Hydrolysis of peptide hormones by endothelin-converting enzyme-1. A comparison with neprilysin

Endothelin and endothelin converting enzyme-1 in the fish gill:evolutionary and physiological perspectives

In euryhaline fishes like the killifish (Fundulus heteroclitus)that experience daily fluctuations in environmental salinity, endothelin 1(EDN1) may be an important regulator molecule necessary to maintain ion homeostasis. The purpose of this study was to determine if EDN1 and the endothelin converting enzyme (ECE1; the enzyme necessary for cleaving the precursor proendothelin-1 to EDN1) are present in the killifish, to determine if environmental salinity regulates their expression, and to examine the phylogenetic relationships among the EDNs and among the ECEs. We sequenced killifish gill cDNA for two EDN1 orthologues, EDN1A and EDN1B, and also sequenced a portion of ECE1 cDNA. EDN1A and ECE1 mRNA are expressed ubiquitously in the killifish while EDN1B mRNA has little expression in the killifish opercular epithelium or gill. Using in situ hybridization and immunohistochemistry, EDN1 was localized to large round cells adjacent to the mitochondrion-rich cells of the killifish gill, and to lamellar pillar cells. In the gill, EDN1A and EDN1B mRNA levels did not differ with acute (<24 h) or chronic (30 days) acclimation to seawater (SW); however, EDN1B levels increased threefold post SW to freshwater (FW) transfer,and ECE1 mRNA levels significantly increased twofold over this period. ECE1 mRNA levels also increased sixfold over 24 h post FW to SW transfer. Chronic exposure to SW or FW had little effect on ECE1mRNA levels. Based upon our cellular localization studies, we modeled EDN1 expression in the fish gill and conclude that it is positioned to act as a paracrine regulator of gill functions in euryhaline fishes. It also may function as an autocrine on pillar cells, where it is hypothesized to regulate local blood flow in the lamellae. From our phylogenetic analyses, ECE is predicted to have an ancient origin and may be a generalist endoprotease in non-vertebrate organisms, while EDNs are vertebrate-specific peptides and may be key characters in vertebrate evolution.

Endothelin-converting enzyme-1 regulates endosomal sorting of calcitonin receptor-like receptor and beta-arrestins

Although cell surface metalloendopeptidases degrade neuropeptides in the extracellular fluid to terminate signaling, the function of peptidases in endosomes is unclear. We report that isoforms of endothelin-converting enzyme-1 (ECE-1a–d) are present in early endosomes, where they degrade neuropeptides and regulate post-endocytic sorting of receptors. Calcitonin gene-related peptide (CGRP) co-internalizes with calcitonin receptor-like receptor (CLR), receptor activity-modifying protein 1 (RAMP1), β-arrestin2, and ECE-1 to early endosomes, where ECE-1 degrades CGRP. CGRP degradation promotes CLR/RAMP1 recycling and β-arrestin2 redistribution to the cytosol. ECE-1 inhibition or knockdown traps CLR/RAMP1 and β-arrestin2 in endosomes and inhibits CLR/RAMP1 recycling and resensitization, whereas ECE-1 overexpression has the opposite effect. ECE-1 does not regulate either the resensitization of receptors for peptides that are not ECE-1 substrates (e.g., angiotensin II), or the recycling of the bradykinin B₂ receptor, which transiently interacts with β-arrestins. We propose a mechanism by which endosomal ECE-1 degrades neuropeptides in endosomes to disrupt the peptide/receptor/β-arrestin complex, freeing internalized receptors from β-arrestins and promoting recycling and resensitization.

Characterisation of endothelin converting enzyme-1 shedding from endothelial cells

The aim of this study was to determine if endothelin converting enzyme-1 (ECE-1) like other members of this metalloprotease family undergoes ectodomain shedding. The release/shedding of catalytically active ECE-1 was measured by monitoring the fluorescence resulting from the cleavage of a specific quenched fluorescent substrate. Catalytically active ECE-1 was detected in the media of human umbilical vein endothelial cells, and was confirmed by mass spectrometry based assays. Specificity of cleavage was confirmed by using both narrow and broad specificity inhibitors. In conclusion we demonstrate and characterize for the first time, ECE-1 shedding from the surface of endothelial cells.

Endothelin-converting enzyme 1 degrades neuropeptides in endosomes to control receptor recycling

Neuropeptide signaling requires the presence of G protein-coupled receptors (GPCRs) at the cell surface. Activated GPCRs interact with beta-arrestins, which mediate receptor desensitization, endocytosis, and mitogenic signaling, and the peptide-receptor-arrestin complex is sequestered into endosomes. Although dissociation of beta-arrestins is required for receptor recycling and resensitization, the critical event that initiates this process is unknown. Here we report that the agonist availability in the endosomes, controlled by the membrane metalloendopeptidase endothelin-converting enzyme 1 (ECE-1), determines stability of the peptide-receptor-arrestin complex and regulates receptor recycling and resensitization. Substance P (SP) binding to the tachykinin neurokinin 1 receptor (NK1R) induced membrane translocation of beta-arrestins followed by trafficking of the SP-NK1R-beta-arrestin complex to early endosomes containing ECE-1a-d. ECE-1 degraded SP in acidified endosomes, disrupting the complex; beta-arrestins returned to the cytosol, and the NK1R, freed from beta-arrestins, recycled and resensitized. An ECE-1 inhibitor, by preventing NK1R recycling in endothelial cells, inhibited resensitization of SP-induced inflammation. This mechanism is a general one because ECE-1 similarly regulated NK3R resensitization. Thus, peptide availability in endosomes, here regulated by ECE-1, determines the stability of the peptide-receptor-arrestin complex. This mechanism regulates receptor recycling, which is necessary for sustained signaling, and it may also control beta-arrestin-dependent mitogenic signaling of endocytosed receptors. We propose that other endosomal enzymes and transporters may similarly control the availability of transmitters in endosomes to regulate trafficking and signaling of GPCRs. Antagonism of these endosomal processes represents a strategy for inhibiting sustained signaling of receptors, and defects may explain the tachyphylaxis of drugs that are receptor agonists.

Neuropeptides of the islets of Langerhans: a peptidomics study

Neuropeptides from the endocrine pancreas (the islets of Langerhans) play an important role in the regulation of blood glucose levels. Therefore, our aim is to identify the "peptidome" (the in vivo peptide profile at a certain time) of the pancreatic islets, which is beneficial for medical progress related to the treatment of diabetes. So far, there are few neuropeptides isolated and sequenced from the endocrine pancreas and mainly in situ hybridisation and immunocytochemical techniques have been used to demonstrate the occurrence of peptides in the pancreas. These techniques do not allow for unequivocal identification of peptides. In contrary, mass spectrometry identifies peptides unambiguously. We have analysed the peptidome of the islets using peptidomics, i.e. a combination of liquid chromatography, mass spectrometry and bioinformatics. We are able to identify the peptidome of islets extracts. We not only confirm the presence of peptides with a well-known effect on blood glucose levels, but also identify new peptides, which are unknown to affect blood glucose levels.

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.

Neuronal Control of Skin Function: The Skin as a Neuroimmunoendocrine Organ

This review focuses on the role of the peripheral nervous system in cutaneous biology and disease. During the last few years, a modern concept of an interactive network between cutaneous nerves, the neuroendocrine axis, and the immune system has been established. We learned that neurocutaneous interactions influence a variety of physiological and pathophysiological functions, including cell growth, immunity, inflammation, pruritus, and wound healing. This interaction is mediated by primary afferent as well as autonomic nerves, which release neuromediators and activate specific receptors on many target cells in the skin. A dense network of sensory nerves releases neuropeptides, thereby modulating inflammation, cell growth, and the immune responses in the skin. Neurotrophic factors, in addition to regulating nerve growth, participate in many properties of skin function. The skin expresses a variety of neurohormone receptors coupled to heterotrimeric G proteins that are tightly involved in skin homeostasis and inflammation. This neurohormone-receptor interaction is modulated by endopeptidases, which are able to terminate neuropeptide-induced inflammatory or immune responses. Neuronal proteinase-activated receptors or transient receptor potential ion channels are recently described receptors that may have been important in regulating neurogenic inflammation, pain, and pruritus. Together, a close multidirectional interaction between neuromediators, high-affinity receptors, and regulatory proteases is critically involved to maintain tissue integrity and regulate inflammatory responses in the skin. A deeper understanding of cutaneous neuroimmunoendocrinology may help to develop new strategies for the treatment of several skin diseases.

The influence of the endothelin-converting enzyme-1 gene polymorphism on the progression of autosomal dominant polycystic kidney disease

BACKGROUND; A significant phenotypical variability is observed in autosomal dominant polycystic kidney disease (ADPKD), the most common renal hereditary disease. Endothelin-1 (ET-1) has been suggested to be an important disease-promoting factor of the kidney. Endothelin-converting enzyme-1 (ECE-1) is the main protease responsible for ET-1 generation by cleavage of its functionally inactive precursor. We examined the influence of the ECE-1b C-338A polymorphism on the progression of ADPKD toward end-stage renal disease (ESRD). The A allele was suggested to be associated with higher plasma level of ET-1.

METHODS

200 ADPKD patients (107 males, 93 females) who had reached ESRD were analyzed. Patients were divided into three groups: (1) 47 patients (23 males, 24 females) with ESRD later than in 63 yr (slow progressors); (2) 71 patients (38 males, 33 females) with ESRD before 45 yr (rapid progressors); and (3) 82 patients (46 males, 36 females) with ESRD between 45-63 yr. Moreover, we analyzed 160 genetically unrelated healthy Czech subjects as the control group (82 males, 78 females, mean age 51.4 +/- 8.2 yr). DNA samples from collected blood were genotyped for ECE-1b C-338A polymorphism using described polymerase chain reaction (PCR) followed by restriction enzyme digestion. We compared the frequencies of different genotypes between the groups of slow and rapid progressors and the ages of ESRD with regard to different genotypes.

RESULTS

The ECE-1b C-338A genotype distribution showed no differences among the groups of slow progressors, rapid progressors, ADPKD group with ESRD between 45-63 yr and control group. Comparing the ages of ESRD of all patients, we did not find significant differences in the ages with regard to different genotypes: CC (51.5 +/- 10.1 yr), AC (51.6 +/- 11.4 yr), AA (48.2 +/- 5.9 yr). There was a tendency to lower age of ESRD in AA homozygotes in comparison with other genotypes (t-test, p = 0.12). We found no influence of gender.

CONCLUSION

We excluded the effect of ECE-1b C-338A polymorphism on the progression of ADPKD. We could observe a mild tendency toward faster decline of renal function in AA homozygous individuals.

Endothelin-converting Enzyme-1, Abundance of Isoforms a-d and Identification of a Novel Alternatively Spliced Variant Lacking a Transmembrane Domain

Endothelin-converting enzyme-1 (ECE-1) cleaves big endothelins, as well as bradykinin and beta-amyloid peptide. Several isoforms of ECE-1 (a-d) have been identified to date; they differ only in their NH(2) terminus but share the catalytic domain located in the COOH-terminal end. Using quantitative PCR, we found ECE-1d to be the most abundant type in several endothelial cells (EC) types. In addition to full-length ECE-1 forms we have identified novel, alternatively spliced mRNAs of ECE-1 b-d. These splice variants (SVs) lack exon 3', which codes for the transmembrane region and is present in full-length forms. SVs mRNA were highly expressed in EC derived from macro and microvascular beds but much less so in other, non-endothelial cells expressing ECE-1, which suggests that the splicing mechanism is cell-specific. Analyses of ECE-1d and its SV form in stably transfected HEK-293 cells revealed that both proteins were recognized by anti COOH-terminal ECE-1 antibodies, but anti NH(2)-terminal antibodies only bound ECE-1d. The novel protein, designated ECE-1 sv, has an apparent molecular mass of 75 kDa; by using site-directed mutagenesis its start site was identified in a region common to all ECE-1 forms suggesting that ECE-1 b-d SV mRNAs are translated into the same protein. In agreement with the findings demonstrating common COOH terminus for ECE-1sv and ECE-1d, both exhibited a similar catalytic activity. However, immunofluorescence staining and differential centrifugation revealed a distinct intracellular localization for these two proteins. The presence of ECE-1sv in different cellular compartments than full-length forms of the enzyme may suggest a distinct physiological role for these proteins.

Aβ-degrading enzymes: modulators of Alzheimer's disease pathogenesis and targets for therapeutic intervention

The accumulation of Aβ (amyloid β-protein) peptides in the brain is a pathological hallmark of all forms of AD (Alzheimer's disease) and reducing Aβ levels can prevent or reverse cognitive deficits in mouse models of the disease. Aβ is produced continuously and its concentration is determined in part by the activities ofseveral degradative enzymes, including NEP (neprilysin), IDE (insulin-degrading enzyme), ECE-1 (endothelinconverting enzyme 1) and ECE-2, and probably plasmin. Decreased activity of any of these enzymes due to genetic mutation, or age- or disease-related alterations in gene expression or proteolytic activity, may increase the risk for AD. Conversely, increased expression of these enzymes may confer a protective effect. Increasing Aβ degradation through gene therapy, transcriptional activation or even pharmacological activation of the Aβ-degrading enzymes represents a novel therapeutic strategy for the treatment of AD that is currently being evaluated in cell-culture and animal models. In this paper, we will review the roles of NEP, IDE, ECE and plasmin in determining endogenous Aβ concentration, highlighting recent results concerning the regulation of these enzymes and their potential as therapeutic targets.

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.

Metabolism of amyloid-beta peptide and Alzheimer's disease

The accumulation of amyloid-beta peptide (Abeta), a physiological peptide, in the brain is a triggering event leading to the pathological cascade of Alzheimer's disease (AD) and appears to be caused by an increase in the anabolic activity, as seen in familial AD cases or by a decrease in catabolic activity. Neprilysin is a rate-limiting peptidase involved in the physiological degradation of Abeta in the brain. As demonstrated by reverse genetics studies, disruption of the neprilysin gene causes elevation of endogenous Abeta levels in mouse brain in a gene-dose-dependent manner. Thus, the reduction of neprilysin activity will contribute to Abeta accumulation and consequently to AD development. Evidence that neprilysin in the hippocampus and cerebral cortex is down-regulated with aging and from an early stage of AD development supports a close association of neprilysin with the etiology and pathogenesis of AD. Therefore, the up-regulation of neprilysin represents a promising strategy for therapy and prevention. Recently, somatostatin, which acts via a G-protein-coupled receptor (GPCR), has been identified as a modulator that increases brain neprilysin activity, resulting in a decrease of Abeta levels. Thus, it may be possible to pharmacologically control brain Abeta levels with somatostatin receptor agonists.

Gender differences in small intestinal perfusion following trauma hemorrhage: the role of endothelin-1

Although gender differences in intestinal perfusion exist following trauma-hemorrhage (T-H), it remains unknown whether endothelin-1 (ET-1) plays any role in these dimorphic responses. To study this, male, proestrus female (female), and 17 beta-estradiol (E2)-treated male rats underwent midline laparotomy, hemorrhagic shock (blood pressure 40 mmHg, 90 min), and resuscitation (Ringer lactate, 4X shed blood volume, 1 h). Two hours thereafter, intestinal perfusion flow (IPF) was measured using isolated intestinal perfusion. The IPF in sham-operated males was significantly lower than those in other groups and decreased markedly following T-H. In contrast, no significant decrease in IPF was observed in females and E2 males following T-H. The lower IPF in sham-operated males was significantly elevated by ET(A) receptor antagonist (BQ-123) administration and was similar to that seen in sham-operated females. The decreased IPF in males after T-H was also attenuated by BQ-123 administration. The intestinal ET-1 levels in sham-operated males were significantly higher than in other groups. Although plasma and intestinal ET-1 levels increased significantly after T-H in all groups, they were highest in males. Plasma E2 levels in females and E2 males were significantly higher than in males; however, they were not affected by T-H. There was a negative correlation between plasma ET-1 and E2 following T-H. Thus ET-1 appears to play an important role in intestinal perfusion failure following T-H in males. Because E2 can modulate this vasoconstrictor effect of ET-1, these findings may partially explain the previously observed salutary effect of estrogen in improving intestinal perfusion following T-H in males.

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.

À quoi sert le système endothéline ?

Endothelins are a family of three peptides of 21 amino acids with strong vasoconstrictor effects. The three peptides are encoded by three different genes and derived from precursors (" big endothelins") which are cleaved by metalloproteases, named endothelin-converting enzyme. Two receptors have been cloned, ET-A and ET-B which bind the three endothelins with various affinities. The diverse expression pattern of the endothelin system (ET) components is associated with a complex pharmacology and its counteracting physiological actions. New modulators of the ET system have been described : retinoic acid, leptin, prostaglandins, hypoxia. Endothelins can be considered as regulators working in paracrine and autocrine fashion in a variety of organs in different cellular types. The ET system has beneficial and detrimental roles in mammals. The different components have been shown to be essential for a normal embryonic and neonatal development, for renal homeostasis and maintenance of basal vascular tone. They are involved in physiological and tumoral angiogenesis. They affect the physiology and pathophysiology of the liver, muscle, skin, adipose tissue and reproductive tract. The endothelin system participates in the development of atherosclerosis as well as pulmonary hypertension, and mediates cardiac remodeling in heart failure. Elaboration of new animal models (knock-out, pathophysiological models em leader ) will allow the clear genetic dissection of physiological and pathophysiological roles of the endothelin system.

Ace revisited: a new target for structure-based drug design

Angiotensin-converting enzyme (ACE) is a chloride-dependent metalloenzyme that catalyses the hydrolytic cleavage of dipeptides from the carboxyl terminus of many regulatory oligopeptides. ACE is central to the renin–angiotensin system that regulates blood pressure, fluid homeostasis, and renal and vascular function. It is therefore a major target for cardiovascular therapies.

ACE inhibitors (for example, captopril, enalaprilat and lisinopril) have been on the market for more than 20 years. Side effects of treatment with ACE inhibitors include cough and angioedema.

ACE comprises an N- and a C-domain, each containing an active site with distinct substrates and activation properties. The design of domain-selective inhibitors might produce new drugs with improved safety and efficacy — this endeavour will be facilitated by the recent determination of the three-dimensional structure of ACE.

The C-domain seems to be primarily responsible for the regulation of blood pressure. Data indicate that C-domain-selective inhibitors will have less severe side effects than current-generation inhibitors, which generally target both the N- and C-domains.

In contrast to the C-domain, the N-domain seems to have relatively low affinity for the peptides that control blood pressure. It preferentially hydrolyses at least three other physiologically important peptides, so targeted inhibition of the N-domain might have novel therapeutic applications.

Current-generation angiotensin-converting enzyme (ACE) inhibitors are widely used for cardiovascular diseases, including high blood pressure, heart failure, heart attack and kidney failure, and have combined annual sales in excess of US $6 billion. However, the use of these ACE inhibitors, which were developed in the late 1970s and early 1980s, is hampered by common side effects. Moreover, we now know that ACE actually consists of two parts (called the N- and C-domains) that have different functions. Therefore, the design of specific domain-selective ACE inhibitors is expected to produce next-generation drugs that might be safer and more effective. Here we discuss the structural features of current inhibitors and outline how next-generation ACE inhibitors could be designed by using the three-dimensional molecular structure of human testis ACE. The ACE structure provides a unique opportunity for rational drug design, based on a combination of in silico modelling using existing inhibitors as scaffolds and iterative lead optimization to drive the synthetic chemistry.

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.

Active amino acids of the Kell blood group protein and model of the ectodomain based on the structure of neutral endopeptidase 24.11

In addition to its importance in transfusion, Kell protein is a member of the M13 family of zinc endopeptidases and functions as an endothelin-3-converting enzyme. To obtain information on the structure of Kell protein we built a model based on the crystal structure of the ectodomain of neutral endopeptidase 24.11 (NEP). Similar to NEP, the Kell protein has 2 globular domains consisting mostly of alpha-helical segments. The domain situated closest to the membrane contains both the N- and C-terminal sequences and the enzyme-active site. The outer domain contains all of the amino acids whose substitutions lead to different Kell blood group phenotypes. In the model, the zinc peptidase inhibitor, phosphoramidon, was docked in the active site. Site-directed mutagenesis of amino acids in the active site was performed and the enzymatic activities of expressed mutant Kell proteins analyzed and compared with NEP. Our studies indicate that Kell and NEP use the same homologous amino acids in the coordination of zinc and in peptide hydrolysis. However, Kell uses different amino acids than NEP in substrate binding and appears to have more flexibility in the composition of amino acids allowed in the active site.

Increased angiotensin-(1-7)-forming activity in failing human heart ventricles: evidence for upregulation of the angiotensin-converting enzyme Homologue ACE2

BACKGROUND

The formation of angiotensin-(1-7) from either angiotensin (Ang) I or Ang II in failing human hearts is not well understood.

METHODS AND RESULTS

Angiotensinase activity in left and right ventricular membranes from 14 idiopathic dilated cardiomyopathy (IDC), 8 primary pulmonary hypertension (PPH), and 13 nonfailing human hearts was measured with either 125I-Ang I or 125I-Ang II as substrate. Ang-(1-7)-forming activity from 125I-Ang I was inhibited by thiorphan. With 125I-Ang II as substrate, Ang-(1-7) formation was inhibited by the ACE2-specific inhibitor C16. Western blotting with an anti-ACE2 antibody confirmed the presence of ACE2. Angiotensinase activity with 125I-Ang I as substrate was increased in failing IDC left ventricles (LVs) compared with nonfailing LVs (P<0.001). Ang-(1-7)-forming activity with 125I-Ang II as substrate was increased in both failing LVs and right ventricles (RVs) of IDC hearts and only in failing RVs of PPH hearts (PPH LV, 51.12+/-5.25; PPH RV, 89.97+/-11.21; IDC LV, 139.7+/-21.96; and IDC RV, 192.7+/-5.43; NF LV, 32.89+/-5.38; NF RV 40.49+/-10.66 fmol/min per milligram (P<0.05 PPH RV versus PPH LV; P<0.05 PPH RV versus NF RV; P<0.001 IDC LV versus NF LV; P<0.001 IDC RV versus NF RV).

CONCLUSIONS

Ang-(1-7)-forming activity from both Ang I and Ang II was increased in failing human heart ventricles but was mediated by at least two different angiotensinases. The first, which demonstrated substrate preference for Ang I, was neutral endopeptidase (NEP)-like. The second was ACE2, as demonstrated by Western blotting and inhibition of activity with C16.

[Amyloid-beta peptide metabolism and Alzheimer's disease]

The deposition of amyloid-beta peptide (Abeta) causes the long-term pathological cascade of Alzheimer's disease (AD). Neprilysin is a rate-limiting peptidase, which participates in Abeta degradation in brain. As demonstrated by reverse genetics, the disruption of neprilysin gene causes an elevation in endogenous Abeta levels in the mouse brain in a gene-dose-dependent manner. Therefore, a reduction of neprilysin activity will contribute to Abeta deposition and thus to AD development. Neprilysin is localized at presynapses and on axons, and its expression levels are decreased at the terminal zones and on axons of the lateral perforant pathway and the mossy fibers with aging in mice, suggesting that local concentrations of Abeta are likely to be elevated at the sites, which play crucial roles on certain forms of learning and memory and are highly vulnerable to AD. Overexpression of neprilysin decreased both extracellular and intracellular Abeta levels in primary cortical neurons. These results indicate that up-regulation of neprilysin activity would be a relevant strategy for therapy and prevention through reduction of the Abeta levels. Recently, we have found that a certain neuropeptide regulates the expression of neprilysin in primary neurons. Since a number of receptors for neuropeptides are G-protein-coupled receptors, we would control brain Abetalevels pharmacologically by the manipulation of neprilysin activity.

An endothelin-converting enzyme homologue in the locust, Locusta migratoria: functional activity, molecular cloning and tissue distribution

Endothelin-converting enzyme is the key enzyme in the process of endothelin production. Endothelin is a peptide that plays an important role in vasoconstriction and the development of neural crest-derived cells in vertebrates. Activity assays performed on membrane extracts from Locusta migratoria brain revealed the existence of a protease activity responsible for the formation of mature endothelin-1 from its precursor, big endothelin. Cloning experiments led to a cDNA sequence (Lom ECE) with an open reading frame of 727 amino acid residues displaying all the characteristic ECE features. A comparison of ECE activity levels among different tissues of the locust showed a high enzyme activity in the gonads and midgut. RT-PCR experiments showed a wide tissue distribution of Lom ECE mRNA, with transcription being most abundant in brain tissue.

Synthesis and degradation of endothelin-1

The endothelin-converting enzyme (ECE) is the main enzyme responsible for the genesis of the potent pressor peptide endothelin-1 (ET-1). It is suggested that the ECE is pivotal in the genesis of ET-1, considering that the knockout of both genes generates the same lethal developments during the embryonic stage. Several isoforms of the ECE have been disclosed, namely ECE-1, ECE-2, and ECE-3. Within each of the first two groups, several sub-isoforms derived through splicing of single genes have also been identified. In this review, the characteristics of each sub-isoform for ECE-1 and 2 will be discussed. It is important to mention that the ECE is, however, not the sole enzyme involved in the genesis of endothelins. Indeed, other moieties, such as chymase and matrix metalloproteinase II, have been suggested to be involved in the production of ET intermediates, such as ET-1 (1-31) and ET-1 (1-32), respectively. Other enzymes, such as the neutral endopeptidase 24-11, is curiously not only involved in the degradation and inactivation of ET-1, but is also responsible for the final production of the peptide via the hydrolysis of ET-1 (1-31). In this review, we will attempt to summarize, through the above-mentioned characteristics, the current wisdom on the role of these different enzymes in the genesis and termination of effect of the most potent pressor peptide reported to date.

Characterization of endothelin-converting enzyme-2. Implication for a role in the nonclassical processing of regulatory peptides

Most neuroendocrine peptides are generated by proteolysis of the precursors at basic residue cleavage sites. Prohormone convertases belonging to the subtilisin family of serine proteases are primarily responsible for processing at these "classical sites." In addition to the classical cleavages, a subset of bioactive peptides is generated by processing at "nonclassical" sites. The proteases responsible for these cleavages have not been well explored. Members of several metalloprotease families have been proposed to be involved in nonclassical processing. Among them, endothelin-converting enzyme-2 (ECE-2) is a good candidate because it exhibits a neuroendocrine distribution and an acidic pH optimum. To examine the involvement of this protease in neuropeptide processing, we purified the recombinant enzyme and characterized its catalytic activity. Purified ECE-2 efficiently processes big endothelin-1 to endothelin-1 by cleavage between Trp(21) and Val(22) at acidic pH. To characterize the substrate specificity of ECE-2, we used mass spectrometry with a panel of 42 peptides as substrates to identify the products. Only 10 of these 42 peptides were processed by ECE-2. A comparison of residues around the cleavage site revealed that ECE-2 exhibits a unique cleavage site selectivity that is related to but distinct from that of ECE-1. ECE-2 tolerates a wide range of amino acids in the P1-position and prefers aliphatic/aromatic residues in the P1'-position. However, only a small fraction of the aliphatic/aromatic amino acid-containing sites were cleaved, indicating that there are additional constraints beyond the P1- and P1'-positions. The enzyme is able to generate a number of biologically active peptides from peptide intermediates, suggesting an important role for this enzyme in the biosynthesis of regulatory peptides. Also, ECE-2 processes proenkephalin-derived bovine adrenal medulla peptides, and this processing leads to peptide products known to have differential receptor selectivity. Finally, ECE-2 processes PEN-LEN, an endogenous inhibitor of prohormone convertase 1, into products that do not inhibit the enzyme. Taken together, these results are consistent with an important role for ECE-2 in the processing of regulatory peptides at nonclassical sites.

Alzheimer's disease beta-amyloid peptide is increased in mice deficient in endothelin-converting enzyme

The abnormal accumulation of beta-amyloid (Abeta) in the brain is an early and invariant feature in Alzheimer's disease (AD) and is believed to play a pivotal role in the etiology and pathogenesis of the disease. As such, a major focus of AD research has been the elucidation of the mechanisms responsible for the generation of Abeta. As with any peptide, however, the degree of Abeta accumulation is dependent not only on its production but also on its removal. In cell-based and in vitro models we have previously characterized endothelin-converting enzyme-1 (ECE-1) as an Abeta-degrading enzyme that appears to act intracellularly, thus limiting the amount of Abeta available for secretion. To determine the physiological significance of this activity, we analyzed Abeta levels in the brains of mice deficient for ECE-1 and a closely related enzyme, ECE-2. Significant increases in the levels of both Abeta40 and Abeta42 were found in the brains of these animals when compared with age-matched littermate controls. The increase in Abeta levels in the ECE-deficient mice provides the first direct evidence for a physiological role for both ECE-1 and ECE-2 in limiting Abeta accumulation in the brain and also provides further insight into the factors involved in Abeta clearance in vivo.

Alzheimer's disease, neuropeptides, neuropeptidase, and amyloid-beta peptide metabolism

Amyloid-beta peptide (Abeta), the pathogenic agent of Alzheimer's disease (AD), is a physiological metabolite in the brain. We have focused our attention and effort on elucidating the unresolved aspect of Abeta metabolism: proteolytic degradation. Among a number of Abeta-degrading enzyme candidates, we used a novel in vivo paradigm to identify a member of the neutral endopeptidase family, neprilysin, as the major Abeta catabolic enzyme. Neprilysin deficiency results in defects in the metabolism of endogenous Abeta 40 and 42 in a gene dose-dependent manner. Our observations suggest that even partial down-regulation of neprilysin activity, which could be caused by aging, can contribute to AD development by promoting Abeta accumulation. Moreover, we discuss the fact that an aging-dependent decline of neprilysin activity, which leads to elevation of Abeta concentrations in the brain, is a natural process that precedes AD pathology. In this Perspective, we hypothesize that neprilysin down-regulation has a role in sporadic AD (SAD) pathogenesis, and we propose that this knowledge be used for developing preventive and therapeutic strategies through use of a G protein-coupled receptor (GPCR).

Constitutive Phosphorylation of Human Endothelin-converting Enzyme-1 Isoforms

To investigate the phosphorylation of human endothelin-converting enzyme-1 (hECE-1) and identify potential residues involved, both in vivo and in vitro phosphorylation labeling assays of hECE-1 isoforms were performed in combination with site-directed mutagenesis and mass spectrometric analyses. Initial studies found that endogenous hECE-1 was constitutively phosphorylated in a primary endothelial cell line. The four known isoforms of hECE-1 expressed in this cell line (1a, 1b, 1c, and 1d) were then cloned by reverse transcription-PCR to determine which isoform(s) may be phosphorylated. The isoforms differ only in the first portion of their short amino-terminal cytoplasmic domains whereas their transmembrane domains and ectodomains of the proteins are identical. Isoforms 1b, 1c, and 1d but not 1a, were constitutively phosphorylated in vivo when expressed in Chinese hamster ovary cells and casein kinase I readily phosphorylated the immunopurified isoforms in vitro. Site-directed mutagenesis established that two conserved serine residues, Ser(18) and Ser(20), (numbering based on isoform 1c) form at least one phosphorylation site in these three isoforms. Mutant forms of 1b, 1c, and 1d were constructed in which a single alanine was introduced at either serine residue and a double mutant for each isoform was constructed as well in which both serines were replaced with alanine. Phosphorylation of the single mutants was greatly reduced and was nearly abolished in the double mutants in both in vivo and in vitro labeling assays. Analysis by MALDI-MS of (32)P-labeled proteolytic peptides derived from wild type 1c and the 1c mutants supported both Ser(18) and Ser(20) as phosphorylated residues. These data demonstrate the first finding that hECE-1 is constitutively phosphorylated within its cytoplasmic domain in an isoform-specific manner.

Decompensation characterized by decreased perfusion of the heart and brain during hemorrhagic shock: role of endothelin-1

BACKGROUND

Endothelin-1 (ET-1), a 21-amino-acid peptide produced by vascular endothelium, is a potent vasoconstrictor and a component of local regulation of vascular tone through its effect on underlying vascular smooth muscle. Hemorrhagic shock (HS) is characterized by compensatory regional vasoconstriction to decrease peripheral tissue perfusion and to maintain core organ perfusion. Decompensation occurs with prolonged duration of HS. In the present study, we hypothesized that systemic and vital organ tissue ET-1 concentrations would correlate with changes in systemic and vital organ perfusion associated with compensatory and decompensatory states of HS.

METHODS

After surgical instrumentation, HS was induced in male Sprague-Dawley rats by withdrawing blood via femoral artery to a mean arterial pressure of 35 to 40 mm Hg that was maintained for either 30 minutes or 90 minutes in separate groups of male Sprague-Dawley rats. Systemic hemodynamics and regional blood flow were measured using a radioactive microsphere technique. In separate groups of animals, sham, 30 minutes of HS, or 90 minutes of HS, plasma and tissue concentrations of ET-1 were determined using a radioimmunoassay technique.

RESULTS

HS maintained for 90 minutes was associated with increased arterial base deficit from 3.6 +/- 0.53 mEq/L to 13 +/- 0.37 mEq/L, decreased cardiac output from 79 +/- 18 mL/min to 18 +/- 5 mL/min, and increased systemic vascular resistance from 1,004 +/- 102 mm Hg/L. min to 2,392 +/- 447 mm. Hg/L min as compared with baseline values. With 90 minutes of HS as compared with 30 minutes of HS, perfusion was significantly decreased in brain (72 +/- 11 vs. 29 +/- 6 mL/min. 100 g tissue) and heart (483 +/- 30 vs. 173 +/- 38 mL/min. 100 g tissue) and kidney perfusion was decreased (from 114 +/- 28 mL/min/100 g tissue to 29 +/- 2 mL/min. 100 g tissue), and ET-1 concentration was increased significantly in brain (cerebral cortex, 89 +/- 14 pg/100 g tissue to 144 +/- 19 pg/100 g tissue; midbrain, 172 +/- 15 pg/100 g tissue to 211 +/- 10 pg/100 g tissue), heart (left ventricle, 312 +/- 11 pg/100 g tissue to 360 +/- 14 pg/100 g tissue), kidney (medulla, 857 +/- 61 pg/100 g tissue to 1,277 +/- 41 pg/100 g tissue), and plasma (5.31 +/- 0.6 pg/100 g tissue to 21.26 +/- 2.9 pg/mL).

CONCLUSION

Decreased vital organ and peripheral tissue perfusion, a primary decompensation effect of HS, was apparent with 90 minutes of HS but not with 30 minutes, and was associated with increased vital organ tissue and plasma ET-1 concentrations. These data suggest a role for ET-1 in control mechanisms of progressive vasoconstriction that occurs with prolonged duration of HS.

Beta-amyloid catabolism: roles for neprilysin (NEP) and other metallopeptidases?

The steady-state level of amyloid beta-peptide (Abeta) represents a balance between its biosynthesis from the amyloid precursor protein (APP) through the action of the beta- and gamma-secretases and its catabolism by a variety of proteolytic enzymes. Recent attention has focused on members of the neprilysin (NEP) family of zinc metalloproteinases in amyloid metabolism. NEP itself degrades both Abeta(1-40) and Abeta(1-42) in vitro and in vivo, and this metabolism is prevented by NEP inhibitors. Other NEP family members, for example endothelin-converting enzyme, may contribute to amyloid catabolism and may also play a role in neuroprotection. Another metalloproteinase, insulysin (insulin-degrading enzyme) has also been advocated as an amyloid-degrading enzyme and may contribute more generally to metabolism of amyloid-forming peptides. Other candidate enzymes proposed include angiotensin-converting enzyme, some matrix metalloproteinases, plasmin and, indirectly, thimet oligopeptidase (endopeptidase-24.15). This review critically evaluates the evidence relating to proteinases implicated in amyloid catabolism. Therapeutic strategies aimed at promoting A,beta degradation may provide a novel approach to the therapy of Alzheimer's disease.

Enhancement of neurokinin A-induced smooth muscle contraction in human urinary bladder by mucosal removal and phosphoramidon: relationship to peptidase inhibition

Neurokinin A (NKA) is potent in contracting the human detrusor muscle. Here, we have investigated whether these contractile responses are influenced by the presence of the mucosa, by the peptidase inhibitor phosphoramidon or by possible modulators, prostaglandins and nitric oxide. Contractile responses to neurokinin A were unaffected by indomethacin or N-omega-nitro-L-arginine, but were significantly reduced in strips containing mucosa. Phosphoramidon, an inhibitor of neutral endopeptidase 24.11 (neprilysin, CD10), was ineffective at 10 microM, but at 100 microM, significant increase in the maximum response was achieved by neurokinin A in detrusor strips with and without mucosa. In immunohistochemical studies, neutral endopeptidase immunoreactivity occurred in peripheral nerve trunks in the detrusor and in a fibrous meshwork in the subepithelial lamina propria. Our data indicate that neutral endopeptidase is present in bladder mucosa and detrusor, and support the concept that this metalloprotease and/or related enzymes are important in regulating the actions of tachykinins.

Mapping the active site of endothelin-converting enzyme-1 through subsite specificity and mutagenesis studies: a comparison with neprilysin

Endothelin-converting enzyme-1 (ECE-1) is a membrane-bound zinc metallopeptidase that is homologous to neprilysin in amino acid sequence. A major in vivo function of ECE-1 is the generation of endothelin-1, a potent vasoconstrictor, from big endothelin-1. ECE-1 is also potentially involved in the processing or degradation of other peptide hormones. In this study we have used substrates based on the sequence of the COOH-terminal half of big endothelin-1 to examine the subsite specificity of recombinant ECE-1. The big endothelin-1 [16-38] peptides were systematically varied at either position 21 (P(1)) or position 22 (P'(1)) and used in steady-state kinetic analyses of ECE-1. The results indicate that the S(1) pocket of ECE-1 is relatively nonselective, but that the S'(1) subsite of ECE-1 has a preference for large hydrophobic side chains. The peptidyl carboxydipeptidase activity of ECE-1 was also characterized, revealing that substrates with COOH-terminal carboxylates are highly preferred over the cognate amides and esters. A site-directed mutagenesis study was carried out to identify the active-site amino acid residues specifically involved in binding to the COOH-terminal carboxylate of substrates. The data indicate that Arg(133) of ECE-1, which corresponds to Arg(102) of neprilysin that has been identified as an active-site residue of neprilysin involved in binding to the free carboxylate of some substrate peptides, may not play the same role. However, the low activity observed for an ECE-1 Arg(726) mutant is consistent with a role for this arginine residue in the binding of substrates, a role which has been ascribed to arginine residues in both thermolysin (Arg(203)) and neprilysin (Arg(717)).

Biodistribution and catabolism of (18)F-labeled neurotensin(8-13) analogs

4-([(18)F]fluoro)benzoyl-neurotensin(8-13) ((18)FB-Arg(8)-Arg(9)-Pro(10)-Tyr(11)- Ile(12)-Leu(13)-OH, 1) and two analogs stabilized in one and two positions ((18)FB-Arg(8)psi(CH(2)NH)Arg(9)-Pro(10)-Tyr(11)- Ile(12)-Leu(13)-OH, 2, (18)FB-Arg(8)psi(CH(2)NH)Arg(9)-Pro(10)-Tyr(11)-Tle(12)-Leu(13)-OH, 3) were synthesized in a radiochemical yield of 25-36% and a specific activity of 5-15 GBq/mmol. The peptides were evaluated in vitro and in vivo for their potential to image tumors overexpressing neurotensin receptor 1 (NTR1) by positron emission tomography (PET). All analogs exhibited in vitro binding affinity in the low nanomolar range to NTR1-expressing human tumors, measured by quantitative receptor autoradiography, HT-29 and WiDr cells, and to sections of tumors derived from these cell lines in mice. The radiotracers were internalized in the cells in vitro, and the fluorinated peptides were able to mobilize intracellular Ca(2+) of WiDr cells. In in vivo studies in rats and in mice bearing HT-29 cell tumors, only a moderate uptake of the radioligands into the studied tumors was observed, presumed to be due to degradation in vivo and fast elimination by the kidneys. In comparison with the other analogs, the specific tumor uptake expressed as tumor-to-muscle relation was highest for the radioligand 3. The blood clearance of 3 was reduced by co-injection of peptidase inhibitors. The catabolic pathways of the radiofluorinated peptides were elucidated. The results suggest that the high binding affinity to NTR1 and the stabilization against proteolytic degradation are not yet sufficient for tumor imaging by PET.

Diagnostic and therapeutic potential of the endothelin system in patients with chronic heart failure

There is now considerable evidence to support a role for the endothelin (ET) system in the pathogenesis and progression of chronic heart failure (CHF). As such, the potential exists for this system to be useful in both diagnosis (by measurement of peptide levels in plasma and other body fluids) and treatment (by pharmacological blockade) of this condition. Plasma levels of endothelin-1 (ET-1) are elevated in CHF and the magnitude of elevation correlates with disease severity. ET-1 levels in plasma predict subsequent mortality in patients with CHF. ET-1 may also contribute to symptoms associated with CHF, such as exercise intolerance. In the diagnosis of CHF, plasma levels of ET-1 appear to be a less powerful discriminator between patients with mild disease and control subjects with normal ventricular function on multivariate analyses, compared to brain natriuretic peptide (BNP), or its N-terminal fragment. ET-1 concentrations are also elevated in the saliva of patients with CHF and may represent an alternative approach to assessment of the status of the ET system in these patients. Specific ET receptor antagonists (both mixed and ET(A)-selective) have been developed. Studies with these agents in animal models of CHF have demonstrated beneficial effects via both haemodynamic and non-haemodynamic pathways. A number of short-term clinical studies have been performed demonstrating improvements in haemodynamic parameters without neurohormonal activation. Long-term clinical studies with ET receptor antagonists are currently underway to definitively test the impact of blockade of this system on mortality and major cardiovascular endpoints. Endothelin converting enzyme (ECE) inhibitors represent an alternative strategy of ET blockade, and early data from animal models suggest these agents may be of clinical utility, either alone or, more likely, in combination with other zinc metallopeptidases.

Localization of the endothelin system in aldosterone-producing adenomas

Endothelin-1 (ET-1) could play a role in the regulation of aldosterone secretion of the human adrenal gland. The presence of the endothelin-converting enzyme 1 (ECE-1) and ET-1 suggests that there is a local ET system in the adrenal cortex, but the in situ synthesis of ET-1 remains to be confirmed. The cellular distribution of the whole ET system was evaluated in 20 cases of aldosterone-producing adenomas. Polymerase chain reaction studies gave strong signals for ECE-1 mRNA and the mRNAs for endothelin type A (ET(A)) and B (ET(B)) receptors and faint signals for prepro-ET-1 mRNA. In situ hybridization showed ET(A) receptors scattered throughout the adenoma, in both secretory cells and vascular structures (score, +). There were more ET(B) receptors (score, ++), but they were restricted mainly to the endothelium. ECE-1 mRNA and protein were ubiquitous and abundant in secretory cells (score, +++) and vascular structures (score, ++); the enzyme was active on big ET-1. There was no prepro-ET-1 mRNA in the cortex, except in the thickened precapillary arterioles present in only 30% of the aldosterone-producing adenomas studied. ET-1 immunoreactivity was detected in vascular structures (score, +), probably bound to receptors, suggesting that ET-1 has an endocrine action. The low concentrations of ET-1 could also indicate that it acts in a paracrine-autocrine fashion to control adrenal blood flow. The discrepancy between the concentrations of ECE-1 and its substrate suggests that ECE-1 has another role in the adrenal secretory cells. Our data indicate that ET probably is not a primary cause of the development or maintenance of the adenoma.

Conserved cysteine and tryptophan residues of the endothelin-converting enzyme-1 CXAW motif are critical for protein maturation and enzyme activity

The neprilysin (NEP)/endothelin-converting enzyme (ECE) family of metalloproteases contains a highly conserved carboxyl-terminal tetrapeptide sequence, CXAW, where "C" is cysteine, "X" is a polar amino acid, "A" is an aliphatic residue, and "W" is tryptophan. Although this sequence strongly resembles a prenylation motif, human ECE-1 did not appear to be prenylated when labeled in vivo using various isoprenoid precursors in cell lines expressing ECE-1. We used site-directed mutagenesis to investigate the role of the CXAW motif and determined that the conserved cysteine residue of the CXAW motif in ECE-1, Cys(755), is critical for proper folding of the enzyme, its export from the endoplasmic reticulum, and its maturation in the secretory pathway. In addition, site-directed mutagenesis revealed that the conserved tryptophan residue of the sequence CEVW appears to be important for endoplasmic reticulum export and is essential for enzyme activity. Deletion of Trp(758) or substitution with alanine greatly slowed maturation of the enzyme, and resulted in more than a 90% loss of enzyme activity relative to the wild type. Conservative substitution of the tryptophan with phenylalanine did not reduce activity, whereas replacement with tyrosine, methionine, or leucine reduced enzyme activity by 50%, 75%, and 85%, respectively. Together, these data indicate that the conserved CEVW sequence does not serve as a prenylation signal and that both the conserved cysteine and tryptophan residues are necessary for proper folding and maturation of the enzyme. Furthermore, the conserved tryptophan appears to be critical for enzyme activity.

Identification of peptides from brain and pituitary of Cpe(fat)/Cpe(fat) mice

Cpe(fat)/Cpe(fat) mice have a naturally occurring point mutation within the carboxypeptidase E gene that inactivates this enzyme, leading to an accumulation of many neuroendocrine peptides containing C-terminal basic residues. These processing intermediates can be readily purified on an anhydrotrypsin affinity resin. Using MS to obtain molecular mass and partial sequence information, more than 100 peptides have been identified. These peptides represent fragments of 16 known secretory pathway proteins, including proenkephalin, proopiomelanocortin, protachykinins A and B, chromogranin A and B, and secretogranin II. Many of the identified peptides represent previously uncharacterized fragments of the precursors. For example, 12 of the 13 chromogranin B-derived peptides found in the present study have not been previously reported. Of these 13 chromogranin B-derived peptides, only five contain consensus cleavage sites for prohormone convertases at both the C and N termini. Two distinct chromogranin B-derived peptides result from cleavage at Trp-Trp bonds, a site not typically associated with neuropeptide processing. An RIA was used to confirm that one of these peptides, designated WE-15, exists in wild-type mouse brain, thus validating the approach to identify peptides in Cpe(fat)/Cpe(fat) mice. These "orphan" peptides are candidate ligands for orphan G protein-coupled receptors. In addition, the general technique of using affinity chromatography to isolate endogenous substrates from a mutant organism lacking an enzyme should be applicable to a wide range of enzyme-substrate systems.

Degradation of the Alzheimer's amyloid beta peptide by endothelin-converting enzyme

Deposition of beta-amyloid (Abeta) peptides in the brain is an early and invariant feature of all forms of Alzheimer's disease. As with any secreted protein, the extracellular concentration of Abeta is determined not only by its production but also by its catabolism. A major focus of Alzheimer's research has been the elucidation of the mechanisms responsible for the generation of Abeta. Much less, however, is known about the mechanisms responsible for Abeta removal in the brain. In this report, we describe the identification of endothelin-converting enzyme-1 (ECE-1) as a novel Abeta-degrading enzyme. We show that treatment of endogenous ECE-expressing cell lines with the metalloprotease inhibitor phosphoramidon causes a 2-3-fold elevation in extracellular Abeta concentration that appears to be due to inhibition of intracellular Abeta degradation. Furthermore, we show that overexpression of ECE-1 in Chinese hamster ovary cells, which lack endogenous ECE activity, reduces extracellular Abeta concentration by up to 90% and that this effect is completely reversed by treatment of the cells with phosphoramidon. Finally, we show that recombinant soluble ECE-1 is capable of hydrolyzing synthetic Abeta40 and Abeta42 in vitro at multiple sites.

Highly sensitive and selective fluorescence assays for rapid screening of endothelin-converting enzyme inhibitors

The highly potent vasoconstrictor peptide endothelin (ET) is generated from an inactive precursor, big endothelin (bET), by endothelin-converting enzyme (ECE). ECE is a phosphoramidon-sensitive zinc metallopeptidase, which is closely related to neprilysin (neutral endopeptidase). It is possible that compounds which inhibit the formation of ET may be used as new drugs for the treatment of cardiovascular diseases. Such an approach requires a fast, simple and selective assay to measure ECE activity, allowing rapid screening of inhibitors. We describe here two new ECE substrates based on the concept of 'intramolecularly quenched fluorescence' which may fulfill this aim. One, S(1) [Pya(21)-Nop(22)-bET-1(19--35)], is the (19--35) fragment of the natural peptide big-ET-1(1--38), which is modified by introducing the fluorescent amino acid, pyrenylalanine (Pya), in position 21 and a quencher, p-nitrophenylalanine (Nop), in position 22. The second substrate (S(2)) is a small peptide, Ac-Ser-Gly-Pya-Lys-Ala-Phe-Ala-Nop-Gly-Lys-NH(2), from a biased substrate peptide library. The recombinant, hECE-1c, cleaved both Pya(21)-Nop(22)-bET-1(19--35) and the natural substrate selectively between residues 21 and 22, whereas cleavage occurred between alanine and phenylalanine in the small peptide. In both cases, this generated intense fluorescence emission. The synthesis and kinetic parameters of these substrates are described. These assays, which can be used directly on tissue homogenates, are the most sensitive and selective described to date for ECE, and are easily automated for a high-throughput screening of inhibitors.

Characterization of PHEX endopeptidase catalytic activity: identification of parathyroid-hormone-related peptide107-139 as a substrate and osteocalcin, PPi and phosphate as inhibitors

Mutations in the PHEX gene (phosphate-regulating gene with homologies to endopeptidases on the X chromosome) are responsible for X-linked hypophosphataemia, and studies in the Hyp mouse model of the human disease implicate the gene product in the regulation of renal phosphate (P(i)) reabsorption and bone mineralization. Although the mechanism for PHEX action is unknown, structural homologies with members of the M13 family of endopeptidases suggest a function for PHEX protein in the activation or degradation of peptide factors involved in the control of renal P(i) transport and matrix mineralization. To determine whether PHEX has endopeptidase activity, we generated a recombinant soluble, secreted form of human PHEX (secPHEX) and tested the activity of the purified protein with several peptide substrates, including a variety of bone-related peptides. We found that parathyroid-hormone-related peptide(107-139) is a substrate for secPHEX and that the enzyme cleaves at three positions within the peptide, all located at the N-terminus of aspartate residues. Furthermore, we show that osteocalcin, PP(i) and P(i), all of which are abundant in bone, are inhibitors of secPHEX activity. Inhibition of secPHEX activity by osteocalcin was abolished in the presence of Ca(2+). We suggest that PHEX activity and mineralization may be controlled in vivo by PP(i)/P(i) and Ca(2+) and, in the latter case, the regulation requires the participation of osteocalcin.

The neprilysin (NEP) family of zinc metalloendopeptidases: genomics and function

Neprilysin (NEP), a thermolysin-like zinc metalloendopeptidase, plays an important role in turning off peptide signalling events at the cell surface. It is involved in the metabolism of a number of regulatory peptides of the mammalian nervous, cardiovascular, inflammatory and immune systems. Examples include enkephalins, tachykinins, natriuretic and chemotactic peptides. NEP is an integral plasma membrane ectopeptidase of the M13 family of zinc peptidases. Other related mammalian NEP-like enzymes include the endothelin-converting enzymes (ECE-1 and ECE-2), KELL and PEX. A number of novel mammalian homologues of NEP have also recently been described. NEP family members are potential therapeutic targets, for example in cardiovascular and inflammatory disorders, and potent and selective inhibitors such as phosphoramidon have contributed to understanding enzyme function. Inhibitor design should be facilitated by the recent three-dimensional structural solution of the NEP-phosphoramidon complex. For several of the family members, however, a well-defined physiological function or substrate is lacking. Knowledge of the complete genomes of Caenorhabditis elegans and Drosophila melanogaster allows the full complement of NEP-like activities to be analysed in a single organism. These model organisms also provide convenient systems for examining cell-specific expression, developmental and functional roles of this peptidase family, and reveal the power of functional genomics.

The neprilysin family in health and disease

The mammalian neprilysin (NEP) family comprises at least seven members: NEP itself, Kell blood group antigen (KELL), the endothelin-converting enzymes (ECE-1 and ECE-2), the enzyme PEX, associated with X-linked hypophosphataemia, "X-converting enzyme" (XCE) a CNS-expressed orphan peptidase and a soluble, secreted endopeptidase (SEP). These zinc metallopeptidases are all type II integral membrane proteins. Where identified, these enzymes have roles in the processing or metabolism of regulatory peptides and therefore represent potential therapeutic targets. A distinct feature of ECE-1 species is their existence as distinct isoforms differing in their N-terminal cytoplasmic tails. These tails play a role in enzyme targeting and turnover with di-leucine and tyrosine-based motifs affecting localization. Additional anchorage of these enzymes can also occur through palmitoylation. Bacterial homologues of the neprilysin family exist, for example the products of the pepO genes from L. lactis and S. parasanguis, and a recently described gene product of P. gingivalis which is an ECE-1 homologue that can catalyse the conversion of big endothelin to endothelin. A genomics based approach to understanding the functions of this proteinase family is aided by the completion of the C. elegans and Drosophila genomes, both of which encode multiple copies of NEP-like enzymes.

Effect of an antisense oligodeoxynucleotide to endothelin-converting enzyme-1c (ECE-1c) on ECE-1c mRNA, ECE-1 protein and endothelin-1 synthesis in bovine pulmonary artery smooth muscle cells

Endothelin-1 (ET-1) is secreted from endothelial and vascular smooth muscle cells (VSMC) after intracellular hydrolysis of big ET-1 by endothelin converting enzyme (ECE). The metallopeptidase called ECE-1 is widely thought to be the physiological ECE, but unequivocal evidence of this role has yet to be provided. Endothelial cells express four isoforms of ECE-1 (ECE-1a, ECE-1b, ECE-1c, and ECE-1d), but the identity of ECE-1 isoforms expressed in VSMC is less clear. Here, we describe the characterization of ECE-1 isoforms in bovine pulmonary artery smooth muscle cells (BPASMC) and the effect on ET-1 synthesis of an antisense oligodeoxynucleotide (ODN) to ECE-1c. Reverse transcriptase-polymerase chain reaction (RT-PCR) evaluation of total RNA from BPASMC showed that ECE-1a and ECE-1d were not expressed. Sequencing of cloned ECE-1 cDNA products and semiquantitative RT-PCR demonstrated that ECE-1b and ECE-1c were expressed in BPASMC, with ECE-1c being the predominant isoform. Basal release of ET-1 from BPASMC was low. Treatment for 24 h with tumor necrosis factor-alpha (TNFalpha) stimulated ET-1 production by up to 10-fold with parallel increases in levels of preproET-1 mRNA. Levels of ECE-1c mRNA were also raised after TNFalpha, whereas amounts of ECE-1b mRNA were decreased significantly. Treatment of BPASMC with a phosphorothioate antisense ODN to ECE-1c caused a marked reduction in ECE-1c mRNA levels and ECE-1 protein levels. However, basal and TNFalpha-stimulated ET-1 release were largely unaffected by the ECE-1c antisense ODN despite the inhibition of ECE-1c synthesis. Hence, an endopeptidase distinct from ECE-1 is mainly responsible big ET-1 processing in BPASMC.

Endothelins and endothelin receptor antagonists: therapeutic considerations for a novel class of cardiovascular drugs

The 21-amino acid peptide endothelin-1 (ET-1) is the predominant isoform of the endothelin peptide family, which includes ET-2, ET-3, and ET-4. It exerts various biological effects, including vasoconstriction and the stimulation of cell proliferation in tissues both within and outside of the cardiovascular system. ET-1 is synthesized by endothelin-converting enzymes (ECE), chymases, and non-ECE metalloproteases; it is regulated in an autocrine fashion in vascular and nonvascular cells. ET-1 acts through the activation of G(i)-protein-coupled receptors. ET(A) receptors mediate vasoconstriction and cell proliferation, whereas ET(B) receptors are important for the clearance of ET-1, endothelial cell survival, the release of nitric oxide and prostacyclin, and the inhibition of ECE-1. ET is activated in hypertension, atherosclerosis, restenosis, heart failure, idiopathic cardiomyopathy, and renal failure. Tissue concentrations more reliably reflect the activation of the ET system because increased vascular ET-1 levels occur in the absence of changes in plasma. Experimental studies using molecular and pharmacological inhibition of the ET system and the first clinical trials have demonstrated that ET-1 takes part in normal cardiovascular homeostasis. Thus, ET-1 plays a major role in the functional and structural changes observed in arterial and pulmonary hypertension, glomerulosclerosis, atherosclerosis, and heart failure, mainly through pressure-independent mechanisms. ET antagonists are promising new agents in the treatment of cardiovascular diseases.

Development of an internally quenched fluorescent substrate selective for endothelin-converting enzyme-1

Endothelin-converting enzyme-1 (ECE-1) is a membrane-bound zinc-metallopeptidase that is related to neprilysin in amino acid sequence. A major in vivo function of ECE-1 is the proteolytic conversion of big endothelin-1 to endothelin-1, one of the most potent vasconstricting peptides known. Although ECE-1 was once thought to be specific for the processing of endothelin precursors, it is now known that the enzyme hydrolyzes a number of peptide hormones. We have incorporated knowledge gained from recent studies of ECE-1 substrate specificity to aid the design of internally-quenched fluorescent substrates derived from bradykinin. The best of these substrates, (7-methoxycoumarin-4-yl)acetyl-Arg-Pro-Pro-Gly-Phe-Ser-Ala-Phe-Lys(2, 4-dinitrophenyl), is hydrolyzed by ECE-1 with a k(cat)/K(m) value of 1.9 x 10(7) M(-1) s(-1), making it the most sensitive substrate yet described for ECE-1. The substrate is suitable for the rapid, continuous assay of the enzyme using a microplate format in a fluorescence plate reader, thereby simplifying both the purification of ECE-1 and the characterization of its inhibitors. It is demonstrated that (7-methoxycoumarin-4-yl)acetyl-Arg-Pro-Pro-Gly-Phe-Ser-Ala-Phe-Lys(2, 4-dinitrophenyl) is also a substrate for neprilysin, but is hydrolyzed 10-fold more efficiently by ECE-1, making this substrate selective for ECE-1. Furthermore, this synthetic peptide is a poor substrate for the matrix metalloproteinases.

The endothelin system in normal human colon

Endothelin (ET)-1 is a potent vasoconstrictor and mitogenic peptide that has a variety of biological effects in noncardiovascular tissues. The precise cellular distribution of the ET-1 system in the wall of the normal human colon was studied to identify the physiological role of ET in the gut. In situ hybridization revealed ET-converting enzyme-1 (ECE-1) mRNA in all vessels, the colon epithelium, and macrophages. Prepro-ET-1 (PPET-1) mRNA had a similar distribution except for a scattered signal in mucosal microvessels. ET(A) and ET(B) receptor mRNAs were mainly in the lamina propria, pericryptal myofibroblasts, microvessels, and mononuclear cells, with ET(A) mRNA more abundant than ET(B) mRNA. (125)I-ET-1 binding showed ET(B) along the crypts and in nerve fibers descending from the ganglionic plexus that contained PPET-1, ECE-1, and ET(B) transcripts, whereas glia contained ET(A) receptors. The finding of the entire ET system in the normal mucosa suggests its implication in some characteristic functions of the colon and its secretion as both a neuroactive and a vasoactive peptide.

Molecular cloning and biochemical characterization of a new mouse testis soluble-zinc-metallopeptidase of the neprilysin family

Because of their roles in controlling the activity of several bio-active peptides, members of the neprilysin family of zinc metallopeptidases have been identified as putative targets for the design of therapeutic agents. Presently, six members have been reported, these are: neprilysin, endothelin-converting enzyme (ECE)-1 and ECE-2, the Kell blood group protein, PHEX (product of the phosphate-regulating gene with homologies to endopeptidase on the X chromosome) and X-converting enzyme (XCE). In order to identify new members of this important family of peptidases, we designed a reverse transcriptase-PCR strategy based on conserved amino acid sequences of neprilysin, ECE-1 and PHEX. We now report the cloning from mouse testis of a novel neprilysin-like peptidase that we called NL1. NL1 is a glycoprotein that, among the members of the family, shows the strongest sequence identity with neprilysin. However, in contrast with neprilysin and other members of the family which are type II integral membrane proteins, NL1 was secreted when expressed in cultured mammalian cells, likely due to cleavage by a subtilisin-like convertase at a furin-like site located 22 amino acid residues in the C-terminus of the transmembrane domain. The recombinant enzyme exhibited neprilysin-like peptidase activity and was efficiently inhibited by phosphoramidon and thiorphan, two inhibitors of neprilysin. Northern blot analysis and in situ hybridization showed that NL1 mRNA was found predominantly in testis, specifically in round and elongated spermatids. This distribution of NL1 mRNA suggests that it could be involved in sperm formation or other processes related to fertility.

Damage-induced neuronal endopeptidase (DINE) is a unique metallopeptidase expressed in response to neuronal damage and activates superoxide scavengers

We isolated a membrane-bound metallopeptidase, DINE (damage-induced neuronal endopeptidase), by differential display PCR using rat normal and axotomized hypoglossal nuclei. The most marked properties of DINE were neuron-specific expression and a striking response to axonal injury in both the central nervous system and peripheral nervous system. For instance, cranial and spinal nerve transection, ischemia, corpus callosum transection, and colchicine treatment increased DINE mRNA expression in the injured neurons, whereas kainate-induced hyperexcitation, immobilization, and osmotic stress failed to up-regulate DINE mRNA. Expression of DINE in COS cells partially inhibited C2-ceramide-induced apoptosis, probably because of the activation of antioxidant enzymes such as Cu/Zn-superoxide dismutase, Mn-superoxide dismutase, and glutathione peroxidase through the proteolytic activity of DINE. These data provide insight into the mechanism of how injured neurons protect themselves against neuronal death.

MALDI-TOF mass spectrometry analysis of substrate specificity of lebetase, a direct-acting fibrinolytic metalloproteinase from Vipera lebetina snake venom

Lebetase is a direct-acting fibrinolytic zinc metalloendopeptidase related in amino acid sequence to reprolysins which include both hemorrhagic and non-hemorrhagic proteinases. Despite apparent structural similarities, fibrinolytic and hemorrhagic proteinases differ significantly in substrate specificity. In this study, we have examined the activity of lebetase I against biologically active peptides (bradykinin, kallidin, substance P) and 6-10 amino acid residues containing peptides synthesized according to cleavage regions of alpha(2)-macroglobulin, pregnancy zone protein (PZP) and fibrinogen. Lebetase was found to have no activity against studied hexapeptides. Surprisingly, the best substrates for lebetase were substance P, and peptide fragment of PZP, both were cleaved at position Pro-Gln. Identification of the hydrolysis products of 15 peptides by MALDI-TOF mass spectrometry analysis indicates that lebetase possesses broad substrate specificity. The MALDI-TOF MS technique was proven to be highly efficient for the recovery and identification of the peptides released by lebetase hydrolysis.

Endothelial dysfunction and atherosclerosis: endothelin receptor antagonists as novel therapeutics

Atherosclerosis, a chronic systemic disease of the vasculature with an inflammatory component, is the primary cause of cardiovascular morbidity and mortality in industrialized countries. It is associated with the impairment of endothelium-dependent relaxation in the coronary, systemic circulation due to decreased bioavailability of nitric oxide, and increased release oxygen-derived free radicals, thus promoting vasoconstriction, leukocyte adhesion, thrombosis, inflammation, and cell proliferation. Expression of endothelin (ET)-1, a 21-amino acid peptide and major isoform of the endothelin peptide family, is produced by endothelial, vascular smooth muscle cells, and macrophages and acts through Gi-protein-coupled ET(A) and ET(B) receptors. Endothelin-1 increases in hypercholesterolemia and atherosclerosis in humans and experimental animals. This paper reviews current experimental and clinical evidence for the involvement of ET-1 in atherogenesis. Furthermore, the effects of ET receptor blockade on experimental hypercholesterolemia and atherosclerosis will be discussed. As chronic endothelin blockade inhibits fatty streak formation and improves vascular function in experimental hypercholesterolemia, hypertension, and heart failure, and as it restores nitric oxide (NO)-mediated endothelial function and reduces atheroma formation in animals with atherosclerosis, endothelin receptor blockade may therefore offer a novel approach for the treatment of atherosclerosis and its vascular complications.