Molecular cloning and characterization of a cDNA encoding the cerebrovascular and the neuritic plaque amyloid peptides

Toxicity of non-abeta component of Alzheimer's disease amyloid, and N-terminal fragments thereof, correlates to formation of beta-sheet structure and fibrils

The non-Abeta component of Alzheimer's disease amyloid (NAC) and its precursor alpha-synuclein have been linked to amyloidogenesis in Alzheimer's disease (AD), Parkinson's disease (PD) and dementia with Lewy bodies (DLB). Previously we have shown that NAC forms beta-sheet structures and fibrils [El-Agnaf, O.M.A., Bodles, A.M., Guthrie, D.J.S., Harriott, P. & Irvine, G.B. (1998) Eur. J. Biochem. 258, 157-163]. As a measure of their neurotoxic potential we have examined the ability of fresh and aged NAC and fragments thereof to inhibit the reduction of the redox dye 3-(4, 5-dimethylthiazol-2-yl)-2,5 diphenyltetrazolium bromide by rat pheochromocytoma PC12 cells. Micromolar concentrations of NAC and fragments thereof display varying degrees of toxicity. On immediate dissolution and after an incubation period for 3 days at 37 degrees C the full-length peptide and fragments NAC(3-18) and NAC(1-18) scrambled sequence [NAC(1-18 s)] were toxic, whereas fragments NAC(1-13) and NAC(6-14) were not. CD indicates that NAC(3-18) and NAC(1-18 s) exhibit beta-sheet secondary structure in aqueous solution, whereas NAC(1-13) and NAC(6-14) do not. NAC(3-18) aggregates, as indicated by concentration of peptide remaining in solution after 3 days measured by an HPLC assay, and forms fibrils, as determined by electron microscopy. However, although some fibrils were detected for NAC(1-18 s) it does not come out of solution to a significant degree. Fragments NAC(1-13) and NAC(6-14) form few fibrils and remain in solution. These findings indicate that the ability of the central region of NAC to form beta-sheet secondary structures is important for determining the toxicity of the peptide. This contrasts with what has been reported previously for most Abeta peptides as their toxicity appears to require the peptide to have formed fibrillary aggregates as well as displaying beta-sheet. These results suggest that an intermediate, which exhibits beta-sheet structure, may be responsible for the toxic properties of NAC and provides further evidence for the role of NAC in the pathogenesis of AD, PD and DLB.

Agrin binds to beta-amyloid (Abeta), accelerates abeta fibril formation, and is localized to Abeta deposits in Alzheimer's disease brain

Agrin is an extracellular matrix heparan sulfate proteoglycan (HSPG) well known for its role in modulation of the neuromuscular junction during development. Although agrin is one of the major HSPGs of the brain, its function there remains elusive. Here we provide evidence suggesting a possible function for agrin in Alzheimer's disease brain. Agrin protein binds the amyloidogenic peptide Abeta (1-40) in its fibrillar state via a mechanism that involves the heparan sulfate glycosaminoglycan chains of agrin. Furthermore, agrin is able to accelerate Abeta fibril formation and protect Abeta (1-40) from proteolysis, in vitro. Supporting a biological significance for these in vitro data, immunocytochemical studies demonstrate agrin's presence within senile plaques and cerebrovascular amyloid deposits, and agrin immunostained capillaries exhibit pathological alterations in AD brain. These data therefore suggest that agrin may be an important factor in the progression of Abeta peptide aggregation and/or its persistence in Alzheimer's disease brain.

A unifying hypothesis of Alzheimer's disease. III. Risk factors

Normal ageing and Alzheimer's disease (AD) have many features in common and, in many respects, both conditions only differ by quantitative criteria. A variety of genetic, medical and environmental factors modulate the ageing-related processes leading the brain into the devastation of AD. In accordance with the concept that AD is a metabolic disease, these risk factors deteriorate the homeostasis of the Ca(2+)-energy-redox triangle and disrupt the cerebral reserve capacity under metabolic stress. The major genetic risk factors (APP and presenilin mutations, Down's syndrome, apolipoprotein E4) are associated with a compromise of the homeostatic triangle. The pathophysiological processes leading to this vulnerability remain elusive at present, while mitochondrial mutations can be plausibly integrated into the metabolic scenario. The metabolic leitmotif is particularly evident with medical risk factors which are associated with an impaired cerebral perfusion, such as cerebrovascular diseases including stroke, cardiovascular diseases, hypo- and hypertension. Traumatic brain injury represents another example due to the persistent metabolic stress following the acute event. Thyroid diseases have detrimental sequela for cerebral metabolism as well. Furthermore, major depression and presumably chronic stress endanger susceptible brain areas mediated by a host of hormonal imbalances, particularly the HPA-axis dysregulation. Sociocultural and lifestyle factors like education, physical activity, diet and smoking may also modulate the individual risk affecting both reserve capacity and vulnerability. The pathophysiological relevance of trace metals, including aluminum and iron, is highly controversial; at any rate, they may adversely affect cellular defences, antioxidant competence in particular. The relative contribution of these factors, however, is as individual as the pattern of the factors. In familial AD, the genetic factors clearly drive the sequence of events. A strong interaction of fat metabolism and apoE polymorphism is suggested by intercultural epidemiological findings. In cultures, less plagued by the 'blessings' of the 'cafeteria diet-sedentary' Western lifestyle, apoE4 appears to be not a risk factor for AD. This intriguing evidence suggests that, analogous to cardiovascular diseases, apoE4 requires a hyperlipidaemic lifestyle to manifest as AD risk factor. Overall, the etiology of AD is a key paradigm for a gene-environment interaction. Copyright 2000 John Wiley & Sons, Ltd.

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

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

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

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

Chromosome missegregation and trisomy 21 mosaicism in Alzheimer's disease

A connection between Alzheimer's disease (AD) and Down syndrome (trisomy 21) is indicated by the fact that all Down syndrome individuals develop Alzheimer's disease neuropathology by the 4th decade of life. Previous studies have examined the frequency of aneuploidy and other chromosomal defects in cells from familial Alzheimer's disease (FAD) patients, with varying results. To investigate the possibility that a specific type of aneuploidy--trisomy 21 mosaicism--may contribute to Alzheimer's disease, we used quantitative fluorescence in situ hybridization to measure the number of trisomy 21 cells in primary fibroblast cultures from AD and unaffected subjects. The 27 AD cultures, including 15 that were derived from individuals carrying FAD mutations in presenilin 1 or 2, exhibited a significant approximately twofold increase in the number of trisomy 21 cells compared to 13 control cultures. A small double-hybridization experiment suggested that the aneuploidy in AD cells was not limited to chromosome 21 but extended at least to chromosome 18 as well. In a parallel study, the endogenous presenilin proteins in fibroblasts were localized to the centrosomes, the nuclear envelope, and its associated interphase kinetochores. Together these results indicate that the presenilin proteins may be involved in mitosis and that FAD mutations in the presenilin genes may predispose to chromosome missegregation (nondisjunction). The data reported here also suggest that trisomy 21 mosaicism may contribute to other forms of AD that are not caused by a presenilin mutation.

Alzheimer's disease: genetic studies and transgenic models

Recent advances in a variety of areas of research, particularly in genetics and in transgenic (Tg)/gene targeting approaches, have had a substantial impact on our understanding of Alzheimer's disease (AD) and related disorders. After briefly reviewing the progress that has been made in diagnostic assessments of patients with senile dementia and in investigations of the neuropathology of AD, we discuss some of the genes/proteins that are causative or risk factors for this disease, including those encoding amyloid precursor protein, presenilin 1 and 2, and apolipoprotein E. In addition, we comment on several potential new candidate loci/genes. Subsequently, we review selected recent reports of analyses of a variety of lines of Tg mice that show several neuropathological features of AD, including A beta-amyloid deposits and dystrophic neurites. Finally, we discuss the several important issues in future investigations of Tg mice, with particular emphasis on the influences of genetic strains on phenotype, especially behavior, and strategies for making new models of neurodegenerative disorders. We believe that investigations of these Tg models will (a) enhance understanding of the relationships between impaired performance on memory tasks and the pathological/biochemical abnormalities in brain, (b) help to clarify pathogenic mechanisms in vivo, (c) lead to identification of new therapeutic targets, and (d) allow testing of new treatment strategies first in mice and then, if successful, in humans with AD.

The kunitz protease inhibitor form of the amyloid precursor protein (KPI/APP) inhibits the proneuropeptide processing enzyme prohormone thiol protease (PTP). Colocalization of KPI/APP and PTP in secretory vesicles

Proteolytic processing of proenkephalin and proneuropeptides is required for the production of active neurotransmitters and peptide hormones. Variations in the extent of proenkephalin processing in vivo suggest involvement of endogenous protease inhibitors. This study demonstrates that "protease nexin 2 (PN2)," the secreted form of the kunitz protease inhibitor (KPI) of the amyloid precursor protein (APP), potently inhibited the proenkephalin processing enzyme known as prohormone thiol protease (PTP), with a Ki,app of 400 nM. Moreover, PTP and PN2 formed SDS-stable complexes that are typical of kunitz protease inhibitor interactions with target proteases. In vivo, KPI/APP (120 kDa), as well as a truncated form of KPI/APP that resembles PN2 in apparent molecular mass (110 kDa), were colocalized with PTP and (Met)enkephalin in secretory vesicles of adrenal medulla (chromaffin granules). KPI/APP (110-120 kDa) was also detected in pituitary secretory vesicles that contain PTP. In chromaffin cells, calcium-dependent secretion of KPI/APP with PTP and (Met)enkephalin demonstrated the colocalization of these components in functional secretory vesicles. These results suggest a role for KPI/APP inhibition of PTP in regulated secretory vesicles. In addition, these results are the first to identify an endogenous protease target of KPI/APP, which is developmentally regulated in aging and Alzheimer's disease.

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

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

Sporadic inclusion-body myositis and its similarities to Alzheimer disease brain. Recent approaches to diagnosis and pathogenesis, and relation to aging

Sporadic inclusion-body myositis (s-IBM) is the most common, debilitating and progressive muscle disease beginning at the age 50 or later. The most characteristic pathologic feature is vacuolar degeneration of muscle fibers accompanied by intrafiber congophilia and clusters ("tangles") of paired-helical filaments, containing phosphorylated tau. An unusual feature of sporadic inclusion-body myositis is accumulation within its abnormal muscle fibers of several proteins that are characteristic of Alzheimer disease brain, including epitopes of beta-amyloid precursor protein (betaAPP), phosphorylated tau, alpha-1-antichymotrypsin, apolipoprotein E, and presenilin-1. Indicators of oxidative stress are also present within abnormal s-IBM muscle fibers. In this review, we describe new advances seeking the pathogenic mechanism of sporadic inclusion-body myositis. We hypothesize on the possible pathogenic role of abnormally accumulated proteins, and we propose that important contributory factors leading to inclusion-body myositis are the milieu of muscle-fiber aging and oxidative stress. In addition, we present evidence that overexpression of adenovirus-transferred betaAPP gene in cultured human muscle fibers induces aspects of the inclusion-body myositis phenotype, and suggest that betaAPP-overexpression is an early event in the pathogenic cascade causing inclusion-body myositis.

Genetic neurodegenerative diseases: the human illness and transgenic models

Review The neurodegenerative disorders, a heterogeneous group of chronic progressive diseases, are among the most puzzling and devastating illnesses in medicine. Some of these disorders, such as Alzheimer's disease, amyotrophic lateral sclerosis, the prion diseases, and Parkinson's disease, can occur sporadically and, in some instances, are caused by inheritance of gene mutations. Huntington's disease is acquired in an entirely genetic manner. Transgenic mice that express disease-causing genes recapitulate many features of these diseases. This review provides an overview of transgenic mouse models of familial amyotrophic lateral sclerosis, familial Alzheimer's disease, and Huntington's disease and the emerging insights relevant to the underlying molecular mechanisms of these diseases.

Drug delivery of antisense molecules to the brain for treatment of Alzheimer's disease and cerebral AIDS

Antisense oligonucleotides (ODNs) and peptide nucleic acids (PNAs) are potential therapeutics for eradication of malignancies, viral infections, and other pathologies. However, ODNs and PNAs in general are unable to cross cellular membranes and blood-tissue barriers, such as the blood-brain barrier (BBB), which is only permeable to lipophilic molecules of molecular weight <600 Da. Cellular delivery systems based on conjugates of streptavidin (SA) and the OX26 monoclonal antibody directed to the transferrin receptor may be employed as a universal carrier for the transport of mono-biotinylated peptides, ODNs, or PNAs. 3'-Biotinylation of phosphodiester (PO)-ODN produces complete protection of ODN against serum and cellular 3'-exonucleases, facilitating the conjugation to avidin-based delivery systems and maintaining the activation of RNase H. These delivery systems markedly increased the cellular uptake and antisense efficacy of 3'-biotinylated ODNs in models of Alzheimer's disease and HIV-AIDS. In vivo brain delivery studies demonstrated that 3'-protected PO-ODNs and PO-phosphorothioate(PS)-ODN hybrids containing a single PO linkage are subjected to endonuclease degradation in vivo. On the contrary PS-ODNs, which were also protected at 3'-terminus by biotinylation, are metabolically stable in vivo and resistant to exo/endonuclease degradation. However, because of the strong binding of these oligomers to plasma protein, PS-ODNs are poorly transported into the brain through the BBB by the OX26-SA delivery vector following intravenous administration. PNAs are also resistant to exo/endonuclease and protease degradation, and these molecules biotinylated at the amino terminal group were transported into the brain by the OX26-SA delivery system with brain uptake levels comparable to that of morphine. Using the rev gene of HIV as a model target, RNase protection assays and cell-free translation arrest showed that the PNA-OX26-SA conjugate maintained active recognition and inactivation of target mRNA, respectively. The overall experimental evidence suggests that PNA-OX26-SA conjugates represent optimal antisense molecules for drug delivery to the brain.

Experimental brain injury induces expression of amyloid precursor protein, which may be related to neuronal loss in the hippocampus

Previous reports have demonstrated that some focal brain injuries increase amyloid precursor protein (APP) immunoreactivity in the region surrounding the injury where it was localized, in damaged axons and in pre-alpha 2 cells of the entorhinal cortex. However, to date, APP expression in the hippocampus remote from the impact site has not been comprehensively studied. Therefore, we have evaluated APP expression not only in the locally injured cerebral cortex but also in the hippocampus remote from the impact site. In the present paper, diffuse axonal injury was induced in rats in midline fluid percussion injury. APP expression was examined post injury using Western blot analysis and immunohistochemistry. Western blot analysis demonstrated that the expression of 100-kd APP was increased in both the cerebral cortex and hippocampus 24 h after injury. It then decreased in the hippocampus, but did not change in the cerebral cortex, 7 days after injury. Immunohistochemical studies showed increased immunoreactivity of APP in the neuronal perikarya and reactive astrocytes near the region of injury in the cerebral cortex 24 h to 7 days after injury. In the hippocampus, APP accumulated in the CA3 neurons 24 h and 3 days after injury, although no hemorrhagic lesions were seen at that site. The APP positive neurons in CA3 showed shrunken cell bodies and pyknotic nuclei 3 days after injury, and some of the neurons in CA3 had disappeared by 7 days postinjury. The results of present study suggest that traumatic brain injury induces overexpression and accumulation of APP in neuronal perikarya and that these events are followed by degeneration of CA3 neurons. Further, the decline in APP expression in the hippocampus is thought to be due to neuronal loss in CA3 subsector.

A basic amino acid in the cytoplasmic domain of Alzheimer's beta-amyloid precursor protein (APP) is essential for cleavage of APP at the alpha-site

In Alzheimer's disease (AD), the beta-amyloid peptide (Abeta) is thought to be produced as a result of the aberrant metabolism of beta-amyloid precursor protein (APP). We report that the APP cytoplasmic domain contains a novel and important signal for APP metabolism. A single amino acid mutation that changed arginine at amino acid 747 of APP770 (corresponding to position 672 of APP695) to a non-basic amino acid greatly increased the production of intracellular APP carboxyl-terminal fragment(s) cleaved at beta-site(s) (CTFbeta), but did not result in increased secretion of Abeta40 and Abeta42. This was not due to a simple intracellular accumulation of CTFbeta resulting from a lack of gamma-secretase. CTFbeta derived from this mutant APP was generated and degraded as efficiently as CTFbeta derived from wild-type APP. This result indicates that the increase in the quantity of CTFbeta does not always give rise to more Abeta production, as was previously suggested by studies of a familial AD mutation of APP. These findings suggest that APP carrying the substitution mutation at this basic amino acid may be metabolized by another protein secretory pathway. Although these results have not completely elucidated why CTFbeta derived from the mutant APP escapes from subsequent cleavage by gamma-secretase, analysis of the processing pathway of this mutant APP should provide insights into the pathogenesis of the sporadic type of AD.

Structural and kinetic features of amyloid beta-protein fibrillogenesis

Alzheimer's disease (AD) is an archetype of a class of diseases characterized by abnormal protein deposition. In each case, deposition manifests itself in the form of amyloid deposits composed of fibrils of otherwise normal, soluble proteins or peptides. An ever-increasing body of genetic, physiologic, and biochemical data supports the hypothesis that fibrillogenesis of the amyloid beta-protein is a seminal event in Alzheimer's disease. Inhibiting A beta fibrillogenesis is thus an important strategy for AD therapy. However, before this strategy can be implemented, a mechanistic understanding of the fibrillogenesis process must be achieved and appropriate steps selected as therapeutic targets. Following a brief introduction to AD, I review here the current state of knowledge of A beta fibrillogenesis. Special emphasis is placed on the morphologic, structural, and kinetic aspects of this complex process.

Evidence of oxidative stress and in vivo neurotoxicity of beta-amyloid in a transgenic mouse model of Alzheimer's disease: a chronic oxidative paradigm for testing antioxidant therapies in vivo

Increased expression of antioxidant enzymes and heat-shock proteins are key markers of oxidative stress. Such proteins are abnormally present within the neuropathological lesions of Alzheimer's disease (AD), suggesting that oxidative stress may play significant but yet undefined roles in this disorder. To gain further insight into the role of oxidative stress in AD, we studied the expression of CuZn superoxide dismutase (SOD) and hemoxygenase-1 (HO-1), two established markers of oxidative stress, in a transgenic mouse model of AD. Immunohistochemistry with anti-SOD and anti-HO-1 antibodies revealed a very pronounced increase of these proteins only in aged transgene-positive mice. Interestingly, the distribution of the oxidative burden was largely overlapping with dystrophic neuritic elements in the mice as highlighted with anti-ubiquitin antibodies. Because the most conspicuous alterations were identified around amyloid (Abeta) deposits, our results provide strong support for the hypothesis that Abeta is neurotoxic in vivo and that such toxicity is mediated by free radicals. To obtain additional experimental evidence for such an interpretation (ie, a cause-effect relationship between Abeta and oxidative neurotoxicity), PC12 cells were exposed to increasing concentrations of Abeta or to oxidative stress. In agreement with the in vivo findings, either treatment caused marked induction of SOD or HO-1 in a dose-dependent fashion. These results validate the transgenic approach for the study of oxidative stress in AD and for the evaluation of antioxidant therapies in vivo.

Cleavage of Alzheimer's amyloid precursor protein (APP) by secretases occurs after O-glycosylation of APP in the protein secretory pathway. Identification of intracellular compartments in which APP cleavage occurs without using toxic agents that interfere with protein metabolism

beta-Amyloid peptide (Abeta) is a principal component of parenchymal amyloid deposits in Alzheimer's disease. Abeta is derived from amyloid precursor protein (APP) by proteolytic cleavage. APP is subject to N- and O-glycosylation and potential tyrosine sulfation, following protein synthesis, and is then thought to be cleaved in an intracellular secretory pathway after or during these post-translational modifications. Studies utilizing agents that affect a series of steps in the protein secretory pathway have identified the possible intracellular sites of APP cleavage and Abeta generation within the protein secretory pathway. In the present study, using cells with normal protein metabolism, but expressing mutant APP with defective O-glycosylation, we demonstrated that the majority of APP cleavage by alpha-, beta-, and gamma-secretases occurs after O-glycosylation. Cells expressing the mutant APP noticeably decreased the generation of the intracellular APP carboxyl-terminal fragment (alphaAPPCOOH), a product of alpha-secretase, and both Abeta40 and Abeta42 in medium, a product of beta- and gamma-secretases. Furthermore, we found that the cells accumulate the mutant APP in intracellular reticular compartments such as the endoplasmic reticulum. Agents that could ambiguously affect the function of specific intracellular organelles and that may be toxic were not used. The present results indicate that APP is cleaved by alpha-, beta-, and gamma-secretases in step(s) during the transport of APP through Golgi complex, where O-glycosylation occurs, or in compartments subsequent to trans-Golgi of the APP secretory pathway.

From differentiation to proliferation: the secretory amyloid precursor protein as a local mediator of growth in thyroid epithelial cells

In various species, thyrotropin (TSH) is known to stimulate both differentiation and proliferation of thyroid follicle cells. This cell type has also been shown to express members of the Alzheimer amyloid precursor (APP) protein family and to release the secretory N-terminal domain of APP (sAPP) in a TSH-dependent fashion. In this study on binding to the cell surfaces, exogenously added recombinant sAPP stimulated phosphorylation mediated by mitogen-activated protein kinase and effectively evoked proliferation in the rat thyroid epithelial cell line FRTL-5. To see whether this proliverative effect of sAPP is of physiological relevance, we used antisense techniques to selectively inhibit the expression of APP and the proteolytic release of sAPP by cells grown in the presence of TSH. The antisense-induced inhibition was detected by immunoblot, immunoprecipitation, and immunocytochemical analyses. After the reduced APP expression and sAPP secretion, we observed a strong suppression of the TSH-induced cell proliferation down to 35%. Recombinant sAPP but not TSH was able to overcome this antisense effect and to completely restore cell proliferation, indicating that sAPP acts downstream of TSH, in that it is released from thyroid epithelial cells during TSH-induced differentiation. We propose that sAPP operates as an autocrine growth factor mediating the proliferative effect of TSH on neighboring thyroid epithelial cells.

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

Alzheimer's amyloid precursor protein (APP) is a transmembrane protein containing three phosphorylation sites in its cytoplasmic domain. In the present study, we isolated cDNA of APP from electric ray electric lobe (elAPP). This APP (elAPP699) consists of 699 amino acids, contains the beta-amyloid domain and has 80.7% similarity with the human APP695 isoform. The cytoplasmic domain, including three phosphorylation sites, was completely conserved. In the nerve terminals of the cholinergic neuron from the electric ray electric organ, we found elAPP699 existed exclusively in the mature form. We found the phosphorylated form of mature elAPP699 in the nerve terminal as well as in cell body. Immature elAPP699 was not subject to phosphorylation. Our findings indicate that, in neurons, the phosphorylation of APP occurs after maturation.

Biology of A beta amyloid in Alzheimer's disease

The genetic associations with the pathological features of AD are diverse: A rapidly growing number of mutations in presenilin 1 and 2 on chromosomes 14 and 1, respectively, are found in many early-onset FAD patients (Lendon et al., 1997). In addition, beta PP mutations are found in a small percentage of early-onset FAD kindreds. The apoE4 allele on chromosome 19 is associated with the presence of the most common form of AD, sporadic AD (Wisniewski & Frangione, 1992; Namba et al., 1991). However, it is clear that other proteins are also involved in the pathogenesis of AD, since some early-onset FAD kindreds do not have linkage to PS1, PS2, apoE, or beta PP, while at least 50% of late-onset AD is unrelated to apoE. Other proteins which have been implicated in the formation of senile plaques, but so far are not known to have any genetic linkage to AD, include proteoglycans (Snow et al., 1987), apoA1 (Wisniewski et al., 1995a), alpha 1-antichymotrypsin (Abraham et al., 1988), HB-GAM (Wisniewski et al., 1996a), complement components (McGeer & Rogers, 1992), acetylcholinesterase (Friede, 1965), and NAC (Ueda et al., 1993). Which of these proteins will be the most important for the etiology of the most common form of AD, late-onset sporadic AD, remains an open question. Three of the genes which are now known to be linked to AD, including PS1, beta PP, and apoE, have been established immunohistochemically and biochemically to be components of senile plaques (see Fig. 1). This raises at least two possibilities: either each of these proteins is part of one pathway with A beta-related amyloid formation as a final causative pathogenic event or amyloid deposition in AD is a reactive process related to dysfunction of a number of different CNS proteins. Whether or not amyloid formation is directly causative in the pathogenesis of AD, current data suggest that new therapeutic approaches which may inhibit the aggregation and/or the conformational change of sA beta to A beta fibrils (Soto et al., 1996) have the greatest likelihood to make a significant impact on controlling amyloid accumulation in AD.

Secreted form of beta-amyloid precursor protein activates protein kinase C and phospholipase Cgamma1 in cultured embryonic rat neocortical cells

The secreted form of beta-amyloid precursor protein (sAPP) has been reported to exert various biological activities in cultured neurons. The signal transduction mechanisms underlying these physiological functions of sAPP remain unclear. We now report that treatment of neural cells with the secreted form of APP695 (sAPP695) leads to dose- and time-dependent increase in phosphorylation of the endogenous substrates with a molecular mass of 80, 57 and 43 kDa. Pretreatment of cells with protein kinase C (PKC) inhibitor H-7 reduced phosphorylation of the 80- and 43-kDa proteins in a dose-dependent manner. The effect of sAPP695 on the phosphorylation is mimicked by phorbol 12-myristate-13-acetate (PMA). Downregulation of PKC by prolonged treatment of cells with PMA abolished sAPP695-enhanced phosphorylation of the 80- and 43-kDa proteins, indicating PKC is involved in the sAPP695-enhanced phosphorylation of these proteins in the cells. We also suggest that the 80- and 43-kDa proteins phosphorylated by sAPP695-stimulation are the major PKC substrates myristoylated alanine-rich C-kinase substrate and growth-associated protein-43. Furthermore, we demonstrate that tyrosine phosphorylation of phospholipase Cgamma1 and formation of inositol 1,4,5-trisphosphate were increased by sAPP695-stimulation. These observations suggest that sAPP695 induces the activation of the signaling pathways through a stimulation of phosphoinositide-PKC cascade.

Differential effects of the swedish mutant amyloid precursor protein on beta-amyloid accumulation and secretion in neurons and nonneuronal cells

Expression of the Swedish DeltaNL mutation in the beta-amyloid precursor protein (APPDeltaNL) dramatically increases Abeta generation in nonneuronal cell lines, although it is unclear whether intracellular levels of beta-amyloid (Abeta) are also elevated after APPDeltaNL expression. Furthermore, the effects of expressing APPDeltaNL in neurons on the production and secretion of Abeta-(1-40) and Abeta-(1-42) are unknown. To address these issues, we examined the generation of both intracellular and secreted Abeta-(1-40) and Abeta-(1-42) in human neuronal NT2N cells, in primary rat astrocytes, and in Chinese hamster ovary cells engineered to express wild-type APP or APPDeltaNL using a recombinant Semliki Forest virus expression system. Expression of APPDeltaNL led to a marked increase in APPbeta and the C-terminal fragment containing the entire Abeta sequence (C99) in all cells tested. However, a dramatic elevation of intracellular and secreted Abeta-(1-40) and Abeta-(1-42) was seen only in astrocytes and Chinese hamster ovary cells. The DeltaNL mutation did not cause a significant increase in intracellular or secreted Abeta-(1-40) or Abeta-(1-42) in NT2N cells. Since NT2N cells expressing APPDeltaNL accumulate much higher levels of C99 than cells expressing wild-type APP, we conclude that the rate-limiting step in Abeta production could be the further processing of C99 by gamma-secretase in these cells. These results show that the Swedish DeltaNL mutation causes nonneuronal cells to process APP via pathways more in common with the metabolism of wild-type APP in neurons.

Purification and spectroscopic characterization of beta-amyloid precursor protein from porcine brains

Soluble and membrane-bound isoforms of beta-amyloid protein precursor (APP) of Alzheimer's disease were extracted and purified from porcine brains. At least three types of soluble APP and membrane-bound APP with different molecular masses, ranging from 86 kDa to 116 kDa, were obtained. CD and infrared spectroscopies were used to determine the overall secondary-structure content of APP. The infrared spectra of soluble and membrane-bound APP (in dry and hydrated states) were similar in the amide-I and amide-II regions, suggesting that the overall secondary structures of the soluble and membrane isoforms were roughly identical. The amide-I band is composed of at least five component bands, located at 1694, 1674, 1652, 1637 and 1618 cm(-1) for soluble APP, and located at 1687, 1674, 1651, 1637 and 1614-1606 cm(-1) for membrane-bound APP, as evidenced by their respective second-derivative infrared spectra. The 1651-1652-cm(-1) band was associated with alpha-helix structures, while two types of beta-sheet structures are evidenced by two characteristic pairs of component bands. The 1674-cm(-1) and 1637-cm(-1) bands for soluble APP and membrane-bound APP were tentatively associated to beta-sheet structures. The second pair of bands, located at 1694 cm(-1) and at 1618 cm(-1) for soluble APP and at 1687 cm(-1) and 1614-1606 cm(-1) for membrane-bound APP, were associated with intermolecular beta-sheet structures or aggregated strands, as confirmed by heat denaturation. CD spectra indicated the presence of alpha-helix structures in soluble and membrane-bound APP. The secondary-structure content, estimated from CD spectra, was about 40-45% alpha-helix and 15-20% beta-sheet structures for soluble and membrane-bound APP.

Reversible ischemia increases levels of Alzheimer amyloid protein precursor without increasing levels of mRNA in the rabbit spinal cord

In a rabbit spinal cord ischemia model (RSCIM), the time courses of neuropathological damage of the spinal cord and neurological impairment of the motor functions are well established, demonstrating that the extent of neuropathological damage and the severity of neurological impairment are closely correlated. We used the RSCIM to elucidate the effects of reversible (15 min) and irreversible (60 min) ischemia on the endogenous levels of amyloid protein precursors (APPs) at both the mRNA and protein levels in the caudolumbar/sacral region of the spinal cord. We speculate that endogenous APPs are induced by ischemia as either trophic factors or stress-induced proteins in the RSCIM. A 15-min occlusion transiently increased the APP protein levels in neurons, which returned to the original levels by the end of 60 min occlusion. The increase in APP protein levels during 15-min ischemic insult does not appear to involve regulation at the mRNA level. The increased level of APPs, particularly of the soluble form, could support the possibility that APPs play a neuroprotective role in the RSCIM as stress-induced proteins. In contrast, failure to maintain the increased APP protein levels or to increase the mRNA, as seen in the 60-min ischemia samples, may be one of the causal factors that induce necrosis and neuronal cell death leading to irreversible neurological impairment.

Melatonin prevents death of neuroblastoma cells exposed to the Alzheimer amyloid peptide

Studies from several laboratories have generated evidence suggesting that oxidative stress is involved in the pathogenesis of Alzheimer's disease (AD). The finding that the amyloid beta protein (Abeta) has neurotoxic properties and that such effects are, in part, mediated by free radicals has provided insights into mechanisms of cell death in AD and an avenue to explore new therapeutic approaches. In this study we demonstrate that melatonin, a pineal hormone with recently established antioxidant properties, is remarkably effective in preventing death of cultured neuroblastoma cells as well as oxidative damage and intracellular Ca2+ increases induced by a cytotoxic fragment of Abeta. The effects of melatonin were extremely reproducible and corroborated by multiple quantitative methods, including cell viability studies by confocal laser microscopy, electron microscopy, and measurements of intracellular calcium levels. The importance of this finding is that, in contrast to conventional antioxidants, melatonin has a proposed physiological role in the aging process. Secretion levels of this hormone are decreased in aging and more severely reduced in AD. The reported phenomenon may be of therapeutic relevance in AD.

Melatonin prevents death of neuroblastoma cells exposed to the Alzheimer amyloid peptide

Studies from several laboratories have generated evidence suggesting that oxidative stress is involved in the pathogenesis of Alzheimer's disease (AD). The finding that the amyloid beta protein (Abeta) has neurotoxic properties and that such effects are, in part, mediated by free radicals has provided insights into mechanisms of cell death in AD and an avenue to explore new therapeutic approaches. In this study we demonstrate that melatonin, a pineal hormone with recently established antioxidant properties, is remarkably effective in preventing death of cultured neuroblastoma cells as well as oxidative damage and intracellular Ca2+ increases induced by a cytotoxic fragment of Abeta. The effects of melatonin were extremely reproducible and corroborated by multiple quantitative methods, including cell viability studies by confocal laser microscopy, electron microscopy, and measurements of intracellular calcium levels. The importance of this finding is that, in contrast to conventional antioxidants, melatonin has a proposed physiological role in the aging process. Secretion levels of this hormone are decreased in aging and more severely reduced in AD. The reported phenomenon may be of therapeutic relevance in AD.

Sequestration of RNA in Alzheimer's disease neurofibrillary tangles and senile plaques

The polypeptide composition of neurofibrillary tangles (NFTs) and senile plaques (SPs) has been characterized extensively within the Alzheimer's disease (AD) brain. Because few data exist on the nonproteinaceous components of these lesions, we sought to determine if NFTs, neuropil threads (NTs), and SPs contain RNA species. To accomplish this, acridine orange (AO) histofluorescence was employed, alone or in combination with thioflavine S (TS) staining and immunohistochemistry to identify RNAs in paraffin-embedded tissue sections of hippocampus and entorhinal cortex. Postmortem brain samples came from 32 subjects including AD and elderly Down's syndrome (DS) patients, age-matched normal controls, and non-AD diseased controls. AO stained the cytoplasm of normal hippocampal and entorhinal neurons in all of the cases, while NFTs, NTs, and SPs were AO-positive in the same regions of AD and DS brains. Cytoplasmic AO histofluorescence was abolished with RNase, but not DNase or proteinase K, indicating the relative specificity of AO for RNA species. Quantitative analysis of double-labeled sections demonstrated that approximately 80% of TS-positive NFTs also were AO-positive, whereas approximately 55% of TS-stained SPs contained AO labeling. These novel observations demonstrate the presence of RNAs in NFTs, NTs, and SPs.

The cytoplasmic domain of Alzheimer's amyloid precursor protein is phosphorylated at Thr654, Ser655, and Thr668 in adult rat brain and cultured cells

BACKGROUND

The cytoplasmic domain of the Alzheimer's disease amyloid precursor protein (APP) is phosphorylated in vitro at Thr654 and Ser655, and both in vitro and in intact cells at Thr668 (numbering for APP695 isoform).

MATERIALS AND METHODS

We have developed phosphorylation state-specific antibodies to each of the sites, and we have used these to analyze the phosphorylation of APP in adult rat brain and in cultured cell lines.

RESULTS

We demonstrate that all three sites in APP are phosphorylated in adult rat brain. Phosphorylation at Thr654, Ser655, and Thr668 was also observed in several cultured cell lines. In PC12 cells, phosphorylation at Ser655 was increased more than 10-fold by treatment with okadaic acid, a specific inhibitor of protein phosphatases 1 and 2A, but was not affected by activators of protein kinase C. In HeLa cells, phosphorylation at Thr668 was regulated in a cell cycle-dependent manner with near-stoichiometric phosphorylation being observed at the G2/M phase of the cell cycle. In general, phosphorylation at Ser655 was found to be highest in mature APP isoforms, whereas phosphorylation of Thr668 was highest in immature APP isoforms in cultured cells.

CONCLUSIONS

The results demonstrate that phosphorylation of the cytoplasmic domain of APP occurs at Thr654, Ser655, and Thr668 under physiological conditions. The further characterization of APP phosphorylation using phosphorylation-specific antibodies may help in the elucidation of the biological function of APP.

A new quantitative solution hybridisation-RNase protection assay for APP and APLP2 mRNA

Amyloid precursor protein (APP) and amyloid precursor-like protein 2 (APLP2) are members of a multigene family of proteins implicated in the pathogenesis of Alzheimer's disease. We describe the development of an RNA-RNA solution hybridisation-RNase protection assay to quantify APP mRNA. APP mRNA splice forms containing the Kunitz-type protease inhibitor (KPI) insert, and APLP2 mRNA in total nucleic acid extracts from a range of tissue types. Solution hybridisation-RNase protection assay enables absolute quantification of target mRNA, by conversion of the hybridisation signal to pg mRNA using a standard curve. The assay is sensitive, capable of detecting 1 pg target mRNA, and reproducible, with an inter-assay variability of less than 10% and an intra-assay variability of 3-4%. We quantified APP and APLP2 mRNA in cell lines and post-mortem human brain tissue samples. To test whether we could detect physiological differences in APP mRNA levels, a fibroblast cell line with a paternal chromosome 21 deletion of the region including the APP gene was analysed and found to express half as much APP mRNA as control fibroblasts. In addition, a reversible, approx. 30% increase in APP mRNA levels was detected in human lymphoblastoid cell lines following heat shock, a physical stimulus previously shown to increase APP expression. Regional differences in the expression of APP and APLP2 were seen in human post-mortem cerebral cortex and cerebellum. Levels of APP and APLP2 mRNA were highest in the temporal cortex, slightly lower in frontal and occipital cortices, and lowest in the cerebellum. The highest proportion of KPI-containing APP was seen in the frontal and temporal cortices. The ratio of APP:APLP2 mRNA was 1:0.3 in the cortical tissue and 1:0.8 in the cerebellum. In conclusion, quantitative solution hybridisation-RNase protection assay of total APP. APP KPI and APLP2 mRNA provides a new tool to improve the resolution of studies of potentially subtle alterations in the expression of these genes in both cell culture model systems and Alzheimer's disease post-mortem human brain tissue.

Quantification of APP and APLP2 mRNA in APOE genotyped Alzheimer's disease brains

Amyloid precursor protein (APP) is metabolised to produce A beta, a peptide found aggregated in Alzheimer's disease neuritic plaques. APP is a member of a multigene protein family which includes amyloid precursor-like protein 2 (APLP2). Since A beta accumulation can be triggered by factors acting up- or downstream of APP processing, we investigated whether APP mRNA expression was altered in Alzheimer's disease post-mortem cerebral cortex. In addition, we characterised cortical APLP2 mRNA levels. Quantitative RNA-RNA solution hybridisation-RNase protection was used to assay total APP. APP containing the Kunitz-type protease inhibitor (KPI) insert and APLP2 mRNA in mid-temporal and superior frontal cortices from apolipoprotein E-genotyped subjects with Alzheimer's disease, other neurological diseases and non-demented controls. Approximately 3 times more APP than APLP2 mRNA was detected and about 70% of total APP mRNA contained the KPI insert in the control subjects. Total APP and APLP2 mRNA levels were significantly reduced in Alzheimer's disease mid-temporal, but not superior frontal cortex, suggesting that regional reductions in these mRNA correlate with severity of disease pathology. A small significant increase in the proportion of APP KPI mRNA was seen in both cortical regions in Alzheimer's disease. Apolipoprotein E genotype did not influence cortical levels of total APP, APP KPI or APLP2 mRNA. Alzheimer's disease-related increases in tissue DNA content were seen in both regions studied, while tissue RNA levels were reduced in the positive disease controls. In summary, these results indicate that Alzheimer's disease is not associated with over-expression of either APP or APLP2 mRNA. Our findings reveal a disease-associated increase in the proportion of APP KPI-containing isoforms, and further investigation should clarify whether this predisposes affected individuals to A beta production and aggregation, or reflects later events such as gliosis and neuronal cell death.

APP-BP1, a novel protein that binds to the carboxyl-terminal region of the amyloid precursor protein

beta-Amyloid protein precursors (APPs, 695-770 amino acids) are the source of the 39-43 amino acid beta-amyloid (A beta) peptides that comprise diffuse and fibrillar deposits in the cerebral cortex and vasculature of Alzheimer's disease brains. A beta is thought to play a role in the pathogenesis of Alzheimer's disease, and, hence, considerable effort has been invested in defining the means by which A beta is generated from the APPs. Knowledge of the normal function of the APPs is sure to provide insights into the genesis and pathological persistence of A beta in Alzheimer's disease. APP is a cell surface protein with a large extracellular amino-terminal domain, a single transmembrane segment, and a short cytoplasmic tail. Its location and structural features characteristic of a receptor for signal transduction led us to search for potential effector proteins capable of binding and interacting with its cytoplasmic domain. Here, we report the cloning of a cDNA encoding one such protein. This ubiquitously expressed 59-kDa APP-binding protein, called APP-BP1, is 61% similar to a protein encoded by the Arabidopsis AXR1 gene, required for normal response to the hormone auxin, and is a relative of the ubiquitin activating enzyme E1.

The interaction between apolipoprotein E and Alzheimer's amyloid beta-peptide is dependent on beta-peptide conformation

An important feature of Alzheimer's disease (AD) is the cerebral deposition of amyloid. The main component of the amyloid is a 39-44-amino acid residue protein called amyloid beta (A beta), which also exists as a normal protein in biological fluids, known as soluble A beta. A major risk factor for late-onset AD is the inheritance of the apolipoprotein (apo) E4 isotype of apoE. How apoE is involved in the pathogenesis of AD is unclear; however, evidence exists for a direct apoE/A beta interaction. We and others have shown that apoE copurifies with A beta from AD amyloid plaques and that under certain in vitro conditions apoE promotes a beta-sheet structure in A beta peptides. Currently we document the high affinity binding of A beta peptides to both human recombinant apoE3 and -E4 with a KD of 20 nM. This interaction is greatly influenced by the conformational state of the A beta peptide used. Furthermore, we show that the fibril modulating effect of apoE is also influenced by the initial secondary structure of the A beta peptide. The preferential binding of apoE to A beta peptides with a beta-sheet conformation can in part explain the copurification of A beta and apoE from AD amyloid plaques.

Two-hybrid system as a model to study the interaction of beta-amyloid peptide monomers

The kinetics of amyloid fibril formation by beta-amyloid peptide (Abeta) are typical of a nucleation-dependent polymerization mechanism. This type of mechanism suggests that the study of the interaction of Abeta with itself can provide some valuable insights into Alzheimer disease amyloidosis. Interaction of Abeta with itself was explored with the yeast two-hybrid system. Fusion proteins were created by linking the Abeta fragment to a LexA DNA-binding domain (bait) and also to a B42 transactivation domain (prey). Protein-protein interactions were measured by expression of these fusion proteins in Saccharomyces cerevisiae harboring lacZ (beta-galactosidase) and LEU2 (leucine utilization) genes under the control of LexA-dependent operators. This approach suggests that the Abeta molecule is capable of interacting with itself in vivo in the yeast cell nucleus. LexA protein fused to the Drosophila protein bicoid (LexA-bicoid) failed to interact with the B42 fragment fused to Abeta, indicating that the observed Abeta-Abeta interaction was specific. Specificity was further shown by the finding that no significant interaction was observed in yeast expressing LexA-Abeta bait when the B42 transactivation domain was fused to an Abeta fragment with Phe-Phe at residues 19 and 20 replaced by Thr-Thr (AbetaTT), a finding that is consistent with in vitro observations made by others. Moreover, when a peptide fragment bearing this substitution was mixed with native Abeta-(1-40), it inhibited formation of fibrils in vitro as examined by electron microscopy. The findings presented in this paper suggest that the two-hybrid system can be used to study the interaction of Abeta monomers and to define the peptide sequences that may be important in nucleation-dependent aggregation.

Coagulation factor XIa cleaves the RHDS sequence and abolishes the cell adhesive properties of the amyloid beta-protein

Amyloid beta-protein (A beta) is the major constituent of senile plaques and cerebrovascular amyloid deposits in Alzheimer's disease and is proteolytically derived from its transmembrane parent protein the amyloid beta-protein precursor (A beta PP). Although the physiological role(s) of secreted A beta PPs are not fully understood, several potential functions have been described including the regulation of hemostatic enzymes factors XIa and IXa and a role in cell adhesion. In the present study, we investigated the proteolytic processing of A beta PP by factor XIa (FXIa). Incubation of the human glioblastoma cell line U138 stably transfected to overexpress the 695 isoform of A beta PP with FXIa (2.5-5 nM) resulted in proteolytic cleavage of secreted A beta PP. Higher concentrations of FXIa (> 25 nM) resulted in loss in cell adherence. Coincubation of FXIa with purified, recombinant Kunitz protease inhibitor domain of A beta PP blocked both the proteolytic processing of A beta PP and the loss of cell adhesion. The RHDS cell adhesion site of A beta PP resides within residues 5-8 of the A beta domain. Incubation of synthetic A beta 1-40 peptide with increasing concentrations of FXIa resulted in cleavage of A beta between Arg5 and His6 within the cell adhesion domain of the peptide. FXIa-digested A beta 1-40 or A beta PP695 lost their abilities to serve as cell adhesion substrates consistent with cleavage through this cell adhesion site. Together, these results suggest a new potential biological function for FXIa in the modulation of cell adhesion. In addition, we have shown that FXIa can proteolytically alter A beta and therefore possibly modify its physiological and perhaps pathological properties.

Enhanced plasmin inhibition by a reactive center lysine mutant of the Kunitz-type protease inhibitor domain of the amyloid beta-protein precursor

The Alzheimer's disease related protein, amyloid beta-protein precursor (A beta PP), contains a domain homologous to Kunitz-type serine protease inhibitors (KPI). The recombinant KPI domain of A beta PP is a potent inhibitor of coagulation factors XIa and IXa and functions as an anticoagulant in vitro. Here we report the expression, purification, and characterization of a reactive center lysine mutant of the KPI domain of A beta PP (KPI-Lys17). An expression plasmid for the KPI-Lys17 domain of A beta PP encoded amino acids 285-345 of the A beta PP cDNA containing a lysine substitution at arginine 17 in the KPI domain. The secreted 61-amino acid product was purified to homogeneity and functionally characterized. The protease inhibitory properties of the KPI-Lys17 domain were compared to those of the native KPI domain of A beta PP. Both KPI domains equally inhibited trypsin, chymotrypsin, and coagulation factors IXa and Xa. However, the KPI-Lys17 domain was an approximately 25-fold less effective inhibitor of coagulation factor XIa resulting in markedly less prolongation of the activated partial thromboplastin time compared to the native KPI domain of A beta PP. On the other hand, the KPI-Lys17 domain was an approximately 10- and 5-fold better inhibitor of plasmin in a chromogenic substrate assay and in a fibrinolytic assay, respectively, than the native KPI domain of A beta PP. Together, these studies suggest that the KPI-Lys17 domain has enhanced anti-fibrinolytic and diminished factor XIa inhibitory properties compared to the native KPI domain of A beta PP.

Amyloid beta-protein aggregation nullifies its pathologic properties in cultured cerebrovascular smooth muscle cells

Alzheimer's disease and related disorders are characterized by deposition of aggregated amyloid beta-protein (A beta) and accompanying pathologic changes in the neuropil and in the walls of cerebral blood vessels. A beta induces neurotoxicity in vitro, and this effect is markedly enhanced when the peptide is preaggregated. Recently, we reported that freshly solubilized A beta 1-42 can induce cellular degeneration and a striking increase in the levels of cellular amyloid beta-protein precursor and soluble A beta peptide in cultured cerebrovascular smooth muscle cells (Davis-Salinas, J., Saporito-Irwin, S. M., Cotman, C. W., and Van Nostrand, W. E. (1995) J. Neurochem. 65, 931-934). In the present study, we show that preaggregation of A beta 1-42 abolishes the ability of the peptide to induce these cellular pathologic responses in these cells in vitro. These findings suggest that distinct mechanisms for A beta-induced cytotoxicity exist for cultured neurons and cerebrovascular smooth muscle cells, supporting that different processes may be involved in the parenchymal and cerebrovascular pathology of Alzheimer's disease and related disorders.

Proteoglycan-mediated inhibition of A beta proteolysis. A potential cause of senile plaque accumulation

Senile plaques of Alzheimer's disease brain contain, in addition to beta amyloid peptide (A beta), multiple proteoglycans. Systemic amyloidotic deposits also routinely contain proteoglycan, suggesting that these glycoconjugates are generally involved in amyloid plaque formation and/or persistence. We demonstrate that heparan sulfate proteoglycan (HSPG) and chondroitin sulfate proteoglycan (CSPG) inhibit the proteolytic degradation of fibrillar, but not non-fibrillar, A beta at physiological pH. In accordance with the proteolysis studies, high affinity binding of proteoglycans to fibrillar A beta(1-40) and A beta(1-42) is observed from pH 4 to 9, whereas appreciable binding of HSPG or CSPG to non-fibrillar peptide is only seen at pH < 6. This differing pH dependence of binding suggests that a lysine residue is involved in proteoglycan association with fibrillar A beta, whereas a protonated histidine appears to be needed for binding of the glycoconjugates to non-fibrillar peptide. Scatchard analysis of fibrillar A beta association with proteoglycans indicates a single affinity interaction, and the binding of both HSPG and CSPG to fibrillar A beta is completely inhibited by free glycosaminoglycan chains. This implies that these sulfated carbohydrate moieties are primarily responsible for proteoglycan.A beta interaction. The ability of proteoglycans to bind fibrillar A beta and inhibit its proteolytic degradation suggests a possible mechanism of senile plaque accumulation and persistence in Alzheimer's disease.

The Alzheimer amyloid precursor proteoglycan (appican) is present in brain and is produced by astrocytes but not by neurons in primary neural cultures

Recent studies showed that the Alzheimer amyloid precursor (APP) occurs as the core protein of a chondroitin sulfate proteoglycan (appican) in C6 glioma cells. In the present study we show that appican is present in both human and rat brain tissue. Cortical rat brain cell cultures were used to identify appican-producing cells. Soluble secreted and cell-associated appican was produced by mixed glial cultures but not by primary neuronal cultures. Among the three major glial cell types, astrocytes produced high levels of appican, while oligodendrocytes failed to produce any. Only low levels of this molecule were occasionally detected in microglial cultures. Expression of appican in astrocyte cultures was regulated by the composition of the growth media. N2a neuroblastoma cells also produced appican; however, treatment with dibutyryl cAMP which promotes neuronal differentiation in these cells inhibited its production without inhibiting synthesis of APP. In contrast to the restricted expression of appican, APP was present in all cultures, and its production was independent of appican synthesis. Neuronal cultures produced mainly APP695 while glial cultures produced the Kunitz type protease inhibitor containing APP. The astrocyte-specific expression of appican suggests a function distinct from the function of APP. Brain appicans may play a role in the development of Alzheimer disease neuropathology.

Ca(2+)-dependent binding of human serum amyloid P component to Alzheimer's beta-amyloid peptide

Serum amyloid P component (SAP), a normal glycoprotein, is universally found in amyloid deposits, including cerebrovascular amyloid of Alzheimer's disease. This paper describes the Ca(2+)-dependent binding of human SAP to Alzheimer's beta-amyloid peptide (A beta). 125I-SAP binds to synthetic human A beta-(1-40) immobilized on microtiter plates at a dissociation constant of 6.0 x 10(-9) M in 0.01 M Tris-HCl, 0.15 M NaCl, pH 7.5, containing 2 mM Ca2+, 1% bovine serum albumin, and 0.05% Tween 20. Binding inhibition assay has shown that soluble A beta-(1-40) and A beta-(1-28) also bind to SAP. Since SAP is resistant to proteases in the presence of calcium, the Ca(2+)-dependent binding of SAP to soluble A beta and to beta-amyloid fibrils would give pathological effects on fibril formation and persistence of beta-amyloid in Alzheimer's disease.

The chondroitin sulfate attachment site of appican is formed by splicing out exon 15 of the amyloid precursor gene

Appicans are secreted and cell-associated chondroitin sulfate proteoglycans containing Alzheimer amyloid precursor (APP) as their core protein. Appicans are found in brain tissue, and in cell cultures their expression depends on both cell type and growth conditions. Here we report that the core protein of appicans derives from an APP mRNA lacking exon 15. Splicing out of this exon creates a new consensus sequence for the attachment of a chondroitin sulfate chain in the resulting APP product. Transfection of C6 glioma or 293 kidney fibroblast cells with APP cDNAs containing exon 15 produced no appican, while transfection with an APP cDNA lacking this exon induced high levels of appican production. Polymerase chain reactions indicated that appican-producing cells contained an APP mRNA species without exon 15, whereas cells without this mRNA produced no appican. Site-directed mutagenesis combined with immunoreactivity experiments showed that the chondroitin sulfate chain is attached to a serine residue 16 amino acids upstream of the amino terminus of the A beta sequence of APP. The attachment of a glycosaminoglycan chain close to the A beta sequence of APP may affect the proteolytic processing of APP and production of A beta. The proteoglycan nature of APP suggests that addition of the chondroitin sulfate glycosaminoglycan is important for the implementation of the biological function of these proteins.

Transgenic mice and modeling Alzheimer's disease

The etiology of Alzheimer's disease (AD) is poorly understood, and no effective therapies are available. Although histopathology of the disease has been studied thoroughly, the relationship of various AD lesions to pathological processes and to dementia are debated. Progress would be greatly enhanced by existence of manipulable small animal models of the disease. Recently, transgenic strategies to developing such a model have been extensively explored. The approach has proved to be difficult and has yielded some disappointments, but also some encouraging results. Transgenic strategies for obtaining a model for AD are surveyed in this review and, as an illustration, early AD-like features of transgenic mice produced in our laboratory are described.

A potential role for apoptosis in neurodegeneration and Alzheimer's disease

Previous studies have shown that beta-amyloid (A beta) peptides are neurotoxic. Recent data suggest that neurons undergoing A beta-induced cell death exhibit characteristics that correspond to the classical features of apoptosis, suggesting that these cells may initiate a program of cell death. This chapter explores the criteria and precautions that must be applied to evaluate mechanisms of cell death in vitro and in vivo, discusses the evidence supporting an apoptotic mechanism of cell death in response to A beta in cultured neurons, and describes potential correlations for these findings in the Alzheimer's disease brain. In addition, cellular signaling pathways that may be associated with apoptosis in response to A beta are examined, and support for apoptosis as a mechanism of cell death for other neurodegeneration-inducing stimuli (e.g., oxidative injury) is described. The connection of multiple stimuli that induce neuronal cell death to an apoptotic mechanism suggests that apoptosis could play a central role in neurodegeneration in the brain.