Nerve growth factor does not influence the expression of beta amyloid precursor protein mRNA in rat brain: in vivo and in vitro studies

Beta-amyloid expression, release and extracellular deposition in aged rat brain slices

Alzheimer's disease (AD) is characterized by beta-amyloid plaques, tau pathology, cholinergic cell death and inflammation. The aim of this study was to investigate whether beta-amyloid is generated, released and extracellularly deposited in organotypic brain slices. In developing slices, no amyloid-precursor protein (APP) was detectable; however, there was a strong upregulation in aging slices. In such slices, rat beta-amyloid(1-42) and -(1-40) peptides were found using four sequence-specific antibodies. APP and beta-amyloid were expressed in neurons and to a lesser extent in astrocytes. Beta-amyloid was secreted into the medium. Beta-amyloid was located extracellularly when aging slices were incubated with medium at pH 6.0 including apolipoprotein E4 (ApoE4). It is concluded that aging organotypic brain slices express beta-amyloid and that acidosis induces cell death with efflux of beta-amyloid and extracellular depositions, which is triggered by ApoE4. This novel in vitro model may enable us to investigate further the pathological cascade for AD and may be useful to explore future therapeutics.

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.

Copper modulation of ion channels of PrP[106-126] mutant prion peptide fragments

We have shown previously that the protease-resistant and neurotoxic prion peptide fragment PrP[106-126] of human PrP incorporates into lipid bilayer membranes to form heterogeneous ion channels, one of which is a Cu(2+)-sensitive fast cation channel. To investigate the role of PrP[106-126]'s hydrophobic core, AGAAAAGA, on its ability to form ion channels and their regulation with Cu(2+), we used the lipid-bilayer technique to examine membrane currents induced as a result of PrP[106-126] (AA/SS) and PrP[106-126] (VVAA/SSSS) interaction with lipid membranes and channel formation. Channel analysis of the mutant (VVAAA/SSS), which has a reduced hydrophobicity due to substitution of hydrophobic residues with the hydrophilic serine residue, showed a significant change in channel activity, which reflects a decrease in the beta-sheet structure, as shown by CD spectroscopy. One of the channels formed by the PrP[106-126] mutant has fast kinetics with three modes: burst, open and spike. The biophysical properties of this channel are similar to those of channels formed with other aggregation-prone amyloids, indicating their ability to form the common beta sheet-based channel structure. The current-voltage (I-V) relationship of the fast cation channel, which had a reversal potential, E(rev), between -40 and -10 mV, close to the equilibrium potential for K(+) ( E(K) = -35 mV), exhibited a sigmoidal shape. The value of the maximal slope conductance (g(max)) was 58 pS at positive potentials between 0 and 140 mV. Cu(2+) shifted the kinetics of the channel from being in the open and "burst" states to the spike mode. Cu(2+) reduced the probability of the channel being open (P(o)) and the mean open time (T(o)) and increased the channel's opening frequency (F(o)) and the mean closed time (T(c)) at a membrane potential ( V(m)) between +20 and + 140 mV. The fact that Cu(2+) induced changes in the kinetics of this channel with no changes in its conductance, indicates that Cu(2+) binds at the mouth of the channel via a fast channel block mechanism. The Cu(2+)-induced changes in the kinetic parameters of this channel suggest that the hydrophobic core is not a ligand Cu(2+) site, and they are in agreement with the suggestion that the Cu(2+)-binding site is located at M(109) and H(111) of this prion fragment. Although the data indicate that the hydrophobic core sequence plays a role in PrP[106-126] channel formation, it is not a binding site for Cu(2+). We suggest that the role of the hydrophobic region in modulating PrP toxicity is to influence PrP assembly into neurotoxic channel conformations. Such conformations may underlie toxicity observed in prion diseases. We further suggest that the conversions of the normal cellular isoform of prion protein (PrP(c)) to abnormal scrapie isoform (PrP(Sc)) and intermediates represent conversions to protease-resistant neurotoxic channel conformations.

Cortico-hippocampal APP and NGF levels are dynamically altered by cholinergic muscarinic antagonist or M1 agonist treatment in normal mice

To determine whether altered cholinergic neurotransmission can modify the long-term secretion of amyloid precursor protein (APP), endogenous levels of APP and nerve growth factor (NGF), we administered a selective M1 muscarinic receptor agonist (RS86) or the muscarinic antagonist, atropine, for 7 days in vivo into young adult mice (C57BL/6j). The levels of NGF and total APP in the hippocampus, frontal cortex, striatum, parietal cortex and cerebrospinal fluid (CSF) were examined by ELISA and Western blot. We found that this repeated i.m. administration of M1 receptor agonist resulted in decreased total APP levels in the hippocampus, frontal cortex and parietal cortex, and increased secreted alpha-APPs levels in the CSF. M1 agonist treatment also resulted in decreased NGF levels in the hippocampus and CSF. These effects of the M1 muscarinic agonist could be blocked by atropine, which by itself elevated tissue levels of total APP. Interestingly, we found that the decrease of total APP in the hippocampus and striatum after M1 agonist treatment inversely correlated with the change in NGF levels. These data suggest that a sustained increased cholinergic, M1-mediated neurotransmission will enhance secretion of alpha-APPs in CSF and adaptively reduce the levels of total APP and NGF in the corticohippocampal regions of normal mice. The dynamic and adaptive regulation linking total APP and NGF levels in normal adult mice is relevant for understanding the pathophysiology of conditions with cholinergic and APP related pathologies, like Alzheimer's disease and Down's syndrome.

Brain-derived neurotrophic factor stimulates beta-amyloid gene promoter activity by a Ras-dependent/AP-1-independent mechanism in SH-SY5Y neuroblastoma cells

The beta-amyloid peptide, the major component of Alzheimer-associated plaques, derives from a larger beta-amyloid precursor protein (APP), that is expressed in both neural and non-neural cells. Overexpression of APP actively contributes to the development of senile plaques and is considered a risk factor for the disease. APP expression is regulated by a variety of cellular mediators, among them ligands of tyrosine kinase receptors. In this study, we present evidence that brain-derived neurotrophic factor (BDNF) modulates, in a dose- and time-dependent fashion, APP promoter activity in SH-SY5Y neuroblastoma cells transiently expressing the receptor TrkB. The APP promoter contains two potential AP-1 sites, and we examined whether or not protein kinase C (PKC) and the AP-1 sites of the promoter mediate the BDNF-induced stimulation of APP. Stimulation of APP promoter activity by BDNF was not affected by the PKC inhibitor bisindolylmaleimide, or by dominant negative mutants of the AP-1 components Fos and Jun, which, however, blocked the response to phorbol esters. These results suggest that activation of the APP promoter by BDNF is largely independent of PKC and AP-1. In contrast, activated Ras increased APP promoter activity in SH-SY5Y cells, and a dominant negative mutant of Ras abolished BDNF-mediated promoter stimulation. Taken together, our results suggest a mechanism that involves activation of the Ras/MAP kinase signaling pathway, and phosphorylation of as yet unidentified effectors which in turn can activate response elements within the APP promoter.

The functions of the amyloid precursor protein gene

The amyloid precursor protein (APP) gene and its protein products have multiple functions in the central nervous system and fulfil criteria as neuractive peptides: presence, release and identity of action. There is increased understanding of the role of secretases (proteases) in the metabolism of APP and the production of its peptide fragments. The APP gene and its products have physiological roles in synaptic action, development of the brain, and in the response to stress and injury. These functions reveal the strategic importance of APP in the workings of the brain and point to its evolutionary significance.

Nerve growth factor treatment in dementia

Millions of people are affected by Alzheimer disease. As longevity increases, so will the number of patients with dementia. This has led to an intense search for successful treatment strategies. One area of interest is neurotrophic factors. Brain development and neuronal maintenance, as well as protective efforts, are mediated by a large number of different neurotrophic factors acting on specific receptors. In neurodegenerative disorders, there may be a possibility of rescuing degenerating neurons and stimulating terminal outgrowth with use of neurotrophic factors. The first neurotrophic factor discovered was nerve growth factor (NGF). A wealth of animal studies have shown that cholinergic neurons are NGF sensitive and NGF dependent, which is especially interesting in cognitive disorders, in which central cholinergic projections are important for cognitive function. In Alzheimer disease, cholinergic neurons have been shown to degenerate. This suggests that NGF may be used to pharmacologically counteract cholinergic degeneration and/or induce terminal sprouting in Alzheimer disease. Data from animal studies, as well as from the author's recent clinical trial, in which NGF was infused to the lateral ventricle in patients with Alzheimer disease, will be presented. Effects of NGF on cognition, as well as issues regarding dosage, side effects, and alternative ways of administering NGF, will be discussed.

Analysis of the 5'-flanking region of the beta-amyloid precursor protein gene that contributes to increased promoter activity in differentiated neuronal cells

To study the transcription control of the beta-amyloid precursor protein (betaAPP) in Alzheimer's disease (AD), we functionally characterized the betaAPP gene promoter in differentiated cells. PC12 cells were first differentiated with nerve growth factor (NGF) and then transient transfection analysis was done with a series of 5'-deletion constructs, that extended as far upstream as -7900 down to +104 base pair (bp) relative to the transcription start site (+1). The truncated regions of the promoter were linked upstream to a reporter gene, chloramphenicol acetyl transferase (CAT). The CAT assay was performed to compare promoter activity of different 5'-flanking and intronic regions of the betaAPP gene. Our results suggest that the longest (-7900/+104) and one of the shortest (-47/+104) regions possessed significantly higher levels of promoter activity than the promoterless vector in NGF-differentiated PC12 cells. A deletion of about 7600 bp region from the -7900 to +104 construct resulted in 50% loss of original promoter activity. A deletion of all but 47 bp from the -7900 to +104 construct resulted in the loss of 66% (and retention of 34%) promoter activity. The region -3416/+104 bp displayed the strongest promoter activity whereas +1/+104 bp showed the least activity among all deletion constructs studied. The upstream region -5529 to -3416 contains a negative regulatory element and -3416 to -1131 contains a positive regulatory element. The very upstream region, -7900 to -3411, lacks independent functional activity. The 5'-UTR region (+1 to +104) showed minimum activity and the -75 to +104 region constitutes the basic promoter element. The first exon or a large part of the first intron (+99 to +6200) did not display any significant promoter activity. Thus, several positive and negative regulatory elements influence the basal level of betaAPP promoter activity in NGF-differentiated PC12 cells. We speculate that any structural alteration(s) due to a specific mutation in these regulatory regions can potentially alter the transcriptional machinery, and that can perhaps affect the level of beta-amyloid protein involved in AD.

Neurotrophin binding to the p75 neurotrophin receptor is necessary but not sufficient to mediate NGF-effects on APP secretion in PC-12 cells

In the present study the pheochromocytoma cell line (PC-12) was used as a model system to determine the role of the two neurotrophin receptors in the regulation of amyloid precursor protein (APP) secretion by nerve growth factor (NGF). To stimulate TrkA and/or p75NTR signaling in PC-12 cells, we used NGF, brain-derived neurotrophic factor (BDNF), and NGF in the presence of an excess of BDNF or the monoclonal antibody 192IgG, to block p75NTR binding to NGF. Our results demonstrate that NGF stimulates APP secretion in a dose dependent fashion with maximum effects at 10 ng/ml, known to saturate high-affinity NGF binding sites. Treatment of PC-12 cells with varying concentrations of BDNF, 1-1,000 ng/ml, did not alter APP secretion, suggesting that binding to p75NTR alone is not sufficient to affect APP secretion. When blocking NGF binding to p75NTR with BDNF or 192IgG, on the other hand, NGF effects on APP secretion were abolished. These findings suggest that in cells expressing p75NTR and TrkA receptors, binding of NGF to the p75NTR is required to mediate NGF effects on APP secretion. Our data are also consistent with a proposed function of the p75NTR in receptor recruitment and "presentation" of NGF to receptors.

Targeted intraparenchymal delivery of human NGF by gene transfer to the primate basal forebrain for 3 months does not accelerate beta-amyloid plaque deposition

Nerve growth factor therapy has been proposed as a potential means of preventing degeneration of basal forebrain cholinergic neurons in Alzheimer's disease and thereby improving cognition. However, NGF has been reported to upregulate expression of the beta-amyloid precursor protein, which in turn could accelerate deposition of "mature" beta-amyloid in the brain. To address this possibility, the brains of 16 adult and aged rhesus monkeys were examined for beta-amyloid plaque deposition in the presence or absence of NGF treatment. Six aged monkeys received intraparenchymal grafts into the cholinergic basal forebrain of autologous cells genetically modified to secrete NGF, six aged monkeys received intraparenchymal grafts of autologous control cells expressing the reporter gene beta-galactosidase, and four adult nonoperated monkeys served as additional controls. All brains were examined for expression of mature beta-amyloid using an antibody recognizing amino acids 1-40 of the beta-amyloid peptide. Amyloid plaques were systematically quantified in representative sections of the temporal, frontal, cingulate, insular, and parietal cortices and in the amygdala and hippocampus. Results disclosed that aging resulted in an increase in amyloid plaque formation: no plaques at all were detected in nonaged monkeys, whereas a mean of 20 +/- 13 plaques per section were present in control-aged monkeys. Aged subjects with intraparenchymal NGF-secreting grafts for 3 months contained a mean of 29 +/- 14 plaques per section, an amount that did not differ significantly from control-aged monkeys (P = 0.66). Thus, 3 months of intraparenchymal NGF delivery did not significantly increase beta-amyloid deposition.

The amyloid precursor protein gene: a neuropeptide gene with diverse functions in the central nervous system

The amyloid precursor protein (APP) is a member of a family of proteins found in the central nervous system with a fundamental role in the pathogenesis of Alzheimer's disease. This review describes the experimental evidence that has provided functional insights into this protein and emphasizes the importance of APP in many neurobiological processes.

Amyloid in Alzheimer's disease and prion-related encephalopathies: studies with synthetic peptides

Deposition of amyloid-beta protein (beta A) in brain parenchyma and vessel walls is a major pathological feature of Alzheimer's disease (AD). In prion-related encephalopathies (PRE), too, an altered form of prion protein (PrPsc) forms amyloid fibrils and accumulates in the brain. In both conditions the amyloid deposition is accompanied by nerve cell loss, whose pathogenesis and molecular basis are not understood. Neuropathological, genetic and biochemical studies indicate a central role of beta A in the AD pathogenesis. Synthetic peptides homologous to beta A and its fragments contribute to investigate the mechanisms of beta A deposit formation and the role played by beta A in AD pathogenesis. The physicochemical studies on the beta-sheet conformation and self-aggregation properties of beta A peptides indicate the conditions and the factors influencing the formation of beta A deposits. The neurotoxic activity of beta A and its fragments support the causal relationship between beta A deposits and the neuropathological events in AD. Numerous studies were performed to clarify the mechanism of neuronal death induced by exposure to beta A peptides. A similar approach has been used to investigate the role of PrPsc in PRE; in these diseases, the association between accumulation of PrPsc and neuropathology is evident and numerous data indicate that PrPsc itself might be the infectious agent responsible for disease transmission. Thus, PrP peptides were used to investigate the pathogenic role of PrPsc in PRE and the conformational change responsible for the conversion PrPc to PrPsc that makes the molecule apparently infectious. In particular, we synthesized a peptide homologous to residues 106-126, an integral part of all abnormal PrP isoforms that accumulate in the brain of subjects' PRE. This peptide is fibrillogenic, has secondary structure largely composed of beta-sheet and proteinase-resistant properties, is neurotoxic and induces astrogliosis. In this review, we summarize and compare the data obtained with beta A and PrP peptides and analyze the significance in terms of amyloidogenic proteins and neurodegeneration.

A comprehensive study of the spatiotemporal pattern of beta-amyloid precursor protein mRNA and protein in the rat brain: lack of modulation by exogenously applied nerve growth factor

Nerve growth factor (NGF) is a neurotrophic factor for basal forebrain cholinergic neurons, a population that degenerates and dies in Alzheimer's disease (AD). It has been suggested that NGF be used to treat AD patients. However, in vivo administration of NGF to the developing hamster brain was shown to induce the expression of the beta-amyloid precursor protein (beta APP) gene. The association of alterations in beta APP gene expression with AD-like neuropathological changes and cognitive impairment in animals, and with AD-like neurodegeneration in Down syndrome patients suggests that NGF-mediated increases in beta APP expression could negate or attenuate NGF's neurotrophic activity in AD treatment trials. The present study was undertaken to explore further the influence of NGF on beta APP expression, and to determine which, if any, of the beta APP mRNAs is altered in response to NGF treatment. We first examined the spatiotemporal pattern of beta APP-695 and Kunitz protease inhibitor (KPI)-containing beta APP mRNA expression in the rat brain. Specific oligonucleotide probes were used to show that these mRNAs are present during embryonic development. In addition, we evaluated postnatal expression in nine brain regions and showed that beta APP mRNAs were readily detected in all regions at postnatal day 2. In human brain, the relative levels of beta APP-695 and beta APP-KPI mRNA and their protein are discordant, in that the level of beta APP-695 mRNA is slightly higher than that of beta APP-KPI, but beta APP-KPI protein predominates. In contrast, the several-fold excess of beta APP-695 mRNA relative to beta APP-KPI mRNA in the rat brain was also reflected at the protein level. Surprisingly, administration of exogenous NGF failed to affect rat beta APP mRNA levels either in vitro or during postnatal development in vivo.

Promoter activity of the gene encoding the beta-amyloid precursor protein is up-regulated by growth factors, phorbol ester, retinoic acid and interleukin-1

Abnormalities in gene regulation of the beta-amyloid precursor protein (beta APP) might be an important factor in the neuropathology of Alzheimer's disease. We analyzed the effects of nerve growth factor (NGF), basic fibroblast growth factor (bFGF), phorbol 12-myristate 13-acetate (PMA), interleukin-1 (IL-1) and retinoic acid (RA) on promoter activity of the beta APP gene. To investigate the effect of these factors on promoter activity, we used two fusion plasmids which contain sequences of -489 and -415 base pairs (bp), respectively, from the transcription start site of the beta APP gene. The truncated regions of the promoter wer linked upstream to a reporter gene, chloramphenicol acetyl transferase (CAT). Promoter activity was tested by transient transfection of fusion plasmids in PC12 cells using the electroporation method (960 microF at 350 V). We report that the treatment of PC12 cells with either NGF, bFGF, PMA, IL-1 or RA stimulated the activity of the beta APP promoter. The treatment of cells with either NGF or bFGF resulted in a higher degree of stimulation in the basal level of promoter activity than when cells were treated with either PMA, IL-1 or RA. The deletion of sequences between -489 to -416 bp had no significant effect on promoter activity. The treatment of cells with these factors for a duration of 4 days prior to transfection with the plasmids is necessary for the stimulatory effect. The cells that were only treated with any of these factors after transfection showed no significant change in the basal level of promoter activity. We conclude that certain growth factors and a cytokine could enhance the basal level of promoter activity of the beta APP gene, suggesting a possible participation of a growth-factor(s)-mediated transcription element in the control of gene expression of beta APP.

Nerve growth factor and Alzheimer's disease

Nerve growth factor (NGF) is a well-characterized protein that exerts pharmacological effects on a group of cholinergic neurons known to atrophy in Alzheimer's disease (AD). Considerable evidence from animal studies suggests that NGF may be useful in reversing, halting, or at least slowing the progression of AD-related cholinergic basal forebrain atrophy, perhaps even attenuating the cognitive deficit associated with the disorder. However, many questions remain concerning the role of NGF in AD. Levels of the low-affinity receptor for NGF appear to be at least stable in AD basal forebrain, and the recent finding of AD-related increases in cortical NGF brings into question whether endogenous NGF levels are related to the observed cholinergic atrophy and whether additional NGF will be useful in treating this disorder. Evidence regarding the localization of NGF within the central nervous system and its presumed role in maintaining basal forebrain cholinergic neurons is summarized, followed by a synopsis of the relevant aspects of AD neuropathology. The available data regarding levels of NGF and its receptor in the AD brain, as well as potential roles for NGF in the pathogenesis and treatment of AD, are also reviewed. NGF and its low affinity receptor are abundantly present within the AD brain, although this does not rule out an NGF-related mechanism in the degeneration of basal forebrain neurons, nor does it eliminate the possibility that exogenous NGF may be successfully used to treat AD. Further studies of the degree and distribution of NGF within the human brain in normal aging and in AD, and of the possible relationship between target NGF levels and the status of basal forebrain neurons in vivo, are necessary before engaging in clinical trials.