Molecular cloning and expression of rat brain endopeptidase 3.4.24.16

Structural basis of divergent substrate recognition and inhibition of human neurolysin

A zinc metallopeptidase neurolysin (Nln) processes diverse bioactive peptides to regulate signaling in the mammalian nervous system. To understand how Nln interacts with various peptides with dissimilar sequences, we determined crystal structures of Nln in complex with diverse peptides including dynorphins, angiotensin, neurotensin, and bradykinin. The structures show that Nln binds these peptides in a large dumbbell-shaped interior cavity constricted at the active site, making minimal structural changes to accommodate different peptide sequences. The structures also show that Nln readily binds similar peptides with distinct registers, which can determine whether the peptide serves as a substrate or a competitive inhibitor. We analyzed the activities and binding of Nln toward various forms of dynorphin A peptides, which highlights the promiscuous nature of peptide binding and shows how dynorphin A (1-13) potently inhibits the Nln activity while dynorphin A (1-8) is efficiently cleaved. Our work provides insights into the broad substrate specificity of Nln and may aid in the future design of small molecule modulators for Nln.

Neurolysin Knockout Mice in a Diet-Induced Obesity Model

Neurolysin oligopeptidase (E.C.3.4.24.16; Nln), a member of the zinc metallopeptidase M3 family, was first identified in rat brain synaptic membranes hydrolyzing neurotensin at the Pro-Tyr peptide bond. The previous development of C57BL6/N mice with suppression of Nln gene expression (Nln-/-), demonstrated the biological relevance of this oligopeptidase for insulin signaling and glucose uptake. Here, several metabolic parameters were investigated in Nln-/- and wild-type C57BL6/N animals (WT; n = 5-8), male and female, fed either a standard (SD) or a hypercaloric diet (HD), for seven weeks. Higher food intake and body mass gain was observed for Nln-/- animals fed HD, compared to both male and female WT control animals fed HD. Leptin gene expression was higher in Nln-/- male and female animals fed HD, compared to WT controls. Both WT and Nln-/- females fed HD showed similar gene expression increase of dipeptidyl peptidase 4 (DPP4), a peptidase related to glucagon-like peptide-1 (GLP-1) metabolism. The present data suggest that Nln participates in the physiological mechanisms related to diet-induced obesity. Further studies will be necessary to better understand the molecular mechanism responsible for the higher body mass gain observed in Nln-/- animals fed HD.

Identification and Characterization of Two Structurally Related Dipeptides that Enhance Catalytic Efficiency of Neurolysin

Neurolysin (Nln) is a recently recognized endogenous mechanism functioning to preserve the brain from ischemic injury. To further understand the pathophysiological function of this peptidase in stroke and other neurologic disorders, the present study was designed to identify small molecule activators of Nln. Using a computational approach, the structure of Nln was explored, which was followed by docking and in silico screening of ∼140,000 molecules from the National Cancer Institute Developmental Therapeutics Program database. Top ranking compounds were evaluated in an Nln enzymatic assay, and two hit histidine-dipeptides were further studied in detail. The identified dipeptides enhanced the rate of synthetic substrate hydrolysis by recombinant (human and rat) and mouse brain-purified Nln in a concentration-dependent manner (micromolar A50 and Amax ≥ 300%) but had negligible effect on activity of closely related peptidases. Both dipeptides also enhanced hydrolysis of Nln endogenous substrates neurotensin, angiotensin I, and bradykinin and increased efficiency of the synthetic substrate hydrolysis (Vmax/Km ratio) in a concentration-dependent manner. The dipeptides and competitive inhibitor dynorphin A (1-13) did not affect each other's affinity for Nln, suggesting differing nature of their respective binding sites. Lastly, drug affinity responsive target stability (DARTS) and differential scanning fluorimetry (DSF) assays confirmed concentration-dependent interaction of Nln with the activator molecule. This is the first study demonstrating that Nln activity can be enhanced by small molecules, although the peptidic nature and low potency of the activators limit their application. The identified dipeptides provide a chemical scaffold to develop high-potency, drug-like molecules as research tools and potential drug leads. SIGNIFICANCE STATEMENT: This study describes discovery of two molecules that selectively enhance activity of peptidase Nln-a newly recognized cerebroprotective mechanism in the poststroke brain. The identified molecules will serve as a chemical scaffold for development of drug-like molecules to further study Nln and may become lead structures for a new class of drugs. In addition, our conceptual and methodological framework and research findings might be used for other peptidases and enzymes, the activation of which bears therapeutic potential.

Between two storms, vasoactive peptides or bradykinin underlie severity of COVID-19?

Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), continues to be a world-wide pandemic with overwhelming socioeconomic impact. Since inflammation is one of the major causes of COVID-19 complications, the associated molecular mechanisms have been the focus of many studies to better understand this disease and develop improved treatments for patients contracting SARS-CoV-2. Among these, strong emphasis has been placed on pro-inflammatory cytokines, associating severity of COVID-19 with so-called "cytokine storm." More recently, peptide bradykinin, its dysregulated signaling or "bradykinin storm," has emerged as a primary mechanism to explain COVID-19-related complications. Unfortunately, this important development may not fully capture the main molecular players that underlie the disease severity. To this end, in this focused review, several lines of evidence are provided to suggest that in addition to bradykinin, two closely related vasoactive peptides, substance P and neurotensin, are also likely to drive microvascular permeability and inflammation, and be responsible for development of COVID-19 pathology. Furthermore, based on published experimental observations, it is postulated that in addition to ACE and neprilysin, peptidase neurolysin (Nln) is also likely to contribute to accumulation of bradykinin, substance P and neurotensin, and progression of the disease. In conclusion, it is proposed that "vasoactive peptide storm" may underlie severity of COVID-19 and that simultaneous inhibition of all three peptidergic systems could be therapeutically more advantageous rather than modulation of any single mechanism alone.

Increased Stability of Oligopeptidases Immobilized on Gold Nanoparticles

The metallopeptidases thimet oligopeptidase (THOP, EC 3.4.24.25) and neurolysin (NEL, EC 3.4.24.26) are enzymes that belong to the zinc endopeptidase M13 family. Numerous studies suggest that these peptidases participate in the processing of bioactive peptides such as angiotensins and bradykinin. Efforts have been conducted to develop biotechnological tools to make possible the use of both proteases to regulate blood pressure in mice, mainly limited by the low plasmatic stability of the enzymes. In the present study, it was investigated the use of nanotechnology as an efficient strategy for to circumvent the low stability of the proteases. Recombinant THOP and NEL were immobilized in gold nanoparticles (GNPs) synthesized in situ using HEPES and the enzymes as reducing and stabilizing agents. The formation of rTHOP-GNP and rNEL-GNP was characterized by the surface plasmon resonance band, zeta potential and atomic force microscopy. The gain of structural stability and activity of rTHOP and rNEL immobilized on GNPs was demonstrated by assays using fluorogenic substrates. The enzymes were also efficiently immobilized on GNPs fabricated with sodium borohydride. The efficient immobilization of the oligopeptidases in gold nanoparticles with gain of stability may facilitate the use of the enzymes in therapies related to pressure regulation and stroke, and as a tool for studying the physiological and pathological roles of both proteases.

Preparation and preliminary characterization of recombinant neurolysin for in vivo studies

The goal of this study was to produce milligram quantities of pure, catalytically active, endotoxin-free recombinant neurolysin (rNln) in standard laboratory conditions for use as a research tool. To this end, we transformed E. coli cells with a plasmid construct for polyhistidine-tagged rNln, selected a high-expressing clone and determined the optimal time-point for translation of rNln. rNln was purified to homogeneity from the soluble pool of the cell lysate using Ni-NTA affinity and size-exclusion chromatography, followed by removal of endotoxins. Using this protocol ∼3mg pure, catalytically active and nearly endotoxin-free (≈0.003EU/μg protein) rNln was reproducibly obtained from 1l of culture. Lack of cytotoxicity of rNln preparation was documented in cultured mouse cells, whereas stability in whole mouse blood. Intraperitonealy administered rNln in mice reached the systemic circulation in intact and enzymatically active form with Tmax of 1h and T1/2 of ∼30min. Administration of rNln (2 and 10mg/kg) did not alter arterial blood pressure, heart rate, body temperature and blood glucose levels in mice. These studies demonstrate that the rNln preparation is suitable for cell culture and in vivo studies and can serve as a research tool to investigate the (patho)physiological function of this peptidase.

Recent insights and therapeutic perspectives of angiotensin-(1-9) in the cardiovascular system

Chronic RAS (renin-angiotensin system) activation by both AngII (angiotensin II) and aldosterone leads to hypertension and perpetuates a cascade of pro-hypertrophic, pro-inflammatory, pro-thrombotic and atherogenic effects associated with cardiovascular damage. In 2000, a new pathway consisting of ACE2 (angiotensin-converting enzyme2), Ang-(1-9) [angiotensin-(1-9)], Ang-(1-7) [angiotensin-(1-7)] and the Mas receptor was discovered. Activation of this novel pathway stimulates vasodilation, anti-hypertrophy and anti-hyperplasia. For some time, studies have focused mainly on ACE2, Ang-(1-7) and the Mas receptor, and their biological properties that counterbalance the ACE/AngII/AT1R (angiotensin type 1 receptor) axis. No previous information about Ang-(1-9) suggested that this peptide had biological properties. However, recent data suggest that Ang-(1-9) protects the heart and blood vessels (and possibly the kidney) from adverse cardiovascular remodelling in patients with hypertension and/or heart failure. These beneficial effects are not modified by the Mas receptor antagonist A779 [an Ang-(1-7) receptor blocker], but they are abolished by the AT2R (angiotensin type 2 receptor) antagonist PD123319. Current information suggests that the beneficial effects of Ang-(1-9) are mediated via the AT2R. In the present review, we summarize the biological effects of the novel vasoactive peptide Ang-(1-9), providing new evidence of its cardiovascular-protective activity. We also discuss the potential mechanism by which this peptide prevents and ameliorates the cardiovascular damage induced by RAS activation.

Peptidomic analysis of the neurolysin-knockout mouse brain

A large number of intracellular peptides are constantly produced following protein degradation by the proteasome. A few of these peptides function in cell signaling and regulate protein-protein interactions. Neurolysin (Nln) is a structurally defined and biochemically well-characterized endooligopeptidase, and its subcellular distribution and biological activity in the vertebrate brain have been previously investigated. However, the contribution of Nln to peptide metabolism in vivo is poorly understood. In this study, we used quantitative mass spectrometry to investigate the brain peptidome of Nln-knockout mice. An additional in vitro digestion assay with recombinant Nln was also performed to confirm the identification of the substrates and/or products of Nln. Altogether, the data presented suggest that Nln is a key enzyme in the in vivo degradation of only a few peptides derived from proenkephalin, such as Met-enkephalin and octapeptide. Nln was found to have only a minor contribution to the intracellular peptide metabolism in the entire mouse brain. However, further studies appear necessary to investigate the contribution of Nln to the peptide metabolism in specific areas of the murine brain.

BIOLOGICAL SIGNIFICANCE

Neurolysin was first identified in the synaptic membranes of the rat brain in the middle 80's by Frederic Checler and colleagues. Neurolysin was well characterized biochemically, and its brain distribution has been confirmed by immunohistochemical methods. The neurolysin contribution to the central and peripheral neurotensin-mediated functions in vivo has been delineated through inhibitor-based pharmacological approaches, but its genuine contribution to the physiological inactivation of neuropeptides remains to be firmly established. As a result, the main significance of this work is the first characterization of the brain peptidome of the neurolysin-knockout mouse. This article is part of a Special Issue entitled: Proteomics, mass spectrometry and peptidomics, Cancun 2013. Guest Editors: César López-Camarillo, Victoria Pando-Robles and Bronwyn Jane Barkla.

Neurolysin knockout mice generation and initial phenotype characterization

The oligopeptidase neurolysin (EC 3.4.24.16; Nln) was first identified in rat brain synaptic membranes and shown to ubiquitously participate in the catabolism of bioactive peptides such as neurotensin and bradykinin. Recently, it was suggested that Nln reduction could improve insulin sensitivity. Here, we have shown that Nln KO mice have increased glucose tolerance, insulin sensitivity, and gluconeogenesis. KO mice have increased liver mRNA for several genes related to gluconeogenesis. Isotopic label semiquantitative peptidomic analysis suggests an increase in specific intracellular peptides in gastrocnemius and epididymal adipose tissue, which likely is involved with the increased glucose tolerance and insulin sensitivity in the KO mice. These results suggest the exciting new possibility that Nln is a key enzyme for energy metabolism and could be a novel therapeutic target to improve glucose uptake and insulin sensitivity.

Functional up-regulation of endopeptidase neurolysin during post-acute and early recovery phases of experimental stroke in mouse brain

In this study, we provide evidence for the first time that membrane-bound endopeptidase neurolysin is up-regulated in different parts of mouse brain affected by focal ischemia-reperfusion in a middle cerebral artery occlusion model of stroke. Radioligand binding, enzymatic and immunoblotting experiments in membrane preparations of frontoparietal cortex, striatum, and hippocampus isolated from the ischemic hemisphere of mouse brain 24 h after reperfusion revealed statistically significant increase (≥ twofold) in quantity and activity of neurolysin compared with sham-operated controls. Cerebellar membranes isolated from the ischemic hemisphere served as negative control supporting the observations that up-regulation of neurolysin occurs in post-ischemic brain regions. This study also documents sustained functional up-regulation of neurolysin in frontoparietal cortical membranes for at least 7 days after stroke, which appears not to be transcriptionally or translationally regulated, but rather depends on translocation of cytosolic neurolysin to the membranes and mitochondria. Considering diversity of endogenous neurolysin substrates (neurotensin, bradykinin, angiotensins I/II, substance P, hemopressin, dynorphin A(1-8), metorphamide, somatostatin) and the well-documented role of these peptidergic systems in pathogenesis of stroke, resistance to ischemic injury and/or post-stroke brain recovery, our findings suggest that neurolysin may play a role in processes modulating the brain's response to stroke and its recovery after stroke.

The effects of para-chloromercuribenzoic acid and different oxidative and sulfhydryl agents on a novel, non-AT1, non-AT2 angiotensin binding site identified as neurolysin

A novel, non-AT1, non-AT2 brain binding site for angiotensin peptides that is unmasked by p-chloromercuribenzoate (PCMB) has been identified as a membrane associated variant of neurolysin. The ability of different organic and inorganic oxidative and sulfhydryl reactive agents to unmask or inhibit 125I-Sar1Ile8 angiotensin II (SI-Ang II) binding to this site was presently examined. In tissue membranes from homogenates of rat brain and testis incubated in assay buffer containing losartan (10 μM) and PD123319 (10 μM) plus 100 μM PCMB, 5 of the 39 compounds tested inhibited 125I-SI Ang II binding in brain and testis. Mersalyl acid, mercuric chloride (HgCl2) and silver nitrate (AgNO3) most potently inhibited 125I-SI Ang II binding with IC50s ~1-20 μM. This HgCl2 inhibition was independent of any interaction of HgCl2 with angiotensin II (Ang II) based on the lack of effect of HgCl2 on the dipsogenic effects of intracerebroventricularly administered Ang II and 125I-SI Ang II binding to AT1 receptors in the liver. Among sulfhydryl reagents, cysteamine and reduced glutathione (GSH), but not oxidized glutathione (GSSG) up to 1mM, inhibited PCMB-unmasked 125I-SI Ang II binding in brain and testis. Thimerosal and 4-hydroxymercuribenzoate moderately inhibited PCMB-unmasked 125I-SI Ang II binding in brain and testis at 100 μM; however, they also unmasked non-AT1, non-AT2 binding independent of PCMB. 4-Hydroxybenzoic acid did not promote 125 I-SI Ang II binding to this binding site indicating that only specific organomercurial compounds can unmask the binding site. The common denominator for all of these interacting substances is the ability to bind to protein cysteine sulfur. Comparison of cysteines between neurolysin and the closely related enzyme thimet oligopeptidase revealed an unconserved cysteine (cys650, based on the full length variant) in the proposed ligand binding channel (Brown et al., 2001) [45] near the active site of neurolysin. It is proposed that the mercuric ion in PCMB and closely related organomercurial compounds binds to cys650, while the acidic anion forms an ionic bond with a nearby arginine or lysine along the channel to effect a conformational change in neurolysin that promotes Ang II binding.

Acute cocaine treatment increases thimet oligopeptidase in the striatum of rat brain

Many studies indicate that thimet oligopeptidase (EC3.4.24.15; TOP) can be implicated in the metabolism of bioactive peptides, including dynorphin 1-8, α-neoendorphin, β-neoendorphin and GnRH. Furthermore, the higher levels of this peptidase are found in neuroendocrine tissue and testis. In the present study, we have evaluated the effect of acute cocaine administration in male rats on TOP specific activity and mRNA levels in prosencephalic brain areas related with the reward circuitry; ventral striatum, hippocampus, and frontal cortex. No significant differences on TOP specific activity were detected in the hippocampus and frontal cortex of cocaine treated animals compared to control vehicle group. However, a significant increase in activity was observed in the ventral striatum of cocaine treated-rats. The increase occurred in both, TOP specific activity and TOP relative mRNA amount determined by real time RT-PCR. As TOP can be implicated in the processing of many neuropeptides, and previous studies have shown that cocaine also alters the gene expression of proenkephalin and prodynorphin in the striatum, the present findings suggest that TOP changes in the brain could play important role in the balance of neuropeptide level correlated with cocaine effects.

Identification of membrane-bound variant of metalloendopeptidase neurolysin (EC 3.4.24.16) as the non-angiotensin type 1 (non-AT1), non-AT2 angiotensin binding site

Recently, we discovered a novel non-angiotensin type 1 (non-AT1), non-AT2 angiotensin binding site in rodent and human brain membranes, which is distinctly different from angiotensin receptors and key proteases processing angiotensins. It is hypothesized to be a new member of the renin-angiotensin system. This study was designed to isolate and identify this novel angiotensin binding site. An angiotensin analog, photoaffinity probe 125I-SBpa-Ang II, was used to specifically label the non-AT1, non-AT2 angiotensin binding site in mouse forebrain membranes, followed by a two-step purification procedure based on the molecular size and isoelectric point of the photoradiolabeled binding protein. Purified samples were subjected to two-dimensional gel electrophoresis followed by mass spectrometry identification of proteins in the two-dimensional gel sections containing radioactivity. LC-MS/MS analysis revealed eight protein candidates, of which the four most abundant were immunoprecipitated after photoradiolabeling. Immunoprecipitation studies indicated that the angiotensin binding site might be the membrane-bound variant of metalloendopeptidase neurolysin (EC 3.4.24.16). To verify these observations, radioligand binding and photoradiolabeling experiments were conducted in membrane preparations of HEK293 cells overexpressing mouse neurolysin or thimet oligopeptidase (EC 3.4.24.15), a closely related metalloendopeptidase of the same family. These experiments also identified neurolysin as the non-AT1, non-AT2 angiotensin binding site. Finally, brain membranes of mice lacking neurolysin were nearly devoid of the non-AT1, non-AT2 angiotensin binding site, further establishing membrane-bound neurolysin as the binding site. Future studies will focus on the functional significance of this highly specific, high affinity interaction between neurolysin and angiotensins.

Molecular Analysis of Neurolysin Expression in the Rat and Bovine Ciliary Body

PURPOSE

This paper deals with the capability of the ciliary epithelium to express neurolysin, involved in the inactivation of numerous neuropeptides.

METHODS

Total RNAs from ciliary body (CB) were processed for RT-PCR, and the amplification products were sequenced. The whole-protein extracts of CBs were analyzed using the Western blot. The CBs were processed for neurolysin immunolocalization.

RESULTS

The RT-PCR detected the presence of neurolysin mRNA in the ciliary body. The Western blot assays demonstrated immunochemical cross-reactivity with neurolysin. The immunoreactivity to neurolysin was observed in ciliary epithelium.

CONCLUSIONS

These results indicate that the ciliary epithelium expresses neurolysin.

Hydrogen bond residue positioning in the 599-611 loop of thimet oligopeptidase is required for substrate selection

Thimet oligopeptidase (EC 3.4.24.15) is a zinc(II) endopeptidase implicated in the processing of numerous physiological peptides. Although its role in selecting and processing peptides is not fully understood, it is believed that flexible loop regions lining the substrate-binding site allow the enzyme to conform to substrates of varying structure. This study describes mutant forms of thimet oligopeptidase in which Gly or Tyr residues in the 599-611 loop region were replaced, individually and in combination, to elucidate the mechanism of substrate selection by this enzyme. Decreases in k(cat) observed on mutation of Tyr605 and Tyr612 demonstrate that these residues contribute to the efficient cleavage of most substrates. Modeling studies showing that a hinge-bend movement brings both Tyr612 and Tyr605 within hydrogen bond distance of the cleaved peptide bond supports this role. Thus, molecular modeling studies support a key role in transition state stabilization of this enzyme by Tyr605. Interestingly, kinetic parameters show that a bradykinin derivative is processed distinctly from the other substrates tested, suggesting that an alternative catalytic mechanism may be employed for this particular substrate. The data demonstrate that neither Tyr605 nor Tyr612 is necessary for the hydrolysis of this substrate. Relative to other substrates, the bradykinin derivative is also unaffected by Gly mutations in the loop. This distinction suggests that the role of Gly residues in the loop is to properly orientate these Tyr residues in order to accommodate varying substrate structures. This also opens up the possibility that certain substrates may be cleaved by an open form of the enzyme.

The role of Tyr605 and Ala607 of thimet oligopeptidase and Tyr606 and Gly608 of neurolysin in substrate hydrolysis and inhibitor binding

The physicochemical properties of TOP (thimet oligopeptidase) and NEL (neurolysin) and their hydrolytic activities towards the FRET (fluorescence resonance energy transfer) peptide series Abz-GFSXFRQ-EDDnp [where Abz is o-aminobenzoyl; X=Ala, Ile, Leu, Phe, Tyr, Trp, Ser, Gln, Glu, His, Arg or Pro; and EDDnp is N-(2,4-dinitrophenyl)-ethylenediamine] were compared with those of site-mutated analogues. Mutations at Tyr605 and Ala607 in TOP and at Tyr606 and Gly608 in NEL did not affect the overall folding of the two peptidases, as indicated by their thermal stability, CD analysis and the pH-dependence of the intrinsic fluorescence of the protein. The kinetic parameters for the hydrolysis of substrates with systematic variations at position P1 showed that Tyr605 and Tyr606 of TOP and NEL respectively, played a role in subsite S1. Ala607 of TOP and Gly608 of NEL contributed to the flexibility of the loops formed by residues 600-612 (GHLAGGYDGQYYG; one-letter amino acid codes used) in NEL and 599-611 (GHLAGGYDAQYYG; one-letter amino acid codes used) in TOP contributing to the distinct substrate specificities, particularly with an isoleucine residue at P1. TOP Y605A was inhibited less efficiently by JA-2 {N-[1-(R,S)-carboxy-3-phenylpropyl]Ala-Aib-Tyr-p-aminobenzoate}, which suggested that the aromatic ring of Tyr605 was an important anchor for its interaction with wild-type TOP. The hydroxy groups of Tyr605 and Tyr606 did not contribute to the pH-activity profiles, since the pKs obtained in the assays of mutants TOP Y605F and NEL Y606F were similar to those of wild-type peptidases. However, the pH-kcat/Km dependence curve of TOP Y605A differed from that of wild-type TOP and from TOP Y606F. These results provide insights into the residues involved in the substrate specificities of TOP and NEL and how they select cytosolic peptides for hydrolysis.

Metallopeptidase, neurolysin, as a novel molecular tool for analysis of properties of cancer-producing matrix metalloproteinases-2 and -9

To compare the substrate preferences of rat brain neurolysin and cancer-producing matrix metalloproteinases (MMPs), which have the same architecture in their catalytic domains, the cleavage activity of neurolysin toward MMP-specific fluorescence-quenching peptides was quantitatively measured. The results show that neurolysin effectively cleaved MOCAc [(7-methoxy coumarin-4-yl) acetyl]-RPKPYANvaWMK(Dnp[2,4-dinitrophenyl])-NH(2), a specific substrate of MMP-2 and MMP-9, but hardly cleaved MOCAc-RPKPVENvaWRK(Dnp)-NH(2), a specific substrate of MMP-3, suggesting that neurolysin has a similar substrate preference to MMP-2 and MMP-9. A structural comparison between neurolysin and MMP-9 showed the similar key amino acid residues for substrate recognition. The possible application of neurolysin displayed on the yeast cell surface, as a safe protein alternative to MMP-2 and MMP-9 which induce cancer cell growth, invasion, and metastasis, to analysis of properties of the MMPs, including the screening of inhibitors and analysis of inhibition mechanism etc., are also discussed.

Enzymatic pathways of the brain renin-angiotensin system: unsolved problems and continuing challenges

The brain renin-angiotensin system continues to be enigmatic more than 40 years after the brain was first recognized to be a site of action of angiotensin II. This review focuses on the enzymatic pathways for the formation and degradation of the growing number of active angiotensins in the brain. A brief description and nomenclature of the peptidases involved in the processing of angiotensin peptides in the brain is given. Of primary interest is the array of enzymes that degrade radiolabeled angiotensins in receptor binding assays. This poses major challenges to studies of brain angiotensin receptors and it is debatable whether an accurate determination of brain angiotensin receptor binding kinetics has yet been made. The quandary facing the investigator of brain angiotensin receptors is the need to protect the radioligand from metabolic alteration while maintaining the characteristics of the receptors in situ. It is the tenet of this review that we have yet to fully understand the binding characteristics of brain angiotensin receptors and the extent of their distribution in the brain because of our inability to fully protect the angiotensins from metabolic alteration until equilibrium binding conditions can be attained.

Swapping the Substrate Specificities of the Neuropeptidases Neurolysin and Thimet Oligopeptidase

Thimet oligopeptidase (EC 3.4.24.15) and neurolysin (EC 3.4.24.16) are closely related zinc-dependent metallopeptidases that metabolize small bioactive peptides. They cleave many substrates at the same sites, but they recognize different positions on others, including neurotensin, a 13-residue peptide involved in modulation of dopaminergic circuits, pain perception, and thermoregulation. On the basis of crystal structures and previous mapping studies, four sites (Glu-469/Arg-470, Met-490/Arg-491, His-495/Asn-496, and Arg-498/Thr-499; thimet oligopeptidase residues listed first) in their substrate-binding channels appear positioned to account for differences in specificity. Thimet oligopeptidase mutated so that neurolysin residues are at all four positions cleaves neurotensin at the neurolysin site, and the reverse mutations in neurolysin switch hydrolysis to the thimet oligopeptidase site. Using a series of constructs mutated at just three of the sites, it was determined that mutations at only two (Glu-469/Arg-470 and Arg-498/Thr-499) are required to swap specificity, a result that was confirmed by testing the two-mutant constructs. If only either one of the two sites is mutated in thimet oligopeptidase, then the enzyme cleaves almost equally at the two hydrolysis positions. Crystal structures of both two-mutant constructs show that the mutations do not perturb local structure, but side chain conformations at the Arg-498/Thr-499 position differ from those of the mimicked enzyme. A model for differential recognition of neurotensin based on differences in surface charge distribution in the substrate binding sites is proposed. The model is supported by the finding that reducing the positive charge on the peptide results in cleavage at both hydrolysis sites.

On the role of prolyl oligopeptidase in health and disease

Prolyl oligopeptidase (POP) is a serine peptidase which digests small peptide-like hormones, neuroactive peptides, and various cellular factors. Therefore, this peptidase has been implicated in many physiological processes as well as in some psychiatric disorders, most probably through interference in inositol cycle. Intense research has been performed to elucidate, on the one hand, the basic structure, ligand binding, and kinetic properties of POP, and on the other, the pharmacology of its inhibitors. There is fairly strong evidence of in vivo importance of POP on substance P, arginine vasopressin, thyroliberin and gonadoliberin metabolism. However, information about the biological relevance of POP is not yet conclusive. Evidence regarding the physiological role of POP is lacking, which is surprising considering that peptidase inhibitors have been exploited for drug development, some of which are currently in clinical trials as memory enhancers for the aged and in a variety of neurological disorders. Here we review the recent progress on POP research and evaluate the relevance of the peptidase in the metabolism of various neuropeptides. The recognition of novel forms and relatives of POP may improve our understanding of how this family of proteins functions in normal and in neuropathological conditions.

Inactivation of neurotensin and neuromedin N by Zn metallopeptidases

The two related peptides neurotensin (NT) and neuromedin N (NN) are efficiently inactivated by peptidases in vitro. Whereas NT is primarily degraded by a combination of three Zn metallo-endopeptidases, namely endopeptidases 24.11, 24.15 and 24.16, in all systems examined, NN is essentially inactivated by the Zn metallo-exopeptidase aminopeptidase M. In this paper we review the work that has led to the identification of the NT- and NN-degrading enzymes and to the purification and cloning of EP 24.16, a previously unidentified peptidase. We provide a brief description of the three NT-inactivating endopeptidases and of their specific and mixed inhibitors, some of them developed in the course of studying NT degradation. Finally, we review in vivo data obtained with these inhibitors that strongly support a physiological role for EP 24.11, 24.15 and 24.16 in the termination of NT-generated signals and for aminopeptidase in terminating NN action. Knowledge of the NT and NN inactivation mechanisms offers the perspective to develop metabolically stable analogs of these peptides with potential therapeutic value.

Modulation of bradykinin signaling by EP24.15 and EP24.16 in cultured trigeminal ganglia

Metalloendopeptidases expressed in neural tissue are characterized in terms of their neuropeptide substrates. One such neuropeptide, bradykinin (BK), is an important inflammatory mediator that activates the type-2 BK receptor (B2R) on the terminal endings of specialized pain-sensing neurons known as nociceptors. Among several metalloendopeptidases that metabolize and inactivate BK, EP24.15 and EP24.16 are known to associate with the plasma membrane in several immortalized cell lines. Potentially, the colocalization of EP24.15/16 and B2R at plasma membrane microdomains known as lipid rafts in a physiologically relevant nociceptive system would allow for discrete, peptidase regulation of BK signaling. Western blot analysis of crude subcellular fractions and lipid raft preparations of cultured rat trigeminal ganglia demonstrate similar expression profiles between EP24.15/16 and B2R on a subcellular level. Furthermore, the treatment of primary cultures of trigeminal ganglia with inhibitors of EP24.15/16 led to the potentiation of several bradykinin-induced events that occur downstream of B2R activation. EP24.15/16 inhibition by N-[1(R,S)-carboxy-3-phenylpropyl]-Ala-AlalTyr-p-aminobenzoate (cFP) resulted in a 1000-fold increase in B2R sensitivity to BK as measured by inositol phosphate accumulation. In addition, cFP treatment resulted in a 31.1+/-5.0% potentiation of the ability of BK to inhibit protein kinase B (Akt) activity. Taken together, these data demonstrate that EP24.15/16 modulate intracellular, peptidergic signaling cascades through B2R in a physiologically relevant nociceptive system.

Substrate phosphorylation affects degradation and interaction to endopeptidase 24.15, neurolysin, and angiotensin-converting enzyme

Recent findings from our laboratory suggest that intracellular peptides containing putative post-translational modification sites (i.e., phosphorylation) could regulate specific protein interactions. Here, we extend our previous observations showing that peptide phosphorylation changes the kinetic parameters of structurally related endopeptidase EP24.15 (EC 3.4.24.15), neurolysin (EC 3.4.24.16), and angiotensin-converting enzyme (EC 3.4.15.1). Phosphorylation of peptides that are degraded by these enzymes leads to reduced degradation, whereas phosphorylation of peptides that interacted as competitive inhibitors of these enzymes alters only the K(i)'s. These data suggest that substrate phosphorylation could be one of the mechanisms whereby some intracellular peptides would escape degradation and could be regulating protein interactions within cells.

MHC class I alleles and their exploration of the antigen-processing machinery

At the cell surface, major histocompatibility complex (MHC) class I molecules present fragments of intracellular antigens to the immune system. This is the end result of a cascade of events initiated by multiple steps of proteolysis. Only a small part of the fragments escapes degradation by interacting with the peptide transporter associated with antigen presentation and is translocated into the endoplasmic reticulum lumen for binding to MHC class I molecules. Subsequently, these newly formed complexes can be transported to the plasma membrane for presentation. Every step in this process confers specificity and determines the ultimate result: presentation of only few fragments from a given antigen. Here, we introduce the players in the antigen processing and presentation cascade and describe their specificity and allelic variation. We highlight MHC class I alleles, which are not only different in sequence but also use different aspects of the antigen presentation pathway to their advantage: peptide acquaintance.

Crystal structure of human thimet oligopeptidase provides insight into substrate recognition, regulation, and localization

Thimet oligopeptidase (TOP) is a zinc metallopeptidase that metabolizes a number of bioactive peptides and degrades peptides released by the proteasome, limiting antigenic presentation by MHC class I molecules. We present the crystal structure of human TOP at 2.0-A resolution. The active site is located at the base of a deep channel that runs the length of the elongated molecule, an overall fold first seen in the closely related metallopeptidase neurolysin. Comparison of the two related structures indicates hinge-like flexibility and identifies elements near one end of the channel that adopt different conformations. Relatively few of the sequence differences between TOP and neurolysin map to the proposed substrate-binding site, and four of these variable residues may account for differences in substrate specificity. In addition, a loop segment (residues 599-611) in TOP differs in conformation and degree of order from the corresponding neurolysin loop, suggesting it may also play a role in activity differences. Cysteines thought to mediate covalent oligomerization of rat TOP, which can inactivate the enzyme, are found to be surface-accessible in the human enzyme, and additional cysteines (residues 321,350, and 644) may also mediate multimerization in the human homolog. Disorder in the N terminus of TOP indicates it may be involved in subcellular localization, but a potential nuclear import element is found to be part of a helix and, therefore, unlikely to be involved in transport. A large acidic patch on the surface could potentially mediate a protein-protein interaction, possibly through formation of a covalent linkage.

pH dependence studies provide insight into the structure and mechanism of thimet oligopeptidase (EC 3.4.24.15)

Thimet oligopeptidase (EC 3.4.24.15; TOP) is a Zn(II) endopeptidase implicated in physiological regulation of processes involving neuropeptides. The present study clarifies the active site structure and mechanism of catalysis of TOP. The enzyme exhibited a bell-shaped pH dependence of activity having an acidic limb due to a protonation event with a pK(a) of 5.7 and a basic limb with pK(a) of 8.8. The acidic limb can be attributed to protonation of a residue affecting k(cat) while the alkaline limb may be due to conformational change. Mutation of Tyr612 to Phe resulted in more than 400-fold decrease in activity. This result, supported by modeling studies, implicates Tyr612 in transition state stabilization analogous to the role of His231 of thermolysin.

Transcriptional control of nonfermentative metabolism in the yeast Saccharomyces cerevisiae

Although sugars are clearly the preferred carbon sources of the yeast Saccharomyces cerevisiae, nonfermentable substrates such as ethanol, glycerol, lactate, acetate or oleate can also be used for the generation of energy and cellular biomass. Several regulatory networks of glucose repression (carbon catabolite repression) are involved in the coordinate biosynthesis of enzymes required for the utilization of nonfermentable substrates. Positively and negatively acting complexes of pleiotropic regulatory proteins have been characterized. The Snf1 (Cat1) protein kinase complex, together with its regulatory subunit Snf4 (Cat3) and alternative beta-subunits Sip1, Sip2 or Gal83, plays an outstanding role for the derepression of structural genes which are repressed in the presence of a high glucose concentration. One molecular function of the Snf1 complex is deactivation by phosphorylation of the general glucose repressor Mig1. In addition to regulation of alternative sugar fermentation, Mig1 also influences activators of respiration and gluconeogenesis, although to a lesser extent. Snf1 is also required for conversion of specific regulatory factors into transcriptional activators. This review summarizes regulatory cis-acting elements of structural genes of the nonfermentative metabolism, together with the corresponding DNA-binding proteins (Hap2-5, Rtg1-3, Cat8, Sip4, Adr1, Oaf1, Pip2), and describes the molecular interactions among general regulators and pathway-specific factors. In addition to the influence of the carbon source at the transcriptional level, mechanisms of post-transcriptional control such as glucose-regulated stability of mRNA are also discussed briefly.

A structure-based site-directed mutagenesis study on the neurolysin (EC 3.4.24.16) and thimet oligopeptidase (EC 3.4.24.15) catalysis

Neurolysin (EP24.16) and thimet oligopeptidase (EP24.15) are closely related metalloendopeptidases. Site-directed mutagenesis of Tyr(613) (EP24.16) or Tyr(612) (EP24.15) to either Phe or Ala promoted a strong reduction of k(cat)/K(M) for both enzymes. These data suggest the importance of both hydroxyl group and aromatic ring at this specific position during substrate hydrolysis by these peptidases. Furthermore, the EP24.15 A607G mutant showed a k(cat)/K(M) of 2x10(5) M(-1) s(-1) for the Abz-GFSIFRQ-EDDnp substrate, similar to that of EP24.16 (k(cat)/K(M)=3x10(5) M(-1) s(-1)) which contains Gly at the corresponding position; the wild type EP24.15 has a k(cat)/K(M) of 2.5x10(4) M(-1) s(-1) for this substrate.

Novel natural peptide substrates for endopeptidase 24.15, neurolysin, and angiotensin-converting enzyme

Endopeptidase 24.15 (EC; ep24.15), neurolysin (EC; ep24.16), and angiotensin-converting enzyme (EC; ACE) are metallopeptidases involved in neuropeptide metabolism in vertebrates. Using catalytically inactive forms of ep24.15 and ep24.16, we have identified new peptide substrates for these enzymes. The enzymatic activity of ep24.15 and ep24.16 was inactivated by site-directed mutagenesis of amino acid residues within their conserved HEXXH motifs, without disturbing their secondary structure or peptide binding ability, as shown by circular dichroism and binding assays. Fifteen of the peptides isolated were sequenced by electrospray ionization tandem mass spectrometry and shared homology with fragments of intracellular proteins such as hemoglobin. Three of these peptides (PVNFKFLSH, VVYPWTQRY, and LVVYPWTQRY) were synthesized and shown to interact with ep24.15, ep24.16, and ACE, with K(i) values ranging from 1.86 to 27.76 microm. The hemoglobin alpha-chain fragment PVNFKFLSH, which we have named hemopressin, produced dose-dependent hypotension in anesthetized rats, starting at 0.001 microg/kg. The hypotensive effect of the peptide was potentiated by enalapril only at the lowest peptide dose. These results suggest a role for hemopressin as a vasoactive substance in vivo. The identification of these putative intracellular substrates for ep24.15 and ep24.16 is an important step toward the elucidation of the role of these enzymes within cells.

Soluble metalloendopeptidases and neuroendocrine signaling

Peptidases play a vital and often highly specific role in the physiological and pathological generation and termination of peptide hormone signals. The thermolysin-like family of metalloendopeptidases involved in the extracellular processing of neuroendocrine and cardiovascular peptides are of particular significance, reflecting both their specificity for particular peptide substrates and their utility as therapeutic targets. Although the functions of the membrane-bound members of this family, such as angiotensin-converting enzyme and neutral endopeptidase, are well established, a role for the predominantly soluble family members in peptide metabolism is only just emerging. This review will focus on the biochemistry, cell biology, and physiology of the soluble metalloendopeptidases EC 3.4.24.15 (thimet oligopeptidase) and EC 3.4.24.16 (neurolysin), as well as presenting evidence that both peptidases play an important role in such diverse functions as reproduction, nociception, and cardiovascular homeostasis.

Comparative fine structural distribution of endopeptidase 24.15 (EC3.4.24.15) and 24.16 (EC3.4.24.16) in rat brain

Endopeptidase 24.15 (EP24.15) and 24.16 (EP24.16) are closely related metalloendopeptidases implicated in the metabolism of several neuropeptides and widely expressed in mammalian brain. To gain insight into the functional role of these two enzymes in the central nervous system, we examined their cellular and subcellular distribution in rat brain by using electron microscopic immunogold labeling. In all areas examined, EP24.15 and EP24.16 immunoreactivity were observed in selective subpopulations of neuronal and glial cells. Subcellular localization of EP24.15 in neurons revealed that this enzyme was predominantly concentrated in the nucleus, whereas EP24.16 was almost exclusively cytoplasmic. The amount of EP24.15 found in the nucleus was inversely correlated with that found in the cytoplasm, suggesting that the enzyme could be mobilized from one compartment to the other. Within the cytoplasm, EP24.15 and EP24.16 immunoreactivity showed comparable distributional patterns. Both enzymes were detected throughout perikarya and dendrites, as well as within axons and axon terminals. In all neuronal compartments, EP24.15 and EP24.16 showed a major association with membranes of neurosecretory elements, including Golgi cisternae, tubulovesicular organelles, synaptic vesicles, and endosomes. However, whereas EP24.15 always faced the cytoplasmic face of the membranes, EP24.16 was observed on both cytoplasmic and luminal sides, suggesting that the latter was more likely to contribute to the processing of peptides or to the degradation of internalized ligands. Taken together, the present results suggest that EP24.15 could play a major role in the hydrolysis of intranuclear substrates, whereas EP24.16 would be predominantly involved in the processing and inactivation of signaling peptides.

Substrate specificity characterization of recombinant metallo oligo-peptidases thimet oligopeptidase and neurolysin

We report a systematic and detailed analysis of recombinant neurolysin (EC 3.4.24.16) specificity in parallel with thimet oligopeptidase (TOP, EC 3.4.24.15) using Bk sequence and its C- and N-terminal extensions as in human kininogen as motif for synthesis of internally quenched fluorescent substrates. The influence of the substrate size was investigated, and the longest peptide susceptible to TOP and neurolysin contains 17 amino acids. The specificities of both oligopeptidases to substrate sites P(4) to P(3)' were also characterized in great detail using seven series of peptides based on Abz-GFSPFRQ-EDDnp taken as reference substrate. Most of the peptides were hydrolyzed at the bond corresponding to P(4)-F(5) in the reference substrate and some of them were hydrolyzed at this bond or at F(2)-S(3) bond. No restricted specificity was found for P(1)' as found in thermolysin as well for P(1) substrate position, however the modifications at this position (P(1)) showed to have large influence on the catalytic constant and the best substrates for TOP contained at P(1), Phe, Ala, or Arg and for neurolysin Asn or Arg. Some amino acid residues have large influence on the K(m) constants independently of its position. On the basis of these results, we are hypothesizing that some amino acids of the substrates can bind to different sub-sites of the enzyme fitting P-F or F-S bond, which requires rapid interchange for the different forms of interaction and convenient conformations of the substrate in order to expose and fit the cleavage bonds in correct position for an efficient hydrolysis. Finally, this plasticity of interaction with the substrates can be an essential property for a class of cytosolic oligopeptidases that are candidates to participate in the selection of the peptides to be presented by the MHC class I.

Differential subcellular distribution of neurolysin (EC 3.4.24.16) and thimet oligopeptidase (EC 3.4.24.15) in the rat brain

Immunohistochemistry was used to analyze the rat brain distribution of thimet oligopeptidase and neurolysin. Both enzymes appear ubiquitously distributed within the entire rat brain. However, neuronal perikarya and processes stained for neurolysin, while intense nuclear labeling was only observed for thimet oligopeptidase. These data suggest that neurolysin and thimet oligopeptidase, endopeptidases sharing several functional and structural similarities, are present in distinctive intracellular compartments in neuronal cells.

Deregulation of gluconeogenic structural genes by variants of the transcriptional activator Cat8p of the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, growth with a non-fermentable carbon source requires co-ordinate transcriptional activation of gluconeogenic structural genes by an upstream activation site (UAS) element, designated CSRE (carbon source-responsive element). The zinc cluster protein encoded by CAT8 is necessary for transcriptional derepression mediated by a CSRE. Expression of CAT8 as well as transcriptional activation by Cat8p is regulated by the carbon source, requiring a functional Cat1p (= Snf1p) protein kinase. The importance of both regulatory levels was investigated by construction of CAT8 variants with a constitutive transcriptional activation domain (INO2TAD) and/or a carbon source-independent promoter (MET25 ). Whereas a reporter gene driven by a CSRE-dependent synthetic minimal promoter showed a 40-fold derepression with wild-type CAT8, an almost constitutive expression was found with a MET25-CAT8-INO2TAD fusion construct due to a dramatically increased gene activation under conditions of glucose repression. Similar results were obtained with the mRNA of the isocitrate lyase gene ICL1 and at the level of ICL enzyme activity. Taking advantage of a Cat8p size variant, we demonstrate its binding to the CSRE. Our data show that carbon source-dependent transcriptional activation by Cat8p is the most important mechanism affecting the regulated expression of gluconeogenic structural genes.

The association of metalloendopeptidase EC 3.4.24.15 at the extracellular surface of the AtT-20 cell plasma membrane

Endopeptidase EC 3.4.24.15 (EP24.15) is a soluble, neuropeptide-degrading metalloenzyme, widely expressed in the brain, pituitary and gonads. For the physiological metabolism of neuropeptides, the enzyme should be located extracellularly, either associated with the plasma membrane or in the extracellular milieu. Western immunoblot analyses of crude cytosolic and post-nuclear membrane fractions prepared by differential centrifugation revealed a slightly smaller molecular mass ( approximately 2 kDa) for EP24.15 in the post-nuclear membrane fraction. This smaller EP24.15 species was also present in an enriched fraction of plasma membrane prepared by Percoll gradient centrifugation. To ascertain whether EP24.15 is associated with the extracellular surface of plasma membrane, two sets of experiments were carried out. First, Western immunoblot analysis of AtT-20 cells treated with the membrane-impermeable, thiol-cleavable cross-linker, 3, 3'-dithio-bis(sulpho-succinimidyl-propionate) (DTSSP), indicated an extracellular membrane association. After cross-linking and thiol-reduction, a distinct band corresponding to EP24.15 was significantly diminished under non-reducing conditions. Second, immunocytochemical studies performed at 4 degrees C on non-permeabilized AtT-20 cells (i.e., non-fixed to prevent antibody internalization), indicated that EP24.15 was expressed on the surface of the AtT-20 cells. We furthermore determined that EP24.15 enzymatic activity is present on the extracellular surface of the cell discernable from the secreted enzyme. These results suggest that the EP24.15 is associated with the extracellular surface of the AtT-20 cell plasma membrane and is enzymatically active. Taken together, the results are consistent with a putative role in the degradation of neuropeptides acting at the external cell surface.

Zinc coordination and substrate catalysis within the neuropeptide processing enzyme endopeptidase EC 3.4.24.15. Identification of active site histidine and glutamate residues

Endopeptidase EC 3.4.24.15 (EP24.15) is a zinc metalloendopeptidase that is broadly distributed within the brain, pituitary, and gonads. Its substrate specificity includes a number of physiologically important neuropeptides such as neurotensin, bradykinin, and gonadotropin-releasing hormone, the principal regulatory peptide for reproduction. In studying the structure and function of EP24.15, we have employed in vitro mutagenesis and subsequent protein expression to genetically dissect the enzyme and allow us to glean insight into the mechanism of substrate binding and catalysis. Comparison of the sequence of EP24.15 with bacterial homologues previously solved by x-ray crystallography and used as models for mammalian metalloendopeptidases, indicates conserved residues. The active site of EP24.15 exhibits an HEXXH motif, a common feature of zinc metalloenzymes. Mutations have confirmed the importance, for binding and catalysis, of the residues (His473, Glu474, and His477) within this motif. A third putative metal ligand, presumed to coordinate directly to the active site zinc ion in concert with His473 and His477, has been identified as Glu502. Conservative alterations to these residues drastically reduces enzymatic activity against both a putative physiological substrate and a synthetic quenched fluorescent substrate as well as binding of the specific active site-directed inhibitor, N-[1-(RS)-carboxy-3-phenylpropyl]-Ala-Ala-Tyr-p-aminobenzoate, the binding of which we have shown to be dependent upon the presence, and possibly coordination, of the active site zinc ion. These studies contribute to a more complete understanding of the catalytic mechanism of EP24.15 and will aid in rational design of inhibitors and pharmacological agents for this class of enzymes.

Confocal microscopy reveals thimet oligopeptidase (EC 3.4.24.15) and neurolysin (EC 3.4.24.16) in the classical secretory pathway

Thimet oligopeptidase (EC 3.4.24.15; EP24.15) and neurolysin (EC 3.4.24.16; EP24.16) are closely related enzymes involved in the metabolic inactivation of bioactive peptides. Both of these enzymes were previously shown to be secreted from a variety of cell types, although their primary sequence lacks a signal peptide. To investigate the mechanisms responsible for this secretion, we examined by confocal microscopy the subcellular localization of these two enzymes in the neuroendocrine cell line AtT20. Both EP24.15 and EP24.16 were found by immunohistochemistry to be abundantly expressed in AtT20 cells. Western blotting experiments confirmed that the immunoreactivity detected in the soma of these cells corresponded to previously cloned isoforms of the enzymes. At the subcellular level, both enzymes colocalized extensively with the integral trans-Golgi network protein, syntaxin-6, in the juxtanuclear region. In addition, both EP24.15 and EP24.16 were found within small vesicular organelles distributed throughout the cell body. Some, but not all, of these organelles also stained positively for ACTH. These results demonstrate that both EP24.15 and EP24.16 are present within the classical secretory pathway. Their colocalization with ACTH further suggests that they may be targeted to the regulated secretory pathway, even in the absence of a signal peptide.

Possible involvement of proteases in the regulation of spermatogenesis

Proteolytic enzymes, which are synthesized and secreted by cells of the seminiferous tubule of the testis, have important functions in spermatogenesis. We performed metabolic studies using small peptide hormones as a substrate to investigate the activity of proteases in cultured Sertoli cells of the rat. High-performance liquid chromatographic analysis of the cell culture supernatants showed cleavage of met- and leu-enkephalin, substance P, and bradykinin. No peptidolysis was observed for the cyclic peptide oxytocin. The hormone cleavage pattern and the use of specific protease inhibitors in peptide degradation experiments demonstrated activities of several proteases in Sertoli cells. These are mainly metalloproteinases including neutral metalloendopeptidases, angiotensin-converting enzyme and aminopeptidases. In addition, activities of serine and aspartic proteases were detected. Only marginal proteolytic activities were observed in Sertoli cell conditioned supernatants, indicating that the investigated proteases are mainly located on Sertoli cell membranes. The peptide hormones used in this study have been found to play a potential role in the endocrine, paracrine or autocrine regulation of testicular cells. The membrane-associated proteases reported here may therefore be involved in the metabolism and inactivation of these peptides.

Protease-protease inhibitor interactions in Sertoli cell-germ cell crosstalk

Peritubular cells, Sertoli cells, and germ cells of the seminiferous tubule synthesize and secrete several proteases and protease inhibitors. Experimental evidence suggests that the complex network of proteolytic enzyme activity and their regulation by protease inhibitors play an important role in male reproduction. Interaction between protease and protease inhibitors seems to play an important role in remodeling and restructuring of the seminiferous tubule during spermatogenesis. Controlled proteolytic activity is also involved in the migration of germ cells from the basal compartment to the lumen of the seminiferous epithelium, and in the release of spermatids during spermiation. The recently reported occurrence of Sertoli cell membrane-associated proteases indicate the possible involvement of regulatory peptide systems within the testis. This view is supported by the detection of all components of one of these paracrine systems, the kallikrein-kinin system, in cells of the seminiferous tubule.

Contribution of endopeptidase 3.4.24.15 to central neurotensin inactivation

The tridecapeptide, neurotensin elicits naloxone-insensitive analgesia after its intracebroventricular administration in mice. We used this central pharmacological effect to assess the putative contribution of the endopeptidase 3.4.24.15 to central inactivation of the peptide. By means of combinatorial chemistry, we previously designed the first potent endopeptidase 3.4.24.15 inhibitor. This agent, Z-(L,D)Phe psi(PO2CH2)(L,D)Ala-Lys-Met (phosphodiepryl 21), is shown here to behave as a fully specific endopeptidase 3.4.24.15 inhibitor, as demonstrated by the absence of effect on a series of other exo- and endopeptidases belonging to various classes of proteolytic activities present in murine brain membranes. Furthermore, central administration of phosphodiepryl 21 drastically prolongs the forepaw licking latency of mice tested on the hot plate and injected with sub-maximally active doses of neurotensin. Altogether, our results demonstrated that, in addition to endopeptidase 3.4.24.16, endopeptidase 3.4.24.15 likely contributes to the physiological termination of the neurotensinergic message in murine brain.

Characterization and localization of mitochondrial oligopeptidase (MOP) (EC 3.4.24.16) activity in the human cervical adenocarcinoma cell line HeLa

In this study we describe the partial purification and characterization of the HeLa cell oligopeptidase M or endopeptidase 3.4.24.16. The HeLa enzyme was isolated initially by its ability to hydrolyse a nonapeptide substrate (P9) which was cognate to the N-terminal cleavage site of preproTGF alpha. The enzyme was shown to be a metalloprotease as it was inhibited by Zn(2+)-chelating agents and DTT, and had an approximate molecular weight of 55-63 kD determined by gel filtration. Neurotensin, dynorphin A1-17 and GnRH1-9 were rapidly degraded by the enzyme while GnRH1-10 and somatostatin were not. Neurotensin was cleaved at the Pro10-Tyr11 bond, leading to the formation of neurotensin (1-10) and neurotensin (11-13). The K(m) for neurotensin cleavage was 7 microM and the Ki for the specific 24.16 dipeptide inhibitor (Pro-ile) was 140 microM which were similar to those observed from the human brain enzyme [Vincent et al. (1996): Brain Res 709:51-58]. Through the use of specific antibodies, the purified HeLa enzyme was shown to be oligopeptidase M. This enzyme and its closely related family member thimet oligopeptidase were shown to co-elute during the isolation procedure but were finally separated using a MonoQ column. Oligopeptidase M is located mainly in mitochondria though it was detected on the plasma membrane in an inactive form. The results obtained demonstrate the first recorded instance of this enzyme in human tissue cultured cells, and raise the issue of its function therein.

Thiol activation of endopeptidase EC 3.4.24.15. A novel mechanism for the regulation of catalytic activity

Endopeptidase EC 3.4.24.15 (EP24.15) is a thermolysin-like metalloendopeptidase involved in the regulated metabolism of a number of neuropeptides. Unlike other thermolysin-like peptidases EP24.15 displays a unique thiol activation, a mechanism that is not clearly understood. In this study we show that both recombinant and tissue-derived EP24.15 are activated up to 8-fold by low concentrations (0.1 mM) of dithiothreitol. Additionally, under non-reducing conditions, recombinant and native EP24.15 forms multimers that can be returned to the monomeric form by reduction. We have also shown that competitive inhibitor binding occurs only to the monomeric form, which indicates that catalytic site access is restricted in the multimeric forms. Through systematic site-directed mutagenesis we have identified that cysteine residues 246, 253, and possibly 248 are involved in the formation of these multimers. Furthermore, both a double mutant (C246S/C253S) and a triple mutant (C246S/C248S/C253S) are fully active in the absence of reducing agents, as measured by both inhibitor binding and hydrolysis. The formation and disruption of disulfide bonds involving these cysteine residues may be a mechanism by which EP24.15 activity is regulated through changes in intra- and extracellular redox potential.

Y-receptor subtypes--how many more?

The Y-receptors belong to the G protein-coupled receptor superfamily and mediate a wide variety of physiological effects, such as regulation of blood pressure, anxiety, memory retention, hormone release and food intake. Since the first human Y-receptor was cloned in 1992, the search for additional subtypes has been an area of intense study. Recently four new NPY-receptor subtypes have been isolated, revealing surprisingly limited sequence identity with values as low as 30%. Several reports indicate further heterogeneity of this receptor family, for example a peripheral Y2 receptor. However, since many studies have been carried out with different peptide analogs and radioligands in different species, there is substantial confusion regarding the pharmacological profile of the receptors. This may have led to an exaggeration of the potential number of discrete receptors.