Alzheimer's amyloid precursor protein produced by recombinant baculovirus expression. Proteolytic processing and protease inhibitory properties

The Maze of APP Processing in Alzheimer's Disease: Where Did We Go Wrong in Reasoning?

Why has Alzheimer’s disease (AD) remained a conundrum today? The main reason is the stagnation in understanding the origins of plaques and tangles. While they are widely thought to be the products of the “aberrant” pathways, we believe that plaques and tangles result from natural aging. From this new perspective, we have proposed that age-related inefficiency of α-secretase is the underpinning for Aβ overproduction. This view contrasts sharply with the current doctrine that Aβ overproduction is the product of the “overactivated” β- and γ-secretases. Following this doctrine, it has been claimed that the two secretases are “positively identified” and that their inhibitors have “successfully reduced Aβ levels.” But, why have these studies not led to the understanding of AD or successful clinical trials? And if so, where did they go off course in reasoning? These questions may touch the basics of biological science and must be answered. In this paper, I dissected several prevailing assumptions and some influential reports with an attempt to trace the origins of the conundrum. This work led me to an original model for Aβ overproduction and also to a serious question: given the universal knowledge that boosting α-secretase reduces Aβ, a straightforward highway for intervention, then why is there such an obsession on “inhibiting β- and γ-secretases,” a much more costly and twisting road even if possible? This issue requires the attention of policymakers and all researchers. I therefore call for a game change in AD study.

Biogenesis and metabolism of Alzheimer's disease Abeta amyloid peptides

Biochemical and genetic evidence indicates the balance of biogenesis/clearance of Abeta amyloid peptides is altered in Alzheimer's disease. Abeta is derived, by two sequential cleavages, from the receptor-like amyloid precursor protein (APP). The proteases involved are beta-secretase, identified as the novel aspartyl protease BACE, and gamma-secretase, a multimeric complex containing the presenilins (PS). Gamma-secretase can release either Abeta40 or the more aggregating and cytotoxic Abeta42. Secreted Abeta peptides become either degraded by the metalloproteases insulin-degrading enzyme (IDE) and neprilysin or metabolized through receptor uptake mediated by apolipoprotein E. Therapeutic approaches based on secretase inhibition or amyloid clearance are currently under development.

Proteolysis of chimeric beta-amyloid precursor proteins containing the Notch transmembrane domain yields amyloid beta-like peptides

gamma-Secretase is an unusual intramembranous protease that has been reported to cleave the beta-amyloid precursor protein (APP) near the middle of its transmembrane domain (TMD) but cleave Notch near the cytoplasmic end of its TMD. To ascertain whether the TMD sequence of the substrate determines where gamma-secretase cleaves and whether the region just before the TMD participates in recognition by the enzyme, we expressed chimeric human APP molecules containing either the TMD or pre-TMD regions of Notch or other transmembrane proteins. APP chimeras bearing either the Notch or the amyloid precursor-like protein-2 TMD released similar amounts of approximately 4-kDa amyloid beta-peptide (Abeta)-like peptides as did intact APP. Mass spectrometry revealed that the principal Abeta-like peptide ended at residue 40, indicating cleavage at the middle of the Notch TMD in the chimera. Generation of Abeta-like peptides was significantly decreased when the APP TMD was replaced by those of SREBP-1 or human epithelial growth factor receptor 3. Replacement of the APP pre-TMD region (Abeta 10-28) with that of SREBP-1 increased generation of Abeta-like peptides, while those of human epithelial growth factor receptor 3 or amyloid precursor-like protein-2 decreased it. We conclude that gamma-secretase can cleave near the middle of the Notch TMD, that Abeta-like peptides may arise during Notch processing, and that the pre-TMD sequence of the substrate influences recognition or binding by the enzyme.

Developmental regulation of amyloid precursor protein at the neuromuscular junction in mouse skeletal muscle

Amyloid precursor protein (APP), associated with Alzheimer's disease plaques, is known to be present in synapses of the brain and in the adult neuromuscular junction (NMJ). In the present study we examined protein and gene expression of APP during the development of mouse skeletal muscle. Using immunocytochemical approaches, we found that APP is first detected in myotube cytoplasm at embryonic day 16 and becomes progressively concentrated at the NMJ beginning at birth until adulthood. The colocalization between APP and acetylcholine receptors at the NMJ is only partial at birth, but becomes complete upon reaching adulthood. We observed that all APP isoforms, including the Kunitz-containing (protease inhibitor or KPI) forms, are up-regulated from birth to postnatal day 5 and then decreased to reach the low levels observed in the adult. This suggests the involvement of APP during the events which lead to a mature mono-innervated synapse. A 92-kDa band, characteristic of a cleaved APP695 isoform and not due to a new muscle-specific alternative spliced form, was observed from postnatal day 15 following completion of polyneuronal synapse elimination. Taken together, these data suggest that skeletal muscle APP may well play a role in the differentiation of skeletal muscle and in the formation and maturation of NMJs.

Suppression of an amyloid beta peptide-mediated calcium channel response by a secreted beta-amyloid precursor protein

Secreted isoforms of the beta-amyloid precursor protein potently enhance neuronal survival in cell cultures exposed to toxic amyloid beta peptide. Lowering of intracellular calcium levels to offset the increases in intraneuronal calcium caused by amyloid beta peptide is thought to underly this neuroprotection. Because we have shown previously that an amyloid beta peptide-mediated potentiation of calcium channel currents may contribute to this cytosolic calcium overload, the present study examined the effects of a secreted beta-amyloid precursor protein on the calcium channel response to amyloid beta peptide. When compared with untreated cultured rat hippocampal neurons, cells that underwent a 24 h preincubation with beta-amyloid precursor protein 751 displayed decreases in the relative size of the calcium channel response to amyloid beta peptide. A membrane-permeable analog of cyclic GMP, a second messenger believed to be involved in the calcium regulation process mediated by beta-amyloid precursor proteins, also attenuated the modulatory calcium channel response. Co-application of beta-amyloid precursor protein 751 with amyloid beta peptide did not alter calcium channel response to amyloid beta peptide. Taken together, these findings suggest that secreted beta-amyloid precursor proteins can suppress a calcium channel response to amyloid beta peptide that is potentially injurious to the cell, and as such, may define a neuroprotective mechanism that is specific for amyloid beta toxicity.

Transgenic Drosophila expressing human amyloid precursor protein show gamma-secretase activity and a blistered-wing phenotype

The importance of the amyloid precursor protein (APP) in the pathogenesis of Alzheimer's disease (AD) became apparent through the identification of distinct mutations in the APP gene, causing early onset familial AD with the accumulation of a 4-kDa peptide fragment (betaA4) in amyloid plaques and vascular deposits. However, the physiological role of APP is still unclear. In this work, Drosophila melanogaster is used as a model system to analyze the function of APP by expressing wild-type and various mutant forms of human APP in fly tissue culture cells as well as in transgenic fly lines. After expression of full-length APP forms, secretion of APP but not of betaA4 was observed in both systems. By using SPA4CT, a short APP form in which the signal peptide was fused directly to the betaA4 region, transmembrane domain, and cytoplasmic tail, we observed betaA4 release in flies and fly-tissue culture cells. Consequently, we showed a gamma-secretase activity in flies. Interestingly, transgenic flies expressing full-length forms of APP have a blistered-wing phenotype. As the wing is composed of interacting dorsal and ventral epithelial cell layers, this phenotype suggests that human APP expression interferes with cell adhesion/signaling pathways in Drosophila, independently of betaA4 generation.

Processing of the Alzheimer's disease amyloid precursor protein in Pichia pastoris: immunodetection of alpha-, beta-, and gamma-secretase products

betaA4 (Abeta) amyloid peptide, a major component of Alzheimer's disease (AD) plaques, is a proteolytic product of the amyloid precursor protein (APP). Endoproteases, termed beta- and gamma-secretase, release respectively the N- and C-termini of the peptide. APP default secretion involves cleavage within the betaA4 domain by alpha-secretase. To study the conservation of APP processing in lower eukaryotes, the yeast Pichia pastoris was transfected with human APP695 cDNA. In addition to the full-length integral transmembrane protein found in the cell lysate, soluble/secreted APP (sAPP) was detected in the culture medium. Most sAPP comprised the N-terminal moiety of betaA4 and corresponds to sAPPalpha, the product of alpha-secretase. The culture medium also contained minor secreted forms detected by a monoclonal antibody specific for sAPPbeta (the ectodomain released by beta-secretase cleavage). Analysis of the cell lysates with specific antibodies also detected membrane-associated C-terminal fragments corresponding to the products of alpha and beta cleavages. Moreover, immunoprecipitation of the culture medium with three antibodies directed at distinct epitopes of the betaA4 domain yielded a 4 kDa product with the same electrophoretic mobility as betaA4 synthetic peptide. These results suggest that the alpha-, beta-, and gamma-secretase cleavages are conserved in yeast and that P. pastoris may offer an alternative to mammalian cells to identify the proteases involved in the generation of AD betaA4 amyloid.

Alzheimer's alpha-secretase may be a calcium-dependent protease

Proteolytic processing of beta-amyloid precursor protein (APP) is believed to be fundamental to the understanding of Alzheimer's disease. The identities and the regulatory elements of the proteases involved in the process, known as alpha/beta/gamma secretases, are unclear. In this study, by examining reported data, we found some indications suggesting that the putative alpha-secretase may be a calcium-dependent protease, and that this enzyme may play a primary role in the regulation of APP processing. Based on this, we proposed a model for the membrane orientations of the secretases for further discussions.

Amyloidogenic processing of human amyloid precursor protein in hippocampal neurons devoid of cathepsin D

betaA4-Amyloid peptide, the main component of the amyloid plaques in the brain of Alzheimer's disease patients is produced from amyloid precursor protein (APP) by proteolytical processing. Several lines of evidence suggest a direct role for cathepsin D, the major endosomal/lysosomal aspartic endopeptidase, in betaA4-amyloid peptide generation. Here we tested this hypothesis using primary cultures of hippocampal neurons derived from cathepsin D-deficient (knock out) mice and expressing wild-type human APP and two clinical APP variants via recombinant Semliki Forest virus. We demonstrate APP secretory processing, production of carboxyl-terminal amyloid fragments, and secretion of the betaA4-amyloid peptide in the complete absence of cathepsin D. The results rule out cathepsin D as a critical component of alpha-, beta-, or gamma-secretase and therefore as a primary target for drugs aimed at decreasing the betaA4-amyloid peptide burden in Alzheimer's disease.

Proteolytic release of membrane-bound angiotensin-converting enzyme: role of the juxtamembrane stalk sequence

Many structurally and functionally diverse membrane proteins are solubilized by a specific proteolytic cleavage in the stalk sequence adjacent to the membrane anchor, with release of the extracellular domain. Examples are the amyloid precursor protein, membrane-bound growth factors, and angiotensin-converting enzyme (ACE). The identities and characteristics of the responsible proteases remain elusive. We have studied this process in Chinese hamster ovary (CHO) cells stably expressing wild-type ACE (WT-ACE; human testis isozyme) or one of four juxtamembrane (stalk) mutants containing either deletions of 17, 24, and 47 residues (ACE-JM delta 17, -JM delta 24, and -JM delta 47, respectively) or a substitution of 26 stalk residues with a 20-residue sequence from the stalk of the low-density lipoprotein receptor (ACE-JMLDL). The C termini of released, soluble WT-ACE and ACE-JM delta 17 and -JMLDL were determined by MALDI-TOF mass spectrometry analyses of C-terminal peptides generated by CNBr cleavage. Observed masses of 4264 (WT-ACE) and 4269 (ACE-JM delta 17) are in good agreement with an expected mass of 4262 for the C-terminal CNBr peptide ending at Arg-627, indicating cleavage at the Arg-627/Ser-628 bond in both WT-ACE and ACE-JM delta 17, at distances of 24 and 10 residues from the membrane, respectively. Data for ACE-JM delta 24 are also consistent with cleavage at or near Arg-627. For ACE-JMLDL, in which the native cleavage site is absent, observed masses of 4372 and 4542 are in close agreement with expected masses of 4371 and 4542 for peptides ending at Ala-628 and Gly-630, respectively, indicating cleavages at 17 or 15 residues from the membrane. These data indicate that the membrane-protein-solubilizing protease (MPSP) in CHO cells is not constrained by a particular cleavage site motif or by a specific distance from the membrane but instead may position itself with respect to the putative proximal, folded extracellular domain adjacent to the stalk. Nevertheless, cleavage at a distance of 10 residues from the membrane is more favorable, as ACE-JM delta 17 is cleaved 12-fold faster than WT-ACE. In contrast, ACE-JM delta 24 is released 17-fold slower, suggesting that a minimum distance from the membrane must be preserved. This is supported by results with the ACE-JM delta 47 mutant, which is membrane-bound but not cleaved, likely because the entire stalk has been deleted. Finally, soluble full-length (anchor-plus) WT-ACE is not cleaved when incubated with various CHO cell fractions or intact CHO cells. On the basis of these and other data, we propose that the CHO cell MPSP that solubilizes ACE (1) only cleaves proteins embedded in a membrane; (2) requires an accessible stalk and cleaves at a minimum distance from both the membrane and proximal extracellular domain; (3) positions itself primarily with respect to the proximal extracellular domain; and (4) may have a weak preference for cleavage at Arg/Lys-X bonds.

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.

Artifactual strain-specific signs of incipient brain amyloidosis in APP transgenic mice

In an attempt to generate transgenic mice modeling Alzheimer-type amyloidogenesis, the COOH-terminal 103 residue human APP segment was expressed in brain regions known to be vulnerable in AD. Transfected cells overexpressing this transgene were previously shown to develop intracytoplasmic inclusions that were immunoreactive with antibodies to the APP COOH-terminus. Transgenic C57B6/SJL mice produced transgene-coded mRNA in their brains at levels up to sixfold above endogenous APP, most abundantly within cortical and hippocampal pyramidal neurons. Immunocytochemistry with anti-A beta antibodies revealed occasional structures that resembled diffuse amyloid, but which could not be detected on serial sections. Immunolabeling with antibodies to APP regions NH2-terminal to the transgene-coded domain revealed elevated immunoreactivity within perikarya and neurites in regions expressing the highest transgene and endogenous APP mRNA levels, similar to observations previously reported within vulnerable neurons in AD brain. However, subsequent breeding revealed that this phenotype segregated with the B6/SJL background rather than the transgene, thus emphasizing the importance of genetic background to observations of putative AD-type pathology in transgenic animals.

Serine proteinase inhibitors in human skeletal muscle: expression of beta-amyloid protein precursor and alpha 1-antichymotrypsin in vivo and during myogenesis in vitro

The balance of serine proteases and inhibitors in nerve and muscle is altered during programmed- and injury-induced remodeling. A serpin, alpha 1-antichymotrypsin (alpha 1-ACT), and Kunitz-inhibitor containing forms of the beta-amyloid precursor protein (beta APP) may be important components of this balance. In the present study, we analyzed their expression in primary cultures of human myogenic (satellite) cells that mimic myogenic differentiation using Western blotting and immunocytochemistry. In vitro results were compared to in vivo results from normal adult human skeletal muscle biopsies. Using an anti-alpha 1-ACT polyclonal antibody, we detected a 62 kDa immunoreactive band both in cultured human myogenic cells (mononucleated myoblasts as well as multi-nucleated myotubes) and in extracts of human muscle biopsies. With a polyclonal anti-beta APP antibody we found two bands (105 and 120 kDa) in myoblasts and myotubes in culture. However, the same antibody recognized only a single band at 92 kDa in biopsies. By immunocytochemistry, both alpha 1-ACT and beta APP were indistinctly present on localized to the surface of myoblasts in culture. In contrast, these inhibitors were dense on myotube surfaces, where they often formed distinct aggregates and frequently co-localized. In permeabilized muscle cells, alpha 1-ACT and beta APP appeared to be localized to the perikarya of both myoblasts and myotubes. Confirming previous results, both alpha 1-ACT and beta APP were present at the neuromuscular junction in human muscle sections. These developmental changes found during in vitro myogenesis for alpha 1-ACT and beta APP, both serine protease inhibitors, reinforce the hypothesis that regulation of the serine proteases and serine protease inhibitors plays an important role in neuromuscular differentiation.

Beta-Amyloid peptide and amyloid precursor proteins in olfactory mucosa of patients with Alzheimer's disease, Parkinson's disease, and Down syndrome

Dystrophic neurites are present in olfactory epithelium (OE) of patients with Alzheimer's disease (AD), Parkinson's disease (PD), and Down syndrome (DS) and occasionally in normal individuals. Cultured olfactory neuroblasts from AD patients generate carboxy terminal amyloid precursor protein (APP) fragments that contain beta-amyloid (A beta), but it is not known if deposits of A beta and/or APP fragments occur in the OE of individuals with or without AD, PD, or DS. To determine if A beta accumulates in the OE in situ, we probed postmortem samples of olfactory mucosa from patients with AD, PD, and AD (PD/AD), and DS and AD (DS/AD), as well as from controls, using polyclonal and monoclonal antibodies to A beta and flanking sequences in APPs. Samples of OE also were examined by thioflavin-S and electron microscopy. Labeling of A beta was observed in 10 of 12 AD cases, 2 of 3 PD/AD cases, 3 of 4 DS/AD cases, 3 of 10 adult controls, and 4 of 6 fetal cases. The A beta staining was seen in the basal third of the OE, in axons projecting through the lamina propria, and in metaplastic respiratory epithelium within the OE. Antibodies to other APP domains stained the OE of patients and controls. Thioflavin-S staining was present in the basal third of the OE of 8 of 9 AD patients and several PD/AD and DS/AD patients, but only in rare cells of 3 controls. Electron microscopy did not reveal amyloid fibrils in the OE.(ABSTRACT TRUNCATED AT 250 WORDS)

Two-dimensional gel mapping of the processing of the human amyloid precursor protein in rat hippocampal neurons

The proteolytic fragments derived from the amyloid precursor protein (APP) in primary cultures of rat hippocampal neurons were analyzed by two-dimensional gel electrophoresis. The Semliki Forest Virus expression vector was used to express human APP695 and a mutant form associated with familial Alzheimer's disease (APP-FAD670/671). Hippocampal neurons expressing wtAPP695 or APP-FAD670/671 secrete at least six APP fragments of 100-110 kDa with isoelectric focusing points ranging from 4.5 to 4.0. The heterogeneity of the secreted APP forms is shown to be in part due to differences in glycosylation. In contrast to wtAPP695, neurons producing the APP-FAD670/671 variant did not secrete detectable amounts of secretory APP derived from cleavage within the amyloid beta A4 domain. This result suggests that there is little alpha-secretase cleavage in neurons expressing the APP-FAD670/671 mutant.

Cathepsin G: localization in human cerebral cortex and generation of amyloidogenic fragments from the beta-amyloid precursor protein

Amyloid deposits in Alzheimer's disease, Down's syndrome and aged brain are composed largely of A beta protein, which is generated by proteolytic processing of beta-amyloid precursor protein. Proteases responsible for liberating the A beta protein from the precursor have not yet been identified. Here, we examined the ability of cathepsin G, a chymotrypsin-like protease, to cleave two protease substrates: (i) a fluorogenic hexapeptide, whose sequence spans the cleavage site in the precursor for generating the A beta NH2-terminus, and (ii) recombinant human beta-amyloid precursor protein purified from a baculovirus expression system. Unlike two other members of the chymotrypsin family, cathepsin G readily degraded the hexapeptide. Furthermore, cathepsin G cleaved the beta-amyloid precursor protein to generate several breakdown products, including a prominent 11,500 mol. wt fragment immunoreactive with antibodies directed against the COOH-terminus of the protein. This COOH-terminal fragment co-migrated using two-dimensional isoelectric focusing/sodium dodecyl sulfate-polyacrylamide gel electrophoresis with C-100, a recombinant COOH-terminal segment of the beta-amyloid precursor, whose NH2-terminus is one residue upstream of the NH2-terminus of the A beta domain. We also examined the localization of cathepsin G in human brain. The distribution of cathepsin G-containing cells was examined by immunohistochemistry in the temporal cortex of both Alzheimer's and aged control samples. Cathepsin G-like immunoreactivity was contained specifically within neutrophils. As visualized by double-labeling with antibodies to cathepsin G and Factor VIII, neutrophils were most frequently found within meningeal or cortical blood vessels. In addition, occasional neutrophils could be identified without an apparent vascular surround, in the brain parenchyma. By simultaneous labeling with antibodies to cathepsin G and A beta protein, neutrophils were also sometimes found associated with both parenchymal and vessel amyloid deposits; however, these associations were rare. These findings indicate that cathepsin G is capable of cleaving the beta-amyloid precursor protein to liberate the free NH2-terminus of the A beta protein and may have access to areas where this material is deposited in Alzheimer's disease. However, since there is no physical association between neutrophils and deposited amyloid and no increase in the number of neutrophils in an Alzheimer's brain, cathepsin G seems to be an unlikely mediator of amyloid deposition in this disease.

Cholinesterase activity in the plaques, tangles and angiopathy of Alzheimer's disease does not emanate from amyloid

Previous histochemical observations in our laboratory have demonstrated the presence of butyrylcholinesterase and an enzymatically altered form of acetylcholinesterase activity in the plaques, tangles and amyloid-containing vessels of Alzheimer's disease. These findings suggested possible interactions between amyloid and cholinesterases. In this study we employed a cholinesterase biochemical assay to determine whether the amyloid precursor protein either had cholinesterase activity itself or influenced the enzymatic activity of cholinesterases. None of the three amyloid precursor sequences used (695, 751, 770, up to 16 micrograms/ml) exhibited any acetylcholinesterase or butyrylcholinesterase activity that could be detected by our method. In addition, none of the amyloid precursor proteins influenced the enzymatic activity of purified acetylcholinesterase or butyrylcholinesterase in a specific manner. It is therefore quite unlikely that amyloid can, by itself, account for the intense cholinesterase activity associated with the pathological lesions of AD.

Neurotrophic regulation of mouse muscle beta-amyloid protein precursor and alpha 1-antichymotrypsin as revealed by axotomy

Kunitz-inhibitor containing forms of the beta-amyloid precursor protein (beta APP), known also as protease nexin II (PNII), and alpha 1-antichymotrypsin (alpha 1-ACT), a serpin, are important components of the serine protease and inhibitor balance in many tissues. In the nervous system, this balance may have trophic or growth factor activity at different stages of development, after injury and in disease states. In the current study, using immunocytochemistry and Western blotting with antibodies against the human homologues, we analyzed whether denervation affected the localization of beta APP and alpha 1-ACT in adult mouse muscle following axotomy. In mouse muscle, anti-human alpha 1-ACT antibody detected a 60 kD immunoreactive band and anti-human beta APP antibody a band at 92 kD in both normal and denervated extracts. beta APP was present in normal mouse muscle at both neuromuscular junctions and within intramuscular nerves. alpha 1-ACT was also detected at neuromuscular junctions, on the perineurium and endothelial cell surfaces. Following axotomy, both beta APP and alpha 1-ACT disappeared from intramuscular nerves simultaneously. However, at the neuromuscular junction, alpha 1-ACT decreased more rapidly with beta APP lingering before disappearing. Since both alpha 1-ACT as well as beta APP are present within senile plaques in Alzheimer's disease brains such experiments with the nicotinic, cholinergic neuromuscular synapse in denervated muscle may help to focus experiments on the mechanism of synapse loss as well as plaque deposition in this disease.

Expression of beta-amyloid precursor protein in the developing human spinal cord

Human fetal spinal cords and other non-neural tissues from cases with gestational age from 6 to 21 weeks were examined with a panel of antibodies to different domains of beta-amyloid precursor proteins (beta-APPs). In the early developmental stages, the beta-APPs were expressed in three distinct layers, i.e., primitive neuroepithelial cell layer, mantle layer and marginal layer. beta-APP immunoreactivity was most prominent in cell bodies of putative neuroblasts located in the outer ventral part of the mantle layer. beta-APP expression diminished as the spinal cord matured and a weak residual immunoreactivity was detected exclusively in a subset of the anterior horn cells by 21 weeks gestational age. Throughout the gestational ages examined, no convincing beta/A4 immunostaining was seen in any of the spinal cord regions. Outside the spinal cord, beta-APP immunostaining was consistently present in (1) cell bodies and proximal nerves of immature neurons of dorsal root ganglia and in (2) myotubules, although these cells were devoid of beta/A4 immunoreactivity. Western blot analysis of fetal spinal cord revealed immunoreactive bands with apparent molecular weight between 100 and 140 kDa in the membrane-associated fraction, while soluble proteins with a molecular mass centered on 115 kDa were detected in the cytosolic fraction. Our results indicate that: (1) one or more isoforms of full length beta-APPs are expressed at very early gestational ages in the developing human spinal cord; (2) the normal metabolism of beta-APPs does not result in accumulations of beta/A4 fragments.

Binding of vascular heparan sulfate proteoglycan to Alzheimer's amyloid precursor protein is mediated in part by the N-terminal region of A4 peptide

The exact mechanisms of deposition and accumulation of amyloid in senile plaques and in blood vessels in Alzheimer's disease remain unknown. Heparan sulfate proteoglycans may play an important role in amyloid deposition in Alzheimer's disease. Previous investigations have demonstrated high affinity binding between heparan sulfate proteoglycans and the amyloid precursor, as well as with the A4 peptide. In the current studies, a specific vascular heparan sulfate proteoglycan found in senile plaques bound with high affinity to two amyloid protein precursors (APP695 and APP770). Vascular heparan sulfate proteoglycan also bound the Alzheimer's amyloid A4 peptide, and not other amyloid protein precursor regions studied, with high affinity. Both heparan sulfate glycosaminoglycan chains and chemically deglycosylated vascular heparan sulfate proteoglycan protein core bound to A4. High affinity interactions between vascular heparan sulfate proteoglycan and the A4 peptide may play a role in the process of amyloidogenesis in Alzheimer's disease, by localizing the site of deposition of A4, protecting A4 from further proteolysis, or by promoting aggregation and fibril formation.

Morphological and biochemical analyses of amyloid plaque core proteins purified from Alzheimer disease brain tissue

Amyloid plaque cores were purified from Alzheimer disease brain tissue. Plaque core proteins were solubilized in formic acid which upon dialysis against guanidinium hydrochloride (GuHCl) partitioned into soluble (approximately 15%) and insoluble (approximately 85%) components. The GuHCl-soluble fraction contained beta-amyloid1-40, whereas the GuHCl-insoluble fraction was fractionated into six components by size exclusion HPLC: S1 (> 200 kDa), S2 (200 kDa), S3 (45 kDa), S4 (15 kDa), S5 (10 kDa), and S6 (5 kDa). Removal of the GuHCl reconstituted 10-nm filaments composed of two intertwined 5-nm strands. Fractions S5 and S6 also yielded filamentous structures when treated similarly, whereas fractions S1-S4 yielded amorphous aggregates. Chemical analysis identified S4-S6 as multimeric and monomeric beta-amyloid. Immunochemical analyses revealed alpha 1-antichymotrypsin and non-beta-amyloid segments of the beta-amyloid precursor protein within fractions S1 and S2. Several saccharide components were identified within plaque core protein preparations by fluorescence and electron microscopy, as seen with fluorescein isothiocyanate- and colloidal gold-conjugated lectins. We have shown previously that this plaque core protein complex is more toxic to neuronal cultures than beta-amyloid. The non-beta-amyloid components likely mediate this additional toxicity, imposing a significant influence on the pathophysiology of Alzheimer disease.

Degeneration of vascular muscle cells in cerebral amyloid angiopathy of Alzheimer disease

In cerebral amyloid angiopathy, the amyloid-beta (A beta) deposits lie primarily in the tunica media suggesting that smooth muscle cells play an important role in A beta deposition. To define this role, we conducted an immunocytochemical study of brain tissue from cases of Alzheimer disease with extensive cerebral amyloid angiopathy and cerebral hemorrhage. Antibodies specific to recombinant beta protein precursor (beta PP) and synthetic peptides homologous to various beta PP sequences from residue 18 to 689 of beta PP695 were used. Antibodies to actin, tropomyosin, alpha-actinin or desmin were used to label muscle cells. Antibodies to A beta sequences intensely recognized the extracellular amyloid deposit. Antibodies raised against beta PP sequences other than the A beta domain recognized smooth muscle cells. beta PP-immunoreactivity was reduced in regions of A beta deposits, since no muscle cells were recognized by cytoskeletal markers or observed ultrastructurally. In order to assess why A beta is deposited in the tunica media, we used biotin-labelled beta PP to determine if beta PP can be locally retained. We found beta PP bound to the tunica media of vessels but not other brain elements. These findings suggest A beta in blood vessels derives from degenerating beta PP-containing smooth muscle cells.

The influence of denervation on beta-amyloid protein precursor and alpha 1-antichymotrypsin in mouse skeletal muscle

A serpin, alpha 1-antichymotrypsin (alpha 1-ACT), and Kunitz inhibitor containing forms of the beta-amyloid precursor protein (beta APP) may be important components of the balance between serine proteases and inhibitors in the nervous system. In the current report we studied whether axotomy affected the localization of beta APP and alpha 1-ACT in adult mouse muscle. Immunocytochemical experiments indicated that beta APP was present in normal muscle both at neuromuscular junctions and within intramuscular nerves. alpha 1-ACT was also present at neuromuscular junctions, on the perineurium of nerves and endothelial cell surfaces. Following axotomy, both beta APP and alpha 1-ACT disappeared from intramuscular nerves simultaneously. However, at the neuromuscular junction alpha 1-ACT decreased more rapidly with beta APP lingering before disappearing.

Rimmed vacuoles of inclusion body myositis and oculopharyngeal muscular dystrophy contain amyloid precursor protein and lysosomal markers

Rimmed vacuoles are small areas of focal destruction of muscle fibres, found in inclusion body myositis, oculopharyngeal muscular dystrophy and other muscle disorders. They are known to contain amyloid proteins, probably of beta-amyloid type. We examined rimmed vacuoles immunohistochemically in 12 patients with inclusion body myositis and two patients with oculopharyngeal muscular dystrophy with antibodies to beta-amyloid precursor protein and cathepsin B and D. We found evidence for the presence of all these markers in rimmed vacuoles. These results confirm the presence of beta-amyloid in rimmed vacuoles, and provide additional support for the hypotheses that rimmed vacuoles are of lysosomal origin and that lysosomes are probably important in the metabolism of amyloid precursor protein.

An alternative secretase cleavage produces soluble Alzheimer amyloid precursor protein containing a potentially amyloidogenic sequence

Cell culture studies have shown that the Alzheimer amyloid precursor protein (APP) is secreted after full-length APP is cleaved by a putative secretase at the Lys16-Leu17 bond (secretase cleavage I) of the amyloid peptide sequence. Because this cleavage event is incompatible with amyloid production, it has been assumed that secreted APP cannot serve as a precursor of the amyloid depositions observed in Alzheimer's disease. Here we show that in neuronally differentiated PC12 cells and human kidney 293 cell cultures a portion of the secreted extracytoplasmic APP reacted specifically with both a monoclonal antibody recognizing amyloid protein residues Leu17-Val24 and a polyclonal antiserum directed against amyloid protein residues Ala21-Lys28. Furthermore, this APP failed to react with antisera recognizing the cytoplasmic domain of the full-length protein. These data indicate the presence of an alternative APP secretase cleavage site (secretase cleavage II), C-terminal to the predominant secretase cleavage I. Depending on the exact location of cleavage site II, potentially amyloidogenic secreted APP species may be produced.

Alzheimer's disease and control brain contain soluble derivatives of the amyloid protein precursor that end within the beta amyloid protein region

The 39-43 amino acid beta amyloid protein (A beta) that deposits as amyloid in the brains of patients with Alzheimer's disease (AD) is encoded as an internal sequence within a larger membrane-associated protein known as the amyloid protein precursor (APP). In cultured cells, the APP is normally cleaved within the A beta to generate a large secreted derivative and a small membrane-associated fragment. Neither of these derivatives can produce amyloid because neither contains the entire A beta. Our study was designed to determine whether the soluble APP derivatives in human brain end within the A beta as described in cell culture or whether AD brain produces potentially amyloidogenic soluble derivatives that contain the entire A beta. We find that both AD and control brain contain nonamyloidogenic soluble derivatives that end at position 15 of the A beta. We have been unable to detect any soluble derivatives that contain the entire A beta in either the AD or control brain.

Human amyloid precursor protein ameliorates behavioral deficit of flies deleted for Appl gene

Drosophila amyloid precursor protein-like (Appl) gene encodes a protein product (APPL) similar to beta-amyloid precursor protein (APP) associated with Alzheimer's disease. To understand the in vivo function of APPL protein, we have generated flies deleted for the Appl gene. These flies are viable, fertile, and morphologically normal, yet they exhibit subtle behavioral deficits. We show that a fast phototaxis defect in Appl- flies is partially rescued by transgenes expressing the wild-type, but not a mutant, APPL protein. We further demonstrate a functional homology between APPL and APP, since transgenes expressing human APP show a similar level of rescue as transgenes expressing fly APPL.