Low concentrations of aggregated β-amyloid induce neurite formation via the neurotrophin receptor p75

Rac1 and Akt Exhibit Distinct Roles in Mediating Aβ-Induced Memory Damage and Learning Impairment

Accumulated beta-amyloid (Aβ) in the brain is the hallmark of Alzheimer's disease (AD). Despite Aβ accumulation is known to trigger cellular dysfunctions and learning and memory damage, the detailed molecular mechanism remains elusive. Recent studies have shown that the onset of memory impairment and learning damage in the AD animal is different, suggesting that the underlying mechanism of the development of memory impairment and learning damage may not be the same. In the current study, with the use of Aβ42 transgenic flies as models, we found that Aβ induces memory damage and learning impairment via differential molecular signaling pathways. In early stage, Aβ activates both Ras and PI3K to regulate Rac1 activity, which affects mostly on memory performance. In later stage, PI3K-Akt is strongly activated by Aβ, which leads to learning damage. Moreover, reduced Akt, but not Rac1, activity promotes cell viability in the Aβ42 transgenic flies, indicating that Akt and Rac1 exhibit differential roles in Aβ regulating toxicity. Taken together, different molecular and cellular mechanisms are involved in Aβ-induced learning damage and memory decline; thus, caution should be taken during the development of therapeutic intervention in the future.

Signaling via the p75 neurotrophin receptor facilitates amyloid-β-induced dendritic spine pathology

Synapse and dendritic spine loss induced by amyloid-β oligomers is one of the main hallmarks of the early phases of Alzheimer's disease (AD) and is directly correlated with the cognitive decline typical of this pathology. The p75 neurotrophin receptor (p75NTR) binds amyloid-β oligomers in the nM range. While it was shown that µM concentrations of amyloid-β mediate cell death, the role and intracellular signaling of p75NTR for dendritic spine pathology induced by sublethal concentrations of amyloid-β has not been analyzed. We describe here p75NTR as a crucial binding partner in mediating effects of soluble amyloid-β oligomers on dendritic spine density and structure in non-apoptotic hippocampal neurons. Removing or over-expressing p75NTR in neurons rescues or exacerbates the typical loss of dendritic spines and their structural alterations observed upon treatment with nM concentrations of amyloid-β oligomers. Moreover, we show that binding of amyloid-β oligomers to p75NTR activates the RhoA/ROCK signaling cascade resulting in the fast stabilization of the actin spinoskeleton. Our results describe a role for p75NTR and downstream signaling events triggered by binding of amyloid-β oligomers and causing dendritic spine pathology. These observations further our understanding of the molecular mechanisms underlying one of the main early neuropathological hallmarks of AD.

Cysteine-Rich Repeat Domains 2 and 4 are Amyloid-β Binding Domains of Neurotrophin Receptor p75NTR and Potential Targets to Block Amyloid-β Neurotoxicity

The p75 neurotrophin receptor (p75NTR) is an amyloid-β (Aβ) receptor that both mediates Aβ neurotoxicity and regulates Aβ production and deposition, thus playing an important role in the pathogenesis of Alzheimer's disease (AD). The extracellular domain of p75NTR (p75ECD), consisting of four cysteine-rich repeat domains (CRDs), was recently reported to be an endogenous anti-Aβ scavenger to block p75NTR-mediated neuronal death and neurite degeneration signaling of Aβ and pro-neurotrophins. Identification of the specific Aβ binding domains of p75NTR is crucial for illuminating their interactions and the etiology of AD. CRDs of p75ECD were obtained by expression of recombinant plasmids or direct synthesis. Aβ aggregation inhibiting test and immunoprecipitation assay were applied to locate the specific binding domains of Aβ to p75ECD. The Aβ neurotoxicity antagonistic effects of different CRDs were examined by cytotoxicity experiments including neurite outgrowth assay, propidium iodide (PI) staining, and MTT assay. In the Aβ aggregation inhibiting test, the fluorescence intensity in the CRD2 and CRD4 treatment groups was significantly lower than that in the CRD1 and CRD3 treatment groups. Immunoprecipitation assay and western blot confirmed that Aβ could bind to CRD2 and CRD4. Besides, CRD2 and CRD4 antagonized Aβ neurotoxicity suggested by longer neurite length, less PI labelled cells, and higher cell viability than the control group. Our results indicate that CRD2 and CRD4 are Aβ binding domains of p75NTR and capable of antagonizing Aβ neurotoxicity, and therefore are potential therapeutic targets to block the interaction of Aβ and p75NTR in the pathogenesis of AD.

Neonatal Neurodegeneration in Alzheimer's Disease Transgenic Mouse Model

Alzheimer’s disease (AD) is a progressive disorder characterized by a variety of molecular pathologies causing cortical dementia with a prominent memory deficit. Formation of the pathology, which begins decades before the diagnosis of the disease, is highly correlated with the clinical symptoms. Several proteomics studies were performed using animal models to monitor the alterations of the brain tissue proteome at different stages of AD. However, proteome changes in the brain regions of newborn transgenic mouse model have not been investigated yet. To this end, we analyzed protein expression alterations in cortex, hippocampus and cerebellum of transgenic mice carrying five familial AD mutations (5XFAD) at neonatal day-1. Our results indicate a remarkable difference in protein expression profile of newborn 5XFAD brain with region specific variations. Additionally, the proteins, which show similar expression alteration pattern in postmortem human AD brains, were determined. Bioinformatics analysis showed that the molecular alterations were mostly related to the cell morphology, cellular assembly and organization, and neuroinflammation. Moreover, morphological analysis revealed that there is an increase in neurite number of 5XFAD mouse neurons in vitro. We suggest that, molecular alterations in the AD brain exist even at birth, and perhaps the disease is silenced until older ages when the brain becomes vulnerable.

The Binding Receptors of Aβ: an Alternative Therapeutic Target for Alzheimer's Disease

Alzheimer's disease (AD) is one of the most common neurodegenerative disorders, which causes the deterioration of memory and other cognitive abilities of the elderly. Previous lines of research have shown that Aβ is an essential factor in AD pathology and the soluble oligomeric species of Aβ peptide is presumed to be the drivers of synaptic impairment in AD. However, the exact mechanisms underlying Aβ-induced synapse dysfunction are still not fully understood. Recently, increasing evidence suggests that some potential receptors which bind specifically with Aβ may play important roles in inducing the toxicity of the neurons in AD pathology. These receptors include the cellular prion protein (PrPc), the α7 nicotinic acetylcholine receptor (α7nAChR), the p75 neurotrophin receptor (p75(NTR)), the beta-adrenergic receptors (β-ARs), the Eph receptors, the paired immunoglobulin-like receptor B (PirB), the PirB's human ortholog receptor (LilrB2), and the Fcγ receptor II-b (FcγRIIb). This review summarizes the characters of these prominent receptors and how the bindings of them with Aβ inhibit the LTP, decrease the number of dendritic spine, damage the neurons, and so on in AD pathogenesis. Blocking or rescuing these receptors may have significant importance for AD treatments.

Computational Studies of Beta Amyloid (Aβ42) with p75NTR Receptor: A Novel Therapeutic Target in Alzheimer's Disease

Alzheimer's disease is a neurodegenerative disorder characterized by the accumulation of beta amyloid plaques (Aβ) which can induce neurite degeneration and progressive dementia. It has been identified that neuronal apoptosis is induced by binding of Aβ42 to pan neurotrophin receptor (p75NTR) and gave the possibility that beta amyloid oligomer is a ligand for p75NTR. However, the atomic contact point responsible for molecular interactions and conformational changes of the protein upon binding was not studied in detail. In view of this, we conducted a molecular docking and simulation study to investigate the binding behaviour of Aβ42 monomer with p75NTR ectodomain. Furthermore, we proposed a p75NTR-ectodomain-Aβ42 complex model. Our data revealed that, Aβ42 specifically recognizes CRD1 and CRD2 domains of the receptor and formed a “cap” like structure at the N-terminal of receptor which is stabilized by a network of hydrogen bonds. These findings are supported by molecular dynamics simulation that Aβ42 showed distinct structural alterations at N- and C-terminal regions due to the influence of the receptor binding site. Overall, the present study gives more structural insight on the molecular interactions of beta amyloid protein involved in the activation of p75NTR receptor.

The role of nerve growth factor in neuropathic pain inhibition produced by resiniferatoxin treatment in the dorsal root ganglia

BACKGROUND

Resiniferatoxin (RTX), an excitotoxic agonist for vanilloid receptor 1, is a promising candidate for intractable pain treatment.

OBJECTIVE

We evaluated the effects of RTX, applied to dorsal root ganglia (DRG) at high doses (1200 ng), in sensory-motor function and nerve growth factor (NGF) alterations in a photochemical sciatic nerve injury rat model.

METHODS

Following RTX injection into the L3-6 DRG at high doses and behavioral evaluation, the rats were sacrificed and the DRG were tested by immunohistochemistry and mRNA analysis for NGF and its' receptors, tyrosine kinase A (TrkA) and p75. The correlation between neuropathic pain and NGF, TrkA, and p75 expression was analyzed.

RESULTS

The treated rats had preserved touch, cold, pain, and high-heat sensations, and exhibited hypoalgesia to low-heat stimulation. After RTX treatment, TrkA and p75 altered their expressions from one neuronal type to another in the DRG. NGF and p75 expression changed from the small-size neurons in neuropathic rat DRG to the large- and medium-size neurons in non-neuropathic and RTX-treated animals, concomitantly with neuropathic pain suppression. TrkA was expressed in the small-size neurons in neuropathic rat DRG, and was drastically reduced in all size neurons after RTX treatment. NGF, TrkA, and p75 mRNA expression supported these phenotypic changes in the DRG.

CONCLUSION

The pathway of NGF-TrkA expressed in the small-size neurons, associated with neuropathic pain, was shifted to the NGF-p75 pathway expressed in the large-size neurons after RTX treatment, in association with neuropathic pain inhibition. These findings may play an important role in clinical trial designs.

Aβ40 modulates GABA(A) receptor α6 subunit expression and rat cerebellar granule neuron maturation through the ERK/mTOR pathway

In addition to their neurotoxic role in Alzheimer's disease (AD), β-amyloid peptides (Aβs) are also known to play physiological roles. Here, we show that recombinant Aβ40 significantly increased the outward current of the GABA(A) receptor containing (GABA(A)α6) in rat cerebellar granule neurons (CGNs). The Aβ40-mediated increase in GABA(A)α6 current was mediated by an increase in GABA(A)α6 protein expression at the translational rather than the transcriptional level. The exposure of CGNs to Aβ40 markedly induced the phosphorylation of ERK (pERK) and mammalian target of rapamycin (pmTOR). The increase in GABA(A)α6 current and expression was attenuated by specific inhibitors of ERK or mTOR, suggesting that the ERK and mTOR signaling pathways are required for the effect of Aβ40 on GABA(A)α6 current and expression in CGNs. A pharmacological blockade of the p75 neurotrophin receptor (p75(NTR)), but not the insulin or α7-nAChR receptors, abrogated the effect of Aβ40 on GABA(A)α6 protein expression and current. Furthermore, the expression of GABA(A)α6 was lower in CGNs from APP(-/-) mice than in CGNs from wild-type mice. Moreover, the internal granule layer (IGL) in APP(-/-) mice was thinner than the IGL in wild-type mice. The injection of Aβ40 into the cerebellum reversed this effect, and the application of p75(NTR) blocking antibody abolished the effects of Aβ40 on cerebellum morphology in APP(-/-) mice. Our results suggest that low concentrations of Aβ40 play a role in regulating CGN maturation through p75(NTR).

An overview of brain-derived neurotrophic factor and implications for excitotoxic vulnerability in the hippocampus

The present paper examines the nature and function of brain-derived neurotrophic factor (BDNF) in the hippocampal formation and the consequences of changes in its expression. The paper focuses on literature describing the role of BDNF in hippocampal development and neuroplasticity. BDNF expression is highly sensitive to developmental and environmental factors, and increased BDNF signaling enhances neurogenesis, neurite sprouting, electrophysiological activity, and other processes reflective of a general enhancement of hippocampal function. Such increases in activity may mediate beneficial effects such as enhanced learning and memory. However, the increased activity also comes at a cost: BDNF plasticity renders the hippocampus more vulnerable to hyperexcitability and/or excitotoxic damage. Exercise dramatically increases hippocampal BDNF levels and produces behavioral effects consistent with this phenomenon. In analyzing the literature regarding exercise-induced regulation of BDNF, this paper provides a theoretical model for how the potentially deleterious consequences of BDNF plasticity may be modulated by other endogenous factors. The peptide galanin may play such a role by regulating hippocampal excitability.

Increased dendrite branching in AbetaPP/PS1 mice and elongation of dendrite arbors by fasudil administration

Amyloid-beta (Abeta) overproduction and dendrite arbor atrophy are hallmarks of Alzheimer's disease. The RhoA GTPase (Rho) signals through Rho kinase (ROCK) to control cytoskeletal dynamics and regulate neuron structure. Hyperactive Rho signaling destabilizes neurons leading to dendritic regression that can be rescued by genetic or pharmacological reduction of ROCK signaling. To understand what effect reduced ROCK signaling has on the dendrite arbors of mice that overproduce Abeta, we administered the ROCK inhibitor fasudil to AbetaPP/PS1 transgenic mice. We report that increased dendrite branching occurs in AbetaPP/PS1 mice and that fasudil promotes lengthening of the dendrite arbors of CA1 pyramidal neurons.

Nerve growth factor as a paradigm of neurotrophins related to Alzheimer's disease

Converging lines of evidence on the possible connection between NGF signaling and Alzheimer's diseases (AD) are unraveling new facets which could depict this neurotrophin (NTF) in a more central role. AD animal models have provided evidence that a shortage of NGF supply may induce an AD-like syndrome. In vitro experiments, moreover, are delineating a possible temporal and causal link between APP amiloydogenic processing and altered post-translational tau modifications. After NGF signaling interruption, the pivotal upstream players of the amyloid cascade (APP, beta-secretase, and active form of gamma-secretase) are up-regulated, leading to an increased production of amyloid beta peptide (Abeta) and to its intracellular aggregation in molecular species of different sizes. Contextually, the Abeta released pool generates an autocrine toxic loop in the same healthy neurons. At the same time tau protein undergoes anomalous, GSKbeta-mediated, phosphorylation at specific pathogenetic sites (Ser262 and Thr 231), caspase(s) and calpain- I- mediated truncation, detachment from microtubules with consequent cytoskeleton collapse and axonal transport impairment. All these events are inhibited when the amyloidogenic processing is reduced by beta and gamma secretase inhibitors or anti-Abeta antibodies and appear to be causally correlated to TrkA, p75CTF, Abeta, and PS1 molecular association in an Abeta-mediated fashion. In this scenario, the so-called trophic action exerted by NGF (and possibly also by other neurotrophins) in these targets neurons is actually the result of an anti-amyloidogenic activity.

The p75 neurotrophin receptor promotes amyloid-beta(1-42)-induced neuritic dystrophy in vitro and in vivo

Oligomeric forms of amyloid-beta (Abeta) are thought to play a causal role in Alzheimer's disease (AD), and the p75 neurotrophin receptor (p75(NTR)) has been implicated in Abeta-induced neurodegeneration. To further define the functions of p75(NTR) in AD, we examined the interaction of oligomeric Abeta(1-42) with p75(NTR), and the effects of that interaction on neurite integrity in neuron cultures and in a chronic AD mouse model. Atomic force microscopy was used to ascertain the aggregated state of Abeta, and fluorescence resonance energy transfer analysis revealed that Abeta oligomers interact with the extracellular domain of p75(NTR). In vitro studies of Abeta-induced death in neuron cultures isolated from wild-type and p75(NTR-/-) mice, in which the p75(NTR) extracellular domain is deleted, showed reduced sensitivity of mutant cells to Abeta-induced cell death. Interestingly, Abeta-induced neuritic dystrophy and activation of c-Jun, a known mediator of Abeta-induced deleterious signaling, were completely prevented in p75(NTR-/-) neuron cultures. Thy1-hAPP(Lond/Swe) x p75(NTR-/-) mice exhibited significantly diminished hippocampal neuritic dystrophy and complete reversal of basal forebrain cholinergic neurite degeneration relative to those expressing wild-type p75(NTR). Abeta levels were not affected, suggesting that removal of p75(NTR) extracellular domain reduced the ability of excess Abeta to promote neuritic degeneration. These findings indicate that although p75(NTR) likely does not mediate all Abeta effects, it does play a significant role in enabling Abeta-induced neurodegeneration in vitro and in vivo, establishing p75(NTR) as an important therapeutic target for AD.

Beta-amyloid toxicity and reversal in embryonic rat septal neurons

Alzheimer's disease is characterized mainly by loss of neurons from the septal nucleus. In this study, neurons from the septal nucleus of the embryonic day 16 (E16) rat were grown in culture with a plane of astrocytes from the embryonic rat and in a defined medium in the absence of serum. Neurons were treated with beta-amyloid (Abeta: 0.1, 1 and 10 microM) on day in vitro (DIV) 1 and DIV 4 and fluorescent microscopy was used to measure survival and apoptosis following exposure of the treated cells on DIV 7. Reversal of neurotoxicity was studied using the potentially neuroprotective agents nerve growth factor (NGF, 100 ng/ml), basic fibroblast growth factor (bFGF, 5 ng/ml), insulin-like growth factors (IGF1 and IGF2, 10 ng/ml) and estrogen (10 nM), administered on DIV 4 and DIV 5, that is, subsequent to the Abeta (10 microM)-induced neurotoxicity. Abeta caused a significant decrease in survival at 10 microM, and a significant increase in apoptosis at 0.1 and 10 microM. IGF1, IGF2 and bFGF all caused a reversal of the Abeta-induced neurotoxic effect on survival while NGF and estrogen did not under these experimental conditions.

Beta/A4-Amyloid increases nerve growth factor production in rat primary hippocampal astrocyte cultures

Nerve growth factor (NGF), a member of the neurotrophin family, is an essential mediator of neuronal activity and synaptic plasticity of basal forebrain cholinergic neurons (BFCN). In processes of chronic degeneration of BFCN like in Alzheimer's disease (AD), characterized among others by amyloid containing plaques, NGF has been shown to improve cognitive decline and rescue BFCN but also to reduce survival of hippocampal neurons via p75 neurotrophin receptor (p75). Little is known about the mechanisms of NGF regulation in glial cells under pathological conditions in AD. This study investigates the influence of amyloid administration on the NGF protein secretion in rat primary hippocampal astrocytes. Astrocytes were stimulated with "aged" beta/A4-Amyloid (1-40), and NGF was measured in different fractions, such as supernatant, vesicles, and cytosol fraction. Treatment with amyloid at a final concentration of 10 microM for 72 h led to increased NGF protein levels up to 30-fold increase compared to unstimulated controls. This observation may be an endogenous neuroprotective mechanism possibly contributing to a delay of amyloid-dependent loss of cholinergic neurons or contribute to accelerated neuronal death by activation of p75 within Alzheimer pathology.

Antagonistic effects of TrkB and p75NTRon NMDA receptor currents in post-synaptic densities transplanted into Xenopus oocytes

Brain-derived neurotrophic factor (BDNF) and its receptor TrkB are essential regulators of synaptic function in the adult CNS. A TrkB-mediated effect at excitatory synapses is enhancement of NMDA receptor (NMDA-R)-mediated currents. Recently, opposing effects of TrkB and the pan-neurotrophin receptor p75(NTR) on long-term synaptic depression and long-term potentiation have been reported in the hippocampus. To further study the regulation of NMDA-Rs by neurotrophin receptors in their native protein environment, we micro-transplanted rat forebrain post-synaptic densities (PSDs) into Xenopus oocytes. One-minute incubations of oocytes with BDNF led to dual effects on NMDA-R currents: either TrkB-dependent potentiation or TrkB-independent inhibition were observed. Pro-nerve growth factor, a ligand for p75(NTR) but not for TrkB, produced a reversible, dose-dependent, TrkB-independent and p75(NTR)-dependent inhibition of NMDA-Rs. Fractionation experiments showed that p75(NTR) is highly enriched in the PSD protein fraction. Immunoprecipitation and pull-down experiments further revealed that p75(NTR) is a core component of the PSD, where it interacts with the PDZ3 domain of the scaffolding protein SAP90/PSD-95. Our data provide striking evidence for a rapid inhibitory effect of p75(NTR) on NMDA-R currents that antagonizes TrkB-mediated NMDA-R potentiation. These opposing mechanisms might be present in a large proportion of forebrain synapses and may contribute importantly to synaptic plasticity.

A cell-biological model of p75NTR signaling

Neurotrophin stimulation of tropomyosin-related kinase (Trk) and p75 receptors influences cellular processes such as proliferation, growth, differentiation, and other cell-specific functions, as well as regeneration. In contrast to Trk receptors, which have a well-defined trophic role, p75 has activities ranging from trophism to apoptosis. Continued neurotrophin stimulation of differentiating neurons transforms the initially trophic character of p75 signaling into negative growth control and overstimulation leads to apoptosis. This function shift reflects the signaling effects of ceramide that is generated upon stimulation of p75. The use of ceramide signaling by p75 may provide a key to understanding the cell-biological role of p75. The review presents arguments that the control of cell shape formation and cell selection can serve as an organizing principle of p75 signaling. Concurrent stimulation by neurotrophins of p75 and Trk receptors constitutes a dual growth control with antagonistic and synergistic elements aimed at optimal morphological and functional integration of cells and cell populations into their context.

Beta-amyloid toxicity in embryonic rat astrocytes

The senile plaques of Alzheimer's disease contain a high concentration of beta-amyloid (betaA) protein, which may affect the glial population in the septal nucleus, an area of increased risk in AD. BetaA toxicity was measured in septal glia, via a dose-response experiment, by quantifying the effects of three different doses (0.1, 1, and 10 microM) of betaA on cell survival. Astrocytes from embryonic day-16 rats were grown in serum-free media in a single layer culture. Cells were treated on day in vitro (DIV)1 and survival was determined on DIV3 to ascertain which concentration was most toxic. In a separate set of experiments, an attempt was made to protect glial cells from the degenerative effects of betaA, with treatments of growth factors and estrogen. BetaA (10 microM) treatment was administered on DIV1, on DIV2 the cells were treated with estrogen (EST, 10 nM), insulin-like growth factors (IGF1 and IGF2, each 10 ng/ml), basic fibroblast growth factor (bFGF, 5 ng/ml) or nerve growth factor (NGF, 100 ng/ml), and on DIV3 the cells were visualized and quantified by fluorescence microscopy with DAPI (4,6-diamidino-2-phenylindole). In addition to dose-response and glial protection, experiments were also conducted to determine whether toxic effects were due to apoptosis. Our results suggest that the survival of glial populations is significantly affected in all three concentrations (0.1, 1.0, and 10 microM) of betaA. Glial protection was evident in the presence of NGF, for it showed the significantly highest survival rate relative to the betaA treatment alone. Furthermore, toxic effects of betaA appear to be due primarily to apoptosis. Significant reversal of betaA-induced apoptosis was seen with bFGF and IGF1.

Mitochondrial c-Jun NH2-terminal kinase prevents the accumulation of reactive oxygen species and reduces necrotic damage in neural tumor cells that lack trophic support

In response to different stress signals, the c-Jun NH(2)-terminal kinase (JNK) can trigger cell death. However, JNK also facilitates the survival and cell cycle progression of tumor cells by mechanisms that are poorly defined. Here, we show that schwannoma RN22 cells can survive and proliferate under serum-free conditions although serum withdrawal rapidly induces mitochondrial fission and swelling. Although the morphologic changes observed in the mitochondria did not trigger cytochrome c release, they were accompanied by an increase in the mitochondrial membrane potential (DeltaPsi(M)) and of immunoreactivity for active JNK in these organelles. Pharmacologic inhibition of JNK provoked a further increase of the DeltaPsi(M), an increase in reactive oxygen species (ROS) production, and a sustained decrease in cell viability due to necrosis. This increase in necrosis was prevented by the presence of ROS scavengers. Immunoreactivity for active JNK was also observed in the mitochondria of neuroblastoma 1E-115 and neuroblastoma 2a neuroblastoma cell lines on serum withdrawal, whereas active JNK was barely detected in serum-deprived fibroblasts. Accordingly, the reduction in neural tumor cell viability induced by JNK inhibition was largely attenuated in serum-deprived fibroblasts. These data indicate that local activation of JNK in the mitochondria can protect against necrotic cell death associated with ROS production, facilitating the growth of neural tumor cells subjected to serum deprivation.

Does the p75 neurotrophin receptor mediate Abeta-induced toxicity in Alzheimer's disease?

Alzheimer's disease is characterized by the over-production and accumulation of amyloidogenic Abeta peptide, which can induce cell death in vitro. It has been suggested that the death signal could be transduced by the pan neurotrophin receptor (p75NTR). p75NTR is well known for its ability to mediate neuronal death in neurodegenerative conditions and is inextricably linked with changes that occur in Alzheimer's disease. Moreover, Abeta binds to p75NTR, activating signalling cascades. However, the complexity of p75NTR-mediated signalling, which does not always promote cell death, leaves open the possibly of Abeta promoting death via an alternative signalling pathway or the regulation of other p75NTR-mediated actions. This review focuses on the interactions between Abeta and p75NTR in the context of the broader p75NTR signalling field, and offers alternative explanations for how p75NTR might contribute to the aetiology of Alzheimer's disease.

Nerve growth factor, neural stem cells and Alzheimer's disease

The protein family of the neurotrophins (NTs) comprises structurally and functionally related molecules such as nerve growth factor (NGF) which influences the proliferation, differentiation, survival and death of neuronal cells. In addition to their established functions for cell survival, NTs also mediate higher brain activities such as learning and memory. Changes in NT expression levels have thus been implicated in neurological diseases such as Alzheimer's disease (AD), an age-related neurodegenerative disorder that is characterized by progressive loss of memory and deterioration of higher cognitive functions. The present review provides an overview of the functional role of NGF in neural stem cells and AD while pointing to a potential application of this peptide for the treatment of AD.