Degradation of soluble amyloid beta-peptides 1-40, 1-42, and the Dutch variant 1-40Q by insulin degrading enzyme from Alzheimer disease and control brains

Chronic exogenous corticosterone administration generates an insulin-resistant brain state in rats

We investigated whether long-term administration of exogenous corticosterone (CST) or vehicle as daily treatment induces changes in rat behavior and in gene expression of the rat brain insulin signaling pathway and the formation of tau protein. Two groups of male adult rats received daily subcutaneous injections of 26.8 mg/kg CST (CST stress group) or vehicle-sesame oil (injection stress group) for 60 days while the third group was taken as untreated controls (n = 8 each). Body weight and plasma CST were measured and psychometric investigations were conducted using a rat holeboard test system before and after the treatment. Gene expression analyzes were performed by RT-PCR in cerebral cortical tissue for insulin genes 1 and 2, insulin receptor (IR), insulin degrading enzyme (IDE), and tau protein. Daily injections of CST for 60 days induced a significant, 2-fold increase in rat plasma CST concentrations in comparison to untreated controls. Significantly reduced behavioral abilities in CST-treated rats were associated with reduced gene expression of insulin 1 ( - 20%), IDE ( - 23%), and IR ( - 26%), indicating an insulin-resistant brain state, followed by increased tau protein (+28%) gene expression. In summary, chronic CST administration affects gene expression in the brain IR signaling cascade and increases tau gene expression, which is associated with reductions in cognition capacity in rats.

Changes with age in the activities of beta-secretase and the Abeta-degrading enzymes neprilysin, insulin-degrading enzyme and angiotensin-converting enzyme

We recently found that insoluble Abeta increases, but soluble Abeta decreases with age in normal brains. We now report the changes in activities of beta-secretase (BACE-1) and Abeta-degrading enzymes with age, and their relationships to concentrations of soluble and insoluble Abeta. We measured BACE-1 activity and the levels and activities of neprilysin (NEP), insulin-degrading enzyme (IDE) and angiotensin-converting enzyme (ACE) in normal control brains (16 years-95 years). We also compared the measurements to those in AD. BACE-1 activity correlated closely with age in controls and was significantly higher in AD. In controls, NEP and IDE activities (but not protein levels) increased with age but ACE activity and level did not. BACE-1 activity correlated directly with insoluble but inversely with soluble Abeta. IDE activity correlated directly with insoluble Abeta and NEP activity was inversely related to soluble Abeta. ACE level correlated directly with insoluble and inversely with soluble Abeta in controls but not AD. Both Abeta-synthesizing and -degrading enzyme activities increase with age, coinciding with declining soluble Abeta and increasing insoluble Abeta. Further research is needed to establish whether these changes in enzyme activity and Abeta levels are causally related and if so how.

Effects of 4-hydroxy-nonenal and Amyloid-beta on expression and activity of endothelin converting enzyme and insulin degrading enzyme in SH-SY5Y cells

The cerebral accumulation of amyloid-beta (Abeta) is a consistent feature of and likely contributor to the development of Alzheimer's disease (AD). In addition to dysregulated production, increasing experimental evidence suggests reduced catabolism plays an important role in Abeta accumulation. Although endothelin converting enzyme (ECE) and insulin degrading enzyme (IDE) degrade and thus contribute to regulating the steady-state levels of Abeta, how these enzymes are regulated remain poorly understood. In this study, we investigated the effects of 4-hydroxy-nonenal (HNE) and Abeta on the expression and activity of ECE-1 and IDE in human neuroblastoma SH-SY5Y cells. Treatment with HNE or Abeta upregulated ECE-1 mRNA and protein, while IDE was unchanged. Although both ECE-1 and IDE were oxidized within 24 h of HNE or Abeta treatment, ECE-1 catalytic activity was elevated while IDE specific activity was unchanged. The results demonstrated for the first time that both ECE-1 and IDE are substrates of HNE modification induced by Abeta. In addition, the results suggest complex mechanisms underlying the regulation of their enzymatic activity.

Type 1 diabetes exaggerates features of Alzheimer's disease in APP transgenic mice

A number of studies suggest an association between Alzheimer's disease (AD) and diabetes: AD patients show impaired insulin function, whereas cognitive deficits and increased risk of developing AD occur in diabetic patients. The reasons for the increased risk are not known. Recent studies of disturbances in the insulin-signaling pathway have revealed new perspectives on the links between AD and Type 1 diabetes with a particular focus on glycogen synthase-kinase-3 (GSK3). We have therefore characterized a mouse model of combined insulin-deficient diabetes and AD and find that diabetes exaggerated defects in the brain of APP transgenic mice. Mice with combined APP overexpression and diabetes showed a decreased insulin receptor activity and an increased GSK3beta activity. Concomitantly, tau phosphorylation and number of Abeta plaques, the two pathologic hallmarks of AD, were increased in the brain of diabetic-APP transgenic mice. Our results indicate that the pathologic features of AD are exaggerated in the brain of APP transgenic mice that have concurrent insulin-deficient diabetes, and underscore a possible mechanism of brain dysfunction common to AD and diabetes.

Involvement of insulin-degrading enzyme in the clearance of beta-amyloid at the blood-CSF barrier: Consequences of lead exposure

Background

Alzheimer's disease (AD) is characterized by the deposition of beta-amyloid (Aβ) peptides in the brain extracellular matrix, resulting in pathological changes and neurobehavioral deficits. Previous work from this laboratory demonstrated that the choroid plexus (CP) possesses the capacity to remove Aβ from the cerebrospinal fluid (CSF), and exposure to lead (Pb) compromises this function. Since metalloendopeptidase insulin-degrading enzyme (IDE), has been implicated in the metabolism of Aβ, we sought to investigate whether accumulation of Aβ following Pb exposure was due to the effect of Pb on IDE.

Methods

Rats were injected with a single dose of Pb acetate or an equivalent concentration of Na-acetate; CP tissues were processed to detect the location of IDE by immunohistochemistry. For in vitro studies, choroidal epithelial Z310 cells were treated with Pb for 24 h in the presence or absence of a known IDE inhibitor, N-ethylmaleimide (NEM) to assess IDE enzymatic activity and subsequent metabolic clearance of Aβ. Additionally, the expression of IDE mRNA and protein were determined using real time PCR and western blots respectively.

Results

Immunohistochemistry and confocal imaging revealed the presence of IDE towards the apical surface of the CP tissue with no visible alteration in either its intensity or location following Pb exposure. There was no significant difference in the expressions of either IDE mRNA or protein following Pb exposure compared to controls either in CP tissues or in Z310 cells. However, our findings revealed a significant decrease in the IDE activity following Pb exposure; this inhibition was similar to that seen in the cells treated with NEM alone. Interestingly, treatment with Pb or NEM alone significantly increased the levels of intracellular Aβ, and a greater accumulation of Aβ was seen when the cells were exposed to a combination of both.

Conclusion

These data suggest that Pb exposure inhibits IDE activity but does not affect its expression in the CP. This, in turn, leads to a disrupted metabolism of Aβ resulting in its accumulation at the blood-CSF barrier.

Reduced neuronal expression of insulin-degrading enzyme in the dorsolateral prefrontal cortex of patients with haloperidol-treated, chronic schizophrenia

Insulin-degrading enzyme (IDE) is a neutral thiol metalloprotease, which cleaves insulin with high specificity. Additionally, IDE hydrolyzes Abeta, glucagon, IGF I and II, and beta-endorphin. We studied the expression of IDE protein in postmortem brains of patients with schizophrenia and controls because: (1) the gene encoding IDE is located on chromosome 10q23-q25, a gene locus linked to schizophrenia; (2) insulin resistance with brain insulin receptor deficits/receptor dysfunction was reported in schizophrenia; (3) the enzyme cleaves IGF-I and IGF-II which are implicated in the pathophysiology of the disease; and (4) brain gamma-endorphin levels, liberated from beta-endorphin exclusively by IDE, have been reported to be altered in schizophrenia. We counted the number of IDE immunoreactive neurons in the dorsolateral prefrontal cortex, the hypothalamic paraventricular and supraoptic nuclei, and the basal nucleus of Meynert of 14 patients with schizophrenia and 14 matched control cases. Patients had long-term haloperidol treatment. In addition, relative concentrations of IDE protein in the dorsolateral prefrontal cortex were estimated by Western blot analysis. There was a significantly reduced number of IDE expressing neurons and IDE protein content in the left and right dorsolateral prefrontal cortex in schizophrenia compared with controls, but not in other brain areas investigated. Results of our studies on the influence of haloperidol on IDE mRNA expression in SHSY5Y neuroblastoma cells, as well as the effect of long-term treatment with haloperidol on the number of IDE immunoreactive neurons in rat brain, indicate that haloperidol per se, is not responsible for the decreased neuronal expression of the enzyme in schizophrenics. Haloperidol however, might exert some effect on IDE, through changes of the expression levels of its substrates IGF-I and II, insulin and beta-endorphin. Reduced cortical IDE expression might be part of the disturbed insulin signaling cascades found in schizophrenia. Furthermore, it might contribute to the altered metabolism of certain neuropeptides (IGF-I and IGF-II, beta-endorphin), in schizophrenia.

Endothelin-converting enzyme-2 is increased in Alzheimer's disease and up-regulated by Abeta

Alzheimer's disease (AD) is thought to be caused by the accumulation of amyloid beta (Abeta) peptide within the brain. Endothelin-converting enzyme-2 (ECE-2), which is expressed in neural tissues, cleaves 'big endothelin' to produce the vasoconstrictor endothelin-1. ECE-2 also degrades Abeta. We have examined ECE-2 expression in the temporal cortex of brain tissue from patients with AD, vascular dementia, and controls. Immunohistochemistry with specific antibodies showed ECE-2 to be abundant within pyramidal neurons in both the hippocampus and neocortex, but also to be present in certain astrocytes and microglia, particularly in AD brains. Quantitative real-time PCR showed ECE-2 mRNA to be markedly elevated in AD but not in vascular dementia. ECE-2 protein concentration, measured by sandwich enzyme-linked immunosorbent assay, was also significantly elevated in AD but not in vascular dementia. Exposure of SH-SY5Y human neuroblastoma cells to monomeric or oligomeric Abeta(1-42) caused an initial decrease in ECE-2 mRNA at 4 hours, but a marked increase by 24 hours. Our findings indicate that Abeta accumulation in AD is unlikely to be caused by ECE-2 deficiency. However, ECE-2 expression is up-regulated, perhaps to minimize Abeta accumulation, but this may also be a mechanism through which endothelin-1 production is increased and cerebral blood flow is reduced in AD. Our findings suggest that endothelin-1 receptor antagonists, already licensed for treating other diseases, could be of benefit in AD therapies.

Inhibition of gamma-secretase activity reduces Abeta production, reduces oxidative stress, increases mitochondrial activity and leads to reduced vulnerability to apoptosis: Implications for the treatment of Alzheimer's disease

It has been argued that gamma-secretase should be considered as a pharmacological target, as there are few mechanism-based experimental and clinical studies on gamma-secretase treatment. In this study, we found that N2a cells bearing APP695 or its Swedish mutant exhibited increased basal levels of ROS, nitric oxide (NO), protein carbonyls, MDA and intracellular calcium, as well as reduced level of the mitochondrial membrane potential and ATP. When the activity of gamma-secretase was inhibited by expression of the D385A PS1 variant, cells (N2a/Swe.D385A) showed reduced basal levels of ROS, nitric oxide (NO), protein carbonyls, MDA and intracellular calcium, as well as increased mitochondrial membrane potential and ATP level. In addition, N2a/Swe.D385A cells showed reduced vulnerability to H(2)O(2)-induced apoptosis. The Bcl-2 and JNK/ERK pathways were proven to be involved in the change of vulnerability to H(2)O(2)-induced apoptosis. Moreover, we discovered that inhibition of gamma-secretase by DAPT would lead to a reduction of ROS levels and stabilization of mitochondrial function in APP (N2a/APP695) and APP Swedish mutant (N2a/APPswe) transfected cells. At last, it was shown that Abeta antibody and antiserum prevented increase of ROS and reduction of mitochondrial membrane potential in N2a/Swe.DeltaE9 cells but not in N2a/Swe.D385A cells, which indicated that reduced formation of Abeta was the reason for reduction of ROS formation and increase of mitochondrial membrane potential when PS-1 activity was impaired in N2a/Swe.D385A cells. We concluded that neurotoxicity was positively correlated with the activity of gamma-secretase, which suggested inhibition of gamma-secretase is a rational pharmacological target for Alzheimer's disease treatment.

Clearance mechanisms of Alzheimer's amyloid-beta peptide: implications for therapeutic design and diagnostic tests

Currently, the 'amyloid hypothesis' is the most widely accepted explanation for the pathogenesis of Alzheimer's disease (AD). According to this hypothesis, altered metabolism of the amyloid-beta (Abeta) peptide is central to the pathological cascade involved in the pathogenesis of AD. Although Abeta is produced by almost every cell in the body, a physiological function for the peptide has not been determined, and the pathways by which Abeta leads to cognitive dysfunction and cell death are unclear. Numerous therapeutic approaches that target the production, toxicity and removal of Abeta are being developed worldwide. Although therapeutic treatment for AD may be imminent, the value and effectiveness of such treatment are largely dependent on early diagnosis of the disease. This review summarizes current knowledge of Abeta clearance, transport and degradation, and evaluates the use of such information in the development of diagnostic tools. The conflicting results of plasma Abeta ELISAs are discussed, as are the more promising results of Abeta imaging by positron emission tomography. Current knowledge of Abeta-binding proteins and Abeta-degrading enzymes is analysed in the context of a potential therapy for AD. Transport across the blood-brain barrier by the receptor for advanced glycation end products and efflux via the multi-ligand lipoprotein receptor LRP-1 is also reviewed. Enhancing clearance and degradation of Abeta remains an attractive therapeutic strategy, and improved understanding of Abeta clearance may lead to advances in diagnostics and interventions designed to prevent or delay the onset of AD.

Predictors of progression of cognitive decline in Alzheimer's disease: the role of vascular and sociodemographic factors

Rates of disease progression differ among patients with Alzheimer’s disease, but little is known about prognostic predictors. The aim of the study was to assess whether sociodemographic factors, disease severity and duration, and vascular factors are prognostic predictors of cognitive decline in Alzheimer’s disease progression. We conducted a longitudinal clinical study in a specialized clinical unit for the diagnosis and treatment of dementia in Rome, Italy. A total of 154 persons with mild to moderate Alzheimer’s disease consecutively admitted to the dementia unit were included. All patients underwent extensive clinical examination by a physician at admittance and all follow-ups. We evaluated the time-dependent probability of a worsening in cognitive performance corresponding to a 5-point decrease in Mini-Mental State Examination (MMSE) score. Survival analysis was used to analyze risk of faster disease progression in relation to age, education, severity and duration of the disease, family history of dementia, hypertension, hypercholesterolemia, and type 2 diabetes. Younger and more educated persons were more likely to have faster Alzheimer’s disease progression. Vascular factors such as hypertension and hypercholesterolemia were not found to be significantly associated with disease progression. However, patients with diabetes had a 65% reduced risk of fast cognitive decline compared to Alzheimer patients without diabetes. Sociodemographic factors and diabetes predict disease progression in Alzheimer’s disease. Our findings suggest a slower disease progression in Alzheimer’s patients with diabetes. If confirmed, this result will contribute new insights into Alzheimer’s disease pathogenesis and lead to relevant suggestions for disease treatment.

Defective insulin signaling pathway and increased glycogen synthase kinase-3 activity in the brain of diabetic mice: parallels with Alzheimer's disease and correction by insulin

We have evaluated the effect of peripheral insulin deficiency on brain insulin pathway activity in a mouse model of type 1 diabetes, the parallels with Alzheimer's disease (AD), and the effect of treatment with insulin. Nine weeks of insulin-deficient diabetes significantly impaired the learning capacity of mice, significantly reduced insulin-degrading enzyme protein expression, and significantly reduced phosphorylation of the insulin-receptor and AKT. Phosphorylation of glycogen synthase kinase-3 (GSK3) was also significantly decreased, indicating increased GSK3 activity. This evidence of reduced insulin signaling was associated with a concomitant increase in tau phosphorylation and amyloid beta protein levels. Changes in phosphorylation levels of insulin receptor, GSK3, and tau were not observed in the brain of db/db mice, a model of type 2 diabetes, after a similar duration (8 weeks) of diabetes. Treatment with insulin from onset of diabetes partially restored the phosphorylation of insulin receptor and of GSK3, partially reduced the level of phosphorylated tau in the brain, and partially improved learning ability in insulin-deficient diabetic mice. Our data indicate that mice with systemic insulin deficiency display evidence of reduced insulin signaling pathway activity in the brain that is associated with biochemical and behavioral features of AD and that it can be corrected by insulin treatment.

Detergent resistant membrane-associated IDE in brain tissue and cultured cells: Relevance to Abeta and insulin degradation

Background

Insulin degrading enzyme (IDE) is implicated in the regulation of amyloid β (Aβ) steady-state levels in the brain, and its deficient expression and/or activity may be a risk factor in sporadic Alzheimer's disease (AD). Although IDE sub-cellular localization has been well studied, the compartments relevant to Aβ degradation remain to be determined.

Results

Our results of live immunofluorescence, immuno gold electron-microscopy and gradient fractionation concurred to the demonstration that endogenous IDE from brain tissues and cell cultures is, in addition to its other localizations, a detergent-resistant membrane (DRM)-associated metallopeptidase. Our pulse chase experiments were in accordance with the existence of two pools of IDE: the cytosolic one with a longer half-life and the membrane-IDE with a faster turn-over. DRMs-associated IDE co-localized with Aβ and its distribution (DRMs vs. non-DRMs) and activity was sensitive to manipulation of lipid composition in vitro and in vivo. When IDE was mis-located from DRMs by treating cells with methyl-β-cyclodextrin (MβCD), endogenous Aβ accumulated in the extracellular space and exogenous Aβ proteolysis was impaired. We detected a reduced amount of IDE in DRMs of membranes isolated from mice brain with endogenous reduced levels of cholesterol (Chol) due to targeted deletion of one seladin-1 allele. We confirmed that a moderate shift of IDE from DRMs induced a substantial decrement on IDE-mediated insulin and Aβ degradation in vitro.

Conclusion

Our results support the notion that optimal substrate degradation by IDE may require its association with organized-DRMs. Alternatively, DRMs but not other plasma membrane regions, may act as platforms where Aβ accumulates, due to its hydrophobic properties, reaching local concentration close to its Km for IDE facilitating its clearance. Structural integrity of DRMs may also be required to tightly retain insulin receptor and IDE for insulin proteolysis. The concept that mis-location of Aβ degrading proteases away from DRMs may impair the physiological turn-over of Aβ in vivo deserves further investigation in light of therapeutic strategies based on enhancing Aβ proteolysis in which DRM protease-targeting may need to be taken into account.

Differential cerebral deposition of IDE and NEP in sporadic and familial Alzheimer's disease

Alzheimer's disease (AD) is characterized by amyloid beta (A beta) accumulation in the brain and is classified as familial early-onset (FAD) or sporadic late-onset (SAD). Evidences suggest that deficits in the brain expression of insulin degrading enzyme (IDE) and neprilysin (NEP), both proteases involved in amyloid degradation, may promote A beta deposition in SAD. We studied by immunohistochemistry IDE and NEP cortical expression in SAD and FAD samples carrying the E280A presenilin-1 missense mutation. We showed that IDE, a soluble peptidase, is linked with aggregated A beta 40 isoform while NEP, a membrane-bound protease, negatively correlates with amyloid angiopathy and its expression in the senile plaques is independent of aggregated amyloid and restricted to SAD cases. NEP, but not IDE, is over-expressed in dystrophic neurites, both proteases are immunoreactive in activated astrocytes but not in microglia and IDE was the only one detected in astrocytes of white matter from FAD cases. Collectively, our results support the notion that gross conformational changes involved in the modification from "natively folded-active" to "aggregated-inactive" IDE and NEP may be a relevant pathogenic mechanism in SAD.

Association of duration and severity of diabetes mellitus with mild cognitive impairment

BACKGROUND

It remains unknown whether diabetes mellitus (DM) is a risk factor for mild cognitive impairment (MCI).

OBJECTIVE

To investigate the association of DM with MCI using a population-based case-control design.

DESIGN

Population-based case-control study.

SETTING

Academic research.

PARTICIPANTS

Our study was conducted, among subjects aged 70 to 89 years on October 1, 2004, who were randomly selected from the Olmsted County (Minnesota) population. Main Outcome Measure We administered to all participants a neurologic examination, the Clinical Dementia Rating Scale, and a neuropsychological evaluation (including 9 tests in 4 cognitive domains) to diagnose normal cognition, MCI, or dementia. We assessed history of DM, DM treatment, and DM complications by interview, and we measured fasting blood glucose levels. History of DM was also confirmed using a medical records linkage system.

RESULTS

We compared 329 subjects having MCI with 1640 subjects free of MCI and dementia. The frequency of DM was similar in subjects with MCI (20.1%) and in subjects without MCI (17.7%) (odds ratio [OR], 1.16; 95% confidence interval [CI], 0.85-1.57). However, MCI was associated with onset of DM before age 65 years (OR, 2.20; 95% CI, 1.29-3.73), DM duration of 10 years or longer (OR, 1.76; 95% CI, 1.16-2.68), treatment with insulin (OR, 2.01; 95% CI, 1.22-3.31), and the presence of DM complications (OR, 1.80; 95% CI, 1.13-2.89) after adjustment for age, sex, and education. Analyses using alternative definitions of DM yielded consistent findings.

CONCLUSION

These findings suggest an association of MCI with earlier onset, longer duration, and greater severity of DM.

Do amyloid oligomers act as traps for misfolded proteins? A hypothesis

Mounting evidence points to soluble peptide oligomers as the primary agents in various amyloid and prion diseases. Multiple mechanisms appear to contribute to the cytotoxic effects of these oligomers. Here, an additional, general mechanism is proposed - that soluble amyloid peptide oligomers serve as "all-purpose"beta strands that can interact with transiently unfolded or nascent proteins where interior beta-sheet edges are exposed. The proteins, trapped in misfolded states through this interaction, become substrates for ubiquitination, targeting them for proteasomal degradation. The increased load of ubiquitinated proteins could contribute to the impairment of the ubiquitin/proteasome system (UPS) seen in many amyloid-related diseases. This "misfolding trap" mechanism could be especially stressful in the endoplasmic reticulum, where the amyloid oligomers would compete with chaperones for nascent beta-sheet proteins. If the bound amyloid oligomer dissociates at some point after the misfolded protein is committed to the UPS pathway, the oligomer could then repeat the process, adding a catalytic aspect to the misfolding mechanism. Direct proof of this proposed mechanism requires detection of amyloid oligomer-beta-sheet protein complexes, and a co-immunoprecipitation experiment is proposed. This hypothesis supports therapies that increase amyloid oligomer degradation or sequestration, as well as therapies that upregulate chaperone activity, for combating amyloid-related diseases.

Abeta-degrading enzymes in Alzheimer's disease

In Alzheimer's disease (AD) Abeta accumulates because of imbalance between the production of Abeta and its removal from the brain. There is increasing evidence that in most sporadic forms of AD, the accumulation of Abeta is partly, if not in some cases solely, because of defects in its removal--mediated through a combination of diffusion along perivascular extracellular matrix, transport across vessel walls into the blood stream and enzymatic degradation. Multiple enzymes within the central nervous system (CNS) are capable of degrading Abeta. Most are produced by neurons or glia, but some are expressed in the cerebral vasculature, where reduced Abeta-degrading activity may contribute to the development of cerebral amyloid angiopathy (CAA). Neprilysin and insulin-degrading enzyme (IDE), which have been most extensively studied, are expressed both neuronally and within the vasculature. The levels of both of these enzymes are reduced in AD although the correlation with enzyme activity is still not entirely clear. Other enzymes shown capable of degrading Abetain vitro or in animal studies include plasmin; endothelin-converting enzymes ECE-1 and -2; matrix metalloproteinases MMP-2, -3 and -9; and angiotensin-converting enzyme (ACE). The levels of plasmin and plasminogen activators (uPA and tPA) and ECE-2 are reported to be reduced in AD. Reductions in neprilysin, IDE and plasmin in AD have been associated with possession of APOEepsilon4. We found no change in the level or activity of MMP-2, -3 or -9 in AD. The level and activity of ACE are increased, the level being directly related to Abeta plaque load. Up-regulation of some Abeta-degrading enzymes may initially compensate for declining activity of others, but as age, genetic factors and diseases such as hypertension and diabetes diminish the effectiveness of other Abeta-clearance pathways, reductions in the activity of particular Abeta-degrading enzymes may become critical, leading to the development of AD and CAA.

Tg-SwDI transgenic mice exhibit novel alterations in AbetaPP processing, Abeta degradation, and resilient amyloid angiopathy

Alzheimer's disease (AD) is characterized by the accumulation of extracellular insoluble amyloid, primarily derived from polymerized amyloid-beta (Abeta) peptides. We characterized the chemical composition of the Abeta peptides deposited in the brain parenchyma and cerebrovascular walls of triple transgenic Tg-SwDI mice that produce a rapid and profuse Abeta accumulation. The processing of the N- and C-terminal regions of mutant AbetaPP differs substantially from humans because the brain parenchyma accumulates numerous, diffuse, nonfibrillar plaques, whereas the thalamic microvessels harbor overwhelming amounts of compact, fibrillar, thioflavine-S- and apolipoprotein E-positive amyloid deposits. The abundant accretion of vascular amyloid, despite low AbetaPP transgene expression levels, suggests that inefficient Abeta proteolysis because of conformational changes and dimerization may be key pathogenic factors in this animal model. The disruption of amyloid plaque cores by immunotherapy is accompanied by increased perivascular deposition in both humans and transgenic mice. This analogous susceptibility and response to the disruption of amyloid deposits suggests that Tg-SwDI mice provide an excellent model in which to study the functional aftermath of immunotherapeutic interventions. These mice might also reveal new avenues to promote amyloidogenic AbetaPP processing and fundamental insights into the faulty degradation and clearance of Abeta in AD, pivotal issues in understanding AD pathophysiology and the assessment of new therapeutic agents.

The irreversible binding of amyloid peptide substrates to insulin-degrading enzyme: a biological perspective

Insulin-degrading enzyme (IDE) is a conserved Zn(2+)metalloendopeptidase involved in insulin degradation and in the maintenance of brain steady-state levels of amyloid beta peptide (Abeta) of Alzheimer's disease (AD). Our recent demonstration that IDE and Abeta are capable of forming a stoichiometric and extremely stable complex raises several intriguing possibilities regarding the role of this unique protein-peptide interaction in physiological and pathological conditions. These include a protective cellular function of IDE as a "dead-end chaperone" alternative to its proteolytic activity and the potential impact of the irreversible binding of Abeta to IDE upon its role as a varicella zoster virus receptor. In a pathological context, the implications for insulin signaling and its relationship to AD pathogenesis are discussed. Moreover, our findings warrant further research regarding a possible general and novel interaction between amyloidogenic peptides and other Zn(2+)metallopeptidases with an IDE-like fold and a substrate conformation-dependent recognition mechanism.

Heterologous amyloid seeding: revisiting the role of acetylcholinesterase in Alzheimer's disease

Neurodegenerative diseases associated with abnormal protein folding and ordered aggregation require an initial trigger which may be infectious, inherited, post-inflammatory or idiopathic. Proteolytic cleavage to generate vulnerable precursors, such as amyloid-beta peptide (Abeta) production via beta and gamma secretases in Alzheimer's Disease (AD), is one such trigger, but the proteolytic removal of these fragments is also aetiologically important. The levels of Abeta in the central nervous system are regulated by several catabolic proteases, including insulysin (IDE) and neprilysin (NEP). The known association of human acetylcholinesterase (hAChE) with pathological aggregates in AD together with its ability to increase Abeta fibrilization prompted us to search for proteolytic triggers that could enhance this process. The hAChE C-terminal domain (T40, AChE(575-614)) is an exposed amphiphilic alpha-helix involved in enzyme oligomerisation, but it also contains a conformational switch region (CSR) with high propensity for conversion to non-native (hidden) beta-strand, a property associated with amyloidogenicity. A synthetic peptide (AChE(586-599)) encompassing the CSR region shares homology with Abeta and forms beta-sheet amyloid fibrils. We investigated the influence of IDE and NEP proteolysis on the formation and degradation of relevant hAChE beta-sheet species. By combining reverse-phase HPLC and mass spectrometry, we established that the enzyme digestion profiles on T40 versus AChE(586-599), or versus Abeta, differed. Moreover, IDE digestion of T40 triggered the conformational switch from alpha- to beta-structures, resulting in surfactant CSR species that self-assembled into amyloid fibril precursors (oligomers). Crucially, these CSR species significantly increased Abeta fibril formation both by seeding the energetically unfavorable formation of amyloid nuclei and by enhancing the rate of amyloid elongation. Hence, these results may offer an explanation for observations that implicate hAChE in the extent of Abeta deposition in the brain. Furthermore, this process of heterologous amyloid seeding by a proteolytic fragment from another protein may represent a previously underestimated pathological trigger, implying that the abundance of the major amyloidogenic species (Abeta in AD, for example) may not be the only important factor in neurodegeneration.

Decreased Catalytic Activity of the Insulin-degrading Enzyme in Chromosome 10-Linked Alzheimer Disease Families

Insulin-degrading enzyme (IDE) is a zinc metalloprotease that degrades the amyloid beta-peptide, the key component of Alzheimer disease (AD)-associated senile plaques. We have previously reported evidence for genetic linkage and association of AD on chromosome 10q23-24 in the region harboring the IDE gene. Here we have presented the first functional assessment of IDE in AD families showing the strongest evidence of the genetic linkage. We have examined the catalytic activity and expression of IDE in lymphoblast samples from 12 affected and unaffected members of three chromosome 10-linked AD pedigrees in the National Institute of Mental Health AD Genetics Initiative family sample. We have shown that the catalytic activity of cytosolic IDE to degrade insulin is reduced in affected versus unaffected subjects of these families. Further, we have shown the decrease in activity is not due to reduced IDE expression, suggesting the possible defects in IDE function in these AD families. In attempts to find potential mutations in the IDE gene in these families, we have found no coding region substitutions or alterations in splicing of the canonical exons and exon 15b of IDE. We have also found that total IDE mRNA levels are not significantly different in sporadic AD versus age-matched control brains. Collectively, our data suggest that the genetic linkage of AD in this set of chromosome 10-linked AD families may be the result of systemic defects in IDE activity in the absence of altered IDE expression, further supporting a role for IDE in AD pathogenesis.

Brain mitochondrial dysfunction as a link between Alzheimer's disease and diabetes

It has been argued that in late-onset Alzheimer's disease a disturbance in the control of neuronal glucose metabolism consequent to impaired insulin signalling strongly resembles the pathophysiology of type 2 diabetes in non-neural tissue. The fact that mitochondria are the major generators and direct targets of reactive oxygen species led several investigators to foster the idea that oxidative stress and damage in mitochondria are contributory factors to several disorders including Alzheimer's disease and diabetes. Since brain possesses high energetic requirements, any decline in brain mitochondria electron chain could have a severe impact on brain function and particularly on the etiology of neurodegenerative diseases. This review is primarily focused in the discussion of brain mitochondrial dysfunction as a link between diabetes and Alzheimer's disease.

Central insulin resistance as a trigger for sporadic Alzheimer-like pathology: an experimental approach

A growing body of evidence implicates impairments in brain insulin signaling in early sporadic Alzheimer disease (sAD) pathology. However, the most widely accepted hypothesis for AD aetiology stipulates that pathological aggregations of the amyloid beta (Abeta) peptide are the cause of all forms of Alzheimer's disease. Streptozotocin-intracerebroventricularly (STZ-icv) treated rats are proposed as a probable experimental model of sAD. The current work reviews evidence obtained from this model indicating that central STZ administration induces brain pathology and behavioural alterations resembling those in sAD patients. Recently, alterations of the brain insulin system resembling those in sAD have been found in the STZ-icv rat model and are associated with tau protein hyperphosphorylation and Abeta-like aggregations in meningeal vessels. In line with these findings the hypothesis has been proposed that insulin resistance in the brain might be the primary event which precedes the Abeta pathology in sAD.

Molecular connexions between dementia and diabetes

Recent evidence suggests that the molecular defects associated with the development of diabetes also contribute to an increased risk of all types of dementia, including Alzheimer's disease, vascular dementia and Pick's disease. Indeed, the presence of type II diabetes mellitus results in a two to three fold higher risk of developing dementia [Fontbonne et al., 2001. Changes in cognitive abilities over a 4-year period are unfavourably affected in elderly diabetic subjects: results of the Epidemiology of Vascular Aging Study. Diabetes Care 24, 366-370; Gregg et al., 2000. Is diabetes associated with cognitive impairment and cognitive decline among older women? Study of Osteoporotic Fractures Research Group. Archives of Internal Medicine 160, 174-180; Peila et al., 2002. Type 2 diabetes, APOE gene, and the risk for dementia and related pathologies: The Honolulu-Asia Aging Study. Diabetes 51, 1256-1262]. There are currently 250 million people worldwide (>2 million in the UK) diagnosed with diabetes, and this number is predicted to double within the next 20 years, therefore the associated risk translates into a potential explosion in the appearance of dementia in the population. This review primarily focuses on the proposed molecular links between insulin action, Diabetes and Alzheimer's disease, while discussing the potential for therapeutic intervention to alleviate these disorders. In particular, we will review the regulation of glycogen synthase kinase-3 (GSK-3) and its neuronal substrates.

Aluminum inhibits proteolytic degradation of amyloid beta peptide by cathepsin D: a potential link between aluminum accumulation and neuritic plaque deposition

Neuritic plaques are the key pathological feature of Alzheimer's disease, and amyloid beta (Abeta) peptides are major component of these plaques. In this study, we demonstrated the influence of aluminum (Al) on the Abeta peptide degradation by cathepsin D. Al did not directly affect the cathepsin D activity using small synthetic substrate. However, when Abeta peptides were used as substrate, the apparent inhibitory effect of Al on cathepsin D activity was observed. This inhibitory effect disappeared by treatment of desferrioxamine. These results indicate that Al has the potential to interact and disrupt Abeta peptide catabolism via the inhibition of proteolytic degradation.

Alois Alzheimer revisited: differences in origin of the disease carrying his name

Based on the means of his time, Alois Alzheimer supposed that the disease, later carrying his name, is a disease of older age, and that the pathomorphological structures he described are due to disturbances in brain metabolism. In this contribution, it is discussed which cellular metabolic abnormalities may be representative for age-related sporadic Alzheimer disease (SAD) the predominant form of SAD in contrast to the very rare hereditary early-onset form. In focus are disturbances in glucose/energy metabolism which involve the deficits in acetylcholine, cholesterol and UDP-N-acetylglucosamine beside ATP. Another leading abnormality is the defect in cell membrane composition. The interrelation between abnormal glucose/energy metabolism and membrane defect may be assumed to form the basis for the induction of both the perturbed metabolism of the amyloid precursor protein leading to increased formation of beta-amyloid and hyperphosphorylation of tau-protein destroying cell structures. Alois Alzheimer may have been so prescient to assume most of this 100 years ago.

Plaque-Associated Overexpression of Insulin-Degrading Enzyme in the Cerebral Cortex of Aged Transgenic Tg2576 Mice With Alzheimer Pathology

It was proposed that insulin-degrading enzyme (IDE) participates in the clearance of amyloid beta (Abeta) in the brain, and its low expression or activity may be relevant for the progression of Alzheimer disease. We performed a longitudinal study of brain level, activity, and distribution of IDE in transgenic mice (Tg2576) expressing the Swedish mutation in human Abeta precursor protein. At 16 months of age, Tg2576 showed a significant 2-fold increment in IDE protein level as compared with 4.5- and 11-month-old animals. The peak of IDE was in synchrony with the sharp accumulation of sodium dodecyl sulfate-soluble Abeta and massive Abeta deposition into plaques. At this stage, IDE appeared surrounding Abeta fibrillar deposits within glial fibrillar acidic protein-positive astrocytes, suggesting that it was locally overexpressed during the Abeta-mediated inflammation process. When primary astrocytes were exposed to fibrillar Abeta in vitro, IDE protein level increased as compared with control, and this effect was reduced by the addition of U0126, a specific inhibitor of the ERK1/2 mitogen-activated protein kinase cascade. We propose that in Tg2576 mice and in contrast to its behavior in Alzheimer brains, active IDE increases with age around plaques as a component of astrocyte activation as a result of Abeta-triggered inflammation.

Brain aging in very old men with type 2 diabetes: the Honolulu-Asia Aging Study

OBJECTIVE

Type 2 diabetes leads to cognitive impairment and dementia, which may reflect microvascular and macrovascular complications as well as neurodegenerative processes. There are few studies on the anatomical basis for loss of cognitive function in type 2 diabetes. The objective of this study was to investigate the association between type 2 diabetes and markers of brain aging on magnetic resonance images, including infarcts, lacunes, and white matter hyperintensities as markers of vascular damage and general and hippocampal atrophy as markers of neurodegeneration in Japanese-American men born between 1900 and 1919 and followed since 1965 in the Honolulu-Asia Aging Study.

RESEARCH DESIGN AND METHODS

Prevalent and incident dementia was assessed. Associations between magnetic resonance imaging markers and diabetic status were estimated with logistic regression, controlling for sociodemographic and other vascular factors.

RESULTS

The prevalence of type 2 diabetes in the cohort is 38%. Subjects with type 2 diabetes had a moderately elevated risk for lacunes (odds ratio [OR] 1.6 [95% CI 1.0-2.6]) and hippocampal atrophy (1.7 [0.9-2.9]). The risk for both hippocampal atrophy and lacunes/infarcts was twice as high in subjects with compared with those without type 2 diabetes. Among the group with type 2 diabetes, those with the longest duration of diabetes, those taking insulin, and those with complications had relatively more pathologic brain changes.

CONCLUSIONS

There is evidence that older individuals with type 2 diabetes have an elevated risk for vascular brain damage and neurodegenerative changes. These pathological changes may be the anatomical basis for an increased risk of cognitive impairment or dementia in type 2 diabetes.

Convergence of genes implicated in Alzheimer's disease on the cerebral cholesterol shuttle: APP, cholesterol, lipoproteins, and atherosclerosis

Polymorphic genes associated with Alzheimer's disease (see ) delineate a clearly defined pathway related to cerebral and peripheral cholesterol and lipoprotein homoeostasis. They include all of the key components of a glia/neurone cholesterol shuttle including cholesterol binding lipoproteins APOA1, APOA4, APOC1, APOC2, APOC3, APOD, APOE and LPA, cholesterol transporters ABCA1, ABCA2, lipoprotein receptors LDLR, LRP1, LRP8 and VLDLR, and the cholesterol metabolising enzymes CYP46A1 and CH25H, whose oxysterol products activate the liver X receptor NR1H2 and are metabolised to esters by SOAT1. LIPA metabolises cholesterol esters, which are transported by the cholesteryl ester transport protein CETP. The transcription factor SREBF1 controls the expression of most enzymes of cholesterol synthesis. APP is involved in this shuttle as it metabolises cholesterol to 7-betahydroxycholesterol, a substrate of SOAT1 and HSD11B1, binds to APOE and is tethered to LRP1 via APPB1, APBB2 and APBB3 at the cytoplasmic domain and via LRPAP1 at the extracellular domain. APP cleavage products are also able to prevent cholesterol binding to APOE. BACE cleaves both APP and LRP1. Gamma-secretase (PSEN1, PSEN2, NCSTN) cleaves LRP1 and LRP8 as well as APP and their degradation products control transcription factor TFCP2, which regulates thymidylate synthase (TS) and GSK3B expression. GSK3B is known to phosphorylate the microtubule protein tau (MAPT). Dysfunction of this cascade, carved out by genes implicated in Alzheimer's disease, may play a major role in its pathology. Many other genes associated with Alzheimer's disease affect cholesterol or lipoprotein function and/or have also been implicated in atherosclerosis, a feature of Alzheimer's disease, and this duality may well explain the close links between vascular and cerebral pathology in Alzheimer's disease. The definition of many of these genes as risk factors is highly contested. However, when polymorphic susceptibility genes belong to the same signaling pathway, the risk associated with multigenic disease is better related to the integrated effects of multiple polymorphisms of genes within the same pathway than to variants in any single gene [Wu, X., Gu, J., Grossman, H.B., Amos, C.I., Etzel, C., Huang, M., Zhang, Q., Millikan, R.E., Lerner, S., Dinney, C.P., Spitz, M.R., 2006. Bladder cancer predisposition: a multigenic approach to DNA-repair and cell-cycle-control genes. Am. J. Hum. Genet. 78, 464-479.]. Thus, the fact that Alzheimer's disease susceptibility genes converge on a clearly defined signaling network has important implications for genetic association studies.

A common genetic system for functional studies of pitrilysin and related M16A proteases

Pitrilysin is a bacterial protease that is similar to the mammalian insulin-degrading enzyme, which is hypothesized to protect against the onset of Alzheimer's disease, and the yeast enzymes Axl1p and Ste23p, which are responsible for production of the a-factor mating pheromone in Saccharomyces cerevisiae. The lack of a phenotype associated with pitrilysin deficiency has hindered studies of this enzyme. Herein, we report that pitrilysin can be heterologously expressed in yeast such that it functionally substitutes for the shared roles of Axl1p and Ste23p in pheromone production, resulting in a readily observable phenotype. We have exploited this phenotype to conduct structure-function analyses of pitrilysin and report that residues within four sequence motifs that are highly conserved among M16A enzymes are essential for its activity. These motifs include the extended metalloprotease motif, a second motif that has been hypothesized to be important for the function of M16A enzymes, and two others not previously recognized as being important for pitrilysin function. We have also established that the two self-folding domains of pitrilysin are both required for its proteolytic activity. However, pitrilysin does not possess all the enzymatic properties of the yeast enzymes since it cannot substitute for the role of Axl1p in the repression of haploid invasive growth. These observations further support the utility of the yeast system for structure-function and comparative studies of M16A enzymes.

George-Hyslop P, Checler F. Presenilin-dependent gamma-secretase-mediated control of p53-associated cell death in Alzheimer's disease

Presenilins (PSs) are part of the gamma-secretase complex that produces the amyloid beta-peptide (Abeta) from its precursor [beta-amyloid precursor protein (betaAPP)]. Mutations in PS that cause familial Alzheimer's disease (FAD) increase Abeta production and trigger p53-dependent cell death. We demonstrate that PS deficiency, catalytically inactive PS mutants, gamma-secretase inhibitors, and betaAPP or amyloid precursor protein-like protein 2 (APLP2) depletion all reduce the expression and activity of p53 and lower the transactivation of its promoter and mRNA expression. p53 expression also is diminished in the brains of PS- or betaAPP-deficient mice. The gamma- and epsilon-secretase-derived amyloid intracellular C-terminal domain (AICD) fragments (AICDC59 and AICDC50, respectively) of betaAPP trigger p53-dependent cell death and increase p53 activity and mRNA. Finally, PS1 mutations enhance p53 activity in human embryonic kidney 293 cells and p53 expression in FAD-affected brains. Thus our study shows that AICDs control p53 at a transcriptional level, in vitro and in vivo, and that FAD mutations increase p53 expression and activity in cells and human brains.

Insulin resistance, inflammation, and cognition in Alzheimer's Disease: Lessons for multiple sclerosis

Insulin resistance (reduced ability of insulin to stimulate glucose utilization) is common in North American and Europe, where as many as one third of all older adults suffer from prodromal or clinical type 2 diabetes mellitus. It has long been known that insulin-resistant conditions adversely affect general health status. A growing body of findings suggests that insulin contributes to normal brain functioning and that peripheral insulin abnormalities increase the risk for memory loss and neurodegenerative disorders such as Alzheimer's disease. Potential mechanisms for these effects include insulin's role in cerebral glucose metabolism, peptide regulation, modulation of neurotransmitter levels, and modulation of many aspects of the inflammatory network. An intriguing question is whether insulin abnormalities also influence the pathophysiology of multiple sclerosis (MS), an autoimmune disorder characterized by elevated inflammatory biomarkers, central nervous system white matter lesions, axonal degeneration, and cognitive impairment. MS increases the risk for type 1 diabetes mellitus. Furthermore, the lack of association between MS and type 2 diabetes may suggest that insulin resistance affects patients with MS and the general population at the same alarming rate. Therefore, insulin resistance may exacerbate phenomena that are common to MS and insulin-resistant conditions, such as cognitive impairments and elevated inflammatory responses. Interestingly, the thiazolidinediones, which are used to treat patients with type 2 diabetes, have been proposed as potential therapeutic agents for both Alzheimer's disease and MS. The agents improve insulin sensitivity, reduce hyperinsulinemia, and exert anti-inflammatory actions. Ongoing studies will determine whether thiazolidinediones improve cognitive functioning for patients with type 2 diabetes or Alzheimer's disease. Future studies are needed to examine the effects of thiazolidinediones on patients with MS.

Rosiglitazone attenuates learning and memory deficits in Tg2576 Alzheimer mice

The thiazolidinediones, such as rosiglitazone, increase peripheral insulin sensitivity and their use is proposed for the treatment of Alzheimer's disease. However, the mechanisms underlying the potential beneficial effects of rosiglitazone in Alzheimer's disease remain unclear. In previous studies, we observed that Tg2576 Alzheimer mice develop peripheral insulin resistance with age and have much higher serum corticosterone levels than wild-type mice when fasted overnight. We further showed that both of these defects can be ameliorated by rosiglitazone administration. Here, we report that during behavioral testing which involves repetitive overnight fasting, Tg2576 mice administered rosiglitazone exhibited better spatial learning and memory abilities and had lower serum corticosterone levels than untreated Tg2576 mice. When untreated Tg2576 mice were administered metyrapone, a drug that blocks glucocorticoid production, their spatial learning and memory abilities and serum corticosterone levels were similar to those of rosiglitazone-treated mice. We further report here that rosiglitazone attenuated reductions in insulin-degrading enzyme (IDE) mRNA and activity, and reduced amyloid beta-peptide (Abeta)42 levels without affecting amyloid deposition, in the brains of Tg2576 mice. These results demonstrate that rosiglitazone attenuates learning and memory deficits in Tg2576 mice and suggest that the effects of the drug on learning and memory, brain IDE levels, and brain Abeta42 levels in the mice may be due to its glucocorticoid-lowering actions.

Hereditary cerebral hemorrhage with amyloidosis-Dutch type

The amyloid beta-protein (Abeta) E22Q mutation of the rare disorder hereditary cerebral hemorrhage with amyloidosis-Dutch type (HCHWA-D) causes severe cerebral amyloid angiopathy (CAA) with hemorrhagic strokes of mid-life onset and dementia. The mutation does not affect total Abeta production but may alter the Abeta1-42:Abeta1-40 ratio, and affect the proteolytic degradation of Abeta and its transport across the blood-brain barrier. Abeta E22Q aggregates faster into more stable amyloid-like fibrils than wild-type Abeta. Non-fibrillar Abeta(x-42) deposits precede the appearance of fibrils and the deposition of Abeta(x-40) in the vascular basement membrane. CAA severity tends to increase with age but may vary greatly among patients of comparable ages. Lumenal narrowing of affected blood vessels, leukoencephalopathy, CAA-associated vasculopathies, and perivascular astrocytosis, microgliosis, and neuritic degeneration complicate the development of HCHWA-D CAA. Parenchymal Abeta deposition is also enhanced in the HCHWA-D brain with non-fibrillar membrane-bound Abeta(x-42) deposits evolving into relatively fibrillar diffuse plaques variously associated with reactive astrocytes, activated microglia, and degenerating neurites. Plaque density tends to decrease with age. Neurofibrillary degeneration is absent or limited. HCHWA-D dementia is associated with CAA severity independently of Braak stage, age, and plaque density. Particularly, microaneurysms may contribute to the development of (small) hemorrhages/infarcts and the latter to cognitive decline in affected subjects. However, the relative importance of cerebral hemorrhages/infarcts, white matter damage and/or other CAA- or Abeta-related factors for cognitive deterioration in HCHWA-D remains to be determined.

Amyloidogenecity and pitrilysin sensitivity of a lysine-free derivative of amyloid beta-peptide cleaved from a recombinant fusion protein

The progressive cerebral deposition of a 40-42 residues amyloid beta-peptide (Abeta) is regarded as a major factor in the onset of the Alzheimer's disease. It has recently been shown that Abeta(1-40) is cleaved by Escherichia coli pitrilysin, a homologue of insulysin, at a specific site. To facilitate the studies on a recognition mechanism of Abeta by pitrilysin, an overproduction system of Abeta(1-40) as a fusion protein with E. coli RNase HI was constructed. This fusion protein was designed such that an Abeta(1-40) derivative, Abeta(1-40)*, in which Lys16 and Lys28 of Abeta(1-40) are simultaneously replaced by Ala, is attached to the C-terminus of E. coli RNase HI and Abeta(1-40)* is separated from RNase HI upon cleavage with lysyl endopeptidase. The fusion protein was overproduced in E. coli in inclusion bodies, solubilized and purified in the presence of guanidine hydrochloride, and cleaved by lysyl endopeptidase. Abeta(1-40)* was purified from the resultant peptide fragments by reverse-phase HPLC. Measurement of the far-UV CD spectra suggests that Abeta(1-40)* is conformationally similar to Abeta(1-40). However, the thioflavin T binding assay suggests that Abeta(1-40)* is more amyloidogenic than Abeta(1-40). Nevertheless, Abeta(1-40)* was cleaved by pitrilysin at the site identical to that in Abeta(1-40).

Proteolytic mechanisms in amyloid-beta metabolism: therapeutic implications for Alzheimer's disease

The accumulation of the amyloid-beta peptide, the main constituent of the "amyloid plaque", is widely considered to be the key pathological event in Alzheimer's disease. Amyloid-beta is produced from the amyloid precursor protein through the action of the proteases beta-secretase and gamma-secretase. Alternative cleavage of amyloid precursor protein by the enzyme alpha-secretase precludes amyloid-beta production. In addition, several proteases are involved in the degradation of amyloid-beta. This review focuses on the proteolytic mechanisms of amyloid-beta metabolism. An increasingly detailed understanding of proteolysis in both amyloid-beta deposition and clearance has identified some of these proteases as potential therapeutic targets for Alzheimer's disease. A more complex knowledge of these proteases takes us one step closer to developing "disease-modifying" therapies, but these advances also emphasize that significant challenges must be overcome before clinically effective drugs to treat Alzheimer's disease become a reality.

Connective tissue growth factor (CTGF) expression in the brain is a downstream effector of insulin resistance‐associated promotion of Alzheimer's disease β‐amyloid neuropathology

The goal of this study was to further explore potential mechanisms through which diabetogenic dietary conditions that result in promotion of insulin resistance (IR), a feature of non-insulin dependant diabetes mellitus (type-2 diabetes), may influence Alzheimer's disease (AD). Using genome-wide array technology, we found that connective tissue growth factor (CTGF), a gene product described previously for its involvement in diabetic fibrosis, is elevated in brain tissue in an established mouse model of diet-induced IR. With this evidence we continued to explore the regulation of CTGF in postmortem AD brain tissue and found that CTGF expression correlated with the progression of AD clinical dementia and amyloid neuritic plaque (NP) neuropathology, but not neurofibrillary tangle (NFT) deposition. Consistent with this evidence, we also found that exposure of Tg2576 mice (a model AD-type amyloid neuropathology) to a diabetogenic diet that promotes IR results in a ~2-fold elevation in CTGF steady-state levels in the brain, coincident with a commensurate promotion of AD-type amyloid plaque burden. Finally, using in vitro cellular models of amyloid precursor protein (APP)-processing and Abeta generation/clearance, we confirmed that human recombinant (hr)CTGF may increase Abeta1-40 and Abeta1-42 peptide steady-state levels, possibly through a mechanism that involves gamma-secretase activation and decreased insulin-degrading enzyme (IDE) steady-state levels in a MAP kinase (MAPK)/ phosphatidylinositol 3-kinase (PI-3K)/protein kinase-B (AKT)1-dependent manner. The findings in this study tentatively suggest that increased CTGF expression in the brain might be a novel biological predicative factor of AD clinical progression and neuropathology in response to dietary regimens promoting IR conditions.

Androgen- and estrogen-dependent regulation of insulin-degrading enzyme in subcellular fractions of rat prostate and uterus

Innumerous data support the fact that insulin-degrading enzyme (IDE) is the primary enzymatic mechanism for initiating and controlling cellular insulin degradation. Nevertheless, insulin degradation is unlikely to be the only cellular function of IDE, because it appears that some cellular effects of insulin are mediated by IDE as a regulatory protein. Insulin-degrading enzyme shows a significant correlation with various cellular functions, such as cellular growth and differentiation, and the expression of IDE is developmentally regulated. Besides insulin, other substrates are also degraded by IDE, including various growth-promoting peptides. It has also been shown that IDE enhances the binding of androgen to DNA in the nuclear compartment. It is also known that the androgen hormones have a stimulatory effect on prostate growth, and that estradiol stimulates uterine growth. To establish whether IDE is regulated by a cellular prostate/uterine growth stimulus, the present study assessed whether IDE was modified in quantity and activity during proliferative conditions (castration + testosterone in the male rat, or castration + estradiol or the proestrus phase of the estrous cycle in the female rat) and autolysis (castration or the metestrus phase of the estrous cycle) using cytosolic and nuclear fractions of rat prostate and cytosolic fractions of rat uterus. The activity and amount of IDE decreased in the cytosolic fraction with castration and during metestrus, and increased with testosterone or estradiol treatment and during proestrus. In the nuclear fraction, the quantity of the IDE followed the same pattern observed in the cytosolic fraction, although without degradative activity. The data presented here suggest that IDE may participate in prostatic and uterine growth and that the testosterone or estradiol and/or prostate and uterus insulin-like growth factors may be important factors for the expression and regulation of IDE in the prostate and uterus.

Insulin-degrading enzyme degrades amyloid peptides associated with British and Danish familial dementia

Familial British dementia (FBD) and familial Danish dementia (FDD) are autosomal dominant disorders characterized by cerebrovascular and parenchymal amyloid deposition and neurofibrillary degeneration. In both conditions, the genetic defects cause the loss of the normal stop codon in the precursor BRI, generating novel 34-residue peptides named ABri and ADan in FBD and FDD, respectively. ABri and ADan show a strong tendency to aggregate into non-fibrillar and fibrillar structures at neutral pH and this property seems to be directly related to neurotoxicity. Here we report that a recombinant insulin-degrading enzyme (rIDE) was capable of degrading monomeric ABri and ADan in vitro more efficiently than oligomeric species. These peptides showed high beta-structure content and were more resistant to proteolysis as compared to the BRI wild-type product of 23 amino acids. Specific sites of cleavage within the C-terminal pathogenic extensions raise the possibility that proteolysis of monomeric soluble precursors by IDE may delay ABri and ADan aggregation in vivo.

Brain insulin impairs amyloid-beta(1-40) clearance from the brain

Cerebral amyloid-beta peptide (Abeta) clearance plays a key role in determining the brain level of Abeta; however, its mechanism remains unclear. In this study, we investigated cerebral Abeta clearance across the blood-brain barrier (BBB) by using the Brain Efflux Index method. [125I]Abeta(1-40) was eliminated from rat brain to circulating blood with a half-life of 48.8 min and a half-saturation concentration of 8.15 nm. The Abeta(1-40) elimination rate was reduced by 30.5% in 23-month-old rats compared with 7-week-old rats. The intact form of Abeta(1-40) was detected in plasma after intracerebral administration, indicating the occurrence of efflux transport of intact Abeta(1-40). The Abeta(1-40) elimination rate was significantly inhibited by coadministration of 100 microg/ml insulin and 1 mm thiorphan by 44.6 and 34.0%, respectively. The level of intact [125I]Abeta(1-40) in the brain was increased by coadministration of insulin. Among insulin-degrading enzyme inhibitors, bacitracin inhibited the elimination rate, whereas N-ethylmaleimide and metal chelators had no effect. Receptor-associated protein, fucoidan, 3-bromo-5-t-butyl-4-hydroxy-benzylidenemalonitrile, anti-IGF-I receptor antibody, and l-tyrosine did not affect the Abeta(1-40) elimination rate, suggesting that the relevant receptors or transporters are not likely to be involved in the clearance. In conclusion, the present study has demonstrated the involvement of a proteolytic degradation process and an insulin-sensitive process in cerebral Abeta(1-40) clearance in the rat.

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.

Amyloid beta degradation: a challenging task for brain peptidases

Amyloid beta (Abeta) accumulates in the neuropil and within the walls of cerebral vessels in association with normal aging, dementia or stroke. Abeta is released from its precursor protein as soluble monomeric species yet, under pathological conditions, it self-aggregates to form soluble oligomers or insoluble fibrils that may be toxic to neurons and vascular cells. Abeta levels could be lowered by inhibiting its generation or by promoting its clearance by transport or degradation. Here we will summarize recent findings on brain proteases capable of degrading Abeta, with a special focus on those enzymes for which there is genetic, transgenic or biochemical evidence supporting a role in the proteolysis of Abeta in vivo.

Insulin-degrading Enzyme in Brain Microvessels

The accumulation of amyloid beta (Abeta) in the walls of small vessels in the cerebral cortex is associated with diseases characterized by dementia or stroke. These include Alzheimer's disease, Down syndrome, and sporadic and hereditary cerebral amyloid angiopathies (CAAs) related to mutations within the Abeta sequence. A higher tendency of Abeta to aggregate, a defective clearance to the systemic circulation, and insufficient proteolytic removal have been proposed as mechanisms that lead to Abeta accumulation in the brain. By using immunoprecipitation and mass spectrometry, we show that insulin-degrading enzyme (IDE) from isolated human brain microvessels was capable of degrading (125)I-insulin and cleaved Abeta-(1-40) wild type and the genetic variants Abeta A21G (Flemish), Abeta E22Q (Dutch), and Abeta E22K (Italian) at the predicted sites. In microvessels from Alzheimer's disease cases with CAA, IDE protein levels showed a 44% increase as determined by sandwich enzyme-linked immunosorbent assay and Western blot. However, the activity of IDE upon radiolabeled insulin was significantly reduced in CAA as compared with age-matched controls. These results support the notion that a defect in Abeta proteolysis by IDE contributes to the accumulation of this peptide in the cortical microvasculature. Moreover they raise the possibility that IDE inhibition or inactivation is a pathogenic mechanism that may open novel strategies for the treatment of cerebrovascular Abeta amyloidoses.