Multiple mechanisms underlie neurotoxicity by different types of Alzheimer's disease mutations of amyloid precursor protein

We examined a neuronal cell system in which single-cell expression of either familial Alzheimer's disease (FAD) gene V642I-APP or K595N/M596L-APP (NL-APP) in an inducible plasmid was controlled without affecting transfection efficiency. This system revealed that (i) low expression of both mutants exerted toxicity sensitive to both Ac-DEVD-CHO (DEVD) and glutathione ethyl ester (GEE), whereas wild-type APP (wtAPP) only at higher expression levels caused GEE/DEVD-resistant death to lesser degrees; (ii) toxicity by the V642I mutation was entirely GEE/DEVD sensitive; and (iii) toxicity by higher expression of NL-APP was GEE/DEVD resistant. The GEE/DEVD-sensitive death was sensitive to pertussis toxin and was due to G(o)-interacting His(657)-Lys(676) domain. The GEE/DEVD-resistant death was due to C-terminal Met(677)-Asn(695). APP mutants lacking either domain unraveled elaborate intracellular cross-talk between these domains. E618Q-APP, responsible for non-AD type of a human disease, only exerted GEE/DEVD-resistant death at higher expression. Therefore, (i) different FAD mutations in APP cause neuronal cell death through different cytoplasmic domains via different sets of mechanisms; (ii) expression levels of FAD genes are critical in activating specific death mechanisms; and (iii) toxicity by low expression of both mutants most likely reflects the pathogenetic mechanism of FAD.     

G protein betagamma complex-mediated apoptosis by familial Alzheimer''s disease mutant of APP.

In familial Alzheimer's disease (FAD), three missense mutations, V642I, V642F and V642G, that co-segregate with the disease phenotype have been discovered in the 695 amino acid form of the amyloid precursor protein APP. Expression of these mutants causes a COS cell NK1 clone to undergo pertussis toxin-sensitive apoptosis in an FAD trait-linked manner by activating the G protein Go, which consists of G alpha(o) and G betagamma subunits. We investigated which subunit was responsible for the induction of apoptosis by V642I APP in NK1 cells. In the same system, expression of mutationally activated G alpha(o) or G alpha(i) induced little apoptosis. Apoptosis by V642I APP was antagonized by the overexpression of the carboxy-terminal amino acids 495-689 of the beta-adrenergic receptor kinase-1, which blocks the specific functions of G betagamma. Co-transfection of G beta2gamma2 cDNAs, but not that of other G beta(x)gamma(z) (x = 1-3; z = 2, 3), induced DNA fragmentation in a manner sensitive to bcl-2. These data implicate G betagamma as a cell death mediator for the FAD-associated mutant of APP.     

Expression of V642 APP mutant causes cellular apoptosis as Alzheimer trait-linked phenotype.

APP is a transmembrane precursor of beta-amyloid. In dominantly inherited familial Alzheimer's disease (FAD), point mutations V6421, V642F and V642G have been discovered in APP695. Here we show that expression of these mutants (FAD-APPs) causes a clone of COS cells to undergo apoptosis associated with DNA fragmentation. Apoptosis by the three FAD-APPs was the highest among all possible V642 mutants; normal APP695 had no effect on apoptosis, suggesting that apoptosis by APP mutants in this system is phenotypically linked to the FAD trait. FAD-APP-induced apoptosis was sensitive to bcl-2 and most probably mediated by heteromeric G proteins. This study presents a model system allowing analysis of the mechanism for FAD-APP-induced cytotoxicity.     

Transforming growth factor beta2 is a neuronal death-inducing ligand for amyloid-beta precursor protein

APP, amyloid beta precursor protein, is linked to the onset of Alzheimer's disease (AD). We have here found that transforming growth factor beta2 (TGFbeta2), but not TGFbeta1, binds to APP. The binding affinity of TGFbeta2 to APP is lower than the binding affinity of TGFbeta2 to the TGFbeta receptor. On binding to APP, TGFbeta2 activates an APP-mediated death pathway via heterotrimeric G protein G(o), c-Jun N-terminal kinase, NADPH oxidase, and caspase 3 and/or related caspases. Overall degrees of TGFbeta2-induced death are larger in cells expressing a familial AD-related mutant APP than in those expressing wild-type APP. Consequently, superphysiological concentrations of TGFbeta2 induce neuronal death in primary cortical neurons, whose one allele of the APP gene is knocked in with the V642I mutation. Combined with the finding indicated by several earlier reports that both neural and glial expression of TGFbeta2 was upregulated in AD brains, it is speculated that TGFbeta2 may contribute to the development of AD-related neuronal cell death.     

Alternative, non-secretase processing of Alzheimer's beta-amyloid precursor protein during apoptosis by caspase-6 and -8.

Alzheimer's disease (AD) is a progressive neurodegenerative disorder. Although the pathogenesis of AD is unknown, it is widely accepted that AD is caused by extracellular accumulation of a neurotoxic peptide, known as Abeta. Mutations in the beta-amyloid precursor protein (APP), from which Abeta arises by proteolysis, are associated with some forms of familial AD (FAD) and result in increased Abeta production. Two other FAD genes, presenilin-1 and -2, have also been shown to regulate Abeta production; however, studies examining the biological role of these FAD genes suggest an alternative theory for the pathogenesis of AD. In fact, all three genes have been shown to regulate programmed cell death, hinting at the possibility that dysregulation of apoptosis plays a primary role in causing neuronal loss in AD. In an attempt to reconcile these two hypotheses, we investigated APP processing during apoptosis and found that APP is processed by the cell death proteases caspase-6 and -8. APP is cleaved by caspases in the intracellular portion of the protein, in a site distinct from those processed by secretases. Moreover, it represents a general effect of apoptosis, because it occurs during cell death induced by several stimuli both in T cells and in neuronal cells.     

APP-BP1 mediates APP-induced apoptosis and DNA synthesis and is increased in Alzheimer's disease brain

APP-BP1, first identified as an amyloid precursor protein (APP) binding protein, is the regulatory subunit of the activating enzyme for the small ubiquitin-like protein NEDD8. We have shown that APP-BP1 drives the S- to M-phase transition in dividing cells, and causes apoptosis in neurons. We now demonstrate that APP-BP1 binds to the COOH-terminal 31 amino acids of APP (C31) and colocalizes with APP in a lipid-enriched fraction called lipid rafts. We show that coexpression of a peptide representing the domain of APP-BP1 that binds to APP, abolishes the ability of overexpressed APP or the V642I mutant of APP to cause neuronal apoptosis and DNA synthesis. A dominant negative mutant of the NEDD8 conjugating enzyme hUbc12, which participates in the ubiquitin-like pathway initiated by APP-BP1, blocks neuronal apoptosis caused by APP, APP(V642I), C31, or overexpression of APP-BP1. Neurons overexpressing APP or APP(V642I) show increased APP-BP1 protein levels in lipid rafts. A similar increase in APP-BP1 in lipid rafts is observed in the Alzheimer's disease brain hippocampus, but not in less-affected areas of Alzheimer's disease brain. This translocation of APP-BP1 to lipid rafts is accompanied by a change in the subcellular localization of the ubiquitin-like protein NEDD8, which is activated by APP-BP1.     

SILKWORM 30K PROTEIN INHIBITS ECDYSONE‐INDUCED APOPTOSIS BY BLOCKING THE BINDING OF ULTRASPIRACLE TO ECDYSONE RECEPTOR‐B1 IN CULTURED Bm5 CELLS

This study investigates the mechanism through which increased 30K protein inhibits ecdysone‐induced apoptosis in the Bm5 silkworm ovarian cell line. Treatment of Bm5 cells with 20‐hydroxyecdysone (20E) after transfection with the pIZT/V5‐His control vector triggered apoptosis, but 20E treatment did not trigger apoptosis in Bm5 cells transfected with the pIZT/30K/V5‐His vector. To confirm its inhibitory effect on apoptosis, 30K protein was first purified from Escherichia coli transformed with a 30K expression vector and used to generate specific antibodies in mice. Anti‐30K antiserum was used to confirm synthesis of the 30K protein in pIZT/30K/V5‐His‐transfected Bm5 cells and to detect 30K protein binding to the ecdysone receptor‐B1 (EcR‐B1). Anti‐30K antiserum was used to immunoprecipitate protein complexes containing 30K from Bm5 cells transfected with pIZT/30K/V5‐His vector and treated with 20E. We observed that 30K proteins bound primarily to the EcR‐B1 and not to ultraspiracle (USP). Reciprocal immunoprecipitation of EcR‐B1‐containing complexes from Bm5 cells transfected with control pIZT/V5‐His vector and treated with 20E showed that EcR‐B1 bound to USP in the absence of 30K but did not bind to USP in pIZT/30K/V5‐His‐transfected Bm5 cells. These results demonstrate that 30K proteins block USP binding to EcR‐B1 through formation of a 30K/EcR‐B1 complex, resulting in inhibition of 20E‐induced Bm5 cell apoptosis.     

Highly flexible ligand binding pocket of ecdysone receptor: a single amino acid change leads to discrimination between two groups of nonsteroidal ecdysone agonists

The insect steroid hormone 20-hydroxyecdysone works through a ligand-activated nuclear receptor, the ecdysone receptor (EcR), which plays critical roles in insect development and reproduction. The EcR has been exploited to develop insecticides to control pests and gene switches for gene regulation. Recently reported crystal structures of the EcR protein show different but partially overlapping binding cavities for ecdysteroid (ECD) and diacylhydrazine (DAH) ligands, providing an explanation for the differential activity of DAH ligands in insects. 1-Aroyl-4-(arylamino)-1,2,3,4-tetrahydroquinoline (THQ) ligands were recently discovered as ecdysone agonists. Mutagenesis of the EcR (from Choristoneura fumiferana, CfEcR) ligand binding domain followed by screening in a reporter assay led to the identification of CfEcR mutants, which responded well to THQ ligands but poorly to both ECD and DAH ligands. These mutants were further improved by introducing a second mutation, A110P, which was previously reported to cause ECD insensitivity. Testing of these V128F/A110P and V128Y/A110P mutants in a C57BL/6 mouse model coactivator interaction assay and in insect cells showed that this mutant EcR is activated by THQ ligands but not by ECD or DAH ligands. The CfEcR and its V128F/A110P mutant were used to demonstrate simultaneous regulation of two reporter genes using THQ and DAH ligands.     

Intrinsic signaling function of APP as a novel target of three V642 mutations linked to familial Alzheimer's disease.

APP695 is a transmembrane precursor of Abeta amyloid. In familial Alzheimer's disease (FAD), three mutations V642I/F/G were discovered in APP695, which has been suggested by multiple studies to be a cell surface signaling receptor. We previously reported that normal APP695 encodes a potential GO-linked receptor with ligand-regulated function and that expression of the three FAD mutants (FAD-APPs), not normal APP, induces cellular outputs by GO-dependent mechanisms. This suggests that FAD-APPs are constitutively active GO-linked receptors. Here, we provide direct evidence for this notion. Reconstitution of either recombinant FAD-APP with GO vesicles induced activation of GO, which was inhibitable by pertussis toxin, sensitive to Mg2+ and proportional in quantity to the reconstituted amounts of FAD-APP. Consistent with the dominant inheritance of this type of FAD, this function was dominant over normal APP, because little activation was observed in APP695-GO vesicles. Experiments with antibody competition and sequence deletion indicated that His657-Lys676 of FAD-APP, which has been specified as the ligand-dependent GO-coupling domain of normal APP, was responsible for this constitutive activation, confirming that the three FAD-APPs are mutationally activated APP695. This study identifies the intrinsic signaling function of APP to be a novel target of hereditary Alzheimer's disease mutations, providing an in vitro system for the screening of potential FAD inhibitors.     

MOCA is an integrator of the neuronal death signals that are activated by familial Alzheimer's disease-related mutants of amyloid β precursor protein and presenilins

The death of cholinergic neurons in the cerebral cortex and certain subcortical regions is linked to irreversible dementia relevant to AD (Alzheimer's disease). Although multiple studies have shown that expression of a FAD (familial AD)-linked APP (amyloid β precursor protein) or a PS (presenilin) mutant, but not that of wild-type APP or PS, induced neuronal death by activating intracellular death signals, it remains to be addressed how these signals are interrelated and what the key molecule involved in this process is. In the present study, we show that the PS1-mediated (or possibly the PS2-mediated) signal is essential for the APP-mediated death in a γ-secretase-independent manner and vice versa. MOCA (modifier of cell adhesion), which was originally identified as being a PS- and Rac1-binding protein, is a common downstream constituent of these neuronal death signals. Detailed molecular analysis indicates that MOCA is a key molecule of the AD-relevant neuronal death signals that links the PS-mediated death signal with the APP-mediated death signal at a point between Rac1 [or Cdc42 (cell division cycle 42)] and ASK1 (apoptosis signal-regulating kinase 1).     

Intracellularly generated amyloid-beta peptide counteracts the antiapoptotic function of its precursor protein and primes proapoptotic pathways for activation by other insults in neuroblastoma cells

Most mutations in amyloid precursor proteins (APPs) linked to early onset familial Alzheimer's disease (FAD) increase the production of amyloid-beta peptides ending at residue 42 (Abeta42), which are released from APP by beta- and gamma-secretase cleavage. Stably transfected cells expressing wild-type human APP (APP(WT)) were more resistant to apoptosis-inducing treatments than cells expressing FAD-mutant human APP (APP(FAD)). Preventing Abeta42 production with an M596I mutation (beta-), which blocks beta-secretase cleavage of APP, or by treatment with a gamma-secretase inhibitor increased the resistance of APP(FAD)-expressing cells to apoptosis. Exposing hAPP(FAD/beta-) cells to exogenous Abeta42 or conditioned medium from Abeta42-producing APP(FAD) cells did not diminish their resistance to apoptosis. Preventing APP from entering the distal secretory pathway, where most Abeta peptides are generated, by retaining APP in the endoplasmic reticulum (ER)/intermediate compartment (IC) increased the resistance of APP(FAD)-expressing cells to apoptosis and did not alter the resistance of APP(WT)-expressing cells. p53-mediated gene transactivation after apoptosis-inducing treatments was much stronger in APP(FAD) cells than in hAPP(WT) or hAPP(FAD/beta-) cells. In contrast, upon induction of ER stress, cells expressing APP(FAD), hAPP(FAD/beta-), or APP(WT) had comparable levels of glucose-regulated protein-78 mRNA, an unfolded protein response indicator. We conclude that Abeta, especially intracellular Abeta, counteracts the antiapoptotic function of its precursor protein and predisposes cells to p53-mediated, and possibly other, proapoptotic pathways.     

Functional characterization of ecdysone receptor gene switches in mammalian cells

Regulated expression of transgene is essential in basic research as well as for many therapeutic applications. The main purpose of the present study is to understand the functioning of the ecdysone receptor (EcR)-based gene switch in mammalian cells and to develop improved versions of EcR gene switches. We utilized EcR mutants to develop new EcR gene switches that showed higher ligand sensitivity and higher magnitude of induction of reporter gene expression in the presence of ligand. We also developed monopartite versions of EcR gene switches with reduced size of the components that are accommodated into viral vectors. Ligand binding assays revealed that EcR alone could not bind to the nonsteroidal ligand, RH-2485. The EcR's heterodimeric partner, ultraspiracle, is required for efficient binding of EcR to the ligand. The essential role of retinoid X receptor (RXR) or its insect homolog, ultraspiracle, in EcR function is shown by RXR knockdown experiments using RNAi. Chromatin immunoprecipitation assays demonstrated that VP16 (activation domain, AD):GAL4(DNA binding domain, DBD):EcR(ligand binding domain, LBD) or GAL4(DBD):EcR(LBD) fusion proteins can bind to GAL4 response elements in the absence of ligand. The VP16(AD) fusion protein of a chimera between human and locust RXR could heterodimerize with GAL4(DBD):EcR(LBD) in the absence of ligand but the VP16(AD) fusion protein of Homo sapiens RXR requires ligand for its heterodimerization with GAL4(DBD):EcR(LBD).     

Intracellular amyloid-beta 1-42, but not extracellular soluble amyloid-beta peptides, induces neuronal apoptosis

Alzheimer disease (AD), the most frequent cause of dementia, is characterized by an important neuronal loss. A typical histological hallmark of AD is the extracellular deposition of beta-amyloid peptide (A beta), which is produced by the cleavage of the amyloid precursor protein (APP). Most of the gene mutations that segregate with the inherited forms of AD result in increasing the ratio of A beta 42/A beta 40 production. A beta 42 also accumulates in neurons of AD patients. Altogether, these data strongly suggest that the neuronal production of A beta 42 is a critical event in AD, but the intraneuronal A beta 42 toxicity has never been demonstrated. Here, we report that the long term expression of human APP in rat cortical neurons induces apoptosis. Although APP processing leads to production of extracellular A beta 1-40 and soluble APP, these extracellular derivatives do not induce neuronal death. On the contrary, neurons undergo apoptosis as soon as they accumulate intracellular A beta 1-42 following the expression of full-length APP or a C-terminal deleted APP isoform. The inhibition of intraneuronal A beta 1-42 production by a functional gamma-secretase inhibitor increases neuronal survival. Therefore, the accumulation of intraneuronal A beta 1-42 is the key event in the neurodegenerative process that we observed.     

The Gtx homeodomain transcription factor exerts neuroprotection using its homeodomain

Certain cases of familial Alzheimer's disease are caused by mutants of amyloid-beta precursor protein (AbetaPP), including V642I-AbetaPP, K595N/M596L-AbetaPP (NL-AbetaPP), A617G-AbetaPP, and L648P-AbetaPP. By using an unbiased functional screening with transfection and expression of a human brain cDNA library, we searched for genes that protect neuronal cells from toxicity by V642I-AbetaPP. One protective clone was identical to the human GTX, a neuronal homeobox gene. Human Gtx (hGtx) inhibited caspase inhibitor-sensitive neuronal cell death not only by V642I-AbetaPP but also by L648P-, NL-, A617G-AbetaPP, apolipoprotein E4, and Abeta. The region of hGtx responsible for this rescue function was specified to be its homeodomain (Lys148-His207). The rescue function was shared by DLX4, a distal-less family gene with a homeodomain only 38.3% homologous to that of hGtx, suggesting that this function would be generally shared by homeodomains. The neuroprotective function of hGtx was attributable to hGtx-stimulated production and secretion of insulin-like growth factor-I. This study provides molecular clues to understand how neuronal cells developmentally regulate themselves against cell death as well as to develop reagents effective in curative therapeutics of Alzheimer's disease.     

Assessment of amyloid β-protein precursor gene mutations in a large set of familial and sporadic Alzheimer disease cases

A genetic locus associated with familial Alzheimer disease (FAD) and a candidate gene, APP, encoding the amyloid protein precursor have both been assigned previously to chromosome 21, and, in a few FAD families, mutations of APP have been detected. However, obligate crossovers between APP and FAD have also been reported in several FAD pedigrees, including FAD4, a large kindred showing highly suggestive evidence for linkage of the disorder to chromosome 21. In case the apparent APP crossover in FAD4 actually represented an intragenic recombination event or segregation of different mutations in different family branches, we have performed a more detailed assessment of APP as a candidate gene in this family. The entire coding region of the APP gene was sequenced for FAD4 and for FAD1, a second large kindred. No mutations were found, indicating that, in at least one chromosome 21-linked FAD pedigree, the gene defect is not accounted for by a mutation in the known coding region of the APP gene. A total of 25 well-characterized early- and late-onset FAD pedigrees were typed for genetic linkage to APP, to assess the percentage of FAD families predicted to carry mutations in the APP gene. None of the FAD families yielded positive lod scores at a recombination fraction of 0.0. To estimate the overall prevalence of FAD-associated mutations in the beta A4 domain of APP, we sequenced exons 16 and 17 in 30 (20 early- and 10 late-onset) FAD kindreds and in 11 sporadic AD cases, and we screened 56 FAD kindreds and 81 cases of sporadic AD for the presence of the originally reported FAD-associated mutation, APP717 Val----Ile (by BclI digestion). No APP gene mutations were found in any of the FAD families or sporadic-AD samples examined in this study, suggesting that the mutations in exons 16 and 17 are a rare cause of FAD. Overall, these data suggest that APP gene mutations account for a very small portion of FAD.     

Improvement of a monopartite ecdysone receptor gene switch and demonstration of its utility in regulation of transgene expression in plants

In plants, regulation of transgene expression is typically accomplished through the use of inducible promoter systems. The ecdysone receptor (EcR) gene switch is one of the best inducible systems available to regulate transgene expression in plants. However, the monopartite EcR gene switches developed to date require micromolar concentrations of ligand for activation. We tested several EcR mutants that were generated by changing one or two amino acid residues in the highly flexible ligand-binding domain of Choristoneura fumiferana EcR (CfEcR). Based on the transient expression assays, we selected a double mutant, V395I + Y415E (VY), of CfEcR (CfEcR(VY)) for further testing in stable transformation experiments. The CfEcR(VY) mutant only slightly improved the induction characteristics of the two-hybrid gene switch, whereas the CfEcR(VY) mutant significantly improved the induction characteristics of the monopartite gene switch (VGCfE(VY)). The ligand sensitivity of the VGCfE(VY) switch was improved by 125-15 625-fold in different transgenic lines analyzed, compared to the VGCfE(Wt) switch. The utility of the VGCfE(VY) switch was tested by regulating the expression of an Arabidopsis zinc finger protein gene (AtZFP11) in both tobacco and Arabidopsis plants. These data showed that the VGCfE(VY) switch efficiently regulated the expression of AtZFP11 and that the phenotype of AtZFP11 could be induced by the application of ligand. In addition, the affected plants recovered after withdrawal of the ligand, demonstrating the utility of this gene switch in regulating the expression of critical transgenes in plants.     

Brain Gene Expression of a Sporadic (icv-STZ Mouse) and a Familial Mouse Model (3xTg-AD Mouse) of Alzheimer’s Disease

Alzheimer’s disease (AD) can be divided into sporadic AD (SAD) and familial AD (FAD). Most AD cases are sporadic and may result from multiple etiologic factors, including environmental, genetic and metabolic factors, whereas FAD is caused by mutations of presenilins or amyloid-β (Aβ) precursor protein (APP). A commonly used mouse model for AD is 3xTg-AD mouse, which is generated by over-expression of mutated presenilin 1, APP and tau in the brain and thus represents a mouse model of FAD. A mouse model generated by intracerebroventricular (icv) administration of streptozocin (STZ), icv-STZ mouse, shows many aspects of SAD. Despite the wide use of these two models for AD research, differences in gene expression between them are not known. Here, we compared the expression of 84 AD-related genes in the hippocampus and the cerebral cortex between icv-STZ mice and 3xTg-AD mice using a custom-designed qPCR array. These genes are involved in APP processing, tau/cytoskeleton, synapse function, apoptosis and autophagy, AD-related protein kinases, glucose metabolism, insulin signaling, and mTOR pathway. We found altered expression of around 20 genes in both mouse models, which affected each of above categories. Many of these gene alterations were consistent with what was observed in AD brain previously. The expression of most of these altered genes was decreased or tended to be decreased in the hippocampus of both mouse models. Significant diversity in gene expression was found in the cerebral cortex between these two AD mouse models. More genes related to synaptic function were dysregulated in the 3xTg-AD mice, whereas more genes related to insulin signaling and glucose metabolism were down-regulated in the icv-STZ mice. The present study provides important fundamental knowledge of these two AD mouse models and will help guide future studies using these two mouse models for the development of AD drugs.     

The Drosophila EcR gene encodes an ecdysone receptor, a new member of the steroid receptor superfamily

The steroid hormone ecdysone triggers coordinate changes in Drosophila tissue development that result in metamorphosis. To advance our understanding of the genetic regulatory hierarchies controlling this tissue response, we have isolated and characterized a gene, EcR, for a new steroid receptor homolog and have shown that it encodes an ecdysone receptor. First, EcR protein binds active ecdysteroids and is antigenically indistinguishable from the ecdysone-binding protein previously observed in extracts of Drosophila cell lines and tissues. Second, EcR protein binds DNA with high specificity at ecdysone response elements. Third, ecdysone-responsive cultured cells express EcR, whereas ecdysone-resistant cells derived from them are deficient in EcR. Expression of EcR in such resistant cells by transfection restores their ability to respond to the hormone. As expected, EcR is nuclear and found in all ecdysone target tissues examined. Furthermore, the EcR gene is expressed at each developmental stage marked by a pulse of ecdysone.     

APP induces neuronal apoptosis through APP-BP1-mediated downregulation of β-catenin

Alzheimer's disease (AD) is a neurodegenerative disease associated with progressive dementia. This mini-review focuses on how the amyloid precursor protein (APP) plays a central role in AD and Down syndrome as the regulator of the APP-BP1/hUba3 activated neddylation pathway. It is argued that the physiological function of APP is to downregulate the level of beta-catenin. However, this APP function is abnormally amplified in patients with familial AD (FAD) mutations in APP and presenilins, resulting in the hyperactivation of neddylation and the decrease of beta-catenin below a threshold level. Evidence in the literature is summarized to show that dysfunction of APP in downregulating beta-catenin may underlie the mechanism of neuronal death in AD and Down syndrome.