Solubilization and purification of a high affinity neurotensin receptor from newborn human brain

Neurotensin and Its Involvement in Reproductive Functions: An Exhaustive Review of the Literature

Neurotensin (NTS) is a peptide discovered in 1973, which has been studied in many fields and mainly in oncology for its action in tumor growth and proliferation. In this review of the literature, we wanted to focus on its involvement in reproductive functions. NTS participates in an autocrine manner in the mechanisms of ovulation via NTS receptor 3 (NTSR3), present in granulosa cells. Spermatozoa express only its receptors, whereas in the female reproductive system (endometrial and tube epithelia and granulosa cells), we find both NTS secretion and the expression of its receptors. It consistently enhances the acrosome reaction of spermatozoa in mammals in a paracrine manner via its interaction with NTSR1 and NTSR2. Furthermore, previous results on embryonic quality and development are discordant. NTS appears to be involved in the key stages of fertilization and could improve the results of in vitro fertilization, especially through its effect on the acrosomal reaction.

The Neurotensinergic System: A Target for Cancer Treatment

BACKGROUND

The scientific interest regarding the involvement of peptides in cancer has increased in the last years. In tumor cells the overexpression of peptides and their receptors is known and new therapeutic targets for the treatment of cancer have been suggested. The overexpression of the neurotensinergic system has been associated with poor prognosis, tumor size, higher tumor aggressiveness, increased relapse risk and worse sensitivity to chemotherapy agents.

OBJECTIVE

The aim of this review is to update the findings regarding the involvement of the neurotensinergic system in cancer to suggest anticancer therapeutic strategies targeting this system. The neurotensin (NT) precursor, NT and its receptors (NTR) and the involvement of the neurotensinergic system in lung, breast, prostate, gastric, colon, liver and pancreatic cancers, glioblastoma, neuroendocrine tumors and B-cell leukemia will be mentioned and discussed as well as the signaling pathways mediated by NT. Some research lines to be developed in the future will be suggested such as: molecules regulating the expression of the NT precursor, influence of the diet in the development of tumors, molecules and signaling pathways activated by NT and antitumor therapeutic strategies targeting the neurotensinergic system.

CONCLUSION

NT, via the NTR, exerts oncogenic (tumor cell proliferation, invasion, migration, angiogenesis) and antiapoptotic effects, whereas NTR antagonists inhibit these effects. NTR expression can be used as a diagnostic tool/therapeutic target and the administration of NTR antagonists as antitumor drugs could be a therapeutic strategy to treat tumors overexpressing NTR.

Investigating the Conformational Response of the Sortilin Receptor upon Binding Endogenous Peptide- and Protein Ligands by HDX-MS

Sortilin is a multifunctional neuronal receptor involved in sorting of neurotrophic factors and apoptosis signaling. So far, structural characterization of sortilin and its endogenous ligands has been limited to crystallographic studies of sortilin in complex with the neuropeptide neurotensin. Here, we use hydrogen/deuterium exchange mass spectrometry to investigate the conformational response of sortilin to binding biological ligands including the peptides neurotensin and the sortilin propeptide and the proteins progranulin and pro-nerve growth factor-β. The results show that the ligands use two binding sites inside the cavity of the β-propeller of sortilin. However, ligands have distinct differences in their conformational impact on the receptor. Interestingly, the protein ligands induce conformational stabilization in a remote membrane-proximal domain, hinting at an unknown conformational link between the ligand binding region and this membrane-proximal region of sortilin. Our findings improve our structural understanding of sortilin and how it mediates diverse ligand-dependent functions important in neurobiology.

Inhibition of insulin/PI3K/AKT signaling decreases adipose Sortilin 1 in mice and 3T3-L1 adipocytes

Sortilin 1(Sort1) is a vesicle trafficking receptor that mediates protein sorting in the endocytic and exocytic pathways. Sort1 is a component of the GLUT4 storage vesicles in adipocytes and is also involved in the regulation of adipogenesis. Sort1 protein is reduced in adipose of obese mice and humans, but the underlying cause is not fully understood. Here we report that insulin/PI3K/AKT signaling cascade critically regulates adipose Sort1 protein abundance. Administration of a PI3K inhibitor rapidly decreased Sort1 protein but not mRNA in adipose of chow-fed mice. In 3T3-L1 adipocytes, serum-starvation or inhibition of the PI3K/AKT signaling also decreased Sort1 protein without affecting Sort1 mRNA expression. Sort1 protein downregulation upon PI3K inhibition was blocked by pretreatment of MG132 but not Bafilomycin A1, suggesting that PI3K inhibition caused Sort1 degradation via the proteasome pathway. Using a phospho-specific Sort1 antibody, we showed that endogenous Sort1 was phosphorylated at S825 adjacent to the DXXLL sorting motif on the cytoplasmic tail. We demonstrated that mutagenesis that abolished Sort1 S825 phosphorylation decreased insulin-stimulated Sort1 localization on the plasma membrane and Sort1 protein stability in 3T3-L1 adipocytes. However, endogenous Sort1 phosphorylation at S825 was not affected by insulin stimulation or by inhibition of PI3K. In conclusion, this study revealed an important role of insulin signaling in regulating adipose Sort1 protein stability, and further suggests that impaired insulin signaling may underlie reduced adipose Sort1 in obesity. The cellular events downstream of insulin/PI3K/AKT signaling that mediates insulin regulation of Sort1 stability requires further investigation.

Increased Brain Neurotensin and NTSR2 Lead to Weak Nociception in NTSR3/Sortilin Knockout Mice

The neuropeptide neurotensin (NT) elicits numerous pharmacological effects through three different receptors (NTSR1, NTSR2, and NTSR3 also called sortilin). Pharmacological approaches and generation of NTSR1 and NTSR2-deficient mice allowed to determine the NT-induced antipsychotic like behavior, the inhibitory of weak fear memory and the nociceptive signaling in a rat formalin tonic pain model to NTSR1. Conversely, the effects of NT on thermal and tonic nociceptions were mediated by NTSR2. However, the role of NTSR3/sortilin on the neurotensinergic system was not investigated. Here, by using C57Bl/6J mouse model in which the gene coding for NTSR3/sortilin has been inactivated, we observed a modification of the expression of both NTSR2 and NT itself. Quantitative PCR and protein expression using Western blot analyses and AlphaLisa™ technology resulted in the observation that brain NTSR2 as well as brain and blood NT were 2-fold increased in KO mice leading to a resistance of these mice to thermal and chemical pain. These data confirm that NTSR3/sortilin interacts with other NT receptors (i.e., NTSR2) and that its deletion modifies also the affinity of this receptor to NT.

Revisiting the structure of the Vps10 domain of human sortilin and its interaction with neurotensin

Sortilin is a multifunctional receptor involved in sorting and apoptosis. We have previously reported a 2.0-Å structure of the Vps10 ectodomain in complex with one of its ligands, the tridecapeptide neurotensin. Here we set out to further characterize the structural properties of sortilin and its interaction with neurotensin. To this end, we have determined a new 2.7 Å structure using a crystal grown with a 10-fold increased concentration of neurotensin. Here a second peptide fragment was observed within the Vps10 β-propeller, which may in principle either represent a second molecule of neurotensin or the N-terminal part of the molecule bound at the previously identified binding site. However, in vitro binding experiments strongly favor the latter hypothesis. Neurotensin thus appears to bind with a 1:1 stoichiometry, and whereas the N-terminus does not bind on its own, it enhances the affinity in context of full-length neurotensin. We conclude that the N-terminus of neurotensin probably functions as an affinity enhancer for binding to sortilin by engaging the second binding site. Crystal packing differs partly from the previous structure, which may be due to variations in the degree and pattern of glycosylations. Consequently, a notable hydrophobic loop, not modeled previously, could now be traced. A computational analysis suggests that this and a neighboring loop may insert into the membrane and thus restrain movement of the Vps10 domain. We have, furthermore, mapped all N-linked glycosylations of CHO-expressed human sortilin by mass spectrometry and find that their locations are compatible with membrane insertion of the hydrophobic loops.

Neurotensin receptor type 1 regulates ethanol intoxication and consumption in mice

Neurotensin receptor type 1 (NTS1) is known to mediate a variety of biological functions of neurotensin (NT) in the central nervous system. In this study, we found that NTS1 null mice displayed decreased sensitivity to the ataxic effect of ethanol on the rotarod and increased ethanol consumption when given a free choice between ethanol and tap water containing bottles. Interestingly, the administration of NT69L, a brain-permeable NT analog, increased ethanol sensitivity in wild-type littermates but had no such effect in NTS1 null mice, suggesting that NTS1 contributes to NT-mediated ethanol intoxication. Furthermore, the daily treatment of NT69L, for 4 consecutive days, significantly reduced alcohol preference and consumption in wild-type littermates but had no such effects in NTS1 null mice in a two-bottle drinking experiment. Our study provides evidence for possible pharmacological roles of NT69L in which it increases sensitivity to the ataxic effect, and decreases voluntary consumption, of ethanol. Our study also demonstrates NTS1-mediated behavioral effects of NT69L. Therefore, our findings will be useful for understanding some aspects of alcoholism as well as to develop novel pharmacological therapeutic options for humans.

Cancer, chemistry, and the cell: molecules that interact with the neurotensin receptors

The literature covering neurotensin (NT) and its signalling pathways, receptors, and biological profile is complicated by the fact that the discovery of three NT receptor subtypes has come to light only in recent years. Moreover, a lot of this literature explores NT in the context of the central nervous system and behavioral studies. However, there is now good evidence that the up-regulation of NT is intimately involved in cancer development and progression. This Review aims to summarize the isolation, cloning, localization, and binding properties of the accepted receptor subtypes (NTR1, NTR2, and NTR3) and the molecules known to bind at these receptors. The growing role these targets are playing in cancer research is also discussed. We hope this Review will provide a useful overview and a one-stop resource for new researchers engaged in this field at the chemistry-biology interface.

Thermostabilization of the neurotensin receptor NTS1

Structural studies on G protein-coupled receptors (GPCRs) have been hampered for many years by their instability in detergent solution and by the number of potential conformations that receptors can adopt. Recently, the structures of the β₁ and β₂ adrenergic receptors and the adenosine A_2a receptor were determined with antagonist bound, a receptor conformation that is thought to be more stable than the agonist-bound state. In contrast to these receptors, the neurotensin receptor NTS1 is much less stable in detergent solution. We have therefore used a systematic mutational approach coupled to activity assays to identify receptor mutants suitable for crystallisation, both alone and in complex with the peptide agonist, neurotensin. The best receptor mutant, NTS1-7m, contained 4 point mutations. It showed increased stability compared to the wild type receptor, in the absence of ligand, after solubilisation with a variety of detergents. In addition, NTS1-7m bound to neurotensin was more stable than unliganded NTS1-7m. Of the four thermostabilising mutations, only one residue (A86L) is predicted to be in the lipid environment. In contrast, I260A appears to be buried within the transmembrane helix bundle, F342A may form a distant part of the putative ligand binding site, whereas F358A is likely to be in a region important for receptor activation. NTS1-7m binds neurotensin with a similar affinity to the wild-type receptor. However, agonist dissociation was slower, and NTS1-7m activated G proteins poorly. The affinity of NTS1-7m for the antagonist SR48692 was also lower than that of the wild-type receptor. Thus we have successfully stabilised NTS1 in an agonist-binding conformation that does not efficiently couple to G proteins.

Role of neuropeptides in appetite regulation and obesity--a review

Obesity represents the most prevalent nutritional problem worldwide which in the long run predisposes to development of diabetes mellitus, hypertension, endometrial carcinoma, osteoarthritis, gall stones and cardiovascular diseases. Despite significant reductions in dietary fat consumption, the prevalence of obesity is on a rise and is taking on pandemic proportions. Obesity develops when energy intake exceeds energy expenditure over time. Recently, a close evolutionary relationship between the peripheral and hypothalamic neuropeptides has become apparent. The hypothalamus being the central feeding organ mediates regulation of short-term and long-term dietary intake via synthesis of various orexigenic and anorectic neuropeptides. The structure and function of many hypothalamic peptides (neuropeptide Y (NPY), melanocortins, agouti-related peptide (AGRP), cocaine and amphetamine regulated transcript (CART), melanin concentrating hormone (MCH), orexins have been characterized in rodent models The peripheral neuropeptides such as cholecystokinin (CCK), ghrelin, peptide YY (PYY3-36), amylin, bombesin regulate important gastrointestinal functions such as motility, secretion, absorption, provide feedback to the central nervous system on availability of nutrients and may play a part in regulating food intake. The pharmacological potential of several endogenous peripheral peptides released prior to, during and/or after feeding are being explored. Long-term regulation is provided by the main circulating hormones leptin and insulin. These systems implicated in hypothalamic appetite regulation provide potential targets for treatment of obesity which could potentially pass into clinical development in the next 5 years. This review summarizes various effects and interrelationship of these central and peripheral neuropeptides in metabolism, obesity and their potential role as targets for treatment of obesity.

Brain neurotensin, psychostimulants, and stress--emphasis on neuroanatomical substrates

Neurotensin (NT) is a peptide that is widely distributed throughout the brain. NT is involved in locomotion, reward, stress and pain modulation, and in the pathophysiology of drug addiction and depression. In its first part this review brings together relevant literature about the neuroanatomy of NT and its receptors. The second part focuses on functional-anatomical interactions between NT, the mesotelencephalic dopamine system and structures targeted by dopaminergic projections. Finally, recent data about the actions of NT in processes underlying behavioral sensitization to psychostimulant drugs and the involvement of NT in the regulation of the hypothalamo-pituitary-adrenal gland axis are considered.

The role of neurotensin in the pathophysiology of schizophrenia and the mechanism of action of antipsychotic drugs

It has become increasingly clear that schizophrenia does not result from the dysfunction of a single neurotransmitter system, but rather pathologic alterations of several interacting systems. Targeting of neuropeptide neuromodulator systems, capable of concomitantly regulating several transmitter systems, represents a promising approach for the development of increasingly effective and side effect-free antipsychotic drugs. Neurotensin (NT) is a neuropeptide implicated in the pathophysiology of schizophrenia that specifically modulates neurotransmitter systems previously demonstrated to be dysregulated in this disorder. Clinical studies in which cerebrospinal fluid (CSF) NT concentrations have been measured revealed a subset of schizophrenic patients with decreased CSF NT concentrations that are restored by effective antipsychotic drug treatment. Considerable evidence also exists concordant with the involvement of NT systems in the mechanism of action of antipsychotic drugs. The behavioral and biochemical effects of centrally administered NT remarkably resemble those of systemically administered antipsychotic drugs, and antipsychotic drugs increase NT neurotransmission. This concatenation of findings led to the hypothesis that NT functions as an endogenous antipsychotic. Moreover, typical and atypical antipsychotic drugs differentially alter NT neurotransmission in nigrostriatal and mesolimbic dopamine (DA) terminal regions, and these effects are predictive of side effect liability and efficacy, respectively. This review summarizes the evidence in support of a role for the NT system in both the pathophysiology of schizophrenia and the mechanism of action of antipsychotic drugs.

Sortilin/neurotensin receptor-3: a new tool to investigate neurotensin signaling and cellular trafficking?

The identification of gp95sortilin, a sorting protein, as being the 100 kDa neurotensin (NT) receptor, a non-G-protein coupled receptor, constitutes a new and interesting but intriguing step in the neuropeptide signaling as well as in cellular trafficking. The isolation of the same protein by three different experimental approaches sum up the complexity for researchers involved in the functional significance of the so-called sortilin/neurotensin receptor 3 (NTR3). This review will concentrate on the putative physiological and cellular roles of sortilin/NTR3 as most results so far have proposed hypothetical conclusions rather than concrete evidence.

Effects of intra-VTA injection of neurotensin on local cerebral glucose utilization in freely moving rats

The [14C]2-deoxyglucose method was applied to measure the effects of the injection of neurotensin (7 microg) in the ventral tegmental area on local cerebral glucose utilization in the rat. Injection of neurotensin produced significant increases of glucose utilization in the shell of the nucleus accumbens and in the olfactory tubercle. These results indicate that stimulation of neurotensin receptors in the ventral tegmental area produces functional changes that are confined to the regions receiving mesolimbic projections within the rostral extended amygdaloid complex. These findings extend our understanding on the effects of neurotensin in the limbic system, with particular regard to reward pathways.

Neurotensin and neurotensin receptors

Neurotensin is a brain and gastrointestinal peptide that fulfils many central and peripheral functions through its interaction with specific receptors. Three subtypes of neurotensin receptors have been cloned. Two of them belong to the family of G protein-coupled receptors, whereas the third one is an entirely new type of neuropeptide receptor and is identical to gp95/sortilin, a 100 kDa-protein with a single transmembrane domain. In this review, the present knowledge regarding the molecular and pharmacological properties of the three cloned neurotensin receptors is summarized and the relationship between these receptors and the known pharmacological effects of neurotensin is discussed.

The 100-kDa neurotensin receptor is gp95/sortilin, a non-G-protein-coupled receptor

In this work, the 100-kDa neurotensin (NT) receptor previously purified from human brain by affinity chromatography (Zsürger, N., Mazella, J., and Vincent, J. P. (1994) Brain Res. 639, 245-252) was cloned from a human brain cDNA library. This cDNA encodes a 833-amino acid protein 100% identical to the recently cloned gp95/sortilin and was then designated NT3 receptor-gp95/sortilin. The N terminus of the purified protein is identical to the sequence of the purified gp95/sortilin located immediately after the furin cleavage site. The binding of iodinated NT to 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid-solubilized extracts of COS-7 cells transfected with the cloned cDNA was saturable and reversible with an affinity of 10-15 nM. The localization of the NT3 receptor-gp95/sortilin into intracellular vesicles was in agreement with previous results obtained with the purified receptor and with gp95/sortilin. Affinity labeling and binding experiments showed that the 110-kDa NT3 receptor can be partly transformed into a higher affinity (Kd = 0.3 nM) 100-kDa protein receptor by cotransfection with furin. This 100-kDa NT receptor corresponded to the mature form of the receptor. The NT3/gp95/sortilin protein is the first transmembrane neuropeptide receptor that does not belong to the superfamily of G-protein-coupled receptors.

Mechanisms of regulation of neurotensin receptors

Since its discovery in 1973, the neuropeptide neurotensin has been demonstrated to be involved in the control of a broad variety of physiological activities in both the central nervous system and in the periphery. Pharmacological studies have shown that the biological effects elicited by neurotensin result from its specific binding to cell membrane neurotensin receptors that have been characterized in various tissue and in cell preparations. In addition, it is now well documented that most of these responses are subject to rapid desensitization. Such desensitization results in transient responses to sustained peptide applications, or to tachyphylaxis during successive stimulations in the same conditions. More recently, desensitization of neurotensin signalling was investigated at the cellular and molecular levels. In cultured cells, regulation at the second messenger level, receptor internalization, and receptor down-regulation processes have been reported. These are proposed to play a critical role in the control of cell responsiveness to neurotensin. This review aims to compile recent data on the different biochemical processes involved in the regulation of the neurotensin receptor and to discuss the physiological consequences of this regulation in vivo.

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.

Purification of a rat neurotensin receptor expressed in Escherichia coli

A truncated rat neurotensin receptor (NTR), expressed in Escherichia coli with the maltose-binding protein fused to its N-terminus and the 13 amino acid Bio tag fused to its C-terminus, was purified to apparent homogeneity in two steps by use of the monomeric avidin system followed by a novel neurotensin column. This purification protocol was developed by engineering a variety of affinity tags on to the C-terminus of NTR. Surprisingly, expression levels varied considerably depending on the C-terminal tag used. Functional expression of NTR was highest (800 receptors/cell) when thioredoxin was placed between the receptor C-terminus and the tag, indicating a stabilizing effect of the thioredoxin moiety. Several affinity chromatography methods were tested for purification. NTR with the in vivo-biotinylated Bio tag was purified with the highest efficiency compared with NTR with the Strep tag or a hexa-histidine tail. Co-expression of biotin ligase improved considerably the in vivo biotinylation of the Bio tag and, therefore, the overall purification yield. Proteolysis of the NTR fusion protein was prevented by removing a protease-sensitive site discovered at the N-terminus of NTR. The ligand binding properties of the purified receptor were similar to those of the membrane-bound protein and the native receptor. The scale-up of this purification scheme, to provide sufficient protein for biophysical studies, is in progress.

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.