Cerebral Aβ Levels Research Articles

Intracellular cleavage of amyloid β by a viral protease NIa prevents amyloid β-mediated cytotoxicity.

Nuclear inclusion a (NIa) of turnip mosaic virus is a cytosolic protease that cleaves amyloid β (Aβ) when heterologously overexpressed. Lentivirus-mediated expression of NIa in the brains of APP(sw)/PS1 mice significantly reduces cerebral Aβ levels and plaque depositions, and improves behavioral deficits. Here, the effects of NIa and neprilysin (NEP), a well-known Aβ-cleaving protease, on oligomeric Aβ-induced cell death were evaluated. NIa cleaved monomeric and oligomeric Aβ at a similar rate, whereas NEP only cleaved monomeric Aβ. Oligomeric Aβ-induced cytotoxicity and mitochondrial dysfunction were significantly ameliorated by NIa, but not by NEP. Endocytosed fluorescently-labeled Aβ localized to mitochondria, and this was significantly reduced by NIa, but not by NEP. These data suggest that NIa may exerts its protective roles by degrading Aβ and thus preventing mitochondrial deposition of Aβ.

Substrate ectodomain is critical for substrate preference and inhibition of γ-secretase

Understanding the substrate recognition mechanism of γ-secretase is a key step for establishing substrate-specific inhibition of amyloid β-protein (Aβ) production. However, it is widely believed that γ-secretase is a promiscuous protease and that its substrate-specific inhibition is elusive. Here we show that γ-secretase distinguishes the ectodomain length of substrates and preferentially captures and cleaves substrates containing a short ectodomain. We also show that a subset of peptides containing the CDCYCxxxxCxCxSC motif binds to the amino terminus of C99 and inhibits Aβ production in a substrate-specific manner. Interestingly, these peptides suppress β-secretase-dependent cleavage of APP, but not that of sialyltransferase 1. Most importantly, intraperitoneal administration of peptides into mice results in a significant reduction in cerebral Aβ levels. This report provides direct evidence of the substrate preference of γ-secretase and its mechanism. Our results demonstrate that the ectodomain of C99 is a potent target for substrate-specific anti-Aβ therapeutics to combat Alzheimer’s disease.

In Vivo Uptake of β-Amyloid by Non-Plaque Associated Microglia

The role of microglia in β-amyloid (Aβ) deposition or clearance in the Alzheimer's disease (AD) brain remains unclear. Previous in vivo studies have focused primarily on the association of microglia with Aβ-positive parenchymal plaques, but have given little consideration to the possible interaction between Aβ and non-plaque associated microglia. Further, it is not known if microglia play a direct role in mediating Aβ uptake following anti-aggregant treatment. We report here the identification of Aβ-positive processes throughout the cortex and hippocampus of TgCRND8 mice expressing the human Swedish (KM670/671NL) and Indiana (V717F) amyloid precursor protein mutations, which localized to ionized calcium binding protein-1-positive resident microglia that were not associated with extracellular plaques. Oral administration of 1-deoxy-1-fluoro-scyllo-inositol, a scyllo-inositol analogue, to TgCRND8 mice improved spatial memory impairments and suppressed amyloid pathology in a dose-dependent manner. Further, treatment with 1-deoxy-1- fluoro-scyllo-inositol significantly increased hippocampal intra-microglial Aβ levels without stimulating microglial proliferation or peripheral macrophage recruitment. These results reveal a novel, beneficial role for non-plaque associated microglia in the regulation of cerebral Aβ levels in a mouse model of AD.

Proteolytic Degradation of Amyloid -Protein

The amyloid β-protein (Aβ) is subject to proteolytic degradation by a diverse array of peptidases and proteinases, known collectively as Aβ-degrading proteases (AβDPs). A growing number of AβDPs have been identified, which, under physiological and/or pathophysiological conditions, contribute significantly to the determination of endogenous cerebral Aβ levels. Despite more than a decade of investigation, the complete set of AβDPs remains to be established, and our understanding of even well-established AβDPs is incomplete. Nevertheless, the study of known AβDPs has contributed importantly to our understanding of the molecular pathogenesis of Alzheimer disease (AD) and has inspired the development of several novel therapeutic approaches to the regulation of cerebral Aβ levels. In this article, we discuss the general features of Aβ degradation and introduce the best-characterized AβDPs, focusing on their diverse properties and the numerous conceptual insights that have emerged from the study of each.

Multi-faced neuroprotective effects of Ginsenoside Rg1 in an Alzheimer mouse model

There has been no extensive characterization of the effects of Ginsenoside Rg1, a pharmacological active component purified from the nature product ginseng, in an Alzheimer's disease mouse model. The well-characterized transgenic Alzheimer disease (AD) mice over expressing amyloid precursor protein (APP)/Aβ (Tg mAPP) and nontransgenic (nonTg) littermates at age of 6 and 9 months were treated with Rg 1 for three months via intraperitoneal injection. Mice were then evaluated for changes in amyloid pathology, neuropathology and behavior. Tg mAPP treated with Rg1 showed a significant reduction of cerebral Aβ levels, reversal of certain neuropathological changes, and preservation of spatial learning and memory, as compared to vehicle-treated mice. Rg1 treatment inhibited activity of γ-secretase in both Tg mAPP mice and B103-APP cells, indicating the involvement of Rg1 in APP regulation pathway. Furthermore, administration of Rg1 enhanced PKA/CREB pathway activation in mAPP mice and in cultured cortical neurons exposed to Aβ or glutamate-mediated synaptic stress. Most importantly, the beneficial effects on attenuation of cerebral Aβ accumulation, improvement in neuropathological and behavioral changes can be extended to the aged mAPP mice, even to 12–13 months old mice that had extensive amyloid pathology and severe neuropathological and cognitive malfunction. These studies indicate that Rg1 has profound multi-faced and neuroprotective effects in an AD mouse model. Rg1 induces neuroprotection through ameliorating amyloid pathology, modulating APP process, improving cognition, and activating PKA/CREB signaling. These findings provide a new perspective for the treatment of AD and demonstrate potential for a new class of drugs for AD treatment.

Direct Exposure of Guinea Pig CNS to Human Luteinizing Hormone Increases Cerebrospinal Fluid and Cerebral Beta Amyloid Levels

Background/Aims: Luteinizing hormone (LH) has been shown to alter the metabolism of beta amyloid (Aβ), a key protein in Alzheimer’s disease (AD) pathogenesis. While LH and components required for LH receptor signalling are present in the brain, their role in the CNS remains unclear. In vitro, LH has been shown to facilitate neurosteroid production and alter Aβ metabolism. However, whether LH can directly modulate cerebral Aβ levels in vivo has not previously been studied. In this study, we investigated the effect of chronic administration of LH to the guinea pig CNS on cerebral Aβ levels. Methods: Gonadectomised male animals were administered, via cortical placement, either placebo or LH slow-release pellets. At 14 and 28 days after treatment, animals were sacrificed. Brain, plasma and CSF were collected and Aβ levels measured via ELISA. Levels of the Aβ precursor protein (APP) and the neurosteroidogenic enzyme cytochrome P450 side-chain cleavage enzyme (P450scc) were also assayed. Results: An increase in CSF Aβ40 levels was observed 28 days following treatment. These CSF data also reflected changes in Aβ40 levels observed in brain homogenates. No change was observed in plasma Aβ40 levels but APP and its C-terminal fragments (APP-CTF) were significantly increased in response to LH exposure. Protein expression of P450scc was increased after 28 days of LH exposure, suggesting activation of the LH receptor. Conclusion: These data indicate that direct exposure of guinea pig CNS to LH results in altered brain Aβ levels, perhaps due to altered APP expression/metabolism.

Cerebral amyloid-β proteostasis is regulated by the membrane transport protein ABCC1 in mice

In Alzheimer disease (AD), the intracerebral accumulation of amyloid-β (Aβ) peptides is a critical yet poorly understood process. Aβ clearance via the blood-brain barrier is reduced by approximately 30% in AD patients, but the underlying mechanisms remain elusive. ABC transporters have been implicated in the regulation of Aβ levels in the brain. Using a mouse model of AD in which the animals were further genetically modified to lack specific ABC transporters, here we have shown that the transporter ABCC1 has an important role in cerebral Aβ clearance and accumulation. Deficiency of ABCC1 substantially increased cerebral Aβ levels without altering the expression of most enzymes that would favor the production of Aβ from the Aβ precursor protein. In contrast, activation of ABCC1 using thiethylperazine (a drug approved by the FDA to relieve nausea and vomiting) markedly reduced Aβ load in a mouse model of AD expressing ABCC1 but not in such mice lacking ABCC1. Thus, by altering the temporal aggregation profile of Aβ, pharmacological activation of ABC transporters could impede the neurodegenerative cascade that culminates in the dementia of AD.

Impaired amyloid β-degrading enzymes in brain of streptozotocin-induced diabetic rats

Enzymes that degrade the amyloid β-peptide (Aβ) are important regulators of cerebral Aβ levels. High level of Aβ was found in the brain of diabetic patients and diabetic animals. Aim of the study was to investigate whether activities of Aβ-degrading enzymes neprilysin (NEP), endothelin-converting enzyme 1 (ECE-1) and insulin-degrading enzyme (IDE) were impaired in the brain of diabetic rats. Diabetes was induced in rats by ip administration of 65 mg/kg streptozotocin. The temporal cortex and hippocampus were obtained for activity and mRNA level assays of the three enzymes on the 35th day after induction. ECE-1 activity was significantly decreased both in the hippocampus and cortex of diabetic rats, while for IDE significantly lower activity occurred only in the cortex. NEP activity was slightly decreased in both brain regions. The hippocampus of diabetic rats showed significant decrease in mRNA levels of NEP and ECE-1 and moderate increase in IDE mRNA level. The cortex of diabetic rats showed slight decrease in mRNA levels of the three enzymes. The results indicated that the three Aβ-degrading enzymes were damaged to different extents in the brain of diabetic rats, and impairment of ECE-1 and IDE partly contributed to the elevated Aβ(1-40) levels in brain of diabetic rats.

Intranasal Phosphoramidon Increases Beta-Amyloid Levels in Wild-Type and NEP/NEP2-Deficient Mice

Intranasal administration is emerging as a reliable and non-invasive method to bypass the blood-brain barrier and deliver drugs to the brain. This approach has been primarily used to explore therapeutic avenues for neurological diseases. However, intranasal administration could also be used to create animal models of brain disease. Beta-amyloid peptide (Aβ) accumulation is a key feature of Alzheimer's disease (AD), and the most common models of AD are transgenic mice expressing mutant human genes linked to familial AD. An alternative model of amyloidosis utilizes intracerebroventricular infusion of thiorphan or phosphoramidon to block the activity of key Aβ degrading enzymes (NEP, NEP2) resulting in accumulation of Aβ. Here, we demonstrate that intranasal administration of phosphoramidon produces significantly elevated cerebral Aβ levels in wild-type mice. Furthermore, intranasal phosphoramidon administration in double knockout mice lacking NEP and NEP2 also showed increased levels of Aβ(40). These data show that intranasal delivery of drugs can be used to model AD and suggest that other phosphoramidon-sensitive peptidases are degrading Aβ in NEP/NEP2-deficient mice.

Synthesis and Biological Evaluation of Analogues of a Novel Inhibitor of β-Amyloid Secretion

A drug library of 17,200 compounds was screened to select small molecules that inhibit the secretion of amyloid beta peptide (Abeta), the major component of Alzheimer disease senile plaques, from a human neuronal cell line. Twenty-nine hits were validated that decreased Abeta secretion by >40% at 10 microM, for a 0.17% hit rate. A lead hit was selected for further study based on its activity and low cytotoxicity, and it was found to inhibit Abeta secretion through activation of the alpha-secretase pathway. Twenty-four commercially available and 53 synthesized analogues were analyzed for activity. Selected analogues were evaluated for biological stability by incubation with hepatoma cells and for transcellular permeability using Caco-2 cell monolayers. The analogue with the best permeability was evaluated in 2-month old amyloid precursor protein transgenic mice and found to acutely reduce cerebral Abeta levels by 40% after a single iv administration.

NFκB-dependent Control of BACE1 Promoter Transactivation by Aβ42

β-Amyloid (Aβ) peptides that accumulate in Alzheimer disease are generated from the β-amyloid precursor protein (βAPP) by cleavages by β-secretase BACE1 and by presenilin-dependent γ-secretase activities. Very few data document a putative cross-talk between these proteases and the regulatory mechanisms underlying such interaction. We show that presenilin deficiency lowers BACE1 maturation and affects both BACE1 activity and promoter transactivation. The specific γ-secretase inhibitor DFK167 triggers the decrease of BACE1 activity in wild-type but not in presenilin-deficient fibroblasts. This decrease is also elicited by catalytically inactive γ-secretase. The overexpression of APP intracellular domain (AICD), the γ/ϵ-secretase-derived C-terminal product of β-amyloid precursor protein, does not modulate BACE1 activity or promoter transactivation in fibroblasts and does not alter BACE1 expression in AICD transgenic brains of mice. A DFK167-sensitive increase of BACE1 activity is observed in cells overexpressing APPϵ (the N-terminal product of βAPP generated by ϵ-secretase cleavage harboring the Aβ domain but lacking the AICD sequence), suggesting that the production of Aβ could account for the modulation of BACE1. Accordingly, we show that HEK293 cells overexpressing wild-type βAPP exhibit a DFK167-sensitive increase in BACE1 promoter transactivation that is increased by the Aβ-potentiating Swedish mutation. This effect was mimicked by exogenous application of Aβ42 but not Aβ40 or by transient transfection of cDNA encoding Aβ42 sequence. The IκB kinase inhibitor BMS345541 prevents Aβ-induced BACE1 promoter transactivation suggesting that NFκB could mediate this Aβ-associated phenotype. Accordingly, the overexpression of wild-type or Swedish mutated βAPP does not modify the transactivation of BACE1 promoter constructs lacking NFκB-responsive element. Furthermore, APP/β-amyloid precursor protein-like protein deficiency does not affect BACE1 activity and expression. Overall, these data suggest that physiological levels of endogenous Aβ are not sufficient per se to modulate BACE1 promoter transactivation but that exacerbated Aβ production linked to wild-type or Swedish mutated βAPP overexpression modulates BACE1 promoter transactivation and activity via an NFκB-dependent pathway.

Genomic analysis of response to traumatic brain injury in a mouse model of Alzheimer's disease (APPsw)

Numerous studies have shown that the β-amyloid peptide (Aβ) or β-amyloid deposits impact many processes that can contribute to neurodegeneration, ranging from immune and inflammatory processes to cell death and apoptosis, processes characteristic of both Alzheimer's disease and head injury. Human and animal studies of traumatic brain injury (TBI) have shown that Aβ production is increased acutely following injury, and there is evidence for increased amyloid deposition and risk for Alzheimer's disease following TBI. Given the poorer outcome after injury observed both in transgenic mice overproducing Aβ, as well as in humans subjected to repetitive head injury, one may conclude that the presence of elevated brain levels of Aβ, whether endogenous or as a consequence of previous injury, exacerbates many of the deleterious processes triggered by TBI. We sought to test this hypothesis by examining the genomic response to injury in wild-type mice and in transgenic mice (APPsw) overexpressing and accumulating cerebral Aβ/β-amyloid. Gene expression was investigated by microarray 24 h after controlled cortical impact (CCI) injury or sham injury in aged APPsw transgenic mice and wild-type controls. Stringent statistical analysis revealed differential expression of a total of 129 genes in the transgenic TBI vs. sham comparison and 119 genes in the wild-type TBI vs. sham comparison. Of these, only 28 genes were common to both comparisons, suggesting considerable differences in response to injury in the Alzheimer models compared to wild-type mice. We focused our analyses by creating a “genotype-dependent” data set of response to injury which contained the genes that were uniquely altered in response to injury in either wild-type or APPsw mice, as well as those which were significantly differently modulated following TBI in one genotype compared to the other. The cellular functions predicted to be influenced by these changes in gene expression thus indicate the adverse pathways triggered by increased levels of Aβ, and the potentially favorable (recovery) pathways which are activated in wild-type mice but suppressed when Aβ levels are high. The results show that the cellular functions most influenced by the cerebral Aβ levels following TBI include inflammation, immune response, and cell death, which suggest a particular vulnerability to head injury in the Alzheimer brain.

Effects of prolonged angiotensin-converting enzyme inhibitor treatment on amyloid β-protein metabolism in mouse models of Alzheimer disease

Genetic and pathologic studies have associated angiotensin-converting enzyme (ACE) with Alzheimer disease. Previously, we and others have reported that ACE degrades in vitro the amyloid beta-protein (Abeta), a putative upstream initiator of Alzheimer disease. These studies support the hypothesis that deficiency in ACE-mediated Abeta proteolysis could increase Alzheimer disease risk and raise the question of whether ACE inhibitors, a commonly prescribed class of anti-hypertensive medications, can elevate Abeta levels in vivo. To test this hypothesis, we administered the ACE inhibitor captopril to two lines of APP transgenic mice harboring either low levels of Abeta or high levels of Abeta with associated plaque deposition. In both models, we show that captopril does not affect cerebral Abeta levels in either soluble or insoluble pools. Furthermore, we find no change in plaque deposition or in peripheral Abeta levels. Data from these Alzheimer models suggest that captopril and similar ACE inhibitors do not cause Abeta accumulation in vivo.

O4-06-06: Endogenous neprilysin modulates cerebral Aβ levels, amyloid plaque burden, and amyloid angiopathy in human APP transgenic mice

O4-06-06: Endogenous neprilysin modulates cerebral Aβ levels, amyloid plaque burden, and amyloid angiopathy in human APP transgenic mice

P1-039: Hierarchical deposition of amyloid β-protein (Aβ) deposits in the brain results from increasing cerebral Aβ-levels

P1-039: Hierarchical deposition of amyloid β-protein (Aβ) deposits in the brain results from increasing cerebral Aβ-levels

Neprilysin-sensitive Synapse-associated Amyloid-β Peptide Oligomers Impair Neuronal Plasticity and Cognitive Function

Neprilysin-sensitive Synapse-associated Amyloid-β Peptide Oligomers Impair Neuronal Plasticity and Cognitive Function

Dendrimeric Aβ1–15 is an effective immunogen in wildtype and APP-tg mice

Immunization of humans and APP-tg mice with full-length β-amyloid (Aβ) results in reduced cerebral Aβ levels. However, due to adverse events in the AN1792 trial, alternative vaccines are required. We investigated dendrimeric Aβ1–15 (dAβ1–15), which is composed of 16 copies of Aβ1–15 peptide on a branched lysine core and thus, includes an Aβ-specific B cell epitope but lacks the reported T cell epitope. Immunization by subcutaneous, transcutaneous, and intranasal routes of B6D2F1 wildtype mice led to anti-Aβ antibody production. Antibody isotypes were mainly IgG1 for subcutaneous or transcutaneous immunization and IgG2b for intranasal immunization, suggestive of a Th2-biased response. All Aβ antibodies preferentially recognized an epitope in Aβ1–7. Intranasal immunization of J20 APP-tg mice resulted in a robust humoral immune response with a corresponding significant reduction in cerebral plaque burden. Splenocyte proliferation against Aβ peptide was minimal indicating the lack of an Aβ-specific cellular immune response. Anti-Aβ antibodies bound monomeric, oligomeric, and fibrillar Aβ. Our data suggest that dAβ1–15 may be an effective and potentially safer immunogen for Alzheimer's disease (AD) vaccination.

Short Amyloid-β (Aβ) Immunogens Reduce Cerebral Aβ Load and Learning Deficits in an Alzheimer's Disease Mouse Model in the Absence of an Aβ-Specific Cellular Immune Response

Amyloid-beta (Abeta) immunotherapy lowers cerebral Abeta and improves cognition in mouse models of Alzheimer's disease (AD). A clinical trial using active immunization with Abeta1-42 was suspended after approximately 6% of patients developed meningoencephalitis, possibly because of a T-cell reaction against Abeta. Nevertheless, beneficial effects were reported in antibody responders. Consequently, alternatives are required for a safer vaccine. The Abeta1-15 sequence contains the antibody epitope(s) but lacks the T-cell reactive sites of full-length Abeta1-42. Therefore, we tested four alternative peptide immunogens encompassing either a tandem repeat of two lysine-linked Abeta1-15 sequences (2xAbeta1-15) or the Abeta1-15 sequence synthesized to a cross-species active T1 T-helper-cell epitope (T1-Abeta1-15) and each with the addition of a three-amino-acid RGD (Arg-Gly-Asp) motif (R-2xAbeta1-15; T1-R-Abeta1-15). High anti-Abeta antibody titers were observed in wild-type mice after intranasal immunization with R-2xAbeta1-15 or 2xAbeta1-15 plus mutant Escherichia coli heat-labile enterotoxin LT(R192G) adjuvant. Moderate antibody levels were induced after immunization with T1-R-Abeta1-15 or T1-Abeta1-15 plus LT(R192G). Restimulation of splenocytes with the corresponding immunogens resulted in moderate proliferative responses, whereas proliferation was absent after restimulation with full-length Abeta or Abeta1-15. Immunization of human amyloid precursor protein, familial AD (hAPP(FAD)) mice with R-2xAbeta1-15 or 2xAbeta1-15 resulted in high anti-Abeta titers of noninflammatory T-helper 2 isotypes (IgG1 and IgG2b), a lack of splenocyte proliferation against full-length Abeta, significantly reduced Abeta plaque load, and lower cerebral Abeta levels. In addition, 2xAbeta1-15-immunized hAPP(FAD) animals showed improved acquisition of memory compared with vehicle controls in a reference-memory Morris water-maze behavior test that approximately correlated with anti-Abeta titers. Thus, our novel immunogens show promise for future AD vaccines.

Amyloid β-Protein Is Degraded by Cellular Angiotensin-converting Enzyme (ACE) and Elevated by an ACE Inhibitor

Human genetic data have associated angiotensin-converting enzyme (ACE) with Alzheimer disease (AD), and purified ACE has been reported to cleave synthetic amyloid beta-protein (Abeta) in vitro. Whether deficiency in ACE activity, arising from genetic alteration or pharmacological inhibition, can decrease Abeta degradation and allow Abeta accumulation in intact cells is unknown. We cloned ACE from human neuroblastoma cells and showed that it had posttranslational processing and enzymatic activity typical of the endogenous protease. Cellular expression of ACE promoted degradation of naturally secreted Abeta40 and Abeta42, leading to significant clearance of both species. Using site-directed mutagenesis, we determined that both active sites within ACE contribute to Abeta clearance, and an ACE construct bearing mutations in each catalytic domain had no effect on Abeta levels. Pharmacological inhibition of ACE with a widely prescribed drug, captopril, promoted the accumulation of cell-derived Abeta in the media of beta-amyloid precursor-protein expressing cells. Together, these results show that ACE can lower the levels of secreted Abeta in living cells and that this effect is blocked by inhibiting the protease's activity with an ACE inhibitor. This work, combined with the genetic studies, supports the hypothesis that ACE may modulate the susceptibility to and progression of AD via degradation of Abeta. Our data encourage further analyses of the ACE gene for disease association and raise the question of whether currently prescribed ACE inhibitors could elevate cerebral Abeta levels in humans.

Lack of protective effect of estrogens on cerebral Aβ levels in intact female and male APP transgenic mice

AbstractIn previous experiments, treatment with either 17β‐estradiol or 17α‐estradiol decreased cerebral β‐amyloid peptide (Aβ) levels in ovariectomized female APPSWE transgenic mice, a mouse model for Alzheimer disease‐like pathology. However, no effect of ovariectomy itself on cerebral Aβ levels was observed. In the current study, intact female and male APPIND,SWE transgenic mice were treated chronically with oral 17β‐estradiol or 17α‐estradiol. These treatments did not reduce cerebral Aβ levels or amyloid plaque counts vs. vehicle treated mice. In fact, 17β‐estradiol treatment of the intact female mice led to a significant increase in cerebral Aβ levels. Drug Dev. Res. 66:136–141, 2006. © 2006 Wiley‐Liss, Inc.