Evidence that a novel somatostatin receptor couples to an inward rectifier potassium current in AtT-20 cells

Truncated variants of pig somatostatin receptor subtype 5 (sst5) act as dominant-negative modulators for sst2-mediated signaling

Somatostatin (SST) and its related peptide cortistatin (CORT) exert their multiple actions through binding to the SST receptor (sst) family, generally considered to comprise five G protein-coupled receptors with seven transmembrane domains (TMD), named sst1-sst5, plus a splice sst2B variant. However, we recently discovered that human and rodent sst5 gene expression also generates, through noncanonical alternative splicing, novel truncated albeit functional sst5 variants with less than seven TMD. Here, we cloned and characterized for the first time the porcine wild-type sst5 (psst5, full-length) and identified two novel truncated psst5 variants with six and three TMD, thus termed psst5TMD6 and psst5TMD3, respectively. In line with that observed in human and rodent truncated sst5 variants, psst5TMD6 and psst5TMD3 are functional (e.g., activate calcium signaling), selectively respond to SST and CORT, respectively, and exhibit specific tissue expression profiles that differ from full-length psst5 and often overlaps with psst2 expression. Moreover, fluorescence resonance energy transfer analysis shows that psst5 truncated variants physically interact with psst2, thereby altering their localization at the plasma membrane and specifically disrupting the cellular response to SST and/or CORT. These results represent the first characterization of a key porcine SST receptor, psst5, and, together with our previous results, provide strong evidence that alternative splicing-derived, truncated sst5 variants with distinct functional capacities exist in the mammalian lineage, where they can act as dominant-negative receptors, by interacting directly with long, seven TMD variants, potentially contributing to modulate normal and pathological SST and CORT signaling.

Dependence of the excitability of pituitary cells on cyclic nucleotides

Cyclic 3',5'-adenosine monophosphate and cyclic 3',5'-guanosine monophosphate are intracellular (second) messengers that are produced from the nucleotide triphosphates by a family of enzymes consisting of adenylyl and guanylyl cyclases. These enzymes are involved in a broad array of signal transduction pathways mediated by the cyclic nucleotide monophosphates and their kinases, which control multiple aspects of cell function through the phosphorylation of protein substrates. We review the findings and working hypotheses on the role of the cyclic nucleotides and their kinases in the control of electrical activity of the endocrine pituitary cells and the plasma membrane channels involved in this process.

Truncated somatostatin receptors as new players in somatostatin-cortistatin pathophysiology

Somatostatin (SST) and cortistatin (CORT) act through a family of seven transmembrane domain (TMD) receptors (sst1-5) to govern multiple functions, from growth hormone (GH) secretion to neurotransmission, metabolic homeostasis, gastrointestinal and immune function, and tumor cell growth. Thus, SST analogs are used to treat endocrine/tumoral pathologies. Yet, some SST/CORT actions cannot be explained by their interaction with known ssts. We recently identified novel sst5 variants in human, pig, mouse, and rat that lack one or more TMDs and display unique molecular/functional features: they exhibit distinct tissue distribution, divergent responses to SST/CORT, and intracellular localization as opposed to the typical plasma-membrane distribution of full-length ssts. When coexpressed in the same cell, truncated sst5 variants colocalize and physically interact with full-length ssts, providing a molecular basis to disrupt normal sst2/sst5 functioning. This may explain the inverse correlation between hsst5TMD4 expression in pituitary tumors and octreotide responsiveness in acromegaly. Discovery of these new truncated sst5 variants provides novel insights on SST/CORT/sst pathophysiology and suggests new research avenues for the therapeutic potential of this system.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

Identification and characterization of new functional truncated variants of somatostatin receptor subtype 5 in rodents

Somatostatin and cortistatin exert multiple biological actions through five receptors (sst1-5); however, not all their effects can be explained by activation of sst1-5. Indeed, we recently identified novel truncated but functional human sst5-variants, present in normal and tumoral tissues. In this study, we identified and characterized three novel truncated sst5 variants in mice and one in rats displaying different numbers of transmembrane-domains [TMD; sst5TMD4, sst5TMD2, sst5TMD1 (mouse-variants) and sst5TMD1 (rat-variant)]. These sst5 variants: (1) are functional to mediate ligand-selective-induced variations in [Ca(2+)]i and cAMP despite being truncated; (2) display preferential intracellular distribution; (3) mostly share full-length sst5 tissue distribution, but exhibit unique differences; (4) are differentially regulated by changes in hormonal/metabolic environment in a tissue- (e.g., central vs. systemic) and ligand-dependent manner. Altogether, our results demonstrate the existence of new truncated sst5-variants with unique ligand-selective signaling properties, which could contribute to further understanding the complex, distinct pathophysiological roles of somatostatin and cortistatin.

Identification and characterization of two novel truncated but functional isoforms of the somatostatin receptor subtype 5 differentially present in pituitary tumors

CONTEXT

Somatostatin and its related peptide cortistatin exert multiple actions on normal and tumoral tissue targets through a family of receptors termed somatostatin receptor (sst)1-5. Despite the considerable advances in the knowledge on these receptors and their (patho)physiological roles, there is still evidence that additional receptors for these peptides should exist to fully explain their actions.

OBJECTIVE

The growing number of spliced variants found in similar receptor families, often present in tumors, and results from our group obtained on sst5 from other species (pig) led us to explore the existence of new human sst5 isoforms.

DESIGN AND RESULTS

A rapid amplification of cDNA ends PCR approach on samples from a human pituitary tumor and a cell line enabled identification of two novel alternatively spliced sst5 receptor variants. The sequences obtained encode putative proteins that correspond to truncated isoforms of five and four transmembrane domains (TMDs), accordingly named sst5TMD5 and sst5TMD4, respectively. Both novel receptors show a differential expression pattern in normal tissues and are also present in pituitary tumors of diverse etiology including nonfunctioning adenomas, corticotropinomas, somatotropinomas, and a prolactinoma. In contrast to the predominant plasma membrane localization of full-length sst5, both sst5TMD5 and sst5TMD4 show a preferentially intracellular localization. Despite their truncated nature, both receptors are functional, as shown by their ability to mediate selective, ligand-induced rises in free cytosolic calcium concentration. Specifically, whereas sst5TMD5 is selectivity activated by somatostatin compared with cortistatin, cells transfected with sst5TMD4 almost exclusively respond to cortistatin and not to somatostatin.

CONCLUSIONS

Our results demonstrate the existence of two previously unidentified sst5 spliced variants with distinct distribution in normal tissues and pituitary tumors, unique ligand-selective signaling properties, and subcellular distribution, which could contribute to somatostatin and cortistatin signaling in normal and tumoral cells.

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.

An update on somatostatin receptor signaling in native systems and new insights on their pathophysiology

The peptide somatostatin (SRIF) has important physiological effects, mostly inhibitory, which have formed the basis for the clinical use of SRIF compounds. SRIF binding to its 5 guanine nucleotide-binding proteins-coupled receptors leads to the modulation of multiple transduction pathways. However, our current understanding of signaling exerted by receptors endogenously expressed in different cells/tissues reflects a rather complicated picture. On the other hand, the complexity of SRIF receptor signaling in pathologies, including pituitary and nervous system diseases, may be studied not only as alternative intervention points for the modulation of SRIF function but also to exploit new chemical space for drug-like molecules.

Binding and functional properties of the novel somatostatin analogue KE 108 at native mouse somatostatin receptors

Clinically used somatostatin (SRIF) analogs, octreotide and lanreotide, act primarily by binding to SRIF receptor subtype 2 (sst2). In contrast, the recently described multiligand SOM230 binds with high affinity to sst(1-3) and sst5 and KE 108 is characterised as a high affinity ligand for all five SRIF receptors. In tumoural mouse corticotrophs (AtT-20 cells) and in mouse hippocampus, binding and functional features of KE 108 were examined and compared to SRIF-14, octreotide and SOM230. In AtT-20 cells, KE 108 bound with high affinity at [125I]LTT-SRIF-28-labelled sites similarly to SRIF-14, octreotide and SOM230. At the functional level, all four ligands increased guanosine-5'-O-(3-[35S]thio)-triphosphate binding and decreased cAMP accumulation or intracellular Ca2+ concentration through G(i/o) proteins. In hippocampal slices, KE 108, octreotide and SOM230 also bound with high affinity at [125I]LTT-SRIF-28-labelled sites similarly to SRIF-14, but KE 108, octreotide or SOM230 did not influence spontaneous epileptiform activity which was, in contrast, inhibited by SRIF-14. In conclusion, this study demonstrates that KE 108 has high affinity for native mouse SRIF receptors. Functionally, KE 108 mediates SRIF action at sst(2/5) in corticotrophs whereas it does not mimic the SRIF-induced inhibition of hippocampal excitation suggesting that the high potency and efficacy of a synthetic ligand to all known SRIF receptors may not reproduce entirely the effects of the natural SRIF.

Somatostatin receptor type 5 modulates somatostatin receptor type 2 regulation of adrenocorticotropin secretion

Somatostatin inhibits adrenocorticotropin (ACTH) secretion from pituitary tumor cells. To assess the contribution of somatostatin receptor subtype 5 (SST5) to somatostatin receptor subtype 2 (SST2) action in these cells, we assessed multipathway responses to novel highly monoreceptor-selective peptide agonists and multireceptor agonists, including octreotide and somatostatin-28. Octreotide and somatostatin-28 cell membrane binding affinities correlated with their respective SST2-selective peptide ligand. Although octreotide had similar inhibiting potency (picomolar) for cAMP accumulation and ACTH secretion as an SST2-selective agonist, somatostatin-28 exhibited a higher potency (femtomolar). Baseline spontaneous calcium oscillations assessed by fluorescent confocal microscopy revealed two distinct effects: SST2 activation reduced oscillations at femtomolar concentrations reflected by high inhibiting potency of averaged normalized oscillation amplitude, whereas SST5 activation induces brief oscillation pauses and increased oscillation amplitude. Octreotide exhibits an integrated effect of both receptors; however, somatostatin-28 exhibited a complex response with two separate inhibitory potencies. SST2 internalization was visualized with SST2-selective agonist at lower concentrations than for octreotide or somatostatin-28, whereas SST5 did not internalize. Using monoreceptor-selective peptide agonists, the results indicate that, in AtT-20 cells, SST5 regulates the dominant SST2 action, attenuating SST2 effects on intracellular calcium oscillation and internalization. This may explain superior somatostatin-28 potency and provides a rationale for somatostatin ligand design to treat ACTH-secreting pituitary tumors.

Induction of galanin receptor-1 (GalR1) expression in external granule cell layer of post-natal mouse cerebellum

Galanin is a modulator of fast transmission in adult brain and recent evidence suggests that it also acts as a trophic factor during neurogenesis and neural injury and repair. Previous studies in our laboratory have identified galanin mRNA in Purkinje cells of adult and developing rat (but not adult mouse) cerebellum; and galanin-binding sites in adult mouse (but not rat) cerebellum. The post-natal development of the cerebellum provides a unique and convenient model for the investigation of developmental processes and to learn more about putative cerebellar galanin systems, the current study examined the presence and distribution of galanin-like-immunoreactivity (- LI), [(125)I]-galanin binding sites and galanin receptor-1 (GalR1) mRNA in post-natal mouse cerebellum. Using autoradiography and in situ hybridization, [(125)I]-galanin binding sites and GalR1 mRNA were first detected on post-natal day 10 (P10) in the external germinal layer of all lobes and high levels were maintained until P14. Quantitative real-time PCR assays detected GalR1 mRNA in whole cerebellum across the post-natal period, with a strong induction and peak of expression at P10. Assessment of galanin levels in whole cerebellum by radioimmunoassay revealed relatively similar concentrations from P5 to P20 and in adult mice (80-170 pg/mg protein), with a significantly higher concentration (250 pg/mg, p < 0.01) detected at P3. These concentrations were some four- to six-fold lower than those in adult forebrain samples. Using immunohistochemistry, galanin-like-immuno-reactivity was observed in prominent fibrous elements within the white matter tracts of the cerebellum at P3-5 and in more punctate elements in the internal granule cell layer and associated with the Purkinje cell layer at P12 and P20. Increased levels of GalR1 mRNA and galanin binding (attributed to GalR1) in the external granule cell layer at P10-12/(14) coincide with granule cell migration from the external to the inner granule cell layer and together with demonstrated effects of other neuropeptide-receptor systems suggest a role for GalR1 signalling in regulating this or related developmental processes.

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.

Multiple genes for neuropeptides and their receptors: co-evolution and physiology

It is now well established that neuropeptide receptors, which are present throughout the CNS and in peripheral tissues, frequently exist in a variety of different forms (called subtypes), each of which is encoded by a distinct gene. With the recent identification of new neuropeptide genes, it has become clear that families of neuropeptides also occur, which raises the possibility that specific peptide ligands activate particular receptor subtypes preferentially. This article reviews some of the recent advances in the neuropeptide field and provides evidence in support of three ideas: (1) that different receptor subtypes for a given ligand can be distinguished physiologically; (2) that neuropeptide genes probably arose before the corresponding receptor genes; and (3) that, despite the current wealth of information on neuropeptides and neuropeptide receptors, several new members are likely to be discovered before the beginning of the next millennium.

Differential regulation of the cloned kappa and mu opioid receptors

To directly compare the regulation of the cloned kappa and mu opioid receptor, we expressed them in the same cells, the mouse anterior pituitary cell line AtT-20. The coupling of an endogenous somatostatin receptor to adenylyl cyclase and an inward rectifier K+ current has been well characterized in these cells, enabling us to do parallel studies comparing the regulation of both the kappa and the mu receptor to this somatostatin receptor. We show that the kappa receptor readily uncoupled from the K+ current and from adenylyl cyclase after a 1 h pretreatment with agonist, as indicated by the loss in the ability of the agonist to induce a functional response. The desensitization of the kappa receptor was homologous, as the ability of somatostatin to mediate inhibition of adenylyl cyclase or potentiation of the K+ current was not altered by kappa receptor desensitization. The mu receptor uncoupled from the K+ current but not adenylyl cyclase after a 1 h pretreatment with agonist. Somatostatin was no longer able to potentiate the K+ current after mu receptor desensitization, thus this desensitization was heterologous. Interestingly, pretreatment with a somatostatin agonist caused uncoupling of the mu receptor but not the kappa receptor from the K+ current. These results show that in the same cell line, after a 1 h pretreatment with agonist, the kappa receptor displays homologous regulation, whereas the mu receptor undergoes only a heterologous form of desensitization. mu receptor desensitization may lead to the alterations of diverse downstream events, whereas kappa receptor regulation apparently occurs at the level of the receptor itself. Broad alterations of non-opioid systems by the mu receptor could be relevant to the addictive properties of mu agonists. Comparison of kappa and mu receptor regulation may help define the properties of the mu receptor which are important in the development of addiction, tolerance, and withdrawal to opioid drugs. These are the first studies to directly compare the coupling of the kappa and mu receptors to two different effectors in the same mammalian expression system.

Cloning, expression, pharmacology and tissue distribution of the mouse somatostatin receptor subtype 5

The gene encoding the mouse somatostatin receptor subtype 5 has been isolated from a genomic library and the mRNA start point mapped to position -95 relative to the translational start codon. The promoter region is devoid of TATA and CAAT boxes but contains putative binding sites for AP-1, AP-2 and SP1 and response elements for glucocorticoids (GRE) and phorbol esters (TRE). The encoded receptor protein with a predicted molecular weight of 42.5 kDa is comprised of 385 amino acids and thus contains 22 and 21 amino acids more than rat and human counterparts. The extra amino acids are caused by another translational initiation codon located further upstream. In the region of overlap the mouse somatostatin receptor subtype 5 displays 96.7% sequence identity to the rat and 81.7% to the human homologue. Application of somatostatin-14 and -28 to human embryonic kidney cells expressing the recombinant receptor resulted in the inhibition of forskolin-stimulated adenylyl cyclase with comparable EC50 values. Consistent with the observed sequence relationship, the mouse somatostatin receptor subtype 5 displays a pharmacological profile that resembles the rat homologue more closely than the human counterpart. mRNA for the mouse somatostatin type 5 receptor has been detected in pituitary, kidney, spleen and ovary and, to a lesser extent, in brain, stomach, intestine and thymus but was not observed in heart, pancreas and liver.

Different G proteins mediate somatostatin-induced inward rectifier K+ currents in murine brain and endocrine cells

1. Types of G proteins (G protein alpha-subunit subtypes) which mediate the activation of inward rectifier K+ currents by somatostatin (somatotrophin release-inhibiting factor, SRIF) were determined in cultured locus coeruleus neurones from newborn rats and in AtT-20 cells (a mouse pituitary cell line). 2. The whole-cell patch clamp technique was used together with injection of antibodies against pertussis toxin (PTX)-sensitive G protein alpha-subunits or with injection of antisense (or sense) oligonucleotides against these G proteins. 3. In locus coeruleus neurones, the SRIF-induced activation of inward rectifier K+ currents was inhibited by anti-G alpha i1/G alpha i2 antibody injection, but not by anti-G alpha i3 or by anti-G alpha o/G alpha i3 antibody injection, suggesting that the SRIF response is mediated through G alpha i1 and/or G alpha i2. 4. The SRIF-induced activation of the inward rectifier was suppressed in locus coeruleus neurones after injection of antisense oligonucleotides against G alpha i2, but not by injection of sense oligonucleotides against G alpha i2. Injection of antisense (or sense) oligonucleotides against G alpha i1, G alpha i3 and G alpha O (common) had no effect. These results suggest that G alpha i2 is involved in this SRIF response. 5. In AtT-20 cells, the SRIF-induced activation of inward rectifier K+ currents was suppressed by injection of anti-G alpha i3 antibody, but not by injection of anti-G alpha i1/G alpha i2 antibody. 6. The above results indicate that Gi mediates the SRIF effects on inward rectifier K+ currents. However, different subtypes of Gi are involved in the brain neurones and in the endocrine cells: Gi2 in locus coeruleus neurones and Gi3 in AtT-20 cells.