Contribution of endopeptidase 3.4.24.15 to central neurotensin inactivation

Phosphinic Peptides as Tool Compounds for the Study of Pharmacologically Relevant Zn-Metalloproteases

Phosphinic peptides constitute an important class of bioactive compounds that have found a wide range of applications in the field of biology and pharmacology of Zn-metalloproteases, the largest family of proteases in humans. They are designed to mimic the structure of natural substrates during their proteolysis, thus acting as mechanism-based, transition state analogue inhibitors. A combination of electrostatic interactions between the phosphinic acid group and the Zn cation as well as optimal noncovalent enzyme-ligand interactions can result in both high binding affinity for the desired target and selectivity against other proteases. Due to these unique properties, phosphinic peptides have been mainly employed as tool compounds for (a) the purposes of rational drug design by serving as ligands in X-ray crystal structures of target enzymes and allowing the identification of crucial interactions that govern optimal molecular recognition, and (b) the delineation of biological pathways where Zn-metalloproteases are key regulators. For the latter objective, inhibitors of the phosphinopeptidic type have been used either unmodified or after being transformed to probes of various types, thus expanding the arsenal of functional tools available to researchers. The aim of this review is to summarize all recent research achievements in which phosphinic peptides have played a central role as tool compounds in the understanding of the mechanism and biological functions of Zn-metalloproteases in both health and disease.

CPP-Ala-Ala-Tyr-PABA inhibitor analogs with improved selectivity for neurolysin or thimet oligopeptidase

Thimet oligopeptidase (TOP, EC 3.4.24.15) and neurolysin (NEL, EC 3.4.24.16) are closely related zinc-dependent metalo-oligopeptidases, which take part in the metabolism of oligopeptides (from 5 to 17 amino acid residues) inside and outside cells. Both peptidases are ubiquitously distributed in tissues. TOP is one of the main intracellular peptide-processing enzymes being important for the antigen selection in the MHC Class I presentation route, while NEL function has been more associated with the extracellular degradation of neurotensin. Despite efforts being made to develop specific inhibitors for these peptidases, the most used are: CPP-Ala-Ala-Tyr-PABA, described by Orlowski et al. in 1988, and CPP-Ala-Aib-Tyr-PABA (JA-2) that is an analog more resistant to proteolysis, which development was made by Shrimpton et al. in 2000. In the present work, we describe other analogs of these compounds but, with better discriminatory capacity to inhibit specifically NEL or TOP. The modifications introduced in these new analogs were based on a key difference existent in the extended binding sites of NEL and TOP: the negatively charged Glu⁴⁶⁹ residue of TOP corresponds to the positively charged Arg⁴⁷⁰ residue of NEL. These residues are in position to interact with the residue at the P₁' and/or P₂' of their substrates (mimicked by the Ala-Ala/P1'-P2' residues of the CPP-Ala-Ala-Tyr-PABA). Therefore, exploring this single difference, the following compounds were synthesized: CPP-Asp-Ala-Tyr-PABA, CPP-Arg-Ala-Tyr-PABA, CPP-Ala-Asp-Tyr-PABA, CPP-Ala-Arg-Tyr-PABA. Confirming the predictions, the replacement of each non-charged residue of the internal portion Ala-Ala by a charged residue Asp or Arg resulted in compounds with higher selectivity for NEL or TOP, especially due to the electrostatic attraction or repulsion by the NEL Arg⁴⁷⁰ or TOP Glu⁴⁶⁹ residue. The CPP-Asp-Ala-Tyr-PABA and CPP-Ala-Asp-Tyr-PABA presented higher affinities for NEL, and, the CFP-Ala-Arg-Tyr-PABA showed higher affinity for TOP.

Sleep deprivation changes thimet oligopeptidase (THOP1) expression and activity in rat brain

The consequences of sleep deprivation on memory, cognition, nociception, stress, and endocrine function are related to the balance of neuropeptides, with peptidases being particularly essential. Thimet oligopeptidase (THOP1) is a metallopeptidase implicated in the metabolism of many sleep-related peptides, including angiotensin I, gonadotropin releasing hormone (GnRH), neurotensin, and opioid peptides. In the present study, we evaluated the effect of sleep deprivation and sleep recovery in male rats on THOP1 expression and specific activity in the central nervous system. In the striatum and hypothalamus, THOP1 activity decreased following sleep deprivation and a recovery period. Meanwhile, THOP1 activity and immunoexpression increased in the hippocampal dentate gyrus during the sleep recovery period. Changes in THOP1 expression after sleep deprivation and during sleep recovery can potentially alter the processing of neuropeptides. In particular, processing of opioid peptides may be related to the known increase in pain sensitivity in this model. These results suggest that THOP1 may be an important player in the effects of sleep deprivation.

Neurotensin Analogues Containing Cyclic Surrogates of Tyrosine at Position 11 Improve NTS2 Selectivity Leading to Analgesia without Hypotension and Hypothermia

Neurotensin (NT) exerts its analgesic effects through activation of the G protein-coupled receptors NTS1 and NTS2. This opioid-independent antinociception represents a potential alternative for pain management. While activation of NTS1 also induces a drop in blood pressure and body temperature, NTS2 appears to be an analgesic target free of these adverse effects. Here, we report modifications of NT at Tyr¹¹ to increase selectivity toward NTS2, complemented by modifications at the N-terminus to impair proteolytic degradation of the biologically active NT(8-13) sequence. Replacement of Tyr¹¹ by either 6-OH-Tic or 7-OH-Tic resulted in a significant loss of binding affinity to NTS1 and subsequent NTS2 selectivity. Incorporation of the unnatural amino acid β³hLys at position 8 increased the half-life to over 24 h in plasma. Simultaneous integration of both β³hLys⁸ and 6-OH-Tic¹¹ into NT(8-13) produced a potent and NTS2-selective analogue with strong analgesic action after intrathecal delivery in the rat formalin-induced pain model with an ED₅₀ of 1.4 nmol. Additionally, intravenous administration of this NT analogue did not produce persistent hypotension or hypothermia. These results demonstrate that NT analogues harboring unnatural amino acids at positions 8 and 11 can enhance crucial pharmacokinetic and pharmacodynamic features for NT(8-13) analogues, i.e., proteolytic stability, NTS2 selectivity, and improved analgesic/adverse effect ratio.

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.

To ingest or rest? Specialized roles of lateral hypothalamic area neurons in coordinating energy balance

Survival depends on an organism’s ability to sense nutrient status and accordingly regulate intake and energy expenditure behaviors. Uncoupling of energy sensing and behavior, however, underlies energy balance disorders such as anorexia or obesity. The hypothalamus regulates energy balance, and in particular the lateral hypothalamic area (LHA) is poised to coordinate peripheral cues of energy status and behaviors that impact weight, such as drinking, locomotor behavior, arousal/sleep and autonomic output. There are several populations of LHA neurons that are defined by their neuropeptide content and contribute to energy balance. LHA neurons that express the neuropeptides melanin-concentrating hormone (MCH) or orexins/hypocretins (OX) are best characterized and these neurons play important roles in regulating ingestion, arousal, locomotor behavior and autonomic function via distinct neuronal circuits. Recently, another population of LHA neurons containing the neuropeptide Neurotensin (Nts) has been implicated in coordinating anorectic stimuli and behavior to regulate hydration and energy balance. Understanding the specific roles of MCH, OX and Nts neurons in harmonizing energy sensing and behavior thus has the potential to inform pharmacological strategies to modify behaviors and treat energy balance disorders.

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.

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.

Flexibility in substrate recognition by thimet oligopeptidase as revealed by denaturation studies

Thimet oligopeptidase (TOP) is a soluble metalloendopeptidase belonging to a family of enzymes including neurolysin and neprilysin that utilize the HEXXH metal-binding motif. TOP is widely distributed among cell types and is able to cleave a number of structurally unrelated peptides. A recent focus of interest has been on structure-function relationships in substrate selectivity by TOP. The enzyme's structural fold comprises two domains that are linked at the bottom of a deep substrate-binding cleft via several flexible loop structures. In the present study, fluorescence spectroscopy has been used to probe structural changes in TOP induced by the chemical denaturant urea. Fluorescence emission, anisotropy and collisional quenching data support a two-step unfolding process for the enzyme in which complete loss of the tertiary structure occurs in the second step. Complete loss of activity and loss of catalytic Zn(II) from the active site, monitored by absorption changes of the metal chelator 4-(2-pyridylazo)-resorcinol, are also connected with the second step. In contrast, the first unfolding event, which is linked to changes in the non-catalytic domain, leads to a sharp increase in kcat towards a 9-residue substrate and a sharp decrease in kcat for a 5-residue substrate. Thus a conformational change in TOP has been directly correlated with a change in substrate selectivity. These results provide insight into how the enzyme can process the range of structurally unrelated peptides necessary for its many physiological roles.

Interaction with calmodulin is important for the secretion of thimet oligopeptidase following stimulation

Thimet oligopeptidase (EC 3.4.24.15; EP24.15) was originally described as a neuropeptide-metabolizing enzyme, highly expressed in the brain, kidneys and neuroendocrine tissue. EP24.15 lacks a typical signal peptide sequence for entry into the secretory pathway and is secreted by cells via an unconventional and unknown mechanism. In this study, we identified a novel calcium-dependent interaction between EP24.15 and calmodulin, which is important for the stimulated, but not constitutive, secretion of EP24.15. We demonstrated that, in vitro, EP24.15 and calmodulin physically interact only in the presence of Ca2+, with an estimated Kd value of 0.52 mum. Confocal microscopy confirmed that EP24.15 colocalizes with calmodulin in the cytosol of resting HEK293 cells. This colocalization markedly increases when cells are treated with either the calcium ionophore A23187 or the protein kinase A activator forskolin. Overexpression of calmodulin in HEK293 cells is sufficient to greatly increase the A23187-stimulated secretion of EP24.15, which can be inhibited by the calmodulin inhibitor calmidazolium. The specific inhibition of protein kinase A with KT5720 reduces the A23187-stimulated secretion of EP24.15 and inhibits the synergistic effects of forskolin with A23187. Treatment with calmidazolium and KT5720 nearly abolishes the stimulatory effects of A23187 on EP24.15 secretion. Together, these data suggest that the interaction between EP24.15 and calmodulin is regulated within cells and is important for the stimulated secretion of EP24.15 from HEK293 cells.

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.

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.

Neurotensin activates GABAergic interneurons in the prefrontal cortex

Converging data suggest a dysfunction of prefrontal cortical GABAergic interneurons in schizophrenia. Morphological and physiological studies indicate that cortical GABA cells are modulated by a variety of afferents. The peptide transmitter neurotensin may be one such modulator of interneurons. In the rat prefrontal cortex (PFC), neurotensin is exclusively localized to dopamine axons and has been suggested to be decreased in schizophrenia. However, the effects of neurotensin on cortical interneurons are poorly understood. We used in vivo microdialysis in freely moving rats to assess whether neurotensin regulates PFC GABAergic interneurons. Intra-PFC administration of neurotensin concentration-dependently increased extracellular GABA levels; this effect was impulse dependent, being blocked by treatment with tetrodotoxin. The ability of neurotensin to increase GABA levels in the PFC was also blocked by pretreatment with 2-[1-(7-chloro-4-quinolinyl)-5-(2,6-dimethoxyphenyl)pyrazole-3-yl)carbonylamino]tricyclo(3.3.1.1 [EC] .3.7)decan-2-carboxylic acid (SR48692), a high-affinity neurotensin receptor 1 (NTR1) antagonist. This finding is consistent with our observation that NTR1 was localized to GABAergic interneurons in the PFC, particularly parvalbumin-containing interneurons. Because neurotensin is exclusively localized to dopamine axons in the PFC, we also determined whether neurotensin plays a role in the ability of dopamine agonists to increase extracellular GABA levels. We found that D2 agonist-elicited increases in PFC GABA levels were blocked by pretreatment with SR48692, consistent with data indicating that D2 autoreceptor agonists increase neurotensin release from dopamine-neurotensin axons in the PFC. These findings suggest that neurotensin plays an important role in regulating prefrontal cortical interneurons and that it may be useful to consider neurotensin agonists as an adjunct in the treatment of schizophrenia.

14-3-3 epsilon modulates the stimulated secretion of endopeptidase 24.15

Endopeptidase 24.15 (ep24.15: EC3.4.24.15), a secreted protein involved in peptide metabolism, is unusual in that it does not contain a signal peptide sequence. In this work, we describe the physical interaction between ep24.15 and 14-3-3 epsilon, one isoform of a family of ubiquitous phosphoserine/threonine-scaffold proteins that organizes cell signaling and is involved in exocytosis. The interaction between ep24.15 and 14-3-3 epsilon increased following phosphorylation of ep24.15 at Ser(644) by protein kinase A (PKA). The co-localization of ep24.15 and 14-3-3 epsilon was increased by exposure of HEK293 cells (human embryonic kidney cells) to forskolin (10 microm). Overexpression of 14-3-3 epsilon in HEK293 cells almost doubled the secretion of ep24.15 stimulated by A23187 (7.5 microm) from 10%[1.4 +/- 0.24 AFU/(min 10(6) cells)] to 19%[2.54 +/- 0.24 AFU/(min 10(6) cells)] (p < 0.001) of the total intracellular enzyme activity. Treatment with forskolin had a synergistic effect on the A23187-stimulated secretion of ep24.15 that was totally blocked by the PKA inhibitor KT5720. The ep24.15 point mutation S644A reduced the co-localization of ep24.15 and 14-3-3 in stably transfected HEK293 cells. Indeed, secretion of the ep24.15 S644A mutant from these cells was only slightly stimulated by A23187 and insensitive to forskolin, in contrast to that of the wild type enzyme. Together, these data suggest that prior interaction with 14-3-3 is an important step in the unconventional stimulated secretion of ep24.15.

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.

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.

The cytosolic endopeptidase, thimet oligopeptidase, destroys antigenic peptides and limits the extent of MHC class I antigen presentation

Most antigenic peptides presented on MHC class I molecules are generated by proteasomes during protein breakdown. It is unknown whether these peptides are protected from destruction by cytosolic peptidases. In cytosolic extracts, most antigenic peptides are degraded by the metalloendopeptidase, thimet oligopeptidase (TOP). We therefore examined whether TOP destroys antigenic peptides in vivo. When TOP was overexpressed in cells, class I presentation of antigenic peptides was reduced. In contrast, TOP overexpression didn't reduce presentation of peptides generated in the endoplasmic reticulum or endosomes. Conversely, preventing TOP expression with siRNA enhanced presentation of antigenic peptides. TOP therefore plays an important role in vivo in degrading peptides released by proteasomes and is a significant factor limiting the extent of antigen presentation.

Anterograde axonal transport of endopeptidase 24.15 in rat sciatic nerves

Axonal transport of endopeptidase 24.15 (EP24.15), a putative neuropeptide degrading-enzyme, was examined in the proximal, middle, and distal segments of rat sciatic nerves using a double ligation technique. At 48h after ligation, a significant amount of the axonal transport of EP24.15 activity was found in the proximal segment, while axonal transport of deamidase activity, a lysosomal enzyme, increased in both proximal and distal segments. Western blot analysis of EP24.15 showed that EP24.15 immunoreactivity in the proximal segment was 1.8-fold higher than that in the middle segment. The immunohistochemical analysis of the segments also showed an increase in the immunoreactive EP24.15 in the proximal segment in comparison with that in the middle segment. In the distal segment, no axonal transport of EP24.15 was found in all methods examined, indicating that EP24.15 is mainly transported by an anterograde axonal flow. These observations suggest that EP24.15 may be involved in the metabolism of neuropeptides in nerve terminals or synaptic clefts.

Localization of EP24.15, a major liver kininase

The liver is important for the kallikrein-kinin system modulation. This system plays a role in the inflammatory cascade with anticoagulant, profibrinolytic, and anti-adhesive attributes. The metalloendopeptidase EP24.15 is a major hepatic kininase. We studied the tissue distribution and subcellular localization of this enzyme in rat liver by cell fractionation and immunohistochemistry. Our results showed that EP24.15 is predominant in the soluble fraction of the liver homogenate and is present in the cytoplasm of hepatocytes, particularly in the perivenous zone (Z3). This localization is relevant because most hepatotoxin-induced necrosis, as well as ischemic hepatocellular injury, is predominant in Z3.

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.

Temperature and salts effects on the peptidase activities of the recombinant metallooligopeptidases neurolysin and thimet oligopeptidase

We report the recombinant neurolysin and thimet oligopeptidase (TOP) hydrolytic activities towards internally quenched fluorescent peptides derived from the peptide Abz-GGFLRRXQ-EDDnp (Abz, ortho-aminobenzoicacid; EDDnp, N-(2,4-dinitrophenyl) ethylenediamine), in which X was substituted by 11 different natural amino acids. Neurolysin hydrolyzed these peptides at R-R or at R-X bonds, and TOP hydrolyzed at R-R or L-R bonds, showing a preference to cleave at three or four amino acids from the C-terminal end. The kinetic parameters of hydrolysis and the variations of the cleavage sites were evaluated under different conditions of temperature and salt concentration. The relative amount of cleavage varied with the nature of the substitution at the X position as well as with temperature and NaCl concentration. TOP was activated by all assayed salts in the range 0.05-0.2 m for NaCl, KCl, NH4Cl and NaI, and 0.025-0.1 m for Na2SO4. Concentration higher than 0.2 N NH4Cl and NaI reduced TOP activity, while 0.5 N or higher concentration of NaCl, KCl and Na2SO4 increased TOP activity. Neurolysin was strongly activated by NaCl, KCl and Na2SO4, while NH4Cl and NaI have very modest effect. High positive values of enthalpy (DeltaH*) and entropy (DeltaS*) of activation were found together with an unusual temperature dependence upon the hydrolysis of the substrates. The effects of low temperature and high NaCl concentration on the hydrolytic activities of neurolysin and TOP do not seem to be a consequence of large secondary structure variation of the proteins, as indicated by the far-UV CD spectra. However, the modulation of the activities of the two oligopeptidases could be related to variations of conformation, in limited regions of the peptidases, enough to modify their activities.

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.

The neuropeptide processing enzyme EC 3.4.24.15 is modulated by protein kinase A phosphorylation

The metalloendopeptidase EC (EP24.15) is a neuropeptide-metabolizing enzyme expressed predominantly in brain, pituitary, and testis, and is implicated in several physiological processes and diseases. Multiple putative phosphorylation sites in the primary sequence led us to investigate whether phosphorylation effects the specificity and/or the kinetics of substrate cleavage. Only protein kinase A (PKA) treatment resulted in serine phosphorylation with a stoichiometry of 1.11 +/- 0.12 mol of phosphate/mol of recombinant rat EP24.15. Mutation analysis of each putative PKA site, in vitro phosphorylation, and phosphopeptide mapping indicated serine 644 as the phosphorylation site. Phosphorylation effects on catalytic activity were assessed using physiological (GnRH, GnRH(1-9), bradykinin, and neurotensin) and fluorimetric (MCA-PLGPDL-Dnp and orthoaminobenzoyl-GGFLRRV-Dnp-edn) substrates. The most dramatic change upon PKA phosphorylation was a substrate-specific, 7-fold increase in both K(m) and k(cat) for GnRH. In both rat PC12 and mouse AtT-20 cells, EP24.15 was serine-phosphorylated, and EP24.15 phosphate incorporation was enhanced by forskolin treatment, and attenuated by H89, consistent with PKA-mediated phosphorylation. Cloning of the full-length mouse EP24.15 cDNA revealed 96.7% amino acid identity to the rat sequence, and conservation at serine 644, consistent with its putative functional role. Therefore, PKA phosphorylation is suggested to play a regulatory role in EP24.15 enzyme activity.

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.

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.