Evidence that gamma-secretase mediates oxidative stress-induced beta-secretase expression in Alzheimer's disease

Beta-secretase (BACE1), an enzyme responsible for the production of amyloid beta-peptide (Abeta), is increased by oxidative stress and is elevated in the brains of patients with sporadic Alzheimer's disease (AD). Here, we show that oxidative stress fails to induce BACE1 expression in presenilin-1 (gamma-secretase)-deficient cells and in normal cells treated with gamma-secretase inhibitors. Oxidative stress-induced beta-secretase activity and sAPPbeta levels were suppressed by gamma-secretase inhibitors. Levels of gamma- and beta-secretase activities were greater in brain tissue samples from AD patients compared to non-demented control subjects, and the elevated BACE1 level in the brains of 3xTgAD mice was reduced by treatment with a gamma-secretase inhibitor. Our findings suggest that gamma-secretase mediates oxidative stress-induced expression of BACE1 resulting in excessive Abeta production in AD.     

BACE knockout mice are healthy despite lacking the primary beta-secretase activity in brain: implications for Alzheimer's disease therapeutics.

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by accumulation of amyloid plaques and neurofibrillary tangles in the brain. The major components of plaque, beta-amyloid peptides (Abetas), are produced from amyloid precursor protein (APP) by the activity of beta- and gamma-secretases. beta-secretase activity cleaves APP to define the N-terminus of the Abeta1-x peptides and, therefore, has been a long- sought therapeutic target for treatment of AD. The gene encoding a beta-secretase for beta-site APP cleaving enzyme (BACE) was identified recently. However, it was not known whether BACE was the primary beta-secretase in mammalian brain nor whether inhibition of beta-secretase might have effects in mammals that would preclude its utility as a therapeutic target. In the work described herein, we generated two lines of BACE knockout mice and characterized them for pathology, beta-secretase activity and Abeta production. These mice appeared to develop normally and showed no consistent phenotypic differences from their wild-type littermates, including overall normal tissue morphology and brain histochemistry, normal blood and urine chemistries, normal blood-cell composition, and no overt behavioral and neuromuscular effects. Brain and primary cortical cultures from BACE knockout mice showed no detectable beta-secretase activity, and primary cortical cultures from BACE knockout mice produced much less Abeta from APP. The findings that BACE is the primary beta-secretase activity in brain and that loss of beta-secretase activity produces no profound phenotypic defects with a concomitant reduction in beta-amyloid peptide clearly indicate that BACE is an excellent therapeutic target for treatment of AD.     

Toward prevention of Alzheimers disease--potential nutraceutical strategies for suppressing the production of amyloid beta peptides

Alzheimers disease (AD) can be viewed as a vicious cycle in which excess production and deposition of amyloid beta (Abeta) peptides promote microglial activation, and the resultant production of inflammatory mediators further boosts Abeta production while inducing death and dysfunction of neurons. Abeta production is mediated by beta- and gamma-secretase activities; it is prevented by alpha-secretase activity, and insulin-degrading enzyme (IDE) catabolizes Abeta. High cellular cholesterol content increases Abeta synthesis by boosting beta-secretase activity; inhibition of cholesterol syntheses and/or stimulation of cholesterol export thus diminishes Abeta production. PPARgamma activity decreases Abeta production by promoting harmless catabolism of amyloid precursor protein while blocking the up-regulatory impact of cytokines on beta-secretase expression. Nitric oxide produced by the healthy cerebral microvasculature can suppress Abeta production by boosting expression of alpha-secretase while suppressing that of beta-secretase; conversely, cerebral ischemia provokes increased APP expression. Good insulin sensitivity and efficient brain insulin function protect by inhibiting gamma-secretase activity and increasing expression of IDE. The DHA provided by fish oil diminishes cerebral Abeta deposition in rodent AD models, for unclear reasons. Various measures which oppose microglial activation can inhibit up-regulation of beta-secretase and gamma-secretase by oxidants and cytokines, respectively. These considerations suggest that a number of nutraceutical or lifestyle measures may have potential for preventing or slowing AD: policosanol; 9-cis-beta-carotene; isomerized hops extract; DHA; measures which promote efficient endothelial NO generation, such as low-salt/potassium-rich diets, exercise training, high-dose folate, and flavanol-rich cocoa; chromium picolinate and cinnamon extract as aids for insulin sensitivity; and various agents which can oppose microglial activation, including vitamin D, genistein, and sesamin. The impact of these measures on Abeta production in rodent models of AD should be evaluated, with the intent of defining practical strategies for AD prevention.     

Beta-site APP cleaving enzyme 1 (BACE1) is increased in remaining neurons in Alzheimer's disease brains

Alzheimer's disease (AD) is characterized by the extensive deposition of amyloid beta protein (Abeta) in the brain cortex. Abeta is produced from beta-amyloid precursor protein (APP) by beta-secretase and gamma-secretase. beta-Secretase has been identified as beta-site APP cleaving enzyme1 (BACE1). We produced rabbit polyclonal antibodies against the amino and the carboxyl terminals of BACE1. Using these antibodies, BACE1 was characterized in temporal lobe cortices by Western blotting and immunohistochemistry. Immunohistochemical studies employing anti-GFAP and anti-MAP2 antibodies as well as anti-BACE1 antibodies showed that BACE1 was expressed exclusively in neurons but not in glial cells. Brain samples were directly extracted by 0.5% SDS and analyzed by Western blotting and densitometer. Although the mean level of BACE1/mg protein in AD brains was not increased, the ratio of BACE1 to MAP2 or to NSE was significantly increased compared with that in control brains. Taken together, these findings suggest that those neurons that survive in AD brains might generate more BACE1 than normal neurons in control brains, indicating that increased BACE1 activity could be one of the causes of AD. This could justify the development of anti-BACE1 drugs for AD treatment.     

The normal equilibrium between CSF and plasma amyloid beta levels is disrupted in Alzheimer's disease

Amyloid-beta (Abeta) with 40 (Abeta40) and 42 (Abeta42) amino acids, the main components of amyloid plaques in the Alzheimer's disease (AD) brain, can be measured in human cerebrospinal fluid (CSF) and plasma. Whereas CSF Abeta42 is decreased in AD, some studies have reported changed plasma Abeta levels in AD and in subjects with mild cognitive impairment (MCI). To this date it is unclear if and how CSF and plasma levels of Abeta correlate with each other in healthy individuals, albeit earlier studies on AD patients found no correlation between CSF and plasma Abeta. We have measured Abeta40 and Abeta42 in paired CSF and plasma samples from patients with AD (n=39), MCI (n=29) and healthy control subjects (n=18). We observed a clear correlation between CSF and plasma levels for both Abeta40 and Abeta42 in healthy individuals, whereas no such correlation could be seen for AD or MCI cases. Similarly to other studies we also found low levels of Abeta42 in AD CSF, whereas there were no significant differences in plasma Abeta levels between the diagnostic groups. Our findings suggest that the normal equilibrium between CSF and plasma Abeta may be disrupted with the initiation of amyloid deposition in the brain.     

The association study between DHCR24 polymorphisms and Alzheimer's disease.

DHCR24 gene in chromosome 1 encodes seladin 1, a cholesterol synthesizing enzyme. Seladin 1 protects neurons from Abeta(42) mediated toxicity and participates in regulation of Abeta(42) formation by organizing the placement of APP cleaving beta-secretase in cholesterol-rich detergent-resistant membrane domains (DRMs). In Alzheimer's disease (AD) the level of seladin 1 in affected neurons is reduced, DRMs are disorganized and Abeta(42) formation is increased. To examine genetic association of the DHCR24 with AD, we genotyped four single nucleotide polymorphism (SNP) sites (rs638944, rs600491, rs718265, and rs7374) in 414 Finnish AD cases and 459 controls and calculated the allelic and genotypic distribution of both cases and controls. The single locus association analysis indicated that men carrying the T allele of rs600491 had an increased risk of AD (OR 1.7 95% CI 1.2-2.4; P = 0.004, Bonferroni corrected P = 0.048 with 12 tests). We estimated haplotypes of SNPs rs638944 and rs600491 between cases and controls and found overall distribution of haplotypes highly significant (P < 0.001). There was a common protective haplotype TC with frequency of 0.22 in cases and 0.30 in controls (P < 0.001) and a risk haplotype GC with frequency of 0.10 in cases and 0.05 in controls (P < 0.001). We also measured CSF Abeta(42), tau and phosphorylated tau (ptau) levels in a subgroup of AD cases (n = 44) and controls (n = 10) and found that AD cases that carry rs718265 GG had lower levels of Abeta(42) than other genotype carriers. Our findings indicate that DHCR24 gene may be associated with AD risk.     

Alpha- and beta-secretase activity as a function of age and beta-amyloid in Down syndrome and normal brain

Aged individuals with Down syndrome (DS) develop Alzheimer's disease (AD) neuropathology by the age of 40 years. The purpose of the current study was to measure age-associated changes in APP processing in 36 individuals with DS (5 months-69 years) and in 26 controls (5 months-100 years). Alpha-secretase significantly decreased with age in DS, particularly in cases over the age of 40 years and was stable in controls. The levels of C-terminal fragments of APP reflecting alpha-secretase processing (CTF-alpha) decreased with age in both groups. In both groups, there was significant increase in beta-secretase activity with age. CTF-beta remained constant with age in controls suggesting compensatory increases in turnover/clearance mechanisms. In DS, young individuals had the lowest CTF-beta levels that may reflect rapid conversion of beta-amyloid (Abeta) to soluble pools or efficient CTF-beta clearance mechanisms. Treatments to slow or prevent AD in the general population targeting secretase activity may be more efficacious in adults with DS if combined with approaches that enhance Abeta degradation and clearance.     

Intracellular domain generation of amyloid precursor protein by epsilon-cleavage depends on C-terminal fragment by alpha-secretase cleavage

A significant accumulation of Abeta is major pathological feature in the brain in patients with Alzheimer's disease (AD). Amyloid precursor protein (APP) is cleaved by alpha- and beta-secretase, resulting in secretion of extracellular domain. Then the remaining membrane-anchored C-terminal fragments (CTFalpha or CTFbeta) are cleaved by gamma-secretase. Therefore, Abeta is derived from APP by sequential proteolytic processing involving 2 proteases, called beta- and gamma-secretase. Gamma-secretase cleavage results in the generation of the APP intracellular domain (AICD), as well as Abeta. Recently, AICD is generated by epsilon-cleavage site near the cytoplasmic membrane boundary of APP, not by gamma-cleavage site in the middle of transmembrane domain. We show that elevated beta-secretase levels induced the increase in CTFbeta and Abeta generation and reduction of CTFalpha and AICD generation in vitro. Furthermore, elevated alpha-secretase levels induced an increase of AICD. The results suggest that generation of AICD by epsilon-cleavage depends on CTFalpha and that gamma- and epsilon-cleavage are differentially regulated.     

Neurochemical diagnosis of Alzheimer's dementia by CSF Abeta42, Abeta42/Abeta40 ratio and total tau

Cerebrospinal fluid (CSF) concentrations of amyloid beta peptides ending at positions 42 and 40 (Abeta42 and Abeta40, respectively), and total tau (tTau) protein were measured by ELISA in order to compare their accuracy in discriminating patients with Alzheimer's disease (AD, n = 22), non-Alzheimer dementia (nAD, n = 11) and control subjects (CON, n=35). As compared to the other groups, the concentrations of Abeta42 and tTau were decreased (P<0.001) and increased (P<0.001) in AD, respectively, while Abeta40 did not differ significantly among the groups. Receiver operating characteristic (ROC) analysis was performed to define cut-off values for maximized sensitivity and specificity. For all groups compared the Abeta peptide ratio 42/40 classified more patients correctly, as compared to the concentration of Abeta42 alone: AD versus controls, 94 and 86.7%; AD versus nAD, 90 and 85% and AD versus nAD plus controls, 90.8 and 87%, respectively. The percentage of correctly classified patients was further improved when the Abeta ratio was combined with the analysis of the tTau concentration. Presence of the apolipoprotein E 4 allele, age or degree of mental disability did not significantly influence the parameters studied.     

Antisense inhibition at the beta-secretase-site of beta-amyloid precursor protein reduces cerebral amyloid and acetyl cholinesterase activity in Tg2576

Misprocessing of beta-amyloid precursor protein (APP) leading to the formation of elevated quantities of beta-amyloid peptide (Abeta), derived by a cleavage at the beta-secretase site (N-671/673aa) and by a cleavage at the gamma-secretase site (C-711/713aa) of APP, is considered a key event in the pathogenesis of Alzheimer disease (AD). Point mutations near the beta-secretase site in the human gene for APP, such as in the Swedish mutation-KM670/671NL, lead to a form of dominantly inherited AD. These mutations are known to promote beta-site cleavage and to increase levels of Abeta. Abeta has been shown previously to increase acetyl cholinesterase (AChE) activity in vitro. We wished to test whether translational blocking of APP-mRNA at the mutated beta-site by antisense (AS) oligodeoxynucleotides (ODNs) directed to the mutated site will reduce cerebral amyloid in the Swedish transgenic mouse model (Tg2576). Mice were injected i.c.v. with AS-ODNs directed at the mutated beta-site (AS-beta site) or with AS-ODNs directed at the normal gamma-site (AS-gamma site) of human APP-mRNA, and compared with procedural controls that received i.c.v. injections of sense ODNs at the beta-site (S-beta site), sense ODNs at the gamma-site (S-gamma site) or mismatched ODNs, and with untreated littermates (Lt) and untreated transgenic mice (Tgs). ODNs were injected into the 3rd ventricle once a week for 4 weeks. Brains were processed for enzyme-linked immunosorbent assay analysis of beta- and gamma-cleaved soluble Abeta40 (sAbeta40), beta- and gamma-cleaved soluble Abeta42 (sAbeta42) and alpha-cleaved soluble beta-amyloid precursor protein (sAPPalpha). The physiological relevance of AS ODNs was tested by evaluating the cerebral distribution of AChE before and after the treatment. AChE was found increased about fivefold in Tg cortex as compared with control brain. Results show that compared with untreated and procedural controls, AS-beta increased cerebral levels of sAPPalpha by 43% and reduced sAbeta40/42 by approximately 39%; while simultaneously reducing the cortical density of AChE by approximately fourfold in the treated Tg animals, almost to the level found in the control brain (all values P<0.0001, analysis of variance, unpaired two-tailed Student's t-test), while AS-gamma did not have any effect. These results indicate that AS directed to the mutated beta-site may be an effective approach to treat familial AD.     

Age-dependent differential expression of BACE splice variants in brain regions of tg2576 mice

Plaques found in the brains of patients suffering from Alzheimer's disease (AD) mainly consist of beta-amyloid (Abeta), which is produced by sequential cleaving of amyloid precursor protein (APP) by two proteolytic enzymes, beta- and gamma-secretases. Any change in the fine balance between these enzymes and their substrate may contribute to the etio-pathogenesis of AD. Indeed, the protein level and enzymatic activity of beta-secretase (BACE), but not its mRNA level, were found elevated in brain areas of AD patients who suffer a high load of Abeta plaque formation. Similarly, increased BACE activity but no mRNA change was observed in a transgenic mouse model of AD, tg2576, in which over expression of the Swedish mutated human APP leads to Abeta plaque formation and learning deficits. Based on the recent demonstration of four BACE splice variants with different enzymatic activity, the discrepancy between BACE activity and mRNA expression may be explained by the altered BACE alternative splicing. To test this hypothesis, we studied the expression of all BACE splice variants in different brain areas of tg2576 mice at age of 4 months and 1 year old. We found developmental and regional differences between wild-type and tg2576 mice. Our results indicate that over expression of APP in tg2576 mice leads to the altered alternative splicing of BACE and the increase of its enzymatically more active splice variant (I-501).     

Stable beta-secretase activity and presynaptic cholinergic markers during progressive central nervous system amyloidogenesis in Tg2576 mice

We examined presynaptic cholinergic markers and beta-secretase activity during progressive central nervous system amyloidogenesis in Tg2576 Alzheimer mice (transgenic for human amyloid precursor protein Swedish mutation; hAPPswe). At 14, 18, and 23 months of age there were no significant differences between wild-type and transgenic mice in four distinct central nervous system cholinergic indices--choline acetyltransferase and acetylcholinesterase activities, and binding to vesicular acetylcholine transporter and Na(+)-dependent high-affinity choline uptake sites. A novel enzyme-linked immunosorbent assay measuring only the secreted human beta-secretase cleavage product (APPsbetaswe) of APPswe also revealed no change with aging in Tg2576 mouse brain. In contrast, transgenic but not wild-type mice exhibited an age-dependent increase in soluble Abeta40 and Abeta42 levels and progressive amyloid deposition in brain. Thus, aging Tg2576 mice exhibited presynaptic cholinergic integrity despite progressively increased soluble Abeta40 and Abeta42 levels and amyloid plaque density in brain. Older Tg2576 mice may best resemble preclinical or early stages of human Alzheimer's disease with preserved presynaptic cholinergic innervation. Homeostatic APPsbetaswe levels with aging suggest that progressive amyloid deposition in brain results not from increased beta-secretase cleavage of APP but from impaired Abeta/amyloid clearance mechanisms.     

Analysis of APBB2 gene polymorphisms in sporadic Alzheimer's disease

The accumulation of beta-amyloid (Abeta) in the brain plays a central role in the pathogenesis of Alzheimer's disease (AD). The processing of Abeta precursor protein to Abeta is modulated by binding proteins including APBB2 [amyloid beta precursor protein-binding family B member 2, FE65-like, FE65L1]. We investigated two intronic SNPs within the APBB2 gene: rs13133980 and hCV1558625 (rs17443013), among Polish AD patients and healthy controls (n=213, 171). The frequencies of rs13133980 alleles and genotypes did not differ between cases and controls, irrespective of age of onset or APOE epsilon4 carrier status. The hCV1558625 G allele was over-represented in patients with onset under age 70 compared to controls in the same age range (57% vs. 43%, p=0.03). The association between the hCV1558625 G allele and susceptibility for AD at relatively young ages needs to be confirmed in other samples.     

Increased cerebrospinal fluid levels of transforming growth factor-beta1 in Alzheimer's disease

Accumulation of beta-amyloid (Abeta) in senile plaques in specific brain regions is a key event in the development of Alzheimer's disease (AD). Expression of transforming growth factor-beta1 (TGF-beta1), a regulator of brain responses to inflammation and injury, has been correlated with Abeta accumulation, aggregation and clearance in transgenic mice and increased production of amyloid precursor protein (APP) followed by Abeta generation in murine and human astrocyte cultures. Here, we compared TGF-beta1 levels in cerebrospinal fluid (CSF) from 20 AD patients and 20 healthy controls and correlated TGF-beta1 to intrathecal levels of the amyloidogenic 42-amino acid fragment of Abeta (Abeta42). AD patients had higher concentration of TGF-beta1 than controls (P = 0.002). Moreover, TGF-beta1 levels were negatively correlated to Abeta42 levels in the whole material (cases and controls, r = -0.35; P = 0.020), although this correlation failed to reach significance in the AD group alone (r = -0.38; P = 0.099). Taken together, the data indicate that TGF-beta1 plays a role in the processes that affect amyloid metabolism in AD.     

Deregulation of sphingolipid metabolism in Alzheimer's disease

Abnormal sphingolipid metabolism has been previously reported in Alzheimer's disease (AD). To extend these findings, several sphingolipids and sphingolipid hydrolases were analyzed in brain samples from AD patients and age-matched normal individuals. We found a pattern of elevated acid sphingomyelinase (ASM) and acid ceramidase (AC) expression in AD, leading to a reduction in sphingomyelin and elevation of ceramide. More sphingosine also was found in the AD brains, although sphingosine-1-phosphate (S1P) levels were reduced. Notably, significant correlations were observed between the brain ASM and S1P levels and the levels of amyloid beta (Abeta) peptide and hyperphosphorylated tau protein. Based on these findings, neuronal cell cultures were treated with Abeta oligomers, which were found to activate ASM, increase ceramide, and induce apoptosis. Pre-treatment of the neurons with purified, recombinant AC prevented the cells from undergoing Abeta-induced apoptosis. We propose that ASM activation is an important pathological event leading to AD, perhaps due to Abeta deposition. The downstream consequences of ASM activation are elevated ceramide, activation of ceramidases, and production of sphingosine. The reduced levels of S1P in the AD brain, together with elevated ceramide, likely contribute to the disease pathogenesis.     

Modifications of platelet from Alzheimer disease patients: a possible relation between membrane properties and NO metabolites

Alzheimer disease (AD) is a chronic neurodegenerative disorder characterised by a progressive loss of memory and cognitive functions. The formation of nitric oxide (NO), by astrocytes, has been suggested to contribute to the neurodegnerative process. Some studies have described the participation of different isoforms of NOS in the progression of AD. The present work was conducted in order to investigate the role played by NO and peroxynitrite in platelets from AD patients, the possible correlation with Na(+)/K(+)-ATPase activity and the intracellular Ca(2+) in the same group of patients as well as the expression of NOS isoenzymes and nitrotyrosine as markers of NO synthesis and reactive protein nitration. NO production was significantly elevated in the platelets from AD patients compared to controls as well as l-arginine/NO-dependent ONOO(-). Membrane Na(+)/K(+)-ATPase activity was significantly decreased in patients than in controls while intracellular Ca(2+) concentration shows an opposite trend. Platelet from AD patients showed a significantly increased 1-[4-(trimethylammonio)phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH) and 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescence anisotropy compared with controls. Western blot analysis using anti-iNOS and eNOS monoclonal antibodies demonstrated that both isoforms were detectable in cell lysates as well as nitrotyrosine more pronounced in the cell lysates from AD patients than controls. In conclusion, the increased expression and activity of nitrergic system may produce platelet membrane alteration or vice versa. These modifications may contribute further to the neurodegenerative process in AD because the abnormal function of Alzheimer platelets play a very important role in the pathogenesis of the disease.     

Cytochrome c oxidase is decreased in Alzheimer's disease platelets

Cytochrome c oxidase (COX) activity reportedly is reduced in Alzheimer's disease (AD) brain and platelets. The reasons for the defect in either tissue are unknown, but its presence in a non-degenerating tissue suggests it is not simply a consequence of neurodegeneration. We now offer confirmation of the AD platelet COX defect. Compared to age-matched controls, in mitochondria isolated from AD platelets there was a 15% decrease in COX activity despite the fact that COX subunits were present at normal levels. Platelet ATP levels were diminished in AD (from 11.33 +/- 0.52 to 9.11 +/- 0.72 nmol/mg), while reactive oxygen species (ROS) were increased (from 97.03 +/- 25.9 to 338.3 +/- 100 K/mg). Platelet membrane fluidity, Vitamin E, and cholesterol content were similar between groups. We conclude that COX catalytic activity is indeed diminished in AD platelet mitochondria, does not result from altered membrane fluidity, and is associated with ROS overproduction and ATP under-production.     

Does increased platelet release of Abeta peptide contribute to brain abnormalities in individuals with depression?

Increased platelet activation with release of procoagulant factors from their alpha granules has been demonstrated in individuals with major depression. Platelet activation has also been shown to be associated with release of beta-amyloid peptides, which have been implicated in Alzheimer's disease. Thus, we are hypothesizing that sustained elevations of Abeta peptides might occur in individuals with recurrent depression. We further hypothesize that such elevations contribute to brain abnormalities in depressed individuals through the formation of neurotoxic oligomeric forms of Abeta peptides and amyloid deposition. We also propose that increased amyloid Abeta peptides from platelet activation may be a mechanism underlying the increased risk for cognitive impairment in nondepressed patients who have other reasons for such activation. If true, our hypothesis would imply that platelet inhibitors may have a role in preventing or delaying the neuronal consequences of disorders characterized by activated platelets.     

Platelet as a physiological model to investigate apoptotic mechanisms in Alzheimer beta-amyloid peptide production

Although neuronal apoptosis in Alzheimer's disease is generally interpreted as the consequence of the toxicity of extracellular beta-amyloid (Abeta) peptide aggregates, some experimental results provide evidence that the Abeta overproduction can be the result of a primary neuronal degeneration. As platelets are considered a good model where to study proteolytic processing of the amyloid precursor protein (APP), we exposed platelets to the proapoptotic agent ionomycin and analyzed Abeta40 and Abeta42 levels in the intracellular and extracellular compartments. The activation of apoptotic pathways in platelets has been verified by mitochondrial membrane depolarization, exposure of phosphatidylserine, protease activation and morphological changes. A significative increase in intraplatelet Abeta40, but not Abeta42, was observed after 10 min treatment with ionomycin. Thus, the activation of apoptotic pathways in platelets determines an altered processing of APP leading to elevated levels of intracellular Abeta40. The specific intracellular production of Abeta40 represents a potential threat to the cells since very high local Abeta40 concentration increases the risk of its aggregation and toxicity. As a result, Abeta40 might be dangerous even before it becomes secreted rendering neurons highly vulnerable.     

Differential processing and secretion of Abeta peptides and sAPPalpha in human platelets is regulated by thrombin and prostaglandine 2

Metabolic and functional studies of the amyloid precursor protein (APP) in platelets have advanced our understanding of Alzheimer's disease (AD). Here we report that human platelets contain Abeta peptides, process and secrete them constitutively. Platelets generate formerly unkown Abeta-species by differential processing of APP. Release of Abeta peptides were also increased by platelet activation with thrombin, indicating the existence of a regulated exocytotic pathway. We showed that Abeta-levels, Abeta-processing patterns and Abeta-release kinetics were regulated by thrombin. In controls, release of Abeta peptide species (Abeta 1-40/42 and 1-37/38/39/) continued for more than 4 h, while thrombin activated cells ceased secretion after 1 h at large. Treatment of platelets with prostaglandine 2 slowed this process down. Intracellular Abeta peptide concentrations decreased steadily until no peptides could be detected after 20 h (control) or after 4 h (thrombin) in cultured platelets.