cDNA isolation of Alzheimer's amyloid precursor protein from cholinergic nerve terminals of the electric organ of the electric ray

Alternative Selection of β-Site APP-Cleaving Enzyme 1 (BACE1) Cleavage Sites in Amyloid β-Protein Precursor (APP) Harboring Protective and Pathogenic Mutations within the Aβ Sequence

β-Site APP-cleaving enzyme 1 (BACE1) cleaves amyloid β-protein precursor (APP) at the bond between Met⁶⁷¹ and Asp⁶⁷² (β-site) to generate the carboxyl-terminal fragment (CTFβ/C99). BACE1 also cleaves APP at another bond between Thr⁶⁸¹ and Gln⁶⁸² (β'-site), yielding CTFβ'/C89. Cleavage of CTFβ/C99 by γ-secretase generates Aβ(1-XX), whereas cleavage of CTFβ'/C89 generates Aβ(11-XX). Thus, β'-site cleavage by BACE1 is amyloidolytic rather than amyloidogenic. β' cleavage of mouse APP is more common than the corresponding cleavage of human APP. We found that the H684R substitution within human Aβ, which replaces the histidine in the human protein with the arginine found at the corresponding position in mouse, facilitated β' cleavage irrespective of the species origin of BACE1, thereby significantly increasing the level of Aβ(11-XX) and decreasing the level of Aβ(1-XX). Thus, amino acid substitutions within the Aβ sequence influenced the selectivity of alternative β- or β'-site cleavage of APP by BACE1. In familial Alzheimer's disease (FAD), the APP gene harbors pathogenic variations such as the Swedish (K670N/M671L), Leuven (E682K), and A673V mutations, all of which decrease Aβ(11-40) generation, whereas the protective Icelandic mutation (A673T) increases generation of Aβ(11-40). Thus, A673T promotes β' cleavage of APP and protects subjects against AD. In addition, CTFβ/C99 was cleaved by excess BACE1 activity to generate CTFβ'/C89, followed by Aβ(11-40), even if APP harbored pathogenic mutations. The resultant Aβ(11-40) was more metabolically labile in vivo than Aβ(1-40). Our analysis suggests that some FAD mutations in APP are amyloidogenic and/or amyloidolytic via selection of alternative BACE1 cleavage sites.

Positive evolutionary selection of an HD motif on Alzheimer precursor protein orthologues suggests a functional role

HD amino acid duplex has been found in the active center of many different enzymes. The dyad plays remarkably different roles in their catalytic processes that usually involve metal coordination. An HD motif is positioned directly on the amyloid beta fragment (Aβ) and on the carboxy-terminal region of the extracellular domain (CAED) of the human amyloid precursor protein (APP) and a taxonomically well defined group of APP orthologues (APPOs). In human Aβ HD is part of a presumed, RGD-like integrin-binding motif RHD; however, neither RHD nor RXD demonstrates reasonable conservation in APPOs. The sequences of CAEDs and the position of the HD are not particularly conserved either, yet we show with a novel statistical method using evolutionary modeling that the presence of HD on CAEDs cannot be the result of neutral evolutionary forces (p<0.0001). The motif is positively selected along the evolutionary process in the majority of APPOs, despite the fact that HD motif is underrepresented in the proteomes of all species of the animal kingdom. Position migration can be explained by high probability occurrence of multiple copies of HD on intermediate sequences, from which only one is kept by selective evolutionary forces, in a similar way as in the case of the “transcription binding site turnover.” CAED of all APP orthologues and homologues are predicted to bind metal ions including Amyloid-like protein 1 (APLP1) and Amyloid-like protein 2 (APLP2). Our results suggest that HDs on the CAEDs are most probably key components of metal-binding domains, which facilitate and/or regulate inter- or intra-molecular interactions in a metal ion-dependent or metal ion concentration-dependent manner. The involvement of naturally occurring mutations of HD (Tottori (D7N) and English (H6R) mutations) in early onset Alzheimer's disease gives additional support to our finding that HD has an evolutionary preserved function on APPOs.

HD amino acid duplex can be found in the active center of different metallo-enzymes. An HD motif is positioned directly on the amyloid beta (Aβ) fragment and on the carboxy-terminal region of the extracellular domain of the human amyloid precursor protein (APP) and a taxonomically well defined group of APP orthologues (APPOs). The conservation of the HD dyad is not position specific and it cannot be seen in a multiple alignment. Yet we show with a novel statistical method using evolutionary modeling that HD motif is positively selected by evolution on APPOs, despite the fact that HD dyad is underrepresented in the proteomes of all species of the animal kingdom. CAED of all APP orthologues and homologues are predicted to bind metal ions including Amyloid-like protein 1 (APLP1) and Amyloid-like protein 2 (APLP2). Our results suggest that HDs on the APPOs are most probably key components of metal-binding domains, which facilitate and/or regulate inter- or intra-molecular interactions in a metal ion-dependent or metal ion concentration-dependent manner. The involvement of naturally occurring mutations of HD (Tottori (D7N) and English (H6R)) in early onset Alzheimer's disease gives additional support to our finding that HD has an evolutionary preserved function on APPOs.

Possible mechanisms of APP-mediated oxidative stress in Alzheimer's disease

Oxidative stress was presented to play an important role in the pathogenesis of Alzheimer's disease (AD), especially in the early evolution of AD amyloidogenesis and not only as a consequence thereof. The effect of oxidative stress catalysed by transition metals appears to have a critical relevance in AD. Metal-ion homeostasis is severely dysregulated in AD and it was found that experimentally induced disturbances in the homeostasis of Zn(II) and Cu(II) affect the amyloid precursor protein (APP) metabolism. APP itself binds Zn(II) and Cu(II) at nanomolar concentrations and an altered APP metabolism or expression level is believed to result in neurotoxic processes.

Contrasting, species-dependent modulation of copper-mediated neurotoxicity by the Alzheimer's disease amyloid precursor protein

The amyloid precursor protein (APP) of Alzheimer's disease (AD) has a copper binding domain (CuBD) located in the N-terminal cysteine-rich region that can strongly bind copper(II) and reduce it to Cu(I) in vitro. The CuBD sequence is similar among the APP family paralogs [amyloid precursor-like proteins (APLP1 and APLP2)] and its orthologs (including Drosophila melanogaster, Xenopus laevis, and Caenorhabditis elegans), suggesting an overall conservation in its function or activity. The APP CuBD is involved in modulating Cu homeostasis and amyloid beta peptide production. In this paper, we demonstrate for the first time that Cu-metallated full-length APP ectodomain induces neuronal cell death in vitro. APP Cu neurotoxicity can be induced directly or potentiated through Cu(I)-mediated oxidation of low-density lipoprotein, a finding that may have important implications for the role of lipoproteins and membrane cholesterol composition in AD. Cu toxicity induced by human APP, Xenopus APP, and APLP2 CuBDs is dependent on conservation of histidine residues at positions corresponding to 147 and 151 of human APP. Intriguingly, APP orthologs with different amino acid residues at these positions had dramatically altered Cu phenotypes. The corresponding C. elegans APL-1 CuBD, which has tyrosine and lysine residues at positions 147 and 151, respectively, strongly protected against Cu-mediated lipid peroxidation and neurotoxicity in vitro. Replacement of histidines 147 and 151 with tyrosine and lysine residues conferred this neuroprotective Cu phenotype to human APP, APLP2, and Xenopus APP CuBD peptides. Moreover, we show that the toxic and protective CuBD phenotypes are associated with differences in Cu binding and reduction. These studies identify a significant evolutionary change in the function of the CuBD in modulating Cu metabolism. Our findings also suggest that targeting of inhibitors to histidine residues at positions 147 and 151 of APP could significantly alter the oxidative potential of APP.

Contrasting, species-dependent modulation of copper-mediated neurotoxicity by the Alzheimer's disease amyloid precursor protein

The amyloid precursor protein (APP) of Alzheimer's disease (AD) has a copper binding domain (CuBD) located in the N-terminal cysteine-rich region that can strongly bind copper(II) and reduce it to Cu(I) in vitro. The CuBD sequence is similar among the APP family paralogs [amyloid precursor-like proteins (APLP1 and APLP2)] and its orthologs (including Drosophila melanogaster, Xenopus laevis, and Caenorhabditis elegans), suggesting an overall conservation in its function or activity. The APP CuBD is involved in modulating Cu homeostasis and amyloid beta peptide production. In this paper, we demonstrate for the first time that Cu-metallated full-length APP ectodomain induces neuronal cell death in vitro. APP Cu neurotoxicity can be induced directly or potentiated through Cu(I)-mediated oxidation of low-density lipoprotein, a finding that may have important implications for the role of lipoproteins and membrane cholesterol composition in AD. Cu toxicity induced by human APP, Xenopus APP, and APLP2 CuBDs is dependent on conservation of histidine residues at positions corresponding to 147 and 151 of human APP. Intriguingly, APP orthologs with different amino acid residues at these positions had dramatically altered Cu phenotypes. The corresponding C. elegans APL-1 CuBD, which has tyrosine and lysine residues at positions 147 and 151, respectively, strongly protected against Cu-mediated lipid peroxidation and neurotoxicity in vitro. Replacement of histidines 147 and 151 with tyrosine and lysine residues conferred this neuroprotective Cu phenotype to human APP, APLP2, and Xenopus APP CuBD peptides. Moreover, we show that the toxic and protective CuBD phenotypes are associated with differences in Cu binding and reduction. These studies identify a significant evolutionary change in the function of the CuBD in modulating Cu metabolism. Our findings also suggest that targeting of inhibitors to histidine residues at positions 147 and 151 of APP could significantly alter the oxidative potential of APP.

Phosphorylation of thr(668) in the cytoplasmic domain of the Alzheimer's disease amyloid precursor protein by stress-activated protein kinase 1b (Jun N-terminal kinase-3)

Threonine(668) (thr(668)) within the carboxy-terminus of the Alzheimer's disease amyloid precursor protein (APP) is a known in vivo phosphorylation site. Phosphorylation of APPthr(668) is believed to regulate APP function and metabolism. Thr(668) precedes a proline, which suggests that it is targeted for phosphorylation by proline-directed kinase(s). We have investigated the ability of four major neuronally active proline-directed kinases, cyclin dependent protein kinase-5, glycogen synthase kinase-3 beta, p42 mitogen-activated protein kinase and stress-activated protein kinase-1b, to phosphorylate APPthr(668) and report here that SAPK1b induces robust phosphorylation of this site both in vitro and in vivo. This finding provides a molecular framework to link cellular stresses with APP metabolism in both normal and disease states.

Neuron-specific phosphorylation of Alzheimer's beta-amyloid precursor protein by cyclin-dependent kinase 5

The mature form of Alzheimer's beta-amyloid precursor protein (APP) is phosphorylated specifically at Thr(668) in neurons. In mature neurons, phosphorylated APP is detected in neurites, with dephosphorylated APP being found mostly in the cell body. In vitro, active cyclin-dependent kinase 5 (Cdk5) phosphorylated the cytoplasmic domain of APP at Thr(668). Treatment of mature neurons with an antisense oligonucleotide to Cdk5 suppressed Cdk5 expression and significantly diminished the level of phosphorylated APP. The expression of APP was unaffected in antisense-treated neurons. These results indicate that in neurons APP is phosphorylated by Cdk5, and that this may play a role in its localization.

Overexpression of human amyloid precursor protein in Drosophila

Amyloid precursor protein (APP) is the precursor of the beta-amyloid peptide which is associated with Alzheimer's disease. The physiological function of APP is not well understood. We have established model system for the analysis of APP function in Drosophila. In neural cells, overexpressed human APP was transported to the synaptic terminal in a manner similar to its localization in human neurons, which suggested that the Drosophila protein transport system localizes human APP appropriately. Expression of APP in imaginal discs resulted in a defect in adult cuticle secretion and a blistered wing phenotype. The severity of the wing blister phenotype was proportional to the APP expression level. These results suggested the presence in Drosophila wing tissue of a protein or protein(s) which can interact with APP.

Distribution of beta-amyloid and amyloid precursor protein in the brain of spawning (senescent) salmon: a natural, brain-aging model

Brain amyloid precursor protein (APP), a normal constituent of neurons, glial cells and cerebrospinal fluid, has several proposed functions (e.g., in neuronal growth and survival). It appears, however, that altered processing of APP is an initial or downstream step in the neuropathology of brain aging, Alzheimer's disease (AD), and Down's syndrome (DS). Some studies suggest that proteolytic cleavage of APP, producing beta-amyloid (Abeta(1-42)), could have neurotoxic or neuroprotective effects. In this study, we utilized antibodies to human APP(695) and Abeta(1-42,) and Congo red staining, to search for amyloid deposition in the brain of semelparous spawning kokanee salmon (Oncorhynchus nerka kennerlyi). Intracellular APP(695) immunoreactivity (APP-ir) was observed in brain regions involved in gustation (glomerulosus complex), olfaction (putative hippocampus, olfactory bulb), vision (optic tectum), the stress response (nucleus preopticus and nucleus lateralis tuberis), reproductive behavior (nucleus preopticus magnocellularis, nucleus preopticus periventricularis, ventral telencephalon), and coordination (cerebellum). Intra- and extra-neuronal Abeta(1-42) immunoreactivity (Abeta-ir) were present in all APP-ir regions except the nucleus lateralis tuberis and Purkinje cells of the cerebellum (coordination). Thus, the relationship between APP and Abeta deposition during brain aging could shed light on the processing of APP into Abeta, neurodegeneration, and possible protection of neurons that are functioning in spawning but senescent salmon. Pacific salmon, with their predictable and synchronized life history, could provide research options not available with the existing models for studies of brain aging and amyloidosis.

What the evolution of the amyloid protein precursor supergene family tells us about its function

The Alzheimer's disease amyloid protein precursor (APP) gene is part of a multi-gene super-family from which sixteen homologous amyloid precursor-like proteins (APLP) and APP species homologues have been isolated and characterised. Comparison of exon structure (including the uncharacterised APL-1 gene), construction of phylogenetic trees, and analysis of the protein sequence alignment of known homologues of the APP super-family were performed to reconstruct the evolution of the family and to assess the functional significance of conserved protein sequences between homologues. This analysis supports an adhesion function for all members of the APP super family, with specificity determined by those sequences which are not conserved between APLP lineages, and provides evidence for an increasingly complex APP superfamily during evolution. The analysis also suggests that Drosophila APPL and Caenorhabditis elegans APL-1 may be a fourth APLP lineage indicating that these proteins, while not functional homologues of human APP, are similarly likely to regulate cell adhesion. Furthermore, the betaA4 sequence is highly conserved only in APP orthologues, strongly suggesting this sequence is of significant functional importance in this lineage.

The evolution of Alzheimer disease, the reproductive schedule, and apoE isoforms

Alzheimer disease (AD)-like neuropathology increases progressively during aging in most primates, and, in some species, is concurrent with reproductive decline in females and cognitive impairments. We consider how the schedule of AD may have evolved in early humans in relation to the apolipoprotein E (apoE) allele system, which is not found in other primates, and to the increasing duration of postnatal care. The delay of independence and the increasing length of maturation required that the schedule of AD-like neurodegeneration be slowed, otherwise parental caregivers would already have become impaired. We hypothesize that the uniquely human apoE epsilon3 allele evolved from the epsilon4 of primate ancestors during human evolution in relation to the rapid increases of brain size and the emergence of grandmothering. In discussing theses possibilities, we review the diverse bioactivities of apoE, which include involvement in hormone systems. The evolution of menopause is also considered in relation to the protective effect of estrogen on AD.

Role of phosphorylation of Alzheimer's amyloid precursor protein during neuronal differentiation

Alzheimer's amyloid precursor protein (APP), the precursor of beta-amyloid (Abeta), is an integral membrane protein with a receptor-like structure. We recently demonstrated that the mature APP (mAPP; N- and O-glycosylated form) is phosphorylated at Thr668 (numbering for APP695 isoform), specifically in neurons. Phosphorylation of mAPP appears to occur during, and after, neuronal differentiation. Here we report that the phosphorylation of mAPP begins 48-72 hr after treatment of PC12 cells with NGF and that this correlates with the timing of neurite outgrowth. The phosphorylated form of APP is distributed in neurites and mostly in the growth cones of differentiating PC12 cells. PC12 cells stably expressing APP with Thr668Glu substitution showed remarkably reduced neurite extension after treatment with NGF. These observations suggest that the phosphorylated form of APP may play an important role in neurite outgrowth of differentiating neurons.

Role of phosphorylation of Alzheimer's amyloid precursor protein during neuronal differentiation

Alzheimer's amyloid precursor protein (APP), the precursor of beta-amyloid (Abeta), is an integral membrane protein with a receptor-like structure. We recently demonstrated that the mature APP (mAPP; N- and O-glycosylated form) is phosphorylated at Thr668 (numbering for APP695 isoform), specifically in neurons. Phosphorylation of mAPP appears to occur during, and after, neuronal differentiation. Here we report that the phosphorylation of mAPP begins 48-72 hr after treatment of PC12 cells with NGF and that this correlates with the timing of neurite outgrowth. The phosphorylated form of APP is distributed in neurites and mostly in the growth cones of differentiating PC12 cells. PC12 cells stably expressing APP with Thr668Glu substitution showed remarkably reduced neurite extension after treatment with NGF. These observations suggest that the phosphorylated form of APP may play an important role in neurite outgrowth of differentiating neurons.

Phosphorylation of the cytoplasmic domain of Alzheimer's beta-amyloid precursor protein at Ser655 by a novel protein kinase

The cytoplasmic domain of Alzheimer's beta-amyloid precursor protein (APP) can be phosphorylated at Thr654, Ser655, and Thr668 (APP695 isoform numbering). Previous studies demonstrated that Ser655 of APP was phosphorylated by protein kinase C (PKC) and calmodulin-dependent protein kinase II (CaMKII) in vitro and by unidentified protein kinase(s) in vivo. We report here the characterization of a novel protein kinase (designated APP kinase I) which phosphorylates Ser655 of APP. APP kinase I was partially purified over 7,000-fold from rat brain and identified as a approximately 43 kDa protein that is distinct from a number of known protein kinases, including PKC and extracellular signal-regulated kinases (ERKs). The identification of a novel protein kinase that phosphorylates Ser655 will hopefully contribute to our understanding of the metabolism and/or function of APP in the pathogenesis of Alzheimer's disease (AD).

Interaction of a neuron-specific protein containing PDZ domains with Alzheimer's amyloid precursor protein

A novel protein, human X11-like (human X11L), contains a phosphotyrosine interaction (PI) domain and two PDZ domains and displays 55.2% amino acid homology with the human X11 (human X11). The PI domain of human X11L interacts with a sequence containing the NPXY motif found in the cytoplasmic domain of Alzheimer's amyloid precursor protein. A construct lacking the carboxyl-terminal domain, which comprises two PDZ domains (N + PI), enhances PI binding to APP, whereas another construct lacking an amino-terminal domain relative to PI domain (PI + C) suppresses PI binding to APP. Overexpression of full-length human X11L (N + PI + C) in cells that express APP695 stably decreased the secretion of Abeta40 but not that of Abeta42. However, overexpression of the PI domain alone and the N + PI construct in cells did not affect the secretion of Abeta despite their ability to bind to the cytoplasmic domain of Alzheimer's amyloid precursor protein. These observations suggest that the amino-terminal domain regulates PI binding to APP and that the carboxyl-terminal domain containing PDZ motifs is essential to modulate APP processing. Because expression of the human X11L gene is specific to brain, the present observations should contribute to shedding light on the molecular mechanism of APP processing in Alzheimer's disease.