The cholinergic system, nerve growth factor and the cytoskeleton

Cholinergic neurons of the basal forebrain provide the major cholinergic innervation to the cortex and hippocampus, and play a key role in memory and attentional processes. Dysfunction of basal forebrain cholinergic neurons (BFCN) is a cardinal feature of Alzheimer's disease (AD) and correlates with cognitive decline. Survival of BFCN neurons depends upon binding of nerve growth factor (NGF), which is synthesized and secreted by cells in the cortex and hippocampus, with high-affinity (TrkA) and low-affinity (p75^NTR) neurotrophin receptors produced within BFCN neurons. NGF released from target cells activates TrkA on axon terminals and triggers activation of PI3K/Akt, MEK/ERK, and PLCγ (phospholipase C) signaling pathways. The signal then travels retrogradely along axon to cell body to promote neuronal survival. However, the nature of the retrograde signal remains mysterious. p75^NTR receptors could mediate a fundamentally different signaling pathway leading to apoptic cell death. Dysfunction of NGF and its receptors has been suggested to underlie the selective degeneration of the BFCN in end stage Alzheimer disease. In this regard, NGF, the founding member of the neurotrophin family, has generated great interest as a potential target for the treatment of AD. This review focuses on NGF-cholinergic dependency, NGF/receptor binding, signal transduction, retrograde transport, regulation of specific cellular endpoints, and the potential involvement of cytoskeleton dysfunction in defected NGF signaling.     

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

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

Impairment of the nerve growth factor pathway driving amyloid accumulation in cholinergic neurons the incipit of the Alzheimer's disease story?

The current idea behind brain pathology is that disease is initiated by mild disturbances of common physiological processes. Overtime, the disruption of the neuronal homeostasis will determine irreversible degeneration and neuronal apoptosis. hTis could be also true in the case of nerve growth factor (NGF) al-terations in sporadic Alzheimer’s disease (AD), an age-related pathology characterized by cholinergic loss, amyloid plaques and neurofibrillary tangles. In fact, the pathway activated by NGF, a key neurotrophin for the metabolism of basal forebrain cholinergic neurons (BFCN), is one of the ifrst homeostatic systems affected in prodromal AD. NGF signaling dysfunctions have been thought for decades to occur in AD late stages, as a mere consequence of amyloid-driven disruption of the retrograde axonal transport of neuro-trophins to BFCN. Nowadays, a wealth of knowledge is potentially opening a new scenario: NGF signaling impairment occurs at the onset of AD and correlates better than amyloid load with cognitive decline. hTe recent acceleration in the characterization of anatomical, functional and molecular proifles of early AD is aimed at maximizing the efficacy of existing treatments and setting novel therapies. Accordingly, the elucidation of the molecular events underlying APP metabolism regulation by the NGF pathway in the sep-to-hippocampal system is crucial for the identiifcation of new target molecules to slow and eventually halt mild cognitive impairment (MCI) and its progression toward AD.     

The precursor pro-nerve growth factor is the predominant form of nerve growth factor in brain and is increased in Alzheimer's disease

Nerve growth factor (NGF) is important for regulation, differentiation, and survival of peripheral and central nervous system neurons, including basal forebrain cholinergic neurons (BFCN) which degenerate in Alzheimer's disease (AD). Mature NGF protein is processed from a larger precursor, proNGF. We demonstrate that proNGF is the predominant form of NGF in mouse, rat, and human brain tissue, whereas little or no mature NGF is detected. Previous reports showed NGF protein, measured by ELISA, is increased in AD BFCN target regions such as hippocampus and cortex. Using Western blotting, we demonstrate a twofold increase in proNGF in AD parietal cortex compared to controls, indicating that it is this precursor form, proNGF, that accumulates in AD. This increase may reflect either a role for biologically active proNGF or posttranslational disturbances in NGF biosynthesis that decrease the processing of proNGF to mature NGF in AD.     

Selective antibody-induced cholinergic cell and synapse loss produce sustained hippocampal and cortical hypometabolism with correlated cognitive deficits.

The physiological interrelationships between cognitive impairments, neurotransmitter loss, amyloid processing and energy metabolism changes in AD, cholinergic dementia and Down's syndrome are largely unknown to date. This report contains novel studies into the association between cognitive function and cerebral metabolism after long-term selective CNS cholinergic neuronal and synaptic loss in a rodent model. We measured local cerebral rates of glucose utilization ((14)C-2-deoxyglucose) throughout the brains of awake rats 4.5 months after bilateral intraventricular injections of a cholinotoxic antibody directed against the low-affinity NGF receptor (p75 NGF) associated with cholinergic neurons (192 IgG-saporin). Permanent cholinergic synapse loss was demonstrated by [(3)H]-vesamicol in vitro autoradiography defining presynaptic vesicular acetylcholine (ACh) transport sites. While other metabolic studies have defined acute and transient glucose use changes after relatively nonspecific lesions of anatomical regions containing cholinergic neurons, our results show sustained reductions in glucose utilization in brain regions impacted by cholinergic synapse loss, including frontal cortical and hippocampal regions, relative to glucose use levels in control rats. In the same animals, impaired cognitive spatial performance in a Morris water maze was correlated with reduced glucose use rates in the cortex and hippocampus at this time point, which is consistent with increased postmortem cortical and hippocampal amyloid precursor protein (APP) levels (45, 46). These results are consistent with the view of cholinergic influence over metabolism, APP processing, and cognition in the cortex and hippocampus.     

NGF-cholinergic dependency in brain aging, MCI and Alzheimer's disease

Forebrain cholinergic neurons are highly dependent on nerve growth factor (NGF) for phenotype maintenance. We have established that in addition to "target-derived" NGF neurotrophic stimulation, cholinergic neurons also respond dose-dependently, to intra-parenchymal NGF administration in the somato-dendritic region of the nucleus Basalis, thus illustrating the potential of alternative reparative therapies which would by-pass the undesirable effects of diffuse neurotrophin application. Moreover, our lab has also observed that the steady-state number of cortical cholinergic synapses is dependent on continuous NGF supply, as anti-NGF monoclonal antibodies and TrkA receptor antagonists deplete pre-existing cholinergic bouton numbers. Furthermore, the application of either NGF or TrkA NGF-mimetic agonists successfully rescues the age-dependent loss of cortical cholinergic boutons in aged-impaired rats. The vulnerability of the cortical cholinergic system has also been demonstrated in transgenic animal models of the Alzheimer's disease (AD) amyloid pathology. It is of interest to note however, that an up-regulation of cholinergic presynaptic boutons has been observed in certain transgenic mouse models prior to plaque formation. This observation is similar to the visibly increased immunoreactivity of cortical and hippocampal choline acetyltransferase (ChAT) fibers in patients with Mild Cognitive Impairment (MCI). A series of ex-vivo experiments conducted by our group have demonstrated that contrary to popular belief, proNGF, as opposed to mature NGF, is released from the cerebral cortex in an activity-dependent manner. In addition, proNGF appears to be released with a series of pro-enzymes and enzymes, which are involved in its subsequent maturation to NGF and degradation in the extracellular space. Given that proNGF is known to be upregulated in AD patients a dysregulation in the maturation or degradation of mature NGF might explain the preferential vulnerability of the cholinergic system in the AD pathology.     

Alzheimer’s disease and NGF signaling

Summary. Age-related degeneration of basal forebrain cholinergic neurons (BFCNs) occurs early and contributes significantly to cognitive decline in Alzheimers disease (AD). Proper function and morphology of BFCNs depends on the supply of nerve growth factor (NGF) from the cortex and the hippocampus. A large number of experiments have shown that decreased supply of NGF at the level of BFCN cell bodies leads to loss of neuronal markers and shrinkage, mimicking what is observed in AD.The delivery of sufficient amounts of NGF signal to BFCN cell bodies depends on the effective participation of several factors including sufficient synthesis and release of NGF, adequate synthesis and expression of NGF receptors by BFCNs, normal signaling and retrograde transport of NGF-receptor complex, and finally effective induction of gene expression by NGF.In the past few years it has become clear that decreased amounts of NGF at the level of BFCN cell bodies is largely due to failed retrograde transport rather than decreased synthesis, binding or expression of NGF receptors in the BFCN terminals. We will discuss in vivo evidence supporting decreased retrograde transport of NGF in a mouse model with BFCN degeneration, and will attempt to match these findings with our studies in postmortem human AD brain. We will speculate about the possible mechanisms of failed NGF retrograde transport and its relationship to AD pathology.     

Loss of medial septum cholinergic neurons in THY-Tau22 mouse model: what links with tau pathology?

Alzheimer's disease (AD) is a neurodegenerative disorder histologically defined by the cerebral accumulation of amyloid deposits and neurofibrillary tangles composed of hyperphosphorylated tau proteins. Loss of basal forebrain cholinergic neurons is another hallmark of the disease thought to contribute to the cognitive dysfunctions. To this date, the mechanisms underlying cholinergic neurons degeneration remain uncertain. The present study aimed to investigate the relationship between neurofibrillary degeneration and cholinergic defects in AD using THY-Tau22 transgenic mouse model exhibiting a major hippocampal AD-like tau pathology and hyperphosphorylated tau species in the septohippocampal pathway. Here, we report that at a time THY-Tau22 mice display strong reference memory alterations, the retrograde transport of fluorogold through the septohippocampal pathway is altered. This impairment is associated with a significant reduction in the number of choline acetyltransferase (ChAT)-immunopositive cholinergic neurons in the medial septum. Analysis of nerve growth factor (NGF) levels supports an accumulation of the mature neurotrophin in the hippocampus of THY-Tau22 mice, consistent with a decrease of its uptake or retrograde transport by cholinergic terminals. Finally, our data strongly support that tau pathology could be instrumental in the cholinergic neuronal loss observed in AD.     

APP, NGF & the 'Sunday-driver' in a Trolley on the Road

Alzheimer's disease (AD) is one of the most common and challenging neurodegenerative diseases in humans and is characterized by: progressive impairment in cognitive function, degeneration of cholinergic neurons of the basal forebrain (CBF), neurofibrillary tangles and amyloid beta-peptide (Abeta) depositions. The amyloid precursor protein (APP) is a transmembrane protein of which abnormal processing produces Abeta that is associated with the pathogenesis of AD. Neurotrophic factors have attracted much attention for their potential as a remedy for neurological disorders. In this regard, nerve growth factor (NGF) has generated a great interest as a potential target for the treatment of AD. This interest is based on the observation that CBF neurons, which provide the major source of cholinergic innervation to the cerebral cortex and hippocampus, undergo selective and severe degeneration in advanced AD and that the survival of CBF neurons depends upon NGF and its receptors, namely, trkA and p75NTR. This review focuses on recent findings about APP, NGF and their potential signaling-connections to the protein encoded by the 'Sunday-driver' (SYD) gene.     

A dual mechanism linking NGF/proNGF imbalance and early inflammation to Alzheimer's disease neurodegeneration in the AD11 anti-NGF mouse model

The neurotrophin Nerve Growth Factor (NGF) is essential for the maintenance and differentiation of basal forebrain cholinergic neurons. Since basal forebrain cholinergic neurons represent one major neuronal population affected and progressively degenerating in Alzheimer's disease (AD), interest has grown for NGF as a potential therapeutic agent in neurodegenerative disorders linked to aging, particularly for AD. However, no evidence was available, to link, in a cause-effect manner, deficits in NGF signalling to the broader activation in the Alzheimer's cascade, besides cholinergic deficits. The phenotypic analysis of the AD11 anti-NGF transgenic mouse, obtained by the "neuroantibodies" phenotypic protein knock out strategy, allowed demonstrating a direct causal link between NGF deprivation and AD pathology. Since then, extensive mechanistic studies on the AD11 model provided a new twist to the concept that alterations in NGF transport and signalling play a crucial role in sporadic Alzheimer's neurodegeneration, leading to the hypothesis of "Neurotrophic imbalance" as an upstream driver for sporadic AD. The results obtained with the AD11 anti-NGF mice highlight the fact that the particular mode of NGF neutralization, with an NGF antibody expressed in the brain, selectively interfering with mature NGF versus unprocessed proNGF, plays a major role in the mechanism of neurodegeneration, and could lead to new insights into the mechanisms of human sporadic AD. Here, we will review (1) the renewed neurotrophic imbalance hypothesis for AD and (2) the mechanisms underlying the neurodegenerative phenotype of AD11 anti-NGF mice.     

Cholinotrophic molecular substrates of mild cognitive impairment in the elderly

Cholinergic nucleus basalis (NB) neurons provide the major cholinergic innervation to the cortical mantle, are selectively vulnerable in late stage Alzheimer's disease (AD) and require the neurotrophin, nerve growth factor (NGF) and its receptors (TrkA and p75(NTR)), for their survival. The molecular events underlying the demise of these neurons in AD were investigated using tissue harvested from participants in a longitudinal clinical pathological study of aging and AD who agreed to an annual clinical evaluation providing a categorization of no cognitive impairment (NCI), mild cognitive impairment (MCI) or AD and postmortem brain donation. Although the number of choline acetyltransferase (ChAT)-positive neurons was unchanged, TrkA and p75(NTR) receptor-containing neurons, which co-localize with ChAT, were significantly reduced in the NB of subjects with MCI and AD compared to those with NCI. These observations indicate a phenotypic down-regulation rather than frank NB neuronal degeneration in MCI. Expression profiling of single cholinergic NB neurons revealed TrkA but not p75(NTR) mRNA is reduced in MCI, suggesting that decreased neurotrophin responsiveness may be an early biomarker for AD. The NGF precursor molecule, proNGF, is increased in the cortex in MCI and AD. Since proNGF accumulates in the presence of reduced cortical TrkA and sustained levels of p75(NTR), a shift in the balance between cell survival and death molecules may occur in prodromal AD. Coincident with these phenomena, brain derived neurotrophic factor (BDNF) and its precursor molecule, proBDNF, are reduced in the MCI cortex, potentially depriving CBF neurons of additional trophic factor support. Moreover, there is a shift in the ratio of 3 repeat tau to 4 repeat tau gene expression, whereas total tau message is stable in NB neurons during the disease process. These data suggest there is a shift in cholinotrophic molecular events in MCI and early AD which may lead to cell dysfunction and eventual cell death over the course of the disease. These findings support the concept that from a neurotrophic pathobiologic perspective, MCI is already early AD.     

Cerebrolysin modulates pronerve growth factor/nerve growth factor ratio and ameliorates the cholinergic deficit in a transgenic model of Alzheimer's disease

Alzheimer's disease (AD) is characterized by degeneration of neocortex, limbic system, and basal forebrain, accompanied by accumulation of amyloid‐β and tangle formation. Cerebrolysin (CBL), a peptide mixture with neurotrophic‐like effects, is reported to improve cognition and activities of daily living in patients with AD. Likewise, CBL reduces synaptic and behavioral deficits in transgenic (tg) mice overexpressing the human amyloid precursor protein (hAPP). The neuroprotective effects of CBL may involve multiple mechanisms, including signaling regulation, control of APP metabolism, and expression of neurotrophic factors. We investigate the effects of CBL in the hAPP tg model of AD on levels of neurotrophic factors, including pro‐nerve growth factor (NGF), NGF, brain‐derived neurotrophic factor (BDNF), neurotropin (NT)‐3, NT4, and ciliary neurotrophic factor (CNTF). Immunoblot analysis demonstrated that levels of pro‐NGF were increased in saline‐treated hAPP tg mice. In contrast, CBL‐treated hAPP tg mice showed levels of pro‐NGF comparable to control and increased levels of mature NGF. Consistently with these results, immunohistochemical analysis demonstrated increased NGF immunoreactivity in the hippocampus of CBL‐treated hAPP tg mice. Protein levels of other neurotrophic factors, including BDNF, NT3, NT4, and CNTF, were unchanged. mRNA levels of NGF and other neurotrophins were also unchanged. Analysis of neurotrophin receptors showed preservation of the levels of TrKA and p75^NTR immunoreactivity per cell in the nucleus basalis. Cholinergic cells in the nucleus basalis were reduced in the saline‐treated hAPP tg mice, and treatment with CBL reduced these cholinergic deficits. These results suggest that the neurotrophic effects of CBL might involve modulation of the pro‐NGF/NGF balance and a concomitant protection of cholinergic neurons. © 2012 Wiley Periodicals, Inc.     

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.     

sAPPalpha enhances the transdifferentiation of adult bone marrow progenitor cells to neuronal phenotypes

The remediation of neurodegeneration and cognitive decline in Alzheimer's Disease (AD) remains a challenge to basic scientists and clinicians. It has been suggested that adult bone marrow stem cells can transdifferentiate into different neuronal phenotypes. Here we demonstrate that the alpha-secretase-cleaved fragment of the amyloid precursor protein (sAPPalpha), a potent neurotrophic factor, potentiates the nerve growth factor (NGF)/retinoic acid (RA) induced transdifferentiation of bone marrow-derived adult progenitor cells (MAPCs) into neural progenitor cells and, more specifically, enhances their terminal differentiation into a cholinergic-like neuronal phenotype. The addition of sAPPalpha to NGF/RA-stimulated MAPCs resulted in their conversion to neuronal-like cells as evidenced by the extension of neurites and the appearance of immature synaptic complexes. MAPCs differentiated in the presence of sAPPalpha and NGF/RA exhibited a 40% to as much as 75% increase in neuronal proteins including NeuN, beta-tubulin III, NFM, and synaptophysin, compared to MAPCs differentiated by NGF/RA alone. This process was accompanied by an increase in the levels of choline acetyltransferase, a marker of cholinergic neurons, compared to those of GABAergic and dopaminergic neuronal subtypes. MAPCs immunopositive for sAPPalpha were identified within the septohippocampal system of transgenic PS/APP mice injected intravenously with sAPPalpha-transfected MAPCs and found in close proximity to the cerebral vasculature. Given that in AD cholinergic neurons are severely vulnerable to neurodegeneration and that the levels of sAPPalpha are significantly reduced, these findings suggest the combined use of sAPPalpha and MAPCs offers a new and potentially powerful therapeutic strategy for AD treatment.     

Cyto- and chemoarchitecture of basal forebrain cholinergic neurons in the common marmoset (Callithrix jacchus)

The cyto- and chemoarchitecture of basal forebrain cholinergic neurons (BFCN) was investigated in the lower primate, the common marmoset (Callithrix jacchus). A large population of magnocellular, hyperchromic, and choline acetyltransferase (ChAT)-positive neurons was detected in the marmoset basal forebrain. The distribution of these neurons was similar to those in higher primates. Thus, ChAT-positive neurons were observed in the medial septum (Ch2), the vertical (Ch2) and horizontal (Ch3) limbs of the diagonal band of Broca, and the nucleus basalis of Meynert (Ch4). The Ch4 complex was relatively well differentiated and displayed distinct sectors. We detected anterior (Ch4a, with a medial and a lateral subdivision), intermediate (Ch4i, with a dorsal and a ventral subdivision), and posterior (Ch4p) sectors in the marmoset Ch4. The Ch4i was relatively small while the Ch4p was large. Similar to the rodent, the marmoset Ch1 extended quite a distance posteriorly, and the Ch4p displayed a major interstitial component distributed within the globus pallidus, its medullary laminae, and the internal capsule. Virtually all of the marmoset BFCN displayed acetylcholinesterase activity, and low affinity (p75(NTR)) and high affinity (Trk) neurotrophin receptor immunoreactivity. A majority contained immunoreactivity for calbindin-D(28K) and calretinin. Many of the Ch4 neurons also displayed tyrosine hydroxylase immunoreactivity. The BFCN lacked galanin immunoreactivity, but were innervated by galanin-positive fibers. None of the marmoset BFCN were NADPH-d-positive. Thus, the BFCN display major anatomical and biochemical differences in the marmoset when compared with higher primates. The marmoset BFCN also display many characteristics common to other primates. This fact, combined with the relatively short life span of the marmoset, indicates that this species may be ideal for studies of age-related changes in the BFCN.     

Evidence that toxicity of lipopolysaccharide upon cholinergic basal forebrain neurons requires the presence of glial cells in vitro

The cholinergic system of the basal forebrain is affected in brains of dementia patients and during neuroinflammation. The aim of this study was to establish a method to cultivate basal forebrain cholinergic neurons in dissociated, pure neuronal cultures and to apply this method to study the effect of acute and chronic experimentally-induced inflammation using lipopolysaccharide. Purity of the cultures, degrees of neuronal dissociation, connectivity and neuronal survival were investigated by immunocytochemistry for microtubule-associated protein-2 (neurons), glial fibrillary acidic protein (astroglia), complement receptor 3 (microglia), choline acetyltransferase and the neurotrophin receptor p75 (cholinergic neurons). Neuronal cultures only contained <7% astrocytes and <1% microglia when using a "sandwich-technique". Acute (1, 10 microg/ml) as well as chronic (0.1, 1 microg/ml) treatment with lipopolysaccharide did neither affect total number of neurons, nor number of p75-positive neurons or enhance expression of major histocompatibility complex I or II. Our results suggest that lipopolysaccharide-induced degeneration of both microtubule-associated protein-2-like immunoreactive as well as specific killing of cholinergic forebrain neurons in vitro are mediated by glial cells.     

β-amyloid binds to p75NTR and activates NFκB in human neuroblastoma cells

Amyloid β peptide (Aβ), a proteolytic fragment of the amyloid precursor protein (APP), is a major component of the plaques found in the brain of Alzheimer's disease (AD) patients. These plaques are thought to cause the observed loss of cholinergic neurons in the basal forebrain of AD patients. In these neurons, particularly those of the nucleus basalis of Meynert, an up-regulation of 75kD-neurotrophin receptor (p75^NTR), a nonselective neurotrophin receptor belonging to the death receptor family, has been reported. p75^NTR expression has been described to correlate with β-amyloid sensitivity in vivo and in vitro, suggesting a possible role for p75^NTR as a receptor for Aβ. Here we used a human neuroblastoma cell line to investigate the involvement of p75^NTR in Aβ-induced cell death. Aβ peptides were found to bind to p75^NTR resulting in activation of NFκB in a time- and dose-dependent manner. Blocking the interaction of Aβ with p75^NTR using NGF or inhibition of NFκB activation by curcumin or NFκB SN50 attenuated or abolished Aβ-induced apoptotic cell death. The present results suggest that p75^NTR might be a death receptor for Aβ, thus being a possible drug target for treatment of AD. J. Neurosci. Res. 54:798–804, 1998. © 1998 Wiley-Liss, Inc.     

Susceptibility to seizure-induced injury and acquired microencephaly following intraventricular injection of saporin-conjugated 192 IgG in developing rat brain

To study the role of neurotrophin-responsive neurons in brain growth and developmental resistance to seizure-induced injury, we infused saporin-conjugated 192-IgG (192 IgG-saporin), a monoclonal antibody directed at the P75 neurotrophin receptors (p75(NTR)), into the ventricles of postnatal day 8 (P8) rat pups. 7-10 days after immunotoxin treatment, loss of p75(NTR) immunoreactivity was associated with depletion of basal forebrain cholinergic projection to the neocortex and hippocampus. Kainic acid (KA)-induced seizures on P15 resulted in hippocampal neuronal injury in the majority of toxin-treated animals (13/16), but only rarely in saline-injected controls (2/25) (P < 0.001). In addition, widespread cerebral atrophy and a significant decrease in brain weight with preserved body weight were observed. Volumetric analysis of the hippocampal hilar region revealed a 2-fold reduction in perikaryal size and a 1.7-fold increase in cell packing density after 192 IgG-saporin injection. These observations indicate that neurotrophin-responsive neurons including basal forebrain magnocellular cholinergic neurons may be critical for normal brain growth and play a protective role in preventing excitotoxic neuronal injury during development.     

Maternal choline supplementation differentially alters the basal forebrain cholinergic system of young‐adult Ts65Dn and disomic mice

Down syndrome (DS), trisomy 21, is a multifaceted condition marked by intellectual disability and early presentation of Alzheimer's disease (AD) neuropathological lesions including degeneration of the basal forebrain cholinergic neuron (BFCN) system. Although DS is diagnosable during gestation, there is no treatment option for expectant mothers or DS individuals. Using the Ts65Dn mouse model of DS that displays age‐related degeneration of the BFCN system, we investigated the effects of maternal choline supplementation on the BFCN system in adult Ts65Dn mice and disomic (2N) littermates at 4.3–7.5 months of age. Ts65Dn dams were maintained on a choline‐supplemented diet (5.1 g/kg choline chloride) or a control, unsupplemented diet with adequate amounts of choline (1 g/kg choline chloride) from conception until weaning of offspring; post weaning, offspring were fed the control diet. Mice were transcardially perfused with paraformaldehyde, and brains were sectioned and immunolabeled for choline acetyltransferase (ChAT) or p75‐neurotrophin receptor (p75^NTR). BFCN number and size, the area of the regions, and the intensity of hippocampal labeling were determined. Ts65Dn‐unsupplemented mice displayed region‐ and immunolabel‐dependent increased BFCN number, larger areas, smaller BFCNs, and overall increased hippocampal ChAT intensity compared with 2N unsupplemented mice. These effects were partially normalized by maternal choline supplementation. Taken together, the results suggest a developmental imbalance in the Ts65Dn BFCN system. Early maternal‐diet choline supplementation attenuates some of the genotype‐dependent alterations in the BFCN system, suggesting this naturally occurring nutrient as a treatment option for pregnant mothers with knowledge that their offspring is trisomy 21. J. Comp. Neurol. 522:1390–1410, 2014. © 2013 Wiley Periodicals, Inc.     

Loss of basal forebrain P75(NTR) immunoreactivity in subjects with mild cognitive impairment and Alzheimer's disease.

The long-held belief that degeneration of the cholinergic basal forebrain was central to Alzheimer's disease (AD) pathogenesis and occurred early in the disease process has been questioned recently. In this regard, changes in some cholinergic basal forebrain (CBF) markers (e.g. the high affinity trkA receptor) but not others (e.g., cortical choline acetyltransferase [ChAT] activity, the number of ChAT and vesicular acetylcholine transporter-immunoreactive neurons) suggest specific phenotypic changes, but not frank neuronal degeneration, early in the disease process. The present study examined the expression of the low affinity p75 neurotrophin receptor (p75(NTR)), an excellent marker of CBF neurons, in postmortem tissue derived from clinically well-characterized individuals who have been classified as having no cognitive impairment (NCI), mild cognitive impairment (MCI), and mild AD. Relative to NCI individuals, a significant and similar reduction in the number of nucleus basalis p75(NTR)-immunoreactive neurons was seen in individuals with MCI (38%) and mild AD (43%). The number of p75(NTR)-immunoreactive nucleus basalis neurons was significantly correlated with performance on the Mini-Mental State Exam, a Global Cognitive Test score, as well as some individual tests of working memory and attention. These data, together with previous reports, support the concept that phenotypic changes, but not frank neuronal degeneration, occur early in cognitive decline. Although there was no difference in p75(NTR) CBF cell reduction between MCI and AD, it remains to be determined whether these findings lend support to the hypothesis that MCI is a prodromal stage of AD.