High-affinity receptor sites and rapid proteolytic inactivation of neurotensin in primary cultured neurons

Retroinverso analogs of spadin display increased antidepressant effects

RATIONALE

Although depression is the most common mood disorder, only one third of patients are treated with success. Finding new targets, new drugs, and also new drug intake way are the main challenges in the depression field. Several years ago, we identified a new target with the TWIK-related potassium channel-1 (TREK-1) potassium channel, and more recently, we have discovered a peptide of 17 amino acids with antidepressant properties. This peptide, that we called spadin, can be considered as a new concept in antidepressant drug design. Spadin derives from a larger peptide resulting to a posttranslational maturation of sortilin; consequently, spadin can be considered as a natural molecule. Moreover, spadin acts more rapidly than classical antidepressants and does not induce side effects.

OBJECTIVES

In this work, we sought analogs of spadin displaying a better affinity on TREK-1 channels and an increased action duration.

METHODS

Analogs were characterized by electrophysiology measurements, by behavioral tests, and by their ability to induce neurogenesis.

RESULTS

We identified two retro-inverso peptides that have kept the antidepressant properties of spadin; particularly, they increased the hippocampal neurogenesis after a 4-day treatment. As spadin, these analogs did not induce side effects on either pain, epilepsy processes, or at the cardiac level.

CONCLUSIONS

Together, our results indicated that spadin retro-inverso peptides could represent new potent antidepressant drugs. As exemplified by spadin in the field of depression, retro-inverso strategies could represent a useful technique for developing new classes of drugs in a number of pathologies.

Neurotensin inversely modulates maternal aggression

Neurotensin (NT) is a versatile neuropeptide involved in analgesia, hypothermia, and schizophrenia. Although NT is released from and acts upon brain regions involved in social behaviors, it has not been linked to a social behavior. We previously selected mice for high maternal aggression (maternal defense), an important social behavior that protects offspring, and found significantly lower NT expression in the CNS of highly protective females. Our current study directly tested NT's role in maternal defense. Intracerebroventricular (i.c.v.) injections of NT significantly impaired defense in terms of time aggressive and number of attacks at all doses tested (0.05, 0.1, 1.0, and 3.0 microg). Other maternal behaviors, including pup retrieval, were unaltered following NT injections (0.05 microg) relative to vehicle, suggesting specificity of NT action on defense. Further, i.c.v. injections of the NT receptor 1 (NT1) antagonist, SR 48692 (30 microg), significantly elevated maternal aggression in terms of time aggressive and attack number. To understand where NT may regulate aggression, we examined Fos following injection of either 0.1 microg NT or vehicle. Thirteen of 26 brain regions examined exhibited significant Fos increases with NT, including regions expressing NT1 and previously implicated in maternal aggression, such as lateral septum, bed nucleus of stria terminalis, paraventricular nucleus, and central amygdala. Together, our results indicate that NT inversely regulates maternal aggression and provide the first direct evidence that lowering of NT signaling can be a mechanism for maternal aggression. To our knowledge, this is the first study to directly link NT to a social behavior.

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.

A highly sensitive and selective radioimmunoassay for the measurement of neurotensin

A highly selective and sensitive radioimmunoassay (RIA) for the detection of endogenous neurotensin (NT) has been developed. We have raised a C-terminally-directed antibody (CAb) that specifically binds 'biologically active' NT (NT and NT(8-13)) and that does not significantly cross-react with inactive NT metabolites or other bioactive peptides in the CNS. By reducing the volume of the assay to a low volume-RIA (30 microl), such that in vivo measurements can be made, we have increased the sensitivity (<0.3 fmol per tube), with inter- and intra-assay variations of 11.2 and 5.8%, respectively. Comparisons with similar methods of detecting NT have demonstrated that this RIA has a higher sensitivity than previously used RIA's and ELISA's. The data presented suggests that this sensitive RIA is a reliable method ideal for the detection of small quantities of biologically active NT.

Developmental changes in neurotensin and its metabolites in the neonatal rat

Neurotensin-like immunoreactivity (NT-LI) was measured in the di-, tel- and mesencephalon of rats from embryonic day 15 (E15) through birth ( approximately E22) until postnatal day 5 (P5) using radioimmunoassay (RIA) and an N-terminal directed polyclonal antibody. NT-LI and NT metabolite-like immunoreactivities (NT 1-8, NT 1-10, NT 1-11 and NT 1-12-LI) were also similarly determined using high performance liquid chromatography (HPLC) coupled with RIA. NT-LI was low at E15 but increased to peak levels at around E20 or birth in the di- and telencephalon, after which the levels declined. Similar, but lower, changes were observed with NT 1-10-LI but not other metabolites while much lower NT-LI and metabolites were observed in the mesencephalon where no transitory changes occurred. The changes in neonatal rat brain NT and metabolites are discussed with respect to the possible neonatal trophic roles of these peptides.

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.

Effect of a novel selective and potent phosphinic peptide inhibitor of endopeptidase 3.4.24.16 on neurotensin-induced analgesia and neuronal inactivation

1. We have examined a series of novel phosphinic peptides as putative potent and selective inhibitors of endopeptidase 3.4.24.16. 2. The most selective inhibitor, Pro-Phe-psi(PO2CH2)-Leu-Pro-NH2 displayed a Ki value of 12 nM towards endopeptidase 3.4.24.16 and was 5540 fold less potent on its related peptidase endopeptidase 3.4.24.15. Furthermore, this inhibitor was 12.5 less potent on angiotensin-converting enzyme and was unable to block endopeptidase 3.4.24.11, aminopeptidases B and M, dipeptidylaminopeptidase IV and proline endopeptidase. 3. The effect of Pro-Phe-psi(PO2CH2)-Leu-Pro-NH2, in vitro and in vivo, on neurotensin metabolism in the central nervous system was examined. 4. Pro-Phe-psi(PO2CHH2)-Leu-Pro-NH2 dose-dependently inhibited the formation of neurotensin 1-10 and concomittantly protected neurotensin from degradation by primary cultured neurones from mouse embryos. 5. Intracerebroventricular administration of Pro-Phe-psi(PO2CH2)-Leu-Pro-NH2 significantly potentiated the neurotensin-induced antinociception of mice in the hot plate test. 6. Altogether, our study has established Pro-Phe-psi(PO2CH2)-Leu-Pro-NH2 as a fully selective and highly potent inhibitor of endopeptidase 3.4.24.16 and demonstrates, for the first time, the contribution of this enzyme in the central metabolism of neurotensin.

Neurotensin and neuroendocrine regulation

More than two decades of research indicate that the peptide neurotensin (NT) and its cognate receptors participate to a remarkable extent in the regulation of mammalian neuroendocrine systems, potentially at multiple levels in a given system. NT-synthesizing neurons appear to exert a direct or indirect stimulatory influence on neurosecretory cells that synthesize gonadotropin-releasing hormone, dopamine (DA), somatostatin, and corticotropin-releasing hormone (CRH). In addition, context-specific synthesis of NT occurs in hypothalamic neurosecretory cells located in the arcuate nucleus and parvocellular paraventricular nucleus, including distinct subsets of cells which release DA, CRH, or growth hormone-releasing hormone into the hypophysial portal circulation. At the level of the anterior pituitary, NT stimulates secretion of prolactin and occurs in subsets of gonadotropes and thyrotropes. Moreover, circulating hormones influence NT synthesis in the hypothalamus and anterior pituitary, raising the possibility that NT mediates certain feedback effects of the hormones on neuroendocrine cells. Gonadal steroids alter NT levels in the preoptic area, arcuate nucleus, and anterior pituitary; adrenal steroids alter NT levels in the hypothalamic periventricular nucleus and arcuate nucleus; and thyroid hormones alter NT levels in the hypothalamus and anterior pituitary. Finally, clarification of the specific neuroendocrine roles subserved by NT should be greatly facilitated by the use of newly developed agonists and antagonists of the peptide.

Distinct properties of neuronal and astrocytic endopeptidase 3.4.24.16: a study on differentiation, subcellular distribution, and secretion processes

Endopeptidase 3.4.24.16 belongs to the zinc-containing metalloprotease family and likely participates in the physiological inactivation of neurotensin. The peptidase displays distinct features in pure primary cultured neurons and astrocytes. Neuronal maturation leads to a decrease in the proportion of endopeptidase 3.4.24.16-bearing neurons and to a concomitant increase in endopeptidase 3.4.24.16 activity and mRNA content. By contrast, there is no change with time in endopeptidase 3.4.24.16 activity or content in astrocytes. Primary cultured neurons exhibit both soluble and membrane-associated endopeptidase 3.4.24.16 activity. The latter behaves as an ectopeptidase on intact plated neurons and resists treatments with 0.2% digitonin and Na2CO3. Further evidence for an association of the enzyme with plasma membranes was provided by cryoprotection experiments and electron microscopic analysis. The membrane-associated form of endopeptidase 3.4.24.16 increased during neuronal differentiation and appears to be mainly responsible for the overall augmentation of endopeptidase 3.4.24.16 activity observed during neuronal maturation. Unlike neurons, astrocytes only contain soluble endopeptidase 3.4.24.16. Astrocytes secrete the enzyme through monensin, brefeldin A, and forskolin-independent mechanisms. This indicates that endopeptidase 3.4.24.16 is not released by classical regulated or constitutive secreting processes. However, secretion is blocked at 4 degrees C and by 8 bromo cAMP and is enhanced at 42 degrees C, two properties reminiscent of that of other secreted proteins lacking a classical signal peptide. By contrast, neurons appear unable to secrete endopeptidase 3.4.24.16.

Neurotensin receptors: binding properties, transduction pathways, and structure

Neurotensin is a 13-amino acid peptide (pGlu-Leu-Tyr-Glu-Asn-Lys-Pro-Arg-Arg-Pro-Tyr-Ile-Leu) originally isolated from hypothalami (Carraway and Leeman, 1973) and later from intestines (Kitabgi et al., 1976) of bovine. The peptide is present throughout the animal kingdom, suggesting its participation to important processes basic to animal life (Carraway et al., 1982). Neurotensin and its analogue neuromedin-N (Lys-Ile-Pro-Tyr-Ile-Leu) (Minamino et al., 1984) are synthesized by a common precursor in mammalian brain (Kislauskis et al., 1988) and intestine (Dobner et al., 1987). The central and peripheral distribution and effects of neurotensin have been extensively studied. In the brain, neurotensin is exclusively found in nerve cells, fibers, and terminals (Uhl et al., 1979), whereas the majority of peripheral neurotensin is found in the endocrine N-cells located in the intestinal mucosa (Orci et al., 1976; Helmstaedter et al., 1977). Central or peripheral injections of neurotensin produce completely different pharmacological effects (Table I) indicating that the peptide does not cross the blood-brain barrier. Many of the effects of centrally administered neurotensin are similar to those of neuroleptics or can be antagonized by simultaneous administration of TRH (Table I). The recently discovered nonpeptide antagonist SR 48692 (Gully et al., 1993) can inhibit several of the central and peripheral effects of neurotensin (Table I). Like many other neuropeptides, neurotensin is a messenger of intracellular communication working as a neurotransmitter or neuromodulator in the brain (Nemeroff et al., 1982) and as a local hormone in the periphery (Hirsch Fernstrom et al., 1980). Thus, several pharmacological, morphological, and neurochemical data suggest that one of the functions of neurotensin in the brain is to regulate dopamine neurotransmission along the nigrostriatal and mesolimbic pathways (Quirion, 1983; Kitabgi, 1989). On the other hand, the likely role of neurotensin as a parahormone in the gastrointestinal tract has been well documented (Rosell and Rökaeus, 1981; Kitabgi, 1982). Both central and peripheral modes of action of neurotensin imply as a first step the recognition of the peptide by a specific receptor located on the plasma membrane of the target cell. Formation of the neurotensin-receptor complex is then translated inside the cell by a change in the activity of an intracellular enzyme. This paper describes the binding and structural properties of neurotensin receptors as well as the signal transduction pathways that are activated by the peptide in various target tissues and cells.

Phosphorus-containing peptides as mixed inhibitors of endopeptidase 3.4.24.15 and 3.4.24.16: effect on neurotensin degradation in vitro and in vivo

1. We have examined several phosphorus-containing peptides as potential mixed inhibitors of two neurotensin-degrading zinc metallopeptidases, endopeptidase 3.4.24.15 and endopeptidase 3.4.24.16. 2. Among a series of 13 phosphonamide peptides, N-(2-(2-naphtyl)ethylphosphonyl-glycyl-prolyl-norleucine (phosphodiepryl 08) was found to inhibit potently the hydrolysis of neurotensin by purified endopeptidase 3.4.24.15 and 3.4.24.16 with an identical Ki value of 0.4 nM. 3. Phosphodiepryl 08 displayed a strong selectivity towards the two peptidases since it failed to inhibit several other zinc-containing peptidases such as endopeptidase 3.4.24.11, angiotensin-converting enzyme, aminopeptidase M, leucine aminopeptidase and carboxypeptidases A and B. 4. The protective effect of phosphodiepryl 08 on neurotensin degradation was examined in vitro and in vivo in central and peripheral bioassays. 5. Phosphodiepryl 08 virtually abolished neurotensin degradation by 4-day-old plated pure cultured neurones from mouse embryos and greatly potentiated neurotensin-induced antinociception in the mouse hot plate test. 6. In the periphery, phosphodiepryl 08 inhibited neurotensin degradation by membranes prepared from isolated longitudinal smooth muscle of guinea-pig ileum and greatly potentiated the neurotensin-induced contraction of the same longitudinal smooth muscle preparation. 7. Our study indicates that phosphodiepryl 08 behaves as a potent and selective mixed inhibitor of endopeptidase 3.4.24.15 and 3.4.24.16 and can be used as a powerful agent to prevent neurotensin degradation, in vitro and in vivo, in central and peripheral assays.

Binding and internalization of neurotensin in hybrid cells derived from septal cholinergic neurons

Autoradiographic studies from our laboratory have previously demonstrated a selective association of high affinity neurotensin (NT) binding sites with basal forebrain cholinergic neurons. In search of an in vitro model for further characterization of the role and regulation of these sites, we have examined the binding and internalization of 125I-Tyr3-NT (125I-NT) and fluorescein isothiocyanate (FITC)-conjugated NT (fluo-NT) on SN17 hybrid cells, produced by fusion of embryonic murine septal cells with neuroblastoma. 125I-NT binding to SN17 membrane preparations was specific and saturable. Scatchard analysis of the data was suggestive of an interaction with a single population of sites, the affinity (Kd = 1.7 nM) and pharmacological profile of which were comparable to those of neural NT receptors. No specific binding was observed on the parent neuroblastoma cell line, confirming that the expression of those sites is a neuronal trait. Incubation of whole SN17 cells with 125I-NT resulted in a time- and temperature-dependent internalization of the specifically bound peptide. The t1/2 of this internalization was estimated at 13 min, a value nearly identical to that reported for neurons in culture. Confocal microscopic analyses using fluo-NT indicated that the internalization process was endocytic in nature in that: 1) it was entirely blocked by the endocytosis inhibitor phenylarsine oxide; and 2) it was mediated through small intracytoplasmic particles the size and maturation of which corresponded to that of endosomes. It is proposed that the expression and internalization of NT receptors by SN17 hybrid cells represent a new facet of these cells' cholinergic phenotype that makes them amenable to the study of NT interactions with cholinergic cells.

Changes in mouse brain neurotensin receptor density following chronic infusion of neurotensin

Many centrally acting drugs affect neurotensin (NT) systems by increasing levels of the peptide in specific brain regions. If these changes represent increases in extracellular NT levels, then changes in NT receptors would be expected. The focus of this study was to examine the effects of continuous exposure of NT receptors to agonist. Continuous infusion of NT (0.6 or 6 nmol/h) into the lateral ventricle via an osmotic minipump for 3 days caused a significant increase (over saline infusion) in total and low-affinity NT receptor density in the cerebellum of LS mice. High-affinity NT receptor density was increased in the frontal cortex. Seven days of NT infusion (6 nmol/h) caused no changes in NT receptor density.

Neurotensin and neuromedin N undergo distinct catabolic processes in murine astrocytes and primary cultured neurons

We examined the occurrence of various endopeptidases and exopeptidases and their subcellular partition within soluble and membrane-associated compartments of 15-day-old astrocytes and 4-day-old primary cultured neurons. Peptidases were monitored with chromogenic or fluorimetric substrates and identified by means of specific inhibitors. We assessed the contribution of these peptidases in the catabolism of two related neuropeptides, neurotensin and neuromedin N. Metabolites were separated by HPLC and the identity of the proteolytic activities involved in their formation was established using specific inhibitors. Neuromedin N and neurotensin undergo both quantitative and qualitative differential proteolysis. Initial maximal rates of neuromedin N degradation were higher than those of neurotensin in both cell types. Furthermore, the two peptides were inactivated much more rapidly by the soluble than by the membrane-associated fractions prepared from both cell cultures. Neuromedin N was rapidly broken down by an aminopeptidase M/leucine aminopeptidase attack, leading to the functionally silent Des-Lys1-neuromedin N metabolite. In the astrocytic membrane-associated fraction, neuromedin N underwent an additional minor endoproteolytic cleavage at the Pro3-Tyr4 bond elicited by endopeptidase 24.11, as suggested by the protective effect of its blocking agent phosphoramidon. Unlike neuromedin N, neurotensin totally resisted hydrolysis by aminopeptidases. Primary inactivating cleavages detected in both cell types appeared mainly located at the Arg8-Arg9 and Pro10-Tyr11 bonds, leading to the formations of neurotensin-(1-8) and neurotensin-(1-10) as the major biologically inactive neurotensin catabolites. Endopeptidase 24.15 appeared mainly responsible for neurotensin-(1-8) formation by the soluble fraction of neurons and astrocytes. In contrast, endopeptidase 24.16 was involved in neurotensin-(1-10) formation by both soluble and membrane-associated fractions of the two cell types. An additional cleavage leading to neurotensin-(1-11) formation and ascribed to endopeptidase 24.11 was detected mainly in the membrane-associated fraction from astrocytes. Finally, the secondary processing of neurotensin degradation products indicated that: (a) neurotensin-(1-11) was converted into neurotensin-(1-8) in the membrane fraction prepared from astrocytes; (b) neurotensin-(1-10) was transformed into neurotensin-(1-8) by an unidentified peptidase belonging to the class of metalloenzymes. The significance of distinct quantitative and qualitative catabolic fates of neuromedin N and neurotensin in cultured astrocytes and neurons is discussed.

Rat kidney endopeptidase 24.16. Purification, physico-chemical characteristics and differential specificity towards opiates, tachykinins and neurotensin-related peptides

Endopeptidase 24.16 was purified from rat kidney homogenate on the basis of its ability to generate the biologically inactive degradation products neurotensin (1-10) and neurotensin (11-13). On SDS gels of the proteins pooled after the last purification step, the enzyme appeared homogeneous and behaved as a 70-kDa monomer. The peptidase was not sensitive to specific inhibitors of aminopeptidases, pyroglutamyl aminopeptidase I, endopeptidase 24.11, endopeptidase 24.15, proline endopeptidase and angiotensin-converting enzyme but was potently inhibited by several metal chelators such as o-phenanthroline and EDTA and was blocked by divalent cations. The specificity of endopeptidase 24.16 towards peptides of the tachykinin, opioid and neurotensin families was examined by competition experiments of tritiated neurotensin hydrolysis as well as HPLC analysis. These results indicated that endopeptidase 24.16 could discriminate between peptides belonging to the same family. Neurotensin, Lys8-Asn9-neurotensin(8-13) and xenopsin were efficiently hydrolysed while neuromedin N and kinetensin underwent little if any proteolysis by the peptidase. Analogously, substance P and dynorphins (1-7) and (1-8) were readily proteolysed by endopeptidase 24.16 while neurokinin A, amphibian tachykinins and leucine or methionine enkephalins totally resisted degradation. By Triton X-114 phase separation, 15-20% of endopeptidase 24.16 partitioned in the detergent phase, indicating that renal endopeptidase 24.16 might exist in a genuine membrane-bound form. The equipotent solubilization of the enzyme by seven detergents of various critical miscellar concentrations confirmed the occurrence of a membrane-bound counterpart of endopeptidase 24.16. Furthermore, the absence of release elicited by phosphatidylinositol-specific phospholipase C suggested that the enzyme was not attached by a glycosyl-phosphatidylinositol anchor in the membrane of renal microvilli. Finally, endopeptidase 24.16 could not be released from these membranes upon trypsinolysis.

Ontogenesis and binding properties of high-affinity neurotensin receptors in human brain

The ontogenesis of neurotensin binding sites was studied in human brain of subjects deceased from Sudden Infant Death Syndrome. Monoiodo-Tyr3 neurotensin specifically recognized 2 distinct classes of binding sites in human brain homogenate. The high affinity sites were already present at birth and increased to a maximal level of 240 fmol/mg protein 1 month after birth. Thereafter, the density of these sites decreased to reach a value of 8 fmol/mg protein in 15-month-old brain, a value similar to that found in adult brain. The dissociation constant of the high-affinity sites (about 0.3 nM) did not vary from birth to adulthood. The high-affinity binding sites were sensitive to GTP which decreased their affinity for neurotensin by a factor of 3, indicating that these sites are functional receptors coupled to GTP-binding proteins. By contrast, the low-affinity sites were insensitive to GTP and could be partly blocked by the antihistaminic drug levocabastine. These sites were absent in human brain during the first post-natal year and could be detected only in brain homogenate of 15-month-old infants. The transient increase in high-affinity neurotensin binding sites after birth suggests that neurotensin could act as a regulatory peptide during brain development.

Rapid agonist-induced decrease of neurotensin receptors from the cell surface in rat cultured neurons

The regulation of neurotensin receptors was studied in vitro in primary cultures of neuronal cells. High affinity receptors for [3H]neurotensin were found in homogenates and at the cell surface of intact neurons cultured from the brain of rat embryos. When intact cells were incubated with 3 nM neurotensin (1-13), a rapid decrease in [3H]neurotensin binding was observed; about 60% of neurotensin receptors disappeared from the cell surface in less than 15 min. This corresponded to a reduction of the Bmax value without a change in the binding affinity. The decrease in neurotensin receptor number was also induced by the active fragment (8-13) of neurotensin but not by its inactive fragment (1-8). It was partially inhibited by bacitracin, at concentrations which are known to interact with receptor internalization, and was not detected when intact cells were incubated at 0-4 degrees with the unlabeled peptide. When intact neurons were incubated with [3H]neurotensin, there was a rapid ligand uptake and the kinetics of endocytosis were similar to those of the cell surface receptor disappearance. Once endocytosed, [3H]neurotensin could not be released (or displaced) from either intact neurons or homogenates, suggesting the sequestration of the labeled peptide in vesicles or other subcellular structures. Therefore, the present results suggest that the rapid agonist-induced decrease in the number of neurotensin receptors from the cell surface corresponds to an internalization process which involves a simultaneous receptor-mediated peptide endocytosis.

Binding and internalization of iodinated neurotensin in neuronal cultures from embryonic mouse brain

The binding and internalization of labeled neurotensin were studied by means of biochemical and light microscopic radioautography techniques in primary cultures of neurons from whole cerebral hemispheres of mouse embryos. Saturable, high affinity neurotensin binding was detected 5-7 days postplating in cells incubated with 0.1 nM 125I-Tyr3-neurotensin at 37 degrees C or 10 degrees C. The binding capacity at equilibrium was 3 times higher at 37 degrees C than at 10 degrees C. Moreover, whereas virtually all the radioactivity bound at 10 degrees C was membrane-bound (i.e. was readily washable by a hypertonic, high pH, NaCl solution), more than 70% of the radioactivity bound at 37 degrees C was intracellular (i.e. resisted the same treatment). Light microscopic radioautograms of whole cells revealed that approximately 16% of neurons were labeled with 125I-Tyr3-neurotensin at either 37 degrees C or 10 degrees C. The labeling was observed over cell bodies and processes, and the density of silver grains associated with perikarya, as compared to processes, was proportionally higher at 37 degrees C than at 10 degrees C. Semi-thin (1 micron thick) sections through cells incubated at 37 degrees C confirmed that a major fraction of the radioactivity was intracellular and showed that it was mainly confined to the cytoplasm. These results indicate that 125I-Tyr3-neurotensin binds to a distinct subset of primary cultured neurons and that a large proportion of the bound radioactivity undergoes rapid internalization in a temperature-dependent manner. It is proposed that this internalization is ligand-induced and that it may play a role in the modulation of central neurotensin receptor levels.

Characterization of neurotensin binding sites on rat mesencephalic cells in primary culture

Many studies have reported the presence of high amounts of neurotensin (NT) binding sites in the mesencephalon of adult rat, and their possible role in mediating the effects of the peptide on the activity of mesencephalic dopaminergic neurons. In the present study, we demonstrate the presence of NT sites in primary cultures of embryonic rat mesencephalic cells. On these cells, a single class of high affinity 125I-NT binding sites was observed. The value of the apparent affinity constant (0.3 nM) did not show any significant change throughout time, from 3 to 14 days in culture. The number of sites, however, increased until day 11 and decreased thereafter. Acetylneurotensin (8-13), NT and neuromedin N were potent competitors of 125I-NT binding, while NT (1-10), NT (1-11) and levocabastine were uneffective. These results indicate that the sites detected in the mesencephalic cultures share common binding properties with the high-affinity NT sites already described in adult rat brain. The neuronal localization of the NT sites was suggested by their presence in neuron-enriched serum-free cultures and their absence in glial cultures. Autoradiographic studies confirmed the cellular localization of NT sites and indicated that, under our experimental conditions, cells labeled by 125I-NT represented 0.14% of the initially plated cell number. Taken together, these results show that the development of mesencephalic neurons in primary culture is associated with an increased expression of NT binding sites. Since such cultures have been shown previously to contain functional dopaminergic neurons, we suggest that they could provide a good model to investigate the modulation of the activity of these neurons by NT.

Tissue distribution of a novel neurotensin-degrading metallopeptidase. An immunological approach using monospecific polyclonal antibodies

A monospecific polyclonal antiserum was raised against a recently purified rat brain neurotensin-degrading metallopeptidase. The purified IgG fraction immunoprecipitated the peptidase and inhibited its proteolytic activity. Western blot analyses revealed that the immune fraction recognizes only one protein in rat brain homogenates, and this corresponds closely to the purified enzyme. The IgG displayed a restricted specificity towards the peptidase from murine origin. In the rat, the neurotensin-degrading enzyme was widely distributed throughout peripheral organs with the noticeable exception of the duodenum. In addition, the peptidase was detected in various cell lines or membrane preparations of neural or extraneural origin in which it had been previously characterized by means of biochemical methods. In light of this widespread distribution, the putative role of the peptidase in the metabolism of neuropeptides is discussed.

Peripheral inactivation of neurotensin. Isolation and characterization of a metallopeptidase from rat ileum

A peptidase that inactivated neurotensin by cleaving the peptide at the Pro10-Tyr11 bond, generating the biologically inactive fragments neurotensin(1-10) and neurotensin(11-13) was purified from whole rat ileum homogenate. The purified enzyme behaved as a 70-75-kDa monomer as determined by SDS-PAGE analysis in reducing or non-reducing conditions and gel permeation on Ultrogel AcA34. The peptidase was insensitive to thiol-blocking agents and acidic and serine protease inhibitors but could be strongly inhibited by 1,10-phenanthroline, EDTA, dithiothreitol and heavy metal ions such as zinc, copper and cobalt. Zinc was the only divalent cation able potently to reactivate the apoenzyme. This enzyme could be distinguished from endopeptidases EC 3.4.24.15 and EC 3.4.24.11, angiotensin-converting enzyme, proline endopeptidase, aminopeptidase and pyroglutamyl-peptide hydrolase since it was not affected by micromolar concentrations of their specific inhibitors. The peptidase displayed a high affinity for neurotensin (1.6 microM). Studies concerning the specificity of the enzyme towards the sequence of neurotensin established the following. (a) Neurotensin(9-13) was the shortest partial sequence that fully inhibited tritiated neurotensin degradation; shortening the C-terminal part of the neurotensin molecule led to inactive fragments. (b) Amidation of the C-terminal end of the peptide did not prevent the recognition by the peptidase. (c) There existed a strong stereospecificity of the peptidase for the residues in positions 8, 9 and 11 of the neurotensin molecule. (d) Pro-Xaa dipeptides (where Xaa represented aromatic or hydrophobic residues) were the most potent inhibitors of tritiated neurotensin degradation while all the Xaa-Pro dipeptides tested were totally ineffective. (e) The neurotensin-related peptides: neuromedin N, xenopsin and [Lys8-Asn9]neurotensin(8-13), as well as angiotensins I and II and dynorphins(1-8) and (1-13) were as potent as neurotensin in inhibiting [3H]neurotensin hydrolysis.

Characterization of neurotensin binding sites in intact and solubilized bovine brain membranes

Analysis of the equilibrium binding of [3H]-neurotensin(1-13) at 25 degrees C to its receptor sites in bovine cortex membranes indicated a single population of sites with an apparent equilibrium dissociation constant (KD) of 3.3 nM and a density (Bmax) of 350 fmol/mg protein (Hill coefficient nH = 0.97). Kinetic dissociation studies revealed the presence of a second class of sites comprising less than 10% of the total. KD values of 0.3 and 2.0 nM were obtained for the higher and lower affinity classes of sites, respectively, from association-dissociation kinetic studies. The binding of [3H]neurotensin was decreased by cations (monovalent and divalent) and by a nonhydrolysable guanine nucleotide analogue. Competition studies gave a potency ranking of [Gln4]neurotensin greater than neurotensin(8-13) greater than neurotensin(1-13). Smaller neurotensin analogues and neurotensin-like peptides were unable to compete with [3H]neurotensin. Stable binding activity for [3H]neurotensin in detergent solution (Kd = 5.5 nM, Bmax = 250 fmol/mg protein, nH = 1.0) was obtained in 2% digitonin/1 mM Mg2+ extracts of membranes which had been preincubated (25 degrees C, 1 h) with 1 mM Mg2+ prior to solubilization. Association-dissociation kinetic studies then revealed the presence of two classes of sites (KD1 = 0.5 nM, KD2 = 3.6 nM) in a similar proportion to that found in the membranes. The solubilized [3H]-neurotensin activity retained its sensitivity to cations and guanine nucleotide.

Neurotensin metabolism in various tissues of central and peripheral origins: ubiquitous involvement of a novel neurotensin degrading metalloendopeptidase

The metabolism of neurotensin in vitro, in various membrane preparations and cell lines of central and peripheral origins was studied. Neurotensin degradation products were separated by HPLC and identified by either amino acid analysis or by their retention times. Peptidases responsible for the cleavages were identified by means of specific fluorigenic substrates or inhibitors. Although the patterns of neurotensin inactivation varied according to the tissue source in all cases, a major primary cleavage occurred at the Pro10-Tyr11 bond, leading to the biologically inactive fragments NT1-10 and NT11-13. A novel neurotensin-degrading metallopeptidase was responsible for this cleavage. Interestingly, it was the only peptidase that was ubiquitously detected. In addition, endopeptidase 24.11 (EC 3.4.24.11) contributed to this cleavage in rat brain synaptic membranes as well as in circular and longitudinal smooth muscle plasma membranes from dog ileum.

Rapid degradation of neurotensin by intact murine neuroblastoma cells (clone N1E-115)

Murine neuroblastoma clone N1E-115, which possesses receptors for neurotensin mediating the formation of intracellular cyclic GMP and the stimulation of inositol phospholipid hydrolysis, exhibited only partial desensitization to neurotensin. This result led to the observation that neurotensin was very rapidly degraded by intact N1E-115 cells. In experiments measuring the time course of [3H]neurotensin degradation, a minimum of six major tritiated products were found, with the breakdown peptides formed and the degree of proteolysis of [3H]neurotensin being dependent upon the length of incubation and the concentration of cells. Clone N1E-115 degraded [3H]neurotensin in an apparently sequential fashion; the primary initial cleavage of intact neurotensin was at the peptide bond between residues Arg8 and Arg9. Initial degradation peptides from the active carboxyl-terminal portion of neurotensin were more rapidly degraded, after formation, than were the peptides from the inactive amino-terminal half of neurotensin. The final two degradation products found were tyrosine, from the carboxyl-terminal portion of neurotensin, and an as yet unidentified peptide from the amino-terminal half of neurotensin. [3H]Neurotensin(8-13) was more rapidly hydrolyzed under identical conditions than was [3H]neurotensin itself. A combination of the protease inhibitors 1,10-phenanthroline and Z-Pro-Prolinal was able to inhibit almost completely the degradation of neurotensin by clone N1E-115.

The role of neuropeptides in regulating airway function. Postsecretory metabolism of peptides

Peptide hormones and neurotransmitters play an essential role in regulation of cellular metabolism. Once released from an endocrine cell or nerve ending, peptides encounter membrane-bound and soluble peptidases. The peptidases inactivate peptides or form fragments with novel biologic activity. Therefore, peptidases must play a major role in homeostatic control, but this aspect of regulation has been a neglected area. This review examines the postsecretory metabolism of biologically active peptides in the brain and alimentary tract, 2 organs in which peptide regulation is of crucial importance.

Neurotensin-metabolizing peptidases in rat fundus plasma membranes

The mechanisms by which neurotensin (NT) was inactivated by rat fundus plasma membranes were characterized. Primary inactivating cleavages occurred at the Arg8-Arg9, Pro10-Tyr11, and Ile12-Leu13 peptidyl bonds. Hydrolysis at the Arg8-Arg9 bond was fully abolished by the use of N-[1(R,S)-carboxy-2-phenylethyl]-alanyl-alanyl-phenylalanine-p- aminobenzoate, a result indicating the involvement at this site of a recently purified soluble metallopeptidase. Hydrolysis of the Pro10-Tyr11 bond was totally resistant to N-benzyloxycarbonyl-prolyl-prolinal and thiorphan, an observation suggesting that the peptidase responsible for this cleavage was different from proline endopeptidase and endopeptidase 24.11 and might correspond to a NT-degrading neutral metallopeptidase recently isolated from rat brain synaptic membranes. The enzyme acting at the Ile12-Leu13 bond has not yet been identified. Secondary cleavages occurring on NT degradation products were mainly generated by bestatin-sensitive aminopeptidases and post-proline dipeptidyl aminopeptidase. The content in NT-metabolizing peptidases present in rat fundus plasma membranes is compared with that previously established for purified rat brain synaptic membranes.

Peptidases in dog-ileum circular and longitudinal smooth-muscle plasma membranes. Their relative contribution to the metabolism of neurotensin

We established the content in neuropeptide-metabolizing peptidases present in highly purified plasma membranes prepared from the circular and longitudinal muscles of dog ileum. Activities were measured by the use of fluorigenic substrates and the identities of enzymes were confirmed by the use of specific peptidase inhibitors. Endopeptidase 24.11, angiotensin-converting enzyme, post-proline dipeptidyl aminopeptidase and aminopeptidases were found in both membrane preparations. Proline endopeptidase was only detected in circular smooth muscle plasma membranes while pyroglutamyl-peptide hydrolase was not observed in either tissue. The relative contribution of these peptidases to the inactivation of neurotensin was assessed. The enzymes involved in the primary inactivating cleavages occurring on the neurotensin molecule were as follows. In both membrane preparations, endopeptidase 24.11 was responsible for the formation of neurotensin-(1-11) and contributed to the formation of neurotensin-(1-10); a recently purified neurotensin-degrading neutral metallopeptidase was also involved in the formation of neurotensin-(1-10). A carboxypeptidase-like activity hydrolysed neurotensin at the Ile12-Leu13 peptide bond, leading to the formation of neurotensin-(1-12). Proline endopeptidase and endopeptidase 24.15 only occurred in circular muscle plasma membranes, yielding neurotensin-(1-7) and neurotensin-(1-8), respectively. In addition, the secondary processing of neurotensin degradation products was catalyzed by the following peptidases. In circular and longitudinal muscle membranes, angiotensin-converting enzyme converted neurotensin-(1-10) into neurotensin-(1-8) and tyrosine resulted from the rapid hydrolysis of neurotensin-(11-13) by bestatin-sensitive aminopeptidases. A post-proline dipeptidyl aminopeptidase activity converted neurotensin-(9-13) into neurotensin-(11-13) in circular muscle plasma membranes. The mechanism of neurotensin inactivation occurring in these membranes will be compared to that previously established for membranes from central origin.

Neurotensin receptors on the rat liver plasma membranes

Neurotensin (NT) is now classified as a brain-gut peptide in the central nervous system and gastrointestinal tract. In the present study, we characterized the NT receptors on the rat liver plasma membranes. The specific binding of [3H]NT was time dependent, reversible, and saturable. Scatchard analysis of the specific binding data yielded two classes of binding sites, a high affinity site and a low affinity site. The average maximum number of binding sites (Bmax) amounted to 13.3 +/- 1.1 fmol/mg protein at high affinity site and 122.3 +/- 21.5 fmol/mg protein at low affinity site, respectively. The dissociation constant (Kd) had values of 0.39 +/- 0.01 nM at high affinity site and 8.1 +/- 1.1 nM at low affinity site, respectively. The amount of specifically bound [3H]NT was significantly reduced in the presence of mono and divalent cations, EDTA, EGTA and a peptidase inhibitor bacitracin, NT1-13 competed with [3H]NT for its binding site with an IC50 of 0.19 nM at high affinity site (0.2 nM concentration of [3H]NT) and 0.7 nM at low affinity site (4.0 nM concentration of [3H]NT). Xenopsin, a NT analogue separated from the skin of Xenopus laevis, was equipotent (IC50 0.75 nM) with NT1-13 at 4.0 nM concentration of [3H]NT. C-terminal sequence of NT contains the structure necessary for interaction with NT binding sites whereas N-terminal sequence had no binding activity. Since NT has a hyperglysemic and a hypercholesterolemic effects in rats, these NT receptors on the rat liver plasma membranes may be involved in the hyperglycemia and/or hypercholesteroremia induced by NT.