Beta APP Gene Research Articles

Trisomy 21 and the brain.

In fetuses with Down syndrome, neurons fail to show normal dendritic development, yielding a "tree in winter" appearance. This developmental failure is thought to result in mental retardation. In adults with Down syndrome, neuronal loss is dramatic and neurofibrillary and neuritic Abeta plaque pathologies are consistent with Alzheimer disease. These pathological changes are thought to underlie the neuropsychological and physiological changes in older individuals with Down syndrome. Two chromosome 21-based gene products, beta-amyloid precursor protein (betaAPP) and S100B, have been implicated in these neuronal and interstitial changes. Although not necessary for mental retardation and other features, betaAPP gene triplication is necessary, although perhaps not sufficient, for development of Alzheimer pathology. S100B is overexpressed throughout life in Down patients, and mice with extra copies of the S100B gene have dendritic abnormalities. S100B overexpression correlates with Alzheimer pathology in post-adolescent Down syndrome patients and has been implicated in Abeta plaque pathogenesis. Interleukin-1 (IL-1) is a non-chromosome-21-based cytokine that is also overexpressed throughout life in Down syndrome. IL-1 upregulates betaAPP and S100B expression and drives numerous neurodegenerative and self-amplifying cascades that support a neuroinflammatory component in the pathogenesis of sporadic and Down syndrome-related Alzheimer disease.

Neuroinflammatory signaling upregulation in Alzheimer's disease.

Alzheimer's disease (AD) is a progressive, neurodestructive process of the human neocortex, characterized by the deterioration of memory and higher cognitive function. A progressive and irreversible brain disorder, AD is characterized by three major pathogenic episodes involving (a) an aberrant processing and deposition of beta-amyloid precursor protein (betaAPP) to form neurotoxic beta-amyloid (betaA) peptides and an aggregated insoluble polymer of betaA that forms the senile plaque, (b) the establishment of intraneuronal neuritic tau pathology yielding widespread deposits of agyrophilic neurofibrillary tangles (NFT) and (c) the initiation and proliferation of a brain-specific inflammatory response. These three seemingly disperse attributes of AD etiopathogenesis are linked by the fact that proinflammatory microglia, reactive astrocytes and their associated cytokines and chemokines are associated with the biology of the microtubule associated protein tau, betaA speciation and aggregation. Missense mutations in the presenilin genes PS1 and PS2, implicated in early onset familial AD, cause abnormal betaAPP processing with resultant overproduction of betaA42 and related neurotoxic peptides. Specific betaA fragments such as betaA42 can further potentiate proinflammatory mechanisms. Expression of the inducible oxidoreductase cyclooxygenase-2 and cytosolic phospholipase A2 (cPLA2) are strongly activated during cerebral ischemia and trauma, epilepsy and AD, indicating the induction of proinflammatory gene pathways as a response to brain injury. Neurotoxic metals such as aluminum and zinc, both implicated in AD etiopathogenesis, and arachidonic acid, a major metabolite of brain cPLA2 activity, each polymerize hyperphosphorylated tau to form NFT-like bundles. Further, epidemiological and longitudinal studies have identified a reduced risk for AD in patients (<70 yrs) previously treated with non-steroidal anti-inflammatory drugs for non-CNS afflictions that include arthritis. This review will focus on the interrelationships between the mechanisms of PS1, PS2 and betaAPP gene expression, tau and betaA deposition and the induction, regulation and proliferation in AD of the neuroinflammatory response. Novel therapeutic interventions in AD are discussed.

Genes and mechanisms involved in beta-amyloid generation and Alzheimer's disease.

Alzheimer's disease is characterized by the invariable accumulation of senile plaques that are predominantly composed of amyloid beta-peptide (Abeta). Abeta is generated by proteolytic processing of the beta-amyloid precursor protein (betaAPP) involving the combined action of beta- and gamma-secretase. Cleavage within the Abeta domain by alpha-secretase prevents Abeta generation. In some very rare cases of familial AD (FAD), mutations have been identified within the betaAPP gene. These mutations are located close to or at the cleavage sites of the secretases and pathologically effect betaAPP processing by increasing Abeta production, specifically its highly amyloidogenic 42 amino acid variant (Abeta42). Most of the mutations associated with FAD have been identified in the two presenilin (PS) genes, particularly the PS1 gene. Like the mutations identified within the betaAPP gene, mutations in PS1 and PS2 cause the increased generation of Abeta42. PS1 has been shown to be functionally involved in Notch signaling, a key process in cellular differentation, and in betaAPP processing. A gene knock out of PS1 in mice leads to an embryonic lethal phenotype similar to that of mice lacking Notch. In addition, absence of PS1 results in reduced gamma-secretase cleavage and leads to an accumulation of betaAPP C-terminal fragments and decreased amounts of Abeta. Recent work may suggest that PS1 could be the gamma-secretase itself, exhibiting the properties of a novel aspartyl protease. Mutagenesis of either of two highly conserved intramembraneous aspartate residues of PS1 leads to reduced Abeta production as observed in the PS1 knockout. A corresponding mutation in PS2 interfered with betaAPP processing and Notch signaling suggesting a functional redundancy of both presenilins. In this issue, some of the recent work on the molecular mechanisms involved in Alzheimer's disease (AD) as well as novel diagnostic approaches and risk factors for AD will be discussed. In the first article, we like to give an overview on mechanisms involved in the proteolytic generation of Amyloid beta-peptide (Abeta), the major pathological player of this devastating disease. In the second part of this article recent results will be described, which demonstrate an unexpected biological and pathological function of an AD associated gene.

The biological and pathological function of presenilin proteins – simple cell systems and a worm in Alzheimer’s disease research

Alzheimer's disease (AD) is the most common form of dementia. In a small number of cases AD is genetically inherited. Mutations are associated with so far three genes. These genes encode the beta-amyloid precursor protein (beta APP), as well as presenilin (PS) 1 and -2. Mutations in all three genes affect the generation of amyloid beta-peptide (A beta), which is the major component of senile plaques. Mutations in the PS genes occur much more frequently as those associated with the beta APP gene and can cause the earliest onset of AD ever recorded. PS genes are not only involved in familial AD but also play a functional role in the general production of A beta. Therefore PS proteins are key molecules, which will allow us to understand fundamental aspects of the molecular mechanisms involved in AD. Here we will summarize the pathological as well as biological function of PS and demonstrate that simple systems, such as cultured cells and the worm Caenorhabditis elegans can be used for modern AD research.

Enhancer function and novel DNA binding protein activity in the near upstream betaAPP gene promoter.

The role of betaAPP gene transcription and promoter regulation in modifying amyloid beta-peptide (Abeta) levels is not well understood. Increased production of Abeta or changes in Abeta42/Abeta40 ratio by fibroblasts occurs in the presence of mutant presenilin or betaAPP alleles in familial Alzheimer's disease subjects. Both betaAPP mRNA and Abeta levels are increased in trisomy 21. The APP gene promoter is in a class of housekeeping genes and contains two putative consensus sites for the binding of transcription factor AP1. Electrophoretic mobility shift (EMSA) and DNase protection assays using human fibroblast and HeLa nuclear extract identified specific protein binding with novel Sp1-like properties to both a near-upstream and a downstream domain of the betaAPP promoter. The upstream binding activity was localized to a putative AP1 consensus site and its immediate 5'-adjacent GC-rich element. However, c-Jun antibody and competition experiments had no effect on binding to this domain. A series of 5'-deleted betaAPP promoter-reporter gene transfections in HeLa and fibroblast cells showed that the domain-containing region, n.t. -383 to -348, exerts a 2.9-fold activating influence on basal pbetaAPP-reporter transcription. When subcloned to test enhancer function, the 5'-GC element/'AP1 site' tandem construct conferred four-fold greater activity than either element alone and two-fold greater than the more 3'-situated HSE consensus sequence. Phorbol ester treatment had no effect in these reporter assays. This element shares homology and binding properties with a domain immediately 5' to the downstream E-box/USF element. An interaction model involving both domains and looping of interjacent DNA is proposed. We conclude that this newly described binding protein-enhancer complex is required for full betaAPP promoter activation.

Isolation of the genomic clone of the rhesus monkey beta-amyloid precursor protein.

A change in the regulation of the beta-amyloid precursor protein (beta APP) gene may be a factor for Alzheimer's disease (AD). We have screened a rhesus monkey genomic library and pulled out a approximately 17 kb genomic DNA region which contains the 5'-flanking region (promoter), first exon and intron of the beta APP gene of the Rhesus monkey (rh beta APP). We have partially characterized the genomic clone by selective restriction enzyme digestion followed by Southern blotting against the beta APP gene-specific DNA probes. The identity and authenticity of the different bands of the clone were also confirmed by Southern blot analysis of the rhesus monkey genomic DNA. Functional identification of the genomic fragment was determined by a promoter assay using the chloramphenicol acetyltransferase (CAT) enzyme as a reported gene. Our results showed that a 1.2 kb fragment from the 5'-flanking region of the rh beta APP gene possessed strong promoter activity. The isolation of this genomic clone will enable characterization of the structure and function of the promoter of the rh beta APP gene. Our initial results indicate a high degree of homology between the structure of the rhesus monkey and human beta APP promoter.

Hypersensitivity to seizures in beta-amyloid precursor protein deficient mice.

Secreted forms of the beta-amyloid precursor protein (beta-APP) have neuroprotective properties in vitro and may be involved in the containment of neuronal excitation. To test whether loss of secreted forms of beta-APP (sAPPs) may enhance excitotoxic responses, we injected mice homozygous for a targeted mutation of the beta-APP gene (beta-APPDelta/Delta) intraperitoneally with kainic acid. We found that in these mice, kainic acid induced seizures initiated earlier, and acute mortality was enhanced compared to isogenic wild-type mice independently from the callosal agenesis phenotype observed to occur at increased frequency in APP mutant mice. Expression of c-fos in cortex and cingulate gyrus was enhanced in beta-APPDelta/Delta mice, although the amount of structural damage and apoptosis in the hippocampal pyramidal cell layer and cortex was similar to that of controls. When cerebellar granule cell cultures and cortical neuronal cultures were challenged with glutamate receptor agonists, the rates of cell death and apoptosis of beta-APPDelta/Delta mice were indistinguishable from those of controls. Therefore, deficiency of sAPPs causes facilitation of seizure activity in the absence of enhanced cell death. Since enhanced seizures were observed also in mice homozygous for a deletion of the entire beta-APP gene, this phenotype results from a loss of APP rather than from a dominant effect of APPDelta.

Functional identification of the promoter of the gene encoding the Rhesus monkey beta-amyloid precursor protein.

Misregulation of the transcription of the beta-amyloid precursor protein (betaAPP) gene is implicated in the pathogenesis of Alzheimer's disease (AD). Here we characterize the 5'-flanking region, the first exon and intron of the betaAPP gene of the Rhesus monkey (rhbetaAPP). For functional analysis, transient transfection in PC12 cells was performed with a series of 5'-deletion constructs (fused with a reporter gene), that extended as far upstream as -7900 down to -1bp. Chloramphenicol acetyltransferase/promoter fusion assays showed that both -7900/+104 and -75/+104-bp regions possessed strong promoter activity. However, -2542/+104bp had the strongest promoter activity, whereas -204/+104bp showed a major reduction in activity and -47/+104bp showed almost a complete loss of activity. A region from -75 to +104bp was essential for minimal basic promoter activity because mutation at the activating site of an upstream stimulator factor (USF) within this region abolished the promoter activity. The very upstream region (-5529/-3416bp) displayed a negative effect on promoter activity. Two blocks of the sequence, 641bp (-1131 /-490) and 105bp (-309/-204), acted as positive regulators for promoter activity. Another 61-bp block (-204/-143) acted as a negative regulator. Gel shift assay indicated that the -249-242-bp region contains a binding domain for the AP-2 transcription factor. No second promoter or bidirectional promoter was observed. A region spanning the first exon and part of the first intron (+99 to +6800bp) acted as a negative regulator. These results suggest that a region of -75 to +104bp, which contains the pyrimidine-rich initiator element, the 5'-untranslated region and the binding site for USF, constitute the minimal promoter element and that interactions between multiple positive and negative elements, the USF and initiator element are crucial for transcription of the TATA-less betaAPP promoter.

Proteolytic processing of Alzheimer's disease associated proteins.

Amyloid beta-peptide (A beta), the major component of senile plaques, is generated by proteolytic processing from the beta-amyloid precursor protein (beta APP). Mutations within the beta APP gene cause early onset familial AD (FAD) by affecting A beta generation. Interestingly, the much more abundant mutations within the presenilin (PS) genes also result in the abnormal generation of a 42 residue A beta (A beta 42), thus clearly supporting a pivotal role of A beta for the pathology of AD. PS proteins are proteolytically processed into stable 30 kDa N-terminal fragments (NTF) and 20 kDa C-terminal fragments (CTF). Beside the conventional proteolytic pathway. PS proteins can also be cleaved further C-terminal by proteases of the caspase superfamily. PS proteins were localized within the endoplasmic reticulum (ER) and early Golgi, compartments which we have demonstrated to be involved in A beta 42 generation and intracellular accumulation. Using Caenorhabditis elegans as a simple animal model, we demonstrate that PS proteins are involved in NOTCH signaling FAD causing mutations interfere with the biological function of PS proteins in NOTCH signaling.

Sporadic Inclusion-body Myositis and Its Similarities to Alzheimer Disease Brain: Recent Approaches to Diagnosis and Pathogenesis, and Relation to Aging

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

Dementia--causes, diagnosis, treatment, and care

The prevalence of dementia in the elderly (65 years old and over) was estimated at 6.3% (men 5.8%, women 6.7%) in Japan in 1985. Epidemiological studies done in several prefectures in 1989 and later showed a tendency for patients with Alzheimer's disease (AD) to outnumber those with vascular dementia (VD); the VD/AD ratio was less than 1.0 in over half of the surveys. A pathologic study (Kosaka 1996) of 79 patients with dementia revealed that AD was more common than VD, although clinical diagnoses were the reverse, which indicated that VD is still overdiagnosed in Japan. Diffuse Lewy body disease was observed in 15% of those patients without correct clinical diagnosis. Many biological markers for AD have been reported. We used 1H-magnetic resonance spectroscopy of the brain and found that the ratio of N-acetyl aspartate to creatine in AD patients was significantly smaller than that in age-matched controls without dementia. Based on genetic studies, AD is classified into five types. These are related to chromosomes 14 (presenilin-1), 21 (beta APP gene), 1 (presenilin-2), 19 (epsilon 4 alleles), and other. The causes of most sporadic cases remain unclear. Tacrine is the only drug authorized in the U.S.A. for treatment of AD, but it is not used in Japan because of its side effects. Many other drugs to treat dementia are now in nationwide clinical trials although only four are in phase III. Therefore, rehabilitation therapy is mandatory and details of that therapy should be individualized. A new system of public insurance for nursing care may be implemented by the government.

A comprehensive study of the spatiotemporal pattern of beta-amyloid precursor protein mRNA and protein in the rat brain: lack of modulation by exogenously applied nerve growth factor.

Nerve growth factor (NGF) is a neurotrophic factor for basal forebrain cholinergic neurons, a population that degenerates and dies in Alzheimer's disease (AD). It has been suggested that NGF be used to treat AD patients. However, in vivo administration of NGF to the developing hamster brain was shown to induce the expression of the beta-amyloid precursor protein (beta APP) gene. The association of alterations in beta APP gene expression with AD-like neuropathological changes and cognitive impairment in animals, and with AD-like neurodegeneration in Down syndrome patients suggests that NGF-mediated increases in beta APP expression could negate or attenuate NGF's neurotrophic activity in AD treatment trials. The present study was undertaken to explore further the influence of NGF on beta APP expression, and to determine which, if any, of the beta APP mRNAs is altered in response to NGF treatment. We first examined the spatiotemporal pattern of beta APP-695 and Kunitz protease inhibitor (KPI)-containing beta APP mRNA expression in the rat brain. Specific oligonucleotide probes were used to show that these mRNAs are present during embryonic development. In addition, we evaluated postnatal expression in nine brain regions and showed that beta APP mRNAs were readily detected in all regions at postnatal day 2. In human brain, the relative levels of beta APP-695 and beta APP-KPI mRNA and their protein are discordant, in that the level of beta APP-695 mRNA is slightly higher than that of beta APP-KPI, but beta APP-KPI protein predominates. In contrast, the several-fold excess of beta APP-695 mRNA relative to beta APP-KPI mRNA in the rat brain was also reflected at the protein level. Surprisingly, administration of exogenous NGF failed to affect rat beta APP mRNA levels either in vitro or during postnatal development in vivo.

Transfer of beta-amyloid precursor protein gene using adenovirus vector causes mitochondrial abnormalities in cultured normal human muscle.

As in Alzheimer-disease (AD) brain, vacuolated muscle fibers of inclusion-body myositis (IBM) contain abnormally accumulated beta-amyloid precursor protein (beta APP), including its beta-amyloid protein epitope, and increased beta APP-751 mRNA. Other similarities between IBM muscle and AD brain phenotypes include paired helical filaments, hyperphosphorylated tau protein, apolipoprotein E, and mitochondrial abnormalities, including decreased cytochrome-c oxidase (COX) activity. The pathogenesis of these abnormalities in IBM muscle and AD brain is not known. We now report that direct transfer of the beta APP gene, using adenovirus vector, into cultured normal human muscle fibers causes structural abnormalities of mitochondria and decreased COX activity. In this adenovirus-mediated beta APP gene transfer, we demonstrated that beta APP overproduction can induce mitochondrial abnormalities. The data suggest that excessive beta APP may be responsible for mitochondrial and COX abnormalities in IBM muscle and perhaps AD brain.

The role of presenilin 1 in the genetics of Alzheimer's disease.

Approximately 75% of AD patients have an onset of the disease after the age of 60 years, and 60% of AD patients have no family history of the disease. Some cases of EOAD are clearly inherited in an autosomal-dominant manner. The beta APP gene on chromosome 21, the PS-1 gene on chromosome 14, and the PS-2 gene on chromosome 1 have all been characterized as genes in which mutations lead to familial EOAD. For LOAD, the work on ApoE indicates that the epsilon 4 allele is a risk factor for developing AD. However, 35-50% of all AD patients do not have an epsilon 4 allele. Other loci contributing to LOAD remain to be mapped and characterized. As in other complex disorders, these additional loci may involve genetic interactions with the known AD loci. Identification of all susceptibility loci for AD is a major goal in resolving the pathogenesis of AD.

Systemic overexpression of a C-terminal fragment of human amyloid beta-protein precursor causes accumulation of Alzheimer beta-amyloid fibrils in pancreas of transgenic mice.

The deposition of amyloid beta-protein (A beta), derived from amyloid beta-protein precursor (beta APP), is a specific and early event in development of Alzheimer's disease. Transgenic mice carrying the carboxyl-terminus of beta APP gene linked to the cytomegalovirus enhancer/chicken beta-actin promoter sequence were studied. Deposition of amyloid fibrils, composing A beta, was observed in the transgenic pancreas, accompanied with cell degeneration. This result will provide a model to investigate the beta APP processing mechanism in vivo and a clue to generate possible A beta amyloidosis in animal brains.

Heat shock factor-1 mediates the transcriptional activation of Alzheimer's beta-amyloid precursor protein gene in response to stress.

Stress may be involved in the pathogenesis of Alzheimer's disease. There is a heat shock element located at position -317 bp on the beta-amyloid precursor protein (beta-APP) gene promoter. Recently we demonstrated [4] that stress, in the form of heat shock, ethanol and sodium arsenite treatment, transcriptionally activates the beta-APP gene. In this report we demonstrate that the nuclear factor that mediates this activation is heat shock factor-1.

Promoter activity of the gene encoding the beta-amyloid precursor protein is up-regulated by growth factors, phorbol ester, retinoic acid and interleukin-1.

Abnormalities in gene regulation of the beta-amyloid precursor protein (beta APP) might be an important factor in the neuropathology of Alzheimer's disease. We analyzed the effects of nerve growth factor (NGF), basic fibroblast growth factor (bFGF), phorbol 12-myristate 13-acetate (PMA), interleukin-1 (IL-1) and retinoic acid (RA) on promoter activity of the beta APP gene. To investigate the effect of these factors on promoter activity, we used two fusion plasmids which contain sequences of -489 and -415 base pairs (bp), respectively, from the transcription start site of the beta APP gene. The truncated regions of the promoter wer linked upstream to a reporter gene, chloramphenicol acetyl transferase (CAT). Promoter activity was tested by transient transfection of fusion plasmids in PC12 cells using the electroporation method (960 microF at 350 V). We report that the treatment of PC12 cells with either NGF, bFGF, PMA, IL-1 or RA stimulated the activity of the beta APP promoter. The treatment of cells with either NGF or bFGF resulted in a higher degree of stimulation in the basal level of promoter activity than when cells were treated with either PMA, IL-1 or RA. The deletion of sequences between -489 to -416 bp had no significant effect on promoter activity. The treatment of cells with these factors for a duration of 4 days prior to transfection with the plasmids is necessary for the stimulatory effect. The cells that were only treated with any of these factors after transfection showed no significant change in the basal level of promoter activity. We conclude that certain growth factors and a cytokine could enhance the basal level of promoter activity of the beta APP gene, suggesting a possible participation of a growth-factor(s)-mediated transcription element in the control of gene expression of beta APP.

The molecular genetics of Alzheimer's disease.

The major pathological characteristic of Alzheimer's disease (AD) is the abnormal deposition of beta-amyloid peptide (A beta) in the brain. In some early onset cases, the disease develops because of mutations in the gene coding for beta-amyloid precursor protein (beta APP). However, the majority of AD families in the early onset subgroup are linked to a locus on chromosome 14. The genetic analysis and age of onset correlates of both the beta APP gene and the chromosome 14 locus are discussed. We speculate on the mechanisms by which the beta APP mutations cause the disease and discuss recent advances in beta APP processing that may be relevant to the pathogenesis of the late-onset (common) form of the disease. In addition, we review the association of the APOE locus with late-onset familial and nonfamilial disease. Further work is required to establish the effects of this locus on disease occurrence, age of onset, and progression. The molecular pathology of ApoE in relation to AD development and the identification of the chromosome 14 gene will greatly contribute to a general pathogenic model of AD, and will clarify the role of beta APP and its derivatives.

A direct role for protein kinase C and the transcription factor Jun/AP-1 in the regulation of the Alzheimer's beta-amyloid precursor protein gene.

Overexpression of the beta-amyloid precursor protein gene (beta-APP) may contribute to the abnormal generation of beta-amyloid protein in Alzheimer's disease. We demonstrate using a human glial cell line (1321N1) that activation of protein kinase C (PKC) with phorbol 12-myristate 13-acetate (PMA) increases beta-APP mRNA levels, induces known components of the transcription factor activator protein-1 (AP-1), and increases protein-DNA binding activity to AP-1 sequences within the beta-APP promoter. A beta-APP promoter-luciferase reporter gene is transcriptionally activated by PMA, as well as by expression of constitutively activated PKC or by expression of c-Jun. Further characterization suggests that the distal but not the proximal AP-1 recognition site binds nuclear proteins regulated by PKC, and that the AP-1 binding activity is likely to be composed of Jun-Jun homodimers rather than Jun-Fos heterodimers. Additional studies demonstrate that a single copy of the distal AP-1 site fused to a heterologous promoter is sufficient to confer a response to PMA. Mutagenesis of this site in the beta-APP promoter renders it unresponsive to c-Jun and attenuates transcriptional activation by PMA. We suggest that cellular mediators that activate PKC, particularly those that induce significant increases in c-Jun, may up-regulate expression of the beta-APP gene and consequently affect production and processing of this protein.

The amyloid beta protein precursor mutations linked to familial Alzheimer's disease alter processing in a way that fosters amyloid deposition.

Normal processing of the amyloid beta protein precursor (BAPP) results in secretion of a soluble 4 kD protein essentially identical to the amyloid beta protein (A beta) that forms insoluble fibrillar deposits in Alzheimer's disease (AD). Strong evidence that amyloid deposition plays a critical role in the development of AD has come from the identification of familial AD (FAD) kindreds in which the AD phenotype cosegregates with mutations in the beta APP gene that are located close to the NH2 or COOH end of the A beta peptide. The location of these mutations immediately suggests that they may cause AD by altering beta APP processing in a way that is amyloidogenic. In a previous study, we found that transfected cells expressing the NH2 side mutant secrete 6-fold more 4 kD A beta than those expressing wild type beta APP or COOH side mutants. We have now shown that the mutations on the COOH side of A beta alter processing to increase secretion of the more amyloidogenic A beta 1-42 form which constitutes only a small percentage of the total 4 kD A beta produced. Thus our data show that all of the FAD-linked beta APP mutations alter beta APP processing in a way that increases the likelihood of amyloid formation.