Molecular identification and characterization of novel membrane-bound metalloprotease, the soluble secreted form of which hydrolyzes a variety of vasoactive peptides

The Drosophila melanogaster Neprilysin Nepl15 is involved in lipid and carbohydrate storage

The prototypical M13 peptidase, human Neprilysin, functions as a transmembrane "ectoenzyme" that cleaves neuropeptides that regulate e.g. glucose metabolism, and has been linked to type 2 diabetes. The M13 family has undergone a remarkable, and conserved, expansion in the Drosophila genus. Here, we describe the function of Drosophila melanogaster Neprilysin-like 15 (Nepl15). Nepl15 is likely to be a secreted protein, rather than a transmembrane protein. Nepl15 has changes in critical catalytic residues that are conserved across the Drosophila genus and likely renders the Nepl15 protein catalytically inactive. Nevertheless, a knockout of the Nepl15 gene reveals a reduction in triglyceride and glycogen storage, with the effects likely occurring during the larval feeding period. Conversely, flies overexpressing Nepl15 store more triglycerides and glycogen. Protein modeling suggests that Nepl15 is able to bind and sequester peptide targets of catalytically active Drosophila M13 family members, peptides that are conserved in humans and Drosophila, potentially providing a novel mechanism for regulating the activity of neuropeptides in the context of lipid and carbohydrate homeostasis.

Molecular insights into information processing and developmental and immune regulation of Eriocheir sinensis megalopa under hyposaline stress

Eriocheir sinensis is an important euryhaline catadromous crustacean of the Yangtze River and an important commercial species for breeding in China. However, wild E. sinensis have suffered serious damage attributed to overfishing, climate change, etc. The Ministry of Agriculture of China issued a notice banning the commercial fishing of wild E. sinensis. E. sinensis megalopa migrates upriver into fresh water for growth and fattening, which creates optimal conditions to experimentally explore its hyposaline osmoregulation mechanism. We performed comparative transcriptome analyses of E. sinensis megalopae under hyposaline stress. The results suggest that KEGG pathways and genes related to genetic information processing, developmental regulation, immune and anti-stress responses were differentially expressed. The present study reveals the most significantly enriched pathways and functional gene groups, and explores the hyposaline osmoregulation mode of E. sinensis megalopae. This study lays a theoretical foundation for further studies on the osmoregulation and developmental mechanisms of E. sinensis.

Placental endothelin-converting enzyme-1 is decreased in preeclampsia

Endothelin-converting enzyme-1(ECE-1) is a key regulatory enzyme in the processing of endothelin-1 (ET-1). We quantified and localized ECE-1 in normal and preeclamptic placentas. Normal (n=6) and preeclamptic (n=6) placentas were serially sectioned for immunofluorescence (IF). Cell type specific markers identified endothelial, trophoblast, macrophage, smooth muscle, and fibroblast cells. Quantitative analyses were performed by western blot and ELISA. IF identified ECE-1 expression within the stroma and villous space. Cellular localization of ECE-1 was limited to endothelial membranes. There was significantly less ECE-1 in preeclamptic placentas, suggesting ECE-1 is important for proper regulation of ET-1 within the placenta.

In silico based investigation of dynamic variations in neprilysin (NEP and NEP2) proteins for extracting the point of specificity

Neprilysin-2 (NEP2) in the central nervous system controls Alzheimer's protein (amyloid-β) deposition, and prevents its occurrence. However, in the peripheral system, its closest homolog, neutral endopeptidase (NEP), regulates hypertension and heart related diseases. Inhibitors of NEP with a lesser degree of specificity can treat hypertension with an increased risk of cerebral deposition of amyloid-β. In order to rationalize the point of selectivity, the dynamic behavior of human NEP and NEP2 proteins was monitored by conducting molecular dynamics (MD) simulations. A computationally reliable model of NEP2 was achieved with 79.9%, 19.1% and 0.2% residues in the allowed, additionally allowed and disallowed regions respectively, using as a reference protein. Additionally, molecular docking studies were carried out for a set of five already known inhibitors of NEP and modeled NEP2 to obtain the comparative behaviors of the complexes. MD results highlighted their different responses along with important residues having a part in ligand-protein binding. For substrate and inhibitor binding, Arg664/661 and Zn697/694 were identified as the most conserved residues. High degree flexible transitions during the MD simulations were also observed in loop areas along with active site residues. Energy calculations, hydrogen bonds and their occupancy rates helped to conclude each ligand's potency towards a particular target. In most complexes of hNEP2, the ligands showed weak interactions which might be due to its larger pocket size or huge conformational variations in active site residues upon complexation. In the case of inhibitors of a small size like thiorphan, Arg49 and Arg664 are found to be acting to support the ligand binding in NEP while only Arg661 is acting in NEP2.

Characterization of a New M13 Metallopeptidase from Deep-Sea Shewanella sp. E525-6 and Mechanistic Insight into Its Catalysis

Bacterial extracellular peptidases are important for bacterial nutrition and organic nitrogen degradation in the ocean. While many peptidases of the M13 family from terrestrial animals and bacteria are studied, there has been no report on M13 peptidases from marine bacteria. Here, we characterized an M13 peptidase, PepS, from the deep-sea sedimentary strain Shewanella sp. E525-6, and investigated its substrate specificity and catalytic mechanism. The gene pepS cloned from strain E525-6 contains 2085 bp and encodes an M13 metallopeptidase. PepS was expressed in Escherichia coli and purified. Among the characterized M13 peptidases, PepS shares the highest sequence identity (47%) with Zmp1 from Mycobacterium tuberculosis, indicating that PepS is a new member of the M13 family. PepS had the highest activity at 30°C and pH 8.0. It retained 15% activity at 0°C. Its half life at 40°C was only 4 min. These properties indicate that PepS is a cold-adapted enzyme. The smallest substrate for PepS is pentapeptide, and it is probably unable to cleave peptides of more than 30 residues. PepS prefers to hydrolyze peptide bonds with P1′ hydrophobic residues. Structural and mutational analyses suggested that His531, His535 and Glu592 coordinate the catalytic zinc ion in PepS, Glu532 acts as a nucleophile, and His654 is probably involved in the transition state stabilization. Asp538 and Asp596 can stablize the orientations of His531 and His535, and Arg660 can stablize the orientation of Asp596. These results help in understanding marine bacterial peptidases and organic nitrogen degradation.

Genetic homogeneity but IgG subclass-dependent clinical variability of alloimmune membranous nephropathy with anti-neutral endopeptidase antibodies

Alloimmune antenatal membranous nephropathy (MN) during pregnancy results from antibodies produced by a neutral endopeptidase (NEP)-deficient mother. Here we report two recent cases that provide clues to the severity of renal disease. Mothers of the two children had circulating antibodies against NEP showing the characteristic species-dependent pattern by immunofluorescence on kidney slices. A German mother produced predominantly anti-NEP IgG4 accompanied by a low amount of IgG1. Her child recovered renal function within a few weeks. In sharp contrast, an Italian mother mainly produced complement-fixing anti-NEP IgG1, which also inhibits NEP enzymatic activity, whereas anti-NEP IgG4 has a weak inhibitory potency. Her child was dialyzed for several weeks. A kidney biopsy performed at 12 days of age showed MN, ischemic glomeruli, and arteriolar and tubular lesions. A second biopsy performed at 12 weeks of age showed aggravation with an increased number of collapsed capillary tufts. Both mothers were homozygous for the truncating deletion mutation 466delC and were thus NEP deficient. The 466delC mutation, identified in three previously described families, suggests a founder effect. Because of the potential severity of alloimmune antenatal MN, it is essential to identify families at risk by the detection of anti-NEP antibodies and NEP antigen in urine. On the basis of the five families identified to date, we propose an algorithm for the diagnosis of the disease and the prevention of complications.

Degradation of amyloid beta by human induced pluripotent stem cell-derived macrophages expressing Neprilysin-2

The purpose of this study was to evaluate the therapeutic potential of human induced pluripotent stem (iPS) cell-derived macrophage-like cells for Alzheimer's disease (AD). In previous studies, we established the technology to generate macrophage-like myeloid lineage cells with proliferating capacity from human iPS cells, and we designated the cells iPS-ML. iPS-ML reduced the level of Aβ added into the culture medium, and the culture supernatant of iPS-ML alleviated the neurotoxicity of Aβ. We generated iPS-ML expressing the Fc-receptor-fused form of a single chain antibody specific to Aβ. In addition, we made iPS-ML expressing Neprilysin-2 (NEP2), which is a protease with Aβ-degrading activity. In vitro, expression of NEP2 but not anti-Aβ scFv enhanced the effect to reduce the level of soluble Aβ oligomer in the culture medium and to alleviate the neurotoxicity of Aβ. To analyze the effect of iPS-ML expressing NEP2 (iPS-ML/NEP2) in vivo, we intracerebrally administered the iPS-ML/NEP2 to 5XFAD mice, which is a mouse model of AD. We observed significant reduction in the level of Aβ in the brain interstitial fluid following administration of iPS-ML/NEP2. These results suggested that iPS-ML/NEP2 may be a potential therapeutic agent in the treatment of AD.

Amyloid-beta and Alzheimer's disease: the role of neprilysin-2 in amyloid-beta clearance

Accumulation of the amyloid-beta (Aβ) peptide is a central factor in Alzheimer's disease (AD) pathogenesis as supported by continuing evidence. This review concisely summarizes this evidence supporting a critical role for Aβ in AD before discussing the clearance of this peptide. Mechanisms of clearance of Aβ are critical for preventing pathological elevations in Aβ concentration. Direct degradation of Aβ by endopeptidases has emerged as one important pathway for clearance. Of particular interest are endopeptidases that are sensitive to the neprilysin (NEP) inhibitors thiorphan and phosphoramidon (i.e., are "NEP-like") as these inhibitors induce a dramatic increase in Aβ levels in rodents. This review will focus on neprilysin-2 (NEP2), a NEP-like endopeptidase which cooperates with NEP to control Aβ levels in the brain. The evidence for the involvement of NEP2 in AD is discussed as well as the therapeutic relevance with regards to gene therapy and the development of molecular markers for the disease.

Endothelin-converting enzymes and related metalloproteases in Alzheimer's disease

The efficient clearance of amyloid-β (Aβ) is essential to modulate levels of the peptide in the brain and to prevent it from accumulating in senile plaques, a hallmark of Alzheimer's disease (AD) pathology.We and others have shown that failure in Aβ catabolism can produce elevations in Aβ concentration similar to those observed in familial forms of AD. Based on the available evidence, it remains plausible that in late-onset AD, disturbances in the activity of Aβ degrading enzymes could induce Aβ accumulation, and that this increase could result in AD pathology. The following review presents a historical perspective of the parallel discovery of three vasopeptidases (neprilysin and endothelin-converting enzymes-1 and -2) as important Aβ degrading enzymes. The recognition of the role of these vasopeptidases in Aβ degradation, beyond bringing to light a possible explanation of how cardiovascular risk factors may influence AD risk, highlights a possible risk of the use of inhibitors of these enzymes for other clinical indications such as hypertension. We will discuss in detail the experiments conducted to assess the impact of vasopeptidase deficiency (through pharmacological inhibition or genetic mutation) on Aβ accumulation, as well as the cooperative effect of multiple Aβ degrading enzymes to regulate the concentration of the peptide at multiple sites, both intracellular and extracellular, throughout the brain.

Dynamic changes in the secondary structure of ECE-1 and XCE account for their different substrate specificities

Background

X-converting enzyme (XCE) involved in nervous control of respiration, is a member of the M13 family of zinc peptidases, for which no natural substrate has been identified yet. In contrast, it’s well characterized homologue endothelin-converting enzyme-1 (ECE-1) showed broad substrate specificity and acts as endopeptidase as well as dipeptidase. To explore the structural differences between XCE and ECE-1, homology model of XCE was built using the complex structure of ECE-1 with phosphoramidon (pdb-id: 3DWB) as template. Phosphoramidon was docked into the binding site of XCE whereas phosphate oxygen of the inhibitor was used as water molecule to design the apo forms of both enzymes. Molecular dynamics simulation of both enzymes was performed to analyze the dynamic nature of their active site residues in the absence and presence of the inhibitor.

Results

Homology model of XCE explained the role of non-conserved residues of its S2’ subsite. Molecular dynamics (MD) simulations identified the flexible transitions of F149/I150, N566/N571, W714/W719, and R145/R723 residues of ECE-1/XCE for the strong binding of the inhibitor. Secondary structure calculations using DSSP method reveals the folding of R145/R723 residue of ECE-1/XCE into β-sheet structure while unfolding of the S2’ subsite residues in aECE-1 and sustained compact folding of that of aXCE. The results evaluated are in good agreement with available experimental data, thus providing detailed molecular models which can explain the structural and specificities differences between both zinc peptidases.

Conclusions

Secondary structure changes of both enzymes during the simulation time revealed the importance of β-sheet structure of R145/R723 for its binding with the terminal carboxylate group of the inhibitor. Unfolding of the α-helix comprising the S2’ subsite residues in aECE-1 correlate well with its endopeptidase activity while their compact folding in aXCE may account for the inactivity of the enzyme towards large C-terminal containing substrates.

The Alzheimer's amyloid-degrading peptidase, neprilysin: can we control it?

The amyloid cascade hypothesis of Alzheimer's disease (AD) postulates that accumulation in the brain of amyloid β-peptide (Aβ) is the primary trigger for neuronal loss specific to this pathology. In healthy brain, Aβ levels are regulated by a dynamic equilibrium between Aβ release from the amyloid precursor protein (APP) and its removal by perivascular drainage or by amyloid-degrading enzymes (ADEs). During the last decade, the ADE family was fast growing, and currently it embraces more than 20 members. There are solid data supporting involvement of each of them in Aβ clearance but a zinc metallopeptidase neprilysin (NEP) is considered as a major ADE. NEP plays an important role in brain function due to its role in terminating neuropeptide signalling and its decrease during ageing or after such pathologies as hypoxia or ischemia contribute significantly to the development of AD pathology. The recently discovered mechanism of epigenetic regulation of NEP by the APP intracellular domain (AICD) opens new avenues for its therapeutic manipulation and raises hope for developing preventive strategies in AD. However, consideration needs to be given to the diverse physiological roles of NEP. This paper critically evaluates general biochemical and physiological functions of NEP and their therapeutic relevance.

Aβ-degrading enzymes: potential for treatment of Alzheimer disease

There is increasing evidence that deficient clearance of β-amyloid (Aβ) contributes to its accumulation in late-onset Alzheimer disease (AD). Several Aβ-degrading enzymes, including neprilysin (NEP), insulin-degrading enzyme, and endothelin-converting enzyme reduce Aβ levels and protect against cognitive impairment in mouse models of AD. The activity of several Aβ-degrading enzymes rises with age and increases still further in AD, perhaps as a physiological response to minimize the buildup of Aβ. The age- and disease-related changes in expression of more recently recognized Aβ-degrading enzymes (e.g. NEP-2 and cathepsin B) remain to be investigated, and there is strong evidence that reduced NEP activity contributes to the development of cerebral amyloid angiopathy. Regardless of the role of Aβ-degrading enzymes in the development of AD, experimental data indicate that increasing the activity of these enzymes (NEP in particular) has therapeutic potential in AD, although targeting their delivery to the brain remains a major challenge. The most promising current approaches include the peripheral administration of agents that enhance the activity of Aβ-degrading enzymes and the direct intracerebral delivery of NEP by convection-enhanced delivery. In the longer term, genetic approaches to increasing the intracerebral expression of NEP or other Aβ-degrading enzymes may offer advantages.

Are amyloid-degrading enzymes viable therapeutic targets in Alzheimer's disease?

: The amyloid cascade hypothesis of Alzheimer's disease envisages that the initial elevation of amyloid β-peptide (Aβ) levels, especially of Aβ(1-42) , is the primary trigger for the neuronal cell death specific to onset of Alzheimer's disease. There is now substantial evidence that brain amyloid levels are manipulable because of a dynamic equilibrium between their synthesis from the amyloid precursor protein and their removal by amyloid-degrading enzymes (ADEs) providing a potential therapeutic strategy. Since the initial reports over a decade ago that two zinc metallopeptidases, insulin-degrading enzyme and neprilysin (NEP), contributed to amyloid degradation in the brain, there is now an embarras de richesses in relation to this category of enzymes, which currently number almost 20. These now include serine and cysteine proteinases, as well as numerous zinc peptidases. The experimental validation for each of these enzymes, and which to target, varies enormously but up-regulation of several of them individually in mouse models of Alzheimer's disease has proved effective in amyloid and plaque clearance, as well as cognitive enhancement. The relative status of each of these enzymes will be critically evaluated. NEP and its homologues, as well as insulin-degrading enzyme, remain as principal ADEs and recently discovered mechanisms of epigenetic regulation of NEP expression potentially open new avenues in manipulation of AD-related genes, including ADEs.

Crystal structure of Mycobacterium tuberculosis zinc-dependent metalloprotease-1 (Zmp1), a metalloprotease involved in pathogenicity

Mycobacterium tuberculosis, the causative agent of tuberculosis, parasitizes host macrophages. The resistance of the tubercle bacilli to the macrophage hostile environment relates to their ability to impair phagosome maturation and its fusion with the lysosome, thus preventing the formation of the phago-lysosome and eventually arresting the process of phagocytosis. The M. tuberculosis zinc-dependent metalloprotease Zmp1 has been proposed to play a key role in the process of phagosome maturation inhibition and emerged as an important player in pathogenesis. Here, we report the crystal structure of wild-type Zmp1 at 2.6 Å resolution in complex with the generic zinc metalloprotease inhibitor phosphoramidon, which we demonstrated to inhibit the enzyme potently. Our data represent the first structural characterization of a bacterial member of the zinc-dependent M13 endopeptidase family and revealed a significant degree of conservation with eukaryotic enzymes. However, structural comparison of the Zmp1-phosphoramidon complex with homologous human proteins neprilysin and endothelin-converting enzyme-1 revealed unique features of the Zmp1 active site to be exploited for the rational design of specific inhibitors that may prove useful as a pharmacological tool for better understanding Zmp1 biological function.

Neprilysin-2 is an important β-amyloid degrading enzyme

Proteases that degrade the amyloid-β peptide (Aβ) are important in protecting against Alzheimer's disease (AD), and understanding these proteases is critical to understanding AD pathology. Endopeptidases sensitive to inhibition by thiorphan and phosphoramidon are especially important, because these inhibitors induce dramatic Aβ accumulation (∼30- to 50-fold) and pathological deposition in rodents. The Aβ-degrading enzyme neprilysin (NEP) is the best known target of these inhibitors. However, genetic ablation of NEP results in only modest increases (∼1.5- to 2-fold) in Aβ, indicating that other thiorphan/phosphoramidon-sensitive endopeptidases are at work. Of particular interest is the NEP homolog neprilysin 2 (NEP2), which is thiorphan/phosphoramidon-sensitive and degrades Aβ. We investigated the role of NEP2 in Aβ degradation in vivo through the use of gene knockout and transgenic mice. Mice deficient for the NEP2 gene showed significant elevations in total Aβ species in the hippocampus and brainstem/diencephalon (∼1.5-fold). Increases in Aβ accumulation were more dramatic in NEP2 knockout mice crossbred with APP transgenic mice. In NEP/NEP2 double-knockout mice, Aβ levels were marginally increased (∼1.5- to 2-fold), compared with NEP(-/-)/NEP2(+/+) controls. Treatment of these double-knockout mice with phosphoramidon resulted in elevations of Aβ, suggesting that yet other NEP-like Aβ-degrading endopeptidases are contributing to Aβ catabolism.

Identification of an endothelin-converting enzyme-2-specific fluorigenic substrate and development of an in vitro and ex vivo enzymatic assay

Endothelin-converting enzyme-2 (ECE-2) is a membrane-bound zinc-dependent metalloprotease that shares a high degree of sequence homology with ECE-1, but displays an acidic pH optimum characteristic of maturing enzymes acting late in the secretory pathway. Although ECE-2, like ECE-1, can cleave the big endothelin intermediate to produce the vasoconstrictive endothelin peptide, its true physiological function remains to be elucidated, a task that is hampered by the lack of specific tools to study and discriminate ECE-2 from ECE-1, i.e. specific substrates and/or specific inhibitors. To fill this gap, we searched for novel ECE-specific peptide substrates. To this end, peptides derived from the big endothelin intermediate were tested using ECE-1 and ECE-2, leading to the identification of an ECE-1-specific substrate. Moreover, screening of our proprietary fluorigenic peptide Fluofast® libraries using ECE-1 and ECE-2 allowed the identification of Ac-SKG-Pya-F-W-Nop-GGK-NH(2) (PL405), as a specific and high affinity ECE-2 substrate. Indeed, ECE-2 cleaved PL405 at the Pya-F amide bond with a specificity constant (k(cat)/K(m)) of 8.1 ± 0.9 × 10(3) M(-1) s(-1). Using this novel substrate, we also characterized the first potent (K(i) = 7.7 ± 0.3 nM) and relatively selective ECE-2 inhibitor and developed a quantitative fluorigenic ECE-2 assay. The assay was used to study the ex vivo ECE-2 activity in wild type and ECE-2 knock-out tissues and was found to truly reflect ECE-2 expression patterns. The PL405 assay is thus the first tool to study ECE-2 inhibition using high throughput screening or for ex vivo ECE-2 quantification.

Neprilysin 4, a novel endopeptidase from Drosophila melanogaster, displays distinct substrate specificities and exceptional solubility states

Proteins belonging to the family of neprilysins are typically membrane bound M13 endopeptidases responsible for the inactivation and/or activation of peptide signaling events on cell surfaces. Mammalian neprilysins are known to be involved in the metabolism of various regulatory peptides especially in the nervous, immune, cardiovascular and inflammatory systems. Although there is still much to learn about their participation in various diseases, they are potential therapeutic targets. Here we report on the identification and first characterization of neprilysin 4 (NEP4) from Drosophila melanogaster. Reporter lines as well as in situ hybridization combined with immunolocalization demonstrated NEP4 expression during embryogenesis in pericardial cells, muscle founder cells, glia cells and male gonads. Western blot analysis confirmed the prediction of one membrane bound and one soluble isoform, a finding quite unusual among neprilysins with presumably strong physiological relevance. At least one NEP4 isoform was found in every developmental stage indicating protein activities required throughout the whole life cycle of Drosophila. Heterologously expressed NEP4 exhibited substrate preferences comparable to human neprilysin 2 with distinct cleavage of substance P and angiotensin I.

Endothelin receptor antagonists as antihypertensives: the next frontier

The endothelin system is a pivotal player along the continuum of cardiovascular disease. There is convincing evidence that the system not only exerts a potent pressor effect but also promotes end-organ damage independent from blood pressure changes. The role of endothelin receptor antagonists (ERAs) in the treatment of hypertension is rapidly evolving. Recent studies demonstrate a formidable antihypertensive effect. Utility of ERAs is likely to be greatest in patients with resistant hypertension. Beyond blood pressure lowering, ERAs exert several properties that may confer additional protection, including effects on endothelial function, atherosclerosis, arterial stiffening, renal function and proteinuria. The full potential of this class will only be realized when the results of ongoing and future studies in hypertension, heart failure and other forms of cardiovascular disease are completed.

Endothelin-converting enzyme inhibition in the rat model of acute heart failure: heart function and neurohormonal activation

Endothelin-1 (ET-1) has been implicated in many cardiovascular diseases, including acute heart failure (AHF) due to myocardial ischemia. Previously we described the oral endothelin-converting enzyme (ECE) inhibitor, PP36, and in this study, we investigated its cardioprotective effect in more detail, and examined the role of PP36 in the neurohormonal activation in rats that had been subjected to acute myocardial ischemia due to the microsphere embolization of coronary microcirculation. PP36 treatment (3.5 x 10(-5) M/kg/day) led to a significant fourfold decrease in hypertensive response when big-ET-1 was administered to healthy, conscious rats. ECE inhibition did not affect mortality during the first 48 hours after ischemia initiation. Systemic hemodynamic, heart function, and neurohormonal activation were analyzed in the healthy control group, the AHF group, and the AHF+PP36 group two days after AHF induction. In conscious rats in the AHF+PP36 group, mean arterial pressure (MAP) was restored and became similar to that of the MAP of the control group. In anesthetized rats, in the AHF+PP36 group, MAP was not restored and was 22% lower than the MAP of the control group. Myocardial contractility was partially restored and cardiac relaxation significantly improved after PP36 application. Further analysis of cardiac output and peripheral resistance in anesthetized rats revealed no differences between the AHF group and the AHF+PP36 group. There were no differences in plasma ET-1 concentration, serum angiotensin converting enzyme activity, and in the adrenal glands' catecholamine content between the AHF group and the AHF+PP36 group. However, rats in the AHF+PP36 group demonstrated a 60% decrease in cardiac endothelial nitric oxide synthase (eNOS) protein expression, and a 56% reduction of myocardial norepinephrine release, when compared with the AHF group's animals. These results suggest that PP36 can preserve heart function during the recovery from acute ischemic injury, and may modulate the cardiac norepinephrine release and eNOS protein level.

Identification of ARIA regulating endothelial apoptosis and angiogenesis by modulating proteasomal degradation of cIAP-1 and cIAP-2

Endothelial apoptosis is a pivotal process for angiogenesis during embryogenesis as well as postnatal life. By using a retrovirus-mediated signal sequence trap method, we identified a previously undescribed gene, termed ARIA (apoptosis regulator through modulating IAP expression), which regulates endothelial apoptosis and angiogenesis. ARIA was expressed in blood vessels during mouse embryogenesis, as well as in endothelial cells both in vitro and in vivo. ARIA is a unique protein with no homology to previously reported conserved domain structures. Knockdown of ARIA in HUVECs by using small interfering RNA significantly reduced endothelial apoptosis without affecting either cell migration or proliferation. ARIA knockdown significantly increased inhibitor of apoptosis (cIAP)-1 and cIAP-2 protein expression, although their mRNA expression was not changed. Simultaneous knockdown of cIAP-1 and cIAP-2 abolished the antiapoptotic effect of ARIA knockdown. Using yeast 2-hybrid screening, we identified the interaction of ARIA with 20S proteasome subunit alpha-7. Thereafter, we found that cIAP-1 and cIAP-2 were degraded by proteasomes in endothelial cells under normal condition. Overexpression of ARIA significantly reduced cIAP-1 expression, and this reduction was abolished by proteasomal inhibition in BAECs. Also, knockdown of ARIA demonstrated an effect similar to proteasomal inhibition with respect to not only expression but also subcellular localization of cIAP-1 and cIAP-2. In vivo angiogenesis studied by Matrigel-plug assay, mouse ischemic retinopathy model, and tumor xenograft model was significantly enhanced by ARIA knockdown. Together, our data indicate that ARIA is a unique factor regulating endothelial apoptosis, as well as angiogenesis, presumably through modulating proteasomal degradation of cIAP-1 and cIAP-2 in endothelial cells.

[Anti-CD10 fetomaternal alloimmunisation]

Fetomaternal alloimmunization with antenatal glomerulopathies (FMAIG) is a recently described alloimmune disorder, which results from the production of maternal antibodies that cross the placenta, bind to fetal glomerular podocytes, and mediate renal disease. The pathogenic antibodies are directed against CD10/neutral endopeptidase (NEP). The infant's mother is NEP-deficient and thus she becomes immunized during the first pregnancy against CD10/NEP expressed by placental cells. Because future pregnancies in CD10/NEP-immunized mothers are at high risk for the fetus, detection of anti-NEP antibodies in pregnant mothers and antigen-driven therapies including induction of tolerance, are urgently needed. This ideally requires identification of the pathogenic epitopes born by the antigen and specifically recognized by B- and T-cells. We have recently characterized such epitopes that will be used in diagnostic tests (ELISA) and for new therapeutic approaches based on peptide-specific immune intervention. For this purpose, we have developed an experimental model by crossing NEP/CD10-deficient female mice to wild-type males. The females develop an alloimmune reaction against NEP, which is a prerequisite for tolerance induction experiments. Although NEP/CD10 does not seem to be involved in common idiopathic forms of membranous nephropathy in the adult, alloimmune antibodies may be implicated in de novo membranous nephropathy that develop in the kidney graft and after alloimmune bone marrow transplantation.

Endothelin: 20 years from discovery to therapy

Since its identification as an endothelial cell-derived vasoconstrictor peptide in 1988, endothelin-1, the predominant member of the endothelin peptide family, has received considerable interest in basic medical science and in clinical medicine, which is reflected by more than 20 000 scientific publications on endothelin research in the past 20 years. The story of endothelin is unique as the gene sequences of endothelin receptors and the first receptor antagonists became available within only 4 years of the identification of the peptide sequence. The first clinical study in patients with congestive heart failure was published only 3 years thereafter. Yet, despite convincing experimental evidence of a pathogenetic role for endothelin in development, cell function, and disease, many initial clinical studies on endothelin antagonism were negative. In many of these studies, study designs or patient selection were inadequate. Today, for diseases such as pulmonary hypertension, endothelin antagonist treatment has become reality in clinical medicine, and ongoing clinical studies are evaluating additional indications, such as renal disease and cancer. Twenty years after the discovery of endothelin, its inhibitors have finally arrived in the clinical arena and are now providing us with new options to treat disease and prolong the lives of patients. Possible future indications include resistant arterial hypertension, proteinuric renal disease, cancer, and connective tissue diseases.

Structure of human endothelin-converting enzyme I complexed with phosphoramidon

Endothelin-converting enzyme I (ECE-1) is a mammalian type II integral membrane zinc-containing endopeptidase. ECE-1 catalyzes the final step in the biosynthesis of endothelins in a rate-limiting fashion, through post-translational conversion of the biologically inactive big endothelins. Endothelin-1 overproduction has been implicated in a heterogeneous list of diseases including systemic and pulmonary hypertension, stroke and asthma, cardiac and renal failure. Therefore, ECE-1 is a prime therapeutic target for the regulation of endothelin-1 production in vivo and there is considerable interest in selective inhibitors of this enzyme. Here, we present the crystal structure of the extracellular domain (residues 90-770) of human ECE-1 (C428S) with the generic metalloprotease inhibitor phosphoramidon determined at 2.38 A resolution. The structure is closely related to that of human NEP, providing essential information for a detailed understanding of ligand-binding, specificity determinants as well as selectivity criteria. Selective inhibitors of ECE-1s should have beneficial effects for the treatment of diseases in which an overproduction of ETs plays a pathogenic role.

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.

Fetomaternal alloimmunization with antenatal glomerulopathies

Fetomaternal alloimmunization with antenatal glomerulopathies (FMAIG) is a recently described alloimmune disorder, which results from maternal antibodies that cross the placenta, bind to fetal glomerular podocytes, and mediate renal disease. The pathogenic antibodies are directed against neutral endopeptidase (NEP). The infant's mother is NEP-deficient and thus she becomes immunized during pregnancy against NEP expressed by placental cells. Because future pregnancies in NEP-immunized mothers are at high risk for the fetus, detection of anti-NEP antibodies in pregnant mothers and antigen-driven therapies including induction of mucosal tolerance, are urgently needed. This ideally requires identification of the pathogenic epitopes born by the antigen. We have recently characterized two linear B cell epitopes on the NEP that are specifically recognized by the mother's antibodies. The identification of these B cell epitopes is useful for diagnostic tests and may lead to future development of new therapeutic approaches based on peptide-specific immune intervention.

Target antigens and nephritogenic antibodies in membranous nephropathy: of rats and men

Membranous nephropathy, a disease characterized by an accumulation of immune deposits on the outer aspect of the glomerular basement membrane, is the most common cause of idiopathic nephrotic syndrome in white adults. In the rat model of Heymann nephritis, the target antigen of antibodies is megalin, a multiligand receptor expressed at the podocyte cell surface. This review summarizes key findings provided by this experimental model and by our discovery of neutral endopeptidase being the alloantigen involved in neonatal cases of membranous nephropathy. We discuss the role of alloimmunization as a new mechanism of renal disease and the approach that we use to identify new podocyte antigens. We also summarize current knowledge on the mechanism of proteinuria, with special emphasis on the role of complement. In conclusion, substantial progresses have been made in understanding molecular mechanisms of membranous nephropathy, which should lead to novel therapeutic approaches.

Expression of NEP2, a soluble neprilysin-like endopeptidase, during embryogenesis in Drosophila melanogaster

Members of the neprilysin family of neutral endopeptidases (M13) are typically membrane-bound enzymes known to be involved in the extra-cellular metabolism of signalling peptides and have important roles during mammalian embryogenesis. In this study we show that membranes prepared from embryos of Drosophila melanogaster possess neprilysin-like activity that is inhibited by phosphoramidon and thiorphan, both inhibitors of mammalian neprilysin. Unexpectedly, we also found strong neprilysin-like neutral endopeptidase activity in a soluble embryo fraction, which we identify as NEP2 by Western blot and immunoprecipitation experiments using NEP2 specific antibodies. NEP2 is a soluble secreted member of the neprilysin family that has been shown previously to be expressed in larval and adult Malpighian tubules and in the testes of adult males. In situ hybridization studies reveal expression at stage 10-11 in a pattern similar to that previously described for stellate cell progenitors of the caudal visceral mesoderm. In later stages of embryogenesis, some of these cells appear to migrate into the growing Malpighian tubule. Recombinant NEP2 protein is N-glycosylated and displays optimum endopeptidase activity at neutral pH, consistent with a role as an extracellular peptidase. The recombinant enzyme hydrolyses Drosophila tachykinin peptides (DTK) at peptide bonds N-terminal to hydrophobic residues. DTK2, like Locusta tachykinin-1, was cleaved at the penultimate peptide bond (Gly(7)-Leu(8)), whereas the other Drosophila peptides were cleaved centrally at Xxx-Phe bonds. However, the rates of hydrolysis of the latter substrates were much slower than the hydrolysis rates of DTK2 and Locusta tachykinin-1, suggesting that the interaction of the bulky side-chain of phenylalanine at the S'(1) sub-site is less favorable for peptide bond hydrolysis. The secretion of NEP2 from tissues during embryogenesis suggests a possible developmental role for this endopeptidase in peptide signalling in D. melanogaster.

Regional Hemodynamic Effects of Neutral Endopeptidase Inhibition and Angiotensin (AT1) Receptor Antagonism Alone or in Combination in Conscious Spontaneously Hypertensive Rats

We tested the hypothesis that angiotensin (AT(1)) receptor antagonism (with losartan) would enhance the cardiovascular actions of neutral endopeptidase (NEP) inhibition [with candoxatrilat or (2S)-2-{[1-({[(1S)-1-carboxy-2-(5-phenyl-1,3-oxazol-2-yl)ethyl]amino}carbonyl)cyclopentyl]methyl}-4-methoxybutanoic acid (UK-489,329)] in conscious spontaneously hypertensive rats (SHR). Four-day continuous intravenous infusion of candoxatrilat (1.9 microg kg(-1) min(-1)) or UK-489,329 (0.15 microg kg(-1) min(-1)) had no significant cardiovascular effects, whereas candoxatrilat (6.4 microg kg(-1) min(-1)) had a modest antihypertensive effect (-10.9 mm Hg on day 4) but no significant sustained effects on regional hemodynamics. Losartan caused a fall in blood pressure (maximum -29.2 mm Hg on day 4) that was associated with renal, mesenteric, and, to a lesser extent, hindquarters vasodilatation. The combination of losartan with either dose of candoxatrilat had no greater antihypertensive or vasodilator effects than losartan alone, with the exception of the increase in renal vascular conductance, which was greater with the combination of the drugs than with either drug alone (significant only in the lower dose study). Losartan combined with UK-489,329 showed a greater antihypertensive effect than losartan alone (-14.6 mm Hg greater on day 4), although the effects of the combination were not significantly greater than the sum of the effects of both agents administered separately. However, losartan combined with UK-489,329 caused increases in renal and hindquarters vascular conductance that were significantly greater with the combination than with either agent given alone. Thus, in conscious SHR, the renin-angiotensin system may act to oppose a vasodilator action of NEP inhibition, particularly in the renal vascular bed.

Mechanisms of Disease: alloimmunization in renal diseases

Graft rejection has long been considered the paradigm of renal diseases induced by alloimmunization, particularly alloimmunization directed against HLA antigens. Accumulating evidence indicates that non-HLA immunity also has an important role in clinical transplantation. Targets of alloimmunization include antigens of tubular basement membrane, tubular epithelial cells and endothelial cells. They can be polymorphic allovariants (as shown in the rat) or 'hidden' antigens exposed when the graft is damaged. Alloimmunization can also occur when a person genetically deficient in a renal protein (e.g. the alpha5 (IV) collagen chain in X-linked Alport's syndrome or nephrin in Finnish-type nephrotic syndrome) is transplanted to treat end-stage renal failure. The non-mutated protein in the donor kidney is recognized as a foreign antigen, and the resulting alloimmune response can damage the graft. We have demonstrated that alloimmunity can also affect the native kidney. We have characterized a novel fetomaternal disease in which a genetic defect in the MME gene encoding neutral endopeptidase (NEP) in the mother leads to the development of membranous nephropathy in her fetus (maternal anti-NEP antibodies bind to NEP on fetal podocytes). Our findings raise the possibility that mutations or genetic polyporphisms in MME or other genes expressed by the podocyte are involved in alloimmune-mediated development of membranous nephropathy after kidney or bone marrow transplantation.

Localization and ontogeny of damage-induced neuronal endopeptidase mRNA-expressing neurons in the rat nervous system

Neuropeptides are crucial mediators in nervous and endocrine systems. Processing and degradation, the major regulatory mechanisms, of enzymes are essential for the control of these peptidergic intercellular signaling systems. Damage-induced neuronal endopeptidase (or endothelin converting enzyme-like1), a member of the neprilysin family, has recently been identified as an M13 zinc metalloprotease. Damage-induced neuronal endopeptidase mRNA expression is strikingly restricted to neurons, and is remarkably induced in response to various types of neuronal injuries, although its function and substrate remain unknown. To clarify the role of damage-induced neuronal endopeptidase, we examined the localization and ontogeny of damage-induced neuronal endopeptidase mRNA expression in the rat nervous system using in situ hybridization. Damage-induced neuronal endopeptidase mRNA was detected at embryonic day 12, and its expression restricted to the ventral region of the neural tube. Subsequently, expression was also apparent in primordia of the striatum, hypothalamus, and cranial motor nuclei during neural development. This specific distribution was relatively maintained in the adult brain, although expression levels became weaker. Expression of damage-induced neuronal endopeptidase was absent in the cerebral cortex, hippocampus, and cerebellum. In addition to prominent expression in CNS, intestinal and sensory ganglia and retina demonstrated transient intense damage-induced neuronal endopeptidase mRNA expression during the embryonic period that then declined, and disappeared after birth. The results indicated that damage-induced neuronal endopeptidase might play an important role in embryonic neural development, in particular in peripheral ganglia derived from the neural crest, and in some neurons originating from the basal plate such as the hypothalamus and cranial motor neurons.

Endothelin-3-converting enzyme activity of the KEL1 and KEL6 phenotypes of the Kell blood group system

The Kell blood group protein is a metalloendopeptidase that preferentially cleaves a Trp(21)-Ile(22) bond of big endothelin-3 producing bioactive endothelin-3. Kell is a polymorphic protein, and 25 different phenotypes, because of point mutations resulting in single amino acid substitutions, have been described. It was recently reported that a recombinant form of KEL1 (K, K1) phenotype, expressed in K562 and HEK293 cells, had no endothelin-3-converting activity, in contrast to the common KEL2 (k, K2) phenotype. We demonstrate that KEL1 red blood cells and also a soluble recombinant form of KEL1 protein (s-Kell KEL1) have similar enzymatic activity as the common Kell phenotype. In addition we show that KEL6 red blood cells, which are more prevalent in persons of African heritage than in Caucasians also have endothelin-3-converting enzyme activity and that the recombinant soluble form of KEL6 protein (s-Kell KEL6) has similar K(m) values as the wild-type.

Brain neprilysin activity and susceptibility to transgene-induced Alzheimer amyloidosis

Neprilysin (NEP) is a zinc metalloproteinase that degrades enkephalins, endothelins, and the Alzheimer's disease amyloid beta (Abeta) peptides. NEP-deficient mice possess increased levels of brain Abeta(1-40) and Abeta(1-42). The objective of this study was to determine whether tissue NEP specific activity differs according to age and/or across mouse strains, especially those strains predisposed toward formation of Abeta-amyloid plaques following overexpression of the human Alzheimer amyloid precursor protein (APP). The C57Bl/6J mouse strain appears to be relatively susceptible to cerebral amyloidosis, whereas the Swiss Webster (SW) strain appears more resistant. We investigated whether NEP specific activity in brain and kidney homogenates from SW and C57 mice of 6, 40, and 80 weeks old varied according to mouse strain, age, and gender. Among the variables tested, NEP specific activity varied most dramatically across mouse strain, with the kidney and brain of SW mice displaying the highest activities. Aging was associated with a reduction in brain NEP specific activity in both strains. Gender-specific differences were identified in kidney but not in brain. We conclude that aging- and strain-dependent differences in NEP specific activity may play a role in the differential susceptibility of some mouse strains for developing cerebral amyloidosis following human APP overexpression.

The ultimate tryptophan residue of neprilysin 2 is not involved in protein maturation and enzymatic activity

Modeling the three-dimensional structure of neprilysin 2 (NEP2) using the crystal structure of neprilysin as template revealed that their active sites share many common features, though slight differences therein cannot completely account for their specific pharmacological profiles. Recent evidence also suggest that residues outside the active site can play crucial functions in the maturation and enzymatic activity of these metalloproteases. To further explore the functions of amino acids in the acquisition and maintenance of the NEP2 structure, site-directed mutagenesis of conserved residues involved in the enzymatic activity of ECE-1 was performed. In particular, the ultimate tryptophan residue of ECE-1 was recently shown to be important in its activation. This residue was thus mutated in the secreted isoform of NEP2, as were proline residues located in its vicinity. Expression of these mutants in AtT20 cells and study of their secretion and catalytic activities shows that while the ultimate tryptophan residue of the NEP2 sequence is not essential to its proper and activity, structural changes in its vicinity can have a severe impact on the maturation processes involved in the activation of NEP2.

Drosophila melanogaster NEP2 is a new soluble member of the neprilysin family of endopeptidases with implications for reproduction and renal function

The mammalian neprilysin (NEP) family members are typically type II membrane endopeptidases responsible for the activation/inactivation of neuropeptides and peptide hormones. Differences in substrate specificity and subcellular localization of the seven mammalian NEPs contribute to their functional diversity. The sequencing of the Drosophila melanogaster genome has revealed a large expansion of this gene family, resulting in over 20 fly NEP-like genes, suggesting even greater diversity in structure and function than seen in mammals. We now report that one of these genes (Nep2) codes for a secreted endopeptidase with a highly restricted pattern of expression. D. melanogaster NEP2 is expressed in the specialized stellate cells of the renal tubules and in the cyst cells that surround the elongating spermatid bundles in adult testis, suggesting roles for the peptidase in renal function and in spermatogenesis. D. melanogaster NEP2 was found in vesicle-like structures in the syncytial cytoplasm of the spermatid bundles, suggesting that the protein was acquired by endocytosis of protein secreted from the cyst cells. Expression of NEP2 cDNA in D. melanogaster S2 cells confirmed that the peptidase is secreted and is only weakly inhibited by thiorphan, a potent inhibitor of human NEP. D. melanogaster NEP2 also differs from human NEP in the manner in which the peptidase cleaves the tachykinin, GPSGFYGVR-amide. Molecular modelling suggests that there are important structural differences between D. melanogaster NEP2 and human NEP in the S1' and S2' ligand-binding subsites, which might explain the observed differences in inhibitor and substrate specificities. A soluble isoform of a mouse NEP-like peptidase is strongly expressed in spermatids, suggesting an evolutionarily conserved role for a soluble endopeptidase in spermatogenesis.

Molecular pathomechanisms of membranous nephropathy: from Heymann nephritis to alloimmunization

Membranous nephropathy (MN), the most common cause of idiopathic nephrotic syndrome in white adults, is characterized by an accumulation of immune deposits on the outer aspect of the glomerular basement membrane. In Heymann nephritis, the rat experimental model for MN, megalin--the target antigen of the nephritogenic antibodies--is expressed on the surface of podocytes, where immune complexes are formed, leading to complement activation and nephrotic-range proteinuria. However, megalin cannot be held responsible for human MN because it has not been found in human podocytes or detected in subepithelial immune deposits in patients with MN. Several potential antigens have been identified in so-called secondary forms of MN, but there is no real proof that these antigens are pathogenic. In a subgroup of infants with antenatal MN, neutral endopeptidase (NEP) has been identified as the first protein target on human podocytes of nephritogenic antibodies. The infants' mothers became immunized during pregnancy against NEP expressed on syncytiotrophoblastic cells because they were NEP deficient as a result of truncating mutations in the MME gene. Severity of neonatal renal disease was determined by the mothers' IgG response that led to the formation of the membrane attack complex of complement in the subepithelial deposits. Alloimmunization against NEP is a novel pathomechanism of MN that might also account for some cases of MN after renal or bone marrow transplantation. Other types of alloimmunization should be investigated in MN but also in other renal and nonrenal diseases, particularly those that affect the pediatric age.

The Kell protein of the common K2 phenotype is a catalytically active metalloprotease, whereas the rare Kell K1 antigen is inactive. Identification of novel substrates for the Kell protein

The Kell blood group is a highly polymorphic system containing over 20 different antigens borne by the protein Kell, a 93-kDa type II glycoprotein that displays high sequence homology with members of the M13 family of zinc-dependent metalloproteases whose prototypical member is neprilysin. Kell K1 is an antigen expressed in 9% of the Caucasian population, characterized by a point mutation (T193M) of the Kell K2 antigen, and located within a putative N-glycosylation consensus sequence. Recently, a recombinant, non-physiological, soluble form of Kell was shown to cleave Big ET-3 to produce the mature vasoconstrictive peptide. To better characterize the enzymatic activity of the Kell protein and the possible differences introduced by antigenic point mutations affecting post-translational processing, the membrane-bound forms of the Kell K1 and Kell K2 antigens were expressed either in K562 cells, an erythroid cell line, or in HEK293 cells, a non-erythroid system, and their pharmacological profiles and enzymatic specificities toward synthetic and natural peptides were evaluated. Results presented herein reveal that the two antigens possess considerable differences in their enzymatic activities, although not in their trafficking pattern. Indeed, although both antigens are expressed at the cell surface, Kell K1 protein is shown to be inactive, whereas the Kell K2 antigen binds neprilysin inhibitory compounds such as phosphoramidon and thiorphan with high affinity, cleaves the precursors of the endothelin peptides, and inactivates members of the tachykinin family with enzymatic properties resembling those of other members of the M13 family of metalloproteases to which it belongs.

Structure, evolutionary conservation, and functions of angiotensin- and endothelin-converting enzymes

Angiotensin-converting enzyme, a member of the M2 metalloprotease family, and endothelin-converting enzyme, a member of the M13 family, are key components in the regulation of blood pressure and electrolyte balance in mammals. From this point of view, they serve as important drug targets. Recently, the involvement of these enzymes in the development of Alzheimer's disease was discovered. The existence of homologs of these enzymes in invertebrates indicates that these enzyme systems are highly conserved during evolution. Most invertebrates lack a closed circulatory system, which excludes the need for blood pressure regulators. Therefore, these organisms represent excellent targets for gaining new insights and revealing additional physiological roles of these important enzymes. This chapter reviews the structural and functional aspects of ACE and ECE and will particularly focus on these enzyme homologues in invertebrates.

Endothelin-converting enzyme-like 1 (ECEL1) is present both in the plasma membrane and in the endoplasmic reticulum

Enzymes of the M13 family of zinc-containing endopeptidases are recognized as important regulators of neuropeptide and peptide hormone activity. Peptidases of this family are type II integral-membrane proteins characterized by short cytosolic domains and large extracellular domains containing the active site. The M13 family has, at present, seven members, including ECEL1 (endothelin-converting enzyme-like 1), one of the newest members. ECEL1 is expressed predominantly in the central nervous system. It has been proposed that the enzyme has a role in the nervous regulation of the respiratory system. No physiological substrate has been identified yet. To better understand the function(s) of this enzyme, we have expressed human ECEL1 in cultured cells and monitored its biosynthesis and subcellular localization. Immunoblot and cell-surface biotinylation analysis of transfected cells expressing ECEL1 showed that only a fraction of the protein travelled to the cell surface, while most of the enzyme was present in an intracellular compartment identified by confocal immunofluorescence microscopy and cell fractionation as the ER (endoplasmic reticulum). Pulse-chase experiments showed that ER-localized ECEL1 was stable, with a half-life of more than 3 h. Endogenous ECEL1 from mouse pituitary gland had a similar distribution between the cell surface and the ER. Finally, using domain-swapping experiments with neprilysin, another member of the M13 family, we showed that localization of ECEL1 to the ER requires both the transmembrane and cytoplasmic domains. It thus appears that ECEL1 may have functions both at the cell surface and in the ER.

Molecular isolation and characterization of a soluble isoform of activated leukocyte cell adhesion molecule that modulates endothelial cell function

Cell adhesion molecules regulate a variety of endothelial cell functions such as migration, response to inflammation, and angiogenesis. Recently, activated leukocyte cell adhesion molecule (ALCAM), a member of the Ig superfamily, has been detected in the primitive subsets of hematopoietic cells and endothelial cells during embryogenesis. ALCAM supports the development of hematopoietic cells as well as enhancing capillary tube formation in vitro. Here, we isolated a novel soluble isoform of ALCAM (sALCAM) that is produced via alternative splicing. sALCAM contains the single amino-terminal Ig-like domain of ALCAM and lacks a transmembrane domain. When expressed in cultured cells, sALCAM was properly secreted into the media. Both ALCAM and sALCAM are expressed in a variety of cultured human endothelial cells. Notably, their transcripts were differentially regulated in human microvascular endothelial cells (HMVEC) upon tumor necrosis factor-alpha stimulation. ALCAM significantly enhanced tube formation of endothelial-like yolk sac cells on Matrigel, whereas it inhibited their migration in vitro. sALCAM completely abolished these effects of ALCAM. Furthermore, sALCAM enhanced migration of mock-transfected endothelial-like yolk sac cells that do not express ALCAM, indicating that sALCAM has an independent effect on cell migration in addition to modulating ALCAM function. In addition, sALCAM significantly enhanced migration of HMVEC, whereas it inhibited tube formation of HMVEC on Matrigel. sALCAM demonstrated an ability to bind ALCAM and partially inhibited ALCAM-ALCAM homophilic interactions. Taken together, these data characterize a novel soluble isoform of ALCAM that may have ALCAM-dependent and ALCAM-independent functions, providing further insights regarding the role of this adhesion molecule in the regulation of endothelial cell function.

Role of truncating mutations in MME gene in fetomaternal alloimmunisation and antenatal glomerulopathies

BACKGROUND

Membranous glomerulonephritis is an immune-mediated disease. In a recent case of antenatal membranous glomerulonephritis, we identified neutral endopeptidase (NEP) as the podocyte target antigen of circulating antibodies produced by the mother who failed to express NEP on granulocytes. We aimed to investigate whether the disease could affect other families, to search for mutations in the metallomembrane endopeptidase (MME) gene for NEP, and to analyse the outcome of the antenatal renal insult.

METHODS

From three families with a case of neonatal membranous glomerulopathy, we detected mutations by direct sequencing of genomic PCR products. Single nucleotide polymorphism (SNP) analysis was undertaken with five SNPs located in the MME gene. IgG subclasses with anti-NEP activity were determined by western blotting.

FINDINGS

In five mothers, we identified two compound heterozygous or homozygous mutations in the MME gene. The first, a 1342C-->T nonsense mutation, was detected in one family. The second, 446delC, was detected in all three families; all chromosomes bearing this mutation had the same alleles for the five SNPs. Severity of neonatal renal disease was determined by the mothers' IgG response to fetal NEP antigens expressed on glomerular podocytes. The oldest affected individual, now aged 20 years, has developed severe chronic renal failure.

INTERPRETATION

Truncating mutations in the MME gene are the cause of alloimmunisation during pregnancy. Idiopathic renal failure in early adulthood might be caused by immune-mediated fetal nephron loss. We show that disease caused by fetomaternal alloimmunisation secondary to a genetic defect is not restricted to blood cells.

RELEVANCE TO CLINICAL PRACTICE

During pregnancy, the absence of the NEP protein induces an alloimmunisation process against NEP presented by fetal cells, including syncytiotrophoblasts. The fetal podocyte insult and ensuing nephron loss could lead to chronic renal failure in early adulthood. Alloimmunisation against NEP should be considered as a leading cause of membranous glomerulopathy early in life. Concentrations of circulating anti-NEP antibodies should be carefully monitored during subsequent pregnancies, and specific therapeutic approaches developed. This new disease might also account for idiopathic chronic renal failure detected during adolescence, in individuals who can be identified by searching for anti-NEP antibodies in their mother and by MME gene mutation analysis. NEP deficiency should also be considered in patients developing de-novo membranous glomerulopathy after renal transplantation.

A Three-dimensional Model of the Neprilysin 2 Active Site Based on the X-ray Structure of Neprilysin

Neprilysin 2 (NEP2), a recently identified member of the M13 subfamily of metalloproteases, shares the highest degree of homology with the prototypical member of the family neprilysin. Whereas the study of the in vitro enzymatic activity of NEP2 shows that it resembles that of NEP as it cleaves the same substrates often at the same amide bonds and binds the same inhibitory compounds albeit with different potencies, its physiological role remains elusive because of the lack of selective inhibitors. To aid in the design of these novel compounds and better understand the different inhibitory patterns of NEP and NEP2, the x-ray structure of NEP was used as a template to build a model of the NEP2 active site. The results of our modeling suggest that the overall structure of NEP2 closely resembles that of NEP. The model of the active site reveals a 97% sequence identity with that of NEP with differences located within the S'(2) subsite of NEP2 where Ser(133) and Leu(739) replace two glycine residues in NEP. To validate the proposed model, site-directed mutagenesis was performed on a series of residues of NEP2, mutants expressed in AtT20 cells, and their ability to bind various substrates and inhibitory compounds was tested. The results confirm the involvement of the conserved Arg(131) and Asn(567) in substrate binding and catalytic activity of NEP2 and further show that the modifications in its S'(2) pocket, particularly the presence therein of Leu(739), account for a number of differences in inhibitor binding between NEP and NEP2.