Nigrostriatal dysfunction in familial Alzheimer's disease-linked APPswe/PS1DeltaE9 transgenic mice

Alzheimer's disease (AD) is often accompanied by extrapyramidal signs attributed to nigrostriatal dysfunction. The association between amyloid deposition and nigrostriatal degeneration is essentially unknown. We showed previously that the striatum and the substantia nigra of transgenic mice harboring familial AD (FAD)-linked APPswe/PS1DeltaE9 mutants exhibit morphological alterations accompanied by amyloid-beta (Abeta) deposition (Perez et al., 2004). In the present study, we further investigated the interaction between Abeta deposition and dopaminergic nigrostriatal dysfunction, by correlating morphological and biochemical changes in the nigrostriatal pathway with amyloid deposition pathology in the brains of 3- to 17-month-old APPswe/PS1DeltaE9 transgenic mice and age-matched wild-type controls. We show that Abeta deposition is pronounced in the striatum of APPswe/PS1DeltaE9 mice at 6 months of age, and the extent of deposition increases in an age-dependent manner. Tyrosine hydroxylase (TH)-positive dystrophic neurites with rosette or grape-like cluster disposition are observed adjacent to Abeta plaques and display multilaminar, multivesicular, and dense-core bodies as well as mitochondria. In addition, an age-dependent increase of TH protein levels are shown in nigral cells in these mutant mice. Using HPLC analysis, we found a reduction in the dopamine metabolite DOPAC in the striatum of these mice. These findings show a close association between amyloid deposition and nigrostriatal pathology and suggest that altered FAD-linked amyloid metabolism impairs, at least in part, the function of dopaminergic neurons.

Familial Alzheimer's disease presenilin 1 mutations cause alterations in the conformation of presenilin and interactions with amyloid precursor protein

Presenilin 1 (PS1) is a critical component of the gamma-secretase complex, an enzymatic activity that cleaves amyloid beta (Abeta) from the amyloid precursor protein (APP). More than 100 mutations spread throughout the PS1 molecule are linked to autosomal dominant familial Alzheimer's disease (FAD). All of these mutations lead to a similar phenotype: an increased ratio of Abeta42 to Abeta40, increased plaque deposition, and early age of onset. We use a recently developed microscopy approach, fluorescence lifetime imaging microscopy, to monitor the relative molecular distance between PS1 N and C termini in intact cells. We show that FAD-linked missense mutations located near the N and C termini, in the mid-region of PS1, and the exon 9 deletion mutation all change the spatial relationship between PS1 N and C termini in a similar way, increasing proximity of the two epitopes. This effect is opposite of that observed by treatment with Abeta42-lowering nonsteroidal anti-inflammatory drugs (NSAIDs) (Lleo et al., 2004b). Accordingly, treatment of M146L PS1-overexpressing neurons with high-dose NSAIDs somewhat offsets the conformational change associated with the mutation. Moreover, by monitoring the relative distance between a PS1 loop epitope and the APP C terminus, we demonstrate that the FAD PS1 mutations are also associated with a consistent change in the configuration of the PS1-APP complex. The nonpathogenic E318G PS1 polymorphism had no effect on PS1 N terminus-C terminus proximity or PS1-APP interactions. We propose that the conformational change we observed may therefore provide a shared molecular mechanism for FAD pathogenesis caused by a wide range of PS1 mutations.

SorLA signaling by regulated intramembrane proteolysis

The single-transmembrane receptor SorLA/LR11 contains binding domains typical for lipoprotein receptors and a VPS10 domain, which binds the neuropeptide head-activator. This undecapeptide enhances proliferation of neuronal precursor cells in a SorLA-dependent manner. Using specific inhibitors we found previously that head activator activates shedding of SorLA by the metalloprotease TACE close to the transmembrane domain releasing the large extra-cellular part of the receptor. Here we show that the remaining COOH-terminal membrane fragment of SorLA is processed by gamma-secretase. Inhibition of gamma-secretase by specific inhibitors or overexpression of dominant negative presenilin mutants and knock out of the presenilin genes led to accumulation of the SorLA membrane fragment and also of full-length SorLA in the membrane. In an in vitro assay we observed the gamma-secretase-dependent release of the two soluble cleavage products, the SorLA cytoplasmic domain and the SorLA beta-peptide. These processing steps are reminiscent of a novel signaling pathway that has been described for the notch receptor. Here, the notch cytoplasmic domain is released into the cytoplasm by the gamma-secretase and migrates to the nucleus where it acts as a transcriptional regulator. In parallel we found that a fusion protein of the released cytoplasmic tail of SorLA with EGFP located to the nucleus only if the nuclear localization signal of SorLA was intact. In a reporter gene assay the cytoplasmic domain of SorLA acted as a transcriptional activator indicating that SorLA might directly regulate transcription after activation by gamma-secretase.

Frameshift proteins in autosomal dominant forms of Alzheimer disease and other tauopathies

Frameshift (+1) proteins such as APP(+1) and UBB(+1) accumulate in sporadic cases of Alzheimer disease (AD) and in older subjects with Down syndrome (DS). We investigated whether these proteins also accumulate at an early stage of neuropathogenesis in young DS individuals without neuropathology and in early-onset familial forms of AD (FAD), as well as in other tauopathies, such as Pick disease (PiD) or progressive supranuclear palsy (PSP). APP(+1) is present in many neurons and beaded neurites in very young cases of DS, which suggests that it is axonally transported. In older DS patients (>37 years), a mixed pattern of APP(+1) immunoreactivity was observed in healthy looking neurons and neurites, dystrophic neurites, in association with neuritic plaques, as well as neurofibrillary tangles. UBB(+1) immunoreactivity was exclusively present in AD type of neuropathology. A similar pattern of APP(+1) and UBB(+1) immunoreactivity was also observed for FAD and much less explicit in nondemented controls after the age of 51 years. Furthermore, we observed accumulation of +1 proteins in other types of tauopathies, such as PiD, frontotemporal dementia, PSP and argyrophylic grain disease. These data suggest that accumulation of +1 proteins contributes to the early stages of dementia and plays a pathogenic role in a number of diseases that involve the accumulation of tau.

Amyloidogenic processing of beta-amyloid precursor protein in intracellular compartments

Trafficking and proteolytic processing of amyloid precursor protein (APP) have been the focus of numerous investigations in the past two decades, since the identification of Abeta as the principal component of brain senile plaques and the cloning of APP cDNA. Tremendous progress has been made in the recent past toward the characterization of beta- and gamma-secretases. Here, we review the salient features of Alzheimer disease amyloidogenesis, and discuss the current knowledge on APP trafficking and amyloidogenic processing of APP in intracellular membrane compartments and microdomains.

Antibody-bound amyloid precursor protein upregulates ornithine decarboxylase expression

Alzheimer's disease is a neurodegenerative disorder characterised by extracellular accumulation of the Abeta peptide, derived from the amyloid precursor protein (APP). The function of APP as a cell surface receptor was examined by ligand-mimicking using an antibody against the APP extracellular domain. Alterations in gene expression evoked by antibody-bound APP were analysed using human pathway-finder gene arrays and the largest change in expression levels was found for ornithine decarboxylase (ODC). These results were confirmed by Western blotting which showed even higher upregulation on the protein level. APP knockdown by RNAi verified that upregulation of ODC was APP-mediated. This APP signalling event did not require gamma-secretase cleavage, as it was independent of the presence of presenilin-1 or -2. The induced ODC expression was rapid and biphasic, resembling growth-factor stimulated signalling events. This study shows that antibody-bound APP leads to altered gene expression that may be relevant to AD.

Presenilin-1 controls the growth and differentiation of endothelial progenitor cells through its β-catenin-binding region

Presenilin-1 (PS1) is a gene responsible for the development of early-onset familial Alzheimer's disease. Targeted disruption of the PS1 gene in mice suggested that PS1 might be involved in angiogenesis. We have used an in vitro embryonic stem (ES) cell culture system to prepare endothelial progenitor cells (EPC) lacking PS1 and investigated the roles of PS1 in endothelial cell lineage. With this system, Flk-1+ E-cadherin- EPC were generated from PS1-deficient ES cells, and the EPC lacking PS1 as well as wild-type EPC grew to form VE-cadherin+ endothelial colonies supported by a layer of OP9 stromal cells. Although the endothelial colonies from PS1-deficient EPC showed morphology similar to those from wild-type EPC, the PS1-deficient EPC formed a large number of the colonies compared to wild-type EPC. The enhanced colony-forming ability of PS1-deficient EPC was attenuated by the inductions of wild-type human PS1. To differentiate multiple activities of PS1 for colony-forming ability, we used two types of human PS1 mutants: one (hPS1D257A) with the aspartate to alanine mutation at residue 257 that impairs the proteolytic activity of PS1, and the other (hPS1Deltacat) deleting amino acids 340-371 of the cytosolic loop sequence essential for beta-catenin binding. hPS1D257A showed activity to regulate the colony-forming ability of PS1-deficient EPC, while hPS1Deltacat failed to exhibit this activity. These results suggest that PS1 regulates the growth and differentiation of endothelial progenitor cells through its beta-catenin-binding region and that the defect of PS1 function in endothelial cell lineage could contribute to the induction of vascular pathology.

Aging sensitizes toward ROS formation and lipid peroxidation in PS1M146L transgenic mice

Mutations in the presenilins (PS) account for the majority of familial Alzheimer disease (FAD) cases. To test the hypothesis that oxidative stress can underlie the deleterious effects of presenilin mutations, we analyzed lipid peroxidation products (4-hydroxynonenal (HNE) and malondialdehyde) and antioxidant defenses in brain tissue and levels of reactive oxygen species (ROS) in splenic lymphocytes from transgenic mice bearing human PS1 with the M146L mutation (PS1M146L) compared to those from mice transgenic for wild-type human PS1 (PS1wt) and nontransgenic littermate control mice. In brain tissue, HNE levels were increased only in aged (19-22 months) PS1M146L transgenic animals compared to PS1wt mice and not in young (3-4 months) or middle-aged mice (13-15 months). Similarly, in splenic lymphocytes expressing the transgenic PS1 proteins, mitochondrial and cytosolic ROS levels were elevated to 142.1 and 120.5% relative to controls only in cells from aged PS1M146L animals. Additionally, brain tissue HNE levels were positively correlated with mitochondrial ROS levels in splenic lymphocytes, indicating that oxidative stress can be detected in different tissues of PS1 transgenic mice. Antioxidant defenses (activities of antioxidant enzymes Cu/Zn-SOD, GPx, or GR) or susceptibility to in vitro oxidative stimulation was unaltered. In summary, these results demonstrate that the PS1M146L mutation increases mitochondrial ROS formation and oxidative damage in aged mice. Hence, oxidative stress caused by the combined effects of aging and PS1 mutations may be causative for triggering neurodegenerative events in FAD patients.

Differences in regional brain atrophy in genetic forms of Alzheimer's disease

Multiple degenerative hallmarks characterize Alzheimer's disease: insoluble protein deposition, neuronal loss and cortical atrophy. Atrophy begins in the medial temporal lobe and becomes global by end stage. In a small proportion of cases, these tissue changes are caused by mutations in three known genes. These cases are affected earlier in life and have more abundant protein deposition, which may indicate greater tissue atrophy and degeneration. This issue remains unresolved. Grey matter atrophy in different cortical regions was determined in genetic cases of Alzheimer's disease (N = 13) and compared to sporadic cases (N = 13) and non-diseased controls (N = 23). Genetic mutations were found to influence the degree and regional pattern of atrophy. The majority of cases had greater medial temporal atrophy than sporadic disease, suggesting that abnormalities affecting Abeta metabolism selectively increase hippocampal degeneration. Cases with mutations in presenilin-1 demonstrated additional increased frontotemporal atrophy. This effect may be due to the influence of presenilin-1 on tau phosphorylation and metabolism. These differences may explain the earlier onset ages in these different forms of Alzheimer's disease.

[Genes in Alzheimer's disease]

INTRODUCTION

Alzheimer's disease (AD) is the most frequent degenerative dementia among the elderly population. Families that have an autosomal dominant pattern for AD constitute about 13% of early cases (< or = 65 years) and less than 0.01% of the total number of patients.

DEVELOPMENT

Molecular analysis of families with early onset AD has made it possible to identify mutations in three different genes that are responsible for the disease: the gene encoding for the amyloid precursor protein peptide (APP), and the presenilin 1 (PSEN1) and presenilin 2 (PSEN2) genes. Yet, these genes are involved in less than 5% of the total number of cases of AD. The remaining AD patients are mostly cases of late or familial onset, where the disease appears as a result of a complex interaction among environmental factors and individual predisposing genetic traits. A large number of molecular genetics studies have clearly implicated the APOE epsilon4 allele as a proven risk factor for the late form of AD in almost all the populations that have been studied.

CONCLUSIONS

Although the APOE epsilon4 allele is the only proven genetic risk factor for the late form of the disease, genetic epidemiological studies suggest that other loci are also involved.

Environmental enrichment mitigates cognitive deficits in a mouse model of Alzheimer's disease

Epidemiological studies suggest that individuals with greater education or more cognitively demanding occupations have diminished risk of developing dementia. We wanted to test whether this effect could be recapitulated in rodents using environmental enrichment, a paradigm well documented to attenuate behavioral deficits induced by various pathological insults. Here, we demonstrate that learning and memory deficits observed in a transgenic mouse model of Alzheimer's disease can be ameliorated by enrichment. Female transgenic mice overexpressing amyloid precursor protein and/or presenilin-1 and nontransgenic controls were placed into enriched or standard cages at 2 months of age and tested for cognitive behavior after 6 months of differential housing. Enrichment significantly improved performance of all genotypes in the radial water maze and in the classic and repeated-reversal versions of the Morris water maze. However, enrichment did not benefit all genotypes equally. Mice overproducing amyloid-beta (Abeta), particularly those with amyloid deposits, showed weaker memory for the platform location in the classic Morris water maze and learned new platform positions in the repeated-reversals task less quickly than their nontransgenic cagemates. Nonetheless, enrichment normalized the performance of Abeta-overproducing mice to the level of standard-housed nontransgenic mice. Moreover, this functional preservation occurred despite increased neuritic plaque burden in the hippocampus of double-transgenic animals and elevated steady-state Abeta levels, because both endogenous and transgene-derived Abeta are increased in enriched animals. These results demonstrate that the generation of Abeta in vivo and its impact on the function of the nervous system can be strongly modulated by environmental factors.

Withdrawal: Pigment epithelium-derived factor inhibits angiogenesis via regulated intracellular proteolysis of vascular endothelial growth factor receptor 1

Pigment epithelium-derived factor (PEDF) has been identified as one of the most potent of endogenous negative regulators of blood vessel growth in the body. Here we report that PEDF is able to inhibit growth factor-induced angiogenesis in microvascular endothelial cells through a novel pathway requiring cleavage and intracellular translocation of the transmembrane domain of the VEGFR-1. Analysis of the subcellular distribution of VEGFR-1 revealed the appearance of an 80-kDa C-terminal domain in the cytosol of cells treated with VEGF and PEDF that correlated with a decrease of the full-length receptor in the nuclear and cytoskeletal fractions. This regulated intramembrane proteolysis is dependent on gamma-secretase because inhibition of gamma-secretase abolished the inhibitory effect of PEDF on VEGF-induced angiogenesis as well as VEGFR-1 cleavage. The addition of PEDF to microvascular endothelial cells significantly increases gamma-secretase activity even in the absence of VEGF, showing that VEGF binding to VEGF-R1 is essential for substrate availability. This increase in activity was associated with translocation of presenilin 1 from the perinuclear region to the cell membrane. PEDF was also able to inhibit VEGF-induced phosphorylation of VEGFR-1. Taken together we have identified two novel pathways by which PEDF inhibits VEGF-induced angiogenesis: regulated intramembrane proteolysis and inhibition of phosphorylation. This confirms the importance of PEDF and VEGFR-1 in the negative regulation of angiogenesis.

Ubiquilin regulates presenilin endoproteolysis and modulates gamma-secretase components, Pen-2 and nicastrin

Mutations in presenilin proteins (PS1 and PS2) lead to early-onset Alzheimer's disease. PS proteins are endoproteolytically cleaved into two main fragments: the NTF (PS N-terminal fragment) and the CTF (PS C-terminal fragment). The two fragments are believed to constitute the core catalytic enzyme activity called gamma-secretase, which is responsible for cleaving beta-amyloid precursor protein to release Abeta. Thus, studying factors that modulate PS fragment levels could provide important information about gamma-secretase. Previously, we demonstrated that the protein, ubiquilin-1, interacts both in vivo and in vitro with PS and that overexpression of ubiquilin-1 or -2 leads to increased accumulation of full-length PS proteins. Using wild-type HEK-293 cells (human embryonic kidney 293 cells) and PS-inducible cells, we now show that overexpression of either ubiquilin-1 or -2 decreases the PS NTF and CTF levels. Conversely, siRNA (small interfering RNA)-mediated knockdown of ubiquilin-1 and -2 proteins increased the PS NTF and CTF levels. We considered that ubiquilin might alter PS fragment accumulation by acting as a shuttle factor escorting PS fragments to the proteasome for degradation. However, through proteasome inhibition studies, we show that this does not occur. Instead, our results suggest that ubiquilin regulates PS fragment production. We also examined whether other components of the gamma-secretase complex are affected by ubiquilin expression. Interestingly, overexpression of ubiquilin resulted in a decrease in Pen-2 and nicastrin levels, two essential components of the gamma-secretase complex. In contrast, knockdown of ubiquilin-1 and -2 protein expression by RNAi (RNA interference) increased Pen-2 and nicastrin levels. Finally, we show that inhibition of the proteasome results in decreased PS fragment production and that reversal of proteasome inhibition restores PS fragment production, suggesting that the proteasome may be involved in PS endoproteolysis. These studies implicate ubiquilin as an important factor in regulating PS biogenesis and metabolism.

A Partial Failure of Membrane Protein Turnover May Cause Alzheimers Disease: A New Hypothesis

The amyloid hypothesis has dominated the thinking in our attempts to understand, diagnose and develop drugs for Alzheimer's disease (AD). This article presents a new hypothesis that takes into account the numerous familial AD (FAD) mutations in the amyloid precursor protein (APP) and its processing pathways, but suggests a new perspective beyond toxicity of forms of the amyloid beta-peptide (Abeta). Clearly, amyloid deposits are an invariable feature of AD. Moreover, although APP is normally processed to secreted and membrane-bound fragments, sAPPbeta and CTFbeta, by BACE, and the latter is subsequently processed by gamma-secretase to Abeta and CTFgamma, this pathway mostly yields Abeta of 40 residues, and increases in the levels of the amyloidogenic 42-residue Abeta (Abeta42) are seen in the majority of the mutations linked to the disease. The resulting theory is that the disease is caused by amyloid toxicity, which impairs memory and triggers deposition of the microtubule associated protein, Tau, as neurofibrillary tangles. Nevertheless, a few exceptional FAD mutations and the presence of large amounts of amyloid deposits in a group of cognitively normal elderly patients suggest that the disease process is more complex. Indeed, it has been hard to demonstrate the toxicity of Abeta42 and the actual target has been shifted to small oligomers of the peptide, named Abeta derived diffusible ligands (ADDLs). Our hypothesis is that the disease is more complex and caused by a failure of APP metabolism or clearance, which simultaneously affects several other membrane proteins. Thus, a traffic jam is created by failure of important pathways such as gamma-secretase processing of residual intramembrane domains released from the metabolism of multiple membrane proteins, which ultimately leads to a multiple system failure. In this theory, toxicity of Abeta42 will only contribute partially, if at all, to neurodegeneration in AD. More significantly, this theory would predict that focussing on specific reagents such as gamma-secretase inhibitors that hamper metabolism of APP, may initially show some beneficial effects on cognitive performance by elimination of acutely toxic ADDLs, but over the longer term may exacerbate the disease process by reducing membrane protein turnover.

Real time PCR assay with fluorescent hybridization probes for genotyping intronic polymorphism in presenilin-1 gene

BACKGROUND

Identification of all susceptibility loci for Alzheimer's disease has been a major goal in resolving the pathogenesis of this disease.

METHODS

A PCR assay with fluorescently labeled oligonucleotide hybrinization probes with subsequent fluorescent probe melting point analysis was developed.

RESULTS

Allelic discrimination of intronic polymorphism of presenilin-1 gene and the restriction fragment length polymorphism method yielded identical results, proving its usefulness for genotyping PS1 gene.

CONCLUSIONS

This method provides excellent robustness, speed, and accuracy, and is well suited for determination of the polymorphism in both small and large numbers of samples. This assay could help to overcome the controversy regarding the association between the PS1 s165932 intronic polymorphism and Alzheimer's disease.

Presenilin clinical mutations can affect gamma-secretase activity by different mechanisms

Mutations in human presenilin (PS) genes cause aggressive forms of familial Alzheimer's disease. Presenilins are polytopic proteins that harbour the catalytic site of the gamma-secretase complex and cleave many type I transmembrane proteins including beta-amyloid precursor protein (APP), Notch and syndecan 3. Contradictory results have been published concerning whether PS mutations cause 'abnormal' gain or (partial) loss of function of gamma-secretase. To avoid the possibility that wild-type PS confounds the interpretation of the results, we used presenilin-deficient cells to analyse the effects of different clinical mutations on APP, Notch, syndecan 3 and N-cadherin substrate processing, and on gamma-secretase complex formation. A loss in APP and Notch substrate processing at epsilon and S3 cleavage sites was observed with all presenilin mutants, whereas APP processing at the gamma site was affected in variable ways. PS1-Delta9 and PS1-L166P mutations caused a reduction in beta-amyloid peptide Abeta40 production whereas PS1-G384A mutant significantly increased Abeta42. Interestingly PS2, a close homologue of PS1, appeared to be a less efficient producer of Abeta than PS1. Finally, subtle differences in gamma-secretase complex assembly were observed. Overall, our results indicate that the different mutations in PS affect gamma-secretase structure or function in multiple ways.

Down-regulation of Endogenous Amyloid Precursor Protein Processing due to Cellular Aging

Processing of amyloid precursor protein (APP) is a well acknowledged central pathogenic mechanism in Alzheimer disease. However, influences of age-associated cellular alterations on the biochemistry of APP processing have not been studied in molecular detail so far. Here, we report that processing of endogenous APP is down-regulated during the aging of normal human fibroblasts (IMR-90). The generation of intracellular APP cleavage products C99, C83, and AICD gradually declines with increasing life span and is accompanied by a reduced secretion of soluble APP (sAPP) and sAPPalpha. Further, the maturation of APP was reduced in senescent cells, which has been shown to be directly mediated by age-associated increased cellular cholesterol levels. Of the APP processing secretases, protein levels of constituents of the gamma-secretase complex, presenilin-1 (PS1) and nicastrin, were progressively reduced during aging, resulting in a progressive decrease in gamma-secretase enzymatic activity. ADAM10 (a disintegrin and metalloprotease 10) and BACE (beta-site APP-cleaving enzyme) protein levels exhibited no age-associated regulation, but interestingly, BACE enzymatic activity was increased in aged cells. PS1 and BACE are located in detergent-resistant membranes (DRMs), well structured membrane microdomains exhibiting high levels of cholesterol, and caveolin-1. Although total levels of both structural components of DRMs were up-regulated in aged cells, their particular DRM association was decreased. This age-dependent membrane modification was associated with an altered distribution of PS1 and BACE between DRM and non-DRM fractions, very likely affecting their APP processing potential. In conclusion, we have found a significant modulation of endogenous APP processing and maturation in human fibroblasts caused by age-associated alterations in cellular biochemistry.

Frontotemporal dementia—A brief review

Frontotemporal dementia (FTD) is the second most common type of presenile dementia and the forth most common type of senile dementia, but probably the most costly due to its florid symptom characteristics. Clinically, it often presents with changes of personality, restlessness, disinhibition, and impulsiveness and the clinical features can be complicated by neurological signs, such as motor neuron signs, parkinsonism, and gait disturbances. Syndromatically, FTD can be subdivided into a group with predominating behavioural disturbances (frontal variant) and another with predominating language deterioration (temporal variant). Based on the underlying pathological changes, FTD is nosologically divided into disorders such as Pick's disease, frontotemporal lobar degeneration, corticobasal degeneration, progressive supranuclear palsy, and frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). The cause in sporadic FTD is most often unknown, but in FTDP-17, one of the hereditary FTDs, there is a causative mutation in the tau gene. The frequency of tau-gene mutations is low in sporadic FTD and present in about 10-40% of hereditary FTD. Other types of hereditary FTD have been described, such as FTD caused by mutations in chromosome 3, chromosome 9, and a FTD syndrome can also be caused by mutations in the presenilin-1 gene. Since there is no curative, treatment of prevailing symptoms is the given alternative. Serotonergic acting drugs have been shown to alleviate behavioural symptoms.

An early specific cell-cell interaction occurs in the production of beta-amyloid in cell cultures

We have earlier proposed that a cell-cell interaction, mediated by the specific binding of molecules of the beta-amyloid precursor protein (beta-APP) on one cell surface with molecules of presenilin (PS) on the other cell surface, is a required initial step in the ultimate production of beta-amyloid (Abeta) from beta-APP. Abeta is widely believed to be the neurotoxic agent in Alzheimer's disease. In this paper, we test this proposal by modifying cells to express surface beta-APP but no PS, and other cells to express surface PS but no beta-APP. Coculturing these two cell populations at appropriate cell densities produces substantial amounts of Abeta that appear both in cell extracts and culture media. Such Abeta production could occur only if the two cell types interacted with one another to provide the beta-APP and the PS required for the generation of Abeta. The addition to the coculture, from the start, of the soluble specific N-terminal domain of the appropriate PS significantly reduces the amount of Abeta produced. These and related experiments, therefore, suggest a very different mechanism for Abeta production than the one that is currently widely accepted.

Presenilin-1 uses phospholipase D1 as a negative regulator of beta-amyloid formation

Presenilin (PS1/PS2) is a major component of gamma-secretase, the activity that mediates proteolysis of beta-amyloid precursor protein to generate beta-amyloid (Abeta). Here we demonstrate that PS1, through its loop region, binds to phospholipase D1 (PLD1), thereby recruiting it to the Golgi/trans-Golgi network. Overexpression of wild-type PLD1 reduces Abeta generation. Conversely, down-regulation of endogenous PLD1 by small hairpin RNA elevates Abeta production. The Abeta-lowering effect of PLD1 is independent of its ability to promote vesicular budding of beta-amyloid precursor protein. The data indicate that overexpression of PLD1 decreases, and down-regulation of PLD1 increases, the catalytic activity, and the association of the subunits, of gamma-secretase.

Phospholipase D1 corrects impaired betaAPP trafficking and neurite outgrowth in familial Alzheimer's disease-linked presenilin-1 mutant neurons

Presenilins (PS1/PS2) regulate proteolysis of beta-amyloid precursor protein (betaAPP) and affect its intracellular trafficking. Here, we demonstrate that a PS1-interacting protein, phospholipase D1 (PLD1), affects intracellular trafficking of betaAPP. Overexpression of PLD1 in PS1wt cells promotes generation of betaAPP-containing vesicles from the trans-Golgi network. Conversely, inhibition of PLD1 activity by 1-butanol decreases betaAPP trafficking in both wt and PS1-deficient cells. The subcellular localization of PLD1 is altered, and PLD enzymatic activity is reduced in cells expressing familial Alzheimer's disease (FAD) PS1 mutations compared with PS1wt cells. Overexpression of wt, but not catalytically inactive, PLD1 increases budding of betaAPP-containing vesicles from the trans-Golgi network in FAD mutant cells. Surface delivery of betaAPP is also increased by PLD1 in these cells. The impaired neurite outgrowth capacity in FAD mutant neurons was corrected by introducing PLD1 into these cells. The results indicate that PLD1 may represent a therapeutic target for rescuing compromised neuronal function in AD.

Reelin-immunoreactivity in the hippocampal formation of 9-month-old wildtype mouse: effects of APP/PS1 genotype and ovariectomy

Reelin, an extracellular matrix protein has an important role in the migration, correct positioning and maturation of neurons during development. Though it is generally down-regulated in the postnatal period, expression of this large glycoprotein continues in the adult brain in some cell populations. In the present study, we examined the distribution of reelin-immunoreactivity (-ir) in the hippocampal formation of 9-month-old wildtype mice (WT). Then, reelin-ir in normal mice was compared to that of transgenic mice (APP/PS1) carrying mutated human APP and PS1 genes, which are linked to the familial form of Alzheimer's disease (AD). The APP/PS1 mice were additionally burdened with a second risk factor for AD, namely depletion of circulating gonadal hormones by ovariectomy (APP/PS1 + OVX). The analyses revealed that in adult WT reelin-ir is expressed by Cajal-Retzius cells and a subgroup of interneurons throughout the hippocampal formation. In addition, layer II projection neurons in the lateral entorhinal subfields are reelin-ir. Interestingly, ovariectomy decreases the number of reelin-ir cells in the hilus in WT mice, whereas AD-related genotype alone induces only a non-significant reduction. Unexpectedly, additional stress, e.g., depletion of gonadal hormones, does not aggravate the slight reduction in the reelin cell number in the APP/PS1 mice. We propose that the changes in normal reelin-ir are linked to disturbances in repair mechanisms in which APP/PS1 and gonadal hormones are involved and which are perturbed in neurodegenerative conditions, namely AD.

Neuropsychological function in nondemented carriers of presenilin-1 mutations

BACKGROUND

Prospective and case-control studies have demonstrated that memory loss and executive dysfunction occur early in Alzheimer disease (AD).

OBJECTIVE

To investigate these observations by the study of persons at risk for autosomal dominant forms of AD.

METHODS

Neuropsychological and genetic tests were performed on 51 nondemented at-risk members of 10 Mexican families with two distinct presenilin-1 (PS1) mutations. Test scores were compared between PS1 mutation carriers (MCs; n = 30) and noncarriers (NCs; n = 21) by analyses of variance, co-varying for family and specific mutation. Regression analyses were performed, taking into account age relative to the median age at dementia diagnosis in the family (adjusted age), gender, Beck Depression Inventory (BDI) scores, education, and number of APOE epsilon4 alleles. Subjects were divided into age tertiles and scores compared within these groups. Composite scores for Verbal Memory, Executive Function/Working Memory, Language, and Visuospatial Function were created, and these scores compared between MCs and NCs.

RESULTS

MCs performed worse than NCs on the Mini-Mental State Examination, Trails Making Tests A and B, Delayed Recall of a 10-Word List, and Wechsler Adult Intelligence Scale WAIS Block Design. In multiple linear regression analyses, BDI score, gender, and number of APOE epsilon4 alleles did not consistently affect test scores. The differences seen between MCs and NCs were due to differences in the oldest tertile. MCs had lower Visuospatial and Executive Function/Working Memory but not Verbal Memory or Language composite scores.

CONCLUSIONS

This study is consistent with findings in sporadic Alzheimer disease of early problems with memory, visuospatial function, and particularly with executive function in PS1 mutation carriers. Depression, gender, and presence of an APOE epsilon4 allele did not demonstrate large influences on neuropsychological performance.

Secretion of the Notch-1 Abeta-like peptide during Notch signaling

The canonical pathway of Notch signaling is mediated by regulated intramembrane proteolysis (RIP). In the pathway, ligand binding results in sequential proteolysis of the Notch receptor, and presenilin (PS)-dependent intramembrane proteolysis at the interface between the membrane and cytosol liberates the Notch-1 intracellular domain (NICD), a transcription modifier. Because the degradation of the Notch-1 transmembrane domain is thought to require an additional cleavage near the middle of the transmembrane domain, extracellular small peptides (Notch-1 Abeta-like peptide (Nbeta)) should be produced. Here we showed that Nbeta species are indeed secreted during the process of Notch signaling. We identified mainly two distinct molecular species of novel Nbeta, Nbeta21 and C-terminally elongated Nbeta25, which were produced in an approximately 5:1 ratio. This process is reminiscent of the production of Alzheimer disease-associated Abeta. PS pathogenic mutants increased the production of the longer species of Abeta (Abeta42) from beta-amyloid protein precursor. We revealed that several Alzheimer disease mutants also cause a parallel increase in the secretion of the longer form of Nbeta. Strikingly, chemicals that modify the Abeta42 level caused parallel changes in the Nbeta25 level. These results demonstrated that the characteristics of C-terminal elongation of Nbeta and Abeta are almost identical. In addition, because many other type 1 membrane-bound receptors release intracellular domains by PS-dependent intramembrane proteolysis, we suspect that the release of Abeta- or Nbeta-like peptides is a common feature of the proteolysis during RIP signaling. We anticipate that this study will open the door to searches for markers of RIP signaling and surrogate markers for Abeta42 production.