Distinct secretases, a cysteine protease and a serine protease, generate the C termini of amyloid beta-proteins Abeta1-40 and Abeta1-42, respectively

Protein truncation as a common denominator of human neurodegenerative foldopathies

Neurodegenerative foldopathies are characterized by aberrant folding of diseased modified proteins, which are major constituents of the intracellular and extracellular lesions. These lesions correlate with the cognitive and/or motor impairment seen in these diseases. The majority of the disease modified proteins in neurodegenerative foldopathies belongs to the group of proteins termed as intrinsically disordered proteins (IDPs). Several independent studies have showed that abnormal protein processing constitutes the key pathological feature of these disorders. The current review focuses on protein truncation as a common denominator of neurodegenerative foldopathies, which is considered to be the major driving force behind the pathological metamorphosis of brain IDPs. The aim of the review is to emphasize the key role of the protein truncation in the pathogenic pathways of neurodegenerative diseases. A deeper understanding of the complex downstream processing of the IDPs, resulting in the generation of pathologically modified proteins might be a prerequisite for the successful therapeutic strategies of several fatal neurodegenerative diseases.

zVLL-CHO at low concentrations acts as a calpain inhibitor to protect neurons against okadaic acid-induced neurodegeneration

There is evidence that β-secretase and amyloid precursor protein β-C-terminal fragments (APP-CTF) are involved in the pathogenesis of Alzheimer's disease (AD). Previously, we have reported that N-benzyloxycarbonyl-Val-Leu-leucinal (zVLL-CHO) reduced APP β-CTF accumulation in axonal swellings of degenerating neurons. Here, in an effort to discover more effective neuroprotective agents, we examined the effects of the β-secretase inhibitors, H-KTEEISEVN-stat-VAEF-OH (VAEF) and H-EVNstatineVAEF-NH2 (GL-189) as well as zVLL-CHO on OA (okadaic acid)-induced neurodegeneration. Unexpectedly, we found that pretreatment with zVLL-CHO (1 μM) protected neurons after OA treatment, whereas both VAEF and GL-189 lacked neuroprotective effects. Interestingly, 1 μM zVLL-CHO did not inhibit β-secretase. We previously reported that calpain is activated by OA treatment and calpain inhibitors protect against OA-induced neurodegeneration. The data presented here show that pretreatment with 1 μM zVLL-CHO decreased the levels of calpain-cleaved α-spectrin with a concomitant decrease in LDH release and an increase in average dendritic branch length compared to neurons treated with OA alone. These findings suggest that zVLL-CHO protects against OA-induced neurodegeneration via calpain inactivation.

Targets for AD treatment: conflicting messages from γ-secretase inhibitors

Current evidence suggests that Alzheimer's disease (AD) is a multi-factorial disease that starts with accumulation of multiple proteins. We have previously proposed that inhibition of γ-secretase may impair membrane recycling causing neurodegeneration starting at synapses (Sambamurti K., Suram A., Venugopal C., Prakasam A., Zhou Y., Lahiri D. K. and Greig N. H. A partial failure of membrane protein turnover may cause Alzheimer's disease: a new hypothesis. Curr. Alzheimer Res., 3, 2006, 81). We also proposed familal AD mutations increase Aβ42 by inhibiting γ-secretase. Herein, we discuss the failure of Eli Lilly's γ-secretase inhibitor, semagacestat, in clinical trials in the light of our hypothesis, which extends the problem beyond toxicity of Aβ aggregates. We elaborate that γ-secretase inhibitors lead to accumulation of amyloid precursor protein C-terminal fragments that can later be processed by γ-secretase to yields bursts of Aβ to facilitate aggregation. Although we do not exclude a role for toxic Aβ aggregates, inhibition of γ-secretase can affect numerous substrates other than amyloid precursor protein to affect multiple pathways and the combined accumulation of multiple peptides in the membrane may impair its function and turnover. Taken together, protein processing and turnover pathways play an important role in maintaining cellular homeostasis and unless we clearly see consistent disease-related increase in their levels or activity, we need to focus on preserving their function rather than inhibiting them for treatment of AD and similar diseases.

Secretases as therapeutic targets for Alzheimer's disease

Accumulation of amyloid-β (Aβ) is widely accepted as the key instigator of Alzheimer's disease (AD). The proposed mechanism is that accumulation of Aβ results in inflammatory responses, oxidative damages, neurofibrillary tangles and, subsequently, neuronal/synaptic dysfunction and neuronal loss. Given the critical role of Aβ in the disease process, the proteases that produce this peptide are obvious targets. The goal would be to develop drugs that can inhibit the activity of these targets. Protease inhibitors have proved very effective for treating other disorders such as AIDS and hypertension. Mutations in APP (amyloid-β precursor protein), which flanks the Aβ sequence, cause early-onset familial AD, and evidence has pointed to the APP-to-Aβ conversion as a possible therapeutic target. Therapies aimed at modifying Aβ-related processes aim higher up the cascade and are therefore more likely to be able to alter the progression of the disease. However, it is not yet fully known whether the increases in Aβ levels are merely a result of earlier events that were already causing the disease.

Formation and participation of nano-amyloids in pathogenesis of Alzheimer's disease and other amyloidogenic diseases

Studies of neurodegenetrative disorders have become particularly actual attracting the attention of researchers from over the world because of the dissemination of Alzheimer's disease. The reason for such pathogenesis is the transition of a "healthy" molecule or peptide from the native conformation into a very stable "pathological" isoform. During this, molecules in the "pathological" conformation aggregate, forming amyloid fibrils that can increase without any control. Novel knowledge is required on sporadic isoforms of Alzheimer's disease, on the nature of triggering mechanisms of conformational transitions of beta-amyloid fragments from normally functioning proteins into new formations - nano-beta-amyloids - that spiral out of control of neurons and organism which leads to the loss of neurons. Herein we review studies devoted to the formation of amyloid fibrils and their role in pathogenesis of amyloid diseases.

BACE and gamma-secretase characterization and their sorting as therapeutic targets to reduce amyloidogenesis

Secretases are named for enzymes processing amyloid precursor protein (APP), a prototypic type-1 membrane protein. This led directly to discovery of novel Aspartyl proteases (beta-secretases or BACE), a tetramer complex gamma-secretase (gamma-SC) containing presenilins, nicastrin, aph-1 and pen-2, and a new role for metalloprotease(s) of the ADAM family as a alpha-secretases. Recent advances in defining pathways that mediate endosomal-lysosomal-autophagic-exosomal trafficking now provide targets for new drugs to attenuate abnormal production of fibril forming products characteristic of AD. A key to success includes not only characterization of relevant secretases but mechanisms for sorting and transport of key metabolites to abnormal vesicles or sites for assembly of fibrils. New developments we highlight include an important role for an 'early recycling endosome' coated in retromer complex containing lipoprotein receptor LRP-II (SorLA) for switching APP to a non-amyloidogenic pathway for alpha-secretases processing, or to shuttle APP to a 'late endosome compartment' to form Abeta or AICD. LRP11 (SorLA) is of particular importance since it decreases in sporadic AD whose etiology otherwise is unknown.

Cathepsins B and L differentially regulate amyloid precursor protein processing

Previous studies have shown that cathepsins control amyloid beta (Abeta) levels in chromaffin cells via a regulated secretory pathway. In the present study, this concept was extended to investigations in primary hippocampal neurons to test whether Abeta release was coregulated by cathepsins and electrical activity, proposed components of a regulated secretory pathway. Inhibition of cathepsin B (catB) activity with CA074Me or attenuation of catB expression through small interfering RNA produced decreases in Abeta release, similar to levels produced with suppression of beta-site APP-cleaving enzyme 1 (BACE1) expression. To test whether the catB-dependent release of Abeta was linked to ongoing electrical activity, neurons were treated with tetrodotoxin (TTX) and CA074Me. These comparisons demonstrated no additivity between decreases in Abeta release produced by TTX and CA074Me. In contrast, pharmacological inhibition of cathepsin L (catL) selectively elevated Abeta42 levels but not Abeta40 or total Abeta. Mechanistic studies measuring C-terminal fragments of amyloid precursor protein (APP) suggested that catL elevated alpha-secretase activity, thereby suppressing Abeta42 levels. The mechanism of catB-mediated regulation of Abeta release remains unclear but may involve elevation of beta-secretase. In summary, these studies provide evidence for a significant alternative pathway for APP processing that involves catB and activity-dependent release of Abeta in a regulated secretory pathway for primary neurons.

Neurosecretases provide strategies to treat sporadic and familial Alzheimer disorders

Recent discoveries on neurosecretases and their trafficking to release fibril-forming neuropeptides or other products, are of interest to pathology, cell signaling and drug discovery. Nomenclature arose from the use of amyloid precursor protein (APP) as a prototypic type-1 substrate leading to the isolation of beta-secretase (BACE), multimeric complexes (gamma-secretase, gamma-SC) for intramembranal cleavage, and attributing a new function to well-characterized metalloproteases of the ADAM family (alpha-secretase) for normal APP turnover. While purified alpha/beta-secretases facilitate drug discovery, gamma-SC presents greater challenges for characterization and mechanisms of catalysis. The review comments on links between mutation or polymorphisms in relation to enzyme mechanisms and disease. The association between lipoprotein receptor LRP11 variants and sporadic Alzheimer's disease (SAD) offers scope to integrate components of pre- and post-Golgi membranes, or brain clathrin-coated vesicles within pathways for trafficking as targets for intervention. The presence of APP and metabolites in brain clathrin-coated vesicles as significant cargo with lipoproteins and adaptors focuses attention as targets for therapeutic intervention. This overview emphasizes the importance to develop new therapies targeting neurosecretases to treat a major neurological disorder that has vast economic and social implications.

Novel processing of Notch 1 within its intracellular domain by a cysteine protease

In order to study N1 processing, we expressed human N1 (hN1) in HEK293 cells (293-hN1). Following Western blot analysis of 293-hN1 extracts, we detected, in addition to full-length hN1 and the N1 extracellular domain truncated form (N1-TM), a novel extracellular domain truncated form of hN1 with a COOH-terminal deletion, designated hN1-TMdeltaCT. Treatment of cells with the gamma-secretase inhibitor L-685,458 resulted in an accumulation of hN1-TMdeltaCT suggesting that this fragment is a gamma-secretase substrate. To identify the proteolytic activity(ies) that generates hN1-TMdeltaCT, we treated 293-hN1 cells with inhibitors of proteasome, calpains, caspases, serine and cysteine proteases. Despite the presence of a caspase-3 cleavage site within hN1 intracellular domain, none of the caspase inhibitors inhibited hN1-TMdeltaCT production. The proteasomal inhibitors used had also no effect. Incubation of cells with the cysteine protease inhibitor E64d resulted in the accumulation of hN1-TM and the inhibition of hN1-TMdeltaCT production suggesting a precursor-product relationship and that a cysteine protease is involved. Similarly, treatment of cells expressing amyloid precursor protein or E-cadherin with E-64d resulted in the accumulation of COOH-terminal fragments suggesting that these proteins are also processed within their intracellular domain by a cysteine protease. Processing towards hN1-TMdeltaCT requires maturation and transport of hN1 to the cell surface since treatment with brefeldin A inhibited its production and resulted in accumulation of hN1. Processing of hN1 within its intracellular domain could generate fragments that can exert novel functions and/or interfere with the function of hN1 intracellular domain.

Inhibition of gamma-secretase as a therapeutic intervention for Alzheimer's disease: prospects, limitations and strategies

Genetic and experimental evidence points to amyloid-beta (Abeta) peptide as the culprit in Alzheimer's disease pathogenesis. This protein fragment abnormally accumulates in the brain cortex and hippocampus of patients with Alzheimer's disease, and self-aggregates to form toxic oligomers causing neurodegeneration.Abeta is heterogeneous and produced from a precursor protein (amyloid precursor protein [APP]) by two sequential proteolytic cleavages that involve beta- and gamma-secretases. This latter enzyme represents a potentially attractive drug target since it dictates the solubility of the generated Abeta fragment by creating peptides of various lengths, namely Abeta(40) and Abeta(42), the longest being the most aggregating. gamma-Secretase comprises a molecular complex of four integral membrane proteins - presenilin, nicastrin, APH-1 and PEN-2 - and its molecular mechanism remains under extensive scrutiny. The ratio of Abeta(42) over Abeta(40) is increased by familial Alzheimer's disease mutations occurring in the presenilin genes or in APP, near the gamma-secretase cleavage site. Potent gamma-secretase inhibitors have been identified by screening drug libraries or by designing aspartyl protease transition-state analogues based on the APP substrate cleavage site. Most of these compounds are not specific for gamma-secretase cleavage of APP, and equally inhibit the processing of other gamma-secretase substrates, such as Notch and a subset of cell-surface receptors and proteins involved in embryonic development, haematopoiesis, cell adhesion and cell/cell contacts. Therefore, current research aims at finding compounds that show selectivity for APP cleavage, and particularly that inhibit the formation of the aggregating form, Abeta(42). Compounds that target the substrate docking site rather than the enzyme active site are also being investigated as an alternative strategy. The finding that some NSAID analogues preferentially inhibit the formation of Abeta(42) over Abeta(40) and do not affect Notch processing has opened a new therapeutic window. The progress in design of selective inhibitors as well as recent results obtained in animal studies prove that gamma-secretase remains among the best targets for the therapeutic control of amyloid build-up in Alzheimer's disease. The full understanding of gamma-secretase regulation may yet uncover new therapeutic leads.

Neuronal dysfunction in Down syndrome: contribution of neuronal models in cell culture

Down syndrome (DS) in humans, or trisomy of autosome 21, represents the hyperdiploidy that most frequently survives gestation, reaching an incidence of 1 in 700 live births. The condition is associated with multisystemic anomalies, including those affecting the central nervous system (CNS), determining a characteristic mental retardation. At a neuronal level, our group and others have shown that the condition determines marked alterations of action potential and ionic current kinetics, which may underlie abnormal processing of information by the CNS. Since the use of human tissue presents both practical and ethical problems, animal models of the human condition have been sought. Murine trisomy 16 (Ts16) is a model of the human condition, due to the great homology between human autosome 21 and murine 16. Both conditions share the same alterations of electrical membrane properties. However, the murine Ts16 condition is unviable (animals die in utero), thus limiting the quantity of tissue procurable. To overcome this obstacle, we have established immortal cell lines from normal and Ts16 mice with a method developed by our group that allows the stable in vitro immortalization of mammalian tissue, yielding cell lines which retain the characteristics of the originating cells. Cell lines derived from cerebral cortex, hippocampus, spinal cord and dorsal root ganglion of Ts16 animals show alterations of intracellular Ca2+ signals in response to several neurotransmitters (glutamate, acetylcholine, and GABA). Gene overdose most likely underlies these alterations in cell function, and the identification of the relative contribution of DS associated genes on such specific neuronal dysfunction should be investigated. This could enlighten our understanding on the contribution of these genes in DS, and identify new therapeutic targets.

Calpain inhibitor MDL28170 modulates Abeta formation by inhibiting the formation of intermediate Abeta46 and protecting Abeta from degradation

The observations that three major cleavages within the transmembrane domain of APP, namely, the gamma-cleavage, -cleavage, and the newly identified zeta-cleavage, are involved in the generation of secreted Abeta40 and Abeta42 prompted us to determine how the calpain inhibitor III MDL 28170 influences these three cleavages and Abeta formation. With the use of a cell culture system, our data demonstrate that 1) at either high concentrations, or at a low range of concentrations, at early time points, MDL 28170 inhibits the formation of secreted Abeta40 and Abeta42. However, this effect is due to inhibition of the intermediate Abeta46 generation by zeta-cleavage and not due to direct inhibition of the gamma-cleavage that produces Abeta40/42 from Abeta46; 2) at low range of concentrations and at late time points, MDL 28170 causes an increase in secreted Abeta40/42 that likely results from inhibition of degradation of both the initial substrate, CTFbeta, and the final product, Abeta40/42, of gamma-secretase. These data strongly suggest that formation of Abeta46 is a key step in the gamma-secretase mediated generation of Abeta40/42 and provide a new target for the development of Abeta inhibitors. These data also suggest that calpain and related proteases, which are sensitive to MDL 28170, play an important role in the accumulation of secreted Abeta.

Expression of tau reduces secretion of Aβ without altering the amyloid precursor protein content in CHOsw cells

Insoluble deposits of tau and amyloid precursor protein (APP) peptides Abeta characterize Alzheimer's disease. We studied the role of tau in the metabolism of APP in cells stably expressing APP Swedish mutation (CHOsw). Transient expression of tau in CHOsw cells caused morphological changes, bundling of microtubules and perinuclear aggregation of Golgi-derived vesicles. It also reduced the secretion of Abeta(1-40) and Abeta(1-42) without altering the APP steady state levels. This was accompanied by a reduction in the gamma-secretase and an increase in the insulin degrading enzyme activities. Our results suggest that tau may play an inhibitory role in the amyloidogenic activity of APP.

Obesity‐related leptin regulates Alzheimer's Aβ

Abeta peptide is the major proteinateous component of the amyloid plaques found in the brains of Alzheimer's disease (AD) patients and is regarded by many as the culprit of the disorder. It is well documented that brain lipids are intricately involved in Abeta-related pathogenic pathways. An important modulator of lipid homeostasis is the pluripotent peptide leptin. Here we demonstrate leptin's ability to modify Abeta levels in vitro and in vivo. Similar to methyl-beta-cyclodextrin, leptin reduces beta-secretase activity in neuronal cells possibly by altering the lipid composition of membrane lipid rafts. This phenotype contrasts treatments with cholesterol and etomoxir, an inhibitor of carnitine-palmitoyl transferase-1. Conversely, inhibitors of acetyl CoA carboxylase and fatty acid synthase mimicked leptin's action. Leptin was also able to increase apoE-dependent Abeta uptake in vitro. Thus, leptin can modulate bidirectional Abeta kinesis, reducing its levels extracellularly. Most strikingly, chronic administration of leptin to AD-transgenic animals reduced the brain Abeta load, underlying its therapeutic potential.

Presenilin-1, nicastrin, amyloid precursor protein, and gamma-secretase activity are co-localized in the lysosomal membrane

Alzheimer's disease (AD) is caused by the cerebral deposition of beta-amyloid (Abeta), a 38-43-amino acid peptide derived by proteolytic cleavage of the amyloid precursor protein (APP). Initial studies indicated that final cleavage of APP by the gamma-secretase (a complex containing presenilin and nicastrin) to produce Abeta occurred in the endosomal/lysosomal system. However, other studies showing a predominant endoplasmic reticulum localization of the gamma-secretase proteins and a neutral pH optimum of in vitro gamma-secretase assays have challenged this conclusion. We have recently identified nicastrin as a major lysosomal membrane protein. In the present work, we use Western blotting and immunogold electron microscopy to demonstrate that significant amounts of mature nicastrin, presenilin-1, and APP are co-localized with lysosomal associated membrane protein-1 (cAMP-1) in the outer membranes of lysosomes. Furthermore, we demonstrate that these membranes contain an acidic gamma-secretase activity, which is immunoprecipitable with an antibody to nicastrin. These experiments establish APP, nicastrin, and presenilin-1 as resident lysosomal membrane proteins and indicate that gamma-secretase is a lysosomal protease. These data reassert the importance of the lysosomal/endosomal system in the generation of Abeta and suggest a role for lysosomes in the pathophysiology of AD.

Partial purification and characterization of gamma-secretase from post-mortem human brain

One characteristic feature of Alzheimer's disease is the deposition of amyloid beta-peptide (Abeta) as amyloid plaques within specific regions of the human brain. Abeta is derived from the amyloid beta-peptide precursor protein (beta-APP) by the intramembranous cleavage activity of gamma-secretase. Studies in cells have revealed that gamma-secretase is a large multimeric membrane-bound protein complex that is functionally dependent on several proteins, including presenilin, nicastrin, Aph-1, and Pen-2. However, the precise biochemical and molecular nature of gamma-secretase is as yet to be fully elucidated, and no investigations have analyzed gamma-secretase in human brain. To address this we have developed a novel in vitro gamma-secretase activity assay using detergent-solubilized cell membranes and a beta-APP-derived fluorescent probe. We report that human brain-derived gamma-secretase activity co-purifies with a high molecular weight protein complex comprising presenilin, nicastrin, Aph-1, and Pen-2. The inhibitor profile and solubility characteristics of brain-derived gamma-secretase are similar to those described in cells, and proteolysis occurs at the Abeta40- and Abeta42-generating cleavage sites. The ability to isolate gamma-secretase from post-mortem human brain may facilitate the identification of brain-specific modulators of beta-APP processing and provide new insights into the biology of this important factor in the pathogenesis of Alzheimer's disease.

The calpain system

The calpain system originally comprised three molecules: two Ca2+-dependent proteases, mu-calpain and m-calpain, and a third polypeptide, calpastatin, whose only known function is to inhibit the two calpains. Both mu- and m-calpain are heterodimers containing an identical 28-kDa subunit and an 80-kDa subunit that shares 55-65% sequence homology between the two proteases. The crystallographic structure of m-calpain reveals six "domains" in the 80-kDa subunit: 1). a 19-amino acid NH2-terminal sequence; 2). and 3). two domains that constitute the active site, IIa and IIb; 4). domain III; 5). an 18-amino acid extended sequence linking domain III to domain IV; and 6). domain IV, which resembles the penta EF-hand family of polypeptides. The single calpastatin gene can produce eight or more calpastatin polypeptides ranging from 17 to 85 kDa by use of different promoters and alternative splicing events. The physiological significance of these different calpastatins is unclear, although all bind to three different places on the calpain molecule; binding to at least two of the sites is Ca2+ dependent. Since 1989, cDNA cloning has identified 12 additional mRNAs in mammals that encode polypeptides homologous to domains IIa and IIb of the 80-kDa subunit of mu- and m-calpain, and calpain-like mRNAs have been identified in other organisms. The molecules encoded by these mRNAs have not been isolated, so little is known about their properties. How calpain activity is regulated in cells is still unclear, but the calpains ostensibly participate in a variety of cellular processes including remodeling of cytoskeletal/membrane attachments, different signal transduction pathways, and apoptosis. Deregulated calpain activity following loss of Ca2+ homeostasis results in tissue damage in response to events such as myocardial infarcts, stroke, and brain trauma.

Microtubular interactions of presenilin direct kinesis of Abeta peptide and its precursors

In our previous study we demonstrated that presenilin 1 (PS1) interacts with cytoplasmic linker protein 170/Restin (CLIP-170/Restin). Herein we show that disruption of the interaction of these proteins within neuronal cell-lines (SY5Y and N2a) can be accomplished by the transfection of vectors that drive the expression of peptide fragments corresponding to their binding domains (BDPs). Interestingly, the disruption of the PS1/CLIP-170 complex is associated with both decreased secretion of endogenous Abeta and decreased uptake of exogenous Abeta from the medium. BDP-expressing cells were also more resistant to surges of Abeta secretion induced by thapsigargin and ionomycin (that elevate intracellular calcium concentrations) and mutations in PS1 linked to familial Alzheimer's disease. Uptake of Abeta by SY5Y cells was amplified when preincubated with ApoE and was mediated through lipoprotein receptor-related protein (LRP). BDP-expressing cells or cells treated with PS1 anti-sense oligonucleotides were less capable of taking up Abeta from the medium compared with controls, indicating that the PS1/CLIP-170 interaction is involved and that PS1 cannot be substituted. In this study, we also mapped the minimum binding domains (mBDPs) of PS1 and CLIP-70 to regions corresponding to the N-terminal end of the large cytoplasmic loop of PS1 and the metal binding motif-containing C-terminal end of CLIP-170. Further, our data obtained from experiments involving in vitro taxol-polymerization of tubulin and confocal immunofluorescence suggest that PS1, via CLIP-170, may serve as an anchor to the microtubules for specific subcellular fractions containing amyloidogenic fragments. Interestingly, Notch is absent from this population of microtubule binding subcellular fractions and its cleavage was unaffected in cells transfected with the PS1-based BDP. This raises the possibility that the interaction of PS1 with CLIP-170 could provide the conceptual basis for anti-amyloidogenic therapeutic strategies with improved specificity. However, this approach may be hampered by a low efficiency, because it may also block Abeta clearance from the interstitial space of the CNS.

Cystatin C colocalizes with amyloid-beta and coimmunoprecipitates with amyloid-beta precursor protein in sporadic inclusion-body myositis muscles

Cystatin C (CC), an endogenous cysteine protease inhibitor, is accumulated within amyloid-beta (A beta) amyloid deposits in Alzheimer's disease (AD) brain and was proposed to play a role in the AD pathogenesis. Because the chemo-morphologic muscle phenotype of sporadic inclusion-body myositis (s-IBM) has several similarities with the phenotype of AD brain, including abnormal accumulation of A beta deposits, we studied expression and localization of CC in muscle biopsies of 10 s-IBM, and 16 disease- and five normal-control muscle biopsies. Physical interaction of CC with amyloid-beta precursor protein (A beta PP) was studied by a combined immunoprecipitation/immunoblotting technique in the s-IBM muscle biopsies and in A beta PP-overexpressing cultured human muscle fibers. In all s-IBM muscle biopsies, CC-immunoreactivity either colocalized with, or was adjacent to, the A beta-immunoreactive inclusions in 80-90% of the vacuolated muscle fibers, mostly in non-vacuolated regions of their cytoplasm. Ultrastructurally, CC immunoreactivity-colocalized with A beta on 6-10 nm amyloid-like fibrils and floccular material. By immunoblotting, CC expression was strongly increased in IBM muscle as compared to the controls. By immunoprecipitation/immunoblotting experiments, CC coimmunoprecipitated with A beta PP, both in s-IBM muscle and in A beta PP-overexpressing cultured normal human muscle fibers. Our studies (i) demonstrate for the first time that CC physically associates with A beta PP, and (ii) suggest that CC may play a novel role in the s-IBM pathogenesis, possibly by influencing A beta PP processing and A beta deposition.

Gamma-secretase inhibitors and Alzheimer's disease

The existence of pathogenic mutations in beta-APP and the presenilin genes provides strong support for the hypothesis that Abeta production and deposition contribute to the etiology of Alzheimer's disease (AD). The heterogeneous carboxyl termini of Abeta molecules deposited in the hippocampus, cortex and cerebrovasculature of AD patients are generated by gamma-secretase. The gamma-secretase that generates the termini in vivo is a complex of proteins containing presenilin as an integral component. Drugs that modulate the production of Abeta by inhibiting gamma-secretase could provide an effective therapy for AD, but like most disease targets, the gamma-secretase appears to have more than a single function. The use of potent inhibitors has aided the discovery and characterization of gamma-secretase functions and reinforced the concept that a successful drug must demonstrate selectivity for lowering Abeta without disrupting the function of gamma-secretase targets. The discovery of drugs that can selectively inhibit beta-APP cleavage is an important objective. This review focuses on studies that enhance our understanding of the effects of inhibiting gamma-secretase and provide direction for developing effective and selective gamma-secretase inhibitors as drugs to treat AD.

N-acetylcysteine and celecoxib lessen cadmium cytotoxicity which is associated with cyclooxygenase-2 up-regulation in mouse neuronal cells

In many neurodegenerative disorders, aggregates of ubiquitinated proteins are detected in neuronal inclusions, but their role in neurodegeneration remains to be defined. To identify intracellular mechanisms associated with the appearance of ubiquitin-protein aggregates, mouse neuronal HT4 cells were treated with cadmium. This heavy metal is a potent cell poison that mediates oxidative stress and disrupts the ubiquitin/proteasome pathway. In the current studies, the following intracellular events were found to be also induced by cadmium: (i) a specific rise in cyclooxygenase-2 (COX-2) gene expression but not COX-1; (ii) an increase in the extracellular levels of the proinflammatory prostaglandin E2, a product of COX-2; and (iii) production of 4-hydroxy-2-nonenal-protein adducts, which result from lipid peroxidation. In addition, cadmium treatment led to the accumulation of high molecular weight ubiquitin-COX-2 conjugates and perturbed COX-2 glycosylation. The thiol-reducing antioxidant N-acetylcysteine, and, to a lesser extent, the COX-2 inhibitor celecoxib, attenuated the loss of cell viability induced by cadmium demonstrating that oxidative stress and COX-2 activation contribute to cadmium cytotoxicity. These findings establish that disruption of the ubiquitin/proteasome pathway is not the only event triggered by cadmium. This oxidative stressor also activates COX-2 function. Both events could be triggered by formation of 4-hydroxy-2-nonenal as a result of cadmium-induced lipid peroxidation. Proinflammatory responses stimulated by oxidative stressors that mimic the cadmium effects may, therefore, be important initiators of the neurodegenerative process and exacerbate its progress.

A novel mechanism for the regulation of amyloid precursor protein metabolism

Modifier of cell adhesion protein (MOCA; previously called presenilin [PS] binding protein) is a DOCK180-related molecule, which interacts with PS1 and PS2, is localized to brain areas involved in Alzheimer's disease (AD) pathology, and is lost from the soluble fraction of sporadic Alzheimer's disease (AD) brains. Because PS1 has been associated with gamma-secretase activity, MOCA may be involved in the regulation of beta-amyloid precursor protein (APP) processing. Here we show that the expression of MOCA decreases both APP and amyloid beta-peptide secretion and lowers the rate of cell-substratum adhesion. In contrast, MOCA does not lower the secretion of amyloid precursor-like protein (APLP) or several additional type 1 membrane proteins. The phenotypic changes caused by MOCA are due to an acceleration in the rate of intracellular APP degradation. The effect of MOCA expression on the secretion of APP and cellular adhesion is reversed by proteasome inhibitors, suggesting that MOCA directs nascent APP to proteasomes for destruction. It is concluded that MOCA plays a major role in APP metabolism and that the effect of MOCA on APP secretion and cell adhesion is a downstream consequence of MOCA-directed APP catabolism. This is a new mechanism by which the expression of APP is regulated.

Advances in the cellular and molecular biology of the beta-amyloid protein in Alzheimer's disease

Alzheimer's disease (AD) is a progressive senile dementia characterized by deposition of a 4 kDa peptide of 39-42 residues known as amyloid beta-peptide (Abeta) in the form of senile plaques and the microtubule associated protein tau as paired helical filaments. Genetic studies have identified mutations in the Abeta precursor protein (APP) as the key triggers for the pathogenesis of AD. Other genes such as presenilins 1 and 2 (PS1/2) and apolipoprotein E (APOE) also play a critical role in increased Abeta deposition. Several biochemical and molecular studies using transfected cells and transgenic animals point to mechanisms by which Abeta is generated and aggregated to trigger the neurodegeneration that may cause AD. Three important enzymes collectively known as "secretases" participate in APP processing. An enzymatic activity, beta-secretase, cleaves APP on the amino side of Abeta producing a large secreted derivative, sAPPbeta, and an Abeta-bearing membrane-associated C-terminal derivative, CTFbeta, which is subsequently cleaved by the second activity, gamma-secretase, to release Abeta. Alternatively, a third activity, alpha-secretase, cleaves APP within Abeta to the secreted derivative sAPPalpha and membrane-associated CTFalpha. The predominant secreted APP derivative is sAPPalpha in most cell-types. Most of the secreted Abeta is 40 residues long (Abeta40) although a small percentage is 42 residues in length (Abeta42). However, the longer Abeta42 aggregates more readily and was therefore considered to be the pathologically important form. Advances in our understanding of APP processing, trafficking, and turnover will pave the way for better drug discovery for the eventual treatment of AD. In addition, APP gene regulation and its interaction with other proteins may provide useful drug targets for AD. The emerging knowledge related to the normal function of APP will help in determining whether or not the AD associated changes in APP metabolism affect its function. The present review summarizes our current understanding of APP metabolism and function and their relationship to other proteins involved in AD.

Ageing research in Greece

Ageing research in Greece is well established. Research groups located in universities, research institutes or public hospitals are studying various and complementary aspects of ageing. These research activities include (a) functional analysis of Clusterin/Apolipoprotein J, studies in healthy centenarians and work on protein degradation and the role of proteasome during senescence at the National Hellenic Research Foundation; (b) regulation of cell proliferation and tissue formation, a nationwide study of determinants and markers of successful ageing in Greek centenarians and studies of histone gene expression and acetylation at the National Center for Scientific Research, Demokritos; (c) work on amyloid precursor protein and Presenilin 1 at the University of Athens; (d) oxidative stress-induced DNA damage and the role of oncogenes in senescence at the University of Ioannina; (e) studies in the connective tissue at the University of Patras; (f) proteomic studies at the Biomedical Sciences Research Center Alexander Fleming; (g) work on Caenorhabditis elegans at the Foundation for Research and Technology; (h) the role of ultraviolet radiation in skin ageing at Andreas Sygros Hospital; (i) follow-up studies in healthy elderly at the Athens Home for the Aged; and (j) socio-cultural aspects of ageing at the National School of Public Health. These research activities are well recognized by the international scientific community as it is evident by the group's very good publication records as well as by their direct funding from both European Union and USA. This article summarizes these research activities and discuss future directions and efforts towards the further development of the ageing field in Greece.

Secretases as therapeutic targets for the treatment of Alzheimer's disease

The extracellular deposition of short amyloid peptides in the brain of patients is thought to be a central event in the pathogenesis of Alzheimer's Disease. The generation of the amyloid peptide occurs via a regulated cascade of cleavage events in its precursor protein, A beta PP. At least three enzymes are responsible for A beta PP proteolysis and have been tentatively named alpha-, beta- and gamma-secretases. The recent identification of several of these secretases is a major leap in the understanding how these secretases regulate amyloid peptide formation. Members of the ADAM family of metalloproteases are involved in the non-amyloidogenic alpha-secretase pathway. The amyloidogenic counterpart pathway is initiated by the recently cloned novel aspartate protease named BACE. The available data are conclusive and crown BACE as the long-sought beta-secretase. This enzyme is a prime candidate drug target for the development of therapy aiming to lower the amyloid burden in the disease. Finally, the gamma-secretases are intimately linked to the function of the presenilins. These multi-transmembrane domain proteins remain intriguing study objects. The hypothesis that the presenilins constitute a complete novel type of protease family, and are cleaving A beta PP within the transmembrane region, remains an issue of debate. Several questions remain unanswered and direct proof that they exert catalytic activity is still lacking. The subcellular localization of presenilins in neurons, their integration in functional multiprotein complexes and the recent identification of additional modulators of gamma-secretase, like nicastrin, indicate already that several players are involved. Nevertheless, the rapidly increasing knowledge in this area is already paving the road towards selective inhibitors of this secretase as well. It is hoped that such drugs, possibly in concert with the experimental vaccination therapies that are currently tested, will lead to a cure of this inexorable disease.

Functional gamma-secretase inhibitors reduce beta-amyloid peptide levels in brain

Converging lines of evidence implicate the beta-amyloid peptide (Ass) as causative in Alzheimer's disease. We describe a novel class of compounds that reduce A beta production by functionally inhibiting gamma-secretase, the activity responsible for the carboxy-terminal cleavage required for A beta production. These molecules are active in both 293 HEK cells and neuronal cultures, and exert their effect upon A beta production without affecting protein secretion, most notably in the secreted forms of the amyloid precursor protein (APP). Oral administration of one of these compounds, N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester, to mice transgenic for human APP(V717F) reduces brain levels of Ass in a dose-dependent manner within 3 h. These studies represent the first demonstration of a reduction of brain A beta in vivo. Development of such novel functional gamma-secretase inhibitors will enable a clinical examination of the A beta hypothesis that Ass peptide drives the neuropathology observed in Alzheimer's disease.

The pathogenic activation of calpain: a marker and mediator of cellular toxicity and disease states

Over-activation of calpain, a ubiquitous calcium-sensitive protease, has been linked to a variety of degenerative conditions in the brain and several other tissues. Dozens of substrates for calpain have been identified and several of these have been used to measure activation of the protease in the context of experimentally induced and naturally occurring pathologies. Calpain-mediated cleavage of the cytoskeletal protein spectrin, in particular, results in a set of large breakdown products (BDPs) that are unique in that they are unusually stable. Over the last 15 years, measurements of BDPs in experimental models of stroke-type excitotoxicity, hypoxia/ischemia, vasospasm, epilepsy, toxin exposure, brain injury, kidney malfunction, and genetic defects, have established that calpain activation is an early and causal event in the degeneration that ensues from acute, definable insults. The BDPs also have been found to increase with normal ageing and in patients with Alzheimer's disease, and the calpain activity may be involved in related apoptotic processes in conjunction with the caspase family of proteases. Thus, it has become increasingly clear that regardless of the mode of disturbance in calcium homeostasis or the cell type involved, calpain is critical to the development of pathology and therefore a distinct and powerful therapeutic target. The recent development of antibodies that recognize the site at which spectrin is cleaved has greatly facilitated the temporal and spatial resolution of calpain activation in situ. Accordingly, sensitive spectrin breakdown assays now are utilized to identify potential toxic side-effects of compounds and to develop calpain inhibitors for a wide range of indications including stroke, cerebral vasospasm, and kidney failure.

Proteasome inhibition in neuronal cells induces a proinflammatory response manifested by upregulation of cyclooxygenase-2, its accumulation as ubiquitin conjugates, and production of the prostaglandin PGE(2)

Inclusions containing ubiquitin-protein aggregates appear in neurons of patients with neurodegenerative disorders such as Alzheimer's disease and Parkinson's disease. The relationship between inclusion production and cell viability is not understood. To address this issue, we investigated the response of an established mouse neuronal cell line and of embryonic rat mesencephalic cultures to inhibition of the ubiquitin/proteasome pathway. Two proteasome inhibitors, a peptidyl aldehyde and an epoxy ketone, which cause accumulation of ubiquitinated proteins, were found to enhance expression of stress-inducible genes, including HSP70i and the polyubiquitin genes UbB and UbC. Under these conditions, mRNA and protein levels of the inducible form of cyclooxygenase (COX-2) were upregulated together with its product, PGE(2), a proinflammatory prostaglandin. Proteasomal inhibition also led to stabilization of COX-2 as ubiquitin conjugates, suggesting that the ubiquitin/proteasome pathway contributes to the regulation of COX-2 protein levels. Treatment with antioxidants known to inhibit NFkappaB and AP-1 transcriptional activation failed to abrogate COX-2 upregulation. Instead, these inhibitors exacerbated the stress response by potentiating HSP70i levels while eliciting a decrease in PGE(2) production. These findings suggest that the accumulation of ubiquitinated proteins resulting from proteasome inhibition in neuronal cells is associated with a proinflammatory response that may be an important contributor to neurodegeneration.

Altered binding of mutated presenilin with cytoskeleton-interacting proteins

The majority of familial Alzheimer's disease (AD) cases are linked to mutations on presenilin 1 and 2 genes (PS1 and PS2). The normal function of the proteins and the mechanisms underlying early-onset AD are currently unknown. To address this, we screened an expression library for proteins that bind differentially to the wild-type PS1 and mutant in the large cytoplasmic loop (PS1L). Thus we isolated the C-terminal tail of the 170 kDa cytoplasmic linker protein (CLIP-170) and Reed-Sternberg cells of Hodgkin's disease-expressed intermediate filament-associated protein (Restin), cytoplasmic proteins linking vesicles to the cytoskeleton. PS1L binding to CLIP-170/restin requires Ca(2+). Treating cells with thapsigargin or ionomycin increased the mutated PS1 in CLIP-170 immunoprecipitates. Further, PS1 and CLIP-170 co-localize in transfected cells and neuronal cultures.