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

Neurotrophin dependence domain: a domain required for the mediation of apoptosis by the p75 neurotrophin receptor

The mechanisms underlying neurotrophin dependence, and cellular dependent states in general, are unknown. We show that a 29 amino acid region in the intracellular domain of the common neurotrophin receptor, p75NTR, is required for the mediation of apoptosis by p75NTR. Furthermore, contrary to results obtained with Fas, monomeric p75NTR is required for apoptosis induction, whereas multimerization inhibits the pro-apoptotic effect. Within the 29-residue domain required for apoptosis induction by p75NTR, a 14-residue region is sufficient as a peptide inducer of apoptosis. This 14-residue peptide requires the positively charged carboxyterminal residues for its effect on cell death, and these same residues are required by the full-length p75NTR. These studies define a novel type of domain that mediates neurotrophin dependence, and suggest that other cellular dependent states may be mediated by proteins displaying similar domains.

Absence of p75(NTR) expression reduces nerve growth factor immunolocalization in cholinergic septal neurons

Septal axons provide a cholinergic innervation to the nerve growth factor (NGF)-producing neurons of the mammalian hippocampus. These cholinergic septal afferents are capable of responding to target-derived NGF because they possess trkA and p75(NTR), the two transmembrane receptors that bind NGF and activate ligand-mediated intracellular signaling. To assess the relative importance of p75(NTR) expression for the responsiveness of cholinergic septal neurons to hippocampally derived NGF, we used three lines of mutant and/or transgenic mice: p75(-/-) mice (having two mutated alleles of the p75(NTR) gene), NGF/p75(+/+) mice (transgenic animals overexpressing NGF within central glial cells and having two normal alleles of the p75(NTR) gene), and NGF/p75(-/-) mice (NGF transgenic animals having two mutated alleles of the p75(NTR) gene). BALB/c and C57B1/6 mice (background strains for the mutant and transgenic lines of mice) were used as controls. Both lines of NGF transgenic mice possess elevated levels of NGF protein in the hippocampus and septal region, irrespective of p75(NTR) expression. BALB/c and C57Bl/6 mice display comparably lower levels of NGF protein in both tissues. Despite differing levels of NGF protein, the ratios of hippocampal to septal NGF levels are similar among BALB/c, C57B1/6, and NGF/p75(+/+) mice. Both p75(-/-) and NGF/p75(-/-) mice, on the other hand, have markedly elevated ratios of NGF protein between these two tissues. The lack of p75(NTR) expression also results in a pronounced absence of NGF immunoreactivity in cholinergic septal neurons of p75(-/-) and NGF/p75(-/-) mice. BALB/c, C57B1/6, and NGF/p75(+/+) mice, on the other hand, display NGF immunoreactivity that appears as discrete granules scattered through the cytoplasm of cholinergic septal neurons. Elevated levels of NGF in the hippocampus and septal region coincide with hypertrophy of cholinergic septal neurons of NGF/p75(+/+) mice but not of NGF/p75(-/-) mice. Levels of choline acetyltransferase (ChAT) enzyme activity are, however, elevated in the septal region and hippocampus of both NGF/p75(+/+) and NGF/p75(-/-) mice, compared with control mice. These data indicate that an absence of functional p75(NTR) expression disrupts the normal cellular immunolocalization of NGF by cholinergic septal neurons but does not affect the ability of these neurons to respond to elevated levels of NGF, as determined by ChAT activity.

NF-kappa B signaling promotes both cell survival and neurite process formation in nerve growth factor-stimulated PC12 cells

Nerve growth factor binds to the TrkA and p75(NTR) (p75) and generates signals leading to neuronal cell survival, differentiation, and programmed cell death. Here we describe a series of experiments involving selective activation of either TrkA or p75 in which distinct cell-signaling intermediates promote different cellular consequences. We analyzed pheochromocytoma 12 (PC12) cells stably expressing chimeras consisting of the extracellular domain of PDGF receptor (PDGFR) fused to the transmembrane and cytoplasmic segments of p75 or TrkA. Because PC12 cells lack endogenous PDGFR, addition of PDGF to these cell lines permits selective activation of the p75 or TrkA responses without stimulating endogenous receptors. Although both p75 and TrkA activated nuclear factor-kappaB (NF-kappaB), we show that distinct proximal-signaling intermediates are used by each receptor. A dominant-negative mutant of TRAF6 blocked p75- but not TrkA-mediated induction of NF-kappaB. Conversely a dominant-negative mutant of Shc inhibited TrkA but not p75 activation of NF-kappaB. Both of these distinct signaling pathways subsequently converge, leading to activation of the IkappaB kinase complex. Moreover, the activation of NF-kappaB by these distinct pathways after stimulation of either TrkA or p75 leads to different physiological consequences. Blocking p75-mediated activation of NF-kappaB by ecdysone-inducible expression of a nondegradable mutant of IkappaBalpha significantly enhanced apoptosis. In contrast, blocking NF-kappaB induction via TrkA significantly inhibited neurite process formation in PC12 cells. Together these findings indicate that, although both of these receptors lead to the activation of NF-kappaB, they proceed via distinct proximal-signaling intermediates and contribute to different cellular outcomes.

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

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

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.

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).

BDNF is needed for postnatal maturation of basal forebrain and neostriatum cholinergic neurons in vivo

Neurotrophins regulate survival, neurite outgrowth, and phenotypic maturation of developing neurons. Brain-derived neurotrophic factor (BDNF) can promote the survival of developing cholinergic forebrain neurons in vitro and reduce their degeneration following injury in adult rats. We investigated the role of endogenous BDNF during postnatal development of these cholinergic neurons by analyzing homozygous BDNF-deficient (-/-) mice and their littermates (+/+, +/-). At P6, the number of choline acetyltransferase- (ChAT) positive neurons in the medial septum was approximately 23% lower in BDNF-/- mice, although their brain and body weight was normal. At P15, control (+/+) littermates had approximately 45% more and approximately 45% larger ChAT-positive neurons and a much denser cholinergic hippocampal innervation than at P6, indicative of maturation of the septohippocampal system. In BDNF-/- mice, the number, size, and ChAT-immunostaining intensity of the cholinergic neurons remained the same between P6 and P15 (few mice survive longer). BDNF-/- mice had about three times more TUNEL-labeled (a marker of apoptosis) cells in the medial septum at P6, consistent with (but not proof of) the possibility that the cholinergic neurons were dying. The cholinergic hippocampal innervation in BDNF-/- mice expanded to a lesser extent than in controls and had reduced levels of acetylcholinesterase staining at P15. The developmental deficits were largely similar in the neostriatum of BDNF-/- mice. These findings suggest that BDNF is critical for postnatal development and maturation of cholinergic forebrain neurons.

Hepatic stellate cells express the low affinity nerve growth factor receptor p75 and undergo apoptosis in response to nerve growth factor stimulation

We have examined the expression of p75, a member of the TNF receptor superfamily in hepatic stellate cells (HSC) and pancreatic stellate cells (PSC). Activated HSC and PSC were demonstrated by Western blot analysis to express p75. p75 was immunolocalized to cells with a myofibroblast-like morphology in the fibrotic bands of six fibrotic and cirrhotic liver biopsies and three biopsies of fibrotic human pancreas. Immunostaining of parallel sections indicated that these cells were alpha-smooth muscle actin-positive, identifying them as activated HSC and PSC, respectively. HSC apoptosis in tissue culture in the presence of serum was quantified after addition of 0.1 to 100 ng/ml of nerve growth factor (NGF) a ligand for p75, by in situ counting of apoptotic bodies after addition of acridine orange. HSC demonstrated a significant increase in apoptosis in response to 100 ng/ml NGF (0.05 > P by Wilcoxon's rank; n = 7) after 24 hours. NGF 100 ng/ml had no effect on HSC proliferation, but reduced total HSC DNA by 19% relative to control after 24 hours (n = 3). These data demonstrate that activated HSC express p75 and respond to NGF stimulation by undergoing apoptosis. We therefore report p75 as a novel marker of activated HSC and suggest that signaling via ligand binding to p75 may provide a mechanism for selective apoptosis of HSC.

Nerve growth factor activation of nuclear factor kappaB through its p75 receptor is an anti-apoptotic signal in RN22 schwannoma cells

Recent evidence indicates that nerve growth factor (NGF) produces its effects through signaling contributions from both TrkA and the p75 receptor. In contrast to its trophic actions through TrkA, NGF binding to p75 has been shown to activate programmed cell death through a mechanism involving the stress kinase JNK. However, this receptor also activates nuclear factor kappaB (NF-kappaB), the role of which has yet to be determined. We investigated the function of p75-mediated NF-kappaB stimulation in regulating cell survival in the rat schwannoma cell line RN22, which expresses p75, but not TrkA. Gel shift assays demonstrated activation of NF-kappaB in response to NGF within 30 min and lasting at least 4 h. NGF also stimulated JNK in the cells (detected by in vitro kinase assays) with a similar time course. Preventing activation of NF-kappaB with the specific inhibitor SN50 resulted in NGF-induced cell loss. Similarly, transfection of the cells with a mutant form of the endogenous NF-kappaB inhibitor (IkappaBalphaDeltaN), which cannot be degraded and therefore remains bound to NF-kappaB, preventing its activation, resulted in a significant increase in the number of apoptotic cells following NGF treatment. These results suggest that NGF activation of NF-kappaB through the p75 receptor promotes survival, counterbalancing the pro-apoptotic signal.

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.

P75 nerve growth factor receptor is important for retrograde transport of neurotrophins in adult cholinergic basal forebrain neurons

The role of the p75 nerve growth factor receptor in the retrograde transport of neurotrophins in the adult CNS was investigated by comparing the transport of 125I-labeled neurotrophins by normal and p75 nerve growth factor receptor-deficient cholinergic septohippocampal neurons. In control mice, nerve growth factor was selectively transported from the hippocampal formation to the cholinergic neurons in the septum. Nerve growth factor labeling was found in three to four times as many septal cholinergic neuronal cell bodies than labeling for neurotrophin-3 or neurotrophin-4/5, and transported brain-derived neurotrophic factor was barely detectable. Cells were considered as labeled when the number of grains per cell exceeded five times background. In p75 nerve growth factor receptor-deficient mice, the number of cholinergic neurons labeled with each of the neurotrophins was reduced by 85-95%. Retrograde labeling of septohippocampal neurons with Fluorogold was not obviously reduced in p75 nerve growth factor receptor-deficient mice, suggesting that general transport mechanisms were not impaired. Despite the reduced neurotrophin transport, cholinergic neurons of p75 nerve growth factor receptor-deficient mice were larger than controls and had an apparently normal density of immunostaining for choline acetyltransferase. Since nerve growth factor is reportedly involved in size regulation and choline acetyltransferase expression, this raises the possibility that the retrograde transport itself is not essential for these events. Thus, p75 nerve growth factor receptor plays an important, although not exclusive, role in the transport of neurotrophins by cholinergic basal forebrain neurons, and retrograde transport of nerve growth factor may not be needed for regulating certain cellular processes.

Overexpression of neurotrophin receptor p75 contributes to the excitotoxin-induced cholinergic neuronal death in rat basal forebrain

Both excitotoxicity and altered trophic factor support have been implicated in the pathogenesis of Alzheimer's disease. To determine whether stimulation of p75, the low-affinity receptor for nerve growth factor, contributes to the excitotoxin-induced apoptotic death of cholinergic neurons, we examined the effect of unilateral kainic acid (KA; PBS vehicle, 1.25, 2.5 and 5.0 nmol) administration into rat basal forebrain on neuronal loss and p75 expression. KA (2. 5 nmol) destroyed 43% of Nissl-stained neurons and 70% of choline acetyltransferase (ChAT)-positive neurons 5 days after injection. Agarose gel electrophoresis revealed that KA (2.5 nmol) induced local internucleosomal DNA fragmentation after 6-48 h. Immunohistochemical analysis further showed that KA (2.5 nmol) augmented p75 immunoreactivity at a time when terminal transferase-mediated deoxyuridine trophosphate (d-UTP)-digoxigenin nick end labeling (TUNEL)-positive nuclei were increased. Many fragmented nuclei were co-labeled with ChAT antibody. The chronic administration of anti-rat p75 or the protein synthesis inhibitor, cycloheximide, but not anti-human p75, substantially reduced the KA-induced destruction of cholinergic neurons and the induction of internucleosomal DNA fragmentation. Anti-rat p75, but not cycloheximide, also reversed the spatial memory impairment produced by KA. These findings suggest that overexpression of p75 contributes to the excitotoxin-induced death of rat basal forebrain cholinergic neurons by an apoptotic-like mechanism.

Signaling pathway for apoptosis: a racetrack for life or death

Apoptosis, or programmed cell death, is a gene-directed mechanism activated as a suicidal event to get rid of excess, damaged, or infected cells. The recent astounding pace of research in this area has expanded our horizon of understanding that this mechanism is regulated largely by pro- and anti-apoptosis factors acting for or against the final death event. The driving force behind these factors, either pro-apoptosis or pro-survival, is largely determined by signal transduction pathways, starting with the initiation of a death signal at the plasma membrane, and following through a complex cytoplasmic network before reaching the end point of cell demise. Enmeshed in this intricate cytoplasmic network are many checkpoints, where complexes of pro- and anti-apoptosis factors function to facilitate or deter the death signals. The culmination of the balancing act between these two camps of factors at these signal transduction checkpoints may then result in the final decision to die or to live. Thus, the eventual death of a cell may require successful passage through all the checkpoints, a mechanism Nature has provided as a safeguard to prevent erroneous triggering of death. With the advent of a new biotechnology revolution at the dawn of the new millenium, we look forward to an exciting era when we can gain fuller understanding of the operation of all these checkpoints. Ultimately, this gain will pave the way to control the apoptosis event at the checkpoints, and to support the organism's functionality as long as possible. J. Cell. Biochem. Suppls. 32/33:95-102, 1999.

Early postnatal Müller cell death leads to retinal but not optic nerve degeneration in NSE-Hu-Bcl-2 transgenic mice

Topographically localized over-expression of the human Bcl-2 protein in retinal glial Müller cells of a transgenic mice (line 71) leads to early postnatal apoptotic Müller cell death and retinal degeneration. Morphological, immunohistological and confocal laser microscopic examination of transgenic and wild-type retinas were achieved on paraffin retinal sections, postnatally. Apoptosis occurs two to three days earlier in the internal nuclear layer of transgenic retinae, than in wild-type littermates. In parallel there was a progressive disappearance of transgenic Hu-Bcl-2 over-expression, as well as of the Müller cell markers, cellular retinaldehyde-binding protein and glutamine synthetase. This phenomenon led to retinal dysplasia, photoreceptor apoptosis and then retinal degeneration and proliferation of the retinal pigment epithelium. The optic nerve, however, remains intact. Two complementary observations confirm the pro-apoptotic action of Bcl-2 over-expression in Müller cells: (i) in the peri-papillary and peripheral regions where the transgene Bcl-2 is not expressed, cellular retinaldehyde-binding protein or glutamine synthetase immunostaining persist and Müller glia do not die; and (ii) the retina conserves a normal organisation in these two regions in spite of total retinal degeneration elsewhere. We conclude that retinal dysplasia and degeneration are linked to primary Müller cell disruption. Besides its generally accepted anti-apoptotic function, over-expression of Bcl-2 also exerts a pro-apoptotic action, at least in immature Müller glia. One may suppose that Bcl-2 translocation resulting in its over-expression in retinal Müller cells could be a putative mechanism for early retinal degeneration.

Cholinergic medial septum neurons do not degenerate in aged 129/Sv control or p75(NGFR)-/-mice

Cholinergic medial septum neurons express TrkA and p75 nerve growth factor receptor (p75(NGFR)) and interactions between TrkA and p75(NGFR) are necessary for high-affinity binding and signaling of nerve growth factor (NGF) through TrkA. In adult p75(NGFR)-deficient (-/-) mice, retrograde transport of NGF and other neurotrophins by these neurons is greatly reduced, however, these neurons maintain their cholinergic phenotype and size. Reduced transport of NGF has been proposed to play a role in Alzheimer's disease. Here, we investigated whether chronic and long-term absence of p75(NGFR) (and possibly reduced NGF transport and TrkA binding) would affect the cholinergic septohippocampal system during aging in mice. In young (6-8 months), middle aged (12-18 months), and aged (19-23 months) 129/Sv control mice the total number of choline acetyltransferase-positive medial septum neurons and the mean diameter and cross sectional area of the cholinergic cell bodies were similar. The cholinergic hippocampal innervation, as measured by the density of acetylcholinesterase-positive fibers in the outer molecular layer of the dentate gyrus was also similar across all ages. These parameters also did not change during aging in p75(NGFR) -/- mice and the number and size of the choline acetyltransferase-positive neurons and the cholinergic innervation density were largely similar as in control mice at all ages. These results suggest that p75(NGFR) does not play a major role in the maintenance of the number or morphology of the cholinergic basal forebrain neurons during aging of these mice. Alternatively, p75(NGFR) -/- mice may have developed compensatory mechanisms in response to the absence of p75(NGFR).

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.

p75(NGFR) and cholinergic neurons in the developing forebrain: a re-examination

The low-affinity nerve growth factor receptor (p75(NGFR)) apparently can mediate apoptosis in a variety of cells in vitro and in vivo. Previously, our laboratory suggested that p75(NGFR) induced apoptosis in a subpopulation of cholinergic forebrain neurons during postnatal development, i.e., the number of choline acetyltransferase (ChAT)-positive neurons in a control strain of mice decreased whereas it remained higher in p75(NGFR)-deficient (-/-) mice. Discrepancies with subsequent data sets in our laboratory caused us to thoroughly re-analyze the fate of these cholinergic medial septum and neostriatal neurons in new sets of p75(NGFR) -/- and two DNA control strains of mice during development. Between postnatal day (P)6 and P15 the number of ChAT-positive neurons detected in the medial septum of 129/Sv mice and Balb/c mice increased by approximately 64% and approximately 62%, respectively. This increase is contrary to previous reports from our laboratory and indicative of normal postnatal development (including an increase in ChAT-enzyme) of the cholinergic forebrain neurons. In p75(NGFR) -/- mice the number of ChAT-positive neurons in the medial septum remained constant between P6 and P15 and was approximately 31% and approximately 56% higher at P6 than 129/Sv and Balb/c mice, respectively. At P15 and adulthood, p75(NGFR) -/- mice had similar numbers of cholinergic neurons as control mice. In the developing neostriatum, the number of ChAT-positive neurons increased by approximately 56% between P6 and P15 and did not differ between p75(NGFR) -/- and control mice at any time. Analyses for apoptotic DNA fragmentation (TUNEL labeling) at P8 revealed no differences between p75(NGFR) -/- and control mice in 12 forebrain regions, including the septum and neostriatum. At all times, all mice had similar levels of acetylcholinesterase-positive cholinergic innervation of the molecular layer in the dorsal dentate gyrus. These findings suggest that the p75(NGFR) does not necessarily mediate apoptosis in medial septum or neostriatal cholinergic neurons during the postnatal time period. The discrepant results of the previous study are most likely due to a less rigorous application of criteria for data acquisition, including anatomical boundaries that define the nucleus.

Gene expression along the cerebral-spinal axis after regional gene delivery

We demonstrate here that intracerebroventricular or spinal cord (intrathecal) injection of either plasmid DNA alone or cationic liposome: DNA complexes (CLDCs) produces significant levels of expression of both reporter genes and biologically relevant genes in nonparenchymal cells lining both the brain and the spinal cord. Gene expression was identified both within the spinal cord and the brain after intracerebroventricular or intrathecal injection of either CLDCs or plasmid DNA alone. Intracerebroventricular or intrathecal injection of CLDCs containing the beta-galactosidase (beta-Gal) gene produced patchy, widely scattered areas of beta-Gal expression. The chloramphenicol acetyltransferase (CAT) reporter gene product reached peak levels between 24 hr and 1 week postinjection, and was still present at significant levels 3 weeks after a single intracerebroventricular or intrathecal injection. Intrathecal injection of the human granulocyte colony-stimulating factor (G-CSF) gene produced high levels of hG-CSF activity in both the spinal cord and the brain. Intracerebroventricular injection of CLDCs containing the murine nerve growth factor (NGF) gene increased mNGF levels in the hippocampus, a target region for cholinergic neurons in the medial septum, and increased cholinergic neurotransmitter synthetic enzyme choline acetyltransferase (ChAT) activity within the brain, a well-characterized effect of both purified and recombinant NGF protein. These findings indicate that intracerebroventricular or intrathecal injection of CLDCs can produce significant levels of expression of biologically and therapeutically relevant genes within the CNS. Efficient gene transfer into the CNS will facilitate the evaluation of gene function and regulation within the brain and spinal cord. We attempted to transfer and express genes within the brain and spinal cord by direct CNS injection of either DNA alone or CLDCs into the intraventricular and subarachnoid compartments. We show that intracerebroventricular or spinal cord (intrathecal) injection of either plasmid DNA alone or CLDCs produces significant levels of expression of both reporter genes and biologically relevant genes in nonparenchymal cells lining both the brain and the spinal cord. Intrathecal injection of the hG-CSF gene produced high levels of hG-CSF activity in both the spinal cord and the brain. Intracerebroventricular injection of CLDCs containing the murine NGF gene increased mNGF levels in the hippocampus, and increased cholinergic neurotransmitter synthetic enzyme ChAT activity within the brain. Locoregional diffusion of gene products expressed by transfected meningeal lining cells into brain and spinal cord parenchyma could potentially target secreted proteins within brain and spinal cord regions relevant to neuropathological states while limiting peripheral side effects.

Functional interaction of Fas-associated phosphatase-1 (FAP-1) with p75(NTR) and their effect on NF-kappaB activation

The common neurotrophin receptor p75(NTR), a member of the tumor necrosis factor (TNF) receptor superfamily, plays an important role in several cellular signaling cascades, including that leading to apoptosis. FAP-1 (Fas-associated phosphatase-1), which binds to the cytoplasmic tail of Fas, was originally identified as a negative regulator of Fas-mediated apoptosis. Here we have shown by co-immunoprecipitation that FAP-1 also binds to the p75(NTR) cytoplasmic domain in vivo through the interaction between the third PDZ domain of FAP-1 and C-terminal Ser-Pro-Val residues of p75(NTR). Furthermore, cells expressing a FAP-1/green fluorescent protein showed intracellular co-localization of FAP-1 and p75(NTR) at the plasma membrane. To elucidate the functional role of this physical interaction, we examined TRAF6 (TNF receptor-associated factor 6)-mediated NF-kappaB activation and tamoxifen-induced apoptosis in 293T cells expressing p75(NTR). The results revealed that TRAF6-mediated NF-kappaB activation was suppressed by p75(NTR) and that the p75(NTR)-mediated NF-kappaB suppression was reduced by FAP-1 expression. Interestingly, a mutant of the p75(NTR) intracellular domain with a single substitution of a Met for Val in its C-terminus, which cannot interact with FAP-1, displayed enhanced pro-apoptotic activity in 293T transfected cells. Thus, similar to Fas, FAP-1 may be involved in suppressing p75(NTR)-mediated pro-apoptotic signaling through its interaction with three C-terminal amino acids (tSPV). Thus, FAP-1 may regulate p75(NTR)-mediated signal transduction by physiological interaction through its third PDZ domain.

TRAF family proteins interact with the common neurotrophin receptor and modulate apoptosis induction

The common neurotrophin receptor, p75(NTR), has been shown to signal in the absence of Trk tyrosine kinase receptors, including induction of neural apoptosis and activation of NF-kappaB. However, the mechanisms by which p75(NTR) initiates these intracellular signal transduction pathways are unknown. Here we report interactions between p75(NTR) and the six members of TRAF (tumor necrosis factor receptor-associated factors) family proteins. The binding of different TRAF proteins to p75(NTR) was mapped to distinct regions in p75(NTR). Furthermore, TRAF4 interacted with dimeric p75(NTR), whereas TRAF2 interacted preferentially with monomeric p75(NTR). TRAF2-p75(NTR), TRAF4-p75(NTR), and TRAF6-p75(NTR) interactions modulated p75(NTR)-induced cell death and NF-kappaB activation with contrasting effects. Coexpression of TRAF2 with p75(NTR) enhanced cell death, whereas coexpression of TRAF6 was cytoprotective. Furthermore, overexpression of TRAF4 abrogated the ability of dimerization to prevent the induction of apoptosis normally mediated by monomeric p75(NTR). TRAF4 also inhibited the NF-kappaB response, whereas TRAF2 and TRAF6 enhanced p75(NTR)-induced NF-kappaB activation. These results demonstrate that TRAF family proteins interact with p75(NTR) and differentially modulate its NF-kappaB activation and cell death induction.

NGF facilitates the developmental maturation of the previously committed cholinergic interneurons in the striatal matrix

Although all of the cholinergic interneurons of the striatum are generated early in development, the maturation of these neurons depends on striatal compartmental localization. The majority of the cholinergic neurons in the patches turn on choline acetyltransferase (CHAT) embryonically, whereas the majority of cholinergic neurons in the matrix turn on CHAT postnatally. To determine whether CHAT expression can be induced earlier in the cholinergic neurons and whether the facilitation is compartment specific, we infused nerve growth factor (NGF) into the lateral ventricle of either embryonic day 19 embryos or postnatal day 1 pups. We simultaneously marked the patch compartment by injecting the retrograde fluorescent tracer True Blue into the substantia nigra at the times of the NGF infusions. After a 2-day survival time, NGF induced a dramatic increase in the number of CHAT-immunoreactive neurons in the matrix compartment (up to adult levels), whereas the NGF infusions did not increase the number of CHAT neurons in the patch compartment. Analyses of the compartmental distributions of the p75 and trkA NGF receptors themselves do not provide an explanation for the differential cholinergic maturation in the compartments of the control striatum or for the upregulation of CHAT in the striatal matrix after the NGF infusion. We conclude that NGF infusion is capable of facilitating the normally slow cholinergic maturation of the cholinergic neurons in the matrix, whereas the cholinergic maturation of the CHAT cells in the patch compartment seems to be largely independent of NGF signalling.

The anti-p75 antibody, MC192, and brain-derived neurotrophic factor inhibit nerve growth factor-dependent neurite growth from adult sensory neurons

We have investigated nerve growth factor-dependent neurite growth from adult sensory neurons using the compartmented culture system. The requirement of both TrkA and the p75 neurotrophin receptors in neurite growth was examined using several experimental interventions. Inhibition of TrkA activation using K252a resulted in a total block of distal neurite extension into nerve growth factor-containing compartments. Brain-derived neurotrophic factor and the anti-p75 monoclonal antibody MC192 have been shown to interfere with the binding of nerve growth factor to p75. Brain-derived neurotrophic factor, which binds p75 but not TrkA, competes with nerve growth factorforp75, while the anti-p75 antibody MC192 has been shown to decrease the interaction of nerve growth factor with TrkA. The addition of brain-derived neurotophic factor to nerve growth factor-containing distal compartments inhibited, but did not totally block, distal neurite extension. MC192, on the other hand, totally inhibited nerve growth factor-dependent neurite growth. To test whether MC192 and brain-derived neurotrophic factor might be influencing Trk activation, TrkA phosphorylation was examined biochemically. Both compounds were found to attenuate nerve growth factor-induced Trk phosphorylation, although neither inhibited the activation completely. The possibility that MC192 or brain-derived neurotrophic factor might activate p75 signaling directly (and potentially antagonize TrkA signaling) was also investigated. This was assessed by quantitating the activation and nuclear translocation of the transcription factor NFkB using immunocytochemistry. Only treatment with the anti-p75 antibody MC192 resulted in prolonged and significant increase in the number of neurons displaying nuclear staining for NFkB. Our results demonstrate that both TrkA and p75 play a role in neurite growth response to nerve growth factor, and further suggest that any alteration in optimal TrkA-p75 interactions, or direct activation of p75 at the expense of TrkA, results in an inhibition of nerve growth factor-dependent neurite growth in adult sensory neurons.

Selective regulation of trkC expression by NT3 in the developing peripheral nervous system

We have studied the influence of neurotrophin-3 (NT3) on the expression of its receptor tyrosine kinase, trkC, in embryonic mice. The expression of trkC transcripts encoding full-length and kinase-deficient receptors was almost entirely restricted to neurons in the trigeminal ganglion and increased markedly throughout development. In NT3(+/-) embryos, the level of trkC mRNA in the trigeminal ganglion was much lower than that in wild-type embryos, although there was no significant reduction in the total number of neurons in the ganglion. This demonstrates that endogenous NT3 regulates trkC expression in trigeminal neurons independently of changes in population size. In NT3(-/-) embryos, the number of neurons in the trigeminal ganglion was much lower than in wild-type embryos, and there was a further reduction in the mean neuronal level of trkC mRNA. Direct regulation of trkC mRNA expression in cultured trigeminal neurons was also observed, although the finding that trkC mRNA levels were sustained better in explant cultures than in dissociated cultures irrespective of the presence of NT3 suggests that trkC mRNA expression is regulated by additional factors within the ganglion. In contrast to trigeminal neurons, the level of trkC mRNA was sustained at normal levels in neurons of the sympathetic chain of NT3(-/-) embryos and was not increased by NT3 in sympathetic neuron cultures. TrkC mRNA expression in developing cutaneous tissues was also unaffected by the NT3 null mutation. In summary, our findings provide the first clear evidence that the expression of a trk receptor, tyrosine kinase, is regulated by physiological levels of its ligand in vivo and show that regulation by NT3 is cell type-specific.

Signaling of neuronal cell death by the p75NTR neurotrophin receptor

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

p75-mediated NF-kappaB activation enhances the survival response of developing sensory neurons to nerve growth factor

We have investigated whether the transcription factor NF-kappaB plays a role in regulating neuronal survival by manipulating NF-kappaB activation in the nerve growth factor (NGF)-dependent sensory neurons of the embryonic mouse trigeminal ganglion. Overexpression of either the p65 or the p50 NF-kappaB subunits resulted in NF-kappaB activation and promoted in vitro survival as effectively as NGF. Expression of a superrepressor IkappaB-alpha protein prevented NF-kappaB activation in p65/p50-overexpressing neurons and caused the neurons to die as rapidly as NGF-deprived neurons. NGF treatment also activated NF-kappaB, and preventing this activation with superrepressor IkappaB-alpha reduced the NGF survival response. Antibodies that block binding of NGF to the p75 receptor prevented NGF-induced NF-kappaB activation and reduced the NGF survival response to the same extent as superrepressor IkappaB-alpha. Trigeminal neurons cultured from p65(-/-) embryos showed a reduced survival response to NGF compared with neurons from wild-type embryos and there was increased apoptosis of neurons in the trigeminal ganglia of p65(-/-) embryos in vivo. However, as with p75-deficient sensory neurons, p65-deficient sensory neurons showed a normal survival response to BDNF. These results reveal a role for NF-kappaB in regulating neuronal survival during embryonic development and suggest that in addition to the well-established Trk receptor tyrosine kinase signaling cascade, NGF enhances neuronal survival by signaling via a p75-mediated pathway.

Functionally antagonistic interactions between the TrkA and p75 neurotrophin receptors regulate sympathetic neuron growth and target innervation

In this report, we provide evidence that NGF and BDNF have functionally antagonistic actions on sympathetic neuron growth and target innervation, with NGF acting via TrkA to promote growth and BDNF via p75NTR to inhibit growth. Specifically, in cultured sympathetic neurons that themselves synthesize BDNF, exogenous BDNF inhibits and function-blocking BDNF antibodies enhance process outgrowth. Both exogenous and autocrine BDNF mediate this effect via p75NTR because (1) BDNF does not inhibit growth of neurons lacking p75NTR, (2) function-blocking p75NTR antibodies enhance NGF-mediated growth, and (3) p75NTR-/- sympathetic neurons grow more robustly in response to NGF than do their wild-type counterparts. To determine the physiological relevance of this functional antagonism, we examined the pineal gland, a well defined sympathetic target organ. BDNF is present in the pineal gland during target innervation, and incoming sympathetic axons are p75NTR positive. Moreover, the pineal glands of BDNF+/- and BDNF-/- mice are hyperinnervated with sympathetic fibers, and tyrosine hydroxylase (TH) levels are elevated. Increased tyrosine hydroxylase is also observed in the BDNF+/- carotid artery, another sympathetic neuron target. Thus, BDNF, made by sympathetic neurons and/or their target organs, acts via p75NTR to antagonize NGF-mediated growth and target innervation, suggesting that sympathetic target innervation is determined by the balance of positively and negatively acting neurotrophins present in developing and potentially mature targets.

Septal cholinergic deafferentation of the dentate gyrus results in a loss of a subset of neuropeptide Y somata and an increase in synaptic area on remaining neuropeptide Y dendrites

Removal of cholinergic septal inputs using the immunotoxin 192 IgG-saporin reduces the number of interneurons containing neuropeptide Y (NPY)-immunoreactivity in the rat dentate gyrus by approximately 30% [Milner et al., J. Comp. Neurol. 386 (1997) 48-59]. The goal of the present study was to determine if NPY-containing neurons that survive deafferentation have any distinguishing morphological and/or microenvironmental features. For this, 2 or 24 weeks after intracerebroventricular injections of 192 IgG-saporin, NPY-immunolabeled neurons in the hilus of the dentate gyrus were examined by electron microscopy. Neither the size nor morphological traits of NPY-labeled perikaryal or dendritic profiles from lesioned compared to control rats at either time-point differed significantly. However, at both time-points, NPY-containing somatal profiles from immunolesioned rats compared to controls had a reduced percentage of their plasmalemmal surface apposed to unmyelinated axon profiles and an increased percentage of their surface occupied by astrocytic profiles. At the 24 week time-point, these differences were statistically significant. The primary contributing factor for these changes was the absence of a subgroup of NPY-labeled somatal profiles in lesioned rats compared to controls which was: (a) distinguished by frequent appositions of unmyelinated axons (from 15 to 35%) to the plasmalemmal surface; and (b) located primarily in the central hilar region. Unlike NPY-containing somata, changes associated with NPY-labeled dendritic profiles were exclusively related to associated presynaptic profiles at the 24 week time-point. In lesioned rats compared to controls at this time-point, NPY-containing dendritic profiles had a concurrent increase in the percentage of the plasmalemmal surface occupied by active zones and the size of terminals contacting them. The present results combined with those of our earlier study suggest that septal cholinergic deafferentation results in: (a) the loss of a distinct subpopulation of hippocampal NPY-containing neurons; and (b) an increase in total active zone area suggesting a strengthening of synaptic connections to the surviving population of NPY-containing neurons in the long term.

p75 neurotrophin receptor as a modulator of survival and death decisions

The p75 receptor is the founding member of the TNF receptor superfamily. Members in this receptor family share a common cysteine motif repeated two to six times that serves as the ligand binding domain. In addition, several members contain a cytoplasmic region designated the death domain. The neurotrophins NGF, BDNF, NT-3, and NT-4 each bind to the p75 receptor and also more selectively to members of the Trk family of receptor tyrosine kinases. Although the biological functions of p75 have been elusive, recent experimental evidence supports an involvement of this receptor in apoptosis. This presents a counter-intuitive function for neurotrophins, which are normally required for the survival of neurons during development. The life-and-death decisions by neurotrophins appear to be governed by the level of expression and signaling activities of the p75 and Trk tyrosine kinase receptors and their downstream effector molecules. The generation of the correct number of cells in the nervous system is a highly controlled and coordinated process that is the consequence of cell proliferation and cell death decisions. The appropriate number of neuronal and glial cells formed during development guarantees the establishment of proper innervation and functional synaptic connections. One common mechanism to account for the number of viable cells is the ability to form ligand-receptor complexes that promote cell survival under conditions of limiting concentrations of trophic factors. Another diametrically opposed mechanism is to produce ligand-receptor interactions that can activate programmed cell death directly.

The role of p75NTR in modulating neurotrophin survival effects in developing motoneurons

Neurotrophins exert their biological functions on neuronal cells through two types of receptors, the trk tyrosine kinases and the low-affinity neurotrophin receptor (p75NTR), which can bind all neurotrophins with similar affinity. The p75NTR is highly expressed in developing motoneurons and in adult motoneurons after axotomy, suggestive of a physiological role in mediating neurotrophin responses under such conditions. In order to characterize this specific function of p75NTR, we have tested the effects of nerve growth factor (NGF) on embryonic motoneurons from control and p75NTR-deficient mice. NGF antagonizes brain-derived neurotrophic factor (BDNF)- and neurotrophin-3 (NT-3)-mediated survival in control but not p75NTR-deficient motoneurons. Survival of cultured motoneurons in the presence of 0.5 ng/mL of either ciliary neurotrophic factor (CNTF) or glial-derived neurotrophic factor (GDNF) was not reduced by 20 ng/mL NGF. Dose-response investigations revealed that five times higher concentrations of BDNF are required for half-maximal survival of p75NTR-deficient motoneurons in comparison to motoneurons from wild-type controls. After facial nerve lesion in newborn wild-type mice, local administration of NGF reduced survival of corresponding motoneurons to less than 2% compared to the unlesioned control side. In p75NTR-deficient mice, the same treatment did not enhance facial motoneuron death on the lesioned side. In the facial nucleus of 1-week-old p75NTR -/- mice, a significant reduction of motoneurons was observed at the unlesioned side in comparison to p75NTR +/+ mice. The observation that motoneuron cell numbers are reduced in the facial nucleus of newborn p75NTR-deficient mice suggests that p75NTR might not function as a physiological cell death receptor in developing motoneurons.

p75NTR immunoreactivity in the rat dentate gyrus is mostly within presynaptic profiles but is also found in some astrocytic and postsynaptic profiles

To localize neurotrophin binding sites within the rat dentate gyrus, the distribution of low-affinity p75 neurotrophin receptor (p75NTR) immunoreactivity (IR) was examined by using antiserum raised against the cytoplasmic domain of the receptor. Semiquantitative electron microscopic examination of p75NTR-labeled sections showed that most p75NTR-labeled profiles were axons and axon terminals (72% from a total of 3,975); p75NTR-IR was observed throughout the extent of these structures and was not limited to the plasmalemmal surface. Axons and axon terminals containing p75NTR-IR were distributed in approximately equal proportions across the hilus, infragranular zone, and the inner, middle, and outer molecular layers; significantly fewer p75NTR-labeled profiles were observed in the granule cell layer. Axon terminals containing p75NTR-IR, which made synapses (296 of 552), formed equal proportions of symmetric and asymmetric synapses, primarily with the shafts and spines of dendrites. The remainder of the p75NTR-labeled terminals apposed unlabeled somata and dendrites without forming synapses in the single sections analyzed. In addition, p75NTR-IR was contained within some astrocytes (17.5% of 3,975) and dendritic shafts (3%) and spines (5%). Within dendritic spines, p75NTR-IR was most often associated with the plasmalemmal surface near postsynaptic densities; in dendritic shafts, p75NTR labeling was associated with microfilaments distant from the plasmalemma. Most p75NTR-labeled dendritic profiles were located in the molecular layer, and some originated from granule cells. Moreover, in some granule cell somata (<1% of 3,975), p75NTR-IR was associated with endosomes. The primary localization of p75NTR-IR to presynaptic structures in the dentate gyrus, presumably arising from medial septal/diagonal band neurons, agrees with previous reports. However, p75NTR-IR within some astrocytes, somata, and dendritic structures suggests that this receptor may also be involved in controlling local neurotrophin levels and possibly modulating the viability of local hippocampal cell populations.

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.

Progress toward valid transgenic mouse models for Alzheimer's disease

A transgenic mouse model for Alzheimer's disease (AD) should mimic the age-dependent accumulation of beta-amyloid plaques, neurofibrillary tangles, neuronal cell death as well as display memory loss and behavioral deficits. Age-dependent accumulation of A beta deposits in mouse brain has been achieved in mice overexpressing mutant alleles of the amyloid precursor protein (APP). In contrast, mice bearing mutant alleles of the presenilin genes show increased production of the A beta42 peptide, but do not form amyloid deposits unless mutant alleles of APP are also overproduced. Furthermore, the onset of A beta deposition is greatly accelerated, paralleling the involvement of presenilins in early onset AD. Studies of APP and presenilin transgenic mice have shown 1) the absence of a requirement for a maturation step in dense core plaque formation, 2) evidence that beta-amyloid deposition is directed by regional factors, and 3) behavioral deficits are observed before A beta deposition. Crosses of APP transgenic mice with mice modified for known AD risk factors and "humanizing" the mouse may be necessary for complete replication of AD.

Genetic evidence for cell death mediated by nerve growth factor and the neurotrophin receptor p75 in the developing mouse retina and spinal cord

The role of nerve growth factor (NGF) and of the neurotrophin receptor p75 (p75(NTR)) in programmed cell death was investigated in the retina and the spinal cord of mouse embryos. Large numbers of cells express p75(NTR) in and along the developing optic nerve and in the mantle zone of the spinal cord. In embryos carrying deletions in the ngf or the p75(NTR) gene, cell death was reduced in the retina and in the spinal cord. Increased numbers of Islet-1-immunoreactive cells were detected in the dorsal spinal cord, and the mantle zone was enlarged in both mutants. These results indicate that NGF/p75(NTR)-dependent mechanisms are used to remove cells when axonal tracts elongate in developing neuroepithelia.

Central neuronal loss and behavioral impairment in mice lacking neurotrophin receptor p75

The neurotrophin receptor p75 is a low-affinity receptor that binds neurotrophins. To investigate the role of p75 in the survival and function of central neurons, p75 null-mutant and wild type litter mate mice were tested on behavioral tasks. Null mutants showed significant performance deficits on water maze, inhibitory avoidance, motor activity, and habituation tasks that may be attributed to cognitive dysfunction or may represent a global sensorimotor impairment. The p75 null-mutant and wild type litter mate mice were assessed for central cholinergic deficit by using quantitative stereology to estimate the total neuronal number in basal forebrain and striatum and for subpopulations expressing the high-affinity tyrosine receptor kinase A (trkA) neurotrophin receptor and choline acetyltransferase (ChAT). In the adult brain, cholinergic neurons of the basal forebrain receive target-derived trophic support, whereas cholinergic striatal neurons do not. Adult p75 null-mutant mice had significant reduction of basal forebrain volume by 25% and had a corresponding significant loss of 37% of total basal forebrain neurons. The basal forebrain population of ChAT-positive neurons in p75-deficient mice declined significantly by 27%, whereas the trkA-positive population did not change significantly. There was no significant change in striatal volume or in striatal neuronal number either in total or by cholinergic subpopulation. These results demonstrate vulnerability to the lack of p75 in adult central neurons that are neurotrophin dependent. In addition, the loss of noncholinergic central neurons in mice lacking p75 suggests a role for p75 in cell survival by an as yet undetermined mechanism. Possible direct and indirect effects of p75 loss on neuronal survival are discussed.

Association of the p75 neurotrophin receptor with TRAF6

In addition to the Trk tyrosine kinase receptors, neurotrophins also bind to a second receptor, p75, a member of the tumor necrosis factor receptor superfamily. Several signaling pathways have been implicated for p75 in the absence of Trk receptors, including induction of NF-kappaB and c-Jun kinase activities and increased production of ceramide. However, to date, the mechanisms by which the p75 receptor initiates intracellular signal transduction have not been defined. Here we report a specific interaction between p75 and TRAF6 (tumor necrosis factor receptor-associated factor-6) after transient transfection in HEK293T cells. The interaction was ligand-dependent and maximal at 100 ng/ml of nerve growth factor (NGF). Other neurotrophins also promoted the association of TRAF6 with p75 but to a lesser extent. The binding of TRAF6 was localized to the juxtamembrane region of p75 by co-immunoprecipitation and Western blotting. To assess the functional significance of this interaction, we have tested responses in cultured Schwann cells that express p75 and TRAF6. An NGF-mediated increase in the nuclear localization of the p65 subunit of NF-kappaB could be blocked by the introduction of a dominant negative form of TRAF6 in Schwann cells. These results indicate that TRAF6 can potentially function as a signal transducer for NGF actions through the p75 receptor.

Dexamethasone induces hypertrophy of developing medial septum cholinergic neurons: potential role of nerve growth factor

Glucocorticoid hormones influence neuronal plasticity during development; however little is known about the mechanisms of this trophic activity. Because glucocorticoids increase nerve growth factor (NGF) synthesis in selected brain areas and NGF plays a role in the development of basal forebrain cholinergic neurons, we tested the hypothesis that glucocorticoids may foster maturation of the cholinergic phenotype during postnatal development via the induction of NGF biosynthesis. The synthetic glucocorticoid dexamethasone (DEX) was injected systemically (0.5 mg/kg, s.c.) once a day for 1 week in 7-d-old (P7) rats. DEX elicited an increase in NGF mRNA and protein levels in the cerebral cortex and hippocampus as well as specific NGF responses, such as TrkA tyrosine phosphorylation in the septum, choline acetyltransferase (ChAT) and p75 neurotrophin receptor (p75NTR) immunoreactivity, and a relative number of cholinergic neurons in the medial septum. To examine whether the effect of DEX is age-related, we treated 1- and 14-d-old rats with DEX for 1 week. DEX increased NGF expression in rats treated from P1 to P8 but not in those treated from P14 to P21. The age-related increased expression of NGF correlated with the induction of ChAT immunoreactivity in the medial septum. Moreover, in the spinal cord, neither NGF nor ChAT levels were increased by DEX, suggesting that the glucocorticoid-mediated changes seen in the basal forebrain are associated with specific NGF responses. Our data suggest that by increasing NGF levels, glucocorticoids may play a role in the maturation of postnatal cholinergic neurons.

Anterograde transport of neurotrophin proteins in the CNS--a reassessment of the neurotrophic hypothesis

The basic tenets of the neurotrophic hypothesis are that i) limiting quantities of a given factor are produced in specific target tissues; ii) responsive neurons projecting to these targets compete for the limiting amounts of the factor; iii) the factor is bound within the target by selective receptors on afferent terminals, internalized, and retrogradely transported to the neuronal cell body where it provides signals affecting neuronal survival and differentiation. Although originally formulated on the basis of evidence for NGF's actions on peripheral sensory and sympathetic neurons, the neurotrophic hypothesis appeared to be upheld for CNS neuronal systems as well, where NGF was found to function primarily as a target-derived trophic factor for basal forebrain cholinergic neurons. With the discovery of additional neurotrophins sharing considerable structural homology with NGF, the question arose of whether the neurotrophic hypothesis held true for all members of this protein family. Recent investigations into the localization and function of neurotrophins other than NGF, particularly BDNF and NT-3, have provided evidence indicating that these molecules may not act in a manner consistent with the neurotrophic hypothesis, as originally postulated. Numerous studies in the peripheral and central nervous systems have now demonstrated that BDNF (and NT-3) may be preferentially trafficked anterogradely along axonal processes and stored within pre-synaptic terminals. Other studies have suggested that these factors may be released in an activity-dependent, rather than constitutive, manner and can act in autocrine or paracrine fashions to subserve an assortment of biological functions including anterograde effects on cell survival and differentiation, as well as more novel roles in synaptic transmission. These recent findings strongly suggest that, while the various neurotrophin proteins may be grouped into a single family based upon their structural homology, they should be considered as a heterogeneous group of trophic factors based upon function and mode of action.

TrkB signalling inhibits p75-mediated apoptosis induced by nerve growth factor in embryonic proprioceptive neurons

Neurotrophins mediate their effects by binding to members of the Trk family of receptor tyrosine kinases and the neurotrophin receptor p75 [1]. Whereas Trks are essential for the trophic effects of neurotrophins [1], p75 has distinct functions in different cells. For example, it enhances the survival response of certain neurons to nerve growth factor (NGF) [2], but mediates a cytotoxic response to NGF in certain other cell types and neurons [3] [4] [5] [6]. We investigated whether the p75-mediated responses to NGF can be modulated through the activation of different signalling pathways in the same neurons. Neurons of the embryonic trigeminal mesencephalic nucleus (TMN) are supported in culture by brain-derived neurotrophic factor (BDNF) and an unrelated neurotrophic factor, ciliary neurotrophic factor (CNTF), but not by NGF [7] [8] [9]. We found that NGF killed TMN neurons that were grown in the presence of CNTF; this effect of NGF was inhibited by anti-p75 antibodies and therefore occurred via a p75-dependent mechanism. NGF did not affect the survival of neurons grown in the presence of BDNF, and very low concentrations of BDNF inhibited NGF cytotoxicity. These results indicate that the activation of different signalling pathways in TMN neurons influences their susceptibility to p75-mediated NGF cytotoxicity.

p75NGFR mediates death of cholinergic neurons during postnatal development of the neostriatum in mice

We have previously shown that p75 nerve growth factor receptor (p75NGFR) mediates apoptosis of approximately 25% of the cholinergic basal forebrain neurons in normal control mice between postnatal day 6 and 15, but only of cholinergic neurons that lacked the nerve growth factor receptor TrkA. Here, we investigated whether and when the cholinergic neurons of the neostriatum, which express TrkA and p75NGFR during early postnatal times, undergo p75NGFR-mediated death. The cholinergic neurons in the lateral neostriatal regions expressed choline acetyltransferase (ChAT) earlier (postnatal day 3-6) than those of the medial regions and TrkA appeared before ChAT in all regions. Between postnatal day 6 and 10, approximately 40% of the ChAT-positive neurons in the most lateral regions disappeared in control mice but not in p75NGFR-deficient mice. During this time, the neostriatum of control, but not p75NGFR-deficient, mice contained many apoptotic cells. This suggests that, similar to the cholinergic neurons of the basal forebrain, the neostriatal cholinergic neurons of control mice die and that this process is mediated by p75NGFR. However, the roles of p75NGFR and TrkA appear to be more complicated in the neostriatum where relatively few neurons express p75NGFR during the death phase (and predominantly in the lateral neostriatum where the neuronal loss is greatest), and TrkA-positive as well as TrkA-negative neurons may be lost.

Evidence for normal aging of the septo-hippocampal cholinergic system in apoE (-/-) mice but impaired clearance of axonal degeneration products following injury

The association of the epsilon4 allele of apoE with increased risk for Alzheimer's disease (AD) and with poor clinical outcome after certain acute brain injuries has sparked interest in the neurobiology of apoE. ApoE (-/-) mice provide a tool to investigate the role of apoE in the nervous system in vivo. Since integrity of the basal forebrain cholinergic system is severely compromised in AD, with severity of dysfunction correlating with apoE4 gene dosage, the present study tested the hypothesis that apoE is required to maintain the normal integrity of basal forebrain cholinergic neurons (BFCNs). Histological and biochemical analyses of the septo-hippocampal cholinergic system were performed in apoE (-/-) mice during aging and following injury. Using unbiased quantitative methods, there was little or no evidence for defects in the septo-hippocampal cholinergic system, as assessed by p75(NTR)-immunoreactive neuron number and size in the medial septum, cholinergic fiber density in the hippocampus, and choline acetyltransferase activity in the hippocampus, cortex, and striatum in aged apoE (-/-) mice (up to 24 months of age) as compared to age-matched wild-type mice of the same strain. In addition, cholinergic neuronal survival and size following fimbria-fornix transection in apoE (-/-) mice did not differ from controls. However, following entorhinal cortex lesion, there was persistence of degeneration products in the deafferented hippocampus in apoE (-/-) mice. These data suggest that although apoE is not required for the maintenance of BFCNs in vivo, it may play a role in the clearance of cholesterol-laden neurodegeneration products following brain injury.

Competitive signaling between TrkA and p75 nerve growth factor receptors determines cell survival

In addition to its role as a survival factor, nerve growth factor (NGF) has been implicated in initiating apoptosis in restricted cell types both during development and after terminal cell differentiation. NGF binds to the TrkA tyrosine kinase and the p75 neurotrophin receptor, a member of the tumor necrosis factor cytokine family. To understand the mechanisms underlying survival versus death decisions, the TrkA receptor was introduced into oligodendrocyte cell cultures that undergo apoptosis in a p75-dependent manner. Here we report that activation of the TrkA NGF receptor in oligodendrocytes negates cell death by the p75 receptor. TrkA-mediated rescue from apoptosis correlated with mitogen-activated protein kinase activation. Concurrently, activation of TrkA in oligodendrocytes resulted in suppression of c-jun kinase activity initiated by p75, whereas induction of NFkappaB activity by p75 was unaffected. These results indicate that TrkA-mediated rescue involves not only activation of survival signals but also simultaneous suppression of a death signal by p75. The selective interplay between tyrosine kinase and cytokine receptors provides a novel mechanism that achieves alternative cellular responses by merging signals from different ligand-receptor systems.

The p75 neurotrophin receptor mediates neuronal apoptosis and is essential for naturally occurring sympathetic neuron death

To determine whether the p75 neurotrophin receptor (p75NTR) plays a role in naturally occurring neuronal death, we examined neonatal sympathetic neurons that express both the TrkA tyrosine kinase receptor and p75NTR. When sympathetic neuron survival is maintained with low quantities of NGF or KCl, the neurotrophin brain-derived neurotrophic factor (BDNF), which does not activate Trk receptors on sympathetic neurons, causes neuronal apoptosis and increased phosphorylation of c-jun. Function-blocking antibody studies indicate that this apoptosis is due to BDNF-mediated activation of p75NTR. To determine the physiological relevance of these culture findings, we examined sympathetic neurons in BDNF-/- and p75NTR-/- mice. In BDNF-/- mice, sympathetic neuron number is increased relative to BDNF+/+ littermates, and in p75NTR-/- mice, the normal period of sympathetic neuron death does not occur, with neuronal attrition occurring later in life. This deficit in apoptosis is intrinsic to sympathetic neurons, since cultured p75NTR-/- neurons die more slowly than do their wild-type counterparts. Together, these data indicate that p75NTR can signal to mediate apoptosis, and that this mechanism is essential for naturally occurring sympathetic neuron death.

Microglia-derived nerve growth factor causes cell death in the developing retina

While nerve growth factor (NGF) is best known for its trophic functions, recent experiments indicate that it can also cause cell death during development by activating the neurotrophin receptor p75. We now identify microglial cells as the source of NGF as a killing agent in the developing eye. When the retina is separated from the surrounding tissue before colonization by microglial cells, no NGF can be detected, and cell death is dramatically reduced. It is restored by the addition of microglial cells, an effect that is blocked by NGF antibodies. NGF adsorbed at the surface of beads, but not soluble NGF, mimics the killing action of microglial cells. These results indicate an active role for macrophages in neuronal death.