Familial Alzheimer's disease cells abnormally accumulate beta-amyloid-harbouring peptides preferentially in cytosol but not in extracellular fluid

Heat shock proteins 70 and 90 inhibit early stages of amyloid beta-(1-42) aggregation in vitro

Alzheimer disease is a neurological disorder that is characterized by the presence of fibrils and oligomers composed of the amyloid beta (Abeta) peptide. In models of Alzheimer disease, overexpression of molecular chaperones, specifically heat shock protein 70 (Hsp70), suppresses phenotypes related to Abeta aggregation. These observations led to the hypothesis that chaperones might interact with Abeta and block self-association. However, although biochemical evidence to support this model has been collected in other neurodegenerative systems, the interaction between chaperones and Abeta has not been similarly explored. Here, we examine the effects of Hsp70/40 and Hsp90 on Abeta aggregation in vitro. We found that recombinant Hsp70/40 and Hsp90 block Abeta self-assembly and that these chaperones are effective at substoichiometric concentrations (approximately 1:50). The anti-aggregation activity of Hsp70 can be inhibited by a nonhydrolyzable nucleotide analog and encouraged by pharmacological stimulation of its ATPase activity. Finally, we were interested in discerning what type of amyloid structures can be acted upon by these chaperones. To address this question, we added Hsp70/40 and Hsp90 to pre-formed oligomers and fibrils. Based on thioflavin T reactivity, the combination of Hsp70/40 and Hsp90 caused structural changes in oligomers but had little effect on fibrils. These results suggest that if these chaperones are present in the same cellular compartment in which Abeta is produced, Hsp70/40 and Hsp90 may suppress the early stages of self-assembly. Thus, these results are consistent with a model in which pharmacological activation of chaperones might have a favorable therapeutic effect on Alzheimer disease.

Pathophysiological roles of amyloidogenic carboxy-terminal fragments of the beta-amyloid precursor protein in Alzheimer's disease

Several lines of evidence suggest that some of the neurotoxicity in Alzheimer's disease (AD) is attributed to proteolytic fragments of amyloid precursor protein (APP) and beta-amyloid (Abeta) may not be the sole active component involved in the pathogenesis of AD. The potential effects of other cleavage products of APP need to be explored. The CTFs, carboxy-terminal fragments of APP, have been found in AD patients' brain and reported to exhibit much higher neurotoxicity in a variety of preparations than Abeta. Furthermore CTFs are known to impair calcium homeostasis and learning and memory through blocking LTP, triggering a strong inflammatory reaction through MAPKs- and NF-kappaB-dependent astrocytosis and iNOS induction. Recently, it was reported that CTF translocated into the nucleus, binding with Fe65 and CP2, and in turn, affected transcription of genes including glycogen synthase kinase-3beta, which results in the induction of tau-rich neurofibrillary tangles and subsequently cell death. Spatial memory of transgenic (Tg) mice overexpressing CT100 was significantly impaired and CTFs were detected in the neurons as well as in plaques of the Tg mice and double Tg mice carrying CT100 and mutant tau. In this review, we summarize observations indicating that both CTF and Abeta may participate in the neuronal degeneration in the progress of AD by differential mechanisms.

Effects of the beta-amyloid and carboxyl-terminal fragment of Alzheimer's amyloid precursor protein on the production of the tumor necrosis factor-alpha and matrix metalloproteinase-9 by human monocytic THP-1

To explore the direct role of beta-amyloid (Abeta) and carboxyl-terminal fragments of amyloid precursor protein in the inflammatory processes possibly linked to neurodegeneration associated with Alzheimer's disease, the effects of the 105-amino acid carboxyl-terminal fragment (CT(105)) of amyloid precursor protein on the production of tumor necrosis factor-alpha (TNF-alpha) and matrix metalloproteinase-9 (MMP-9) were examined in a human monocytic THP-1 cell line and compared with that of Abeta. CT(105) elicited a marked increase in TNF-alpha and MMP-9 production in the presence of interferon-gamma in a dose- and time-dependent manner. Similar patterns were obtained with Abeta despite its low magnitude of induction. Autocrine TNF-alpha is likely to be a main mediator of the induction of MMP-9 because the neutralizing antibody to TNF-alpha inhibits MMP-9 production. Genistein, a specific inhibitor of tyrosine kinase, dramatically diminished both TNF-alpha secretion and subsequent MMP-9 release in response to CT(105) or Abeta. Furthermore, PD98059 and SB202190, specific inhibitors of ERK or p38 MAPK respectively, efficiently suppressed CT(105)-induced effects whereas only PD98059 was effective at reducing Abeta-induced effects. Our results suggest that CT(105) in combination with interferon-gamma might serve as a more potent activator than Abeta in triggering inflammatory processes and that both tyrosine kinase and MAPK signaling pathways may represent potential therapeutic targets for the control of Alzheimer's disease progression.

The functions of the amyloid precursor protein gene

The amyloid precursor protein (APP) gene and its protein products have multiple functions in the central nervous system and fulfil criteria as neuractive peptides: presence, release and identity of action. There is increased understanding of the role of secretases (proteases) in the metabolism of APP and the production of its peptide fragments. The APP gene and its products have physiological roles in synaptic action, development of the brain, and in the response to stress and injury. These functions reveal the strategic importance of APP in the workings of the brain and point to its evolutionary significance.

Effects of nicotine on APP secretion and Abeta- or CT(105)-induced toxicity

Several lines of evidence indicated that overexpression or aberrant processing of amyloid precursor protein (APP) is causally related to Alzheimer's disease (AD). Amyloid precursor protein is principally cleaved within the amyloid beta protein domain to release a large soluble ectodomain (APPs), known to have a wide range of trophic functions. The central hypothesis guiding this review is that nicotine may play an important role in APP secretion and protection against toxicity induced by APP metabolic fragments (beta-amyloid [Abeta], carboxyl terminal [CT]). Findings from our experiments have shown that nicotine enhances the release of APPs, which has neurotrophic and neuroprotective activities in concentration-dependent (>50 micromol/L) and time-dependent (>2 hours) manners. In addition, pretreatment of nicotine (>10 micromol/L for 24 hours) partially prevented Abeta or CT(105)-induced cytotoxicity in primary cultured neuron cells, and the effects of nicotine-induced protection were inhibited by the pretreatment with a nicotine alpha-bungarotoxin. Nicotine (>10 micromol/L for 24 hours) partially inhibited CT(105)-induced cytotoxicity when PC12 cells was transfected with CT(105). From these results, we proposed that nicotine or nicotinic receptor agonist treatment might improve the cognitive functions not only by supplementation of cholinergic neurotransmission, but also by protecting Abeta- or CT(105)-induced neurotoxicity probably through the increased release of APPs and the activation of nicotinic receptors.

APP carboxyl-terminal fragment without or with abeta domain equally induces cytotoxicity in differentiated PC12 cells and cortical neurons

Mutations in the beta-amyloid precursor protein (APP) gene cause familial Alzheimer's disease (AD). Although amyloid beta peptide (Abeta) is the principal constituent of senile plaques in AD, other cleavage products of APP are also implicated in playing a role in the pathogenesis of AD. C-terminal fragments of APP (APP-CTs), that contain complete Abeta sequence, are found in neuritic plaques, neurofibrillary tangles and the cytosol of lymphoblastoid cells obtained from AD patients. Our previous report demonstrated that APP-CT105 causes death of differentiated PC12 cells and cultured rat cortical neurons (Kim and Suh [1996] J. Neurochem. 67:1172-1182) and induces strong inward currents in Xenopus oocyte (Fraser et al., [1996] J. Neurochem. 66:2034-2040). In the present study, to investigate which domain of APP-CT105 is responsible for the neurotoxicity, we have made deletion mutants of APP-CT105 without Abeta and transmembrane domain (TM) or without NPTY domain, a putative endocytosis signaling sequence, using the PCR-amplified strategy and the recombinant GST-fusion protein strategy. The effect on cell survival of the deletion mutants of APP-CT105 (8 microM) was then determined by the LDH and MTT assay. We found that C-terminal fragment without NPTY significantly causes cell death in NGF-differentiated PC12 cells and cultured rat cortical neurons. This finding suggests that NPTY may not play an important role in APP-CT105 mediated neurotoxicity. We found, however, that C-terminal fragment without Abeta and TM significantly induces neuronal cell death. Our results suggest that in addition to Abeta, C-terminal fragment of APP without Abeta and TM domain itself may also participate in the neuronal degeneration in AD.

Roles of A beta and carboxyl terminal peptide fragments of amyloid precursor protein in Alzheimer disease

Several lines of evidence indicate that A beta may play an important role in the pathogenesis of AD. However, there are several discrepancies between the production of A beta and the development of the disease. Thus, A beta may not be the sole active fragment of beta-amyloid precursor protein (betaAPP) in the neurotoxicity assiciated with AD. We focused on the amyloidegenic carboxyl terminal fragments of betaAPP containing the full length of A beta (CT105). We synthesized a recombinant carboxyl-terminal 105 amino acid fragment of betaAPP and examined the effects of CT105 and A beta on cultured neurons, Ca++ uptake into rat brain microsomes, Na+-Ca++ exchange activity, ion channel forming activity in lipid bilayers and passive avoidance performance of mice. Our results suggest that the cytotoxic and channel inducing effects of CT105 are much more potent than that of A beta and toxic mechanisms of CT105 are different from those of A beta. Taken together, these lines of evidence postulate that CT is an alternative toxic element important in the generation of the symptoms common to AD.

A human brain proteolytic activity capable of cleaving natural beta-amyloid precursor protein is affected by its substrate glycoconjugates

Human brain proteins were partially purified by using arginine-Sepharose 4B affinity chromatography, which traps proteins having an affinity to certain groups of arginine residue, such as serine proteases and zymogens. Bound proteins were analyzed for binding and cleavage related to the brain beta-amyloid precursor protein (APP). They were then further separated and isolated using a preparative gel system having a liquid-phase collection apparatus, using a non-denaturing gel system. Each fractionated protein was also analyzed for the above activity using natural APP. Among these, we found several fractions that bind preferentially to APP treated with chondroitinase ABC but not to intact APP, and that also generate particular beta-amyloid containing C-terminal peptides of APP via proteolysis. Our results suggest that sulfated glycoconjugates attached to APP play a role in the substrate specificity of APP for proteases, and also that the nature of natural APP processing mechanisms in vivo is very complex.

The beta-amyloid epitope masking activity in human brain is identified as albumin

Human brain homogenate proteins were analyzed for binding and processing activity in relation to brain beta-amyloid precursor protein (APP). The homogenate was purified by arginine-Sepharose 4B affinity chromatography, which traps proteins with affinity to certain groups of arginine residue, such as serine proteases and zymogens. A 69 kDa protein that masks epitope(s) of brain APP was found in a weakly bound fraction. The nature of the 69 kDa brain protein was identified as albumin by N-terminal amino acid sequencing and Western blot analysis using anti-human albumin antibody. Western blot analysis with domain-specific anti-APP antibodies revealed that the masking activity is complete for beta-amyloid epitope(s), but incomplete for cytoplasmic and extracellular domain epitopes, suggesting that the interaction site of the albumin is beta-amyloid itself. Therefore, it seems that brain albumin is not merely a carrier protein for beta-amyloid in cerebrospinal fluid, but also a modulator which interferes with processing of beta-amyloid precursor protein and its peptides.

Enhanced aggregation of beta-amyloid-containing peptides by extracellular matrix and their degradation by the 68 kDa serine protease prepared from human brain

To explore whether extracellular matrix components in human brain affect the deposition and aggregation of beta-amyloid containing peptides, human brain samples from patients with sporadic Alzheimer's disease and normal aged were analyzed by Western blot analysis. All major beta-amyloid-containing peptides contained epitope(s) which is recognized by anti heparan sulfate antibody. Incubation of brain beta-amyloid-containing peptides with human collagen type IV in neutral pH efficiently generated a high molecular weight aggregated band, approximately 5-fold that of the control sample. We have previously found a serine protease which is capable of cleaving an oligopeptide at the N-terminus of beta-amyloid. In this study, the protease, which also contains heparan sulfate glycoconjugates, degraded the above brain peptides as natural substrates, although with different efficiency. These findings suggest that extra-cellular matrix components affect the processing and aggregation of beta-amyloid-containing peptides in human brain.

Human brain beta-secretase contains heparan sulfate glycoconjugates

A polyclonal antibody against the 68 kDa beta-secretase was established, which recognizes a single 68 kDa band in brain homogenate of Alzheimer's disease patients and normal aged. Western analysis revealed that the protease is an acidic glycoprotein with negative charge on its glycoconjugate(s). Sensitiveness to heparitinase and glycopeptidase A indicates that the protease contains asparagine-linked oligosaccharide with heparan sulfate moieties. Specific detection of the 68 kDa band in the analysis using anti-heparan sulfate antibody, and its time-course-dependent degradation, also confirm the above results. It seems that, like human blood coagulation factors IXa and XIa, the glycoconjugate(s) attached to the protease interfere with substrate specificity, stability and topological restriction of proteolysis in brain extracellular matrix, where diffuse plaque formation is taking place.

A serine protease in Alzheimer's disease cells cleaves a 16K-peptide with flanking residues upstream to beta-amyloid-N-terminus as natural substrate

A serine protease which cleaves an oligopeptide at the beta-amyloid (beta A4) N-terminus was identified and purified from extracellular fluid of familial Alzheimer's disease (FAD) lymphoblastoid cells. In order to search for a natural substrate that stands for a direct precursor of beta A4, C-terminal fragments of beta-amyloid precursor protein (APP) were prepared by immunoprecipitation of cytosol proteins with beta A4-specific antibody. The 16 kDa peptide with N-terminus 30 amino acids upstream from the beta A4-N-terminus, also existing in other hematopoietic cells, was proved to be a natural substrate for the protease in human lymphoid cells. Its cleaved fragment with beta A4-N-terminus was thought to be less amyloidogenic on the basis of its property of self-aggregation in acidic pH. The results suggest the significance of the unique cleavage site at beta A4-upstreams in generation and accumulation of beta A4.

Molecular cloning of human cDNA with a sequence highly similar to that of the dihydrofolate reductase gene in brain libraries derived from Alzheimer's disease patients

A polyclonal antibody was raised against a serine protease purified from the extracellular fluid of familial Alzheimer's disease lymphoblastoid cells. Using this antibody, a cDNA library from familial Alzheimer's disease cells and two cDNA libraries from the brains of two Alzheimer's disease patients were screened independently. The familial Alzheimer's disease protein 1 (FADP1) cDNA clones isolated from these three libraries were subjected to DNA sequence analysis. The nucleotide sequence of FADP1 cDNA is highly similar to the 5' portion of the human dihydrofolate reductase (DHFR) gene, however, the sequence corresponding to exon 1 of the DHFR gene is completely disrupted and contains a 247-bp DNA insert with a sequence unique to FADP1. Moreover, FADP1 cDNA harbours a large open reading frame, including the unique insert, which has the potential to code an approximately 50-kDa protein. The deduced amino acid sequence of this protein contains 12 cysteine residues potentially involved in six disulfide bonds, a proline-rich segment and a hydrophobic segment. Northern-blot analysis with the unique insert DNA probe verified that FADP1 protein is expressed in both lymphoblastoid and brain cells derived from Alzheimer's disease patients.