Identification of a stable fragment of the Alzheimer amyloid precursor containing the beta-protein in brain microvessels

P25/CDK5-mediated Tau Hyperphosphorylation in Both Ipsilateral and Contralateral Cerebra Contributes to Cognitive Deficits in Post-stroke Mice

OBJECTIVE

Post-stroke cognitive impairment (PSCI) develops in approximately one-third of stroke survivors and is associated with ingravescence. Nonetheless, the biochemical mechanisms underlying PSCI remain unclear. The study aimed to establish an ischemic mouse model by means of transient unilateral middle cerebral artery occlusions (MCAOs) and to explore the biochemical mechanisms of p25/cyclin-dependent kinase 5 (CDK5)-mediated tau hyperphosphorylation on the PSCI behavior.

METHODS

Cognitive behavior was investigated, followed by the detection of tau hyperphosphorylation, mobilization, activation of kinases and/or inhibition of phosphatases in the lateral and contralateral cerebrum of mice following ischemia in MACO mice. Finally, we treated HEK293/tau cells with oxygen-glucose deprivation (OGD) and a CDK5 inhibitor (Roscovitine) or a GSK3β inhibitor (LiCl) to the roles of CDK5 and GSK3β in mediating ischemia-reperfusion-induced tau phosphorylation.

RESULTS

Ischemia induced cognitive impairments within 2 months, as well as causing tau hyperphosphorylation and its localization to neuronal somata in both ipsilateral and contralateral cerebra. Furthermore, p25 that promotes CDK5 hyperactivation had significantly higher expression in the mice with MCAO than in the shamoperation (control) group, while the expression levels of protein phosphatase 2 (PP2A) and the phosphorylation level at Tyr307 were comparable between the two groups. In addition, the CDK5 inhibitor rescued tau from hyperphosphorylation induced by OGD.

CONCLUSION

These findings demonstrate that upregulation of CDK5 mediates tau hyperphosphorylation and localization in both ipsilateral and contralateral cerebra, contributing to the pathogenesis of PSCI.

Endothelial expression of human amyloid precursor protein leads to amyloid β in the blood and induces cerebral amyloid angiopathy in knock-in mice

The deposition of amyloid β (Aβ) in blood vessels of the brain, known as cerebral amyloid angiopathy (CAA), is observed in most Alzheimer's disease (AD) patients. Compared with the pathology of CAA in humans, the pathology in most mouse models of AD is not as evident, making it difficult to examine the contribution of CAA to the pathogenesis of AD. On the basis of biochemical analyses that showed blood levels of soluble amyloid precursor protein (sAPP) in rats and mice were markedly lower than those measured in human samples, we hypothesized that endothelial APP expression would be markedly lower in rodents, and subsequently generated mice that specifically express human wild type APP (APP770) in endothelial cells. The resulting EC-APP770⁺ mice exhibited increased levels of serum Aβ and sAPP, indicating that endothelial APP makes a critical contribution to blood Aβ levels. Even though aged EC-APP770⁺ mice did not exhibit Aβ deposition in the cortical blood vessels, crossing these animals with APP knock-in mice (App^NL-F/NL-F) led to an expanded CAA pathology, as evidenced by increased amounts of amyloid accumulated in the cortical blood vessels. These results highlight an overlooked interplay between neuronal and endothelial APP in brain vascular Aβ deposition. We propose that these EC-APP770⁺:App^NL-F/NL-F mice may be useful to study the basic molecular mechanisms behind the possible breakdown of the blood-brain barrier upon administration of anti-Aβ antibodies.

Non-specific Detection of a Major Western Blotting Band in Human Brain Homogenates by a Multitude of Amyloid Precursor Protein Antibodies

The use of human post-mortem brain material is of great value when investigating which pathological mechanisms occur in human brain, and to avoid translational problems which have for example been evident when translating animal research into Alzheimer disease (AD) clinical trials. The amyloid β (Aβ)-peptide, its amyloid precursor protein (APP) and the intermediate APP-c-terminal fragments (APP-CTFs) are all important players in AD pathogenesis. In order to elucidate which APP CTF that are the most common in brain tissue of different species and developmental stages, and whether there are any differences in these fragments between AD and control brain, we investigated the occurrence of these fragments using different APP c-terminal antibodies. We noticed that whereas the conventional APP-CTFα and CTFβ fragments were most prominent in rat and mouse brain tissue, the major western blotting band detected in human, macaque and guinea pig was of approximately 20 kDa in size, possibly corresponding to the newly discovered APP-CTFη. However, this band was also intensely stained with a total protein stain, as well as by several other antibodies. The staining intensity of the 20 kDa band by the APP antibodies varied considerably between samples and correlated with the staining intensity of this band by the total protein stain. This could potentially be due to non-specific binding of the antibodies to another protein of this size. In-gel digestion and mass spectrometry confirmed that small amounts of APP were present in this band, but many other proteins were identified as well. The major hit of the mass spectrometry analysis was myelin basic protein (MBP) and a myelin removal protocol removed proportionally more of the 20 kDa APP band than the full-length APP and APP-CTFα/β bands. However, the signal could not be immunodepleted with an MBP antibody. In summary, we report on a potentially non-specific western blotting band of approximately 20 kDa and call for precaution when analyzing proteins of this size in human brain tissue.

α-Synuclein pre-formed fibrils impair tight junction protein expression without affecting cerebral endothelial cell function

Recently it has been shown that there is impaired cerebral endothelial function in many chronic neurodegenerative disorders including Alzheimer's and Huntington's disease. Such problems have also been reported in Parkinson's disease, in which α-synuclein aggregation is the pathological hallmark. However, little is known about the relationship between misfolded α-synuclein and endothelial function. In the present study, we therefore examined whether α-synuclein preformed fibrils affect endothelial function in vitro. Using a well-established endothelial cell model, we found that the expression of tight junction proteins, in particular zona occludens-1 and occludin, was significantly perturbed in the presence of fibril-seeded neurotoxicity. Disrupted expression of these proteins was also found in the postmortem brains of patients dying with Parkinson's disease. There was though little evidence in vitro of functional impairments in endothelial cell function in terms of transendothelial electrical resistance and permeability. This study therefore shows for the first time that misfolded α-synuclein can interact and affect the cerebral endothelial system, although its relevance to the pathogenesis of Parkinson's disease remains to be elucidated.

Insights into the oxygen-based ligand of the low pH component of the Cu(2+)-amyloid-β complex

In spite of significant experimental effort dedicated to the study of Cu(2+) binding to the amyloid beta (Aβ) peptide, involved in Alzheimer's disease, the nature of the oxygen-based ligand in the low pH component of the Cu(2+)-Aβ(1-16) complex is still under debate. This study reports density-functional-theory-based calculations that explore the potential energy surface of Cu(2+) complexes including N and O ligands at the N-terminus of the Aβ peptide, with a focus on evaluating the role of Asp1 carboxylate in copper coordination. Model conformers including 3, 6, and 17 amino acids have been used to systematically study several aspects of the Cu(2+)-coordination such as the Asp1 side chain conformation, local peptide backbone geometry, electrostatic and/or hydrogen bond interactions, and number and availability of Cu(2+) ligands. Our results show that the Asp1 peptide carbonyl binds to Cu(2+) only if the coordination number is less than four. In contrast, if four ligands are available, the most stable structures include the Asp1 carboxylate in equatorial position instead of the Asp1 carbonyl group. The two lowest energy Cu(2+)-Aβ(1-17) models involve Asp1 COO(-), the N-terminus, and His6 and His14 as equatorial ligands, with either a carbonyl or a water molecule in the axial position. These models are in good agreement with experimental data reported for component I of the Cu(2+)-Aβ(1-16) complex, including EXAFS- and X-ray-derived Cu(2+)-ligand distances, Cu(2+) EPR parameters, and (14)N and (13)C superhyperfine couplings. Our results suggest that at low pH, Cu(2+)-Aβ species with Asp1 carboxylate equatorial coordination coexist with species coordinating the Asp1 carbonyl. Understanding the bonding mechanism in these species is relevant to gain a deeper insight on the molecular processes involving copper-amyloid-β complexes, such as aggregation and redox activity.

Enhanced generation of Alzheimer's amyloid-beta following chronic exposure to phorbol ester correlates with differential effects on alpha and epsilon isozymes of protein kinase C

Alzheimer's amyloid precursor protein (APP) sorting and processing are modulated through signal transduction mechanisms regulated by protein phosphorylation. Notably, protein kinase C (PKC) appears to be an important component in signaling pathways that control APP metabolism. PKCs exist in at least 11 conventional and unconventional isoforms, and PKCalpha and PKCepsilon isoforms have been specifically implicated in controlling the generation of soluble APP and amyloid-beta (Abeta) fragments of APP, although identification of the PKC substrate phospho-state-sensitive effector proteins remains challenging. In the current study, we present evidence that chronic application of phorbol esters to cultured cells in serum-free medium is associated with several phenomena, namely: (i) PKCalpha down-regulation; (ii) PKCepsilon up-regulation; (iii) accumulation of APP and/or APP carboxyl-terminal fragments in the trans Golgi network; (iv) disappearance of fluorescence from cytoplasmic vesicles bearing a green fluorescent protein tagged form of APP; (v) insensitivity of soluble APP release following acute additional phorbol application; and (vi) elevated cellular APP mRNA levels and holoprotein, and secreted Abeta. These data indicate that, unlike acute phorbol ester application, which is accompanied by lowered Abeta generation, chronic phorbol ester treatment causes differential regulation of PKC isozymes and increased Abeta generation. These data have implications for the design of amyloid-lowering strategies based on modulating PKC activity.

Selective apoptosis induction in the hippocampal mossy fiber pathway by exposure to CT105, the C-terminal fragment of Alzheimer's amyloid precursor protein

Beta-amyloid protein (Abeta), a proteolytic byproduct of Alzheimer's amyloid precursor protein (APP), has been shown to play a central role in the development of Alzheimer's disease (AD). In addition, recent studies strongly suggest that other byproducts of proteolysis, such as C-terminal fragments of APP (APP-CTF), are also critically involved in the AD pathology. To explore this possibility, we investigated the histopathological changes induced by repeated low-dose intrahippocampal injection of a recombinant 105 amino acid C-terminal fragment of APP (CT105). First, we carried out a behavioral analysis by using the three-panel runway task, and found that the working memory was significantly impaired by CT105 exposure. Then, via propidium iodide staining, we encountered a number of cells exhibiting fragmented or shrank nuclei in the mossy fiber pathway (stratum lucidum and dentate hilus) in CT105-treated rats. These cells were positive for single-stranded DNA (ssDNA), an apoptosis-specific marker, and thus were considered to be apoptotic. Some of the ssDNA-positive cells were also positive for somatostatin. But neither ionized calcium-binding adapter molecule 1 (Iba1) nor S100beta occurred in ssDNA-positive cells. These findings suggest that CT105 induces apoptotic changes in cells of neuronal origin. Quantitative analysis showed that the densities of ssDNA-positive cells in the mossy fiber pathway were significantly higher in CT105-treated rats than in control animals. The present results suggest that CT105 causes dysfunction in the hippocampal mossy fiber system, and also provide some key to understand the relationship between APP-CTF and glutamatergic synaptic dysregulation in AD.

Cerebral vascular accumulation of Dutch-type Abeta42, but not wild-type Abeta42, in hereditary cerebral hemorrhage with amyloidosis, Dutch type

Hereditary cerebral hemorrhage with amyloidosis, Dutch type (HCHWA-D), is an autosomal dominant disorder caused by the Dutch mutation (E693Q) in the beta-amyloid precursor protein. This mutation produces an aberrant amyloid beta (Abeta) species (AbetaE22Q) and causes severe meningocortical vascular Abeta deposition. We analyzed the Abeta composition of the vascular amyloid in the brains of HCHWA-D patients. Immunohistochemistry demonstrated that the vascular amyloid contained both Abeta40 and Abeta42, with a high Abeta40/Abeta42 ratio. In Western blotting of cerebral microvessel fractions isolated from the brains, both wild-type and Dutch-type Abeta40 were observed as major species. Reverse-phase HPLC-mass spectrometric analysis of the fractions revealed both wild-type and Dutch-type Abeta38 as the other main components of the vascular amyloid. Moreover, we detected peaks corresponding to Dutch-type Abeta42 but not to wild-type Abeta42. These results suggest a pathogenic role for the mutant Abeta42 in addition to the mutant Abeta40 in the cerebral amyloid angiopathy of HCHWA-D.

Pro-inflammatory conditions promote neuronal damage mediated by Amyloid Precursor Protein and decrease its phagocytosis and degradation by microglial cells in culture

Aberrant handling of Amyloid Precursor Protein (APP) and beta-amyloid (Abeta), glial activation and inflammation are key events in Alzheimer's disease. We set out to determine the role of inflammation on microglial reactivity against APP. We studied microglia-mediated neurotoxicity, uptake and degradation of a biotinylated APP construct (biotin-APP-C-244). APP, in contrast to Abeta, only induced mild activation of glial cells. However, under pro-inflammatory conditions, APP induced microglial-mediated cytotoxicity. Biotin-APP-C-244 or lipopolysaccharide and interferon-gamma (LPS+IFNgamma), administered separately, did not change reduction metabolism of microglia. However, biotin-APP-C-244+(LPS+IFNgamma) increased microglial reactivity and decreased reduction metabolism by 75% (P<0.001). Biotin-APP-C-244 was readily taken up by microglial cells; 80% was phagocytosed at 2 h. In the presence of LPS+IFNgamma, phagocytosis of biotin-APP-C-244 was reduced at 2 h; and cell damage was evident after 4 h. Our results support our hypothesis that, in neuroinflammation, microglial scavenger function is impaired and reactivity against APP enhanced as an initial step for neurodegeneration.

C31 enhances voltage-gated calcium channel currents in undifferentiated PC12 cells

C31, consisting of 31 amino acid residues, is generated from the carboxyl terminal fragments (CTFs) of amyloid precursor protein (APP). It has been shown that C31 causes apoptosis in neurons and is present in brains of Alzheimer disease (AD) patients. Using whole-cell patch clamp techniques, we investigated effects of C31 on voltage-gated calcium channel (VGCC) currents and the protective effects of beta-estradiol on PC12 cells. The results demonstrated that C31 induced a significant increase of the VGCC currents in PC12 cells, which was blocked by beta-estradiol. These results suggest that modulation of intracellular calcium levels by VGCC may in part be involved in C31 induced neuronal death associated with AD.

Pathophysiological roles of amyloidogenic carboxy-terminal fragments of the beta-amyloid precursor protein in Alzheimer's disease

Several lines of evidence suggest that some of the neurotoxicity in Alzheimer's disease (AD) is attributed to proteolytic fragments of amyloid precursor protein (APP) and beta-amyloid (Abeta) may not be the sole active component involved in the pathogenesis of AD. The potential effects of other cleavage products of APP need to be explored. The CTFs, carboxy-terminal fragments of APP, have been found in AD patients' brain and reported to exhibit much higher neurotoxicity in a variety of preparations than Abeta. Furthermore CTFs are known to impair calcium homeostasis and learning and memory through blocking LTP, triggering a strong inflammatory reaction through MAPKs- and NF-kappaB-dependent astrocytosis and iNOS induction. Recently, it was reported that CTF translocated into the nucleus, binding with Fe65 and CP2, and in turn, affected transcription of genes including glycogen synthase kinase-3beta, which results in the induction of tau-rich neurofibrillary tangles and subsequently cell death. Spatial memory of transgenic (Tg) mice overexpressing CT100 was significantly impaired and CTFs were detected in the neurons as well as in plaques of the Tg mice and double Tg mice carrying CT100 and mutant tau. In this review, we summarize observations indicating that both CTF and Abeta may participate in the neuronal degeneration in the progress of AD by differential mechanisms.

Extracellular Deposits of Aβ Produced in Cultures of Alzheimer Disease Brain Vascular Smooth Muscle Cells

Alzheimer disease (AD) and Down syndrome (DS) brains contain deposits of amyloid-beta peptide that are located extracellularly in the neuropil and in blood vessels walls. A small fraction of brain Abeta is detected intracellularly in neurons, smooth muscle cells, and microglia. The roles of these extracellular and intracellular pools of Abeta in pathogenesis of AD-type dementia are controversial. Cell culture models of vascular amyloidosis-beta revealed intracellular, but not extracellular deposition of Abeta. Here we demonstrate for the first time, formation of extracellular deposits of Abeta in primary cultures of vascular smooth muscle cells isolated from AD cases with cerebrovascular amyloid angiopathy. Extracellular Abeta deposition required the use of cultures that produced high quantities of Abeta, which contained at least 50% of cells forming intracellular Abeta deposits, and providing extracellular matrix proteins. During 12 days of culture in this system, we observed accumulation of nonfibrillar, granular deposits in extracellular matrix, similar to early stages of vascular amyloidogenesis in vivo. This is a valuable system to study the effects of various potential amyloidogenic factors on formation of extracellular Abeta deposits.

Molecular mechanisms underlying cyclic AMP inhibition of macrophage dependent TNF-alpha production and neurotoxicity in response to amyloidogenic C-terminal fragment of Alzheimer's amyloid precursor protein

In the present study, we characterized the intracellular pathway involved in the macrophage production of tumor necrosis factor-alpha (TNF-alpha) and the molecular mechanisms by which cyclic AMP (cAMP) regulates the neurotoxic inflammatory signaling cascade in response to the 105 amino acid carboxyl-terminal fragment (CT105) of amyloid precursor protein, a candidate of alternative toxic elements in Alzheimer's disease (AD) pathology. CT105 in combination with interferon-gamma (IFN-gamma) elicited a robust and sustained increase of TNF-alpha production due to enhanced TNF-alpha mRNA transcription, mediated via increased nuclear factor-kappaB (NF-kappaB) in human macrophages derived from monocytic THP-1 cells. A mechanistic analysis revealed that the cAMP analog, dibutyryl cyclic AMP (dbcAMP), or the adenyl cyclase activator, forskolin, effectively suppressed the stimulant-induced TNF-alpha production by reducing the nuclear translocation and DNA binding activity of NF-kappaB. The inhibitory mechanisms manifested by dbcAMP included the decreased phosphorylation/degradation of NF-kappaB inhibitor (IkappaB) followed by its increased synthesis/stability. Importantly, this macrophage derived TNF-alpha appears to be a key pathological mediator of the resultant neurotoxicity, which was attenuated by increased cAMP levels during macrophage stimulation with CT105. These findings provide evidence, which supports an important role of CT105 as a potent macrophage stimulator eliciting NF-kappaB-mediated inflammatory signals for excess TNF-alpha production, which in turn ultimately leads to the neurotoxicity. In addition, the detailed inhibitory mechanism of cAMP action implies that an increased cAMP level could be benefit against AD progression.

Effects of intrahippocampal CT105, a carboxyl terminal fragment of beta-amyloid precursor protein, alone/with inflammatory cytokines on working memory in rats

In this study, we examined the effects of a 105 amino acid carboxyl terminal fragment of beta-amyloid precursor protein (CT105) and inflammatory cytokines on working memory in rats, by using a three-panel runway set-up. CT105 at 10 nmol/side significantly impaired working memory when it was administered bilaterally into the hippocampus. Furthermore, to elucidate the interaction of CT105 with inflammatory cytokines, we co-administered tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) in combination with CT105. Concurrent injections of CT105 (1.0 nmol/side) and TNF-alpha (100 ng/side) produced a synergistic deficit of working memory, whereas IL-1beta (100 ng/side) combined with CT105 (1.0 nmol/side) did not affect the working memory performance. These results indicate that the CT105-induced impairment of working memory is strongly aggravated by an increase in the level of the inflammatory cytokine TNF-alpha, which may occur in the brains of patients with Alzheimer's disease.

Carboxyl-terminal peptide of beta-amyloid precursor protein blocks inositol 1,4,5-trisphosphate-sensitive Ca2+ release in Xenopus laevis oocytes

The effects of Alzheimer's disease-related amyloidogenic peptides on inositol 1,4,5-trisphosphate receptor-mediated Ca(2+) mobilization were examined in Xenopus laevis oocytes. Intracellular Ca(2+) was monitored by electrophysiological measurement of the endogenous Ca(2+)-activated Cl(-) current. Application of a hyperpolarizing pulse released intracellular Ca(2+) in oocytes primed by pre-injection of a non-metabolizable inositol 1,4,5-trisphosphate analogue. The carboxyl terminus of the amyloid precursor protein inhibited inositol 1,4,5-trisphosphate receptor-mediated intracellular Ca(2+) release in a dose-dependent manner. Equimolar beta-amyloid peptides Abeta(1-40) or Abeta(1-42) had no effect, and whereas a truncated carboxyl terminus lacking the Abeta domain was equipotent to the full-length one, a carboxyl terminus fragment lacking the NPTY sequence was less effective than the full-length fragment. The inhibition induced by the carboxyl terminus was not associated with the block of the Ca(2+)-dependent Cl(-) channel itself or compromised Ca(2+) influx. We conclude that the carboxyl terminus of the amyloid precursor protein inhibits inositol 1,4,5-trisphosphate-sensitive Ca(2+) release and could thus disrupt Ca(2+) homeostasis and that the carboxyl terminus is much more effective than the beta-amyloid fragments used. By perturbing the coupling of inositol 1,4,5-trisphosphate and Ca(2+) release, the carboxyl terminus of the amyloid precursor protein can potentially be involved in inducing the neural toxicity characteristic of Alzheimer's disease.

Accumulation of the amyloid-beta precursor protein in multivesicular body-like organelles

It has been suggested that the successive proteolytic events leading to the production of the amyloid-beta protein from its precursor may take place at different intracellular locations. Using cultured human leptomeningeal smooth muscle cells and brain pericytes, we modulated the intracellular localization of the amyloid-beta precursor protein (APP) to study possible effects on its processing. By using immunofluorescence and immunoelectron microscopy we demonstrated that, under normal conditions, the APP is found in small intracellular vesicles, some of which were characterized as lysosomes. Both the cytokine interferon-gamma and the lysosomotropic drug chloroquine, but not the cytokines interleukin (IL)-1, IL-6, or tumor necrosis factor-alpha (TNF-alpha), induced an accumulation of APP in newly formed multivesicular body-like organelles. The secretion of the amyloid-beta precursor protein was slightly reduced by interferon-gamma or chloroquine. Double-labeling and tracer molecule uptake experiments showed that the multivesicular body-like organelles were part of the endocytic pathway. Our findings suggest that the multivesicular body-like organelles function as an intermediate organelle in the intracellular trafficking of the APP. Accumulation of the APP in this organelle is reflected by its reduced secretion from the cell.

Beta-amyloid deposition in the brains of rats chronically infused with thiorphan or lipopolysaccharide: the role of ascorbic acid in the vehicle

Beta-amyloid deposition and neuroinflammation are two important features of Alzheimer's disease (AD) that may influence its progression. Chronic infusion of lipopolysaccharide (LPS) into the rodent 4th ventricle reproduces many of the neurobiological changes seen in AD. Chronic infusion of ascorbic acid containing thiorphan, an inhibitor of the enzyme neprilysin that catabolizes beta-amyloid, into the hippocampus induces extracellular deposition of beta-amyloid fibrils. We investigated whether the combined presence of chronic neuroinflammation could exacerbate the deposition of beta-amyloid induced by thiorphan. The infusion of any solution containing ascorbic acid alone or with thiorphan or LPS increased the level of intraneuronal beta-amyloid immunoreactivity. Solutions that did not contain ascorbic acid were not associated with increased intraneuronal beta-amyloid immunoreactivity. The role of neprilysin in the deposition of beta-amyloid in AD brains remains undetermined.

Effects of the beta-amyloid and carboxyl-terminal fragment of Alzheimer's amyloid precursor protein on the production of the tumor necrosis factor-alpha and matrix metalloproteinase-9 by human monocytic THP-1

To explore the direct role of beta-amyloid (Abeta) and carboxyl-terminal fragments of amyloid precursor protein in the inflammatory processes possibly linked to neurodegeneration associated with Alzheimer's disease, the effects of the 105-amino acid carboxyl-terminal fragment (CT(105)) of amyloid precursor protein on the production of tumor necrosis factor-alpha (TNF-alpha) and matrix metalloproteinase-9 (MMP-9) were examined in a human monocytic THP-1 cell line and compared with that of Abeta. CT(105) elicited a marked increase in TNF-alpha and MMP-9 production in the presence of interferon-gamma in a dose- and time-dependent manner. Similar patterns were obtained with Abeta despite its low magnitude of induction. Autocrine TNF-alpha is likely to be a main mediator of the induction of MMP-9 because the neutralizing antibody to TNF-alpha inhibits MMP-9 production. Genistein, a specific inhibitor of tyrosine kinase, dramatically diminished both TNF-alpha secretion and subsequent MMP-9 release in response to CT(105) or Abeta. Furthermore, PD98059 and SB202190, specific inhibitors of ERK or p38 MAPK respectively, efficiently suppressed CT(105)-induced effects whereas only PD98059 was effective at reducing Abeta-induced effects. Our results suggest that CT(105) in combination with interferon-gamma might serve as a more potent activator than Abeta in triggering inflammatory processes and that both tyrosine kinase and MAPK signaling pathways may represent potential therapeutic targets for the control of Alzheimer's disease progression.

Effects of nicotine on APP secretion and Abeta- or CT(105)-induced toxicity

Several lines of evidence indicated that overexpression or aberrant processing of amyloid precursor protein (APP) is causally related to Alzheimer's disease (AD). Amyloid precursor protein is principally cleaved within the amyloid beta protein domain to release a large soluble ectodomain (APPs), known to have a wide range of trophic functions. The central hypothesis guiding this review is that nicotine may play an important role in APP secretion and protection against toxicity induced by APP metabolic fragments (beta-amyloid [Abeta], carboxyl terminal [CT]). Findings from our experiments have shown that nicotine enhances the release of APPs, which has neurotrophic and neuroprotective activities in concentration-dependent (>50 micromol/L) and time-dependent (>2 hours) manners. In addition, pretreatment of nicotine (>10 micromol/L for 24 hours) partially prevented Abeta or CT(105)-induced cytotoxicity in primary cultured neuron cells, and the effects of nicotine-induced protection were inhibited by the pretreatment with a nicotine alpha-bungarotoxin. Nicotine (>10 micromol/L for 24 hours) partially inhibited CT(105)-induced cytotoxicity when PC12 cells was transfected with CT(105). From these results, we proposed that nicotine or nicotinic receptor agonist treatment might improve the cognitive functions not only by supplementation of cholinergic neurotransmission, but also by protecting Abeta- or CT(105)-induced neurotoxicity probably through the increased release of APPs and the activation of nicotinic receptors.

Beta amyloid fragments derived from activated platelets deposit in cerebrovascular endothelium: usage of a novel blood brain barrier endothelial cell model system

Amyloid precursor protein (A betaPP) processing results in generation of amyloid beta peptide (A beta) which deposits in the brain parenchyma and cerebrovasculature of patients with Alzheimer's disease (AD). Evidence that the vascular deposits derive in part from A betaPP fragments originating from activated platelets includes findings that individuals who have had multiple small strokes have a higher prevalence of AD compared to individuals who have taken anti-platelet drugs. Thus, determination of whether platelet A betaPP fragments are capable of traversing the blood-brain barrier (BBB) is critical. We have established that activated platelets from patients with AD retain more surface transmembrane-bound A betaPP (mA betaPP) than control platelets. We report here that this mA betaPP can be cleaved to A beta-containing fragments which pass through a novel BBB model system. This model utilizes human BBB endothelial cells (BEC) isolated from brains of patients with AD. These BEC, after exposure to activated platelets which have been surface-labeled with fluorescein and express surface-retained mA betaPP, cleave fluorescein-tagged surface proteins, including mA betaPP, resulting in passage to the BEC layer The data confirm that BEC contribute to processing of platelet-derived mA betaPP and show that the processing yields A beta containing fragments which could potentially contribute to cerebrovascular A beta deposition.

Cerebral amyloid angiopathy: an overview

Cerebral amyloid angiopathy (CAA) is characterized by amyloid deposition in cortical and leptomeningeal vessels. Several cerebrovascular amyloid proteins (amyloid beta-protein (Abeta), cystatin C (ACys), prion protein (AScr), transthyretin (ATTR), gelsolin (AGel), and ABri (or A-WD)) have been identified, leading to the classification of several types of CAA. Sporadic CAA of Abeta type is commonly found in elderly individuals and patients with Alzheimer's disease. Cerebral amyloid angiopathy is an important cause of cerebrovascular disorders including lobar cerebral hemorrhage, leukoencephalopathy, and small cortical hemorrhage and infarction. We review the clinicopathological and molecular aspects of CAA and discuss the pathogenesis of CAA with future perspectives.

Roles of A beta and carboxyl terminal peptide fragments of amyloid precursor protein in Alzheimer disease

Several lines of evidence indicate that A beta may play an important role in the pathogenesis of AD. However, there are several discrepancies between the production of A beta and the development of the disease. Thus, A beta may not be the sole active fragment of beta-amyloid precursor protein (betaAPP) in the neurotoxicity assiciated with AD. We focused on the amyloidegenic carboxyl terminal fragments of betaAPP containing the full length of A beta (CT105). We synthesized a recombinant carboxyl-terminal 105 amino acid fragment of betaAPP and examined the effects of CT105 and A beta on cultured neurons, Ca++ uptake into rat brain microsomes, Na+-Ca++ exchange activity, ion channel forming activity in lipid bilayers and passive avoidance performance of mice. Our results suggest that the cytotoxic and channel inducing effects of CT105 are much more potent than that of A beta and toxic mechanisms of CT105 are different from those of A beta. Taken together, these lines of evidence postulate that CT is an alternative toxic element important in the generation of the symptoms common to AD.

Agrin is a major heparan sulfate proteoglycan accumulating in Alzheimer's disease brain

Heparan sulfate proteoglycans (HSPGs) have been suggested to play an important role in the formation and persistence of senile plaques and neurofibrillary tangles in dementia of the Alzheimer's type (DAT). We performed a comparative immunohistochemical analysis of the expression of the HSPGs agrin, perlecan, glypican-1, and syndecans 1-3 in the lesions of DAT brain neocortex and hippocampus. Using a panel of specific antibodies directed against the protein backbone of the various HSPG species and against the glycosaminoglycan (GAG) side-chains, we demonstrated the following. The basement membrane-associated HSPG, agrin, is widely expressed in senile plaques, neurofibrillary tangles and cerebral blood vessels, whereas the expression of the other basement membrane-associated HSPG, perlecan, is lacking in senile plaques and neurofibrillary tangles and is restricted to the cerebral vasculature. Glypican and three different syndecans, all cell membrane-associated HSPG species, are also expressed in senile plaques and neurofibrillary tangles, albeit at a lower frequency than agrin. Heparan sulfate GAG side chains are also associated with both senile plaques and neurofibrillary tangles. Our results suggest that glycosaminoglycan side chains of the HSPGs agrin, syndecan, and glypican, but not perlecan, may play an important role in the formation of both senile plaques and neurofibrillary tangles. In addition, we speculate that agrin, because it contains nine protease-inhibiting domains, may protect the protein aggregates in senile plaques and neurofibrillary tangles against extracellular proteolytic degradation, leading to the persistence of these deposits.

Effects of the amyloid precursor protein Glu693-->Gln 'Dutch' mutation on the production and stability of amyloid beta-protein

Hereditary cerebral haemorrhage with amyloidosis, Dutch type (HCHWA-D), is a cerebral amyloidosis characterized by prominent vascular deposits and fatal haemorrhages. The disorder is caused by a point mutation in codon 693 of the gene encoding the amyloid precursor protein (APP), resulting in a Glu-->Gln amino acid substitution at position 22 of the amyloid beta-protein (Abeta) region. The pathogenetic mechanisms of HCHWA-D are unknown but could involve alterations in the proteolytic processing of APP and in amyloid fibril formation. We examined Abeta production and stability by using cultured human embryonic kidney 293 cells stably expressing wild-type or 'Dutch' APP. Radiosequencing and quantitative immunoprecipitation experiments showed that cells expressing Dutch APP secreted increased quantities of Abeta peptides beginning at Asp1, and of truncated peptides beginning at Val18 and Phe19. The ratio of levels of 4 kDa (Abeta) to 3 kDa (p3) peptides remained constant due to co-ordinate decreases in other peptide species. Novel truncated or elongated peptides were not observed. Pulse-chase experiments showed that the Dutch mutation did not affect the stability of the Abeta or p3 populations. These results are consistent with a disease process in which the Dutch mutation results in the production of Abeta peptides with enhanced propensities for fibrillogenesis, leading to accelerated vascular deposition and disease.

Biochemical assay for amyloid beta deposits to distinguish Alzheimer's disease from other dementias

Biochemical markers for Alzheimer's disease (AD) are of great value for precise diagnosis and in studies of the pathogenetic processes of this disease. A new biochemical assay allowing to differentiate AD from other forms of dementia is described. The assay is based on the extraction of amyloid beta (A beta) from milligram amounts of brain tissue by using 20% acetonitrile in 0.1% trifluoroacetic acid and its detection by Western blotting. The presence of the 4 kDa A beta was demonstrated in all cases of AD (n = 8) that were diagnosed by the independent histopathological examination of the postmortem tissues. No A beta was found in tissue extracts from seven out of eight cases of other forms of dementia. In contrast to other biochemical techniques of A beta detection in brain, the developed assay is simple; it does not require any special equipment and allows detection of A beta using milligram amounts of brain tissue.

Blood brain barrier endothelial cells express candidate amyloid precursor protein-cleaving secretases

Proteolytic cleavage of the amyloid precursor protein (A beta PP) results in the generation of the amyloidogenic fragment known as amyloid beta peptide (A beta). Deposition of A beta in the brain parenchyma and cerebrovasculature is a feature of Alzheimer's disease (AD). To date, the process whereby A beta is generated and deposited remains unclear. We have previously established that activated platelets from AD patients retain more A beta PP on their surface than control platelets. We report here that an endothelial cell-derived enzyme can cleave this surface platelet A beta PP. Human blood brain barrier endothelial cells from brains of AD patients were assayed for potential A beta PP-cleaving enzymes using synthetic peptide substrates encompassing the A beta N-terminus cleavage site. A protease activity capable of cleaving A beta PP on the surface of AD platelets was noted. The A beta PP cleavage is partially inhibited by EDTA, by ZincOV, as well as by a specific inhibitor of the Zn metalloprotease E.C.3.4.24.15. Furthermore, the protease is recognized by an antibody directed against it, using immunohistochemistry, Western blot analysis and flow cytometry. The protease is not secreted, but rather resides intracellularly as well as on the surface of the endothelial cells. The data suggest that E.C.3.4.24.15 synthesized by brain endothelial cells may process the platelet-derived A beta PP, yielding fragments which could contribute to cerebrovascular A beta deposits.

Metabotropic glutamate receptor activation in vivo induces intraneuronal amyloid immunoreactivity in guinea pig hippocampus

Stimulation of metabotropic glutamate receptors in vitro has been shown to accelerate the breakdown of amyloid precursor protein (APP) to form increased production of non-amyloidogenic secreted APP (sAPP). The mechanism whereby this occurs is not entirely clear but it is presumed to be linked to generation of diacylglycerol and activation of protein kinase C because other neurotransmitter receptors such as m1 and m3 muscarinic receptors, known to be coupled to this second messenger cascade, likewise increase sAPP production. Although it is presumed that a reciprocal relationship exists between the formation of amyloid beta protein (Abeta) and the production of sAPP, recent evidence suggests alternative processing can occur. Given the fact that much of the observations on APP metabolism have been made in vitro we sought to investigate the effect of metabotropic receptor activation on Abeta in vivo in a species known to contain the same amino acid sequence of Abeta as found in humans. Intrahippocampal injection of the mGluR agonist 1S,3R-ACPD in guinea pigs produced neurodegeneration of CA1 hippocampal pyramidal neurons at 12 h postinjection. Immunocytochemistry of sections from ACPD injected animals using selective antibodies to Abeta revealed the presence of punctate intraneuronal granules in pyramidal neurons of the hippocampus. These structures appeared to be localized within the nucleus and were particularly prominent in neurons within the region of neurodegeneration. Immunoreactivity was not observed in vehicle injected controls nor in sections from ACPD injected animals stained with preadsorbed antiserum. Abeta immunodetection was correlated with the onset of neurodegeneration since animals evaluated at 1 h and 4 h postinjection lacked both Abeta immunoreactivity as well as neurodegeneration. Evaluation of animals injected with NMDA revealed neurodegeneration but no Abeta immunoreactivity suggesting Abeta formation did not appear to be due to non-selective excitotoxicity. Staining of sections with antibodies directed to various regions of APP demonstrated increased C-terminal APP immunoreactivity in pyramidal neurons in the vicinity of degeneration. These data support recent in vitro studies illustrating that Abeta can be found intracellularly within neurons.

Evidence for presenilin-1 involvement in amyloid angiopathy in the Alzheimer's disease-affected brain

Presenilin-1 (PS-1) has been identified as the protein encoded by the chromosome 14 locus that, when mutated, leads to familial Alzheimer's disease (FAD). The role PS-1 plays in the pathogenesis of Alzheimer's disease (AD) remains unclear. Using a set of antibodies raised against PS-1 synthetic peptides, polyclonal antibody to amyloid beta protein (Abeta) and end-specific antibodies against Abeta40, and Abeta42, immunohistochemical studies were performed on brain sections obtained from AD cases and controls. The PS-1 antibodies clearly stained amyloid angiopathies in AD-affected brains, but no recognizable immunoreactions were observed in any other vessels free from amyloid involvement in either AD-affected brains or controls. Abeta antibodies and the end-specific antibody against Abeta40 also decorated amyloid angiopathies, showing localization similar to that of PS-1. Western blot analyses predominantly detected protein band polypeptide species of a 50 kDa, band, presumably full-length PS-1 protein with N-terminus antisera, since these antibodies turned out to recognize a 50-kDa full-length band in cell lysate of transfected HeLa cell overexpressing PS-1. In addition, we recognized 30, 27 and 25 kDa proteins in both AD and control brain homogenate with these antibodies. In microvessel fractions extracted from brain homogenates, the 50, and 27 kDa fragments were observed in AD-affected brains but not in those of controls. C-terminus rabbit antisera reacted strongly with the 33 and 27 kDa bands, and additionally detected a small amount of full-length PS-1 protein in extracts from AD and control brains. Our present data indicate that PS-1 might be involved in the pathogenesis of amyloid angiopathy in the AD brain.

Characterization of endogenous APP processing in a cell-free system

We have developed a simple in vitro assay using tissue homogenates that allows detection and characterization of several endogenous proteolytic activities which convert Alzheimer's amyloid precursor protein (APP) to the smaller, carboxy-terminal fragments, postulated to be intermediates in the formation of β-amyloid peptide (Aβ). Incubation at 37°C results in the degradation of transmembrane APP and formation of a mixture of carboxy-terminal containing peptides with mass values of 9-12 kDa. Epitope mapping and electrophoretic comparison with a truncated APP standard showed one of these peptides to contain the entire Aβ sequence. Analysis of pH dependence shows that formation of this carboxy-terminal product as well as another fragment, that is the likely product of 'secretase' activity, requires acidic pH. This suggests that cleavage of full-length APP to secreted forms may take place in an acidic intracellular compartment.

Preparation and purification of antisera against different regions or isoforms of beta-amyloid precursor protein

We describe a procedure for the production and peptide affinity purification of polyclonal antisera against synthetic peptides representing different domains of beta-amyloid precursor protein (APP). Rabbits were immunised with keyhole limpet haemocyanin coupled to synthetic peptides representing the amino-terminal APP18-32, Kunitz-type protease inhibitor (KPI) region APP301-316, the A beta region APP670-686, and the carboxy-terminal APP756-770 of APP770 for the production of antisera anti-AP-1, anti-AP-2, anti-AP-4 and anti-AP-5, respectively. Each antiserum was purified to specific antibody using the respective cognate peptides immobilised on affinity columns as ligand, using the 1-ethyl-3-(dimethylaminopropyl)carbodiimide-diaminodipropylamine method. Purified antibodies of these four antisera were highly specific and in enzyme-linked immunosorbent assays (ELISA) reacted only to the corresponding peptide. These purified antisera have been used in Western blot, immunohistochemical and immunoprecipitation techniques to facilitate the understanding of the regulation of APP and amyloid beta-protein (A beta). The A beta is formed by proteolysis of APP, and its deposition leading to the formation of senile plaques in the brain is considered to be a key step in the pathogenesis of Alzheimer's disease.

Butyrylcholinesterase in the life cycle of amyloid plaques

Deposits of diffuse beta-amyloid (Abeta) may exist in the brain for many years before leading to neuritic degeneration and dementia. The factors that contribute to the putative transformation of the Abeta amyloid from a relatively inert to a pathogenic state remain unknown and may involve interactions with additional plaque constituents. Matching brain sections from 2 demented and 4 nondemented subjects were processed for the demonstration of Abeta immunoreactivity, butyrylcholinesterase (BChE) enzyme activity, and thioflavine S binding. Additional sections were processed for the concurrent demonstration of two or three of these markers. A comparative analysis of multiple cytoarchitectonic areas processed with each of these markers indicated that Abeta plaque deposits are likely to undergo three stages of maturation, ie, a "diffuse" thioflavine S-negative stage, a thioflavine S-positive (ie, compact) but nonneuritic stage, and a compact neuritic stage. A multiregional analysis showed that BChE-positive plaques were not found in cytoarchitectonic areas or cortical layers that contained only the thioflavine S-negative, diffuse type of Abeta plaques. The BChE-positive plaques were found only in areas containing thioflavine S-positive compact plaques, both neuritic and nonneuritic. Within such areas, almost all (>98%) BChE-containing plaques bound thioflavine S, and almost all (93%) thioflavine S plaques contained BChE. These results suggest that BChE becomes associated with amyloid plaques at approximately the same time that the Abeta deposit assumes a compact beta-pleated conformation. BChE may therefore participate in the transformation of Abeta from an initially benign form to an eventually malignant form associated with neuritic tissue degeneration and clinical dementia.

Abeta25-35 induces rapid lysis of red blood cells: contrast with Abeta1-42 and examination of underlying mechanisms

Amyloid beta-peptide (A beta) is produced by many different cell types and circulates in blood and cerebrospinal fluid in a soluble form. In Alzheimer's disease (AD), A beta forms insoluble fibrillar aggregates that accumulate in association with cells of the brain parenchyma and vasculature. Both full-length A beta (A beta1-40/42) and the A beta25-35 fragment can damage and kill neurons by a mechanism that may involve oxidative stress and disruption of calcium homeostasis. Circulating blood cells are exposed to soluble A beta1-40/42 and may also be exposed to A beta aggregates associated with the luminal surfaces of cerebral microvessels. We therefore examined the effects of A beta25-35 and A beta1-42 on human red blood cells (RBCs) and report that A beta25-35, in contrast to A beta1-42, induces rapid (10-60 min) lysis of RBCs. The mechanism of RBC lysis by A beta25-35 involved ion channel formation and calcium influx, but did not involve oxidative stress because antioxidants did not prevent cell lysis. In contrast, A beta1-42 induced a delayed (4-24 h) damage to RBCs which was attenuated by antioxidants. The damaging effects of both A beta25-35 and A beta1-42 towards RBCs were completely prevented by Congo red indicating a requirement for peptide fibril formation. A beta1-42 induced membrane lipid peroxidation in RBC, and basal levels of lipid peroxidation in RBCs from AD patients were significantly greater than in age-matched controls, suggesting a possible role for A beta1-42 in previously reported alterations in RBCs from AD patients.

Zyme, a novel and potentially amyloidogenic enzyme cDNA isolated from Alzheimer's disease brain

The deposition of the beta amyloid peptide in neuritic plaques and cerebral blood vessels is a hallmark of Alzheimer's disease (AD) pathology. The major component of the amyloid deposit is a 4.2-kDa polypeptide termed amyloid beta-protein of 39-43 residues, which is derived from processing of a larger amyloid precursor protein (APP). It is hypothesized that a chymotrypsin-like enzyme is involved in the processing of APP. We have discovered a new serine protease from the AD brain by polymerase chain reaction amplification of DNA sequences representing active site homologous regions of chymotrypsin-like enzymes. A cDNA clone was identified as one out of one million that encodes Zyme, a serine protease. Messenger RNA encoding Zyme can be detected in some mammalian species but not in mice, rats, or hamster. Zyme is expressed predominantly in brain, kidney, and salivary gland. Zyme mRNA cannot be detected in fetal brain but is seen in adult brain. The Zyme gene maps to chromosome 19q13.3, a region which shows genetic linkage with late onset familial Alzheimer's disease. When Zyme cDNA is co-expressed with the APP cDNA in 293 (human embryonic kidney) cells, amyloidogenic fragments are detected using C-terminal antibody to APP. These co-transfected cells release an abundance of truncated amyloid beta-protein peptide and shows a reduction of residues 17-42 of Abeta (P3) peptide. Zyme is immunolocalized to perivascular cells in monkey cortex and the AD brain. In addition, Zyme is localized to microglial cells in our AD brain sample. The amyloidogenic potential and localization in brain may indicate a role for this protease in amyloid precursor processing and AD.

Cerebrovascular degeneration is related to amyloid-beta protein deposition in Alzheimer's disease

Current evidence is not inconsistent with the suggestion that cerebrovascular functions decline during normal aging with pronounced effects in both sporadic and familial Alzheimer's disease (AD). The primary causes of these changes remain unknown. It is possible that amyloid beta (A beta) protein is involved in the degeneration of both the larger penetrating vessels as well as the cerebral capillaries that represent the blood-brain barrier (BBB). A beta-induced endothelial changes could also alter muscular tone, resulting not only in increased expression of vascular amyloid precursor protein (APP) and production of A beta, but also in oxidative injury. We used immunochemical methods to examine the status of the perfusing cerebral vessels and the microvascular endothelium in relation to deposition of A beta in AD and non-AD aging control subjects. Double-immunostaining with antibodies to vascular markers revealed marked loss of smooth muscle in larger vessels and absence or attenuation of the endothelium in capillary profiles that still appeared to retain their basement membranes. These vascular changes were predominantly restricted to neocortical regions abundant in A beta deposits. Quantitative studies showed that the microvascular abnormalities were correlated to A beta deposition rather than neurofibrillary tangles or neuronal numbers. Our studies suggest that A beta, irrespective of its origin within vascular myocytes or brain parenchyma, is responsible not only for cerebral amyloid angiopathy, but also for the degeneration of the cerebral microvasculature, which may profoundly affect brain perfusion and BBB functions.

Ionic effects of the Alzheimer's disease beta-amyloid precursor protein and its metabolic fragments

Alzheimer's disease is a progressive dementia characterized in part by deposition of proteinaceous plaques in various areas of the brain. The main plaque protein component is beta-amyloid, a metabolic product of the beta-amyloid precursor protein. Substantial evidence has implicated beta-amyloid (and other amyloidogenic fragments of the precursor protein) with the neurodegeneration observed in Alzheimer's disease. Recently, beta-amyloid precursor protein and its amyloidogenic metabolic fragments have been shown to alter cellular ionic activity, either through interaction with existing channels or by de novo channel formation. Such alteration in ionic homeostasis has also been linked with cellular toxicity and might provide a molecular mechanism underlying the neurodegeneration seen in Alzheimer's disease.

Expression of Kunitz protease inhibitor--containing forms of amyloid beta-protein precursor within vascular thrombi

BACKGROUND

The presence of patent neovessels within vascular occlusions in chronic thromboembolic pulmonary hypertension suggests that local mechanisms exist to regulate the coagulation system. This study investigated the expression of a potent inhibitor of Factor IXa and Factor XIa (ie, protease nexin-2/ amyloid beta-protein precursor, A beta PP) in the organized vascular occlusions harvested from patients with this disease.

METHODS AND RESULTS

Immunohistochemical analysis revealed intense immunoreactivity for A beta PP in the single layer of cells that line the neovessels. A positive signal was also detected by in situ hybridization analysis with the use of a 35S-UTP-labeled antisense riboprobe that recognizes the various alternatively spliced mRNA forms of this molecule. To identify the forms of A beta PP produced within the thrombi, total RNA was extracted from the thrombi, reverse transcribed, and subjected to amplification with the use of the polymerase chain reaction (PCR) and primers that flank the region encoding the alternatively spliced 56-amino acid Kunitz-type protease inhibitor (KPI) domain. The major PCR products consisted of 255 bp and 312 bp and corresponded to transcripts encoding this domain (ie, A beta PP751 and A beta PP770). In situ hybridization analysis with the use of a 35S-UTP-labeled antisense riboprobe complementary to the region encoding the KPI domain confirmed the presence of these mRNA species in nucleated cells lining the neovessels.

CONCLUSIONS

The expression of KPI-containing isoforms of A beta PP in thrombus endothelial cells may represent one mechanism utilized in this disease to shift the local hemostatic balance and preserve regional vessel patency.

Inhibition of amyloid beta-protein production in neural cells by the serine protease inhibitor AEBSF

Cerebral deposition of amyloid beta protein (A beta) is an early and critical feature of Alzheimer's disease. A beta production requires the proteolytic release of A beta from the beta-amyloid precursor protein (beta APP). Thus, inhibition of A beta release is a prime therapeutic goal. Here, we show that the broad spectrum, irreversible serine protease inhibitor, AEBSF, inhibits the constitutive production of A beta in five different human cell lines, both neural and nonneural. AEBSF also stabilizes full-length beta APP and enhances alpha-secretion, as shown by an increase in the proteolytic derivative, alpha-APPS. Further, we demonstrate that the inhibitory effect of AEBSF is specific for A beta proteins starting at Aspartate 1, suggesting that AEBSF directly inhibits beta-secretase, the Methionine-Aspartate (Met-Asp)-cleaving enzyme. These results indicate that specific inhibition of this A beta-generating protease is possible in living human neural cells and provide information about the characteristics of this as yet unidentified enzyme.

Enhanced release of amyloid beta-protein from codon 670/671 "Swedish" mutant beta-amyloid precursor protein occurs in both secretory and endocytic pathways

The mutation at codons 670/671 of beta-amyloid precursor protein (betaPP) dramatically elevates amyloid beta-protein (Abeta) production. Since increased Abeta may be responsible for the disease phenotype identified from a Swedish kindred with familial Alzheimer's disease, evaluation of the cellular mechanism(s) responsible for the enhanced Abeta release may suggest potential therapies for Alzheimer's disease. In this study, we analyzed Chinese hamster ovary cells stably transfected with either wild type betaPP (betaPP-wt) or "Swedish" mutant betaPP (betaPP-sw) for potential differences in betaPP processing. We confirmed that increased amounts of Abeta and a beta-secretase-cleaved COOH-terminally truncated soluble betaPP (betaPPs) were secreted from betaPP-sw cells. As shown previously for betaPP-wt cells, Abeta was released more slowly than the secretion of betaPPs from surface-labeled betaPP-sw cells, indicating that endocytosis of cell surface betaPP is one source of Abeta production. In contrast, by [35S]methionine metabolic labeling, the rates of Abeta and betaPPs release were virtually identical for both cell lines. In addition, the identification of intracellular betaPPs and Abeta shortly after pulse labeling suggests that Abeta is produced in the secretory pathway. Interestingly, more Abeta was present in medium from betaPP-sw cells than betaPP-wt cells after either cell surface iodination or [35S]methionine labeling, indicating that betaPP-sw cells have enhanced Abeta release in both the endocytic and secretory pathways. Furthermore, a variety of drug treatments known to affect protein processing similarly reduced Abeta release from both betaPP-wt and betaPP-sw cells. Taken together, the data suggest that the processing pathway for betaPP is similar for both betaPP-wt and betaPP-sw cells and that increased Abeta production by betaPP-sw cells arises from enhanced cleavage of mutant betaPP by beta-secretase, the as-yet unidentified enzyme(s) that cleaves at the NH2 terminus of Abeta.

Extracellular matrix regulates the amount of the beta-amyloid precursor protein and its amyloidogenic fragments

We have studied the influence of the extracellular matrix (ECM) on the amount of beta-amyloid precursor protein (APP) and C-terminal amyloid-bearing fragments in 313 fibroblasts. After incubation with ECM components, the cellular APP content of 3T3 cells changed. Besides, different substrata including collagen, fibronectin, laminin, vitronectin, and heparin, determined changes in the amount of a C-terminal 22 kDa-fragment. The regulation of amyloidogenic fragments by the ECM was transient; in fact, when 3T3 cells were plated on tissue culture dishes coated with collagen or vitronectin, maximal levels of the 22 kDa fragment were observed 12 h after plating; in the presence of fibronectin, the maximum level of the amyloidogenic fragment was obtained 36 h after plating. These results indicate that the ECM modulates in a transient way the generation of APP-derived polypeptides containing the amyloid-beta-peptide (A beta). The ECM does not have a generalized effect on 3T3 fibroblasts, because no significant differences in cell attachment, growth rate, whole-cell polypeptide pattern beta 1 integrin and alpha-tubulin levels were observed on cells grown on various matrix proteins. Laminin, collagen, and heparin also influence the level of an amyloidogenic fragment of 35 kDa in Neuro 2A neuronal cells, without a significant change in the neuronal marker acetylcholinesterase. In this case, however, a long-lasting response to ECM molecules was observed. These observations provide evidence that ECM molecules influence APP biogenesis, including the generation of amyloidogenic fragments containing the A beta peptide. Our studies might prove significant to understand the localized increment of beta-amyloid deposition in selected areas of the brain of Alzheimer's patients.

Altered expression of amyloid beta precursor mRNAs in cerebral vessels, meninges, and choroid plexus in Alzheimer's disease

Altered tissue-specific processing or production of the amyloid precursor protein (APP) is thought to be central to amyloid deposition in cerebrovascular and the neocortical tissues in Alzheimer's disease (AD). We demonstrate that A beta peptide(s) is readily detectable and increased in cerebral vessels, meninges and choroid plexus obtained at autopsy from AD subjects compared to age-matched controls. Using the reverse transcription (RT)-polymerase chain reaction (PCR), we further found that A beta transcripts encoding the A beta sequence in all forms of APP containing exons 16 and 17 (of APP770) were significantly increased in vessel samples in AD subjects. This was also evident in the neocortical samples and not related to pre-mortem factors or postmortem interval. It is possible that the increased A beta mRNAs reflect enhanced expression of the L-APP isoform (APP770 without exon 15) expressed in leukocytes and glia alike. We also found evidence for changed proportions of APP 770, 756 and 695 mRNAs in cerebral vessel samples from AD subjects compared to controls. Whereas APP770 and APP751, the predominant forms, were significantly decreased, APP695 transcript was increased in vessel samples from AD subjects. Such changes were not evident in neocortical samples from the same subjects. These observations suggest tissue-specific changes in expression of APP isoforms implicating one of the mechanisms for increased accumulation of A beta in cerebrovascular tissues in AD.