Signal transduction. Fishing at the cell surface

Roles of Different Signaling Pathways in Cryptococcus neoformans Virulence

Cryptococcus neoformans is a widespread fungal pathogen that can infect the human central nervous system (CNS) and cause fungal meningitis, leading to hundreds of thousands of deaths worldwide each year. Previous studies have demonstrated that many signal transduction pathways are crucial for the morphological development and virulence of C. neoformans. In this review, data from over 116 research articles have been compiled to show that many signaling pathways control various characteristics of C. neoformans, individually or in association with other pathways, and to establish strong links among them to better understand C. neoformans pathogenesis. Every characteristic of C. neoformans is closely linked to these signaling pathways, making this a rich area for further research. It is essential to thoroughly explore these pathways to address questions that remain and apply a molecular mechanistic approach to link them. Targeting these pathways is crucial for understanding the exact mechanism of infection pathogenesis and will facilitate the development of antifungal drugs as well as the diagnosis and prevention of cryptococcosis.

Upstream Regulation of Development and Secondary Metabolism in Aspergillus Species

In filamentous fungal Aspergillus species, growth, development, and secondary metabolism are genetically programmed biological processes, which require precise coordination of diverse signaling elements, transcription factors (TFs), upstream and downstream regulators, and biosynthetic genes. For the last few decades, regulatory roles of these controllers in asexual/sexual development and primary/secondary metabolism of Aspergillus species have been extensively studied. Among a wide spectrum of regulators, a handful of global regulators govern upstream regulation of development and metabolism by directly and/or indirectly affecting the expression of various genes including TFs. In this review, with the model fungus Aspergillus nidulans as the central figure, we summarize the most well-studied main upstream regulators and their regulatory roles. Specifically, we present key functions of heterotrimeric G proteins and G protein-coupled receptors in signal transduction), the velvet family proteins governing development and metabolism, LaeA as a global regulator of secondary metabolism, and NsdD, a key GATA-type TF, affecting development and secondary metabolism and provide a snapshot of overall upstream regulatory processes underlying growth, development, and metabolism in Aspergillus fungi.

Transcription Factor VdCf2 Regulates Growth, Pathogenicity, and the Expression of a Putative Secondary Metabolism Gene Cluster in Verticillium dahliae

Transcription factors (TFs) bind to the promoters of target genes to regulate gene expression in response to different stimuli. The functions and regulatory mechanisms of transcription factors (TFs) in Verticillium dahliae are, however, still largely unclear. This study showed that a C2H2-type zinc finger TF, VdCf2 (V. dahliae chorion transcription factor 2), plays key roles in V. dahliae growth, melanin production, and virulence. Transcriptome sequencing analysis showed that VdCf2 was involved in the regulation of expression of genes encoding secreted proteins, pathogen-host interaction (PHI) homologs, TFs, and G protein-coupled receptors (GPCRs). Furthermore, VdCf2 positively regulated the expression of VdPevD1 (VDAG_02735), a previously reported virulence factor. VdCf2 thus regulates the expression of several pathogenicity-related genes that also contribute to virulence in V. dahliae. VdCf2 also inhibited the transcription of the Vd276-280 gene cluster and interacted with two members encoding proteins (VDAG_07276 and VDAG_07278) in the gene cluster. IMPORTANCE Verticillium dahliae is an important soilborne phytopathogen which can ruinously attack numerous host plants and cause significant economic losses. Transcription factors (TFs) were reported to be involved in various biological processes, such as hyphal growth and virulence of pathogenic fungi. However, the functions and regulatory mechanisms of TFs in V. dahliae remain largely unclear. In this study, we identified a new transcription factor, VdCf2 (V. dahliae chorion transcription factor 2), based on previous transcriptome data, which participates in growth, melanin production, and virulence of V. dahliae. We provide evidence that VdCf2 regulates the expression of the pathogenicity-related gene VdPevD1 (VDAG_02735) and Vd276-280 gene cluster. VdCf2 also interacts with VDAG_07276 and VDAG_07278 in this gene cluster based on a yeast two-hybrid and bimolecular fluorescence complementation assay. These results revealed the regulatory mechanisms of a pivotal pathogenicity-related transcription factor, VdCf2 in V. dahliae.

AaPKAc Regulates Differentiation of Infection Structures Induced by Physicochemical Signals From Pear Fruit Cuticular Wax, Secondary Metabolism, and Pathogenicity of Alternaria alternata

Alternaria alternata, the casual agent of black rot of pear fruit, can sense and respond to the physicochemical cues from the host surface and form infection structures during infection. To evaluate the role of cyclic AMP-dependent protein kinase (cAMP-PKA) signaling in surface sensing of A. alternata, we isolated and functionally characterized the cyclic adenosine monophosphate-dependent protein kinase A catalytic subunit gene (AaPKAc). Gene expression results showed that AaPKAc was strongly expressed during the early stages of appressorium formation on hydrophobic surfaces. Knockout mutants ΔAaPKAc were generated by replacing the target genes via homologous recombination events. We found that intracellular cAMP content increased but PKA content decreased in ΔAaPKAc mutant strain. Appressorium formation and infection hyphae were reduced in the ΔAaPKAc mutant strain, and the ability of the ΔAaPKAc mutant strain to recognize and respond to high hydrophobicity surfaces and different surface waxes was lower than in the wild type (WT) strain. In comparison with the WT strain, the appressorium formation rate of the ΔAaPKAc mutant strain on high hydrophobicity and fruit wax extract surface was reduced by 31.6 and 49.3% 4 h after incubation, respectively. In addition, AaPKAc is required for the hypha growth, biomass, pathogenicity, and toxin production of A. alternata. However, AaPKAc negatively regulated conidia formation, melanin production, and osmotic stress resistance. Collectively, AaPKAc is required for pre-penetration, developmental, physiological, and pathological processes in A. alternata.

G-protein-coupled Receptors in Fungi

G-protein-coupled receptors (GPCRs) are the largest family of transmembrane receptors in fungi. These receptors have an important role in the transduction of extracellular signals into intracellular sites in response to diverse stimuli. They enable fungi to coordinate cell function and metabolism, thereby promoting their survival and propagation, and sense certain fundamentally conserved elements, such as nutrients, pheromones, and stress, for adaptation to their niches, environmental stresses, and host environment, causing disease and pathogen virulence. This chapter highlights the role of GPCRs in fungi in coordinating cell function and metabolism. Fungal cells sense the molecular interactions between extracellular signals. Their respective sensory systems are described here in detail.

Sensing and transduction of nutritional and chemical signals in filamentous fungi: Impact on cell development and secondary metabolites biosynthesis

Filamentous fungi respond to hundreds of nutritional, chemical and environmental signals that affect expression of primary metabolism and biosynthesis of secondary metabolites. These signals are sensed at the membrane level by G protein coupled receptors (GPCRs). GPCRs contain usually seven transmembrane domains, an external amino terminal fragment that interacts with the ligand, and an internal carboxy terminal end interacting with the intracellular G protein. There is a great variety of GPCRs in filamentous fungi involved in sensing of sugars, amino acids, cellulose, cell-wall components, sex pheromones, oxylipins, calcium ions and other ligands. Mechanisms of signal transduction at the membrane level by GPCRs are discussed, including the internalization and compartmentalisation of these sensor proteins. We have identified and analysed the GPCRs in the genome of Penicillium chrysogenum and compared them with GPCRs of several other filamentous fungi. We have found 66 GPCRs classified into 14 classes, depending on the ligand recognized by these proteins, including most previously proposed classes of GPCRs. We have found 66 putative GPCRs, representatives of twelve of the fourteen previously proposed classes of GPCRs, depending on the ligand recognized by these proteins. A staggering fortytwo putative members of the new GPCR class XIV, the so-called Pth11 sensors of cellulosic material as reported for Neurospora crassa and some other fungi, were identified. Several GPCRs sensing sex pheromones, known in yeast and in several fungi, were also identified in P. chrysogenum, confirming the recent unravelling of the hidden sexual capacity of this species. Other sensing mechanisms do not involve GPCRs, including the two-component systems (HKRR), the HOG signalling system and the PalH mediated pH transduction sensor. GPCR sensor proteins transmit their signals by interacting with intracellular heterotrimeric G proteins, that are well known in several fungi, including P. chrysogenum. These G proteins are inactive in the GDP containing heterotrimeric state, and become active by nucleotide exchange, allowing the separation of the heterotrimeric protein in active Gα and Gβγ dimer subunits. The conversion of GTP in GDP is mediated by the endogenous GTPase activity of the G proteins. Downstream of the ligand interaction, the activated Gα protein and also the Gβ/Gγ dimer, transduce the signals through at least three different cascades: adenylate cyclase/cAMP, MAPK kinase, and phospholipase C mediated pathways.

Genome-wide comparative analysis of putative Pth11-related G protein-coupled receptors in fungi belonging to Pezizomycotina

Background

G-protein coupled receptors (GPCRs) are the largest family of transmembrane receptors in fungi, where they play important roles in signal transduction. Among them, the Pth11-related GPCRs form a large and divergent protein family, and are only found in fungi in Pezizomycotina. However, the evolutionary process and potential functions of Pth11-related GPCRs remain largely unknown.

Results

Twenty genomes of fungi in Pezizomycotina covering different nutritional strategies were mined for putative Pth11-related GPCRs. Phytopathogens encode much more putative Pth11-related GPCRs than symbionts, saprophytes, or entomopathogens. Based on the phylogenetic tree, these GPCRs can be divided into nine clades, with each clade containing fungi in different taxonomic orders. Instead of fungi from the same order, those fungi with similar nutritional strategies were inclined to share orthologs of putative Pth11-related GPCRs. Most of the CFEM domain-containing Pth11-related GPCRs, which were only included in two clades, were detected in phytopathogens. Furthermore, many putative Pth11-related GPCR genes of phytopathogens were upregulated during invasive plant infection, but downregulated under biotic stress. The expressions of putative Pth11-related GPCR genes of saprophytes and entomopathogens could be affected by nutrient conditions, especially the carbon source. The gene expressions revealed that Pth11-related GPCRs could respond to biotic/abiotic stress and invasive plant infection with different expression patterns.

Conclusion

Our results indicated that the Pth11-related GPCRs existed before the diversification of Pezizomycotina and have been gained and/or lost several times during the evolutionary process. Tandem duplications and trophic variations have been important factors in this evolution.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-017-1076-5) contains supplementary material, which is available to authorized users.

WISH, a novel CFEM GPCR is indispensable for surface sensing, asexual and pathogenic differentiation in rice blast fungus

We have selected and characterized a unique Conserved Fungal-specific Extra-cellular Membrane-spanning (CFEM) domain containing PTH11 like G-protein coupled receptor (GPCR), which is responsible for Water wettability, Infection, Surface sensing and Hyper-conidiation (WISH). The pathogenicity gene WISH is predicted to encode a novel seven transmembrane protein in the rice blast fungus, Magnaporthe oryzae, one of the deadliest pathogens of rice. We generated knockout mutants through a homologous recombination-based method to understand the function of the gene. These mutants are nonpathogenic due to a defect in sensing hydrophobic surface and appressorium differentiation. The mutant failed to undergo early events of pathogenesis, and appressorium development is diminished on inductive hydrophobic surface and was unable to penetrate susceptible rice leaves. The Δwish mutant did not develop any appressorium, suggesting that WISH protein is required for appressorium morphogenesis and is also involved in host surface recognition. We examined various aspects of pathogenesis and the results indicated involvement of WISH in preventing autolysis of vegetative hyphae, determining surface hydrophobicity and maintenance of cell-wall integrity. WISH gene from M. oryzae strain B157 complemented the Δwish mutant, indicating functional authenticity. Exogenous activation of cellular signaling failed to suppress the defects in Δwish mutants. These findings suggest that WISH GPCR senses diverse extracellular signals to play multiple roles and might have effects on PTH11 and MPG1 genes especially as an upstream effector of appressorium differentiation. It is for the first time that a typical GPCR containing seven transmembrane helices involved in the early events of plant pathogenesis of M. oryzae has been functionally characterized.

Metabolic activity in dormant conidia of Aspergillus niger and developmental changes during conidial outgrowth

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Resting conidia are demonstrated to be metabolically active.

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After triggering of conidial outgrowth fermentation occurs, followed by respiration.

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Sorbic acid inhibits O₂ uptake and delays the onset of respiration.

The early stages of development of Aspergillus niger conidia during outgrowth were explored by combining genome-wide gene expression analysis (RNAseq), proteomics, Warburg manometry and uptake studies. Resting conidia suspended in water were demonstrated for the first time to be metabolically active as low levels of oxygen uptake and the generation of carbon dioxide were detected, suggesting that low-level respiratory metabolism occurs in conidia for maintenance. Upon triggering of spore germination, generation of CO₂ increased dramatically. For a short period, which coincided with mobilisation of the intracellular polyol, trehalose, there was no increase in uptake of O₂ indicating that trehalose was metabolised by fermentation. Data from genome-wide mRNA profiling showed the presence of transcripts associated with fermentative and respiratory metabolism in resting conidia. Following triggering of conidial outgrowth, there was a clear switch to respiration after 25 min, confirmed by cyanide inhibition. No effect of SHAM, salicylhydroxamic acid, on respiration suggests electron flow via cytochrome c oxidase. Glucose entry into spores was not detectable before 1 h after triggering germination. The impact of sorbic acid on germination was examined and we showed that it inhibits glucose uptake. O₂ uptake was also inhibited, delaying the onset of respiration and extending the period of fermentation. In conclusion, we show that conidia suspended in water are not completely dormant and that conidial outgrowth involves fermentative metabolism that precedes respiration.

G-protein coupled receptor-mediated nutrient sensing and developmental control in Aspergillus nidulans

Nutrient sensing and utilisation are fundamental for all life forms. As heterotrophs, fungi have evolved a diverse range of mechanisms for sensing and taking up various nutrients. Despite its importance, only a limited number of nutrient receptors and their corresponding ligands have been identified in fungi. G-protein coupled receptors (GPCRs) are the largest family of transmembrane receptors. The Aspergillus nidulans genome encodes 16 putative GPCRs, but only a few have been functionally characterised. Our previous study showed the increased expression of an uncharacterised putative GPCR, gprH, during carbon starvation. GprH appears conserved throughout numerous filamentous fungi. Here, we reveal that GprH is a putative receptor involved in glucose and tryptophan sensing. The absence of GprH results in a reduction in cAMP levels and PKA activity upon adding glucose or tryptophan to starved cells. GprH is pre-formed in conidia and is increasingly active during carbon starvation, where it plays a role in glucose uptake and the recovery of hyphal growth. GprH also represses sexual development under conditions favouring sexual fruiting and during carbon starvation in submerged cultures. In summary, the GprH nutrient-sensing system functions upstream of the cAMP-PKA pathway, influences primary metabolism and hyphal growth, while represses sexual development in A. nidulans.

Global gene expression and focused knockout analysis reveals genes associated with fungal fruiting body development in Neurospora crassa

Fungi can serve as highly tractable models for understanding genetic basis of sexual development in multicellular organisms. Applying a reverse-genetic approach to advance such a model, we used random and multitargeted primers to assay gene expression across perithecial development in Neurospora crassa. We found that functionally unclassified proteins accounted for most upregulated genes, whereas downregulated genes were enriched for diverse functions. Moreover, genes associated with developmental traits exhibited stage-specific peaks of expression. Expression increased significantly across sexual development for mating type gene mat a-1 and for mat A-1 specific pheromone precursor ccg-4. In addition, expression of a gene encoding a protein similar to zinc finger, stc1, was highly upregulated early in perithecial development, and a strain with a knockout of this gene exhibited arrest at the same developmental stage. A similar expression pattern was observed for genes in RNA silencing and signaling pathways, and strains with knockouts of these genes were also arrested at stages of perithecial development that paralleled their peak in expression. The observed stage specificity allowed us to correlate expression upregulation and developmental progression and to identify regulators of sexual development. Bayesian networks inferred from our expression data revealed previously known and new putative interactions between RNA silencing genes and pathways. Overall, our analysis provides a fine-scale transcriptomic landscape and novel inferences regarding the control of the multistage development process of sexual crossing and fruiting body development in N. crassa.

Genome-wide prediction of G protein-coupled receptors in Verticillium spp

G protein-coupled receptors (GPCRs) are critical factors in regulating morphogenesis, mating, infection and virulence in fungi. In this study, various computational strategies were applied to identify GPCR-like proteins from the genomes of both Verticillium dahliae and Verticillium albo-atrum. The putative GPCRs were distributed over 13 classes, and significantly, three of those represented novel classes of GPCR-like proteins in fungi. The three novel GPCRs had high levels of identity to their counterparts in higher eukaryotes, including Homo sapiens. The numbers of GPCR-like proteins in the two Verticillium spp. were similar to those seen in other filamentous fungi, such as Magnaporthe grisea, Neurospora crassa and Fusarium graminearum. Additionally, the carbon/amino acid receptors were divided into three different subclasses, indicating that differences among the GPCRs existed not only among different classes but also within classes. In conclusion, the identification and classification of GPCRs and their homology to some well-studied fungi will be an important starting point for future research in Verticillium spp.

Membrane progestin receptor beta (mPR-beta): a protein related to cumulus expansion that is involved in in vitro maturation of pig cumulus-oocyte complexes

A new group of putative membrane receptors have now been isolated from fish and other vertebrates, including human. These proteins are classified into three groups known as membrane progestin receptor alpha, beta and gamma (mPR-alpha, -beta and -gamma). In the present study we have investigated the role of mPR-beta in regulating in vitro maturation (IVM) of pig cumulus-oocyte complexes (COCs). RT-PCR and Western blot analysis indicated that COCs contain transcripts and proteins for mPR-beta. The levels of both transcripts and proteins increased between 0 and 20h IVM, but then decreased between 20 and 44h. The luteinizing hormone (LH) and follicle-stimulating hormone (FSH) did not affect mPR-beta expression during IVM. Immunofluorescence analysis indicated that the mPR-beta was localized in the plasma membrane of cumulus cell. However, in mouse embryonic fibroblasts (MEFs), mPR-beta was detected at the endoplasmic reticulum (ER) rather than the plasma membrane. Cumulus expansion was impaired significantly (P<0.05) when COCs were incubated in maturation medium containing 10% (v/v) anti-mPR-beta serum during IVM. Bioinformatics analysis predicted that mPR-beta had an ER retention motif and an endocytosis internalization motif. These results suggest that the mPR-beta is a molecule related to cumulus expansion and it might function by regulation of exocytosis. In conclusion, this is the first description of the expression patterns and subcellular localization of mPR-beta in COCs and might shed light on the function of the protein.

Heterologous expression of human mPRalpha, mPRbeta and mPRgamma in yeast confirms their ability to function as membrane progesterone receptors

The nuclear progesterone receptor (nPR) mediates many of the physiological effects of progesterone by regulating the expression of genes, however, progesterone also exerts non-transcriptional (non-genomic) effects that have been proposed to rely on a receptor that is distinct from nPR. Several members of the progestin and AdipoQ-Receptor (PAQR) family were recently identified as potential mediators of these non-genomic effects. Membranes from cells expressing these proteins, called mPRalpha, mPRbeta and mPRgamma, were shown to specifically bind progesterone and have G-protein coupled receptor (GPCR) characteristics, although other studies dispute these findings. To clarify the role of these mPRs in non-genomic progesterone signaling, we established an assay for PAQR functional evaluation using heterologous expression in Saccharomyces cerevisiae. Using this assay, we demonstrate unequivocally that mPRalpha, mPRbeta and mPRgamma can sense and respond to progesterone with EC(50) values that are physiologically relevant. Agonist profiles also show that mPRalpha, mPRbeta and mPRgamma are activated by ligands, such as 17alpha-hydroxyprogesterone, that are known to activate non-genomic pathways but not nPR. These results strongly suggest that these receptors may indeed function as the long-sought-after membrane progesterone receptors. Additionally, we show that two uncharacterized PAQRs, PAQR6 and PAQR9, are also capable of responding to progesterone. These mPR-like PAQRs have been renamed as mPRdelta (PAQR6) and mPRvarepsilon (PAQR9). Additional characterization of mPRgamma and mPRalpha indicates that their progesterone-dependent signaling in yeast does not require heterotrimeric G-proteins, thus calling into question the characterization of the mPRs as a novel class of G-protein coupled receptor.

Magnificent seven: roles of G protein-coupled receptors in extracellular sensing in fungi

G protein-coupled receptors (GPCRs) represent the largest family of transmembrane receptors and are responsible for transducing extracellular signals into intracellular responses that involve complex intracellular-signaling networks. This review highlights recent research advances in fungal GPCRs, including classification, extracellular sensing, and G protein-signaling regulation. The involvement of GPCRs in pheromone and nutrient sensing has been studied extensively over the past decade. Following recent advances in fungal genome sequencing projects, a panoply of GPCR candidates has been revealed and some have been documented to play key roles sensing diverse extracellular signals, such as pheromones, sugars, amino acids, nitrogen sources, and even photons. Identification and deorphanization of additional putative GPCRs may require the development of new research tools. Here, we compare research on GPCRs in fungi with information derived from mammalian systems to provide a useful road map on how to better understand ligand-GPCR-G protein interactions in general. We also emphasize the utility of yeast as a discovery tool for systemic studies of GPCRs from other organisms.

Estradiol and progesterone regulate HIV type 1 replication in peripheral blood cells

Endogenous levels of estradiol and progesterone fluctuate in the peripheral blood of premenopausal women during the reproductive cycle. We studied the effects of these sex hormones on HIV-1 replication in peripheral blood mononuclear cells (PBMCs). We compared HIV-1 replication in PBMCs infected in the presence of mid-secretory (high concentrations) and mid-proliferative (low concentrations) or in the absence of sex hormones. With PBMCs from men, we used concentrations of estradiol and progesterone that are normally present in their plasma. Our findings demonstrate that mid-proliferative phase conditions increased, and mid-secretory phase conditions decreased, HIV-1 replication. To determine if sex hormones affect specific stages of the viral life cycle we performed real-time PCR assays and found decreased levels of HIV-1 integration in the mid-secretory phase and increased levels viral transcription in the mid-proliferative phase. No significant effects on HIV-1 reverse transcription or on CCR5 expression were found. In addition, we assessed hormonal regulation of the HIV-1 LTR in the absence of the viral regulatory protein Tat. We observed that mid-proliferative hormone levels enhanced, whereas mid-secretory hormone concentrations reduced, the activity of the LTR. These findings demonstrate that in HIV-1-infected cells, estradiol and progesterone regulate HIV-1 replication most likely by directly altering HIV-1 transcriptional activation. An additional indirect mechanism of sex hormone regulation of cytokine and chemokine secretion cannot be excluded.

Propofol depresses angiotensin II-induced cardiomyocyte hypertrophy in vitro

Cardiomyocyte hypertrophy is formed in response to pressure or volume overload, injury, or neurohormonal activation. The most important vascular hormone that contributes to the development of hypertrophy is angiotensin II (Ang II). Accumulating studies have suggested that reactive oxygen species (ROS) may play an important role in cardiac hypertrophy. Propofol is a general anesthetic that possesses antioxidant action. We therefore examined whether propofol inhibited Ang II-induced cardiomyocyte hypertrophy. Our results showed that both ROS formation and hypertrophic responses induced by Ang II in cardiomyocytes were partially blocked by propofol. Further studies showed that propofol inhibited the phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) and mitogen-activated protein kinase/ERK kinase 1/2 (MEK1/2) induced by Ang II via a decrease in ROS production. In addition, propofol also markedly attenuated Ang II-stimulated nuclear factor-kappaB (NF-kappaB) activation via a decrease in ROS production. In conclusion, propofol prevents cardiomyocyte hypertrophy by interfering with the generation of ROS and involves the inhibition of the MEK/ERK signaling transduction pathway and NF-kappaB activation.

Expression of progesterone and oestrogen receptors by early intrauterine equine conceptuses

Progesterone and oestrogen play essential roles in the maintenance of pregnancy in eutherian mammals and are thought to exert their effects on the developing conceptus indirectly, via the endometrium. In some species, early embryos have themselves been shown to express steroid receptors, thereby suggesting that reproductive steroids may also influence embryonic development directly. The aim of this study was to determine whether early intrauterine equine conceptuses express either the classical intracellular progesterone (PR) and oestrogen receptors (ERalpha and ERbeta) or the more recently characterised membrane-bound progesterone receptors (PGRMC1 and mPR). Horse conceptuses recovered on days 7, 10 and 14 after ovulation (n=8 at each stage) were examined for steroid receptor mRNA expression using quantitative rtPCR. Where commercial antibodies were available (PR, ERbeta), receptor localisation was examined immunohistochemically in day 10, 12, 14, 15 and 16 conceptuses (n=2 at each stage). mRNA for PR, PGRMC1 and mPR was detected at all stages examined, but while PGRMC1 and mPR expression increased during the day 7-14 period, PR expression decreased. ERalpha mRNA was not detected at any stage examined, whereas ERbeta mRNA was detected in all day 14, some day 10 and no day 7 conceptuses. Immunoreactive ERbeta receptors were localised to the trophectoderm of day 14-16 conceptuses; PR were not detected immunohistochemically in conceptus tissue. In summary, this study demonstrates that equine conceptuses express mRNA and, in the case of ERbeta, protein for steroid hormone receptors during the period encompassing rapid conceptus growth, differentiation and maternal pregnancy recognition.

Steroid and G protein binding characteristics of the seatrout and human progestin membrane receptor alpha subtypes and their evolutionary origins

A novel progestin receptor (mPR) with seven-transmembrane domains was recently discovered in spotted seatrout and homologous genes were identified in other vertebrates. We show that cDNAs for the mPR alpha subtypes from spotted seatrout (st-mPRalpha) and humans (hu-mPRalpha) encode progestin receptors that display many functional characteristics of G protein-coupled receptors. Flow cytometry and immunocytochemical staining of whole MDA-MB-231 cells stably transfected with the mPRalphas using antibodies directed against their N-terminal regions show the receptors are localized on the plasma membrane and suggest the N-terminal domain is extracellular. Both recombinant st-mPRalpha and hu-mPRalpha display high affinity (Kd 4.2-7.8 nm), limited capacity (Bmax 0.03-0.32 nm), and displaceable membrane binding specific for progestins. Progestins activate a pertussis toxin-sensitive inhibitory G protein (G(i)) to down-regulate membrane-bound adenylyl cyclase activity in both st-mPRalpha- and hu-mPRalpha-transfected cells. Coimmunoprecipitation experiments demonstrate the receptors are directly coupled to the G(i) protein. Similar to G protein-coupled receptors, dissociation of the receptor/G protein complex results in a decrease in ligand binding to the mPRalphas and mutation of the C-terminal, and third intracellular loop of st-mPRalpha causes loss of ligand-dependent G protein activation. Phylogenetic analysis indicates the mPRs are members of a progesterone and adipoQ receptor (PAQR) subfamily that is only present in chordates, whereas other PAQRs also occur in invertebrates and plants. Progesterone and adipoQ receptors are related to the hemolysin3 family and have origins in the Eubacteria. Thus, mPRs arose from Eubacteria independently from members of the GPCR superfamily, which arose from Archeabacteria, suggesting convergent evolution of seven-transmembrane hormone receptors coupled to G proteins.

ClassicalXenopus laevis progesterone receptor associates to the plasma membrane through its ligand-binding domain

During the last decade, considerable evidence is accumulating that supports the view that the classic progesterone receptor (xPR-1) is mediating Xenopus laevis oocyte maturation through a non-genomic mechanism. Overexpression and depletion of oocyte xPR-1 have been shown to accelerate and to block progesterone-induced oocyte maturation, respectively. In addition, rapid inhibition of plasma membrane adenylyl cyclase (AC) by the steroid hormone, supports the idea that xPR-1 should be localized at the oocyte plasma membrane. To test this hypothesis, we transiently transfected xPR-1 cDNA into Cos-7 cells and analyzed its subcellular distribution. Through Western blot and immunofluorescence analysis, we were able to detect xPR-1 associated to the plasma membrane of transfected Cos-7 cells. Additionally, using Progesterone-BSA-FITC, we identified specific progesterone-binding sites at the cell surface of xPR-1 expressing cells. Finally, we found that the receptor ligand-binding domain displayed membrane localization, in contrast to the N-terminal domain, which expressed in similar levels, remained cytosolic. Overall, these results indicate that a fraction of xPR-1 expressed in Cos-7 cells, associates to the plasma membrane through its LBD.

The role of Xenopus membrane progesterone receptor beta in mediating the effect of progesterone on oocyte maturation

Rapid, nongenomic membranal effects of progesterone were demonstrated in amphibian oocytes more than 30 y ago. Recently, a distinct family of membrane progestin receptors (mPRs) has been cloned in fish and other vertebrate species. In this study we explore the role of mPR in promoting oocyte maturation in Xenopus laevis. RT-PCR analysis indicates that Xenopus oocytes contain transcripts for the mPRbeta ortholog, similar to what has been reported in zebrafish oocytes, and Western blotting shows that the protein is expressed on the oocyte plasma membrane. Microinjection of mPRbeta-specific antibodies into oocytes resulted in a dramatic inhibition of progesterone-dependent oocyte maturation, whereas microinjection of mRNA encoding Myc-Xenopus mPR (XmPR)beta resulted in an accelerated rate of progesterone-induced oocyte maturation, concomitant with membranal localization of the protein. Binding studies in mammalian cells expressing XmPRbeta confirmed specific binding of progesterone by the expressed protein. These results suggest that XmPRbeta is a physiological progesterone receptor involved in initiating the resumption of meiosis during maturation of Xenopus oocytes.

Human Homologs of the Putative G Protein-Coupled Membrane Progestin Receptors (mPRα, β, and γ) Localize to the Endoplasmic Reticulum and Are Not Activated by Progesterone

The steroid hormone progesterone exerts pleiotrophic functions in many cell types. Although progesterone controls transcriptional activation through binding to its nuclear receptors, it also initiates rapid nongenomic signaling events. Recently, three putative membrane progestin receptors (mPRα, β, and γ) with structural similarity to G protein-coupled receptors have been identified. These mPR isoforms are expressed in a tissue-specific manner and belong to the larger, highly conserved family of progestin and adiponectin receptors found in plants, eubacteria, and eukaryotes. The fish mPRα has been reported to mediate progesterone-dependent MAPK activation and inhibition of cAMP production through coupling to an inhibitory G protein. To functionally characterize the human homologs, we established human embryonic kidney 293 and MDA-MB-231 cell lines that stably express human mPRα, β, or γ. For comparison, we also established cell lines expressing the mPRα cloned from the spotted seatrout (Cynoscion nebulosus) and Japanese pufferfish (Takifugu rubripes). Surprisingly, we found no evidence that human or fish mPRs regulate cAMP production or MAPK (ERK1/2 or p38) activation upon progesterone stimulation. Furthermore, the mPRs did not couple to a highly promiscuous G protein subunit, Gαq5i, in transfection studies or provoke Ca2+ mobilization in response to progesterone. Finally, we demonstrate that transfected mPRs, as well as endogenous human mPRα, localize to the endoplasmic reticulum, and that their expression does not lead to increased progestin binding either in membrane preparations or in intact cells. Our results therefore do not support the concept that mPRs are plasma membrane receptors involved in transducing nongenomic progesterone actions.

Two unrelated putative membrane-bound progestin receptors, progesterone membrane receptor component 1 (PGMRC1) and membrane progestin receptor (mPR) beta, are expressed in the rainbow trout oocyte and exhibit similar ovarian expression patterns

BACKGROUND

In lower vertebrates, steroid-induced oocyte maturation is considered to involve membrane-bound progestin receptors. Two totally distinct classes of putative membrane-bound progestin receptors have been reported in vertebrates. A first class of receptors, now termed progesterone membrane receptor component (PGMRC; subtypes 1 and 2) has been studied since 1996 but never studied in a fish species nor in the oocyte of any animal species. A second class of receptors, termed membrane progestin receptors (mPR; subtypes alpha, beta and gamma), was recently described in vertebrates and implicated in the progestin-initiated induction of oocyte maturation in fish.

METHODS

In the present study, we report the characterization of the full coding sequence of rainbow trout PGMRC1 and mPR beta cDNAs, their tissue distribution, their ovarian expression profiles during oogenesis, their hormonal regulation in the full grown ovary and the in situ localization of PGMRC1 mRNA in the ovary.

RESULTS

Our results clearly show, for the first time in any animal species, that rainbow trout PGMRC1 mRNA is present in the oocyte and has a strong expression in ovarian tissue. In addition, we show that both mPR beta and PGMRC1, two members of distinct membrane-bound progestin receptor classes, exhibit highly similar ovarian expression profiles during the reproductive cycle with maximum levels during vitellogenesis and a down-expression during late vitellogenesis. In addition, the mRNA abundance of both genes is not increased after in vitro hormonal stimulation of full grown follicles by maturation inducing hormones.

CONCLUSION

Together, our findings suggest that PGMRC1 is a new possible participant in the progestin-induced oocyte maturation in fish. However, its participation in the process of oocyte maturation, which remains to be confirmed, would occur at post-transcriptional levels.

B-Raf and C-Raf are required for Ras-stimulated p42 MAP kinase activation in Xenopus egg extracts

During mitosis, a select pool of MEK1 and p42/p44 MAPK becomes activated at the kinetochores and spindle poles, without substantial activation of the bulk of the cytoplasmic p42/p44 MAPK. Recently, we set out to identify the MAP kinase kinase kinase (MAPKKK) responsible for this mitotic activation, using cyclin-treated Xenopus egg extracts as a model system, and presented evidence that Mos was the relevant MAPKKK . However, a second MAPKKK distinct from Mos was readily detectable as well. Here, we partially purify this second MAPKKK and identify it as B-Raf. No changes in the activity of B-Raf were detectable during progesterone-induced oocyte maturation, after egg fertilization, or during the early embryonic cell cycle, arguing against a role for B-Raf in the mitotic activation of MEK1 and p42 MAPK. Ras proteins can bring about activation of MEK1 and p42 MAPK in extracts, and Ras may contribute to signaling from the classical progesterone receptor during oocyte maturation and from receptor tyrosine kinases during early embryogenesis. We found that both B-Raf and C-Raf, but not Mos, are required for Ras-induced MEK1 and p42 MAPK activation. These data indicate that two upstream stimuli, active Ras and active Cdc2, utilize different MAPKKKs to activate MEK1 and p42 MAPK.

Expression and regulation of progestin membrane receptors in the rat corpus luteum

Despite evidence strongly supporting progesterone's autocrine actions in the rat corpus luteum (CL), classical progesterone receptors (PR) have not been detected in this gland. Alternatively, in several other systems, progestins have been reported to activate nongenomic pathways via putative progestin membrane receptors (PMRs). The aim of this investigation was to determine whether rat CL membranes bind progestins and contain PMR homologs and whether these proteins are expressed during CL development in a manner that parallels luteal function. We found that luteal cell membranes specifically bind progesterone. Low levels of progesterone and 20alpha-dihydroprogesterone decreased binding of [(3)H]progesterone, whereas androstenedione, 17alpha-hydroxyprogesterone, and pregnenolone were less potent. Other steroids, including corticosterone, mifepristone, and estradiol, were ineffective. We found that the rat CL expresses five genes previously postulated to encode for putative PMRs: PMRalpha, PMRbeta, PMRgamma, PR membrane component 1 (PRMC1), and Rda288. Pmralpha, Pmrgamma, and Prmc1 transcripts rose steadily during pregnancy whereas Pmrbeta and Rda288 remained constant. Just before parturition, concomitant with falling progesterone levels, Pmralpha, Pmrbeta, and Prmc1 decreased. Luteal PMRalpha and PRMC1 protein levels were lower in samples taken at the end of pregnancy compared with midpregnancy samples. Ergocriptine, which inhibits the secretion of prolactin, the primary luteotrophic hormone in the rat CL, reduced Pmralpha, Pmrbeta, and Prmc1 expression significantly. Ergocriptine effects were prevented by coadministration of prolactin. These findings provide evidence for the expression and regulation of putative membrane-bound progestin-binding proteins in the rat CL, a tissue that does not express detectable levels of nuclear progesterone receptors.

G protein-coupled receptor Gpr4 senses amino acids and activates the cAMP-PKA pathway in Cryptococcus neoformans

The Galpha protein Gpa1 governs the cAMP-PKA signaling pathway and plays a central role in virulence and differentiation in the human fungal pathogen Cryptococcus neoformans, but the signals and receptors that trigger this pathway were unknown. We identified seven putative proteins that share identity with known G protein-coupled receptors (GPCRs). One protein, Gpr4, shares limited sequence identity with the Dictyostelium discoideum cAMP receptor cAR1 and the Aspergillus nidulans GPCR protein GprH and also shares structural similarity with the Saccharomyces cerevisiae receptor Gpr1. gpr4 mutants exhibited reduced capsule production and mating defects, similar to gpa1 mutants, and exogenous cAMP suppressed both gpr4 mutant phenotypes. Epistasis analysis provides further evidence that Gpr4 functions upstream of the Galpha subunit Gpa1. Gpr4-Gpr4 homomeric interactions were observed in the yeast two-hybrid assay, and Gpr4 was shown to physically interact with Gpa1 in the split-ubiquitin system. A Gpr4::DsRED fusion protein was localized to the plasma membrane and methionine was found to trigger receptor internalization. The analysis of intracellular cAMP levels showed that gpr4 mutants still respond to glucose but not to certain amino acids, such as methionine. Amino acids might serve as ligands for Gpr4 and could contribute to engage the cAMP-PKA pathway. Activation of the cAMP-PKA pathway by glucose and amino acids represents a nutrient coincidence detection system shared in other pathogenic fungi.

Membrane 5alpha-pregnane-3,20-dione (5alphaP) receptors in MCF-7 and MCF-10A breast cancer cells are up-regulated by estradiol and 5alphaP and down-regulated by the progesterone metabolites, 3alpha-dihydroprogesterone and 20alpha-dihydroprogesterone, with associated changes in cell proliferation and detachment

Previous studies have shown that the progesterone metabolite, 5alpha-pregnane-3,20-dione (5alphaP), exhibits mitogenic and metastatic activity in breast cell lines and that specific, high affinity receptors for 5alphaP are located in the plasma membrane fractions of tumorigenic (ER/PR-positive) MCF-7 cells. The aim of this study was to determine the effects of the mitogenic (estradiol; 5alphaP) and anti-mitogenic (3alpha-hydroxy-4-pregnen-20-one, 3alphaHP; 20alpha-hydroxy-4-pregnen-3-one, 20alphaHP) endogenous steroid hormones on 5alphaP receptor (5alphaP-R) numbers and on cell proliferation and adhesion of MCF-7 and MCF-10A cells. Exposure of MCF-7 cells for 24h to estradiol or 5alphaP resulted in significant (p < 0.05-0.001) dose-dependent increases in 5alphaP-R levels. Conversely, treatment with 3alphaHP or 20alphaHP resulted in significant (p < 0.05-0.01) dose-dependent decreases in 5alphaP-R levels. Treatment with one mitogenic and one anti-mitogenic hormone resulted in inhibition of the mitogen-induced increases, whereas treatment with two mitogenic or two anti-mitogenic hormones resulted in additive effects on 5alphaP-R numbers. Treatments with cycloheximide and actinomycin D indicate that changes in 5alphaP-R levels depend upon transcription and translation. The non-tumorigenic breast cell line, MCF-10A, was also shown to posses specific, high affinity plasma membrane receptors for 5alphaP that were up-regulated by estradiol and 5alphaP and down-regulated by 3alphaHP. Estradiol binding was demonstrated in MCF-10A cell membrane fractions and may explain the estradiol action in these cells that lack intracellular ER. In both MCF-7 and MCF-10A cells, the increases in 5alphaP-R due to estradiol or 5alphaP, and decreases due to 3alphaHP or 20alphaHP correlate with respective increases and decreases in cell proliferation as well as detachment. These results show distribution of 5alphaP-R in several cell types and they provide further evidence of the significance of progesterone metabolites and their novel membrane-associated receptors in breast cancer stimulation and control. The findings that 3alphaHP and 20alphaHP down-regulate 5alphaP-R and suppress mitogenic and metastatic activity suggest that these endogenous anti-mitogenic progesterone metabolites deserve considerations in designing new breast cancer therapeutic agents.

Transcription-dependent and transcription-independent functions of the classical progesterone receptor in Xenopus ovaries

Two forms of the classical progesterone receptors (PR), XPR-1 and XPR-2, have been cloned in Xenopus laevis. Their relative roles in mediating progesterone action in the ovaries are not clear. Using antibodies generated against the cloned XPR-2, we demonstrated here that the somatic follicle cells expressed an 80-kDa PR protein, termed XPR-1. Treatment of follicle cells with progesterone resulted in disappearance of this protein, consistent with proteosome-mediated XPR-1 protein degradation. A smaller (approximately 70 kDa) PR protein, termed XPR-2, was expressed in the oocytes, but not in follicle cells. XPR-2 underwent progesterone-induced phosphorylation but not protein degradation. Treating isolated ovarian fragments with progesterone caused oocyte maturation and the release of the mature oocytes from the ovarian tissues ("ovulation"). Inhibition of transcription, with actinomycin D, did not interfere with progesterone-induced oocyte maturation but blocked "ovulation" so that mature oocytes were trapped in the follicles. These results suggest that the dual functions of progesterone, transcription-dependent follicle rupture and transcription-independent oocyte maturation, are mediated by the two forms of PR proteins differentially expressed in the follicle cells and the oocytes, respectively.

A serotonin receptor antagonist induces oocyte maturation in both frogs and mice: evidence that the same G protein-coupled receptor is responsible for maintaining meiosis arrest in both species

Accumulating evidence has indicated that vertebrate oocytes are arrested at late prophase (G2 arrest) by a G protein coupled receptor (GpCR) that activates adenylyl cyclases. However, the identity of this GpCR or its regulation in G2 oocytes is unknown. We demonstrated that ritanserin (RIT), a potent antagonist of serotonin receptors 5-HT2R and 5-HT7R, released G2 arrest in denuded frog oocytes, as well as in follicle-enclosed mouse oocytes. In contrast to RIT, several other serotonin receptor antagonists (mesulergine, methiothepine, and risperidone) had no effect on oocyte maturation. The unique ability of RIT, among serotonergic antagonists, to induce GVBD did not match the antagonist profile of any known serotonin receptors including Xenopus 5-HT7R, the only known G(s)-coupled serotonin receptor cloned so far in this species. Unexpectedly, injection of x5-HT7R mRNA in frog oocytes resulted in hormone-independent frog oocyte maturation. The addition of exogenous serotonin abolished x5-HT7R-induced oocyte maturation. Furthermore, the combination of x5-HT7R and exogenous serotonin potently inhibited progesterone-induced oocyte maturation. These results provide the first evidence that a G-protein coupled receptor related to 5-HT7R may play a pivotal role in maintaining G2 arrest in vertebrate oocytes.

A glance at G-protein-coupled receptors for lipid mediators: a growing receptor family with remarkably diverse ligands

A plethora of lipid-like molecules known to act as intracellular second messengers are now recognized to signal cells through plasma membrane 7 transmembrane G-protein-coupled receptors (GPCRs). This has been the result of a decade-long genetic hunt for novel sequences encoding 7 transmembrane receptor proteins and the efforts to pair novel sequences with biologically active substances of (partly) unknown molecular mechanism of action. Identification of novel GPCR ligand pairs represents the first step to shed more light into the mode of action of novel cellular signaling molecules in human health and disease and might represent a fruitful source for the development of new drugs, judged on the successful history of GPCR as drug targets. Since 2000, more than 16 reports became available on lipid mediators--as diverse as lysophospholipids, arachidonic acid metabolites, short-, medium-, and long-chain fatty acids as well as steroid-like molecules--exerting their effects as extracellular mediators via rhodopsin-like family GPCRs. These reports have opened new avenues for research in human lipid receptor physiology and pharmacology. Here, the current knowledge on the recently deorphanized lipid receptors, including their isolation, expression pattern, function, and possible physiological or pathological roles will be reviewed.

Mechanisms of thyroid hormone receptor-specific nuclear and extra nuclear actions

Triiodothyronine (T3) classically regulates gene expression by binding to high-affinity thyroid hormone receptors (TR) that recognize specific response elements in the promoters of T3-target genes and activate or repress transcription in response to hormone. However, a number of thyroid hormone effects occur rapidly and are unaffected by inhibitors of transcription and translation, suggesting that thyroid hormones may also mediate non-genomic actions. Such actions have been described in many tissues and cell types, including brown adipose tissue, the heart and pituitary. The site of non-genomic hormone action has been localized to the plasma membrane, cytoplasm and cellular organelles. These non-genomic actions include the regulation of ion channels, oxidative phosphorylation and mitochondrial gene transcription and involve the generation of intracellular secondary messengers and induction of [Ca(2+)](I), cyclic AMP or protein kinase signalling cascades. These observations have been interpreted to imply the presence of a specific, membrane associated, TR isoform or an unrelated high affinity membrane receptor for thyroid hormone. The recent identification of a progestin membrane receptor and the sub cellular targeted nuclear receptor isoforms ER46, mtRXR, mtPPAR, p28 and p46, has highlighted the potential importance of non-genomic actions of steroid hormones. Here we compare these recently identified receptors with the genomic, non-genomic and mitochondrial actions of thyroid hormones and consider their implications.