The role of cell-derived oligomers of Abeta in Alzheimer's disease and avenues for therapeutic intervention

Burgeoning evidence suggests that soluble oligomers of Abeta (amyloid beta-protein) are the earliest effectors of synaptic compromise in Alzheimer's disease. Whereas most other investigators have employed synthetic Abeta peptides, we have taken advantage of a beta-amyloid precursor protein-overexpressing cell line (referred to as 7PA2) that secretes sub-nanomolar levels of low-n oligomers of Abeta. These are composed of heterogeneous Abeta peptides that migrate on SDS/PAGE as dimers, trimers and tetramers. When injected into the lateral ventricle of rats in vivo, these soluble oligomers inhibit hippocampal long-term potentiation and alter the memory of a complex learned behaviour. Biochemical manipulation of 7PA2 medium including immunodepletion with Abeta-specific antibodies and fractionation by size-exclusion chromatography allowed us to unambiguously attribute these effects to low-n oligomers. Using this paradigm we have tested compounds directed at three prominent amyloid-based therapeutic targets: inhibition of the secretases responsible for Abeta production, inhibition of Abeta aggregation and immunization against Abeta. In each case, compounds capable of reducing oligomer production or antibodies that avidly bind Abeta oligomers also ameliorate the synaptotoxic effects of these natural, cell-derived oligomers.

Certain inhibitors of synthetic amyloid beta-peptide (Abeta) fibrillogenesis block oligomerization of natural Abeta and thereby rescue long-term potentiation

Recent studies support the hypothesis that soluble oligomers of amyloid beta-peptide (Abeta) rather than mature amyloid fibrils are the earliest effectors of synaptic compromise in Alzheimer's disease. We took advantage of an amyloid precursor protein-overexpressing cell line that secretes SDS-stable Abeta oligomers to search for inhibitors of the pathobiological effects of natural human Abeta oligomers. Here, we identify small molecules that inhibit formation of soluble Abeta oligomers and thus abrogate their block of long-term potentiation (LTP). Furthermore, we show that cell-derived Abeta oligomers can be separated from monomers by size exclusion chromatography under nondenaturing conditions and that the isolated, soluble oligomers, but not monomers, block LTP. The identification of small molecules that inhibit early Abeta oligomer formation and rescue LTP inhibition offers a rational approach for therapeutic intervention in Alzheimer's disease and highlights the utility of our cell-culture paradigm as a useful secondary screen for compounds designed to inhibit early steps in Abeta oligomerization under biologically relevant conditions.

Deciphering the genetic basis of Alzheimer's disease

A remarkable rise in life expectancy during the past century has made Alzheimer's disease (AD) the most common form of progressive cognitive failure in humans. Compositional analyses of the classical brain lesions, the senile (amyloid) plaques and neurofibrillary tangles, preceded and has guided the search for genetic alterations. Four genes have been unequivocally implicated in inherited forms of AD, and mutations or polymorphisms in these genes cause excessive cerebral accumulation of the amyloid beta-protein and subsequent neuronal and glial pathology in brain regions important for memory and cognition. This understanding of the genotype-to-phenotype conversions of familial AD has led to the development of pharmacological strategies to lower amyloid beta-protein levels as a way of treating or preventing all forms of the disease.

The oligomerization of amyloid beta-protein begins intracellularly in cells derived from human brain

The progressive aggregation and deposition of amyloid beta-protein (Abeta) in brain regions subserving memory and cognition is an early and invariant feature of Alzheimer's disease, the most common cause of cognitive failure in aged humans. Inhibiting Abeta aggregation is therapeutically attractive because this process is believed to be an exclusively pathological event. Whereas many studies have examined the aggregation of synthetic Abeta peptides under nonphysiological conditions and concentrations, we have detected and characterized the oligomerization of naturally secreted Abeta at nanomolar levels in cultures of APP-expressing CHO cells [Podlisny, M. B., Ostaszewski, B. L., Squazzo, S. L., Koo, E. H., Rydell, R. E., Teplow, D. B., and Selkoe, D. J. (1995) J. Biol. Chem. 270, 9564-9570 (1); Podlisny, M. B., Walsh, D. M., Amarante, P., Ostaszewski, B. L., Stimson, E. R., Maggio, J. E., Teplow, D. B., and Selkoe, D. J. (1998) Biochemistry 37, 3602-3611 (2)]. To determine whether similar species occur in vivo, we probed samples of human cerebrospinal fluid (CSF) and detected SDS-stable dimers of Abeta in some subjects. Incubation of CSF or of CHO conditioned medium at 37 degrees C did not lead to new oligomer formation. This inability to induce oligomers extracellularly as well as the detection of oligomers in cell medium very early during the course of pulse-chase experiments suggested that natural Abeta oligomers might first form intracellularly. We therefore searched for and detected intracellular Abeta oligomers, principally dimers, in primary human neurons and in neuronal and nonneural cell lines. These dimers arose intracellularly rather than being derived from the medium by reuptake. The dimers were particularly detectable in neural cells: the ratio of intracellular to extracellular oligomers was much higher in brain-derived than nonbrain cells. We conclude that the pathogenically critical process of Abeta oligomerization begins intraneuronally.

Insulin-degrading enzyme regulates extracellular levels of amyloid beta-protein by degradation

Excessive cerebral accumulation of the 42-residue amyloid beta-protein (Abeta) is an early and invariant step in the pathogenesis of Alzheimer's disease. Many studies have examined the cellular production of Abeta from its membrane-bound precursor, including the role of the presenilin proteins therein, but almost nothing is known about how Abeta is degraded and cleared following its secretion. We previously screened neuronal and nonneuronal cell lines for the production of proteases capable of degrading naturally secreted Abeta under biologically relevant conditions and concentrations. The major such protease identified was a metalloprotease released particularly by a microglial cell line, BV-2. We have now purified and characterized the protease and find that it is indistinguishable from insulin-degrading enzyme (IDE), a thiol metalloendopeptidase that degrades small peptides such as insulin, glucagon, and atrial natriuretic peptide. Degradation of both endogenous and synthetic Abeta at picomolar to nanomolar concentrations was completely inhibited by the competitive IDE substrate, insulin, and by two other IDE inhibitors. Immunodepletion of conditioned medium with an IDE antibody removed its Abeta-degrading activity. IDE was present in BV-2 cytosol, as expected, but was also released into the medium by intact, healthy cells. To confirm the extracellular occurrence of IDE in vivo, we identified intact IDE in human cerebrospinal fluid of both normal and Alzheimer subjects. In addition to its ability to degrade Abeta, IDE activity was unexpectedly found be associated with a time-dependent oligomerization of synthetic Abeta at physiological levels in the conditioned media of cultured cells; this process, which may be initiated by IDE-generated proteolytic fragments of Abeta, was prevented by three different IDE inhibitors. We conclude that a principal protease capable of down-regulating the levels of secreted Abeta extracellularly is IDE.

Oligomerization of endogenous and synthetic amyloid beta-protein at nanomolar levels in cell culture and stabilization of monomer by Congo red

Amyloid beta-proteins (A beta) are proteolytic fragments of the beta-amyloid precursor protein (beta APP) that are secreted by mammalian cells throughout life but also accumulate progressively as insoluble cerebral aggregates in Alzheimer's disease (AD). Because mounting evidence indicates that A beta aggregation and deposition are early, critical features of AD leading to neurotoxicity, many studies of A beta aggregation have been conducted using synthetic peptides under generally nonphysiological conditions and concentrations. We recently described the oligomerization of A beta peptides secreted by beta APP-expressing cells at low nanomolar (20-30 ng/mL) levels into sodium dodecyl sulfate- (SDS-) stable oligomers of 6-16 kDa. Here, we extensively characterize this in vitro system and show that the amyloid binding dye, Congo red, acts to markedly decrease oligomer/monomer ratios by stabilizing the 4 kDa A beta monomers (ID50 approximately equal to 3.4 microM). Addition of radioiodinated synthetic A beta 1-40 to the cultures or to their conditioned media at physiological concentrations (0.25-2.5 nM) reveals that it undergoes progressive aggregation into SDS-stable oligomers of 6-25 kDa during brief (approximately 4 h) incubation at 37 degrees C, and this is inhibitable by Congo red. The level of A beta oligomers can be quantitated in the Chinese hamster ovary (CHO) conditioned medium by size-exclusion chromatography as well as by SDS-polyacrylamide gel electrophoresis (PAGE), and comparison of these two methods suggests that aggregation of A beta into higher molecular weight polymers that are not detectable by SDS-PAGE occurs in the cultures. We conclude that both endogenous and synthetic A beta can assemble into stable oligomers at physiological concentrations in cell culture, providing a manipulable system for studying the mechanism of early A beta aggregation and identifying inhibitors thereof under biologically relevant conditions.

Enhanced production and oligomerization of the 42-residue amyloid beta-protein by Chinese hamster ovary cells stably expressing mutant presenilins

Mutations in the presenilin 1 (PS1) and presenilin 2 (PS2) genes cause the most common and aggressive form of early onset familial Alzheimer's disease. To elucidate their pathogenic mechanism, wild-type (wt) or mutant (M146L, C410Y) PS1 and wt or mutant (M239V) PS2 genes were stably transfected into Chinese hamster ovary cells that overexpress the beta-amyloid precursor protein (APP). The identity of the 43-45-kDa PS1 holoproteins was confirmed by N-terminal radiosequencing. PS1 was rapidly processed (t1/2 = 40 min) in the endoplasmic reticulum into stable fragments. Wild-type and mutant PS2 holoproteins exhibited similar half lives (1.5 h); however, their endoproteolytic fragments showed both mutation-specific and cell type-specific differences. Mutant PS1 or PS2 consistently induced a 1.4-2.5-fold increase (p < 0.001) in the relative production of the highly amyloidogenic 42-residue form of amyloid beta-protein (Abeta42) as determined by quantitative immunoprecipitation and by enzyme-linked immunosorbent assay. In mutant PS1 and PS2 cell lines with high increases in Abeta42/Abetatotal ratios, spontaneous formation of low molecular weight oligomers of Abeta42 was observed in media, suggesting enhanced Abeta aggregation from the elevation of Abeta42. We conclude that mutant PS1 and PS2 proteins enhance the proteolysis of beta-amyloid precursor protein by the gamma-secretase cleaving at Abeta residue 42, thereby promoting amyloidogenesis.

Presenilin proteins undergo heterogeneous endoproteolysis between Thr291 and Ala299 and occur as stable N- and C-terminal fragments in normal and Alzheimer brain tissue

Humans inheriting missense mutations in the presenilin (PS)1 and -2 genes undergo progressive cerebral deposition of the amyloid beta-protein at an early age and develop a clinically and pathologically severe form of familial Alzheimer's disease (FAD). Because PS1 mutations cause the most aggressive known form of AD, it is important to elucidate the structure and function of this multitransmembrane protein in the brain. Using a panel of region-specific PS antibodies, we characterized the presenilin polypeptides in mammalian tissues, including brains of normal, AD, and PS1-linked FAD subjects, and in transfected and nontransfected cell lines. Very little full-length PS1 or -2 was detected in brain and untransfected cells; instead the protein occurred as a heterogeneous array of stable N- and C-terminal proteolytic fragments that differed subtly among cell types and mammalian tissues. Sequencing of the major C-terminal fragment from PS1-transfected human 293 cells showed that the principal endoproteolytic cleavage occurs at and near Met298 in the proximal portion of the large hydrophilic loop. Full-length PS1 in these cells is quickly turned over (T1/2 approximately 60 min), in part to the two major fragments. The sizes and amounts of the PS fragments were not significantly altered in four FAD brains with the Cys410Tyr PS1 missense mutation. Our results indicate that presenilins are rapidly processed to N- and C-terminal fragments in both neural and nonneural cells and that interference with this processing is not an obligatory feature of FAD-causing mutations.

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

BACKGROUND

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by the deposition of extracellular senile plaques composed of amyloid beta-peptide (A beta). Whereas most cases of AD occur sporadically, about 10% of AD cases are inherited as a fully penetrant autosomal dominant trait. Mutations in the recently cloned Presenilin genes (PS-1 and PS-2) are by far the most common cause of early onset familial AD.

MATERIALS AND METHODS

Cellular expression of endogenous and overexpressed PS proteins was analyzed by immunocytochemistry and metabolic labeling followed by immunoprecipitation. In vivo phosphorylation sites of PS proteins were analyzed by extensive mutagenesis.

RESULTS

PS-1 as well as PS-2 proteins were localized predominantly within the endoplasmic reticulum (ER). However, small amounts of the PS proteins were detected within the Golgi compartment, where they colocalize with the beta-amyloid precursor protein (beta APP). The PS-2 protein was found to be highly phosphorylated, whereas very little phosphorylation was observed for PS-1. The selective phosphorylation of PS-2 occurs exclusively on serine residues. In vivo phosphorylation of PS-2 was mapped to serine residues 7, 9, and 19 within an acidic stretch at the N terminus, which is absent in PS-1. casein kinase (CK)-1 and CK-2 were shown to phosphorylate the N terminus of PS-2 in vitro.

CONCLUSIONS

The majority of PS proteins were detected in the ER where little if any proteolytic processing of beta APP was reported. ER retention of PS proteins might occur by intramolecular aggregation. Small amounts of PS proteins were also detected in the Golgi where they colocalized with beta APP. This might suggest that potential interactions between PS proteins and beta APP could occur within the Golgi. Selective phosphorylation of PS-2 proteins within the acidic domain missing in PS-1 indicates differences in the biological functions and regulation of the two highly homologous proteins.

Diffuse plaques contain C-terminal A beta 42 and not A beta 40: evidence from cats and dogs

Recent reports have suggested that beta-amyloid (A beta) species of variable length C-termini are differentially deposited within early and late-stage plaques and the cerebrovasculature. Specifically, longer C-terminal length A beta 42/3 fragments (i.e., A beta forms extending to residues 42 and/or 43) are thought to be predominant within diffuse plaques while both A beta 42/3 and A beta 40 (A beta forms terminating at residue 40) are present within a subset of neuritic plaques and cerebrovascular deposits. We sought to clarify the issue of differential A beta deposition using aged canines, a partial animal model of Alzheimer's disease that exhibits extensive diffuse plaques and frequent vascular amyloid, but does not contain neuritic plaques or neurofibrillary tangles. We examined the brains of 20 aged canines, 3 aged felines, and 17 humans for the presence of A beta immunoreactive plaques, using antibodies to A beta 1(-17), A beta 17(-24), A beta 1(-28), A beta 40, and A beta 42. We report that plaques within the canine and feline brain are immunopositive for A beta 42 but not A beta 40. This is the first observation of nascent AD pathology in the aged feline brain. Canine plaques also contained epitopes within A beta 1(-17), A beta 17(-24), and A beta 1(-28). In all species examined, vascular deposits were immunopositive for both A beta 40 and A beta 42. In the human brain, diffuse plaques were preferentially A beta 42 immunopositive, while neuritic plaques and vascular deposits were both A beta 40 and A beta 42 immunopositive. However, not all neuritic plaques contain A beta 40 epitopes.

The role of APP processing and trafficking pathways in the formation of amyloid beta-protein

The amyloid beta-protein (A beta) is a proteolytic fragment of the beta-amyloid precursor protein (beta APP). We previously reported the constitutive secretion of A beta peptides from a variety of cells expressing beta APP under normal culture conditions. These endogenously produced A beta peptides have heterogeneous N- and C-termini that vary as a function of beta APP missense mutations. Treatment of A beta-secreting cells with agents that alter intravesicular pH showed that an acidic compartment is required for proper A beta generation. One such compartment appears to be the endosome. Immunolabeling of cell-surface beta APP in living neurons and non-neuronal cells directly demonstrated the endocytosis of the protein and its rapid recycling (within 5-10 minutes) to the cell surface, as well as the trafficking of some beta APP to lysosomes. Expression of beta APP with various deletions of the cytoplasmic domain, including the NPTY motif, leads to decreased internalization and an associated decrease in the production of A beta peptides that begin at the usual asp1 start site. These and other data suggest that A beta production begins with cleavage of beta APP by a still unknown protease(s) (beta-secretase[s]) at the met-asp bond proceeding the A beta N-terminus and that this occurs in part in early endosomes. To characterize the substrate requirements of beta-secretase, beta APP was mutagenized by placing stop codons within or at the end of the transmembrane domain or substituting other amino acids for the wild-type met and asp at the P1 and P1' positions. These experiments showed that proper beta-secretase cleavage requires the precursor to be membrane-anchored and is highly sequence specific; most substitutions at met or asp substantially decrease A beta production. Analogous mutagenesis experiments around the A beta C-terminus revealed that the unknown protease(s) cleaving here ("gamma-secretase[s]") does not show such specificity. Cells secreting A beta may also be useful for examining the critical issue of the aggregation of A beta into its neurotoxic polymeric form under physiological conditions. In this regard, we have found that beta APP-expressing CHO cells show aggregation of > or = 10-20% of their secreted A beta peptides into SDS-stable dimers, trimers and sometimes tetramers under normal culture conditions. The identity of these small multimers was confirmed by extensive immunochemical characterization and radiosequencing. They are present at approximately 100-500 pM levels in conditioned medium of CHO transfectants. Using this endogenous A beta aggregating system, we have begun to examine variables that influence aggregation and compounds which may retard it. In conclusion, studies of the regulation of A beta production and aggregation in cell culture can provide information under physiological conditions that can complement analyses of these processes in vivo.

Aggregation of secreted amyloid beta-protein into sodium dodecyl sulfate-stable oligomers in cell culture

Filamentous aggregates of the 40-42-residue amyloid beta-protein (A beta) accumulate progressively in the limbic and cerebral cortex in Alzheimer's disease, where they are intimately associated with neuronal and glial cytopathology. Attempts to model this cytotoxicity in vitro using synthetic peptides have shown that monomeric A beta is relatively inert, whereas aggregated A beta reproducibly exerts a variety of neurotoxic effects. The processes that mediate the conversion of monomeric A beta into a toxic aggregated state are thus of great interest. Previous studies of this conversion have employed high concentrations (10(-5)-10(-3) M) of synthetic A beta peptides under nonbiological conditions. We report here the detection of small amounts (< 10(-9) M) of SDS-stable A beta oligomers in the culture media of Chinese hamster ovary cells expressing endogenous or transfected amyloid beta-protein precursor genes. The identity of these oligomers (primarily dimers and trimers) was established by immunoprecipitation with a panel of A beta antibodies, by electrophoretic comigration with synthetic A beta oligomers, and by amino acid sequencing. The oligomeric A beta species comprised approximately 10-20% of the total immunoprecipitable A beta in these cultures. A truncated A beta species beginning at Arg 5 was enriched in the oligomers, suggesting that amino-terminal heterogeneity can influence A beta oligomerization in this system. Addition of Congo red (10 microM) during metabolic labeling of the cells led to increased monomeric and decreased oligomeric A beta. The ability to detect and quantitate oligomers of secreted A beta peptides in cell culture should facilitate dynamic studies of the critical process of initial A beta aggregation under physiological conditions.

Passage of human amyloid beta-protein 1-40 across the murine blood-brain barrier

Previous studies have suggested that the amyloid beta-protein present in the brains of patients with Alzheimer's disease may be derived in part from peripheral blood. We determined that after IV injection of synthetic amyloid beta-protein 1-40 (A beta), labeled with radioactive 125I (I-A beta), radioactivity accumulated in the brains of mice by a nonsaturable mechanism. Radioactivity also accumulated in the brain after the i.v. injection of radioiodinated reverse amyloid beta-protein 40-1 (I-rA beta). Capillary depletion techniques, however, showed I-A beta to have a much greater degree of association with brain capillaries than I-rA beta. Acid precipitation of radioactivity in CSF samples and recovery from cortical homogenates suggested the presence of intact I-A beta within the CNS after peripheral administration. HPLC analysis of cortical homogenates confirmed the presence of intact I-A beta. Gel electrophoresis of the CSF acid precipitates and of the HPLC fractions further verified the presence of intact blood-derived I-A beta peptide in CNS. These results suggest that endogenous bloodborne A beta can enter the CNS after associating with the capillary endothelium to accumulate intact within the parenchymal and CSF spaces of the brain.

Microinjection of synthetic amyloid beta-protein in monkey cerebral cortex fails to produce acute neurotoxicity

The cerebral deposition of amyloid beta protein (A beta P) is an early pathogenetic event in Alzheimer's disease (AD). Recent studies suggest both neurotoxic and neurotrophic effects of A beta P in vitro. Because progressive A beta P deposition and surrounding neuritic dystrophy occur spontaneously in primates, we evaluated the in vivo effects of synthetic A beta P in monkey cortex. Experimental and control (reverse or substituted) peptides were stereotactically injected into multiple neocortical sites of adult rhesus monkeys in a vehicle of either artificial cerebrospinal fluid or acetonitrile. After 2 weeks, all injection sites were identified and characterized. A beta P antibodies specifically detected the injected A beta P1-40 peptide. Serial sections stained with silver and antineurofilament protein demonstrated comparable degrees of degenerating neurons, dystrophic neurites, and axonal spheroids associated with both experimental and control peptide injections. Alz 50 staining was sparse or absent in all sites. Similar results were obtained in an animal killed 3 months after injection. We conclude that specific cellular changes closely resembling the pathology of Alzheimer's disease were not detected in these acute experiments, and that control and experimental A beta P peptides produced indistinguishable effects.

Fibril formation by primate, rodent, and Dutch-hemorrhagic analogues of Alzheimer amyloid beta-protein

Deposition of extraneuronal fibrils that assemble from the 39-43 residue beta/A4 amyloid protein is one of the earliest histopathological features of Alzheimer's disease. We have used negative-stain electron microscopy, Fourier-transform infrared (FT-IR) spectroscopy, and fiber X-ray diffraction to examine the structure and properties of synthetic peptides corresponding to residues 1-40 of the beta/A4 protein of primate [Pm(1-40); human and monkey], rodent [Ro(1-40); with Arg5-->Gly, Tyr10-->Phe, and His13-->Arg], and hereditary cerebral hemorrhage with amyloidosis of the Dutch type (HCHWA-D) [Du(1-40); with Glu22-->Gln]. As controls, we examined a reverse primate sequence [Pm*(40-1)] and an extensively substituted primate peptide [C(1-40); with Glu3-->Arg, Arg5-->Glu, Asp7-->Val, His13-->Lys, Lys16-->His, Val18-->Asp, Phe19-->Ser, Phe20-->Tyr, Ser26-->Pro, Ala30-->Val, Ile31-->Ala, Met35-->norLeu, Gly38-->Ile, Val39-->Ala, and Val40-->Gly]. The assembly of these peptides was studied to understand the relationship between species-dependent amyloid formation and beta/A4 sequence and the effect of a naturally occurring point mutation of fibrillogenesis. The three N-terminal amino acid differences between Pm(1-40) and Ro(1-40) had virtually no effect on the morphology or organization of the fibrils formed by these peptides, indicating that the lack of amyloid deposits in rodent brain is not due directly to specific changes in its beta/A4 sequence. beta-Sheet and fibril formation, judged by FT-IR, was maximal within the pH range 5-8 for Pm(1-40), pH 5-10.5 for Du(1-40), and pH 2.5-8 for Ro(1-40).(ABSTRACT TRUNCATED AT 250 WORDS)

Synthetic amyloid beta-protein fails to produce specific neurotoxicity in monkey cerebral cortex

Because progressive amyloid beta-protein (A beta P) deposition and surrounding neuritic dystrophy occur spontaneously in primates, we evaluated the in vivo effects of synthetic A beta P in monkey cortex. Experimental and control A beta P were stereotactically injected into multiple neocortical sites of adult rhesus monkeys in a vehicle of either artificial cerebrospinal fluid or acetonitrile. After 2 weeks or 3 months, injection sites were identified and characterized histologically and immunocytochemically. A beta P antibodies specifically detected the injected A beta P1-40 peptide. Serial sections stained with silver and antineurofilament protein demonstrated comparable degrees of degenerating neurons, dystrophic neurites, and axonal spheroids associated with both experimental and control peptide injections. Alz 50 staining was sparse or absent in all sites. We conclude that specific cellular changes closely resembling AD pathology were not detected in these experiments, and that control and experimental A beta P peptides produced indistinguishable effects. Methodological concerns regarding the in vivo modeling of A beta P bioactivity are discussed.

Homology of the amyloid beta protein precursor in monkey and human supports a primate model for beta amyloidosis in Alzheimer's disease

Progressive cerebral deposition of the amyloid beta-protein (A beta P) occurs in Alzheimer's disease and during aging of certain mammals (eg, human, monkey, dog) but not others (eg, mouse, rat). The authors cloned and sequenced a full-length cDNA encoding the beta-protein precursor (beta APP) of cynomolgus monkey. The predicted amino acid sequence of the 695-residue protein is completely homologous to that of human. The alternatively transcribed exons encoding the Kunitz protease inhibitor region in monkey were cloned, showing only a single conservative amino acid substitution in the 751-residue form of beta APP and four substitutions in beta APP770. Immunoblots of cerebral cortex with antibodies to various beta APP domains showed highly similar beta APP polypeptides in human and monkey, in contrast to those of mouse and rat. The latter differences reflect sequence substitutions, transcriptional regulation, and possibly post-translational modifications that may decrease the amyloidogenic potential of rodent beta APP. Immunocytochemistry of aged cynomolgus brain showed A beta P deposited in blood vessels and diffuse and compacted plaques closely resembling those of humans, and the presence of beta-amyloid-associated proteins (alpha 1-antichymotrypsin; complements C1q and C3c) characteristic of A beta P deposits in Alzheimer's disease. The authors' findings demonstrate that cynomolgus monkey and perhaps other primates provide a close animal model for examining the early transcriptional and post-translational processing of beta APP that precedes A beta P deposition during aging and in Alzheimer's disease.

Transforming growth factor-beta bound to soluble derivatives of the beta amyloid precursor protein of Alzheimer's disease

Transforming growth factors beta (TGF beta) are multifunctional polypeptides that participate in regulation of growth, differentiation and function of many cell types. The mature TGF beta molecule is a 25 kDa protein composed of two 12.5 kDa monomers linked by disulfide bonds. Human glioblastoma cells secrete biologically active TGF beta 2. Here we report that in addition to the free form of TGF beta 2, a stable complex between a approximately 110 kDa binding protein and TGF beta 2 was isolated from glioblastoma cell supernatant. This binding protein was purified and was found to show sequence identity to part of the beta amyloid precursor protein (beta APP), to be specifically labeled by several different antisera to beta APP, and to be affinity labeled with TGF beta by crosslinking. The complex formation between TGF beta and beta APP may have important implications in regulation of biological activity of the two proteins and in delivery or clearance of TGF beta and beta APP in the brain and other compartments.

Detection of soluble forms of the beta-amyloid precursor protein in human plasma

A approximately 40-residue fragment of the beta-amyloid precursor protein (APP) is progressively deposited in the extracellular spaces of brain and blood vessels in Alzheimer's disease (AD), Down's syndrome and aged normal subjects. Soluble, truncated forms of APP lacking the carboxyl terminus are normally secreted from cultured cells expressing this protein and are found in cerebrospinal fluid. Here, we report the detection of a similar soluble APP isoform in human plasma. This approximately 125 kDa protein, which was isolated from plasma by Affi-Gel Blue chromatography or dialysis-induced precipitation, comigrates with the larger of the two major soluble APP forms present in spinal fluid and contains the Kunitz protease inhibitor insert. It thus derives from the APP751 and APP770 precursors; a soluble form of APP695 has not yet been detected in plasma. The approximately 125 kDa plasma form lacks the C-terminal region and is unlikely to serve as a precursor for the beta-protein that forms the amyloid in AD.

Soluble derivatives of the beta amyloid protein precursor of Alzheimer's disease are labeled by antisera to the beta amyloid protein

The amyloid deposited in Alzheimer's disease (AD) is composed primarily of a 39-42 residue polypeptide (beta AP) that is derived from a larger beta amyloid protein precursor (beta APP). In previous studies, we and others identified full-length, membrane-associated forms of the beta APP and showed that these forms are processed into soluble derivatives that lack the carboxyl-terminus of the full-length forms. In this report, we demonstrate that the soluble approximately 125 and approximately 105 kDa forms of the beta APP found in human cerebrospinal fluid are specifically labeled by several different antisera to the beta AP. This finding indicates that both soluble derivatives contain all or part of the beta AP sequence, and it suggests that one or both of these forms may be the immediate precursor of the amyloid deposited in AD.

The beta-amyloid protein precursor of Alzheimer disease has soluble derivatives found in human brain and cerebrospinal fluid

In this study, we use antisera to synthetic beta-amyloid protein precursor (beta APP) peptides to identify, in human brain and cerebrospinal fluid (CSF), soluble approximately 125- and approximately 105-kDa derivatives of the beta APP that lack the carboxyl terminus of the full-length, membrane-associated forms. We show that the soluble approximately 125-kDa beta APP derivative contains the Kunitz protease inhibitor domain, whereas the approximately 105-kDa form does not, and we confirm that these two proteins are soluble beta APP derivatives by purifying each from human CSF and directly sequencing its amino terminus.

Antisera to an amino-terminal peptide detect the amyloid protein precursor of Alzheimer's disease and recognize senile plaques

The cerebral amyloid deposited in Alzheimer's disease (AD) contains a 4.2 kDa beta amyloid polypeptide (beta AP) that is derived from a larger beta amyloid protein precursor (beta APP). Three beta APP mRNAs encoding proteins of 695, 751, and 770 amino acids have previously been identified. In each of these, there is a single membrane-spanning domain close to the carboxyl-terminus of the beta APP, and the 42 amino acid beta AP sequence extends from within the membrane-spanning domain into the large extracellular region of the beta APP. We raised rabbit antisera to a peptide corresponding to amino acids 45-62 near the amino-terminus of the beta APP. We show that these antisera detect the beta APP by demonstrating that they (i) label a set of approximately 120 kDa membrane-associated proteins in human brain previously detected by antisera to the carboxyl-terminus of beta APP and (ii) label a set of approximately 120 kDa membrane-associated proteins that are selectively overexpressed in cells transfected with a full length beta APP expression construct. The beta APP45-62 antisera specifically stain senile plaques in AD brains. This finding, along with the previous demonstration that antisera to the carboxyl-terminus of the beta APP label senile plaques, indicates that both near amino-terminal and carboxyl-terminal domains of the beta APP are present in senile plaques and suggests that proteolytic processing of the full length beta APP molecule into insoluble amyloid fibrils occurs in a highly localized fashion at the sites of amyloid deposition in AD brains.

Beta-amyloid precursor protein of Alzheimer disease occurs as 110- to 135-kilodalton membrane-associated proteins in neural and nonneural tissues

Progressive cerebral deposition of extracellular filaments composed of the beta-amyloid protein (beta AP) is a constant feature of Alzheimer disease (AD). Since the gene on chromosome 21 encoding the beta AP precursor (beta APP) is not known to be altered in AD, transcriptional or posttranslational changes may underlie accelerated beta AP deposition. Using two antibodies to the predicted carboxyl terminus of beta APP, we have identified the native beta APP in brain and nonneural human tissues as a 110- to 135-kDa protein complex that is insoluble in buffer and found in various membrane-rich subcellular fractions. These proteins are relatively uniformly distributed in adult brain, abundant in fetal brain, and detected in nonneural tissues that contain beta APP mRNA. Similarly sized proteins occur in rat, cow, and monkey brain and in cultured human HL-60 and HeLa cells; the precise patterns in the 110- to 135-kDa range are heterogeneous among various tissues and cell lines. Confirmation that the immunodetected tissue proteins are forms of beta APP was obtained when mammalian cells transfected with a full-length beta APP cDNA showed selectively augmented expression of 110- to 135-kDa proteins and specific immunocytochemical staining. Unexpectedly, the antibodies to the carboxyl terminus of beta APP labeled amyloid-containing senile plaques in AD brain. We conclude that the highly conserved beta APP molecule occurs in mammalian tissues as a heterogeneous group of membrane-associated proteins of approximately 120 kDa. Detection of the nonamyloidogenic carboxyl terminus within plaques suggests that proteolytic processing of the beta APP into insoluble filaments occurs locally in cortical regions that develop beta-amyloid deposits with age.

Gene dosage of the amyloid beta precursor protein in Alzheimer's disease

The progressive deposition in the human brain of amyloid filaments composed of the amyloid beta protein is a principal feature of Alzheimer's disease (AD). Densitometric analysis of Southern blots probed with a complementary DNA for the amyloid protein has been carried out to determine the relative dosage of this gene in genomic DNA of 14 patients with AD, 12 aged normal subjects, and 10 patients with trisomy 21 (Down syndrome). Whereas patients in the last group showed the expected 1.5-fold increase in dosage of this gene, none of the patients with AD had a gene dosage higher than that of the normal controls. These results do not support the hypothesis that the genetic defect in AD involves duplication of a segment of chromosome 21 containing the amyloid gene. Alternative mechanisms for the brain-specific increase in amyloid protein deposition in AD should be considered.

Conservation of brain amyloid proteins in aged mammals and humans with Alzheimer's disease

The formation of clusters of altered axons and dendrites surrounding extracellular deposits of amyloid filaments (neuritic plaques) is a major feature of the human brain in both aging and Alzheimer's disease. A panel of antibodies against amyloid filaments and their constituent proteins from humans with Alzheimer's disease cross-reacted with neuritic plaque and cerebrovascular amyloid deposits in five other species of aged mammals, including monkey, orangutan, polar bear, and dog. Antibodies to a 28-amino acid peptide representing the partial protein sequence of the human amyloid filaments recognized the cortical and microvascular amyloid of all of the aged mammals examined. Plaque amyloid, plaque neurites, and neuronal cell bodies in the aged animals showed no reaction with antibodies to human paired helical filaments. Thus, with age, the amyloid proteins associated with progressive cortical degeneration in Alzheimer's disease are also deposited in the brains of other mammals. Aged primates can provide biochemically relevant models for principal features of Alzheimer's disease: cerebrovascular amyloidosis and neuritic plaque formation.

Isolation of low-molecular-weight proteins from amyloid plaque fibers in Alzheimer's disease

During aging of the human brain, and particularly in Alzheimer's disease, progressive neuronal loss is accompanied by the formation of highly stable intra- and extraneuronal protein fibers. Using fluorescence-activated particle sorting, a method has been developed for purifying essentially to homogeneity the extracellular amyloid fibers that form the cores of senile plaques. The purified plaque cores each contain 60-130 pg of protein. Their amino acid composition shows abundant glycine, trace proline, and approximately 50% hydrophobic residues; it resembles that of enriched fractions of the paired helical filaments (PHF) that accumulate intraneuronally in Alzheimer's disease. Senile plaque amyloid fibers share with PHF insolubility in numerous protein denaturants and resistance to proteinases. However, treatment of either fiber preparation with concentrated (88%) formic acid or saturated (6.8 M) guanidine thiocyanate followed by sodium dodecyl sulfate causes disappearance of the fibers and releases proteins migrating at 5-7,000 and 11-15,000 Mr which appear to be dimerically related. Following their separation by size-exclusion HPLC, the proteins solubilized from plaque amyloid and PHF-enriched fractions have highly similar compositions and, on dialysis, readily aggregate into higher Mr polymers. Antibodies raised to the major low-Mr protein selectively label both plaque cores and vascular amyloid deposits in Alzheimer brain but do not stain neurofibrillary tangles, senile plaque neurites, or any other neuronal structure. Thus, extraneuronal amyloid plaque filaments in Alzheimer's disease are composed of hydrophobic low-Mr protein(s) which are also present in vascular amyloid deposits. Current evidence suggests that such protein(s) found in PHF-enriched fractions may derive from copurifying amyloid filaments rather than from PHF.