Polyribosome and ribonucleoprotein complex redistribution of mRNA induced by GnRH involves both EIF2AK3 and MAPK signaling

Lipopolysaccharide-induced chronic inflammation increases female serum gonadotropins and shifts the pituitary transcriptomic landscape

Introduction

Female reproductive function depends on a choreographed sequence of hormonal secretion and action, where specific stresses such as inflammation exert profound disruptions. Specifically, acute LPS-induced inflammation inhibits gonadotropin production and secretion from the pituitary, thereby impacting the downstream production of sex hormones. These outcomes have only been observed in acute inflammatory stress and little is known about the mechanisms by which chronic inflammation affects reproduction. In this study we seek to understand the chronic effects of LPS on pituitary function and consequent luteinizing and follicle stimulating hormone secretion.

Methods

A chronic inflammatory state was induced in female mice by twice weekly injections with LPS over 6 weeks. Serum gonadotropins were measured and bulk RNAseq was performed on the pituitaries from these mice, along with basic measurements of reproductive biology.

Results

Surprisingly, serum luteinizing and follicle stimulating hormone was not inhibited and instead we found it was increased with repeated LPS treatments.

Discussion

Analysis of bulk RNA-sequencing of murine pituitary revealed paracrine activation of TGFβ pathways as a potential mechanism regulating FSH secretion in response to chronic LPS. These results provide a framework with which to begin dissecting the impacts of chronic inflammation on reproductive physiology.

SRXN1 Is Necessary for Resolution of GnRH-Induced Oxidative Stress and Induction of Gonadotropin Gene Expression

A defining characteristic of the hypothalamus-pituitary-gonad reproductive endocrine axis is the episodic secretion of the pituitary gonadotropin hormones LH and FSH by the anterior pituitary gonadotropes. Hormone secretion is dictated by pulsatile stimulation, with GnRH released by hypothalamic neurons that bind and activate the G protein-coupled GnRH receptor expressed by gonadotropes. Hormone secretion and synthesis of gonadotropins are influenced by the amplitude and frequency of GnRH stimulation; variation in either affects the proportion of LH and FSH secreted and the differential regulation of hormone subunit gene expression. Therefore, proper decoding of GnRH signals is essential for appropriate gonadotropin synthesis and secretion. The GnRH receptor robustly activates downstream signaling cascades to facilitate exocytosis and stimulate gene expression and protein synthesis. It is necessary to rapidly quench signaling to preserve sensitivity and adaptability to changing pulse patterns. Reactive oxygen species (ROS) generated by receptor-activated oxidases fulfill the role of rapid signaling intermediates that facilitate robust and transient signaling. However, excess ROS can be detrimental and, unchecked, can confuse signal interpretation. We demonstrate that sulfiredoxin (SRXN1), an ATP-dependent reductase, is essential for normal responses to GnRH receptor signaling and plays a central role in resolution of ROS induced by GnRH stimulation. SRXN1 expression is mitogen-activated protein kinase dependent, and knockdown reduces Lhb and Fshb glycoprotein hormone subunit mRNA and promoter activity. Loss of SRXN1 leads to increased basal and GnRH-stimulated ROS levels. We conclude that SRXN1 is essential for normal responses to GnRH stimulation and plays an important role in ROS management.

The RNA-Binding Protein ELAVL1 Regulates GnRH Receptor Expression and the Response to GnRH

Gonadotropin secretion, which is elicited by GnRH stimulation of the anterior pituitary gonadotropes, is a critical feature of reproductive control and the maintenance of fertility. In addition, activation of the GnRH receptor (GnRHR) regulates transcription and translation of multiple factors that regulate the signaling response and synthesis of gonadotropins. GnRH stimulation results in a broad redistribution of mRNA between active and inactive polyribosomes within the cell, but the mechanism of redistribution is not known. The RNA-binding protein embryonic lethal, abnormal vision, Drosophila-like 1 (ELAVL1) binds to AU-rich elements in mRNA and is one of the most abundant mRNA-binding proteins in eukaryotic cells. It is known to serve as a core component of RNA-binding complexes that direct the fate of mRNA. In LβT2 gonadotropes, we showed that ELAVL1 binds to multiple mRNAs encoding factors that are crucial for gonadotropin synthesis and release. Association with some mRNAs is GnRH sensitive but does not correlate with abundance of binding. We also showed MAPK-dependent changes in intracellular localization of ELAVL1 in response to GnRH stimulation. Knockdown of ELAVL1 gene expression resulted in reduced Lhb and Gnrhr mRNA levels, reduced cell surface expression of GnRHR, and reduced LH secretion in response to GnRH stimulation. Overall, these observations not only support the role of ELAVL1 in GnRHR-mediated regulation of gene expression and LH secretion but also indicate that other factors may contribute to the precise fate of mRNA in response to GnRH stimulation of gonadotropes.

Inducible Genome Editing with Conditional CRISPR/Cas9 Mice

Genetically engineered mouse models (GEMMs) are powerful tools by which to probe gene function in vivo, obtain insight into disease etiology, and identify modifiers of drug response. Increased sophistication of GEMMs has led to the design of tissue-specific and inducible models in which genes of interest are expressed or ablated in defined tissues or cellular subtypes. Here we describe the generation of a transgenic mouse harboring a doxycycline-regulated Cas9 allele for inducible genome engineering. This model provides a flexible platform for genome engineering since editing is achieved by exogenous delivery of sgRNAs and should allow for the modeling of a range of biological and pathological processes.

Induction of Stress Signaling In Vitro and Suppression of Gonadotropin Secretion by Free Fatty Acids in Female Mouse Gonadotropes

An emerging body of evidence supports the concept that the pituitary is a site for integration of multiple physiological and metabolic signals that inform and modulate endocrine pathways. Multiple endocrine mediators of energy balance and adiposity are known to impinge on the neuroendocrine axis regulating reproduction. Observations in humans show that obesity is correlated with decreased gonadotropin secretion, and studies have also suggested that pituitary sensitivity to stimulation by gonadotropin-releasing hormone (GnRH) is decreased in obese individuals. Free fatty acids are a potential mediator of adiposity and energy balance, but their impact as an endocrine modulator of pituitary function has not been closely examined. We evaluated the impact of free fatty acids on a pituitary gonadotrope cell line and in primary pituitary cultures of female mice. We show that increasing physiologically relevant doses of the monounsaturated ω-9 fatty acid oleate induces cellular stress and increases production of reactive oxygen species in a mouse gonadotrope cell line. In contrast, the unsaturated ω-3 α-linolenic and ω-6 linoleic fatty acids do not have this effect. Additionally, oleate can activate immediate-early gene expression independent of GnRH stimulation but has a negative impact on GnRH induction and expression of the gonadotropin subunit gene Lhb. Further, oleate suppresses gonadotropin secretion in response to pulsatile stimulation by GnRH. These results indicate that free fatty acids can directly alter gonadotropin gene expression and secretion in response to GnRH and may provide a link between energy sensing and reproduction.

[Investigation methods to explore G protein-coupled receptor-regulated translatome]

With the advent of next-generation sequencing technologies, identifying the translatome, which includes genome-wide ribosome-associated mRNAs, provides new opportunities to define faithfully the protein repertoire of a cell, as opposed to transcriptomic approaches. In addition, the role that extracellular signals such as hormonal modulations could play on the translatome remains to be deciphered. In particular, the regulation of the translatome by G protein-coupled receptors (GPCR) is still poorly described, albeit the trophic role that many receptors of this family play in their target cells. Here, we provide an overview of the current methods that are used to study the translatome, applied to the GPCR receptor family.

G Protein-Coupled Receptors As Regulators of Localized Translation: The Forgotten Pathway?

G protein-coupled receptors (GPCRs) exert their physiological function by transducing a complex signaling network that coordinates gene expression and dictates the phenotype of highly differentiated cells. Much is known about the gene networks they transcriptionally regulate upon ligand exposure in a process that takes hours before a new protein is synthesized. However, far less is known about GPCR impact on the translational machinery and subsequent mRNA translation, although this gene regulation level alters the cell phenotype in a strikingly different timescale. In fact, mRNA translation is an early response kinetically connected to signaling events, hence it leads to the synthesis of a new protein within minutes following receptor activation. By these means, mRNA translation is responsive to subtle variations of the extracellular environment. In addition, when restricted to cell subcellular compartments, local mRNA translation contributes to cell micro-specialization, as observed in synaptic plasticity or in cell migration. The mechanisms that control where in the cell an mRNA is translated are starting to be deciphered. But how an extracellular signal triggers such local translation still deserves extensive investigations. With the advent of high-throughput data acquisition, it now becomes possible to review the current knowledge on the translatome that some GPCRs regulate, and how this information can be used to explore GPCR-controlled local translation of mRNAs.

Gonadotropin releasing hormone activation of the mTORC2/Rictor complex regulates actin remodeling and ERK activity in LβT2 cells

The mammalian target of rapamycin (mTOR) assembles into two different multi-protein complexes, mTORC1 and mTORC2. The mTORC2 complex is distinct due to the unique expression of the specific core regulatory protein Rictor (rapamycin-insensitive companion of mTOR). mTORC2 has been implicated in regulating actin cytoskeletal reorganization but its role in gonadotrope function is unknown. Using the gonadotrope-derived LβT2 cell line, we find that the GnRH agonist buserelin (GnRHa) phosphorylates both mTOR and Rictor. Interestingly, inhibition of mTORC2 blunts GnRHa-induced cyto-architectural rearrangements. Coincident with blunting of actin reorganization, inhibition of mTORC2 also attenuates GnRHa-mediated activation of both protein kinase C (PKC) and extracellular signal regulated kinase (ERK). Collectively, our data suggests that GnRHa-mediated mTORC2 activation is important in facilitating actin reorganization events critical for initiating PKC activity and subsequent ERK phosphorylation in the gonadotrope-derived LβT2 cell line.

Reactive Oxygen Species Link Gonadotropin-Releasing Hormone Receptor Signaling Cascades in the Gonadotrope

Biological rhythms lie at the center of regulatory schemes that control many aspects of living systems. At the cellular level, meaningful responses to external stimuli depend on propagation and quenching of a signal to maintain vigilance for subsequent stimulation or changes that serve to shape and modulate the response. The hypothalamus-pituitary-gonad endocrine axis that controls reproductive development and function relies on control through rhythmic stimulation. Central to this axis is the pulsatile stimulation of the gonadotropes by hypothalamic neurons through episodic release of the neuropeptide gonadotropin-releasing hormone. Alterations in pulsatile stimulation of the gonadotropes result in differential synthesis and secretion of the gonadotropins LH and FSH and changes in the expression of their respective hormone subunit genes. The requirement to amplify signals arising from activation of the gonadotropin-releasing hormone (GnRH) receptor and to rapidly quench the resultant signal to preserve an adaptive response suggests the need for rapid activation and feedback control operating at the level of intracellular signaling. Emerging data suggest that reactive oxygen species (ROS) can fulfill this role in the GnRH receptor signaling through activation of MAP kinase signaling cascades, control of negative feedback, and participation in the secretory process. Results obtained in gonadotrope cell lines or other cell models indicate that ROS can participate in each of these regulatory cascades. We discuss the potential advantage of reactive oxygen signaling for modulating the gonadotrope response to GnRH stimulation and the potential mechanisms for this action. These observations suggest further targets of study for regulation in the gonadotrope.

Intrinsic and Regulated Gonadotropin-Releasing Hormone Receptor Gene Transcription in Mammalian Pituitary Gonadotrophs

The hypothalamic decapeptide gonadotropin-releasing hormone (GnRH), acting via its receptors (GnRHRs) expressed in pituitary gonadotrophs, represents a critical molecule in control of reproductive functions in all vertebrate species. GnRH-activated receptors regulate synthesis of gonadotropins in a frequency-dependent manner. The number of GnRHRs on the plasma membrane determines the responsiveness of gonadotrophs to GnRH and varies in relation to age, sex, and physiological status. This is achieved by a complex control that operates at transcriptional, translational, and posttranslational levels. This review aims to overview the mechanisms of GnRHR gene (Gnrhr) transcription in mammalian gonadotrophs. In general, Gnrhr exhibits basal and regulated transcription activities. Basal Gnrhr transcription appears to be an intrinsic property of native and immortalized gonadotrophs that secures the presence of a sufficient number GnRHRs to preserve their functionality independently of the status of regulated transcription. On the other hand, regulated transcription modulates GnRHR expression during development, reproductive cycle, and aging. GnRH is crucial for regulated Gnrhr transcription in native gonadotrophs but is ineffective in immortalized gonadotrophs. In rat and mouse, both basal and GnRH-induced Gnrhr transcription rely primarily on the protein kinase C signaling pathway, with subsequent activation of mitogen-activated protein kinases. Continuous GnRH application, after a transient stimulation, shuts off regulated but not basal transcription, suggesting that different branches of this signaling pathway control transcription. Pituitary adenylate cyclase-activating polypeptide, but not activins, contributes to the regulated transcription utilizing the protein kinase A signaling pathway, whereas a mechanisms by which steroid hormones modulate Gnrhr transcription has not been well characterized.

GnRH Regulates Gonadotropin Gene Expression Through NADPH/Dual Oxidase-Derived Reactive Oxygen Species

The appropriate control of synthesis and secretion of the gonadotropin hormones LH and FSH by pituitary gonadotropes is essential for the regulation of reproduction. The hypothalamic neuropeptide GnRH is the central regulator of both processes, coordinating secretion with transcription and translation of the gonadotropin hormone subunit genes. The MAPK family of second messengers is strongly induced in gonadotropes upon GnRH stimulation, and multiple pathways activate these kinases. Intracellular reactive oxygen species participate in signaling cascades that target MAPKs, but also participate in signaling events indicative of cell stress. The NADPH oxidase (NOX)/dual oxidase (DUOX) family is a major enzymatic source of intracellular reactive oxygen, and we show that GnRH stimulation of mouse primary pituitary cells and the LβT2 gonadotrope cell line elevates intracellular reactive oxygen via NOX/DUOX activity. Mouse pituitary and LβT2 cells abundantly express NOX/DUOX and cofactor mRNAs. Pharmacological inhibition of NOX/DUOX activity diminishes GnRH-stimulated activation of MAPKs, immediate-early gene expression, and gonadotropin subunit gene expression. Inhibitor studies implicate the calcium-activated DUOX family as a major, but not exclusive, participant in GnRH signaling. Knockdown of DUOX2 in LβT2 cells reduces GnRH-induced Fshb, but not Lhb mRNA levels, suggesting differential sensitivity to DUOX activity. Finally, GnRH pulse-stimulated FSH and LH secretion are suppressed by inhibition of NOX/DUOX activity. These results indicate that reactive oxygen is a potent signaling intermediate produced in response to GnRH stimulation and further suggest that reactive oxygen derived from other sources may influence the gonadotrope response to GnRH stimulation.