Pharmacological characterisation of human cerebral cortex somatostatin SRIF1 and SRIF2 receptors

Molecular-Level Understanding of the Somatostatin Receptor 1 (SSTR1)-Ligand Binding: A Structural Biology Study Based on Computational Methods

Somatostatin receptor 1 (SSTR1), a subtype of somatostatin receptors, is involved in various signaling mechanisms in different parts of the human body. Like most of the G-protein-coupled receptors (GPCRs), the available information on the structural features of SSTR1 responsible for the biological activity is scarce. In this study, we report a molecular-level understanding of SSTR1-ligand binding, which could be helpful in solving the structural complexities involved in SSTR1 functioning. Based on a three-dimensional quantitative structure-activity relationship (3D-QSAR) study using comparative molecular field analysis (CoMFA) and comparative molecular similarity index analysis (CoMSIA), we have identified that an electronegative, less-bulkier, and hydrophobic atom substitution can substantially increase the biological activity of SSTR1 ligands. A density functional theory (DFT) study has been followed to study the electron-related properties of the SSTR1 ligands and to validate the results obtained via the 3D-QSAR study. 3D models of SSTR1-ligand systems have been embedded in lipid-lipid bilayer membranes to perform molecular dynamics (MD) simulations. Analysis of the MD trajectories reveals important information about the crucial residues involved in SSTR1-ligand binding and various conformational changes in the protein that occur after ligand binding. Additionally, we have identified the probable ligand-binding site of SSTR1 and validated it using MD. We have also studied the favorable conditions that are essential for forming the most stable and lowest-energy bioactive conformation of the ligands inside the binding site. The results of the study could be useful in constructing more potent and novel SSTR1 antagonists and agonists.

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 sst1 receptor: an autoreceptor for somatostatin in brain and retina?

The sst1 receptor was the first of the 5 somatostatin receptors to be cloned by homology with the glucagon receptor 13 years ago. It is a 7-transmembrane domain G-protein-coupled receptor that is negatively coupled to adenylyl cyclase, but can also trigger other transduction pathways. The distribution of sst1 mRNA, immunolabeling, and radioligand binding are not entirely overlapping, but the recent availability of knockout (KO) mice and a (still limited) number of selective agonists/antagonists has increased our knowledge about this receptor. These new tools have helped to reveal a role for the sst1 receptor in hippocampal, hypothalamic, basal ganglia, and retinal functions. In at least the latter 3 structures, the sst1 receptor appears to act as an inhibitory autoreceptor located on somatostatin neurons, whereas in the hippocampus such a role is still based on circumstantial evidence.

Somatostatin receptor 1 selective analogues: 2. N(alpha)-Methylated scan

Des-AA(1,2,5)-[d-Trp(8)/d-Nal(8),IAmp(9)]SRIF (AA = amino acid, Nal = 3-(2-naphthyl)-alanine, IAmp = 4-(N-isopropyl)-aminomethylphenylalanine, SRIF = somatostatin), with or without a tyrosine or monoiodotyrosine, were scanned with the introduction of a backbone N-methyl group and tested for binding affinity at the five human somatostatin receptors (sst(1)(-)(5)). N(alpha)-Methylation resulted in loss of sst affinity (2- to >5-fold) when introduced at residues Lys(4) (6), Phe(6) (7), Phe(7) (8), Thr(10) (11), and Phe(11) (12) of the parent compound Des-AA(1,2,5)-[d-Nal(8),IAmp(9)]SRIF (4). N(alpha)-Methylation was tolerated at residues Cys(3) (5), d-Nal(8) (9), Thr(12) (13), and Cys(14) (15) with retention of binding sst affinity and selectivity and resulted in an increase in sst binding affinity at positions IAmp(9) (10) and Ser(13) (14). In these series, the d-Trp(8) substitution versus d-Nal(8) is clearly superior. C-Terminally lysine-extended analogues (21-25) retained sst(1) selectivity and binding affinity when compared to their d-Nal(8)- (4) or d-Trp(8)- (3) containing parent. Des-AA(1,2,5)-[d-Trp(8), (N(alpha)Me)IAmp(9)]SRIF (17), Des-AA(1,2,5)-[d-Trp(8),IAmp(9),(N(alpha)Me)Ser(13)]SRIF (19), Des-AA(1,2,5)-[d-Trp(8),IAmp(9),(N(alpha)Me)Cys(14)]SRIF (20), Des-AA(1,2,5)-[d-Trp(8),(N(alpha)Me)IAmp(9),Tyr(11)]SRIF (34), and Des-AA(1,2,5)-[d-Agl(8)(N(beta)Me,2-naphthoyl),IAmp(9),Tyr(11)]SRIF (42) (Agl = aminoglycine) are sst(1) agonists in their ability to inhibit forskolin-induced cAMP production.

Pharmacological characterisation of native somatostatin receptors in AtT-20 mouse tumour corticotrophs

1. The mouse corticotroph tumour cell line AtT-20 is a useful model to investigate the physiological role of native somatostatin (SRIF, Somatotropin release inhibitory factor) receptor subtypes (sst(1) - sst(5)). The objective of this study was to characterise the pharmacological features and the functional effects of SRIF receptors expressed by AtT-20 cells using radioligand binding and cAMP accumulation. 2. [(125)I]LTT-SRIF-28, [(125)I]CGP 23996, [(125)I]Tyr(10)-cortistatin-14 and [(125)I]Tyr(3)-octreotide labelled SRIF receptor binding sites with high affinity and in a saturable manner (B(max)=315, 274, 239 and 206 fmol mg(-1), respectively). [(125)I]LTT-SRIF-28 labels significantly more sites than [(125)I]Tyr(10) -cortistatin-14 and [(125)I]Tyr(3) -octreotide as seen previously in cells expressing pure populations of sst(2) or sst(5) receptors. 3. SRIF analogues displaced the binding of the four radioligands. sst(2/5) receptor-selective ligands showed much higher affinity than sst(1/3/4) receptor-selective ligands. The binding profile of [(125)I]Tyr(3)-octreotide was different from that of [(125)I]LTT-SRIF-28, [(125)I]CGP 23996 and [(125)I]Tyr(10)-cortistatin-14. The sst(5/1) receptor-selective ligand L-817,818 identified two binding sites, one with subnanomolar affinity (sst(5) receptors) and one with micromolar affinity (sst(2) receptors); however, the proportions were different: 70 - 80% high affinity with [(125)I]LTT-SRIF-28, [(125)I]CGP 23996, [(125)I]Tyr(10)-cortistatin-14, but only 20% with [(125)I]Tyr(3)-octreotide. 4. SRIF analogues inhibited the forskolin-stimulated cAMP levels depending on concentration. sst(2/5) receptor-selective ligands were highly potent, whereas sst(1/3/4) receptor-selective ligands had no significant effects. The sst(2) receptor antagonist D-Tyr(8)-CYN 154806 competitively antagonised the effects of SRIF-14 and sst(2) receptor-preferring agonists, but not those of L-817,818. 5. The complex binding properties of SRIF receptor analogues indicate that sst(2) and sst(5) receptors are the predominant SRIF receptors expressed on AtT-20 cell membranes with no or only negligible presence of sst(1), sst(3) and sst(4) receptors. In the functional studies using cAMP accumulation, only sst(2) and sst(5) receptors appear to play a role. However, the "predominant" receptor appears to be the sst(2) receptor, although sst(5) receptors can also mediate the effect, when the ligand is not able to activate sst(2) receptors. This clearly adds flexibility to SRIF-mediated functional effects and suggests that the physiological role of SRIF and its analogues may be mediated preferentially via one subtype over another.

Fluorescent somatostatin receptor probes for the intraoperative detection of tumor tissue with long-wavelength visible light

Targeted fluorescent dyes are of substantial value for the intraoperative delineation of primary tumors and metastatic lesions. For this purpose long-wavelength red light (lambda=550-650 nm) offers advantages because of good tissue penetration and direct visibility. Since somatostatin receptors (SSTR) are overexpressed in a number of tumors, a series of potentially tumor-selective peptide-dye conjugates were synthesized by solid-phase peptide synthesis (SPPS). The octapeptides octreotate, Tyr(3)-octreotate and Tyr(3)-octreotide were employed and exhibited high affinity for somatostatin receptors (SSTR). The fluorescent dyes rhodamine 101, sulforhodamine B acid chloride, sulforhodamine 101 or rhodamine B isothiocyanate were conjugated either directly or via spacers, for example the peptidase-labile pentapeptide sequence Ala-Leu-Ala-Leu-Ala. The conjugates were completely assembled on the solid support: Fmoc-SPPS, cyclization via a disulfide linkage, N-terminal attachment of a spacer, and linkage to the fluorescent dye. An in vitro competition assay revealed that the conjugates bind to SSTRs with IC(50) values between 0.7 and 89 nM. The conjugates were generally stable to hydrolysis at pH 7-8 in buffer or serum. However, the rhodamine 101 conjugates revealed a loss of absorption at alkaline pH due to conversion to a neutral spirolactam form, as characterized by NMR.

Somatostatin sst2 receptor knock-out mice: localisation of sst1-5 receptor mRNA and binding in mouse brain by semi-quantitative RT-PCR, in situ hybridisation histochemistry and receptor autoradiography

The peptide hormone/neurotransmitter somatostatin (somatotropin release inhibiting factor; SRIF) and its receptors (sst(1)-sst(5)) appear to regulate many physiological functions in the CNS. Semi-quantitative analysis of the densities of mRNA expression for sst(1-5) receptors and SRIF receptor binding sites were established in sst(2) receptor knock-out (KO) mice. Patterns of sst(1-5) receptor mRNA expression were largely conserved for sst(1,3,4) and sst(5) selective oligonucleotide probes; whereas sst(2) signals were completely absent in KO mouse brain. Autoradiographic analysis demonstrated [(125)I]LTT SRIF(28), [(125)I]CGP 23996 (two radioligands known to label all five recombinant SRIF receptors) and [(125)I]Tyr(3)-octreotide (sst(2) and sst(5) receptor selective) binding in wild type (WT) mouse brain sections; yet no specific binding of [(125)I]Tyr(3)-octreotide in KO mice. In contrast, [(125)I]LTT SRIF(28) and [(125)I]CGP 23996 binding was still present in a number of brain areas in KO mice, although to a lesser degree than in those regions where [(125)I]Tyr(3)-octreotide binding was found, in WT animals. The present data suggest first, that both sst(2) receptor protein and mRNA were completely absent in the brain of these KO animals. Second, there was little evidence of compensatory regulation, at the mRNA level, of the other SRIF receptors as a consequence of the sst(2) KO. Third, the absence of any [(125)I]Tyr(3)-octreotide binding, in KO mice, suggests that this particular ligand is selective for the sst(2) receptor subtype (under the conditions utilised); or that sst(5) receptors are only marginally expressed in brain. Fourth, there were regions where the binding of [(125)I]LTT SRIF(28) and [(125)I]CGP 23996 were moderately affected by the sst(2) KO, suggesting that additional SRIF receptors may well contribute to the binding of the aforementioned radioligands. Finally, since the relative distribution of these two ligands were not entirely superimposable, it suggests that their respective selectivity profiles towards the different SRIF receptor subtypes in situ are not identical.

Distribution and characterisation of somatostatin receptor mRNA and binding sites in the brain and periphery

The distribution and nature of (somatostatin) SRIF receptors and receptor mRNAs was studied in the brain and periphery of various laboratory animals using in situ hybridisation, autoradiography and radioligand binding. The messenger RNA (mRNA) expression of SRIF receptors msst1, msst2, msst3, msst4 and msst5 was studied in the adult mouse brain by in situ hybridisation histochemistry using specific oligonucleotide probes and compared to that of adult rats. As observed in rat brain, sst3 receptor mRNA is prominently expressed across the mouse brain, although equivalent binding has not yet been identified in situ. Sst1 and sst2 receptor mRNA expression, was prominent and again comparable to that observed in rat brain, whereas sst4 and especially sst5 receptor mRNA show comparatively low levels, although the former appears to be widely distributed while the latter could only be identified in a few nuclei. Altogether, the data are compatible with current knowledge, i.e. sst1 and sst2 receptor mRNA is prominent (both receptors have been functionally identified in the brain and for sst2 in the periphery), sst3 mRNA is highly expressed but in the absence of any functional correlate remains elusive. The expression of sst4 mRNA is comparatively low (especially when compared to what is seen in the lung, where high densities of sst4 receptors are present) and it remains to be seen whether sst5 receptor mRNA, which is confined to a few nuclei, will play a role in the brain, keeping in mind that high levels are found in the pituitary. Radioligand binding studies were performed in CCL39 cells expressing the five human recombinant receptors and compared to binding in membranes of rat cerebral cortex with [125I]Tyr11-SRIF14 which in the presence of 120 mM labels primarily sst1 receptor as suggested by the better correlation hsst1 and similar rank order of potency. The profile of [125I]Tyr3-octreotide labelled sites in rat cortex correlates better with recombinant sst2 than sst3 or sst5 binding profiles. Finally, [125I]LTT-SRIF28-labelled sites in rat lung express a sst4 receptor profile in agreement with previous findings. SRIF receptor autoradiography was performed in the brain and peripheral tissue of rat and/or guinea-pig using a number of ligands known to label recombinant SRIF receptors: [125I]LTT-SRIF28, [125I]CGP 23996, [125I]Tyr10-CST, or [125I]Tyr3-octreotide. Although, [125I]Tyr10-CST has been shown to label all five recombinant SRIF receptors, it is apparent that this radioligand is not useful for autoradiographic studies. By contrast, the other three ligands show good signal to noise ratios in rat or guinea-pig brain, rat lung, rat pancreas, or guinea-pig ileum. In most tissues, [125I]Tyr3-octreotide represents a prominent part of the binding (when compared to [125I]LTT-SRIF28 and [125I]CGP 23996), suggesting that sst2 receptors are strongly expressed in most tissues; it is only in rat lung that [125I]LTT-SRIF28 and [125I]CGP 23996 show marked binding, whereas [125I]Tyr3-octreotide does apparently label no sites, in agreement with the sole presence of sst4 receptors in this tissue.

Brain somatostatin: a candidate inhibitory role in seizures and epileptogenesis

Recent evidence shows that neuropeptide expression in the CNS is markedly affected by seizure activity, particularly in the limbic system. Changes in neuropeptides in specific neuronal populations depend on the type and intensity of seizures and on their chronic sequelae (i.e. neurodegeneration and spontaneous convulsions). This paper reviews the effects of seizures on somatostatin-containing neurons, somatostatin mRNA and immunoreactivity, the release of this peptide and its receptor subtypes in the CNS. Differences between kindling and status epilepticus in rats are emphasized and discussed in the light of an inhibitory role of somatostatin on hippocampal excitability. Pharmacological studies show that somatostatin affects electrophysiological properties of neurons, modulates classical neurotransmission and has anticonvulsant properties in experimental models of seizures. This peptidergic system may be an interesting target for pharmacological attempts to control pathological hyperactivity in neurons, thus providing new directions for the development of novel anticonvulsant treatments.

Characterisation of the fish sst3 receptor, a member of the SRIF1 receptor family: atypical pharmacological features

The first cloned non-mammalian somatostatin (somatostatin release-inhibiting factor = SRIF) receptor previously obtained from the teleost fish Apteronotus albifrons and generically named somatostatin receptor 3 (fsst3), was stably expressed and characterised in Chinese hamster lung fibroblast (CCL39) cells. Radioligand binding studies were performed with four radioligands selective for SRIF receptors in CCL39 cells expressing the fsst3 receptors; [125I]LTT-SRIF28 ([Leu8, D-Trp22, 125I-Tyr25]-SRIF28), [125I]Tyr10-cortistatin, [125I]CGP 23996, and [125I]Tyr3-octreotide labelled the fsst3 receptor with high affinity (pKd values: 10.47, 10.87, 9.59 and 9.57) and in a saturable manner, but defined different Bmax values; 4500, 4000, 3400 and 1500 fmol/mg, respectively. The affinities of SRIF peptides and analogues determined for fsst3 receptors displayed the following rank order of potency: seglitide = SRIF25 > SRIF14 = SRIF28 > cortistatin 14 > BIM 23014 > RC160 = L361,301 = octreotide > or = BIM 23052 > or = L362,855 > CGP23996 > BIM 23056 > BIM 23030 = cycloantagonist > SRIF22. The pharmacological profiles determined with [125I]LTT-SRIF28, [125I]CGP 23996 and [125I]Tyr10-cortistatin correlated highly significantly (r = 0.96-0.99), whereas [125I]Tyr3-octreotide binding was rather divergent (r = 0.78-0.81). Further, [125I]Tyr3-octreotide- and [125I]CGP 23996-labelled sites showed higher affinity for the various peptides than [125I]LTT-SRIF28 and [125I]Tyr10-cortistatin-labelled sites, although there were exceptions. [125I]LTT-SRIF28-binding to fsst3 receptors and human sst1-5 receptors was compared; the fsst3 binding profile correlated better with the hsst5- than with the hsst3 receptor profile. SRIF inhibited potently forskolin-stimulated adenylate cyclase activity in fsst3 transfected CCL39 cells; this effect was blocked by pertussis toxin, suggesting coupling of the fsst3 receptor to Gialpha and/or Goalpha. [125I]LTT-SRIF28 binding was detected in fish brain, liver, heart, spleen, and stomach, but not in gut. The pharmacological profile of [125I]LTT-SRIF28-labelled sites in brain, but not in liver, correlated significantly with the recombinant fsst3 receptor, in agreement with expression of the fsst3 receptor gene found by RT-PCR in the brain. However, biphasic binding curves obtained with two SRIF-analogues in brain, as well as the distinct pharmacological profile of the liver SRIF receptor, suggest the existence of several yet to be defined SRIF receptor subtypes in fish. The present data demonstrate that the recombinantly expressed fsst3 receptor has a pharmacological profile compatible with that of a SRIF1 receptor, although the rank order of affinity of fsst3 is closer to that of hsst5 than hsst3 receptors, as may be found when comparing very distantly related species. The fsst3 receptor expressed in CCL39 cells, is negatively coupled to adenylate cyclase activity via pertussis toxin-sensitive G-proteins, like mammalian sst3 receptors. Radioligand binding performed with fish tissue suggests the presence of a native sst3 receptor in brain as well as other yet to be defined SRIF receptor subtypes.

[125I][Tyr3]octreotide labels human somatostatin sst2 and sst5 receptors

Human somatostatin (somatotropin release inhibiting factor = SRIF) receptor subtypes sst2 and sst5 were stably expressed in Chinese hamster lung fibroblast (CCL39) cells. [125I][Tyr3]octreotide labelled with high affinity and in a saturable manner both sst2 (pKd = 9.89+/-0.02, Bmax = 210+/-10 fmol/mg, n = 3) and sst5 sites (pKd = 9.64+/-0.04, Bmax = 920+/-170 fmol/mg, n = 3). The pharmacological profile of sst2 sites established in CCL39 cells using SRIF and various peptide analogues was very similar to that described previously in CHO cells and in human cortex: SRIF14 = SRIF28 > or = seglitide > BIM 23014 = RC 160 > octreotide > CGP 23996 > or = L362,855 > BIM 23052 > L361,301 = cortistatin14 > BIM 23030 > BIM 23056 > cycloantagonist SA. However, peptides classically perceived as sst2 receptor selective (e.g., seglitide, octreotide, vapreotide) showed also high affinity for human sst5 receptors labelled with [125I][Tyr3]octreotide: SRIF28 > seglitide > SRIF14 > L361,301 = octreotide > cortistatin14 = BIM 23014 = BIM 23052 > L362,855 = RC160 > CGP 23996 > BIM 23056 > cycloantagonist SA > BIM 23030. Further radioligand binding studies were performed with [Leu8,D-Trp22,125I-Tyr25]SRIF28 ([125I]LTT-SRIF28) and [125I]CGP 23996. At sst2 receptors, Bmax values determined with [125I][Tyr3]octreotide, [125I]LTT-SRIF28 and [125I]CGP 23996 were in the same range (180-370 fmol/mg). 5'-Guanylyl-imidodiphosphate (GppNHp) displaced all three radioligands to the same extent (85%) and the pharmacological profiles were superimposable. By contrast, at sst5 receptors Bmax values were very different: [125I][Tyr3]octreotide (920 fmol/mg), [125I]CGP 23996 (3530 fmol/mg) and [125I]LTT-SRIF28 (6950 fmol/mg). GppNHp affected [125I][Tyr3]octreotide more than [125I]CGP 23996 binding, whereas [125I]LTT-SRIF28 was much less affected. In addition, the affinity values determined in competition experiments at sst5 receptors, varied markedly; whereas SRIF14, cortistatin14 and SRIF28 showed 2-, 4- and 8-fold differences in affinity at sst5 receptors labelled with [125I][Tyr3]octreotide and [125I]LTT-SRIF28 compounds such as RC160, L363,301, L362,855, octreotide or CGP 23996 showed between 42- and 123-fold lower affinity when sst5 sites were labelled with [125I]LTT-SRIF28. The present data suggest caution to be used when comparing affinity profiles determined in binding studies using different radioligands. In addition, the present results suggest that effects produced by octreotide and related short chain SRIF analogues on hormone release, modulation of tumour growth and central effects may be mediated by either sst2 and/or sst5 receptors.