Maho Morishima-Kawashima

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.

gamma-Secretase: successive tripeptide and tetrapeptide release from the transmembrane domain of beta-carboxyl terminal fragment.

Amyloid β protein (Aβ), a pathogenic molecule associated with Alzheimers disease, is produced by γ-secretase, which cleaves the β-carboxyl terminal fragment (βCTF) of β-amyloid precursor protein in the middle of its transmembrane domain. How the cleavage proceeds within the membrane has long been enigmatic. We hypothesized previously that βCTF is cleaved first at the membrane–cytoplasm boundary, producing two long Aβs, Aβ48 and Aβ49, which are processed further by releasing three residues at each step to produce Aβ42 and Aβ40, respectively. To test this hypothesis, we used liquid chromatography tandem mass spectrometry (LC-MS/MS) to quantify the specific tripeptides that are postulated to be released. Using CHAPSO (3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxyl-1-propanesulfonate)-reconstituted γ-secretase system, we confirmed that Aβ49 is converted to Aβ43/40 by successively releasing two or three tripeptides and that Aβ48 is converted to Aβ42/38 by successively releasing two tripeptides or these plus an additional tetrapeptide. Most unexpectedly, LC-MS/MS quantification revealed an induction period, 3–4 min, in the generation of peptides. When extrapolated, each time line for each tripeptide appears to intercept the same point on the x-axis. According to numerical simulation based on the successive reaction kinetics, the induction period exists. These results strongly suggest that Aβ is generated through the stepwise processing of βCTF by γ-secretase.

Ubiquitin is conjugated with amino-terminally processed tau in paired helical filaments

We have investigated ubiquitinated paired helical filaments, which produce a proteinaceous smear in SDS-polyacrylamide gel electrophoresis and immunoblotting. The smear consisted largely of the carboxy-terminal portion of tau and ubiquitin. The ubiquitin-targeted protein was identified as tau in paired helical filaments, and the conjugation sites were localized to the microtubule-binding region. Most ubiquitin in paired helical filaments occurred as a monoubiquitinated form, and only a small proportion of ubiquitin formed multiubiquitin chains. There was a ubiquitin-negative smear, in which tau was much less processed in the amino-terminal portion. This strongly suggests that the amino-terminal processing of tau in paired helical filaments precedes its ubiquitination.

Hyperphosphorylation of Tau in PHF

Tau in PHF is known to be highly phosphorylated and immunochemical study has indicated the similarity of the phosphorylation between PHF-tau and fetal tau. We have determined the exact phosphorylation sites in both PHF-tau and fetal rat tau by ion-spray mass spectrometry and sequencing of ethanethiol-modified peptides. In PHF-tau, 19 sites have been identified; all the phosphorylation sites except for Ser-262 are localized to the amino- and carboxyl-terminal flanking regions of the microtubule-binding domain. Half of them are shared by fetal tau. Thus, PHF-tau is much more phosphorylated. Whereas most of the sites in fetal tau are proline-directed, half of them in PHF-tau are nonproline-directed. Overall, the hyperphosphorylation of PHF-tau can be considered to consist of fetal-type phosphorylation and additional proline-directed and nonproline-directed phosphorylation. This extraphosphorylation may provide PHF-tau with the unusual characteristics including assembly incompetence.

Effect of Apolipoprotein E Allele ε4 on the Initial Phase of Amyloid β-Protein Accumulation in the Human Brain

Deposition of amyloid β-protein (Aβ), a hallmark of Alzheimers disease, occurs to some extent in the brains of most elderly individuals. We sought to learn when Aβ deposition begins and how deposition is affected by apolipoprotein E allele e4, a strong risk factor for late-onset Alzheimers disease. Using an improved extraction protocol and specific enzyme-linked immunosorbent assay, we quantified the levels of Aβ40 and Aβ42 in the insoluble fractions of brains from 105 autopsy cases, aged 22 to 81 years at death, who showed no signs of dementia. Aβ40 and Aβ42 were detected in the insoluble fractions from all of the brains examined; low levels were even found in the brains of patients as young as 20 to 30 years of age. The incidence of significant Aβ accumulation increased age-dependently, with Aβ42 levels beginning to rise steeply in some patients in their late 40s, accompanied by much smaller increases in Aβ40 levels. The presence of the apolipoprotein E e4 allele was found to significantly enhance the accumulation of Aβ42 and, to a lesser extent, that of Aβ40. These findings strongly suggest that the presence of e4 allele results in an earlier onset of Aβ42 accumulation in the brain.

The Ubiquitin–Proteasome System and the Autophagic–Lysosomal System in Alzheimer Disease

As neurons age, their survival depends on eliminating a growing burden of damaged, potentially toxic proteins and organelles-a capability that declines owing to aging and disease factors. Here, we review the two proteolytic systems principally responsible for protein quality control in neurons and their important contributions to Alzheimer disease pathogenesis. In the first section, the discovery of paired helical filament ubiquitination is described as a backdrop for discussing the importance of the ubiquitin-proteasome system in Alzheimer disease. In the second section, we review the prominent involvement of the lysosomal system beginning with pathological endosomal-lysosomal activation and signaling at the very earliest stages of Alzheimer disease followed by the progressive failure of autophagy. These abnormalities, which result in part from Alzheimer-related genes acting directly on these lysosomal pathways, contribute to the development of each of the Alzheimer neuropathological hallmarks and represent a promising therapeutic target.

Mutant Presenilin 2 Transgenic Mouse: Effect on an Age-Dependent Increase of Amyloid β-Protein 42 in the Brain†

Abstract: The N141I missense mutation in presenilin (PS) 2 is tightly linked with a form of autosomal dominant familial Alzheimers disease (AD) in the Volga German families. We have generated transgenic mouse lines overexpressing human wild‐type or mutant PS2 under transcriptional control of the chicken β‐actin promoter. In the brains of transgenic mice, the levels of human PS2 mRNA were found to be five‐ to 15‐fold higher than that of endogenous mouse PS2 mRNA. The amyloid β‐protein (Aβ) 42 levels in the brains of mutant PS2 transgenic mice were higher than those in wild‐type PS2 transgenic mice at the age of 2, 5, or 8 months. In addition, the Aβ42 levels appeared to increase steadily in the mutant PS2 transgenic mouse brains from 2 to 8 months of age, whereas there was only a small increase in wild‐type transgenic mice between the ages of 5 and 8 months. There was no definite difference in the levels of N‐terminal and C‐terminal fragments between wild‐type and mutant PS2 transgenic mice at the age of 2, 5, or 8 months. These data show a definite effect of the PS2 mutation on an age‐dependent increase of Aβ42 content in the brain.

Appearance of Sodium Dodecyl Sulfate-Stable Amyloid β-Protein (Aβ) Dimer in the Cortex During Aging

We previously noted that some aged human cortical specimens containing very low or negligible levels of amyloid β-protein (Aβ) by enzyme immunoassay (EIA) provided prominent signals at 6∼8 kd on the Western blot, probably representing sodium dodecyl sulfate (SDS)-stable Aβ dimer. Re-examination of the specificity of the EIA revealed that BAN50- and BNT77-based EIA, most commonly used for the quantitation of Aβ, capture SDS-dissociable Aβ but not SDS-stable Aβ dimer. Thus, all cortical specimens in which the levels of Aβ were below the detection limits of EIA were subjected to Western blot analysis. A fraction of such specimens contained SDS-stable dimer at 6∼8 kd, but not SDS-dissociable Aβ monomer at ∼4 kd, as judged from the blot. This Aβ dimer is unlikely to be generated after death, because (i) specimens with very short postmortem delay contained the Aβ dimer, and (ii) until 12 hours postmortem, such SDS-stable Aβ dimer is detected only faintly in PDAPP transgenic mice. The presence of Aβ dimer in the cortex may characterize the accumulation of Aβ in the human brain, which takes much longer than that in PDAPP transgenic mice.

Presence of Sodium Dodecyl Sulfate-Stable Amyloid β-Protein Dimers in the Hippocampus CA1 Not Exhibiting Neurofibrillary Tangle Formation

The amyloid cascade hypothesis of Alzheimers disease postulates that accumulation of amyloid beta-protein (Abeta) precedes neurofibrillary tangle formation or neuronal loss in the cortex. Although this temporal profile has been proved in the neocortex by silver staining and immunocytochemical methods, CA1 of the hippocampus exhibits a distinct temporal profile during normal aging: the formation of neurofibrillary tangles precedes senile plaque formation. This temporal profile has been further confirmed by two-site enzyme immunoassay (EIA) quantitation of sodium dodecyl sulfate (SDS)-dissociable Abeta42; neurofibrillary tangles are already present despite undetectable levels of SDS-dissociable Abeta42. However, when the same specimens were subjected to Western blotting, many cases with or without neurofibrillary tangles showed some accumulation of SDS-stable Abeta dimers that cannot be detected by EIA. Thus, the temporal profile prerequisite for the hypothesis is still valid in CA1, and this finding also suggests that SDS-stable Abeta dimers have some significant effects on CA1 pyramidal neurons, which are most vulnerable to neurofibrillary tangle formation.

Molecular Analysis of Mutant and Wild-Type Tau Deposited in the Brain Affected by the FTDP-17 R406W Mutation

Frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) is a familial neurological disorder, characterized genetically by autosomal dominant inheritance, clinically by behavioral abnormalities and parkinsonism, and neuropathologically by tauopathy. Linkage analyses of affected families have led to identification of several exonic and intronic mutations in the tau gene. In this study, we analyzed molecular species of tau in the soluble and insoluble fractions of brain affected by the FTDP-17 R406W mutation. Protein chemical analysis and Western blotting using site-specific antibodies indicated that almost equal amounts of wild-type and mutant tau were present in the Sarkosyl-insoluble fraction of the R406W brain. Consistent with this, wild-type and mutant tau colocalized in neurofibrillary tangles in the frontal cortex and hippocampus of the R406W brain. In contrast to soluble R406W tau, which was less phosphorylated than soluble wild-type tau, the Sarkosyl-insoluble mutant tau was highly phosphorylated as well as the insoluble wild-type tau.