Georgia Dolios

Aβ42 Is Essential for Parenchymal and Vascular Amyloid Deposition in Mice

Considerable circumstantial evidence suggests that Abeta42 is the initiating molecule in Alzheimers disease (AD) pathogenesis. However, the absolute requirement for Abeta42 for amyloid deposition has never been demonstrated in vivo. We have addressed this by developing transgenic models that express Abeta1-40 or Abeta1-42 in the absence of human amyloid beta protein precursor (APP) overexpression. Mice expressing high levels of Abeta1-40 do not develop overt amyloid pathology. In contrast, mice expressing lower levels of Abeta1-42 accumulate insoluble Abeta1-42 and develop compact amyloid plaques, congophilic amyloid angiopathy (CAA), and diffuse Abeta deposits. When mice expressing Abeta1-42 are crossed with mutant APP (Tg2576) mice, there is also a massive increase in amyloid deposition. These data establish that Abeta1-42 is essential for amyloid deposition in the parenchyma and also in vessels.

Longer Forms of Amyloid β Protein: Implications for the Mechanism of Intramembrane Cleavage by γ-Secretase

γ-Cleavage of β-amyloid precursor protein (APP) in the middle of the cell membrane generates amyloid β protein (Aβ), and ϵ-cleavage, ∼10 residues downstream of the γ-cleavage site, releases the APP intracellular domain (AICD). A significant link between generation of Aβ and AICD and failure to detect AICD41-99 led us to hypothesize that ϵ-cleavage generates longer Aβs, which are then processed to Aβ40/42. Using newly developed gel systems and an N-end-specific monoclonal antibody, we have identified the longer Aβs (Aβ1-43, Aβ1-45, Aβ1-46, and Aβ1-48) within the cells and in brain tissues. The production of these longer Aβs as well as Aβ40/42 is presenilin dependent and is suppressed by {1S-benzyl-4R-[1S-carbamoyl-2-phenylethylcarbamoyl-1S-3-methylbutylcarbamoyl]-2R-hydroxy-5-phenylpentyl}carbamic acid tert-butyl ester, a transition state analog inhibitor for aspartyl protease. In contrast, N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester, a potent dipeptide γ-secretase inhibitor, builds up Aβ1-43 and Aβ1-46 intracellularly, which was also confirmed by mass spectrometry. Notably, suppression of Aβ40 appeared to lead to an increase in Aβ43, which in turn brings an increase in Aβ46, in a dose-dependent manner. We therefore propose an α-helical model in which longer Aβ species generated by ϵ-cleavage is cleaved at every three residues in its carboxyl portion.

Caloric restriction attenuates β-amyloid neuropathology in a mouse model of Alzheimer's disease

This study was designed to explore the possibility that caloric restriction (CR) may benefit Alzheimers disease (AD) by preventing β‐amyloid (Aβ) neuropathology pivotal to the initiation and progression of the disease. We report that a CR dietary regimen prevents Aβ peptides generation and neuritic plaque deposition in the brain of a mouse model of AD neuropathology through mechanisms associated with promotion of anti‐amyloidogenic α‐secretase activity. Study findings support existing epidemiological evidence indicating that caloric intake may influence risk for AD and raises the possibility that CR may be used in preventative measures aimed at delaying the onset of AD amyloid neuropathology.

CUL7: A DOC domain-containing cullin selectively binds Skp1⋅Fbx29 to form an SCF-like complex

Selective protein degradation targeted by members of the F-box protein family plays pivotal roles in cell biology. It is widely accepted that an F-box protein directs substrate ubiquitination within a Skp1⋅CUL1⋅F-box protein⋅ROC1 (SCF-ROC1) E3 ubiquitin ligase complex. This assembly utilizes the CUL1 molecular scaffold, allowing the F-box protein to position its bound substrate for ubiquitination by a ROC1-recruited E2-conjugating enzyme. Here, we describe an alternative mechanism for assembling an F-box protein-based E3 complex through a previously uncharacterized cullin, CUL7, identified by mass spectrometry as a ROC1-interacting protein. CUL7 is a large polypeptide containing a cullin domain, which is responsible for ROC1 binding, and a DOC domain, which is also present in the anaphase-promoting complex. Remarkably, CUL7 assembles an SCF-ROC1-like E3 ubiquitin ligase complex consisting of Skp1, CUL7, the Fbx29 F-box protein, and ROC1. In contrast to CUL1 that binds Skp1 by itself, CUL7 interacts with the Skp1⋅Fbx29 complex, but not with Skp1 alone. Strikingly, CUL7 selectively interacts with Skp1⋅Fbx29 but not with Skp1⋅βTRCP2 or Skp1⋅Skp2. Thus, CUL7 may define a previously uncharacterized, Fbx29-mediated, and ubiquitin-dependent proteolysis pathway.

Morphine Administration Alters the Profile of Hippocampal Postsynaptic Density-associated Proteins A Proteomics Study Focusing on Endocytic Proteins

Numerous studies have shown that drugs of abuse induce changes in protein expression in the brain that are thought to play a role in synaptic plasticity. Drug-induced plasticity can be mediated by changes at the synapse and more specifically at the postsynaptic density (PSD), which receives and transduces synaptic information. To date, the majority of studies examining synaptic protein profiles have focused on identifying the synaptic proteome. Only a handful of studies have examined the changes in synaptic profile by drug administration. We applied a quantitative proteomics analysis technique with the cleavable ICAT reagent to quantitate relative changes in protein levels of the hippocampal PSD in response to morphine administration. We identified a total of 102 proteins in the mouse hippocampal PSD. The majority of these were signaling, trafficking, and cytoskeletal proteins involved in synaptic plasticity, learning, and memory. Among the proteins whose levels were found to be altered by morphine administration, clathrin levels were increased to the largest extent. Immunoblotting and electron microscopy studies showed that this increase was localized to the PSD. Morphine treatment was also found to lead to a local increase in two other components of the endocytic machinery, dynamin and AP-2, suggesting a critical involvement of the endocytic machinery in the modulatory effects of morphine. Because α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are thought to undergo clathrin-mediated endocytosis, we examined the effect of morphine administration on the association of the AMPA receptor subunit, GluR1, with clathrin. We found a substantial decrease in the levels of GluR1 associated with clathrin. Taken together, these results suggest that, by causing a redistribution of endocytic proteins at the synapse, morphine modulates synaptic plasticity at hippocampal glutamatergic synapses.

Latrepirdine improves cognition and arrests progression of neuropathology in an Alzheimer's mouse model

Latrepirdine (Dimebon) is a pro-neurogenic, antihistaminic compound that has yielded mixed results in clinical trials of mild to moderate Alzheimers disease, with a dramatically positive outcome in a Russian clinical trial that was unconfirmed in a replication trial in the United States. We sought to determine whether latrepirdine (LAT)-stimulated amyloid precursor protein (APP) catabolism is at least partially attributable to regulation of macroautophagy, a highly conserved protein catabolism pathway that is known to be impaired in brains of patients with Alzheimers disease (AD). We utilized several mammalian cellular models to determine whether LAT regulates mammalian target of rapamycin (mTOR) and Atg5-dependent autophagy. Male TgCRND8 mice were chronically administered LAT prior to behavior analysis in the cued and contextual fear conditioning paradigm, as well as immunohistological and biochemical analysis of AD-related neuropathology. Treatment of cultured mammalian cells with LAT led to enhanced mTOR- and Atg5-dependent autophagy. Latrepirdine treatment of TgCRND8 transgenic mice was associated with improved learning behavior and with a reduction in accumulation of Aβ42 and α-synuclein. We conclude that LAT possesses pro-autophagic properties in addition to the previously reported pro-neurogenic properties, both of which are potentially relevant to the treatment and/or prevention of neurodegenerative diseases. We suggest that elucidation of the molecular mechanism(s) underlying LAT effects on neurogenesis, autophagy and behavior might warranty the further study of LAT as a potentially viable lead compound that might yield more consistent clinical benefit following the optimization of its pro-neurogenic, pro-autophagic and/or pro-cognitive activities.

Degradation of fibrillar forms of Alzheimer's amyloid β-peptide by macrophages

Cultured microglia internalize fibrillar amyloid Aβ (fAβ) and deliver it to lysosomes. Degradation of fAβ by microglia is incomplete, but macrophages degrade fAβ efficiently. When mannose-6 phosphorylated lysosomal enzymes were added to the culture medium of microglia, degradation of fAβ was increased, and the increased degradation was inhibited by excess mannose-6-phosphate, which competes for binding and endocytic uptake. This suggests that low activity of one or more lysosomal enzymes in the microglia was responsible for the poor degradation of fAβ. To further characterize the degradation of fAβ in late endosomes and lysosomes, we analyzed fAβ-derived intracellular degradation products in macrophages and microglia by mass spectrometry. Fragments with truncations in the first 12 N-terminal residues were observed in extracts from both cell types. We also analyzed material released by the cells. Microglia released mainly intact Aβ1–42, whereas macrophages released a variety of N-terminal truncated fragments. These results indicate that initial proteolysis near the N-terminus is similar in both cell types, but microglia are limited in their ability to make further cuts in the fAβ.

Aggregation and catabolism of disease-associated intra-Aβ mutations: reduced proteolysis of AβA21G by neprilysin☆

Five point mutations within the amyloid beta-protein (Abeta) sequence of the APP gene are associated with hereditary diseases which are similar or identical to Alzheimers disease and encode: the A21G (Flemish), E22G (Arctic), E22K (Italian), E22Q (Dutch) and the D23N (Iowa) amino acid substitutions. Although a substantial body of data exists on the effects of these mutations on Abeta production, whether or not intra-Abeta mutations alter degradation and how this relates to their aggregation state remain unclear. Here we report that the E22G, E22Q and the D23N substitutions significantly increase fibril nucleation and extension, whereas the E22K substitution exhibits only an increased rate of extension and the A21G substitution actually causes a decrease in the extension rate. These substantial differences in aggregation together with our observation that aggregated wild type Abeta(1-40) was much less well degraded than monomeric wild type Abeta(1-40), prompted us to assess whether or not disease-associated intra-Abeta mutations alter proteolysis independent of their effects on aggregation. Neprilysin (NEP), insulin degrading enzyme (IDE) and plasmin play a major role in Abeta catabolism, therefore we compared the ability of these enzymes to degrade wild type and mutant monomeric Abeta peptides. Experiments investigating proteolysis revealed that all monomeric peptides are degraded similarly by IDE and plasmin, but that the Flemish peptide was degraded significantly more slowly by NEP than wild type Abeta or any of the other mutant peptides. This finding suggests that resistance to NEP-mediated proteolysis may underlie the pathogenicity associated with the A21G mutation.

Mutations in APP have independent effects on Aβ and CTFγ generation

Understanding the molecular mechanism of beta-amyloid (Abeta) generation is crucial for Alzheimers disease pathogenesis as well as for normal APP function. The transmembrane domain (TM) of APP appears to undergo presenilin-dependent gamma-secretase cleavage at two topologically distinct sites: a site in the middle of the TM domain that is crucial for the generation of Abeta-peptides, and a site close to the cytoplasmic border (S3-like/epsilon site) of the TM domain that leads to production of the APP intracellular domain (CTFgamma/AICD). We demonstrate that, in contrast to the unique effect of familial Alzheimers disease (FAD) mutations in APP on Abeta42 production, some but not all FAD mutations also affect CTFgamma generation. Furthermore, changes in total CTFgamma levels do not correlate with either an increase or a decrease of any Abeta species, and inhibition of Abeta-peptide formation starting from position +1 (Abeta1-x) does not affect CTFgamma production. These results suggest that cleavage at the gamma40/42- and the S3-like sites can be dissociated, and that APP signaling and Abeta production are not tightly linked.

Presenilin-1 Mutation L271V Results in Altered Exon 8 Splicing and Alzheimer's Disease with Non-cored Plaques and No Neuritic Dystrophy

The mutation L271V in exon 8 of the presenilin-1 (PS-1) gene was detected in an Alzheimers disease pedigree. Neuropathological examination of affected individuals identified variant, large, non-cored plaques without neuritic dystrophy, reminiscent of cotton wool plaques. Biochemical analysis of L271V mutation showed that it increased secretion of the 42-amino acid amyloid-β peptide, suggesting a pathogenic mutation. Analysis of PS-1 transcripts from the brains of two mutation carriers revealed a 17–50% increase in PS-1 transcripts with deletion of exon 8 (PS-1Δexon8) compared with unrelated Alzheimers disease brains. Exon trapping analysis confirmed that L271V mutation enhanced the deletion of exon 8. Western blots of brain lysates indicated that PS-1Δexon8 was overexpressed in an affected individual. Biochemical analysis of PS-1Δexon8 in COS and BD8 cells indicate the splice isoform is not intrinsically active but interacts with wild-type PS-1 to generate amyloid-β. Western blots of cell lysates immunoprecipitated with anti-Tau or anti-GSK-3β antibodies indicated that PS-1Δexon8, unlike wild-type PS-1, does not interact directly with Tau or GSK-3β, potential modifiers of neuritic dystrophy. We postulate that variant plaques observed in this family are due in part to the effects of PS-1Δexon8 and that interaction between PS-1 and various protein complexes are necessary for neuritic plaque formation.