Expression and function of the luteinizing hormone choriogonadotropin receptor in human endometrial stromal cells

The human luteinising hormone choriogonadotropin receptor (LHCGR) is a G-protein coupled receptor activated by both human chorionic gonadotropin (hCG) and luteinizing hormone (LH), two structurally related gonadotropins with essential roles in ovulation and maintenance of the corpus luteum. LHCGR expression predominates in ovarian tissues where it elicits functional responses through cyclic adenosine mononucleotide (cAMP), Ca²⁺ and extracellular signal-regulated kinase (ERK) signalling. LHCGR expression has also been localized to the human endometrium, with purported roles in decidualization and implantation. However, these observations are contentious. In this investigation, transcripts encoding LHCGR were undetectable in bulk RNA sequencing datasets from whole cycling endometrial tissue and cultured human endometrial stromal cells (EnSC). However, analysis of single-cell RNA sequencing data revealed cell-to-cell transcriptional heterogeneity, and we identified a small subpopulation of stromal cells with detectable LHCGR transcripts. In HEK-293 cells expressing recombinant LHCGR, both hCG and LH elicited robust cAMP, Ca²⁺ and ERK signals that were absent in wild-type HEK-293 cells. However, none of these responses were recapitulated in primary EnSC cultures. In addition, proliferation, viability and decidual transformation of EnSC were refractory to both hCG and LH, irrespective of treatment to induce differentiation. Although we challenge the assertion that LHCGR is expressed at a functionally active level in the human endometrium, the discovery of a discrete subpopulation of EnSC that express LHCGR transcripts may plausibly account for the conflicting evidence in the literature.

Reduced FSH and LH action: implications for medically assisted reproduction

Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) play complementary roles in follicle development and ovulation via a complex interaction in the hypothalamus, anterior pituitary gland, reproductive organs, and oocytes. Impairment of the production or action of gonadotropins causes relative or absolute LH and FSH deficiency that compromises gametogenesis and gonadal steroid production, thereby reducing fertility. In women, LH and FSH deficiency is a spectrum of conditions with different functional or organic causes that are characterized by low or normal gonadotropin levels and low oestradiol levels. While the causes and effects of reduced LH and FSH production are very well known, the notion of reduced action has received less attention by researchers. Recent evidence shows that molecular characteristics, signalling as well as ageing, and some polymorphisms negatively affect gonadotropin action. These findings have important clinical implications, in particular for medically assisted reproduction in which diminished action determined by the afore-mentioned factors, combined with reduced endogenous gonadotropin production caused by GnRH analogue protocols, may lead to resistance to gonadotropins and, thus, to an unexpected hypo-response to ovarian stimulation. Indeed, the importance of LH and FSH action has been highlighted by the International Committee for Monitoring Assisted Reproduction Technologies (ICMART) in their definition of hypogonadotropic hypogonadism as gonadal failure associated with reduced gametogenesis and gonadal steroid production due to reduced gonadotropin production or action. The aim of this review is to provide an overview of determinants of reduced FSH and LH action that are associated with a reduced response to ovarian stimulation.

The direct and indirect effects of kisspeptin-54 on granulosa lutein cell function

STUDY QUESTION

What are the in vivo and in vitro actions of kisspeptin-54 on the expression of genes involved in ovarian reproductive function, steroidogenesis and ovarian hyperstimulation syndrome (OHSS) in granulosa lutein (GL) cells when compared with traditional triggers of oocyte maturation?

SUMMARY ANSWER

The use of kisspeptin-54 as an oocyte maturation trigger augmented expression of genes involved in ovarian steroidogenesis in human GL cells including, FSH receptor (FSHR), LH/hCG receptor (LHCGR), steroid acute regulatory protein (STAR), aromatase, estrogen receptors alpha and beta (ESR1, ESR2), 3-beta-hydroxysteroid dehydrogenase type 2 (3BHSD2) and inhibin A (INHBA), when compared to traditional maturation triggers, but did not alter markers of OHSS.

WHAT IS KNOWN ALREADY

hCG is the most widely used trigger of oocyte maturation, but is associated with an increased risk of OHSS. The use of GnRH agonists to trigger oocyte maturation is a safer alternative to hCG. More recently, kisspeptin-54 has emerged as a novel therapeutic option that safely triggers oocyte maturation even in women at high risk of OHSS. Kisspeptin indirectly stimulates gonadotropin secretion by acting on hypothalamic GnRH neurons. Kisspeptin and its receptor are also expressed in the human ovary, but there is limited data on the direct action of kisspeptin on the ovary.

STUDY DESIGN SIZE, DURATION

Forty-eight women undergoing IVF treatment for infertility consented to kisspeptin-54 triggering and/or granulosa cell collection and were included in the study. Twelve women received hCG, 12 received GnRH agonist and 24 received kisspeptin-54 to trigger oocyte maturation. In the kisspeptin-54 group, 12 received one injection of kisseptin-54 (9.6 nmol/kg) and 12 received two injections of kisspeptin-54 at a 10 h interval (9.6 nmol/kg × 2).

PARTICIPANTS/MATERIALS, SETTING, METHODS

Follicular fluid was aspirated and pooled from follicles during the retrieval of oocytes for IVF/ICSI. GL cells were isolated and either RNA extracted immediately or cultured in vitro ± kisspeptin or hCG.

MAIN RESULTS AND THE ROLE OF CHANCE

GL cells from women who had received kisspeptin-54 had a 14-fold and 8-fold higher gene expression of FSHR and a 2-fold (ns) and 2.5-fold (P < 0.05) higher expression of LHCGR than GL cells from women who had received hCG or GnRH agonist, respectively. CYP19A1 expression was 3.6-fold (P < 0.05) and 4.5-fold (P < 0.05) higher, STAR expression was 3.4-fold (P < 0.01) and 1.8-fold (P < 0.05) higher, HSD3B2 expression was 7.5- (P < 0.01) and 2.5-fold higher (P < 0.05), INHBA was 2.5-fold (P < 0.01) and 2.5-fold (P < 0.01) higher in GL cells from women who had received kisspeptin-54 than hCG or GnRHa, respectively. ESR1 (P < 0.05) and ESR2 (P < 0.05) both showed 3-fold higher expression in cells from kisspeptin treated than GnRHa treated women. Markers of vascular permeability and oocyte growth factors were unchanged (VEGFA, SERPINF1, CDH5, amphiregulin, epiregulin). Gene expression of kisspeptin receptor was unchanged. Whereas treating GL cells in vitro with hCG induced steroidogenic gene expression, kisspeptin-54 had no significant direct effects on either OHSS genes or steroidogenic genes.

LIMITATIONS REASONS FOR CAUTION

Most women in the study had PCOS, which may limit applicability to other patient groups. For the analysis of the in vitro effects of kisspeptin-54, it is important to note that GL cells had already been exposed in vivo to an alternate maturation trigger.

WIDER IMPLICATIONS OF THE FINDINGS

The profile of serum gonadotropins seen with kisspeptin administration compared to other triggers more closely resemble that of the natural cycle as compared with hCG. Thus, kisspeptin could potentially permit an ovarian environment augmented for steroidogenesis, in particular progesterone synthesis, which is required for embryo implantation.

STUDY FUNDING/COMPETING INTEREST(S)

Dr Owens is supported by an Imperial College London PhD Scholarship. Dr Abbara is supported by an National Institute of Health Research Academic Clinical Lectureship. The authors do not have any conflict of interest to declare.

TRIAL REGISTRATION NUMBER

ClinicalTrials.gov NCT01667406.

Temporal reprogramming of calcium signalling via crosstalk of gonadotrophin receptors that associate as functionally asymmetric heteromers

Signal crosstalk between distinct G protein-coupled receptors (GPCRs) is one mechanism that underlies pleiotropic signalling. Such crosstalk is also pertinent for GPCRs activated by gonadotrophic hormones; follicle-stimulating hormone (FSH) and luteinising hormone (LH), with specific relevance to female reproduction. Here, we demonstrate that gonadotrophin receptor crosstalk alters LH-induced Gαq/11-calcium profiles. LH-induced calcium signals in both heterologous and primary human granulosa cells were prolonged by FSHR coexpression via influx of extracellular calcium in a receptor specific manner. LHR/FSHR crosstalk involves Gαq/11 activation as a Gαq/11 inhibitor abolished calcium responses. Interestingly, the enhanced LH-mediated calcium signalling induced by FSHR co-expression was dependent on intracellular calcium store release and involved Gβγ. Biophysical analysis of receptor and Gαq interactions indicated that ligand-dependent association between LHR and Gαq was rearranged in the presence of FSHR, enabling FSHR to closely associate with Gαq following LHR activation. This suggests that crosstalk may occur via close associations as heteromers. Super-resolution imaging revealed that LHR and FSHR formed constitutive heteromers at the plasma membrane. Intriguingly, the ratio of LHR:FSHR in heterotetramers was specifically altered following LH treatment. We propose that functionally significant FSHR/LHR crosstalk reprograms LH-mediated calcium signalling at the interface of receptor-G protein via formation of asymmetric complexes.

Impact of G protein-coupled receptor heteromers in endocrine systems

The fine-tuning of endocrine homeostasis is regulated by dynamic receptor mediated processes. The superfamily of G protein-coupled receptors (GPCRs) have diverse roles in the modulation of all endocrine axes, thus understanding the mechanisms underpinning their functionality is paramount for treatment of endocrinopathies. Evidence over the last 20 years has highlighted homo and heteromerization as a key mode of mediating GPCR functional diversity. This review will discuss the concept of GPCR heteromerization and its relevance to endocrine function, detailing in vitro and in vivo evidence, and exploring current and potential pharmacological strategies for specific targeting of GPCR heteromers in endocrine heath and disease.

Identification of transmembrane domains that regulate spatial arrangements and activity of prokineticin receptor 2 dimers

The chemokine prokineticin 2 (PK2) activates its cognate G protein-coupled receptor (GPCR) PKR2 to elicit various downstream signaling pathways involved in diverse biological processes. Many GPCRs undergo dimerization that can modulate a number of functions including membrane delivery and signal transduction. The aim of this study was to elucidate the interface of PKR2 protomers within dimers by analyzing the ability of PKR2 transmembrane (TM) deletion mutants to associate with wild type (WT) PKR2 in yeast using co-immunoprecipitation and mammalian cells using bioluminescence resonance energy transfer. Deletion of TMs 5-7 resulted in a lack of detectable association with WT PKR2, but could associate with a truncated mutant lacking TMs 6-7 (TM1-5). Interestingly, TM1-5 modulated the distance, or organization, between protomers and positively regulated Gαs signaling and surface expression of WT PKR2. We propose that PKR2 protomers form type II dimers involving TMs 4 and 5, with a role for TM5 in modulation of PKR2 function.

The short chain fatty acid propionate stimulates GLP-1 and PYY secretion via free fatty acid receptor 2 in rodents

Background and Objectives:

The gut hormones peptide YY (PYY) and glucagon-like peptide 1 (GLP-1) acutely suppress appetite. The short chain fatty acid (SCFA) receptor, free fatty acid receptor 2 (FFA2) is present on colonic enteroendocrine L cells, and a role has been suggested for SCFAs in appetite regulation. Here, we characterise the in vitro and in vivo effects of colonic propionate on PYY and GLP-1 release in rodents, and investigate the role of FFA2 in mediating these effects using FFA2 knockout mice.

Methods:

We used Wistar rats, C57BL6 mice and free fatty acid receptor 2 knockout (FFA^−/−) mice on a C57BL6 background to explore the impact of the SCFA propionate on PYY and GLP-1 release. Isolated colonic crypt cultures were used to assess the effects of propionate on gut hormone release in vitro. We subsequently developed an in vivo technique to assess gut hormone release into the portal vein following colonic infusion of propionate.

Results:

Propionate stimulated the secretion of both PYY and GLP-1 from wild-type primary murine colonic crypt cultures. This effect was significantly attenuated in cultures from FFA2^−/− mice. Intra-colonic infusion of propionate elevated PYY and GLP-1 levels in jugular vein plasma in rats and in portal vein plasma in both rats and mice. However, propionate did not significantly stimulate gut hormone release in FFA2^−/− mice.

Conclusions:

Intra-colonic administration of propionate stimulates the concurrent release of both GLP-1 and PYY in rats and mice. These data demonstrate that FFA2 deficiency impairs SCFA-induced gut hormone secretion both in vitro and in vivo.

Regulation of GPCR signal networks via membrane trafficking

G-protein-coupled receptors (GPCRs) are a superfamily of cell surface signaling proteins that act as central molecular activators and integrators in all endocrine systems. Membrane trafficking of GPCRs is a fundamental process in shaping extensive signaling networks activated by these receptors. Mounting evidence has identified an increasingly complex network of pathways and protein interactions that a GPCR can traverse and associate with, indicating a multi-level system of regulation. This review will discuss the recent developments in how GPCRs are trafficked to the cell surface as newly synthesized receptors, their recruitment to the clathrin-mediated pathway for endocytosis, and their sorting to subsequent divergent post-endocytic fates, focusing primarily on hormone-activated GPCRs. Current models depicting the classic roles membrane trafficking plays in GPCR signaling have evolved to a highly regulated and complex system than previously appreciated. These developments impart key mechanistic information on how spatial and temporal aspects of GPCR signaling may be integrated and could provide pathway-specific targets to be exploited for therapeutic intervention.

Casein kinase II sites in the intracellular C-terminal domain of the thyrotropin-releasing hormone receptor and chimeric gonadotropin-releasing hormone receptors contribute to beta-arrestin-dependent internalization

We have previously shown that the mammalian gonadotropin-releasing hormone receptor (GnRHR), a unique G-protein-coupled receptor (GPCR) lacking an intracellular carboxyl tail (C-tail), does not follow a beta-arrestin-dependent internalization pathway. However, internalization of a chimeric GnRHR with the thyrotropin-releasing hormone receptor (TRHR) C-tail does utilize beta-arrestin. Here, we have investigated the sites within the intracellular C-tail domain that are important for conferring beta-arrestin-dependent internalization. In contrast to the chimeric GnRHR with a TRHR C-tail, a chimeric GnRHR with the catfish GnRHR C-tail is not beta-arrestin-dependent. Sequence comparisons between these chimeric receptors show three consensus phosphorylation sites for casein kinase II (CKII) in the TRHR C-tail but none in the catfish GnRHR C-tail. We thus investigated a role for CKII sites in determining GPCR internalization via beta-arrestin. Sequential introduction of three CKII sites into the chimera with the catfish C-tail (H354D,A366E,G371D) resulted in a change in the pattern of receptor phosphorylation and beta-arrestin-dependence, which only occurred when all three sites were introduced. Conversely, mutation of the putative CKII sites (T365A,T371A,S383A) in the C-tail of a beta-arrestin-sensitive GPCR, the TRHR, resulted in decreased receptor phosphorylation and a loss of beta-arrestin-dependence. Mutation of all three CKII sites was necessary before a loss of beta-arrestin-dependence was observed. Visualization of beta-arrestin/GFP redistribution confirmed a loss or gain of beta-arrestin sensitivity for receptor mutants. Internalization of receptors without C-tail CKII sites was promoted by a phosphorylation-independent beta-arrestin mutant (R169E), suggesting that these receptors do not contain the necessary phosphorylation sites required for beta-arrestin-dependent internalization. Apigenin, a specific CKII inhibitor, blocked the increase in receptor internalization by beta-arrestin, thus providing further support for the involvement of CKII. This study presents evidence of a novel role for C-tail CKII consensus sites in targeting these GPCRs to the beta-arrestin-dependent pathway.

Constitutive and agonist-dependent homo-oligomerization of the thyrotropin-releasing hormone receptor. Detection in living cells using bioluminescence resonance energy transfer

The ability of G-protein-coupled receptors (GPCRs) to interact to form new functional structures, either forming oligomers with themselves or forming associations with other intracellular proteins, has important implications for the regulation of cellular events; however, little is known about how this occurs. Here, we have employed a newly emerging technology, bioluminescence resonance energy transfer (BRET), used to study protein-protein interactions in living cells, to demonstrate that the thyrotropin-releasing hormone receptor (TRHR) forms constitutive homo-oligomers. This formation of TRHR homo-oligomers in the absence of ligand was shown by demonstration of an energy transfer between TRHR molecules fused to either donor, Renilla luciferase (Rluc) or acceptor, enhanced yellow fluorescent protein (EYFP) molecules. This interaction was shown to be specific, since energy transfer was not detected between co-expressed tagged TRHRs and either complementary tagged gonadotropin-releasing hormone (GnRH) or beta(2)-adrenergic receptors. Furthermore, generation of a BRET signal between the TRHRs could only be inhibited by co-expression of the wild-type TRHR and not by other GPCRs. Agonist stimulation led to a time- and dose-dependent increase in the amount of energy transfer. Inhibition of receptor internalization by co-expression of dynamin mutant K44A did not affect the interaction between TRHRs, suggesting that clustering of receptors within clathrin-coated pits is not sufficient for energy transfer to occur. BRET also provided evidence for the agonist-induced oligomerization of another GPCR, the GnRH receptor (GnRHR), and the presence of an agonist-induced interaction of the adaptor protein, beta-arrestin, with TRHR and the absence of an interaction of beta-arrestin with GnRHR. This study supports the usefulness of BRET as a powerful tool for studying GPCR aggregations and receptor/protein interactions in general and presents evidence that the functioning unit of TRHRs exists as homomeric complexes.

The rat gonadotropin-releasing hormone receptor internalizes via a beta-arrestin-independent, but dynamin-dependent, pathway: addition of a carboxyl-terminal tail confers beta-arrestin dependency

This study examined the mechanism underlying the rat GnRH receptor (GnRH-R) internalization pathway by investigating the role of added/extended C-terminal tails and the effect of beta-arrestins and dynamin. The internalization of the wild-type (WT) rat GnRH-R, stop codon mutants, GnRH-R/TRH receptor (TRH-R) chimera, rat TRH-R, and catfish GnRH-R was examined using radioligand binding assay. Overexpression of beta-arrestin in COS-7 cells expressing each of the receptor constructs substantially increased endocytosis rate constants (k(e)) of the TRH-R, catfish GnRH-R, and GnRH-R/TRH-R chimera, but not of the WT rat GnRH-R and stop codon mutants. The beta-arrestin-promoted increase in the k(e) value was diminished by cotransfecting cells with the dominant negative beta-arrestin-(319-418) mutant, whereas WT GnRH-R and stop codon mutant internalization were unaffected. Additionally, confocal microscopy showed that activated GnRH-Rs failed to induce time-dependent redistribution of either beta-arrestin-1- or beta-arrestin-2-green fluorescent protein conjugate to the plasma membrane. However, the dominant negative dynamin (DynK44A) mutant impaired internalization of all of the receptors regardless of their beta-arrestin dependency, indicating that they internalize via a clathrin-mediated pathway. We conclude that the mammalian GnRH-R uses a beta-arrestin-independent, dynamin-dependent internalization mechanism distinct from that employed by the other receptors studied.