FE65 interaction with the ApoE receptor ApoEr2

The adaptor protein FE65 interacts with the beta-amyloid precursor protein (APP) via its C-terminal phosphotyrosine binding (PTB) domain and affects APP processing and Abeta production. Our previous data demonstrate that the apoE receptor ApoEr2 co-precipitated with APP and suggest that there are extracellular and intracellular interactions between these two transmembrane proteins. We hypothesized that FE65 acts as an intracellular link between ApoEr2 and APP. Co-immunoprecipitation experiments in COS7 cells demonstrated an interaction between ApoEr2 and FE65 that depended on the N-terminal PTB domain of FE65. Full-length FE65 increased co-immunoprecipitation of ApoEr2 and APP. Full-length FE65 also increased surface expression of ApoEr2, as determined by surface protein biotinylation and live cell surface staining. Constructs containing both the C- and N-terminal PTB domains of FE65 increased secreted APP, secreted ApoEr2, APP C-terminal fragment, and ApoEr2 C-terminal fragment, but constructs containing only single PTB domains did not affect APP or ApoEr2 processing. In addition, full-length FE65 decreased Abeta to a significantly greater extent than individual FE65 domains. These data suggest that FE65 can bind APP and ApoEr2 at the same time and affect the processing of each.     

DAB1 and Reelin effects on amyloid precursor protein and ApoE receptor 2 trafficking and processing

Numerous cytoplasmic adaptor proteins, including JIP1, FE65, and X11alpha, affect amyloid precursor protein (APP) processing and Abeta production. Dab1 is another adaptor protein that interacts with APP as well as with members of the apoE receptor family. We examined the effect of Dab1 on APP and apoEr2 processing in transfected cells and primary neurons. Dab1 interacted with APP and apoEr2 and increased levels of their secreted extracellular domains and their cytoplasmic C-terminal fragments. These effects depended on the NPXY domains of APP and apoEr2 and on the phosphotyrosine binding domain of Dab1 but did not depend on phosphorylation of Dab1. Dab1 decreased the levels of APP beta-C-terminal fragment and secreted Abeta. Full-length Dab1 or its phosphotyrosine binding domain alone increased surface levels of APP, as determined by surface protein biotinylation and live cell staining. A ligand for apoEr2, the extracellular matrix protein Reelin, significantly increased the interaction of apoEr2 with Dab1. Surprisingly, we also found that Reelin treatment significantly increased the interaction of APP and Dab1. Moreover, Reelin treatment increased cleavage of APP and apoEr2 and decreased production of the beta-C-terminal fragment of APP and Abeta. Together, these data suggest that Dab1 alters trafficking and processing of APP and apoEr2, and this effect is influenced by extracellular ligands.     

Apolipoprotein E receptor 2 interactions with the N-methyl-D-aspartate receptor

In our previous studies we showed that apoE treatment of neurons activated ERK 1/2 signaling, and activation was blocked by treatment with inhibitors of the low density lipoprotein receptor family, the N-methyl-d-aspartate (NMDA) receptor antagonist MK 801, and calcium channel blockers. We hypothesized an interaction between the low density lipoprotein receptor family members and the NMDA receptor. In the present study, we confirmed through co-immunoprecipitation experiments an interaction between the apoE receptor, ApoEr2, and NMDAR1 through their extracellular domains. We also found that the PDZ1 domain of PSD95, a postsynaptic scaffolding protein, interacted with the C terminus of ApoEr2 via an alternatively spliced, intracellular exon. This interaction between ApoEr2 and PSD95 in neurons was modulated by NMDA receptor activation and an ApoEr2 ligand. We also found that the PDZ2 domain of PSD95 interacted with the NR2A and NR2B subunits of NMDA receptors. Full-length PSD95 increased cell surface levels of ApoEr2 and its cleavage, resulting in increases in secreted ApoEr2 and C-terminal fragments of ApoEr2. These studies suggest that ApoEr2 can form a multiprotein complex with NMDA receptor subunits and PSD95.     

Ligand-induced Homotypic and Heterotypic Clustering of Apolipoprotein E Receptor 2

ApoE Receptor 2 (ApoER2) and the very low density lipoprotein receptor (VLDLR) are type I transmembrane proteins belonging to the LDLR family of receptors. They are neuronal proteins found in synaptic compartments that play an important role in neuronal migration during development. ApoER2 and VLDLR bind to extracellular glycoproteins, such as Reelin and F-spondin, which leads to phosphorylation of adaptor proteins and subsequent activation of downstream signaling pathways. It is thought that ApoER2 and VLDLR undergo clustering upon binding to their ligands, but no direct evidence of clustering has been shown. Here we show strong clustering of ApoER2 induced by the dimeric ligands Fc-RAP, F-spondin, and Reelin but relatively weak clustering with the ligand apoE in the absence of lipoproteins. This clustering involves numerous proteins besides ApoER2, including amyloid precursor protein and the synaptic adaptor protein PSD-95. Interestingly, we did not observe strong clustering of ApoER2 with VLDLR. Clustering was modulated by both extracellular and intracellular domains of ApoER2. Together, our data demonstrate that several multivalent ligands for ApoER2 induce clustering in transfected cells and primary neurons and that these complexes included other synaptic molecules, such as APP and PSD-95.     

Functional interactions of APP with the apoE receptor family

The beta-amyloid precursor protein (APP) is central to the pathogenesis of Alzheimer's disease, but its normal functions in the brain are poorly understood. A number of APP-interacting proteins have been identified: intracellularly, APP interacts with adaptor proteins through its conserved NPXY domain; extracellularly, APP interacts with a component of the extracellular matrix, F-spondin. Interestingly, many of these APP-interacting proteins also interact with the family of receptors for apolipoprotein E (apoE), the Alzheimer's disease risk factor. apoE receptors also share with APP the fact that they are cleaved by the same secretase activities. apoE receptors are shed from the cell surface, a cleavage that is regulated by receptor-ligand interactions, and C-terminal fragments of apoE receptors are cleaved by gamma-secretase. Functionally, both APP and apoE receptors affect neuronal migration and synapse formation in the brain. This review summarizes these numerous interactions between APP and apoE receptors, which provide clues about the normal functions of APP.     

Apolipoprotein E modulates gamma-secretase cleavage of the amyloid precursor protein

Polymorphisms in the apolipoprotein E (APOE) gene affect the risk of Alzheimer disease and the amount of amyloid beta-protein (Abeta) deposited in the brain. The apoE protein reduces Abeta levels in conditioned media from cells in culture, possibly through Abeta clearance mechanisms. To explore this effect, we treated multiple neural and non-neural cell lines for 24 h with apoE at concentrations similar to those found in the cerebrospinal fluid (1-5 microg/mL). The apoE treatment reduced Abeta40 by 60-80% and Abeta42 to a lesser extent (20-30%) in the conditioned media. Surprisingly, apoE treatment resulted in an accumulation of amyloid precursor protein (APP)-C-terminal fragments in cell extracts and a marked reduction of APP intracellular domain-mediated signaling, consistent with diminished gamma-secretase processing of APP. All three isoforms of apoE, E2, E3 and E4, had similar effects on Abeta and APP-C-terminal fragments, and the effects were independent of the low-density lipoprotein receptor family. Apolipoprotein E had minimal effects on Notch cleavage and signaling in cell-based assays. These data suggest that apoE reduces gamma-secretase cleavage of APP, lowering secreted Abeta levels, with stronger effects on Abeta40. The apoE modulation of Abeta production and APP signaling is a potential mechanism affecting Alzheimer disease risk.     

Regulated Proteolysis of APP and ApoE Receptors

The beta-amyloid precursor protein (APP) shares intracellular and extracellular-binding partners with the family of receptors for apolipoprotein E (apoE). Binding of APP and apoE receptors to specific extracellular matrix proteins (F-spondin and Reelin) promotes their presence on the cell surface and influences whether they will interact with specific cytoplasmic adaptor proteins. Cleavage of APP and apoE receptors at the cell surface occurs by alpha-secretase activities; thus, the processing of these proteins can be regulated by their trafficking either to or from the cell surface. Their cleavages can also be regulated by tissue inhibitor of metalloproteinase-3 (TIMP-3), a metalloprotease inhibitor in the extracellular matrix. ApoE receptors have functions in neuronal migration during development and in proper synaptic function in the adult. Thus, the functions of apoE receptors and by analogy of APP will be modified by the various extracellular and intracellular interactions reviewed in this paper.     

Adaptor protein sorting nexin 17 regulates amyloid precursor protein trafficking and processing in the early endosomes

Accumulation of extracellular amyloid beta peptide (Abeta), generated from amyloid precursor protein (APP) processing by beta- and gamma-secretases, is toxic to neurons and is central to the pathogenesis of Alzheimer disease. Production of Abeta from APP is greatly affected by the subcellular localization and trafficking of APP. Here we have identified a novel intracellular adaptor protein, sorting nexin 17 (SNX17), that binds specifically to the APP cytoplasmic domain via the YXNPXY motif that has been shown previously to bind several cell surface adaptors, including Fe65 and X11. Overexpression of a dominant-negative mutant of SNX17 and RNA interference knockdown of endogenous SNX17 expression both reduced steady-state levels of APP with a concomitant increase in Abeta production. RNA interference knockdown of SNX17 also decreased APP half-life, which led to the decreased steady-state levels of APP. Immunofluorescence staining confirmed a colocalization of SNX17 and APP in the early endosomes. We also showed that a cell surface adaptor protein, Dab2, binds to the same YXNPXY motif and regulates APP endocytosis at the cell surface. Our results thus provide strong evidence that both cell surface and intracellular adaptor proteins regulate APP endocytic trafficking and processing to Abeta. The identification of SNX17 as a novel APP intracellular adaptor protein highly expressed in neurons should facilitate the understanding of the relationship between APP intracellular trafficking and processing to Abeta.     

Low density lipoprotein receptor-related protein (LRP) interacts with presenilin 1 and is a competitive substrate of the amyloid precursor protein (APP) for gamma-secretase

Presenilin 1 (PS1) is a critical component of the gamma-secretase complex, which is involved in the cleavage of several substrates including the amyloid precursor protein (APP) and the Notch receptor. Recently, the low density receptor-related protein (LRP) has been shown to be cleaved by a gamma-secretase-like activity. We postulated that LRP may interact with PS1 and tested its role as a competitive substrate for gamma-secretase. In this report we show that LRP colocalizes and interacts with endogenous PS1 using coimmunoprecipitation and fluorescence lifetime imaging microscopy. In addition, we found that gamma-secretase active site inhibitors do not disrupt the interaction between LRP and PS1, suggesting that the substrate associates with a gamma-secretase docking site located in close proximity to PS1. This is analogous to APP-gamma-secretase interactions. Finally, we show that LRP competes with APP for gamma-secretase activity. Overexpression of a truncated LRP construct consisting of the C terminus, the transmembrane domain, and a short extracellular portion leads to a reduction in the levels of the Abeta40, Abeta42, and p3 peptides without changing the total level of APP expression. In addition, transfection with the beta-chain of LRP causes an increase in uncleaved APP C-terminal fragments and a concomitant decrease in the signaling effects of the APP intracellular domain. In conclusion, LRP is a PS1 interactor and can compete with APP for gamma-secretase enzymatic activity.     

The neuronal adaptor protein X11beta reduces amyloid beta-protein levels and amyloid plaque formation in the brains of transgenic mice

Accumulation of cerebral amyloid beta-protein (Abeta) is believed to be part of the pathogenic process in Alzheimer's disease. Abeta is derived by proteolytic cleavage from a precursor protein, the amyloid precursor protein (APP). APP is a type-1 membrane-spanning protein, and its carboxyl-terminal intracellular domain binds to X11beta, a neuronal adaptor protein. X11beta has been shown to inhibit the production of Abeta in transfected non-neuronal cells in culture. However, whether this is also the case in vivo in the brain and whether X11beta can also inhibit the deposition of Abeta as amyloid plaques is not known. Here we show that transgenic overexpression of X11beta in neurons leads to a decrease in cerebral Abeta levels in transgenic APPswe Tg2576 mice that are a model of the amyloid pathology of Alzheimer's disease. Moreover, overexpression of X11beta retards amyloid plaque formation in these APPswe mice. Our findings suggest that modulation of X11beta function may represent a novel therapeutic approach for preventing the amyloid pathology of Alzheimer's disease.     

Internalized antibodies to the Abeta domain of APP reduce neuronal Abeta and protect against synaptic alterations

Immunotherapy against beta-amyloid peptide (Abeta) is a leading therapeutic direction for Alzheimer disease (AD). Experimental studies in transgenic mouse models of AD have demonstrated that Abeta immunization reduces Abeta plaque pathology and improves cognitive function. However, the biological mechanisms by which Abeta antibodies reduce amyloid accumulation in the brain remain unclear. We provide evidence that treatment of AD mutant neuroblastoma cells or primary neurons with Abeta antibodies decreases levels of intracellular Abeta. Antibody-mediated reduction in cellular Abeta appears to require that the antibody binds to the extracellular Abeta domain of the amyloid precursor protein (APP) and be internalized. In addition, treatment with Abeta antibodies protects against synaptic alterations that occur in APP mutant neurons.     

Role of ABCG1 and ABCA1 in regulation of neuronal cholesterol efflux to apolipoprotein E discs and suppression of amyloid-beta peptide generation

Maintenance of an adequate supply of cholesterol is important for neuronal function, whereas excess cholesterol promotes amyloid precursor protein (APP) cleavage generating toxic amyloid-beta (Abeta) peptides. To gain insights into the pathways that regulate neuronal cholesterol level, we investigated the potential for reconstituted apolipoprotein E (apoE) discs, resembling nascent lipoprotein complexes in the central nervous system, to stimulate neuronal [3H]cholesterol efflux. ApoE discs potently accelerated cholesterol efflux from primary human neurons and cell lines. The process was saturable (17.5 microg of apoE/ml) and was not influenced by APOE genotype. High performance liquid chromatography analysis of cholesterol and cholesterol metabolites effluxed from neurons indicated that <25% of the released cholesterol was modified to polar products (e.g. 24-hydroxycholesterol) that diffuse from neuronal membranes. Thus, most cholesterol (approximately 75%) appeared to be effluxed from neurons in a native state via a transporter pathway. ATP-binding cassette transporters ABCA1, ABCA2, and ABCG1 were detected in neurons and neuroblastoma cell lines and expression of these cDNAs revealed that ABCA1 and ABCG1 stimulated cholesterol efflux to apoE discs. In addition, ABCA1 and ABCG1 expression in Chinese hamster ovary cells that stably express human APP significantly reduced Abeta generation, whereas ABCA2 did not modulate either cholesterol efflux or Abeta generation. These data indicate that ABCA1 and ABCG1 play a significant role in the regulation of neuronal cholesterol efflux to apoE discs and in suppression of APP processing to generate Abeta peptides.     

Intracellular apolipoprotein E affects Amyloid Precursor Protein processing and amyloid Aβ production in COS-1 cells

The apoE gene has been identified as a major susceptibility locus for late-onset Alzheimer's disease (LOAD). The epsilon4 allele greatly reduces age of onset of LOAD as compared to the wild-type 3 allele. The molecular mechanism(s) underlying the association has not yet been fully elucidated. The apoE protein has been shown to physically interact with the Abeta region of the Amyloid Precursor Protein (APP), but also with the ectodomain of the APP holoprotein itself. In this study we have used apoE fusion proteins containing either the ER retention sequence KDEL or trans-Golgi network (TGN) signal sequence in order to define potential apoE-mediated alterations in APP protein processing. Co-expression and pulse-chase experiments showed that a functional apoE:APP interaction occurs intracellularly which directly affects maturation and subsequently the secretion kinetics of APP. In addition, an epsilon4 allele-specific induction of Abeta production has been demonstrated. apoE3 resulted in increased Abeta production only when targeted to the ER, as observed in cells transfected with an apoE3KDEL fusion protein as well as following treatment with brefeldin A. The findings suggest that in cells that express both apoE and APP, such as astrocytes and microglia, a functional apoE:APP interaction may occur which modulates APP processing and Abeta production.     

Molecular dissection of the interaction between amyloid precursor protein and its neuronal trafficking receptor SorLA/LR11

SorLA/LR11 is a sorting receptor that regulates the intracellular transport and processing of the amyloid precursor protein (APP) in neurons. SorLA/LR11-mediated binding results in sequestration of APP in the Golgi and in protection from processing into the amyloid-beta peptide (Abeta), the principal component of senile plaques in Alzheimer's disease (AD). To gain insight into the molecular mechanisms governing sorLA and APP interaction, we have dissected the respective protein interacting domains. Using a fluorescence resonance energy transfer (FRET) based assay of protein proximity, we identified binding sites in the extracellular regions of both proteins. Fine mapping by surface plasmon resonance analysis and analytical ultracentrifugation of recombinant APP and sorLA fragments further narrowed down the binding domains to the cluster of complement-type repeats in sorLA that forms a 1:1 stoichiometric complex with the carbohydrate-linked domain of APP. These data shed new light on the molecular determinants of neuronal APP trafficking and processing and on possible targets for intervention with senile plaque formation in patients with AD.     

Characterization of AmphiF-spondin reveals the modular evolution of chordate F-spondin genes

The F-spondin genes are a family of extracellular matrix molecules united by two conserved domains, FS1 and FS2, at the amino terminus plus a variable number of thrombospondin repeats at the carboxy terminus. Currently, characterized members include a single gene in Drosophila and multiple genes in vertebrates. The vertebrate genes are expressed in the midline of the developing embryo, primarily in the floor plate of the neural tube. To investigate the evolution of chordate F-spondin genes, I have used the basal position in chordate phylogeny of the acraniate amphioxus. A single F-spondin-related gene, named AmphiF-spondin, was isolated from amphioxus. Based on molecular phylogenetics, AmphiF-spondin is closely related to a particular subgroup of vertebrate F-spondin genes that encode six thrombospondin repeats. However, unlike these genes, expression of AmphiF-spondin is not confined to the midline but is found through most of the central nervous system. Additionally, AmphiF-spondin has lost three thrombospondin repeats and gained two fibronectin type III repeats, one of which has strong identity to a fibronectin type III repeat from Deleted in Colorectal Cancer (DCC). Taken together, these results suggest a complex evolutionary history for chordate F-spondin genes that includes (1) domain loss, (2) domain gain by tandem duplication and divergence of existing domains, and (3) gain of heterologous domains by exon shuffling.      

Structural studies of the transmembrane C-terminal domain of the amyloid precursor protein (APP): does APP function as a cholesterol sensor?

The amyloid precursor protein (APP) is subject to alternative pathways of proteolytic processing, leading either to production of the amyloid-beta (Abeta) peptides or to non-amyloidogenic fragments. Here, we report the first structural study of C99, the 99-residue transmembrane C-terminal domain of APP liberated by beta-secretase cleavage. We also show that cholesterol, an agent that promotes the amyloidogenic pathway, specifically binds to this protein. C99 was purified into model membranes where it was observed to homodimerize. NMR data show that the transmembrane domain of C99 is an alpha-helix that is flanked on both sides by mostly disordered extramembrane domains, with two exceptions. First, there is a short extracellular surface-associated helix located just after the site of alpha-secretase cleavage that helps to organize the connecting loop to the transmembrane domain, which is known to be essential for Abeta production. Second, there is a surface-associated helix located at the cytosolic C-terminus, adjacent to the YENPTY motif that plays critical roles in APP trafficking and protein-protein interactions. Cholesterol was seen to participate in saturable interactions with C99 that are centered at the critical loop connecting the extracellular helix to the transmembrane domain. Binding of cholesterol to C99 and, most likely, to APP may be critical for the trafficking of these proteins to cholesterol-rich membrane domains, which leads to cleavage by beta- and gamma-secretase and resulting amyloid-beta production. It is proposed that APP may serve as a cellular cholesterol sensor that is linked to mechanisms for suppressing cellular cholesterol uptake.     

The amyloid beta-protein precursor of Alzheimer's disease is degraded extracellularly by a Kunitz protease inhibitor domain-sensitive trypsin-like serine protease in cultures of chick sympathetic neurons.

The amyloid beta-protein precursor (APP) of Alzheimer's disease (AD) is cleaved either by alpha-secretase to generate an N-terminally secreted fragment, or by beta- and gamma-secretases to generate the beta-amyloid protein (Abeta). The accumulation of Abeta in the brain is an important step in the pathogenesis of AD. Alternative mRNA splicing can generate isoforms of APP which contain a Kunitz protease inhibitor (KPI) domain. However, little is known about the physiological function of this domain. In the present study, the metabolic turnover of APP was examined in cultured chick sympathetic neurons. APP was labelled by incubating neurons for 5 h with [35S]methionine and [35S]cysteine. Intracellular labelled APP decayed in a biphasic pattern suggesting that trafficking occurs through two metabolic compartments. The half-lives for APP in each compartment were 1.5 and 5.7 h, respectively. A small fraction (10%) of the total APP was secreted into the culture medium where it was degraded with a half-life of 9 h. Studies using specific protease inhibitors demonstrated that this extracellular breakdown was due to cleavage by a trypsin-like serine protease that was secreted into the culture medium. Significantly, this protease was inhibited by a recombinant isoform of APP (sAPP751), which contains a region homologous to the Kunitz protease inhibitor (KPI) domain. These results suggest that KPI forms of APP regulate extracellular cleavage of secreted APP by inhibiting the activity of a secreted APP-degrading protease.     

Blocking the cleavage at midportion between gamma- and epsilon-sites remarkably suppresses the generation of amyloid beta-protein

To examine how gamma- and epsilon-cleavages of beta-amyloid precursor protein (APP) are related, each cleavage site was replaced with a stretch of Trp that cannot be cleaved by gamma-secretase. Replacement of the gamma- or epsilon-site significantly suppressed secretion of amyloid beta-protein (Abeta), and produced longer Abeta or longer APP intracellular domain, respectively. This cleavage at the midportion between gamma- and epsilon-sites was also gamma-secretase-dependent. Blocking this cleavage with a Trp stretch remarkably suppressed Abeta generation, indicating that the midportion cleavage is required for the generation of Abeta.