Reduced retinal function in amyloid precursor protein-over-expressing transgenic mice via attenuating glutamate-N-methyl-d-aspartate receptor signaling

Here, we examined whether amyloid-beta (Abeta) protein participates in cell death and retinal function using three types of transgenic (Tg) mice in vivo [human mutant amyloid precursor protein (APP) Tg (Tg 2576) mice, mutant presenilin-1 (PS-1) knock-in mice, and APP/PS-1 double Tg mice]. ELISA revealed that the insoluble form of Abeta(1-40) was markedly accumulated in the retinas of APP and APP/PS-1, but not PS-1 Tg, mice (vs. wild-type mice). In APP Tg and APP/PS-1 Tg mice, immunostaining revealed accumulations of intracellular Abeta(1-42) in retinal ganglion cells and in the inner and outer nuclear layers. APP Tg and APP/PS-1 Tg, but not PS-1 Tg, mice had less NMDA-induced retinal damage than wild-type mice, and the reduced damage in APP/PS-1 Tg mice was diminished by the pre-treatment of N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester, a gamma-secretase inhibitor. Furthermore, the number of TUNEL-positive cells was significantly less in ganglion cell layer of APP/PS-1 Tg mice than PS-1 Tg mice 24 h after NMDA injection. The phosphorylated form of calcium/calmodulin-dependent protein kinase IIalpha (CaMKIIalpha), but not total CaMKIIalpha or total NMDA receptor 1 (NR1) subunit, in total retinal extracts was decreased in non-treated retinas of APP/PS-1 Tg mice (vs. wild-type mice). CaMKIIalpha and NR2B proteins, but not NR1, in retinal membrane fraction were significantly decreased in APP/PS-1 Tg mice as compared with wild-type mice. The NMDA-induced increase in p-CaMKIIalpha in the retina was also lower in APP/PS-1 Tg mice than in wild-type mice. In electroretinogram and visual-evoked potential recordings, the implicit time to each peak from a light stimulus was prolonged in APP/PS-1 mice versus wild-type mice. Hence, Abeta may impair retinal function by reducing activation of NMDA-receptor signaling pathways.     

The Alzheimer's disease-associated presenilins are differentially phosphorylated proteins located predominantly within the endoplasmic reticulum.

BACKGROUND: Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by the deposition of extracellular senile plaques composed of amyloid beta-peptide (A beta). Whereas most cases of AD occur sporadically, about 10% of AD cases are inherited as a fully penetrant autosomal dominant trait. Mutations in the recently cloned Presenilin genes (PS-1 and PS-2) are by far the most common cause of early onset familial AD. MATERIALS AND METHODS: Cellular expression of endogenous and overexpressed PS proteins was analyzed by immunocytochemistry and metabolic labeling followed by immunoprecipitation. In vivo phosphorylation sites of PS proteins were analyzed by extensive mutagenesis. RESULTS: PS-1 as well as PS-2 proteins were localized predominantly within the endoplasmic reticulum (ER). However, small amounts of the PS proteins were detected within the Golgi compartment, where they colocalize with the beta-amyloid precursor protein (beta APP). The PS-2 protein was found to be highly phosphorylated, whereas very little phosphorylation was observed for PS-1. The selective phosphorylation of PS-2 occurs exclusively on serine residues. In vivo phosphorylation of PS-2 was mapped to serine residues 7, 9, and 19 within an acidic stretch at the N terminus, which is absent in PS-1. casein kinase (CK)-1 and CK-2 were shown to phosphorylate the N terminus of PS-2 in vitro. CONCLUSIONS: The majority of PS proteins were detected in the ER where little if any proteolytic processing of beta APP was reported. ER retention of PS proteins might occur by intramolecular aggregation. Small amounts of PS proteins were also detected in the Golgi where they colocalized with beta APP. This might suggest that potential interactions between PS proteins and beta APP could occur within the Golgi. Selective phosphorylation of PS-2 proteins within the acidic domain missing in PS-1 indicates differences in the biological functions and regulation of the two highly homologous proteins.     

Mutations in amyloid precursor protein and presenilin-1 genes increase the basal oxidative stress in murine neuronal cells and lead to increased sensitivity to oxidative stress mediated by amyloid beta-peptide (1-42), HO and kainic acid: implications for Alzheimer's disease

Oxidative stress is observed in Alzheimer's disease (AD) brain, including protein oxidation and lipid peroxidation. One of the major pathological hallmarks of AD is the brain deposition of amyloid beta-peptide (Abeta). This 42-mer peptide is derived from the beta-amyloid precursor protein (APP) and is associated with oxidative stress in vitro and in vivo. Mutations in the PS-1 and APP genes, which increase production of the highly amyloidogenic amyloid beta-peptide (Abeta42), are the major causes of early onset familial AD. Several lines of evidence suggest that enhanced oxidative stress, inflammation, and apoptosis play important roles in the pathogenesis of AD. In the present study, primary neuronal cultures from knock-in mice expressing mutant human PS-1 and APP were compared with those from wild-type mice, in the presence or absence of various oxidizing agents, viz, Abeta(1-42), H2O2 and kainic acid (KA). APP/PS-1 double mutant neurons displayed a significant basal increase in oxidative stress as measured by protein oxidation, lipid peroxidation, and 3-nitrotyrosine when compared with the wild-type neurons (p < 0.0005). Elevated levels of human APP, PS-1 and Abeta(1-42) were found in APP/PS-1 cultures compared with wild-type neurons. APP/PS-1 double mutant neuron cultures exhibited increased vulnerability to oxidative stress, mitochondrial dysfunction and apoptosis induced by Abeta(1-42), H2O2 and KA compared with wild-type neuronal cultures. The results are consonant with the hypothesis that Abeta(1-42)-associated oxidative stress and increased vulnerability to oxidative stress may contribute significantly to neuronal apoptosis and death in familial early onset AD.     

Proteomic analysis of brain proteins in APP/PS-1 human double mutant knock-in mice with increasing amyloid β-peptide deposition: insights into the effects of in vivo treatment with N-acetylcysteine as a potential therapeutic intervention in mild cognitive impairment and Alzheimer's disease

Proteomics analyses were performed on the brains of wild-type (WT) controls and an Alzheimer's disease (AD) mouse model, APP/PS-1 human double mutant knock-in mice. Mice were given either drinking water or water supplemented with N-acetylcysteine (NAC) (2 mg/kg body weight) for a period of five months. The time periods of treatment correspond to ages prior to Aβ deposition (i.e. 4-9 months), resembling human mild cognitive impairment (MCI), and after Aβ deposition (i.e. 7-12 months), more closely resembling advancing stages of AD. Substantial differences exist between the proteomes of WT and APP/PS-1 mice at 9 or 12 months, indicating that Aβ deposition and oxidative stress lead to downstream changes in protein expression. Altered proteins are involved in energy-related pathways, excitotoxicity, cell cycle signaling, synaptic abnormalities, and cellular defense and structure. Overall, the proteomic results support the notion that NAC may be beneficial for increasing cellular stress responses in WT mice and for influencing the levels of energy- and mitochondria-related proteins in APP/PS-1 mice.     

Presenilin-1-mediated retention of APP derivatives in early biosynthetic compartments

Processing of the amyloid precursor protein (APP) leads to the production of amyloid-beta (Abeta), the major component of extracellular plaques in the brains of Alzheimer's disease (AD) patients. Presenilin-1 (PS-1) plays a key role in the final step of Abeta formation, the gamma-secretase cleavage. Previously, we showed that PS-1 is retained in pre-Golgi compartments by incorporation into COPI-coated membranes of the vesicular tubular clusters (VTCs) between endoplasmic reticulum (ER) and Golgi complex. Here, we show that PS-1 also mediates the retention of the beta-cleavage-derived APP-C-terminal fragment (CTFbeta) and/or Abeta in pre-Golgi membranes. Overexpression of PS-1 increased the percentage of CTFbeta and/or Abeta in VTCs as well as their distribution to COPI-coated VTC membranes. By contrast, overexpression of the dominant-negative aspartate mutant PS-1(D257A) or PS-knockout decreased incorporation of these APP derivatives into COPI-coated membranes. Sorting of APP derivatives to COPI-coated VTC membranes was not depending on the APP cytosolic tail. In post-Golgi compartments, PS-1 expression enhanced the association of full-length APP/APPs with endosomal compartments at the expense of plasma membrane-bound APP. We conclude that PS-1, in addition to its role in gamma-secretase cleavage, is also required for the subcellular routing of APP and its derivatives. Malfunctioning of PS-1 in this role may have important consequences for the progress of AD.     

Proteomic identification of specifically carbonylated brain proteins in APP(NLh)/APP(NLh) × PS-1(P264L)/PS-1(P264L) human double mutant knock-in mice model of Alzheimer disease as a function of age

Alzheimer disease (AD) is the most common type of dementia and is characterized pathologically by the presence of neurofibrillary tangles (NFTs), senile plaques (SPs), and loss of synapses. The main component of SP is amyloid-beta peptide (Aβ), a 39 to 43 amino acid peptide, generated by the proteolytic cleavage of amyloid precursor protein (APP) by the action of beta- and gamma-secretases. The presenilins (PS) are components of the γ-secretase, which contains the protease active center. Mutations in PS enhance the production of the Aβ42 peptide. To date, more than 160 mutations in PS1 have been identified. Many PS mutations increase the production of the β-secretase-mediated C-terminal (CT) 99 amino acid-long fragment (CT99), which is subsequently cleaved by γ-secretase to yield Aβ peptides. Aβ has been proposed to induce oxidative stress and neurotoxicity. Previous studies from our laboratory and others showed an age-dependent increase in oxidative stress markers, loss of lipid asymmetry, and Aβ production and amyloid deposition in the brain of APP/PS1 mice. In the present study, we used APP (NLh)/APP(NLh) × PS-1(P246L)/PS-1(P246L) human double mutant knock-in APP/PS-1 mice to identify specific targets of brain protein carbonylation in an age-dependent manner. We found a number of proteins that are oxidatively modified in APP/PS1 mice compared to age-matched controls. The relevance of the identified proteins to the progression and pathogenesis of AD is discussed.     

Amino acid sequence of the proteolipid subunit of the proton-translocating ATPase complex from the thermophilic bacterium PS-3

The proteolipid subunit of the ATPase complex was identified in whole membranes of the thermophilic bacterium PS-3 by means of a covalent modification with the 14C-labelled inhibitor dicyclohexylcarbodiimide. The proteolipid could be purified from the membrane in free and carbodiimide-modified form by extraction with chloroform/methanol and subsequent carboxymethylcellulose chromatography in mixtures of chloroform/methanol/water. The complete amino acid sequence of the 72-residue polypeptide could be determined by automated solid-phase Edman degradation of the whole protein, and of fragments obtained after cleavage with cyanogen bromide and N-bromosuccinimide. Chemical cleavages and separations of the resulting fragments by gel chromatography were performed in 80% formic acid. The amino acid sequence shows a concentration of hydrophobic amino acids in two segments of about 25 residues at the amino-terminal and carboxy-terminal ends. The polar residues are clustered in the middle of the polypeptide chain. The bound [14C]dicyclohexylcarbodiimide label is recovered exclusively at position 56, which is occupied by a glutamyl residue. The proteolipid from PS-3 exhibits homology to the corresponding ATPase subunit from mitochondria. The carbodiimide-reactive glutamyl residue occurs at the position as in the mitochondrial proteins.     

Promotion of β-amyloid production by C-reactive protein and its implications in the early pathogenesis of Alzheimer's disease

C-reactive protein (CRP) and β-amyloid protein (Aβ) are involved in the development of Alzheimer's disease (AD). However, the relationship between CRP and Aβ production is unclear. In vitro and in vivo experiments were performed to investigate the association of CRP with Aβ production. Using the rat adrenal pheochromocytoma cell line (PC12 cells) to mimic neurons, cytotoxicity was evaluated by cell viability and supernatant lactate dehydrogenase (LDH) activity. The levels of amyloid precursor protein (APP), beta-site APP cleaving enzyme (BACE-1), and presenilins (PS-1 and PS-2) were investigated using real-time polymerase chain reaction and Western blotting analysis. Aβ1-42 was measured by enzyme-linked immunosorbent assay. The relevance of CRP and Aβ as well as potential mechanisms were studied using APP/PS1 transgenic (Tg) mice. Treatment with 0.5-4.0 μM CRP for 48 h decreased cell viability and increased LDH leakage in PC12 cells. Incubation with CRP at a sub-toxic concentration of 0.2 μM increased the mRNA levels of APP, BACE-1, PS-1, and PS-2, as well as Aβ1-42 production. CRP inhibitor reversed the CRP-induced upregulations of the mRNA levels of APP, BACE-1, PS-1, and PS-2, and the protein levels of APP, BACE-1, PS-1, and Aβ1-42, but did not reversed Aβ1-42 cytotoxicity. The cerebral levels of CRP and Aβ1-42 in APP/PS1 Tg mice were positively correlated, accompanied with the elevated mRNA expressions of serum amyloid P component (SAP), complement component 1q (C1q), and tumor necrosis factor-α (TNF-α). These results suggest that CRP cytotoxicity is associated with Aβ formation and Aβ-related markers expressions; CRP and Aβ were relevant in early-stage AD; CRP may be an important trigger in AD pathogenesis.     

Regulation of amyloid precursor protein processing by presenilin 1 (PS1) and PS2 in PS1 knockout cells

The presenilin 1 (PS1) and PS2 proteins are thought to play roles in processing of amyloid precursor protein (APP), but the nature of this role is not fully understood. Recent studies have shown that PS1 is necessary for cleavage of APP at the gamma-secretase site. We now show that PS1 and PS2 participate in other aspects of APP processing. Fibroblasts generated from PS1 knockout mice have increased levels of the APP cleavage products, secreted APP (APPs), and APP C-terminal fragments, but lower secretion of APPs and Abeta. We have also observed that loss of PS1 prevents protein kinase C or extracellular regulated kinase from increasing production of the APP cleavage products, APPs, and APP C-terminal fragments. Transfection of PS1 -/- cells with PS1 restores the responsiveness of APP processing to protein kinase C and extracellular regulated kinase. This suggests that the changes in APP processing in PS1 -/- cells result strictly from the absence of PS1. Transfection of PS1 -/- cells with PS2 is also able to correct the deficits in APP secretion, which suggests that the PS2 also has the ability to regulate APP processing. Finally, transfection of the truncated PS2 construct, Alg3, into cells lacking PS1 increases APP C-terminal fragments. This suggests that Alg3 can interfere with the processing of APP by PS2. These data point to roles for both PS1 and PS2 in regulating APP processing and suggest that the role of these proteins also includes coupling APP to signal transduction pathways.     

Oxidative damage in brain from human mutant APP/PS-1 double knock-in mice as a function of age

Oxidative stress is strongly implicated in the progressive decline of cognition associated with aging and neurodegenerative disorders. In the brain, free radical-mediated oxidative stress plays a critical role in the age-related decline of cellular function as a result of the oxidation of proteins, lipids, and nucleic acids. A number of studies indicate that an increase in protein oxidation and lipid peroxidation is associated with age-related neurodegenerative diseases and cellular dysfunction observed in aging brains. Oxidative stress is one of the important factors contributing to Alzheimer's disease (AD), one of whose major hallmarks includes brain depositions of amyloid beta-peptide (Abeta) derived from amyloid precursor protein (APP). Mutation in APP and PS-1 genes, which increases production of the highly amyloidogenic amyloid beta-peptide (Abeta42), is the major cause of familial AD. In the present study, protein oxidation and lipid peroxidation in the brain from knock-in mice expressing human mutant APP and PS-1 were compared with brain from wild type, as a function of age. The results suggest that there is an increased oxidative stress in the brain of wild-type mice as a function of age. In APP/PS-1 mouse brain, there is a basal increase (at 1 month) in oxidative stress compared to the wild type (1 month), as measured by protein oxidation and lipid peroxidation. In addition, age-related elevation of oxidative damage was observed in APP/PS-1 mice brain compared to that of wild-type mice brain. These results are discussed with reference to the importance of Abeta42-associated oxidative stress in the pathogenesis of AD.     

Oxidative Stress in Synaptosomal Proteins from Mutant Presenilin-1 Knock-in Mice: Implications for Familial Alzheimer's Disease

Presenilin-1 (PS-1) is a transmembrane protein that may be involved in the processing of amyloid precursor protein (APP). Mutations in PS-1 are the major cause of familial Alzheimer's disease (AD). AD brain is under significant oxidative stress, including protein oxidation. In the present study, protein oxidation was compared in synaptosomes from knock-in mice expressing mutant human PS-1 (M146V mutation) and from wild-type mice expressing non-mutant human PS-1. Synaptosomal membrane protein conformational alterations associated with oxidative stress were measured using electron paramagnetic resonance (EPR) in conjunction with a protein-specific spin-label. Direct synaptosomal protein oxidation was assessed by a carbonyl detection assay. Synaptosomal proteins from PS-1 mutant mice displayed increased oxidative stress as measured by both techniques, compared with synaptosomal proteins from wild type mice. These data suggest that PS-1 mutations cause oxidative alterations in synaptosomal membrane protein structure and oxidative modification of synaptosomal proteins. Our findings suggest that familial AD may be associated with oxidative stress that may play a pivotal role in neuronal dysfunction and death.     

BRI2 interacts with amyloid precursor protein (APP) and regulates amyloid beta (Abeta) production

Transmembrane proteins BRI2 and amyloid precursor protein (APP) co-localize with amyloid beta (Abeta) lesions in sporadic Alzheimer disease and mutations in both precursor proteins are linked to early-onset familial cases of cerebral amyloidosis associated with dementia and/or cerebral hemorrhage. A specific interaction between BRI2 and APP was unveiled by immunoprecipitation experiments using transfected and non-transfected cells. The use of deletion mutants further revealed that stretches 648-719 of APP751 and 46-106 of BRI2, both inclusive of the full transmembrane domains, are sufficient for the interaction. Removal of most of the APP and BRI2 extracellular domains without affecting the interaction implies that both proteins interact when are expressed on the same cell membrane (cis) rather than on adjacent cells (trans). The presence of BRI2 had a modulatory effect on APP processing, specifically increasing the levels of cellular APP as well as beta-secretase-generated COOH-terminal fragments while decreasing the levels of alpha-secretase-generated COOH-terminal fragments as well as the secretion of total APP and Abeta peptides. Determining the precise molecular pathways affected by the specific binding between APP and BRI2 could result in the identification of common therapeutic targets for these sporadic and familial neurodegenerative disorders.     

Combined solid-phase and solution approach for the synthesis of large peptides or proteins.

In the synthesis of large peptides or proteins, highly homogeneous segments are indispensable for a convergent strategy either on a solid-phase resin or in solution. Employing Boc/Bzl chemistry to prepare fully protected segments with a free alpha-carboxyl group from the solid support, base-labile linkers are profitable for practical peptide synthesis since they require no special equipment. For this purpose, an N-[9-(hydroxymethyl)-2-fluorenyl]succinamic acid (HMFS) linker was adopted. Consequently, there must be high compatibility between the protecting groups of the segment and the anchoring group which is cleavable by treatment with morpholine or piperidine in DMF. Instead of using the 2-bromobenzyloxycarbonyl (BrZ) group for the Tyr residue and the formyl (For) group for the Trp residue, both of which are the most susceptible protecting groups under these base-catalysed conditions, the base-resistant 3-pentyl (Pen) and cyclohexyloxycarbonyl (Hoc) groups were introduced to the respective side-chain functional groups. By applying the present strategy, the authors were able to rapidly synthesize homogeneous protected segments for use in the subsequent segment coupling in solution. In the present paper, the utility of the combined solid-phase and solution approach is demonstrated by synthesizing muscarinic toxin 1 (MTX1) which binds to the muscarinic acetylcholine receptors.     

X11alpha impairs gamma- but not beta-cleavage of amyloid precursor protein

The phosphotyrosine binding domain of the neuronal protein X11alpha/mint-1 binds to the C-terminus of amyloid precursor protein (APP) and inhibits catabolism to beta-amyloid (Abeta), but the mechanism of this effect is unclear. Coexpression of X11alpha or its PTB domain with APPswe inhibited secretion of Abeta40 but not APPsbetaswe, suggesting inhibition of gamma- but not beta-secretase. To further probe cleavage(s) inhibited by X11alpha, we coexpressed beta-secretase (BACE-1) or a component of the gamma-secretase complex (PS-1Delta9) with APP, APPswe, or C99, with and without X11alpha, in HEK293 cells. X11alpha suppressed the PS-1Delta9-induced increase in Abeta42 secretion generated from APPswe or C99. However, X11alpha did not impair BACE-1-mediated proteolysis of APP or APPswe to C99. In contrast to impaired gamma-cleavage of APPswe, X11alpha or its PTB domain did not inhibit gamma-cleavage of NotchDeltaE to NICD (the Notch intracellular domain). The X11alpha PDZ-PS.1Delta9 interaction did not affect gamma-cleavage activity. In a cell-free system, X11alpha did not inhibit the catabolism of APP C-terminal fragments. These data suggest that X11alpha may inhibit Abeta secretion from APP by impairing its trafficking to sites of active gamma-secretase complexes. By specifically targeting substrate instead of enzyme X11alpha may function as a relatively specific gamma-secretase inhibitor.     

Developments in peptide and amide synthesis

The solid-phase methodology is key for an effective synthesis of peptides, from a milligram scale for research to a multi-kilo scale for drug production. Indeed, small peptides containing up to 20-30 amino acids are most readily synthesized by a solid-phase strategy. Larger peptides (up to 60 amino acids) should be synthesized by a convergent approach (i.e. synthesis of protected constituent peptides in solid-phase and combination of these units in solution). Larger peptides and proteins are prepared by chemical ligation, where unprotected segments have been prepared in solid-phase.     

Presenilin 1 mutations activate gamma 42-secretase but reciprocally inhibit epsilon-secretase cleavage of amyloid precursor protein (APP) and S3-cleavage of notch

The presenilin 1 (PS1) and presenilin 2 (PS2) proteins are necessary for proteolytic cleavage of the amyloid precursor protein (APP) within its transmembrane domain. One of these cleavage events (termed gamma-secretase) generates the C-terminal end of the Abeta-peptide by proteolysis near residue 710 or 712 of APP(770). Another event (termed gamma-like or epsilon-secretase cleavage) cleaves near residue 721 at approximately 2-5 residues inside the cytoplasmic membrane boundary to generate a series of stable, C-terminal APP fragments. This latter cleavage is analogous to S3-cleavage of Notch. We report here that specific mutations in the N terminus, loop, or C terminus of PS1 all increase the production of Abeta(42) but cause inhibition of both epsilon-secretase cleavage of APP and S3-cleavage of Notch. These data support the hypothesis that epsilon-cleavage of APP and S3-cleavage of Notch are similar events. They also argue that, although both the gamma-site and the epsilon-site cleavage of APP are presenilin-dependent, they are likely to be independent catalytic events.     

Imaging the Intracellular Trafficking of APP with Photoactivatable GFP

Beta-amyloid (Aβ) is the major constituent of senile plaques found in the brains of Alzheimer’s disease patients. Aβ is derived from the sequential cleavage of Amyloid Precursor Protein (APP) by β and γ-secretases. Despite the importance of Aβ to AD pathology, the subcellular localization of these cleavages is not well established. Work in our laboratory and others implicate the endosomal/lysosomal system in APP processing after internalization from the cell surface. However, the intracellular trafficking of APP is relatively understudied.While cell-surface proteins are amendable to many labeling techniques, there are no simple methods for following the trafficking of membrane proteins from the Golgi. To this end, we created APP constructs that were tagged with photo-activatable GFP (paGFP) at the C-terminus. After synthesis, paGFP has low basal fluorescence, but it can be stimulated with 413 nm light to produce a strong, stable green fluorescence. By using the Golgi marker Galactosyl transferase coupled to Cyan Fluorescent Protein (GalT-CFP) as a target, we are able to accurately photoactivate APP in the trans-Golgi network. Photo-activated APP-paGFP can then be followed as it traffics to downstream compartments identified with fluorescently tagged compartment marker proteins for the early endosome (Rab5), the late endosome (Rab9) and the lysosome (LAMP1). Furthermore, using inhibitors to APP processing including chloroquine or the γ-secretase inhibitor L685, 458, we are able to perform pulse-chase experiments to examine the processing of APP in single cells.We find that a large fraction of APP moves rapidly to the lysosome without appearing at the cell surface, and is then cleared from the lysosome by secretase-like cleavages. This technique demonstrates the utility of paGFP for following the trafficking and processing of intracellular proteins from the Golgi to downstream compartments.