Antibody-regulated neurotoxic function of cell-surface beta-amyloid precursor protein

Humanin and Alzheimer's disease: The beginning of a new field

BACKGROUND

Humanin (HN) is an endogenous peptide factor and known as a member of mitochondrial-derived peptides. We first found the gene encoding this novel 24-residue peptide in a brain of an Alzheimer's disease (AD) patient as an antagonizing factor against neuronal cell death induced by AD-associated insults.

SCOPE OF REVIEW

This review presents an overview of HN actions in AD-related conditions among its wide range of action spectrum as well as a brief history of the discovery.

MAJOR CONCLUSIONS

HN exhibits multiple intracellular and extracellular anti-cell death actions and antagonizes various AD-associated pathomechanisms including amyloid plaque accumulation.

GENERAL SIGNIFICANCE

This review concisely reflects accumulated knowledge on HN since the discovery focusing on its functions related to AD pathogenesis and provides a perspective to its potential contribution in AD treatments.

NRBP1-Containing CRL2/CRL4A Regulates Amyloid β Production by Targeting BRI2 and BRI3 for Degradation

Alzheimer's disease (AD) is a progressive neurodegenerative disease caused by accumulations of Aβ peptides. Production and fibrillation of Aβ are downregulated by BRI2 and BRI3, which are physiological inhibitors of amyloid precursor protein (APP) processing and Aβ oligomerization. Here, we identify nuclear receptor binding protein 1 (NRBP1) as a substrate receptor of a Cullin-RING ubiquitin ligase (CRL) that targets BRI2 and BRI3 for degradation. Moreover, we demonstrate that (1) dimerized NRBP1 assembles into a functional Cul2- and Cul4A-containing heterodimeric CRL through its BC-box and an overlapping cryptic H-box, (2) both Cul2 and Cul4A contribute to NRBP1 CRL function, and (3) formation of the NRBP1 heterodimeric CRL is strongly enhanced by chaperone-like function of TSC22D3 and TSC22D4. NRBP1 knockdown in neuronal cells results in an increase in the abundance of BRI2 and BRI3 and significantly reduces Aβ production. Thus, disrupting interactions between NRBP1 and its substrates BRI2 and BRI3 may provide a useful therapeutic strategy for AD.

C9-ALS/FTD-linked proline-arginine dipeptide repeat protein associates with paraspeckle components and increases paraspeckle formation

A GGGGCC hexanucleotide repeat expansion in the C9ORF72 gene has been identified as the most common genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia. The repeat expansion undergoes unconventional translation to produce five dipeptide repeat proteins (DPRs). Although DPRs are thought to be neurotoxic, the molecular mechanism underlying the DPR-caused neurotoxicity has not been fully elucidated. The current study shows that poly-proline-arginine (poly-PR), the most toxic DPR in vitro, binds to and up-regulates nuclear paraspeckle assembly transcript 1 (NEAT1) that plays an essential role as a scaffold non-coding RNA during the paraspeckle formation. The CRISPR-assisted up-regulation of endogenous NEAT1 causes neurotoxicity. We also show that the poly-PR modulates the function of several paraspeckle-localizing heterogeneous nuclear ribonucleoproteins. Furthermore, dysregulated expression of TAR DNA-binding protein 43 (TDP-43) up-regulates NEAT1 expression and induces neurotoxicity. These results suggest that the increase in the paraspeckle formation may be involved in the poly-PR- and TDP-43-mediated neurotoxicity.

Gi/o-Protein Coupled Receptors in the Aging Brain

Cells translate extracellular signals to regulate processes such as differentiation, metabolism and proliferation, via transmembranar receptors. G protein-coupled receptors (GPCRs) belong to the largest family of transmembrane receptors, with over 800 members in the human species. Given the variety of key physiological functions regulated by GPCRs, these are main targets of existing drugs. During normal aging, alterations in the expression and activity of GPCRs have been observed. The central nervous system (CNS) is particularly affected by these alterations, which results in decreased brain functions, impaired neuroregeneration, and increased vulnerability to neuropathologies, such as Alzheimer's and Parkinson diseases. GPCRs signal via heterotrimeric G proteins, such as G_o, the most abundant heterotrimeric G protein in CNS. We here review age-induced effects of GPCR signaling via the G_i/o subfamily at the CNS. During the aging process, a reduction in protein density is observed for almost half of the G_i/o-coupled GPCRs, particularly in age-vulnerable regions such as the frontal cortex, hippocampus, substantia nigra and striatum. G_i/o levels also tend to decrease with aging, particularly in regions such as the frontal cortex. Alterations in the expression and activity of GPCRs and coupled G proteins result from altered proteostasis, peroxidation of membranar lipids and age-associated neuronal degeneration and death, and have impact on aging hallmarks and age-related neuropathologies. Further, due to oligomerization of GPCRs at the membrane and their cooperative signaling, down-regulation of a specific G_i/o-coupled GPCR may affect signaling and drug targeting of other types/subtypes of GPCRs with which it dimerizes. G_i/o-coupled GPCRs receptorsomes are thus the focus of more effective therapeutic drugs aiming to prevent or revert the decline in brain functions and increased risk of neuropathologies at advanced ages.

The physiological role of the amyloid precursor protein as an adhesion molecule in the developing nervous system

The amyloid precursor protein (APP) is a type I transmembrane glycoprotein better known for its participation in the physiopathology of Alzheimer disease as the source of the beta amyloid fragment. However, the physiological functions of the full length protein and its proteolytic fragments have remained elusive. APP was first described as a cell-surface receptor; nevertheless, increasing evidence highlighted APP as a cell adhesion molecule. In this review, we will focus on the current knowledge of the physiological role of APP as a cell adhesion molecule and its involvement in key events of neuronal development, such as migration, neurite outgrowth, growth cone pathfinding, and synaptogenesis. Finally, since APP is over-expressed in Down syndrome individuals because of the extra copy of chromosome 21, in the last section of the review, we discuss the potential contribution of APP to the neuronal and synaptic defects described in this genetic condition. Read the Editorial Highlight for this article on page 9. Cover Image for this issue: doi. 10.1111/jnc.13817.

Amyloid Precursor Protein family as unconventional Go-coupled receptors and the control of neuronal motility

Cleavage of the Amyloid Precursor Protein (APP) generates amyloid peptides that accumulate in Alzheimer Disease (AD), but APP is also upregulated by developing and injured neurons, suggesting that it regulates neuronal motility. APP can also function as a G protein-coupled receptor that signals via the heterotrimeric G protein Gαo, but evidence for APP-Gαo signaling in vivo has been lacking. Using Manduca as a model system, we showed that insect APP (APPL) regulates neuronal migration in a Gαo-dependent manner. Recently, we also demonstrated that Manduca Contactin (expressed by glial cells) induces APPL-Gαo retraction responses in migratory neurons, consistent with evidence that mammalian Contactins also interact with APP family members. Preliminary studies using cultured hippocampal neurons suggest that APP-Gαo signaling can similarly regulate growth cone motility. Whether Contactins (or other APP ligands) induce this response within the developing nervous system, and how this pathway is disrupted in AD, remains to be explored.

Role of APP Interactions with Heterotrimeric G Proteins: Physiological Functions and Pathological Consequences

Following the discovery that the amyloid precursor protein (APP) is the source of β-amyloid peptides (Aβ) that accumulate in Alzheimer’s disease (AD), structural analyses suggested that the holoprotein resembles a transmembrane receptor. Initial studies using reconstituted membranes demonstrated that APP can directly interact with the heterotrimeric G protein Gαo (but not other G proteins) via an evolutionarily G protein-binding motif in its cytoplasmic domain. Subsequent investigations in cell culture showed that antibodies against the extracellular domain of APP could stimulate Gαo activity, presumably mimicking endogenous APP ligands. In addition, chronically activating wild type APP or overexpressing mutant APP isoforms linked with familial AD could provoke Go-dependent neurotoxic responses, while biochemical assays using human brain samples suggested that the endogenous APP-Go interactions are perturbed in AD patients. More recently, several G protein-dependent pathways have been implicated in the physiological roles of APP, coupled with evidence that APP interacts both physically and functionally with Gαo in a variety of contexts. Work in insect models has demonstrated that the APP ortholog APPL directly interacts with Gαo in motile neurons, whereby APPL-Gαo signaling regulates the response of migratory neurons to ligands encountered in the developing nervous system. Concurrent studies using cultured mammalian neurons and organotypic hippocampal slice preparations have shown that APP signaling transduces the neuroprotective effects of soluble sAPPα fragments via modulation of the PI3K/Akt pathway, providing a mechanism for integrating the stress and survival responses regulated by APP. Notably, this effect was also inhibited by pertussis toxin, indicating an essential role for Gαo/i proteins. Unexpectedly, C-terminal fragments (CTFs) derived from APP have also been found to interact with Gαs, whereby CTF-Gαs signaling can promote neurite outgrowth via adenylyl cyclase/PKA-dependent pathways. These reports offer the intriguing perspective that G protein switching might modulate APP-dependent responses in a context-dependent manner. In this review, we provide an up-to-date perspective on the model that APP plays a variety of roles as an atypical G protein-coupled receptor in both the developing and adult nervous system, and we discuss the hypothesis that disruption of these normal functions might contribute to the progressive neuropathologies that typify AD.

PAT1 inversely regulates the surface Amyloid Precursor Protein level in mouse primary neurons

Background

The amyloid precursor protein (APP) is a key molecule in Alzheimer disease. Its localization at the cell surface can trigger downstream signaling and APP cleavages. APP trafficking to the cell surface in neurons is not clearly understood and may be related to the interactions with its partners. In this respect, by having homologies with kinesin light chain domains and because of its capacity to bind APP, PAT1 represents a good candidate.

Results

We observed that PAT1 binds poorly APP at the cell surface of primary cortical neurons contrary to cytoplasmic APP. Using down and up-regulation of PAT1, we observed respectively an increase and decrease of APP at the cell surface. The increase of APP at the cell surface induced by low levels of PAT1 did not trigger cell death signaling.

Conclusions

These data suggest that PAT1 slows down APP trafficking to the cell surface in primary cortical neurons. Our results contribute to the elucidation of mechanisms involved in APP trafficking in Alzheimer disease.

Amyloid β precursor protein as a molecular target for amyloid β--induced neuronal degeneration in Alzheimer's disease

A role of amyloid β (Aβ) peptide aggregation and deposition in Alzheimer's disease (AD) pathogenesis is widely accepted. Significantly, abnormalities induced by aggregated Aβ have been linked to synaptic and neuritic degeneration, consistent with the "dying-back" pattern of degeneration that characterizes neurons affected in AD. However, molecular mechanisms underlying the toxic effect of aggregated Aβ remain elusive. In the last 2 decades, a variety of aggregated Aβ species have been identified and their toxic properties demonstrated in diverse experimental systems. Concurrently, specific Aβ assemblies have been shown to interact and misregulate a growing number of molecular effectors with diverse physiological functions. Such pleiotropic effects of aggregated Aβ posit a mayor challenge for the identification of the most cardinal Aβ effectors relevant to AD pathology. In this review, we discuss recent experimental evidence implicating amyloid β precursor protein (APP) as a molecular target for toxic Aβ assemblies. Based on a significant body of pathologic observations and experimental evidence, we propose a novel pathologic feed-forward mechanism linking Aβ aggregation to abnormalities in APP processing and function, which in turn would trigger the progressive loss of neuronal connectivity observed early in AD.

Cross-linking of cell surface amyloid precursor protein leads to increased β-amyloid peptide production in hippocampal neurons: implications for Alzheimer's disease

The accumulation of the β-amyloid peptide (Aβ) in Alzheimer's disease (AD) is thought to play a causative role in triggering synaptic dysfunction in neurons, leading to their eventual demise through apoptosis. Aβ is produced and secreted upon sequential cleavage of the amyloid precursor protein (APP) by β-secretases and γ-secretases. However, while Aβ levels have been shown to be increased in the brains of AD patients, little is known about how the cleavage of APP and the subsequent generation of Aβ is influenced, or whether the cleavage process changes over time. It has been proposed that Aβ can bind APP and promote amyloidogenic processing of APP, further enhancing Aβ production. Proof of this idea has remained elusive because a clear mechanism has not been identified, and the promiscuous nature of Aβ binding complicates the task of demonstrating the idea. To work around these problems, we used an antibody-mediated approach to bind and cross-link cell-surface APP in cultured rat primary hippocampal neurons. Here we show that cross-linking of APP is sufficient to raise the levels of Aβ in viable neurons with a concomitant increase in the levels of the β-secretase BACE1. This appears to occur as a result of a sorting defect that stems from the caspase-3-mediated inactivation of a key sorting adaptor protein, namely GGA3, which prevents the lysosomal degradation of BACE1. Together, our data suggest the occurrence of a positive pathogenic feedback loop involving Aβ and APP in affected neurons possibly allowing Aβ to spread to nearby healthy neurons.

Neuronal cell death in Alzheimer's disease and a neuroprotective factor, humanin

Brain atrophy caused by neuronal loss is a prominent pathological feature of Alzheimer's disease (AD). Amyloid beta (Abeta), the major component of senile plaques, is considered to play a central role in neuronal cell death. In addition to removal of the toxic Abeta, direct suppression of neuronal loss is an essential part of AD treatment; however, no such neuroprotective therapies have been developed. Excess amount of Abeta evokes multiple cytotoxic mechanisms, involving increase of the intracellular Ca(2+) level, oxidative stress, and receptor-mediated activation of cell-death cascades. Such diversity in cytotoxic mechanisms induced by Abeta clearly indicates a complex nature of the AD-related neuronal cell death. We have identified a 24-residue peptide, Humanin (HN), which suppresses in vitro neuronal cell death caused by all AD-related insults, including Abeta, so far tested. The anti-AD effect of HN has been further confirmed in vivo using mice with Abeta-induced amnesia. Altogether, such potent neuroprotective activity of HN against AD-relevant cytotoxicity both in vitro and in vivo suggests the potential clinical applications of HN in novel AD therapies aimed at controlling neuronal death.

Antibody binding to cell surface amyloid precursor protein induces neuronal injury by deregulating the phosphorylation of focal adhesion signaling related proteins

The biological function of full-length amyloid-beta protein precursor (APP), the precursor of Abeta, is not fully understood. Mounting studies reported that antibody binding to cell surface APP causes neuronal injury. However, the mechanism of cell surface APP mediating neuronal injury remains to be determined. Colocalization of APP with integrin on cell surface leads us to suppose that focal adhesion (FA) related mechanism is involved in surface APP-mediated neuronal injury. In the present study, results demonstrated that primary cultured neurons treated with antibody against APP-N-terminal not only caused neuronal injury and aberrant morphologic changes of neurite, but also induced reaction of FA proteins appearing an acute increase then decrease pattern. Moreover, the elevation of tyrosine phosphorylation of FA proteins including paxillin and focal adhesion kinase (FAK), and down-regulated expression of protein tyrosine phosphatase (PTP1B) induced by APP antibody were prevented by inhibitor of Src protein kinases 4-amino-5-(4-chlorophenyl)-7(t-butyl) pyrazol (3,4-D) pyramide (PP2) and G protein inhibitor pertussis toxin (PTX), implying that Src family kinase and G protein play roles in APP-induced FA signals. In addition, pretreatment with PTX and PP2 was able to suppress APP-antibody induced neuronal injury. Taken together, the results suggest a novel mechanism for APP mediating neuronal injury through deregulating FA signals.

PAT1 induces cell death signal and SET mislocalization into the cytoplasm by increasing APP/APLP2 at the cell surface

The cleavage of amyloid precursor protein (APP) by caspases unmasks a domain extending from membrane to caspase cleavage site. This domain induces apoptosis in vitro and in vivo when overexpressed in neurons through the help of an internalization vector. In this model, we previously showed that SET rapidly binds to the internalized domain and is involved in downstream deleterious effects. Under these conditions SET mislocalizes from the nucleus to the cytoplasm, as in Alzheimer's disease (AD). In this report using the same model, we show that PAT1 attaches to the internalized domain earlier than SET and that this binding causes an increase in the levels of APP and APLP2 at the cell surface. Down regulation experiments of PAT1 and of APP and APLP2 show that the increase of the levels of APP and APLP2 at the cell surface triggers the cell death signal and SET mislocalization into the cytoplasm. In the context of AD these data suggest that mislocalization of SET into the cytoplasm may occur downstream of first cell death signal events involving PAT1 protein.

Neuroprotective secreted amyloid precursor protein acts by disrupting amyloid precursor protein dimers

The amyloid precursor protein (APP) is implied both in cell growth and differentiation and in neurodegenerative processes in Alzheimer disease. Regulated proteolysis of APP generates biologically active fragments such as the neuroprotective secreted ectodomain sAPPalpha and the neurotoxic beta-amyloid peptide. Furthermore, it has been suggested that the intact transmembrane APP plays a signaling role, which might be important for both normal synaptic plasticity and neuronal dysfunction in dementia. To understand APP signaling, we tracked single molecules of APP using quantum dots and quantitated APP homodimerization using fluorescence lifetime imaging microscopy for the detection of Förster resonance energy transfer in living neuroblastoma cells. Using selective labeling with synthetic fluorophores, we show that the dimerization of APP is considerably higher at the plasma membrane than in intracellular membranes. Heparan sulfate significantly contributes to the almost complete dimerization of APP at the plasma membrane. Importantly, this technique for the first time structurally defines the initiation of APP signaling by binding of a relevant physiological extracellular ligand; our results indicate APP as receptor for neuroprotective sAPPalpha, as sAPPalpha binding disrupts APP dimers, and this disruption of APP dimers by sAPPalpha is necessary for the protection of neuroblastoma cells against starvation-induced cell death. Only cells expressing reversibly dimerized wild-type, but not covalently dimerized mutant APP are protected by sAPPalpha. These findings suggest a potentially beneficial effect of increasing sAPPalpha production or disrupting APP dimers for neuronal survival.

Amyloid-beta precursor protein mediates neuronal toxicity of amyloid beta through Go protein activation

Amyloid beta (Abeta) is a metabolic product of amyloid-beta precursor protein (APP). Deposition of Abeta in the brain and neuronal degeneration are characteristic hallmarks of Alzheimer's disease (AD). Abeta induces neuronal degeneration, but the mechanism of neurotoxicity remains elusive. Here we show that overexpression of APP renders hippocampal neurons vulnerable to Abeta toxicity. Deletion of the extracellular Abeta sequence of APP prevents binding of APP to Abeta, and abolishes toxicity. Abeta toxicity is also abrogated by deletion of the cytoplasmic domain of APP, or by deletions comprising the Go protein-binding sequence of APP. Treatment with Pertussis toxin (PTX) abrogates APP-dependent toxicity of Abeta. Overexpression of PTX-insensitive Galpha-o subunit, but not Galpha-i subunit, of G protein restores Abeta toxicity in the presence of PTX, and this requires the integrity of APP-binding site for Go protein. Altogether, these experiments indicate that interaction of APP with toxic Abeta-species promotes toxicity in hippocampal neurons by a mechanism that involves APP-mediated Go protein activation, revealing an Abeta-receptor-like function of APP directly implicated in neuronal degeneration in AD.

Immunotherapy against APP β-Secretase Cleavage Site Improves Cognitive Function and Reduces Neuroinflammation in Tg2576 Mice without a Significant Effect on Brain Aβ Levels

BACKGROUND/OBJECTIVES

Active and passive immunization methodologies against amyloid-beta (Abeta) are employed to clear and reduce cerebral Abetatowards treatment of Alzheimer's disease (AD) patients. The therapeutic potential of these antibodies in AD patients is limited because of adverse inflammatory reactions and cerebral hemorrhage, which are associated with the treatment. We propose a novel approach to inhibit Abeta production via antibodies against the beta-secretase cleavage site of the amyloid precursor protein (APP). Such an approach limits APP processing by beta-secretase, mainly through the endocytic pathway, and overcomes some of the limitations of BACE inhibition. Anti-APP beta-site antibodies, tested in a cellular model expressing wild-type APP, were found to bind full-length APP, internalize into the cells and interfere with BACE activity, inhibiting both intra- and extracellular Abeta peptide formation.

METHODS

We investigated the effect of anti-beta-site antibodies in an AD animal model regarding antibody efficacy, as well as possible adverse effects in the brain and periphery that may result from antibody treatment.

RESULTS/CONCLUSIONS

Here, we show that long-term systemic administration of anti-APP beta-site antibodies to Tg2576 transgenic mice improved mouse cognitive functions associated with a reduction in both brain inflammation and the incidence of microhemorrhage. Furthermore, antibody treatment did not induce any peripheral autoimmunity responses. In spite of the beneficial effects observed in antibody-treated mice, brain Abeta levels were not altered as a result of antibody treatment.

Amyloid precursor protein cytoplasmic domain antagonizes reelin neurite outgrowth inhibition of hippocampal neurons

The function of the amyloid precursor protein (APP), a key molecule in Alzheimer's disease (AD) remains unknown. Among the proteins that interact with the APP cytoplasmic domain in vitro and in heterologous systems is Disabled-1, a signaling molecule of the reelin pathway. The physiological consequence of this interaction is unknown. Here we used an in vitro model of hippocampal neurons grown on a reelin substrate that inhibits neurite outgrowth. Our results show that an excess of APP cytoplasmic domain internalized by a cell permeable peptide, is able to antagonize the neurite outgrowth inhibition of reelin. The APP cytoplasmic domain binds Disabled-1 and retains it in the cytoplasm, preventing it from reaching the plasma membrane and sequesters tyrosine phosphorylated Disabled-1, both of which disrupt reelin signaling. In the context of AD, increased formation of APP cytoplasmic domain in the cytosol released after cleavage of the A beta peptide, could then inhibit reelin signaling pathway in the hippocampus and thus influence synaptic plasticity.

Dysfunction of amyloid precursor protein signaling in neurons leads to DNA synthesis and apoptosis

The classic neuropathological diagnostic markers for AD are amyloid plaques and neurofibrillary tangles, but their role in the etiology and progression of the disease remains incompletely defined. Research over the last decade has revealed that cell cycle abnormalities also represent a major neuropathological feature of AD. These abnormalities appear very early in the disease process, prior to the appearance of plaques and tangles; and it has been suggested that neuronal cell cycle regulatory failure may be a significant component of the pathogenesis of AD. The amyloid precursor protein (APP) is most commonly known as the source of the beta-amyloid (Abeta) peptides that accumulate in the brains of patients with AD. However, a large body of work supports the idea that APP is also a signaling receptor. Most recently, it has been shown that familial AD (FAD) mutations in APP or simple overexpression of wild type APP cause dysfunction of APP signaling, resulting in initiation of DNA synthesis in neurons and consequent apoptosis. In this article, we review the evidence that APP has the potential to activate aberrant neuronal cell cycle re-entry in AD, and we describe a signal transduction pathway that may mediate this abnormal activation of the cell cycle.

Development of a femtomolar-acting humanin derivative named colivelin by attaching activity-dependent neurotrophic factor to its N terminus: characterization of colivelin-mediated neuroprotection against Alzheimer's disease-relevant insults in vitro and in vivo

Alzheimer's disease (AD) is the most common cause of dementia. Humanin (HN) is a short bioactive peptide abolishing neuronal cell death induced by various familial AD (FAD)-causative genes and amyloid-beta (Abeta) in vitro. It has been shown that HN suppresses memory impairment of mice induced by intracerebroventricular administration of Abeta. To potentiate the neuroprotective effect of HN, we synthesized a hybrid peptide named Colivelin composed of activity-dependent neurotrophic factor (ADNF) C-terminally fused to AGA-(C8R)HNG17, a potent HN derivative. Colivelin completely suppresses death induced by overexpressed FAD-causative genes and Abeta1-43 at a concentration of 100 fM, whereas AGA-(C8R)HNG17 does so at a concentration of 10 pM. Colivelin-induced neuroprotection has been confirmed to occur via two neuroprotective pathways: one mediated by Ca2+/calmodulin-dependent protein kinase IV, triggered by ADNF, and one mediated by signal transducer and activator of transcription 3, triggered by HN. In vivo animal studies have further indicated that intracerebroventricular administration of Colivelin not only completely suppresses impairment in spatial working memory induced by repetitive intracerebroventricular injection of Abeta25-35 or Abeta1-42, but also it antagonizes neuronal loss in the CA1 region of hippocampus induced by hippocampal injection of Abeta1-42. In addition, intraperitoneally administered Colivelin suppresses memory impairment caused by a muscarinic acetylcholine receptor antagonist, 3-quinuclidinyl benzilate, indicating that a substantial portion of intraperitoneally administered Colivelin passes through the blood-brain barrier and suppresses functional memory deficit. Thus, Colivelin might serve as a novel drug candidate for treatment of AD.

Physiological regulation of the beta-amyloid precursor protein signaling domain by c-Jun N-terminal kinase JNK3 during neuronal differentiation

Beta-amyloid precursor protein (APP) is a conserved and ubiquitous transmembrane glycoprotein strongly implicated in the pathogenesis of Alzheimer's disease but whose normal biological function is unknown. Analogy to the Notch protein suggests that APP is a cell-surface receptor that signals via sequential proteolytic cleavages that release its intracellular domain (AICD) to the nucleus. Because these cleavages are major targets for therapeutic inhibition, it is critical to elucidate their physiological function. AICD is stabilized by Fe65, interacts with the transcriptional factor Tip60, and translocates to the nucleus. Here, we show that endogenous AICD in primary neurons is detectable only during a short period of time during differentiation in culture. During this transient rise, a portion of AICD localizes to the nucleus. Subsequently, phosphorylation of the APP cytoplasmic domain at threonine 668 appears to disrupt the stabilizing interaction with Fe65 and thus downregulate AICD-mediated signaling. Furthermore, we find that the neuron-specific c-Jun N-terminal kinase JNK3, but not JNK1 or JNK2, mediates a substantial portion of this phosphorylation. We conclude that endogenous AICD undergoes tight temporal regulation during the differentiation of neurons and is negatively regulated by JNK3 via phosphorylation of APP at Thr668.

SET protein (TAF1beta, I2PP2A) is involved in neuronal apoptosis induced by an amyloid precursor protein cytoplasmic subdomain

When overexpressed, a short cytoplasmic domain of the amyloid precursor protein (APP), normally unmasked in the brain of Alzheimer's disease patients, activates caspase-3 and induces neuronal death. Death induction by this "Jcasp" domain is lost when tyrosine 653 is changed into an aspartate, suggesting specific interactions with unknown partners. To identify these putative partners and start to elucidate the mechanisms involved in Jcasp-induced cell death, we internalized a biotinylated version of the peptide into primary neurons and analyzed intracellular interacting proteins by pull-down and mass spectrometry. We find that SET protein, also called template-activating factor (TAF1beta) or phosphatase 2A inhibitor 2 (I2(PP2A)), specifically binds Jcasp early after internalization and that SET and Jcasp interact directly in vitro. Down-regulation of SET reduces Jcasp-induced cell death, confirming a role of this protein in Jcasp-induced apoptosis. Conversely, SET gain of function increases cell death, which suggests that SET level is crucial for neuronal survival/death. Taken together, these results suggest that SET is part of a neuronal apoptotic pathway related to Alzheimer's disease and provides a new entry in the analysis of this pathology.

Antibody-bound β-amyloid precursor protein stimulates the production of tumor necrosis factor-α and monocyte chemoattractant protein-1 by cortical neurons

Alzheimer's disease (AD) is an age-related neurodegenerative disorder characterized by the accumulation of extracellular depositions of fibrillar beta-amyloid (A beta), which is derived from the alternative processing of beta-amyloid precursor protein (APP). Although APP is thought to function as a cell surface receptor, its mode of action still remains elusive. In this study, we found that the culture medium derived from cortical neurons treated with an anti-APP antibody triggers the death of naive neurons. Biochemical and immunocytochemical analyses revealed the presence, both in the conditioned medium and in neurons, of increased levels of tumor necrosis factor-alpha and monocyte chemoattractant protein-1. Furthermore, the expression of these proinflammatory mediators occurred through a c-Jun N-terminal protein kinase/c-Jun-dependent mechanism. Taken together, our findings provide evidence for a novel mechanism whereby neuronal APP in its full-length configuration induces neuronal death. Such a mechanism might be relevant to neuroinflammatory processes as those observed in AD.

Humanin: after the discovery

Humanin (HN) is a novel neuroprotective factor that consists of 24 amino acid residues. HN suppresses neuronal cell death caused by Alzheimer's disease (AD)-specific insults, including both amyloid-beta (betaAbeta) peptides and familial AD-causative genes. Cerebrovascular smooth muscle cells are also protected from Abeta toxicity by HN, suggesting that HN affects both neuronal and non-neuronal cells when they are exposed to AD-related cytotoxicity. HN peptide exerts a neuroprotective effect through the cell surface via putative receptor(s). HN activates a cellular signaling cascade that intervenes (at least) in activation of c-Jun N-terminal kinase. The highly selective effect of HN on AD-relevant cell death indicates that HN is promising for AD therapy. Additionally, a recent study showed that intracellularly overexpressed HN suppressed mitochondria-mediated apoptosis by inhibiting Bax activity.

Cytotoxic mechanisms by M239V presenilin 2, a little-analyzed Alzheimer's disease-causative mutant

Although neurotoxic functions are well characterized in familial Alzheimer's disease (FAD)-linked N141I mutant of presenilin (PS)2, little has been known about M239V-PS2, another established FAD-causative mutant. We found that expression of M239V-PS2 caused neuronal cytotoxicity. M239V-PS2 exerted three forms of cytotoxicity: one was sensitive to both an antioxidant glutathione-ethyl-ester (GEE) and a caspase inhibitor Ac-DEVD-CHO (DEVD); the second was sensitive to GEE but resistant to DEVD; and the third was resistant to both. The GEE/DEVD-sensitive cytotoxicity by M239V-PS2 was likely through NADPH oxidase and the GEE-sensitive/DEVD-resistant cytotoxicity through xanthine oxidase (XO). Both mechanisms by M239V-PS2 were suppressed by pertussis toxin (PTX) and were mediated by Galpha(o), but not by Galpha(i). Although Abeta1-43 itself induced no cytotoxicity, Abeta1-43 potentiated all three components of M239V-PS2 cytotoxicity. As these cytotoxic mechanisms by M239V-PS2 are fully shared with N141I-PS2, they are most likely implicated in the pathomechanism of FAD by PS2 mutations. Notably, cytotoxicity by M239V-PS2 could be inhibited by the combination of two clinically usable inhibitors of superoxide-generating enzymes, apocynin and oxypurinol.

Deposition of amyloid fibrils promotes cell-surface accumulation of amyloid β precursor protein

Amyloid beta protein (Abeta) deposition and neuronal degeneration are characteristic pathological features of Alzheimer's disease (AD). In vitro, Abeta fibrils (fAbeta) induce neuronal degeneration reminiscent to AD, but the mechanism of neurotoxicity is unknown. Here we show that amyloid fibrils increase the level of cell-surface full-length amyloid beta precursor protein (h-AbetaPP) and secreted AbetaPP (s-AbetaPP). Pulse-chase analysis indicated that fAbeta selectively inhibited the turnover of cell-surface AbetaPP, without altering its intracellular levels. FAbeta-induced AbetaPP accumulation was not abrogated by cycloheximide, suggesting that increased protein synthesis is not critically required. Abeta fibrils sequester s-AbetaPP from the culture medium and promote its accumulation at the cell surface, indicating that binding of Abeta fibrils mediates AbetaPP accumulation. A time course analysis of Abeta treatment showed that AbetaPP level is elevated before significant cell death can be detected, while other toxic insults do not augment AbetaPP level, suggesting that AbetaPP may be specifically involved in early stages of Abeta-induced neurodegeneration. Finally, Abeta fibrils promote clustering of h-AbetaPP in abnormal focal adhesion-like (FA-like) structures that mediate neuronal dystrophy, increasing its association with the cytoskeleton. These results indicate that the interaction of Abeta fibrils with AbetaPP is an early event in the mechanism of Abeta-induced neurodegeneration that may play a significant role in AD pathogenesis.

Humanin antagonists: mutants that interfere with dimerization inhibit neuroprotection by Humanin

The 24-residue peptide Humanin (HN) protects neuronal cells from insults of various Alzheimer's disease (AD) genes and Abeta by forming a homodimer. We have previously shown that P3A, S7A, C8A, L9A, L12A, T13A, S14A and P19A mutations nullify the neuroprotective function of HN [Yamagishi, Y., Hashimoto, Y., Niikura, T. & Nishimoto, I. (2003) Peptides, 24, 585-595]. Here we examined whether any of these 'null' mutants could function as dominant-negative mutants. Homodimerization-defective mutants, P3A-, L12A-, S14A- and P19A-HN, specifically blocked neuroprotection by HN, but not by activity-dependent neurotrophic factor. Furthermore, insertion of S7A, the mutation that blocks the homodimerization of HN, but not insertion of G5A abolished the antagonizing function of L12A-HN. While L12A-HN and G5A/L12A-HN actually inhibited HN homodimerization, S7A/L12A-HN had no effect. These data indicate that P3A-, L12A-, S14A- and P19A-HN function as HN antagonists by forming an inactive dimer with HN. This study provides a novel insight into the understanding of the in vivo function of HN, as well as into the development of clinically applicable HN neutralizers.

Amyloid beta protein toxicity mediated by the formation of amyloid-beta protein precursor complexes

The amyloid-beta protein precursor, a type 1 transmembrane protein, gives rise to the amyloid beta-protein, a neurotoxic peptide postulated to be involved in the pathogenesis of Alzheimer's disease. Here, we show that soluble amyloid beta protein accelerates amyloid precursor protein complex formation, a process that contributes to neuronal cell death. The mechanism of cell death involves the recruitment of caspase-8 to the complex, followed by intracytoplasmic caspase cleavage of amyloid precursor protein. In vivo, the levels of soluble amyloid beta protein correlated with caspase-cleaved fragments of the amyloid precursor protein in brains of Alzheimer's disease subjects. These findings suggest that soluble amyloid beta protein-induced multimerization of the amyloid precursor protein may be another mechanism by which amyloid beta protein contributes to synapse loss and neuronal cell death seen in Alzheimer's disease.

Characterization of V642I-A?PP-induced cytotoxicity in primary neurons

Amyloid precursor protein (AbetaPP), a precursor of amyloid beta (Abeta) peptide, is one of the molecules involved in the pathogenesis of Alzheimer's disease (AD). Specific mutations in AbetaPP have been found in patients inheriting familial AD (FAD). These mutant AbetaPP proteins cause cell death in neuronal cell lines in vitro, but the molecular mechanism of cytotoxicity has not yet been clarified completely. We analyzed the cytotoxic mechanisms of the London-type AbetaPP mutant, V642I-AbetaPP, in primary cortical neurons utilizing an adenovirus-mediated gene transfer system. Expression of V642I-AbetaPP protein induced degeneration of the primary neurons. This cytotoxicity was blocked by pertussis toxin, a specific inhibitor for heterotrimeric G proteins, Go/i, and was suppressed by an inhibitor of caspase-3/7 and an antioxidant, glutathione ethyl ester. A specific inhibitor for NADPH oxidase, apocynin, but not a xanthine oxidase inhibitor or a nitric oxide inhibitor, blocked V642I-AbetaPP-induced cytotoxicity. Among mitogen-activated protein kinase (MAPK) family proteins, c-Jun N-terminal kinase (JNK) and p38MAPK, but not extracellular regulated kinase (ERK), were involved in this cytotoxic pathway. The V642I-AbetaPP-induced cytotoxicity was not suppressed by two secretase inhibitors, suggesting that Abeta does not play a major role in this cytotoxicity. Two neuroprotective factors, insulin-like growth factor I (IGF-I) and Humanin, protected these primary neurons from V642I-AbetaPP-induced cytotoxicity. Furthermore, interleukin-6 and -11 also attenuated this cytotoxicity. This study demonstrated that the signaling pathway activated by mutated AbetaPP in the primary neurons is the same as that by the other artificial insults such as antibody binding to AbetaPP and the artificial dimerization of cytoplasmic domain of AbetaPP. The potential of neurotrophic factors and cytokines in AD therapy is also indicated.

Amino- and carboxyl-terminal mutants of presenilin 1 cause neuronal cell death through distinct toxic mechanisms: Study of 27 different presenilin 1 mutants

Presenilin (PS)1 and its mutants, which consist of the N-terminal and C-terminal fragments, cause certain familial forms of Alzheimer's disease (FAD). Our earlier studies found that FAD-linked M146L-PS1 causes neuronal cell death through nitrogen oxide synthase (NOS) and that FAD-linked N141I-PS2, another member of the PS family, causes neuronal cell death through NADPH oxidase. In this study, we examined 27 different FAD-linked mutants of PS1, and found that PS1 mutants with mutations in the N-terminal fragment caused NOS inhibitor (NOSI)-sensitive neuronal cell death; in contrast, the PS1 mutants with mutations in the C-terminal fragment caused NOSI-resistant neuronal cell death. The former toxicity was resistant to the specific NADPH oxidase inhibitor apocynin and was inhibited by Humanin (HN), a newly identified neuroprotective factor against Alzheimer's disease (AD)-relevant insults, but not by insulin-like growth factor-I (IGF-I). In contrast, the latter toxicity was sensitive to apocynin and inhibited by both IGF-I and HN. This study indicates for the first time that N- and C-terminal fragment PS1 mutants can generate distinct neurotoxic signals, which will provide an important clue to the understanding of the entire array of neurotoxic signals generated by FAD-causative mutations of PS1.

Analysis of neurons created from wild-type and Alzheimer's mutation knock-in embryonic stem cells by a highly efficient differentiation protocol

It is impossible to obtain and amplify live neurons from Alzheimer's disease (AD) patients. To establish the neurons harboring AD abnormality, we constructed mouse embryonic stem (ES) cells, in which the AD-causative V642I mutation was introduced to the endogenous amyloid precursor protein (APP) gene, in combination with a protocol to efficiently differentiate ES cells into postmitotic neurons without using a cell sorter. By this protocol, ES cells differentiated into >90% of the central type of adult postmitotic neurons. Neurons derived from V642I-APP knock-in ES cells were indistinguishable from wild-type ES-derived neurons, as determined by the expression of various markers for neuronal differentiation. Notably, V642I-APP knock-in ES cell-derived neurons exhibited significantly increased secretion of Abeta42 without AD-related hyperphosphorylation of tau, indicating that the direct output of the AD-causative mutation is increased Abeta42 secretion. In this study, we analyze created neurons with wild-type and AD genotypes and propose a new strategy for generating neurons for any dominantly inherited neurodegenerative diseases. The strategy can be applied to create human neurons with AD or any other neurodegenerative disease by using human ES cells.

Characterization of the toxic mechanism triggered by Alzheimer's amyloid-beta peptides via p75 neurotrophin receptor in neuronal hybrid cells

Neuronal pathology of the brain with Alzheimer's disease (AD) is characterized by numerous depositions of amyloid-beta peptides (Abeta). Abeta binding to the 75-kDa neurotrophin receptor (p75NTR) causes neuronal cell death. Here we report that Abeta causes cell death in neuronal hybrid cells transfected with p75NTR, but not in nontransfected cells, and that p75NTR(L401K) cannot mediate Abeta neurotoxicity. We analyzed the cytotoxic pathway by transfecting pertussis toxin (PTX)-resistant G protein alpha subunits in the presence of PTX and identified that Galpha(o), but not Galpha(i), proteins are involved in p75NTR-mediated Abeta neurotoxicity. Further investigation suggested that Abeta neurotoxicity via p75NTR involved JNK, NADPH oxidase, and caspases-9/3 and was inhibited by activity-dependent neurotrophic factor, insulin-like growth factor-I, basic fibroblast growth factor, and Humanin, as observed in primary neuron cultures. Understanding the Abeta neurotoxic mechanism would contribute significantly to the development of anti-AD therapies.

The cytoplasmic domain of Alzheimer's amyloid-beta protein precursor causes sustained apoptosis signal-regulating kinase 1/c-Jun NH2-terminal kinase-mediated neurotoxic signal via dimerization

The biological function of full-length amyloid-beta protein precursor (AbetaPP), the precursor of Abeta, is not fully understood. Multiple laboratories have reported that antibody binding to cell surface AbetaPP causes neuronal cell death. Here we examined whether induced dimerization of the cytoplasmic domain of AbetaPP (AbetaPPCD) triggers neuronal cell death. In neurohybrid cells expressing fusion constructs of the epidermal growth factor (EGF) receptor with AbetaPPCD (EGFR/AbetaPP hybrids), EGF drastically enhanced neuronal cell death in a manner sensitive to acetyl-l-aspartyl-l-glutamyl-l-valyl-l-aspartyl-aldehyde (Ac-DEVD-CHO; DEVD), GSH-ethyl ester (GEE), and pertussis toxin (PTX). Dominant-negative apoptosis signal-regulating kinase 1 (ASK1) blocked this neuronal cell death, but not alpha-synuclein-induced cell death. Constitutively active ASK1 (caASK1) caused DEVD/GEE-sensitive cell death in a manner resistant to PTX and sensitive to Humanin, which also suppressed neuronal cell death by EGFR/AbetaPP hybrid. ASK1 formed a complex with AbetaPPCD via JIP-1b, the c-Jun N-terminal kinase (JNK)-interacting protein. EGFR/AbetaPP hybrid-induced and caASK1-induced neuronal cell deaths were specifically blocked by SP600125 (anthra[1,9-cd]pyrazol-6(2H)-one), a specific JNK inhibitor. Combined with our earlier study, these data indicate that dimerization of AbetaPPCD triggers ASK1/JNK-mediated neuronal cell death. We also noticed a potential role of ASK1/JNK in sustaining the activity of this mechanism after initial activation by AbetaPP, which allows for the achievement of cell death by short-term anti-AbetaPP antibody treatment. Understanding the function of AbetaPPCD and its downstream pathway should lead to effective anti-Alzheimer's disease therapeutics.

Upregulation and antiapoptotic role of endogenous Alzheimer amyloid precursor protein in dorsal root ganglion neurons

The amyloid precursor protein (APP) is a transmembrane protein whose abnormal processing is associated with the pathogenesis of Alzheimer's disease. In this study, we examined the expression and role of cell-associated APP in primary dorsal root ganglion (DRG) neurons. When dissociated DRG cells prepared from mouse embryos were treated with nerve growth factor (NGF), neuronal APP levels were transiently elevated. DRG neurons treated with an antibody against cell surface APP failed to mature and underwent apoptosis. When NGF was withdrawn from the cultures after a 36-h NGF treatment, virtually all neurons underwent apoptosis by 48 h. During the course of apoptosis, some neurons with intact morphology contained increased levels of APP immunoreactivity, whereas the APP levels were greatly reduced in apoptotic neurons. Furthermore, affected neurons contained immunoreactivities for activated caspase-3, a caspase-cleaved APP fragment (APPDeltaC31), and Abeta. Downregulation of endogenous APP expression by treatment with an APP antisense oligodeoxynucleotide significantly increased the number of apoptotic neurons in NGF-deprived DRG cultures. Furthermore, overexpression of APP by adenovirus vector-mediated gene transfer reduced the number of apoptotic neurons deprived of NGF. These results suggest that endogenous APP is upregulated to exert an antiapoptotic effect on neurotrophin-deprived DRG neurons and subsequently undergoes caspase-dependent proteolysis.

Two serine residues distinctly regulate the rescue function of Humanin, an inhibiting factor of Alzheimer's disease-related neurotoxicity: functional potentiation by isomerization and dimerization

The 24-residue peptide Humanin (HN), containing two Ser residues at positions 7 and 14, protects neuronal cells from insults of various Alzheimer's disease (AD) genes and A beta. It was not known why the rescue function of (S14G)HN is more potent than HN by two to three orders of magnitude. Investigating the possibility that the post-translational modification of Ser14 might play a role, we found that HN with D-Ser at position 14 exerts neuroprotection more potently than HN by two to three orders of magnitude, whereas D-Ser7 substitution does not affect the rescue function of HN. On the other hand, S7A substitution nullified the HN function. Multiple series of experiments indicated that Ser7 is necessary for self-dimerization of HN, which is essential for neuroprotection by this factor. These findings indicate that the rescue function of HN is quantitatively modulated by d-isomerization of Ser14 and Ser7-relevant dimerization, allowing for the construction of a very potent HN derivative that was fully neuroprotective at 10 pM against 25 microM A beta1-43. This study provides important clues to the understanding of the neuroprotective mechanism of HN, as well as to the development of novel AD therapeutics.

Identification of essential amino acids in Humanin, a neuroprotective factor against Alzheimer's disease-relevant insults

Humanin (HN) is a secretory peptide that inhibits neurotoxicity by various Alzheimer's disease-relevant insults. We have so far identified that the substitution of Leu9 for Arg nullifies the extracellular secretion of HN. Here we comprehensively investigate the amino acid requirement of HN essential for its secretion and for its neuroprotective function. Intracellulary expressed HN-EGFP (EGFP N-terminally fused with HN) was extracellularly secreted, whereas neither EGFP nor (L9R)HN-EGFP was secreted at all. While Ala substitution of neither residue affected HN secretion, Arg substitution revealed that the two structures-Leu9-Leu11 and Pro19-Va120-were essential for the secretion of full-length HN. In the Leu9-Leu11 domain, the Leu10 residue turned out to play a central role in this function, because the Asp substitution of Leu10, but not Leu9 or Leu11, nullified the secretion of HN. Utilizing Ala-scanned HN constructs, we also investigated a comprehensive structure-function relationship for the neuroprotective function of full-length HN, which revealed (i) that Pro3, Ser7, Cys8, Leu9, Leu12, Thr13, Ser14, and Pro19 were essential for this function and (ii) that Ser7 and Leu9 were essential for self-dimerization of HN. These findings indicate that HN has activity similar to a signal peptide, for which the Leu9-Leu11 region, particularly Leu10, functions as a core domain, and suggest that self-dimerization of HN is a process essential for its neuroprotective function.

Beta-amyloid and cholinergic neurons

It is generally accepted that the crucial events in the pathogeny of Alzheimer's disease (AD) are the increased accumulation of amyloidogenic peptides derived from amyloid precursor protein and the harmful actions of these peptides on neurons, which bring about neurodegeneration. The enhanced beta-amyloid accumulation is known to be caused by mutations of specific genes in patients who suffer from the familial (hereditary) form of AD but who represent just a minor group within the total population of AD patients. The reasons for beta-amyloid accumulation are not known in the much larger group of patients with the sporadic form of the disease. A biochemical feature common to either form of the disease is the preferential atrophy and degeneration of cholinergic neurons, which is probably responsible for much of the cognitive decline characteristic of the disease. We present an overview of recent investigations on the interactions between beta-amyloid and cholinergic neurons.

Involvement of c-Jun N-terminal kinase in amyloid precursor protein-mediated neuronal cell death

Amyloid precursor protein (APP), the precursor of Abeta, has been shown to function as a cell surface receptor that mediates neuronal cell death by anti-APP antibody. The c-Jun N-terminal kinase (JNK) can mediate various neurotoxic signals, including Abeta neurotoxicity. However, the relationship of APP-mediated neurotoxicity to JNK is not clear, partly because APP cytotoxicity is Abeta independent. Here we examined whether JNK is involved in APP-mediated neuronal cell death and found that: (i) neuronal cell death by antibody-bound APP was inhibited by dominant-negative JNK, JIP-1b and SP600125, the specific inhibitor of JNK, but not by SB203580 or PD98059; (ii) constitutively active (ca) JNK caused neuronal cell death and (iii) the pharmacological profile of caJNK-mediated cell death closely coincided with that of APP-mediated cell death. Pertussis toxin (PTX) suppressed APP-mediated cell death but not caJNK-induced cell death, which was suppressed by Humanin, a newly identified neuroprotective factor which inhibits APP-mediated cytotoxicity. In the presence of PTX, the PTX-resistant mutant of Galphao, but not that of Galphai, recovered the cytotoxic action of APP. These findings demonstrate that JNK is involved in APP-mediated neuronal cell death as a downstream signal transducer of Go.

Death and survival of neuronal cells exposed to Alzheimer's insults

Neuronal cell death is the central abnormality occurring in brains suffering from Alzheimer's disease (AD). The notion that AD is a disease caused by loss of neurons points toward suppression of neuronal death as the most important therapeutic target. Nevertheless, the mechanisms for neuronal death in AD are still relatively unclear. Three known mutant genes cause familial AD (FAD): amyloid precursor protein, presenilin 1, and presenilin 2. Detailed analysis of cytotoxic mechanisms of the FAD-linked mutant genes reveals that they cause neuronal cell death at physiologically low expression levels. Unexpectedly, cytotoxic mechanisms vary depending on the type of mutations and genes, suggesting that various mechanisms for neuronal cell death are involved in AD patients. In support of this, activity-dependent neurotrophic factor, basic fibroblast growth factor, and insulin-like growth factor-I can completely protect neurons from beta-amyloid (A beta) cytotoxicity but exhibit incomplete or little effect on cytotoxicity by FAD mutant genes. By contrast, Humanin, a newly identified 24-residue peptide, suppresses neuronal cell death by various FAD mutants and A beta, whereas this factor has no effect on cytotoxicity from AD-irrelevant insults. Studies investigating death and survival of neuronal cells exposed to AD insults will open a new horizon in developing therapy aimed at neuroprotection.

Advances in the cellular and molecular biology of the beta-amyloid protein in Alzheimer's disease

Alzheimer's disease (AD) is a progressive senile dementia characterized by deposition of a 4 kDa peptide of 39-42 residues known as amyloid beta-peptide (Abeta) in the form of senile plaques and the microtubule associated protein tau as paired helical filaments. Genetic studies have identified mutations in the Abeta precursor protein (APP) as the key triggers for the pathogenesis of AD. Other genes such as presenilins 1 and 2 (PS1/2) and apolipoprotein E (APOE) also play a critical role in increased Abeta deposition. Several biochemical and molecular studies using transfected cells and transgenic animals point to mechanisms by which Abeta is generated and aggregated to trigger the neurodegeneration that may cause AD. Three important enzymes collectively known as "secretases" participate in APP processing. An enzymatic activity, beta-secretase, cleaves APP on the amino side of Abeta producing a large secreted derivative, sAPPbeta, and an Abeta-bearing membrane-associated C-terminal derivative, CTFbeta, which is subsequently cleaved by the second activity, gamma-secretase, to release Abeta. Alternatively, a third activity, alpha-secretase, cleaves APP within Abeta to the secreted derivative sAPPalpha and membrane-associated CTFalpha. The predominant secreted APP derivative is sAPPalpha in most cell-types. Most of the secreted Abeta is 40 residues long (Abeta40) although a small percentage is 42 residues in length (Abeta42). However, the longer Abeta42 aggregates more readily and was therefore considered to be the pathologically important form. Advances in our understanding of APP processing, trafficking, and turnover will pave the way for better drug discovery for the eventual treatment of AD. In addition, APP gene regulation and its interaction with other proteins may provide useful drug targets for AD. The emerging knowledge related to the normal function of APP will help in determining whether or not the AD associated changes in APP metabolism affect its function. The present review summarizes our current understanding of APP metabolism and function and their relationship to other proteins involved in AD.

Amyloid precursor protein family-induced neuronal death is mediated by impairment of the neuroprotective calcium/calmodulin protein kinase IV-dependent signaling pathway

The aberrant metabolism of beta-amyloid precursor protein (APP) and the progressive deposition of its derived fragment beta-amyloid peptide are early and constant pathological hallmarks of Alzheimer's disease. Because APP is able to function as a cell surface receptor, we investigated here whether a disruption of the normal function of APP may contribute to the pathogenic mechanisms in Alzheimer's disease. To this aim, we generated a specific chicken polyclonal antibody directed against the extracellular domain of APP, which is common with the beta-amyloid precursor-like protein type 2. Exposure of cultured cortical neurons to this antibody (APP-Ab) induced cell death preceded by neurite degeneration, oxidative stress, and nuclear condensation. Interestingly, caspase-3-like protease was not activated in this neurotoxic action suggesting a different mode of cell death than classical apoptosis. Further analysis of the molecular mechanisms revealed a calpain- and calcineurin-dependent proteolysis of the neuroprotective calcium/calmodulin-dependent protein kinase IV and its nuclear target protein cAMP responsive element binding protein. These effects were abolished by the G protein inhibitor pertussis toxin, strongly suggesting that APP binding operates via a GTPase-dependent pathway to cause neuronal death.

Neurotoxic mechanisms by Alzheimer's disease-linked N141I mutant presenilin 2

Although it has been established that oxidative stress mediates cytotoxicity by familial Alzheimer's disease (FAD)-linked mutants of presenilin (PS)1 and that pertussis toxin inhibits cytotoxicity by FAD-linked N141I-PS2, it has not been determined whether oxidative stress is involved in cytotoxicity by N141I-PS2 or which pertussis toxin-sensitive proteins mediate the cytotoxicity. Here we report that low expression of N141I-PS2 caused neuronal cell death, whereas low expression of wild-type PS2 did not. Cytotoxicities by low and high expression of N141I-PS2 occurred through dissimilar mechanisms: the former cytotoxicity was blocked by a cell-permeable caspase inhibitor, and the latter was not. Since both mechanisms were sensitive to a cell-permeable antioxidant, we examined potential sources of reactive oxygen species in each mechanism, and found that the caspase inhibitor-sensitive neurotoxicity by N141I-PS2 was likely through NADPH oxidase and the caspase inhibitor-resistant neurotoxicity by N141I-PS2 through xanthine oxidase. Pertussis toxin greatly suppressed both toxic mechanisms by N141I-PS2, and only Galpha(o), a neuron-enriched pertussis toxin-sensitive G protein, was involved in both mechanisms. We therefore conclude that N141I-PS2 is capable of triggering multiple neurotoxic mechanisms, which can be inhibited by the combination of clinically usable inhibitors of NADPH oxidase and xanthine oxidase. This study thus provides a novel insight into the therapeutic intervention of PS2 mutant-associated FAD.

Cell death induced by a caspase-cleaved transmembrane fragment of the Alzheimer amyloid precursor protein

The Alzheimer amyloid precursor protein (APP) is a transmembrane protein whose abnormal processing is associated with the pathogenesis of Alzheimer's disease. Activated caspases cleave APP and generate its carboxyl-terminally truncated fragment (APPdeltaC31). We have previously reported that overexpression of wild-type APP induces caspase-3 activation and apoptosis in postmitotic neurons. We now report that APPdeltaC31 potentially plays pathophysiological roles in neuronal death. Adenovirus-mediated overexpression of wild-type APP695 induced activation of caspase-3 and accumulation of APPdeltaC31 in postmitotic neurons derived from human NT2 embryonal carcinoma cells, whereas an APP mutant lacking the Abeta(1-20) region induced neither caspase-3 activation nor APPdeltaC31 generation. Inhibition of caspase-3 suppressed the generation of APPdeltaC31 in APP-overexpressing neurons. Forced expression of APPdeltaC31 induced apoptotic changes of neurons and non-neuronal cells, but failed to activate caspase-3. The cytotoxicity of APPdeltaC31 was also dependent on the Abeta(1-20) region. These results suggest that accumulation of wild-type APP activates neuronal caspase-3 to generate APPdeltaC31 that mediates caspase-3-independent cell death.

Detailed characterization of neuroprotection by a rescue factor humanin against various Alzheimer's disease-relevant insults

A novel factor, termed Humanin (HN), antagonizes against neurotoxicity by various types of familial Alzheimer's disease (AD) genes [V642I and K595N/M596L (NL) mutants of amyloid precursor protein (APP), M146L-presenilin (PS) 1, and N141I-PS2] and by Abeta1-43 with clear action specificity ineffective on neurotoxicity by polyglutamine repeat Q79 or superoxide dismutase 1 mutants. Here we report that HN can also inhibit neurotoxicity by other AD-relevant insults: other familial AD genes (A617G-APP, L648P-APP, A246E-PS1, L286V-PS1, C410Y-PS1, and H163R-PS1), APP stimulation by anti-APP antibody, and other Abeta peptides (Abeta1-42 and Abeta25-35). The action specificity was further indicated by the finding that HN could not suppress neurotoxicity by glutamate or prion fragment. Against the AD-relevant insults, essential roles of Cys(8) and Ser(14) were commonly indicated, and the domain from Pro(3) to Pro(19) was responsible for the rescue action of HN, in which seven residues turned out to be essential. We also compared the neuroprotective action of S14G HN (HNG) with that of activity-dependent neurotrophic factor, IGF-I, or basic FGF for the antagonism against various AD-relevant insults (V642I-APP, NL-APP, M146L-PS1, N141I-PS2, and Abeta1-43). Although all of these factors could abolish neurotoxicity by Abeta1-43, only HNG could abolish cytotoxicities by all of them. HN and HN derivative peptides may provide a new insight into the study of AD pathophysiology and allow new avenues for the development of therapeutic interventions for various forms of AD.

c-Jun N-terminal kinase (JNK)-interacting protein-1b/islet-brain-1 scaffolds Alzheimer's amyloid precursor protein with JNK

Using a yeast two-hybrid method, we searched for amyloid precursor protein (APP)-interacting molecules by screening mouse and human brain libraries. In addition to known interacting proteins containing a phosphotyrosine-interaction-domain (PID)-Fe65, Fe65L, Fe65L2, X11, and mDab1, we identified, as a novel APP-interacting molecule, a PID-containing isoform of mouse JNK-interacting protein-1 (JIP-1b) and its human homolog IB1, the established scaffold proteins for JNK. The APP amino acids Tyr(682), Asn(684), and Tyr(687) in the G(681)YENPTY(687) region were all essential for APP/JIP-1b interaction, but neither Tyr(653) nor Thr(668) was necessary. APP-interacting ability was specific for this additional isoform containing PID and was shared by both human and mouse homologs. JIP-1b expressed by mammalian cells was efficiently precipitated by the cytoplasmic domain of APP in the extreme Gly(681)-Asn(695) domain-dependent manner. Reciprocally, both full-length wild-type and familial Alzheimer's disease mutant APPs were precipitated by PID-containing JIP constructs. Antibodies raised against the N and C termini of JIP-1b coprecipitated JIP-1b and wild-type or mutant APP in non-neuronal and neuronal cells. Moreover, human JNK1beta1 formed a complex with APP in a JIP-1b-dependent manner. Confocal microscopic examination demonstrated that APP and JIP-1b share similar subcellular localization in transfected cells. These data indicate that JIP-1b/IB1 scaffolds APP with JNK, providing a novel insight into the role of the JNK scaffold protein as an interface of APP with intracellular functional molecules.

A rescue factor abolishing neuronal cell death by a wide spectrum of familial Alzheimer's disease genes and Abeta

Through functional expression screening, we identified a gene, designated Humanin (HN) cDNA, which encodes a short polypeptide and abolishes death of neuronal cells caused by multiple different types of familial Alzheimer's disease genes and by Abeta amyloid, without effect on death by Q79 or superoxide dismutase-1 mutants. Transfected HN cDNA was transcribed to the corresponding polypeptide and then was secreted into the cultured medium. The rescue action clearly depended on the primary structure of HN. This polypeptide would serve as a molecular clue for the development of new therapeutics for Alzheimer's disease targeting neuroprotection.

Mechanisms of neuroprotection by a novel rescue factor humanin from Swedish mutant amyloid precursor protein

We report a novel gene, designated Humanin (HN) cDNA, that suppresses neuronal cell death by K595N/M596L-APP (NL-APP), a mutant causing familial Alzheimer's disease (FAD), termed Swedish mutant. Transfection of neuronal cells with HN cDNA or treatment with the coding HN polypeptide abrogated cytotoxicity by NL-APP. HN suppressed neurotoxicity by Abeta1-43 in the absence of N2 supplement, but could not inhibit Abeta secretion from NL-APP. HN could also protect neuronal cells from death by NL-APP lacking the 41st and 42nd residues of the Abeta region. Therefore, HN suppressed neuronal cell death by NL-APP not through inhibition of Abeta42 secretion, but with two targets for its inhibitory action: (i) the intracellular toxic mechanism directly triggered by NL-APP and (ii) neurotoxicity by Abeta. HN will contribute to the development of curative therapy of AD, especially as a novel reagent that could mechanistically supplement Abeta-production inhibitors.

Secreted Abeta does not mediate neurotoxicity by antibody-stimulated amyloid precursor protein

Antibodies against APP, a precursor of Abeta deposited in Alzheimer's disease brain, have been shown to cause neuronal death. Therefore, it is important to determine whether Abeta mediates antibody-induced neurotoxicity. When primary neurons were treated with anti-APP antibodies, Abeta40 and Abeta42 in the cultured media were undetectable by an assay capable of detecting 100 nM Abeta peptides. However, exogenously treated Abeta1-42 or Abeta1-43 required >3 microM to exert neurotoxicity, and 25 microM Abeta1-40 was not neurotoxic. Glutathione-ethyl-ester inhibited neuronal death by anti-APP antibody, but not death by Abeta1-42, whereas serum attenuated toxicity by Abeta1-42, but not by anti-APP antibody. Using immortalized neuronal cells, we specified the domain responsible for toxicity to be cytoplasmic His(657)-Lys(676), but not the Abeta1-42 region, of APP. This indicates that neuronal cell death by anti-APP antibody is not mediated by secreted Abeta.