 or p38, indicating the survival was not mediated by the trk A receptor. Taken together, the data indicate that the p75(NTR) mediates survival of myoblasts prior to differentiation and that the activity of this receptor during myogenesis is important for developing muscle.

Apoptosis and Dependence Receptors: A Molecular Basis for Cellular Addiction

Bredesen, Dale E., Patrick Mehlen, and Shahrooz Rabizadeh. Apoptosis and Dependence Receptors: A Molecular Basis for Cellular Addiction. Physiol Rev 84: 411–430, 2004; 10.1152/physrev.00027.2003.—Classical signal transduction is initiated by ligand-receptor interactions. We have described an alternative form of signal transduction that is initiated by the withdrawal of ligands from specific receptors referred to as dependence receptors. This process is widespread, featuring in developmental cell death, carcinogenesis (especially metastasis), neurodegeneration, and possibly subapoptotic events such as neurite retraction and somal atrophy. Initial mechanistic studies of dependence receptors suggest that these receptors form complexes that include specific caspases. Complex formation appears to be a function of ligand-receptor interaction, and dependence receptors appear to exist in at least two conformational states. Complex formation in the absence of ligand leads to caspase activation by a mechanism that in at least some cases is dependent on caspase cleavage of the receptor itself, releasing proapoptotic peptides. Thus these receptors may serve in caspase amplification, and in so doing create cellular states of dependence on their respective ligands.

Nerve growth factor-dependent regulation of NADE-induced apoptosis

The p75 neurotrophin receptor (p75NTR) is a member of the tumor necrosis factor receptor (TNFR) superfamily, and can mediate both cell survival and cell death in response to nerve growth factor (NGF). Based on the structural and functional differences between p75NTR and the related receptors Fas or TNFR, it has been suggested that these receptors have distinct signaling functions. NADE (p75NTR-associated cell death executor) is a p75NTR-associated protein that mediates apoptosis in response to NGF by interacting with the cell death domain of p75NTR. NADE has at least four isoforms, designated as NADE2, NADE3, NADE4/Bex1, and NADE5/Bex2. NADE plays a role in NGF-induced apoptosis in oligodendrocytes and in zinc-induced neuronal death. In this review, we focus on the proapoptotic actions of NADE that regulate p75NTR signaling in response to NGF.

Ten years on: mediation of cell death by the common neurotrophin receptor p75(NTR)

The common neurotrophin receptor p75(NTR) remains one of the most enigmatic of the tumor necrosis factor receptor (TNFR) superfamily: on the one hand, it displays a death domain and has been shown to be capable of mediating programmed cell death (PCD) upon ligand binding; on the other hand, its death domain is of type II (unlike that of Fas or TNFR I), and it has also been shown to be capable of mediating cell death in response to the withdrawal of ligand. Thus, p75(NTR) may function as a death receptor-similar to Fas or TNFR I-or a dependence receptor-similar to deleted in colorectal cancer (DCC) or uncoordinated gene-5 homologues 1-3 (UNC5H1-3). Here, we review the data relating to the mediation of PCD by p75(NTR), and suggest that one reasonable model for the apparently paradoxical effects of p75(NTR) is that this receptor functions as a "quality control" in that it is capable of mediating PCD in at least four situations: (1). withdrawal of neurotrophins; (2). exposure to mismatched neurotrophins; (3). exposure to unprocessed neurotrophins; and (4). exposure of inappropriately immature cells to neurotrophins. Results to date suggest that these functions are mediated through different underlying mechanisms, and that their respective signaling pathways are cell type and co-receptor dependent.

In vivo cellular and molecular mechanisms of neuronal apoptosis in the mammalian CNS

Apoptosis has been recognized to be an essential process during neural development. It is generally assumed that about half of the neurons produced during neurogenesis die before completion of the central nervous system (CNS) maturation, and this process affects nearly all classes of neurons. In this review, we discuss the experimental data in vivo on naturally occurring neuronal death in normal, transgenic and mutant animals, with special attention to the cerebellum as a study model. The emerging picture is that of a dual wave of apoptotic cell death affecting central neurons at different stages of their life. The first wave consists of an early neuronal death of proliferating precursors and young postmitotic neuroblasts, and appears to be closely linked to cell cycle regulation. The second wave affects postmitotic neurons at later stages, and is much better understood in functional terms, mainly on the basis of the neurotrophic concept in its broader definition. The molecular machinery of late apoptotic death of postmitotic neurons more commonly follows the mitochondrial pathway of intracellular signal transduction, but the death receptor pathway may also be involved.Undoubtedly, analysis of naturally occurring neuronal death (NOND) in vivo will offer a basis for parallel and future studies aiming to elucidate the mechanisms of pathologic neuronal loss occurring as the result of conditions such as neurodegenerative disorders, trauma or ischemia.

PLAIDD, a type II death domain protein that interacts with p75 neurotrophin receptor

We describe the cloning and characterization of a rat single transmembrane protein that is homologous to the common neurotrophin receptor p75NTR in its death domain and the transmembrane region but dissimilar outside these regions. We have dubbed this protein PLAIDD, for p75-like apoptosis-inducing death domain protein. PLAIDD messenger RNA, which is ubiquitously distributed, is highly expressed in the embryo, but downregulated in adult tissues. Alternative splicing within the extracellular region of PLAIDD generates four RNA species, but only two of them are translated, PLAIDD_L and PLAIDD_S (long and short isoforms, respectively). While the amino acid sequence of the intracellular region of PLAIDD displays 41% identity with the intracellular region of p75NTR, the extracellular region of PLAIDD does not reveal any homology with p75NTR. Overexpression of each isoform of PLAIDD led to cytotoxicity in superior cervical ganglion neurons and in human embryonic kidney 293T cells. Both isoforms of PLAIDD could be co-immunoprecipitated with p75NTR, suggesting an interaction between these molecules.

Mechanisms of p75-mediated death of hippocampal neurons. Role of caspases

Neurotrophins support neuronal survival and differentiation via Trk receptors, yet can also induce cell death via the p75 receptor. In these studies, we investigated signaling mechanisms governing p75-mediated death of hippocampal neurons, specifically the role of caspases. Although p75 is structurally a member of the Fas/TNFR1 receptor family, caspase-8 was not required for p75-mediated death, unlike other members of this receptor family. In contrast, p75-mediated neuronal death was associated with mitochondrial loss of cytochrome c and required Apaf-1 and caspase-9, -6, and -3. In particular, caspase-6 plays a central role in mediating neurotrophin-induced death, illuminating a novel role for this caspase. Inhibition of DIABLO/Smac, which blocks inhibitor of apoptosis proteins, protected cells from death, whereas simultaneous inhibition of both DIABLO/Smac and MIAP3 allowed trophin-induced death to proceed. In vivo, pilocarpine-induced seizures, previously shown to up-regulate p75 expression and increase neurotrophin production, caused activation of caspase-6 and -3 and cleavage of poly(ADP-ribose) polymerase in p75-expressing hippocampal neurons. In p75(-/-) mice, no activated caspase-3 was detected, and there was a marked reduction in the number of dying neurons after pilocarpine treatment compared with wild type mice. Neurotrophin-induced p75-mediated death is likely to play an important role in mediating neuronal loss consequent to brain injury.

Dual role for p75(NTR) signaling in survival and cell death: can intracellular mediators provide an explanation?

Several recent reports support a dual role of p75(NTR) in cell death, as well as survival, depending on the physiological or developmental stage of the cells. Coexpression of the TrkA receptor with p75(NTR) further enhances the complexity of nerve growth factor (NGF) signaling. Recent identification of serine/threonine kinases that interact with the p75(NTR) provides an explanation for the lack of an apparent kinase domain needed for signaling. In this report, we review the possible roles of the intracellular proteins that directly interact with the p75(NTR), atypical protein kinase C (PKC) binding protein, p62 and second messengers in the functional antagonism exhibited by TrkA and p75(NTR) with an emphasis on the nuclear factor-kappa B activation pathway.

Structure-function analysis of NADE: identification of regions that mediate nerve growth factor-induced apoptosis

Nerve growth factor (NGF) can induce apoptosis in neural cells via activation of the low affinity neurotrophin receptor p75NTR. NADE (p75NTR-associated cell death executor) is a p75NTR-associated protein that mediates apoptosis in response to NGF by interacting with the death domain of p75NTR in 293T, PC12, and nnr5 cells (Mukai, J., Hachiya, T., Shoji-Hoshino, S., Kimura, M. T., Nadano, D., Suvanto, P., Hanaoka, T., Li, Y., Irie, S., Greene, L. A., and Sato, T. A. (2000) J. Biol. Chem. 275, 17566-17570). We performed extensive mutational analysis on NADE, to better characterize its structural and functional features. Truncation of a minimal region, including amino acid residues 41-71 of NADE, was found to be sufficient to induce apoptosis. The designated regulatory region includes the C-terminal amino acid residues (72-112) and is essential for NGF-dependent regulation of NADE-induced apoptosis. Furthermore, the mutants with amino acid substitutions in the leucine-rich nuclear export signal (NES) sequence (residues 90-100) abolished the export of NADE from the nucleus to the cytoplasm. Mutation of the NES also abolished self-association of NADE, its interaction with p75NTR, and NGF-dependent apoptosis. Expression of a fragment of NADE (amino acid residues 81-124) blocked NGF-induced apoptosis in oligodendrocytes, suggesting that this region has a dominant negative effect on NGF/p75NTR-induced apoptosis. These studies identify distinct regions of NADE that are involved in regulating specific functions involved in p75NTR signal transduction.

Excitotoxic and metabolic damage to the rodent striatum: role of the P75 neurotrophin receptor and glial progenitors

After injury, the striatum displays several morphologic responses that may play a role in both regenerative and degenerative events. One such response is the de novo expression of the low-affinity p75 neurotrophin receptor (p75(NTR)), a gene that plays critical roles in central nervous system (CNS) cell death pathways. The present series of experiments sought to elucidate the cellular origins of this p75(NTR) response, to define the conditions under which p75(NTR) is expressed after striatal injury, and how this receptor expression is associated with neuronal plasticity. After chemical lesions, by using either the excitotoxin quinolinic acid (QA) or the complex II mitochondria inhibitor 3-nitropropionic acid (3-NP), we compared the expression of the p75(NTR) receptor within the rat striatum at different survival times. Intrastriatal administration of QA between 7 days and 21 days postlesion induced p75(NTR) expression in astrocytes that was preferentially distributed throughout the lesion core. P75(NTR) immunoreactivity within astrocytes was seen at high (100-220 nmol) but not low (50 nmol) QA doses. Seven and 21 days after 3-NP lesions, p75(NTR) expression was present in astrocytes at all doses tested (100-1,000 nmol). However, in contrast to QA, these cells were located primarily around the periphery of the lesion and not within the lesion core. At the light microscopic level p75(NTR) immunoreactive elements resembled vasculature: but did not colocalize with the pan endothelium cell marker RecA-1. In contrast, p75(NTR)-containing astrocytes colocalized with nestin, vimentin, and 5-bromo-2-deoxyuridine, indicating that these cells are newly born astrocytes. Additionally, striatal cholinergic neurons were distributed around the lesion core expressed p75(NTR) 3-5 days after lesion in both QA and 3-NP lesions. These cells did not coexpress the pro-apoptotic degradation enzyme caspase-3. Taken together, these data indicate that striatal lesions created by means of excitotoxic or metabolic mechanisms trigger the expression of p75(NTR) in structures related to progenitor cells. The expression of the p75(NTR) receptor after these chemical lesions support the concept that this receptor plays a role in the initiation of endogenous cellular events associated with CNS injury.

14-3-3 is involved in p75 neurotrophin receptor-mediated signal transduction

The low affinity neurotrophin receptor (p75NTR) has been shown to mediate the apoptosis signaling to neural cells. However, the specific mechanisms of intracellular signal transduction of this process are largely unknown. To understand p75NTR-mediated signal transduction, we previously identified a protein that interacts with the intracellular domain of p75NTR, and we named it p75NTR-associated cell death executor (NADE). To elucidate further the signaling mechanisms utilized by p75NTR and NADE, we screened for NADE-binding protein(s) with the yeast two-hybrid method, and we identified 14-3-3epsilon as a NADE-binding protein in vivo. To examine whether 14-3-3epsilon affects the induction of p75NTR-mediated apoptosis, wild type or various deletion mutant forms of 14-3-3epsilon were co-expressed in HEK293, PC12nnr5, and oligodendrocytes. Interestingly, transient expression of the mutant form of 14-3-3epsilon lacking the 208-255 amino acid region blocked nerve growth factor-dependent p75NTR/NADE-mediated apoptosis, although this mutant form of 14-3-3epsilon continued to associate with NADE. These results suggest that 14-3-3epsilon plays an important role in the modulation of nerve growth factor-dependent p75NTR/NADE-mediated apoptosis.

Downregulation of the p75 neurotrophin receptor in tissue culture and in vivo, using beta-cyclodextrin-adamantane-oligonucleotide conjugates

Formation of complexes with beta-cyclodextrin derivatives via adamantyl groups was found to enhance the uptake and antisense efficacy of phosphorothioate oligos targeted to the p75 neurotrophin receptor in neuronally differentiated PC12 cells. After a 2-week course of systemic administration to mice (by intraperitoneal injection), there was evidence of a pronounced uptake of these oligos by the dorsal root ganglia (DRG), as well as by liver and kidney. There was no uptake by the brain. Consistent with uptake of antisense oligos by the DRG, systemic administration resulted in marked and consistent downregulation of p75 in DRG neurons. These results indicate that cyclodextrin-adamantane-oligo conjugates have great potential as agents to downregulate target genes in neurons, particularly in vivo in the peripheral nervous system.

Neurotrophins induce death of hippocampal neurons via the p75 receptor

Nerve growth factor (NGF) and related neurotrophins influence neuronal survival and differentiation via interactions with the trk family of receptors. Recent studies have demonstrated that neurotrophins may also induce cell death via the p75 receptor. The importance and generality of neurotrophin-induced death in the brain have not been defined but may play a critical role during development and in disease-associated neuronal death. Here we demonstrate for the first time that all four members of the neurotrophin family directly elicit the death of hippocampal neurons via the p75 receptor. The hippocampus is a complex structure with many different neuronal subpopulations, and signals that influence neuronal death during development may have a critical impact on the mature function of this structure. In these studies we show that each neurotrophin causes the death of hippocampal neurons expressing p75 but lacking the cognate trk receptor. Neurotrophin-induced neuronal death is mediated by activation of Jun kinase. These studies demonstrate that neurotrophins can regulate death as well as survival of CNS neurons.

Dimerization-dependent block of the proapoptotic effect of p75(NTR)

The biochemical mechanism by which neurons become dependent on neurotrophins for survival is unknown. We found previously that the common neurotrophin receptor, p75(NTR), is a mediator of neurotrophin dependence and that this effect requires a novel type of domain dubbed a neurotrophin dependence domain. We report here that, in contrast to other proapoptotic receptors such as Fas, apoptosis induction by p75(NTR) requires monomerization, with dimerization inhibiting the effect. Blocking the proapoptotic effect of the monomer by dimerization requires a distinct domain that lies at the carboxyterminus of p75(NTR). These results define a novel type of domain required for inhibiting apoptosis induction by p75(NTR).

Enlarged cholinergic forebrain neurons and improved spatial learning in p75 knockout mice

The p75 low affinity neurotrophin receptor (p75) can induce apoptosis in various neuronal and glial cell types. Because p75 is expressed in the cholinergic neurons of the basal forebrain, p75 knockout mice may be expected to show an increased number of neurons in this region. Previous studies, however, have produced conflicting results, suggesting that genetic background and choice of control mice are critical. To try to clarify the conflicting results from previous reports, we undertook a further study of the basal forebrain in p75 knockout mice, paying particular attention to the use of genetically valid controls. The genetic backgrounds of p75 knockout and control mice used in this study were identical at 95% of loci. There was a small decrease in the number of cholinergic basal forebrain neurons in p75 knockout mice at four months of age compared with controls. This difference was no longer apparent at 15 months due to a reduction in numbers in control mice between the ages of 4 and 15 months. Cholinergic cell size in the basal forebrain was markedly increased in p75 knockout mice compared with controls. Spatial learning performance was consistently better in p75 knockout mice than in controls, and did not show any deterioration with age. The results indicate that p75 exerts a negative influence on the size of cholinergic forebrain neurons, but little effect on neuronal numbers. The markedly better spatial learning suggests that the function, as well as the size, of cholinergic neurons is negatively modulated by p75.

Differential actions of neurotrophins on apoptosis mediated by the low affinity neurotrophin receptor p75NTR in immortalised neuronal cell lines

The low affinity neurotrophin receptor (p75NTR) mediates apoptosis of a number of neuronal and non-neuronal cells but the signals leading to the apoptosis remain obscure. To reveal the mechanism of p75NTR-mediated apoptosis, a neural cell line expressing human p75NTR was established. The human cDNA fragment encoding for p75NTR was PCR-amplified, cloned into the retrovirus expression vector pXT-1 and transfected into the rat cerebellum cell line R2. The expression of p75NTR in the R2 cell line was demonstrated by both Northern blotting analysis and immunocytochemistry. Serum withdrawal induced dramatic apoptosis in p75NTR-expressing R2 cells (R2L1) but not in pXT-1 transfected control R2 cells (R2P). Reverse transcription polymerase chain reaction (RT-PCR) revealed that these cell lines express trkA and trkB but not trkC. The apoptosis of R2L1 cells triggered by the serum deprivation for 48 h was completely prevented by neurotrophin-3 and the antibody to p75NTR but only partially prevented by the nerve growth factor and brain derived neurotrophic factor. We conclude that the p75NTR mediates apoptosis of R2L1 cells by its intrinsic receptor effects requiring an unbound status of this receptor and that the apoptosis is prevented by neurotrophins or the antibody to p75NTR through distinct mechanisms.

Function for NF-kB in neuronal survival: regulation by atypical protein kinase C

Activation of the transcription factor nuclear factor kappa B (NF-kB) has been intensely studied in the past several years due to its role as an inducible regulator of inflammation, apoptosis, transformation, and oncogenesis. Recently, increasing evidence supports a role for NF-kB in regulation of anti-apoptotic gene expression and promotion of cell survival (May and Ghosh [1999] Science 284:272-273). Studies in the past 5 years have provided evidence that NF-kB regulates neuronal survival as well. Moreover, atypical protein kinase (aPKC) has been shown to play a novel role in modulating the NF-kB pathway. In this review, I focus on neurons and the factors that contribute to regulation of NF-kB via aPKC.

Neurotrophin signaling via Trks and p75

This review focuses on recent advances in our understanding of receptor-mediated signaling by the neurotrophins NGF, BDNF, NT3, and NT4/5. Two distinct receptor types have been distinguished, Trks and p75. The Trks are receptor tyrosine kinases that utilize a complex set of substrates and adapter proteins to activate defined secondary signaling cascades required for neurotrophin-promoted neuronal differentiation, plasticity, and survival. A specialized aspect of Trk/neurotrophin action in neurons is the requirement for retrograde signaling from the distal periphery to the cell body. p75 is a universal receptor for neurotrophins that is a member of the TNF receptor/Fas/CD40 superfamily. p75 appears to modify Trk signaling when the two receptor types are coexpressed. When expressed in the absence of Trks, p75 mediates responses to neurotrophins including promotion of apoptotic death. The mechanisms of p75 receptor signaling remain to be fully understood.

Olfactory receptor neurons exist as distinct subclasses of immature and mature cells in primary culture

The processes of neuronal differentiation and survival are key questions in neurobiology. The olfactory system possesses unique regenerative capacity, as its neurons are continually replaced throughout adulthood from a maintained population of precursor cells. Primary cultures of olfactory epithelium enriched in olfactory neurons would provide a useful model to study the processes of neurogenesis, differentiation and senescence. To determine whether immature olfactory neurons could be isolated in primary culture and to investigate the mechanisms underlying these processes, culture conditions which selectively favored the presence of immature olfactory neurons were optimized. Using low plating densities, a population of cells was identified which, by reverse transcription-polymerase chain reaction, demonstrated messages for olfactory neuronal markers, including Golf, olfactory cyclic nucleotide-gated channel and olfactory marker protein, as well as the p75 low-affinity nerve growth factor receptor. Immunocytochemical analysis showed that these putative immature olfactory neurons possessed immunoreactivity to G(olf), neuron-specific tubulin, neural cell adhesion molecule, synaptophysin and neurofilament. These neurons were defined as olfactory receptor neuron-1 cells. Under these conditions, a separate class of rarely occurring cells with different morphology demonstrated immunoreactivity to mature markers, such as adenylyl cyclase III and olfactory marker protein. Electrophysiologically, these cells displayed properties consistent with those of acutely dissociated olfactory receptor neurons. Another class of rarer cells which represented less than 2% of cells in culture demonstrated immunoreactivity to glial fibrillary acidic protein. These cultures can serve as a model for in vitro analysis of olfactory receptor neuronal development and maintenance, and provide a potential substrate for the development of cell lines.

Regulation of the p75 neurotrophin receptor in a rat myogenic cell line (L6)

Neurotrophins are expressed in muscle cells both during development and postnatally. Furthermore, during development muscle cells express high levels of the common p75 neurotrophin receptor, which binds all neurotrophins. Only fragmentary and controversial data are available regarding the responsiveness of muscle cells to neurotrophins and the importance of low-affinity p75 receptor in muscle development. The present study investigates in vitro the immunocytochemical expression of p75 in a rat myogenic cell line (L6) at various time points and in response to different coating substrates as a first step in elucidating the regulation of p75 in muscle. We found that in L6 myoblasts, p75 is expressed only at very early stages of maturation and its levels of expression are regulated by the nature of the coating substrates. p75 expression decreases in cells growing on substrates more suitable for myoblast fusion into myotubes. Time course analysis indicates a reverse correlation between myoblast fusion into myotubes and the levels of p75 expression. Myotubes were always p75 negative. Substrates not suitable for the fusion process induced a prolonged presence of p75 in myoblasts with an increase of their apoptosis. We conclude that expression of p75, at least in this in vitro condition, is regulated by the stages of myoblast differentiation and the nature of the coating substrates. According to the observed time- and substrate-related evidences, future studies should investigate in vivo both the regulation of p75 in the myoblast fusion and the effects and the importance of neurotrophins binding during myoblast differentiation.

Focally administered nerve growth factor suppresses molecular regenerative responses of axotomized peripheral afferents in rats

Effects of delivery of nerve growth factor, from a catheterized osmotic mini-pump to the proximal stump of a transected sciatic nerve, were compared with the effects of normal saline. A pilot measured retrograde axonal transport of nerve growth factor to determine a pump concentration which raised axonal transport ipsilaterally, but not contralaterally. The effects of this delivery over 12 days were then determined on expression of growth-associated protein-43, trkA, p75NTR and preprotachykinin A ipsilateral and contralateral to the pump in dorsal root ganglia at L4 and L5 (pooled). Ganglionic expression was measured both as messenger RNA and protein. Axotomy (saline pumps) increased growth-associated protein-43 messenger RNA (318 +/- 14%: all changes are percent of contralateral, non-axotomized ganglia with saline pumps) and immunoreactivity (431 +/- 43%). The increase was significantly less (P < 0.001) ipsilateral to nerve growth factor pumps (191 +/- 45%). Axotomy reduced expression of p75NTR (messenger RNA: 52 +/- 17%, P < 0.01; immunoreactivity: 74 +/- 3%, P < 0.05). These decreases were converted to increases by nerve growth factor delivery (respectively 143 +/- 40% and 281 +/- 67%; both P < 0.01). With trkA, axotomy decreased the expression of the messenger RNA (68 +/- 40%, P < 0.01) and of the primary translation product--110,000 mol. wt protein (55 +/- 12%, P < 0.01)--but not the fully glycosylated trkA protein (mol. wt 145,000). Nerve growth factor delivery did not affect trkA expression. Axotomy reduced messenger RNA for the substance P precursor, preprotachykinin A, to 42 +/- 17% (P < 0.01) and this reduction was prevented by nerve growth factor treatment. We suggest that the primary effect of nerve growth factor on axotomized C-fibres is not to promote regeneration, although that may be its secondary effect via an action on Schwann cells. It is possible that reduced neuronal sensitivity to nerve growth factor during regeneration is advantageous in suppressing nociception.

Differential effects of olanzapine on the gene expression of superoxide dismutase and the low affinity nerve growth factor receptor

Neuroanatomical studies of schizophrenia suggest that progressive neuropathological changes (such as neuronal atrophy and/or cell death) occur over the lifetime course of the disease. Early intervention with atypical neuroleptics has been shown to prevent progression of at least some symptoms, although the mechanisms by which neuroleptics may do this remain unknown. In this study, PC12 cells were used to determine the effects of the new atypical antipsychotic olanzapine on the gene expression of superoxide dismutase (SOD1) and the low affinity nerve growth factor receptor (p75). The results show that olanzapine increases SOD1 at concentrations of 10 and 100 microM after 48 hr of incubation in PC12 cultures. The treatment decreases p75 gene expression at concentrations 100 microM after 48 hr of incubation. Since both the upregulation of SOD1 mRNA and the antisense blockade of p75 mRNA have been associated with reduced cell death, our results suggest that olanzapine has neuroprotective potential and thus may be useful in preventing further neurodegeneration accompanying schizophrenia.

The DCC gene product induces apoptosis by a mechanism requiring receptor proteolysis

The development of colonic carcinoma is associated with the mutation of a specific set of genes. One of these, DCC (deleted in colorectal cancer), is a candidate tumour-suppressor gene, and encodes a receptor for netrin-1, a molecule involved in axon guidance. Loss of DCC expression in tumours is not restricted to colon carcinoma, and, although there is no increase in the frequency of tumour formation in DCC hemizygous mice, reestablishment of DCC expression suppresses tumorigenicity. However, the mechanism of action of DCC is unknown. Here we show that DCC induces apoptosis in the absence of ligand binding, but blocks apoptosis when engaged by netrin-1. Furthermore, DCC is a caspase substrate, and mutation of the site at which caspase-3 cleaves DCC suppresses the pro-apoptotic effect of DCC completely. These results indicate that DCC may function as a tumour-suppressor protein by inducing apoptosis in settings in which ligand is unavailable (for example, during metastasis or tumour growth beyond local blood supply) through functional caspase cascades by a mechanism that requires cleavage of DCC at Asp 1,290.

Nerve growth factor and ceramides modulate cell death in the early developing inner ear

Regulation of normal development involves a dynamic balance of the mechanisms regulating cell division, differentiation and death. We have investigated the signalling mechanisms involved in regulation of the balance between cell proliferation and apoptotic cell death in the otic vesicle. The sphingomyelin pathway signals apoptosis for nerve growth factor upon binding to p75 receptors. It is initiated by sphingomyelin hydrolysis to generate the second messenger ceramide. In the present study, we show that nerve growth factor stimulates sphingomyelin hydrolysis and the concomitant ceramide release in organotypic cultures of otic vesicles. Both nerve growth factor and ceramide induce apoptotic responses to a different extent. Ceramide-induced apoptosis was suppressed by insulin-like growth factor-I which is a strong promoter of cell growth and morphogenesis for the developing inner ear. In contrast, ceramide-1-phosphate protected the explants from apoptosis induced by serum withdrawal but did not antagonise ceramide-induced cell death. This study suggests that sphingomyelin-derived second messengers might be key modulators of programmed cell death during development.

Expression of CD40 induces neural apoptosis

The tumor necrosis factor receptor superfamily includes 12 members, some of which (e.g., tumor necrosis factor receptor I and FAS) induce cell death triggered by ligand binding. Another member of the superfamily, the neurotrophin receptor p75NTR, induces neural apoptosis, with apoptosis being inhibited by binding of ligand to the receptor. As such, it is a candidate for the mediation of neurotrophin dependence. Here, we show that CD40, a superfamily member that is closely related to p75NTR, also induces neural apoptosis, but apoptosis is inhibited by binding of the G28-5 monoclonal antibody to CD40. These results provide further support for a model in which some members of the tumor necrosis factor receptor superfamily induce apoptosis triggered by ligand binding, whereas other members may, at least under certain conditions, induce apoptosis in the absence of ligand binding, with apoptosis being inhibited by binding of ligand or monoclonal antibody.

A role for TrkA during maturation of striatal and basal forebrain cholinergic neurons in vivo

Nerve growth factor (NGF), acting via the TrkA receptor, has been shown to regulate the survival and maturation of specific neurons of the peripheral nervous system. Furthermore, exogenous NGF has potent actions on TrkA-expressing cholinergic neurons of the basal forebrain (BFCNs) and striatum. However, initial analysis of mice lacking NGF or TrkA revealed that forebrain cholinergic neurons were present in these animals through the fourth postnatal week. Because of the potential effects of NGF/TrkA interactions on these developing neurons, we have analyzed quantitatively the striatal and basal forebrain cholinergic neurons in trkA knock-out mice. By postnatal day (P) 7/8, forebrain cholinergic neurons are smaller in trkA (-/-) mice than those in wild-type littermate controls. However, cholinergic neuron number and fiber density in the hippocampus, a target region of BFCNs, are grossly intact. Interestingly, by P20-P25 trkA knock-outs contain significantly fewer (20-36%) and smaller cholinergic neurons in both the striatum and septal regions, as compared with controls. Cholinergic fiber density within the hippocampus also is depleted in knock-outs by the end of the second postnatal week. Contrary to some predictions, despite expression of p75(NTR) in the absence of trkA in BFCNs of these knock-out mice, many cells, although smaller, are still alive at P25. Our data suggest that, in the absence of NGF/TrkA signaling, striatal cholinergic neurons and BFCNs do not mature fully and that BFCNs begin to atrophy and/or die surrounding the time of target innervation.

Absence of p75NTR causes increased basal forebrain cholinergic neuron size, choline acetyltransferase activity, and target innervation

Emerging evidence suggests that the p75 neurotrophin receptor (p75NTR) mediates cell death; however, it is not known whether p75NTR negatively regulates other neuronal phenotypes. We found that mice null for p75NTR displayed highly significant increases in the size of basal forebrain cholinergic neurons, including those that are TrkA-positive. Cholinergic hippocampal target innervation also was increased significantly. Activity of the cholinergic neurotransmitter synthetic enzyme choline acetyltransferase (ChAT) was increased in both the medial septum and hippocampus. Upregulation of these cholinergic features was not associated with increased basal forebrain or hippocampal target NGF levels. In contrast, striatal cholinergic neurons, which do not express p75NTR, showed no difference in neuronal number, size, or ChAT activity between wild-type and p75NTR null mutant mice. These findings indicate that p75NTR negatively regulates cholinergic neuronal phenotype of the basal forebrain cholinergic neurons, including cell size, target innervation, and neurotransmitter synthesis.

p75NTR and apoptosis: Trk-dependent and Trk-independent effects

The ongoing dissection of the roles of p75NTR and TrkA, -B and -C in neurotrophin signaling has generated a number of apparent paradoxes. Limiting consideration to the role of p75NTR in cell death, a theory is proposed that is based on the following postulates: (1) that p75NTR displays a pro-apoptotic intrinsic (ligand-independent, Trk-independent) receptor effect (IRE), which is inhibited by ligand binding; (2) that p75NTR and TrkA exhibit mutual repression of signaling; and (3) that p75NTR and TrkA are required for the efficient generation of high-affinity NGF binding sites.

A new member of the tumor necrosis factor/nerve growth factor receptor family inhibits T cell receptor-induced apoptosis

By comparing untreated and dexamethasone-treated murine T cell hybridoma (3DO) cells by the differential display technique, we have cloned a new gene, GITR (glucocorticoid-induced tumor necrosis factor receptor family-related gene) encoding a new member of the tumor necrosis factor/nerve growth factor receptor family. GITR is a 228-amino acids type I transmembrane protein characterized by three cysteine pseudorepeats in the extracellular domain and similar to CD27 and 4-1BB in the intracellular domain. GITR resulted to be expressed in normal T lymphocytes from thymus, spleen, and lymph nodes, although no expression was detected in other nonlymphoid tissues, including brain, kidney, and liver. Furthermore, GITR expression was induced in T lymphocytes upon activation by anti-CD3 mAb, Con A, or phorbol 12-myristate 13-acetate plus Ca-ionophore treatment. The constitutive expression of a transfected GITR gene induced resistance to anti-CD3 mAb-induced apoptosis, whereas antisense GITR mRNA expression lead to increased sensitivity. The protection toward T cell receptor-induced apoptosis was specific, because other apoptotic signals (Fas triggering, dexamethasone treatment, or UV irradiation) were not modulated by GITR transfection. Thus, GITR is a new member of tumor necrosis factor/nerve growth factor receptor family involved in the regulation of T cell receptor-mediated cell death.

Low-affinity nerve growth factor receptor is associated with motoneuron axonal pathways

The unidentified cell-surface antigen recognized by monoclonal antibody M7412 is distributed along motoneuron axonal outgrowth pathways in chicken embryos. To better characterize its role in motoneuron development, the M7412 antigen was purified from chicken embryos by immunoaffinity chromatography. Its N-terminal amino acid sequence corresponded to that predicted for chicken low-affinity nerve growth factor receptor (LNGFR). Experiments were performed to confirm that LNGFR was indeed the antigen recognized by M7412. First, M7412 bound to recombinant chicken LNGFR expressed in mammalian cells. Second, a rabbit serum raised to the purified antigen showed the same staining pattern in chicken embryos as did M7412. Lastly, a novel method for direct detection of nerve growth factor (NGF) bound to its receptors was used to show that in mixed spinal cord cultures, only neurons that expressed M7412 antigen had low-affinity binding sites for NGF. Furthermore, at the subcellular level, M7412 labeling was co-localized with bound NGF. The M7412 antigen is thus chicken LNGFR, whose role in motoneuron outgrowth pathways is discussed.

Nerve growth factor and neurotrophin-3 differentially regulate the proliferation and survival of developing rat brain oligodendrocytes

There is increasing evidence that the neurotrophins, particularly nerve growth factor (NGF) and neurotrophin-3 (NT-3), play a role in the regulation of glial development in the CNS. Recent studies have shown that the proliferation of optic nerve-derived O2A progenitors (OLPs) is potentiated by NT-3 in combination with platelet-derived growth factor, whereas NT-3 alone supports the survival of their differentiated progeny (Barres et al., 1994). In this study, we have examined the expression of the high-affinity neurotrophin receptors (trks) and the low-affinity nerve growth factor receptor p75 in developing oligodendrocytes (OLs). In addition, we have examined the effects of NGF and NT-3 on proliferation and survival of OLPs and OLs, respectively. TrkC, the high-affinity NT-3 receptor, and trkA, the high-affinity NGF receptor, are both expressed from the early OLP through the mature OL stage. The truncated form of trkB, lacking the tyrosine kinase domain, and the low-affinity neurotrophin receptor p75 are expressed at low levels in OLPs and are upregulated in mature OLs. NGF and NT-3 both induced the phosphorylation of mitogen-activated protein kinase (MAPK) in OLPs and in OLs. In both OLPs and OLs, NT-3 sustained the activation of MAPK more than NGF. NT-3 enhanced the proliferation of OLPs and supported the survival of OLs. By contrast, unless coadministered with FGF-2, NGF did not exhibit mitogenic effects on OLPs but did enhance the survival of differentiated OLs. Our data demonstrate the presence of functional trkA and trkC in developing OLs and indicate that both NGF and NT-3 have a broad spectrum of developmental actions on cells of the OL lineage.

Herpes simplex virus-1 entry into cells mediated by a novel member of the TNF/NGF receptor family

We identified and cloned a cellular mediator of herpes simplex virus (HSV) entry. Hamster and swine cells resistant to viral entry became susceptible upon expression of a human cDNA encoding this protein, designated HVEM (for herpesvirus entry mediator). HVEM was shown to mediate the entry of several wild-type HSV strains of both serotypes. Anti-HVEM antibodies and a soluble hybrid protein containing the HVEM ectodomain inhibited HVEM-dependent infection but not virus binding to cells. Mutations in the HSV envelope glycoprotein gD significantly reduced HVEM-mediated entry. The contribution of HVEM to HSV entry into human cells was demonstrable in activated T cells. HVEM, the first identified mediator of HSV entry, is a new member of the TNF/NGF receptor family.

Selective activation of NF-kappa B by nerve growth factor through the neurotrophin receptor p75

Nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) selectively bind to distinct members of the Trk family of tyrosine kinase receptors, but all three bind with similar affinities to the neurotrophin receptor p75 (p75NTR). The biological significance of neurotrophin binding to p75NTR in cells that also express Trk receptors has been difficult to ascertain. In the absence of TrkA, NGF binding to p75NGR activated the transcription factor nuclear factor kappa B (NF-kappa B) in rat Schwann cells. This activation was not observed in Schwann cells isolated from mice that lacked p75NTR. The effect was selective for NGF; NF-kappa B was not activated by BDNF or NT-3.

Neural apoptosis

Apoptosis is a mode of cell death in which the cell plays an active role in its own demise. The study of neural apoptosis, the identification of genes controlling apoptosis, and the examination of the mechanisms by which these genes achieve their effects have assumed increasing importance over the past few years. This is because (1) neural apoptosis occurs not only in development, but also in pathophysiological states such as stroke, glutamate toxicity, and beta-amyloid peptide toxicity; (2) genes that control apoptotic cell death, such as bcl-2, p35, p53, and p75NTR, also modulate necrotic neural death in some cases; (3) the emerging mechanisms by which these genes control apoptosis may be relevant for understanding neurodegenerative processes, and for the design of therapeutic agents; and (4) the findings that the cell plays an active role in its own demise, and that specific gene products are involved, suggest that therapeutic intervention may be feasible.