Cell-surface beta-amyloid precursor protein stimulates neurite outgrowth of hippocampal neurons in an isoform-dependent manner

beta-Amyloid precursor protein (beta APP) is an integral membrane polypeptide expressed in many neural and non-neural cells. beta APP occurs in part at the cell surface and undergoes proteolytic processing to release the large soluble ectodomain (APPs) and the amyloid beta-peptide (A beta), both of which have apparent trophic activity in vitro. Despite intense interest in beta APP expression and metabolism, there is limited knowledge about the function mediated by beta APP inserted at the cell-surface. We established a coculture system in which beta APP-transfected CHO cells serve as a substrate for the growth of primary rat hippocampal neurons. Compared to nontransfected CHO cells, the increased surface beta APP of the transfectants stimulated short-term neuronal adhesion and longer-term neurite outgrowth, whereas the increased amount of secreted APPs and A beta in conditioned medium produced no such effects when neurons were grown either on untransfected CHO cells or on a polylysine substrate. Moreover, a peptide which has been shown to block the trophic effects of secreted APPs (Ninomiya et al., 1993) failed to interrupt the neurite promoting activity mediated by the surface-expressed beta APP. Surface-expressed beta APP751 or beta APP770 isoforms mediated more neurite outgrowth than did the beta APP695 isoform. Antibody blocking and regional deletion experiments indicated that the mid-region of the beta APP ectodomain (residues 361-648) is involved in promoting neurite outgrowth. We conclude that surface-expressed cellular beta APP has a neurite-promoting function which is distinct from the trophic function of the secreted beta APP derivatives and may have special significance during brain development.

beta-Amyloid, protein processing and Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder resulting in the deposition of amyloid beta-peptide (A beta) in senile plaques in cerebral and limbic corteces and the walls of meningeal and cerebral blood vessels. A beta is a proteolytic break-down product of a membrane bound precursor, the beta-amyloid precursor protein (beta APP). Conventional secretory processing of beta APP prevents A beta formation. An additional processing pathway of beta APP involving endosomal/lysosomal targeting is described. Within isolated lysosomes amygloidogenic fragments are found which might serve as precursors for A beta production. From such precursors A beta might be proteolytically processed. Indeed, secreted A beta was identified in the media of cultured cells. A beta is also secreted in vivo and can be detected in human plasma and cerebral spinal fluid. These findings provide a cellular system to analyze the molecular mechanism and the biological regulation of A beta generation. Furthermore, the effect of inherited mutations within the beta APP gene in some cases of familial AD can now be analyzed in such tissue culture cells transfected with the mutant cDNA constructs. A model will be presented proposing that A beta generation might occur during reinternalization of the full-length molecule.

Polarized sorting of beta-amyloid precursor protein and its proteolytic products in MDCK cells is regulated by two independent signals

Progressive cerebral deposition of the amyloid (A beta) beta-protein is an early and invariant feature of Alzheimer's disease. A beta is derived by proteolysis from the membrane-spanning beta-amyloid precursor protein (beta APP). beta APP is processed into various secreted products, including soluble beta APP (APPs), the 4-kD A beta peptide, and a related 3-kD peptide (p3). We analyzed the mechanisms regulating the polarized basolateral sorting of beta APP and its proteolytic derivatives in MDCK cells. Deletion of the last 32 amino acids (residues 664-695) of the beta APP cytoplasmic tail had no influence on either the constitutive approximately 90% level of basolateral sorting of surface beta APP, or the strong basolateral secretion of APPs, A beta, and p3. However, deleting the last 42 amino acids (residues 654-695) or changing tyrosine 653 to alanine altered the distribution of cell surface beta APP so that approximately 40-50% of the molecules were inserted apically. In parallel, A beta was now secreted from both surfaces. Surprisingly, this change in surface beta APP had no influence on the basolateral secretion of APPs and p3. This result suggests that most beta APP molecules which give rise to APPs in MDCK cells are cleaved intracellularly before reaching the surface. Consistent with this conclusion, we readily detected intracellular APPs in carbonate extracts of isolated membrane vesicles. Moreover, ammonium chloride treatment resulted in the equal secretion of APPs into both compartments, as occurs with other non-membranous, basolaterally secreted proteins, but it did not influence the polarity of cell surface beta APP. These results demonstrate that in epithelial cells two independent mechanisms mediate the polarized trafficking of beta APP holoprotein and its major secreted derivative (APPs) and that A beta peptides are derived in part from beta APP holoprotein targeted to the cell surface by a signal that includes tyrosine 653.

Beta APP mRNA transcription is increased in cultured fibroblasts from the familial Alzheimer's disease-1 family

Familial (autosomal dominant) Alzheimer's disease (FAD) is a genetically heterogeneous disorder. Mutations in exons 16 and 17 of the amyloid beta-protein precursor (beta PP) gene currently account for less than 2% of FAD kindreds. No known defect in beta PP quantity, structure, or processing accounts for disease-associated beta-amyloid deposition in the majority of early-onset FAD kindreds. Only two out of a sample of 48 pedigrees (particularly the early onset FAD 4 kindred) contributed noticeably to evidence of linkage at the D21S16/13 and S1/S11 loci in the chromosomal region 21q21 [75]. Many early onset FAD pedigrees (including the FAD 1 and FAD 4 kindreds) show strong evidence of linkage to markers in the chromosome 14q24.3 region. Patients with trisomy 21 (Down's syndrome, DS) virtually always develop a histopathological phenotype indistinguishable from FAD, presumably on the basis of increased beta PP gene dosage and transcription. Whereas no beta PP gene duplication has been found in FAD, other mechanisms that augment beta PP production by effects at the transcriptional level could explain some FAD cases. Here, we report that cultured fibroblasts from affected members of the FAD 1 pedigree show a approximately 1.9 fold increase (P = 0.007) in beta PP mRNA levels compared to unaffected members when the cells are grown under stressed conditions in 0.5% serum. The elevated levels of beta PP mRNA in cells cultured in 0.5% serum also cosegregate with haplotypes in the 14q24.3 region when analyzed by linkage methods (LOD score = 3.26 at theta = 0.001). This is the chromosomal region to which FAD in this family has previously been mapped. As expected, fibroblasts from patients with DS used as a control show a similar beta PP mRNA increase. Fibroblasts from the FAD 4 pedigree did not show this defect under the conditions utilized here. beta PP and A beta protein levels were determined quantitatively after metabolic labeling and immunoprecipitation and found to increase 2.0 and 2.5 fold, respectively, in the fibroblasts from affected FAD 1 members. Finally, transient transfections of a beta PP promoter/chloramphenicol acetyl transferase reporter gene construct demonstrated a approximately 3-4 fold increase in beta PP promoter activity in affected fibroblasts from the FAD 1 but not the FAD 4 pedigree. Taken together, these data raise the possibility that an increase in beta PP transcription may underlie the AD phenotype in at least some of the chromosome 14-linked FAD families.

Excessive production of amyloid beta-protein by peripheral cells of symptomatic and presymptomatic patients carrying the Swedish familial Alzheimer disease mutation

The 39- to 43-amino acid amyloid beta-protein (A beta), which is progressively deposited in cerebral plaques and blood vessels in Alzheimer disease (AD), is secreted by cultured human cells during normal metabolism. In studies of cell lines transfected with beta-amyloid precursor protein (beta APP) cDNAs, the beta APP mutation K670N/M671L found in a Swedish familial AD (FAD) pedigree has previously been shown to cause a marked augmentation of A beta secretion. Here, we have conducted blinded analyses of beta APP metabolism in primary skin fibroblasts from affected members of the Swedish FAD pedigree and their unaffected siblings or spouses. These fibroblasts continuously secrete a homogenous population of A beta molecules starting at Asp-1 (D672 of beta APP). We found a consistent and significant approximately 3-fold elevation of A beta release from all biopsied skin fibroblasts bearing the FAD mutation. No significant alterations of other metabolic derivatives of beta APP were detected. The elevated A beta levels were found in cells from both patients with clinical AD and presymptomatic subjects. Thus, A beta overproduction in this FAD pedigree is not a secondary event but is consistent with a causal role in the development of the disease. Increased A beta secretion can begin many years prior to onset of symptoms, even in peripheral tissues, indicating that it does not require preexisting neural abnormalities.

Amyloid beta-protein precursor: new clues to the genesis of Alzheimer's disease

Research into the cellular processing and functional properties of the amyloid beta-protein precursor (beta PP) and its secreted derivatives is rapidly accelerating and drawing interest from many investigators outside the field of Alzheimer's disease pathobiology. Recent in vitro studies suggest multiple activities for the major secreted product, beta PPs, including protease inhibition, growth promotion, neuroprotection and stimulation of a signal transduction pathway. Surprisingly, the hydrophobic amyloid beta-protein fragment is also constitutively secreted. Its generation occurs in part during the endocytosis of cell-surface beta PP molecules. Processing of beta PP via amyloidogenic versus non-amyloidogenic pathways is under complex regulation by various first and second messengers. Analyses of beta PP missense mutations and of the effects of apolipoprotein E genotype lend new support to the hypothesis that accelerated amyloid beta protein deposition plays a pivotal role in the genesis of Alzheimer's disease.

Secreted beta-amyloid precursor protein stimulates mitogen-activated protein kinase and enhances tau phosphorylation

Biological effects related to cell growth, as well as a role in the pathogenesis of Alzheimer disease, have been ascribed to the beta-amyloid precursor protein (beta-APP). Little is known, however, about the intracellular cascades that mediate these effects. We report that the secreted form of beta-APP potently stimulates mitogen-activated protein kinases (MAPKs). Brief exposure of PC-12 pheochromocytoma cells to beta-APP secreted by transfected Chinese hamster ovary cells stimulated the 43-kDa form of MAPK by > 10-fold. Induction of a dominant inhibitory form of ras in a PC12-derived cell line prevented the stimulation of MAPK by secreted beta-APP, demonstrating the dependence of the effect upon p21ras. Because the microtubule-associated protein tau is hyperphosphorylated in Alzheimer disease, we sought and found a 2-fold enhancement in tau phosphorylation associated with the beta-APP-induced MAPK stimulation. In the ras dominant inhibitory cell line, beta-APP failed to enhance phosphorylation of tau. The data presented here provide a link between secreted beta-APP and the phosphorylation state of tau.

Mutations associated with a locus for familial Alzheimer's disease result in alternative processing of amyloid beta-protein precursor

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by the extracellular deposition of amyloid beta-protein (A beta), a molecule produced by post-translational processing of the beta-amyloid precursor protein (beta APP). Mutations within the gene encoding beta APP have been linked to early onset forms of AD, but the pathogenetic mechanism(s) producing the phenotype are unknown. We analyzed the effects on beta APP processing in vitro of a naturally occurring Ala-->Gly mutation at position 692 of beta APP770 (A692G) (Hendriks, L., Duijin, C., Cras, P., Cruts, M., van Hul, W., van Harskamp, F., Warren, A., McInnis, M., Antonarakis, S., Martin, J.-J., Hofman, A., and van Broeckhoven, C. (1992) Nature Genet. 1, 218-221), as well as the effects of five genetically engineered mutations at or near this site. Substitution of glycine or proline for Ala692, or for Phe690, produced relative increases in secretion of A beta and relative decreases in secretion of the p3 peptide(s) arising after alpha-secretase generation of soluble APP (APPs). The Phe690-->Pro substitution also resulted in the synthesis of truncated APPs molecules. The structurally conservative substitutions Ala692-->Val and Phe690-->Tyr did not exhibit these effects. Certain of the substitutions also resulted in the production of a minor peptides, previously undescribed in vitro, beginning at Ala2, Lys16, and Phe19 of A beta. These data show that beta APP mutations carboxyl-terminal to alpha-secretase and beta-secretase cleavage sites can exert strong control over beta APP processing. Increased secretion of A beta may accelerate amyloidogenesis by providing more precursors for aggregation. It is also possible that truncated A beta peptides resulting from several of these mutations may accelerate amyloidogenesis through self-aggregation and/or seeding the fibrillogenesis of longer, more abundant A beta species.

Calcium ionophore increases amyloid beta peptide production by cultured cells

Amyloid beta peptide (A beta) is released into the media of a variety of cells in culture during normal metabolism. The discovery of several missense mutations within or flanking the A beta region of the beta amyloid precursor protein (beta APP) in familial Alzheimer's disease provides strong evidence for a role of altered processing of beta APP in the pathogenesis of this disorder. The cellular mechanisms that regulate the relative utilization of the secretory pathway, which causes beta APP to be cleaved within the A beta domain, and the alternative proteolytic pathway, which produces intact A beta, are unknown. It is hypothesized that a number of neurodegenerative diseases, including Alzheimer's disease, are characterized by abnormal calcium metabolism. We investigated the effect of disordered calcium homeostasis on A beta production in human kidney 293 cells transfected with beta APP cDNA. A beta immunoprecipitated from the conditioned media of cells was compared to immunoprecipitated full-length and secreted forms of beta APP in both metabolic labeling and pulse-chase labeling paradigms. The calcium ionophore A23187 consistently increased the production of A beta approximately 3-fold. This effect was dependent on the presence of extracellular calcium in intact cells. Caffeine also increased A beta production, possibly through release of calcium from intracellular stores. The increase in A beta was cAMP-independent, and it was not mediated by a protein kinase C-dependent pathway, as treatment with phorbol esters decreased A beta levels. The effects of the ionophore on beta APP maturation and phosphorylation were also established. We conclude that elevation of intracellular calcium levels has an important effect on beta APP maturation and proteolytic processing and substantially enhances the production and release of the amyloidogenic A beta peptide.

Polarized secretion of beta-amyloid precursor protein and amyloid beta-peptide in MDCK cells

The beta-amyloid precursor protein (beta APP) is a widely expressed integral membrane protein that is proteolytically processed to yield several secreted derivatives, including soluble APP (APPs), the 4-kDa amyloid beta-peptide (A beta), and a related 3-kDa peptide (p3). To understand beta APP trafficking and processing, we analyzed the sorting of beta APP in Madin-Darby canine kidney (MDCK) cells, an epithelial cell known to possess physiologically distinct apical and basolateral plasma membranes. Processing of beta APP resulted in highly polarized secretion of APPs. More than 90% of APPs was detected in the basolateral compartment, and less than 10% was found in the apical compartment. This was associated with a preferential localization of beta APP on the basolateral cell surface. Activation of protein kinase C, which is known to enhance the secretion of APPs, did not change the polarity of APPs release but significantly increased the amount secreted. A beta and p3 peptides were also secreted predominantly basolaterally. In addition, MDCK cells secreted a truncated form of A beta beginning at Arg-5. These data show that the proteolytic processing products of beta APP undergo polarized secretion. Moreover, the results suggest that the amyloidogenic A beta peptide is generated following the polarized sorting of beta APP. The polarized basolateral secretion of A beta in these epithelial cells provides a potential mechanism for the accumulation of A beta in the abluminal basement membrane of brain microvessels during Alzheimer disease.

Selective ectodomain phosphorylation and regulated cleavage of beta-amyloid precursor protein

The beta-amyloid precursor protein (beta APP) is a highly conserved integral membrane protein expressed in most mammalian tissues and found at highest levels in the nervous system. Cerebral deposition of the amyloid beta-peptide (A beta), derived by proteolysis of beta APP, is an early and invariant feature of Alzheimer's disease. Protein phosphorylation by protein kinase C (PKC) has been found to regulate the metabolism of beta APP into nonamyloidogenic and amyloidogenic derivatives, but both the mechanism of these effects and the nature of beta APP phosphorylation are unknown. When labeled in vivo with [32P]orthophosphate, beta APP was phosphorylated only on serine residues in the N-terminal half of the extracellular domain, resulting in the secretion of phosphorylated soluble beta APP. PKC-mediated stimulation of beta APP secretion and concurrent inhibition of A beta release did not involve enhanced phosphorylation of beta APP and proceeded in the absence of cytoplasmic or extracellular phosphorylation of the precursor. The region of beta APP required for this indirect regulation by PKC was largely restricted to a 64 amino acid stretch around the secretory cleavage site. Moreover, in a truncated molecule designed to release soluble beta APP without the need for proteolytic cleavage, secretion was no longer regulated by PKC. Our data indicate that PKC-mediated pathways play a pivotal role in the control of beta APP metabolism and amyloid formation. However, in contrast to current postulates, this regulation is independent of beta APP phosphorylation and instead involves phosphorylation of other substrates that alter beta APP processing, such as beta APP-cleaving proteases.

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.

Twisted tubulofilaments of inclusion body myositis muscle resemble paired helical filaments of Alzheimer brain and contain hyperphosphorylated tau

We immunostained muscle biopsies of 8 patients with sporadic inclusion body myositis (S-IBM), 7 patients with autosomal recessive hereditary inclusion body myopathy (H-IBM) (both diseases being characterized by similar muscle fiber vacuoles containing inclusions), and 11 normal and disease controls. We used the following well-characterized antibodies against tau protein: Tau-1, Alz-50, and anti-paired helical filament (PHF) antiserum. By light microscopy, in all S-IBM muscle biopsies virtually all vacuoles immunoreactive for ubiquitin and beta-amyloid protein also contained inclusions immunoreactive with Alz-50 and anti-PHF antiserum. With tau-1 antibody, strong immunoreactivity in the vacuoles was obtained only after dephosphorylation of muscle sections. By electronmicroscopy, all three antibodies immunodecorated exclusively cytoplasmic twisted tubulofilaments (TTFs). In H-IBM, virtually all ubiquitin and beta-amyloid-positive muscle fiber vacuoles contained inclusions immunoreactive with anti-PHF antiserum, but in only 40% of those fibers were the inclusions immunoreactive with Alz-50. In six H-IBM patients there were no tau-1 immunoreactive inclusions in any of their vacuolated muscle fibers; in one patient, 24% of the vacuolated fibers had tau-1 immunoreactivity. By demonstrating that hyperphosphorylated tau, which is characteristic of Alzheimer brain PHFs, is a component of S-IBM-muscle TTFs (which are also ultrastructurally similar to PHFs), our study: 1) provides the first demonstration of abnormally accumulated tau in nonneural tissue and 2) suggests that the cytopathogenesis in Alzheimer brain and S-IBM muscle may share some similar mechanisms. Whether the difference in tau immunoreactivity between S-IBM and most of the H-IBM patients reflects a difference in genetically determined transcriptional or posttranslational modifications of tau protein or other factors remains to be determined.

Activation of protein kinase C inhibits cellular production of the amyloid beta-protein

The 39-43-amino acid amyloid beta-protein (A beta), which is progressively deposited in cerebral plaques and blood vessels in Alzheimer's disease (AD), is released by cultured human cells during normal metabolism. Here we show that agents which activate protein kinase C or otherwise enhance protein phosphorylation caused a substantial decrease in A beta production in vitro. Protein kinase C activation also markedly decreased A beta release from cells that express mutant forms of the beta-amyloid precursor protein genetically linked to familial AD. Inhibition of A beta secretion could also be effected by direct stimulation of m1 muscarinic acetylcholine receptors with carbachol. These results demonstrate that activation of the protein kinase C signal transduction pathways down-regulates the generation of the amyloidogenic A beta peptide. Pharmacologic agents that activate this system, including a variety of first messengers, could potentially slow the development or growth of some A beta plaques during the early stages of AD.

Production of amyloid-beta-peptide by cultured cells: no evidence for internal initiation of translation at Met596

It has been suggested that the A beta fragment of the amyloid precursor protein in Alzheimer's disease arises from internal translation initiation at Met596 (1). Here we use the recently described in vitro model of A beta production and secretion (2) to examine this hypothesis. We show that A beta is no longer detectable when the beta APP reading frame is destroyed by introduction of frame shift mutations that leave the A beta coding region intact. This result strongly suggests that internal initiation at Met596 does not contribute significantly to the amount of A beta observed.

Physiological production of the beta-amyloid protein and the mechanism of Alzheimer's disease

The progressive deposition of the beta-amyloid peptide in the brain and its microvasculature is an invariant feature of Alzheimer's disease that appears to precede the onset of dementia by many years. It had been assumed that the proteolytic release of beta-amyloid peptide from the transmembrane region of its large precursor protein was an aberrant event, requiring prior membrane injury. However, it has recently been shown that beta-amyloid peptide is continuously secreted from healthy neural and non-neural cells in culture and circulates in human CSF and blood. The finding that beta-amyloid peptide is a normal, soluble product of cellular metabolism has led to many dynamic studies of its formation and clearance in health and in genetic forms of Alzheimer's disease, and should facilitate the design of amyloid-inhibiting therapeutics.

Normal cellular processing of the beta-amyloid precursor protein results in the secretion of the amyloid beta peptide and related molecules

Alzheimer's disease is characterized by the extracellular deposition in the brain and its blood vessels of insoluble aggregates of the amyloid beta peptide (A beta). This peptide is derived from a large integral membrane protein, the beta-amyloid precursor protein (beta APP), by proteolytic processing. The A beta has previously been found only in the brains of patients with Alzheimer's disease or advanced aging. We describe here the finding that A beta is produced continuously by normal processing in tissue culture cells. A beta and closely related peptides were identified in the media of cells transfected with cDNAs coding for beta APP in a variety of cell lines and primary tissue cultured cells. The identity of these peptides was confirmed by epitope mapping and radiosequencing. Peptides of a molecular weight of approximately 3 and approximately 4 kDa are described. The 4 kDa range contains mostly the A beta and two related peptides starting N-terminal to the beginning of A beta. In the 3 kDa range, the majority of peptides start at the secretase site; in addition, two longer peptides were found starting at amino acid F(4) and E(11) of the A beta sequence. To identify the processing pathways which lead to the secretion of these peptides, we used a variety of drugs known to interfere with certain cell biological pathways. We conclude that lysosomes may not play a predominant role in the formation of 3 and 4 kDa peptides. We show that an acidic environment is necessary to create the N-terminus of the A beta and postulate that alternative secretory cleavage might result in the formation of the N-terminus of A beta and related peptides. This cleavage takes place either in the late Golgi, at the cell-surface or in early endosomes, but not in lysosomes. The N-terminus of most of the 3 kDa peptides is created by secretory cleavage on the cell surface or within late Golgi.

Amyloid beta-protein as a substrate interacts with extracellular matrix to promote neurite outgrowth

Progressive deposition of amyloid beta-protein (A beta) in brain parenchyma and blood vessels is a characteristic feature of Alzheimer disease. Recent evidence suggests that addition of solubilized synthetic A beta to medium may produce toxic or trophic effects on cultured hippocampal neurons. Because soluble A beta may not accumulate in significant quantities in brain, we asked whether immobilized A beta peptide as a substrate alters neurite outgrowth from cultured rat peripheral sensory neurons. This paradigm may closely mimic the conditions in Alzheimer disease brain tissue, in which neurites contact insoluble, extracellular aggregates of beta-amyloid. We detected no detrimental effects of A beta substrate on neurite outgrowth. Rather, A beta in combination with low doses of laminin or fibronectin enhanced neurite out-growth from these neuronal explants. Our results suggest that insoluble A beta in the cerebral neuropil may serve as a neurite-promoting matrix, perhaps explaining the apparent regenerative response of neurites observed around amyloid plaques in Alzheimer disease. Moreover, in concert with the recent discovery of A beta production by cultured neurons, our data suggest that A beta plays a normal physiological role in brain by complexing with the extracellular matrix.