Nerve growth factor (NGF) induces motoneuron apoptosis in rat embryonic spinal cord in vitro

Infusions of Nerve Growth Factor Into the Developing Frontal Cortex Leads to Deficits in Behavioral Flexibility and Increased Perseverance

In the pursuit of further establishing a neurodevelopmental animal model to investigate the mechanisms underlying impaired executive function, a core and severely debilitating symptom of schizophrenia, we sought to characterize the deficits in behavioral flexibility in adult rats following neonatal infusions of nerve growth factor (NGF) into the medial part of the developing frontal cortex. Our previous studies using this neonatal frontal cortical lesion model have shown that it leads to adult-onset positive and negative symptom-like features, and several neuropathological abnormalities of schizophrenia. In the present study, we used operant conditioning-based paradigms to investigate set-shifting ability and reversal learning performance in adult rats that received infusions of NGF into the developing frontal cortex on post-natal day 1. NGF-infusion caused apoptosis of cells in the subplate layer. Adult rats that received neonatal infusions of NGF showed decreased grey matter thickness, and decreased levels of parvalbumin in prelimbic and infralimbic areas of the medial prefrontal cortex (mPFC). NGF-treated rats had difficulty completing the set-shifting and reversal learning tasks due to increased perseverance (ie, a failure to disengage from the previously-learned strategy once the rule contingencies were changed) compared to the control group. Collectively, these results identify the crucial role of the frontal cortical subplate layer in the structural and functional development of the mPFC relevant to schizophrenia. Furthermore, the present findings substantially advance the face and construct validity of this putative preclinical model of schizophrenia based on developmental disruption of the frontal cortical subplate.

Beyond good and evil: A putative continuum-sorting hypothesis for the functional role of proBDNF/BDNF-propeptide/mBDNF in antidepressant treatment

Depression and posttraumatic stress disorder are assumed to be maladaptive responses to stress and antidepressants are thought to counteract such responses by increasing BDNF (brain-derived neurotrophic factor) levels. BDNF acts through TrkB (tropomyosin-related receptor kinase B) and plays a central role in neuroplasticity. In contrast, both precursor proBDNF and BDNF propeptide (another metabolic product from proBDNF cleavage) have a high affinity to p75 receptor (p75R) and usually convey apoptosis and neuronal shrinkage. Although BDNF and proBDNF/propeptide apparently act in opposite ways, neuronal turnover and remodeling might be a final common way that both act to promote more effective neuronal networking, avoiding neuronal redundancy and the misleading effects of environmental contingencies. This review aims to provide a brief overview about the BDNF functional role in antidepressant action and about p75R and TrkB signaling to introduce the "continuum-sorting hypothesis." The resulting hypothesis suggests that both BDNF/proBDNF and BDNF/propeptide act as protagonists to fine-tune antidepressant-dependent neuroplasticity in crucial brain structures to modulate behavioral responses to stress.

Retrograde apoptotic signaling by the p75 neurotrophin receptor

Neurotrophins are target-derived factors necessary for mammalian nervous system development and maintenance. They are typically produced by neuronal target tissues and interact with their receptors at axonal endings. Therefore, locally generated neurotrophin signals must be conveyed from the axon back to the cell soma. Retrograde survival signaling by neurotrophin binding to Trk receptors has been extensively studied. However, neurotrophins also bind to the p75 receptor, which can induce apoptosis in a variety of contexts. Selective activation of p75 at distal axon ends has been shown to generate a retrograde apoptotic signal, although the mechanisms involved are poorly understood. The present review summarizes the available evidence for retrograde proapoptotic signaling in general and the role of the p75 receptor in particular, with discussion of unanswered questions in the field. In-depth knowledge of the mechanisms of retrograde apoptotic signaling is essential for understanding the etiology of neurodegeneration in many diseases and injuries.

Tumor Necrosis Factor-α and Neuronal Development

Tumor necrosis factor-α (TNFα) is a prototypic inflammatory cytokine up-regulated in most if not all neurodegenerative diseases. Many studies have reported variable roles in the adult or pathological brain. In contrast, the implication of TNFα in developmental neuronal cell death has been well documented in few studies. In sympathetic and trigeminal neurons, TNFα acts in an autocrine manner to induce immediate cell death on neurotrophic factor deprivation. In the spinal cord, TNFα is transiently produced by macrophages and commits motoneurons to become competent to die 2 days later. TNFα is also likely to induce immediate and delayed prodeath effects in adult and pathological tissues. Data obtained in embryonic systems will thus help to develop new therapeutic approaches to pathological neuronal death in adults.

Inhibition of motor neuron death in vitro and in vivo by a p75 neurotrophin receptor intracellular domain fragment

The p75 neurotrophin receptor (p75(NTR); also known as NGFR) can mediate neuronal apoptosis in disease or following trauma, and facilitate survival through interactions with Trk receptors. Here we tested the ability of a p75(NTR)-derived trophic cell-permeable peptide, c29, to inhibit p75(NTR)-mediated motor neuron death. Acute c29 application to axotomized motor neuron axons decreased cell death, and systemic c29 treatment of SOD1(G93A) mice, a common model of amyotrophic lateral sclerosis, resulted in increased spinal motor neuron survival mid-disease as well as delayed disease onset. Coincident with this, c29 treatment of these mice reduced the production of p75(NTR) cleavage products. Although c29 treatment inhibited mature- and pro-nerve-growth-factor-induced death of cultured motor neurons, and these ligands induced the cleavage of p75(NTR) in motor-neuron-like NSC-34 cells, there was no direct effect of c29 on p75(NTR) cleavage. Rather, c29 promoted motor neuron survival in vitro by enhancing the activation of TrkB-dependent signaling pathways, provided that low levels of brain-derived neurotrophic factor (BDNF) were present, an effect that was replicated in vivo in SOD1(G93A) mice. We conclude that the c29 peptide facilitates BDNF-dependent survival of motor neurons in vitro and in vivo.

Use of a Human Artificial Chromosome for Delivering Trophic Factors in a Rodent Model of Amyotrophic Lateral Sclerosis

A human artificial chromosome (HAC) is maintained as an episome within a cell and avoids random integration into the host genome. It can transfer multiple and/or large transgenes along with their regulatory elements thereby resembling native chromosomes. Using this HAC system, we established mesenchymal stem cells (MSCs) that simultaneously expressed hepatocyte growth factor, glial cell line-derived neurotrophic factor, and insulin-like growth factor 1, termed HAC-MSCs. This cell line provides an opportunity for stable transplantation and thorough analyses. We then introduced the cells for the treatment of a neurodegenerative disorder, amyotrophic lateral sclerosis. The HAC-MSCs were transplanted via the fourth cerebral ventricle (CV) or intravenous (i.v.) infusion at various ages of recipient mice. Littermate- and sex-matched mice underwent a sham procedure. Compared to the controls, there was an encouraging trend of increased life span via CV transplantation and delayed onset in i.v. infusion 60 days after transplantation. Further, we confirmed a statistically significant increase in life span via CV transplantation at 100 days. This effect was not seen in mice transplanted with MSCs lacking the HAC. We successfully enhanced the trophic potential of the MSCs using the HAC. This strategy could be a promising direction for the treatment of neurodegenerative disorders.

Organotypic Spinal Cord Culture: a Proper Platform for the Functional Screening

Recent improvements in organotypic slice culturing and its accompanying technological innovations have made this biological preparation increasingly useful ex vivo experimental model. Among organotypic slice cultures obtained from various central nervous regions, spinal cord slice culture is an absorbing model that represents several unique advantages over other current in vitro and in vivo models. The culture of developing spinal cord slices, as allows real-time observation of embryonic cells behaviors, is an instrumental platform for developmental investigation. Importantly, due to the ability of ex vivo models to recapitulate different aspects of corresponding in vivo conditions, these models have been subject of various manipulations to derive disease-relevant slice models. Moreover spinal cord slice cultures represent a potential platform for screening of different pharmacological agents and evaluation of cell transplantation and neuroregenerative materials. In this review, we will focus on studies carried out using the ex vivo model of spinal cord slice cultures and main advantages linked to practicality of these slices in both normal and neuropathological diseases and summarize them in different categories based on application.

Organotypic brain slice cultures: A review

In vitro cell cultures are an important tool for obtaining insights into cellular processes in an isolated system and a supplement to in vivo animal experiments. While primary dissociated cultures permit a single homogeneous cell population to be studied, there is a clear need to explore the function of brain cells in a three-dimensional system where the main architecture of the cells is preserved. Thus, organotypic brain slice cultures have proven to be very useful in investigating cellular and molecular processes of the brain in vitro. This review summarizes (1) the historical development of organotypic brain slices focusing on the membrane technology, (2) methodological aspects regarding culturing procedures, age of donors or media, (3) whether the cholinergic neurons serve as a model of neurodegeneration in Alzheimer’s disease, (4) or the nigrostriatal dopaminergic neurons as a model of Parkinson’s disease and (5) how the vascular network can be studied, especially with regard to a synthetic blood–brain barrier. This review will also highlight some limits of the model and give an outlook on future applications.

P75 and phosphorylated c-Jun are differentially regulated in spinal motoneurons following axotomy in rats

The neurotrophin receptor (p75) activates the c-Jun N-terminal kinase (JNK) pathway. Activation of JNK and its substrate c-Jun can cause apoptosis. Here we evaluate the role of p75 in spinal motoneurons by comparing immunoreactivity for p75 and phosphorylated c-Jun (p-c-Jun), the production of JNK activation in axotomized motoneurons in postnatal day (PN)1, PN7, PN14 and adult rats. Intensive p-c-Jun was induced in axotomized motoneurons in PN1 and PN7. In PN14, p-c-Jun expression was sharply reduced after the same injury. The decreased expression of p-c-Jun at this age coincided with a developmental switch of re-expression of p75 in axotomized cells. In adult animals, no p-c-Jun but intensive p75 was detected in axotomized motoneurons. These results indicate differential expression or turnover of phosphorylation of c-Jun and p75 in immature versus mature spinal motoneurons in response to axonal injury. The non-co-occurrence of p75 and p-c-Jun in injured motoneurons indicated that p75 may not activate JNK pathway, suggesting that the p75 may not be involved in cell death in axotomized motoneurons.

The biological functions and signaling mechanisms of the p75 neurotrophin receptor

The p75 neurotrophin receptor (p75(NTR)) regulates a wide range of cellular functions, including programmed cell death, axonal growth and degeneration, cell proliferation, myelination, and synaptic plasticity. The multiplicity of cellular functions governed by the receptor arises from the variety of ligands and co-receptors which associate with p75(NTR) and regulate its signaling. P75(NTR) promotes survival through interactions with Trk receptors, inhibits axonal regeneration via partnerships with Nogo receptor (Nogo-R) and Lingo-1, and promotes apoptosis through association with Sortilin. Signals downstream of these interactions are further modulated through regulated intramembrane proteolysis (RIP) of p75(NTR) and by interactions with numerous cytosolic partners. In this chapter, we discuss the intricate signaling mechanisms of p75(NTR), emphasizing how these signals are differentially regulated to mediate these diverse cellular functions.

Neurotrophins in the regulation of cellular survival and death

The neurotrophins play crucial roles regulating survival and apoptosis in the developing and injured nervous system. The four neurotrophins exert profound and crucial survival effects on developing peripheral neurons, and their expression and action is intimately tied to successful innervation of peripheral targets. In the central nervous system, they are dispensable for neuronal survival during development but support neuronal survival after lesion or other forms of injury. Neurotrophins also regulate apoptosis of both peripheral and central neurons, and we now recognize that there are regulatory advantages to having the same molecules regulate life and death decisions. This chapter examines the biological contexts in which these events take place and highlights the specific ligands, receptors, and signaling mechanisms that allow them to occur.

Temporal profile of nerve growth factor expression in the partial central nervous system of the Yangtze alligator Alligator sinensis (Reptilia,Crocodylia) during early postnatal growth

Expression of nerve growth factor (NGF) in structures of the partial central nervous system of the Yangtze alligator, Alligator sinensis (Reptilia, Crocodylia) was examined during early postnatal growth using immunohistochemistry and Western blot assays. In animals 0-2 years of age NGF-positive cells in the cerebral cortex increased gradually in number and size, and were predominantly distributed in the molecular layer. NGF-positive cells in the midbrain showed similar increases but with predominant distribution in the ependymal layer. NGF-positive cells increased in the cerebellum between 0 and 1 years of age, with increased NGF expression being seen during the first 2 years of life mostly in the ependymal layer. NGF-positive cells were mainly found in the gray matter of the spinal cord with decreasing cell numbers, NGF expression levels being seen from 0 to 2 years and small processes without synaptic connection from 1 to 2 years. These results suggest that NGF is involved in the early postnatal growth of several structures of Yangtze alligator partial central nervous system, suggesting a possible role of NGF in the Yangtze alligator partial central nervous system.

Spatiotemporal changes of NGF, BDNF and NT-3 in the developing spinal cords of embryonic chicken

Spatiotemporal changes of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) in the spinal cords of chick embryonic stage day 7 (E7) and day 14 (E14) were examined by using immunohistochemistry and Western blot. Intensive NGF immunoreaction (IR) was detected in the white matter of the spinal cords, while BDNF-IR in perikaryon and neurite, and NT-3-IR in the nucleus and cytoplasm were seen in the neurons of the ventral horn in the gray matter. Comparatively, the expressions for three growth factors have expanded largely into the dorsal horn at E14, and the level of proteins for these growth factors increased significantly in the spinal cords from E7 to E14. Morphological observation showed that the lumbar spinal cords of E7 appeared rectangular, whereas it gave a butterfly shape in the gray matter consisting of the typical ventral horn, dorsal horn and intermediate zone at E14. The present findings indicated that the spatiotemporal changes of NGF, BDNF and NT-3 could be associated to the morphological changes of developing spinal cords, suggesting the possible roles of three growth factors in the development of spinal cords.

p75NTR-dependent modulation of cellular handling of reactive oxygen species

Our previous studies demonstrated that p75NTR confers protection against oxidative stress-induced apoptosis upon PC12 cells; however, the mechanisms responsible for this effect are not known. The present studies reveal decreased mitochondrion membrane potential and increased generation of reactive oxygen species (ROS) in p75NTR-deficient PC12 cells as well as diminution of ROS generation after transfection of a full-length p75NTR construct into these cells. They also show that p75NTR deficiency attenuates activation of the phosphatidylinositol 3-kinase --> phospho-Akt/protein kinase B pathway in PC12 cells by oxidative stress or neurotrophic ligands and inhibition of Akt phosphorylation decreases the glutathione (GSH) content in PC12 cells. In addition, decreased de novo GSH synthesis and increased GSH consumption are observed in p75NTR-deficient cells. These findings indicate that p75NTR regulates cellular handling of ROS to effect a survival response to oxidative stress.

NRAGE, a p75NTR adaptor protein, is required for developmental apoptosis in vivo

NRAGE (also known as Maged1, Dlxin) is a member of the MAGE gene family that may play a role in the neuronal apoptosis that is regulated by the p75 neurotrophin receptor (p75NTR). To test this hypothesis in vivo, we generated NRAGE knockout mice and found that NRAGE deletion caused a defect in developmental apoptosis of sympathetic neurons of the superior cervical ganglia, similar to that observed in p75NTR knockout mice. Primary sympathetic neurons derived from NRAGE knockout mice were resistant to apoptosis induced by brain-derived neurotrophic factor (BDNF), a pro-apoptotic p75NTR ligand, and NRAGE-deficient sympathetic neurons show attenuated BDNF-dependent JNK activation. Hair follicle catagen is an apoptosis-like process that is dependent on p75NTR signaling; we show that NRAGE and p75NTR show regulated co-expression in the hair follicle and that identical defects in hair follicle catagen are present in NRAGE and p75NTR knockout mice. Interestingly, NRAGE knockout mice have severe defects in motoneuron apoptosis that are not observed in p75NTR knockout animals, raising the possibility that NRAGE may facilitate apoptosis induced by receptors other than p75NTR. Together, these studies demonstrate that NRAGE plays an important role in apoptotic-signaling in vivo.

Identification and kainic acid-induced up-regulation of low-affinity p75 neurotrophin receptor (p75NTR) in the nigral dopamine neurons of adult rats

Parkinson's disease is a common and severe debilitating neurological disease that results from massive and progressive degenerative death of dopamine neurons in the substantia nigra, but the mechanisms of neuronal degeneration and disease progression remains largely obscure. We are interested in possible implications of low-affinity p75 neurotrophin receptor (p75NTR), which may mediate neuronal apoptosis in the central nervous system, in triggering cell death of the nigral dopamine neurons. The RT-PCR and immunohistochemistry were carried out to detect if p75NTR is expressed in these nigral neurons and up-regulated by kainic acid (KA) insult in adult rats. It revealed p75NTR-positive immunoreactivity in the substantia nigra, and co-localization of p75NTR and tyrosine hydroxylase (TH) was found in a large number of substantia nigra neurons beside confirmation of p75NTR in the choline acetyltransferase (ChAT)-positive forebrain neurons. Cell count data further indicated that about 47-100% of TH-positive nigral neurons and 98-100% of ChAT-positive forebrain neurons express p75NTR. More interestingly, significant increasing in both p75NTR mRNA and p75NTR-positive neurons occurred rapidly following KA insult in the substantia nigra of animal model. The present study has provided first evidence on p75NTR expression and KA-inducing p75NTR up-regulation in substantia nigra neurons in rodent animals. Taken together with previous data on p75NTR functions in neuronal apoptosis, this study also suggests that p75NTR may play important roles in neuronal cell survival or excitotoxic degeneration of dopamine neurons in the substantia nigra in pathogenesis of Parkinson's disease in human beings.

Modulation of Fas-induced apoptosis by p75 neurotrophin receptor in a human neuroblastoma cell line

Fas and p75 neurotrophin receptors (p75(NTR)) are death receptors that alone induce apoptosis of SH-SY5Y neuroblastoma cell line respectively by Fas ligand or brain-derived neurotrophic factor (BDNF, a p75(NTR) ligand). We report on the modulation of Fas-mediated apoptosis by concomitant p75(NTR) activation. The exposure to both ligands suppressed the apoptotic effect. A co-localisation of Fas and p75(NTR) receptors was evidenced by co-capping and immunoprecipitation assays. Moreover, a caspase-8 inhibitor suppressed the protective effect of the concomitant BDNF and Fas ligand stimulation, suggesting that p75(NTR) and Fas receptors could share common signalling pathways.

Astrocyte and muscle-derived secreted factors differentially regulate motoneuron survival

During development, motoneurons (MNs) undergo a highly stereotyped, temporally and spatially defined period of programmed cell death (PCD), the result of which is the loss of 40-50% of the original neuronal population. Those MNs that survive are thought to reflect the successful acquisition of limiting amounts of trophic factors from the target. In contrast, maturation of MNs limits the need for target-derived trophic factors, because axotomy of these neurons in adulthood results in minimal neuronal loss. It is unclear whether MNs lose their need for trophic factors altogether or whether, instead, they come to rely on other cell types for nourishment. Astrocytes are known to supply trophic factors to a variety of neuronal populations and thus may nourish MNs in the absence of target-derived factors. We investigated the survival-promoting activities of muscle- and astrocyte-derived secreted factors and found that astrocyte-conditioned media (ACM) was able to save substantially more motoneurons in vitro than muscle-conditioned media (MCM). Our results indicate that both ACM and MCM are significant sources of MN trophic support in vitro and in ovo, but only ACM can rescue MNs after unilateral limb bud removal. Furthermore, we provide evidence suggesting that MCM facilitates the death of a subpopulation of MNs in a p75(NTR) - and caspase-dependent manner; however, maturation in ACM results in MN trophic independence and reduced vulnerability to this negative, pro-apoptotic influence from the target.

Pro-NGF secreted by astrocytes promotes motor neuron cell death

It is well established that motor neurons depend for their survival on many trophic factors. In this study, we show that the precursor form of NGF (pro-NGF) can induce the death of motor neurons via engagement of the p75 neurotrophin receptor. The pro-apoptotic activity was dependent upon the presence of sortilin, a p75 co-receptor expressed on motor neurons. One potential source of pro-NGF is reactive astrocytes, which up-regulate the levels of pro-NGF in response to peroxynitrite, an oxidant and producer of free radicals. Indeed, motor neuron viability was sensitive to conditioned media from cultured astrocytes treated with peroxynitrite and this effect could be reversed using a specific antibody against the pro-domain of pro-NGF. These results are consistent with a role for activated astrocytes and pro-NGF in the induction of motor neuron death and suggest a possible therapeutic target for the treatment of motor neuron disease.

Cholesterol biosynthesis and the pro-apoptotic effects of the p75 nerve growth factor receptor in PC12 pheochromocytoma cells

Neocarzinostatin (NCS), an enediyne antimitotic agent, induces cell death in both p75NTR neurotrophin receptor (NTR)-positive and p75NTR-negative PC12 cells in a concentration-dependent fashion. However, p75NTR-positive cells demonstrate a higher susceptibility to NCS-induced cell damage. Furthermore, treatment of p75NTR-positive cells with the p75NTR-specific ligand, MC192, resulted in apoptosis, while treatment of these cells with the TrkA-specific ligand, NGF-mAbNGF30, protected them from NCS-induced death, implying that both the naked and liganded p75NTR receptors have a pro-apoptotic effect on PC12 cells. Microarray studies aimed at examining differential gene expression between p75NTR-positive and p75NTR-negative cells suggested that enzymes of the cholesterol biosynthetic pathway are differentially expressed. We therefore tested the hypothesis that altered cholesterol biosynthesis contributes directly to the pro-apoptotic effects of p75NTR in this PC12 cell-NCS model. Subsequent Northern blotting studies confirmed that the expression of p75NTR is associated with the upregulation of cholesterol biosynthetic enzymes including 3-hydroxy-3-methylglutaryl CoA reductase (HMG CoA reductase), farnesyl-diphosphate synthase, and 7-dehydro-cholesterol reductase. Mevastatin, an HMG CoA reductase inhibitor, converts the apoptosis susceptibility of p75NTR-positive cells to that of p75NTR-negative cells. It does so at concentrations that do not themselves alter cell survival. These studies provide evidence that the pro-apoptotic effects of p75NTR in PC12 cells are related to the upregulation of cholesterol biosynthetic enzymes and consequent increased cholesterol biosynthesis.

Differential effects of endogenous brain-derived neurotrophic factor on the survival of axotomized sensory neurons in dorsal root ganglia: a possible role for the p75 neurotrophin receptor

After peripheral nerve injury, axotomized sensory neurons in dorsal root ganglia (DRG) undergo apoptosis and up-regulate brain-derived neurotrophic factor (BDNF). We tested whether endogenous BDNF plays any role in the survival of axotomized sensory neurons using in vitro and in vivo models. In the in vitro model, treatment with BDNF antibody significantly reduced apoptosis of sensory neurons in DRG explants from both adult and neonate rats and adult mice cultured for 48 h. Consistently, exogenous BDNF increased the percentage of apoptotic neurons in the DRGs from mice. The effects of the BDNF antibody and BDNF were not seen in DRGs from p75NTR(-/-) mice. In the in vivo model, sciatic nerve transection in neonatal rats decreased the total number of neurons in the injured DRG and treatment with antiserum to BDNF significantly exaggerated the loss of DRG neurons. Numbers of sensory neurons expressing BDNF and p75NTR in cultured DRGs increased but that expressing TrkB decreased. In contrast, sciatic nerve transection in vivo reduced the numbers of neurons expressing both p75NTR and TrkB but increased the numbers of cells expressing BDNF, 1 and 7 days after the surgery. These results suggest that BDNF may have differential effects on the survival of sensory neurons depending on the expression of p75NTR. While endogenous BDNF induced apoptosis of axotomized sensory neurons through p75NTR in vitro where more neurons expressed p75NTR, it prevented apoptosis in vivo where fewer neurons expressed p75NTR after sciatic nerve transection.

The intracellular domain of p75NTR as a determinant of cellular reducing potential and response to oxidant stress

The low-affinity neurotrophin receptor, p75NTR, has been found to be pro- or anti-apoptotic depending upon the cell in which it is expressed. Reactive oxygen species play a major role in apoptosis induction and enactment. Using two polyclonal PC12 populations that, respectively, do or do not express p75NTR, this paper demonstrates that p75NTR expression confers resistance to oxidant stress upon PC12 cells maintained in serum-containing medium. The effect of p75NTR on cell survival is mimicked in p75-negative cells by expression of constructs that produce the p75NTR intracellular domain (ICD) or p75NTR with the extracellular domain deleted (DeltaECD), suggesting that binding of an extracellular ligand to p75NTR is not required. Our studies further document that the differential sensitivity to oxidant stress is serum-dependent and associated with differential oxidation of glutathione between p75-positive and p75-negative cells. These results suggest that the role of p75NTR in determining the consequences and treatment of age-related disorders and conditions in which reactive oxygen species are involved may require neither the extracellular receptor domain nor, by inference, the cognate extracellular ligands of this neurotrophin receptor.

Rat embryonic motoneurons in long-term co-culture with Schwann cells—a system to investigate motoneuron diseases on a cellular level in vitro

Investigations of motoneuron diseases on a cellular and molecular level require long-term cultivation of primary cells. Here we present a new culture system in which matured motoneurons interact with their physiological partners like interneurons, astroglia and peripheral glia cells. This enables motoneuron-maturation for up to 3 weeks, while motoneurons consistently reached large diameters of their somata of 30-45 microm, occasionally more than 80 microm. Dissociated rat embryonic ventral spinal cord cells were enriched for motoneurons by density gradient centrifugation and seeded on a non-confluent mono-layer of highly enriched neonatal rat Schwann cells. Immunocytochemical visualization of neuron specific betaIII-tubulin in all neurons and of motoneuron specific non-phosphorylated neurofilament H/M, respectively, revealed that after 3 days in vitro >70% of all neurons were motoneurons. After 20 days in vitro, a motoneuron fraction of 12% was maintained. Motoneurons were susceptible to transient transfection with green fluorescent protein cDNA when liposomal transfection and an enhancer substance were combined. Synaptic connections enabled formation of spontaneously active neuronal networks which provide a culture model to study glutamate excitotoxicity and calcium deregulation on a molecular level. Both mechanisms are implied in the pathophysiology of amyotrophic lateral sclerosis, a neurodegenerative motoneuron disorder.

Motoneuronal death during spinal cord development is mediated by oxidative stress

The involvement of reactive oxygen species (ROS) in neuronal death has been determined in culture, and in association with several neurodegenerative disorders. We examined whether ROS participate in the cell death observed during spinal cord development. We found that the general pattern of high ROS levels, gene expression for some antioxidant enzymes, and motoneuron death correlated positively along spinal cord development. ROS were reduced in spinal cords cultured in the presence of a synthetic superoxide dismutase and catalase mimetic, with a concomitant reduction in cell death and an increase in the number of motoneurons. The number of motoneurons was higher in spinal cords treated with the antioxidant than in those treated with caspase inhibitors. In general, the increase in motoneuron survival did not correlate with the reduction in cells undergoing DNA degradation in the motoneuronal region. These results suggest that ROS are signaling molecules controlling caspase-dependent and caspase-independent programmed motoneuron death, and support the hypothesis that this mechanism is abnormally turned on in some neurodegenerative disorders and aging.

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.

Macrophage-derived tumor necrosis factor alpha, an early developmental signal for motoneuron death

Mechanisms inducing neuronal death at defined times during embryogenesis remain enigmatic. We show in explants that a developmental switch occurs between embryonic day 12 (E12) and E13 in rats that is 72-48 hr before programmed cell death. Half the motoneurons isolated from peripheral tissues at E12 escape programmed cell death, whereas 90% of motoneurons isolated at E13 enter a death program. The surrounding somite commits E12 motoneurons to death. This effect requires macrophage cells, is mimicked by tumor necrosis factor alpha (TNFalpha), and is inhibited by anti-TNFalpha antibodies. In vivo, TNFalpha is detected within somite macrophages, and TNF receptor 1 (TNFR1) is detected within motoneurons precisely between E12 and E13. Although motoneuron cell death occurs normally in TNFalpha-/- mice, this process is significantly reduced in explants from TNFalpha-/- and TNFR1-/- mice. Thus, embryonic motoneurons acquire the competence to die, before the onset of programmed cell death, from extrinsic signals such as macrophage-derived TNFalpha

Astrocytic production of nerve growth factor in motor neuron apoptosis: implications for amyotrophic lateral sclerosis

Reactive astrocytes frequently surround degenerating motor neurons in patients and transgenic animal models of amyotrophic lateral sclerosis (ALS). We report here that reactive astrocytes in the ventral spinal cord of transgenic ALS-mutant G93A superoxide dismutase (SOD) mice expressed nerve growth factor (NGF) in regions where degenerating motor neurons expressed p75 neurotrophin receptor (p75(NTR)) and were immunoreactive for nitrotyrosine. Cultured spinal cord astrocytes incubated with lipopolysaccharide (LPS) or peroxynitrite became reactive and accumulated NGF in the culture medium. Reactive astrocytes caused apoptosis of embryonic rat motor neurons plated on the top of the monolayer. Such motor neuron apoptosis could be prevented when either NGF or p75(NTR) was inhibited with blocking antibodies. In addition, nitric oxide synthase inhibitors were also protective. Exogenous NGF stimulated motor neuron apoptosis only in the presence of a low steady state concentration of nitric oxide. NGF induced apoptosis in motor neurons from p75(NTR +/+) mouse embryos but had no effect in p75(NTR -/-) knockout embryos. Culture media from reactive astrocytes as well as spinal cord lysates from symptomatic G93A SOD mice-stimulated motor neuron apoptosis, but only when incubated with exogenous nitric oxide. This effect was prevented by either NGF or p75(NTR) blocking-antibodies suggesting that it might be mediated by NGF and/or its precursor forms. Our findings show that NGF secreted by reactive astrocytes induce the death of p75-expressing motor neurons by a mechanism involving nitric oxide and peroxynitrite formation. Thus, reactive astrocytes might contribute to the progressive motor neuron degeneration characterizing ALS.

The p75 neurotrophin receptor: multiple interactors and numerous functions

The neurotrophin receptor p75 (p75NTR), is involved in a diverse array of cellular responses, including apoptosis, neurite outgrowth and myelination. Stimulation of p75NTR with neurotrophin can activate multiple downstream signals, including the small GTP binding protein Rac, the transcription factor NF-kappa B and the stress activated kinase, JNK. How these signals are generated and regulated to produce a specific cellular effect has yet to be fully elucidated. A number of proteins have recently been shown to interact with the intracellular domain of p75NTR. Here, we review these p75NTR interacting factors and the current evidence as to how they contribute to the functional effects of p75NTR activation.

Characterization of NADE, NRIF and SC-1 gene expression during mouse neurogenesis

The p75 neurotrophin receptor (p75NTR) is a member of the tumor necrosis factor receptor superfamily. p75NTR signaling events have been implicated in both cell cycle arrest and apoptosis depending on which effector molecules are associated with its intracellular domain after ligand binding. Two such effector proteins, p75NTR-associated cell death executor (NADE) and neurotrophin receptor interacting factor (NRIF) promote p75NTR-mediated apoptosis, whereas Schwann cell factor-1 (SC-1) mediates neurotrophin-dependent withdrawal from the cell cycle. An understanding of the expression profiles of these three interacting proteins and p75NTR during embryogenesis is critical for addressing whether these effector proteins might function outside of p75NTR-mediated signaling events. The distribution of NADE, NRIF and SC-1 mRNAs during murine development suggests that the action of these genes is in fact not limited to regions of p75NTR expression. Specifically, a detailed comparison of the spatial and temporal expression domains of NADE, NRIF and SC-1 during brain development revealed regions of co-expression with p75NTR but also illustrates a distinct and discordant spatial and temporal expression. These results yield novel insights into the unique developmental characteristics of the three p75NTR-interacting proteins, thus revealing their diverse signaling potential during embryonic development.

Glial cell line-derived neurotrophic factor and target-dependent regulation of large-conductance KCa channels in developing chick lumbar motoneurons

The functional expression of large-conductance Ca2+-activated K+ (K(Ca)) channels in lumbar motoneurons (LMNs) of the developing chick embryo is regulated in part by interactions with striated muscle target tissues. Here we show that the functional expression of K(Ca) channels in LMNs developing in vitro can be stimulated by application of a skeletal muscle extract (MEX) or by coculture with hindlimb myotubes. A similar stimulation of K(Ca) channels in vitro can be produced by the trophic factors glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor but not by neurotrophin (NT)-3 or NT-4. The actions of MEX and hindlimb myotubes are blocked by a GDNF-neutralizing antiserum. Moreover, injection of this same antiserum into the embryonic hindlimb reduced the functional expression of K(Ca) channels in vivo to levels seen in LMNs deprived of interactions with the hindlimb. The effects of GDNF on K(Ca) channel expression in LMNs require 24 hr of continuous exposure to reach maximum and are blocked by the translation inhibitor anisomycin, indicating the need for synthesis of new proteins. GDNF actions are also blocked by the farnesyl transferase inhibitor manumycin, suggesting a role for Ras in the actions of GDNF. Finally, the actions of GDNF are inhibited by PP2, an inhibitor of Src family tyrosine kinases, and by LY29003, an inhibitor of phosphatidylinositol 3 kinases, but not by PD98059, an inhibitor of the Erk signaling cascade. None of these treatments alter expression of voltage-activated Ca2+ channels. Thus, the actions of GDNF on LMN K(Ca) channel expression appear to use a transduction pathway similar to that used for regulation of apoptosis.

Brain-derived neurotrophic factor can act as a pronecrotic factor through transcriptional and translational activation of NADPH oxidase

Several lines of evidence suggest that neurotrophins (NTs) potentiate or cause neuronal injury under various pathological conditions. Since NTs enhance survival and differentiation of cultured neurons in serum or defined media containing antioxidants, we set out experiments to delineate the patterns and underlying mechanisms of brain-derived neurotrophic factor (BDNF)-induced neuronal injury in mixed cortical cell cultures containing glia and neurons in serum-free media without antioxidants, where the three major routes of neuronal cell death, oxidative stress, excitotoxicity, and apoptosis, have been extensively studied. Rat cortical cell cultures, after prolonged exposure to NTs, underwent widespread neuronal necrosis. BDNF-induced neuronal necrosis was accompanied by reactive oxygen species (ROS) production and was dependent on the macromolecular synthesis. cDNA microarray analysis revealed that BDNF increased the expression of cytochrome b558, the plasma membrane-spanning subunit of NADPH oxidase. The expression and activation of NADPH oxidase were increased after exposure to BDNF. The selective inhibitors of NADPH oxidase prevented BDNF-induced ROS production and neuronal death without blocking antiapoptosis action of BDNF. The present study suggests that BDNF-induced expression and activation of NADPH oxidase cause oxidative neuronal necrosis and that the neurotrophic effects of NTs can be maximized under blockade of the pronecrotic action.

Motoneuron-derived neurotrophin-3 is a survival factor for PAX2-expressing spinal interneurons

Rat spinal cord interneurons undergo programmed cell death shortly after birth. We investigated here whether cell death of interneurons could be regulated by trophic factors produced by motoneurons, one of their main targets. To test this hypothesis, we studied the effect of the selective destruction of motoneurons on the survival of interneurons in organotypic cultures of embryonic rat spinal cords. Motoneurons were eliminated by an anti-p75(NTR)-specific immunotoxin (192 IgG-saporin). We then observed a decrease of 28% in the number of ventral spinal interneurons immunoreactive (IR) for the homeoprotein PAX2. This was correlated with an increase in the number of apoptotic nuclei in the same area. Because neurotrophin-3 (NT-3) is specifically produced by motoneurons and because interneurons express the NT-3 high-affinity receptor trkC, we examined the role of NT-3 in the survival of PAX2-IR interneurons. Addition of NT-3 to 192 IgG-saporin-treated explants rescued ventral PAX2-IR interneurons. Depletion of secreted NT-3 by anti-NT-3 antibodies induced 66% loss of ventral PAX2-IR interneurons. We conclude that motoneuron-derived NT-3 is a trophic factor for ventral PAX2-IR interneurons.

Nerve growth factor alters p75 neurotrophin receptor-induced effects in mouse facial motoneurons following axotomy

The p75 neurotrophin receptor (p75(NTR)) has been implicated as being detrimental for cell survival in facial motoneurons following injury. Although facial motoneurons do not respond to nerve growth factor (NGF) under normal circumstances, this study shows that NGF can interfere with p75(NTR)-mediated cell survival effects on motoneurons following injury. Twenty-five days following injury, the proportion of surviving axotomized neurons in NGF/p75(+/+) mice, which overexpress NGF, was significantly higher compared to wild-type mice, while NGF/p75(-/-) mice, which overexpress NGF but carry two mutated alleles for p75(NTR), had fewer neurons compared to wild-type and p75(-/-) mice, which carry two mutated alleles for p75(NTR), resulting in a lack of functional expression of this receptor. Sympathetic axons sprouted into the axotomized facial nucleus of both NGF/p75(+/+) and NGF/p75(-/-) following injury, due to transgene expression of NGF in reactive astrocytes. Removal of these sympathetic axons enhanced the number of surviving axotomized neurons in NGF/p75(-/-) mice but not in NGF/p75(+/+) mice. Although motoneurons do not express trkA and should therefore be unresponsive to NGF, our results reveal that NGF can influence p75-mediated motoneuron survival following axotomy.

Expression analysis of a novel p75(NTR) signaling protein, which regulates cell cycle progression and apoptosis

Neurotrophin receptor-interacting MAGE (NRAGE) is the most recently identified p75 neurotrophin receptor (p75(NTR)) intracellular binding protein. Previously, NRAGE over-expression was shown to mediate cell cycle arrest and facilitate nerve growth factor (NGF) dependent apoptosis of sympathetic neuroblasts in a p75(NTR) specific manner. Here we have examined the temporal and spatial expression patterns of NRAGE over the course of murine embryogenesis to determine whether NRAGE's expression is consistent with its proposed functions. We demonstrate that NRAGE mRNA and protein are expressed throughout embryonic and adult tissues. The mRNA is constitutively expressed within each tissue across development. However, expression of NRAGE protein displays a tight temporal tissue specific regulation. During early CNS development, NRAGE protein is expressed throughout the neural tube, but by later stages of neurogenesis, NRAGE protein is restricted within the ventricular zone, subplate and cortical plate. Moreover, NRAGE protein expression is limited to proliferative neural subpopulations as we fail to detect NRAGE expression co-localized with mature/differentiation associated neuronal markers. Interestingly, NRAGE's expression is not restricted solely to areas of p75(NTR) expression suggesting that NRAGE may mediate proliferation and/or apoptosis from other environmental signals in addition to NGF within the CNS. Our data support previously characterized roles for NRAGE as a mediator of precursor apoptosis and a repressor of cell cycle progression in neural development.

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.

p75-nerve growth factor as an antiapoptotic complex: independence versus cooperativity in protection from enediyne chemotherapeutic agents

Growth factors, including nerve growth factor (NGF), have been hypothesized to play a role in resistance to chemotherapeutic agent-induced apoptosis. Induction by NGF of resistance to apoptosis is primarily thought to be the result of its binding to its high-affinity receptor, TrkA. The low-affinity NGF receptor, p75, has long been thought merely to facilitate NGF binding to TrkA. However, we have previously shown that the binding of NGF to its low-affinity receptor, p75, protects neuroblastoma cells that do not express TrkA against apoptosis induced by enediyne chemotherapeutic agents. In cells that express both receptors, it is not clear what determines which receptor is responsible for the protective effect of NGF. We now show that, in enediyne-treated SH-SY5Y neuroblastoma transfectants with native levels of p75 and a low TrkA/p75 ratio (1/100), the anti-apoptotic effect of NGF requires binding to p75. In contrast, in transfectants with native levels of p75 and a high TrkA/p75 ratio (100/100), NGF treatment prevents enediyne-induced apoptosis by a mechanism independent of p75 binding. Treatment of low TrkA/p75 ratio cells with NGF results in activation and nuclear translocation of NF-kappaB and tyrosine phosphorylation of TrkA. Analogous treatment of high TrkA/p75 ratio cells results only in phosphorylation of TrkA even though nuclear factor (NF)-kappaB signaling is not inactive and can be initiated by other ligands. The ratio of TrkA/p75 in cells that express both receptors probably contributes to the determination of which of the two known roles of p75 (i.e., TrkA independent or TrkA facilitatory) are responsible for NGF-mediated protection from enediyne-induced apoptosis.

Neuronal survival and cell death signaling pathways

Neuronal viability is maintained through a complex interacting network of signaling pathways that can be perturbed in response to a multitude of cellular stresses. A shift in the balance of signaling pathways after stress or in response to pathology can have drastic consequences for the function or the fate of a neuron. There is significant evidence that acutely injured and degenerating neurons may die by an active mechanism of cell death. This process involves the activation of discrete signaling pathways that ultimately compromise mitochondrial structure, energy metabolism and nuclear integrity. In this review we examine recent evidence pertaining to the presence and activation of anti- and pro-cell death regulatory pathways in nervous system injury and degeneration.

Low-affinity nerve growth factor receptor (p75) in dermatofibrosarcoma protuberans and other nonneural tumors: a study of 1,150 tumors and fetal and adult normal tissues

Low-affinity nerve growth factor receptor (p75) is a member of the tumor necrosis factor receptor family. It may modulate the binding of nerve growth factor (NGF) to the functional high-affinity receptor tyrosine kinase (trk) A. NGF is thought to be responsible for growth, apoptosis, and function of the nervous system. The presence of this receptor (p75) was determined in a large group of neural and nonneural tumors and fetal and adult tissues. One thousand one hundred fifty tumors were analyzed with monoclonal antibody for p75, along with selected normal fetal and adult tissues. Immunoreactivity for p75 was present in adult pericytes, perivascular fibroblasts, basal cells of several types of epithelia, perineurial cells, and dendritic reticulum cells. Additionally, a wide zone of subepithelial mesenchyme and skeletal muscle were positive in the first-trimester fetus, but were diminished or negative in the adult. Consistently positive nonneural mesenchymal tumors included dermatofibrosarcoma protuberans (DFSP), embryonal and alveolar rhabdomyosarcoma, synovial sarcoma, and spindle cell hemangio(endotheli)oma. Schwann cell tumors, ganglioneuroma, granular cell tumor, and malignant peripheral nerve sheath tumor (MPNST) were also p75 positive. Mesenchymal nonneural tumors that were variably positive (32% to 69%) for p75 included fibrosarcoma variants, solitary fibrous tumor, hemangiopericytoma, spindle cell lipoma, Ewing's sarcoma, mesenchymal chondrosarcoma, and malignant melanoma. Nervous system tumors such as paragangliomas, neuroblastoma, meningioma, and perineurioma and nonneural mesenchymal tumors, including extraskeletal osteosarcoma, benign fibrous histiocytomas, fibromas, alveolar soft part sarcoma, epithelioid sarcoma, smooth muscle and gastrointestinal stromal tumors, and angiosarcomas, were almost always negative for p75. Epithelial tumors that were consistently positive included mixed tumor and adenoid cystic carcinoma, whereas mesothelioma, adenocarcinomas, and most squamous cell carcinomas were negative. p75 is not a specific marker for nerve sheath tumors. It is present in a variety of other mesenchymal tumors including synovial sarcoma and in CD34-positive tumors such as DFSP, spindle cell lipoma, and hemangiopericytoma. The presence of p75 in nonneural tumors such as DFSP and rhabdomyosarcoma mimic its presence in early fetal mesenchyme and skeletal muscle, suggesting oncofetal expression in these tumors. p75 may be useful to distinguish DFSP from benign fibrous histiocytoma.

The binding of zinc and copper ions to nerve growth factor is differentially affected by pH: implications for cerebral acidosis

It has recently been shown that transition metal cations Zn2+ and Cu2+ bind to histidine residues of nerve growth factor (NGF) and other neurotrophins (a family of proteins important for neuronal survival) leading to their inactivation. Experimental data and theoretical considerations indicate that transition metal cations may destabilize the ionic form of histidine residues within proteins, thereby decreasing their pK(a) values. Because the release of transition metal cations and acidification of the local environment represent important events associated with brain injury, the ability of Zn2+ and Cu2+ to bind to neurotrophins in acidic conditions may alter neuronal death following stroke or as a result of traumatic injury. To test the hypothesis that metal ion binding to neurotrophins is influenced by pH, the effects of Zn2+ and Cu2+ on NGF conformation, receptor binding and NGF tyrosine kinase (trkA) receptor signal transduction were examined under conditions mimicking cerebral acidosis (pH range 5.5-7.4). The inhibitory effect of Zn2+ on biological activities of NGF is lost under acidic conditions. Conversely, the binding of Cu2+ to NGF is relatively independent of pH changes within the studied range. These data demonstrate that Cu2+ has greater binding affinity to NGF than Zn2+ at reduced pH, consistent with the higher affinity of Cu2+ for histidine residues. These findings suggest that cerebral acidosis associated with stroke or traumatic brain injury could neutralize the Zn2+-mediated inactivation of NGF, whereas corresponding pH changes would have little or no influence on the inhibitory effects of Cu2+. The importance of His84 of NGF for transition metal cation binding is demonstrated, confirming the involvement of this residue in metal ion coordination.

Zinc inhibits p75NTR-mediated apoptosis in chick neural retina

It has previously been documented that Zn2+ inhibits TrkA-mediated effects of NGF. To evaluate the ability of Zn2+ to attenuate the biological activities of NGF mediated by p75NTR, we characterized the effects of this transition metal cation on both binding and the pro-apoptotic properties of the NGF-p75NTR interaction. Binding of NGF to p75NTR displayed higher affinity in embryonic chick retinal cells than in PC12 cells. NGF induced apoptosis in dissociated cultures of chick neural retina. The addition of 100 microM Zn2+ inhibited binding and chemical cross-linking of 125I-NGF to p75NTR, and also attenuated apoptosis mediated by this ligand-receptor interaction. These studies lead to the conclusion that Zn2+ antagonizes NGF/p75NTR-mediated signaling, suggesting that the effect of this transition metal cation can be either pro- or anti-apoptotic depending on the cellular context.

p75(NTR) mediates neurotrophin-induced apoptosis of vascular smooth muscle cells

The development of atherosclerotic lesions results from aberrant cell migration, proliferation, and extracellular matrix production. In advanced lesions, however, cellular apoptosis, leading to lesion remodeling, predominates. During lesion formation, the neurotrophins and the neurotrophin receptor tyrosine kinases, trks B and C, are induced and mediate smooth muscle cell migration. Here we demonstrate that a second neurotrophin receptor, p75(NTR), is expressed by established human atherosclerotic lesions and late lesions that develop after balloon injury of the rat thoracic aorta. The p75(NTR), a member of the tumor necrosis factor/FAS receptor family, can modulate trk receptor function as well as initiate cell death when expressed in cells of the nervous system that lack kinase-active trk receptors. p75(NTR) expression colocalizes to neointimal cells, which express smooth muscle cell alpha-actin and are expressed by cultured human endarterectomy-derived cells (HEDC). Areas of the plaque expressing p75(NTR) demonstrate increased TUNEL positivity, and HEDC undergo apoptosis in response to the neurotrophins. Finally, neurotrophins also induced apoptosis of a smooth muscle cell line genetically manipulated to express p75(NTR), but lacking trk receptor expression. These studies identify the regulated expression of neurotrophins and p75(NTR) as an inducer of smooth muscle cell apoptosis in atherosclerotic lesions.

Neurotrophin signal transduction in the nervous system

Neurotrophins use two types of receptors, the Trk tyrosine kinase receptors and the p75 neurotrophin receptor (p75NTR), to regulate the growth, development, survival and repair of the nervous system. These receptors can either collaborate with or inhibit each other's actions to mediate neurotrophin effects. The development and survival of neurons is thus based upon the functional interplay of the signals generated by Trk and p75NTR. In the past two years, the signaling pathways used by these receptors, including Akt and MAPK-induced signaling via Trk, and JNK, p53, and NF-kappaB signaling via p75NTR, have been identified. In addition, a number of novel p75NTR-interacting proteins have been identified that transmit growth, survival, and apoptotic signals.

Active killing of neurons during development and following stress: a role for p75(NTR) and Fas?

Evidence for active triggering of neuronal death continues to accumulate. The transmembrane receptors p75(NTR) and Fas can trigger (and in some cases are required for) programmed cell death of the neurons that express them, through signalling pathways that are regulated by a variety of cytoplasmic effectors. Neuronal death induced by trophic deprivation often requires Fas signalling, further blurring the boundaries between naturally occurring and stress-induced neuronal death.