Parkinson's disease (PD) and multiple system atrophy (MSA) are distinct clinical syndromes characterized by the pathological accumulation of α-synuclein (α-syn) protein fibrils in neurons and glial cells. These disorders and other neurodegenerative diseases may progress via prion-like mechanisms. The prion model of propagation predicts the existence of “strains” that link pathological aggregate structure and neuropathology. Prion strains are aggregated conformers that stably propagate in vivo and cause disease with defined incubation times and patterns of neuropathology. Indeed, tau prions have been well defined, and research suggests that both α-syn and β-amyloid may also form strains. However, there is a lack of studies characterizing PD- versus MSA-derived α-syn strains or demonstrating stable propagation of these unique conformers between cells or animals. To fill this gap, we used an assay based on FRET that exploits a HEK293T “biosensor” cell line stably expressing α-syn (A53T)-CFP/YFP fusion proteins to detect α-syn seeds in brain extracts from PD and MSA patients. Both soluble and insoluble fractions of MSA extracts had robust seeding activity, whereas only the insoluble fractions of PD extracts displayed seeding activity. The morphology of MSA-seeded inclusions differed from PD-seeded inclusions. These differences persisted upon propagation of aggregation to second-generation biosensor cells. We conclude that PD and MSA feature α-syn conformers with very distinct biochemical properties that can be transmitted to α-syn monomers in a cell system. These findings are consistent with the idea that distinct α-syn strains underlie PD and MSA and offer possible directions for synucleinopathy diagnosis.
It has been reported previously that ATP inhibits the insulysin reaction (Camberos, M. C., Perez, A. A., Udrisar, D. P., Wanderley, M. I., and Cresto, J. C. (2001) Exp. Biol. Med. 226, 334 -341). We report here that with 2-aminobenzoyl-GGFLRKHGQ-ethylenediamine-2,4-dinitrophenyl as substrate, ATP and other nucleotides increase the rate >20-fold in Tris buffer. There is no specificity with respect to the nucleotide; however, ATP is more effective than ADP, which is more effective than AMP. Triphosphate itself was as effective as ATP, indicating it is this moiety that is responsible for activation. The binding of triphosphate was shown to be at a site distinct from the active site, thus acting as a noncompetitive activator. With the physiological substrates insulin and amyloid  peptide, nucleotides and triphosphate were without effect. However, with small physiological peptides such as bradykinin and dynorphin B-9, ATP and triphosphate increased the rate of hydrolysis ϳ10-fold. Triphosphate and ATP shifted the oligomeric state of the enzyme from primarily dimertetramers to a monomer. These data suggest the presence of an allosteric regulatory site on insulysin that may shift its specificity toward small peptide substrates.Insulysin (insulin-degrading enzyme (IDE), 1 EC 3.4.24.56) in conjunction with neprilysin, endothelin-converting enzyme, plasmin, and possibly other peptidases provides a major catabolic pathway for amyloid  peptides in the brain (1-3). A decrease in the activity of one or more of these peptidases would lead to increased A and its subsequent oligomerization and deposition. The most convincing evidence that IDE participates in A clearance is the finding of elevated levels of both A 1-40 and A 1-42 in the brains of mice in which the IDE gene was disrupted (4, 5). Recent genetic evidence suggests, but has yet to be firmly established, that IDE is linked to late onset Alzheimer's disease (6 -8). Any such linkage between IDE and late onset Alzheimer's disease is likely through the ability of IDE to degrade amyloid  peptides.We recently reported that IDE displays the characteristics of an allosteric enzyme, whereby peptide substrates increase the rate of hydrolysis of other peptide substrates 2-7-fold (9). This behavior could be attributed to two effects as follows: a peptidedependent shift in the monomer-oligomer equilibrium of IDE to favor a dimer, and a conformational change produced by the binding of peptide to one subunit that increased the activity of the adjacent subunit.The finding that IDE displays allosteric properties led us to investigate further the reported effect of ATP and other nucleotides acting as noncompetitive inhibitors of the IDE-dependent hydrolysis of insulin (10). The results of this study show that nucleotide triphosphates can increase the rate of cleavage of small peptide substrates of IDE, and that the effect of nucleotide triphosphates can be attributed primarily to the triphosphate moiety. Triphosphates bind to a site distinct from the substrate-binding site. Thus ...
Insulysin (IDE) and neprilysin (NEP) were found to be inactivated by oxidation with hydrogen peroxide, an iron-ascorbate oxidation system, and by treatment with 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH). In each case reaction led to the introduction of protein carbonyl groups as judged by reaction with 2,4-dintrophenylhydrazine. IDE was inactivated by reaction with 4-hydroxy-2-nonenal (HNE) with the concomitant formation of protein adducts. NEP was not inactivated to a significant extent by HNE, but some HNE-adduct formation did occur. Prior reaction with hydrogen peroxide or AAPH led to enhanced formation of HNE adducts. Treatment of IDE with AAHP or hydrogen peroxide increased its susceptibility to proteolysis, while treatment of NEP with iron/ascorbate or hydrogen peroxide increased its susceptibility to proteolysis. Since IDE and NEP play a prominent role in the clearance of amyloid beta peptides, their oxidative inactivation and enhanced proteolysis can contribute to the onset and/or progression of Alzheimer's disease.
Insulin-degrading enzyme (IDE) (insulysin) is a zinc metallopeptidase that metabolizes several bioactive peptides, including insulin and the amyloid  peptide. IDE is an unusual metallopeptidase in that it is allosterically activated by both small peptides and anions, such as ATP. Here, we report that the ATPbinding site is located on a portion of the substrate binding chamber wall arising largely from domain 4 of the four-domain IDE. Two variants having residues in this site mutated, IDE K898A,K899A,S901A and IDE R429S , both show greatly decreased activation by the polyphosphate anions ATP and PPP i . IDE K898A,K899A,S901A is also deficient in activation by small peptides, suggesting a possible mechanistic link between the two types of allosteric activation. Sodium chloride at high concentrations can also activate IDE. There are no observable differences in average conformation between the IDE-ATP complex and unliganded IDE, but regions of the active site and C-terminal domain do show increased crystallographic thermal factors in the complex, suggesting an effect on dynamics. Activation by ATP is shown to be independent of the ATP hydrolysis activity reported for the enzyme. We also report that IDE K898A,K899A,S901A has reduced intracellular function relative to unmodified IDE, consistent with a possible role for anion activation of IDE activity in vivo. Together, the data suggest a model in which the binding of anions activates by reducing the electrostatic attraction between the two halves of the enzyme, shifting the partitioning between open and closed conformations of IDE toward the open form.
Mid-life obesity and type 2 diabetes mellitus (T2DM) confer a modest, increased risk for Alzheimer’s disease (AD), though the underlying mechanisms are unknown. We have created a novel mouse model that recapitulates features of T2DM and AD by crossing morbidly obese and diabetic db/db mice with APPΔNL/ΔNLx PS1P264L/P264L knock-in mice. These mice (db/AD) retain many features of the parental lines (e.g. extreme obesity, diabetes, and parenchymal deposition of β-amyloid (Aβ)). The combination of the two diseases led to additional pathologies-perhaps most striking of which was the presence of severe cerebrovascular pathology, including aneurysms and small strokes. Cortical Aβ deposition was not significantly increased in the diabetic mice, though overall expression of presenilin was elevated. Surprisingly, Aβ was not deposited in the vasculature or removed to the plasma, and there was no stimulation of activity or expression of major Aβ-clearing enzymes (neprilysin, insulin degrading enzyme, or endothelin-converting enzyme). The db/AD mice displayed marked cognitive impairment in the Morris Water Maze, compared to either db/db or APPΔNLx PS1P264L mice. We conclude that the diabetes and/or obesity in these mice leads to a destabilization of the vasculature, leading to strokes and that this, in turn, leads to a profound cognitive impairment and that this is unlikely to be directly dependent on Aβ deposition. This model of mixed or vascular dementia provides an exciting new avenue of research into the mechanisms underlying the obesity-related risk for age-related dementia, and will provide a useful tool for the future development of therapeutics.
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