Lack of receptor-selective effects of either RGS2, RGS3 or RGS4 on muscarinic M3- and gonadotropin-releasing hormone receptor-mediated signalling through G alpha q/11

Mathematical models in GnRH research

Mathematical modelling is an indispensable tool in modern biosciences, enabling quantitative analysis and integration of biological data, transparent formulation of our understanding of complex biological systems, and efficient experimental design based on model predictions. This review article provides an overview of the impact that mathematical models had on GnRH research. Indeed, over the last 20 years mathematical modelling has been used to describe and explore the physiology of the GnRH neuron, the mechanisms underlying GnRH pulsatile secretion, and GnRH signalling to the pituitary. Importantly, these models have contributed to GnRH research via novel hypotheses and predictions regarding the bursting behaviour of the GnRH neuron, the role of kisspeptin neurons in the emergence of pulsatile GnRH dynamics, and the decoding of GnRH signals by biochemical signalling networks. We envisage that with the advent of novel experimental technologies, mathematical modelling will have an even greater role to play in our endeavour to understand the complex spatiotemporal dynamics underlying the reproductive neuroendocrine system.

Gonadotropin-Releasing Hormone and GnRH Receptor: Structure, Function and Drug Development

BACKGROUND

Gonadotropin-Releasing Hormone (GnRH) is a key element in sexual maturation and regulation of the reproductive cycle in the human organism. GnRH interacts with the pituitary cells through the activation of the Gonadotropin Releasing Hormone Receptors (GnRHR). Any impairments/dysfunctions of the GnRH-GnRHR complex lead to the development of various cancer types and disorders. Furthermore, the identification of GnRHR as a potential drug target has led to the development of agonist and antagonist molecules implemented in various treatment protocols. The development of these drugs was based on the information derived from the functional studies of GnRH and GnRHR.

OBJECTIVE

This review aims at shedding light on the versatile function of GnRH and GnRH receptor and offers an apprehensive summary regarding the development of different agonists, antagonists and non-peptide GnRH analogues.

CONCLUSION

The information derived from these studies can enhance our understanding of the GnRH-GnRHR versatile nature and offer valuable insight into the design of new more potent molecules.

Transcriptomic analysis of the porcine anterior pituitary gland during the peri-implantation period

The peri-implantation period is controlled by signals originating from hypothalamic-pituitary-ovarian axis, uterus and developing embryos. The transcriptomic activity of the anterior pituitary gland may be important for the control of the peri-implantation period. The aim of this study was to determine the alternations in the transcriptomic profile of porcine anterior pituitary gland during the peri-implantation period (days 15-16 of pregnancy) in comparison with established for the respective days of the oestrous cycle. Analysis using a microarray approach indicated that the 651 genes (fold-change ˂1.2; p ≤ .05) were differentially expressed (DEGs) in the anterior pituitary of pigs during the peri-implantation period when compared to cyclic females. Of these DEGs, 404 were upregulated and 247 downregulated. Analysis of occurred relationships among DEGs revealed that some of them are involved in steroid-response and oestrogen synthesis, FSH secretion, immune response, PPAR signalling pathway and the potential for DNA methylation. In conclusion, the altered transcriptomic profile of the porcine pituitary gland in pigs during the peri-implantation period indicates the role of embryos presence in the creation of transcriptomic activity of the pituitary gland in pigs.

A bronchoprotective role for Rgs2 in a murine model of lipopolysaccharide-induced airways inflammation

Background

Asthma exacerbations are associated with the recruitment of neutrophils to the lungs. These cells release proteases and mediators, many of which act at G protein-coupled receptors (GPCRs) that couple via Gq to promote bronchoconstriction and inflammation. Common asthma therapeutics up-regulate expression of the regulator of G protein signalling (RGS), RGS2. As RGS2 reduces signaling from Gq-coupled GPCRs, we have defined role(s) for this GTPase-activating protein in an acute neutrophilic model of lung inflammation.

Methods

Wild type and Rgs2^−/− C57Bl6 mice were exposed to nebulized lipopolysaccharide (LPS). Lung function (respiratory system resistance and compliance) was measured using a SCIREQ flexivent small animal ventilator. Lung inflammation was assessed by histochemistry, cell counting and by cytokine and chemokine expression in bronchoalveolar lavage (BAL) fluid.

Results

Lipopolysaccharide inhalation induced transient airways hyperreactivity (AHR) and neutrophilic lung inflammation. While AHR and inflammation was greatest 3 h post-LPS exposure, BAL neutrophils persisted for 24 h. At 3 h post-LPS inhalation, multiple inflammatory cytokines (CSF2, CSF3, IL6, TNF) and chemokines (CCL3, CCL4, CXCL1, CXCL2) were highly expressed in the BAL fluid, prior to declining by 24 h. Compared to wild type counterparts, Rgs2^−/− mice developed significantly greater airflow resistance in response to inhaled methacholine (MCh) at 3 h post-LPS exposure. At 24 h post-LPS exposure, when lung function was recovering in the wild type animals, MCh-induced resistance was increased, and compliance decreased, in Rgs2^−/− mice. Thus, Rgs2^−/− mice show AHR and stiffer lungs 24 h post-LPS exposure. Histological markers of inflammation, total and differential cell counts, and major cytokine and chemokine expression in BAL fluid were similar between wild type and Rgs2^−/− mice. However, 3 and 24 h post-LPS exposure, IL12B expression was significantly elevated in BAL fluid from Rgs2^−/− mice compared to wild type animals.

Conclusions

While Rgs2 is bronchoprotective in acute neutrophilic inflammation, no clear anti-inflammatory effect was apparent. Nevertheless, elevated IL12B expression in Rgs2^−/− animals raises the possibility that RGS2 could dampen Th1 responses. These findings indicate that up-regulation of RGS2, as occurs in response to inhaled corticosteroids and long-acting β₂-adrenoceptor agonists, may be beneficial in acute neutrophilic exacerbations of airway disease, including asthma.

Electronic supplementary material

The online version of this article (10.1186/s13223-018-0266-5) contains supplementary material, which is available to authorized users.

Mathematical modeling of gonadotropin-releasing hormone signaling

Gonadotropin-releasing hormone (GnRH) acts via G-protein coupled receptors on pituitary gonadotropes to control reproduction. These are G_q-coupled receptors that mediate acute effects of GnRH on the exocytotic secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), as well as the chronic regulation of their synthesis. GnRH is secreted in short pulses and GnRH effects on its target cells are dependent upon the dynamics of these pulses. Here we overview GnRH receptors and their signaling network, placing emphasis on pulsatile signaling, and how mechanistic mathematical models and an information theoretic approach have helped further this field.

Modeling and high-throughput experimental data uncover the mechanisms underlying Fshb gene sensitivity to gonadotropin-releasing hormone pulse frequency

Neuroendocrine control of reproduction by brain-secreted pulses of gonadotropin-releasing hormone (GnRH) represents a longstanding puzzle about extracellular signal decoding mechanisms. GnRH regulates the pituitary gonadotropin's follicle-stimulating hormone (FSH) and luteinizing hormone (LH), both of which are heterodimers specified by unique β subunits (FSHβ/LHβ). Contrary to Lhb, Fshb gene induction has a preference for low-frequency GnRH pulses. To clarify the underlying regulatory mechanisms, we developed three biologically anchored mathematical models: 1) parallel activation of Fshb inhibitory factors (e.g. inhibin α and VGF nerve growth factor-inducible), 2) activation of a signaling component with a refractory period (e.g. G protein), and 3) inactivation of a factor needed for Fshb induction (e.g. growth differentiation factor 9). Simulations with all three models recapitulated the Fshb expression levels obtained in pituitary gonadotrope cells perifused with varying GnRH pulse frequencies. Notably, simulations altering average concentration, pulse duration, and pulse frequency revealed that the apparent frequency-dependent pattern of Fshb expression in model 1 actually resulted from variations in average GnRH concentration. In contrast, models 2 and 3 showed "true" pulse frequency sensing. To resolve which components of this GnRH signal induce Fshb, we developed a high-throughput parallel experimental system. We analyzed over 4,000 samples in experiments with varying near-physiological GnRH concentrations and pulse patterns. Whereas Egr1 and Fos genes responded only to variations in average GnRH concentration, Fshb levels were sensitive to both average concentration and true pulse frequency. These results provide a foundation for understanding the role of multiple regulatory factors in modulating Fshb gene activity.

Potent inhibition of angiotensin AT1 receptor signaling by RGS8: importance of the C-terminal third exon part of its RGS domain

R4/B subfamily RGS (regulator of G protein signaling) proteins play roles in regulation of many GPCR-mediated responses. Multiple RGS proteins are usually expressed in a cell, and it is difficult to point out which RGS protein species are functionally important in the cell. To evaluate intrinsic potency of these RGS proteins, we compared inhibitory effects of RGS1, RGS2, RGS3, RGS4, RGS5, RGS8 and RGS16 on AT1 receptor signaling. Intracellular Ca(2+) responses to angiotensin II were markedly attenuated by transiently expressed RGS2, RGS3 and RGS8, compared to weak inhibition by RGS1, RGS4, RGS5 and RGS16. N-terminally deleted RGS2 (RGS2 domain) lost this potent inhibitory effect, whereas RGS domains of RGS3 and RGS8 showed strong inhibition similar to those of the full-length proteins. To investigate key determinants that specify the differences in potency, we constructed chimeric domains by replacing one or two of three exon parts of RGS8 domain with the corresponding part of RGS5. The chimeric RGS8 domains containing the first or the second exon part of RGS5 showed strong inhibitory effects similar to that of wild type RGS8, but the chimeric domain with the third exon part of RGS5 lost its activity. On the contrary, replacement of the third exon part of RGS5 with the corresponding residues of RGS8 increased the inhibitory effect. The role of the third exon part of RGS8 domain was further confirmed with the chimeric RGS8/RGS4 domains. These results indicate the potent inhibitory activity of RGS8 among R4/B subfamily proteins and importance of the third exon.

PDZ Protein Regulation of G Protein-Coupled Receptor Trafficking and Signaling Pathways

G protein-coupled receptors (GPCRs) contribute to the regulation of every aspect of human physiology and are therapeutic targets for the treatment of numerous diseases. As a consequence, understanding the myriad of mechanisms controlling GPCR signaling and trafficking is essential for the development of new pharmacological strategies for the treatment of human pathologies. Of the many GPCR-interacting proteins, postsynaptic density protein of 95 kilodaltons, disc large, zona occludens-1 (PDZ) domain-containing proteins appear most abundant and have similarly been implicated in disease mechanisms. PDZ proteins play an important role in regulating receptor and channel protein localization within synapses and tight junctions and function to scaffold intracellular signaling protein complexes. In the current study, we review the known functional interactions between PDZ domain-containing proteins and GPCRs and provide insight into the potential mechanisms of action. These PDZ domain-containing proteins include the membrane-associated guanylate-like kinases [postsynaptic density protein of 95 kilodaltons; synapse-associated protein of 97 kilodaltons; postsynaptic density protein of 93 kilodaltons; synapse-associated protein of 102 kilodaltons; discs, large homolog 5; caspase activation and recruitment domain and membrane-associated guanylate-like kinase domain-containing protein 3; membrane protein, palmitoylated 3; calcium/calmodulin-dependent serine protein kinase; membrane-associated guanylate kinase protein (MAGI)-1, MAGI-2, and MAGI-3], Na(+)/H(+) exchanger regulatory factor proteins (NHERFs) (NHERF1, NHERF2, PDZ domain-containing kidney protein 1, and PDZ domain-containing kidney protein 2), Golgi-associated PDZ proteins (Gα-binding protein interacting protein, C-terminus and CFTR-associated ligand), PDZ domain-containing guanine nucleotide exchange factors (GEFs) 1 and 2, regulator of G protein signaling (RGS)-homology-RhoGEFs (PDZ domain-containing RhoGEF and leukemia-associated RhoGEF), RGS3 and RGS12, spinophilin and neurabin-1, SRC homology 3 domain and multiple ankyrin repeat domain (Shank) proteins (Shank1, Shank2, and Shank3), partitioning defective proteins 3 and 6, multiple PDZ protein 1, Tamalin, neuronal nitric oxide synthase, syntrophins, protein interacting with protein kinase C α 1, syntenin-1, and sorting nexin 27.

Molecular mechanisms of gonadotropin-releasing hormone signaling: integrating cyclic nucleotides into the network

Gonadotropin-releasing hormone (GnRH) is the primary regulator of mammalian reproductive function in both males and females. It acts via G-protein coupled receptors on gonadotropes to stimulate synthesis and secretion of the gonadotropin hormones luteinizing hormone and follicle-stimulating hormone. These receptors couple primarily via G-proteins of the Gq/ll family, driving activation of phospholipases C and mediating GnRH effects on gonadotropin synthesis and secretion. There is also good evidence that GnRH causes activation of other heterotrimeric G-proteins (Gs and Gi) with consequent effects on cyclic AMP production, as well as for effects on the soluble and particulate guanylyl cyclases that generate cGMP. Here we provide an overview of these pathways. We emphasize mechanisms underpinning pulsatile hormone signaling and the possible interplay of GnRH and autocrine or paracrine regulatory mechanisms in control of cyclic nucleotide signaling.

RGS3 controls T lymphocyte migration in a model of Th2-mediated airway inflammation

T cell migration toward sites of antigen exposure is mediated by G protein signaling and is a key function in the development of immune responses. Regulators of G protein signaling (RGS) proteins modulate G protein signaling; however, their role in the regulation of adaptive immune responses has not been thoroughly explored. Herein we demonstrated abundant expression of the Gi/Gq-specific RGS3 in activated T cells, and that diminished RGS3 expression in a T cell thymoma increased cytokine-induced migration. To examine the role of endogenous RGS3 in vivo, mice deficient in the RGS domain (RGS3(ΔRGS)) were generated and tested in an experimental model of asthma. Compared with littermate controls, the inflammation in the RGS3(ΔRGS) mice was characterized by increased T cell numbers and the striking development of perivascular lymphoid structures. Surprisingly, while innate inflammatory cells were also increased in the lungs of RGS3(ΔRGS) mice, eosinophil numbers and Th2 cytokine production were equivalent to control mice. In contrast, T cell numbers in the draining lymph nodes (dLN) were reduced in the RGS3(ΔRGS), demonstrating a redistribution of T cells from the dLN to the lungs via increased RGS3(ΔRGS) T cell migration. Together these novel findings show a nonredundant role for endogenous RGS3 in controlling T cell migration in vitro and in an in vivo model of inflammation.

Gonadotrophin-releasing hormone signalling downstream of calmodulin

Gonadotrophin-releasing hormone (GnRH) regulates reproduction via binding a G-protein coupled receptor on the surface of the gonadotroph, through which it transmits signals, mostly via the mitogen-activated protein (MAPK) cascade, to increase synthesis of the gonadotrophin hormones: luteinising hormone (LH) and follicle-stimulating hormone (FSH). Activation of the MAPK cascade requires an elevation in cytosolic Ca(2+) levels, which is a result of both calcium influx and mobilisation from intracellular stores. However, Ca(2+) also transmits signals via an MAPK-independent pathway, through binding calmodulin (CaM), which is then able to bind a number of proteins to impart diverse downstream effects. Although the ability of GnRH to activate CaM was recognised over 20 years ago, only recently have some of the downstream effects been elucidated. GnRH was shown to activate the CaM-dependent phosphatase, calcineurin, which targets gonadotrophin gene expression both directly and indirectly via transcription factors such as nuclear factor of activated T-cells and Nur77, the Transducer of Regulated CREB (TORC) co-activators and also the prolyl isomerase, Pin1. Gonadotrophin gene expression is also regulated by GnRH-induced CaM-dependent kinases (CaMKs); CaMKI is able to derepress the histone deacetylase-inhibition of β-subunit gene expression, whereas CaMKII appears to be essential for the GnRH-activation of all three subunit genes. Asides from activating gonadotrophin gene expression, GnRH also exerts additional effects on gonadotroph function, some of which clearly occur via CaM, including the proliferation of immature gonadotrophs, which is dependent on calcineurin. In this review, we summarise these pathways, and discuss the additional functions that have been proposed for CaM with respect to modifying GnRH-induced signalling pathways via the regulation of the small GTP-binding protein, Gem, and/or the regulator of G-protein signalling protein 2.

Role of Cholinergic Innervation and RGS2 in Atrial Arrhythmia

The heart receives sympathetic and parasympathetic efferent innervation as well as the ability to process information internally via an intrinsic cardiac autonomic nervous system (ICANS). For over a century, the role of the parasympathetics via vagal acetylcholine release was related to controlling primarily heart rate. Although in the late 1800s shown to play a role in atrial arrhythmia, the myocardium took precedence from the mid-1950s until in the last decade a resurgence of interest in the autonomics along with signaling cascades, regulators, and ion channels. Originally ignored as being benign and thus untreated, recent emphasis has focused on atrial arrhythmia as atrial fibrillation (AF) is the most common arrhythmia seen by the general practitioner. It is now recognized to have significant mortality and morbidity due to resultant stroke and heart failure. With the aging population, there will be an unprecedented increased burden on health care resources. Although it has been known for more than half a century that cholinergic stimulation can initiate AF, the classical concept focused on the M2 receptor and its signaling cascade including RGS4, as these had been shown to have predominant effects on nodal function (heart rate and conduction block) as well as contractility. However, recent evidence suggests that the M3 receptor may also playa role in initiation and perpetuation of AF and thus RGS2, a putative regulator of the M3 receptor, may be a target for therapeutic intervention. Mice lacking RGS2 (RGS2^−/−), were found to have significantly altered electrophysiological atrial responses and were more susceptible to electrically induced AF. Vagally induced or programmed stimulation-induced AF could be blocked by the selective M3R antagonist, darifenacin. These results suggest a potential surgical target (ICANS) and pharmacological targets (M3R, RGS2) for the management of AF.

Using automated imaging to interrogate gonadotrophin-releasing hormone receptor trafficking and function

Gonadotrophin-releasing hormone (GnRH) acts via seven transmembrane receptors on gonadotrophs to stimulate gonadotrophin synthesis and secretion, and thereby mediates central control of reproduction. Type I mammalian GnRHR are unique, in that they lack C-terminal tails. This is thought to underlie their resistance to rapid homologous desensitisation as well as their slow rate of internalisation and inability to provoke G-protein-independent (arrestin-mediated) signalling. More recently it has been discovered that the vast majority of human GnRHR are actually intracellular, in spite of the fact that they are activated at the cell surface by a membrane impermeant peptide hormone. This apparently reflects inefficient exit from the endoplasmic reticulum and again, the absence of the C-tail likely contributes to their intracellular localisation. This review is intended to cover some of these novel aspects of GnRHR biology, focusing on ways that we have used automated fluorescence microscopy (high content imaging) to explore GnRHR localisation and trafficking as well as spatial and temporal aspects of GnRH signalling via the Ca(2+)/calmodulin/calcineurin/NFAT and Raf/MEK/ERK pathways.

Pulsatile and sustained gonadotropin-releasing hormone (GnRH) receptor signaling: does the Ca2+/NFAT signaling pathway decode GnRH pulse frequency?

Gonadotropin-releasing hormone (GnRH) acts via 7 transmembrane region receptors on gonadotrophs to stimulate synthesis and secretion of the luteinizing hormone and follicle-stimulating hormone. It is secreted in pulses, and its effects depend on pulse frequency, but decoding mechanisms are unknown. Here we have used (nuclear factor of activated T-cells 2 (NFAT2)-emerald fluorescent protein) to monitor GnRH signaling. Increasing [Ca²⁺]_i causes calmodulin/calcineurin-dependent nuclear NFAT translocation, a response involving proteins (calmodulins and NFATs) that decode frequency in other systems. Using live cell imaging, pulsatile GnRH caused dose- and frequency-dependent increases in nuclear NFAT2-emerald fluorescent protein, and at low frequency, translocation simply tracked GnRH exposure (albeit with slower kinetics). At high frequency (30-min intervals), failure to return to basal conditions before repeat stimulation caused integrative tracking, illustrating how the relative dynamics of up- and downstream signals can increase efficiency of GnRH action. Mathematical modeling predicted desensitization of GnRH effects on [Ca²⁺]_i and that desensitization would increase with dose, frequency, and receptor number, but no such desensitization was seen in HeLa and/or LβT2 cells possibly because pulsatile GnRH did not reduce receptor expression (measured by immunofluorescence). GnRH also caused dose- and frequency-dependent activation of αGSU, luteinizing hormone β, and follicle-stimulating hormone β luciferase reporters, effects that were blocked by calcineurin inhibition. Pulsatile GnRH also activated an NFAT-responsive luciferase reporter, but this response was directly related to cumulative pulse duration. This together with the lack of desensitization of translocation responses suggests that NFAT may mediate GnRH action but is not a genuine decoder of GnRH pulse frequency.

Regulators of G protein signaling proteins as central components of G protein-coupled receptor signaling complexes

The regulators of G protein signaling (RGS) proteins bind directly to G protein alpha (Gα) subunits to regulate the signaling functions of Gα and their linked G protein-coupled receptors (GPCRs). Recent studies indicate that RGS proteins also interact with GPCRs, not just G proteins, to form preferred functional pairs. Interactions between GPCRs and RGS proteins may be direct or indirect (via a linker protein) and are dictated by the receptors, rather than the linked G proteins. Emerging models suggest that GPCRs serve as platforms for assembling an overlapping and distinct constellation of signaling proteins that perform receptor-specific signaling tasks. Compelling evidence now indicates that RGS proteins are central components of these GPCR signaling complexes. This review will outline recent discoveries of GPCR/RGS pairs as well as new data in support of the idea that GPCRs serve as platforms for the formation of multiprotein signaling complexes.

Assembly of high order G alpha q-effector complexes with RGS proteins

Transmembrane signaling through G alpha(q)-coupled receptors is linked to physiological processes such as cardiovascular development and smooth muscle function. Recent crystallographic studies have shown how G alpha(q) interacts with two activation-dependent targets, p63RhoGEF and G protein-coupled receptor kinase 2 (GRK2). These proteins bind to the effector-binding site of G alpha(q) in a manner that does not appear to physically overlap with the site on G alpha(q) bound by regulator of G-protein signaling (RGS) proteins, which function as GTPase-activating proteins (GAPs). Herein we confirm the formation of RGS-G alpha(q)-GRK2/p63RhoGEF ternary complexes using flow cytometry protein interaction and GAP assays. RGS2 and, to a lesser extent, RGS4 are negative allosteric modulators of Galpha(q) binding to either p63RhoGEF or GRK2. Conversely, GRK2 enhances the GAP activity of RGS4 but has little effect on that of RGS2. Similar but smaller magnitude responses are induced by p63RhoGEF. The fact that GRK2 and p63RhoGEF respond similarly to these RGS proteins supports the hypothesis that GRK2 is a bona fide G alpha(q) effector. The results also suggest that signal transduction pathways initiated by GRK2, such as the phosphorylation of G protein-coupled receptors, and by p63RhoGEF, such as the activation of gene transcription, can be regulated by RGS proteins via both allosteric and GAP mechanisms.