Methods for the investigation of neuropeptide catabolism and stability in vitro

Improved identification and quantitation of mature endogenous peptides in the rodent hypothalamus using a rapid conductive sample heating system

Measurement, identification, and quantitation of endogenous peptides in tissue samples by mass spectrometry (MS) contribute to our understanding of the complex molecular mechanisms of numerous biological phenomena. For accurate results, it is essential to arrest the postmortem degradation of ubiquitous proteins in samples prior to performing peptidomic measurements. Doing so ensures that the detection of endogenous peptides, typically present at relatively low levels of abundance, is not overwhelmed by protein degradation products. Heat stabilization has been shown to inactivate the enzymes in tissue samples and minimize the presence of protein degradation products in the subsequent peptide extracts. However, the efficacy of different heat treatments to preserve the integrity of full-length endogenous peptides has not been well documented; prior peptidomic studies of heat stabilization methods have not distinguished between the full-length (mature) and numerous truncated (possible artifacts of sampling) forms of endogenous peptides. We show that thermal sample treatment via rapid conductive heat transfer is effective for detection of mature endogenous peptides in fresh and frozen rodent brain tissues. Freshly isolated tissue processing with the commercial Stabilizor T1 heat stabilization system resulted in the confident identification of 65% more full-length mature neuropeptides compared to widely used sample treatment in a hot water bath. This finding was validated by a follow-up quantitative multiple reaction monitoring MS analysis of select neuropeptides. The rapid conductive heating in partial vacuum provided by the Stabilizor T1 effectively reduces protein degradation and decreases the chemical complexity of the sample, as assessed by determining total protein content. This system enabled the detection, identification, and quantitation of neuropeptides related to 22 prohormones expressed in individual rat hypothalami and suprachiasmatic nuclei.

Strengthening peptide-based drug activity with novel glyconanoparticle

The therapeutic application of peptide-based drugs is significantly limited by the rapid proteolytic degradation that occurs when in blood. Encapsulation of these peptide structures within a delivery system, such as liposomes, can greatly improve both stability and target delivery. As part of our work focused on novel ambiphilic mannosylated neoglycolipids as targeted drug delivery systems, we have developed a C14-alkyl-mannopyranoside that forms self-assembled monodisperse liposomes. Herein, these glycoliposomes are investigated as a potential method to improve the plasma stability of peptide-based drugs. Reversed phase high-performance liquid chromatography (RP-HPLC) and mass spectrometry (MS) methods were developed to assess the in vitro plasma stability of two structurally diverse peptides, including the kappa opioid receptor selective antagonist dynantin, and the NOD2 innate immune receptor ligand muramyl dipeptide (MDP). The RP-HPLC methods developed were able to resolve the peptides from background plasma contaminants and provided suitable response levels and linearity over an appropriate concentration range. Both compounds were found to be significantly degraded in rat plasma. Increasing degrees of both entrapment and stabilization were noted when dynantin was combined with the C14-alkyl-mannopyranoside in increasing peptide:glycoside ratios. The combination of MDP with the glycolipid also led to peptide entrapment, which greatly improved the plasma stability of the peptide. Overall, the results clearly indicate that the stability of peptide-based structures, which are subject to degradation in plasma, can be greatly improved via entrapment within C14-alkyl-mannopyranoside-bearing glycoliposomes.

Mechanisms underlying the rapid degradation and elimination of the incretin hormones GLP-1 and GIP

The incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP, gastric inhibitory peptide) are secreted from intestinal L and K cells and stimulate insulin secretion from pancreatic beta cells. However, they are immediately inactivated mainly via N-terminal degradation by dipeptidyl peptidase IV (DPP IV, CD26), a specialised enzyme located on the cell surface enzyme of endothelial, epithelial and some other cell types. Cleavage by neprilysin (neutral endopeptidase) is a minor degradation route, and renal clearance eliminates incretin/fragments, but appears of less importance for regulating incretin bioactivities. Based on these observations two novel types of drugs for the treatment of type 2 diabetes have been developed: DPP IV inhibitors and DPP IV-resistant incretin analogues. Both have distinct advantages and disadvantages. Potential side effects of DPP IV inhibitors may result from affecting the bioactivity of other hormones, neuropeptides or chemokines and also by their cross-reactivity with DPP IV-related enzymes.

Cell-surface peptidases

The cell surface has various functions: communicating with other cells, integrating into the tissue, and interacting with the extracellular matrix. Proteases play a key role in these processes. This review focuses on cell-surface peptidases (ectopeptidases, oligopeptidases) that are involved in the inactivation or activation of extracellular regulatory peptides, hormones, paracrine peptides, cytokines, and neuropeptides. The nomenclature of cell-surface peptidases is explained in relation to other proteases, and information is provided on membrane anchoring, catalytic sites, regulation, and, in particular, on their physiological and pharmacological importance. Furthermore, nonenzymatic (binding) functions and participation in intracellular signal transduction of cell surfaces peptidases are described. An overview on the different cell-surface peptidases is given, and their divergent functions are explained in detail. An example of actual pharmacological importance, dipeptidyl-peptidase IV (CD26), is discussed.

Techniques for neuropeptide determination

Because immunoassay responds to epitopes, and many molecules share the same peptide epitope, it is very difficult to obtain an accurate understanding of peptides, their creation and hydrolysis, in biological systems. Separate-and-detect approaches have merit in that the many active peptides and inactive fragments of a particular system can be separately determined. This review discusses the separation, by chromatography and capillary electrophoresis, and detection, by absorbance, fluorescence, electrochemistry, and immunoassay techniques. When separation pre-concentration is accompanied by laser-induced fluorescence or biuret-based electrochemical detection, nM-pM detection limits are obtained.

Metabolism of dynorphins by peptidases of pulmonary artery endothelial cells

Degradation of several dynorphins by peptidases expressed in cultured porcine pulmonary artery endothelial cells was studied by incubation of the peptide in cell suspensions followed by electrospray ionization and tandem mass spectrometric analyses. Under the in vitro conditions applied, only the metabolism of dynorphin A1-8 occurred in a significant extent. Studies involving specific peptidase inhibitors indicated that mainly bestatin-sensitive aminopeptidases, thiorphan-sensitive endopeptidases, and cFPAAF-pAB-sensitive endopeptidases expressed by the endothelial cells were involved in the process that converted dynorphin A1-8 to dynorphin A2-8, dynorphin A1-6, and leucine enkephalin (dynorphin A1-5), respectively. These peptidases may form a metabolic barrier for the cellular penetration of intact dynorphin A1-8 and/or control effects of the circulating peptide on endothelial opioid receptors of the cells.

In vitro quantitative study of the degradation of endomorphins

The catabolism of the endomorphins was investigated in detail. The endomorphins were degraded relatively slowly in the rat brain homogenate (t1/2(endomorphin-1)=4.94 min; t1/2(endomorphin-2)=3.81 min). The inhibition of metalloproteases and aminopeptidases stabilised the endomorphins to the greatest extent. The digestion of endomorphins tritiated specifically on Tyr(1), Pro(2) or Phe(3) established also that only the aminopeptidase pathways were essential for inactivation of the endomorphins, and that the tetrapeptides were degraded by cleavage of the Pro(2)-Trp(3) or Pro(2)-Phe(3) bond. The end-products of the catabolism were amino acids; the fragments Tyr-Pro-OH and Pro-Trp-Phe-NH2 were present as intermediates. Metabolites produced by brain carboxypeptidases were not detected.

Characterization of human nasal primary culture systems to investigate peptide metabolism

The objectives of this study were to validate and compare the suitability of different primary cell culture systems as models to investigate peptide enzymatic stability following nasal administration. The degradation kinetics of a model peptide, leucine enkephalin (Tyr-Gly-Gly-Phe-Leu, Leu-Enk), was determined in four nasal cell culture systems: immersion, air-liquid interface, sequential monolayer-suspension, floating collagen. The influence of enzyme inhibitors (bestatin, puromycin) and Leu-Enk metabolite analogs (Tyr-Gly, Phe-Leu, Tyr-Gly-Gly, Gly-Phe-Leu) on the Leu-Enk degradation profile was also investigated. The disappearance of Leu-Enk in all the cell culture systems followed first order kinetics. The specific activity in the cell culture systems followed the rank: sequential monolayer-suspension (32.60 microM min(-1) mg(-1)) >air-liquid interface (15.19 microM min(-1) mg(-1)) >immersion (11.49 microM min(-1) mg(-1)) >floating collagen (4.57 microM min(-1) mg(-1)). At equimolar concentration, bestatin had a higher inhibitory effect than puromycin. The rate of hydrolysis of Leu-Enk was reduced significantly by co-incubation with Leu-Enk metabolite analogs. This study showed that immersion, sequential monolayer-suspension and air-liquid interface culture systems may be potentially suitable for further studies on peptide enzymatic stability following nasal administration.

Proline-specific dipeptidyl peptidase activity in the cockroach brain and intestine: partial characterization, distribution, and inactivation of tachykinin-related peptides

Proline-specific dipeptidyl peptidase (DPP IV) is an established enzyme known to degrade neuropeptides and peptide hormones in vertebrate tissues. DPP IV cleaves peptides at the Pro2 residue. Because several neuropeptides of the cockroach Leucophaea maderae, such as LemTRP-1 (APSGFLGVRamide), are potential substrates for this peptidase, we investigated the occurrence of proline-specific DPP activity in cockroach tissues. Partly purified DPP activity was characterized from the brain and midgut of L. maderae by using Gly-Pro-4-nitroanilide as a substrate. The highest activity was obtained from the membrane fraction of intestine; about 10 times less activity (per milligram protein) was obtained from brain membranes. A smaller amount of soluble DPP activity could also be identified in both tissues. Gel chromatography of the solubilized intestinal DPP activity revealed a molecular mass of about 75 kDa. The enzyme had a pH optimum of 8.5. Diprotin A (Ile-Pro-Ile) was an efficient competitive inhibitor of the cockroach DPP, whereas other known DPP inhibitors were found to be less potent. When incubated with human and cockroach DPP IV, the cleavage products of LemTRP-1 were AP and SGFLGVRamide (des-AP-LemTRP-1) as determined by mass spectrometry of high-performance liquid chromatography (HPLC)-purified peptide fragments. The AP fragment was biologically inactive and the des-AP fragment had a drastically reduced myostimulatory activity on the hindgut of L. maderae. The blowfly TRP callitachykinin-I (CavTK-I; APTAFYGVRamide) was cleaved in two steps to des-AP-CavTK-I and desAPTA-CavTK-I, showing that cockroach DPP does not only liberate Xaa-Pro, but also Xaa-Ala dipeptides. The fragment desAPTA-CavTK-I was completely inactive on the cockroach hindgut. To compare, LemTRP-3 and CavTK-II, which lack a Pro2, were not cleaved by DPP IV. Enzyme histochemistry for DPP IV was performed on cryostat sections of brain and intestine with Gly-Pro-4-methoxy-2-naphthylamide as the substrate and Fast Blue B as the chromogen. Strong histochemical labeling was seen in specific neuropils of the brain such as the calyces of the mushroom bodies, the antennal glomeruli, and the central body. Also, the inner lining of the midgut (the peritrophic membrane) and the malpighian tubules were strongly labeled by reaction product. In both the brain and intestine, the enzyme-histochemical reaction was inhibited by diprotin A.

GLP-1-analogues resistant to degradation by dipeptidyl-peptidase IV in vitro

Glucagon-like peptide-1 (GLP-1) stimulates insulin secretion and improves glycemic control in type 2 diabetes. In serum the peptide is degraded by dipeptidyl peptidase IV (DPP IV). The resulting short biological half-time limits the therapeutic use of GLP-1. DPP IV requires an intact alpha-amino-group of the N-terminal histidine of GLP-1 in order to perform its enzymatic activity. Therefore, the following GLP- analogues with alterations in the N-terminal position 1 were synthesized: N-methylated- (N-me-GLP-1), alpha-methylated (alpha-me-GLP-1), desamidated- (desamino-GLP-1) and imidazole-lactic-acid substituted GLP-1 (imi-GLP-1). All GLP-1 analogues except alpha-me-GLP-1 were hardly degraded by DPP IV in vitro. The GLP-1 analogues showed receptor affinity and in vitro biological activity comparable to native GLP-1 in RINm5F cells. GLP-1 receptor affinity was highest for imi-GLP-1, followed by alpha-me-GLP-1 and N-me-GLP-1. Only desamino-GLP-1 showed a 15-fold loss of receptor affinity compared to native GLP-1. All analogues stimulated intracellular cAMP production in RINm5F cells in concentrations comparable to GLP-1. N-terminal modifications might therefore be useful in the development of long-acting GLP-1 analogues for type 2 diabetes therapy.

Liquid chromatographic study of the enzymatic degradation of endomorphins, with identification by electrospray ionization mass spectrometry

The recently discovered native endomorphins play an important role in opioid analgesia, but their metabolic fate in the organism remains relatively little known. This paper describes the application of high-performance liquid chromatography combined with electrospray ionization mass spectrometry to identify the degradation products resulting from the incubation of endomorphins with proteolytic enzymes. The native endomorphin-1, H-Tyr-Pro-Trp-Phe-NH2 (1), and endomorphin-2, H-Tyr-Pro-Phe-Phe-NH2 (2), and an analog of endomorphin-2, H-Tyr-Pro-Phe-Phe-OH (3), were synthetized, and the levels of their resistance against carboxypeptidase A, carboxypeptidase Y, aminopeptidase M and proteinase A were determined. The patterns of peptide metabolites identified by this method indicated that carboxypeptidase Y first hydrolyzes the C-terminal amide group to a carboxy group, and then splits the peptides at the Trp3-Phe4 or Phe3-Phe4 bond. The remaining fragment peptides are stable against the enzymes investigated. Carboxypeptidase A degrades only analog 3 at the Phe3-Phe4 bond. Aminopeptidase M cleaves the peptides at the Pro2-Trp3 or Pro2-Phe3 bond. The C-terminal fragments hydrolyze further, giving amino acids and Phe-NH2-s while the N-terminal part displays a resistance to further aminopeptidase M digestion. Proteinase A exhibits a similar effect to carboxypeptidase Y: the C-terminal amide group is first converted to a carboxy group, and one amino acid is then split off from the C-terminal side.

Biological activity of GLP-1-analogues with N-terminal modifications

Glucagon-like peptide-1 (GLP-1) stimulates insulin secretion and improves glycemic control in type 2 diabetes. In serum the peptide is degraded by dipeptidyl peptidase IV (DPP IV). The resulting short biological half-time limits the therapeutic use of GLP-1. Therefore, various GLP-1 analogues with alterations in cleavage positions were synthesized. GLP-1-receptor binding was investigated in RINm5F cells. Biological activity of the GLP-1 analogues was investigated in vitro by measuring cAMP production in RINm5F cells. GLP-1 analogues with modifications in position 2 were not cleaved by DPP IV and showed receptor affinity and in vitro biological activity comparable to native GLP-1. Analogues with alterations in positions 2 and 8, 2 and 9 or 8 and 9 showed a significant decrease in receptor affinity and biological activity. In vivo biological activity was tested in pigs. GLP-1 analogues were administered subcutaneously followed by an intravenous bolus injection of glucose. Plasma glucose and insulin were monitored over 4 h. Compared to native GLP-1, analogues with an altered position 2 showed similar or increased potency and biological half-time. Other GLP-1 analogues were less active. Despite the lack of degradation of these GLP-1 analogues by DPP IV in vitro, their biological action is as short as that of GLP-1, except for desamino-GLP-1, indicating that other degradation enzymes are important in vivo. Alterations of GLP-1 in positions 8 or 9 result in a loss of biological activity without extending biological half-time.

Receptors and effects of the inhibitory neuropeptide somatostatin in microglial cells

The expression of receptors for the neuropeptide somatostatin was investigated in cultured immunocytochemically pure rat microglial cells. By the reverse transcriptase-polymerase chain reaction, the mRNAs for the receptor subtypes sst2, sst3 and sst4, but not sst1 and sst5 could be detected. To show that these receptors were functionally active, the effects of somatostatin and the metabolically stable, receptor subtype (2, 3 and 5) selective derivative octreotide (SMS 201-995, Sandostatin) on protein phosphorylation and proliferation were evaluated. Somatostatin induced the tyrosine phosphorylation of a 95 kDa protein in microglia. Furthermore, somatostatin or octreotide inhibited the basal as well as the GM-CSF-(granulocyte macrophage colony-stimulating factor) or the IL-3-(interleukin-3)-stimulated proliferation of microglial cells. This effect was dose-dependent, with a half maximum activity of about 0.2-0.3 nM. Somatostatin was relatively stable in the cultures due to protease inhibitors in the serum. The results indicate that microglial cells are targets for the widespread neuropeptide somatostatin and that its receptors can transduce complex signals to microglia.

Proline-specific dipeptidyl peptidase from the blue blowfly Calliphora vicina hydrolyzes in vitro the ecdysiostatic peptide trypsin-modulating oostatic factor (Neb-TMOF)

To elucidate the mechanisms of inactivation of the ecdysiostatic peptide trypsin-modulating oostatic factor (Neb-TMOF) in the blue blowfly Calliphora vicina, we investigated its proteolytic degradation. In homogenates and membrane and soluble fractions, this hexapeptide (sequence: NPTNLH) was hydrolyzed into two fragments, NP and TNLH, suggesting the involvement of a proline-specific dipeptidyl peptidase. The dipeptidyl peptidase activity was highest in the late larval stage. It was purified 240-fold from soluble fractions of pupae of mixed age and classified on the basis of several catalytic properties as an invertebrate homologue of mammalian dipeptidyl peptidase IV (EC 3.4.14.5). Fly dipeptidyl peptidase IV has a molecular mass of 200 kDa, showed a pH optimum of 7.5-8.0 with the chromogenic substrate Gly-Pro-4-nitroanilide, and cleaved other chromogenic substrates with penultimate Pro or, with lower activity, Ala. It liberated Xaa-Pro dipeptides from the N-terminus of several bioactive peptides including substance P, neuropeptide Y, and peptide YY but not from bradykinin, indicating that the peptide bond between the two proline residues was resistant to cleavage. Fly dipeptidyl peptidase belongs to the serine class of proteases as the mammalian enzyme does; the fly enzyme, however, is not inhibited by several selective or nonselective inhibitors of its mammalian counterpart. It is suggested that dipeptidyl peptidases exert a regulatory role for the clearance not only of TMOF in files but for other bioactive peptides in various invertebrates.

Meningeal cells are targets and inactivation sites for the neuropeptide somatostatin

Transcripts of the somatostatin receptor subtypes sst3 and sst2 are expressed in meninges from rat brain as well as in immunocytochemical pure rat meningeal cells and rat fibroblasts in culture. mRNA of three other subtypes tested are absent or detected in trace amounts by reverse transcription-polymerase chain reaction. Presence of active receptors on the surface of meningeal cells and fibroblasts could be verified by direct visualisation of binding sites by affinity labelling with a somatostatin gold conjugate. The metabolically stable somatostatin agonist SMS 201-995 (octreotide) had a time-dependent effect on the [3H]thymidine incorporation by meningeal cells: after 2-5 h, the agonist inhibited cell proliferation to about 80% of controls, after 24 h proliferation was stimulated to about 150% of controls. Apart from being targets for somatostatin, meningeal cells had a high capacity to inactivate the peptide by proteolytic degradation. By analysis of cleavage sites and use of specific inhibitors, endopeptidase-24.11 ('enkephalinase', neutral endopeptidase, neprilysin, EC 3.4.24.11) was identified to be responsible for the initial catabolism of the peptide whereas aminopeptidase(s) truncated the fragments. Thus, meningeal cells express transcripts of multiple somatostatin receptor subtypes and produce peptidases that inactivate the neuropeptide somatostatin.