Presynaptic localization of neprilysin contributes to efficient clearance of amyloid-beta peptide in mouse brain

Statins promote the degradation of extracellular amyloid {beta}-peptide by microglia via stimulation of exosome-associated insulin-degrading enzyme (IDE) secretion

Epidemiological studies indicate that intake of statins decrease the risk of developing Alzheimer disease. Cellular and in vivo studies suggested that statins might decrease the generation of the amyloid β-peptide (Aβ) from the β-amyloid precursor protein. Here, we show that statins potently stimulate the degradation of extracellular Aβ by microglia. The statin-dependent clearance of extracellular Aβ is mainly exerted by insulin-degrading enzyme (IDE) that is secreted in a nonconventional pathway in association with exosomes. Stimulated IDE secretion and Aβ degradation were also observed in blood of mice upon peripheral treatment with lovastatin. Importantly, increased IDE secretion upon lovastatin treatment was dependent on protein isoprenylation and up-regulation of exosome secretion by fusion of multivesicular bodies with the plasma membrane. These data demonstrate a novel pathway for the nonconventional secretion of IDE via exosomes. The modulation of this pathway could provide a new strategy to enhance the extracellular clearance of Aβ.

N-acetylcysteine prevents 4-hydroxynonenal- and amyloid-beta-induced modification and inactivation of neprilysin in SH-SY5Y cells

As one of the dominant amyloid-beta peptide (Abeta) proteases, neprilysin (NEP) plays a crucial role in maintaining a physiologic balance between Abeta production and catabolism. We have previously shown that NEP is modified by 4-hydroxynonenal (HNE) adducts, resulting in decreased activity in the brain of AD patients and cultured cells. In order to determine whether antioxidants can rescue NEP, SH-SY5Y cells were treated with HNE or Abeta, together with N-acetylcysteine for 24 hours, prior to analysis of NEP protein levels, activity, and oxidative modifications. Intracellular NEP developed HNE adducts after 24 hours of HNE or Abeta treatment as determined by immunoprecipitation, immunoblotting, and double immunofluorescence staining. N-acetylcysteine at 10 to 100 microM alleviated HNE adduction after HNE or Abeta treatment. In keeping with previous reports, HNE-modified NEP showed decreased catalytic activity. The present study demonstrates that antioxidants can be used to spare NEP from oxidative modification, suggesting a potential mechanism underlying the neuroprotective effects of antioxidants in aging or Alzheimer's disease.

Current therapeutic targets for the treatment of Alzheimer's disease

Alzheimer's disease is a progressive neurodegenerative disease for which no cure exists. There is a substantial need for new therapies that offer improved symptomatic benefit and disease-slowing capabilities. In recent decades there has been substantial progress in understanding the molecular and cellular changes associated with Alzheimer's disease pathology. This has resulted in identification of a large number of new drug targets. These targets include, but are not limited to, therapies that aim to prevent production of or remove the amyloid-beta protein that accumulates in neuritic plaques; to prevent the hyperphosphorylation and aggregation into paired helical filaments of the microtubule-associated protein tau; and to keep neurons alive and functioning normally in the face of these pathologic challenges. We provide a review of these targets for drug development.

Gene therapy in Alzheimer's disease - potential for disease modification

Alzheimer's disease (AD) is the major cause of dementia in the elderly, leading to memory loss and cognitive decline. The mechanism underlying onset of the disease has not been fully elucidated. However, characteristic pathological manifestations include extracellular accumulation and aggregation of the amyloid beta-peptide (Abeta) into plaques and intracellular accumulation and aggregation of hyperphosphorylated tau, forming neurofibrillary tangles. Despite extensive research worldwide, no disease modifying treatment is yet available. In this review, we focus on gene therapy as a potential treatment for AD, and summarize recent work in the field, ranging from proof-of-concept studies in animal models to clinical trials. The multifactorial causes of AD offer a variety of possible targets for gene therapy, including two neurotrophic growth factors, nerve growth factor and brain-derived neurotrophic factor, Abeta-degrading enzymes, such as neprilysin, endothelin-converting enzyme and cathepsin B, and AD associated apolipoprotein E. This review also discusses advantages and drawbacks of various rapidly developing virus-mediated gene delivery techniques for gene therapy. Finally, approaches aiming at down-regulating amyloid precursor protein (APP) and beta-site APP cleaving enzyme 1 levels by means of siRNA-mediated knockdown are briefly summarized. Overall, the prospects appear hopeful that gene therapy has the potential to be a disease modifying treatment for AD.

Ginsenoside Rg3 promotes beta-amyloid peptide degradation by enhancing gene expression of neprilysin

Objectives

It has been hypothesized that the accumulation of beta-amyloid peptide (Aβ) in the brain is a triggering event leading to the pathological cascade of Alzheimer's disease. The steady-state levels of Aβ are determined by the metabolic balance between anabolic and catabolic activity and the dysregulation of this activity leads to Alzheimer's disease. Recent evidence has shown that neprilysin (NEP) is the rate-limiting enzyme in the Aβ degradation in the brain. Ginseng, the root of Panax ginseng C.A. Meyer, is widely used as a tonic for the prevention and treatment of age-related disorders in China. We aimed to investigate the basis of this use.

Methods

In this study, we investigated the effect of ginsenoside Rg3, one of the major active components of ginseng, on the metabolism of Aβ40 and Aβ42 in SK-N-SH cells transfected with Swedish mutant β-amyloid precursor protein (SweAPP).

Results

The ELISA result showed that Rg3 significantly reduced the levels of Aβ40 and Aβ42, 19.65 ± 6.05%, 23.61 ± 6.74%, respectively (P < 0.01). The Western blot analysis showed that Rg3 reduced the levels of Aβ40 and Aβ42 through enhancing NEP gene expression, and real-time PCR assay showed that 50 μM Rg3 could significantly enhance NEP gene expression (2.9 fold at 48 h).

Conclusions

Our findings suggest that the Rg3 compound of ginseng may be useful for treating patients suffering with Alzheimer's disease.

Targeted chemotherapy in drug-resistant tumors, noninvasive imaging of P-glycoprotein-mediated functional transport in cancer, and emerging role of Pgp in neurodegenerative diseases

Multidrug resistance (MDR) mediated by overexpression of P-glycoprotein (Pgp) is one of the best characterized transporter-mediated barriers to successful chemotherapy in cancer patients and is also a rapidly emerging target in the progression of neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. Therefore, strategies capable of delivering chemotherapeutic agents into drug-resistant tumors and targeted radiopharmaceuticals acting as ultrasensitive molecular imaging probes for detecting functional Pgp expression in vivo could be expected to play a vital role in systemic biology as personalized medicine gains momentum in the twenty-first century. While targeted therapy could be expected to deliver optimal doses of chemotherapeutic drugs into the desired targets, the interrogation of Pgp-mediated transport activity in vivo via noninvasive imaging techniques (SPECT and PET) would be beneficial in stratification of patient populations likely to benefit from a given therapeutic treatment, thereby assisting management of drug resistance in cancer and treatment of neurodegenerative diseases. Both strategies could play a vital role in advancement of personalized treatments in cancer and neurodegenerative diseases. Via this tutorial, authors make an attempt in outlining these strategies and discuss their strengths and weaknesses.

Transgenic Drosophila models of Alzheimer's disease and tauopathies

Alzheimer's disease (AD) is the most common form of senile dementia. Aggregation of the amyloid-beta42 peptide (Abeta42) and tau proteins are pathological hallmarks in AD brains. Accumulating evidence suggests that Abeta42 plays a central role in the pathogenesis of AD, and tau acts downstream of Abeta42 as a modulator of the disease progression. Tau pathology is also observed in frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17) and other related diseases, so called tauopathies. Although most cases are sporadic, genes associated with familial AD and FTDP-17 have been identified, which led to the development of transgenic animal models. Drosophila has been a powerful genetic model system used in many fields of biology, and recently emerges as a model for human neurodegenerative diseases. In this review, we will summarize key features of transgenic Drosophila models of AD and tauopathies and a number of insights into disease mechanisms as well as therapeutic implications gained from these models.

Feline immunodeficiency virus as a gene transfer vector in the rat nucleus tractus solitarii

Gene transfer has been used to examine the role of putative neurotransmitters in the nucleus tractus solitarii (NTS). Most such studies used adenovirus vector-mediated gene transfer although adenovirus vector transfects both neuronal and non-neuronal cells. Successful transfection in the NTS has also been reported with lentivirus as the vector. Feline immunodeficiency virus (FIV), a lentivirus, may preferentially transfect neurons and could be a powerful tool to delineate physiological effects produced by altered synthesis of transmitters in neurons. However, it has not been studied in NTS. Therefore, we sought to determine whether FIV transfects rat NTS cells and to define the type of cell transfected. We found that injection of FIV encoding LacZ gene (FIVLacZ) into the NTS led to transfection of numerous NTS cells. Injection of FIVLacZ did not alter immunoreactivity (IR) for neuronal nitric oxide synthase, which we have shown resides in NTS neurons. A majority (91.7 +/- 3.9%) of transfected cells contained IR for neuronal nuclear antigen, a neuronal marker; 2.1 +/- 3.8% of transfected cells contained IR for glial fibrillary acidic protein, a glial marker. No transfected neurons or fibers were observed in the nodose ganglion, which sends afferents to the NTS. We conclude that FIV almost exclusively transfects neurons in the rat NTS from which it is not retrogradely transported. The cell-type specificity of FIV in the NTS may provide a molecular method to study local physiological functions mediated by potential neurotransmitters in the NTS.

Versatile somatic gene transfer for modeling neurodegenerative diseases

A growing variety of technical approaches allow control over the expression of selected genes in living organisms. The ability to deliver functional exogenous genes involved in neurodegenerative diseases has opened pathological processes to experimental analysis and targeted therapeutic development in rodent and primate preclinical models. Biological adaptability, economic animal use, and reduced model development costs complement improved control over spatial and temporal gene expression compared with conventional transgenic models. A review of viral vector studies, typically adeno-associated virus or lentivirus, for expression of three proteins that are central to major neurodegenerative diseases, will illustrate how this approach has powered new advances and opportunities in CNS disease research.

Amyloid Beta annular protofibrils in cell processes and synapses accumulate with aging and Alzheimer-associated genetic modification

Amyloid beta (Abeta) annular protofibrils (APFs) have been described where the structure is related to that of beta barrel pore-forming bacterial toxins and exhibits cellular toxicity. To investigate the relationship of Abeta APFs to disease and their ultrastructural localization in brain tissue, we conducted a pre-embedding immunoelectron microscopic study using anti-annular protofibril antiserum. We examined brain tissues of young- and old-aged amyloid precursor protein transgenic mice (APP23), neprilysin knockout APP23 mice, and nontransgenic littermates. alphaAPF-immunoreactions tended to be found (1) on plasma membranes and vesicles inside of cell processes, but not on amyloid fibrils, (2) with higher density due to aging, APP transgene, and neprilysin deficiency, and (3) with higher positive rate at synaptic compartments in aged APP23, especially in neprilysin knockout APP23 mice. These findings imply that APFs are distinct from amyloid fibrils, interact with biological membranes, and might be related to synaptic dysfunction in Alzheimer model mouse brains.

Environmental experience modulates ischemia-induced amyloidogenesis and enhances functional recovery

In this study, we examined whether ischemia-induced amyloidogenesis could be modulated by environmental "experience," and whether this modulation is associated with improved cognitive functioning. Rats were subjected to either global ischemia or sham surgery and then were randomly assigned to either enriched environment housing (EE) or socially paired housing (controls). After 14 days of differential environmental housing, the rats were tested in the water maze. Our results show decreased C-terminal fragments of the beta-amyloid precursor protein (betaAPP) and decreased amyloid beta (Abeta) load in the ischemic EE rats compared to the ischemic control animals. In addition, Abeta oligomerization was significantly decreased in the ischemic EE animals compared to the ischemic control rats. Further, significantly increased levels of neprilysin, but not insulin-degrading enzyme, amyloid-degrading enzymes, were seen in the ischemic EE rats compared to the ischemic control animals. Behavioral analyses showed that ischemic EE rats performed significantly better on the memory task compared to the ischemic control group. These results suggest that use of multi-sensory environmental enrichment following cerebral ischemia may reduce the accumulation of Abeta peptide in the more pathologic oligomeric form, and consequently may enhance functional recovery.

Insulin receptor dysfunction impairs cellular clearance of neurotoxic oligomeric a{beta}

Accumulation of amyloid beta (Abeta) oligomers in the brain is toxic to synapses and may play an important role in memory loss in Alzheimer disease. However, how these toxins are built up in the brain is not understood. In this study we investigate whether impairments of insulin and insulin-like growth factor-1 (IGF-1) receptors play a role in aggregation of Abeta. Using primary neuronal culture and immortal cell line models, we show that expression of normal insulin or IGF-1 receptors confers cells with abilities to reduce exogenously applied Abeta oligomers (also known as ADDLs) to monomers. In contrast, transfection of malfunctioning human insulin receptor mutants, identified originally from patient with insulin resistance syndrome, or inhibition of insulin and IGF-1 receptors via pharmacological reagents increases ADDL levels by exacerbating their aggregation. In healthy cells, activation of insulin and IGF-1 receptor reduces the extracellular ADDLs applied to cells via seemingly the insulin-degrading enzyme activity. Although insulin triggers ADDL internalization, IGF-1 appears to keep ADDLs on the cell surface. Nevertheless, both insulin and IGF-1 reduce ADDL binding, protect synapses from ADDL synaptotoxic effects, and prevent the ADDL-induced surface insulin receptor loss. Our results suggest that dysfunctions of brain insulin and IGF-1 receptors contribute to Abeta aggregation and subsequent synaptic loss.

Low levels of amyloid-beta and its transporters in neonatal rats with and without hydrocephalus

Background

Previous studies in aging animals have shown that amyloid-beta protein (Aβ) accumulates and its transporters, low-density lipoprotein receptor-related protein-1 (LRP-1) and the receptor for advanced glycation end products (RAGE) are impaired during hydrocephalus. Furthermore, correlations between astrocytes and Aβ have been found in human cases of normal pressure hydrocephalus (NPH) and Alzheimer's disease (AD). Because hydrocephalus occurs frequently in children, we evaluated the expression of Aβ and its transporters and reactive astrocytosis in animals with neonatal hydrocephalus.

Methods

Hydrocephalus was induced in neonatal rats by intracisternal kaolin injections on post-natal day one, and severe ventriculomegaly developed over a three week period. MRI was performed on post-kaolin days 10 and 21 to document ventriculomegaly. Animals were sacrificed on post-kaolin day 21. For an age-related comparison, tissue was used from previous studies when hydrocephalus was induced in a group of adult animals at either 6 months or 12 months of age. Tissue was processed for immunohistochemistry to visualize LRP-1, RAGE, Aβ, and glial fibrillary acidic protein (GFAP) and with quantitative real time reverse transcriptase polymerase chain reaction (qRT-PCR) to quantify expression of LRP-1, RAGE, and GFAP.

Results

When 21-day post-kaolin neonatal hydrocephalic animals were compared to adult (6–12 month old) hydrocephalic animals, immunohistochemistry demonstrated levels of Aβ, RAGE, and LRP-1 that were substantially lower in the younger animals; in contrast, GFAP levels were elevated in both young and old hydrocephalic animals. When the neonatal hydrocephalic animals were compared to age-matched controls, qRT-PCR demonstrated no significant changes in Aβ, LRP-1 and RAGE. However, immunohistochemistry showed very small increases or decreases in individual proteins. Furthermore, qRT-PCR indicated statistically significant increases in GFAP.

Conclusion

Neonatal rats with and without hydrocephalus had low expression of Aβ and its transporters when compared to adult rats with hydrocephalus. No statistical differences were observed in Aβ and its transporters between the control and hydrocephalic neonatal animals.

AAV1/2-mediated CNS gene delivery of dominant-negative CCL2 mutant suppresses gliosis, beta-amyloidosis, and learning impairment of APP/PS1 mice

Accumulation of aggregated amyloid-beta (Abeta) peptide was studied as an initial step for Alzheimer's disease (AD) pathogenesis. Following amyloid plaque formation, reactive microglia and astrocytes accumulate around plaques and cause neuroinflammation. Here brain chemokines play a major role for the glial accumulation. We have previously shown that transgenic overexpression of chemokine CCL2 in the brain results in increased microglial accumulation and diffuse amyloid plaque deposition in a transgenic mouse model of AD expressing Swedish amyloid precursor protein (APP) mutant. Here, we report that adeno-associated virus (AAV) serotype 1 and 2 hybrid efficiently deliver 7ND gene, a dominant-negative CCL2 mutant, in a dose-response manner and express >1,000-fold higher recombinant CCL2 than basal levels after a single administration. AAV1/2 hybrid virus principally infected neurons without neuroinflammation with sustained expression for 6-months. 7ND expressed in APP/presenilin-1 (APP/PS1) bigenic mice reduced astro/microgliosis, beta-amyloidosis, including suppression of both fibrillar and oligomer Abeta accumulation, and improved spatial learning. Our data support the idea that the AAV1/2 system is a useful tool for CNS gene delivery, and suppression of CCL2 may be a therapeutic target for the amelioration of AD-related neuroinflammation.

Effects of HNE-modification induced by Abeta on neprilysin expression and activity in SH-SY5Y cells

The cerebral accumulation of beta-amyloid (Abeta) is a consistent feature of and likely contributor to the development of Alzheimer's disease. In addition to dysregulated production, increasing experimental evidence suggests reduced catabolism also plays an important role in Abeta accumulation. We have previously shown that neprilysin (NEP), the major protease which cleaves Abetain vivo, is modified by 4-hydroxy-nonenal (HNE) adducts in the brain of Alzheimer's disease patients. To determine if these changes affected Abeta, SH-SY5Y cells were treated with HNE or Abeta, and then NEP mRNA, protein levels, HNE adducted NEP, NEP activity and secreted Abeta levels were determined. Intracellular NEP developed HNE adducts after 24 h of HNE treatment as determined by immunoprecipitation, immunoblotting, and double immunofluorescence staining. HNE-modified NEP showed decreased catalytic activity, which was associated with elevations in Abeta1-40 in SH-SY5Y and H4 APP695wt cells. Incubation of cells with Abeta1-42 also induced HNE adduction of NEP. In an apparent compensatory response, Abeta-treated cells showed increased NEP mRNA and protein expression. Despite elevations in NEP protein, the activity was significantly lower compared with the NEP protein level. This study demonstrates that NEP can be inactivated by HNE-adduction, which is associated with, at least partly, reduced Abeta cleavage and enhanced Abeta accumulation.

Neprilysin overexpression inhibits plaque formation but fails to reduce pathogenic Abeta oligomers and associated cognitive deficits in human amyloid precursor protein transgenic mice

The accumulation of amyloid-beta (Abeta) peptides in the brain of patients with Alzheimer's disease (AD) may arise from an imbalance between Abeta production and clearance. Overexpression of the Abeta-degrading enzyme neprilysin in brains of human amyloid precursor protein (hAPP) transgenic mice decreases overall Abeta levels and amyloid plaque burdens. Because AD-related synaptic and cognitive deficits appear to be more closely related to Abeta oligomers than to plaques, it is important to determine whether increased neprilysin activity also diminishes the levels of pathogenic Abeta oligomers and related neuronal deficits in vivo. To address this question, we crossed hAPP transgenic mice with neprilysin transgenic mice and analyzed their offspring. Neprilysin overexpression reduced soluble Abeta levels by 50% and effectively prevented early Abeta deposition in the neocortex and hippocampus. However, it did not reduce levels of Abeta trimers and Abeta*56 or improve deficits in spatial learning and memory. The differential effect of neprilysin on plaques and oligomers suggests that neprilysin-dependent degradation of Abeta affects plaques more than oligomers and that these structures may form through distinct assembly mechanisms. Neprilysin's inability to prevent learning and memory deficits in hAPP mice may be related to its inability to reduce pathogenic Abeta oligomers. Reduction of Abeta oligomers will likely be required for anti-Abeta treatments to improve cognitive functions.

Neuropeptide Y fragments derived from neprilysin processing are neuroprotective in a transgenic model of Alzheimer's disease

The endopeptidase neprilysin (NEP) is a major amyloid-beta (Abeta) degrading enzyme and has been implicated in the pathogenesis of Alzheimer's disease. Because NEP cleaves substrates other than Abeta, we investigated the potential role of NEP-mediated processing of neuropeptides in the mechanisms of neuroprotection in vivo. Overexpression of NEP at low levels in transgenic (tg) mice affected primarily the levels of neuropeptide Y (NPY) compared with other neuropeptides. Ex vivo and in vivo studies in tg mice and in mice that received lentiviral vector injections showed that NEP cleaved NPY into C-terminal fragments (CTFs), whereas silencing NEP reduced NPY processing. Immunoblot and mass spectrometry analysis showed that NPY 21-36 and 31-36 were the most abundant fragments generated by NEP activity in vivo. Infusion of these NPY CTFs into the brains of APP (amyloid precursor protein) tg mice ameliorated the neurodegenerative pathology in this model. Moreover, the amidated NPY CTFs protected human neuronal cultures from the neurotoxic effects of Abeta. This study supports the possibility that the NPY CTFs generated during NEP-mediated proteolysis might exert neuroprotective effects in vivo. This function of NEP represents a unique example of a proteolytic enzyme with dual action, namely, degradation of Abeta as well as processing of NPY.

X11 proteins regulate the translocation of amyloid beta-protein precursor (APP) into detergent-resistant membrane and suppress the amyloidogenic cleavage of APP by beta-site-cleaving enzyme in brain

X11 and X11-like proteins (X11L) are neuronal adaptor proteins whose association to the cytoplasmic domain of amyloid beta-protein precursor (APP) suppresses the generation of amyloid beta-protein (Abeta) implicated in Alzheimer disease pathogenesis. The amyloidogenic, but not amyloidolytic, metabolism of APP was selectively increased in the brain of mutant mice lacking X11L (Sano, Y., Syuzo-Takabatake, A., Nakaya, T., Saito, Y., Tomita, S., Itohara, S., and Suzuki, T. (2006) J. Biol. Chem. 281, 37853-37860). To reveal the actual role of X11 proteins (X11s) in suppressing amyloidogenic cleavage of APP in vivo, we generated X11 and X11L double knock-out mice and analyzed the metabolism of APP. The mutant mice showed enhanced beta-site cleavage of APP along with increased accumulation of Abeta in brain and increased colocalization of APP with beta-site APP-cleaving enzyme (BACE). In the brains of mice deficient in both X11 and X11L, the apparent relative subcellular distributions of both mature APP and its beta-C-terminal fragment were shifted toward the detergent-resistant membrane (DRM) fraction, an organelle in which BACE is active and both X11s are not nearly found. These results indicate that X11s associate primarily with APP molecules that are outside of DRM, that the dissociation of APP-X11/X11L complexes leads to entry of APP into DRM, and that cleavage of uncomplexed APP by BACE within DRM is enhanced by X11s deficiency. Present results lead to an idea that the dysfunction of X11L in the interaction with APP may recruit more APP into DRM and increase the generation of Abeta even if BACE activity did not increase in brain.

Long-term neprilysin gene transfer is associated with reduced levels of intracellular Abeta and behavioral improvement in APP transgenic mice

Background

Proteolytic degradation has emerged as a key pathway involved in controlling levels of the Alzheimer's disease (AD)-associated amyloid-β (Aβ) peptide in the brain. The endopeptidase, neprilysin, has been implicated as a major Aβ degrading enzyme in mice and humans. Previous short and intermediate term studies have shown the potential therapeutic application of neprilysin by delivering this enzyme into the brain of APP transgenic mice using gene transfer with viral vectors. However the effects of long-term neprilysin gene transfer on other aspects of Aβ associated pathology have not been explored yet in APP transgenic mice.

Results

We show that the sustained expression of neprilysin for up to 6 months lowered not only the amyloid plaque load but also reduced the levels of intracellular Aβ immunoreactivity. This was associated with improved behavioral performance in the water maze and ameliorated the dendritic and synaptic pathology in the APP transgenic mice.

Conclusion

These data support the possibility that long-term neprilysin gene therapy improves behavioral and neurodegenerative pathology by reducing intracellular Aβ.

Adeno-associated viral (AAV) serotype 5 vector mediated gene delivery of endothelin-converting enzyme reduces Abeta deposits in APP + PS1 transgenic mice

Reduction of Abeta deposition is a major therapeutic strategy in Alzheimer's disease (AD). The concentration of Abeta in the brain is modulated not only by Abeta production but also by its degradation. One of the proteases involved in the degradation of Abeta peptides is endothelin-converting enzyme (ECE). In this study, we investigated the effects of an intracranial administration of a seroptype 5 recombinant adeno-associated viral vector (rAAV) containing the ECE-1 synthetic gene on amyloid deposition in amyloid precursor protein (APP) plus presenilin-1 (PS1) transgenic mice. The rAAV vector was injected unilaterally into the right anterior cortex and hippocampus of 6-month-old mice, while control mice received an AAV vector expressing green fluorescent protein (GFP). Immunohistochemical testing for the hemagglutinin (HA) tag appended to ECE revealed strong expression in areas surrounding the injection sites but minimal expression in the contralateral regions. Immunohistochemical tests showed that Abeta decreases in the anterior cortex and hippocampus in mice receiving the ECE synthetic gene. Further, decreases in Congo red positive deposits were also observed in both regions. These results indicate that increasing the expression of beta-amyloid degrading enzymes through gene therapy is a promising approach to the treatment of AD.

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.

Overexpression of neprilysin reduces alzheimer amyloid-beta42 (Abeta42)-induced neuron loss and intraneuronal Abeta42 deposits but causes a reduction in cAMP-responsive element-binding protein-mediated transcription, age-dependent axon pathology, and premature death in Drosophila

The amyloid-beta42 (Abeta42) peptide has been suggested to play a causative role in Alzheimer disease (AD). Neprilysin (NEP) is one of the rate-limiting Abeta-degrading enzymes, and its enhancement ameliorates extracellular amyloid pathology, synaptic dysfunction, and memory defects in mouse models of Abeta amyloidosis. In addition to the extracellular Abeta, intraneuronal Abeta42 may contribute to AD pathogenesis. However, the protective effects of neuronal NEP expression on intraneuronal Abeta42 accumulation and neurodegeneration remain elusive. In contrast, sustained NEP activation may be detrimental because NEP can degrade many physiological peptides, but its consequences in the brain are not fully understood. Using transgenic Drosophila expressing human NEP and Abeta42, we demonstrated that NEP efficiently suppressed the formation of intraneuronal Abeta42 deposits and Abeta42-induced neuron loss. However, neuronal NEP overexpression reduced cAMP-responsive element-binding protein-mediated transcription, caused age-dependent axon degeneration, and shortened the life span of the flies. Interestingly, the mRNA levels of endogenous fly NEP genes and phosphoramidon-sensitive NEP activity declined during aging in fly brains, as observed in mammals. Taken together, these data suggest both the protective and detrimental effects of chronically high NEP activity in the brain. Down-regulation of NEP activity in aging brains may be an evolutionarily conserved phenomenon, which could predispose humans to developing late-onset AD.

Aggregation and catabolism of disease-associated intra-Abeta mutations: reduced proteolysis of AbetaA21G by neprilysin

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

Tau expression levels from various adeno-associated virus vector serotypes produce graded neurodegenerative disease states

Neurodegenerative diseases involving neurofibrillary tangle pathology are pernicious. By expressing the microtubule-associated protein tau, a major component of tangles, with a viral vector, we induce neuropathological sequelae in rats that are similar to those seen in human tauopathies. We tested several variants of the adeno-associated virus (AAV) vector for tau expression in the nigrostriatal system in order to develop models with graded onset and completeness. Whereas previous studies with AAV2 tau vectors produced partial lesions of the nigrostriatal system, AAV9 or AAV10 tau vectors were more robust. These vectors had formidable efficacy relative to 6-hydroxydopamine for dopamine loss in the striatum. Time-courses for tau transgene expression, dopamine loss and rotational behavior tracked the disease progression with the AAV9 tau vector. There was a nearly complete lesion over a delayed time-course relative to 6-hydroxydopamine, with a sequence of tau expression by 1 week, dopamine loss by 2 weeks and then behavior effect by 3-4 weeks. Relative to AAV2 or AAV8, tau expression from AAV9 or AAV10 peaked earlier and caused more dopamine loss. Varying vector efficiencies produced graded states of disease up to nearly complete. The disease models stemming from the AAV variants AAV9 or AAV10 may be useful for rapid drug screening, particularly for tau diseases that affect the nigrostriatal system, such as progressive supranuclear palsy.

Somatostatin and Alzheimer's disease

Alzheimer's disease (AD) is characterized by the cerebral deposition of senile plaques that are mainly composed of a set of peptides referred to as amyloid beta-peptides (Abeta). Among the numerous neuropeptides produced in intrinsic cortical and hippocampal neurons, somatostatin (SRIF) has been found to be the most consistently reduced in the brain and cerebrospinal fluid of AD patients. SRIF receptors (SSTR), which mediate the neuromodulatory signals of SRIF, are also markedly depleted in the AD brain, there being subtype-selective alterations in cortical areas. In the rat temporal cortex, we have shown that intracerebroventricular infusion of Abeta25-35 results in a decrease in SRIF-like immunoreactivity and in SRIF receptor subtype 2 (SSTR2) mRNA and protein levels, in correlation with a decrease in SSTR functionality. Insulin-like growth factor-I prevents the reduction in these parameters induced by Abeta25-35. Abeta has recently been demonstrated to be degraded primarily by a neutral endopeptidase, neprilysin, in the brain. SRIF regulates brain Abeta levels via modulation of neprilysin activity. Because SRIF expression in the brain declines upon aging in various mammals, including rodents, apes and humans, the aging-dependent reduction of SRIF has been hypothesized to trigger accumulation of Abeta in the brain by suppressing neprilysin action. Here we present an overview of recent advances on the role of SRIF in AD and its relationship with Abeta peptides.

Neprilysin: an enzyme candidate to slow the progression of Alzheimer's disease

It is well established that the extracellular deposition of amyloid beta (Abeta) peptide plays a central role in the development of Alzheimer's disease (AD). Therefore, either preventing the accumulation of Abeta peptide in the brain or accelerating its clearance may slow the rate of AD onset. Neprilysin (NEP) is the dominant Abeta peptide-degrading enzyme in the brain; NEP becomes inactivated and down-regulated during both the early stages of AD and aging. In this study, we investigated the effect of human (h)NEP gene transfer to the brain in a mouse model of AD before the development of amyloid plaques, and assessed how this treatment modality affected the accumulation of Abeta peptide and associated pathogenetic changes (eg, inflammation, oxidative stress, and memory impairment). Overexpression of hNEP for 4 months in young APP/DeltaPS1 double-transgenic mice resulted in reduction in Abeta peptide levels, attenuation of amyloid load, oxidative stress, and inflammation, and improved spatial orientation. Moreover, the overall reduction in amyloidosis and associated pathogenetic changes in the brain resulted in decreased memory impairment by approximately 50%. These data suggest that restoring NEP levels in the brain at the early stages of AD is an effective strategy to prevent or attenuate disease progression.

Novel strategies for Alzheimer's disease treatment

Considerable progress has been made in recent years towards better understanding the pathogenesis of Alzheimer's disease (AD), a dementing neurodegenerative disorder that affects > 10 million individuals in the US and Europe combined. Recent studies suggest that alterations in the processing of amyloid precursor protein (APP), resulting in the accumulation of amyloid-beta protein (Abeta) and the formation of oligomers leads to synaptic damage and neurodegeneration. Therefore, strategies for treatment development have been focused on reducing Abeta accumulation using, among other approaches, antiaggregation molecules, regulators of the APP proteolysis and processing, reducing APP production (e.g., small-interfering RNA), and increasing Abeta clearance with antibodies, apolipoprotein E and Abeta-degrading enzymes (e.g., neprilysin). The main focus of this review is on novel treatments for AD with a special emphasis on delivering neuroprotective and antiamyloidogenic molecules by gene therapy and by promoting neurogenesis.

In vitro and in vivo degradation of Abeta peptide by peptidases coupled to erythrocytes

It is generally believed that amyloid beta peptides (Abeta) are the key mediators of Alzheimer's disease. Therapeutic interventions have been directed toward impairing the synthesis or accelerating the clearance of Abeta. An equilibrium between blood and brain Abeta exists mediated by carriers that transport Abeta across the blood-brain barrier. Passive immunotherapy has been shown to be effective in mouse models of AD, where the plasma borne antibody binds plasma Abeta causing an efflux of Abeta from the brain. As an alternative to passive immunotherapy we have considered the use of Abeta-degrading peptidases to lower plasma Abeta levels. Here we compare the ability of three Abeta-degrading peptidases to degrade Abeta. Biotinylated peptidases were coupled to the surface of biotinylated erythrocytes via streptavidin. These erythrocyte-bound peptidases degrade Abeta peptide in plasma. Thus, peptidases bound to or expressed on the surface of erythroid cells represent an alternative to passive immunotherapy.

Involvement of prostaglandin E2 in production of amyloid-beta peptides both in vitro and in vivo

Amyloid-beta peptides (Abeta), generated by proteolysis of the beta-amyloid precursor protein (APP) by beta- and gamma-secretases, play an important role in the pathogenesis of Alzheimer disease (AD). Inflammation is also believed to be integral to the pathogenesis of AD. Here we show that prostaglandin E(2) (PGE(2)), a strong inducer of inflammation, stimulates the production of Abeta in cultured human embryonic kidney (HEK) 293 or human neuroblastoma (SH-SY5Y) cells, both of which express a mutant type of APP. We have demonstrated using subtype-specific agonists that, of the four main subtypes of PGE(2) receptors (EP(1-4)), EP(4) receptors alone or EP(2) and EP(4) receptors together are responsible for this PGE(2)-stimulated production of Abeta in HEK293 or SH-SY5Y cells, respectively. An EP(4) receptor antagonist suppressed the PGE(2)-stimulated production of Abeta in HEK293 cells. This stimulation was accompanied by an increase in cellular cAMP levels, and an analogue of cAMP stimulated the production of Abeta, demonstrating that increases in the cellular level of cAMP are responsible for the PGE(2)-stimulated production of Abeta. Immunoblotting experiments and direct measurement of gamma-secretase activity suggested that PGE(2)-stimulated production of Abeta is mediated by activation ofgamma-secretase but not of beta-secretase. Transgenic mice expressing the mutant type of APP showed lower levels of Abeta in the brain, when they were crossed with mice lacking either EP(2) or EP(4) receptors, suggesting that PGE(2)-mediated activation of EP(2) and EP(4) receptors is involved in the production of Abeta in vivo and in the pathogenesis of AD.

Loss of neprilysin function promotes amyloid plaque formation and causes cerebral amyloid angiopathy

Cerebral deposition of the amyloid beta protein (Abeta), an invariant feature of Alzheimer's disease, reflects an imbalance between the rates of Abeta production and clearance. The causes of Abeta elevation in the common late-onset form of Alzheimer's disease (LOAD) are largely unknown. There is evidence that the Abeta-degrading protease neprilysin (NEP) is down-regulated in normal aging and LOAD. We asked whether a decrease in endogenous NEP levels can prolong the half-life of Abeta in vivo and promote development of the classic amyloid neuropathology of Alzheimer's disease. We examined the brains and plasma of young and old mice expressing relatively low levels of human amyloid precursor protein and having one or both NEP genes silenced. NEP loss of function 1) elevated whole-brain and plasma levels of human Abeta(40) and Abeta(42), 2) prolonged the half-life of soluble Abeta in brain interstitial fluid of awake animals, 3) raised the concentration of Abeta dimers, 4) markedly increased hippocampal amyloid plaque burden, and 5) led to the development of amyloid angiopathy. A approximately 50% reduction in NEP levels, similar to that reported in some LOAD brains, was sufficient to increase amyloid neuropathology. These findings demonstrate an important role for proteolysis in determining the levels of Abeta and Abeta-associated neuropathology in vivo and support the hypothesis that primary defects in Abeta clearance can cause or contribute to LOAD pathogenesis.

Reducing amyloid plaque burden via ex vivo gene delivery of an Abeta-degrading protease: a novel therapeutic approach to Alzheimer disease

BACKGROUND

Understanding the mechanisms of amyloid-beta protein (Abeta) production and clearance in the brain has been essential to elucidating the etiology of Alzheimer disease (AD). Chronically decreasing brain Abeta levels is an emerging therapeutic approach for AD, but no such disease-modifying agents have achieved clinical validation. Certain proteases are responsible for the catabolism of brain Abeta in vivo, and some experimental evidence suggests they could be used as therapeutic tools to reduce Abeta levels in AD. The objective of this study was to determine if enhancing the clearance of Abeta in the brain by ex vivo gene delivery of an Abeta-degrading protease can reduce amyloid plaque burden.

METHODS AND FINDINGS

We generated a secreted form of the Abeta-degrading protease neprilysin, which significantly lowers the levels of naturally secreted Abeta in cell culture. We then used an ex vivo gene delivery approach utilizing primary fibroblasts to introduce this soluble protease into the brains of beta-amyloid precursor protein (APP) transgenic mice with advanced plaque deposition. Brain examination after cell implantation revealed robust clearance of plaques at the site of engraftment (72% reduction, p = 0.0269), as well as significant reductions in plaque burden in both the medial and lateral hippocampus distal to the implantation site (34% reduction, p = 0.0020; and 55% reduction, p = 0.0081, respectively).

CONCLUSIONS

Ex vivo gene delivery of an Abeta-degrading protease reduces amyloid plaque burden in transgenic mice expressing human APP. These results support the use of Abeta-degrading proteases as a means to therapeutically lower Abeta levels and encourage further exploration of ex vivo gene delivery for the treatment of Alzheimer disease.

Evidence of a role for lactadherin in Alzheimer's disease

Lactadherin is a secreted extracellular matrix protein expressed in phagocytes and contributes to the removal of apoptotic cells. We examined lactadherin expression in brain sections of patients with or without Alzheimer's disease and studied its role in the phagocytosis of amyloid beta-peptide (Abeta). Cells involved in Alzheimer's disease, including vascular smooth muscle cells, astrocytes, and microglia, showed a time-related increase in lactadherin production in culture. Quantitative analysis of the level of lactadherin showed a 35% reduction in lactadherin mRNA expression in the brains of patients with Alzheimer's disease (n = 52) compared with age-matched controls (n = 58; P = 0.003). Interestingly, lactadherin protein was detected in the brains of patients with Alzheimer's disease and controls, with low expression in areas rich in senile plaques and marked expression in areas without Abeta deposition. Using surface plasmon resonance, we observed a direct protein-protein interaction between recombinant lactadherin and Abeta 1-42 peptide in vitro. Lactadherin deficiency or its neutralization using specific antibodies significantly prevented Abeta 1-42 phagocytosis by murine and human macrophages. In conclusion, lactadherin plays an important role in the phagocytosis of Abeta 1-42 peptide, and its expression is reduced in Alzheimer's disease. Alterations in lactadherin production/function may contribute to the initiation and/or progression of Alzheimer's disease.

Exercise reverses preamyloid oligomer and prolongs survival in alphaB-crystallin-based desmin-related cardiomyopathy

The R120G mutation in the small heat shock-like protein alphaB-crystallin (CryAB(R120G)) causes desmin-related myopathy (DRM), which is characterized by the formation of desmin- and CryAB-containing aggregates within muscle fibers. Mice with cardiac-specific overexpression of CryAB(R120G) develop cardiomyopathy at 3 months and die at 6-7 months from heart failure (HF). Previous studies showed that overexpression of CryAB(R120G) results in accumulation of preamyloid oligomer (PAO). PAO is considered to be the cytotoxic entity in many of the protein misfolding-based neurodegenerative diseases. On the basis of data from mouse models of neurodegenerative diseases showing that exercise or environmental enrichment reduces the amyloid oligomer level and improves cognitive ability, we hypothesized that CryAB(R120G)-induced DRM would also respond favorably to prolonged voluntary exercise, reducing HF symptoms and rescuing the mice from premature death. Six months of voluntary exercise in CryAB(R120G) animals resulted in 100% survival at a time when all unexercised mice had died. After 22 weeks of exercise, PAO levels were decreased by 47% compared with the unexercised CryAB(R120G) control mice (P = 0.00001). Although CryAB(R120G) expression led to decreased levels of the metallomembrane endopeptidase neprilysin, normal levels were maintained in the exercised CryAB(R120G) mice, and in vitro loss-of-function and gain-of-function experiments using adenovirus-infected cardiomyocytes confirmed the importance of neprilysin in ameliorating PAO accumulation. The data demonstrate that voluntary exercise slows the progression to HF in the CryAB(R120G) DRM model and that PAO accumulation is mediated, at least in part, by decreased neprilysin activity.

Single photon emission computed tomography and positron emission tomography imaging of multi-drug resistant P-glycoprotein--monitoring a transport activity important in cancer, blood-brain barrier function and Alzheimer's disease

Overexpression of multi-drug resistant P-glycoprotein (Pgp) remains an important barrier to successful chemotherapy in cancer patients and impacts the pharmacokinetics of many important drugs. Pgp is also expressed on the luminal surface of brain capillary endothelial cells wherein Pgp functionally comprises a major component of the blood-brain barrier by limiting central nervous system penetration of various therapeutic agents. In addition, Pgp in brain capillary endothelial cells removes amyloid-beta from the brain. Several single photon emission computed tomography and positron emission tomography radiopharmaceutical have been shown to be transported by Pgp, thereby enabling the noninvasive interrogation of Pgp-mediated transport activity in vivo. Therefore, molecular imaging of Pgp activity may enable noninvasive dynamic monitoring of multi-drug resistance in cancer, guide therapeutic choices in cancer chemotherapy, and identify transporter deficiencies of the blood-brain barrier in Alzheimer's disease.

[Utility of AAV vectors derived from novel serotypes]

AAV vector is derived from nonpathogenic virus and has a number of attractive features as a vector for human gene transfer including safety, broad tissue specificity, and low immunogenicity following gene transfer. Moreover, persistent transgene expression (for years) was demonstrated in multiple animal experiments. For these reasons, applications to a wide spectrum of diseases are expected, and several clinical trials have been conducted. Although it is too early to conclude the outcome, the efficacy of treatment was not sufficiently substantiated in most of the trials despite confirming the safety of the vector. These results are primarily due to low levels of transgene expression. One of the approaches to improve this situation is the use of alternative serotypes of AAV. Traditionally, serotype 2 was considered to be a prototype of AAV, and the majority of studies including human clinical trials have been conducted using this serotype. On the other hand, there are five "classical" serotypes, and several have been additionally discovered from tissues of primates including humans. These serotypes are considered to be valuable resources for vector development to overcome the shortcomings of serotype 2. This review focuses on the difference in expression levels and tissue specificity of various serotype-derived vectors and summarizes current status in the treatment of candidate diseases.

Enhanced Amyloidogenic Metabolism of the Amyloid β-Protein Precursor in the X11L-deficient Mouse Brain

X11L, a neuronal adaptor protein, associates with the cytoplasmic domain of APP and suppresses APP cellular metabolism. APP is the precursor of Abeta, whose metabolism is strongly implicated in Alzheimer disease pathogenesis. To examine the roles of X11L function in APP metabolism, including the generation of Abeta in the brain, we produced X11L-deficient mutant mice on the C57BL/6 background. The mutant mice did not exhibit histopathological alterations or compensatory changes in the expression of other X11 family proteins, X11 and X11L2. The expression level and distribution of APP in the brain of mutant mice were also identical to those in wild-type mice. However, in the hippocampus, where substantial levels of X11L and APP are expressed, the mutant mice exhibited a significant increase in the level of the C-terminal fragments of APP produced by cleavage with beta-secretase but not alpha-secretase. The levels of Abeta were increased in the hippocampus of aged mutant mice as compared with age-matched controls. These observations clearly indicate that X11L suppresses the amyloidogenic but not amyloidolytic processing of APP in regions of the brain such as the hippocampus, which express significant levels of X11L.

Neuronal neprilysin overexpression is associated with attenuation of Aβ-related spatial memory deficit

Converging evidence links abnormally high brain concentrations of amyloid-beta peptides (Abeta) to the pathology of Alzheimer's disease (AD). Lowering brain Abeta levels, therefore, is a therapeutic strategy for the treatment of AD. Neuronal neprilysin upregulation led to increased degradation of Abeta, reduced the formation of Abeta-plaques and the associated cytopathology, but whether overexpression of neprilysin can improve cognition is unknown. We show that neuronal overexpression of neprilysin improved the Morris water maze memory performance in mice with memory deficits resulting from overexpression of the AD-causing mutated human amyloid precursor protein (APP). This improvement was associated with decreased brain levels of Abeta and with unchanged endoproteolytic processing of APP. These results provide the evidence that lowering of brain Abeta levels by increasing its degradation can improve cognitive functions in vivo, and suggest that increasing the activity of neprilysin in brain may be effective in preventing cognitive decline in AD.

Age-dependent decline of neprilysin in Alzheimer's disease and normal brain: inverse correlation with A beta levels

Brain deposition of amyloid-beta (A beta) is a pathological hallmark of Alzheimer disease (AD) but A beta is also detected in non-demented elderly individuals. Neprilysin has been shown to be an important enzyme to degrade A beta in brain. We investigated whether decreased neprilysin levels contributes to the accumulation of A beta in AD and in normal aging. No difference in neprilysin protein and mRNA levels were found between AD subjects and age-matched controls. Protein levels of neprilysin were reduced with age in the temporal and frontal cortex of AD and normal brain. A significant positive correlation between insoluble A beta 40 and A beta 42 with age was found in cortex of normal brain whereas in AD brain the correlation between age and A beta was weaker. Our findings of an inverse correlation between neprilysin and insoluble A beta levels in both groups suggest that neprilysin is involved in the clearance of A beta. The observed age-dependent decline in neprilysin may be related to the increased A beta levels during normal aging. The similar rate of decline in neprilysin with age may not be the major cause of the high levels of A beta associated with AD but is likely to be a trigger of AD pathology.

Antiamyloidogenic and neuroprotective functions of cathepsin B: implications for Alzheimer's disease

Alzheimer's disease (AD) may result from the accumulation of amyloid-beta (Abeta) peptides in the brain. The cysteine protease cathepsin B (CatB) is associated with amyloid plaques in AD brains and has been suspected to increase Abeta production. Here, we demonstrate that CatB actually reduces levels of Abeta peptides, especially the aggregation-prone species Abeta1-42, through proteolytic cleavage. Genetic inactivation of CatB in mice with neuronal expression of familial AD-mutant human amyloid precursor protein (hAPP) increased the relative abundance of Abeta1-42, worsening plaque deposition and other AD-related pathologies. Lentivirus-mediated expression of CatB in aged hAPP mice reduced preexisting amyloid deposits, even thioflavin S-positive plaques. Under cell-free conditions, CatB effectively cleaved Abeta1-42, generating C-terminally truncated Abeta peptides that are less amyloidogenic. Thus, CatB likely fulfills antiamyloidogenic and neuroprotective functions. Insufficient CatB activity might promote AD; increasing CatB activity could counteract the neuropathology of this disease.

Decreased Expression and Activity of Neprilysin in Alzheimer Disease Are Associated With Cerebral Amyloid Angiopathy

Neprilysin (NEP) degrades amyloid-beta (Abeta) and is thought to contribute to its clearance from the brain. In Alzheimer disease (AD), downregulation of NEP has been suggested to contribute to the development of cerebral amyloid angiopathy (CAA). We examined the relationship among NEP, CAA, and APOE status in AD and elderly control cases. NEP was most abundant in the tunica media of cerebrocortical blood vessels and in pyramidal neurons. In homogenates of the frontal cortex, NEP protein levels were reduced in AD but not significantly; NEP enzymatic activity was significantly reduced in AD. Immunohistochemistry revealed a reduction of both vascular and parenchymal NEP. The loss of vessel-associated NEP in AD was inversely related to the severity of CAA, and analysis of cases with severe CAA showed that levels of vascular NEP were reduced to the same extent in Abeta-free and Abeta-laden vessels, strongly suggesting that the reduction in NEP is not simply secondary to CAA. Possession of APOE epsilon4 was associated with significantly lower levels of both parenchymal and vascular NEP. Colinearity of epsilon4 with the presence of moderate to severe CAA precluded assessment of the independence of this association from NEP levels. However, logistic regression analysis showed low NEP levels to be a significant independent predictor of moderate to severe CAA.

Clearance of amyloid-beta in Alzheimer's disease: progress, problems and perspectives

Alzheimer's disease (AD) is the most common form of senile dementia and the fourth highest cause of disability and death in the elderly. Amyloid-beta (Abeta) has been widely implicated in the etiology of AD. Several mechanisms have been proposed for Abeta clearance, including receptor-mediated Abeta transport across the blood-brain barrier and enzyme-mediated Abeta degradation. Moreover, pre-existing immune responses to Abeta might also be involved in Abeta clearance. In AD, such mechanisms appear to have become impaired. Recently, therapeutic approaches for Abeta clearance, targeting immunotherapy and molecules binding Abeta, have been developed. In this review, we discuss recent progress and problems with respect to Abeta clearance mechanisms and propose strategies for the development of therapeutics targeting Abeta clearance.

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

A subtle but chronic alteration in metabolic balance between amyloid-beta peptide (Abeta) anabolic and catabolic activities is thought to cause Abeta accumulation, leading to a decade-long pathological cascade of Alzheimer disease. However, it is still unclear whether a reduction of the catabolic activity of Abeta in the brain causes neuronal dysfunction in vivo. In the present study, to clarify a possible connection between a reduction in neprilysin activity and impairment of synaptic and cognitive functions, we cross-bred amyloid precursor protein (APP) transgenic mice (APP23) with neprilysin-deficient mice and biochemically and immunoelectron-microscopically analyzed Abeta accumulation in the brain. We also examined hippocampal synaptic plasticity using an in vivo recording technique and cognitive function using a battery of learning and memory behavior tests, including Y-maze, novel-object recognition, Morris water maze, and contextual fear conditioning tests at the age of 13-16 weeks. We present direct experimental evidence that reduced activity of neprilysin, the major Abeta-degrading enzyme, in the brain elevates oligomeric forms of Abeta at the synapses and leads to impaired hippocampal synaptic plasticity and cognitive function before the appearance of amyloid plaque load. Thus, reduced neprilysin activity appears to be a causative event that is at least partly responsible for the memory-associated symptoms of Alzheimer disease. This supports the idea that a strategy to reduce Abeta oligomers in the brain by up-regulating neprilysin activity would contribute to alleviation of these symptoms.

BACE1, a major determinant of selective vulnerability of the brain to amyloid-beta amyloidogenesis, is essential for cognitive, emotional, and synaptic functions

A transmembrane aspartyl protease termed beta-site APP cleavage enzyme 1 (BACE1) that cleaves the amyloid-beta precursor protein (APP), which is abundant in neurons, is required for the generation of amyloid-beta (Abeta) peptides implicated in the pathogenesis of Alzheimer's disease (AD). We now demonstrate that BACE1, enriched in neurons of the CNS, is a major determinant that predisposes the brain to Abeta amyloidogenesis. The physiologically high levels of BACE1 activity coupled with low levels of BACE2 and alpha-secretase anti-amyloidogenic activities in neurons is a major contributor to the accumulation of Abeta in the CNS, whereas other organs are spared. Significantly, deletion of BACE1 in APPswe;PS1DeltaE9 mice prevents both Abeta deposition and age-associated cognitive abnormalities that occur in this model of Abeta amyloidosis. Moreover, Abeta deposits are sensitive to BACE1 dosage and can be efficiently cleared from the CNS when BACE1 is silenced. However, BACE1 null mice manifest alterations in hippocampal synaptic plasticity as well as in performance on tests of cognition and emotion. Importantly, memory deficits but not emotional alterations in BACE1(-/-) mice are prevented by coexpressing APPswe;PS1DeltaE9 transgenes, indicating that other potential substrates of BACE1 may affect neural circuits related to emotion. Our results establish BACE1 and APP processing pathways as critical for cognitive, emotional, and synaptic functions, and future studies should be alert to potential mechanism-based side effects that may occur with BACE1 inhibitors designed to ameliorate Abeta amyloidosis in AD.

Capsaicin-induced inactivation of sensory neurons promotes a more aggressive gene expression phenotype in breast cancer cells

Capsaicin-induced inactivation of sensory neurons has been reported to enhance metastasis of a murine breast cancer cell line, specifically enhancing myocardial metastases. Here we characterized changes in gene expression patterns in primary tumors which developed in capsaicin-treated vs. control mice. We identified a small cohort of genes (17) which all showed significant decreases in expression levels. All of the identified genes have been linked to cell growth, differentiation, and/or cancer progression. Three representative genes, Caspase-7 (an executor of apoptosis), ADAM-10 (A Disintegrin and Metalloprotease), and Elk-3 (a transcriptional repressor of the ternary factor subfamily of the Ets factors) were further investigated. All three showed dramatic downregulation at the protein level in primary tumors from capsaicin-treated animals compared with control (vehicle-treated) animals, and their expression was also lost in cell culture. Elk-3 and Caspase-7 were not expressed in vitro in cultured cell lines, suggesting that their expression was induced by the tumor microenvironment. Loss of Caspase-7 expression can be expected to result in loss of function of apoptotic pathways. At first glance, loss of ADAM-10 expression would be expected to result in decreased invasive capability, due to loss of matrix metalloprotease activity. However, just the opposite appears to be true. We found that ADAM-10 actually hydrolyzes Substance P. Specifically ADAM-10 produces the same growth-inhibitory products from Substance P (i.e., SP (1-7)) that Neprilysin does, so that loss of ADAM-10 expression actually results in loss of production of growth inhibitory peptides from Substance P. Similarly, ADAM-10 also efficiently hydrolyzes Calcitonin Gene-Related Peptide, which may act in concert with Substance P. Finally, overactivity of Ets transcriptional suppressor functions has been linked to inhibition of tumorigenesis (e.g., Erf and Mef), and in addition loss of Elk-3 expression might also be be linked to tumorigenesis via loss of its putative anti-inflammatory activities. There is anecdotal evidence in the literature to indicate that the rest of the down-regulated genes may also contribute to development of a more aggressive phenotype in this breast cancer model.

New prospects and strategies for drug target discovery in neurodegenerative disorders

The future of neurodegenerative therapeutics development depends upon effective disease modification strategies centered on carefully investigated targets. Pharmaceutical research endeavors that probe for a much deeper understanding of disease pathogenesis, and explain how adaptive or compensatory mechanisms might be engaged to delay disease onset or progression, will produce the needed breakthroughs. Below, we discuss the prospects for new targets emerging out of the study of brain disease genes and their associated pathogenic pathways. We describe a general experimental paradigm that we are employing across several mouse models of neurodegenerative disease to elucidate molecular determinants of selective neuronal vulnerability. We outline key elements of our target discovery program and provide examples of how we integrate genomic technologies, neuroanatomical methods, and mouse genetics in the search for neurodegenerative disease targets.