Probing cell type-specific functions of Gi in vivo identifies GPCR regulators of insulin secretion

Rgs16 and Rgs8 in embryonic endocrine pancreas and mouse models of diabetes

Diabetes is characterized by the loss, or gradual dysfunction, of insulin-producing pancreatic beta-cells. Although beta-cells can replicate in younger adults, the available diabetes therapies do not specifically target beta-cell regeneration. Novel approaches are needed to discover new therapeutics and to understand the contributions of endocrine progenitors and beta-cell regeneration during islet expansion. Here, we show that the regulators of G protein signaling Rgs16 and Rgs8 are expressed in pancreatic progenitor and endocrine cells during development, then extinguished in adults, but reactivated in models of both type 1 and type 2 diabetes. Exendin-4, a glucagon-like peptide 1 (Glp-1)/incretin mimetic that stimulates beta-cell expansion, insulin secretion and normalization of blood glucose levels in diabetics, also promoted re-expression of Rgs16::GFP within a few days in pancreatic ductal-associated cells and islet beta-cells. These findings show that Rgs16::GFP and Rgs8::GFP are novel and early reporters of G protein-coupled receptor (GPCR)-stimulated beta-cell expansion after therapeutic treatment and in diabetes models. Rgs16 and Rgs8 are likely to control aspects of islet progenitor cell activation, differentiation and beta-cell expansion in embryos and metabolically stressed adults.

The Gq/G11-mediated signaling pathway is critical for autocrine potentiation of insulin secretion in mice

A variety of neurotransmitters, gastrointestinal hormones, and metabolic signals are known to potentiate insulin secretion through GPCRs. We show here that beta cell-specific inactivation of the genes encoding the G protein alpha-subunits Galphaq and Galpha11 resulted in impaired glucose tolerance and insulin secretion in mice. Interestingly, the defects observed in Galphaq/Galpha11-deficient beta cells were not restricted to loss of muscarinic or metabolic potentiation of insulin release; the response to glucose per se was also diminished. Electrophysiological recordings revealed that glucose-induced depolarization of isolated beta cells was impaired in the absence of Galphaq/Galpha11, and closure of KATP channels was inhibited. We provide evidence that this reduced excitability was due to a loss of beta cell-autonomous potentiation of insulin secretion through factors cosecreted with insulin. We identified as autocrine mediators involved in this process extracellular nucleotides such as uridine diphosphate acting through the Gq/G11-coupled P2Y6 receptor and extracellular calcium acting through the calcium-sensing receptor. Thus, the Gq/G11-mediated signaling pathway potentiates insulin secretion in response to glucose by integrating systemic as well as autocrine/paracrine mediators.

RGS4 is a negative regulator of insulin release from pancreatic beta-cells in vitro and in vivo

Therapeutic strategies that augment insulin release from pancreatic beta-cells are considered beneficial in the treatment of type 2 diabetes. We previously demonstrated that activation of beta-cell M(3) muscarinic receptors (M3Rs) greatly promotes glucose-stimulated insulin secretion (GSIS), suggesting that strategies aimed at enhancing signaling through beta-cell M3Rs may become therapeutically useful. M3R activation leads to the stimulation of G proteins of the G(q) family, which are under the inhibitory control of proteins known as regulators of G protein signaling (RGS proteins). At present, it remains unknown whether RGS proteins play a role in regulating insulin release. To address this issue, we initially demonstrated that MIN6 insulinoma cells express functional M3Rs and that RGS4 was by far the most abundant RGS protein expressed by these cells. Strikingly, siRNA-mediated knockdown of RGS4 expression in MIN6 cells greatly enhanced M3R-mediated augmentation of GSIS and calcium release. We obtained similar findings using pancreatic islets prepared from RGS4-deficient mice. Interestingly, RGS4 deficiency had little effect on insulin release caused by activation of other beta-cell GPCRs. Finally, treatment of mutant mice selectively lacking RGS4 in pancreatic beta-cells with a muscarinic agonist (bethanechol) led to significantly increased plasma insulin and reduced blood glucose levels, as compared to control littermates. Studies with beta-cell-specific M3R knockout mice showed that these responses were mediated by beta-cell M3Rs. These findings indicate that RGS4 is a potent negative regulator of M3R function in pancreatic beta-cells, suggesting that RGS4 may represent a potential target to promote insulin release for therapeutic purposes.

Transport of thyroid hormones is selectively inhibited by 3-iodothyronamine

Thyroid hormone transporters are responsible for the cellular uptake of thyroid hormones, which is a prerequisite for their subsequent metabolism and action at nuclear thyroid hormone receptors. A recently discovered thyroid hormone derivative, 3-iodothyronamine (T(1)AM), has distinct biological effects that are opposite those of thyroid hormone. Here we investigate the effects of T(1)AM on thyroid hormone transporters using COS-1 cells transfected with the multispecific organic anion transporting polypeptides (OATPs) 1A2, 1B3, and 1C1, as well as the specific thyroid hormone transporters MCT8 and MCT10, and show that T(1)AM displays differential inhibition of T(3) and T(4) cellular uptake by these transporters. T(1)AM inhibits T(3) and T(4) transport by OATP1A2 with IC(50) values of 0.27 and 2.1 microM, respectively. T(4) transport by OATP1C1, which is thought to play a key role in thyroid hormone transport across the blood-brain barrier, is inhibited by T(1)AM with an IC(50) of 4.8 microM. T(1)AM also inhibits both T(3) and T(4) uptake via MCT8, the most specific thyroid hormone transporter identified to date, with IC(50) values of 95 and 31 microM, respectively. By contrast, T(1)AM has no effect on thyroid hormone transport by OATP1B3 and MCT10. Given that OATP1A2, OATP1C1, and MCT8 are all present in the brain, T(1)AM may play an important role in modulating thyroid hormone delivery and activity in specific target regions in the central nervous system.

Local protease signaling contributes to neural tube closure in the mouse embryo

We report an unexpected role for protease signaling in neural tube closure and the formation of the central nervous system. Mouse embryos lacking protease-activated receptors 1 and 2 showed defective hindbrain and posterior neuropore closure and developed exencephaly and spina bifida, important human congenital anomalies. Par1 and Par2 were expressed in surface ectoderm, and Par2 was expressed selectively along the line of closure. Ablation of G(i/z) and Rac1 function in these Par2-expressing cells disrupted neural tube closure, further implicating G protein-coupled receptors and identifying a likely effector pathway. Cluster analysis of protease and Par2 expression patterns revealed a group of membrane-tethered proteases often coexpressed with Par2. Among these, matriptase activated Par2 with picomolar potency, and hepsin and prostasin activated matriptase. Together, our results suggest a role for protease-activated receptor signaling in neural tube closure and identify a local protease network that may trigger Par2 signaling and monitor and regulate epithelial integrity in this context.

Structure, biological functions and applications of the AB5 toxins

AB(5) toxins are important virulence factors for several major bacterial pathogens, including Bordetella pertussis, Vibrio cholerae, Shigella dysenteriae and at least two distinct pathotypes of Escherichia coli. The AB(5) toxins are so named because they comprise a catalytic A-subunit, which is responsible for disruption of essential host functions, and a pentameric B-subunit that binds to specific glycan receptors on the target cell surface. The molecular mechanisms by which the AB(5) toxins cause disease have been largely unravelled, including recent insights into a novel AB(5) toxin family, subtilase cytotoxin (SubAB). Furthermore, AB(5) toxins have become a valuable tool for studying fundamental cellular functions, and are now being investigated for potential applications in the clinical treatment of human diseases.

G protein coupled receptors in embryonic stem cells: a role for Gs-alpha signaling

Background

Identification of receptor mediated signaling pathways in embryonic stem (ES) cells is needed to facilitate strategies for cell replacement using ES cells. One large receptor family, largely uninvestigated in ES cells, is G protein coupled receptors (GPCRs). An important role for these receptors in embryonic development has been described, but little is known about GPCR expression in ES cells.

Methodology/Principal Findings

We have examined the expression profile of 343 different GPCRs in mouse ES cells demonstrating for the first time that a large number of GPCRs are expressed in undifferentiated and differentiating ES cells, and in many cases at high levels. To begin to define a role for GPCR signaling in ES cells, the impact of activating Gs-alpha, one of the major alpha subunits that couples to GPCRs, was investigated. Gs-alpha activation resulted in larger embryoid bodies (EBs), due, in part, to increased cell proliferation and prevented the time-related decline in expression of transcription factors important for maintaining ES cell pluripotency.

Significance/Conclusions

These studies suggest that Gs-alpha signaling contributes to ES cell proliferation and pluripotency and provide a framework for further investigation of GPCRs in ES cells.

G(i)-coupled GPCR signaling controls the formation and organization of human pluripotent colonies

Background

Reprogramming adult human somatic cells to create human induced pluripotent stem (hiPS) cell colonies involves a dramatic morphological and organizational transition. These colonies are morphologically indistinguishable from those of pluripotent human embryonic stem (hES) cells. G protein-coupled receptors (GPCRs) are required in diverse developmental processes, but their role in pluripotent colony morphology and organization is unknown. We tested the hypothesis that G_i-coupled GPCR signaling contributes to the characteristic morphology and organization of human pluripotent colonies.

Methodology/Principal Findings

Specific and irreversible inhibition of G_i-coupled GPCR signaling by pertussis toxin markedly altered pluripotent colony morphology. Wild-type hES and hiPS cells formed monolayer colonies, but colonies treated with pertussis toxin retracted inward, adopting a dense, multi-layered conformation. The treated colonies were unable to reform after a scratch wound insult, whereas control colonies healed completely within 48 h. In contrast, activation of an alternative GPCR pathway, G_s-coupled signaling, with cholera toxin did not affect colony morphology or the healing response. Pertussis toxin did not alter the proliferation, apoptosis or pluripotency of pluripotent stem cells.

Conclusions/Significance

Experiments with pertussis toxin suggest that G_i signaling plays a critical role in the morphology and organization of pluripotent colonies. These results may be explained by a G_i-mediated density-sensing mechanism that propels the cells radially outward. GPCRs are a promising target for modulating the formation and organization of hiPS and hES cell colonies and may be important for understanding somatic cell reprogramming and for engineering pluripotent stem cells for therapeutic applications.

The selective antagonist EPPTB reveals TAAR1-mediated regulatory mechanisms in dopaminergic neurons of the mesolimbic system

Trace amine-associated receptor 1 (TAAR1) is a G protein-coupled receptor (GPCR) that is nonselectively activated by endogenous metabolites of amino acids. TAAR1 is considered a promising drug target for the treatment of psychiatric and neurodegenerative disorders. However, no selective ligand to identify TAAR1-specific signaling mechanisms is available yet. Here we report a selective TAAR1 antagonist, EPPTB, and characterize its physiological effects at dopamine (DA) neurons of the ventral tegmental area (VTA). We show that EPPTB prevents the reduction of the firing frequency of DA neurons induced by p-tyramine (p-tyr), a nonselective TAAR1 agonist. When applied alone, EPPTB increases the firing frequency of DA neurons, suggesting that TAAR1 either exhibits constitutive activity or is tonically activated by ambient levels of endogenous agonist(s). We further show that EPPTB blocks the TAAR1-mediated activation of an inwardly rectifying K(+) current. When applied alone, EPPTB induces an apparent inward current, suggesting the closure of tonically activated K(+) channels. Importantly, these EPPTB effects were absent in Taar1 knockout mice, ruling out off-target effects. We additionally found that both the acute application of EPPTB and the constitutive genetic lack of TAAR1 increase the potency of DA at D2 receptors in DA neurons. In summary, our data support that TAAR1 tonically activates inwardly rectifying K(+) channels, which reduces the basal firing frequency of DA neurons in the VTA. We hypothesize that the EPPTB-induced increase in the potency of DA at D2 receptors is part of a homeostatic feedback mechanism compensating for the lack of inhibitory TAAR1 tone.

A chemical-genetic approach to study G protein regulation of beta cell function in vivo

Impaired functioning of pancreatic beta cells is a key hallmark of type 2 diabetes. beta cell function is modulated by the actions of different classes of heterotrimeric G proteins. The functional consequences of activating specific beta cell G protein signaling pathways in vivo are not well understood at present, primarily due to the fact that beta cell G protein-coupled receptors (GPCRs) are also expressed by many other tissues. To circumvent these difficulties, we developed a chemical-genetic approach that allows for the conditional and selective activation of specific beta cell G proteins in intact animals. Specifically, we created two lines of transgenic mice each of which expressed a specific designer GPCR in beta cells only. Importantly, the two designer receptors differed in their G protein-coupling properties (G(q/11) versus G(s)). They were unable to bind endogenous ligand(s), but could be efficiently activated by an otherwise pharmacologically inert compound (clozapine-N-oxide), leading to the conditional activation of either beta cell G(q/11) or G(s) G proteins. Here we report the findings that conditional and selective activation of beta cell G(q/11) signaling in vivo leads to striking increases in both first- and second-phase insulin release, greatly improved glucose tolerance in obese, insulin-resistant mice, and elevated beta cell mass, associated with pathway-specific alterations in islet gene expression levels. Selective stimulation of beta cell G(s) triggered qualitatively similar in vivo metabolic effects. Thus, this developed chemical-genetic strategy represents a powerful approach to study G protein regulation of beta cell function in vivo.

Sphingosine-1-phosphate in the plasma compartment regulates basal and inflammation-induced vascular leak in mice

Maintenance of vascular integrity is critical for homeostasis, and temporally and spatially regulated vascular leak is a central feature of inflammation. Sphingosine-1-phosphate (S1P) can regulate endothelial barrier function, but the sources of the S1P that provide this activity in vivo and its importance in modulating different inflammatory responses are unknown. We report here that mutant mice engineered to selectively lack S1P in plasma displayed increased vascular leak and impaired survival after anaphylaxis, administration of platelet-activating factor (PAF) or histamine, and exposure to related inflammatory challenges. Increased leak was associated with increased interendothelial cell gaps in venules and was reversed by transfusion with wild-type erythrocytes (which restored plasma S1P levels) and by acute treatment with an agonist for the S1P receptor 1 (S1pr1). S1pr1 agonist did not protect wild-type mice from PAF-induced leak, consistent with plasma S1P levels being sufficient for S1pr1 activation in wild-type mice. However, an agonist for another endothelial cell Gi-coupled receptor, Par2, did protect wild-type mice from PAF-induced vascular leak, and systemic treatment with pertussis toxin prevented rescue by Par2 agonist and sensitized wild-type mice to leak-inducing stimuli in a manner that resembled the loss of plasma S1P. Our results suggest that the blood communicates with blood vessels via plasma S1P to maintain vascular integrity and regulate vascular leak. This pathway prevents lethal responses to leak-inducing mediators in mouse models.

Molecular pharmacology of promiscuous seven transmembrane receptors sensing organic nutrients

A number of highly promiscuous seven transmembrane (7TM) receptors have been cloned and characterized within the last few years. It is noteworthy that many of these receptors are activated broadly by amino acids, proteolytic degradation products, carbohydrates, or free fatty acids and are expressed in taste tissue, the gastrointestinal tract, endocrine glands, adipose tissue, and/or kidney. These receptors thus hold the potential to act as sensors of food intake, regulating, for example, release of incretin hormones from the gut, insulin/glucagon from the pancreas, and leptin from adipose tissue. The promiscuous tendency in ligand recognition of these receptors is in contrast to the typical specific interaction with one physiological agonist seen for most receptors, which challenges the classic "lock-and-key" concept. We here review the molecular mechanisms of nutrient sensing of the calcium-sensing receptor, the G protein-coupled receptor family C, group 6, subtype A (GPRC6A), and the taste1 receptor T1R1/T1R3, which are sensing L-alpha-amino acids, the carbohydrate-sensing T1R2/T1R3 receptor, the proteolytic degradation product sensor GPR93 (also termed GPR92), and the free fatty acid (FFA) sensing receptors FFA1, FFA2, FFA3, GPR84, and GPR120. The involvement of the individual receptors in sensing of food intake has been validated to different degrees because of limited availability of specific pharmacological tools and/or receptor knockout mice. However, as a group, the receptors represent potential drug targets, to treat, for example, type II diabetes by mimicking food intake by potent agonists or positive allosteric modulators. The ligand-receptor interactions of the promiscuous receptors of organic nutrients thus remain an interesting subject of emerging functional importance.

International Union of Pharmacology. LXXII. Recommendations for trace amine receptor nomenclature

Trace amines such as p-tyramine and beta-phenylethylamine are found endogenously as well as in the diet. Concomitant ingestion of these foodstuffs with monoamine oxidase inhibitors may result in the hypertensive crisis known as the "beer, wine, and cheese effect" attributed to their sympathomimetic action. Trace amines have been shown to act on one of a novel group of mammalian seven transmembrane spanning G protein-coupled receptors belonging to the rhodopsin superfamily, cloned in 2001. This receptor encoded by the human TAAR1 gene is also present in rat and mouse genomes (Taar1) and has been shown to be activated by endogenous trace amine ligands, including p-tyramine and beta-phenylethylamine. A number of drugs, most notably amphetamine and its derivatives, act as agonists at this receptor. This review proposes an official nomenclature designating TAAR1 as the trace amine 1 receptor following the convention of naming receptors after the endogenous agonist, abbreviated to TA(1) where necessary. It goes on to discuss briefly the significance of the receptor, agents acting upon it, its distribution, and currently hypothesized physiological and pathophysiological roles. In humans, a further five genes are thought to encode functional receptors (TAAR2, TAAR5, TAAR6, TAAR8, and TAAR9). TAAR3 seems to be a pseudogene in some individuals but not others. TAAR4 is a pseudogene in humans, but occurs with TAAR3 as a functional gene in rodents. Nine further genes are present in rats and mice. The endogenous ligands are not firmly established but some may respond to odorants consistent with their expression in olfactory epithelium.

Islet G protein-coupled receptors as potential targets for treatment of type 2 diabetes

Islet dysfunction - characterized by a combination of defective insulin secretion, inappropriately high glucagon secretion and reduced beta-cell mass - has a central role in the pathophysiology of type 2 diabetes. Several G protein-coupled receptors (GPCRs) expressed in islet beta-cells are known to be involved in the regulation of islet function, and therefore are potential therapeutic targets. This is evident from the recent success of glucagon-like peptide 1 (GLP1) mimetics and dipeptidyl peptidase 4 (DPP4) inhibitors, which promote activation of the GLP1 receptor to stimulate insulin secretion and inhibit glucagon secretion, and also have the potential to increase beta-cell mass. Other islet beta-cell GPCRs that are involved in the regulation of islet function include the glucose-dependent insulinotropic peptide (GIP) receptor, lipid GPCRs, pleiotropic peptide GPCRs and islet biogenic amine GPCRs. This Review summarizes islet GPCR expression, signalling and function, and highlights their potential as targets for the treatment of type 2 diabetes.

Identification and characterization of 3-iodothyronamine intracellular transport

3-Iodothyronamine (T(1)AM) is a naturally occurring thyroid hormone metabolite with distinct biological effects that are opposite those of thyroid hormone. The known molecular targets of T(1)AM include both plasma membrane and intracellular proteins, suggesting that intracellular transport of T(1)AM may be an important component of its action, although no uptake mechanism has yet been described. Using various human cell lines, we show that, indeed, cellular uptake of T(1)AM occurs in multiple cell types and that this process involves specific, saturable, and inhibitable transport mechanisms. These mechanisms are sodium and chloride independent, pH dependent, thyronamine specific, and do not involve the likely candidate transporters of other monoamines, organic cations, or thyroid hormones. A large-scale RNA interference screen targeting the entire solute carrier superfamily of transporter genes reveals that the transport of T(1)AM into cells involves multiple transporters, and we identify eight transporters that may contribute to the uptake of T(1)AM in HeLa cells. This type of transporter small interfering RNA screening approach can be used in general to identify the constellation of transporters that participate in the intracellular disposition of compounds.

The molecular basis of species-specific ligand activation of trace amine-associated receptor 1 (TAAR(1))

The trace amine-associated receptor 1 (TAAR(1)) is an aminergic G protein-coupled receptor (GPCR) potently activated by 3-iodothyronamine (1), an endogenous derivative of thyroid hormone. Structure-activity relationship studies on 1 and related agonists showed that the rat and mouse species of TAAR(1) accommodated structural modifications and functional groups on the ethylamine portion and the biaryl ether moiety of the molecule. However, the two receptors clearly exhibited distinct, species-specific ligand preferences despite being remarkably similar with 93% sequence similarity. In this study, we generated single and double mutants of rat and mouse TAAR(1) to probe the molecular recognition of agonists and the underlying basis for the ligand selectivity of rat and mouse TAAR(1). Key, nonconserved specificity determinant residues in transmembranes helices 4 and 7 within the ligand binding site appear to be the primary source of a number of the observed ligand preferences. Residue 7.39 in transmembrane 7 dictated the preference for a beta-phenyl ring, while residue 4.56 in transmembrane 4 was partially responsible for the lower potency of 1 and tyramine for the mouse receptor. Additionally, 1 and tyramine were found to have the same binding mode in rat TAAR(1) despite structure-activity relationship data suggesting the possibility of each molecule having different binding orientations. These findings provide valuable insights into the critical binding site residues involved in the ligand-receptor interaction that can influence compound selectivity and functional activity of aminergic GPCRs.

Identification of a transient subpial neurogenic zone in the developing dentate gyrus and its regulation by Cxcl12 and reelin signaling

One striking feature of dentate gyrus development, distinct from the other cortical structures, is the relocation of neural precursors from the ventricular zone to the forming dentate pole to produce a lifelong neurogenic subgranular zone (SGZ). In this study, we demonstrate that dentate progenitors first dwell for up to 1 week in a previously unrecognized neurogenic zone intimately associated with the pial meningeal surface lining the outer edge of the forming dentate. This zone also serves as the organizational matrix for the initial formation of the dentate glial scaffolding. Timely clearance of neural precursors from their transient location depends on reelin, whereas initial formation of this transient stem cell niche requires Cxcl12-Cxcr4 signaling. The final settlement of the neural precursors at the subgranular zone relies on a pertussis toxin-sensitive pathway independent of Cxcl12-Cxcr4 signaling. Furthermore, genetic fate-mapping analysis suggests that subpial precursors contribute to the SGZ formation. These results demonstrate that the relocation of neural precursors in the dentate gyrus consists of discrete steps regulated by multiple pathways.

Iodothyronamines are oxidatively deaminated to iodothyroacetic acids in vivo

3-Iodothyronamine (T(1)AM) and 3,3',5-triiodothyroacetic acid (Triac) are bioactive metabolites of the hormone thyroxine (T(4)). In the present study, the ability of T(1)AM and 3,3',5-triiodothyronamine (T(3)AM) to be metabolized to 3-iodothyroacetic acid (TA(1)) and Triac, respectively, was investigated. Both T(1)AM and T(3)AM were converted to their respective iodinated thyroacetic acid analogues in both cell and tissue extracts. This conversion could be significantly inhibited with the monamine oxidase (MAO) and semicarbazide-sensitive amine oxidase (SSAO) inhibitor iproniazid. TA(1) was found to be present in trace quantities in human serum and in substantial levels in serum from T(1)AM-treated rats. These results demonstrate that iodothyronamines are substrates for amine oxidases and that this metabolism may be the source of the corresponding endogenous arylacetic acid products Triac and TA(1).

Minireview: 3-Iodothyronamine (T1AM): a new player on the thyroid endocrine team?

3-Iodothyronamine (T(1)AM) is an endogenous compound with chemical features that are similar to thyroid hormone. T(1)AM has a carbon skeleton identical to that of T(4) and contains a single carbon-iodine bond. Theoretically, T(1)AM could be produced from T(4) by enzymatic decarboxylation and deiodination. Recent studies show that T(1)AM and higher iodinated thyronamines are subject to similar metabolic processing as iodothyronines such as T(4), suggesting a biological linkage between iodothyronines and iodothyronamines. In addition, single doses of T(1)AM administered to rodents induce a hypometabolic state that in certain ways resembles hibernation and is opposite to the effects of excess T(4). This review will discuss the latest developments on this recently discovered thyroid hormone derivative.

A role for von Hippel-Lindau protein in pancreatic beta-cell function

OBJECTIVE

The Vhlh gene codes for the von Hippel-Lindau protein (VHL), a tumor suppressor that is a key player in the cellular response to oxygen sensing. In humans, a germline mutation in the VHL gene leads to the von Hippel-Lindau disease, a familial syndrome characterized by benign and malignant tumors of the kidney, central nervous system, and pancreas.

RESEARCH DESIGN AND METHODS

We use Cre-lox recombination to eliminate Vhlh in adult mouse pancreatic beta-cells. Morphology of mutant islets is assessed by immunofluorescence analysis. To determine the functional state of Vhlh(-/-) islets, insulin secretion is measured in vivo and in vitro, and quantitative PCR is used to identify changes in gene expression.

RESULTS

Loss of VHL in beta-cells leads to a severe glucose-intolerant phenotype in adult animals. Although VHL is not required for beta-cell specification and development, it is critical for beta-cell function. Insulin production is normal in beta-cells lacking VHL; however, insulin secretion in the presence of high concentrations of glucose is impaired. Furthermore, the loss of VHL leads to dysregulation of glycolytic enzymes, pointing to a perturbation of the intracellular energy homeostasis.

CONCLUSIONS

We show that loss of VHL in beta-cells leads to defects in glucose homeostasis, indicating an important and previously unappreciated role for VHL in beta-cell function. We believe that the beta-cell-specific Vhlh-deficient mice might be a useful tool as a "genetic hypoxia" model, to unravel the possible link between hypoxia signaling and impairment of beta-cell function.

Anatomical profiling of G protein-coupled receptor expression

G protein-coupled receptors (GPCRs) comprise the largest family of transmembrane signaling molecules and regulate a host of physiological and disease processes. To better understand the functions of GPCRs in vivo, we quantified transcript levels of 353 nonodorant GPCRs in 41 adult mouse tissues. Cluster analysis placed many GPCRs into anticipated anatomical and functional groups and predicted previously unidentified roles for less-studied receptors. From one such prediction, we showed that the Gpr91 ligand succinate can regulate lipolysis in white adipose tissue, suggesting that signaling by this citric acid cycle intermediate may regulate energy homeostasis. We also showed that pairwise analysis of GPCR expression across tissues may help predict drug side effects. This resource will aid studies to understand GPCR function in vivo and may assist in the identification of therapeutic targets.

Validation of a liquid chromatography-tandem mass spectrometry method to enable quantification of 3-iodothyronamine from serum

There is great interest lately in the availability of analytical methods for quantification of 3-iodothyronamine from blood and tissues. To date, no validated method for determination of 3-iodothyronamine from biological matrices has been described. Detailed in this report is an LC-MS/MS method that permits accurate and reproducible quantification of pharmacological concentrations of 3-iodothyronamine from rat serum, with a 0.0008 M lower limit of quantification. Endogenous 3-iodothyronamine was observed from rodent and human serum (0.2 mL) at the method limit of detection. In summary, the LC-MS/MS method enables quantification of circulating 3-iodothyronamine to allow examination of a relationship with biological activity.

The role of vasoactive intestinal peptide (VIP) in megakaryocyte proliferation

Megakaryocytopoiesis is a multistage process that involves differentiation of hematopoietic stem cells through the myeloid lineage, ultimately producing megakaryocytes and platelets. Vasoactive intestinal peptide (VIP) stimulates adenylate cyclase and induces differentiation in multiple cell types; VIP is expressed in hematopoietic stem cells and in megakaryocytes, but its function in these cells has not yet been delineated. The present study was designed to investigate whether the type 1 VIP receptor, VPAC1, mediates VIP effects on megakaryocytopoiesis. The human megakaryoblastic leukemia cell line (CMK) was transfected with VPAC1 and the transgene expression was confirmed by qualitative polymerase chain reaction and immunohistochemistry. The rate of proliferation and the patterns of differentiation were then compared for CMK and CMK/VPAC1 through multiple growth cycles. Upregulation of VPAC1 expression resulted in a decreased proliferation rate (p = 0.0003) and enhanced differentiation with CMK/VPAC1 cells having twice the cell surface area of control CMK cells (p = 0.001), thus increasing potential for proplatelet formation. These results suggest that VIP acts in an autocrine fashion via VPAC1 to inhibit megakaryocyte proliferation and induce proplatelet formation.