Cellular biology of somatostatin receptors

Activity-regulated somatostatin expression reduces dendritic spine density and lowers excitatory synaptic transmission via postsynaptic somatostatin receptor 4

Neuronal activity regulates multiple aspects of the morphological and functional development of neural circuits. One mechanism by which it achieves this is through regulation of gene expression. In a screen for activity-induced genes, we identified somatostatin (SST), a neuropeptide secreted by the SST subtype of interneurons. Using real time quantitative PCR and ELISA, we showed that persistent elevation of neuronal activity increased both the gene expression and protein secretion of SST over a relatively prolonged time course of 48 h. Using primary hippocampal neuronal cultures, we found that SST treatment for 1 day significantly reduced the density of dendritic spines, the morphological bases of excitatory synapses. Furthermore, the density of pre- and postsynaptic markers of excitatory synapses was significantly lowered following SST treatment, whereas that of inhibitory synapses was not affected. Consistently, SST treatment reduced the frequency of miniature excitatory postsynaptic currents, without affecting inhibition. Finally, lowering the endogenous level of SST receptor subtype 4 in individual hippocampal pyramidal neurons significantly blocked the effect of SST in reducing spine density and excitatory synaptic transmission in a cell autonomous fashion, suggesting that the effect of SST in regulating excitatory synaptic transmission is mainly mediated by SST receptor subtype 4. Together, our results demonstrated that activity-dependent release of SST reduced the density of dendritic spines and the number of excitatory synapses through postsynaptic activation of SST receptor subtype 4 in pyramidal neurons. To our knowledge, this is the first demonstration of the long term effect of SST on neuronal morphology.

Tlx1/3 and Ptf1a control the expression of distinct sets of transmitter and peptide receptor genes in the developing dorsal spinal cord

Establishing the pattern of expression of transmitters and peptides as well as their receptors in different neuronal types is crucial for understanding the circuitry in various regions of the brain. Previous studies have demonstrated that the transmitter and peptide phenotypes in mouse dorsal spinal cord neurons are determined by the transcription factors Tlx1/3 and Ptf1a. Here we show that these transcription factors also determine the expression of two distinct sets of transmitter and peptide receptor genes in this region. We have screened the expression of 78 receptor genes in the spinal dorsal horn by in situ hybridization. We found that receptor genes Gabra1, Gabra5, Gabrb2, Gria3, Grin3a, Grin3b, Galr1, and Npy1r were preferentially expressed in Tlx3-expressing glutamatergic neurons and their derivatives, and deletion of Tlx1 and Tlx3 resulted in the loss of expression of these receptor genes. Furthermore, we obtained genetic evidence that Tlx3 uses distinct pathways to control the expression of receptor genes. We also found that receptor genes Grm3, Grm4, Grm5, Grik1, Grik2, Grik3, and Sstr2 were mainly expressed in Pax2-expressing GABAergic neurons in the spinal dorsal horn, and their expression in this region was abolished or markedly reduced in Ptf1a and Pax2 deletion mutant mice. Together, our studies indicate that Tlx1/3 and Ptf1a, the key transcription factors for fate determination of glutamatergic and GABAergic neurons in the dorsal spinal cord, are also responsible for controlling the expression of two distinct sets of transmitter and peptide receptor genes.

[111In-DOTA]Somatostatin-14 analogs as potential pansomatostatin-like radiotracers - first results of a preclinical study

Background

In this study, we report on the synthesis, radiolabeling, and biological evaluation of two new somatostatin-14 (SS14) analogs, modified with the universal chelator DOTA. We were interested to investigate if and to what extent such radiotracer prototypes may be useful for targeting sst_1-5-expressing tumors in man but, most importantly, to outline potential drawbacks and benefits associated with their use.

Methods

AT1S and AT2S (DOTA-Ala¹-Gly²-c[Cys³-Lys⁴-Asn⁵-Phe⁶-Phe⁷-Trp⁸/DTrp⁸-Lys⁹-Thr¹⁰-Phe¹¹-Thr¹²-Ser¹³-Cys¹⁴-OH], respectively) were synthesized on the solid support and labeled with ¹¹¹In. The sst_1-5 affinity profile of AT1S/AT2S was determined by receptor autoradiography using [Leu⁸,dTrp²²,¹²⁵I-Tyr²⁵]SS28 as radioligand. The ability of AT2S to stimulate sst₂ or sst₃ internalization was qualitatively analyzed by an immunofluorescence-based internalization assay using hsst₂- or hsst₃-expressing HEK293 cells. Furthermore, the internalization of the radioligands [¹¹¹In]AT1S and [¹¹¹In]AT2S was studied at 37 °C in AR4-2J cells endogenously expressing sst₂. The in vivo stability of [¹¹¹In]AT1S and [¹¹¹In]AT2S was tested by high-performance liquid chromatography analysis of mouse blood collected 5 min after radioligand injection, and biodistribution was studied in normal mice. Selectively for [¹¹¹In]AT2S, biodistribution was further studied in SCID mice bearing AR4-2J, HEK293-hsst_2A⁺, -hsst₃⁺ or -hsst₅⁺ tumors.

Results

The new SS14-derived analogs were obtained by solid phase peptide synthesis and were easily labeled with ¹¹¹In. Both SS14 conjugates, AT1S, and its DTrp⁸ counterpart, AT2S, showed a pansomatostatin affinity profile with the respective hsst_1-5 IC₅₀ values in the lower nanomolar range. In addition, AT2S behaved as an agonist for sst₂ and sst₃ since it stimulated receptor internalization. The ¹¹¹In radioligands effectively and specifically internalized into rsst_2A-expressing AR4-2J cells with [¹¹¹In]AT2S internalizing faster than [¹¹¹In]AT1S. Ex vivo mouse blood analysis revealed a rapid degradation of both radiopeptides in the bloodstream with the DTrp⁸ analog showing higher stability. Biodistribution results in healthy mice were consistent with these findings with only [¹¹¹In]AT2S showing specific uptake in the sst₂-rich pancreas. Biodistribution of [¹¹¹In]AT2S in tumor-bearing mice revealed receptor-mediated uptake in the AR4-2J (1.82 ± 0.36 %ID/g - block 0.21 ± 0.17 %ID/g at 4 h post injection (pi)), the HEK293-hsst_2A⁺ (1.49 ± 0.2 %ID/g - block 0.27 ± 0.20 %ID/g at 4 h pi), the HEK293-hsst₃⁺ (1.24 ± 0.27 %ID/g - block 0.32 ± 0.06 %ID/g at 4 h pi), and the HEK293-hsst₅⁺ tumors (0.41 ± 0.12 %ID/g - block 0.22 ± 0.006 %ID/g at 4 h pi). Radioactivity washed out from blood and background tissues via the kidneys.

Conclusions

This study has revealed that the native SS14 structure can indeed serve as a motif for the development of promising pansomatostatin-like radiotracers. Further peptide stabilization is required to increase in vivo stability and, consequently, to enhance in vivo delivery and tumor targeting.

Sstr2A: a relevant target for the delivery of genes into human glioblastoma cells using fiber-modified adenoviral vectors

Glioblastomas are the most aggressive of the brain tumors occurring in adults and children. Currently available chemotherapy prolongs the median survival time of patients by only 4 months. The low efficiency of current treatments is partly owing to the blood-brain barrier, which restricts the penetration of most drugs into the central nervous system. Locoregional treatment strategies thus become mandatory. In this context, viral tools are of great interest for the selective delivery of genes into tumoral cells. Gliomas express high levels of type 2 somatostatin receptors (sstr2A), pinpointing them as suitable targets for the improvement of transduction efficiency in these tumors. We designed a new adenoviral vector based on the introduction of the full-length somatostatin (SRIF (somatotropin release-inhibiting factor)) sequence into the HI loop of the HAdV fiber protein. We demonstrate that (i) HAdV-5-SRIF uptake into cells is mediated by sstr2A, (ii) our vector drives high levels of gene expression in cells expressing endogenous sstr2A, with up to 65-fold enhancement and (iii) low doses of HAdV-5-SRIF are sufficient to infect high-grade human primary glioblastoma cells. Adenoviral vectors targeting SRIF receptors might thus represent a promising therapeutic approach to brain tumors.

The Concept of Divergent Targeting through the Activation and Inhibition of Receptors as a Novel Chemotherapeutic Strategy: Signaling Responses to Strong DNA-Reactive Combinatorial Mimicries

Recently, we reported the combination of multitargeted ErbB1 inhibitor-DNA damage combi-molecules with OCT in order to downregulate ErbB1 and activate SSTRs. Absence of translation to cell kill was believed to be partially due to insufficient ErbB1 blockage and DNA damage. In this study, we evaluated cell response to molecules that damage DNA more aggressively and induce stronger attenuation of ErbB1 phosphorylation. We used three cell lines expressing low levels (U87MG) or transfected to overexpress wildtype (U87/EGFR) or a variant (U87/EGFRvIII) of ErbB1. The results showed that Iressa ± HN2 and the combi-molecules, ZRBA4 and ZR2003, significantly blocked ErbB1 phosphorylation in U87MG cells. Addition of OCT significantly altered cell cycle distribution. Analysis of the DNA damage response pathway revealed strong upregulation of p53 by HN2 and the combi-molecules. Apoptosis was only induced by a 48 h exposure to HN2. All other treatments resulted in cell necrosis. This is in agreement with Akt-Bad pathway activation and survivin upregulation. Despite strong DNA damaging properties and downregulation of ErbB1 phosphorylation by these molecules, the strongest effect of SSTR activation was on cell cycle distribution. Therefore, any enhanced antiproliferative effects of combining ErbB1 inhibition with SSTR activation must be addressed in the context of cell cycle arrest.

Bak Foong Pills induce an analgesic effect by inhibiting nociception via the somatostatin pathway in mice

Dysmenorrhoea, defined as cramping pain in the lower abdomen occurring before or during menstruation, affects, to varying degrees, up to 90% of women of child-bearing age. We investigated whether BFP (Bak Foong Pills), a traditional Chinese medicine treatment for dysmenorrhoea, possesses analgesic properties. Results showed that BFP was able to significantly reduce pain responses following subchronic treatment for 3 days, but not following acute (1 h) treatment in response to acetic acid-induced writhing in C57/B6 mice. The analgesic effect was not due to inhibition of COX (cyclo-oxygenase) activity, evidenced by the lack of inhibition of prostacyclin and PGE2 (prostaglandin E2) production. Molecular analysis revealed that BFP treatment modulated the expression of a number of genes in the spinal cord of mice subjected to acetic acid writhing. RT-PCR (reverse transcription-PCR) analysis of spinal cord samples showed that both sst4 (somatostatin receptor 4) and sst2 receptor mRNA, but not μOR (μ-opiate receptor) and NK1 (neurokinin-1) receptor mRNA, were down-regulated following BFP treatment, thus implicating somatostatin involvement in BFP-induced analgesia. Administration of c-som (cyclo-somatostatin), a somatostatin antagonist, prior to acetic acid-induced writhing inhibited the analgesic effect. Thus subchronic treatment with BFP has anti-nociceptive qualities mediated via the somatostatin pathway.

Rat hippocampal somatostatin sst3 and sst4 receptors mediate anticonvulsive effects in vivo: indications of functional interactions with sst2 receptors

Somatostatin-14 (SRIF) is a potent anticonvulsant in rodent models of limbic seizures in which the hippocampus is its major site of action. However, the distribution of hippocampal sst receptors and their role in the anticonvulsant effects of SRIF remain controversial. Moreover, striking differences have been described between mice and rats. In rats, sst(2) but not sst(1) receptors play a critical role in the anticonvulsant effects of SRIF. At present, the role of rat sst(3) and sst(4) receptors in these anticonvulsive effects remains unknown. Here we demonstrate in vivo anticonvulsive actions of rat hippocampal sst(3) and sst(4) receptors. Using microdialysis and telemetry-based electroencephalographic recordings we show that intrahippocampal administration of the sst(2) agonist L-779,976 (500 nM), the sst(3) agonist L-796,778 (100 nM) or the sst(4) agonist L-803,087 (100 nM) protects rats against focal pilocarpine-induced seizures. SRIF (1 μM)-, sst(3)- and sst(4)-mediated anticonvulsive actions are reversed by the selective sst(2) receptor antagonist cyanamid 154806 (100 nM). Moreover, the selective sst(3) antagonist SST3-ODN-8 (100 nM) blocks the sst(4)-mediated anticonvulsant effect. Sst(3) antagonism does not reverse the sst(2)- or SRIF-mediated anticonvulsant effects. Our findings provide the first in vivo evidence for potent anticonvulsive properties of sst(3) and sst(4) receptors in the rat hippocampus. Nevertheless, selective sst(2) receptor antagonism prevented these sst(3)- or sst(4) receptor-mediated anticonvulsant effects, suggesting a functional cooperation with rat hippocampal sst(2) receptors.

Somatostatin receptor biology in neuroendocrine and pituitary tumours: part 1--molecular pathways

Neuroendocrine tumours (NETs) may occur at many sites in the body although the majority occur within the gastroenteropancreatic axis. Non-gastroenteropancreatic NETs encompass phaeochromocytomas and paragangliomas, medullary thyroid carcinoma, anterior pituitary tumour, broncho-pulmonary NETs and parathyroid tumours. Like most endocrine tumours, NETs also express somatostatin (SST) receptors (subtypes 1–5) whose ligand SST is known to inhibit endocrine and exocrine secretions and have anti-tumour effects. In the light of this knowledge, the idea of using SST analogues in the treatment of NETs has become increasingly popular and new studies have centred upon the development of new SST analogues. We attempt to review SST receptor (SSTR) biology primarily in neuroendocrine tissues, focusing on pituitary tumours. A full data search was performed through PubMed over the years 2000–2009 with keywords ‘somatostatin, molecular biology, somatostatin receptors, somatostatin signalling, NET, pituitary’ and all relevant publications have been included, together with selected publications prior to that date. SSTR signalling in non-neuroendocrine solid tumours is beyond the scope of this review. SST is a potent anti-proliferative and anti-secretory agent for some NETs. The successful therapeutic use of SST analogues in the treatment of these tumours depends on a thorough understanding of the diverse effects of SSTR subtypes in different tissues and cell types. Further studies will focus on critical points of SSTR biology such as homo- and heterodimerization of SSTRs and the differences between post-receptor signalling pathways of SSTR subtypes.

Somatostatin inhibits cell migration and reduces cell counts of human keratinocytes and delays epidermal wound healing in an ex vivo wound model

The peptide hormone somatostatin (SST) and its five G protein-coupled receptors (SSTR1-5) were described to be present in the skin, but their cutaneous function(s) and skin-specific signalling mechanisms are widely unknown. By using receptor specific agonists we show here that the SSTRs expressed in keratinocytes are functionally coupled to the inhibition of adenylate cyclase. In addition, treatment with SSTR4 and SSTR5/1 specific agonists significantly influences the MAP kinase signalling pathway. As epidermal hormone receptors in general are known to regulate re-epithelialization following skin injury, we investigated the effect of SST on cell counts and migration of human keratinocytes. Our results demonstrate a significant inhibition of cell migration and reduction of cell counts by SST. We do not observe an effect on apoptosis and necrosis. Analysis of signalling pathways showed that somatostatin inhibits cell migration independent of its effect on cAMP. Migrating keratinocytes treated with SST show altered cytoskeleton dynamics with delayed lamellipodia formation. Furthermore, the activity of the small GTPase Rac1 is diminished, providing evidence for the control of the actin cytoskeleton by somatostatin receptors in keratinocytes. While activation of all receptors leads to redundant effects on cell migration, only treatment with a SSTR5/1 specific agonist resulted in decreased cell counts. In accordance with reduced cell counts and impaired migration we observe delayed re-epithelialization in an ex vivo wound healing model. Consequently, our experiments suggest SST as a negative regulator of epidermal wound healing.

Resistance to somatostatin analogs in acromegaly

Somatostatin analogs (SA) are widely used in acromegaly, either as first-line or adjuvant treatment after surgery. First-line treatment with these drugs is generally used in the patients with macroadenomas or in those with clinical conditions so severe as to prevent unsafe reactions during anesthesia. Generally, the response to SA takes into account both control of GH and IGF-I excess, with consequent improvement of clinical symptoms directly related to GH and IGF-I excess, and tumor shrinkage. This latter effect is more prominent in the patients treated first-line and bearing large macroadenomas, but it is also observed in patients with microadenomas, even with little clinical implication. Predictors of response are patients' gender, age, initial GH and IGF-I levels, and tumor mass, as well as adequate expression of somatostatin receptor types 2 and 5, those with the highest affinity for octreotide and lanreotide. Only sporadic cases of somatostatin receptor gene mutation or impaired signaling pathways have been described in GH-secreting tumors so far. The response to SA also depends on treatment duration and dosage of the drug used, so that a definition of resistance based on short-term treatments using low doses of long-acting SA is limited. Current data suggest that response to these drugs is better analyzed taking together biochemical and tumoral effects because only the absence of both responses might be considered as a poor response or resistance. This latter evidence seems to occur in 25% of treated patients after 12 months of currently available long-acting SA.

Somatostatin stimulates intestinal NHE8 expression via p38 MAPK pathway

Diarrhea is a common manifestation of gastrointestinal disorders. Diarrhea-induced losses of fluid and electrolyte could lead to dehydration and electrolyte imbalances, resulting in significant morbidity and mortality, especially in children living in developing countries. Somatostatin, a peptide hormone secreted by D-cells, plays an important role in regulating motility and intestinal Na(+) absorption. Although octreotide, a somatostatin analog, is used to treat diarrhea, its mechanisms of action are unclear. Here we showed that octreotide increased brush-border membrane Na(+)/H(+) exchanger 8 (NHE8) expression in the small intestine to the exclusion of other NHEs that participate in Na(+) absorption. The same effect also occurred in human intestinal cells (Caco-2). We found that the increase of NHE8 expression by somatostatin required p38 mitogen-activated protein kinase (MAPK) activation. Furthermore, the somatostatin receptor SSTR2 antagonist CYN154806 could abolish somatostatin-induced NHE8 expression and p38 MAPK phosphorylation. Thus our data provided the first concrete evidence indicating that somatostatin stimulates intestinal Na(+) absorption by increasing intestinal NHE8 expression through the SSTR2-p38 MAPK pathway.

Reprogramming into pancreatic endocrine cells based on developmental cues

Due to the increasing prevalence of type 1 diabetes and the complications arising from actual therapies, alternative treatments need to be established. In order to compensate the beta-cell deficiency associated with type 1 diabetes, current researches focus on new strategies to generate insulin-producing beta cells for transplantation purpose, including the differentiation of stem or progenitor cells, as well as the transdifferentiation of dispensable mature cell types. However, to successfully force any cell to adopt a functional beta-cell fate or phenotype, a better understanding of the molecular mechanisms underlying the genesis of these in vivo is required. The present short review summarizes the hitherto known functions and interplays of several key factors involved in the differentiation of the endocrine cell lineages during pancreas morphogenesis, as well as there potential in generating beta cells. Furthermore, an emphasize is made on beta-cell regeneration and the determinants implicated.

Somatostatin inhibition of gonadotropin-releasing hormone neurons in female and male mice

Previous studies indicate that somatostatin regulates gonadotropin secretion. We investigated here whether somatostatin has direct effects on GnRH neurons in the adult male and female mice. Dual-labeling immunofluorescence experiments revealed the presence of somatostatin-immunoreactive fibers adjacent to GnRH neurons, and three-dimensional confocal reconstructions demonstrated apparent somatostatin fiber appositions with 50-60% of GnRH neurons located throughout the brain in both male and female mice. Perforated patch-clamp recordings from GnRH-green fluorescent protein neurons revealed that approximately 70% of GnRH neurons responded in a dose-dependent manner to 10-300 nm somatostatin with an acute membrane hyperpolarization and cessation of firing. This effect persisted in the presence of tetrodotoxin and amino acid receptor antagonists, indicating a direct postsynaptic site of action on the GnRH neuron. The identity of the somatostatin receptors underlying this action was assessed using GnRH neuron single-cell RT-PCR. Of the somatostatin receptor subtypes, the sstr2 transcript was the most prevalent and detected in both males and females. The expression of sstr2 by GnRH neurons was confirmed in the sstr2 knockout/LacZ knock-in mouse line. Electrophysiological studies demonstrated that the sstr2-selective agonist seglitide exerted acute hyperpolarizing actions on GnRH neurons identical to those of somatostatin. Together, these studies reveal somatostatin, acting through sstr2, to be one of the most potent inhibitors of electrical excitability of male and female GnRH neurons identified thus far.

Somatostatin interneurons delineate the inner part of the external plexiform layer in the mouse main olfactory bulb

Neuropeptides play a major role in the modulation of information processing in neural networks. Somatostatin, one of the most concentrated neuropeptides in the brain, is found in many sensory systems including the olfactory pathway. However, its cellular distribution in the mouse main olfactory bulb (MOB) is yet to be characterized. Here we show that approximately 95% of mouse bulbar somatostatin-immunoreactive (SRIF-ir) cells describe a homogeneous population of interneurons. These are restricted to the inner lamina of the external plexiform layer (iEPL) with dendritic field strictly confined to the region. iEPL SRIF-ir neurons share some morphological features of Van Gehuchten short-axon cells, and always express glutamic acid decarboxylase, calretinin, and vasoactive intestinal peptide. One-half of SRIF-ir neurons are parvalbumin-ir, revealing an atypical neurochemical profile when compared to SRIF-ir interneurons of other forebrain regions such as cortex or hippocampus. Somatostatin is also present in fibers and in a few sparse presumptive deep short-axon cells in the granule cell layer (GCL), which were previously reported in other mammalian species. The spatial distribution of somatostatin interneurons in the MOB iEPL clearly outlines the region where lateral dendrites of mitral cells interact with GCL inhibitory interneurons through dendrodendritic reciprocal synapses. Symmetrical and asymmetrical synaptic contacts occur between SRIF-ir dendrites and mitral cell dendrites. Such restricted localization of somatostatin interneurons and connectivity in the bulbar synaptic network strongly suggest that the peptide plays a functional role in the modulation of olfactory processing.

Receptor activation and inhibition in cellular response to chemotherapeutic combinational mimicries: the concept of divergent targeting

The antiproliferative effect of tandem somatostatin receptor (SSTR) activation, epidermal growth factor receptor (EGFR) inhibition, and induction of DNA damage was analyzed using octreotide (OCT), a SSTR agonist, the clinical DNA methylating agent temozolomide (TMZ), Iressa, an EGFR inhibitor, and dual EGFR-DNA targeting agents termed "combi-molecules". Using SSTR-expressing glioma cells harbouring low levels of EGFR (U87MG) or transfected to overexpress EGFR (U87/EGFR) or a variant (U87/EGFRvIII), we showed that Iressa, alone or in combination with the DNA damaging agent TMZ, and combi-molecules RA2 and RA5 inhibited EGF-induced phosphorylation of EGFR in U87MG and more moderately in U87/EGFR and U87/EGFRvIII transfected cells. This translated into equivalent levels of Erk 1/2 inhibition. Activation of SSTRs with OCT did not modulate the effects of the various treatments on Erk 1/2 phosphorylation. Likewise, SSTR activation did not alter TMZ- or DNA-damaging combi-molecules, RA2 and RA5, induced p53 activation nor upregulation. However, SSTR activation significantly shifted TMZ-, RA2- and RA5-induced cell-cycle arrest to earlier phases (i.e., G2/M to late S, late S to S, S to G1). Further analysis showed that apoptosis was not induced. This was in agreement with the fact that p53 activation did not induce Bax upregulation nor did EGFR inhibition promote Bad dephosphorylation. Moreover, enhancement of survivin, an anti-apoptotic protein, expression was observed. The results in toto suggest that the combination of SSTR activation with EGFR inhibition and DNA damage affects cell-cycle progression but a disconnection between the targeted signalling pathways in these brain tumour cells precludes synergistic cell-killing by the triple growth inhibitory events.

The somatostatin receptor subtype 5 in neuroendocrine tumours

IMPORTANCE OF THE FIELD

In recent years, scientific work has been intensified to unravel new (patho-) physiological insights, particularly regarding the functional role of somatostatin (SRIF) receptor subtype 5 (sst) and the development of novel sst(5)-targeted SRIF analogues, in order to broaden medical therapeutic opportunities in patients suffering from neuroendocrine diseases.

AREAS COVERED IN THIS REVIEW

The scope of this review is primarily focused upon recent insights in sst(5)-receptor physiology, novel sst(5)-targeted treatment options predominantly directed towards pituitary adenomas, and gastroenteropancreatic neuroendocrine tumours.

WHAT THE READER WILL GAIN

An understanding of the potential that novel sst(5)-targeted SRIF analogues might have in the medical treatment of Cushing's disease and acromegaly, as demonstrated by translational research, based on pathophysiological data combined with results from clinical trials.

TAKE HOME MESSAGE

The role of targeting sst(5) in gastroenteropancreatic neuroendocrine tumours remains to be established. The sst(5) subtype might function as sst(2) modulator in terms of receptor internalization and desensitization, and seems less important compared with sst(2)-preferring SRIF analogues in the regulation of human insulin secretion by the pancreas. Finally, absence of sst(5) in corticotroph adenomas could be related to tumour aggressiveness in Cushing's disease.

Reprogramming into pancreatic endocrine cells based on developmental cues

Due to the increasing prevalence of type 1 diabetes and the complications arising from actual therapies, alternative treatments need to be established. In order to compensate the beta-cell deficiency associated with type 1 diabetes, current research focuses on new strategies to generate insulin-producing beta-cells for transplantation purpose, including the differentiation of stem or progenitor cells, as well as the transdifferentiation of dispensable mature cell types. However, to successfully force specific cells to adopt a functional beta-cell fate or phenotype, a better understanding of the molecular mechanisms underlying beta-cell genesis is required. The present short review summarizes the hitherto known functions and interplays of several key factors involved in the development of the different endocrine cell lineages during pancreas morphogenesis, as well as their potential to direct the generation of beta-cells. Furthermore, an emphasis is made on beta-cell regeneration and the determinants implicated.

Mechanisms underlying modulation of monocarboxylate transporter 1 (MCT1) by somatostatin in human intestinal epithelial cells

Somatostatin (SST), an important neuropeptide of the gastrointestinal tract has been shown to stimulate sodium chloride absorption and inhibit chloride secretion in the intestine. However, the effects of SST on luminal butyrate absorption in the human intestine have not been investigated. Earlier studies from our group and others have shown that monocarboxylate transporter (MCT1) plays an important role in the transport of butyrate in the human intestine. The present studies were undertaken to examine the effects of SST on butyrate uptake utilizing postconfluent human intestinal epithelial Caco2 cells. Apical SST treatment of Caco-2 cells for 30-60 min significantly increased butyrate uptake in a dose-dependent manner with maximal increase at 50 nM ( approximately 60%, P < 0.05). SST receptor 2 agonist, seglitide, mimicked the effects of SST on butyrate uptake. SST-mediated stimulation of butyrate uptake involved the p38 MAP kinase-dependent pathway. Kinetic studies demonstrated that SST increased the maximal velocity (V(max)) of the transporter by approximately twofold without any change in apparent Michaelis-Menten constant (K(m)). The higher butyrate uptake in response to SST was associated with an increase in the apical membrane levels of MCT1 protein parallel to a decrease in the intracellular MCT1 pool. MCT1 has been shown to interact specifically with CD147 glycoprotein/chaperone to facilitate proper expression and function of MCT1 at the cell surface. SST significantly enhanced the membrane levels of CD147 as well as its association with MCT1. This association was completely abolished by the specific p38 MAP kinase inhibitor, SB203580. Our findings demonstrate that increased MCT1 association with CD147 at the apical membrane in response to SST is p38 MAP kinase dependent and underlies the stimulatory effects of SST on butyrate uptake.

The beta-D-glucose scaffold as a beta-turn mimetic

Activity and selectivity are typically the first considerations when designing a drug. However, absorption, distribution, metabolism, excretion, and toxicity (ADMET) are equally important considerations. Peptides can provide a combination of potent binding and exquisite selectivity, as evidenced by their pervasive use as enzymes, hormones, and signaling agents within living systems. In particular, peptidic turn motifs are key elements of molecular recognition. They may be found at the exposed surfaces of globular proteins, where they are available for binding interactions with other peptides and small molecules. However, despite these advantages, peptides often make poor drugs. The amide backbone is subject to rapid enzymatic proteolysis, resulting in short half-lives. Furthermore, the ability of the amide backbone to hydrogen bond with water restricts its ability to cross membranes and, consequentially, results in poor oral bioavailability. Accordingly, the development of nonpeptidic scaffolds that mimic peptidic turn motifs represents a promising means of converting peptidic agents into more drugable molecules. In this Account, we describe the design and synthesis of beta-turn mimetics that use a beta-D-glucose scaffold, the first use of a sugar scaffold for this purpose. Somatostatin (SRIF) is a small protein (14 amino acid residues) human hormone; a shorter (6 amino acid residues) synthetic peptide, L-363,301, is a fully peptidal agonist. These two cyclic peptides share the beta-turn motif comprising Phe(7)-Trp(8)-Lys(9)-Thr(10) (d-Trp(8) in the case of L-363,301), of which the tryptophan and lysine residues in the i + 1 and i + 2 positions, respectively, are critical for binding. In 1988, we initiated a program that tested and validated the then-novel proposition that the beta-D-glucose scaffold can mimic the beta-turn in L-363,301. The beta-D-glucose scaffold proved to be an attractive mimic of a beta-turn in part because it permits the convenient attachment of amino acid side chains via facile etherification reactions, rather than carbon-carbon bond formations; it is also an inexpensive starting material with well-defined stereochemistry. From the beginning, biological assays were used alongside physical measurements to assess the relevance of the design. Our first two synthetic targets, compounds 6 and 7, bound the SRIF receptors on benchmark (AtT-20) cells, albeit weakly, consistent with the objective of the design. Subsequently, a better ligand (8) and two congeners were found to be agonists at the SRIF receptors, providing convincing evidence that the peptide backbone is not required for receptor binding or signal transduction. The unexpectedly high level of receptor affinity of selected analogs, as well as the fortuitous discovery that our peptidomimetics were active against several chemically distinct receptors, led us to hypothesize that these monosaccharides could access multiple potential binding modes. Our later studies of this sugar scaffold confirmed this property, which we termed pseudosymmetry, whereby multiple similar but nonidentical motifs are displayed within a single analog. We propose the presence of pseudosymmetry to be an element of privilege and an advantage for lead discovery.

Subcellular dynamics of somatostatin receptor subtype 1 in the rat arcuate nucleus: receptor localization and synaptic connectivity vary in parallel with the ultradian rhythm of growth hormone secretion

Growth hormone (GH) secretion in male rats exhibits a 3.3 h ultradian rhythm generated by the reciprocal interaction of GH-releasing hormone (GHRH) and somatostatin (SRIF). SRIF receptor subtypes sst(1) and sst(2) are highly expressed in GHRH neurons of the hypothalamic arcuate nucleus (ARC). We previously demonstrated an ultradian oscillation in binding of SRIF analogs to the ARC in relation to GH peaks and troughs. Here we tested the hypothesis that these ultradian changes in SRIF binding are due to differential plasma membrane targeting of sst(1) receptors in ARC neurons using immunocytochemistry and electron microscopy. We found that 87% of sst(1)-positive ARC neurons also synthesized GHRH. Subcellularly, 80% of sst(1) receptors were located intracellularly and 20% at the plasma membrane regardless of GH status. However, whereas 30% of the cell-surface sst(1) receptors were located perisynaptically or subsynaptically following exposure to high GH secretion, this fraction was increased to 42% following a GH trough period (p = 0.05). Furthermore, the relative abundance of symmetric and asymmetric synapses on sst(1)-positive dendrites also varied significantly, depending on the GH cycle, from approximately equal numbers following GH troughs to 70:30 in favor of symmetric, i.e., inhibitory, inputs after GH peaks (p < 0.02). These findings suggest that postsynaptic localization of sst(1) receptors and synaptic connectivity in the ARC undergo pronounced remodeling in parallel with the GH rhythm. Such synaptic plasticity may be an important mechanism by which sst(1) mediates SRIF's cyclical effects on ARC GHRH neurons to generate the ultradian rhythm of GH secretion.

Characterization of intracellular signaling mediated by human somatostatin receptor 5: role of the DRY motif and the third intracellular loop

Somatostatin (SST) exerts inhibitory effects on hormone secretion and cell proliferation by interacting with five different receptors (SST1-SST5) linked to multiple cellular effectors. The receptor structural domains involved in these effects have been only partially elucidated. The aim of the study was to investigate the molecular determinants mediating the interaction of the human SST5 with intracellular signaling in the pituitary cell line GH3, focusing on the BBXXB domain in the third intracellular loop and the DRY motif in the second intracellular loop. We analyzed the effects of the SST5 agonist BIM23206 on cAMP accumulation, intracellular calcium, GH secretion, cell proliferation, and ERK1/2 phosphorylation in cells expressing either wild-type SST5 or mutant receptors, in particular the naturally occurring mutant R240W in the BBXXB domain and the D136A and R137A mutants in the DRY motif. We found that residues D136 and R137 were critical for SST5 signaling because their substitutions abolished all the intracellular responses. Conversely, third intracellular loop mutations resulted in receptor that inhibited intracellular cAMP levels similar to the wild-type (50 +/- 9 vs. 53 +/- 12% inhibition) but failed to mediate the other responses elicited by wild-type SST5, i.e. reduction of intracellular calcium levels as well as inhibition of ERK1/2. These events resulted in an absent inhibition of GH release and an impaired reduction of cell proliferation (38 +/- 7 vs. 76 +/- 6% inhibition in wild type, P < 0.05). These data indicate that different regions of SST5 are required for the activation of different signaling pathways.

Novel, potent, and radio-iodinatable somatostatin receptor 1 (sst1) selective analogues

The proposed sst(1) pharmacophore (J. Med. Chem. 2005, 48, 523-533) derived from the NMR structures of a family of mono- and dicyclic undecamers was used to design octa-, hepta-, and hexamers with high affinity and selectivity for the somatostatin sst(1) receptor. These compounds were tested for their in vitro binding properties to all five somatostatin (SRIF) receptors using receptor autoradiography; those with high SRIF receptor subtype 1 (sst(1)) affinity and selectivity were shown to be agonists when tested functionally in a luciferase reporter gene assay. Des-AA(1,4-6,10,12,13)-[DTyr(2),DAgl(NMe,2naphthoyl)(8),IAmp(9)]-SRIF-Thr-NH(2) (25) was radio-iodinated ((125)I-25) and specifically labeled sst(1)-expressing cells and tissues. 3D NMR structures were calculated for des-AA(1,4-6,10,12,13)-[DPhe(2),DTrp(8),IAmp(9)]-SRIF-Thr-NH(2) (16), des-AA(1,2,4-6,10,12,13)-[DAgl(NMe,2naphthoyl)(8),IAmp(9)]-SRIF-Thr-NH(2) (23), and des-AA(1,2,4-6,10,12,13)-[DAgl(NMe,2naphthoyl)(8),IAmp(9),Tyr(11)]-SRIF-NH(2) (27) in DMSO. Though the analogues have the sst(1) pharmacophore residues at the previously determined distances from each other, the positioning of the aromatic residues in 16, 23, and 27 is different from that described earlier, suggesting an induced fit mechanism for sst(1) binding of these novel, less constrained sst(1)-selective family members.

The somatostatin 2A receptor is enriched in migrating neurons during rat and human brain development and stimulates migration and axonal outgrowth

The neuropeptide somatostatin has been suggested to play an important role during neuronal development in addition to its established modulatory impact on neuroendocrine, motor and cognitive functions in adults. Although six somatostatin G protein-coupled receptors have been discovered, little is known about their distribution and function in the developing mammalian brain. In this study, we have first characterized the developmental expression of the somatostatin receptor sst2A, the subtype found most prominently in the adult rat and human nervous system. In the rat, the sst2A receptor expression appears as early as E12 and is restricted to post-mitotic neuronal populations leaving the ventricular zone. From E12 on, migrating neuronal populations immunopositive for the receptor were observed in numerous developing regions including the cerebral cortex, hippocampus and ganglionic eminences. Intense but transient immunoreactive signals were detected in the deep part of the external granular layer of the cerebellum, the rostral migratory stream and in tyrosine hydroxylase- and serotonin- positive neurons and axons. Activation of the sst2A receptor in vitro in rat cerebellar microexplants and primary hippocampal neurons revealed stimulatory effects on neuronal migration and axonal growth, respectively. In the human cortex, receptor immunoreactivity was located in the preplate at early development stages (8 gestational weeks) and was enriched to the outer part of the germinal zone at later stages. In the cerebellum, the deep part of the external granular layer was strongly immunoreactive at 19 gestational weeks, similar to the finding in rodents. In addition, migrating granule cells in the internal granular layer were also receptor-positive. Together, theses results strongly suggest that the somatostatin sst2A receptor participates in the development and maturation of specific neuronal populations during rat and human brain ontogenesis.

alpha-Tocopherol decreases the somatostatin receptor-effector system and increases the cyclic AMP/cyclic AMP response element binding protein pathway in the rat dentate gyrus

Neuronal survival has been shown to be enhanced by alpha-tocopherol and modulated by cyclic AMP (cAMP). Somatostatin (SST) receptors couple negatively to adenylyl cyclase (AC), thus leading to decreased cAMP levels. Whether alpha-tocopherol can stimulate neuronal survival via regulation of the somatostatinergic system, however, is unknown. The aim of this study was to investigate the effects of alpha-tocopherol on the SST signaling pathway in the rat dentate gyrus. To that end, 15-week-old male Sprague-Dawley rats were treated daily for 1 week with (+)-alpha-tocopherol or vehicle and sacrificed on the day following the last administration. No changes in either SST-like immunoreactivity (SST-LI) content or SST mRNA levels were detected in the dentate gyrus as a result of alpha-tocopherol treatment. A significant decrease in the density of the SST binding sites and an increase in the dissociation constant, however, were detected. The lower SST receptor density in the alpha-tocopherol-treated rats correlated with a significant decrease in the protein levels of the SST receptor subtypes SSTR1-SSTR4, whereas the corresponding mRNA levels were unaltered. G-protein-coupled-receptor kinase 2 expression was decreased by alpha-tocopherol treatment. This vitamin induced a significant increase in both basal and forskolin-stimulated AC activity, as well as a decrease in the inhibitory effect of SST on AC. Whereas the protein levels of AC type V/VI were not modified by alpha-tocopherol administration, ACVIII expression was significantly enhanced, suggesting it might account for the increase in AC activity. In addition, this treatment led to a reduction in Gialpha1-3 protein levels and in Gi functionality. alpha-Tocopherol did not affect the expression of the regulator of G-protein signaling 6/7 (RGS6/7). Finally, alpha-tocopherol induced an increase in the levels of phosphorylated cAMP response element binding protein (p-CREB) and total CREB in the dentate gyrus. Since CREB synthesis and phosphorylation promote the survival of many cells, including neurons, whereas SST inhibits the cAMP-PKA pathway, which is known to be involved in CREB phosphorylation, the alpha-tocopherol-induced reduction of SSTR observed here might possibly contribute, via increased cAMP levels and CREB activity, to the mechanism by which this vitamin promotes the survival of newborn neurons in the dentate gyrus.

N-type Ca(2+) -channels in murine pancreatic beta-cells are inhibited by an exclusive coupling with somatostatin receptor subtype 1

Somatostatin (SRIF) is a well-established inhibitor of insulin secretion, an effect in part mediated by a direct inhibition of voltage-operated Ca(2+)-channels. However, the identity of the somatostatin receptor subtypes (SSTRs) and voltage-operated Ca(2+)-channels involved in this process are unknown. Whole-cell perforated patch-clamp methods were applied to the murine pancreatic beta-cell line, MIN6, to explore the molecular pharmacology of this problem. SRIF-14 inhibited voltage-gated Ca(2+) currents (ICa(2+)) by 19 +/- 3% (n=24) with a pEC(50) = 9.05 (95% confidence limits 9-9.1). This action was mimicked solely by 100 nm CH-275, a selective agonist at the somatostatin type 1 receptor (SSTR1), but not by 100 nm BIM-23027, L-362855, or NNC-269100; agonists selective for the other four SSTRs known to exist in MIN6. The inhibition of ICa(2+) produced by SRIF and CH-275 was insensitive to pertussis toxin but was reversed by a prepulse to +100 mV. The inhibition of ICa(2+) by SRIF-14 was unaffected by 20 microm nifedipine, an inhibitor of L-type Ca(2+) channels. Application of the specific N-type Ca(2+) channel (Ca(v)2.2) inhibitor omega-conotoxin GV1A at 100 nm mimicked, and as a consequence abolished, the inhibitory effect of SRIF-14 on ICa(2+). SRIF selectively inhibits N-type Ca(2+)-channels in murine pancreatic beta-cells via exclusive coupling with SSTR1. These findings help explain how SSTR1 activation can inhibit insulin secretion in pancreatic beta-cells and suggest a possible new therapeutic lead for treatment of hyperinsulinemia.

Hippocampal SSTR4 somatostatin receptors control the selection of memory strategies

RATIONALE

Somatostatin (SS14) has been implicated in various cognitive disorders, and converging evidence from animal studies suggests that SS14 neurons differentially regulate hippocampal- and striatal-dependent memory formation. Four SS14 receptor subtypes (SSTR1-4) are expressed in the hippocampus, but their respective roles in memory processes remain to be determined.

OBJECTIVES

In the present study, effects of selective SSTR1-4 agonists on memory formation were assessed in a water-maze task which can engage either hippocampus-dependent "place" and/or striatum-dependent "cue" memory formation.

MATERIALS AND METHODS

Mice received an intrahippocampal injection of one of each of the selective agonists and were then trained to locate an escape platform based on either distal cues (place memory) or a visible proximal cue (cue memory). Retention was tested 24 h later on probe trials aimed at identifying which memory strategy was preferentially retained.

RESULTS

Both SS14 and the SSTR4 agonist (L-803,087) dramatically impaired place memory formation in a dose-dependent manner, whereas SSTR1 (L-797,591), SSTR2 (L-779,976), or SSTR3 (L-796,778) agonists did not yield any behavioral effects. However, unlike SS14, the SSTR4 agonist also dose-dependently enhanced cue-based memory formation. This effect was confirmed in another striatal-dependent memory task, the bar-pressing task, where L-803,087 improved memory of the instrumental response, whereas SS14 was once again ineffective.

CONCLUSIONS

These data suggest that hippocampal SSTR4 are selectively involved in the selection of memory strategies by switching from the use of hippocampus-based multiple associations to the use of simple dorsal striatum-based behavioral responses. Possible neural mechanisms and functional implications are discussed.

Acromegaly: correlation between expression of somatostatin receptor subtypes and response to octreotide-lar treatment

About one-third of acromegalics are resistant to the clinically available somatostatin analogs (SA). The resistance is related to density reduction or different expression of somatostatin receptor subtypes (SSTR). This study analyzes SSTR's expression in somatotrophinomas, comparing to SA response, hormonal levels, and tumor volume. We analyzed 39 somatotrophinomas; 49% were treated with SA. The most expressed SSTR was SSTR5, SSTR3, SSTR2, SSTR1, and SSTR4, respectively. SSTR1 and SSTR2 had higher expression in patients that had normalized GH and IGF-I. SSTR3 was more expressed in patients with tumor reduction. There was a positive correlation between the percentage of tumor reduction and SSTR1, SSTR2 and SSTR3 expression. Also, a positive correlation between SSTR2 mRNA expression and the immunohistochemical reactivity of SSTR2 was found. Our study confirmed the association between the SA response to GH and IGF-I and the SSTR2. Additionally, this finding was also demonstrated in relation to SSTR1.

Somatostatin receptors 1 and 5 heterodimerize with epidermal growth factor receptor: agonist-dependent modulation of the downstream MAPK signalling pathway in breast cancer cells

The role of somatostatin (SST) and epidermal growth factor (EGF) in breast cancer is undisputed; however, the molecular mechanisms underlying their antiproliferative or proliferative effects are not well understood. We initially confirmed that breast tumour tissues express all five somatostatin receptors (SSTR1-5) and four epidermal growth factor receptors (ErbB1-4). Subsequently, to gain insight into the function of SSTRs and ErbBs in oestrogen receptor (ER)-positive (MCF-7) or ERalpha-negative (MDA-MB-231) breast cancer cells, we defined SSTR1, SSTR5 and ErbB1 mRNA and protein expression in these two tumour cell lines. Consistent with previous studies showing SSTR1/SSTR5 heterodimerization and having seen cell-specific and ligand-selective alterations in receptor expression, we next elucidated whether SSTR1 and SSTR5 functionally interact with ErbB1 using pbFRET analysis. We subsequently determined the effects of SST and EGF either alone, or in combination, on selected downstream signalling molecules such as erk1/2, p38 and JNK. Here, we showed that both SST and EGF influenced erk1/2 phosphorylation and that SST modulated the effects of EGF in a cell-specific manner. We also demonstrated agonist-, time and cell-dependent regulation of p38 phosphorylation. We further investigated modulation of Grb2, SOS, Shc, SH-PTP1 and SH-PTP2. ErbB1 adaptor proteins known to play a role in MAPK activation, Shc, Grb2 and SOS, changed in an agonist- and cell-specific manner whereas, SH-PTP1 and SH-PTP2, adaptor proteins reported to interact with SSTRs, translocated from the cytosol to membrane in a cell-specific manner following SST and/or EGF treatment. Although several previous studies have shown crosstalk between RTKs and GPCRs, there are no reports describing SSTR (GPCR) modulation of ErbBs (RTK) in breast cancer. To the best of our knowledge, this is the first report describing crosstalk/interactions between SSTRs and ErbBs.

Somatostatin receptor subtype 1 is a PDZ ligand for synapse-associated protein 97 and a potential regulator of growth cone dynamics

We report that somatostatin receptor subtype 1 (sst1) associates in vivo and in vitro with synapse-associated protein SAP) 97, a membrane-associated guanylate kinase homolog implicated as a scaffolding protein in the structural organization of specialized membrane complexes in various tissues, including the CNS. SAP97 and sst1 were coimmuno-precipitated from rodent brain and from transfected human embryonic kidney (HEK) 293 cells, and pull-down experiments demonstrated that the interaction is dependent on the class I PDZ binding motif in sst1 carboxyterminus. Calorimetric titration indicated that the postsynaptic density-95/discs large/zona occludens-1 (PDZ) 2 domain of SAP97 provides the main contribution to the interaction. We noticed substantial sst1 immunoreactivity in differentiating cortical neurons in culture which declined as the cultures matured. The sst1 immunoreactivity extended, together with SAP97 to neuronal growth cones. Somatostatin (1 microM) triggered retraction of the filopodia and lamellipodia in the growth cones. This growth cone collapse was enhanced by overexpression of green fluorescent protein-tagged sst1, whereas sst1 mutant lacking the PDZ binding motif had no effect. These findings suggest a role for somatostatin signaling in the regulation of growth cone stability, which may involve PDZ domain proteins interacting with sst1 and/or other somatostatin receptors. Consistent with a developmental role, sst1 immunoreactivity was present transiently in the developing mouse cortex, peaking at postnatal day 5 and declining thereafter to low levels in the adult cortex.

Effect of cortistatin on tau phosphorylation at Ser262 site

The development of intraneuronal lesions as a result of the progressive deposition of hyperphosphorylated tau at specific brain regions (such as hippocampus and cortex) plays a key role in the pathological process of Alzheimer's disease. However, the mechanisms by which tau phosphorylation is regulated, mainly in the pathology found in the cortex, are still poorly understood. Here, we analyzed the effect of cortistatin, a cortical neuropeptide related to somatostatin, on tau phosphorylation at Ser262 in cultures of murine cortical neurons. Both somatostatin and cortistatin induce tau phosphorylation at Ser262, a site modified in Alzheimer's disease, although with different kinetics in cortex. The effect of cortistatin likely is mediated by heterodimeric receptors composed of somatostatin receptor subtypes 2 and 4 and also by protein kinase C signaling. Cortistatin-deficient mice show decreased tau phosphorylation at Ser262 in the cortex but not in other brain regions tested. Our results suggest an important role for cortistatin in the regulation of tau phosphorylation that may be associated with the pathophysiology of Alzheimer's disease in regions such as the cerebral cortex.

Intracellular degradation of somatostatin-14 following somatostatin-receptor3-mediated endocytosis in rat insulinoma cells

Somatostatin receptor (SSTR) endocytosis influences cellular responsiveness to agonist stimulation and somatostatin receptor scintigraphy, a common diagnostic imaging technique. Recently, we have shown that SSTR1 is differentially regulated in the endocytic and recycling pathway of pancreatic cells after agonist stimulation. Additionally, SSTR1 accumulates and releases internalized somatostatin-14 (SST-14) as an intact and biologically active ligand. We also demonstrated that SSTR2A was sequestered into early endosomes, whereas internalized SST-14 was degraded by endosomal peptidases and not routed into lysosomal degradation. Here, we examined the fate of peptide agonists in rat insulinoma cells expressing SSTR3 by biochemical methods and confocal laser scanning microscopy. We found that [(125)I]Tyr11-SST-14 rapidly accumulated in intracellular vesicles, where it was degraded in an ammonium chloride-sensitive manner. In contrast, [(125)I]Tyr1-octreotide accumulated and was released as an intact peptide. Rhodamine-B-labeled SST-14, however, was rapidly internalized into endosome-like vesicles, and fluorescence signals colocalized with the lysosomal marker protein cathepsinD. Our data show that SST-14 was cointernalized with SSTR3, was uncoupled from the receptor, and was sorted into an endocytic degradation pathway, whereas octreotide was recycled as an intact peptide. Chronic stimulation of SSTR3 also induced time-dependent downregulation of the receptor. Thus, the intracellular processing of internalized SST-14 and the regulation of SSTR3 markedly differ from the events mediated by the other SSTR subtypes.

Dynamics of somatostatin type 2A receptor cargoes in living hippocampal neurons

Despite the large number of G-protein-coupled receptor (GPCR) types expressed in the CNS, little is known about their dynamics in neuronal cells. Dynamic properties of the somatostatin type 2A receptor were therefore examined in resting conditions and after agonist activation in living hippocampal neurons. Using fluorescence recovery after photobleaching experiments, we found that, in absence of ligand, the sst(2A) receptor is mobile and laterally and rapidly diffuse in neuronal membranes. We then observed by live-cell imaging that, after agonist activation, membrane-associated receptors induce the recruitment of beta-arrestin 1-enhanced green fluorescent protein (EGFP) and beta-arrestin 2-EGFP to the plasma membrane. In addition, beta-arrestin 1-EGFP translocate to the nucleus, suggesting that this protein could serve as a nuclear messenger for the sst(2A) receptor in neurons. Receptors are then recruited to preexisting clathrin coated pits, form clusters that internalize, fuse, and move to a perinuclear compartment that we identified as the trans-Golgi network (TGN), and recycle. Receptor cargoes are transported through a microtubule-dependent process directly from early endosomes/recycling endosomes to the TGN, bypassing the late endosomal compartment. Together, these results provide a comprehensive description of GPCR trafficking in living neurons and provide compelling evidence that GPCR cargoes can recycle through the TGN after endocytosis, a phenomenon that has not been anticipated from studies of non-neuronal cells.

Null mutant mouse models of somatostatin and cortistatin, and their receptors

Somatostatin (somatotropin release inhibitory factor, SRIF) and the related cortistatin (CST) are multifunctional peptide molecules attributed with neurohormone, neurotransmitter/modulator, and autocrine/paracrine actions. The physiological responses of SRIF and CST are mediated by five widely distributed G protein-coupled receptors (sst1-5) which have been implicated in regulating numerous biological processes. Much of the information on the effects of somatostatin has been gained through pharmacological studies with analogs and antagonists. The possibility of targeted mutagenesis in the mouse has resulted, over the last 10 years, in the generation of mouse models which genetically lack somatostatin ligands or receptors. We will review here the mouse models generated, the studies undertaken with them, and what has been learned so far.

Intracellular trafficking of somatostatin receptors

The somatostatin receptor subtypes 1-5 (sst(1)-sst(5)) exhibit different intracellular trafficking and endosomal sorting after agonist exposure. The internalization of the somatostatin receptor subtypes sst(2), sst(3) and sst(5) occurs to a much higher extent after somatostatin exposure than of sst(1) or sst(4). After endocytosis, sst(2) and sst(5) recycle to the plasma membrane, whereas sst(3) is predominantly down-regulated. This review will focus on the molecular mechanisms of the differential intracellular trafficking of sst(2), sst(3) and sst(5), and discusses our current knowledge on somatostatin receptor interacting proteins.

Somatostatinergic systems in brain: networks and functions

Somatostatin is abundantly expressed in mammalian brain. The peptide binds with high affinity to six somatostatin receptors, sst1, sst2A and B, sst3 to 5, all belonging to the G-protein-coupled receptor family. Recent advances in the neuroanatomy of somatostatin neurons and cellular distribution of sst receptors shed light on their functional roles in the neuronal network. Beside their initially described neuroendocrine role, somatostatin systems subserve neuromodulatory roles in the brain, influencing motor activity, sleep, sensory processes and cognitive functions, and are altered in brain diseases like affective disorders, epilepsia and Alzheimer's disease.

Biology of somatostatin in breast cancer

The biological effects of the neuropeptide somatostatin (SST) are mediated via a family of five somatostatin receptors (SSTRs) belonging to a family of G-protein-coupled receptors (GPCRs). SSTR regulate the secretion of hormones, growth factors, neurotransmission and cell growth in receptor-specific manner. In addition, SST plays an inhibitory role in several mammary cancer models. These effects are mediated both indirectly through inhibition of hormones and growth factors which promote tumor growth as well as directly via SSTRs present on tumor cells to inhibit mitogenic signaling of growth factor receptor kinases leading to growth arrest and induction of apoptosis. Here, we present an overview on the role of SST and its analogs in breast cancer.

Selective agonism in somatostatin receptor signaling and regulation

The five somatostatin receptor subtypes, named sst1-sst5, activate both distinct and common signaling pathways and exhibit different patterns of receptor regulation. Until recently it was believed that once a particular somatostatin receptor was activated by an agonist, all the down-stream signaling and regulatory effects characteristic of that receptor subtype in that cellular environment would be triggered. Thus, differences in the actions of somatostatin analogs between tissues were attributed to variability in the nature and concentration of the sst receptor subtypes and effectors expressed in different targets. However, agonists have recently been shown to exhibit functional selectivity at individual sst receptors such that they can elicit a subset of that receptor's potential effects, a property known as biased agonism. This review will summarize the evidence for functionally selective somatostatin receptor agonists and discuss the implications and promise of these new findings.

Cortistatin--functions in the central nervous system

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

Novel signals mediating the functions of somatostatin: the emerging role of NO/cGMP

The neuropeptide somatostatin is a cyclic tetradecapeptide, which is widely distributed in the peripheral and central nervous system. It mediates a plethora of physiological actions and functions as a neurotransmitter, neuromodulator or trophic factor. Somatostatin activates six receptor subtypes that are expressed differentially in different tissues and are coupled to diverse signalling pathways. In order to elucidate the functional role of the individual receptor subtypes, many investigations focused on the assignment of each receptor to a particular signalling pathway. Signalling pathways involving enzyme (adenylate cyclase, phospholipases, phosphatases) and ion channel systems in native and recombinant receptor systems have been extensively studied. A one to one situation (receptor/pathway) has yet to be established, thus justifying the diverse actions of somatostatin. Recently, a NO/cGMP pathway has been shown to mediate the functions of somatostatin and its receptors. This review will present the findings that support the emerging role of NO/cGMP as a novel signal in SRIF's actions in retinal physiology and somatotroph release.

Ring size of somatostatin analogues (ODT-8) modulates receptor selectivity and binding affinity

The synthesis, biological testing, and NMR studies of several analogues of H-c[Cys (3)-Phe (6)-Phe (7)-DTrp (8)-Lys (9)-Thr (10)-Phe (11)-Cys (14)]-OH (ODT-8, a pan-somatostatin analogue, 1) have been performed to assess the effect of changing the stereochemistry and the number of atoms in the disulfide bridge on binding affinity. Cysteine at positions 3 and/or 14 (somatostatin numbering) were/was substituted with d-cysteine, norcysteine, D-norcysteine, homocysteine, and/or D-homocysteine. The 3D structure analysis of selected partially selective, bioactive analogues (3, 18, 19, and 21) was carried out in dimethylsulfoxide. Interestingly and not unexpectedly, the 3D structures of these analogues comprised the pharmacophore for which the analogues had the highest binding affinities (i.e., sst 4 in all cases).

17β-Estradiol protects depletion of rat temporal cortex somatostatinergic system by β-amyloid

Estradiol prevents amyloid-beta peptide (Abeta)-induced cell death through estrogen receptors (ERs) and modulates somatostatin (SRIF) responsiveness in the rat brain. As intracerebroventricular (ICV) Abeta25-35 administration reduces SRIFergic tone in the temporal cortex of ovariectomized (Ovx) rats, we asked whether 17beta-estradiol (E2) treatment can restore the Abeta25-35 induced changes in SRIF content, SRIF receptor density and adenylyl cyclase (AC) activity, as well as if these effects are mediated by ERs. E2 treatment did not change Abeta25-35 levels in the temporal cortex, but partially restored the SRIFergic parameters affected by Abeta insult and decreased cell death, which was correlated with Akt activation. The ER antagonist ICI 182,780 prevented the protective effect of E2 on sst2 levels, but did not modify SRIF levels. Furthermore, ICI 182,780 treatment further decreased sst2 protein and mRNA levels when administered alone to Abeta25-35-treated rats, suggesting that it may block the effects of endogenous estrogens. These findings indicate that E2 protects the temporal cortical SRIFergic system from Abeta-induced depletion independently of Abeta accumulation.

Expression and localisation of somatostatin receptor subtypes sst1-sst5 in areas of the rat medulla oblongata involved in autonomic regulation

Somatostatin is known to modulate the activity of neurones of the medulla oblongata involved in autonomic regulation, mediated through five subtypes of G protein-coupled receptors, sst1-sst5. This study utilises reverse transcription polymerase chain reaction and immunohistochemistry to investigate the expression of sst1-sst5, including the sst2(A)/sst2(B) isoforms, in the main autonomic centres of the rat medulla oblongata: nucleus of the solitary tract (NTS), dorsal motor vagal nucleus (DVN) and ventrolateral medulla (VLM). In tissue from the cerebral cortex, hippocampus and cerebellum all subtype mRNAs were detected, but sst5 signals were weak, and the distribution of sst1-sst5 immunoreactivities was consistent with previous reports. In the medulla, all sst mRNAs gave clear amplicons and subtype-specific antibodies produced characteristic patterns of immunolabelling, frequently in areas of somatostatinergic innervation. Anti-sst1 labelled beaded fibres, sst2(A), sst2(B), sst4 and sst5 gave somatodendritic labelling and sst3 labelled presumptive neuronal cilia. In NTS tissue, sst1, sst2(A), sst4 and sst5 mRNAs were strongly expressed, while in VLM tissue sst1, sst2(A), sst2(B) and sst4 predominated. In both areas of the medulla, neurones with intense somatodendritic sst2(A) immunoreactivity were principally catecholaminergic in phenotype, being double labelled for tyrosine hydroxylase (TH) and phenylethanolamine-N-methyl-transferase (PNMT). Some TH/PNMT positive neurones were also sst2(B) and sst4 immunoreactive. Cholinergic parasympathetic neurones in the DVN were immunoreactive for the sst2(A), sst2(B), sst4 and sst5 subtypes. These observations are consistent with the proposal that multiple somatostatin receptor subtypes, possibly combining as heterodimers, are involved in mediating the modulatory effects of somatostatin on autonomic function, including cardiovascular, respiratory and gastrointestinal reflex activity.

Embryonic endocrine pancreas and mature beta cells acquire alpha and PP cell phenotypes upon Arx misexpression

Aristaless-related homeobox (Arx) was recently demonstrated to be involved in pancreatic alpha cell fate specification while simultaneously repressing the beta and delta cell lineages. To establish whether Arx is not only necessary, but also sufficient to instruct the alpha cell fate in endocrine progenitors, we used a gain-of-function approach to generate mice conditionally misexpressing this factor. Mice with forced Arx expression in the embryonic pancreas or in developing islet cells developed a dramatic hyperglycemia and eventually died. Further analysis demonstrated a drastic loss of beta and delta cells. Concurrently, a remarkable increase in the number of cells displaying alpha cell or, strikingly, pancreatic polypeptide (PP) cell features was observed. Notably, the ectopic expression of Arx induced in embryonic or adult beta cells led to a loss of the beta cell phenotype and a concomitant increase in a number of cells with alpha or PP cell characteristics. Combining quantitative real-time PCR and lineage-tracing experiments, we demonstrate that, in adult mice, the misexpression of Arx, rather than its overexpression, promotes a conversion of beta cells into glucagon- or PP-producing cells in vivo. These results provide important insights into the complex mechanisms underlying proper pancreatic endocrine cell allocation and cell identity acquisition.