Amyloid Precursor Protein Holoprotein Research Articles

Expression and proteolytic processing of the amyloid precursor protein is unaffected by the expression of the three human apolipoprotein E alleles in the brains of mice

The 3 human apolipoprotein E (APOE) gene alleles modify an individual's risk of developing Alzheimer's disease (AD): compared to the risk-neutral APOE ε3 allele, the ε4 allele (APOE4) is strongly associated with increased AD risk while the ε2 allele is protective. Multiple mechanisms have been shown to link APOE4 expression and AD risk, including the possibility that APOE4 increases the expression of the amyloid precursor protein (APP) (Y-W.A. Huang, B. Zhou, A.M. Nabet, M. Wernig, T.C. Südhof, 2019). In this study, we investigated the impact of APOE genotype on the expression, and proteolytic processing of endogenously expressed APP in the brains of mice humanized for the 3 APOE alleles. In contrast to prior studies using neuronal cultures, we found in the brain that both App gene expression, and the levels of APP holoprotein were not affected by APOE genotype. Additionally, our analysis of APP fragments showed that APOE genotype does not impact APP processing in the brain: the levels of both α- and β-cleaved soluble APP fragments (sAPPs) were similar across genotypes, as were the levels of the membrane-associated α- and β-cleaved C-terminal fragments (CTFs) of APP. Lastly, APOE genotype did not impact the level of soluble amyloid beta (Aβ). These findings argue that the APOE-allele-dependent AD risk is independent of the brain expression and processing of APP.

The cellular expression and proteolytic processing of the amyloid precursor protein is independent of TDP-43.

Alzheimer’s disease (AD) is a neurodegenerative condition, of which one of the cardinal pathological hallmarks is the extracellular accumulation of amyloid β (Aβ) peptides. These peptides are generated via proteolysis of the amyloid precursor protein (APP), in a manner dependent on the β-secretase, BACE1 and the multicomponent γ-secretase complex. Recent data also suggest a contributory role in AD of transactive response DNA binding protein 43 (TDP-43). There is little insight into a possible mechanism linking TDP-43 and APP processing. To this end, we used cultured human neuronal cells to investigate the ability of TDP-43 to interact with APP and modulate its proteolytic processing. Immunocytochemistry showed TDP-43 to be spatially segregated from both the extranuclear APP holoprotein and its nuclear C-terminal fragment. The latter (APP intracellular domain) was shown to predominantly localise to nucleoli, from which TDP-43 was excluded. Furthermore, neither overexpression of each of the APP isoforms nor siRNA-mediated knockdown of APP had any effect on TDP-43 expression. Doxycycline-stimulated overexpression of TDP-43 was explored in an inducible cell line. Overexpression of TDP-43 had no effect on expression of the APP holoprotein, nor any of the key proteins involved in its proteolysis. Furthermore, increased TDP-43 expression had no effect on BACE1 enzymatic activity or immunoreactivity of Aβ1-40, Aβ1-42 or the Aβ1-40:Aβ1-42 ratio. Also, siRNA-mediated knockdown of TDP-43 had no effect on BACE1 immunoreactivity. Taken together, these data indicate that TDP-43 function and/or dysfunction in AD is likely independent from dysregulation of APP expression and proteolytic processing and Aβ generation.

APP-Mediated Signaling Prevents Memory Decline in Alzheimer’s Disease Mouse Model

Amyloid precursor protein (APP) and its metabolites play key roles in Alzheimer's disease (AD) pathophysiology. Whereas short amyloid-β (Aβ) peptides derived from APP are pathogenic, the APP holoprotein serves multiple purposes in the nervous system through its cell adhesion and receptor-like properties. Our studies focused on the signaling mediated by the APP cytoplasmic tail. We investigated whether sustained APP signaling during brain development might favor neuronal plasticity and memory process through a direct interaction with the heterotrimeric G-protein subunit GαS (stimulatory G-protein alpha subunit). Our results reveal that APP possesses autonomous regulatory capacity within its intracellular domain that promotes APP cell surface residence, precludes Aβ production, facilitates axodendritic development, and preserves cellular substrates of memory. Altogether, these events contribute to strengthening cognitive functions and are sufficient to modify the course of AD pathology.

The Effects of Extracellular Serum Concentration on APP Processing in Npc1-Deficient APP-Overexpressing N2a Cells.

Amyloid precursor protein (APP) is cleaved by a set of proteases including α-/β-/γ- and recently identified η-secretases, generating C-terminal fragments (CTFs) of varying lengths and amyloid β (Aβ) peptides, which are considered to play a pivotal role in Alzheimer's disease (AD) pathogenesis. Cellular cholesterol content/distribution can regulate the production/clearance of APP metabolites and hence modify AD pathology. To determine the functional relation between endosomal-lysosomal (EL) cholesterol sequestration and APP metabolism, we used our recently developed mouse N2a-ANPC cells that overexpress Swedish mutant human APP in the absence of cholesterol-trafficking Niemann-Pick type C1 (Npc1) protein. Here, we report that neither increased levels nor EL cholesterol sequestration altered APP holoprotein levels but caused the intracellular accumulation of APP α-/β-/η-CTFs and Aβ1-40/42 peptides. The levels of APP-cleaved products increased as a function of extracellular serum concentration in N2a-ANPC cells, which are more vulnerable to death than the control cells. Additionally, we show that pH of the lysosomal vesicles in N2a-ANPC cells shifted to a less acidic range with increasing serum concentrations, thus making them less efficient functionally. Interestingly, the addition of cholesterol to the culture media not only increased the levels of cellular cholesterol and APP-cleaved products but also rendered the cells more vulnerable to toxicity. Collectively, our results suggest that extracellular cholesterol concentration in serum under conditions of Npc1 deficiency can influence intracellular cholesterol content/distribution and lysosomal efficacy, triggering the accumulation of toxic APP-cleaved products, eventually leading to cell death.

Effects of cholesterol transport inhibitor U18666A on APP metabolism in rat primary astrocytes.

Amyloid β (Aβ) peptides generated from the amyloid precursor protein (APP) play an important role in the degeneration of neurons and development of Alzheimer's disease (AD). Current evidence indicates that high levels of cholesterol-which increase the risk of developing AD-can influence Aβ production in neurons. However, it remains unclear how altered level/subcellular distribution of cholesterol in astrocytes can influence APP metabolism. In this study, we evaluated the effects of cholesterol transport inhibitor U18666A-a class II amphiphile that triggers redistribution of cholesterol within the endosomal-lysosomal (EL) system-on APP levels and metabolism in rat primary cultured astrocytes. Our results revealed that U18666A increased the levels of the APP holoprotein and its cleaved products (α-/β-/η-CTFs) in cultured astrocytes, without altering the total levels of cholesterol or cell viability. The cellular levels of Aβ1-40 were also found to be markedly increased, while secretory levels of Aβ1-40 were decreased in U18666A-treated astrocytes. We further report a corresponding increase in the activity of the enzymes regulating APP processing, such as α-secretase, β-secretase, and γ-secretase as a consequence of U18666A treatment. Additionally, APP-cleaved products are partly accumulated in the lysosomes following cholesterol sequestration within EL system possibly due to decreased clearance. Interestingly, serum delipidation attenuated enhanced levels of APP and its cleaved products following U18666A treatment. Collectively, these results suggest that cholesterol sequestration within the EL system in astrocytes can influence APP metabolism and the accumulation of APP-cleaved products including Aβ peptides, which can contribute to the development of AD pathology.

OP07 Neuroprotective effect of hydrogen sulfide: Regulation of amyloidosis and inflammation in SH-SY5Y neuroblastoma and BV-2 microglia cells

The excessive accumulation of amyloid peptide and chronic neuroinflammation have been shown to play indisputable roles in pathogenesis of Alzheimer’s disease (AD). In the process of ongoing neurological damage in AD, increased extracellular levels of ATP and its metabolites (e.g. adenosine) in brain worsen the scenario by evoking the cascade of harmful events. The goal of this project was to investigate anti-amyloidogenic and anti-inflammatory effects of hydrogen sulfide (H 2 S) in various in vitro experimental models used for studying AD pathology. We found that NaHS attenuated HENECA (selective A2A adenosinergic receptor agonist) induced amyloid β -42 (A β 42) production in SH-SY5Y cells. NaHS interfered with the production and post-translational modification of amyloid precursor protein (APP) by inhibiting APP maturation. While NaHS did not affect β -secretase activity, it significantly inhibited γ -secretase activity. NaHS induced-reductions were accompanied by similar decreases in intracellular cAMP levels and phosphorylation of cAMP responsive element binding protein (CREB). Moreover, studies using forskolin (adenylyl cyclase agonist) and IBMX (phosphodiesterase inhibitor) suggested that NaHS may preferentially suppress adenylyl cyclase activity when it is stimulated. Therefore, H 2 S protects SH-SY5Y cells against HENECA induced amyloidogenesis by inhibiting γ -secretase via cAMP dependent pathway. Consistent with its effect in SH-SY5Y cells, NaHS significantly reduced ATP induced APP holoprotein production and A β 42 levels in murine microglial BV-2 cells. We found that NaHS suppressed ATP induced production of pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF- α ) and interleukin - 1 beta (IL-1 β ). NaHS reversed the upregulation of oxidative stress markers like nitric oxide (NO) and reactive oxygen species (ROS) accompanied by decreases in the protein expression of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2). Furthermore, NaHS suppressed ATP augmentation of extracellular signal-regulated protein kinases 1 and 2 (ERK 1/2) expression as P2 purinergic receptor couple to the ERK signaling pathway. We also found that inhibition of ERK pathway by NaHS blocked specific serine phosphorylation of signal transducer and activator of transcription 3 (STAT3) transcription factor which regulates genes participating in inflammation and amyloidogenesis. To this end, we found that H 2 S inhibits ATP induced inflammation and amyloidogenesis in BV-2 cells via regulating ERK1/2 and STAT3 signaling pathway. Overall, our results demonstrate that by combination of anti-amyloidogenic and anti-inflammatory properties, H 2 S can be of immense therapeutic value in treatment of AD.

The ATP-Binding Cassette Transporter-2 (ABCA2) Increases Endogenous Amyloid Precursor Protein Expression and Abeta Fragment Generation

The ATP binding cassette transporter-2 (ABCA2) has been genetically linked to Alzheimer's disease but the molecular mechanisms are unknown. In this study, the effects of expression of human ABCA2 on endogenous amyloid precursor protein (APP) expression, trafficking and processing were examined in mouse N2a neuronal cells. ABCA2 expression increased the steady-state APP mRNA levels through transcription. ABCA2 also induced increased synthesis of APP holoprotein and altered APP processing and metabolite generation. ABCA2 expression promoted b-secretase (BACE1) cleavage of APP not at the common Asp1 amino acid site (β-site) of Aβ in APP but at the Glu11 site (β'-site) to increase C89 carboxyl-terminal fragment levels (β'-CTF/C89). The levels of N-terminally truncated Aβ11-40 peptides were also increased by ABCA2 expression. The delivery of newly synthesized APP to the cell surface through the secretary pathway was not perturbed by ABCA2 expression; however, ABCA2 expression increased the amount of APP in early-endosomal compartments, which also contained the highest levels of β'-CTF/C89 and is likely the site of increased BACE1 processing of APP. This report identifies ABCA2 as a key regulator of endogenous APP expression and processing and suggests a possible biochemical mechanism linking ABCA2 expression, APP processing and Alzheimer's disease.

Amyloidogenic Processing of APP in Lipid Rafts

Increased generation of amyloid β peptide (Aβ) derived from amyloid precursor protein (APP) is the primary pathological characteristic of Alzheimer’s disease (AD). However, the sub cellular compartment in which APP undergoes cleavage by secretases to generate Aβ is not precisely known. Compelling evidences suggest that amyloidogenic processing of APP occurs in lipid rafts. An indirect support for lipid raft processing of APP includes the localization of Aβ, APP C-terminal fragments (CTFs), APP holoprotein and secretases in the lipid raft microdomains, although few studies failed to find APP in the lipid rafts. The indirect support also comes from both experimental and clinical studies involving modulation of cholesterol levels and its effect on Aβ generation. Moderate depletion of cholesterol results in significant reduction in Aβ levels and increased dietary intake of cholesterol leads to higher levels of Aβ production suggesting that amyloidogenic processing of APP strongly depends on cholesterol levels and therefore on lipid raft integrity. More convincing evidence that lipid rafts are critical for amyloidogenic processing of APP comes from studies using antibody-mediated co-patching of APP and BACE1 which results in lipid raft association of APP and BACE1 and increased Aβ generation. Further, an endosome/lipid raft targeting of β-secretase inhibitor by sterol-mediated anchoring leading to reduced Aβ generation also suggests that lipid rafts are pivotal for amyloidogenic processing of APP. In the absence of an effective therapy for AD, proteins responsible for delivery of APP to lipid rafts including LRP, RanBP9 and ApoER2 may be excellent therapeutic targets in AD.

Novel Role of RanBP9 in BACE1 Processing of Amyloid Precursor Protein and Amyloid β Peptide Generation

Accumulation of the amyloid beta (Abeta) peptide derived from the proteolytic processing of amyloid precursor protein (APP) is the defining pathological hallmark of Alzheimer disease. We previously demonstrated that the C-terminal 37 amino acids of lipoprotein receptor-related protein (LRP) robustly promoted Abeta generation independent of FE65 and specifically interacted with Ran-binding protein 9 (RanBP9). In this study we found that RanBP9 strongly increased BACE1 cleavage of APP and Abeta generation. This pro-amyloidogenic activity of RanBP9 did not depend on the KPI domain or the Swedish APP mutation. In cells expressing wild type APP, RanBP9 reduced cell surface APP and accelerated APP internalization, consistent with enhanced beta-secretase processing in the endocytic pathway. The N-terminal half of RanBP9 containing SPRY-LisH domains not only interacted with LRP but also with APP and BACE1. Overexpression of RanBP9 resulted in the enhancement of APP interactions with LRP and BACE1 and increased lipid raft association of APP. Importantly, knockdown of endogenous RanBP9 significantly reduced Abeta generation in Chinese hamster ovary cells and in primary neurons, demonstrating its physiological role in BACE1 cleavage of APP. These findings not only implicate RanBP9 as a novel and potent regulator of APP processing but also as a potential therapeutic target for Alzheimer disease.

Differential effects of interleukin-1β and S100B on amyloid precursor protein in rat retinal neurons

Interleukin-1beta (IL-1beta) and S100B calcium binding protein B (S100B) have been implicated in the pathogenesis of Alzheimer's disease. Both are present in and around senile plaques and have been shown to increase levels of amyloid precursor protein (APP) mRNA in vitro. However, it is not known how either of these substances affects APP in vivo. We have studied the effects of IL-1beta and S100B on the expression and processing of APP using a retinal-vitreal model. We have also investigated the effect of amyloid beta peptide (Abeta) on APP in the same system and the regulation of S100B production by Abeta and IL-1beta from retinal glial cells. Retinal ganglion cells constitutively express APP. However, after intravitreal injection of IL-1beta or Abeta there was a marked reduction in APP levels as detected by Western blotting and IL-1beta produced a decrease in APP immunoreactivity (IR). Nissl staining showed that the integrity of the injected retinas was unchanged after injection. Two days after S100B injection, there was a small reduction in APP-IR but this was accompanied by the appearance of some intensely stained large ganglion cells and there was some up-regulation in APP holoprotein levels on Western blot. Seven days post-S100B injection, these large, highly stained cells had increased in number throughout the retina. Injection of Abeta and IL-1beta also caused an increase in S100B production within the retinal Müller glial cells. These results support the hypothesis that S100B (a glial-derived neurotrophic factor) and IL-1beta (a pro-inflammatory cytokine) can modulate the expression and processing of APP in vivo and so may contribute to the progression of Alzheimer's disease.

The Cholesterol Transport Inhibitor U18666a Regulates Amyloid Precursor Protein Metabolism and Trafficking in N2aAPP “Swedish” Cells

Cholesterol transport is a key regulator of amyloid precursor protein (APP) processing and beta-amyloid (Abeta production, implicated in Alzheimer's disease. Perturbation of cholesterol transport can be pharmacologically induced by the class II amphiphile 3-beta-[2-(diethylamino)ethoxy]androst-5-en-17-one, U18666a; however, the mechanisms by which U18666a controls APP metabolism and trafficking have not been elucidated. We proposed to determine how U18666a regulates APP holoprotein metabolism and trafficking in N2a mouse neuroblastoma cells stably expressing the human APP protein. Secretion of Abeta1-40 was reduced in U18666a-treated cells. U18666a elevated the steady state level of the APP holoprotein but not APP mRNA levels. U18666a increased sAPPalpha secretion and intracellular alpha-CTF/C83 levels but intracellular betaCTF/C99 levels were reduced. The increase in APP protein level was due to decreased catabolism rather than increased APP synthesis. Interestingly, U18666a regulated APP trafficking and increased the level of the holoprotein at the cell surface for alpha-secretase processing and reduced internalization for beta-secretase processing. These data demonstrate that U18666a effects on cholesterol transport function to regulate amyloid precursor protein metabolism and trafficking.

Novel N-terminal Cleavage of APP Precludes Aβ Generation in ACAT-Defective AC29 Cells

A common pathogenic event that occurs in all forms of Alzheimer's disease is the progressive accumulation of amyloid beta-peptide (Abeta) in brain regions responsible for higher cognitive functions. Inhibition of acyl-coenzyme A: cholesterol acyltransferase (ACAT), which generates intracellular cholesteryl esters from free cholesterol and fatty acids, reduces the biogenesis of the Abeta from the amyloid precursor protein (APP). Here we have used AC29 cells, defective in ACAT activity, to show that ACAT activity steers APP either toward or away from a novel proteolytic pathway that replaces both alpha and the amyloidogenic beta cleavages of APP. This alternative pathway involves a novel cleavage of APP holoprotein at Glu281, which correlates with reduced ACAT activity and Abeta generation in AC29 cells. This sterol-dependent cleavage of APP occurs in the endosomal compartment after internalization of cell surface APP. The resulting novel C-terminal fragment APP-C470 is destined to proteasomal degradation limiting the availability of APP for the Abeta generating system. The proportion of APP molecules that are directed to the novel cleavage pathway is regulated by the ratio of free cholesterol and cholesteryl esters in cells. These results suggest that subcellular cholesterol distribution may be an important regulator of the cellular fate of APP holoprotein and that there may exist several competing proteolytic systems responsible for APP processing within the endosomal compartment.

Alzheimer's disease: advances in trafficking

Alzheimer's disease: advances in trafficking

The Common Inhalation Anesthetic Isoflurane Induces Apoptosis and Increases Amyloid β Protein Levels

The common inhalation anesthetic isoflurane has previously been reported to enhance the aggregation and cytotoxicity of the Alzheimer disease-associated amyloid beta protein (Abeta), the principal peptide component of cerebral beta-amyloid deposits. H4 human neuroglioma cells stably transfected to express human full-length wild-type amyloid precursor protein (APP) were exposed to 2% isoflurane for 6 h. The cells and conditioned media were harvested at the end of the treatment. Caspase-3 activation, processing of APP, cell viability, and Abeta levels were measured with quantitative Western blotting, cell viability kit, and enzyme-linked immunosorbent assay sandwich. The control condition consisted of 5% CO2 plus 21% O2 and balanced nitrogen, which did not affect caspase-3 activation, cell viability, APP processing, or Abeta generation. Two percent isoflurane caused apoptosis, altered processing of APP, and increased production of Abeta in H4 human neuroglioma cell lines. Isoflurane-induced apoptosis was independent of changes in Abeta and APP holoprotein levels. However, isoflurane-induced apoptosis was potentiated by increased levels of APP C-terminal fragments. A clinically relevant concentration of isoflurane induces apoptosis, alters APP processing, and increases Abeta production in a human neuroglioma cell line. Because altered processing of APP leading to accumulation of Abeta is a key event in the pathogenesis of Alzheimer disease, these findings may have implications for use of this anesthetic agent in individuals with excessive levels of cerebral Abeta and elderly patients at increased risk for postoperative cognitive dysfunction.

Focally Elevated Creatine Detected in Amyloid Precursor Protein (APP) Transgenic Mice and Alzheimer Disease Brain Tissue

The creatine/phosphocreatine system, regulated by creatine kinase, plays an important role in maintaining energy balance in the brain. Energy metabolism and the function of creatine kinase are known to be affected in Alzheimer diseased brain and in cells exposed to the beta-amyloid peptide. We used infrared microspectroscopy to examine hippocampal, cortical, and caudal tissue from 21-89-week-old transgenic mice expressing doubly mutant (K670N/M671L and V717F) amyloid precursor protein and displaying robust pathology from an early age. Microcrystalline deposits of creatine, suggestive of perturbed energetic status, were detected by infrared microspectroscopy in all animals with advanced plaque pathology. Relatively large creatine deposits were also found in hippocampal sections from post-mortem Alzheimer diseased human brain, compared with hippocampus from non-demented brain. We therefore speculate that this molecule is a marker of the disease process.

The Integrated Role of Desferrioxamine and Phenserine Targeted to an Iron‐Responsive Element in the APP‐mRNA 5′‐Untranslated Region

The Alzheimer's amyloid precursor protein (APP) is the metalloprotein that is cleaved to generate the pathogenic Abeta peptide. We showed that iron closely regulated the expression of APP by 5'-untranslated region (5'-UTR) sequences in APP mRNA. Iron modulated APP holoprotein expression by a pathway similar to iron control of the translation of the ferritin-L and -H mRNAs by iron-responsive elements in their 5'-UTRs. APP gene transcription is also responsive to copper deficit where the Menkes protein depleted fibroblasts of copper to suppress transcription of APP through metal regulatory and copper regulatory sequences upstream of the APP 5' cap site. APP is a copper-zinc metalloprotein and chelation of Fe(3+) by desferrioxamine and Cu(2+) by clioquinol appeared to provide effective therapy for the treatment of AD in limited patient studies. We have introduced an RNA-based screen for small APP 5'-UTR binding molecules to identify leads that limit APP translation (but not APLP-1 and APLP-2) and amyloid Abeta peptide production. A library of 1200 drugs was screened to identify lead drugs that limited APP 5'-UTR-directed translation of a reporter gene. The efficacy of these leads was confirmed for specificity in a cell-based secondary assay to measure the steady-state levels of APP holoprotein relative to APLP-1/APLP-2 by Western blotting. Several chelators were identified among the APP 5'-UTR directed leads consistent with the presence of an IRE stem-loop in front of the start codon of the APP transcript. The APP 5'-UTR-directed drugs--desferrioxamine (Fe(3+) chelator), tetrathiomolybdate (Cu(2+) chelator), and dimercaptopropanol (Pb(2+) and Hg(2+) chelator)--each suppressed APP holoprotein expression (and lowered Abeta peptide secretion). The novel anticholinesterase phenserine also provided "proof of concept" for our strategy to target the APP 5'-UTR sequence to identify "anti-amyloid" drugs. We further defined the interaction between iron chelation and phenserine action to control APP 5'-UTR-directed translation in neuroblastoma (SY5Y) transfectants. Phenserine was most efficient to block translation under conditions of intracellular iron chelation with desferrioxamine suggesting that this anticholinesterase operated through an iron (metal)-dependent pathway at the APP 5'-UTR site.

Amyloid precursor protein (APP) and its derivatives change after cellular energy depletion. An in vitro-study

To study the relationship between the metabolism of amyloid precursor protein (APP) and cellular energy failure, HEK 293 cells stably transfected with betaAPP 695 underwent graded energy failure induced either by i) hypoxia (pO(2) 25 mm Hg), ii) inhibition of the respiratory chain by sodium azide (NaN(3)), or iii) by combined glucose deprivation/hypoxia of different duration and severity. Secreted APP (APPs) and the derivative betaA4 were quantified autoradiographically by immunoprecipitation, and [(35)S] methionine labeling. APP holoprotein (APPh) was determined by Western blot analysis. The concentrations of the energy-rich metabolites ATP, ADP, creatine phosphate (CrP), and adenosine were measured by high performance liquid chromatography. Mild to moderate energy failure after NaN(3) treatment (2h, 4h) and hypoxia (2h, 8h) was characterized by normal ATP concentration but also by a high reduction in CrP. A stress condition indicated by an increased ATP turnover and adenosine increase was obtained. Intracellular APPh increased but its metabolites APPs and betaA4 as measured in the extracellular compartment decreased. These changes may point to a compensatory response of APP but also to a initial disturbance in intracellular APP metabolism. Severe abnormalities in both energy formation and utilization after 8h NaN(3) and hypoxia glucose deprivation were found to be accompanied by a drastic fall in intracellular APPh concentration by at least 50%, paralleled by an accelerating reduction in the extracellular concentrations of both APPs and betaA4.A significant linear correlation between APPh and ATP and between CrP and betaA4 became obvious. The data of the present study indicate that abnormalities in APP metabolism were generated in an energy-dependent manner. The obvious similarities to sporadic Alzheimer s disease are discussed.

Prostaglandin E 2 regulates amyloid precursor protein expression via the EP2 receptor in cultured rat microglia

We investigated the effects of prostaglandin E 2 (PGE 2) on amyloid precursor protein (APP) expression in cultured rat microglia. PGE 2 treatment significantly increased the expression of APP holoprotein and was associated with an elevation in cyclic AMP (cAMP). Direct activation of adenylate cyclase with forskolin also increased APP expression. Co-treatment of microglia with PGE 2 and the PKA inhibitor H-89 suppressed the overexpression of APP caused by PGE 2 alone. The prostaglandin EP2 receptor is known to be positively coupled to cAMP production. Stimulation of the EP2 receptor with butaprost increased APP holoprotein, whereas co-incubation of the cells with PGE 2 and the EP2 receptor antagonist AH-6809 blocked the effect of PGE 2 on APP expression. These data suggest that PGE 2 is able to regulate the expression of APP, and that this effect may be mediated by the EP2 receptor and the cAMP signaling cascade.

FDA-preapproved drugs targeted to the translational regulation and processing of the amyloid precursor protein.

The 5' untranslated region (5'UTR) of the transcript encoding the Alzheimer's amyloid precursor protein (APP) is a key regulatory sequence that determines the amount of intracellular APP holoprotein present in brain derived cells. Using neuroblastoma cells (SY5Y) we developed a transfection based screen of a library of FDA drugs to identify compounds that limited APP luciferase reporter expression translated from the APP 5'UTR. Paroxetine (Paxil trade mark ), dimercaptopropanol, phenserine, desferrioxamine, tetrathiolmobdylate, and azithromycin were six leads that were subsequently found to also suppress APP holoprotein levels or to alter APP cleavage (azithromycin). Since APP holoprotein levels are proportionate to Abeta peptide output in many systems we tested the efficacy of paroxetine and dimercaptopropanol to limit Abeta secretion as measured by ELISA assays. Paroxetine and dimercaptopropanol limited Abeta peptide secretion from lens epithelial cells (B3 cells). Interestingly, paroxetine changed the steady-state levels of transferrin receptor mRNAs. These data suggested that this serotonin reuptake inhibitor (SSRI) provided extra pharmacological action to chelate interacellular iron or change the intracellular iron distribution. An altered iron distribution would be predicted to indirectly limit APP holoprotein expression and Abeta peptide secretion.

Rab5-stimulated Up-regulation of the Endocytic Pathway Increases Intracellular β-Cleaved Amyloid Precursor Protein Carboxyl-terminal Fragment Levels and Aβ Production

We previously identified abnormalities of the endocytic pathway in neurons as the earliest known pathology in sporadic Alzheimer's disease (AD) and Down's syndrome brain. In this study, we modeled aspects of these AD-related endocytic changes in murine L cells by overexpressing Rab5, a positive regulator of endocytosis. Rab5-transfected cells exhibited abnormally large endosomes immunoreactive for Rab5 and early endosomal antigen 1, resembling the endosome morphology seen in affected neurons from AD brain. The levels of both Abeta40 and Abeta42 in conditioned medium were increased more than 2.5-fold following Rab5 overexpression. In Rab5 overexpressing cells, the levels of beta-cleaved amyloid precursor protein (APP) carboxyl-terminal fragments (betaCTF), the rate-limiting proteolytic intermediate in Abeta generation, were increased up to 2-fold relative to APP holoprotein levels. An increase in beta-cleaved soluble APP relative to alpha-cleaved soluble APP was also observed following Rab5 overexpression. BetaCTFs were co-localized by immunolabeling to vesicular compartments, including the early endosome and the trans-Golgi network. These results demonstrate a relationship between endosomal pathway activity, betaCTF generation, and Abeta production. Our findings in this model system suggest that the endosomal pathology seen at the earliest stage of sporadic AD may contribute to APP proteolysis along a beta-amyloidogenic pathway.