Reduced neprilysin in high plaque areas of Alzheimer brain: a possible relationship to deficient degradation of beta-amyloid peptide

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.

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.

Lack of association of neprilysin gene polymorphisms with Alzheimer's disease in a southern Chinese community

BACKGROUND

Increasing evidence suggests that neprilysin (NEP) may be the major degrading enzyme of amyloid beta (Abeta) in the brain and the NEP gene has been proposed as a candidate gene for Alzheimer's disease (AD). Association results between the NEP gene and AD are still preliminary. This study investigates the effect of the polymorphisms of -204G/C and 159C/T in the NEP gene on the development of sporadic Alzheimer's disease (SAD) in a southern Chinese community.

METHOD

236 sporadic late-onset AD patients were recruited from Guangzhou Psychiatric Hospital in southern China, and 332 healthy elderly controls were enrolled from three old age homes in suburban Guangzhou. NEP and ApoE genotype were determined by PCR-RFLP.

RESULTS

No differences in genotypic and allelic frequencies of -204G/C and 159C/T polymorphisms in NEP were found between AD and control group (for -204G/C genotype: chi2 = 2.34, P > 0.05; for allele: chi2 = 2.31, P > 0.05; for 159C/T genotype: chi2 = 1.34, P > 0.05; for allele: chi2 = 0.88, P > 0.05). Neither was any difference found in genotypic and allelic frequency when stratified by sex or by ApoE epsilon4 allele (P > 0.05).

CONCLUSIONS

Our results suggest that -204G/C and 159C/T polymorphisms of the NEP gene may not be associated with SAD. Moreover, both sex and ApoE epsilon4 allele do not affect the distribution of NEP gene polymorphisms.

Neprilysin deficiency-dependent impairment of cognitive functions in a mouse model of amyloidosis

Alzheimer's disease, responsible for the vast majority of dementia cases in the elderly population, is caused by accumulation of toxic levels of amyloid beta peptide (A beta) in the brain. Neprilysin is a major enzyme responsible for the degradation of A beta in vivo. We have previously shown that elevation of neprilysin levels in the brain delays the deposition of A beta-plaques in a mouse model of amyloidosis and that lack of neprilysin leads to increased A beta generation and to signs of incipient neurodegeneration in mouse brains. This study was designed to test whether low brain levels of neprilysin affect the amyloid pathology or perturb the learning and memory performance of mice. Double-mutated mice carrying a targeted depletion of one allele of Mme, the gene encoding neprilysin, and over-expressing human amyloid precursor protein (APP), exhibited a reinforced amyloid pathology in comparison with their APP transgenic littermates. Moreover, in contrast to their parental lines, these mice were impaired in the Morris water maze learning and memory paradigm and showed facilitated extinction in the conditioned taste aversion test. These data suggest that even a partial neprilysin deficiency, as is found during aging, exacerbates amyloid pathology and may impair cognitive functions.

Immunohistochemical characterisation of cell-type specific expression of CK1delta in various tissues of young adult BALB/c mice

BACKGROUND

Casein kinase 1 delta (CK1delta) phosphorylates many key proteins playing important roles in such biological processes as cell growth, differentiation, apoptosis, circadian rhythm and vesicle transport. Furthermore, deregulation of CK1delta has been linked to neurodegenerative diseases and cancer. In this study, the cell specific distribution of CK1delta in various tissues and organs of young adult BALB/c mice was analysed by immunohistochemistry.

METHODOLOGY/PRINCIPAL FINDINGS

Immunohistochemical staining of CK1delta was performed using three different antibodies against CK1delta. A high expression of CK1delta was found in a variety of tissues and organ systems and in several cell types of endodermal, mesodermal and ectodermal origin.

CONCLUSIONS

These results give an overview of the cell-type specific expression of CK1delta in different organs under normal conditions. Thus, they provide evidence for possible cell-type specific functions of CK1delta, where CK1delta can interact with and modulate the activity of key regulator proteins by site directed phosphorylation. Furthermore, they provide the basis for future analyses of CK1delta in these tissues.

Estrogen regulation of the neprilysin gene through a hormone-responsive element

Our previous data showed that neprilysin (NEP), a zinc metalloendopeptidase, which can degrade amyloid-beta peptide (Abeta) whose central nerve system accumulation is the primary cause of Alzheimer's disease (AD), responds to estrogen in the brain. Recently, it has been shown that the transcription of the neprilysin gene can be up regulated by progesterone, androgens, and glucocorticoids through two androgen response elements within the NEP gene--an androgen response region (ARR) and an androgen response element (ARE). Through a yeast report gene system, we now find that the ARR but not the ARE respond to estrogen. However, androgen could efficiently enhance the expression of the report gene mainly through ARE. Our results indicate that the decrease of NEP, caused by the decline of estrogen or androgen with aging, may be an important factor leading to Abeta accumulation and AD.

Altered expression of Abeta metabolism-associated molecules from D-galactose/AlCl(3) induced mouse brain

Cerebral deposition of amyloid-beta peptide (Abeta) is a critical feature of Alzheimer's disease (AD). Either aluminium trichloride (Al) or D-galactose (D-gal) induces Abeta overproduction in rat or mouse brain and has been used to produce models of aging and AD. Here it is shown that mice treated with Al plus D-gal represent a good model of AD with altered expression of Abeta metabolism-associated molecules. The work shows that Al/D-gal causes memory impairment and high Abeta levels in the cortex (Co) and hippocampus (Hi). Then, we found that beta-site APP cleavage enzyme 1 (BACE1) was increased in mouse Co and Hi. Al or Al plus D-gal suppressed mRNA of the low-density lipoprotein receptor-related protein 1(LRP1). D-gal also decreased the LRP expression in Hi, but not in Co. However, Al/D-gal did not affect the receptor for advanced glycation end products (RAGE) expression in mouse brains. Furthermore, Al/D-gal reduced the expression of neprilysin (NEP), but not the insulin degrading enzyme (IDE). This study indicates that Al/D-gal affects the expression of Abeta metabolism-associated molecules that are responsible for Abeta deposition during AD, suggesting that this mouse model can be a useful model for studying the mechanisms and biomarkers of AD and for drug screening.

Pharmacotherapeutic targets in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder which is characterized by an increasing impairment in normal memory and cognitive processes that significantly diminishes a person's daily functioning. Despite decades of research and advances in our understanding of disease aetiology and pathogenesis, there are still no effective disease-modifying drugs available for the treatment of AD. However, numerous compounds are currently undergoing pre-clinical and clinical evaluations. These candidate pharma-cotherapeutics are aimed at various aspects of the disease, such as the microtubule-associated τ-protein, the amyloid-β (Aβ) peptide and metal ion dyshomeostasis – all of which are involved in the development and progression of AD. We will review the way these pharmacological strategies target the biochemical and clinical features of the disease and the investigational drugs for each category.

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

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

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.

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

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

Amyloid beta-degrading cryptidases: insulin degrading enzyme, presequence peptidase, and neprilysin

The accumulation of aggregates of amyloidogenic peptides is associated with numerous human diseases. One well studied example is the association between deposition of amyloid beta (Abeta) and Alzheimer's disease. Insulin degrading enzyme and neprilysin are involved in the clearance of Abeta, and presequence peptidase is suggested to play a role in the degradation of mitochondrial Abeta. Recent structural analyses reveal that these three peptidases contain a catalytic chamber (crypt) that selectively encapsulates and cleaves amyloidogenic peptides, hence the name cryptidase. The substrate selectivity of these cryptidases is determined by the size and charge distribution of their crypt as well as the conformational flexibility of substrates. The interaction of Abeta with the catalytic core of these cryptidases is controlled by conformational changes that make the catalytic chambers accessible for Abeta binding. These new structural and biochemical insights into cryptidases provide potential therapeutic strategies for the control of Abeta clearance.

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.

Complementary Distributions of Amyloid-β and Neprilysin in the Brains of Dogs and Cats

Neprilysin is an amyloid-β-degrading enzyme localized in the brain parenchyma. The involvement of neprilysin in the pathogenesis of Alzheimer's disease has recently received much attention. We examined the localization of neprilysin and amyloid-β, as well as the activity of neprilysin, in the brains of dogs and cats of various ages to clarify the relationship between neprilysin activity and amyloid-β deposition. The distribution of neprilysin was almost identical in dogs and cats, being high in the striatum, globus pallidus, and substantia nigra, but very low in the cerebral cortex. The white matter and hippocampus were negative. Neprilysin activity in the brain regions in dogs and cats was ranked from high to low as follows: thalamus/striatum > cerebral cortex > hippocampus > white matter. Amyloid-β deposition was first detected at 7 and 10 years of age in dogs and cats, respectively, and both the quantity and frequency of deposition increased with age. In both species, amyloid-β deposition appeared in the cerebral cortex and the hippocampus. In summary, the localization of neprilysin and neprilysin activity, and that of amyloid-β, were complementary in the brains of dogs and cats.

Human membrane metallo-endopeptidase-like protein degrades both beta-amyloid 42 and beta-amyloid 40

Beta-amyloid (Abeta) degrading endopeptidases are thought to protect against Alzheimer's disease (AD) and are potentially therapeutic. Of particular interest are endopeptidases that are blocked by thiorphan and phosphoramidon (T/P), as these inhibitors rapidly induce Abeta deposition in rodents. Neprilysin (NEP) is the best known target of T/P; however neprilysin knockout results in only modest Abeta increases insufficient to induce deposition. Therefore, other endopeptidases targeted by T/P must be critical for Abeta catabolism. Another candidate is the T/P sensitive membrane metallo-endopeptidase-like protein (MMEL), a close homolog of neprilysin. The endopeptidase properties of beta and gamma splice forms of human MMEL were determined in HEK293T cells transduced with the human cDNAs for the two splice forms; this showed degradation of both Abeta(42) and Abeta(40) by hMMEL-beta but not hMMEL-gamma. hMMEL-beta activity was found at the extracellular surface with no significant secreted activity. hMMEL-gamma was not expressed at the extracellular surface. Finally, it was found that hMMEL cleaves Abeta near the alpha-secretase site (producing Abeta(1-17)>>Abeta(1-16)). These data establish hMMEL as a mediator of Abeta catabolism and raise the possibility of its involvement in the etiology of AD and as a target for intervention.

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.

Catabolic attacks of membrane-bound angiotensin-converting enzyme on the N-terminal part of species-specific amyloid-beta peptides

Catabolic processes play a crucial role in the steady state of the amyloid-beta peptide (Abeta). Neprilysin (NEP) and angiotensin-converting enzyme (ACE), two transmembranal enzymes with greatest importance in peptide pharmacology, are known to play a role in Abeta catabolism. This paper focuses on the N-terminal part of Abeta. This region contains the three amino acid residues that determine the differences between human (hAbeta) and murine Abeta (mAbeta). Moreover, the N-terminal part of Abeta contains the zinc-binding site of the molecule. Consequently, all hydrolytic attacks on this part of the Alzheimer peptide should be of exceptional interest. We investigated domain-selective forms of ACE in HPLC-monitored peptide degradation studies and used mass spectrometry for product analyses. We found that ACE-evoked a hydrolysis of the N-terminal part of m- and hAbeta. The hAbeta sequence hAbeta (4-15) was found to be a better substrate for ACE compared to the corresponding murine form. Moreover, we localized the corresponding cleavage sites in the N-terminal part of Abeta as well as in the full-length molecule and identified new sites of endopeptidolytic attack by ACE. Finally, we demonstrate that both catalytic domains of mACE have similar hydrolytic activity on the N-terminal part of Abeta. Our results show that ACE besides its typical function as a dipeptidyl-carboxypeptidase has also unequivocal endopeptidolytic activities.

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.

Calpastatin levels affect calpain activation and calpain proteolytic activity in APP transgenic mouse model of Alzheimer's disease

The intracellular Ca(2+)-dependent protease calpain and the specific calpain endogenous inhibitor calpastatin are widely distributed, with the calpastatin/calpain ratio varying among tissues and species. Increased Ca(2+) and calpain activation have been implicated in Alzheimer's disease (AD), with scant data available on calpastatin/calpain ratio in AD. Information is lacking on calpain activation and calpastatin levels in transgenic mice that exhibit AD-like pathology. We studied calpain and calpastatin in Tg2576 mice and in their wild type littermates (control mice). We found that in control mice calpastatin level varies among brain regions; it is significantly higher in the cerebellum than in the hippocampus, frontal and temporal cortex, whereas calpain levels are similar in all these regions. In the Tg2576 mice, calpain is activated, calpastatin is diminished, and calpain-dependent proteolysis is observed in brain regions affected in AD and in transgenic mice (especially hippocampus). In contrast, no differences are observed between the Tg2576 and the control mice in the cerebellum, which does not exhibit AD-like pathology. The results are consistent with the notion that a high level of calpastatin in the cerebellum renders the calpain in this brain region less liable to be activated; in the other brain parts, in which calpastatin is low, calpain is more easily activated in the presence of increased Ca(2+), and in turn the activated calpain leads to further diminution in calpastatin (a known calpain substrate). The results indicate that calpastatin is an important factor in the regulation of calpain-induced protein degradation in the brains of the affected mice, and imply a role for calpastatin in attenuating AD pathology. Promoting calpastatin expression may be used to ameliorate some manifestations 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.

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

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

Enviromimetics: exploring gene environment interactions to identify therapeutic targets for brain disorders

There is a growing awareness of the central role played by environmental factors in many of the most debilitating neural disorders. Epidemiological studies have suggested a complex balance between genetic and environmental factors in the pathogenesis of neurological and psychiatric conditions. The use of accurate animal models, combined with experimental manipulations such as environmental enrichment, has shown that increased sensory, cognitive and motor stimulation has beneficial effects in a range of CNS disorders, including Huntington's, Alzheimer's, Parkinson's and other neurodegenerative diseases. Various studies have identified molecular, structural and functional correlates of this experience-dependent plasticity. The authors propose that the molecular systems which mediate the therapeutic effects of environmental enrichment may provide novel targets for pharmacotherapies. More specifically, they elaborate a theoretical framework for the development of 'enviromimetics', therapeutics that mimic or enhance the beneficial effects of environmental stimulation, targeted towards a wide range of nervous system disorders.

N-truncated amyloid-beta oligomers induce learning impairment and neuronal apoptosis

N-terminal-truncated forms of amyloid-beta (A beta) peptide have been recently suggested to play a pivotal role early in Alzheimer's disease (AD). Among them, A beta 3(pE)-42 peptide, starting with pyroglutamyl at residue Glu-3, is considered as the predominant A beta species in AD plaques and pre-amyloid lesions. Its abundance is reported to be directly proportional to the severity of the clinical phenotype. The present study investigates the effects of soluble oligomeric A beta 3(pE)-42 after intracerebroventricular injection on mice learning ability and the molecular mechanisms of its in vitro neurotoxicity. Mice injected with soluble A beta 3(pE)-42 or A beta(l-42) displayed impaired spatial working memory and delayed memory acquisition in Y-maze and Morris water maze tests, while those injected with soluble A beta(42-1) showed no effect. These cognitive alterations were associated with free radical overproduction in the hippocampus and olfactory bulbs, but not in the cerebral cortex or cerebellum. In vitro, A beta 3(pE)-42 oligomers induced a redox-sensitive neuronal apoptosis involving caspase activation and an arachidonic acid-dependent pro-inflammatory pathway. These data suggest that A beta 3(pE)-42 could mediate the neurodegenerative process and subsequent cognitive alteration occurring in preclinical AD stages.

Lack of neprilysin suffices to generate murine amyloid-like deposits in the brain and behavioral deficit in vivo

Accumulation of the beta-amyloid peptide (Abeta) in the brain is a major pathological hallmark of Alzheimer's disease (AD), leading to synaptic dysfunction, neuronal death, and memory impairment. The levels of neprilysin, a major Abeta-degrading enzyme, are decreased in AD brains and during aging. Because neprilysin cleaves Abeta in vivo, its down-regulation may contribute to the pathophysiology of AD. The aim of this study was to assess the consequences of neprilysin deficiency on accumulation of murine Abeta in brains and associated pathologies in vivo by investigating neprilysin-deficient mice on biochemical, morphological, and behavioral levels. Aged neprilysin-deficient mice expressed physiological amyloid precursor protein (APP) levels and exhibited elevated brain Abeta concentrations and amyloid-like deposits in addition to signs of neuronal degeneration in their brains. Behaviorally, neprilysin-deficient mice acquired a significantly weaker conditioned taste aversion that extinguished faster than the aversion of age-matched controls. Our data establish that, under physiological APP expression levels, neprilysin deficiency is associated with increased Abeta accumulation in the brain and leads to deposition of amyloid-like structures in vivo as well as with signs of AD-like pathology and with behavioral deficits.

Identification of the amyloid-degrading enzyme neprilysin in mouse islets and potential role in islet amyloidogenesis

Islet amyloid contributes to loss of beta-cell mass and function in type 2 diabetes. It is poorly understood how the building block of amyloid, islet amyloid polypeptide (IAPP), misfolds and accumulates within the islet to contribute to cellular dysfunction. We sought to determine whether neprilysin, an amyloid-degrading enzyme, is present in islets and plays a role in the accumulation of amyloid fibrils. Human IAPP (hIAPP) transgenic mice, a model of islet amyloid in which primarily male mice develop amyloid by 12 months of age, were studied at 10 weeks and 6 months of age, enabling investigation of islet changes before and during early amyloidogenesis. Neprilysin was present in islets, including beta-cells, and islet neprilysin mRNA and activity were found to decline with age in nontransgenic mice as well as in hIAPP transgenic female mice. In contrast, neprilysin mRNA and activity did not decrease in amyloid-prone hIAPP transgenic male mice at 6 months compared with nontransgenic mice and female hIAPP transgenic mice. Islet amyloid was detected in 43% of the 6-month-old hIAPP transgenic male mice only, suggesting the sustained elevation of islet neprilysin in these mice was a compensatory mechanism aimed at preventing amyloid accumulation. In keeping with amyloid formation, the proportion of insulin-positive area to islet area was significantly reduced in 6-month-old hIAPP transgenic male mice, which also displayed mild fasting hyperglycemia compared with age-matched transgenic female and nontransgenic mice. Together, these findings demonstrate that neprilysin is a factor associated with islet amyloid accumulation and subsequent deterioration of beta-cell function in hIAPP transgenic male mice.

Subcutaneous Nogo receptor removes brain amyloid-beta and improves spatial memory in Alzheimer's transgenic mice

The production and aggregation of cerebral amyloid-beta (Abeta) peptide are thought to play a causal role in Alzheimer's disease (AD). Previously, we found that the Nogo-66 receptor (NgR) interacts physically with both Abeta and the amyloid precursor protein (APP). The inverse correlation of Abeta levels with NgR levels within the brain may reflect regulation of Abeta production and/or Abeta clearance. Here, we assess the potential therapeutic benefit of peripheral NgR-mediated Abeta clearance in APPswe/PSEN-1deltaE9 transgenic mice. Through site-directed mutagenesis, we demonstrate that the central 15-28 aa of Abeta associate with specific surface-accessible patches on the leucine-rich repeat concave side of the solenoid structure of NgR. In transgenic mice, subcutaneous NgR(310)ecto-Fc treatment reduces brain Abeta plaque load while increasing the relative levels of serum Abeta. These changes in Abeta are correlated with improved spatial memory in the radial arm water maze. The benefits of peripheral NgR administration are evident when therapy is initiated after disease onset. Thus, the peripheral association of NgR(310)ecto-Fc with central Abeta residues provides an effective therapeutic approach for AD.

Acyl peptide hydrolase, a serine proteinase isolated from conditioned medium of neuroblastoma cells, degrades the amyloid-? peptide

Considerable evidence indicates that the amyloid-beta (Abeta) peptide, a proteolytic fragment of the amyloid precursor protein, is the pathogenic agent in Alzheimer's disease (AD). A number of proteases have been reported as capable of degrading Abeta, among them: neprilysin, insulin-degrading enzyme, endothelin-converting enzyme-1 and -2, angiotensin-converting enzyme and plasmin. These proteases, originating from a variety of cell types, degrade Abeta of various conformational states and in different cellular locations. We report here the isolation of a serine protease from serum-free conditioned medium of human neuroblastoma cells. Tandem mass spectrometry (MS/MS)-based sequencing of the isolated protein identified acyl peptide hydrolase (APH; EC3.4.19.1) as the active peptidase. APH is one of four members of the prolyl oligopeptidase family of serine proteases expressed in a variety of cells and tissues, including erythrocytes, liver and brain, but its precise biological activity is unknown. Here, we describe the identification of APH as an Abeta-degrading enzyme, and we show that the degradation of Abeta by APH isolated from transfected cells is inhibited by APH-specific inhibitors, as well as by synthetic Abeta peptide. In addition, we cloned APH from human brain and from neuroblastoma cells. Most importantly, our results indicate that APH expression in AD brain is lower than in age-matched controls.

Somatostatin in the dentate gyrus

The neuropeptide somatostatin (SST) is expressed in a discrete population of interneurons in the dentate gyrus. These interneurons have their soma in the hilus and project to the outer molecular layer onto dendrites of dentate granule cells, adjacent to perforant path input. SST-containing interneurons are very sensitive to excitotoxicty, and thus are vulnerable to a variety of neurological diseases and insults, including epilepsy, Alzheimer's disease, traumatic brain injury, and ischemia. The SST gene contains a prototypical cyclic AMP response element (CRE) site. Such a regulatory site confers activity-dependence to the gene, such that it is turned on when neuronal activity is high. Thus SST expression is increased by pathological conditions such as seizures and by natural stimulation such as environmental enrichment. SST may play an important role in cognition by modulating the response of neurons to synaptic input. In the dentate, SST and the related peptide cortistatin (CST) reduce the likelihood of generating long-term potentiation, a cellular process involved in learning and memory. Thus these neuropeptides would increase the threshold of input required for acquisition of new memories, increasing "signal to noise" to filter out irrelevant environmental cues. The major mechanism through which SST inhibits LTP is likely through inhibition of voltage-gated Ca(2+) channels on dentate granule cell dendrites. Transgenic overexpression of CST in the dentate leads to profound deficits in spatial learning and memory, validating its role in cognitive processing. A reduction of synaptic potentiation by SST and CST in dentate may also contribute to the well-characterized antiepileptic properties of these neuropeptides. Thus SST and CST are important neuromodulators in the dentate gyrus, and disruption of this signaling system may have major impact on hippocampal function.

New insights into the roles of metalloproteinases in neurodegeneration and neuroprotection

Proteolytic enzymes constitute around 2% of the human genome and are involved in many stages of cell development from fertilization to death (apoptosis). The identification of many novel proteases from genome-sequencing programs has suggested them as potential new therapeutic targets. In addition, several well-characterized metallopeptidases were recently shown to possess new biological roles in neuroinflammation and neurodegeneration. As a result of these studies, metabolism of the neurotoxic and inflammatory amyloid peptide (Abeta) is considered as a physiologically relevant process with several metallopeptidases being suggested for the role of amyloid-degrading enzymes. These include the neprilysin (NEP) family of metalloproteinases (including its homologue endothelin-converting enzyme), insulin-degrading enzyme, angiotensin-converting enzyme, plasmin, and, possibly, some other enzymes. NEP also has a role in metabolism of sensory and inflammatory neuropeptides such as tachykinins and neurokinins. The existence of natural enzymatic mechanisms for removal of amyloid peptides has extended the therapeutic avenues in Alzheimer's disease (AD) and neurodegeneration. The proteolytic events underlying AD are highly compartmentalized in the cell and formation of amyloid peptide from its precursor molecule APP (amyloid precursor protein) takes place both within intracellular compartments and in the plasma membrane, especially in lipid raft domains. Degradation of amyloid peptide by metallopeptidases can also be both intra- and extracellular depending on the activity of membrane-bound enzymes and their soluble partners. Soluble forms of proteases can be secreted or released from the cell surface through the activity of "sheddases"-another group of proteolytic enzymes involved in key cellular regulatory functions. The activity of proteases involved in amyloid metabolism depends on numerous factors (e.g., genetic, environmental, age), and some conditions (e.g., hypoxia and ischemia) shift the balance of amyloid metabolism toward accumulation of higher concentrations of Abeta. In this regard, regulation of the activity of amyloid-degrading enzymes should be considered as a viable strategy in neuroprotection.

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.

G protein-coupled receptor systems and their lipid environment in health disorders during aging

Cells, tissues and organs undergo phenotypic changes and deteriorate as they age. Cell growth arrest and hyporesponsiveness to extrinsic stimuli are all hallmarks of senescent cells. Most such external stimuli received by a cell are processed by two different cell membrane systems: receptor tyrosine kinases (RTKs) and G protein-coupled receptors (GPCRs). GPCRs form the largest gene family in the human genome and they are involved in most relevant physiological functions. Given the changes observed in the expression and activity of GPCRs during aging, it is possible that these receptors are directly involved in aging and certain age-related pathologies. On the other hand, both GPCRs and G proteins are associated with the plasma membrane and since lipid-protein interactions regulate their activity, they can both be considered to be sensitive to the lipid environment. Changes in membrane lipid composition and structure have been described in aged cells and furthermore, these membrane changes have been associated with alterations in GPCR mediated signaling in some of the main health disorders in elderly subjects. Although senescence could be considered a physiologic process, not all aging humans develop the same health disorders. Here, we review the involvement of GPCRs and their lipid environment in the development of the major human pathologies associated with aging such as cancer, neurodegenerative disorders and cardiovascular pathologies.

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.

Herpes simplex virus RNAi and neprilysin gene transfer vectors reduce accumulation of Alzheimer's disease-related amyloid-β peptide in vivo

Accumulation of insoluble aggregates of amyloid-beta peptide (Abeta), a cleavage product of amyloid precursor protein (APP), is thought to be central to the pathogenesis of Alzheimer's disease (AD). Consequently, downregulation of APP, or enhanced clearance of Abeta, represent possible therapeutic strategies for AD. We generated replication-defective herpes simplex virus (HSV) vectors that inhibit Abeta accumulation, both in vitro and in vivo. In cell culture, HSV vectors expressing either (i) short hairpin RNA directed to the APP transcript (HSV-APP/shRNA), or (ii) neprilysin, an endopeptidase that degrades Abeta (HSV-neprilysin), substantially inhibited accumulation of Abeta. To determine whether these vectors showed similar activity in vivo, we developed a novel mouse model, in which overexpression of a mutant form of APP in the hippocampus, using a lentiviral vector (LV-APP(Sw)), resulted in rapid Abeta accumulation. Co-inoculation of LV-APP(Sw) with each of the HSV vectors showed that either HSV-APP/shRNA or HSV-neprilysin inhibited Abeta accumulation in this model, whereas an HSV control vector did not. These studies demonstrate the utility of HSV vectors for reducing Abeta accumulation in the brain, thus providing useful tools to clarify the role of Abeta in AD that may facilitate the development of novel therapies for this important disease.

Inhibition of neprilysin by thiorphan (i.c.v.) causes an accumulation of amyloid β and impairment of learning and memory

An accumulation of amyloid beta peptide (Abeta) due to an imbalance between anabolism and catabolism triggers Alzheimer's disease (AD). Neprilysin is a rate-limiting peptidase, which participates in the catabolism of Abeta in brain. We investigated whether rats continuously infused with thiorphan, a specific inhibitor for neprilysin, into the cerebral ventricle cause cognitive dysfunction, with an accumulation of Abeta in the brain. Thiorphan-infused rats displayed significant cognitive dysfunction in the ability to discriminate in the object recognition test and spatial memory in the water maze test, but not in other hippocampus-dependent learning and memory tasks. Thiorphan infusion also elevated the Abeta40 level in the insoluble fraction of the cerebral cortex, but not that of the hippocampus. There was no significant difference in the nicotine-stimulated release of acetylcholine in the hippocampus between vehicle- and thiorphan-infused rats. These results indicate that continuous infusion of thiorphan into the cerebral ventricle causes cognitive dysfunction by raising the level of Abeta in the cerebral cortex, and suggest that a reduction of neprilysin activity contribute to the deposition of Abeta and development of AD.

Metabolism of amyloid beta peptide and pathogenesis of Alzheimer's disease. Towards presymptomatic diagnosis, prevention and therapy

The conversion of what has been interpreted as "normal brain aging" to Alzheimer's disease (AD) via a transition state, i.e. mild cognitive impairment, appears to be a continuous process caused primarily by aging-dependent accumulation of amyloid beta peptide (Abeta) in the brain. This notion give us a hope that, by manipulating the Abeta levels in the brain, we may be able not only to prevent and cure the disease but also to partially control some very significant aspects of brain aging. Abeta is constantly produced from its precursor and immediately catabolized under normal conditions, whereas dysmetabolism of Abeta seems to lead to pathological deposition upon aging. We have focused our attention on elucidation of the unresolved mechanism of Abeta catabolism in the brain. In this review, we describe a new approach to prevent AD development by reducing Abeta burdens in aging brains through up-regulation the catabolic mechanism involving neprilysin that can degrade both monomeric and oligomeric forms of Abeta. The strategy of combining presymptomatic diagnosis with preventive medicine seems to be the most pragmatic in both medical and socio-economical terms. We also introduce a novel non-invasive amyloid imaging approach using a high-power magnetic resonance imaging (MRI) for the presymptomatic diagnosis of AD.

Neprilysin participates in skeletal muscle regeneration and is accumulated in abnormal muscle fibres of inclusion body myositis

Neprilysin (NEP, EP24.11), a metallopeptidase originally shown to modulate signalling events by degrading small regulatory peptides, is also an amyloid-beta- (Abeta) degrading enzyme. We investigated a possible role of NEP in inclusion body myositis (IBM) and other acquired and hereditary muscle disorders and found that in all myopathies NEP expression was directly associated with the degree of muscle fibre regeneration. In IBM muscle, NEP protein was also strongly accumulated in Abeta-bearing abnormal fibres. In vitro, during the experimental differentiation of myoblasts, NEP protein expression was regulated at the post-transcriptional level with a rapid increase in the early stage of myoblast differentiation followed by a gradual reduction thereafter, coincident with the progression of the myogenic programme. Treatment of differentiating muscle cells with the NEP inhibitor dl-3-mercapto-2-benzylpropanoylglycine resulted in impaired differentiation that was mainly associated with an abnormal regulation of Akt activation. Therefore, NEP may play an important role during muscle cell differentiation, possibly through the regulation, either directly or indirectly, of the insulin-like growth factor I-driven myogenic programme. In IBM muscle increased NEP may be instrumental in (i) reducing the Abeta accumulation in vulnerable fibres and (ii) promoting a repair/regenerative attempt of muscle fibres possibly through the modulation of insulin-like growth factor I-dependent pathways.

Inhibition of Neprilysin by Infusion of Thiorphan into the Hippocampus Causes an Accumulation of Amyloid β and Impairment of Learning and Memory

An imbalance between anabolism and catabolism causes an accumulation of amyloid beta-peptide (Abeta), which is a proposed trigger of the onset of Alzheimer's disease. Neprilysin is a rate-limiting peptidase that participates in the catabolism of Abeta in the brain. We examined whether rats continuously infused with thiorphan, a specific neprilysin inhibitor, into the hippocampus develop cognitive impairments through accumulation of Abeta. Thiorphan infusion elevated hippocampal Abeta40 and Abeta42 levels in the insoluble but not the soluble fraction. Thiorphan-infused rats displayed cognitive impairments in the ability to discriminate in the object recognition test, associative learning in the conditioned fear learning test, and spatial memory in the water maze test, tasks that depend on the hippocampus. These cognitive abilities in the battery of behavioral tasks inversely correlated with insoluble Abeta contents in the hippocampus. The nicotine-stimulated release of acetylcholine in the hippocampus of thiorphan-infused rats was significantly lower than that in vehicle-infused rats. These results indicate that continuous infusion of thiorphan into the hippocampus causes cognitive dysfunction and reduces cholinergic activity by raising the level of Abeta in the hippocampus and suggest that a reduction of neprilysin activity contributes to the deposition of Abeta and development of Alzheimer's disease.

Brain estrogen deficiency accelerates Abeta plaque formation in an Alzheimer's disease animal model

Much evidence indicates that women have a higher risk of developing Alzheimer's disease (AD) than do men. The reason for this gender difference is unclear. We hypothesize that estrogen deficiency in the brains of women with AD may be a key risk factor. In rapidly acquired postmortem brains from women with AD, we found greatly reduced estrogen levels compared with those from age- and gender-matched normal control subjects; AD and control subjects had comparably low levels of serum estrogen. We examined the onset and severity of AD pathology associated with estrogen depletion by using a gene-based approach, by crossing the estrogen-synthesizing enzyme aromatase gene knockout mice with APP23 transgenic mice, a mouse model of AD, to produce estrogen-deficient APP23 mice. Compared with APP23 transgenic control mice, estrogen-deficient APP23 mice exhibited greatly reduced brain estrogen and early-onset and increased beta amyloid peptide (Abeta) deposition. These mice also exhibited increased Abeta production, and microglia cultures prepared from the brains of these mice were impaired in Abeta clearance/degradation. In contrast, ovariectomized APP23 mice exhibited plaque pathology similar to that observed in the APP23 transgenic control mice. Our results indicate that estrogen depletion in the brain may be a significant risk factor for developing AD neuropathology.

Neprylisin decreases uniformly in Alzheimer's disease and in normal aging

The proteolysis of beta-amyloid (Abeta) requires neprylisin, an enzyme that has been shown as reduced in Alzheimer's disease (AD). We investigated whether a decrease in neprilysin levels contributes to the accumulation of amyloid deposits not only in AD but also in the normal aging. We analyzed neprilysin mRNA and protein levels in cerebral cortex from 10 cognitively normal elderly subjects with amyloid plaques (NA), 10 cases of AD, and 10 control cases free of amyloid plaques. We found a significant decrease in neprilysin mRNA levels in both AD and NA compared to control cases. Thereby, the defect of neprilysin appears to correlate with Abeta deposition but not with degeneration and dementia.