Oligodendrocyte apoptosis mediated by caspase activation.
Treatment with NGF causes long-term cultures of oligodendrocytes to die via a yet undefined mechanism mediated by the p75 neurotrophin receptor. The p75 receptor belongs to the TNF receptor superfamily of molecules, which includes Fas and p55 TNF receptors. The Fas and TNF receptors use adaptor molecules to recruit and activate caspase-8 to the receptor. Using a combination of immunohistochemical and Western blotting assays, we have examined caspase activity during NGF-induced apoptosis. Interestingly, although caspase-1 [interleukin-1beta-converting enzyme (ICE)], caspase-2, caspase-3, and caspase-8 were expressed in oligodendrocytes, only caspase-1, -2, and -3 were activated after NGF treatment, whereas caspase-8 was not. These data suggest that the mechanism of apoptosis by NGF through the p75 receptor is different from TNF and Fas-mediated killing. gamma Radiation of oligodendrocytes also activated a similar subset of caspases as NGF, indicating that NGF-induced oligodendrocyte apoptosis uses a similar cell death execution mechanism as injury models. This consolidates a potential role of the p75 neurotrophin receptor during stress and inflammatory conditions. (+info)
p75NTR and the concept of cellular dependence: seeing how the other half die.
Cells depend on specific stimuli, such as trophic factors, for survival and in the absence of such stimuli, undergo apoptosis. How do cells initiate apoptosis in response to the withdrawal of trophic factors or other dependent stimuli? Recent studies of apoptosis induction by neurotrophin withdrawal argue for a novel form of pro-apoptotic signal transduction - 'negative signal transduction' - in which the absence of ligand-receptor interaction induces cell death. We have found that the prototype for this form of signaling - the common neurotrophin receptor, p75NTR - creates a state of cellular dependence (or addiction) on neurotrophins, and that this effect requires an 'addiction/dependence domain' (ADD) in the intracytoplasmic region of p75NTR. We have recently found other receptors that include dependence domains, arguing that dependence receptors, and their associated dependence domains, may be involved in a rather general mechanism to create cellular states of dependence on trophic factors, cytokines, adhesion, electrical activity and other dependent stimuli. (+info)
Distribution of the low-affinity neurotrophin receptor (p75) in the developing trigeminal brainstem complex in the rat.
The low-affinity neurotrophin receptor (p75) binds all members of the neurotrophin family. In the rat, during the first week postpartum, dense p75-immunoreactivity (IR) is present throughout all components of the trigeminal brainstem complex (TBC), largely associated with primary sensory afferents. Within subnucleus caudalis (SpC) of the TBC, intense p75-IR is present in all laminae at birth. During the second and third postnatal weeks, p75-IR in SpC gradually fades within the deeper laminae, becoming generally restricted in the adult to laminae I and II. Similar declines in p75-IR intensity occur in the subnucleus oralis (SpO); in the SpO in the adult, p75-IR is confined to the dorsalmost portion of SpO. In subnucleus interpolaris, an emerging, vibrissa-related pattern of p75-IR is detectable on PD0 (first 24 hr postpartum), which becomes fully differentiated during PD4-PD7. However, this pattern gradually disappears during the third postnatal week. Ventrally in the nucleus principalis (PrV), a pattern of p75-IR that mirrors the topographical arrangement of the vibrissae is detectable by PD0-PD1, is fully differentiated by the end of the first postnatal week, and persists into adulthood. Perinatal unilateral sectioning of the infraorbital nerve on PD0-PD1, but not as late as PD4, disrupts p75-IR patterning in the adult PrV. Although p75 appears to be associated with primary afferent pattern formation, to determine whether it is essential, we examined mutant mice unable to form functional p75. In the TBC of these knockout mice, examined as adults, patterns of cytochrome oxidase staining (which parallel those of p75-IR) appeared to be normal. In summary, during early development, p75 is widely expressed in the TBC during periods of active synaptogenesis and pattern formation, whereas in the adult, its expression is restricted to association with populations of primary sensory afferents. However, the absence of functional p75 in genetically altered mice does not appear to prevent primary afferent pattern formation. (+info)
Transcriptional targeting of retroviral vectors to the erythroblastic progeny of transduced hematopoietic stem cells.
Targeted expression to specific tissues or cell lineages is a necessary feature of a gene therapy vector for many clinical applications, such as correction of hemoglobinopathies or thalassemias by transplantation of genetically modified hematopoietic stem cells. We developed retroviral vectors in which the constitutive viral enhancer in the U3 region of the 3' LTR is replaced by an autoregulatory enhancer of the erythroid-specific GATA-1 transcription factor gene. The replaced enhancer is propagated to the 5' LTR upon integration into the target cell genome. The modified vectors were used to transduce human hematopoietic cell lines, cord blood-derived CD34(+) stem/progenitor cells, and murine bone marrow repopulating stem cells. The expression of appropriate reporter genes (triangle upLNGFR, EGFP) was analyzed in the differentiated progeny of transduced stem cells in vitro, in liquid culture as well as in clonogenic assay, and in vivo, after bone marrow transplantation in lethally irradiated mice. The GATA-1 autoregulatory enhancer effectively restricts the expression of the LTR-driven proviral transcription unit to the erythroblastic progeny of both human progenitors and mouse-repopulating stem cells. Packaging of viral particles, integration into the target genome, and stability of the integrated provirus are not affected by the LTR modification. Enhancer replacement is therefore an effective strategy to target expression of a retroviral transgene to a specific progeny of transduced hematopoietic stem cells. (+info)
Enhanced neurotrophin-induced axon growth in myelinated portions of the CNS in mice lacking the p75 neurotrophin receptor.
Axonal growth in the adult mammalian CNS is limited because of inhibitory influences of the glial environment and/or a lack of growth-promoting molecules. Here, we investigate whether supplementation of nerve growth factor (NGF) to the CNS during postnatal development and into adulthood can support the growth of sympathetic axons within myelinated portions of the maturing brain. We have also asked whether p75(NTR) plays a role in this NGF-induced axon growth. To address these questions we used two lines of transgenic mice overexpressing NGF centrally, with or without functional expression of p75(NTR) (NGF/p75(+/+) and NGF/p75(-/-) mice, respectively). Sympathetic axons invade the myelinated portions of the cerebellum, beginning shortly before the second week of postnatal life, in both lines of NGF transgenic mice. Despite the presence of central myelin, these sympathetic axons continue to sprout and increase in density between postnatal days 14 and 100, resulting in a dense plexus of sympathetic fibers within this myelinated environment. Surprisingly, the growth response of sympathetic fibers into the cerebellar white matter of NGF/p75(-/-) mice is enhanced, such that both the density and extent of axon ingrowth are increased, compared with age-matched NGF/p75(+/+) mice. These dissimilar growth responses cannot be attributed to differences in cerebellar levels of NGF protein or sympathetic neuron numbers between NGF/p75(+/+) and NGF/p75(-/-) mice. Our data provide evidence demonstrating that growth factors are capable of overcoming the inhibitory influences of central myelin in the adult CNS and that neutralization of the p75(NTR) may further enhance this growth response. (+info)
The p75(NTR)-induced apoptotic program develops through a ceramide-caspase pathway negatively regulated by nitric oxide.
SK-N-BE neuroblastoma cell clones transfected with p75(NTR) and lacking Trk neurotrophin receptors, previously reported to undergo extensive spontaneous apoptosis and to be protected by nerve growth factor (NGF) (Bunone, G., Mariotti, A., Compagni, A., Morandi, E., and Della Valle, G. (1997) Oncogene 14, 1463-1470), are shown to exhibit (i) increased levels of the pro-apoptotic lipid metabolite ceramide and (ii) high activity of caspases, the proteases of the cell death cascade. In the p75(NTR)-expressing cells, these parameters were partially normalized by prolonged NGF treatment, which, in addition, decreased apoptosis, similar to caspase blockers. Conversely, exogenous ceramide increased caspase activity and apoptosis in both wild-type and p75(NTR)-expressing cells. A new p75(NTR)-expressing clone characterized by low spontaneous apoptosis exhibited high endogenous ceramide and low caspase levels. A marked difference between the apoptotic and resistant clones concerned the very low and high activities of nitric-oxide (NO) synthase, respectively. Protection from apoptosis by NO was confirmed by results with the NO donor S-nitrosoacetylpenicillamine and the NO-trapping agent hemoglobin. We conclude that the p75(NTR) receptor, while free of NGF, triggers a cascade leading to apoptosis; the cascade includes generation of ceramide and increased caspase activity; and the protective role of NO occurs at step(s) in between the latter events. (+info)
p75 neurotrophin receptor-mediated neuronal death is promoted by Bcl-2 and prevented by Bcl-xL.
The p75 neurotrophin receptor (p75NTR) has been shown to mediate neuronal death through an unknown pathway. We microinjected p75NTR expression plasmids into sensory neurons in the presence of growth factors and assessed the effect of the expressed proteins on cell survival. We show that, unlike other members of the TNFR family, p75NTR signals death through a unique caspase-dependent death pathway that does not involve the "death domain" and is differentially regulated by Bcl-2 family members: the anti-apoptotic molecule Bcl-2 both promoted, and was required for, p75NTR killing, whereas killing was inhibited by its homologue Bcl-xL. These results demonstrate that Bcl-2, through distinct molecular mechanisms, either promotes or inhibits neuronal death depending on the nature of the death stimulus. (+info)
Suppression of telomerase, reexpression of KAI1, and abrogation of tumorigenicity by nerve growth factor in prostate cancer cell lines.
Nerve growth factor (NGF) is expressed in the prostate, where it appears to be involved in the control of epithelial cell growth and differentiation. NGF production is decreased in prostate tumors. However, the role of this neurotrophin in the control of proliferation and progression of prostate cancers is still a matter of investigation. Prostate adenocarcinomas are telomerase-positive tumors. Chronic exposure of DU145 and PC3 prostate tumor cell lines to NGF resulted in a dramatic down-regulation of telomerase activity. This effect was correlated in terms of concentrations and time with a remarkable down-regulation of cell proliferation both in vitro and in vivo but was not secondary to NGF-induced quiescence. No down-regulation of telomerase activity was, in fact, detectable during serum starvation-induced quiescence. LNCaP cells, which do not express NGF receptors, appear to be insensitive to the actions of NGF. DU145 and PC3 cells do not express the KAI1 metastasis suppressor gene, which is present in the prostate and is progressively lost during the progression of prostate cancers. Chronic NGF treatment strongly induced the reexpression of this gene in these cell lines, and this effect was correlated with the suppression of their invasive potential in vitro. The data presented here suggest that NGF reverts two metastatic prostate cancer cell lines to slowly proliferating, noninvasive phenotypes characterized by a very low telomerase activity and by the expression of the KAI1 metastasis suppressor gene. (+info)