Protease inhibitors generate cytotoxic fragments from Alzheimer amyloid protein precursor in cDNA-transfected glioma cells

Overexpression of TDP-43 causes partially p53-dependent G2/M arrest and p53-independent cell death in HeLa cells

It has been hypothesized that the dysregulation of transactive response DNA-binding protein-43 (TDP-43) in neurons is closely linked to the pathogenesis of amyotrophic lateral sclerosis and frontotemporal lobar degeneration with ubiquitinated inclusions. However, it remains undefined whether the dysregulation of TDP-43 in non-neuronal cells, such as glial cells, contributes to the pathogenesis of these neurodegenerative diseases. Primarily using HeLa cells, we show that a low-grade overexpression of TDP-43, 2- to 5-fold greater than endogenous expression, which is thought to mimic the gain of function of TDP-43, induced cell cycle arrest at the G2/M phase and cell death in cultured non-neuronal cells. Since the activation of p53 may induce G2/M arrest and/or cell death in many abnormal situations, we examined the mechanism underlying G2/M arrest from the standpoint of p53 regulation. It was determined that the TDP-43-induced G2/M arrest was attenuated, while TDP-43-induced death was not attenuated, in cells in which the p53 function was compromised. These data collectively indicate that TDP-43 causes G2/M arrest in a partially p53-dependent manner and it causes cell death in a p53-independent manner in cycling cells. Because it is likely that the impaired proliferation in glial cells causes a decrease in the neuron-supporting ability, these findings further suggests that the gain of function of TDP-43 may cause neurotoxicity by inducing cell cycle arrest and death in glial cells.

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.

Intracellular deposition, microtubule destabilization, and transport failure: an "early" pathogenic cascade leading to synaptic decline

Protein deposition is a common event in age-related neurological diseases that are characterized by neuronal dysfunction and eventual cell death. Here, cultured hippocampal slices were infused with the lysosomal disrupter chloroquine to examine the link between abnormal protein processing/deposition and early synaptopathogenesis. Tau species of 55 to 69 kDa increased over several days of treatment with chloroquine, while the protein and message levels of synaptic markers were selectively reduced. Neurons of subfields CA1, CA3, and dentate gyrus accumulated protein deposits recognized by antibodies against paired helical filaments and ubiquitin, and this was accompanied by tubulin fragmentation and deacetylation. The deposition filled the basal pole of pyramidal neurons, encompassing the area of the axon hillock and initial dendritic branching but without causing overt neuronal atrophy. Neurons containing the polar aggregates exhibited severely impaired transport along basal dendrites. Transport capability was also lost along apical dendrites, the opposite direction of deposited material in the basal pole; thus, perpetuating the problem beyond physical blockage must be the associated loss of microtubule integrity. These data indicate that transport failure forms a link between tau deposition and synaptic decline, thus shedding light on how protein aggregation events disrupt synaptic and cognitive functions before the ensuing cellular destruction.

Neuronal apoptosis by apolipoprotein E4 through low-density lipoprotein receptor-related protein and heterotrimeric GTPases

The epsilon4 genotype of apolipoprotein E (apoE4) is the most established predisposing factor in Alzheimer's disease (AD); however, it remains unclear how apoE4 contributes to the pathophysiology. Here, we report that the apoE4 protein (ApoE4) evokes apoptosis in neuronal cells through the low-density lipoprotein receptor-related protein (LRP) and heterotrimeric GTPases. We examined neuron/neuroblastoma hybrid F11 cells and found that these cells were killed by 30 microg/ml ApoE4, but not by 30 microg/ml ApoE3. ApoE4-induced death occurred with typical features for apoptosis in time- and dose-dependent manners, and was observed in SH-SY5Y neuroblastomas, but not in glioblastomas or non-neuronal Chinese hamster ovary cells. Activated, but not native, alpha2-macroglobulin suppressed this ApoE4 toxicity. Suppression by the antisense oligonucleotide to LRP and inhibition by low nanomolar concentrations of LRP-associated protein RAP provided evidence for the involvement of LRP. The involvement of heterotrimeric GTPases was demonstrated by the findings that (1) ApoE4-induced death was suppressed by pertussis toxin (PTX), but not by heat-inactivated PTX; and (2) transfection with PTX-resistant mutant cDNAs of Galpha(i) restored the toxicity of ApoE4 restricted by PTX. We thus conclude that one of the neurotoxic mechanisms triggered by ApoE4 is to activate a cell type-specific apoptogenic program involving LRP and the G(i) class of GTPases and that the apoE4 gene may play a direct role in the pathogenesis of AD and other forms of dementia.

Antibody-regulated neurotoxic function of cell-surface beta-amyloid precursor protein

APP is a transmembrane precursor of beta-amyloid, and its mutations cause early-onset familial Alzheimer's disease. We report a toxic function of normal wild-type APP (wtAPP). Treatment of neuronal F11 cells, immortalized embryonic day 13 neurons, overexpressing wtAPP with anti-APP antibodies caused death. Death was not induced by antibody in parental F11 cells. Death by antibody occurred through cell-surface APP, not through secreted APP, in a pertussis toxin-sensitive manner and was typical apoptosis, not observed in primary astrocytes or glioma cells overexpressing wtAPP, but observed in primary cortical neurons. Cell-surface APP thus performs a toxic function as an extracellularly controllable regulator of neuronal death. This study provides a novel insight into the normal and pathological functions of cell-surface wtAPP.

A combinatorial approach to the identification of dipeptide aldehyde inhibitors of beta-amyloid production

In an effort to rapidly identify potent inhibitors of Abeta production and to probe the amino acid sequence specificity of the protease(s) responsible for the production of this peptide, a large number of dipeptide aldehydes were combinatorially synthesized and manually evaluated for their inhibitory properties. The starting point for this study was the dipeptide aldehyde carbobenzoxyl-valinyl-phenylalanal previously shown to inhibit the production of Abeta in CHO cells stably transfected with the cDNA encoding betaAPP695. Pools of related dipeptide aldehydes were combinatorially synthesized, and the most active pool was deconvoluted, resulting in the identification of the most active inhibitor of this pool. Systematic optimization of this inhibitor resulted in a series of dipeptide aldehydes with enhanced potencies relative to carbobenzoxyl-valinyl-phenylalanal. The most active dipeptide aldehydes were those that possessed hydrophobic amino acids at both the P1 and P2 positions. The most potent compound identified in this study was 3, 5-dimethoxycinnamamide-isoleucinyl-leucinal with an IC(50) of 9.6 microM, approximately 10-fold more active than carbobenzoxyl-valinyl-phenylalanal. In immunoprecipitation experiments using antibodies directed toward either Abeta1-40 or Abeta1-42, 3,5-dimethoxycinnamamide-isoleucinyl-leucinal, like carbobenzoxyl-valinyl-phenylalanal, preferentially inhibited the shorter 1-40 form of Abeta, whereas the longer 1-42 form was not as strongly inhibited. These results suggest that dipeptide aldehydes related to carbobenzoxyl-valinyl-phenylalanal inhibit Abeta through similar mechanisms and demonstrate the utility of a combinatorial synthesis approach to rapidly identify potent inhibitors of Abeta production.

Caveolin-3 upregulation activates beta-secretase-mediated cleavage of the amyloid precursor protein in Alzheimer's disease

Here, we investigate the involvement of caveolins in the pathophysiology of Alzheimer's disease (AD). We show dramatic upregulation of caveolin-3 immunoreactivity in astroglial cells surrounding senile plaques in brain tissue sections from authentic AD patients and an established transgenic mouse model of AD. In addition, we find that caveolin-3 physically interacts and biochemically colocalizes with amyloid precursor protein (APP) both in vivo and in vitro. Interestingly, recombinant overexpression of caveolin-3 in cultured cells stimulated beta-secretase-mediated processing of APP. Immunoreactivities of APP and presenilins were concomitantly increased in caveolin-3-positive astrocytes. Because the presenilins also form a physical complex with caveolin-3, caveolin-3 may provide a common platform for APP and the presenilins to associate in astrocytes. In AD, augmented expression of caveolin-3 and presenilins in reactive astrocytes may alter APP processing, leading to the overproduction of its toxic amyloid metabolites.

Regulation of brain G-protein go by Alzheimer's disease gene presenilin-1

To investigate a possible association between G-proteins and presenilin-1 (PS-1), a series of glutathione S-transferase-fusion proteins containing portions of PS-1 were prepared and used in vitro in binding experiments with tissue and recombinant G-proteins. The results demonstrate that the 39 C-terminal amino acids of PS-1 selectively bind the brain G-protein, Go. Addition of guanosine 5'-3-O-(thio)triphosphate promoted Go dissociation from PS-1, indicating that this domain mimics the function of G-protein-coupling domains found in receptors. The 39-amino acid synthetic polypeptide activated Go in a magnesium ion-dependent manner. Physical interaction of full-length PS-1 and Go was also demonstrated. Following transfection of Goalpha and N-terminally FLAG-tagged PS-1 in COS-7 cells, Go was immunoprecipitated by FLAG antibodies. In addition, endogenous PS-1 and Goalpha were colocalized immunocytochemically in human glioma cell lines. The results indicate that PS-1 regulates Go activities in living cells.

Caveolae, plasma membrane microdomains for alpha-secretase-mediated processing of the amyloid precursor protein

Caveolae are plasma membrane invaginations where key signaling elements are concentrated. In this report, both biochemical and histochemical analyses demonstrate that the amyloid precursor protein (APP), a source of Abeta amyloid peptide, is enriched within caveolae. Caveolin-1, a principal component of caveolae, is physically associated with APP, and the cytoplasmic domain of APP directly participates in this binding. The characteristic C-terminal fragment that results from APP processing by alpha-secretase, an as yet unidentified enzyme that cleaves APP within the Abeta amyloid sequence, was also localized within these caveolae-enriched fractions. Further analysis by cell surface biotinylation revealed that this cleavage event occurs at the cell surface. Importantly, alpha-secretase processing was significantly promoted by recombinant overexpression of caveolin in intact cells, resulting in increased secretion of the soluble extracellular domain of APP. Conversely, caveolin depletion using antisense oligonucletotides prevented this cleavage event. Our current results indicate that caveolae and caveolins may play a pivotal role in the alpha-secretase-mediated proteolysis of APP in vivo.

Degeneration in vivo of rat hippocampal neurons by wild-type Alzheimer amyloid precursor protein overexpressed by adenovirus-mediated gene transfer

In an attempt to elucidate the pathological implications of intracellular accumulation of the amyloid precursor protein (APP) in postmitotic neurons in vivo, we transferred APP695 cDNA into rat hippocampal neurons by using a replication-defective adenovirus vector. We first improved the efficiency of adenovirus-mediated gene transfer into neurons in vivo by using hypertonic mannitol. When a beta-galactosidase-expressing recombinant adenovirus suspended in 1 M mannitol was injected into a dorsal hippocampal region, a number of neurons in remote areas were positively stained, presumably owing to increased retrograde transport of the virus. When an APP695-expressing adenovirus was injected into the same site, part of the infected neurons in the hippocampal formation underwent severe degeneration in a few days, whereas astrocytes near the injection site showed no apparent degeneration. These degenerating neurons accumulated different epitopes of APP, and beta/A4 protein (Abeta)-immunoreactive materials were undetected in the extracellular space. A small number of degenerating neurons showed nuclear DNA fragmentation. Electron microscopic examinations demonstrated that degenerating neurons had shrunken perikarya along with synaptic abnormalities. Microglial cells/macrophages were often found in close proximity to degenerating neurons, and in some cases they phagocytosed these neurons. These results suggest that intracellular accumulation of wild-type APP695 causes a specific type of neuronal degeneration in vivo in the absence of extracellular Abeta deposition.

Oligodendrocytes express different isoforms of beta-amyloid precursor protein in chemically defined cell culture conditions: in situ hybridization and immunocytochemical detection

The expression of beta-amyloid precursor protein (betaAPP) by astrocytes is well documented; however, data concerning oligodendrocytes remain controversial. The main goal of the present study was to determine whether or not oligodendrocytes in culture constitutively express the different betaAPP isoforms. Oligodendrocytes were cultured in a chemically defined medium that avoids putative effects of unknown serum factors on oligodendrocyte development. We have employed immunocytochemistry and in situ hybridization with antibodies and synthetic oligonucleotides recognizing, respectively, specific protein epitopes and mRNA transcripts of rat betaAPP isoforms. Oligodendrocytes, in both mixed primary cultures in the presence of serum or in secondary cultures in defined medium, were clearly labeled by antibodies directed to different betaAPP sequences. Antibodies against the serine protease inhibitor domain of betaAPP, also strongly labelled oligodendrocytes. Immunohistochemistry and in situ hybridization were combined to determine precisely the expression of different isoforms of betaAPP. In situ hybridization revealed the presence in oligodendrocytes of mRNA transcripts coding not only for betaAPP695 but also for betaAPP770 and betaAPP751. This indicates that betaAPP immunoreactivity found in oligodendrocytes corresponds to constitutive expression of betaAPP. Oligodendrocyte cultured in chemically defined medium are able to express not only betaAPP695 but also betaAPP770, betaAPP751 isoforms containing the Kunitz protease inhibitor domain. Although the role of betaAPP in the pathological processes of Alzheimer's disease (AD) remains unknown, possible disturbances of betaAPP processing and/or synthesis in oligodendrocytes may account for some myelin disorders observed in AD and other senile dementias.

Composition of white matter bovine brain coated vesicles: evidence that several components influence beta-amyloid peptide to form oligomers and aggregates in vitro

Clathrin coated vesicles (CVs) purified from white matter of human or bovine brain contain amyloid precursor protein (APP), several C-terminal fragments encompassing the beta-amyloid domain (betaA), the alpha-secretase 11-12 kDa intermediate, ApoE and tau. The convergence of these components implicates CVs as potential sites for their interaction, yielding products linked to fibrillogenesis in Alzheimer's disease (AD). Analysis of components co-reactive with both anti-ApoE and betaA suggested presence of stable intravesicular conjugates. To evaluate these interactions in vitro, mixtures containing betaA(1-40), ApoE4 or E3 isoforms with and without lipid added as dymyristoyl phosphatidylcholine liposomes were co-incubated from 5 h to 7 days at 37 degrees C and analyzed on Western blots using a panel of antibodies recognizing betaA and ApoE. Data showed ApoE4 plus lipid induced betaA to form oligomers, conjugates and high Mr aggregates. The rates of formation for these products varied significantly with the ApoE isoform. E3 formed conjugates more rapidly, but these levels were exceeded by those of E4 at 7 days. ApoE4 plus lipid facilitated more rapid formation of higher Mr betaA aggregates which appeared in parallel with betaA oligomers containing up to seven molecules of betaA. Data suggest that the native ApoE, as found in CVs which contain lipid, can be an effective agent for promoting formation of betaA oligomers or other complexes that may be linked to formation of abnormal deposits in AD.

Ionic effects of the Alzheimer's disease beta-amyloid precursor protein and its metabolic fragments

Alzheimer's disease is a progressive dementia characterized in part by deposition of proteinaceous plaques in various areas of the brain. The main plaque protein component is beta-amyloid, a metabolic product of the beta-amyloid precursor protein. Substantial evidence has implicated beta-amyloid (and other amyloidogenic fragments of the precursor protein) with the neurodegeneration observed in Alzheimer's disease. Recently, beta-amyloid precursor protein and its amyloidogenic metabolic fragments have been shown to alter cellular ionic activity, either through interaction with existing channels or by de novo channel formation. Such alteration in ionic homeostasis has also been linked with cellular toxicity and might provide a molecular mechanism underlying the neurodegeneration seen in Alzheimer's disease.

Processing of beta-amyloid precursor protein by cathepsin D

The events leading to the formation of beta-amyloid (betaA4) from its precursor (betaAPP) involve proteolytic cleavages that produce the amino and carboxyl termini of betaA4. The enzyme activities responsible for these cleavages have been termed beta- and gamma-secretase, respectively, although these protease(s) have not been identified. Since betaA4 is known to possess heterogeneity at both the amino and carboxyl termini, beta- and gamma-secretases may actually be a collection of proteolytic activities or perhaps a single proteolytic enzyme with broad amino acid specificity. We investigated the role of cathepsin D in the processing of betaAPP since this enzyme has been widely proposed as a gamma-secretase candidate. Treatment of a synthetic peptide that spans the gamma-secretase site of betaAPP with human cathepsin D resulted in the cleavage of this substrate at Ala42-Thr43. A sensitive liquid chromatography/mass spectrometry technique was also developed to further investigate the ability of cathepsin D to process longer recombinant betaAPP substrates (156 and 100 amino acids of betaAPP carboxyl terminus) in vitro. The precise cathepsin D cleavage sites within these recombinant betaAPP substrates were identified using this technique. Both recombinant substrates were cleaved at the following sites: Leu49-Val50, Asp68-Ala69, Phe93-Phe94. No cleavages were observed at putative gamma-secretase sites: Val40-Ile41 or Ala42-Thr43, suggesting that cathepsin D is not gamma-secretase as defined by these betaA4 termini. Under conditions where the betaAPP156 substrate was first denatured prior to cathepsin D digestion, two additional cleavage sites near the amino terminus of betaA4, Glu-3-Val-2 and Glu3-Phe4, were observed, indicating that cathepsin D cleavage of betaAPP is influenced by the structural integrity of the substrate. Taken together, these results indicate that in vitro, cathepsin D is unlikely to function as gamma-secretase; however, the ability of this enzyme to efficiently cleave betaAPP substrates at nonamyloidogenic sites within the molecule may reflect a role in betaAPP catabolism.

The possible place of cathepsins and cystatins in the puzzle of Alzheimer disease: a review

Lysosomal proteinases (cathepsins) and their endogenous inhibitors (cystatins) have been found to be closely associated with senile plaques, cerebrovascular amyloid deposits, and neurofibrillary tangles in Alzheimer disease (AD). Further, profound changes in the lysosomal system seem to be an early event in "at-risk" neurons of AD brains. There is an ongoing controversy as to whether lysosome-associated proteolytic mechanisms are causally related to the development and/or further progression of the disease. The present article deals with some arguments "pro" and "contra" an involvement of the endosomal/lysosomal pathway in amyloidogenesis as a cardinal process in AD. Other putative targets of acidic proteinases and their natural inhibitors in the pathogenesis of AD (such as formation of neurofibrillary tangles and regulation of apolipoprotein E) are also discussed.

Long-term hippocampal slices: a model system for investigating synaptic mechanisms and pathologic processes

Organotypic cultures provide a unique strategy with which to examine many aspects of brain physiology and pathology. Long-term slice cultures from the hippocampus, a region involved in memory encoding and one that exhibits early degeneration in Alzheimer's disease and ischemia, are particularly valuable in this regard due to their expression of synaptic plasticity mechanisms (e.g., long-term potentiation) and responsiveness to pathological insults (e.g., excitotoxicity). Long-term slices can be prepared from hippocampi at the second or third postnatal week of development and thus incorporate a number of relatively mature features; further signs of maturation and the preservation of adult-like characteristics occur over succeeding weeks. The stability of the cultured slice renders it an appropriate model for studying 1) prolonged regulation/stabilization events linked to synaptogenesis and certain forms of plasticity, 2) temporal patterns of cellular atrophy associated with pathogenic conditions such as ischemia and epilepsia, and 3) slow processes associated with aging and age-related pathologies.

Brain cathepsin B but not metalloendopeptidases degrade rAPP751 with production of amyloidogenic fragments. Comparison with synthetic peptides emulating beta- and gamma-secretase sites

Lysosomal cathepsin B but not L degraded rAPP751 to yield C-terminal 19-25 kDa fragments containing beta A4, reinforcing the view that acidic proteases participate in endosomal-lysosomal processing to yield amyloidogenic fragments in situ. This mechanism is consistent with fragmentation of endogenous APPs within clathrin-coated vesicles (CVs) by vesicular hydrolases, with the appearance of C-terminal amyloidogenic fragments following incubation at pH 6.5. A neutral endopeptidase resembling NEP 24.11 (PS-NEP) purified from detergent extracts of human brain degraded rAPP751; however, breakdown was not blocked robustly by metal chelators or phosphoramidon, suggesting the presence of an alternative processing enzyme. Effects of other inhibitors showed that breakdown was mediated by serine-protease-like component(s). A phosphoramidon-insensitive metalloendopeptidase (PI-NEP) partially purified from rat brain P2 using detergents, and resembling NEP 24.15, showed no activity towards rAPP751. Peptides containing putative beta- or gamma-secretase sites were synthesized for purposes of examining their metabolism by the brain enzymes. Those containing beta-secretase sites were hydrolysed at one or more sites by the four enzymes, but only PI- and PS-NEP acted at the Met-Asp site of Ac-Val-Lys-Met-Asp-Ala-Glu-Phe-Arg.NH2. In the case of substrates containing the gamma-site, these two categories of enzymes were the only ones degrading N-Ac-Ile-Ala.NH2. These data imply that the brain metalloendopeptidases, while inactive towards intact precursors, may be involved in turnover of intermediates containing beta- or gamma-sites.

Gene expression and cellular content of cathepsin D in Alzheimer's disease brain: evidence for early up-regulation of the endosomal-lysosomal system

In Alzheimer's disease brains, more than 90% of pyramidal neurons in lamina V and 70% in lamina III displayed 2- to 5-fold elevated levels of cathepsin D (Cat D) mRNA by in situ hybridization compared with neurologically normal controls. Most of these cells appeared histologically normal. The less vulnerable nonpyramidal neuron population in lamina IV had relatively normal message levels. Neuronal populations expressing more Cat D mRNA also displayed quantitatively increased Cat D immunoreactive protein. Cat D mRNA expression was only moderately increased in astrocytes. Degenerating neurons exhibited intense immunoreactivity but lowered Cat D mRNA levels. The upregulation of Cat D synthesis and accumulation of hydrolase-laden lysosomes indicate an early activation of the endosomal-lysosomal system in vulnerable neuronal populations, possibly reflecting early regenerative or repair processes. These abnormalities also represent a basis for altered regulation of amyloid precursor protein processing.

Inhibition of beta-amyloid formation identifies proteolytic precursors and subcellular site of catabolism

Cerebral deposition of beta-amyloid protein is a pathological feature central to Alzheimer's disease. Production of beta-amyloid by proteolytic processing of the beta-amyloid precursor protein (beta APP) is a critical initial step in beta-amyloidogenesis. We use an inhibitor of beta APP processing to block beta-amyloid peptide formation. Application of the inhibitor to cultured cells results in an accumulation of proteolytic intermediates of beta APP, enabling a precursor-product relationship between beta APP carboxy-terminal fragments and beta-amyloid peptides to be demonstrated directly. In the presence of inhibitor, these amyloidogenic carboxy-terminal fragments can be degraded to nonamyloidogenic products. The catabolism of beta APP carboxy-terminal intermediates and the formation of beta-amyloid peptides are likely to involve an early endosomal compartment as the subcellular site of processing.

Altered processing of a mutant amyloid precursor protein in neuronal and endothelial cells

Altered proteolysis of the amyloid precursor protein (APP) may play an important role in Alzheimer disease (AD). To better understand the role of mutant APP in the pathogenesis of the disease, we stably overexpressed the mutant APP717F approximately twofold vs. the endogenous wild-type gene in several cell types. The processing of APP was examined by Western blot analysis and immunoprecipitation. We observed distinctive patterns of APP metabolites among various cell lines. Neuronal and endothelial cells expressing mutant APP717F generated higher levels of large, potentially amyloidogenic carboxyl terminal fragments, which were enhanced upon treatment of the cells with leupeptin. These results suggest that mutations in the APP gene shift the protein processing towards the amyloidogenic pathway in neuronal and endothelial cells possibly involving the endosomal-lysosomal system.

The Alzheimer's A beta peptide induces neurodegeneration and apoptotic cell death in transgenic mice

To test whether the hypothesis that the Alzheimer's A beta peptide is neurotoxic, we introduced a transgene into mice to direct expression of this peptide to neurons. We show that the transgene is expressed in brain regions which are severely affected in Alzheimer's disease resulting in extensive neuronal degeneration. Morphological and biochemical evidence indicates that the eventual death of these cells occurs by apoptosis. Coincident with the cell degeneration and cell death is the presence of a striking reactive gliosis. Over 50% of the transgenic mice die by 12 months of age, half the normal life span of control mice. These data show that A beta is neurotoxic in vivo and suggest that apoptosis may be responsible for the accompanying neuronal loss, the principal underlying cellular feature of Alzheimer's disease.

Molecular biological approaches to aging in Japan: an overview

Progress in molecular biological investigations of aging in Japan is overviewed. Emphasis is put on studies which, in the author's opinion, appear to have considerable relevance to a definition of aging, i.e. functional decline of cells and tissues with advancing age. (1) Changes in the nuclear DNA, most significantly in methylation, and deletions of mitochondrial DNA have been shown to occur with age. (2) Various aspects of protein metabolism have been investigated, i.e. the fidelity of translation, accumulation of altered enzymes, oxidative damage, and half-lives and degradation of proteins. (3) Japanese researchers have made significant contributions to the understanding of the chemical structure and the mechanisms of generation of paired helical filaments in Alzheimer's disease. Also, studies on beta-amyloid peptide are noteworthy. (4) Murine models of aging were developed and molecular biological investigations on them are progressing. Nematodes and fruit flies are also used as models.(5) Changes in gene expression with age have attracted considerable interest.

Hydrolysis of amyloid precursor protein-derived peptides by cysteine proteinases and extracts of rat brain clathrin-coated vesicles

Amyloid precursor proteins (APPs) and C-terminal fragments were colocalized with cysteine proteinase-like enzymes in purified rat brain clathrin-coated vesicles. Vesicular extracts degraded beta A4(12-28), yielding a product profile similar to that of purified rat brain cathepsin B. Cathepsin B degraded this peptide sequentially, with initial cleavage occurring at Val18-Phe19 and Phe19-Phe20 followed by release of dipeptides. Enzyme also hydrolyzed beta A4(1-40) at Phe19-Phe20 bond but at lower rates, likely due to aggregate formation. An octapeptide analogue of the domain adjacent to beta A4 (N-Ac-Val-Lys-Met-Asp-Ala-Glu-Phe-NH2) was also hydrolyzed by brain cathepsins B and L, and metalloendopeptidase 24.11. Enzymes acted at multiple sites, but only 24.11 cleaved the Met-Asp bond, thus resembling a proposed beta-secretase. Data imply that clathrin-coated vesicles contain cysteine-like proteinases capable of initiating the processing of APP or its fragments.

The Alzheimer beta-amyloid protein precursor/protease nexin-II is cleaved by secretase in a trans-Golgi secretory compartment in human neuroglioma cells

Alzheimer beta-amyloid protein precursor (beta APP) is expressed endogenously and abundantly by human neuroglioma (H4) cells. Its secretory processing has been shown to involve discrete proteolysis within the beta A4 region, thus preventing beta-amyloid formation, by an enzyme which has been referred to as 'beta APP secretase'. This cleavage results in secretion of a soluble N-terminal 135 kDa protein and retention of an integral membrane C-terminal fragment within the cell. The membrane-associated C-terminal fragment is sorted to lysosomes where it undergoes limited degradation. We show here that most newly synthesized beta APP is degraded via a non-lysosomal pathway before maturation in H4 cells, and most mature beta APP is processed predominantly by the so-called secretase. The rapid kinetics of appearance/disappearance of a cleaved 135 kDa protein within a microsomal fraction and the slow accumulation of this form in the extracellular medium indicated that secretase cleaves beta APP in an intracellular compartment. Low-temperature block (20 degrees C) was used to demonstrate that beta APP is cleaved within a late Golgi compartment after sulphation which occurs in the trans-Golgi network (TGN). This is consistent with (1) the immunolocalization of most of the beta APP within a Golgi compartment that reacts with wheat germ agglutinin, (2) the fact that less than 1.5% of the total mature full-length beta APP is present at the plasma membrane and (3) subcellular fractionation studies which showed that the mature full-length and intracellular cleaved beta APPs co-sediment with a membrane fraction that is slightly more dense than the plasma membrane. This study provides evidence that most of the beta APP secretase in H4 cells is intracellular, and confirms that the resulting C-terminal fragment is delivered to lysosomes immediately after cleavage. These results are discussed with regard to the possibility that mature full-length beta APP escapes secretase cleavage and is delivered directly from the TGN to the lysosome without passing through the plasma membrane. Either pathway will result in the generation of amyloidogenic fragments.