Identification, transmembrane orientation and biogenesis of the amyloid A4 precursor of Alzheimer's disease

Increased processing of APLP2 and APP with concomitant formation of APP intracellular domains in BDNF and retinoic acid-differentiated human neuroblastoma cells

The amyloid precursor protein (APP) belongs to a conserved gene family, also including the amyloid precursor-like proteins, APLP1 and APLP2. We have previously shown that all members of the APP protein family are up-regulated upon retinoic acid (RA)-induced neuronal differentiation of SH-SY5Y neuroblastoma cells. Here, we demonstrate that RA also affects the processing of APLP2 and APP, as shown by increased shedding of both sAPLP2 and sAPPalpha, as well as elevated levels of the APP intracellular domains (AICDs). Brain-derived neurotrophic factor (BDNF) has been reported to induce APP promoter activity and RA induces expression of the tyrosine kinase receptor B (TrkB) in neuroblastoma cells. We show that the increase in shedding of both APLP2 and APP in response to RA is not mediated through the TrkB receptor. However, BDNF concomitant with RA increased the expression of APP even further. In addition, the secretion of sAPLP2 and sAPPalpha as well as the levels of AICDs were increased in response to BDNF. In contrast, the levels of membrane-bound APP C-terminal fragment C99 significantly decreased. Our results suggest that RA and BDNF shifts APP processing towards the alpha-secretase pathway. In addition, we show that RA and BDNF regulate N-linked glycosylation of APLP1.

Proteomic analysis of glial fibrillary acidic protein in Alzheimer's disease and aging brain

Chronic inflammation is known to play an important role in the heterogeneous pathogenesis of Alzheimer's disease (AD). Activated astrocytes expressing glial fibrillary acidic protein (GFAP) are closely associated with AD pathology, such as tangles, neuritic plaques and amyloid depositions. Altogether, 46 soluble isoforms of GFAP were separated and most of them quantified by two-dimensional immunoblotting in frontal cortices of AD patients and age-matched controls. A 60% increase in the amount of more acidic isoforms of GFAP was observed in AD and these isoforms were both phosphorylated and N-glycosylated, while more basic isoforms were O-glycosylated and exhibited no quantitative differences between post-mortem AD and control brains. These data highlight the importance of exploring isoform-specific levels of proteins in pathophysiological conditions since modifications of proteins determine their activity state, localization, turnover and interaction with other molecules. Mechanisms, structures and functional consequences of modification of GFAP isoforms remain to be clarified.

Fast anterograde transport of herpes simplex virus: role for the amyloid precursor protein of alzheimer's disease

Anterograde transport of herpes simplex virus (HSV) from its site of synthesis in the neuronal cell body out the neuronal process to the mucosal membrane is crucial for transmission of the virus from one person to another, yet the molecular mechanism is not known. By injecting GFP-labeled HSV into the giant axon of the squid, we reconstitute fast anterograde transport of human HSV and use this as an assay to uncover the underlying molecular mechanism. HSV travels by fast axonal transport at velocities four-fold faster (0.9 microm/sec average, 1.2 microm/sec maximal) than that of mitochondria moving in the same axon (0.2 microm/sec) and ten-fold faster than negatively charged beads (0.08 microm/sec). Transport of HSV utilizes cellular transport mechanisms because it appears to be driven from inside cellular membranes as revealed by negative stain electron microscopy and by the association of TGN46, a component of the cellular secretory pathway, with GFP-labeled viral particles. Finally, we show that amyloid precursor protein (APP), a putative receptor for the microtubule motor, kinesin, is a major component of viral particles, at least as abundant as any viral encoded protein, while another putative motor receptor, JIP 1/2, is not detected. Conventional kinesin is also associated with viral particles. This work links fast anterograde transport of the common pathogen, HSV, with the neurodegenerative Alzheimer's disease. This novel connection should prompt new ideas for treatment and prevention strategies.

Accumulation of the amyloid precursor-like protein APLP2 and reduction of APLP1 in retinoic acid-differentiated human neuroblastoma cells upon curcumin-induced neurite retraction

Amyloid precursor protein (APP) belongs to a conserved gene family, also including the amyloid precursor-like proteins, APLP1 and APLP2. The function of these three proteins is not yet fully understood. One of the proposed roles of APP is to promote neurite outgrowth. The aim of this study was to investigate the regulation of the expression levels of APP family members during neurite outgrowth. We observed that retinoic acid (RA)-induced neuronal differentiation of human SH-SY5Y cells resulted in increased expression of APP, APLP1 and APLP2. We also examined the effect of the NFkappaB, AP-1 and c-Jun N-terminal kinase inhibitor curcumin (diferuloylmethane) on the RA-induced expression levels of these proteins. We found that treatment with curcumin counteracted the RA-induced mRNA expression of all APP family members. In addition, we observed that curcumin treatment resulted in neurite retraction without any effect on cell viability. Surprisingly, curcumin had differential effects on the APLP protein levels in RA-differentiated cells. RA-induced APLP1 protein expression was blocked by curcumin, while the APLP2 protein levels were further increased. APP protein levels were not affected by curcumin treatment. We propose that the sustained levels of APP and the elevated levels of APLP2, in spite of the reduced mRNA expression, are due to altered proteolytic processing of these proteins. Furthermore, our results suggest that APLP1 does not undergo the same type of regulated processing as APP and APLP2.

Identification of a novel alternative splicing isoform of human amyloid precursor protein gene, APP639

Senile plaques, neurofibrillary tangles and amyloid-laden cerebral vessels are characteristic features in the brains of individuals with Alzheimer's disease (AD). The principal component of amyloid in senile plaque and amyloid-laden cerebral vessels is beta-amyloid (Abeta), a peptide proteolytically derived from a large amyloid precursor protein (APP). To date, several alternatively spliced human APP transcripts have been described. Here, we report the identification of a novel alternative splicing isoform of the human APP gene, APP639, which excludes exon 2 as well as exons 7 and 8. RT-PCR and Southern blot analysis show that APP639 mRNA is expressed in many human fetal tissues. In contrast, the APP639 transcript is hardly detected in the aged human cerebral cortex from both pathologically confirmed sporadic AD cases and nondemented controls. However, APP639 mRNA exists in the adult human liver. Western blot analysis shows that the protein product produced from the APP639 cDNA could be recognized by the APP antibody, and it does lack the exon 2 coding region. These results suggest that APP639, a novel alternative splicing isoform of human APP gene, does exist in human tissues in vivo.

Multiple immunostaining methods to detect traumatic axonal injury in the rat fluid-percussion brain injury model

Immunohistochemistry using beta-amyloid precursor protein (APP) N-terminus antibodies is routinely used to detect traumatic axonal injury (TAI). The temporal and regional distributions of APP C- and N-terminus immunoreactivity were investigated in rats with experimental brain injury and compared to distribution of neurofilament (NF) immunoreactivity. Male Sprague-Dawley rats underwent right lateral fluid-percussion (FP) brain injury or sham injury. Six FP injury rats and two control rats were transcardially fixed with 10% formalin at 1, 6, 24, and 48 hours, and 1 and 2 weeks after injury and serial 6 microm-thick paraffin sections were prepared. At 6 hours after injury, APP C-terminus immunostaining labeled small neurons and axonal deposits in the injured parasagittal cortex, striatum, thalami, and dorsal medulla, whereas APP N-terminus and NF immunostaining showed few axonal deposits in the parasagittal cortex. At 24-48 hours post-injury, marked axonal damage, including axonal swelling and bulbs, was observed in the injured cerebral hemisphere, cerebellar white matter, and medulla. NF immunostaining was most sensitive for axonal damage in the injured deep cortical layers, cerebellum, and medulla. At 1-2 weeks after injury, APP N-terminus immunostaining mainly showed dot-like axonal profiles in the injured thalamus. APP C-terminus immunoreactivity may serve as an early marker of TAI, and the C-terminal fragments of APP may be involved in the evolution of TAI because C-terminal fragments of APP are neurotoxic and pro-apoptotic. Multiple immunostaining methods may be required to fully recognize the extent of TAI.

The transmembrane domain of the amyloid precursor protein in microsomal membranes is on both sides shorter than predicted

The amyloid precursor protein is cleaved within its ectodomain by beta-amyloid-converting enzyme (BACE) yielding C99, which is further cleaved by gamma-secretase within its putative transmembrane domain (TMD). Because it is difficult to envisage how a protease may cleave within the membrane, alternative mechanisms have been proposed for gamma-cleavage in which the TMD is shorter than predicted or positioned such that the gamma-cleavage site is accessible to cytosolic proteases. Here, we have biochemically determined the length of the TMD of C99 in microsomal membranes. Using a single cysteine mutagenesis scan of C99 combined with cysteine modification with a membrane-impermeable labeling reagent, we identified which residues are accessible to modification and thus located outside of the membrane. We find that in endoplasmic reticulum-derived microsomes the TMD of C99 consists of 12 residues that span from residues 37 to 48, which is N- and C-terminally shorter than predicted. Thus, the gamma-cleavage sites are positioned around the middle of the lipid bilayer and are unlikely to be accessible to cytosolic proteases. Moreover, the center of the TMD is positioned at the gamma-cleavage site at residue 42. Our data are consistent with a model in which gamma-secretase is a membrane protein that cleaves at the center of the membrane.

Characterization of an O-glycosylated plaque-associated protein from Alzheimer disease brain

In this work we characterized a 90-kDa glycoprotein from Alzheimer disease (9OAzgp) brain extracts that is recognized by the GalNAc-specific lectin from Amaranthus leucocarpus (ALL), as determined through Western blot. The 90Azgp was purified by electro-elution, and its amino acid sequence determined from peptides obtained after trypsin digestion through MALDI-TOF (Matrix-assisted laser desorption ionization-time of flight), and compared with the relative values obtained from the NCBInr (Swiss-Prot 10/01/2001) database. The 90Azgp showed 32% and 42% homology with the KIAA0310 protein from human brain and the human gastric mucin, respectively. Presence of O-glycosidically linked glycans in the proteins recognized by ALL was confirmed by inhibition of the lectin-glycoprotein interaction through hapten-inhibition assays and also by elimination of the O-glycosidically linked glycans after treatment with O-glycanase from Diplococcus pneumoniae. Electron transmission microscopy confirmed that the receptor recognized by the lectin is processed in the Golgi apparatus of AD neurons. Although the specific role of this glycoprotein has not been identified, considering that the presence of this lectin receptor co-localized with neuritic plaques and in AD sprouting neurons, it could suggest that the O-glycosyl-protein identified by the A. leucocarpus lectin participates in the pathogenesis of neurodegenerative diseases.

MENTHO, a MLN64 homologue devoid of the START domain

MLN64 is a late endosomal membrane protein containing a carboxyl-terminal cholesterol binding START domain and is presumably involved in intracellular cholesterol transport. In the present study, we have cloned a human cDNA encoding a novel protein that we called MENTHO as an acronym for MLN64 N-terminal domain homologue because this protein is closely related to the amino-terminal half of MLN64. MLN64 and MENTHO share 70% identity and 83% similarity in an original protein domain encompassing 171 amino acids that we designated as the MENTAL (MLN64 N-terminal) domain. By translation initiation scanning MENTHO is synthesized as two isoforms of 234 (alpha) and 227 (beta) amino acids that can be phosphorylated. As MLN64, MENTHO is ubiquitously expressed and is located in the membrane of late endosomes, its amino and carboxyl-terminal extremities projecting toward the cytoplasm. We show that MENTHO overexpression does not rescue the Niemann-Pick type C lipid storage phenotype. However, MENTHO overexpression alters severely the endocytic compartment by leading at steady state to the accumulation of enlarged endosomes. These results indicate that in addition to its previously established function in addressing and anchoring proteins to the membrane of late endosomes, the MENTAL domain possesses an intrinsic biological function in endocytic transport.

Metastatic Lymph Node 51, a novel nucleo-cytoplasmic protein overexpressed in breast cancer

Metastatic Lymph Node 51 (MLN51) cDNA was isolated by differential screening of a human breast cancer metastasis cDNA library. MLN51 cDNA encodes a novel human protein of 703 residues that shares no significant homology to any known protein. However MLN51 is well conserved between vertebrate and invertebrate species suggesting an important biological function. The amino terminal half of the protein contains a coiled-coil domain and two potential nuclear localization signals (NLS). The carboxy terminal half contains one SH2 and four SH3 binding motifs. The coiled-coil domain promotes MLN51 oligomerization in transfected cells. When transiently expressed, the MLN51 protein is mainly found in the cytoplasm with a weak nuclear staining. However, deletion of the carboxy terminal half of the protein allows the targeting of the protein to the nucleus, demonstrating that the NLSs are functional. MLN51 is ubiquitously expressed in normal tissues. Human breast carcinomas show MLN51 overexpression in malignant epithelial cells. The uncommon association of protein-protein interaction domains often found either in nuclear or in cytoplasmic signaling proteins raises a possible nucleo-cytoplasmic function for MLN51.

Oxidation of Abeta and plaque biogenesis in Alzheimer's disease and Down syndrome

The processes involved with beta-amyloid (Abeta) degradation and clearance in human brain are not well understood. We hypothesized that the distribution of oxidatively modified Abeta, as determined by an affinity-purified antibody in the entorhinal and frontal cortices of Alzheimer's disease (AD), Down syndrome (DS), nondemented elderly control cases, and canine brain, would provide insight into the mechanisms of Abeta accumulation. Based upon plaque counts, oxidized Abeta was present within 46-48% of diffuse and primitive plaques and 98% of cored plaques. Dense punctate deposits of oxidized Abeta were distributed throughout the neuropil in AD and DS brains but were also present within controls with mild neuropathology and isolated cognitive impairments. Confocal studies indicate that punctate oxidized Abeta deposits were within activated microglia. Oxidatively modified Abeta may reflect the efforts of microglial cells to take up and degrade Abeta. Oxidative modification of Abeta may be an early event in Abeta pathogenesis and may be important for plaque biogenesis.

Homodimerization of amyloid precursor protein and its implication in the amyloidogenic pathway of Alzheimer's disease

We reported previously that the carbohydrate domain of the amyloid precursor protein is involved in amyloid precursor protein (APP)-APP interactions. Functional in vitro studies suggested that this interaction occurs through the collagen binding site of APP. The physiological significance remained unknown, because it is not understood whether and how APP dimerization occurs in vivo. Here we report that cellular APP exists as homodimers matching best with a two-site model. Consistent with our published crystallographic data, we show that a deletion of the entire sequence after the kunitz protease inhibitor domain did not abolish APP homodimerization, suggesting that two domains are critically involved but that neither is essential for homodimerization. Finally, we generated stabilized dimers by expressing mutant APP with a single cysteine in the ectodomain juxtamembrane region. Mutation of Lys(624) to cysteine produced approximately 6-8-fold more A beta than cells expressing normal APP. Our results suggest that amyloid A beta production can in principle be positively regulated by dimerization in vivo. We suggest that dimerization could be a physiologically important mechanism for regulating the proposed signal activity of APP.

Oxidative and hydrolytic properties of beta-amyloid

beta-Amyloid protein is the major component of senile plaques found in the brains of Alzheimer's patients. Previously, a new biochemical property of amyloid, its ability to disrupt ester and peptide bonds, was described [Elbaum, D., Brzyska, M., Bacia, A. & Alkon, D. (2000) Biochem. Biophys. Res. Commun. 267, 733-738]. In the present work we compare the ability of beta-amyloid to hydrolyse and oxidize model fluorescent derivatives of dichlorofluorescein [dichlorodihydrofluorescein (H2DCF) or dichlorofluorescein diacetate (DCF-DA), respectively] to the same final product (dichlorofluorescein). Although there is accumulating evidence of oxidative properties of beta-amyloid, little is known about its hydrolytic abilities. Chemical modification studies revealed that hydrolytic properties are related to a His, Ser and Asp/Glu triad, while residues of His, Tyr and Met are involved in the oxidative activity of amyloid. Studies with the rat homologue of human beta-amyloid (1-40), containing three amino-acid substitutions (Arg5-->Gly, Tyr10-->Phe and His13-->Arg) confirmed a role of His in the studied processes. Reduction of the hydrolysis product caused by inhibitors of Ser esterases (phenylmethylsulphonyl fluoride and eserine) suggests that beta-amyloid-mediated hydrolysis is Ser sensitive. Antioxidants and metal chelators that reduced H2DCF oxidation did not change or increase DCF-DA hydrolysis. Solvent isotope effects suggest the involvement of hydrogen bonds in the hydrolysis reaction. Hydrolysis was inhibited by redox-active metal ions and was practically oxygen independent while the oxidation process was redox-active-metal enhanced [Cu(II) and Fe(II) primarily], and oxygen dependent. Product formation was significantly inhibited by catalase and superoxide dismutase as well as benzoquinone, a specific superoxide anion radical scavenger. Increase of fluorescence by oxidation was strongly inhibited by azide and His and enhanced in samples prepared with deuterated phosphate buffer, suggesting singlet oxygen intermediacy. Our data are consistent with superoxide-mediated singlet oxygen intermediate in this Fenton mechanism-driven reaction. These results indicate that hydrolytic and oxidative properties of beta-amyloid are distinct features of this peptide and probably require different mechanisms to occur, but both of them may contribute to beta-amyloid toxicity.

A dinucleotide deletion in amyloid precursor protein (APP) mRNA associated with sporadic Alzheimer's disease results in efficient secretion of truncated APP isoforms from neuroblastoma cell cultures

Recently, two dinucleotide deletions were detected in the mRNA of the amyloid precursor protein (APP) from cerebral cortex neurons of patients with sporadic Alzheimer's disease (AD) or Down's syndrome. These deletions resulted in truncation of APP, producing an APP isoform with a 38-kDa N-terminus and a novel carboxyl terminus (APP+1). We investigated the subcellular localization and the processing of APP+1 in the neuroblastoma cell line B103. cDNA constructs were generated encoding fusion proteins of APP+1 or full-length APP with the enhanced green fluorescent protein (eGFP). In transient transfection experiments using B103 cells, the APP+1-eGFP fusion protein showed a reticular localization with intense staining in the Golgi complex. Unlike full-length APP fused to eGFP, the APP+1-eGFP fusion protein did not localize to the perinuclear area or to the plasma membrane. Western blot analysis of cell extracts confirmed the translation of the expected fusion proteins. Analysis of the supernatant by western blot indicated that the APP+1-eGFP fusion protein was efficiently secreted from B103 cells, whereas the secreted form of full-length APP fusion protein (APPs) was hardly detectable. Thus, both dinucleotide deletions in the APP mRNA result in truncated APP+1 that is not membrane associated and is readily secreted from neurons.

Alzheimer's disease: a dysfunction of the amyloid precursor protein(1)

In this review, we argue that at least one insult that causes Alzheimer's disease (AD) is disruption of the normal function of the amyloid precursor protein (APP). Familial Alzheimer's disease (FAD) mutations in APP cause a disease phenotype that is identical (with the exception that they cause an earlier onset of the disease) to that of 'sporadic' AD. This suggests that there are molecular pathways common to FAD and sporadic AD. In addition, all individuals with Down syndrome, who carry an extra copy of chromosome 21 and overexpress APP several-fold in the brain, develop the pathology of AD if they live past the age of 40. These data support the primacy of APP in the disease. Although APP is the source of the beta-amyloid (Abeta) that is deposited in amyloid plaques in AD brain, the primacy of APP in AD may not lie in the production of Abeta from this molecule. We suggest instead that APP normally functions in the brain as a cell surface signaling molecule, and that a disruption of this normal function of APP is at least one cause of the neurodegeneration and consequent dementia in AD. We hypothesize in addition that disruption of the normal signaling function of APP causes cell cycle abnormalities in the neuron, and that these abnormalities constitute one mechanism of neuronal death in AD. Data supporting these hypotheses have come from investigations of the molecular consequences of neuronal expression of FAD mutants of APP or overexpression of wild type APP, as well as from identification of binding proteins for the carboxyl-terminus (C-terminus) of APP.

Proton MR spectroscopy of the brain

Magnetic resonance spectroscopy (MRS) has been shown to be an effective noninvasive diagnostic tool that can be used to monitor serially biochemical and metabolic changes in serial disease processes that affect the brain. MRS is now a clinical tool that is reimbursable. This article reviews MRS physics, techniques, pulse sequences, and the clinical applications of MRS.

Alternate aggregation pathways of the Alzheimer beta-amyloid peptide. An in vitro model of preamyloid

Deposition of amyloid-beta (Abeta) aggregates in the brain is a defining characteristic of Alzheimer's disease (AD). Fibrillar amyloid, found in the cores of senile plaques, is surrounded by dystrophic neurites. In contrast, the amorphous Abeta (also called preamyloid) in diffuse plaques is not associated with neurodegeneration. Depending on the conditions, Abeta will also form fibrillar or amorphous aggregates in vitro. In this present study, we sought to characterize the properties of the amorphous aggregate and determine whether we could establish an in vitro model for amorphous Abeta. CD data indicated that Abeta40 assembled to form either a beta-structured aggregate or an unfolded aggregate with the structured aggregate forming at high peptide concentrations and the unstructured aggregate forming at low Abeta40 levels. The critical concentration separating these two pathways was 10 microm. Fluorescence emission and polarization showed the structured aggregate was tightly packed containing peptides that were not accessible to water. Peptides in the unstructured aggregate were loosely packed, mobile, and accessible to water. When examined by electron microscopy, the structured aggregate appeared as protofibrillar structures and formed classic amyloid fibrils over a period of several weeks. The unstructured aggregate was not visible by electron microscopy and did not generate fibrils. These findings suggest that the unstructured aggregate shares many properties with the amorphous Abeta of AD and that conditions can be established to form amorphous Abeta in vitro. This would allow for investigations to better understand the relationship between fibrillar and amorphous Abeta and could have significant impact upon efforts to find therapies for AD.

Antibodies to the C-terminus of the beta-amyloid precursor protein (APP): a site specific marker for the detection of traumatic axonal injury

Antibodies to the amyloid precursor protein (APP) are commonly used to detect traumatic axonal injury (TAI). Carried by fast anterograde axoplasmic transport, APP will pool at regions of impaired transport associated with TAI. Based primarily upon commercial antibody availability, previous studies have targeted the N-terminus of APP, which, with respect to antigen detection, is suboptimally located within anterogradely transported vesicles. Recently, antibodies to the APP C-terminus, located on the external surface of anterogradely transported vesicles, have become available, allowing for the exploration of their utility in detecting TAI. To this end, rats were subjected to an impact acceleration injury, surviving 30 min to 24 h post-injury. They were then perfused, their brains sectioned and prepared for dual label immunofluorescent microscopy, single label bright field microscopy, and electron microscopy (EM). Antibodies to the APP C-terminus yielded the ready detection of intensely labeled TAI with significantly reduced diffuse background staining in comparison to antibodies to the APP N-terminus in both dual label immunofluorescent and single label bright-field approaches. EM examination of antibodies to the APP C-terminus in TAI revealed intense labeling of pooled intra-axonal vesicular profiles, confirming the anterogradely transported vesicular source of the APP seen in TAI. Interestingly, in addition to providing a technically superior approach and new detailed information on the subcellular localization of APP, antibodies to the APP C-terminus also proved more cost effective. Immunofluorescent studies of APP C-terminus immunoreactivity involved 1/3 the cost of targeting the N-terminus, while bright field APP C-terminus studies were performed for 1/20 the cost.

Identification of cis-elements regulating exon 15 splicing of the amyloid precursor protein pre-mRNA

Alternative splicing of exon 15 of the amyloid precursor protein (APP) pre-mRNA generates two APP isoform groups APP(ex15) (containing exon 15) and L-APP (without exon 15), which show a cell-specific distribution in non-neuronal cells and neurons of rat. Both APP isoforms differ in regard to functional properties like post-translational modification, APP secretion, and proteolytic production of Abeta peptide from APP molecules. Since Abeta generation is an important factor in the development of Alzheimer's disease, one could anticipate that these major APP isoforms might contribute differentially to the mechanisms underlying neurodegeneration in Alzheimer's disease. In this study, we established an APP minigene system in a murine cell system to identify cis-acting elements controlling exon 15 recognition. A 12. 5-kilobase pair genomic fragment of the murine APP gene contained all cis-regulatory elements to reproduce the splicing pattern of the endogenous APP transcripts. By using this approach, two intronic cis-elements flanking exon 15 were identified that block the inclusion of exon 15 in APP transcripts of non-neuronal cells. Point mutation analysis of these intronic regions indicated that pyrimidine-rich sequences are involved in the splice repressor function. Finally, grafting experiments demonstrated that these regulatory regions cell-specifically enhance the blockage of a chimeric exon in the non-neuronal splicing system.

The amyloid beta-protein precursor of Alzheimer's disease is degraded extracellularly by a Kunitz protease inhibitor domain-sensitive trypsin-like serine protease in cultures of chick sympathetic neurons

The amyloid beta-protein precursor (APP) of Alzheimer's disease (AD) is cleaved either by alpha-secretase to generate an N-terminally secreted fragment, or by beta- and gamma-secretases to generate the beta-amyloid protein (Abeta). The accumulation of Abeta in the brain is an important step in the pathogenesis of AD. Alternative mRNA splicing can generate isoforms of APP which contain a Kunitz protease inhibitor (KPI) domain. However, little is known about the physiological function of this domain. In the present study, the metabolic turnover of APP was examined in cultured chick sympathetic neurons. APP was labelled by incubating neurons for 5 h with [35S]methionine and [35S]cysteine. Intracellular labelled APP decayed in a biphasic pattern suggesting that trafficking occurs through two metabolic compartments. The half-lives for APP in each compartment were 1.5 and 5.7 h, respectively. A small fraction (10%) of the total APP was secreted into the culture medium where it was degraded with a half-life of 9 h. Studies using specific protease inhibitors demonstrated that this extracellular breakdown was due to cleavage by a trypsin-like serine protease that was secreted into the culture medium. Significantly, this protease was inhibited by a recombinant isoform of APP (sAPP751), which contains a region homologous to the Kunitz protease inhibitor (KPI) domain. These results suggest that KPI forms of APP regulate extracellular cleavage of secreted APP by inhibiting the activity of a secreted APP-degrading protease.

Neuronal overexpression of mutant amyloid precursor protein results in prominent deposition of cerebrovascular amyloid

Transgenic mice that overexpress mutant human amyloid precursor protein (APP) exhibit one hallmark of Alzheimer's disease pathology, namely the extracellular deposition of amyloid plaques. Here, we describe significant deposition of amyloid beta (Abeta) in the cerebral vasculature [cerebral amyloid angiopathy (CAA)] in aging APP23 mice that had striking similarities to that observed in human aging and Alzheimer's disease. Amyloid deposition occurred preferentially in arterioles and capillaries and within individual vessels showed a wide heterogeneity (ranging from a thin ring of amyloid in the vessel wall to large plaque-like extrusions into the neuropil). CAA was associated with local neuron loss, synaptic abnormalities, microglial activation, and microhemorrhage. Although several factors may contribute to CAA in humans, the neuronal origin of transgenic APP, high levels of Abeta in cerebrospinal fluid, and regional localization of CAA in APP23 mice suggest transport and drainage pathways rather than local production or blood uptake of Abeta as a primary mechanism underlying cerebrovascular amyloid formation. APP23 mice on an App-null background developed a similar degree of both plaques and CAA, providing further evidence that a neuronal source of APP/Abeta is sufficient to induce cerebrovascular amyloid and associated neurodegeneration.

Glial HO-1 expression, iron deposition and oxidative stress in neurodegenerative diseases

The mechanisms responsible for the pathological deposition of brain iron in Parkinson's disease, Alzheimer's disease and other human neurodegenerative disorders remain poorly understood. In rat primary astrocyte cultures, we demonstrated that dopamine, cysteamine, H(2)O(2) and menadione rapidly induce heme oxygenase-1 (HO-1) expression (mRNA and protein) followed by sequestration of non-transferrin-derived (55)Fe by the mitochondrial compartment. The effects of dopamine on HO-1 expression were inhibited by ascorbate implicating a free radical mechanism of action. Dopamine-induced mitochondrial iron trapping was abrogated by administration of the heme oxygenase inhibitors, tin mesoporphyrin (SnMP) or dexamethasone (DEX) indicating that HO-1 upregulation is necessary for subsequent mitochondrial iron deposition in these cells. Overexpression of the human HO-1 gene in cultured rat astroglia by transient transfection also stimulated mitochondrial (55)Fe deposition, an effect that was again preventible by SnMP or DEX administration. We hypothesize that free ferrous iron and carbon monoxide generated by HO-1-mediated heme degradation promote mitochondrial membrane injury and the deposition of redox-active iron within this organelle. We have shown that the percentages of GFAP-positive astrocytes that co-express HO-1 in Parkinson-affected substantia nigra and Alzheimer-diseased hippocampus are significantly increased relative to age-matched controls. Stress-induced up-regulation of HO-1 in astroglia may be responsible for the abnormal patterns of brain iron deposition and mitochondrial insufficiency documented in various human neurodegenerative disorders.

A novel substrate for analyzing Alzheimer's disease gamma-secretase

Proteolytic processing of Alzheimer's disease amyloid precursor protein (APP) by beta-secretase leads to A4CT (C99), which is further cleaved by the as yet unknown protease called gamma-secretase. To study the enzymatic properties of gamma-secretase independently of beta-secretase, A4CT together with an N-terminal signal peptide (SPA4CT) may be expressed in eukaryotic cells. However, in all existing SPA4CT proteins the signal peptide is not correctly cleaved upon membrane insertion. Here, we report the generation of a mutated SPA4CT protein that is correctly cleaved by signal peptidase and, thus, identical to the APP-derived A4CT. This novel SPA4CT protein is processed by gamma-secretase in the same manner as APP-derived A4CT and might be valuable for the generation of transgenic animals showing amyloid pathology.

Death of PC12 cells and hippocampal neurons induced by adenoviral-mediated FAD human amyloid precursor protein gene expression

We used adenoviral-mediated gene transfer of human amyloid precursor proteins (h-APPs) to evaluate the role of various h-APPs in causing neuronal cell death. We were able to infect PC12 cells with very high efficiency because approximately 90% of the cells were cytochemically positive for beta-galactosidase activity when an adenoviral vector containing LacZ cDNA was used to infect cells. Cells infected with adenovirus containing h-APP cDNA showed high-level transcription and expression of h-APP as measured by reverse transcriptase-polymerase chain reaction and Western immunoblot analyses, respectively. Intracellular and extracellular levels of h-APP were elevated approximately 17-and 24-fold in cultures infected with recombinant adenovirus containing wild-type mutant and 13- and 17-fold with V642F mutant. No elevation in h-APP was seen in cultures infected with antisense h-APP or null adenovirus. H-APP levels were maximal 3 days after infection. Overexpression of V642F mutant h-APP in PC12 cells and hippocampal neurons resulted in about a twofold increase in death compared with overexpression of wild-type h-APP. These results demonstrate the usefulness of recombinant adenoviral mediated gene transfer in cell culture studies and suggest that overexpression of a familial Alzheimer's disease mutant APP may be toxic to neuronal cells.

Expression of amyloid precursor protein (beta-APP) in the neonatal brain following hypoxic ischaemic injury

Perinatal hypoxic ischaemic brain injury (HII) is a major cause of neonatal mortality and long-term neurological morbidity. An understanding of the molecular events which follow HII may lead to novel treatments to improve the final outcome for affected infants. The beta-amyloid precursor protein (beta-APP) is a widely expressed transmembrane protein whose proposed functions include stabilization of neuronal calcium fluxes, inhibition of the clotting cascade and cell-cell or cell-matrix adhesion. Normally present at low levels in neurons its expression is induced as part of the acute response of the adult brain to HII. This study aimed to determine whether beta-APP is also part of the acute adaptive response of the infant brain to HII. Immunohistochemistry and Western blotting were used to assess cerebral beta-APP expression in 14-day-old rat pups subjected to unilateral HII, and in 10 term human infants, who died between 12 h and 16 months after severe perinatal HII. In the rat pups beta-APP expression was increased by 2 h post-injury, peaked, fourfold above control levels, at 24 h and gradually declined over the following 4 days. Expression was induced bilaterally, but was greater on the side of injury. In the human infants, increased, predominantly neuronal expression of beta-APP, was detectable immunohistochemically within 24 h of injury and was greatest in those infants dying within 3 days. Expression was particularly strong in the areas showing histological evidence of injury, but was also seen in apparently undamaged areas. We conclude that beta-APP induction is part of the the acute adaptive response of the neonatal brain to HII.

Pulse-chase experiments revealed beta-secretase cleavage from immature full-length amyloid precursor protein harboring the Swedish mutation. Implications for distinct pathways

The molecular mechanisms of the nonamyloidogenic and the amyloidogenic pathways of the amyloid precursor protein (APP) are unknown, but proteolysis of APP is essential for the generation of beta-amyloid. To study the time-course of C-terminal fragment generation by alpha- and beta-secretase, we expressed the APP751 isoform with the Swedish mutation in the human neuroblastoma cell line SY5Y as previously described (Urmoneit et al., 1995). We show in pulse-chase experiments that the C-terminal fragments, CT, generated by alpha-secretase and A4CT, generated by beta-secretase, could be generated from immature full-length APP before O-glycosylation is completed. Thus beta A4 may be generated from immature APP that has not passed through the trans-Golgi-network (TGN), which presents experimental evidence for the intracellular localization of beta-secretase activity in an earlier Golgi complex.

The role of A beta 42 in Alzheimer's disease

Our recent studies of plasma, fibroblasts, transfected cells and transgenic mice show that a fundamental effect of the mutations linked to familial Alzheimer's disease (FAD) is to increase the extracellular concentration of A beta 42. This effect of the FAD-linked mutations is likely to be directly related to the pathogenesis of Alzheimer's disease (AD) because A beta 42 is deposited early and selectively in the senile plaques that are an invariant feature of all forms of AD. Thus our results provide strong evidence that the FAD-linked mutations all cause AD by increasing the extracellular concentration of A beta 42 (43), thereby fostering A beta deposition, and they support the hypothesis that cerebral A beta deposition is an essential early event in the pathogenesis of all forms of AD. Interactions between the basal forebrain cholinergic system and A beta that could influence AD pathogenesis are discussed.

Post-translational processing and turnover kinetics of presynaptically targeted amyloid precursor superfamily proteins in the central nervous system

The amyloid precursor superfamily is composed of three highly conserved transmembrane glycoproteins, the amyloid precursor protein (APP) and amyloid precursor-like proteins 1 and 2 (APLP1, APLP2), whose functions are unknown. Proteolytic cleavage of APP yields the betaA4 peptide, the major component of cerebral amyloid in Alzheimer's disease. Here we show that five post-translationally modified, full-length species of APP and APLP2 (but not APLP1) arrive at the mature presynaptic terminal in the fastest wave of axonal transport and are subsequently rapidly cleared (mean half-life of 3.5 h). Rapid turnover of presynaptic APP and APLP2 occurs independently of visual activity. Turnover of the most rapidly arriving APP species was accompanied by a delayed accumulation of a 120-kDa, APP fragment lacking the C terminus, consistent with presynaptic APP turnover via constitutive proteolysis. Turnover of APLP2 was not accompanied by detectable APLP2 fragment peptides, suggesting either that APLP2 either is more rapidly degraded than is APP or is retrogradely transported shortly after reaching the terminus. A single 150-kDa APLP2 species containing the Kunitz protease inhibitor domain is the major amyloid precursor superfamily protein transported to the presynapse. Presynaptic APP and APLP2 are sialylated and N- and O-glycosylated, and some also carry chondroitin sulfate glycosaminoglycan and/or dermatan sulfate glycosaminoglycan. The rapid kinetics for turnover of APP and APLP2 predict a sensitive balance of synthesis, transport, and elimination rates that may be critical to normal neuronal functions and metabolic fates of these proteins.

A beta amyloidogenesis: unique, or variation on a systemic theme?

For more than a century amyloid was considered to be an interesting, unique, but inconsequential pathologic entity that rarely caused significant clinical problems. We now recognize that amyloid is not one entity. In vivo it is a uniform organization of a disease, or process, specific protein co-deposited with a set of common structural components. Amyloid has been implicated in the pathogenesis of diseases affecting millions of patients. These range from Alzheimer's disease, adult-onset diabetes, consequences of prolonged renal dialysis, to the historically recognized systemic forms associated with inflammation and plasma cell disturbances. Strong evidence is emerging that even when deposited in local organ sites significant physiologic effects may ensue. With emphasis on A beta amyloid, we review the present definition, classification, and general in vivo pathogenetic events believed to be involved in the deposition of amyloids. This encompasses the need for an adequate amyloid precursor protein pool, whether precursor proteolysis is required prior to deposition, amyloidogenic amino acid sequences, fibrillogenic nucleating particles, and an in vivo microenvironment conducive to fibrillogenesis. The latter includes several components that seem to be part of all amyloids. The role these common components may play in amyloid accumulation, why amyloids tend to be associated with basement membranes, and how one may use these findings for anti-amyloid therapeutic strategies is also examined.

Profiles of amyloid precursor and presenilin 2-like proteins are correlated during development of the mouse hypothalamus

The amyloid precursor protein (APP) and APP-like (APLP) material, as visualized with the Mab22C11 antibody, have previously been shown to be associated with radial glia in hypothalamus, which are known to promote neurite outgrowth. By Northern blot analysis, APP 695 mRNA levels increased steadily over hypothalamic development, APP 770 mRNA was transiently expressed at 12 days postnatally, and APLP mRNA was only weakly expressed in the hypothalamus. The developmental pattern of APP moeities in mouse hypothalamus and in fetal hypothalamic neurons in culture was compared with a presenilin 2 (PS2) related protein using an antibody developed against the N-terminal part of PS2. By Western blot analysis, APP and PS2-like immunoreactivity were visualized as a 100-130 and 52 kDa bands, respectively. An APP biphasic increase was observed during hypothalamic development in vivo. APP immunoreactivity was equally detected in neuronal and glial cultures, while PS2-like material was more concentrated in neurons. A correlation between APP/APP-like and PS2-like levels was observed during development in vivo. While APP was mostly associated with membrane fractions, a significant portion of PS2-like material was also recovered from cytosolic fractions in vitro. In contrast to native PS2 in COS-transfected cells, the PS2-like material did not aggregate after heating for 90 s at 90 degrees C. These results indicate a close association between APP and PS2-like material during hypothalamic development in vivo, and suggest that neuronal and glial cultures may provide appropriate models to test their interactions.

A new paradigm for neurotoxicity by FAD mutants of betaAPP: a signaling abnormality

We have demonstrated that normal betaAPP695 behave as a signaling receptor and indicated that point mutations at V642 create autoactive betaAPP in signal transduction. Cellular expression of those familial Alzheimer's disease-associated mutants causes neuronal cells to undergo apoptotic death; and procedures inhibiting the signal of normal betaAPP block the mutant-induced apoptosis. We have also shown that the mutant-induced death is mediated by intracellular G protein activity but not by secretion of Abeta peptides. Accordingly, the mutant-induced death requires a cytoplasmic domain but not the 41st and 42nd residues of the Abeta region. These studies provide a novel insight that betaAPP may play a normal role as a death receptor and that Alzheimer's disease-relevant abnormality occurred in this function may lead neurons to suicidal degeneration.

Purification and spectroscopic characterization of beta-amyloid precursor protein from porcine brains

Soluble and membrane-bound isoforms of beta-amyloid protein precursor (APP) of Alzheimer's disease were extracted and purified from porcine brains. At least three types of soluble APP and membrane-bound APP with different molecular masses, ranging from 86 kDa to 116 kDa, were obtained. CD and infrared spectroscopies were used to determine the overall secondary-structure content of APP. The infrared spectra of soluble and membrane-bound APP (in dry and hydrated states) were similar in the amide-I and amide-II regions, suggesting that the overall secondary structures of the soluble and membrane isoforms were roughly identical. The amide-I band is composed of at least five component bands, located at 1694, 1674, 1652, 1637 and 1618 cm(-1) for soluble APP, and located at 1687, 1674, 1651, 1637 and 1614-1606 cm(-1) for membrane-bound APP, as evidenced by their respective second-derivative infrared spectra. The 1651-1652-cm(-1) band was associated with alpha-helix structures, while two types of beta-sheet structures are evidenced by two characteristic pairs of component bands. The 1674-cm(-1) and 1637-cm(-1) bands for soluble APP and membrane-bound APP were tentatively associated to beta-sheet structures. The second pair of bands, located at 1694 cm(-1) and at 1618 cm(-1) for soluble APP and at 1687 cm(-1) and 1614-1606 cm(-1) for membrane-bound APP, were associated with intermolecular beta-sheet structures or aggregated strands, as confirmed by heat denaturation. CD spectra indicated the presence of alpha-helix structures in soluble and membrane-bound APP. The secondary-structure content, estimated from CD spectra, was about 40-45% alpha-helix and 15-20% beta-sheet structures for soluble and membrane-bound APP.

Mutations in the transmembrane domain of APP altering gamma-secretase specificity

Alzheimer's disease (AD) beta-amyloid peptide (Abeta and betaA4) is derived from the amyloid precursor protein (APP) by the subsequent action of the so-far unidentified beta- and gamma-secretases. gamma-secretase, which generates the C-terminus of Abeta, cleaves within the transmembrane domain of APP, preferentially after Abeta-residue 40 (Abeta 40) but also after residue 42 (Abeta 42). This Abeta 42 represents the major subunit of the plaques in AD. Since the position of gamma-secretase cleavage is crucial for understanding the pathogenic pathway, we investigated the effect of different point mutations at Thr43 on gamma-secretase specificity in SPA4CT (SPC99)-expressing COS7 cells. These constructs only require gamma-cleavage for Abeta release. We observed that all Thr43 mutations altered the specificity of gamma-secretase. Small hydrophobic residues favored the generation of Abeta 42, leading to an increase in the 42/40 ratio of Abeta (1.6-2.8-fold). The increase was even stronger (5.6-5.8-fold) when combined with the familial mutation Val46Phe. Thus, these constructs might be highly valuable for the generation of animal models for AD. Processing of full-length APP or SPA4CT yielded the same 42/40 ratio of Abeta (4. 7%). Both constructs, bearing the familial AD mutation Val46Phe, led to a similar increase in the 42/40 ratio (3.3- versus 3.6-fold). The p3 fragment, produced by alpha- and gamma-secretase, showed 42/40 ratios similar to Abeta when derived from wild-type and mutant proteins. These results suggest that the different Abeta- and p3-species are generated by gamma-cleavage activities with a similar enzymatic mechanism.

Small heat shock proteins inhibit in vitro A beta(1-42) amyloidogenesis

We demonstrate that small heat shock proteins (sHsp) inhibit in vitro amyloid formation by the Alzheimer's A beta(1-42) polypeptide as detected by a thioflavine T fluorescence assay and electron microscopy. Human sHsp27 (0.50-3.0 microM) inhibited amyloid formation from 20 microM A beta(1-42) by 23-75%, in 24 h. In contrast, treatment of pre-formed amyloid with 0.5-3.0 microM sHsp27 only reduced the fluorescence signal by 6-36%. The data suggest that ordered fibril formation may represent a form of off-pathway aggregation that can be prevented by chaperone action. The data raise the possibility that age-related changes in chaperone function could contribute toward the pathogenesis of Alzheimer's and other amyloid-associated diseases.

Neuronal expression of beta-amyloid precursor protein Alzheimer mutations causes intracellular accumulation of a C-terminal fragment containing both the amyloid beta and cytoplasmic domains

Five different Alzheimer mutations of the beta-amyloid precursor protein (APP) were expressed in neurons via recombinant herpes simplex virus (HSV) vectors, and the levels of APP metabolites were quantified. The predominant intracellular accumulation product was a C-terminal fragment of APP that co-migrated with the protein product of an HSV recombinant expressing the C-terminal 100 amino acids (C100) of APP, which is known to cause neurodegeneration. Fractionation studies revealed that the C-terminal fragment generated by expression of the Alzheimer mutations, like C100, partitioned into membrane fractions and was particularly enriched in synaptosomes. The processing abnormality caused by expression of the Alzheimer mutations occurs predominantly in neurons. Expression of these mutations or of C100 alone in neurons caused increased secretion of Abeta relative to that of neurons infected with wild type APP recombinant vectors. These data show that expression of APP mutations that cause familial Alzheimer's disease increases the intracellular accumulation of potentially amyloidogenic and neurotoxic C-terminal fragments of APP in neurons.

Vesicular neurotransmitter transporters. Potential sites for the regulation of synaptic function

Neurotransmission depends on the regulated release of chemical transmitter molecules. This requires the packaging of these substances into the specialized secretory vesicles of neurons and neuroendocrine cells, a process mediated by specific vesicular transporters. The family of genes encoding the vesicular transporters for biogenic amines and acetylcholine have recently been cloned. Direct comparison of their transport characteristics and pharmacology provides information about vesicular transport bioenergetics, substrate feature recognition by each transporter, and the role of vesicular amine storage in the mechanism of action of psychopharmacologic and neurotoxic agents. Regulation of vesicular transport activity may affect levels of neurotransmitter available for neurosecretion and be an important site for the regulation of synaptic function. Gene knockout studies have determined vesicular transport function is critical for survival and have enabled further evaluation of the role of vesicular neurotransmitter transporters in behavior and neurotoxicity. Molecular analysis is beginning to reveal the sites involved in vesicular transporter function and the sites that determine substrate specificity. In addition, the molecular basis for the selective targeting of these transporters to specific vesicle populations and the biogenesis of monoaminergic and cholinergic synaptic vesicles are areas of research that are currently being explored. This information provides new insights into the pharmacology and physiology of biogenic amine and acetylcholine vesicular storage in cardiovascular, endocrine, and central nervous system function and has important implications for neurodegenerative disease.

In vivo magnetic resonance spectroscopy of human brain: the biophysical basis of dementia

Nuclear magnetic resonance spectroscopy (MRS) in low and medium magnetic fields yields well-resolved natural abundance proton and decoupled phosphorus spectra from small (1-10 cc) volumes of brain in vivo in minutes. With this tool, neurochemical research has advanced through identification and non-invasive assay of specific neuronal--(N-acetylaspartate), glial (myo-inositol)--markers, energetics and osmolytes, and neurotransmitters (glutamate, GABA). From these simple measurements, several dozen disease states are recognized, including birth injury, and white matter and Alzheimer disease. Addition of stable isotopes of carbon (in man) or nitrogen (in experimental animals) has provided in vivo assays of enzyme flux through glucose transport, glycolysis, TCA-cycle, and the glutamine-glutamate-GABA system. Finally, a number of xenobiotics are recognized with heteronuclear NMR techniques. Together, these tools are having a major impact on neuroscience and clinical medicine. Through diagnosis and therapeutic monitoring, a new generation of in vivo metabolite imaging is expected with the advent of conforming RF coils and higher field NMR systems.

The role of thrombin-like (serine) proteases in the development, plasticity and pathology of the nervous system

There is increasing evidence suggesting that members of the serine protease family, including thrombin, chymotrypsin, urokinase plasminogen activator, and kallikrein, may play a role in normal development and/or pathology of the nervous system. Serine proteases and their cognate inhibitors have been shown to be increased in the neural parenchyma and cerebrospinal fluid following injury to the blood brain barrier. Zymogen precursors of thrombin and thrombin-like proteases as well as their receptors have also been localized in several distinct regions of the developing or adult brain. Thrombin-like proteases have been shown to exert deleterious effects, including neurite retraction and death, on different neuronal and non-neuronal cell populations in vitro. These effects appear to be mediated through cell surface receptors and can be prevented or reversed with specific serine protease inhibitors (serpins). Furthermore, we have recently shown that treatment with protease nexin-1 (a serpin that inhibits thrombin-like proteases) promotes the survival and growth of spinal motoneurons during the period of programmed cell death and following injury. Taken together, these observations suggest that thrombin-like proteases play a deleterious role, whereas serpins promote the development and maintenance of neuronal cells. Thus, changes in the balance between serine proteases and their cognate inhibitors may lead to pathological states similar to those associated with some neurodegenerative diseases such as Alzheimer's disease. The present review summarizes the current state of research involving such serine proteases and speculates on the possible role of these thrombin-like proteases in the development, plasticity and pathology of the nervous system.

The recognition of haemoglobin by antibodies raised for the immunoassay of beta-amyloid

Canine and porcine cerebrospinal fluid (CSF) were fractionated by size exclusion chromatography and analysed by a luminescence enzyme linked immunosorbent assay (ELISA) configured to detect beta-amyloid. A peak of activity was observed in the CSF consistent with the molecular weight of beta-amyloid. When CSF contaminated with blood was analysed an additional peak of immunoreactivity at a higher molecular weight was observed. The peak of activity was found to be derived from cross-reactivity of the immunoglobulins employed in the ELISA with haemoglobin. These findings are discussed with reference to primary and structural sequence homology between beta-amyloid and haemoglobin from a number of species, the known properties of beta-amyloid and recent clinical reports.

Preferential adsorption, internalization and resistance to degradation of the major isoform of the Alzheimer's amyloid peptide, A beta 1-42, in differentiated PC12 cells

A central question in Alzheimer's disease (AD) is the role of amyloid in pathogenesis. Recent discoveries implicating the longer A beta 1-42 form of amyloid in pathogenesis led us to characterize the interaction of A beta with cells to elucidate differences that might account for these observations. We characterized the adsorption, internalization and degradation of radiolabeled A beta in NGF-differentiated PC12 cells under conditions that are not acutely toxic. All A beta peptides examined absorb to the surface of PC12 cells and are internalized; however the adsorption and internalization of A beta 1-42 is significantly greater than that of A beta 1-40 and A beta 1-28. The adsorption of A beta 1-42 is decreased by treatment of the cells with neuraminidase, but not heparitinase. The fate of the internalized A beta 1-42 is also very different than shorter A beta peptides; a fraction of the internalized A beta 1-42 accumulates intracellularly and is resistant to degradation for at least 3 days while A beta 1-40 and shorter peptides are eliminated with a half life of about 1 h. A beta 1-42 does not appear to inhibit lysosomal hydrolases, since A beta 1-28 is degraded at the same rate in the presence or absence of A beta 1-42. The intracellular A beta 1-42 is located in a dense organellar compartment and colocalizes with the lysosomal markers Lucifer Yellow and horseradish peroxidase. These data indicate that there are significant differences in the cell surface adsorption, internalization and catabolism of A beta 1-42 compared to A beta 1-40 and A beta 1-28. These differences may be important for the preferential accumulation of the longer A beta 1-42 isoform and its association with AD pathogenesis.

Generation of Alzheimer amyloid beta peptide through nonspecific proteolysis

Polymerization of Alzheimer amyloid beta peptide (Abeta) into amyloid fibrils is associated with resistance to proteolysis and tissue deposition. Here, it was investigated whether Abeta might be generated as a protease-resistant core from a polymerized precursor. A 100-amino acid C-terminal fragment of the Alzheimer beta-amyloid precursor protein (C100), containing the Abeta and cytoplasmic domains, polymerized both when inserted into membranes and after purification. When subjected to digestion using the nonspecific enzyme proteinase K, the cytoplasmic domain of C100 was degraded, whereas the Abeta domain remained intact. In contrast, dissociated C100 polymers were almost completely degraded by proteinase K. Mammalian cells transfected with the human Alzheimer beta-amyloid precursor gene contained a fragment corresponding to C100, which needed similar harsh conditions to be dissolved, as did polymers formed by purified C100. Hence, it was concluded that C100 polymers are formed in mammalian cells. These results suggest that the C terminus of Abeta can be generated by nonspecific proteases, acting on a polymerized substrate, rather than a specific gamma-secretase. This offers an explanation of how the Abeta peptide can be formed in organelles containing proteases capable of cleaving most peptide bonds.

The beta-amyloid domain is essential for axonal sorting of amyloid precursor protein

We have analysed the axonal sorting signals of amyloid precursor protein (APP). Wild-type and mutant versions of human APP were expressed in hippocampal neurons using the Semliki forest virus system. We show that wild-type APP and mutations implicated in Alzheimer's disease and another brain beta-amyloidosis are sorted to the axon. By analysis of deletion mutants we found that the membrane-inserted APP ectodomain but not the cytoplasmic tail is required for axonal sorting. Systematic deletions of the APP ectodomain identified two regions required for axonal delivery: one encoded by exons 11-15 in the carbohydrate domain, the other encoded by exons 16-17 in the juxtamembraneous beta-amyloid domain. Treatment of the cells with the N-glycosylation inhibitor tunicamycin induced missorting of wild-type APP, supporting the importance of glycosylation in axonal sorting of APP. The data revealed a hierarchy of sorting signals on APP: the beta-amyloid-dependent membrane proximal signal was the major contributor to axonal sorting, while N-glycosylation had a weaker effect. Furthermore, recessive somatodendritic signals, most likely in the cytoplasmic tail, directed the protein to the dendrites when the ectodomain was deleted. Analysis of detergent solubility of APP and another axonally delivered protein, hemagglutinin, demonstrated that only hemagglutinin formed CHAPS-insoluble complexes, suggesting distinct mechanisms of axonal sorting for these two proteins. This study is the first delineation of sorting requirements of an axonally targeted protein in polarized neurons and indicates that the beta-amyloid domain plays a major role in axonal delivery of APP.

Alzheimer's disease betaA4 protein release and amyloid precursor protein sorting are regulated by alternative splicing

We show here that alternative splicing influences the polarized secretion of amyloid precursor protein (APP) as well as the release of its proteolytic 3-4-kDa fragments betaA4 and p3. In Madin-Darby canine kidney II cells stably transfected with various APP isoforms and APP mutants, APPsec was consistently secreted basolaterally. In contrast, Madin-Darby canine kidney II cells transfected with L-APP677, which occurs naturally by alternative splicing of exon 15, secreted this isoform both apically and basolaterally, while maintaining the basolateral sorting of endogenous APPsec. This suggests that the alternative splicing of APP exon 15 modulates the polarized sorting of secretory APP. The same alternative splicing event also decreased the production of betaA4 relative to p3. This is the first example of alternative splicing regulating polarized trafficking of a secretory protein.

Negative transactivation of cAMP response element by familial Alzheimer's mutants of APP

In familial Alzheimer's disease (FAD), missense point mutations V642I/F/G, which co-segregate with the disease phenotype, have been discovered in amyloid precursor APP695. Here, we report that three FAD mutants (FAD-APPs) negatively regulated the transcriptional activity of cAMP response element (CRE) by a G(o)-dependent mechanism, but expression of wildtype APP695 had no effect on CRE. Experiments with various Galpha(s) chimeras demonstrated that Phe-APP coupled selectively to the C-terminus of Galpha(0). Again, wild-type APP695 had no effect on its C-terminus. These data indicate that FAD-APPs are gain-of-function mutants of APP695 that negatively regulate the CRE activity through G(o). This negative transactivation of CRE is the first biochemically analyzed signal evoked by the three FAD-APPs, but not by wild-type APP695, in a whole-cell system. We discuss the significance of constitutive CRE suppression by FAD-APPs, which is potentially relevant to synaptic malplasticity or memory disorders.

APP-BP1, a novel protein that binds to the carboxyl-terminal region of the amyloid precursor protein

beta-Amyloid protein precursors (APPs, 695-770 amino acids) are the source of the 39-43 amino acid beta-amyloid (A beta) peptides that comprise diffuse and fibrillar deposits in the cerebral cortex and vasculature of Alzheimer's disease brains. A beta is thought to play a role in the pathogenesis of Alzheimer's disease, and, hence, considerable effort has been invested in defining the means by which A beta is generated from the APPs. Knowledge of the normal function of the APPs is sure to provide insights into the genesis and pathological persistence of A beta in Alzheimer's disease. APP is a cell surface protein with a large extracellular amino-terminal domain, a single transmembrane segment, and a short cytoplasmic tail. Its location and structural features characteristic of a receptor for signal transduction led us to search for potential effector proteins capable of binding and interacting with its cytoplasmic domain. Here, we report the cloning of a cDNA encoding one such protein. This ubiquitously expressed 59-kDa APP-binding protein, called APP-BP1, is 61% similar to a protein encoded by the Arabidopsis AXR1 gene, required for normal response to the hormone auxin, and is a relative of the ubiquitin activating enzyme E1.