Alzheimer beta/A4-amyloid precursor protein: evidence for putative amyloidogenic fragment

Mild oxidative stress induces redistribution of BACE1 in non-apoptotic conditions and promotes the amyloidogenic processing of Alzheimer's disease amyloid precursor protein

BACE1 is responsible for β-secretase cleavage of the amyloid precursor protein (APP), which represents the first step in the production of amyloid β (Aβ) peptides. Previous reports, by us and others, have indicated that the levels of BACE1 protein and activity are increased in the brain cortex of patients with Alzheimer's disease (AD). The association between oxidative stress (OS) and AD has prompted investigations that support the potentiation of BACE1 expression and enzymatic activity by OS. Here, we have established conditions to analyse the effects of mild, non-lethal OS on BACE1 in primary neuronal cultures, independently from apoptotic mechanisms that were shown to impair BACE1 turnover. Six-hour treatment of mouse primary cortical cells with 10-40 µM hydrogen peroxide did not significantly compromise cell viability but it did produce mild oxidative stress (mOS), as shown by the increased levels of reactive radical species and activation of p38 stress kinase. The endogenous levels of BACE1 mRNA and protein were not significantly altered in these conditions, whereas a toxic H2O2 concentration (100 µM) caused an increase in BACE1 protein levels. Notably, mOS conditions resulted in increased levels of the BACE1 C-terminal cleavage product of APP, β-CTF. Subcellular fractionation techniques showed that mOS caused a major rearrangement of BACE1 localization from light to denser fractions, resulting in an increased distribution of BACE1 in fractions containing APP and markers for trans-Golgi network and early endosomes. Collectively, these data demonstrate that mOS does not modify BACE1 expression but alters BACE1 subcellular compartmentalization to favour the amyloidogenic processing of APP, and thus offer new insight in the early molecular events of AD pathogenesis.

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

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

Identification of novel genes that modify phenotypes induced by Alzheimer's beta-amyloid overexpression in Drosophila

Sustained increases in life expectancy have underscored the importance of managing diseases with a high incidence in late life, such as various neurodegenerative conditions. Alzheimer's disease (AD) is the most common among these, and consequently significant research effort is spent on studying it. Although a lot is known about the pathology of AD and the role of beta-amyloid (Abeta) peptides, the complete network of interactions regulating Abeta metabolism and toxicity still eludes us. To address this, we have conducted genetic interaction screens using transgenic Drosophila expressing Abeta and we have identified mutations that affect Abeta metabolism and toxicity. These analyses highlight the involvement of various biochemical processes such as secretion, cholesterol homeostasis, and regulation of chromatin structure and function, among others, in mediating toxic Abeta effects. Several of the mutations that we identified have not been linked to Abeta toxicity before and thus constitute novel potential targets for AD intervention. We additionally tested these mutations for interactions with tau and expanded-polyglutamine overexpression and found a few candidate mutations that may mediate common mechanisms of neurodegeneration. Our data offer insight into the toxicity of Abeta and open new areas for further study into AD pathogenesis.

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

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

Baculoviruses expressing the human familial Alzheimer's disease presenilin 1 mutation lacking exon 9 increase levels of an amyloid beta-like protein in Sf9 cells

Presenilin 1 (PS1) plays a pivotal role in the production of the amyloid-beta protein (Abeta) that is central to the pathogenesis of Alzheimer's disease. PS1 regulates the intramembranous proteolysis of a 99-amino-acid C-terminal fragment of the amyloid precursor protein (APP-C99), a cleavage event that releases Abeta following a reaction catalyzed by an enzyme termed 'gamma-secretase'. The molecular mechanism of PS1-mediated, gamma-secretase cleavage remains largely unresolved. In particular, controversy surrounds whether PS1 includes the catalytic site of the gamma-secretase protease or whether instead PS1 mediates gamma-secretase activity indirectly, perhaps by regulating the trafficking or presentation of substrates to the 'authentic' protease, which may be a molecule distinct from PS1. To address this issue, the baculovirus expression system was used to co-express: (i) APP-C99; (ii) a pathogenic, constitutively active mutant form of PS1 lacking exon 9 (PS1DeltaE9); (iii) nicastrin and (iv) tropomyosin in Spodoptera frugiperda (Sf9) cells. Cells infected with APP-C99 alone produced an Abeta-like species, and levels of this species were enhanced by the addition of baculoviruses bearing the PS1DeltaE9 mutation. The addition to APP-C99-infected cells of baculoviruses bearing nicastrin, also a transmembrane protein, had a neutral or inhibitory effect on the reaction; tropomyosin viruses had the same effect as nicastrin viruses. These results suggest that PS1DeltaE9 molecules expressed in Sf9 cells retain the ability to modulate Abeta levels. Baculoviral-expressed PS1DeltaE9 provides a source of microgram quantities of bioactive molecules for use as starting material for purifying and reconstituting gamma-secretase activity from its individual purified component parts.

Signal transduction through tyrosine-phosphorylated C-terminal fragments of amyloid precursor protein via an enhanced interaction with Shc/Grb2 adaptor proteins in reactive astrocytes of Alzheimer's disease brain

The proteolytic processing of amyloid precursor protein (APP) through the formation of membrane-bound C-terminal fragments (CTFs) and of soluble beta-amyloid peptides likely influences the development of Alzheimer's disease (AD). We show that in human brain a subset of CTFs are tyrosine-phosphorylated and form stable complexes with the adaptor protein ShcA. Grb2 is also part of these complexes, which are present in higher amounts in AD than in control brains. ShcA immunoreactivity is also greatly enhanced in patients with AD and occurs at reactive astrocytes surrounding cerebral vessels and amyloid plaques. A higher amount of phospho-ERK1,2, likely as result of the ShcA activation, is present in AD brains. In vitro experiments show that the ShcA-CTFs interaction is strictly confined to glial cells when treated with thrombin, which is a well known ShcA and ERK1,2 activator and a regulator of APP cleavage. In untreated cells ShcA does not interact with either APP or CTFs, although they are normally generated. Altogether these data suggest that CTFs are implicated in cell signaling via Shc transduction machinery, likely influencing MAPK activity and glial reaction in AD patients.

Intracellular accumulation of detergent-soluble amyloidogenic A beta fragment of Alzheimer's disease precursor protein in the hippocampus of aged transgenic mice

To study amyloid beta-protein (A beta) production and aggregation in vivo, we created two transgenic (Tg) mouse lines expressing the C-terminal 100 amino acids of human amyloid precursor protein (APP): Tg C100.V717F and Tg C100.WT. Western blot analysis showed that human APP-C100 and A beta were produced in brain and some peripheral tissues and A beta was produced in serum. Using antibodies specific for the A beta C terminus we found that Tg C100.V717F produced a 1.6-fold increase in A beta42/A beta40 compared with Tg C100.WT. Approximately 30% of total brain A beta (approximately 122 ng/g of wet tissue) was water-soluble. The remaining 70% of A beta partitioned into the particulate fraction and was completely sodium dodecyl sulfate-soluble. In contrast, human Alzheimer's disease brain has predominantly sodium dodecyl sulfate-insoluble A beta. Immunohistochemistry with an A beta(5-8) antibody showed that A beta or A beta-containing fragments accumulated intracellularly in the hippocampus of aged Tg C100.V717F mice. The soluble A beta levels in Tg brain are similar to those in normal human brain, and this may explain the lack of microscopic amyloid deposits in the Tg mice. However, this mouse model provides a system to study the intracellular processing and accumulation of A beta or A beta-containing fragments and to screen for compounds directed at the gamma-secretase activity.

Stable association of presenilin derivatives and absence of presenilin interactions with APP

Mutations in two related genes, presenilin 1 and 2 presenilin 2 (PS1 and PS2), cosegregate with Alzheimer's disease. PS1 and PS2 are highly homologous polytopic membrane proteins that are subject to endoproteolytic cleavage in vivo. The resulting N- and C-terminal derivatives are the preponderant PS-related species that accumulate in cultured cells and tissue. In earlier studies, we demonstrated that PS1 N- and C-terminal derivatives accumulate to 1:1 stoichiometry and that the absolute levels of fragments are established by a tightly regulated and saturable mechanism. These findings led to the suggestion that the levels of PS1 derivatives might be determined by their association with limiting cellular components. In this study, we use in situ chemical cross-linking and coimmunoprecipitation analyses to document that the N- and C-terminal derivatives of either PS1 or PS2 can be coisolated. Moreover, and in contrast to published reports which documented that PS1 and PS2 form stable heteromeric assemblies with the beta-amyloid precursor protein (APP), we have failed to provide evidence for physiological complexes between PS1 and PS2 holoproteins or their derivatives with APP.

Human A beta-amyloid and amyloid precursor protein accumulates in rat brain cells after cultured human leptomeningeal fibroblast implants

Cultured human leptomeningeal fibroblasts grafted into rat frontal cortex were localized to the implant pocket and to adjacent host leptomeninges. Immunohistochemical studies using a panel of human-specific and domain-specific APP antibodies revealed that all grafted cells expressed both APP and A beta in situ. Remarkably, these antibodies also labeled rat pial and ependymal cells as well as reactive astrocytes adjacent to vessels. In addition, apical projections and cell bodies of many cortical pyramidal neurons contained human-specific APP immunoreactive material. Groups of subcortical neurons, particularly those of the amygdala, hippocampal formation and suprachiasmatic nuclei, were similarly labeled. The presence of human APP in host brains was confirmed by immunoblotting. Birefringent Congo Red staining was observed in the cortical neuropil and in leptomeningeal vessels. These data indicate that grafted leptomeningeal fibroblasts hyperexpress APP and A beta which can diffuse into parenchyma and be taken up by specific rat cells.

A novel metalloprotease in rat brain cleaves the amyloid precursor protein of Alzheimer's disease generating amyloidogenic fragments

The amyloid protein (A beta or beta A4) is the major constituent of amyloid plaques in the Alzheimer's disease brain. A beta is cleaved from the amyloid precursor protein (APP) by a mechanism which is poorly understood. Cell culture studies suggest that APP may be cleaved by secretases within the late Golgi compartment. Studies performed so far have mainly used exogenous APP and synthetic peptides as substrates. For this study, a Golgi and plasma membrane-enriched fraction was isolated from rat brain and incubated at 37 degrees C at pH 7.2 to study the degradation of endogenous APP. The breakdown of APP was accompanied by the concomitant generation of A beta-containing C-terminal fragments, in a time-dependent fashion. The metal ion chelators EDTA and 1,10-phenanthroline inhibited this degradation. The inhibition by EDTA was reversed by 50 microM Zn2+ but not by other metal ions. The protease activity was not inhibited by cysteine, serine or aspartic protease inhibitors nor was it inhibited by compounds which are inhibitors of known metalloendopeptidases and matrix metalloproteinases (cFP, phosphoramidon and TIMP-2). Our data suggest that a novel Zn(2+)-dependent metalloprotease activity associated with a Golgi and plasma membrane-enriched fraction can degrade endogenous APP to generate A beta containing C-terminal fragments. This protease may generate amyloidogenic fragments of APP which may serve as precursors for A beta.

Cerebrovascular transport of Alzheimer's amyloid beta and apolipoproteins J and E: possible anti-amyloidogenic role of the blood-brain barrier

It is uncertain whether soluble circulating amyloid beta (sA beta) is the precursor of amyloid beta (A beta) found in cerebrovascular and parenchymal amyloid lesions in Alzheimer's Disease, and if so, how the transition to the filamentous form is brought about. Several lines of evidence suggest that apolipoprotein E (apoE) and apolipoprotein J (apoJ) may be involved in the regulation of amyloidogenesis. They both bind sA beta/A beta in vivo and in vitro. It has been suggested that apoE may modulate beta-pleated conformation of A beta and therefore act as a proamyloidogenic factor. On the other hand, apoJ as a major carrier protein of sA beta in body fluids may keep the peptide in a soluble form, thus having an anti-amyloidogenic effect. Using a well established guinea-pig brain perfusion model we have studied the blood-brain barrier (BBB) processes involved in the regulation of cerebral capillary sequestration, transport and metabolism of i) sA beta 1-40 and sA beta 1-42, synthetic peptides identical to the 40 and 42 residue forms of A beta, found primarily in vascular deposits and senile plaques, respectively; and ii) apoJ, apoE3 and apoE4 alone, and in a complex with sA beta. Specific saturable BBB luminal binding of both peptides was followed by transport into brain parenchyma and metabolism at the abluminal side of the BBB and/or in brain. The capillary sequestration of sA beta 1-40 was significant, while retention by the microvasculature of sA beta 1-42 was negligible. Binding to microvessels and blood-to-brain transport of both intact apoJ and sA beta 1-40 apoJ complexes were among the highest ever recorded for peptides and proteins at the BBB in vivo. These processes appear to be mediated by glycoprotein 330 (gp330/megalin), a receptor for multiple ligands, including apoJ. In contrast, capillary retention and transport of apoE3, apoE4 and sA beta 1-40-apoE3 complex were low to negligible, while blood-brain transport of sA beta 1-40-apoE4 was moderate. It is suggested that normal BBB may have predominantly anti-amyloidogenic functions by i) degrading sA beta during blood-to-brain transport; ii) favoring sequestration and transport of apoJ alone and in complex with sA beta via gp330 receptor-mediated mechanism and iii) excluding apoE3 and apoE4 isoforms from brain.

Baculovirus-infected cells do not produce the amyloid peptide of Alzheimer's disease from its precursor

The amyloid peptide (Abeta) of Alzheimer's disease (AD) is produced by proteolytic cleavage of a larger precursor, the amyloid peptide precursor or APP. The discovery of pathogenic mutations in the APP gene provides strong evidence for the hypothesis that APP metabolism is involved in the etiology of AD. To study the metabolism of the protein, human APP has been expressed in several mammalian cell types. Insect cells, infected by a recombinant baculovirus carrying the human APP sequence, also provide an interesting expression system because these cells do not produce endogenous APP. Baculovirus-infected cells synthesize very high amounts of extracellular soluble APP, after cleavage of the transmembrane protein, as described for mammalian cells. However, we demonstrate here that insect cells do not produce Abeta from APP. These results suggest that while the enzymatic activity needed for the production of soluble APP is conserved between insect and mammalian cells, the enzymes required for the production of Abeta from APP are only expressed in mammalian cells.

Induction of connexin43 and gap junctional communication in PC12 cells overexpressing the carboxy terminal region of amyloid precursor protein

Previous studies have shown that PC12 cells overexpressing beta/A4 amyloid peptide display altered morphology characterized by pronounced membrane ruffling and extensive intercellular appositions. Having observed other cell types in which these features accompany increased connexin43 (Cx43) production and gap junctional communication, we examined Cx43 in normal and beta/A4-transfected PC12 cells. Studies of two beta/A4-transfected PC12 clones revealed an induction of Cx43 expression by Western blotting, intracellular and plasma membrane-associated Cx43 in some cells of cultures processed by immunofluorescence, dye-transfer between some cells microinjected with Lucifer Yellow, and gap junctions between cells examined by EM. Normal and vector-transfected PC12 cells exhibited none of these properties. Increased immunofluorescence in some clusters of beta/A4-transfected cells was also observed with a monoclonal antibody against connexin32. The results suggest that beta/A4 amyloid peptide may cause aberrant intercellular communication and gap junction formation through induction or increased expression of connexins in cells that are not normally coupled or only poorly coupled by gap junctions.

Production and increased detection of amyloid beta protein and amyloidogenic fragments in brain microvessels, meningeal vessels and choroid plexus in Alzheimer's disease

Recent advances indicate soluble amyloid beta (A beta) protein is produced constitutively during normal metabolism of the amyloid precursor protein (APP). This has not been directly examined in human brain vascular tissues. Using a panel of well-characterized antibodies, here we show that increased amounts of soluble A beta were found in isolated vascular tissues from AD subjects compared to age-matched controls without significant Alzheimer pathology. Immunocytochemical analyses of isolated vessel preparations showed characteristic transverse patterns of A beta deposits in large vessels with smooth muscle, however, fine A beta deposits were apparent even in capillaries. A proportion of such A beta protein and potentially amyloidogenic carboxyl terminal fragments were released by solubilization and disruption of the vascular basement membrane by collagenase treatments. We further demonstrated by in vitro metabolic labelling that soluble A beta or an A beta-like peptide is associated and produced by cerebral microvessels, meningeal vessels and the choroid plexus isolated postmortem from human as well as rat brain. Compared to those from young rats, cerebral microvessels from aging rats showed increased release of carboxyl terminal fragments of APP and A beta-like peptide. Our observations provide the first direct demonstration that human vascular tissues produce soluble A beta, a product of the secretory pathway in APP processing. Our findings also suggest that aging associated alterations in the basement membranes are a factor in A beta accumulation that results in vascular amyloid deposition, the principal feature of cerebral amyloid angiopathy.

Accumulation of amyloid precursor protein-like immunoreactivity in rat brain in response to thiamine deficiency

Thiamine deficiency (TD) is a classical model of impaired cerebral oxidation. As in Alzheimer's disease (AD), TD is characterized by selective neuronal loss, decreased activities of thiamine pyrophosphate-dependent enzymes, cholinergic deficits and memory loss. Amyloid beta-protein (A beta), a approximately 4 kDa fragment of the beta-amyloid precursor protein (APP), accumulates in the brains of patients with AD or Down's syndrome. In the current study, we examined APP and A beta immunoreactivity in the brains of thiamine-deficient rats. Animals received thiamine-deficient diet ad libitum and daily injections of the thiamine antagonist, pyrithiamine. Immunocytochemical staining and immunoblotting utilized a rabbit polyclonal antiserum against human APP645-694 (numbering according to APP695 isoform). Three, 6 and 9 days of TD did not appear to damage any brain region nor change APP-like immunoreactivity. However, 13 days of TD led to pathological lesions mainly in the thalamus, mammillary body, inferior colliculus and some periventricular areas. While immunocytochemistry and thioflavine S histochemistry failed to show fibrillar beta-amyloid, APP-like immunoreactivity accumulated in aggregates of swollen, abnormal neurites and perikarya along the periphery of the infarct-like lesion in the thalamus and medial geniculate nucleus. Immunoblotting of the thalamic region around the lesion revealed increased APP-like holoprotein immunoreactivity. APP-like immunoreactive neurites were scattered in the mammillary body and medial vestibular nuclei where the lesion did not resemble infarcts. In the inferior colliculus, increased perikaryal APP-like immunostaining occurred in neurons surrounding necrotic areas. Regions without apparent pathological lesions showed no alteration in APP-like immunoreactivity. Thus, the oxidative insult associated with cell loss, hemorrhage and infarct-like lesions during TD leads to altered APP metabolism. This is the first report to show a relationship between changes in APP expression, oxidative metabolism and selective cell damage caused by nutritional/cofactor deficiency. This model appears useful in defining the role of APP in the reponse to central nervous system injury, and may also be relevant to the pathophysiology of Wernicke-Korsakoff syndrome and AD.

Secretory cleavage of beta-amyloid precursor protein in the cerebral white matter produces amyloidogenic carboxyl-terminal fragments

To elucidate the metabolic process generating amyloid-beta protein (A beta) from beta-amyloid precursor protein (APP) in human brain, we partially purified secretory forms and carboxyl-terminal fragments (CTFs) of APP from the white matter of a Down's syndrome brain. We obtained secretory forms of APP which lack the entire A beta sequence and CTFs which contain the full-length A beta from the cerebral white matter. Some A beta-lacking secretory APP isoforms in the white matter were derived from APP695. These results suggest that amyloidogenic CTFs can be produced by secretory cleavage of APP which is anterogradely transported through the axon in human brain.

Synaptotrophic effects of human amyloid beta protein precursors in the cortex of transgenic mice

The amyloid precursor protein (APP) is involved in Alzheimer's disease (AD) because its degradation products accumulate abnormally in AD brains and APP mutations are associated with early onset AD. However, its role in health and disease appears to be complex, with different APP derivatives showing either neurotoxic or neurotrophic effects in vitro. To elucidate the effects APP has on the brain in vivo, cDNAs encoding different forms of human APP (hAPP) were placed downstream of the neuron-specific enolase (NSE) promoter. In multiple lines of NSE-hAPP transgenic mice neuronal overexpression of hAPP was accompanied by an increase in the number of synaptophysin immunoreactive (SYN-IR) presynaptic terminals and in the expression of the growth-associated marker GAP-43. In lines expressing moderate levels of hAPP751 or hAPP695, this effect was more prominent in homozygous than in heterozygous transgenic mice. In contrast, a line with several-fold higher levels of hAPP695 expression showed less increase in SYN-IR presynaptic terminals per amount of hAPP expressed than the lower expressor lines and a decrease in synaptotrophic effects in homozygous compared with heterozygous offspring. Transgenic mice (2-24 months of age) showed no evidence for amyloid deposits or neurodegeneration. These findings suggest that APP may be important for the formation/maintenance of synapses in vivo and that its synaptotrophic effects may be critically dependent on the expression levels of different APP isoforms. Alterations in APP expression, processing or function could contribute to the synaptic pathology seen in AD.

Immunocytochemical localization of amyloid precursor protein in rat brain

The localization of amyloid precursor protein (APP) in rat brain was studied with a cytoplasmic domain-specific antibody. Light microscopic immunocytochemistry demonstrated that APP is present in most neurons, in some oligodendrocytes, and in a population of cells with diameters less than 10 microns that may be glial. Marked differences in immunoreactivity among neurons were observed, and the strongest immunoreactivity was contained in larger neurons. Neurons with scant cytoplasm, such as granule cells in the olfactory bulb, dentate gyrus, and cerebellum, were weakly immunoreactive. Differences in neuropil immunoreactivity were also observed; this type of staining was strongest in the caudatoputamen, lateral septum, medial habenula, nucleus reticularis of the dorsal thalamus, and the lateral portion of the ventroposterior nucleus. Neuropil immunostaining was weakest in layer IV of cortex and in areas containing granule cells. The fact that APP seems to be present in the vast majority of neurons suggests that this protein plays a role common to all neurons. The fact that there is a great difference in the steady-state amount of APP among different types of neurons suggests that APP may play a specific role in the function of certain classes of neurons.

Potentially amyloidogenic fragment of 50 kDa and intracellular processing of amyloid precursor protein in cells cultured under leupeptin

The principal neuropathological feature of Alzheimer's disease is extracellular deposition of approximately 4-kDa proteinous fragment, designated as beta-amyloid peptides (beta/A4 peptides) derived by proteolytic cleavage from amyloid precursor protein (APP), a large cell-surface receptor-like protein. There has been evidence that APP is proteolytically degraded in the secretory and endosomal/lysosomal pathways. The pathway in which APP is cleaved to generate beta/A4 peptides is still not identified. To clarify the intracellular processing of APP into the generation of beta/A4 peptides, we detected and characterized potentially amyloidogenic or non-amyloidogenic fragments using newly established monoclonal and polyclonal antibodies in the cultured cells with or without leupeptin, potent lysosomal protease inhibitor of lysosome. APP fragments of 50 and 20 kDa containing full-length beta/A4 peptides were identified in the cultured cells. Immunoblot analysis, biochemical study for specific marker enzyme activity of the fractions obtained from subcellular fractionation, sucrose density gradient centrifugation indicated that the 50-kDa APP fragment was produced in the compartment closely related to endosomal/lysosomal system. Our data suggest that the endosomal/lysosomal pathway is involved in the processing and generation of beta/A4 peptides.

Characterization of a novel processing pathway for Alzheimer's amyloid beta precursor protein

Amyloid beta (A beta) is a normal proteolytic fragment of a large precursor protein (beta PP) which undergoes altered conformation, leading to fibril formation. Two main beta PP processing pathways have been described, and we are now reporting the characterization of a third beta PP pathway. A membrane-associated 16 kDa component identified in human platelets isolated from normal donors. Based on size, immunoreactivity and amino acid sequence analysis, the fragment is a C-terminal beta PP component which starts at position 642 (APP770 numbering) and contains the intact A beta sequence. The presence of this novel pathway of beta PP processing in resting platelets suggest that it occurs as a normal event.

Alzheimer's disease and soluble A beta

The discovery of soluble amyloid beta (sA beta) suggests that the role of amyloid in Alzheimer's disease (AD) is similar to the previously studied systemic amyloidoses and alters the notion that membrane damage is the initial event in AD. The disease state is characterized by the abnormal accumulation of a normal degradative peptide, which becomes resistant to further proteolysis due to a conformational change. Mutations in the beta PP gene have been found in a very small percentage of AD cases; hence other factors, both genetic and environmental, need to be identified. Priority needs to be given to detailed studies of the structural differences between sA beta and the A beta in amyloid deposits. This will help uncover the determining factors governing the aggregation of sA beta. These structural alterations may be critical for the possible toxic effects A beta and/or associated proteins (molecular chaperones, e.g., apolipoprotein E) have on brain cell function.

Regulated cleavage of the Alzheimer amyloid precursor protein: molecular and cellular basis

The relative utilization of alternative processing pathways for APP can be regulated by the activation state of certain protein phosphorylation signal transduction pathways. For example, activation of protein kinase C (PKC), or inactivation of protein phosphatases 1 and 2A, leads to a relative increase in utilization of the nonamyloidogenic, 'alpha-secretase' cleavage pathway for APP processing at the expense of other pathways. The molecular and cellular basis for this regulatory event is unknown. The possible mechanisms of regulated APP cleavage include (either singly or in combination): 1) substrate (ie APP) activation; 2) substrate redistribution; 3) enzyme (ie alpha-secretase) activation; or 4) enzyme redistribution. APP is a phosphoprotein; however, recent evidence from studies of the metabolism of mutant APP molecules suggests that changes in the APP cytoplasmic tail phosphorylation state may not be necessary for the phosphorylation-dependent activation of 'alpha-secretase' cleavage. Further, indirect immunofluorescent studies of the subcellular distribution of APP in the absence or presence of phorbol esters (PKC activators) fail to disclose obvious phorbol-induced redistribution of APP immunoreactivity. Taken together, current data suggest that major candidate phosphorylation-state sensitive targets relevant to the molecular basis of PKC-activated processing (or 'regulated cleavage') of APP include the APP ectodomain as well as secretase enzymes and/or other components of the APP trafficking/processing apparatus. Progress in distinguishing among these possibilities is discussed.

Bacterial expression, purification of full length and carboxyl terminal fragment of Alzheimer amyloid precursor protein and their proteolytic processing by thrombin

Human amyloid protein precursor(APP770) and its carboxyl terminal portion (CT105) including beta/A4 domain were highly expressed using strong expression systems in E. coli. These recombinant APP peptides were purified with a combination of urea solubilization and ion-exchange chromatography and used for proteolytic processing by thrombin. Three thrombin cleavage sites were predicted by the decrease of APP770 and the appearance of M(r) 56, 27 and 18 kDa fragments containing beta/A4 domain on SDS-PAGE gel and on the immunoblot. A similar but limited proteolysis of platelet APPs exposed to thrombin resulted in the stimulated production of 60 and 27 KDa carboxyl terminal peptides containing the intact beta/A4. This thrombin mediated proteolysis was completely blocked by hirudin, the specific thrombin inhibitor. These results suggest that thrombin may play a role in altered processing of APP to generate potentially amyloidogenic intermediates in vivo leading to amyloid deposition.

beta-Amyloid precursor protein binds to the neurite-promoting IKVAV site of laminin

We previously characterized a 110-kDa membrane-associated laminin-binding protein (LBP110) from brain which binds the laminin A chain -Ile-Lys-Val-Ala-Val-(IKVAV) site and increases in injury. Here we demonstrate that antisera directed against different epitopes of beta-amyloid precursor protein (APP) recognize LBP110 and that APP is recognized by LBP110 antiserum. APP specifically binds IKVAV and not another biologically active laminin-derived peptide containing the amino acid sequence -Tyr-Ile-Gly-Ser-Arg-. PC-12 cells transfected with antisense APP RNA produce less APP and LBP110, and they form fewer processes when cultured on either laminin or the IKVAV peptide. Thus, LBP110 is a member of the APP family and a function for APP in neurite outgrowth is now defined.

The Alzheimer beta-amyloid protein precursor/protease nexin-II is cleaved by secretase in a trans-Golgi secretory compartment in human neuroglioma cells

Alzheimer beta-amyloid protein precursor (beta APP) is expressed endogenously and abundantly by human neuroglioma (H4) cells. Its secretory processing has been shown to involve discrete proteolysis within the beta A4 region, thus preventing beta-amyloid formation, by an enzyme which has been referred to as 'beta APP secretase'. This cleavage results in secretion of a soluble N-terminal 135 kDa protein and retention of an integral membrane C-terminal fragment within the cell. The membrane-associated C-terminal fragment is sorted to lysosomes where it undergoes limited degradation. We show here that most newly synthesized beta APP is degraded via a non-lysosomal pathway before maturation in H4 cells, and most mature beta APP is processed predominantly by the so-called secretase. The rapid kinetics of appearance/disappearance of a cleaved 135 kDa protein within a microsomal fraction and the slow accumulation of this form in the extracellular medium indicated that secretase cleaves beta APP in an intracellular compartment. Low-temperature block (20 degrees C) was used to demonstrate that beta APP is cleaved within a late Golgi compartment after sulphation which occurs in the trans-Golgi network (TGN). This is consistent with (1) the immunolocalization of most of the beta APP within a Golgi compartment that reacts with wheat germ agglutinin, (2) the fact that less than 1.5% of the total mature full-length beta APP is present at the plasma membrane and (3) subcellular fractionation studies which showed that the mature full-length and intracellular cleaved beta APPs co-sediment with a membrane fraction that is slightly more dense than the plasma membrane. This study provides evidence that most of the beta APP secretase in H4 cells is intracellular, and confirms that the resulting C-terminal fragment is delivered to lysosomes immediately after cleavage. These results are discussed with regard to the possibility that mature full-length beta APP escapes secretase cleavage and is delivered directly from the TGN to the lysosome without passing through the plasma membrane. Either pathway will result in the generation of amyloidogenic fragments.

High-level expression and in vitro mutagenesis of a fibrillogenic 109-amino-acid C-terminal fragment of Alzheimer's-disease amyloid precursor protein

We amplified DNA encoding the 3' 109 codons of Alzheimer's-disease amyloid precursor protein (APP) inclusive of the beta protein (A beta) and cytoplasmic domains from cDNA using oligonucleotide primers designed to facilitate cloning into the T7 expression vector pT7Ad23K13. We also modified this construct to generate recombinant molecules incorporating two recently described APP mutants by site-directed mutagenesis. Both native C109 (deletion construct inclusive of the C-terminal 109 residues of APP) and constructs with a single mutation at codon 642 (T-->G, resulting in a substitution of glycine for valine) or a double mutation at codons 595 (G-->T, substituting asparagine for lysine) and 596 (A-->C, substituting leucine for methionine) were expressed in Escherichia coli to levels of 5-20% of total bacterial protein after induction. The major constituent of expressed C109 protein had an apparent molecular mass of 16-18 kDa by SDS/PAGE and appeared to be the full-length construct by size and N-terminal microsequencing. Also present was a 4-5 kDa species that co-purified with C109, constituting only approximately 1% of expressed protein, which was revealed by Western-blot analysis with antibodies specific for A beta epitopes and after biotinylation of purified recombinant C109. This fragment shared N-terminal sequence with, and appeared to arise by proteolysis of, full-length C109 in biosynthetic labelling experiments. C109 spontaneously precipitated after dialysis against NaCl or water, and with prolonged (> 20 weeks) standing was found by electron microscopy to contain a minor (< 5%) fibrillar component that was reactive with antibodies to a C-terminal epitope of APP. Recombinant C109 appears to duplicate some of the biochemical and physicochemical properties of C-terminal A beta-inclusive fragments of APP that have been found in transfected cells, brain cortex and cerebral microvessels.

The cerebrospinal-fluid soluble form of Alzheimer's amyloid beta is complexed to SP-40,40 (apolipoprotein J), an inhibitor of the complement membrane-attack complex

The amyloid fibrils deposited in Alzheimer's neuritic plaque cores and cerebral blood vessels are mainly composed of aggregated forms of a unique peptide, 39-42 amino acids long, named amyloid beta (A beta). A similar, although soluble, A beta ('sA beta') has been identified in cerebrospinal fluid, plasma and cell supernatants, indicating that it is normally produced by proteolytic processing of its precursor protein, amyloid precursor protein (APP). Using direct binding experiments we have isolated and characterized an 80 kDa circulating protein that specifically interacts with a synthetic peptide identical with A beta. The protein was unmistakably identified as SP-40,40 or ApoJ, a cytolytic inhibitor and lipid carrier, by means of amino acid sequence and immunoreactivity with specific antibodies. Immunoprecipitation with anti-SP-40,40 retrieved soluble A beta from cerebrospinal fluid, indicating that the interaction occurs in vivo.

A 109-amino-acid C-terminal fragment of Alzheimer's-disease amyloid precursor protein contains a sequence, -RHDS-, that promotes cell adhesion

Amyloid beta (A beta), the major constituent of the fibrils composing senile plaques and vascular amyloid deposits in Alzheimer's disease (AD) and related disorders, is a 39-42-residue self-aggregating degradation peptide of a larger multidomain membrane glycoprotein designated amyloid precursor protein (APP). An array of biological functions has been assigned to different APP domains, including growth regulation, neurotoxicity, inhibitory activity of serine proteinases and promotion of cell-cell and cell-matrix interactions. A beta is generated through an as-yet-unknown catabolic pathway that by-passes or inhibits the cleavage of APP within the A beta sequence. We have identified a 16 kDa intermediate APP C-terminal fragment containing A beta in leptomeningeal vessels of aged normal individuals and AD patients by means of its immunoreactivity with a panel of four different anti-(APP C-terminal) antibodies, indicating a different pathway of APP processing. Previous studies have indicated that the APP C-terminal domain is the most likely to be involved in cell-matrix interactions. A 109-amino-acid construct C109 with a sequence analogous to the C-terminal of APP (positions 587-695 of APP695), similar in length and immunoreactivity to the 16 kDa fragment, was found to promote cell adhesion. By use of synthetic peptides, this activity was initially located to the extracellular 28 residues of A beta. Inhibition studies demonstrated that the sequence RHDS (amino acids 5-8 of A beta, corresponding to residues 601-604 of APP695 was responsible for the adhesion-promoting activity. The interaction is dependent on bivalent cations and can be blocked either by the tetrapeptides RHDS and RGDS or by an anti-(beta 1 integrin) antibody. Thus, through integrin-like surface receptors, APP or its derivative proteolytic fragments containing the sequence RHDS may modulate cell-cell or cell-matrix interactions.

Amyloidogenesis in Alzheimer's disease: basic biology and animal models

A principal neuropathological hallmark of Alzheimer's disease is deposition of beta-amyloid, composed primarily of a 4 kD peptide, A beta. This peptide is derived from larger amyloid precursor proteins. The mechanisms that are responsible for A beta formation in vivo are unknown. Recently, transgenic strategies have been employed to test several hypothetical mechanisms in order to reproduce Alzheimer's disease-specific pathology in rodents.

The nature and metabolism of potentially amyloidogenic carboxyl-terminal fragments of the Alzheimer beta/A4-amyloid precursor protein: some technical notes

The proteolytic processing and secretion of APP are regulated by protein phosphorylation, especially via protein kinase C and protein phosphatases 1 and/or 2A. Our studies of these regulatory mechanisms have led us to perform extensive experimentation on the metabolism of APP carboxyl-terminal fragments, using as our system either untransfected, undifferentiated rat pheochromocytoma (PC12) cells or APP-baculovirus infected Sf9 cells. We have not assayed APP fragments for biological activity in either system. However, we have made potentially relevant observations regarding APP carboxyl-terminal fragment trafficking. In this note, we review our published and unpublished data in relation to published reports from other laboratories using related systems.

Molecular biology of Alzheimer's amyloid--Dutch variant

Hereditary cerebral hemorrhage with amyloidosis, Dutch type (HCHWA-D) (or familial cerebral amyloid angiopathy) and familial Alzheimer's disease (FAD) share several properties. Both are autosomal dominant forms of cerebral amyloidosis characterized by beta-amyloid (A beta) deposition. In HCHWA-D the A beta is predominantly found in blood vessels and in early parenchymal plaques, whereas in AD parenchymal A beta deposits in the form of senile plaques and neurofibrillary tangles are a more prominent finding. Point mutations in the amyloid precursor protein (APP) have recently been described, in both conditions. A G to C transversion at codon 618 (extracellular portion of APP695), producing a single amino acid substitution of glutamine instead of glutamine acid, occurs in HCHWA-D; whereas mutations at codon 642 in the intramembrane region of APP695 (phenylalanine, isoleucine, or glycine instead of valine) are associated with early onset FAD. This suggests that the site of particular mutations in the APP gene and the type of amino acid substitution in the APP holoprotein are more important in determining clinicopathological phenotype and age at which A beta is deposited. Thus FAD and HCHWA-D can be regarded as two sides of the same coin.