Receptor-mediated internalization of [3H]-neurotensin in synaptosomal preparations from rat neostriatum

Engineering of targeted nanoparticles for cancer therapy using internalizing aptamers isolated by cell-uptake selection

One of the major challenges in the development of targeted nanoparticles (NPs) for cancer therapy is to discover targeting ligands that allow for differential binding and uptake by the target cancer cells. Using prostate cancer (PCa) as a model disease, we developed a cell-uptake selection strategy to isolate PCa-specific internalizing 2'-O-methyl RNA aptamers (Apts) for NP incorporation. Twelve cycles of selection and counter-selection were done to obtain a panel of internalizing Apts, which can distinguish PCa cells from nonprostate and normal prostate cells. After Apt characterization, size minimization, and conjugation of the Apts with fluorescently labeled polymeric NPs, the NP-Apt conjugates exhibit PCa specificity and enhancement in cellular uptake when compared to nontargeted NPs lacking the internalizing Apts. Furthermore, when docetaxel, a chemotherapeutic agent used for the treatment of PCa, was encapsulated within the NP-Apt, a significant improvement in cytotoxicity was achieved in targeted PCa cells. Rather than isolating high-affinity Apts as reported in previous selection processes, our selection strategy was designed to enrich cancer cell-specific internalizing Apts. A similar cell-uptake selection strategy may be used to develop specific internalizing ligands for a myriad of other diseases and can potentially facilitate delivering various molecules, including drugs and siRNAs, into target cells.

The potential use of the neurotensin high affinity receptor 1 as a biomarker for cancer progression and as a component of personalized medicine in selective cancers

A growing challenge in medicine today, is the need to improve the suitability of drug treatments for cancer patients. In this field, biomarkers have become the "flags" to provide additional information in tumor biology. They are a relay between the patient and practitioner and consequently, aid in the diagnosis, providing information for prognosis, or in some cases predicting the response to specific therapies. In addition to being markers, these tumor "flags" can also be major participants in the process of carcinogenesis. Neurotensin receptor 1 (NTSR1) was recently identified as a prognosis marker in breast, lung, and head and neck squamous carcinomas. Neurotensin (NTS) was also shown to exert numerous oncogenic effects involved in tumor growth and metastatic spread. These effects were mostly mediated by NTSR1, making the NTS/NTSR1 complex an actor in cancer progression. In this review, we gather information on the oncogenic effects of the NTS/NTSR1 complex and its associated signaling pathways in order to illuminate its significant role in tumor progression and its potential as a biomarker and a therapeutic target in some tumors.

Validation of the Transferrin Receptor for Drug Targeting to Brain Capillary Endothelial CellsIn Vitro

Recently, we have shown that transferrin (Tf) is actively endocytosed by the Tf R on primary cultured bovine brain capillary endothelial cells (BCEC). The objective of this investigation is to determine whether the Tf R can facilitate endocytosis of a (protein) model drug, using Tf as a targeting vector. Secondly, the mechanism of endocytosis was investigated. Horseradish peroxidase (HRP, 40 kDa) was chosen as a model drug, since it normally does not cross the blood-brain barrier (BBB) and its concentration in biological media can be easily quantified. Tf-HRP conjugates (1:1) are actively and specifically endocytosed by BCEC in vitro in a concentration and time-dependent manner. At an applied concentration of 3 microg/ml, association (a combination of binding and endocytosis) of Tf-HRP reached equilibrium at a concentration of 2 ng/mg cell protein after 1 h of incubation at 37 degree C. This was approximately 3-fold higher compared to binding at 4 degree C (0.6 ng/mg cell protein). Association of Tf-HRP was compared to BSA-HRP. After 2 h of incubation at 37 degree C association levels were 5.2 and 2.5 ng/mg cell protein, for Tf-HRP and BSA-HRP, respectively. Under those conditions, association of Tf-HRP could be inhibited to approximately 30% of total association by an excess of non-conjugated Tf, but not with BSA, while association of BSA-HRP could be inhibited by both proteins. Furthermore, by using specific inhibitors of endocytotic processes, it was shown that association of Tf-HRP is via clathrin-coated vesicles. Association of Tf-HRP is inhibited by phenylarsine oxide (an inhibitor of clathrin-mediated endocytosis) to 0.4 ng/mg cell protein, but not by indomethacin, which inhibits formation of caveolae. Finally, following iron scavenging by deferoxamine mesylate (DFO, resulting in a higher Tf R expression) a 5-fold increase in association of Tf-HRP to 15.8 ng/mg cell protein was observed. In conclusion, the Tf R is potentially suitable for targeting of a (protein) cargo to the BBB and to facilitate its endocytosis by the BCEC.

Stress-induced intracellular trafficking of corticotropin-releasing factor receptors in rat locus coeruleus neurons

Corticotropin-releasing factor (CRF) activates locus coeruleus (LC)-norepinephrine neurons during stress. Previous stress or CRF administration attenuates the magnitude of this response by decreasing postsynaptic sensitivity to CRF. Here we describe the fate of CRF receptors (CRFr) in LC neurons after stress. Rats were exposed to swim stress or handling and perfused 1 or 24 h later. Sections through the LC were processed for immunogold-silver labeling of CRFr. CRFr in LC dendrites was present on the plasma membrane and within the cytoplasm. In control rats, the ratio of cytoplasmic to total dendritic labeling was 0.55 +/- 0.01. Swim stress increased this ratio to 0.77 +/- 0.01 and 0.80 +/- 0.02 at 1 and 24 h after stress, respectively. Internalized CRFr was associated with different organelles at different times after stress. At 1 h after stress, CRFr was often associated with early endosomes in dendrites and perikarya. By 24 h, more CRFr was associated with multivesicular bodies, suggesting that some of the internalized receptor is targeted for degradation. In perikarya, more internalized CRFr was associated with Golgi apparatus 24 vs. 1 h after stress. This is suggestive of changes in CRFr synthesis. Alternatively, this may indicate communication between multivesicular bodies and Golgi apparatus in the process of recycling. Administration of the selective CRF(1) antagonist, antalarmin, before swim stress attenuated CRFr internalization. The present demonstration of stress-induced internalization of CRFr in LC neurons provides evidence that CRF is released in the LC during swim stress to activate this system and initiate cellular trafficking of the receptor that determines subsequent sensitivity of LC neurons to CRF.

Molecular and cellular regulation of neurotensin receptor under acute and chronic agonist stimulation

Neurotensin is a tridecapteptide acting mostly in the brain and gastrointestinal tract. NT binds two G protein coupled receptors (GPCR), NTS1 and NTS2, and a single transmembrane domain receptor, NTS3/gp95/sortilin receptor. NTS1 mediates the majority of NT action in neurons and the periphery. Like many other GPCRs, upon agonist stimulation, NTS1 is internalized, endocytosed, and the cells are desensitized. It is tacitly acknowledged that the intensity and the lasting of cellular responses to NT are dependent on free and functional NTS1 at the cell surface. Understanding how NTS1 expression is regulated at the membrane should provide a better comprehension towards its function. This review analyzes and discusses the current cellular and molecular mechanisms affecting the expression of NTS1 at the cellular membrane upon acute and chronic NT stimulation.

Dendrite-selective redistribution of the chemokine receptor CXCR4 following agonist stimulation

The chemokine SDF-1 is a secreted protein that plays a critical role in several aspects of neuron development through interaction with its unique receptor CXCR4. A key mechanism that controls neuron responsiveness to extracellular signals during neuronal growth is receptor endocytosis. Since we previously reported that SDF-1 regulates axon development without affecting the other neurites, we asked whether this could correlate with a compartment-selective trafficking of CXCR4. We thus studied CXCR4 behavior upon SDF-1 exposure in rat hippocampus slices and in transfected neuron cultures. A massive agonist-induced redistribution of CXCR4 in endosomes was observed in dendrites whereas no modification was evidenced in axons. Our data suggest that CXCR4 trafficking may play a role in mediating selective effects of SDF-1 on distinct neuronal membrane subdomains.

Neurotensin: role in psychiatric and neurological diseases

Neurotensin (NT), an endogenous brain-gut peptide, has a close anatomical and functional relationship with the mesocorticolimbic and neostriatal dopamine system. Dysregulation of NT neurotransmission in this system has been hypothesized to be involved in the pathogenesis of schizophrenia. Additionally, NT containing circuits have been demonstrated to mediate some of the mechanisms of action of antipsychotic drugs, as well as the rewarding and/or sensitizing properties of drugs of abuse. NT receptors have been suggested to be novel targets for the treatment of psychoses or drug addiction.

Agonist-induced internalization of corticotropin-releasing factor receptors in noradrenergic neurons of the rat locus coeruleus

Corticotropin-releasing factor (CRF) acts within the locus coeruleus (LC), to modulate activity of the LC-norepinephrine (NE) system. Combining molecular and cellular approaches, we demonstrate CRF receptor (CRFr) mRNA expression in Sprague-Dawley rat LC and provide the first in vivo evidence for agonist-induced internalization of CRFr. CRFr mRNA was detected in LC micropunches by RT-PCR. In dual labelling immunofluorescence studies, tyrosine hydroxylase (TH) containing neurons exhibited CRFr labelling. At the ultrastructural level, immunogold-silver labelling for CRFr was localized to the plasma membrane of TH-immunoperoxidase labelled dendrites. CRF (100 ng) injection into the LC produced a robust neuronal activation that peaked 10-15 min after injection and was maintained for the duration of the recording. This was associated with CRFr internalization in LC neurons that was apparent at 5 and 30 min after injection. By 5 min after injection the ratio of cytoplasmic to total dendritic CRFr-labelling was 0.81 +/- 0.01 in rats injected with CRF and 0.59 +/- 0.02 in rats injected with artificial cerebrospinal fluid (ACSF; P < 0.0001). Enhanced internalization of CRFr was maintained at 30 min after CRF injection, with the ratio being 0.86 +/- 0.02 for CRF-injected cases and 0.57 +/- 0.03 for ACSF-injected cases (P < 0.0001). Internalized CRFr was associated with early endosomes, indicative of degradation or recycling. Agonist-induced CRFr internalization in LC neurons may underlie acute desensitization to CRF or stress. This process may be a pivotal target by which stressors or pharmacological agents regulate the sensitivity of the LC-NE system to CRF and subsequent stressors.

Homologous down-regulation of histamine H3 receptors in rat striatal slices

Preincubation of striatal slices with the selective histamine H3-receptor agonist immepip (100 nM) decreased the specific binding of N-alpha-[methyl-3H]-histamine ([3H]-NMHA) to membranes obtained from the treated slices. The binding decrease was significant after 5 min, remained at similar reduced levels between 5- and 30-min incubations with agonist, and only a partial recovery was observed after 90-min washout (34, 41, and 44% at 90, 120, and 150 min, respectively). Saturation analysis showed a significant decrease in both receptor density (-44% +/- 9%) and affinity (dissociation constant, Kd 1.15 +/- 0.23 nM from 0.59 +/- 0.17 nM). The effect of immepip was mimicked by histamine and the H3 agonists imetit and R-alpha-methylhistamine, and was blocked by the H3 antagonist thioperamide. The reduction in [3H]-NMHA binding was fully and partially prevented by incubation at 4 degrees C and in hypertonic medium, respectively, but not by the endocytosis inhibitor phenylarsine oxide (10 microM). None of the following protein kinase inhibitors, Ro-318220 and Gö-6976 (PKC), H-89 (PKA) and staurosporine (general inhibitor) prevented the effect of immepip. In [3H]-adenine-labeled slices the preincubation with immepip (100 nM, 15 min) prevented the inhibitory effect of H3 receptor activation on forskolin-induced [3H]-cAMP accumulation (99% +/- 9% vs. 76% +/- 4% of control values). Taken together our results indicate that agonist binding promotes the down-regulation of striatal H3 receptors resulting in a significant loss of function.

Intraneuronal trafficking of G-protein-coupled receptors in vivo

In vitro studies have widely demonstrated that the abundance and availability of G-protein-coupled receptors (GPCRs) at the cell surface is regulated by the neuronal environment and is the result of complex intraneuronal trafficking. However, this regulation is still poorly understood in vivo. Modulation of receptor availability at the neuronal membrane is a key event in the regulation of neuronal functions (e.g. neurotransmitter release or neuronal excitability in physiological, pathological or therapeutic conditions). We discuss the effects of duration of receptor stimulation (acute versus chronic) on the intraneuronal trafficking of GPCRs in vivo, and we show that this trafficking might differ according to subcellular compartment (soma, dendrites or axon terminals).

Morphine-induced changes in delta opioid receptor trafficking are linked to somatosensory processing in the rat spinal cord

An in vivo fluorescent deltorphin (Fluo-DLT) internalization assay was used to assess the distribution and regulation of pharmacologically available delta opioid receptors (deltaORs) in the rat lumbar (L4-5) spinal cord. Under basal conditions, intrathecal injection of Fluo-DLT resulted in the labeling of numerous deltaOR-internalizing neurons throughout dorsal and ventral horns. The distribution and number of Fluo-DLT-labeled perikaryal profiles were consistent with that of deltaOR-expressing neurons, as revealed by in situ hybridization and immunohistochemistry, suggesting that a large proportion of these cells was responsive to intrathecally administered deltaOR agonists. Pretreatment of rats with morphine for 48 hr resulted in a selective increase in Fluo-DLT-labeled perikaryal profiles within the dorsal horn. These changes were not accompanied by corresponding augmentations in either deltaOR mRNA or (125)I-deltorphin-II binding levels, suggesting that they were attributable to higher densities of cell surface deltaOR available for internalization rather than to enhanced production of the receptor. Unilateral dorsal rhizotomy also resulted in increased Fluo-DLT internalization in the ipsilateral dorsal horn when compared with the side contralateral to the deafferentation or to non-deafferented controls, suggesting that deltaOR trafficking in dorsal horn neurons may be regulated by afferent inputs. Furthermore, morphine treatment no longer increased Fluo-DLT internalization on either side of the spinal cord after unilateral dorsal rhizotomy, indicating that microOR-induced changes in the cell surface availability of deltaOR depend on the integrity of primary afferent inputs. Together, these results suggest that regulation of deltaOR responsiveness through microOR activation in this region is linked to somatosensory information processing.

The Caenorhabditis elegans orthologue of mammalian puromycin-sensitive aminopeptidase has roles in embryogenesis and reproduction

Mammals possess membrane-associated and cytosolic forms of the puromycin-sensitive aminopeptidase (PSA; EC 3.4.11.14). Increasing evidence suggests the membrane PSA is involved in neuromodulation within the central nervous system and in reproductive biology. The functional roles of the cytosolic PSA are less clear. The genome of the nematode Caenorhabditis elegans encodes an aminopeptidase, F49E8.3 (PAM-1), that is orthologous to PSA, and sequence analysis predicts it to be cytosolic. We have determined the spatio/temporal gene expression pattern of pam-1 by using the promoter region of F49E8.3 to control expression in the nematode of a second exon translational fusion of the aminopeptidase to green fluorescent protein. Cytosolic fluorescence was observed throughout development in the intestine and nerve cells of the head. Neuronal expression was also observed in the tail of adult males. Recombinant PAM-1, expressed and purified from Escherichia coli, hydrolyzed the N-terminal amino acid from peptide substrates. Favored substrates had positively charged or small neutral amino acids in the N-terminal position. Peptide hydrolysis was inhibited by the metal-chelating agent 1,10-phenanthroline and by the aminopeptidase inhibitors actinonin, amastatin, and leuhistin. However, the enzyme was approximately 100-fold less sensitive toward puromycin (IC50, 135 mum) than other PSA homologues. Following inactivation of the enzyme, aminopeptidase activity was recovered with Zn2+, Co2+, and Ni2+. Silencing expression of pam-1 by RNA interference resulted in 30% embryonic lethality. Surviving adult hermaphrodites deposited large numbers of oocytes throughout the self-fertile period. The overall brood size was, however, unaffected. We conclude that pam-1 encodes an aminopeptidase that clusters phylogenetically with the PSAs, despite attenuated sensitivity toward puromycin, and that it functions in embryo development and reproduction of the nematode.

Pharmacology and Functional Properties of NTS2 Neurotensin Receptors in Cerebellar Granule Cells

The binding and signaling properties of neuronal NTS2 neurotensin (NT) receptors were examined in cultured rat cerebellar granule cells. As shown by reverse transcription-PCR, receptor autoradiography, and confocal microscopic localization of fluorescent NT, these cells selectively express the NTS2 receptor subtype. Accordingly, a single apparent class of (125)I-NT-binding sites, with an affinity of 3.1 nm, was detected in cerebellar granule cell cultures. This binding was competed for with high affinity (IC(50) = 5.7 nm) by the NTS2 ligand levocabastine and with low affinity (IC(50) = 203 nm) by the NTS1 antagonist SR48692. Hypertonic acid stripping of surface-bound ligand and hyperosmolar sucrose treatment revealed that 64% of specifically bound (125)I-NT was internalized at equilibrium via a clathrin-dependent pathway. In cells loaded with the Ca(2+)-sensitive fluorescent dye Fluo4, SR48692, but neither NT nor levocabastine, triggered a marked increase in cytosolic [Ca(2+)](i). By contrast, both NT and levocabastine, but not SR48692, induced a sustained (>60 min) activation of the mitogen-activated protein kinases, p42/p44, indicating functional coupling of NTS2 receptors. Complementary experiments carried out on synaptosomes from adult rat cerebellum demonstrated the presence of presynaptic NTS2 receptors. However, in contrast to perikaryal NTS2 sites, these presynaptic receptors did not internalize in response to NT stimulation. Taken together, the present results demonstrate that NTS2 receptors are present both presynaptically and postsynaptically in central neurons and that NT and levocabastine act as agonists on these receptors.