Dimerization leads to changes in APP (amyloid precursor protein) trafficking mediated by LRP1 and SorLA

Proteolytic cleavage of the amyloid precursor protein (APP) by α-, β- and γ-secretases is a determining factor in Alzheimer's disease (AD). Imbalances in the activity of all three enzymes can result in alterations towards pathogenic Aβ production. Proteolysis of APP is strongly linked to its subcellular localization as the secretases involved are distributed in different cellular compartments. APP has been shown to dimerize in cis-orientation, affecting Aβ production. This might be explained by different substrate properties defined by the APP oligomerization state or alternatively by altered APP monomer/dimer localization. We investigated the latter hypothesis using two different APP dimerization systems in HeLa cells. Dimerization caused a decreased localization of APP to the Golgi and at the plasma membrane, whereas the levels in the ER and in endosomes were increased. Furthermore, we observed via live cell imaging and biochemical analyses that APP dimerization affects its interaction with LRP1 and SorLA, suggesting that APP dimerization modulates its interplay with sorting molecules and in turn its localization and processing. Thus, pharmacological approaches targeting APP oligomerization properties might open novel strategies for treatment of AD.

Amiodarone Offsets the Cardioprotective Effects of Ischaemic Preconditioning against Ischaemia/Reperfusion Injury

Both ischaemic preconditioning (IPC) and amiodarone protect against myocardial ischaemia. We examined whether a combination of IPC and amiodarone demonstrated an additive protective effect in isolated rat hearts ( n = 40). The controls (group I) were subjected to ischaemia/reperfusion injury; group II was subjected to cycles of IPC prior to ischaemia/reperfusion injury; group III was subjected to ischaemia in the presence of amiodarone (10−10 mol/l); and group IV was subjected to IPC followed by ischaemia in the presence of amiodarone (10−10 mol/l). Amiodarone produced the best preserved left ventricular end-systolic pressure and dP/dtmax, less developed ventricular stiffness, the shortest arrhythmia duration, and the smallest infarct size among the groups. All of the myocardial protective effects against ischaemia/reperfusion injury were diminished or abolished when IPC and amiodarone were applied sequentially.

Abeta induces cell death by direct interaction with its cognate extracellular domain on APP (APP 597-624)

Amyloid beta-peptide (Abeta) is postulated to play a central role in the pathogenesis of Alzheimer's disease. We recently proposed a pathway of Abeta-induced toxicity that is APP dependent and involves the facilitation of APP complex formation by Abeta. The APP-dependent component requires cleavage of APP at position 664 in the cytoplasmic domain, presumably by caspases or caspase-like proteases, with release of a potentially cytotoxic C31 peptide. In this study we show that Abeta interacted directly and specifically with membrane-bound APP to facilitate APP homo-oligomerization. Using chimeric APP molecules, this interaction was shown to take place between Abeta and its homologous sequence on APP. Consistent with this finding, we demonstrated that Abeta also facilitated the oligomerization of beta-secretase cleaved APP C-terminal fragment (C99). We found that the YENPTY domain in the APP cytoplasmic tail and contained within C31 is critical for this cell death pathway. Deletion or alanine- scanning mutagenesis through this domain significantly attenuated cell death apparently without affecting either APP dimerization or cleavage at position 664. This indicated that sequences within C31 are required after its release from APP. As the YENPTY domain has been shown to interact with a number of cytosolic adaptor molecules, it is possible that the interaction of APP, especially dimeric forms of APP, with these molecules contribute to cell death.

A subset of NSAIDs lower amyloidogenic Abeta42 independently of cyclooxygenase activity

Epidemiological studies have documented a reduced prevalence of Alzheimer's disease among users of nonsteroidal anti-inflammatory drugs (NSAIDs). It has been proposed that NSAIDs exert their beneficial effects in part by reducing neurotoxic inflammatory responses in the brain, although this mechanism has not been proved. Here we report that the NSAIDs ibuprofen, indomethacin and sulindac sulphide preferentially decrease the highly amyloidogenic Abeta42 peptide (the 42-residue isoform of the amyloid-beta peptide) produced from a variety of cultured cells by as much as 80%. This effect was not seen in all NSAIDs and seems not to be mediated by inhibition of cyclooxygenase (COX) activity, the principal pharmacological target of NSAIDs. Furthermore, short-term administration of ibuprofen to mice that produce mutant beta-amyloid precursor protein (APP) lowered their brain levels of Abeta42. In cultured cells, the decrease in Abeta42 secretion was accompanied by an increase in the Abeta(1-38) isoform, indicating that NSAIDs subtly alter gamma-secretase activity without significantly perturbing other APP processing pathways or Notch cleavage. Our findings suggest that NSAIDs directly affect amyloid pathology in the brain by reducing Abeta42 peptide levels independently of COX activity and that this Abeta42-lowering activity could be optimized to selectively target the pathogenic Abeta42 species.

Microarray analysis of gene expression in human donor corneas

OBJECTIVES

To use microarray analysis to identify genes expressed in human donor corneas and to create a preliminary, comprehensive database of human corneal gene expression.

METHODS

A complementary DNA (cDNA) library was constructed from transplant-quality, human donor corneas. Biotin-labeled RNA was transcribed from the cDNA library and hybridized in duplicate to microarrays containing approximately 5600 human genes. Results were analyzed using a gene database of the National Institutes of Health, Bethesda, Md. Reverse transcriptase polymerase chain reaction analysis was performed to confirm corneal expression of genes identified by microarray analysis.

RESULTS

Duplicate microarrays identified the expression of 1200 genes in human donor corneas. Chromosomal loci had been assigned to 1025 (85%) of these genes. A preliminary database of human corneal gene expression was compiled. A Web site containing these genes was created. Six collagen genes were identified that had not previously been localized within the cornea. Five apoptosis-related genes were identified, 4 of which had not previously been localized within the cornea. Three genes previously shown to cause corneal diseases were identified. Reverse transcriptase polymerase chain reaction analysis of genes identified by microarray analysis confirmed the corneal expression of 2 apoptosis-related genes and 1 collagen gene.

CONCLUSIONS

Microarray analysis of healthy human donor corneas has produced a preliminary, comprehensive database of corneal gene expression. Large-scale analysis of gene expression has the potential to generate large amounts of data, which should be made readily accessible to the scientific community. The Internet offers many potential advantages as a medium for the maintenance of these large data sets.

CLINICAL RELEVANCE

Identification of structural, apoptosis-related, and disease-causing genes within the cornea by microarrays may increase the understanding of normal and abnormal corneal function with likely relevance to corneal diseases and transplants.

Loss of presenilin 1 is associated with enhanced beta-catenin signaling and skin tumorigenesis

Presenilin 1 (PS1) is required for the proteolytic processing of Notch and the beta-amyloid precursor protein (APP), molecules that play pivotal roles in cell-fate determination during development and Alzheimer's disease pathogenesis, respectively. In addition, PS1 interacts with beta-catenin and promotes its turnover through independent mechanisms. Consistent with this activity, we report here that PS1 is important in controlling epidermal cell proliferation in vivo. PS1 knockout mice that are rescued through neuronal expression of human PS1 transgene develop spontaneous skin cancers. PS1-null keratinocytes exhibit higher cytosolic beta-catenin and beta-catenin/lymphoid enhancer factor-1/T cell factor (beta-catenin/LEF)-mediated signaling. This effect can be reversed by reintroducing wild-type PS1, but not a PS1 mutant active in Notch processing but defective in beta-catenin binding. Nuclear beta-catenin protein can be detected in tumors. Elevated beta-catenin/LEF signaling is correlated with activation of its downstream target cyclin D1 and accelerated entry from G(1) into S phase of the cell cycle. This report demonstrates a function of PS1 in adult tissues, and our analysis suggests that deregulation of beta-catenin pathway contributes to the skin tumor phenotype.

The amyloidogenic pathway of amyloid precursor protein (APP) is independent of its cleavage by caspases

Amyloid beta-protein (A beta) is the main constituent of senile plaques in Alzheimer's disease and is derived by proteolysis from the amyloid precursor protein (APP). Generation and secretion of both A beta 40 and A beta 42 isoforms depend largely on internalization of APP and occurs mainly in the endocytic pathway. Evidence has also been presented (Gervais, F. G., Xu, D., Robertson, G. S., Vaillancourt, J. P., Zhu, Y., Huang, J., LeBlanc, A., Smith, D., Rigby, M., Shearman, M. S., Clarke, E. E., Zheng, H., Van der Ploeg, L. H. T., Ruffolo, S. C., Thornberry, N. A., Xanthoudakis, S., Zamboni, R. J., Roy, S., and Nicholson, D. W. (1999) Cell, 97, 395--406) that caspase cleavage of APP at its cytosolic tail affects its processing such that it is redirected to a more amyloidogenic pathway, resulting in enhanced A beta generation. However, caspase cleavage of APP also results in loss of its endocytosis signal (YENP), an event that would predict a decline in internalization and a concomitant decrease, not an increase, in A beta generation. In the present work, we examined whether caspase cleavage of APP is relevant to amyloidogenesis. We found that 1) caspase cleavage of APP results in reduced internalization and, accordingly, a decline in A beta secretion; 2) masking of the caspase site in APP did not affect A beta levels and, 3) caspase activation in cells by serum withdrawal did not increase A beta secretion. Thus, caspase cleavage of APP is unlikely to play a direct role in amyloidogenesis.

Protein kinase C epsilon suppresses Abeta production and promotes activation of alpha-secretase

Deposition of plaques containing Abeta is considered important in the pathogenesis of Alzheimer's disease. Phorbol esters that activate protein kinase C (PKC) promote alpha-secretase-mediated processing of the beta amyloid precursor protein (APP), which generally reduces formation of Abeta. To determine which PKC isozymes mediate this process, we studied CHO cells that express human APP751. Phorbol 12-myristate, 13-acetate (PMA)-stimulated APP secretion, which was reduced by a general PKC inhibitor bisindoylmaleimide I, but not by Gö 6976, which inhibits PKCalpha, beta, gamma, and mu. Since PKCdelta and epsilon were the only other PMA-sensitive isozymes present, we studied cells that express selective peptide inhibitors of these isozymes. Expression of the PKCepsilon inhibitor inhibited PMA-induced APPs secretion and suppression of Abeta production. In contrast, the PKCdelta inhibitor had no effect. These results provide evidence that PKCepsilon decreases Abeta production by promoting alpha-secretase mediated cleavage of APP.

Modulation of amyloid beta-protein clearance and Alzheimer's disease susceptibility by the LDL receptor-related protein pathway

Susceptibility to Alzheimer's disease (AD) is governed by multiple genetic factors. Remarkably, the LDL receptor-related protein (LRP) and its ligands, apoE and alpha2M, are all genetically associated with AD. In this study, we provide evidence for the involvement of the LRP pathway in amyloid deposition through sequestration and removal of soluble amyloid beta-protein (Abeta). We demonstrate in vitro that LRP mediates the clearance of both Abeta40 and Abeta42 through a bona fide receptor-mediated uptake mechanism. In vivo, reduced LRP expression is associated with LRP genotypes and is correlated with enhanced soluble Abeta levels and amyloid deposition. Although LRP has been proposed to be a clearance pathway for Abeta, this work provides the first in vivo evidence that the LRP pathway may modulate Abeta deposition and AD susceptibility by regulating the removal of soluble Abeta.

LDL receptor-related protein (LRP) in Alzheimer's disease: towards a unified theory of pathogenesis

To date, mutations in three genes, beta-amyloid precursor protein (APP), presenilin 1 (PS1), and presenilin 2 (PS2), have been found to be causally related to familial Alzheimer's disease (AD). In addition, polymorphisms in three other genes (among others), apolipoprotein E (apoE), alpha2-macroglobulin (alpham), and the low density lipoprotein receptor-related protein (LRP), are implicated to contribute to AD pathogenesis. Interestingly, the encoded gene products are all functionally related in various ways to LRP. Specifically apoE, alpha2m, secreted APP, and amyloid beta-protein (Abeta) complexed to either apoE or alpha2m are ligands of LRP. Furthermore, over-expression of presenilin 1 results in decreased expression of LRP. Since levels of many LRP ligands are increased in Alzheimer's disease and LRP and its ligands are present in senile plaques, decreased LRP function may be a central component in AD pathogenesis. This review explores the current knowledge of LRP in AD and its relationship to the other known AD susceptibility markers.

The nonconserved hydrophilic loop domain of presenilin (PS) is not required for PS endoproteolysis or enhanced abeta 42 production mediated by familial early onset Alzheimer's disease-linked PS variants

Presenilin 1 (PS1) and presenilin 2 (PS2) are polytopic membrane proteins that are mutated in the majority of early onset familial Alzheimer's disease (FAD) cases. Two lines of evidence establish a critical role for PS in the production of beta-amyloid peptides (Abeta). FAD-linked PS mutations elevate the levels of highly amyloidogenic Abeta ending at residue 42 (Abeta42), and cells with ablated PS1 alleles secrete low levels of Abeta. Several recent reports have shown that the hydrophilic loop (HL) domain, located between transmembrane domains 6 and 7, contains sites for phosphorylation, caspase cleavage, and sequences that bind several PS-interacting proteins. In the present report, we examined the metabolism of PS polypeptides lacking the HL domain and the influence of these molecules on Abeta production. We report that the deletion of the HL domain does not have a deleterious effect on the regulated endoproteolysis of PS, saturable accumulation of PS fragments, or the self-association of PS fragments. Abeta production was not significantly altered in cells expressing HL-deleted PS polypeptides compared with cells expressing full-length PS. Importantly, deletion of the HL domain did not affect FAD mutation-mediated elevation in the production of Abeta42. Furthermore, the deletion of the HL domain did not impair the role of PS1 or PS2 in facilitating Notch processing. Thus, our results argue against a biologically or pathologically relevant role for the HL domain phosphorylation and caspase cleavage and the association of PS HL domain-interacting proteins, in amyloid precursor protein metabolism and Abeta production or Notch cleavage.

A second cytotoxic proteolytic peptide derived from amyloid beta-protein precursor

The amyloid beta-protein precursor gives rise to the amyloid beta-protein, the principal constituent of senile plaques and a cytotoxic fragment involved in the pathogenesis of Alzheimer disease. Here we show that amyloid beta-protein precursor was proteolytically cleaved by caspases in the C terminus to generate a second unrelated peptide, called C31. The resultant C31 peptide was a potent inducer of apoptosis. Both caspase-cleaved amyloid beta-protein precursor and activated caspase-9 were present in brains of Alzheimer disease patients but not in control brains. These findings indicate the possibility that caspase cleavage of amyloid beta-protein precursor with the generation of C31 may be involved in the neuronal death associated with Alzheimer disease.

Amyloid-beta peptides interact with plasma proteins and erythrocytes: implications for their quantitation in plasma

Amyloid beta peptides are bound rapidly in the plasma complicating an accurate assessment of their in vivo abundance by immunoassay procedures. The extent of Abeta immunoassay interference was used to estimate the Abeta binding capacity of purified plasma proteins, erythrocytes and whole plasma. Human serum albumin bound Abeta peptides rapidly with a 1:1 stoichiometry and at physiological concentrations was capable of binding over 95% of an input of 5 ng/ml Abeta. Purified alpha2-macroglobulin was able to bind Abeta peptides and at physiological concentration bound 73% of 5 ng/ml of Abeta. Erythrocytes also sequestered the Abeta peptides, showing a preference for binding Abeta 1-42. Incubation of 5 ng/ml of Abeta in plasma revealed that about 30% of the peptides were still detectable by immunoassay, presumably reflecting the binding of Abeta peptides with albumin and other plasma molecules. Thus, our studies reveal that both the soluble and formed elements of the blood are capable of sequestering Abeta peptides. To avoid underestimating plasma Abeta values, we employed an improved column chromatography method under denaturing conditions to liberate Abeta from its associations with plasma proteins. Quantification of Abeta 40 and 42 levels in plasma from both normal and AD individuals after chromatography showed a large overlap between AD and control groups, despite the very large pool of Abeta present in the AD brains. The potential origins of the plasma Abeta pool are discussed.

Expression of beta-amyloid precursor protein-CD3gamma chimeras to demonstrate the selective generation of amyloid beta(1-40) and amyloid beta(1-42) peptides within secretory and endocytic compartments

Amyloid beta-protein (Abeta) is the main constituent of amyloid fibrils found in senile plaques and cerebral vessels in Alzheimer's disease (AD) and is derived by proteolysis from the beta-amyloid precursor protein (APP). We have analyzed the amyloidogenic processing of APP using chimeric proteins stably transfected in Chinese hamster ovary cells. The extracellular and transmembrane domains of APP were fused to the cytoplasmic region derived from the CD3 gamma chain of the T cell antigen receptor (CD3gamma). CD3gamma contains an endoplasmic reticulum (ER) retention motif (RKK), in the absence of which the protein is targeted to lysosomes without going through the cell surface (Letourneur, F., and Klausner, R.D. (1992) Cell 69, 1143-1157). We used the wild-type sequence of CD3gamma to create an APP chimera predicted to remain in the ER (gammaAPP(ER)). Deletion of the RKK motif at the C terminus directed the protein directly to the lysosomes (gammaAPP(LYS)). A third chimera was created by removing both lysosomal targeting signals in addition to RKK (gammaAPP(DeltaDelta)). This last construct does not contain known targeting signals and consequently accumulates at the cell surface. We show by immunofluorescence and by biochemical methods that all three APP chimeras localize to the predicted compartments within the cell, thus providing a useful model to study the processing of APP. We found that Abeta(1-40) is generated in the early secretory and endocytic pathways, whereas Abeta(1-42) is made mainly in the secretory pathway. More importantly, we provide evidence that, unlike in neuronal models, both ER/intermediate compartment- and endocytic-derived Abeta forms can enter the secretable pool. Finally, we directly demonstrate that lysosomal processing is not involved in the generation or secretion of either Abeta(1-40) or Abeta(1-42).

Mutagenesis identifies new signals for beta-amyloid precursor protein endocytosis, turnover, and the generation of secreted fragments, including Abeta42

It has long been assumed that the C-terminal motif, NPXY, is the internalization signal for beta-amyloid precursor protein (APP) and that the NPXY tyrosine (Tyr743 by APP751 numbering, Tyr682 in APP695) is required for APP endocytosis. To evaluate this tenet and to identify the specific amino acids subserving APP endocytosis, we mutated all tyrosines in the APP cytoplasmic domain and amino acids within the sequence GYENPTY (amino acids 737-743). Stable cell lines expressing these mutations were assessed for APP endocytosis, secretion, and turnover. Normal APP endocytosis was observed for cells expressing Y709A, G737A, and Y743A mutations. However, Y738A, N740A, and P741A or the double mutation of Y738A/P741A significantly impaired APP internalization to a level similar to that observed for cells lacking nearly the entire APP cytoplasmic domain (DeltaC), arguing that the dominant signal for APP endocytosis is the tetrapeptide YENP. Although not an APP internalization signal, Tyr743 regulates rapid APP turnover because half-life increased by 50% with the Y743A mutation alone. Secretion of the APP-derived proteolytic fragment, Abeta, was tightly correlated with APP internalization, such that Abeta secretion was unchanged for cells having normal APP endocytosis but significantly decreased for endocytosis-deficient cell lines. Remarkably, secretion of the Abeta42 isoform was also reduced in parallel with endocytosis from internalization-deficient cell lines, suggesting an important role for APP endocytosis in the secretion of this highly pathogenic Abeta species.

Presenilin 1 facilitates the constitutive turnover of beta-catenin: differential activity of Alzheimer's disease-linked PS1 mutants in the beta-catenin-signaling pathway

Although an association between the product of the familial Alzheimer's disease (FAD) gene, presenilin 1 (PS1), and beta-catenin has been reported recently, the cellular consequences of this interaction are unknown. Here, we show that both the full length and the C-terminal fragment of wild-type or FAD mutant PS1 interact with beta-catenin from transfected cells and brains of transgenic mice, whereas E-cadherin and adenomatous polyposis coli (APC) are not detected in this complex. Inducible overexpression of PS1 led to increased association of beta-catenin with glycogen synthase kinase-3beta (GSK-3beta), a negative regulator of beta-catenin, and accelerated the turnover of endogenous beta-catenin. In support of this finding, the beta-catenin half-life was dramatically longer in fibroblasts deficient in PS1, and this phenotype was completely rescued by replacement of PS1, demonstrating that PS1 normally stimulates the degradation of beta-catenin. In contrast, overexpression of FAD-linked PS1 mutants (M146L and DeltaX9) failed to enhance the association between GSK-3beta and beta-catenin and interfered with the constitutive turnover of beta-catenin. In vivo confirmation was demonstrated in the brains of transgenic mice in which the expression of the M146L mutant PS1 was correlated with increased steady-state levels of endogenous beta-catenin. Thus, our results indicate that PS1 normally promotes the turnover of beta-catenin, whereas PS1 mutants partially interfere with this process, possibly by failing to recruit GSK-3beta into the PS1-beta-catenin complex. These findings raise the intriguing possibility that PS1-beta-catenin interactions and subsequent activities may be consequential for the pathogenesis of AD.

Recombinant adenovirus is an appropriate vector for endocytotic protein trafficking studies in cultured neurons

Endocytosis of full-length beta-amyloid precursor protein (APP) from the plasma membrane contributes to beta-amyloid peptide (Abeta) secretion, and, hence, potentially contributes to the molecular pathogenesis of Alzheimer's disease. We recently have demonstrated that central neuronal APP is endocytosed in a common vesicular compartment with recycling synaptic vesicle integral membrane proteins, but is then sorted away from synaptic vesicles for retrograde transport to the neuronal soma. For this report, we explore whether recombinant adenovirus can be used to modulate APP expression in cultured central neurons to study APP processing by the endocytotic pathway in these cells. Using a replication-deficient recombinant adenovirus that expresses a lacZ reporter (Ad5/CMV-lacZ), we demonstrate high efficiency of transfection (30-35%) at low viral titer (10-20 MOI), with no significant neuronal toxicity or cytoarchitectural change. In addition, we demonstrate that infection with the control virus does not result in re-direction of endogenous neuronal APP from usual endocytotic pathways. We have prepared, using the same genomic background as the control virus, an adenoviral vector that expresses the neuronal isoform of human APP (Ad5/CMV-APP). Infection with Ad5/CMV-APP at 10-20 MOI results in significantly increased immunoreactivity for endocytosed APP with preservation of usual endocytotic trafficking. These results demonstrate that recombinant adenovirus at low titer is an appropriate and effective vector for protein trafficking/processing studies in cultured central neurons.

Contrasting role of presenilin-1 and presenilin-2 in neuronal differentiation in vitro

Presenilin-1 (PS1) and presenilin-2 (PS2), the major genes of familial Alzheimer's disease, are homologous to sel-12, a Caenorhabditis elegans gene involved in cell fate decision during development. Recently, wild-type and mutant presenilins have been associated also with apoptotic cell death. By using stable transfection of antisense cDNAs, we studied the functions of PS1 and PS2 during neuronal differentiation in the NTera2 human teratocarcinoma (NT2) cell line. Expression of antisense PS1 resulted in a failure of the clones to differentiate into neurons after retinoic acid induction, whereas cells transfected with antisense PS2 differentiated normally. Concomitantly, antisense PS1 clones were associated with increased apoptosis both under basal conditions and during the early period of neuronal differentiation after retinoic acid treatment. Overexpression of bcl-2 in antisense PS1 clones reduced cell death and resulted in a recovery of neuronal differentiation. These studies suggest that PS1 plays a role in differentiation and cell death and that PS1 and PS2 have differing physiological roles in this experimental paradigm.

Presenilin 1 regulates the processing of beta-amyloid precursor protein C-terminal fragments and the generation of amyloid beta-protein in endoplasmic reticulum and Golgi

Progressive cerebral deposition of the amyloid beta-protein (Abeta) is believed to play a pivotal role in the pathogenesis of Alzheimer's disease (AD). The highly amyloidogenic 42-residue form of Abeta (Abeta42) is the first species to be deposited in both sporadic and familial AD. Mutations in two familial AD-linked genes, presenilins 1 (PS1) and 2 (PS2), selectively increase the production of Abeta42 in cultured cells and the brains of transgenic mice, and gene deletion of PS1 shows that it is required for normal gamma-secretase cleavage of the beta-amyloid precursor protein (APP) to generate Abeta. To establish the subcellular localization of the PS1 regulation of APP processing to Abeta, fibroblasts from PS1 wild-type (wt) or knockout (KO) embryos as well as Chinese hamster ovary (CHO) cells stably transfected with wt or mutant PS1 were subjected to subcellular fractionation on discontinuous Iodixanol gradients. APP C-terminal fragments (CTF) were markedly increased in both endoplasmic reticulum- (ER-) and Golgi-rich fractions of fibroblasts from KO mice; moreover, similar increases were documented directly in KO brain tissue. No change in the subcellular distribution of full-length APP was detectable in fibroblasts lacking PS1. In CHO cells, a small portion of APP, principally the N-glycosylated isoform, formed complexes with PS1 in both ER- and Golgi-rich fractions, as detected by coimmunoprecipitation. When the same fractions were analyzed by enzyme-linked immunosorbent assays for Abetatotal and Abeta42, Abeta42 was the major Abeta species in the ER fraction (Abeta42:Abetatotal ratio 0.5-1.0), whereas absolute levels of both Abeta42 and Abeta40 were higher in the Golgi fraction and the Abeta42:Abetatoal ratio was 0.05-0.16 there. Mutant PS1 significantly increased Abeta42 levels in the Golgi fraction. Our results indicate PS1 and APP can interact in the ER and Golgi, where PS1 is required for proper gamma-secretase processing of APP CTFs, and that PS1 mutations augment Abeta42 levels principally in Golgi-like vesicles.

Subcellular distribution and turnover of presenilins in transfected cells

The mechanisms by which mutations in presenilin-1 (PS1) and presenilin-2 (PS2) result in the Alzheimer's disease phenotype are unclear. Full-length PS1 and PS2 are each processed into stable proteolytic fragments after their biosynthesis in transfected cells. PS1 and PS2 have been localized by immunocytochemistry to the endoplasmic reticulum (ER) and Golgi compartments, but previous studies could not differentiate between the full-length presenilin proteins and their fragments. We carried out subcellular fractionation of cells stably transfected with PS1 or PS2 to determine the localization of full-length presenilins and their fragments. Full-length PS1 and PS2 were principally distributed in ER fractions, whereas the N- and C-terminal fragments were localized predominantly to the Golgi fractions. In cells expressing the PS1 mutant lacking exon 9 (DeltaE9), we observed only full-length molecules that were present in the ER and Golgi fractions. The turnover rate was considerably slower for the DeltaE9 holoprotein, apparently due to decreased degradation within the ER. Our results suggest that that full-length presenilin proteins are primarily ER resident molecules and undergo endoproteolysis within the ER. The fragments are subsequently transported to the Golgi compartment, where their turnover rate is much slower than that of the full-length presenilin in the ER.

The beta-amyloid precursor protein of Alzheimer's disease enhances neuron viability and modulates neuronal polarity

beta-Amyloid precursor protein (betaPP) can reside at neuron and glial cell surfaces or undergo proteolytic processing into secreted fragments. Although betaPP has been studied extensively, its precise physiological role is unknown. A line of transgenic knock-out mice selectively deficient in betaPP survive and breed but exhibit motor dysfunction and brain gliosis, consistent with a physiological role for betaPP in neuron development. To elucidate these functions, we cultured hippocampal neurons from wild-type and betaPP-deficient mice and compared their ability to attach, survive, and develop neurites. We found that hippocampal neurons from betaPP-deficient mice had diminished viability and retarded neurite development. We also compared the effects of betaPP secretory products, released from wild-type astrocytes, on process outgrowth from wild-type and betaPP-deficient hippocampal neurons. Outgrowth was enhanced at 1 d in the presence of wild-type astrocytes, as compared with betaPP-deficient astrocytes. However, by 3 d, neurons had shorter axons but more minor processes with more branching when cocultured with wild-type astrocytes, as compared with betaPP-deficient astrocytes. Our data demonstrate that cell-associated neuronal betaPP contributes to neuron viability, axonogenesis, and arborization and that betaPP secretory products modulate axon growth, dendrite branching, and dendrite numbers.

Characterization of amyloid beta protein species in cerebral amyloid angiopathy of a squirrel monkey by immunocytochemistry and enzyme-linked immunosorbent assay

We analyzed the composition of amyloid beta protein (A beta) species in cerebral amyloid angiopathy (CAA) of an aged squirrel monkey. Immunocytochemistry demonstrated that the cerebral cortex contained no lesions other than widespread CAA with A beta40 as its apparent major component. However, enzyme-linked immunosorbent assay revealed that A beta42(43) predominated over A beta40 in a formic acid-extracted cortical fraction. These findings suggest possible underestimation of A beta42(43) levels in some previous immunocytochemical investigations.

Interaction between amyloid precursor protein and presenilins in mammalian cells: implications for the pathogenesis of Alzheimer disease

Mutations in the presenilin 1 (PS1) and presenilin 2 (PS2) genes increase the production of the highly amyloidogenic 42-residue form of amyloid beta-protein (Abeta42) in a variety of cell lines and transgenic mice. To elucidate the molecular mechanism of this effect, wild-type (wt) or mutant PS1 and PS2 genes were stably transfected into Chinese hamster ovary cells expressing endogenous or transfected beta-amyloid precursor protein (APP). By immunoprecipitation/Western blot analysis, APP was consistently found to coimmunoprecipitate with PS1 or PS2 proteins. Several distinct PS1, PS2, or APP antibodies precipitated PS-APP complexes that were detectable by blotting with either APP or PS antibodies. Importantly, complex formation could be detected at endogenous protein levels in nontransfected cells. In various Chinese hamster ovary cell lines, the amounts of APP coprecipitated by PS antibodies were proportional to the expression levels of both APP and PS. APP-PS complexes also were recovered from human 293 and HS683 cells. Full maturation of APP was not required for the interaction; most APP molecules complexed with PS were solely N-glycosylated. Treatment of cells with brefeldin A or incubation at 20 degrees C did not block complex formation, suggesting that the association between APP and PS occurs in part in the endoplasmic reticulum. Complex formation was detected for both wt and mutant PS and APP proteins. Deletion of the APP C-terminal domain did not abrogate complex formation, suggesting that the interaction does not occur in the cytoplasmic domains of the proteins. Our results demonstrate that wt and mutant PS1 and PS2 proteins form complexes with APP in living cells, strongly supporting the hypothesis that mutant PS interacts with APP in a way that enhances the intramembranous proteolysis of the latter by a gamma-secretase cleaving at Abeta42.

Enhanced production and oligomerization of the 42-residue amyloid beta-protein by Chinese hamster ovary cells stably expressing mutant presenilins

Mutations in the presenilin 1 (PS1) and presenilin 2 (PS2) genes cause the most common and aggressive form of early onset familial Alzheimer's disease. To elucidate their pathogenic mechanism, wild-type (wt) or mutant (M146L, C410Y) PS1 and wt or mutant (M239V) PS2 genes were stably transfected into Chinese hamster ovary cells that overexpress the beta-amyloid precursor protein (APP). The identity of the 43-45-kDa PS1 holoproteins was confirmed by N-terminal radiosequencing. PS1 was rapidly processed (t1/2 = 40 min) in the endoplasmic reticulum into stable fragments. Wild-type and mutant PS2 holoproteins exhibited similar half lives (1.5 h); however, their endoproteolytic fragments showed both mutation-specific and cell type-specific differences. Mutant PS1 or PS2 consistently induced a 1.4-2.5-fold increase (p < 0.001) in the relative production of the highly amyloidogenic 42-residue form of amyloid beta-protein (Abeta42) as determined by quantitative immunoprecipitation and by enzyme-linked immunosorbent assay. In mutant PS1 and PS2 cell lines with high increases in Abeta42/Abetatotal ratios, spontaneous formation of low molecular weight oligomers of Abeta42 was observed in media, suggesting enhanced Abeta aggregation from the elevation of Abeta42. We conclude that mutant PS1 and PS2 proteins enhance the proteolysis of beta-amyloid precursor protein by the gamma-secretase cleaving at Abeta residue 42, thereby promoting amyloidogenesis.

Phorbol esters affect multiple steps in beta-amyloid precursor protein trafficking and amyloid beta-protein production

BACKGROUND

Amyloid beta-protein (A beta), the major constituent of amyloid deposits found in Alzheimer's disease, is derived from the beta-amyloid precursor protein (beta PP). Constitutive proteolysis by alpha-secretase and secretion of soluble beta PP (beta PPs) are stimulated by protein kinase C (PKC) activation, whereas A beta production and release are inhibited. The cellular mechanism that underlies the PKC-mediated down-regulation of A beta generation is unclear. Because endocytic processing of beta PP from the cell surface is a major pathway of A beta production, the effect of PKC activation by phorbol 12,13-dibutyrate (PDBu) on endocytic trafficking of beta PP was examined.

MATERIALS AND METHODS

In this study, trafficking of beta PP was assayed in Chinese hamster ovary cells (CHO) cells stably transfected with full-length beta PP751.

RESULTS

Treatment with PDBu resulted in a rapid and striking reduction of up to 80% in the amount of beta PP at the cell surface. This loss of cell-surface molecules could not be accounted for by changes in the trafficking of cell-surface beta PP molecules, as determined by a radiolabeled antibody assay. Rather, the decrease in beta PP was due primarily to a reduction in the sorting of beta PP to the cell surface. This alteration was correlated with accelerated intracellular alpha-secretase-mediated beta PP cleavage and accelerated beta PP trafficking in the exocytic pathway.

CONCLUSIONS

The data suggest that the displacement of beta PP away from the cell surface after phorbol ester treatment reduces the substrate available for endocytic processing and in turn, results in the inhibition of A beta production.

Isolation and characterization of Drosophila presenilin homolog

Presenilin-1 (PS1) and presenilin-2 (PS2) are associated with a majority of early onset familial Alzheimer's disease (FAD). Sequence analysis of PS1/2 has revealed integral transmembrane proteins which are highly homologous to the protein coded by sel-12, a Caenorhabditis elegans gene involved in the lin-12/Notch signaling pathway. The normal function of PS1/2, as well as the pathogenesis caused by mutations of these genes in FAD, are unknown however. We have identified a Drosophila presenilin homolog (DPS) and mapped the chromosomal location of this gene. DPS shows 53% amino acid identity to PS1/2 and 45% to the sel-12 product. Strong amino acid conservations appear at the position associated with FAD. In embryonic stages, DPS is expressed primarily in the CNS.

Cell surface amyloid beta-protein precursor colocalizes with beta 1 integrins at substrate contact sites in neural cells

Amyloid beta-protein (A beta), the principal constituent of the senile plaques seen in Alzheimer's disease (AD), is derived by proteolysis from the beta-amyloid precursor protein (beta PP). The distribution and trafficking of cell surface beta PP are of particular interest because some of these molecules are direct precursors of secreted A beta and because the localization of beta PP at the cell surface may be related directly to its physiological functions. Recently, we reported that, in cultured hippocampal neurons, cell surface beta PP is preferentially expressed on axons in a striking discontinuous pattern. In this study, we describe the colocalization of cell surface beta PP and integrins in primary cultured cells. In rat hippocampal neurons, cell surface beta PP was colocalized selectively with alpha 1 beta 1 and alpha 5 beta 1 integrin heterodimers at these characteristic segmental locations. In rat cortical astrocytes, both cell surface beta PP and beta 1 integrin were located at the cell periphery in the "spreading" stage shortly after plating. In "flattened" astrocytes cultured for several days, beta PP was found in punctate deposits called point contacts. In these sites, beta PP was colocalized with alpha 1 beta 1, but not with alpha 5 beta 1 integrin heterodimers, the latter of which were situated at focal contact sites. In both neurons and astrocytes examined after shearing, clathrin and alpha-adaptin were colocalized with beta PP on the surface that directly contacts the substratum. These results are consistent with the putative role of beta PP in cell adhesion and suggests that beta PP either interacts with selected integrins or shares similar cellular machinery to promote cell adhesion.

Trafficking of cell-surface beta-amyloid precursor protein: evidence that a sorting intermediate participates in synaptic vesicle recycling

We recently demonstrated that the Alzheimer's beta-amyloid precursor protein (APP) is internalized from the axonal cell surface. In this study, we use biochemical and cell biological methods to characterize endocytotic compartments that participate in trafficking of APP in central neurons. APP is present in presynaptic clathrin-coated vesicles purified from bovine brain, together with the recycling synaptic vesicle integral membrane proteins synaptophysin, synaptotagmin, and SV2. In contrast, APP is largely excluded from synaptic vesicles purified from rat brain. In primary cerebellar macroneurons, cell-surface APP is internalized with recycling synaptic vesicle integral membrane proteins but is subsequently sorted away from synaptic vesicles and transported retrogradely to the neuronal soma. Internalized APP partially co-localizes with rab5a-containing compartments in axons and with V-ATPase-containing compartments in both axons and neuronal soma. These results provide direct biochemical evidence that an obligate sorting compartment participates in the regeneration of synaptic vesicles during exo/endocytotic recycling at nerve terminals but do not preclude concurrent "kiss-and-run" recycling. Moreover, APP is now, to our knowledge, the first demonstrated example of an axonal cell-surface protein that is internalized with recycling synaptic vesicle membrane proteins but is subsequently sorted away from synaptic vesicles.

Presenilin proteins undergo heterogeneous endoproteolysis between Thr291 and Ala299 and occur as stable N- and C-terminal fragments in normal and Alzheimer brain tissue

Humans inheriting missense mutations in the presenilin (PS)1 and -2 genes undergo progressive cerebral deposition of the amyloid beta-protein at an early age and develop a clinically and pathologically severe form of familial Alzheimer's disease (FAD). Because PS1 mutations cause the most aggressive known form of AD, it is important to elucidate the structure and function of this multitransmembrane protein in the brain. Using a panel of region-specific PS antibodies, we characterized the presenilin polypeptides in mammalian tissues, including brains of normal, AD, and PS1-linked FAD subjects, and in transfected and nontransfected cell lines. Very little full-length PS1 or -2 was detected in brain and untransfected cells; instead the protein occurred as a heterogeneous array of stable N- and C-terminal proteolytic fragments that differed subtly among cell types and mammalian tissues. Sequencing of the major C-terminal fragment from PS1-transfected human 293 cells showed that the principal endoproteolytic cleavage occurs at and near Met298 in the proximal portion of the large hydrophilic loop. Full-length PS1 in these cells is quickly turned over (T1/2 approximately 60 min), in part to the two major fragments. The sizes and amounts of the PS fragments were not significantly altered in four FAD brains with the Cys410Tyr PS1 missense mutation. Our results indicate that presenilins are rapidly processed to N- and C-terminal fragments in both neural and nonneural cells and that interference with this processing is not an obligatory feature of FAD-causing mutations.

The E280A presenilin 1 Alzheimer mutation produces increased A beta 42 deposition and severe cerebellar pathology

Missense mutations in the presenilin 1 (PS1) gene cause the most common form of dominant early-onset familial Alzheimer's disease (FAD) and are associated with increased levels of amyloid beta-peptides (A beta) ending at residue 42 (A beta 42) in plasma and skin fibroblast media of gene carriers. A beta 42 aggregates readily and appears to provide a nidus for the subsequent aggregation of A beta 40 (ref. 4), resulting in the formation of innumerable neuritic plaques. To obtain in vivo information about how PS1 mutations cause AD pathology at such early ages, we characterized the neuropathological phenotype of four PS1-FAD patients from a large Colombian kindred bearing the codon 280 Glu to Ala substitution (Glu280Ala) PS1 mutation. Using antibodies specific to the alternative carboxy-termini of A beta, we detected massive deposition of A beta 42, the earliest and predominant form of plaque A beta to occur in AD (ref. 6-8), in many brain regions. Computer-assisted quantification revealed a significant increase in A beta 42, but not A beta 40, burden in the brains from 4 PS1-FAD patients compared with those from 12 sporadic AD patients. Severe cerebellar pathology included numerous A beta 42-reactive plaques, many bearing dystrophic neurites and reactive glia. Our results in brain tissue are consistent with recent biochemical evidence of increased A beta 42 levels in PS1-FAD patients and strongly suggest that mutant PS1 proteins alter the proteolytic processing of the beta-amyloid precursor protein at the C-terminus of A beta to favor deposition of A beta 42.

Expression and distribution of amyloid precursor protein-like protein-2 in Alzheimer's disease and in normal brain

Amyloid precursor-like protein-2 (APLP-2) belongs to a family of homologous amyloid precursor-like proteins. In the present study we report on the expression and distribution of APLP-2 in fetal and adult human brain and in brains of patients with Alzheimer's disease. We demonstrate that APLP-2 mRNAs encoding isoforms predicted to undergo post-translational modification by chondroitin sulfate glycosaminoglycans are elevated in fetal and aging brains relative to the brains of young adults. Immunocytochemical labeling with APLP-2-specific antibodies demonstrates APLP-2 immunoreactivity in cytoplasmic compartments in neurons and astrocytes, in large part overlapping the distribution of the amyloid precursor protein. In Alzheimer's disease brain, APLP-2 antibodies also label a subset of neuritic plaques. APLP-2 immunoreactivity is particularly conspicuous in large dystrophic neurites that also label with antibodies specific for APP and chromogranin A. In view of the age-dependent increase in levels of chondroitin sulfate glycosaminoglycan-modified forms of APLP-2 in aging brain and the accumulation of APLP-2 in dystrophic presynaptic elements, we suggest that APLP-2 may play roles in neuronal sprouting or in the aggregation, deposition, and/or persistence of beta-amyloid deposits.

Trafficking of cell-surface amyloid beta-protein precursor. I. Secretion, endocytosis and recycling as detected by labeled monoclonal antibody

Amyloid beta-protein, the principal constituent of amyloid fibrils found in senile plaques and blood vessels in Alzheimer's disease, is constitutively produced and released into medium of cultured cells. Amyloid beta-protein is derived by proteolysis of the beta-amyloid precursor protein by unclear mechanisms. Beta-amyloid precursor protein is a transmembrane protein which can be processed to release a large secretory product or processed in the endosomal/lysosomal pathway without secretion. Previous studies have shown that from the cell surface, beta-amyloid precursor protein may be released after cleavage or internalized without cleavage, the latter in a pathway that both produces amyloid beta-protein and also targets some molecules to the lysosomal compartment. Analysis of beta-amyloid precursor protein trafficking is confounded by the concomitant secretion and internalization of molecules from the cell surface. To address this issue, we developed an assay, based on the binding of radioiodinated monoclonal antibody, to measure the release and internalization of cell surface beta-amyloid precursor protein in transfected cells. With this approach, we showed that surface beta-amyloid precursor protein is either rapidly released or internalized, such that the duration at the cell surface is very short. Approximately 30% of cell surface beta-amyloid precursor protein molecules are released. Following internalization, a fraction of molecules are recycled while the majority of molecules are rapidly sorted to the lysosomal compartment for degradation When the C terminus of beta-amyloid precursor protein is deleted, secretion is increased by approximately 2.5-fold as compared to wild-type molecules. There is concomitant decrease in internalization in these mutant molecules as well as prolongation of the resident time on the cell surface. This observation is consistent with recent evidence that signals within the cytoplasmic domain mediate beta-amyloid precursor protein internalization.

Trafficking of cell-surface amyloid beta-protein precursor. II. Endocytosis, recycling and lysosomal targeting detected by immunolocalization

Amyloid beta-protein (A beta) is a proteolytic fragment of the amyloid beta-protein precursor (beta PP). Progressive cerebral deposition of A beta is an early and invariant feature of Alzheimer's disease. The cellular trafficking of beta PP is of particular interest because understanding the production of A beta requires a comprehensive elucidation of the metabolic pathways of this protein. In addition, beta PP is a type I integral membrane glycoprotein that belongs to a class of molecules with both full length and secreted products. Recent evidence suggests that beta PP can be processed in an endosomal/lysosomal pathway. In the latter organelles, a number of beta PP carboxy-terminal derivatives are found, but the precise pathway and kinetics of beta PP trafficking from the cell surface remain unclear. To address these questions, we visualized directly the beta PP internalization pathway by following the localization and distribution of beta PP monoclonal antibodies added to intact beta PP-transfected Chinese hamster ovary cells. Using immunofluorescence and immunoelectron microscopy, beta PP was shown to be rapidly internalized via coated pits and vesicles, after which the molecules were transported to endosomes, prelysosomes, and lysosomes. Using a modified immunodetection protocol, we demonstrated the rapid recycling of endocytosed beta PP to the cell surface and its ultimate targeting to lysosomes. Because we recently found that endocytosis of cell surface beta PP is one route for the constitutive production of A beta, the recycling pathway for cell surface beta PP demonstrated here is a probable route for production of the critical A beta fragment.

Enhanced release of amyloid beta-protein from codon 670/671 "Swedish" mutant beta-amyloid precursor protein occurs in both secretory and endocytic pathways

The mutation at codons 670/671 of beta-amyloid precursor protein (betaPP) dramatically elevates amyloid beta-protein (Abeta) production. Since increased Abeta may be responsible for the disease phenotype identified from a Swedish kindred with familial Alzheimer's disease, evaluation of the cellular mechanism(s) responsible for the enhanced Abeta release may suggest potential therapies for Alzheimer's disease. In this study, we analyzed Chinese hamster ovary cells stably transfected with either wild type betaPP (betaPP-wt) or "Swedish" mutant betaPP (betaPP-sw) for potential differences in betaPP processing. We confirmed that increased amounts of Abeta and a beta-secretase-cleaved COOH-terminally truncated soluble betaPP (betaPPs) were secreted from betaPP-sw cells. As shown previously for betaPP-wt cells, Abeta was released more slowly than the secretion of betaPPs from surface-labeled betaPP-sw cells, indicating that endocytosis of cell surface betaPP is one source of Abeta production. In contrast, by [35S]methionine metabolic labeling, the rates of Abeta and betaPPs release were virtually identical for both cell lines. In addition, the identification of intracellular betaPPs and Abeta shortly after pulse labeling suggests that Abeta is produced in the secretory pathway. Interestingly, more Abeta was present in medium from betaPP-sw cells than betaPP-wt cells after either cell surface iodination or [35S]methionine labeling, indicating that betaPP-sw cells have enhanced Abeta release in both the endocytic and secretory pathways. Furthermore, a variety of drug treatments known to affect protein processing similarly reduced Abeta release from both betaPP-wt and betaPP-sw cells. Taken together, the data suggest that the processing pathway for betaPP is similar for both betaPP-wt and betaPP-sw cells and that increased Abeta production by betaPP-sw cells arises from enhanced cleavage of mutant betaPP by beta-secretase, the as-yet unidentified enzyme(s) that cleaves at the NH2 terminus of Abeta.

The role of APP processing and trafficking pathways in the formation of amyloid beta-protein

The amyloid beta-protein (A beta) is a proteolytic fragment of the beta-amyloid precursor protein (beta APP). We previously reported the constitutive secretion of A beta peptides from a variety of cells expressing beta APP under normal culture conditions. These endogenously produced A beta peptides have heterogeneous N- and C-termini that vary as a function of beta APP missense mutations. Treatment of A beta-secreting cells with agents that alter intravesicular pH showed that an acidic compartment is required for proper A beta generation. One such compartment appears to be the endosome. Immunolabeling of cell-surface beta APP in living neurons and non-neuronal cells directly demonstrated the endocytosis of the protein and its rapid recycling (within 5-10 minutes) to the cell surface, as well as the trafficking of some beta APP to lysosomes. Expression of beta APP with various deletions of the cytoplasmic domain, including the NPTY motif, leads to decreased internalization and an associated decrease in the production of A beta peptides that begin at the usual asp1 start site. These and other data suggest that A beta production begins with cleavage of beta APP by a still unknown protease(s) (beta-secretase[s]) at the met-asp bond proceeding the A beta N-terminus and that this occurs in part in early endosomes. To characterize the substrate requirements of beta-secretase, beta APP was mutagenized by placing stop codons within or at the end of the transmembrane domain or substituting other amino acids for the wild-type met and asp at the P1 and P1' positions. These experiments showed that proper beta-secretase cleavage requires the precursor to be membrane-anchored and is highly sequence specific; most substitutions at met or asp substantially decrease A beta production. Analogous mutagenesis experiments around the A beta C-terminus revealed that the unknown protease(s) cleaving here ("gamma-secretase[s]") does not show such specificity. Cells secreting A beta may also be useful for examining the critical issue of the aggregation of A beta into its neurotoxic polymeric form under physiological conditions. In this regard, we have found that beta APP-expressing CHO cells show aggregation of > or = 10-20% of their secreted A beta peptides into SDS-stable dimers, trimers and sometimes tetramers under normal culture conditions. The identity of these small multimers was confirmed by extensive immunochemical characterization and radiosequencing. They are present at approximately 100-500 pM levels in conditioned medium of CHO transfectants. Using this endogenous A beta aggregating system, we have begun to examine variables that influence aggregation and compounds which may retard it. In conclusion, studies of the regulation of A beta production and aggregation in cell culture can provide information under physiological conditions that can complement analyses of these processes in vivo.

Aggregation of secreted amyloid beta-protein into sodium dodecyl sulfate-stable oligomers in cell culture

Filamentous aggregates of the 40-42-residue amyloid beta-protein (A beta) accumulate progressively in the limbic and cerebral cortex in Alzheimer's disease, where they are intimately associated with neuronal and glial cytopathology. Attempts to model this cytotoxicity in vitro using synthetic peptides have shown that monomeric A beta is relatively inert, whereas aggregated A beta reproducibly exerts a variety of neurotoxic effects. The processes that mediate the conversion of monomeric A beta into a toxic aggregated state are thus of great interest. Previous studies of this conversion have employed high concentrations (10(-5)-10(-3) M) of synthetic A beta peptides under nonbiological conditions. We report here the detection of small amounts (< 10(-9) M) of SDS-stable A beta oligomers in the culture media of Chinese hamster ovary cells expressing endogenous or transfected amyloid beta-protein precursor genes. The identity of these oligomers (primarily dimers and trimers) was established by immunoprecipitation with a panel of A beta antibodies, by electrophoretic comigration with synthetic A beta oligomers, and by amino acid sequencing. The oligomeric A beta species comprised approximately 10-20% of the total immunoprecipitable A beta in these cultures. A truncated A beta species beginning at Arg 5 was enriched in the oligomers, suggesting that amino-terminal heterogeneity can influence A beta oligomerization in this system. Addition of Congo red (10 microM) during metabolic labeling of the cells led to increased monomeric and decreased oligomeric A beta. The ability to detect and quantitate oligomers of secreted A beta peptides in cell culture should facilitate dynamic studies of the critical process of initial A beta aggregation under physiological conditions.

Trafficking of cell surface beta-amyloid precursor protein: retrograde and transcytotic transport in cultured neurons

Amyloid beta-protein (A beta), the principal constituent of senile plaques seen in Alzheimer's disease (AD), is derived by proteolysis from the beta-amyloid precursor protein (beta PP). The mechanism of A beta production in neurons, which are hypothesized to be a rich source of A beta in brain, remains to be defined. In this study, we describe a detailed localization of cell surface beta PP and its subsequent trafficking in primary cultured neurons. Full-length cell surface beta PP was present primarily on perikarya and axons, the latter with a characteristic discontinuous pattern. At growth cones, cell surface beta PP was inconsistently detected. By visualizing the distribution of beta PP monoclonal antibodies added to intact cultures, beta PP was shown to be internalized from distal axons or terminals and retrogradely transported back to perikarya in organelles which colocalized with fluid-phase endocytic markers. Retrograde transport of beta PP was shown in both hippocampal and peripheral sympathetic neurons, the latter using a compartment culture system that isolated cell bodies from distal axons and terminals. In addition, we demonstrated that beta PP from distal axons was transcytotically transported to the surface of perikarya from distal axons in sympathetic neurons. Indirect evidence of this transcytotic pathway was obtained in hippocampal neurons using antisense oligonucleotide to the kinesin heavy chain to inhibit anterograde beta PP transport. Taken together, these results demonstrate novel aspects of beta PP trafficking in neurons, including retrograde axonal transport and transcytosis. Moreover, the axonal predominance of cell surface beta PP is unexpected in view of the recent report of polarized sorting of beta PP to the basolateral domain of MDCK cells.

Cell-surface beta-amyloid precursor protein stimulates neurite outgrowth of hippocampal neurons in an isoform-dependent manner

beta-Amyloid precursor protein (beta APP) is an integral membrane polypeptide expressed in many neural and non-neural cells. beta APP occurs in part at the cell surface and undergoes proteolytic processing to release the large soluble ectodomain (APPs) and the amyloid beta-peptide (A beta), both of which have apparent trophic activity in vitro. Despite intense interest in beta APP expression and metabolism, there is limited knowledge about the function mediated by beta APP inserted at the cell-surface. We established a coculture system in which beta APP-transfected CHO cells serve as a substrate for the growth of primary rat hippocampal neurons. Compared to nontransfected CHO cells, the increased surface beta APP of the transfectants stimulated short-term neuronal adhesion and longer-term neurite outgrowth, whereas the increased amount of secreted APPs and A beta in conditioned medium produced no such effects when neurons were grown either on untransfected CHO cells or on a polylysine substrate. Moreover, a peptide which has been shown to block the trophic effects of secreted APPs (Ninomiya et al., 1993) failed to interrupt the neurite promoting activity mediated by the surface-expressed beta APP. Surface-expressed beta APP751 or beta APP770 isoforms mediated more neurite outgrowth than did the beta APP695 isoform. Antibody blocking and regional deletion experiments indicated that the mid-region of the beta APP ectodomain (residues 361-648) is involved in promoting neurite outgrowth. We conclude that surface-expressed cellular beta APP has a neurite-promoting function which is distinct from the trophic function of the secreted beta APP derivatives and may have special significance during brain development.

Polarized sorting of beta-amyloid precursor protein and its proteolytic products in MDCK cells is regulated by two independent signals

Progressive cerebral deposition of the amyloid (A beta) beta-protein is an early and invariant feature of Alzheimer's disease. A beta is derived by proteolysis from the membrane-spanning beta-amyloid precursor protein (beta APP). beta APP is processed into various secreted products, including soluble beta APP (APPs), the 4-kD A beta peptide, and a related 3-kD peptide (p3). We analyzed the mechanisms regulating the polarized basolateral sorting of beta APP and its proteolytic derivatives in MDCK cells. Deletion of the last 32 amino acids (residues 664-695) of the beta APP cytoplasmic tail had no influence on either the constitutive approximately 90% level of basolateral sorting of surface beta APP, or the strong basolateral secretion of APPs, A beta, and p3. However, deleting the last 42 amino acids (residues 654-695) or changing tyrosine 653 to alanine altered the distribution of cell surface beta APP so that approximately 40-50% of the molecules were inserted apically. In parallel, A beta was now secreted from both surfaces. Surprisingly, this change in surface beta APP had no influence on the basolateral secretion of APPs and p3. This result suggests that most beta APP molecules which give rise to APPs in MDCK cells are cleaved intracellularly before reaching the surface. Consistent with this conclusion, we readily detected intracellular APPs in carbonate extracts of isolated membrane vesicles. Moreover, ammonium chloride treatment resulted in the equal secretion of APPs into both compartments, as occurs with other non-membranous, basolaterally secreted proteins, but it did not influence the polarity of cell surface beta APP. These results demonstrate that in epithelial cells two independent mechanisms mediate the polarized trafficking of beta APP holoprotein and its major secreted derivative (APPs) and that A beta peptides are derived in part from beta APP holoprotein targeted to the cell surface by a signal that includes tyrosine 653.

Secreted beta-amyloid precursor protein stimulates mitogen-activated protein kinase and enhances tau phosphorylation

Biological effects related to cell growth, as well as a role in the pathogenesis of Alzheimer disease, have been ascribed to the beta-amyloid precursor protein (beta-APP). Little is known, however, about the intracellular cascades that mediate these effects. We report that the secreted form of beta-APP potently stimulates mitogen-activated protein kinases (MAPKs). Brief exposure of PC-12 pheochromocytoma cells to beta-APP secreted by transfected Chinese hamster ovary cells stimulated the 43-kDa form of MAPK by > 10-fold. Induction of a dominant inhibitory form of ras in a PC12-derived cell line prevented the stimulation of MAPK by secreted beta-APP, demonstrating the dependence of the effect upon p21ras. Because the microtubule-associated protein tau is hyperphosphorylated in Alzheimer disease, we sought and found a 2-fold enhancement in tau phosphorylation associated with the beta-APP-induced MAPK stimulation. In the ras dominant inhibitory cell line, beta-APP failed to enhance phosphorylation of tau. The data presented here provide a link between secreted beta-APP and the phosphorylation state of tau.

Evidence that production and release of amyloid beta-protein involves the endocytic pathway

Amyloid beta-protein (A beta), the 40-43-amino acid polypeptide that is the principal constituent of senile plaques found in Alzheimer's disease, is constitutively produced and released into medium of cultured cells by an unclear mechanism. In this study, we report that one route of A beta generation involves the internalization of cell surface amyloid precursor protein (beta PP) via the coated pit-mediated endocytic pathway. Radiolabeled A beta can be recovered in medium following selective cell surface radioiodination, indicating that cell surface beta PP is a direct precursor to A beta. In addition, deletion of the cytoplasmic domains of beta PP or depletion of potassium in medium, both of which resulted in reduced beta PP internalization, significantly diminished A beta release. Moreover, pulse-chase experiments after surface radioiodination showed that the kinetics of beta PP secretion and A beta release was different, with the latter occurring at a significantly slower rate. We therefore hypothesize that the internalization of cell surface beta PP via coated pit-mediated endocytosis is one pathway leading to A beta generation and release into medium.

Polarized secretion of beta-amyloid precursor protein and amyloid beta-peptide in MDCK cells

The beta-amyloid precursor protein (beta APP) is a widely expressed integral membrane protein that is proteolytically processed to yield several secreted derivatives, including soluble APP (APPs), the 4-kDa amyloid beta-peptide (A beta), and a related 3-kDa peptide (p3). To understand beta APP trafficking and processing, we analyzed the sorting of beta APP in Madin-Darby canine kidney (MDCK) cells, an epithelial cell known to possess physiologically distinct apical and basolateral plasma membranes. Processing of beta APP resulted in highly polarized secretion of APPs. More than 90% of APPs was detected in the basolateral compartment, and less than 10% was found in the apical compartment. This was associated with a preferential localization of beta APP on the basolateral cell surface. Activation of protein kinase C, which is known to enhance the secretion of APPs, did not change the polarity of APPs release but significantly increased the amount secreted. A beta and p3 peptides were also secreted predominantly basolaterally. In addition, MDCK cells secreted a truncated form of A beta beginning at Arg-5. These data show that the proteolytic processing products of beta APP undergo polarized secretion. Moreover, the results suggest that the amyloidogenic A beta peptide is generated following the polarized sorting of beta APP. The polarized basolateral secretion of A beta in these epithelial cells provides a potential mechanism for the accumulation of A beta in the abluminal basement membrane of brain microvessels during Alzheimer disease.

beta-Amyloid protein is higher in Alzheimer's disease brains: description of a quantitative biochemical assay

Deposition of beta-amyloid protein (A beta) in senile plaques and in the walls of cerebral vessels is a pathologic hallmark of Alzheimer's disease (AD). The current diagnostic criteria for AD requires the presence of neurofibrillary tangles and a minimum number of senile plaques in cortex. Senile plaques are readily visualized by silver staining or immunocytochemistry using antibodies raised to A beta. Available histochemical and immunocytochemical methods are sensitive but the results may occasionally be variable and sampling from many brain regions is difficult and impractical. This study describes a simple biochemical method for quantifying the A beta load in unfixed brain homogenates. The immunoassay recognizes all forms of A beta deposits (neuritic and diffuse plaques, and cerebrovascular amyloid) and has a sensitivity and specificity comparable to immunocytochemistry. In direct comparisons, results from the dot blot method correspond well with both Western blot analysis of partially purified A beta and plaque counting by immunocytochemistry. In a retrospective series of 39 postmortem AD and control cases, the amount of A beta in brain by dot blot immunoreactivity effectively separated the two groups. Therefore, this method provides a rapid, sensitive, and accurate quantitation of A beta in postmortem brain tissue and represents an alternative approach for studying A beta deposition in aging and AD.

Protein phosphorylation inhibits production of Alzheimer amyloid beta/A4 peptide

The major component of amyloid plaque cores and cerebrovascular amyloid deposits found in Alzheimer disease is the beta/A4 peptide, which is derived from the Alzheimer amyloid protein precursor (APP). Recent evidence suggests that abnormalities in beta/A4 peptide production or beta/A4 peptide aggregation may underlie cerebral amyloidosis. In the present study, treatment of cells with phorbol dibutyrate, which activates protein kinase C, and/or okadaic acid, which inhibits protein phosphatases 1 and 2A, reduced beta/A4 peptide production by 50-80%. These effects were observed with APP695 and APP751 expressed in stably transfected CHO cells, as well as with endogenous APP in human glioma (Hs 683) cells. Phorbol dibutyrate also decreased beta/A4 peptide production in cells expressing various mutant forms of APP associated with familial Alzheimer disease, one of which was reported to manifest greatly increased beta/A4 peptide production in cultured cells. Mastoparan and mastoparan X, compounds which can activate phospholipase C and hence protein kinase C, also decreased beta/A4 peptide production in CHO cells stably transfected with APP695. A model is presented in which decreases in beta/A4 peptide production can be achieved by accelerating the metabolism of APP through a nonamyloidgenic secretory pathway.

Identification and transport of full-length amyloid precursor proteins in rat peripheral nervous system

Amyloid deposits are a characteristic feature of the senile plaques identified in the brains of aged primates, individuals with Down's syndrome, and cases of Alzheimer's disease. The beta-amyloid protein (A beta), the principal component of amyloid, is a 4 kDa peptide derived from larger amyloid precursor protein(s) (APP). Four mRNAs, generated by alternative splicing of pre-mRNA derived from a single gene, encode A beta-containing membrane glycoproteins termed APP-695, -714, -751, and -770; the latter two isoforms contain a domain homologous to Kunitz protease inhibitors (KPI). The present study uses in vitro and in vivo strategies to examine the expression of APP in neurons of the dorsal root ganglia and the nature of APP transported in sciatic nerves of rats. Using quantitative in situ hybridization and semiquantitative PCR analysis, we document that mRNAs encoding APP-695 are expressed preferentially over transcripts that encode KPI-containing isoforms in rat sensory ganglia. Furthermore, we provide compelling evidence that APP-695 is the predominant isoform synthesized in sensory neurons of the rat PNS and that full-length APP-695 and, to a lesser extent, APP-751/770 are rapidly transported anterogradely in axons.

Amyloid beta-protein as a substrate interacts with extracellular matrix to promote neurite outgrowth

Progressive deposition of amyloid beta-protein (A beta) in brain parenchyma and blood vessels is a characteristic feature of Alzheimer disease. Recent evidence suggests that addition of solubilized synthetic A beta to medium may produce toxic or trophic effects on cultured hippocampal neurons. Because soluble A beta may not accumulate in significant quantities in brain, we asked whether immobilized A beta peptide as a substrate alters neurite outgrowth from cultured rat peripheral sensory neurons. This paradigm may closely mimic the conditions in Alzheimer disease brain tissue, in which neurites contact insoluble, extracellular aggregates of beta-amyloid. We detected no detrimental effects of A beta substrate on neurite outgrowth. Rather, A beta in combination with low doses of laminin or fibronectin enhanced neurite out-growth from these neuronal explants. Our results suggest that insoluble A beta in the cerebral neuropil may serve as a neurite-promoting matrix, perhaps explaining the apparent regenerative response of neurites observed around amyloid plaques in Alzheimer disease. Moreover, in concert with the recent discovery of A beta production by cultured neurons, our data suggest that A beta plays a normal physiological role in brain by complexing with the extracellular matrix.