Inhibitors of neprilysin: design, pharmacological and clinical applications

Neprilysin impedes islet amyloid formation by inhibition of fibril formation rather than peptide degradation

Deposition of islet amyloid polypeptide (IAPP) as islet amyloid in type 2 diabetes contributes to loss of beta-cell function and mass, yet the mechanism for its occurrence is unclear. Neprilysin is a metallopeptidase known to degrade amyloid in Alzheimer disease. We previously demonstrated neprilysin to be present in pancreatic islets and now sought to determine whether it plays a role in degrading islet amyloid. We used an in vitro model where cultured human IAPP (hIAPP) transgenic mouse islets develop amyloid and thereby have increased beta-cell apoptosis. Islet neprilysin activity was inhibited or up-regulated using a specific inhibitor or adenovirus encoding neprilysin, respectively. Following neprilysin inhibition, islet amyloid deposition and beta-cell apoptosis increased by 54 and 75%, respectively, whereas when neprilysin was up-regulated islet amyloid deposition and beta-cell apoptosis both decreased by 79%. To determine if neprilysin modulated amyloid deposition by cleaving hIAPP, analysis of hIAPP incubated with neprilysin was performed by mass spectrometry, which failed to demonstrate neprilysin-induced cleavage. Rather, neprilysin may act by reducing hIAPP fibrillogenesis, which we showed to be the case by fluorescence-based thioflavin T binding studies and electron microscopy. In summary, neprilysin decreases islet amyloid deposition by inhibiting hIAPP fibril formation, rather than degrading hIAPP. These findings suggest that targeting the role of neprilysin in IAPP fibril assembly, in addition to IAPP cleavage by other peptidases, may provide a novel approach to reduce and/or prevent islet amyloid deposition in type 2 diabetes.

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

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

Neutral Endopeptidase Up-Regulation in Isolated Human Umbilical Artery: Involvement in Desensitization of Bradykinin-Induced Vasoconstrictor Effects

Previous reports show that bradykinin B(2) receptors mediate contractile responses induced by bradykinin (BK) in human umbilical artery (HUA). However, although it has been reported that BK-induced responses can desensitize in several inflammatory models, the effects of prolonged in vitro incubation on BK-induced vasoconstriction in HUA have not been studied. In isolated HUA rings, BK-induced responses after a 5-h in vitro incubation showed a marked desensitization compared with responses at 2 h. Inhibition of either angiotensin-converting enzyme (ACE) or neutral endopeptidase (NEP), both BK-inactivating enzymes, failed to modify responses to BK at 2 h. After 5 h, ACE inhibition produced only a slight potentiation of BK-induced responses. In contrast, BK-induced vasoconstriction at 5 h was markedly potentiated by NEP inhibition. Moreover, NEP activity, measured by hydrolysis of its synthetic substrate (Z-Ala-Ala-Leu-p-nitroanilide), showed a 2.4-fold increase in 5-h incubated versus 2-h incubated tissues, which was completely reversed by cycloheximide (CHX) treatment. Furthermore, CHX significantly potentiated BK-induced responses, suggesting that NEP-mediated kininase activity increase at 5 h depends on de novo protein synthesis. In addition, under NEP inhibition, CHX treatment failed to produce an additional potentiation of BK-induced vasoconstriction. Still, NEP up-regulation was confirmed by Western blot, showing a 2.1-fold increase in immunoreactive NEP in 5-h incubated versus 2-h incubated HUA. In summary, the present study provides strong pharmacological evidence that NEP is up-regulated and plays a key role in desensitization of BK-induced vasoconstriction after prolonged in vitro incubation in HUA. Our results provide new insights into the possible mechanisms involved in BK-induced response desensitization during sustained inflammatory conditions.

Aminopeptidase-N/CD13 (EC 3.4.11.2) inhibitors: chemistry, biological evaluations, and therapeutic prospects

Aminopeptidase N (APN)/CD13 (EC 3.4.11.2) is a transmembrane protease present in a wide variety of human tissues and cell types (endothelial, epithelial, fibroblast, leukocyte). APN/CD13 expression is dysregulated in inflammatory diseases and in cancers (solid and hematologic tumors). APN/CD13 serves as a receptor for coronaviruses. Natural and synthetic inhibitors of APN activity have been characterized. These inhibitors have revealed that APN is able to modulate bioactive peptide responses (pain management, vasopressin release) and to influence immune functions and major biological events (cell proliferation, secretion, invasion, angiogenesis). Therefore, inhibition of APN/CD13 may lead to the development of anti-cancer and anti-inflammatory drugs. This review provides an update on the biological and pharmacological profiles of known natural and synthetic APN inhibitors. Current status on their potential use as therapeutic agents is discussed with regard to toxicity and specificity.

Human neprilysin is capable of degrading amyloid beta peptide not only in the monomeric form but also the pathological oligomeric form

Amyloid beta-peptide (Abeta) is widely believed to play a central role in Alzheimer's disease (AD). Coordinate regulation of cerebral Abeta level is important in the pathogenesis of AD since either increased production of Abeta from amyloid precursor protein or decreased degradation causes elevated levels of Abeta, leading to accumulation of cerebral plaque formation or amyloid angiopathy. Here we studied neprilysin, a putative proteolytic enzyme for Abeta, and found that it degraded not only monomeric but also oligomeric forms of Abeta1-40. Moreover, neprilysin was found to be capable of degradation of the oligomeric form of Abeta1-42, a significant Abeta species in early pathogenesis. Neprilysin to decrease cerebral Abeta is suggested to be inevitable factor as a vital therapeutic target.

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.

M13 endopeptidases: New conserved motifs correlated with structure, and simultaneous phylogenetic occurrence of PHEX and the bony fish

M13 endopeptidase alignments have focused mainly on mammalian sequences and on the active site region defining the catalytic sequence signatures. Aligning all available M13 from bacteria to human on a full-length basis, we have performed a sequence analysis. This enabled us to highlight the origin and function of the M13 PHEX subtype family endopeptidase (phosphate regulating gene with homologies to endopeptidases on the X chromosome). New evolutionary conserved regions in both prokaryotes and eukaryotes have been detected and eukaryotic-specific regions clearly delineated. Using the recently solved neprilysin structure, we have observed that all new motifs, except one, localize in the spatial vicinity of the previously reported catalytic signatures. Interestingly, a highly hydrophobic pocket containing three newly reported motifs is centered by the C-terminal tryptophan residue. Extensive M13 searches in complete and in progress higher eukaryotic genomes have lead to the identification of Danio rerio as the simplest organism having PHEX. Finally, the human PHEX substrate, the parathyroid hormone-related peptide, PTHrP(107-139), is absent in bony fish: this suggests the existence of further PHEX substrates common to both bony fishes and higher vertebrates.

Expression of neuropeptide-degrading enzymes in alopecia areata: an immunohistochemical study

BACKGROUND

Much clinical evidence suggests that the nervous system, including psychological factors, can influence the course of alopecia areata (AA). However, there has been little substantial evidence of specific participation of cutaneous neurogenic factors in the disease process.

OBJECTIVES

As previous studies have demonstrated that stress elicits the release of the neuropeptide substance P (SP) from peripheral nerves and that some patients with AA show prominent SP expression in nerves surrounding their hair follicles, we aimed to evaluate the role of SP in AA.

METHODS

We used immunohistochemistry to examine the expression of SP and SP-degrading enzymes in scalp biopsies from patients with AA and from healthy controls.

RESULTS

Affected hair follicles in the centre of the areas of hair loss of patients with AA were richly innervated by SP-staining nerve fibres. Strong expression of the SP-degrading enzyme, neutral endopeptidase (NEP), was observed in hair follicles not only in the acute progressive phase of AA but also in the chronic stable phase. Expression of NEP in hair follicles from the margins of areas of hair loss was stronger than in normal controls, but was weaker than in the centre of the areas of hair loss. In addition, endothelial immunoreactivity for angiotensin-converting enzyme (also capable of degrading SP) was not observed in the centre of the areas of hair loss, which was in significant contrast to normal controls as well as to the margins of areas of hair loss where it was expressed. Further, intense expression of endothelial leucocyte adhesion molecule-1 on vessels and many degranulating mast cells was observed adjacent to affected hair follicles in AA, in admixture with dense lymphocytic inflammation.

CONCLUSIONS

These findings suggest that SP is endogenously released by dermal nerve fibres around hair follicles and that it may play an important part in epithelial-mesenchymal-neuroectodermal interactions in AA. This study reveals that SP and its degrading enzymes are involved in the pathogenesis of AA, which in turn might explain the pathological significance of neurogenic and psychogenic aspects in the disease process.

Pathways for angiotensin-(1---7) metabolism in pulmonary and renal tissues

Two of the primary sites of actions for angiotensin (ANG)-(1---7) are the vasculature and the kidney. Because little information exists concerning the metabolism of ANG-(1---7) in these tissues, we investigated the hydrolysis of the peptide in rat lung and renal brush-border membrane (BBM) preparations. Radiolabeled ANG-(1---7) was hydrolyzed primarily to ANG-(1---5) by pulmonary membranes. The ANG-converting enzyme (ACE) inhibitor lisinopril abolished the generation of ANG-(1---5), as well as that of smaller metabolites. Kinetic studies of the hydrolysis of ANG-(1---7) to ANG-(1---5) by somatic (pulmonary) and germinal (testes) forms of rat ACE yielded similar values, suggesting that the COOH-domain is responsible for the hydrolysis of ANG-(1---7). Pulmonary metabolism of ANG-(1---5) yielded ANG-(3---5) and was independent of ACE but may involve peptidyl or dipeptidyl aminopeptidases. In renal cortex BBM, ANG-(1---7) was rapidly hydrolyzed to mono- and dipeptide fragments and ANG-(1---4). Aminopeptidase (AP) inhibition attenuated the hydrolysis of ANG-(1---7) and increased ANG-(1---4) formation. Combined treatment with AP and neprilysin (Nep) inhibitors abolished ANG-(1---4) formation and preserved ANG-(1---7). ACE inhibition had no effect on the rate of hydrolysis or the metabolites formed in the BBM. In conclusion, ACE was the major enzymatic activity responsible for the metabolism of ANG-(1---7) in the lung, which is consistent with the ability of ACE inhibitors to increase the half-life of circulating ANG-(1---7) and raise endogenous levels of the peptide. An alternate pathway of metabolism was revealed in the renal cortex, where increased AP and Nep activities, relative to ACE activity, promote conversion of ANG-(1---7) to ANG-(1---4) and smaller fragments.

Identification of the major Abeta1-42-degrading catabolic pathway in brain parenchyma: suppression leads to biochemical and pathological deposition

Alzheimer amyloid beta-peptide (Abeta) is a physiological peptide constantly anabolized and catabolized under normal conditions. We investigated the mechanism of catabolism by tracing multiple-radiolabeled synthetic peptide injected into rat hippocampus. The Abeta1-42 peptide underwent full degradation through limited proteolysis conducted by neutral endopeptidase (NEP) similar or identical to neprilysin as biochemically analyzed. Consistently, NEP inhibitor infusion resulted in both biochemical and pathological deposition of endogenous Abeta42 in brain. This NEP-catalyzed proteolysis therefore limits the rate of Abeta42 catabolism, up-regulation of which could reduce the risk of developing Alzheimer's disease by preventing Abeta accumulation.

Enkephalin-processing oligopeptidases in cobra venom: inhibition by thiorphan and bestatin reveals co-operative actions

The peptidase inhibitors thiorphan and bestatin were tested for their ability to inhibit the actions of the oligopeptidases contained in the venom of the Taiwan cobra (Naja naja atra). With methionine enkephalin (TyrGlyGlyPheMet) as substrate, thiorphan was an effective inhibitor of cleavage of the GlyPhe peptide bond while bestatin inhibited cleavage of the TyrGly peptide bond. Thiorphan and bestatin also inhibited subsequent cleavage of the fragments GlyGlyPheMet and TyrGlyGly respectively. These inhibitors reveal an interplay between the venom oligopeptidases in which the enzymes provide additional substrates for each other following their initial competitive attack on the neuropeptide. A possible explanation is that the system is intended to ensure a steady release of Tyr, GlyGly and PheMet over time. Significantly, Tyr is the favoured substrate of the L-amino acid oxidase present in the venom, which rapidly transforms this aromatic amino acid into phenolic derivatives. The efficacies of these inhibitors also suggest that there are similarities between the venom oligopeptidases and the peptidases associated with the processing of enkephalin in its normal contexts.

Neuropeptides in the skin: interactions between the neuroendocrine and the skin immune systems

The interaction between components of the nervous system and multiple target cells in the cutaneous immune system has been receiving increasing attention. It has been observed that certain skin diseases such as psoriasis and atopic dermatitis have a neurogenic component. Neuropeptides released by sensory nerves that innervate the skin and often contact epidermal and dermal cells can directly modulate functions of keratinocytes, Langerhans cells (LC), mast cells, dermal microvascular endothelial cells and infiltrating immune cells. Among these neuropeptides the tachykinins substance P (SP) and neurokinin A (NKA), calcitonin gene-related peptide (CGRP), vasoactive intestinal peptide (VIP) and somatostatin (SOM) have been reported to effectively modulate skin and immune cell functions such as cell proliferation, cytokine production or antigen presentation under physiological or pathophysiological conditions. Expression and regulation of their corresponding receptors that are expressed on a variety of skin cells as well as the presence of neuropeptide-specific peptidases such as neutral endopeptidase (NEP) or angiotensin-converting enzyme (ACE) determine the final biological response mediated by these peptides on the target cell or tissue. Likewise, skin cells like keratinocytes or fibroblasts are a source for neurotrophins such as nerve growth factor that are required not only for survival and regeneration of sensory neurons but also to control responsiveness of these neurons to external stimuli. Therefore, neuropeptides, neuropeptide receptors, neuropeptide-degrading enzymes and neurotrophins participate in a complex, interdependent network of mediators that modulate skin inflammation, wound healing and the skin immune system. This review will focus on recent studies demonstrating the role of tachykinins, CGRP, SOM and VIP and their receptors and neuropeptide-degrading enzymes in mediating neurogenic inflammation in the skin.

Angiotensin-(1-7) contributes to the antihypertensive effects of blockade of the renin-angiotensin system

Angiotensin-converting enzyme (ACE) inhibition alone or in combination with the angiotensin type-I receptor (AT1) antagonist losartan augments circulating levels of the bioactive peptide angiotensin-(1-7) [Ang-(1-7)]. Hence, we determined whether Ang-(1-7) contributes to the hypotensive effects produced by the combined administration of lisinopril and losartan in spontaneously hypertensive rats by blocking the peptide's synthesis with either of two structurally different neprilysin inhibitors. Intravenous administration of CGS 24592 (30 mg/kg) to rats in which blood pressure was normalized by 9 days of therapy with lisinopril and losartan elicited an elevation of mean arterial pressure that was sustained throughout the infusion period and for 20 minutes thereafter. The hypertensive response was associated with a 62% reduction in circulating levels of Ang-(1-7) and no change in plasma angiotensin II (Ang II). Intravenous infusion of one other neprilysin inhibitor (SCH 39370, 30 mg/kg) produced an increase in mean blood pressure of a magnitude similar to that found with CGS 24592. Pretreatment with the nonselective antagonist [Sar1,Thr8]-Ang II abolished any additional pressor effects of either neprilysin inhibitor in spontaneously hypertensive rats treated with lisinopril or losartan. However, neither the endothelin A antagonist BQ123 nor the kinin B2 antagonist HOE 140 had an effect on basal blood pressure or altered the pressor or heart rate effects of the neprilysin inhibitors. These data suggest that inhibition of Ang-(1-7) formation in rats exposed to the combined blockade of Ang II production and activity is associated with a reversal of the antihypertensive actions produced by these therapies. Thus, endogenous Ang-(1-7) functions as a vasodilator hormone in this form of genetic hypertension.

Prospects for rational approaches to anthelmintic discovery

Rational approaches to anthelmintic discovery include the design of screens for compounds directed at specific proteins in helminths that are pharmacologically distinguishable from their vertebrate homologues. The existence of several anthelmintics that selectively target the neuromusculature of helminths (e.g. levamisole, ivermectin, praziquantel, metrifonate), together with recent basic research in helminth physiology, have contributed to the recognition that neurobiology distinguishes these organisms from their vertebrate hosts. In this survey, we focus on mechanism-based screening and its application to anthelmintic discovery, with particular emphasis on targets in the neuromusculature of helminths. Few of these proteins have been exploited in chemotherapy. However, recent studies in comparative pharmacology and molecular biology, including the C. elegans genome project, have provided insights on potential new targets and, in some cases, molecular probes useful for their incorporation in mechanism-based screens.