Enzymes processing somatostatin precursors: an Arg-Lys esteropeptidase from the rat brain cortex converting somatostatin-28 into somatostatin-14

Paracrine crosstalk between intestinal L- and D-cells controls secretion of glucagon-like peptide-1 in mice

DPP-4 inhibitors, used for treatment of type 2 diabetes, act by increasing the concentrations of intact glucagon-like peptide-1 (GLP-1), but at the same time, they inhibit secretion of GLP-1, perhaps by a negative feedback mechanism. We hypothesized that GLP-1 secretion is feedback regulated by somatostatin (SS) from neighboring D-cells, and blocking this feedback circuit results in increased GLP-1 secretion. We used a wide range of experimental techniques, including gene expression analysis, immunohistochemical approaches, and the perfused mouse intestine to characterize the paracrine circuit controlling GLP-1 and SS. We show that 1) antagonizing the SS receptor (SSTr) 2 and SSTr5 led to increased GLP-1 and SS secretion in the mouse, 2) SS exhibits strong tonic inhibition of GLP-1 secretion preferentially through SSTr5, and 3) the secretion of S was GLP-1 receptor dependent. We conclude that SS is a tonic inhibitor of GLP-1 secretion, and interventions in the somatostain-GLP-1 paracrine loop lead to increased GLP-1 secretion.

Nardilysin in human brain diseases: both friend and foe

Nardilysin is a metalloprotease that cleaves peptides, such as dynorphin-A, α-neoendorphin, and glucagon, at the N-terminus of arginine and lysine residues in dibasic moieties. It has various functionally important molecular interaction partners (heparin-binding epidermal growth factor-like growth factor, tumour necrosis factor-α-converting enzyme, neuregulin 1, beta-secretase 1, malate dehydrogenase, P42(IP4)/centaurin-α1, the histone H3 dimethyl Lys4, and others) and is involved in a plethora of normal brain functions. Less is known about possible implications of nardilysin for brain diseases. This review, which includes some of our own recent findings, attempts to summarize the current knowledge on possible roles of nardilysin in Alzheimer disease, Down syndrome, schizophrenia, mood disorders, alcohol abuse, heroin addiction, and cancer. We herein show that nardilysin is a Janus-faced enzyme with regard to brain pathology, being probably neuropathogenic in some diseases, but neuroprotective in others.

Cortistatin--functions in the central nervous system

Cortistatin (CST) is a neuropeptide from the somatostatin (SRIF)/urotensin (UII) family named after its predominantly cortical expression and ability to depress cortical activity, which was discovered a decade ago. In vitro assays show CST is able to bind all five cloned somatostatin receptors and shares many pharmacological and functional properties with SRIF. However, distinct from SRIF, CST has been shown to induce slow-wave sleep, reduce locomotor activity, and activate cation selective currents not responsive to somatostatin. Different lines of evidence also indicate that CST, like SRIF, is involved in learning and memory processes. CST-14 may also function as an endogenous anti-convulsant. In addition to its role in cortical synchronization, CST-14 has emerged as an important mediator of immunity and inflammation. This review will cover some of the basic properties of CST in the brain, and will discuss new data on the role of CST in cortical activity.

The metalloendopeptidase nardilysin (NRDc) is potently inhibited by heparin-binding epidermal growth factor-like growth factor (HB-EGF)

Nardilysin (N-arginine dibasic convertase, or NRDc) is a cytosolic and cell-surface metalloendopeptidase that, in vitro, cleaves substrates upstream of Arg or Lys in basic pairs. NRDc differs from most of the other members of the M16 family of metalloendopeptidases by a 90 amino acid acidic domain (DAC) inserted close to its active site. At the cell surface, NRDc binds heparin-binding epidermal growth factor-like growth factor (HB-EGF) and enhances HB-EGF-induced cell migration. An active-site mutant of NRDc fulfills this function as well as wild-type NRDc, indicating that the enzyme activity is not required for this process. We now demonstrate that NRDc starts at Met(49). Furthermore, we show that HB-EGF not only binds to NRDc but also potently inhibits its enzymic activity. NRDc-HB-EGF interaction involves the 21 amino acid heparin-binding domain (P21) of the growth factor, the DAC of NRDc and most probably its active site. Only disulphide-bonded P21 dimers are inhibitory. We also show that Ca(2+), via the DAC, regulates both NRDc activity and HB-EGF binding. We conclude that the DAC is thus a key regulatory element for the two distinct functions that NRDc fulfills, i.e. as an HB-EGF modulator and a peptidase.

Gene expression of the dibasic-pair cleaving enzyme NRD convertase (N-arginine dibasic convertase) is differentially regulated in the GH3 pituitary and Mat-Lu prostate cell lines

NRD convertase (N-arginine dibasic convertase, NRD-C) is a dibasic selective metalloprotease which cleaves on the N-terminal side of an arginine residue in a dibasic pair. Abundant in endocrine tissues, the highest levels are found in testis. The mechanism whereby NRD-C expression is regulated at the transcriptional level has been examined by reporter-gene assay and electrophoretic-mobility-shift assays. Analysis of the rat and human promoters show that they are highly conserved, containing a number of motifs which may correspond to transcription-factor binding sites. The rat promoter has been cloned into a luciferase reporter vector and analysed in a number of cell lines. Full functionality of the promoter is observed with 5' deletions to 411 bp upstream of the transcriptional start site in spermatid, prostate and pituitary cell lines. Further deletion to 101 bp causes a complete loss of activity in spermatid and prostate lines. By contrast, GH3 pituitary cells display no reduction in promoter activity with deletion to 101 bp of upstream sequence. A number of transcription-factor binding sites have been identified by electrophoretic-mobility-shift assays in the region 411-101; however, no differences in binding between the cell lines were observed.

Human and rat testis express two mRNA species encoding variants of NRD convertase, a metalloendopeptidase of the insulinase family

Rat testis NRD convertase (EC 3.4.24.61) is a Zn2+-dependent endopeptidase that cleaves, in vitro, peptide substrates at the N-terminus of Arg residues in dibasic sites. This putative processing enzyme of the insulinase family of metallopeptidases exhibits a significant degree of similarity to insulinase and two yeast processing enzymes, Axl1 and Ste23. We report the cloning of two human testis cDNA species encoding isoforms of NRD convertase, hNRD1 and hNRD2. Whereas the hNRD1 transcript (3.7 kb) is equivalent to the previously characterized rat cDNA (rNRD1), hNRD2 and rNRD2 are 3.9 kb novel forms containing a nucleotide insertion encoding a 68-residue segment. This motif, which is inserted N-terminal of the Zn2+-binding site, HXXEH, is contained within the most conserved region among the insulinase family members. Analysis of the deduced primary sequences revealed 92% identity between rat and human orthologues. The human gene encoding NRD convertase was localized to chromosome 1p32.1-p32.2. Whereas NRD convertase is mostly expressed in testis and in 24 cell lines, low mRNA levels were detected in most of the 27 other tissues tested.

A cortical neuropeptide with neuronal depressant and sleep-modulating properties

Acetylcholine (ACh) plays a key role in the transitions between the different phases of sleep: Slow-wave sleep requires low ACh concentrations in the brain, whereas rapid-eye-movement (REM) sleep is associated with high levels of ACh. Also, these phases of sleep are differentially sensitive to a number of endogenous neuropeptides and cytokines, including somatostatin, which has been shown to increase REM sleep without significantly affecting other phases. Here we report the cloning and initial characterization of cortistatin, a neuropeptide that exhibits strong structural similarity to somatostatin, although it is the product of a different gene. Administration of cortistatin depresses neuronal electrical activity but, unlike somatostatin, induces low-frequency waves in the cerebral cortex and antagonizes the effects of acetylcholine on hippocampal and cortical measures of excitability. This suggests a mechanism for cortical synchronization related to sleep.

Colocalization of prosomatostatin-derived peptides in the caudate-putamen of the rat

In the striatum of rat, somatostatin 14, somatostatin 28, and somatostatin 28(1-12) have previously been localized within a small population of medium aspiny local circuit neurons. Because all three peptide fragments are generated through the cleavage of prosomatostatin by different converting enzymes, the possibility for differential expression of these peptides exists. In order to investigate this possibility, frozen sections were collected from the brains of adult female Wistar rats fixed with 4% paraformaldehyde and double labelled using immunocytochemistry and in situ hybridization. Sections were first processed for somatostatin 14, somatostatin 28, or somatostatin 28(1-12) by using the avidin-biotin complex immunocytochemical technique followed by in situ hybridization using 35S-labelled antisense riboprobes to somatostatin mRNA. The results of such analysis revealed that somatostatin 28 and somatostatin mRNA are 100% colocalized. Somatostatin 14 and somatostatin 28(1-12), in contrast, are only present within 66% of the neurons that express somatostatin mRNA. Examination of the anatomical distribution of neurons that express both somatostatin mRNA and somatostatin 14 or somatostatin 28(1-12) protein reveals that these neurons are present throughout the caudate-putamen of rat but are more prevalent in the ventromedial regions. Neurons that express somatostatin mRNA but not somatostatin 14 or somatostatin 28(1-12) are also present throughout the caudate-putamen but are most numerous within a dorsolateral strip just beneath the corpus callosum. These results suggest that the somatostatin neuron population within the rat caudate-putamen is actually composed of two smaller subpopulations based on neuropeptide content. The first subpopulation contains somatostatin 28 and constitutes one-third of the total somatostatin population, whereas the other contains somatostatin 28, somatostatin 14, and somatostatin 28(1-12) and represents the remaining two-thirds of the cells that express somatostatin mRNA.

Kinetic analysis of the type-1 proinsulin endopeptidase by a monoclonal antibody-based immunoadsorbent assay

A simple, rapid and sensitive assay for the type-1 endopeptidase (Arg-Arg cleaving) was developed by using an antiproinsulin monoclonal immunoadsorbent to separate reaction products from the substrate. The values obtained by this assay were identical with those obtained by an h.p.l.c.-based procedure and yielded similar values for the pH optimum (5.6) and Ca2+ activation (K0.5 = 2 mM). It was shown that the type-1 endopeptidase was readily solubilized by Triton X-114 (87 +/- 3%, n = 12) and partitioned principally into the aqueous phase at 30 degrees C (90.1 +/- 2.6%, n = 12). Activity was lost on gel filtration, but could be restored by adenosine 5'-[gamma-thio]triphosphate (K0.5 = 6 microM), 50 microM-dithiothreitol or 50 microM-Ca(2+)-trans-1,2-diaminocyclohexane-NNN'N'-tetra-acetic acid (CDTA), indicating that the enzyme was particularly sensitive to heavy metal ions. The Km obtained with proinsulin as substrate (13 +/- 1.7 microM) indicated that the enzyme works at close to its Vmax. in the nascent secretory granule. The Vmax. of the enzyme prepared from insulin granules (0.6% proinsulin converted/min) corresponded closely to the rate measured in vivo in rat islets. The type-1 endopeptidase also appears to be capable of binding to proinsulin in the region of the C-peptide/A-chain junction, since a peptide spanning this region was found to inhibit the 125I-proinsulin processing measured by this assay.

Somatostatin-14 and somatostatin-28 induce opposite effects on potassium currents in rat neocortical neurons

The prosomatostatin-derived peptides somatostatin-14 (Som-14) and somatostatin-28 (Som-28) are believed to act as neurotransmitters in the central nervous system. To examine possible mechanisms by which these peptides induce their physiological actions in brain, the effects of Som-14 and Som-28 on voltage-dependent K+ currents in rat cerebral cortical neurons in culture were examined by using whole-cell patch-clamp techniques. Som-14 increased a delayed rectifier K+ current (IK) in the cortical neurons, while Som-28 reduced IK in the neurons, both in a concentration-dependent manner. Som-14 and Som-28 could induce opposite changes in IK in the same neurons. Elevating intracellular cAMP in the cortical neurons did not modify the effects of Som-14 or Som-28 on IK, indicating that the peptides can regulate this ionic current through cAMP-independent mechanisms. Pretreatment of the neocortical cells with pertussis toxin, which inactivates inhibitory GTP-binding proteins, abolished both Som-14 and Som-28 modulation of IK, indicating that Som-14 and Som-28 receptors are coupled to IK via GTP-binding proteins. These studies show that Som-14 and Som-28 can induce opposite biological effects, suggesting that Som-14 and Som-28, acting through distinct receptors, may function as different neurotransmitters or neuromodulators.

Analysis of the relationship between cleavability of a paramyxovirus fusion protein and length of the connecting peptide

The relationship between the length of the connecting peptide in a paramyxovirus F0 protein and cleavage of F0 into the F1 and F2 subunits has been examined by constructing a series of mutant F proteins via site-directed mutagenesis of a cDNA clone encoding the simian virus 5 F protein. The mutant F proteins had one to five arginine residues deleted from the connecting peptide. The minimum number of arginine residues required for cleavage-activation of the simian virus 5 F0 protein by host cell proteases was found to be four. F proteins with two or three arginine residues in the connecting peptide were not cleaved by host cell proteases but could be cleaved by exogenously added trypsin. The mutant F protein possessing a connecting peptide consisting of one arginine residue was not cleaved by trypsin. The altered F proteins were all transported to the infected-cell plasma membrane as shown by cell surface immunofluorescence or cell surface trypsinization. However, the only mutant F protein found to be biologically active as detected by syncytium formation was the F protein which has four arginine residues at the cleavage site. The results presented here suggest that in the paramyxovirus F protein the number of basic amino acid residues in the connecting peptide is important for cleavage of the precursor protein by host cell proteases but is not the only structural feature involved. In addition, the data indicate that cleavage of F0 into F1 and F2 does not necessarily result in biological activity and that the connecting peptide may affect the local conformation of the F polypeptide.

Sensitivity of rat pancreatic A and B cells to somatostatin

Islet A and B cells were purified from the rat pancreas and examined for their respective sensitivity to somatostatin. Both somatostatin-14 (S14) and -28 (S28) inhibited glucagon and insulin release through direct interactions with the corresponding cell types. A dose-dependent suppression of the secretory activities was paralleled by a reduction in cellular cyclic AMP formation with similar ED50 values for both actions. The somatostatin effects on pancreatic hormone release may thus be mediated via an inhibition of adenylate cyclase activity. In pancreatic A cells, S14 and S28 were equally potent inhibitors with ED50 values ranging from 2 x 10(-12) to 2 x 10(-11) mol/l. Pancreatic B cells exhibited a similar sensitivity to S28 as the A cells (ED50 of 2 to 5 x 10(-11) mol/l), but not to S14 (ED50 of 2 x 10(-9) mol/l). Extrapolation of these in vitro sensitivities of islet A and B cells to the in vivo situation suggests that both cell types can respond to circulating S28 levels and that A cells are sensitive to both locally and distally released S14. Islet B cells appear insensitive to the normal peripheral S14 levels but could respond to locally released somatostatin. The marked difference in the sensitivities of islet A and B cells to S14 suggest that these cell types are equipped with different somatostatin receptors. This notion was further supported by the cell-selective actions of the synthetic S14 analogues [D-Trp8, D-Cys14]S14 and desAsn5[D-Trp8, D-Ser13]S14.

Relationship between endo- and exopeptidases in a processing enzyme system: activation of an endoprotease by the aminopeptidase B-like activity in somatostatin-28 convertase

The somatostatin-28 convertase activity involved in vitro in the processing of somatostatin-28 into the neuropeptides somatostatin-28-(1-12) and somatostatin-14 is composed of an endoprotease and a basic aminopeptidase. We report herein on the purification to apparent homogeneity of these two constituents and on their functional interrelationship. In particular we observed that after various physicochemical treatments, the 90-kDa endoprotease activity was recovered both at this molecular mass and as a 45-kDa entity. Moreover, the production of [Arg-2,Lys-1]somatostatin-14 from somatostatin-28 by the action of the endoprotease was activated in a cooperative manner by the aminopeptidase B-like enzyme. A 10-fold activation occurred when the exopeptidase was inhibited by 6.5 mM diisopropyl fluorophosphate and allowed the determination of a half-maximal activation constant (K1/2) of approximately equal to 13 nM. These observations strongly suggest that both enzymes act in a concerted manner in vitro and that they may form a complex in vivo.

Role of peptide substrate structure in the selective processing of peptide prohormones at basic amino acid pairs by endoproteases

Three putative processing enzymes, each with defined action in a prohormone system, a 'pro-ocytocin-neurophysin convertase' from bovine neurohypophysis secretory granules, a 'Leu-enkephalin Arg6 generating enzyme' from human CSF and the endoprotease from the 'S-28 convertase' complex of rat brain cortex, were tested for their ability to hydrolyze peptides deriving from pro-ocytocin, pro-enkephalin B and pro-somatostatin, respectively at pairs of basic amino acids. The observations suggest that structural parameters specified by the peptide region around the dibasic moieties govern recognition by the enzyme and define which peptide bond is hydrolyzed.

Proteolytic enzymes in the post-translational processing of polypeptide hormone precursors

Selective and limited proteolysis is a key step in the post-translational modification of peptide hormone precursors. This process appears to involve a proteolytic machinery including highly specific endoproteases. Some of the enzyme systems possibly involved in the processing of pro-neuropeptides will be described and their mechanism of action discussed. Special emphasis will be on the following: i) the physico-chemical characteristics of proteolytic enzymes which are believed to be involved in the processing of some of these polypeptide hormone precursors; ii) the bio-specificity of these enzymes toward the substrates; iii) the importance of both secondary and tertiary structures of the cleavage domain in recognition by the selective proteases. These properties will be discussed in connection with the possible importance of the maturation enzymes in the in vivo regulation of hormone biosynthesis.

Mutations within the proteolytic cleavage site of the Rous sarcoma virus glycoprotein that block processing to gp85 and gp37

We have investigated the specificity of the proteolytic cleavage of the Rous sarcoma virus glycoprotein precursor by introducing two mutations into the putative cleavage region (Arg-Arg-Lys-Arg). We show that neither a deletion of the cleavage sequence nor a glutamic acid for lysine substitution altered intracellular transport or surface expression of the env gene products. However, both the four-amino-acid deletion and the glutamic acid substitution block processing of the env precursor. Susceptibility of the glutamic acid-substituted env precursor to proteases indicated that tertiary protein structure was unaffected. While inhibitor experiments suggested that more than one endopeptidase might be capable of mediating the proteolytic cleavage, the results presented here point to the presence in the Golgi apparatus of a novel endopeptidase, required for retroviral glycoprotein cleavage, that has a high specificity for lysine-containing peptides.

Reactivity of consecutive basic amino acid residues in peptides

Different tetrapeptides of general formula L-Ala-X-X-Gly, possessing a basic doublet in the second and third position (X = Arg or Lys), have been synthesized as free or N-acetylated molecules. The chemical reactivity of the arginine guanidino group and of the lysine epsilon-amino group were studied using respectively the Sakaguchi and the ortho-diacetylbenzene reactions, in the tetrapeptides as well as in related molecules. In both cases, the colour yield is markedly influenced by the length of the polypeptide chain and by the relative positions of the arginine and lysine residues, suggesting the occurrence of intramolecular bonds within the tetrapeptide molecule. Tryptic hydrolysis of the tetrapeptides was followed by evaluating the amino acids or peptides which appear to be specific for the different possible cleavages at the arginyl or at the lysyl bonds. The susceptibility to trypsin of the carboxylic group of the second basic amino acid decreases progressively in the order Lys-Arg greater than Arg-Arg much greater than Lys-Lys greater than Arg-Lys, which shows a fair correlation with the intra-cellular cleavage of the bonds observed during the processing of preproteins of of the precursors of several physiologically active peptides.

Precursors for peptide hormones share common secondary structures forming features at the proteolytic processing sites

We have analyzed the amino acid sequences situated around the putative proteolytic cleavage sites in twenty different biosynthetic precursors of peptide hormones by processing enzymes. The prediction of the probability for forming secondary structures around the basic amino acids, constituting the cleavage sites, was made using the modified method of Chou and Fasman. The results indicate that the processing sequences which are cleaved in vivo, are in all cases located inside regions with high beta-turn formation probability or else immediately adjacent to these structures. The beta-turn forming region at the cleavage locus, is flanked on both sides by amino acid sequences with a high probability for forming highly ordered structures, either beta-sheet or alpha-helix. These conformational features are not found in precursors around dibasic pairs, i.e. putative cleavage loci, but which are not cleaved in vivo and appear to be conserved. We hypothesize that beta-turns including the basic amino acids doublets, flanked by highly ordered secondary structures (either beta-sheet or alpha-helix) may constitute a minimal requirement for the recognition by the endoproteases involved in the processing of these precursors.

In vivo synthesis and processing of rat hypothalamic prosomatostatin

The in vivo incorporation of [3H]phenylalanine into an apparent 15 kDa prosomatostatin was observed in the hypothalamus of rats injected with the labeled amino acid in the third ventricle. Precursor-product relationships were established between this newly synthesized material and both somatostatin-28 and -14.

Proteolytic events in the post-translational processing of somatostatin precursors from rat brain cortex and anglerfish pancreatic islets

An Arg-Lys esteropeptidase which converts somatostatin-28 (S-28) into somatostatin-14 (S-14) was detected in rat brain cortical extracts using a synthetic undecapeptide substrate mimicking the octacosapeptide sequence at the restriction site. This enzyme system was unable to release either the octacosapeptide or S-14 from the 15,000 mol wt (15K) rat hypothalamic precursor. This argues in favor of sequential degradation of the precursor into S-14 via S-28 as an obligatory intermediate. Another in vivo processing system was analyzed in the anglerfish pancreatic Brockmann organs. Here, cloning of two cDNA corresponding to two mRNA species predicts two distinct somatostatins precursors, called prosomatostatins I and II (Hobart et al., Nature 288:137, 1980). While a single S-14 can be detected in extracts made from this pancreatic tissue, indistinguishable from the mammalian species, two S-28 species could be separated by HPLC. Immunochemical and biochemical evidence indicates that the second species should correspond to anglerfish S-28 (AF S-28), the product of prosomatostatin-II processing in vivo. Amino acid analysis, together with the determined complete amino acid sequence of this peptide, demonstrates that this is indeed the case and that AF S-28 contains in its C-terminal half the [Tyr7, Gly10] derivative of S-14. These observations give an example of a AF S-28 being a terminal active product of prosomatostatin processing. They suggest that this octacosapeptide, which is potent on the inhibition of growth hormone release by anterior pituitary cells, may play such a role in the gastrointestinal tract of the anglerfish. These results, while not excluding alternative routes, give support to a sequential processing of the 15 K precursor----S-28----S-14.

Characterization of a somatostatin-28 containing the (Tyr-7, Gly-10) derivative of somatostatin-14: a terminal active product of prosomatostatin II processing in anglerfish pancreatic islets

Anglerfish (Lophius piscatorius) Brockmann organs contain a form of somatostatin-14, identical to the hypothalamic tetradecapeptide, and two distinct forms of somatostatin-28, which can be separated by reversed-phase high-pressure liquid chromatography (HPLC). Analysis of the NH2-terminal amino acid sequence and comparison of the ability to incorporate 125I indicate that one of these forms corresponds to an octacosapeptide including in its sequence the (Tyr-7, Gly-10) derivative of somatostatin-14 (somatostatin II). Exposure of this somatostatin-28 species to an endopeptidase activity from the rat brain cortex generates a peptide immunologically related to somatostatin and undistinguishable from synthetic (Tyr-7, Gly-10) somatostatin-14 II by HPLC. This somatostatin-28 II exhibits a potent inhibitory effect on growth hormone release by rat anterior pituitary cells, comparable to the other somatostatin-28 form. Since (Tyr-7, Gly-10) somatostatin-14 II cannot be detected in anglerfish pancreatic islets, these results indicate that somatostatin-28 II represents the terminal active product of prosomatostatin II processing, whose structure was predicted from the cDNA nucleotide sequence corresponding to the second mRNA cloned from anglerfish Brockmann organs [Hobart, P., Crawford, R., Shen, L. P., Pictet, R. & Rutter, W. J. (1980) Nature (London) 288, 137-141].